Identification, genetic variations and expression profiles of the FRGs
Cytotoxic T cell-driven immunity promotes ferroptosis in cancer cells. For example, the anti-PD-L1 antibody induces lipid peroxidation-dependent ferroptosis in tumor cells (Fig. 1A), making ferroptosis-related anti-cancer therapy a promising target [24]. In this study, we aimed to explore the role of ferroptosis in molecular subtype identification of TNBC and construct a FerrScore model to screen promising drugs and predict response to immunotherapy (Supplementary Fig. 1). Initially, we identified 125 ferroptosis-related genes (FRGs) from the FerrDb, and intersected them with 841 survival-related genes screened by Kaplan-Meier survival analysis (Fig. 1B, Supplementary Fig. 2A-J, Supplementary table 1). In the FUSCC TNBC cohort, EMC2, FTH1, HMOX1, LPCAT3, NOX4, and SOCS1 were up-regulated in tumors, while BAP1 and ISCU were down-regulated (Fig. 1C). Eight hub genes were identified, while ALOX15B was excluded due to no difference between normal and tumor samples. The expression of the eight FRGs distinguished tumors from normal tissues as shown by principal component analysis (PCA) (Fig. 1D). Additionally, the expression of the eight FRGs varied across different grades, T and N stages of TNBC (Supplementary Fig. 3). The coding sequence (CDS) length and chromosomal location of the eight FRGs were obtained from NCBI (Supplementary table 2). In pan-cancer analysis, ISCU was significantly down-regulated in breast invasive carcinoma, and EMC2 showed the highest frequency of copy number variation (CNV) (Supplementary Fig. 4A-B). NOX4 exhibited the highest frequency of methylation, and the expression of SOCS1 positively correlated with promoter methylation across 20 types of cancer in TCGA (Supplementary Fig. 4C-D).
The protein-protein interaction (PPI) network revealed that HMOX1 interacted with FTH1 and NOX4, and the correlation of the eight FRGs was visualized in the network plot (Fig. 1E-G). Somatic mutation analysis of the eight FRGs showed that 6 (4.41%) of 136 samples had genetic variations, and EMC2 had a significantly high frequency (32.37%) of copy number gain in the TCGA cohort (Fig. 1H-J, Supplementary table 2). In the GSE118389 single-cell RNA-seq dataset, SOCS1 was primarily expressed in T cells, while HMOX1 was predominantly expressed in macrophages at the single-cell level (Fig. 1K-T), which was further validated in four additional datasets in TISCH (Supplementary Fig. 5A-H). At the protein level, FTH1, ISCU, and SOCS1 showed moderate staining in breast cancer tissues on the Human Protein Atlas portal (HPA) (Supplementary Fig. 5I). These results strongly indicated that dysregulation and imbalance of the FRGs contributed to the progression and malignancy of TNBC.
Ferroptosis-related subtypes identification and pathway enrichment analysis
We performed consensus clustering to identify ferroptosis-related molecular subtypes based on the mRNA expression of the eight FRGs in the FUSCC TNBC cohort. This analysis divided the patients into two distinct groups (K = 2) with high and low intergroup correlations (Fig. 2A-E). PCA analysis demonstrated that the distinct subgroups could be differentiated based on the expression of the eight FRGs (Fig. 2F). Kaplan-Meier analysis revealed that patients in cluster 1 had better overall survival (OS) compared to those in cluster 2 (Fig. 2G). Tumors in cluster 2 were primarily composed of immunomodulatory (IM) and BLIS mRNA subtypes, which were associated with a younger age distribution (Supplementary Fig. 6A-C, Supplementary table 3). Research from MD Anderson Cancer Center has suggested that the BLIS mRNA subtype has the worst outcome [11]. The expression of LPCAT3 and NOX4 was higher in cluster 2, while EMC2 showed no difference, and the remaining FRGs exhibited an opposite profile (Fig. 2H-I). Univariate Cox regression analysis indicated that cluster 2 was a risk factor (HR = 2.47) associated with worse OS (Fig. 2J). Consistent with this, validation using the NMF Clustering algorithm showed that patients in the FUSCC TNBC cohort could also be divided into two distinct subgroups with significantly different OS probabilities (Supplementary Fig. 7A-C). Similarly, unsupervised clustering analysis in the GSE76124 and GSE21653 datasets clearly divided TNBC patients into two groups with different OS, and PCA revealed the distribution of FRG-related subtypes (Supplementary Fig. 7D-K).
