In the present study, we explored the roles of SLC25 family in the genesis and development of cancer. According to current publications, a total of 15 SLC25 family genes were screened out. Multi-dimensional analyses were systematically performed based on the transcriptome and genome data of SLC25 family from a variety of online databases for the association among their expression, mutation and copy number variation, cancer prognosis, signaling pathways and immune cell infiltration. Validation by qPCR was further conducted for the expression status of partial SLC25 family members in some tumor tissue.
Our study demonstrated that SLC25 family had differential expression in both tissue mRNA and protein as well as cell lines. At tissue mRNA level, SLC25A5, SLC25A27, SLC25A23, SLC25A4 and SLC25A25 were down-regulated in some cancer including kidney chromophobe, head and neck squamous cell carcinoma, breast invasive carcinoma and lung adenocarcinoma. At tissue protein level, the majority of SLC25 family genes had medium to high expression levels in 10 common cancer types such as lung adenocarcinoma, colorectal cancer, breast cancer and other tissue. For detailed results at tissue mRNA level, SLC25A4 was low expressed in stomach adenocarcinoma, head and neck squamous cell carcinoma, colon adenocarcinoma and kidney renal clear cell carcinoma. SLC25A7 was highly expressed in stomach adenocarcinoma, kidney chromophobe and colon adenocarcinoma but low in lung adenocarcinoma, thyroid cancer and breast cancer. SLC25A23 had low expression in colon cancer as well as head and neck squamous cell carcinoma. Therefore, SLC24A4, SLC25A7 and SLC25A23 were selected for subsequent validation of in situ expression in fresh cancer tissue by qPCR. We found that SLC25A4 and SLC25A23 were respectively decreased in gastric cancer and colon cancer, while SLC25A7 was increased in gastric cancer, which was consistent with our bioinformatic prediction. It has been shown that the ANT1 protein encoded by SLC25A4 was low expressed in rhabdomyosarcoma [7]. Combining with this, SLC25A4 is located in mitochondria and involved in metabolic process by regulating ATP/ADP. Thus the decreased expression of SLC25A4 might lead to aberrant metabolism, participating in tumor progression. As a well-known uncoupling protein, SLC25A7 also functions in important metabolic process in mitochondria, while its effect in tumor has been rarely studied. Similar findings could be observed in previous research for the expression of SLC25A7 in tumor. Alexandra et al. reported that SLC25A7 was highly expressed in squamous cell carcinoma of non-small cell lung cancer and closely related to glycolysis [8]. SLC25A23 is a Ca2 + sensitive mitochondrial carrier with the ability to effectively transport ATP, ADP and AMP except for Ca2+. Its expression in cancer might be associated with abnormal ATP levels. The study would provide theoretical basis for their application as potential targets of cancer therapy including SLC25A4, SLC25A7 and SLC25A23.
The association analysis of SLC25s expression with tumor-related pathways revealed 35 pathways associated with SLC25s such as proliferation-related pathways, Kas and MYC pathways. The expression of SLC25 genes varied with each pathway, suggesting that different SLC25 genes might exert diverse roles in tumor. Among them, SLC25A5 was more likely to be involved in carcinogenic pathways. It has been reported that the SLC25A5 gene is up-regulated in most tumor[9–12], mainly encoding the ANT2 protein specifically expressed in proliferative tissue[13]. Several pathways shown to be positively associated with SLC25A5 were common tumor-related pathways regulating proliferation, including PI3K_AKT_MTOR, MYC_TARGETS_V2 and E2F_TARGETS[14–17]. ANT inhibitors have been considered as potential therapeutic targets for cancer [18]. Therefore, SLC25A5 might participate in cancer initiation and development via these oncogenic pathways. To elucidate the association hidden in SLC25 family genes differentially expressed in tumor could not only benefit the identification of novel molecules involved in the regulation of tumor progression and better insights of pathogenesis, but also provide clues for candidate biomarkers used for cancer diagnosis, prognosis and treatment.
