Unmasking of Insulin Receptors in Fat Cells and Fat Cell Membranes: PERTURBATION OF MEMBRANE LIPIDS

Abstract

Digestion of isolated fat cells, fat cell membranes, and liver cell membranes with phospholipase C and phospholipase A from various sources leads to a 3- to 6-fold increase in the specific binding of 125I-insulin to receptors in the structures. Calcium is required for digestion with phospholipase A, and digestion with phospholipase D is without effect. In contrast to the studies with insulin, the specific binding of glucagon to its receptor in fat cells or fat cell membranes is diminished or destroyed after digestion with phospholipase C. The increase in the specific binding of insulin to fat cells or membranes after phospholipase digestion reflects an increase in the total quantity of receptor available for interaction with this hormone, and is not accompanied by a change in the affinity of the receptor-insulin complex. The effects of digestion with phospholipase C are observed even after severely modifying the normally exposed receptors by tryptic digestion. The rates of association and dissociation of the insulin-receptor complex, and the equilibrium constants of the complex, are nearly identical in native and phospholipase C-treated cells. Extraction of membrane lipids with organic solvents increases the capacity of the membrane to specifically bind insulin. Addition of the extracted lipids, or of certain phospholipids, such as phosphatidylethanolamine and phosphatidylserine (dipalmityl), to phospholipase C-treated (and washed) membranes results in a partial reversal of the effects of phospholipase digestion. Certain lipid compounds (digitonin, vitamin K5) can partially mimic the effects of phospholipase digestion. The principal protein component of bee venom, mellitin, which can interact strongly with membrane phospholipids, increases the binding of insulin to fat cell membranes. Certain polyene antibiotics (nystatin, amphotericin B) which probably selectively disrupt sterol components of membranes, are without effect; whereas filipin, which can disrupt phospholipid structures, enhances the specific binding of insulin to membranes. The present studies indicate that displacement of phospholipids from liver and fat cell membranes can result in the unmasking of substantial quantities of insulin receptor which is probably identical with that normally exposed to the solvent in these cells.