The Transcription Factor OCT1 in Metabolism and Malignancy

The transition of a cell from a normal to a cancerous state requires a series of alterations within the cell that often start with genetic changes to tumor suppressors and oncogenes. In some cases, however, the proteins encoded by such genes are altered without concomitant modification to their mRNA sequence, and hence are missed from screens that look only for genetic alterations. The transcription factor Oct1 is heavily modified at the protein level, and most of these modifications have been studied extensively. Oct1 has been shown to play key roles in malignant transformation, such as promoting genotoxic and oxidative stress resistance, inducing a metabolic switch to glycolysis, and increasing expression of cell transporters, among others. These characteristics, however, are also shared by stem cells, and thus Oct1 is an important transcription factor in stem cell biology. In this work, I have analyzed the consequence of ubiquitination on the Oct1 protein, which leads to its degradation via the proteasome. I have studied the stability of Oct1 protein in breast cancer as well as in intestinal stem cells and intestinal and colon cancer. In Chapter 2, I summarize what’s currently known about Oct1 in epithelial malignancies, including those of the breast and colon. Chapter 3 describes a study where I identify the tumor suppressor BRCA1 as a ubiquitin E3 ligase that targets Oct1 for proteasomal degradation, leading to a metabolic switch in the cell to a more glycolytic state and higher rates of transformation in a xenograft assay. Lastly, I study the roles of Oct1 in intestinal and colonic stem cells and identify a role for Oct1 in intestinal epithelium regeneration, but not homeostasis. I extend these findings by studying the loss of Oct1 in two different models of colon cancer, which yield opposing results reconciled by the fact that Oct1 can also play tumor suppressing roles such as helping to maintain normal mitoses. The results from the studies described here contribute to what is currently known about this transcription factor, and add complexity to its biology and function

Oct1/Pou2f1 is selectively required for colon regeneration and regulates colon malignancy.

The transcription factor Oct1/Pou2f1 promotes poised gene expression states, mitotic stability, glycolytic metabolism and other characteristics of stem cell potency. To determine the effect of Oct1 loss on stem cell maintenance and malignancy, we deleted Oct1 in two different mouse gut stem cell compartments. Oct1 deletion preserved homeostasis in vivo and the ability to establish organoids in vitro, but blocked the ability to recover from treatment with dextran sodium sulfate, and the ability to maintain organoids after passage. In a chemical model of colon cancer, loss of Oct1 in the colon severely restricted tumorigenicity. In contrast, loss of one or both Oct1 alleles progressively increased tumor burden in a colon cancer model driven by loss-of-heterozygosity of the tumor suppressor gene Apc. The different outcomes are consistent with prior findings that Oct1 promotes mitotic stability, and consistent with differentially expressed genes between the two models. Oct1 ChIPseq using HCT116 colon carcinoma cells identifies target genes associated with mitotic stability, metabolism, stress response and malignancy. This set of gene targets overlaps significantly with genes differentially expressed in the two tumor models. These results reveal that Oct1 is selectively required for recovery after colon damage, and that Oct1 has potent effects in colon malignancy, with outcome (pro-oncogenic or tumor suppressive) dictated by tumor etiology