Cholesterol, Triglycerides and Disease of Lipid Metabolism

Lipoproteins

DefinitionLipoproteins are particles that transport cholesterol and triglycerides in the bloodstream, composed of apoproteins, phospholipids, triglycerides, and cholesterol. They play a crucial role in lipid metabolism, crucial for maintaining cellular function and energy supply.

ImportanceCholesterol and triglycerides are hydrophobic molecules and must be transported via lipoproteins due to their insolubility in the aqueous environment of blood. The esterification of cholesterol enhances its hydrophobicity, facilitating its integration into the lipoprotein core, which is essential for maintaining the structural integrity of these particles.

Apolipoproteins

ClassificationApolipoproteins are divided into several classes and subclasses, including:

  • Apolipoprotein A: apoA1, apoA2, etc., primarily involved in HDL function

  • Apolipoprotein B: apoB48, apoB100, significant for LDL and chylomicron structure

  • Apolipoprotein C: apoC-I, apoC-II, etc., critical in lipoprotein metabolism

  • Apolipoproteins D, E, F, H, L, M, (a)

Roles

  • Enzymatic Functions: Facilitate the transfer of fatty acids and the esterification of cholesterol, important for energy storage and mobilization.

  • Ligands for Receptors: Assist in the recognition of lipoprotein particles by target cells, initiating endocytosis and metabolism of lipoproteins.

  • Regulatory Functions: Control lipoprotein metabolism and synthesis, directly influencing lipid levels in circulation.

  • Structural Components: Stabilize lipoprotein particles, ensuring their functionality in the bloodstream.

  • Cofactors: Activate enzymes within cells, impacting lipid and carbohydrate metabolism.

Lipoprotein Groups

Classes of Lipoproteins and Their Density:

  • Chylomicrons: Size ~500 nm, Density ~0.95; transports dietary lipids.

  • VLDL (Very Low-Density Lipoprotein)

  • IDL (Intermediate Density Lipoprotein)

  • LDL (Low-Density Lipoprotein)

  • HDL (High-Density Lipoprotein)

Formation of Chylomicrons

Source and FunctionChylomicrons are vital for transporting dietary triacylglycerides (TAGs), cholesterol, and fat-soluble vitamins. They are majorly composed of 90% TAGs, allowing efficient delivery of energy and nutrients.

Formation Steps

  1. Nascent Chylomicron Formation: Formed in the endoplasmic reticulum of enterocytes; they are immature and contain TAGs and apolipoproteins (apo B-48, apo A). Dietary TAGs are transferred via microsomal transfer protein (MTP).

  2. Maturation: In circulation, chylomicrons acquire apoC-II and ApoE from HDL, becoming mature chylomicrons that can efficiently deliver lipids.

Lipid Delivery by Chylomicrons Mechanism

  • Deliver lipids to Adipose Tissue: Triggered by lipoprotein lipase (LPL), which is activated by ApoC-II; this enzyme breaks down TAG into free fatty acids and glycerol for storage.

  • Skeletal Muscles and Liver: As TAGs are hydrolyzed, chylomicrons shrink, becoming chylomicron remnants that are absorbed by the liver for further processing.

Formation and Metabolism of VLDL

VLDL CompositionVLDL is synthesized in the liver and contains approximately 15-20% cholesterol and 55-65% TAGs, serving as a crucial carrier of endogenous lipids.

Lipoprotein Metabolism Pathway

  • VLDL Exportation & Degradation: Interacts with tissue lipase, transforming into IDL and then LDL as TAGs are progressively lost.

  • Cholesterol Transport: LDL functions primarily to transport cholesterol from the liver to peripheral tissues, playing a key role in maintaining cellular cholesterol homeostasis.

Mechanism of LDL Uptake

  • LDL Receptor Binding: LDL binds to receptors via ApoB100 or ApoE, crucial for cellular uptake.

  • Uptake Process: LDL is processed in a clathrin-coated vesicle; after LDL extraction, the receptor is recycled to the cell surface.

  • Effects of Cholesterol Levels: Intracellular cholesterol levels dictate the synthesis and number of LDL receptors, thus regulating cholesterol homeostasis.

HDL Functionality

HDL CharacteristicsHDL is synthesized in the liver and intestines, consisting of 40-55% protein and around 20% cholesterol. It is often referred to as "good cholesterol" due to its cardioprotective effects.

Functions

  • Cholesterol Uptake: Removes free cholesterol from cells (particularly foam cells); this process is esterified via LCAT (Lecithin-Cholesterol Acyltransferase).

  • Reverse Transport: Transfers cholesterol back to the liver for processing and influences the risk of atheroma formation positively by preventing plaque buildup.

Lipid Profile Measurement Parameters

To assess lipid metabolism, the following parameters are important:

  • Total Cholesterol

  • HDL

  • Triglycerides calculated to assess LDL levels.Recommended levels:

  • HDL > 40 mg/dL

  • LDL < 100 mg/dL

  • Total Cholesterol < 200 mg/dL

  • Triglycerides < 150 mg/dL

Hypercholesterolemias

Types

  • Primary Hypercholesterolemias: Includes Classic Familial Hypercholesterolemia (autosomal dominant disorders affecting LDL receptor function), leading to high LDL levels.

  • Secondary Hypercholesterolemias: Result from other diseases (e.g., diabetes, hypothyroidism) that affect lipoprotein metabolism, contributing to increased cardiovascular risk.

Atheromatous Plaque Formation

Steps in formation are linked to LDL deposition, leading to chronic inflammation and plaque development, which increases the risk of cardiovascular diseases.

Treatments of Hypercholesterolemia

Pharmacological Interventions

  • Statins: HMG CoA reductase inhibitors (e.g., Atorvastatin), reduce cholesterol synthesis.

  • PCSK9 Inhibitors: (e.g., Alirocumab) increase LDL receptor availability, lowering LDL levels.

  • Nicotinic Acid: Increases HDL levels, improving lipid profiles.

  • Plant Sterols: Lower LDL by inhibiting cholesterol absorption.

Dietary AdjustmentsFocus on reducing saturated fats and increasing dietary fiber to improve lipid profiles; inclusion of whole grains, fruits, vegetables, and healthy fats (like those from fish and nuts) can enhance cardiovascular health and optimize lipid metabolism.