Plasma Lipoproteins and the Molecular Basis of Atherosclerosis

Overview of Plasma Lipoproteins

  • Definition: Plasma lipoproteins are spherical macromolecular complexes composed of lipids and specific proteins known as apoproteins (or apolipoproteins).
  • Primary Function: They function to keep lipids soluble as they transport them through the plasma and provide an efficient mechanism for transporting their lipid contents to (and from) the tissues.
  • General Structure:
    • Surface Layer (Hydrophilic): Composed of proteins, a phospholipid monolayer, and free cholesterol. The polar heads of phospholipids and the free hydroxyl group of cholesterol face the aqueous environment.
    • Core (Hydrophobic): Composed of triacylglycerols (TAG) and cholesteryl esters. Although cholesterol is physically a surface component, its esters are sequestered in the interior of the particle.

Classification and Physical Properties

Lipoproteins are classified based on their density, which is determined by the ratio of lipid to protein. As the lipid content decreases and protein content increases, the density of the particle increases, while its size decreases.

  • Chylomicrons (CM):
    • Size: 70−1000 nm70-1000\,\text{nm}.
    • Density: <0.95 g/mL< 0.95\,\text{g/mL}.
    • Composition: Highest percentage of lipids (90% triacylglycerol90\%\,\text{triacylglycerol}) and lowest percentage of protein (1−2%1-2\%\text{proteins}, 2−5%2-5\%\text{cholesterol and esters}, 3−8%3-8\%\text{phospholipids}).
    • Origin: Formed in the intestinal mucosal cells from dietary (exogenous) lipids.
  • Very-Low-Density Lipoproteins (VLDL):
    • Size: 30−80 nm30-80\,\text{nm}.
    • Density: 0.95−1.006 g/mL0.95-1.006\,\text{g/mL}.
    • Composition: High triacylglycerol content (60%60\%\text{TAG}, 10−15%10-15\%\text{cholesterol}, 15−20%15-20\%\text{phospholipids}, 5−10%5-10\%\text{protein}).
    • Origin: Produced in the liver (endogenous lipids).
  • Low-Density Lipoproteins (LDL):
    • Size: 20−26 nm20-26\,\text{nm}.
    • Density: 1.019−1.063 g/mL1.019-1.063\,\text{g/mL}.
    • Composition: Very high cholesterol and cholesteryl ester content (50%50\%\text{cholesterol/esters}, 8%8\%\text{TAG}, 22%22\%\text{phospholipids}, 20%20\%\text{protein}).
    • Origin: Derived from the metabolism of VLDL (specifically and intermediate stage known as IDL).
  • High-Density Lipoproteins (HDL):
    • Size: 6−12.5 nm6-12.5\,\text{nm}.
    • Density: 1.063−1.21 g/mL1.063-1.21\,\text{g/mL}.
    • Composition: Highest protein content (40−55%40-55\%\text{protein}, 25−30%25-30\%\text{phospholipids}, 15−20%15-20\%\text{cholesterol/esters}, 5%5\%\text{TAG}).
    • Origin: Synthesized in the liver and intestine.
  • Electrophoretic Mobility: At pH 8.6\text{pH 8.6}, lipoproteins migrate toward the anode (++). The order of mobility from slowest to fastest is: Chylomicrons (remain at origin) → LDL\rightarrow\,\text{LDL}  (Beta-lipoprotein) → VLDL\rightarrow\,\text{VLDL}  (Pre-beta-lipoprotein) → HDL\rightarrow\,\text{HDL}  (Alpha-lipoprotein).

Apolipoproteins: Structure and Function

Apolipoproteins (Apo) serve three main functions: acting as structural components, acting as enzyme cofactors, and serving as ligands for cell-surface receptors.

  • Major Classes: Classes A through E, often with specific subclasses.
  • Specific Apoproteins and Their Roles:
    • Apo A-I: Found in HDL; acts as an activator for the enzyme LCAT (Lecithin-cholesterol acyltransferase) and serves as a ligand for the HDL receptor.
    • Apo B-48: Specific to chylomicrons; essential for the assembly and secretion of chylomicrons from the small intestine.
    • Apo B-100: Found in VLDL and LDL; recognized by the LDL receptor for cellular uptake.
    • Apo C-II: Acts as a mandatory cofactor to activate lipoprotein lipase (LPL).
    • Apo E: Recognized by the hepatic remnant receptor (LRP) and is essential for the clearance of chylomicron remnants and IDL.

