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Disperse Lipids (insoluble) in blood
- foriming micelles in hydophillicblood,
proteins attached to lipoprotein shell that regulate enzymes, transfer lipids, bind to cell receptors
Chylomicrons and VLDL
known collectively as triglyceride-rich lipoproteins) transport dietary and endogenous triglyceride respectively.
After their triglyceride has been depleted, they are known as chylomicron remnants and intermittent density lipoproteins (IDL) respectively. IDL are precursors for low-density lipoproteins (LDL) which transport cholesterol to the periphery
Lipoprotein (a) which is distinguished by the presence of a unique apolipoprotein that resists LDL receptor uptake
The smallest particles consist of cholesterol and phospholipid rich high-density lipoproteins (HDL) which are responsible for reverse cholesterol transport
The apolipoprotein B100 containing lipoproteins (VLDL, IDL, LDL and lipoprotein (a) represent the lipoproteins which are most likely to contribute to atherosclerotic cardiovascular disease.
AB100 artery damage
Intestinal Absorption of DietaryLipids and Biliary Cholesterol:
Drugs Acting Here
- bile acid squestrants
-Plant stanols and sterols
bile acid squestrants
inhibit bile acid reabsorption in the ileum, causing hepatic bile acid deficiency. This deficiency promotes compensatory increase in bile acid synthesis from hepatic cholesterol, which is replenished through increased hepatic uptake of LDL/chylomicron from plasma and increased hepatic cholesterol synthesis
Plant stanols and sterols
are thought to displace cholesterol from micelles, preventing its uptake at the brush border membrane and reducing the amount of cholesterol transported to the liver. Reduced delivery of dietary/biliary cholesterol to the liver effectively increases clearance of LDL (and LDL-C) particles from plasma.
selectively inhibits the uptake of micellar cholesterol into intestinal epithelial cells and substantially reduces the amount of cholesterol from diet and bile that is transported to the liver, thereby resulting in compensatory increase in LDL clearance by the liver and reduction in plasma LDL-C levels. The mechanism of action of ezetimibe is thought to involve selective inhibition of the putative sterol transporter on the brush border surface of intestinal epithelial cells.
Intestinal Absorption of DietaryLipids and Biliary Cholesterol
1. Cholestrol from the diet (1/3 or 400mg/day) and Bile (2/3 1200mg/day) enter small intestine
2. Cholestrol gets emulsified by bile salts into micelles
3. Micelles deliver cholestrol to brush border
4. Cholestrol goes from brush border into duodenal and jejunal enterocytes (sterol transporter NPC1L1)
5. After abs. it can be exported back from the enterocyte into the intestinal lumen by the ABC transporter ABCG5/G8
6. Cholestrol that remains is esterified and packaged into chylomicrons and released into the lymohatics
7. Bile salts are mostly reabs. by the IBAT (intestinal bile acid transporter) and transported back to the liver (hepatic portal)
diet enriched in n-3 fatty acids decreases the gene expression of NPC1L1 in duodenum and jejunum
Exogenous TG metabolism
1. Dietary fat broken down into constituent fatty acids
2. TG combined with Apoprotein B and secreted into the the thoracic duct as chylomicrons
3. Activate by a Apoprotein C to mediate TG hydrolysis fatty acids released and feuse to nearby tissue
Cholestrol Delivery via LDL
The expression of LDLRs is also affected by low hepatocyte cholesterol levels, which activate a set of transcription factors called sterol regulatory element-binding proteins (SREBPs). While SREBPs increase the uptake of LDL by increasing the overall expression of LDLRs, they also increase PCSK9 expression, resulting in increased degradation of LDLRs and limited clearance of LDL-cholesterol from the blood.
NORMALLY VERY TIGHTLY REGULATED
Cholesterol delivery via LDL-R alters intracellular membrane cholesterol and SREBP, which
# Reduces cholesterol synthesis via HMGCoAR
# Reduces LDL-R synthesis
# Increases storage as Ester
HDL and reverse Cholesterol Transport
- excess free cholestrol, accumulate in cell membrane regions aka "lipid rafts"
- these iteractwith apoAI in ECF
- the free cholestrol goes then to the apoAI and amkes Nascent dicoidal HDL (transport mediated by ABC A1
- the dic HDL converted to mature spheres by LCAT, by esterifying it to form hydrophobic ester core. the sphere can also recieve more cholestrol from other rafts by diffusion, SRB1 and other membrane transports
- transports to liver to dispose in bile
FOAMY cells and lipid m/p give up their cholestrol to HDL (atheroma) by ABC-A1
LDL in Artery Walls
1. LDL through endothelium into sub-endo space
2. chemical modification (oxidation)
4. m/p uptake
5. foam cell formation
6. inflammation INC. permeability of endothelium thus more LDL, thus VICIOUS CYCLE
CELLS DON'T NORMALLY GET OVERLOADED BY CHOLESTROL BUT THESE OXIDISED FATS GET UP TAKEN BY SCAVENGER CELLS AND SO NO -VE FEEDBACK PATHWAYS
How does obesity and insulin resistance cause atheroma?
