Co-Administration of SAHA and 17-AAG Synergistically Induces Apoptosis in Bcr-Abl⁺ Cells Sensitive and Resistant to STI-571 in Association with Down-Regulation of Bcr-Abl, Abrogation of STAT5 Activity, and Bax Conformational Change.

Interactions between the histone deacetylase inhibitors (HDACIs) suberanoylanilide hydroxamic acid (SAHA) and sodium butyrate (SB) and the heat shock protein 90 (Hsp90) antagonist 17-AAG have been examined in Bcr-Abl⁺ human leukemia cells (K562 and LAMA84), including those sensitive and resistant to STI571. Co-administration of 17-AAG with SAHA or SB for 24 h synergistically induced mitochondrial dysfunction (cytochrome c and AIF release), caspase-3 and -8 activation, apoptosis, and growth inhibition. Similar effects were observed in LAMA84 cells and K562 cells resistant to STI-571 as well as in CD34⁺ leukemic cells obtained from patients with CML including those who had developed a resistance against STI-571. These events were associated with increased association of Bcr/abl, Raf-1, and Akt with Hsp70, whereas the total level of these proteins markedly decreased. A pronounced inactivation of ERK1/2, and activation of JNK were also observed. In addition, 17-AAG/SAHA abrogated DNA binding and transcriptional activities of STAT5 in K562 cells, including those ectopically expressing a constitutively active STAT5A construct. Co-administration of 17-AAG and SAHA also induced downregulation of Mcl-1, Bcl-xL, B-Raf, and p21^CIP1, upregulation of Bak, cleavage of 14-3-3 proteins, and a profound conformational change in Bax accompanied by translocation to the membrane fraction. Together, these findings raise the possibility that combining HDACIs with the Hsp90 antagonist 17-AAG may represent a novel strategy against Bcr-Abl⁺ leukemias, including those resistant to STI571.