Nucleotide sequence of the haptoglobin and haptoglobin-related gene pair. The haptoglobin-related gene contains a retrovirus-like element

Simultaneous Genotyping of Three SNVs, rs5471, rs5472, and rs2000999 Involved in Serum Haptoglobin Levels by Fluorescent Probe-Based Melting Curve Analysis

Haptoglobin (Hp) is a hemoglobin-binding acute-phase serum protein. Several single nucleotide variations (SNVs) within the Hp gene (HP) or Hp-related protein gene (HPR), such as rs5471 (A > C) and rs5472 (A > G) in HP promoter region and rs2000999 (G > A) in intron 2 of HRP, are suggested to correlate with the serum Hp levels. To determine these three SNVs simultaneously, a genotyping assay based on duplex dual-labeled fluorescent probes was developed. The method was then validated by analyzing genomic DNA from 121 Ghanaian and two Japanese subjects who had been previously genotyped for rs5471, rs5472, and rs2000999. Both rs5471 and rs5472 could be determined as haplotypes with a single FAM-labeled fluorescent probe, and rs2000999 could be genotyped with a HEX-labeled fluorescent probe. The results obtained with the present method were consistent with the previous results except for those of three Ghanaian subjects. All three subjects appear to have multiple HPR copy number variants characteristic of African populations, which may have led to incorrect results during previous genotyping. This method allows us to genotype these three SNVs in a relatively large number of samples, especially in African populations where rs5471 is uniquely distributed.

Identification and Diagnosis of Complete Haptoglobin Gene Deletion, One of the Genes Responsible for Adverse Posttransfusion Reactions

Allergic reactions are the most frequent adverse events in blood transfusion, and anaphylactic shock, although less frequent, is systemic and serious. The cause of allergic reactions to blood transfusions are largely unknown, but deficiencies in serum proteins such as haptoglobin (Hp) can lead to anaphylactic shock. A complete deletion of the haptoglobin gene (HPdel) was first identified in families with anomalous inheritance and then verified as a genetic variant that can cause anaphylactic shock because homozygotes for HPdel have complete Hp deficiency. Thereby, they may produce antibodies against Hp from blood transfusions. HPdel is found in East and Southeast Asian populations, with a frequency of approximately 0.9% to 4%, but not in other populations. Diagnosis of Hp deficiency due to HPdel prior to transfusion is advisable because severe adverse reactions can be prevented by washing the red blood cells and/or platelets with saline or by administering plasma products obtained from an Hp-deficient donor pool. This review outlines the background of the identification of HPdel and several genetic and immunological methods developed for diagnosing Hp deficiency caused by HPdel.

Duplex dual-labeled fluorescence probe-based melting curve and endpoint genotyping assays for genotyping of rs2000999 and haptoglobin gene deletion

Haptoglobin (Hp) is a hemoglobin-binding serum glycoprotein. Some variations in the Hp gene (HP) or Hp-related gene (HPR), including a single-nucleotide polymorphism in intron 2 of HRP, rs2000999, and a complete deletion of the HP gene (HP^{de l} ), one of the rare variants of HP, have been reported to correlate with the serum cholesterol concentration as well as the serum Hp concentration. In this study, we developed a duplex dual-labeled fluorescence probe-based method to simultaneously determine the rs2000999 G > A polymorphism by melting curve genotyping and the zygosity of HP^{de l} by endpoint genotyping. This method was then validated by using the genomic DNA from 94 Japanese subjects for whom genotypes of rs2000999 and HP^del zygosity had already been determined. The results obtained with this method were in perfect agreement with the previous ones. Thus, the present method enables us to estimate these two polymorphisms in relatively large-scale groups of subjects, especially in Asian populations where the HP^del is distributed.

Re-evalution of the measurement of haptoglobin in human plasma samples

Haptoglobin (Hp) is an abundant plasma protein scavenging hemoglobin (Hb) via CD163 on macrophages. This process consumes Hp, which therefore negatively correlates to hemolysis. However, exact measurements of Hp plasma levels are complicated by different phenotypes (Hp1-1, Hp2-1, and Hp2-2) forming different oligomeric states with differences in immunoreactivity. In addition, humans have an immune-cross-reactive Hp-related protein. In the present study, we developed Hp-specific monoclonal antibodies for an accurate Hp analysis of the different Hp phenotypes in a panel of 112 anonymous samples from hospitalized individuals subjected to routine Hp immunoturbidimetric measurements. The data revealed immunoturbidimetry as a reliable method in most cases but also that the use of non-phenotype-specific calibrators leads to substantial bias in the measurement of the Hp-concentration, non at least in Hp1-1 individuals. Furthermore, analysis of the Hb-dependence of the CD163 interaction with Hp1-1 and Hp2-2 showed that a higher 'cost-effectiveness' in the consumption of dimeric Hp1-1 versus multimeric Hp phenotypes is a likely contribution to the observed differences in the plasma levels of the Hp phenotypes. In conclusion, the determination of Hp phenotype and the use of phenotype-specific calibrators are essential to obtain a precise estimate of the Hp level in healthy and diseased individuals.

Haptoglobin: From hemoglobin scavenging to human health

Haptoglobin (Hp) belongs to the family of acute-phase plasma proteins and represents the most important plasma detoxifier of hemoglobin (Hb). The basic Hp molecule is a tetrameric protein built by two α/β dimers. Each Hp α/β dimer is encoded by a single gene and is synthesized as a single polypeptide. Following post-translational protease-dependent cleavage of the Hp polypeptide, the α and β chains are linked by disulfide bridge(s) to generate the mature Hp protein. As human Hp gene is characterized by two common Hp1 and Hp2 alleles, three major genotypes can result (i.e., Hp1-1, Hp2-1, and Hp2-2). Hp regulates Hb clearance from circulation by the macrophage-specific receptor CD163, thus preventing Hb-mediated severe consequences for health. Indeed, the antioxidant and Hb binding properties of Hp as well as its ability to stimulate cells of the monocyte/macrophage lineage and to modulate the helper T-cell type 1 and type 2 balance significantly associate with a variety of pathogenic disorders (e.g., infectious diseases, diabetes, cardiovascular diseases, and cancer). Alternative functions of the variants Hp1 and Hp2 have been reported, particularly in the susceptibility and protection against infectious (e.g., pulmonary tuberculosis, HIV, and malaria) and non-infectious (e.g., diabetes, cardiovascular diseases and obesity) diseases. Both high and low levels of Hp are indicative of clinical conditions: Hp plasma levels increase during infections, inflammation, and various malignant diseases, and decrease during malnutrition, hemolysis, hepatic disease, allergic reactions, and seizure disorders. Of note, the Hp:Hb complexes display heme-based reactivity; in fact, they bind several ferrous and ferric ligands, including O₂, CO, and NO, and display (pseudo-)enzymatic properties (e.g., NO and peroxynitrite detoxification). Here, genetic, biochemical, biomedical, and biotechnological aspects of Hp are reviewed.

