TGF-beta Signaling in Prostate Cancer

In the first part of this dissertation I examine functions of the proteins Meis2 and Pbx1. Meis2 and Pbx1 are homeodomain containing transcription factors that function cooperatively, both with each other and with other proteins to regulate gene transcription. Meis2 and Pbx1 are both important regulators of development and cancer. Due to the presence of potential Meis/TGIF DNA binding sites in the promoter of the important cyclin dependent kinase p15 (CDKN2B), we hypothesized that Meis2 and Pbx1 regulated its transcription. p15 is an important inhibitor of the cell cycle and a well characterized tumor suppressor. I show that Meis2 and Pbx1 function to regulate transcription of p15 and E-cadherin. The regulation of p15 was mapped to a region in its promoter containing GC boxes previously reported to bind Sp1. However, I found that Meis2 and Pbx1 were regulating p15 through a newly described interaction with Klf4. I show that Klf4 functions to recruit Meis2 and Pbx1 to DNA, where they help to activate gene transcription. Further, the DNA binding sequences for these components were used in a bioinformatics approach to find additional genes regulated in a cooperative manner by Meis2, Pbx1 and Klf4. Since we demonstrated that Meis2 and Pbx1 activated expression of the important tumor suppressors p15 and E-cadherin, we hypothesized that expression of Meis2 and Pbx1 would be decreased in cancer. Consequently, we found that expression of Meis2 and Pbx1 were decreased in prostate cancer. These experiments further the knowledge of Meis2 and Pbx1’s functions and help explain some of the many different roles played by these proteins.
In the second part of this dissertation I describe novel mouse models of prostate cancer. Prostate cancer is the second most common cancer in men in the US and causes the second most cancer deaths in men in the US. Mouse models that mimic human disease are a necessity in the field of prostate cancer research for studying tumor growth and for testing new drugs. Genetic deletion of genes that function in the TGF-β signaling pathway are common in prostate cancer. Therefore, we hypothesized that deletion of the TGF-β type II receptor (Tgfbr2), which effectively blocks activation of the TGF-β signaling pathway, would lead to increased tumor formation in combination with other oncogenic stimuli. I describe how loss of TGF-β signaling by knockout of Tgfbr2 cooperates with loss of the tumor suppressor Pten to cause a rapid progression to adenocarcinoma. This model also results in a significant number of metastases to the lymph nodes and lungs. The cancer cells are both highly proliferative, as shown by staining for Cyclin D & Ki67, and resistant to androgen deprivation by castration. In this model, we also discovered activation of TGF-β signaling downstream of loss of Pten. We think the induction of TGF-β signaling functions as a tumor suppressive feedback mechanism in the event of loss of Pten. Furthermore, this activation of TGF-β signaling is demonstrated to be downstream of the oncogenic kinase Akt. Accordingly, deletion of Tgfbr2 results in cancer when combined with a constitutively active Akt1. I also found that deletion of Tgfbr2 cooperates with loss of Apc (Adenomatous polyposis coli), a tumor suppressor and important inhibitor of Wnt signaling, resulting in adenosquamous carcinoma in the prostate. Subsequently, it was discovered that in all of these models loss of Tgfbr2 causes growth of the basal cell and stem-like cell populations, which we believe is driving tumor formation. Therefore, we hypothesize that the TGF-β pathway is important for inhibiting growth of the basal lineage and for promoting differentiation of cells in the prostate. These models help further the knowledge of TGF-β signaling in prostate cancer, which should lead to better ideas of critical components to target and thereby to better therapies in the future.