THE ROLE OF POU5F1B IN PROSTATE CANCER

Accounting for 14% of all new cancer diagnosis in the United States, prostate cancer is the most commonly diagnosed cancer and the second leading cause of cancer related death in the United States. Prognosis for patients diagnosed with metastatic disease is especially poor, since no effective treatments have been developed (1). In this study, we examined the expression and function of POU5F1B, a protein-encoding pseudogene of the homeodomain transcription factor Oct4, in prostate cancer. POU5F1B is located at 8q24, a gene desert containing numerous alleles associated with prostate cancer risk. A recent study has indicated that a number of these risk alleles are correlated with POU5F1B expression and prostate cancer susceptibility. The role of POU5F1B in prostate cancer carcinogenesis and progression, however, is not known. In our study, we found that POU5F1B expression is upregulated in prostate cancers and highly overexpressed by high grade (Gleason ≥8) and metastatic prostate cancers. We cloned POU5F1B from prostate cancer cell lines, which contains prostate cancer risk associated SNPs, including a missense mutation inside the homeobox DNA binding domain, to study the functional effects of POU5F1B overexpression in prostate cancers. Here, we report that POU5F1B from prostate tumor encodes functional proteins that exhibit gene transactivation activity comparable to its parent gene, Oct4. Further, we report that POU5F1B overexpression in prostate cancer cell lines increases prostate cancer cell proliferation, migration, anchorage independent growth, and drug resistance in vitro and tumor xenograft growth in vivo. Conversely, shRNA mediated knockdown of endogenous POU5F1B expression in prostate cancer cells inhibit cell proliferation in vitro and tumor growth in vivo, as well as prolong tumor free survival in animal models. The data provide compelling evidence that POU5F1B is an important mediator of prostate cancer progression. We further examined the molecular mechanism behind POU5F1B driven prostate cancer progression. Our studies found that POU5F1B overexpression suppresses E-Cadherin expression at both mRNA and protein levels. Our studies further found POU5F1B overexpression in prostate cancer cells increases Wnt1, TCF1, and TCF4 expression, as well as increased Wnt/β-Catenin signaling - indicating the induction of epithelial-to-mesenchymal transition (EMT) in POU5F1B overexpressing cells(2). Consistently, qPCR analysis found that POU5F1B overexpression significantly increased the expressions of numerous EMT related genes and prostate cancer stem cell markers. Functional studies further confirmed that the transactivation activity of Nanog, another stem cell related transcription factor, is dramatically increased in POU5F1B overexpressing cells. Taken together, our data strongly suggests that POU5F1B overexpression drives prostate cancer progression through the induction of EMT and conferment of stem-cell properties to tumor cells. In summary, our data demonstrated that POU5F1B is overexpressed in prostate tumors, especially high-grade and metastatic tumors, and is a functional driver of prostate cancer progression by inducing EMT in prostate cancer cells. Our study also showed that POU5F1B can potentially be targeted to treat prostate cancer. Based on our findings, depletion of POU5F1B may reduce the risk of metastatic disease or tumor recurrence when used with concurrent therapies in early state tumors and may attenuate treatment resistance in diseases at advanced stages