Iron and Cancer.

In vivo , cancer cells inhabit a microenvironment composed of many different cell types, each of which can have positive or negative effects on tumor growth. A growing number of studies on the role of iron in cancer have begun to focus on cells that constitute the tumor microenvironment ( Figure 3 ). Such studies have examined not only the “iron phenotype” of the cellular microenvironment (i.e. expression of genes, proteins and metabolites important to iron metabolism as well as iron content itself), but how cells in the microenvironment interact with cancer cells to either provide or compete for iron. Although multiple cell types are present in the tumor microenvironment, including endothelial cells, fibroblasts, adipocytes and immune cells, here we briefly examine tumor-associated fibroblasts (also termed cancer-associated fibroblasts) and cells of the immune system as examples. Our group found that cancer-associated fibroblasts stimulate hepcidin synthesis in breast cancer cells grown in 3D spheroid culture, reducing levels of the iron efflux pump ferroportin in cancer cells and potentially contributing to increased cancer cell iron retention ( Blanchette-Farra et al. 2018 ). A complementary mechanism was suggested in pancreatic cancer cells, which depend on ferritinophagy to provide sufficient iron to sustain iron-sulfur cluster biogenesis and mitochondrial function ( Mukhopadhyay et al. 2023 ; Santana-Codina et al. 2022 ). Co-culture experiments revealed that this requirement can be overridden by co-culture of pancreatic cancer cells with CAFS. In this scenario, elevated levels of ferroportin in CAFS enables them to efflux and supply iron to pancreatic cancer cells under conditions in which cancer cell ferritinophagy is blocked ( Mukhopadhyay et al. 2023 ). Accordingly, restriction of dietary iron synergizes with autophagy inhibition in reducing growth of pancreatic tumors ( Mukhopadhyay et al. 2023 ). Fibroblasts in the tumor microenvironment also contribute to the production of a collagenous matrix that stiffens the stroma and activates tumor cell invasion, a process facilitated by iron-dependent lysyl hydroxylase ( Guo et al. 2018 ). The matrix may also contribute to cancer invasiveness by influencing the iron regulatory state of tumor cells. Thus, sequencing analysis of triple-negative breast cancer (TNBC) cells implanted in a dense collagen matrix mimicking the extracellular tumor microenvironment revealed an induction of heme oxygenase 1 and polarization of cancer cells into a non-invasive state, characterized by expression of genes associated with iron sequestration, or an invasive state, characterized by upregulation of genes involved in iron uptake such as TFRC and SLC39A14 (Zip14). These alternative iron regulatory states were associated with patient survival outcomes ( Leineweber et al. 2024 ). Tumors are invested with multiple different immune cell types, which can exert either pro- or anti-tumor effects. These include innate immune cells such as macrophages, NK cells, neutrophils and dendritic cells, as well as adaptive B and T cells. Many immune cells influence iron pathways in tumor cells ( Zhang et al. 2024c ). For example, alternatively activated macrophages (“M2-like”) can deliver iron to tumor cells in vitro ( Recalcati et al. 2019 ; Recalcati et al. 2010 ). Iron-laden tumor associated macrophages were observed in breast tumors ( Marques et al. 2016 ) and their depletion was associated with tumor reduction in animal models ( Leftin et al. 2019 ). Both ferroportin ( Recalcati et al. 2019 ; Recalcati et al. 2010 ) and lipocalin ( Duan et al. 2018 ) have been implicated in mechanisms of iron delivery from macrophages to tumor cells. In sarcomas, an iron-rich subset of macrophages characterized by high iron content and a unique genetic signature was induced by heme and contributed to tumor growth and vascular density ( Folkert et al. 2024 ). In non-small cell lung cancer, single-cell transcriptomics revealed a transcriptional reprogramming of macrophages in tumors that promoted iron efflux within the tumor microenvironment ( De Zuani et al. 2024 ). In contrast, in hepatocellular carcinoma, reduced levels of iron in TAMs were observed and attributed to TFRC-mediated iron uptake by tumor cells ( Sun et al. 2021 ). Iron provided by hemolysis or synthetic iron-containing nanoparticles can also directly polarize myeloid cells to a proinflammatory, tumor suppressive phenotype ( Ludwig et al. 2024 ), as well as modulate cell plasticity ( Muller et al. 2024 ), lending further nuance to the conversation between tumor and myeloid cells in the tumor microenvironment. Cytokines released by T cells of the adaptive immune system, in particular CTLs activated by immune checkpoint blockade, can both promote and inhibit ferroptosis of cancer cells ( Zhang et al. 2024c ) ( Figure 3 ). Overall, immune cells play an active and significant role in tumor iron biology that is nevertheless complex, nuanced, and may differ among tumor types.

