Absence of iron-regulatory protein Hfe results in hyperproliferation of retinal pigment epithelium: role of cystine/glutamate exchanger

Abstract

Summary Hemochromatosis is an iron overload disorder with age-dependent oxidative stress and dysfunction in a variety of tissues. Mutations in HFE are responsible for most cases with hemochromatosis. We recently demonstrated that Hfe is expressed exclusively in the basal membrane of retinal pigment epithelium (RPE). Here 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 to 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 upregulated in Hfe−/− RPE cells was Slc7a11 that 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-PCR and also by measuring transport function. We also found increased levels of glutathione and the transcription factor/cell cycle promoter AP1 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.

mTOR-mediated dedifferentiation of the retinal pigment epithelium initiates photoreceptor degeneration in mice

“…Injection of oligomeric amyloid β peptide into the subretinal space of B6 mice causes RPE hypertrophy and hypopigmentation (but not apoptosis) and diminished expression of RLBP1, RPE65, and tight junction proteins (74). Mice homozygous for a null mutation in the hemochromatosisassociated gene, Hfe, exhibit late-onset (18 months) RPE hypertrophy and hyperplasia, presumably as a result of iron accumulation (75). Similarly, iron buildup in the RPE of ceruloplasmin/hephaestin double-knockout mice leads to RPE hyperplasia and hypertrophy, with increased autofluorescent deposits in 7- to 9-month-old animals (76).…”

Biometals and Their Therapeutic Implications in Alzheimer's Disease

“…Neuronal iron deposition causes oxidative stress via the Fenton reaction, which might contribute to elevated oxidative stress observed in the AD brain [109]. Iron-induced oxidative stress has been shown to initiate several apoptotic signaling pathways in neurons [110], and damage proteins such as Ca 2+ -ATPase [111][112][113][114], glutamate transporter [115][116][117], apolipoprotein E [118,119], and Na + /K + -ATPase [111,114,120,121], as well as N-methyl-D-aspartate (NMDA) receptor [122][123][124], and lipids such as cholesterol [125][126][127], ceramides [128,129], and unsaturated fatty acids [130][131][132][133], as well as sphingomyelin [134,135]. Oxidative damage to proteins and lipids by iron can cause synaptic dysfunction and neuronal cell death [136].…”

The Cystine/Glutamate Antiporter System xc−in Health and Disease: From Molecular Mechanisms to Novel Therapeutic Opportunities

“…In the retina, HFE is predominantly expressed in RPE (174). HFE -/ -mice undergo RPE hypertrophy and hyperplasia (74). An analysis of cultured HFE -/ -primary RPE cells revealed increased expression of xCT protein and mRNA and system x c -activity (74).…”