Ailanthone: a new potential drug for castration-resistant prostate cancer

© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Prostate cancer (PCa) is the most common male cancer [1, 2]. PCa initially depends on androgen receptor (AR) signaling for growth and survival. Androgen deprivation therapy causes a temporary reduction in PCa tumor burden, but the tumor eventually develops into castrationresistant prostate cancer (CRPC) with the ability to grow again in the absence of androgens [3]. Mechanisms of CRPC progression include AR amplification and overexpression [4], AR gene rearrangement promoting synthesis of constitutively-active truncated AR splice variants (ARVs) [4], and induction of intracrine androgen metabolic enzymes [3]. Current anti-androgen therapies including MDV3100 (Enzalutamide) and abiraterone have focused on the androgen-dependent activation of AR through its ligand-binding domain (LBD), but do not provide a continuing clinical benefit for patients with CRPC and presumably fail due to multiple mechanisms including the expression of AR-Vs lacking the LBD [5]. These AR-Vs signal in the absence of ligand and are therefore resistant to LBD-targeting AR antagonists or agents that repress androgen biosynthesis [6]. To identify compounds that block the transcriptional activities of both ligand-dependent full-length AR (AR-FL) and AR-Vs, we used the MMTV-luciferase (MMTV-luc) reporter system that contains AR-binding elements [7] to screen approximately 100 compounds from a library of natural compounds from Traditional Chinese Medicine and identified a small-molecule compound termed ailanthone, which is extracted from the whole seedlings of Ailanthus altissima (Simaroubaceae) that has antimalarial and antitumor activities [8]. In our work recently reported in the paper entitled “Ailanthone targets p23 to overcome MDV3100 resistance in castration-resistant prostate cancer” in Nature Communications [9], ailanthone not only blocked the ligand-induced activities of full-length AR but also inhibited AR-V which lacks the LBD at low concentrations (AR-FL half maximal inhibitory concentration [IC50] = 69 nmol/L, 95% confidence interval [CI] = 53–89 nmol/L; AR1–651 [a constructed AR splice variant] IC50 = 309 nmol/L, 95% CI = 236–687 nmol/L in 22RV1 cells). Ailanthone decreased the androgen-dependent induction of endogenous AR downstream genes PSA, TMPRSS2, FKBP5, SLC45A3, and NDRG1 mRNA expression in LNCaP cells. To investigate whether ailanthone suppressed the functioning of AR-V which lacks the LBD, we performed RNA sequencing after treating LNCaP cells with or without ailanthone in the absence or presence of AR1–651. Indeed, ailanthone strongly suppressed AR1–651-induced expression of some genes, such as PSA and TMPRSS2, supporting the potential therapeutic use of ailanthone in CRPC. Moreover, ailanthone reduced the expression of both the full-length AR and the AR-Vs in vitro and in vivo. Using the sulforhodamine B colorimetric (SRB) assay, we confirmed that ailanthone potently inhibited the growth of several PCa cell lines including LNCaP, c4–2b, 22RV1, and LAPC4. Interestingly, ailanthone more potently inhibited the growth of AR-positive PCa cells than either AR-negative tumor cell lines or normal prostate cell lines. Notably, not only intraperitoneal injection administration but also per os administration of ailanthone had excellent efficiency for blocking the growth and metastasis of CRPC in LNCaP, 22RV1, and VCaP subcutaneous xenografts and 22RV1-luc orthotopic xenografts. Our study also explored the mechanism of ailanthoneinduced AR degradation [9]. We found that ailanthone disrupted the interaction between AR and the chaperones HSP90, HSP70, and HSP40, and consequently AR was ubiquitinated and degradated through the proteasome-mediated pathway. When not bound to ligand, AR Open Access Chinese Journal of Cancer