Expression of haptoglobin receptors in human hepatoma cells

Harnessing molecular recognition for localized drug delivery

A grand challenge in drug delivery is providing the right dose, at the right anatomic location, for the right duration of time to maximize therapeutic efficacy while minimizing off-target toxicity and other deleterious side-effects. Two general modalities are receiving broad attention for localized drug delivery. In the first, referred to as "targeted accumulation", drugs or drug carriers are engineered to have targeting moieties that promote their accumulation at a specific tissue site from circulation. In the second, referred to as "local anchoring", drugs or drug carriers are inserted directly into the tissue site of interest where they persist for a specified duration of time. This review surveys recent advances in harnessing molecular recognition between proteins, peptides, nucleic acids, lipids, and carbohydrates to mediate targeted accumulation and local anchoring of drugs and drug carriers.

Innate Nutritional Immunity

Iron (Fe) is an essential micronutrient for both microbes and their hosts. The biologic importance of Fe derives from its inherent ability to act as a universal redox catalyst, co-opted in a variety of biochemical processes critical to maintain life. Animals evolved several mechanisms to retain and limit Fe availability to pathogenic microbes, a resistance mechanism termed "nutritional immunity." Likewise, pathogenic microbes coevolved to deploy diverse and efficient mechanisms to acquire Fe from their hosts and in doing so overcome nutritional immunity. In this review, we discuss how the innate immune system regulates Fe metabolism to withhold Fe from pathogenic microbes and how strategies used by pathogens to acquire Fe circumvent these resistance mechanisms.

Haptoglobin and hemopexin inhibit vaso-occlusion and inflammation in murine sickle cell disease: Role of heme oxygenase-1 induction

During hemolysis, hemoglobin and heme released from red blood cells promote oxidative stress, inflammation and thrombosis. Plasma haptoglobin and hemopexin scavenge free hemoglobin and heme, respectively, but can be depleted in hemolytic states. Haptoglobin and hemopexin supplementation protect tissues, including the vasculature, liver and kidneys. It is widely assumed that these protective effects are due primarily to hemoglobin and heme clearance from the vasculature. However, this simple assumption does not account for the consequent cytoprotective adaptation seen in cells and organs. To further address the mechanism, we used a hyperhemolytic murine model (Townes-SS) of sickle cell disease to examine cellular responses to haptoglobin and hemopexin supplementation. A single infusion of haptoglobin or hemopexin (± equimolar hemoglobin) in SS-mice increased heme oxygenase-1 (HO-1) in the liver, kidney and skin several fold within 1 hour and decreased nuclear NF-ĸB phospho-p65, and vaso-occlusion for 48 hours after infusion. Plasma hemoglobin and heme levels were not significantly changed 1 hour after infusion of haptoglobin or hemopexin. Haptoglobin and hemopexin also inhibited hypoxia/reoxygenation and lipopolysaccharide-induced vaso-occlusion in SS-mice. Inhibition of HO-1 activity with tin protoporphyrin blocked the protections afforded by haptoglobin and hemopexin in SS-mice. The HO-1 reaction product carbon monoxide, fully restored the protection, in part by inhibiting Weibel-Palade body mobilization of P-selectin and von Willebrand factor to endothelial cell surfaces. Thus, the mechanism by which haptoglobin and hemopexin supplementation in hyperhemolytic SS-mice induces cytoprotective cellular responses is linked to increased HO-1 activity.

One ring to rule them all: trafficking of heme and heme synthesis intermediates in the metazoans

The appearance of heme, an organic ring surrounding an iron atom, in evolution forever changed the efficiency with which organisms were able to generate energy, utilize gasses and catalyze numerous reactions. Because of this, heme has become a near ubiquitous compound among living organisms. In this review we have attempted to assess the current state of heme synthesis and trafficking with a goal of identifying crucial missing information, and propose hypotheses related to trafficking that may generate discussion and research. The possibilities of spatially organized supramolecular enzyme complexes and organelle structures that facilitate efficient heme synthesis and subsequent trafficking are discussed and evaluated. Recently identified players in heme transport and trafficking are reviewed and placed in an organismal context. Additionally, older, well established data are reexamined in light of more recent studies on cellular organization and data available from newer model organisms. This article is part of a Special Issue entitled: Cell Biology of Metals.

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.

Can haptoglobin be an indicator for the early diagnosis of neonatal jaundice?

