Haemochromatosis is an
iron-overload disorder with age-dependent oxidative stress and dysfunction in a variety of tissues. Mutations in HFE (histocompatability leucocyte
antigen class I-like
protein involved in
iron homoeostasis) are responsible for most cases of
haemochromatosis. We demonstrated recently that HFE is expressed exclusively in the basal membrane of RPE (retinal pigment epithelium). In the present study, we used Hfe-/- mice to examine
ferritin levels (an indirect readout for
iron levels) and morphological changes in retina. We found increased
ferritin accumulation in retina in 18-month-old, but not in 2-month-old, mice with considerable morphological damage compared with age-matched controls. The
retinal phenotype included
hypertrophy and
hyperplasia of RPE. RPE cells isolated from Hfe-/- mice exhibited a hyperproliferative phenotype. We also compared the gene expression profile between wild-type and Hfe-/- RPE cells by microarray analysis. These studies showed that many cell cycle-related genes were differentially regulated in Hfe-/- RPE cells. One of the genes up-regulated in Hfe-/- RPE cells was Slc7a11 (where Slc is solute carrier) which codes for the 'transporter proper' xCT in the heterodimeric
cystine/
glutamate exchanger (xCT/4F2hc). This transporter plays a critical role in cellular
glutathione status and cell-cycle progression. We confirmed the microarrray data by monitoring xCT
mRNA levels by RT (reverse transcription)-PCR and also by measuring transport function. We also found increased levels of
glutathione and the
transcription factor/cell-cycle promoter AP1 (
activator protein 1) in Hfe-/- RPE cells. Wild-type mouse RPE cells and human RPE cell lines, when loaded with
iron by exposure to
ferric ammonium citrate, showed increased expression and activity of xCT, reproducing the biochemical phenotype observed with Hfe-/- RPE cells.