Insulin-dependent protein kinases

It usually contains some phosphoserine and phospho-threonine residues but they apparently have little effect on activity.222 Phosphorylation is catalyzed by cAMP-dependent and by insulin-dependent protein kinases.223 224 A related reaction is the ATP-dependent cleavage of malate to acetyl-CoA and glyoxylate. It requires two enzymes, malyl-CoAbeing an intermediate.225... 

The disturbances in protein phosphorylation patterns in Cr(VI) treated cells are considered among the possible reasons for Cr(VI) toxicity and carcinogenicity (295, 626). The question then arises, as to whether the proposed beneficial action of Cr(III) in activation of insulin receptor tyrosine kinase (496,497) is, in fact, a sign of Cr(III) toxicity (5). Unpredictable changes in the concentrations of phosphorylated proteins in the presence of excess Cr(III) may lead to abnormalities in the cell signaling pathways and ultimately to cancer (5). An answer to this dilemma may lie in selectivity studies (which are yet to be performed) of different types of Cr(III) complexes toward various kinases or phosphatases. Clearly, the Cr(III) complexes of potential use as anti-diabetics should be highly selective in the activation of protein tyrosine kinase of the p-subunit of the insulin receptor (496, 497). On the other hand, the potential ability of some Cr(III) complexes to selectively activate non-insulin dependent protein kinases may lead to beneficial effects, such as stimulation of immune responses or antitumor activity (627, 628). 

Earner, J., 1990. Insulin and the stimulation of glycogen synthesis The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators. Advances in Enzymology 63 173-231. 

Both phosphorylase a and phosphorylase kinase a are dephosphorylated and inactivated by protein phos-phatase-1. Protein phosphatase-1 is inhibited by a protein, inhibitor-1, which is active only after it has been phosphorylated by cAMP-dependent protein kinase. Thus, cAMP controls both the activation and inactivation of phosphorylase (Figure 18-6). Insulin reinforces this effect by inhibiting the activation of phosphorylase b. It does this indirectly by increasing uptake of glucose, leading to increased formation of glucose 6-phosphate, which is an inhibitor of phosphorylase kinase. 

Figure 21-6. Regulation of acetyl-CoA carboxylase by phosphorylation/dephosphorylation.The enzyme is inactivated by phosphorylation by AMP-activated protein kinase (AMPK), which in turn is phosphorylated and activated by AMP-activated protein kinase kinase (AMPKK). Glucagon (and epinephrine), after increasing cAMP, activate this latter enzyme via cAMP-dependent protein kinase. The kinase kinase enzyme is also believed to be activated by acyl-CoA. Insulin activates acetyl-CoA carboxylase, probably through an "activator" protein and an insulin-stimulated protein kinase.

Petruzzelli, L., Herrer, R., Garcia-Arenas, R., and Rosen, R.M. (1985) Acquisition of insulin-dependent protein tyrosine kinase activity during Drosophila embryogenesis.J. Biol. Chem. 226,16072-16075. 

Insulin activates PFK-2 (via the tyrosine kinase receptor and activation of protein phosphatases), which converts a tiny amovmt of fructose 6-phosphate to fructose 2,6-bisphosphate (F2,6-BP). F2,6-BP activates PFK-1. Glucagon inhibits PFK-2 (via cAMP-dependent protein kinase A), lowering F2,6 BP and thereby inhibiting PPK-1. 

Figure 7.15 Inhibition of acetyl-CoA carboxylase by cyclic AMP dependent protein kinase and AMP dependent protein kinase the dual effect of glucagon. Phosphorylation of acetyl-CoA carboxylase by either or both enzymes inactivates the enzyme which leads to a decrease in concentration of malonyl-CoA, and hence an increase in activity of carnitine palmitoyltransferase-I and hence an increase in fatty acid oxidation. Insulin decreases the cyclic AMP concentration maintaining an active carboxylase and a high level of malonyl-CoA to inhibit fatty acid oxidation.

Considerable cross-talk also occurs between different hormones at the level of receptor function. For instance, the diacylglycerol-activated protein kinase C phosphory-lates and thereby inhibits the activity of insulin and epidermal growth factor receptor kinases (epidermal growth factor is discussed in a later section). Likewise, when cAMP-dependent protein kinases are active they can inhibit receptors for epinephrine by phosphorylating them. 

Regulation The concentration of free fatty acids in the blood is controlled by the rate at which hormone-sensitive triacylglycerol lipase hydrolyzes the triacylglycerols stored in adipose tissue. Glucagon, epinephrine and norepinephrine cause an increase in the intracellular level of cAMP which allosterically activates cAMP-dependent protein kinase. The kinase in turn phosphorylates hormone-sensitive lipase, activating it, and leading to the release of fatty acids into the blood. Insulin has the opposite effect it decreases the level of cAMP which leads to the dephosphorylation and inactivation of hormone-sensitive lipase. 

Borthwick, A.C., Edgell, N.J., Denton, R.M. 1987. Use of rapid gel-permeation chromatography to explore the inter-relationships between polymerization, phosphorylation and activity of acetyl-CoA carboxylase. Effects of insulin and phosphorylation by cyclic AMP-dependent protein kinase. Biochem. J. 241, 773-782. 

If beta cells are incubated in media containing 2 mM glucose and then treated with forskolin and/or tolbutamide, there is a small transient increase in insulin secretion. The subsequent addition of CCK8S leads to a very marked first phase of insulin secretion, but causes no sustained increase or second phase of insulin secretion. These results mean that an increase in cAMP alters the Ca2+ sensitivity of the response elements underlying the first phase of secretion. These elements, presumed to be Ca2+-calmodulin-dependent processes including CaM-dependent protein kinases, become more sensitive to activation by Ca2+ either because cAMP acts to enhance the sensitivity of CaM-dependent kinases to Ca2+, or because cAMP inhibits, by an unknown mechanism, the activity of phosphoprotein phosphatases. 

Other evidence for the involvement of a G-protein in the action of insulin has come from studies by Walaas and co-workers [104]. They have demonstrated that insulin stimulated the activity of a cyclic AMP-dependent protein kinase activity in sarcolemma membranes. As this effect of insulin was enhanced if micromolar concentrations of GTP-binding protein were present, they suggested that a guanine nucleotide regulatory protein was involved in the hormonal control of this kinase. Indeed, cholera toxin also appeared to obliterate this action of insulin, as it did the effect of insulin on liver adenylate cyclase and the peripheral plasma membrane cyclic AMP phosphodiesterase in liver. 

PKB, insulin-activated protein kinase, Akt PKG, Ca2+- and phospholipid-dependent... 

Because the primary eveuts between insulin binding to its receptor and glucose transport are signal transduction events, a role for chromodulin in these events has been probed. Chromoduhn activates the tyrosine kinase activity of insulin-activated insulin receptor and activates a membrane phosphotyrosine phosphatase in adipocyte membranes. The addition of bovine liver chromodulin to rat adipocytic membranes in the presence of insuhn results in a concentration-dependent eightfold stimulation of insulin-dependent protein tyrosine kinase activity (while no activation... 

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