Erythropoietin, iron, and erythropoiesis

The clinical relevance of detectable plasma iron species in iron overload states and subsequent to intravenous iron-carbohydrate administration

Many disorders of iron homeostasis (e.g., iron overload) are associated with the dynamic kinetic profiles of multiple non-transferrin bound iron (NTBI) species, chronic exposure to which is associated with deleterious end-organ effects. Here we discuss the chemical nature of NTBI species, challenges with measuring NTBI in plasma, and the clinical relevance of NTBI exposure based on source (iron overload disorder vs. intravenous iron-carbohydrate complex administration). NTBI is not a single entity but consists of multiple, often poorly characterized species, some of which are kinetically non-exchangeable while others are relatively exchangeable. Prolonged presence of plasma NTBI is associated with excessive tissue iron accumulation in susceptible tissues, with consequences, such as endocrinopathy and heart failure. In contrast, intravenous iron-carbohydrate nanomedicines administration leads only to transient NTBI appearance and lacks evidence for association with adverse clinical outcomes. Assays to measure plasma NTBI are typically technically complex and remain chiefly a research tool. There have been two general approaches to estimating NTBI: capture assays and redox-activity assays. Early assays could not avoid capturing some iron from transferrin, thus overestimating NTBI. By contrast, some later assays may have promoted the donation of NTBI species to transferrin during the assay procedure, potentially underestimating NTBI levels. The levels of transferrin saturation at which NTBI species have been detectable have varied between different methodologies and between patient populations studied.

Clinical significance of a novel reticulocyte-based erythropoietin resistance index in HD patients: A retrospective study

INTRODUCTION

The erythropoietin resistance index (ERI) is an indicator of erythropoiesis-stimulating agent (ESA) responsiveness and is typically calculated using Hb. However, Hb does not directly reflect ESA-induced erythropoiesis because of its long-term nature. We thus designed a novel ERI calculated with reticulocyte Hb (RetHb), a real-time index, and investigated its association with mortality in HD patients.

METHODS

We calculated the ERI using the change in RetHb before and after ESA administration (ERI_ΔRetHb ) and retrospectively analyzed its association with 3-year all-cause mortality using Kaplan-Meier survival curves and Cox regression analyses.

RESULTS

A total of 102 patients were included. Patients with the highest ERI_ΔRetHb had the worst prognosis according to the Kaplan-Meier survival curves (Log-rank p = 0.02). Multivariate Cox regression analysis showed that the ERI_ΔRetHb was significantly and independently associated with all-cause mortality (hazard ratio: 9.82, 95% CI [1.50, 64.41], p = 0.02).

CONCLUSION

The ERI_ΔRetHb was significantly and independently associated with all-cause mortality in HD patients.

Limiting hepatic Bmp-Smad signaling by matriptase-2 is required for erythropoietin-mediated hepcidin suppression in mice

Hepcidin, the main regulator of iron homeostasis, is repressed when erythropoiesis is acutely stimulated by erythropoietin (EPO) to favor iron supply to maturing erythroblasts. Erythroferrone (ERFE) has been identified as the erythroid regulator that inhibits hepcidin in stress erythropoiesis. A powerful hepcidin inhibitor is the serine protease matriptase-2, encoded by TMPRSS6, whose mutations cause iron refractory iron deficiency anemia. Because this condition has inappropriately elevated hepcidin in the presence of high EPO levels, a role is suggested for matriptase-2 in EPO-mediated hepcidin repression. To investigate the relationship between EPO/ERFE and matriptase-2, we show that EPO injection induces Erfe messenger RNA expression but does not suppress hepcidin in Tmprss6 knockout (KO) mice. Similarly, wild-type (WT) animals, in which the bone morphogenetic protein-mothers against decapentaplegic homolog (Bmp-Smad) pathway is upregulated by iron treatment, fail to suppress hepcidin in response to EPO. To further investigate whether the high level of Bmp-Smad signaling of Tmprss6 KO mice counteracts hepcidin suppression by EPO, we generated double KO Bmp6-Tmprss6 KO mice. Despite having Bmp-Smad signaling and hepcidin levels that are similar to WT mice under basal conditions, double KO mice do not suppress hepcidin in response to EPO. However, pharmacologic downstream inhibition of the Bmp-Smad pathway by dorsomorphin, which targets the BMP receptors, improves the hepcidin responsiveness to EPO in Tmprss6 KO mice. We concluded that the function of matriptase-2 is dominant over that of ERFE and is essential in facilitating hepcidin suppression by attenuating the BMP-SMAD signaling.

