molecules Review The Biological Effects of Forsythia Leaves Containing the Cyclic AMP Phosphodiesterase 4 Inhibitor Phillyrin Sansei Nishibe 1,*, Kumiko Mitsui-Saitoh 2, Junichi Sakai 2 and Takahiko Fujikawa 3,* 1 Faculty of Pharmaceutical Sciences, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293, Japan 2 Faculty of Health and Sport, Nagoya Gakuin University, 1350 Kamishinano, Seto, Aichi 480-1298, Japan; [email protected] (K.M.-S.); [email protected] (J.S.) 3 Faculty of Pharmaceutical Sciences, Suzuka University of Medical Science, 3500-3 Minamitamagaki-cho, Suzuka-City, Mie 513-8670, Japan * Correspondence: [email protected] (S.N.); [email protected] (T.F.); Fax: +81-11-812-5460 (S.N.); +81-59-340-0595 (T.F.) Abstract: Forsythia fruit (Forsythia suspensa Vahl (Oleaceae)) is a common component of Kampo medicines for treating the common cold, influenza, and allergies. The main polyphenolic compounds in the leaves of F. suspensa are pinoresinol β-D-glucoside, phillyrin and forsythiaside, and their levels are higher in the leaves of the plant than in the fruit. It is known that polyphenolic compounds stimulate lipid catabolism in the liver and suppress dyslipidemia, thereby attenuating diet-induced obesity and polyphenolic anti-oxidants might attenuate obesity in animals consuming high-fat diets. Recently, phillyrin was reported as a novel cyclic AMP phosphodiesterase 4 (PDE4) inhibitor derived from forsythia fruit. It was expected that the leaves of F. suspensa might display anti-obesity effects and Citation: Nishibe, S.; Mitsui-Saitoh, K.; Sakai, J.; Fujikawa, T. The serve as a health food material. In this review, we summarized our studies on the biological effects of Biological Effects of Forsythia Leaves forsythia leaves containing phillyrin and other polyphenolic compounds, particularly against obesity, Containing the Cyclic AMP atopic dermatitis, and influenza A virus infection, and its potential as a phytoestrogen. Phosphodiesterase 4 Inhibitor Phillyrin. Molecules 2021, 26, 2362. Keywords: forsythia leaves; polyphenolic compound; forsythiaside; phillyrin; PDE4 inhibitor; https://doi.org/10.3390/ anti-obesity; atopic dermatitis; influenza A virus infection; phytoestrogen molecules26082362 Academic Editors: Ihsan˙ Çalı¸s, Horváth Györgyi and 1. Introduction Agnieszka Ludwiczuk Obesity has become an urgent worldwide public health problem in recent decades. Diet-induced obesity is closely associated with lifestyle-related diseases (e.g., diabetes, Received: 23 March 2021 hyperlipidemia, hypertension), which are primary risk factors for cardiovascular dis- Accepted: 14 April 2021 Published: 19 April 2021 ease [1]. Therefore, preventing obesity is strongly encouraged to alleviate various lifestyle- related diseases. Forsythia suspensa Publisher’s Note: MDPI stays neutral Vahl (Oleaceae) is listed in Japanese Pharmacopoeia as the original with regard to jurisdictional claims in plant of the crude drug “forsythia fruit” [2], a common component in Kampo medicines published maps and institutional affil- for treating the common cold, influenza, and allergies. With at least 3000 years of con- iations. tinuous use, forsythia fruit is traditionally considered a detoxicant for treating so-called toxic and hot conditions. In addition, forsythia fruit is one of the main components of “Bofutsushosan” (BOFU), an extremely popular Kampo medicine in Japan with anti-obesity effects [3]. Wang et al. reported review with the title of “Phytochemistry, pharmacology, quality control, and future research of Forsythia suspensa (Thunb.) Vahl: A review” [4]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. However, we could not find a description of the anti-obesity effects of the forsythia fruit or This article is an open access article leaves alone in the review. distributed under the terms and We found that the main polyphenolic compounds in leaves of F. suspensa are pinoresinol conditions of the Creative Commons β-D-glucoside, phillyrin and forsythiaside and their levels are higher in the leaves of the Attribution (CC BY) license (https:// plant than in its fruit [5,6]. Polyphenolic compounds stimulate lipid catabolism in the creativecommons.org/licenses/by/ liver and suppress dyslipidemia, thereby attenuating diet-induced obesity, and polyphe- 4.0/). nolic anti-oxidants can attenuate obesity in animals fed a high-fat diets (HFD) [7,8]. We Molecules 2021, 26, 2362. https://doi.org/10.3390/molecules26082362 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 20 We found that the main polyphenolic compounds in leaves of F. suspensa are pi- noresinol β-D-glucoside, phillyrin and forsythiaside and their levels are higher in the leaves of the plant than in its fruit [5,6]. Polyphenolic compounds stimulate lipid catabo- Molecules 2021, 26, 2362 2 of 21 lism in the liver and suppress dyslipidemia, thereby attenuating diet-induced obesity, and polyphenolic anti-oxidants can attenuate obesity in animals fed a high-fat diets (HFD) [7,8]. We found that pinoresinol β-D-glucoside is a strong cyclic AMP phosphodiesterase (PDE)found inhibit that pinoresinolor in vitro β[9].