Comparison of Insulin Binding Proteins In Plasma and Nuclear Membranes of Obese and Lean Mouse Liver

Abstract

Plasma membranes obtained from obese (ob/ob) and lean (+/+ or +/ob) mouse livers were chemically crosslinked to [¹²⁵I] -insulin and examined by electrophoresis and autoradiography. The pattern of crosslinked hormone was qualitatively similar in obese and lean plasma membranes. A major insulin binding protein of approximately M 120,000 was observed. Two additional bands were apparent, one which remained near the top of the gel and one about M 90,000. A minor band at approximately M 50,000 was also detected. For each of the insulin binding proteins a reduction in the amount of [¹²⁵I]-insulin bound was observed with obese plasma membranes as compared with lean. For all proteins the insulin binding was specific as determined by competition with unlabeled hormone. In addition to plasma membrane receptors, insulin has also been reported to bind to nuclear membranes. The autoradiographic patterns of gels of [¹²⁵]-insulin bound and crosslinked to nuclear membranes from obese and lean mouse livers indicated the presence of proteins of the same M as those described for plasma membranes. Nuclear membrane proteins bound less insulin than plasma membranes and, again, the obese was decreased relative to the lean. Contamination of the nuclear membrane fraction by plasma membranes was ruled out. Scatchard analyses of [¹²⁵]-insul in bound to plasma and nuclear membranes indicated that the decrease in hormone binding in the obese mouse is a result of a reduction in the absolute number of receptors. The findings presented in this study provide additional support for this conclusion by demonstrating that membranes from obese mice are comprised of the same set of apparently unaltered insulin binding proteins. Further, the presence of similar insulin binding proteins in both nuclear and plasma membranes suggests a physiological relationship between these structures with respect to hormone binding and/or in the mechanism of action of insulin.