lecture 4

Question 1

What are other names for an obese hypertensive diabetic?

Answer 1

• syndrome X/syndrome X plus
• dysmetabolic syndrome
• insulin resistance syndrome
• plurimetabolic syndrome
• cardiometabolic syndrome
• dyslipidaemic hypertension
• hypertriglyceridemic waist
• deadly quartet

Question 2

What are the core components of the metabolic syndrome?

Answer 2

• obesity
• insulin resistance/glucose intolerance
• hypertension
• dyslipidemia
• however key criteria differ between groups

Question 3

What is the WHO definition of metabolic syndrome?

Answer 3

• mandatory component: high insulin levels, an elevated fasting blood glucose or an elevated post meal glucose
• with at least 2 of the following criteria:
• abdominal obesity as define by:
  
  • a waist to hip ratio of greater than 0.9
• BMI of at least 30 kg/m2
• waist measurement over 94cm
• triglyceride level of at least 1.7 mmol/L
• HDL cholesterol lower than 0.9 mmol/L
• blood pressure of 140/90 or above (or on treatment for high blood pressure)

Question 4

What is the NCEP - ATP III criteria for metabolic syndrome?

Answer 4

Three or more of the following five risk factors:

• central obesity (men: waist circumference greater than 102 cm , women: greater than 88 cm)
• triglycerides greater than 150 mg/dL (1.7mmol/L)
• HDL cholesterol (men: less than 40mg/dL, women less than 50)
• blood pressure: greater than 130/ greater than 85
• fasting glucose: greater than 110 mg/dL (6.1mmol/L)

Question 5

What is the more generally accepted definition of metabolic syndrome?

Answer 5

IDF Criteria (International Diabetes Foundation)

• mandatory component: central obesity

– waist circumference, ethnicity specific

– europid: greater than or equal to 94cm in men, 80cm in women

– asian (not japanese): 90cm men, 90cm women

– japanese: 85 cm men, 90cm women

• plus two or more of other criteria

– triglycerides greater than 1.7 mmol/L or on specific treatment

– HDL cholesterol less than 1.03 mmol/: in men, <1.29 in women or on specific treatment

– blood pressure greater than or equal to 130/85 or on treatment

– FBG greater than or equal to 5.6mmol/L or previously diagnosed type II diabetes

Question 6

Is the metabolic syndrome a useful syndrome to name? (case study)

Answer 6

Mr McDonalds

• 65 yo male
• BP 180/100, PR 90 AF
• waist circumference 103cm
• total cholesterol 6.5, LDL 4.5, HDL 0.6, TG 2.3
• diabetic with FBG 15 and an HbA1c 12%
• smokes 20/day with 50 year pack history
• family history of Diabetes and IHD Mr Smith
• 40 yo male
• BP 138/90, PR 60 SR
• waist circumference 103cm
• total cholesterol 4.5, LDL 2.0, HDL 1.02, TG 1.8
• FBG 5.6
• non smoker
• no family history of diabetes and IHD
• using each of the criteria both have the metabolic syndrome
• but they do not have the same risk:

– Framingham 10 year risk CHD case 1 : >30%

– Framingham 10 year risk CHD case 2: 1%

• Apart from lifestyle modification, treatment would be vastly different for each case and would involve treatment of individual risk factors

Question 7

Why so many definitions?

Answer 7

• because we don’t know what the basis of the syndrome or biological cause is: what is the mandatory component?
• because each group has its own interest – both in the etiology of the syndrome, the outcomes it predicts and the threatments we should be instigating –– NCEP and ATPIII are lipid centric –– IDF and WHO definitions are insulin resistance centric
• because we are still determining its role as a predictive tool

Question 8

So is it a syndrome?

Answer 8

• does the diagnosis of the metabolic syndrome add anything on top of the sum of its parts? – does it change the future risk? – which risk does it predict? – does it change our management
• joint statement from the american diabetes association and the european association for the study of diabetes
• the term “metabolic syndrome” refers to a clustering of specific cardiovascular disease (CVD) risk factors whose underlying pathophysiology is thought to be related to insulin resistance
• since the term is widely used in research and clinical practice, we undertook an extensive review of the literature in relation to the syndrome’s definition, underlying pathogenesis, and association with CVD and to the goals and impact of treatment
• while there is no question that certain CVD risk factors are prone to cluster, we found that the metabolic syndrome has been:

– imprecisely defined

– there is a lack of certainty regarding its pathogenesis, and

– there is considerable doubt regarding its value as a CVD risk marker

• our analysis indicates that too much critically important information is missing to warrant its designation as a “syndrome”
• until much needed research is completed, clinicians should evaluate and treat all CVD risk factors without regard to whether a patient meets the criteria for diagnosis of the “metabolic syndrome”

Question 9

Is there one biological basis?

