Differential regulation of lubricin/superficial zone protein by transforming growth factor β/bone morphogenetic protein superfamily members in articular chondrocytes and synoviocytes

Objective. To investigate the role of transforming growth factor beta (TGF beta)/bone morphogenetic protein (BMP) superfamily members on accumulation of superficial zone protein (SZP) in articular chondrocytes and synoviocytes. Methods. Chondrocytes and synoviocytes were isolated from articular cartilage and synovium from calf stifle joints and cultured as monolayers in serum-free chemically defined medium. Articular chondrocytes were isolated from 3 distinct zones of the cartilage: superficial, middle, and deep. Accumulation of SZP in the culture medium in response to various members of the TGF beta/BMP superfamily was demonstrated by immunoblotting and quantified by enzyme-linked immunosorbent assay. Results. TGF beta stimulated SZP accumulation in both superficial zone chondrocytes and synoviocytes. The 3 isoforms of TGF beta elicited a similar dose response. Inhibition of TGF beta receptor type I kinase by the specific inhibitor SB431542 abolished the TGF beta-stimulated accumulation of SZP. BMPs up-regulated SZP accumulation in the superficial zone; however, the magnitude of the effects was not as great as was observed with TGFJ3. There was an additive action between TGF beta and BMP on SZP accumulation. The response of synoviocytes to BMP was stronger than that of superficial zone chondrocytes. Activin up-regulated SZP accumulation in synoviocytes, but not in chondrocytes. Conclusion. TGF beta is a critical regulator of SZP accumulation in both superficial zone articular chondrocytes and synoviocytes. TGF beta and BMP have an additive effect. Synoviocytes are more sensitive to BMP family members and activins than are superficial zone articular chondrocytes. Thus, regulation of SZP accumulation by TGF beta/BMP superfamily members is regulated differently in articular chondrocytes and synoviocytes.