Introduction
Ovarian cancer (OV) is the second most common gynecological disease and the most fatal gynecological malignant tumor worldwide, thus seriously threatening women’s safety and health [
1]. As reported, there were 19,880 new cases and 12,810 deaths related to OV, estimated for 2022 in the United States [
2]. Owing to the lack of specific early symptoms, over 70% OV cases were first diagnosed at late period, which led to a poor 5-year overall survival rate of 35% [
3,
4]. After the initial therapy of surgery combined with platinum-based chemotherapy, approximately 80% OV patients finally suffer recurrence and progression [
5]. Accordingly, to improve survival, identifying a promising prognostic signature is of great urgency.
During the past decades, cell death, one of the most fundamental issues for life sciences, has been defined as a hallmark of cancer [
6]. Recently, increasing researches have been focused on pyroptosis, a newly-acknowledged inflammatory form of cell death [
7]. Pyroptosis was usually caused by certain inflammasomes, which could lead to the cleavage of Gasdermin D (GSDMD) and maturation of pro-inflammatory cytokines, such as interleukin-18 (IL-18) and interleukin-1β (IL-1β) [
8]. With the deepening of studies, the essential role of pyroptosis has been proved in various aspects, including tumor origin, tumor progression, and therapy-resistance, etc. [
9]. As for OV and pyroptosis, Berkel and colleagues pointed out that the expression of GSDMD and GSDMC was up-regulated, whereas GSDME was downregulated in OV tissue, which was associated with poor prognosis [
10]. In this regard, it is of great importance to explore the underlying mechanisms of pyroptosis-related genes (PRGs) in the process of OV progression, which has guiding significance in the treatment and prevention of cancer [
11].
Recently, immunotherapy has become a hotspot in OV studies, though the effective rate for immunotherapy in OV is still limited [
12,
13]. Up till now, emerging evidence has suggested the crosstalk between tumor immune microenvironment and pyroptosis [
14]. Most researches focused on only one or two pyroptosis-related genes (PRGs) and several cell types in the microenvironment, however, the tumor progression process is characterized by numerous genes and cell types interacting in a high-coordinated manner, which haven’t been fully understood yet [
15,
16]. Hence, the in-depth mechanisms of pyroptosis along with the tumor immune microenvironment in OV progression could be instrumental in developing efficacious immunotherapy to overcome drug resistance [
17].
Accordingly, in our research, we comprehensively evaluated the importance of PRGs in OV, and filtered 6 PRGs to build a prognostic signature. Moreover, we assessed the difference of methylation N6 adenosine (m6A) level, tumor immune microenvironment, and sensitivity towards chemotherapy/immunotherapy between risk groups classified via the pyroptosis-related signature.
Discussion
OV is the most fatal gynecological malignant tumors worldwide, mainly due to inefficient biomarkers and high recurrence rates [
1,
5]. Therefore, identifying a promising prognostic signature is of great urgency to improve OV survival. Latterly, pyroptosis, a newly-discovered inflammatory form of cell death caused by certain inflammasomes, has been demonstrated to play vital roles in the regulation of tumor progression, thus be considered a potential strategy for tumor treatment [
8,
33]. As for OV, previous studies indicated that regulation of PRGs, including HOTTIP [
34], α-NETA [
35], and LncRNA GAS5 [
36] in tumor cells could promote pyroptosis by inducing inflammasome formation, in order to inhibit OV progression, which could be used as a potential target for tumor therapy [
7,
37]. Therefore, in this study, we aimed to identify a pyroptosis-related signature and evaluated prognostic potential, tumor immune microenvironment, and sensitivity to treatments related to ferroptosis patterns.
Recently, few current studies focused on pyroptosis, especially on its mechanism in OC. Zhou and colleagues constructed and validated a pyroptosis-related 8-gene signature (including CD44, EPB41L3, FCN1, IRF4, ISG20, LYN, SLC31A2, and VSIG4), which could be used to predict OV prognosis [
38]. However, the study only included 25 PRGs for signature identification, which could limit the accuracy and integrality of the research. Another research from Ye and colleagues defined another prognostic signature, which consisted of 8 PRGs including AIM2, CASP3, CASP6, ELANE, GSDMA, PLCG1, and PJVK, though with limited ROC-AUC for 1-year, 2-year, and 3-year OS prediction of 0.628, 0.662, and 0.607, respectively [
11]. Up till now, none of the pyroptosis-related prognostic signatures have been standardized and applied to OV clinical practice yet, which might be caused by the limited prognosis value. Accordingly, in our study, we aimed to identify a satisfactory pyroptosis-related signature from 106 potential PRGs obtained from the Genecards database. Through integrative analysis, we distinguished a 6-gene signature (CITED2, EXOC6B, MIA2, NRAS, SETBP1, and TRPV4), which had a promising prognostic value among both training cohorts (TCGA-OV, p-value < 0.0001) and validation cohort (ICGC-OV, p-value = 0.0002). To the best of our knowledge, this is the first study identifying the 6-gene pyroptosis-related OV signature with satisfactory prognostic value, in order to guide clinical decision-making for OV patients.
