Duncan, Kelly

Abstract Recently, we have identified serum response factor (SRF) as a mediator of clinically relevant androgen receptor (AR) action in prostate cancer (PCa). Genes that rely on SRF for androgen-responsiveness represent a small fraction of androgen-regulated genes, but distinguish benign from malignant prostate and correlate with aggressive disease. Moreover, the SRF-AR expression signature is associated more strongly with biochemical recurrence than similarly sized sets of AREdriven AR target genes. Thus, understanding the mechanism(s) by which SRF conveys androgen effects to its target genes may provide novel opportunities to target clinically relevant androgen signaling. Here, we explore the role of the RhoA signaling axis, a well-known activating pathway upstream of SRF, in androgen activation of SRF action. PCa cells were treated with inhibitors of RhoA signaling or transfected with siRNAs targeting RhoA, the critical SRF cofactor MAL or control siRNAs in the presence or absence of the synthetic androgen R1881. Real-time RT-PCR and western blotting was done to assess effects on expression of SRF target genes. Cell fractionation experiments evaluated the cellular localization of AR, MAL and SRF. ChIP was performed to assess recruitment of MAL to SRF target genes. Wild-type, dominant-active and dominant-negative versions of RhoA were overexpressed to determine effects on androgen regulation of SRF target genes. Immunohistochemistry was done to evaluate RhoA expression in clinical PCa samples and a tissue microarray derived from a CWR22 PCa xenograft model that mimics progression from androgen-stimulated (AS) to castration-recurrent (CR) disease. Effects on AS and CR PCa cell biology were assessed using proliferation and migration assays. Here, we show that the small GTPase RhoA mediates androgen responsiveness of more than half of SRF target genes tested. Interference with expression of RhoA, activity of the RhoA effector RhoA-associated kinase (ROCK), and actin polymerization necessary for nuclear translocation of the SRF cofactor MAL, all prevented full androgen-regulation of SRF target genes. Androgen treatment increased the nuclear content of MAL and led to MAL recruitment to the promoter of the SRF target gene FHL2. Moreover, in clinical specimens RhoA expression was higher in PCa cells than benign prostate cells, and elevated RhoA expression levels were associated with aggressive disease features and decreased disease-free survival following radical prostatectomy. Overexpression of RhoA markedly increased the androgen responsiveness of select SRF target genes, in a manner that depends on its GTPase activity. Cell lines and a xenograft model that mimics the transition from AS to CR PCa indicate that RhoA levels are not altered during disease progression, suggesting that RhoA expression levels in the primary tumor determine disease aggressiveness. Silencing of RhoA, use of ROCK inhibitors or an inhibitor of SRF-MAL interaction blunted (androgen-regulated) cell viability and attenuated AS and CR PCa cell migration. Taken together, our work demonstrates that the RhoA signaling axis mediates clinically relevant AR action in PCa. In addition, these studies are the first to report on overexpression of RhoA in PCa and its association with aggressive disease and poor outcome. Note: This abstract was not presented at the conference because the presenter was unable to attend. Citation Format: Lucy Schmidt, Thomas Sebo, Donald Tindall, Hannelore Heemers, Kelly Duncan, Neelu Yadav, Kevin Regan, Christine Lohse, Elena Pop, Kristopher Attwood, Gregory Wilding, James Mohler. Androgen regulation of SRF target genes in prostate cancer cells is mediated in part by the small GTPase RhoA [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr A8.

AbstractRecently, we have identified serum response factor (SRF) as a mediator of clinically relevant androgen receptor (AR) action in prostate cancer (PCa). Genes that rely on SRF for androgen responsiveness represent a small fraction of androgen-regulated genes, but distinguish benign from malignant prostate, correlate with aggressive disease, and are associated with biochemical recurrence. Thus, understanding the mechanism(s) by which SRF conveys androgen regulation to its target genes may provide novel opportunities to target clinically relevant androgen signaling. Here, we show that the small GTPase ras homolog family member A (RhoA) mediates androgen-responsiveness of more than half of SRF target genes. Interference with expression of RhoA, activity of the RhoA effector Rho-associated coiled-coil containing protein kinase 1 (ROCK), and actin polymerization necessary for nuclear translocation of the SRF cofactor megakaryocytic acute leukemia (MAL) prevented full androgen regulation of SRF target genes. Androgen treatment induced RhoA activation, increased the nuclear content of MAL, and led to MAL recruitment to the promoter of the SRF target gene FHL2. In clinical specimens RhoA expression was higher in PCa cells than benign prostate cells, and elevated RhoA expression levels were associated with aggressive disease features and decreased disease-free survival after radical prostatectomy. Overexpression of RhoA markedly increased the androgen-responsiveness of select SRF target genes, in a manner that depends on its GTPase activity. The use of isogenic cell lines and a xenograft model that mimics the transition from androgen-stimulated to castration-recurrent PCa indicated that RhoA levels are not altered during disease progression, suggesting that RhoA expression levels in the primary tumor determine disease aggressiveness. Androgen-responsiveness of SRF target genes in castration-recurrent PCa cells continued to rely on AR, RhoA, SRF, and MAL and the presence of intact SRF binding sites. Silencing of RhoA, use of Rho-associated coiled-coil containing protein kinase 1 inhibitors, or an inhibitor of SRF-MAL interaction attenuated (androgen-regulated) cell viability and blunted PCa cell migration. Taken together, these studies demonstrate that the RhoA signaling axis mediates clinically relevant AR action in PCa.

Abstract The androgen receptor (AR) is the principal target for treatment of non–organ-confined prostate cancer (PCa). Androgen deprivation therapies (ADT) directed against the AR ligand–binding domain do not fully inhibit androgen-dependent signaling critical for PCa progression. Thus, information that could direct the development of more effective ADTs is desired. Systems and bioinformatics approaches suggest that considerable variation exists in the mechanisms by which AR regulates expression of effector genes, pointing to a role for secondary transcription factors. A combination of microarray and in silico analyses led us to identify a 158-gene signature that relies on AR along with the transcription factor SRF (serum response factor), representing less than 6% of androgen-dependent genes. This AR-SRF signature is sufficient to distinguish microdissected benign and malignant prostate samples, and it correlates with the presence of aggressive disease and poor outcome. The AR-SRF signature described here associates more strongly with biochemical failure than other AR target gene signatures of similar size. Furthermore, it is enriched in malignant versus benign prostate tissues, compared with other signatures. To our knowledge, this profile represents the first demonstration of a distinct mechanism of androgen action with clinical relevance in PCa, offering a possible rationale to develop novel and more effective forms of ADT. Cancer Res; 71(5); 1978–88. ©2011 AACR.