PTEN is one of the most commonly deleted tumor suppressor genes in human prostate cancer. Our group previously demonstrated that Pten deletion in the murine prostate epithelium recapitulates the disease progression seen in human prostate cancer, culminating in invasive adenocarcinoma. In addition to Pten loss endowing prostate cells with enhanced proliferative capacity, we found that Pten loss also led to the upregulation of inflammatory pathways, including Csf-1 and Il1b expression, within the prostate epithelium. These inflammatory cytokines recruit myeloid-derived suppressor cells (MDSCs) into the prostate, which subsequently promote an immune-suppressive tumor microenvironment and thereby facilitate tumor progression. Targeting immune-responsive pathways with the CSF-1R inhibitor GW2580 successfully inhibits MDSC infiltration and delays tumor progression.
As Pten deletion alone does not produce distant macrometastasis, we surveyed additional pathways altered in human metastatic prostate cancer, and found that the RAS/MAPK pathway was significantly elevated in metastatic lesions. Indeed, when we combined Pten deletion with Kras activation in the prostate epithelium (Pb-Cre+/-;PtenL/L;KrasG12D/+) (CPK), we observed macrometastasis to the lungs and liver. Interestingly, within the prostate, we observed an epithelial-mesenchymal transition (EMT) phenotype, accompanied by significant upregulation of the EMT transcription factor Snail. Importantly, genetic deletion of Snail in CPK mice prevented distant macrometastasis, providing a mechanistic link between EMT and metastasis.
To study the dynamic regulation of the EMT process, we crossed CPK mice with Vimentin-GFP reporter mice (CPKV), and were able to isolate populations of epithelial, EMT, and mesenchymal-like prostate tumor cells. We demonstrate that EMT and mesenchymal-like tumor cells have enhanced stem-like and tumor-initiating capacities and exhibit cellular plasticity in vivo. HMGA2, a chromatin remodeling protein, is significantly upregulated in EMT and mesenchymal-like tumor cells, as well as in human metastatic castration-resistant prostate cancer (mCRPC). Knockdown of Hmga2, or suppressing Hmga2 expression with the HDAC inhibitor LBH589, inhibits epithelial-mesenchymal plasticity and stemness activities in vitro and dramatically reduces prostate tumor burden and distant metastasis in vivo. Importantly, LBH589 in combination with castration significantly prolongs survival by targeting castration-resistant mesenchymal-like tumor cells and preventing mCRPC. LBH589 treatment in combination with androgen deprivation therapy may therefore be a promising treatment for patients with mCRPC.