Chapter Summary, Questions Answers - Cholesterol, Lipoprotein, and Steroid Metabolism

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 acetyl coenzyme A (CoA), and nicotinamide adenine dinucleotide phosphate provides the reducing equivalents. The pathway is driven by hydrolysis of the high-energy thioester bond of acetyl CoA and the terminal phosphate bond of adenosine triphosphate. Cholesterol is synthesized in the cytoplasm. The rate-limiting and regulated step in cholesterol synthesis is catalyzed by the smooth endoplasmic reticulum–membrane protein, hydroxymethylglutaryl coenzyme A (HMG CoA) reductase, which produces mevalonate from HMG CoA. The enzyme is regulated by a number of mechanisms: 1)  Expression of the gene for the reductase is activated when cholesterol levels are low, via the transcription factor, sterol regulatory element–binding protein-2 (SREBP-2), bound to a sterol regulatory element (SRE), resulting in increased enzyme and, therefore, more cholesterol, synthesis; 2) degradation of the reductase is accelerated when cholesterol levels are high; 3) reductase activity is controlled by adenosine monophosphate (AMP)–activated protein kinase ([AMPK], which phosphorylates and inactivates the reductase) and an insulin-activated protein phosphatase (which dephosphorylates and activates it); and 4) expression of the gene for the reductase is upregulated by insulin and downregulated by glucagon. Statins are competitive inhibitors of HMG CoA reductase. These drugs are used to decrease plasma cholesterol in patients with hypercholesterolemia. The ring structure of cholesterol cannot be degraded in humans.

Cholesterol can be eliminated from the body either by conversion to bile salts or b y secretion into the bile. Bile salts and phosphatidylcholine are quantitatively the most important organic components of bile. The rate-limiting step in bile acid synthesis is catalyzed by cholesterol-7-gα-hydroxylase, which is inhibited by bile acids. Before the bile acids leave the liver, they are conjugated to a molecule of either glycine or taurine, producing the conjugated bile salts glycocholic or taurocholic acid and glycochenodeoxycholic or taurochenodeoxycholic acid. Bile salts (deprotonated) are more amphipathic than bile acids (protonated) and, therefore, are more effective emulsifiers of dietary fat. In the intestine, bacteria can remove the glycine and taurine and can remove a hydroxyl group from the steroid nucleus, producing the secondary bile salts, deoxycholic and lithocholic acids. More than 95% of the bile salts are efficiently reabsorbed in the intestinal ileum by a sodium–bile salt cotransporter, returned to the blood, and carried by albumin back to the liver where they are taken up by the hepatic form of the cotransporter and reused (enterohepatic circulation, which bile acid sequestrants reduce). 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-low-density lipoproteins (VLDLs) , low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs). They function to keep lipids (primarily triacyl-glycerol [TAG] and cholesteryl esters) soluble as they transport them between tissues. Lipoproteins are composed of a neutral lipid (TAG, cholesteryl esters, or both) core surrounded by a shell of amphipathic apolipoproteins, phospholipid, and unesterified cholesterol. Chylomicrons are assembled in intestinal mucosal cells from dietary lipids (primarily TAG). 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. Apo C-II activates endothelial lipoprotein lipase (LPL), which degrades the TAG in chylomicrons 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 LPL or apo C-II show a dramatic accumulation of chylomicrons in the plasma (type I hyperlipoproteinemia, or familial LPL deficiency) even if fasted. After most of the TAG 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. Defective uptake of these remnants causes type III hyperlipoproteinemia. Nascent VLDLs are produced in the liver and are composed predominantly of TAG. They contain a single molecule of apo B-100. Like nascent chylomicrons, VLDLs receive apo C-II and apo E from HDL in the plasma. The function of VLDL is to carry hepatic TAG to the peripheral tissues where LPL degrades the lipid. Additionally, the VLDL particle receives cholesteryl esters from HDL in exchange for TAG. This process is accomplished by cholesteryl ester transfer protein (CETP). 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 o n peripheral tissues and the liver. LDLs undergo receptor-mediated endocytosis, and their contents are degraded in the lysosomes. A deficiency of functional LDL receptors causes type II hyperlipoproteinemia (familial hypercholesterolemia). The endocytosed cholesterol decreases synthesis of HMG CoA reductase (and of LDL receptors) through prevention of SREBP-2 binding to the SRE. Some of it can also be esterified by acyl CoA:cholesterol acyltransferase (ACAT) 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 from peripheral tissues via ABCA1and esterifying it using lecithin:cholesterol acyl transferase(LCAT), a liver-synthesized plasma enzyme that is activated by apo A-1; and 3) delivering these cholesteryl esters to the liver (reverse cholesterol transport) for uptake via scavenger receptor-B1(SR-B1).

Cholesterol is the precursor of all classes of steroid hormones, which include glucocorticoids, mineralocorticoids, and the sex hormones (androgens, estrogens, and progestins) . Synthesis, using primarily cytochrome P450 mixed-function oxidases, occurs in the adrenal cortex (cortisol, aldosterone, and androgens), ovaries and placenta (estrogens and progestins), and testes (testosterone). The initial and rate-limiting step is the conversion of cholesterol to pregnenolone by the side-chain cleavage enzyme P450scc. Deficiencies in synthesis Iead to congenital adrenal hyperplasia. 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 18.29 Concept map for cholesterol and the lipoproteins. HMG CoA = hydroxymethylglutaryl coenzyme A; SREBP = sterol regulatory element-binding protein; HDL = high-density lipoprotein; VLDL = very-low-density lipoprotein; LDL = lowdensity lipoprotein; TAG = triacylglycerol; NADP(H) = nicotinamide adenine dinucleotide phosphate.

