BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

CHAPTER 8. LIPID METABOLISM

IX. Cholesterol: Functions and Metabolism

Cholesterol is a sterol found exclusively in animal organisms. Although it is synthesized in many human Tissues, the Liver is the primary site of synthesis, accounting for over 50% of total cholesterol production. The Small Intestine contributes another 15–20%, while the remainder is synthesized in the Skin, adrenal cortex, and Gonads. The Human Body synthesizes about 1 g of cholesterol per day, supplemented by 300–500 mg obtained from the diet (Fig. 8-65). Cholesterol performs numerous vital Functions: it is an essential structural component of all Cell membranes, where it modulates fluidity and permeability, and it serves as the precursor for the synthesis of Bile acids and Steroid Hormones. Furthermore, intermediates in the cholesterol biosynthetic pathway are converted into ubiquinone (a component of the Respiratory Chain) and dolichol (which participates in glycoprotein synthesis). Through its hydroxyl group, cholesterol can form esters with Fatty acids. Cholesteryl esters predominate in the Blood and are stored in small amounts in specific cell types that utilize them as substrates for the Synthesis of Other molecules. Because cholesterol and its esters are hydrophobic, they are transported in the bloodstream exclusively as components of various Lipoproteins. Cholesterol METABOLISM is exceptionally complex, requiring approximately 100 sequential reactions just for its synthesis, and involves roughly 300 different Proteins in total. Disruptions in cholesterol metabolism lead to one of the most widespread pathologies: atherosclerosis. Mortality from the consequences of atherosclerosis (such as myocardial infarction and stroke) ranks first in overall population mortality statistics. Atherosclerosis is a "polygenic

disease," meaning that many factors contribute to its development, with hereditary factors playing a paramount role. The accumulation of cholesterol in the body also leads to The Development of another common disorder: cholelithiasis (gallstone disease).

Class="center">Fig. 8-65. The body's cholesterol pool, pathways of its utilization, and excretion.

A. Cholesterol Synthesis and Its Regulation

The reactions of cholesterol synthesis take place in the Cytosol of Cells. This constitutes one of the longest Metabolic Pathways in the human body.

Formation of Mevalonate

The complex pathway of cholesterol synthesis can be divided into three stages (Fig. 8-66). The First stage concludes with The formation of mevalonate (mevalonic acid). Two molecules of acetyl-CoA are condensed by the enzyme thiolase to form acetoacetyl-CoA. The enzyme hydroxymethylglutaryl-CoA synthase then adds a third acetyl residue to yield HMG-CoA (3-hydroxy-3-methylglutaryl-CoA). This sequence of reactions is similar to the Initial Stages of Ketone Body Synthesis (see Fig. 8-33). However, ketone body synthesis occurs in the Mitochondria of the liver, whereas cholesterol synthesis takes place in The Cell cytosol.

Fig. 8-66. Cholesterol synthesis. C₅ — isopentenyl pyrophosphate; C₁₅ — farnesyl pyrophosphate. All carbon atoms of cholesterol are derived from acetyl-CoA. Squalene, a linear hydrocarbon, is converted by the enzyme cyclase into lanosterol, which contains four fused rings and a hydroxyl group. Lanosterol is subsequently converted into cholesterol through a series of sequential reactions (I, II, III — stages of synthesis).

The next reaction, catalyzed by HMG-CoA reductase, is the rate-limiting regulatory step in the cholesterol biosynthetic pathway. In this reaction, HMG-CoA is reduced to mevalonate utilizing two molecules of NADPH. The enzyme HMG-CoA reductase is a glycoprotein spanning The Endoplasmic reticulum (ER) membrane, with its Active Site protruding into the cytosol.

Formation of Squalene

During the Second Stage of synthesis, mevalonate is converted into a five-carbon isoprenoid Structure containing a pyrophosphate group—isopentenyl pyrophosphate. The Condensation of two isoprene units yields geranyl pyrophosphate. The addition of yet another isoprene unit produces farnesyl pyrophosphate, a 15-carbon compound. Two molecules of farnesyl pyrophosphate then condense to form squalene, a linear hydrocarbon consisting of 30 carbon atoms.

Formation of Cholesterol

In the Third Stage of cholesterol synthesis, squalene is converted via an epoxide intermediate by the enzyme cyclase into a lanosterol molecule, which contains four fused rings and 30 carbon atoms. This is followed by 20 sequential reactions that transform lanosterol into cholesterol. During these final stages, three carbon atoms are removed from lanosterol, leaving cholesterol with 27 carbon atoms. Cholesterol features a saturated, branched 8-carbon side chain at position 17, a double bond in ring B between carbon atoms 5 and 6, and a hydroxyl group at position 3.

In the human body, isopentenyl pyrophosphate also serves as a precursor for ubiquinone (CoQ) and dolichol, which is involved in glycoprotein synthesis.

