A complex transcriptional circuitry is essential in maintaining the self-renewal capacity and the undifferentiated state of embryonic stem cells (ESCs). Key factors in this pluripotency network include Sox2, Nanog, and Oct3/4, which form multiple positive and negative feedback loops to regulate an array of downstream genes. Although it was previously thought that pluripotency factors are downregulated upon differentiation, the phenotypic similarities between ESCs and cancer cells have led to speculation that common underlying regulatory mechanisms may exist. For example, the cancer phenotype includes a block in differentiation, limitless replicative potential and increased proliferation, characteristics that are also observed in pluripotent embryonic stem cells. Taken together, these observations regarding similarities in tumor cells and embryonic stem cells suggest regulatory mechanisms that may be shared in development and cancer. In the studies presented here, I investigated the expression, localization, and function of Nanog, Sox2, Oct4 and particularly the Oct3 splice variant protein in cancer.
I performed a variety of biochemical, genetic and pharmacological experiments to elucidate potential roles of these pluripotency factors in maintaining the cancer phenotype, using a panel of breast cancer cells as a model system. Expression of Sox2 and the Oct3 isoform was detected in all breast cancer cell lines investigated, whereas Nanog and Oct4 expression were not detected. Moreover, Sox2 was localized to both the cytoplasm and nucleus, but Oct3 expression was detected exclusively the cytoplasm of breast cancer cells. This study also shows that Oct3 and Sox2 are upregulated in a number of breast cancer cell lines when compared to expression levels in mammary epithelial cells. I also determined that Src is an upstream regulator of Oct3 and Sox2 expression.
Given the known function of Sox2 as a regulator of the cell cycle, I focused subsequent studies on the characterization and function of Oct3. Results showed that the Oct3 N-terminal domain contains an auto-inhibitory sequence that blocks its nuclear translocation despite the presence of a functional nuclear localization signal. Additional experiments to investigate Oct3 function suggested that Oct3 may be involved in regulating the cell cycle and that this regulation may be in concert with Sox2 and β-catenin to modulate cyclin D1 expression. However, studies of Oct3 function in cancer remain inconclusive due to technical difficulties in Oct3 RNAi that precluded achieving consistent Oct3 knockdown. Despite these difficulties, the data presented here suggest that Oct3 may play a role in maintaining the cancer phenotype, warranting further functional studies.