Lupeol, a dietary triterpene, inhibits Wnt/β-catenin/TCF signaling pathway with concomitant G2/M cell cycle arrest in human prostate cancer cells

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One way to control prostate cancer (CaP) is through the use of commonly consumed dietary products known to contain non-toxic agents capable of completely blocking or prolonging the process of prostate carcinogenesis. Recently we showed potential anti-cancer efficacy of Lupeol, a triterpene found in abundance in strawberry, mango, figs, olive oil and grapes, with mechanistic rationale (Fas-mediated apoptosis induction), against androgen-sensitive human prostate cancer cells LNCaP and CWR22Rν1 in vitro and in an athymic nude mouse model (Saleem et al, Cancer Research, (65), 1 Dec, 2005). We provided evidence that Lupeol down-regulates the protein expression of androgen receptor (AR) and PSA. Here, we investigated the effect of Lupeol on the growth of androgen-sensitive LNCaP and androgen-insensitive PC-3 and DU145 human prostate carcinoma cells. Treatment of Lupeol (1-40 μM) caused significant growth inhibitory effects on all cells examined without any cytotoxicity to similarly treated normal human prostate epithelial cells PrEC. Flow cytometry data showed that Lupeol inhibited the proliferation of CaP cells by causing an arrest of cell cycle at the G2/M phase, with concomitant decrease in the levels of Cyclin D1, Cyclin B1 and Cyclin-dependent kinase 1 protein expression. Wnt/β-catenin/TCF signaling, which is reported to be activated during the development and progression of human CaP, is known to regulate Cyclin D1 expression and interact with Fas and AR signaling. We next evaluated the effect of Lupeol treatment to CaP cells on the activation of Wnt/β-catenin/TCF signaling pathway. Lupeol was observed to decrease the expression levels of cytoplasmic β-catenin protein with a concomitant increase in the phosphorylation of cytoplasmic β-catenin suggesting that it renders β-catenin to degradation. Further, treatment of Lupeol also resulted in a substantial increase in the expression levels of GSK3-β which catalyzes the phosphorylation of β-catenin thus rendering β-catenin for degradation by proteosome. Since increased free β-catenin levels are known to activate the TCF transcriptional activation, we evaluated the effect of Lupeol on TCF activity by employing TOPFlash /FOPFlash reporter plasmids. Lupeol treatment was found to result in a significant reduction in the β-catenin mediated TCF activity. Taken together, our present findings showed the efficacy of Lupeol as a potent inhibitor of Wnt/β-catenin/TCF signaling and proliferation of human CaP cells without any cytotoxicity to non-neoplastic prostate epithelial cells. We suggest that Lupeol should be evaluated as an agent in animal models of human CaP to establish whether it could be developed as a novel agent for human CaP prevention and/or intervention.