Ionone Is More Than a Violet's Fragrance

molecules

Review Ionone Is More than a Violet’s Fragrance: A Review

Lujain Aloum 1 , Eman Aleﬁshat 1,2 , Abdu Adem 1 and Georg Petroianu 1,*

1 Department of Pharmacology, College of Medicine and Health Sciences, Khalifa University of Science and Technology, Abu Dhabi 127788, UAE; [email protected] (L.A.); Eman.aleﬁ[email protected] (E.A.); [email protected] (A.A.) 2 Center for Biotechnology, Khalifa University of Science and Technology, Abu Dhabi 127788, UAE * Correspondence: [email protected]; Tel.: +971-50-413-4525

Academic Editor: Domenico Montesano Received: 25 November 2020; Accepted: 8 December 2020; Published: 10 December 2020

Abstract: The term ionone is derived from “iona” (Greek for violet) which refers to the violet scent and “ketone” due to its structure. Ionones can either be chemically synthesized or endogenously produced via asymmetric cleavage of β-carotene by β-carotene oxygenase 2 (BCO2). We recently proposed a possible metabolic pathway for the conversion of α-and β-pinene into α-and β-ionone. The differences between BCO1 and BCO2 suggest a unique physiological role of BCO2; implying that β-ionone (one of BCO2 products) is involved in a prospective biological function. This review focuses on the effects of ionones and the postulated mechanisms or signaling cascades involved mediating these effects. β-Ionone, whether of an endogenous or exogenous origin possesses a range of pharmacological effects including anticancer, chemopreventive, cancer promoting, melanogenesis, anti-inflammatory and antimicrobial actions. β-Ionone mediates these effects via activation of olfactory receptor (OR51E2) and regulation of the activity or expression of cell cycle regulatory proteins, pro-apoptotic and anti-apoptotic proteins, HMG-CoA reductase and pro-inflammatory mediators. α-Ionone and β-ionone derivatives exhibit anti-inflammatory, antimicrobial and anticancer effects, however the corresponding structure activity relationships are still inconclusive. Overall, data demonstrates that ionone is a promising scaffold for cancer, inflammation and infectious disease research and thus is more than simply a violet’s fragrance.

Keywords: ionone; biological activity; BCO2; ionone derivatives; cancer; inﬂammation; OR51E2

1. Chemical Structure, Physicochemical Properties and Natural Occurrence Ionone is a ketone compound composed of 13 carbons with a monocyclic terpenoid backbone [1]. The term ionone derived from “iona” (Greek for violet) referring to the violet scent and “ketone” in relation to its structure [2]. Several isomeric ionones exist in Nature, including α-ionone, β-ionone and others. Ionones are found as secondary plant metabolism products that share mevalonic acid as a common precursor [3]. They are widely present in fruits and vegetables comprising β-carotene and are found in plant oils, for example oils from Petunia hybrida, Boronia megastigma Nees and especially Viola odorata [4–7]. It can be found in cow’s milk where is passively transferred after consumption of alfalfa pasture [8,9]. α-Ionone and β-ionone are 3-buten-2-ones substituted by 2,6,6-trimethyl-2-cyclohexen-1-yl and 2,6,6-trimethyl-1-cyclohexen-1-yl groups respectively, at position 4 (Table1)[ 10,11]. The ionone stereoisomers, α-ionone and β-ionone, are pale yellow-to-yellow liquids [12] with a woody ﬂoral scent. However, the former has an extra honey olfactory aspect [13]. α-Ionone and β-ionone share similar physicochemical properties [10–12,14–18]. They are both soluble in most ﬁxed oils, alcohol and propylene glycol but insoluble/very slightly soluble in glycerin/water, respectively [12].

Molecules 2020, 25, 5822; doi:10.3390/molecules25245822 www.mdpi.com/journal/molecules Molecules 2020, 25, 5822 2 of 25 MoleculesMolecules 2020, 202025, x, FOR25, x PEERFOR PEER REVIEW REVIEW 2 of 262 of 26

Molecules 2020, 25, x FOR PEER REVIEW 2 of 26 Table 1. TheTable chemicalTable 1. The 1. chemicalThe structure chemical structure of structureα-ionone of α-ionone of and α-iononeβ and-ionone. βand-ionone. β-ionone. Table 1. Theα chemical-Iononeα-Ionone structure of α-iononeβ -Iononeandβ -Iononeβ-ionone. ReferenceReference α-Ionone β-Ionone Reference

α-Ionone O O β-Ionone O O Reference ChemicalChemical O Chemicalstructure structure depiction O [10,11[10,11]] [10,11] Chemical depictiondepiction structure [10,11]

depiction

α-Ionone andα-Iononeβα-ionone-Ionone and are andβ-ionone both β-ionone widely are areboth used both widely as aromawidely used ingredientsused as aromaas aroma iningredients various ingredients industries, in variousin variousincluding industries, industries, includingincluding cosmetics cosmetics (e.g., (e.g., shampoos, shampoos, soaps) soaps) and andnon-cosmetic non-cosmetic (e.g., (e.g., household household detergents detergents and and cosmetics (e.g., shampoos, soaps) and non-cosmetic (e.g., household detergents and cleaners) products. α-Iononecleaners)cleaners) and products. β-ionone products. The are TheFlavor both Flavor andwidely andExtract Extractused Manufa as Manufa aromacturer’scturer’s ingredients Association Association in(FEMA) various (FEMA) declares industries, declares that thatboth both The Flavor and Extract Manufacturer’s Association (FEMA) declares that both ionones are Generally includingionones cosmeticsionones are Generallyare(e.g., Generally shampoos, Recognized Recognized soaps) As Safe Asand Safe(GRAS) non-cosmetic (GRAS) when when used (e.g., used as flavoring householdas flavoring agents. detergentsagents. The TheUS FDA andUS FDA has has Recognized As Safe (GRAS) when used as flavoring agents. The US FDA has also approved their use cleaners) alsoproducts. alsoapproved approved The their Flavor their use andasuse flavoring asExtract flavoring agentsManufa agents [10,11].cturer’s [10,11]. Association (FEMA) declares that both asionones flavoring are agents Generally [10,11 Recognized]. As Safe (GRAS) when used as flavoring agents. The US FDA has also approved2. Synthesis2. their Synthesis use as flavoring agents [10,11]. 2. Synthesis 2.1. Chemical2.1. Chemical Synthesis Synthesis 2.1.2. ChemicalSynthesis Synthesis VioletViolet scent scent (ionone) (ionone) is currently is currently used used in many in many commercial commercial products products as its as price its price is low. is low. However, However, 2.1.Violet Chemical scentbefore Synthesisbefore (ionone)the 1890s the 1890s isviolet currently violet flower flower usedoil was oil in wasconsidered many considered commercial the mo thest mo precious productsst precious of asall of itsessential all price essential isoils. low. oils.This However, This is because is because the productionthe production of one of onekilogram kilogram of violet of violet oil requoil requires ires33,000 33,000 kg ofkg violet of violet flowers flowers [19], [19], which which cost cost before theViolet 1890s scent violet (ionone) flower is currently oil was considered used in many the commercial most precious products of all as essential its price oils. is low. This However, is because approximatelyapproximately 82,500 82,500 German German gold gold marks marks for raw for rawmaterials. materials. This This costly costly procedure procedure motivated motivated many many thebefore production the 1890s of violet one kilogramflower oil was of violet considered oil requires the most 33,000 precious kg of of all violet essential flowers oils. This [19], is which because cost approximatelychemistschemists 82,500 and German andindustries industries gold to find marks to finda cost-efficient fora cost-efficient raw materials. way way to synthesize to This synthesize costly this procedure oilthis [20]. oil [20]. motivated many the productionThus ofThus onein 1893, inkilogram 1893, Tiemann Tiemann of violetand andKrueger oil Krueger requ investigatires investigat 33,000ed thekged ofcompoundthe violet compound flowers responsible responsible [19], whichfor thefor costscentthe scent of of chemists and industries to find a cost-efficient way to synthesize this oil [20]. approximatelyvioletsviolets 82,500 by studying by German studying orris gold orris root marks rootoil; aoil;for much araw much cheaper ma terials.cheaper oil Thiswith oil with costlya similar a similarprocedure odour odour principle motivated principle [20,21]. many [20,21]. They They Thus in 1893, Tiemann and Krueger investigated the compound responsible for the scent of violets chemists achievedand achievedindustries the synthesisthe to synthesis find ofa cost-efficient ionone of ionone in two in way twosteps. tosteps. synthesizeFirs t,Firs theyt, they performed this performed oil [20]. an aldol an aldol condensation condensation of citral of citral by studyingThuswith orrisin with1893, acetone root acetone Tiemann oil; (dimethyl a (dimethyl much and ketone) cheaper Krueger ketone) in oilan ininvestigat alkaline with an alkaline a similarmeeddium methedium odour compoundto produce to principle produce pseudoionone. responsible pseudoionone. [20,21]. Theyfor Then the Then achieved pseudoionone scent pseudoionone of the synthesisviolets by ofwas iononestudying wascyclized cyclized in orris twoto ionone to steps.root ionone oil;upon First, aupon exposuremuch they exposure cheaper performed to acidic to oilacidic co with annditions co aldol nditionsa similar (Scheme condensation (Scheme odour 1). The 1).principle Thetype of citral type and [20,21]. andconcentration with concentration acetoneThey of of (dimethylachieved ketone) acidthe acidsynthesisused in used andetermines alkaline determinesof ionone the medium in ratiothe two ratio of tosteps. α produceof-ionone αFirs-iononet, and pseudoionone.they andβ -iononeperformed β-ionone produced Then anproduced aldol pseudoionone in thiscondensation in thisreaction. reaction. was For of cyclized citralexample,For example, towith ionone acetone uponphosphoric, (dimethylphosphoric, exposure fumaric ketone) tofumaric acidic and in and otheran conditions alkaline other weaker weaker me (Schemeacidsdium acids mainly to 1 mainlyproduce). Theyield yield typeα pseudoionone.-ionone α and-ionone while concentration while concentrated Then concentrated pseudoionone of acidsulfuric sulfuric used acid acid determineswas cyclizedpreferentially the topreferentially ratio ionone of α uponyields-ionone yields exposure β-ionone and β-iononeβ to -ionone[22,23]. acidic [22,23]. co producedThisnditions This synthesis synthesis (Scheme in this marked reaction. marked1). The the type Forthestart and example,start of concentration fragranceof fragrance phosphoric, chemistry ofchemistry fumaricacid used and(Duftstoff-Chemie) otherdetermines(Duftstoff-Chemie) weaker the acids ratio [2]. mainly To[2].ofday αTo-ionone daythe yield globalthe αand global-ionone usage β-ionone usage of while α -iononeof produced α concentrated-ionone and inandβ-ionone this β sulfuric-ionone reaction. is approximately is acid approximately For preferentially example, between between yieldsphosphoric,β-ionone100 fumaricand100 [22 and1000,23]. and1000 metric This other metric synthesis tons weaker tons yearly yearly marked acids[10,11]. [10,11]. mainly the start yield of fragranceα-ionone while chemistry concentrated (Duftsto sulfuricff-Chemie) acid [ 2]. Todaypreferentially the global yields usage ofβ-iononeα-ionone [22,23]. and β This-ionone synthesis is approximately marked the between start of 100 fragrance and 1000 chemistry metric tons yearly(Duftstoff-Chemie) [10,11]. [2]. Today the global usage of α-ionone and β-ionone is approximately between 100 and 1000 metric tons yearly [10,11].

SchemeScheme 1. The 1. chemicalThe chemical synthesis synthesis of ionones of ionones via the via Ti theemann-Krueger Tiemann-Krueger synthesis. synthesis. Citral Citral and acetoneand acetone are condensedare condensed in an in alkaline an alkaline medium medium to yield to yield pseudoionone. pseudoionone. Pseudoionone Pseudoionone is cyclized is cyclized to ionone to ionone uponupon exposure exposure to acidic to acidic conditions. conditions. Phosphoric, Phosphoric, fumaric fumaric and otherand other weaker weaker acids acids mainly mainly yield yield α- α-

ionone,ionone, while while concentrated concentrated sulfur sulfuric acidic acidpreferentially preferentially yields yields β-ionone. β-ionone. SchemeScheme 1. 1.The The chemical chemical synthesis synthesis ofof iononesionones via the Ti Tiemann-Kruegeremann-Krueger synthesis. synthesis. Citral Citral and and acetone acetone are condensed in an alkaline medium to yield pseudoionone. Pseudoionone is cyclized to ionone upon are condensed in an alkaline medium to yield pseudoionone. Pseudoionone is cyclized to ionone exposureupon exposure to acidic to conditions. acidic conditions. Phosphoric, Phosphoric, fumaric fumaric and other and other weaker weaker acids acids mainly mainly yield yieldα-ionone, α- whileionone, concentrated while concentrated sulfuric acid sulfur preferentiallyic acid preferentially yields β -ionone.yields β-ionone.

