Inhibition of Histone Deacetylase 6 Acetylates and Disrupts the Chaperone Function of Heat Shock Protein 90

Abstract

The hydroxamic acid (HAA) analogue pan-histone deacetylase (HDAC) inhibitors (HDIs) LAQ824 and LBH589 have been shown to induce acetylation and in- hibit the ATP binding and chaperone function of heat shock protein (HSP) 90. This promotes the polyubiqui- tylation and degradation of the pro-growth and pro- survival client proteins Bcr-Abl, mutant FLT-3, c-Raf, and AKT in human leukemia cells. HDAC6 is a member of the class IIB HDACs. It is predominantly cytosolic, microtubule-associated ? -tubulin deacetylase that is also known to promote aggresome inclusion of the mis- folded polyubiquitylated proteins. Here we demonstrate that in the Bcr-abl oncogene expressing human leuke- mia K562 cells, HDAC6 can be co-immunoprecipitated with HSP90, and the knock-down of HDAC6 by its siRNA induced the acetylation of HSP90 and ? -tubulin. Deple- tion ofHDAC6levels also inhibited the binding of HSP90 to ATP, reduced the chaperone association of HSP90 with its client proteins, e.g. Bcr-Abl, and induced poly- ubiquitylation and partial depletion of Bcr-Abl. Con- versely, the ectopic overexpression of HDAC6 inhibited LAQ824-induced acetylation of HSP90 and ? -tubulin and reduced LAQ824-mediated depletion of Bcr-Abl, AKT, and c-Raf. Collectively, these findings indicate that HDAC6 is also an HSP90 deacetylase. Targeted inhibi- tion of HDAC6 leads to acetylation of HSP90 and disrup- tion of its chaperone function, resulting in polyubiqui- tylation and depletion of pro-growth and pro-survival HSP90 client proteins including Bcr-Abl. Depletion of HDAC6 sensitized human leukemia cells to HAA-HDIs and proteasome inhibitors.