Exhaustive Notes on Dyslipidemia and Lipoprotein Metabolism

Overview and Learning Outcomes of Lipoprotein Metabolism

The study of dyslipidemia begins with an understanding of lipoprotein metabolism and its associated disorders. The primary educational objectives focusing on the spring 2025-2026 curriculum at Abu Dhabi University include outlining the specific roles and functions of lipoproteins, identifying disorders arising from abnormalities in lipoprotein metabolism, and understanding the clinical significance of these metabolic diseases. Furthermore, the course covers diagnostic tests and preventive measures necessary for managing lipid-related conditions.

Transport of Lipids in the Blood and Structural Design

Lipids absorbed from the diet or synthesized by the liver must be transported between various tissues and organs for utilization and storage. However, since lipids are insoluble in water, they cannot travel freely in the aqueous environment of the blood. Consequently, they are transported in the form of lipoproteins. The transport process involves the villus, short-chain fatty acids entering the blood capillary, and chylomicrons entering the lacteal and lymphatic vessels. These eventually reach the heart via the thoracic duct and the left subclavian vein. Simultaneously, amino acids and monosaccharides are transported via the hepatic portal vein to the liver.

Lipoproteins are complex particles formed by combining lipids and specific proteins known as apolipoproteins or apoproteins. The structural design of a lipoprotein ensures its solubility in serum. The core of the lipoprotein is composed of hydrophobic lipids, such as triglycerides and cholesteryl esters. This hydrophobic interior is surrounded by a shell of amphipathic lipids, including phospholipids and free cholesterol, which interface with the aqueous blood environment. Specific proteins, or apoproteins, are embedded in or associated with this surface layer to provide structural stability and biological functionality.

Classification and Composition of Human Plasma Lipoproteins

Plasma lipoproteins are classified into four major categories based on their density and composition: Chylomicrons (CM), Very low-density lipoproteins (VLDL), Low-density lipoproteins (LDL), and High-density lipoproteins (HDL). These particles vary significantly in their size, density, and lipid-to-protein ratios.

Chylomicrons have a diameter ranging from $70\text{ to }1200\,nm$ and a density of less than $0.95\,g/ml$. Their lipid-to-protein ratio is approximately $99:1$. Compositionally, they are $1\%$ protein and $99\%$ lipid, with triglycerides making up $88\%$ of the total mass, while phospholipids account for $7\%$ and cholesterol accounts for $4\%$.

VLDL particles range from $25\text{ to }70\,nm$ in diameter with a density between $0.95\text{ to }1.006\,g/ml$. They have a lipid-to-protein ratio of $90:10$. Their composition includes $9\%$ protein and $91\%$ lipid, specifically $50\%$ triglycerides, $18\%$ phospholipids, and $23\%$ cholesterol.

LDL particles have a diameter of $20\text{ to }25\,nm$ and a density of $1.019\text{ to }1.063\,g/ml$. The lipid-to-protein ratio is $80:20$. They consist of $20\%$ protein and $80\%$ lipid, characterized by a high cholesterol content of $48\%$, alongside $24\%$ phospholipids and $8\%$ triglycerides.

HDL particles are the smallest and densest, with a diameter of $5\text{ to }10\,nm$ and a density of $1.063\text{ to }1.210\,g/ml$. They have a lipid-to-protein ratio of $50:50$. Their composition is $56\%$ protein and $44\%$ lipid, comprised of $21\%$ cholesterol, $20\%$ phospholipids, and only $3\%$ triglycerides.

Characteristics and Functions of Apolipoproteins

Apolipoproteins (Apoproteins) serve three critical functions: they provide structural stability to the lipoprotein, act as ligands for cell receptors to promote the transport and uptake of lipids, and serve as cofactors or inhibitors for enzymes involved in lipid metabolism.

Apo A-I is produced in the liver and intestine and is the major protein component of HDL and chylomicrons. It serves as a structural protein for HDL and activates the enzyme Lecithin-cholesterol acyltransferase (LCAT). Its molecular weight is $28,000$ and its normal blood level is approximately $150\,mg/dl$. It is considered anti-atherogenic. Apo A-II, also from the liver and intestine, is a structural protein for HDL but inhibits LCAT and stimulates lipase, with a blood level of $30\,mg/dl$.

Apo B exists in two forms: B-48 and B-100. Apo B-48 is synthesized in the intestine and is essential for the formation and secretion of chylomicrons; it represents $48\%$ of the size of B-100 with a molecular weight of $250,000$. Apo B-100 is synthesized in the liver and is found in VLDL, IDL, and LDL. It serves as the ligand for the LDL receptor, enabling the uptake of cholesterol by tissues. Its molecular weight is approximately $550,000$ and its blood level is $100\,mg/dl$.

