CircCDK8 regulates osteogenic differentiation and apoptosis of PDLSCs by inducing ER stress/autophagy during hypoxia

Mounting evidence indicates that circular RNAs (circRNAs) have essential roles in several diseases, including periodontitis. Periodontal ligament stem cells (PDLSCs) exhibit potential for treating periodontitis accompanied by hypoxia. However, it is unclear how circRNA affects the osteogenesis of PDLSCs under hypoxia. In this study, a novel circRNA, hsa_circ_0003489, was found located at the gene for cyclin-dependent kinase 8 (CDK8) and referred to as circCDK8. The expression levels of circCDK8 and hypoxia-inducible factor-1α were significantly increased in periodontitis tissues, and the expression of circCDK8 was further confirmed in a hypoxia model using cobalt chloride (CoCl₂). Interestingly, the results showed that the expression levels of osteoblast markers (RUNX2, ALP, OCN, and COL1A1) were increased in CoCl₂-treated PDLSCs at 6 and 12 h, but decreased at 24, 48, and 72 h. On the basis of bioinformatics and functional experiments, CoCl₂ also induced endoplasmic reticulum stress, autophagy, and apoptosis of PDLSCs; the inhibition of autophagy promoted the osteogenic differentiation of CoCl₂-treated PDLSCs. Furthermore, circCDK8 overexpression induced autophagy and apoptosis through mTOR signaling, and circCDK8 silencing reversed the inhibitory effects of CoCl₂ on osteogenic differentiation of PDLSCs. In conclusion, our results indicate that circCDK8 represses the osteogenic differentiation of PDLSCs by triggering autophagy activation in a hypoxic microenvironment. CircCDK8 could be a new therapeutic target of periodontitis.