Differential regulation of androgen and glucocorticoid receptors by retinoblastoma protein

Predicting abiraterone efficacy in advanced prostate cancer: Insights from marker of proliferation Ki67

BACKGROUND

KI67 is a well-known biomarker reflecting cell proliferation. We aim to elucidate the predictive role of KI67 in the efficacy of abiraterone for patients with advanced prostate cancer (PCa).

METHODS

Clinicopathological data of 152 men with metastatic PCa, who received abiraterone therapy were retrospectively collected. The KI67 positivity was examined by immunohistochemistry using the prostate biopsy specimen. The predictive value of KI67 on the therapeutic efficacy of abiraterone was explored using Kaplan-Meier curve and Cox regression analysis. The endpoints included prostate-specific antigen (PSA) progression-free survival (PSA-PFS), radiographic PFS (rPFS), and overall survival (OS).

RESULTS

In total, 85/152 (55.9%) and 67/152 (44.1%) cases, respectively, received abiraterone at metastatic hormone-sensitive (mHSPC) and castration-resistant PCa (mCRPC) stage. The median KI67 positivity was 20% (interquartile range: 10%-30%). Overall, KI67 rate was not correlated with PSA response. Notably, an elevated KI67-positive rate strongly correlated with unfavorable abiraterone efficacy, with KI67 ≥ 30% and KI67 ≥ 20% identified as the optimal cutoffs for prognosis differentiation in mHSPC (median PSA-PFS: 11.43 Mo vs. 26.43 Mo, p < 0.001; median rPFS: 16.63 Mo vs. 31.90 Mo, p = 0.003; median OS: 21.77 Mo vs. not reach, p = 0.005) and mCRPC (median PSA-PFS: 7.17 Mo vs. 12.20 Mo, p = 0.029; median rPFS: 11.67 Mo vs. 16.47 Mo, p = 0.012; median OS: 21.67 Mo vs. not reach, p = 0.073) patients, respectively. Multivariate analysis supported the independent predictive value of KI67 on abiraterone efficacy. In subgroup analysis, an elevated KI67 expression was consistently associated with unfavorable outcomes in the majority of subgroups. Furthermore, data from another cohort of 79 PCa patients with RNA information showed that those with KI67 RNA levels above the median had a significantly shorter OS than those below the median (17.71 vs. 30.72 Mo, p = 0.035).

CONCLUSIONS

This study highlights KI67 positivity in prostate biopsy as a strong predictor of abiraterone efficacy in advanced PCa. These insights will assist clinicians in anticipating clinical outcomes and refining treatment decisions for PCa patients.

ARe we there yet? Understanding androgen receptor signaling in breast cancer

The role of androgen receptor (AR) activation and expression is well understood in prostate cancer. In breast cancer, expression and activation of AR is increasingly recognized for its role in cancer development and its importance in promoting cell growth in the presence or absence of estrogen. As both prostate and breast cancers often share a reliance on nuclear hormone signaling, there is increasing appreciation of the overlap between activated cellular pathways in these cancers in response to androgen signaling. Targeting of the androgen receptor as a monotherapy or in combination with other conventional therapies has proven to be an effective clinical strategy for the treatment of patients with prostate cancer, and these therapeutic strategies are increasingly being investigated in breast cancer. This overlap suggests that targeting androgens and AR signaling in other cancer types may also be effective. This manuscript will review the role of AR in various cellular processes that promote tumorigenesis and metastasis, first in prostate cancer and then in breast cancer, as well as discuss ongoing efforts to target AR for the more effective treatment and prevention of cancer, especially breast cancer.

Combined TP53 and RB1 Loss Promotes Prostate Cancer Resistance to a Spectrum of Therapeutics and Confers Vulnerability to Replication Stress

Prostate cancers (PCs) with loss of the potent tumor suppressors TP53 and RB1 exhibit poor outcomes. TP53 and RB1 also influence cell plasticity and are frequently lost in PCs with neuroendocrine (NE) differentiation. Therapeutic strategies that address these aggressive variant PCs are urgently needed. Using deep genomic profiling of 410 metastatic biopsies, we determine the relationships between combined TP53 and RB1 loss and PC phenotypes. Notably, 40% of TP53/RB1-deficient tumors are classified as AR-active adenocarcinomas, indicating that NE differentiation is not an obligate consequence of TP53/RB1 inactivation. A gene expression signature reflecting TP53/RB1 loss is associated with diminished responses to AR antagonists and reduced survival. These tumors exhibit high proliferation rates and evidence of elevated DNA repair processes. While tumor cells lacking TP53/RB1 are highly resistant to all single-agent therapeutics tested, the combination of PARP and ATR inhibition is found to produce significant responses, reflecting a clinically exploitable vulnerability resulting from replication stress.

