What is cholesterol?
Living cells are bounded by a cell membrane composed of a double layer of phospholipid, which is traversed by proteins that have catalytic, transportation, and signaling functions. Variable amounts of sterol are interspersed among the molecules of phospholipid in each membrane layer. Sterols are essential components in the membranes of fungal, plant, and animal cells (but not in bacteria). In vertebrates, the predominant sterol is cholesterol. There is little or no cholesterol in plant cell membranes; its place is taken by chemically related substances, chiefly sitosterol. This fact is of nutritional significance; only animal products add cholesterol to the diet.
In mammals, cholesterol is the precursor of steroid hormones, which are essential for mineral balance, adjustment of the body to stress, and normal reproductive function. It is also the precursor of bile acids, which are required for the absorption of dietary lipids. Bile acids play a role in cholesterol balance; the body can dissolve excess cholesterol in bile so that it may be expelled as waste, or the body may convert cholesterol to produce more bile acid.
Cholesterol itself is required for normal functioning of the mammalian cell membrane. Cholesterol alters the membrane’s fluidity—the ease with which proteins embedded in the membrane can move about and interact with one another—and also affects the activity of enzymes and transport proteins embedded in the membrane. Cultured cells that are prevented from making their own cholesterol will not grow unless it is provided in the medium.
Cholesterol may either be free or esterified (cholesterol becomes esterified when an ester bond binds a fatty acid to the hydroxyl group of cholesterol). Free cholesterol is confined to cellular membranes. Cholesterol content is highest (about one molecule of cholesterol for every one or two molecules of phospholipid) in the outer cell membrane that forms the boundary between a cell and its environment. It is much lower in intracellular membranes. Although cholesterol synthesis is completed within the endoplasmic reticulum, the ratio of cholesterol to phospholipid in this membrane is less than one to twenty. In a typical cell, it is estimated that between 80 and 90 percent of the total free cholesterol is located in the outer cell membrane.
Cholesterol in excess of normal proportions is coupled (esterified) to long-chain fatty acids. The resulting cholesteryl ester accumulates, along with triglyceride, in lipid droplets in the cytoplasm. The activity of the enzyme catalyzing this esterification is increased when cholesterol is imported into the cell in excess of its needs. The ability to esterify and sequester cholesterol defends the cell against excessive membrane concentration of the free sterol.
Free cholesterol and cholesteryl ester are carried in plasma in lipoproteins. Lipoproteins are aggregates of several thousand molecules of lipid and one or more molecules of specific proteins (apolipoproteins). Each lipoprotein particle consists of a core of triglyceride and cholesteryl ester surrounded by a single layer of phospholipid and free cholesterol. The apolipoproteins are embedded in the surface of the particle. Lipoproteins differ in size and relative lipid composition. In humans, about two-thirds of circulating cholesterol is contained in low-density lipoprotein (LDL), a core of cholesteryl ester with a single molecule of apolipoprotein B on the surface. LDL transports cholesterol from the liver to peripheral tissues. Most of the remaining circulating cholesterol is carried in high-density lipoprotein (HDL), which transports cholesterol from peripheral tissues back to the liver for disposal. Under normal conditions, LDL is the principal source of exogenous cholesterol available to a cell.
Uptake of LDL is mediated by a specific protein on the cell’s surface, the LDL receptor, which is concentrated in pockets in the cell membrane termed coated pits. LDL receptors bind apolipoprotein B and particles such as LDL that contain this protein. Coated pits continually pinch off to form sealed vesicles (endosomes) inside the cell. At the same time, new coated pits form on the cell surface. When a coated pit pinches off, it carries with it LDL receptors and associated LDL. The fluid within the endosome is acidified, which causes LDL to dissociate from its receptor and float free. The LDL is transferred to lysosomes, where cholesteryl ester is cleaved to liberate cholesterol; along with the free cholesterol that formed part of the LDL surface, this liberated cholesterol is now available for use within the cell. The LDL receptor is returned to the cell surface, where it can again become associated with a coated pit and participate in a new round of LDL uptake.
