The low-density lipoprotein receptor family: genetics, function, and evolution

Lipoprotein receptors in ovary of eel, Anguilla australis: molecular characterisation of putative vitellogenin receptors

Lipoprotein receptors, including low-density lipoprotein receptor (LDLr) relatives (Lrs) and LDLr-related proteins (Lrps), belong to the LDLr supergene family and participate in diverse physiological functions. In this study, novel sequences of lr and lrp genes expressed in the ovary of the short-finned eel, Anguilla australis, during early gonadal development are presented. The genes encoding the LDLr-like, Lrp1-like, Lrp1b-like, Lrp3, Lrp4-like, Lrp5-like, Lrp6, Lrp10, Lrp11, Lrp12-like, and Lr11-like proteins were found and identified by sequence and structure analysis, in addition to phylogenetic analysis. Genes encoding proteins previously implicated in follicle development and vitellogenin (Vtg) uptake in oviparous vertebrates were also identified, i.e. lr8 (including lr8 + and lr8- variants) and lrp13; their identification was reinforced by conserved synteny with orthologues in other teleost fish. Compared to other lr/lrp genes, the genes encoding Lr8 + , Lr8-, and Lrp13 were highly expressed in ovary during early development, decreasing as oocyte development advanced when induced by hypophysation. Furthermore, lr8 + , lr8-, and lrp13 were dominantly expressed in the ovary when compared with 17 other tissues. Finally, this study successfully detected the expression of both lr8 variants, which showed different expression patterns to those reported in other oviparous vertebrates and provided the first characterisation of Lrp13 in Anguilla sp. We propose that lr8 + , lr8-, and lrp13 encode putative Vtg receptors in anguillid eels.

Changes in lipid biology during ovarian development in farmed beluga sturgeon, Huso huso L

The present study was conducted to understand key biochemical, physiological, and molecular changes associated with ovarian growth and with lipid transfer and/or accumulation into the ovary during oogenesis in captive beluga sturgeon. Plasma levels of triacylglycerides, cholesterol, phospholipid, and sex steroid hormones were determined and all were found to increase notably throughout development from the perinucleolar to the tertiary yolk stage. Using fast protein liquid chromatography, we recognized three major lipoprotein peaks in chromatograms from all samples. These peaks were characterized as containing very low-density lipoprotein (Vldl), low-density lipoprotein/high-density lipoprotein (Ldl/Hdl), and plasma proteins. While Ldl/Hdl represented the most abundant lipoprotein fraction, the relative abundance of different lipoprotein classes did not change with the stage of oogenesis. Eluted lipoproteins were separated using sodium dodecyl-sulfate polyacrylamide gel electrophoresis and sequenced. The peptide sequence spectra for 66-kDa, 205-kDa, 29-kDa, and 70-kDa bands matched with albumin, vitellogenin (Vtg) AB2b, immunoglobulin light-chain precursor, and immunoglobulin heavy-chain, respectively. The large amount of albumin in the plasma protein peak and the confined presence of Vtg AB2b to within Ldl/Hdl reinforce the lipoprotein classification. Lastly, transcript levels of genes encoding ovarian lipoprotein lipase (lpl), apolipoprotein E (apoe), very low-density lipoprotein receptors (vldlr), and low-density lipoprotein receptor-related protein 8-like (lrp8) were estimated using quantitative RT-PCR. The high mRNA levels of lpl, apoe, and lipoprotein receptors vldlr and lrp8 in previtellogenic females suggest that sturgeon oocytes need to be prepared to accept and traffic Vtg and lipids internally, before the start of vitellogenesis.

Vitellogenin receptor and fatty acid profiles of individual lipid classes of oocytes from wild and captive-reared greater amberjack (Seriola dumerili) during the reproductive cycle

The greater amberjack Seriola dumerili (Risso, 1810) is a large migratory pelagic fish occurring in tropical and temperate waters with a great potential for the world aquaculture industry. Previous studies showed that wild-caught female greater amberjack reared in sea cages and handled during the reproductive season, underwent extensive ovarian atresia. This atresia, however, was not related to an insufficient liver transcription or oocyte uptake of vitellogenin (Vtg). In the present study, the structure of two greater amberjack vitellogenin receptors, namely Vtgr (Lr8-) and Lrp13, was characterized. Moreover, vtgr and lrp13 gene expression and the fatty acid profiles of specific phospholipids and neutral lipids were compared in the ovaries of wild and captive-reared greater amberjack during different phases of the reproductive cycle (i.e. early gametogenesis, advanced gametogenesis and spawning). Ovarian vtgr and lrp13 transcription was more active during early gametogenesis, suggesting that vitellogenin receptor transcripts were synthesized by previtellogenic oocytes and remained in the cellular mRNA pool until oocytes resumed meiosis and entered into secondary growth (i.e. vitellogenesis). Rearing of wild-caught greater amberjack in captivity together with handling during the reproductive season was associated with a reduced vtgr and lrp13 transcription and with a diminished capacity of oocytes in the early phase of gametogenesis (primary oocyte growth) to enter into vitellogenesis. During early gametogenesis, remarkable differences in the fatty acid composition were observed between wild and captive-reared individuals: all phospholipids of captive fish displayed dramatic increases of saturates (16:0 and 18:0) and decreases of arachidonic acid (ARA) and docosahexaenoic acid (DHA). The present study confirms the susceptibility of greater amberjack reproductive function to handling stress and suggests that the consequent extensive atresia of vitellogenic follicles originated during the primary oocytes growth when the capacity of oocytes to synthesize vitellogenin receptors was reduced. The study also suggests that this reduced capacity was associated with an altered oocyte phospholipid fatty acid composition during early gametogenesis.