The Role of RB in Prostate Cancer Progression

The RB tumor suppressor is one of the most commonly deleted/mutated genes in human cancers. In prostate cancer specifically, mutation of RB is most frequently observed in aggressive, metastatic disease. As one of the earliest tumor suppressors to be identified, the molecular functions of RB that are lost in tumor development have been studied for decades. Earlier work focused on the canonical RB pathway connecting mitogenic signaling to the cell cycle via Cyclin/CDK inactivation of RB, thereby releasing the E2F transcription factors. More in-depth analysis revealed that RB-E2F complexes regulate cellular processes beyond proliferation. Most recently, "non-canonical" roles for RB function have been expanded beyond its E2F interactions, which may play a particular role in advanced prostate cancer. For example, in mouse models of prostate cancer, loss of RB has been shown to induce lineage plasticity, which enables resistance to androgen deprivation therapy. This increased understanding of the potential downstream functions of RB in prostate cancer may lead the way to identifying therapeutic vulnerabilities in cells following RB loss.

Androgen Receptor Tumor Suppressor Function Is Mediated by Recruitment of Retinoblastoma Protein

Although well characterized as a transcriptional activator that drives prostate cancer (PCa) growth, androgen receptor (AR) can function as a transcriptional repressor, and high-level androgens can suppress PCa proliferation. The molecular basis for this repression activity remains to be determined. Genes required for DNA replication are highly enriched among androgen-repressed genes, and AR is recruited to the majority of these genes, where it rapidly represses their transcription. This activity is enhanced in PCa cells expressing high AR levels and is mediated by recruitment of hypophosphorylated retinoblastoma protein (Rb). Significantly, AR also indirectly increases the expression of DNA replication genes through stimulatory effects on other metabolic genes with subsequent CDK activation and Rb hyperphosphorylation. In castration-resistant PCa cells, which are dependent on high-level AR expression, this anti-proliferative repression function might be exploited through treatment with androgen in combination with agents that suppress AR-driven metabolic functions or cell cycle progression.

Androgen receptor-related diseases: what do we know?

The androgen receptor (AR) and the androgen-AR signaling pathway play a significant role in male sexual differentiation and the development and function of male reproductive and non-reproductive organs. Because of AR's widely varied and important roles, its abnormalities have been identified in various diseases such as androgen insensitivity syndrome, spinal bulbar muscular atrophy, benign prostatic hyperplasia, and prostate cancer. This review provides an overview of the function of androgens and androgen-AR mediated diseases. In addition, the diseases delineated above are discussed with respect to their association with mutations and other post-transcriptional modifications in the AR. Finally, we present an introduction to the potential therapeutic application of most recent pharmaceuticals including miRNAs in prostate cancer that specifically target the transactivation function of the AR at post-transcriptional stages.

Proto-oncogene activity of melanoma antigen-A11 (MAGE-A11) regulates retinoblastoma-related p107 and E2F1 proteins

Melanoma antigen-A11 (MAGE-A11) is a low-abundance, primate-specific steroid receptor coregulator in normal tissues of the human reproductive tract that is expressed at higher levels in prostate cancer. Increased expression of MAGE-A11 enhances androgen receptor transcriptional activity and promotes prostate cancer cell growth. Further investigation into the mechanisms of MAGE-A11 function in prostate cancer demonstrated interactions with the retinoblastoma-related protein p107 and Rb tumor suppressor but no interaction with p130 of the Rb family. MAGE-A11 interaction with p107 was associated with transcriptional repression in cells with low MAGE-A11 and transcriptional activation in cells with higher MAGE-A11. Selective interaction of MAGE-A11 with retinoblastoma family members suggested the regulation of E2F transcription factors. MAGE-A11 stabilized p107 by inhibition of ubiquitination and linked p107 to hypophosphorylated E2F1 in association with the stabilization and activation of E2F1. The androgen receptor and MAGE-A11 modulated endogenous expression of the E2F1-regulated cyclin-dependent kinase inhibitor p27(Kip1). The ability of MAGE-A11 to increase E2F1 transcriptional activity was similar to the activity of adenovirus early oncoprotein E1A and depended on MAGE-A11 interactions with p107 and p300. The immunoreactivity of p107 and MAGE-A11 was greater in advanced prostate cancer than in benign prostate, and knockdown with small inhibitory RNA showed that p107 is a transcriptional activator in prostate cancer cells. These results suggest that MAGE-A11 is a proto-oncogene whose increased expression in prostate cancer reverses retinoblastoma-related protein p107 from a transcriptional repressor to a transcriptional activator of the androgen receptor and E2F1.

