INTRODUCTION
One of the main functions of the liver is the synthesis of glycoproteins; one of the most important of which is haptoglobin (HP), which eliminates iron from physiological hemolysis through binding to free hemoglobin. HP also has immunomodulatory properties because it is synthesized during the acute phase of inflammation[1]. Its scavenging function, related to the elimination of free hemoglobin (Hb) from hemolysis also gives it an important antioxidant role[2]. Its concentration is significantly increased during inflammatory processes. Rodrigues et al[3] reported that HP levels were higher in type 2 diabetes mellitus patients compared with the general population (P = 0.005). Hypertensive diabetic patients had higher HP levels than those without comorbidities (P = 0.021). Obese type 2 diabetic patients had a higher HP rate than the control population (P = 0.009) and those with type 2 and non-obese diabetes (P = 0.003)[3]. However, it has other properties, depending on its genotype.
The HP molecule is composed of two peptide chains, alpha and beta, linked together by disulfide bonds[4,5]. Both chains are encoded by the same gene, located on chromosome 16q22.2, as a preprotein that undergoes post-translational modifications, followed by proteolytic cleavage of the two subunits[6]. The HP1 allele is conserved among species and consists of five exons. In comparison, the HP2 allele is human-specific and contains seven exons; the product of a duplication involving exons 3 and 4 of the HP1 allele[6]. The two codominant alleles lead to synthesis of three distinct phenotypes. However, hemoglobin-binding affinity and plasma concentration vary by phenotype. HP2-2 has reduced overall affinity for Hb compared with other phenotypes[7], reduced antioxidant capacity[7,8], and reduced clearance of HP2–Hb complexes by less-efficient macrophages[9]. The consequence of this is an increase in nitric oxide uptake by HP2–Hb complexes[10], with endocytosis by macrophages of HP2–Hb complexes followed by a reduction in downstream anti-inflammatory signaling[11], a greater angiogenic effect and reduced inhibition of prostaglandin synthesis[12,13]. Studies have shown the association between HP genotypes and disease pathophysiology. Adekile and Haider[14] reported that sickle cell patients with the HP2-2 genotype tended to have more frequent vaso-occlusive episodes. Willen et al[15] and several other authors have reported that HP genotype can predict acute vaso-occlusive seizures in children with sickle cell disease. Cox et al[2] reported that the association between HP2 genotype and the SS gene has a favorable effect on the risk of high cerebral blood flow velocity (CBFv). Suchdev et al[16] reported a decrease in Hb levels in children with malaria compared with those with HP genotype 1-2 or 2-2 (P = 0.018).
New claims indicate that HP may also be involved in the immune response. The influence of HP2-2 on high genetic pressure suggests its important role in human pathology[17]. In addition to the above beneficial properties, HP also has negative effects on health as is associated with worsening of diseases, and this is also a function of its genotype. Thus, there is a need to study the genetic polymorphism of HP and see how it is associated with pathophysiology. This review aims to elucidate the properties of HP and present how they are involved in disease.
PROPERTIES OF HP
HP structure and generalities
HP is a glycoprotein of hepatocyte origin that is present in serum, electrophoretically migrating to the α2-globulin zone, and is one of the proteins of the acute phase of inflammation. Its structure includes two heavy β chains, common to all molecules, and two α light chains, of which there are two types. The genetic polymorphism of the chains leads to three phenotypes/proteins, the difference of which lies in the number, molecular weight and chain type, namely HP1-1 (85 kDa), HP1-2 (120 kDa) and HP2-2 (170 kDa). They are located on the long arm of chromosome 16, carried by the HP gene[6]. The HP1 allele, conserved among species, consists of five exons, whereas the HP2 allele is human-specific and contains seven exons whose origin is probably exons 3 and 4 of the HP1 allele, implying a duplication event[6].
These proteins are probably not equivalent, as patients with HP2-2 have more vascular complications[18]; however, this risk appears to vary depending on the presence or absence of underlying disease[9]. The distribution of the different forms of HP in the population is 15% 1-1, 50% 1-2 and 35% 2-2[19]. The half-life of the molecule is 3–5 d. HP participates in the neutralization of Hb released into plasma by physiological intravascular hemolysis (which accounts for two-thirds of HP catabolism) by constituting a stable and soluble HP–Hb complex, not filterable by the glomeruli. The complexes thus formed quickly disappear from the circulation (complexes have a half-life < 10 min), and are catabolized by the hepatic reticuloendothelial system, which allows the recovery of iron from Hb and degradation of Hb into bilirubin[20].
HP also has antioxidant properties because its prevents iron produced by hemoglobin catabolism from facilitating the formation of free radicals[21]. These properties vary with the type of HP[22]. HP is involved in the inflammatory response and oxidative stress of atherosclerotic plaques and HP2-2 promotes lipid peroxidation and accumulation of macrophages within these plaques[23].
