Androgen receptor: a key molecule in the progression of prostate cancer to hormone independence

Despite earlier detection and recent advances in surgery and radiation, prostate cancer is second only to lung cancer in male cancer deaths in the United States. Hormone therapy in the form of medical or surgical castration remains the mainstay of systemic treatment in prostate cancer. Over the last 15 years with the clinical use of prostate specific antigen (PSA), there has been a shift to using hormone therapy earlier in the disease course and for longer duration. Despite initial favorable response to hormone therapy, over a period of time these tumors will develop androgen‐independence that results in death. The androgen receptor (AR) is central to the initiation and growth of prostate cancer and to its response to hormone therapy. Analyses have shown that AR continues to be expressed in androgen‐independent tumors and AR signaling remains intact as demonstrated by the expression of the AR regulated gene, PSA. Androgen‐independent prostate cancers have demonstrated a variety of AR alterations that are either not found in hormone naïve tumors or found at lower frequency. These changes include AR amplification, AR point mutation, and changes in expression of AR co‐regulatory proteins. These AR changes result in a “super AR” that can respond to lower concentrations of androgens or to a wider variety of agonistic ligands. There is also mounting evidence that AR can be activated in a ligand independent fashion by compounds such as growth factors or cytokines working independently or in combination. These growth factors working through receptor tyrosine kinase pathways may promote AR activation and growth in low androgen environments. The clinical significance of these AR alterations in the development and progression of androgen‐independent prostate cancer remains to be determined. Understanding the changes in AR signaling in the evolution of androgen‐independent prostate cancer will be key to the development of more effective hormone therapy. © 2003 Wiley‐Liss, Inc.     

雄激素受体信号转导途径及其在前列腺癌发展中的作用

前列腺癌是西方男性发病率最高的癌症之一,在采用雄激素阻断疗法后,大部分患者的病情可得到控制,但经过一段时间又会转变为雄激素非依赖型前列腺癌。雄激素受体(AR)在前列腺细胞中扮演重要的角色,它可调节大量基因的表达。在前列腺癌由雄激素依赖型向雄激素非依赖型的转变过程中,AR及其信号途径通过多种方式发挥作用,AR基因的扩增、AR的突变,以及与共激活子之间作用的改变都可能使细胞获得雄激素非依赖型的生长能力。此外,AR还受到多肽生长因子和细胞因子等的调节,表现激素非依赖型的转录激活活性。AR在前列腺癌中作用的阐明对前列腺癌的诊断与治疗有着重大的意义。     

Androgens as therapy for androgen receptor-positive castration-resistant prostate cancer

Prostate cancer is the most frequently diagnosed non-cutaneous tumor of men in Western countries. While surgery is often successful for organ-confined prostate cancer, androgen ablation therapy is the primary treatment for metastatic prostate cancer. However, this therapy is associated with several undesired side-effects, including increased risk of cardiovascular diseases. Shortening the period of androgen ablation therapy may benefit prostate cancer patients. Intermittent Androgen Deprivation therapy improves quality of life, reduces toxicity and medical costs, and delays disease progression in some patients. Cell culture and xenograft studies using androgen receptor (AR)-positive castration-resistant human prostate cancers cells (LNCaP, ARCaP, and PC-3 cells over-expressing AR) suggest that androgens may suppress the growth of AR-rich prostate cancer cells. Androgens cause growth inhibition and G1 cell cycle arrest in these cells by regulating c-Myc, Skp2, and p27^Kip via AR. Higher dosages of testosterone cause greater growth inhibition of relapsed tumors. Manipulating androgen/AR signaling may therefore be a potential therapy for AR-positive advanced prostate cancer.     

Androgen signaling and its interactions with other signaling pathways in prostate cancer

Prostate cancer is the most frequently diagnosed non-skin cancer and the third leading cause of cancer mortality in men. In the initial stages, prostate cancer is dependent on androgens for growth, which is the basis for androgen ablation therapy. However, in most cases, prostate cancer progresses to a hormone refractory phenotype for which there is no effective therapy available at present. The androgen receptor (AR) is required for prostate cancer growth in all stages, including the relapsed, "androgen-independent" tumors in the presence of very low levels of androgens. This review focuses on AR function and AR-target genes and summarizes the major signaling pathways implicated in prostate cancer progression, their crosstalk with each other and with AR signaling. This complex network of interactions is providing a deeper insight into prostate carcinogenesis and may form the basis for novel diagnostic and therapeutic strategies.     

