A murine B16 melanoma parental line (B16) and two clones either pigmented (B16P) or non-pigmented (B16NP) were cocultured as aggregates with mouse 3T3 fibroblasts. In vivo, these mixed aggregates developed more aggressive tumors in the mouse than corresponding pure aggregates. In vitro, the three pure cell lines secreted TIMP-2 and tissue-type plasminogen activator (t-PA) activities. Coculture conditioned media contained gelatinases (A and B), their inhibitors (TIMP-1 and TIMP-2), both plasminogen activators (u-PA and t-PA) and a plasminogen activator inhibitor (PAI) which formed high MW t-PA/PAI complexes. In all cases, cell-associated extracts contained essentially u-PA activities. These proteolytic activities are involved in extracellular matrix degradation and could contribute to the greater tumorigenic and invasive capacities of these B16 lines when previously cultured with fibroblasts in 3 dimensions.

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