Melanoma-mediated Dissolution of Extracellular Matrix: Contribution of Urokinase-dependent and Metalloproteinase-dependent Proteolytic Pathways1

Constitutive overexpression of both urokinase and matrix metalloproteinase (MMP) activity is frequently observed in individual malignant tumors. In this study we describe the combined contribution of these distinct enzyme systems to the invasive phenotype of a highly metastatic human melanoma cell line (M24met). \124im-t cells were found to secrete a spectrum of MMl's, including interstitial collagenase, type IV collagenases (M, 92,000 and 72,000 progelatinases), and stromelysin. Urokinase, but not tissue-type plasminogen activator, was detected in M24met-conditioned media and on cell surfaces. The contribution of these enzymes to extracellular matrix dissolution was determined by exploiting specific inhibitors, namely tissue inhibitor of the metalloproteinases-2 and plasminogen activator inhibitor-2. Due to the coexpression of urokinase and MMP-dependent activity, M24met cells were observed to degrade multiple components of the extracellular matrix and to significantly degrade both interstitial and basement membrane matrices. Urokinase-dependent removal of matrix glycoprotein was ob served to precede MMP-dependent collagenolysis as a prerequisite ratelimiting step. We present evidence which suggests that this temporal re lationship is imposed by the structural architecture of the matrix such that matrix glycoprotein serves to protect associated collagen from MMPdependent degradation. In addition to mediating significant collagen olysis, MMP activity was further implicated in the dissolution of matrix tropoelastin. Urokinase/plasmin activity was not found to be required for MMP-zymogen activation.