Iron plays an important role in DNA synthesis, mitochondrial respiration, and neurotransmitter synthesis, and changes in iron homeostasis in patients with neurodegenerative diseases (ND) are associated with decreased autophagy. Defects in the autophagy lysosomal pathway may be associated with the pathogenesis of ND, oxidative stress induces lysosome rupture in ferroptosis, whereas phagocytic vesicles formed during autophagy are bound to lysosomes and degraded. Currently, activating or inhibiting ferroptosis may be exploited to achieve therapeutic effects in ND (Tang et al., 2018).

The death of dopaminergic neurons in the substantia nigra during PD is associated with the accumulation of iron (Jiang et al., 2017; Zucca et al., 2017; Song and Xie, 2018). PD is characterized by the depletion of dopamine neurons in the substantia nigra of the brain and abnormal aggregation of Lewy bodies, in which α-synuclein (α-syn) is regulated by iron during translation (Febbraro et al., 2012). Iron directly interacts with α-syn in Lewy bodies, and α-syn is mainly digested through an autophagy lysosomal pathway, which not only responds to iron chelators and channels but also participates in the cellular iron cycle through the degradation of ferritin and other iron-containing components. These studies suggest that there is a relationship between iron deposition and autophagy injury during PD (Chen L. L et al., 2019). Baksi and Singh (2017) demonstrated that α-syn affects ferritinophagy by disrupting lysosome activity, thus leading to PD. This suggests that ferritinophagy may be involved in the progression of PD. Several PD causative or associated mutations involve proteins whose functions are associated with autophagy. PD-associated mutations in genes encoding parkin, PTEN-induced putative kinase 1, Parkinson protein 7, and HtrA serine peptidase 2 result in deficits in mitochondrial homeostasis and mitophagy pathway. RNA sequencing (RNA-seq) revealed that FTH1 was abnormally expressed in 6-hydroxydopamine (6-OHDA)–induced rat PD. In PC-12 cells, FTH1 overexpression downregulates NCOA4, inhibits ferritinophagy and ferroptosis, and ameliorates cell death. The autophagy inhibitors, chloroquine and bafilomycin A (1), can significantly inhibit ferritinophagy and ferroptosis in PC-12 cells treated with 6-OHDA (Tian et al., 2020). Since redox-active iron is associated with the generation of ROS, inappropriate iron accumulation may be a key factor in the pathogenesis of PD.

HD (Huntington's disease) is an autosomal dominant ND characterized by a loss of motor control, loss of neurons in the striatum and cortex, and cognitive decline that leads to dementia. Both autophagy and CMA are either activated or impaired in patients with HD and animal models (McColgan and Tabrizi, 2018). Koga observed an increased CMA activity in different cell and mouse models of HD.

AD is characterized by abnormal concentrations of β-amyloid and tau proteins inside and outside cells, which leads to neuronal dysfunction and neurodegeneration. Recent studies have shown that iron deposited in brain tissue is a pathogenic factor of AD and induces oxidative stress that leads to the death of nerve cells (Mena et al., 2015). However, the relationship between iron homeostasis and the pathogenesis of AD remains unclear. Proteins involved in the pathogenesis of AD may contribute to the autophagy pathway or iron homeostasis. The autophagy lysosomal pathway appears to be significantly involved in the pathogenesis of AD, playing a pivotal role in the production and accumulation of amyloid plaques and neurofibrillary tangles. Furthermore, the stimulation of the autophagic clearance of protein aggregates is a potential therapeutic strategy to restore behavioral, learning, and memory deficits and limit AD progression (Biasiotto et al., 2016).

With the reduction in global autophagy observed in some ND, ferritin autophagy flow is also reduced. In vitro models of NCOA4 depletion indicate that bioavailable iron decreases as a result of reduced ferritin degradation, which leads to impaired iron release (Bellelli et al., 2016). Studies on the role of autophagy in iron processing provide a new perspective on the role of iron in neurodegeneration, suggesting that autophagy dysregulation may lead to the dysregulation of iron homeostasis. Overall, further studies are needed to understand the role of NCOA4 in central nervous system development and the pathophysiology of iron-related ND.

Ferroptosis induced by autophagy is an important regulatory pathway associated with vascular endothelial damage caused by inflammatory inducers (Flores-Mireles et al., 2015). Urinary tract infections (UTIs) are generally caused by uropathogenic Escherichia coli (UPEC) that persist in the urinary tract epithelial cells. Ferritin-bound iron to UPEC shuttles between the autophagosome and lysosomal compartments in the urinary tract epithelium. Iron is very important for the survival of host cells. Increasing the content of free iron in cells significantly increases the growth rate of the host cells. Therefore, a high dependence of UPEC on iron may be used as a novel strategy to defend against pathogens (Khasheii et al., 2016).

