Production of biologically active forms of recombinant hepcidin, the iron-regulatory hormone.

Hepcidin is a liver produced cysteine-rich peptide hormone that acts as the central regulator of body iron metabolism. Hepcidin is synthesized under the form of a precursor, prohepcidin, which is processed to produce the biologically active mature 25 amino acid peptide. This peptide is secreted and acts by controlling the concentration of the membrane iron exporter ferroportin on intestinal enterocytes and macrophages. Hepcidin binds to ferroportin, inducing its internalization and degradation, thus regulating the export of iron from cells to plasma. The aim of the present study was to develop a novel method to produce human and mouse recombinant hepcidins, and to compare their biological activity towards their natural receptor ferroportin. Hepcidins were expressed in Escherichia coli as thioredoxin fusion proteins. The corresponding peptides, purified after cleavage from thioredoxin, were properly folded and contained the expected four-disulfide bridges without the need of any renaturation or oxidation steps. Human and mouse hepcidins were found to be biologically active, promoting ferroportin degradation in macrophages. Importantly, biologically inactive aggregated forms of hepcidin were observed depending on purification and storage conditions, but such forms were unrelated to disulfide bridge formation.

Pro-hepcidin is unable to degrade the iron exporter ferroportin unless maturated by a furin-dependent process.

BACKGROUND/AIMS: The iron-regulatory peptide hepcidin is synthesized in the liver as an 84-aa pre-pro-hormone maturated by proteolysis through a consensus furin cleavage site to generate the bioactive 25-aa peptide secreted in the circulation. This peptide regulates iron export from enterocytes and macrophages by binding the membrane iron exporter, ferroportin, leading to its degradation. Whether pro-hepcidin could be secreted and reflect hepcidin levels remains an open question. However, the activity of the pro-peptide on ferroportin degradation has never been addressed. METHODS: To answer this question, we produced recombinant pro-hepcidin, both the wild-type form and a furin cleavage site mutant, and tested their activity on ferroportin levels in macrophagic J774 cells. Furin activity was also modulated using furin inhibitor or siRNA-mediated furin mRNA knockdown. RESULTS: We found that pro-hepcidin could fully induce ferroportin degradation, but only when processed by furin to generate the mature hepcidin-25 form. Pro-hepcidin activity was abolished in the presence of furin inhibitor and diminished after siRNA-mediated knockdown of furin mRNA. Furthermore, the mutated version of pro-hepcidin was completely inefficient at degrading ferroportin in macrophages. CONCLUSIONS: Our results demonstrate that pro-hepcidin lacks biological activity, unless fully maturated by a furin-dependent process to yield the bioactive 25-aa peptide.