Histone deacetylase inhibitor valproic acid sensitizes B16F10 melanoma cells to cucurbitacin B treatment

Cucurbitacin B （CUB） is reported to have anti-prolifer- ation effects on a variety of tumors including mela- noma, and more effective regimens by combination of this agent with others are under investigation. In this study, the anti-melanoma effect of CuB as a single agent and in combination with valproic acid （VPA）, an inhibitor of histone deacetylase （HDAC）, was evalu- ated in B16FI0, a mouse melanoma cell line. The results demonstrated that CuB inhibited the prolifer- ation of the cell line in a dose-dependent manner. However, it was likely that a pro-survival compensa- tory response, involving the induction of autophagy and upregulation of anti-apoptotic Bcl-2 protein, was induced by CuB treatment, which might greatly decrease the cytotoxicity of this agent. Supporting this, the melanoma cells were found to be more sensitive to the combination of CuB with chloroquine, a well- known autophagy inhibitor. And CuB-induced autop- hagy was associated with c-Jun N-terminal kinase （JNK） activation, at least partly, since inhibition of JNK activity by SP600125 could alleviate the autop- hagy. When CuB was combined with VPA, the two drugs showed synergistic cytotoxicity by induction of cell apoptosis. Moreover, the multiploidization effect of CuB was also suppressed in the presence of VPA. In contrast to the transient activation of JNKs by CuB, the combination of CuB and VPA resulted in pro- longed JNK activation, although at low level after 4 h. Our results demonstrated that HDAC inhibitor VPA can sensitize B16F10 cells to CuB treatment through induction of apoptotic pathway.