To further explore the involved Gene Ontology (GO) processes and enriched pathways contributing to the progression of the distinct subgroups, we identified differentially expressed genes (DEGs) using a fold change cutoff of 1.5, as illustrated in the volcano plot (Fig. 2K). GO analysis of the up-regulated DEGs showed a significant enrichment of iron ion binding molecular function (MF) in cluster 2, while the down-regulated DEGs were predominantly enriched in T cell activation and leukocyte proliferation biological processes (BP) (Fig. 3A-B). Collectively, tumors in cluster 2, which exhibited worse OS, showed an increased iron ion overload and decreased T cell activation, strongly suggesting their increased sensitivity to ferroptosis. Furthermore, using Gene Set Variation Analysis (GSVA), we investigated the impact of tumor cell ferroptosis on the reprogramming of the tumor microenvironment (TME). Tumors in cluster 2 exhibited higher enrichment scores for programmed cell death, metal ion SLC transporters, drug metabolism cytochrome p450, and epithelial-mesenchymal transition (EMT) pathways (Supplementary Fig. 8). Previous studies have confirmed that tumor cells in a high-mesenchymal state are more sensitive to ferroptosis due to enhanced iron endocytosis during EMT [15, 25]. Additionally, tumors in cluster 2 showed lower enrichment scores for immune-related pathways, such as natural killer cell-mediated cytotoxicity, leukocyte transendothelial migration, and T cell and B cell receptor signaling pathways (Fig. 3D-E, Supplementary table 4). Overall, tumors in cluster 2 represented a subgroup of cancers that were more sensitive to ferroptosis, exhibited an immunosuppressive phenotype, and had worse OS compared to those in cluster 1.
Immune landscape characterization within ferroptosis-related subtypes
Ferroptotic tumor cells release damage-associated molecular patterns (DAMPs) that can impact the infiltration, differentiation, and function of immune cells in the tumor microenvironment (TME), thereby influencing tumor growth [18]. To comprehensively illustrate the immune signature of the two ferroptosis-related subtypes and the role of ferroptosis in TME remodeling, we performed a comprehensive analysis. We observed marked differences in the expression of chemokines, interleukins, interferons, and receptors between the two distinct subgroups (Fig. 4A). Tumors in cluster 2 exhibited lower immune score, stromal score, ESTIMATE score, dysfunction score, and TIS score, as well as higher tumor purity, TIDE score, and exclusion score (Fig. 4B-I). Furthermore, by employing Cibersort, we inferred the immune infiltration in tumors and found lower abundance of resting NK cells and activated CD4 memory T cells in cluster 2 (Fig. 4J). This observation may be attributed to the low expression of SOCS1, which showed a positive correlation with the number of resting NK cells based on Spearman correlation analysis (Fig. 4K). Additionally, the network plot illustrated the complex communications among immune cells in the TME of TNBC (Fig. 4L). Finally, tumors in cluster 2 exhibited lower expression of the major histocompatibility complex (MHC) complex and higher expression of immune checkpoints (Fig. 4M-N). In summary, tumors in cluster 2 were more likely to display a cold tumor phenotype characterized by reduced immune infiltration and increased expression of immune checkpoints, thus establishing an immunosuppressive TME.
Construction of the FerrScore model in TNBC
We developed the FerrScore model based on the expression of eight FRGs to investigate how ferroptosis affects the tumor phenotype and TME remodeling, and to assess the predictive accuracy of the FRGs signature in chemotherapy and immunotherapy response in TNBC. By applying a cutoff value of 0.03, we identified high and low FerrScore groups with significantly different overall survival (OS), and the FerrScore model achieved an area under the curve (AUC) of 0.70 for 3-year survival prediction (Fig. 5A-C, Supplementary table 5). The two groups differed in terms of age distribution, mRNA type, intrinsic subtype, paclitaxel treatment status, tumor grade, and relapse-free survival (RFS) status of patients (Supplementary table 6). Among the two distinct TNBC subtypes, cluster 2 exhibited significantly higher FerrScore than cluster 1, and patients in cluster 2 were classified into the high FerrScore group with worse OS (Fig. 5D-E). Moreover, the high FerrScore group had a higher proportion of luminal androgen receptor (LAR) mRNA subtype and a lower proportion of basal intrinsic subtype (Fig. 5F). Among the eight FRGs, the expression of EMC2 positively correlated with tumor FerrScore, whereas HMOX1 showed a distinct negative correlation (Fig. 5G). Additionally, the expression of LPCAT3, EMC2, and NOX4 was higher in the high FerrScore group, while the other five FRGs were down-regulated in this group (Fig. 5H).
