Effects of sorafenib on C-terminally truncated androgen receptor variants in human prostate cancer cells

[The impact of the androgen receptor splice variant AR-V7 on the prognosis and treatment of advanced prostate cancer]

A recently discovered mechanism enabling prostate cancer cells to escape the effects of endocrine therapies consists in the synthesis of C-terminally truncated, constitutively active androgen receptor (AR) splice variants (AR-V). Devoid of a functional C-terminal hormone/ligand binding domain, various AR-Vs are insensitive to therapies targeting the androgen/AR signalling axis. Preliminary studies suggest that AR-V7, the most common AR-V, is a promising predictive tumour marker and a relevant selection marker for the treatment of advanced prostate cancer. This review critically outlines recent advances in AR-V7 diagnostics and presents an overview of current AR-V7 targeted therapies.

Raddeanin A down-regulates androgen receptor and its splice variants in prostate cancer

Castration-resistant progression of prostate cancer is a major cause of prostate cancer mortality, and increased expression and activity of the full-length and the splice variants of androgen receptor (AR) have been indicated to drive castration resistance. Consequently, there is an urgent need to develop agents that can target both the full-length and the splice variants of AR for more effective treatment of prostate cancer. In the present study, we showed that raddeanin A (RA), an oleanane-type triterpenoid saponin, suppresses the transcriptional activities of both the full-length and the splice variants of AR. This is attributable to their decreased expression as a result of RA induction of proteasome-mediated degradation and inhibition of the transcription of the AR gene. We further showed the potential of using RA to enhance the growth inhibitory efficacy of docetaxel, the first-line chemotherapy for prostate cancer. This study identifies RA as a new agent to target both the full-length and the splice variants of AR and provides a rationale for further developing RA for prostate cancer treatment.

C-terminally truncated constitutively active androgen receptor variants and their biologic and clinical significance in castration-resistant prostate cancer

A mechanism allowing castration resistant prostate cancer cells to escape the effects of conventional anti-hormonal treatments is the synthesis of constitutively active, C-terminally truncated androgen receptor (AR)-variants. Lacking the entire or vast parts of the ligand binding domain, the intended target of traditional endocrine therapies, these AR-variants (termed ARΔLBD) are insensitive to all traditional treatments including second generation compounds like abiraterone, enzalutamide or ARN-509. Although ARΔLBD are predominantly products of alternative splicing, they can also be products of nonsense mutations or proteolytic cleavage. In this review, we will discuss the etiology and function of c-terminally truncated AR-variants and their clinical significance as markers/targets for the treatment of castration resistant prostate cancer.

Emerging data on androgen receptor splice variants in prostate cancer

Androgen receptor splice variants are alternatively spliced variants of androgen receptor, which are C-terminally truncated and lack the canonical ligand-binding domain. Accumulating evidence has indicated a significant role of androgen receptor splice variants in mediating resistance of castration-resistant prostate cancer to current therapies and in predicting therapeutic responses. As such, there is an urgent need to target androgen receptor splicing variants for more effective treatment of castration-resistant prostate cancer. Identification of precise and critical targeting points to deactivate androgen receptor splicing variants relies on a deep understanding of how they are generated and the mechanisms of their action. In this review, we will focus on the emerging data on their generation, clinical significance and mechanisms of action as well as the therapeutic influence of these findings.

Adaptive responses of androgen receptor signaling in castration-resistant prostate cancer

Prostate Cancer (PCa) is an important age-related disease being the most common cancer malignancy and the second leading cause of cancer mortality in men in Western countries. Initially, PCa progression is androgen receptor (AR)- and androgen-dependent. Eventually advanced PCa reaches the stage of Castration-Resistant Prostate Cancer (CRPC), but remains dependent on AR, which indicates the importance of AR activity also for CRPC. Here, we discuss various pathways that influence the AR activity in CRPC, which indicates an adaptation of the AR signaling in PCa to overcome the treatment of PCa. The adaptation pathways include interferences of the normal regulation of the AR protein level, the expression of AR variants, the crosstalk of the AR with cytokine tyrosine kinases, the Src-Akt-, the MAPK-signaling pathways and AR corepressors. Furthermore, we summarize the current treatment options with regard to the underlying molecular basis of the common adaptation processes of AR signaling that may arise after the treatment with AR antagonists, androgen deprivation therapy (ADT) as well as for CRPC, and point towards novel therapeutic strategies. The understanding of individualized adaptation processes in PCa will lead to individualized treatment options in the future.

