Gastroenterology

Gastroenterology

Volume 141, Issue 5, November 2011, Pages 1907-1914
Gastroenterology

Original Research
Basic and Translationalā€”Liver
Iron Regulation of Hepcidin Despite Attenuated Smad1,5,8 Signaling in Mice Without Transferrin Receptor 2 or Hfe

https://doi.org/10.1053/j.gastro.2011.06.077Get rights and content

Background & Aims

HFE and transferrin receptor 2 (TFR2) are each necessary for the normal relationship between body iron status and liver hepcidin expression. In murine Hfe and Tfr2 knockout models of hereditary hemochromatosis (HH), signal transduction to hepcidin via the bone morphogenetic protein 6 (Bmp6)/Smad1,5,8 pathway is attenuated. We examined the effect of dietary iron on regulation of hepcidin expression via the Bmp6/Smad1,5,8 pathway using mice with targeted disruption of Tfr2, Hfe, or both genes.

Methods

Hepatic iron concentrations and messenger RNA expression of Bmp6 and hepcidin were compared with wild-type mice in each of the HH models on standard or iron-loading diets. Liver phospho-Smad (P-Smad)1,5,8 and Id1 messenger RNA levels were measured as markers of Bmp/Smad signaling.

Results

Whereas Bmp6 expression was increased, liver hepcidin and Id1 expression were decreased in each of the HH models compared with wild-type mice. Each of the HH models also showed attenuated P-Smad1,5,8 levels relative to liver iron status. Mice with combined Hfe/Tfr2 disruption were most affected. Dietary iron loading increased hepcidin and Id1 expression in each of the HH models. Compared with wild-type mice, HH mice demonstrated attenuated (Hfe knockout) or no increases in P-Smad1,5,8 levels in response to dietary iron loading.

Conclusions

These observations show that Tfr2 and Hfe are each required for normal signaling of iron status to hepcidin via the Bmp6/Smad1,5,8 pathway. Mice with combined loss of Hfe and Tfr2 up-regulate hepcidin in response to dietary iron loading without increases in liver Bmp6 messenger RNA or steady-state P-Smad1,5,8 levels.

Section snippets

Animal Care

Hfe knockout mice33 and Tfr2Y245X mice34 were bred to uniformity on an FVB background for more than 7 generations. The Tfr2Y245X mice have no detectable Tfr2 or truncated form of the protein in hepatocellular membrane preparations and are a functional knockout.34 These 2 mouse lines were crossed with each other and bred to homozygosity for each mutant allele. Colonies were maintained as homozygotes for each allele individually (hereafter referred to as Hfe mice or Tfr2 mice) and as compound

Elevated Bmp6 mRNA Expression is Associated With Hepatic Iron Loading in Mice With Loss of Hfe and/or Tfr2

Functional loss of Hfe or Tfr2 is known to result in inappropriately low hepatic expression of hepcidin and consequent iron overload. Several lines of evidence suggest that up-regulation of Bmp6 contributes to iron-dependent regulation of hepcidin. We measured the hepatic expression of Bmp6 mRNA in Hfe, Tfr2, and Hfe/Tfr2 mice to assess if the decreased hepcidin expression could be attributed to decreased hepatic Bmp6 expression. To provide a comparison group for the degree of hepatic iron

Discussion

The normal relationship between body iron status and liver hepcidin expression requires the action of several identified genes, including HFE, TFR2, HJV, BMP6, and SMAD4.26, 37 Mutations in any one of these genes generate the classic HH phenotype, that is, excess dietary iron absorption, elevated serum iron concentrations, hepatocellular iron loading, and macrophage iron sparing.1, 2 TFR2 is the only one of these gene products known to directly interact with an iron-containing protein, that is,

Acknowledgments

The authors thank Rosemary O'Neill for excellent technical assistance.

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    Conflicts of interest The authors disclose no conflicts.

    Funding Supported by National Institutes of Health grants (R01 DK063016 to R.E.F., K08 DK075846 to J.L.B., and R01 DK069533 and R01 DK071837 to H.Y.L.) and a Claflin Distinguished Scholar Award from the Massachusetts General Hospital (to J.L.B).

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