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Molecules 2020, 25, x FOR PEER REVIEW 3 of 26 2.2. Endogenous Synthesis 2.2. Endogenous Synthesis 2.2.1. β-Carotene Oxygenase 1 (BCO1) 2.2.1. β-Carotene Oxygenase 1 (BCO1) Carotenoids are naturally occurring compounds found in fruits and vegetables. They are isoprenoids with a characteristicCarotenoids are long naturally conjugated occurring double-bond compounds chain found in their in structures fruits and [24 vegetables.]. In humans, They the are main carotenoidsisoprenoids naturally with a characteristic found are α -carotene,long conjugatedβ-carotene, double-bondβ-cryptoxanthin, chain in their lycopene structures and lutein[24]. In [25 ]. Thehumans, central the oxidative main caroteno cleavageids ofnaturallyβ-carotene found at are the α 15,15-carotene,0 double β-carotene, bond results β-cryptoxanthin, in two all-trans lycopeneretinal entitiesand lutein (Scheme [25].2 ).The The central all- trans oxidativeretinal cleavage can then eitherof β-carotene be oxidized at the to 15,15 all-trans′ double-retinoic bond acid results or reduced in two to all-all-transtrans-retinol retinal (vitamin entities A),(Scheme respectively. 2). The all- Thistrans process retinal is can carried then outeither by be the oxidized cytosolic to enzymeall-trans-retinoicβ-carotene acid or reduced to all-trans-retinol (vitamin A), respectively. This process is carried out by the oxygenase 1 (BCO1), also identiﬁed as β-carotene-15,150-mono-oxygenase 1 (BCMO1) [24,26]. It was cytosolic enzyme β-carotene oxygenase 1 (BCO1), also identified as β-carotene-15,15′-mono- concluded that for the carotenoid to be converted to vitamin A, the carotenoid should contain a oxygenase 1 (BCMO1) [24,26]. It was concluded that for the carotenoid to be converted to vitamin A, minimum of one non-substituted β-ionone ring [27,28], thus BCO1 is only able to facilitate the cleavage the carotenoid should contain a minimum of one non-substituted β-ionone ring [27,28], thus BCO1 is of β-cryptoxanthin, α-carotene and β-carotene [28]. only able to facilitate the cleavage of β-cryptoxanthin, α-carotene and β-carotene [28].

SchemeScheme 2. 2.The The enzymatic enzymatic cleavage cleavage ofof ββ-carotene-carotene by β-carotene-carotene oxygenase oxygenase 1 1(BCO1) (BCO1) and and β-caroteneβ-carotene

oxygenaseoxygenase 2 (BCO2).2 (BCO2). The The central central cleavagecleavage ofof β-carotene catalyzed catalyzed by by BCO1 BCO1 at at the the 15,15 15,15′ double0 double bond bond resultsresults in in the the formation formation of of two two molecules molecules of of all- all-transtransretinal. retinal. The The asymmetric asymmetric cleavagecleavage ofof β-carotene at

theat 9,10 the 9,10 double double bond bond facilitated facilitated by by BCO2 BCO2 results results in in generation generation of ββ-ionone-ionone and and β-10β-10′-apocarotenal.0-apocarotenal. AnotherAnother cleavage cleavage at at the the 9 09,10′,100′ doubledouble bond yields an an additional additional ββ-ionone-ionone moiety moiety and and rosafluene. rosaﬂuene.

2.2.2.2.2.2.β -Caroteneβ-Carotene Oxygenase Oxygenase 22 (BCO2)-Mediated(BCO2)-Mediated Synthesis Synthesis VogtVogt and and von von Lintig, Lintig, at theat the University University of Freiburg of Freiburg in Germany, in Germany, first describedfirst described endogenous endogenous ionone synthesisionone synthesis by the asymmetric by the asymmetric cleavage cleavage of β-carotene of β-carotene mediated mediated by an enzyme. by an enzyme. However, However, von Lintig von at CaseLintig Western at Case Reserve Western University Reserve in University the United in States the Un carriedited States further carried investigations further investigations on this enzyme on [this29,30 ]. β-Iononeenzyme and [29,30].β-apo-10 β-Ionone0-carotenal and β are-apo-10 the′ products-carotenal of are asymmetric the productscleavage of asymmetric of β-carotene cleavage (Scheme of β-2). carotene (Scheme 2). This cleavage at the 9′,10′ double bond is facilitated by β-carotene oxygenase 2 This cleavage at the 90,100 double bond is facilitated by β-carotene oxygenase 2 (BCO2), also known as (BCO2), also known as β-carotene-9′,10′-dioxygenase 2 (BCDO2). Another cleavage at the 9′,10′ β-carotene-90,100-dioxygenase 2 (BCDO2). Another cleavage at the 90,100 double bond yields an additional β-iononedouble moietybond yields and rosafluene,an additional a compound β-ionone moiety naturally and occurringrosafluene, in a rosescompound (Scheme naturally2)[24,29 occurring,31,32]. in rosesThe di (Schemefference 2) between [24,29,31,32]. the BCO1 and BCO2 enzymes is not confined to their mechanism of cleavage The difference between the BCO1 and BCO2 enzymes is not confined to their mechanism of (Table2). BCO2 can asymmetrically cleave carotenoids that contain hydroxylated ionones and acyclic cleavage (Table 2). BCO2 can asymmetrically cleave carotenoids that contain hydroxylated ionones carotenoids including non-provitamin A carotenoids, for instance lycopene and xanthophylls and and acyclic carotenoids including non-provitamin A carotenoids, for instance lycopene and thus has broader substrate specificity [31]. Furthermore, BCO2 is localized in the inner mitochondrial xanthophylls and thus has broader substrate specificity [31]. Furthermore, BCO2 is localized in the membrane [33] while BCO1 is a cytoplasmic protein [28]. These differences between BCO1 and BCO2 inner mitochondrial membrane [33] while BCO1 is a cytoplasmic protein [28]. These differences suggestbetween unique BCO1 physiological and BCO2 suggest roles [un32ique]. physiological roles [32].

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Table 2. β-carotene oxygenase 1 (BCO1) and β-carotene oxygenase 2 (BCO2) enzymes have diﬀerent site and products of β-carotene cleavage, substrate speciﬁcity, cellular compartmentalization and human tissue expression.

β-Carotene Oxygenase 1 (BCO1) β-Carotene Oxygenase 2 (BCO2) Site of Cleavage Symmetric–central Asymmetric–eccentric Products of the cleavage of β-carotene Two entities of all-trans retinal β-ionone and β-apo-100-carotenal Substrate speciﬁcity Pro-vitamin carotenoids Pro-vitamin and non-pro-vitamin carotenoids Cellular compartmentalization Cytoplasm Inner mitochondrial membrane Tissues that are expressing BCO1 were also found to express BCO2 however only Human tissue expression BCO2 was detected in endometrial connective tissue, endocrine pancreas and cardiac and skeletal muscle

The central cleavage (via BCO1) of β-carotene represents the primary pathway for retinoid production. It was believed that the asymmetric cleavage (via BCO2) of β-carotene is a salvage pathway for retinoid production [34]. Later it was reported that BCO1-knockout mice with intact BCO2 were vitamin A deficient [35]. In addition, only BCO2 but not BCO1 is expressed in prostate and endometrial connective tissue, endocrine pancreas and cardiac and skeletal muscle. Notably, some of these tissues are not sensitive to vitamin A deficit, thus BCO2 is suggested to play a physiological role independent of vitamin A synthesis [36]; implying that β-ionone (one of BCO2 products) may be involved in other prospective biological functions [37]. BCO2 may play a role in the prevention of hepatic steatosis [32]. Deficiency/mutations in BCO2 led to buildup of carotenoids in adipose tissue in various animals and in mitochondria in a mouse model [32,33,38,39]. Carotenoids in turn can cause oxidative stress and thus apoptosis. Thus, BCO2 was found to keep cells protected from apoptosis brought by carotenoid-induced oxidative stress [40]. The association between the level of β-ionone and BCO2 mutations has not been investigated. It appears likely that mutations or absence of BCO2 would result in reduced levels of β-ionone and might contribute in the pathogenesis of the mentioned diseases. In addition, the discovery of endogenous production of β-ionone indicates that β-ionone might have physiological roles that are yet to be revealed. This is supported by the fact that high intake of fruits and vegetables, which contain volatile isoprenoids such as ionones, is associated with lower risk of cancer [41]. Studies have focused to understand the biological role of apocarotenoids generated by BCO2 [42], β-ionone has been less studied. Thus, in this review, we will present the data describing the physiological effects of ionone.

2.3. Possible Ionone Synthesis from Pinene Pinene is the major constituent of turpentine oil. The turpentine derived from the resin of terbenith tree had been widely used as a medicine and in wine as a preservative agent and taste enhancer. It was noticed that the consumption of turpentine oil alters the urine scent into violet. Anecdotally, turpentine oil was allegedly used by Cleopatra for this purpose. However, mixing turpentine oil with urine doesn’t confer the urine violet-like scent. It was assumed that ionone is the compound accountable for the scent of violet. Thus, it was concluded that pinene (the most prevalent compound in turpentine) should be converted into ionone and then the latter must be renally excreted to produce the violet scent of urine. Hepatic enzymes (cytochrome P450) might be responsible for the conversion of pinene to ionone [2]. Al-Tel and his colleagues suggested a possible metabolic pathway for the conversion of α-and β-pinene into α-and β-ionone (Scheme3). First, the CYP enzyme would oxidize α-and β-pinene generating intermediates (1) and (a), respectively. The intermediate generated from β-pinene (a) will undergo free radical rearrangement forming intermediate (b). The latter will abstract a hydrogen radical from CYP to yield intermediate (c). Molecules 2020, 25, 5822 5 of 25 Molecules 2020, 25, x FOR PEER REVIEW 5 of 26

SchemeScheme 3. 3.Possible Possible metabolic metabolic conversionconversion of α-- and and ββ--pinene pinene to to αα- -and and ββ- ionone-ionone (modified (modiﬁed from from [43] [43).]).

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This is followed by CYP abstracting the hydrogen radical from intermediate (c). At this point, it is postulated that the two pathways meet via the formation of intermediate 1. Subsequent free radical rearrangement and cleavage of the cyclobutyl ring should yield intermediate 2. The latter undergoes oxidation into alcohol 3 and then into aldehyde 4. Intermediate 4 undergoes a Knoevenagel condensation with acetone to produce 5. 1,5-Sigmatropic rearrangement of intermediate 5 will yield intermediate 6. The latter can rearrange to form intermediate 7, which can later further rearrange into either α- or β-ionone. Acetone is a critical player in this suggested metabolic conversion of pinene to ionone. It could be generated in creatures either via decarboxylation of acetoacetate or dehydrogenation of 2-propanol. The former route constitutes the main source of acetone in mammals [43]. The proposed catalytic reactions were supported by calculated physicochemical properties. The α- and β-ionones had higher hydrophilicty than α- and β-pinenes, which suggests pinene conversion to ionone to facilitate renal excretion. Predicted sites of pinene metabolism via human CYP isoform were reported and CYP isoforms with high probabilities of involvement were: 1A2, 3A4, 2C19 and 2D6 [43].

