Endoglin differentially regulates TGF-β-induced Smad2/3 and Smad1/5 signalling and its expression correlates with extracellular matrix production and cellular differentiation state in human chondrocytes

Objective: Transforming growth factor-beta (TGF-beta) plays a critical role in cartilage homeostasis and deregulation of its signalling is implicated in osteoarthritis (OA). TGF-beta isoforms signal through a pair of transmembrane serine/threonine kinases known as the type I and type II TGF-beta receptors. Endoglin is a TGF-beta co-receptor that binds TGF-beta with high affinity in the presence of the type II TGF-beta receptor. We have previously shown that endoglin is expressed in human chondrocytes and that it forms a complex with the TGF-beta signalling receptors. However, the functional significance of endoglin expression in chondrocytes is unknown. Our objective was to determine whether endoglin regulates TGF-beta/Smad signalling and extracellular matrix (ECM) production in human chondrocytes and whether its expression varies with chondrocyte differentiation state. Method: Endoglin function was determined by overexpression or antisense morpholino/siRNA knockdown of endoglin in human chondrocytes and measuring TGF-beta-induced Smad phosphorylation, transcriptional activity and ECM production. Alterations in endoglin expression levels were determined during subculture-induced dedifferentiation of human chondrocytes and in normal vs OA cartilage samples. Results: Endoglin enhances TGF-beta 1-induced Smad1/5 phosphorylation and inhibits TGF-beta 1-induced Smad2 phosphorylation, Smad3-driven transcriptional activity and ECM production in human chondrocytes. In addition, the enhancing effect of endoglin siRNA knockdown on TGF-beta 1-induced Smad3-driven transcription is reversed by ALK1 overexpression. Furthermore, endoglin levels are increased in chondrocytes following subculture-induced dedifferentiation and in OA cartilage as compared to normal cartilage. Conclusion: Together, our results suggest that endoglin regulates the balance between TGF-beta/ALK1/Smad1/5 and ALK5/Smad2/3 signalling and ECM production in human chondrocytes and that endoglin may represent a marker for chondrocyte phenotype. (C) 2010 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.