Novel genetic loci associated with long-term deterioration in blood lipid concentrations and coronary artery disease in European adults

Background

Cross-sectional genome-wide association studies have identified hundreds of loci associated with blood lipids and related cardiovascular traits, but few genetic association studies have focused on long-term changes in blood lipids.

Methods

Participants from the GLACIER Study (Nmax = 3492) were genotyped with the MetaboChip array, from which 29 387 SNPs (single nucleotide polymorphisms; replication, fine-mapping regions and wildcard SNPs for lipid traits) were extracted for association tests with 10-year change in total cholesterol (ΔTC) and triglycerides (ΔTG). Four additional prospective cohort studies (MDC, PIVUS, ULSAM, MRC Ely; Nmax = 8263 participants) were used for replication. We conducted an in silico look-up for association with coronary artery disease (CAD) in the Coronary ARtery DIsease Genome-wide Replication and Meta-analysis (CARDIoGRAMplusC4D) Consortium (N ∼ 190 000) and functional annotation for the top ranking variants.

Results

In total, 956 variants were associated (P < 0.01) with either ΔTC or ΔTG in GLACIER. In GLACIER, chr19:50121999 at APOE was associated with ΔTG and multiple SNPs in the APOA1/A4/C3/A5 region at genome-wide significance (P < 5 × 10-8), whereas variants in four loci, DOCK7, BRE, SYNE1 and KCNIP1, reached study-wide significance (P < 1.7 × 10-6). The rs7412 variant at APOE was associated with ΔTC in GLACIER (P < 1.7 × 10-6). In pooled analyses of all cohorts, 139 SNPs at six and five loci were associated with ΔTC and for ΔTG, respectively (P < 10-3). Of these, a variant at CAPN3 (P = 1.2 × 10-4), multiple variants at HPR (Pmin = 1.5 × 10-6) and a variant at SIX5 (P = 1.9 × 10-4) showed evidence for association with CAD.

Conclusions

We identified seven novel genomic regions associated with long-term changes in blood lipids, of which three also raise CAD risk.

Haptoglobin

Haptoglobin (Hp) is an abundant human plasma protein that tightly captures hemoglobin (Hb) during hemolysis. The Hb-Hp complex formation reduces the oxidative properties of heme/Hb and promotes recognition by the macrophage scavenger receptor CD163. This leads to Hb-Hp breakdown and heme catabolism by heme oxygenase and biliverdin reductase. Gene duplications of a part of or the entire Hp gene in the primate evolution have led to variant Hp gene products that collectively may be designated "the haptoglobins (Hps)" as they all bind Hb. These variant products include the human-specific multimeric Hp phenotypes in individuals, which are hetero- or homozygous for an Hp² gene allele. The Hp-related protein (Hpr) is another Hp duplication product in humans and other primates. Alternative functions of the variant Hps are indicated by numerous reports on association between Hp phenotypes and disease as well as the elucidation of a specific role of Hpr in the innate immune defense. Recent Advances: Recent functional and structural information on Hp and receptor systems for Hb removal now provides insight on how Hp carries out essential functions such as the Hb detoxification/removal, and how Hpr, by acting as an Hp-lookalike, can sneak a lethal toxin into trypanosome parasites that cause mammalian sleeping sickness. Critical Issues and Future Directions: The new structural insight may facilitate ongoing attempts of developing Hp derivatives for prevention of Hb toxicity in hemolytic diseases such as sickle cell disease and other hemoglobinopathies. Furthermore, the new structural knowledge may help identifying yet unknown functions based on other disease-relevant biological interactions involving Hps. Antioxid. Redox Signal. 26, 814-831.

Immune Evasion Strategies of Trypanosoma brucei within the Mammalian Host: Progression to Pathogenicity

The diseases caused by African trypanosomes (AT) are of both medical and veterinary importance and have adversely influenced the economic development of sub-Saharan Africa. Moreover, so far not a single field applicable vaccine exists, and chemotherapy is the only strategy available to treat the disease. These strictly extracellular protozoan parasites are confronted with different arms of the host's immune response (cellular as well as humoral) and via an elaborate and efficient (vector)-parasite-host interplay they have evolved efficient immune escape mechanisms to evade/manipulate the entire host immune response. This is of importance, since these parasites need to survive sufficiently long in their mammalian/vector host in order to complete their life cycle/transmission. Here, we will give an overview of the different mechanisms AT (i.e. T. brucei as a model organism) employ, comprising both tsetse fly saliva and parasite-derived components to modulate host innate immune responses thereby sculpturing an environment that allows survival and development within the mammalian host.

Molecular functions of human endogenous retroviruses in health and disease

Human endogenous retroviruses (HERVs) and related genetic elements form 504 distinct families and occupy ~8% of human genome. Recent success of high-throughput experimental technologies facilitated understanding functional impact of HERVs for molecular machinery of human cells. HERVs encode active retroviral proteins, which may exert important physiological functions in the body, but also may be involved in the progression of cancer and numerous human autoimmune, neurological and infectious diseases. The spectrum of related malignancies includes, but not limits to, multiple sclerosis, psoriasis, lupus, schizophrenia, multiple cancer types and HIV. In addition, HERVs regulate expression of the neighboring host genes and modify genomic regulatory landscape, e.g., by providing regulatory modules like transcription factor binding sites (TFBS). Indeed, recent bioinformatic profiling identified ~110,000 regulatory active HERV elements, which formed at least ~320,000 human TFBS. These and other peculiarities of HERVs might have played an important role in human evolution and speciation. In this paper, we focus on the current progress in understanding of normal and pathological molecular niches of HERVs, on their implications in human evolution, normal physiology and disease. We also review the available databases dealing with various aspects of HERV genetics.

Plasma clearance of hemoglobin and haptoglobin in mice and effect of CD163 gene targeting disruption

AIM

In humans, plasma haptoglobin (Hp) and the macrophage receptor CD163 promote a fast scavenging of hemoglobin (Hb). In the present study, we have compared the mouse and human CD163-mediated binding and uptake of Hb and HpHb complex in vitro and characterized the CD163-mediated plasma clearance of Hb in CD163 gene knockout mice and controls.

