Scientific Abstract

Proposal No. IBD-0253
Principal Investigator: Bobby J. Cherayil, M.D.
Applicant Organization: Massachusetts General Hospital (Charlestown, U.S.A.)
Project Title: Iron-based modulation of cytokine biosynthesis for treatment of intestinal inflammation
Period of Award: November 1, 2008 – February 28, 2011

Chronic inflammatory states and infections are associated with disturbances of iron metabolism that lead to anemia. The underlying mechanism involves decreased iron absorption from the gut, and sequestration of iron within macrophages of the reticuloendothelial system. Recent advances in our understanding of the molecular biology of iron homeostasis have revealed that these abnormalities result from down-regulation of the enterocyte and macrophage iron exporter ferroportin (FPN) by a peptide – hepcidin – that is secreted by hepatocytes in response to the pro-inflammatory cytokine IL-6. Thus, inflammation leads to increased hepcidin production by the liver, with consequent lowering of FPN expression and decreased release of iron into the circulation from macrophages and enterocytes. This mechanism is likely to contribute to the anemia that is frequently seen in inflammatory bowel disease (IBD) since recent studies have documented that patients with Crohn’s disease have increases in IL-6 and hepcidin levels.

Our preliminary experiments with Hfe knock-out mice, a model of the human iron metabolism disorder type I hemachromatosis, provide several lines of evidence to indicate that the intracellular concentration of free iron within macrophages has a clear impact on inflammatory responses. Specifically, our studies show that macrophages from the Hfe-deficient mice produce significantly reduced levels of the pro-inflammatory cytokines TNFα and IL-6 in response to either Salmonella infection or lipopolysaccharide (LPS) treatment. This abnormality is the outcome of decreased intracellular iron in these cells, which in turn is caused by the elevated FPN expression that results from abnormally low circulating levels of hepcidin. The reduced cytokine production by the Hfe knock-out macrophages is associated in vivo with a clear attenuation of intestinal inflammation caused by Salmonella infection or by oral administration of dextran sulphate sodium (DSS). These findings raise the possibility that elevated hepcidin levels, like those observed in IBD, may have the opposite effect to that of Hfe deficiency, i.e., they may promote pro-inflammatory responses in macrophages by down-regulating FPN and thereby raising intracellular iron. Thus, we hypothesize that blocking hepcidin expression in IBD may help to reduce intestinal inflammation by lowering intracellular iron levels in macrophages and decreasing the amount of pro-inflammatory cytokines produced by these cells. We will test this novel concept in mouse models of IBD by using two reagents that have been shown recently by our collaborators to inhibit the IL-6-induced up-regulation of hepcidin. We expect that treatment with these reagents will reduce the severity of intestinal inflammation, and thus provide proof of principle for a new therapeutic approach to treating IBD by manipulating iron homeostasis. Demonstration of the anti-inflammatory efficacy of the hepcidin blocking reagents will pave the way for the development of a new class of drugs for IBD treatment based on this concept. This strategy has the added potential benefit of alleviating the anemia caused by hepcidin-mediated down-regulation of FPN expression.