The prognostic and clinicopathological significance of SLC7A11 in human cancers: a systematic review and meta-analysis

Introduction

Cancer is the leading cause of death worldwide, and has been a severe public health burden with increased mortality and morbidity. Thus, it is of great importance to identify biomarkers for early diagnosis, prognosis assessment and precision treatment for cancer patients (Siegel et al., 2021). Therefore, SLC7A11 could be a potential biomarker for human cancer prognosis.

The solute carrier (SLC) series includes more than 60 gene families with a total of more than 400 members, encoding most human transporters, of which solute carrier family 7 (SLC7) is mainly involved in the transport of amino acids on the plasma membrane (Lin et al., 2020). The SLC7A11 gene is located on human chromosome 4 and contains 14 exons, and is widely expressed in tissues and cells such as brain, liver, macrophages and retinal pigment cells (Hong et al., 2021). The SLC7A11 gene encodes SLC7A11 (also known as xCT) protein, which acts as a light chain subunit and forms the Xc⁻ cystine/glutamate antiporter with the heavy chain subunit SLC3A2 (also known as 4F2hc) (Koppula et al., 2018). As an antiporter, SLC7A11 mediates the intake of cystine and the exchange of glutamate (Luo et al., 2022). After taken into the cell, cystine would be reduced into cysteine, which plays an important role in protecting cells from oxidative stress (Shao et al., 2022). SLC7A11 has been observed to express in various cancer tissues and plays different functions in a number of pathophysiological processes, including ferroptosis, immune system function, metabolic flexibility/nutrient dependency and redox homeostasis (Jyotsana, Ta & DelGiorno, 2022). For example, tumor cells can maintain high levels of glutathione by upregulating the expression of the catalytic subunit SLC7A11 of the Xc⁻ system to counteract the oxidative stress caused by the increased rate of their own metabolism (Koppula, Zhuang & Gan, 2021). Gout et al. (2001)’s research pointed out that the growth of lymphocytes depends on the uptake of cystine/cysteine from the microenvironment by systems such as Xc⁻, and the ability to increase cystine uptake is a potential process for the progression of T-cell malignancies. The activity of the Xc⁻ system and intracellular glutathione levels are closely related to lymphoma cell growth (Guan et al., 2009). By constructing SLC7A11 knockout in U251 glioma cells, Polewski et al. (2017; 2016) found that SLC7A11 knockout could increase basal reactive oxygen species levels, reduce glutathione generation, and promote oxidative stress and genotoxic stress. SLC7A11 overexpression increases glioma cell resistance to oxidative stress and reduces sensitivity to temozolomide (Polewski et al., 2017; Polewski et al., 2016).

However, the association between solute carrier family 7 member 11 (SLC7A11) and prognosis is still controversial. Therefore, we conducted this systematic review and meta-analysis to identify the prognostic and clinicopathological significance of SLC7A11 in human cancers.

Methods

The protocol of this systematic review and meta-analysis has been registered on PROSPERO website (http://www.crd.york.ac.uk/PROSPERO). The ID number is CRD42022321708. We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (Moher et al., 2009). And we referred to the methods used in our previously published paper (Wang et al., 2022).

Results

Study characteristics

The characteristics of included studies were exhibited in Table 2. There were 10 types of cancer, including esophageal squamous cell carcinoma, non-small cell lung cancer, hepatocellular carcinoma, oral cavity squamous cell carcinoma, laryngeal squamous cell carcinoma, glioma, colorectal cancer, glioblastoma, tongue cancer, and liver carcinoma. SLC7A11 expression was assessed by RT-qPCR or IHC. The follow-up duration and outcomes were also exhibited.

Table 2:

Our search strategy of PubMed, the Cochrane Library, Embase, Web of Science and Scopus.

The search time limit is from the establishment of databases to March 19th 2022. Characteristics of the included studies.

