molecules Article Ellagitannins of Davidia involucrata. I. Structure of Davicratinic Acid A and Effects of Davidia Tannins on Drug-Resistant Bacteria and Human Oral Squamous Cell Carcinomas Yuuki Shimozu 1, Yuriko Kimura 1, Akari Esumi 1, Hiroe Aoyama 1, Teruo Kuroda 2, Hiroshi Sakagami 3 and Tsutomu Hatano 1,* 1 Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, Okayama 700-8530, Japan; [email protected] (Y.S.); [email protected] (Y.K.); [email protected] (A.E.); [email protected] (H.A.) 2 Department of Molecular Microbiology and Biotechnology, Graduate School of Biomedical and Health Sciences, Hiroshima University, Hiroshima 734-8553, Japan; [email protected] 3 Division of Pharmacology, Department of Diagnostic and Therapeutic Sciences, School of Dentistry, Meikai University, Saitama 350-0283, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-86-251-7936; Fax: +81-86-251-7926 Academic Editor: Derek J. McPhee Received: 11 February 2017; Accepted: 12 March 2017; Published: 15 March 2017 Abstract: We isolated a new ellagitannin, davicratinic acid A (5), together with four known ellagitannins, davidiin (1), granatin A (2), pedunculagin (3), and 3-O-galloylgranatin A (4), from an aqueous acetone extract of dried Davidia involucrata leaves. The known ellagitannins were identified based on spectroscopic data. The structure of davicratinic acid A (5), a monomeric ellagitannin possessing a unique, skew-boat glucopyranose core, was established based on spectroscopic data. Additionally, we examined the effects of several tannins with good yields from this plant on drug-resistant bacteria and human oral squamous cell carcinomas, and found that davidiin (1) exhibited the most potent antibacterial and antitumor properties among the tannins examined. Keywords: Davidia involucrata; ellagitannin; anti-bacterial effect; antitumor effect; davicratinic acid A 1. Introduction Davidia involucrata Baill., known as the dove tree or handkerchief tree, is the sole species of the genus Davidia of the family Davidiaceae (as per the Engler system) [1]. Although it has been cultivated as an ornamental tree, its natural habitat is limited to a region in southern China. A number of constituents including phenolics [2] and triterpenes [3,4] have been identified from its branch bark. More significantly, this plant has been shown to contain ellagitannins [5]. Ellagitannins, a large group of polyphenolic compounds widely distributed in plants, are often encountered as the constituents of food, beverages, and medicinal plants [6]. Ellagitannins from various plant sources have been demonstrated to have notable antiviral, antimicrobial, and antitumor properties [7]. The isolation of davidiin (1) and granatin A (2) (Figure1) as major tannins from D. involucrata leaves was reported in 1982 [5]. Among them, 1 is a unique ellagitannin possessing a skew-boat glucopyranose core. Recently, this compound has been identified as an inhibitor of the protein modification called SUMOylation in a post-translational process in which the inhibitor suppresses the formation of the small ubiquitin-like modifier (SUMO)-activating enzyme-SUMO-1 intermediate [8]. Despite this discovery, there are few reports of additional chemical research on its constituents, probably due to the scarcity of this plant species. Molecules 2017, 22, 470; doi:10.3390/molecules22030470 www.mdpi.com/journal/molecules Molecules 2017, 22, 470 2 of 9 MoleculesInfectious 2017, 22, 470 diseases caused by multidrug-resistant bacteria, including methicillin-resistant2 of 9 Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE), have become serious problemsworldwide worldwide [9]. To date, [9]. researchTo date, has research been hasfocused been on focused developing on developing antibacterial antibacterial agents against agents againstMRSA andMRSA VRE. and The VRE. synergistic The synergistic effects effects of ellagitannins of ellagitannins with antibiotics with antibiotics against against drug-resistant drug-resistant bacteria bacteria are amongare among the most the most notable notable antimicrobial antimicrobial activities activities of tannins of tannins [10]. [ Our10]. Ourrecent recent studies studies have have revealed revealed that severalthat several phenolic phenolic compounds compounds exhibited exhibited potent potent anti-VRE anti-VRE activity activity [11,12]. [11,12 ].Accordingly, Accordingly, it it is is expected expected that ellagitannins fromfrom DD.. involucratainvolucrata alsoalso havehavepotent potentantibacterial antibacterial effects effects against against MRSA MRSA and and VRE. VRE. Several lines lines of of evidence evidence suggest suggest that thattannins tannins have important have important antitumor antitumor properties, properties, such as inhibition such as ofinhibition tumor promotion of tumor promotion[13,14] and [inhibition13,14] and of inhibition carcinogen of mutagenicity carcinogen mutagenicity [15]. Recent evidence [15]. Recent includes evidence