Among the 6 identified PRGs, only NRAS and SETBP1 have definite functions reported in OV progression. Dariush and colleagues demonstrated that NRAS, an oncogenic driver in serous ovarian carcinomas, could co-expressed with EIF1AX, which promoted clonogenicity and proliferation in OV [
24,
25]. SETBP1 was an oncoprotein that directly binds to SET, which could protect it from proteasome degradation [
39]. As for OV, Qiao and colleague reported that SETBP1 could maintain the Cancer Stem Cell (CSC)-like phenotype of tumor cells via the SET/PP2A axis [
23]. Previous studies identified EXOC6B as a gene involved in the Notch signaling pathway, a key pathway in tumor progression, though haven’t been validated in OV yet [
27,
28]. In breast cancer, researchers indicated that CITED2, as a transcriptional coactivator, could modulate the metastatic ability of tumor cells through the regulation on IKKα [
29]. Kurihara etc., claimed that MIA2 could regulate the infiltration of lymphocytes via a variety of integrins and subtypes of mitogen-activated protein kinase in oral squamous cell carcinoma [
26]. As for TRPV4, researchers found that TRPV4 could promote breast cancer metastasis by regulating cell extravasation, stiffness, and actin cortex [
22]. Interestingly, in our study, the Hazard ratios of CITED2 and EXOC6B were greater than 1 in Fig.
2C, and the higher expression of CITED2 and EXOC6B, the poorer survival in Fig.
2F. The results indicated that CITED2 and EXOC6B were risk factors related to OV progression. However, CITED2 and EXOC6B were highly expressed in normal group in Fig.
2E, which demonstrated that CITED2 and EXOC6B were negatively related to oncogenesis. The underlying mechanism for the opposite role of CITED2 and EXOC6B in OV progression and oncogenesis needs further validation and investigation.
Nowadays, owing to the increasing breakthroughs in immune checkpoint inhibitors, the crosstalk between immune environment and tumor has gained increasing attention [
40]. Current studies have reported that tumor cells could release signals that recruited anti-tumor immune cells through the pyroptosis process, while the immune cells could also induce pyroptosis in tumor cells, thus causing a positive feedback loop [
41,
42]. For instance, Wang and colleagues concluded that pyroptosis of less than 15% of tumor cells was sufficient to eliminate the entire mammary tumor graft, partly due to anti-tumor immunity. In tumors that underwent pyroptosis, the number of CD4 + T cells, CD8 + T cells, NK cells, and M1 macrophages largely increased, while the number of M2 macrophages, monocytes, and neutrophils decreased [
43]. Another study by Zhang and colleagues reported that CD8 + T cells and NK cells could evoke pyroptosis of tumor cells independent of caspases through the GSDME-GZMB axis, which is induced by interferon-γ (IFNγ) [
44]. Nevertheless, the correlations between immune cell infiltration and pyroptosis patterns in OV remains to be further explored.
Accordingly, we evaluated the landscape of immune infiltration in OV. According to the CIBERSORT analysis, 2 out of the 22 immune cells, including activated DCs and M1 macrophages, were up-regulated in the low-risk group compared to the high-risk group. Lee et al. concluded that activated DCs were essential for T cell recruitment into the tissue, the initiation of T cell responses, and maintenance of effector memory T cells [
45]. In this regard, activated DCs played an essential role in the immune responses in the process of OV progression [
45,
46]. Most previous studies have reported anti-tumor effects of M1 macrophages, which was consistent with our findings [
47]. Surprisingly, Untack Cho and colleagues indicated that M1 macrophages could promote OV metastasis by activating the NF-κB signaling pathway [
48]. Interestingly, we also found that Follicular Helper T cells and M2 Macrophage had the most substantial negative relationship, while CD8 + T cells and Macrophage M1 had the strongest positive relationship. However, these findings need validation and exploration for the underlying mechanism in the future study.
Nowadays, regardless of the recent advances in immunotherapy and chemotherapy, clinical treatments for OV face bottlenecks, with a high recurrence rate of approximately 80%, [
5,
49]. Emerging evidence demonstrated that pyroptosis, a programmed cell death (PCD) process mediated by gasdermin (GSDM), was a new bridge to tumor immunity, which could influence sensitivity to immunotherapy and chemotherapy [
50,
51]. Accordingly, we tried to explore the relationship between pyroptosis patterns and sensitivity to immunotherapy and chemotherapy based on the 6-gene signature. According to the evaluation through the GDSC dataset, high-risk patients were more sensitive to chemotherapy, including Vinblastine, Docetaxel, and Sorafenib. Besides, our results revealed that high-risk patients were more likely to benefit from the immunotherapies based on immune checkpoint molecules, including CTLA4, PDCD1LG2, and HAVCR2. However, OV patients with high risk-score had higher TIDE scores, which indicated poorer efficacy and shorter survival after ICB therapy. Previous researches showed that some PD-L1-positive patients could be insensitive to PD-L1/PD-1 immunotherapy in clinical practice of OV therapy [
52]. Hence, our findings might hint that the underlying mechanism of immune checkpoint inhibitors in OV could be more complicated than directly targeting the related immune checkpoints.
However, there remained several limitations of the study. Firstly, the underlying mechanism of the 6 identified PGRs, especially CITED2, EXOC6B, MIA2, and TRPV4, in OV progression and tumor immune microenvironment remained largely unknown, which needs stepwise investigation. Moreover, the pyroptosis-related signature should be further validated in more populations, in order to apply to clinical practice and improve OV survival in the future.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.