NCOA4 is Regulated by HIF and Mediates Mobilization of Murine Hepatic Iron Stores After Blood Loss

Abstract

The mechanisms by which phlebotomy promotes the mobilization of hepatic iron stores are not well understood. NCOA4 (nuclear receptor coactivator 4) is a widely-expressed intracellular protein previously shown to mediate the autophagic degradation of ferritin. Here, we investigate a local requirement for NCOA4 in the regulation of hepatic iron stores and examine mechanisms of NCOA4 regulation. Hepatocyte-targeted Ncoa4 knockdown in non-phlebotomized mice had only modest effects on hepatic ferritin subunit levels and non-heme iron concentration. After phlebotomy, mice with hepatocyte-targeted Ncoa4 knockdown exhibited anemia and hypoferremia similar to control mice with intact Ncoa4 regulation, but showed a markedly impaired ability to lower hepatic ferritin subunit levels and hepatic non-heme iron concentration. This impaired hepatic response was observed even when dietary iron was limited. In both human and murine hepatoma cell lines, treatment with chemicals that stabilize hypoxia inducible factor (HIF), including desferrioxamine, cobalt chloride, and dimethyloxalylglycine, raised NCOA4 mRNA. This NCOA4 mRNA induction occurred within 3 hours, preceded a rise in NCOA4 protein, and was attenuated in the setting of dual HIF-1a and HIF-2a knockdown. In summary, we show for the first time that NCOA4 plays a local role in facilitating iron mobilization from the liver after blood loss and that HIF regulates NCOA4 expression in cells of hepatic origin. Because the prolyl hydroxylases that regulate HIF stability are oxygen and iron-dependent enzymes, our findings suggest a novel mechanism by which hypoxia and iron deficiency may modulate NCOA4 expression to impact iron homeostasis.

Intestinal ferritinophagy is regulated by HIF-2α and is essential for systemic iron homeostasis

“…Subsequent NCOA4 promoter activity analysis in human intestinal cell lines established that intestinal NCOA4 is a specifically dependent on HIF-2α. In contrast to a recent report, where it was demonstrated that hepatic NCOA4 is a target of both HIF-1α and HIF-2α, we show that intestinal NCOA4 expression is specifically dependent on HIF-2α, but not on HIF-1α (17). This exemplifies that ferritinophagy is differentially regulated across different tissues.…”

Iron Availability in Tissue Microenvironment: The Key Role of Ferroportin

“…A recent study showed that hepatocyte-specific expression of NCOA4, the protein that directs ferritin to autophagosomes and lysososomes, is required for erythropoiesis [65]. Obviously, in order to be used by the erythron, iron must exit the liver; therefore, it can be inferred that FPN expression in hepatocytes is also necessary for a prompt iron mobilization in response to increased demand.…”

Macrophages and Iron: A Special Relationship

“…NCOA4 specifically binds ferritin and delivers it to lysosomes where it is degraded, thereby increasing iron availability [ 29 ]. This mechanism, which is controlled by iron and oxygen levels through hypoxia inducible factors [ 30 ], may be beneficial in macrophages where it is accompanied by iron release, as it would sustain erythropoiesis, particularly under conditions of iron deficiency [ 31 ].…”