Cellular signalling in diabetes (IRS 1)

Peresented by:Mohsen Koolivand

MSC Clinical of Biochemistry

Supervised by:dr.moein

introductionNo cell lives in isolation. Adjacent cells often communicate by direct cell-cell contact. For

example, gap junctions in the plasma membranes of adjacent cells permit them to exchange small molecules and to coordinate metabolic responses.

Extracellular signaling molecules are synthesized and released by signaling cells and produce a specific response only in target cells that have receptors for the signaling molecules.

In multicellular organisms, an enormous variety of chemicals, including small molecules

(e.g., amino acid or lipid derivatives, acetylcholine) peptides, and proteins, are used in this type of cell-to-cell communication.

Some signaling molecules, especially hydrophobic molecules such as steroids, retinoids,

and thyroxine, spontaneously diffuse through the plasma membrane and bind to intracellular receptors.

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A fundamental mechanism for the maintenance of glucose homeostasis is the rapid action of insulin to stimulate glucose uptake and metabolism in peripheral tissues.

Skeletal muscle is the primary site of glucose disposal in the insulin-stimulated state . Resistance to the actions of insulin in skeletal muscle is a major pathogenic factor in type 2 or type 1 diabetes mellitus ; it also contributes to the morbidity of obesity, and complicates poorly controlled type 1 (autoimmune) diabetes. The ability of insulin to increase glucose transport in skeletal muscle is elicited by the translocation of glucose transporter 4 (Glut4), the major insulin regulated glucose transporter, from intracellular vesicles to the plasma membrane and transverse tubules .

In muscle of type 2 diabetic subjects, the expression of the Glut4 gene is normal, and impairedm glucose uptake by insulin action most likely results from altered trafficking or impaired function of Glut4.

Because glucose transport in response to other stimuli that use different signaling pathways is normal in muscle of type 2 diabetic subjects , the resistance to insulin stimulation may be due to impaired insulin signal transduction.

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Diabetes mellitus is taking its place as one of the most important diseases in the world.

Diabetes mellitus is a group of metabolic diseases characterized by hyperglycemia

resulting from defects of insulin secretion, insulin action, or both.

There are two main types of diabetes: type-1 and type-2, the worldwide diabetes

epidemic relates particularly to type-2 diabetes.

Type-2 diabetes is characterized by impaired insulin action

and/or abnormal insulin secretion.

Insulin resistance leads to profound decreases in glucose uptake and glycogen synthesis

in peripheral tissues. Insulin resistance yields to defective suppression of hepatic

glucose output.

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Resistance to the antilipolytic action of insulin also favors triglyceride breakdown

in adipose tissue and the generation of free fatty acids, which interfere with

insulin receptor signals.

It is now well accepted that defective postreceptor insulin signaling is the main

feature involved in insulin resistance of type-2 diabetes.

A number of molecules associated to insulin resistance such as free fatty acids,

interleukin-6, TNF-alpha, affect insulin receptor signaling, interestingly most

of them are related to adipose tissue.

Transcriptional factors such as peroxisome proliferator activated receptor

gamma (PPAR gamma) and peroxisome proliferator activated receptor

gamma coactivator 1 alpha (PGC-1 alpha) have also been found to be

associated with insulin resistance.

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Molecular mechanisms involved in the regulation of metabolism under normal conditions Molecular mechanisms of insulin secretion

Insulin secretion in response to glucose is a complex, multistep process that requires transport and oxidation of glucose, electrophysiological changes and fusion of insulin-containing secretory granules with the beta-cell plasma membrane.

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Inhibition of insulin signalingTermination of the insulin signal is critical for the maintenance of metabolic

control.They act to terminate insulin’s effects through activation of lipid or protein

phosphatases and through the induction of serine/threonine kinases that phosphorylate and uncouple various elements along the insulin signaling pathways .

Because Ser/Thr phosphorylation of IRS proteins is stimulated by insulin treatment and by inducers of insulin resistance, it suggests that the same Ser kinases might be utilized to phosphorylate the IRS proteins under both physiological and pathological conditions. Protein-tyrosine phosphatase IB (PTP1B), phosphatase and tensin homologue protein (PTEN) and SHIP2, which inactivate the lipid products of PK3I, are involved in the termination of insulin signaling.

Other negative feedback control mechanisms operate on a longer time scale, and involve a reduction in the cellular content of insulin receptor and its substrates.

Inhibition of insulin signal represents a new area of research that might shed light to the molecular mechanisms involved in the physiopathology of type-2 diabetes.

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Molecular mechanisms affected in type-2 diabetes Insulin resistance

At molecular level, elevated FFAs are associated with a reduction in insulin-stimulated

IRS-1 phosphorylation and IRS-1-associated PI3K activity.

Free fatty acids activate cellular kinases, including atypical protein kinase C isoforms by

increasing cellular diacylglycerol concentrations which can activate the inflammatory

kinase inhibitor kB (IKK) and c.jun Nterminal kinases, increasing serine/threonine

phosphorylation of IRS-1 and reducing downstream IRS-1 signalling.

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MUTATIONS IN IRS PROTEINSIn humans, rare mutations of the IRS-1 protein are associatedwith insulin resistance. Disruption of the IRS-1 gene in mice results in insulin resistance,

mainly of muscle and fat. Interesting results are obtained by studying Irs in knockout mice.

Heterozygous knockout mice lacking a single allele of IRS-1 gene lack any significant phenotype, whereas homozygous disruption of the IRS-1 gene results in a mild form of insulin resistance.

