Suberoylanilide hydroxamic acid (vorinostat) represses androgen receptor expression and acts synergistically with an androgen receptor antagonist to inhibit prostate cancer cell proliferation

Molecular hybridization: a powerful tool for multitarget drug discovery

INTRODUCTION

The current drug discovery paradigm of 'one drug, multiple targets' has gained attention from both the academic medicinal chemistry community and the pharmaceutical industry. This is in response to the urgent need for effective agents to treat multifactorial chronic diseases. The molecular hybridization strategy is a useful tool that has been widely explored, particularly in the last two decades, for the design of multi-target drugs.

AREAS COVERED

This review examines the current state of molecular hybridization in guiding the discovery of multitarget small molecules. The article discusses the design strategies and target selection for a multitarget polypharmacology approach to treat various diseases, including cancer, Alzheimer's disease, cardiac arrhythmia, endometriosis, and inflammatory diseases.

EXPERT OPINION

Although the examples discussed highlight the importance of molecular hybridization for the discovery of multitarget bioactive compounds, it is notorious that the literature has focused on specific classes of targets. This may be due to a deep understanding of the pharmacophore features required for target binding, making targets such as histone deacetylases and cholinesterases frequent starting points. However, it is important to encourage the scientific community to explore diverse combinations of targets using the molecular hybridization strategy.

Molecular Targeted Therapies in Metastatic Prostate Cancer: Recent Advances and Future Challenges

Prostate cancer is the most frequent malignant tumor in men, and, despite the great improvements in survival in patients with localized cancer, the prognosis for metastatic disease remains poor. Novel molecular targeted therapies, which block specific molecules or signaling pathways in tumor cells or in their microenvironment, have shown encouraging results in metastatic castration-resistant prostate cancer. Among these therapeutic approaches, prostate-specific membrane antigen-targeted radionuclide therapies and DNA repair inhibitors represent the most promising ones, with some therapeutic protocols already approved by the FDA, whereas therapies targeting tumor neovascularization and immune checkpoint inhibitors have not yet demonstrated clear clinical benefits. In this review, the most relevant studies and clinical trials on this topic are illustrated and discussed, together with future research directions and challenges.

Enhancer profiling identifies epigenetic markers of endocrine resistance and reveals therapeutic options for metastatic castration-resistant prostate cancer patients

Androgen Receptor (AR) signaling inhibitors, including enzalutamide, are treatment options for patients with metastatic castration-resistant prostate cancer (mCRPC), but resistance inevitably develops. Using metastatic samples from a prospective phase II clinical trial, we epigenetically profiled enhancer/promoter activities with H3K27ac chromatin immunoprecipitation followed by sequencing, before and after AR-targeted therapy. We identified a distinct subset of H3K27ac-differentially marked regions that associated with treatment responsiveness. These data were successfully validated in mCRPC patient-derived xenograft models (PDX). In silico analyses revealed HDAC3 as a critical factor that can drive resistance to hormonal interventions, which we validated in vitro . Using cell lines and mCRPC PDX tumors in vitro , we identified drug-drug synergy between enzalutamide and the pan-HDAC inhibitor vorinostat, providing therapeutic proof-of-concept. These findings demonstrate rationale for new therapeutic strategies using a combination of AR and HDAC inhibitors to improve patient outcome in advanced stages of mCRPC.

Seleno-aspirin compound AS-10 promotes histone acetylation ahead of suppressing androgen receptor transcription, G1 arrest, and apoptosis of prostate cancer cells

BACKGROUND

The novel selenium-aspirin compound AS-10 was recently reported by us with a cancer cell killing potency three orders of magnitude greater than aspirin in pancreatic cancer cell lines with caspase-mediated apoptosis and a reasonable selectivity against malignant cells. Although we also observed its cytocidal activity against PC-3 and DU145 androgen receptor (AR)-negative and P53-null/mutant aggressive human prostate cancer (PCa) cell lines in NCI-60 screen, the potential involvement and targeting of AR and P53 pathways that are intact in early-stage prostate carcinogenesis has not been examined, nor its primary molecular signaling after exposure.

METHODS

Human LNCaP PCa cells with functional AR and intact P53 were used to examine their cell cycle and cell fate responses to AS-10 exposure and upstream molecular signaling events including histone acetylation as a known aspirin effect. The AR-positive 22Rv1 human PCa cells were used to validate key findings.

RESULTS

In addition to confirming AS-10's superior cytocidal potency than aspirin against all four PCa cell lines, we report a rapid (within 5 min) promotion of histone acetylation several hours ahead of the suppression of AR and prostate-specific antigen (PSA, coded by KLK3 gene) in LNCaP and 22Rv1 cells. AS-10 decreased AR and KLK3 mRNA levels without impacting pre-existing AR protein degradation or nuclear translocation in LNCaP cells. Sustained exposure to AS-10 arrested cells predominantly in G1 , and induced caspase-mediated apoptosis without necrosis. The death induced by AS-10 in LNCaP cells was attenuated by nontranscriptional activation of P53 protein or Jun N-terminal Kinase cellular stress signaling and was mitigated modestly by glutathione-boosting antioxidant N-acetylcysteine. AS-10 synergized with histone deacetylase inhibitor SAHA to suppress AR/PSA abundance and kill LNCaP cells. RNA-seq confirmed AR suppression at the transcriptional level and suggested multiple oncogene, cyclin, and CDK/CKI transcriptional actions to contribute to the cellular consequences.

CONCLUSIONS

AS-10 promotes histone acetylation as its probable primary mechanism of action to induce PCa cell-cycle arrest and apoptosis, regardless of AR and P53 status. Nevertheless, the inhibition of AR signaling through mechanisms distinct from canonical AR antagonists may hold promise for combinatorial use with androgen deprivation therapy regimens or AR-axis targeting drugs.

Insights into the therapeutic potential of histone deacetylase inhibitor/immunotherapy combination regimens in solid tumors

Solid tumors including skin, lung, breast, colon, and prostate cancers comprise the most diagnosed cancers worldwide. Treatment of such cancers is still challenging specially in the advanced/metastatic setting. The growing understanding of the tumor microenvironment has revolutionized the cancer therapy paradigms. Targeting programmed death-1 (PD-1)/PD-L1 immune checkpoint has been extensively studied over this decade as a new trend in the management of hard-to-treat cancers by harnessing the power of the immune system to eradicate the tumors. Yet, low response rate and resistance were observed when immunotherapies were tested as monotherapy. This urged the need to develop combinatorial regimens of immunotherapy with other immune modulatory agents to enhance its therapeutic potential and help in reverting the resistance. Epigenetic modifiers such as histone deacetylase inhibitors (HDACIs) showed favorable effects on modulating the tumor microenvironment along with the host immune cells. This qualified HDACIs as an attractive candidate class to be tested in combination with immunotherapy. In this review we cover the ongoing clinical trials that investigate the safety and/or the efficacy of HDACI/immunotherapy combinations in solid tumors including skin cancer, prostate cancer, breast cancer, colorectal cancer, lung cancer and recapitulates areas for future research.

From Therapy Resistance to Targeted Therapies in Prostate Cancer

Prostate cancer (PCa) is the second most common malignancy among men worldwide. Although early-stage disease is curable, advanced stage PCa is mostly incurable and eventually becomes resistant to standard therapeutic options. Different genetic and epigenetic alterations are associated with the development of therapy resistant PCa, with specific players being particularly involved in this process. Therefore, identification and targeting of these molecules with selective inhibitors might result in anti-tumoral effects. Herein, we describe the mechanisms underlying therapy resistance in PCa, focusing on the most relevant molecules, aiming to enlighten the current state of targeted therapies in PCa. We suggest that selective drug targeting, either alone or in combination with standard treatment options, might improve therapeutic sensitivity of resistant PCa. Moreover, an individualized analysis of tumor biology in each PCa patient might improve treatment selection and therapeutic response, enabling better disease management.

