Transforming viruses directly reduce the cellular growth requirement for a platelet derived growth factor

Abstract Early passage mouse embryo fibroblasts, mouse 3T3 cell lines, and early passage diploid human fibroblasts grew to higher cell densities in tissue culture medium supplemented with serum than in medium supplemented with defibrinogenated platelet‐poor plasma (PPP). Unlike the mouse cells, the human fibroblasts displayed this differential growth response only in the presence of hypophysiologic concentrations of calcium. The addition of heat‐treated extracts of human platelets to PPP‐supplemented medium stimulated the replication of both the normal mouse cells and early passage human embryo fibroblasts. Human or mouse fibroblasts transformed by either retroviruses or by SV40, including SV40 infected “serum revertants” and “flat transformants,” grew to equal cell densities in medium supplemented with either serum or PPP. Infection of Balb/c‐3T3 cells with SV40 rapidly induced them to grow in PPP‐supplemented medium demonstrating that the ability of SV40‐transformed cell lines to proliferate in PPP‐supplemented medium does not arise from the cell culture selection procedures usually employed to obtain stable virus‐transformed cell lines. 3T3 cells infected but not transformed by retroviruses do not replicate in PPP‐supplemented medium demonstrating that reduction of the growth requirement for the platelet growth factor(s) by retroviruses is a transformation‐specific response. Cell cultures that did not proliferate well in PPP‐supplemented medium did not form tumors when inoculated into athymic nude mice. Many, although not all, of the lines which grew well in PPP medium were tumorigenic in nude mice. Together, these findings indicate that: (1) normal fibroblast‐like cells display a growth requirement for factor(s) present in serum but not found in PPP; (2) this serum specific growth factor is derived from platelets; (3) a primary response to viral transforming genes is a reduction in the growth requirement for these platelet‐derived factors; and (4) cells that have a reduced requirement for the platelet‐derived growth factor are often tumorigenic.