The proteolytic potential of normal human melanocytes: Comparison with other skin cells and melanoma cell lines

To understand the contribution of epidermal melanocytes in the proteolytic potential of human skin, we have studied melanocytes grown in a low-serum medium deprived of phorbol esters, cholera toxin, and other non-physiological supplements. We focused on the plasminogen activation system and certain matrix metalloproteinases (gelatinases). Supposing that the proteolytic activity of cells can influence binding to collagen matrix and its reorganization, we have analyzed these parameters as well. We found that human melanocytes secreted tissue-type plasminogen activator and utilised it to generate cell-bound plasmin. No urokinase-type plasminogen activator was detected in the cultures but its receptor was found in cell extracts. Both the 72 kDa and 92 kDa gelatinases were secreted by the cells and in equal amounts. In addition, melanocytes secreted the wide-spectrum proteinase inhibitor alpha-2-macroglobulin. Melanocytes cast into collagen matrices retained a rounded morphology, did not extend processes, and were unable to contract collagen lattices. As a control, these parameters were investigated in parallel in cultures of human keratinocytes, dermal fibroblasts, and two melanoma cell lines. The obtained characteristics suggest that normal human melanocytes are proteolytically active cells. This function may pertain to skin physiology and pathophysiology.