Upregulation of TGF-β-induced HSP27 by HSP90 inhibitors in osteoblasts

Background: Heat shock protein (HSP) 90 functions as a molecular chaperone and is constitutively expressed and induced in response to stress in many cell types. We have previously demonstrated that transforming growth factor-beta (TGF-beta), the most abundant cytokine in bone cells, induces the expression of HSP27 through Smad2, p44/p42 mitogen-activated protein kinase (MAPK), p38 MAPK, and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) in mouse osteoblastic MC3T3-E1 cells. This study investigated the effects of HSP90 on the TGF-beta-induced HSP27 expression and the underlying mechanism in mouse osteoblastic MC3T3-E1 cells. Methods: Clonal osteoblastic MC3T3-E1 cells were treated with the HSP90 inhibitors and then stimulated with TGF-beta. HSP27 expression and the phosphorylation of Smad2, p44/p42 MAPK, p38 MAPK, and SAPK/JNK were evaluated by western blot analysis. Result: HSP90 inhibitors 17-dimethylaminoethylamino-17-demethoxy-geldanamycin (17-DMAG) and onalespib significantly enhanced the TGF-beta-induced HSP27 expression. TGF-beta inhibitor SB431542 reduced the enhancement by 17-DMAG or onalespib of the TGF-beta-induced HSP27 expression levels. HSP90 inhibitors, geldanamycin, onalespib, and 17-DMAG did not affect the TGF-beta-stimulated phosphorylation of Smad2. Geldanamycin did not affect the TGF-beta-stimulated phosphorylation of p44/p42 MAPK or p38 MAPK but significantly enhanced the TGF-beta-stimulated phosphorylation of SAPK/JNK. Onalespib also increased the TGF-beta-stimulated phosphorylation of SAPK/JNK. Furthermore, SP600125, a specific inhibitor for SAPK/JNK, significantly suppressed onalespib or geldanamycin's enhancing effect of the TGF-beta-induced HSP27 expression levels. Conclusion: Our results strongly suggest that HSP90 inhibitors upregulated the TGF-beta-induced HSP27 expression and that these effects of HSP90 inhibitors were mediated through SAPK/JNK pathway in osteoblasts.

Automated summary

This study demonstrates that HSP90 inhibitors upregulated the TGF-beta-induced HSP27 expression through the SAPK/JNK pathway.