UNIT III Lipid Metabolism Cholesterol and Steroid Metabolism

UNIT III: Lipid Metabolism Cholesterol and Steroid Metabolism

I. Overview • Cholesterol, the characteristic steroid alcohol of animal tissues, performs a number of essential functions in the body. • For example, cholesterol is a structural component of all cell membranes, modulating their fluidity, and, in specialized tissues, cholesterol is a precursor of bile acids, steroid hormones, and vitamin D. • It is therefore of critical importance that the cells of the body be assured an appropriate supply of cholesterol. • To meet this need, a complex series of transport, biosynthetic, and regulatory mechanisms has evolved. • The liver plays a central role in the regulation of the body's cholesterol homeostasis.

• For example, cholesterol enters the liver's cholesterol pool from a number of sources including dietary cholesterol, as well as cholesterol synthesized de novo by extrahepatic tissues and by the liver itself. • Cholesterol is eliminated from the liver as unmodified cholesterol in the bile, or it can be converted to bile salts that are secreted into the intestinal lumen. • It can also serve as a component of plasma lipoproteins sent to the peripheral tissues. • In humans, the balance between cholesterol influx and efflux is not precise, resulting in a gradual deposition of cholesterol in the tissues, particularly in the endothelial linings of blood vessels. • This is a potentially life-threatening occurrence when the lipid deposition leads to plaque formation, causing the narrowing of blood vessels (atherosclerosis) and increased risk of coronary artery disease.

The major sources of liver cholesterol and the routes by which cholesterol leaves the liver

II. Structure of Cholesterol • Cholesterol is a very hydrophobic compound. It consists of four fused hydrocarbon rings (A, B, C, and D, called the “steroid nucleus”), and it has an eight-carbon, branched hydrocarbon chain attached to C-17 of the D ring. • Ring A has a hydroxyl group at C-3, and ring B has a double bond between C-5 and C-6 (Figure 18. 2).

• Figure 18. 2 Structure of cholesterol and its ester.

A. Sterols • • • Steroids with 8– 10 carbon atoms in the side chain at C 17 and a hydroxyl group at C-3 are classified as sterols. Cholesterol is the major sterol in animal tissues. [Note: Plant sterols, such as β-sitosterol are poorly absorbed by humans. After entering the enterocytes, they are actively transported back into the intestinal lumen. Because some cholesterol is transported as well, plant sterols appear to block the absorption of dietary cholesterol. This has led to clinically useful dietary treatment of hypercholesteremia. Daily ingestion of plant steroid esters (in the form of commercially available trans fatty acid–free margarine) is one of a number of dietary strategies leading to the reduction of plasma cholesterol levels]

B. Cholesteryl esters • Most plasma cholesterol is in an esterified form (with a fatty acid attached at C-3), which makes the structure even more hydrophobic than free cholesterol. • Cholesteryl esters are not found in membranes, and are normally present only in low levels in most cells. • Because of their hydrophobicity, cholesterol and its esters must be transported in association with protein as a component of a lipoprotein particle or be solubilized by phospholipids and bile salts in the bile.

III. Synthesis of Cholesterol • Cholesterol is synthesized by virtually all tissues in humans, although liver, intestine, adrenal cortex, and reproductive tissues, including ovaries, testes, and placenta, make the largest contributions to the body's cholesterol pool. • As with fatty acids, all the carbon atoms in cholesterol are provided by acetate, and NADPH provides the reducing equivalents. • The pathway is endergonic, being driven by hydrolysis of the high-energy thioester bond of acetyl coenzyme A (Co. A) and the terminal phosphate bond of adenosine triphosphate (ATP).

• Synthesis occurs in the cytoplasm, with enzymes in both the cytosol and the membrane of the endoplasmic reticulum. • The pathway is responsive to changes in cholesterol concentration, and regulatory mechanisms exist to balance the rate of cholesterol synthesis within the body against the rate of cholesterol excretion. • An imbalance in this regulation can lead to an elevation in circulating levels of plasma cholesterol, with the potential for coronary artery disease.

A. Synthesis of 3 -hydroxy-3 -methylglutaryl )HMG) Co. A • The first two reactions in the cholesterol synthetic pathway are similar to those in the pathway that produces ketone bodies. They result in the production of HMG Co. A. First, two acetyl Co. A molecules condense to form acetoacetyl Co. A. Next, a third molecule of acetyl Co. A is added, producing HMG Co. A, a six-carbon compound. [Note: Liver parenchymal cells contain two isoenzymes of HMG Co. A synthase. The cytosolic enzyme participates in cholesterol synthesis, whereas the mitochondrial enzyme functions in the pathway for ketone body synthesis. ] • •

• Figure 18. 3 Synthesis of HMG Co. A.

B. Synthesis of mevalonic acid (mevalonate( • The next step, the reduction of HMG Co. A to mevalonic acid, is catalyzed by HMG Co. A reductase, and is the rate-limiting and key regulated step in cholesterol synthesis. • It occurs in the cytosol, uses two molecules of NADPH as the reducing agent, and releases Co. A, making the reaction irreversible. • ]Note: HMG Co. A reductase is an intrinsic membrane protein of the endoplasmic reticulum (ER), with the enzyme's catalytic domain projecting into the cytosol.

C. Synthesis of cholesterol • The reactions and enzymes involved in the synthesis of cholesterol from mevalonate are illustrated in Figure 18. 5. • ]Note: The numbers shown in brackets below correspond to numbered reactions shown in this figure[.

[1]Mevalonic acid is converted to 5 pyrophosphomevalonate in two steps, each of which transfers a phosphate group from ATP. [2]A five-carbon isoprene unit—isopentenyl pyrophosphate (IPP)—is formed by the decarboxylation of 5 -pyrophosphomevalonate. The reaction requires ATP. • ]Note: IPP is the precursor of a family of molecules with diverse functions, the isoprenoids. Cholesterol is a sterol isoprenoid. Nonsterol isoprenoids include dolichol and ubiquinone[.

• Figure 18. 4 Synthesis of mevalonic acid.

[3]IPP is isomerized to 3, 3 -dimethylallyl pyrophosphate (DPP. ( [4]IPP and DPP condense to form ten-carbon geranyl pyrophosphate (GPP. ( [5]A second molecule of IPP then condenses with GPP to form 15 -carbon farnesyl pyrophosphate (FPP. ( • ]Note: Covalent attachment of farnesyl to proteins, a process known as “prenylation, ” is one mechanism for anchoring proteins to plasma membranes[. [6]Two molecules of FPP combine, releasing pyrophosphate, and are reduced, forming the 30 -carbon compound squalene. • ]Note: Squalene is formed from six isoprenoid units. Because three ATP are hydrolyzed per mevalonic acid residue converted to IPP, a total of 18 ATP are required to make the polyisoprenoid squalene[.

