"Posttranskriptionelle Regulierung des Eisenmetabolismus: Entschlüsselung der biologischen Funktionen des Iron Response Elements in der mRNA des Transportproteins DMT1"
Biochemie
Zusammenfassung der Projektergebnisse
Iron is an integral component of heme and must be supplied in adequate amounts to prevent anemia. Anemia due to iron deficiency is highly prevalent, and is often caused by insufficient dietary iron intake. Iron overload, e.g. due to mutations in iron homeostasis genes, is also frequent, damaging organs and promoting infections. Dietary iron is transported from the intestinal lumen into absorptive enterocytes by divalent-metal transporter 1 (DMT1, also SLC11A2), and is then exported into the circulation via ferroportin (FPN). Tight regulation of DMT1 and FPN is essential to avoid iron imbalances. Intestinal iron absorption is systemically regulated by the hormone Hepcidin, which inhibits FPN. It is also controlled locally by mechanisms acting within enterocytes, including regulation of Dmt1 transcription by hypoxia inducible factor 2 (HIF2). Importantly, half of the Dmt1 mRNA isoforms harbor an iron-responsive element (IRE) in their 3’ untranslated region (UTR). IREs are cis-regulatory RNA structures that bind iron regulatory proteins (IRPs) to modulate either RNA decay or translation. IRE-containing Dmt1 mRNA variants predominate in the intestine, suggesting possible involvement of the IRP/IRE system in local regulation of DMT1. However, the role of the Dmt1 3’IRE has been questioned, in part because classical reporter assays with the Dmt1 3’IRE failed to exhibit iron-dependent regulation in cultured cells. In this IREΔ project, we generated a novel mouse line with targeted disruption of the Dmt1 3’IRE (Dmt1 ) to address three main questions: I) how does the Dmt1 3’IRE influence baseline iron homeostasis throughout life; II) is the Dmt1 3’IRE important to adapt to dietary iron insufficiency; III) what is the role of the Dmt1 3’IRE in the response to stress erythropoiesis. Dmt1 IRE-/- mice were viable and fertile and did not exhibit any gross abnormality, showing that while DMT1 is vital, its 3’IRE is not essential. Surprisingly, disruption of the Dmt1 3’IRE led to distinct alterations in iron IRE-/- metabolism depending on the life stage. During postnatal growth, when iron needs are high, Dmt1 mice displayed hypoferremia, associated with decreased intestinal DMT1 expression due to Dmt1 mRNA destabilization. During adult life, they instead showed systemic iron accumulation, associated with increased expression of DMT1 in the gut, due to stimulation of Dmt1 mRNA translation. The Dmt1 3’IRE thus exerts age-dependent effects on Dmt1 in the intestine. It inhibits Dmt1 mRNA decay and promotes iron uptake during early life, but impairs Dmt1 mRNA translation in adults to avoid iron overload. The intestinal iron absorption machinery, including DMT1, is stimulated when body iron levels are low. We assessed how loss of the Dmt1 3’IRE affects Dmt1 regulation in conditions of chronic dietary iron deficiency. Feeding mice with an iron-poor diet resulted in iron deficiency anemia. This treatment strongly enhanced intestinal DMT1 expression at the mRNA and protein level in wild-type animals. Disruption of the Dmt1 3’IRE did not interfere with upregulation of Dmt1 transcription. However, it impaired the posttranscriptional stimulation of Dmt1, resulting in a blunted DMT1 protein response. These results suggested that upregulation of Dmt1 transcription by HIF2 is not sufficient, and that post-transcriptional stimulation of Dmt1 through the Dmt1 3’IRE is necessary to achieve full stimulation of DMT1 in conditions of chronic dietary iron deficiency. As iron is mostly used for the hemoglobinization of erythrocytes, stimulation of erythropoiesis increases intestinal iron absorption and DMT1 expression. We treated mice with erythropoietin to stimulate erythrocyte production directly. We also subjected mice to hemolytic anemia, a condition that enhances erythropoiesis to replace damaged red blood cells. Both treatments raised DMT1 levels in the intestine. However, the Dmt1 3’IRE appeared to be dispensable for adequate regulation of Dmt1 and other iron absorption molecules. DMT1 regulation in response to acute erythropoietic stress is thus largely IRP/IRE independent, and is most likely mediated by HIF2. Overall, this study demonstrates that the 3’IRE of Dmt1 is important for the regulation of DMT1 expression and for maintaining baseline iron homeostasis. However, the 3’IRE exerts tissue- and age-specific effects on DMT1. It also influences DMT1 regulation in a stimulus-dependent manner, being required to adapt during chronic dietary iron deficiency but not during acute erythropoietic stress. This suggests that additional factors (e.g. miRNA, protein factors) may influence the activity of the Dmt1 3’IRE. Although UTRs are usually not included in genetic diagnostic, our study suggests that variants in the Dmt1 3’IRE could be considered as potential causes of iron imbalance not attributable to known pathogenic mutations in iron genes.
Projektbezogene Publikationen (Auswahl)
- (2020) Control of Systemic Iron Homeostasis by the 3’ Iron-Responsive Element of Divalent Metal Transporter 1 in Mice. HemaSphere 4(5):e459
Tybl E, Gunshin H, Gupta S, Barrientos T, Bonadonna M, Nos FC, Palais G, Karim Z, Sanchez M, Andrews NC, Galy B
(Siehe online unter https://doi.org/10.1097/hs9.0000000000000459) - (2022) IRE-dependent Regulation of Intestinal Dmt1 Prevails During Chronic Dietary Iron Deficiency but is Dispensable in Conditions of Acute Erythropoietic Stress, HemaSphere: 6(3):e693
Qatato M, Bonadonna M, Palais, G, Ertl A, Schmidt G, Polycarpou-Schwarz Maria, Karim Zoubida, Galy B
(Siehe online unter https://doi.org/10.1097/hs9.0000000000000693)
