Krupple-Like Factor 4 Regulation in Intestinal Epithelial Differentiation, Proliferation and Polarity
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The zinc finger protein and transcription factor, KLF4, plays a vital role in the regulation of lineage differentiation during development, as well as the maintenance of epithelial homeostasis in the intestine. KLF4 is predominantly expressed in the epithelial cells that are fully differentiated in normal intestine. In colorectal cancer, KLF4 was identified as a tumor suppressor; it is also a stem cell factor that along with other factors can induce pluripotent stem cells. KLF4 knockout mice demonstrate decrease in goblet cell number in the colon; conditional KLF4 ablation from the intestinal epithelium leads to changes in epithelial homeostasis. This dissertation defines the role of KLF4 in normal intestinal cells and in colon cancer cells, as well as the mechanism by which KLF4 regulates intestinal homeostasis and represses tumorigenesis. I demonstrate the role of KLF4 in maintaining numbers, positions and polarity of intestinal epithelial cells, changes of which result in morphology alterations. I further delineate the mechanisms of KLF4 regulation in the intestinal stem cells and colorectal cancers, which mainly focuses on its target Bmi1, a Polycomb group protein, and an intestinal stem cell marker. Human colorectal cancers tissue array staining showed that Bmi1 levels are significantly increased in the tumor tissues from colon cancer patients compared with adjacent normal tissues; and expression of Bmi1 is significantly associated with nuclear β-catenin. Bmi1 knockdown by lentivirus-mediated Bmi1-shRNA repressed growth of different colon cancer cell lines, inhibited xenograft tumor growth, and increased differentiation of colon cancer cells in mouse xenograft. Dominant negative TCF (dnTCF), which blocks Wnt signaling, only marginally inhibits Bmi1 expression. However, the expression of Bmi1 was directly inhibited by KLF4 in LS174T colon cancer cells, and its function in H2A ubiquitination is inhibited by KLF4. In addition, using a three-dimensional (3D) culture system for colon cancer cell lines, I demonstrate that KLF4 is important for cell polarity and position; also it is essential for epithelial crypt-cyst structure formation in vitro. This research provides novel insights into the mechanisms under which KLF4 functions and regulate the intestinal stem cells and colorectal cancers, and will prove useful in development of targeting therapeutics of colon cancer, as well as understanding of tumor initiation in the background of stem cell regulation.