The Roles of Low-Density Lipoprotein Receptor-Related Proteins 5, 6, and 8 in Cancer: A Review

Low-density lipoprotein receptor (LDLR) has been an object of research since the 1970s because of its role in various cell functions. The LDLR family members include LRP5, LRP6, and LRP8. Even though LRP5, 6, and 8 are in the same family, intriguingly, these three proteins have various roles in physiological events, as well as in regulating different mechanisms in various kinds of cancers. LRP5, LRP6, and LRP8 have been shown to play important roles in a broad panel of cancers. LRP5 is highly expressed in many tissues and is involved in the modulation of glucose-induced insulin secretion, bone development, and cholesterol metabolism, as well as cancer progression. Recently, LRP5 has also been shown to play a role in chondroblastic subtype of osteosarcoma (OS) and prostate cancer and also in noncancer case such as osteoporosis. LRP6, which has been previously discovered to share the same structures as LRP5, has also been associated with many cancer progressions such as human triple negative breast cancer (TNBC), primary chronic lymphocytic leukemia (CLL), nonsmall cell lung cancer (NSCL), lung squamous cell carcinoma (LSCC), and hepatocellular carcinoma (HCC). In addition to its role in cancer progression, LRP8 (apolipoprotein E receptor 2 [APOER2]) has also been demonstrated to regulate canonical Wnt/β-catenin signaling pathway whereby this pathway plays a role in cell migration and development. Therefore, this review aimed to elucidate the role of LRP 5, 6, and 8 in regulating the cancer progression.

Two novel linkage SNPs of VLDLR gene intron 11 are associated with laying traits in two quail populations

Abstract. The very low density lipoprotein receptor (VLDLR) is an important multifunctional receptor and plays a key role in chicken reproduction. This study is designed to investigate the effect of variants in the VLDLR gene on quail laying traits. Two quail populations were studied – yellow feather quail and chestnut feather quail; 384 individuals per breed were used. The laying traits (the weight of the first egg, the age of the first egg, egg weight, the weight of 20-week-old and the egg number of 20-week-old) were measured and recorded. The polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP) method was developed to genotype those individuals. The results showed two novel polymorphisms, i.e. two linkage variations in intron 11 of the VLDLR gene (363T > C and 392C > T). They are associated with the weight of the first egg, the age of the first egg and egg number of 20-week-old in yellow feather and chestnut feather quail. The two variations in intron 11 of VLDLR may be linked with potential major loci or genes affecting some laying traits.

Expression of vitellogenin receptor gene in the ovary of wild and captive Atlantic bluefin tuna (Thunnus thynnus)

The cDNA sequences of vitellogenin receptor proteins (VgR(+) and VgR(-)), containing or lacking the O-linked sugar domain, were determined in Atlantic bluefin tuna (Thunnus thynnus L.). VgR(-) gene expression in the ovary was compared in captive-reared and wild Atlantic bluefin tuna during the reproductive cycle. Gonad samples from adult fish were sampled from 2008 to 2010 from stocks reared in captivity at different commercial fattening operations in the Mediterranean Sea and from wild individuals caught either by traditional tuna traps during their migration towards the spawning grounds in the Mediterranean Sea or by the long-line artisanal fishery. In addition, juvenile male and female Atlantic bluefin tuna were sampled from a farming facility, to obtain baseline information and pre-adulthood amounts of VgR(-). The total length of VgR(+) cDNA was 4006 nucleotides (nt) and that of VgR(-) was 3946 nt. Relative amounts of VgR(-) were greater in juvenile females and in those adults having only previtellogenic oocytes (119 ± 55 and 146 ± 26 folds more than juvenile males, respectively). Amounts of VgR(-) were less in individuals with yolked oocytes (ripening stage, May-June) and increased after spawning in July (92 ± 20 and 113 ± 13 folds more than juvenile males in ripening and post-spawning fish, respectively). These data suggest that regulation of VgR(-) is not under oestrogen control. During the ripening period, greater VgR(-) gene expression was observed in wild fish than in fish reared in captivity, possibly because of (a) differences in water temperature exposure and/or energy storage, and/or (b) an inadequate diet in reared Atlantic bluefin tuna.

