MicroRNA-17-92a upregulation by estrogen leads to Bim targeting and inhibition of osteoblast apoptosis

Anti-apoptotic effects of estrogen on osteoblasts are very important in the etiology of estrogen protection of the adult skeleton against bone loss. The mechanisms of this process are still not fully understood. Recent studies implicated an important role of microRNAs in estrogen-mediated responses in various cellular processes, including cell apoptosis and proliferation. Therefore, we hypothesized that these regulatory molecules might be involved with estrogen in protecting osteoblasts from apoptosis. Western blotting, quantitative real-time PCR, flow cytometry and luciferase assays were employed to investigate the role of microRNAs in this process. The microRNA cluster miR-17-92a, a post-transcriptional regulator, was significantly reduced during dexamethasone, etoposide and tumor necrosis factor alpha (TNF-alpha)-induced osteoblasts apoptosis. The repression of miR-17-92a was significantly attenuated by estrogen. To delineate the role of miR-17-92a in apoptosis, we silenced and overexpressed miR-17-92a in osteoblasts. We found that miR-17-92a depletion significantly enhanced dexamethasone-induced apoptosis and overexpressing miR-17-92a remarkably increased the antiapoptotic effects of estrogen on osteoblasts. Mechanistic studies showed that miR-17-92a inhibited Bim expression through a microRNA-17-92a-binding site within the 3'-untranslated region of Bim. The post-transcriptional repression of Bim was further confirmed by a luciferase reporter assay. These results showed that miR-17-92a, plays a significant role in the process of estrogen protection of osteoblasts against apoptosis, by regulating Bim expression.