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 beta-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 H2O2 formation and stimulation of the pentose phosphate pathway activity were: insulin (EC50: 2.5 x 10(-11) M), desalanine-insulin (EC50: 2.5 x 10(-10) M), proinsulin (EC50: 8 x 10(-9) M), and NGF (EC50: 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 EC50 of 5 x 10(-7) M, indicating a suboptimal level of H2O2 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: H2O2 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 H2O2 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.