Metabolism of Chylomicrons

  1. Synthesis and Assembly: Chylomicrons are assembled in the intestinal mucosal cells, carrying dietary triacylglycerols, cholesterol, fat-soluble vitamins, and cholesteryl esters.
  2. Nascent Chylomicrons: These initially contain only Apo B-48. They are secreted into the lymphatic system and then enter the blood.
  3. Maturation: In the blood, they receive Apo C-II and Apo E from HDL particles, becoming "mature" chylomicrons.
  4. Degradation: Lipoprotein Lipase (LPL), an extracellular enzyme anchored by heparan sulfate to capillary walls (predominantly in adipose, heart, and skeletal muscle), is activated by Apo C-II. LPL hydrolyzes the TAG in the chylomicron into free fatty acids and glycerol.
  5. Fate of Components: Free fatty acids are taken up by tissues for energy (muscle) or storage (adipose). Glycerol is transported to the liver.
  6. Remnant Formation: As the chylomicron loses TAG, it becomes smaller and relatively enriched in cholesterol. It returns Apo C-II to HDL. The resulting chylomicron remnant binds to the liver via Apo E and is endocytosed.

Lipoprotein Lipase (LPL) Regulation

  • Synthesis: LPL is synthesized by adipose tissue and muscle (cardiac and skeletal).
  • Insulin Regulation:
    • Fed State: High insulin increases LPL synthesis in adipose tissue (favoring storage) but decreases it in muscle.
    • Fasting State: Low insulin favors LPL synthesis in muscle tissue (favoring energy use).
  • Tissue Specificity: The highest concentration of LPL is in cardiac muscle, reflecting the heart's continuous need for fatty acids as a primary energy source.
  • Clinical Note: Type 1 Hyperlipoproteinemia: A deficiency in LPL or Apo C-II leads to massive accumulation of chylomicron-TAG (>1,000 mg/dl> 1,000\,\text{mg/dl}) even when fasting. This condition, also called familial LPL deficiency, increases the risk of acute pancreatitis.

Metabolism of VLDL and LDL

  • VLDL Production: VLDL is produced in the liver to carry endogenous triacylglycerols to peripheral tissues. Like chylomicrons, nascent VLDL receives Apo C-II and Apo E from HDL.
  • VLDL to LDL Conversion: As LPL degrades TAG in VLDL, the particles become smaller (Intermediate-Density Lipoprotein or IDL). Eventually, Apo C-II and Apo E are returned to HDL, and the particle is converted to LDL.
  • CETP Function: Cholesteryl Ester Transfer Protein (CETP) facilitates an exchange reaction: it moves some TAG from VLDL to HDL in exchange for cholesteryl esters from HDL to VLDL.
  • Abetalipoproteinemia: A rare condition caused by a defect in Microsomal TAG Transfer Protein (MTP), preventing the loading of Apo B with lipid. This leads to an inability to form VLDLs or chylomicrons, resulting in TAG accumulation in the liver and intestine.
  • Hepatic Steatosis (Fatty Liver): Occurs when there is an imbalance between hepatic TAG synthesis and VLDL secretion, commonly seen in obesity and Type 2 diabetes.

Cellular Uptake and Degradation of LDL

  • LDL Core: LDL particles contain a high concentration of cholesteryl esters and provide peripheral tissues with cholesterol.
  • Receptor-Mediated Endocytosis:
    1. LDL binds to high-affinity LDL receptors concentrated in clathrin-coated pits.
    2. The complex is internalized as a coated vesicle, which fuses with an endosome.
    3. The LDL receptor is recycled back to the cell membrane.
    4. The LDL remnant moves to a lysosome, where it is degraded into amino acids, fatty acids, and free cholesterol.
  • Regulation of Intracellular Cholesterol: An oversupply of cholesterol within the cell triggers:
    • Downregulation of HMG CoA reductase (the rate-limiting enzyme of cholesterol synthesis).
    • Downregulation of new LDL receptor synthesis to prevent further uptake.
    • Activation of ACAT (Acyl-CoA:cholesterol acyltransferase) to convert free cholesterol into cholesteryl esters for storage.