[7] Squalene is converted to the sterol lanosterol by a sequence of reactions that use molecular oxygen and NADPH. The hydroxylation of squalene triggers the cyclization of the structure to lanosterol. [8] The conversion of lanosterol to cholesterol is a multistep process, resulting in the shortening of the carbon chain from 30 to 27 carbons, removal of the two methyl groups at C-4, migration of the double bond from C-8 to C-5, and reduction of the double bond between C-24 and C-25. • [Note: This pathway includes more than 19 different enzymatic reactions. Smith-Lemli-Opitz syndrome (SLOS), a relatively common autosomal recessive disorder of cholesterol biosynthesis, is caused by a partial deficiency in 7 -dehydrocholesterol-7 -reductase— an enzyme involved in the migration of the double bond. SLOS is one of several multisystem, embryonic malformation syndromes associated with impaired cholesterol synthesis. ]

• Figure 18. 5 Synthesis of cholesterol from mevalonic acid.

D. Regulation of cholesterol synthesis • HMG Co. A reductase, the rate-limiting enzyme, is the major control point for cholesterol biosynthesis, and is subject to different kinds of metabolic control. 1. Sterol-dependent regulation of gene expression: • Expression of the HMG Co. A reductase gene is controlled by the transcription factor, SREBP (sterol regulatory element–binding protein) that binds DNA at the cis-acting sterol regulatory element (SRE) of the reductase gene. • SREBP is an integral protein of the ER membrane, and associates with a second ER membrane protein, SCAP (SREBP cleavage–activating protein).

• Figure 18. 6 Regulation of HMG Co. A reductase. SRE = sterol regulatory element; SREBP = sterol regulatory element binding protein; SCAP = SREBP cleavage-activating protein

2. Sterol-accelerated enzyme degradation: • The reductase itself is an integral protein of the ER membrane. When sterol levels in the cell are high, the reductase binds to insig proteins. This binding leads to ubiquitination and proteasomal degradation of the reductase. . 3 Sterol-independent phosphorylation/dephosphorylation : • HMG Co. A reductase activity is controlled covalently through the actions of adenosine monophosphate (AMP) –activated protein kinase (AMPK), and a phosphoprotein phosphatase. • The phosphorylated form of the enzyme is inactive, whereas the dephosphorylated form is active.

• [Note: AMPK is activated by AMP, so cholesterol synthesis, like fatty acid synthesis, is decreased when ATP availability is decreased. ] 4. Hormonal regulation: • The amount (and, therefore, the activity) of HMG Co. A reductase is controlled hormonally. • An increase in insulin favors up-regulation of the expression of the HMG Co. A reductase gene. Glucagon has the opposite effect.

5. Inhibition by drugs: • The statin drugs (atorvastatin, fluvastatin, lovastatin, pravastatin, rosuvastatin, and simvastatin) are structural analogs of HMG Co. A, and are (or are metabolized to) reversible, competitive inhibitors of HMG Co. A reductase. • They are used to decrease plasma cholesterol levels in patients with hypercholesterolemia

• Figure 18. 7 Structural similarity of HMG and simvastatin, a clinically useful cholesterollowering drug of the “statin” family.

IV. Degradation of Cholesterol • The ring structure of cholesterol cannot be metabolized to CO 2 and H 2 O in humans. • Rather, the intact sterol nucleus is eliminated from the body by conversion to bile acids and bile salts, which are excreted in the feces, and by secretion of cholesterol into the bile, which transports it to the intestine for elimination. • Some of the cholesterol in the intestine is modified by bacteria before excretion. The primary compounds made are the isomers coprostanol and cholestanol, which are reduced derivatives of cholesterol. • Together with cholesterol, these compounds make up the bulk of neutral fecal sterols.

V. Bile Acids and Bile Salts • Bile consists of a watery mixture of organic and inorganic compounds. • Phosphatidylcholine (lecithin) and bile salts (conjugated bile acids) are quantitatively the most important organic components of bile. • Bile can either pass directly from the liver where it is synthesized into the duodenum through the common bile duct, or be stored in the gallbladder when not immediately needed for digestion.

A. Structure of the bile acids • The bile acids contain 24 carbons, with two or three hydroxyl groups and a side chain that terminates in a carboxyl group. • The carboxyl group has a p. Ka of about six and, therefore, is not fully ionized at physiologic p. H—hence, the term “bile acid. ” • The bile acids are amphipathic in that the hydroxyl groups are α in orientation (they lie “below” the plane of the rings) and the methyl groups are β (they lie “above” the plane of the rings). • Therefore, the molecules have both a polar and a nonpolar face, and can act as emulsifying agents in the intestine, helping prepare dietary triacylglycerol and other complex lipids for degradation by pancreatic digestive enzymes.

B. Synthesis of bile acids • Bile acids are synthesized in the liver by a multistep, multiorganelle pathway in which hydroxyl groups are inserted at specific positions on the steroid structure, the double bond of the cholesterol B ring is reduced, and the hydrocarbon chain is shortened by three carbons, introducing a carboxyl group at the end of the chain. • The most common resulting compounds, cholic acid (a triol) and chenodeoxycholic acid (a diol), are called “primary” bile acids. • [Note: The rate-limiting step in bile acid synthesis is the introduction of a hydroxyl group at carbon 7 of the steroid nucleus by cholesterol-7 -α-hydroxylase, an ERassociated cytochrome P 450 (CYP) enzyme found only in liver. The enzyme is down-regulated by cholic acid and up-regulated by cholesterol]

• Figure 18. 8 Bile acids.

• Figure 18. 9 Synthesis of cholic acid, a bile acid.

C. Synthesis of bile salts • Before the bile acids leave the liver, they are conjugated to a molecule of either glycine or taurine (an endproduct of cysteine metabolism) by an amide bond between the carboxyl group of the bile acid and the amino group of the added compound. • These new structures are called bile salts and include glycocholic and glycochenodeoxycholic acids, and taurocholic and taurochenodeoxycholic acids. • The ratio of glycine to taurine forms in the bile is approximately 3: 1. • Addition of glycine or taurine results in the presence of a carboxyl group with a lower p. Ka (from glycine) or a sulfonate group (from taurine), both of which are fully ionized (negatively charged) at physiologic p. H.

• Bile salts are more effective detergents than bile acids because of their enhanced amphipathic nature. • Therefore, only the conjugated forms—that is, the bile salts—are found in the bile. • Individuals with genetic deficiencies in the conversion of cholesterol to bile acids are treated with exogenously supplied chenodeoxycholic acid. [Bile salts provide the only significant mechanism for cholesterol excretion, both as a metabolic product of cholesterol and as a solubilizer of cholesterol in bile].