RESULTS

Contrary to human Hp, mouse Hp did not promote high-affinity binding to CD163. This difference between mouse and man was evident both by analysis of the binding of purified proteins and by ligand uptake studies in CD163-transfected cells. Plasma clearance studies in mice showed a fast clearance (half-life few minutes) of fluorescently labeled mouse Hb with the highest uptake in the kidney and liver. HPLC analysis of serum showed that the clearance curve exhibited a two-phase decay with a faster clearance of Hb than plasma-formed HpHb. In CD163-deficient mice, the overall clearance of Hb was slightly slower and followed a one-phase decay.

INNOVATION AND CONCLUSION

In conclusion, mouse Hp does not promote high-affinity binding of mouse Hb to CD163, and noncomplexed mouse Hb has a higher CD163 affinity than human Hb has. Moreover, CD163-mediated uptake in mice seems to only account for a part of the Hb clearance. The new data further underscore the fact that the Hp system in man seems to have a broader and more sophisticated role. This has major implications in the translation of data on Hb metabolism from mouse to man.

CD163 binding to haptoglobin-hemoglobin complexes involves a dual-point electrostatic receptor-ligand pairing

Formation of the haptoglobin (Hp)-hemoglobin (Hb) complex in human plasma leads to a high affinity recognition by the endocytic macrophage receptor CD163. A fast segregation of Hp-Hb from CD163 occurs at endosomal conditions (pH <6.5). The ligand binding site of CD163 has previously been shown to involve the scavenger receptor cysteine-rich (SRCR) domain 3. This domain and the adjacent SRCR domain 2 of CD163 contain a consensus motif for a calcium-coordinated acidic amino acid triad cluster as originally identified in the SRCR domain of the scavenger receptor MARCO. Here we show that site-directed mutagenesis in each of these acidic triads of SRCR domains 2 and 3 abrogates the high affinity binding of recombinant CD163 to Hp-Hb. In the ligand, Hp Arg-252 and Lys-262, both present in a previously identified CD163 binding loop of Hp, were revealed as essential residues for the high affinity receptor binding. These findings are in accordance with pairing of the calcium-coordinated acidic clusters in SRCR domains 2 and 3 with the two basic Arg/Lys residues in the Hp loop. Such a two-point electrostatic pairing is mechanistically similar to the pH-sensitive pairings disclosed in crystal structures of ligands in complex with tandem LDL receptor repeats or tandem CUB domains in other endocytic receptors.

Individual variation in levels of haptoglobin-related protein in children from Gabon

Background

Haptoglobin related protein (Hpr) is a key component of trypanosome lytic factors (TLF), a subset of high-density lipoproteins (HDL) that form the first line of human defence against African trypanosomes. Hpr, like haptoglobin (Hp) can bind to hemoglobin (Hb) and it is the Hpr-Hb complexes which bind to these parasites allowing uptake of TLF. This unique form of innate immunity is primate-specific. To date, there have been no population studies of plasma levels of Hpr, particularly in relation to hemolysis and a high prevalence of ahaptoglobinemia as found in malaria endemic areas.

Methods and Principal Findings

We developed a specific enzyme-linked immunosorbent assay to measure levels of plasma Hpr in Gabonese children sampled during a period of seasonal malaria transmission when acute phase responses (APR), malaria infection and associated hemolysis were prevalent. Median Hpr concentration was 0.28 mg/ml (range 0.03–1.1). This was 5-fold higher than that found in Caucasian children (0.049 mg/ml, range 0.002–0.26) with no evidence of an APR. A general linear model was used to investigate associations between Hpr levels, host polymorphisms, parasitological factors and the acute phase proteins, Hp, C-reactive protein (CRP) and albumin. Levels of Hpr were associated with Hp genotype, decreased with age and were higher in females. Hpr concentration was strongly correlated with that of Hp, but not CRP.

Conclusions/Significance

Individual variation in Hpr levels was related to Hp level, Hp genotype, demographics, malaria status and the APR. The strong correlations between plasma levels of Hp and Hpr suggest that they are regulated by similar mechanisms. These population-based observations indicate that a more dynamic view of the relative roles of Hpr and Hpr-Hb complexes needs to be considered in understanding innate immunity to African trypanosomes and possibly other pathogens including the newly discovered Plasmodium spp of humans and primates.

Trypanosome resistance to human innate immunity: targeting Achilles' heel

Trypanosome lytic factors (TLFs) are powerful, naturally occurring toxins in humans that provide sterile protection against infection by several African trypanosomes. These trypanocidal complexes predominantly enter the parasite by binding to the trypanosome haptoglobin/hemoglobin receptor (HpHbR), trafficking to the lysosome, causing membrane damage and, ultimately, cell lysis. Despite TLF-mediated immunity, the parasites that cause human African Trypanosomiasis (HAT), Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense, have developed independent mechanisms of resistance to TLF killing. In this review we describe the parasite defenses that allow trypanosome infections of humans and discuss how targeting these apparent strengths of the parasite may reveal their Achilles' heel, leading to new approaches in the treatment of HAT.

A genome-wide association study identifies rs2000999 as a strong genetic determinant of circulating haptoglobin levels

Haptoglobin is an acute phase inflammatory marker. Its main function is to bind hemoglobin released from erythrocytes to aid its elimination, and thereby haptoglobin prevents the generation of reactive oxygen species in the blood. Haptoglobin levels have been repeatedly associated with a variety of inflammation-linked infectious and non-infectious diseases, including malaria, tuberculosis, human immunodeficiency virus, hepatitis C, diabetes, carotid atherosclerosis, and acute myocardial infarction. However, a comprehensive genetic assessment of the inter-individual variability of circulating haptoglobin levels has not been conducted so far.

We used a genome-wide association study initially conducted in 631 French children followed by a replication in three additional European sample sets and we identified a common single nucleotide polymorphism (SNP), rs2000999 located in the Haptoglobin gene (HP) as a strong genetic predictor of circulating Haptoglobin levels (P_overall = 8.1×10⁻⁵⁹), explaining 45.4% of its genetic variability (11.8% of Hp global variance). The functional relevance of rs2000999 was further demonstrated by its specific association with HP mRNA levels (β = 0.23±0.08, P = 0.007). Finally, SNP rs2000999 was associated with decreased total and low-density lipoprotein cholesterol in 8,789 European children (P_{total cholesterol} = 0.002 and P_LDL = 0.0008).

