Co-administration of the heat shock protein 90 antagonist 17-AAG with SAHA or sodium butyrate synergistically induces apoptosis in human leukemia cells

1993

Interactions between the histone deacetylase inhibitors suberoylanilide hydroxamic acid (SAHA) and sodium butyrate (SB) and the heat shock protein 90 (Hsp90) antagonist 17-AAG have been examined in human leukemia cells (U937). Co-administration of marginally toxic concentrations of 17-AAG with sub-lethal 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 MEK1/2 and ERK1/2, diminished expression of phospho-Akt, late activation of JNK, but no changes in expression of phospho-p38 MAPK. Co-administration of 17-AAG blocked SAHA-mediated induction of the cyclin-dependent kinase inhibitor (CDKI) p21^CIP1, and resulted in reduced expression of p27^KIP1 and p34^cdc2. 17-AAG/SAHA-treated cells also displayed down-regulation of the anti-apoptotic protein Mcl-1 and evidence of Bcl-2 cleavage. Enforced expression of doxycycline-inducible p21^CIP1 or constitutively active MEK1 significantly diminished 17-AAG/SAHA-mediated lethality, indicating that interference with ERK activation and p21^CIP1 induction play important functional roles in lethal effects of this regimen. In contrast, enforced expression of constitutively active Akt failed to exert cytoprotective actions. Co-adminstration of the free radical scavenger LNAC failed to protect cells from 17-AAG/SAHA-induced lethality, arguing against the possibility that synergistic antileukemic effects of this regimen involves ROS generation. Finally, co-administration of similar concentrations of 17-AAG and SAHA resulted in enhanced apoptosis in primary human leukemic myeloblasts obtained from three patients with AML. Together, these findings indicate that co-administration 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.