The rate at which LDL is removed from the circulation depends on the concentration of LDL receptors in cells of the liver and other tissues. (Although there are other mechanisms for removing LDL, these are less efficient than uptake via the LDL receptor.) This process is subject to feedback regulation. When a cell contains an adequate supply of cholesterol, the synthesis of new LDL receptors is inhibited. This inhibition occurs at the transcriptional level: The rate at which the gene for the LDL receptor is copied into messenger ribonucleic acid (mRNA) is reduced. With less mRNA for this protein reaching the cytoplasm, the rate at which new copies of the protein are made also falls. Following normal turnover of existing receptors, the concentration of LDL receptors at the cell surface declines and the uptake of cholesterol is accordingly reduced.
The gene for the LDL receptor contains short stretches of deoxyribonucleic acid (DNA), termed sterol response elements (SREs), that make transcription sensitive to cholesterol-induced “down-regulation.” These can be spliced out and inserted into another gene that codes for a bacterial protein. The foreign gene can be inserted into a mammalian cell, which will then begin to produce the corresponding protein. If the foreign gene does not contain an SRE, the rate of synthesis of the coded protein is unaffected by the cholesterol content of the recipient cell. When an SRE has been inserted into the gene in an appropriate location, however, excess cholesterol down-regulates transcription of the foreign gene just as it does that for the LDL receptor.
The signal that causes down-regulation of the cholesterol receptor has not been clearly identified, although some researchers have suggested it may be mediated by excess lipoprotein cholesterol. It is also possible that an internal regulatory pool of cholesterol—such as the cholesterol content of certain internal membranes or the concentration of individual cholesterol molecules bound to some cytoplasmic protein carrier—is responsible. Some evidence points to oxysterol analogues of cholesterol, rather than to cholesterol itself, as mediators of down-regulation. These analogues arise as minor metabolites of cholesterol and as intermediates and by-products of cholesterol synthesis. They are much more potent than cholesterol itself in inhibiting the synthesis of LDL receptors. A cytoplasmic oxysterol-binding protein has been identified whose affinity for different oxysterols parallels their relative potency in down-regulating the transcription of LDL receptor genes.
Mammalian cells also have the capacity to synthesize their own cholesterol. The enzymes catalyzing the successive reactions along this pathway are located in the cytoplasm and in the membranes of the endoplasmic reticulum. The starting material from which cholesterol is made is acetic acid in the form of acetyl coenzyme A (acetyl CoA). This material is generated in mitochondria as an intermediate in the oxidation of glucose and of fatty acids. To be utilized for synthesis of cholesterol, it must first be transported from the mitochondria to the cytoplasm; a specific shuttle exists for this purpose. The synthesis of fatty acids from acetyl CoA also takes place in the cytoplasm, and more of the translocated precursor is used for this purpose than to make cholesterol.
An early step on the pathway to cholesterol is the reduction of hydroxymethylglutaryl coenzyme A (HMG CoA) to mevalonic acid, catalyzed by HMG CoA reductase. The enzymatic capacity to catalyze this reaction is substantially lower than that for other steps on the pathway, so that the overall rate of cholesterol synthesis is largely determined by the activity of this enzyme. Both amount and activity are regulated to match the rates of cholesterol synthesis to the needs of the cell. When the cell has adequate supplies of cholesterol, HMG CoA reductase activity is low and, simultaneously, uptake via the LDL receptor is reduced. Conversely, when more cholesterol is needed, HMG CoA reductase activity and overall cholesterol synthesis are increased, as are expression of the LDL receptor and the uptake of cholesterol.
HMG CoA reductase activity is regulated at several points, including transcription of the gene, efficiency with which its mRNA is translated into protein, turnover of the enzyme protein, and inactivation by chemical modification. Regulation of the transcription of the HMG CoA reductase gene and regulation of the LDL receptor gene appear to have the same fundamental mechanism. The gene for HMG CoA reductase contains an SRE similar in sequence to those in the gene for the LDL receptor. Oxysterols that repress transcription of the LDL receptor exert the same effect on transcription of the gene for HMG CoA reductase, and with the same relative potency. Mutant cells that have lost the capacity to respond to oxysterols by repressing the synthesis of LDL receptors also fail to respond by repressing the synthesis of HMG CoA reductase.
