1. Insulin Resistance and Obesity
Orla Mahony
Sept 2013

2. Insulin resistance and obesity
Insulin signaling
Definitions; insulin resistance, T2DM
Hypotheses for why IR develops in obesity
Unifying hypothesis
Measuring insulin resistance
IR in the liver/hepatic steatosis
Metabolic syndrome
Other theories of IR: TNFα, adiponectin, leptin
Veterinary literature

3. Insulin Signaling
In muscle insulin binds to R, activates tyrosine kinase, phosphorylates and activates IRS1 (insulin receptor substrate 1)
IRS1 activates PI3K (1 phosphatidylinositol 3- kinase), which activates Akt2, which phosphorylates and inactivates AS160, a protein that prevents translocation of GLUT4
Insulin thus promotes docking and fusion of GLUT4 vesicles to the plasma membrane

4. Figure 2. Mechanisms of insulin sensitivity and resistance in muscle and liver(A) Insulin-sensitive muscle. (B) Insulin-resistant muscle. (C) Insulin-sensitive liver. (D) Insulin-resistant liver. IRS=insulin-receptor substrate. IR=insulin receptor. PI3K=1-phosphatidylinositol 3-kinase. GLUT4=glucose transporter 4. DAG=diacylglycerol. PKC=protein kinase C. Ser=serine. Thr=threonine. FOX01=forkhead box O1. FOXA2=forkhead box A2. G6P=glucose-6-phosphate. GS=glycogen synthase. GSK=glycogen synthase kinase. Green circle with plus sign represents activation. Red circle with minus sign represents inactivation. Solid line with arrowhead represents increase or accumulation of substrate. Dotted line indicates inhibition of pathway.

5. Insulin resistance
Insulin resistance is defined as a subnormal glucose response to both endogenous and exogenous insulin

6. Insulin resistance in people has been seen in association with
Inherited disorders e.g. lipodystrophy
Secondary to obesity, drugs, stress, infection, acromegaly, glucocorticoid excess, pregnancy
Impaired glucose tolerance and type 2 DM
Hypertension, hyperlipidemia, polycystic ovary disease, coronary heart disease; MOA of insulin resistance unknown
Lipodystrophy; no peripheral fat stores

7. Consequences Type 2 DM Cardiovascular disease
Neoplasia (colon, breast and endometrial cancers associated with obesity and IR)

8. T2DM
T2DM is characterized by hyperglycemia, impaired insulin secretion and insulin resistance
It can arise through genetic and environmental influences
IR is characterized by impairment of insulin-mediated translocation of GLUT4 to the muscle sarcolemmal membrane

9. Hypotheses for why IR develops in obesity
Accumulation of fatty acid metabolites (DAG) within insulin-responsive tissues
Obesity is a proinflammatory state with increases in TNFα and IL6, and activation of PKC’s and serine phosphorylation of IRS1
Changes in adipokines and lipokines

10. Link between lipid and IR
Tyrosine phosphorylation of IRS1 and activation of PI3K impaired in rodent models of IR and in muscles of volunteers given lipid infusions, implying a lipid-induced reduction in insulin-stimulated glucose transport is due to a defect in insulin signaling
PKC serine threonine kinases are the link between lipid accumulation and impaired insulin action

11. In obesity, delivery of fatty acids overwhelm capacity of cells to oxidize them or convert DAG’s to TG’s (via diacylglycerol acyl transferase 1, DAGT1)
DAG activates protein kinase C serine-threonine kinases which lead to serine phosphorylation of IRS1, preventing it from interacting with insulin R and GLUT4 and taking up glucose

12. Figure 2. Mechanisms of insulin sensitivity and resistance in muscle and liver(A) Insulin-sensitive muscle. (B) Insulin-resistant muscle. (C) Insulin-sensitive liver. (D) Insulin-resistant liver. IRS=insulin-receptor substrate. IR=insulin receptor. PI3K=1-phosphatidylinositol 3-kinase. GLUT4=glucose transporter 4. DAG=diacylglycerol. PKC=protein kinase C. Ser=serine. Thr=threonine. FOX01=forkhead box O1. FOXA2=forkhead box A2. G6P=glucose-6-phosphate. GS=glycogen synthase. GSK=glycogen synthase kinase. Green circle with plus sign represents activation. Red circle with minus sign represents inactivation. Solid line with arrowhead represents increase or accumulation of substrate. Dotted line indicates inhibition of pathway.

