Effects of iron overload on transferrin secretion by cultured fetal rat hepatocytes

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

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Transferrin mRNA in relation to liver iron storage in farmed Atlantic salmonSalmo salar

Transferrin mRNA in liver was quantified in farmed Atlantic salmon (Salmo salar) fed three different levels (310-, 65-, and 40 ppm) of dietary iron for 26 weeks. At the time of sampling, the fish fed 40 ppm iron showed symptoms of iron deficiency. Mean values of liver storage iron was reduced from 84 to 12 mg kg(-1) and plasma iron from 2.3 to 1.6 mg l(-1) compared with fish fed 310 ppm iron. No significant difference in transferrin mRNA was observed between normal and iron deficient salmon; the amount of Tf mRNA was not related to the amount of stored iron, either in groups or individual fish. This suggests that there is no iron-modulated transcriptional regulation of liver Tf expression in salmon as there is in chickens and rats.

Expression of some hepatocyte-like functional properties of WRL-68 cells in culture

Some morphologic and functional characteristics of an hepatic fetal human epithelial cell line (WRL-68 cells) were determined to validate the use of these cells as an in vitro hepatic model. WRL-68 cells have a morphologic structure similar to hepatocytes and hepatic primary cultures. They secrete alpha-feto protein and albumin and exhibit a cytokeratin pattern similar to other hepatic cultures. WRL-68 cells preserve the activity of some characteristic or specific liver enzymes or both used in clinical chemistry for the diagnosis of hepatic disorders, i.e. alanine amino transferase, aspartate amino transferase, gamma-glutamyl transpeptidase, and alkaline phosphatase.

Iron(III)-binding polypeptide in human cord and adult serum: isolation, purification and partial characterization

A low-molecular-weight, non-transferrin-bound, Fe(III)-binding polypeptide has been isolated and purified from normal human cord and adult sera by gel-filtration and high-performance liquid chromatography. The polypeptide was traced isotopically with 59Fe(III)-chloride and high voltage paper electrophoresis. This Fe(III)-binding polypeptide has been partially characterized, and found to be biochemically distinct from other known Fe(III)-binding proteins, such as transferrin, lactoferrin and ferritin. Furthermore, the electrophoretic mobility and amino-acid composition distinguish the polypeptide from other physiological iron chelators such as a previously described Fe(III)-citrate complex. The polypeptide is highly hydrophilic, rich in lysine residues and phosphorylated. The observed positive charge of the polypeptide is suggested to originate from these lysine residues. The molecular mass of this polypeptide is estimated to be approx. 2500 Da, which is in close agreement with previous reports and is consistent with amino-acid analysis data. Cord serum levels of the polypeptide were significantly higher than adult serum levels. The degree of Fe(III) specificity and the function of the Fe(III)-polypeptide have not been ascertained at the present time. It is possible that the polypeptide may compete for Fe(III) with transferrin and is involved in the mobilization and transportation of iron by some as yet unknown mechanism.