Introduction: TGF‐β is a multifunctional regulator of chondrocyte proliferation, differentiation, and extracellular matrix production. Dysregulation of TGF‐β action has been implicated in cartilage diseases such as osteoarthritis. TGF‐β signaling is transduced through a pair of transmembrane serine/threonine kinases, known as the type I (ALK5) and type II receptors. However, recent studies on endothelial cells have identified ALK1 as a second type I TGF‐β receptor and have shown that ALK1 and ALK5 have opposing functions in these cells. Here we examined ALK1 expression and its regulation of TGF‐β signaling and responses in human chondrocytes.

Materials and Methods: ALK1 expression in human chondrocytes was examined by RT‐PCR and Western blot. The ability of ALK1 to form complexes with other TGF‐β receptors was determined by affinity labeling/immunoprecipitation and by immunoprecipitation followed by Western blot. The effect of ALK1 on TGF‐β1–induced signaling and responses was determined by varying ALK1 expression levels and measuring transcriptional activity using promoter/luciferase assays, Smad1/5 and Smad3 phosphorylation, and expression of type II collagen, PAI‐1, and fibronectin.

Results: Our results indicate that ALK1 is expressed in human chondrocytes and that it is a component of the TGF‐β receptor system, associating with ALK5, type II TGF‐β receptor, endoglin, and betaglycan. Furthermore, we show that both ALK1 and ALK5 are needed for TGF‐β–induced phosphorylation of intracellular mediators Smad1/5, whereas only ALK5 is essential for TGF‐β1–induced phosphorylation of Smad3. In addition, our results show that ALK1 inhibits, whereas ALK5 potentiates, TGF‐β–induced Smad3‐driven transcriptional activity and the expression of PAI‐1, fibronectin, and type II collagen in chondrocytes.

Conclusions: Our results suggest that ALK1 and ALK5 display opposing functions in human chondrocytes, implicating an essential role for ALK1 in the regulation of TGF‐β signaling and function in these cells.