C2 (Lipoproteins)

Introduction to Lipoproteins

Lipids constitute a heterogeneous family of largely hydrophobic and often amphipathic molecules. Because of their hydrophobic nature, lipids cannot be transported freely in the bloodstream or other aqueous compartments. Consequently, they are transported as complex macromolecular assemblies known as lipoproteins. These assemblies facilitate the movement of hydrophobic lipids through the aqueous environment of the circulatory system.

Structure of Lipoproteins

Lipoproteins are heterogeneous macromolecular complexes composed of lipids and specific proteins called apolipoproteins. Structurally, lipoproteins are micelle-like spherical particles consisting of two primary regions: a hydrophobic core and an outer shell. The hydrophobic core contains triglycerides ($TG$), cholesteryl esters, and fat-soluble vitamins. The outer shell is a thin monolayer composed of phospholipids and free cholesterol associated with apolipoproteins. These components are amphipathic, meaning they possess both hydrophilic and hydrophobic properties, which ensures the solubility of the entire lipoprotein particle in aqueous biological fluids.

Classification of Lipoproteins According to Density

Plasma lipoproteins are classified according to their density, which is determined by their relative proportions of lipid and protein. There are four primary classes: Chylomicrons, which are of intestinal origin; Very low-density lipoproteins ($VLDL$), which are of hepatic origin; Low-density lipoproteins ($LDL$), which are formed from the catabolism of $VLDL$ via intermediary density lipoproteins ($IDL$); and High-density lipoproteins ($HDL$), which have both hepatic and intestinal origins. A critical relationship exists between the size of lipoprotein particles and their density: the larger the lipoprotein particle, the lower its density; conversely, the smaller the particle, the higher its density.

Classification According to Electrophoretic Migration

Plasma lipoproteins are heterogeneous in their apolipoprotein and lipid composition. This diversity results in different net electrical charges for each lipoprotein class. Because of these charge differences, they can be separated using agarose gel electrophoresis. Under standard conditions, four distinct bands are typically observed. These include Chylomicrons, which remain at the origin; $\beta$-lipoproteins (associated with $LDL$); pre-$\beta$-lipoproteins (associated with $VLDL$); and $\alpha$-lipoproteins (associated with $HDL$).

Composition and Functional Roles of Lipoprotein Classes

The main classes of lipids carried by lipoproteins vary depending on the particle type. Chylomicrons are primarily responsible for the transport of exogenous triglycerides, particularly during the postprandial period. $VLDL$ particles are involved in the transport of endogenous triglycerides. $LDL$ particles function in the transport of cholesterol, primarily in the form of cholesteryl esters, toward peripheral tissues. Finally, $HDL$ particles are responsible for the transport of cholesterol from peripheral tissues back to the liver, a process known as reverse cholesterol transport, as well as the exchange of phospholipids and apolipoproteins.

Study of Apolipoproteins

Apolipoproteins are divided into five major classes: $A$, $B$, $C$, $D$, and $E$, each containing various subclasses. These proteins are categorized into two functional groups based on their size and exchangeability. The classes $A$, $C$, and $E$ are small, exchangeable proteins with a molecular weight of $< 100\,\text{kDa}$. The $B$ class, including $B-48$ and $B-100$, consists of very large, non-exchangeable structural proteins with a molecular weight of $> 500\,\text{kDa}$. These large proteins serve as essential structural components for Chylomicrons, $VLDL$, and $LDL$.

Apolipoproteins serve a dual role in metabolism. Their structural role involves stabilizing the lipoprotein particle and ensuring its solubilization in aqueous fluids, which allows for lipid transport from synthesis sites to utilization sites. Their metabolic role involves acting as ligands for cellular receptors, such as the $ApoE$ receptor or the $ApoB/E$ (also known as the $LDL$) receptor. Additionally, they function as activators or inhibitors of enzymes involved in lipoprotein metabolism, such as lipoprotein lipase and lecithin-cholesterol acyltransferase ($LCAT$).

Characteristics of Specific Apolipoproteins

Apolipoprotein $B-48$ is the main structural apolipoprotein of Chylomicrons. Apolipoprotein $B-100$ is the main apolipoprotein of $VLDL$ and $LDL$; it is an integral, non-exchangeable component secreted by hepatocytes. Apolipoproteins $A-I$ and $A-II$ are the primary apolipoproteins found in $HDL$. In contrast, apolipoproteins $E$ and $C$ are exchangeable. They are synthesized mainly in the liver, though $ApoE$ is also produced in macrophages. These exchangeable apolipoproteins circulate largely on $HDL$ particles and are transferred between different lipoprotein particles during the course of their metabolism.