Dual-function antiandrogen/HDACi hybrids based on enzalutamide and entinostat

The combination of androgen receptor antagonists with histone deacetylase inhibitors (HDACi) has been shown to be more effective than antiandrogens alone in halting growth of prostate cancer cell lines. Here we have designed, synthesized and assessed a series of antiandrogen/HDACi hybrids by combining structural features of enzalutamide with either SAHA or entinostat. The hybrids are demonstrated to maintain bifunctionality using a fluorometric HDAC assay and a bioluminescence resonance energy transfer (BRET) antiandrogen assay. Antiproliferative assays showed that hybrids bearing o-aminoanilide-based HDACi motifs outperformed hydroxamic acid based HDACi's. The hybrids demonstrated selectivity for epithelial cell lines vs. stromal cell lines, suggesting a potentially useful therapeutic window.

Shifting the paradigm in treating multi-factorial diseases: polypharmacological co-inhibitors of HDAC6

Multi-factorial diseases are illnesses that exploit multiple cellular processes, or stages within one process, and thus highly targeted therapies often succumb to the disease, losing efficacy as resistance sets in. Combination therapies have become a mainstay to battle these diseases, however these regimens are plagued with caveats. An emerging avenue to treat multi-factorial diseases is polypharmacology, wherein a single drug is rationally designed to bind multiple targets, and is widely touted to be superior to combination therapy by inherently addressing the latter's shortcomings, which include poor patient compliance, narrow therapeutic windows and spiraling healthcare costs. Through its roles in intracellular trafficking, cell motility, mitosis, protein folding and as a back-up to the proteasome pathway, HDAC6 has rapidly become an exciting new target for therapeutics, particularly in the discovery of new drugs to treat Alzheimer's disease and cancer. Herein, we describe recent efforts to marry together HDAC pharmacophores, with a particular emphasis on HDAC6 selectivity, with those of other targets towards the discovery of potent therapeutics to treat these evasive diseases. Such polypharmacological agents may supercede combination therapies through inherent synergism, permitting reduced dosing, wider therapeutic windows and improved compliance.

Liver-Targeting Class I Selective Histone Deacetylase Inhibitors Potently Suppress Hepatocellular Tumor Growth as Standalone Agents

Dysfunctions in epigenetic regulation play critical roles in tumor development and progression. Histone deacetylases (HDACs) and histone acetyl transferase (HAT) are functionally opposing epigenetic regulators, which control the expression status of tumor suppressor genes. Upregulation of HDAC activities, which results in silencing of tumor suppressor genes and uncontrolled proliferation, predominates in malignant tumors. Inhibition of the deacetylase activity of HDACs is a clinically validated cancer therapy strategy. However, current HDAC inhibitors (HDACi) have elicited limited therapeutic benefit against solid tumors. Here, we disclosed a class of HDACi that are selective for sub-class I HDACs and preferentially accumulate within the normal liver tissue and orthotopically implanted liver tumors. We observed that these compounds possess exquisite on-target effects evidenced by their induction of dose-dependent histone H4 hyperacetylation without perturbation of tubulin acetylation status and G0/G1 cell cycle arrest. Representative compounds 2 and 3a are relatively non-toxic to mice and robustly suppressed tumor growths in an orthotopic model of HCC as standalone agents. Collectively, our results suggest that these compounds may have therapeutic advantage against HCC relative to the current systemic HDACi. This prospect merits further comprehensive preclinical investigations.

A New Histone Deacetylase Inhibitor, MHY4381, Induces Apoptosis via Generation of Reactive Oxygen Species in Human Prostate Cancer Cells

Histone deacetylase (HDAC) inhibitors represent a novel class of anticancer agents, which can be used to inhibit cell proliferation and induce apoptosis in several types of cancer cells. In this study, we investigated the anticancer activity of MHY4381, a newly synthesized HDAC inhibitor, against human prostate cancer cell lines and compared its efficacy with that of suberoylanilide hydroxamic acid (SAHA), a well-known HDAC inhibitor. We assessed cell viability, apoptosis, cell cycle regulation, and other biological effects in the prostate cancer cells. We also evaluated a possible mechanism of MHY4381 on the apoptotic cell death pathway. The IC₅₀ value of MHY4381 was lower in DU145 cells (IC₅₀=0.31 μM) than in LNCaP (IC₅₀=0.85 μM) and PC-3 cells (IC₅₀=5.23 μM). In addition, the IC₅₀ values of MHY4381 measured in this assay were significantly lower than those of SAHA against prostate cancer cell lines. MHY4381 increased the levels of acetylated histones H3 and H4 and reduced the expression of HDAC proteins in the prostate cancer cell lines. MHY4381 increased G2/M phase arrest in DU145 cells, and G1 arrest in LNCaP cells. It also activated reactive oxygen species (ROS) generation, which induced apoptosis in the DU145 and LNCaP cells by increasing the ratio of Bax/Bcl-2 and releasing cytochrome c into the cytoplasm. Our results indicated that MHY4381 preferentially results in antitumor effects in DU145 and LNCaP cells via mitochondria-mediated apoptosis and ROS-facilitated cell death pathway, and therefore can be used as a promising prostate cancer therapeutic.

Induction of apoptosis and modulation of homologous recombination DNA repair pathway in prostate cancer cells by the combination of AZD2461 and valproic acid

Cancer therapies using defects in homologous recombination (HR) DNA repair pathway of tumor cells are not yet approved to be applicable in patients with malignancies other than BRCA1/2-mutated tumors. This study was designed to determine the efficacy of combination therapy of a histone deacetylase inhibitor, valproic acid (VPA) and a novel PARP inhibitor AZD2461 in both PC-3 (PTEN-mutated) and DU145 (PTEN-unmutated) prostate cancer cell lines. The Trypan blue dye exclusion assay and the tetrazolium-based colorimetric (MTT) assay were performed to measure the cytotoxicity while combination effects were assessed based on Chou-Talalay's principles. Flow-cytometric assay determined the type of cell death. The real-time PCR analysis was used to evaluate the alterations in mRNA levels of HR-related genes while their protein levels were measured using the ELISA method. γ-H2AX levels were determined as a marker of DNA damage. We observed a synergistic relationship between VPA and AZD2461 in all affected fractions of PC-3 cells (CI<0.9), but not in DU145 cells (CI>1.1). Annexin-V staining analysis revealed a significant induction of apoptosis when PC-3 cells were treated with VPA+AZD2461 (p<0.05). Both mRNA and protein levels of Rad51 and Mre11 were significantly decreased in PC-3 cells co-treated with VPA+AZD2461 while enhanced H2AX phosphorylation was found in PC-3 cells after 12 and 24 hours of co-treatment (p<0.05). Our findings established a preclinical rationale for selective targeting of HR repair pathways by a combination of VPA and AZD2461 as a mechanism for reducing the HR pathway sufficiency in PTEN-mutated prostate cancer cells.