Excess cholesterol also increases the rate at which HMG CoA reductase is broken down. This might be a response to changes in cholesterol concentration in the internal membranes in which HMG CoA reductase is embedded. When the gene for HMG CoA reductase is altered to remove the sequences that anchor it to the membrane and the altered gene is inserted into a recipient cell, the mutant protein is located unattached in the cytoplasm. In contrast to the native enzyme, the rate of turnover of the altered gene product is not affected by the cholesterol content of the recipient cell.
Phosphorylation (attachment of a phosphate to specific amino acid residues) of HMG CoA reductase is a third mechanism by which the synthesis of cholesterol is altered. This is best documented in the liver (which is also the principal site within the body for the synthesis of cholesterol). Liver cells contain an enzyme that phosphorylates and inactivates HMG CoA reductase; the same enzyme also phosphorylates and inactivates the enzyme catalyzing the rate-determining step in the synthesis of fatty acids, acetyl CoA carboxylase. The phosphorylation of both enzymes is promoted, and the synthesis of fatty acids and cholesterol is correspondingly inhibited, under fasting conditions. In the fed state, the reverse is true. These changes appear to be a response to circulating levels of the hormone insulin.
Most of the mevalonic acid that is produced by HMG CoA reductase activity is converted to cholesterol. Since its formation is the rate-determining step on the pathway, the addition of mevalonic acid itself to the cell results in much higher rates of cholesterol synthesis than can be achieved with acetyl CoA as starting material. Moreover, the synthesis of cholesterol from mevalonic acid is not controlled by feedback inhibition. Under these circumstances, expression of HMG CoA reductase and expression of the LDL receptor are maximally repressed.
Mevalonic acid is also the precursor of other substances needed by the cell. These include dolichol (a coenzyme needed for the addition of carbohydrate residues to proteins), ubiquinone (a participant in electron transport reactions in mitochondria), and isoprenyl side chains that are attached to specific proteins. Although their synthesis consumes only a minor portion of the mevalonic acid produced by HMG CoA reductase, these substances are essential to normal cell function. When HMG CoA reductase is inhibited by lovastatin, a drug that competes with the substrate HMG CoA for binding to the enzyme, cell growth is inhibited even when supplies of cholesterol in the medium are adequate. Resumption of cell growth requires the addition of small amounts of mevalonic acid as well as cholesterol. Mutant cell lines have been obtained that, even without lovastatin, are unable to grow unless mevalonic acid is present. The requirements of these cells can be met with a large amount of mevalonic acid in the medium or with a small amount of mevalonic acid plus a large amount of cholesterol.
The multiple roles of mevalonic acid are also reflected in the way that HMG CoA reductase activity is regulated. Although transcription of the HMG CoA reductase gene is reduced when cholesterol levels in the cell are high, some transcription continues to allow enough mevalonic acid to be produced to meet other needs of the cell. Adding small amounts of mevalonic acid further decreases the synthesis of HMG CoA reductase by decreasing the rate at which its mRNA is translated into protein. In contrast, expression of the LDL receptor is not under dual regulation; maximal suppression can be obtained with cholesterol alone.
Excessive levels of LDL cholesterol are associated with an increased risk of coronary heart disease and stroke. Efforts to reduce LDL cholesterol through changes in diet or through drugs take advantage of what is known about cholesterol balance in individual cells and in the body as a whole. The latter is determined by three factors: the dietary intake of cholesterol, the rate of cholesterol synthesis within the body (principally by the liver), and the rate of cholesterol disposal (also principally by the liver, through secretion of free sterol into the bile and by the conversion of cholesterol to bile acids). Accordingly, levels of LDL cholesterol can be diminished by limiting the intake and synthesis of new cholesterol, reducing cholesterol secretion by the liver (in the form of an LDL precursor particle), promoting LDL uptake by the liver (mediated by the LDL receptor), and increasing the formation and secretion of bile acids and free cholesterol.