Answer 9

maybe, if one could be found it would warrant it’s definition as a syndrome

Question 10

How does insulin signal?

Answer 10

• insulin signalling is central to insulin resistance
• insulin binds to its receptor, which is a two chain molecule, tyrosine kinase • when insulin binds to the receptor on the outside it signals through two main pathways
• first pathway is the PI3 kinase pathway which mitigates insulin’s metabolic effects: the increase in glucose uptake in muscles, the inhibition of hepatic glucose output etc
• other arm is the MAPK pathway: mediates growth signals onto the nucleus (mainly)

Question 11

What is the hypothesis?

Answer 11

• obesity causes a defect in one pathway only - the Pi3 Kinase signalling
• leads to transiently increased glucose due to insulin not being able to stimulate its metabolic arm
• causes pancreas to release insulin
• insulin causes hypertension and hyper stimulates the MAPK signalling pathway
• this leads to an increase in cytokine production
• this has two effects: acts directly on the liver to increase VLDL and triglyceride production, and upregulates an enzyme called 11 beta-HSD-1 which then converts inactive to active cortisol, excess corticosterone (cortisol) also acts on the liver to increase VLDL and plasma triglyceride production

Question 12

What is the evidence for obesity causing impaired PI3 kinase signalling?

Answer 12

• paper from 1999: insulin resistance differentially affects the PI 3-kinase and MAP kinase-mediated signalling in human muscle
• paper from 2000: “in skeletal muscle from type 2 diabetic subjects, IRS-1 phosphorylation, PI-3 kinase activity and glucose transport activity were impaired, whereas insulin receptor tyrposine phosphorylation, MAP kinase phosphorylation and glycogen synthase activity were normal”

Question 13

What is the evidence for insulin causing hypertension?

Answer 13

• Randeree HA et al: Effect of insulin therapy on blood pressure in NIDDM patients with secondary failure
• studied 160 patients with secondary failure to oral agents
• half were started on insulin
• weight, glucose and blood pressure were measured before and three months after starting insulin
• found that weight increased
• glucose deceased
• systolic BP increased
• diastolic BP increased

another study

• Genev NM et al: Does insulin therapy have hypertensive effect in type 2 diabetes?
• prospective study measuring 24 hour BP profile in 12 patients before, 6 and 12 weeks after starting insulin
• at 12 weeks there was an increase in BP
• systolic: 135 to 145
• diastolic: 72 to 75
• there was an increase in BMI: 27.2 to 29.6
• multiple regression analysis showed insulin to be a significant independent factor (p=0.0003)

another study:

• the effect of insulin on renal sodium metabolism. a review with clinical implications
• increases in plasma insulin concentration within the physiological range stimulate sodium reabsorption by the distal nephron segments
• retention of salt leads to increase in blood pressure
• insulin also activates the sympathetic nervous system

Question 14

What is the evidence for increased MAPK activity leading to increased cytokine production?

Answer 14

• Lee Y.B et al: map kinase regulates TNF-alpha production in human astrocytes and microglia by multiple mechanisms
• Gon T et al: common and distinct signalling cascades in the production of TNF-alpha and interleukin-13 induced by lipopolysaccharide in RBL-2H3 cells
• Shen J et al: Leptin enhances TNF-alpha production via p38 and JNK MAPK in LPS-stimulated Kupffer cells

Question 15

What is evidence for cytokine production leading to increased 11beta-HSD-1 expression?

Answer 15

• Cooper MS et al: Modulation of 11beta-hydroxysteroid dehydrogenase isozymes by proinflammatory cytokines in osteoblasts: an autocrine switch from glucocorticoid inactivation to activation
• Cai TQ et al: induction of 11beta-HSD type 1 but not -2 in human aortic smooth muscle cells by inflammatory stimuli
• Sun K, Myatt L: enhancement of glucocorticoid induced 11beta-HSD-1 expression by pro-inflammatory cytokines in cultured human amnion fibroblasts
• Odile Paulmyer-Lacroix et al: expression of the mRNA coding for 11beta-HSD-1 in adipose tissue from obese patients: an in situ hybridisation study
• fairly clear evidence in multiple different tissues that increased cytokines leads to increase 11 beta-HSD-1

Question 16

What is the activity of 11beta-HSD-1?