Esterification of Cholesterol

In certain tissues, the hydroxyl group of cholesterol is esterified to form more hydrophobic molecules known as cholesteryl esters. This reaction is catalyzed by the intracellular enzyme ACAT (acyl-CoA:cholesterol acyltransferase).

Esterification also occurs in the blood within HDL particles, which contain the enzyme LCAT (lecithin:cholesterol acyltransferase). Cholesteryl esters represent the form in which cholesterol is stored intracellularly or transported through the bloodstream. In the blood, approximately 75% of cholesterol is in the form of esters.

Regulation of Cholesterol Synthesis

The key enzyme of cholesterol synthesis (HMG-CoA reductase) is regulated through multiple mechanisms.

Phosphorylation/Dephosphorylation of HMG-CoA Reductase (Fig. 8-67). When the Insulin/Glucagon ratio increases, this enzyme is dephosphorylated and transitions into the active state. The Action of Insulin is mediated via two Enzymes: HMG-CoA reductase kinase phosphatase, which converts the kinase into an inactive dephosphorylated state, and HMG-CoA reductase phosphatase, by converting it into a dephosphorylated active state. The net result of these reactions is the Formation of the dephosphorylated, active form of HMG-CoA reductase.

Fig. 8-67. Regulation of HMG-CoA reductase activity in the liver. Cholesterol and Bile acids decrease The rate of Transcription and, consequently, enzyme synthesis. Insulin stimulates dephosphorylation, whereas glucagon stimulates phosphorylation of HMG-CoA reductase. Insulin activates 2 Phosphatases: HMG-CoA reductase kinase phosphatase* and the phosphatase that directly dephosphorylates HMG-CoA reductase. Glucagon stimulates the phosphorylation and inactivation of these 2 phosphatases, as well as the phosphorylation and activation of HMG-CoA reductase kinase.

Consequently, cholesterol synthesis increases during the absorptive period. This period is also characterized by an increased availability of the primary substrate for cholesterol synthesis—acetyl-CoA (resulting from the intake of carbohydrate- and fat-containing foods, since acetyl-CoA is produced during The breakdown of glucose and fatty acids).

In the postabsorptive state, glucagon acts via protein kinase A to stimulate the phosphorylation of HMG-CoA reductase, converting it into an inactive form. This effect is reinforced because glucagon simultaneously promotes the phosphorylation and inactivation of HMG-CoA reductase phosphatase, alongside the phosphorylation of HMG-CoA reductase kinase, thereby keeping HMG-CoA reductase in a phosphorylated, inactive state. As a result, cholesterol synthesis is inhibited during the postabsorptive period and fasting.

Inhibition of HMG-CoA reductase synthesis.

The end product of the metabolic pathway (cholesterol) decreases the transcription rate of the HMG-CoA reductase Gene, thus suppressing its own synthesis. The liver actively synthesizes bile acids from cholesterol; therefore, bile acids (as the End products of this pathway) also suppress The activity of the HMG-CoA reductase gene (Fig. 8-67). Given that the HMG-CoA reductase molecule has a half-life of about 3 hours after synthesis, end-product inhibition of this enzyme by cholesterol serves as an highly efficient regulatory mechanism.

B. Transport of cholesterol by blood lipoproteins

Cholesterol is transported in the bloodstream exclusively as a component of lipoproteins (LPs). LPs ensure the delivery of exogenous cholesterol to tissues, regulate cholesterol fluxes among Organs, and mediate the removal of excess cholesterol from the body.

Transport of exogenous cholesterol

Dietary cholesterol is ingested in amounts of 300–500 mg/day, primarily in the form of esters. Following Hydrolysis, micellar absorption, and re-esterification in intestinal mucosal cells, cholesterol esters and a small amount of free cholesterol are incorporated into chylomicrons (CMs) and released into the blood. Once triglycerides are cleared from CMs by lipoprotein lipase, the remaining cholesterol within remnant CMs is delivered to the liver. These remnant CMs bind to hepatic cell receptors and are internalized via endocytosis. Lysosomal enzymes then hydrolyze the components of remnant CMs, yielding free cholesterol. Exogenous cholesterol delivered to hepatocytes in this manner can inhibit endogenous cholesterol synthesis by slowing down the rate of HMG-CoA reductase synthesis.

