Lipoproteins are the transport vehicles that move fats through your blood. Because lipids like cholesterol and triglycerides can't dissolve in water, they need these protein-wrapped particles to travel through the aqueous bloodstream. Understanding how lipoproteins work is foundational to pharmacology in this unit, since every lipid-lowering drug targets some part of this transport system.
The balance between different lipoprotein types directly affects cardiovascular risk. Too much LDL cholesterol promotes plaque buildup in arteries, while HDL cholesterol helps remove it. Grasping these relationships will help you understand why specific drugs are prescribed and what lab values you're monitoring in patients.
Lipoproteins and Apolipoproteins
Role of lipoproteins
Lipoproteins solve a basic chemistry problem: lipids (cholesterol and triglycerides) are hydrophobic, meaning they can't dissolve in the watery environment of blood. Lipoproteins wrap these fats in a shell of phospholipids and proteins so they can be transported where the body needs them.
Lipoproteins are classified by their density, size, and composition. The four major classes are:
- Chylomicrons (largest, least dense)
- Very low-density lipoproteins (VLDL)
- Low-density lipoproteins (LDL)
- High-density lipoproteins (HDL) (smallest, most dense)
A helpful pattern: as the particle gets smaller and denser, it carries proportionally more cholesterol relative to triglycerides.
Apolipoproteins are the protein components embedded in the lipoprotein shell. They do two critical things: they stabilize the particle's structure, and they act as ligands that bind to specific receptors on cell surfaces. This receptor binding is what directs lipid delivery and uptake. Two you should know:
- ApoB-100: found on LDL particles; binds to LDL receptors on cells
- ApoA-I: found on HDL particles; activates the enzyme LCAT, which is essential for reverse cholesterol transport
Abnormal lipid levels and atherosclerosis
Three lipid abnormalities raise cardiovascular risk: elevated LDL cholesterol (LDL-C), elevated triglycerides, and low HDL cholesterol (HDL-C). Together or individually, these promote atherosclerosis, the buildup of plaque within arterial walls that causes narrowing and hardening. When atherosclerosis affects the arteries supplying the heart (such as the left anterior descending or right coronary artery), it's called coronary artery disease (CAD).
Here's how LDL drives plaque formation, step by step:
- LDL particles infiltrate the arterial wall through the endothelium.
- Once trapped in the wall, LDL undergoes oxidation.
- Oxidized LDL triggers an inflammatory response, attracting macrophages.
- Macrophages engulf the oxidized LDL and become foam cells.
- Foam cells accumulate, forming fatty streaks that progress into fibrous plaques over time.
Two additional risk factors make this worse:
- High triglycerides increase the production of small, dense LDL particles. These are more atherogenic than larger, buoyant LDL because they penetrate the arterial wall more easily and are more susceptible to oxidation.
- Low HDL-C means less reverse cholesterol transport, the process by which HDL removes excess cholesterol from peripheral tissues and the arterial wall and carries it back to the liver. With less HDL doing this cleanup, cholesterol accumulates where it shouldn't.
Major lipoprotein types and functions
Chylomicrons
Chylomicrons are produced by the intestines after you eat a meal containing fat. Their job is to transport dietary triglycerides and cholesterol from the gut into the bloodstream. An enzyme called lipoprotein lipase (found on the surface of capillaries in adipose tissue and muscle) breaks down the triglycerides so those tissues can take them up for storage or energy. What's left after triglyceride removal is a chylomicron remnant, which the liver clears from the blood via the ApoE receptor.
Very Low-Density Lipoproteins (VLDL)
VLDL is produced by the liver and carries endogenous (internally made) triglycerides and cholesterol out to peripheral tissues. Like chylomicrons, VLDL delivers triglycerides through the action of lipoprotein lipase. As triglycerides are progressively removed, the VLDL particle shrinks and becomes denser, first becoming an intermediate-density lipoprotein (IDL), and eventually becoming LDL. This is why high triglyceride production by the liver leads to more LDL in the blood.
Low-Density Lipoproteins (LDL)
LDL is derived from VLDL and IDL and is the primary carrier of cholesterol to peripheral tissues. Cells take up LDL by binding its ApoB-100 protein to LDL receptors on their surface. The cholesterol delivered is used for cell membrane synthesis and steroid hormone production (particularly in the adrenal glands and gonads). Elevated LDL-C is the single most significant modifiable risk factor for atherosclerosis, which is why it's the primary target of lipid-lowering therapy.
High-Density Lipoproteins (HDL)
HDL is produced by both the liver and intestines and plays a protective role. Its main function is reverse cholesterol transport: HDL picks up excess cholesterol from peripheral tissues and the arterial wall and carries it back to the liver. Once at the liver, that cholesterol can be excreted in bile acids or recycled into new VLDL particles. Higher HDL-C levels are associated with reduced risk of cardiovascular events like myocardial infarction and stroke. Think of HDL as the cleanup crew working against the damage that excess LDL causes.