Conditions Inducing Excessive O-GlcNAcylation Inhibit BMP2-Induced Osteogenic Differentiation of C2C12 Cells

Hyperglycemic conditions in diabetic patients can affect various cellular functions, including the modulation of osteogenic differentiation. However, the molecular mechanisms by which hyperglycemia affects osteogenic differentiation are yet to be clarified. This study aimed to investigate whether the aberrant increase in protein O-linked--N-acetylglucosamine glycosylation (O-GlcNAcylation) contributes to the suppression of osteogenic differentiation due to hyperglycemia. To induce osteogenic differentiation, C2C12 cells were cultured in the presence of recombinant human bone morphogenetic protein 2 (BMP2). Excessive protein O-GlcNAcylation was induced by treating C2C12 cells with high glucose, glucosamine, or N-acetylglucosamine concentrations or by O-GlcNAc transferase (OGT) overexpression. The effect of O-GlcNAcylation on osteoblast differentiation was then confirmed by examining the expression levels of osteogenic marker gene mRNAs, activity of alkaline phosphatase, and transcriptional activity of Runx2, a critical transcription factor for osteoblast differentiation and bone formation. Cell treatment with high glucose, glucosamine or N-acetylglucosamine increased O-GlcNAcylation of Runx2 and the total levels of O-GlcNAcylated proteins, which led to a decrease in the transcriptional activity of Runx2, expression levels of osteogenic marker genes (Runx2, osterix, alkaline phosphatase, and type I collagen), and activity of alkaline phosphatase. These inhibitory effects were rescued by lowering protein O-GlcNAcylation levels by adding STO45849, an OGT inhibitor, or by overexpressing -N-acetylglucosaminidase. Our findings suggest that excessive protein O-GlcNAcylation contributes to high glucose-suppressed osteogenic differentiation.