Transport of endogenous cholesterol by VLDL (pre-$eta$-lipoproteins)

The liver is the primary site of cholesterol synthesis. Endogenous cholesterol, synthesized from the starting substrate acetyl-CoA, and exogenous cholesterol, delivered via remnant CMs, together form the total hepatic cholesterol pool. In hepatocytes, triacylglycerols and cholesterol are packaged into very-low-density lipoproteins (VLDL). In addition, VLDL contain apolipoprotein B-100 and Phospholipids. VLDL are secreted into the bloodstream, where they acquire apolipoproteins E and C-II from HDL. In the Circulation, VLDL are acted upon by lipoprotein lipase, which—similarly to its action on CMs—is activated by apoC-II and hydrolyzes fats into glycerol and fatty acids. As the triacylglycerol content of VLDL decreases, they are converted into intermediate-density lipoproteins (IDL). As the lipid content of IDL further diminishes, apoC-II proteins are transferred back to HDL. The content of cholesterol and its esters in IDL reaches up to 45%; a fraction of these lipoproteins is captured by liver cells via LDL receptors, which interact with both apoE and apoB-100.

Transport of cholesterol by LDL.

LDL Receptors

IDL remaining in the circulation continue to be acted upon by lipoprotein lipase, converting them into low-density lipoproteins (LDL), which contain up to 55% cholesterol and its esters. Apolipoproteins E and C-II are transferred back to HDL, leaving apoB-100 as the primary apolipoprotein in LDL. Apolipoprotein B-100 interacts with LDL receptors, thereby determining the metabolic fate of cholesterol. LDL serve as the primary transport vehicle for delivering cholesterol to peripheral tissues. Approximately 70% of blood cholesterol and its esters circulate as part of LDL. From the bloodstream, LDL are taken up by the liver (up to 75%) and other extrahepatic tissues that display LDL receptors on their surfaces.

The LDL receptor is a complex protein consisting of 5 domains and containing a carbohydrate moiety (Fig. 8-68).

Fig. 8-68. STRUCTURE OF THE LDL receptor. The receptor protein consists of 5 domains. The N-terminal domain directly binds LDL. Two other domains (linked to Oligosaccharides) project from the cell surface and ensure the proper conformation of the LDL-binding N-terminal domain.

LDL receptors are synthesized in the Endoplasmic reticulum and the Golgi apparatus, and are subsequently displayed on the cell surface within specialized pits coated with the protein clathrin. These invaginations are referred to as coated pits (Fig. 8-69). The outwardly projecting N-terminal domain of the receptor interacts with apoB-100 and apoE; consequently, it can bind not only LDL but also IDL, VLDL, and remnant CMs containing these apolipoproteins. Peripheral cells maintain A large number of LDL receptors on their surfaces—for instance, a single fibroblast can carry from 20,000 to 50,000 receptors. This demonstrates that cells acquire the bulk of their blood-borne cholesterol via LDL.

Fig. 8-69. Synthesis of LDL receptors and their subsequent intracellular journey. Following the binding of LDL to the receptor (1), the coated pit, along with the receptor-LDL complex, is internalized via endocytosis (2). The resulting endosome undergoes acidification driven by an ATP-dependent proton pump. Upon acidification, LDL receptors dissociate from LDL (3), and the majority of the receptors are recycled back to The Plasma Membrane (5). Thus, LDL receptors can be utilized by the cell multiple times. After receptor detachment, the endosome fuses with Lysosomes, and lysosomal hydrolytic enzymes degrade the endosomal components (4). This releases free cholesterol, which can be utilized for membrane biogenesis, converted into bile acids in hepatocytes, or used for steroid hormone synthesis in endocrine cells.

When the influx of cellular cholesterol exceeds metabolic demand, the synthesis of LDL receptors is downregulated, which reduces the inward flow of cholesterol from the bloodstream. Conversely, a drop in intracellular free cholesterol concentrations stimulates the synthesis of both HMG-CoA reductase and LDL receptors.

The regulation of LDL receptor synthesis involves several hormones, including insulin, triiodothyronine (T₃), and Sex Hormones, all of which upregulate LDL receptor expression, whereas glucocorticoids (primarily cortisol) downregulate it. The effects of insulin and T₃ likely explain the mechanisms behind hypercholesterolemia and the elevated risk of atherosclerosis observed in Diabetes Mellitus or hypothyroidism.

Alternative pathways of cellular cholesterol uptake

In addition to LDL receptors, The surface of cells in various organs (such as the liver, Brain, and Placenta) features another type of receptor known as the "LDL receptor-related protein" (LRP). This receptor interacts with apoE and mediates the uptake of remnant CMs and IDL. The primary function of these receptors is presumably the clearance of remnant particles from Blood Plasma. Because remnant particles are rich in cholesterol, this receptor class also contributes to tissue cholesterol supply.

In addition to the uptake of cholesterol into tissues via lipoprotein endocytosis, a certain amount of cholesterol enters cells through diffusion from LDL and other lipoproteins upon their contact with cell membranes.