Inhibition of BET Proteins and Histone Deacetylase (HDACs): Crossing Roads in Cancer Therapy

Histone DeACetylases (HDACs) are enzymes that remove acetyl groups from histones and other proteins, regulating the expression of target genes. Pharmacological inhibition of these enzymes re-shapes chromatin acetylation status, confusing boundaries between transcriptionally active and quiescent chromatin. This results in reinducing expression of silent genes while repressing highly transcribed genes. Bromodomain and Extraterminal domain (BET) proteins are readers of acetylated chromatin status and accumulate on transcriptionally active regulatory elements where they serve as scaffold for the building of transcription-promoting complexes. The expression of many well-known oncogenes relies on BET proteins function, indicating BET inhibition as a strategy to counteract their activity. BETi and HDACi share many common targets and affect similar cellular processes to the point that combined inhibition of both these classes of proteins is regarded as a strategy to improve the effectiveness of these drugs in cancer. In this work, we aim to discuss the molecular basis of the interplay between HDAC and BET proteins, pointing at chromatin acetylation as a crucial node of their functional interaction. We will also describe the state of the art of their dual inhibition in cancer therapy. Finally, starting from their mechanism of action we will provide a speculative perspective on how these drugs may be employed in combination with standard therapies to improve effectiveness and/or overcome resistance.

Cancer epigenetics and the potential of epigenetic drugs for treating solid tumors

Introduction: Epigenetic modification without DNA sequence mutation plays an important role in cancer development. Some small molecular inhibitors targeting key epigenetic molecules have been approved by the Food and Drug Administration to treat hematological malignancies. However, the anticancer effects of these drugs on solid tumors are not satisfactory, and the mechanisms of action remain largely unknown. Areas covered: The review summarizes the latest research on cancer epigenetics and discusses the potentials and limitations of using epigenetic drugs to treat solid tumors. An analysis of possible reasons for epigenetic drug treatment failure in solid tumors in some clinical trials is discussed along with prospects for future development. Expert commentary: Next-generation small molecule inhibitors will target novel epigenetic regulators with high cancer specificity. Combined modalities exploiting epigenetic drugs with chemo-/radiotherapy, molecular-targeting drugs, and immunotherapy will be able to effectively treat solid tumors in the near future.

New Opportunities for Targeting the Androgen Receptor in Prostate Cancer

Recent genomic analyses of metastatic prostate cancer have provided important insight into adaptive changes in androgen receptor (AR) signaling that underpin resistance to androgen deprivation therapies. Novel strategies are required to circumvent these AR-mediated resistance mechanisms and thereby improve prostate cancer survival. In this review, we present a summary of AR structure and function and discuss mechanisms of AR-mediated therapy resistance that represent important areas of focus for the development of new therapies.

Combination Therapy With Histone Deacetylase Inhibitors (HDACi) for the Treatment of Cancer: Achieving the Full Therapeutic Potential of HDACi

Genetic and epigenetic changes in DNA are involved in cancer development and tumor progression. Histone deacetylases (HDACs) are key regulators of gene expression that act as transcriptional repressors by removing acetyl groups from histones. HDACs are dysregulated in many cancers, making them a therapeutic target for the treatment of cancer. Histone deacetylase inhibitors (HDACi), a novel class of small-molecular therapeutics, are now approved by the Food and Drug Administration as anticancer agents. While they have shown great promise, resistance to HDACi is often observed and furthermore, HDACi have shown limited success in treating solid tumors. The combination of HDACi with standard chemotherapeutic drugs has demonstrated promising anticancer effects in both preclinical and clinical studies. In this review, we summarize the research thus far on HDACi in combination therapy, with other anticancer agents and their translation into preclinical and clinical studies. We additionally highlight the side effects associated with HDACi in cancer therapy and discuss potential biomarkers to either select or predict a patient's response to these agents, in order to limit the off-target toxicity associated with HDACi.

Validation of histone deacetylase 3 as a therapeutic target in castration-resistant prostate cancer

BACKGROUND

Whereas the androgen receptor (AR) signaling axis remains a therapeutic target in castration-resistant prostate cancer (CRPC), the emergence of AR mutations and splice variants as mechanisms underlying resistance to contemporary inhibitors of this pathway highlights the need for new therapeutic approaches to target this disease. Of significance in this regard is the considerable preclinical data, indicating that histone deacetylase (HDAC) inhibitors may have utility in the treatment of CRPC. However, the results of clinical studies using HDAC inhibitors (directed against HDAC1, 2, 3, and 8) in CRPC are equivocal, a result that some have attributed to their ability to induce an epithelial to mesenchymal transition (EMT) and neuroendocrine differentiation. We posited that it might be possible to uncouple the beneficial effects of HDAC inhibitors on AR signaling from their undesired activities by targeting specific HDACs as opposed to using the pan-inhibitor strategy that has been employed to date.

METHODS

The relative abilities of pan- and selective-Class I HDAC inhibitors to attenuate AR-mediated target gene expression and proliferation were assessed in several prostate cancer cell lines. Small interfering RNA (siRNA)-mediated knockdown approaches were used to confirm the importance of of HDAC 1, 2, and 3 expression in these processes. Further, the ability of each HDAC inhibitor to induce the expression of EMT markers (RNA and protein) and EMT-like phenotype(s) (migration) were also assessed. The anti-tumor efficacy of a HDAC3-selective inhibitor, RGFP966, was compared to the pan-HDAC inhibitor Suberoylanilide Hydroxamic Acid (SAHA) in the 22Rv1 xenograft model.

RESULTS

Using genetic and pharmacological approaches we demonstrated that a useful inhibition of AR transcriptional activity, absent the induction of EMT, could be achieved by specifically inhibiting HDAC3. Significantly, we also determined that HDAC3 inhibitors blocked the activity of the constitutively active AR V7-splice variant and inhibited the growth of xenograft tumors expressing this protein.

CONCLUSIONS

Our studies provide strong rationale for the near-term development of specific HDAC3 inhibitors for the treatment of CRPC.

Dysregulated fibronectin trafficking by Hsp90 inhibition restricts prostate cancer cell invasion

The molecular chaperone Hsp90 is overexpressed in prostate cancer (PCa) and is responsible for the folding, stabilization and maturation of multiple oncoproteins, which are implicated in PCa progression. Compared to first-in-class Hsp90 inhibitors such as 17-allylamino-demethoxygeldanamycin (17-AAG) that were clinically ineffective, second generation inhibitor AUY922 has greater solubility and efficacy. Here, transcriptomic and proteomic analyses of patient-derived PCa explants identified cytoskeletal organization as highly enriched with AUY922 treatment. Validation in PCa cell lines revealed that AUY922 caused marked alterations to cell morphology, and suppressed cell motility and invasion compared to vehicle or 17-AAG, concomitant with dysregulation of key extracellular matrix proteins such as fibronectin (FN1). Interestingly, while the expression of FN1 was increased by AUY922, FN1 secretion was significantly decreased. This resulted in cytosolic accumulation of FN1 protein within late endosomes, suggesting that AUY922 disrupts vesicular secretory trafficking pathways. Depletion of FN1 by siRNA knockdown markedly reduced the invasive capacity of PCa cells, phenocopying AUY922. These results highlight a novel mechanism of action for AUY922 beyond its established effects on cellular mitosis and survival and, furthermore, identifies extracellular matrix cargo delivery as a potential therapeutic target for the treatment of aggressive PCa.

