Liver iron modulates hepcidin expression during chronically elevated erythropoiesis in mice

Abstract

The liver-derived peptide hepcidin controls the balance between iron demand and iron supply. By inhibiting the iron export activity of ferroportin, hepcidin modulates iron absorption and delivery from the body's stores. The regulation of hepcidin, however, is not completely understood and includes a variety of different signals. We studied iron metabolism and hepcidin expression in mice constitutively overexpressing erythropoietin (Epo) (Tg6 mice), which leads to excessive erythropoiesis. We observed a very strong down-regulation of hepcidin in Tg6 mice that was accompanied by a strong increase in duodenal expression of ferroportin and divalent metal tranporter-1, as well as enhanced duodenal iron absorption. Despite these compensatory mechanisms, Tg6 mice displayed marked circulating iron deficiency and low levels of iron in liver, spleen, and muscle. To elucidate the primary signal affecting hepcidin expression during chronically elevated erythropoiesis, we increased iron availability by either providing iron (thus further increasing the hematocrit) or reducing erythropoiesis-dependent iron consumption by means of splenectomy. Both treatments increased liver iron and up-regulated hepcidin expression and the BMP6/SMAD pathway despite continuously high plasma Epo levels and sustained erythropoiesis. This suggests that hepcidin expression is not controlled by erythropoietic signals directly in this setting. Rather, these results indicate that iron consumption for erythropoiesis modulates liver iron content, and ultimately BMP6 and hepcidin. Analysis of the BMP6/SMAD pathway targets showed that inhibitor of DNA binding 1 (ID1) and SMAD7, but not transmembrane serine protease 6 (TMPRSS6), were up-regulated by increased iron availability and thus may be involved in setting the upper limit of hepcidin. Conclusion: We provide evidence that under conditions of excessive and effective erythropoiesis, liver iron regulates hepcidin expression through the BMP6/SMAD pathway. (HEPATOLOGY 2013;58:2122-2132

Erythropoietin's inhibiting impact on hepcidin expression occurs indirectly

“…1, D and E). Therefore, prolonged administration of high Epo doses leads to Fpn-mediated liver iron mobilization, obviously to support the erythroid demand, an observation in agreement with our previous results in Epo-overexpressing mice (3). In line with these findings, the hepatic mRNA levels of the iron stores regulator BMP6, which is key in the control of hepcidin expression (8,11), were significantly decreased only after 10 days of treatment (Fig.…”

“…Mice were maintained under standard conditions with free access to water and food (170 mg iron/kg) in compliance with the "Principles of Laboratory Animal Care," as described previously (3). We used 6-to 8-wk-old male ICR CD1 and C57BL/6 wild-type mice, as well as TgEpoR mice (TgEPOR), with EpoR expression restricted to hematopoietic tissue (erythroid GATA-1 promoter/EpoR cDNA transgene) established on the EpoR Ϫ/Ϫ background (18).…”

“…Hepcidin, BMP6, and ERFE mRNA levels were measured by quantitative RT-PCR, as previously described (3). Total RNA isolated from liver and spleen using TRI reagent (Sigma) was reverse transcribed into cDNA with Proto Script M-MuLV First Strand cDNA Synthesis Kit (New England Biolabs), and the obtained cDNA served as a template for RT-PCR, based on the TaqMan methodology (Life Technologies).…”

Identification of erythroferrone as an erythroid regulator of iron metabolism

“…Hemorrhage/ERFE-mediated suppression of hepcidin mRNA was observed over a large range of iron loads and baseline hepcidin mRNA concentrations, and the ERFE signal appeared to act on the hepcidin gene independently from the iron-regulated BMP/Smad pathway. A recent study of hepcidin regulation by iron in an Epo-overproducing mouse model 25 also supports the existence of independent pathways regulating hepcidin in response to hepatic iron load and erythropoietic activity.…”

Erythroferron: Modern Concepts of Its Role in Iron Metabolism Regulation

“…Установлено существо-вание независимых путей, регулирующих уровень гепсидина в ответ на нагрузку печени железом и актив-ность эритропоэтина [22]. Подавление секреции геп-сидина на фоне повышения активности эритропоэза лежит в основе развития перегрузки железом при наследственных анемиях с неэффективным эритро-поэзом.…”