Connective tissue growth factor (CTGF/CCN2) is a downstream mediator for TGF-β1-induced extracellular matrix production in osteoblasts

Connective tissue growth factor-(CTGF/CCN2) is a cysteine-rich, extra cellular matrix (ECM) protein that acts as an anabolic growth factor to regulate osteoblast differentiation and function. Recent studies have identified CTGF as a downstream effector of transforming growth factor-beta 1 (TGF-beta 1) for certain functions in specific cell types. In this study, we examined the role of CTGF as a downstream mediator of TGF-beta 1-induced ECM production and cell growth in osteoblasts. Using primary Cultures, we demonstrated that TGF-beta 1 is a potent inducer of CTGF expression in osteoblasts, and that this induction Occurred at all stakes of osteoblast differentiation from the proliferative through mineralization stages. TGF-beta 1 treatment of osteoblasts increased the expression and synthesis of the FCM components, collagen and fibronectin. When CTGF-specific siRNA was used to prevent TGF-beta 1 induction of CTGF expression, it also inhibited collagen and fibronectin production, thereby demonstrating the requirement of CTGF for their upregulation. To examine the effects of TGF-beta 1 on osteoblast cell growth, cultures were treated with TGF-beta 1 during the proliferative stage. Cell number was significantly reduced and the cells exhibited a decrease in G(1) cyclin expression, consistent with TGF-beta 1-induced cell-cycle arrest. cultures transfected with CTGF siRNA prior to TGF-beta 1 treatment showed all even greater reduction in cell number, suggesting that TGF-beta 1 -induced growth arrest is independent of CTGF in osteoblasts. Collectively, these data demonstrate for the first time that CTGF is an essential downstream mediator for TGF-beta 1 -induced FCM production in osteoblasts, but these two growth factors function independently regarding their opposing effects on osteoblast proliferation.