Case report of severe Cushing's syndrome in medullary thyroid cancer complicated by functional diabetes insipidus, aortic dissection, jejunal intussusception, and paraneoplastic dysautonomia: remission with sorafenib without reduction in cortisol concentration

Background

Normalization of cortisol concentration by multikinase inhibitors have been reported in three patients with medullary thyroid cancer-related Cushing’s syndrome. Aortic dissection has been reported in three patients with Cushing’s syndrome. Diabetes insipidus without intrasellar metastasis, intestinal intussusception, and paraneoplastic dysautonomia have not been reported in medullary thyroid cancer.

Case presentation

An adult male with metastatic medullary thyroid cancer presented with hyperglycemia, hypernatremia, hypokalemia, hypertension, acne-like rash, and diabetes insipidus (urine volume >8 L/d, osmolality 190 mOsm/kg). Serum cortisol, adrenocorticoitropic hormone, dehydroepiandrostenedione sulfate, and urinary free cortisol were elevated 8, 20, 4.4, and 340 folds, respectively. Pituitary imaging was normal. Computed tomography scan revealed jejunal intussusception and incidental abdominal aortic dissection. Sorafenib treatment was associated with Cushing’s syndrome remission, elevated progesterone (>10 fold), normalization of dehydroepiandrostenedione sulfate, but persistently elevated cortisol concentration. Newly-developed proximal lower limb weakness and decreased salivation were associated with elevated ganglionic neuronal acetylcholine receptor (alpha-3) and borderline P/Q type calcium channel antibodies.

Conclusion

Extreme cortisol concentration may have contributed to aortic dissection and suppressed antidiuretic hormone secretion; which combined with hypokalemia due cortisol activation of mineralocorticoid receptors, manifested as diabetes insipidus. This is the first report of paraneoplastic dysautonomia and jejunal intussusception in medullary thyroid cancer, they may be related to medullary thyroid cancer’s neuroendocrine origin and metastasis, respectively. Remission of Cushing’s syndrome without measurable reduction in cortisol concentration suggests a novel cortisol-independent mechanism of action or assay cross-reactivity. Normalization of dehydroepiandrostenedione sulfate and elevation of progesterone suggest inhibition of 17-hydroxylase and 21-hydroxylase activities by sorafenib.

Inhibition of IGF-1R diminishes transcriptional activity of the androgen receptor and its constitutively active, C-terminally truncated counterparts Q640X and AR-V7

PURPOSE

Failure of endocrine treatment in castration-resistant prostate cancer (CRPC) is often associated with the emergence of C-terminally truncated androgen receptor variants that function as constitutively active transcription factors (i.e., AR∆LBD). The mechanisms involved in the regulation of AR∆LBD signaling are largely unknown. Since the IGF-1 pathway was repeatedly shown to affect AR function, we studied whether an inhibition of IGF-1R could also affect AR∆LBD signaling.

METHODS

Regulation of androgen receptor (AR) and AR∆LBD signaling was analyzed by reporter gene assays, immunoblotting, ELISA and quantitative RT-PCR.

RESULTS

Inhibition of IGF-1R with the small-molecule inhibitor NVP-AEW541 reduced the transcriptional activity of the AR and its truncated counterparts Q640X and AR-V7. As shown in Q640X, the inhibition of transcriptional activity was paralleled by a decreased receptor phosphorylation.

CONCLUSIONS

Inhibition of IGF-1R leads to a down-regulation of AR∆LBD signaling and provides a rationale for CRPC therapies targeting growth factor receptors.

Methylselenocysteine preventing castration-resistant progression of prostate cancer

BACKGROUND

Castration-resistant progression of prostate cancer after androgen deprivation therapy remains a critical challenge in the clinical management of prostate cancer. Resurgent androgen receptor activity is an established driver of castration-resistant progression, and upregulation of androgen receptor expression has been implicated to contribute to the resurgent androgen receptor activity. We reported previously that methylselenocysteine can decrease the expression and activity of androgen receptor. Here we investigated the ability of methylselenocysteine to inhibit castration-resistant progression of prostate cancer.

