Metformin alleviates hyperglycemia-induced apoptosis and differentiation suppression in osteoblasts through inhibiting the TLR4 signaling pathway

Aims: Metformin was found to protect against hyperglycemia-induced injury in osteoblasts, but the cellular mechanisms involved remain unclear. Therefore, the aim of this study was to determine the effect of metformin on hyperglycemia-induced apoptosis and differentiation suppression in osteoblasts and to explore its relationships with the TLR4 signaling pathway. Main methods: A mouse osteoblast cell line, MC3T3-E1, and a diabetic rat model were used to survey the protective effects of metformin on hyperglycemia-induced injury. TLR4 expression was altered using small interfering (si) RNA and lentivirus-mediated TLR4 overexpression. LPS was used as a specific TLR4 activator, and CLI095 was used as a TLR4 inhibitor. Key findings: Metformin improved osteoblast differentiation, reduced apoptosis in hyperglycemic osteoblasts, and inhibited TLR4, MyD88 and NF-kappa B expression in a dose-dependent manner. Down-regulating the expression or inhibiting the activity of TLR4 enhanced these protective effects of metformin on osteoblast differentiation, cell viability and cell apoptosis in hyperglycemic conditions, whereas up-regulating the expression or activating the activity of TLR4 had the opposite effects. Activating NF-kappa B suppressed the protective effects of metformin, while inhibiting NF-kappa B activity had the opposite effects. Metformin increased ALP and OCN secretion, enhanced BMP-2 expression, improved bone mineral density (BMD), and decreased TLR4, MyD88 and NF-kappa B levels in the femur tissues of diabetic rats. Significance: Taken together our experimentation support the hypothesis that metformin may alleviate hyperglycemia-induced apoptosis and differentiation suppression in osteoblasts by inhibiting the TLR4/MyD88/NF-kappa B signaling pathway.