Serum iron overload activates the SMAD pathway and hepcidin expression of hepatocytes via SMURF1

Background and Aims

The liver orchestrates systemic iron balance by producing and secreting hepcidin. Hepcidin binds to the nonheme iron exporter ferroportin 1, induces its internalization and degradation, and is vital for maintaining iron homeostasis throughout the body. Hepcidin deficiency leads to excess iron deposition in parenchymatous organs and ferroptosis and is the common pathogenic mechanism for iron overload disorders, such as hereditary hemochromatosis.

 

The most commonly studied pathway that influences hepcidin transcription in response to an increase in body iron levels is the bone morphogenetic protein (BMP) and the SMA and mothers against decapentaplegic homolog (SMAD) pathway, and BMP6 is the predominant BMP ligand responsible for hepcidin regulation in vivo. Circulating (serum) and tissue (liver) iron levels have been suggested to regulate hepcidin expression. Recent studies have reported that serum and liver iron overload differentially regulates the BMP/SMAD pathway, and the liver iron (deposited iron) content is correlated with the hepatic BMP6 mRNA level and subsequently regulates the BMP/SMAD pathway and hepcidin expression. However, serum iron (transferrin-bound iron) overload does not induce BMP6 expression but can still regulate the BMP/SMAD pathway and hepcidin expression. Thus, the mechanism by which serum iron overload activates SMAD and hepcidin expression remains unknown.

 

In this study, we investigated the molecular mechanism of the serum iron overload-induced BMP/SMAD pathway and hepcidin expression using transferrin-bound iron (Holo-transferrin, Holo-Tf) and revealed the master regulatory function of SMURF1 in serum iron overload.

 

Methods

A cell model of serum iron overload was established by treating hepatocytes with 2 mg/mL of holo-transferrin (Holo-Tf). A serum iron overload mouse model and a liver iron overload mouse model were established by intraperitoneally injecting 10 mg of Holo-Tf into C57BL/6 mice and administering a high-iron diet for 1 week followed by a low-iron diet for 2 days. Western blotting and real-time PCR were performed to evaluate the activation of the BMP/SMAD pathway and the expression of hepcidin.

 

Results

Holo-Tf augmented the sensitivity and responsiveness of hepatocytes to BMP6. The E3 ubiquitin-protein ligase SMURF1 mediated Holo-Tf-induced SMAD1/5 activation and hepcidin expression; specifically, SMURF1 expression dramatically decreased when the serum iron concentration was increased. Additionally, the expression of SMURF1 substrates, which are important molecules involved in the transduction of BMP/SMAD signaling, was significantly upregulated. Furthermore, in vivo analyses confirmed that SMURF1 specifically regulated the BMP/SMAD pathway during serum iron overload.

 

Conclusions

Holo-Tf activates SMAD1/5 and increases hepcidin expression by augmenting the sensitivity and responsiveness of hepatocytes to BMPs by reducing SMURF1 expression. The inhibition of SMURF1, such as by the inhibitor Smurf1-IN-A01, may represent a therapeutic strategy for iron overload-related diseases such as hemochromatosis.

 

https://www.xiahepublishing.com/2310-8819/JCTH-2023-00440

 

The study was recently published in the Journal of Clinical and Translational Hepatology.

Journal of Clinical and Translational Hepatology (JCTH) publishes high quality, peer reviewed studies in the translational and clinical human health sciences of liver diseases.

JCTH is currently indexed in Science Citation Index Expanded (SCIE) ,  PubMed and Scopus.

 

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