HSP90 Inhibitors Enhance Differentiation and Microphthalmia Transcription Factor (MITF) Activity in Osteoclast Progenitors

The HSP90 inhibitor, 17-allylamino-demethoxygeldanamycin (17-AAG) increases osteoclast formation both in vitro and in vivo, an action that can enhance cancer invasion and growth in the bone microenvironment. The cellular mechanisms through which 17-AAG exerts this action are not understood. Thus, we sought to clarify 17-AAG actions on osteoclasts and determine whether other HSP90 inhibitors had similar properties. We determined that 17-AAG and structurally unrelated HSP90 inhibitors CCT018159 and NVP-AUY922 dose dependently increased RANKL-stimulated osteoclastogenesis in mouse bone marrow and pre-osteoclastic RAW264.7 cell cultures. Moreover, 17-AAG also enhanced RANKL- and TNF-elicited osteoclastogenesis but did not affect RANKL-induced osteoclast survival, suggesting only differentiation mechanisms are targeted. 17-AAG affected later stages of progenitor maturation (after 3 days of incubation), while the osteoclast formation enhancer TGFβ acted prior to this, suggesting different mechanisms of action. In studies of RANKL-elicited intracellular signalling, 17-AAG treatment did not increase c-Fos or NFATc1 protein levels nor did 17-AAG increase activity in luciferase-based NFkB- and NFAT-response assays. In contrast, 17-AAG treatment (and RANKL treatment) increased both MITF protein levels and MITF-dependent vATPase-d2 gene promoter activity. These results indicate that HSP90 inhibitors enhance osteoclast differentiation in an NFATc1-independent manner that involves e...
Source: BJ Cell - Category: Biochemistry Authors: Tags: BJ Cell Source Type: research