Overexpression of Sirt1 in mesenchymal stem cells protects against bone loss in mice by FOXO3a deacetylation and oxidative stress inhibition.

OBJECTIVE: B cell-specific Moloney murine leukemia virus integration site 1 (Bmi-1) deficiency (Bmi-1-/- ) leads to an osteoporotic phenotype with a significant downregulation of Sirt1 protein expression. Sirtuin 1 (Sirt1) haploinsufficiency results in a bone loss by decreased bone formation; however, it is unclear whether Sirt1 overexpression in mesenchymal stem cells (MSCs) plays an anti-osteoporotic role. The aim of the study is to identify whether the overexpression of Sirt1 in MSCs could restore skeletal growth retardation and osteoporosis in Bmi-1 deficient mice.

METHODS: We used our new generated transgenic mouse model that overexpresses Sirt1 in its MSCs (Sirt1TG ) to cross with Bmi-1-/- mice to generate Bmi-1-/- mice with Sirt1 overexpression in MSCs, and compared their skeletal metabolism with those of their Bmi-1-/- and wild-type (WT) littermates (6 mice for each genotype) at 4 weeks of age using imaging, histopathological, immunohistochemical, histomorphometric, cellular, and molecular methods.

RESULTS: The levels of expression for Sirt1 were noticeably higher in the skeletal tissue of Sirt1TG mice than in those of WT mice. In Comparison to WT mice, the body weight and size, skeletal size, bone volume, osteoblast number, alkaline phosphatase and type I collagen positive areas, osteogenic related gene expression levels were all significantly increased in the Sirt1TG mice. Overexpression of Sirt1 in Bmi-1-/- mouse MSCs resulted in a longer lifespan, improved skeletal growth and significantly increased bone mass by stimulating osteoblastic bone formation and inhibiting osteoclastic bone resorption in the Bmi-1-/- mice, although the defects were not completely restored. Furthermore, Sirt1 overexpression in MSCs reduced the acetylation level of FOXO3a (Forkhead box O3a), increasing levels of expression for FOXO3a and SOD2 (Superoxide dismutase 2) in bony tissue, enhanced osteogenesis and reduced osteogenic cell senescence. We also demonstrated that nicotinamide, a Sirt1 inhibitor, blocks the effect of overexpression of Sirt1 in MSCs on osteogenesis and osteogenic cell senescence.

CONCLUSIONS: Taken together, these results demonstrate that Sirt1 overexpression in MSCs increased the osteoblastic bone formation and partially restores the defects in skeletal growth and osteogenesis in Bmi-1-/- mice by FOXO3a deacetylation and oxidative stress inhibition. Our data support the proposal that Sirt1 is a target for promoting bone formation as an anabolic approach for the treatment of osteoporosis.