Coincident increase in periportal expression of iron proteins in the iron-loaded rat liver

The liver is the major iron storage organ in the body and, as a result, total body iron stores closely regulate hepatocyte iron uptake, storage and release. Transferrin, transferrin receptor and ferritin facilitate these processes.

Expression of the three proteins was localized by immunohistochemistry and in situ hybridization on normal, iron-loaded and iron-deficient rat livers. Gel shift assays were used to determine iron regulatory protein (IRP) binding activity.

In the normal rat liver, all three proteins and mRNA were evenly distributed throughout the hepatic lobule. In iron-loaded liver, increased iron stores were found in a periportal distribution, coinciding with increased periportal protein levels of each protein. Periportal transferrin and ferritin mRNA levels were also increased. Hepatic transferrin and transferrin receptor expression was increased in iron deficiency compared with controls; however, despite no change in ferritin mRNA levels being found, ferritin protein was not detected. Hepatic IRP2 binding activity was decreased in iron loading and increased in iron deficiency.

The combined findings of this study were that, in the dietary iron-loaded rat model, increased iron stores were localized to periportal hepatocytes and that these same hepatocytes also had increased ferritin, transferrin receptor and transferrin protein expression. This response suggests that additional, non-IRP control mechanisms may be involved in the regulation or stability of these proteins. In iron deficiency the inverse post-transcriptional regulation of ferritin and transferrin receptor was consistent with IRP regulation.