Fructose-2,6-bisphosphate in control of hepatic gluconeogenesis. From metabolites to molecular genetics

Abstract

Hormonal regulation of hepatic gluconeogenic pathway flux is brought about by phosphorylation/dephosphorylation and control of gene expression of several key regulatory enzymes. Regulation by cAMP-dependent phosphorylation occurs at the level of pyruvate kinase and 6-phosphofructo-2-kinase (6PF-1-K)/fructose-2,6-bisphosphatase (Fru-2,6-P2ase). The latter is a unique bifunctional enzyme that catalyzes both the synthesis and degradation of fructose-2,6-bisphosphate (Fru-2,6-P2), which is an activator of 6PF-1-K and an inhibitor of Fru-1,6-P2ase. The bifunctional enzyme is a homodimer whose activities are regulated by cAMP-dependent protein kinase-catalyzed phosphorylation at a single NH2-terminal seryl residue/subunit, which results in activation of the Fru-2,6-P2ase and inhibition of the PF-1-K reactions. Hormone-mediated changes in the phosphorylation state of the bifunctional enzyme are responsible for acute regulation of Fru-2,6-P2 levels. 6PF-2-K/Fru-2,6-P2ase thus provides a switching mechanism between glycolysis and gluconeogenesis in mammalian liver. Pyruvate kinase is regulated by both phosphorylation and allosteric effectors. Fru-1,6-P2, an allosteric activator, also inhibits cAMP-dependent enzyme phosphorylation, and its steady-state concentration is indirectly determined by the level of Fru-2,6-P2. Therefore, acute regulation of both pyruvate kinase and the bifunctional enzyme provide coordinated control at both the pyruvate/phosphoenolpyruvate and Fru-6-P/Fru-1,6-P2 substrate cycles. The Fru-2,6-P2 system is also subject to complex multihormonal long-term control through regulation of 6 PF-2-K/Fru-2,6-P2ase gene expression. Glucocorticoids are the major factor in turning on this gene in liver, but insulin is also a positive effector. cAMP prevents the effects of glucocorticoids and insulin. Although Fru-2,6-P2 plays a key role in the regulation of carbon flux in the gluconeogenic pathway, the regulation of this flux depends on several factors and regulation of other key enzymes whose importance varies depending on the dietary and hormonal status of the animal. Molecular cloning of the cDNA encoding PF-2-K/Fru-2,6-P2ase has elucidated its structure and permitted analysis of its evolutionary origin as well as its tissue distribution and control of its gene expression. The rat liver and skeletal muscle isoforms arose by alternative splicing of a single gene. The muscle form differs from the liver form only at the NH2-terminal and does not have a cAMP-dependent protein kinase phosphorylation site. The hepatic enzyme subunit consists of 470 amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)