3. Olfactory Receptors Olfactory receptors (OR) are G-protein coupled receptors that detect volatile substances [44,45]. In olfactory sensory neurons, OR bind to Golf [46] (G protein similar to Gαs) followed by activation of an adenylate cyclase and production of cAMP which would result in an increase in intracellular calcium [47–50]. In 1992, Parmentier and his colleagues recognized the first OR gene transcript in mammalian germ cells [51]. Further studies displayed that OR expression is not constrained to olfactory epithelium, however they are unexpectedly expressed in all human tissues studied to date such as colon, lung, ovary, liver, kidney, lymph node, heart, blood, testis, skeletal muscle, skin, adipose, adrenal, brain, breast, prostate, and thyroid. Over 40 different OR have been identified, which are found to be expressed in more than 45 different human tissues. This ectopic expression of OR suggests that the function of OR is more than odor detection and discrimination [45,52–54]. Studies reported various functions of ectopically expressed OR such as regulation of serotonin release in enterochromaffin cells [55], cancer cell proliferation [56], cytokinesis [57], blood pressure, enzyme secretion in kidneys [58,59] and sperm chemotaxis [60–63]. Several studies showed that ionone binds to OR51E2 [45,49,56,64–68]. Essentially OR51E2 is identified as ionone receptor belonging to OR family 51 and subfamily E where is it member 2 [30]. OR51E2 is not only one of the most expressed ORs at mRNA level; it is found in a multitude of tissues. OR51E2 is most abundant in prostate tissue (3- FPKM (fragments per kilobase per million) in ∞ comparison to other tissues where it is less abundant such as liver (0–0.01 FPKM), brain (0.01–0.1 FPKM), testis (0.1–1 FPKM) and colon (1–3 FPKM) [54]. OR51E2 expression is augmented in prostate cancer [54,69]. Thus, it is also identified as prostate- specific G-protein-coupled receptor (PSGR) [45,54] and considered a marker for prostate tumor [70]. However, Cao et al. reported that PSGR expression was high in prostate intraepithelial neoplasia but decreased as the disease progressed to prostate cancer. In prostate cancer patients, low PSGR expression was found to be correlated with poor overall survival [66]. The number of OR51E2 is higher in human melanoma cells in comparison to melanocytes [49,68]. In addition, another recent study showed that OR51E2 is the most expressed OR transcript in human adult and fetal retinal pigment epithelium. Interestingly, OR51E2 was not only localized in the plasma membrane but also in the cytosol of benign and cancer prostate epithelial cells, retinal pigment epithelium and melanocytes; specifically in the early endosome antigen 1 of melanocytes [37,49,71].

4. Ionone Eﬀects

4.1. β-Ionone β-Ionone has been shown to exert a variety of pharmacological eﬀects, including anticancer, chemopreventive, cancer-promoting, melanogenesis, anti-inﬂammatory and antimicrobial activity. Molecules 2020, 25, 5822 7 of 25 Molecules 2020, 25, x FOR PEER REVIEW 7 of 26

SeveralSeveral mechanismsmechanisms and intracellular signaling cascadescascades have been reported explaining these effects. eﬀects. ββ-Ionone activates OR51E2OR51E2 [[37,49,50,56,65–68,70]37,49,50,56,65–68,70] andand regulatesregulates thethe activityactivity oror expressionexpression ofof cellcell cyclecycle regulatoryregulatory proteinsproteins [[3,72–76],3,72–76], pro-apoptoticpro-apoptotic andand anti-apoptoticanti-apoptotic proteinsproteins [[3,9,73,76–79],3,9,73,76–79], HMG-CoAHMG-CoA reductasereductase [[3,74,80–82]3,74,80–82] andand pro-inﬂammatorypro-inflammatory mediatorsmediators [[73,83].73,83].

4.1.1.4.1.1. OR51E2-Mediated EEffectsﬀects StudiesStudies showed showed that thatβ-ionone β-ionone inhibits inhibits the proliferation the proliferation [50,56 ,66[50,56,66,67],67] and induced and induced invasiveness invasiveness [65,66,70] of[65,66,70] prostate of cancer prostate cells cancer via activation cells via activation of OR51E2, of whereasOR51E2,β whereas-ionone β mediated-activation-ionone mediated-activation of OR51E2 of inhibitedOR51E2 inhibited proliferation proliferation and migration and ofmigration melanocytes of melanocytes and induced and melanogenesis induced melanogenesis [49,68]. Surprisingly, [49,68]. βSurprisingly,-ionone-mediated β-ionone-mediated activation of OR51E2activation was of foundOR51E2 to was increase found the to proliferation increase the andproliferation metastasis and of retinalmetastasis pigment of retinal cells, pigment activating cells, p44 activating/42 and protein p44/42 kinase and protein B (AKT) kinase proteins B (AKT) [37]. proteins [37]. ββ-Ionone activationactivation of OR51E2of OR51E2 resulted resulted in modulation in modu oflation kinases of activitykinases and activity increase and in intracellularincrease in calciumintracellular (Figure calcium1). The (Figure Neuhaus 1). team The showed Neuhaus that teamβ-ionone showed activated that β OR51E2-ionone (notactivated androgen OR51E2receptors (not (AR))androgen causing receptors an increase (AR)) causing in intracellular an increase calcium in intracellular concentration calcium in bothconcentration primary in prostate both primary cancer epithelialprostate cancer cells and epithelial prostate cells cancer and cell prostate line (LNCaP). cancer cell Further line (LNCaP). testing elucidated Further thattestingβ-ionone elucidated exert that the anti-proliferativeβ-ionone exert the eﬀ ectanti-proliferative via phosphorylating effect p38 via and phosphorylating stress-activated proteinp38 and kinase stress-activated/Jun amino-terminal protein kinasekinase/Jun (SAPK amino-terminal/JNK) which are kinase members (SAPK/JNK) of the mitogen-activated which are members protein of the kinase mitogen-activated (MAPK) family protein [56]. Anotherkinase (MAPK) study found family that [56].β-ionone Another inhibited study the found proliferation that β of-ionone mammary inhibited cancer the cells proliferation via modulating of themammary MAPK pathwaycancer cells [84 via]. modulating the MAPK pathway [84].

FigureFigure 1. AA proposed proposed scheme scheme illustrating illustrating the the effect eﬀect of ofβ-iononeβ-ionone mediated mediated activation activation of membrane of membrane and andcytosolic cytosolic OR51E2 OR51E2 receptor receptor leading leading to toan anincrease increase inin calcium calcium and and regulation regulation of of various kinaseskinases modulatingmodulating cellcell proliferation, metastasismetastasis andand melanogenesis.melanogenesis. (OR51E2, olfactory receptor 51E2; BCO2, ββ-carotene oxygenaseoxygenase 2; 2; cAMP, cAMP, cyclic cyclic adenosine adenosine monophosphate; monophosphate; PKA, PKA, protein protein kinase A;kinase TRP calciumA; TRP channel,calcium channel, transient transient receptor receptor potential potential calcium channel;calcium channel; Pyk2, proline-rich Pyk2, proline-rich tyrosine tyrosine kinase 2; kinase MAPK, 2; mitogen-activatedMAPK, mitogen-activated protein kinase;protein SAPKkinase;/JNK, SAPK/J stress-activatedNK, stress-activated protein kinasesprotein/ Junkinases/Jun amino-terminal amino- kinases;terminal NDRG1, kinases; N-myc NDRG1, downstream N-myc downstream regulated gene regulated 1; AR, androgen gene 1; receptor;AR, androgen AKT, proteinreceptor; kinase AKT, B; PI3KproteinG, phosphoinositide kinase B; PI3Kɣ, phosphoinositide 3-kinase gamma). 3-kinase gamma).

WieseWiese etet al.al. showed that proline-rich tyrosinetyrosine kinasekinase 22 (Pyk2)(Pyk2) playsplays aa criticalcritical rolerole inin the signaling cascadescascades initiatedinitiated after after OR51E2 OR51E2 activation activation (Figure (Figure1). β -Ionone1). β-Ionone (via OR51E2) (via OR51E2) activated activated Pyk2 in prostatePyk2 in prostate cancer cells, possibly mediated via an increase in intracellular calcium concentration. Pyk2 increases the phosphorylation of p38 MAPK and dephosphorylation of N-myc downstream

Molecules 2020, 25, 5822 8 of 25 cancer cells, possibly mediated via an increase in intracellular calcium concentration. Pyk2 increases the phosphorylation of p38 MAPK and dephosphorylation of N-myc downstream regulated gene 1 (NDRG1). The latter along with Pyk2 are thought to be responsible for the β-ionone mediated anti-proliferative effect in prostate cancer cells. In addition, this study revealed that β-ionone activating OR51E2 resulted in regulation of various proteins involved in processes such as signaling and ion transport in prostate cancer cells [67]. Jones et al. supported the idea that anti-proliferative effect of β-ionone is AR independent [74]. However, a recent study by Xie et al. revealed that the presence of AR is essential for the β-ionone activation of OR51E2. OR51E2 stimulation by β-ionone induced calcium influx in prostate cancer cell lines (LNCaP, C4-2 and DU145). However, it only inhibited prostate cancer proliferation in LNCaP and C4-2 cells, which are expressing AR but not in DU145 cells, which do not express AR. Notably β-ionone-induced activation of OR51E2 inhibited AR nuclear translocation via p38 and JNK-mediated AR phosphorylation (Figure1). The suppressed AR translocation caused a reduction in AR transactivation and thus retardation of prostate cancer cell growth. This was further supported by in vivo studies that showed that β-ionone suppressed tumor growth in mice and caused an increase in the phosphorylation in AR, p38, and JNK [50]. However, Sanz reported that β-ionone induced prostate cancer (LNCaP) cell invasiveness via activation of phosphatidylinositol-4,5-bisphosphate 3-kinase-G(PI3KG) mediated by OR51E2 stimulation (Figure1). The βGsubunit of G protein was responsible to activate PI3KG [65]. A study from Cao et al. showed that β-ionone mediated activation of OR51E2 inhibited the proliferation of C4-2 cells but promoted their invasion consistent with reported data from LNCaP cells [66]. Notably, β-ionone administered to mice with prostate tumors also induced cell metastasis [65,70]. On the other hand, another study found that β-ionone combined with sorafenib (a multi kinase inhibitor) exhibit a synergistic inhibition of metastasis of human hepatoma cells [85]. This was mediated via β-ionone-induced upregulation of the expression of tissue inhibitor matrix metalloproteinase-1 and -2 [85,86], which is a well-accepted approach for metastasis inhibition [85]. Studies showed that β-ionone also inhibited the proliferation in melanocytes via activation of OR51E2 [49]. However, β-ionone decreased the LPA-induced migration and retarded the growth of vertical-growth phase (VGP) melanoma cells by induction of apoptosis [68]. On the other hand, β-ionone increased the melanin level in melanocytes and the percentage of cells with more than two dendrites. Melanogenesis is found to be mediated by β-ionone activation of OR51E2 resulting in activation of adenylate cyclase and thus increase in cAMP and activation of protein kinase A (PKA). PKA might be responsible to upregulate the melanogenic enzyme, tyrosinase (Figure1). Moreover, activated MAPK (p44/42 and p38) have been reported which could also be the likely regulator of melanogenesis. It was shown that the surface OR51E2 and not cytosolic OR51E2 receive the external signal. On this basis, OR51E2 are assumed to be inserted into the membrane with repetitive β-ionone stimulation of melanocytes [49]. On that note, studies investigated the mechanism of calcium increase in prostate cancer cells upon OR51E2 stimulation. It was found that transient receptor potential vanilloid type 6 (TRPV6) channels are activated downstream of β-ionone-mediated activation of OR51E2. Src kinases play a critical role in the β-ionone mediated influx of calcium; OR51E2 directly activates Src kinase independently of G protein activation (Figure1)[64]. β-Ionone also caused an increase in intracellular calcium concentration in melanocytes and cultured cells derived from metastatic and VGP as reported in prostate tumor cells [49,68]. This process is also mediated via the activation of surface OR51E2. Calcium from the extracellular space and intracellular stores account for the elevation of calcium levels however, the former was found to be the main significant contributor (Figure1). Extracellular calcium is influxed via transient receptor potential channels specifically TRPM and not TRPV as reported in prostate cancer cells [49]. Another study showed that β-ionone induced an increase in intracellular calcium in retinal pigment epithelial cells; calcium originated from extracellular space. The increase in calcium was Molecules 2020, 25, 5822 9 of 25 mediated via activation of the cAMP pathway. Even though no channel was identified, L-type calcium or TRP channels (as in prostate and melanocytes) were suggested to be involved [37]. It appears that exogenous β-ionone, provided via diet and fragrance, might activate OR51E2 found in the plasma membrane. However, it is proposed that β-ionone, endogenously produced from BCO2 Molecules 2020, 25, x FOR PEER REVIEW 9 of 26 in the mitochondria, might activate intracellular OR51E2 [37]. Intracellular localization of OR51E2 has beenBCO2 presented in the in mitochondria, the aforementioned might activate cells [37 intrace,49,71].llular Based OR51E2 on β-ionone [37]. Intracellular hydrophobic localization properties of that allowsOR51E2 it to traversehas been cellularpresented membrane in the afor [ementioned49], it appears cells that [37,49,71]. exogenous, Based endogenous on β-ionone βhydrophobic-ionone might activateproperties plasma that membrane, allows it to or trav cytosolerse cellular localized membrane OR51E2. [49], it appears that exogenous, endogenous β-ionone might activate plasma membrane, or cytosol localized OR51E2. 4.1.2. Cell Cycle Regulatory Protein-Mediated Effects 4.1.2. Cell Cycle Regulatory Protein-Mediated Effects β-Ionone has been shown to inhibit the proliferation of breast cancer [3,72,73], prostate cancer [74], human colonβ-Ionone cancer has [75 been], human shown gastric to inhibit adenocarcinoma the proliferatio [79],n murineof breast B16 cancer melanoma [3,72,73], cells, prostate human cancer leukemia and[74], human human colon colon adenocarcinoma cancer [75], human cell lines gastric [3] aden and ocarcinomain vivo in mammary [79], murine cancerous B16 melanoma glands cells, [76] via inductionhuman ofleukemia cell cycle and arrest. human colon adenocarcinoma cell lines [3] and in vivo in mammary cancerous glands [76]via induction of cell cycle arrest. β-ionone has been shown to suppress cell proliferation via arresting the cell cycle at the G1/G0 β-ionone has been shown to suppress cell proliferation via arresting the cell cycle at the G1/G0 phase in breast cancer cell line [73], G1 phase in human colon cancer [75] and prostate cancer cell line phase in breast cancer cell line [73], G1 phase in human colon cancer [75] and prostate cancer cell line (DU145, PC-3 cells) [74] and G1/G0 and G2/M phases in human gastric cancer and breast cancer cell (DU145, PC-3 cells) [74]and G1/G0 and G2/M phases in human gastric cancer and breast cancer cell lineslines [72 ,[72,79].79]. CellCell proliferation proliferation is regulatedis regulated by by the the cell cell cyclecycle [[79].79]. Cyclin, Cyclin, cyclin cyclin dependent-kinases dependent-kinases (CDK) (CDK) and and CDKCDK inhibitors inhibitors (CdkI) (CdkI) play play critical critical role inrole modulating in modulating the cell the cycle. cell cycle. Cell cycle Cell arrestcycle arrest was explained was explained (Figure 2) by a(Figure reduction 2) by in a cyclinreduction D1 [in72 cyclin–74,76 D1,79 ],[72–74,76,79],cyclin E, cyclin cyclin B1, E, cyclincyclin B1, A, Cdkcyclin 2, A, [72 Cdk], Cdk4, 2, [72], [72 Cdk4,–74, 79]. Cdk[72–74,79]. phosphorylate Cdk phosphorylate proteins involved proteins in involved cell cycle in cell progression. cycle progression. Cdk become Cdk become activated activated only only when complexedwhen complexed with cyclins with to cyclins form Cdk-cyclinto form Cdk-cyclin complexes. complexes. Cdk4-Cyclin Cdk4-Cyclin D, Cdk2-cyclin D, Cdk2-cyclin E, Cdk2-cyclin E, Cdk2- A, Cdk1-cyclincyclin A, ACdk1-cyclin and Cdk1-cyclin A and BCdk1-cyclin regulate early B regulate G1 phase, early late G1 G1 phase, phase, late progression G1 phase, through progression S phase, G2/entrythrough of S M phase, phase G2/entry and progression of M phase through and progressio M phase,n through respectively M phase, [87 ]respectively (Figure2). [87] (Figure 2).

