A role of the clock gene Period1 in chondrogenic differentiation

The view that circadian rhythm involves different proteins relevant to clock genes, including Period (Per), Cryptochrome, Clock, Bmal1 and Dec, in the suprachiasmatic nucleus in mammals is prevailing. In line with recent studies on expression of these clock genes in particular peripheral tissues such as heart and liver, we have shown transient promotion by parathyroid hormone (PTH) of Per1 mRNA expression in cultured chondrocytes. In order to elucidate a possible functional role of Per1 in chondrocytes, in the present study, we have attempted to establish the mouse chondrogenic cell line ATDC5 cells stably transfected with Per1 expression vector. For this purpose, ATDC5 cells were transfected with pcDNA3.1 containing the full-length coding region of Per1 (ATDC5-Per1) or empty vector (ATDC5-EV), followed by selection of particular clones with G418. Pools of clones of ATDC5-Per1 cells were isolated for determination of expression levels of Per1 by real-time-PCR. In ATDC5 cells stably overexpressing Per1, significant inhibition was found with the maturation-dependent temporal increases in Alcian blue staining and mRNA expression of type II and type X collagens. An attempt was next made to elucidate underlying mechanisms for the suppression by Per1 overexpression of chondrogenic differentiation with the reporter assay using a 4x48-p89-Luc, 4 tandem copies of Sox9 allies (Sox5, Sox6 and Sox9) binding site linked to the minimal Col II gene promoter in the luciferase reporter plasmid. The 4x48-p89-Luc plasmid was transiently transfected into ATDC5-Per1 and ATDC5-EV cells, followed by determination of luciferase activity 48 h after transfection. Luciferase activity was significantly inhibited in ATDC5-Per1 cells compared with that in ATDC5-EV cells. Moreover, overexpression of Per1 decreased the Sox6 mRNA level along with a decreased Sox6 promoter activities required for Sox6 mRNA expression in ATDC5 cells. These results suggest that PTH may suppress chondrogenesis through the promotion of Per1 expression toward repression of Sox6 mRNA expression.