Iron overload inhibits BMP/SMAD and IL-6/STAT3 signaling to hepcidin in cultured hepatocytes

Abstract

Hepcidin is a peptide hormone that targets the iron exporter ferroportin, thereby limiting iron entry into the bloodstream. It is generated in hepatocytes mainly in response to increased body iron stores or inflammatory cues. Iron stimulates expression of bone morphogenetic protein 6 (BMP6) from liver sinusoidal endothelial cells, which in turn binds to BMP receptors on hepatocytes and induces the SMAD signaling cascade for transcriptional activation of the hepcidin-encoding HAMP mRNA. SMAD signaling is also essential for inflammatory HAMP mRNA induction by the IL-6/STAT3 pathway. Herein, we utilized human Huh7 hepatoma cells and primary murine hepatocytes to assess the effects of iron perturbations on signaling to hepcidin. Iron chelation appeared to slightly impair signaling to hepcidin. Subsequent iron supplementation not only failed to reverse these effects, but drastically reduced basal HAMP mRNA and inhibited HAMP mRNA induction by BMP6 and/or IL-6. Thus, treatment of cells with excess iron inhibited basal and BMP6-mediated SMAD5 phosphorylation and induction of HAMP, ID1 and SMAD7 mRNAs in a dose-dependent manner. Iron also inhibited IL-6-mediated STAT3 phosphorylation and induction of HAMP and SOCS3 mRNAs. These responses were accompanied by induction of GCLC and HMOX1 mRNAs, known markers of oxidative stress. We conclude that hepatocellular iron overload suppresses hepcidin by inhibiting the SMAD and STAT3 signaling pathways downstream of their respective ligands.

A crosstalk between hepcidin and IRE/IRP pathways controls ferroportin expression and determines serum iron levels in mice

“…While experimental iron loading of cultured cells is rapid, dietary iron loading of mice is gradual (20) and most of excess iron is effectively detoxified within ferritin, which is highly induced (30). By contrast, the suppression of hepcidin preceded ferritin induction in cultured cells (10), which may explain the discrepancy with the in vivo data.…”

“…We sought to analyze how iron overload affects hepcidin-mediated inflammatory responses. We and others reported that excess iron inhibits the major hepcidin signaling pathways (BMP/SMAD and IL-6/STAT3) in cultured cells (10,11). To explore the physiological relevance of these findings, wt mice were subjected to variable degrees of dietary iron loading and then treated with LPS.…”

“…Thus, Hjv-/- mice, a model of juvenile hemochromatosis characterized by severe iron overload and hepcidin deficiency (8), exhibit blunted inflammatory induction of hepcidin and fail to mount a hypoferremic response following LPS treatment or infection with E. coli (9). Excess iron inhibits hepcidin induction via the BMP/SMAD and IL-6/STAT3 signaling pathways in cultured cells (10, 11), but the in vivo relevance of these findings is not known.…”

Genome-wide analysis in over 1 million individuals reveals over 2,000 independent genetic signals for blood pressure

“…Genetic variation at TMPRSS6 is also associated with biomarkers of iron overload 53 . SMAD7 (rs72917789, SBP p = 1.14x10 − 8 ) has been shown to modulate expression of hepcidin, a key regulator of intestinal iron absorption 54,55 . Additionally, GSMT1 (rs36209093, DBP p = 9.94x10 − 15 ), encoding glutathione S-transferase Mu 1, has been implicated in cardiomyopathy due to iron overload 56,57 .…”

Epigallocatechin-3-Gallate Suppresses BMP-6-Mediated SMAD1/5/8 Transactivation of Hepcidin Gene by Inducing SMILE in Hepatocytes

“…For example, activated NFE2-related factor 2, by sensing systemic iron accumulation or by treatment with EGCG, can induce BMP-6 and hepcidin expression by binding to the antioxidant response element on the gene promoter, subsequently decreasing serum iron levels and iron toxicity [51,52]. Interestingly, it is reported that hepatocellular iron accumulation inhibited hepcidin expression by suppressing BMP/SMAD and IL-6/STAT3 signaling [53]. Our previous findings have shown that EGCG at a high concentration can diminish IL-6-induced hepcidin expression and secretion through the induction of FOXO1-dependent SMILE expression, which, in turn, can increase serum iron levels [20].…”