The body can meet its need for cholesterol through synthesis; there is no dietary requirement. Cholesterol deficiency does not arise in humans even on a purely vegetarian (cholesterol-free) diet. The average Western diet, rich in meat and dairy products, contains between 250 and 400 milligrams of cholesterol per day. Small amounts of cholesterol in the diet are fairly well absorbed, but efficiency declines with larger quantities; on average, about half of the cholesterol consumed per day is assimilated. Absorption of cholesterol and other lipids in the small intestine requires the presence of bile acids. These mix with cholesterol and partially degraded dietary triglyceride to form small droplets that facilitate the absorption of lipids by the cells lining the small intestine. In the process, most of the bile acid (as well as cholesterol secreted in the bile) is reabsorbed and returned to the liver. The absorbed cholesterol is esterified and secreted into the lymph, along with triglyceride, in the core of chylomicrons. These large lipoproteins are reduced in size by removal of triglyceride in capillary beds, and the remnants, containing all the original cholesteryl ester, are taken up by the liver. Thus, the cholesterol absorbed in the intestine passes initially to the liver.
The liver is the most important site for cholesterol synthesis within the body. As explained above, hepatic synthesis of cholesterol is under feedback control. HMG CoA reductase activity and cholesterol synthesis are suppressed when large amounts of dietary cholesterol reach the liver and are augmented when the diet is cholesterol-free. Drugs such as lovastatin that inhibit HMG CoA reductase are useful in reducing cholesterol levels in the circulation, since they limit the ability of the liver to respond by making more cholesterol when the dietary intake is reduced.
Cholesterol within the liver may be incorporated into very low-density lipoprotein (VLDL) and secreted into the circulation. VLDL is secreted primarily to transport triglyceride to other tissues for use as a metabolic fuel or for storage. After serving this function, most of the VLDL remnants return to the liver. Those that escape hepatic reabsorption are transformed in the circulation into LDL. LDL is also taken up by the liver (and elsewhere), but at a much slower rate than VLDL remnant particles, so that this lipoprotein accumulates in the circulation. It is deposition of LDL in the lining of blood vessels that initiates the formation of atherosclerotic plaques—the beginning of atherosclerotic disease. Since cholesterol is required for the secretion of VLDL, HMG CoA reductase inhibitors that cause a partial depletion of cholesterol in liver cells reduce the rate at which it enters the bloodstream. By the same means, they also increase expression of LDL receptors in liver cells, which results in a more rapid removal of LDL from the circulation. Both mechanisms reduce circulating levels of LDL cholesterol.
Cholesterol in the liver may also be converted to bile acids and secreted in the bile. Although most of the secreted bile acid comes back to the liver, some escapes reabsorption in the small intestine. The bile acid that is lost must be replaced by the metabolization of more cholesterol. Bile acid sequestrants such as cholestyramine are also used to reduce circulating cholesterol. They act by forming complexes with bile acids in the small intestine and interfere with their reabsorption. A larger fraction of the secreted bile acid is thus lost by excretion, and the rate of conversion of cholesterol to bile acid in the liver is correspondingly increased.
The liver secretes a large amount of free cholesterol into the bile. Most of this biliary cholesterol is reabsorbed and returned to the liver. Since bile acids are required for the intestinal uptake of cholesterol, a second beneficial action of bile acid sequestrants is to increase the fraction of biliary cholesterol that escapes reabsorption and is excreted. The combination of bile acid sequestrant and HMG CoA reductase inhibitor is especially effective in lowering blood levels of LDL cholesterol by limiting the uptake of cholesterol from the diet, limiting the synthesis of cholesterol in the liver, increasing the clearance of cholesterol from the blood by the liver, and increasing the conversion of hepatic cholesterol to bile acids.
