The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids (after removal of 21-amino acid signal peptide) [5] that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor that recognizes apolipoprotein B100 (ApoB100), which is embedded in the outer phospholipid layer of very low-density lipoprotein (VLDL), their remnants—i.e. intermediate-density lipoprotein (IDL), and LDL particles. The receptor also recognizes apolipoprotein E (ApoE) which is found in chylomicron remnants and IDL. In humans, the LDL receptor protein is encoded by the LDLR gene on chromosome 19. [6] [7] [8] It belongs to the low density lipoprotein receptor gene family. [9] It is most significantly expressed in bronchial epithelial cells and adrenal gland and cortex tissue. [10]
Michael S. Brown and Joseph L. Goldstein were awarded the 1985 Nobel Prize in Physiology or Medicine for their identification of LDL-R [11] and its relation to cholesterol metabolism and familial hypercholesterolemia. [12] Disruption of LDL-R can lead to higher LDL-cholesterol as well as increasing the risk of related diseases. Individuals with disruptive mutations (defined as nonsense, splice site, or indel frameshift) in LDLR have an average LDL-cholesterol of 279 mg/dL, compared with 135 mg/dL for individuals with neither disruptive nor deleterious mutations. Disruptive mutations were 13 times more common in individuals with early-onset myocardial infarction or coronary artery disease than in individuals without either disease. [13]
The LDLR gene resides on chromosome 19 at the band 19p13.2 and is split into 18 exons. [8] Exon 1 contains a signal sequence that localises the receptor to the endoplasmic reticulum for transport to the cell surface. Beyond this, exons 2-6 code the ligand binding region; 7-14 code the epidermal growth factor (EGF) domain; 15 codes the oligosaccharide rich region; 16 (and some of 17) code the membrane spanning region; and 18 (with the rest of 17) code the cytosolic domain.
This gene produces 6 isoforms through alternative splicing. [14]
This protein belongs to the LDLR family and is made up of a number of functionally distinct domains, including 3 EGF-like domains, 7 LDL-R class A domains, and 6 LDL-R class B repeats. [14]
The N-terminal domain of the LDL receptor, which is responsible for ligand binding, is composed of seven sequence repeats (~50% identical). Each repeat, referred to as a class A repeat or LDL-A, contains roughly 40 amino acids, including 6 cysteine residues that form disulfide bonds within the repeat. Additionally, each repeat has highly conserved acidic residues which it uses to coordinate a single calcium ion in an octahedral lattice. Both the disulfide bonds and calcium coordination are necessary for the structural integrity of the domain during the receptor's repeated trips to the highly acidic interior of the endosome. The exact mechanism of interaction between the class A repeats and ligand (LDL) is unknown, but it is thought that the repeats act as "grabbers" to hold the LDL. Binding of ApoB requires repeats 2-7 while binding ApoE requires only repeat 5 (thought to be the ancestral repeat).
Next to the ligand binding domain is an EGF precursor homology domain (EGFP domain). This shows approximately 30% homology with the EGF precursor gene. There are three "growth factor" repeats; A, B and C. A and B are closely linked while C is separated by the YWTD repeat region, which adopts a beta-propeller conformation (LDL-R class B domain). It is thought that this region is responsible for the pH-dependent conformational shift that causes bound LDL to be released in the endosome.
A third domain of the protein is rich in O-linked oligosaccharides but appears to show little function. Knockout experiments have confirmed that no significant loss of activity occurs without this domain. It has been speculated that the domain may have ancestrally acted as a spacer to push the receptor beyond the extracellular matrix.
The single transmembrane domain of 22 (mostly) non-polar residues crosses the plasma membrane in a single alpha helix.
The cytosolic C-terminal domain contains ~50 amino acids, including a signal sequence important for localizing the receptors to clathrin-coated pits and for triggering receptor-mediated endocytosis after binding. Portions of the cytosolic sequence have been found in other lipoprotein receptors, as well as in more distant receptor relatives. [15] [16] [17]
Loss-of-function mutations in the gene encoding the LDL receptor are known to cause familial hypercholesterolaemia.
