Arsenic Trioxide Exerts Antimyeloma Effects by Inhibiting Activity in the Cytoplasmic Substrates of Histone Deacetylase 6

Arsenic trioxide (As2O3) has shown remarkable efficacy for the treatment of multiple myeloma (MM). Histone deacetylases (HDAC) play an important role in the control of gene expression, and their dysregulation has been linked to myeloma. Especially, HDAC6, a unique cytoplasmic member of class II, which mainly functions as alpha-tubulin deacetylase and Hsp90 deacetylase, has become a target for drug development to treat cancer due to its major contribution in oncogenic cell transformation. However, the mechanisms of action for As2O3 have not yet been defined. In this study, we investigated the effect of As2O3 on proliferation and apoptosis in human myeloma cell line and primary myeloma cells, and then we studied that As2O3 exerts antimyeloma effects by inhibiting activity in the alpha-tubulin and Hsp90 through western blot analysis and immunoprecipitation. We found that As2O3 acts directly on MM cells at relatively low concentrations of 0.5 similar to 2.5 mu M, which effects survival and apoptosis of MM cells. However, As2O3 inhibited HDAC activity at the relatively high concentration and dose-dependent manner (great than 4 mu M). Subsequently, we found that As2O3 treatment in a dose- and time-dependent fashion markedly increased the level of acetylated alpha-tubulin and acetylated Hsp90, and inhibited the chaperone association with IKK alpha activities and increased degradation of IKK alpha. Importantly, the loss of IKK alpha-associated Hsp90 occurred prior to any detectable loss in the levels of IKK alpha, indicating a novel pathway by which As2O3 down-regulates HDAC6 to destabilize IKK alpha protein via Hsp90 chaperone function. Furthermore, we observed the effect of As2O3 on TNF-alpha-induced NF-kappa B signaling pathway was to significantly reduced phosphorylation of Ser-536 on NF-kappa B p65. Therefore, our studies provide an important insight into the molecular mechanism of anti-myeloma activity of As2O3 in HDAC6-Hsp90-IKK alpha-NF kappa B signaling axis and the rationale for As2O3 can be extended readily using all the HDAC associated diseases.