A single entity, the AMP-activated protein kinase (AMPK), phosphorylates and regulates in vivo hydroxymethylglutaryl-CoA reductase and acetyl-CoA carboxylase (key regulatory enzymes of sterol synthesis and fatty acid synthesis, respectively), and probably many additional targets. The kinase is activated by high AMP and low ATP via a complex mechanism, which involves allosteric regulation, promotion of phosphorylation by an upstream protein kinase (AMPK kinase), and inhibition of dephosphorylation. This protein-kinase cascade represents a sensitive system, which is activated by cellular stresses that deplete ATP, and thus acts like a cellular fuel gauge. Our central hypothesis is that, when it detects a 'low-fuel' situation, it protects the cell by switching off ATP-consuming pathways (e.g. fatty acid synthesis and sterol synthesis) and switching on alternative pathways for ATP generation (e.g. fatty acid oxidation). Native AMP-activated protein kinase is a heterotrimer consisting of a catalytic alpha subunit, and beta and gamma subunits, which are also essential for activity. All three subunits have homologues in budding yeast, which are components of the SNF1 protein-kinase complex. SNF1 is activated by glucose starvation (which in yeast leads to ATP depletion) and genetic studies have shown that it is involved in derepression of glucose-repressed genes. This raises the intriguing possibility that AMPK may regulate gene expression in mammals. AMPK/SNF1 homologues are found in higher plants, and this protein-kinase cascade appears to be an ancient system which evolved to protect cells against the effects of nutritional or environmental stress.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1997.00259.x

The AMP‐Activated Protein Kinase

The cloning is described of two related human complementary DNAs encoding polypeptides that interact specifically with the translation initiation factor eIF-4E, which binds to the messenger RNA 5'-cap structure. Interaction of these proteins with eIF-4E inhibits translation but treatment of cells with insulin causes one of them to become hyperphosphorylated and dissociate from eIF-4E, thereby relieving the translational inhibition. The action of this new regulator of protein synthesis is therefore modulated by insulin, which acts to stimulate the overall rate of translation and promote cell growth.

Insulin-dependent stimulation of protein synthesis by phosphorylation of a regulator of 5'-cap function

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have ma...

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Mechanisms of Insulin Action and Insulin Resistance

Loss of Insulin Signaling in Hepatocytes Leads to Severe Insulin Resistance and Progressive Hepatic Dysfunction

Lipids form a diverse group of water-insoluble molecules that include triacylglycerides, phosphoglycerides, sterols and sphingolipids. They play several important roles at cellular and organismal levels. Fatty acids are the major building blocks for the synthesis of triacylglycerides, which are mainly used for energy storage. Phosphoglycerides, together with sterols and sphingolipids, represent the major structural components of biological membranes. Lipids can also have important roles in signalling, functioning as second messengers and as hormones. There is increasing evidence that cancer cells show specific alterations in different aspects of lipid metabolism. These alterations can affect the availability of structural lipids for the synthesis of membranes, the synthesis and degradation of lipids that contribute to energy homeostasis and the abundance of lipids with signalling functions. Changes in lipid metabolism can affect numerous cellular processes, including cell growth, proliferation, differentiation and motility. This review will examine some of the alterations in lipid metabolism that have been reported in cancer, at both cellular and organismal levels, and discuss how they contribute to different aspects of tumourigenesis.

https://febs.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1742-4658.2012.08644.x

Lipid metabolism in cancer

Plasma insulin concentrations in fed rats were altered acutely by administration of glucose or anti-insulin serum. Rates of fatty acid synthesis in adipose tissue and liver were estimated from the incorporation of 3H from 3H2O. In the adipose tissue dehydrogenase and acetyl-CoA carboxylase were evident. In liver, although changes in rates of fatty acid synthesis were found, the initial activity of pyruvate dehydrogenase did not alter, but small parallel changes in acetyl-CoA carboxylase activity were observed.

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Acute effects in vivo of anti-insulin serum on rates of fatty acid synthesis and activities of acetyl-coenzyme A carboxylase and pyruvate dehydrogenase in liver and epididymal adipose tissue of fed rats.

