Insulin Signaling

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Insulin Signaling

• Type 2 Diabetes
• And
• Insulin Resistance

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Insulin

• Pancreatic hormone
• Essential for growth development, and control
  of energy metabolism
• Homeostasis of
• Glucose metabolism
• Lipid metabolism
• Protein metabolism
• Examples
• translocation of GLUT4 vesicles to plasma
  membrane
• stimulation of glycogen and protein synthesis
• initiation of gene transcription
• Signals the "fed state" promoting storage of
  carbohydrates and fats.

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Adipose Tissue as an Endocrine Organ

• Secretion of 'adipocyte factors' involved in
  regulation of metabolism.
• Leptin - polypeptide associated with appetite
  control system.
• Adiponectin and resistin - peptide hormones
  associated with insulin resistance.
• Dysfunctions in adipose metabolism can induce
  insulin resistance in muscle and liver.
• About 75 of glucose uptake occurs in skeletal
  muscle adipose tissue accounts for only a small
  fraction of glucose disposal. Yet
• Mouse KO of muscle insulin receptor shows normal
  glucose tolerance.
• Mouse KO of adipose insulin-sensitive glucose
  transporter show impaired glucose tolerance.

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• Insulin Receptor
• (a2b2) subunits
• a-subunits suppress b activity.
• Insulin binds to the a-subunits and un-suppresses
  b activity.
• Intracellular portion of b subunit is a tyrosine
  kinase

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Targets of the Insulin Receptor

• Insulin Receptor Substrates (IRS proteins IRS-1,
  IRS-2, IRS-3, IRS-4)
• Shc proteins
• Cbl / CAP proteins
• Gab proteins

All of these proteins undergo tyrosine
phosphorylation by the activated insulin receptor
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IRS Proteins and Shc Proteins

• Major targets of IR
• Mediate metabolic functions and growth-related
  functions
• Activate two major kinase cascades
• Phosphatidylinositol-3-kinase (PI3K) cascade
• Mitogen-activated protein kinase (MAP kinase)
  cascade

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IRS Proteins

• Act as docking sites for SH2 proteins
• Studies show
• IRS-1 has more mitogenic activity than IRS-2
• IRS-2 knockout cells are defective in
  insulin-stimulated glucose transport
• Roles of IRS-3 and IRS-4 are less defined

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SH2 ProteinsProteins that contain src-homology
region (domains similar to that found in src
protein).

• SH2 proteins bind phospho-Tyr residues with high
  affinity.
• Show a variety of functions
• Some are kinases
• Some are phosphatases
• Some activate G-proteins (exchange GTP for GDP)

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MAP Kinase Pathway
Shows the redundancy in the signaling mechanism
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Phospholipids can generate second messenger
molecules
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The Phosphatidylinositol-3-Kinase Pathway
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PI3K Pathway

• The 3-phosphorylated lipid activates
  phosphatidylinositol-dependent kinase-1 (PDK1)
• This initiates a signal cascade causing

Glycogen synthesis
Activation of Factors affecting Cell growth
differentiation
Translocation of GLUT4 to Plasma membrane
Glucose handling
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Cbl / CAP Signal Cascade Cbl protein and
Cbl-associated protein (CAP)

• Function through lipid rafts and caveolea in the
  plasma membrane
• Activates GLUT4 (translocation to the membrane)
• Mediates glucose transport in a PDK1 independent
  manner

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Regulation of Insulin Signaling

• There are multiple levels and mechanisms of
  regulation
• Autoregulation downstream enzymes inhibit
  upstream elements in the same pathway (feedback
  inhibition).
• Unrelated Receptor Pathways molecular signals
  that inhibit insulin signaling (desensitization).

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IRS Proteins canPropagate the Insulin
SignalandProvide Feedback Regulation

• Such Regulation Involves
• Proteosome-mediated degradation
• Phosphatase-mediated dephosphorylation
• Serine/Threonine (Ser/Thr) phosphorylation

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Positive and NegativeFeedback Control

• The insulin receptor and IRS proteins contain
• Tyrosine phosphorylation sites
• Provides mechanism of insulin signaling
• Serine/Threonine phosphorylation sites
• Provides modulation of insulin signaling
• Different Ser/Thr sites on the protein serve
  different purposes.