FigureFigure 2. β 2.-Ionone β-Ionone modulates modulates several several cell cell cyclecycle regulatory proteins proteins causing causing cell cell cycle cycle arrest arrest in various in various cancercancer cells cells (Cdk-1, (Cdk-1, cyclin-dependent cyclin-dependent kinase-1; kinase-1; Cdk-2, Cd cyclin-dependentk-2, cyclin-dependent kinase-2; kinase-2;Cdk-4, cyclin-dependentCdk-4, cyclin- kinase-4;dependent p27, cyclin-dependentkinase-4; p27, cyclin-dependent kinase inhibitor; kinase p21, inhibitor; cyclin-dependent p21, cyclin-depen kinasedent inhibitor). kinase inhibitor).

Molecules 2020, 25, 5822 10 of 25

Moreover, β-ionone augmentation in the expression of p27 was reported that could also explain the cell cycle arrest and thus cell growth retardation [79]. It is believed that β-ionone targets p53 in persistent preneoplastic lesions at early stages of heptocarcinognesis [88]. P53 regulate cell cycle arrest primarily via transcriptional activation of p21 [89]. P21 and p27 are CdkI that bind to and blockMolecules Cdk-cyclin 2020, 25, x complex FOR PEER activityREVIEW speciﬁcally complex involving cyclin A, B, D and E (Figure102 of)[ 2690 ]. In addition, it is proposed that cell cycle arrest may be modulated via MAPK pathway; as Dong et al. showedaddition, that itβ is-ionone proposed inhibited that cell the cycle expression arrest ma ofy extracellularbe modulated signal-regulated via MAPK pathway; kinase as Dong and induced et al. showed that β-ionone inhibited the expression of extracellular signal-regulated kinase and induced the expression of p38 and SAPK/JNK [79]. the expression of p38 and SAPK/JNK [79]. 4.1.3. Apoptotic Eﬀect 4.1.3. Apoptotic Effect β-Ionone induces apoptosis in mammary cancer glands [76] and breast cancer [73], leukemia [3] β-Ionone induces apoptosis in mammary cancer glands [76] and breast cancer [73], leukemia [3] and human gastric adenocarcinoma cell lines [9,77–79] via induction of apoptosis. β-Ionone-induced and human gastric adenocarcinoma cell lines [9,77–79] via induction of apoptosis. β-Ionone-induced apoptosis was mediated via reducing the expression of anti-apoptotic protein, Bcl-2, and increasing the apoptosis was mediated via reducing the expression of anti-apoptotic protein, Bcl-2, and increasing expression of pro-apoptotic protein, Bax (Figure3) [9,76,79]. In addition, β-ionone increased the expression of the expression of pro-apoptotic protein, Bax (Figure 3) [9,76,79]. In addition, β-ionone increased the cleaved caspase-3 in breast cancer and human gastric adenocarcinoma cell lines [9,73,78,79]. It is believed expression of cleaved caspase-3 in breast cancer and human gastric adenocarcinoma cell lines β that[9,73,78,79].-ionone targetsIt is believed p53 in that persistent β-ionone preneoplastic targets p53 in lesionspersistent at earlypreneoplastic stages of lesions hepatocarcinogenesis at early stages (Figureof hepatocarcinogenesis3)[ 88]. However, the (Figureβ-ionone 3) [88]. induction However, of the apoptosis β-ionone was induction previously of apoptosis shown to was be previously independent of mutatedshown to p53be independent function [3]. of mutated p53 function [3].

FigureFigure 3. 3.A A proposed proposed mechanism mechanism ofof ββ-ionone-ionone modulating modulating pro-apoptoti pro-apoptoticc and and anti-apoptotic anti-apoptotic pathways. pathways. (OR51E2,(OR51E2, olfactory olfactory receptor receptor 51E2; 51E2; BCO2, BCO2,β β-carotene-carotene oxygenase 2; 2; JNK, JNK, Jun Jun amino-terminal amino-terminal kinases; kinases; AKT,AKT, protein protein kinase kinase B; B; PI3K, PI3K, phosphoinositide phosphoinositide 3-kinase;3-kinase; Apaf-1, apop apoptotictotic protease protease activating activating factor factor 1; TRAIL,1; TRAIL, tumor tumor necrosis necrosis factor factor related related apoptosis-inducingapoptosis-inducing ligand; ligand; DR4/5, DR4/5, death death receptor receptor 4/5; 4/ 5;NF- NF-кB,к B, nuclearnuclear factorfactor-κB;κB; transcription transcription factorfactor SP1; specificity speciﬁcity protein protein 1; 1;DR5 DR5 promoter, promoter, death death receptor receptor 5 5 promoter).promoter).

InIn addition, addition, reduction reduction in in the the level level ofof phosphorylatedphosphorylated PI3K PI3K (phosphoinositide (phosphoinositide 3-kinases) 3-kinases) proteins proteins andand AKT AKT expression expression has has been been reported.reported. The The PI3K-A PI3K-AKTKT pathway pathway could could also also explain explain the apoptosis the apoptosis of ofcells as as the the pathway pathway is involved is involved in cell in proliferation, cell proliferation, survival survival and cell and signaling cell signaling (Figure 3)(Figure [9]. Further3)[ 9]. Furtherinvestigations investigations supported supported the role the of MAPK role of pathway MAPK pathway in cell proliferation in cell proliferation and apoptosis and as apoptosis an increase asan in phosphorylated-p38 was documented [73]. These studies reveal that apoptosis might be mediated via MAPK or PI3K-AKT pathway. Liu et al. found that β-ionone caused a dose-dependent inhibition of mammary carcinogenesis in rats; tumor multiplicity decreased and time to tumor appearance time increased. Furthermore, a decrease in proliferating cell nuclear antigen and an increase in nuclear fragmentation was found

Molecules 2020, 25, 5822 11 of 25 increase in phosphorylated-p38 was documented [73]. These studies reveal that apoptosis might be mediated via MAPK or PI3K-AKT pathway. Liu et al. found that β-ionone caused a dose-dependent inhibition of mammary carcinogenesis in rats; tumor multiplicity decreased and time to tumor appearance time increased. Furthermore, a decrease in proliferating cell nuclear antigen and an increase in nuclear fragmentation was found [76]. Additionally, β-ionone was found to sensitize the cells to TRAIL (Tumor necrosis factor related apoptosis-inducing ligand)-stimulated apoptosis. This is mediated via increasing the direct binding of transcription factor Sp1 to the DR5 promoter site, abolishing NF-κB activation and decreasing the expression of anti-apoptotic proteins (Figure3)[ 91] Another study found that β-ionone targets NF-кB in persistent preneoplastic lesions at early stages of hepatocarcinogenesis [88]. Janakiram et al. found that β-ionone induces apoptosis possibly via acting as a ligand at the retinoid X receptor-α (RXR-α) in human colon cancer cells. β-ionone was found to increase the levels of expression of RXR-α mRNA dose dependently in cells. In addition, RXR-α expression is found to be lessened in other cancer cells [75]. Notably, the expression of RXR-α mRNA was decreased not only in rat colonic adenocarcinoma but also in basal cells in prostate cancer; proposing its influence in cancer progression [75,92]. This is further supported by the structure similarity depicted between β-ionone and β-carotene/vitamin A, recognized to bind to RXRs [75]. However, β-ionone in vivo effect on rat colonic aberrant crypt focci was controversial as studies reported inhibition or no effect on aberrant crypt focci [75,93].

4.1.4. HMG CoA Reductase-Mediated Eﬀect 3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase is responsible for the synthesis of mevalonate [3]. Mevalonate is a precursor of sterol molecules such as cholesterol and nonsterol isoprenoids such as dolichol, ubiquinone, farnesyl pyrophosphate and geranylgeranyl pyrophosphate [94]. While cholesterol plays an important role in membrane integrity, nonsterol isoprenoids are involved in various cellular processes. The geranylgeranyl pyrophosphate and farnesyl pyrophosphate prenylate proteins (post-translational modiﬁcation) such as Ras, Rho and Rac GTPase family [95,96], which in turn play vital role in cell proliferation, angiogenesis and survival [97,98]. The inhibition of HMG CoA reductase has been associated with cell apoptosis, cell cycle arrest and proliferation suppression [99–101]. Statins, which are well-established inhibitors of HMG-CoA reductase, are reported to possess anticancer activity. In tumor cells, the HMG-CoA reductase is resistant to sterol-regulated feedback inhibition (Figure4)[ 8,102] and thus its upregulated in various types of cancers [103–107]. However, HMG CoA reductase in tumor cells is sensitive to posttranscriptional inhibition mediated by nonsterol compounds. Isoprenoids were also found to post transcriptionally suppress HMG-CoA reductase activity [100,108]. Dietary β-ionone suppressed serum total cholesterol level and HMG CoA activity in chickens, while increasing the HDL level [80,81]. β-Ionone-induction of chemoprevention in rat mammary carcinogenesis [76,109,110] was parallel to its inhibition of HMG-CoA reductase [109]. Espindola et al. reported that the higher doses of β-ionone (16 mg/100 g) inhibited cell proliferation of preneoplastic lesions and caused smaller visible hepatocyte nodules in initial phases of hepatocarcinogenesis. Notably, the level of total plasma cholesterol decreased with administration of higher doses of β-ionone [82]. Other studies found that β-ionone suppression of the growth of Murine B16 melanoma, breast cancer, human leukemia, human colon adenocarcinoma [3] and prostate tumor cell lines was conveyed via reducing HMG CoA reductase activity [74]. This was further supported by the synergism anti-proliferative activity reported between β-ionone and either lovastatin or trans, trans-farnesol, compounds involved in reductase suppression [3,74]. β-Ionone suppression of HMG-CoA activity is mediated via posttranscriptional action. The β-ionone inhibition of HMG CoA activity was explained by indirect facilitation of HMG CoA degradation. This is mediated via induction in the activity of prenyl pyrophosphate pyrophosphatase [81], which will in turn increase the level of farnesol, a nonsterol mevalonate product that degrades HMG CoA reductase Molecules 2020, 25, 5822 12 of 25

