Renal Handling of Circulating and Renal-Synthesized Hepcidin and Its Protective Effects against Hemoglobin–Mediated Kidney Injury

Abstract

Urinary hepcidin may have protective effects against AKI. However, renal handling and the potential protective mechanisms of hepcidin are not fully understood. By measuring hepcidin levels in plasma and urine using mass spectrometry and the kidney using immunohistochemistry after intraperitoneal administration of human hepcidin-25 (hhep25) in C57Bl/6N mice, we showed that circulating hepcidin is filtered by the glomerulus and degraded to smaller isoforms detected in urine but not plasma. Moreover, hepcidin colocalized with the endocytic receptor megalin in proximal tubules, and compared with wild-type mice, megalin-deficient mice showed higher urinary excretion of injected hhep25 and no hepcidin staining in proximal tubules that lack megalin. This indicates that hepcidin is reaborbed in the proximal tubules by megalin dependent endocytosis. Administration of hhep25 concomitant with or 4 hours after a single intravenous dose of hemoglobin abolished hemoglobin-induced upregulation of urinary kidney injury markers (NGAL and KIM-1) and renal Interleukin-6 and Ngal mRNA observed 24 hours after administration but did not affect renal ferroportin expression at this point. Notably, coadministration of hhep25 and hemoglobin but not administration of either alone greatly increased renal mRNA expression of hepcidin-encoding Hamp1 and hepcidin staining in distal tubules. These findings suggest a role for locally synthesized hepcidin in renal protection. Our observations did not support a role for ferroportin in hhep25-mediated protection against hemoglobin-induced early injury, but other mechanisms of cellular iron handling may be involved. In conclusion, our data suggest that both systemically delivered and locally produced hepcidin protect against hemoglobin-induced AKI.

Association of hepcidin‐25 with survival after kidney transplantation

“…This study, in stable RTR patients with a large variation in kidney function, did not show an association of serum hepcidin with graft failure, cardiovascular mortality and all‐cause mortality in age‐ and sex‐adjusted analysis. This is in contrast with results from other populations where serum hepcidin has been shown to be associated with increased risk for cardiovascular events as well as all‐cause mortality, and with a protective effect on kidney function .…”

Systemic and local hepcidin as emerging and important peptides in renal homeostasis and pathology

“…Veuthey et al have suggested that prohepcidin is produced by proximal tubule, though it has to be mentioned that authors of this study did not measure mRNA expression, but rather immunohistochemical expression of this peptide, which could mean that the presence of the peptide inside proximal tubular cells is due to absorption from the blood filtrate which contains the systemic form of the peptide . van Swelm et al have shown that renal hepcidin is secreted in distal tubule, and one of the causes of increased local hepcidin expression occurs via hemoglobin‐induced kidney injury (Table ). But, hepcidin expression is not confined to tubular cells, because there is evidence of hepcidin expression in the leukocytes of renal interstitium during inflammation .…”

“…Local hepcidin could affect renal iron homeostasis by regulating the expression of DMT1 and/or TFR1, which are known renal iron carriers [40,43,44]. Interestingly, local hepcidin does not seem to affect FPN expression in all the portions of the nephron, which is the classical mode of action of hepcidin in other organs [21,37,40]. In addition, renal FPN seems to be affected by cellular iron load not by increasing its expression, but by being distributed to the surface of the basolateral membrane [45].…”

“…According to Kulaksiz et al, renal hepcidin is mostly located in the epithelial cells of the cortical thick ascending limb of nephron tubules and in the connecting tubules [34]. Local renal hepcidin produced by these cells is most likely secreted in the luminal surface of the epithelium [21,34,35], though other authors suggest that the local hepcidin could be secreted in bloodstream via basal membrane [36]. Veuthey et al have suggested that prohepcidin is produced by proximal tubule, though it has to be mentioned that authors of this study did not measure mRNA expression, but rather immunohistochemical expression of this peptide, which could mean that the presence of the peptide inside proximal tubular cells is due to absorption from the blood filtrate which contains the systemic form of the peptide [36].…”

“…In addition, renal FPN seems to be affected by cellular iron load not by increasing its expression, but by being distributed to the surface of the basolateral membrane [45]. Finally, studies show that hepcidin might affect renal iron metabolism independently from FPN [21,37,42]. On the other hand, similarly to liver hepcidin, renal hepcidin could be induced through BMP6/SMAD pathway [35].…”