Another prediction of a link between iron and cancer is that levels of dietary iron in patient populations would affect cancer risk. There have been many attempts to address this prediction using epidemiological tools, but they have produced a stubbornly mixed picture, with some studies pointing to positive associations and others finding no correlation or even a protective effect of iron (reviewed in ( Zeidan et al. 2024 ; Charlebois and Pantopoulos 2023 )). The form of dietary iron (heme vs non-heme iron) may be important, with a trend towards a more positive association seen with heme iron intake and cancer risk. For example, in a meta-analysis of 23 studies, high dietary heme iron significantly increased the risk of breast cancer (RR 1.12, 95% CI:1.04–1.22) whereas total iron intake did not (1.01, 95% CI: 0.89–1.15)( Chang et al. 2019 ). Similarly, in a meta-analysis of 20 studies of esophageal cancer, a 1 mg/day increase in heme iron intake was associated with a 21% increase in cancer risk (OR = 1.21, 95% CI: 1.02–1.45) whereas total iron intake reduced risk (OR = 0.81, 95% CI: 0.70–0.94) ( Ma et al. 2018 ). Heme iron intake also increased risk of lung cancer ( Ward et al. 2019 ) in the European Prospective Investigation into Cancer and Nutrition (EPIC) study of 16,746 individuals from 10 countries (HR 1.16, 95% CI 1.02–1.32, highest versus lowest quintile), whereas non-heme iron intake did not. However red and processed meats are the most prevalent dietary forms of heme, and it is difficult to dissociate the contribution of heme iron from other potentially cancer-promoting components of these foods. For example, in a study of meat consumption of over 175,000 US men, a heightened risk of prostate cancer was associated with increased heme iron intake (HR = 1.09 (95% CI: 1.02, 1.17) ( Sinha et al. 2009 ), but was also positively associated with other aspects of dietary red meat, including cooking method (grilling/barbecuing), nitrite/nitrates, and benzo[a]pyrene. Other complexities associated with assessing the contribution of dietary iron to cancer risk include the influence of sex and variations among different malignancies ( Zeidan et al. 2024 ). Disentangling these variables remains a challenge in assessing the role of dietary iron in cancer risk. A related approach to assessing the relationship between body iron and cancer risk has been to evaluate cancer risk in patients with hemochromatosis, a disorder that results in iron overload, particularly in the liver (see Chapter 9). Although the increased risk of untreated hemochromatotic individuals for liver cancer has been known for decades ( Niederau et al. 1985 ), whether hemochromatosis increases overall cancer risk is less clear. A study of approximately 28,000 individuals in the Melbourne Collaborative Cohort Study identified subjects with HFE C282Y homozygous mutations as having twice the risk of colorectal and breast cancer ( Osborne et al. 2010 ) when compared to individuals without the mutation. However, a recent study that evaluated this question using a large community sample of more than 450,000 individuals of European ancestry in Great Britain ( Atkins et al. 2022 ) provided suggestive evidence of increased prostate cancer incidence (14.4% of male p.C282Y homozygotes projected to develop prostate cancer versus 10.7% without mutations, 95% CI, 1.3–6.8), but no evidence of excess risk in other cancers. The “flip side of the coin” ( Aksan et al. 2021 ) has also been considered: i.e. whether Iron insufficiency may contribute to cancer ( Phipps et al. 2021 ; Aksan et al. 2021 ). Supporting this possibility, in a population-based study in Taiwan, an approximately 2-fold increased incidence of gastrointestinal and other cancers in patients with iron deficiency anemia was observed ( Hung et al. 2015 ). In another population-based cohort study, the proportion of men and postmenopausal women developing GI malignancy was 31 times greater (95% confidence interval 9 to 107) in those with iron deficiency anemia compared with those with normal iron levels ( Ioannou et al. 2002 ). These results are congruent with a study demonstrating a U-shaped association between iron intake and development of adenomatous polyps (a precursor lesion to colorectal cancer), which demonstrated that consumption of either high or low amounts of iron led to increased risk of developing polyps relative to iron-adequate diets ( Bird et al. 1996 ). To explain the apparent ability of iron insufficiency to increase cancer incidence, it has been argued that iron is required for proper immunosurveillance, and that lack of iron may therefore create a more permissive environment for cancer initiation and progression ( Phipps et al. 2021 ). Overall, these demonstrations of the potentially dual role of iron excess and deficiency in alternative facets of cancer risk are both a reminder of the need to consider the critical role of iron in the tumor microenvironment and a potential explanation for the mixed picture that sometimes emerges when evaluating iron and cancer risk in patient populations.

The ability of iron to cycle between reduced and oxidized states endows it with valuable properties in catalysis but also renders it capable of inducing oxidative stress (Chapter 1). Maintaining a balance between these features of iron – i.e. maintaining the bioavailability of iron for essential processes while preventing undesirable induction of oxidative stress -- represents the major challenge in organismal and cellular handling of iron. Many of the fundamental proteins and processes used to meet this challenge have become well understood, primarily through the study of non-cancer cells and patients with iron overload disorders (see Figure 1 and ( Katsarou and Pantopoulos 2020 ; Galy et al. 2024 ) for excellent reviews of this topic). Here we focus on modulations of iron metabolism that have been described in cancer. An early molecular insight into differences in iron handling between normal and neoplastic cells came from the study of the regulation of ferritin, a protein with a major role in the sequestration, storage and recycling of intracellular iron. Ferritin is a ubiquitously expressed 24 subunit protein composed of two subunit types, termed H and L (reviewed in ( Arosio et al. 2024 )). The H and L subunits of this protein are subject to coordinate post-transcriptional regulation ( Arosio et al. 2024 ). In a departure from this paradigm, early studies showed that the H chain of ferritin could be transcriptionally regulated by the adenoviral E1A oncogene ( Tsuji et al. 1993 ) and c-m yc ( Wu et al. 1999 ), both of which were found to suppress ferritin H, reducing iron storage and enhancing its intracellular availability. The discovery of the iron efflux pump ferroportin and its negative regulation by the liver peptide hormone hepcidin (( Nemeth et al. 2004 ) and Chapter 3) provided a new dimension to our understanding of the role of iron in cancer. Following this seminal discovery in non-cancer cells, the study of ferroportin in breast cancer revealed that ferroportin downregulation was frequently observed in breast cancer cells and contributed to iron retention and tumor growth in mice ( Pinnix et al. 2010 ). Directly tying this observation to human disease, transcriptional downregulation of ferroportin in tumors was associated with poor patient prognosis ( Miller et al. 2011 ). Subsequently, it was observed that hepcidin was also synthesized by cancer cells and that its up-regulation was linked to cancer cell growth, exemplifying another mechanism for