The mRNA expression of SLC7A5 is upregulated in colorectal cancer
To investigate the potential association between SLC7A5 and colorectal cancer development, firstly, the gene expression of it in several GEO datasets of colorectal cancer and TCGA database was investigated. It was found that the mRNA expression of SLC7A5 was significantly increased in the cancer samples compared to that in the non-tumor controls for the datasets GSE8671, GSE9348, GSE20916, and GSE5206 (Fig. 1a-d) with all the P-value<0.0001, and the result was validated by TCGA database (Fig. 1e) (P<0.0001), with a 7.5 folds difference between colorectal cancers and non-tumor controls.
To further verify the results discovered in the samples of public datasets, the mRNA expression of SLC7A5 were investigated in 40 pairs of colorectal cancer and adjacent mucosa samples from Xiangya hospital (Table. 1) by real-time PCR. It was showed that the relative expression of SLC7A5 was significantly upregulated in the colorectal cancer tissues compared with the non-tumor tissues (Fig. 1f) (P=0.0001), which is consistent with the previous results of TCGA and GEO databases.
SLC7A5 may affect cell cycle and migration in colorectal cancer
In order to investigate the potential mechanism that SLC7A5 affect the etiology of colorectal cancer, co-expression network of the gene was mined in the public database. PanCancer datasets, Provision datasets and Nature2012 datasets from TCGA were used to propose genes co-expressing with SLC7A5. A total of 238 genes were found in these three datasets (Figure. 2a), and FunRich was used to functional enrichment analysis of those genes. FunRich analysis showed that the pathways enriched most were cell cycle and EMT (Figure. 2c), and Cyclin D1 (CCND1) were the only gene that had a significant positive correlation with SLC7A5 (Figure 2b) in TCGA COADREAD gene expression dataset.
To validate the results got from public dataset analysis, the loss of SLC7A5 function experiments were performed in colorectal cancer cell lines, and factors important for cell cycle and migration were examined.
Knockdown of SLC7A5 mRNA may arrest cell cycle progression by downregulation of cyclin D1 and CDK2 in SW480 and HCT116 cell lines
Western Blot Assays were used in different colorectal cancer cells to evaluate the expression levels of SLC7A5 in SW480, Caco 2, HCT116, and HT-29 cell lines. The expression levels of SLC7A5 in SW480 and HCT116 cells were much higher than that in Caco 2 and HT-29 cells (Fig. S1). Therefore, HCT116 and SW480 cell lines were used for the following loss-of-function assays.
In SW480 cells (Fig. 3b), the cell percentages of si SLC7A5- 1 and si SLC7A5- 2 groups were raised significantly compared with normal control (NC) group at G0/G1 phase, while the cell percentage of NC group was considerably higher than the si SLC7A5- 1 and si SLC7A5- 2 groups at G2/M phase. Moreover, the S phases cell percentages of si SLC7A5- 1 and si SLC7A5- 2 groups were reduced significantly in comparison to the NC group. Coincidentally, the similar result showed in HCT116 cells (Fig. 3c). These results suggest that knockdown of SLC7A5 might arrest cell cycle at G1 phase in colorectal cancer cells.
In SW480 cells (Fig. 3e), cyclin D1 and CDK2 protein expression were down-regulated when SLC7A5 was dramatically knockdown in si SLC7A5- 1 and si SLC7A5- 2 group. Also, the same situation occurred in HCT116 cells (Fig. 3d) as well. These similar tendencies might suggest cyclin D1 and CDK2 were regulated by SLC7A5 in colorectal cancer cells with a positive correlation, which is consistent with the results got from public dataset mining for co-expression genes for SLC7A5 in TCGA.
Knockdown of SLC7A5 mRNA limits migration in colorectal cancer cells
To examine whether SLC7A5 affect the cell migration, the wound healing assay was performed in SW480 cells after knocking down of the gene. The gap area of NC group was much smaller than that of si SLC7A5- 1 and si SLC7A5- 2 groups (Fig. 4a), which revealed that loss of SLC7A5 function on mRNA level could significantly weaken the ability of migration of SW480 cells.
To further confirm the change of migration in SW480 cells, the protein expression levels of zonula occludens (ZO)-1 and E-Cadherin in SW480 cells were detected. When SLC7A5 was knock down, the epithelial marker E-Cadherin was upregulated, while the mesenchymal marker ZO-1 was downregulated (Fig. 4c). Similar changes were also observed in HCT116 cells (Fig. 4b). Therefore, SLC7A5 may regulated migration of colorectal cancer cells by reversing EMT pathway.