Impaired Iron Uptake in Mdr2 -/ Mice results in Hepatic Iron Deficiency: Implications for
Patients with Cholestatic Liver Injury.
Introduction: Hepatic iron accumulation occurs in up to sixty per cent of patients with advanced hepatocellular liver disease. However, iron accumulation in liver diseases of biliary origin is very uncommon and occurs in less than eight per cent of affected subjects
1,2
. We have previously shown that Mdr2
-/-
mice have reduced hepatic iron levels despite lower Hamp1 and prohepcidin expression, increased iron absorption and serum iron and increased hepatic expression of transferrin receptor 1 (Tfr1). We hypothesized that the hepatic iron deficiency seen in Mdr2 -/mice is due to impaired hepatocyte iron uptake.
Methods: Wild type and Mdr2
-/mice were challenged with either an iron deficient diet from 3 to 9 weeks of age or a 1% carbonyl iron diet from 5 to 9 weeks of age (n = 6). Serum iron indices were measured and full blood counts were performed. Liver and spleen sections were stained with Perls’
Prussian blue, and hepatic and splenic iron concentrations were determined. qRT-PCR was used to assess mRNA expression.
Results: Wild type and Mdr2
-/mice fed an iron deficient diet developed severe systemic anaemia evidenced by reduced haemoglobin (Hgb) (WT: 67 ± 11; Mdr2
-/: 84 ± 10 g/dL). No significant differences in haematological parameters were seen in wild type or Mdr2
-/mice following 1% carbonyl iron feeding however Mdr2 -/ mice had significantly elevated serum iron levels ( Mdr2 -/:
463 ± 30 vs WT: 252 ± 14 µg/dL).
Mdr2
-/-
mice fed a control diet had a lower hepatic iron concentration than wild type mice (7.3 ± 1.8 vs 11.2 ± 1.8 µmol Fe/g dry wt, P = 0.08) with redistribution of stainable iron to the reticuloendothelial system. Both wild type and Mdr2 -/mice fed 1% carbonyl iron had significant hepatic iron accumulation however the degree of iron loading was greater in wild type mice (HIC: 57 ± 4 vs 23 ± 2 µmol Fe/g dry wt, P < 0.001). Perls’ Prussian blue staining indicated that iron accumulation was predominantly in reticuloendothelial macrophages in contrast to a hepatocellular distribution seen in wild types. Splenic iron concentration was similar in wild type and Mdr2
-/mice fed a control diet (38.5 ± 3.0 vs 36.7 ± 1.8
µmol Fe/g dry wt) however was significantly higher in
Mdr2
-/-
when compared to wild type mice following 1% carbonyl iron (64.4 ± 2.0 vs 54.3 ± 2.4 µmol Fe/g dry wt). Both hepatic and splenic iron concentration were significantly lower in wild type and Mdr2
-/mice following an iron deficient diet. Hamp1 mRNA was upregulated in both wild type and Mdr2
-/-
mice following 1% carbonyl iron feeding and downregulated following an iron deficient diet. Mdr2 -/had 2.8-fold higher Tfr1 expression than wild type mice when fed a control diet. Tfr1 was further upregulated in both wild type and Mdr2
-/mice following an iron deficient diet.
Discussion/Conclusions: The reduced hepatic iron concentration, redistribution of iron to the reticuloendothelial system and resistance to hepatic iron loading when fed a 1% carbonyl iron diet suggests that hepatocyte iron uptake is impaired in Mdr2 -/ mice.
This work may explain the paucity of iron loading in biliary cirrhosis and suggests that iron homeostasis depends upon an appropriately functioning biliary system.
References:
1. Stuart et al. Hepatology 2000;32(6):1200-7.
2. Ludwig et al. Gastroenterology 1997;112:882-888.