Bioluminescent mouse models of prostate cancer progression and therapy

Prostate cancer is the most common visceral neoplasm and second leading cause of cancer death in US men. It is a complex heterogeneous and multi-factorial disease whose mechanistic basis is poorly understood. Furthermore, treatment options for advanced metastatic prostate cancer are limited and do not impart significant survival benefits, highlighting the need for novel and more effective therapeutic strategies. To this end we examined the use of RNAi therapy based on the systemic delivery of optimized small interfering RNAs (siRNAs) targeting the androgen receptor (AR). siRNA treatment induced massive cell death in vitro but was unable to induce tumor regression or improve survival in an animal model of advanced human prostate cancer due to inefficient cellular delivery and uptake of siRNAs at metastatic sites. To explore further, we developed a bioluminescent animal model that enables the high throughput screening of siRNA pharmacodynamic and biodistribution properties. Systemically delivered siRNAs, although potent, were only effective at inhibiting gene expression in the liver.

Reliable and tractable animal models of prostate cancer are needed in order to help better understand the molecular mechanisms of disease progression. Here we describe an improvement on the previously characterized Pten mutant model of prostate cancer by incorporation of a luciferase reporter allele. Specifically, the cancer initiating event is coupled to activation of firefly luciferase from the endogenous ROSA26 promoter in the same prostate epithelial cells. Additionally, we have extensively backcrossed our model onto the albino C57BL6TYRC2J and BALB/c backgrounds which enhances bioluminescence imaging (BLI) sensitivity. Consistent with previous studies, we show that progression of prostate cancer from 3-12 weeks is rapid and correlates with an increase in bioluminescence. However, longitudinal BLI from 12-40 weeks reveals a plateau in bioluminescence signal that is correlated with a halt in cellular proliferation as evidenced by Ki67 staining at necropsy. In contrast to other studies, prostate cancer never progressed beyond a high grade prostatic intraepithelial neoplasm (HGPIN). Furthermore, we demonstrate for the first time that cancer progression is accompanied by increased inflammation, characterized by enhanced recruitment of myeloid derived suppressor cells (MDSCs). We further demonstrate the utility of B6-Luc mice in monitoring response to prostate cancer therapy.

Relatively few established risk factors for prostate cancer have been identified. Obesity has been linked to increased mortality rates in prostate cancer patients. Using B6-Luc mice we demonstrate that obesity induced a prostatic inflammatory response characterized by increased expression of IL-6 and IL-1β. However, the effects of obesity on prostate cancer progression were inconclusive due to the small number of animals enrolled in the study. Taken together, we have developed novel model systems that will enable the mechanistic basis of prostate cancer progression and response to therapy to be evaluated in the appropriate experimental context.