Action of insulin modulated by pertussis toxin in rat adipocytes

Abstract

We studied the effect of pertussis toxin (PT) treatment on the ability of insulin to inhibit lipolysis and to stimulate glucose oxidation in isolated rat adipocytes. In cells maximally modified by PT (100% ADP ribosylation of a 41-kdalton protein in membranes), the ability of insulin to inhibit lipolysis stimulated either by PT alone or in combination with a catecholamine was abolished. In cells wherein ADP ribosylation was submaximal (about 67% modification), a small but variable antilipolytic action of insulin could still be detected. In cells maximally modified by PT, both basal and insulin-stimulated glucose oxidation were markedly reduced (to 10-15% of control levels). However, relative to the basal oxidation level, the fold stimulation by insulin in PT-treated cells was equivalent to the fold stimulation in control cells. Nonetheless, PT treatment caused a rightward shift in the dose-response curve for insulin-stimulated glucose oxidation as well as a small reduction in insulin binding. Our results point strongly not only to a link between the inhibitory guanine nucleotide regulatory complex (Gi) and the antilipolytic action of insulin but also to a link between the Gi complex and the overall regulation of glucose metabolism in adipocytes.

GTP‐binding protein‐activator sequences in the insulin receptor

“…Lutterel et al [18] found that insulin promotes guanine nucleotide-binding to the membranes of BC3-H-1 myocytes, where they had earlier reported that pertussis toxin inhibits DNA synthesis and glucose uptake promoted by insulin [10]. Similar findings have been reported using adipocytes [11][12][13]19]. It is also reported that in hepatocyte membranes, insulin attenuates the pertussis-toxin sensitivity of a 40-kDa Gi-like protein [20,21], suggesting that the insulin-bound insR activates the pertussis-toxin-sensitive G protein.…”

“…Receptors activate G proteins by stimulating the GDP/GTP exchange of G proteins. It has been reported that pertussis toxin inhibits insulin-stimulated metabolic action as well as mitogenic action in intact cells [9][10][11][12][13][14]. Pertussis toxin ADPribosylates Gt, Gi, and Go and uncouples them from receptors [15].…”

Regulation of cardiac insulin receptor function by guanosine nucleotides

“…In the past few years evidence has accumulated which suggests that insulin receptor signalling may involve coupling to GTP-binding proteins [1][2][3][4][5][6][7]. This view is mainly supported by the effects of cholera and pertussis toxin on insulin action.…”

“…This view is mainly supported by the effects of cholera and pertussis toxin on insulin action. Thus, cholera toxin was found to modify insulin signalling in the liver [1] and in the heart [7], whereas pertussis toxin attenuates the activation of hexose transport and the generation of inositolglycan mediators in BC3H-I myocytes [3], and the antilipolytic activity of insulin in adipocytes [2]. More recently it was found that insulin increases GTP-binding to plasma membranes from fat and muscle cells [8,9] and that this binding occurs at a 40/41 kDa membrane protein very similar or identical to G~ [9,10].…”

Guanosine 5'‐(γ‐thio) triphosphate (GTPγS) inhibits phosphorylation of insulin receptor and a novel GTP‐binding protein, Gir, from human placenta

“…Cholera toxin, which ADP-ribosylates primarily the stimulatory G-proteins (G,), has been shown to increase insulin-stimulated protein synthesis [4], and insulin inhibits stimulation of adenylate cyclase by this toxin [5]. Pertussis toxin, which ADP-ribosylates the inhibitory type of G-proteins (Gi) and G,, inhibits a number of insulin-stimulated cellular events such as glucose transport [6] and its metabolism [7], protein synthesis [4], and activation of cyclic AMP phosphodiesterase [8]. Interestingly, insulin inhibits pertussis toxin catalyzed ADP-ribosylation of Gi [9] and this toxin also attenuates insulin-induced inhibition of adenyl cyclase [8].…”