Comprehensive Study Notes on Cholesterol and Plasma Lipoproteins

Cholesterol and Plasma Lipoproteins Notes

Introduction to Cholesterol and Lipoproteins

Clinical Case: Ahmed

Ahmed, a healthy, middle-aged, and not overweight man, presents with elevated serum cholesterol and moderate hypertension. This raises concerns about potential cardiovascular risks.

  • Diet: He consumes a low-fat diet, rich in polyunsaturated fatty acids, fruits, and vegetables, which is generally beneficial for heart health.
  • Plant Sterols/Stanols: His GP suggests incorporating plant sterols and stanols, which help reduce cholesterol absorption.
  • Statin Drug: A statin is prescribed to inhibit cholesterol synthesis when diet adjustments are insufficient.
Key Questions Addressed
  • What is cholesterol and its uses?
  • Where does cholesterol come from?

Lecture Objectives

  • Describe the main roles of cholesterol in the body.
  • Outline cholesterol synthesis and explain the clinical relevance of HMG CoA reductase inhibitors.
  • Describe plasma lipoprotein classes, their interactions, and metabolism.
  • Explain the importance of lipoproteins in cardiovascular disease and the stages of atherosclerosis development.
  • Explain why a high-fat diet and poorly controlled diabetes are risk factors for atherosclerosis, and why unsaturated fatty acids, antioxidants, and non-starch polysaccharides are protective.

Cholesterol and Coronary Heart Disease

  • Discusses the relationship between cholesterol levels (mg/dL) and coronary heart disease.

The Role of Cholesterol

Structural Component

Cholesterol is a structural component of all cell membranes, modulating their fluidity.

Precursor Molecule

It serves as a precursor for:

  • Bile acids
  • Steroid hormones
  • Vitamin D
Liver's Central Role

The liver is central in regulating the body's cholesterol homeostasis.

Cholesterol Synthesis

  • Synthesis: Occurs in all tissues, utilizing acetate as the carbon source and requiring NADPH.
  • Balance: The balance of cholesterol depends on the rates of cholesterol excretion and synthesis. An imbalance leads to elevated plasma cholesterol, causing gradual deposition in tissues, particularly in blood vessel linings, leading to atherosclerosis and coronary artery disease.

Steps in Cholesterol Synthesis

1. Synthesis of 3-hydroxy-3-methylglutaryl (HMG) CoA

The first two steps are similar to ketone body (KB) synthesis.

  • Isoenzymes: HMG CoA synthase has cytosolic and mitochondrial isoenzymes.
    • The cytosolic enzyme is involved in cholesterol synthesis.
    • The mitochondrial enzyme is involved in ketone body synthesis.
2. Synthesis of Mevalonic Acid (Mevalonate)

The reduction of HMG CoA to mevalonic acid is catalyzed by HMG CoA reductase.

  • HMG CoA Reductase: This is the rate-limiting and key regulatory step in cholesterol synthesis.
  • Irreversible Reaction: The reaction is irreversible.
  • Statins: Statins inhibit HMG CoA reductase, reducing cholesterol synthesis.

Degradation of Cholesterol

The ring structure of cholesterol cannot be metabolized to CO<em>2CO<em>2 and H</em>2OH</em>2O. Thus, it is excreted in bile as is or in the form of bile acids or salts. It can also be excreted in stools.

Bile Acid Synthesis

  • Primary Bile Acids: Cholic acid and chenodeoxycholic acid are synthesized in the liver.
  • Conjugation: These are conjugated with glycine or taurine to form glycocholic acid, taurocholic acid, glycochenodeoxycholic acid, and taurochenodeoxycholic acid.
  • Secondary Bile Acids: Intestinal bacteria deconjugate and metabolize primary bile acids into secondary bile acids like deoxycholic acid and lithocholic acid.

Enterohepatic Circulation

  • Reabsorption: Bile salts secreted into the intestine are efficiently reabsorbed (>95%) and reused.
  • Secretion: The liver actively secretes bile salts into the bile.
  • Reabsorption Location: Bile salts are reabsorbed in the terminal ileum and returned to the blood.
  • Non-Starch Polysaccharides (NSP): NSP binds bile salts and reduces their reabsorption.
  • Excretion: Bile salts are the main route for cholesterol excretion.

