A functional assessment of insulin/insulin-like growth factor-I hybrid receptors

Insulin/insulin-like growth factor-I (IGF-I) hybrid receptors are composed of an alpha beta-heterodimer from an insulin receptor and an alpha beta-heterodimer from an IGF-I receptor. In this study, we evaluate the effect of insulin receptor overexpression on hybrid formation. The more human insulin receptors expressed in rodent fibroblasts, the greater the percentage of endogenous rat IGF-I receptors that form hybrid receptors. The IGF-I receptor in rodent fibroblasts has two receptor isoforms, one with a 95-kilodalton (kDa) beta-subunit and one with an 105 kDa beta-subunit. A truncated mutant insulin receptor was used to demonstrate that only activated IGF-I receptors with the 105-kDa beta-subunit form hybrid receptors with the insulin receptor. Insulin/IGF-I hybrid receptors with a kinase-defective insulin heterodimer undergo trans and a small amount of cis autophosphorylation, but overall autophosphorylation is markedly decreased from that seen in hybrids with a kinase-competent insulin receptor. The kinase-defective insulin receptor heterodimer functions as a dominant-negative, inhibiting phosphorylation by the kinase-competent IGF-I receptor heterodimer. The kinase-defective hybrid receptors are, however, able to undergo internalization. Despite an increasing percentage of insulin/IGF-I hybrid receptors in the three cell lines studied, the rates of IGF-I internalization and degradation remain similar to those mediated by the IGF-I receptor and distinct from those of insulin receptor heterotetramers. In conclusion, IGF-I-stimulated insulin/IGF-I hybrid receptors function like IGF-I receptors, rather than like insulin receptors.