Additionally, immune cell infiltration also contributed to the effects of SLC25 family in tumor. SLC25 family was previously reported to be relevant with inflammation[19]. Kai Yasukawa et al. found that SLC25A12 was associated with the innate immunity of body through mitochondrial metabolism[20]. Marie-Clotilde et al. showed that SLC25A8 was rapidly activated and mobilized during the anesthesia of immune cells [21]. Hence, the SLC25 family was related to immunity more or less. In our research, the association between SLC25s and immune cell infiltration was mainly manifested in Macrophage M2. The role of macrophages in tumorigenesis and development has already been confirmed. They had a two-way interaction with cancer cells, especially M2 polarization. Cancer cells induced M2 polarity to promote a series of cancer deterioration[22]. Various metabolites in tumor microenvironment promoted the M2 polarization of macrophages to accelerate tumor progression [23]. These may provide corresponding thoughts for the diagnosis and treatment of cancer with respect to SLC25 family and tumor microenvironment.
Concerning the association of SLC25s expression with cancer prognosis, SLC25A8 was associated with poor prognosis in cervical squamous cell carcinoma, endocervical adenocarcinoma, cholangiocarcinoma and thyroid cancer, while better prognosis in membranous melanoma and brain low-grade glioma. SLC25A8 was involved in many diseases such as cancer and cardiovascular diseases, etc. [24], and its expression was connected with chemoresistance[25]. Masaru Kawanishi et al. found that in ovarian serous carcinoma, patients with high expression of SLC25A8 were less sensitive to platinum-based chemotherapy and developed drug resistance[26]. A latest study by Wang et al. demonstrated that knocking down SLC25A8 in cervical cancer cell lines could delay or reduce the proliferation, migration and invasion of cervical cancer cells via Ras/MAPK/ERK signaling pathways. Thus it could be applied to predicting the prognosis of cervical cancer[27], which was consistent with our findings. Similarly, a research for breast cancer indicated that SLC25A8 overexpression might directly affect the mitochondrial membrane potential of tumor cells, inhibit cell apoptosis and promote tumor metastasis through the TGF-β pathway, resulting in the poor prognosis [28]. Kuai et al. also believed that SLC25A8 was highly expressed in human colon cancer tissue associated with the metastasis and poor prognosis of colon cancer[29]. Furthermore, we found that SLC25A8 made discrepant effects on the prognosis of different cancer, with the potential to be a versatile biomarker for cancer prognosis. Above all, the expression of SLC25 family showed diverse roles in cancer prognosis, possibly owing to the various substrates transported by SLC25 genes affecting cancer genesis and development.
Currently, numerous studies have proved that the occurrence and progression of tumor were closely related to genetic variation including mutation and copy number variation. The mutation and copy number variation of SLC25 family were also investigated based on TCGA data. It was shown that almost all SLC25 genes had high mutation frequency in uterine corpus endometrial carcinoma, which was a well-accepted tumor with extensive gene mutation [30]. Notably, SLC25A4 had more deletion of copy number in cholangiocarcinoma, sarcoma and lung squamous cell carcinoma. For its reason, the expression of ANT1 protein encoded by SLC25A4 might reduce ATP transport and promote Rax release, easily leading to cell apoptosis and tumorigenesis[31–33]. Moreover, the mutation analysis of SLC25 family in tumor cell lines revealed that the mutation rate of SLC25A25 was highest in breast cancer, suggesting that SLC25A25 was more apt to mutate in the environment of breast tissue. The SLC25A25 gene could control the dynamic balance of ATP, and the missing of SLC25A25 may reduce the metabolic efficiency in mice[34]. We also analyzed the influence of mutation and CNV in SLC25 genes on their expression and cancer prognosis. SLC25A4 and SLC25A6 mutation mainly affect their expression in uterine corpus endometrial carcinoma and ovarian serous cystadenocarcinoma, respectively. It has been reported that cells pretreated with ANT inhibitors can attenuate the release of Cytc connected with mitochondrial bioenergy. Therefore, ANT mutations are involved in many human diseases [35], which is consistent with our research. As for the impacts of mutation on cancer prognosis, SLC25A24 mutation was quite probably associated with a high risk in the prognosis of BRCA. Besides, SLC25A24 could protect tumor cells from death. Thus it might promote tumor growth and metastasis causing poor prognosis when mutated[36]. Taken overall, genetic variation may be the key to the alteration of gene expression and cancer prognosis. The variation in SLC25 genes might play critical roles in the regulation of their expression and cancer prognosis. However, the specific mechanism needs to be further explored.