BBA - Molecular and Cell Biology of Lipids (v.1529, #1-3)

Cholesterol in the year 2000 by Dennis E. Vance; Henk Van den Bosch (1-8).
Cholesterol research was one of the key areas of scientific investigation in the 20th century. Little was known about the structure of cholesterol until the pioneering research of A. Windaus and H. Wieland in the first part of the century. The structure of cholesterol was completely elucidated in 1932. With the development of isotopic tracers in the 1930s studies on cholesterol biosynthesis were initiated. In 1942 K. Bloch and D. Rittenberg showed that deuterium-labeled acetate was incorporated into the ring structure and side chain of cholesterol. Another important discovery from Bloch’s laboratory was that squalene was a precursor of cholesterol. In 1956, the main elements of the biosynthetic pathway became known when isopentenyl pyrophosphate was discovered as a precursor. In 1966, J. Cornforth and G. Popjak predicted that there were 16 234 possible stereochemical pathways by which mevalonate could be converted into squalene. They subsequently showed which of these pathways was correct. In the 1970s and 1980s K. Bloch was able to provide intriguing evidence for an evolutionary advantage of cholesterol over lanosterol or some of the intermediates in the conversion of lanosterol to cholesterol. The last quarter of the 20th century was when M. Brown and J. Goldstein showed that the low density lipoprotein receptor was a key regulator of cholesterol homeostasis. They have also demonstrated that cholesterol balance in the cell is transcriptionally regulated via the sterol regulatory element binding protein. In the later part of the 20th century drugs were developed that effectively lower plasma cholesterol and lessen the risk of atherosclerosis and cardiovascular disease.
Keywords: Cholesterol; Konrad Bloch; Sterol regulatory element binding protein; Cholesterol biosynthesis; Atherosclerosis; Evolution;

The structure of the catalytic portion of human HMG-CoA reductase by Eva S. Istvan; Johann Deisenhofer (9-18).
In higher plants, fungi, and animals isoprenoids are derived from the mevalonate pathway. The carboxylic acid mevalonate is formed from acetyl-CoA and acetoacetyl-CoA via the intermediate 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA). The four-electron reduction of HMG-CoA to mevalonate, which utilizes two molecules of NADPH, is the committed step in the biosynthesis of isoprenoids. This reaction is catalyzed by HMG-CoA reductase (HMGR). The activity of HMGR is controlled through synthesis, degradation and phosphorylation. The human enzyme has also been targeted successfully by drugs, known as statins, in the clinical treatment of high serum cholesterol levels. The crystal structure of the catalytic portion of HMGR has been determined recently with bound reaction substrates and products. The structure illustrates how HMG-CoA and NADPH are recognized and suggests a catalytic mechanism. Catalytic portions of human HMGR form tight tetramers, explaining the influence of the enzyme’s oligomeric state on the activity and suggesting a mechanism for cholesterol sensing.
Keywords: 3-Hydroxy-3-methylglutaryl coenzyme A; Nicotinamide adenine dinucleotide phosphate; Oxidoreductase; Cholesterol biosynthesis; Enzyme mechanism;

Biochemical and genetic aspects of mevalonate kinase and its deficiency by Sander M Houten; Ronald J.A Wanders; Hans R Waterham (19-32).
Mevalonate kinase (MK) is an essential enzyme in the mevalonate pathway which produces numerous cellular isoprenoids. The enzyme has been characterized both at the biochemical and the molecular level in a variety of organisms. Despite the fact that mevalonate kinase is not the rate-limiting enzyme in isoprenoid biosynthesis, its activity is subject to feedback regulation by the branch-point intermediates geranyldiphosphate, farnesyldiphosphate and geranylgeranyldiphosphate. Recently, the importance of mevalonate kinase was demonstrated by the identification of its deficiency as the biochemical and molecular cause of the inherited human disorders mevalonic aciduria and hyperimmunoglobulinemia D and periodic fever syndrome. The pathophysiology of these disorders is not yet understood, but eventually will give insight into the in vivo role of mevalonate kinase and isoprenoid biosynthesis with respect to the acute phase response and fever. The subcellular localization of mevalonate kinase is still a matter of debate. The enzyme could be localized predominantly in the cytosol, or in peroxisomes, or it is associated differentially with peroxisomes. Here we review the biochemical and molecular properties of MK, and discuss its biological significance, the regulation of its enzyme activity and finally its subcellular localization.
Keywords: Mevalonate kinase; Biochemical characterization; Inherited disorder; Mevalonic aciduria; Hyperimmunoglobulinemia D and periodic fever syndrome; Subcellular localization;