Study	Cancer type	Expression	Detecting method	Sample size	Mean age (range)	Follow-up (month)	Cut-off	High expression (%)	Endpoints
Feng et al. (2021)	Esophageal squamous cell carcinoma	Protein	IHC	127	66(43–84)	168	staining intensity scores 2-3	49	OS, PFS
Ji et al. (2018)	Non-small cell lung cancer	Protein	IHC	254	64.8	60	positive cells scored more than 3	43	OS
Kinoshita et al. (2013)	Hepatocellular carcinoma	mRNA	RT-qPCR	130	66	60	based on expression status	26	OS, RFS
Lee et al. (2018)	Oral cavity squamous cell carcinoma	Protein	IHC	231	57(23–88)	180	percentage of immunepositive cells more than 20%	58	OS, RFS,
Ma et al. (2017)	Laryngeal squamous cell carcinoma	Protein	IHC	327	65	125	positive cells scored 3-7	54	OS, RFS
Namikawa et al. (2015)	Hepatocellular carcinoma	Protein	IHC	84	62(29–81)	133	Stained tumour cells score more than 1	65	OS, RFS
Shiozaki et al. (2014)	Esophageal squamous cell carcinoma	Protein	IHC	70	61.7	60	xCT positivity rate more than 20%	50	OS
Sorensen et al. (2018)	Glioma	Protein	IHC	215	61.9	60	staining intensity scored 1-3	21	OS
Sugano et al. (2015)	Colorectal cancer	Protein	IHC	304	65.8	60	immunopositive cells more than 25%	68	OS
Takeuchi et al. (2013)	Glioblastoma	Protein	IHC	40	63(21–79)	150	50%	50	OS, PFS
Toyoda et al. (2014)	Tongue cancer	Protein	IHC	85	69(33–92)	115	stained tumour cells score more than 3	21	OS, PFS
Zhang et al. (2018a) and Zhang et al. (2018b)	Liver carcinoma	Protein	IHC	88	55(28–76)	50	intensity scores more than 6	50	OS

DOI: 10.7717/peerj.14931/table-2

Notes:

IHC

Immunohistochemical

RT-qPCR

Real Time quantitative Polymerase Chain Reaction

OS

Overall Survival

PFS

Progression-free Survival

RFS

Recurrence-free Survival

Correlation between SLC7A11 expression and OS

Ten studies reported the correlation between SLC7A11 expression and OS (Feng et al., 2021; Ji et al., 2018; Kinoshita et al., 2013; Lee et al., 2018; Ma et al., 2017; Shiozaki et al., 2014; Sorensen et al., 2018; Takeuchi et al., 2013; Toyoda et al., 2014; Zhang et al., 2018a). The pooled HR showed that the high expression of SLC7A11 was statistically associated with unfavorable univariate OS (HR =1.93, 95% CI [1.60–2.30], I² = 0%, fixed effect model) (Fig. 2A). The multivariate OS was also combined (Feng et al., 2021; Ji et al., 2018; Kinoshita et al., 2013; Lee et al., 2018; Shiozaki et al., 2014; Zhang et al., 2018a). The results indicated that high expression of SLC7A11 was significantly associated with poor multivariate OS (HR = 1.63, 95% CI [1.29–2.05], I² = 35%, random effect model) (Fig. 2B).

Cumulative meta-analysis and sensitivity analysis

As shown in the Fig. 3A, with the addition of large samples, the HR values decreased and the 95% confidence interval became narrower. The cumulative meta-analysis reflected the association between SLC7A11 and unfavorable prognosis more objectively. Considering the heterogeneity detected in the meta-analysis of OS, sensitivity analysis was applied to explore the stability of meta-analysis. Each included studies reported OS was omitted one by one, in order to detect their influence to the result respectively. The result of sensitivity analysis indicate that our result was robust, which would not statistically significant change when omitting included studies (Fig. 3B).

Further bioinformatics verification

With the aim to verify our results, we explored the expression and prognosis significance in TCGA database. SLC7A11 expression was significant high in 18 kinds of human cancers (Fig. 4A). Then, we explored the expression of SLC7A11 in TCGA database. We combined all kinds of human cancers together, and the result indicated that high expression of SLC7A11 was significantly associated with unfavorable OS (Fig. 4B) and RFS (Fig. 4C).

Discussion

In this study, we performed a systematic review and meta-analysis with a total of 1955 patients from 12 studies included. Our results demonstrated that SLC7A11 expression was correlated with more unfavorable prognosis including OS, RFS and PFS. Then, we assessed the correlation between SLC7A11expresssion and clinicopathological features. The result indicated that high SLC7A11 expression was statistically significant related to advanced tumor stage. In addition, closely correlation between SLC7A11 and advanced stage indicated that tumor progression mediated by SLC7A11 might contribute to SLC7A11 caused worse prognosis. Considering the heterogeneity, we conducted the sensitivity analysis which showed our result was stable and robust. Also, further bioinformatics results further supported our conclusion.