Enzymes Involved in Lipoprotein Metabolism

Lipoprotein lipase ($LPL$) is abundant in adipose tissue as well as skeletal and cardiac muscle. It is bound to the luminal surface of capillary endothelial cells. The function of $LPL$ is to hydrolyze triglycerides within Chylomicrons and $VLDL$, resulting in chylomicron remnants and $VLDL$ remnants ($IDL$) that are depleted of triglycerides. The released fatty acids are then taken up by tissues for oxidation (energy production) or storage. In this process, $ApoC-II$ serves as a required activator or cofactor, while $ApoC-III$ inhibits the activity of $LPL$.

Hepatic lipase ($HL$) shares a structure similar to that of lipoprotein lipase. It is synthesized by the liver and bound to the endothelial surface of hepatic capillaries. Hepatic lipase hydrolyzes triglycerides and phospholipids in $IDL$, contributing to their conversion into $LDL$. It also participates in the remodeling of $HDL$ by converting $HDL_2$ into smaller particles such as $HDL_3$ or pre-$\beta_1$ $HDL$.

Lecithin-cholesterol acyltransferase ($LCAT$) is associated with $HDL$. It catalyzes the esterification of free cholesterol, which mainly originates from peripheral tissues. This is achieved by transferring a fatty acid from lecithin (phosphatidylcholine). $ApoA-I$ is the major activator of $LCAT$, and $ApoA-IV$ may also contribute. The resulting cholesteryl esters migrate into the core of the $HDL$ particle, promoting its maturation and expansion.

Chylomicron Metabolism

Chylomicrons are formed in enterocytes following a meal. They are rich in triglycerides and contain $apoB-48$ as their structural protein. Once in circulation, they acquire $apoC-II$ and $apoE$ from $HDL$. Lipoprotein lipase ($LPL$) then acts on the particles to hydrolyze the triglycerides, releasing free fatty acids. The remaining chylomicron remnants are subsequently taken up by the liver. The primary role of chylomicrons is the transport of dietary (exogenous) triglycerides.

VLDL, IDL, and LDL Metabolism

$VLDL$ particles are formed in the liver and are composed of lipids (endogenous triglycerides and cholesterol) and apolipoprotein $B100$. After being released into the circulation, they acquire $apoC-II$ and $apoE$ from $HDL$. In the capillaries, they undergo the action of lipoprotein lipase ($LPL$), where triglycerides are hydrolyzed to release free fatty acids to the tissues; during this step, $apoC-II$ is returned to $HDL$. The particles are then converted into $IDL$. A portion of $IDL$ is taken up by the liver via $LDL$ receptors ($B/E$) and $LRP$ (which is $apoE$-dependent). The remaining $IDL$ lose their $apoE$, become enriched in cholesteryl esters, and are converted into $LDL$.

$LDL$ are formed in the plasma from $VLDL$ via $IDL$ after the removal of triglycerides and the loss of $apoC$ and $apoE$. This process results in particles that are rich in cholesteryl esters. The fate of $LDL$ involves three pathways. First, in peripheral tissues, they are taken up by receptor-mediated endocytosis via $LDL$ receptors to deliver cholesterol to cells. Second, the liver provides major clearance through $LDL$ receptor-mediated endocytosis. Third, macrophages can take up modified or oxidized $LDL$ via scavenger receptors, leading to the formation of foam cells. Intracellularly, the cholesterol from $LDL$ is used for membranes and steroid synthesis, while excess cholesterol is esterified by $ACAT$. The primary role of $LDL$ is the transport of cholesterol to peripheral tissues.

Atherosclerosis and LDL

Atherosclerosis is characterized as a chronic inflammatory disease affecting the intima of large- and medium-caliber arteries. It is promoted by oxidized $LDL$ and results from a cascade of complex, multifactorial interactions. Clinical complications associated with atherosclerosis include endothelial dysfunction, stenosis (narrowing of the arteries), and thrombosis.

HDL Metabolism and Reverse Cholesterol Transport

$HDL$ is synthesized in the liver and intestine as lipid-poor $apoA-I$ particles, known as nascent or discoidal $HDL$. These particles collect free cholesterol and phospholipids from peripheral cell membranes. $ApoA-I$ activates the enzyme $LCAT$, which esterifies the free cholesterol into cholesteryl esters ($CE$). This process allows the $HDL$ to mature from a discoidal shape into a spherical shape. $HDL$ also acts as a reservoir of apolipoproteins, mainly $apoC-II$ and $apoE$, which are exchanged with chylomicrons and $VLDL$. Through exchange mediated by $CETP$ (cholesteryl ester transfer protein), $HDL$ transfers cholesteryl esters to $apoB$-containing lipoproteins and receives triglycerides in return. Cholesteryl esters are finally delivered to the liver and steroidogenic tissues primarily via $SR-B1$ (selective uptake). The role of $HDL$ is to ensure reverse cholesterol transport by removing excess cholesterol from peripheral tissues to the liver and to