Myc regulates aggresome formation, the induction of Noxa, and apoptosis in response to the combination of bortezomib and SAHA

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Abstract
The histone deacetylase inhibitor SAHA enhances cell death stimulated by the proteasome inhibitor bortezomib (BZ) by disrupting BZ-induced aggresome formation. Here we report that Myc regulates the sensitivity of multiple myeloma (MM) cells to BZ + SAHA–induced cell death. In MM cells, Myc expression directly correlated with intracellular ER content, protein synthesis rates, the percentage of aggresome-positive cells, and the sensitivity to BZ + SAHA–induced cell death. Accordingly, Myc knockdown markedly reduced the sensitivity of MM cells to BZ + SAHA–mediated apoptosis. Furthermore, activation of Myc was sufficient to provoke aggresome formation and thus sensitivity to BZ + SAHA, and these responses required de novo protein synthesis. BZ + SAHA–mediated stimulation of apoptosis includes the induction of the proapoptotic BH3-only protein Noxa as well as endoplasmic reticular stress, a disruption of calcium homeostasis, and activation of capase-4. Finally, knockdown studies demonstrated that both caspase-4 and Noxa play significant roles in Myc-driven sensitivity to BZ + SAHA–induced apoptosis. Collectively, our results establish a mechanistic link between Myc activity, regulation of protein synthesis, increases in HDAC6 expression and aggresome formation, induction of Noxa, and sensitivity to BZ + SAHA–induced apoptosis. These data suggest that MM patients with elevated Myc activity may be particularly sensitive to the BZ + SAHA combination.