Suppression of fibrinolysin T activity fails to restore density-dependent growth inhibition to SV3T3 cells

Abstract

SMALL quantities of proteolytic enzymes added to cultures can cause untransformed fibroblastic cells to manifest transiently the altered growth potential^1,2, enhanced lectin-mediated agglutinability³ and enhanced glucose transport⁴ characteristic of fibroblast cultures transformed by oncogenic viruses. Thus, the enhanced proteolytic activity of cells transformed by both oncogenic DNA viruses^5,6 and RNA viruses⁷ may play an important role in the loss of density-dependent growth inhibition exhibited by virus-transformed cells. In particular, Reich et al. have shown that cells transformed by simian virus 40 (SV40) or Rous or murine sarcoma virus produce increased levels of a proteolytic activity (fibrinolysin T) which hydrolyses ¹²⁵I-fibrin^8,9. This fibrinolysin T activity is the result of the conversion of serum plasminogen to plasmin by a plasminogen activator (cell factor)^10,11. In addition, using plasminogendeficient serum prepared by affinity chromatography, Reich et al. have demonstrated plasminogen dependence of several phenotypic parameters associated with viral transformation¹². We have used ε-aminocaproic acid (EACA), an inhibitor of plasminogen activation¹³ and fibrinolysin T activity⁸, and found that suppression of the fibrinolysin T activity of SV40-transformed 3T3 (SV3T3) cell cultures does not restore density-dependent growth inhibition to these cells. This suggests that the enhanced plasmin level in SV3T3 cell cultures is not responsible for loss of density-dependent growth inhibition in this cell line.