• Figure 18. 10 Bile salts. [Note “cholic” in the names. ]

D. Action of intestinal flora on bile salts • Bacteria in the intestine can remove glycine and taurine from bile salts, regenerating bile acids. • They can also convert some of the primary bile acids into “secondary” bile acids by removing a hydroxyl group, producing deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid (Figure 18. 11. (

Figure 18. 11 Enterohepatic circulation of bile salts and bile acids.

E. Enterohepatic circulation • Bile salts secreted into the intestine are efficiently reabsorbed (greater than 95%) and reused. • The mixture of primary and secondary bile acids and bile salts is absorbed primarily in the ileum. • They are actively transported from the intestinal mucosal cells into the portal blood, and are efficiently removed by the liver parenchymal cells. • ]Note: Bile acids are hydrophobic and require a carrier in the portal blood. Albumin carries them in a noncovalent complex, just as it transports fatty acids in blood [. • The liver converts both primary and secondary bile acids into bile salts by conjugation with glycine or taurine, and secretes them into the bile.

• The continuous process of secretion of bile salts into the bile, their passage through the duodenum where some are converted to bile acids, and their subsequent return to the liver as a mixture of bile acids and salts is termed the enterohepatic circulation. • Between 15 and 30 g of bile salts are secreted from the liver into the duodenum each day, yet only about 0. 5 g is lost daily in the feces. • Approximately 0. 5 g/day is synthesized from cholesterol in the liver to replace the lost bile acids. Bile acid sequestrants, such as cholestyramine, bind bile acids in the gut, prevent their reabsorption, and so promote their excretion. • They are used in the treatment of hypercholesterolemia because the removal of bile acids relieves the inhibition on bile acid synthesis in the liver, thereby diverting additional cholesterol into that pathway. • ]Note: Dietary fiber also binds bile acids and increases their excretion[.

Figure 18. 11 Enterohepatic circulation of bile salts and bile acids.

F. Bile salt deficiency: cholelithiasis • The movement of cholesterol from the liver into the bile must be accompanied by the simultaneous secretion of phospholipid and bile salts. • If this dual process is disrupted and more cholesterol enters the bile than can be solubilized by the bile salts and lecithin present, the cholesterol may precipitate in the gallbladder, initiating the occurrence of cholesterol gallstone disease—cholelithiasis. • This disorder is typically caused by a decrease of bile acids in the bile, which may result from : (1 gross malabsorption of bile acids from the intestine, as seen in patients with severe ileal disease ; (2 obstruction of the biliary tract, interrupting the enterohepatic circulation ;

3) severe hepatic dysfunction, leading to decreased synthesis of bile salts, or other abnormalities in bile production; or 4) excessive feedback suppression of bile acid synthesis as a result of an accelerated rate of recycling of bile acids. • Cholelithiasis also may result from increased biliary cholesterol excretion, as seen with the use of fibrates. • [Note: Fibrates, such as gemfibrozil, are derivatives of fibric acid. They are used to reduce triacylglycerol levels in blood through up-regulation of fatty acid β-oxidation. ] • Laparoscopic cholecystectomy (surgical removal of the gallbladder through a small incision) is currently the treatment of choice. • However, for patients who are unable to undergo surgery, administration of chenodeoxycholic acid to supplement the body's supply of bile acids results in a gradual (months to years) dissolution of the gallstones.

• Figure 18. 12 Gallbladder with gallstones

VI. Plasma Lipoproteins • The plasma lipoproteins are spherical macromolecular complexes of lipids and specific proteins (apolipoproteins or apoproteins). • The lipoprotein particles include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). • They differ in lipid and protein composition, size, density, and site of origin.

• Lipoproteins function both to keep their component lipids soluble as they transport them in the plasma and to provide an efficient mechanism for transporting their lipid contents to (and from) the tissues. • In humans, the transport system is less perfect than in other animals and, as a result, humans experience a gradual deposition of lipid— especially cholesterol—in tissues. • This is a potentially life-threatening occurrence when the lipid deposition contributes to plaque formation, causing the narrowing of blood vessels (atherosclerosis).

Figure 18. 13 Approximate size and density of serum lipoproteins. Each family of lipoproteins exhibits a range of sizes and densities; this figure shows typical values. The width of the rings approximates the amount of each component. [Note: Although cholesterol and its esters are shown as one component in the center of each particle, physically cholesterol is a surface component whereas cholesteryl esters are located in the interior of the lipoproteins. ]

A. Composition of plasma lipoproteins • Lipoproteins are composed of a neutral lipid core (containing triacylglycerol, and cholesteryl esters) surrounded by a shell of amphipathic apolipoproteins, phospholipid, and nonesterified cholesterol. • These amphipathic compounds are oriented so that their polar portions are exposed on the surface of the lipoprotein, thus making the particle soluble in aqueous solution. • The triacylglycerol and cholesterol carried by the lipoproteins are obtained either from the diet (exogenous source) or from de novo synthesis (endogenous source. ( • ]Note: Lipoprotein particles constantly interchange lipids and apolipoproteins with each other; therefore, the actual apolipoprotein and lipid content of each class of particles can be somewhat variable[.

1. Size and density of lipoprotein particles: • Chylomicrons are the lipoprotein particles lowest in density and largest in size, and contain the highest percentage of lipid and the lowest percentage of protein. • VLDLs and LDLs are successively denser, having higher ratios of protein to lipid. HDL particles are the densest. • Plasma lipoproteins can be separated on the basis of their electrophoretic mobility, as shown in Figure 18. 15, or on the basis of their density by ultracentrifugation.

• Figure 18. 14 Structure of a typical lipoprotein particle.

• Figure 18. 15 Electrophoretic mobility of plasma lipoproteins. The order of LDL and VLDL is reversed if ultracentrifugation is used as the separation technique.

2. Apolipoproteins: • The apolipoproteins associated with lipoprotein particles have a number of diverse functions, such as providing recognition sites for cell-surface receptors, and serving as activators or coenzymes for enzymes involved in lipoprotein metabolism. • Some of the apolipoproteins are required as essential structural components of the particles and cannot be removed (in fact, the particles cannot be produced without them), whereas others are transfered freely between lipoproteins. • Apolipoproteins are divided by structure and function into five major classes, A through E, with most classes having subclasses, for example, apolipoprotein (or apo) A-I and apo C-II. • [Note: Functions of all of the apolipoproteins are not yet known. ]

B. Metabolism of chylomicrons • Chylomicrons are assembled in intestinal mucosal cells and carry dietary triacylglycerol, cholesterol, fat-soluble vitamins, and cholesteryl esters (plus additional lipids made in these cells) to the peripheral tissues. • ]Note: TAGs account for close to 90% of the lipids in a chylomicron[.

