Lipoproteins are transport vehicles for lipids in the blood, consisting of a hydrophobic core and a hydrophilic shell containing apolipoproteins. They are classified by density and composition, ensuring the delivery of fats and cholesterol to tissues or their reverse transport to the liver.
Primary atherogenic carrierLDLs transport cholesterol to peripheral tissues via Apo B-100.
Protective factorHDLs mediate the reverse transport of cholesterol to the liver, protecting blood vessels.
Particle structureThe core contains cholesteryl esters and triglycerides, while the shell contains phospholipids and proteins.
Exogenous transportChylomicrons transport dietary lipids from the intestine into the bloodstream and tissues.
Structure of a Lipoprotein Particle
Each transport particle has a specialized structure adapted for circulation in the aqueous environment of the blood:
Hydrophobic core: The central region containing cholesteryl esters and triglycerides.
Shell (surface monolayer): A less hydrophobic zone of contact with blood, composed of free cholesterol and phospholipids.
Apolipoproteins: Protein components on the surface that serve as ligands for cellular receptors and activate enzymes.
Classification and Functions of Classes
The blood lipid profile is represented by several classes of particles:
Chylomicrons (CMs): Formed in enterocytes from dietary fats, containing up to 95% triglycerides. Markers include Apo B-48 and Apo C.
VLDLs (Very Low-Density Lipoproteins): Transport endogenous triglycerides, contain Apo B-100 and Apo E, and exhibit atherogenic activity.
IDLs (Intermediate-Density Lipoproteins): A transient, unstable form with an equal ratio of cholesterol to triglycerides, rapidly converting into LDLs.
LDLs (Low-Density Lipoproteins): The main carriers of cholesterol to the periphery (up to 55%), representing the key atherogenic fraction.
HDLs (High-Density Lipoproteins): Rich in phospholipids and proteins, responsible for the reverse cholesterol transport to the liver.
Metabolism and Enzymatic Cleavage
The transformation of particles in the bloodstream involves specific enzymes and receptors:
In tissue capillaries, triglycerides in chylomicrons and VLDLs are hydrolyzed by endothelial lipoprotein lipase.
The key activator of this lipase is Apo C, located on the particle surface.
As a result of lipolysis, chylomicrons are converted into remnants, while VLDLs transform into IDLs and subsequently into LDLs.
The uptake of remnant particles by the liver occurs via receptor-mediated endocytosis.
Atherogenic and Anti-Atherogenic Pathways
The role of lipoproteins in vascular pathology is divided into two categories:
Atherogenic pathway: All classes containing Apo B-100 (VLDLs, IDLs, LDLs) transport cholesterol from the liver to tissues and can deposit within the vascular wall.
Anti-atherogenic pathway: HDLs do not contain Apo B-100, remove excess cholesterol from tissues, and deliver it to the liver for excretion, protecting blood vessels.
Mnemonic
The Apo B Rule: Particles with Apo B-100 travel from the liver to the periphery and promote atherosclerosis (VLDLs, IDLs, LDLs), whereas HDLs lack this protein and clear the vessels.
Frequently asked questions
How do hepatocytes uptake chylomicron remnants?
The uptake of chylomicron remnants by the liver is carried out by hepatocytes via receptor-mediated endocytosis. Particle binding occurs with the participation of scavenger receptors and receptors for Apo B and E, after which the chylomicron remnants are degraded in cellular lysosomes.
Which lipoproteins transport exogenous lipids?
Exogenous (dietary) lipids are transported by chylomicrons, which are synthesized in the intestinal enterocytes.
Why are LDLs referred to as the atherogenic fraction?
LDLs contain the highest amount of cholesterol and the protein Apo B-100, through which they deliver cholesterol to peripheral tissues via receptor-mediated endocytosis.
What is the function of HDLs?
HDLs perform reverse cholesterol transport, removing excess cholesterol from peripheral tissues and delivering it to the liver, making them the primary anti-atherogenic factor.
Which enzyme hydrolyzes triglycerides within chylomicrons and VLDLs?
Triglycerides in tissue capillaries are hydrolyzed by endothelial lipoprotein lipase, which is activated by apolipoprotein C.
Go deeper
Role of hepatocyte scavenger receptors in chylomicron remnant clearance
Synthesis of bile acids from cholesterol in the liver
Molecular mechanisms of LDL receptor-mediated endocytosis
Role of lecithin-cholesterol acyltransferase (LCAT) in HDL metabolism
Pathophysiology of cholesteryl ester accumulation in the vascular wall