Bauckman and Mysorekar (2016) first reported that NCOA4-dependent ferritinophagy triggers excessive proliferation and persistence of UPEC in host cells, which is caused by increased intracellular iron availability. Iron overload in urinary epithelial cells induces ferritinophagy in an NCOA4-dependent manner. This results in increased iron availability of UPEC, leading to excessive bacterial proliferation and host cell death. In addition, the lysosome damage caused by iron overload represents a specific mechanism that causes host cell death.

Ferritin phagocytosis promotes erythropoiesis and the development of UTIs as follows: 1) During iron deficiency, NCOA4 selectively interacts with the FTH1 subunit of ferritin through its C-terminal domain and arginine residues on FTH1. NCOA4 binds to PCBP1 and mediates ferritin flux, which is crucial for regulating heme synthesis and further erythrocyte differentiation. Concurrently, NCOA4-mediated ferritinophagy accelerates UTIs by increasing the availability of iron, thus enabling the persistence of UPEC in host cells. 2) In the case of adequate iron, HERC2 mediates the degradation of NCOA4 in a ubiquitin-dependent manner through the CUL7 homologous domain of HERC2 and the C-terminal domain of NCOA4, thus preventing ferritinophagy (Tang et al., 2018). In addition, iron can promote the survival and growth of UPEC in host cells. (Table 1) Khasheii et al. (2016) demonstrated that free iron improves the growth rate of UPEC. Moreover, Bauckman et al. (2015) found that an increase in free iron caused by ferritinophagy also promotes the growth and survival of UPEC. UPEC may directly or indirectly drive ferritinophagy by depleting unstable iron pools and forcing UPEC to catabolize ferritin to provide iron for basic cellular functions. ATG16L1 protein is an autophagy-related protein. Studies have found that ATG16L1-deficient mice exhibit strong resistance to UPEC infection (Wang et al., 2012). When free iron is removed by an iron-chelating agent, the growth rate of UPEC is significantly inhibited (Brumbaugh et al., 2013). UPEC quiescent intracellular reservoirs in autophagosomes in vivo may lead to repeated infections as a result of UPEC following iron into autophagosomes, whereas UPEC is able to chelate iron through its siderophores.

Therefore, the regulation of ferritinophagy may have certain clinical value to reduce the persistence of UPEC and prevent the recurrence of UTI (Bauckman and Mysorekar, 2016). Interrupting ferritinophagy to control iron levels may provide a potentially therapeutic avenue to suppress UTI.

Both iron deficiency and iron overload are associated with cardiomyopathy an heart failure through complex mechanisms (Fang et al., 2020). Excessive iron accumulation in the heart leads to iron overload cardiomyopathy, the leading cause of death in patients with hemochromatosis (Sumneang et al., 2020). In a doxorubicin (Dox)-induced cardiomyopathy mouse model, RNA-seq analysis revealed differentially expressed genes, including HO-1 significantly upregulated (Conrad and Proneth, 2019). Iron overload was found in liver and kidney in HO-1 knockout mice, and the inhibition of HO-1 by zinc protoporphyrin IX reversed Dox-induced ferroptosis (Fang et al., 2019). Previous studies have reported that iron overload leads to cardiac mitochondrial dysfunction, which is primarily manifested by reduced mitochondrial respiration, increased mitochondrial ROS levels, and low mitochondrial swelling (Khamseekaew et al., 2017). Dox-induced cardiomyopathy mainly triggered a significant increase in the level of peroxidized phosphatidylethanolamine in mitochondria (Tadokoro et al., 2020). The above result further highlights the heart mitochondria in the role of lipid peroxide produced (Li et al., 2019; Jang et al., 2021). Studies have shown that lipopolysaccharide (LPS)-induced myocardial injury in a mouse sepsis model is closely associated with ferroptosis. Li N et al. (2020) found that after LPS treatment, the level of ferritinophagy was enhanced, and the mRNA expression and protein expression of sideroflexin 1 (SFXN1) were increased. A large amount of Fe²⁺ binds with SFXN1 and is transported to mitochondria, where a large amount of ROS is produced by the Fenton reaction and promotes the occurrence of ferroptosis. In in vitro experiments, LPS can increase the expression of NOCA4, promote ferritinophagy, and increase the release of free iron to cause lipid peroxidation, which may be one of the ferroptosis mechanisms that occurs in myocardial cells (Li J et al., 2020). The deletion of NCOA4 in mouse hearts improved cardiac function along with the attenuation of the upregulation of ferritinophagy-mediated ferritin degradation after stress overload (Li N et al., 2020; Ito et al., 2021). Ferrostatin-1 did not provide additional protection from pressure overload–induced cardiac remodeling in NCOA4^−/− mice, suggesting that iron-dependent cardiomyocyte death is downstream of NCOA4-mediated ferritinophagy. Toll-like receptor 4 and NADPH oxidase 4 knockdown can delay hyperactivated autophagy and ferroptosis (Chen X et al., 2019).

