Abstract 1313: The androgen metabolite-dependent growth of hormone receptor-positive breast cancer as a possible aromatase inhibitor resistance mechanism.

Introduction: Aromatase inhibitors (AIs) have been reported to exert their anti-proliferative effects not only by reducing estrogen production but also by unmasking the inhibitory effect of androgens such as testosterone (TS) or dihydrotestosterone (DHT) in postmenopausal women with hormone receptor-positive breast cancer. The behavior of androgens in AI-resistance mechanisms is not sufficiently understood. 5α-Androstane-3β,17β-diol (3β-diol) generated from DHT by 3β-hydroxysteroid dehydrogenase type 1 (3β-HSD type 1: HSD3B1) has androgenic activity and substantial estrogenic activity, representing a potential mechanism of AI resistance. Methods: To investigate these issues, ERE-GFP-transfected MCF-7 cells (E10 cells) were cultured for 3 months under steroid-depleted, TS supplemented conditions. Among the surviving cells, two stable variants (V1 and V2 cells) showing ER activity depending on androgen metabolites were selected by monitoring GFP expression. Using these variant cell lines, we investigated the process of adaptation to androgen-abundant conditions and the role of androgens in AI-resistance mechanisms. Results: The variant cell lines showed increased growth and induction of estrogen-responsive genes rather than androgen-responsive genes by androgens or 3β-diol. Further analysis of these cell lines suggested that increased expression of HSD3B1 and reduced expression of androgen receptor (AR) provide adaptability to androgen-abundant conditions, as proved by increased conversion of DHT into 3β-diol by 3β-HSD type 1 and AR signal reduction. Corresponding to these hypotheses, in parental E10 cells, ectopic expression of HSD3B1 or inhibition of AR resulted in adaptation to androgen-abundant conditions. In coculture with stromal cells mimicking the local estrogen production from androgen, these cell lines showed lower sensitivity to an AI compared with parental E10 cells. Immunohistochemistry on 9 pairs of primary and recurrent tissue samples from AI-resistant breast cancer revealed the decrease of AR protein expression in all cases. Conclusion:In the present study, we successfully cloned two stable variants that show ER activity depending on androgen metabolites. Investigation of these cell lines suggested that the increased expression of HSD3B1 and reduced expression of AR might decrease the sensitivity to aromatase inhibitors as proved by enhancement of androgen metabolite-induced ER activation and growth mechanism. The androgen metabolite-dependent growth of hormone receptor positive breast cancer possibly plays a certain role as an aromatase inhibitor-resistance mechanism. Citation Format: Toru Hanamura, Toshifumi Niwa, Sayo Nishikawa, Hiromi Konno, Tatsuyuki Gohno, Chika Chika Tazawa, Yasuhito Kobayashi, Masafumi Kurosumi, Hiroyuki Takei, Yuri Yamaguchi, Ken-ichi Ken-ichi Ito, Shin-ichi Hayashi. The androgen metabolite-dependent growth of hormone receptor-positive breast cancer as a possible aromatase inhibitor resistance mechanism. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1313. doi:10.1158/1538-7445.AM2013-1313