SALL level in gastric cancer tissue and its correlation with TNM stage from TCGA dataset.
In order to explore the expression level of SALL4 gene in GC, we analyzed the TCGA dataset (GSE79973) to evaluate the distribution of gene expression in GC and its adjacent normal gastric mucosal tissues, the results showed that SALL4 gene level in GC was upregulated in comparison to the adjacent non-tumor mucosa (Fig. 1A). Based on this, we dissected the expression level of SALL family member (including SALL1, SALL2, SALL3, and SALL4) in GC. Firstly, we found only SALL4 levels in SALL family members were higher in nonpaired and paired GC tissues than those in corresponding normal tissues in TCGA dataset (Fig. 1B); simultaneously, compared to the normal tissues, only SALL4 levels were elevated not only in various pathological types (Fig. 1C) and stages of gastric cancer (Fig. 1D). Then, in respect of T, N, and M stages, which concern the tumor local extension, spread-to-nearby lymph nodes and long-distance metastasis respectively, also only the higher levels of SALL4 got involved in the different status compared to the normal stages (Fig. 1E). These results suggest that SALL4 in the SALL family should be frequently linked with GC as a research biomarker, and deserved more and more focalization.
SALL4 level in gastric cancer cells and its prognostic analysis from TCGA dataset
Encouraged by the above results, to further confirm SALL4 level in GC cells. we analyzed SALL4 expression in four GC cell lines (HGC-27, AGS, SGC-7901, and MGC803) and normal GES-1 Via real-time RT-PCR (Fig. 2A) and western blotting (Fig. 2B), we found that SALL4 levels were notably higher in AGS, SGC-7901, and MGC-803 than that in GES-1, especially in SGC-7901 and MGC-803. Owing to this, we selected these two cell lines as the subsequent experiments. Then we verified that the aggregation extent of SALL4 in the cell nucleus in SGC-7901 and MGC-803 cells was remarkably stronger than that in GES-1 cell (Fig. 2C) by immunofluorescent staining. To facilitate the succeeding study of SALL4 phenotype, here we focus on its prognostic association analysis in TCGA. We found patients with GC having SALL4-high expression illustrated a worse overall survival (OS) in the Kaplan–Meier survival analysis (Fig. 3D), which also happened in pathological stages of gastric cancer, especially in advanced gastric cancer, but not in the early-stage GC(Fig. 3E). In the further analysis when stratifying patients by the TNM stage, the OS remained significantly discrepancy. In N3 stage and T4 stage GC, patients with SALL4-high GC showed shorter OS than those with SALL4-low GC (Fig. 2G and 2F). These results indicated that the SALL4 level was more correlated with the OS of GC patients with the highest T and N stages, implying whether SALL4 was involved in the metastasis and local invasion of gastric cancer.
Phenotypic functions of SALL4 gene in gastric cancer cell.
The high expression of SALL4 in GC cells and its prognostic survival analysis in patients with high-SALL4 levels prompted us to focalize its phenotype. We investigated the transfection efficiency of SALL4 siRNA (siSALL4) in SGC-7901 and MGC-803 cells (Fig. 3A). Then, we suppressed SALL4 expression in SGC-7901 and MGC-803 cells by using siSALL4, in contrast to control groups, silencing of the SALL4 had conspicuously repression on cancer cell growth (Fig. 3B-1# SGC-7901, 3B-2# MGC-803 cells, red vs. blue dots/lines); In addition, the migration of the cancer cells was affected after siSALL4 transfection (Fig. 3C-1# SGC-7901, 3C-2# MGC-803 cells). Furthermore, knockdown of SALL4 expression level by siSALL4 was sufficient to inhibit the invasiveness of cancer cells in a Boyden chamber assay (Fig. 3D). These results further supported why the high-SALL4 level was expressed in cancer cells compared with normal gastric mucosa cells, and also bolstered our above results that SALL4 could be involved in the metastasis and local invasion of GC patients and be closely related to the tumor-malignancy degree and long-term prognosis.
