![]() ![]() This effect does not appear to involve changes in the level of cAMP because the hormone also acts in cultured cells when added alone or in the presence of dexamethasone. Insulin also opposes the action of cAMP on gene transcription by an as yet unknown mechanism. However, in the systems in which this has been studied, the cAMP-independent effects on gluconeogenic/glycolytic pathway flux are small in comparison to cAMP-dependent regulation. Insulin also opposes the action of hormones (alpha-adrenergic agonists, angiotensin, vasopressin) that act in liver via cAMP-independent phosphorylation. Insulin acts to oppose the acute actions of cAMP on enzyme phosphorylation, presumably by activating a phosphodiesterase and thereby lowering cAMP levels. Insulin is the most important hormone for restraining the level of cAMP. Its role in the regulation of gene transcription of the bifunctional enzyme and 6-phosphofructo 1-kinase remains to be defined. Cyclic AMP acts to induce synthesis of mRNA for phosphoenolpyruvate carboxykinase and probably fructose 1, 6-bisphosphatase while it suppresses transcription of the genes for pyruvate kinase and glucokinase. Regulation of cAMP levels by glucagon, insulin, and catecholamines accounts in large part for minute-to-minute hormonal control of pathway flux in fed animals and during the transition from fed to starved second, cAMP plays a key role in regulation of gene transcription of phosphoenolpyruvate carboxykinase, pyruvate kinase, glucokinase, and probably 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. First, cAMP levels determine the phosphorylation state of key regulatory enzymes including pyruvate kinase and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. The cyclic nucleotide acts on two levels. Cyclic AMP plays a major, if not primary, role in the regulation of hepatic gluconeogenesis. ![]()
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