Relationship between serum response factor and androgen receptor in prostate cancer

SRF inhibitors reduce prostate cancer cell proliferation through cell cycle arrest in an isogenic model of castrate-resistant prostate cancer

Castrate-resistant prostate cancer (CRPC) is challenging to treat, despite improvements with next-generation anti-androgens such as enzalutamide, due to acquired resistance. One of the mechanisms of such resistance includes aberrant activation of co-factors of the androgen receptor (AR), such as the serum response factor (SRF), which was associated with prostate cancer progression and resistance to enzalutamide. Here, we show that inhibition of SRF with three small molecules (CCG-1423, CCG-257081 and lestaurtinib), singly and in combination with enzalutamide, reduces cell viability in an isogenic model of CRPC. The effects of these inhibitors on the cell cycle, singly and in combination with enzalutamide, were assessed with western blotting, flow cytometry and β-galactosidase staining. In the androgen deprivation-sensitive LNCaP parental cell line, a synergistic effect between enzalutamide and all three inhibitors was demonstrated, while the androgen deprivation-resistant LNCaP Abl cells showed synergy only with the lestaurtinib and enzalutamide combination, suggesting a different mechanism of action of the CCG series of compounds in the absence and presence of androgens. Through analysis of cell cycle checkpoint proteins, flow cytometry and β-galactosidase staining, we showed that all three SRF inhibitors, singly and in combination with enzalutamide, induced cell cycle arrest and decreased S phase. While CCG-1423 had a more pronounced effect on the expression of cell cycle checkpoint proteins, CCG-257081 and lestaurtinib decreased proliferation also through induction of cellular senescence. In conclusion, we show that inhibition of an AR co-factors, namely SRF, provides a promising approach to overcoming resistance to AR inhibitors currently used in the clinic.

Emerging role for the Serum Response Factor (SRF) as a potential therapeutic target in cancer

INTRODUCTION

The Serum Response Factor (SRF) is a transcription factor involved in three hallmarks of cancer: the promotion of cell proliferation, cell death resistance and invasion and metastasis induction. Many studies have demonstrated a leading role in the development and progression of multiple cancer types, thus highlighting the potential of SRF as a prognostic biomarker and therapeutic target, especially for cancers with poor prognosis.

AREAS COVERED

This review examines the role of SRF in several cancers in promoting cellular processes associated with cancer development and progression. SRF co-factors and signalling pathways are discussed as possible targets to inhibit SRF in a tissue and cancer-specific way. Small-molecule inhibitors of SRF, such as the CCGs series of compounds and lestaurtinib, which could be used as cancer therapeutics, are also discussed.

EXPERT OPINION

Targeting of SRF and its co-factors represents a promising therapeutic approach. Further understanding of the molecular mechanisms behind the action of SRF could provide a pipeline of novel molecular targets and therapeutic combinations for cancer. Basket clinical trials and the use of SRF immunohistochemistry as companion diagnostics will help testing of these new targets in patients.

Inhibition of Serum Response Factor Improves Response to Enzalutamide in Prostate Cancer

Castrate-resistant prostate cancer (CRPC) is challenging to treat with the androgen receptor (AR), the main target and key focus of resistance. Understanding the mechanisms of AR interaction with co-regulators will identify new therapeutic targets to overcome AR resistance mechanisms. We previously identified the serum response factor (SRF) as a lead target in an in vitro model of CRPC and showed that SRF expression in tissues of CRPC patients was associated with shorter survival. Here, we tested SRF inhibition in vitro and in vivo to assess SRF as a potential target in CRPC. Inhibition of SRF with the small-molecule inhibitor CCG1423 resulted in enhanced response to enzalutamide in vitro and reduced tumour volume of LuCaP 35CR, a CRPC patient-derived xenograft model. Nuclear localisation of AR post-CCG1423 was significantly decreased and was associated with decreased α-tubulin acetylation in vitro and decreased prostate specific antigen (PSA) levels in vivo. SRF immunoreactivity was tested in metastatic tissues from CRPC patients to investigate its role in enzalutamide response. Kaplan-Meier curves showed that high SRF expression was associated with shorter response to enzalutamide. Our study supports the use of SRF inhibitors to improve response to enzalutamide.