The Role of HDL in Cholesterol Metabolism

HDLs perform two main functions: they supply apoproteins to other lipoproteins in the blood and participate in the so-called "reverse cholesterol transport." HDLs are synthesized in The Liver and, in small amounts, in the small intestine as "immature lipoproteins"—precursors to HDL. They have a discoid shape, a small size, and contain a high percentage of proteins and phospholipids. In the liver, apoproteins A, E, C-II, and the enzyme LCAT are incorporated into HDLs. In the blood, apoC-II and apoE are transferred from HDL to chylomicrons and VLDLs. HDL precursors contain practically no cholesterol or TAGs and become enriched with cholesterol in the bloodstream by acquiring it from other lipoproteins and cell membranes.

There is a complex mechanism for transferring cholesterol to HDL. The enzyme LCAT (lecithin-cholesterol acyltransferase) is located On the surface of HDL. This enzyme converts cholesterol, which has a hydroxyl group protruding onto the surface of lipoproteins or cell membranes, into cholesterol esters. A fatty acid radical is transferred from phosphatidylcholine (lecithin) to the hydroxyl group of cholesterol. The reaction is activated by apoprotein A-I, which is a component of HDL.

The hydrophobic cholesterol ester molecule moves into the interior of the HDL. Thus, HDL particles become enriched with cholesterol esters. HDLs increase in size, transforming from small discoid particles into spherical particles known as HDL₃, or "mature HDL." HDL₃ partially exchanges cholesterol esters for triacylglycerols contained in VLDL, IDL, and chylomicrons (Fig. 8-70). This transfer is mediated by the cholesteryl ester transfer protein (also referred to as apoD). Consequently, some cholesterol esters are transferred to VLDL and IDL, while HDL₃ particles, through the accumulation of triacylglycerols, increase in size and transform into HDL₂. Under the action of lipoprotein lipase, VLDLs are first converted into IDLs and then into LDLs. LDLs and IDLs are taken up by cells via LDL receptors.

Fig. 8-70. The role of HDL and LDL in the reverse transport of cholesterol to the liver. Immature HDL precursors are enriched with cholesterol, which enters HDL with the participation of the enzyme LCAT from the surface of cells and other cholesterol-containing lipoproteins. As immature HDLs become enriched with cholesterol, they transform into HDL₃—spherical particles of larger size. HDL₃ particles exchange cholesterol esters for triacylglycerols contained in VLDL and IDL with the participation of the cholesteryl ester transfer protein*. HDL₃ is converted into HDL₂, which increases in size due to the accumulation of triacylglycerols. Under the action of lipoprotein lipase, VLDLs and IDLs are converted into LDLs, which deliver cholesterol to the liver. A portion of HDL is taken up by liver cells by interacting with specific HDL receptors for apoA-I. On the surface of liver cells, the phospholipids and triacylglycerols of IDL and HDL₂ are hydrolyzed by hepatic lipase**, which destabilizes the surface structure of the lipoproteins and facilitates the diffusion of cholesterol into hepatocytes. As a result, HDL₂ particles are converted back into HDL₃ and return to the bloodstream. C — cholesterol, CE — cholesterol esters, PL — phospholipids, LCAT — lecithin-cholesterol acyltransferase, A-I — apoprotein, activator of LCAT.

Thus, cholesterol from all tissues returns to the liver mainly as part of LDL, though IDL and HDL₂ also participate in this process. Practically all cholesterol destined for elimination from the body enters the liver and is subsequently excreted from this organ as derivatives in feces. The pathway of cholesterol return to the liver is termed "reverse cholesterol transport."

Excretion of Cholesterol from the Body

The structural core of cholesterol—the cyclopentanoperhydrophenanthrene ring—cannot be degraded to CO₂ and Water like other organic components derived from diet or synthesized within the body. Therefore, the bulk of cholesterol is excreted in the form of bile acids.

A certain amount of bile acids is excreted unchanged, while a portion undergoes the action of bacterial enzymes in the intestine. The breakdown products of these acids (primarily secondary bile acids) are eliminated from the body.

In the intestine, a fraction of cholesterol molecules undergoes bacterial enzymatic reduction at the double bond in ring B, resulting in the formation of Two Types of molecules—cholestanol and coprostanol—which are excreted in feces. Between 1.0 g and 1.3 g of cholesterol is eliminated from the body daily, with the major portion removed via feces.

C. Synthesis of Bile Acids from Cholesterol and Its Regulation

Bile acids are synthesized in the liver from cholesterol. A portion of bile acids undergoes conjugation in the liver—a reaction combining them with hydrophilic molecules (Glycine and taurine). Bile acids facilitate fat emulsification, the absorption of Digestion products, and certain hydrophobic dietary substances, such as Fat-soluble Vitamins and cholesterol. Bile acids are also reabsorbed, transported back to the liver via the portal vein, and repeatedly utilized for fat emulsification. This pathway is known as the enterohepatic circulation of bile acids.

