Roger W. Brownsey

Regulation of acetyl-CoA carboxylase: identity of sites phosphorylated in intact cells treated with adrenaline and in vitro by cyclic AMP-dependent protein kinase

The rate of fatty acid synthesis in mammals and birds is rapidly depressed by hormones which raise the concentration of cyclic AMP in target cells, i.e., adrenaline in adipose tissue [ 1 ] or glucagon in the liver [Z-4]. This decrease correlates with a decrease in the activity of acetyl-CoA carboxylase measured in a freshly-prepared extract [4-91. However there has been some controversy as to the mechanism by which an increase in cyclic AMP concentration leads to a decrease in the activity of acetyl-CoA carboxylase. This decrease in activity may be the result of a decrease in the cytosolic concentration of the allosteric activator, citrate [3]. According to this hypothesis, the decrease in the rate of fatty acid synthesis is secondary to the decrease in the rate of glycolysis produced by inhibition of phosphofructokinase [lo]. An alternative hypothe~s is that cyclic AMP brings about a direct inactivation of acetyl-CoA carboxylase through a phosphorylation mechanism. The phosphorylation of acetyl-CoA carboxylase in intact cells was first demonstrated in [ If.] and it has been found that the phosphorylation of the enzyme is increased by adrenaline in fat cells [S ,7] or glucagon in hepatocytes [9]. In [ 121 it was reported that puritied rabbit mamma gland acetyl-CoA carboxylase could be phosphorylated in vitro by cyclic AMPdependent protein kinase and at least one cyclic UP-independent protein kinase. In [ 131 it was shown that phosphorylation of rat mammary acetylCoA carboxylase by cyclic AMP-dependent protein

Anti-Insulin Receptor Antibodies Mimic the Effects of Insulin on the Activities of Pyruvate Dehydrogenase and AcetylCoA Carboxylase and on Specific Protein Phosphorylation in Rat Epididymal Fat Cells

SummaryPrevious studies have shown that auto-antibodies against insulin receptors found in certain patients with severe insulin resistance stimulate glucose transport and metabolism in fat cell and muscle preparations. The present studies show that preincubation of rat epididymal adipose tissue with 1∶1000 dilution of one such serum results in a two fold increase in the initial activities of pyruvate dehydrogenase and acetylCoA carboxylase. These increases are similar to the maximum effects of insulin. Incubation of isolated fat cells with the serum at the same concentration also resulted in the increased phosphorylation of three intracellular proteins with subunit molecular weights of 130,000, 35,000 and 22,000 to the same extent as observed with insulin. These findings lend further support to the view that the short term effects of insulin do not involve the entry of the insulin molecule (or part thereof) into cells of target tissues.

Evidence for phosphorylation and activation of acetyl CoA carboxylase by a membrane-associated cyclic AMP-independent protein kinase: Relationship to the activation of acetyl CoA carboxylase by insulin

Exposure of rat epididymal adipose tissue or fat cells to insulin for a few minutes results in a marked increase in the activity of acetyl CoA carboxylase assayed immediately upon extraction in the absence of added citrate [l-4]. In contrast, treatment of tissue or cells with adrenaline leads to a marked decrease in this initial activity [2-51. The effects of adrenaline are probably brought about by increased phosphorylation of acetyl CoA carboxylase by cyclic AMPdependent protein kinase. Acetyl CoA carboxylase from a number of sources has been shown to become less active following phosphorylation by cyclic AMPdependent protein kinase [3,6-71 while significant increases in the overall level of phosphorylation have been observed under conditions where cellular cyclic AMP levels are raised in both fat and liver cells [3,5,8]. Other studies have indicated that acetyl CoA carboxylase is phosphorylated at multiple sites both with purified preparations [4,6,9,10] and within intact fat cells [ 111. Analysis of tryptic phosphopeptides has indicated that adrenaline brings about the phosphorylation within fat cells of sites on acetyl CoA carboxylase which correspond closely with those phosphorylated on purified mammary gland enzyme with cyclic AMPdependent protein kinase [ 111. The activating effects of insulin are not simply the converse of the inhibitory actions of adrenaline [3]. In particular, adrenaline results in a decrease in the activity of the enzyme even after incubation of cell extracts with citrate to induce maximum polymerisation of the enzyme [3,5] whereas the effects of insulin are no longer apparent after incubation with

ACUTE HORMONAL REGULATION OF FATTY ACID SYNTHESIS IN MAMMALIAN TISSUES

Publisher Summary This chapter discusses the acute hormonal regulation of fatty acid synthesis in mammalian tissues. Exposure of white adipose tissue such as rat epididymal fat pads to insulin leads within a minute or so to a large increase in the rate of conversion of glucose to fatty acids. The increase in the rate of fatty acid synthesis in the presence of insulin is brought about by activation not only of glucose transport but also of pyruvate dehydrogenase and acetyl CoA carboxylase. Both these enzymes exist in interconvertible forms. After exposure to insulin, the proportion of both enzymes in their respective active forms is increased 2–3 fold. These parallel activations do appear to offer a satisfactory explanation of the preferential conversion of glucose carbon into fatty acids that is so characteristic of the response of white adipose tissue to insulin.