Changes in primary lymphoid organs with aging

Aging is associated with decreased immune function that leads to increased morbidity and mortality in the elderly. Immune senescence is accompanied by age-related changes in two primary lymphoid organs, bone marrow and thymus, that result in decreased production and function of B and T lymphocytes. In bone marrow, hematopoietic stem cells exhibit reduced self-renewal potential, increased skewing toward myelopoiesis, and decreased production of lymphocytes with aging. These functional sequelae of aging are caused in part by increased oxidative stress, inflammation, adipocyte differentiation, and disruption of hypoxic osteoblastic niches. In thymus, aging is associated with tissue involution, exhibited by a disorganization of the thymic epithelial cell architecture and increased adiposity. This dysregulation correlates with a loss of stroma-thymocyte 'cross-talk', resulting in decreased export of naïve T cells. Mounting evidence argues that with aging, thymic inflammation, systemic stress, local Foxn1 and keratinocyte growth factor expression, and sex steroid levels play critical roles in actively driving thymic involution and overall adaptive immune senescence across the lifespan. With a better understanding of the complex mechanisms and pathways that mediate bone marrow and thymus involution with aging, potential increases for the development of safe and effective interventions to prevent or restore loss of immune function with aging.

The AR dependent cell cycle: mechanisms and cancer relevance

Prostate cancer cells are exquisitely dependent on androgen receptor (AR) activity for proliferation and survival. As these functions are critical targets of therapeutic intervention for human disease, it is imperative to delineate the mechanisms by which AR engages the cell cycle engine. More than a decade of research has revealed that elegant intercommunication between AR and the cell cycle machinery governs receptor-dependent cellular proliferation, and that perturbations in this process occur frequently in human disease. Here, AR-cell cycle interplay and associated cancer relevance will be reviewed.

RB1, development, and cancer

The RB1 gene is the first tumor suppressor gene identified whose mutational inactivation is the cause of a human cancer, the pediatric cancer retinoblastoma. The 25 years of research since its discovery has not only illuminated a general role for RB1 in human cancer, but also its critical importance in normal development. Understanding the molecular function of the RB1 encoded protein, pRb, is a long-standing goal that promises to inform our understanding of cancer, its relationship to normal development, and possible therapeutic strategies to combat this disease. Achieving this goal has been difficult, complicated by the complexity of pRb and related proteins. The goal of this review is to explore the hypothesis that, at its core, the molecular function of pRb is to dynamically regulate the location-specific assembly or disassembly of protein complexes on the DNA in response to the output of various signaling pathways. These protein complexes participate in a variety of molecular processes relevant to DNA including gene transcription, DNA replication, DNA repair, and mitosis. Through regulation of these processes, RB1 plays a uniquely prominent role in normal development and cancer.

Liver kinase B1 (LKB1) in the pathogenesis of epithelial cancers

LKB1 acts as a master kinase, with its major protein targets being the family of AMPKs. Through activation of multiple signaling pathways, LKB1's main physiologic functions involve regulating cellular growth, metabolism, and polarity. Germline mutations in LKB1 result in Peutz-Jeghers Syndrome, a rare cancer susceptibility syndrome. In addition, multiple LKB1 mutations have been identified in sporadic cancers, especially those of the lung. Recent studies from a variety of murine models have helped characterize LKB1's role in the pathogenesis of epithelial cancers. In some tumor types, LKB1 might function chiefly to suppress cell growth or invasion, while in other cases, it may serve to prevent metastasis. Moreover, molecular signatures of individual tumors likely influence LKB1's operational role, as multiple studies have shown that LKB1 can synergize with other tumor suppressors and/or oncogenes to accelerate tumorigenesis. To date, LKB1 has been considered mainly a tumor suppressor; however, some studies have suggested its potential oncogenic role, mainly through the suppression of apoptosis. In short, LKB1 is a tissue and context-specific kinase. This review aims to summarize our current understanding of its role in the pathogenesis of epithelial cancers.

Retinoblastoma protein plays multiple essential roles in the terminal differentiation of Sertoli cells

Retinoblastoma protein (RB) plays crucial roles in cell cycle control and cellular differentiation. Specifically, RB impairs the G(1) to S phase transition by acting as a repressor of the E2F family of transcriptional activators while also contributing towards terminal differentiation by modulating the activity of tissue-specific transcription factors. To examine the role of RB in Sertoli cells, the androgen-dependent somatic support cell of the testis, we created a Sertoli cell-specific conditional knockout of Rb. Initially, loss of RB has no gross effect on Sertoli cell function because the mice are fertile with normal testis weights at 6 wk of age. However, by 10-14 wk of age, mutant mice demonstrate severe Sertoli cell dysfunction and infertility. We show that mutant mature Sertoli cells continue cycling with defective regulation of multiple E2F1- and androgen-regulated genes and concurrent activation of apoptotic and p53-regulated genes. The most striking defects in mature Sertoli cell function are increased permeability of the blood-testis barrier, impaired tissue remodeling, and defective germ cell-Sertoli cell interactions. Our results demonstrate that RB is essential for proper terminal differentiation of Sertoli cells.