Synthesis of Bile Acids

The body synthesizes 200–600 mg of bile acids per day. The initial reaction of synthesis—the formation of 7-α-hydroxycholesterol—is regulatory. The enzyme 7-α-hydroxylase, which catalyzes this reaction, is inhibited by the end product, bile acids. 7-α-Hydroxylase is a form of cytochrome P₄₅₀ and utilizes oxygen as one of its substrates. One oxygen atom from O₂ is incorporated into the hydroxyl group at the 7-position, while the other is reduced to water. Subsequent synthetic reactions lead to the formation of two types of bile acids: cholic and chenodeoxycholic acids (Fig. 8-71), which are referred to as "primary bile acids."

Fig. 8-71. Synthesis of primary bile acids and its regulation. During bile acid synthesis, cholesterol undergoes hydroxylation, reduction of the double bond at positions 5 and 6, and oxidation of the side chain. Two types of bile acids are formed: one with hydroxyl groups at positions 3 and 7, and another with hydroxyl groups at positions 3, 7, and 12.

Conjugation of Bile Acids

Conjugation is the attachment of ionized molecules of glycine or taurine to the carboxyl group of bile acids; it enhances their detergent properties by increasing the amphiphilicity of the molecules.

Conjugation occurs within liver cells and begins with the formation of the active form of bile acids—CoA derivatives (Fig. 8-72).

Fig. 8-72. Conjugation of bile acids in the liver and their degradation in the intestine. A — conjugation products exhibit superior detergent properties due to a lower dissociation constant, ensuring the molecules are fully dissociated at pH 6 in the intestine. Cholic and chenodeoxycholic acids undergo conjugation; B — in the intestine, a small amount of bile acids is converted into lithocholic and deoxycholic acids through the action of bacterial enzymes.

Subsequently, taurine or glycine is attached, resulting in four types of conjugates: taurocholing, taurochenodeoxycholic, glycocholic, or glycochenodeoxycholic acids (which are significantly stronger emulsifiers than the original bile acids).

Glycine conjugates are formed in amounts three times greater than taurine conjugates because the availability of taurine is limited.

Enterohepatic circulation of bile acids. Transformations of bile acids in the intestine

Fat hydrolysis products are absorbed mainly in the upper small intestine, whereas bile salts are absorbed in the ileum. About 95% of the bile acids entering the intestine return to the liver via the portal vein, are then re-secreted into bile, and are reused in fat emulsification (Fig. 8–73). This pathway of bile acids is called the enterohepatic circulation. A total of 12–32 g of bile salts are reabsorbed per day; since the body contains 2–4 g of bile acids, each bile acid molecule completes this cycle 6–8 times.

Fig. 8-73. Enterohepatic circulation of bile acids. Light circles represent bile micelles; dark circles represent mixed micelles of bile and triacylglycerol hydrolysis products.

Some bile acids in the intestine are acted upon by bacterial enzymes that cleave glycine and taurine, as well as the hydroxyl group at position 7 of the bile acids. Bile acids lacking this hydroxyl group are called secondary bile acids. Secondary bile acids include deoxycholic acid (formed from cholic acid) and lithocholic acid (formed from deoxycholic acid); they are less soluble and more slowly absorbed in the intestine than primary bile acids. Therefore, secondary bile acids are primarily eliminated with feces. However, reabsorbed secondary bile acids are converted back into primary bile acids in the liver and participate in fat emulsification. About 500–600 mg of bile acids are excreted from the body per day. This pathway of bile acid excretion also serves as the primary route for eliminating cholesterol from the body. To compensate for the loss of bile acids with feces, the liver continuously synthesizes bile acids from cholesterol in an amount equivalent to the excreted bile acids. As a result, the bile acid pool (2–4 g) remains constant.

Regulation of bile acid synthesis

The regulatory enzymes for bile acid synthesis (7-α-hydroxylase) and cholesterol synthesis (HMG-CoA reductase) are inhibited by bile acids. Throughout the day, the activity of both enzymes changes in a similar manner; that is, an increase in The amount of bile acids in the liver leads to a decrease in the synthesis of both bile acids and cholesterol. The return of bile acids to the liver via enterohepatic circulation exerts an important regulatory effect; interruption of this circulation leads to the activation of 7-α-hydroxylase and increased uptake of cholesterol from the blood. This mechanism underlies one of the Methods for lowering blood cholesterol concentrations in the Treatment of hypercholesterolemia. In this case, drugs that adsorb cholesterol and bile acids in the intestine and prevent their absorption are used.

The regulation of 7-α-hydroxylase is also carried out by other mechanisms: phosphorylation/dephosphorylation, with the phosphorylated form being active (unlike HMG-CoA reductase); changes in enzyme quantity; cholesterol induces gene transcription, whereas bile acids repress it.

