Diabetes Flashcards
(22 cards)
Insulin: discovery, structure, signalling (draw), how its secreted
Discovered in 1921
One of most important discoveries in pharmacology
Previously diabetes meant death
Insulin:
51 aa
Dimer of B and (shorter) A chain that are held together via interchain disulphide bonds
Signals by binding the RTK insulin receptor (on target cells; liver, muscle adipose tissue) alpha subunits, causing dimerization, beta subunit autophosphorylation, leading to phosphorylation of downstream signalling -
Insulin receptor substrate; IRS phosphorylation leads to stimulation of PI3K leading to phosphorylation of PDK1 then PKC (regulates glucose uptake, glycogen/lipid/protein synthesis), then phosphorylation of GSK3 (inhibits) mTOR (stimulates for protein synthesis), and JNK)
Shc/Ras leads to phosphorylation of ERK1/2 and survival pathways
Negative feedback loop of PKC, JNK, ERK1/2 phosphorylating S on IRS1, turning off
Glucose transported into pancreatic beta cell via GLUT-2 (and GLUT-1 in humans)
Glucose metabolization via glycolysis and citric acid cycle increases ATP/ADP ratio
ATP dependent K+ channels close causing membrane depolarisation
Ca2+ voltage dependent channels open, causing influx into beta cell
Triggers exocytosis of stored insulin vesicles
First phase of a rapid release of pre-stored insulin, then a slower, sustained release involving new synthesis and secretion
Augmentation of insulin secretion (GSIS) through gastrointestinal hormones incretins - secretion of Glucagon-like peptide-1 (GLP1) in response to food intake and glucose dependent insulinotropic peptide (GIP) in response to glucose/fat intake, both to increase insulin secretion in a glucose dependent manner
Ghrelin secretion: inhibits insulin secretion
Effects of insulin secretion and metabolic consequence of type 2 diabetes
Peripheral effects of insulin: most classical effects
Skeletal muscle: promotion of glucose uptake via GLUT4
Liver: prevents gluconeogenesis, increases glycogen synthesis
White adipose tissue: suppresses lipolysis and increases lipogenesis
Central effects of insulin: through brain the peripheral effects are augmented
Muscle and liver: whole body insulin sensitivity improved
Adipose tissue: decreases lipolysis and increases storage
Food intake: Suppresses appetite to reduce food intake
Metabolic consequence of insulin resistance:
Pancreas: hypersecretion of insulin then decreased in later insulin resistance with more beta cell apoptosis, less beta cell mass, hyperglucagonemia
Adipocytes: more circulating fatty acids, hyperlipidemia
Muscle: insulin resistance
Liver: insulin resistance, more hepatic glucose output
Gut: impaired incretin effect
Type 2 diabetes: prevalence, how it develops, risk
Affects millions in UK and costs NHS £10bil on diabetes and diabetes related disease
Develop insulin resistance in peripheral tissue (mainly liver, adipose tissue, skeletal muscle) preventing glucose uptake and energy storage
Loss of pancreatic beta cell function
More insulin produced to compensate and over time when this becomes inefficient patient is diagnosed as type 2 diabetes
Characterised by high blood glucose level
Obesity is around 80% of someone’s risk for developing Type 2 diabetes - beta cells increase in size and increase more insulin to compensate for metabolic load. Higher BMI means increased load on beta cells and so more insulin produced
Complications of Type 2 diabetes
Range of health issues in patient
Microvascular complications:
Diabetic retinopathy - leading cause of blindness in working-age adults
Diabetic nephropathy - leading cause of end-stage renal disease
Diabetic neuropathy - leading cause of nontraumatic lower extremity amputations
Macrovascular complications:
Stroke - 2-4fold increase in cardiovascular mortality and stroke
Heart disease
Peripheral vascular diseases
Strategies to treat T2D
Drugs must be in conjunction with change in diet and exercise, and requires regular monitoring, and ongoing care - eye exams, blood pressure, blood glucose and cholesterol monitoring, etc
Need safer, and better drug treatments, though lots on the market
-Reduce obesity
-Improve pancreatic function/increase insulin secretion
GLP1 receptor agonist drugs (supresses appetite for weight loss; ozempic)
TZD drugs (activates PPARgamma to improve insulin sensitivity and increase fat storage in adipose tissue lowering blood glucose levels)
Islet cell transplant for severe cases, expensive
-Improve skeletal muscle insulin sensitivity
Exercise encourages glucose uptake by muscle cells lowering blood sugar
TZDs
Metformin improves insulin sensitivity and helps muscles better absorb glucose
-Increase incretin release from the gut
Drugs targeting GLP1 developed (ex. semaglutide agonist) and are safe and effective, was very difficult to develop
-Reduce inflammation
