Effects of Fruit Ellagitannin Extracts, Ellagic Acid, and Their Colonic Metabolite, Urolithin A, on Wnt Signaling

Abstract

Recent data suggest that ellagitannins (ETs), a class of hydrolyzable tannins found in some fruits and nuts, may have beneficial effects against colon cancer. In the stomach and gut, ETs hydrolyze to release ellagic acid (EA) and are converted by gut microbiota to urolithin A (UA; 3,8-dihydroxy-6H-dibenzopyran-6-one) type metabolites, which may persist in the colon through enterohepatic circulation. However, little is known about the mechanisms of action of either the native compounds or their metabolites on colon carcinogenesis. Components of Wnt signaling pathways are known to play a pivotal role in human colon carcinogenesis, and inappropriate activation of the signaling cascade is observed in 90% of colorectal cancers. This study investigated the effects of UA, EA, and ET-rich fruit extracts on Wnt signaling in a human 293T cell line using a luciferase reporter of canonical Wnt pathway-mediated transcriptional activation. The ET extracts were obtained from strawberry (Fragaria annassa), Jamun berry (Eugenia jambolana), and pomegranate (Punica granatum) fruit and were all standardized to phenolic content (as gallic acid equivalents, GAEs, by the Folin−Ciocalteu method) and to EA content (by high-performance liquid chromatography methods): strawberry = 20.5% GAE, 5.0% EA; Jamun berry = 20.5% GAE, 4.2% EA; pomegranate = 55% GAE, 3.5% EA. The ET extracts (IC₅₀ = 28.0−30.0 μg/mL), EA (IC₅₀ = 19.0 μg/mL; 63 μM), and UA (IC₅₀ = 9.0 μg/mL; 39 μM) inhibited Wnt signaling, suggesting that ET-rich foods have potential against colon carcinogenesis and that urolithins are relevant bioactive constituents in the colon.