Lipoprotein Metabolism: Structure, Classification, Steps

Lipoproteins are complex biological particles composed of a core of triglycerides and cholesterol esters, surrounded by a shell of free cholesterol, phospholipids, and apolipoproteins.

• Lipoproteins are formed in the liver and intestine, arise from metabolic changes of precursor lipoproteins, or are assembled at cell membranes using lipids and apolipoproteins.
• They function as transport vehicles of lipids in blood plasma.

Structure of Lipoproteins

• The core of lipoprotein contains hydrophobic lipids such as triglycerides and cholesterol esters, which are protected from the aqueous environment.
• This core is surrounded by a monolayer of amphipathic molecules, consisting of phospholipids, free cholesterol, and apolipoproteins. 
• Apolipoproteins contribute to both structural integrity and biological function.

Classification of Lipoproteins

Lipoproteins are a heterogeneous group, distinguished by differences in density and by the specific roles they perform in the circulatory system.

They can be classified into five main categories:

• Chylomicrons
  • Produced in the intestinal mucosa, these are very low-density and large-sized particles.
  • Mostly made of dietary triglycerides (approximately 84 %) and minimal protein.
  • Its main role is to deliver exogenous triglycerides to the peripheral tissues.
• Very low-density lipoproteins (VLDL)
  • Produced in the liver and enriched with triglycerides produced endogenously.
  • It contains Apo B-100 as the structural apoprotein, along with Apo C and Apo E.
  • Primarily involved in the transportation of endogenous triglycerides in the liver to tissues.
• Intermediate-density lipoproteins (IDL)
  • This is formed as temporary particles when the VLDL is catabolized in the circulation. 
  • Exhibit intermediate density of mixed triglyceride and cholesterol. 
  • Act as a metabolic precursor of LDL or are taken up by the liver through Apo E-mediated receptors.
• Low-density lipoproteins (LDL)
  • Formed as a result of additional metabolism of IDL.
  • High in cholesterol and cholesterol ester, Apo B-100 is the only apoprotein.
  • The key role includes transporting cholesterol to the peripheral tissues.
• High-density lipoproteins (HDL)
  • Synthesized in the liver and intestine, the smallest and densest lipoprotein class.
  • Characterized by high protein content (30–60%), mainly Apo A-I.
  • Central role in reverse cholesterol transport, returning excess cholesterol to the liver.

The Role of Apoproteins: Functions and Types (ApoA, ApoB, ApoE)

• Apoproteins, also known as apolipoproteins, are the protein components of lipoproteins that bind with lipids such as cholesterol, triglycerides, and phospholipids.
• They stabilize lipoprotein structure, and this enables the movement of lipids in the blood.
• They are essential for the transport of hydrophobic lipids in the aqueous environment of the circulatory system.
• Apoproteins act as enzyme activators, receptor ligands, and play a role in lipoprotein metabolism and clearance.

Key Apoprotein Types

• ApoA (Apolipoprotein A)
  • The main protein component of HDL is synthesized within the liver (70%) and intestine (30%).
  • Promotes the removal of extra cholesterol from peripheral tissues and returns it to the liver for excretion by driving reverse cholesterol transport.
  • Cardiovascular protection is linked to elevated ApoA levels.
• Apolipoprotein B (ApoB)
  • Structural protein of VLDL, IDL, and LDL; present as ApoB-100 in liver-derived particles and ApoB-48 in intestinal chylomicrons.
  • Vital for ligand binding to LDL receptors and lipoprotein assembly and secretion, which facilitates the delivery of cholesterol to cells.
  • A higher number of atherogenic lipoprotein particles and a higher risk of cardiovascular disease are indicated by elevated ApoB.
• Apolipoprotein E (ApoE)
  • present on HDL, VLDL, and chylomicron remnants.
  • Act as a ligand for lipoprotein receptors, especially in the liver, facilitating clearance of remnant particles from the circulation.
  • Additionally, ApoE plays a role in lipid transport in tissues other than the liver, such as the brain.

Exogenous Pathway: Metabolism of Chylomicrons from Diet

Formation of Chylomicrons

• In the intestinal lumen, pancreatic lipase breaks down dietary triglycerides (TG) and cholesterol esters into free fatty acids, monoacylglycerols, and glycerol. 
• These elements in enterocytes are resynthesized into cholesterol esters and triglycerides. 
• The nascent chylomicrons are assembled with triglycerides, cholesterol esters, phospholipids, and apolipoproteins (apo A and apo B-48).

