Systemic iron homeostasis is maintained by regulation of iron absorption in the duodenum, iron recycling from erythrocytes, and iron mobilization from the liver and is controlled by the hepatic hormone hepcidin. Hepcidin expression is induced via the bone morphogenetic protein (BMP) signaling pathway that preferentially uses two type I (ALK2 and ALK3) and two type II (ActRIIA and BMPR2) BMP receptors. Hemojuvelin (HJV), HFE, and transferrin receptor-2 (TfR2) facilitate this process presumably by forming a plasma membrane complex with BMP receptors. Matriptase-2 (MT2) is a protease and key suppressor of hepatic hepcidin expression and cleaves HJV. Previous studies have therefore suggested that MT2 exerts its inhibitory effect by inactivating HJV. Here, we report that MT2 suppresses hepcidin expression independently of HJV. In mice, increased expression of exogenous MT2 in the liver significantly reduced hepcidin expression similarly as observed in wild-type mice. Exogenous MT2 could fully correct abnormally high hepcidin expression and iron deficiency in mice. In contrast to MT2, increased Hjv expression caused no significant changes in wild-type mice, suggesting that Hjv is not a limiting factor for hepcidin expression. Further studies revealed that MT2 cleaves ALK2, ALK3, ActRIIA, Bmpr2, Hfe, and, to a lesser extent, Hjv and Tfr2. MT2-mediated Tfr2 cleavage was also observed in HepG2 cells endogenously expressing MT2 and TfR2. Moreover, iron-loaded transferrin blocked MT2-mediated Tfr2 cleavage, providing further insights into the mechanism of Tfr2's regulation by transferrin. Together, these observations indicate that MT2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway.

Hereditary hemochromatosis (HH), a disease marked by chronic iron overload from insufficient expression of the hormone hepcidin, is one of the most common genetic diseases. One form of HH (Type III) results from mutations in the transferrin receptor-2 (TfR2). TfR2 is postulated to be a part of signaling system that is capable of modulating hepcidin expression. The molecular details of TfR2’s role in this system remain unclear, however. TfR2 is predicted to bind the iron carrier transferrin (Tf) when the iron-saturation of Tf is high. To better understand the nature of these TfR-Tf interactions, a binding study with the full-length receptors was conducted. In agreement with previous studies with truncated forms of the receptors, holo-Tf binds to the homolog, TfR1, significantly stronger than TfR2. However, the binding constant for Tf-TfR2 is still far above that of physiological holo-Tf levels, inconsistent with the hypothetical model and suggests that other factors mediate the interaction. One possible factor, Apo-Tf, only weakly binds TfR2 at serum pH, thus will not be able to effectively compete with holo-Tf. Tf-binding to a TfR2 chimera containing the TfR1-helical domain indicates that the differences in the helical domain account for differences in on-rate of Tf, and non-conserved inter-receptor interactions are necessary for the stabilization of the complex. Conserved residues at one possible site of stabilization, the apical arm junction, are not important for TfR1-Tf binding, but are critical for the TfR2-Tf interaction. Our results highlight the differences in Tf interactions with the two TfRs.

Matriptase-2 (MT2), encoded by TMPRSS6, is a membrane-anchored serine protease, which plays a key role in suppressing hepatic hepcidin expression. MT2 is synthesized as a zymogen and undergoes autocleavage for activation. Previous studies suggest that MT2 suppresses hepcidin by cleaving hemojuvelin (HJV) and other components of the BMP-signaling pathway. However, the underlying mechanism is still debatable. Here we dissected the contributions of the non-proteolytic and proteolytic activities of Mt2 by taking advantage of Mt2 mutants and Tmprss6-/- mice. Studies of the protease-dead full-length Mt2 (Mt2S762A) and the truncated Mt2 that lacks the catalytic domain (Mt2mask) indicate that the catalytic domain, but not its proteolytic activity, was required for Mt2 to suppress hepcidin expression. This process was likely accomplished by the binding of Mt2 ectodomain to Hjv and Hfe. We found that Mt2 specifically cleaved the key components of the hepcidin induction pathway including Hjv, Alk3, ActRIIA, and Hfe when overexpressed in hepatoma cells. Nevertheless, studies of a murine IRIDA-causing mutant (Mt2I286F) in the CUB domain indicate that Mt2I286F can be activated but it displayed a largely compromised ability to suppress hepcidin expression. Co-immunoprecipitation analysis revealed that Mt2I286F, but not Mt2S762A, had reduced interactions with Hjv, ActRIIA, and Hfe. Additionally, increased expression of the serine protease inhibitor Hai-2 in the liver failed to alter hepcidin. Together these observations support the idea that the substrate interaction with Mt2 plays a determinant role, and suggest that the proteolytic activity is not an appropriate target to modulate the function of MT2 for clinical applications.

Loss of p53’s proper function accounts for over half of identified human cancers. We identified the metal transporter ZIP14 (Zinc-regulated transporter (ZRT) and Iron-regulated transporter (IRT)-like Protein 14) as a p53-regulated protein. ZIP14 protein levels were upregulated by lack of p53 and downregulated by increased p53 expression. This regulation did not fully depend on the changes in ZIP14’s mRNA expression. Co-precipitation studies indicated that p53 interacts with ZIP14 and increases its ubiquitination and degradation. Moreover, knockdown of p53 resulted in higher non-transferrin-bound iron uptake, which was mediated by increased ZIP14 levels. Our study highlights a role for p53 in regulating nutrient metabolism and provides insight into how iron and possibly other metals such as zinc and manganese could be regulated in p53-inactivated tumor cells.

Intraflagellar transport (IFT) is a microtubule based system that supports the assembly and maintenance of cilia. Genetic and biochemical studies have identified two distinct complexes containing multiple proteins that are part of the IFT machinery. In this study we prepared mouse pituitary cells that expressed an epitope-tagged IFT protein and immuno-purified the IFT B complex from these cells. Mass spectrometry analysis of the isolated complex led to identification of a number of well known components of the IFT B complex. In addition, peptides corresponding to mouse tetratricopeptide repeat proteins, TTC30A1, TTC30A2 and TTC30B were identified. The mouse Ttc30A1, Ttc30A2, Ttc30B genes are orthologs of Caenorhabditis elegans dyf-1, which is required for assembly of the distal segment of the cilia. We used co-immunoprecipitation studies to provide evidence that, TTC30A1, TTC30A2 or TTC30B can be incorporated into a complex with a known IFT B protein, IFT52. We also found that TTC30B can interact with mouse KIF17, a kinesin which participates in IFT. In vitro expression in a cell-free system followed by co-immunoprecipitation also provided evidence that TTC30B can directly interact with several different IFT B complex proteins. The findings support the view that mouse TTC30A1, TTC30A2 and TTC30B can contribute to the IFT B complex, likely through interactions with multiple IFT proteins and also suggest a possible link to the molecular motor, KIF17 to support transport of cargo during IFT.