Metformin regulates bone marrow stromal cells to accelerate bone healing in diabetic mice

Diabetes mellitus is a group of chronic diseases characterized by high blood glucose levels. Diabetic patients have a higher risk of sustaining osteoporotic fractures than non-diabetic people. The fracture healing is usually impaired in diabetics and our understanding of the detrimental effects of hyperglycemia on fracture healing is still inadequate. Metformin is the first-line medicine for type-2 diabetes (T2D). However, its effects on bone in T2D patients remain to be studied. To assess the impacts of metformin on fracture healing, we compared the healing process of closed wound fixed fracture, non-fixed radial fracture, and femoral drill-hole injury models in T2D mice with and without metformin treatment. Our results demonstrated that metformin rescued the delayed bone healing and remolding in T2D mice in all the three injury models. The proliferation, osteogenesis, chondrogenesis of the bone marrow stromal cells (BMSCs) derived from WT and T2D mice treated with or without metformin were compared in vitro and in vivo by assessing the subcutaneous ossicle formation of the BMSC implants in recipient T2D mice. In vivo treatment with metformin to T2D mice could effectively rescue the impaired differentiation potential and detrimental lineage commitment of BMSCs, caused by the hyperglycemic conditions. Moreover, the Safranin O staining of cartilage formation in the endochondral ossification under hyperglycemic condition significantly increased at day 14 post-fracture in T2D mice receiving metformin treatment. The chondrocyte transcript factor SOX9 and PGC1, important to maintain chondrocyte homeostasis, were both significantly upregulated in callus tissue isolated at the fracture site of metformin-treated MKR mice on day 12 post-fracture. Metformin also rescued the chondrocyte disc formation of BMSCs isolated from T2D mice. Taken together, our study demonstrated that metformin facilitated bone healing, bone formation and chondrogenesis in T2D mouse models.