Isoprenyl diphosphate synthases by Kevin C Wang; Shin-ichi Ohnuma (33-48).
Isoprenyl diphosphate synthases catalyze consecutive condensations of isopentenyl diphosphates with allylic primer substrates to form linear backbones for all isoprenoid compounds including cholesterol. These synthases are classified according to the final chain length of their end products and the stereochemistry of the newly formed double bonds. Mutagenesis and X-ray crystallography data have uncovered the basic catalytic and chain length determination mechanisms of E-isoprenyl diphosphate synthases and shed light on their possible evolutionary course. Although much less is known about the Z-isoprenyl diphosphate synthase family, successful cloning and subsequent crystallizations in the near future will no doubt bring more insight as researchers begin to unravel the essential components and precise reaction mechanisms of this cellular machinery.
Keywords: Isoprenyl diphosphate synthase; Prenyltransferase; Isoprenoid; Molecular evolution; Chain length determination; Cholesterol;

Structure and regulation of mammalian squalene synthase by Terese R Tansey; Ishaiahu Shechter (49-62).
Mammalian squalene synthase (SQS) catalyzes the first reaction of the branch of the isoprenoid metabolic pathway committed specifically to sterol biosynthesis. SQS produces squalene in an unusual two-step reaction in which two molecules of farnesyl diphosphate are condensed head-to-head. Recent studies have advanced understanding of the reaction mechanism, the functional domains of the enzyme, and transcriptional regulation of the gene. Site-directed mutagenesis has identified conserved Asp, Tyr, and Phe residues that are essential for SQS activity. The Asp residues are hypothesized to be required for substrate binding; the Tyr and Phe residues may stabilize carbocation reaction intermediates. The elucidation of SQS crystal structure will most likely direct future research on the relationship between enzyme structure and function. SQS activity, protein, and mRNA levels are regulated by cholesterol status and by the cytokines TNF-α and IL-1β. Activation of the SQS promoter in response to cholesterol deficit is mediated by sterol regulatory element binding proteins SREBP-1a and SREBP-2. The precise contributions made by individual SREBPs and accessory transcription factors to SQS transcriptional control, and the mechanisms underlying cytokine regulation of SQS are major foci of current research.
Keywords: Squalene synthase; Cholesterol biosynthesis; Isoprenoid metabolism; Sterol regulatory element binding protein; Cytokine;

Sterol C-methylations catalyzed by the (S)-adenosyl-l-methionine: Δ24-sterol methyl transferase (SMT) have provided the focus for study of electrophilic alkylations, a reaction type of functional importance in C–C bond formation of natural products. SMTs occur generally in nature, but do not occur in animal systems, suggesting that the difference in sterol synthetic pathways can be exploited therapeutically and in insect–plant interactions. The SMT genes from several plants and fungi have been cloned, sequenced and expressed in bacteria or yeast and bioengineered into tobacco or tomato plants. These enzymes share significant amino acid sequence similarity in the putative sterol and AdoMet binding sites. Investigations of the molecular recognition of sterol fitness and studies with stereospecifically labeled substrates as well as various sterol analogs assayed with native or mutant SMTs from fungi and plants have been carried out recently in our own and other laboratories. These analyses have led to an active-site model, referred to as the ‘steric-electric plug’ model, which is consistent with a non-covalent mechanism involving the intermediacy of a 24β-methyl (or ethyl) sterol bound to the ternary complex. Despite the seeming differences between fungal and plant SMT activities the recent data indicate that a distinct SMT or family of SMTs exist in these organisms which bind and transform sterols according to a similar mechanistic plan. Vascular plants have been found to express different complements of C1/C2-activities in the form of at least three SMT isoforms. This enzyme multiplicity can be a target of regulatory control to affect phytosterol homeostasis in transgenic plants. The state of our current understanding of SMT enzymology and inhibition is presented.
Keywords: Sterol methyl transferase; Inhibitor; C-Methyl transfer mechanism; Kinetic isotope effect; Ergosterol; Site-directed mutagenesis; Bioengineering phytosterol synthesis;

