Enhancement of Plasminogen Activator Activity by the Culture Medium of Rous Sarcoma Virus-transformed Cells

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Abstract
The effect of the culture medium of chicken embryo fibroblasts (CEF) transformed by Rous sarcoma virus (RSV) on plasminogen activator (PA) activity of normal and RSV-infected cells was tested. Fibrin digestion by cultures of normal uninfected and RSV-infected cells directly attached to a fibrin substrate was increased when the cells were exposed to the supernatant of an 18 h culture of CEF transformed by RSV. Addition of cycloheximide to the supernatant completely abolished this effect. Exposure of both normal and RSV-infected cells to the culture medium of RSV-transformed CEF resulted in a marked increase of the soluble PA activity present in the supernatant. This effect was not abolished by inhibition of protein synthesis. For both the cell-bound and soluble PA activities of normal and RSV-infected cells the stimulatory effect was transient, being maximal between 8-12 h of exposure to the transformed cell-conditioned medium and disappearing after 24 h. A possible correlation with transformation-enhancing factor(s), previously reported to be present in the culture medium of transformed cells, is discussed.