FBXL5 Is Required for the Maintenance of Cellular Andsystemic Iron Homeostasis

Iron is an essential element for most living organisms. Due to its chemical properties, iron plays an important role in many vital biochemical processes. Both iron excess and deficiency have detrimental effects in human health. Therefore, iron metabolism must be tight regulated. Maintenance of cellular iron homeostasis requires coordinate posttranscriptional regulation of iron metabolism genes by Iron Regulatory Proteins 1 and 2 (IRP1 and IRP2). IRP2 is targeted for proteasomal degradation in iron replete cells by the E3 ubiquitin ligase complex containing F-box and Leucine-rich Repeat Protein 5 (FBXL5). Depletion of FBXL5 leads to aberrant accumulation of IRP2 and misregulation of IRP2 under high iron conditions, underscoring FBXL5 importance in regulation of iron metabolism. Interestingly, FBXL5 is regulated in an inverse fashion to IRP2 as it is stabilized under iron-replete conditions and preferentially degraded when iron or oxygen becomes limiting. However, FBXL5Õs iron- and oxygen-dependent regulation and its role in the maintenance of systemic iron homeostasis are poorly understood. Biochemical and molecular biology assays revealed that FBXL5 features a hemerythrin-like domain that serves as a direct sensor of cellular iron as well as oxygen availability and subsequently governs FBXL5Õs own stability. Importantly, in vivo deletion of the ubiquitously-expressed murine Fbxl5 gene results in a failure to sense increased cellular iron availability, accompanied by constitutive IRP2 accumulation and misexpression of IRP2 target genes. FBXL5-null mice die during embryogenesis, though viability is restored by simultaneous deletion of the IRP2, but not IRP1, gene. Fbxl5 heterozygous mice behave like their wild type littermates when fed an iron-sufficient diet. However, unlike wild type mice that manifest decreased hematocrit and hemoglobin levels when fed a low-iron diet, Fbxl5 heterozygotes maintain normal hematologic values due to increased iron absorption. IRP2Õs responsiveness to low iron is specifically enhanced in the duodena of the heterozygotes and is accompanied by increased expression of the Divalent Metal Transporter-1. These results confirm FBXL5Õs role in the in vivo maintenance of cellular and systemic iron homeostasis and reveal a privileged role for the intestine in their regulation by virtue of its unique FBXL5 iron sensitivity.