Ironing out the role of hepcidin in infection

In this issue of Blood, Stefanova et al provide direct proof (which was previously lacking) that hepcidin is a major component of innate immunity.¹ 

Shortly after hepcidin was identified and isolated^2,3  but before it was elucidated as a master regulator of iron metabolism,⁴  it became clear that inflammation was one of the principal regulators of hepcidin. Since then, it has been thought that hepcidin may protect against some infections. Proliferation in some so-called siderophilic bacteria is limited by access to iron, whereas other bacteria are unaffected by iron status. The article by Stefanova took advantage of that fact and established the basis for differential parameters of bacterial virulence directly regulated by hepcidin. It was essential for these investigators to establish crucial experimental conditions. First, they needed to show that only the unbound iron from transferrin was essential for the virulence of those bacterial strains that required iron. The first requirement was to show that iron loading leads to free or unbound iron (not transferrin-bound) and that this free, unbound iron is essential for the virulence of those bacterial strains. Thus, they developed an assay for free plasma iron (ie, the redox-active, chelatable component of non–transferrin-bound iron [NTBI]). The second requirement was to use the same conditions in situations in which hepcidin was absent, and they had a suitable animal model: a mouse with a knockout hepcidin gene (HKO mouse).⁵  It was also helpful that 2 strains of Yersinia enterocolitica were available to them, one siderophilic and the other nonsiderophilic. Third, if they could show that hepcidin plays a major role in the virulence of siderophilic bacteria, then the hepcidin analog they previously made and verified, which they termed minihepcidin PR73,⁶  might revert bacterial virulence in animals with iron overload. They could then proceed with their well-designed experiments.

In the initial experiments, Stefanova et al indeed showed that iron makes siderophilic Y enterocolitica more virulent than the nonsiderophilic strain. They demonstrated that the iron-overloaded HKO mouse had 100% mortality upon infection with the siderophilic bacteria, which was reversed by iron depletion and, importantly, by minihepcidin. Minihepcidin was also protective in wild-type mice that were iron overloaded. In additional experiments, they demonstrated that the virulence of these siderophilic bacteria was entirely the result of the presence of NTBI rather than neutrophil recruitment and bacterial killing. These manipulations were relevant only to the siderophilic bacteria, because hepcidin and iron manipulation had no effect on the virulence of Staphylococcus aureus or Mycobacterium tuberculosis.

Along with this important improvement in our understanding of the role of hepcidin in protection from certain types of infections, more work in divergent hepcidin roles in other pathophysiological areas is needed. Clearly, inflammation is not the only regulator of hepcidin expression, and hepcidin is regulated by other pathophysiological situations that contribute to numerous hematologic diseases. Although the relationship of iron status and regulation of hepcidin is clear, another important question was only recently answered by Park et al³ : Why is hyperactive erythropoiesis in chronic hemolysis resulting from deficiencies in red cell enzymes (such as pyruvate kinase deficiency) or inefficient erythropoiesis in dyserythropoietic anemias and thalassemia frequently accompanied by, and at times diagnosed by, clinically significant iron overload? The answer was that the erythropoietin-regulated hormone erythroferrone downregulates hepcidin transcripts in the liver.⁷  Hepcidin is also regulated independently by hypoxia, which mediates the expression of many genes by modulating the levels of hypoxia-induced master transcription factors (HIFs). HIFs are dimers of α subunits and the common β subunit; there are 3 α homologs, HIF-1α, HIF-2α, and HIF-3α, that constitute HIF-1, HIF-2, and HIF-3. Of these, HIF-1 controls hepcidin expression. In the HIF-1α knockout mouse, which is embryonically lethal, hepcidin transcription was increased in these embryos, proving that hypoxia via HIF-1 downregulates hepcidin.⁸  However, the molecular mechanism of HIF-1 hepcidin downregulation still remains to be deciphered.

There may be other pleotropic roles of hepcidin. For example, there are reports of the presence and production of hepcidin in astrocytes and glial cells in the brain.^9,10  Much more work remains to be done to understand its function, regulation, and significance in neurocognition and brain pathophysiology. Thus, more investigative work on the role of hepcidin in health and disease is awaited. Nevertheless, the article by Stefanova et al represents a great step forward in our understanding of the role of hepcidin in protecting against certain infections and provides tempting potential therapeutic applications for hepcidin analogs.