Alterations of androgen receptor in prostate cancer

The significance of androgens in the development of prostate cancer has been known for more than half century. During the last decade, a lot of effort has been put to study the significance of the specific nuclear receptor of the hormone, androgen receptor (AR). It has been suggested that polymorphisms, especially the length of CAG repeat in exon 1 of the gene, are associated with the risk of prostate cancer. However, not all studies have confirmed the association. Most surprisingly, it has now become clear that prostate carcinomas emerging during the androgen withdrawal therapy (i.e. hormone-refractory tumors) are capable of reactivating the AR-mediated signalling despite of the low levels of androgens. In addition, it has been shown that AR gene itself is genetically targeted. One-third of the hormone-refractory prostate carcinomas contains amplification of the gene. In addition, 10-30% of prostate carcinomas treated by antiandrogens acquire point mutation in the AR gene. The genetic alterations in AR indicate that receptor should be considered as putative treatment target. Evidently, the currently available antiandrogens are not capable to abolish the AR-mediated signalling efficiently enough.     

Androgen receptor action in hormone-dependent and recurrent prostate cancer

The importance of androgens and androgen receptors (AR) in primary prostate cancer is well established. Metastatic disease is usually treated with some form of androgen ablation, which is effective for a limited amount of time. The role of AR in prostate cancers that recur despite androgen ablation therapy is less certain. Most of these tumors express prostate specific antigen (PSA), an androgen-regulated gene; moreover, AR is generally highly expressed in recurrent prostate cancer. We propose that AR continues to play a role in many of these tumors and that it is not only the levels of AR, ligands, and co-regulators, but also the changes in cell signaling that induce AR action in recurrent prostate cancer. These pathways are, therefore, potential therapeutic targets.     

前列腺癌雄激素受体和PI3K/Akt信号通路相互作用研究进展

前列腺癌的发生、进展依赖于雄激素,因此去势手术成为治疗晚期前列腺癌的标准疗法。但是去势后大多前列腺癌最终将转化为雄激素非依赖性前列腺癌,甚至进展为激素难治性前列腺癌,使得肿瘤的进展不受低水平雄激素的影响。即使如此,大多数激素非依赖性前列腺癌,依然阳性表达雄激素受体。因而雄激素受体在前列腺癌发生发展中起着重要作用。而PI3K/Akt信号通路能够通过维持细胞生存、抑制细胞凋亡、促进细胞代谢及血管生成等促进前列腺癌进展。本综述旨在总结前人研究,阐述雄激素受体和PI3K/Akt信号通路之间相互作用关系。研究表明Akt信号通路能够正性或者负性调控AR蛋白表达、蛋白的稳定性及其转录活性,从而维持细胞的生存、代谢。而AR即可以通过基因转录途径抑制Akt活化又能通过非转录基因途径激活Akt及其下游蛋白。因此,AR和Akt信号通路相互协同促进前列腺癌的发生及其向雄激素非依赖性前列腺癌进展。     

Expression and Functional Role of Orphan Receptor GPR158 in Prostate Cancer Growth and Progression

Prostate cancer (PCa) is the second-leading cause of cancer-related mortality, after lung cancer, in men from developed countries. In its early stages, primary tumor growth is dependent on androgens, thus generally can be controlled by androgen deprivation therapy (ADT). Eventually however, the disease progresses to castration-resistant prostate cancer (CRPC), a lethal form in need of more effective treatments. G-protein coupled receptors (GPCRs) comprise a large clan of cell surface proteins that have been implicated as therapeutic targets in PCa growth and progression. The findings reported here provide intriguing evidence of a role for the newly characterized glutamate family member GPR158 in PCa growth and progression. We found that GPR158 promotes PCa cell proliferation independent of androgen receptor (AR) functionality and that this requires its localization in the nucleus of the cell. This suggests that GPR158 acts by mechanisms different from other GPCRs. GPR158 expression is stimulated by androgens and GPR158 stimulates AR expression, implying a potential to sensitize tumors to low androgen conditions during ADT via a positive feedback loop. Further, we found GPR158 expression correlates with a neuroendocrine (NE) differentiation phenotype and promotes anchorage-independent colony formation implying a role for GPR158 in therapeutic progression and tumor formation. GPR158 expression was increased at the invading front of prostate tumors that formed in the genetically defined conditional Pten knockout mouse model, and co-localized with elevated AR expression in the cell nucleus. Kaplan-Meier analysis on a dataset from the Memorial Sloan Kettering cancer genome portal showed that increased GPR158 expression in tumors is associated with lower disease-free survival. Our findings strongly suggest that pharmaceuticals targeting GPR158 activities could represent a novel and innovative approach to the prevention and management of CRPC.     