• Figure 18. 16 Metabolism of chylomicrons. CM = chylomicron; TAG = triacylglycerol; C = cholesterol; CE = cholesteryl esters. Apo B-48, apo C-II, and apo E are apolipoproteins found as specific components of plasma lipoproteins. The lipoproteins are not drawn to scale (see Figure 18. 13 for details of the size and density of lipoproteins).

1. • • • Synthesis of apolipoproteins: Apolipoprotein B-48 is unique to chylomicrons. Its synthesis begins on the rough ER; it is glycosylated as it moves through the RER and Golgi. [Note: Apo B-48 is so named because it constitutes the N-terminal, 48% of the protein coded for by the apo B gene. Apo B-100, which is synthesized by the liver and found in VLDL and LDL, represents the entire protein coded for by the apo B gene. Posttranscriptional editing of a cytosine to a uracil in intestinal apo B-100 m. RNA creates a nonsense codon, allowing translation of only 48% of the m. RNA. ].

2. Assembly of chylomicrons: • The enzymes involved in triacylglycerol, cholesterol, and phospholipid synthesis are located in the smooth ER. • Assembly of the apolipoproteins and lipid into chylomicrons requires microsomal triacylglycerol transfer protein, which loads apo B-48 with lipid. • This occurs before transition from the ER to the Golgi, where the particles are packaged in secretory vesicles. • These fuse with the plasma membrane releasing the lipoproteins, which then enter the lymphatic system and, ultimately, the blood.

3. Modification of nascent chylomicron particles: • The particle released by the intestinal mucosal cell is called a “nascent” chylomicron because it is functionally incomplete. • When it reaches the plasma, the particle is rapidly modified, receiving apo E (which is recognized by hepatic receptors) and C apolipoproteins. • The latter include apo C-II, which is necessary for the activation of lipoprotein lipase, the enzyme that degrades the triacylglycerol contained in the chylomicron. • The source of these apolipoproteins is circulating HDL.

• Figure 18. 16 Metabolism of chylomicrons. CM = chylomicron; TAG = triacylglycerol; C = cholesterol; CE = cholesteryl esters. Apo B-48, apo C-II, and apo E are apolipoproteins found as specific components of plasma lipoproteins. The lipoproteins are not drawn to scale (see Figure 18. 13 for details of the size and density of lipoproteins).

4. Degradation of triacylglycerol by lipoprotein lipase: • Lipoprotein lipase is an extracellular enzyme that is anchored by heparan sulfate to the capillary walls of most tissues, but predominantly those of adipose tissue and cardiac and skeletal muscle. • Adult liver does not have this enzyme. • ]Note: A hepatic lipase is found on the surface of endothelial cells of the liver. However, it does not significantly attack chylomicrons or VLDL triacylglycerol, but rather assists with HDL metabolism [ • Lipoprotein lipase, activated by apo C-II on circulating lipoprotein particles, hydrolyzes the triacylglycerol contained in these particles to yield fatty acids and glycerol.

• The fatty acids are stored (by the adipose) or used for energy (by the muscle). If they are not immediately taken up by a cell, the long-chain fatty acids are transported by serum albumin until their uptake does occur. • Glycerol is used by the liver, for example, in lipid synthesis, glycolysis, or gluconeogenesis. • [Note: Patients with a deficiency of lipoprotein lipase or apo C-II (Type 1 hyperlipoproteinemia, or familial lipoprotein lipase deficiency) show a dramatic accumulation (1000 mg/dl or greater) of chylomicrons in the plasma (hypertriacylglycerolemia), even in the fasted state. ]

5. Regulation of lipoprotein lipase activity: • Lipoprotein lipase synthesis and transfer to the luminal surface of the capillary is stimulated by insulin (signifying a fed state. ( • Furthermore, isomers of lipoprotein lipase have different Km values for triacylglycerol. • For example, the adipose enzyme has a large Km, allowing the removal of fatty acids from circulating lipoprotein particles and their storage as triacylglycerols only when plasma lipoprotein concentrations are elevated. • Conversely, heart muscle lipoprotein lipase has a small Km, allowing the heart continuing access to the circulating fuel, even when plasma lipoprotein concentrations are low. • ]Note: The highest concentration of lipoprotein lipase is in cardiac muscle, reflecting the use of fatty acids to provide much of the energy needed for cardiac function [.

6. Formation of chylomicron remnants: • As the chylomicron circulates and more than 90% of the triacylglycerol in its core is degraded by lipoprotein lipase, the particle decreases in size and increases in density. • In addition, the C apoproteins (but not apo E) are returned to HDL. • The remaining particle, called a “remnant, ” is rapidly removed from the circulation by the liver, whose cell membranes contain lipoprotein receptors that recognize apo E. • Chylomicron remnants bind to these receptors and are taken into the hepatocytes by endocytosis. • The endocytosed vesicle then fuses with a lysosome, and the apolipoproteins, cholesteryl esters, and other components of the remnant are hydrolytically degraded, releasing amino acids, free cholesterol, and fatty acids. • The receptor is recycled.

• Figure 18. 16 Metabolism of chylomicrons. CM = chylomicron; TAG = triacylglycerol; C = cholesterol; CE = cholesteryl esters. Apo B-48, apo C-II, and apo E are apolipoproteins found as specific components of plasma lipoproteins. The lipoproteins are not drawn to scale (see Figure 18. 13 for details of the size and density of lipoproteins).

C. Metabolism of VLDL • VLDLs are produced in the liver. • They are composed predominantly of triacylglycerol (approximately 60%), and their function is to carry this lipid from the liver to the peripheral tissues. • There, the triacylglycerol is degraded by lipoprotein lipase, as discussed for chylomicrons. • [Note: “Fatty liver” (hepatic steatosis) occurs in conditions in which there is an imbalance between hepatic triacylglycerol synthesis and the secretion of VLDL. • Such conditions include obesity, uncontrolled diabetes mellitus, and chronic ethanol ingestion. ]

• Figure 18. 17 Metabolism of VLDL and LDL. TAG = triacylglycerol; VLDL = very-low-density lipoprotein; LDL = low-density-lipoprotein; IDL = intermediate-density lipoprotein; C = cholesterol; CE = cholesteryl esters. Apo B-100, apo C-II, and apo E are apolipoproteins found as specific components of plasma lipoproteins. Lipoproteins are not drawn to scale (see Figure 18. 13 for details of the size and density of lipoproteins).

