The chemical mediators of insulin action: possible targets for postreceptor defects

Defective insulin response of cyclic adenosine monophosphate-dependent protein kinase in insulin-resistant humans

Insulin-stimulated glycogen synthase activity in human muscle correlates with insulin-mediated glucose disposal and is reduced in insulin-resistant subjects. Inhibition of the cyclic AMP-dependent protein kinase (A-kinase) is considered as a possible mechanism of insulin action for glycogen synthase activation. In this study, we investigated the time course of insulin action on human muscle A-kinase activity during a 2-h insulin infusion in 13 insulin-sensitive (group S) and 7 insulin-resistant subjects (group R). Muscle biopsies were obtained from quadriceps femoris muscle at times 0, 10, 20, 40, and 120 min. Insulin infusion resulted in significant inhibition of A-kinase activity at 20 and/or 40 min using 0.2, 0.6, and 1.0 microM cyclic AMP in group S. A-kinase activities both before and after insulin administration were lower in group S than in group R using 0.6 microM cyclic AMP. The decrease in apparent affinity for cyclic AMP during insulin infusion was larger for group S compared with group R. Glycogen synthase activity increased significantly after insulin infusion in both groups and was higher in group S compared with group R. The data suggest that a defective response of A-kinase to insulin in insulin-resistant subjects could contribute to their reduced insulin stimulation of skeletal muscle glycogen synthase.

Insulin stimulates generation of intracellular mediators in rat heart

Insulin treatment of rats results in an increased amount or activity of insulin mediators in heart muscle. The mediators stimulated mitochondrial pyruvate dehydrogenase and inhibited glucagon-stimulated adenylate cyclase. The mediators were copurified by ultrafiltration, ethanol extraction, Dowex cation-exchange, and QAE-Sephadex anion-exchange chromatography. The activities of the two mediators were separated by Sephadex G-10 chromatography. Fasting rats for 72 h diminished the mediator response to insulin treatment. These results, taken together with previous reports, indicate that insulin generates a number of mediators which have a ubiquitous tissue distribution. The activity of these mediators, like insulin responsiveness, is altered by the metabolic state of the animal.

Insulin mediators from rat skeletal muscle have differential effects on insulin-sensitive pathways of intact adipocytes

The effects of partially purified insulin-generated mediators from rat skeletal muscle were compared to those of insulin on intact adipocytes. Insulin and insulin mediator stimulated both pyruvate dehydrogenase and glycogen synthase activity of intact adipocytes. In contrast, insulin stimulated glucose oxidation and 3-O-methylglucose transport, whereas insulin-generated mediators had no effect. Insulin-generated mediators cannot account for all the pleiotropic effects of insulin, especially membrane-controlled processes.

Insulin-dependent production of low-molecular-weight compounds that modify key enzymes in metabolism

The mechanism by which the metabolic effects of insulin are transmitted is yet to be resolved. Second messengers mediating the action of insulin have been proposed and recently an insulin-dependent, peptide-like, heat- and acid-stable substance (Mr approx. 1000-3000) released from plasma membranes has been described which regulates the activity of key enzymes such as pyruvate dehydrogenase by altering its state of phosphorylation. It has been suggested that this material is the elusive second messenger of insulin and its discovery, generation, properties, isolation and mode of action are reviewed.

Insulin secretion and action

Recent advances in insulin secretion indicate that pertussis toxin abolishes the inhibition by alpha 2 adrenoceptor activation of insulin release by the pancreas. Pertussis toxin adenosine diphosphate (ADP) ribosylates an inhibitory guanine nucleotide-binding protein (Ni) involved in inhibition of adenylate cyclase. The decrease in cyclic adenosine monophosphate (AMP) by epinephrine may account for its inhibition of insulin release. Insulin interaction with its receptor results in an increase in the tyrosine protein kinase activity of the receptor. Second messengers for insulin are generated, hexose transport is accelerated, and a cyclic AMP-independent protein kinase is activated that phosphorylates at serinethreonine residues. The activity of membrane-bound enzymes such as adenylate cyclase and Ca2+-Mg2+-ATPase is affected. The relative importance of these effects of insulin in its regulation of cellular metabolism remains to be established.

Insulin stimulation of phospholipid methylation in isolated rat adipocyte plasma membranes

Partially purified plasma membranes prepared from rat adipocytes contain N-methyltransferase(s) that utilize(s) S-adenosyl-L-methionine to synthesize phosphatidylcholine from phosphatidylethanolamine. The incorporation of [3H]methyl from S-adenosyl-L-[methyl-3H]methionine into plasma membrane phospholipids was linear with incubation time and plasma membrane protein concentration and was inhibited in a dose-dependent manner by both S-adenosyl-L-homocysteine and 3-deazadenosine. The addition of insulin to plasma membranes stimulated the methylation of endogenous phosphatidylethanolamine, as evidenced by an increase in the levels of phosphatidyl-N-monomethylethanolamine, phosphatidyl-N, N-dimethylethanolamine, and phosphatidylcholine. The effect of insulin was rapid and concentration-dependent, with 100 microunits/ml providing near maximal stimulation. The incorporation of [3H]methyl into phospholipids of control and insulin-stimulated plasma membranes was enhanced by the addition of exogenous methyltransferase substrates phosphatidylethanolamine, phosphatidyl-N-monomethylethanolamine, and phosphatidyl-N,N-dimethylethanolamine. The stimulatory effect of insulin on adipocyte plasma membrane phospholipid methylation may have a physiological role in insulin action.

Inhibition of transcription of the phosphoenolpyruvate carboxykinase gene by insulin

Insulin regulates the synthesis of several proteins in a variety of tissues. Before techniques were available to quantify the amount of specific mRNAs, insulin was thought to regulate the synthesis of proteins by influencing the rate of translation of a fixed amount of mRNA. A very different interpretation is called for by experiments which show that insulin alters the amount of several specific mRNAs, but little is known about the mechanism. Insulin decreases the rate of synthesis of the critical gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in both liver and H4IIE heptoma cells. We recently showed that insulin acts directly on H4IIE cells to decrease mRNAPEPCK activity without any other hormone intermediaries. This effect is mediated by the insulin receptor and occurs at insulin concentrations which are well within the physiological range range (10(-12)--10(-9) M). Here we extend these studies to show that insulin specifically inhibits transcription of the PEPCK gene. This inhibition results in a rapid decrease in the concentration of nuclear PEPCK transcripts which is followed, in turn, by a proportionate decline in cytoplasmic mRNAPEPCK and synthesis of the protein.