3.1.1. Ferroptosis in LUAD tumorigenesis
About 25-30% of lung adenocarcinomas develop KRAS mutations, with the KRAS-G12C point mutation being the most common[
6,
26]. When G12C mutation occurs in
KRAS, its GTP hydrolase activity will be lost, which will activate several signaling pathways such as RAF-MEK-ERK, PI3K-AKT-mTOR and Ral-GDS, and then the cells will have malignant potential and gradually become cancerous, which will eventually lead to the development of LUAD[
27]. It is established that KRAS-mediated cellular transformation requires the generation of reactive oxygen species (ROS) due to elevated expression of NADPH oxidase 1(Nox1)[
28]. So how do cells expressing mutant KRAS alleviate ROS-induced cell death? That is, FSP1 expression was sufficient in KRAS-mutated LUAD to significantly promote 3D spheroid growth
in vitro and accelerate tumor onset
in vivo by protecting from ferroptosis[
25]. Consistent with this finding, Zang
et al showed that KRAS-driven LUAD has a greater resistance to ferroptosis owing to a reprogrammed lipid metabolism by a higher level of acyl-coenzyme A synthetase long-chain family member 3 (ACSL3) expression[
29]. Due to the small size and smooth surface of the KRAS protein, there is no suitable binding pocket for other small molecules except the one for GTP/GDP, and the affinity of RAS for GTP is very high, which makes it difficult for drugs to compete with the substrate[
7]. The treatment of KRAS-mutated LUAD has been a challenge. Intervention of FSP1 and expression in the early stage of atypical adenomatoid hyperplasia (AAH) to adenocarcinoma in situ (AIS) or to micro infiltrating adenocarcinoma (MIA) and thus intervention in ferroptosis tumor initiation, may be a novel therapeutic direction for KRAS-mutated LUAD.
3.1.2. Ferroptosis in LUAD progression
Ferroptosis is also linked to the growth and development of LUAD. Researches found many ferroptosis inhibitors act as indicators of poor prognosis, promotes tumor cells growth in LUAD, such as CCT3 and NFS1 [
30,
31]. In addition, bioinformatics analyses have revealed a potential clinical connection between the ferroptosis-related genes (FRGs) and LUAD patients. A prognostic model integrating several FRGs was used to predict the prognosis, mutation burden, TME cell infiltration characteristics and immunotherapy effects, chemotherapy sensitivities in patients with LUAD[
32,
33,
34,
35,
36,
37].
Epithelial-mesenchymal transition (EMT) and ferroptosis are two important processes in tumor progression and recent studies reported that they may form a positive feedback loop to a certain extent in LUAD. On the one hand, it has been known that highly mesenchymal-like tumor cells are indeed more sensitive to ferroptosis inducers. The E3 Ligase MIB1 can promote EMT, while at the same time, promote ferroptosis by NRF2 degradation[
38]. ZEB1 is one of the major transcription factors that regulate EMT by binding to the E-box in E-cadherin[
39]. In LUAD, ZEB1 correlates with the transcription of LPCAT3 and thus increases the susceptibility to ferroptosis[
19]. On the other hand, ferroptosis tendency also enhances the development of EMT. Erastin, which is an inducer of ferroptosis, can reduces the expression of E-cadherin and causes de-epithelialization. While Ferrostatin-1(Fer-1), an inhibitor of ferroptosis, can partially inhibit EMT induced by TGF-β1[
40]. Not coincidentally, some ferroptosis markers (GPX4, SCP2, CAV1) again suggest that ferroptosis can positively regulate the occurrence of EMT
in vivo [
19]. However, there is also negative feedback loop between EMT and ferroptosis. High ARNTL2 expression was associated with EMT and lymph node metastasis in patients with LUAD, while it plays an inhibiting role in ferroptosis[
41].
Epidermal growth factor receptor (EGFR) is the most common mutation in LUAD, with a prevalence of 15% of Caucasians and 50% of Asians[
42,
43]. EGFR tyrosine kinase inhibitors (EGFR-TKIs) are used to treat EGFR mutant LUAD, first- to third-generation EGFR-TKIs have been approved both domestically and internationally, and fourth-generation EGFR-TKIs such as BLU-945 have entered clinical studies[
44,
45]. EGFR-mutant LUAD cells also increase cellular sensitivity to ferroptosis. Low-dose selenite synergized with osimertinib in EGFR-mutant H1975[
46]. Acquired resistance is still inevitable with the use of EGFR-TKIs, and about 20%-30% of EGFR mutant LUADs are intrinsically resistant to EGFR-TKIs[
47,
48,
49]. In addition to mesenchymal state and EGFR-mutant tumor cells mentioned above, EGFR-TKI resistant LUAD cells also increase cellular sensitivity to ferroptosis, and the histone deacetylase inhibitor Vorinostat can further downregulate the expression of xCT in EGFR mutant LUAD cells and enhance the effect of ferroptosis induction therapy[
50]. Further work on the clinical effect of these drugs and their combination with TKI on EGFR mutation-LUAD are warranted. Besides, chemoresistance also makes LUAD cells more sensitive to ferroptosis, and promoting ferroptosis can overcome or reverse the resistance of tumor cells to cisplatin, pemetrexed and Lapatinib[
51,
52,
53].
The tumor microenvironment constitutes the balance between tumor cells and immune cells and can have both adverse and beneficial consequences during tumor progression. Ribonucleotide reductase subunit M2 (RRM2) not only affects tumor cells proliferation but also regulates immune cells infiltration, thereby influence lung cancer progression in a ferroptosis-dependent manner. For one thing, depletion or silencing RRM2 inhibited the proliferation and induce ferroptosis in H1975 and H358 LUAD cells[
54,
55]. For another, RRM2 effectively promoted M2 macrophage polarization, facilitating tissue repair and LUAD development
in vitro and
in vivo [
56]. RRM2 also regulated the infiltration levels of activated mast cells and activated CD4 memory T cells, again suggesting that RRM2 may be engaged in immune infiltration[
54]. Not coincidentally, Bioinformatics has demonstrated that others FRGs can modify the behavior of TME cells, and that these subtypes of TME cells exhibited distinct biological features and communicate extensively with tumor epithelial cells. Patients with a higher abundance of these ferroptosis-related TME cell subtypes have a better clinical outcome[
53]. In addition, Ferroptosis-related prognostic signatures, such as prognostic ferroptosis-related lncRNA signature and GPX4-related prognostic signature, are not only correlated with multiple tumor-infiltrating immune cells and immune-associated processes and pathways in TME, but also with the response to immunotherapy, chemotherapy, and targeted therapy[
57,
58].
Figure 1.
Ferroptosis in LUAD tumorigenesis and progression. RRM2, ribonucleotide reductase subunit M2;CCT3, chaperonin containing TCP1 subunit 3;NFS1, nitrogen fixation 1;ZEB1, zinc finger E-box binding homeobox 1;Fer-1, ferrostatin-1;TKI, tyrosine kinase inhibitor.
Figure 1.
Ferroptosis in LUAD tumorigenesis and progression. RRM2, ribonucleotide reductase subunit M2;CCT3, chaperonin containing TCP1 subunit 3;NFS1, nitrogen fixation 1;ZEB1, zinc finger E-box binding homeobox 1;Fer-1, ferrostatin-1;TKI, tyrosine kinase inhibitor.