Association between increasing inflammation (particularly in adipose from infiltrating macrophages) with insulin resistance and onset of diabetes
-Dietary change
Reason why people become obese
Fish oil omega 3 supplements to improve metabolic health
Dietary fibre, it’s fermentation in gut produces short chain fatty acids (anti-inflammatory effects and improve insulin sensitivity)
Fatty acids: types, length, receptors
Recommended dietary fat is 20-35% of energy intake (majority is LCFA)
Saturated fatty acids (SFA) from animal fat and tropical oils
Short chain fatty acids (SCFA, 2-6 C) from fermentation of fiber by gut microbiota
Medium (MCFA, 7-12 C)
Long (LCFA, 13-22 C) from fish, animal fat, vegetable oil
Very long: >22 C
Monounsaturated fatty acids (MUFA): Oleic palmitoleic
Polyunsaturated fatty acids (PUFA): Omega-3, 6, 9
-Receptors
Long chain FA: FFA1 (GPR40), FFA4
Short chain FA: FFA2, FFA3
FFA1 and FFA4 expression profile and function for each
FFA1 expression profile:
Mostly expressed in pancreas beta and alpha cells
Enteroendocrine L, I and K cells (produce GLP1, CKK and GIP respectively)
Skeletal muscle, Heart, Liver, Bone, Brain, Monocytes
FFA4 expression profile:
Lower intestine (enteroendocrine cells): L cells secreting GLP1 and I cells secreteing CKK
Not pancreatic beta islets: low/negligible
Pancreatic alpha: Increases glucagon secretion
Pancreatic delta: Inhibits somatostatin secretion
Adipose tissue: anti-inflammatory effect, insulin sensitisation (increases glucose uptake)
Taste buds: fatty tase perception
Intestinal epithelial cell: nutrient sensing
Macrophages: anti-inflammatory
Hypothalamus: decrease appetite and weight
FFA2 and FFA3 expression profile, signalling conjugated
Both: immune cells and gut and pancreatic beta cells and adipose (although for FFA3 controversial)
FFA2: Gq, Gi and Arrestin
Highly expressed in monocytes and polymorphonuclear cells
FFA3: Gi
Least well charecterised
FFA1: Signalling pathway, ligands, ligand effects
FFA1 receptor/GPR40
Signalling:
Activated by MCFAs, LCFAs
Highest potency for long chain PUFA
Can couple to Gq (primary pathway), i and s in a ligand dependent manner
Little evidence it can recruit arrestin (desensitisation and internalisation of GPCRs to regulate cycling of FFA1)
FFA1 contributes 50% of total effect of LCFAs on enhancing glucose stimulated insulin secretion (GSIS)
Chronic effect inhibits GSIS and increase lipotoxicity (TAK-875 induced liver toxicity)
However recent evidence shows FFA1 is protective of beta cell (reduces apoptosis or enhances function under stress)
Previous evidence indicating lipotoxicity may be due to non-selective lipid toxicity due to chronic exposure of LCFA (regardless of receptor involvement)
Agonists:
Synthetic agonists: TAK-875 and AMG-837 (partial agonists), AMG-1638 (full agonist) which are all ago-allosteric modulators (amplify agonistic activity of endodgenous ligand alpha linolenic acid)
All bind FFA1 to increase Gq signalling and calcium driven augmentation of insulin secretion in pancreatic beta cell
AMG-1638 also increases insulin secretion and increases incretin release (GLP1 and GIP) through activation of Gs in full agonism (dual effect), so larger insulin secretion
Synthetic antagonists also available
Incretin: secreting cells, target, effect,
Gut derived hormones secreted from enteroendocrine L, I and K cells (produce GLP1, CKK and GIP respectively) into blood within mins of eating to regulate insulin secretion
GLP1
Targets pancreas to augment insulin secretion and decrease glucagon secretion from pancreas (which then decreases glucose production in liver), decreases gastric emptying in stomach and increases satiety in brain
FFA1: current drugs on market, future of agonist development
Current drugs:
TAK-875 reduced hyperglycemia and improved GSIS in type 2 diabetic patients
Reached phase 3 clinical trials but found it induced liver toxicity
Excessive FA in liver (steatosis) disrupts normal function (insulin resistance caused) and can exacerbate conditions like non-alcoholic fatty liver disease; NAFLD
No current FFA1 drugs on the market as others withdrawn due to concerns on liver toxicity and safety profile long term
Withdrawing the drug from clinical trials allowed research to be released for public access, so lots known
Future:
FFA1 potentially has multiple allosteric binding sites to target as therapeutic agent
Understand how different FFA1 ligands potentially interact with each other to consider the effect of drug in the presence of high endogenous FA
Unclear how natural ligands bind so further research to clarify the most effective way to target
Understand mechanism behind hepatoxicity to produce safer drugs (drug chemical structure, not receptor activation, is causing toxicity since larger molecules are more toxic in metabolism)
FFA4: activators, signalling, structure, variants, KO studies
FFA4 (GRP120)
Activated by wide range of LCFA (particularly PUFAs)
