Abstract
Introduction: Prior studies have identified two iron signals regulating liver hepcidin expression (PMID 21488083, 21480335). One is mediated by liver sinusoidal BMP6 (and BMP2) and is reflective of liver iron concentration (LIC). The other is independent of BMP expression and reflective of transferrin saturation. The mechanism by which TF regulates hepcidin is unclear but appears to involve hepatocellular transferrin receptor 2 (TFR2). We previously generated mice with TF mutations that block iron binding to either N-lobe (Tf N-bl) or C-lobe (Tf C-bl). These mice demonstrated differences in Epo sensitivity and hepcidin regulation (PMID 31434707). To characterize the differential regulation of Hamp1 in these mice we analyzed the effect of dietary iron in juvenile mice. This model advantages a physiologic low iron state with very low basal Hamp1 and Bmp6 expression.
Methods: Twelve-day old WT, Tf N-bl, Tf C-bland Tfr2 Y245X (functional knockout) mice were gavaged with 4 mg/kg/d FeSO 4 (or saline) in 2 divided doses for 3 days and sacrificed on the fourth day. The primary outcome was liver Hamp1 relative to Bmp6 expression by RT-PCR. Other iron-related parameters included liver Bmp2, serum and tissue non-heme iron, and TIBC. Erythroid parameters included splenic Fam132b (Erfe gene) expression, kidney Epo, blood packed cell volume (PCV), splenic weight index (SWI).
Results: Iron-treated mice demonstrated increased serum iron, TF saturation, LIC, liver Bmp6 in all strains compared to untreated. Liver Hamp1 was increased in all strains with iron-treatment; however, the magnitude was least in the Tf N-bland greatest in the Tf C-bl mice. Splenic Fam132b was unchanged. The iron-treated Tf C-bl had ~16 fold greater Hamp1 compared with iron-treated Tf N-bl mice (P=0.0005), despite similar Bmp6, Bmp2, and splenic Fam132b expression. The iron-treated Tf C-bl mice also had modestly higher splenic index and PCV compared with iron-treated Tf N-bl mice, despite similar kidney Epo. In Tfr2 Y245X mice Hamp1 and Hamp1 indexed to Bmp6 expression was intermediate between Tf N-bland Tf C-bl mice (figure).
Conclusion: Iron treatment increased liver Bmp6 expression as well as Bmp6 sensitivity (Hamp:Bmp6), in association with increases in LIC and in TF saturation. The effect of iron on Bmp6 sensitivity is markedly influenced by which TF lobe iron occupies, as well as the presence of TFR2. These observations support a model in which TF modulates the hepcidin response to BMP6 in either direction, via interaction with TFR2. We speculate that iron occupancy of the N-lobe of transferrin confers an upregulatory signal to hepcidin expression in the hepatocyte, and that its absence confers a downregulatory signal to BMP6 sensitivity in the hepatocyte.
No relevant conflicts of interest to declare.
