Background: Increasing evidence suggests that SALL4 plays oncogenic roles in cancer development and progression. We have previously shown that SALL4 is highly expressed in gastric cancer, and its upregulation is associated with lymph node metastasis and poor prognosis. The role of SALL4 in gastric cancer metastasis and the underlying mechanism remain unclear.
Materials and methods: The biological roles of SALL4 in gastric cancer cell mobility, migration, and invasion were investigated by wound healing, transwell migration assay, and Matrigel invasion assay. The effects of SALL4 on epithelial–mesenchymal transition (EMT) in gastric cancer cells were examined by quantitative real-time PCR and Western blot. The downstream target genes of SALL4 were identified by microarray. The regulation of TGF-β1 by SALL4 in gastric cancer cells was analyzed by luciferase reporter assay and chromatin immunoprecipitation assay.
Results: SALL4 knockdown inhibited, while SALL4 overexpression promoted the motility, migration, and invasion abilities of gastric cancer cells in vitro. SALL4 knockdown also suppressed the peritoneal metastasis of gastric cancer cells in nude mice. SALL4 knockdown suppressed, while SALL4 overexpression induced the activation of TGF-β/SMAD signaling pathway and triggered EMT in gastric cancer cells. TGF-β1 was identified as a direct target gene of SALL4. The results of chromatin immunoprecipitation study and luciferase reporter assay further confirmed that SALL4 bound to the promoter of TGF-β1  gene and activated its expression. TGF-β1 knockdown reversed SALL4-mediated promotion of gastric cancer cell motility, migration, and invasion, indicating that TGF-β1 acts as a downstream effector of SALL4. Furthermore, the expression of TGF-β1 was found to be closely associated with that of SALL4 in gastric cancer tissues.
Conclusion: SALL4 promotes the metastasis of gastric cancer, at least partly, by directly activating TGF-β1, suggesting that SALL4 may serve as a new target for gastric cancer therapy.
Keywords: gastric cancer, SALL4, TGF-β1, EMT, metastasis