CCN family 2/connective tissue growth factor (CCN2/CTGF) regulates the expression of Vegf through Hif-1α expression in a chondrocytic cell line, HCS-2/8, under hypoxic condition

Vascular endothelial growth factor (VEGF) is essential for establishing vascularization and regulating chondrocyte development and survival. We have demonstrated that VEGF regulates the expression of CCN2/connective tissue growth factor (CCN2/CTGF) an essential mediator of cartilage development and angiogenesis, suggesting that CCN2 functions in down-stream of VEGF, and that VEGF function is mediated in part by CCN2. On the other hand, the phenotype of Ccn2 mutant growth plates, which exhibit decreased expression of VEGF in the hypertrophic zone, indicates that Vegf expression is dependent on Ccn2 expression as well. Therefore, we investigated the molecular mechanisms underlying the induction of VEGF by CCN2 using a human chondrocytic cell line, HCS-2/8. Hypoxic stimulation (5% O₂) of HCS-2/8 cells increased VEGF mRNA levels by ∼ 8 fold within 6 h as compared with the cells cultured under normoxia. In addition, VEGF expression was further up-regulated under hypoxia in HCS-2/8 cells transfected with a Ccn2 expression plasmid. Hypoxia-inducible factor (HIF)-1α mRNA and protein levels were increased by stimulation with recombinant CCN2 (rCCN2). Furthermore, the activity of a VEGF promoter that contained a HIF-1 binding site was increased in HCS-2/8, when the cells were stimulated by rCCN2. These results suggest that CCN2 regulates the expression of VEGF at a transcriptional level by promoting HIF-1α activity. In fact, HIF-1α was detected in the nuclei of proliferative and pre-hypertrophic chondrocytes of wild-type mice, whereas it was not detected in Ccn2 mutant chondrocytes in vivo. This activation cascade from CCN2 to VEGF may therefore play a critical role in chondrocyte development and survival.