Distribution of insulin/insulin-like growth factor-I hybrid receptors in human tissues

Abstract

Insulin receptors (IR) and type 1 IGF receptors (IGF-IR) have been shown to form insulin/IGF-I hybrid receptors in tissues expressing both molecules. The biological function of hybrid receptors is still undefined. To date there is no information about the distribution of hybrid receptors in human tissues. We have applied two microwell-based immunoassays which are capable of quantitating hybrid receptors in small samples of human tissues and cells. Results demonstrated that the proportion of total IGF-IR assembled as hybrids varied between 40 and 60%, thus indicating that hybrid receptors account for a large fraction of total IGF-I binding in human tissues. A significant fraction of total IR was assembled as hybrids in the tissues examined, varying from 37% in placenta to 45% in hepatoma, with the exception of adipose tissue where the fraction of insulin receptors forming hybrids was 17%. Because hybrid receptors bind IGF-I, but not insulin, with high affinity, it is likely that in human tissues hybrid receptors may be primarily activated by IGF-I rather than insulin under physiological conditions. Therefore, differences in hybrid receptors distribution may contribute to regulate tissue sensitivity to insulin and IGF-I by sequestering insulin receptor αβ-heterodimer in an IGF-I responsive form.

Introduction

Type 1 insulin-like growth factor (IGF) receptors (IGF-IR) and insulin receptors (IR) are highly homologous tyrosine kinase receptors, each consisting of two αβ-heterodimers linked together by disulfide bonds to yield the mature α2β2 heterotetrameric receptor 1, 2, 3. The α-subunits are entirely extracellular and contain the high affinity hormone binding site(s). The transmembrane β-subunits possess the hormone-stimulated tyrosine kinase activity in their cytoplasmic domain which plays a crucial role in signal transduction. There is evidence that hybrid receptors comprised of an insulin receptor αβ-heterodimer and a type 1 IGF receptor αβ-heterodimer are formed in tissues co-expressing both molecules 4, 5, 6. Insulin/IGF-I hybrid receptors are also assembled in vitro under defined ligand incubation conditions [7]. The functional significance of hybrid receptors is yet unclear. Studies with transfected cells overexpressing the human insulin receptor or with affinity-purified hybrid receptors have shown that hybrid receptors bind IGF-I with an affinity similar to that of type 1 IGF receptors, but bind insulin with lower affinity than classic insulin receptors 8, 9. Furthermore, hybrid receptors behave as type 1 IGF receptors rather than an insulin receptor in terms of receptor autophosphorylation, hormone internalization and degradation 9, 10, 11. Therefore, the presence of insulin/IGF-I hybrid receptors would be expected to affect insulin binding, and, thereby, insulin sensitivity in tissues co-expressing both receptors. To date there is limited information about the distribution of hybrid receptors in human tissues. To address this, we have developed and applied two microwell-based immunoassays which are capable of determining the distribution of insulin/IGF-I hybrid receptors in various human tissues and cells.