[Mitogenic responsiveness to growth factor and tumor promoters and regulation of cell growth]

Most of our current understanding of the molecular action of mitogens such as insulin, EGF and tumor promoters is derived from studies using biochemical, pharmacological and immunological methods. In the present study, however, this important problem has been approached from a genetic point of view. To facilitate genetic studies, attempts have been made to isolate variants having receptors and/or post-receptor functions which are deficient or extraordinary in their cellular responsiveness. The first selection method was to use toxic hybrid proteins such as insulin crosslinked to the A fragment of diphtheria toxin (DTa), EGF crosslinked to the A chain of toxic ricin (RICa) and monoclonal anti-EGF receptor antibody (IgG) crosslinked to the RICa. These conjugates have provided us with a variety of cell variants which are unable to either bind the ligand or take up the bound ligand and process it for mitogenic response. The second selection procedure utilized the mechanical removal of cells whose growth was stimulated by the mitogens but was arrested at mitosis by vinblastine sulphate. Variants of mouse 3T3-L1 cells which no longer responded to the indole alkaloid tumor promoter dihydroteleocidin B exhibited various characteristics in terms of the receptors for EGF, insulin and the tumor promoters. The results obtained from these cell variants are discussed in relation to the role of different receptor systems in the regulation of DNA replication, cell growth and differentiation.