Exposure of Cryptic Domains in the !1-chain of Laminin-1 by Elastase Stimulates Macrophages Urokinase and Matrix Metalloproteinase-9 Expression*

Degradation of the extracellular matrix leads to the release of fragments, which elicit biological responses distinct from intact molecules. We have reported that !1:Ser2091–Arg2108, a peptide derived from the !1-chain of laminin-1, triggers protein kinase C-dependent activation of MAPKerk1/2, leading to the up-regulation of macrophage urokinase type plasminogen activator and matrix metalloproteinase (MMP)-9 expression. Since intact laminin-1 failed to trigger these events, we hypothesized that !1:Ser2091–Arg2108 is cryptic or assumes a conformation not recognized by macrophages. Here we demonstrate that elastase cleavage of laminin-1 generates fragments, which stimulate proteinase expression by RAW264.7 macrophages and peritoneal macrophages. In contrast, fragments generated by MMP-2, MMP-7, or plasmin had no effect on macrophage proteinase expression. Elastase-generated laminin-1 fragments were fractionated by heparin-Sepharose chromatography. Heparinbinding fragments stimulated macrophages’ proteinase expression severalfold greater than nonbinding fragments. The heparin binding fragments reacted with antibodies directed against regions of the !1-chain including !1:Ser2091– Arg2108 and the globular domain. A peptide from the first loop of the globular domain (!1:Ser2179–Ser2198) triggered the phosphorylation of MAPKerk1/2 and stimulated the expression of macrophage urokinase type plasminogen activator and MMP-9. Moreover, a heparin-binding fraction isolated from an aortic aneurysm contained fragments of !1-chain and stimulated macrophages’ proteinase expression. Based on these data, we conclude that cryptic domains in the COOH-terminal portion of the !1-chain of laminin are exposed by proteolysis and stimulate macrophages’ proteinase expression.