Cancer cells need extracellular nutrient to support their survival and proliferation. Among them amino acids were the most important by supplying building blocks, energy as well as redox hemostasis. Cysteine is a very important amino acid with multiple roles in protein synthesis, post-translational modifications and redox maintenance (Liu et al., 2021). Cysteine is the rate-limiting precursor of glutathione, a tripeptide composed of three amino acids (cysteine, glutamate and glycine) that is the most abundant intracellular antioxidant. Intracellular cysteine can be recovered by ab initio biosynthesis or by protein degradation, and most cancer cells rely primarily on the cystine transporter protein system Xc- (consisting of the catalytic subunit SLC7A11 and the chaperone subunit SLC3A2) to obtain cystine from the extracellular environment, which is then converted to cysteine in the cytoplasm via a reduction reaction that consumes NADPH; cysteine is then used for the synthesis of glutathione (and other biomolecules) (Liu, Xia & Huang, 2020). A growing number of studies suggest that SLC7A11-mediated cystine uptake plays a key role in inhibiting oxidative responses and maintaining cell survival under conditions of oxidative stress (Hémon et al., 2020). When it comes to its association with the prognosis of cancers, a pan-cancer analysis used survival data from The Cancer Genome Atlas (TCGA) database reached a conclusion which was similar with our systematic review and meta-analysis. Through their bioinformatics approaches, Lin et al. (2022) concluded that SLC7A11 was found to be highly expressed in the 20 types of cancer and the up-regulated expression of SLC7A11 was related to poor prognosis in a variety of cancer including, including adrenocortical carcinoma, cervical squamous cell carcinoma and endocardial adenocarcinoma, and so on (Lin et al., 2022). The potential mechanism of SLC7A11 in tumor development and progression include resistance to anti-cancer drugs and inhibition of ferroptosis. On the one hand, SLC7A11 overexpression has been shown to correlate with or functionally promote resistance to various anti-cancer drugs, such as cisplatin, gemcitabine, and MAPK pathway inhibitors. SLC7A11 overexpression promotes radio-resistance, whereas SLC7A11 inhibition enhances radio-sensitivity (Shen et al., 2018). On the other hand, SLC7A11 is able to promote tumor progression by inhibiting ferroptosis. Studies have shown that loss of tumor suppressors (e.g., p53 and BAP1), mutations in proto-oncogenes (e.g., KRAS) or overexpression of pro-tumor function proteins (e.g., OTUB1) increase the levels of SLC7A11 by upregulating its transcriptional levels or stabilizing its protein, thereby suppressing iron death and promoting tumor development (Koppula, Zhuang & Gan, 2021). Studies have found that SLC7A11 is a key target gene of BAP1, and BAP1 inhibits the expression of SLC7A11 by reducing H2Aub on the SLC7A11 promoter, promoting the occurrence of ferroptosis, and then inhibiting tumor growth. Mutations in BAP1 can cause various types of cancer, such as renal cell carcinoma and mesothelioma, and when BAP1 is mutated, the inhibition of SLC7A11 and the promotion of ferroptosis are lost (Zhang et al., 2018b). The results of our systematic review and meta-analysis also indicated that high expression of SLC7A11 was statistically significant related to unfavorable OS, RFS, PFS and clinicopathological features in a number of human cancers. Taking these results together, we could infer that SLC7A11 is a potential oncogene in human cancers.

Normal cells or tissues can compensate for the loss of SLC7A11, cysteine synthesis, or cystine (or cysteine) uptake via other transporter proteins by acquiring intracellular cysteine. In contrast, cancer cells are more dependent on SLC7A11-mediated cysteine uptake for cysteine acquisition and maintenance of redox homeostasis than normal tissues. These two features of SLC7A11, its non-essential nature in normal physiology and its high expression in cancer, suggest that targeting SLC7A11 may selectively kill tumor cells and impair tumor growth while preserving normal cells or tissues. Therefore, tumor suppression could be achieved by the following methods: (1) Directly blocking the activity of SLC7A11 cystine transporter protein using various inhibitors. These drugs inhibit SLC7A11 uptake of cystine, which induces lipid peroxidation and iron death. (2) Targeting the glucose dependence of SLC7A11 high expressing cancer cells by inhibiting glucose uptake. Reduction of available glucose in SLC7A11 high expressing cancer cells induces disulfide bond stress, leading to rapid cell death. (3) Targeting glutamine dependence in SLC7A11 high expressing cancer cells by using glutaminase inhibitors (e.g., CB-839) to inhibit cancer cell growth.

However, there also existed some limitations in our study. Firstly, the number of subjects of some included studies was limited. Secondly, the included studies were all retrospective studies. No prospective studies reached the inclusion criteria. Thirdly, the cut-off values of SLC7A11 expression were different among the included studies, which may be a resource of heterogeneity. Therefore, further multi-center prospective studies are still needed.

Conclusion

Our study demonstrated that high SLC7A11 expression is associated with worse prognosis in patients with cancer and more advanced tumor stage. Thus, SLC7A11 expression could be a biomarker for the prognosis of cancers.

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