(Figure4) [ 111]. Moreover, β-ionone and lovastatin affected the post-translational modification of lamin B, which is an important action for the assembly of nuclear membrane during interphase. Thus, it is suggested that β-ionone mediated inhibition of HMG CoA-reductase inhibits lamin B nuclear localization and thus causes the DNA to be vulnerable to endonucleases. This action will result in apoptosis as depicted with β-ionone and lovastatin. Moreover, β-ionone and lovastatin caused cell cycle arrest at G1 phase [3]. Therefore, the β-ionone and statin shared HMG-CoA regulation and synergistic anti-proliferative actions support β-ionone usage as adjunct cancer therapy to reduce statins’ adverseMolecules 2020 effects, 25, [x96 FOR]. PEER REVIEW 12 of 26

FigureFigure 4.4. ββ-Ionone-mediated-Ionone-mediated inhibition inhibition of of HMG-CoA HMG-CoA reductase reductase resulting resulting in anti-proliferative in anti-proliferative eﬀect effect and reductionand reduction in the levelin the of level cholesterol. of cholesterol. (OR51E2, (OR51E2, olfactory receptorolfactory 51E2; receptor BCO2, 51E2;β-carotene BCO2, oxygenase-2; β-carotene HMGoxygenase-2; CoA, 3-hydroxy-3-methylglutaryl HMG CoA, 3-hydroxy-3-methylgl coenzymeutaryl A; PP,coenzyme pyrophosphate). A; PP, pyrophosphate).

β WeDietary suggest β-ionone that -ionone suppressed might serum be directly total inhibitingcholesterol HMG-CoA level and reductaseHMG CoA in activity a similar in manner chickens, to β statin,while asincreasing-ionone the looks HDL like level it is sharing[80,81]. aβ similar-Ionone-induction backbone to of statin chemoprevention pharmacophore in (Figure rat mammary4)[ 112]. Incarcinogenesis addition, a recent [76,109,110] study showed was parallel that compounds to its inhibition that areof HMG-CoA completely reductase distinct from [109]. statin Espindola structure et wereal. reported able to inhibit that the HMG-CoA higher reductasedoses of [113β-ionone]. (16 mg/100 g) inhibited cell proliferation of β preneoplasticNevertheless, lesions Duncan and andcaused his colleaguessmaller visibl showede hepatocyte that -ionone nodules inhibit in breastinitial cancerphases cells’ of β proliferationhepatocarcinogenesis. and cell cycle Notably, without the a levelffecting of total HMG pl CoAasma reductase cholesterol activity decreased [72]. with-Ionone administration was found toof havehigher a chemoprotectivedoses of β-ionone e ff[82].ect asOther it decreased studies found the mean that numberβ-ionone of suppression visible hepatocyte of the growth nodules of [ 82Murine,114], andB16 persistentmelanoma, preneoplastic breast cancer, lesions human [ leukemia,88,114] and huma causedn colon DNA adenocarcinoma damage throughout [3] and the prostate initiation tumor of β hepatocarcinogenesiscell lines was conveyed in via rats. reducing Notably, HMG-ionone CoA increasedreductase theactivity hepatic [74]. HMG-CoA This was further reductase supported mRNA levelsby the butsynergism decreased anti-proliferative the level of total activity plasma reported cholesterol between in rats β-ionone with hepaticand either cancer. lovastatin This increaseor trans, intrans-farnesol, the mRNA levelscompounds of the involved reductase in couldreductase be a suppression compensatory [3,74]. mechanism subsequent to HMG CoAβ degradation-Ionone suppression [114]. Farnesol of HMG-CoA had similar activity effects is mediated on HMG CoAvia posttranscriptional reductase and cholesterol action. The [115 β],- β whichionone supports inhibition the of documented HMG CoA-ionone activity modulation was explained of farnesol by indirect activity. facilitation of HMG CoA degradation. This is mediated via induction in the activity of prenyl pyrophosphate pyrophosphatase 4.1.5. Antioxidant-Mediated Effect [81], which will in turn increase the level of farnesol, a nonsterol mevalonate product that degrades HMGAsokkumar CoA reductase proposed (Figure that β4)-ionone [111]. anti-proliferativeMoreover, β-ionone activity and is attributablelovastatin affected to its antioxidant the post- properties.translational Benzo(a)pyrene, modification of a well-knownlamin B, which carcinogen, is an important was chosen action to induce for the lung assembly cancer in of mice nuclear as it membrane during interphase. Thus, it is suggested that β-ionone mediated inhibition of HMG CoA- reductase inhibits lamin B nuclear localization and thus causes the DNA to be vulnerable to endonucleases. This action will result in apoptosis as depicted with β-ionone and lovastatin. Moreover, β-ionone and lovastatin caused cell cycle arrest at G1 phase [3]. Therefore, the β-ionone and statin shared HMG-CoA regulation and synergistic anti-proliferative actions support β-ionone usage as adjunct cancer therapy to reduce statins’ adverse effects [96]. We suggest that β-ionone might be directly inhibiting HMG-CoA reductase in a similar manner to statin, as β-ionone looks like it is sharing a similar backbone to statin pharmacophore (Figure 4) [112]. In addition, a recent study showed that compounds that are completely distinct from statin structure were able to inhibit HMG-CoA reductase [113].

Molecules 2020, 25, x FOR PEER REVIEW 13 of 26

Nevertheless, Duncan and his colleagues showed that β-ionone inhibit breast cancer cells’ proliferation and cell cycle without affecting HMG CoA reductase activity [72]. β-Ionone was found to have a chemoprotective effect as it decreased the mean number of visible hepatocyte nodules [82,114], and persistent preneoplastic lesions [88,114] and caused DNA damage throughout the initiation of hepatocarcinogenesis in rats. Notably, β-ionone increased the hepatic HMG-CoA reductase mRNA levels but decreased the level of total plasma cholesterol in rats with hepatic cancer. This increase in the mRNA levels of the reductase could be a compensatory mechanism subsequent to HMG CoA degradation [114]. Farnesol had similar effects on HMG CoA reductase and cholesterol [115], which supports the documented β-ionone modulation of farnesol activity.

4.1.5. Antioxidant-Mediated Effect Molecules 2020, 25, 5822 13 of 25 Asokkumar proposed that β-ionone anti-proliferative activity is attributable to its antioxidant properties. Benzo(a)pyrene, a well-known carcinogen, was chosen to induce lung cancer in mice as it isis believedbelieved to generateto generate reactive reactive oxygen oxygen species specie and initiates and proliferativeinitiate proliferative changes. Thechanges. anti-proliferative The anti- eproliferativeﬀect of β-ionone effect was of β shown-ionone via was a decrease shown via in proliferating a decrease in cell proliferating nuclear antigen cell expressionnuclear antigen and restorationexpression ofand normal restoration levels ofof cancer normal marker levels enzymes of cancer (carcinoembryonic marker enzymes antigen(carcinoembryonic and neuron-speciﬁc antigen enolase).and neuron-specificβ-ionone administeredenolase). β-ionone to mice administered restored the to activitiesmice rest ofored enzymatic the activities (such of as enzymatic catalase, glutathione(such as catalase, peroxidase glutathione (GPx), glutathione-S-transferase, peroxidase (GPx), glutathione-S-transferase, glutathione reductase glutathione (GR) and superoxide reductase dismutase(GR) and superoxide (SOD)) and dismutase non-enzymatic (SOD)) antioxidants and non-enzyma (suchtic as glutathioneantioxidants (GSH), (such as vitamins glutathione E and (GSH), C) to levelsvitamins detected E and prior C) to benzo(a)pyrene levels detected treatmentprior benzo(a)pyrene (Figure5)[116 treatmen]. t (Figure 5) [116].

Figure 5. β-Ionone inhibition of pro-inflammatory mediators and induction/restoration of antioxidants Figure 5. β-Ionone inhibition of pro-inflammatory mediators and induction/restoration of causing anti-proliferative and anti-inflammatory effects. (OR51E2, olfactory receptor 51E2; BCO2, antioxidants causing anti-proliferative and anti-inflammatory effects. (OR51E2, olfactory receptor β-carotene oxygenase-2; MAPK, mitogen-activated protein kinase; JNK, Jun amino-terminal kinases; 51E2; BCO2, β-carotene oxygenase-2; MAPK, mitogen-activated protein kinase; JNK, Jun amino- AKT, protein kinase B; NF-кB, nuclear factor-κB; Erk1/2, extracellular signal-regulated kinase 1/2; SOD, terminal kinases; AKT, protein kinase B; NF-кB, nuclear factor-κB; Erk1/2, extracellular signal- superoxide dismutase; CAT, catalase; GPx, glutathione peroxidase; GST, glutathione-S-transferase; GR, regulated kinase 1/2; SOD, superoxide dismutase; CAT, catalase; GPx, glutathione peroxidase; GST, glutathione reductase; GSH, glutathione). glutathione-S-transferase; GR, glutathione reductase; GSH, glutathione). Another study showed that β-ionone had chemoprotective effects and suppressed the 7,12- dimethylbenz(a)anthracene-inducedAnother study showed that β-ionone mammary had carcinogenesischemoprotective in rats.effects It increasedand suppressed the activities the 7,12- of antioxidantdimethylbenz(a)anthracene-induced enzymes (GPx, GR, SOD) mammary and GSH. carcinogen The antioxidantesis in rats. effect It increased of β-ionone the wasactivities further of supportedantioxidant by enzymes a reduction (GPx, in lipidGR, peroxidation,SOD) and GSH. malondialdehyde The antioxidant (marker effect forof β oxidative-ionone was stress) further and nitric oxide (a reactive nitrogen species) [110,116]. Dong et al. showed that β-ionone application on Hepa1c1c7 cells augmented the quinone reductase activity, which is a phase II detoxifcation enzyme [73]. The exact mechanism of how β-ionone imparts an antioxidant effect is still not clear. However, the effect on quinone reductase might be explained via modulation of p38, which is a negative modulator of the activation of phase II detoxifying enzymes [117].

4.1.6. Pro-Inflammatory Molecules-Mediated Effects β-Ionone inhibits COX-2 expression in human gastric cancer cells and rat mammary tumor tissues, which might explain its anti-proliferative effect. Moreover, the release of prostaglandin E2 (PGE2) in human gastric cancer cells was reduced [73]. Kang supported the β-ionone inhibitory activity on the expression of lipopolysaccharide (LPS)-induced pro-inflammatory mediators. It was Molecules 2020, 25, 5822 14 of 25 reported that β-ionone attenuated the release of nitric oxide (NO), prostaglandin E2 (PGE2) and tumor necrosis factor-α (TNF-α) and the protein and mRNA expression of inducible NO synthesis (iNOS), cyclooxygenase-2 and TNF-α in LPS-induced BV2 microglia cells. It was found that β-ionone regulates these inflammatory mediators via inhibition of NF-κB and MAPK pathway (Figure5). The authors showed that β-ionone decreased the DNA-binding activity of NF-κB by retardation of Akt activity. In addition, β-ionone inhibited the phosphorylation of MAPKs (Erk, p38 and JNK) which are important regulators of the release of pro-inflammatory mediators [83].

4.1.7. Antimicrobial Effects Griffin and his colleagues studied the role of structural groups in determining the antimicrobial activity of several terpenoids. β-ionone inhibitory activity was found against Escherichia coli and Candida albicans [118]. β-Ionone inhibited the growth of Aspergillus flavus and sporulation of both A. flavus and A. parasiticus. Furthermore, the morphology of asexual reproductive structures were altered upon β-ionone exposure. Notably, the aflatoxin accumulation was dramatically reduced in spores of A. parasiticus [119].

