Inhibitory effects of hypoxia on metabolic switch and osteogenic differentiation of human mesenchymal stem cells.

We previously demonstrated that metabolic switch and mitochondrial activation are required for osteogenic differentiation of human mesenchymal stem cells (hMSCs). However, stem cells in niches or transplanted into injured tissues constantly encounter hypoxic stress that hinders aerobic metabolism. Therefore, we investigated the effects of oxygen tension (1% vs. 21%) on metabolism and osteogenic differentiation of hMSCs. We found that hypoxia impaired osteogenic differentiation as indicated by attenuation of alkaline phosphatase activity and expression of osteogenic markers core binding factor a-1 and osteopontin. In addition, differentiation-induced mitochondrial activation was compromised as shown by the decrease in the expression of respiratory enzymes and oxygen consumption rate. On the contrary, anaerobic metabolism was augmented as revealed by the upregulation of glycolytic enzymes and increase of lactate production, rendering the cells to rely more on anaerobic glycolysis for energy supply. Moreover, administration of 2-deoxyglucose (a glycolytic inhibitor) but not antimycin A (a respiratory inhibitor) significantly decreased intracellular ATP levels of hMSCs differentiating under hypoxia. Treatment with cobalt chloride, a hypoxia-inducible factor-1α (HIF-1α) stabilizer, recapitulated the inhibitory effects of hypoxia, suggesting that HIF-1α is involved in the compromise of hMSCs differentiation. These results suggest that hypoxia inhibits metabolic switch and mitochondrial function and therefore suppresses osteogenic differentiation of hMSCs.