IRS-1 homozygous null mice (IRS-1-/-) do not show a clear diabetic phenotypic expression, presumably because of pancreatic β cell compensation.

IRS-2-/- mice, on the other hand, developed diabetes as a result of severe insulin resistance paired with β cell failure.

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Recent studies have made it apparent that serine phosphorylation of IRS proteins

can reduce the ability of IRS proteins to attract PI3-kinase, thereby minimizing its

activation , and can also lead to an accelerated degradation of the IRS-1 protein.

This serine phosphorylation in turn decreases IRS-1 tyrosine phosphorylation,

impairing downstream effectors.

Serine phosphorylation of IRS proteins can occur in response to a number of

intracellular serine kinases.

Recent studies have demonstrated hyper-serine phosphorylationof IRS-1 on Ser302, Ser307, Ser612, and Ser632 in several insulin-resistant rodent

models as well as in young lean insulin-resistant offspring of type 2 diabetic parents.

Further evidence for this hypothesis stems from recent studies in a muscle-specific triple serine to alanine mutant mouse (IRS-1 Ser → Ala302, Ser → Ala307, and Ser → Ala612), which has been shown to be protected from high-fat diet-induced insulin resistance in vivo.

Based on in vitro studies, serine phosphorylation may lead to dissociation between the insulin receptor/IRS-1 and/or IRS-1/PI3-kinase, preventing PI3-kinase activation or increased degradation of IRS-1. Furthermore, there are data linking IRS dysfunction in skeletal muscle to adipocyte biology and lipotoxicity.

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OTHER CAUSES OF INSULIN RESISTANCE

Mitochondrial dysfunction

It has been known for many years that severe mitochondrial dysfunction can result in diabetes.

it was found that mitochondrial density was reduced by 38%, intramyocellular lipid content was increased by 60% and serine phosphorylation of IRS-1 was increased by 50% in the young insulin-resistant offspring of type 2 diabetes parents.Adipokines

Insulin has three major target tissues-skeletal muscle, adipose tissue and the liver. Not only is IR overexpressed in the cells of these tissues, but these are also the three places where glucose is deposited and stored; no other tissue can store glucose.

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About 75% of insulin-dependent postprandial glucose disposal occurs into the skeletal muscle; it is therefore the major target organ. Patients suffering from insulin resistance and type 2 diabetes frequently display signs of abnormal lipid metabolism, increased circulatory concentration and elevated deposition of lipids in the skeletal muscle.

Increase in plasma FFA reduces insulin-stimulated glucose uptake, whereas a decrease in plasma lipid content improves insulin activity in the skeletal muscle cells, adipocytes and liver.

Studies have shown that raising plasma fatty acids in both rodents and humans abolishes insulin activation of IRS-1-associated PI3-kinase activity in skeletal muscle where IRS-1 is most prevalent.

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Rho-kinase and IRS-1 serine phosphorylationRho-kinase (ROCK) is a serine/threonine protein kinase identified as a GTP-

Rho-binding protein . There are two isoforms, ROCK1 (also known as ROKβ) andROCK2 (also known as ROKα). ROCK participates in the insulin signaling network by interacting with IRS-1 .

Our work has demonstrated that inhibiting ROCK decreases insulin-stimulated IRS-1- associated PI3K activity in adipocytes and myotubes.

This effect is mainly due to decreased tyrosine phosphorylation of the YXXM motif in IRS-1, which can lead to reduced interaction of IRS-1 with the p85 subunit of PI3K. Indeed, insulin-stimulated IRS-1 binding to the p85 regulatory subunit of PI3K is impaired in adipocytes expressing dominant negative ROCK .

By mass spectrometry analysis, we identified the serine residues of IRS-1 at serine 632/635, serine 936, and serine 972, all of which are phosphorylated by ROCK.

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ConclusionsClassical physiology studies, biochemical methods, tissue culture technology,

the use of gene targeting approaches in mice, and naturally occurring mutations in patients have shed some light on the molecular causes of type-2 diabetes and have contributed to a deeper understanding of the underlying mechanisms involved in the disease. As to what belongs the cure, it seems a most hard thing of this disease to draw propositions for curing, for this cause lies hid inside the molecular mechanisms of insulin signaling and insulin secretion.

Studies with the skeletal muscle of type 2 diabetic humans demonstrate impaired insulin activation of the IRS-1/PI3K/Akt signaling pathway, which is a critical step in the regulation of glucose transport in response to insulin. These defects are selectively restored by treatment with an insulinsensitizing agent and lifestyle changes, representing the core of insulin signaling components

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REFERENCES1.Kangduk Choi and Young-Bum Kim:Molecular Mechanism of Insulin

Resistance in Obesity and Type 2 Diabetes2. Patricia Joseph-Bravo:Molecular basis of type-2 diabetes3. Joseph C. Koster, M. Alan Permutt, and Colin G. Nichols:The ATP-Sensitive

K Channel (KATP) Connection4. Deborah M. Muoio and Christopher B. Newgard:Molecular and metabolic

mechanisms of insulin resistance and β‑cell failure in type 2 diabetes5. Receptor Tyrosine Kinases : Ahu Karademir & Andrei Vasiliev6. Assessing the potential of glucokinase activators in diabetes therapy:franz

m.matschinsky7.www.youtube.com8.Biochemy lehninger 9.Cell molecular lodish10.Biochemistry harper

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