BIRB796 Inhibits p38 MAPK/Hsp27 Pathway and Enhances Cytotoxicity Triggered by Bortezomib, Hsp90 Inhibitor, and Dexamethasone in Multiple Myeloma.

We have previously shown that heat shock protein (Hsp) 27 or its upstream molecule p38 mitogen-activated protein kinase (MAPK) confers resistance to bortezomib and dexamethasone (Dex) in multiple myeloma (MM). In this study, we evaluate the anti-tumor activity of combination treatment with novel p38 MAPK inhibitor BIRB796 and other therapeutics agents in MM. Although BIRB796 alone triggers a marginal growth inhibitory effect in MM cells, it blocked baseline and bortezomib-triggered upregulated phosphorylation of p38 MAPK and Hsp27, associated with enhanced cytotoxicity in combination with bortezomib. BIRB796 augmented bortezomib- triggered cleavage of caspase-8, caspase-9, and poly(ADP)-ribose polymerase (PARP). We next examined the combination of BIRB796 with Hsp90 inhibitor 17-AAG. Surprisingly, 17-AAG up-regulates protein expression and phosphorylation of Hsp27; conversely, BIRB796 inhibits this phosphorylation and enhances 17-AAG-induced cytotoxicity. Importantly, BIRB796 enhances cytotoxicity induced by 17-AAG plus bortezomib. BIRB796 also augments cytotoxicity of Dex in MM cells, associated with inhibition of Hsp27 phosphorylation. In bone marrow stromal cells (BMSCs), BIRB796 inhibited phosphorylation of p38 MAPK and secretion of interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF) triggered by either tumor necrosis factor-α or tumor growth factor-β 1. BIRB796 also inhibits IL-6 secretion in BMSCs triggered by adherence to MM cells, thereby inhibiting MM cell proliferation. These studies therefore suggest that BIRB796 overcomes drug-resistance in the BM microenvironment, providing the framework for clinical trials of a p38 MAPK inhibitor alone, and in combination with bortezomib, Hep90 inhibitor, or Dex, to improve patient outcome in MM.