A287 Background: Endothelin-1 (ET-1) is a potent vasoconstrictor peptide present as a normal ejaculate protein that we have shown to be elevated in men with prostate adenocarcinoma. The endothelin A receptor (ET A R) subtype is the predominant ET receptor in prostate cancer and functions in cell survival, in part, through PI3-kinase signaling and subsequent Akt phosphorylation. The endothelin B receptor (ET B R) subtype is downregulated in cancer through promoter methylation. ET A R levels are further upregulated during androgen ablation as demonstrated in both an in vitro model of prostate cancer and the prostate cancer tissue of men receiving hormone ablation therapy.
Aims: The present study examined the effects of combining cytotoxic drugs with the specific ET A R antagonist, ZD4054, on prostate cancer cell survival. In addition, this study examined the effects of the androgen receptor antagonist, bicalutamide (Casodex ® ), on ET-1 secretion.
Methods: Human prostate cancer (PCa) cell lines were pretreated with 10 nM ZD4054 followed by different cytotoxic agents: paclitaxel, docetaxel, vinblastine, and doxorubicin (100 pM-1 µM). Survival was measured by MTT assay and apoptosis by an ELISA assay. Modulation of ET-1 secretion in prostate cancer cell lines by bicalutamide treatment (0.1-100 µM) was analyzed using an ELISA-based assay, and the effect of ZD4054 on prostate-specific antigen (PSA) secretion was assessed by the University of Pittsburgh Medical Center’s clinical chemistry laboratory using the Tosoh assay. Immunoblot analysis was used to assess ZD4054’s effect on ET-1-induced Akt phosphorylation.
Results: Treatment of PCa cells with ZD4054, in combination with either paclitaxel or docetaxel, significantly reduced proliferation compared with any treatment alone. ZD4054 also inhibited ET-1-induced Akt phosphorylation in PPC-1 cells. In androgen-sensitive PCa cell lines (LNCaP and LAPC4), androgen receptor blockade with bicalutamide significantly increased ET-1 secretion (LNCaP 2.0-fold at 1 µM; LAPC4 1.9-fold at 100 µM) compared with vehicle-treated cells, while PSA secretion did not change with ZD4054 treatment.
Conclusions : This study has shown that ZD4054, in combination with taxanes, potentiates PCa cell death, in part through inhibiting ET-1-induced Akt phosphorylation. In androgen-sensitive cells, the induction of ET-1 secretion by bicalutamide treatment implies that ET-1 signaling through ET A R may be more efficiently targeted during androgen ablation to inhibit survival of androgen-refractory prostate cancer cells.