Effect of the Membrane Lipid Environment on the Properties of Insulin Receptors

Abstract

We have altered the fatty acid composition of the Friend erythroleukemia cells and studied the effect on the binding parameters of the insulin receptor. Cells were grown for about five generations in media enriched with oleate (a monounsaturated fatty acid) or linoleate (a polyunsaturated fatty acid). The microsomal phospholipids normally contained about 40% saturated, 40% monounsaturated, and 20% polyunsaturated fatty acids. In cells grown with oleate, monounsaturates rose to 63% and in cells grown with linoleate, the polyunsaturated component rose to 52%. Fluidity determination by electron spin resonance similarly indicated that the membranes of cells grown in linoleic acid were more fluid than those grown in oleic. Control cells demonstrated the least membrane fluidity. The cells enriched in unsaturated fatty acids had an increased number of insulin receptors and decreased receptor affinity, but the exact interpretation of the change was dependent on the method of data analysis. When analyzed assuming negative cooperativity among a single class of receptors, there was an increase in receptor number from 23,000/cell in the control cells to 39,000/cell in the oleate-enriched and to 81,000/cell in the linoleate-enriched cells. These changes were accompanied by reciprocal decreases in the affinity of both the unoccupied form of the receptor (Ke) and the occupied form of the receptor (Kf). When analyzed assuming two independent classes of sites, there was no major change in the number or affinity of the high affinity receptors (6000–8000/cell). There was, however, a marked increase in the number of low affinity receptors from 15,000/cell in the control cells to 32,000/cell in the oleate-enriched and 74,000/cell in the linoleate-enriched cells. This increase in receptor concentration was again accompanied by a reciprocal decrease in the affinity of the low affinity receptor. These studies thus demonstrate a relationship between the cellular fatty acid composition and the properties of insulin receptors. The observed changes could be due to alterations in the quaternary structure Of the receptor.