NORMAL GENOMIC

• Yen FT et al.
• Molecular cloning of a lipolysis-stimulated remnant receptor expressed in the liver.
• J Biol Chem. 1999; 274: 13390-8
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• The lipolysis-stimulated receptor (LSR) is a lipoprotein receptor primarily expressed in the liver and activated by free fatty acids. Antibodies inhibiting LSR functions showed that the receptor is a heterotrimer or tetramer consisting of 68-kDa (alpha) and 56-kDa (beta) subunits associated through disulfide bridges. Screening of expression libraries with these antibodies led to identification of mRNAs derived by alternate splicing from a single gene and coding for proteins with molecular masses matching that of LSR alpha and beta. Antibodies directed against a synthetic peptide of LSR alpha and beta putative ligand binding domains inhibited LSR activity. Western blotting identified two liver proteins with the same apparent molecular mass as that of LSR alpha and beta. Transient transfections of LSR alpha alone in Chinese hamster ovary cells increased oleate-induced binding and uptake of lipoproteins, while cotransfection of both LSR alpha and beta increased oleate-induced proteolytic degradation of the particles. The ligand specificity of LSR expressed in cotransfected Chinese hamster ovary cells closely matched that previously described using fibroblasts from subjects lacking the low density lipoprotein receptor. LSR affinity is highest for the triglyceride-rich lipoproteins, chylomicrons, and very low density lipoprotein. We speculate that LSR is a rate-limiting step for the clearance of dietary triglycerides and plays a role in determining their partitioning between the liver and peripheral tissues.

• Dong Y et al.
• Molecular cloning and characterization of LR3, a novel LDL receptor family protein with mitogenic activity.
• Biochem Biophys Res Commun. 1998; 251: 784-90
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• We report molecular cloning and initial functional characterization of a novel member of the low density lipoprotein receptor (LDLR) gene family. The cDNA was isolated from a human osteoblast cDNA library and encoded a 1,615 amino acids protein designated as LR3. It has, in the extracellular region, a cluster of three LDLR ligand binding repeats at a juxtamembrane position and four EGF precursor homology domains separated by YWTD spacer repeats. The entire ectodomain shares the same modular organization with the middle portion of the extracellular regions of two LDLR family members, LDLR-related protein (LRP), and gp330/megalin. LR3 mRNA was expressed in most of the adult and fetal tissues examined. The highest expression level was seen in aorta. In human osteosarcoma cells examined, LR3 mRNA was highly enriched in TE85 cells, moderately expressed in MG63 cells and primary human osteoblasts, and undetectable in SaOS-2 cells. NIH 3T3 cells transfected with either full length LR3 or its ectodomain showed significantly increased proliferation, whereas transfection of intracellular domain had no proliferative effect. We predict that LR3 is a multi-functional protein with potential mitogenic activity.

• Adachi H, Tsujimoto M, Arai H, Inoue K
• Expression cloning of a novel scavenger receptor from human endothelial cells.
• J Biol Chem. 1997; 272: 31217-20
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• Scavenger receptors mediate the endocytosis of chemically modified lipoproteins, such as acetylated low density lipoprotein (Ac-LDL) and oxidized LDL (Ox-LDL), and have been implicated in the pathogenesis of atherosclerosis. The evidence that endothelial cells possess scavenger receptor activity is substantial, and this property is widely used in the isolation of endothelial cells from vascular tissues. In the current study, we have isolated, by expression cloning, the cDNA encoding a novel type of scavenger receptor expressed by endothelial cells (SREC), which mediates the binding and degradation of Ac-LDL. The primary structure of the molecule has no significant homology to other types of scavenger receptors, including the recently cloned endothelial cell Ox-LDL receptor, a member of the C-type lectin family. The cDNA encodes a protein of 830 amino acids with a calculated molecular mass of 85, 735 Da (mature peptide). Chinese hamster ovary cells stably expressing SREC bound 125I-labeled Ac-LDL with high affinity (Kd = 3.0 microg/ml, approximately 1.7 nM) and degraded them via an endocytic pathway. Association of DiII-Ac-LDL were effectively inhibited by Ox-LDL, malondialdehyde-modified LDL, dextran sulfate, and polyinosinic acid, but not by natural LDL and heparin. The cloned receptor has several characteristic domain structures, including an N-terminal extracellular domain with five epidermal growth factor-like cysteine pattern signatures and an unusually long C-terminal cytoplasmic domain (391 amino acids) composed of a Ser/Pro-rich region followed by a Gly-rich region.