1. Release of VLDL: • VLDL are secreted directly into the blood by the liver as nascent VLDL particles containing apo B -100. • They must obtain apo C-II and apo E from circulating HDL (see Figure 18. 17). As with chylomicrons, apo C-II is required for activation of lipoprotein lipase. • [Note: Abetalipoproteinemia is a rare hypolipoproteinemia caused by a defect in microsomal triacylglycerol transfer protein, leading to an inability to load apo B with lipid. As a consequence, no VLDL or chylomicrons are formed, and triacylglycerols accumulate in the liver and intestine. ]

• Figure 18. 18 Transfer of cholesteryl esters (CE) from HDL to VLDL in exchange for triacylglycerol (TAG) or phospholipids (PL).

2. Modification of circulating VLDL: • As VLDL pass through the circulation, triacylglycerol is degraded by lipoprotein lipase, causing the VLDL to decrease in size and become denser. • Surface components, including the C and E apoproteins, are returned to HDL, but the particles retain apo B-100. • Finally, some triacylglycerols are transferred from VLDL to HDL in an exchange reaction that concomitantly transfers some cholesteryl esters from HDL to VLDL. • This exchange is accomplished by cholesteryl ester transfer protein (Figure 18. 18).

• Figure 18. 18 Transfer of cholesteryl esters (CE) from HDL to VLDL in exchange for triacylglycerol (TAG) or phospholipids (PL).

3. Production of LDL from VLDL in the plasma: • With these modifications, the VLDL is converted in the plasma to LDL. • Intermediate-sized particles, the intermediate-density lipoproteins (IDL) or VLDL remnants, are observed during this transition. • IDLs can also be taken up by cells through receptor-mediated endocytosis that uses apo E as the ligand.

[Note: • Apo E is normally present in three isoforms, E-2, E-3, and E-4. • Apo E 2 binds poorly to receptors, and patients who are homozygotic for apo E 2 are deficient in the clearance of chylomicron remnants and IDL. • The individuals have familial Type III hyperlipoproteinemia (familial dysbetalipoproteinemia, or broad beta disease), with hypercholesterolemia and premature atherosclerosis. • Not yet understood is the fact that the E-4 isoform confers increased susceptibility to and decreased age of onset of late-onset Alzheimer disease, doubling the lifetime risk. ]

D. Metabolism of LDL • LDL particles contain much less triacylglycerol than their VLDL predecessors, and have a high concentration of cholesterol and cholesteryl esters (Figure 18. 19). 1. 2. 3. 4. 5. 6. Receptor-mediated endocytosis: The primary function of LDL particles is to provide cholesterol to the peripheral tissues (or return it to the liver). They do so by binding to cell surface membrane LDL receptors that recognize apo B-100 (but not apo B-48). Because these LDL receptors can also bind apo E, they are known as apo B-100/apo E receptors. A summary of the uptake and degradation of LDL particles is presented in Figure 18. 20. [Note: The numbers in brackets below refer to corresponding numbers on that figure. ] A similar mechanism of receptor-mediated endocytosis is used for the cellular uptake and degradation of chylomicron remnants and IDLs by the liver.

Figure 18. 19 Composition of the plasma lipoproteins. Note the high concentration of cholesterol and cholesteryl esters in LDL.

[1]LDL receptors are negatively charged glycoproteins that are clustered in pits on cell membranes. The intracellular side of the pit is coated with the protein clathrin, which stabilizes the shape of the pit. [2]After binding, the LDL-receptor complex is internalized by endocytosis. ]Note: A deficiency of functional LDL receptors causes a significant elevation in plasma LDL and, therefore, of plasma cholesterol. Patients with such deficiencies have Type II hyperlipidemia (familial hypercholesterolemia) and premature atherosclerosis[. [3]The vesicle containing the LDL rapidly loses its clathrin coat and fuses with other similar vesicles, forming larger vesicles called endosomes.

[4] The p. H of the endosome falls (due to the proton-pumping activity of endosomal ATPase), which allows separation of the LDL from its receptor. - The receptors then migrate to one side of the endosome, whereas the LDLs stay free within the lumen of the vesicle. [Note: This structure is called CURL—the Compartment for Uncoupling of Receptor and Ligand. ] [5] The receptors can be recycled, whereas the lipoprotein remnants in the vesicle are transferred to lysosomes and degraded by lysosomal (hydrolytic) enzymes, releasing free cholesterol, amino acids, fatty acids, and phospholipids. - These compounds can be reutilized by the cell. [Note: Rare autosomal recessive deficiencies in the ability to hydrolyze lysosomal cholesteryl esters (Wolman disease), or to transport unesterified cholesterol out of the lysosome (Niemann-Pick disease, type C) have been identified. ]

Figure 18. 20 Cellular uptake and degradation of LDL. ACAT = acyl Co. A: cholesterol acyltransferase.

2. Effect of endocytosed cholesterol on cellular cholesterol homeostasis: -The chylomicron remnant-, IDL-, and LDL-derived cholesterol affects cellular cholesterol content in several ways (see Figure 18. 20. ( -First, HMG Co. A reductase is inhibited by high cholesterol, as a result of which, de novo cholesterol synthesis decreases. -Second, synthesis of new LDL receptor protein is reduced by decreasing the expression of the LDL receptor gene, thus limiting further entry of LDL cholesterol into cells.

[Note: Regulation of the LDL receptor gene involves a SRE and a SREBP, as was seen in the regulation of the gene for HMG Co. A reductase (see p. 223). ] - Third, if the cholesterol is not required immediately for some structural or synthetic purpose, it is esterified by acyl Co. A: cholesterol acyltransferase (ACAT). - ACAT transfers a fatty acid from a fatty acyl Co. A derivative to cholesterol, producing a cholesteryl ester that can be stored in the cell (Figure 18. 21). - The activity of ACAT is enhanced in the presence of increased intracellular cholesterol.

• Figure 18. 21 Synthesis of intracellular cholesteryl ester by ACAT.

3. Uptake of chemically modified LDL by macrophage scavenger receptors: -In addition to the highly specific and regulated receptormediated pathway for LDL uptake described above, macrophages possess high levels of scavenger receptor activity. -These receptors, known as scavenger receptor class A (SR-A), can bind a broad range of ligands, and mediate the endocytosis of chemically modified LDL in which the lipid components or apo B have been oxidized. -Unlike the LDL receptor, the scavenger receptor is not down-regulated in response to increased intracellular cholesterol. -Cholesteryl esters accumulate in macrophages and cause their transformation into “foam” cells, which participate in the formation of atherosclerotic plaque (Figure 18. 22. (

• Figure 18. 22 Role of oxidized lipoproteins in plaque formation in arterial wall.

