The interaction between ferroptosis and inflammatory signaling pathways

Abstract

Ferroptosis is an iron-dependent regulated cell death driven by excessive lipid peroxidation. Inflammation is one common and effective physiological event that protects against various stimuli to maintain tissue homeostasis. However, the dysregulation of inflammatory responses can cause imbalance of the immune system, cell dysfunction and death. Recent studies have pointed out that activation of inflammation, including the activation of multiple inflammation-related signaling pathways, can lead to ferroptosis. Among the related signal transduction pathways, we focused on five classical inflammatory pathways, namely, the JAK-STAT, NF-κB, inflammasome, cGAS-STING and MAPK signaling pathways, and expounded on their roles in ferroptosis. To date, many agents have shown therapeutic effects on ferroptosis-related diseases by modulating the aforementioned pathways in vivo and in vitro. Moreover, the regulatory effects of these pathways on iron metabolism and lipid peroxidation have been described in detail, contributing to further understanding of the pathophysiological process of ferroptosis. Taken together, targeting these pathways related to inflammation will provide appropriate ways to intervene ferroptosis and diseases.

Fig. 1. The mechanisms of ferroptosis.

Iron-mediated lipid peroxidation is the core process in ferroptosis. In brief, TF and LCN2 carry extracellular iron and transfer it into cell via their corresponding receptors, while intracellular iron is mainly exported by ferroportin to maintain iron homeostasis. In the cells, HMOX1 degrades heme to release free iron, whereas the iron storage protein ferritin limits intracellular iron utilization to prevent iron overload and ferroptosis. As a catalyst, ferrous iron converts peroxides into free radicals such as hydroxyl and hydroperoxyl radicals, via the Fenton reaction, which causes excessive lipid peroxidation and ultimately triggers ferroptosis. During this process, the suppression of redox systems is an essential factor in ferroptosis. At present, four antioxidant pathways have been verified to be associated with ferroptosis: The system Xc^--GPX4, FSP1-CoQ₁₀, GCH1-BH₄, and DHODH-CoQ₁₀ pathways. The antiporter System Xc^- containing SLC7A11 and SLC3A2 mediates the uptake of cystine, which is consumed during the synthesis of intracellular GSH. Next, the antioxidant enzyme GPX4 reduces lipid hydroperoxides into lipid alcohols via GSH to protect cells from ferroptosis. Thus, the inhibition of system Xc^--GPX4 by erastin, RSL3, ML162 or FIN56 induces ferroptosis, and therefore, these compounds have been applied to tumor therapy in animal experiments. Furthermore, FSP1, GCH1, DHFR, and DHODH can drive the production of antioxidants, such as BH₄ and CoQ₁₀H₂ to defend against oxidative stress and ferroptotic cell death. Together, the joint dysregulation of iron metabolism and redox systems leads to the accumulation of intracellular lipid hydroperoxides, ultimately causing ferroptosis.

Fig. 2. Ferroptosis is a type of autophagy-dependent cell death.

NCOA4-mediated ferritinophagy induces ferritin degradation and iron overload, finally promoting oxidative injury and ferroptosis. E3 ubiquitin ligase HERC2 drives the proteasomal degradation of NCOA4 and enhances the stabilization of ferritin, which decreases intracellular levels of iron. RAB7A-mediated lipophagy and SQSTM1-mediated clockophagy both contributes to lipid peroxidation in ferroptosis. Erastin not only increases the expression of Lamp-2a, which promotes HSP90-mediated chaperone-mediated autophagy to degrade GPX4, but also promotes the combination of BECN1 and SLC7A11 to block System Xc- activity via AMPK phosphorylation.

Fig. 3. The role of JAK-STAT signaling pathway in ferroptosis.

Cytokines, such as IL-6 and TNF bind to their corresponding receptors and thus induce the phosphorylation and activation of JAKs. In turn, activated JAKs phosphorylate STATs and induce their dimerization and nuclear translocation. Finally, STAT dimers bind to DNA sequences to induce the transcription of target genes and the activation of downstream signaling pathways. As a downstream target of the JAK-STAT pathway, phosphorylated STAT3 can increase hepcidin expression to inhibit iron export, resulting in ferroptosis. Moreover, activated STAT1 inhibits System Xc^- and exerts proferroptosis effects by via modulating IRF1.

Fig. 4. The role of NF-κB signaling pathway in ferroptosis.

The classical signaling pathway is activated by Toll‑like receptor ligands (such as LPS), TNF, IL-1, and other stimuli. Under resting conditions, IκBα combines with NF-κB dimers to sequester NF-κB activity. These signaling molecules combine with their corresponding receptors and mediate the phosphorylation and subsequent degradation of IκBα. Then, the liberated NF-κB dimers are translocated to the nucleus and regulate target gene transcription. On the one hand, NF-κB can reduce the transcription of antioxidant molecules, such as GPX4, NQO1, and HMOX1, indicating the contribution of the NF-κB pathway to oxidative stress. On the other hand, the loss of LIFR enhances IκBα ubiquitination degradation and positively regulates NF-κB activation, which further promotes LCN2 secretion to sequester extracellular iron. In addition, a few agents, including BRD4770 and BAY 11-7082, exert their anti-ferroptosis effects via inhibiting NF-κB pathway.

Fig. 5. The role of inflammasome signaling pathway in ferroptosis.

Membrane receptors sense inflammatory signals and, in turn, activate the NF-κB signaling pathway to induce NLRP3 and IL-1β transcription. NLRP3 recruits ASC and pro-caspase 1 to trigger the assembly of the NLRP3 inflammasome, which mediates the self-cleavage of pro-caspase 1. Activated caspase 1 further cleaves pro-IL-1β and pro-IL-18, which leads to the maturation of these proinflammatory cytokines. In parallel, gasdermin D is cleaved by caspase 1, and its N-terminal domain is transferred to plasma membrane, where it forms pores, mediating the release of mature IL-1β and IL-18 and causing cell lysis (pyroptosis). During this process, iron drives NLRP3 inflammasome formation via cGAS-STING pathway, whereas GPX4 blocks GSDMD cleavage to inhibit the inflammasome pathway. In addition, lipid peroxidation induced by octanal contributes to the production of the NLRP3 inflammasome, but 4-HNE binds to NLRP3 and thereby hinders its interaction with NEK7, suppressing inflammasome activation.