We further investigated the immune signature within the high and low FerrScore groups to examine the impact of FerrScore on TME reprogramming. Interestingly, FerrScore displayed a negative correlation with the expression of MHC complex molecules and immune checkpoints (Fig. 5I-J). Additionally, FerrScore exhibited a positive correlation with metal ion SLC transporters, tumor escape from immune attack, and PD-1 signaling pathways, while showing a negative correlation with T cell and B cell receptor signaling pathways (Fig. 5K). Furthermore, FerrScore was positively associated with stromal score, TIDE score, and exclusion score, while negatively correlated with TIS score and immune score (Fig. 5L). We inferred the immune signatures between the two groups using Cibersort, XCELL, ssGSEA, TIMER, and MCP counter algorithms, and found that tumors with low FerrScore had a higher abundance of immune cells in the TME (Supplementary Fig. 9). Overall, FerrScore served as an effective indicator to determine the immune phenotype of TNBC, which provides a solid foundation for its application in screening promising candidate drugs and predicting response to immune checkpoint inhibitor therapy.
A. Classification of patients into high and low FerrScore groups using the "ggrisk" R package with a cut-off value of 0.03. B. Kaplan-Meier curve demonstrating worse overall survival (OS) in patients with high FerrScore. C. ROC curve illustrating the specificity and sensitivity of FerrScore in predicting 1-, 3-, and 5-year survival of TNBC patients. D. Variations in FerrScore between the two distinct clusters; Wilcoxon test. E. Sankey diagram displaying the correlations among clusters, FerrScore, recurrence-free survival (RFS) status, intrinsic subtype, and mRNA subtype using the "ggalluvial" R package. F. Heatmap showing the correlation between FerrScore and CNA subtype, iCluster subtype, intrinsic subtype, mRNA subtype, mutation subtype, and SNF subtype in TNBC patients. G. Pearson correlation analysis between the expression of the eight FRGs and FerrScore. H. mRNA expression levels of the eight FRGs in the high and low FerrScore groups; Student’s t-test. I-J. Comparison of the expression levels of MHC complexes and immune checkpoints between the high and low FerrScore groups; Student’s t-test, *p < 0.05. K-L. Pearson correlation analysis between FerrScore and immune-related signaling pathways and scores.
Screening Candidate Drugs for Chemotherapy Guided by FerrScore
Large amounts of preclinical evidence have revealed the potential of inducing ferroptosis in tumors to prevent acquired chemotherapy resistance and enhance the efficacy of traditional cancer therapies. Synergistic effects have been observed between ferroptosis inducers and conventional drugs in suppressing tumor growth in mouse models of head and neck cancer [26]. We initially applied the FerrScore scoring model to human breast cancer cell lines in the CCLE database, which also revealed the presence of two distinct clusters (Supplementary Fig. 10A). Cluster 2 tumors exhibited higher sensitivity to NUTLIN-3A(-), AFATINIB, and AZD8055, albeit with lower AUC values, while cluster 1 tumors demonstrated greater sensitivity to KU-55933, MASITINIB, PAC-1, AZD7762, BI-2536, and PELITNIB (Supplementary Fig. 10B-J).
To further investigate the potential role of FerrScore in predicting chemotherapy response, we utilized the FerrScore model to screen candidate compounds that could guide chemotherapy for patients with TNBC. Firstly, we applied the FerrScore model to the CTRP and PRISM datasets, resulting in the identification of five CTRP-derived drugs (BRD5468, ZSTK474, AZD6482, Sildenafil, and Canertinib) and five PRISM-derived drugs (Temsirolimus, Idasanutlin, Everolimus, CGM097, and Nutlin-3) for tumors with high FerrScore, using Pearson correlation analysis (Fig. 6A-D, Supplementary table 7). Secondly, we calculated the fold changes of the target genes of the candidate agents, with higher fold changes indicating greater potential for candidate drugs. Thirdly, to determine the most promising agents for high FerrScore tumors, we calculated the CMap score by submitting the 300 differentially expressed genes (DEGs) with the most significant fold changes to the CMap website. Notably, Everolimus exhibited the highest CMap score of -90.58 (Fig. 6E-F). Finally, we conducted a search to explore published experimental evidence and the clinical trial status of the candidate drugs, which revealed that Everolimus has been recognized as a therapeutic agent for advanced breast cancer (Supplementary table 7), further affirming its status as the most promising compound for patients with higher FerrScore.
