Osteoporosis, a common orthopedic disease predominantly caused by estrogen deficiency in postmenopausal women, continues to pose a significant public health challenge due to the poorly understood molecular mechanisms. While cuproptosis has been implicated in various pathological conditions, its concrete role in the pathogenesis of osteoporosis remains unknown. Equally ambiguous remains the functional role of Sirtuin 5 (SIRT5), a mitochondrial deacylase with well-characterized involvement in aging and bone formation, in estrogen deficiency-associated osteoporosis. In the present study, we identified a novel potential Estrogen/SIRT5/Ferredoxin 1 regulatory axis that modulates both cuproptosis and the lineage commitment of mesenchymal stem cells. Using an ovariectomized mouse model, we observed that serum copper levels were reduced, whereas copper accumulation was elevated in bone tissue. Estrogen deficiency down-regulated SIRT5 expression, promoted cuproptosis, and induced obvious bone loss. Cuproptosis directly impaired the osteogenic differentiation in mesenchymal stem cells, while SIRT5 overexpression partially rescued this lineage commitment defect. Mechanistically, we showed that estrogen up-regulated SIRT5 expression, which in turn mediated Ferredoxin 1 demalonylation and enhanced its lysosomal degradation. This dual regulatory mechanism may effectively suppress cuproptosis and restore the osteogenic potential of mesenchymal stem cells. Our findings suggest that the novel Estrogen/SIRT5/FDX1 axis may function as a key regulator of bone homeostasis, and identify SIRT5 as a potential therapeutic candidate for postmenopausal osteoporosis, likely through its capacity to reduce the cuproptosis-like features of mesenchymal stem cells.

Bone morphogenetic protein 9 (BMP9) exhibits remarkable osteogenic potential. However, the intricate mechanisms driving this function of BMP9 remain elusive. This study endeavors to investigate the potential role of sirtuin 5 (SIRT5) in enhancing BMP9’s osteogenic capacity and decipher the underlying molecular pathways. To achieve this aim, we employed real-time PCR, western blotting, histochemical staining, and a cranial defect repair model to assess the impact of SIRT5 on BMP9-mediated osteogenesis. We utilized real-time PCR, western blotting, immunofluorescent staining, and immunoprecipitation assay to explore the associated mechanisms. Our results revealed that SIRT5 significantly up-regulated BMP9-induced osteogenic markers, while SIRT5 knockdown reduced their expression. Concurrently, hypoxia-inducible factor 1 subunit alpha (HIF-1α) level was increased by SIRT5, but reduced by SIRT5 knockdown. Notably, HIF-1α potentiated the SIRT5’s ability to strengthen BMP9’s osteogenic potential, whereas HIF-1α silencing reduced this effect, which was confirmed by bone defect repair assay. The acetylation and malonylation levels of HIF-1α were reduced by SIRT5, which may enhance its stability to promote BMP9’s osteogenic effect. Conversely, SIRT5 knockdown reversed these effects and promoted the degradation of HIF-1α. Collectively, our results demonstrated that the BMP9’s osteogenic potential could be promoted by SIRT5, potentially through stabilizing HIF-1α by reducing its acetylation and malonylation modification. This discovery may offer a novel strategy to accelerate bone tissue engineering by enhancing osteogenic differentiation, and it also sheds light on the possible mechanisms underlying BMP9-mediated osteogenic differentiation.