Abstract 4081: Taxane sensitivity in prostate cancer is determined by androgen receptor splice variants.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Although its pivotal role in prostate cancer care, androgen deprivation therapy (ADT) is not curative as many patients progress to a castration-resistant stage (CRPC) which remains driven by pathologic reactivation of androgen receptor (AR). Taxanes represent the only chemotherapy class that improves survival in CRPC and as such Docetaxel (DTX) and Cabazitaxel (CTX) are standard of care. Recent evidence revealed the presence of several AR splice variants lacking the ligand binding domain and thus constitutively active in CRPC. Previous data from our lab showed that taxanes impair the ligand-induced nuclear accumulation and transcriptional activity of full-length AR (AR-FL) in taxane treated human prostate cancer cells and CRPC patients. Indeed we showed that AR cytoplasmic sequestration in patient's circulating tumour cells (CTCs) was significantly correlated with clinical response to taxane chemotherapy. This is due to AR binding microtubules (MTs) and using them for nuclear accumulation supported by the dynein motor protein. However, the effect of taxane on the nuclear accumulation and activity of the AR variants is unknown. Thus, we set out to investigate the mechanism underlying AR variant nuclear accumulation and the impact of taxane treatment on AR variant function focusing on the two clinically relevant AR splice variants ARv567 and AR-V7. A microtubule co-sedimentation assay revealed that ARv567 is associated with MTs, in contrast to AR-V7. Serial AR mutagenesis demonstrated that the MT binding domain comprises a region aa 559 - 663 which is missing in AR-V7. Dynamitin overexpression inhibited the nuclear accumulation of AR-FL and ARv567 but had no effect on AR-V7, suggesting that AR-V7’s nuclear translocation is independent of dynein-based MT transport. To examine the impact of taxane treatment on variant activity we microinjected GFP-tagged AR-FL, or AR variants into the nucleus of PC3 cells and monitored the dynamics of AR nuclear translocation using live-cell confocal microscopy. Moreover we stably transfected M12 cells with AR-FL, AR-V7 and AR-v567 and assessed AR's nuclear translocation in response to R1881 and in the presence or absence of DTX. Our data showed that taxanes significantly inhibited the nuclear accumulation and activity of AR-FL and ARv567 but not that of AR-V7 and that this effect is androgen independent. We are currently elucidating the effect of taxanes on AR variant transcriptional activity using luciferase reporter and specific genes expression assays. Taken together our data revealed that functional MTs are required for AR's nuclear transport in a ligand independent manner, as in the case of ARv567. However, this does not apply to AR-V7 which is not under MT control and thus insensitive to taxane treatment. These data suggest that impairment of AR variant expression might be predictive of clinical taxane sensitivity in CRPC. Citation Format: Luigi Portella, Maria Thadani-Mulero, Alexandre Matov, David M. Nanus, Stephen R. Plymate, Paraskevi Giannakakou. Taxane sensitivity in prostate cancer is determined by androgen receptor splice variants. [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 4081. doi:10.1158/1538-7445.AM2013-4081