FerrScore-based prediction of ICIs therapy to optimize immunotherapy
ICIs therapy has revolutionized the clinical treatment of cancer patients. It has been demonstrated that anti-PD-L1 antibodies act synergistically with ferroptosis activators, such as erastin, to suppress tumor cell growth [24]. The release of DAMPs from ferroptotic tumor cells alters the TME and influences the response to ICIs therapy. In light of this, we utilized the FerrScore scoring model to predict the benefits of anti-PD-L1, anti-PD-1, and anti-PD-1&CTLA-4 ICIs immunotherapy, as well as adoptive T cell therapy.
Initially, we examined an immunotherapy cohort (IMvigor210) of urothelial carcinoma treated with anti-PD-L1. Patients were divided into high and low FerrScore groups, with those in the high FerrScore group exhibiting worse overall survival (OS) (Fig. 7A). Notably, patients with low FerrScore were more likely to benefit from anti-PD-L1 therapy (Fig. 7B-C). Furthermore, tumors with lower FerrScore demonstrated higher tumor mutational burden (TMB) and tumor neoantigen burden (TNB) (Fig. 7D-E). Increased TMB leads to a higher TNB, enhancing the chances of T cell recognition and clinically correlating with improved ICIs therapy outcomes [27]. The FerrScore model achieved an area under the curve (AUC) value of 0.56 for predicting anti-PD-L1 ICIs benefits in the IMvigor210 cohort (Fig. 7F).
Next, we applied the FerrScore model to an anti-PD-1 therapy cohort (GSE78220) comprising melanoma patients. Patients with high FerrScore had worse OS (Fig. 7G). Patients with low FerrScore displayed a more favorable response to anti-PD-L1 therapy, and the AUC value of this model for predicting therapy response was 0.74 (Fig. 7H-J). Subsequently, we divided patients with melanoma (PRJEB23709) undergoing anti-PD-1&CTLA-4 therapy into high and low FerrScore groups. Patients with low FerrScore exhibited better OS and derived greater benefits from anti-PD-1&CTLA-4 therapy (Fig. 7K-M). The AUC value of the FerrScore model to predict anti-PD-1&CTLA-4 therapy benefits was 0.61 (Fig. 7N). Lastly, we classified patients with melanoma (GSE100797) undergoing adoptive T cell therapy into high and low FerrScore groups but observed no difference in OS or therapy benefits between the two groups (Fig. 7O-Q). Overall, FerrScore emerges as a promising biomarker for predicting the benefits of ICIs therapy, facilitating the optimization of therapy options for patients.
SOCS1 and NOX4 are associated with immunity to TNBC
Our primary focus was to delve into the role of the FRGs signature in reprogramming the tumor microenvironment (TME). To accomplish this, we conducted a detailed examination of SOCS1 and NOX4, based on the above analysis. Pearson correlation analysis revealed a strong positive correlation between the expression of SOCS1 and the mRNA levels of PDCD1 (PD-1), CD274 (PD-L1), CTLA-4, LAG-3, and HAVCR2 (TIM3). Conversely, the expression of NOX4 showed a negative correlation with the mRNA levels of PDCD1, IDO1, and LAG3 (Fig. 8A-H). Given these findings, PD-1 was selected as the key marker to explore the relationship between the two FRGs and the shaping of immune phenotypes. We categorized TNBC samples into cold and hot immune phenotypes based on CD3 IHC staining and examined the expression of SOCS1 and NOX4 across the two tumor subtypes (Fig. 8I). Our analysis revealed that tumors with high expression of NOX4 and low expression of SOCS1 were characterized as cold tumors, while tumors with high expression of SOCS1 and low expression of NOX4 exhibited phenotypes associated with hot tumors (Fig. 8I-J). These results validate the active involvement of SOCS1 and NOX4 in TME infiltration and remodeling in TNBC.