Hepcidin-Independent Regulation of Dietary Iron Absorption

Abstract

Iron homeostasis in mammals is maintained at the level of iron absorption by the gut. Hepcidin plays a central role in homeostasis by binding to ferroportin and regulating cellular iron export. We found that mice weaned onto diets ranging from 35–350 mg Fe/kg for a period of 4 wk did not change body iron levels as measured by organ iron content and hematological parameters. Direct measure of absorption of 59Fe administered by gavage revealed an inverse correlation between dietary iron content and absorption. Gavage experiments were done following a 4h fast when the stomach and proximal small bowel were free of dietary content. Although iron absorption changed, liver expression of hepcidin mRNA did not. We measured the absorptive response in mice weaned onto diets containing 35 mg Fe/kg for 4 wk and abruptly changed to 350 mg Fe/kg. There was no change in iron absorption at day 1 but by day 3 absorption was reduced nearly 3-fold compared to controls and remained at this level for at least 7d. During this time neither liver nor spleen iron content changed but transferrin saturation increased approximately 1.5-fold. Most importantly, serum hepcidin levels, measured by a competitive binding assay (De Domenico et al. Cell Metab. 2008, 8:146–156), were unchanged. Mice were then changed from diets containing 350 mg Fe/kg to diets containing 35 mg Fe/kg. Within 24h mice increased absorption of 59Fe 3-fold. Elevated absorption continued for at least 3d, declined by 7d and at 14d was at a level found in mice maintained on a diet containing 35 mg Fe/kg. During this period there was no change in organ iron content or in transferrin saturation. Serum hepcidin did not change on day 1, but was reduced by approximately 40% on days 3 through 7. Increased iron absorption could be attributed in part to increased expression of Dmt1 but no change in ferroportin message was detected. Enterocyte ferritin levels doubled on day 1 but returned to control levels on days 3 through 7. Finally, mice with a targeted disruption of the hepcidin gene were challenged with an abrupt reduction in dietary iron content and absorption increased approximately 3-fold. These data suggest that iron absorption can respond to changes in dietary iron content independent of hepcidin and that response to changes in luminal iron content are at the level of uptake and storage in a manner intrinsic to the enterocyte.