In efforts to curb tumor progression androgen ablation remains at the forefront of treating advanced, metastatic prostate cancer; however, treatment failure occurs in many patients resulting in hormone-refractory disease. Previous reports have correlated ETA receptor expression with increasing prostate cancer grade and stage. We have shown in prostate cancer that the ET-axis, in particular endothelin-1 signaling through the ETA receptor, protects against paclitaxel-induced apoptosis. We hypothesize that the ET-axis contributes to the evolution of advanced, hormone-refractory prostate cancer by providing prostate epithelia a means of surviving androgen deprivation.
To examine changes in ETA receptor expression due to long-term androgen deprivation, prostate cancer cells were cultured in androgen-reduced media for 12 months. Real Time-qPCR analysis demonstrated increases in ETA, ET-1 & androgen receptor (AR) mRNA levels as early as four weeks. Affymetrix microarray analysis of three week, one, five and 12 month androgen-deprived cells confirmed the changes in ETA, ET-1 and AR expression and identified additional genes which demonstrate gene expression patterns similar to ETA. Testing the effect of acute AR blockade on ET-1 secretion in prostate cancer cells, an ELISA based assay demonstrated that treatment with the specific AR inhibitor bicalutamide, resulted in significantly elevated ET-1 secretion within 48 hours. To determine the effect of hormone therapy on ETA expression in human prostate tissue we performed immunohistochemistry on short- and long-term androgen ablated human prostate tumor specimens. Prostate tissue from patients receiving long-term ablation (>6 months) revealed significant increases in ETA receptor expression compared with intact patients. We previously demonstrated that ET-1 treatment of prostate cancer cells induces the phosphorylation of AKT (pAKT). ETA over-expression results in greater pAKT and a more rapid induction of pAKT than in wild-type cells. Additionally, ETA blockade, using the specific ETA antagonist atrasentan, reduced pAKT levels in both wild type and ETA over-expressing prostate cancer cells. Matrigel invasion assay results suggest that ETA receptor expression may even play a role in prostate cancer cell invasion. Lastly, in vivo mouse model experiments demonstrate that atrasentan in combination with castration more effectively inhibits xenograft tumor progression than castration or atrasentan alone.
We have shown that androgen deprivation increases ETA and ET-1 expression, which enhances pAKT induction. ET-axis signaling during androgen deprivation may promote prostate cancer cell survival through inhibiting apoptotic mechanisms, therefore setting the stage for progression to advanced, hormone-refractory prostate cancer.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA