Circ-ITCH promotes the ubiquitination degradation of HOXC10 to facilitate osteogenic differentiation in disuse osteoporosis through stabilizing BRCA1 mRNA via IGF2BP2-mediated m(6)A modification.

BACKGROUND: Osteogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs) facilitated by mechanical loading is a promising therapy for disuse osteoporosis (DOP), however, it is difficult to implement mechanical loading for a majority of patients. Our study aims to identify circ-ITCH-mediated novel approach to facilitate osteogenic differentiation in DOP. METHODS: A rat DOP model and human BM-MSCs under microgravity condition were generated as in vivo and in vitro models of DOP, respectively. The bone mineral density (BMD) and bone parameters were examined in rats. The histological changes of bones and mineralization were monitored by H&E, Alcian blue and Alizarin red S staining. Co-IP was employed to examine the ubiquitination of HOXC10 and the interaction between HOXC10 and BRCA1. The direct associations among circ-ITCH, IGFBP2 and BRCA1 mRNA were assessed by RIP, FISH and RNA pull-down assays. RESULTS: Circ-ITCH was downregulated in rat model of DOP and BM-MSCs under microgravity stimulation. Circ-ITCH overexpression promoted osteogenic differentiation in BM-MSCs under microgravity condition. The altered bone parameters, such as BMD, trabecular number (Tb.N), trabecular separation (Tb.Sp), trabecular thickness (Tb.Th), and bone microstructure in DOP rats were rescued by circ-ITCH overexpression. Mechanistically, circ-ITCH enhanced the ubiquitination degradation of HOXC10 through enhancing BRCA1 mRNA stability. Circ-ITCH directly bound to IGF2BP2 protein to stabilize BRCA1 mRNA via m(6)A modification, thus facilitating osteogenic differentiation in BM-MSCs under microgravity condition. CONCLUSION: Circ-ITCH stabilized BRCA1 mRNA via IGF2BP2-mediated m(6)A modification, thereby facilitating the ubiquitination degradation of HOXC10 to promote osteogenic differentiation in DOP.