E. Metabolism of HDL • HDL comprise a heterogeneous family of lipoproteins with a complex metabolism that is not yet completely understood. • HDL particles are formed in blood by the addition of lipid to apo A-1, an apolipoprotein made by the liver and intestine and secreted into blood. • Apo A-1 accounts for about 70% of the apoproteins in HDL. • HDL perform a number of important functions, including the following:

1. 2. 1. 2. HDL is a reservoir of apolipoproteins : HDL particles serve as a circulating reservoir of apo C-II (the apolipoprotein that is transferred to VLDL and chylomicrons, and is an activator of lipoprotein lipase), and apo E (the apolipoprotein required for the receptormediated endocytosis of IDLs and chylomicron remnants. ( HDL uptake of unesterified cholesterol: Nascent HDL are disk-shaped particles containing primarily phospholipid (largely phosphatidylcholine) and apolipoproteins A, C, and E. They are rapidly converted to spherical particles as they accumulate cholesterol (Figure 18. 23). [Note: HDL particles are excellent acceptors of unesterified cholesterol (both from other lipoproteins particles and from cell membranes) as a result of their high concentration of phospholipids, which are important solubilizers of cholesterol. ]

3. Esterification of cholesterol: When cholesterol is taken up by HDL, it is immediately esterified by the plasma enzyme phosphatidylcholine: cholesterol acyltransferase (PCAT, also known as LCAT, in which “L” stands for lecithin). 1. This enzyme is synthesized by the liver. PCAT binds to nascent HDL, and is activated by apo A-I. PCAT transfers the fatty acid from carbon 2 of phosphatidylcholine to cholesterol. 2. This produces a hydrophobic cholesteryl ester, which is sequestered in the core of the HDL, and lysophosphatidylcholine, which binds to albumin. 3. As the nascent HDL accumulates cholesteryl esters, it first becomes a relatively cholesteryl ester–poor HDL 3 and, eventually, a cholesteryl ester–rich HDL 2 particle that carries these esters to the liver.

4. Reverse cholesterol transport : The selective transfer of cholesterol from peripheral cells to HDL, and from HDL to the liver for bile acid synthesis or disposal via the bile, and to steroidogenic cells for hormone synthesis, is a key component of cholesterol homeostasis. 1. This is, in part, the basis for the inverse relationship seen between plasma HDL concentration and atherosclerosis, and for HDL's designation as the “good” cholesterol carrier. 2. Reverse cholesterol transport involves efflux of cholesterol from peripheral cells to HDL, esterification of cholesterol by PCAT, binding of the cholesteryl ester– rich HDL )HDL 2) to liver and steroidogenic cells, the selective transfer of the cholesteryl esters into these cells, and the release of lipid-depleted HDL (HDL 3. (

1. The efflux of cholesterol from peripheral cells is mediated, at least in part, by the transport protein, ABCA 1. 2. ]Note: Tangier disease is a very rare deficiency of ABCA 1, and is characterized by the virtual absence of HDL particles due to degradation of lipid-poor apo A-1[. 3. The uptake of cholesteryl esters by the liver is mediated by a cell-surface receptor, SR-B 1 (scavenger receptor class B type 1) that binds HDL (see p, 234 for SR-A. ( 4. It is not yet clear as to whether the HDL particle itself is taken up, the cholesteryl esters extracted, and the lipidpoor HDL released back into the blood, or if there is selective uptake of the cholesteryl ester alone.

• Figure 18. 23 Metabolism of HDL. PC = phosphatidylcholine; lyso-PC = lysophosphatidylcholine. PCAT = Phosphatidylcholine cholesterol transferase. CETP = cholesteryl ester transfer protein. ABCA 1 = transport protein. [Note: For convenience the size of VLDLs are shown smaller than HDL, whereas VLDLs are larger than HDL. ]

• [Note: Hepatic lipase, with its ability to degrade both TAG and phospholipids, participates in the conversion of HDL 2 to ABCA 1 is an ATP-binding cassette (ABC) protein. ABC proteins use energy from ATP hydrolysis to transport materials, including lipids, in and out of cells and across intracellular compartments. • In addition to Tangier disease, defects in specific ABC proteins result in X-linked adrenoleukodystrophy, respiratory distress syndrome due to decreased surfactant secretion, and cystic fibrosis.

F. Role of lipoprotein (a) in heart disease • Lipoprotein (a), or Lp(a), is a particle that, when present in large quantities in the plasma, is associated with an increased risk of coronary heart disease. • Lp(a) is nearly identical in structure to an LDL particle. • Its distinguishing feature is the presence of an additional apolipoprotein molecule, apo(a), that is covalently linked at a single site to apo B-100. • Circulating levels of Lp(a) are determined primarily by genetics.

• However, factors such as diet may play some role, as trans fatty acids have been shown to increase Lp(a), and estrogen decreases both LDL and Lp(a). • [Note: Apo(a) is structurally homologous to plasminogen—the precursor of a blood protease whose target is fibrin, the main protein component of blood clots. It is hypothesized that elevated Lp(a) slows the breakdown of blood clots that trigger heart attacks because it competes with plasminogen for binding to fibrin. ]

VII. Steroid Hormones • Cholesterol is the precursor of all classes of steroid hormones: glucocorticoids (for example, cortisol), mineralocorticoids (for example, aldosterone), and sex hormones—androgens, estrogens, and progestins (Figure 18. 24). • [Note: Glucocorticoids and mineralocorticoids are collectively called corticosteroids. ] • Synthesis and secretion occur in the adrenal cortex (cortisol, aldosterone, androgens), ovaries and placenta (estrogens and progestins), and testes (testosterone). • Steroid hormones are transported by the blood from their sites of synthesis to their target organs.

• Because of their hydrophobicity, they must be complexed with a plasma protein. • Plasma albumin can act as a nonspecific carrier, and does carry aldosterone. • However, specific steroid-carrier plasma proteins bind the steroid hormones more tightly than does albumin, for example, corticosteroid-binding globulin (transcortin) is responsible for transporting cortisol, and sex hormone–binding protein transports sex steroids. • A number of genetic diseases are caused by deficiencies in specific steps in the biosynthesis of steroid hormones. Some representative diseases are described in Figure 18. 25.

Figure 18. 24 Key steroid hormones.

