The efficacy of cisplatin-based chemotherapy in patients with bladder cancer is often limited due to the development of therapeutic resistance. Our recent findings in bladder cancer suggested that activation of prostaglandin receptors (e.g. EP2, EP4) or cyclooxygenase (COX)-2 induced cisplatin resistance. Meanwhile, emerging evidence indicates the involvement of estrogen receptor-β (ERβ) signals in urothelial cancer progression. In this study, we aimed to investigate whether ERβ activity was associated with cisplatin sensitivity in bladder cancer. Immunohistochemistry in muscle-invasive bladder cancer specimens from 55 patients who had subsequently received at least 3 cycles of cisplatin + gemcitabine neoadjuvant chemotherapy showed that ERβ was positive in 38% of responders vs. 71% of non-responders (P = 0.016), including 42% of male responders vs. 65% of male non-responders (P = 0.142) and 20% of female responders vs. 100% of female non-responders (P = 0.048). Then, cisplatin cytotoxicity was compared in human bladder cancer cell lines. Control sublines endogenously expressing ERβ were significantly more resistant to cisplatin treatment at its pharmacological concentrations, compared with ERβ knockdown sublines via short hairpin RNA virus infection. An ER modulator tamoxifen increased sensitivity to cisplatin in ERα-negative/ERβ-positive cell lines, while, in an estrogen-depleted condition, 17β-estradiol reduced it. Additionally, western blot showed considerable elevation in ERβ expression in cisplatin-resistant bladder cancer sublines, compared with respective controls. Moreover, treatment with tamoxifen or a COX-2 inhibitor celecoxib increased cisplatin sensitivity even in resistant cells, while COX-2/EP2/EP4 inhibitor treatment resulted in reduced expression of ERβ. The expression and activity of β-catenin known to involve cisplatin resistance was also up-regulated in cisplatin-resistant cells, which was further induced by 17β-estradiol treatment. The present results suggest that estrogen-mediated ERβ signaling plays an important role in modulating cisplatin sensitivity in bladder cancer cells. Targeting ERβ during chemotherapy may thus be a useful strategy to overcome cisplatin resistance especially in female patients with ERβ-positive bladder cancer.
