15 - Diabetes

• 5 classes of receptors:

  1. Receptors linked with intrinsic enzyme activity
  2. Receptors link to protein kinases
  3. Receptors coupled to target proteins via a G protein
  4. Intracellular receptors
  5. Receptors that are ion channels
• Receptors linked with intrinsic enzyme activity. Some receptors are enzymes, binding of ligand activates the enzyme activity. The prototype for this group is the insulin receptor (IR).
• Blood glucose levels are ~4.5mM. The combined action of hormones are required for regulation:

  1. insulin: pancreatic: lowers blood sugar levels
  2. glucagon: pancreatic: raises blood sugar levels
  3. epinephrine: adrenal: raises blood sugar levels
  4. cortisol: adrenal: raises blood sugar levels
• Anatomy of the pancreas:

  Gall bladder feeds into the duodenum which feeds into the pancreas.

  The pancreas contains a pancreatic duct which have many acini on the ends.

  Acini are comprised of acinar cells.

  Acini surround the Islet of Langerhans which is comprised of A, B and delta cells.
• Acinar cells have digestive functions
• The Islets of Langerhans secrete hormones:

  A cells: glucagon

  B cells: insulin

  Delta cells: somatostatin
• How the insulin receptor is made:

  Following translation the receptor is inserted into the ER, they accumulate and associate into dimers. They are exported through the Golgi where they are modified by proteolytic cleavage into a (extracellular) and b (intracellular) subunits. At the plasma membrane the recpetor is a trans-membrane protein.
• a subunits and b subunits in the insulin receptor are linked by a disulphide bond
• Insulin signalling starts at the plasma membrane: insulin binding stimulates an allosteric change in IR, bringing the cytosolic domains close, allowing activation.
• First messenger/primary messenger/ligand:

  an extracellular substance (for examples, the hormone epinephrine or the neurotransmitter serotonin) that binds to a cell-surface receptor and initiates signal transduction that results in a change in intracellular activity
• Receptor:

  a protein that binds and responds to the first messenger.  Receptors may be either displayed at the cell-surface (e.g. IR, EGFR, GPCRs) or may be intracellular.
• INSULIN SIGNALING GROWTH:

  1. Activated IR phosphorylates and activates the insulin receptor substrate-1 (IRS-1)
  2. Activated IRS-1 is bound by the adaptor molecules Grb2 and Sos.
  3. Sos converts inactive (GDP-bound) Ras to active (GTP-bound) Ras
  4. Activated Ras recruits Raf kinase to the membrane and activates its protein kinase activity. RAF phosphorylates and activates MEK kinase. MEK phosphorylates and activates mitogen-activated protein kinase (MAPK).
  5. Activated ERK migrates to the nucleus:  turns on insulin genes, as well as causing expression of the cyclins and CDKs required for cell division.
• Sos is a guanine nucleotide exchange factor (GEF)
• the signal has been transduced from the cytosolic face of the plasma membrane and amplified across the cytosol through a MAPK cascade
• The adaptors Grb2 and Sos are common to both EGF and insulin signalling, so activation of EGFR and IR recruits the same MAPK cascade which means the same genes are modulated in the downstream response.
• INSULIN SIGNALING IN GLUCOSE REGULATION:

  • IRS-1 is bi-functional.

  It also recruits and activates phosphoinositide 3-kinase (PI-3K) to the cytosolic face of the plasma membrane

  • PI-3K phosphorylates the membrane lipid phosphotidylinositol 4,5 bisphosphate (PIP2) to produce phosphotidylinositol 3,4,5 trisphosphate (PIP3). Which is a second messenger.
  • PIP3 recruits PDK1 (PIP3-dependent protein kinase).
  • PDK1 activates protein kinase B (PKB).
• Second messenger:

  a small metabolically unique molecule, not a protein,  whose concentrations can change rapidly. Second messengers relay signals from receptors to target molecules in the cytoplasm or nucleus.
• Insulin is a growth factor:

  •Phosphorylation of IRS-1 amplifies the signal.

  •Adaptors recruit and activate Ras.

  •Signal transduction via an amplifying MAPK cascade

   Response: Gene expression changes 
• Insulin is a glucose regulator:

  •Phosphorylation of IRS-1 amplifies the signal.

  •Signal propagation and amplification via conversion of membrane lipids.

  •Amplification via a lipid dependent kinase and activation of PKB.

  Responses: Upregulation of glucose entry into cells and upregulation of glycogen production
• Growth and glucose metabolism can be co-ordinated via insulin signalling, because there is a common intermediate.

  IRS-1.

  This is bcs there is not much point growing of no food supply.
• Ras independent: altered metabolism

  Ras dependant: growth signals
• Terminally-differentiated cells do not respond to the growth signal because of loss of signalling chain components. Fibroblasts regard insulin as a growth factor as they are involved in mitogenesis.
• Cell response to insulin at ~10-9-10-10 M

  within minutes:

  •increased uptake of glucose into muscle cells and adipocytes

  •altered glucose metabolism by modulation of enzyme activities

  within hours of continuous ~10-8 M exposure to insulin:

  •increased expression of liver enzymes that synthesise glycogen

  •increased expression of adipocyte enzymes that synthesise triacylglycerols

  •increased expression of genes involved in mitogenesis in some cell lines
• In diabetes the Ras-independent pathway is terminated.

  A PIP3-specific phosphatase (PTEN) removes the phosphate at the 3 position of PIP3 to convert it into PIP2.

  PDKI and PKB can no longer be recruited to the plasma membrane, shutting off signalling through PKB.
• Type I, Insulin-dependent diabetes mellitus (IDDM)

  •caused by deficiency in insulin production

  •early onset

  •responds to insulin injection
• Type II, Non-insulin-dependent diabetes mellitus (NIDDM), or Insulin-resistant diabetes

  •typically late onset

  •associated with obesity

  •failure to respond to insulin
• Characteristic symptoms of both types of diabetes mellitus:

  •excessive thirst

  •frequent urination (polyuria)

  •excretion of large amounts of glucose in the urine (glucosuria)
• In muscle and adipose tissues, activated PKB stimulates the movement of the glucose transporter GLUT4 from internal membrane vesicles to the PM, increasing uptake of glucose.
• PKB also mediates the conversion of excess glucose to glycogen (in the liver /muscles) and to triacylglycerols (in adipose tissue)
• Diabetic failure to produce insulin or to respond to insulin results in high blood sugar levels.
• Lim et al, (2011) - Reversal of type 2 diabetes: normalisation of beta cell function in association with decreased pancreas and liver triacylglycerol.

  Patients increased their exercise levels and ate a restricted diet of ~600 kcal per day for 2 months.

  In all cases, weight loss was accompanied by

  •   a possible rewiring of the brain
  •   a reduction in diabetic symptoms
  •   a restoration of insulin sensitivity

  This suggests, at least in part, that desensitisation of insulin responses follows prolonged exposure to high sugar levels