Insulin phosphorylation

FIGURE 23.22 The metabolic effects of insulin. As described in Chapter 34, binding of insulin to membrane receptors stimulates the protein kinase activity of the receptor. Subsequent phosphorylation of target proteins modulates the effects indicated. 

Insulin Receptor. Figure 1 Structure and function of the insulin receptor. Binding of insulin to the a-subunits (yellow) leads to activation of the intracellular tyrosine kinase ((3-subunit) by autophosphorylation. The insulin receptor substrates (IRS) bind via a phospho-tyrosine binding domain to phosphorylated tyrosine residues in the juxtamembrane domain of the (3-subunit. The receptor tyrosine kinase then phosphorylates specific tyrosine motifs (YMxM) within the IRS. These tyrosine phosphorylated motifs serve as docking sites for some adaptor proteins with SRC homology 2 (SH2) domains like the regulatory subunit of PI 3-kinase. 

Also, phosphorylation of Akt results in activation of sterol regulatory-element binding protein 1 (SREBP1), a key transcription factor involved in regulation of lipogenic enzymes. In addition, some of the effects of insulin on cell proliferation and survival may be explained by an Akt-dependent inhibition of apoptosis through phosphorylation and inactivation of proa-poptotic proteins (e.g., BAD, Caspase 9). 

Rapamycin is an immunosuppressive diug and an inhibitor of S6K1 (also known as p70S6-kinase) which phosphorylates ribosomal S6 protein. S6K1 is activated in response to insulin via activation of Akt. Rapamycin binds to a specific target protein (mTOR, mammalian target of rapamycin) which is functionally located downstream of Akt, but upstream... 

Family of enzymes phosphorylating phosphatidylinositol (Ptdlns), PtdIns(4)phosphate, and PtdIns(4,5)phosphate in the 3-position. The Ptdlns(3 phospholipids are second messengers in processes like cell growth, cytoskeletal rearrangement, and vesicular transport. PI 3-kinases are heterodimers composed of a catalytic and a regulatory subunit. The enzymes are activated by insulin, many growth factors, and by a variety of cytokines. Their activity can be inhibited by wortmannin and LY294002. 

S6K1 (also known as p70S6 kinase) is a serine/ threonine protein kinase which is involved in the regulation of translation by phosphorylating the 40S ribosomal protein S6. Insulin and several growth factors activate the kinase by phosphorylation in a PI 3-kinase dependent and rapamycin-sensitive manner. Phosphorylation of S6 protein leads to the translation of mRNA with a characteristic 5 polypyrimidine sequence motif. 

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

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.

Lipogenesis is regulated at the acetyl-CoA carboxylase step by allosteric modifiers, phosphorylation/de-phosphorylation, and induction and repression of enzyme synthesis. Citrate activates the enzyme, and long-chain acyl-CoA inhibits its activity. Insulin activates acetyl-CoA carboxylase whereas glucagon and epinephrine have opposite actions. 

Insulin and other growth factors result in the phosphorylation of BP-1 at five unique sites. Phosphorylation of BP-1 results in its dissociation from 4E, and it cannot rebind until critical sites are dephosphorylated. The protein kinase responsible has not been identified, but it appears to be different from the one that phos-phorylates 4E. A kinase in the mammalian target of rapamycin (mTOR) pathway, perhaps mTOR itself, is involved. These effects on the activation of 4E explain in part how insuhn causes a marked posttranscriptional... 

Figure 38-7. Activation of elF-4E by insulin and formation of the cap binding elF-4F complex. The 4F-cap mRNA complex is depicted as in Figure 38-6. The 4F complex consists of elF-4E (4E), elF-4A, and elF-4G. 4E is inactive when bound by one ofa family of binding proteins (4E-BPs). Insulin and mitogenic factors (eg, IGF-1, PDGF, interleukin-2, and angiotensin II) activate a serine protein kinase in the mTOR pathway, and this results in the phosphorylation of 4E-BP. Phosphorylated 4E-BP dissociates from 4E, and the latter is then able to form the 4F complex and bind to the mRNA cap. These growth peptides also phosphorylate 4E itself by activating a component of the MAP kinase pathway. Phosphorylated 4E binds much more avidly to the cap than does nonphosphorylated 4E.

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