Effect of FFA4 is context dependent:
In diet induced obesity mice models, FFA4 is upregulated
Mixed data in FFA4 adipose expression reported in obese individuals (up or down regulation)
High fed FFA4 KO mice developed insulin resistance (suggesting FFA4 protects against diet induced dysfunction) and omega-3 FA (ex. DHA a PUFA) failed to improve insulin resistance (improved in WT)
However other studies show no significant effect of FFA4 KO on insulin sensitity in high fed mice, yet omega-3 improved insulin sensitivity (suggesting FFA4 is not essential for the metabolic effect)
Studies vary in mice strains, housing conditions, microbiome, diet composition, compensatory pathways (others ex. receptors; FFA1, PPAR, etc may contribute to omega 3 effects) etc
Gq - promotes GLP1 secretion
Gi - inhibits ghrelin secretion
Arrestin (key component) - anti-inflammatory effects
Splice variants:
Humans - short (most commonly expressed) and long (16aa insert in ICL3) forms of FFA4
Rodents - short form
Short form - activate Gq and Arrestin mediated pathways
Long form - preferentially recruits beta-Arrestin
May have limited tissue expression (predominantly colon). Physiological relevance not fully understood
FFA4: activation effect, KO effects
In pancreatic alpha-cells - increase glucagon secretion (elevates blood glucose)
In pancreatic delta cells - inhibits release of somatostatin (inhibitor of glucagon and insulin release)
Ability of islets to secrete insulin unaffected by FFA4 KO
However, FFA4 KO studies indicate a cytoprotective role for FFA4, aiding in inhibition of lipotoxicity
Anti-inflammatory in macrophages: FFA4 activation by DHA recruits beta-arrestin which interacts with TAK1 binding protein (TAB1) preventing it from interacting with TAK1 (Transforming Growth Factor Beta Activated Kinase 1), blocking TAK1 activation (typically activated by LPS and TNFalpha) and suppressing its downstream inflammatory pathways (NF-κB and JNK) that would induce insulin resistance in adipocytes
Ghrelin: function, expression profile, regulation
Circulating appetite stimulant
Produced predominantly in endocrine cells (alpha cells) of human gastric mucosa lining the stomach and proximal small intestine
Rise in ghrelin before each meal (signal for meal initiation), decrease after eating
Simulates GHSR to increase food uptake, body weight, insulin resistance, and decrease insulin release
Inhibited by FFA4
FFA1 synthetic ligand binding, key residues, importance
Ligand binding:
Earliest/most charecterised is TAK-875: carboxylate interacts with two Arg. Asparagine formed H bond with one of the Arg to position the residue within the binding pocket
Allosteric binding protruding into the lipid environment of PM (between TM3 and TM4) - similar binding to analogous receptor S1P to the S1P1 receptor (between TMV7 and TM1)
Key moment in FFA research was understanding how TAK-875 bound to improve efficacy, potency, and understand in general how FFA ligands bind (via PM)
Another binding site between TM4 and TM5
Adipocyte differentiation, regulation, experimental evidence
MSCs differentiated into preadipocytes and PPARgamma activation drives into adipocyte
FFA4 has important role in progression - antagonist and siRNA KO mice reduces preadipocyte differentiation into adipocyte and inhibits PPARgamma expression
Agonist in vitro causes slight increase
Measured with Oil red O staining, and ex. RT-qPCR
Characteristics of adipose tissue in diabetes
Macrophages increases their infiltrate adipose tissue as obesity progresses into chronic obesity (associate with diabetes)
Switches from M2 (anti-inflammatory phenotype) into M1 (pro-inflammatory phenotype)
Enlarged adipocytes from excess lipid storage
FFA signalling in adipocytes
In adipocytes, FFA4 activates Gq to increase glucose uptake and inhibit lipolysis
Beta-adreno agonist (isoprenaline) with FFA4 agonist, the Gq pathway will produce PDE3B and inhibit the cAMP produced (inducer of lipolysis)
Short chain FA: receptor, source, involvement in diabetes
SCFA receptor: FFA2 and FFA3
Modulation of gut bacteria (trillions, 1200 different species) researched to improve human health; probiotics, diet, etc
Main source of SFA is the fermentation of non-digestible carbohydrates (dietary fibre) by gut microbiota (ex. propionate, acetate)
Metabolism of ethanol in liver (acetate)
[draw]
SCFA receptors mediate:
Energy regulation
Inflammatory responses
Gut motility
Appetite and glucose homeostasis
So associated with obesity/T2D
FFA2 and FFA3: natural and synthetic ligands
SCFA: propionate, acetate, butyrate
Orthosteric and allosteric agonists and antagonists (ex. compound 6 is an allosteric antagonist for FFA3, and compound 1 is an orthosteric agonist for FFA2)
Often specific for mice or humans
Endogenous ligands for both have an overlap in their preferance:
FFA2: C3/C4/C2 > C5 > C6 > C1
FFA3: C3/C4/C5 > C6 > C2 > C1