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Different Ser/Thr sites on the IRS Protein Serve
Different Purposes

• Block tyrosine phosphatases Effect?

Propagation of insulin signal (positive
feedforward control)

• Decreases tyrosine phosphorylation Effect?

Attenuates or terminates the signal (negative
feedback control)
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Effectors of Insulin Action

• Free fatty acids and tumor necrosis factor-?
  (TNF?)
• Activate Ser/Thr kinases that lead to
  downregulation of IRS.
• Terminates insulin signal or induces insulin
  resistance
• Expression of (TNF?) is increased in obese
  rodents and humans causing insulin resistance

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More Effectors of Insulin Action

• Mammalian target of rapamycin (mTOR)
• Enhances phosphorylation of Ser residues on
  IRS-1, which inhibits tyrosine phosphorylation of
  IRS-1 (Effect?)

Negative feedback inhibition Inhibits ability of
IRS-1 to bind PI3K
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More Effectors of Insulin Action

• Protein Kinase C Family (PKC? and PKC?) and I?B
  kinase-? (IKK?)
• IKK? binds to PKC? and regulates its function
• TNF? has also been shown to activate PKC?

Mediates phosphorylation of IRS proteins
Dissociation of IR-IRS complex
Inhibits Try phosphorylation
Terminates insulin signal
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More Effectors of Insulin Action

• c-jun-N-terminal kinase (JNK)

Phosphorylates a Ser residue on IRS-1
Inhibits Tyr phosphorylation of IRS-1
Insulin resistance
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c-JUN
IKK?
TNF?
mTOR
PKC?
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More Effectors of Insulin Action

• Protein tyrosine phosphatases (PTPase)

Dephosphorylates protein Tyr residues and
attenutates insulin action

• PTP1B knockout mice show increased Tyr
  phosphorylation of IR and IRS proteins and
  improved insulin sensitivity.
• KO mice are resistant to diet-induced obesity.
• PTP1B is a potential therapeutic target in
  diabetes and obesity.

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More Effectors of Insulin Action

• SHIP2
• a protein phosphatase that dephosphorylates
  phosphatidylinositol-3-phosphates

Interrupts or attenuates the insulin signaling
cascade
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The Phosphatidylinositol-3-Kinase Pathway
SHIP2 can dephosphorylate at 3 position
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SHIP2 lowers conc. of PI-3-phosphates and
counteracts activation by PI3K
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Insulin Resistance
Associated with type-2 diabetes, cardiovascular
disease, obesity, hypertension, and chronic
infection.

• Causes
• Genetic factors polymorphisms in the multiple
  genes encoding proteins involved in insulin
  action (likely factor in type-2 diabetes).
• Increased activity of lipid or protein-Tyr
  phosphatases (PTPs) that interfere with insulin
  signaling.
• Ser/Thr phosphorylation of IRS proteins --
  uncouples IRS from upstream and downstream
  effectors.

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Obesity and Insulin Resistance

• Elevated FFAs are characteristics of obesity,
  insulin resistance, and type-2 diabetes.
• FFAs are inhibitors of insulin action
• Possible mechanism
• Obesity and high fat diet activates IKK?

Mediates Ser/Thr phosphorylation via PKC isoforms
Inactivates IRS-1
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Potential Molecular Targetsof Insulin Resistance

• PKC?, mTOR, IKK? (inhibitors of these)
• Peroxisome proliferator-activated receptor-?
  (PPAR?)
• Thaizolidinedione class of antidiabetic drugs
  (i.e., rosiglitazone) are activators of PPAR?
• PPAR??
• Abundant in adipose tissue.
• Modulates lipid metabolism in adipose tissue,
  which may affect the overall profile of
  circulating lipids, and stimulate processing of
  lipids/glucose by muscle and liver.
• Increases expression of CAP

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