Association of polymorphisms of PTEN, AKT1, PI3K, AR, and AMACR genes in patients with prostate cancer

Polymorphic variants in the PTEN (rs2735343), PI3K (rs2699887), AKT1 (rs2494750), AR (rs17302090), and AMACR (rs3195676) genes were evaluated as possible molecular markers of susceptibility, prognosis, and progression of prostate cancer (PCa), in a case-control study. Samples consisted of 277 patients with PCa and 277 controls from Londrina, PR, Brazil. SNPs were analyzed by real-time PCR. A family history of cancer, including PCa, as well as level of schooling were risk factors for PCa. The data were obtained via logistic regression, using odds ratios with a CI 95%. The genotypes of AKT1 and AKT1+AR demonstrated an association with protection for the disease. The combination of SNPs with the histopathological tumor data between allele variants of AMACR, AKT1+AR, and AKT1+AMACR indicated an association with protection against seminal vesicle invasion. The polymorphisms AKT1+AR and PI3K+AR were associated with protection against tumor bilaterality. The genotype combinations PTEN+AMACR and PTEN+AR were associated with the risk of extracapsular extension. Of the five genes studied, two were associated with protection for PCa, four were associated with protection for some prognostic variables, and only one was associated with risk. Thus, these SNPs are candidates for markers to discriminate men with better or worse prognosis for PCa.

Protein Kinase N1 control of androgen-responsive serum response factor action provides rationale for novel prostate cancer treatment strategy

Sustained reliance on androgen receptor (AR) after failure of AR-targeting androgen deprivation therapy (ADT) prevents effective treatment of castration-recurrent (CR) prostate cancer (CaP). Interfering with the molecular machinery by which AR drives CaP progression may be an alternative therapeutic strategy but its feasibility remains to be tested. Here, we explore targeting the mechanism by which AR, via RhoA, conveys androgen-responsiveness to serum response factor (SRF), which controls aggressive CaP behavior and is maintained in CR-CaP. Following a siRNA screen and candidate gene approach, RNA-Seq studies confirmed that the RhoA effector Protein Kinase N1 (PKN1) transduces androgen-responsiveness to SRF. Androgen treatment induced SRF-PKN1 interaction, and PKN1 knockdown or overexpression severely impaired or stimulated, respectively, androgen regulation of SRF target genes. PKN1 overexpression occurred during clinical CR-CaP progression, and hastened CaP growth and shortened CR-CaP survival in orthotopic CaP xenografts. PKN1's effects on SRF relied on its kinase domain. The multikinase inhibitor lestaurtinib inhibited PKN1 action and preferentially affected androgen regulation of SRF over direct AR target genes. In a CR-CaP patient-derived xenograft, expression of SRF target genes was maintained while AR target gene expression declined and proliferative gene expression increased. PKN1 inhibition decreased viability of CaP cells before and after ADT. In patient-derived CaP explants, lestaurtinib increased AR target gene expression but did not significantly alter SRF target gene or proliferative gene expression. These results provide proof-of-principle for selective forms of ADT that preferentially target different fractions of AR's transcriptional output to inhibit CaP growth.

The prognostic utility of the transcription factor SRF in docetaxel-resistant prostate cancer: in-vitro discovery and in-vivo validation

Background

Docetaxel based therapy is one of the first line chemotherapeutic agents for the treatment of metastatic castrate-resistant prostate cancer. However, one of the major obstacles in the treatment of these patients is docetaxel-resistance. Defining the mechanisms of resistance so as to inform subsequent treatment options and combinations represents a challenge for clinicians and scientists.

Previous work by our group has shown complex changes in pro and anti-apoptotic proteins in the development of resistance to docetaxel. Targeting these changes individually does not significantly impact on the resistant phenotype but understanding the central signalling pathways and transcription factors (TFs) which control these could represent a more appropriate therapeutic targeting approach.

Methods

Using a number of docetaxel-resistant sublines of PC-3 cells, we have undertaken a transcriptomic analysis by expression microarray using the Affymetrix Human Gene 1.0 ST Array and in conjunction with bioinformatic analyses undertook to predict dysregulated TFs in docetaxel resistant prostate cancer. The clinical significance of this prediction was ascertained by performing immunohistochemical (IHC) analysis of an identified TF (SRF) in the metastatic sites from men who died of advanced CRPC. Investigation of the functional role of SRF was examined by manipulating SRF using SiRNA in a docetaxel-resistant PC-3 cell line model.

Results

The transcription factors identified include serum response factor (SRF), nuclear factor kappa-B (NFκB), heat shock factor protein 1 (HSF1), testicular receptor 2 & 4 (TR2 &4), vitamin-D and retinoid x receptor (VDR-RXR) and oestrogen-receptor 1 (ESR1), which are predicted to be responsible for the differential gene expression observed in docetaxel-resistance. IHC analysis to quantify nuclear expression of the identified TF SRF correlates with both survival from date of bone metastasis (p = 0.003), survival from androgen independence (p = 0.00002), and overall survival from prostate cancer (p = 0.0044). Functional knockdown of SRF by siRNA demonstrated a reversal of apoptotic resistance to docetaxel treatment in the docetaxel-resistant PC-3 cell line model.

Conclusions

Our results suggest that SRF could aid in treatment stratification of prostate cancer, and may also represent a therapeutic target in the treatment of men afflicted with advanced prostate cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3100-4) contains supplementary material, which is available to authorized users.