13. Defect in insulin signaling
Unifying hypothesis for IR in obesity, lean healthy offspring of T2DM parents, ageing, and disorders such as congenital and acquired lipodystrophy
Increases in intramyocellular lipid content occurs in lean offspring of T2DM patients, and in elderly, maybe because of decreased lipid oxidation and a reduction in mitochondrial content

14. Exercise increases DAGT1 expression in muscle and is associated with increased TG’s, reduced DAG and improved insulin sensitivity
Reductions in DAG also occur with thiazolidinedione therapy

15. Measuring insulin resistance
Euglycemic insulin clamp technique is gold standard
IV glucose tolerance test and the insulin tolerance test/insulin suppression test also used most frequently in research setting
Impractical for clinical use
Glucose to insulin ratios and homeostasis model assessment of insulin resistance (HOMA) used in large population epidemiologic studies

16. Limitations to HOMA & insulin ratios
Changes in beta cell function over time
Lack of standardized insulin assay
No data demonstrating that markers of IR predict response to treatment

17. IR estimation
In nondiabetic, normotensive overweight individuals, serum TG concentration, ratio of TG to HDL cholesterol concentration and fasting insulin concentration used clinically to identify people who may be insulin resistant

18. Hepatic steatosis Non alchoholic fatty liver disease
Most common cause of chronically inc LE’s
Closely assoc with obesity, IR, T2DM
In liver, insulin binds R, activates TK, phosphorylates IRS1 and IRS2, activates PI3K and Akt2, promoting glycogen synthesis and inhibiting gluconeogenesis

19. Effect of IR on the liver
IR in muscle, impairs storage of CHO as glycogen, CHOs redirected to liver for use in de-novo lipogenesis
Increases in DAG activates PKC’s, leading to impaired tyrosine phosphorylation of IRS1 and IRS2 by the insulin R, ultimately impairing ability of insulin to activate hepatic glycogen synthesis and suppress hepatic glucose production

20. Figure 2. Mechanisms of insulin sensitivity and resistance in muscle and liver(A) Insulin-sensitive muscle. (B) Insulin-resistant muscle. (C) Insulin-sensitive liver. (D) Insulin-resistant liver. IRS=insulin-receptor substrate. IR=insulin receptor. PI3K=1-phosphatidylinositol 3-kinase. GLUT4=glucose transporter 4. DAG=diacylglycerol. PKC=protein kinase C. Ser=serine. Thr=threonine. FOX01=forkhead box O1. FOXA2=forkhead box A2. G6P=glucose-6-phosphate. GS=glycogen synthase. GSK=glycogen synthase kinase. Green circle with plus sign represents activation. Red circle with minus sign represents inactivation. Solid line with arrowhead represents increase or accumulation of substrate. Dotted line indicates inhibition of pathway.

21. Omega 3 Fatty Acids Not all fats are bad
Docosahexaenoic acid and eicosapentaenoic acid bind to GPR120 , a cell surface receptor abundant on adipocytes. GPR influences metabolism and reduced activity is assoc. w inflammation, weight gain and poor glucose control
Omega 3 FA’s may promote FA oxidation through activation of PPAR’s, increasing adiponectin, and preventing DAG accumulation

22. Metabolic syndrome/ insulin resistance syndrome
Metabolic syndrome is the co-occurrence of risk factors for T2DM and CVD
Genetic predisposition, lack of exercise and body fat distribution determine whether an obese individual will develop T2DM or CVD
Diagnosed based on
Overall obesity (increased BMI)
Abdominal obesity
Increased BP
Increased fasting glucose and TG levels,
Low HDL

23. Other theories of Insulin Resistance
Proinflammatory state associated with release of TNFα and IL6, activation of PKC and increased serine phosphorylation of IRS1
Effects of adipokines centrally and peripherally

24. TNF-α
Inflammatory cytokine secreted by macrophages, mast cells, neuronal cells, fibroblasts, and adipocytes
Necrosis and apotosis of adipocytes attracts monos and macs leading to inc. TNF α
Causes localized insulin resistance by activating PKC and serine phosphorylation of IRS1

25. TNF-α Impairs TG storage, induces fat lipolysis
High conc’s of serum FFA’s diminish insulin sensitivity in perhipheral tissues
TNF-α is inversely correlated with adiponectin and alters its gene expression

26. The role of key adipokines in obesity and insulin resistance in cats
Adipose tissue is an endocrine organ
Adipokines are proteins, hormones and cytokines derived from adipose tissue
Of over 100, leptin, adiponectin, TNFα best known
Lusby et al JAVMA 2009:325;