Roles of histone H3-lysine 4 methyltransferase complexes in NR-mediated gene transcription

Transcriptional regulation by nuclear hormone receptors (NRs) requires multiple coregulators that modulate chromatin structures by catalyzing a diverse array of posttranslational modifications of histones. Different combinations of these modifications yield dynamic functional outcomes, constituting an epigenetic histone code. This code is inscribed by histone-modifying enzymes and decoded by effector proteins that recognize specific covalent marks. One important modification associated with active chromatin structures is methylation of histone H3-lysine 4 (H3K4). Crucial roles for this modification in NR transactivation have been recently highlighted through our purification and subsequent characterization of a steady-state complex associated with ASC-2, a coactivator of NRs and other transcription factors. This complex, designated ASCOM for ASC-2 complex, contains H3K4-methyltransferase MLL3/HALR or its paralogue MLL4/ALR and represents the first Set1-like H3K4-methyltransferase complex to be reported in vertebrates. This review focuses on recent progress in our understanding of how ASCOM-MLL3 and ASCOM-MLL4 influence NR-mediated gene transcription and of their physiological function.

Up-regulation of circadian clock gene Period 2 in the prostate mesenchymal cells during flutamide-induced apoptosis

Androgen regulates the proper development and physiological function of the prostate. Here, we investigated the modulation of androgen and androgen receptor (AR) antagonist on circadian oscillations of a clock core gene Period 2 (Per2) in rat prostate mesenchymal cells (PMCs). Circadian oscillations were analyzed with the real-time monitoring system of gene expression using transgenic rats introduced with mouse Per2 promoter fused to a destabilized luciferase (Per2-dLuc) reporter gene. Analyses of circadian oscillations, immunofluorescence, and androgen response element (ARE)-luciferase reporter assay revealed that circadian clocks are operative and the AR protein is functional in PMCs in vitro. Androgen such as testosterone (T) and dihydrotestosterone (DHT) did not cause any changes in circadian Per2-dLuc oscillations of confluent cells. Conversely, flutamide (FL) up-regulated the amplitude of circadian Per2-dLuc oscillations in a dose-dependent manner, whereas T antagonized the action of FL. The PER2 protein was markedly accumulated by FL treatment and localized in both the nucleus and cytoplasm during the first peak period of circadian Per2-dLuc oscillations. Simultaneously, FL treatment increased apoptotic cell death. Collectively, the present study demonstrates that a clock gene Per2 is up-regulated in PMCs during FL-induced apoptotic cell death. Thus, circadian oscillations of Per2 gene expression may be closely linked to the cellular states of PMCs such as apoptotic cell death.

A novel androgen receptor-binding element modulates Cdc6 transcription in prostate cancer cells during cell-cycle progression

The androgen receptor (AR) plays a pivotal role in the onset and progression of prostate cancer by promoting cellular proliferation. Recent studies suggest AR is a master regulator of G1-S progression and possibly a licensing factor for DNA replication yet the mechanisms remain poorly defined. Here we report that AR targets the human Cdc6 gene for transcriptional regulation. Cdc6 is an essential regulator of DNA replication in eukaryotic cells and its mRNA expression is inversely modulated by androgen or antiandrogen treatment in androgen-sensitive prostate cancer cells. AR binds at a distinct androgen-response element (ARE) in the Cdc6 promoter that is functionally required for androgen-dependent Cdc6 transcription. We found that peak AR occupancy at the novel ARE occurs during the G1/S phase concomitant with peak Cdc6 mRNA expression. We also identified several of the coactivators and corepressors involved in AR-dependent Cdc6 transcriptional regulation in vivo and further characterized ligand-induced alterations in histone acetylation and methylation at the Cdc6 promoter. Significantly, AR silencing in prostate cancer cells markedly decreases Cdc6 expression and androgen-dependent cellular proliferation. Collectively, our results suggest that Cdc6 is a key regulatory target for AR and provide new insights into the mechanisms of prostate cancer cell proliferation.

Amino-terminus domain of the androgen receptor as a molecular target to prevent the hormonal progression of prostate cancer

Prostate cancer has a propensity to metastasize to the bone. Currently the only effective systemic treatment for these patients is androgen ablation therapy. However, the tumor will invariably progress to an androgen-independent stage and the patient will succumb to his disease within approximately 2 years. The earliest indication of hormonal progression is the rising titer of serum prostate specific antigen. Current evidence implicates the androgen receptor (AR) as a key factor in maintaining the growth of prostate cancer cells in an androgen-depleted state. Under normal conditions, binding of ligand activates the receptor, allowing it to effectively bind to its respective DNA element. However, AR is also transformed in the absence of androgen (ligand-independent activation) in prostate cells via multiple protein kinase pathways and the interleukin-6 (IL-6) pathway that converge upon the N-terminal domain of the AR. This domain is the main region for phosphorylation and is also critical for normal coregulator recruitment. Here we discuss evidence supporting the role of the AR, IL-6 and other protein kinase pathways in the hormonal progression of prostate cancer to androgen independence and the mechanisms involved in activation of the AR by these pathways. Receptor-targeted therapy, especially potential drugs targeting the N-terminal domain, may effectively prevent or delay the hormonal progression of AR-dependent prostate cancer.