4.2. α-Ionone As previously discussed, β-ionone acts as an agonist of OR51E2. On the other hand, α-ionone activity on OR51E2 is not as clear. Several studies reported that α-ionone antagonize OR51E2 which prevented or suppressed the effects of β-ionone, notably it has been utilized to confirm OR51E2 activation by β-ionones [45,49,56,65]. For example, Neuhaus found that α-ionone had no effect on the intracellular calcium concentration and it inhibited the β-ionone induced increase in intracellular calcium and anti-proliferative effect in prostate cancer cells [56]. In contrast, Sanz et al. showed that α-ionone is a real agonist of OR51E2. Notably, α-ionone primarily induced growth of LNCaP cells and prostate tumors in mice whereas β-ionone increased cell invasiveness. These findings are in agreement with biased agonism phenomena reported with GPCR [120]. Thus, it is proposed that each ionone induces a distinct downstream signaling cascade depending on the G protein or β-arrestin coupled [70]. In addition, α-ionone elicited distinct responses depending on its dose; moderate doses augmented LNCaP cell invasiveness while higher doses did not (instead it sustained cell growth). It is proposed that α-ionone modulates different cellular signaling cascades depending on doses. For instance, ionone might be partially or fully activating OR51E2 that triggers distinct downstream signaling cascades. It could also mean that α-ionone is antagonizing OR51E2 at higher doses. Nevertheless, the higher doses of α-ionone activating OR51E2 in transfected HEK293 cells supported the former hypothesis [70].

4.3. Ionone Derivatives Several compounds have been synthesized and shown to exert anticancer, anti-inﬂammatory and antimicrobial activity.

4.3.1. Anti-Cancer 3-Hydroxy-β-ionone (Figure6) retarded the proliferation, colony formation and cell migration of squamous cell carcinoma. In addition, it caused apoptosis and cell cycle arrest at G2/M phase in these cells. Apoptosis was indicated by an increase in cleaved caspases and Bax with a decrease in Bcl-2 (as seen with β-ionone, Figure3)[ 121]. It appears that 3-hydroxy-β-ionone is produced endogenously from BCO2 asymmetric cleavage of zeaxanthin, a carotenoid with a 3-hydroxy-β-ionone ring [40]. β-Ionone was also found to antagonize 12-O-tetradecanoylphorbol-13-acetate (TPA); a tumor-inducing compound. Another derivative, 5,6-epoxy-β-ionone (Figure7), displayed more pronounced TPA inhibition than β-ionone in bovine lymphocytes [122]. MoleculesMolecules 20202020,, 2525,, xx FORFOR PEERPEER REVIEWREVIEW 1515 ofof 2626

4.3.1.4.3.1. Anti-CancerAnti-Cancer 3-Hydroxy-3-Hydroxy-ββ-ionone-ionone (Figure(Figure 6)6) retardedretarded thethe proliferation,proliferation, colonycolony formationformation andand cellcell migrationmigration ofMoleculesof squamoussquamous 2020, 25 cellcell, x FOR carcinoma.carcinoma. PEER REVIEW InIn addition,addition, itit causedcaused apoptosisapoptosis andand cellcell cyclecycle arrestarrest atat G2/MG2/M phasephase15 of inin26 thesethese cells.cells. ApoptosisApoptosis waswas indicatedindicated byby anan increaseincrease inin cleavedcleaved caspasescaspases andand BaxBax withwith aa decreasedecrease inin Bcl-24.3.1.Bcl-2 Anti-Cancer(as(as seenseen withwith ββ-ionone,-ionone, FigureFigure 3)3) [121].[121]. ItIt appearsappears thatthat 3-hydroxy-3-hydroxy-ββ-ionone-ionone isis producedproduced endogenouslyendogenously3-Hydroxy- fromfromβ-ionone BCO2BCO2 (Figure asymmetricasymmetric 6) retarded cleavagecleavage the proliferation,ofof zeaxanthin,zeaxanthin, colony aa carotenoidcarotenoid formation withwith and aa cell3-hydroxy-3-hydroxy- migrationββ-- iononeofionone squamous ringring [40].[40]. cell carcinoma. In addition, it caused apoptosis and cell cycle arrest at G2/M phase in these cells. Apoptosis was indicated by an increase in cleaved caspases and Bax with a decrease in Bcl-2 (as seen with β-ionone, Figure 3) [121]. It appears that 3-hydroxy-β-ionone is produced Moleculesendogenously2020, 25, 5822from BCO2 asymmetric cleavage of zeaxanthin, a carotenoid with a 3-hydroxy-15 ofβ 25- ionone ring [40].

FigureFigure 6.6. 3-Hydroxy-3-Hydroxy-ββ-ionone.-ionone. TheThe ββ-ionone-ionone moietymoiety isis shownshown inin red.red.

ββ-Ionone-Ionone waswas alsoalso foundfound toto antagonizeantagonize 12-O-tetradecanoylphorbol-13-acetate12-O-tetradecanoylphorbol-13-acetate (TPA);(TPA); aa tumor-tumor- inducinginducing compound.compound. AnotherAnother derivative,derivative, 5,6-epoxy-5,6-epoxy-ββ-ionone-ionone (Figure(Figure 7),7), displayeddisplayed moremore pronouncedpronounced TPATPA inhibitioninhibition thanthan ββ-ionone-ionone inin bovinebovine lymphocyteslymphocytes [122].[122]. FigureFigure 6.6. 3-Hydroxy-3-Hydroxy-ββ-ionone.-ionone. The β-ionone moiety is shown in red.

β-Ionone was also found to antagonize 12-O-tetradecanoylphorbol-13-acetate (TPA); a tumor- inducing compound. Another derivative, 5,6-epoxy-β-ionone (Figure 7), displayed more pronounced TPA inhibition than β-ionone in bovine lymphocytes [122].

FigureFigure 7.7.7. 5,6-Epoxy-5,6-Epoxy-ββ-ionone.-ionone. The The ββ-ionone-ionone moietymoiety is is shown shown in in red. red.

SharmaSharma andandand his hishis team teamteam combined combinedcombinedβ-ionone ββ-ionone-ionone with withwith various variousvarious aldehydes aldehydesaldehydes to prepare toto prepareprepare a series ofaa chalcones.seriesseries ofof chalcones.Chalconeschalcones. Chalcones areChalcones plant-derived areare plant-derivedplant-derived ketones composed ketonesketones composed ofcomposed two aromatic ofof twotwo rings aromaticaromatic that possessringsrings thatthat anticancer possesspossess anticancerand antimicrobial and antimicrobial activity. Allactivity. derivatives All derivatives inhibited inhibited the proliferation the proliferation of various of various cancer cancer cell lines cell anticancer and antimicrobialFigure 7. 5,6-Epoxy- activity. Allβ-ionone. derivatives The β-ionone inhibited moiety the proliferationis shown in red. of various cancer cell linesincludinglines includingincluding prostate, prostate,prostate, breast, breast,breast, CNS, CNS,CNS, cervix cervixcervix and and liver.and livliv Compounder.er. CompoundCompound (a) in(a)(a) Figure inin FigureFigure8 was 88 waswas more moremore e ﬀ ectiveeffectiveeffective in ininhibitingin inhibitinginhibitingSharma cell cellcelland proliferation proliferationproliferation his team comparedcombined comparedcompared to βtoto other-ionone otherother derivatives. derivatives. derivatives.with various It ItIt was waswas aldehydes noticed noticednoticed that thatthatto prepare compoundscompoundscompounds a series havinghaving of electron-withdrawingchalcones.electron-withdrawing Chalcones group grouparegroup plant-derived on onon the thethepara paraparaposition ketonespositionposition of the composed of ringof thethe moiety ringring of were moietymoietytwo more aromatic werewere cytotoxic moremore rings than cytotoxiccytotoxic that compounds possess thanthan withcompoundsanticancercompounds electron-donating and withwith antimicrobial electron-donatingelectron-donating group activity. in the ortho groupAllgroup position.derivatives inin thethe Further ortho orthoinhibited position. studiesposition. the usingproliferation FurtherFurther Chinese studiesstudies of hamster various usingusingovary cancer ChineseChinese cells, cell hamstershowedlineshamster including thatovaryovary compound cells,cells,prostate, showedshowed (a)breast, caused thatthat CNS, compoundcompound cytotoxicity cervix and (a)(a) via causlivcaus inductioner.eded Compound cytotoxicitycytotoxicity of apoptosis (a) viavia in Figureinductioninduction and cell 8 was cycleofof apoptosismoreapoptosis arrest effective at andand G0 phasecellincell inhibiting cyclecycle [123 arrestarrest]. cell atat proliferation G0G0 phasephase [123].[123]. compared to other derivatives. It was noticed that compounds having electron-withdrawing group on the para position of the ring moiety were more cytotoxic than compounds with electron-donating group in the ortho position. Further studies using Chinese hamster ovary cells, showed that compound (a) caused cytotoxicity via induction of apoptosis and cell cycle arrest at G0 phase [123].

FigureFigure 8.8.8.(Compound (Compound(Compounda). Raa=).). H,RR R’ == H,H,H, R” R’R’= NO== 2H,H,; [(1 R’’ER’’,4 E )-1-(4-nitrophenyl)-5-(2,6,6-trimethylcyclohex== NONO22;; [(1[(1EE,4,4EE)-1-(4-nitrophenyl)-5-(2,6,6-)-1-(4-nitrophenyl)-5-(2,6,6- -1-en-1-yl)trimethylcyclohex-1-en-1-yl)trimethylcyclohex-1-en-1-yl) penta-1,4-dien-3-one]; penta-1,4-dien-3-one];penta-1,4-dien-3-one]; (compound b). R = H, (compound(compound R’ = CF3, R” bb).).= RRH; == H, [(1H, E R’R’,4 E== )-1-(3-(triﬂuoromethyl) CFCF33,, R’’R’’ == H;H; [(1[(1EE,4,4EE)-)- phenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one]1-(3-(trifluoromethyl)phenyl)-5-(2,6,6-trimethylcycl1-(3-(trifluoromethyl)phenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one]ohex-1-en-1-yl)penta-1,4-dien-3-one] (compound c). R = NO2 , (compound R’(compound= H, R” = cHc).). [(1 RR E== ,4 NONOE)-1-(2-nitrophenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one];22,, R’R’ == H,H, R”R” == HH [(1[(1EE,4,4EE)-1-(2-nitrophenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-)-1-(2-nitrophenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4- (compound d). R = Cl, R’ = H, R” = H [(1E,4E)-1-(2-chlorophenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3- Figure 8. (Compound a). R = H, R’ = H, R’’ = NO2; [(1E,4E)-1-(4-nitrophenyl)-5-(2,6,6- one]; (compound e). R = Br, R’ = H, R” = H; [(1E,4E)-1-(2-bromophenyl)-5-(2,6,6-trimethylcyclohex trimethylcyclohex-1-en-1-yl) penta-1,4-dien-3-one]; (compound b). R = H, R’ = CF3, R’’ = H; [(1E,4E)- -1-en-1-yl)penta-1,4-dien-3-one]. The β-ionone moiety is shown in red. 1-(3-(trifluoromethyl)phenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one] (compound Zhouc). R = etNO al.2, designedR’ = H, R” a = variety H [(1E,4 ofEβ)-1-(2-nitrophenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4--ionone-based chalcones; they exhibited various level of cytotoxic effect on a variety of prostate cancer cell lines, including LNCaP, MDA-PCa-2b, 22Rv1, C4-2B and PC-3. It is believed that compounds with an electron withdrawing group at the meta position of the ring moiety are more potent than compound with an electron donating group at the para position. Compound (b) in Figure8 was found to induce the strongest cytotoxic e ﬀect in androgen dependent and independent prostate cancer cells even in comparison to β-ionone in the context of LNCaP and PC-3 cells. Furthermore, compound (b) was found to elicit anti-androgenic activity; as it inhibited MoleculesMolecules 20202020,, 2525,, xx FORFOR PEERPEER REVIEWREVIEW 1616 ofof 2626

dien-3-one];dien-3-one]; (compound(compound dd).). RR == Cl,Cl, R’R’ == H,H, R”R” == HH [(1[(1EE,4,4EE)-1-(2-chlorophenyl)-5-(2,6,6-)-1-(2-chlorophenyl)-5-(2,6,6- trimethylcyclohex-1-en-1-yl)penttrimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one];a-1,4-dien-3-one]; (compound(compound ee).). RR == Br,Br, R’R’ == H,H, R”R” == H;H; [(1[(1EE,4,4EE)-1-)-1- (2-bromophenyl)-5-(2,6,6-trimethylcyclohex(2-bromophenyl)-5-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-one].-1-en-1-yl)penta-1,4-dien-3-one]. TheThe ββ-ionone-ionone moietymoiety isis shownshown inin red.red.