the inhibitoryincludes the effects inhibitory of 1 effectson cell of 1proliferationon cell proliferation and tumor and tumorgrowth growth in hepatocellular in hepatocellular carcinomas carcinomas by downregulatingby downregulating EZH2 EZH2 via via a aproteasome-depende proteasome-dependentnt mechanism mechanism [16]. [16]. The The cytotoxic cytotoxic effects effects of ellagitannin monomersmonomers and and dimers dimers from fromTamarix Tamarix nilotica niloticaagainst against human oralhuman squamous oral squamous cell carcinomas cell (OSCC)carcinomas [17– (OSCC)19], depending [17–19], ondepending their structures, on their havestructures, been shown have been in our shown previous in our studies. previous studies. In this study, wewe isolatedisolated a newnew ellagitannin, together with four known ones, from the leaves of D. involucratainvolucrata, and determineddetermined thethe antibacterialantibacterial effects ofof DavidiaDavidia tanninstannins onon antibiotic-resistantantibiotic-resistant bacteria (MRSA and VRE) and the cytotoxic effectseffects onon OSCCOSCC (Ca9-22,(Ca9-22, HSC-2,HSC-2, HSC-3,HSC-3, andand HSC-4)HSC-4) cellcell lines.lines. Figure 1. StructuresStructures of hydrolysab hydrolysablele tannins isolated from Davidia involucrata. 2. Results and Discussion 2.1. Elucidation of the Structure of the New Ellagitannin An aqueous acetone homogenate of the dried leaves of D.. involucratainvolucrata was extracted successively with chloroform, ethyl acetate, and n-BuOH. In addition to the four known tannins obtained from the n-BuOH extract, davicratinic acid acid A A ( 5) was isolated as a new ellagitannin from from the H 2OO extract. extract. The known tannins were identified as davidiin (1) [7,20], granatin A (2) [7], pedunculagin (3) [21], and 3-O-galloylgranatin A (helioscopinin A) (4) [22] (Figure 1) based on the 1H-NMR and ESI-MS data. Davicratinic acid A (5) was obtained as a pale yellow amorphous powder. High-resolution electrospray ionization mass spectrometry (HR-ESI-MS) in the positive ion mode showed the [M + H]+ Molecules 2017, 22, 470 3 of 9 Molecules 2017, 22, x 3 of 9 andThe known3-O-galloylgranatin tannins were A identified (helioscopinin as davidiin A) (4 (1) )[[227,]20 (Figure], granatin 1) based A (2)[ on7], pedunculaginthe 1H-NMR and (3)[ 21ESI],- andMS data3-O-galloylgranatin. A (helioscopinin A) (4)[22] (Figure1) based on the 1H-NMR and ESI-MS data. DavicratinicDavicratinic acid acid A A ( 5)) was obtained as a palepale yellowyellow amorphousamorphous powder.powder. High High-resolution-resolution electrosprayelectrospray ionization ionization mass mass spectrometry spectrometry (HR (HR-ESI-MS)-ESI-MS) in the positive ion mode showed the [M + H]+ 11 ionion peak peak corresponding corresponding toto thethe molecularmolecular formulaformula CC4141H3131O28. .The The HH-NMR-NMR spectrum spectrum of of 55 showedshowed that that thethe signals signals were were due due to to a a galloyl galloyl group group ( (δδ 7.03, 2H, s) and a 4,4 ′0,5,5,5,5′0,6,6,6,6′-0hexahydroxydiphenoyl-hexahydroxydiphenoyl (HHDP) (HHDP) groupgroup ( (δδ 6.746.74 and and 6.89, 6.89, each each 1H, 1H, s). s). In In addition, addition, the the spectrum spectrum exhibited exhibited sugar sugar signals signals at at δδ 3.933.93 (1H, (1H, dd, dd, JJ == 4.8, 4.8 ,7.8 7.8 Hz), Hz), 4.04 4.04 (1H, (1H, dd, dd, J =J 4.8,= 4.8, 11.4 11.4 Hz), Hz), 4.26 4.26 (1H, (1H, dt, dt,J = 4.8,J = 4.8,11.4 11.4Hz), Hz), 4.60 4.60(1H, (1H,t, J = t,11.4J = Hz), 11.4 5.13 Hz), (1H,5.13 (1H,dd, J dd,= 3.6,J = 7.8 3.6, Hz), 7.8 Hz),5.37 (1H, 5.37 (1H,t, J = t,7.8J =Hz), 7.8 Hz),and 5.88 and 5.88(1H, (1H,d, J = d, 3.6J = Hz), 3.6 Hz),which which were were assigned assigned to the to glucosethe glucose H-4, H-4, H-6a, H-6a, H-5, H-5, H-6b, H-6b, H-2, H-2, H- H-3,3, and and H- H-11 signals signals based based on on 1H1H--1H1H-COSY.-COSY. The The appearance appearance of of thethe H H-4-4 signal in the higherhigher fieldfield indicatedindicated thatthat the the C-4 C-4 hydroxyl hydroxyl group group was was unacylated. unacylated. According According to tothe the coupling coupling constants constants of of these these glucose glucose proton proton signals, signals, the the glucopyranoseglucopyranose ringring inin 5 was as assignedsigned a a skewskew-boat–type-boat–type conformation. conformation. The The HMBC HMBC spectrum spectrum revealed revealed that that glucose glucose OO--33 was was acylated acylated by by the the galloylgalloyl group group and and the the OO--11 and and OO--66 positions positions of of glucose glucose were were bridged bridged via via the the HHDP HHDP group group (Figure (Figure 2a).2a). TheThe HMBC HMBC spectrum spectrum of of 55 also showed couplings couplings of of the the si singletnglet protons protons at at δδ 6.83 (H (H-5-5′0)) and and 5.49 5.49 (H (H-3-3′0)) withwith a a carboxyl carboxyl carbon carbon signal signal at at δδ 165.5165.5 (C (C-7-7′0)) of of the the remaining remaining acyl acyl group.