SLC25A11 serves as a novel prognostic biomarker in liver cancer

Liver cancer is a disease with high mortality; it is often diagnosed at intermediate and advanced stages and has a high recurrence rate. ROS restriction and adequate energy supply play significant roles in liver cancer. SLC25A11, a member of the malate-aspartate shuttle (MAS), regulates electroneutral exchange between 2-oxoglutarate and other dicarboxylates. It transports glutathione (GSH) from the cytoplasm into mitochondria to maintain GSH levels to limit ROS production. Moreover, SLC25A11 is essential for ATP generation in cancers as it regulates NADH transportation from the cytoplasm to mitochondria. The purpose of this research was to investigate the prognostic value of SLC25A11 in liver cancer. The Cancer Genome Atlas database was used to analyze the levels of SLC25A11 in liver cancer. Fisher’s exact and chi-square tests were used to evaluate the relationship between SLC25A11 expression and clinical characteristics. Finally, we explored the value of SLC25A11 in prognosis by Cox analysis and Kaplan-Meier curves. Our results revealed that SLC25A11 was downregulated in liver cancer compared to normal controls. Low expression of SLC25A11 was associated with clinical stage, vital status, histologic grade, overall survival (OS) and relapse-free survival (RFS). Liver cancer patients with low SLC25A11 expression had shorter OS and RFS than patients with high SLC25A11 expression. Multivariate analysis showed that the expression of SLC25A11 was an independent predictor of RFS and OS. In conclusion, this study identified that SLC25A11 serves as a new prognostic marker for liver cancer.

The diagnostic value of SLC25A11. The diagnostic value of SLC25A11 was evaluated by ROC curve analysis using data from the TCGA. The area below the ROC curve (AUC) was 0.635, suggesting the modest diagnostic value of SLC25A11 (Fig. 2). Subgroup analysis revealed the diagnostic value in different stages of liver cancer with AUC values of 0.593, 0.668, 0.696, and 0.588 for stage I, stage II, stage III, and stage IV, respectively (Fig. 2).
Correlations between SLC25A11 and clinical features with liver cancer. After dividing the patients into low/high SLC25A11 mRNA groups according to the threshold value determined by the ROC curve, the relationships between clinical features and SLC25A11 expression were analyzed (Table 1). Histologic grade (P = 0.0244), clinical stage (P = 0.0428) and vital status (P = 0.0126) were significantly correlated with SLC25A11 expression. www.nature.com/scientificreports www.nature.com/scientificreports/ Low SLC25A11 predicts poor prognosis of OS and RFS. Kaplan-Meier survival curves were constructed to explore the prognostic value of SLC25A11 in OS. The results showed that low expression of SLC25A11 was related to worse OS (P = 0.0088; Fig. 3). In addition, a survival curve was constructed to test the prognostic value of SLC25A11 in RFS. The results showed that low SLC25A11 expression was associated with poorer RFS than high SLC25A11 expression (P = 0.002; Fig. 3), which was also confirmed by using the ICGC and GSE54236 databases (Fig. 3).   www.nature.com/scientificreports www.nature.com/scientificreports/ Subgroup analysis revealed the prognostic value of SLC25A11 in terms of OS. Subgroup analysis revealed that the low expression of SLC25A11 was correlated with poor OS in grade G1 and G2 patients (P = 0.0061), older patients (P = 0.026), and male patients (P = 0.042) (Fig. 4).

Discussion
Among cancers worldwide, liver cancer has one of the worst prognoses. Many studies have investigated novel prognostic biomarkers in different types of cancer [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] . In this study, we demonstrated that SLC25A11 was low expressed in liver cancer by analyzing data from the TCGA. Low expression of SLC25A11 is related to poor prognosis. Both univariate and multivariate Cox analyses revealed that SLC25A11 played an important role in OS and RFS, which means that SLC25A11 is a novel independent prognostic factor in liver cancer.
In recent years, high SLC25A11 has been found in NSCLC and melanoma 8 . However, we found that SLC25A11 was expressed at low levels in liver cancer. ROC analysis suggested that SLC25A11 could be regarded as a marker in the diagnosis and prognosis of liver cancer. Moreover, we observed that histologic grade, clinical stage, and survival status were related to SLC25A11 expression, which confirmed the significance of this molecule. SLC25A11, a member of the malate-aspartate shuttle (MAS), regulates the shuttling of 2-oxoglutarate by dicarboxylates from different tissues and cells using various methods [27][28][29] . These studies also provide evidence that SLC25A11 is a GSH carrier. In addition, SLC25A11 induction is a key strategy for maintaining mtGSH levels to limit ROS production in HCC 9 . Based on the findings that SLC25A11 expression is associated with clinical stage and histologic grade in liver cancer, we postulate that by stabilizing the mitochondrial membrane www.nature.com/scientificreports www.nature.com/scientificreports/ and maintaining sufficient mtGSH to withstand hypoxia-induced ROS production and cell apoptosis, SLC25A11 improves cell growth and tumor progression.
As one of the characteristics of cancer, fast growing cancer cells requires more ATP than normal cells. Because cancer cells rely mainly on glycolysis for energy, they maintain high mitochondrial oxidative phosphorylation for ATP generation and survival 30 . SLC25A11 has been proven to be a crucial transporter of NADH from the cytoplasm to mitochondria in the ATP production process, especially in cancer cells. Therefore, the association of the expression of SLC25A11 with poor survival may be due to the effect of SLC25A11 on tumor energy metabolism. This implies that SLC25A11 is necessary for embryogenesis but not for the proliferation of differentiated cells, which suggests that it can be a good marker for cancer.
To our knowledge, our research is the first to show the significant role of SLC25A11 in the prognosis of liver cancer by mining the TCGA database. We found that SLC25A11 could be regarded as a prognostic factor for poor survival in liver cancer. However, the total number of patients evaluated is a limitation of our research. There was no significant correlation between SLC25A11 expression and overall survival in ICGC and the GSE54236 database in Fig. 3. However, the statistical significance does not represent clinical significance because some variables could influence the P-value, such as sample size. For this study, we observed that low SLC25A11 expression was associated with poor prognosis in the ICGC and GSE54236 cohorts, but the P-value did not indicate significance, perhaps because of the sample size. We need to expand the sample size to confirm the prognostic value of SLC25A11 expression levels in the future. Furthermore, we postulate that SLC25A11 plays an essential role in transporting NADH and GSH from the cytoplasm into mitochondria, providing energy for liver cancer cells to withstand harsh environments. However, clinical trials and further experiments are needed to certify these results and clarify the specific molecular mechanism.