Team: Hypoxia and iron homeostasis
Our team generated new genetic models to investigate the role of HIFs (Hypoxia Inducible Factors), central mediators of cellular adaptation to hypoxia and of hepcidin, the major iron regulatory hormone, in pathophysiological conditions.
Main scientific achievements
- HIF-1 and HIF-2 are heterodimeric transcriptional factors and central mediators of cellular adaptation to hypoxia. In the presence of oxygen, the HIF-α subunit is hydroxylated by oxygen- and iron- dependent prolyl hydroxylases (PHDs) and targeted to the proteasome after the binding to the von Hippel-Lindau (VHL) protein. Upon hypoxia (or iron deficiency), HIF-α is stabilized and binds to the HIF-β constitutive subunit to induce the transcription of target genes implicated in the control of angiogenesis, glycolytic metabolism, apoptosis, cellular stress and other critical processes.
- Hepcidin, a 25 amino acid peptide, has been demonstrated to be the iron regulatory hormone, capable of blocking iron absorption from the intestine and iron release from macrophages. Its expression is induced by iron accumulation and diminished in situations of iron needs (anemia, hypoxia). Hepcidin controls serum iron levels by binding ferroportin, the only known iron exporter, and inducing its degradation. Mutations affecting hepcidin expression can cause hemochromatosis, a common genetic disorder.
Our research group unraveled the role of HIFs in iron metabolism as critical regulators of iron absorption in the intestine and in systemic iron homeostasis by downregulating hepcidin
Role of HIF in iron homeostasis
Iron is an essential nutrient and is critical for a multitude of biological processes including oxygen delivery. Both iron excess and iron scarcity have important consequences. Excess iron accumulation is observed in hereditary hemochromatosis, the most common genetic disorder of humans, while iron deficiency is one of the most frequently observed diseases in the world today, affecting as many as two billion people. As excess iron is not excreted, it use and storage needs to be tightly regulated at the level of intestinal absorption.
We have shown, by using genetic mouse models that HIF-2 played a crucial role in maintaining iron balance in the organism. HIF-2 regulates the expression of iron absorption genes in the intestine: DMT1 and ferroportin (figure) and the consecutive level of serum iron.
To address the contribution of HIF-2 in a model of pathological increased iron absorption, we generated hepcidin KO mice (a murine model of hemochromatosis) lacking HIF-2 in the intestine and showed that the deletion of duodenal HIF-2 was beneficial in the development of the disease by decreasing the severity of the tissue iron loading in the hepcidin KO mice (Mastrogiannaki et al., Blood, 2012). Altogether, these results highlight the essential role of duodenal HIF-2 in regulating iron absorption and tissue iron accumulation in pathophysiology.
We showed that in mice where both liver HIF1 and HIF2 were stabilized (the liver VHL KO mouse model presenting increased erythropoiesis), hepcidin expression was strongly repressed (Peyssonnaux et al., JCI, 2007). We demonstrated that hepcidin repression was due to HIF-2, through EPO-induced erythropoietic expansion, and not to other recently identified HIF-regulated pathways. In VHL/HIF-1a KO mice treated with neutralizing EPO antibody, hematological parameters were corrected and hepcidin gene expression returned to normal value (Mastrogiannaki et al., Hematologica, 2012). Our results highlight the contribution of hepatic HIF-2 to repress hepcidin through EPO-mediated increased erythropoiesis, a result of potential clinical interest.
Generation of tissue specific knockout of Hepcidin
Hepcidin is produced mainly by the liver, but many tissues have been described as being capable of expressing hepcidin: macrophages, brain, heart, retina, kidney, adipocyte, pancreas. However, the contribution of these tissues on circulating hepcidin and the impact of hepcidin deficiency in different tissues with regards to iron homeostasis is unknown. To address these questions, we have developed a tissue specific invalidation of hepcidin using a classical CreloxP strategy.
We first compared a liver-specific knockout mouse model to total knockout mice. We showed that the liver-specific knockout mice fully recapitulate the severe iron overload phenotype observed in the total knockout, with increased plasma iron and massive parenchymal iron accumulation. This result demonstrates that the hepatocyte constitutes the predominant reservoir for systemic hepcidin and that the other tissues are unable to compensate. (Zumerle et al., Blood, 2014).
- Zumerle S, Mathieu JR, Delga S, Heinis M, Viatte L, Vaulont S, Peyssonnaux C. Targeted disruption of hepcidin in the liver recapitulates the hemochromatotic phenotype. Blood. 2014 Jun 5;123(23):3646-50.
- Mastrogiannaki M, Matak P, Delga S, Deschemin JC, Vaulont S, Peyssonnaux C. Deletion of HIF-2α in the enterocytes decreases the severity of tissue iron loading in hepcidin knockout mice. Blood. 2012 Jan 12;119(2):587-90.
- Mastrogiannaki M, Matak P, Keith B, Simon MC, Vaulont S, Peyssonnaux C. HIF-2alpha, but not HIF-1alpha, promotes iron absorption in mice. J Clin Invest. 2009 May;119(5):1159-66.
- Peyssonnaux C, Zinkernagel AS, Schuepbach RA, Rankin E, Vaulont S, Haase VH, Nizet V, Johnson RS. Regulation of iron homeostasis by the hypoxia-inducible transcription factors (HIFs). J Clin Invest. 2007 Jul;117(7):1926-32.
- Peyssonnaux C, Datta V, Cramer T, Doedens A, Theodorakis EA, Gallo RL, Hurtado-Ziola N, Nizet V, Johnson RS. HIF-1alpha expression regulates the bactericidal capacity of phagocytes. J Clin Invest. 2005 Jul;115(7):1806-15.