Phosphorylation of the corepressor TGIF by EGF-activated Ras/MEK signaling has been reported; TGIF phosphorylation resulted in stabilization of the repressor and formation of R-Smad/TGIF transcriptionally suppressive complexes.30 We surmise that HGF may suppress hepcidin induction by BMP through MAPK stabilization of TGIF. HGF is a pleiotropic growth factor that activates a multitude of downstream signaling pathways; many of the mitogenic, morphogenic, and motogenic effects of Met are regulated by more than one of these downstream signals. Our kinase inhibitor screen in primary hepatocytes identified at least two signaling pathways (MEK and PI3K) that appear to regulate hepcidin.

The activity of the MEK1/2 inhibitor U0126 in our studies suggested a role for MEK in HGF suppression. It was previously reported that Ras/MEK activation by EGF results in phosphorylation and stabilization of the Smad Selleck Opaganib transcriptional Proteasome assay corepressor TGIF.30 HGF may cause a similar stabilization of TGIF by way of MEK activation. A more detailed exploration of the similarities and differences between HGF and EGF pathways will be undertaken in a future study. In view of the role of growth factors HGF, EGF,

and transforming growth factor alpha (TGF-α), which also binds to the EGF receptor, as mediators of the hepatic regenerative response,14 the suppression of hepcidin by growth factors may be relevant to hepcidin deficiency and hepatic iron loading in chronic liver diseases. Elevated liver tissue concentrations of growth factors in chronic viral and

alcoholic hepatitis could be repressing maximal hepcidin response to iron, thereby increasing dietary iron absorption and worsening the liver injury. As in hereditary hemochromatosis, the relative lack of hepcidin induction by iron in chronic hepatitis results in chronic hyperabsorption of dietary iron. Excess iron accumulates particularly in the liver due to the avid uptake of non-transferrin-bound iron (NTBI) by hepatocytes, PLEKHM2 as well as the first-pass effect of portal circulation from the gut. The iron deposition is often parenchymal and compounds preexisting liver injury from hepatitis, worsening disease prognosis. In chronic hepatitis C (CHC), iron correlates with development of cirrhosis and hepatocellular carcinoma (HCC).11 The role of iron in disease progression has been supported by studies in which phlebotomy improved disease indices in nonalcoholic steatohepatitis and CHC.31, 32 However, the effects of iron on hepatitis C may be complex; excess iron promotes tissue damage but it also suppresses viral replication, perhaps accounting for the divergent outcomes of phlebotomy interventions.33 Regulation of hepcidin by growth factors may be important for normal iron homeostasis as well. Hepcidin must be physiologically suppressed during early years of life, when continuing growth and development require greater iron absorption than in the mature adult.