Smad7 deficiency decreases iron and haemoglobin through hepcidin up‐regulation by multilayer compensatory mechanisms

Abstract

To maintain iron homoeostasis, the iron regulatory hormone hepcidin is tightly controlled by BMP‐Smad signalling pathway, but the physiological role of Smad7 in hepcidin regulation remains elusive. We generated and characterized hepatocyte‐specific Smad7 knockout mice (Smad7 Alb/Alb), which showed decreased serum iron, tissue iron, haemoglobin concentration, up‐regulated hepcidin and increased phosphor‐Smad1/5/8 levels in both isolated primary hepatocytes and liver tissues. Increased levels of hepcidin lead to reduced expression of intestinal ferroportin and mild iron deficiency anaemia. Interestingly, we found no difference in hepcidin expression or phosphor‐Smad1/5/8 levels between iron‐challenged Smad7 Alb/Alb and Smad7 flox/flox, suggesting other factors assume the role of iron‐induced hepcidin regulation in Smad7 deletion. We performed RNA‐seq to identify differentially expressed genes in the liver. Significantly up‐regulated genes were then mapped to pathways, revealing TGF‐β signalling as one of the most relevant pathways, including the up‐regulated genes Smad6, Bambi and Fst (Follistatin). We found that Smad6 and Bambi—but not Follistatin—are controlled by the iron‐BMP–Smad pathway. Overexpressing Smad6, Bambi or Follistatin in cells significantly reduced hepcidin expression. Smad7 functions as a key regulator of iron homoeostasis by negatively controlling hepcidin expression, and Smad6 and Smad7 have non‐redundant roles. Smad6, Bambi and Follistatin serve as additional inhibitors of hepcidin in the liver.

Therapeutic Advances in Regulating the Hepcidin/Ferroportin Axis

“…Lastly, erythropoiesis has been theorised to reduce hepcidin expression acting through growth differentiation factor 15 (GDF-15), twisted gastrulation factor 1 (TWSG1) or erythroferrone (ERFE), however the exact mechanism is not understood. An et al demonstrated that phosphorylation of SMAD1/5/8 is regulated by the inhibitory SMADs 6 and 7 [24]. Hepatocyte-specific Smad7 knockout mice demonstrated a decrease in non-haem iron within the liver and spleen, in addition to decreases in L-ferritin and FPN levels [24].…”

“…An et al demonstrated that phosphorylation of SMAD1/5/8 is regulated by the inhibitory SMADs 6 and 7 [24]. Hepatocyte-specific Smad7 knockout mice demonstrated a decrease in non-haem iron within the liver and spleen, in addition to decreases in L-ferritin and FPN levels [24]. Conversely, hepcidin and phosphorylated SMAD1/5/8 levels were increased in these mice [24].…”

“…Hepatocyte-specific Smad7 knockout mice demonstrated a decrease in non-haem iron within the liver and spleen, in addition to decreases in L-ferritin and FPN levels [24]. Conversely, hepcidin and phosphorylated SMAD1/5/8 levels were increased in these mice [24]. SMAD6, BMP, activin membrane-bound inhibitor homolog (Bambi) and follistatin have been shown to be inhibitors of hepcidin expression in a Smad7 knockout mouse model fed an iron-rich diet [24].…”

“…Conversely, hepcidin and phosphorylated SMAD1/5/8 levels were increased in these mice [24]. SMAD6, BMP, activin membrane-bound inhibitor homolog (Bambi) and follistatin have been shown to be inhibitors of hepcidin expression in a Smad7 knockout mouse model fed an iron-rich diet [24]. SMAD6 is known to inhibit the phosphorylation of other SMAD proteins while both Bambi and follistatin inhibit the BMP pathway through interacting with the BMPRs and BMPs respectively [24].…”

“…SMAD6, BMP, activin membrane-bound inhibitor homolog (Bambi) and follistatin have been shown to be inhibitors of hepcidin expression in a Smad7 knockout mouse model fed an iron-rich diet [24]. SMAD6 is known to inhibit the phosphorylation of other SMAD proteins while both Bambi and follistatin inhibit the BMP pathway through interacting with the BMPRs and BMPs respectively [24]. Interestingly, An et al found that SMAD6 and Bambi were controlled by the BMP/SMAD pathway, while follistatin was unaffected [24].…”