Lipoproteins and Cholesterol Metabolism

Lipoproteins Overview

• Lipoproteins: Molecules that transport lipids (fats) in the bloodstream.
• Focus on chylomicrons, very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL).
• These lipoproteins are crucial for cholesterol trafficking in the body.

Very Low Density Lipoproteins (VLDL)

• Composition of VLDL:
    - Triacylglycerol (TAG): Approximately 50% of VLDL weight consists of triacylglycerols.
    - Free Cholesterol and Cholesterol Esters: Remaining lipid content includes a significant amount of free cholesterol and cholesterol esters.
• Density Comparison:
    - VLDL has a higher protein content than chylomicrons, resulting in a greater density.
    - Chylomicrons have a low density due to a comparatively low protein content.
    - High Density Lipoproteins (HDL): Have the highest protein content, hence the highest density.
• Structure:
    - Lipoproteins have a phospholipid monolayer (instead of bilayer) due to their hydrophobic internal contents which do not require interaction with polar parts of phospholipids.
  

Chylomicrons

• Description: Lipoproteins created in the intestine, primarily composed of triacylglycerols (TAG).
• Function:
    - Distribute triacylglycerols to various tissues, primarily supplying adipose tissue.
    - Fatty acids from chylomicrons can also be delivered to muscle tissues as free fatty acids.
• Liver Interaction:
    - The remnants of chylomicrons are sent to the liver.
    - The liver processes these remnants and liberates dietary cholesterol, which contributes to the liver's cholesterol pool.

Conversion of VLDL to LDL

• Formation: VLDL is synthesized in the liver with a high content of TAG transformed from dietary intake.
• Function of VLDL:
    - VLDL transports large amounts of TAG and cholesterol esters into the bloodstream.
    - Once in the bloodstream, VLDL loses TAG via hydrolysis, delivering fatty acids to tissues, which converts it into LDL.
• Role of LDL:
    - LDL carries cholesterol to cells that require it for membrane synthesis and hormone production.
• Internalization of LDL:
    - Cells express LDL receptors to bind to ApoB100 protein on LDL particles.
    - Once bound, they enter the cell via endocytosis.
    - Endosomal degradation separates LDL from its receptor, allowing recycling of the receptor to the surface of the cell.

Cholesterol Control in Cells

• What Happens to LDL:
    - The lysosome breaks down the LDL particles into free cholesterol and fatty acids.
• Regulation of LDL Receptors:
    - The quantity of LDL receptors is dependent on cellular cholesterol levels.
    - High cholesterol signals downregulation of LDL receptors (recycling reduced or receptors stored/broken down).

High Density Lipoproteins (HDL)

• Function of HDL:
    - HDL is responsible for transporting excess cholesterol back to the liver, counteracting the cholesterol distribution by LDL.
    - HDL particles serve as scavengers and collect cholesterol from peripheral tissues, including foam cells.
• Mechanism of HDL Action:
    - HDL precursors are produced by the liver.
    - HDL transports cholesterol back to liver through scavenger receptors for processing.

Atherosclerosis

• Definition: A condition characterized by high cholesterol levels leading to arterial disease.
• Initiation: Begins with monocytes responding to an abundance of circulating LDL in blood.
    - Cells differentiate to macrophages that uptake oxidized LDL particles, forming foam cells.
    - Foam cells are lipid-laden macrophages that accumulate in the arterial walls.
• Consequences:
    - Foam cells aggregate, leading to calcification and hardening of arteries (atherosclerosis).
    - Smooth muscle proliferation caused by growth factors released from endothelial cells leads to reduced elasticity and increased blood pressure.
  

Prevention of High Cholesterol Levels

• HMG CoA Reductase Inhibitors: Statins can help lower cholesterol when necessary.
• Lifestyle Changes:
    - Keeping lipid levels low promotes lower LDL and higher HDL levels.
    - Antioxidants (e.g., Vitamin E) sourced from food are essential for preventing oxidation of lipoproteins.

Importance of HDL

• Increased HDL Benefits:
    - High HDL levels are advantageous, potentially even surpassing the importance of keeping total cholesterol below 200 mg/dL.
    - High levels of HDL can help manage and keep LDL levels under control.
• Methods to Elevate HDL:
    - Regular exercise has been consistently shown to increase HDL levels, beneficial for cardiovascular health and longevity.