METHODS

The regrowth of LNCaP prostate cancer xenografts after castration was monitored. The levels of prostate-specific antigen in mouse serum were measured by ELISA. Tumor cell proliferation and apoptosis were analyzed via Ki-67 immunohistochemistry and TUNEL assay, respectively. Intratumoral angiogenesis was assessed by immunohistochemistry staining of vascular endothelial growth factor and CD31.

RESULTS

We showed that methylselenocysteine delayed castration-resistant regrowth of LNCaP xenograft tumors after androgen deprivation. This was accompanied by decreased serum levels of prostate-specific antigen, inhibition of prostate cancer cell proliferation and tumor angiogenesis, as well as downregulation of androgen receptor and induction of apoptosis in the relapsed tumors.

CONCLUSIONS

The present study represents the first to show the preclinical efficacy of methylselenocysteine in delaying castration-resistant progression of prostate cancer. The findings provide a rationale for evaluating the clinical application of combining methylselenocysteine with androgen deprivation therapy for the treatment of advanced prostate cancer.

Evaluation of in vivo responses of sorafenib therapy in a preclinical mouse model of PTEN-deficient of prostate cancer

Background

Despite recent advances in the treatment for advanced prostate cancer, outcomes remain poor. This lack of efficacy has prompted the development of alternative treatment strategies. In the present study we investigate the effects of the multikinase inhibitor sorafenib in a genetically engineered mouse model of prostate cancer and explore the rational combination with the mTOR inhibitor everolimus.

Methods

Conditional prostate specific PTEN-deficient knockout mice were utilized to determine the pharmacodynamic and chemopreventive effects of sorafenib. This mouse model was also used to examine the therapeutic efficacy of sorafenib alone or in combination with everolimus. Preclinical efficacy was assessed by comparing the reduction of tumor burden, proliferation, angiogenesis and the induction of apoptosis. Molecular responses were assessed by immunohistochemical, TUNEL and western blot assays.

Results

Pharmacodynamic analysis revealed that a single dose of sorafenib decreased activation of the PI3K/AKT/mTOR signaling axis at doses of 30–60 mg/kg, but activated JAK/STAT3 signaling. Levels of cleaved casapase-3 increased in a dose dependent manner. Chemoprevention studies showed that chronic sorafenib administration was capable of inhibiting tumor progression through the reduction of cancer cell proliferation, angiogenesis and the induction of apoptosis. In intervention models of established castration-naïve and castration-resistant prostate cancer, treatment with sorafenib provided modest but statistically insignificant reduction in tumor burden. However, sorafenib significantly inhibited cancer cell proliferation and MVD but had minimal effects on the induction of apoptosis. Interestingly, the administration of sorafenib increased the expression levels of the androgen receptor, p-GSK3β and p-ERK1/2 in castration-resistant prostate cancers. In both intervention models, combination therapy demonstrated a clear tendency of enhanced antitumor effects over monotherapy. Notably, the treatment combination of sorafenib and everolimus overcame therapeutic escape from single agent therapy in castration-resistant prostate cancers.

Conclusions

In summary, we provide insights into the molecular responses of sorafenib therapy in a clinically relevant model of prostate cancer and present preclinical evidence for the development of targeted treatment strategies based on the use of multikinase inhibitors in combination with mTOR inhibitors for the treatment of advanced prostate cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0509-x) contains supplementary material, which is available to authorized users.

20(S)-protopanaxadiol inhibition of progression and growth of castration-resistant prostate cancer

Castration-resistant progression of prostate cancer after androgen deprivation therapies remains the most critical challenge in the clinical management of prostate cancer. Resurgent androgen receptor (AR) activity is an established driver of castration-resistant progression, and upregulation of the full-length AR (AR-FL) and constitutively-active AR splice variants (AR-Vs) has been implicated to contribute to the resurgent AR activity. We reported previously that ginsenoside 20(S)-protopanaxadiol-aglycone (PPD) can reduce the abundance of both AR-FL and AR-Vs. In the present study, we further showed that the effect of PPD on AR expression and target genes was independent of androgen. PPD treatment resulted in a suppression of ligand-independent AR transactivation. Moreover, PPD delayed castration-resistant regrowth of LNCaP xenograft tumors after androgen deprivation and inhibited the growth of castration-resistant 22Rv1 xenograft tumors with endogenous expression of AR-FL and AR-Vs. This was accompanied by a decline in serum prostate-specific antigen levels as well as a decrease in AR levels and mitoses in the tumors. Notably, the 22Rv1 xenograft tumors were resistant to growth inhibition by the next-generation anti-androgen enzalutamide. The present study represents the first to show the preclinical efficacy of PPD in inhibiting castration-resistant progression and growth of prostate cancer. The findings provide a rationale for further developing PPD or its analogues for prostate cancer therapy.

