The plasminogen‐activating system in hepatic stellate cells

Urokinase plasminogen activator (uPA) generates plasmin, a process inhibited by plasminogen‐activator inhibitor (PAI)‐1 and localized to the cell surface by binding of uPA to a specific receptor. Plasmin degrades extracellular matrix (ECM) both directly and by activation of matrix metalloproteinases (MMPs). Because stellate cells play a central role in the pathogenesis of liver fibrosis both via production of ECM proteins and through secretion of MMPs, their contribution to plasmin generation was assessed. Stellate cells were prepared from rat liver and cultured on plastic. Northern analysis showed cellular expression of messenger RNA (mRNA) for PAI‐1, uPA, and uPA receptor. Zymography/reverse zymography identified cell‐surface‐associated uPA activity and uPA and PAI‐1 in culture media. Net uPA activity in culture media was maximal after 7 days in culture and then declined, whereas PAI‐1 antigen levels remained consistently elevated between 7 and 21 days in culture. Stellate cell‐mediated plasmin generation was also seen in in vitro cultures supplemented with plasminogen. Because hepatic stellate cells (HSCs) contain retinoids and release them on activation, the effect of retinoic acid on the plasminogen‐activating system was also assessed. Treatment of cultured HSCs with retinoic acid (1 μmol/L) increased uPA secretion 2.6‐fold but did not alter PAI‐1. We conclude that stellate cells synthesize key components of the plasminogen‐activating system and generate plasmin and therefore have the ability to regulate MMP activation. Upregulation of uPA synthesis by retinoic acid may have implications in matrix remodeling in sites of stellate cell activation in which high concentrations of retinoids may be achieved.