Identification of EP4 as a Potential Target for the Treatment of Castration-Resistant Prostate Cancer Using a Novel Xenograft Model

More effective therapeutic approaches for castration-resistant prostate cancer (CRPC) are urgently needed, thus reinforcing the need to understand how prostate tumors progress to castration resistance. We have established a novel mouse xenograft model of prostate cancer, KUCaP-2, which expresses the wild-type androgen receptor (AR) and which produces the prostate-specific antigen (PSA). In this model, tumors regress soon after castration, but then reproducibly restore their ability to proliferate after 1 to 2 months without AR mutation, mimicking the clinical behavior of CRPC. In the present study, we used this model to identify novel therapeutic targets for CRPC. Evaluating tumor tissues at various stages by gene expression profiling, we discovered that the prostaglandin E receptor EP4 subtype (EP4) was significantly upregulated during progression to castration resistance. Immunohistochemical results of human prostate cancer tissues confirmed that EP4 expression was higher in CRPC compared with hormone-naïve prostate cancer. Ectopic overexpression of EP4 in LNCaP cells (LNCaP-EP4 cells) drove proliferation and PSA production in the absence of androgen supplementation in vitro and in vivo. Androgen-independent proliferation of LNCaP-EP4 cells was suppressed when AR expression was attenuated by RNA interference. Treatment of LNCaP-EP4 cells with a specific EP4 antagonist, ONO-AE3-208, decreased intracellular cyclic AMP levels, suppressed PSA production in vitro, and inhibited castration-resistant growth of LNCaP-EP4 or KUCaP-2 tumors in vivo. Our findings reveal that EP4 overexpression, via AR activation, supports an important mechanism for castration-resistant progression of prostate cancer. Furthermore, they prompt further evaluation of EP4 antagonists as a novel therapeutic modality to treat CRPC. 4 samples in each group: androgen-dependent growth (AD), castration-induced regression nadir (ND), and castration-resistant regrowth (CR) stages

临床上迫切需要更有效的去势抵抗性前列腺癌（castration-resistant prostate cancer, CRPC）治疗方案，因此亟需阐明前列腺肿瘤如何进展为去势抵抗表型。我们构建了一种新型前列腺癌小鼠异种移植模型KUCaP-2，该模型表达野生型雄激素受体（androgen receptor, AR），并可分泌前列腺特异性抗原（prostate-specific antigen, PSA）。在该模型中，肿瘤在去势后短期内发生退缩，但在1~2个月后可重现性恢复增殖能力，且未发生AR突变，这一表型模拟了CRPC的临床进程。本研究利用该模型筛选CRPC的新型治疗靶点。通过基因表达谱分析对不同阶段的肿瘤组织进行评估，我们发现前列腺素E受体EP4亚型（prostaglandin E receptor EP4 subtype, EP4）在进展为去势抵抗的过程中显著上调。对人前列腺癌组织的免疫组化结果证实，相较于激素初治前列腺癌，CRPC组织中EP4的表达水平显著升高。在LNCaP细胞中异位过表达EP4（即LNCaP-EP4细胞），可在无雄激素补充的条件下促进细胞增殖与PSA分泌，该现象在体外与体内实验中均得到验证。当通过RNA干扰（RNA interference, RNAi）抑制AR表达后，LNCaP-EP4细胞的雄激素非依赖性增殖受到抑制。使用特异性EP4拮抗剂ONO-AE3-208处理LNCaP-EP4细胞，可降低细胞内环磷酸腺苷（cyclic AMP, cAMP）水平，在体外抑制PSA分泌，并在体内抑制LNCaP-EP4或KUCaP-2肿瘤的去势抵抗性生长。本研究结果表明，EP4过表达通过激活AR通路，为前列腺癌进展为去势抵抗性表型提供了重要的分子机制。此外，本研究提示EP4拮抗剂有望作为治疗CRPC的新型治疗手段，有待进一步评估其临床应用价值。每组包含4例样本：雄激素依赖性生长（androgen-dependent growth, AD）期、去势诱导的退缩低谷期（castration-induced regression nadir, ND）以及去势抵抗性复发生长（castration-resistant regrowth, CR）期。