At least three different subcellular compartments, including peroxisomes, are involved in cholesterol synthesis. Recently, it has been demonstrated that peroxisomes contain a number of enzymes involved in cholesterol biogenesis that previously were considered to be cytosolic or located in the endoplasmic reticulum. Peroxisomes have been shown to contain acetoacetyl-CoA thiolase, HMG-CoA synthase, HMG-CoA reductase, mevalonate kinase, phosphomevalonate kinase, phosphomevalonate decarboxylase, isopentenyl diphosphate isomerase and FPP synthase. Moreover, the activities of these enzymes are also significantly decreased in liver tissue and fibroblast cells obtained from patients with peroxisomal deficiency diseases. In addition, the cholesterol biosynthetic capacity is severely impaired in cultured skin fibroblasts obtained from patients with peroxisomal deficiency diseases. These findings support the proposal that peroxisomes play an essential role in isoprenoid biosynthesis. This paper presents a review of peroxisomal protein targeting and of recent studies demonstrating the localization of cholesterol biosynthetic enzymes in peroxisomes and the identification of peroxisomal targeting signals in these proteins.
Keywords: Peroxisome; Targeting signal; Cholesterol biosynthesis enzyme;

Regulation of gene expression by SREBP and SCAP by Peter A Edwards; David Tabor; Heidi R Kast; Asha Venkateswaran (103-113).
Sterol regulatory element binding proteins (SREBPs) function as transcription factors that activate specific genes involved in cholesterol synthesis, endocytosis of low density lipoproteins, the synthesis of both saturated and unsaturated fatty acids and glucose metabolism. As such, these proteins provide a link between lipid and carbohydrate metabolism. There are three SREBPs, SREBP-1a, SREBP-1c and SREBP-2, that are encoded by two genes. SREBPs are synthesized as 125 kDa precursor proteins that are localized to the endoplasmic reticulum. The precursor is transported to the Golgi by a chaperone protein (SREBP-cleavage activating protein) and then cleaved by two proteases to release the mature, transcriptionally active 68 kDa amino terminal domain. Recent studies have shown that formation of mature SREBP is controlled at multiple levels in response to changes in the levels of oxysterols, insulin/glucose and polyunsaturated fatty acids. These recent findings have important clinical implications relevant to hyperlipidemia and diabetes and are the topic of this review.
Keywords: Sterol regulatory element binding protein; Sterol regulatory element binding protein-cleavage activating protein; Lipid synthesis;

Sterols and gene expression: control of affluence by Kristina Schoonjans; Carole Brendel; David Mangelsdorf; Johan Auwerx (114-125).
Intracellular and extracellular cholesterol levels are tightly maintained within a narrow concentration range by an intricate transcriptional control mechanism. Excess cholesterol can be converted into oxysterols, signaling molecules, which modulate the activity of a number of transcription factors, as to limit accumulation of excess of cholesterol. Two key regulatory pathways are affected by oxysterols. The first pathway involves the uptake and de novo synthesis of cholesterol and is controlled by the family of sterol response element binding proteins, whose activity is regulated by a sterol-dependent feedback mechanism. The second pathway, which only recently has become a topic of interest, involves the activation by a feedforward mechanism of cholesterol utilization for either bile acid or steroid hormone synthesis by oxysterol-activated nuclear receptors, such as liver X receptor and steroidogenic factor-1. Furthermore, biosynthesis and enterohepatic reabsorption of bile acids are regulated by the farnesol X receptor, a receptor activated by bile acids. Both the feedback inhibition of cholesterol uptake and production and the stimulation of cholesterol utilization will ultimately result in a lowering of the intracellular cholesterol concentration and allow for a fine-tuned regulation of the cholesterol concentration.
Keywords: Atherosclerosis; Cholesterol; Farnesol X receptor; Gene expression; Liver receptor homologue 1; Liver X receptor; Nuclear receptor; Oxysterol; 9-cis retinoic acid receptor; Short heterodimeric partner; Steroidogenic factor-1; Sterol response element binding protein;

Oxysterol biosynthetic enzymes by David W Russell (126-135).
Oxysterols, herein defined as derivatives of cholesterol with a hydroxyl group on the side chain, play several roles in lipid metabolism. Members of this class regulate the expression of genes that participate in both sterol and fat metabolism, serve as substrates for the synthesis of bile acids, and are intermediates in the transfer of sterols from the periphery to the liver. Three abundant naturally occurring oxysterols are 24-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol. The cholesterol hydroxylase enzymes that synthesize each of these have been isolated over the last several years and their study has produced insight into the biology of oxysterols. This article focuses on the properties of these enzymes.
Keywords: Cholesterol hydroxylase; Sterol metabolism; Bile acid synthesis; Cytochrome P-450; Diiron cofactor;

