LOSS OF EPIDERMAL GROWTH-FACTOR REQUIREMENT AND MALIGNANT TRANSFORMATION

Serum provides growth factors that regulate and limit the growth of normal cells in tissue culture. Animal cells that are malignantly transformed usually exhibit diminished serum requirements for growth in culture. A defined, serum-free medium was used to determine which of these growth factors becomes dispensable for the growth of transformed Syrian and Chinese hamster fibroblast cells. The medium''s 4 growth factors (epidermal growth factor (EGF), insulin, fibroblast growth factor and transferrin) were added or omitted as desired. A decreased requirement for EGF was most closely related to tumorigenicity of chemically (ethyl methanesulfonate) transformed cells in nude mice. All lines examined retained their requirement for transferrin, which is needed throughout the growth cycle, in contrast to the other factors, which are needed primarily in G1 phase. Lines that had lost their EGF requirement but had retained their insulin requirement were arrested in G1 by insulin deficiency, indicating that their growth control system remained. Mutagenesis with ethyl methanesulfonate can also create requirements of the transformed cells for unknown factors in serum. An initial step that reduces the serum requirement in culture and in tumorigenesis is apparently relaxation of the growth-regulatory function of EGF.