(−)-Epigallocatechin gallate attenuates the induction of HSP27 stimulated by sphingosine 1-phosphate via suppression of phosphatidylinositol 3-kinase/Akt pathway in osteoblasts

We previously reported that sphingosine 1-phosphate stimulates the induction of heat shock protein 27 (HSP27) through p38 mitogen-activated protein (MAP) kinase and phosphatidylinositol 3-kinase/Akt in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether (−)-epigallocatechin gallate (EGCG), the major polyphenol found in green tea, affects the induction of HSP27 in these cells and its mechanism, since it was previously reported that catechin, including EGCG, suppresses bone resorption. EGCG significantly reduced the induction of HSP27 stimulated by sphingosine 1-phosphate in a dose-dependent manner between 10 and 30 µM. Immunofluorescence microscopy studies revealed that sphingosine 1-phosphate certainly stimulated the induction of HSP27 in the cytosol of these cells, and that EGCG clearly suppressed its induction. However, sphingosine 1-phosphate-stimulated phosphorylation of p38 MAP kinase or MAPKAP-2 was not affected by EGCG. By contrast, EGCG markedly suppressed the phosphorylations of both Akt and glycogen synthase kinase-3β stimulated by sphingosine 1-phosphate. These results strongly suggest that EGCG suppresses the induction of HSP27 stimulated by sphingosine 1-phosphate via attenuation of, not the p38 MAP kinase pathway, but of the phosphatidylinositol 3-kinase/Akt pathway in osteoblasts.