HP and toxic free radicals
The Fenton reaction is characterized by the action of free iron with oxygen, leading to the production of superoxide radicals and H2O2-generating hydroxyl radicals[24]. Free iron can also, because of its catalytic action, oxidize low-density lipoprotein (LDL), which could lead to damage to vascular endothelial cells and damage to vascular endothelial cells[25]. The phenotype of HP influences its ability to reduce free radical damage[26,27]. In vitro experiments with purified HP showed better protection against oxidative damage by HP1-1[28,29]. The three main phenotypes of HP have the same binding affinities for Hb[30]. Differences in the size of these proteins affect their ability to prevent the release of heme. Thus, HP2-2, because of its larger size, eliminates iron more slowly in the extravascular space. In people with HP2-2, free Hb therefore remains in the circulation longer, thus promoting greater oxidative stress[30].
The genetic polymorphism of HP is also one of the main factors in the establishment of atherogenesis[7]. Individuals with HP2-2 genotype have a greater risk of developing coronary artery disease[31], peripheral vascular disease[8] and vascular diabetic complications[32]. In addition, it has been reported that people with HP2-2 phenotypes have a significantly higher oxidized LDL/LDL ratio than those with HP1-1, and those with heterozygous phenotypes (HP2-1) have intermediate levels. In contrast, there was no difference in plasma malondialdehyde concentrations between HP phenotypes[33]. There is a link between HP polymorphism and iron in the induction of oxidative stress. HP has an antioxidant function by binding to free Hb that continuously escapes from erythrocytes[7].
GENETIC POLYMORPHISM OF HP AND ITS INVOLVEMENT IN THE ONSET OR WORSENING OF DISEASE
HP, inflammation, iron and oxidative stress
Inflammation is a pathophysiological process involved in the disruption of tissue homeostasis as a result of acute or chronic stimuli from various causes: infections, stress, autoimmune reactions, or mechanical injury[34,35]. Polymorphonuclear leukocytes (PMNs) play an important role in mediating homeostatic immunosurveillance, with disruption causing PMN migration through the TH1/TH2 cytokine profile. HP is actively involved in all processes, from the recruitment of PMNs and the suppression of free radicals to the repair and regeneration of tissues. Thus, in haptoglobinemia or hypohaptoglobinemia, characterized by the absence or reduction of HP protein, transfusion (skin and pulmonary) and allergic anaphylactic reactions are observed, respectively[19]. In the model representation of the role of HP, hazard signals stimulate the expression of HP through interleukin-6 activity. In HP1-1 subjects, HP induces a considerable reduction in reactive oxygen species generation due to its powerful antioxidant role by binding to Hb and its anti-inflammatory role[34].
HP1-1 and HP1-2 genotypes play important roles because HP1-1 is associated with a decrease in dyslipidemia, and therefore prevention of cardiovascular disease, unlike the HP2-2 genotype, which promotes dyslipidemia – a risk factor for cardiovascular disease. For example, a comparative study of HP1-1 and HP2-2 genotypes[23] showed that HP2-2 is the cause of greater peroxidation because it promotes Hb oxidation and therefore oxidative stress, unlike HP1-1, which limits oxidative stress by capturing iron from hemolysis.
Yalcinkaya et al[36] described markers of inflammation in sickle cell patients. C-reactive protein (CRP) and ferritin showed a significant difference in sickle cell patients compared with the normal population[36]. The elevation of CRP suggests its role in the activation of inflammation in sickle cell patients. They also reported the association of other inflammatory parameters with sickle cell disease, including HP[36]. Gurung et al[37] reported that increased risk of myocardial infarction is correlated with low plasma levels of HP.
HP and its involvement in worsening malaria
Several studies have been conducted to evaluate the influence of HP polymorphism on the clinical manifestations of malaria. Cox et al[38] reported that HP2-2 genotype promoted a significant drop in Hb levels compared with other genotypes in children with malaria. Suchdev et al[16] also reported a decrease in Hb levels in children with a 2-2 or 1-2 HP genotype (P = 0.018). They suggest that, due to the low binding capacity of HP2-2 to Hb, it is less effective at trapping free iron Hb after malaria-induced hemolysis (Suchdev et al[16]). Fowkes et al[40] reported a negative correlation between age and HP levels observed at low parasite densities. Moreover, a positive correlation between age and higher parasite densities was observed. They suggested that in the presence of parasitic hyperhemolysis, the production of HP is increased in older children. As in the studies cited above, several authors also reported worsening of malaria in patients with the HP2 allele[41,42].
