Ferritinophagy and Cell Cycle Control

The autophagic degradation of the iron-storage macromolecule ferritin, called ferritinophagy, is critical to restore the appropriate cellular iron levels and influences systemic iron homeostasis. Under low iron conditions, nuclear receptor coactivator 4 (NCOA4) protein accumulates and promotes, as cargo receptor, ferritinophagy. We have recently demonstrated that mice carrying genetic ablation of NCOA4 were unable to mobilize iron from deposits, featuring tissue iron overload as well as mild anemia. Because of impaired ferritinophagy, NCOA4 null mice displayed a severe microcytic hypochromic anemia and ineffective erythropoiesis when fed with an iron low diet. Conversely, they poorly tolerated an iron rich diet, dying prematurely from iron toxicity. Since in previous studies we discovered that nuclear NCOA4 is a chromatin binding protein that acts as a negative regulator of DNA replication origin activation, inhibiting the MCM2-7 DNA helicase, we also investigated whether NCOA4 could regulate DNA replication as a function of iron bioavailability. Treatment with iron chelators promoted a G1 phase cell cycle arrest, blocking DNA replication origins activation. In cell fractionation experiments, we observed that iron depletion induced not only cytosolic but also nuclear NCOA4 stabilization, and by chromatin immunoprecipitation (CHIP) and co-immunoprecipitation assays, we demonstrated that NCOA4 enriches at canonical DNA replication origins increasing the binding to MCM2-7 complex. Silencing of NCOA4 induced an unscheduled activation of DNA replication under iron-depleted conditions that promotes replication stress and impairs cell viability. In conclusion, our data indicate NCOA4 as a novel key iron responsive protein able to couple DNA replication origin activation to cellular iron levels.