Cardiac hypertrophy in anion exchanger 1-null mutant mice with severe hemolytic anemia

Anion exchanger 1 (AE1; SLC4A1), the plasma membrane Cl(-)/HCO(3)(-) exchanger of erythrocytes, is also expressed in heart. The aim of this study was to assess the role of AE1 in heart function through study of AE1-null (AE1(-/-)) mice, which manifest severe hemolytic anemia resulting from erythrocyte fragility. Heart weight-to-body weight ratios were significantly higher in the AE1(-/-) mice than in wild-type (AE1(+/+)) littermates at both 1-3 days postnatal (3.01 +/- 0.38 vs. 1.45 +/- 0.04) and at 7 days postnatal (9.45 +/- 0.53 vs. 4.13 +/- 0.41), indicating that loss of AE1 led to cardiac hypertrophy. Heterozygous (AE1(+/-)) mice had no signs of cardiac hypertrophy. Morphology of the adult AE1(-/-) mutant heart revealed an increased left ventricular mass, accompanied by increased collagen deposition and fibrosis. M-mode echocardiography revealed dysfunction of the AE1(-/-) hearts, including dilated left ventricle end diastole and systole and expanded left ventricular mass compared with AE1(+/+) hearts. Expression of intracellular pH-regulatory mechanisms in the hypertrophic myocardium of neonate AE1(-/-) mutant mice was indistinguishable from AE1(+/-) and AE1(+/+) mice, as assessed by quantitative real-time RT-PCR. Confocal immunofluorescence revealed that, in normal mouse myocardium, AE1 is sarcolemmal, whereas AE3 and slc26a6 are found both at the sarcolemma and in internal membranes (T tubules and sarcoplasmic reticulum). These results indicate that AE1(-/-) mice, which suffer from severe hemolytic anemia and spherocytosis, display cardiac hypertrophy and impaired cardiac function, reminiscent of findings in patients with hereditary abnormalities of red blood cells. No essential role for AE1 in heart function was found.

A randomized, controlled trial of the effects of adding vitamin B12 and folate to erythropoietin for the treatment of anemia of prematurity

BACKGROUND

Premature infants, especially those with birth weights of <1500 g, often suffer from anemia of prematurity and associated problems. Erythropoietin therapy is a safe effective way to prevent and to treat anemia of prematurity. We hypothesized that combined administration of vitamin B12 and folate with erythropoietin and iron would enhance erythropoietin-induced erythropoiesis.

METHODS

In a randomized, controlled trial, 64 premature infants (birth weight: 801-1300 g) receiving erythropoietin and iron supplementation were assigned randomly to receive either vitamin B12 (3 microg/kg per day) and folate (100 microg/kg per day) (treatment group) or a lower dose of folate (60 microg/kg per day) (control group).

RESULTS

During the 4-week observation period, vitamin B12 and folate enhanced erythropoietin-induced erythropoiesis significantly, as indicated by a 10% increase in red blood cell counts, compared with folate alone. Hemoglobin and hematocrit levels remained stable in the treatment group, whereas they decreased in the control group. Vitamin B12 levels in the treatment group increased over baseline and control values, whereas red blood cell folate levels were comparable between the groups. Subsequent analysis showed slight nonsignificant differences in baseline red blood cell count, hemoglobin level, hematocrit level, and mean corpuscular volume values, which must be addressed as a limitation.

CONCLUSIONS

With the limitation of a slight imbalance in baseline data between the study groups, combined therapy with vitamin B12, folate, erythropoietin, and orally and intravenously administered iron seemed more effective in stimulating erythropoiesis among premature infants, compared with erythropoietin, iron, and low-dose folate alone. Additional trials are necessary to confirm these data.

Long-term reversal of chronic anemia using a hypoxia-regulated erythropoietin gene therapy

Anemia is a common clinical problem, and there is much interest in its role in promoting left ventricular hypertrophy through increasing cardiac workload. Normally, red blood cell production is adjusted through the regulation of erythropoietin (Epo) production by the kidney. One important cause of anemia is relative deficiency of Epo, which occurs in most types of renal disease. Clinically, this can be corrected by supplementation with recombinant Epo. Here we describe an oxygen-regulated gene therapy approach to treating homozygous erythropoietin-SV40 T antigen (Epo-TAg(h)) mice with relative erythropoietin deficiency. We used vectors in which murine Epo expression was directed by an Oxford Biomedica hypoxia response element (OBHRE) or a constitutive cytomegalovirus (CMV) promoter. Both corrected anemia, but CMV-Epo-treated mice acquired fatal polycythemia. In contrast, OBHRE-Epo corrected the hematocrit level in anemic mice to a normal physiologic level that stabilized without resulting in polycythemia. Importantly, the OBHRE-Epo vector had no significant effect on the hematocrit of control mice. Homozygous Epo-TAg(h) mice display cardiac hypertrophy, a common adaptive response in patients with chronic anemia. In the OBHRE-Epo-treated Epo-TAg(h) mice, we observed a significant reversal of cardiac hypertrophy. We conclude that the OBHRE promoter gives rise to physiologically regulated Epo secretion such that the hematocrit level is corrected to healthy in anemic Epo-TAg(h) mice. This establishes that a hypoxia regulatory mechanism similar to the natural mechanism can be achieved, and it makes EPO gene therapy more attractive and safer in clinical settings. We envisage that this control system will allow regulated delivery of therapeutic gene products in other ischemic settings.