Hepcidin is an iron-regulated hepatic peptide hormone that controls systemic iron homeostasis. We wish identify and define activators and suppressors of hepcidin synthesis. Two major signaling pathways communicate systemic stimuli to activate hepcidin mRNA expression in hepatocytes. Hepcidin activators include proteins responsible for hereditary hemochromatosis [HFE, TfR2 and hemojuvelin (HFE2)] and the bone morphogenetic proteins (BMPs), a group of cytokines of the TGF family that induce hepcidin transcription via the SMAD signalling pathway. Two sequence motifs (the proximal BMP-RE1 and the distal BMP-RE2) within the human and murine hepcidin promoters are critical for the stimulation of hepcidin via HJV, BMP and SMAD4. The BMP signaling pathway communicates systemic iron levels, maintains steady-state hepcidin expression and contributes to the activation of hepcidin by inflammatory stimuli. In addition, proinflammatory cytokines stimulate hepcidin transcription via the Janus kinase (JAK)/ signal transducer and activator of transcription (STAT) signaling pathway and a STAT binding motif proximal to the transcription start site.
Comparatively little is known about mechanisms that suppress hepcidin synthesis. Hypoxia, high erythropoietic activity and iron deficiency inhibit hepcidin expression by largely unknown mechanisms to mobilize iron stores and increase iron absorption. Growth differentiation factor (GDF)-15, twisted gastrulation (TWSG)1 and erythropoietin have been implicated in mediating hepcidin suppression in response to augmented hematopoietic activity, but their mode of involvement remains to be defined. Sensing of iron deficiency was recently linked to TMPRSS6, a protease shown to cleave hemojuvelin, and to the von Hippel Lindau (VHL)-hypoxia-inducible factor (HIF) pathway. A further study suggested that hypoxia-mediated hepcidin suppression requires 2-oxoglutarate-dependent oxygenases, but is independent of HIF1α. For all of the implicated negative regulators it is unclear how repressive signals reach the hepcidin promoter. We now apply genome-wide siRNA screening to identify novel regulators of hepcidin expression.