enhanced tumor iron retention ( Blanchette et al. 2016 ; Tesfay et al. 2015 ). IRP1 and IRP2 are post-transcriptional regulators of several proteins of iron metabolism, including ferritin, ferroportin and transferrin receptor (reviewed in ( Galy et al. 2024 ) and in Chapter 2), and have also been linked to cancer. IRP2, which represses ferritin translation (thus promoting iron storage) and positively regulates transferrin receptor mRNA stability (thus promoting iron uptake), was also associated with cancer cell growth in vitro and in animal models ( Wang et al. 2014 ). Cancer cells also deploy multiple mechanisms to amplify iron uptake relative to their non-cancer counterparts. For example, lipocalin (LCN2, (also termed 24p3), is upregulated in breast and other cancers ( Yang et al. 2009 ). Lipocalin binds iron-containing siderophores and delivers its iron payload following internalization and binding to its receptor ( Bao et al. 2010 ; Devireddy et al. 2005 ), enabling metastatic cells to acquire iron in nutrient-limited microenvironments ( Chi et al. 2020 ). Another example is CD44, a transmembrane glycoprotein that mediates endocytosis of hyaluronate-bound iron in breast tumors ( Muller et al. 2020 ). Intriguingly, it was recently reported that ether lipids, prevalent in cancer stem cells and cells with a high metastatic capacity, confer a high membrane fluidity that is conducive to CD44-mediated iron uptake. This novel finding represents an entirely new mechanism of iron regulation in cancer cells that operates through lipid-mediated changes in iron uptake ( Henry et al. 2024 ). In addition to mechanisms affecting iron uptake and efflux, it has become apparent that modulation of intracellular pathways of iron trafficking also occurs in cancer cells. NCOA4 (nuclear receptor coactivator 4) is a cargo receptor that mediates delivery of ferritin to the lysosome for degradation and release of iron to the cytosol and mitochondria, a process termed ferritinophagy ( Dowdle et al. 2014 ; Mancias et al. 2014 ). NCOA4 accelerates pancreatic tumorigenesis in mouse models ( Santana-Codina et al. 2022 ). Further, an elevated ferritinophagy expression signature predicts poor prognosis in patients with pancreatic cancer ( Santana-Codina et al. 2022 ). Pancreatic tumors that utilize the ferritinophagy pathway as a source of iron for mitochondrial iron-sulfur cluster protein synthesis become dependent upon this pathway following treatment with RAS-MAPK inhibitors ( Ravichandran et al. 2022 ). Since disruption of ferritinophagy synergizes with KRAS-MAPK blockade to inhibit pancreatic tumor growth, ferritinophagy may represent a new targetable vulnerability in pancreatic cancer, a notoriously treatment-refractory disease ( Ravichandran et al. 2022 ). Notably, targeting ferritinophagy also impaired quiescent cancer stem cells in vivo models of acute myeloid leukemia ( Larrue et al. 2024 ). Iron balance can also be affected through the efflux of ferritin in extracellular vesicles via less well-studied pathways mediated by CD63 ( Yanatori et al. 2021 ; Yanatori et al. 2023 ) and prominin2 ( Brown et al. 2019 ). Ferritin released by these pathways may also impact tumorigenesis, since ferritin has the potential to interact with kininogen to affect angiogenesis ( Coffman et al. 2009 ); in addition, the removal of iron-containing ferritin through this pathway may reduce cancer cell susceptibility to ferroptosis ( Brown et al. 2019 ). Overall, these observations are consistent with a picture in which cancer cells activate a complex variety of mechanisms, ranging from transcriptional regulation to membrane lipid content, that converge on remodeling pathways of iron uptake, efflux and trafficking to optimize iron acquisition and retention while promoting cell survival.

