Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Science

Program

Medical Biophysics

Collaborative Specialization

Molecular Imaging

Supervisor

Goldhawk, Donna

Affiliation

Lawson Health Research Institute

Abstract

Monocytes are an important immune cell type in chronic inflammatory conditions like atherosclerosis and heart failure. The increase in number of monocytes released to the peripheral blood circulation, the differentiation of monocytes to macrophages, and the presence of different macrophage subpopulations during pro- and anti-inflammatory stages of tissue injury may provide markers for monitoring inflammation. In particular, changes in monocyte iron metabolism during an inflammatory response may increase the possibility of tracking these immune cells non-invasively using magnetic resonance imaging (MRI). When secretion of the polypeptide hormone hepcidin is stimulated during inflammation, it binds the iron export protein ferroportin (FPN) on a limited number of cell types, including monocytes and macrophages. This ligand-receptor interaction leads to FPN internalization and downregulation through ubiquitin-mediated degradation. We hypothesized that hepcidin-mediated changes in monocyte iron regulation influence both cellular iron content and magnetic resonance (MR) relaxation rates. In response to varying conditions of extracellular iron supplementation, we observed THP-1 expression of FPN protein and its downregulation following hepcidin treatment. Also, in the presence hepcidin and iron supplementation, we detected a significant increase in the total transverse relaxation rate, R2*, compared to non-supplemented cells. The positive correlation between total cellular iron content and R2* improved from moderate to strong in the presence of hepcidin. These in vitro findings suggest that hepcidin-mediated changes detected in monocytes using MRI could be valuable for cell tracking in vivo during an inflammatory response.

Summary for Lay Audience

Background: Inflammation is important in many diseases like cancer, heart failure and atherosclerosis. Inflammatory responses have many roles, including tissue repair and the elimination of harmful pathogens. This is partly accomplished by limiting the amount of iron required for bacterial growth. Although inflammation is part of wound healing even in conditions like heart disease, excess inflammation can lead to changes in the size, shape and function of the heart and eventually to heart failure. Therefore, clinical management of inflammation by distinguishing between pro- and anti-inflammatory responses is important but remains challenging for many reasons. To understand when interventions should be introduced to prevent excess inflammation, we propose the use of Magnetic Resonance Imaging (MRI) to noninvasively detect the activity of immune cells (like monocytes, a type of white blood cell) that play a major role in inflammation. In our molecular imaging approach, we track changes in cellular iron metabolism using MRI since changes in iron are regulated by the hormone, hepcidin and reflect stage of inflammation.

Hypothesis: Hepcidin increases cellular iron content and MRI contrast in a laboratory model of human monocytes.

Methods: Monocytes are cultured for one week in the absence and presence of iron supplement and hepcidin, to mimic the cellular environment after a heart attack. Small volumes of these cultured cells are then placed in a clinical MRI scanner and imaged.

Results and Significance: When monocytes were exposed to hepcidin, we observed an increase in both cellular iron and MRI contrast. This research may improve how we monitor cells responding to inflammation signals like hepcidin, in patients with conditions like cancer, heart failure and atherosclerosis.

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