Neonatal jaundice is the result of an imbalance between bilirubin production and elimination. Bilirubin conjugation in newborns is significantly impaired in the first few days; even a small increase in the rate of production can contribute to the development of hyperbilirubinemia. Hemolysis has a significant role in bilirubin increase in newborns. Intrauterine is tolerated by the maternal metabolism in life. When hemolysis takes place, a decrease is accepted in the haptoglobin and hemopoexin blood levels binding hemoglobin in the environment. Therefore, it may be considered that haptoglobin and hemopoexin from the early period umbilical cord (UC) blood in newborns may be an indicator in determining jaundice likely to develop in later stages. Babies were called to the control polyclinic in the third and fifth days. Eighty-four babies with normal term birth were included in the study. Gestational age of the mothers was 39.5+/-1.5 weeks in average. A significant negative correlation was found between the haptoglobin level from the UC taken during delivery and the bilirubin value in the fifth day (r=-0.345; P=0.001). The haptoglobin value from the blood of the UC can be used as a guiding indicator to demonstrate the future occurrence of jaundice in newborns. This way, the babies with high jaundice risk may be detected earlier and closer follow-up of these babies can be obtained. As a result, the haptoglobin level of the blood from the UC during delivery allows us to make an early prediction on whether neonatal jaundice will occur.

Functionally distinct NEAT (NEAr Transporter) domains within the Staphylococcus aureus IsdH/HarA protein extract heme from methemoglobin

The pathogen Staphylococcus aureus uses iron-regulated surface determinant (Isd) proteins to scavenge the essential nutrient iron from host hemoproteins. The IsdH protein (also known as HarA) is a receptor for hemoglobin (Hb), haptoglobin (Hp), and the Hb-Hp complex. It contains three NEAT (NEAr Transporter) domains: IsdH(N1), IsdH(N2), and IsdH(N3). Here we show that they have different functions; IsdH(N1) binds Hb and Hp, whereas IsdH(N3) captures heme that is released from Hb. The staphylococcal IsdB protein also functions as an Hb receptor. Primary sequence homology to IsdH indicates that it will also employ functionally distinct NEAT domains to bind heme and Hb. We have used site-directed mutagenesis and surface plasmon resonance methods to localize the Hp and Hb binding surface on IsdH(N1). High affinity binding to these structurally unrelated proteins requires residues located within a conserved aromatic motif that is positioned at the end of the beta-barrel structure. Interestingly, this site is quite malleable, as other NEAT domains use it to bind heme. We also demonstrate that the IsdC NEAT domain can capture heme directly from Hb, suggesting that there are multiple pathways for heme transfer across the cell wall.

Targeted delivery of ribavirin improves outcome of murine viral fulminant hepatitis via enhanced anti-viral activity

Side effects of interferon-ribavirin combination therapy limit the sustained viral response achievable in hepatitis C virus (HCV) patients. Coupling ribavirin to macromolecular carriers that target the drug to the liver would reduce systemic complications. The aim of this study was to evaluate the efficacy of a hemoglobin-ribavirin conjugate (HRC 203) in murine hepatitis virus strain 3 (MHV-3) induced viral hepatitis. HRC 203 had greater anti-viral activity on both isolated hepatocytes and macrophages, whereas both ribavirin and HRC 203 inhibited production of the pro-inflammatory cytokines interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) by macrophages. In vivo, untreated MHV-3-infected mice all developed clinical and biochemical signs of acute viral hepatitis and died by day 4 post infection. Livers recovered from untreated infected mice showed greater than 90% necrosis. In contrast, survival was enhanced in both ribavirin- and HRC 203-treated mice with a marked reduction in biochemical [ALT(max) 964 +/- 128 IU/L (ribavirin); 848 +/- 212 IU/L (HRC 203)] and histological evidence of hepatic necrosis (<10% in ribavirin/HRC 203 vs. 90% in untreated controls). Clinically, HRC 203-treated mice behaved normally, in contrast to ribavirin-treated mice, which developed lethargy and abnormal fur texture. In conclusion, targeted delivery of ribavirin to the liver alters the course of MHV-3 infection as demonstrated by prolonged survival, improved behavior, and reduced signs of histologically evident disease, as well as inhibition of viral replication and production of inflammatory cytokines in vitro.