Via triglyceride and CETP
1. Obesity, insulin resistance and diabetes predispose to overproduction of triglyceride rich VLDL by the liver
2. INC. VLDL enhances action of cholestrol ester transfer protein (CETP)
3. This promotes lipid transfer between TG and lipid rich particels
4. Replacement of LDL with TG and thus removal of TG by lipolysis promotes formation of small dense LDL
5. And the smae process in HDL forms small, unstable particles which are catabolised
TG and Lipoproteins
TG drives cholesterol ester transfer via CETP (> 1.5 mmol/l ?) :
1) TG exchange reduces HDL-C and impairs reverse cholesterol transport. TG and HDL-C are inversely correlated
2) TG exchange causes smaller, denser LDL. When TG is raised, LDL-C underestimates CVD risk.
3) Small, rather than large, TG-rich particles may carry cholesterol into the artery wall. The linear relationship between TG and CVD risk declines at very high levels
Ceiling effect with plasma TG
maximum cardiovascular risk occurs in moderate hypertriglyceridaemia. The risk associated with severe hypertriglyceridaemia is elevated, but not to the same extent.
Everyone has atherosclerosis
The rate of this progression is proportional to the number of risk factors and the severity of each individual risk factor
that factors predisposing a plaque to rupture are
- Lipid accumulation
- Increase in lipid-laden macrophages
- Disruption in reparative smooth muscle proliferation in the cap
Total daily Fat in Diet
American Heart Association Diet. Total fat should be 30% (not the percentage weight but that by energy)
Atherosclerosis Risk factors
IHD Risk Factor Odds Ratio Population attributable risk
ApoB/ApoAI 3.25 49%
Smoking 2.87 36%
Hypertension 1.91 18%
Diabetes 2.37 10%
Abdominal obesity 1.12 - 1.62 20%
Psychosocial 2.67 33%
Diet (fruit & veg) 0.70 14%
Activity 0.86 12%
Alcohol (not binge) 0.91 7%
Laboratory assessment of lipids and lipoproteins.
NHDL=TC - HDL (OK non-fasting)
Measure TC, HDL and fasting TG
Total chol = VLDL + LDL + HDL
VLDL = TG/2.2 unless TG >>4
LDL in mmol/l of chol =
Total chol - HDL - TG/2.2
What is FH?
FH is "Familial Hypercholesterolaemia"
- Co-dominant mutation of the LDL-receptor gene (or up to 5 other genes)
- The cause of metabolic and clinical consequences including precocious cardiovascular disease (CVD)
What does FH cause?
- Increased LDL,
- Reduced clearance of remnants including LDL's precursor, IDL.
- Increased Lp(a)?
- Reduced HDL?
- Premature CHD
- Premature CVD
- Aortic stenosis
- Tendon xanthomas (11%) specific?
- Corneal arcus (27%) non-specific > 40y?
- Xanthelasmas (12%) nonspecific
- No signs highly sensitive
Antilipemic: reduces release of VLDL from liver into circulation. Lowers LDL cholesterol and triglycerides and raises HDL cholesterol. Tox: flushing, pruritus, liver dysfunction, increased risk of myopathy when combined with statins.
LDL receptor Upregulation
1. red. intracellular cholestrol
2. release of SREBPs up-regulate LDL receptor gene
3. more LDL receptors
4. INC. removal of ciculating LDL
the statin also blocks the compensatory stimulation of INC. cholestrol synthesis by SREBPs too
Therapy for Hypertriglyceridaemia
1st line fibrates
fish oil requires modest doses of 4 to 6 g per day or more as these tend to reduce triglyceride with very little effect on HDL
Fenofibrate in Type 2 DM
good in DM for microvascular impairments (lipid levels not predicting these outcomes)
Urban myths about Lipids
Suicide, depression, violent death
Brains need cholesterol from blood
Risk of cancer
Severity of side effects
The relationship between low (?LDL) cholesterol levels and the slight risk of haemorrhagic stroke is the only adverse association not attributable to intercurrent disease.
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