Transforming growth factor-beta modulates the expression of osteoblast and chondroblast phenotypes in vitro

Transforming growth factor β (TGF‐β) has been shown to induce chondrogenesis by embryonic rat mesenchymal cells (Seyedin et al., J. Biol. Chem., 261:5693, 1986). Here we report the effects of bovine TGF‐β on the phenotypic expression of differentiated primary rat osteoblastic and chondroblastic cells. Culture of rat calvarial osteoblasts with TGF‐β resulted in a dose and time‐dependent decrease in alkaline phosphatase activity. Levels of alkaline phosphatase were reduced to less than 10% of control values by 0.4 nM TGF‐β. The decrease became apparent after 24 hours and reached a maximum by 72 hours. Similarly, treatment of chondroblasts with 0.4 nM TGF‐β resulted in decreased production of cartilage‐specific macromolecules: type II collagen and cartilage proteoglycan. Both cell types exhibited dramatic changes in cell shape after treatment with TGF‐β. Modulation of these differentiated markers by TGF‐β could be mimicked, in part, by addition of fibronectin. Addition of dihydrocytochalasin B blocked the inhibition of phenotypic expression by TGF‐β. These results indicate that TGF‐β inhibits phenotypic expression by osteoblasts and chondroblasts in vitro and suggest that this activity of TGF‐β may be mediated through interactions between the extracellular matrix and cytoskeletal elements.