LKB1 catalytic activity contributes to estrogen receptor alpha signaling

The tumor suppressor serine-threonine kinase LKB1 is mutated in Peutz-Jeghers syndrome (PJS) and in epithelial cancers, including hormone-sensitive organs such as breast, ovaries, testes, and prostate. Clinical studies in breast cancer patients show low LKB1 expression is related to poor prognosis, whereas in PJS, the risk of breast cancer is similar to the risk from germline mutations in breast cancer (BRCA) 1/BRCA2. In this study, we investigate the role of LKB1 in estrogen receptor alpha (ERalpha) signaling. We demonstrate for the first time that LKB1 binds to ERalpha in the cell nucleus in which it is recruited to the promoter of ERalpha-responsive genes. Furthermore, LKB1 catalytic activity enhances ERalpha transactivation compared with LKB1 catalytically deficient mutants. The significance of our discovery is that we demonstrate for the first time a novel functional link between LKB1 and ERalpha. Our discovery places LKB1 in a coactivator role for ERalpha signaling, broadening the scientific scope of this tumor suppressor kinase and laying the groundwork for the use of LKB1 as a target for the development of new therapies against breast cancer.

Cadmium--a metallohormone?

Cadmium is a heavy metal that is often referred to as the metal of the 20th century. It is widely used in industry principally in galvanizing and electroplating, in batteries, in electrical conductors, in the manufacture of alloys, pigments, and plastics, and in the stabilization of phosphate fertilizers. As a byproduct of smelters, cadmium is a prevalent environmental contaminant. In the general population, exposure to cadmium occurs primarily through dietary sources, cigarette smoking, and, to a lesser degree, drinking water. Although the metal has no known physiological function, there is evidence to suggest that the cadmium is a potent metallohormone. This review summarizes the increasing evidence that cadmium mimics the function of steroid hormones, addresses our current understanding of the mechanism by which cadmium functions as a hormone, and discusses its potential role in development of the hormone dependent cancers.

Aberrant E2F activation by polyglutamine expansion of androgen receptor in SBMA neurotoxicity

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative disorder caused by a polyglutamine repeat (polyQ) expansion within the human androgen receptor (AR). Unlike other neurodegenerative diseases caused by abnormal polyQ expansion, the onset of SBMA depends on androgen binding to mutant human polyQ-AR proteins. This is also observed in Drosophila eyes ectopically expressing the polyQ-AR mutants. We have genetically screened mediators of androgen-induced neurodegeneration caused by polyQ-AR mutants in Drosophila eyes. We identified Rbf (Retinoblastoma-family protein), the Drosophila homologue of human Rb (Retinoblastoma protein), as a neuroprotective factor. Androgen-dependent association of Rbf or Rb with AR was remarkably potentiated by aberrant polyQ expansion. Such potentiated Rb association appeared to attenuate recruitment of histone deacetyltransferase 1 (HDAC1), a corepressor of E2F function. Either overexpression of Rbf or E2F deficiency in fly eyes reduced the neurotoxicity of the polyQ-AR mutants. Induction of E2F function by polyQ-AR-bound androgen was suppressed by Rb in human neuroblastoma cells. We conclude that abnormal expansion of polyQ may potentiate innate androgen-dependent association of AR with Rb. This appears to lead to androgen-dependent onset of SBMA through aberrant E2F transactivation caused by suppressed histone deacetylation.

AR, the cell cycle, and prostate cancer

The androgen receptor (AR) is a critical effector of prostate cancer development and progression. The dependence of this tumor type on AR activity is exploited in treatment of disseminated prostate cancers, wherein ablation of AR function (achieved either through ligand depletion and/or the use of AR antagonists) is the first line of therapeutic intervention. These strategies are initially effective, and induce a mixed response of cell cycle arrest or apoptosis in prostate cancer cells. However, recurrent, incurable tumors ultimately arise as a result of inappropriately restored AR function. Based on these observations, it is imperative to define the mechanisms by which AR controls cancer cell proliferation. Mechanistic investigation has revealed that AR acts as a master regulator of G1-S phase progression, able to induce signals that promote G1 cyclin-dependent kinase (CDK) activity, induce phosphorylation/inactivation of the retinoblastoma tumor suppressor (RB), and thereby govern androgen-dependent proliferation. These functions appear to be independent of the recently identified TMPRSS2-ETS fusions. Once engaged, several components of the cell cycle machinery actively modulate AR activity throughout the cell cycle, thus indicating that crosstalk between the AR and cell cycle pathways likely modulate the mitogenic response to androgen. As will be discussed, discrete aberrations in this process can alter the proliferative response to androgen, and potentially subvert hormonal control of tumor progression.

