The Role And Molecular Mechanism Of STAT3 Signaling And MiR-508 In Colorectal Cancer Metastasis

Chapter Ⅰ The role and molecular mechanism of STAT3/EZH2/VDR pathway in colorectal cancer metastasisMetastasis is one leading causes of death in CRC patients. The polycomb group proteins(PcGs) play an important role in the development and disease of organisms. As the major subunit of Pc G proteins, EZH2 has histone methyltrasferase(HMT) activity, which mediates silencing expressions of target genes by alteration of the methylation level of histone in the promoter region of these target genes. In our previous research, we found that EZH2 is overexpressed in human CRC. EZH2 could decrease cell apoptosis and increase CRC proliferation/invasion ability in vitro. Downregulation of EZH2 significantly increased the expression of epithelial markers(E-cadherin and VDR) and decreased the expression of mesenchymal marker(N-cadherin). Downregulation of epithelial markers and upregualtion of mesenchymal markers mean the initiation of EMT in tumor development in CRC cells.Therefore, in this study we focused on the role and mechanism by which EZH2 mediates CRC invasion and the regulatory mechanism of EZH2 in vivo. We found increased expression of STAT3, p-STAT3, EZH2, and decreased expression of VDR in CRC patients. Significant inverse correlations were observed between the expression of EZH2, STAT3, p-STAT3 and that of VDR. The expression of STAT3, p-STAT3, EZH2 and VDR were correlated with histological grade, vascular invasion and lymph node metastasis. Downregulation of EZH2 is associated with better survival rate in patients. Overexpression of STAT3 could lead to upregulation of EZH2 expression and downregulation of VDR in a nude mouse CRC cell xenograft model. Knockdown of EZH2 remarkably increased the overall survival time and significantly decreased lung metastasis of tumor cells in our nude mice metastatic tumor model. Above all, we demonstrated the possible mechanism by which EZH2 induced alteration of VDR or other EMT-associated genes expression in response to STAT3 activation in CRC EMT initiation and metastasis; and evaluated the significance of EZH2, VDR and other EMT genes in clinical diagnosis and treatment via in vivo experiments. In a word, our research may provide potential targets and theoretical direction for molecular diagnosis and treatment of CRC invasion and metastasis. Chapter Ⅱ The role and molecular mechanism of miR-508 in colorectal cancer progressionEpithelial-Mesenchymal Transition(EMT) is a vital process in the initiation of tumor metastasis. EMT plays important roles in CRC invasion, however, the key driver moleculars and mechanisms in this process remain largely unknown. According to recent research, micro RNA could regulate EMT and invasion in CRC. Mi R-506, a key family member in miR-506 family, could modulate EMT via targeting TGF-βsignaling in human ovary cancer, and plays vital role in gastric cancer, breast cancer and liver cancer. However, the role of miR-506 and any of miR-506 family members in CRC is unknown.Therefore, in this study we evaluated the role and mechanisms of miR-506 family members in CRC invasion. Our preliminary data revealed that, miR-506 family members differ from each other in CRC. Among them, miR-508-3p/5p showed the most significant expression difference between CRC tissues and normal tissues, and the most significant inhibition of invasion ability. According to data from miR Base and our experiment data, miR-508-3p is a major product of miR-508, the expression level of miR-508-3p is much higher than that of miR-508-5p. So we further narrowed our research on miR-508-3p.We confirmed that miR-508-3p could inhibit EMT in colorectal cancer via upreglulating epithelial markers(CDH1 and Zo-1) and downregulating mesenchymal markers(Vimentin and N-cadherin). Mi R-508-3p could block transforming growth factor β(TGF-β)-induced EMT in CRC via downregulating its putative target genes SALL4/ZEB1/BMI1. We also found that miR-508-3p expression was associated with decreased mesenchymal markers, elevated CDH1, decreased ZEB1, SALL4, BMI1, and longer overall survival duration in CRC patients, suggesting that miR-508-3p could serve as a prognostic factor in CRC. As a key regulator of EMT, the mechanism of the attenuated expression of miR-508-3p in CRC remained unclear. We identified 6 CpG sites in the promoter region of miR-508-3p and validated that miR-508-3p may be regulated by DNA methylation in CRC.