Cholesterol receptor-mediated endocytosis

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

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. 

VLDLs, IDLs, and LDLs are closely related to one another. VLDLs formed in the liver (see p. 312) transport triacylglycerols, cholesterol, and phospholipids to other tissues. Like chylomicrons, they are gradually converted into IDL and LDL under the influence of lipoprotein lipase [1]. This process is also stimulated by HDL. Cells that have a demand for cholesterol bind LDL through an interaction between their LDL receptor and ApoB-100, and then take up the complete particle through receptor-mediated endocytosis. This type of transport is mediated by depressions in the membrane ( coated pits"), the interior of which is lined with the protein clathrin. After LDL binding, clathrin promotes invagination of the pits and pinching off of vesicles ( coated vesicles"). The clathrin then dissociates off and is reused. After fusion of the vesicle with ly-sosomes, the LDL particles are broken down (see p. 234), and cholesterol and other lipids are used by the cells. 

LDL is catabolized chiefly in hepatocytes and other cells by receptor-mediated endocytosis. Cholesteryl esters from LDL are hydrolyzed, yielding free cholesterol for the synthesis of cell membranes. Cells also obtain cholesterol by synthesis via a pathway involving the formation of mevalonic acid by HMG-CoA reductase. Production of this enzyme and of LDL receptors is transcriptionally regulated by the content of cholesterol in the cell. Normally, about 70% of LDL is removed from plasma by hepatocytes. Even more cholesterol is delivered to the liver via IDL and chylomicrons. Unlike other cells,... 

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-... 

FIGURE 21-42 Uptake of cholesterol by receptor-mediated endocytosis. 

The LDL, rich in cholesterol and its esters, is taken up by receptor-mediated endocytosis, in which the apolipoprotein B-100 of LDL is recognized by receptors in the plasma membrane. HDL removes cholesterol from the blood, carrying it to the liver. Dietary conditions or genetic defects in cholesterol metabolism may lead to atherosclerosis and heart disease. 

VLDL in the plasma is converted to LDL—a much smaller, denser particle. Apo CM and apo E are returned to HDLs, but the LDL retains apo B-100, which is recognized by receptors on 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 hyperlipidemia (familial hypercholesterolemia). The endocytosed cholesterol inhibits HMG CoA reductase and decreases synthesis of LDL receptors. Some of it can also be esterified by acyl CoAxholesterol acyltransferase and stored. 

Numerous studies have pointed to an important role for cholesterol during proliferation and progression of cancer (e.g., ref. 612-615). Rapidly dividing cancer cells have two major routes to fulfill their need for cholesterol to form new cell membrane endogenous synthesis of cholesterol and/or receptor-mediated uptake of exogenous LDL particle-associated cholesterol and cholesterol esters (ref. 612,613,615). Each LDL particle contains a cholesterol ester core surrounded by a polar shell of phospholipids (primarily phosphoglycerides), free cholesterol, and apolipoprotein B (ref. 616-618). Once bound to its cell surface receptor, LDL is internalized by receptor-mediated endocytosis and degraded in lysosomes, and the subsequently released cholesterol may be used for membrane synthesis by the tumor (ref. 619). 

Receptor-mediated endocytosis is the selective uptake of extracellular macromolecules (such as cholesterol) through their binding to specific cell-surface receptors. The receptor-macromolecule complex then accumulates in clathrin-coated pits and is endocytosed via a clathrin-coated vesicle. 

Chylomicrons are synthesized in the intestine and transport dietary triacylglycerols to skeletal muscle and adipose tissue, and dietary cholesterol to the liver. At these target tissues the triacylglycerols are hydrolyzed by lipoprotein lipase on the surface of the cells and the released fatty acids are taken up either for metabolism to generate energy or for storage. The resulting cholesterol-rich chylomicron remnants are transported in the blood to the liver where they are taken up by receptor-mediated endocytosis. 

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. 

HDLs are synthesized in the blood and extract cholesterol from cell membranes, converting it into cholesterol esters. Some of the cholesterol esters are then transferred to VLDLs. About half of the VLDLs and all of the HDLs are taken up into the liver cells by receptor-mediated endocytosis and the cholesterol disposed of in the form of bile salts. 

LDLs are then taken up by target cells through receptor-mediated endocytosis (see Topic E4). The LDL receptor, a transmembrane glycoprotein on the surface of the target cells, specifically binds apoB-100 in the LDL coat. The receptors then cluster into clathrin-coated pits and are internalized (see Topic E4, Fig. 3). Once in the lysosomes, the LDLs are digested by lysosomal enzymes, with the cholesterol esters being hydrolyzed by a lysosomal lipase to release the cholesterol (Fig. 1). This is then incorporated into the cell membrane and any excess is re-esterified for storage by acyl CoA cholesterol acyltransferase (ACAT). 

HDLs have the opposite function to that of LDLs in that they remove cholesterol from the tissues. The HDLs are synthesized in the blood mainly from components derived from the degradation of other lipoproteins. HDLs then acquire their cholesterol by extracting it from cell membranes and converting it into cholesterol esters by the action of LCAT (Fig. 1). The HDLs are then either taken up directly by the liver or transfer their cholesterol esters to VLDLs, of which about half are taken up by the liver by receptor-mediated endocytosis (Fig. 1). The liver is the only organ that can dispose of significant quantities of cholesterol, primarily in the form of bile salts (see Topic K5). 

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. 