Androgen receptor (AR) coregulators: a diversity of functions converging on and regulating the AR transcriptional complex

Androgens, acting through the androgen receptor (AR), are responsible for the development of the male phenotype during embryogenesis, the achievement of sexual maturation at puberty, and the maintenance of male reproductive function and behavior in adulthood. In addition, androgens affect a wide variety of nonreproductive tissues. Moreover, aberrant androgen action plays a critical role in multiple pathologies, including prostate cancer and androgen insensitivity syndromes. The formation of a productive AR transcriptional complex requires the functional and structural interaction of the AR with its coregulators. In the last decade, an overwhelming and ever increasing number of proteins have been proposed to possess AR coactivating or corepressing characteristics. Intriguingly, a vast diversity of functions has been ascribed to these proteins, indicating that a multitude of cellular functions and signals converge on the AR to regulate its function. The current review aims to provide an overview of the AR coregulator proteins identified to date and to propose a classification of these AR coregulator proteins according to the function(s) ascribed to them. Taken together, this approach will increase our understanding of the cellular pathways that converge on the AR to ensure an appropriate transcriptional response to androgens.

Structure-function analysis of the retinoblastoma tumor suppressor protein - is the whole a sum of its parts?

Biochemical analysis of the retinoblastoma protein's function has received considerable attention since it was cloned just over 20 years ago. During this time pRB has emerged as a key regulator of the cell division cycle and its ability to block proliferation is disrupted in the vast majority of human cancers. Much has been learned about the regulation of E2F transcription factors by pRB in the cell cycle. However, many questions remain unresolved and researchers continue to explore this multifunctional protein. In particular, understanding how its biochemical functions contribute to its role as a tumor suppressor remains to be determined. Since pRB has been shown to function as an adaptor molecule that links different proteins together, or to particular promoters, analyzing pRB by disrupting individual protein interactions holds tremendous promise in unraveling the intricacies of its function. Recently, crystal structures have reported how pRB interacts with some of its molecular partners. This information has created the possibility of rationally separating pRB functions by studying mutants that disrupt individual binding sites. This review will focus on literature that investigates pRB by isolating functions based on binding sites within the pocket domain. This article will also discuss the prospects for using this approach to further explore the unknown functions of pRB.

Androgen receptor coregulators and their involvement in the development and progression of prostate cancer

The androgen receptor signaling axis plays an essential role in the development, function and homeostasis of male urogenital structures including the prostate gland although the mechanism by which the AR axis contributes to the initiation, progression and metastatic spread of prostate cancer remains somewhat enigmatic. A number of molecular events have been proposed to act at the level of the AR and associated coregulators to influence the natural history of prostate cancer including deregulated expression, somatic mutation, and post-translational modification. The purpose of this article is to review the evidence for deregulated expression and function of the AR and associated coactivators and corepressors and how such events might contribute to the progression of prostate cancer by controlling the selection and expression of AR targets.

The retinoblastoma tumor-suppressor gene, the exception that proves the rule

The retinoblastoma tumor-suppressor gene (Rb1) is centrally important in cancer research. Mutational inactivation of Rb1 causes the pediatric cancer retinoblastoma, while deregulation of the pathway in which it functions is common in most types of human cancer. The Rb1-encoded protein (pRb) is well known as a general cell cycle regulator, and this activity is critical for pRb-mediated tumor suppression. The main focus of this review, however, is on more recent evidence demonstrating the existence of additional, cell type-specific pRb functions in cellular differentiation and survival. These additional functions are relevant to carcinogenesis suggesting that the net effect of Rb1 loss on the behavior of resulting tumors is highly dependent on biological context. The molecular mechanisms underlying pRb functions are based on the cellular proteins it interacts with and the functional consequences of those interactions. Better insight into pRb-mediated tumor suppression and clinical exploitation of pRb as a therapeutic target will require a global view of the complex, interdependent network of pocket protein complexes that function simultaneously within given tissues.

Androgen receptor remains critical for cell-cycle progression in androgen-independent CWR22 prostate cancer cells

The majority of prostate cancers (PCa) that relapse after androgen deprivation therapy (androgen-independent PCa) continue to express androgen receptor (AR). To study the functional importance of AR in these tumors, we derived androgen-independent CWR22 PCa xenografts in castrated mice and generated a cell line from one of these xenografts (CWR22R3). Similarly to androgen-independent PCa in patients, the relapsed xenografts and cell line expressed AR and were resistant to treatment with bicalutamide. However, expression of the AR-regulated PSA gene in the CWR22R3 cell line was markedly decreased compared to the relapsed xenografts in vivo. Transfections with androgen-regulated reporter genes further indicated that the cells lacked androgen-independent AR transcriptional activity and were not hypersensitive to low androgen concentrations despite constitutive activation of the Erk/MAP kinases. Nonetheless, AR remained essential for androgen-independent growth because retroviral shRNA-mediated AR down-regulation resulted in marked long-term growth suppression. This was associated with increased levels of p27(kip1) and hypophosphorylation of retinoblastoma protein but not with decreases in D-type cyclin levels or MAP kinase activation. These results reveal a potentially critical function of AR in androgen-independent PCa that is distinct from its previously described transcriptional or nontranscriptional functions.

