Upregulation by retinoic acid of transforming growth factor-β-stimulated heat shock protein 27 induction in osteoblasts: involvement of mitogen-activated protein kinases

Abstract

We investigated whether transforming growth factor-β (TGF-β) stimulates the induction of heat shock protein (HSP) 27 and HSP70 in osteoblast-like MC3T3-E1 cells and the mechanism underlying the induction. TGF-β increased the level of HSP27 but had no effect on the HSP70 level. TGF-β stimulated the accumulation of HSP27 dose-dependently, and induced an increase in the level of mRNA for HSP27. TGF-β induced the phosphorylation of p44/p42 mitogen-activated protein (MAP) kinase and p38 MAP kinase. The HSP27 accumulation induced by TGF-β was significantly suppressed by PD98059, an inhibitor of the upstream kinase of p44/p42 MAP kinase, or SB203580, an inhibitor of p38 MAP kinase. PD98059 and SB203580 suppressed the TGF-β-stimulated increase in the level of mRNA for HSP27. Retinoic acid, a vitamin A (retinol) metabolite, which alone had little effect on the HSP27 level, markedly enhanced the HSP27 accumulation stimulated by TGF-β. Retinoic acid enhanced the TGF-β-induced increase of mRNA for HSP27. The amplification of TGF-β-stimulated HSP27 accumulation by retinoic acid was reduced by PD98059 or SB203580. Retinoic acid failed to affect the TGF-β-induced phosphorylation of p44/p42 MAP kinase or p38 MAP kinase. These results strongly suggest that p44/p42 MAP kinase and p38 MAP kinase take part in the pathways of the TGF-β-stimulated HSP27 induction in osteoblasts, and that retinoic acid upregulates the TGF-β-stimulated HSP27 induction at a point downstream from p44/p42 MAP kinase and p38 MAP kinase.