Given the central position of haptoglobin in many inflammation-related metabolic pathways, the relevance of rs2000999 genotyping when evaluating haptoglobin concentration should be further investigated in order to improve its diagnostic/therapeutic and/or prevention impact.

Glycoproteomic analysis and molecular modeling of haptoglobin multimers

Extra-thiol groups on the α-subunit allow haptoglobin (Hp) to form a variety of native multimers which influence the biophysical and biological properties of Hp. In this work, we demonstrated how differences of multimeric conformation alter the glycosylation of Hp. The isoform distributions of different multimers were examined by an alternative approach, i.e. 3-D-(Native/IEF/SDS)-PAGE, which revealed differences in N-glycosylation among individual multimers of the same Hp sample. Glycomic mapping of permethylated N-glycan indicated that the assembled monomer and multimeric conformation modulate the degree of glycosylation, especially the reduction in terminal sialic acid residues on the bi-antennary glycan. Loss of the terminal sialic acid in the higher order multimers increases the number of terminal galactose residues, which may contribute to conformation of Hp. A molecular model of the glycosylated Hp multimer was constructed, suggesting that the effect of steric hindrance on multimeric formation is critical for the enlargement of the glycan moieties on either side of the monomer. In addition, N241 of Hp was partially glycosylated, even though this site is unaffected by steric consideration. Thus, the present study provides evidence for the alteration of glycan structures on different multimeric conformations of Hp, improving our knowledge of conformation-dependent function of this glycoprotein.

The trypanolytic factor of human serum: many ways to enter the parasite, a single way to kill

Humans have developed a particular innate immunity system against African trypanosomes, and only two Trypanosoma brucei clones (T. b. gambiense, T. b. rhodesiense) can resist this defence and cause sleeping sickness. The main players of this immunity are the primate-specific apolipoprotein L-I (apoL1) and haptoglobin-related protein (Hpr). These proteins are both associated with two serum complexes, a minor subfraction of HDLs and an IgM/apolipoprotein A-I (apoA1) complex, respectively, termed trypanosome lytic factor (TLF) 1 and TLF2. Although the two complexes appear to lyse trypanosomes by the same mechanism, they enter the parasite through various modes of uptake. In case of TLF1 one uptake process was characterized. When released in the circulation, haemoglobin (Hb) binds to Hpr, hence to TLF1. In turn the TLF1-Hpr-Hb complex binds to the trypanosome haptoglobin (Hp)-Hb receptor, whose original function is to ensure haem uptake for optimal growth of the parasite. This binding triggers efficient uptake of TLF1 and subsequent trypanosome lysis. While Hpr is involved as TLF ligand, the lytic activity is due to apoL1, a Bcl-2-like pore-forming protein. We discuss the in vivo relevance of this uptake pathway in the context of other potentially redundant delivery routes.

Hemoglobin and heme scavenger receptors

Heme, the functional group of hemoglobin, myoglobin, and other hemoproteins, is a highly toxic substance when it appears in the extracellular milieu. To circumvent potential harmful effects of heme from hemoproteins released during physiological or pathological cell damage (such as hemolysis and rhabdomyolysis), specific high capacity scavenging systems have evolved in the mammalian organism. Two major systems, which essentially function in a similar way by means of a circulating latent plasma carrier protein that upon ligand binding is recognized by a receptor, are represented by a) the hemoglobin-binding haptoglobin and the receptor CD163, and b) the heme-binding hemopexin and the receptor low density lipoprotein receptor-related protein/CD91. Apart from the disclosure of the molecular basis for these important heme scavenging systems by identifying the functional link between the carrier proteins and the respective receptors, research over the last decade has shown how these systems, and the metabolic pathways they represent, closely relate to inflammation and other biological events.

Unique functions of repetitive transcriptomes

Repetitive sequences occupy a huge fraction of essentially every eukaryotic genome. Repetitive sequences cover more than 50% of mammalian genomic DNAs, whereas gene exons and protein-coding sequences occupy only ~3% and 1%, respectively. Numerous genomic repeats include genes themselves. They generally encode "selfish" proteins necessary for the proliferation of transposable elements (TEs) in the host genome. The major part of evolutionary "older" TEs accumulated mutations over time and fails to encode functional proteins. However, repeats have important functions also on the RNA level. Repetitive transcripts may serve as multifunctional RNAs by participating in the antisense regulation of gene activity and by competing with the host-encoded transcripts for cellular factors. In addition, genomic repeats include regulatory sequences like promoters, enhancers, splice sites, polyadenylation signals, and insulators, which actively reshape cellular transcriptomes. TE expression is tightly controlled by the host cells, and some mechanisms of this regulation were recently decoded. Finally, capacity of TEs to proliferate in the host genome led to the development of multiple biotechnological applications.

Retroelements and their impact on genome evolution and functioning

Retroelements comprise a considerable fraction of eukaryotic genomes. Since their initial discovery by Barbara McClintock in maize DNA, retroelements have been found in genomes of almost all organisms. First considered as a "junk DNA" or genomic parasites, they were shown to influence genome functioning and to promote genetic innovations. For this reason, they were suggested as an important creative force in the genome evolution and adaptation of an organism to altered environmental conditions. In this review, we summarize the up-to-date knowledge of different ways of retroelement involvement in structural and functional evolution of genes and genomes, as well as the mechanisms generated by cells to control their retrotransposition.

Classification and nomenclature of endogenous retroviral sequences (ERVs): problems and recommendations

The genomes of many species are crowded with repetitive mobile sequences. In the case of endogenous retroviruses (ERVs) there is, for various reasons, considerable confusion regarding names assigned to families/groups of ERVs as well as individual ERV loci. Human ERVs have been studied in greater detail, and naming of HERVs in the scientific literature is somewhat confusing not just to the outsider. Without guidelines, confusion for ERVs in other species will also probably increase if those ERVs are studied in greater detail. Based on previous experience, this review highlights some of the problems when naming and classifying ERVs, and provides some guidance for detecting and characterizing ERV sequences. Because of the close relationship between ERVs and exogenous retroviruses (XRVs) it is reasonable to reconcile their classification with that of XRVs. We here argue that classification should be based on a combination of similarity, structural features, (inferred) function, and previous nomenclature. Because the RepBase system is widely employed in genome annotation, RepBase designations should be considered in further taxonomic efforts. To lay a foundation for a phylogenetically based taxonomy, further analyses of ERVs in many hosts are needed. A dedicated, permanent, international consortium would best be suited to integrate and communicate our current and future knowledge on repetitive, mobile elements in general to the scientific community.

