Cholesterol uptake regulation

We begin with an account of the main steps in the biosynthesis of cholesterol from acetate, then discuss the transport of cholesterol in the blood, its uptake by cells, the normal regulation of cholesterol synthesis, and its regulation in those with defects in cholesterol uptake or transport. We next consider other cellular components derived from cholesterol, such as bile acids and steroid hormones. Finally, an outline of the biosynthetic pathways to some of the many compounds derived from isoprene units, which share early steps with the pathway to cholesterol, illustrates the extraordinary versatility of isoprenoid condensations in biosynthesis. 

The primary function of the pBz receptor is in the regulation of cholesterol uptake and the synthesis of neurosteroids. The latter compounds have an affinity for the GABA-A receptors which provide an indirect coupling between the pBz and the GABA receptors in the brain. 

The level of intracellular cholesterol is regulated through cholesterol-induced suppression of LDL-receptor synthesis and cholesterol-induced inhibition of cholesterol synthesis. The increased level of intracellular cholesterol that results from LDL uptake has the additional effect of activating acyl-CoA cholesteryl acyl transferase (ACAT) (see below), thereby allowing the storage of excess cholesterol within cells. However, the effect of cholesterol-induced suppression of LDL-receptor synthesis is a decrease in the rate at which LDLs and IDLs are removed from the serum. This can lead to excess circulating levels of cholesterol and cholesteryl esters when the dietary intake of fat and cholesterol is excessive. Excess cholesterol tends to be deposited in the skin and tendons and within the arteries, which can lead to atherosclerosis. 

Exposure of cholesterol-starved cells to LDL is followed by massive buildup of intracellular free cholesterol pools. HMG-CoA reductase is suppressed, shutting down cellular cholesterol synthesis [84,85]. ACAT is stimulated as much as 500-fold by a process that apparently is independent of protein synthesis [86]. Subsequently, the number of LDL receptors declines dramatically with a calculated half-life of 15-20 h [29,31]. Since the rate of decline under conditions which block protein synthesis is comparable to the LDL-mediated rate of decline [29,87], down-regulation may be due to suppression of receptor synthesis. Excess eholesterol is esterified and stored in the c)rtoplasm as cholesteryl esters. A steady-state characterized by large cholesteryl ester and free cholesterol pools and by basal levels of both HMG-CoA reductase and LDL receptors is ultimately attained. This regulatory mechanism allows cells to control their rate of cholesterol uptake, synthesis, and storage in response to the available supply of lipoprotein cholesterol. 

While the LDL pathway for control of cholesterol uptake was elucidated using cultured cells, it also appears to be functional in vivo. The pattern of regulation depicted in Fig. 5 has been confirmed in freshly isolated blood leukocytes and lymphocytes [89-92]. In addition, administration of 4-aminopyrazolepyrimidine, a drug which suppresses lipoprotein release from animal liver [93], elevates HMG-CoA reductase levels and cholesterol synthesis in non-hepatic tissues [94,95]. These observations are consistent with the hypothesis that non-hepatic cells exhibit a low rate of cholesterol synthesis because they utilize cholesterol synthesized by the liver and present in the plasma lipoproteins [96],... 

In tissues, cholesterol balance is regulated as follows (Figure 26-5) Cell cholesterol increase is due to uptake of cholesterol-containing Hpoproteins by receptors, eg, the LDL receptor or the scavenger receptor uptake of free cholesterol from cholesterol-rich hpoproteins to the cell... 

The bulk of pinocytosis in the nervous system is mediated by clathrin-mediated endocytosis (CME) [55] and this is the best-characterized pathway. More detail about clathrin-mediated pathways will be given when receptor-mediated endocytosis and the synaptic vesicle cycle pathways are considered. Pinocytosis through CME is responsible for uptake of essential nutrients such as cholesterol bound to low density lipoprotein (LDL) and transferring, but also plays a role in regulating the levels of membrane pumps and channels in neurons. Finally, CME is critical for normal synaptic vesicle recycling. 

An increased rate of metabolic clearance has been observed after removal of sialic acid from human, low-density lipoprotein in vivo.472 Sialic acid controls the receptor-mediated uptake of this lipoprotein by fibroblasts. Removal of sialic acid residues accelerates the rate of internalization of the lipoprotein and, subsequently, the regulation of the metabolism of cellular cholesterol.473... 