Regulatory processes affecting androgen receptor expression, stability, and function: Potential targets to treat hormone-refractory prostate cancer

Prostate cancer cells rely on androgen receptor (AR) for proliferation and survival. Therefore, curing prostate cancer will require elimination of AR. Although androgen is the natural ligand that activates AR, AR activity is also subject to regulation by growth factor/growth factor receptor-stimulated signaling pathways that control the cell cycle. Cell cycle regulatory proteins and protein kinases in signaling pathways affected by growth factors can lead to AR activation in the absence of androgen. While downstream signaling proteins such as cyclins, cyclin-dependent kinases (CDKs), and pRB can modulate AR activity, upstream signaling pathways involving protein kinases such as mitogen-activated protein kinases, protein kinase A, and protein kinase B/Akt can affect post-translational modification of AR to affect not only AR function but also AR stability. Calcium and calmodulin (CaM), essential for proliferation and viability of a number of cells, including prostate cancer cells, play an important role in AR expression, stability, and function. CaM affects AR partly by interacting directly with AR and partly by activating protein kinases such as Akt and DNA-PK that can phosphorylate AR. The ubiquitin/26S proteasome pathway responsible for timely destruction of cell cycle regulatory proteins whose levels impede cell cycle progression also induces AR expression by activating NF-kappaB, and promotes AR activity by participating in the assembly of an AR transcription complex. Maspin, a serine protease inhibitor that is known mostly for its role as a tumor suppressor can also regulate AR intracellular localization and function by competing with AR for binding to the chaperone protein Hsp90 and co-repressor HDAC1, respectively. This perspective reviews the experimental evidence implicating these diverse cellular processes in AR expression, stability, and/or function, and presents a rationale for disrupting these cellular processes as a viable option for the treatment of both the hormone-sensitive and the hormone-insensitive prostate cancer.

A role for cyclin A1 in mediating the autocrine expression of vascular endothelial growth factor in prostate cancer

Elevated levels of cyclin A1 expression have been implicated in acute myeloid leukemia and in male germ cell tumors. However, a role of cyclin A1 in tumorigenesis of prostate cancer has not been reported. In the present study, expression of cyclin A1 in patients with prostate cancer and a role of cyclin A1 in mediating expression of vascular endothelial growth factor (VEGF) were investigated. Cyclin A1 was highly expressed in aggressive tumors and was significantly correlated with VEGF expression in 96 patients with prostate cancer. Treatment of LNCaP cells with R1881, a synthetic androgen resulted in increased cyclin A1 expression. Induction of cyclin A1 expression in LNCaP cells led to an increase in VEGF expression and this effect was manifested upon the R1881 treatment. Cyclin A1 failed to mediate VEGF activation in DU-145 cells lacking a functional Rb and an androgen receptor (AR). Although AR expression was induced into DU-145 cells, cyclin A1 was unable to mediate VEGF expression. However, induced coexpression of cyclin A1, Rb and AR in DU-145 cells in the presence of R1881 greatly promoted VEGF promoter activity. This suggests that cyclin A1 mediates VEGF expression in cooperation with Rb- and androgen-dependent pathways in prostate cancer.

Tumour suppressor retinoblastoma protein Rb: a transcriptional regulator

Rb was the first tumour suppressor identified through human genetic studies. The most significant achievement after almost twenty years since its cloning is the revelation that Rb possesses functions of a transcription regulator. Rb serves as a transducer between the cell cycle machinery and promoter-specific transcription factors. In this capacity, Rb is best known as a repressor of the E2F/DP family of transcription factors, which regulate expression of genes involved in cell proliferation and survival. An equally important aspect of Rb as a transcription regulator is that Rb also activates certain differentiation transcription factors to promote cellular differentiation. The molecular mechanisms behind the repressive effects of Rb on E2Fs have come to light in significant details, while those relating to Rb activation of differentiation transcription factors are much less understood. Finally, it has become clear that there are other aspects to Rb function that are not immediately related to transcription regulation.

A pp32–retinoblastoma protein complex modulates androgen receptor-mediated transcription and associates with components of the splicing machinery

We have previously shown pp32 and the retinoblastoma protein interact. pp32 and the retinoblastoma protein are nuclear receptor transcriptional coregulators: the retinoblastoma protein is a coactivator for androgen receptor, the major regulator of prostate cancer growth, while pp32, which is highly expressed in prostate cancer, is a corepressor of the estrogen receptor. We now show pp32 increases androgen receptor-mediated transcription and the retinoblastoma protein modulates this activity. Using affinity purification and mass spectrometry, we identify members of the pp32-retinoblastoma protein complex as PSF and nonO/p54nrb, proteins implicated in coordinate regulation of nuclear receptor-mediated transcription and splicing. We show that the pp32-retinoblastoma protein complex is modulated during TPA-induced K562 differentiation. Present evidence suggests that nuclear receptors assemble multiprotein complexes to coordinately regulate transcription and mRNA processing. Our results suggest that pp32 and the retinoblastoma protein may be part of a multiprotein complex that coordinately regulates nuclear receptor-mediated transcription and mRNA processing.

