Hepcidin Regulation Mechanisms In Iron Disorders

Iron is the essential element for all organisms. Due to the redox ability of iron, iron is well suited to the catalytic center of many important enzymes and important proteins, such as hemoglobin for oxygen, myoglobin for energy metabolism. Iron deficiency will lead to iron deficiency anemia, in contrast, iron overload will cause damage to the tissues of human body. Therefore, there exists a meticulous regulatory mechanism to maintain iron homeostasis in the body. Actually, two parallel mechanisms exist in iron metabolism. One is on single cell level to constrain cellular iron content, while the other one is systematic control of body iron absorption and mobilization. Iron regulatory hormone "hepcidin", which can interact with the iron export transporter "ferroportin", is the pivotal molecule to control iron metabolism in mammals. Hepcidin can trigger the internalization of ferrotportin on small intestine base membrane to reduce iron absorption and macrophage to reduce iron recycling. Multiple factors generate regulatory effects on hepcidin expression and secretion, such as circulating iron concentration, iron storage in the liver and erythropoietic iron demand. Besides iron, inflammatory cytokines can trigger hepcidin secretion to immobilize iron in order to restrict the iron availability for invading microorganisms. If genetic mutations disturb the hepcidin-ferroportin axis, it will lead to iron disorders such as hereditary hemochromatosis, a disease featured by severe iron deposition in major organs.To investigate the genetic mechanisms of iron deficiency, we analyzed the single nucleotide polymorphisms (SNP) of iron regulatory protein encoding gene TMPRSS6. We tested the association of these SNPs with iron parameters and the risks of iron deficiency and iron deficiency anemia. We found that TMPRSS6 polymorphisms were significantly associated with decreased serum iron and hemoglobin concentration. In addition, these polymorphisms were also genetic risk factors for both iron deficiency and iron deficiency anemia OR=1.78 (95% CI 1.59-1.97). We supposed that SNP rs855791 on TMPRSS6 gene perhaps altered the inhibition effect on hepcidin, which led to decreased iron availability and increased iron deficiency anemia risk.To investigate the genetic mechanisms of iron overload,four domestic sporadic families of hereditary hemochromatosis in China were collected and we identified mutations in HJV, TFR2 and SLC40A1 respectively. Among these mutations, a novel heterozygous c.626C>G (p.Ser209Leu) missense mutation in exon6 of SLC40A1 was firstly identified. Our genetic findings broaden the mutation site list of hereditary hemochromatosis. TFR2 p.Ile238Met and SLC40A1 p.Trp158Cys mutations were also reported for the first time in Chinese. The aim of this study is to attract more attention of native physicians to Chinese hemochromatosis patients and promote the progress of hemochromatosis diagnosis and treatment.To investigate the regulatory mechanism of hepcidin, we constructed hepatocyte-specific deletion of Smad7 gene in mouse. Our study identified that Smad7 played an important role in the maintenance of iron homeostasis through negative controlling the basal expression level of hepcidin. In the absence of Smad7, Increase hepcidin expression and decreased iron parameters were found in mice lack of Smad7. In addition, Smad6 cannot substitute the fundamental function of Smad7. Our findings also indicated that other factors might participate the regulation of hepcidin under iron challenge. Therefore, further investigation of the meticulous coordination of hepcidin regulators will promote the therapeutic treatment of iron disorders.We next screened bioactive extracts from Chinese medicinal plants, which may be efficacious in the inhibition of expression of hepcidin. To test this, we measured theexpression level of hepcidin in cultured cells treated with sixteen different medicinal plant extracts, all of which are used to treat anemia-related disorders in traditional Chinese medicine. Among the extracts tested, that of CaulisSpatholobishowed the most potent inhibitory effect on HAMP expression in the Huh7 cell line and was therefore selected for further mechanistic study. Mice fed diet containing dried CaulisSpatholobi showed a decrease in hepatic HAMP expression an increase in serum iron concentration untreated group. The extract of CaulisSpatholobi as a novel, potent HAMP expression inhibitor, which may be further modified and optimized to become a dietary supplement or a therapeutic option for the amelioration of hepcidin-overexpression related diseases including iron deficiency anemiaIn conclusion, we investigated the genetic mechanisms of iron disorders in our studies using cell, mouse and population based study. We identified the first wide spread genetic risk factor for iron deficiency anemia and a novel mutation site on SLC40A1 in hemochromatosis. We also constructed mice with specific deletion of Smad7 in hepatocytes to demonstrate the negative role of Smad7 in hepcidin regulation. The purpose of our research is to have a better understanding of hepcidin-ferroportin regulation to improve the diagnosis and treatment of iron disorders.