Ferroptosis Meets Cell-Cell Contacts

Abstract

Ferroptosis is a regulated form of cell death characterized by iron dependency and increased lipid peroxidation. Initially assumed to be selectively induced in tumour cells, there is increasing evidence that ferroptosis plays an important role in pathophysiology and numerous cell types and tissues. Deregulated ferroptosis has been linked to human diseases, such as neurodegenerative diseases, cardiovascular disorders, and cancer. Along these lines, ferroptosis is a promising pathway to overcoming therapy resistance of cancer cells. It is therefore of utmost importance to understand the cellular signalling pathways and the molecular mechanisms underlying ferroptosis regulation, including context-specific effects mediated by the neighbouring cells through cell-cell contacts. Here, we give an overview on the molecular events and machinery linked to ferroptosis induction and commitment. We further summarize and discuss current knowledge about the role of cell-cell contacts, which differ in ferroptosis regulation between normal somatic cells and cancer cells. We present emerging concepts on the underlying mechanisms, address open questions, and discuss the possible impact of cell-cell contacts on exploiting ferroptosis in cancer therapy.

Keywords: cancer therapy; cell–cell contacts; epithelial–mesenchymal transition; ferroptosis.

Figure 1

Mechanism of lipid peroxidation. Left: Radical initiation is carried out by a radical (initiator = In^.) abstracting a hydrogen from a bisallylic site, i.e., the carbon between two double bonds of a PUFA, thereby creating a carbon-centred lipid radical (L*). Due to the adjacent double bonds, the radical can delocalize its electron system, resulting in a resonance-stabilized conformation. In a fast reaction, oxygen is then added, and a lipid peroxyl radical (LOO*) is formed. By abstracting a hydrogen from an adjacent PUFA, the lipid peroxyl radical itself reacts to a lipid hydroperoxide (LOOH) and generates the next carbon-centred radical (L*), thereby propagating the chain reaction. The lipid hydroperoxide (LOOH) is readily decomposed, either by ferrous iron (Fe²⁺) to the lipid alkoxyl radical (LO*) or by ferric iron (Fe³⁺) back to the lipid peroxyl radical (LOO*). Both radicals can fuel the chain reaction. The reaction is terminated when two radicals meet each other, e.g., forming a lipid dimer (L-L) or a peroxide-bridged lipid dimer (LOOL) (not shown), or when a radical meets a radical trapping agent, for instance, coenzyme Q10 or alpha-tocopherol (vitamin E), creating a lipid hydroperoxide (LOOH). Right: Alternatively, lipid peroxidation is carried out in a controlled manner by lipoxygenases (LOX). Although initiating the process at the level of lipid hydroperoxide formation, the lipid hydroperoxide will be decomposed by ferrous or ferric iron to lipid alkoxyl and peroxyl radicals, respectively, and the vicious circle of radical formation will be fuelled as described above. Hence, the initial enzymatic reaction is switched in a second step to a radical process [29].

Figure 2

Mechanism of lipid peroxidation. Hypothetical model of phospholipid peroxidation leading to ferroptosis. Phospholipid peroxidation may be triggered either by radical formation due to reactive oxygen species (ROS) derived from cytochrome P450 oxidoreductase (POR), the electron transport chain (ETC), or NADPH oxidase (NOX), or enzymatically by 15-lipoxygenase (15-LOX). Association of 15-LOX with PEPB1 alters its substrate specificity towards membrane-bound PUFAs. Reduction in GSH by any means will increase the redox tone of the cell and thereby activate any LOX isoform. While 15-LOX can directly dioxygenate membrane-bound PUFAs, other isoforms indirectly cause accumulation of phospholipid hydroperoxides (i) by their primary reaction products, which activate 15-LOX, and (ii) by decreasing GSH levels, thereby attenuating GPX4 activity.

Figure 3

Examples of ferroptosis inducers (FINs). Class I FINs act by decreasing GSH levels. Class II FINs directly inhibit GPX4. Class III FINs indirectly block GPX4 activity. Class IV FINs increase the labile iron pool (LIP). See text for details. SQS: squalene synthase, LIP: labile iron pool, LPO: lipid peroxidation, PP: pyrophosphate.

Figure 4

Simplified scheme to present principles of EMT. The core transcription factors inducing EMT are SNAI1/SNAI2, TWIST, and ZEB. They reduce expression of epithelial markers, such as E-cadherin, and induce expression of mesenchymal markers, such as N-cadherin. They are regulated by upstream acting transcription factors, which also support proliferation, survival, and EMT by their own gene expression. (Note: CREB is representative for all transcription factors activated by the MAPK cascade). E-cadherin inhibits EMT by recruiting β-catenin (β), sequestering YAP/TAZ, and by inhibiting activity of receptor tyrosine kinases (RTKs), which, e.g., activate the mitogen-activated protein kinase (MAPK) cascade.

Figure 5

Cell–cell contacts turn on the Hippo pathway. E-cadherin inactivates p21-activated kinase (PAK), thereby blocking phosphorylation and inactivation of merlin (Mer). Active merlin associates with KIBRA and activates LATS1/2, which phosphorylates YAP/TAZ. This leads to cytosolic sequestration and degradation of YAP/TAZ. In addition, merlin inhibits activity of the E3 ubiquitin ligase CRL4^DCAF1 thereby blocking degradation of LATS1/2. YAP/TAZ are also directly sequestered to adherens junctions (AJ) by α-catenin (α), which is associated via β-catenin (β) with E-cadherin. Moreover, YAP/TAZ are bound to tight junctions (TJ) via angiomotin (AMOT) and sequestered in the cytoplasm by the protein tyrosine phosphatase nonreceptor (PTPN)14.

Figure 6

Regulation of ferroptosis by E-cadherin and EMT. Left: E-cadherin activates the Hippo pathway, thereby inactivating YAP- and TAZ-activity. This leads directly or indirectly to decreased expression of proteins that are relevant for ferroptosis (note that decreased levels of NOX4 and NOX2 are due to reduced expression of EMP1 and ANGPTL4, respectively, see text for details). However, involvement of additional genes and/or signalling pathways is likely. Ferroptosis is blocked. Right: EMT—for instance, triggered by activation of ZEB—leads to a loss of E-cadherin expression and inhibition of the Hippo pathway. Although the downstream targets have not been elucidated so far, YAP/TAZ activation supports ferroptosis. Additional pathways remain to be elucidated. However, this can be counterbalanced by phosphorylation of AKT (or, possibly, other mechanisms). Activated AKT may lead to a decreased level of ACSL4 as well as increased formation of MUFAs and CoQ10 via mTORC1 stimulation. Ferroptosis is prevented.