Hepcidin, a small peptide produced by the liver, is the principal iron-regulatory hormone. It acts by binding to the iron exporter ferroportin, inducing its internalization and degradation, thereby blocking cellular iron efflux. The bioactive 25 amino acid peptide has a hairpin structure stabilized by 4 disulfide bonds. We synthesized a series of hepcidin derivatives and determined their bioactivity in a cell line expressing ferroportin-GFP fusion protein, by measuring the degradation of ferroportin-GFP and the accumulation of ferritin after peptide treatment. Bioactivity was also assayed in vivo, by measuring hypoferremia in mice injected with hepcidin derivatives.
Serial deletion of N-terminal amino acids caused progressive decrease in activity which was completely lost when five N-terminal aa were deleted. Synthetic 3-aa and 6-aa N-terminal peptides alone, however, did not internalize ferroportin, and did not interfere with ferroportin internalization by native hepcidin. Deletion of two C-terminal amino acids did not affect peptide activity. Removal of individual disulfide bonds by pairwise substitution of cysteines with alanines also did not impact peptide activity in vitro. However, these peptides were significantly less active in vivo, likely due to their decreased stability in circulation. Peptides with a substitution G71D or K83R, previously described in human subjects, were fully active in vitro and in vivo. Zebrafish hepcidin, which is only 60% similar to human hepcidin, but with conservative substitutions at the N-terminus, was as active as its human counterpart.
Apart from the essential nature of the N-terminus, hepcidin structure appears permissive for mutations. Further studies of hepcidin structure in relation to its function are essential for the design of hepcidin antagonists and agonists which could be used for treatment of iron disorders.