Metabolism of HDL and Reverse Cholesterol Transport

  • HDL Functions:
    • Acts as a reservoir for Apo C-II and Apo E.
    • Reverse Cholesterol Transport: The selective transfer of cholesterol from peripheral cells to HDL and then back to the liver.
  • Mechanism:
    1. Nascent HDL: Discoidal particles produced by the liver and intestine containing Apo A-I.
    2. Cholesterol Uptake: Free cholesterol from peripheral tissues is transferred to HDL via the ABCA1 transporter.
    3. Esterification: Lecithin-cholesterol acyltransferase (LCAT) esterifies this cholesterol, pushing it into the hydrophobic core of the HDL particle (converting HDL3 to HDL2).
    4. Hepatic Return: Cholesteryl esters are delivered to the liver via the SR-B1 scavenger receptor.

Role of Lipoproteins in Heart Disease

  • Lp(a): A variant of LDL that contains an additional protein, Apo(a), linked to Apo B-100 by a disulfide bond.
    • Genetics: Circulating levels of Lp(a) are determined primarily by genetics.
    • Mechanism: Apo(a) is structurally homologous to plasminogen, which binds to fibrin to break down clots. Lp(a) competes with plasminogen for fibrin binding, slowing the breakdown of blood clots and increasing the risk of heart attacks.
  • Oxidized LDL (oxLDL): When LDL is modified (oxidized) in the subendothelial space by reactive oxygen species (superoxide, nitric oxide), it is no longer recognized by high-affinity LDL receptors. Instead, it is taken up by low-affinity scavenger receptors on macrophages.

Biochemical Aspects of Atherosclerosis

Atherosclerosis is an inflammatory process within the arterial wall involving the accumulation of lipids and monocytic cells.

  • Risk Factors: Hypertension, smoking, high circulating levels of LDL or remnants, low HDL, chronic hyperglycemia, and high angiotensin II levels.
  • Development of the Fatty Streak:
    1. Endothelial Injury: Caused by hypertension, toxins (smoking), or hyperlipidemia.
    2. Monocyte Adhesion: Monocytes adhere to endothelial cells and migrate into the subendothelium (intima).
    3. Foam Cell Formation: Monocytes convert into macrophages, which consume modified (oxidized) LDL via scavenger receptors. As they become engorged with lipid, they become "foam cells."
    4. Fatty Streak: An accumulation of lipid-laden foam cells in the subendothelial space.
  • Plaque Progression: Foam cells release cytokines and growth factors that stimulate smooth muscle cells (SMCs) to migrate from the media to the intima. SMCs proliferate and produce collagen, eventually forming a fibrous cap over a lipid core.
  • Therapeutic Suppression: Anti-inflammatory agents like acetylsalicylic acid (aspirin) and HMG-CoA reductase inhibitors (statins) can help suppress this inflammatory cascade.

Key Enzymes and Pathways in Cholesterol Metabolism

  • HMG CoA Reductase Pathway:
    • Acetyl CoA→HMG CoA\text{Acetyl CoA} \rightarrow \text{HMG CoA}
    • HMG CoA→HMG CoA ReductaseMevalonate\text{HMG CoA} \xrightarrow{\text{HMG CoA Reductase}} \text{Mevalonate}
    • Mevalonate→IPP→Geranyl-PP→Farnesyl-PP→Squalene→Cholesterol\text{Mevalonate} \rightarrow \text{IPP} \rightarrow \text{Geranyl-PP} \rightarrow \text{Farnesyl-PP} \rightarrow \text{Squalene} \rightarrow \text{Cholesterol}
    • Inhibitors: Statins target HMG CoA Reductase to lower endogenous cholesterol synthesis.

Questions & Discussion

  • Why is cardiac muscle regulation of LPL unique?
    • Cardiac muscle has the highest concentration of LPL because it relies heavily on fatty acids for energy. Its LPL synthesis is not downregulated during fasting in the same way adipose LPL is; instead, it is optimized to ensure the heart receives energy regardless of the nutritional state.
  • What is the role of the scavenger receptor in foam cell formation?
    • Unlike the LDL receptor, scavenger receptors are not downregulated when the cell has sufficient cholesterol. This allows macrophages to continue taking up oxidized LDL indefinitely, leading to the massive accumulation of lipid that characterizes foam cells.