Roles And Mechanisms Of FOXC1 In Prevention Of Metastasis Of Breast Cancer Cells And P18^INK4C In Differentiation Of K562 Cells

This thesis is composed of two different parts of research work.Part I Roles and mechanisms of PcG target gene FOXC1 in prevention of metastasis of breast cancer cellsMalignant cell metastasis of breast cancer is the most important cause of death. The mechanism of metastasis has been a hot point of cancer researches. Recognition of metastasis-related genes have a great significance in prevention and therapeutics of breast carcinomatous metastasis. Polycomb group (PcG) proteins, which have methyltransferase activity specific to histone H3K27, are central players in various epigenetic phenomena, such as the maintenance of Hox expression patterns during development in organisms ranging from fruit flies to humans; as well as in X chromosome inactivation and cell proliferation in mammals. PcG proteins have recently been shown to be related to cancer development. The polycomb group protein EZH2 is involved in progression of prostate cancer and breast cancer, and has been identified as a molecular marker that distinguishes indolent breast cancer from those at risk of lethal progression, and as a possible target for preventative intervention. Other members of PcG have also been reported to associate with cancer development; for example, Bmi1 was identified as a proto-oncogene that cooperates with Myc to promote the generation of B- and T-cell lymphomas. Nevertheless, the molecular mechanisms by which PcG proteins regulate cancer progression and malignant metastasis are still unclear so far. FOXC1 is a member of the Forkhead box transcription factor family, which play an important role in development and is involved in eye development. Some members of the FOX family play a role in cancer progression, such as FOXO, which has been identified as a potential tumor suppressor. In this study, we discovered that the expression of FOXC1 gene was negatively correlated to that of PcG genes in two different breast cancer cell lines MCF-7 and MDA-MB-231. Moreover, PcG proteins, especially EZH2, could transcriptional regulate the expression of FOXC1 gene. Chromatin immunoprecipitation (ChIP) assays showed that the histone methylation and acetylation modifications played important roles in this regulatory process. When FOXC1 was stably transfected into MDA-MB-231 cell line, it inhibited cell proliferation, induced cell cycle arrest at S phase, prevented cell migration and invasion, and induced tumor cell senescence. Together, our results provide evidence that FOXC1 is a target of PcG proteins. Additionally, our study also demonstrated that FOXC1 expression was related to breast cancer progression, and overexpression of FOXC1 inhibited metastasis. Data presented in this thesis contribute considerably to the understanding of the mechanisms by which EZH2 regulates tumor development.Part II Sodium butyrate-mediated p18^INK4C upregulation participated in K562 cell G0/G1 arrest and erythroid differentiationHistone deacetylase inhibitor sodium butyrate (NaBu) can induce G0/G1 arrest and erythroid differentiation in K562 cells. However, the molecular mechanisms underlying this process are unclear. Here we show that both p18^INK4C mRNA and protein levels were upregulated during K562 cell erythroid differentiation induced by NaBu. Moreover, the NaBu activation of p18^INK4C was dependent on the integrity of Sp1 clusters in the promoter. NaBu caused hyperacetylation of histones H3 and H4 on endogenous p18^INK4C promoter and enhanced binding of transcription factor Sp1 in vivo. Also, overexpression of p18^INK4C in K562 cells resulted in G0/G1 arrest and partial erythroid differentiation. Our results suggested that NaBu-mediated p18^INK4C regulation played a role in cell cycle arrest and erythroid differentiation in K562 cells.