Answer 16

• increases production of active corticosteroid
• in humans it converts cortisone to cortisol
• in rodents it converts 11-dehyrdocorticosterone to corticosterone

Question 17

What is the evidence that increased cytokine production leads to increased VLDL production?

Answer 17

• dyslipidemia and inflammation: an evolutionary conserved mechanism
• Esteve E. et al. Clin Nutr 24:16-31 2005

Question 18

What is the evidence for corticosterone/cortisol leading to elevated hepatic VLDL production?

Answer 18

• Masuzaki H. et al: a transgenic model of visceral obesity and the metabolic syndrome. Science 2001
• they made transgenic mice overexpressing 11 beta HSD selectively in adipose tissue to an extent similar to that found in adipose tissue from obese humans
• these mice developed mild obesity, elevated triglycerides and elevated free fatty acids
• Transgenic amplification of glucocorticoid action in adipose tissue causes high blood pressure in mice, Hiroaki Masuzaki
• improved lipid and lipoprotein profile, hepatic insulin sensitivity, and glucose tolerance in 11-HSD type 1 null mice – the reverse

Question 19

So how did we arrive at this hypothesis?

Answer 19

• this evidence for the first time links one defect in insulin signalling to the metabolic syndrome
• selective defect in signalling through the PI3 kinase pathway
• how did we arrive here:

AIM: to create a transgenic rat model of glucose toxicity

• many years ago wanted to show that glucose itself was damaging and could cause insuling resistance
• to do that they looked at the gluconeogenic pathway (lactate and amino acids are converted to glucose)
• in this pathway there are a number of rate limiting steps
  
  • PEPCK
  • FBPase
  • G6Pase
• chose to over express PEPCK in a rat
  
  • picked it because they felt it was the most rate limiting step
  • controlled at the gene level and not at the protein level
• these transgenic rats developed metabolic syndrome
  
  • features of syndrome X develop in transgenic rats expressing a non-insulin responsive phosphoenolpyruvate carboxykinase gene
  • thorburn AW et al Diabetologia 1999
• develop mild obesity
• subtle change in weight actually represents a substantial change in fat in the body
• change in both males and females
• fasting plasma glucose in control and transgenic rats: significant increase in transgenic
  
  • normal for the first few months but trended upwards as the rats got older
  • insulin was high from early on so developed insulin resistance
  • primary defect was a defect in glucose overproduction
• surprisingly developed high triglycerides and high cholesterol

Question 20

What happens in an insulin resitant muscle?

Answer 20

• insulin stimulates cytokine production
• when they did the clamp they measured glucose uptake in the muscle while the animal was alive and then biopsied the muscle and measure IL-6 RNA
• basal level the same between transgenic and control (2-deoxyglucose uptake measured)
• stimulated with insulin:
  
  • would expect glucose uptake into cell to go up
  • while basal levels of glucose uptake were the same, the transgenic mice had less of an increase in uptake after insulin stimulation
  • this indicates these mice had developed insulin resistance
• surprise came when looking at IL-6 RNA
  
  • in the control when you gave them insulin, not much happened
  • in the PEPCK animal there has a huge fold increase in levels of IL-6 mRNA
• from this experiment came the idea that perhaps in our animal the increase in IL-6 is caused by the fact that when you stimulate with insulin normally you get MAPK and PI3K cancelling each other out (re: cytokine production)
  
  • does large presence of cytokines indicate that the PI3 kinase pathway is faulty and therefore failing to inhibit cytokine production following insulin stimulation?
• set out to try to prove this
• looked at Akt activation in skeletal muscle: surrogate measure of the PI3 kinase signalling
  
  • perhaps lower but no statistical difference for basal level
  • following insulin stimulation there was significantly less pAkt in transgenic rat vs control
  • thus obese animals had developed a defect in PI3 kinase signalling
  • in contrast there was no defect in MAPK signalling
• do they have an increase in more cytokines than just IL-6?
  
  • no
  • also measured TNFalpha and IL-1beta
• 11Beta HSD-1 also goes up in adipose tissue (subcutaneous and visceral fat)

Question 21

What are conclusions of this lecture?

Answer 21

• metabolic syndrome is a prevalent condition that confers high cardiovascular risk
• there is controversy regarding the utility of the concept of metabolic syndrome
• a unifying single defect causing the metabolic syndrome would justify calling this cluster of risk factors a “syndrome”
• an hypothesis is presented linking a specific defect in insulin signallign to the development of the metabolic syndrome