Changed iron metabolism and iron toxicity in scrapie-infected neuroblastoma cells

Abstract: Reactions and interactions of iron and oxygen can be both beneficial and detrimental to cells and tissues. Iron is mainly found in our blood where it functions as a mediator in the transport of oxygen to the cells and is further vital for the cellular respiration reducing the oxygen to water. The flexible redox state of iron makes it ideal to contribute in single electron transfers, but may also catalyze reactions with oxygen resulting in cell damaging reactive oxygen species (ROS). Normally the cells are protected against iron toxicity by controlling iron uptake and storage. When the intracellular demand for iron increases; the iron uptake is promoted by increasing the expression of transferrin receptor (TfR) and by decreasing the expression of the iron storage protein ferritin. Ferritin has a central role in the cellular iron detoxification by keeping it in a non reactive but still bioavailable form. However, in neurodegenerative diseases like in Alzheimer’s and Parkinson’s disease the iron storage capacity is disturbed and iron induced oxidative stress adds to the pathology of the diseases. The role of iron and its possible contribution to the pathology of prion diseases, like Creutzfeldt-Jakob disease, is less explored. In the first three studies of this thesis, the iron metabolism and the mutual relation between iron and oxygen are studied in scrapie-infected mouse neuroblastoma cells (ScN2a) as compared to control cells (N2a). In the fourth study we have analyzed the expression of ferritin and TfR in response to inflammation by treating the cells with the bacterial endotoxin lipopolysaccharide (LPS). LPS promotes the expression of inducible nitric oxide synthase (iNOS), a producer of nitric oxide (NO), a well known regulator of the iron metabolism.In the first study, the scrapie infection was found to reduce the iron levels, to reduce the mRNA and protein levels of ferritin and the TfR. In addition, reduced levels and activities of the iron regulatory proteins 1 and 2 were observed as compared to the uninfected N2a cells.In the second study, the addition of iron to the cell medium strongly increased the level of ROS and decreased the cell viability of the ScN2a cells, whereas the N2a cells were unaffected. The ferritin expression in N2a cells in response to the iron treatment was strongly increased and the concomitant measurement of the labile iron pool (LIP) revealed the LIP to be normalized within four hours. In the ScN2a cells the induction of ferritin expression was lower resulting in elevations in LIP that lasted up to 16 h, indicating that the increased ROS levels were iron catalyzed.In the third study, the cells were challenged with hydrogen peroxide (H2O2) to elevate the oxidative stress and to analyze the effects on the LIP and cell viability. The ScN2a cells were sensitive to the increased oxidative stress according to the cell viability test, and responded to the treatment with marked increase in the LIP levels, probably derived from an intra-cellular source. The cell viability could be reset by the co-addition of an iron chelator to the cell media. The N2a cells did not elevate the LIP and resisted higher concentrations of H2O2 than the ScN2a cells, according to the cell viability assay.In the fourth study, the LPS treatment resulted in increased mRNA levels of the heavy chain of ferritin, increased the protein levels of ferritin light chain and decreased the protein levels of the TfR in N2a cells, but no effects were observed in the ScN2a cells. Co-treatment with LPS and the iNOS inhibitor aminoguanidine did not affect the LPS induced decrease of TfR in N2a cells, whereas the free radical scavenger N-acetyl-L-cysteine reversed the effect of LPS on TfR expression, indicating that the changes were mediated by an oxidative rather than a nitric oxide mechanism in the N2a cells.