Connective tissue growth factor induces osteogenic differentiation of vascular smooth muscle cells through ERK signaling

Connective tissue growth factor (CTGF) plays an important role in the pathogenesis of atherosclerosis by promoting vascular smooth muscle cell (VSMC) growth, migration, apoptosis, adhesion and the secretion of matrix components. The osteogenic differentiation of VSMCs is essential in the development of vascular calcification. However, the role of CTGF in the transdifferentiation and calcification of VSMCs is unclear. In the present study, we examined whether CTGF stimulates VSMC transdifferentiation. Primary VSMCs were obtained from mouse thoracic aortas by enzymatic digestion and identified by immunostaining for smooth muscle specific alpha-actin antibody (alpha-SMA). VSMC calcification was induced by the addition of CTGF to the osteogenic mediaum containing 5-10% FBS in the presence of 0.25 mM ascorbic acid and 10 mM beta-glycerophosphate for 14 days. Calcified cells were determined by Alizarin Red S staining. Our results revealed that CTGF induced the expression of several bone markers, including alkaline phosphatase (ALP), osteocalcin (OC), osteoprotegerin (OPG) and core-binding factor subunit alpha 1 (Cbf alpha 1)/runt-related transcription factor 2 (Runx2), as well as calcification. However, the inhibition of extracellular signal-regulated kinase (ERK) activity using the ERK-specific inhibitor, PD98059, blocked the induction of these proteins and VSMC calcification. Based on these data, we conclude that CTGF stimulates the transdifferentiation of VSMCs into osteoblasts and that the ERK signaling pathway appears to play a critical role in this process.