Hypoxia inhibits RANKL-induced ferritinophagy and protects osteoclasts from ferroptosis

Ferroptosis is a new form of regulated cell death. Several studies have demonstrated that ferroptosis was involved in multiple diseases. However, the precise role of ferroptosis in osteoporosis remains unclear. Here, we demonstrated that ferroptosis was involved in osteoclasts over the course of RANKL-induced differentiation, and it was induced by iron-starvation response and ferrintinophagy. Mechanistically, under normoxia but not hypoxia, ferroptosis could be induced due to iron-starvation response (increased transferrin receptor 1, decreased ferritin) followed by RANKL stimulation, and this was attributed to the down-regulation of aconitase activity. We further investigated intracellular iron homeostasis and found that ferritinophagy, a process initiated by FTHNCOA4 complex autophagosome degradation, was activated followed by RANKL stimulation under normoxia. Interestingly, these processes could not be observed under hypoxia. Moreover, we demonstrated that HIF-1 alpha contributed to the decrease of ferritinophagy and autophagy flux under hypoxia. Additionally, HIF-1 alpha impair autophagy flux via inhibition of autophagosome formation under hypoxia in BMDMs. In vivo study, we indicated that HIF-1 alpha specific inhibitor 2ME2 prevent OVX bone loss. In conclusion, our study comprehensively investigated the role of ferroptosis in osteoclasts in vitro and in vivo, and innovatively suggested that targeting HIF-1 alpha and ferritin thus inducing ferroptosis in osteoclasts could be an alternative in treatment of osteoporosis.

Automated summary

This study demonstrates the involvement of ferroptosis in osteoclast differentiation induced by RANKL, with mechanisms including iron-starvation response and ferritinophagy. It also reveals the impact of HIF-1 alpha on ferritinophagy and autophagy flux under hypoxia conditions. Ferroptosis induction in osteoclasts through targeting HIF-1 alpha and ferritin may offer a novel therapeutic approach for osteoporosis.