/pdf/a-partial-view-of-the-mechanism-of-insulin-action-12118liov4.pdf

A partial view of the mechanism of insulin action

1. A new rapid method for the purification of fat-cell acetyl-CoA carboxylase is described; the key step is sedimentation after specific polymerization by citrate. 2. Incubation of epididymal fat-pads or isolated fat-cells with insulin or adrenaline leads to a rapid increase or decrease respectively in the activity of acetyl-CoA carboxylase measured in fresh tissue extracts. The persistence of the effect of insulin through high dilution of tissue extracts and through purification involving precipitation with (NH4)2SO4 suggests that the enzyme undergoes a covalent modification after exposure of intact tissue to the hormone. The opposed effects of insulin and adrenaline are not adequately explained through modification of a common site on acetyl-CoA carboxylase, since these hormones bring about qualitatively different alterations in the kinetic properties of the enzyme measured in tissue extracts. 3. The state of phosphorylation of acetyl-CoA carboxylase within intact fat-cells exposed to insulin was determined, and results indicate a small but consistent rise in overall phosphorylation of the Mr-230000 subunit after insulin treatment. 4. Acetyl-CoA carboxylase from fat-cells previously incubated in medium containing [32P]phosphate was purified by immunoprecipitation and then digested with performic acid and trypsin before separation of the released phosphopeptides by two-dimensional analysis. Results obtained show that the exposure of fat-cells to insulin leads to a 5-fold increase in incorporation of 32P into a peptide which is different from those most markedly affected after exposure of fat-cells to adrenaline. 5. These studies indicate that the activation of acetyl-CoA carboxylase in cells incubated with insulin is brought about by the increased phosphorylation of a specific site on the enzyme, possibly catalysed by the membrane-associated cyclic AMP-independent protein kinase described by Brownsey, Belsham & Denton [(1981) FEBS Lett. 124, 145-150].

Evidence that insulin activates fat-cell acetyl-CoA carboxylase by increased phosphorylation at a specific site.

1. Exposure of rat epididymal fat-pads or isolated fat-cells to adrenaline results in a decrease in acetyl-CoA carboxylase activity measured both in initial extracts and in extracts incubated with potassium citrate; in addition the concentration of citrate required to give half-maximal activation may also be increased. 2. Incorporation of 32Pi into acetyl-CoA carboxylase within intact fat-cells was investigated and evidence is presented that adrenaline increases the extent of phosphorylation of the enzyme. 3. Dephosphorylation of 32P-labelled acetyl-CoA carboxylase was studied in cell extracts. The rate of release of 32P is increased by 5mM-MgCl2 plus 10--100 microM-Ca2+, whereas it is inhibited by the presence of bivalent metal ion chelators such as EDTA and citrate. 4. The effects of adrenaline on the kinetic properties of acetyl-CoA carboxylase disappear if pad or cell extracts are treated with Mg2+ and Ca2+ under conditions that also lead to dephosphorylation of the enzyme. 5. The results of this study represent convincing evidence that adrenaline inactivates acetyl-CoA carboxylase in adipose-tissue preparations by increasing the degree of phosphorylation of the enzyme.

/pdf/adrenaline-and-the-regulation-of-acetyl-coenzyme-a-5febqfywqj.pdf

Adrenaline and the regulation of acetyl-coenzyme A carboxylase in rat epididymal adipose tissue. Inactivation of the enzyme is associated with phosphorylation and can be reversed on dephosphorylation

Intact rat epididymal fat-cells were incubated with 32Pi and the intracellular proteins separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. One of the phosphorylated proteins has the same RF value as [14C]biotin-labelled acetyl-CoA carboxylase purified from fat-cells and is specifically precipitated after incubation with antiserum raised against acetyl-CoA carboxylase. No significant changes in the extent of phosphorylation of acetyl-CoA carboxylase were detected after exposure of the cells to insulin.

/pdf/demonstration-of-the-phosphorylation-of-acetyl-coenzyme-a-amohlekf7x.pdf

R W Brownsey

Papers

Acute effects in vivo of anti-insulin serum on rates of fatty acid synthesis and activities of acetyl-coenzyme A carboxylase and pyruvate dehydrogenase in liver and epididymal adipose tissue of fed rats.

A partial view of the mechanism of insulin action

Evidence that insulin activates fat-cell acetyl-CoA carboxylase by increased phosphorylation at a specific site.

Adrenaline and the regulation of acetyl-coenzyme A carboxylase in rat epididymal adipose tissue. Inactivation of the enzyme is associated with phosphorylation and can be reversed on dephosphorylation

Demonstration of the phosphorylation of acetyl-coenzyme A carboxylase within intact rat epididymal fat-cells.