Targeting MTA1/HIF-1α signaling by pterostilbene in combination with histone deacetylase inhibitor attenuates prostate cancer progression

The metastasis‐associated protein 1(MTA1)/histone deacetylase (HDAC) unit is a cancer progression‐related epigenetic regulator, which is overexpressed in hormone‐refractory and metastatic prostate cancer (PCa). In our previous studies, we found a significantly increased MTA1 expression in a prostate‐specific Pten‐null mouse model. We also demonstrated that stilbenes, namely resveratrol and pterostilbene (Pter), affect MTA1/HDAC signaling, including deacetylation of tumor suppressors p53 and PTEN. In this study, we examined whether inhibition of MTA1/HDAC using combination of Pter and a clinically approved HDAC inhibitor, SAHA (suberoylanilide hydroxamic acid, vorinostat), which also downregulates MTA1, could block prostate tumor progression in vivo. We generated and utilized a luciferase reporter in a prostate‐specific Pten‐null mouse model (Pb‐Cre⁺; Pten^f/f; Rosa26^Luc/+) to evaluate the anticancer efficacy of Pter/SAHA combinatorial approach. Our data showed that Pter sensitized tumor cells to SAHA treatment resulting in inhibiting tumor growth and additional decline of tumor progression. These effects were dependent on the reduction of MTA1‐associated proangiogenic factors HIF‐1α, VEGF, and IL‐1β leading to decreased angiogenesis. In addition, treatment of PCa cell lines in vitro with combined Pter and low dose SAHA resulted in more potent inhibition of MTA1/HIF‐1α than by high dose SAHA alone. Our study provides preclinical evidence that Pter/SAHA combination treatment inhibits MTA1/HIF‐1α tumor‐promoting signaling in PCa. The beneficial outcome of combinatorial strategy using a natural agent and an approved drug for higher efficacy and less toxicity supports further development of MTA1‐targeted therapies in PCa.

Bifunctional conjugates with potent inhibitory activity towards cyclooxygenase and histone deacetylase

We herein disclose a series of compounds with potent inhibitory activities towards histone deacetylases (HDAC) and cyclooxygenases (COX). These compounds potently inhibited the growth of cancer cell lines consistent with their anti-COX and anti-HDAC activities. While compound 2b showed comparable level of COX-2 selectivity as celecoxib, compound 11b outperformed indomethacin in terms of selectivity towards COX-2 relative to COX-1. An important observation with our lead compounds (2b, 8, 11b, and 17b) is their enhanced cytotoxicity towards androgen dependent prostate cancer cell line (LNCaP) relative to androgen independent prostate cancer cell line (DU-145). Interestingly, compounds 2b and 17b arrested the cell cycle progression of LNCaP in the S-phase, while compound 8 showed a G0/G1 arrest, similar to SAHA. Relative to SAHA, these compounds displayed tumor-selective cytotoxicity as they have low anti-proliferative activity towards healthy cells (VERO); an attribute that makes them attractive candidates for drug development.

A Novel Class of Hsp90 C-Terminal Modulators Have Pre-Clinical Efficacy in Prostate Tumor Cells Without Induction of a Heat Shock Response

BACKGROUND

While there is compelling rationale to use heat shock protein 90 (Hsp90) inhibitors for treatment of advanced prostate cancer, agents that target the N-terminal ATP-binding site of Hsp90 have shown little clinical benefit. These N-terminal binding agents induce a heat shock response that activates compensatory heat shock proteins, which is believed to contribute in part to the agents' lack of efficacy. Here, we describe the functional characterization of two novel agents, SM253 and SM258, that bind the N-middle linker region of Hsp90, resulting in reduced client protein activation and preventing C-terminal co-chaperones and client proteins from binding to Hsp90.

METHODS

Inhibition of Hsp90 activity in prostate cancer cells by SM253 and SM 258 was assessed by pull-down assays. Cell viability, proliferation and apoptosis were assayed in prostate cancer cell lines (LNCaP, 22Rv1, PC-3) cultured with N-terminal Hsp90 inhibitors (AUY922, 17-AAG), SM253 or SM258. Expression of HSR heat shock proteins, Hsp90 client proteins and co-chaperones was assessed by immunoblotting. Efficacy of the SM compounds was evaluated in human primary prostate tumors cultured ex vivo by immunohistochemical detection of Hsp70 and Ki67.

RESULTS

SM253 and SM258 exhibit antiproliferative and pro-apoptotic activity in multiple prostate cancer cell lines (LNCaP, 22Rv1, and PC-3) at low micromolar concentrations. Unlike the N-terminal inhibitors AUY922 and 17-AAG, these SM agents do not induce expression of Hsp27, Hsp40, or Hsp70, proteins that are characteristic of the heat shock response, in any of the prostate cell lines analyzed. Notably, SM258 significantly reduced proliferation within 2 days in human primary prostate tumors cultured ex vivo, without the significant induction of Hsp70 that was caused by AUY922 in the tissues.

CONCLUSIONS

Our findings provide the first evidence of efficacy of this class of C-terminal modulators of Hsp90 in human prostate tumors, and indicate that further evaluation of these promising new agents is warranted. Prostate 76:1546-1559, 2016. © 2016 Wiley Periodicals, Inc.

Nephrotoxicity of epigenetic inhibitors used for the treatment of cancer

This study determined the anti-neoplastic activity and nephrotoxicity of epigenetic inhibitors in vitro. The therapeutic efficacy of epigenetic inhibitors was determined in human prostate cancer cells (PC-3 and LNCaP) using the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-Aza) and the histone deacetylase inhibitor trichostatin A (TSA). Cells were also treated with carbamazepine (CBZ), an anti-convulsant with histone deacetylase inhibitor-like properties. 5-Aza, TSA or CBZ alone did not decrease MTT staining in PC-3 or LNCaP cells after 48 h. In contrast, docetaxel, a frontline chemotherapeutic induced concentration-dependent decreases in MTT staining. Pretreatment with 5-Aza or TSA increased docetaxel-induced cytotoxicity in LNCaP cells, but not PC-3 cells. TSA pretreatment also increased cisplatin-induced toxicity in LNCaP cells. Carfilzomib (CFZ), a protease inhibitor approved for the treatment of multiple myeloma had minimal effect on LNCaP cell viability, but reduced MTT staining 50% in PC-3 cells compared to control, and pretreatment with 5-Aza further enhanced toxicity. Treatment of normal rat kidney (NRK) and human embryonic kidney 293 (HEK293) cells with the same concentrations of epigenetic inhibitors used in prostate cancer cells significantly decreased MTT staining in all cell lines after 48 h. Interestingly, we found that the toxicity of epigenetic inhibitors to kidney cells was dependent on both the compound and the stage of cell growth. The effect of 5-Aza and TSA on DNA methyltransferase and histone deacetylase activity, respectively, was confirmed by assessing the methylation and acetylation of the CDK inhibitor p21. Collectively, these data show that combinatorial treatment with epigenetic inhibitors alters the efficacy of chemotherapeutics in cancer cells in a compound- and cell-specific manner; however, this treatment also has the potential to induce nephrotoxic cell injury.

Targeting prostate cancer with compounds possessing dual activity as androgen receptor antagonists and HDAC6 inhibitors

While enzalutamide and abiraterone are approved for treatment of metastatic castration-resistant prostate cancer (mCRPC), approximately 20-40% of patients have no response to these agents. It has been stipulated that the lack of response and the development of secondary resistance to these drugs may be due to the presence of AR splice variants. HDAC6 has a role in regulating the androgen receptor (AR) by modulating heat shock protein 90 (Hsp90) acetylation, which controls the nuclear localization and activation of the AR in androgen-dependent and independent scenarios. With dual-acting AR-HDAC6 inhibitors it should be possible to target patients who don't respond to enzalutamide. Herein, we describe the design, synthesis and biological evaluation of dual-acting compounds which target AR and are also specific towards HDAC6. Our efforts led to compound 10 which was found to have potent dual activity (HDAC6 IC₅₀=0.0356μM and AR binding IC₅₀=<0.03μM). Compound 10 was further evaluated for antagonist and other cell-based activities, in vitro stability and pharmacokinetics.

