Aim: To investigate the function of Pea3 in colorectal carcinoma (CRC) invasion and metastatic potential.
Methods: The expression of Pea3 during clinical progression of human CRC was investigated using Oncomine Research Edition. To assay Pea3 expression in established CRC cell lines, we performed western blotting of cell lysates. We employed shRNA-mediated knockdown of Pea3 in HCT116 (HCT) and LS174T CRC cells which was confirmed by real-time quantitative PCR (qPCR) and western blotting. Transwell invasion assays, MTS proliferation assays, anoikis assays, and fluorometric matrix metalloprotease (MMP) assays were performed to determine the effects of Pea3 knockdown on invasion, proliferation, anoikis and MMP activity in CRC cells in vitro. Alterations in epithelial-mesenchymal transition (EMT) and matrix metalloprotease (MMP) mRNA levels were determined by qPCR. CRC cells were injected into the flanks of nude mice to generate xenografts and tumor growth monitored with serial calliper measurements. To assay metastatic potential, CRC cells were injected into the spleen of nude mice, and histological analysis performed on the livers 21 d later.
Results: We demonstrated that reduction of Pea3 expression in CRC cells significantly impaired their invasive capacity (HCT.shPea3, 0.28 ± 0.04 fold, P < 0.01; LS.shPea3, 0.15 ± 0.04 fold; SW.shPea3, 0.23 ± 0.03, P < 0.01), reduced anoikis resistance (HCT.shPea3 75.4% ± 1.9% viable cells vs HCT.shCtrl 88.6% ± 0.6% viable cells, P < 0.01; LS.shPea3 71.7% ± 0.5% viable cells vs LS.Ctrl 89.6% ± 0.3% viable cells, P < 0.005, but had no effect on proliferation (HCT.shCtrl AUC 5098 ± 123 vs HCT.shPea3 5689 ± 151, P < 0.05; LS.shCtrl AUC 5600 ± 324.1 vs LS.shPea3 6423 ± 400, P < 0.05). In vivo, HCT.shPea3 and HCT.shCtrl tumour xenografts grew at a similar rate (HCT.shPea3 2.64 ± 0.82 fold vs HCT.shCtrl 2.88 ± 0.80 fold, P > 0.05). In keeping with a pro-metastatic function for Pea3 in CRC, several EMT markers and MMPs were downregulated in shPea3-expressing cells, suggesting that Pea3 may exert its effects through these processes. A reduction in overall MMP activity was observed in HCT.shPea3 cells compared to their control counterparts (HCT.shPea3 0.61 ± 0.04 fold, P < 0.005). This translated in vivo to the complete absence of metastases in the livers of mice that were grafted with CRC cells lacking Pea3. Conversely, CRC cells expressing Pea3 formed liver metastases in all mice.
Conclusion: Our study implicates Pea3 as a mediator of metastases, and provides a biological rationale for the adverse prognosis associated with elevated Pea3 expression in human CRC.
Keywords: Colorectal cancer; Epithelial-mesenchymal transition; Liver metastasis; Pea3.