Abnormal lipid metabolism, oxidative stress, and inflammation are the main features of atherosclerosis (AS) (Zhu et al., 2018). The Nrf2-Keap1 pathway decreases ferroptosis associated with AS by maintaining cellular iron homeostasis, increasing the production of glutathione, GPX4, and NADPH. The overexpression of GPX4 in apolipoprotein E knockout (ApoE^−/−) mice attenuates the upregulation of endothelial cell adhesion molecule and monocyte–endothelial cell adhesion, thereby inhibiting the development of AS (Guo et al., 2008). It has been reported that chronic iron overload could exacerbate the AS in ApoE^−/− mice by inducing oxidative stress and endothelial dysfunction (Habas and Shang, 2018; Marques et al., 2019). Zinc oxide nanoparticles (ZNOPs) exhibit various activities in biomedicine. They can be used as drug carriers but can be cytotoxic to vascular endothelial cells, thus aggravating vascular injury. Studies have shown that ZNOPs can lead to ferroptosis in vascular endothelial cells, whereas NCOA4 knockdown can reduce intracellular iron and lipid peroxidation. It was demonstrated that ZNOPs may lead to ferroptosis of endothelial cells through ferritinophagy (Qin et al., 2021). Iron overload induces endothelial dysfunction by enhancing oxidative and inflammatory responses of endothelial cells (Xu, 2019), revealing the potential relationship between ferritinophagy-induced ferroptosis and AS.

Ischemic stroke is a complex brain disease that is regulated by multiple pathways of cell death, including autophagy and ferroptosis. However, the potential association between autophagy and ferroptosis during ischemic stroke has not been determined. Since the destruction of iron homeostasis and autophagy defects coexist in ischemic stroke and autophagy promotes the clearance of toxic proteins to maintain cell homeostasis, we postulate that abnormal autophagy is likely to mediate the degradation of ferritin, thus promoting iron overload and ferroptosis during ischemic stroke (Liu et al., 2020). NCOA4 may play a role in FTH1 conversion in the brain (Santana-Codina et al., 2019). High serum ferritin content is also associated with poor prognosis during ischemic stroke (García-Yébenes et al., 2012), suggesting that ferritin plays an important role in mediating iron storage and release. TfR1 is a major receptor involved in iron transport to the brain, which plays an important role in maintaining iron homeostasis and regulating iron atonia. Thus, the inhibition of ferroptosis by inhibiting free Fe²⁺, and reducing ROS accumulation and lipid peroxidation may provide a new approach to the treatment of ischemic stroke.

Normal iron metabolism is a marker of abnormal expression in many cancers. Intracellular ferritin imbalance is closely associated with the development of cancer, and there is increasing evidence that ferritinophagy and ferroptosis play an important role in tumor therapy (Zhang et al., 2018, Zhu et al., 2017). NCOA4 may be relevant to carcinogenesis, and the intersection of ferritinophagy and ferroptosis pathways may represent a therapeutic approach (Shaw et al., 2001). A positive correlation between NCOA4 mRNA and NCOA4α protein levels and transformation has been reported in ovarian carcinoma. The expression and function of NCOA4 isoforms have been reported in transformed endometriotic and malignant ovarian cancer cells (Rockfield et al., 2018). The overexpression of several oncogenes (MYC, H-Ras, and p53 inactivation) in normal endometriotic cells to induce transformation resulted in an upregulation of NCOA4α and NCOA4β expression. NCOA4α knockdown in transformed cells decreased survival, whereas NCOA4β overexpression decreased colony formation. In contrast, in studies of prostate cancer, NCOA4α acts as a tumor suppressor, whereas NCOA4β expression correlated with proliferation and invasion. The prostate cancer cell proliferation and invasion was stimulated by the androgen receptor coactivator, androgen receptor-associated protein 70 (Peng et al., 2008). NCOA4-mediated ferritinophagy promotes ferroptosis induced by erastin, but not by RSL3 in HeLa cells. Ferritinophagy is required for the induction of ferroptosis by the bromodomain protein, bromodomain-containing protein 4 inhibitor (+)-JQ1, in cancer cells. Low expression of the ferritinophagy-related NCOA4 gene was related to unfavorable outcomes and defective immune cell infiltration in clear cell renal carcinoma. Lipid peroxidation–mediated ferroptosis has recently attracted widespread attention as a potential therapeutic strategy for cancer. Zeta1 (COPZ1), the complex subunit of the coat protein, is a putative therapeutic target for glioblastoma, which can significantly affect iron metabolism and tumor prognosis. The COPZ1-NCOA4-FTH1 axis may represent a new target for the treatment of glioblastoma (Zhang W et al., 2021). Pancreatic cancer cell lines have increased NCOA4 levels and a corresponding high flux through the ferritinophagy pathway. Quantitative proteomics identifies NCOA4 as the cargo receptor mediating ferritinophagy (Mancias et al., 2014). Direct evidence for the role of NCOA4 in modulating ferroptotic cell death in pancreatic cancer cells was provided by Yang et al. who showed that artesunate-mediated ferroptotic cell death is attenuated by NCOA4 depletion (Eling et al., 2015).

