Cell membranes cholesterol transport

A deepened understanding of the pathogenesis of atherosclerosis came from research addressing the question of how lipophilic compounds were transported in blood. Along with food, humans take up various lipids (fats, cholesterol and its esters), which are important sources of energy, essential precursors for the biogenesis of steroid hormones, and indispensable components of many cell membranes. For transport in a blood vessel, the lipids form non-covalent aggregates with various proteins, known as lipoproteins. 

Although cholesterol has a bad reputation, it serves many important ftmctions in the body. Like phospholipids and giycolipids, cholesterol is part of cell membranes. Cholesterol also serves as a starting material (or precursor) for the body to synthesize other steroids such as testosterone, a principal male hormone, and estrogen, a principal female hormone. Hormones are chemical messengers that regulate many body processes, such as growth and metabolism. They are secreted by specialized tissues and transported in the blood. 

The hypothesis of the participation of those cholesterol transporters (NPCILI and ABCAl) in the carotenoid transport remains to be confirmed, especially at the in vivo human scale. If the mechanism by which carotenoids are transported through the intestinal epithelial membrane seems better understood, the mechanism of intracellular carotenoid transport is yet to be elucidated. The fatty acid binding protein (FABP) responsible for the intracellular transport of fatty acids was proposed earlier as a potential transporter for carotenoids. FABP would transport carotenoids from the epithelial cell membrane to the intracellular organelles such as the Golgi apparatus where CMs are formed and assembled, but no data have illustrated this hypothesis yet. 

The first study was conducted to determine whether carotenoids and cholesterol share common pathways (transporters) for their intestinal absorption (During et al., 2005). Differentiated Caco-2 cells on membranes were incubated (16 h) with a carotenoid (1 pmol/L) with or without ezetimibe (EZ Zetia, an inhibitor of cholesterol transport), and with or without antibodies against the receptors, cluster determinant 36 (CD36) and scavenger receptor class B, type I (SR-BI). Carotenoid transport in Caco-2 cells (cellular uptake + secretion) was decreased by EZ (lOmg/L) as follows P-C and a-C (50% inhibition) P-cryptoxanthin and LYC (20%) LUT ZEA (1 1) (7%). EZ reduced cholesterol transport by 31%, but not retinol transport. P-Carotene transport was also inhibited by anti-SR-BI, but not by anti-CD36. The inhibitory effects of EZ and anti-SR-BI on P-C transport... 

Steroid hormones are produced by the adrenal cortex, testes, ovaries, and placenta. Synthesized from cholesterol, these hormones are lipid soluble therefore, they cross cell membranes readily and bind to receptors found intracellularly. However, because their lipid solubility renders them insoluble in blood, these hormones are transported in the blood bound to proteins. Furthermore, steroid hormones are not typically preformed and stored for future use within the endocrine gland. Because they are lipid soluble, they could diffuse out of the cells and physiological regulation of their release would not be possible. Finally, steroid hormones are absorbed easily by the gastrointestinal tract and therefore may be administered orally. 

The lipid compositions of plasma membranes, endoplasmic reticulum and Golgi membranes are distinct 26 Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues 26 In adult brain most cholesterol synthesis occurs in astrocytes 26 The astrocytic cholesterol supply to neurons is important for neuronal development and remodeling 27 The structure and roles of membrane microdomains (rafts) in cell membranes are under intensive study but many aspects are still unresolved 28... 

Kobayashi T. Late endosomal membranes rich in lysobisphosphatidic acid (LBPA) regulate cholesterol transport. Nat Cell Biol 1999 1 113-118. 

G. HDL particles have several functions, but among the most important is transport of excess cholesterol scavenged from the cell membranes back to the liver, a process called reverse cholesterol transport. 

In the human body choline is needed for the synthesis of phospholipids in cell membranes, methyl metabolism, transmembrane signaling and lipid cholesterol transport and metabolism [169]. It is transported into mammalian cells by a high-affinity sodium-dependent transport system. Intracellular choline is metabolized to phosphorylcholine, the reaction being catalyzed by the enzyme choline... 

The HDL lipids are removed from the circulation by a selective uptake and by an indirect pathway. The selective uptake of cholesterol esters from HDL into he-patocytes and steroidogenic cells is mediated by the binding of HDL to scavenger receptor B1 (SR-BI). This selective uptake by SR-BI may depend on the presence of cofactors such as HL, which hydrolyses phospholipids on the surface of both HDL and plasma membranes and thereby enables the flux of cholesteryl esters from the lipoprotein core into the plasma membrane [42]. The indirect pathway involves the enzyme CETP, which exchanges cholesteryl esters of a-HDL with triglycerides of chylomicrons, VLDL, IDL, and LDL. The a-HDL derived cholesteryl esters are therefore removed via the LDL-receptor pathway. The removal of excess cholesterol from the periphery and the delivery to the liver for excretion in the bile is termed reverse cholesterol transport. 

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

HDL may be taken up in the liver by receptor-mediated endocytosis, but at least some of the cholesterol in HDL is delivered to other tissues by a novel mechanism. HDL can bind to plasma membrane receptor proteins called SR-BI in hepatic and steroidogenic tissues such as the adrenal gland. These receptors mediate not endocytosis but a partial and selective transfer of cholesterol and other lipids in HDL into the cell. Depleted HDL then dissociates to recirculate in the bloodstream and extract more lipids from chylomicron and VLDL remnants. Depleted HDL can also pick up cholesterol stored in extrahepatic tissues and carry it to the liver, in reverse cholesterol transport pathways (Fig. 21-40). In one reverse transport path, interaction of nascent HDL with SR-BI receptors in cholesterol-rich cells triggers passive movement of cholesterol from the cell surface into HDL, which then carries it back to the liver. In a second pathway, apoA-I in depleted HDL in-... 