• Sawamura T et al.
• An endothelial receptor for oxidized low-density lipoprotein.
• Nature. 1997; 386: 73-7
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• Endothelial dysfunction or activation elicited by oxidatively modified low-density lipoprotein (Ox-LDL) has been implicated in the pathogenesis of atherosclerosis, characterized by intimal thickening and lipid deposition in the arteries. Ox-LDL and its lipid constituents impair endothelial production of nitric oxide, and induce the endothelial expression of leukocyte adhesion molecules and smooth-muscle growth factors, which may be involved in atherogenesis. Vascular endothelial cells in culture and in vivo internalize and degrade Ox-LDL through a putative receptor-mediated pathway that does not involve macrophage scavenger receptors. Here we report the molecular cloning, using expression cloning strategy, of an Ox-LDL receptor from vascular endothelial cells. The cloned receptor is a membrane protein that belongs structurally to the C-type lectin family, and is expressed in vivo in vascular endothelium and vascular-rich organs.

• Sappington TW, Kokoza VA, Cho WL, Raikhel AS
• Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor.
• Proc Natl Acad Sci U S A. 1996; 93: 8934-9
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• The mosquito (Aedes aegypti) vitellogenin receptor (AaVgR) is a large membrane-bound protein (214 kDa when linearized) that mediates internalization of vitellogenin, the major yolk-protein precursor, by oocytes during egg development. We have cloned and sequenced two cDNA fragments encompassing the entire coding region of AaVgR mRNA, to our knowledge the first insect VgR sequence to be reported. The 7.3-kb AaVgR mRNA is present only in female germ-line cells and is abundant in previtellogenic oocytes, suggesting that the AaVgR gene is expressed early in oocyte differentiation. The deduced amino acid sequence predicts a 202.7-kDa protein before posttranslational processing. The AaVgR is a member of the low density lipoprotein receptor superfamily, sharing significant homology with the chicken (Gallus gallus) VgR and particularly the Drosophila melanogaster yolk protein receptor, in spite of a very different ligand for the latter. Distance-based phylogenetic analyses suggest that the insect VgR/yolk protein receptor lineage and the vertebrate VgR/low density lipoprotein receptor lineage diverged before the bifurcation of nematode and deuterostome lines.

• Bujo H, Yamamoto T, Hayashi K, Hermann M, Nimpf J, Schneider WJ
• Mutant oocytic low density lipoprotein receptor gene family member causes atherosclerosis and female sterility.
• Proc Natl Acad Sci U S A. 1995; 92: 9905-9
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• The so-called very low density lipoprotein receptors (VLDLRs) are related to the LDLR gene family. So far, naturally occurring mutations have only been described for the prototype LDLR; in humans, they cause familial hypercholesterolemia. Here we describe a naturally occurring mutation in a VLDLR that causes a dramatic abnormal phenotype. Hens of the mutant restricted-ovulator chicken strain carry a single mutation, lack functional oocyte receptors, are sterile, and display severe hyperlipidemia with associated premature atherosclerosis. The mutation converts a cysteine residue into a serine, resulting in an unpaired cysteine and greatly reduced expression of the mutant avian VLDLR on the oocyte surface. Extraoocytic cells in the mutant produce higher than normal amounts of a differentially spliced form of the receptor that is characteristic for somatic cells but absent from germ cells.

• Jackman RW, Beeler DL, VanDeWater L, Rosenberg RD
• Characterization of a thrombomodulin cDNA reveals structural similarity to the low density lipoprotein receptor.
• Proc Natl Acad Sci U S A. 1986; 83: 8834-8
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• We have isolated a partial-length cDNA for bovine thrombomodulin from a lambda gt11 bovine adrenal capillary endothelial cell expression library. This was accomplished by immunoscreening with rabbit anti-thrombomodulin IgG heteroantibody and then rescreening with the initial positive recombinant insert. The cDNA obtained was authenticated by showing that it coded for the primary structure of two separate regions of bovine thrombomodulin. The nucleotide sequence of the largest cDNA allowed us to establish the structure of about 80% of the mature thrombomodulin transcript, which encodes the C-terminal half of the polypeptide. This membrane component is structurally similar to coated-pit receptors and is organized into domains that resemble those of the low density lipoprotein receptor.