Apo C-II is synthesized in the liver and is a vital cofactor for extrahepatic lipoprotein lipase (LPL) in vessel walls, facilitating the clearance of triglycerides from chylomicrons and VLDL. Conversely, Apo C-III, also from the liver, inhibits lipoprotein lipase. Apo E is synthesized in the liver and found in chylomicron remnants, IDL, and HDL. It acts as a ligand for hepatic uptake via the LDL receptor or the LDL receptor-related protein (LRP).

Other apoproteins include Apo(a), which is attached to Apo B-100 via a disulfide bond to form Lipoprotein (a). Apo(a) impairs fibrinolysis and is highly atherogenic. Apo D is present in HDL, while Apo J, Apo L, and Apo M are also associated with HDL with functions that are currently less well-defined or still under investigation.

The Metabolism and Synthesis of Chylomicrons

Chylomicrons are formed in the intestinal mucosal cells and are responsible for the transport of dietary (exogenous) lipids from the intestine to peripheral tissues. They are exceptionally rich in triglycerides ($90\%$) and contain Apo B-48, Apo E, Apo C, and Apo A.

The synthesis of Apo B-48 is a unique process involving RNA editing. In the liver, the single Apo B gene is transcribed into an unedited mRNA that directs the synthesis of the full-length Apo B-100 protein ($4563$ amino acids). The CAA codon at position $2153$ encodes Glutamine. However, in the intestine, the enzyme Cytidine deaminase converts this CAA codon to UAA, which is a stop (termination) signal. This results in the translation of a shorter protein, Apo B-48 ($2152$ amino acids), which is essential for lipid absorption and chylomicron formation.

In the catabolism of chylomicrons, the particles enter the circulation and are acted upon by lipoprotein lipase (LPL) on the endothelial surface of extrahepatic tissues (adipose and muscle). LPL is activated by Apo C-II. This reaction results in the loss of approximately $90\%$ of the triglycerides. The Apo C is returned to HDL, while Apo E is retained. The resulting particle is a chylomicron remnant, which is smaller and enriched in cholesterol. The liver takes up these remnants via receptor-mediated endocytosis, mediated by Apo E and facilitated by hepatic lipase which acts as a ligand. The contents are سپس metabolized or used for VLDL synthesis.

Metabolism of VLDL, IDL, and LDL

VLDL is synthesized in the liver to transport endogenously synthesized triglycerides to peripheral tissues. It contains Apo B-100, Apo C-II, and Apo E. VLDL formation requires Apo B-100. Nascent VLDL acquires the full complement of Apo C and Apo E from HDL once in circulation.

Catabolism of VLDL is similar to chylomicrons. Lipoprotein lipase hydrolyzes the triglycerides into free fatty acids and glycerol. As VLDL loses triglycerides, it is converted into VLDL remnants, also known as Intermediate-density lipoprotein (IDL). IDL can be taken up directly by the liver via the LDL receptor (recognizing Apo B-100 and Apo E) or further processed into LDL. Each LDL particle is derived from a single precursor VLDL particle, as the single molecule of Apo B-100 is conserved during the transformation.

LDL is the primary transporter of cholesterol to peripheral tissues. Approximately $70\%$ of LDL is degraded in the liver, while $30\%$ is degraded in extra-hepatic tissues. The uptake is mediated by the LDL receptor. High plasma concentrations of LDL cholesterol are positively correlated with the incidence of coronary atherosclerosis. Familial hypercholesterolemia is a genetic disorder caused by a defect on chromosome 19 that results in defective LDL receptors, preventing the body from removing LDL cholesterol from the blood.

Metabolism and Function of HDL and Reverse Cholesterol Transport

HDL is synthesized and secreted by both the liver and the intestine. Nascent HDL appears as discoid phospholipid bilayers containing Apo A and free cholesterol. A major function of HDL is to serve as a repository for Apo C and Apo E needed by chylomicrons and VLDL.

HDL is central to Reverse Cholesterol Transport, a mechanism that prevents cholesterol accumulation in peripheral tissues by transporting excess cholesterol back to the liver for excretion. The enzyme Lecithin-cholesterol acyltransferase (LCAT), activated by Apo A-I, catalyzes the esterification of cholesterol:

$\text{Lecithin} + \text{Cholesterol} \rightarrow \text{Lysolecithin} + \text{Cholesteryl Ester}$

As the nonpolar cholesteryl esters move into the hydrophobic interior of the discoid particle, it transforms into a spherical, pseudomicellar HDL. The scavenger receptor B1 (SR-B1) facilitates this process. In the liver, SR-B1 binds HDL via Apo A-I to selectively deliver cholesteryl esters to the cells without internalizing the entire particle. In peripheral tissues, SR-B1 mediates the acceptance of cholesterol by HDL.