Biologic and clinical significance of androgen receptor variants in castration resistant prostate cancer

As prostate cancer (PCa) progresses to the lethal castration resistant and metastatic form, genetic and epigenetic adaptation, clonal selection, and evolution of the tumor microenvironment contribute to the emergence of unique biological characteristics under the selective pressure of external stresses. These stresses include the therapies applied in the clinic or laboratory and the exposures of cancers to hormonal, paracrine, or autocrine stimuli in the context of the tumor micro- and macro-environment. The androgen receptor (AR) is a key gene involved in PCa etiology and oncogenesis, including disease development, progression, response to initial hormonal therapies, and subsequent resistance to hormonal therapies. Alterations in the AR signaling pathway have been observed in certain selection contexts and contribute to the resistance to agents that target hormonal regulation of the AR, including standard androgen deprivation therapy, antiandrogens such as enzalutamide, and androgen synthesis inhibition with abiraterone acetate. One such resistance mechanism is the synthesis of constitutively active AR variants lacking the canonical ligand-binding domain. This review focuses on the etiology, characterization, biological properties, and emerging data contributing to the clinical characteristics of AR variants, and suggests approaches to full-length AR and AR variant biomarker validation, assessment, and systemic targeting in the clinic.

Targeting the androgen receptor in prostate cancer

INTRODUCTION

The androgen receptor (AR) is a ligand-activated transcription factor that is expressed in primary and metastatic prostate cancers. There are advances in endocrine therapy for prostate cancer that are based on improved understanding of AR function.

AREAS COVERED

PubMed has been used to include most important publications on targeting the AR in prostate cancer. AR expression may be downregulated by agents used for chemoprevention of prostate cancer or, in models of advanced prostate cancer, by antisense oligonucleotides. New drugs that inhibit the steroidogenic enzyme CYP17A1 (abiraterone acetate) or diminish nuclear translocation of the AR (enzalutamide) have been shown to improve patients' survival in prostate cancer. However, it is clear that there is a development of resistance to these novel therapies. They may include increased expression of truncated, constitutively active AR or activation of the signaling pathway of signal transducers and activators of transcription.

EXPERT OPINION

Although introduction of novel drugs have improved patients' survival, there is a need to investigate the mechanisms of resistance further. The role of truncated AR and compensatory activation of signaling pathways as well as the development of scientifically justified combination therapies seems to be issues of a high priority.

On the origins of the androgen receptor low molecular weight species

Prostate cancer (CaP), a commonly diagnosed malignancy, is readily treated by androgen ablation. This treatment temporarily halts the disease, but castration-resistant neoplasms that are refractory to current therapies emerge. While these neoplasms are no longer dependent on physiological levels of androgens, they remain reliant on the expression of the androgen receptor (AR). There are multiple mechanisms by which CaP cells circumvent androgen ablation therapies. These include AR mutations that broaden ligand specificity, AR overexpression, AR activation by growth factors and cytokines, overexpression of AR co-activators, altered steroid metabolism, and a locus-wide histone transcriptional activation of some AR targets. This review focuses on a more recently described mechanism: the expression of low molecular weight AR species that are missing the ligand-binding domain and function independently of ligand to drive proliferation. The etiology, biological activity, unique features, predictive value, and therapeutic implication of these androgen receptor isoforms are discussed in depth.

Perspectives on treatment of metastatic castration-resistant prostate cancer

The arrival of several new agents--cabazitaxel, abiraterone acetate, enzalutamide, and radium-223--is changing the treatment options and management of patients with metastatic castration-resistant prostate cancer (mCRPC). Many other novel agents are also being investigated. As new drugs become approved, new treatment strategies and markers to best select which patients will best respond to which drug are needed. This review article is a summary of a European Treatment Practices Meeting, which was convened to discuss these latest data on novel agents and current treatment strategies in the mCRPC setting.