Secretion into Lymph and Blood

• The nascent chylomicrons are discharged into the lymphatic system through the lacteals to be introduced into the bloodstream through the thoracic duct.
• Nascent CM picks up apo E, apo C-II from HDL. 

Triglyceride Hydrolysis by Lipoprotein Lipase

• The Apo C-II stimulates lipoprotein lipase (LPL) in the endothelium of the capillaries.
• Triglycerides in chylomicrons are hydrolyzed by LPL, releasing fatty acids that are absorbed by peripheral tissues.

Formation of Chylomicron Remnants

• After most of the TG is removed, chylomicrons become smaller and are called chylomicron remnants.
• During the process, CM gives some of the phospholipids and apo C and apo A to HDL.

Hepatic uptake and processing 

• Chylomicron remnants are endocytosed by binding to liver receptors (LDL receptor or LRP) through apo E.
•  They bring dietary cholesterol, triglycerides, and HDL-derived cholesterol to the liver to be stored, to form bile salt, or to be excreted.
• The half-life of CM is short, less than 1 hour.

Endogenous Pathway: VLDL Synthesis and Transformation to LDL

• VLDL is made by the liver. The hepatocytes actively synthesize triglycerides (TG) out of fatty acids and glycerol.
• Hepatic cholesterol can either be derived from chylomicron remnants via the exogenous pathway or be synthesized on its own from cholesterol by hepatocytes when dietary cholesterol is insufficient.
• The triglycerides and cholesterol that are formed are packaged together in the liver and released into the circulation as nascent VLDL.
• The VLDL core is made up of triglycerides as the major lipid constituent.
• Nascent VLDL contains apo B-100. 
• In the blood, nascent VLDL pick up apo E and apo C-II from circulating HDL and become mature VLDL.
• Lipoprotein lipase (LPL) breaks down VLDL triglycerides in the capillaries of the peripheral tissues into glycerol and free fatty acids, which are diffused into the tissues. 
• LPL requires Apo C-II in its activation.
• When the triglycerides are removed, VLDL is converted to a VLDL remnant, or intermediate-density lipoprotein (IDL). In the process, the apo C-II is reintroduced to HDL.
• VLDL and HDL exchange lipids, with triglycerides being transferred to HDL and cholesteryl esters being transferred to VLDL, a process mediated by CETP.
• VLDL levels are enriched with cholesteryl esters. 

Low-Density Lipoprotein (LDL): Receptor-Mediated Endocytosis

• The process of LDL removal in the blood is regulated and is mainly done by hepatic cells and other peripheral tissue cells.
• LDL particles bind with certain LDL receptors, which are found on the cell surface.
• These LDL-receptor complexes then gather in specialized regions of the cell membrane called clathrin-coated pits.
• Such pits invert inwards and constrict to create clathrin-coated vesicles.
• Immediately after internalization, the clathrin coat disassembles from the vesicle and fuses with an early endosome.
• LDL is released from the receptor within the acidic environment of the endosome, and the receptor is returned to the plasma membrane.
• The LDL particles are then moved out of the endosome into the lysosome.
• Lysosomes break down the LDL proteins into amino acids, and the cholesterol esters are broken down into free cholesterol and fatty acids.

 Reverse Cholesterol Transport: The Protective Role of HDL

• Reverse cholesterol transport refers to cellular transport of cholesterol from peripheral tissue to the liver, where it is excreted in feces as bile acids, cholesterol, and other catabolic products.
• HDL particles are formed in the blood by the addition of lipid to apo A-1, an apolipoprotein made by the liver and intestine and secreted into the blood.
• Approximately 70 percent of HDL apoproteins are Apo A-I, with Apo C-II and Apo E also present.
• Nascent HDL are disk-shaped, containing a high amount of phospholipids, principally phosphatidylcholine, and little cholesterol esters and triglycerides.
• Nascent HDL takes up cholesterol in chylomicrons, VLDL, and peripheral tissue in plasma through ATP-binding cassette protein-1 (ABCA1)
• Lecithin-cholesterol acyltransferase (LCAT) esterifies the obtained cholesterol immediately into cholesteryl esters of hydrophobic nature, which are transported into the HDL core.
• This is where nascent HDL is transformed into spherical mature HDL.
• HDL transports the cholesterol to the liver either directly through the SR-B1 receptor or indirectly through the transfer to the triglyceride-rich lipoproteins that are removed by the hepatic LDL receptors.
• In the liver, cholesterol is converted to bile acids or excreted into bile, completing elimination from the body.