The synthesis of 7-α-hydroxylase is influenced by hormones: THYROID HORMONES induce its synthesis, whereas estrogens repress it. This effect of estrogens on bile acid synthesis explains why cholelithiasis occurs 3–4 times more frequently in women than in men.

G. Cholelithiasis

Cholelithiasis is a pathological process in which stones form in the Gallbladder, primarily composed of cholesterol.

The excretion of cholesterol into bile must be accompanied by the proportional excretion of bile acids and phospholipids, which maintain hydrophobic cholesterol molecules in bile in a micellar state (Table 8–9).

Table 8-9. Components of bile

In most patients with cholelithiasis, HMG-CoA reductase activity is elevated, resulting in increased cholesterol synthesis, whereas the activity of 7-α-hydroxylase, which is involved in bile acid synthesis, is reduced. Consequently, cholesterol synthesis is increased while its conversion into bile acids is slowed down, leading to a disproportion between the amounts of cholesterol and bile acids secreted into bile.

When these proportions are disrupted, cholesterol begins to precipitate in the gallbladder, initially forming a viscous sediment that gradually becomes harder. Sometimes it becomes impregnated with bilirubin (a heme breakdown product), proteins, and calcium salts. Gallstones may consist entirely of cholesterol (cholesterol stones) or a mixture of cholesterol, bilirubin, proteins, and calcium. Cholesterol stones are usually white, whereas mixed stones range in various shades of brown. There are many causes of altered bile acid-to-cholesterol ratios in bile, including a cholesterol-rich diet, a hypercaloric diet, gallbladder stasis, impaired enterohepatic circulation, defective bile acid synthesis, and gallbladder infections.

If stones begin to migrate from the gallbladder into the bile ducts, they trigger spasms of the gallbladder and ducts, which the patient perceives as a severe pain attack. If a stone blocks a duct for some time, bile flow into the intestine is disrupted, and bile pigments pass through hepatocyte membranes toward the sinusoids and enter the blood, leading to obstructive (posthepatic) jaundice.

Treatment of cholelithiasis. In the Cytology/cytology/16.html">Early stages of stone formation, chenodeoxycholic acid can be used pharmacologically. Upon reaching the gallbladder, this bile acid gradually dissolves the cholesterol sediment (cholesterol stones), although this is a slow process requiring several months.

D. Dyslipoproteinemias. Hypercholesterolemia and the development of atherosclerosis

Dyslipoproteinemias are disorders of plasma lipoprotein (LP) metabolism and, accordingly, of the metabolism of Lipids transported by LPs. Dyslipoproteinemias most commonly manifest as an elevated concentration of a single LP type or a combined increase in the levels of several LP types.

Currently, there are several classifications of dyslipoproteinemias. The main Classification is presented in Table 8–10.

Table 8-10. Dyslipoproteinemias

Disorders of cholesterol and triacylglycerol metabolism are the most common.

Disturbances in cholesterol metabolism most frequently lead to hypercholesterolemia and the subsequent development of atherosclerosis. Atherosclerosis involves the formation of so-called atherosclerotic plaques on arterial walls, which consist primarily of cholesterol deposits. Atherosclerotic plaques damage vascular endothelial cells, and thrombi frequently form at such sites. Atherosclerosis is a polygenic disease. One of the main Causes of atherosclerosis development is an imbalance among dietary cholesterol intake, its synthesis, and its elimination from the body. Cholesterol elimination is limited and does not exceed 1.2–1.5 g/day, whereas improper dietary intake can exceed this threshold, leading to a gradual accumulation of cholesterol in the body with age. Genetic Defects in Proteins and Enzymes involved in cholesterol metabolism represent an important factor in the development of atherosclerosis.

Hypercholesterolemia. The Role of Dietary Factors in the Development of Hypercholesterolemia

The blood cholesterol concentration in healthy adults is 200 ± 50 mg/dL (5.2 ± 1.2 mmol/L) and typically increases with age. Exceeding this normal blood concentration is referred to as hypercholesterolemia.

Hypercholesterolemia frequently develops as a result of the excessive Dietary intake of cholesterol, CARBOHYDRATES, and fats. A hypercaloric diet is one of the most common contributing factors to hypercholesterolemia, since the de novo synthesis of cholesterol requires only acetyl-CoA, ATP, and NADPH. All of these substrates are generated during the Oxidation of glucose and fatty acids; consequently, an excessive intake of these dietary components promotes the development of hypercholesterolemia. Under normal physiological conditions, dietary cholesterol intake downregulates endogenous Cholesterol synthesis in the liver; however, with advancing age, the efficiency of this regulatory mechanism declines in many individuals.

Maintaining a healthy, balanced diet throughout life is a critical factor in the Prevention of hypercholesterolemia. A clear correlation has been proven between elevated plasma cholesterol concentrations and mortality from cardiovascular diseases (CVD)—specifically myocardial infarction and stroke—which develop as a consequence of atherosclerosis (Fig. 8-74).

