Rusty Pipes? Hepcidin’s role in atherosclerosis
ABSTRACT
Iron has long been hypothesized to play a role in heart disease, but both a potential mechanism and experimental evidence have been lacking. Recently, progress has been made on both of these fronts, and the current thinking is that iron accumulation in macrophages leads to the formation of foamy macrophages, an important step in atherogenesis. This is an exciting finding because it may lead to new strategies to prevent heart disease. For example, one way to decrease iron load in macrophages would be to lower hepcidin levels. And in fact, this has been done in a mouse model with promising results. However, lowering hepcidin levels by altering BMP signaling has several effects on the body, not the least of which is increasing circulating iron levels, and therefore it cannot be definitely determined how hepcidin lowered atherogenesis in this mouse model, and one also must question the feasibility of translating this finding to human patients. This grant seeks to clarify hepcidin’s role in atherogenesis by using a mouse model to directly alter hepcidin’s cellular target, ferroportin, in macrophages, thereby eliminating any off target effects of hepcidin.
A. Specific Aims
Current evidence points to a role for hepcidin in atherosclerosis, and indeed, it has recently been shown that pharmacologically lowering hepcidin in apolipoprotein E-/- mice increases macrophage ferroportin expression, lowers macrophage iron load, reduces foamy macrophage generation, and decreases the size of atherosclerotic plaques. The authors attribute this to increased macrophage expression of two ABC transporters that export cholesterol. However, decreasing hepcidin production by inhibiting BMP signaling has multiple effects, and in fact it has independently been shown that affecting BMP signaling decreases inflammation and vascular calcification. This grant proposes that lowering hepcidin levels decreases foamy macrophage generation by increasing iron export from macrophages, and this will be confirmed by directly altering macrophage ferroportin, thus avoiding any off target effects of BMP signaling inhibition.
Aim 1: Reduce macrophage iron store levels without altering BMP signaling, and analyze the effect on plaque size.
Aim 2: Eliminate all possible confounding variables by following individual macrophage lineages in atherosclerotic plaques.
B. bACKGROUND
It is well known that men have a higher risk of heart disease than premenopausal women, and it has been proposed that the monthly menses of women reduce their iron stores and confer a protective effect against atherosclerosis (1). Studies in animals have generally confirmed the “iron hypothesis,” for example, it has been shown that feeding apolipoprotein E-/- mice a low iron diet reduces atherosclerotic lesion size (2). In contrast, studies in humans have provided lukewarm evidence for iron’s role in heart disease. To analyze the effect of lowering iron levels in people, the large FeAST study was undertaken whereby they lowered iron levels through phlebotomy. The study showed no protective benefit of lowering iron levels, but a subgroup analysis showed that in the youngest group of subjects lowering iron did provide a benefit, P<.001 (3). This could possibly indicate that iron levels are more important for earlier stages of atherosclerosis. Like this study, others have also shown conflicting results. It has been shown that high stored iron levels correlate with heart disease in Finnish men, but the NHANES I study did not show a correlation (4,5). One possible reason for the discrepancies seen in many studies is how they are measuring iron levels, i.e. stored iron vs. circulating iron. This discrepancy can best be seen in hemochromatosis. According to the “iron hypothesis” these patients should have a very high risk of heart disease since they have large amounts of stored iron. However, studies have not shown an increased risk of heart disease in hemochromatosis, and in fact studies have shown a protective benefit of having hemochromatosis alleles (6,7). This discrepancy can be explained by hepcidin’s role in iron metabolism (8). In hemochromatosis, hepcidin levels are very low, resulting in high ferroportin levels, high circulating iron, and high iron stores throughout the body. However, ferroportin levels are also high in macrophages, thus sparing them from iron overload. Hemochromatosis is an extreme example, but there are other disease states and environment factors that can cause one to have either high or low hepcidin, and in theory a high or low risk of heart disease. For example, aspirin use is associated with a lower risk of heart disease and chronic aspirin use results in gastrointestinal bleeding which decreases hepcidin levels (9). Similarly, regularly donating blood also lowers hepcidin levels and confers a protection to heart disease (10). Even living at high altitudes has been associated with lower hepcidin and a lower risk of heart disease (11). Conversely, diseases that increase hepcidin levels such as SLE are associated with heart disease (12). Experimental evidence for hepcidin’s role in atherosclerosis was recently provided when scientists experimentally lowered hepcidin levels in apolipoprotein E-/- mice with the BMP signaling inhibitor LDN 193189 (13). However, the authors note that altering BMP signaling has several effects, and therefore it cannot be definitely determined that it was the lowering of hepcidin levels that decreased atherogenesis (14). This grant seeks to show directly that lowering iron load in macrophages decreases atherogenesis.
C. Experimental Design and methods
Specific Aim 1:
Design: Hepcidin affects iron levels by causing the degradation of ferroportin, which is responsible for iron export from a cell. Lowering hepcidin levels results in increased ferroportin levels, and therefore more iron export from macrophages. Instead of altering BMP signaling to affect ferroportin levels, this grant will genetically alter macrophage ferroportin so that it can no longer bind to hepcidin. In fact, hemochromatosis type 4 is characterized by a ferroportin that cannot bind to hepcidin, thereby causing too much iron export. Mice with the altered ferroportin will be compared to mice with wildtype ferroportin.
Methods: The apolipoprotein E-/- and CD11b-Cre mice will be used as starting points. These mice will be crossed to generate an apolipoprotein E-/-, CD11b-Cre mouse. This new strain, named CreApo, will then be used as a starting point for the generation of a mouse with mutant ferroportin. The mutant ferroportin will be inserted into the Rosa26 locus, with a strong CAGG promoter, and a properly floxed neomycin resistance roadblock. Once these mice are generated, they, along with CreApo mice, will be fed a high fat diet for 8 weeks, at which point they will be sacrificed and analyzed. Blood will be collected to measure serum iron and lipid, the aortic arch will be analyzed for plaque formation with oil red staining, and immunohistochemistry will be performed to analyze the cells present in the plaques.
Analysis: Statistical analysis will be performed to identify potential significant differences in all the quantities measured. Any difference with a P value below .05 will be viewed as significant.
Specific Aim 2:
Design: Increasing iron export from macrophages will invariably increase circulating iron levels with unknown consequences, and as a result it would be preferable to analyze different macrophage populations in the same animal. This is possible with a Brainbow inspired construct. Brainbow is often used to distinguish individual cells within an organism using fluorescent proteins, but in this case Brainbow will be used to generate functionally distinct macrophages, which will also be able to be distinguished by immunohistochemistry. Specifically, some macrophages within the same mouse will contain wildtype ferroportin that can bind hepcidin, while others will contain a mutant ferroportin that cannot bind hepcidiin, and each population will be analyzed in plaques.
Methods: The CreApo mice will be used as a starting point. The Brainbow 2.1 construct will be altered such that wildtype ferroportin tagged with a Myc-tag will replace the GFP locus, YFP locus, and CFP locus. Mutant ferroportin with a FLAG-tag will replace the RFP locus. The new construct will be inserted into the Rosa26 locus. This should result in the adult mice having 1 out of 4 macrophages with the mutant ferroportin. The mice will be fed a high fat diet for 8 weeks, sacrificed, and their blood and aortic arch will be analyzed by immunohistochemistry.
Analysis: The fraction of mutant/wildtype macrophages in blood will be compared to that in plaques, and any differences will be subjected to thorough statistical analysis.
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