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Rat intestinal and hepatic ferritin subunit expression during development and after dietary iron feeding.

Ferritin consists of 24 heavy (H) and light (L) subunits in varying proportions in different tissues and plays a significant role in iron metabolism. We studied rat ferritin subunit expression in the duodenum and liver during early life, when a cycle of iron depletion and repletion occurs. In both tissues, ferritin contents decreased to low levels from day 3 to day 12. The ferritin on day 3 had an H/L mRNA ration of 0.9 and an H/L subunit ratio of 0.6. The decrease of tissue ferritin levels, but not mRNA, on day 12 suggests translational repression consistent with iron depletion. In the duodenum, a twofold increase in both H and mRNA and subunit protein occurred on day 18. The subsequent increase of H mRNA was accompanied by a 50% decrease in L mRNA, resulting in the increase of H/L mRNA and subunit ratios to 7.9 and 9, respectively, by day 32. In contrast, liver H/L mRNA and subunit ratios were similar throughout development. The possibility that dietary iron regulates duodenal ferritin subunit expression was investigated. When day 12 rats were fed 6 ml of a milk formula containing 56 microgram/ml iron for 18 h, dietary iron increased the duodenal levels of L mRNA but not H mRNA. In contrast, hepatic H and L mRNA levels did not change. Dietary iron promoted greater increases in ferritin protein than mRNA in both tissues. Thus a shift from L-rich to H-rich ferritin isoforms occurs in the duodenum but not in the liver during neonatal development. This change is regulated at the pretranslational level and is independent of dietary iron.