The long-standing appreciation of links between iron and cancer has been accompanied by an equally long-standing effort to leverage iron in cancer therapy. Two opposing approaches have been taken: to inhibit tumor growth and metastases by depriving tumors of this essential nutrient or, conversely, to provide tumors with sufficient excess iron to drive them into iron-dependent cell death pathways. Use of iron chelators has the longest history in the quest to manipulate iron to target cancer. Iron chelators are small molecules that selectively bind iron; several of these have an established safety record by virtue of their use in treating iron overload disorders in human patients (Chapter 23). Although chelator-mediated iron depletion has met with some efficacy in cancer patients using the iron chelator desferoxamine, responses were modest ( Yamasaki et al. 2011 ), and problems of selective delivery to tumors without disruption of systemic iron homeostasis have yet to be overcome. An alternative strategy has been to design iron chelators that do not produce a chemically inert complex of iron but rather permit bound iron to redox cycle. Well-studied examples are the thiosemicarbazones such as Dp44mT and triapine ( Jansson et al. 2010 ; Lovejoy et al. 2011 ), which also inhibit the essential iron-dependent enzyme ribonucleotide reductase. Dp44mT has the additional unanticipated property of inducing the metastasis suppressor NDRG1 ( Wijesinghe et al. 2021 ). Newly designed derivatives of the Dp44mT chelator family continue to be actively investigated for their anti-cancer properties ( Dharmasivam et al. 2023 ), and a Phase III trial of oral triapine with chemoradiation is currently underway for advanced stage cervical and vaginal cancers ( NCT02466971 ) based on encouraging results obtained in Phase I and Phase II trials ( Taylor et al. 2024 ; Kunos et al. 2019 ). The discovery of ferroptosis and the susceptibility of cancer cells to this mode of cell death has prompted a vigorous effort to identify agents that increase tumor cell iron, foster ferroptosis-mediated cell death, and thus act as anti-tumor agents ( Basuli et al. 2017 ). Iron-containing nanoparticles are a good example of the new candidate drugs being developed to exploit this vulnerability ( Wang et al. 2023b ). Along these lines, preclinical studies with ferumoxytol, a clinically approved iron supplement, showed substantial promise, significantly reducing disease burden and prolonging survival in a murine leukemia model ( Su et al. 2020 ; Trujillo-Alonso et al. 2019 ). Similarly, carbon-nanoparticle loaded iron (CNSI-Fe(II)] is currently being explored in a Phase I study of patients with solid tumors, particularly those with KRAS mutations ( clinical trials.gov NCT06048367 ). Approaches that target iron indirectly are also being explored. One is to use iron markers overexpressed in cancer cells, such as TFRC (TfR1), either as direct targets of anti-TfR1 antibodies ( Daniels-Wells et al. 2020 ) ( Daniels-Wells et al. 2024 ) or as targets for the preferential delivery of cytotoxic agents to tumor cells via conjugation to its ligand TF ( Candelaria et al. 2021 ); however, this approach has been hindered by the presence of TF receptors on non-cancer cells. Gallium is an iron mimetic that can replace iron in key proteins such as iron-dependent mitochondrial enzymes and ribonucleotide reductase to generate non-functional catalytic centers ( Chitambar 2018 ). Gallium maltolate, a contemporary version of gallium nitrate, the form of gallium originally used in this approach, has recently demonstrated promising anti-cancer activity in experimental models of glioblastoma ( Al-Gizawiy et al. 2023 ). In addition to their potential use as direct tumoricidal agents, drugs that target iron metabolism may also be useful in combination chemotherapy. For example, as discussed above, iron sulfur clusters are required for the function of DNA repair enzymes ( Gari et al. 2012 ; Stehling et al. 2012 ). Since blocking DNA repair increases sensitivity to DNA damaging drugs, impeding iron metabolism to block formation of iron-sulfur clusters may increase the activity of DNA-damaging agents used in conventional chemotherapy ( Tesfay et al. 2022 ).