Hemoglobin and heme scavenging

Release of hemoglobin into plasma is a physiological phenomenon associated with intravascular hemolysis. In plasma, stable haptoglobin-hemoglobin complexes are formed and these are subsequently delivered to the reticulo-endothelial system by CD163 receptor-mediated endocytosis. Heme arising from the degradation of hemoglobin, myoglobin, and of enzymes with heme prosthetic groups could be delivered in plasma. Albumin, haptoglobin, hemopexin, and high and low density lipoproteins cooperate to trap the plasma heme, thereby ensuring its complete clearance. Then hemopexin releases the heme into hepatic parenchymal cells only after internalization of the hemopexin-heme complex by CD91 receptor-mediated endocytosis. Moreover, alpha1-microglobulin contributes to heme degradation by a still unknown mechanism, with the concomitant formation of heterogeneous yellow-brown kynurenine-derived chromophores which are very tightly bound to amino acid residues close to the rim of the lipocalin pocket. During hemoglobin synthesis, the erythroid alpha-chain hemoglobin-stabilizing protein specifically binds free alpha-hemoglobin subunits limiting the free protein toxicity. Although highly toxic because capable of catalyzing free radical formation, heme is also a major and readily available source of iron for pathogenic organisms. Gram-negative bacteria pick up the heme-bound iron through the secretion of a hemophore that takes up either free heme or heme bound to heme-proteins and transports it to a specific receptor, which, in turn, releases the heme and hence iron into the bacterium. Here, hemoglobin and heme trapping mechanisms are summarized.

Binding of acellular, native and cross-linked human hemoglobins to haptoglobin: enhanced distribution and clearance in the rat

It is well established that hemoglobin resulting from red cell lysis binds to haptoglobin in plasma to form a complex. The increased molecular size precludes its filtration by the kidneys, redirecting it toward hepatocellular entry. Chemically cross-linked hemoglobins are designed to be resistant to renal excretion, even in the absence of haptoglobin. The manner in which binding to haptoglobin influences the pharmacokinetics of acellular cross-linked and native hemoglobins was investigated after intravenous injection of radiolabeled native human hemoglobin and trimesyl-(Lys82)beta-(Lys82)beta cross-linked human hemoglobin, at trace doses, into rats. Under these conditions, there is sufficient plasma haptoglobin for binding with hemoglobin. In vitro binding assayed by size-exclusion chromatography for bound and free hemoglobin revealed that, at <8 muM hemoglobin, native human hemoglobin was completely bound to rat haptoglobin, whereas only approximately 30% of trimesyl-(Lys82)beta-(Lys82)beta cross-linked hemoglobin was bound. Plasma disappearance of low doses (0.31 mumol/kg) of native and cross-linked hemoglobins was monoexponential (half-life = 23 and 33 min, respectively). The volume of distribution (40 vs. 19 ml/kg) and plasma clearance (1.22 vs. 0.4 ml.min(-1).kg(-1)) were higher for native than for cross-linked hemoglobin. Native and cross-linked human hemoglobins were found primarily in the liver, and not in the kidney, heart, lung, or spleen, mostly as degradation products. These pharmacokinetic findings suggest that the binding of hemoglobin to haptoglobin enhances its hepatocellular entry, clearance, and distribution.

Plasma protein haptoglobin modulates renal iron loading

Haptoglobin is the plasma protein with the highest binding affinity for hemoglobin. The strength of hemoglobin binding and the existence of a specific receptor for the haptoglobin-hemoglobin complex in the monocyte/macrophage system clearly suggest that haptoglobin may have a crucial role in heme-iron recovery. We used haptoglobin-null mice to evaluate the impact of haptoglobin gene inactivation on iron metabolism. Haptoglobin deficiency led to increased deposition of hemoglobin in proximal tubules of the kidney instead of the liver and the spleen as occurred in wild-type mice. This difference in organ distribution of hemoglobin in haptoglobin-deficient mice resulted in abnormal iron deposits in proximal tubules during aging. Moreover, iron also accumulated in proximal tubules after renal ischemia-reperfusion injury or after an acute plasma heme-protein overload caused by muscle injury, without affecting morphological and functional parameters of renal damage. These data demonstrate that haptoglobin crucially prevents glomerular filtration of hemoglobin and, consequently, renal iron loading during aging and following acute plasma heme-protein overload.

Haptoglobin Polymorphism and Iron Homeostasis

Background: There is a marked difference in the degree of expression of the homozygous C282Y HFE genotype that is associated with hereditary hemochromatosis. It has been reported that individuals with the haptoglobin 2-2 type manifest increased iron concentrations, including serum iron, transferrin saturation, and ferritin.

