Insulin receptors in transformed fibroblasts and in adipocytes: a comparative study

Abstract

This study compares the receptor for insulin in rat adipocytes with an insulin receptor from transformed AL/N mouse embryo fibroblasts (SVS AL/N). In many respects the receptors from the two tissue sources are very similar. Crude solubilized receptor preparations from both fat cells and SVS AL/N cells exhibit complicated insulin-binding kinetics (two binding plateaus) as well as an apparent chromatographic heterogeneity (Sepharose 6B); each receptor preparation yields two insulin-binding components having apparent Stokes radii of about 7.2 and 3.8 nm. In contrast, the receptors isolated from both cell types by affinity chromatography behave as single chromatographic components (Sepharose 6B) having apparent stokes radii of 3.8 nm and exhibit simple insulin-binding kinetics without evidence for receptor–ligand co-operativity. The isolated receptor from SVS AL/N cells (KD about 1.2 nM) possesses a slightly lower insulin affinity than does the receptor from fat cells (KD about 0.8 nM); nonetheless, the SVS AL/N receptor exhibits an appropriate ligand specificity for insulin and insulin analogues. Receptors from both cell types are similarly sensitive to trypsin and exhibit an identical dependence on pH for insulin binding. Differences in the two receptor preparations are observed in the effect of NaCl and phospholipase C, reagents which augment insulin binding in adipocyte membranes but do not affect the binding by SVS AL/N cell membranes. Additionally, iodoacetamide, N-ethylmaleimide, and ethanol markedly reduce the binding by SVS AL/N membranes, whereas the binding by adipocyte membranes is unaffected. It is concluded that certain differences between the receptors from the two sources may be attributed to differences in the membrane environment in which the receptors reside. However, in view of the many similarities between the two receptor preparations, it is suggested that the same receptor macromolecule may participate in the response of a variety of cell types not only to insulin but also to other insulin-like polypeptides.