Heterozygous Hfe gene deletion leads to impaired glucose homeostasis, but not liver injury in mice fed a high-calorie diet

Iron depletion attenuates steatosis in a mouse model of non-alcoholic fatty liver disease: Role of iron-dependent pathways

BACKGROUND & AIMS

Iron has been proposed as influencing the progression of liver disease in subjects with non-alcoholic fatty liver disease (NAFLD). We have previously shown that, in the Hfe^-/- mouse model of hemochromatosis, feeding of a high-calorie diet (HCD) leads to increased liver injury. In this study we investigated whether the feeding of an iron deficient/HCD to Hfe^-/- mice influenced the development of NAFLD.

METHODS

Liver histology was assessed in Hfe^-/- mice fed a standard iron-containing or iron-deficient diet plus or minus a HCD. Hepatic iron concentration, serum transferrin saturation and free fatty acid were measured. Expression of genes implicated in iron regulation and fatty liver disease was determined by quantitative real-time PCR (qRT-PCR).

RESULTS

Standard iron/HCD-fed mice developed severe steatosis whereas NAS score was reduced in mice fed iron-deficient HCD. Mice fed iron-deficient HCD had lower liver weights, lower transferrin saturation and decreased ferroportin and hepcidin gene expression than HCD-fed mice. Serum non-esterified fatty acids were increased in iron-deficient HCD-fed mice compared with standard iron HCD. Expression analysis indicated that genes involved in fatty-acid binding and mTOR pathways were regulated by iron depletion.

CONCLUSIONS

Our results indicate that decreasing iron intake attenuates the development of steatosis resulting from a high calorie diet. These results also suggest that human studies of agents that modify iron balance in patients with NAFLD should be revisited.

Ferroportin Expression in Adipocytes Does Not Contribute to Iron Homeostasis or Metabolic Responses to a High Calorie Diet

BACKGROUND & AIMS

Iron has an increasingly recognized role in the regulation of adipose tissue function, including the expression of adipokines involved in the pathogenesis of nonalcoholic fatty liver disease. The cellular iron exporter, ferroportin, has been proposed as being a key determinant of adipocyte iron homeostasis.

METHODS

We studied an adipocyte-specific ferroportin (Fpn1) knockout mouse model, using an Adipoq-Cre recombinase driven Fpn1 deletion and fed mice according to the fast food diet model of nonalcoholic steatohepatitis.

RESULTS

We showed successful selective deletion of Fpn1 in adipocytes, but found that this did not lead to increased adipocyte iron stores as measured by atomic absorption spectroscopy or histologically quantified iron granules after staining with 3,3'-diaminobenzidine-enhanced Perls' stain. Mice with adipocyte-specific Fpn1 deletion did not show dysregulation of adiponectin, leptin, resistin, or retinol-binding protein-4 expression. Similarly, adipocyte-specific Fpn1 deletion did not affect insulin sensitivity during hyperinsulinemic-euglycemic clamp studies or lead to histologic evidence of increased liver injury. We have shown, however, that the fast food diet model of nonalcoholic steatohepatitis generates an increase in adipose tissue macrophage infiltration with crown-like structures, as seen in human beings, further validating the utility of this model.

CONCLUSIONS

Ferroportin may not be a key determinant of adipocyte iron homeostasis in this knockout model. Further studies are needed to determine the mechanisms of iron metabolism in adipocytes and adipose tissue.

Autophagy and hepatic lipid metabolism

Autophagy is initially thought to be a non-selective process in which intracellular proteins or damaged organelles are degraded. It is activated when cells lack nutrients and energy. Autophagy degrades cytoplasmic components within lysosomes and reuses the energy of amino acids to promote cell survival and maintain the cytoplasmic content. Current evidence implicates autophagy in the regulation of lipid stores within the two main organs involved in maintaining lipid homeostasis, the liver and adipose tissue. Upregulation of autophagy may lead to conversion of white adipose tissue into brown adipose tissue, thus regulating energy expenditure and obesity. Discovering new therapeutic interventions to treat lipid and lipoprotein disorders is of great interest and the discovery of autophagy as a regulator of lipid metabolism has opened up a new avenue for this area. In the liver, autophagy can play a role in some common metabolic disorders, which needs further research.

Heterozygous Hfe gene deletion leads to impaired glucose homeostasis, but not liver injury in mice fed a high-calorie diet

Heterozygous mutations of the Hfe gene have been proposed as cofactors in the development and progression of nonalcoholic fatty liver disease (NAFLD). Homozygous Hfe deletion previously has been shown to lead to dysregulated hepatic lipid metabolism and accentuated liver injury in a dietary mouse model of NAFLD. We sought to establish whether heterozygous deletion of Hfe is sufficient to promote liver injury when mice are exposed to a high‐calorie diet (HCD). Eight‐week‐old wild‐type and Hfe^+/− mice received 8 weeks of a control diet or HCD. Liver histology and pathways of lipid and iron metabolism were analyzed. Liver histology demonstrated that mice fed a HCD had increased NAFLD activity score (NAS), steatosis, and hepatocyte ballooning. However, liver injury was unaffected by Hfe genotype. Hepatic iron concentration (HIC) was increased in Hfe^+/− mice of both dietary groups. HCD resulted in a hepcidin‐independent reduction in HIC. Hfe^+/− mice demonstrated raised fasting serum glucose concentrations and HOMA‐IR score, despite unaltered serum adiponectin concentrations. Downstream regulators of hepatic de novo lipogenesis (pAKT, SREBP‐1, Fas, Scd1) and fatty acid oxidation (AdipoR2, Pparα, Cpt1) were largely unaffected by genotype. In summary, heterozygous Hfe gene deletion is associated with impaired iron and glucose metabolism. However, unlike homozygous Hfe deletion, heterozygous gene deletion did not affect lipid metabolism pathways or liver injury in this model.