Epigenetic modulators as therapeutic targets in prostate cancer

Prostate cancer is one of the most common non-cutaneous malignancies among men worldwide. Epigenetic aberrations, including changes in DNA methylation patterns and/or histone modifications, are key drivers of prostate carcinogenesis. These epigenetic defects might be due to deregulated function and/or expression of the epigenetic machinery, affecting the expression of several important genes. Remarkably, epigenetic modifications are reversible and numerous compounds that target the epigenetic enzymes and regulatory proteins were reported to be effective in cancer growth control. In fact, some of these drugs are already being tested in clinical trials. This review discusses the most important epigenetic alterations in prostate cancer, highlighting the role of epigenetic modulating compounds in pre-clinical and clinical trials as potential therapeutic agents for prostate cancer management.

Hybrid Enzalutamide Derivatives with Histone Deacetylase Inhibitor Activity Decrease Heat Shock Protein 90 and Androgen Receptor Levels and Inhibit Viability in Enzalutamide-Resistant C4-2 Prostate Cancer Cells

Histone deacetylase inhibitors (HDACIs) can disrupt the viability of prostate cancer (PCa) cells through modulation of the cytosolic androgen receptor (AR) chaperone protein heat shock protein 90 (HSP90). However, toxicities associated with their pleiotropic effects could contribute to the ineffectiveness of HDACIs in PCa treatment. We designed hybrid molecules containing partial chemical scaffolds of enzalutamide and suberoylanilide hydroxamic acid (SAHA), with weakened intrinsic pan-HDACI activities, to target HSP90 and AR in enzalutamide-resistant PCa cells. The potency of the new molecules, compounds 2-75 [4-(3-(4-cyano-3-(trifluoromethyl)phenyl)-5,5-dimethyl-4-oxo-2-thioxoimidazolidin-1-yl)-2-fluoro-N-(7-(hydroxyamino)-7-oxoheptyl)benzamide] and 1005 [(E)-3-(4-(3-(4-cyano-3-(trifluoromethyl)phenyl)-5,5-dimethyl-4-oxo-2-thioxoimidazolidin-1-yl)-2-fluorophenyl)-N-hydroxyacrylamide], as inhibitors of nuclear and cytosolic histone deacetylases was substantially lower than that of SAHA in cell-free and in situ assays. Compounds 2-75 and 1005 antagonized gene activation by androgen without inducing chromatin association of AR. Enzalutamide had no effect on the levels of AR or HSP90, whereas the hybrid compounds induced degradation of both AR and HSP90, similar to (compound 1005) or more potently than (compound 2-75) SAHA. Similar to SAHA, compounds 2-75 and 1005 decreased the level of HSP90 and induced acetylation in a predicted approximately 55 kDa HSP90 fragment. Compared with SAHA, compound 2-75 induced greater hyperacetylation of the HDAC6 substrate α-tubulin. In contrast with SAHA, neither hybrid molecule caused substantial hyperacetylation of histones H3 and H4. Compounds 2-75 and 1005 induced p21 and caused loss of viability in the enzalutamide-resistant C4-2 cells, with efficacies that were comparable to or better than SAHA. The results suggest the potential of the new compounds as prototype antitumor drugs that would downregulate HSP90 and AR in enzalutamide-resistant PCa cells with weakened effects on nuclear HDACI targets.

A ZEB1-miR-375-YAP1 pathway regulates epithelial plasticity in prostate cancer

MicroRNA-375 (miR-375) is frequently elevated in prostate tumors and cell-free fractions of patient blood, but its role in genesis and progression of prostate cancer is poorly understood. In this study, we demonstrated that miR-375 is inversely correlated with epithelial-mesenchymal transition signatures (EMT) in clinical samples and can drive mesenchymal-epithelial transition (MET) in model systems. Indeed, miR-375 potently inhibited invasion and migration of multiple prostate cancer lines. The transcription factor YAP1 was found to be a direct target of miR-375 in prostate cancer. Knockdown of YAP1 phenocopied miR-375 overexpression, and overexpression of YAP1 rescued anti-invasive effects mediated by miR-375. Furthermore, transcription of the miR-375 gene was shown to be directly repressed by the EMT transcription factor, ZEB1. Analysis of multiple patient cohorts provided evidence for this ZEB1-miR-375-YAP1 regulatory circuit in clinical samples. Despite its anti-invasive and anti-EMT capacities, plasma miR-375 was found to be correlated with circulating tumor cells in men with metastatic disease. Collectively, this study provides new insight into the function of miR-375 in prostate cancer, and more broadly identifies a novel pathway controlling epithelial plasticity and tumor cell invasion in this disease.

Effects of Brassicaceae Isothiocyanates on Prostate Cancer

Despite the major progress made in the field of cancer biology, cancer is still one of the leading causes of mortality, and prostate cancer (PCa) is one of the most encountered malignancies among men. The effective management of this disease requires developing better anticancer agents with greater efficacy and fewer side effects. Nature is a large source for the development of chemotherapeutic agents, with more than 50% of current anticancer drugs being of natural origin. Isothiocyanates (ITCs) are degradation products from glucosinolates that are present in members of the family Brassicaceae. Although they are known for a variety of therapeutic effects, including antioxidant, immunostimulatory, anti-inflammatory, antiviral and antibacterial properties, nowadays, cell line and animal studies have additionally indicated the chemopreventive action without causing toxic side effects of ITCs. In this way, they can induce cell cycle arrest, activate apoptosis pathways, increase the sensitivity of resistant PCa to available chemodrugs, modulate epigenetic changes and downregulate activated signaling pathways, resulting in the inhibition of cell proliferation, progression and invasion-metastasis. The present review summarizes the chemopreventive role of ITCs with a particular emphasis on specific molecular targets and epigenetic alterations in in vitro and in vivo cancer animal models.

IκBα mediates prostate cancer cell death induced by combinatorial targeting of the androgen receptor

Background

Combining different clinical agents to target multiple pathways in prostate cancer cells, including androgen receptor (AR) signaling, is potentially an effective strategy to improve outcomes for men with metastatic disease. We have previously demonstrated that sub-effective concentrations of an AR antagonist, bicalutamide, and the histone deacetylase inhibitor, vorinostat, act synergistically when combined to cause death of AR-dependent prostate cancer cells.

Methods

In this study, expression profiling of human prostate cancer cells treated with bicalutamide or vorinostat, alone or in combination, was employed to determine the molecular mechanisms underlying this synergistic action. Cell viability assays and quantitative real time PCR were used to validate identified candidate genes.

Results

A substantial proportion of the genes modulated by the combination of bicalutamide and vorinostat were androgen regulated. Independent pathway analysis identified further pathways and genes, most notably NFKBIA (encoding IκBα, an inhibitor of NF-κB and p53 signaling), as targets of this combinatorial treatment. Depletion of IκBα by siRNA knockdown enhanced apoptosis of prostate cancer cells, while ectopic overexpression of IκBα markedly suppressed cell death induced by the combination of bicalutamide and vorinostat.

Conclusion

These findings implicate IκBα as a key mediator of the apoptotic action of this combinatorial AR targeting strategy and a promising new therapeutic target for prostate cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2188-2) contains supplementary material, which is available to authorized users.

