Yi Luan,Yi Zhao,Ying Luan,Xing Chen,Yang Yang＊

1Department of Translational Medicine Center,the First Affiliated Hospital of Zhengzhou University,Zhengzhou 450052,China;2Department of Physiology and Neurobiology,School of Basic Medical Sciences,Zhengzhou University,Zhengzhou 450001,China.

Iron is one of the essential trace elements in human body,which plays a critical role in electron respiratory chain,DNA synthesis,and oxygen utilization.The iron content in adult body is about 40 mg/kg [1].When iron content in human body decreases,the normal physiological functions are affected,which causes series of iron deficiency diseases,such as iron deficiency anemia.Iron deficiency is a common nutritional deficiency that affects over 2 billion people worldwide [2].Iron deficiency leads to immune,neurological,respiratory,and other disorders.There is a dynamic balance of iron metabolism in human body,which includes three main processes:iron uptake,storage,and output.It is absorbed by small intestinal epithelial cells,stored in red blood cells,liver,and muscle fibers,and exported by excrement or shedding of small intestinal epithelial cells.

Food iron mainly exists in oxidation state and is uptaken by DYCTB reductase.By the help of ion channel protein DMT1,iron is transported into cells,and through the membrane transporter FPN turning out,rely on the intestinal cells of the basolateral copper oxidase protein HEPH oxidized to ferric iron,again into the blood by transfer ferritin [3].After the transferrin binds iron,it binds to the transferrin receptor TFR of target cells,and absorbs iron through the endocytosis of cells,so as to be utilized by target cells (Figure 1).

Iron is required for cancer cells,and its deficiency suppresses tumor growth [4].Iron not only directly affects DNA synthesis through nucleotide reductase,also affects the expression of cycle-related proteins and cell cycle [5].The main regulatory molecules of cell cycle include cyclins,cyclin-dependent protein kinases,cyclin-dependent protein kinase inhibitors andp53.Interestingly,iron deficiency induced by iron chelating agents leads to changes in the expression of cell cycle-related genes,such asp53,CCND1,p21,andCDK2,stimulating cell cycle arrest at the G1/S phase [6].

Studies demonstrated a positive correlation of increased systemic iron levels with several types of human cancers,and high transferrin saturation is correlated with increased cancer risk [7].Patients with iron overload caused by hemochromatosis presented a higher risk of cancer development.Increasing iron level artificially also presents high cancer risk [8].For an example,patients receiving blood transfusions were more susceptible for cancer development.Cancer cells display peculiar alterations in iron metabolism that depict an overall increased iron turnover,with an enhanced affinity for iron that could be considered as a hallmark of cancer [7].Notably,the iron availability could directly mediate mitochondrial biogenesis and oxidative metabolism.The alteration of iron levels of cells leads to uncontrolled cell proliferation and cancer development.

Several important iron transporters can influence tumor progression by mediating iron content in cells[9].Transferrin receptor (TFR1) overexpression has been widely found in cancer cells and tissues,including leukemia,glioma,prostate,liver,and ovarian cancers.High TFR1 expression was correlated with the prognosis of patients with these cancers [10].However,the depletion of TFR1 also suppressed cell growth and proliferation in several types of cancers,such as pancreatic cancer [10].High expression of TFR1 promoted DSS-induced colon epithelial cell proliferation and apoptosis,and exacerbated tumorigenesis through activating the IL-6/IL-11-STAT3 signaling pathway in the colon [11].As a main iron transporter,DMT1 pharmacological inhibition antagonized colon tumor progression by suppressing JAK-STAT3 signaling in colorectal tumorigenesis [12].Melanotransferrin characterized to participate in delivering iron was found to foster melanoma cell migration,invasion,and angiogenesis[13].The iron-loading process in macrophages was correlated with the improved prognosis of lung cancer patients [14].FPN expression is sharply restrained in a variety of cancer types.Low FPN expression facilitates proliferation and colony formation in non-small cell lung cancer cells,partially by enriched iron concentration for cancer cell proliferation [12].Breast cancer is a disease that needs urgent treatment [15,16].The overexpression of FPN in breast cancer cells showed decreased proliferation,colony formation,and tumor growth as well as liver metastases.FPN overexpression resulted in cell cycle arrest and stress-induced DNA damage via stimulating autophagy and activation of p53 and its downstream target p21 in prostate cancers.Decreased FPN expression augmented E-cadherin and suppressed N-cadherin,twist and slug expression thus to accelerate cell proliferation and epithelial-mesenchymal transition (EMT) in triple-negative breast cancer cells [17].Additionally,FTH overexpression has been correlated to chemotherapy-resistance in both ovarian and breast cancers.These findings broaden our perspective of iron transporters are beneficial to the strategy development of manipulating therapeutically in cancer by means of mediating iron output.

