From Iron Chelation to Overload as a Therapeutic Strategy to Induce Ferroptosis in Leukemic Cells

Abstract

Despite its crucial importance in numerous physiological processes, iron also causes oxidative stress and damage which can promote the growth and proliferation of leukemic cells. Iron metabolism is strictly regulated and the related therapeutic approaches to date have been to restrict iron availability to tumor cells. However, since a new form of iron-catalyzed cell death has been described, termed ferroptosis, and subsequently better understood, iron excess is thought to represent an opportunity to selectively kill leukemic cells and spare normal hematopoietic cells, based on their differential iron needs. This review summarizes the physiology of iron metabolism and its deregulation in leukemia, the known ferrotoposis pathways, and therapeutic strategies to target the altered iron metabolism in leukemia for the purposes of initiating ferroptosis in these cancer cells.

Keywords: acute myeloid leukemia; ferritinophagy; ferroptosis; iron; reactive oxygen species.

Figure 1

The ferroptotic cascade. Accumulation of free iron is a key initiator of ferroptosis. After loading onto Tf, the Fe³⁺/Tf complex binds to TFR1, and iron is thereafter released into endocytosis vesicles under lower pH conditions, where Fe³⁺ is reduced into the ferrous form Fe²⁺ under the action of ferrireductase STEAP3. Iron then exits into the cytosol via DMT1 to form the labile iron pool (LIP). The LIP represents a very slight per se, whereas most iron is either stored in FTH, either used as a cofactor for several iron-containing enzymes, or for the synthesis of heme. Excess iron is carried in the extracellular space through FPN after being reoxydized by hephaestin. FTH autophagic degradation, a process termed ferritinophagy, is mediated by NCOA4, a cargo receptor that binds to the heavy chain of FTH delivering it to the early-stage autophagosome, inducing the subsequent release of iron into the cytosol. Iron release from heme through HO-1 is another way to increase the LIP. Inside mitochondria, CISD1 modulates mitochondrial iron uptake and respiratory capacity and mitigates ferroptosis. The hormone hepcidin, mostly secreted by hepatocytes in the systemic circulation, triggers ferroportin lysosomal degradation, thus hindering iron exit. AA or AdA are key membrane phospholipids esterified by ACSL4 and LPCAT to generate PE-AA/ AdA, and further oxidized to phospholipid hydroperoxides (PE-AA/AdA-OOH) by LOXs. Free iron can also interact with ROS, specifically hydrogen peroxide, to form hydroxyl/peroxyl (LOH/LOOH) toxic radicals via the Fenton reaction. Therefore, iron can then abstract a hydrogen atom from PUFAs, forming a lipid radical which promptly reacts with oxygen to generate a lipid peroxide (PUFA-OOH). In a steady state, lipid peroxides and their degradation products are neutralized by GSH-based redox reactions. The xCT antiporter (consisting of two subunits SLC7A11 and SLC3A2) exports glutamine and imports cystine into the cell. Inside the cell, cystine is reduced to cysteine by cystine reductase and then GCL and GSS add L-glutamate and glycine respectively, to produce GSH. Another way to generate cysteine is via the trans-sulfuration pathway converting methionine in homocystein, and later cysteine by CBS. Many redox enzymes use GSH, including GPX4, which reduces reactive lipid peroxydes to their alcohol counterparts. Additionally, CoQ10, a byproduct of the mevalonate pathway, acts as a complemental RTA to mitigate ferroptosis. CoQ10 is reduced to CoQ1O-H2 by FSP1, enabling its activity. GPX4 and SLC7A11 are target genes of the master antioxidant regulator NRF2, as well as the enzymes contributing to GSH synthesis, GCL, GSS, and CBL. NRF2 additionally mediates iron metabolism by upregulating the transcriptional level of FTH, FPN, and HO-1. In the absence of redox stress, NRF2 is downregulated by the E3-ubiquitine ligase KEAP1. If the key redox regulator is genetically disrupted or pharmacologically inhibited, lipid peroxides and their degradation products accumulate, and thereby initiate ferroptosis through a yet unknown mechanism involving membrane destabilization, cytoskeletal changes, and cell death. AA, arachidonic acid; AdA, adrenic acid; ACSL4, acyi-CoA synthetase long chain family member 4; CBS, cystathionine-13-synthase; CISD1, CDGSH iron sulfur domain 1; CoQ10, coenzyme Q10/Ubiquinone-10; DMTl, divalent metal transporter 1; FSP1, ferroptosis-suppressor-protein 1; FPN, ferroportin; FTH, ferritin; GCL, glutamate-cysteine ligase; GPX4, glutathione peroxidase 4; GSH, glutathione; GSS, glutathione synthetase; HO-1, heme oxygenase 1; KEAP1, Kelch-like ECH-associated protein 1; LOXs, lipoxygenases; LPCAT, lysophosphatidylcholine acyltransferase; NCOA4, nuclear receptor coactivator 4; NRF2, nuclear factor (erythroid-derived 2)-like 2; PE, phosphatidylethanolamine; PUFA, polyunsaturated fatty acids; RTA, radical trapping agent; STEAP3, six-transmembrane epithelial antigen of the prostate 3; TFR1, transferrin receptor 1.

Figure 2

Lipid ROS homeostasis. (A) Schema showing enzymatic or substrate modulators of lipid ROS. The size of the box is proportional to the functional importance of the modulator. (B) Theorical imbalance of lipid ROS homeostasis, in this example caused by enhanced ferritinophagy through NCOA4 upregulation.