Fig. 8-74. Correlation between blood cholesterol concentration and CVD mortality per 1,000 men. Mortality per 1,000 men

LDL Receptor Gene: Structure and Mutation Types

Hereditary factors play a major role in predisposition to atherosclerosis. Mutations in The structure of the LDL receptor gene are among the most frequently observed genetic defects.

The LDL receptor gene is located on chromosome 19 and consists of 18 exons (Fig. 8-75). Different groups of exons encode distinct functional domains within this protein. Mutations in this gene have been thoroughly investigated and classified into 4 distinct classes.

Fig. 8-75. Selection/21.html">Types of mutations in the LDL receptor protein gene. Exons are represented by dark rectangles, introns by lines connecting the exons. The illustration shows the mutation types: <-> — deletion, • — nonsense mutation, ■ — missense mutation; kb — kilobases.

The first class of mutations, which is the most prevalent, results in a complete lack of receptor synthesis; the second class is characterized by the synthesis of a receptor that fails to undergo Intracellular Transport to the cell surface; the third class corresponds to a scenario where the receptor is successfully transported to the cell surface but is incapable of binding LDL; and the fourth class involves a receptor that binds LDL, but fails to undergo receptor-mediated endocytosis. Structural alterations in LDL receptors resulting from all these mutation types lead to hypercholesterolemia, because LDL particles are not internalized by cells, causing LDL-associated cholesterol to accumulate in the bloodstream.

Familial Hypercholesterolemia

Any defect in the LDL receptor or the apolipoprotein B-100 (apoB-100) that interacts with it leads to the Development of the most common hereditary lipid disorder: familial hypercholesterolemia. This autosomal dominant disorder is caused by the aforementioned mutations in the LDL receptor gene. Heterozygotes, who carry one normal and one defective gene, have a prevalence of approximately 1 in 500 individuals, and in certain African populations, it can reach as high as 1 in 100 individuals. The number of cell-surface LDL receptors in heterozygotes is reduced by half, resulting in a proportional doubling of plasma cholesterol concentrations. By age 35–40, heterozygotes experience blood cholesterol levels reaching 400–500 mg/dL, leading to severe atherosclerosis and premature death from myocardial infarction or stroke. Homozygotes are rare, occurring in approximately 1 in 1,000,000 individuals. In such patients, blood cholesterol and LDL concentrations are already elevated 5- to 6-fold in early childhood. LDL particles are scavenged by macrophages via phagocytosis. Macrophages overloaded with an excess of Cholesterol and other lipids derived from LDL accumulate in the skin and even in tendons, forming lesions known as xanthomas. Cholesterol is also deposited within the walls of Arteries, forming atherosclerotic plaques. Without prompt and aggressive treatment, such children typically do not survive past the age of 5 or 6. Treatment for this severe form of the disease involves removing LDL from the blood via plasmapheresis, though the most radical and definitive treatment is liver transplantation. A donor liver possessing a normal Complement of LDL receptors significantly lowers blood cholesterol concentrations and prevents premature death from atherosclerosis.

In addition to genetic defects in the LDL receptor, hypercholesterolemia and subsequent atherosclerosis can be caused by inherited structural defects in apoB-100, as well as by the elevated synthesis or secretion of apoB-100 in familial combined hyperlipidemia, a condition characterized by elevated blood concentrations of both cholesterol and triacylglycerols.

Chemical Modification of LDL Lipids and Proteins, and LDL Receptors

Alterations in the normal lipid and Protein Structure of LDL particles render them foreign to the body, thereby increasing their uptake by phagocytic cells. For example, the activation of free-radical Lipid Peroxidation alters not only the lipid structure within lipoproteins but also the structure of apoB-100. Another factor that modifies the structure of both LDL and the LDL receptor is non-enzymatic protein glycation, which occurs when blood glucose levels are chronically elevated, as in diabetes mellitus. These modified LDL particles are avidly taken up by macrophages via scavenger receptors.

MOLECULAR MECHANISMS OF Atherosclerosis Pathogenesis

The development of atherosclerosis progresses through several distinct stages (Fig. 8-76).

Fig. 8-76. Development of an atherosclerotic plaque in vascular endothelial cells.