Study Questions

Choose the ONE best answer.

18.1 Mice were genetically engineered to contain hydroxymethylglutaryl coenzyme A reductase in which serine 871, a phosphorylation site, was replaced by alanine. Which of the following statements concerning the modified form of the enzyme is most likely to be correct?

A. The enzyme is nonresponsive to adenosine triphosphate depletion.

B. The enzyme is nonresponsive to statin drugs.

C. The enzyme is nonresponsive to the sterol response element–sterol response element–binding protein system.

D. The enzyme is unable to be degraded by the ubiquitin–proteasome system.

Correct answer = A. The reductase is regulated by covalent phosphorylation and dephosphorylation. Depletion of adenosine triphosphate results in a rise in adenosine monophosphate (AMP), which activates AMP kinase (AMPK), thereby phosphorylating and inactivating the enzyme. In the absence of the serine, a common phosphorylation site, the enzyme cannot be phosphorylated by AMPK. The enzyme is also regulated physiologically through changes in transcription and degradation and pharmacologically by statin drugs (competitive inhibitors), but none of these depends on serine phosphorylation.

18.2 Calculate the amount of cholesterol in the low-density lipoproteins in an individual whose fasting blood gave the following lipid-panel test results: total cholesterol = 300 mg/dl, high-density lipoprotein cholesterol = 25 mg/dl, triglycerides = 150 mg/dl.

A. 55 mg/dl

B. 95 mg/dl

C. 125 mg/dl

D. 245 mg/dl

Correct answer = D. The total cholesterol in the blood of a fasted individual is equal to the sum of the cholesterol in low-density lipoproteins plus the cholesterol in high-density lipoproteins plus the cholesterol in very-low-density lipoproteins (VLDLs). This last term is calculated by dividing the triacylglycerol value by 5 because cholesterol accounts for about 1/5 of the volume of VLDL in fasted blood.

For Questions 18.3 and 18.4:

A young girl with a history of severe abdominal pain was taken to her local hospital at 5 a.m. in severe distress. Blood was drawn, and the plasma appeared milky, with the triacylglycerol level in excess of 2,000 mg/dl (normal = 4–150 mg/dl). The patient was placed on a diet extremely limited in fat but supplemented with medium-chain triglycerides.

Correct answer = A. The milky appearance of her blood was a result of triacylglycerol-rich chylomicrons. Because 5 a.m. is presumably several hours after her evening meal, the patient must have difficulty degrading these lipoprotein particles. Intermediate-, low-, and high-density lipoproteins contain primarily cholesteryl esters, and, if one or more of these particles was elevated, it would cause hypercholesterolemia. Very-low-density lipoproteins do not cause the described “milky appearance” in plasma.

18.3 Which of the following lipoprotein particles are most likely responsible for the appearance of the patient’s plasma?

A. Chylomicrons

B. High-density lipoproteins

C. Intermediate-density lipoproteins

D. Low-density lipoproteins

E. Very-low-density lipoproteins

Correct answer = C. The triacylglycerol (TAG) in chylomicrons is degraded by endothelial lipoprotein lipase, which requires apo C-II as a coenzyme. Deficiency of the enzyme or coenzyme results in decreased ability to degrade chylomicrons to their remnants, which get cleared by the liver. Apo A-I is the coenzyme for lecithin:cholesterol acyltransferase; apo B-48 is the ligand for the hepatic receptor that binds chylomicron remnants; cholesteryl ester transfer protein catalyzes the cholesteryl ester–TAG exchange between high-density and very-low-density lipoproteins (VLDLs); and microsomal triglyceride transfer protein is involved in the formation, not degradation, of chylomicrons (and VLDLs).

18.4 Which one of the following proteins is most likely to be deficient in this patient?

A. Apo A-I

B. Apo B-48

C. Apo C-II

D. Cholesteryl ester transfer protein

E. Microsomal triglyceride transfer protein

18.5 Complete the table below for an individual with classic 21-α-hydroxylase deficiency relative to a normal individual.

How might the results be changed if this individual were deficient in 17-α-hydroxylase, rather than 21-α-hydroxylase?

21-α-Hydroxylase deficiency causes mineralocorticoids (aldosterone) and glucocorticoids (cortisol) to be virtually absent. Because aldosterone increases blood pressure, and cortisol increases blood glucose, their deficiencies result in a decrease in blood pressure and blood glucose, respectively. Cortisol normally feeds back to inhibit adrenocorticotropic hormone (ACTH) release by the pituitary, and, so, its absence results in an elevation in ACTH. The loss of 21-α-hydroxylase pushes progesterone and pregnenolone to androgen synthesis, therefore, causes androstenedione levels to rise.

With 17-α-hydroxylase deficiency, sex hormone synthesis would be inhibited. Mineralocorticoid production would be increased, leading to hypertension.