Cross-talk between the mitogen activated protein kinase and bone morphogenetic protein/hemojuvelin pathways is required for the induction of hepcidin by holotransferrin in primary mouse hepatocytes.

BACKGROUND: The circulating hormone hepcidin plays a central role in iron homeostasis. Our goal was to establish an ex vivo iron-sensing model and to characterize the molecular mechanisms linking iron to hepcidin. DESIGN AND METHODS: Murine hepatocytes were isolated by the collagenase method, either from wild type or HFE knockout mice, and cultured 42 h without serum before treatments.
RESULTS: After 42 h of serum-free culture, hepcidin gene expression was undetectable in the hepatocytes. Hepcidin gene expression could, however, be re-activated by an additional 24 h of incubation with 10% serum. Interestingly, addition of 30 microM holotransferrin consistently increased serum-dependent hepcidin levels 3- to 5-fold. The effects of serum and serum+holotransferrin were direct, transcriptional, independent of de novo protein synthesis and required the presence of bone morphogenetic protein. Transferrin receptor-2 activation by its ligand holotransferrin led to extracellular signal regulated kinase (ERK)/mitogen activated protein kinase pathway stimulation and the ERK specific inhibitor U0-126 blunted holotransferrin-mediated induction of hepcidin. ERK activation by holotransferrin provoked increased levels of phospho-Smad1/5/8 highlighting cross-talk between the bone morphogenetic protein/hemojuvelin and ERK1/2 pathways. Finally, we demonstrated, using hepatocytes isolated from Hfe(-/-) mice, that HFE was not critical for the hepcidin response to holotransferrin but important for basal hepcidin expression.
CONCLUSIONS: We demonstrate that hepatocytes are liver iron-sensor cells and that transferrin receptor-2, by signaling through the ERK1/2 pathway, and bone morphogenetic protein/hemojuvelin, by signaling through the Smad pathways, coordinately regulate the iron-sensing machinery linking holotransferrin to hepcidin.