NCL1, a highly selective lysine-specific demethylase 1 inhibitor, suppresses prostate cancer without adverse effect

Herein, we investigated therapeutic potential of a novel histone lysine demethylase 1 (LSD1) inhibitor, NCL1, in prostate cancer. Hormone-sensitive prostate cancer cells, (LNCaP) and castration resistant cancer cells (PC3 and PCai1) were treated with NCL1, and LSD1 expression and cell viability were assessed. Prostate cancer cells showed strong LSD1 expression, and cell viability was decreased by NCL1. ChIP analysis showed that NCL1 induced H3K9me2 accumulation at the promoters of androgen-responsive genes. NCL1 also induced G1 cell cycle arrest and apoptosis. In addition, autophagosomes and autolysosomes were induced by NCL1 treatment in LNCaP. Furthermore, LC3-II expression was significantly increased by NCL1 and chloroquine. In mice injected subcutaneously with PCai1 and intraperitoneally with NCL1, tumor volume was reduced with no adverse effects in NCL1-treated mice. Finally, LSD1 expression in human cancer specimens was significantly higher than that in normal prostate glands. In conclusion, NCL1 effectively suppressed prostate cancer growth without adverse events. We suggest that NCL1 is a potential therapeutic agent for hormone-resistant prostate cancer.

Identification of a group of brominated flame retardants as novel androgen receptor antagonists and potential neuronal and endocrine disrupters

Brominated flame-retardants (BFRs) are used in industrial products to reduce the risk of fire. However, their continuous release into the environment is a concern as they are often persistent, bioaccumulating and toxic. Information on the impact these compounds have on human health and wildlife is limited and only a few of them have been identified to disrupt hormone receptor functions. In the present study we used in silico modeling to determine the interactions of selected BFRs with the human androgen receptor (AR). Three compounds were found to dock into the ligand-binding domain of the human AR and these were further tested using in vitro analysis. Allyl 2,4,6-tribromophenyl ether (ATE), 2-bromoallyl 2,4,6-tribromophenyl ether (BATE) and 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE) were observed to act as AR antagonists. These BFRs have recently been detected in the environment, in house dust and in aquatic animals. The compounds have been detected at high concentrations in both blubber and brain of seals and we therefore also assessed their impact on the expression of L-type amino acid transporter system (LAT) genes, that are needed for amino acid uptake across the blood-brain barrier, as disruption of LAT gene function has been implicated in several brain disorders. The three BFRs down-regulated the expression of AR target genes that encode for prostate specific antigen (PSA), 5α-reductases and β-microseminoprotein. The potency of PSA inhibition was of the same magnitude as the common prostate cancer drugs, demonstrating that these compounds are strong AR antagonists. Western blot analysis of AR protein showed that ATE, BATE and DPTE decreased the 5α-dihydrotestosterone-induced AR protein levels, further confirming that these BFRs act as AR antagonists. The transcription of the LAT genes was altered by the three BFRs, indicating an effect on amino-acid uptake across cellular membranes and blood-brain barrier. This study demonstrated that ATE, BATE and DPTE are potent AR antagonists and the alterations in LAT gene transcription suggest that these compounds can affect neuronal functions and should be considered as potential neurotoxic and endocrine disrupting compounds.

The association between histone 3 lysine 27 trimethylation (H3K27me3) and prostate cancer: relationship with clinicopathological parameters

BACKGROUND

It is well established that genetic and epigenetic alterations are common events in prostate cancer, which may lead to aberrant expression of critical genes. The importance of epigenetic mechanisms in prostate cancer carcinogenesis is increasingly evident. In this study, the focus will be on histone modifications and the primary objectives are to map H3K27me3 marks and quantify RAR beta 2, ER alpha, SRC3, RGMA, PGR, and EZH2 gene expressions in prostate cancer tissues compared to normal tissues. In addition, a data analysis was made in connection with the clinicopathological parameters.

METHODS

71 normal specimens and 66 cancer prostate tissues were randomly selected in order to assess the proportion of the repressive H3K27me3 mark and gene expression. H3K27me3 level was evaluated by ChIP-qPCR and mRNA expression using RT-qPCR between prostate cancer and normal tissues. Subsequently, western-blotting was performed for protein detection. The analysis of variance (ANOVA) was performed, and Tukey's test was used to correct for multiple comparisons (p-value threshold of 0.05). The principal component analysis (PCA) and discriminant factorial analysis (DFA) were used to explore the association between H3K27me3 level and clinicopathological parameters.

RESULTS

The study demonstrated that H3K27me3 level was significantly enriched at the RAR beta 2, ER alpha, PGR, and RGMA promoter regions in prostate cancer tissues compared to normal tissues. After stratification by clinicopathological parameters, the H3K27me3 level was positively correlated with Gleason score, PSA levels and clinical stages for RAR beta 2, ER alpha, PGR, and RGMA. High H3K27me3 mark was significantly associated with decreased RAR beta 2, ER alpha, PGR and RGMA gene expressions in prostate cancer sample compared to the normal one. Moreover, the results showed that mRNA level of EZH2, AR and SRC3 are upregulated in prostate cancer compared to normal prostate tissues and this correlates positively with Gleason score, PSA levels and clinical stages. Obviously, these observations were confirmed by protein level using western-blot.

CONCLUSIONS

This data clearly demonstrated that H3K27me3 level correlated with aggressive tumor features. Also this study revealed that reverse correlation of RAR beta 2, ER alpha, PGR, and RGMA expressions with EZH2, SRC3, and AR expressions in prostate cancer tissues suggests that these genes are the target of EZH2. Therefore, all therapeutic strategies leading to histone demethylation with epigenetic drugs such as histone methyltransferase inhibitor may be relevant treatments against prostate cancer.

Selectively targeting prostate cancer with antiandrogen equipped histone deacetylase inhibitors

Diverse cellular processes relevant to cancer progression are regulated by the acetylation status of proteins. Among such processes is chromatin remodeling via histone proteins, controlled by opposing histone deacetylase (HDAC) and histone acetyltransferase (HAT) enzymes. Histone deacetylase inhibitors (HDACi) show great promise in preclinical cancer models, but clinical trials treating solid tumors have failed to improve patient survival. This is due in part to an inability of HDACi to effectively accumulate in cancerous cells. To address this problem we designed HDACi with secondary pharmacophores to facilitate selective accumulation in malignant cells. We present the first example of HDACi compounds targeted to prostate tumors by equipping them with the additional ability to bind the androgen receptor (AR) with nonsteroidal antiandrogen moieties. Leads among these new dual-acting molecules bind to the AR and halt AR transcriptional activity at lower concentrations than clinical antiandrogens. They inhibit key isoforms of HDAC with low nanomolar potency. Fluorescent microscopy reveals varying degrees of AR nuclear localization in response to these compounds that correlates with their HDAC activity. These biological properties translate into potent anticancer activity against hormone-dependent (AR+) LNCaP and to a lesser extent against hormone-independent (AR-) DU145 prostate cancer, while having greatly reduced toxicity in noncancerous cells. This illustrates that engaging multiple biological targets with a single chemical probe can achieve both potent and cell-type-selective responses.

Histone deacetylase inhibition promotes osteoblast maturation by altering the histone H4 epigenome and reduces Akt phosphorylation

Bone has remarkable regenerative capacity, but this ability diminishes during aging. Histone deacetylase inhibitors (HDIs) promote terminal osteoblast differentiation and extracellular matrix production in culture. The epigenetic events altered by HDIs in osteoblasts may hold clues for the development of new anabolic treatments for osteoporosis and other conditions of low bone mass. To assess how HDIs affect the epigenome of committed osteoblasts, MC3T3 cells were treated with suberoylanilide hydroxamic acid (SAHA) and subjected to microarray gene expression profiling and high-throughput ChIP-Seq analysis. As expected, SAHA induced differentiation and matrix calcification of osteoblasts in vitro. ChIP-Seq analysis revealed that SAHA increased histone H4 acetylation genome-wide and in differentially regulated genes, except for the 500 bp upstream of transcriptional start sites. Pathway analysis indicated that SAHA increased the expression of insulin signaling modulators, including Slc9a3r1. SAHA decreased phosphorylation of insulin receptor β, Akt, and the Akt substrate FoxO1, resulting in FoxO1 stabilization. Thus, SAHA induces genome-wide H4 acetylation and modulates the insulin/Akt/FoxO1 signaling axis, whereas it promotes terminal osteoblast differentiation in vitro.