Additional functions of HDL include competing with LDL for binding sites on arterial wall membranes (aided by Apo E) to prevent LDL internalization, and stimulating prostacyclin synthesis by endothelial cells, which helps prevent thrombus formation.

Clinical Significance: Fatty Liver and Lipotropic Agents

Fatty liver is the abnormal accumulation of triglycerides inside liver cells. This occurs when there is an imbalance between the synthesis of triglycerides and the secretion of VLDL. Fatty livers are generally categorized into two types. The first type involves increased synthesis of triglycerides due to a high carbohydrate diet (providing excess Acetyl CoA), high fat feeding, starvation, or diabetes mellitus. The second type involves defective VLDL synthesis or secretion, which may be caused by a block in apolipoprotein synthesis (e.g., Hypobetalipoproteinemia or impaired absorption), a failure in phospholipid provision (e.g., essential fatty acid deficiency), or impaired secretion due to oxidative stress disrupting membranes.

Alcoholic fatty liver is caused by chronic alcoholism, leading to fat accumulation, hyperlipidemia, and cirrhosis, often by interfering with transcription factors that regulate metabolic enzymes.

Lipotropic agents are substances that help remove fat from liver cells and prevent fatty liver. Examples include choline, methionine and other essential amino acids, essential fatty acids, antioxidant vitamins, Vitamin $B_{12}$, and folic acid.

Clinical Markers: Lipoprotein (a) and Diagnostic Lipid Profiles

Lipoprotein (a), often called the "little rascal," is a highly atherogenic particle consisting of LDL with an additional protein, Apo(a), attached to Apo B-100 by a disulfide bond. It is strongly associated with myocardial infarction (MI), especially in individuals aged $30\text{ to }40\,years$. It is noted that Indian populations tend to have higher levels of Lp(a) than Western populations. Lp(a) contributes to atherosclerosis by impairing fibrinolysis; it competes with plasminogen, preventing its conversion to plasmin by tissue plasminogen activator (tPA), thereby inhibiting clot dissolution.

The diagnostic importance of lipoproteins is assessed through a plasma lipid profile. Normal values for these analytes are:

• Total plasma lipids: $400-600\,mg/dl$
• Total cholesterol: $140-200\,mg/dl$
• HDL cholesterol: $40-60\,mg/dl$
• LDL cholesterol: $80-130\,mg/dl$
• Triglycerides: $50-150\,mg/dl$

Disorders of Lipoprotein Metabolism: Dyslipoproteinaemia

Dyslipoproteinaemia refers to disorders of lipoprotein metabolism, which can be categorized into Hyperlipoproteinemia (increased levels) and Hypolipoproteinemia (decreased levels). Primary conditions are typically due to inherited defects in formation, transport, or degradation.

Hyperlipoproteinemia classifications include:

• Type I: Increased Chylomicrons and triglycerides; cause is a deficiency in lipoprotein lipase; risk of atherosclerosis may increase.
• Type IIa: Increased LDL and cholesterol; cause is a deficiency of LDL receptors; risk of atherosclerosis is very high, mostly in coronary arteries.
• Type IIb: Increased LDL and VLDL, triglycerides, and cholesterol; cause is overproduction of Apo B; very high risk of atherosclerosis.
• Type III: Increased IDL, triglycerides, and cholesterol; cause is an abnormality in Apo E; very high risk of atherosclerosis, mostly in peripheral vessels.
• Type IV: Increased VLDL and triglycerides; cause is overproduction of TG; atherosclerosis risk may or may not increase.
• Type V: Increased Chylomicrons and VLDL and triglycerides.

Hypolipoproteinemia includes conditions where one or more lipoproteins are decreased:

• Abetalipoproteinemia: An autosomal recessive defect in the synthesis of Apo B (both B-48 and B-100). This results in a total absence of LDL, VLDL, and chylomicrons in circulation. Clinical consequences include fat malabsorption, impaired absorption of fat-soluble vitamins, and degenerative changes in the retina that can lead to blindness.
• Familial alpha-lipoprotein deficiency (Tangier disease): An inability to synthesize Apo A (the major protein of HDL). This results in extremely low plasma HDL levels, impaired reverse cholesterol transport, and the accumulation of cholesteryl esters in tissues.