Browse Scientific Research by Tag

Flavanols, a class of polyphenols present in certain plant-based foods, have received increasing attention for their putative anticancer activity. In vitro and in vivo studies, which have compared the effectiveness of various monomer flavanols, indicate that the presence of a galloyl residue on the 3 position on the C-ring enhances the cytotoxicity of these compounds. Procyanidins, oligomerized flavanols, have been reported to be more cytotoxic than monomer flavanols in a variety of human cancer cell lines. Given the above, we evaluated the potential anticancer properties of dimer procyanidins that contain galloyl groups. Specifically, the cytotoxicity of synthetic digalloyl dimer B1 and B2 esters {[3-O-galloyl]-(−)-epicatechin-(4β,8)-(+)-catechin-3-O-gallate (DGB1) and [3-O-galloyl]-(−)-epicatechin-(4β,8)-(+)-epicatechin-3-O-gallate (DGB2), respectively} were tested in a number of in vitro models. DGB1 produced significant cytotoxicity in a number of human cancer cell lines evaluated by three independent methods: ATP content, MTT and MTS assays. For the three most sensitive cell lines, exposure to DGB1 and DGB2 for 24, 48 or 72 h was associated with a reduction in cell number and an inhibition of cell proliferation. Digalloyl dimers exerted significantly higher cytotoxic effects than the structurally related flavanols, (−)-epicatechin, (+)-catechin, (−)-epicatechin gallate, (−)-epigallocatechin gallate, (−)-catechin gallate and dimer B1 and B2. These results support the concept that the incorporation of galloyl groups and the oligomerization of flavanols enhances the cytotoxic effects of typical monomer flavanols. The therapeutic value of these compounds and their derivative forms as anticancer agents merits further investigation in whole animal models.

A naturally occurring, cocoa-derived pentameric procyanidin (pentamer) was previously shown to cause G0/G1 cell cycle arrest in human breast cancer cells by an unknown molecular mechanism. Here, we show that pentamer selectively inhibits the proliferation of human breast cancer cells (MDA MB-231, MDA MB-436, MDA MB-468, SKBR-3, and MCF-7) and benzo(a)pyrene-immortalized 184A1N4 and 184B5 cells. In contrast, normal human mammary epithelial cells in primary culture and spontaneously immortalized MCF-10A cells were significantly resistant. We evaluated whether this differential response to pentamer may involve depolarization of the mitochondrial membrane. Pentamer caused significant depolarization of mitochondrial membrane in MDA MB231 cells but not the more normal MCF-10A cells, whereas other normal and tumor cell lines tested gave variable results. Further investigations, using a proteomics approach with pentamer-treated MDA MB-231, revealed a specific dephosphorylation, without changes in protein expression, of several G1-modulatory proteins: Cdc2 (at Tyr15), forkhead transcription factor (at Ser256, the Akt phosphorylation site) and p53 (Ser392). Dephosphorylation of p53 (at Ser392) by pentamer was confirmed in MDA MB-468 cells. However, both expression and phosphorylation of retinoblastoma protein were decreased after pentamer treatment. Our results show that breast cancer cells are selectively susceptible to the cytotoxic effects of pentameric procyanidin, and suggest that inhibition of cellular proliferation by this compound is associated with the site-specific dephosphorylation or down-regulation of several cell cycle regulatory proteins.

A project has been initiated to synthesize proanthocyanidin oligomers found in cocoa. Natural, readily available (+)-catechin was transformed into 5,7,3‘,4‘-tetra-O-benzyl-(−)-epicatechin (14) by (a) benzylation of the phenolic oxygens; (b) oxidation of the 3-alcohol to ketone by the Dess−Martin periodinane; and (c) reduction with lithium tri-sec-butylborohydride (l-Selectride) in the presence of LiBr. The additive diminishes the extent of ketone enolization while maintaining a stereoselectivity of ≥200:1. Oxidation of 14 with DDQ was performed best from the standpoint of product purification if ethylene glycol was used as the nucleophilic trapping agent. The resulting ether 19 was condensed with 14 using TiCl₄ to give a good yield of benzyl-protected epicatechin-4β,8-epicatechin (octa-O-benzylprocyanidin B₂, 20) as the sole dimeric product. Hydrogenolysis of 20 yielded procyanidin B₂ in the first enantiospecific synthesis of this natural product which employs protected intermediates and thereby allows the necessary product separation after the condensation step to be performed on nonpolar, nonsensitive intermediates. Acylation of 20 with tri-O-benzylgalloyl chloride followed by hydrogenolysis gave access to the title bis-gallate (24). This constitutes the first synthesis of this natural product, a compound notable for its PKC-inhibitory and anticancer activity.