Overexpression of Insulin Receptors in Fibroblast and Ovary Cells Induces a Ligand-Mediated Transformed Phenotype

To investigate whether overexpression of the insulin receptor results in altered cell growth we used NIH 3T3 cells transfected with a bovine papilloma virus/insulin receptor cDNA construct (3T3/HIR). These cells expressed high numbers of insulin receptors (mean ± sd, 631.0 ± 16.7 ng receptors/10⁶ cells). Insulin significantly stimulated the growth of 3T3/HIR cells maintained in serum-free medium. Moreover, in these cells, insulin induced marked phenotypic changes, including alterations in cell shape, loss of contact inhibition, and focal growth. In contrast to 3T3/HIR cells, insulin was without effect in either wild-type 3T3 cells (3T3/wt), 3T3 cells transfected with the neomycin resistance gene (3T3/NEO), or the bovine papilloma virus (3T3/BPV).

To assess the presence of anchorage-independent growth, cells were seeded in soft agar and inspected for colony formation. 3T3/HIR cells showed absent or minimal colony growth in the absence of insulin. However, there was a dose-dependent insulin-stimulated increase in both colony size and number. Insulin-stimulated colony formation was specifically inhibited by an insulin antagonist, monoclonal antibody MA-10. In the presence of 100 nM insulin, about 3% of cells formed large colonies. Insulin neither stimulated growth nor induced colony formation in 3T3/wt cells or 3T3/NEO cells. Insulin also stimulated colony formation in CHO cells transfected with an insulin receptor cDNA construct. In conclusion, overexpression of normal insulin receptors induces a ligand-dependent transformed phenotype. This phenomenon may have clinical relevance by conferring a selective growth advantage to tumor cells with high numbers of insulin receptors.

Author notes

This work was supported in part by a grant from the AIRC (Associazione Italiana per la Ricerca sul Cancro), NIH Grant DK-42171, and the John A. Kerner Foundation.

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Recipients of AIRC fellowships.