Nutrient deprivation and various forms of stress induce cells to activate autophagy, a process by which proteins and organelles are engulfed in intracellular membranous compartments known as autophagosomes, which fuse with lysosomes. Degradation of the contents can provide resources to support cell function or can lead to cell death. Iron is an important cofactor for metalloprotein enzymes involved in the redox reactions required for mitochondrial ATP metabolism and other cellular processes. Several proteins, including transferrin (the iron-binding protein in circulation) and ferritin (an intracellular iron-binding storage protein) manage the transport and storage of iron and shield it from undesirable molecular interactions that could produce damaging free radicals. By analyzing autophagosome-enriched samples using mass spectrometry, Mancias et al. determined that cells use autophagy to regulate the metabolism of iron. Autophagosome-enriched fractions from cancer cell lines contained several known autophagy-associated proteins, as well as NCOA4 (nuclear receptor coactivator 4), a protein that binds to the androgen receptor and coactivates gene transcription. Exposing cells to lysosome inhibitors increased the abundance of NCOA4 and caused NCOA4 to relocalize from the cytosol to autophagosomes. Mass spectrometry analysis of NCOA4-interacting proteins showed that NCOA4 bound to ferritin heavy and light chain polypeptides, which form a multisubunit molecular cage responsible for storing iron. NCOA4 and ferritin colocalized in autophagosomes of cells stimulated with ferric ammonium citrate to induce expression of ferritin-encoding genes or exposed to lysosome inhibitors. Ferritin degradation stimulated by chelation of iron was blocked by knockdown of NCOA4 or autophagy-associated proteins or by inhibition of lysosomes. Knockdown of NCOA4 blocked ferritin localization in lysosomes, increased the abundance of the RNA-binding protein IRP2 (iron-responsive element–binding protein 2), which inversely correlates with the amount of free iron, and prevented cell death induced by exogenous reactive oxygen species. Overexpression of NCOA4 decreased the abundance of ferritin, whereas stimulation of ferritin production reduced the abundance of NCOA4. Thus, NCOA4 is a cargo-recognition receptor for the selective autophagy of ferritin and participates in iron homeostasis.
J. D. Mancias, X. Wang, S. P. Gygi, J. W. Harper, A. C. Kimmelman, Quantitative proteomics identifies NCOA4 as the cargo receptor mediating ferritinophagy. Nature 30 March 2014 (10.1038/nature13148). [PubMed]