Unraveling Hepcidin Plasma Protein Binding: Evidence from Peritoneal Equilibration Testing.

Peptide hormone hepcidin regulates systemic iron metabolism and has been described to be partially bound to α2-macroglobulin and albumin in blood. However, the reported degree of hepcidin protein binding varies between <3% and ≈89%. Since protein-binding may influence hormone function and quantification, better insight into the degree of hepcidin protein binding is essential to fully understand the biological behavior of hepcidin and interpretation of its measurement in patients. Here, we used peritoneal dialysis to assess human hepcidin protein binding in a functional human setting for the first time. We measured freely circulating solutes in blood and peritoneal fluid of 14 patients with end-stage renal disease undergoing a peritoneal equilibration test to establish a curve describing the relation between molecular weight and peritoneal clearance. Calculated binding percentages of total cortisol and testosterone confirmed our model. The protein-bound fraction of hepcidin was calculated to be 40% (±23%). We, therefore, conclude that a substantial proportion of hepcidin is freely circulating. Although a large inter-individual variation in hepcidin clearance, besides patient-specific peritoneal transport characteristics, may have affected the accuracy of the determined binding percentage, we describe an important step towards unraveling human hepcidin plasma protein binding in vivo including the caveats that need further research.

Tubular reabsorption and local production of urine hepcidin-25.

BACKGROUND: Hepcidin is a central regulator of iron metabolism. Serum hepcidin levels are increased in patients with renal insufficiency, which may contribute to anemia. Urine hepcidin was found to be increased in some patients after cardiac surgery, and these patients were less likely to develop acute kidney injury. It has been suggested that urine hepcidin may protect by attenuating heme-mediated injury, but processes involved in urine hepcidin excretion are unknown. METHODS: To assess the role of tubular reabsorption we compared fractional excretion (FE) of hepcidin-25 with FE of ß2-microglobulin (ß(2)m) in 30 patients with various degrees of tubular impairment due to chronic renal disease. To prove that hepcidin is reabsorbed by the tubules in a megalin-dependent manner, we measured urine hepcidin-1 in wild-type and kidney specific megalin-deficient mice. Lastly, we evaluated FE of hepcidin-25 and ß(2)m in 19 patients who underwent cardiopulmonary bypass surgery. Hepcidin was measured by a mass spectrometry assay (MS), whereas ß(2)m was measured by ELISA. RESULTS: In patients with chronic renal disease, FE of hepcidin-25 was strongly correlated with FE of ß(2)m (r = 0.93, P <0.01). In megalin-deficient mice, urine hepcidin-1 was 7-fold increased compared to wild-type mice (p < 0.01) indicating that proximal tubular reabsorption occurs in a megalin- dependent manner. Following cardiac surgery, FE of hepcidin-25 increased despite a decline in FE of ß(2)m, potentially indicating local production at 12-24 hours. CONCLUSIONS: Hepcidin-25 is reabsorbed by the renal tubules and increased urine hepcidin-25 levels may reflect a reduction in tubular uptake. Uncoupling of FE of hepcidin-25 and ß(2)m in cardiac surgery patients suggests local production.

Intra-individual variability of serum hepcidin-25 in haemodialysis patients using mass spectrometry and ELISA.

BACKGROUND: Measurement of serum hepcidin levels may provide a useful alternative to the current methods of determining iron status in chronic haemodialysis (HD) patients. However, the biological variability of this pivotal regulator of iron homeostasis is unclear, and the impact of inflammation, dialysis clearance and iron therapy on hepcidin variability has not been established. METHODS: Two independent studies in chronic HD patients were conducted; serum hepcidin levels were measured at the start of dialysis sessions in 20 UK patients and in 43 Dutch patients by mass spectrometry (MS). Samples from UK patients were also analysed by a competitive enzyme-linked immunosorbent assay (cELISA). Coefficient of variance (CV(1)) was calculated and potential factors affecting CV(1) were also examined. RESULTS: The median CV(1) (inter-quartile range) was 23% (17-28) for the UK MS, 26% (17-48) for the Dutch MS and 23% (17-39) for the UK cELISA. The CV(1) was similar in those patients receiving and those not receiving regular intravenous iron. The CV(1) was not associated with the degree of inflammation. Hepcidin levels were higher following an inter-dialytic period of 3 versus 2 days (P = 0.02). CONCLUSIONS: These findings suggest considerable variability of serum hepcidin levels in HD patients. Inflammation and the use of iron did not impact on the degree of variability, and hepcidin levels were higher after an inter-dialytic period of 3 versus 2 days. These findings need to be taken into account in future studies assessing the utility of serum hepcidin as a guide to the use of iron or erythropoiesis-stimulating agents therapy.

High urinary excretion of kidney injury molecule-1 is an independent predictor of end-stage renal disease in patients with IgA nephropathy.

