Increased renal hypertrophy in diabetic mice genetically modified at the haptoglobin locus

Interaction between the Haptoglobin 2 Phenotype and Diabetes Mellitus on Systolic Pulmonary Arterial Pressure and Nitric Oxide Bioavailability in Hemodialysis Patients

Elevated systolic pulmonary artery pressure (s-PAP, ≥35 mmHg) serves as an independent predictor of mortality in hemodialysis (HD) and diabetic (DM) patients. A polymorphism in the antioxidant Haptoglobin (Hp) gene has been shown to regulate the bioavailability of nitric oxide (NO), a major mediator of pulmonary vascular tone. We therefore set out to test the hypothesis that the Hp polymorphism may be a determinant of developing elevated s-PAP specifically in the DM state due to a decreased bioavailability of NO. To test our hypothesis we Hp typed and performed transthoracic echocardiography on a series of HD patients and stratified them into elevated and normal s-PAP groups and then evaluated whether there was a significant association between the Hp type, elevated s-PAP, and decreased NO bioavailability as defined by low plasma nitrite. We found a statistically significant interaction between the Hp type and DM on the prevalence of elevated s-PAP and lower mean nitrite levels with the combination of elevated s-PAP and low nitrite levels being significantly more prevalent in Hp 2-2 DM individuals. We conclude that the Hp 2 type is associated with elevated s-PAP levels and low plasma nitrite levels in HD patients specifically in the DM state.

Vitamin E and diabetic nephropathy in mice model and humans

Diabetes mellitus (DM) is associated with increased oxidative stress due to elevated glucose levels in the plasma. Glucose promotes glycosylation of both plasma and cellular proteins with increased risk for vascular events. Diabetic patients suffer from a higher incidence of cardiovascular complications such as diabetic nephropathy. Haptoglobin (Hp) is an antioxidant plasma protein which binds free hemoglobin, thus preventing heme-iron mediated oxidation. Two alleles exist at the Hp gene locus (1 and 2) encoding three possible Hp genotypes that differ in their antioxidant ability, and may respond differently to vitamin E treatment. Several clinical studies to have shown that Hp 1-1 genotype is a superior antioxidant to the Hp 2-2 genotype and Hp 2-2 genotype is associated with a higher incidence of cardiovascular disease. Vitamin E was found to have beneficial effect in patient and mice with Hp 2-2 genotype. In this review we have summarized the results of our studies in patients with diabetic nephropathy treated with vitamin E and in diabetic mice with different haptoglobin genotypes.

Is the Hp 2-2 diabetic mouse model a good model to study diabetic nephropathy?

Diabetic nephropathy (DN) is the leading cause of end stage renal disease and dialysis worldwide. Despite aggressive treatment, the number of patients on hemodialysis due to type 1 and type 2 diabetes mellitus is increasing annually. The lack of reliable animal models that mimic human disease has delayed the identification of specific factors that cause or predict DN. Different investigators around the world are testing different murine models. Validation criteria for early and advanced DN, phenotypic methods, background strain have recently been developed. Establishment of an authentic mouse model of DN will undoubtedly facilitate the understanding of the underlying genetic mechanisms that contribute to the development of DN and to study new treatments. Here we describe the characteristics of our new mouse model with type 1 diabetes mellitus and different haptoglobin genotypes that can mimic human DN.

Haptoglobin genotype and its role in determining heme-iron mediated vascular disease

Haptoglobin (Hp) is a hemoglobin (Hb) binding protein whose major function is to prevent heme-iron mediated oxidation. The polymorphic nature of the Hp gene results in varying levels of antioxidant function associated with the protein products. Multiple clinical studies have now determined that the Hp 2-2 genotype is associated with an increased risk of developing vascular complications in patients suffering from diabetes. The mechanism for this phenomenon is a decrease in antioxidant capability associated with the Hp 2-2 protein. Specifically, heme iron associated with the Hp2-2/Hb complex is more redox active than other Hp type complexes and has been shown in a number of systems to lead to increased levels of oxidative stress in the form of oxidized lipids and decreased lipoprotein function. In addition, Hp 2-2/Hb complexes are cleared less efficiently from the circulation, leading to a buildup of iron in the plasma and in tissues. Recent analyses from clinical studies utilizing vitamin E treatment have shown beneficial results specifically in the diabetic Hp 2-2 genotype population. The use of vitamin E in the treatment of Hp 2-2 diabetics has the potential to greatly reduce medical costs and improve quality of life in the ever-growing diabetic population.