System Xc⁻ is the main active site for erastin, which was first screened for its ability to kill tumor cells with almost no toxicity against homologous normal cells with Ras mutations (Wang et al., 2020). SLC7A11 is overexpressed in various malignant tumors and is closely related to the growth, prognosis, metastasis, and treatment of malignant tumors including breast, ovarian, liver, and lung cancers. RNA-binding protein (RBP) plays a role in the regulation of gene expression. Yang et al. found that RBP, RBMS1, affects ferroptosis in lung cancer by regulating the translation of SLC7A11. NTP can lead to the downregulation of RBMS1 in radiotherapy-resistant cells, thus reducing the expression of SLC7A11 and promoting the ferroptosis. This renders radiotherapy-resistant lung cancer cells sensitive to radiotherapy and provides a new treatment for lung cancer patients (Zhang Y et al., 2021). Xue et al. demonstrated an important role of micronutrient iron in colorectal cancer growth. Conversely, ferritinophagy is not required for colon cancer cell growth. Compared with other tissues, the gut is a major tissue that is essential for normal dietary iron absorption. Therefore, it contains several mechanisms that regulate apical iron transport and basolateral outflow. Hasan et al. (2020) demonstrated a marginal role for ferritinophagy in growth under normal and cytotoxic conditions in colon cancer cells, as well as a possible compensatory mechanism in response to ferroptosis. This study demonstrated that ferritinophagy is not required for the basal growth of colorectal cancer (CRC)–derived cells. The main reason is that an increased ability of colon cancer cells to uptake iron from the extracellular environment may compensate for the loss of NCOA4 function through the exclusive use of extracellular iron for survival. In addition, an increase in Ftn protein expression may also serve as a protective mechanism by sequestering the iron needed for cell death and ferroptosis into Ftn. CRC pathogenesis is dependent upon iron. TfR1 expression has also been found to be high in CRC tissues and CRC-derived cell lines. Its downregulation promotes cancer progression, which underscores the role of transferrin-bound iron uptake in tumor growth (Cui et al., 2019).

Intracellular ROS imbalance–mediated oxidative stress defense is also closely related to drug resistance in tumors. Currently, acquired resistance to chemotherapy is a major cause of tumor progression. Iron overload in tumor cells catalyzes the production of ROS and meets the needs of cell proliferation to a certain extent; however, when tumor cells are exposed to chemotherapeutic drugs, ROS production is induced (Galadari et al., 2017). The sensitivity of K562 leukemia cells to doxorubicin may be increased by treating it with the iron-chelating agent, deferoxamine (DFO), to inhibit iron overload. DFO treatment of cervical cancer cells enhances their sensitivity to oxaliplatin, providing direct evidence that iron overload is involved in chemotherapeutic drug resistance (Chen et al., 2016). DFO treatment of different types of ovarian cancer cells significantly increases intracellular iron ions and ROS (Watson, 2013). Recent studies showed that the nuclear transcription factor, Nrf2, is closely associated with the regulation of ferroptosis and the inhibition of the Nrf2-Keap1 signaling pathway activation promotes ferroptosis and reverses drug resistance in cisplatin-resistant head and neck tumors (Roh et al., 2017). Sorafenib, a targeted therapy for liver cancer, is a strong inducer of ferroptosis. The activation of the Nrf2 pathway in liver cancer cells can upregulate the expression of metallothionein-1G (MT-1G) and promotes sorafenib drug resistance by inhibiting ferroptosis (Roh et al., 2017). Therefore, triggering ferroptosis in cancer with high autophagy levels may reveal a potential vulnerability. One possible therapeutic strategy is to increase iron flux through the ferritinophagy pathway, leading to increased labile iron and ROS and sensitizing cancer cells to ferroptosis-inducing agents. Thus, inducing autophagy-dependent ferroptosis may represent a new antitumor strategy.