Role And Mechanism Of Six1 Gene On Biological Behavior Of Cervical Cancer Cells

Background and Objective:Cervical cancer is one of the most frequent malignancies in women, with an estimated worldwide incidence of about 500000 new cases and almost 15% in deaths of all cancers. This malignancy is the second most common cause of death from cancer, after breast carcinoma, among women worldwide. It was demonstrated that persistent high-risk human papillomavirus (HPV) infection is the major cause for cervical cancer; however, only a small percentage of infected women develop cervical cancer. Therefore, there may be other pathogenetic factors that play an independent role or are synergistic with high-risk HPV in the occurrence of cervical cancer, although the precise mechanism is unknown yet. Early-stage cervical cancer can be treated satisfactorily by surgery and/or radiotherapy. However, there are no effective treatments for advanced cervical cancer. Thereby, it is crucial to investigate the molecular mechanism of cervical cancer.The homeobox gene superfamily encodes transcription factors that act as master regulators of development through activating or repressing downstream target genes. Six1, an important member of the homeobox gene family, is mapped to human chromosome 14q23. It is composed of HD and Six domains, and is essential for the development of multiple organs such as the brain, ear, eye, muscle and kidney. Inappropriate expression of Six1 may initiate tumorigenesis and promote metastasis. Alteration of Six1 expression takes place in human breast cancer , ovarian cancer, Wilms’cancer and rhabdomyosarcoma (RMS), which indicates that Six may contribute to tumorigenesis. Overexpression of Six1 is potentially related to the metastatic status of breast cancer, RMS, ovarian cancer, and hepatocellular carcinoma. The precise role of Six1 in tumorigenesis is still unclear. It was demonstrated that as a cell cycle regulator, Six1 can activate cell cycle and promote cell differentiation and proliferation. Six1 itself is regulated in the cell cycle. Six1 is a transcriptional target of E2F1, and its transcript levels increase as cells progress into S-phase. Six1 is degraded at the late stage of mitosis, and this allows activation of the cell cycle regulatory targets of Six1, such as cyclins A1 and D1, thus ensuring proper cell cycle progression. Six overexpression results in accelerated cell cycle progression, indefinite cell proliferation, and tumorigenesis. The action targets of Six1 vary from one tumor type to another. Cyclin A1 is a downstream effector for Six1 in breast cancer where overexpression of Six1 promotes cyclin A1 expression and subsequently increases cell proliferation and cell cycle progression. In rhabdomyosarcoma, Six1 overexpression activates oncogenes such as cyclin D1, Cyclin A1 and Ezrin, and promotes metastasis via Ezrin. Six1 may contribute to ovarian epithelial carcinogenesis by increasing A-type cyclin-mediated proliferation, Six1 expression also correlates with advanced stage and poor survival in ovarian cancer.It has been demonstrated that cyclins A1 and D1 are expressed at low levels or even not expressed in normal cervical tissue, and that they are upregulated in cervical intraepithelial neoplasias and cervical cancer, suggesting their roles in the occurrence and development of cervical cancer. Changes in the levels of these cell cycle regulators may become a useful indicator for early diagnosis and prognosis prediction of cervical cancer. In addition, Overexpression of Ezrin has a positive effect on invasive transition of cervical cancer. These results provide a theoretical basis for further study of the mechanism of Six1 in cervical cancer.The results and conclusions were as follow:1. There was Six1 mRNA and protein overexpression in cervical cancer cell lines CaSki, HeLa, and C33A. The Six1 expression level was higher in CaSki and HeLa cells than in C33A cells. Six1 mRNA and protein expression increased from normal cervical epithelial tissues, to cervical intraepithelial neoplasias, and then to cervical cancer tissue. The status of Six1 overexpression was correlated to clinical staging and lymph node metastasis of cervical cancer, but not to pathological grading, tumor size, and age of the patient.2. A recombinant adenovirus vector for Six1 gene didn’t influence the cellular morphology after transfected, A recombinant adenovirus vector for sense and antisense Six1 gene can up-regulated or down-regulated Six1 expression. It is concluded that the adenovirus vector for Six1 gene has been constructed suecessfully, which provides the material basis for further studying the biologic function and potential application of Six1.3. Overexpression of Six1 shortened G2/M phase, increased cervical cancer cells proliferation and invasion. After the suppression of Six1 expression, the invasion and proliferation ability were decreased, And resulted in an arrest in G2/M phase,. It is suggested that Six1 may be involved in regulation of proliferation and invasiveness of cervical cancer cells.4. The nude mice were subcutaneously inoculated with CaSki cells transfected with antisense Six1 gene by recombinant adenovirus vector. Compared to the two control groups, the tumor growth of antisense Six1 gene group was more slowly, and tumor weight inhibition effects were significantly higher, The results suggested that the down regulation of Six1 expression could suppress the growth of tumor in nude mice.5. The excessive expression of Six1 up-regulated the expression of Cyclin A1、Cyclin D1 and Ezrin in cervical cancer cells, and after the suppression of Six1 expression, the expression of Cyclin A1、Cyclin D1 and Ezrin in cervical cancer cells were inhibited. It is suggested that Cyclin A1、Cyclin D1 and Ezrin could have an important role in target genes in the regulation of biological function of cervical cancer.