Membrane-associated cathepsin L: a role in metastasis of melanomas.

Subcellular distribution of cathepsin L, the major protein released by transformed or ras transfected fibroblasts, was examined in murine liver, murine B16 amelanotic melanoma and human A2058 melanoma after sequential differential and Percoll density gradient centrifugation. In both murine and human melanomas, cathepsin L activity was found to be enriched in plasma membrane fractions; cathepsin L in these fractions was in both native and acid activatable forms. Plasma membrane fractions from B16 melanoma subpopulations of "low" and "high" metastatic potential were assayed for activity of cathepsin L and of heat stable endogenous inhibitors. The relative specific activity of cathepsin L was 7-fold greater in the subpopulation of "high" metastatic potential, whereas cysteine proteinase inhibitory activity was 5-fold less. Since cathepsin L can degrade intact basement membrane, this membrane-associated cathepsin L may well contribute to metastatic spread of melanomas.