The dual regulation of EZH2 and KDM6A to SALL4 in gastric cancer cells
To seek the potential upstream regulatory mechanisms of SALL4, we searched the ARCHS4 dataset (https://maayanlab.cloud/archs4/gene/SALL4) principally invested for mining of available RNA-seq Data. According to the Z-score rating of human RNA-seq in the ARCHS4 dataset, we selected predicted and top-10 ranked upstream regulatory factors of SALL4 from the TCGA dataset, and analyzed their correlation with SALL4 mRNA expression levels in gastric cancer (Fig. 4A), FOXM1, KDM6A, SOX2, and CREB1 mRNA levels were shown positive correlations with SALL4 levels in patients with GC in the TCGA dataset (Fig. 4B). To further confirm the expression levels of SALL4, FOXM1, KDM6A, SOX2, and CREB1 in gastric cancer cells, our real-time qPCR results revealed that Only KDM6A and SALL4 were expressed at higher levels in both SGC-7901 and MGC803 cell lines than those in the GES-1 cell line (Fig. 4C).
Inspirited by these data, we focused on investigating the correlation between KDM6A and SALL4 in patients with GC. From TCGA dataset analysis, our results showed that KDM6A mRNA has higher levels in both unpaired (Fig. 4D-1#) and paired (Fig. 4D-2#) gastric cancer tissues than those in the corresponding normal gastric tissues. Furthermore, the KDM6A and SALL4 mRNA levels presented a positive correlation (Fig. 4E). To clarify KDM6A as the upstream regulator to SALL4, SALl4 expression levels were downregulated after KDM6A siRNA (siKDM6A) transfection treatment in SGC-7901 and MGC-803 analyzing by western blotting (Fig. 4F). KDM6A is the demethylase to H3K27me3, which loss conduces to the SALL4 activation19. Here, we observed the downregulation and upregulation levels of SALL4 in the SGC-7901 cell line after silencing and overexpressing KDM6A expressions by siKDM6A and plasmid transfection, which contrarily induced the activation and suppression of H3K27me3 expression, separately (Fig. 4G). EZH2 is the methyltransferase for catalyzing the H3K27me3, so we hypothesized that EZH2 could also be involved in the GC progression of SALL4 regulation. To validate our hypothesis, we found, compared to the GC patients with low-EZH2 levels, GC patients with high-EZH2 levels have a conspicuously longer overall survival in TCGA dataset (Fig. 4H), which is completely opposite to the SALL4. Moreover, we found H3K27me3 inhibition and SALL4 increase in SGC-7901 after EZH2 silencing by EZH2 siRNA (siEZH2)(Fig. 4I). These results verified that KDM6A and EZH2 respectively positive- or negative-regulated SALL4 expression via H3K27me3 in human GC cell lines, respectively.
SALL4 promoted the tumorigenicity of human gastric cancer cells via Wnt/β-catenin signaling pathway
It had been reported that SALL4 modulated the tumorigenicity of breast cancer cells20, cervical cancer cells21 and esophageal squamous cell carcinoma22, and induced acute myeloid leukemia in transgenic mice via activation of the Wnt/β-catenin pathway23. To indagate whether the function of the Wnt/β-catenin pathway was mediated by SALL4 in GC cells and the potential upstream regulatory mechanisms involving EZH2 and KDM6A.
Here, as shown in Fig. 5A, the positive correlation between SALL4 and CTNNB1 (encoded β-catenin ) was excavated in patients with GC from TCGA dataset (Fig. 5A). And then, our western blotting results detected the downregulation level of β-catenin after siSALL4 transfection in SGC-7901 and MGC-803 (Fig. 5B), which was consistent with the correlation result from the TCGA in human GC patients. Besides, we found that β-catenin levels were dominantly decreased by knockdown of EZH2 and KDM6A upstream of SALL4 (Fig. 5C). Hereby, we proposed that SALL4 modulated the evolution of GC cells that mediated by the upstream modulator including EZH2 and KDM6A via the activation of the Wnt/β-catenin pathway (Fig. 5D). To verify the pertinence between the tumorigenesis and β-catenin in GC, the specific inhibitors of Wnt/β-catenin (FH535 and XAV939) were used, which could stably accelerate the inhibition of β-catenin18,21,24,25. After validating the repressed efficacy of FH535 and XAV939 in SGC-7901 (Fig. 5E) and MGC-803 (Fig. 5H), we found the cell growth and viability of GC were significantly inhibited by FH535 and XAV939 (Fig. 5F, 5G, 5I, 5J). These results indicated that KDM6A/EZH2-H3K27me3-SALL4-β-catenin might be the modulatory pathway for regulating the progression of gastric cancer cell lines.