Receptor targeting of hemoglobin mediated by the haptoglobins: roles beyond heme scavenging

Haptoglobin, the haptoglobin-hemoglobin receptor CD163, and the heme oxygenase-1 are proteins with a well-established function in the clearance and metabolism of "free" hemoglobin released during intravascular hemolysis. This scavenging system counteracts the potentially harmful oxidative and NO-scavenging effects associated with "free" hemoglobin, and, furthermore, elicits an anti-inflammatory response. In the late primate evolution, haptoglobin variants with distinct functions have arisen, including haptoglobin polymers and the haptoglobin-related protein. The latter associates with a subspecies of high-density lipoprotein (HDL) particles playing a crucial role in the innate immunity against certain trypanosome parasites. Recent studies have elucidated this fairly sophisticated immune defense mechanism that takes advantage of a trypanosomal haptoglobin-hemoglobin receptor evolved to supply the parasite with heme. Because of the high resemblance between haptoglobin and haptoglobin-related protein, the receptor also takes up the complex of hemoglobin and the HDL-bound haptoglobin-related protein. This tricks the parasite into internalizing another HDL-associated protein and toxin, apolipoprotein L-I, that kills the parasite. In conclusion, variant human homologous hemoglobin-binding proteins that collectively may be designated the haptoglobins have diverted from the haptoglobin gene. On hemoglobin and receptor interaction, these haptoglobins contribute to different biologic events that go beyond simple removal from plasma of the toxic hemoglobin.

Membrane permeabilization by trypanosome lytic factor, a cytolytic human high density lipoprotein

Trypanosome lytic factor (TLF) is a subclass of human high density lipoprotein (HDL) that mediates an innate immune killing of certain mammalian trypanosomes, most notably Trypanosoma brucei brucei, the causative agent of a wasting disease in cattle. Mechanistically, killing is initiated in the lysosome of the target trypanosome where the acidic pH facilitates a membrane-disrupting activity by TLF. Here we utilize a model liposome system to characterize the membrane binding and permeabilizing activity of TLF and its protein constituents, haptoglobin-related protein (Hpr), apolipoprotein L-1 (apoL-1), and apolipoprotein A-1 (apoA-1). We show that TLF efficiently binds and permeabilizes unilamellar liposomes at lysosomal pH, whereas non-lytic human HDL exhibits inefficient permeabilizing activity. Purified, delipidated Hpr and apoL-1 both efficiently permeabilize lipid bilayers at low pH. Trypanosome lytic factor, apoL-1, and apoA-1 exhibit specificity for anionic membranes, whereas Hpr permeabilizes both anionic and zwitterionic membranes. Analysis of the relative particle sizes of susceptible liposomes reveals distinctly different membrane-active behavior for native TLF and the delipidated protein components. We propose that lysosomal membrane damage in TLF-susceptible trypanosomes is initiated by the stable association of the TLF particle with the lysosomal membrane and that this is a property unique to this subclass of human HDL.

Haptoglobin phenotype is not a predictor of recurrence free survival in high-risk primary breast cancer patients

Background

Better breast cancer prognostication may improve selection of patients for adjuvant therapy. We conducted a retrospective follow-up study in which we investigated sera of high-risk primary breast cancer patients, to search for proteins predictive of recurrence free survival.

Methods

Two sample sets of high-risk primary breast cancer patients participating in a randomised national trial investigating the effectiveness of high-dose chemotherapy were analysed. Sera in set I (n = 63) were analysed by surface enhanced laser desorption ionisation time-of-flight mass spectrometry (SELDI-TOF MS) for biomarker finding. Initial results were validated by analysis of sample set II (n = 371), using one-dimensional gel-electrophoresis.

Results

In sample set I, the expression of a peak at mass-to-charge ratio 9198 (relative intensity ≤ 20 or > 20), identified as haptoglobin (Hp) alpha-1 chain, was strongly associated with recurrence free survival (global Log-rank test; p = 0.0014). Haptoglobin is present in three distinct phenotypes (Hp 1-1, Hp 2-1, and Hp 2-2), of which only individuals with phenotype Hp 1-1 or Hp 2-1 express the haptoglobin alpha-1 chain. As the expression of the haptoglobin alpha-1 chain, determined by SELDI-TOF MS, corresponds to the phenotype, initial results were validated by haptoglobin phenotyping of the independent sample set II by native one-dimensional gel-electrophoresis. With the Hp 1-1 phenotype as the reference category, the univariate hazard ratio for recurrence was 0.87 (95% CI: 0.56 – 1.34, p = 0.5221) and 1.03 (95% CI: 0.65 – 1.64, p = 0.8966) for the Hp 2-1 and Hp 2-2 phenotypes, respectively, in sample set II.

Conclusion

In contrast to our initial results, the haptoglobin phenotype was not identified as a predictor of recurrence free survival in high-risk primary breast cancer in our validation set. Our initial observation in the discovery set was probably the result of a type I error (i.e. false positive). This study illustrates the importance of validation in obtaining the true clinical applicability of a potential biomarker.

TaqMan-based real-time PCR for genotyping common polymorphisms of haptoglobin (HP1 and HP2)

BACKGROUND

The haptoglobin gene (HP) has 2 common codominant alleles (HP(1) and HP(2)) that account for 3 phenotypes. HP(2) is generated by a 1.7-kb intragenic duplication of HP(1).

METHODS

We used the real-time TaqMan PCR system to develop an effective method for HP genotyping that allows us to evaluate the relative number of copies of the HP(2) allele-specific junctional region of the 1.7-kb gene duplication (HP2) by comparing the intensity of the amplification signals to those of the HP promoter region (HP5'), which was used as the internal control. The difference in threshold cycles (DeltaCt) between HP2 and HP5' was used to assess HP(2) copy number. In addition, the assay detects the HP deletion (HP(del)) at the same time.

RESULTS

The mean 2(-DeltaDeltaCt) values (the HP2/HP5' ratio) obtained from 123 samples of known HP genotypes clearly differentiated 2 nonoverlapping intervals that correspond to the HP genotypes. Ratios for HP(2)/HP(1) samples ranged from 0.34-0.50, HP(2)/HP(2) samples ranged from 0.79-0.98, and the absence of an HP(2) allele signal was defined as HP(1)/HP(1). We simultaneously detected HP(del). The assay produces results in <1 h.