-D -glucosideHowever, it is was a strong unclear cyclic whether AMP pinoresinol phosphodiesterase β-D-glucoside (PDE) isinhibitor a selectivein vitro PDE4[9 ].inhibitor However, or not. it was Phillyrin unclear also whether showed pinoresinol a moderateβ -inhibitionD-glucoside in isour a analysis.selective PDE4Phillyrin inhibitor was recently or not. Phillyrinreported alsoas a showednovel selective a moderate PDE4 inhibition inhibitor in [10 our]. As analysis. PDE4 inhibitionPhillyrin was is a recentlytherapeutic reported strategy as a for novel metabolic selective disorders PDE4 inhibitor [11], it is [10 expected]. As PDE4 that inhibition the fruit andis a therapeuticleaves of F. strategysuspensa formight metabolic display disorders anti-obesity [11], effect it is expecteds. that the fruit and leaves of F.However, suspensa might the use display of forsythia anti-obesity fruit effects.as a health food is currently prohibited in Japan by theHowever, Regulatory the uselaw of“Distinction forsythia fruit between as a health drugs food and is food currently stuffs” prohibited from Ministry in Japan of Health,by the RegulatoryLabor, and lawWelfare “Distinction (Japan). Therefore, between drugs we studied and food the stuffs”biological from effects Ministry of for- of sythiaHealth, leaves Labor, for and use Welfare as a health (Japan). food Therefore, material we as studied an alternative the biological to pharmaceuticals. effects of forsythia In addition,leaves for studies use as aof health F. suspensa food materialregarding as its an effects alternative on atopic to pharmaceuticals. dermatitis [12] and In addition, the pre- ventionstudies ofof F.influenza suspensa [13,14]regarding have its been effects reported. on atopic dermatitis [12] and the prevention of influenzaThis review [13,14] summarizes have been reported. our studies on the biological effects of forsythia leaves contain- ing phillyrinThis review and other summarizes polyphenolic our compounds studies on against the biological obesity, effectsatopic dermatitis, of forsythia and leaves influ- enzacontaining A virus phillyrin infection, and as well other as polyphenolic its potential as compounds a phytoestrogen. against obesity, atopic dermatitis, and influenza A virus infection, as well as its potential as a phytoestrogen. 2. Structures of Phillyrin and Forsythiaside 2. Structures of Phillyrin and Forsythiaside Pinoresinol β-D-glucoside, phillyrin and forsythiaside (caffeoyl glycoside of 3,4-di- Pinoresinol β-D-glucoside, phillyrin and forsythiaside (caffeoyl glycoside of 3,4- hydroxy-β-phenethyl alcohol) were isolated as the main polyphenolic compounds from dihydroxy-β-phenethyl alcohol) were isolated as the main polyphenolic compounds from thethe fruit fruit and and leaves leaves of of F. F.suspens suspensaa, respectively, respectively [5,6]. [5 ,The6]. contents The contents of the of compound the compoundss in the leaves are as follows; pinoresinol β-D-glucoside, 0.3–2%, phillyrin, 0.4–3% and forsythi- in the leaves are as follows; pinoresinol β-D-glucoside, 0.3–2%, phillyrin, 0.4–3% and asideforsythiaside,, 0.4–5% [ 0.4–5%6]. [6]. 2.1. Phillyrin Phillyrin PhillyrinPhillyrin,, the glucoside of (+) (+)-epipinoresinol-epipinoresinol monomethyl monomethyl ether ether,, is widely distributed inin the leaves of Forsythia species [[1155]. T Thehe position (C (C-4-4′0 or C C-4-4”)00) of the glucose linkage with the aglycone was previously undetermined. We We establishedestablished the position using the 13CC-NMR-NMR spectra spectra of of aglycone aglycone derivatives derivatives as as shown in Figure 11[ [1166].]. The aglycone was obtained via the hydrolysis of phillyrinphillyrin by β-glucosidase.-glucosidase. Pelter et al. reported that the 11′0 and 1” 100 carboncarbon atoms atoms of the the axial and equatorial aryl groups of 2,6 2,6-diaryl-3,7-dioxabicyclo-diaryl-3,7-dioxabicyclo [3.3.0][3.3.0] octane octane lignans lignans are clearly distinct from each other in thethe 1313CC-NMR-NMR spectra, spectra, that that is, is, thethe signals ofof formerformer areare assigned assigned to to approximately approximately 131 131 ppm, ppm, whereas whereas the the latter latter signals signals are areassigned assigned to approximately to approximately 134 134 ppm ppm for thefor the veratryl veratryl group group [17]. [17]. 1’ RO 1” Phillyrin : R=-D-glucose