The effect of Helicobacter pylori on cellular iron homeostasis

Iron deficiency anaemia (IDA) is a serious health problem worldwide, particularly in developing countries. Infection with the gastric pathogen Helicobacter pylori is more prevalent in developing countries. Additionally, there is strong evidence supporting a link between infection and the development of iron deficiency anaemia. However, the biological mechanisms involved in this process remain an area of controversy. Several mechanisms have been proposed, one of which is that the inflammation induced by the infection might involve the redirection of circulating iron to the gastric epithelium, which is the site of H. pylori colonisation. The research undertaken in this thesis investigated this hypothesis, by first determining whether infection with H. pylori is able to induce alterations in iron homeostasis in gastric epithelial cells. A series of assays were designed to measure changes in total cellular iron (both labile and strongly bound to proteins), the cytosolic labile iron pool (that is responsible for regulating the expression of proteins involved in iron metabolism through the IRP/IRE system) and the total labile iron (that is located mainly in lysosomes, where ferritin is degraded). The results from these experiments indicated that labile iron is significantly increased in gastric epithelial cells in response to H. pylori infection, as is the level of total cellular iron, although this increase is not significant. However, unexpectedly, the cytosolic labile iron pool was reduced under these same conditions. Further experiments assessed the expression of the transferrin receptor (TfR, involved in cellular iron uptake) and H-ferritin (that is in charge of cellular iron storage). Whereas no difference in TfR expression was detected, H-ferritin was clearly overexpressed in response to H. pylori infection. Furthermore, the increased levels of H-ferritin in infected cells localised mainly to subcellular compartment that were most likely lysosomes, a finding that correlated with the observed increase in total labile iron in these cells. Interestingly, while the iii Abstract________________________________________________________________________________ expression of TfR was not increased in infected cells, cellular distribution of the receptor was altered, with notably more cell-surface associated receptor evident in infected cells, and this may account for the increase in total iron detected in these cells. Many H. pylori strains produce two well characterised virulence factors, CagA and VacA, and a role for these proteins in the perturbation of gastric epithelial cell iron homeostasis was investigated through the use of H. pylori isogenic mutant strains. The results of these experiments implicate the CagA protein in the changes in cellular iron homeostasis observed in H. pylori-infected AGS cells. In summary, the finding of this research is that H. pylori are able to interfere with iron homeostasis in gastric cells and that the CagA protein is involved in this process. While these findings remain to be validated in vivo, they provide evidence of a novel mechanism that may link H. pylori infection to host iron deficiency. iv Acknowledgements_____________________________________________________________________