Abstract 728: New derivative of galiellalactone inhibits the STAT3 activity and suppresses ENZ-resistant Prostate Cancer in vitro
Authors: Thaper Daksh , Vahid Sepideh , Bishop Jennifer L. , Johansson Martin , Zoubeidi Amina 
Published: August 2015

Background: The role of signal transducer and activator of transcription (STAT) 3 in the progression of Prostate Cancer (PCa), the most common male cancer in North America, is well established. Although androgen ablation remains the most effective therapy for patients with advanced disease, most progress to castrate resistant prostate cancer (CRPC). Despite the development of new potent anti-androgens, like Enzalutamide (ENZ), which prolongs survival in CRPC patients, ENZ resistance rapidly occurs. Re-activation of the androgen receptor (AR) is a hallmark of CRPC and can occur via different mechanisms, including STAT3 activation. Furthermore, STAT3 can promote CRPC independently of AR activity, however whether STAT3 promotes resistance to ENZ in CRPC remains unknown. We have found that our LNCaP xenograft derived ENZR cell lines exhibit higher activity of the STAT3 pathway compared to CRPC. Additionally, in this model further targeting the AR using novel anti-androgens is short lived or ineffective. Therefore, we propose that targeting STAT3 using the small molecule inhibitor GPA500, alone or in combination with anti-androgens may delay or treat ENZR CRPC.

Methods and Results: We developed a unique model of ENZ-resistance and found that cell lines derived from serially passaged ENZR tumors displayed broad genetic diversity and differential AR activity but consistent STAT3 hyper-activation compared to CRPC controls. WB analysis showed that all ENZR cell lines had increased pSTAT3Y705 compared to CRPC. Accordingly, Crystal Violet and MTT proliferation assays showed that ENZR cell lines were more sensitive to the STAT3 inhibitor GPA500 compared to CRPC control. The reduction in growth of GPA500 treated ENZR cells was independent of PARP cleavage, whereas GPA500 induced PARP cleavage in CRPC cells. Moreover, in ENZR cell lines with high AR activity and CRPC control cells, inhibition of STAT3 reduced AR activity as shown with WB and qRT-PCR for AR downstream targets like PSA and an AR-luciferase reporter assay. In ENZR cells with low AR activity and a stem cell phenotype, GPA500 treatment reduced surface expression of α2β1, CD44 and CD133. Inhibition of STAT3 activity was confirmed by WB as well as qRT-PCR analysis of STAT3 regulated genes like c-Myc, MCL1, BCL-XL.

Conclusion: In this study, we provide pre-clinical proof that targeting the STAT3 pathway using GPA500 in ENZ resistance as well as CRPC reduces cell proliferation and AR activity as well as expression of stem cell markers.

Impact: Exploring mechanisms of ENZR resistance serves a critical unmet need in PCa oncology, as the number of men with ENZ resistant CRPC continues to rise. Targeting STAT3 with the small molecule inhibitor GPA500 may provide an effective method to treat ENZR patients or delay the emergence of ENZ resistance in CRPC via reduction of AR activity and/or by suppressing aggressive tumor cell phenotypes such as cancer stem cells.

Citation Format: Daksh Thaper, Sepideh Vahid, Jennifer L. Bishop, Martin Johansson, Amina Zoubeidi. New derivative of galiellalactone inhibits the STAT3 activity and suppresses ENZ-resistant Prostate Cancer in vitro. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 728. doi:10.1158/1538-7445.AM2015-728