Methods: We studied 232 patients, 115 homozygous for the c.845G→A (C282Y) mutation and 117 matched controls with the wild-type HFE genotype, for haptoglobin phenotypes. Haptoglobin types were determined by electrophoresis of the denatured protein. The HFE genotype was determined by allele-specific oligonucleotide hybridization. Ferritin and transferrin saturation were measured by standard methods.

Results:There was no relationship between haptoglobin type and ferritin concentration or transferrin saturation.

Conclusions: The effect of haptoglobin type on iron homeostasis cannot account for the marked phenotypic variation that is seen in patients homozygous for the HFE C282Y mutation.

Haptoglobin polymorphism and body iron stores

In humans the iron status is influenced by environmental and genetic factors. Among them, the genetic polymorphism of the hemoglobin (Hb)-binding plasma protein haptoglobin (Hp) has been shown to affect iron turnover. The best known biological function of Hp is capture of free Hb in plasma to allow hepatic recycling of heme iron and to prevent kidney damage during hemolysis. In healthy males, but not in females, the Hp 2-2 phenotype is associated with higher serum iron, higher transferrin saturation, and higher ferritin than Hp 1-1 and 2-1. Moreover, serum ferritin correlates with monocyte L-ferritin content, which is also highest in Hp 2-2 subjects due to endocytosis of multimeric Hb-Hp 2-2 complexes by the recently identified Hb scavenger receptor CD163 in macrophages. This iron delocalization pathway, occurring selectively in Hp 2-2 subjects, has important biological and clinical consequences. The Hp polymorphism is related to the prevalence and the outcome of various pathological conditions with altered iron metabolism such as hemochromatosis, infections, and atherosclerotic vascular disease.

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.

The Haptoglobin 2-2 Phenotype Affects Serum Markers of Iron Status in Healthy Males

Background: Human iron status is influenced by environmental and genetic factors. We hypothesized that the genetic polymorphism of haptoglobin (Hp), a hemoglobin-binding plasma protein, could affect iron status.

Methods: Reference values of serum iron status markers were compared according to Hp phenotypes (Hp 1-1, Hp 2-1, Hp 2-2; determined by starch gel electrophoresis) in 717 healthy adults. Iron storage was investigated in peripheral blood monocyte-macrophages by measuring cytosolic L- and H-ferritins and by in vitro uptake of radiolabeled (125I) hemoglobin-haptoglobin complexes.

Results: In males but not in females, the Hp 2-2 phenotype was associated with higher serum iron (P &lt;0.05), transferrin saturation (P &lt;0.05), and ferritin (P &lt;0.01) concentrations than Hp 1-1 and 2-1, whereas soluble transferrin receptor concentrations were lower (P &lt;0.05). Moreover, serum ferritin correlated with monocyte L-ferritin content (r = 0.699), which was also highest in the male Hp 2-2 subgroup (P &lt;0.01). In vitro, monocyte-macrophages took up a small fraction of 125I-labeled hemoglobin complexed to Hp 2-2 but not to Hp 1-1 or 2-1.

Conclusions: The Hp 2-2 phenotype affects serum iron status markers in healthy males and is associated with higher L-ferritin concentrations in monocyte-macrophages because of a yet undescribed iron delocalization pathway, selectively occurring in Hp 2-2 subjects.

Increased Susceptibility in Hp Knockout Mice During Acute Hemolysis

Haptoglobin, a conserved plasma glycoprotein, forms very stable soluble complexes with free plasma hemoglobin. Hemoglobin binding by haptoglobin is thought to be important in the rapid hepatic clearance of hemoglobin from the plasma and in the inhibition of glomerular filtration of hemoglobin. To evaluate these functions,Haptoglobin knockout (−/−) mice were created. These mice were viable but had a small, significant reduction in postnatal viability. Contrary to popular belief, the lack of haptoglobin did not impair clearance of free plasma hemoglobin in −/− mice. Induction of severe hemolysis by phenylhydrazine caused extensive hemoglobin precipitation in the renal tubular cells of both −/− and +/+ mice, with death occurring in 55% of −/− mice and in 18% of +/+ mice. In general, phenylhydrazine-treated −/− mice suffered greater tissue damage, as evidenced by the induction of hepatic acute phase response resulting in increased plasma alpha 1-acid glycoprotein (AGP) levels. Among −/− and +/+ mice that survived, −/− mice tend to suffer greater oxidative damage and failed to repair or regenerate damaged renal tissues, as indicated by their higher plasma malonaldehyde (MDA) and 4-hydroxy-2(E)-nonenal (HNE) levels and lower mitotic indices in their kidneys, respectively. This study suggested that a physiologically important role of hemoglobin-haptoglobin complex formation is the amelioration of tissue damages by hemoglobin-driven lipid peroxidation.

