Prostate epithelium-specific deletion of the selenocysteine tRNA gene Trsp leads to early onset intraepithelial neoplasia

Although various lines of evidence suggest that oxidative stress plays a role in human prostate cancer initiation and progression, there is a paucity of direct evidence for its role in tumor initiation. To begin to address this issue, we developed a novel tumorigenesis model by reducing the expression of multiple selenoproteins (SPs) in mouse prostatic epithelium. This was accomplished via the prostate-specific deletion of Trsp, a gene that encodes a transfer RNA (Sec tRNA) required for the insertion of selenocysteine residues into SPs during their translation. By 6 weeks of age, Trsp-deficient mice exhibited widespread prostatic intraepithelial neoplasia lesions in all prostatic lobes, which then progressed to high-grade dysplasia and microinvasive carcinoma by 24 weeks. In contrast to other murine prostate cancer models, Trsp-deficient mice required neither the deletion of a tumor suppressor nor the transgenic introduction of an oncogene for prostatic intraepithelial neoplasia lesion development. In keeping with the antioxidant functions of several SPs, we found increases in lipid peroxidation markers in Trsp-deficient epithelial cells. This novel model of prostate neoplasia provides evidence for the existence of a selenoprotein or selenoproteins capable of acting as a tumor suppressor in the murine prostate.

The pace of prostatic intraepithelial neoplasia development is determined by the timing of Pten tumor suppressor gene excision

Loss of the PTEN tumor suppressor is a common occurrence in human prostate cancer, particularly in advanced disease. In keeping with its role as a pivotal upstream regulator of the phosphatidylinositol 3-kinase signaling pathway, experimentally-induced deletion of Pten in the murine prostate invariably results in neoplasia. However, and unlike humans where prostate tumorigenesis likely evolves over decades, disease progression in the constitutively Pten deficient mouse prostate is relatively rapid, culminating in invasive cancer within several weeks post-puberty. Given that the prostate undergoes rapid androgen-dependent growth at puberty, and that Pten excisions during this time might be especially tumorigenic, we hypothesized that delaying prostate-specific Pten deletions until immediately after puberty might alter the pace of tumorigenesis. To this end we generated mice with a tamoxifen-inducible Cre recombinase transgene enabling temporal control over prostate-specific gene alterations. This line was then interbred with mice carrying floxed Pten alleles. Despite evidence of increased Akt/mTOR/S6K axis activity at early time points in Pten-deficient epithelial cells, excisions induced in the post-pubertal (6 wk-old) prostate yielded gradual acquisition of a range of lesions. These progressed from pre-malignant changes (nuclear atypia, focal hyperplasia) and low grade prostatic intraepithelial neoplasia (PIN) at 16-20 wks post-tamoxifen exposure, to overtly malignant lesions by approximately 1 yr of age, characterized by high-grade PIN and microinvasive carcinoma. In contrast, when Pten excisions were triggered in the pre-pubertal (2 week-old) prostate, neoplasia evolved over a more abbreviated time-frame, with a spectrum of premalignant lesions, as well as overt PIN and microinvasive carcinoma by 10-12 wks post-tamoxifen exposure. These results indicate that the developmental stage at which Pten deletions are induced dictates the pace of PIN development.

Temporally controlled prostate epithelium-specific gene alterations

Employing the Hprt locus as the site for targeted transgenesis we have developed mice expressing the tamoxifen-inducible Cre-ER(T2) fusion protein under the control of the ARR2-rat probasin promoter. This system enables external control over the timing of prostate epithelial cell-specific gene alterations. Using both the ROSA26-lacZ and ROSA26-EYFP reporter strains to monitor recombinase activity, Cre-ER(T2) was found to be specifically expressed in the prostatic epithelium and was strictly tamoxifen dependent. This strain thus allows precise control over the timing of gene alterations in the mouse prostate, enabling analyses of the phenotypic consequences of gene alterations in mice of any age. It also provides an ideal platform to study the impact of environmental, hormonal, and age-related factors on prostate tumorigenesis. This latter feature will be of particular value given the paucity of murine models that accurately mimic the late onset and prolonged natural history of human prostate cancer.