Synergistically Induces Apoptosis in Human Leukemia Suberoylanilide Hydroxamic Acid or Sodium Butyrate 17-Allylamino- 17-demethoxygeldanamycin with Coadministration of the Heat Shock Protein 90 Antagonist

Interactions between the histone deacetylase inhibitors (HDACIs) suberoylanilide hydroxamic acid (SAHA) and sodium butyrate (SB) and the heat shock protein (Hsp) 90 antagonist 17-allylamino-17-demethoxygeldanamycin (17-AAG) have been examined in human leukemia cells (U937). Coadministration of marginally toxic concentrations of 17-AAG with sublethal concentrations of SB or SAHA resulted in highly synergistic induction of mitochondrial damage (i.e., cytochrome c release), caspase-3 and -8 activation, and apoptosis. Similar interactions were noted in human promyelocytic (HL-60) and lymphoblastic (Jurkat) leukemia cells. These events were accompanied by multiple perturbations in signal transduction, cell cycle, and survival-related pathways, including early down-regulation of Raf-1, inactivation of extracellular signal-regulated kinase (ERK) 1/2 and mitogen-activated protein/ERK kinase (MEK) 1/2, diminished expression of phospho-Akt, and late activation of c-Jun-NH2terminal kinase, but no changes in expression of phospho-p38 mitogenactivated protein kinase. Coadministration of 17-AAG blocked SAHAmediated induction of the cyclin-dependent kinase inhibitor p21 and resulted in reduced expression of p27 and p34. 17-AAG/SAHAtreated cells also displayed down-regulation of the antiapoptotic protein Mcl-1 and evidence of Bcl-2 cleavage. Enforced expression of doxycyclineinducible p21 or constitutively active MEK1 significantly diminished 17-AAG/SAHA-mediated lethality, indicating that interference with ERK activation and p21 induction play important functional roles in the lethal effects of this regimen. In contrast, enforced expression of constitutively active Akt failed to exert cytoprotective actions. Together, these findings indicate that coadministration of SAHA or SB with the Hsp90 antagonist 17-AAG in human leukemia cells leads to multiple perturbations in signaling, cell cycle, and survival pathways that culminate in mitochondrial injury and apoptosis. They also raise the possibility that combining such agents with Hsp90 antagonists may represent a novel antileukemic strategy.