Expression of the gene coding for the synthesis of 25(R),26-hydroxycholesterol in many tissues and the finding that this sterol can be the sole pathway for the production of bile acids have led to a renewed interest in this metabolic pathway. A further impetus for exploring the normal biologic roles that are served by expression of the CYP27A1 gene is the knowledge that mutations in humans are associated with accelerated atherosclerosis and with severe neurologic impairment. The molecular mechanisms governing these phenotypic expressions are not known but in light of the traditional role of steroids as ligands for receptors that regulate gene expression it seems likely that the intermediates in this pathway modulate a number of enzymatic activities that remain to be elucidated.
Keywords: 25(R),26-Hydroxycholesterol; Cerebrotendinous xanthomatosis; Cholestasis; Cholesterol 7α-hydroxylase; Oxysterol 7α-hydroxylase; Bile acid; CYP27A1; CYP7A1; CYP7B1;

Mammalian acyl-CoA:cholesterol acyltransferases by Kimberly F. Buhman; Michel Accad; Robert V. Farese (142-154).
Cholesterol, the chief sterol found in vertebrates, exists both as a free sterol and as a component of cholesterol esters, which are synthesized by acyl-CoA:cholesterol acyltransferase (ACAT) enzymes. Considerable knowledge concerning cholesterol ester metabolism has accumulated during the past century. However, rapid advances have occurred in the past 7 years since the cloning of an ACAT gene, including the discovery that two ACATs function in mammalian biology. A clearer picture of the functions of ACAT enzymes in cellular cholesterol metabolism and physiologic processes is now emerging. These insights may have relevance for the development of ACAT inhibitors for treating hypercholesterolemia or atherosclerosis in humans.
Keywords: Acyl-coenzyme A:cholesterol acyltransferase; Cholesterol; Cholesterol ester; Fatty acid; Lipoprotein; Atherosclerosis;

The model eukaryote Saccharomyces cerevisiae (budding yeast) has provided significant insight into sterol homeostasis. The study of sterol metabolism in a genetically amenable model organism such as yeast is likely to have an even greater impact and relevance to human disease with the advent of the complete human genome sequence. In addition to definition of the sterol biosynthetic pathway, almost to completion, the remarkable conservation of other components of sterol homeostasis are described in this review.
Keywords: Yeast; Ergosterol; Acyl-coenzyme A:cholesterol acyltransferase; Niemann-Pick C;

Cholesterol-loaded macrophages are present at all stages of atherogenesis, and recent in vivo data indicate that these cells play important roles in both early lesion development and late lesion complications. To understand how these cells promote atherogenesis, it is critical that we understand how lesional macrophages interact with subendothelial lipoproteins, the consequences of this interaction, and the impact of subsequent intracellular metabolic events. In the arterial wall, macrophages likely interact with both soluble and matrix-retained lipoproteins, and a new challenge is to understand how certain consequences of these two processes might differ. Initially, the major intracellular metabolic route of the lipoprotein-derived cholesterol is esterification to fatty acids, but macrophages in advanced atherosclerotic lesions progressively accumulate large amounts of unesterified, or free, cholesterol (FC). In cultured macrophages, excess FC accumulation stimulates phospholipid biosynthesis, which is an adaptive response to protect the macrophage from FC-induced cytotoxicity. This phospholipid response eventually decreases with continued FC loading, leading to a series of cellular death reactions involving both death receptor-induced signaling and mitochondrial dysfunction. Because macrophage death in advanced lesions is thought to promote plaque instability, these intracellular processes involving cholesterol, phospholipid, and death pathways may play a critical role in the acute clinical manifestations of advanced atherosclerotic lesions.
Keywords: Foam cell; Atherosclerosis; Lipoprotein; Acyl-coenzyme A:cholesterol acyltransferase; Phosphatidylcholine; Cytidine 5′-triphosphate:phosphocholine cytidylyltransferase; Apoptosis;

