Glucose restriction regulates osteoblasts function by modulating mitochondrial activity and lipolysis

Nutrient availability is fundamental for osteoblasts (OBs) bioenergetics and function. However, OBs metabolic response to glucose (G) restriction and lipid availability remains to be clarified. We studied the effect of G restriction in MC3T3-E1 osteoblastic cells and in primary osteoblasts (pOBs) cultured with G at physiological (PG; 5.5 mM) or low (LG; 1.25 mM) concentrations. We found that LG decreased OBs proliferation, migration, and clonogenicity compared to PG levels, while osteogenesis was enhanced in LG conditions. An increased mitochondrial size associated with increased mitofusin 2 expression and elongated mitochondria in LG was observed compared to PG-treated OBs. These mitochondrial features were associated with increased mitochondrial oxidative capacity and elevated ATP production in LG-treated OBs compared to PG. Etomoxir (Eto) addition to LG negatively affected mitochondrial function and osteogenesis, suggesting lipid utilization by OBs in LG condition. Accordingly, lipid droplet count was lower in LG condition while lipolysis genes were upregulated. Eto addition to lipid-reduced serum induced increased lipid droplets accumulation and downregulation of lipolytic genes in LG environment. Finally, inhibition of lipolysis through ATGListatin reduced osteogenesis in LG-treated compared to PG-treated OBs cell cultures. Altogether, these results show that OBs function is modulated by fuel availability and reinforces the fundamental role for lipid utilization in orchestrating OBs metabolism, during energy demand when tissue glucose availability is limited, such as in pathophysiological conditions, including diabetes.