Plant Sterols and Stanols

Plant sterols and stanols reduce cholesterol absorption from the gastrointestinal tract (GIT).

  • Competition: They compete with cholesterol for esterification in the mucosal cells.
  • Excretion: Plant sterols and stanols, and unesterified cholesterol, are excreted from intestinal mucosal cells via an ATP-dependent transporter (an ABC transporter).
  • Inhibition of Esterification: They inhibit cholesterol esterification, increasing the amount of unesterified cholesterol available for excretion.

Lipid Transport in Circulation

Lipids are insoluble in plasma and are transported via lipoproteins.

  • Lipoproteins: Lipids combine with specific proteins (apoproteins) to form lipoproteins.
  • Components: Lipoproteins consist of a core of nonpolar lipids (triacylglycerols and cholesterol esters) and surface proteins (apoproteins), cholesterol, and phospholipids.

Plasma Lipoproteins

  • Chylomicrons: Derived from the small intestine; carry exogenous dietary triacylglycerols.
  • Very Low-Density Lipoprotein (VLDL): Derived from the liver; carry endogenous triacylglycerols.
  • Intermediate Density Lipoprotein (IDL): Formed from VLDL in circulation.
  • Low-Density Lipoprotein (LDL): Carries cholesterol from the liver to tissues (often referred to as "bad" lipoprotein).
  • High-Density Lipoprotein (HDL): Carries cholesterol from tissues to the liver for excretion in bile (often referred to as "good" lipoprotein).

Ultracentrifugation

  • Separation Technique: This method separates lipoproteins based on their density.
  • Density Correlation: The higher the fat content, the lower the density of the lipoprotein particle.

Image of Ultracentrifugation separating LDL, VLDL, Chylomicrons, and HDL

Chylomicrons (CM)

  • Assembly: Assembled in the intestinal mucosa and contain re-esterified dietary lipids.
  • Triacylglycerol Uptake: Tissues take up triacylglycerol via lipoprotein lipase.
  • Remnant Clearance: Remnants are cleared by the liver.
  • Composition: Consist of triacylglycerol, phospholipid, cholesterol ester, free cholesterol, and proteins.

Metabolism of Chylomicrons

Image of Chylomicron Metabolism

  • Origin and Function: CM are assembled in intestinal mucosal cells, enter the lymphatics, then the blood, and carry TAG of dietary origin to peripheral tissues.
  • Nascent CM: Contain Apo B-48.
  • Mature CM: Contain Apo B-48, Apo C-II, and Apo E (obtained from HDL during circulation).
  • Appearance: CM are responsible for the physiological milky appearance of plasma up to 2 hours after a meal.
  • Lipoprotein Lipase: Required to degrade TAG into glycerol and fatty acids.

Chylomicron Remnants

  • Degradation: As CMs circulate, 90% of their TAG is degraded by lipoprotein lipase, reducing their size.
  • Apo C-II Return: Apo C-II returns to HDL.
  • Remnant Removal: CM remnants are rapidly removed from the blood by the liver via endocytosis, using receptors that recognize Apo E protein.

Image of Lipoprotein Lipase Action

Very Low-Density Lipoprotein (VLDL)

  • Assembly: Assembled in the liver, containing triacylglycerol synthesized in the liver and from chylomicron remnants.
  • Triacylglycerol Uptake: Tissues take up triacylglycerol via lipoprotein lipase.
  • Formation of IDL: As triacylglycerol is removed, VLDL becomes IDL.
  • Composition: Triacylglycerol, phospholipid, cholesterol ester, free cholesterol, and protein.

Metabolism of VLDL

Image of VLDL Metabolism showing LPL action

Intermediate Density Lipoprotein (IDL)

  • Formation: Formed in the circulation by progressive removal of triacylglycerol from VLDL by tissues.
  • Composition: Triacylglycerol, phospholipid, cholesterol ester, free cholesterol, and protein.

