Objective To explore the intrinsic relationship between circadian clock and cell cycle in osteoarthritis(OA)chondrocytes,especially the regulation of cell cycle-related genes by the clock gene Bmal1.Methods The chondroid ATDC5 cells induced by insulin-transfering-selenium(ITS)were divided into control group,OA group and LV-Bmal1 group.The cell viability of each group was detected by CCK8 method.The expression of Bmal1,Per1,Wee1,Cdk1,Ccnb1 and Mmp13 mRNA in each group was detected by RT-qPCR.The expression of BMAL1,PER1,WEE1,CDK1,CCNB1 and MMP13 protein in each group was detected by Western blot.The effects of Bmal1 on different stages of cell cycle and apoptosis was analyzed by flow cytometry.The regulation of Bmal1 on Per1,Wee1,Cdk1,Ccnb1 and Mmp13 and their roles in OA were analyzed.Results Compared with the normal group,the cell viability of the OA group was increased,the relative mRNA expression of Bmal1 and Wee1 in the OA group decreased,and the relative mRNA expression of Per1,Cdk1,Ccnb1 and Mmp13 increased signif-icantly.The cell viability of LV-Bmal1 group decreased,the relative expression of Bmal1 and Wee1 mRNA in-creased,and the relative expression of Per1,Cdk1,Ccnb1 and Mmp13 mRNA decreased(P＜0.05).Correlation analysis showed that Bmal1 was positively correlated with Wee1 and they were negatively correlated with Per1,Cdk1,Ccnb1 or Mmp13.The results of Western blot showed that protein expression in different groups were con-sistent with the trend of PCR.The results of cell cycle and apoptosis showed that compared with the normal group,the S phase and G2/M phase of the OA group were shortened,the proportion of cells decreased significantly,and the proportion of early and late apoptosis increased.The S phase and G2/M phase of the LV-Bmal1 group were prolonged,the proportion of cells was increased,and the proportion of early and late apoptosis was decreased.Conclusions Circadian clock gene Bmal1 in inflammatory chondrocytes might regulate the expression of cell cycle-related genes.