Insulin stimulation of glucose metabolism in rat adipocytes: possible implication of protein kinase C

Abstract

The hypothesis that insulin stimulation of glucose and lipid metabolism in rat adipocytes may involve the activation of protein kinase C was evaluated. 4 beta-Phorbol 12 beta-myristate, 13 alpha-acetate (PMA, 0.1-1000 ng/ml), a potent tumor promoter acting as a substitute for diacylglycerol to activate protein kinase C, stimulated 2-deoxyglucose transport in a time- and dose-dependent manner, without affecting passive glucose diffusion. PMA (0.1-1000 ng/ml) also elicited a dose-dependent activation of lipogenesis from [3-3H] glucose. Maximal PMA effects (100 ng/ml) on both processes were 60% of insulin maximal effects. In contrast, PMA (1-1000 ng/ml) failed to mimic the ability of insulin to stimulate lipogenesis from [3H]acetate. 4 beta-Phorbol 12,13 dibutyrate, mezerein, 1-oleyl-2-acetyl-glycerol, 1,2 diolein, known as protein kinase C activators, also markedly stimulated glucose metabolism whereas 4 alpha-phorbol 12,13 didecanoate and 4 beta-phorbol 13-monoacetate, shown not to activate protein kinase C, were ineffective. Mellitin, a cytotoxin-inhibiting protein kinase C, markedly decreased both PMA and insulin stimulation of glucose metabolism. PMA decreased insulin submaximal effects on 2-deoxyglucose transport without inhibiting insulin binding. Maximal PMA and insulin effects on 2-deoxyglucose transport and lipogenesis from [3-3H]glucose were not additive. Both PMA and insulin activated each metabolic process in a calcium-dependent manner. PMA, like insulin, no longer stimulated 2-deoxyglucose transport in fat cells treated with 2,4-dinitrophenol. These data show that PMA exhibited specific insulin-like properties on glucose metabolism in fat cells, without any effect on lipid synthesis from acetate. They indicate that PMA and insulin bioeffects may share a common step implicating a calcium- and energy-dependent process, distal to the initial insulin binding event. Our results suggest that protein kinase C may play a role in insulin regulation of glucose metabolism.