ZhouZhou etet al.al. designeddesigned aa varietyvariety ofof ββ-ionone-based-ionone-based chalcones;chalcones; theythey exhibitedexhibited variousvarious levellevel ofof cytotoxiccytotoxic effecteffect onon aa varietyvariety ofof prostateprostate cancercancer cellcell lines,lines, includingincluding LNCaP,LNCaP, MDA-PCa-2b,MDA-PCa-2b, 22Rv1,22Rv1, C4-C4- 2B2B andand PC-3.PC-3. ItIt isis believedbelieved thatthat compoundscompounds withwith anan electronelectron withdrawingwithdrawing groupgroup atat thethe metameta positionposition ofof thethe ringring moietymoiety areare moremore potentpotent thanthan compoundcompound withwith anan electronelectron donatingdonating groupgroup atat thethe parapara position.position. CompoundCompound (b)(b) inin FigureFigure 88 waswas foundfound toto ininduceduce thethe strongeststrongest cytotoxiccytotoxic effecteffect inin androgenandrogen Molecules 2020, 25, 5822 16 of 25 dependentdependent andand independentindependent prostateprostate cancercancer cellscells eveneven inin comparisoncomparison toto ββ-ionone-ionone inin thethe contextcontext ofof LNCaPLNCaP andand PC-3PC-3 cells.cells. Furthermore,Furthermore, compoundcompound (b)(b) waswas foundfound toto elicitelicit anti-androgenicanti-androgenic activity;activity; asas itit inhibitedtheinhibited DHT-induced thethe DHT-inducedDHT-induced transactivation transactivationtransactivation of wild type ofof wildwild AR and typetype various ARAR andand mutated variousvarious ARs mutatedmutated (as seen ARsARs with (as(as seenseenβ-ionone, withwith βFigureβ-ionone,-ionone,1)[ 124FigureFigure]. 1)1) [124].[124]. ZhouZhou later later synthesizedsynthesized a aa varietyvarietyvariety of ofof β ββ-ionone-based-ionone-based-ionone-based compounds; compounds;compounds; the thethe compound compoundcompound shown shownshown in inin Figure FigureFigure9 9and9 andand compound compoundcompound (a) (a)(a) in Figureinin FigureFigure 10 were 1010 werewere the most thethe most potentmost popo inhibitorstenttent inhibitorsinhibitors of the proliferationofof thethe proliferationproliferation of various ofof prostatevariousvarious prostatecancerprostate cells cancercancer (including cellscells LNCaP,(includi(including MDA-PCa-2b,ng LNCaP,LNCaP, MDA-PCa-2b,MDA-PCa-2b, C4-2B and 22Rv1). C4-2BC4-2B In anan addition,dd 22Rv1).22Rv1). these InIn compounds addition,addition, these actedthese compoundsascompounds full antagonists actedacted asas of fullfull the antagonistsantagonists wild type of ARof thethe and wildwild a variety typetype ARAR of andand clinically aa varietyvariety important ofof clinicallyclinically mutated importantimportant ARs mutatedmutated in PC-3 ARscellsARs inin [125 PC-3PC-3,126 cellscells]. Molecular [125,126].[125,126]. MolecularMolecular modeling modelingmodeling revealed thatrevealedrevealed the twothatthat the bulkythe twotwo side-chains bulkybulky side-chainsside-chains of compound ofof compoundcompound (a) in (a)Figure(a) inin FigureFigure 10 are 10 critical10 areare criticalcritical for mutated forfor mutatedmutated AR antagonism. ARAR antagonism.antagonism. On the otherOnOn thethe hand, otherother compound hand,hand, compoundcompound (a) elicited (a)(a) cytotoxic elicitedelicited cytotoxicactivitycytotoxic against activityactivity PC-3 againstagainst cells, PC-3PC-3 which cells,cells, lack whichwhich AR, suggestinglacklack AR,AR, suggestingsuggesting that compound thatthat compoundcompound is not only isis targeting notnot onlyonly targetingtargeting AR [125]. ARFurtherAR [125].[125]. investigations FurtherFurther investigationsinvestigations revealed that revealedrevealed the compounds thatthat ththee compoundscompounds in Figures in9in and FiguresFigures 10 inhibited 99 andand 1010 NF-KB inhibitedinhibited activation NF-KBNF-KB activationandactivation interleukin-6 andand interleukin-6interleukin-6 (IL-6) release. (IL-6)(IL-6) Compound release.release. CompoundCompound (b) in Figure (b)(b) 10 inin, a FigureFigure derivatives 10,10, aa derivativesderivatives of the compound ofof thethe compoundcompound in Figure9, inalsoin FigureFigure suppressed 9,9, alsoalso prostatesuppressedsuppressed tumor prostateprostate cell proliferation tumortumor cellcell (C4-2B,proliferationproliferation 22Rv1, (C4-2B,(C4-2B, PC-3 and 22Rv1,22Rv1, DU-145 PC-3PC-3 cells), andand ARDU-145DU-145 and NF- cells),cells),κB ARsignaling.AR andand NF-NF- Inκκ addition,BB signaling.signaling. compound InIn addition,addition, (b) resulted compoundcompound in apoptosis (b)(b) reresultedsulted as it caused inin apoptosisapoptosis caspase asas 3 activationitit causedcaused caspase incaspase LNCaP 33 activationcellsactivation [127]. inin LNCaPLNCaP cellscells [127].[127].

FigureFigure 9.9.9.(1 E(1(1,4EE,4,4)-1-(Benzo[b]thiophen-3-yl)-5-(3-hydroxy-2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dienEE)-1-(Benzo[b]thiophen-3-yl)-5-(3-hydroxy-2,)-1-(Benzo[b]thiophen-3-yl)-5-(3-hydroxy-2,6,6-trimethylcyclohex-1-en-1-yl)penta-6,6-trimethylcyclohex-1-en-1-yl)penta- -3-one.1,4-dien1,4-dien The -3-one.-3-one.β-ionone TheThe ββ moiety-ionone-ionone is moietymoiety shown isis in shownshown red. inin red.red.

FigureFigure 10.10. (Compound(Compound(Compounda a).a).). R RR= ==H; H;H; [(1 [(1[(1E,6EEE,6,6)-4-((EE)-4-(()-4-((Z)-4-hydroxy-3-methoxybenzylidene)-1-(4-hydroxy-3-methoxyZZ)-4-hydroxy-3-methoxybenzylidene)-1-(4-hydroxy-3-)-4-hydroxy-3-methoxybenzylidene)-1-(4-hydroxy-3- methoxyphenyl)-7-(2,6,6-trimethylcyclohex-1-en-1-yl)hepta-1,6-diene-3,5-dione]methoxy phenyl)-7-(2,6,6-trimethylcyclohex-1-ephenyl)-7-(2,6,6-trimethylcyclohex-1-en-1-yl)hepta-1,6-diene-3,5-dione]n-1-yl)hepta-1,6-diene-3,5-dione] (compound (compound(compoundb). R = OH; bb).). [(1 RR E ==,6 E)-1 -(3-hydroxy-2,6,6-trimethylcyclohex-1-en-1-yl)-4-((OH;OH; [(1[(1EE,6,6EE)-1-(3-)-1-(3- hydroxy-2,6,6-trimethylcyclohex-1-en-1-yl)-4-((hydroxy-2,6,6-trimethylcyclohex-1-en-1-yl)-4-((Z)-4-hydroxy-3-methoxybenzylidene)-7-(4-hydroxy-3-ZZ)-4-hydroxy-3-)-4-hydroxy-3- methoxybenzylidene)-7-(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione].methoxyphenyl)hepta-1,6-diene-3,5-dione].methoxybenzylidene)-7-(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione]. The β-ionone moiety is shown in red. TheThe ββ-ionone-ionone moietymoiety isis shownshown inin red.red. 4.3.2. Anti-Inﬂammatory

Balbi and co-workers synthesized heterocyclic ionone-like derivatives. However, they focused on α-ionone derivatives, as they were easier to synthesize. Almost half of the synthesized compounds inhibited superoxide anion production in neutrophils in a dose-dependent manner, in which compounds (a) and (b) (Figure 11) were found to be most active. These two compounds also inhibited the chemotactic response of neutrophils in a dose dependent manner. It was found that these derivatives inhibited neutrophil chemotaxis at lower concentrations compared to nonsteroidal anti-inﬂammatory drugs. It was noticed that a pyrazole ring and its substitution at position 1 seems to be very important for activity whereas an unsubstituted pyrazole (compound (a)) or an aminotriazole (compound (b)) substituent elicited the highest activity [128]. Molecules 2020, 25, x FOR PEER REVIEW 17 of 26

4.3.2. Anti-Inflammatory Balbi and co-workers synthesized heterocyclic ionone-like derivatives. However, they focused on α-ionone derivatives, as they were easier to synthesize. Almost half of the synthesized compounds inhibited superoxide anion production in neutrophils in a dose-dependent manner, in which compounds (a) and (b) (Figure 11) were found to be most active. These two compounds also inhibited the chemotactic response of neutrophils in a dose dependent manner. It was found that these derivatives inhibited neutrophil chemotaxis at lower concentrations compared to nonsteroidal anti- inflammatory drugs. It was noticed that a pyrazole ring and its substitution at position 1 seems to be

Moleculesvery important2020, 25, 5822 for activity whereas an unsubstituted pyrazole (compound (a)) or an aminotriazole17 of 25 (compound (b)) substituent elicited the highest activity [128].

FigureFigure 11.11. (Compound(Compound aa).). RR= =H; H; [( [(E)-5-(2-(2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)-1H-pyrazole]E)-5-(2-(2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)-1H-pyrazole] (compound(compoundb b).). R R= =4-amino-4 4-amino-4HH-1,2,4-triazole-3-thiol;-1,2,4-triazole-3-thiol; [(E)-4-amino-5-(5-(2-(2,6,6-trimethylcyclohex-2-en [(E)-4-amino-5-(5-(2-(2,6,6-trimethylcyclohex-2- -1-yl)vinyl)-1en-1-yl)vinyl)-1H-pyrazol-1-yl)-4H-pyrazol-1 -yl)-4H-1,2,4-triazole-3-thiol]H-1,2,4-triazole-3-thiol] (compound (compoundc). Rc).= Rphenyl = phenyl [(E [()-1-phenyl-5-(2-E)-1-phenyl-5- (2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)-1(2-(2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)-1H-pyrazole].H-pyrazole]. The Theα-ionone α-ionone moiety moiety is shown is shown in blue.in blue.

InIn addition,addition, aa collectioncollection ofof ββ-ionone-derived-ionone-derived curcumin analogs inhibited the secretion of pro- inflammatoryinflammatory cytokinescytokines includingincluding TNF-TNF-α andand IL-6,IL-6, followingfollowing LPSLPS stimulationstimulation inin aa dosedose dependentdependent manner.manner. TheseThese analogsanalogs werewere foundfound toto bebe moremore potentpotent thanthan curcumin.curcumin. The most active compound (c)(c) (Figure(Figure8 11)) inhibited inhibited LPS-induced LPS-induced septic septic death death in in mice. mice. The The study study showed showed that that having having any any substituent substituent atat thethe second second position position of of the the phenyl phenyl group group resulted resulted in normal in normal activity activity against against inflammatory inflammatory cells while cells awhile substituent a substituent at fourth at positionfourth position of the phenyl of the groupphenyl reduced group reduced the activity the of activity the compounds. of the compounds. In addition, In itaddition, showed it that showed having that chloro having group chloro substituents group substituents at the third andat the fourth third position and fourth of the position phenyl groupof the displayedphenyl group normal displayed activity againstnormal inflammatoryactivity against cells inflammatory however replacing cells however the chloro replacing groups with the methylchloro groupsgroups almostwith methyl abolished groups this almost activity abolished [129]. this activity [129].

