TYPE 2 DIABETES PATHOGENESIS Flashcards
In diabetes, what is the lipid profile like overall?
In T2DM, there are high triglyceride concentrations, particularly post-prandially
➢ Low HDL concentrations
➢ Normal total and LDL cholesterol concentrations, but small dense LDL particles
What are the five different types of lipoprotein?
- VLDL (very low density lipoprotein)
- Intermediate LDL
- LDL
- HDL
- Chylomicrons
What is the structure of the lipoprotein like?
➢ The apolipoprotein in the phospholipid layer plays a role in transporting the lipid around the body
➢ The centre of the lipoprotein will consist of cholesterol esters and trigylcerides
➢ The higher of cholesterol ester/triglyceride ratio, the higher density the lipoprotein
➢ Chylomicrons have the apolipoprotein ApoB48
What does insulin do to triglycerides?
Insulin will increase fatty acid and glycerol uptake into cell into trigylcerides (via lipoprotein lipase- HSL), inhibiting the lipolysis of triglycerides into fatty acids and glycerol back into the circulation
What happens to fatty acid metabolism in insulin resistance?
If there is resistance to insulin- there is more fatty acid degradation (because IRS promotes lipoprotein lipase activation and inhibition of hormone sensitive lipase- HSL)
For example with VLDL what happens?
When VLDL passes LPL in circulation (lipoprotein lipase, found in vascular endothelium), this lipase will take out some TG from the lipoprotein → becomes Intermediate LDL (IDL)
➢ This is then further broken down to become LDL (contains even less TG)
80% of this LDL will be transported back into the liver
20% will stay in the circulation can cause deposition
➢ Increased lipid levels in circulation will increase the risk of deposition and atherosclerotic fat
What happens in lipoprotein uptake?
- Will bind to the LDL receptor, and goes into cell by endocytosis
- LDL becomes degraded and used by the liver for cholesterol storage.
If there is an overload of fatty acids in the liver during fasting state, it will leak out more FA (VLDL) into the circulation.
What happens in reverse cholesterol transport?
a multi-step process resulting in the net movement of cholesterol from peripheral tissues back to the liver via the plasma.
- Another Apoprotein on VLDL, Apo-A1, can be given up and will bind onto nascent HDL (can also be secreted in gut and liver)
Nascent HDL can take up unesterified cholesterol in tissue and combine it with phospholipid within the HDL
➢ Becomes a mature HDL particle
Lecithin= cholesterol acetyltransferase
This leads to the production of GOOD CHOLESTEROL
What does cholesterol ester do?
➢ Acts on various lipoproteins
➢ Can transfer TG and cholesterol from one molecule to the other allows interplay between different lipoproteins
What is the role of the liver?
Hepatic lipase takes up TG back into the liver, if CETP removes TG that has been taken up by HDL. This mops up bad cholesterol back into the liver.
➢ HDL becomes smaller, and can go into muscles for oxidation to be used for energy.
When too much TG has been taken up by the liver via hepatic lipase, it releases small dense LDL
➢ This greatly increases the risk of atherosclerosis
➢ High proportion of this lipoprotein found in T2DM
What happens in insulin resistance versus insufficient insulin?
In insulin resistance:
- CETP , hepatic lipase are stimulated
➢ Hypertriglyceridaemia, as CETP promotes movement of TG from one to another tends to go the WRONG way (make lipoproteins lower in density).
Reduced insulin action:
- Inhibits HSL (hormone sensitive lipase)
- Inhibits hepatic VLDL production
- Stimulates lipoprotein lipase
• However, insulin resistance causes hormone sensitive lipase to be activated. Thus, high levels of free fatty acids are released by the adipose cells.
What is fatty liver disease? Causes and characteristics.
Fat deposition in the liver when no other causes can be found. It is a chronic liver disease is related to the increase in the incidence of obesity and T2DM in population
- Less common causes are medicines, antiretroviral drugs, and rapid weight loss
• Steatosis
• Mixed inflammatory cellular infiltration
• Degeneration and hepatocyte necrosis
• Nuclear glycogen, Mallory bodies (damaged intermediate filaments within the hepatocytes) and fibrosis
How does NAFLD fit into insulin resistance and diabetes?
T2DM patients, the incidence of steatosis is up to 50%. It goes up to 76-90% in obese population.