Enzymatic Regulation: Lipoprotein Lipase (LPL) and LCAT

Lipoprotein Lipase (LPL):

• LPL breaks triglycerides (TG) of VLDL and chylomicrons into free fatty acids and glycerol.
• ApoC-II activates LPL, allowing the breakdown of triglycerides, whereas ApoC-III suppresses this effect.
• Insulin increases the LPL activity in the adipose tissue following meals; fasting or low insulin stimulates the LPL activity in muscle to provide energy.
• Repressed by ANGPTL4, which temporarily lowers lipid metabolism by destabilizing the active site of LPL

Lecithin–Cholesterol Acyltransferase (LCAT)

• LCAT is a plasma enzyme secreted by the liver, which esterifies free cholesterol to cholesteryl esters on HDL, leading to HDL maturation and reverse cholesterol transport.
• Activated by apo A-I on the HDL particles, which increases the esterification activity of LCAT.
• The control of hormones is indirect; sex hormones can affect the activity, but no hormone regulates LCAT directly, as LPL is regulated by insulin.

Lipid Transport Disorders: Dyslipidemia and Hypercholesterolemia

Dyslipidemia 

• Dyslipidemia is a metabolic disorder characterized by abnormal quantities and patterns of lipids and lipoproteins in the blood.
•  Typically manifested as Elevated LDL cholesterol, total cholesterol, triglycerides, and low HDL cholesterol concentrations.
• It is a significant risk factor for atherosclerosis and heart diseases such as heart attacks and strokes.
• There are primary causes (genetic predisposition) and secondary (lifestyle/medical) ones, including obesity, diabetes, unhealthy diet, and exercise.

Hypercholesterolemia 

• Hypercholesterolemia is a lipid disorder that is characterized by excessively high levels of low-density lipoprotein cholesterol (LDL-C) in the blood, increasing the risk of atherosclerotic cardiovascular disease.
• Major risk factors include sedentary lifestyle, saturated/trans-fat diets, obesity, diabetes, and genetic predisposition (e.g., familial hypercholesterolemia).
• A condition usually asymptomatic up to the point of advanced atherosclerosis that leads to the occurrence of events like myocardial infarction or stroke.
• The management aims at lifestyle change (diet, exercise, quitting smoking) and pharmacotherapy using evidence-based statins and other lipid-lowering drugs to decrease LDL-C and cardiovascular risk.

Atherosclerosis: The Link Between Oxidized LDL and Plaque Formation

• Atherosclerosis is a condition, which is characterized by the deposition of lipids, fibrous matter, and calcification in the large arteries.
• Endothelial dysfunction initiates the process of atherosclerosis, which permits the entry of LDL into the arterial intima. Modified LDL, especially oxidized LDL (oxLDL), plays a central role.
• Because oxidized LDL avoids normal proteoglycan retention, it is more vulnerable to unregulated uptake by macrophage scavenger receptors.
• After being internalized, oxidized LDL causes the macrophages to express pro-inflammatory molecules, which also leads to chronic inflammation in the vessel wall.
• The macrophages that ingest large volumes of oxidized LDL develop into foam cells, forming the initial fatty streaks in atherosclerotic plaque.
• Further lipid deposition, inflammation, and recruitment of cells cause the further development of the plaque, which constricts the arteries and increases the risk of cardiovascular disease.

Clinical Assessment: Understanding Lipid Profile Tests

• A Lipid Profile Test, commonly referred to as a lipid panel, is a vital diagnostic test that measures the amount of the various fats or lipids in your blood.
• It quantifies the total cholesterol, LDL (bad) cholesterol, HDL (good) cholesterol, and triglycerides, which give an understanding of atherosclerotic risk and management.

Lipid Profile Diagnostic Criteria (mg/dL):

Conclusion 

• Lipoproteins are particles produced in the liver and intestines and carry triglycerides and cholesterol in the blood.
• They contain a hydrophobic core and an apolipoprotein on the surface; they are chylomicrons, VLDL, IDL, LDL, and HDL.
• The ApoA facilitates reverse cholesterol transport in HDL, ApoB facilitates VLDL/LDL structure and receptor binding, and ApoE clears remnants.
• Dietary lipids are transported through chylomicron (exogenous), liver lipids through VLDL-LDL (endogenous).
• LDL transports cholesterol to the cell; HDL cleanses the excess cholesterol.
• LPL and LCAT control the hydrolysis of lipids and the maturation of HDL.
• Lipid profile tests measure dyslipidemia and hypercholesterolemia, which encourage atherosclerosis through oxidized LDL.

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