FIGURE 21-44 Regulation of cholesterol formation balances synthesis with dietary uptake. Glucagon promotes phosphorylation (inactivation) of HMG-CoA reductase insulin promotes dephosphorylation (activation). X represents unidentified metabolites of cholesterol that stimulate proteolysis of HMG-CoA reductase. 

The plasma half-life of LDL is about 2 days, and their primary function is the delivery of cholesterol to the tissues. LDL are taken up into cells by two routes, one that is receptor mediated (and is regulated by the cholesterol requirement of the cell) and one that appears to be nonreceptor mediated (and depends entirely on the extravascular concentrations of LDL). The receptor-mediated uptake occurs by binding of apo-BlOO, which is predominantly present on LDL. Hence these receptors are also known as LDL receptors and have been identified on a variety of cell types. In normal humans, about two thirds of total LDL clearance is mediated by the LDL receptor, and about 80-90% of the receptor-mediated uptake occurs in the liver. However, the relative importance of receptor and nonreceptor-mediated LDL uptake can vary depending on factors including diet and different disease states. 

A close connection exists between the regulation of cholesterol biosynthesis and uptake. When HMG-CoA reductase is inhibited, the cell responds by synthesizing more LDL receptors to ensure the uptake of cholesterol from the serum. When cholesterol is present in a high enough concentration in the cell, LDL receptors are not exported to the cell surface, an example of the phenomenon of down regulation. 

Both IDL and LDL can be removed from the circulation by the liver, which contains receptors for ApoE (IDL) and ApoB-100 (IDL and LDL). After IDL or LDL interacts with these receptors, they are internalized by the process of receptor-mediated endocytosis. Receptors for ApoB-100 are also present in peripheral tissues, so that clearance of LDL occurs one-half by the liver and one-half by other tissues. In the liver or other cells, LDL is degraded to cholesterol esters and its other component parts. Cholesterol esters are hydrolyzed by an acid lipase and may be used for cellular needs, such as the building of plasma membranes or bile salt synthesis, or they may be stored as such. Esterification of intracellular cholesterol by fatty acids is carried out by acyl-CoA-cholesterol acyltransferase (ACAT). Free cholesterol derived from LDL inhibits the biosynthesis of endogenous cholesterol. B-100 receptors are regulated by endogenous cholesterol levels. The higher the latter, the fewer ApoB-100 receptors are on the cell surface, and the less LDL uptake by cells takes place. 

Recently, milk sphingomyelins were reported to interact significantly with the physical state of cholesterol, which correlated positively with reduced uptake and esterification of cholesterol by Caco-2 cells they also significantly reduced cholesterol absorption in mice, even at 0.1% of the diet (Eckhardt et al., 2002). An earlier study showed the regulation of cholesterol absorption by the content of sphingomyelin in intestinal cell membranes (Chen et al., 1992). 

Cholesteryl esters that are internalized via the LDL receptor are hydrolyzed to produce cholesterol and an acyl chain. Cholesterol, in (urn, activates the enzyme acyl-CoA cholesterol acyl-transferase (ACAT) which re-esterifies cholesterol. In an apparently futile cycle, the cholesteryl esters are hydrolyzed by cholesteryl ester hydrolase. The cholesterol moiety has several fates it may leave the cell and bind to an acceptor such as high-density lipoprotein (HDL), it may be converted to steroid hormones, or it may be reesterified by ACAT. When the cellular cholesterol concentration falls, the activity of HMG-CoA reductase is increased, as is the number of LDL receptors, which results in an increase of cellular cholesterol, due both to de novo synthesis and to the uptake of cholesterol-rich lipoproteins in the circulation. An increase in cellular cholesterol results in the rapid decline in the mRNA levels for both HMG-CoA reductase and the LDL receptor. This coordinated regulation is brought about by the presence of an eight nucleotide sequence on the genes which code for both proteins this is termed the sterol regulatory element-1. 

LDL is oxidatively modified when incubated in vitro with three major cellular constituents of the vascular wall endothelial cells [35], vascular smooth muscle cells [35] and macrophages [35-37], The uptake of oxidised LDL occurs via the scavenger-receptor pathway, and expression of scavenger receptors has been demonstrated on macrophages, endothelial cells [38], fibroblasts [39] and smooth muscle cells [39]. Unlike the LDL receptor, expression of the scavenger receptor is not down-regulated by an increase in intracellular cholesterol [40]. Therefore, uptake of Ox-LDL contributes to the accumulation of cholesteryl esters in foam cells of atherosclerotic lesions [40]. Now, the question is Does oxidation of LDL-lipids influence the development of atherosclerosis ... 

---