The process initiates with vascular endothelial injury, which can be driven by various mechanisms. A primary mechanism is endothelial Damage caused by structurally altered LDL—for instance, resulting from the activation of free-radical lipid peroxidation (LPO) within the LDL particle, provoked by free radicals generated during normal metabolic processes or introduced from exogenous sources. During LPO within LDL, not only is the structure of the lipids themselves modified, but the conformation of apoproteins is also disrupted. Oxidized LDL particles are engulfed by macrophages via scavenger receptors. Unlike the tightly regulated uptake of cholesterol via specific high-affinity receptors, this pathway lacks feedback regulation; consequently, macrophages become overloaded with cholesterol and transform into "foam cells," which then migrate into the subendothelial space. This leads to the formation of fatty streaks in the walls of Blood Vessels. At this early stage, the vascular endothelium may retain its structural integrity. As the accumulation of foam cells continues, however, vascular endothelial damage ensues. Under normal conditions, endothelial cells secrete prostaglandin I₂ (prostacyclin I₂), which inhibits platelet aggregation. When endothelial cells are damaged, platelets become activated. First, they secrete thromboxane A2 (TXA2), which stimulates platelet aggregation and can trigger thrombus formation in the region of the atherosclerotic plaque; second, platelets begin producing platelet-derived growth factor, a peptide that stimulates the proliferation of vascular smooth Muscle cells (SMCs). SMCs migrate from the medial layer into the inner layer of the arterial wall, thereby contributing to the growth of the plaque. Subsequently, the plaque becomes infiltrated with fibrous tissue (Collagen,

Elastin); cells underlying the fibrous cap undergo necrosis, and cholesterol is deposited in the extracellular space. At this advanced stage, cholesterol crystals even form within the core of the plaque. During the final Stages of development, the plaque becomes calcified and exceptionally rigid. Thrombi frequently form over the plaque, obstructing the vascular lumen and causing acute disruption of blood flow to the corresponding tissue area, culminating in a myocardial infarction. Because atherosclerotic plaques most commonly develop in the coronary Arteries of the myocardium, myocardial infarction is the most prevalent clinical consequence of atherosclerosis.

Biochemical Basis for the Treatment of Atherosclerosis and the Prevention of Myocardial Infarction

An essential therapeutic measure for reducing the risk of hypercholesterolemia and atherosclerosis is a hypocaloric, cholesterol-lowering diet. Dietary cholesterol intake should not exceed 300 mg/day (Table 8-11).

Table 8-11. Dietary Guidelines for reducing cholesterol and fat intake in humans

Cholesterol is a sterol of animal origin; therefore, it enters the body through the consumption of animal fats and fatty meats. Plant-based foods do not contain cholesterol, making them the dietary foundation for middle-aged and older adults.

Therapeutic and preventive measures include enriching the diet with ω-3 polyunsaturated fatty acids, which reduce the risk of thrombus formation. Unsaturated fatty acids facilitate the more rapid Elimination of Cholesterol from the body, although the exact mechanism of this phenomenon remains fully elucidated. At the same time, it has been proven that polyunsaturated acids suppress the synthesis of platelet-derived growth factor, thereby slowing the progression of atherosclerotic plaques.

Antioxidant vitamins C, E, and A inhibit lipid peroxidation (free-radical oxidation) in LDL, helping to maintain the normal lipid structure and metabolism of LDL.

However, dietary modifications alone are insufficient for treating pronounced hypercholesterolemia and atherosclerosis. The management of hypercholesterolemia is typically multifaceted.

One of the therapeutic principles involves "interrupting" the enterohepatic circulation of bile acids. This is achieved using medications such as cholestyramine—a polymer that binds bile acids in the intestine, is excreted in the feces, and thereby reduces the return of bile acids to the liver. In response, the liver increases the uptake of cholesterol from the bloodstream to synthesize new bile acids. Drugs of this type are referred to as bile acid sequestrants.

The most effective medications used in the treatment of atherosclerosis are HMG-CoA reductase inhibitors. These agents, such as the antibiotic Mevacor, are converted in the liver into their active form (Fig. 8-77) and potently inhibit the key regulatory enzyme of Cholesterol Biosynthesis. Such drugs can virtually halt the body's endogenous cholesterol synthesis. Under these conditions, the liver increases its uptake of circulating cholesterol from the blood. To accomplish this, the synthesis of LDL receptor proteins in liver cells nearly doubles, leading to a corresponding increase in the clearance of LDL from the bloodstream. As a result, blood cholesterol

levels can be reduced to nearly normal, even in patients with heterozygous familial hypercholesterolemia.

Fig. 8-77. Formation of the active form of Mevacor.

Fibrates (such as clofibrate and fenofibrate) accelerate the Catabolism of VLDL by activating lipoprotein lipase. These drugs also stimulate Fatty acid oxidation in the liver, thereby reducing the synthesis of triacylglycerols and cholesterol esters and, consequently, hepatic VLDL secretion. Clofibrate induces the synthesis of peroxisomal enzymes capable of oxidizing fatty acids. Fibrates are generally prescribed when hypertriglyceridemia and hypercholesterolemia occur concurrently. Effective treatment of atherosclerosis typically requires a combined pharmacological approach utilizing multiple medications.