➢ Insulin resistance is an important predicting factor for NAFLD.
➢ NAFLD develops in the majority of patients who exhibit morbid obesity and diabetes
What is the role of ceramides?
Ceramides and T2DM:
- Ceramides are a family of sphingoloid molecules with important structural and functional roles in cell signaling, cell differention, proliferation and apoptosis.
- Ceramides accumulate within tissues and inhibit insulin action and subsequent glucose uptake
- Meditators of lipotoxicity
- Induce inflammation through activation of TNF-alpha
- Facilitate inflammatory signaling pathways which further contribute to the state of insulin resistance
- Although, not much is known about their mechanisms
Ceramides are elevated in patients with T2DM. However, this is just an observation, and relationship may not be directly causal, or just an association. They are also a major component of ectopic fat in obese individuals.
Study:
- 13 patients (10 female) who had a BMI >47kg/m2 underwent gastric bypass
- Showed that plasma ceramide subspecies was reduced, and improvement in insulin sensitivity.
How is hormone sensitive lipase activated?
• Hormone-sensitive lipase is activated by glucagon (fasting) or epinephrine (exercise). Therefore, fat in adipose tissue is hydrolyzed to give glycerol and fatty acids during both fasting and exercise.
• The fatty acids can be used directly as an energy source by most tissues with mitochondria, excluding the brain
However these hormones inhibit lipoprotein lipase
What happens in starvation?
• During prolonged srarvation, the fatty acids can also be converted to ketone bodies in the liver
Overall what happens in insulin resistance?
increased free fatty-acid release from insulin-resistant fat cells. The increased flux of free fatty acids into the liver in the presence of adequate glycogen stores promotes triglyceride production, which in turn stimulates the secretion of apolipoprotein B (ApoB) and VLDL cholesterol.
Decrease the level of HDL cholesterol and increase the concentration of small dense LDL-cholesterol particles how?
The increased number of VLDL cholesterol particles and increased plasma triglyceride levels via several processes: -VLDL-transported triglyceride is exchanged for HDL-transported cholesteryl ester through the action of the chol- esteryl ester transfer protein (CETP), which results in increased amounts of both athero- genic cholesterol-rich VLDL remnant particles and triglyceride-rich, cholesterol-depleted HDL particles.
- The triglyceride-enriched HDL is subsequently hydrolyzed by hepatic lipase or lipoprotein lipase; ApoA-I dissociates from the reduced-size HDL,
- The increased concentration of small dense LDL-cholesterol particles is explained by a similar lipid exchange. Increased levels of VLDL-transported triglyceride enable CETP to promote the transfer of triglyceride into LDL in exchange for LDL-transported cholesteryl ester. The triglyceride-rich LDL undergoes hydrolysis by hepatic lipase or lipo- protein lipase, which results in lipid-depleted small dense LDL particles (Figure 1)
What does adiponectin do?
positive correlation between the levels of HDL chol- esterol and adiponectin
What is the thrifty gene phenotype?
T2DM trait to environment that has changed when it was advantageous to put on weight quickly in times of abundance, especially for child bearing women, to then survive famines
Now in modern times, genetic predisposition to excess calories and diabetes.
James Neels who came up with it when it was clear that obesity had a genetic component
What are problems with the thrift gene hypothesis?
- today’s populations have no history of famine
- evidence that hunter gatherers did get fat in between famines
- no evidence that fat people survived famines any better than thin people. In fact those that didn’t survive famine were the young/old
What is the Barker hypothesis?
BARKER HYPOTHESIS: poor in utero environment indicates a harsh post natal one and so foetus undergoes METABOLLIC changes to prepare itself. If foetus then grows up in plentiful environment then these changes are detrimental leading to obesity and diabetes. The foetus in adult life would be thrifty
What does the Barker hypothesis depend on?
Foetuses plastic and susceptible to environmental changes. The crucial window continues into the immediate post natal period.
What is the el Nino hypothesis?
Susceptibility to central adiposity has evolved to protect against a variable energy supply.
A chronic energy deficiency favours central fat.
May also be beneficial for energy saving in terms of thermogenesis.S. Asians have higher visceral fat because in the absence of weight gain (previous starvation) chronic energy deficiency lead to increased allocation to this fat depot. – S Asians exposed to high levels of heat stress, famine and agiculture is dependent on monsoon cycles.
