UbcH5A, a member of human E2 ubiquitin-conjugating enzymes, is closely related to SFT, a stimulator of iron transport, and is up-regulated in hereditary hemochromatosis

Abstract

SFT, a stimulator of iron (Fe) transport, has been described as a transmembrane protein that facilitates the uptake of ferrous and ferric iron in mammalian cells. This study was initiated to investigate the 5 regulatory region of SFT and its role in the etiology of hereditary hemochromatosis. Sequence analyses of the putative 5 regulatory region revealed that the SFT cDNA sequence corresponds to intron 6/exon 7 of UbcH5A, a member of E2 ubiquitin-conjugating enzymes, which is involved in the iron-dependent ubiquitination of the hypoxia-inducible factor (HIF) by the von Hippel-Lindau tumor suppressor (pVHL) E3 ligase complex. Further mRNA expression studies using a sequence-specific reverse transcriptasepolymerase chain reaction (RT-PCR) assay showed that UbcH5A is significantly upregulated in the liver of iron-overloaded patients with hereditary hemochromatosis, as previously published for SFT. However, in vitro studies on HepG2 cells failed to demonstrate any significant UbcH5A regulation in response to iron loading or iron chelation. In conclusion, in vivo mRNA expression data previously obtained for SFT might be attributed to UbcH5A.

Hepcidin in Iron Metabolism

“…Those results were confirmed and extended in humans. Indeed, two independent reports demonstrated that patients with genetic haemochromatosis related to a C282Y homozygous mutation presented a reduced or even no hepatic hepcidin mRNA increase despite iron overload [42,53]. All these data suggest that a deregulation of hepcidin expression is involved in the development of iron overload in genetic haemochromatosis.…”

“…Others demonstrated that, in human hepatocytes which express only one hepcidin gene, the treatment with ferrous ammonium citrate or transferrin-iron decreased significantly the hepcidin mRNA steady-state level [36]. Finally, using HepG2 cells, a human-derived hepatoblastoma cell line which expresses hepcidin at low level compared to normal liver [26,30], Gehrke et al found that iron-NTA, a non-transferrin-bound form of iron, decreased the expression of hepcidin in a dose-dependent manner [42]. In addition, they found that di-ferric transferrin has no significant effect.…”

“…In addition, they found that di-ferric transferrin has no significant effect. From these data, they postulated that non-transferrin-bound iron decreases hepcidin expression [42].…”

Mammalian iron transport

“…Another candidate is SFT, or stimulator of Fe transport. This protein was isolated as an iron transporter from erythroid leukemia cells and has been found to play a role in both Tf-dependent and independent iron uptake, but its role remains poorly characterized [77][78][79]. Indeed, the original published sequence was incorrect, and the sequence responsible for conferring iron transport activity consists of several smaller open reading frames [77].…”

Hepcidin knockout mice as a model of iron-overload associated liver disease

“…The uptake of NTBI is incompletely understood but seems to involve DMT1 [84,85] (Figure 1.2) and stimulator of iron transport (SFT) which is found on plasma membrane of hepatocytes. Accordingly, expression of DMT1 and SFT is increased in situation of iron overload [84,85,86,87]. …”