The genetics of vascular complications in diabetes mellitus

Prospective identification of which individuals with diabetes mellitus (DM) are at greatest risk for developing cardiovascular disease (CVD) complications would have considerable public health importance by allowing the allocation of limited resources to be focused on those individuals who would most benefit from aggressive intervention. Over the past 20 years genetic disease association studies have demonstrated that polymorphisms at specific genetic loci may identify those individuals at greatest risk for developing CVD in the setting of DM. This article reviews the evidence accumulated to date on four polymorphic loci with the aim of explaining how these polymorphisms modify the risk for CVD in DM by modifying the functional activity of a specific gene. Use of the knowledge of these genetic differences among individuals in targeting drug therapy (pharmacogenomics) is also discussed.

Toxicological consequences of extracellular hemoglobin: biochemical and physiological perspectives

Under normal physiology, human red blood cells (RBCs) demonstrate a circulating lifespan of approximately 100-120 days with efficient removal of senescent RBCs taking place via the reticuloendothelial system, spleen, and bone marrow phagocytosis. Within this time frame, hemoglobin (Hb) is effectively protected by efficient RBC enzymatic systems designed to allow for interaction between Hb and diffusible ligands while preventing direct contact between Hb and the external environment. Under normal resting conditions, the concentration of extracellular Hb in circulation is therefore minimal and controlled by specific plasma and cellular (monocyte/macrophage) binding proteins (haptoglobin) and receptors (CD163), respectively. However, during pathological conditions leading to hemolysis, extracellular Hb concentrations exceed normal plasma and cellular binding capacities, allowing Hb to become a biologically relevant vasoactive and redox active protein within the circulation and at extravascular sites. Under conditions of genetic, drug-induced, and autoimmune hemolytic anemias, large quantities of Hb are introduced into the circulation and often lead to acute renal failure and vascular dysfunction. Interestingly, the study of chemically modified Hb for use as oxygen therapeutics has allowed for some basic understanding of extracellular Hb toxicity, particularly in the absence of functional clearance mechanisms and in circulatory antioxidant depleted states.

Pharmacogenomics in prevention of diabetic cardiovascular disease: utilization of the haptoglobin genotype in determining benefit from vitamin E

Numerous large clinical trials have been carried out over the past several years testing the ability of the antioxidant vitamin E to prevent diabetic cardiovascular disease. Meta-analysis of these studies has demonstrated that vitamin E does not provide any cardiovascular protection and may be associated with an increase in mortality. However, these studies did not address possible benefit to subgroups with increased oxidative stress. In this review we provide supporting clinical evidence and a mechanistic basis for utilizing a genetic marker, the haptoglobin genotype, in determining whether vitamin E therapy may or may not be beneficial for a given patient with diabetes.

Hypothesis--haptoglobin genotype and diabetic nephropathy

Vascular complications cause serious morbidity in patients with diabetes mellitus. Three such complications are nephropathy, retinopathy and accelerated atherosclerotic cardiovascular disease. There is currently scant evidence of a genetic marker that predicts which patients will have vascular complications. Oxidative stress has an important role in the development of diabetic vascular complications. Haptoglobin (Hp) is a hemoglobin-binding protein that has a major role in protecting against heme-driven oxidative stress. There are two common alleles for Hp (1 and 2) and, therefore, three common Hp genotypes: Hp 1-1, Hp 2-1, and Hp 2-2. The antioxidant protection provided by Hp is genotype-dependent; the protein encoded by Hp 1-1 provides superior antioxidant protection compared with that encoded by Hp 2-2. We have shown that diabetic individuals with Hp 2-2 are more likely to develop nephropathy, retinopathy, and cardiovascular disease than those with the Hp 2-1 or Hp 1-1 genotypes.