The steroidogenic acute regulatory (StAR) protein regulates the rate limiting step in steroidogenesis, the transport of cholesterol from the outer to the inner mitochondrial membrane. Insight into the structure and function of StAR was attained through molecular genetic studies of congenital lipoid adrenal hyperplasia, a rare disease caused by mutations in the StAR gene. Subsequent functional analysis defined two major domains within the StAR protein, the N-terminal mitochondrial targeting sequence and the C-terminus, which promotes the translocation of cholesterol between the two mitochondrial membranes. Two models of StAR’s mechanism of action, (1) stimulation of cholesterol desorption from the outer mitochondrial membrane and (2) an intermembrane shuttle hypothesis, are discussed with respect to the known biochemical and biophysical events associated with the process of steroidogenesis and the structure of StAR. StAR gene expression is regulated primarily at the transcriptional level, and the roles of transcription factors that govern basal and cAMP-dependent StAR expression including SF-1, C/EBP β, Sp1 and GATA-4 are reviewed.
Keywords: Cholesterol; Mitochondria; Steroidogenesis; Steroidogenic acute regulatory protein; Transcription;

Cholesterol modification of proteins by Randall K. Mann; Philip A. Beachy (188-202).
The demonstration over 30 years ago that inhibitors of cholesterol biosynthesis disrupt animal development suggested an intriguing connection between fundamental cellular metabolic processes and the more global processes of embryonic tissue patterning. Adding a new dimension to this relationship is the more recent finding that the Hedgehog family of tissue patterning factors are covalently modified by cholesterol. Here we review the mechanism of the Hedgehog autoprocessing reaction that results in this modification, and compare this reaction to that undergone by other autoprocessing proteins. We also discuss the biological consequences of cholesterol modification, in particular the use of cholesterol as a molecular handle in the spatial deployment of the protein signal in developing tissues. Finally, the developmental consequences of chemical and genetic disruption of cholesterol homeostasis are summarized, along with the potential importance of cholesterol-rich lipid rafts in production of and response to the Hh signal.
Keywords: Hedgehog protein; Cholesterol; Autoproteolysis; Hint domain; Holoprosencephaly;

There are now numerous examples of post-translational modification with geranylgeranyl or farnesyl substituents. Once thought of as solely a mechanism for association of proteins with membranes, other functional aspects of protein prenylation have come to be appreciated. Although, in almost all instances, such proteins are membrane associated, they are often found to also engage in protein–protein interactions. In some instances, such interactions are critical aspects of prenylated protein trafficking. In this review, the role of prenylation in mediating protein–protein interactions will be considered. The hypothesis will be developed that such interactions occur through recognition of the prenyl group and a second domain, on the prenylated protein, by a heterodimeric protein partner.
Keywords: Prenylation; Protein–protein interaction; Trafficking;

Cholesterol and caveolae: structural and functional relationships by Christopher J. Fielding; Phoebe E. Fielding (210-222).
Caveolae are free cholesterol (FC)- and sphingolipid-rich surface microdomains abundant in most peripheral cells. Caveolin, a FC binding protein, is a major structural element of these domains. Caveolae serve as portals to regulate cellular FC homeostasis, possibly via their association with ancillary proteins including scavenger receptor B1. The FC content of caveolae regulates the transmission of both extracellular receptor-mediated and endogenous signal transduction via changes in the composition of caveolin-associated complexes of signaling intermediates. By controlling surface FC content, reporting membrane changes by signal transduction to the nucleus, and regulating signal traffic in response to extracellular stimuli, caveolae exert a multifaceted influence on cell physiology including growth and cell division, adhesion, and hormonal response. Cell surface lipid ‘rafts’ may assume many of the functions of caveolae in cells with low levels of caveolin.
Keywords: Caveolae; Caveolin; Free cholesterol; Raft;

Cholesterol and hepatic lipoprotein assembly and secretion by Sohye Kang; Roger A Davis (223-230).
The assembly and secretion of apo B100 containing lipoproteins (i.e., VLDL) by the liver and cholesterol metabolism are interrelated on several different levels and for several different physiologic reasons. Firstly, hepatic VLDL is the major precursor for LDL, which in the human is the major vehicle responsible for transporting cholesterol to peripheral tissues. Secondly, cholesterol is supplied to many tissues by a specific uptake of LDL via LDL receptor, which is expressed in a regulated manner by most mammalian tissues. Thirdly, the rate of hepatic cholesterol biosynthesis and metabolism to bile acids correlates with production of VLDL. This apparent coordinate expression of cholesterol biosynthetic/catabolic enzymes and hepatic VLDL assembly/secretion are mediated at least in part through the sterol response element binding protein (SREBP) transcription factor family. Their gene targets include a plethora of enzymes that regulate glycolysis, energy production, lipogenesis and cholesterol catabolism. Studies of hepatoma cells overexpressing CYP7A1, the rate-limiting enzyme controlling bile acid synthesis, show that as a result of increased mature SREBP1, there is a coordinate induction of lipogenesis and the assembly and secretion of VLDL. These and additional studies show that the bile acid synthetic pathway and the VLDL assembly/secretion pathway are coordinately linked through SREBP-dependent transcription. Based on studies showing that within the liver acinus, the expression of CYP7A1 is mainly in the pericentral region while HMG-CoA reductase is mainly periportal, we propose that a ‘metabolic zonal segregation’ plays an important role in coordinate regulation of cholesterol and VLDL metabolism. This putative ‘metabolic zonal segregation’ may provide segregation of metabolic functions which may be mutually antagonistic. For example, there may be physiologic states in which the bile acid synthetic pathway may compete with the VLDL assembly/secretion pathway for a limited amount of cholesterol. Metabolic antagonism (e.g., competition for cholesterol) may be avoided via inducing SREBP-mediated transcription. Adaptation of catabolic hepatocytes to accommodate the expression of VLDL assembly/secretion may occur in response to activation of SREBP-mediated transcription. Support for these is discussed.
Keywords: Apolipoprotein B; Lipoprotein; Cholesterol; Bile acid; HMG-CoA reductase; Sterol response element binding protein;

