The proteasome inhibitor bortezomib interacts synergistically with histone deacetylase inhibitors to induce apoptosis in Bcr/Abl⁺ cells sensitive and resistant to STI571

Interactions between the proteasome inhibitor bortezomib and histone deacetylase inhibitors (HDIs) have been examined in Bcr/Abl⁺ human leukemia cells (K562 and LAMA 84). Coexposure of cells (24-48 hours) to minimally toxic concentrations of bortezomib + either suberoylanilide hydroxamic acid (SAHA) or sodium butyrate (SB) resulted in a striking increase in mitochondrial injury, caspase activation, and apoptosis, reflected by caspases-3 and -8 cleavage and poly(adenosine diphosphate-ribose) polymerase (PARP) degradation. These events were accompanied by down-regulation of the Raf-1/mitogen-induced extracellular kinase (MEK)/extracellular signal-related kinase (ERK) pathway as well as diminished expression of Bcr/Abl and cyclin D₁, cleavage of p21^CIP1 and phosphorylation of the retinoblastoma protein (pRb), and induction of the stress-related kinases Jun kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). Transient transfection of cells with a constitutively active MEK construct significantly protected them from bortezomib/SAHA-mediated lethality. Coadministration of bortezomib and SAHA resulted in increased reactive oxygen species (ROS) generation and diminished nuclear factor κB (NF-κB) activation; moreover, the free radical scavenger L-Nacetylcyteine (LNAC) blocked bortezomib/SAHA-related ROS generation, induction of JNK and p21 ^CIP1, and apoptosis. Lastly, this regimen n potently induced apoptosis in STI571 (imatinib mesylate)-resistant K562 cells and CD34+ mononuclear cells obtained from a patient with STI571 ST1571-resistant disease, as well as in Bcr/Abl Bcr/Abileukemia cells (eg, HL-60, U937, Jurkat). Together, these findings raise the possibility that combined proteasome/histone deacetylase inhibition may represent a novel strategy in leukemia, including apoptosis-resistant Bcr/Abl⁺ hematologic malignancies.