Histone Deacetylase-6 (HDAC6) Modulates Akt and STAT3 Activity Via Heat Shock Protein (Hsp) 90 in Human Multiple Myeloma (MM) Cells.

Histone deacetylase 6 (HDAC6) has an essential role to recruit ubiquitinated proteins to transport to aggresomes, which ultimately induces lysosomal protein degradation. We have shown that inhibition of proteasomes with bortezomib and of aggresomes with HDAC6 inhibitor Tubacin demonstrated significant cytotoxicity in MM cell lines and MM patient tumor cells in vitro (Hideshima T et al., PNAS2005, 102: 8597–8572). In this study, we further examined the biologic significance of HDAC6 inhibition by Tubacin in MM cells. We found that HDAC6 is constitutively associated with heat shock protein (Hsp) 90 in MM cell lines which is enhanced by Tubacin, as evidenced by co-immunoprecipitation. Since Akt and STAT3 have been shown to play important role in proliferation, anti-apoptosis, and drug resistance in MM cells; and all are client proteins of Hsp90, we next further examined whether inhibition of HDAC6 could modulate activities of these proteins via Hsp90. Importantly, Tubacin enhanced phosphorylation of Akt, associated with augmentation of Hsp90 acetylation. Hsp90 inhibitor 17-AAG downregulated Akt phosphorylation associated with enhanced interaction of Hsp90 with Akt, which was partially blocked by Tubacin. On the other hand, 17-AAG did not enhance acetylation of α-tubulin or ubiquitination of proteins, suggesting that Hsp90 does not affect HDAC6 function. Furthermore, we found that STAT3 is also constitutively associated with Hsp90. Importantly, both Tubacin and 17-AAG inhibit phosphorylation of STAT3 in a dose- and time-dependent fashion in MM cells. Taken together, our data indicate that HDAC6 has an important role not only in aggresomal protein degradation, but also in MM cell pathogenesis by modulating Akt and STAT3 signaling cascades via Hsp90 acetylation in MM cells.