Anionic glycoconjugates from differentiated and dedifferentiated cultures of bovine articular chondrocytes: modulation by TGF-beta

Hyperglycosylation of fibronectin by TGF-β1-stimulated chondrocytes

The addition of TGF-beta1 to bovine articular chondrocytes resulted in increased synthesis and secretion of two anionic glycoproteins, including a previously studied but unidentified high molecular weight anionic glycoconjugate (HMW-AG). Sequencing by mass spectroscopy identified these anionic glycoproteins as fibronectin. Western blot analysis confirmed the identity of these two overexpressed glycoproteins as fibronectin. In the presence and absence of TGF-beta1 both V(+) and V(-) isoforms of fibronectin, which are EDA(-) and EDB(-), are synthesized. Dual labeling experiments suggest that the HMW-AG, the larger of the two overexpressed glycoproteins (apparent molecular weight of monomer approximately 260,000 Da), is more heavily glycosylated than the lower molecular weight anionic glycoprotein. Since fibronectin proteolytic fragments appear to enhance matrix metalloproteinase synthesis, TGF-beta1-mediated hyperglycosylation of fibronectin could regulate cartilage metabolism by providing protection of fibronectin from proteolysis, a mechanism that would also favor articular cartilage health.

TGF beta 1 and biglycan, decorin, and fibromodulin metabolism in canine cartilage

OBJECTIVE

Small proteoglycans (PGs) may accumulate in late stage osteoarthritis even as aggrecan is lost. It is not clear what role transforming growth factor (TGF) beta has in this accumulation. Our goal was to investigate the ability of TGF beta 1 to modulate the synthesis and accumulation of decorin, biglycan, and fibromodulin in cartilage explants cultured under conditions in which aggrecan synthesis remains relatively constant.

DESIGN

Articular cartilage was cultured in the presence or absence of 4 ng/ml TGF beta 1 for up to 16 days. Material extracted from cartilage was assayed for 35SO(4)-large and small PGs and for total endogenous decorin, biglycan and fibromodulin.

RESULTS

The synthesis of 35SO(4)-small PGs increased during the 16 days in culture in response to TGF beta 1, but declined in control cultures. The difference in 35SO(4)-decorin between TGF beta 1 and control samples reached nine-fold after 16 days, while the difference in total endogenous decorin was less than 1.5-fold. 35SO(4)-decorin, which was present in TGF beta 1-treated cultures had an identical core protein, but a longer glycosaminoglycan chain than that of decorin in control cultures. No significant differences in endogenous biglycan were detected, but accumulation of fibromodulin in TGF beta 1 explants exceeded fibromodulin in controls, on average, by 3.8-fold. Fibromodulin was present in cartilage in both keratan sulfate- and non-sulfated oligosaccharide-substituted forms.

CONCLUSIONS

The accumulation of each of the three small PGs was affected to a different extent in response to TGF beta 1. Of the three, fibromodulin content was most rapidly augmented in response to TGF beta 1.