Abstract C128: Reprogramming transcription factors SOX2 and REST modulates SPINK1 expression in governing cellular plasticity in prostate cancer
Authors: Manzar Nishat , Tiwari Ritika , Bhatia Vipul , Yadav Anjali , Carskadon Shannon , Gupta Nilesh , Zoubeidi Amina , Poutanen Matti , Beltran Himisha , Palanisamy Nallasivam , Ateeq Bushra 
Published: March 2020

Abstract
Androgen deprivation therapy (ADT) remains the clinical paradigm for the management of prostate cancer (PCa) patients. However, most of the cases inevitably become resistant to ADT, leading to a more aggressive, hormone-refractory stage known as castrate-resistant prostate cancer (CRPC). A subset of CRPC patients undergoing ADT develop tumors with low AR-signaling dependence where the luminal prostate cancer cells consequently acquire alternative lineage programs, resulting in the development of neuroendocrine prostate cancer (NEPC). In the past decade, several molecular features have been associated with progression of the adenocarcinoma to NEPC, including loss of RE1-silencing transcription factor (REST), N-MYC proto-oncogene amplification, upregulation of Aurora kinase A, neural transcription factor BRN2, and reprogramming factor SRY (sex determining region Y)-box 2 (SOX2). Nonetheless, the underlying molecular mechanism involved in lineage plasticity in neuroendocrine (NE) transdifferentaition is poorly understood and remains a matter of speculation. Overexpression of Serine Peptidase Inhibitor, Kazal type 1 (SPINK1) represents the second major molecular PCa subtype (~15-20% of the cases) associated with aggressive stage and poor patient outcome. Oncogenic role of SPINK1 in PCa has already been established, and recently SPINK1 produced in the tumor stroma was found to act as a senescence-associated secretory factor and contributes to chemoresistance in a paracrine manner. Here, we show that androgen receptor (AR) along with its corepressor REST functions as a direct transcriptional repressor of SPINK1, and blocking AR signaling using anti-androgens relieve this repression, leading to increase in SPINK1 expression. We also show that the lineage reprogramming factor SOX2 binds to the SPINK1 promoter and positively regulate its expression in androgen deprivation induced NE-transdifferentiated PCa cells. Conversely, silencing SPINK1 in the NE-transdifferentiated PCa cells result in reduced expression of epithelial-mesenchymal transition (EMT) and neuroendocrine markers accompanied with a concomitant decrease in the neurite-like projections. Moreover, castration-resistant mice xenografts treated with anti-androgens show increase in SPINK1 levels as well as EMT and neuroendocrine markers. Likewise, higher SPINK1 expression was observed in NE patient-derived organoids and NEPC clinical specimens, indicating its plausible role in cellular plasticity and NE progression. Since, REST along with AR negatively regulates SPINK1 expression, and Casein kinase 1 elicits ubiquitin-mediated proteasomal degradation of REST. Therefore, we treated SPINK1-positive PCa cells with Casein kinase 1 inhibitor (iCK1), which results in restoration of the REST expression, leading to transcriptional repression of SPINK1 as well as decrease in SPINK1-mediated oncogenesis. Taken together, we highlight the clinical complications associated with high SPINK1 levels and discovered its possible role in maintaining cellular plasticity in prostate cancer cells. Notably, stabilization of REST levels using iCK1 suggests a novel therapeutic strategy for the management of SPINK1-positive subtype and also open new avenues for the treatment modalities for CRPC patients.
Citation Format: Nishat Manzar, Ritika Tiwari, Vipul Bhatia, Anjali Yadav, Shannon Carskadon, Nilesh Gupta, Amina Zoubeidi, Matti Poutanen, Himisha Beltran, Nallasivam Palanisamy, Bushra Ateeq. Reprogramming transcription factors SOX2 and REST modulates SPINK1 expression in governing cellular plasticity in prostate cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C128. doi:10.1158/1535-7163.TARG-19-C128