Potentiation of 2-methoxyestradiol-induced cytotoxicity by blocking endothelin A receptor in prostate cancer cells

BACKGROUND. 2-Methoxyestradiol (2ME2) is an antitumoral and antiangiogenic compound that inhibits hypoxia-inducible factor (HIF)-1, a key regulator of the hypoxic response that promotes tumor progression. HIF-1α, the regulated subunit of HIF-1, is overexpressed in premalignant, cancerous and metastatic lesions of prostate. Endothelin (ET)-1 is a HIF target gene and one that plays an important role during prostate bone metastasis via its interaction with endothelin A (ET_A) receptor. We reasoned that 2ME2 combined with an ET_A receptor antagonist would induce potent cytotoxic effects in prostate cancer cells. METHODS. PC-3 and LNCaP cells were grown alone or cocultured with human osteoblasts. The cells were treated with 2ME2, with an ET_A receptor antagonist (BQ-123) or with combinations of both compounds. The cells were then evaluated for cytotoxicity, HIF-1α protein expression and HIF-1 transcriptional activity. RESULTS. The combination of 2ME2 with BQ-123 induced synergistic cytotoxic effects in prostate cancer cells and in their cocultures with osteoblasts. No synergism was observed when 2ME2 was combined with the ET_B selective antagonist, BQ-788. These results correlated with inhibition of HIF-1α protein expression, HIF-1 transcriptional activity, and PSA mRNA expression. CONCLUSIONS. The ET _A receptor antagonist was capable of potentiating the cytotoxic effects of 2ME2 in prostate cancer cells. These effects were apparently mediated through the inhibition of the HIF-1 pathway. Our in vitro data strengthen the rationale for using 2ME2 in combination with ET_A receptor antagonists for the treatment of metastatic prostate cancer.