Most mammalian somatic cells are unable to catabolize cholesterol and therefore need to export it in order to maintain sterol homeostasis. This mechanism may also function to reduce excessively accumulated cholesterol, which would thereby contribute to prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) has been believed to play a main role in this reaction based on epidemiological evidence and in vitro experimental data. At least two independent mechanisms are identified for this reaction. One is non-specific diffusion-mediated cholesterol ‘efflux’ from cell surface. Cholesterol molecules desorbed from cells can be trapped by various extracellular acceptors including various lipoproteins and albumin, and extracellular cholesterol esterification mainly on HDL may provide a driving force for the net removal of cell cholesterol by maintaining a cholesterol gradient between lipoprotein surface and cell membrane. The other is apolipoprotein-mediated process to generate new HDL by removing cellular phospholipid and cholesterol. The reaction is initiated by the interaction of lipid-free or lipid-poor helical apolipoproteins with cellular surface resulting in assembly of HDL particles with cellular phospholipid and incorporation of cellular cholesterol into the HDL being formed. Thus, HDL has dual functions as an active cholesterol acceptor in the diffusion-mediated pathway and as an apolipoprotein carrier for the HDL assembly reaction. The impairment of the apolipoprotein-mediated reaction was found in Tangier disease and other familial HDL deficiencies to strongly suggest that this is a main mechanism to produce plasma HDL. The causative mutations for this defect was identified in ATP binding cassette transporter protein A1, as a significant step for further understanding of the reaction and cholesterol homeostasis.
Keywords: Apolipoprotein; Cholesterol efflux; High density lipoprotein; ATP-binding cassette transporter; Caveolin; Membrane;

Lecithin cholesterol acyltransferase by Ana Jonas (245-256).
Cholesterol transport in circulation and its removal from tissues depends on the activity of lecithin cholesterol acyltransferase (LCAT). LCAT is a soluble enzyme that converts cholesterol and phosphatidylcholines (lecithins) to cholesteryl esters and lyso-phosphatidylcholines on the surface of high-density lipoproteins. This review presents key background information and recent research advances on the structure of human LCAT, its reactions and substrates, and the expression of the LCAT gene. While the three-dimensional structure of LCAT is not yet known, a partial model now exists that facilitates the study of structure–function relationships of the native enzyme, and of natural and engineered mutants. The LCAT reaction on lipoproteins consists of several steps, starting with enzyme binding to the lipoprotein/lipid surface, followed by activation of LCAT by apolipoproteins, binding of lipid substrates and the catalytic steps giving rise to the lipid products. Quantitative data are presented on the kinetic and equilibrium constants of some of the LCAT reaction steps. Finally, overexpression of the human LCAT gene in mice and rabbits has been used to examine the physiologic role of LCAT in vivo and its protective effect against diet induced atherosclerosis.
Keywords: Phosphatidylcholine; Cholesteryl ester; High density lipoprotein; Glycoprotein; α/β hydrolase; Lipase; Apolipoprotein A-I; Small unilamellar vesicle;