Other drugs used to reduce circulating LDL cholesterol levels include nicotinic acid, fibiric acid derivatives, and probucol. Nicotinic acid (niacin) is also a vitamin, but the amounts required to affect plasma cholesterol levels are far in excess of the daily requirement for this compound as an essential nutrient. At these pharmacologic doses, side effects such as itching, facial flushing, and gastric distress are common. Nicotinic acid decreases the formation of VLDL triglyceride in the liver and therefore reduces the formation of LDL; it also promotes the uptake of LDL through LDL receptors and increases the concentration of HDL cholesterol (cholesterol being returned to the liver for disposal). The underlying mechanisms for these actions have not been determined. Fibric acid derivatives, such as gemfibrazole, also reduce VLDL secretion and promote LDL uptake by the liver; again the mechanism of drug action is uncertain. Probucol acts primarily to prevent chemical modification of LDL in the circulation that makes it more likely to be deposited in the walls of blood vessels.
Although high circulating levels of LDL cholesterol call for drug intervention, the risk of atherosclerotic disease can be reduced in most adults by attention to the dietary factors that affect cholesterol balance. Dietary studies have had a controversial history because of frequently contradictory findings. A few principles are, however, well supported by the data. First, eliminating cholesterol from the diet, with no other intervention, produces a significant drop in the levels of cholesterol in circulating LDL. Second, intake of calories beyond actual energy needs raises serum cholesterol. Excess fuel is stored as triglycerides, which to a large extent are formed in the liver and exported in VLDL. Since VLDL contains cholesterol, and since it is the precursor of LDL, high rates of triglyceride formation in the liver promote the accumulation of cholesterol in plasma LDL, which is especially likely to occur if the calories are ingested in the form of triglyceride. Not only does this increase the requirement for the liver to secrete VLDL, but the fatty acids derived from triglyceride also stimulate cholesterol synthesis. Third, saturated fatty acids have the strongest tendency to elevate plasma cholesterol levels. This is the case even when these fatty acids are consumed as vegetable oils (such as palm oil and coconut oil) unaccompanied by cholesterol. The basis for this effect is not well understood but appears to result from decreased expression of LDL receptors in the liver. The American Heart Association recommends that not more than 300 milligrams per day of cholesterol be consumed in a diet which is matched to caloric need and in which not more than 25 to 30 percent of calories are consumed as fat (and not more than 7 percent as saturated fat and not more than 1 percent trans fat). Individuals with heart disease or with LDL cholesterol levels greater than 100 milligrams per deciliter should limit their cholesterol intake to less than 200 milligrams each day.
The average adult body contains about 150 grams of cholesterol. Less than 5 percent of this cholesterol is in circulating lipoproteins or trapped in atherosclerotic lesions. The remainder performs essential functions as a structural component of membranes and as the precursor of other vital substances. Although researchers have learned much about the subcellular distribution, the pathway for biosynthesis, and the mechanisms for transport of cholesterol, important questions remain. It is not clearly understood how cholesterol is transported within the cell or what determines its relative distribution among different cellular membranes. It is not known what signals suppress the synthesis of HMG CoA reductase and LDL receptors or how the message is transmitted to the nucleus to diminish transcription of these genes. Some product of mevalonic acid metabolism other than cholesterol also regulates the expression of HMG CoA reductase, but this factor has not yet been identified. It is not known why cholesterol is an absolute requirement for functioning of mammalian cell membranes. It is not known what determines the fraction of dietary cholesterol that is absorbed across the intestinal lining. Finally, there is much to learn about factors that regulate the secretion and reuptake of lipoproteins by the liver and that control the return to the liver of cholesterol in HDL.
These questions are of practical as well as academic interest. Ischaemic heart disease and other complications of atherosclerosis are the leading causes of death, causing about 12.8 percent of deaths worldwide, according to data from the World Health Organization. Controlling and reversing this process is a serious medical challenge. While other factors such as hypertension, smoking, and diabetes mellitus contribute to the risk of atherosclerosis, reducing the level of cholesterol that circulates as a component of LDL and increasing the level transported in HDL have been shown to provide significant protection. Knowledge of how cholesterol is normally produced and assimilated has contributed to the design of drugs that reduce circulating levels by interfering with the absorption and synthesis of cholesterol and promoting its metabolism and excretion. This field of investigation continues to be a major concern of both basic and pharmaceutical scientists.
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