CONCLUSIONS

The TaqMan-based real-time PCR method was successfully applied to HP genotyping. The method is easy to use in a molecular diagnosis laboratory, and its robustness and rapidity make it suitable for high-throughput analysis of large populations.

Acute appendicitis is characterized by a uniform and highly selective pattern of inflammatory gene expression

Acute appendicitis (AA) is the most common life-threatening surgical emergency in pediatrics. To characterize the nature of the inflammatory response in AA, gene expression profiles were generated. We found remarkable uniformity in the genes that were differentially expressed between patients with appendicitis and control groups. Sixty-four probe sets were differentially expressed in samples from patients with both severe and mild appendicitis compared to control samples, and within this group we were able to identify four dominant clusters. Interestingly, expression levels of interleukin (IL)-8 significantly correlated with histologic score, and expression of IL-8 protein was observed within both neutrophils and mononuclear cells by immunohistochemistry, suggesting a possible role in the etiology of appendicitis. Although there was some overlap between genes reported to be differentially expressed in Crohn's disease (CD) and those observed in AA, differential expression of genes involved in interferon responses that characterize CD was not observed.

A haptoglobin-hemoglobin receptor conveys innate immunity to Trypanosoma brucei in humans

The protozoan parasite Trypanosoma brucei is lysed by apolipoprotein L-I, a component of human high-density lipoprotein (HDL) particles that are also characterized by the presence of haptoglobin-related protein. We report that this process is mediated by a parasite glycoprotein receptor, which binds the haptoglobin-hemoglobin complex with high affinity for the uptake and incorporation of heme into intracellular hemoproteins. In mice, this receptor was required for optimal parasite growth and the resistance of parasites to the oxidative burst by host macrophages. In humans, the trypanosome receptor also recognized the complex between hemoglobin and haptoglobin-related protein, which explains its ability to capture trypanolytic HDLs. Thus, in humans the presence of haptoglobin-related protein has diverted the function of the trypanosome haptoglobin-hemoglobin receptor to elicit innate host immunity against the parasite.

The transcriptional activity of HERV-I LTR is negatively regulated by its cis-elements and wild type p53 tumor suppressor protein

Human endogenous retroviruses (HERVs), abundantly inter-dispersed in the genome, carry long terminal repeats (LTRs) that may potentially retro-transpose to new genomic sites and deregulate the neighboring cellular genes. However, normally HERVs are either structurally defective or inactive due possibly to stringent negative control mechanisms. To study the possible negative regulation of HERV, we isolated the LTR of RTVL-Ia and constructed site-specific mutations for analysis of the promoter and enhancer functions by using chloramphenicol acetyl transferase (CAT) reporter assay. Our results showed that in most transfected human cells the LTR-mediated CAT expression was negligible unless a sequence segment at the AGTAAA polyadenylation site was deleted. In addition, we have found that the wild type p53 may inhibit whereas a p53 mutant (V143A) stimulate the transcriptional activity of HERV-I LTR. Our results imply that HERV-I LTR, while under negative control by its LTR cis-elements and by wild type p53, may become active upon p53 mutation.

A unique loop extension in the serine protease domain of haptoglobin is essential for CD163 recognition of the haptoglobin-hemoglobin complex

Haptoglobin and haptoglobin-related protein are homologous hemoglobin-binding proteins consisting of a complement control repeat (alpha-chain) and a serine protease domain (beta-chain). Haptoglobin-hemoglobin complex formation promotes high affinity binding of hemoglobin to the macrophage scavenger receptor CD163 leading to endocytosis and degradation of the haptoglobin-hemoglobin complex. In contrast, complex formation between haptoglobin-related protein and hemoglobin does not promote high affinity interaction with CD163. To define structural components of haptoglobin important for CD163 recognition, we exploited this functional difference to design and analyze recombinant haptoglobin/haptoglobin-related protein chimeras complexed to hemoglobin. These data revealed that only the beta-chain of haptoglobin is involved in receptor recognition. Substitution of 4 closely spaced amino acid residues of the haptoglobin beta-chain (valine 259, glutamate 261, lysine 262, and threonine 264) abrogated the high affinity receptor binding. The 4 residues are encompassed by a part of the primary structure not present in other serine protease domain proteins. Structural modeling based on the well characterized serine protease domain fold suggests that this sequence represents a loop extension unique for haptoglobin and haptoglobin-related protein. A synthetic peptide representing the haptoglobin loop sequence exhibited a pronounced inhibitory effect on receptor binding of haptoglobin-hemoglobin.

Haptoglobin-related protein is a high-affinity hemoglobin-binding plasma protein

Haptoglobin-related protein (Hpr) is a primate-specific plasma protein associated with apolipoprotein L-I (apoL-I)-containing high-density lipoprotein (HDL) particles shown to be a part of the innate immune defense. Despite the assumption hitherto that Hpr does not bind to hemoglobin, the present study revealed that recombinant Hpr binds hemoglobin as efficiently as haptoglobin (Hp). However, in contrast to Hp, Hpr did not promote any high-affinity binding to the scavenger receptor CD163. Binding of hemoglobin to circulating native Hpr incorporated into the HDL fraction was indicated by hemoglobin-affinity precipitation of plasma Hpr together with apoL-I. In conclusion, plasma has 2 high-affinity hemoglobin-binding haptoglobins instead of one, but only Hp-hemoglobin complexes are efficiently recognized by CD163. Circulating Hpr-bound hemoglobin should therefore be taken into consideration when measuring "free" plasma hemoglobin. Furthermore, Hpr-bound hemoglobin might contribute to the biologic activity of the circulating apoL-I/Hpr-containing HDL particles.

Endogenous retrovirus HERV-I LTR family in primates: sequences, phylogeny, and evolution

A human endogenous retrovirus (HERV-I; RTVL-I) has been located within the first intron of a haptoglobin-related gene. Two members of the HERV-I family were identified in proximal Yq11.2 and caused AZFa microdeletions as a result of intra-chromosomal recombination events in azoospermic patients. Using PCR25 and the sequencing approach with the genomic DNAs of primates, hominoids, Old and New World monkeys, and prosimians, the HERV-I LTR elements were identified and analysed. The LTR elements were detected only in the hominoids and the Old World monkeys, indicating that the HERV-I LTR elements were inserted into the primate genome after the split of the New World monkeys in the Oligocene era, about 33 million years ago. Nineteen members of the HERV-I LTR elements from the hominoids and the Old World monkeys showed multiple insertions or deletions. They showed a 78.6-97.4% sequence similarity to that of Hu-15 (accession no. AF290422; HERV-I LTR on human Yq11.2). The evolutionary relationships within the HERV-I LTR family among hominoids and Old World monkeys showed a random cluster, indicating that HERV-I LTR elements have evolved independently in primate evolution.

