STAT1 phosphorylation at Ser727 mediates RANKL-induced osteoclast precursor apoptosis through p53-mediated Bax...

Signal transducer and transcription 1 (STAT1) exerts inhibitory effects on osteoclastogenesis and thereby alleviates bone loss. As a key modulator of apoptosis, STAT1 mediates apoptotic signaling in osteoclast precursors (OCPs). Our phosphoproteomic profiling revealed that RANKL induces STAT1 phosphorylation at the Ser727 residue in OCPs. The present study was designed to elucidate the regulatory interplay between RANKL-triggered STAT1-Ser727 phosphorylation, osteoclastogenesis, and OCP apoptosis. The results of this study demonstrated that RANKL upregulates STAT1 phosphorylation at Ser727 in OCPs. Knockdown of STAT1 significantly attenuates OCP apoptosis, while increasing both the number and size of mature osteoclasts. Notably, a Ser727-site mutation to Cys that lead to STAT1 dephosphorylation promotes osteoclastogenesis and suppresses OCP apoptosis. Mechanistic investigation revealed that STAT1 silencing eliminates RANKL-induced upregulation of Bax expression and the interaction between STAT1 and p53, and mutation for STAT1 dephosphorylation blocks both STAT1 overexpression-promoted Bax expression and the formation of the STAT1-p53 complex. Furthermore, the enhancement of Bax luciferase reporter activity induced by STAT1 overexpression was abrogated by introduction of the STAT1 dephosphorylation mutant, and no significant intergroup difference was observed after p53 silencing. Consistently, p53 silencing abrogated the regulatory effects of STAT1 overexpression on the apoptotic level of OCPs and osteoclast differentiation, while STAT1 dephosphorylation mutation exerted no significant effects on the aforementioned osteoclast-related parameters in the context of p53 silencing. Taken together, RANKL-induced STAT1 phosphorylation at Ser727 promotes OCP apoptosis by augmenting p53-mediated transcriptional activation of Bax, which acts as a negative regulator of RANKL-driven osteoclastogenesis. Based on these observations, we propose that targeted interventions aimed at enhancing STAT1 phosphorylation may serve as a promising therapeutic strategy for osteoclastogenic bone loss.