Dehydrogenase Deficiency, Biotinidase Deficiency, and Adrenoleukodystrophy. Catabolism of essential amino acid skeletons is discussed in the chapters Phenylketonuria and HMG-CoA Lyase Deficiency. The chapters Inborn Errors of Urea Synthesis and Neonatal Hyperbilirubinemia discuss the detoxification and excretion of amino acid nitrogen and of heme. The chapter Gaucher Disease provides an illustration of the range of catabolic problems that result in lysosomal storage diseases. Several additional chapters deal with key aspects of intracellular transport of enzymes and metabolic intermediates the targeting of enzymes to lysosomes (I-Cell Disease), receptor-mediated endocytosis (Low-Density Lipoprotein Receptors and Familial Hypercholesterolemia) and the role of ABC transporters in export of cholesterol from the cell (Tangier disease). 

LDL complexes are the primary plasma carriers of cholesterol for delivery to all tissues. LDLs are taken up by cells via LDL receptor-mediated endocytosis. The uptake of LDLs occurs predominantly in liver (75%), adrenals and adipose tissue. As with IDLs, the interaction of LDLs with LDL receptors requires the presence of apo-B-100. The endocytosed membrane vesicles (endosomes) fuse with lysosomes, in which the apoproteins are degraded and the cholesterol esters are hydrolysed to yield free cholesterol. 

Figure 5.6 General scheme of lipoprotein metabolism. Triacylglycerols and cholesterol are exported from the liver in VLDLs, containing apolipoprotein B-lOO they further acquire apo-C-I, II, III and apo-E from circulating HDL. Apo-C-II activates lipoprotein lipase to remove fatty acids from VLDLs. As triacylglycerols are removed, VLDLs transform to IDEs and finally LDLs. LDLs are the main vehicle for transfer of cholesterol to the tissues uptake of LDL occurs primarily in the liver through LDL-receptor-mediated endocytosis, which requires the presence of apo-B-100. HDLs are synthesised essentially devoid of cholesterol or triacylglycerol and provide a circulating source of apo-C-I, II and apo-E. HDLs gradually accumulate cholesteryl esters, eventually returning these to the liver, mediated by an apo-A-I receptor this is referred to as reverse cholesterol transport. ...

Chylomicrons are produced from dietary fat by the removal of resynthesised triglycerides from the mucosal cells of the small intestine into the intestinal lumen. These then enter the circulation via the thoracic dncts in the lymphatic system and enter into the subclavian veins, where triglyceride content is reduced by the action of lipoprotein lipases (LPL) on capillary endothelial surfaces in skeletal muscle and fat. The free fatty acids (FFA) from the triglycerides are used by the tissues as an energy source or stored as triglycerides. The chylomicron remnants, stripped of triglyceride and therefore denser, are then taken up by the liver by LDL receptor-mediated endocytosis, thereby delivering cholesterol to the liver. 

Chylomicron remnants and very low density lipoprotein (VLDL) remnants are rapidly removed from the circulation by receptor-mediated endocytosis. ApoE, the major apolipoprotein of the chylomicron in the brain, binds to a specific receptor and is essential for the normal catabolism of triglyceride-rich lipoprotein constituents. Defects in apolipoprotein E result in familial dysbetalipoproteinemia, or type III hyperlipoproteinemia (HLP III), in which increased plasma cholesterol and triglycerides are the consequence of impaired clearance of chylomicron and VLDL remnants (Mahley et al., 1999). In the brain, lipidated apoE binds aggregated in a isoform-speciflc manner, apoE4 being much more effective than the other forms,... 

Figure 12.40. Receptor-Mediated Endocytosis. The process of receptor-mediated endocytosis is illustrated for the cholesterol-carrying complex, low-density lipoprotein (LDL) (1) LDL binds to a specific receptor, the LDL receptor (2) this complex invaginates to form an internal vesicle (3) after separation from its receptor, the LDL-containing vesicle fuses with a lysosome, leading to degradation of the LDL and release of the cholesterol.

Catabolism of chylomicron remnants may be viewed as the second step in the processing of chylomicrons. After the loss of apo C-II and other C and A apoproteins, LPL no longer acts upon the remnants, and they leave the capillary surface. Chylomicron remnants are rapidly removed by uptake into liver parenchymal cells via receptor-mediated endocytosis. Apo E is important in this uptake process. The chylomicron receptors in liver are distinct from the B-E receptor that mediates uptake of LDL. The hepatic receptor for chylomicrons binds with apo E, but not apo B-48. Another receptor, known as the LDL receptor-related protein (LRP), may also function in chylomicron uptake. Chylomicron remnants are transported into the lysosomal compartment where acid lipases and proteases complete their degradation. In the liver, fatty acids so released are oxidized or are reconverted to triacylglycerol, which is stored or secreted as VLDL. The cholesterol may be used in membrane synthesis, stored as cholesteryl ester, or excreted in the bile unchanged or as bile acids. 

Receptor-mediated endocytosis plays a key role in cholesterol metabolism (p. 745). Some cholesterol in the blood is in the form of a lipid—protein complex called low-density lipoprotein (LDL). Low-density lipoprotein... 

LDL Low-density lipoprotein a product of the degradation of very-low-density lipoproteins (VLDLs) by the action of lipoprotein lipase. LDLs are taken up by a receptor-mediated endocytosis by both peripheral tissues and the liver. It is commonly called the bad cholesterol. ... 

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