BACKGROUND: The variable course of immunoglobulin A nephropathy (IgAN) warrants accurate tools for the prediction of progression. Urinary kidney injury molecule-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) are markers for the detection of early tubular damage caused by various renal conditions. We evaluated the prognostic value of these markers in patients with IgAN. METHODS: We included patients (n = 65, 72% male, age 43 ± 13 years) with biopsy-proven IgAN, who were evaluated for proteinuria. Urinary KIM-1 and NGAL were measured by enzyme-linked immunosorbent assay. We analysed data using Cox regression for the outcome end-stage renal disease (ESRD). RESULTS: Median serum creatinine was 142 µmol/L and proteinuria 2.2 g/day. During follow-up (median 75 months), 23 patients (35%) developed ESRD. In patients with IgAN median urinary KIM-1 excretion was 1.7 ng/min and NGAL excretion was 47 ng/min, both significantly higher than in healthy controls. KIM-1 and NGAL were correlated with proteinuria (r = 0.40 and 0.34, respectively, P < 0.01) and each other (r = 0.53, P < 0.01) but not with estimated glomerular filtration rate (eGFR). Interestingly, KIM-1 was not significantly correlated with the excretion of α(1)-microglobulin (α(1)m) and ß(2)-microglobulin (ß(2)m), known markers of tubular injury. Univariate analysis showed that baseline serum creatinine and urinary excretion of total protein, α(1)m, ß(2)m, immunoglobulin G, KIM-1 and NGAL were significantly associated with ESRD. By multivariate analysis, serum creatinine and KIM-1 excretion proved to be significant independent predictors of ESRD. CONCLUSION: KIM-1 and NGAL excretion are increased in patients with IgAN and correlate with proteinuria but not with eGFR. Baseline serum creatinine and urinary KIM-1, but not proteinuria, are independent predictors of ESRD.

Immunochemical and mass-spectrometry-based serum hepcidin assays for iron metabolism disorders.

BACKGROUND: Hepcidin is an iron-regulatory peptide hormone that consists of 3 isoforms: bioactive hepcidin-25, and inactive hepcidin-22 and hepcidin-20. Hepcidin is instrumental in the diagnosis and monitoring of iron metabolism disorders, but reliable methods for its quantification in serum are sparse, as is knowledge of their relative analytical strengths and clinical utility. METHODS: We developed a competitive (c)-ELISA and an immunocapture TOF mass-spectrometry (IC-TOF-MS) assay. Exploiting these 2 methods and our previously described weak cation exchange (WCX)-TOF-MS assay, we measured serum hepcidin concentrations in 186 patients with various disorders of iron metabolism and in 23 healthy controls. RESULTS: We found that (a) the relative differences in median hepcidin concentrations in various diseases to be similar, although the absolute concentrations measured with c-ELISA and WCX-TOF-MS differed; (b) hepcidin isoforms contributed to differences in hepcidin concentrations between methods, which were most prominent in patients with chronic kidney disease; and (c) hepcidin concentrations measured by both the c-ELISA and IC-TOF-MS correlated with ferritin concentrations <60 µg/L, and were suitable for distinguishing between iron deficiency anemia (IDA) and the combination of IDA and anemia of chronic disease. CONCLUSIONS: c-ELISA is the method of choice for the large-scale quantification of serum hepcidin concentrations, because of its low limit of detection, low cost, and high-throughput. Because of its specificity for bioactive hepcidin-25, WCX-TOF-MS can be regarded as a valuable special-purpose assay for disorders with variable concentrations of hepcidin isoforms, such as chronic kidney disease.

Serum hepcidin-25 levels in patients with chronic kidney disease are independent of glomerular filtration rate.

BACKGROUND: Hepcidin is a key regulator of iron homeostasis and levels are elevated in patients with chronic kidney disease (CKD). Hepcidin may explain the often observed imbalance in iron metabolism in patients with CKD. We evaluated the influence of estimated glomerular filtration rate (eGFR) on serum levels of hepcidin-25 and its isoforms in patients with renal dysfunction. METHODS: Serum levels of the biologically active hepcidin-25 and its isoforms were determined in CKD and dialysis patients by a mass spectrometry-based assay. RESULTS: In 83 patients with CKD not requiring dialysis, serum hepcidin-25 levels were not significantly increased (5.1 nM versus 4.2 nM, P = 0.30) and positively correlated with ferritin (r = 0.74, P < 0.01). Multiple regression analysis showed ferritin to be the only significant predictor of hepcidin-25 levels. Serum hepcidin-25 levels were not dependent on eGFR. In contrast, hepcidin-20 and total hepcidin levels showed an independent significant inverse correlation with eGFR. In 48 haemodialysis patients, median hepcidin-25 levels were significantly higher than in CKD patients (9.4 nM versus 5.1 nM, P < 0.001) and again strongly correlated with ferritin (r = 0.79, P < 0.001). CONCLUSIONS: eGFR is not a major determinant of serum hepcidin-25 levels. In contrast, the hepcidin isoforms hepcidin-20 and hepcidin-22 accumulate in patients with renal impairment.