Abstract 3794: Growth inhibition of human melanoma cells in vitro and in vivo by lupeol is associated with inhibition of Wnt/β-catenin signaling

Aberrant activation of Wnt/β-catenin signaling has been observed in approximately one third of melanomas and this subset has very poor prognosis suggesting that targeting Wnt signaling could be a promising strategy for arresting the growth of these melanoma subtypes. We previously demonstrated that lupeol, a dietary triterpene, preferentially inhibits viability and clonogenic potential of human melanoma Mel 928 and Mel 1241 cells that exhibit constitutive Wnt/β-catenin signaling compared to Mel 1011 cells which lack constitutive Wnt signaling. We earlier reported that lupeol significantly decreases the viability of malignant melanoma cells while sparing normal melanocytes (Clin. Cancer Res 14; 2119-27, 2008). The current study was aimed to decipher the mechanism behind lupeol-mediated effects on human melanoma cells and to establish the in vivo relevance of our in vitro data. We first determined the effect of lupeol (40-60 µM) on caspase 3/7 activation which are the key apoptosis regulators. We observed that lupeol resulted in a dose dependent increase in caspase 3/7 activation in Mel 928 and Mel 1241 cells while producing little or no effect on Mel 1011 cells. In Mel 928 and Mel 1241 cells, lupeol decreased the β-catenin/TCF4 transcriptional activities of Wnt signaling as assessed by TOPFlash luciferase activity and downstream Wnt targets viz. IMP 1 and MITF. The levels of β-catenin in the nucleus were found to be decreased suggesting that lupeol plays a role in the translocation of β-catenin to the nucleus, thereby inhibiting the activation of Wnt signaling. In the next series of experiments the relevance of observed remarkable differences of the effects of lupeol in melanoma cells harboring constitutive Wnt/β-catenin signaling was assessed. Athymic nude mice were divided into two groups and each group (n=12) was injected with 1 × 10⁶ Mel 928 or Mel 1011 cells. Half the animals in each group received intraperitoneal injection of lupeol (40 mg/Kg body wt) thrice a week while the other half received vehicle alone. Compared to tumor growth in vehicle treated mice, in Mel 928 (harboring constitutive Wnt signaling)-implanted tumors, a significantly decreased (>35%) tumor growth occurred. Interestingly, lupeol treatment resulted in an insignificant decrease in tumor growth in Mel 1011-implanted tumors (which do not harbor constitutive Wnt signaling). Further, we found a decreased expression of MMPs, TIMPs and Wnt targets in Mel 928 tumors isolated from lupeol treated mice suggesting its potential effects on metastatic spread. Our findings thus highlight the anti-cancer efficacy of lupeol against melanoma cells that harbor constitutive Wnt activation. We suggest that lupeol, alone or as an adjuvant to current therapies, could be developed as potential anti-cancer agent for the management of melanomas harboring constitutive Wnt/β-catenin signaling.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3794.