© 1998 by The American Society of Hematology.

Increased Susceptibility in Hp Knockout Mice During Acute Hemolysis

Haptoglobin, a conserved plasma glycoprotein, forms very stable soluble complexes with free plasma hemoglobin. Hemoglobin binding by haptoglobin is thought to be important in the rapid hepatic clearance of hemoglobin from the plasma and in the inhibition of glomerular filtration of hemoglobin. To evaluate these functions,Haptoglobin knockout (−/−) mice were created. These mice were viable but had a small, significant reduction in postnatal viability. Contrary to popular belief, the lack of haptoglobin did not impair clearance of free plasma hemoglobin in −/− mice. Induction of severe hemolysis by phenylhydrazine caused extensive hemoglobin precipitation in the renal tubular cells of both −/− and +/+ mice, with death occurring in 55% of −/− mice and in 18% of +/+ mice. In general, phenylhydrazine-treated −/− mice suffered greater tissue damage, as evidenced by the induction of hepatic acute phase response resulting in increased plasma alpha 1-acid glycoprotein (AGP) levels. Among −/− and +/+ mice that survived, −/− mice tend to suffer greater oxidative damage and failed to repair or regenerate damaged renal tissues, as indicated by their higher plasma malonaldehyde (MDA) and 4-hydroxy-2(E)-nonenal (HNE) levels and lower mitotic indices in their kidneys, respectively. This study suggested that a physiologically important role of hemoglobin-haptoglobin complex formation is the amelioration of tissue damages by hemoglobin-driven lipid peroxidation.

© 1998 by The American Society of Hematology.

Iron and gene expression: molecular mechanisms regulating cellular iron homeostasis

In recent years, specific post-transcriptional mechanisms in the cytoplasm of vertebrate cells have been elucidated that directly affect the stability and translation of mRNAs coding for central proteins in iron metabolism. This review shall focus primarily on these mechanisms. Other levels of control, either affecting gene transcription and/ or related to the function of iron-capturing substances and transmembrane transport, are also likely to exist and to influence the iron balance and utilization. They are, however, much less clear.

Endothelial-cell heme uptake from heme proteins: induction of sensitization and desensitization to oxidant damage

Iron-derived reactive oxygen species are implicated in the pathogenesis of various vascular disorders including atherosclerosis, vasculitis, and reperfusion injury. The present studies examine whether heme, when liganded to physiologically relevant proteins as in hemoglobin, can provide potentially damaging iron to intact endothelium. We demonstrate that reduced ferrohemoglobin, while relatively innocuous to cultured endothelial cells, when oxidized to ferrihemoglobin (methemoglobin), greatly amplifies oxidant (H2O2)-mediated endothelial-cell injury. Drawing upon our previous observation that free heme similarly primes endothelium for oxidant damage, we posited that methemoglobin, but not ferrohemoglobin, releases its hemes that can then be incorporated into endothelial cells. In support, cultured endothelial cells exposed to methemoglobin--in contrast to exposure to ferrohemoglobin, cytochrome c, or metmyoglobin--rapidly increased their heme oxygenase mRNA and enzyme activity, thereby supporting heme uptake; ferritin production was also markedly increased after such exposure, thus attesting to eventual incorporation of Fe. These cellular methemoglobin effects were inhibited by the heme-scavenging protein hemopexin and by haptoglobin or cyanide, agents that strengthen the liganding between heme and globin. If the endothelium is exposed to methemoglobin for a more prolonged period (16 hr), it accumulates large amounts of ferritin; concomitantly, and presumably associated with iron sequestration by this protein, the endothelium converts from hypersusceptible to hyperresistant to oxidative damage. We conclude that when oxidation of hemoglobin facilitates release of its heme groups, catalytically active iron is provided to neighboring tissue environments. The effect of this relinquished heme on the vasculature is determined both by extracellular factors--i.e., plasma proteins, such as haptoglobin and hemopexin--as well as intracellular factors, including heme oxygenase and ferritin. Acutely, if both extra- and intracellular defenses are overwhelmed, cellular toxicity arises; chronically, when ferritin is induced, resistance to oxidative injury may supervene.