Intracellular transactivation of the insulin-like growth factor I receptor by an epidermal growth factor receptor

Growth factor receptors may be transactivated not only by homologous receptors, but also by heterologous receptors. We have investigated this possibility, using for this purpose R-/EGFR cells, which are mouse embryo cells devoid of IGF-I receptors, but overexpressing the EGF receptor. At variance with mouse embryo cells with a wild-type number of IGF-I receptors and overexpressing the EGF receptor, R-/EGFR cells cannot grow in EGF only, nor can they form colonies in soft agar. However, if a wild type human IGF-I receptor is stably transfected into R-/EGFR cells, growth in EGF and colony formation in soft agar are restored. To determine a possible interaction between the two receptors, we transfected into R-/EGFR cells a number of IGF-I receptor mutants with different impaired functions. The only IGF-I receptor that cannot reverse the growth phenotype of R-/EGFR cells is a receptor with a point mutation at the ATP-binding site. All other mutant receptors, even when incapable of responding to IGF-I with a mitogenic signal, made R-/EGFR cells fully capable of responding with growth to EGF stimulation. IGF-I receptor mutants that are mitogenic but not transforming made R-/EGFR cells grow in EGF only, but were incapable of inducing the transformed phenotype. The mutant IGF-I receptors are activated (tyrosyl phosphorylation of IRS-I) in response to EGF. These experiments indicate that certain IGF-I receptor mutants with loss of function can be reactivated intracellularly by an overexpressed EGF receptor and confirm that the C-terminus of the IGF-IR is required for its transforming activity.