Decades of work firmly support the core concept that iron plays a vital and potentially targetable role in cancer. We now understand at a much more granular (and hence more specifically targetable) level the complex interplay between iron and cancer cell metabolism. The continuing discovery of new players in cancer cell iron uptake and handling such as CD44 and NCOA4, as well as the discovery of new roles for iron in controlling processes as diverse as ferroptotic cell death and mitochondrial glutathione, have further illuminated (and may be expected to continue to illuminate) the multifaceted role of iron in the tumor cell and its environment. These discoveries have already offered new opportunities for therapeutic intervention. However, with the discovery of each new iron-dependent protein or process comes additional complexity. Disentangling the contribution of each of these elements to patient outcome presents a challenge that will ultimately have to be overcome to optimize and tailor therapies to target iron in cancer.

The role of iron in cancer has recently been extended to the study of ferroptosis, a term coined in 2012 ( Dixon et al. 2012 ) to describe an iron-dependent form of cell death (reviewed in detail elsewhere ( Berndt et al. 2024 ); see also Chapter 5). A myriad of studies have associated ferroptosis gene signatures with cancer patient prognosis in a variety of malignancies. It is worth noting that in ovarian and other cancers, “persister” cells and cancer cells with stem cell-like properties – treatment refractory cells thought to give rise to metastases -- are particularly susceptible to ferroptosis due to their enhanced levels of iron ( Rodriguez et al. 2022 ; Basuli et al. 2017 ). Observations such as these have led to the current concept that in some instances, iron repletion or provision of excess iron may push cancer cells into death pathways. This concept is being actively pursued in the design of novel therapeutics, as discussed below.

The relationship between iron and cancer has been the subject of exploration and debate for decades. In the 1940’s and 50’s, exposure to environmental iron dust or the intramuscular injection of iron was shown to induce tumors in rodents ( Campbell 1940 ; Richmond 1959 ), introducing the concept of iron as a tumor initiator. Work performed in ensuing decades showed that iron-rich diets promoted tumor growth in mice; conversely, iron deficient diets reduced tumor growth ( Hann et al. 1988 ; Hann et al. 1991 ; Hann et al. 1992 ; Radulescu et al. 2012 ), suggesting that iron can also serve as a tumor growth factor. Clinical observations that levels of iron in blood are frequently reduced in patients with cancer inspired the idea that iron withholding is an inherent organismal strategy to combat malignancy (and infections) by depriving invaders of this essential nutrient, indirectly supporting the notion that iron is critical to malignant disease. These ideas were supported by many subsequent experiments that contributed to the broad concept (championed by ED Weinberg and others) that iron is an essential tumor nutrient that can be manipulated for therapeutic benefit ( Weinberg 1983 ). An early case report (1987) in which DFO, an iron chelator, induced a partial response in a patient with neonatal leukemia further demonstrated the potential of this idea for translation to human cancer therapy ( Estrov et al. 1987 ). The benefit of manipulation of iron in cancer patients has been supported by a substantial number of more modern studies (see below), although control of iron has yet to advance to a first-line therapy. Nevertheless, the core concept that iron plays a vital and potentially targetable role in cancer still stands.