Low-Density Lipoprotein (LDL)

  • Function: Transfers cholesterol to tissues.
  • LDL Receptor Regulation: The LDL receptor is down-regulated by tissue cholesterol levels.
  • Oxidized LDL: Oxidized LDL is not recognized by the LDL receptor, and is taken up by macrophages, forming foam cells that infiltrate the arterial lining, initiating atherogenesis.
  • Composition: Triacylglycerol, phospholipid, cholesterol ester, free cholesterol, and protein.

Cholesterol and Cholesteryl Esters in the Liver

  • ACAT: Acyl-CoA cholesterol acyltransferase (ACAT) esterifies cholesterol into cholesteryl esters for storage in lipid droplets.
  • LDL Uptake: LDL brings cholesterol to the liver.
  • Regulation: Free cholesterol binds to a protein that downregulates the expression of the LDL receptor.

Cholesterol Esterification

  • ACAT Enzyme: Acyl CoA cholesterol acyltransferase (ACAT) mediates cholesterol esterification.
  • Substrate Preference: Saturated fatty acids are poor substrates, mono-unsaturated are good, and polyunsaturated are very good substrates for ACAT.

cholesterol+acyl CoA→cholesteryl ester+CoAcholesterol + acyl~CoA \rightarrow cholesteryl~ester + CoA

  • Liver Regulation: The liver only takes up LDL when it needs cholesterol.

Effect of Fatty Acids on Serum Cholesterol

  • Saturated Fatty Acids: Cholesterol increases proportionally with saturated fatty acid intake.
  • Polyunsaturated Fatty Acids (PUFA): Cholesterol decreases proportionally with polyunsaturated fatty acid intake.

Graph illustrating the effect of Saturated and PUFA on cholesterol levels.

High Fat Diet and Hypercholesterolemia

  • Chylomicron Remnants: Increased fat consumption leads to more chylomicron remnants.
  • Inhibition of LDL Clearance: Chylomicron remnants are taken up by the liver using the same receptors as LDL, inhibiting LDL clearance.
  • Increased VLDL Export: More chylomicron remnants lead to more fat exported in VLDL, which is a precursor to LDL, increasing LDL levels.

Decreased LDL Clearance by the Liver

  1. Genetic Polymorphisms in Apo-protein E: Some variants have low affinity for the LDL receptor, leading to genetic susceptibility to atherosclerosis.
  2. Familial Hypercholesterolemia: Genetic defects of the LDL receptor.
  3. Chemical Modification of LDL: Reduces its affinity for the LDL receptor.
    • Oxidative Damage: Oxidative damage to unsaturated fatty acids (hence the protective role of antioxidants).
    • Amino Acid Modification: Chemical modification of amino acids in apo-proteins by:
      • Products of fatty acid oxidation
      • Non-enzymic glycation in poorly controlled diabetes
      • Reaction with homocysteine (elevated plasma homocysteine is a factor in atherosclerosis)

High-Density Lipoprotein (HDL)

  • Secretion: Secreted by the liver and small intestine as an "empty" protein shell, taking up cholesterol from tissues for return to the liver.
  • Clearance: HDL is cleared by the liver.
  • Composition: Triacylglycerol, phospholipid, cholesterol ester, free cholesterol, and protein.

HDL Metabolism

  • Reverse Cholesterol Transport: HDL is the main transporter of cholesterol from peripheral tissues to the liver to be excreted through bile.
  • Inverse Relationship: The concentration of HDL in serum is inversely related to the incidence of myocardial infarction.

HDL Formation

  • Nascent HDL: Disk-shaped particles containing phosphatidylcholine (Lecithin) and apoproteins A-1, C-II, and E.
  • Conversion to Spherical Particles: Rapidly converted to spherical particles as they accept cholesterol from membranes of tissue cells.
  • LCAT Action: When Cholesterol is taken up by HDL, it is immediately esterified by the plasma enzyme LCAT (Lecithin: Cholesterol Acyl Transferase).

LCAT and HDL Maturation

  • LCAT Function: LCAT transfers fatty acids from lecithin to cholesterol, producing hydrophobic cholesterol esters, which are sequestered in the core of HDL.
  • HDL Size Increase: Over time, HDL increases in size by accumulating cholesterol esters inside its core.
  • Uptake by Liver: Finally, HDL is taken up by the liver cells via a cell-surface receptor protein.

