Expression of epithelial cell iron-related genes upon infection by Neisseria meningitidis

Infection by the obligate human pathogens Neisseria meningitidis (MC) and Neisseria gonorrhoeae (GC) reduces the expression of host epithelial cell transferrin receptor 1 (TfR-1) (Bonnah et al., 2000, Cellular Microbiology 2: 207-218). In addition, the rate and pattern of TfR-1 cycling is altered, leading to diminished uptake of Tf-iron by infected host cells. As Tf-iron is important for maintaining iron homeostasis in the eukaryotic cell, these findings raised the possibility that Neisseria infection might affect further pathways of epithelial cell iron metabolism. We used a specialized cDNA microarray platform, the 'IronChip', to investigate the expression of genes involved in iron transport, storage and regulation. We show that mRNA expression of several host genes involved in iron homeostasis is altered. Surprisingly, the general mRNA expression profile of infected cells closely resembled that of uninfected cells grown in an iron-limited environment. An important exception to this profile is TfR-1, the mRNA level of which is strongly reduced. Low TfR-1 expression may be explained in part by decreased activity of the iron-regulatory proteins (IRPs) in MC-infected cells, which may result in the destabilization of TfR-1 mRNA. Intriguingly, low IRP activity contrasts with the decrease in H-ferritin protein levels in infected cells. This finding suggests that low IRP activity may be responsible in part for the decrease in TfR-1 mRNA levels. A discussion of these novel findings in relation to MC infection and virulence is provided.

Balancing acts: molecular control of mammalian iron metabolism

Iron is ubiquitous in the environment and in biology. The study of iron biology focuses on physiology and homeostasis-understanding how cells and organisms regulate their iron content, how diverse tissues orchestrate iron allocation, and how dysregulated iron homeostasis leads to common hematological, metabolic, and neurodegenerative diseases. This has provided novel insights into gene regulation and unveiled remarkable links to the immune system.

An Hfe-dependent pathway mediates hyposideremia in response to lipopolysaccharide-induced inflammation in mice

Inflammation influences iron balance in the whole organism. A common clinical manifestation of these changes is anemia of chronic disease (ACD; also called anemia of inflammation). Inflammation reduces duodenal iron absorption and increases macrophage iron retention, resulting in low serum iron concentrations (hyposideremia). Despite the protection hyposideremia provides against proliferating microorganisms, this 'iron withholding' reduces the iron available to maturing red blood cells and eventually contributes to the development of anemia. Hepcidin antimicrobial peptide (Hamp) is a hepatic defensin-like peptide hormone that inhibits duodenal iron absorption and macrophage iron release. Hamp is part of the type II acute phase response and is thought to have a crucial regulatory role in sequestering iron in the context of ACD. Mice with deficiencies in the hemochromatosis gene product, Hfe, mounted a general inflammatory response after injection of lipopolysaccharide but lacked appropriate Hamp expression and did not develop hyposideremia. These data suggest a previously unidentified role for Hfe in innate immunity and ACD.

HFE downregulates iron uptake from transferrin and induces iron-regulatory protein activity in stably transfected cells

Hereditary hemochromatosis (HH) is a common autosomal-recessive disorder of iron metabolism. More than 80% of HH patients are homozygous for a point mutation in a major histocompatibility complex (MHC) class I type protein (HFE), which results in a lack of HFE expression on the cell surface. A previously identified interaction of HFE and the transferrin receptor suggests a possible regulatory role of HFE in cellular iron absorption. Using an HeLa cell line stably transfected with HFE under the control of a tetracycline-sensitive promoter, we investigated the effect of HFE expression on cellular iron uptake. We demonstrate that the overproduction of HFE results in decreased iron uptake from diferric transferrin. Moreover, HFE expression activates the key regulators of intracellular iron homeostasis, the iron-regulatory proteins (IRPs), implying that HFE can affect the intracellular "labile iron pool." The increase in IRP activity is accompanied by the downregulation of the iron-storage protein, ferritin, and an upregulation of transferrin receptor levels. These findings are discussed in the context of the pathophysiology of HH and a possible role of iron-responsive element (IRE)-containing mRNAs.