Emergence of metastatic hormone-refractory disease in prostate cancer after anti-androgen therapy

The anti-androgens used in prostate cancer therapy have been designed to interfere with the normal androgen receptor (AR)-mediated processes that ensure prostate cell survival, triggering tumor cells to undergo programmed cell death. While anti-androgens were originally designed to treat advanced disease, they have recently been used to debulk organ-confined prostate tumors, to improve positive margins prior to surgery, and for chemoprevention in patients at high risk for prostate cancer. However, tumors treated with anti-androgens frequently become hormone refractory and acquire a more aggressive phenotype. Progression toward metastatic hormone-refractory disease has often been regarded as the outgrowth of a small number of hormone-independent cells that emerge from a hormone-dependent tumor during anti-androgen treatment by natural selection. While a number of selective advantages have recently been identified, there is also considerable evidence suggesting that the progression toward metastatic hormone-refractory disease is an dynamic process which involves abrogation of programmed cell death as a result of the attenuation of DNA fragmentation and maintenance of mitochondrial membrane potential in tumor cells; the upregulation of stromal-mediated growth factor signaling pathways; and the upregulation of extracellular matrix (ECM) protease expression.     

Impact of circulating cholesterol levels on growth and intratumoral androgen concentration of prostate tumors

Prostate cancer (PCa) is the second most common cancer in men. Androgen deprivation therapy (ADT) leads to tumor involution and reduction of tumor burden. However, tumors eventually reemerge that have overcome the absence of gonadal androgens, termed castration resistant PCa (CRPC). Theories underlying the development of CRPC include androgen receptor (AR) mutation allowing for promiscuous activation by non-androgens, AR amplification and overexpression leading to hypersensitivity to low androgen levels, and/or tumoral uptake and conversion of adrenally derived androgens. More recently it has been proposed that prostate tumor cells synthesize their own androgens through de novo steroidogenesis, which involves the step-wise synthesis of androgens from cholesterol. Using the in vivo LNCaP PCa xenograft model, previous data from our group demonstrated that a hypercholesterolemia diet potentiates prostatic tumor growth via induction of angiogenesis. Using this same model we now demonstrate that circulating cholesterol levels are significantly associated with tumor size (R = 0.3957, p = 0.0049) and intratumoral levels of testosterone (R = 0.41, p = 0.0023) in LNCaP tumors grown in hormonally intact mice. We demonstrate tumoral expression of cholesterol uptake genes as well as the spectrum of steroidogenic enzymes necessary for androgen biosynthesis from cholesterol. Moreover, we show that circulating cholesterol levels are directly correlated with tumoral expression of CYP17A, the critical enzyme required for de novo synthesis of androgens from cholesterol (R = 0.4073, p = 0.025) Since hypercholesterolemia does not raise circulating androgen levels and the adrenal gland of the mouse synthesizes minimal androgens, this study provides evidence that hypercholesterolemia increases intratumoral de novo steroidogenesis. Our results are consistent with the hypothesis that cholesterol-fueled intratumoral androgen synthesis may accelerate the growth of prostate tumors, and suggest that treatment of CRPC may be optimized by inclusion of cholesterol reduction therapies in conjunction with therapies targeting androgen synthesis and the AR.     

