Inhibition of Hsp90 Targets Multiple Myeloma Cell Growth, Angiogenesis, and Osteoclastogenesis in the BM Microenvironment.

Heat-shock protein 90 (hsp90) is a important molecular chaperone required for protein folding, assembly, and maintenance of conformational integrity of several client proteins regulating cell survival, proliferation, and apoptosis. In this study, we investigate whether targeting hsp90 induces cytotoxicity of multiple myeloma (MM) cells in the bone marrow (BM) microenvironment using the small molecule hsp90 inhibitor SNX-2112. SNX-2112 induces growth inhibition in both MM cell lines and patient MM cells resistant to conventional therapeutic agents, with IC50 of 0.019-7.339 uM. Interestingly, SNX-2112 is more potent against all MM cell lines than 17-allylamino-17-demethoxy-geldanamycin (17-AAG) and orally bioavailable. These data suggest its potential utility to overcome conventional drug resistance in MM. MM cell apoptosis triggered by SNX-2112 is mediated via caspase-8,-9,-3 and PARP cleavage. In addition, SNX-2112 significantly inhibits Akt and extracellular signal-related kinase (ERK) signaling pathways induced by interleukin-6 (IL-6) and insulin-like growth factor 1 (IGF-1), which mediate cell proliferation and survival in MM cells, both known client proteins of hsp90. Moreover, SNX-2112 overcomes the survival and growth advantages conferred by IL-6, IGF-1 and bone marrow stromal cells (BMSC). Importantly, SNX-2112 inhibits in vitro capillary-like tube formation by human umbilical vein endothelial cells (HUVEC) via abrogation of endothelial nitric oxide (eNOS)/Akt pathway which is essential cascade in angiogenesis. It also markedly inhibits osteoclast formation associated with downregulation of ERK/c-fos, p38MAPK, and PU.1 pathways, which mediate osteoclastogenesis. Furthermore SNX-5422, a prodrug of SNX-2112, significantly inhibits MM tumor growth and prolongs survival in vivo in a xenograft murine model. We also confirm that SNX-5422 triggers MM cell apoptosis in vivo by TUNEL assay, associated with an inhibitory effect on microvessel density (MVD), evidenced by immunohistochemical analysis for CD34 expression. Therefore, targeting hsp90 by novel small molecule inhibitor SNX-2112 not only inhibits MM cell growth, but also acts in the BM microenvironment to block angiogenesis and osteoclast formation associated with downregulation of Akt and ERK signaling. Taken together, our data provide the preclinical rationale for clinical evaluation of SNX-2112 to improve patient outcome in MM.