What is the fundamental metabolic need that drives cancer cells to acquire and retain excess iron? This simple question has been surprisingly complex to resolve, likely because there is more than one answer, and because the requirement for iron may be both direct and indirect. Many iron-dependent proteins have been proposed as candidates for drivers of iron dependence. One of the earliest was ribonucleotide reductase, an iron-dependent enzyme that catalyzes the rate-limiting step in DNA synthesis and is required to sustain cancer cell proliferation. This remains a viable notion, and drugs that target this enzyme have been and remain in clinical use in the treatment of cancer patients ( Huff et al. 2022 ). Other iron-containing proteins implicated in the malignant process include replicative DNA polymerases, helicases, primases, DNA-modifying members of the Fe(II) 2-oxoglutarate dependent enzyme family such as histone demethylase JumonjiC domain-containing histone demethylases ( Roatsch et al. 2019 ), and TET (ten-eleven translocation (TET) methylcytosine dioxygenases ( Yang et al. 2024 ). Iron has also been linked to activation of a number of signaling pathways, including WNT, STAT3, EGFR, ERK 1/2, Akt, as reviewed elsewhere ( Torti and Torti 2020 ). In addition to individual proteins and signaling pathways, metabolic processes linked to iron have been implicated in cancer. Mitochondria appear essential to cancer cell growth ( Tan et al. 2015 ), and many of the numerous metabolic processes carried out by this organelle depend on iron, including iron sulfur cluster biogenesis, heme synthesis, maintenance of redox balance, and oxidative phosphorylation ( Figure 2 ). Iron sulfur clusters are present in numerous proteins, including enzymes involved in oxidative phosphorylation and DNA repair. Our group showed that SFXN4, an inner mitochondrial membrane protein upregulated in ovarian cancer cells and tissues, plays a role in iron sulfur cluster biogenesis ( Paul et al. 2019 ). Accordingly, knockdown of SFXN4 not only reduced respiration, but reduced levels of DNA repair and sensitized cells to chemotherapy in ovarian cancer models ( Tesfay et al. 2022 ). Along similar lines, a mitochondrially-targeted iron chelator inhibited iron sulfur cluster as well as heme biogenesis in in vitro and in vivo models of breast cancer, leading to destabilization of proteins containing these prosthetic groups, inhibition of respiration, and inhibition of tumor growth and metastasis ( Sandoval-Acuna et al. 2021 ). NEET proteins, including mitoNEET (CISD1), are a class of mitochondrial 2Fe-2S iron sulfur proteins with a unique sequence motif and labile and redox active cluster. Although a unifying mechanism of action has not yet emerged ( Mittler et al. 2019 ) NEET proteins affect iron/Fe-S homeostasis, protect OXPHOS proteins from oxidative stress, impact various signaling pathways, and have been implicated in tumor growth and metastasis in breast, cervical and other cancers ( Mittler et al. 2019 ). Ligands that stabilize the mitoNEET cluster under oxidative conditions reduce ovarian cancer cell proliferation, implying an important role for the lability of the cluster in mitoNEET function ( Marjault et al. 2022 ). In addition to examples of individual iron sulfur cluster-containing proteins, genes encoding proteins involved in processes of iron sulfur cluster biogenesis are frequently altered in cancer, leading to the suggestion that this may represent a critical target in cancer ( Petronek et al. 2021 ). Intriguingly, a CRISPR-Cas9 screen revealed that genes required for the mitochondrial functions of iron sulfur cluster biogenesis (e.g. NFU1) and oxidative phosphorylation (e.g. NDFUS1) facilitate survival in the acidic environment favored by tumors ( Boedtkjer and Pedersen 2020 ), suggesting it may be possible to selectively target these processes in tumors despite their broad utilization in non-cancer cells ( Michl et al. 2022 ). Iron in iron sulfur clusters can also play important regulatory roles in other metabolic processes. An interesting example is SLC25A39, a protein that was recently identified as a transporter of mitochondrial GSH required for proper mitochondrial respiration ( Shi et al. 2022 ; Wang et al. 2021 ). The stability of this protein depends on an iron sulfur cluster that facilitates its interaction with AFG3L2 ( Shi et al. 2024 ; Liu et al. 2023b ) a protease that degrades SLC25A39 ( Shi et al. 2024 ; Liu et al. 2023b ). Overexpression of mitochondrial iron transporters mitoferrin1 or mitoferrin2 stabilized SLC25A39 iron-sulfur cluster content, increased SLC25A39 degradation, and decreased mitochondrial GSH, thus linking iron metabolism and maintenance of mitochondrial GSH ( Shi et al. 2024 ; Liu et al. 2023b ). Upregulation of SLC25A39 promoted cell growth and metastasis in colorectal cancer models ( Zhang et al. 2024b ), suggesting that iron may be utilized by cancer cells to regulate mitochondrial GSH as well as respiration. These recent observations illustrate the indirect and complex way in which iron can affect cellular function.