INTESTINAL AND HEPATIC FERRITIN SUBUNIT EXPRESSION DURING DEVELOPMENTAND AFTER DIETARY IRON FEEDING

Ferritin consists of 24 heavy (H) and light (L) subunits in varying pr oportions in different tissues and plays a significant role in iron me tabolism. We studied rat ferritin subunit expression in the duodenum a nd liver during early life, when a cycle of iron depletion and repleti on 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 ratio of 0 .9 and an H/L subunit ratio of 0.6. The decrease of tissue ferritin le vels, but not mRNA, on day 12 suggests translational repression consis tent with iron depletion. In the duodenum, a twofold increase in both H 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 t he 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 simila r throughout development. The possibility that dietary iron regulates duodenal ferritin subunit expression was investigated. When day 12 rat s were fed 6 ml of a milk formula containing 56 mu g/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 p romoted greater increases in ferritin protein than mRNA in both tissue s. Thus a shift, from L-rich to H-rich ferritin isoforms occurs in the duodenum but not in the liver during neonatal development. This chang e is regulated at the pretranslational level and is independent of die tary iron.