Cholesterol Ester Transfer Protein (CETP)

Some TAGs are transferred from VLDL to HDL in an exchange reaction that concomitantly transfers cholesteryl esters from HDL to VLDL. This exchange is accomplished by cholesteryl ester transfer protein (CETP).

Atherosclerosis and Coronary Artery Disease

Images illustrating Atherosclerosis and Coronary Artery Disease

Macrophage Infiltration and LDL Uptake

  1. Monocyte Rolling: Monocytes "roll" along vascular endothelium.
  2. Chemoattraction: Attracted by chemoattractants from damaged cells and macrophages under the endothelium.
  3. Macrophage Transformation: They become macrophages, take up LDL through scavenger receptors, and infiltrate between endothelial cells.
  4. Foam Cell Formation: Lipid-engorged macrophages (foam cells) lay down a fatty streak under the endothelium.

Atherosclerotic Plaque Formation

Foam cells accumulate in the sub-endothelial layer, leading to the formation of atherosclerotic plaques. This narrows blood vessels and predisposes them to thrombosis, with further ischemia and infarction.

Development of Atherosclerosis

Image illustrating foam cells dying and forming fatty plaques

Metabolic Syndrome

Abdominal obesity is associated with a cluster of metabolic abnormalities referred to as the metabolic syndrome, including:

  • Glucose intolerance
  • Insulin resistance, hyperinsulinemia
  • Dyslipidemia
  • Hypertension
  • Chronic, systemic inflammation that contributes to the pathogenesis of atherosclerosis

Hypertension as a Risk Factor

The calcified cap over the fatty plaque can rupture due to increased blood pressure, initiating the blood clotting cascade and activating platelets.

Image illustrating plaque rupture due to hypertension

Stages of Atherosclerosis

*Image showing:
* Moderate narrowing of arterial lumen
* Calcification and almost complete occlusion
* Haemorrhage into atheroma – only a slit-like lumen
* Complete occlusion by thrombus formation in narrowed lumen*

Familial Hypercholesterolemia

  • Frederickson classification of Hyperlipoproteinemia phenotypes and chylomicrons

Primary Hyperlipidemia: Familial Hypercholesterolemia (FH)

  • Type II A (most common): Elevated Total Cholesterol (TC, mainly LDL-C) with normal Triglycerides (TGs).
  • Inheritance: Autosomal dominant disorder.
  • Cause: Mutations in LDL receptor gene.
  • Family History: Strong family history of hypercholesterolemia and/or premature cardiovascular disease.
  • Treatment: Statins.

Secondary Hyperlipoproteinemia

  1. Elevated TC:
    • Hypothyroidism
    • Liver disease
    • Nephrotic syndrome
  2. Elevated TGs:
    • Obesity
    • Diabetes Mellitus (DM)
    • Progesterone
    • Acute hepatitis
  3. Low HDL:
    • Malnutrition
    • Obesity
    • Metabolic syndrome

Review Questions

  • The class of lipoproteins that is protective against atherosclerosis is High-density lipoproteins. (c)
  • Genetic deficiency of lipoprotein lipase causes hyperlipoproteinemia of Type I. (a)
  • Familial Hypercholesterolemia is an autosomal dominant genetic disorder caused by a mutation of the gene that encodes for the LDL receptor. (c)
  • Chylomicron is a type of lipoprotein that transports triglycerides from the intestine to peripheral tissues. Apo B48 is an integral apolipoprotein present in a chylomicron (b)
  • Chylomicron remnants are rapidly taken by the liver in a process that requires ApoE (d)

Lecture Objectives Revisited

  • Describe the main roles of cholesterol in the body, and outline its synthesis and explain the clinical relevance of HMG CoA reductase inhibitors.
  • Describe the classes of plasma lipoproteins, their interactions, and metabolism.
  • Explain the importance of lipoproteins in cardiovascular disease and the stages in the initiation and development of atherosclerosis.
  • Explain why a high-fat diet and poorly controlled diabetes are risk factors for atherosclerosis, and why unsaturated fatty acids, antioxidants, and non-starch polysaccharide are protective.