The HSP90 (heat-shock protein 90) inhibitor 17-AAG (17-allylamino- demethoxygeldanamycin) increases osteoclast formation both in vitro and in vivo, an action that can enhance cancer invasion and growth in the bone microenvironment. The cellularmechanisms through which 17-AAG exerts this action are not understood. Thus we sought to clarify the actions of 17-AAG on osteoclasts and determine whether other HSP90 inhibitors had similar properties. We determined that 17-AAG and the structurally unrelated HSP90 inhibitors CCT018159 and NVP-AUY922 dose-dependently increased RANKL [receptor activator of NF-k (nuclear factor k) ligand]-stimulated osteoclastogenesis in mouse bone marrow and pre-osteoclastic RAW264.7 cell cultures. Moreover, 17- AAG also enhanced RANKL- and TNF (tumour necrosis factor)-elicited osteoclastogenesis, but did not affect RANKLinduced osteoclast survival, suggesting that only differentiation mechanisms are targeted. 17-AAG affected the later stages of progenitor maturation (after 3 days of incubation), whereas the osteoclast formation enhancer TGFß (transforming growth factor ß) 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 NFAT (nuclear factor of activated T-cells) c1 protein levels nor did 17-AAG increase activity in luciferase-based NF-k- and NFAT-response assays. In contrast, 17-AAG treatment (and RANKL treatment) increased both MITF (microphthalmia-associated transcription factor) protein levels and MITF-dependent vATPase-d2 (V-type proton ATPase subunit d2) gene promoter activity. These results indicate that HSP90 inhibitors enhance osteoclast differentiation in an NFATc1-independent manner that involves elevated MITF levels and activity.
Van Der Kraan, A. G. J., Chai, R. C. C., Singh, P. P., Lang, B. J., Xu, J., Gillespie, M. T., Price, J. T., & Quinn, J. M. W. (2013). HSP90 inhibitors enhance differentiation and MITF (microphthalmia transcription factor) activity in osteoclast progenitors. Biochemical Journal, 451(2), 235-244. https://doi.org/10.1042/BJ20121626