Effect of Iron Overload and Iron Deficiency on Liver Hemojuvelin Protein

Abstract

IntroductionHemojuvelin (Hjv) is a key component of the signaling cascade that regulates liver hepcidin (Hamp) expression. The purpose of this study was to determine Hjv protein levels in mice and rats subjected to iron overload and iron deficiency.MethodsC57BL/6 mice were injected with iron (200 mg/kg); iron deficiency was induced by feeding of an iron-deficient diet, or by repeated phlebotomies. Erythropoietin (EPO)-treated mice were administered recombinant EPO at 50 U/mouse. Wistar rats were injected with iron (1200 mg/kg), or fed an iron-deficient diet. Hjv protein was determined by immunoblotting, liver samples from Hjv−/− mice were used as negative controls. Mouse plasma Hjv content was determined by a commercial ELISA kit.ResultsLiver crude membrane fraction from both mice and rats displayed a major Hjv-specific band at 35 kDa, and a weaker band of 20 kDa. In mice, the intensity of these bands was not changed following iron injection, repeated bleeding, low iron diet or EPO administration. No change in liver crude membrane Hjv protein was observed in iron-treated or iron-deficient rats. ELISA assay for mouse plasma Hjv did not show significant difference between Hjv+/+ and Hjv−/− mice. Liver Hamp mRNA, Bmp6 mRNA and Id1 mRNA displayed the expected response to iron overload and iron deficiency. EPO treatment decreased Id1 mRNA, suggesting possible participation of the bone morphogenetic protein pathway in EPO-mediated downregulation of Hamp mRNA.DiscussionSince no differences between Hjv protein levels were found following various experimental manipulations of body iron status, the results indicate that, in vivo, substantial changes in Hamp mRNA can occur without noticeable changes of membrane hemojuvelin content. Therefore, modulation of hemojuvelin protein content apparently does not represent the limiting step in the control of Hamp gene expression.

Expression of Human Hemojuvelin (HJV) Is Tightly Regulated by Two Upstream Open Reading Frames in HJV mRNA That Respond to Iron Overload in Hepatic Cells

“…Despite prior studies showing that HJV liver membrane protein expression itself does not appear to be regulated by iron in mice and rats (38), our data have shown that in response to iron overload, translational repression mediated by the human HJV uORFs is significantly relieved in hepatic HepG2 cells (Fig. 5 and 6).…”

Serum hepcidin levels and muscle iron proteins in humans injected with low‐ or high‐dose erythropoietin

“…Insights from pregnancy‐induced erythropoiesis in women also suggest that soluble HJV may act as an hepcidin suppressor . Such hypothesis is however challenged by other data indicating that HJV gene is not modulated by Epo and that hepatocyte membrane HJV is not a critical determinant of hepcidin expression . In line with the latter, our data demonstrate that soluble HJV, which is not influenced by Epo (even at high dose), is not a mediator of Epo‐induced hepcidin suppression in humans.…”

Physiology of Iron Metabolism

“…Furthermore, at least 3 other proteins play roles by interacting between BMPs and the BMP receptor. The first protein is hemojuvelin (HJV) a glycosylphosphatidylinositol-linked membrane protein [41], the second is Mt2 [42], which regulates the levels of membrane-bound HJV, and the third is neogenin, a ubiquitously expressed transmembrane protein with multiple functions [43,44,45,46,47,48]. The gene of Mt2 carries several polymorphisms that have been linked to iron metabolic parameters, notably in patients presenting with iron-refractory iron-deficient anemia [49,50,51,52,53,54,55,56,57,58].…”