The goals of our project are to isolate prostate epithelial populations, including prostate stem cells, and to evaluate the tumorigenic and the metastatic capacity of such populations upon transformation by genetic mutations that are commonly associated with human prostate cancer. Here we describe our progress in characterizing a mouse model of prostate cancer initiated by prostate epithelial cell specific deletion of the tumor suppressors, Pten and Tp53. Loss of PTEN and loss of TP53 are common genetic aberrations occurring in prostate cancer. PTEN and TP53 contribute significantly to the regulation of adult stem/progenitor cells in somatic tissues, and we hypothesized that combined deletion of these tumor suppressors in prostate epithelium would provide insight into the pathophysiology of transformed prostate progenitor cells. Using the PB-Cre4;Ptenfl/fl;TP53fl/fl model of prostate cancer, we describe the histologic and metastatic properties of primary tumors, of transplanted primary tumor cells, and of clonal cell lines established from tumors. Adenocarcinoma was the major tumor type that developed in this mouse model, and in addition, basal carcinomas, prostatic urothelial carcinoma and late-developing sarcomatoid carcinomas were observed. We show here that tumor initiating cells with multi-lineage potential gave rise to orthotopic PIN (oPIN), adenocarcinomas and basal-phenotype carcinomas. CK8+ committed luminal epithelial cells were capable of undergoing epithelial to mesenchymal transition in vivo to sarcomatoid carcinomas that often produced cartilaginous and bone structures. Metastasis was rarely observed from primary tumors, even after secondary orthotopic transplantation, implying that additional selection must occur for metastatic colonization. Upon orthotopic tumor development from cell lines, multi-potential epithelial progenitor cells were highly metastatic to lung and lymph nodes while prostatic sarcomatoid carcinoma cells rarely metastasized. Combined loss of Pten and Tp53 in prostate epithelial progenitors results in lineage plasticity of transformed cells and heterogeneity of resultant tumors. TP53 contributes significantly to the regulation of stem cell self-renewal, and we hypothesized that Pten/TP53 play crucial roles in determining prostate cancer stem/progenitor cell properties. Clonogenic assays that isolate progenitor function in primary prostate epithelial cells were used to measure self-renewal, differentiation, and tumorigenic potential. Pten/TP53 null as compared to wild type protospheres showed increased self-renewal activity and modified lineage commitment. Pten/TP53 null progenitors also demonstrated increased dependence upon the AKT/mTORC1 and androgen receptor pathways for clonogenic and tumorigenic growth. Orthotopic transplantation of Pten/TP53 null cells derived from protospheres produced invasive PIN/adenocarcinoma, recapitulating the pathology seen in primary tumors. These data demonstrate roles for Pten/TP53 in prostate epithelial stem/progenitor cell function, and moreover, as seen in patients with castrate-resistant prostate cancer, suggest a crucial role for an AR-dependent axis in the clonogenic expansion of prostate cancer stem cells. The model system described here has proven to be unique among mouse models of prostate cancer in allowing robust serial transplantation of adenocarcinoma. This feature has allowed us to analyze the characteristics of tumor initiating populations of prostate adenocarcinoma. The characteristics analyzed have included androgen sensitivity and expression of stem cell markers.
