Assembly of insulin/insulin-like growth factor-1 hybrid receptors in vitro

Abstract

Insulin and Mn/MgATP treatment of immunoaffinity-purified alpha beta heterodimeric insulin receptors induced the formation of an alpha 2 beta 2 heterotetrameric insulin receptor complex. In contrast, insulin-like growth factor-1 (IGF-1) treatment was completely ineffective in inducing the association of alpha beta heterodimeric insulin receptors. Similarly, IGF-1 or Mn/MgATP, but not insulin, treatment of immunoaffinity-purified alpha beta heterodimeric IGF-1 receptors induced the formation of an alpha 2 beta 2 heterotetrameric IGF-1 receptor complex. A monoclonal antibody specific for the insulin receptor (MA5) completely immunoprecipitated all the insulin binding activity from both the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric insulin receptor complexes but did not immunoprecipitate IGF-1 receptors. Conversely, the IGF-1 receptor-specific monoclonal antibody (alpha IR-3) immunoprecipitated all the IGF-1 binding activity, but not insulin receptors. The simultaneous treatment of pooled equal amounts of alpha beta heterodimeric insulin and IGF-1 receptors with a combination of insulin and IGF-1 resulted in the formation of alpha 2 beta 2 heterotetrameric insulin and IGF-1 receptor complexes. However, in the mixed alpha 2 beta 2 heterotetrameric receptor fraction MA5 immunoprecipitated 94% of the insulin binding in addition to 27% of the IGF-1 binding activity whereas alpha IR-3 immunoprecipitated 97% of the IGF-1 binding in addition to 38% of the insulin binding activity. Treatment of the mixed alpha beta heterodimeric insulin and IGF-1 receptors with Mn/MgATP also resulted in the formation of cross-immunoreactive (42-46%) alpha 2 beta 2 heterotetrameric receptors. These data directly demonstrate the formation of insulin/IGF-1 hybrid receptors by both a combination of insulin plus IGF-1 or Mn/MgATP treatment of purified human placenta alpha beta heterodimeric insulin and IGF-1 half-receptors in vitro.