Tensile strain increases expression of CCN2 and COL2A1 by activating TGF-β-Smad2/3 pathway in chondrocytic cells

Physiologic mechanical stress stimulates expression of chondrogenic genes, such as multifunctional growth factor CYR61/CTGF/NOV (CCN) 2 and α1(II) collagen (COL2A1), and maintains cartilage homeostasis. In our previous studies, cyclic tensile strain (CTS) induces nuclear translocation of transforming growth factor (TGF)-β receptor-regulated Smad2/3 and the master chondrogenic transcription factor Sry-type HMG box (SOX) 9. However, the precise mechanism of stretch-mediated Smad activation remains unclear in transcriptional regulation of CCN2 and COL2A1. Here we hypothesized that CTS may induce TGF-β1 release and stimulate Smad-dependent chondrogenic gene expression in human chondrocytic SW1353 cells. Uni-axial CTS (0.5. Hz, 5% strain) stimulated gene expression of CCN2 and COL2A1 in SW1353 cells, and induced TGF-β1 secretion. CCN2 synthesis and nuclear translocalization of Smad2/3 and SOX9 were stimulated by CTS. In addition, CTS increased the complex formation between phosphorylated Smad2/3 and SOX9. The CCN2 promoter activity was cooperatively enhanced by CTS and Smad3 in luciferase reporter assay. Chromatin immunoprecipitation revealed that CTS increased Smad2/3 interaction with the CCN2 promoter and the COL2A1 enhancer. Our results suggest that CTS epigenetically stimulates CCN2 transcription via TGF-β1 release associated with Smad2/3 activation and enhances COL2A1 expression through the complex formation between SOX9 and Smad2/3.