Ferroptosis: A flexible constellation of related biochemical mechanisms

Abstract

It is common to think about and depict biological processes as being governed by fixed pathways with specific components interconnected by concrete positive and negative interactions. However, these models may fail to effectively capture the regulation of cell biological processes that are driven by chemical mechanisms that do not rely absolutely on specific metabolites or proteins. Here, we discuss how ferroptosis, a non-apoptotic cell death mechanism with emerging links to disease, may be best understood as a highly flexible mechanism that can be executed and regulated by many functionally related metabolites and proteins. The inherent plasticity of ferroptosis has implications for how to define and study this mechanism in healthy and diseased cells and organisms.

Keywords: PUFA; ROS; cell death; ferroptosis; iron; lipid; pathway; peroxidation.

Figure 1.. A typical pathway map for a biological process.

The regulation of biological processes is often conceived and summarized using pathway maps. In these maps, individual elements like proteins and metabolites are represented as colored shapes and connections between elements are depicted by positive (arrow) or negative (T bar) interactions. Here, different shapes and colors are used to indicate different notional types of molecules. The strength and importance of individual connections are not easily captured or specified in such maps.

Figure 2.. Old and new representations of ferroptosis.

(A) Several key elements of the ferroptosis mechanism, depicted using a traditional pathway map. This pathway map encompasses far less than is known about ferroptosis. Some key inputs are also not easily captured in this map due to conflicting evidence in the literature. This map also contains substantial redundancy, which makes it difficult to intuit how inhibition or activation of any one element modulates ferroptosis. Key small molecule inducers of ferroptosis are indicated in red. NOX: NAD(P)H oxidase, LOX: lipoxygenase, CI/CIII: complex I and complex III of the mitochondrial electron transport chain. POR: NAD(P)H-cytochrome P450 reductase. Notional peroxidized phospholipid acyl chains are denoted in red; purple indicates termination of the phospholipid peroxidation process. (B) A new depiction of ferroptosis as a flexible mechanism involving a constellation of related biochemical mechanisms. The core mechanism of lipid peroxidation (magenta) involves iron, and different configurations of oxidizable lipids and lipid peroxidation initiating reactions and enzymes. The core lipid peroxidation mechanism is surrounded by several biochemically distinct inner defense mechanisms (blue). The core mechanism and the inner defenses are in turn modulated by a regulatory penumbra that may encompass dozens or hundreds of regulatory inputs, only some of which are depicted here (green). (C) Mechanistic details of the core lipid peroxidation mechanism, illustrating the processes of (auto)initiation, propagation, and termination of the chain reaction. Key points of intervention by radical trapping antioxidants (RTAs), peroxidase mimics, and iron chelators, are indicated.

Figure 3.. Examples of complexity and contradiction in ferroptosis regulation.

(A) Several different enzymes and pathways can contribute redundantly to synthesis of the reducing agent NAD(P)H, including the oxidative pentose phosphate pathway (oxPPP), malic enzyme 1 (ME1), and isocitrate dehydrogenase 1 (IDH1). NAD(P)H is used by enzymes that both promote and inhibit ferroptosis. (B) Mechanistic target of rapamycin complex 1 (mTORC1) can regulate ferroptosis both positively and negatively through effects on the expression of glutathione peroxidase 4 (GPX4) or sterol response element binding protein 1 (SREBP1), or by diverting cysteine flux into protein synthesis that competes with the synthesis of glutathione. (C) The p53 transcription factor can inhibit ferroptosis through transcriptional induction of CDKN1A or interaction with the dipeptidyl peptidase 4 (DPP4). P53 can also induce ferroptosis through transcriptional repression of SLC7A11; SLC7A11 protein can inhibit LOX12 activity through direct interaction.

Figure 4.. Visualizing the contribution of different molecules to ferroptosis.

(A) The total inner defense capacity and total lipid peroxidation susceptibility of the cell can be influenced by many metabolites and proteins, some of which are listed here. The relative contribution of each molecule to ferroptosis sensitivity will vary between contexts. (B) Quadrant diagram summarizing four different possible states in which a cell may exist. Cells with low total inner defense capacity and high total lipid peroxidation susceptibility with be most sensitive to ferroptosis. The boundaries between these states are likely to be fluid. (C) Different treatments, such as iron chelation, or overexpression of GPX4, will reduce ferroptosis sensitivity by lowering total lipid peroxidation susceptibility or increasing total inner defense capacity. (D) Cells may exist in different states of ferroptosis sensitivity and resistance over time, for example due to induction of NRF2, epithelial-to-mesenchymal transition (EMT), or transit in the body through a lymph fluid rich in monounsaturated fatty acids.