A. Synthesis of steroid hormones • Synthesis involves shortening the hydrocarbon chain of cholesterol, and hydroxylation of the steroid nucleus. • The initial and rate-limiting reaction converts cholesterol to the 21 -carbon pregnenolone. It is catalyzed by the cholesterol side-chain cleavage enzyme complex (desmolase)—a CYP mixed function oxidase of the inner mitochondrial membrane. • NADPH and molecular oxygen are required for the reaction. • The cholesterol substrate can be newly synthesized, taken up from lipoproteins, or released from cholesteryl esters stored in the cytosol of steroidogenic tissues. • An important control point is the movement of cholesterol into mitochondria. • This process is mediated by St. AR (steroidogenic acute regulatory protein. (

• [Note: Steroid hormone synthesis consumes little cholesterol as compared with that required for bile acid synthesis. ] • Pregnenolone is the parent compound for all steroid hormones (see Figure 18. 25). • Pregnenolone is oxidized and then isomerized to progesterone, a progestin, which is further modified to the other steroid hormones by hydroxylation reactions that occur in the ER and mitochondria. • Like desmolase, the enzymes are CYP proteins.

• A defect in the activity or amount of an enzyme in this pathway can lead to a deficiency in the synthesis of hormones beyond the affected step, and to an excess in the hormones or metabolites before that step. • Because all members of the pathway have potent biologic activity, serious metabolic imbalances occur if enzyme deficiencies are present (see Figure 18. 25). • Collectively these disorders are known as the congenital adrenal hyperplasias.

Figure 18. 25 Steroid hormone synthesis and associated diseases.

B. Secretion of adrenal cortical steroid hormones • Steroid hormones are secreted on demand from their tissues of origin in response to hormonal signals. • The corticosteroids androgens are made in different regions of the adrenal cortex, and are secreted into blood in response to different signals. 1. Cortisol: Its secretion from the middle layer (zona fasciculata) of the adrenal cortex is controlled by the hypothalamus, to which the pituitary gland is attached (Figure 18. 26).

1. In response to severe stress (for example, infection), corticotropin-releasing hormone (CRH), produced by the hypothalamus, travels through a network of capillaries to the anterior lobe of the pituitary, where it induces the production and secretion of adrenocorticotropic hormone (ACTH, or corticotropin). 2. The polypeptide ACTH, often called the “stress hormone, ” stimulates the adrenal cortex to synthesize and secrete the glucocorticoid cortisol. 3. Cortisol allows the body to respond to stress through its effects on intermediary metabolism and the inflammatory response. 4. As cortisol levels rise, the release of CRH and ACTH is inhibited. 5. [Note: ACTH binds to a plasma membrane receptor. Its intracellular effects are mediated through a second messenger, cyclic AMP (c. AMP). ]

2. Aldosterone: 1. This hormone's secretion from the outer layer (zona glomerulosa) of the adrenal cortex is induced by a decrease in the plasma Na+/K+ ratio, and by the hormone, angiotensin II. 2. Angiotensin II is produced from angiotensin I by angiotensin-converting enzyme (ACE), an enzyme found predominantly in the lungs, but which is also distributed widely in the body. 3. [Note: Angiotensin I, an octapeptide, is produced in the blood by cleavage of an inactive precursor, angiotensinogen, secreted by the liver. Cleavage is accomplished by the enzyme renin, made and secreted by the kidney. ] Estrogen is derived from testosterone by aromatase (CYP 19). Aromatase inhibitors are used in the treatment of estrogen-responsive breast cancer in post-menopausal women.

• Angiotensin II binds to cell-surface receptors. • However, in contrast to ACTH, its effects are mediated through the phosphatidylinositol 4, 5 bisphosphate (PIP 2) pathway (see p. 205) and not by c. AMP. • Aldosterone's primary effect is on the kidney tubules, where it stimulates sodium uptake and potassium excretion (Figure 18. 27). • [Note: An effect of aldosterone is an increase in blood pressure. Competitive inhibitors of ACE are used to treat renin-dependent hypertension. ]

3. Androgens: • Both the inner (zona reticularis) and middle layers of the adrenal cortex produce androgens, primarily dehydroepiandrosterone androstenedione. • Although adrenal androgens themselves are weak, they are converted in peripheral tissues to testosterone—a strong androgen—and to estradiol.

Figure 18. 26 Pituitary hormone stimulation of steroid hormone synthesis and secretion.

C. Secretion of steroid hormones from gonads • The testes and ovaries synthesize hormones necessary for physical development and reproduction. • A single hypothalamic-releasing factor, gonadotropinreleasing hormone, stimulates the anterior pituitary to release the glycoproteins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH. ( • Like ACTH, LH and FSH bind to surface receptors and cause an increase in c. AMP. • LH stimulates the testes to produce testosterone and the ovaries to produce estrogens and progesterone (see Figure 18. 27. ( • FSH regulates the growth of ovarian follicles and stimulates testicular spermatogenesis. • ]Note: For maximum effect on the male or female gonad, FSH also requires the presence of LH[.

D. Mechanism of steroid hormone action • Each steroid hormone diffuses across the plasma membrane of its target cell and binds to a specific cytosolic or nuclear receptor. • These receptor–ligand complexes accumulate in the nucleus, dimerize, and bind to specific regulatory DNA sequences (hormone-response elements, HRE) in association with coactivator proteins, thereby causing promoter activation and increased transcription of targeted genes (Figure 18. 28. ( • An HRE is found in the promoter (or an enhancer element, see p. 423) for genes that respond to a specific steroid hormone, thus ensuring coordinated regulation of these genes.

• Hormone–receptor complexes can also inhibit transcription in association with corepressors. • [Note: The binding of a hormone to its receptor causes a conformational change in the receptor that uncovers its DNA-binding domain, allowing the complex to interact through a zinc-finger motif with the appropriate sequence on the DNA. • It is recognized that the receptors for the diverse group of steroid hormones, plus those for thyroid hormone, retinoic acid, and 1, 25 dihydroxycholecalciferol (Vitamin D), are members of a “superfamily” of structurally related gene regulators that function in a similar way. ]

E. Further metabolism of steroid hormones • Steroid hormones are generally converted into inactive metabolic excretion products in the liver. • Reactions include reduction of unsaturated bonds and the introduction of additional hydroxyl groups. • The resulting structures are made more soluble by conjugation with glucuronic acid or sulfate (from 3′phosphoadenosyl-5′-phosulfate, see p. 162. ( • Approximately 20– 30% of these metabolites are secreted into the bile and then excreted in the feces, whereas the remainder are released into the blood and filtered from the plasma in the kidney, passing into the urine. • These conjugated metabolites are fairly water-soluble and do not need protein carriers.

Figure 18. 27 Actions of steroid hormones.

VIII. Chapter Summary • Cholesterol is a hydrophobic compound, with a single hydroxyl group—located at carbon 3 of the A ring—to which a fatty acid can be attached, producing an even more hydrophobic cholesteryl ester. • Cholesterol is synthesized by virtually all human tissues, although primarily by liver, intestine, adrenal cortex, and reproductive tissues (Figure 18. 29). • All the carbon atoms in cholesterol are provided by acetate, and NADPH provides the reducing equivalents. • The pathway is driven by hydrolysis of the high-energy thioester bond of acetyl coenzyme A (Co. A) and the terminal phosphate bond of ATP.

