br Methods and materials br Conflict of interest statement b

Methods and materials
Conflict of interest statement
Introduction Ferroptosis (ferro = “ferrous ion (Fe2+)”, ptosis = “fall”), implicating the crucial role of cellular iron played in cell death, is a new form of nonapoptotic programmed cell death characterized by iron-dependent accumulation of lipid hydroperoxides to lethal levels, which was first named by Stockwell in 2012 [1], [2]. Different from traditional apoptosis, necrosis, and autophagy, ferroptotic cell death is morphologically characterized by cell volume shrinkage, condensed mitochondrial membrane density, reduction or vanishing of mitochondria crista and outer mitochondrial membrane rupture, which can be induced by system Xc− inhibitors and glutathione peroxidase 4 (GPX4) inhibitors [2], [3], [4]. When system Xc− or GPX4 is blocked, the normal metabolism of lipid would be disrupted, resulting in accumulation of lipid peroxide and reactive oxygen species (ROS) under assistance of iron, and then damaging the mitochondria, finally causing cell death. These inhibitors include various drugs and experimental compounds, such as sulfasalazine (SAS), sorafenib, cisplatin, artemisinin and its derivatives, erastin, Ras selective lethal 3 compound (RSL3), DPI2, buthionine sulfoximine, withaferin A and so on [5], [6], [7], [8], [9]. The major mechanisms of ferroptosis are always correlated with lipid peroxidation and iron metabolism [10]. There are two important regulatory targets involved in lipid peroxidation and cell’s anti-oxidation capacity, that are cystine/glutamate antiporter system Xc− and GPX4 [4]. By inhibiting system Xc− activity, on one hand, the uptake of cysteine will be inhibited, which then down regulates the synthesis of glutathione (GSH), resulting in GSH depletion in the cancer cells. Since GSH is an essential intracellular antioxidant that serves as a reducing co-substrate of GPX4 to protect AH 7614 from oxidative damage caused by highly toxic ROS, its depletion will therefore inactivate the activity of GPX4 and facilitate ROS accumulation in cancer cells [10], [11]. On the other hand, system Xc− inhibitors can also inactivate the cystine/cysteine redox cycle to restrict the intracellular supply of cysteine, inhibiting new GSH synthesis and causing ferroptotic cell death [12], [13]. Moreover, GPX4 is another key parameter related with ferroptosis. Inactivation of GPX4 always prohibits decomposition of lipid peroxide and ROS, which induces excessive ROS and extensively damage of cell molecular substances such as proteins, nucleic acids and lipids [14], [15]. Meanwhile, iron metabolism plays an important role in ferroptosis processes. It was reported that overloading iron could induce ferroptotic cell death through the production of ROS and further oxidation of the lipid, protein and DNA via a chemical reaction named Fenton reaction [16], [17]. In this reaction, intracellular H2O2 produced by the mitochondrial respiratory chain is catalyzed by Fe2+/ Fe3+ to produce highly toxic, reactive hydroxyl radicals (OH) or superoxide radicals (OOH) [16], [18]. The equation is illustrated as follows: All these evidences indicate that the ferroptosis is an iron and ROS dependent cell death. And how to decrease GSH concentration and/or to increase the level of iron and H2O2 simultaneously in cells is the key to the occurrence of ferroptosis. For example, Angeli et al. [11] found that cysteine starvation would lead to ferroptotic cell death via limiting the biosynthesis of GSH, while vitamin E and selenium could protect cells from ferroptosis [19], [20], [21], [22]. However, iron physiologically mainly exists in the endosome and is stored in the form of ferritin, which can hardly be used for Fenton reaction. Therefore, to accelerate the Fenton reaction, exogenetic iron is always essential. In the past years, an increasing number of studies have revealed close relationships between ferroptosis and nanomedicine, which was regarded as a new strategy for engineering nanomaterial-based therapeutic reagents for highly effective therapy of various cancers. These nanomaterials include iron-based nanomaterials and nanomaterials without iron. On one hand, these nanomaterials are endowed with the ability to elevate ROS accumulation after cellular uptake, eventually leading to the cell death. On the other hand, due to the nanoscale size of the engineered nanomaterials, they are prone to passively target tumor tissues via enhanced permeability and retention (EPR) effect, thus achieving cancer specific therapy. Recently, there are a couple of reviews focusing on mechanisms or inducers and inhibitions of ferroptosis [11], [23], [24], and nanomaterials have been scarcely reviewed as inducers of ferroptosis except for the recent review summarized by Chen and his co-workers [25]. However, the fundamental molecular mechanisms in the regulation of ferroptosis and some newly developed nanomaterials are not included. In this review, we first summarized the molecular mechanisms and pathways for ferroptosis regulation, then systematically discussed various nanomaterials that induce ferroptosis for cancer specific therapy, and finally provided the current challenges and constructive perspectives in this field.