4.3.3.4.3.3. Antimicrobial EEffectﬀect AA varietyvariety ofof heterocyclicheterocyclic ionone-likeionone-like derivativesderivatives displayeddisplayed moderatemoderate to good activity against C. albicansalbicans.. OnlyOnly compound compound (c) (c) (Figure (Figure 11 )11) and and the compoundthe compound shown shown in Figure in Figure12 exhibited 12 exhibited good activity good againstactivityPseudomonas against Pseudomonas aeruginosa aeruginosa, Propionibacterium, Propionibacterium acnes and acnesE. coli and. Most E. coli of. theMost compounds of the compounds showed goodshowed activity good againstactivityStaphylococcus against Staphylococcus aureus. Theaureus addition. The addition of a phenyl of a phenyl group atgroup the position-1 at the position-1 of the pyrazoleof the pyrazole ring is important ring is important for the antibacterial for the antibact activityerial however, activity the however, substitution the withsubstitution 2,4-dinitrophenyl with 2,4- groupdinitrophenyl in the same group position in the reducedsame position the inhibitory reduced activitythe inhibitory against activityE. coli, againstP. aeruginosa E. coliand, P. aeruginosaS. aureus. Itand was S. concludedaureus. It was that concluded lipophilicity that is anlipophilicity important is factor an important in increasing factor the in activity increasing against the broaderactivity spectrumMoleculesagainst 2020broader of, microbial25, x spectrumFOR PEER species REVIEW of microbial [130 ]. species [130]. 18 of 26

FigureFigure 12.12. ((E)-2-(Methylthio)-4-(2-(2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)pyrimidine.)-2-(Methylthio)-4-(2-(2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)pyrimidine. TheThe αα-ionone-ionone moietymoiety isis shownshown inin blue.blue.

SharmaSharma andand his his team team combined combinedβ-ionone β-ionone with with various various aldehydes aldehydes to prepare to prepare a series ofa chalcones.series of Thechalcones. active compoundsThe active compounds exhibited promising exhibited antimicrobial promising antimicrobial activity against activity a variety against of Gram a variety negative of Gram negative bacteria (E. coli, P. aeruginosa, Salmonella typhimurium), Gram positive bacteria (S. aureus, Bacillus subtilis) and fungal strains (A. niger, Saccharomyces cerevisiae, C. albicans). In addition, further testing showed that some of the active compounds were also active against methicillin resistant S. aureus. Compound (d) in Figure 8 displayed the highest anti-bacterial activity against both Gram negative and positive bacteria while compound (e) showed the uppermost antifungal activity against species including A. niger and S. cerevisiae. It was noticed that compounds with electron withdrawing substituents on the aromatic ring mostly displayed better antibacterial activity in comparison to compounds with electron releasing groups [131]. β-Ionone was also used as a starting material to produce a variety of synthetic halolactones and microbiologically obtained hydroxylactones. All of the derivatives inhibited the growth of B. subtilis, S. aureus and E.coli, however, the chlorolactone (compound (a) in Figure 13) and hydroxylactone (compound (b)) completely inhibited the growth of B. subtilis and S. aureus, respectively. In addition, all of the compounds increased the growth of A. niger and Fusarium linii except the bromolactone (compound (c) in Figure 13) which completely inhibited the growth of A. niger [132].

Figure 13. (Compound a). R = Cl; [10-(11-chloroethyl)-7-(1,1,5-trimethylcyclohex-5-en-6- yl)dihydrofuran-2(3H)-one]; (compound b). R = OH [10-(11-hydroxyethyl)-7-(1,1,5- trimethylcyclohex-5-en-6-yl)dihydrofuran-2(3H)-one]; (compound c). R = Br [10-(11-bromoethyl)-7- (1,1,5-trimethylcyclohex-5-en-6-yl)dihydrofuran-2(3H)-one]. The β-ionone moiety is shown in red.

Suryawanshi and his team synthesized several terpenyl pyrimidines substituted with 4-N in which the starting material (ketene dithioacetal) was made available from β-ionone. The 4- thiomethoxy substituted pyrimidine (Figure 14) caused 66% in vivo inhibition of Leishmania donovani in hamsters. It was noticed that the 4-thiomethoxy group is very important for the anti-

Molecules 2020, 25, x FOR PEER REVIEW 18 of 26

Figure 12. (E)-2-(Methylthio)-4-(2-(2,6,6-trimethylcyclohex-2-en-1-yl)vinyl)pyrimidine. The α-ionone Molecules 2020, 25, 5822 18 of 25 moiety is shown in blue. bacteria (E.Sharma coli, P. and aeruginosa, his team Salmonella combined typhimurium), β-ionone withGram various positive aldehydes bacteria to ( S.prepare aureus ,aBacillus series subtilisof ) chalcones. The active compounds exhibited promising antimicrobial activity against a variety of and fungal strains (A. niger, Saccharomyces cerevisiae, C. albicans). In addition, further testing showed Gram negative bacteria (E. coli, P. aeruginosa, Salmonella typhimurium), Gram positive bacteria (S. that someaureus of, Bacillus the active subtilis compounds) and fungal were strains also (A. active niger, againstSaccharomyces methicillin cerevisiae resistant, C. albicansS. aureus). In addition,. Compound (d) infurther Figure testing8 displayed showed the that highest some anti-bacterialof the active compounds activity against were bothalso Gramactive negativeagainst methicillin and positive bacteriaresistant while S. aureus compound. Compound (e) showed (d) in Figure the uppermost 8 displayed the antifungal highest anti-bacterial activity against activity species against includingboth A. nigerGramand negativeS. cerevisiae and positive. It was bacteria noticed while that compounds compound (e) with showed electron the uppermost withdrawing antifungal substituents activity on the aromaticagainst ring species mostly including displayed A. betterniger and antibacterial S. cerevisiae activity. It was in noticed comparison that compounds to compounds with withelectron electron releasingwithdrawing groups [substituents131]. on the aromatic ring mostly displayed better antibacterial activity in βcomparison-Ionone was to compounds also used aswith a starting electron materialreleasing togroups produce [131]. a variety of synthetic halolactones and β-Ionone was also used as a starting material to produce a variety of synthetic halolactones and microbiologically obtained hydroxylactones. All of the derivatives inhibited the growth of B. subtilis, microbiologically obtained hydroxylactones. All of the derivatives inhibited the growth of B. subtilis, S. aureus and E.coli, however, the chlorolactone (compound (a) in Figure 13) and hydroxylactone S. aureus and E.coli, however, the chlorolactone (compound (a) in Figure 13) and hydroxylactone (compound(compound (b)) (b)) completely completely inhibited inhibited the the growth growth of B.B. subtilis subtilis andand S. S.aureus aureus, respectively., respectively. In addition, In addition, all ofall the of compoundsthe compounds increased increased the the growth growth ofof A.A. niger andand FusariumFusarium linii linii exceptexcept the thebromolactone bromolactone (compound(compound (c) in (c) Figure in Figure 13 )13) which which completely completely inhibitedinhibited the the growth growth of of A.A. niger niger [132].[132 ].

FigureFigure 13. (Compound 13. (Compounda). R = Cl;a). [10-(11-chloroethyl)-7-(1,1,5-trimethylcyclohex-5-en-6-yl)dihydrofuran-2 R = Cl; [10-(11-chloroethyl)-7-(1,1,5-trimethylcyclohex-5-en-6- (3H)-one];yl)dihydrofuran-2(3H (compound b).)-one]; R = OH [10-(11-hydroxyethyl)-7-(1,1,5-trimethylcyclohex-5-en-6-yl)dihydrofuran(compound b). R = OH [10-(11-hydroxyethyl)-7-(1,1,5- -2(3H)-one];trimethylcyclohex-5-en-6-yl)dihy (compound c). R = Br [10-(11-bromoethyl)-7-(1,1,5-trimethylcyclohex-5-en-6-yl)dihydrofuran-2drofuran-2(3H)-one]; (compound c). R = Br [10-(11-bromoethyl)-7- (3H)-one].(1,1,5-trimethylcyclohex-5-en-6- The β-ionone moiety isyl)dihydrofuran-2(3H)-one]. shown in red. The β-ionone moiety is shown in red.

SuryawanshiSuryawanshi and and his his team team synthesized synthesized severalseveral te terpenylrpenyl pyrimidines pyrimidines substituted substituted with with4-N in 4-N in which the starting material (ketene dithioacetal) was made available from β-ionone. The 4- which the starting material (ketene dithioacetal) was made available from β-ionone. The 4-thiomethoxy thiomethoxyMolecules 2020, 25 substituted, x FOR PEER REVIEWpyrimidine (Figure 14) caused 66% in vivo inhibition of Leishmania19 of 26 substituteddonovani pyrimidine in hamsters. (Figure It was 14 noticed) caused that 66% thein 4-thiomethoxy vivo inhibition group of Leishmaniais very important donovani for the in anti- hamsters. It was noticed that the 4-thiomethoxy group is very important for the anti-leishmanial activity as its leishmanial activity as its replacement with various primary and secondary amines resulted in replacementactivity loss with [133]. various primary and secondary amines resulted in activity loss [133].

FigureFigure 14. ( E14.)-4-(methylthio)-6-(2-(2,6,6-trimethylcyclohex-1-en-1-yl)vinyl)pyrimidin-2-amine. (E)-4-(methylthio)-6-(2-(2,6,6-trimethylcyclohex-1-en-1-yl)vinyl)pyrimidin-2-amine. The β The- β- iononeionone moiety moiety is shown is shown in red.in red.

5. Conclusions5. Conclusions and and Outlook Outlook Ionone,Ionone, speciﬁcally specificallyβ-ionone, β-ionone, can can either either bebe endogenouslyendogenously produced produced via via BCO2 BCO2 or exogenously or exogenously suppliedsupplied via thevia diet.the diet.β-Ionone, β-Ionone, whether whether ofof anan endogenousendogenous or or exogenous exogenous origin, origin, possess possess anticancer, anticancer, chemopreventive,chemopreventive, cancer cancer promoting, promoting, melanogenesis, melanogenesis, anti-inflammatory anti-inﬂammatory and and antimicrobial antimicrobial activity. activity. β-Ionone mediates these effects via activation of OR51E2 and modulation of HMG CoA reductase, cell cycle regulatory proteins, pro- and anti- apoptotic pathways, pro-inflammatory cytokines and anti-oxidant enzymes. The activation of OR51E2 causes regulation of the activity of various kinases and an increase in intracellular calcium. OR51E2 activation results in anti-proliferative effect and metastasis in prostate cancer cells, anti-proliferative effect and melanogenesis in melanocytes and proliferation and metastasis in retinal pigment epithelial cells. The wide spectrum of effects and various signaling cascades attributed to β-ionone might be due to the different cell repertoire [37]. In addition, β-ionone might be interacting with a wide range of receptors, not constrained to OR51E2, such as the reported interaction with AR and RXR-α. Moreover, β-ionone might be activating cytosolic OR51E2 in various intracellular compartments. This is based on β-ionone hydrophobic properties that allows it to traverse cellular membrane. β-Ionone and α-ionone derivatives exhibit anti-inflammatory, anti-microbial and anticancer effects. Further studies are recommended to investigate the structure activity relationship between β- and α-ionone derivatives and the various reported physiological effects. However, most of the studies are in agreement that electron-withdrawing substituents on the ring moiety attached to β- ionone yield better anti-cancer and anti-microbial activity. Overall, the data demonstrates that β-ionone is a promising scaffold for cancer, inflammation and infectious disease research and thus is more than a violet’s fragrance.

Funding: This research received no external funding.

Acknowledgments: The structures were drawn using ChemDraw while the other figures where generated using Biorender.

Conflicts of Interest: The authors declare no conflict of interest.

References

Molecules 2020, 25, 5822 19 of 25

β-Ionone mediates these effects via activation of OR51E2 and modulation of HMG CoA reductase, cell cycle regulatory proteins, pro- and anti- apoptotic pathways, pro-inflammatory cytokines and anti-oxidant enzymes. The activation of OR51E2 causes regulation of the activity of various kinases and an increase in intracellular calcium. OR51E2 activation results in anti-proliferative effect and metastasis in prostate cancer cells, anti-proliferative effect and melanogenesis in melanocytes and proliferation and metastasis in retinal pigment epithelial cells. The wide spectrum of effects and various signaling cascades attributed to β-ionone might be due to the different cell repertoire [37]. In addition, β-ionone might be interacting with a wide range of receptors, not constrained to OR51E2, such as the reported interaction with AR and RXR-α. Moreover, β-ionone might be activating cytosolic OR51E2 in various intracellular compartments. This is based on β-ionone hydrophobic properties that allows it to traverse cellular membrane. β-Ionone and α-ionone derivatives exhibit anti-inflammatory, anti-microbial and anticancer effects. Further studies are recommended to investigate the structure activity relationship between β- and α-ionone derivatives and the various reported physiological effects. However, most of the studies are in agreement that electron-withdrawing substituents on the ring moiety attached to β-ionone yield better anti-cancer and anti-microbial activity. Overall, the data demonstrates that β-ionone is a promising scaffold for cancer, inflammation and infectious disease research and thus is more than a violet’s fragrance.

Funding: This research received no external funding. Acknowledgments: The structures were drawn using ChemDraw while the other figures where generated using Biorender. Conflicts of Interest: The authors declare no conflict of interest.

References

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