Luminal hepcidin targets intestinal DMT1 Free

In this issue of Blood, Falabrègue et al¹ identify a new function for the iron-regulatory hormone hepcidin by showing that it acts within the intestinal lumen to target the iron importer divalent metal-ion transporter 1 (DMT1) and block iron absorption.

Iron is an essential nutrient for humans and other mammals. Given that iron deficiency and iron overload are both associated with significant morbidity and mortality, body iron levels must be maintained within narrow physiological limits. Moreover, given that no active, regulated mechanism exists to excrete excess iron, intestinal iron absorption is tightly controlled. Hepcidin is a peptide hormone that is a regulator of systemic iron homeostasis. Hepcidin lowers serum iron by blocking ferroportin (FPN)-mediated iron export from cells that absorb iron (enterocytes) or store/recycle iron (reticuloendothelial macrophages, hepatocytes). Historically, hepcidin was thought to be produced and secreted mainly by the liver, and the iron exporter FPN was presumed to be its major, if not only, target. More recent investigations have revealed that hepcidin is synthesized in many different cell and tissue types, and more localized autocrine/paracrine functions of hepcidin are emerging.² So far, extrahepatic hepcidin has been shown to mainly function the same as liver-derived hepcidin by blocking iron efflux via FPN in various cells.

In this exciting, paradigm-shifting study published by the French research groups of Peyssonnaux and Vaulont, a new function of hepcidin was described in which it does not act via the blood and FPN is not the molecular target.¹ These authors developed a genetically engineered mouse model in which a murine Hamp transgene (encoding hepcidin) was constitutively transactivated by a strong viral (cytomegalovirus) promoter but only in the intestinal epithelium (using the villin1/Cre system). They initially noted that mice with elevated intestinal hepcidin production developed hypoferremia and iron-deficiency anemia within a few weeks of birth. The endogenous hepatic Hamp gene was repressed in these mice, probably because of iron depletion/hypoxia, but, perhaps surprisingly, over-production of intestinal hepcidin increased circulating hepcidin levels by approximately twofold. Despite higher hepcidin levels, intestinal FPN levels did not decrease, which is consistent with previous reports that suggested that intestinal FPN is not regulated by hepcidin.^3,4 Falabrègue et al then made the potentially seminal discovery that hepcidin was also present within the intestinal lumen and that this was associated with decreased iron import into duodenal enterocytes and reduced expression of the main intestinal (nonheme) iron transporter DMT1. Given these unanticipated outcomes, the authors next sought to develop an experimental protocol to deliver hepcidin intraluminally to test the influence of exogenous hepcidin on DMT1 expression and iron absorption. For these studies, they used a mouse model of the iron-loading disorder hereditary hemochromatosis (HH), namely liver-specific, Hamp knockout (KO) mice. The approach was to engineer food-grade Lactococcus lactis to express murine Hamp and secrete hepcidin. Peroral administration of the hepcidin-secreting bacteria decreased DMT1 expression and iron absorption within hours of administration, and daily administration of the bacteria for 4 weeks prevented iron overload in liver-specific Hamp KO mice. The authors postulated that this approach could be effective at mitigating iron loading in humans with HH or other iron-overload disorders.

Hepcidin was previously shown to be produced by intestinal dendritic cells in a mouse model of intestinal inflammation in which it targeted FPN and caused iron sequestration in phagocytes, thereby altering the gut microbiota and facilitating intestinal repair.⁵ The current study by Falabrègue et al expands our knowledge of the possible functions of intestinal hepcidin and has provided critical new insight into the mechanisms that regulate iron homeostasis. The importance of this study comes from 3 key observations, namely (1) when Hamp is constitutively transactivated throughout the epithelium of the small and large intestines, hepcidin can be secreted into the blood in significant amounts and also into the intestinal lumen; (2) luminal hepcidin regulates iron absorption by reducing iron import into duodenal enterocytes via DMT1; and (3) luminal hepcidin blunts excessive iron absorption and mitigates iron loading in a mouse model of HH. The approach and outcomes of this investigation also raise several intriguing questions. First and foremost, do intestinal epithelial cells secrete hepcidin into the intestinal lumen under physiological conditions and, if so, by what cell type(s) and in which gut segment(s)? Second, what about in inflammatory states, when hepatic Hamp is transactivated by proinflammatory cytokines? Does intestinal hepcidin production also increase in response to inflammation and thus contribute to the development of the anemia of inflammation (by blocking iron absorption and reducing serum iron)? Third, could luminal secretion of hepcidin be an artifact of the overexpression system used in which CRE expression, driven by the villin1 promoter, transactivates a Hamp transgene throughout the epithelium of the entire intestinal tract?⁶ This concern, however, is mitigated by the discovery that intraluminal hepcidin downregulates iron absorption via interaction with DMT1, a clearly nonrandom regulatory process that has developed through evolutionary time, thus highlighting the potential physiological relevance of the luminal hepcidin-DMT1 interaction. And finally, how would this new proposed function of hepcidin translate to diets containing both nonheme and heme iron as in the prototypical, omnivorous human diet? This is a particularly important question because, in this study, mice were (presumably) fed diets containing only nonheme iron and DMT1 transports only nonheme iron. It is possible that blunting DMT1 expression could also decrease heme-iron absorption,⁷ but whether DMT1 is important for this process has not been clarified.

In summary, this investigation by the research groups of Peyssonnaux and Vaulont points toward a potential new function of hepcidin in which iron absorption is regulated from the apical surface of enterocytes by the hepcidin-DMT1 interaction as opposed to the traditional function of hepcidin that acts from the blood side on FPN. Although key concepts remain to be tested in future work, the intriguing outcomes of this study could, as postulated by the authors, lead to the development of new therapeutic approaches to prevent the excessive intestinal iron absorption that typifies genetic iron-overload disorders.

Conflict-of-interest disclosure: The author declares no competing financial interests.