Histone deacetylase inhibitors modulate miRNA and mRNA expression, block metaphase, and induce apoptosis in inflammatory breast cancer cells

To develop new therapies for inflammatory breast cancer (IBC) we have compared the effects of two hydroxamic acid-based histone deacetylase (HDAC) inhibitors, CG-1521 and Trichostatin A (TSA) on the biology of two IBC cell lines: SUM149PT and SUM190PT. CG-1521 and TSA induce dose (0−10 µM) and time-dependent (0−96 h) increases in the proportion of cells undergoing cell cycle arrest and apoptosis in the presence or absence of 17β-estradiol. In SUM 149PT cells, both CG-1521 and TSA increase the levels of acetylated α-tubulin; however the morphological effects are different: CG-1521 blocks mitotic spindle formation and prevents abscission during cytokinesis while TSA results in an increase in cell size. In SUM190PT cells CG-1521 does not cause an increase in acetylated-α-tubulin and even though TSA significantly increases the levels of acetylated tubulin, neither inhibitor alters the morphology of the cells. Microarray analysis demonstrates that CG-1521 modulates the expression of 876 mRNAs and 63 miRNAs in SUM149PT cells, and 1227 mRNAs and 35 miRNAs in SUM190PT cells. Only 9% of the genes are commonly modulated in both cell lines, suggesting that CG-1521 and TSA target different biological processes in the two cell lines most likely though the inhibition of different HDACs in these cell lines. Gene ontology (GO) analysis reveals that CG-1521 affects the expression of mRNAs that encode proteins associated with the spindle assembly checkpoint, chromosome segregation, and microtubule-based processes in both cell lines and has cell-type specific effects on lipid biosynthesis, response to DNA damage, and cell death.

Finding the place of histone deacetylase inhibitors in prostate cancer therapy

Histone deacetylase inhibitors (HDACIs) are showing promise as therapeutic agents for hematological malignancies and solid tumors. In the case of prostate cancer, HDACIs are effective at inhibiting proliferation and inducing apoptosis in a range of in vitro and in vivo experimental models. Recent studies have revealed that the actions of HDACIs in prostate cancer cells extend beyond regulation of histone acetylation and affect proteins involved in maintaining cellular homeostasis and tumor progression, including the androgen receptor, p21(WAF1) and VEGF. The broad spectrum of HDACI targets has allowed rational design of combinations with other therapeutic agents to target multiple pathways involved in prostate cancer progression, including angiogenesis and androgen signaling. In particular, synergistic inhibition of prostate cancer cell growth has been demonstrated using HDACIs in combination with radio- and chemo-therapy, Apo2L/TRAIL, angiogenesis inhibitors, heat-shock protein 90 inhibitors and androgen receptor antagonists. This review examines the current understanding of the actions of HDACIs in prostate cancer cells, both in a laboratory and a clinical context and discusses the potential utility of combination strategies for the treatment of prostate cancer.

The emerging role of histone lysine demethylases in prostate cancer

Early prostate cancer (PCa) is generally treatable and associated with good prognosis. After a variable time, PCa evolves into a highly metastatic and treatment-refractory disease: castration-resistant PCa (CRPC). Currently, few prognostic factors are available to predict the emergence of CRPC, and no curative option is available. Epigenetic gene regulation has been shown to trigger PCa metastasis and androgen-independence. Most epigenetic studies have focused on DNA and histone methyltransferases. While DNA methylation leads to gene silencing, histone methylation can trigger gene activation or inactivation, depending on the target amino acid residues and the extent of methylation (me1, me2, or me3). Interestingly, some histone modifiers are essential for PCa tumor-initiating cell (TIC) self-renewal. TICs are considered the seeds responsible for metastatic spreading and androgen-independence. Histone Lysine Demethylases (KDMs) are a novel class of epigenetic enzymes which can remove both repressive and activating histone marks. KDMs are currently grouped into 7 major classes, each one targeting a specific methylation site. Since their discovery, KDM expression has been found to be deregulated in several neoplasms. In PCa, KDMs may act as either tumor suppressors or oncogenes, depending on their gene regulatory function. For example, KDM1A and KDM4C are essential for PCa androgen-dependent proliferation, while PHF8 is involved in PCa migration and invasion. Interestingly, the possibility of pharmacologically targeting KDMs has been demonstrated. In the present paper, we summarize the emerging role of KDMs in regulating the metastatic potential and androgen-dependence of PCa. In addition, we speculate on the possible interaction between KDMs and other epigenetic effectors relevant for PCa TICs. Finally, we explore the role of KDMs as novel prognostic factors and therapeutic targets. We believe that studies on histone demethylation may add a novel perspective in our efforts to prevent and cure advanced PCa.

Elevation of c-FLIP in castrate-resistant prostate cancer antagonizes therapeutic response to androgen receptor-targeted therapy

PURPOSE

To characterize the importance of cellular Fas-associated death domain (FADD)-like interleukin 1β-converting enzyme (FLICE) inhibitory protein (c-FLIP), a key regulator of caspase-8 (FLICE)-promoted apoptosis, in modulating the response of prostate cancer cells to androgen receptor (AR)-targeted therapy.

EXPERIMENTAL DESIGN

c-FLIP expression was characterized by immunohistochemical analysis of prostatectomy tissue. The functional importance of c-FLIP to survival and modulating response to bicalutamide was studied by molecular and pharmacologic interventions.

RESULTS

c-FLIP expression was increased in high-grade prostatic intraepithelial neoplasia and prostate cancer tissue relative to normal prostate epithelium (P < 0.001). Maximal c-FLIP expression was detected in castrate-resistant prostate cancer (CRPC; P < 0.001). In vitro, silencing of c-FLIP induced spontaneous apoptosis and increased 22Rv1 and LNCaP cell sensitivity to bicalutamide, determined by flow cytometry, PARP cleavage, and caspase activity assays. The histone deacetylase inhibitors (HDACi), droxinostat and SAHA, also downregulated c-FLIP expression, induced caspase-8- and caspase-3/7-mediated apoptosis, and increased apoptosis in bicalutamide-treated cells. Conversely, the elevated expression of c-FLIP detected in the CRPC cell line VCaP underpinned their insensitivity to bicalutamide and SAHA in vitro. However, knockdown of c-FLIP induced spontaneous apoptosis in VCaP cells, indicating its relevance to cell survival and therapeutic resistance.

CONCLUSION

c-FLIP reduces the efficacy of AR-targeted therapy and maintains the viability of prostate cancer cells. A combination of HDACi with androgen deprivation therapy may be effective in early-stage disease, using c-FLIP expression as a predictive biomarker of sensitivity. Direct targeting of c-FLIP, however, may be relevant to enhance the response of existing and novel therapeutics in CRPC.

Evidence for efficacy of new Hsp90 inhibitors revealed by ex vivo culture of human prostate tumors

PURPOSE

Targeting Hsp90 has significant potential as a treatment for prostate cancer, but prototypical agents such as 17-allylamino-17 demethoxygeldanamycin (17-AAG) have been ineffective in clinical trials. Recently, a phase I study aimed at defining a biologically active dose reported the first response to an Hsp90 inhibitor in a patient with prostate cancer, which supports the development of new generation compounds for this disease.