Markedly Increased Urinary Preprohaptoglobin and Haptoglobin in Passive Heymann Nephritis: A Differential Proteomics Approach

Membranous nephropathy (MN), a common cause of idiopathic nephrotic syndrome in adults, remains a potentially devastating problem worldwide. At present, there is no reliable noninvasive method for predicting and/or monitoring this glomerular disease, and its pathophysiology remains poorly understood. In the present study, the urinary proteome profile of rats after 10 days of an induction of passive Heymann nephritis (PHN), which resembles human MN, was compared to that of the baseline (control) urine prior to the induction of PHN by anti-Fx1A injection. Each pool of PHN and control urine samples (n = 10 each) was labeled with different fluorescent dyes (Cy3 or Cy5), and equal amounts of the labeled proteins of both pools were resolved in the same 2D gel, together with an internal standard labeled with Cy2. Two-dimensional difference gel electrophoresis revealed a number of protein spots whose expression levels were altered during PHN. Eighteen protein spots with >1.5-fold changes and p < 0.05 were selected for subsequent identification by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. They were successfully identified as serum albumin precursor, alpha-1-antitrypsin, preprohaptoglobin, liver-regeneration-related protein, and transthyretin (which increased during PHN) and E-cadherin, MPP7, tropomyosin beta, kallikrein, and alpha-2u globulin (which decreased in the PHN urine). Among these proteins, the increase in urinary preprohaptoglobin has particularly drawn our attention because of its byproduct, haptoglobin (Hp), which is involved in the protection of tissue damage from hemoglobin-induced oxidative stress. Western blotting and enzyme-linked immunosorbent assay clearly showed a markedly increased level of Hp in the urine, but not in the serum, of the PHN animals. Our findings may lead to a significant advance in the attempt to define a new therapeutic target and/or novel biomarker for human MN.

Haptoglobin genotype modulates the balance of Th1/Th2 cytokines produced by macrophages exposed to free hemoglobin

The haptoglobin genotype has been demonstrated to be an independent risk factor for CVD in multiple epidemiological studies. The primary function of haptoglobin is to mitigate the deleterious effects of extracorpuscular hemoglobin. We sought to determine if the protein products of the two haptoglobin alleles differed in their ability to modulate the cytokine profile produced by macrophages in response to hemoglobin. Peripheral blood mononuclear cells were isolated from normal human volunteers and cultured in the presence of complexes formed by the protein products of the two different haptoglobin alleles with hemoglobin. The release of specific cytokines in the conditioned media of these cells was assessed by ELISA. We found that the haptoglobin 1 allele protein product-hemoglobin complex stimulated the secretion of significantly more Il-6 and Il-10 than the haptoglobin 2 allele protein product-hemoglobin complex. We demonstrate that the release of these cytokines is dependent on the liganding of the haptoglobin-hemoglobin complex to the CD163 receptor and the activity of casein kinase II. Haptoglobin genotype modulates the balance of inflammatory (Th1) and anti-inflammatory (Th2) cytokines produced by macrophages exposed to free hemoglobin. This may have implications in understanding inter-individual differences in the inflammatory response to hemorrhage.

Retinal capillary basement membrane thickness in diabetic mice genetically modified at the haptoglobin locus

BACKGROUND

Individuals with diabetes mellitus (DM) homozygous for the haptoglobin (Hp) 1 allele are at decreased risk of retinopathy as compared to DM individuals with the Hp 2 allele. We sought to recapitulate these findings in DM mice genetically modified at the Hp locus.

METHODS

An early morphological characteristic of the microangiopathy seen in diabetic retinal disease is retinal capillary basement membrane (RCBM) thickening. RCBM thickness as assessed by electron microscopy was performed on a total of 12 eyes taken from three mice in each of the four study groups (three eyes from C57Bl/6 Hp 1 and C57Bl/6 Hp 2 mice with and without streptozotocin-induced diabetes).

RESULTS

The non-parametric Kruskal-Wallis ANOVA test demonstrated that there was a highly significant difference between the four groups of mice (P < 0.0001). Mann-Whitney tests for specific pair-wise comparisons demonstrated that there was no significant difference in the RCBM thickness between Hp 1 and Hp 2 mice (p = 0.70) or between DM Hp 1 and non-DM Hp 1 mice (p = 0.42). However, induction of diabetes resulted in a marked increase in RCBM thickness in Hp 2 mice compared to non-DM Hp 2 mice (p = 0.0004) and compared to DM Hp 1 mice (p = 0.0005).