27. Leptin Leptin regulates body fat through satiety and energy metabolism
As fat increases, leptin secretion from adipocytes increases
Leptin acts on arcuate nucleus of the hypothalamus, inhibits neurotransmitters that increase food intake and lower energy expenditure and activate neurons that suppress appetite and increase energy expenditure (this may indirectly improve insulin sensitivity)

28. Leptin
Obese animals have highest leptin levels because of leptin resistance (may be from defects in signaling or transport across BBB)
Leptin also has effects on immune, cardiovascular and reproductive systems

29. Adiponectin
Released from adipocytes, yet more fat assoc with lower adiponectin levels
As adipocytes increase in size become more IR and secrete less adiponectin
Also TNF-α decrease adiponectin
30-kDa monomers form low molecular wt forms, middle molecular wt and HMW forms
HWF (12 or more) more closely assoc with DM and IR

30. Adionectin cont. Anti-inflammatory and cardioprotective
Insulin sensitizer
Lower TG levels in muscle and liver by moving them to adipose tissue

31. IR in cats and dogs
Mori et al, Japan, compared insulin signaling pathways between a cat and dog to see if any differences since IR more common in cats
Measured IRS1, IRS2, PIK3 mRNA from liver, muscle, abdom fat and leukocytes by qRT-PCR
Also measured enzymes involved in CHO and fat metabolism, MDH (malate dehydrogenas), FAS (fatty acid synthase) and G6PDH (glu 6 phos dehydrogenase)

32. Higher levels of gene expression in dogs
Implying that cats have a lower metabolic rate than dogs and cats may have underlying low insulin sensitivity level and be predisposed to developing insulin resistance
Limitations; 2 animal study
Mori et al Vet Res Commun 2009;33:

33. 4 normal cats, 4 moderately obese cats
Decreased gene expression of insulin signaling genes in insulin sensitive tissues of obese cats
4 normal cats, 4 moderately obese cats
Decreased levels of IRS2 and PI3K in obese cats and MDH and HSL mRNA indicating insulin signaling gene alterations occurring
No studies of glucose tolerance performed
Mori et al. Vet Res Commun 2009:33;

34. Hypothyroidism and IR 16 anestrous female dogs, 8 w expt hypothyr
Insulin-modified frequently sampled IV GGT and minimal model analysis used to determine basal insulin, gluc conc., acute insulin response to gluc, insulin sensitivity, glucose effectiveness, and disposition index
Showed that insulin sensitivity lower, and acute insulin response to glu higher in hpoTh
Hofer-Inteeworn et al. AJVR 2012;73;

35. Hypothyroidism and IR
Hypothyroidism induces insulin resistance, but glucose tolerance preserved by increased insulin secretion
May be due to high GH and IGF-1 conc in hypoth dogs and increased abdominal fat
Relevance for poorly regulated DM and concurrent hypothyroidism

36. Assoc of inc. TG’s with IR in schnauzers
28 min schn’s w hyperlipidemia, 31 controls
Healthy for prior 3 months
Single fasting blood sample, for serum insulin and gluc and the homeostasis model assessment (HOMA) score was calculated; basal serum insulin X basal glu conc (mmol/L)/ 22.5
Basal insulin conc higher and % of dogs with higher HOMA scores and higher insulin conc.’s was greater in hyperlipidemic dogs
Xenoulis et al JAVMA 2011:238:

37. Authors conclude that IR is often present in min Schnauzers with hypertriglyceridemia
May make concurrent DM more difficult to control
IR may play role in vacuolar hepatopathy and gall bladder disease
Limitation; no body scores included and obesity important cause of IR
Hypothyroidism and DM not ruled out although 57/59 had normal BG’s and 8 had normal TT4

38. Insulin resistance and obesity Summary
Accumulation of intramyocellular lipid, (DAG), activates novel PKC’s causing serine phosphorylation of IRS1, preventing it from, interacting with insulin receptor and moving GLUT4 to plasma membrane, for glucose uptake
Proinflammatory state (IL6 and TNFα)
Increased leptin, decreased adiponectin

39. Figure 1. Glucose-fatty-acid cycle proposed by Randle and colleaguesCoA=coenzyme A. PDH=pyruvate dehydrogenase. PFK=phosphofructokinase. G6P=glucose-6-phosphate. HK=hexokinase. Red circle with minus sign represents inhibition. Black line with arrowhead represents increase or accumulation of substrate. Blue dotted line with arrowhead indicates a pathway that is inhibited.