Modulation of fibronectin and proteoglycan synthesis by chondrocytes.

Synthesis of fibronectin and proteoglycan by chondrocytes in both cell and organ culture was affected by the culture conditions. Less than 2% of the fibronectin extracted from cartilage contained the ED-III-A sequence, however, chondrocytes placed in primary culture began to synthesize increased amounts of ED-III-A fibronectin (up to 36% of the total fibronectin). Expression of ED-III-A fibronectin was selectively reduced when the chondrocytes were cultured in the presence of 0.5 mM dibutyryl cyclic AMP. The amount of fibronectin in the media of DBcAMP treated cultures was reduced by 30% but there was no effect on the overall protein biosynthetic rates of the cells. Concomitant with the decrease in fibronectin was an increase in the concentration of keratan sulfate in the media of treated cultures. Treatment with 0.5 mM dibutyryl cAMP caused the cells to adopt a more rounded morphology. Proteoglycan synthesis in cartilage explants was maintained at levels comparable to those at Day 0 for up to twelve days in a medium supplemented with insulin. Fibronectin and collagen synthesis, however, declined relative to Day 0 levels in this medium. The addition of TGF-beta at 2 and 10 ng/ml to the medium not only prevented the decline in fibronectin synthesis but progressively increased its rate of synthesis until the Day 0 levels were exceeded 4-fold. Experiments were initiated to study also the effects of mechanical factors on cartilage explant metabolism.