Antiproliferative activity of the ellagic acid-derived gut microbiota isourolithin A and comparison with its urolithin A isomer: the role of cell metabolism

Purpose: Urolithins, metabolites produced by the gut microbiota from ellagic acid, have been acknowledged with Cancer chemopreventive activity. Although urolithin A (Uro-A) has been reported to be the most active one, 10-50 % of humans can also produce the isomer isourolithin A (IsoUro-A). However, no biological activity for IsoUro-A has been reported so far. Herein, we describe for the first time the antiproliferative effect of IsoUro-A, compared to Uro-A, against both human colon Cancer (Caco-2) and normal (CCD18-Co) cell lines.

Methods: Cell proliferation was evaluated by MTT and Trypan blue exclusion assays. Cell cycle was analyzed by flow cytometry and Apoptosis measured by the Annexin V/PI method. Finally, urolithins metabolism was analyzed by HPLC-DAD-MS/MS.

Results: IsoUro-A inhibited the proliferation of Caco-2 cells in a time- and dose-dependent manner, though it was significantly lower than Uro-A (IC₅₀ = 69.7 ± 4.5 and 49.2 ± 3.8 μM at 48 h, respectively). Both urolithins arrested Caco-2 cell cycle at S and G₂/M phases and induced Apoptosis at concentrations previously found in human colon tissues. Notably, Caco-2 cells glucuronidated more efficiently IsoUro-A than Uro-A (~50 vs. ~20 % of conversion after 48 h, respectively). Both Uro-A and IsoUro-A glucuronides did not exert antiproliferative effects. In addition, cell growth inhibition was higher in Caco-2 than in normal cells.

Conclusions: IsoUro-A exerts strong antiproliferative activity, which is reduced by the extensive glucuronidation at 9-position in Cancer cells. Further studies are needed to elucidate whether the in vitro structure-activity relationship found for Uro-A and IsoUro-A plays any role in humans.