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, extracellular 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‐β1 (TGF‐β1) for certain functions in specific cell types. In this study, we examined the role of CTGF as a downstream mediator of TGF‐β1‐induced ECM production and cell growth in osteoblasts. Using primary cultures, we demonstrated that TGF‐β1 is a potent inducer of CTGF expression in osteoblasts, and that this induction occurred at all stages of osteoblast differentiation from the proliferative through mineralization stages. TGF‐β1 treatment of osteoblasts increased the expression and synthesis of the ECM components, collagen and fibronectin. When CTGF‐specific siRNA was used to prevent TGF‐β1 induction of CTGF expression, it also inhibited collagen and fibronectin production, thereby demonstrating the requirement of CTGF for their up‐regulation. To examine the effects of TGF‐β1 on osteoblast cell growth, cultures were treated with TGF‐β1 during the proliferative stage. Cell number was significantly reduced and the cells exhibited a decrease in G1 cyclin expression, consistent with TGF‐β1‐induced cell‐cycle arrest. Cultures transfected with CTGF siRNA prior to TGF‐β1 treatment showed an even greater reduction in cell number, suggesting that TGF‐β1‐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‐β1‐induced ECM production in osteoblasts, but these two growth factors function independently regarding their opposing effects on osteoblast proliferation. J. Cell. Physiol. 210: 843–852, 2007. © 2006 Wiley‐Liss, Inc.