Human High Density Lipoproteins Are Platforms for the Assembly of Multi-component Innate Immune Complexes

Human innate immunity to non-pathogenic species of African trypanosomes is provided by human high density lipoprotein (HDL) particles. Here we show that native human HDLs containing haptoglobin-related protein (Hpr), apolipoprotein L-I (apoL-I) and apolipoprotein A-I (apoA-I) are the principle antimicrobial molecules providing protection from trypanosome infection. Other HDL subclasses containing either apoA-I and apoL-I or apoA-I and Hpr have reduced trypanolytic activity, whereas HDL subclasses lacking apoL-I and Hpr are non-toxic to trypanosomes. Highly purified, lipid-free Hpr and apoL-I were both toxic to Trypanosoma brucei brucei but with specific activities at least 500-fold less than those of native HDLs, suggesting that association of these apolipoproteins within the HDL particle was necessary for optimal cytotoxicity. These studies show that HDLs can serve as platforms for the assembly of multiple synergistic proteins and that these assemblies may play a critical role in the evolution of primate-specific innate immunity to trypanosome infection.

Characterization of primate trypanosome lytic factors

Humans are one of the few species that resist infection by Trypanosoma brucei brucei because the parasites are killed by lytic factors found in human serum. Trypanosome lytic factors (TLFs) are protein/lipid complexes that contain apolipoprotein A-I (apoA-I), and are therefore a class of high density lipoproteins (HDLs). Haptoglobin-related protein (Hpr) is a unique protein component of TLFs, and its expression has only been demonstrated in humans. Trypanolytic activity has only been found in the sera of five primates: humans, gorillas, mandrills, baboons and sooty mangabeys. We describe here previously unidentified components of highly purified human TLF1: apolipoprotein L-I (apoL-I), human cathelicidin antimicrobial peptide 18 (hCAP18) and glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD). However, we found that hCAP18 and GPI-PLD, along with apoA-I, are common components of both lytic and non-lytic primate HDLs. In contrast, Hpr, which has been previously implicated as the main lytic component of TLF1, was a unique component of all trypanolytic primate HDLs. Furthermore, a polyclonal antiserum to Hpr neutralized the lytic activity from humans and baboons. ApoL-I, a candidate lytic component of human serum, was not immunologically or genetically detectable in two primate species with lytic activity. Polyclonal antiserum to apoL-I also did not neutralize TLF activity in a total human HDL preparation. These findings suggest that apoL-I is not essential in all primate TLFs, and apoL-I alone is not sufficient for optimal trypanosome lytic activity in human TLF.

Convulsive disorder and genetic polymorphism. Association of idiopathic generalized epilepsy with haptoglobin polymorphism

Haptoglobin is a polymorphic protein that is well known for its hemoglobin (Hb)-binding property. The protein shows gross differences in molecular size among genotypes, resulting in different degrees of diffusion in central nervous system tissue. Since the breakdown of erythrocytes in the intracerebral fluid results in Hb-mediated free OH radical formation, lipid peroxidation, and increased neuronal excitability, a differential diffusion of haptoglobin phenotypes in the intracerebral fluid might result in a different degree of protection from oxidative damage. We have studied two samples of children with idiopathic generalized epilepsy from two different Italian populations. In both samples the haptoglobin *1/*1 genotype is much less represented in epileptic children than in controls. These observations suggest that subjects carrying the Hp*1/*1 genotype, that has the lowest molecular size and diffuses more readily in the interstitial cerebral fluid, are more protected against idiopathic generalized epilepsy than those with other haptoglobin genotypes.

A circulating ligand for galectin-3 is a haptoglobin-related glycoprotein elevated in individuals with colon cancer

BACKGROUND & AIMS

Galectin-3 is a beta-galactoside-binding protein implicated in tumor progression and metastasis of colorectal cancers. To determine whether circulating galectin-3 ligands are related to the presence of colon cancer, we sought to identify and quantify ligands in serum that bind to galectin-3.

METHODS

Sera from patients with colon cancer, adenomas, and normal individuals were desialylated, reduced, and separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE) and blots probed with biotinylated galectin-3.

RESULTS

In colon cancer sera, the major galectin-3 ligand was a 40-kilodalton band distinct from mucin, carcinoembryonic antigen, and Mac-2 binding protein. Serum 40-kilodalton ligand was 10- to 30-fold higher in patients with colon cancer than in healthy subjects. Ligand was purified by gel filtration, affinity precipitation on galectin-3/agarose, and SDS-PAGE. When tryptic peptides were analyzed by matrix-assisted laser-desorption ionization mass spectrometry and protein database searching, the 40-kilodalton ligand was identified as haptoglobin beta subunit. In confirmation of this finding, depletion of haptoglobin by immunoprecipitation also eliminated the 40-kilodalton ligand. Colon cancer sera had only a modest increase in total haptoglobin as compared with healthy subjects, suggesting that the structure rather than the amount of haptoglobin is altered in patients with colon cancer. Immunohistochemical staining confirmed the absence of haptoglobin in normal colon and the ectopic expression of haptoglobin in colon cancers and adenomatous polyps.

CONCLUSIONS

A major circulating ligand for galectin-3, which is elevated in the sera of patients with colon cancer, is a cancer-associated glycoform of haptoglobin.

Retroelements and the human genome: new perspectives on an old relation

Retroelements constitute a large portion of our genomes. One class of these elements, the human endogenous retroviruses (HERVs), is comprised of remnants of ancient exogenous retroviruses that have gained access to the germ line. After integration, most proviruses have been the subject of numerous amplifications and have suffered extensive deletions and mutations. Nevertheless, HERV-derived transcripts and proteins have been detected in healthy and diseased human tissues, and HERV-K, the youngest, most conserved family, is able to form virus-like particles. Although it is generally accepted that the integration of retroelements can cause significant harm by disrupting or disregulating essential genes, the role of HERV expression in the etiology of malignancies and autoimmune and neurologic diseases remains controversial. In recent years, striking evidence has accumulated indicating that some proviral sequences and HERV proteins might even serve the needs of the host and are therefore under positive selection. The remarkable progress in the analysis of host genomes has brought to light the significant impact of HERVs and other retroelements on genetic variation, genome evolution, and gene regulation.