High Transcript Level of Fatty Acid-Binding Protein 11 but Not of Very Low-Density Lipoprotein Receptor Is Correlated to Ovarian Follicle Atresia in a Teleost Fish (Solea senegalensis)1

Transcripts encoding a fatty acid-binding protein (FABP), Fabp11, and two isoforms of very low-density lipoprotein receptor (Vldlr; vitellogenin receptor) were characterized from the ovary of Senegalese sole (Solea senegalensis). Phylogenetic analyses of vertebrate FABPs demonstrated that Senegalese sole Fabp11, as zebrafish (Danio rerio) homologous sequences, is part of a newly defined teleost fish FABP subfamily that is a sister clade of tetrapod FABP4/FABP5/FABP8/FABP9. RT-PCR revealed high levels of vldlr transcript splicing variants in the ovaries and, to a lesser extent, in somatic tissues, whereas fabp11 was highly expressed in the ovaries, liver, and adipose tissue. In situ hybridization analysis showed vldlr and fabp11 mRNAs in previtellogenic oocytes, whereas no hybridization signals were detected in the larger vitellogenic oocytes. Transcript expression of fabp11 was strongly upregulated in somatic cells surrounding atretic follicles. Real-time quantitative RT-PCR demonstrated that ovarian transcript levels of vldlr and fabp11 had a significant positive correlation with the percentage of follicles in previtellogenesis and atresia, respectively. These results suggest that the expression level of vldlr transcripts may be used as a precocious functional marker to quantify the number of oocytes recruited for vitellogenesis and that fabp11 mRNA may be a very useful molecular marker for determining cellular events and environmental factors that regulate follicular atresia in fish.

Avian apolipoprotein A-V binds to LDL receptor gene family members

Apolipoprotein A-V (apoA-V) affects plasma triglyceride (TG) levels; however, the properties of apoA-V that mediate its action(s) are still incompletely understood. It is unclear how apoA-V, whose plasma concentration is extremely low, can affect the pronounced TG differences observed in individuals with various apoA-V dysfunctions. To gain novel insights into apoA-V biology, we expanded our previous studies in the chicken to this apolipoprotein. First, we characterized the first avian apoA-V, revealing its expression not only in liver and small intestine but also in brain, kidney, and ovarian follicles and showing its presence in the circulation. Second, we demonstrate directly that galline apoA-V binds to the major LDL receptor family member (LR) of the laying hen and that this interaction does not depend on the association of the apolipoprotein with lipid or lipoproteins. We propose that a direct interaction with LRs may represent a novel, additional mechanism for the modulation of TG levels by apoA-V.

cDNA cloning and transcriptional regulation of the vitellogenin receptor from the imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae)

We describe the cloning of the first hymenopteran vitellogenin receptor (VgR) cDNA from the imported fire ant, Solenopsis invicta, an invasive pest. Using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends, fragments encompassing the entire coding region of a putative VgR were cloned and sequenced. The complete 5764 bp cDNA encodes a 1782 residue protein with a predicted molecular mass of 201.3 kDa (=SiVgR). Northern blot analysis demonstrated that the 7.4 kb SiVgR transcript was present only in ovaries of reproductive females (virgin alates and queens). The temporal profile of transcriptional expression showed that SiVgR mRNA increased with age in virgin alate females and that this was up-regulated by methoprene, a juvenile hormone (JH) analogue. This suggests that the SiVgR gene is JH regulated.

Molecular characterization of the first avian LDL receptor: role in sterol metabolism of ovarian follicular cells

Low levels of expression and sluggish sterol-mediated regulation have been likely reasons for the failure to molecularly characterize a bona fide LDL receptor (LDLR) in egg-laying species to date. The overall structure of the chicken LDLR, delineated here by cDNA cloning, has been conserved in evolution, since hallmark properties of mammalian LDLRs are already present in the avian protein. The chicken receptor appears to prefer LDL over VLDL as ligand, in compliance with its main role in providing lipoprotein-derived cholesterol for steroid production in ovarian follicular cells. This is also compatible with the fact that estrogen administration increased hepatic LDLR expression in roosters despite dramatically stimulated VLDL production. In cultured chicken embryo fibroblasts, expression of the receptor was induced by incubation with cholesterol synthesis inhibitors such as a statin. Furthermore, preincubation of induced cells with a specific anti-receptor antibody blocks LDL endocytosis, demonstrating that the receptor is ligand-endocytosis competent. Finally, the distribution of LDLRs among the extraoocytic cell populations lends support to a three-cell model for estrogen production within the ovarian follicle. In summary, the molecular characterization of the first avian LDLR reveals novel information about evolutionary, structural, and functional aspects of members of the supergene family of LDLR-related proteins.