Comparison of fibrin clot retraction with other transformation parameters after hydridization of normal and established cell lines

The expression of transformation parameters (inhibition of cell division during cell crowding, anchorage dependence, loss of fibrin clot retractile activity and secretion of plasminogen activator) was studied in a hetero‐specific cellular hybrid, made between established L (TK − ) cells and the normal human MRC‐5 cells. The hybrid nature of the cross was confirmed by the ability to incorporate [ 3 H]‐thymidine, by growth in selective HAT medium, by the identification of human chromosomes and by the expression on the surface of 100% of hybrid cells of a human glycoprotein, which is recognized by the 4F2 monoclonal antibody. The hybrid cultures showed cell cycle inhibition which became less stringent with increasing population doublings and the loss of human chromosomes. Fibrin clot retraction and anchorage dependence were absent in spite of the presence of many human chromosomes. The two properties were present or lost simultaneously in the normal parent cells and in the transformed parent or hybrid cells respectively. The human type of plasminogen activator was secreted even with very little human genetic material left, and a complete dissociation between fibrin clot retraction and production of plasminogen activator was observed. The data strengthen the hypothesis that transformation is a multi‐step process that involves complex genetic control and where cells progressively express different phenotypes and escape growth control.