Molecular biology and pathophysiological aspects of plasma cholesteryl ester transfer protein by Shizuya Yamashita; Ken-ichi Hirano; Naohiko Sakai; Yuji Matsuzawa (257-275).
Plasma cholesteryl ester transfer protein (CETP) facilitates the transfer of cholesteryl ester (CE) from high density lipoprotein (HDL) to apolipoprotein B-containing lipoproteins. Since CETP regulates the plasma levels of HDL cholesterol and the size of HDL particles, CETP is considered to be a key protein in reverse cholesterol transport, a protective system against atherosclerosis. CETP, as well as plasma phospholipid transfer protein, belongs to members of the lipid transfer/lipopolysaccharide-binding protein (LBP) gene family, which also includes the lipopolysaccharide-binding protein (LBP) and bactericidal/permeability-increasing protein. Although these four proteins possess different physiological functions, they share marked biochemical and structural similarities. The importance of plasma CETP in lipoprotein metabolism was demonstrated by the discovery of CETP-deficient subjects with a marked hyperalphalipoproteinemia (HALP). Two common mutations in the CETP gene, intron 14 splicing defect and exon 15 missense mutation (D442G), have been identified in Japanese HALP patients with CETP deficiency. The deficiency of CETP causes various abnormalities in the concentration, composition, and functions of both HDL and low density lipoprotein. Although the pathophysiological significance of CETP in terms of atherosclerosis has been controversial, the in vitro experiments showed that large CE-rich HDL particles in CETP deficiency are defective in cholesterol efflux. Epidemiological studies in Japanese-Americans and in the Omagari area where HALP subjects with the intron 14 splicing defect of CETP gene are markedly frequent, have shown an increased incidence of coronary atherosclerosis in CETP-deficient patients. The current review will focus on the recent findings on the molecular biology and pathophysiological aspects of plasma CETP, a key protein in reverse cholesterol transport.
Keywords: Hyperalphalipoproteinemia; Cholesteryl ester transfer protein deficiency; Reverse cholesterol transport; Atherosclerosis;

High-density lipoproteins (HDL) play an important role in protection against atherosclerosis by mediating reverse cholesterol transport – the transport of excess cholesterol from peripheral tissues to the liver for disposal. SR-BI is a cell surface receptor for HDL and other lipoproteins (LDL and VLDL) and mediates the selective uptake of lipoprotein cholesterol by cells. Overexpression or genetic ablation of SR-BI in mice revealed that it plays an important role in HDL metabolism and reverse cholesterol transport and protects against atherosclerosis in mouse models of the disease. If it plays a similar role in humans then it may be an attractive target for therapeutic intervention. We will review some of the recent advances in the understanding of SR-BI’s physiological role and cellular function in lipoprotein metabolism.
Keywords: apoA-I; Atherosclerosis; High density lipoprotein; Low density lipoprotein; Reverse cholesterol transport; Scavenger receptor class B, type I;

The discovery of an ever growing number of low density lipoprotein (LDL) receptor gene family members has triggered research into many different directions. Here we first summarize the results of classical studies on the role of the LDL receptor in cholesterol transport, the structure/function relationships delineated with the help of LDL receptor mutations in familial hypercholesterolemia, and the elegant way in which cells regulate cholesterol at the transcriptional level. The second part deals with a multifunctional, structurally very close relative, the very low density lipoprotein (VLDL) receptor. While it is involved in lipoprotein transport in certain tissues and species, detailed studies on its function have generated new knowledge about the growing spectrum of ligands and about exciting and unexpected aspects of receptor biology. In particular, these investigations have elucidated the roles of LDL receptor gene family members in ligand-mediated signal transduction. In the third part of this review article, we provide first insight into the roles of the VLDL receptor and of another small relative, the so-called apolipoprotein E receptor-2, in such signaling processes. These findings suggest that to date, only the tip of an iceberg has been uncovered.
Keywords: Cholesterol; Low density lipoprotein receptor; Very low density lipoprotein receptor; Apolipoprotein E receptor-2; Signaling;

Cholesterol and atherosclerosis by Peter Libby; Masanori Aikawa; Uwe Schönbeck (299-309).
Keywords: Lipoprotein; Inflammation; Thrombosis; Arteriosclerosis; Plaque; Macrophage;

