The Human Lymphocyte: A Model for the Study of Insulin-Receptor Interaction

Abstract

The circulating lymphocyte because of its ready accessibility provides a useful model for the study of insulin receptor binding in man. Dissenting data exists, however, as to whether circulating lymphocytes are capable of binding insulin. In an effort to reconcile this data, we have examined the possibility that the method of cell preparation is a critical factor in studying receptor function. Consequently, we have studied insulin binding to lymphocytes prepared from Ficoll-Hypaque (F-H) gradients, and from nylon fiber columns (NFC). Our results demonstrate that circulating human lymphocytes prepared from F-H gradients readily bind ¹²⁵I-insulin while lymphocytes prepared from NFC do not bind appreciable amounts of insulin. Thus, it appears that the two cell preparation methods yield different populations of lymphocytes which differ markedly in their ability to bind insulin. The binding of ¹²⁵I-insulin to F-H prepared lymphocytes is highly specific and can be inhibited by small physiologic amounts of unlabeled insulin. The binding of ¹²⁵I-insulin to lymphocytes is a linear function of cell concentration, and a saturable function of ¹²⁵I-insulin concentration. At 15 C the time course of the binding reaction indicates that equilibrium conditions are reached by 75 min, and are maintained for at least 2 hr. Kinetic analysis of this binding reaction reveals two orders of receptor sites of strikingly different affinity. The high affinity site (Kaff =1.4 × 10⁹m⁻¹) has a low capacity (220 binding sites/cell) while the lower affinity site (Kaff = 1.4 × 10⁸M⁻¹) has a higher capacity (1900 binding sites/cell). Insulin binding by human lymphocytes is a highly specific, reproducible finding, and because of their ready accessibility we believe that study of insulin-lymphocyte receptor binding can serve as an important tool for the understanding of human disease states characterized by altered insulin receptor function.