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

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Abstract
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.