Tuesday, August 18, 2009

Insulin Receptor Tyrosine Kinase

The insulin receptor is a transmembrane receptor that is activated by insulin. It belongs to the large class of tyrosine kinase receptors.

Structural Characteristics
The insulin receptor is composed of two a-subunits that are each linked to a b-subunit and to each other by disulfide bonds. Both subunits are derived from a single proreceptor by proteolytic processing at a cleavage site consisting of four basic amino acids. The mature heterotetramer (a2b2) contains complex N-linked carbohydrate side chains capped by terminal sialic acid residues and migrates with a molecular mass of 300400 kDa. The a-subunits are located entirely outside of the cell and contain the insulin binding site(s), whereas the intracellular portion of the b subunit contains the insulin-regulated tyrosine protein kinase. The insulin receptor family contains two other structurally related molecules, the insulin-like growth factor (IGF-1) receptor and the insulin receptor – related receptor, an orphan receptor. This family shares more than 80% amino acid sequence identity in the kinase domain but has low amino acid sequence identity in the extracellular domain1.

Mode of Action
The physiological effects of insulin are mediated by its cell surface receptor, a a2b2 transmembrane glycoprotein with intrinsic protein tyrosine activity. Binding of insulin to the extracellular a-chains results in autophosphorylation of specific tyrosine residues in the portion of the b-chains: two in the juxtamembrane region, three in the kinase (catalytic) domain, and two in the C-terminal tail2. Autophosphorylation of Tyr1158, Tyr1162 and Tyr1163 in the activation loop (A-loop) of the kinase domain is critical for stimulation of kinase activity and function.

The activation loop (A-loop) of the kinase undergoes a major conformational change upon autophosphorylation of Tyr1158, Tyr1162 and Tyr1163 within the loop, resulting in unrestricted access of ATP and protein substrates to the kinase active site3.

An important consequence of activation is the recruitment of insulin receptor substrate-1 (IRS-1) and other effector molecules such as phosphatidylinositol 3'-kinase (PI-3K), Ras GTP activating protein (Ras GAP) and phospholipase C-y (PLCyj and adaptor molecules, including Shc and Grb2 [1]. Each of these effectors and adaptors contain Src homology 2 (SH2) domains that bind phosphotyrosines with high affinity. Importantly, it is IRS-1 that is the main target for these SH2-containing proteins rather than the insulin receptor itself. Eventually these transient complexes formed with IRS-1 activate a number of pathways involving serine/threonine kinases, such as the mitogenactivated- protein (MAP) kinase which ultimately lead to the initiation of protein synthesis and to the translocation of an insulin-sensitive glucose transporter to the cell surface4.

Effects of hypercortisolemia and insulin receptor function - Activation of skeletal muscle insulin receptor tyrosine kinase in vitro and in vivo was studied in two rat models of insulin resistance: insulinopenic diabetes and hypercortisolemia. In control rats, intravenous insulin administration resulted in dose-dependent in vivo activation of the muscle insulin receptor kinase towards histone H2b. Half-maximal and maximal activation were observed 5 min after injecting 0.1 and 0.5 U insulin/100 g, respectively. Diabetes (7 days) induced with streptozotocin did not affect insulin binding affinity of solubilized muscle receptors but depressed receptor kinase activation in vivo by 52 or 40% after intravenous insulin administration. Cortisone treatment (5 days) resulting in weight loss, hyperglycemia, and hyperinsulinemia did not affect the number, insulin binding affinity, or kinase activity of solubilized receptors activated with insulin in vitro or in vivo5.

Insulin Internalization in the Absence of the Insulin Receptor Tyrosine Kinase Domain Is Insufficient for Mediating Intracellular Biological Effects - The intracellular portion of the human insulin receptor (hIR) ß-subunit contains distinct functional domains including the exon 16-encoded juxtamembrane (JM) domain that mediates endocytosis, and the tyrosine kinase (TK) domain that mediates insulin's metabolic and mitogenic actions. To explore the functional relationship between these domains and to determine the role of insulin internalization in insulin action, a study was conducted using hIR mutant, truncated at Glu-1012. This truncation removes the carboxyl-terminal 343 amino acids containing essentially all of the TK domain but leaves behind the exon 16-encoded JM domain that is necessary for endocytosis, plus an additional 35 amino acids downstream. It was found that the wild-type (hIR-WT) and mutant (hIR?343) receptors were stably expressed in CHO cells. In cells expressing hIR-WT, insulin markedly enhanced tyrosine phosphorylation of the ß-subunit and of the endogenous 185 kDa substrate whereas these effects were completely absent in cells expressing hIR?343. The hIR?343 receptors retained the ability to internalize a significant amount of surface-bound insulin at 37°C. However, they were unable to mediate either short or long-term biological effects such as insulin-stimulated glucose uptake and DNA synthesis. Further results indicate that the hIR ß-subunit JM and TK domains can be functionally uncoupled, and that insulin internalization in the absence of hIR TK domain and kinase activity is insufficient for mediating intracellular insulin action6.

1.NQ McDonald, J Murray – Rust and TL Blundell (1995).The first structure of a receptor tyrosine kinase domain: a further step in understanding the molecular basis of insulin action. Structure, 3, 1-6.
2.Feener EP, Backer JM., King GL, Wilden PA, Sun XJ, Kahn CR and White MF (1993). Insulin stimulates serine and tyrosine phosphorylation in the juxtamembrane region of the insulin receptor. J. Biol. Chem., 268, 11256–11264.
3.Stevan R Hubbard (1997). Crystal structure of the activated insulin receptor tyrosine kinase in complex with peptide substrate and ATP analog. The EMBO Journal., 16(18), 5573–5581.
4.White MF & Kahn CR (1994). The insulin signalling system . J. Biol. Chem., 269, 1-4.
5.NE Block and MG Buse (1989). Effects of hypercortisolemia and diabetes on skeletal muscle insulin receptor function in vitro and in vivo. Am. J. Physiol. Endocrino.l Metab., 256, E39-E48.
6.Dorota B. Schranz, Arif MK Rohilla, Celia Anderson, William M Wood and Paulos Berhanu (1996). Insulin Internalization in the Absence of the Insulin Receptor Tyrosine Kinase Domain Is Insufficient for Mediating Intracellular Biological Effects.

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