The emerging role of nuclear receptor coactivator 4 in health and disease: a novel bridge between iron metabolism and immunity

Abstract

Nuclear receptor coactivator 4 (NCOA4) has recently been recognized as a selective cargo receptor of ferritinophagy participating in ferroptosis. However, NCOA4 is also a coactivator that modulates the transcriptional activity of many vital nuclear receptors. Recent novel studies have documented the role of NCOA4 in healthy and pathogenic conditions via its modulation of iron- and non-iron-dependent metabolic pathways. NCOA4 exhibits non-ferritinophagic and iron-independent features such as promoting tumorigenesis and erythropoiesis, immunomodulation, regulating autophagy, and participating in DNA replication and mitosis. Full-length human-NCOA4 is composed of 614 amino acids, of which the N-terminal (1-237) contains nuclear-receptor-binding domains, while the C-terminal (238-614) principally contains a ferritin-binding domain. The exploration of the protein structure of NCOA4 suggests that NCOA4 possesses additional significant and complex functions based on its structural domains. Intriguingly, another three isoforms of NCOA4 that are produced by alternative splicing have been identified, which may also display disparate activities in physiological and pathological processes. Thus, NCOA4 has become an important bridge that encompasses interactions between immunity and metabolism. In this review, we outline the latest advances in the important regulating mechanisms underlying NCOA4 actions in health and disease conditions, providing insights into potential therapeutic interventions.

Fig. 1. The main regulation pathway of intracellular iron metabolism.

The iron metabolism in vivo consists of uptake, utilization, storage, and export. Duodenal cytochrome B (DcytB) restores ferric iron as ferrous iron, the latter then is transported into the cytoplasm via divalent metal-ion transporter-1 (DMT1). As a main form of circulating iron, transferrin (Tf), is responsible for transporting ferric iron via internalization in an endosome pathway after being recognized by the TfR. Then the TfR1-TF complex is processed back to the cell surface for further reuse after iron releasing to cytoplasm. The intracellular labile iron pool is usually used for (i) the synthesis of heme/hemoglobin and iron-sulfur cluster in mitochondria; (ii) it acts as a cofactor for multiple enzymes. Overload iron is combined with ferritin for storage or exported from cells by ferroportin. The output ferrous iron is oxidized by Hephaestin, binding to Tf for circulating transportation. NCOA4 is vital in the mutual transformation between storage iron (ferritin) and the labile iron pool. The Fe(II) is represented in green circle, and the Fe(III) is represented in purple circle shape. The figure is created with BioRender.com.

Fig. 2. Regulation of NCOA4 in orchestrating intracellular iron homeostasis.

Upon the basal or iron depletion conditions, two main mechanisms regulate the cellular iron level (A, B): (A) Classical ferritinophagy pathway. NCOA4 interacts with ferritin-heavy chain (FTH)1, transferring autophagosomes to lysosomes to degrade ferritin and release free iron. VPS34-mediated LC3 lipidation and ATG7 involve in ferritinophagy. B Autophagy-independent lysosomal targeting regulated pathway. The ULK1/2-FIP200 complex regulates the dissociation of NCOA4 from TAX1BP1. ATG9, VPS34, and ESCRT-III are necessary for the trafficking of NCOA4-ferritin complexes to lysosomes to release free iron. ULK1/2-FIP200 loss of function results in aggregates of TAX1BP1-NCOA4. Instead, TAX1BP1 recruits TBK1 to regulate basal ferritin flux. Upon the iron repletion conditions, the main mechanisms that modulate the cellular iron level were shown in (C–E): (C) NCOA4 degradation via proteasome and lysosome pathways. NCOA4 combined with iron provides a target for HERC2 ubiquitination of NCOA4, which is recognized and degraded by proteasomes. NCOA4 can also be directly degraded by lysosomes. D NCOA4 forms condensate via binding with Fe (III) in the early phase of iron repletion. The NCOA4 condensates sequester ferritin, forming a complex for accumulation to avoid iron overload. During prolonged iron supply, TAX1BP1 binds NCOA4 and delivers ferritin to lysosomes to prevent iron deficiency. E NCOA4 regulates the secretion of ferritin in a CD63-dependent pathway. Iron repletion induces IRP dissociating from the IRE of CD63 mRNA, initiating the translation of CD63, increasing the CD63 expression in extracellular vehicles (EVs), as well as the secretion of CD63-EVs that contains ferritin.

Fig. 3. Other functions of NCOA4 under physiological or pathological conditions.

A NCOA4 is a co-regulator of nuclear receptors. Conserved ARA70 domain I (amino acids 37–167) contains a specific motif interacting with PPARγ, TR, and VDR. ARA70 domain II (amino acid 138–332) interacts with AR, PR, and AHR. The interaction between NCOA4 and these nuclear receptors regulates corresponding physiological and pathological processes. B Novel roles of NCOA4 in innate immunity regulation. NCOA4 exerts its anti-tumor, anti-infection, and immunoregulation functions by orchestrating iron metabolism and interacting with proteins. C NCOA4 in tumorigenesis. The NCOA4-RET fusion shows stronger invasiveness in tumors. NCOA4-RET also participates in cell growth, proliferation, and invasion via regulating the initiation of DNA replication, co-localization with tubulin, and mitosis, without clear cognition of molecular mechanism. Abbreviation: CAFs cancer associated fibroblasts, FSTL1 follistatin like protein 1.

Fig. 4. Precise regulation of NCOA4 expression and activity.

In both physiological and pathological conditions, NCOA4 is regulated by specific intracellular proteins, small molecular compounds, and various conditions at the levels of transcription, translation, post-translational modification (including ferritinophagy and self-degradation process). The figure illustrates the regulators of NCOA4, using different colors to distinguish physiological and pathological conditions.