Stabilization of the Ras oncoprotein by the insulin-like growth factor 1 receptor during anchorage-independent growth.

R- cells are 3T3 cells derived from mouse embryos with a targeted disruption of the type 1 insulin-like growth factor receptor (IGF-IR) genes. R- cells are refractory to transformation by a variety of viral and cellular oncogenes, including an activated Ras. R- cells stably transfected with an activated Ha-Ras (R-Ras cells) fail to form colonies in soft agar. An IGF-IR truncated at residue 1245 cannot transform R- cells, even when strongly overexpressed. However, the combination of the truncated IGF-IR and an activated Ras induces transformation of R- cells. We show here that the Ras oncoprotein is rapidly degraded when R-Ras cells are grown under anchorage-independent conditions and that signaling from the truncated IGF-IR stabilizes Ras. In monolayer cultures, Ras levels remain constant regardless of the presence or absence of IGF-IR signaling. These results directly explain why Ras cannot transform mouse embryo fibroblasts devoid of IGF-IR. They also suggest a more generalized, alternative mechanism for transformation by Ras and, implicitly, another possible way for targeting Ras in tumor cells.