Many of the proteins of membranes are enzymes. For example, the entire electron transport system of mitochondria (Chapter 18) is embedded in membranes and a number of highly lipid-soluble enzymes have been isolated. Examples are phosphatidylseiine decarboxylase, which converts phosphatidylserine to phosphatidylethanolamine in biosynthesis of the latter, and isoprenoid alcohol phosphokinase, which participates in bacterial cell wall synthesis (Chapter 20). A number of ectoenzymes are present predominantly on the outsides of cell membranes.329 Enzymes such as phospholipases (Chapter 12), which are present on membrane surfaces, often are relatively inactive when removed from the lipid environment but are active in the presence of phospholipid bilay-ers.330 33 The distribution of lipid chain lengths as well as the cholesterol content of the membrane can affect enzymatic activities.332... 

Unlike fatty acids, cholesterol is not degraded to yield energy. Instead excess cholesterol is removed from tissues by HDL for delivery to the liver from which it is excreted in the form of bile salts into the intestine. The transfer of cholesterol from extrahepatic tissues to the liver is called reverse cholesterol transport. When HDL is secreted into the plasma from the liver, it has a discoidal shape and is almost devoid of cholesteryl ester. These newly formed HDL particles are good acceptors for cholesterol in the plasma membranes of cells and are converted into spherical particles by the accumulation of cholesteryl ester. The cholesteryl ester is derived from a reaction between cholesterol and phosphatidylcholine on the surface of the HDL particle catalyzed by lecithimcholesterol acyltransferase (LCAT) (fig. 20.17). LCAT is associated with FIDL in plasma and is activated by apoprotein A-I, a component of HDL (see table 20.3). Associated with the LCAT-HDL complex is cholesteryl ester transfer protein, which catalyzes the transfer of cholesteryl esters from HDL to VLDL or LDL. In the steady state, cholesteryl esters that are synthesized by LCAT are transferred to LDL and VLDL and are catabolized as noted earlier. The HDL particles themselves turn over, but how they are degraded is not firmly established. 

Genes coding for APO-E are associated with different risks for Alzheimer s disease. There are three alleles (or copies) of the gene coding for this apolipoprotein which are called E2, E3, and E4. For example, a gene on chromosome 19 that codes for APO-E is linked to many cases of late-onset Alzheimer s disease. Moreover, APO-E is associated with cholesterol transport and involved with other neuronal functions, including repair, growth, and maintenance of myelin sheaths and cell membranes. 

Absorption of cholesterol in the small intestine contributes to maintaining whole-body cholesterol homeostasis, yet the mechanisms of absorption have not been completely defined. For many years it was believed that cholesterol, a normal component of cell membranes, simply diffused through the brush border membrane of enterocytes (Grundy, 1983 Westergaard and Dietschy, 1974). However, the discovery of specific transporters, receptors,... 

The apoproteins of HDL are secreted by the liver and intestine. Much of the lipid comes from the surface monolayers of chylomicrons and VLDL during lipolysis. HDL also acquire cholesterol from peripheral tissues in a pathway that protects the cholesterol homeostasis of cells. In this process, free cholesterol is transported from the cell membrane by a transporter protein, ABCA1, acquired by a small particle termed prebeta-1 HDL, and then esterified by lecithin cholesterol acyltransferase (LCAT), leading to the formation of larger HDL species. The cholesteryl esters are transferred to VLDL, IDL, LDL, and chylomicron remnants with the aid of cholesteryl ester transfer protein (CETP). Much of the cholesteryl ester thus transferred is ultimately delivered to the liver by endocytosis of the acceptor lipoproteins. HDL can also deliver cholesteryl esters directly to the liver via a docking receptor (scavenger receptor, SR-BI) that does not endocytose the lipoproteins. 

VLDLs are synthesized in the liver and transport triacylglycerols, cholesterol and phospholipids to other tissues, where lipoprotein lipase hydrolyzes the triacylglycerols and releases the fatty acids for uptake. The VLDL remnants are transformed first to IDLs and then to LDLs as all of their apoproteins other than apoB-100 are removed and their cholesterol esterified. The LDLs bind to the LDL receptor protein on the surface of target cells and are internalized by receptor-mediated endocytosis. The cholesterol, which is released from the lipoproteins by the action of lysosomal lipases, is either incorporated into the cell membrane or re-esterified for storage. High levels of intracellular cholesterol decrease the synthesis of the LDL receptor, reducing the rate of uptake of cholesterol, and inhibit HMG CoA reductase, preventing the cellular synthesis of cholesterol. 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

As shown in figure 7.22(A), increases in membrane static order caused by supplementing the growth medium of Chinese hamster ovary cells with cholesterol led to regular decreases in the specific activity of the Na+-K+-ATPase (Sinensky et al., 1979). Presumably, the increased viscosity of the domain of lipids adjacent to the enzyme hindered the conformational changes required for ion transport, leading to reductions in enzymatic activity. 

Fig. 3. Steroidogenic pathway in granulosa cells. A. Lipoprotein in receptors. B. 3-Hydroxy-3-methyl-glutaryl coenzyme A reductase (HMG-CoA reductase). C. Acyl-coenzyme A (cholesterol acyl transferase). D. Cholesterol esterase. E. Cholesterol transport to the mitochondria. F. Cholesterol side-chain cleavage enzymes (phospholipid membrane environment and enzyme levels). G. 3/3-Hydroxysteroid dehydrogenase (3/3-HSD). H. 20a-Hydroxysteroid dehydrogenase (20a-HSD). I. Aromatases.

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