Identification of liver cell membrane galactoglycoproteins involved in the process of insulin binding

Tunicamycin blocks the emergence and maintenance of insulin receptors on mitogen-activated human T lymphocytes

Treatment of phytohemagglutinin (PHA) activated human T lymphocytes with tunicamycin, an antibiotic that specifically inhibits asparagine-linked N-glycosylation of proteins, totally blocked the normal emergence of insulin receptors on these lymphocytes and their cellular proliferation during culture in a dose-dependent manner. Carbohydrate incorporation into protein was inhibited 82% by 0.5 microgram/mL while leucine incorporation was unaffected. Tunicamycin exposure of activated T lymphocytes, which had acquired insulin receptors during culture, reduced cellular insulin binding by 35% to 84% and reduced PHA binding to 40% of control levels within 24 hours. Scatchard analysis revealed decreases in insulin binding capacity but not affinity. Similar treatment with cycloheximide only decreased insulin binding by 12%. These findings suggest N-glycosylation of proteins is a necessary biochemical event (1) for the emergence and maintenance of insulin receptors on mitogen activated T lymphocytes, and (2) for mitogen activated T lymphocytes to undergo cell division.

Evidence for surface glycoprotein involvement in the intracellular bioactivity of insulin in rat adipocytes

Lectins specific for D-mannose (concanavalin A), N-acetyl-D-glucosamine (wheat-germ agglutinin) or D-galactose (Ricinus communis agglutinin I) inhibited insulin binding and activated glucose transport in rat adipocytes [Cherqui, Caron, Capeau & Picard (1982) Mol. Cell. Endocrinol. 28, 627-643]. In the present investigation, the intracellular activities of insulin and lectins on lipogenesis and protein synthesis were studied under conditions where neither agent had an effect on membrane transport processes. (1) When glucose transport was rate-limiting (0.5 mM-glucose), insulin (0.8 ng/ml) and lectins (20 micrograms/ml) increased lipogenesis by 2.4-3-fold. (2) When passive diffusion of glucose was amplified (10 mM-glucose), insulin (0.8 ng/ml) and lectins (20 micrograms/ml) increased lipogenesis by 1.6-1.8-fold even in the presence of 50 microM-cytochalasin B, which completely blocked glucose transport. (3) Insulin (6 ng/ml), concanavalin A and wheat-germ agglutinin (40 micrograms/ml) stimulated the incorporation of L-[U-14C]leucine into fat-cell protein 1.5-fold but did not modify alpha-aminoisobutyric acid uptake or 14C-labelled protein degradation. (4) Peanut and soya-bean agglutinins (specific for O-glycosidically-linked oligosaccharides), known not to alter insulin binding, were ineffective. (5) Lectin effects were dose-dependent and were markedly inhibited by specific monosaccharides (50 mM). (6) Insulin and lectin maximal effects were not additive and were completely abolished by neuraminidase treatment of fat-cells (0.05 unit/ml). These data indicate involvement of surface sialylated glycoproteins of the complex N-linked type in the insulin stimulation of glucose and amino acid intracellular metabolic processes. They suggest, together with our previous results, that the transmission of the insulin signal for both membrane and intracellular effects occurs via glycosylated effector entities of, or closely linked to, the insulin-receptor complex.

Further evidence for a role of carbohydrates in insulin binding: studies in lectin-purified receptors

Purification of liver membrane insulin receptors on concanavalin A- and ricin I-lectin columns gave a 15-fold enrichment in the insulin binding capacity per milligram of protein. Final receptor and protein recoveries were 53 and 3.8% respectively. Lectin-purification increased the receptor affinity for insulin, as indicated by a left-ward shift in the binding competition curve and a steeper slope in the Scatchard plot. Lectin-purification increased the receptor sensitivity towards the glycosidic probes. The maximal effects of beta-galactosidase, ricin I (galactose-binding lectin) and alpha-mannosidase were markedly amplified: 80, 90 and 60% inhibition, versus 45, 40 and 15% with particulate membranes. The limulus polyphemus (LPA) and wheat germ (WGA) agglutinins (sialic acid- and N-acetyl-glucosaminyl-binding lectins) became effective in modifying the insulin binding: 45 and 80% inhibition, respectively. The effects were dose-dependent, reversed by the monosaccharide competitors (lectin effects) and unrelated to the state of receptor occupancy. These findings indicate that, within the hormone recognition area, peptide chains containing galactose, mannose and N-acetyl-glucosamine are strictly required for insulin-receptor interaction and suggest that change in the receptor affinity is related to the role of carbohydrate in insulin binding.

Carbohydrate determinants involved in both the binding and action of insulin in rat adipocytes

The insulin receptor apparent affinity was markedly decreased in fat cells treated with lectins specific either for D-galactose (Ricinus communis agglutinin I, RCAI), D-mannose (concanavalin A, Con A, Lens culinaris agglutinin, LCA) or N-acetyl-D-glucosamine (wheat germ agglutinin, WGA), as indicated by a rightward shift of the binding competition curves and almost lineared Scatchard plots. Limulus polyphemus agglutinin (LPA), specific for sialic acid, was ineffective. All lectins enhanced 2-deoxy-D-glucose uptake with relative bioactivities (maximal lectin effect/maximal insulin effect) of 68-86%. Insulin and lectin stimulatory effects were antagonized by specific carbohydrates used as competitors and inhibited by cytochalasin B (70 microM). Maximal effects of insulin and lectins were not additive and were completely abolished in neuraminidase-treated fat cells. Lectins did not affect insulin degradation. These data show that sialylated glycosidic moieties containing D-galactose, D-mannose and N-acetyl-D-glucosamine units are involved in both processes of insulin 'high affinity' binding and activation of glucose transport but are not implicated in hormone degradation. They suggest that N-linked carbohydrate chains of the complex type may be essential for functional insulin receptor and post-receptor systems.