Co‐cultivation of tumorigenic mouse melanoma cells with cells of a non‐tumorigenic subclone inhibits plasminogen activator expression by the melanoma cells

Clone B₅ 59 mouse melanoma cells are highly tumorigenic and produce plasminogen activator. Cells of clone C₃471, a line obtained by continued growth of B₅59 cells in medium containing 5‐bromodeoxyuridine (1 μg/ml), have no plasminogen activator and are non‐tumorigenic. When B₅59 cells are co‐cultivated with C₃471 cells, the ability of B₅59 cells to activate plasminogen is suppressed. Under these conditions cell fusion occurs. Lack of expression of plasminogen activators is not a consequence of cell fusion, inhibition of cell division or release of soluble inhibitors of either plasminogen activators or plasmin. No inhibitors of plasminogen activators could be demonstrated in association with subcellular fractions of C₃471 cells or with the C‐type viral particles released from C₃471 cells. Close contact between cells of the two lines is shown to be essential for suppression of plasminogen activation.