Insulin-like Metabolic Effects Research Articles

Similar activities of nerve growth factor and its homologue proinsulin in intracellular hydrogen peroxide production and metabolism in adipocytes: Transmembrane signalling relative to insulin-mimicking cellular effects

Generation of hydrogen peroxide in adipocyte plasma membrane and its intracellular metabolism and regulatory role have been shown by Mukherjee and co-workers to be a major effector system for insulin [ Fedn Proc. 35, 1694 (1976); Archs Biochem. Biophys. 184, 69 (1977); Biochem. Pharmac. 27, 2589 (1978); Fedn Proc. 37, 1689 (1978); and Biochem. Pharmac. 29, 1239 (1980)]. The possible involvement of this mechanism in the action of structurally similar polypeptides having some insulin-like metabolic effects was investigated. The β-subunit of nerve growth factor (2.5 S NGF, mol. wt 13,500) which has a striking structural homology with proinsulin and has been reported to exert certain insulin-like metabolic effects in its own target tissues (e.g. growing neurites and sympathetic ganglia), and the insulin-derived polypeptides, desalanine-insulin and desoctapeptide-insulin, as well as proinsulin, were examined for their effects on rat adipocytes, employing the technique of formate oxidation. Both NGF and proinsulin caused increased [ 14C]formate oxidation, showing similar intrinsic activities, up to a maximum of 140–160% of the basal rate; insulin increased the rate to 190–210% of the basal rate. The relative potencies of the hormones toward H 2O 2 formation and stimulation of the pentose phosphate pathway activity were: insulin ( EC 50: 2.5 × 10 −11 M) , desalanine-insulin ( EC 50: 2.5 × 10 −10 M) , proinsulin ( EC 50: 8 × 10 −9 M) , and NGF ( EC 50: 10 −9 M) . The biologically inactive derivative, desoctapeptide-insulin, did not stimulate glucose oxidation, although it caused a small increase in formate oxidation, with an EC 50 of 5 × 10 −7 M , indicating a suboptimal level of H 2O 2 formation in the elevation of the hexose monophosphate shunt activity. 3-Amino-1,2,4,-triazole (50 mM), which irreversibly decomposes the peroxidatic compound II of the catalase: H 2O 2 complex, inhibited formate oxidation to a greater extent in the hormone-treated cells than in the control cells, whereas sodium azide, an inhibitor of the hemoprotein, catalase, completely inhibited it. The abilities of the polypeptides to stimulate H 2O 2 formation correlated with their abilities to promote lipogenesis from [U- 14C]- D-glucose, as expected of insulin. The cellular GSH/GSSG ratio increased concomitantly with the stimulation of glucose oxidation via the shunt, indicating a tight coupling between these processes. The results confirm that the hydrogen peroxide production is a common basis of the metabolic actions of growth-promoting polypeptide hormones or mitogens beyond their respective receptors.