Figure 1 Mechanism of cellular iron transport

Iron chelator inhibited breast cancer cell proliferation and stimulated cell cycle arrest [18].Iron chelators DFO and DFX have been proved to induct autophagy in multiple myeloma and breast cancer cells through the upregulation and phosphorylation of autophagic proteinsp62/SQSTM1 following iron chelation [19].Conversely and importantly,it induced hypoxia-mediated angiogenesis and EMT in breast cancer cells.A non-taxane,synthetic inhibitor of microtubule dynamics,Eribulin,could suppress the expression of the hypoxia and EMT markers in the presence of iron chelator [20].Iron chelator,as was known,inducing oncogenic effects such as angiogenesis and EMT,may be an effective treatment for breast cancer combined with eribulin [4].Iron is both essential to life whereas the high level of iron is potentially toxic.Previous study based on a comprehensive set of iron biomarkers and breast cancer patients investigated the relationships between iron body stores and breast cancer risk [21].However,they found the circulating iron,ferritin,and transferrin saturation did not support a strong association between elevated iron stores and breast cancer risk.They thought further investigation of low iron was warranted.

Callapina et al.found that low iron can also simulate hypoxic conditions,because Fe2+was necessary for proline hydroxylase activity [22].Intracellular iron deficiency leads to inactivation of proline hydroxylase,which cannot hydroxylate HIF-1,thus increasing HIF-1 stability.Meanwhile,inactivation of proline hydroxylase also prevents the activity of VHL.Finally,HIF-1 enters the nucleus and binds to HIF-1 to form an active dimer that stimulates a series of target gene transcriptions,including the expression of genes associated with angiogenesis [23].Elevated expression of HIF-1α and VEGF were observed in mice fed iron-deficient diets compared to mice fed iron-overloaded diets.Thus iron depletion treatments potentially serve as a novel therapeutic approach with anti-angiogenic drugs in the treatment of cancer.

Angiogenesis is an important link in tumor genesis and development [24].It is the result of a synergistic action of multiple cellular processes,such as proliferation and directed migration [25].Tumor microenvironment in hypoxic state can stimulate the secretion of pro-angiogenic factors such as VEGF.Iron chelators induce upregulation of VEGF,and these chelators promote angiogenesis and tumor growth.Additionally,iron deficiency also has a number of anti-angiogenic effects.In vivo studies provided the evidence that iron chelators inhibit tumor growth suggesting that the dominant anti-tumor effect of iron deficiency in tumor growth [26].Iron supplementation inhibited VEGF pathway in endothelial cells in vitro and reduced lung cancer vascularization in vivo.Low dose of iron inhibited angiogenesis,whereas high dose promoted oxidative stress that was thought to contribute to angiogenesis.Hence,the potential effect of iron depends on the equilibrium of iron levels that satisfy metabolic requirements but not lead to cell damage,impair oncogenic signaling,or induce ferroptosis.The iron treatment also promoted migration and invasion via producing ROS in lung cancer cells.

Iron plays a vital function in matrix degradation and cancer metastasis by means of irritating or stabilizing some metalloprotease activities [27].Tumor metastasis is related to extracellular matrix changes,mainly involving matrix metalloproteinases (MMPs).The culture of head and neck squamous cancer cells with iron solution resulted in increased expression of MMP9.Several studies showed that iron chelators caused a decrease in MMP expression,suggesting that iron chelators suppress tumor metastasis by altering the expression of MMPs associated with extracellular matrix in addition to inhibiting tumor growth.

To maintain the homeostasis of cellular redox balance,iron and ROS levels both need to be manipulated exquisitely.Once the balance was broken,cell go to either tumorigenesis or death.Quite distinct from apoptosis,autophagy,necroptosis and necrosis,ferroptosis,a type of programmed cell death was featured by enrichment of iron-induced lipid peroxidation and metabolic constraints [28].It could be triggered by multiple small molecules such as,erastin,sulfasalazine,and RSL3.A recent research has identified ironomycin,a derivative of salinomycin,as a powerful therapeutics against breast cancer stem cells by inducing an iron-dependent cell death.Ferroptosis was also functional in suppressing hepatocellular carcinoma (HCC),pancreatic carcinoma,breast cancer,and prostate cancer [29].Above all,therapeutics targeting ferroptosis offer a new potential strategy for beneficial outcome of both hematologic and solid malignancies.Ferroptosis was manipulated by the glutathione (GSH) redox system.Modulating the GSH redox system was sufficient to regulate ferroptosis induced cell death.Furthermore,cell sensitivity towards ferroptosis also could be affected by p53 via different mechanisms.

Collectively,iron play diverse functions in tumor growth,angiogenesis,cellular redox balance,and metastasis (Figure 2).Aberration of iron-related proteins,iron-mediated signaling pathway and physiological processes were observed in cancer cells,which further proved the importance of iron in tumorigenesis.Hence medications against iron-related proteins and metabolism serve as promising manner towards cancer therapy.Despite various discovery about iron-related therapies,the long-term effects,metabolic re-programming of immune and cancer cells and tumor vascularization and development of distant metastasis of these therapy are not further studied in cancer patients.Moreover,the efficiency and safety of these iron-related cancer therapy required further improvement.

Figure 2 The regulatory mechanism underlying iron homeostasis affecting cancer progression