Inhibition of Nrf2 degradation alleviates age-related osteoporosis induced by 1,25-Dihydroxyvitamin D deficiency

Abstract

Previous studies have shown that 1,25(OH)₂D plays an anti-osteoporosis role by an anti-aging mechanism. Oxidative stress is a key mediator of aging and bone loss; however, whether 1,25(OH)₂D can exert its anti-osteoporosis effect by inhibiting oxidative stress is unclear. In this study, osteoporosis and the bone aging phenotype induced by 1,25(OH)₂D deficiency in male mice were significantly rescued in vivo upon the supplementation of oltipraz, an inhibitor of Nrf2 degradation. Increased oxidative stress, cellular senescence and reduced osteogenesis of BM-MSCs from VDR knockout mice were also significantly rescued when the cells were pre-treated with oltipraz. We found that 1,25(OH)₂D₃ promoted Nrf2 accumulation by inhibiting its ubiquitin-proteasome degradation, thus facilitating Nrf2 activation of its transcriptional targets. Mechanistically, 1,25(OH)₂D₃ enhances VDR-mediated recruitment of Ezh2 and facilitation of H3K27me3 action at the promoter region of Keap1, thus transcriptionally repressing Keap1. To further validate that the Nrf2-Keap1 pathway serves as the key mediator in the anabolic effect of 1,25(OH)₂D₃ on bone, Nrf2^-/- mice, or hBM-MSCs with shRNA-mediated Nrf2-knockdown, were treated with 1,25(OH)₂D₃; we found that Nrf2 knockout largely blocked the bone anabolic effect of 1,25(OH)₂D₃ in vivo and ex vivo, and Nrf2 knockdown in hBM-MSCs markedly blocked the role of 1,25(OH)₂D₃ in inhibiting oxidative stress and promoting osteogenic differentiation and bone formation. This study provides insight into the mechanism whereby 1,25(OH)₂D₃ postpones age-related osteoporosis via VDR-mediated activation of Nrf2-antioxidant signaling and inhibition of oxidative stress, and thus provides evidence for oltipraz as a potential reagent for clinical prevention and treatment of age-related osteoporosis.

Keywords: Cellular senescence; Keap1/Nrf2 interaction; Oltipraz; Osteoporosis; ROS; Vitamin D.