An intronic endogenous retrovirus-like sequence attenuates human haptoglobin-related gene expression in an orientation-dependent manner

The human haptoglobin-related gene (HPR) gene codes for a haptoglobin-related protein (Hpr), a component of trypanosome lytic factor which circulates in plasma in small quantities. Except for the presence of a retrovirus-like element, RTVL-Ia, in intron 1, HPR is 92% identical in sequence to the closely linked human haptoglobin gene (HP) gene coding for haptoglobin. We have explored experimentally in tissue culture and in vivo in mice and in humans the influence of the retroviral-like sequence type Ia (RTVL-Ia) element on HPR expression. Transient expression in HepG2 cells of plasmids carrying the HPR promoter joined by a shortened version of intron 1 to the chloramphenicol acetyltransferase (CAT) vector showed that fragments containing the 5' long terminal repeat (LTR) had no significant effect. In contrast, a gag-pol related part and a pol-env-3'LTR related part of RTVL-Ia decreased expression to 20% and 40% of that in their absence but only when they were in naturally occurring orientation. The latter fragment that contains sequences reminiscent of elements essential for retrovirus viability, such as a splicing acceptor site, TATA box and polyA addition signal sequence, was further tested in site-specific transgenic mice. Similar to in vitro experiment, insertion of this fragment into an HPR transgene in mice reduced HPR expression to 50% compared to a transgene without the insert, but none of the viral sequence motifs appear to explain this effect. Instead, we found within the fragment two cryptic splicing donor sites whose products were present in transgenic mouse and in human liver RNA. Our data suggest that a combination of multiple small effects of RTVL-Ia including aberrant splicing accounts for the low (6%) expression of the present-day HPR relative to HP.

Insight into the mechanism of trypanosome lytic factor-1 killing of Trypanosoma brucei brucei

It has been known for almost a century that normal human serum can lyse the extracellular blood parasite Trypanosoma brucei brucei. This process is a result of a non-immune killing factor in human sera known as trypanosome lytic factor (TLF). In this work, we demonstrate that killing of T. b. brucei by trypanosome lytic factor-1 (TLF-1) in vitro is inhibited by the lipophyllic iron chelator, LI, the lipophyllic antioxidant DPPD, and the protease inhibitors antipain and E64. Thus TLF-1 killing likely requires iron, oxidants, and serine and cysteine proteases. Furthermore, we demonstrate that TLF-1 mediated lysis causes measurable peroxidation in T. brucei lipids via a reaction that is inhibited by DPPD, weak bases, and human haptoglobin. We hypothesize that TLF-1 lysis requires intracellular factors within the trypanosome including high intracellular H2O2 and high polyenoic lipid concentrations, lysosomal acidification and proteases, and intracellular iron sources. The data presented supports the hypothesis that the combination of these factors with TLF-1 inside the lysosome results in lysosomal membrane breakdown, release of the lysosomal contents, and subsequent autodigestion of the cell.

Activity of human trypanosome lytic factor in mice

The inability of the cattle pathogen Trypanosoma brucei brucei to infect humans is due to an innate factor in human serum termed Trypanosome Lytic Factor (TLF). Human haptoglobin-related protein is the proposed toxin in TLF and can exist either as a component of a minor subclass of high-density lipoprotein (TLF-1) or as a lipid free, high molecular weight protein complex (TLF-2). The trypanolytic activity of both TLF-1 and TLF-2 has been studied in vitro but their relative contributions to protection against T. b. brucei infection in vivo has not been established. In the present studies we show that treatment of T. b. brucei infected mice with TLF-1 resulted in a dose dependent decrease in parasite numbers but did not affect parasite numbers in mice infected with Trypanosoma brucei rhodesiense, the causative agent of the human sleeping sickness. Similarly, pretreatment of mice with TLF-1 resulted in protection against a challenge by T. b. brucei but had no effect on T. b. rhodesiense challenge. Induction of the acute phase protein haptoglobin, a natural antagonist of TLF-1, diminished but did not abolish the protection against trypanosome challenge. In addition, haptoglobin knockout mice showed higher levels of TLF-1 mediated protection against a T. b. brucei challenge. These results suggest that while TLF-1 is active in vivo, even in the presence of elevated levels of haptoglobin, its activity is modulated in a dose dependent fashion by haptoglobin in the circulation.

Haptoglobin-related Protein Mediates Trypanosome Lytic Factor Binding to Trypanosomes

Trypanosome lytic factor (TLF-1) is an unusual high density lipoprotein (HDL) found in human serum that is toxic to Trypanosoma brucei brucei and may be critical in preventing human infections by this parasite. TLF-1 is composed of four major apolipoproteins: apolipoprotein AI, apolipoprotein AII, paraoxonase, and the primate-specific haptoglobin-related protein (Hpr). Hpr is greater than 90% homologous to haptoglobin (Hp), an abundant acute phase serum protein. Killing of trypanosomes by TLF-1 requires cell surface binding, endocytosis, and subsequent lysosomal targeting. Low temperature binding studies reveal two receptors for TLF-1: one that is high affinity/low capacity (K(d) approximately 12 nm, 350 receptors per cell) and another that binds with low affinity/high capacity (K(d) approximately 1 microm, 60,000 receptors per cell). The low affinity binding is competed by nonlytic human HDL and is likely to be apolipoprotein AI-mediated. Purified human Hpr and human Hp bind to trypanosomes, are internalized, and are targeted to the lysosome. Furthermore, Hpr shows competition for TLF-1 binding, and a monoclonal antibody against Hpr prevents both TLF-1 uptake and trypanosome killing. Based on these results, we propose that Hpr mediates the high affinity binding of TLF-1 to T. b. brucei through a haptoglobin-like receptor.

Induction of haptoglobin by all-trans retinoic acid in THP-1 human monocytic cell line

Haptoglobin (Hp) is one of the acute-phase proteins and is mainly synthesized in the liver. During our study on the differentiation of leukemia cells, we have found that Hp is synthesized in human monocytic cells by all-trans retinoic acid (ATRA). The synthesis of Hp by ATRA is induced in a dose- and time-dependent manner. Hp cDNA cloned from ATRA-treated THP-1 cells corresponds to the Hp alpha 2(FS)-beta form. Whereas ATRA acted as a strong inducer in THP-1 cells, IL-1 beta, TNF-alpha, IL-6, and LPS had little effect on Hp gene expression in these cells. These findings suggest that THP-1 cells express the Hp gene through a signal pathway different from hepatocytes, and that ATRA is a potent Hp-inducer in these cells.