CONCLUSIONS

A highly significant increase in RCBM thickness was observed in DM mice with the Hp 2 genotype. These data provide important support for association studies done in humans showing an increased prevalence of diabetic retinopathy in individuals with the Hp 2 genotype.

Application of pharmacogenomics in the prevention of diabetic cardiovascular disease: Mechanistic basis and clinical evidence for utilization of the haptoglobin genotype in determining benefit from antioxidant therapy

Atherosclerotic cardiovascular disease (CVD) and diabetic vascular disease have been associated with an increase in oxidative stress. Mechanistic studies in vitro and in animals have demonstrated a direct role for oxidatively modified protein and lipid molecules in the pathophysiology of these diseases. As a result of this oxidation hypothesis numerous studies have been carried out over the past 5-10 years testing the ability of antioxidant vitamins to decrease the incidence of these diseases. The general consensus from these studies, involving over 200,000 individuals, has been that antioxidant vitamins do not provide any vascular protection. Moreover, several of these studies have demonstrated that antioxidant supplementation may be associated with an increased incidence of disease and mortality. One reason why these antioxidant vitamins may have failed to demonstrate benefit may have been due to inappropriate patient selection. In this review we provide supporting clinical evidence and a mechanistic basis for utilizing a genetic marker, the haptoglobin (Hp) genotype, in determining whether antioxidant vitamin therapy may or may not be beneficial for a given patient with diabetes.

Haptoglobin 2-2 Phenotype is a Risk Factor for Type 2 Diabetes in Ghana

We have investigated the role of haptoglobin gene polymorphisms in 129 type 2 diabetic patients and 87 non-diabetic subjects, classified by the ADA criteria, in Ghana. The diabetic subjects were recruited consecutively from the National Diabetic Management and Research Center of the University of Ghana Medical School, Korle-Bu, Accra, Ghana and were categorized by their haptoglobin phenotypes. The haptoglobin 2 allele was determined to be a risk factor for type 2 diabetes in Ghana (OR = 6.1, 95% CI = 1.8-21.2; P = .0.001) while the Hp1 allele appeared protective (OR = 0.56, 95% CI = 0.31-1.0; P = .06). The deleterious role of the Hp2 allele was further evidenced by the reduced risk associated with Hp2-1M mutant heterozygotes, who produce less Hp2 protein than the normal Hp2-1 heterozygote. (OR = 0.52, 95% CI = 0.27-1.0; P = 0.06). The subjects with the homozygous Hp2 allele were also hypertensive and overweight. There was no difference (p > 0.05) in the levels of triglycerides, total cholesterol, LDL and HDL between diabetic subjects with different haptoglobin phenotypes. We conclude that hypertensive and overweight individuals with the Hp2-2 phenotype in Ghana are at a high risk of developing type 2 diabetes and may require a more aggressive management.

Haptoglobin genotype- and diabetes-dependent differences in iron-mediated oxidative stress in vitro and in vivo

We have recently demonstrated in multiple independent population-based longitudinal and cross sectional analyses that the haptoglobin 2-2 genotype is associated with an increased risk for diabetic cardiovascular disease. The chief function of haptoglobin (Hp) is to bind to hemoglobin and thereby prevent hemoglobin-induced oxidative tissue damage. This antioxidant function of haptoglobin is mediated in part by the ability of haptoglobin to prevent the release of iron from hemoglobin on its binding. We hypothesized that there may be diabetes- and haptoglobin genotype-dependent differences in the amount of catalytically active redox active iron derived from hemoglobin. We tested this hypothesis using several complementary approaches both in vitro and in vivo. First, measuring redox active iron associated with haptoglobin-hemoglobin complexes in vitro, we demonstrate a marked increase in redox active iron associated with Hp 2-2-glycohemoglobin complexes. Second, we demonstrate increased oxidative stress in tissue culture cells exposed to haptoglobin 2-2-hemoglobin complexes as opposed to haptoglobin 1-1-hemoglobin complexes, which is inhibitable by desferrioxamine by either a chelation or reduction mechanism. Third, we demonstrate marked diabetes-dependent differences in the amount of redox active iron present in the plasma of mice genetically modified expressing the Hp 2 allele as compared with the Hp 1 allele. Taken together these data implicate redox active iron in the increased susceptibility of individuals with the Hp 2 allele to diabetic vascular disease.