Cartilage synthesizes the serine protease inhibitor PAI-1: support for the involvement of serine proteases in cartilage remodeling

The work described here demonstrates the synthesis by human articular cartilage of plasminogen activator inhibitor-1 (PAI-1), a potent inhibitor of the serine protease tissue plasminogen activator (tPA). We also present data demonstrating an increase in PAI-1 messenger ribonucleic acid (mRNA) in chondrocytes exposed to the cytokine interleukin-1 (IL-1). Interestingly, this elevation of steady-state mRNA levels does not appear to result in an increase in synthesis of PAI-1 protein. Northern blot analysis reveals that of the two mRNA species (3.4 kb, 2.4 kb) previously reported for PAI-1, only the larger species (3.4 kb) appears to be synthesized by chondrocytes. Our data demonstrate the IL-1-stimulated production by cartilage of tissue plasminogen activator. We also show evidence for the presence of plasminogen in cartilage. A scheme is presented indicating the probable importance of the serine proteases (tPA and plasminogen) and PAI-1 in cartilage degradation.