Is glycosylation in the liver needed for insulin binding, processing, and action? Evidence for heterogeneity

Incubation of primary cultures of rat hepatocytes with tunicamycin (1 microgram/ml) for 16 h inhibits de novo glycosylation by more than 90%, yet 50% of initial 125I-insulin binding is still present. The remaining insulin-binding activity is to glycosylated receptors, since they are adsorbed and specifically desorbed from wheat germ agglutinin-Sepharose. Furthermore, there is no difference in the molecular weight size of the alpha-subunit of the insulin receptors either in normal or tunicamycin-treated hepatocytes as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 125I-insulin receptor cross-linked with disuccinimidyl suberate. This gives us an opportunity to study the function of these glycosylated insulin receptors in cells where de novo glycosylation is inhibited. Inhibition of de novo glycosylation decreases receptor-mediated 125I-insulin internalization and degradation. The tunicamycin-treated cells are totally resistant to insulin with regard to [14C]aminoisobutyric acid uptake. In contrast, there is normal stimulation of [14C] acetate and [14C]glucose incorporation into lipids and glycogen by a maximal concentration of insulin. The interpretation of these data is, however, complicated by the fact that tunicamycin markedly inhibits basal lipid synthesis without altering basal glycogen synthesis or [14C]aminoisobutyric uptake. These results demonstrate the heterogeneity of the postinsulin binding system in the liver. Besides glycosylation of the insulin receptor, which is needed for insulin binding, other cellular glycoproteins are important for insulin processing as well as mediation of some, but not all, of the biological actions of insulin. The nature of these liver glycoproteins and their relationship to the insulin receptor have not yet been defined.