Dietary cholesterol and atherosclerosis by Donald J McNamara (310-320).
The perceived relationship between dietary cholesterol, plasma cholesterol and atherosclerosis is based on three lines of evidence: animal feeding studies, epidemiological surveys, and clinical trials. Over the past quarter century studies investigating the relationship between dietary cholesterol and atherosclerosis have raised questions regarding the contribution of dietary cholesterol to heart disease risk and the validity of dietary cholesterol restrictions based on these lines of evidence. Animal feeding studies have shown that for most species large doses of cholesterol are necessary to induce hypercholesterolemia and atherosclerosis, while for other species even small cholesterol intakes induce hypercholesterolemia. The species-to-species variability in the plasma cholesterol response to dietary cholesterol, and the distinctly different plasma lipoprotein profiles of most animal models make extrapolation of the data from animal feeding studies to human health extremely complicated and difficult to interpret. Epidemiological surveys often report positive relationships between cholesterol intakes and cardiovascular disease based on simple regression analyses; however, when multiple regression analyses account for the colinearity of dietary cholesterol and saturated fat calories, there is a null relationship between dietary cholesterol and coronary heart disease morbidity and mortality. An additional complication of epidemiological survey data is that dietary patterns high in animal products are often low in grains, fruits and vegetables which can contribute to increased risk of atherosclerosis. Clinical feeding studies show that a 100 mg/day change in dietary cholesterol will on average change the plasma total cholesterol level by 2.2–2.5 mg/dl, with a 1.9 mg/dl change in low density lipoprotein (LDL) cholesterol and a 0.4 mg/dl change in high density lipoprotein (HDL) cholesterol. Data indicate that dietary cholesterol has little effect on the plasma LDL:HDL ratio. Analysis of the available epidemiological and clinical data indicates that for the general population, dietary cholesterol makes no significant contribution to atherosclerosis and risk of cardiovascular disease.
Keywords: Dietary cholesterol; Saturated fat; Atherosclerosis; Epidemiology; Plasma cholesterol; Low density lipoprotein; High density lipoprotein;

Tangier disease and ABCA1 by John F. Oram (321-330).
Tangier disease is an autosomal recessive genetic disorder characterized by a severe high-density lipoprotein (HDL) deficiency, sterol deposition in tissue macrophages, and prevalent atherosclerosis. Mutations in the ATP binding cassette transporter ABCA1 cause Tangier disease and other familial HDL deficiencies. ABCA1 controls a cellular pathway that secretes cholesterol and phospholipids to lipid-poor apolipoproteins. This implies that an inability of newly synthesized apolipoproteins to acquire cellular lipids by the ABCA1 pathway leads to their rapid degradation and an over-accumulation of cholesterol in macrophages. Thus, ABCA1 plays a critical role in modulating flux of tissue cholesterol and phospholipids into the reverse cholesterol transport pathway, making it an important therapeutic target for clearing excess cholesterol from macrophages and preventing atherosclerosis.
Keywords: Tangier disease; Adenosine 5′-triphosphate binding cassette transporter A1; Adenosine 5′-triphosphate binding cassette transporter 1; Cholesterol efflux; Apolipoprotein; High-density lipoprotein; Atherosclerosis;

Keywords: Niemann-Pick type C; Cholesterol; Trafficking; Glycolipid; Sterol;

In recent years, several inherited human disorders caused by defects in cholesterol biosynthesis have been identified. These are characterized by malformations, multiple congenital anomalies, mental and growth retardation and/or skeletal and skin abnormalities indicating a pivotal role of cholesterol in morphogenesis and embryonic development. The first recognized and most common of these developmental disorders is Smith-Lemli-Opitz syndrome, an autosomal recessive trait caused by mutations in the DHCR7 gene resulting in a deficiency of the encoded sterol Δ7-reductase, alternatively called 7-dehydrocholesterol reductase (EC 1.3.1.21). This enzyme catalyzes the final step in cholesterol biosynthesis, which is the reduction of the Δ7 double bond of 7-dehydrocholesterol to produce cholesterol.
Keywords: Cholesterol biosynthesis; Metabolic disease; Sterol Δ7-reductase; Embryogenesis; Morphogenesis;

The X-linked dominant male-lethal mouse mutations tattered and bare patches are homologous to human X-linked dominant chondrodysplasia punctata and CHILD syndrome, rare human skeletal dysplasias. These disorders also affect the skin and can cause cataracts and microphthalmia in surviving, affected heterozygous females. They have recently been shown to result from mutations in genes encoding enzymes involved in sequential steps in the conversion of lanosterol to cholesterol. This review will summarize clinical features of the disorders and describe recent biochemical and molecular investigations that have resulted in the elucidation of the involved genes and their metabolic pathway. Finally, speculations about possible mechanisms of pathogenesis will be provided.
Keywords: X-Linked; Chondrodysplasia punctata; Congenital hemidysplasia with ichthyosiform erythroderma and limb defects syndrome; Bare patch; Tattered; Cholesterol biosynthesis;