Androgen receptor represses the neuroendocrine transdifferentiation process in prostate cancer cells

Androgen-ablation therapy is an effective method for treating prostate cancer. However, prostate tumors that survive long-term androgen-ablation therapy are classified as androgen-independent as they proliferate in the absence of androgens, and they tend to be enriched for neuroendocrine (NE) cells. Androgen withdrawal causes androgen-dependent prostate cancer cells to adopt a pronounced NE phenotype, suggesting that androgen receptor (AR) represses an intrinsic NE transdifferentiation process in prostate cancer cells. In this report we show that short interfering RNA-induced AR silencing induced a NE phenotype that manifested itself in the growth of dendritic-like processes in both the androgen-dependent LNCaP and androgen-independent LNCaP-AI human prostate cancer cells. Western blot analysis revealed that neuronal-specific enolase, a marker of the neuronal lineage, was increased by AR knockdown in LNCaP cells. The expression levels of the neuronal-specific cytoskeletal proteins beta-tubulin III, nestin, and glial acidic fibrillary protein were also characterized in AR knockdown cells. Most interestingly, AR silencing induced beta-tubulin III expression in LNCaP cells, while AR knockdown increased glial acidic fibrillary protein levels in both LNCaP and LNCaP-AI cells. Lastly, AR silencing reduced the proliferative capacity of LNCaP and LNCaP-AI cells. Our data demonstrate that AR actively represses an intrinsic NE transdifferentiation process in androgen-responsive prostate cancer cells and suggest a potential link between AR inactivation and the increased frequency of NE cells in androgen-independent tumors.     

Glucocorticoids can promote androgen-independent growth of prostate cancer cells through a mutated androgen receptor

The androgen receptor (AR) is involved in the development, growth and progression of prostate cancer (CaP). CaP often progresses from an androgen-dependent to an androgen-independent tumor, making androgen ablation therapy ineffective. However, the mechanisms for the development of androgen-independent CaP are unclear. More than 80% of clinically androgen-independent prostate tumors show high levels of AR expression. In some CaPs, AR levels are increased because of gene amplification and/or overexpression, whereas in others, the AR is mutated. Nonetheless, the involvement of the AR in the transition of CaP to androgen-independent growth and the subsequent failure of endocrine therapy are not fully understood. Here we show that in CaP cells from a patient who failed androgen ablation therapy, a doubly mutated AR functioned as a high-affinity cortisol/cortisone receptor (ARccr). Cortisol, the main circulating glucocorticoid, and its metabolite, cortisone, both equally stimulate the growth of these CaP cells and increase the secretion of prostate-specific antigen in the absence of androgens. The physiological concentrations of free cortisol and total cortisone in men greatly exceed the binding affinity of the ARccr and would activate the receptor, promoting CaP cell proliferation. Our data demonstrate a previously unknown mechanism for the androgen-independent growth of advanced CaP. Understanding this mechanism and recognizing the presence of glucocorticoid-responsive AR mutants are important for the development of new forms of therapy for the treatment of this subset of CaP.     

Beyond T and DHT - Novel Steroid Derivatives Capable of Wild Type Androgen Receptor Activation

While androgen deprivation therapy (ADT) remains the primary treatment for metastatic prostate cancer (PCa), castration does not eliminate androgens from the prostate tumor microenvironment, and residual intratumoral androgens are implicated in nearly every mechanism by which androgen receptor (AR)-mediated signaling promotes castration-resistant disease. The uptake and intratumoral (intracrine) conversion of circulating adrenal androgens such as dehydroepiandrosterone sulfate (DHEA-S) to steroids capable of activating the wild type AR is a recognized driver of castration resistant prostate cancer (CRPC). However, less well-characterized adrenal steroids, including 11-deoxcorticosterone (DOC) and 11beta-hydroxyandrostenedione (11OH-AED) may also play a previously unrecognized role in promoting AR activation. In particular, recent data demonstrate that the 5α-reduced metabolites of DOC and 11OH-AED are activators of the wild type AR. Given the well-recognized presence of SRD5A activity in CRPC tissue, these observations suggest that in the low androgen environment of CRPC, alternative sources of 5α-reduced ligands may supplement AR activation normally mediated by the canonical 5α-reduced agonist, 5α-DHT. Herein we review the emerging data that suggests a role for these alternative steroids of adrenal origin in activating the AR, and discuss the enzymatic pathways and novel downstream metabolites mediating these effects. We conclude by discussing the potential implications of these findings for CRPC progression, particularly in context of new agents such as abiraterone and enzalutamide which target the AR-axis for prostate cancer therapy.