There are 5 broad classes of mutation of the LDL receptor:
Gain-of-function mutations decrease LDL levels and are a target of research to develop a gene therapy to treat refractory hypercholesterolemia. [19]
LDL receptor mediates the endocytosis of cholesterol-rich LDL and thus maintains the plasma level of LDL. [20] This occurs in all nucleated cells, but mainly in the liver which removes ~70% of LDL from the circulation. LDL receptors are clustered in clathrin-coated pits, and coated pits pinch off from the surface to form coated endocytic vesicles that carry LDL into the cell. [21] After internalization, the receptors dissociate from their ligands when they are exposed to lower pH in endosomes. After dissociation, the receptor folds back on itself to obtain a closed conformation and recycles to the cell surface. [22] The rapid recycling of LDL receptors provides an efficient mechanism for delivery of cholesterol to cells. [23] [24] It was also reported that by association with lipoprotein in the blood, viruses such as hepatitis C virus, Flaviviridae viruses and bovine viral diarrheal virus could enter cells indirectly via LDLR-mediated endocytosis. [25] LDLR has been identified as the primary mode of entry for the Vesicular stomatitis virus in mice and humans. [26] In addition, LDLR modulation is associated with early atherosclerosis-related lymphatic dysfunction. [27] Synthesis of receptors in the cell is regulated by the level of free intracellular cholesterol; if it is in excess for the needs of the cell then the transcription of the receptor gene will be inhibited. [28] LDL receptors are translated by ribosomes on the endoplasmic reticulum and are modified by the Golgi apparatus before travelling in vesicles to the cell surface.
In humans, LDL is directly involved in the development of atherosclerosis, which is the process responsible for the majority of cardiovascular diseases, due to accumulation of LDL-cholesterol in the blood [ citation needed ]. Hyperthyroidism may be associated with reduced cholesterol via upregulation of the LDL receptor, and hypothyroidism with the converse. A vast number of studies have described the relevance of LDL receptors in the pathophysiology of atherosclerosis, metabolic syndrome, and steatohepatitis. [29] [30] Previously, rare mutations in LDL-genes have been shown to contribute to myocardial infarction risk in individual families, whereas common variants at more than 45 loci have been associated with myocardial infarction risk in the population. When compared with non-carriers, LDLR mutation carriers had higher plasma LDL cholesterol, whereas APOA5 mutation carriers had higher plasma triglycerides. [31] Recent evidence has connected MI risk with coding-sequence mutations at two genes functionally related to APOA5, namely lipoprotein lipase and apolipoprotein C-III. [32] [33] Combined, these observations suggest that, as well as LDL cholesterol, disordered metabolism of triglyceride-rich lipoproteins contributes to MI risk. Overall, LDLR has a high clinical relevance in blood lipids. [34] [35]
A multi-locus genetic risk score study based on a combination of 27 loci, including the LDLR gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmö Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22). [36]
Click on genes, proteins and metabolites below to link to respective articles. [§ 1]
Cholesterol is the principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.
HMG-CoA reductase is the rate-controlling enzyme of the mevalonate pathway, the metabolic pathway that produces cholesterol and other isoprenoids. HMGCR catalyzes the conversion of HMG-CoA to mevalonic acid, a necessary step in the biosynthesis of cholesterol. Normally in mammalian cells this enzyme is competitively suppressed so that its effect is controlled. This enzyme is the target of the widely available cholesterol-lowering drugs known collectively as the statins, which help treat dyslipidemia.
Michael Stuart Brown ForMemRS NAS AAA&S APS is an American geneticist and Nobel laureate. He was awarded the Nobel Prize in Physiology or Medicine with Joseph L. Goldstein in 1985 for describing the regulation of cholesterol metabolism.
Scavenger receptors are a large and diverse superfamily of cell surface receptors. Its properties were first recorded in 1970 by Drs. Brown and Goldstein, with the defining property being the ability to bind and remove modified low density lipoproteins (LDL). Today scavenger receptors are known to be involved in a wide range of processes, such as: homeostasis, apoptosis, inflammatory diseases and pathogen clearance. Scavenger receptors are mainly found on myeloid cells and other cells that bind to numerous ligands, primarily endogenous and modified host-molecules together with pathogen-associated molecular patterns(PAMPs), and remove them. The Kupffer cells in the liver are particularly rich in scavenger receptors, includes SR-A I, SR-A II, and MARCO.
Apolipoprotein B (ApoB) is a protein that in humans is encoded by the APOB gene. It is commonly used to detect risk of atherosclerotic cardiovascular disease.
In structural biology, a beta-propeller (β-propeller) is a type of all-β protein architecture characterized by 4 to 8 highly symmetrical blade-shaped beta sheets arranged toroidally around a central axis. Together the beta-sheets form a funnel-like active site.
The low-density lipoprotein receptor gene family codes for a class of structurally related cell surface receptors that fulfill diverse biological functions in different organs, tissues, and cell types. The role that is most commonly associated with this evolutionarily ancient family is cholesterol homeostasis. In humans, excess cholesterol in the blood is captured by low-density lipoprotein (LDL) and removed by the liver via endocytosis of the LDL receptor. Recent evidence indicates that the members of the LDL receptor gene family are active in the cell signalling pathways between specialized cells in many, if not all, multicellular organisms.