EXPERIMENTAL DESIGN

The biological actions of two new synthetic Hsp90 inhibitors, NVP-AUY922 and NVP-HSP990, were evaluated in the prostate cancer cell lines PC-3, LNCaP, and VCaP and in an ex vivo culture model of human prostate cancer.

RESULTS

In cell lines, both NVP-AUY922 and NVP-HSP990 showed greater potency than 17-AAG with regard to modulation of Hsp90 client proteins, inhibition of proliferation, and induction of apoptotic cell death. In prostate tumors obtained from radical prostatectomy that were cultured ex vivo, treatment with 500 nmol/L of NVP-AUY922, NVP-HSP990, or 17-AAG caused equivalent target modulation, determined by the pharmacodynamic marker Hsp70, but only NVP-AUY922 and NVP-HSP990 showed antiproliferative and proapoptotic activity.

CONCLUSIONS

This study provides some of the first evidence that new generation Hsp90 inhibitors are capable of achieving biologic responses in human prostate tumors, with both NVP-AUY922 and NVP-HSP990 showing potent on-target efficacy. Importantly, the ex vivo culture technique has provided information on Hsp90 inhibitor action not previously observed in cell lines or animal models. This approach, therefore, has the potential to enable more rational selection of therapeutic agents and biomarkers of response for clinical trials.

Epigenetics in prostate cancer

Prostate cancer (PC) is the most commonly diagnosed nonskin malignancy and the second most common cause of cancer death among men in the United States. Epigenetics is the study of heritable changes in gene expression caused by mechanisms other than changes in the underlying DNA sequences. Two common epigenetic mechanisms, DNA methylation and histone modification, have demonstrated critical roles in prostate cancer growth and metastasis. DNA hypermethylation of cytosine-guanine (CpG) rich sequence islands within gene promoter regions is widespread during neoplastic transformation of prostate cells, suggesting that treatment-induced restoration of a “normal” epigenome could be clinically beneficial. Histone modification leads to altered tumor gene function by changing chromosome structure and the level of gene transcription. The reversibility of epigenetic aberrations and restoration of tumor suppression gene function have made them attractive targets for prostate cancer treatment with modulators that demethylate DNA and inhibit histone deacetylases.

Castration-resistant prostate cancer: new science and therapeutic prospects

There is a growing number of new therapies targeting different pathways that will revolutionize patient management strategies in castration-resistant prostate cancer (CRPC) patients. Today there are more clinical trial options for CRPC treatment than ever before, and there are many promising agents in late-stage clinical testing. The hypothesis that CRPC frequently remains driven by a ligand-activated androgen receptor (AR) and that CRPC tissues exhibit substantial residual androgen levels despite gonadotropin-releasing hormone therapy, has led to the evaluation of new oral compounds such as abiraterone and MDV 3100. Their results, coupled with promising recent findings in immunotherapy (eg sipuleucel-T) and with agents targeting angiogenesis (while awaiting the final results of the CALGB trial 90401) will most probably impact the management of patients with CRPC in the near future. Other new promising agents need further development. With our increased understanding of the biology of this disease, further trial design should incorporate improved patient selection so that patient populations are those who may be most likely to benefit from treatment.

New insights into the androgen-targeted therapies and epigenetic therapies in prostate cancer

Prostate cancer is the most common cancer in men in the United States, and it is the second leading cause of cancer-related death in American men. The androgen receptor (AR), a receptor of nuclear family and a transcription factor, is the most important target in this disease. While most efforts in the clinic are currently directed at lowering levels of androgens that activate AR, resistance to androgen deprivation eventually develops. Most prostate cancer deaths are attributable to this castration-resistant form of prostate cancer (CRPC). Recent work has shed light on the importance of epigenetic events including facilitation of AR signaling by histone-modifying enzymes, posttranslational modifications of AR such as sumoylation. Herein, we provide an overview of the structure of human AR and its key structural domains that can be used as targets to develop novel antiandrogens. We also summarize recent findings about the antiandrogens and the epigenetic factors that modulate the action of AR.

GSTP1 DNA methylation and expression status is indicative of 5-aza-2'-deoxycytidine efficacy in human prostate cancer cells

DNA methylation plays an important role in carcinogenesis and the reversibility of this epigenetic modification makes it a potential therapeutic target. To date, DNA methyltransferase inhibitors (DNMTi) have not demonstrated clinical efficacy in prostate cancer, with one of the major obstacles being the inability to monitor drug activity during the trial. Given the high frequency and specificity of GSTP1 DNA methylation in prostate cancer, we investigated whether GSTP1 is a useful marker of DNMTi treatment efficacy. LNCaP prostate cancer cells were treated with 5-aza-2′-deoxycytidine (5-aza-CdR) either with a single high dose (5–20 µM), every alternate day (0.1–10 µM) or daily (0.005–2.5 µM). A daily treatment regimen with 5-aza-CdR was optimal, with significant suppression of cell proliferation achieved with doses of 0.05 µM or greater (p<0.0001) and induction of cell death from 0.5 µM (p<0.0001). In contrast, treatment with a single high dose of 20 µM 5-aza-CdR inhibited cell proliferation but was not able to induce cell death. Demethylation of GSTP1 was observed with doses of 5-aza-CdR that induced significant suppression of cell proliferation (≥0.05 µM). Re-expression of the GSTP1 protein was observed only at doses of 5-aza-CdR (≥0.5 µM) associated with induction of cell death. Treatment of LNCaP cells with a more stable DNMTi, Zebularine required at least a 100-fold higher dose (≥50 µM) to inhibit proliferation and was less potent in inducing cell death, which corresponded to a lack of GSTP1 protein re-expression. We have shown that GSTP1 DNA methylation and protein expression status is correlated with DNMTi treatment response in prostate cancer cells. Since GSTP1 is methylated in nearly all prostate cancers, our results warrant its testing as a marker of epigenetic therapy response in future clinical trials. We conclude that the DNA methylation and protein expression status of GSTP1 are good indicators of DNMTi efficacy.

New hormonal therapies for castration-resistant prostate cancer

Continued activation of the androgen receptor (AR) axis despite castration remains a critical force in the development of castration-resistant prostate cancer (CRPC). Therapeutic strategies designed to more effectively ablate tumoral androgen activity are required to improve clinical efficacy and prevent disease progression. Tumor-based alterations in expression and activity of the AR and in steroidogenic pathways mediating ligand generation facilitate the development of CRPC. This article reviews AR and ligand-dependent mechanisms underlying CRPC progression and the status of novel hormonal therapies targeting the AR axis that are currently in clinical and preclinical development.

Increased androgen receptor transcription: a cause of castration-resistant prostate cancer and a possible therapeutic target

Few effective therapies exist for the treatment of castration-resistant prostate cancer (CRPC). Recent evidence suggests that CRPC may be caused by augmented androgen/androgen receptor (AR) signaling, generally involving AR overexpression. Aberrant androgen/AR signaling associated with AR overexpression also plays a key role in prostate carcinogenesis. Although AR overexpression could be attributed to gene amplification, only 10-20% of CRPCs exhibit AR gene amplification, and aberrant AR expression in the remaining instances of CRPC is thought to be attributed to transcriptional, translational, and post-translational mechanisms. Overexpression of AR at the protein level, as well as the mRNA level, has been found in CRPC, suggesting a key role for transcriptional regulation of AR expression. Since the analysis of the AR promoter region in the 1990s, several transcription factors have been reported to regulate AR transcription. In this review, we discuss the molecules involved in the control of AR gene expression, with emphasis on its transcriptional control by transcription factors in prostate cancer. We also consider the therapeutic potential of targeting AR expression.