Lactate inhibits osteogenic differentiation of human periodontal ligament stem cells via autophagy through the MCT1-mTOR signaling pathway

Periodontal ligament stem cells (PDLSCs) play a crucial role in periodontal bone regeneration. Lactate used to be considered a waste product of glucose metabolism. In recent years, a few pieces of evidence revealed its roles in regulating the osteogenic differentiation of stem cells, but the standpoints were controversial. This study aims to investigate the effects and the mechanisms of lactate on the osteogenic differentiation of human periodontal ligament stem cells (hPDLSCs).The hPDLSCs were treated with different concentrations of lactic acid and lactate to differentiate the effects of the acidic PH and ion lactate. Proliferation and cytotoxicity were measured by Cell Counting Kit-8 (CCK8) assay and Live/Dead assay. The osteogenic differentiation of hPDLSCs was analyzed by alizarin red staining, alkaline phosphatase (ALP) staining, and then osteogenic proteins and genes were measured by western blot and reverse transcription-quantitative PCR (qRT-PCR). To investigate the potential signaling pathways, MCT1 inhibitor, G-protein inhibitors, and rapamycin were used, and then autophagy-related proteins and osteogenic proteins were measured by western blot.The inhibition of lactic acid on the osteogenic differentiation of hPDLSCs was more significant than lactate at the same concentration. Lactate inhibited the expression of ALP which can be rescued by Gα inhibitor. Alizarin red staining, the protein expression levels of osteocalcin (OCN), osteoprotegerin (OPN), osterix (OSX), and beclin1, LC3-II/LC3-I were decreased by lactate and partly rescued by MCT1 inhibitor. Rapamycin restored the protein expression levels of beclin1, LC3-II/LC3-I and OCN, OPN, OSX under the high lactate conditions.Lactate inhibits the expression of ALP via Gα subunit signaling, and inhibits mineralized nodules formation and the expression of osteogenic-related proteins via reducing autophagy through the MCT1-mTOR signaling pathway.