Abstract 3745: Inhibition of Protein Kinase N1 prevents control of AR over SRF action in advanced prostate cancer
Authors: Venkadarkrishnan Varadha Balaji , DePriest Adam , Su Yixue , Mudduluru Giridhar , Ben-Salem Salma , Kumari Sangeeta , Hu Qiang , Cortes Eduardo , Dehm Scott , Magi-Galluzzi Cristina , Klein Eric , Sharifi Nima , Liu Song , Heemers Hannelore 
Published: August 2018

Abstract
A major limitation in the management of castration-recurrent prostate cancer (CR-CaP) is the lack of treatments to inhibit androgen receptor (AR) action that is driving CaP growth when androgen-deprivation therapy (ADT) has failed. AR action breaks down in fractions that are regulated differently at the molecular level, control diverse aspects of CaP biology and contribute differentially to CaP progression. Blocking a specific fraction(s) of AR action that mediates CaP progression may serve as alternative treatment strategy, yet evidence for such an approach and drugs that render this approach feasible are not known. Our laboratory has identified a mechanism of AR action in which AR conveys androgen-responsiveness to Serum Response Factor (SRF) target genes via androgen activation of RhoA. Androgen-responsive SRF action mediates cell migration and is associated with aggressive CaP behavior and recurrence. Importantly, control of AR over RhoA-SRF action is maintained in CR-CaP, which renders this signaling a potentially attractive novel therapeutic target. Using an siRNA screen, we identified Protein Kinase N1 (PKN1) as the Rho effector that transduces androgen-responsiveness from RhoA to SRF. In promoter-reporter, qRT-PCR and oligoarray assays, knockdown of PKN1 severely impaired the androgen-regulation of SRF target genes, but affected androgen response of only a few (12 of 452) direct AR target genes. In contrast, transient overexpression of PKN1 preferentially stimulated androgen-responsive SRF target gene expression. Stable overexpression of PKN1 hastened growth and shortened survival in CaP orthotopic xenografts. Site-directed mutagenesis showed that the effect of PKN1 on SRF target genes relied on its kinase activity. Treatment with lestaurtinib, a multikinase inhibitor that is also a potent inhibitor of PKN1, mimicked the effects of PKN1 knockdown on expression of SRF and AR target genes in several CaP cell lines and ex vivo CaP explants. Lestaurtinib also inhibited the proliferation of CaP cells that are either ADT-naïve or -resistant, express only the constitutively active AR variant ARv567es, or are growth-stimulated by the AR-activating ADT drug metabolite 5α-abiraterone. RNA-Seq followed by MSigDB analyses confirmed that lestaurtinib impairs androgen-dependent PKN1 activity and revealed that 100 of the 127 MgSigDB gene sets isolated after PKN1 silencing overlapped with 123 gene sets derived after lestaurtinib treatment. Overlapping gene sets included several related to hormonal carcinogenesis and cancer progression, prostate development, signaling cascades that have been implicated in SRF and RhoA function and/or are relevant to CaP. PKN1 inhibition via lestaurtinib, which is used already to treat human hematologic malignancies and is well tolerated by carcinoma patients, may thus serve as novel alternative treatment strategy to target AR-dependent SRF action in CR-CaP.
Citation Format: Varadha Balaji Venkadarkrishnan, Adam DePriest, Yixue Su, Giridhar Mudduluru, Salma Ben-Salem, Sangeeta Kumari, Qiang Hu, Eduardo Cortes, Scott Dehm, Cristina Magi-Galluzzi, Eric Klein, Nima Sharifi, Song Liu, Hannelore Heemers. Inhibition of Protein Kinase N1 prevents control of AR over SRF action in advanced prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3745.