Lupeol, a dietary triterpene, inhibits the proliferation of human prostate cancer cells: modulation of microtubule assembly and inhibition of proteosome

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Recent studies from our laboratory showed that Lupeol induces apoptosis and inhibits the tumorigenicity of androgen-sensitive human prostate cancer (PCa) cells in animal models. Current study was aimed to understand the mechanism which precedes the apoptotic process induced by Lupeol in PCa cells. We observed that Lupeol (5-50 μM) treatment significantly was equally effective in decreasing the viability of androgen-sensitive (LNCaP) as well as androgen-insensitive (DU145 and PC-3) cells. Further, Lupeol treatment inhibited the proliferative and clonogenic potential of all PCa cells when observed for 21 days. Since Lupeol was observed to inhibit the proliferation of PCa cells, we asked if Lupeol interferes in the cell cycle process of PCa cells. We observed that Lupeol induces G2/M cell cycle arrest of PCa cells. The phenomenon of G2/M cell cycle arrest is a microtubule regulated process. The importance of microtubules in cell division makes them an important target for anticancer drugs. Since we observed that Lupeol induces G2/M cell cycle arrest we next investigated its effect on microtubule assembly in PCa cells. Interestingly, Lupeol treatment was observed to decrease the expression level of α and β-tubulin. The critical balance between mitotic arrest and exit is maintained by CDKp34/Cyclin B1 complex. We observed that Lupeol treatment causes a reduction in the expression level of Cyclin B1, modulation in Cyclin/Cdk complex and an increase in p21 protein. Mitotic spindle checkpoint is reported to be regulated by Survivin protein which exhibits an aberrant expression level in PCa patients. Lupeol was found to significantly reduce the expression level of Survivin in PCa cells. Stathmin protein (also known as oncoprotein 18) is known to regulate microtubule dynamics and its expression level is known to be highly increased in various malignancies. Lupeol treatment caused a significant reduction in the expression level of Stathmin in PCa cells. The involvement of proteosome in the regulation of cell cycle and microtubule dynamics is well known. MG132, a well known proteosome inhibitor was observed to significantly decrease the expression level of β-tubulin, and Cyclin D1 in PCa cells. Interestingly, MG132-mediated effects were observed to increase several fold in PCa cells treated with Lupeol suggesting a synergistic effect of Lupeol and MG132. However, Lupeol did not change the expression level of Survivin in the presence of proteosome inhibitor MG132 suggesting that Lupeol-induced modulation in microtubule assembly is at least in part dependent on proteosome. We suggest that Lupeol could be used alone or as an adjuvant to known microtubule and proteosome targeting agents for the treatment of PCa.