Abstract
The effects of various growth factors on the colony-forming ability were examined in soft-agar culture of thermosensitive (ts) mutants derived from Chinese hamster lung cells that exhibited anchorage-independent growth. Fibroblast growth factor (FGF), a competence factor, and epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I), progression factors, did not stimulate colony formation in soft-agar culture of any of the ts cells. Insulin stimulated colony formation in soft-agar culture of some of the thermosensitive mutants at the nonpermissive temperature. These results suggested that the interruption of growth occurred late in the Gi phase of the cell cycle in these mutant cells. Mutants were classified into two groups: members of Group 1 could be stimulated by insulin but not by FGF, EGF or IGF-I (5 clones); members of Group 2 were not stimulated either by insulin, FGF, and EGF or by IGF-I (4 clones). Insulin-binding capacities of mutants designated cs-17-25 and hs-211 did not differ between cells cultured at a nonpermissive and a permissive temperature. The extent of phosphorylation of the insulin receptors of these clones changed depending upon the dose of insulin. However, there were no differences in the extent of autophosphorylation of receptors between the mutant cells cultured at the permissive and the nonpermissive temperature. Therefore, thermosensitivity of colony-forming ability is not the result of any decrease in insulin-binding capacity or in activity of the receptor kinase. These results suggest that the thermosensitive lesions of these ts mutants occur distal to the transduction of a growth signal that is mediated by the kinase activity of the insulin receptor.
