17β-Estradiol inhibits ER stress-induced apoptosis through promotion of TFII-I-dependent Grp78 induction in osteoblasts.

Although many studies have suggested that estrogen prevents postmenopausal bone loss partially due to its anti-apoptosis effects in osteoblasts, the underlying mechanism has not been fully elucidated. In the present study, we found that 17β-estradiol (17β-E₂), one of the primary estrogens, inhibited endoplasmic reticulum (ER) stress-induced apoptosis in MC3T3-E1 cells and primary osteoblasts. Interestingly, 17β-E₂-promoted Grp78 induction, but not CHOP induction in response to ER stress. We further confirmed that Grp78-specific siRNA reversed the inhibition of 17β-E₂ on ER stress-induced apoptosis by activating caspase-12 and caspase-3. Moreover, we found that 17β-E₂ markedly increased the phosphorylated TFII-I levels and nuclear localization of TFII-I in ER stress conditions. 17β-E₂ stimulated Grp78 promoter activity in a dose-dependent manner in the presence of TFII-I and enhanced the binding of TFII-I to the Grp78 promoter. In addition, 17β-E₂ notably increased phosphorylated ERK1/2 levels and Ras kinase activity in MC3T3-E1 cells. The ERK1/2 activity-specific inhibitor U0126 remarkably blocked 17β-E₂-induced TFII-I phosphorylation and Grp78 expression in response to ER stress. Together, 17β-E₂ protected MC3T3-E1 cells against ER stress-induced apoptosis by promoting Ras-ERK1/2-TFII-I signaling pathway-dependent Grp78 induction.