HP and sickle cell disease
The HP polymorphism is an important marker of vascular disease and may, depending on the gene, confer resistance to the development of some complications[6]. HP2-2 is known for its low oxidative activity in sickle cell patients and higher inflammatory response[43]. These properties, together with other environmental and genetic factors, confer haptoglobin polymorphism, which contributes to varied clinical manifestations in sickle cell patients. Olatunya et al[44] in Nigeria showed that HP2-2 sickle cell patients had more vaso-occlusive seizures and greater effects on laboratory parameters when compared to patients with other haptoglobin genotypes; even if the reported results were not statistically significant due to the small size of their study population. The distribution of HP genotypes among patients was as follows: HP1-1, 43 (42.6%); HP2-1, 40 (39.6%); HP2-1, 40 (39.6%); and HP2-2, 18 (17.8%)[44].
Meher et al[45] reported that there was a significant difference (P < 0.001) in the distribution of phenotypes among sickle cell genotypes, reflecting the association of the HP2-2 genotype with severe forms of the disease (vaso-occlusive seizures). Analysis of HP levels in different genotypes in patients with sickle cell anemia revealed higher levels of HP in those with the HP1-1 genotype, followed by HP1-2, and finally HP2-2, with mean HP levels of 37.78 ± 27.79 mg/dL, 28.28 ± 22.79 mg/dL and 23.49 ± 21.05 mg/dL, respectively (P < 0.05). However, there was no difference between HP1-2 and HP2-2 (P > 0.05)[45]. Mild phenotypes had a higher HP level than severe phenotypes. They explain this by the fact that dimers are the protein product of HP1-1 (HP1), unlike HP1-2 (HP1-2) and HP2-2 (HP2), which can form polymers of up to 10 and 20 repeating units[46]. Perhaps due to the greater molecular weight of HP2, the chances of degradation and decreasing its availability in the circulation are greater, which makes it less antioxidant and more immunogenic[47]. However, the dimer form and low molecular weight of HP1 give it greater antioxidant function, as a result of its greater ability to remove free Hb from the circulation by promoting the uptake of HP–Hb complexes by the CD163 macrophage receptor[9,23].
Willen et al[15] found that vaso-occlusive disease was more common in children with the HP2-2 genotype in contrast to other genotypes. Adekile and Haider[14] also reported that sickle cell patients with the HP2-2 genotype tended to have more frequent vaso-occlusive episodes. The difference in the distribution of the HP allele and genotype may promote clinical complications; the presence of HP2 is a factor in the severity of vaso-occlusive seizures. The HP2-2 genotype is a mediator of oxidative stress and higher tissue damage in contrast to the HP1-1 genotype[9]. Cox et al[2] reported that the association between the HP2 genotype and the SS gene is a factor in favor of the risk of high CBFv. Several other authors[10,15,48] have shown that the haptoglobin genotype predicts clinical manifestations in children with sickle cell anemia.
Haptoglobin association and other diseases
Several studies have reported that the genetic polymorphism of HP 1/2 is associated with an increased prevalence of infections, and aggravation of autoimmune, cardiovascular and other diseases. More specifically, severe forms of myocardial infarction are related to the HP2-2 phenotype. In contrast, the HP1-1 phenotype has recently been reported to protect against critical vascular complications of diabetes mellitus, including restenosis and diabetic nephropathy[9].
Delanghe et al[31] reported an increased risk of coronary artery disease associated with the HP2-2 genotype. In 2004, Asleh et al[9] also evaluated the genetic polymorphisms of HP and their association with coronary heart disease in patients with or without diabetes. In nondiabetic participants, there was an increased prevalence of coronary artery disease associated with the Hp2 genotype, regardless of conventional risk factors. However, although they did not find a significant cardiac risk between the different HP polymorphisms in the combined population of diabetic and nondiabetic subjects, there was evidence that the HP2-2 phenotype was associated with an increased risk of stroke and cardiovascular disease in diabetic patients, as also reported by Vardi et al[49]. There are several reports of the association between the HP2 genotype and the SS as a risk factor for high CBFv[50-52]. The HP2 allele has also been reported to be associated with increased severity and adverse outcomes in various infections, including HIV and malaria[53,54].
HP is also involved in angiogenesis through its involvement in tumor development, inflammatory disorders and wound healing. HP2-2 is reported to be more angiogenic than other phenotypes, suggesting its greater involvement in tumor growth and poor wound healing[55].
CONCLUSION
The aim of this review was to elucidate the role of haptoglobin polymorphism in the development of diseases. It highlight the properties of haptoglobin in aggravation of certain pathologies including inflammation, deregulation of iron metabolism, imbalance of oxidative stress, aggravation of vaso-occlusive crises in sickle cell patients, aggravation of malaria and other diseases. These are mainly mediated by the HP2-2 phenotype. Furthermore, it highlights the role played by HP1-1 phenotype in protection against diseases and the re-establishment of homeostasis. This therefore raises the importance and necessity of systematically determining the haptoglobin genotype in any individual, because knowledge would make it possible to predict complications and adapt appropriate treatment.
