Functional interactions between iron metabolism, nitric oxide pathway and cellular immune effector function

Our project dealt mainly with three topics: 1. We investigated the mechanisms by which iron influences cell mediated immune effector pathways in macrophages. We found that iron inhibits the gene expression of inducible nitric oxide synthase, the protein being responsible for high output formation of the labile radical nitric oxide (NO) in macrophages. The increased synthesis of NO is well known as a major immune effector mechanism directed against invading pathogens and tumor cells. The underlying mechanism by which iron affects iNOS transcription can be referred to direct interaction of iron with the binding affinity of certain transcription factors-i.e. proteins with bind to genes and increase gene expression- being essential of iNOS expression. The latter is also due to the fact that iron interacts with signal transduction processes-i.e the communication system between the cell membrane and the nucleus. Regulatory interactions between iron homeostasis, immune function and the NO pathway are prominently involved in the pathogenesis of anemia of chronic disease, a condition which is frequently observed in patients suffering from chronic infections, tumors or auto-immune diseases. 2. As we have shown that iron reduced the immune effector potential of macrophages we investigated whether or not modulation of cellular iron homeostasis may affect the clinical course of two very important infections. Malaria -due to infection with Plasmodium falciparum- is a global health problem with approximately 300- 500 million new infections causing more than two million death each year. We found that limitation of iron availability by using an iron chelator stimulates the immune effector potential of macrophages directed against Pl. falciparum. Such a conditions has previously been shown to be associated with a more favorable clinical course. Since the chelators currently available have a rather poor potential to penetrate immune cells we tested a number of newly designed siderophores, i.e. iron binding drugs, for their potential to kill Pl. falciparum and/or to modulate immune function. This lead to the identification and selection of a number of favorable drugs which will be further studied for their potential to treat malaria. We also studied the role of iron for the clinical course of hepatitis C virus infection, which is a frequent chronic infectious disease often leading to cirrhosis and hepatic carcinoma. We found that higher iron concentrations in patients` blood lead to an impaired immune effector function and a more progressive clinical course of the infection. Therefore, also in this condition the restriction of iron e.g. by performing phlebotomy or application of iron binding drugs may hold promise in the treatment of this disorder. 3. The third big issue we worked on is genetic or hereditary hemochromatosis (HH). HH is an autosomal recessive disorder and a very frequent condition in Middle and Northern Europe with a carrier rate of approx. 1 in 9-12 people and with 1 in 250-400 people being affected by the disease (i.e. approx. 30 000 in Austria). Although, the genetic locus has been identified a few years ago the pathophysiological mechanisms leading to increased uptake of iron via the duodenum has not been elucidated so far. Since the body has no excretion mechanisms for iron, iron accumulates in parenchymal tissues such as the liver, the heart and the pancreas. Over the years this leads to progressive organ damage which then present clinically as liver cirrhosis, heart insufficiency or diabetes. In duodenal biopsies obtained from patients we could shown that HH is associated with increased expression of an iron transporter, termed DMT-1, a conditions which is normally only seen in sever iron deficiency. The reason for this is likely being due to the fact that because of the genetic defect the sensing mechanisms for iron by duodenal cells are impaired. Therefore duodenal enterocytes can not sufficiently acquire iron from the blood and thus appear to be iron deficiency. As a normal consequence DMT-1 expression is up-regulated and iron is taken up from the alimentary site of the duodenum although iron stores are full. In extending our studies we are now interested to find a specific inhibitor for DMT-1 mediated iron transport in the duodenum in order to come up with a new therapy for HH. During this project we published 24 papers in international "peer-reviewed" journals and also received two of the most prestigious scientific awards in Austria, i.e Novartis Award for Medicine 1998 and the Kraupp award for the best medical "habilitation" (tenure-work) in Austria in 1999).