The very-low-density-lipoprotein receptor (VLDLR) is a transmembrane lipoprotein receptor of the low-density-lipoprotein (LDL) receptor family. VLDLR shows considerable homology with the members of this lineage. Discovered in 1992 by T. Yamamoto, VLDLR is widely distributed throughout the tissues of the body, including the heart, skeletal muscle, adipose tissue, and the brain, but is absent from the liver. This receptor has an important role in cholesterol uptake, metabolism of apolipoprotein E-containing triacylglycerol-rich lipoproteins, and neuronal migration in the developing brain. In humans, VLDLR is encoded by the VLDLR gene. Mutations of this gene may lead to a variety of symptoms and diseases, which include type I lissencephaly, cerebellar hypoplasia, and atherosclerosis.
Foam cells, also called lipid-laden macrophages, are a type of cell that contain cholesterol. These can form a plaque that can lead to atherosclerosis and trigger myocardial infarction and stroke.
Familial hypercholesterolemia (FH) is a genetic disorder characterized by high cholesterol levels, specifically very high levels of low-density lipoprotein cholesterol, in the blood and early cardiovascular diseases. The most common mutations diminish the number of functional LDL receptors in the liver or produce abnormal LDL receptors that never go to the cell surface to function properly. Since the underlying body biochemistry is slightly different in individuals with FH, their high cholesterol levels are less responsive to the kinds of cholesterol control methods which are usually more effective in people without FH. Nevertheless, treatment is usually effective.
Low density lipoprotein receptor-related protein-associated protein 1 also known as LRPAP1 or RAP is a chaperone protein which in humans is encoded by the LRPAP1 gene.
Low-density lipoprotein receptor-related protein 8 (LRP8), also known as apolipoprotein E receptor 2 (ApoER2), is a protein that in humans is encoded by the LRP8 gene. ApoER2 is a cell surface receptor that is part of the low-density lipoprotein receptor family. These receptors function in signal transduction and endocytosis of specific ligands. Through interactions with one of its ligands, reelin, ApoER2 plays an important role in embryonic neuronal migration and postnatal long-term potentiation. Another LDL family receptor, VLDLR, also interacts with reelin, and together these two receptors influence brain development and function. Decreased expression of ApoER2 is associated with certain neurological diseases.
Sterol regulatory element-binding protein cleavage-activating protein, also known as SREBP cleavage-activating protein or SCAP, is a protein that in humans is encoded by the SCAP gene.
The epididymal secretory protein E1, also known as NPC2, is one of two main lysosomal transport proteins that assist in the regulation of cellular cholesterol by exportation of LDL-derived cholesterol from lysosomes. Lysosomes have digestive enzymes that allow it to break down LDL particles to LDL-derived cholesterol once the LDL particle is engulfed into the cell via receptor mediated endocytosis.
Oxidized low-density lipoprotein receptor 1 also known as lectin-type oxidized LDL receptor 1 (LOX-1) is a protein that in humans is encoded by the OLR1 gene.
Low density lipoprotein receptor-related protein 2 also known as LRP-2 or megalin is a protein which in humans is encoded by the LRP2 gene.
Low density lipoprotein receptor-related protein 1 (LRP1), also known as alpha-2-macroglobulin receptor (A2MR), apolipoprotein E receptor (APOER) or cluster of differentiation 91 (CD91), is a protein forming a receptor found in the plasma membrane of cells involved in receptor-mediated endocytosis. In humans, the LRP1 protein is encoded by the LRP1 gene. LRP1 is also a key signalling protein and, thus, involved in various biological processes, such as lipoprotein metabolism and cell motility, and diseases, such as neurodegenerative diseases, atherosclerosis, and cancer.
Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an enzyme encoded by the PCSK9 gene in humans on chromosome 1. It is the 9th member of the proprotein convertase family of proteins that activate other proteins. Similar genes (orthologs) are found across many species. As with many proteins, PCSK9 is inactive when first synthesized, because a section of peptide chains blocks their activity; proprotein convertases remove that section to activate the enzyme. The PCSK9 gene also contains one of 27 loci associated with increased risk of coronary artery disease.
Low-density lipoprotein receptor adapter protein 1 is a protein that in humans is encoded by the LDLRAP1 gene.
YWTD repeats are four-stranded beta-propeller repeats found in low-density lipoprotein receptors (LDLR). The six YWTD repeats together fold into a six-bladed beta-propeller. Each blade of the propeller consists of four antiparallel beta-strands; the innermost strand of each blade is labeled 1 and the outermost strand, 4. The sequence repeats are offset with respect to the blades of the propeller, such that any given 40-residue YWTD repeat spans strands 24 of one propeller blade and strand 1 of the subsequent blade. This offset ensures circularization of the propeller because the last strand of the final sequence repeat acts as an innermost strand 1 of the blade that harbors strands 24 from the first sequence repeat. The repeat is found in a variety of proteins that include, vitellogenin receptor from Drosophila melanogaster, low-density lipoprotein (LDL) receptor, preproepidermal growth factor, and nidogen (entactin).