Lactate induces oxidative phosphorylation in osteoblasts via Gpr81-Stat3 signaling

Lactate has long been regarded as an end product of glycolysis and a metabolic "waste product" under hypoxic conditions, but recent studies have revealed that lactate plays a central role in energy metabolism reprogramming and intercellular communication. However, it remains unknown whether lactate promotes osteogenic differentiation through metabolic reprogramming. Here, we showed that lactate significantly increased the cellular ATP content, activated succinate dehydrogenase activity, and enhanced oxygen consumption rate in pre-osteoblast MC3T3-E1 cells. Moreover, lactate treatment increased oxidative phosphorylation (OXPHOS) in parathyroid hormone (PTH)-treated MC3T3-E1 cells. Microarray and RNA-sequencing analysis revealed that Stat3 signaling was enriched in MC3T3-E1 cells treated with lactate or co-treated with lactate and PTH. Immunoblotting verification analysis further showed that lactate activated the Jak2-Stat3-Y705 and Akt-Stat3-S727 signaling. Inhibition of Jak2-Stat3-Y705 signaling by AG490 interrupted lactate-induced osteoblast differentiation. Inhibition of Gpr81 by 3-OBA or decrease in Gpr81 expression by Gpr81 siRNA, but not the interruption of MCT1 by AZD3965, led to the inhibition of the Gpr81-Jak2-Stat3-Y705 and Gpr81-Akt-Stat3-S727 signaling, and OXPHOS and cell differentiation of MC3T3-E1 cells were also inhibited. Furthermore, we demonstrated that the Gpr81 subunit G_βγ plays a central role in lactate-Gpr81 signaling. Lastly, osteoblast Gpr81-deficient mice showed lower bone formation. Thus, these findings propose a novel signaling mechanism by which lactate regulates cell differentiation as well as OXPHOS through the activation of Stat3 signaling by Gpr81.