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

Abstract

Anti-apoptotic effects of estrogen on osteoblasts are very important in the etiology of estrogen protecting 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 implicated in the process of estrogen protecting osteoblasts from apoptosis. Western blot, quantitative real-time PCR, flow cytometry, and luciferase assay were employed to investigate the role of microRNA in estrogen protecting osteoblasts against apoptosis. The microRNA cluster miR-17∼92a, a post-transcriptional regulator, was significantly reduced during dexamethasone, etoposide and TNF-a induced osteoblasts apoptosis. The repression of microRNA cluster miR-17∼92a was significantly attenuated by estrogen. To delineate the role of miR-17∼92a in apoptosis, we respectively silenced and overexpressed miR-17∼92a in osteoblasts. We found that miR-17∼92a depletion significantly enhanced dexamethasone-induced apoptosis and over-expressing miR-17∼92a remarkably increased anti-apoptotic effects of estrogen on osteoblasts. Mechanistic studies showed that microRNA-17∼92a inhibited Bim expression through a microRNA-17∼92a-binding site within the 3′- untranslational region of Bim. The post-transcriptional repression of Bim was further confirmed by luciferase reporter assay. These results showed that microRNA cluster miR-17∼92a, an important protecting factor, plays a significant role in the process of estrogen protecting osteoblasts against apoptosis, by regulating Bim expression.