• Cholesterol is synthesized in the cytoplasm. • The rate-limiting and regulated step in cholesterol synthesis is catalyzed by the endoplasmic reticulum–-membrane protein, hydroxymethylglutaryl (HMG) Co. A reductase, which produces mevalonic acid from HMG Co. A. • The enzyme is regulated by a number of mechanisms: 1) Expression of the HMG Co. A reductase gene is activated when cholesterol levels are low, resulting in increased enzyme and, therefore, more cholesterol synthesis.

2) HMG Co. A reductase activity is controlled covalently through the actions of an adenosine monophosphate (AMP)–activated protein kinase (AMPK, which phosphorylates and inactivates HMG Co. A reductase) and an insulin-activated protein phosphatase (which activates HMG Co. A reductase). 3) Statins are competitive inhibitors of HMG Co. A reductase. These drugs are used to decrease plasma cholesterol in patients with hypercholesterolemia. The ring structure of cholesterol can not be degraded in humans.

• Cholesterol can be eliminated from the body either by conversion to bile salts or by secretion into the bile. • Intestinal bacteria can reduce cholesterol to coprostanol and cholestanol, which together with cholesterol make up the bulk of neutral fecal sterols. • Bile salts and phosphatidylcholine are quantitatively the most important organic components of bile. • Bile salts are conjugated bile acids produced by the liver and stored in the gallbladder. • The primary bile acids, cholic or chenodeoxycholic acids, are amphipathic, and can serve as emulsifying agents.

• The rate-limiting step in bile acid synthesis is catalyzed by cholesterol-7 -α-hydroxylase, which is activated by cholesterol and inhibited by bile acids. • Before the bile acids leave the liver, they are conjugated to a molecule of either glycine or taurine, producing the primary bile salts: glycocholic or taurocholic acid, and glycochenodeoxycholic or taurochenodeoxycholic acid. • Bile salts are more amphipathic than bile acids and, therefore, are more effective emulsifiers.

• In the intestine, bacteria can remove the glycine and taurine, and can remove a hydroxyl group from the steroid nucleus, producing the secondary bile acids— deoxycholic and lithocholic acids. • Bile is secreted into the intestine, and more than 95% of the bile acids and salts are efficiently reabsorbed. • They are actively transported from the intestinal mucosal cells into the portal blood, where they are carried by albumin back to the liver (enterohepatic circulation). • In the liver, the primary and secondary bile acids are reconverted to bile salts, and secreted into the bile. • If more cholesterol enters the bile than can be solubilized by the available bile salts and phosphatidylcholine, cholesterol gallstone disease (cholelithiasis) can occur.

• The plasma lipoproteins include chylomicrons, very-lowdensity lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). • They function to keep lipids (primarily triacylglycerol and cholesteryl esters) soluble as they transport them between tissues. • Lipoproteins are composed of a neutral lipid core (containing triacylglycerol, cholesteryl esters, or both) surrounded by a shell of amphipathic apolipoproteins, phospholipid, and nonesterified cholesterol. • Chylomicrons are assembled in intestinal mucosal cells from dietary lipids (primarily, triacylglycerol) plus additional lipids synthesized in these cells.

• Each nascent chylomicron particle has one molecule of apolipoprotein (apo) B-48. • They are released from the cells into the lymphatic system and travel to the blood, where they receive apo C -II and apo E from HDLs, thus making the chylomicrons functional. • Apo C-II activates lipoprotein lipase, which degrades the chylomicron's triacylglycerol to fatty acids and glycerol. • The fatty acids that are released are stored (in the adipose) or used for energy (by the muscle). • The glycerol is metabolized by the liver. • Patients with a deficiency of lipoprotein lipase or apo C-II show a dramatic accumulation of chylomicrons in the plasma (Type I hyperlipoproteinemia, familial lipoprotein lipase deficiency, or hypertriacylglycerolemia).

• After most of the triacylglycerol is removed, apo C-II is returned to the HDL, and the chylomicron remnant— carrying most of the dietary cholesterol—binds to a receptor on the liver that recognizes apo E. • The particle is endocytosed and its contents degraded by lysosomal enzymes. • Nascent VLDL are produced in the liver, and are composed predominantly of triacylglycerol. • They contain a single molecule of apo B-100. Like nascent chylomicrons, VLDL receive apo C-II and apo E from HDL in the plasma. • The function of VLDL is to carry triacylglycerol from the liver to the peripheral tissues where lipoprotein lipase degrades the lipid. As triacylglycerol is removed from the VLDL, the particle receives cholesteryl esters from HDL.

• This process is accomplished by cholesteryl ester transfer protein. • Eventually, VLDL in the plasma is converted to LDL—a much smaller, denser particle. • Apo C-II and apo E are returned to HDL, but the LDL retains apo B-100, which is recognized by receptors on peripheral tissues and the liver. • LDL undergo receptor-mediated endocytosis, and their contents are degraded in the lysosomes. • A deficiency of functional LDL receptors causes Type II hyperlipidemia (familial hypercholesterolemia). • The endocytosed cholesterol inhibits HMG Co. A reductase and decreases synthesis of LDL receptors.

• Some of it can also be esterified by acyl Co. A: cholesterol acyltransferase and stored. • HDL are created by lipidation of apo A-1 synthesized in the liver and intestine. • They have a number of functions, including: 1) serving as a circulating reservoir of apo C-II and apo E for chylomicrons and VLDL; 2) removing unesterified cholesterol from cell surfaces and other lipoproteins and esterifying it using phosphatidylcholine: cholesterol acyl transferase, a liver-synthesized plasma enzyme that is activated by apo A-1; and 3) delivering these cholesteryl esters to the liver (“reverse cholesterol transport”).

• Cholesterol is the precursor of all classes of steroid hormones (glucocorticoids, mineralocorticoids, and sex hormones—androgens, estrogens, and progestins). • Synthesis, using primarily cytochrome P 450 (CYP) mixed function oxidases, occurs in the adrenal cortex (cortisol, aldosterone, androgens), ovaries and placenta (estrogens and progestins), and testes (testosterone). • Each steroid hormone diffuses across the plasma membrane of its target cell and binds to a specific cytosolic or nuclear receptor. • These receptor–ligand complexes accumulate in the nucleus, dimerize, and bind to specific regulatory DNA sequences (hormone-response elements) in association with coactivator proteins, thereby causing promoter activation and increased transcription of targeted genes. • In association with corepressors, transcription is decreased.

Figure 18. 28 Activation of transcription by interaction of steroid hormone-receptor complex with hormone response element (HRE).