Molecular markers of prostate cancer outcome

Molecular markers have the potential to serve not only as prognostic factors but may be targets for new therapeutic strategies and predictors of response in a range of cancers. Prostate cancer development and progression is predicated on a series of genetic and epigenetic events within the prostate cell and its milieu. Within this review, we identify candidate molecules involved in diverse processes such as cell proliferation, death and apoptosis, signal transduction, androgen receptor (AR) signalling, cellular adhesion and angiogenesis that are linked to outcome in prostate cancer. Current markers with potential prognostic value include p53, Bcl-2, p16INK4A, p27Kip1, c-Myc, AR, E-cadherin and vascular endothelial growth factor. Evolving technology permits the identification of an increasing number of molecular markers with prognosis and predictive potential. We also review the use of gene microarray analysis in gene discovery as a means of identifying and cosegregating novel markers of prostate cancer outcome. By integrating selected markers into prospective clinical trials, there is potential for us to provide specific targeted therapy tailored for an increasing number of patients.

Rb enhances p160/SRC coactivator-dependent activity of nuclear receptors and hormone responsiveness

The retinoblastoma tumor suppressor protein (Rb) is best known as a repressor of genes involved in cell cycle progression. Rb has also been implicated in activation of transcription, in particular by nuclear receptors (NRs) and by differentiation-related transcription factors, but the relevance of this activity is unclear. We show that Rb and the related proteins p107 and p130 enhance the activity of NRs related to NGFI-B (Nur factors) through direct interactions with NGFI-B and SRC-2. Although recruitment of SRC/p160 coactivators to the NGFI-B AF1 domain is independent of Rb, its presence enhances SRC-dependent transcription. Rb potentiation of SRC coactivators is exerted on a subset (Nur factors, hepatocyte nuclear factor-4 (HNF-4), SF-1, and ER) but not all NRs. The levels of Rb-related proteins modulate hormone responsiveness of the NGFI-B-dependent pituitary proopiomelanocortin gene and HNF-4-dependent transcription during enterocyte differentiation. Increased Rb expression upon cell differentiation may promote differentiated functions, at least in part, by potentiation of NR activity.

Multiple mechanisms are involved in Ah receptor-mediated cell cycle arrest

The liver is the only solid organ that can respond to major tissue loss or damage by regeneration to restore liver biomass. The aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can disrupt the regenerative process, as evidenced by suppression of DNA synthesis in rat primary hepatocytes in culture and in vivo liver regeneration after partial hepatectomy. Independent observations demonstrated that AhR-mediated G(1) phase cell cycle arrest depends on an interaction with the retinoblastoma tumor suppressor protein (pRb), but differences exist regarding proposed mechanisms of action. Two distinct models have been proposed, one supporting the AhR-pRb interaction functioning in corepression of E2F activity and the other favoring an AhR-pRb interaction participating in transcriptional coactivation of genes encoding G(1) phase regulatory proteins. In the present study, experiments in rat hepatoma cells using dominant-negative DNA-binding-defective AhR and Ah receptor nuclear translocator (Arnt) mutants provided evidence that TCDD-induced AhR-mediated G(1) arrest is only partially regulated by direct AhR transcriptional activity, suggesting that both coactivation and corepression are involved. Studies using a small interfering RNA to down-regulate Arnt protein expression revealed that TCDD-induced G(1) arrest is absolutely dependent on the Arnt protein.

The role of the androgen receptor in the development of prostatic hyperplasia and prostate cancer

The androgen receptor (AR) is an androgen-inducible transcription factor characterized by a modular primary structure, with each module representing a distinct functional unit. After its interaction with androgens, the cytoplasmic AR is activated and translocated to the nucleus where it binds to target genes at the androgen responsive element(s) and recruits coregulators to form a multiprotein complex that interacts with transcriptional mediators and the basal transcription machinery to regulate gene transcription. Androgens play an essential role in the morphogenesis and physiology of the normal prostate. The etiology of benign prostatic hyperplasia (BPH) and prostatic neoplasia, which can progress to adenocarcinoma, is androgen-dependent, and reduction/obliteration of androgen action in the prostate has been the therapy of choice for BPH and prostate cancer. After androgen withdrawal and antiandrogen treatment, the androgen responsive prostate cancer cells cease to proliferate and undergo apoptosis, causing tumor regression. However, relapses are seen invariably, when tumors emerge as androgen-independent and apoptosis-resistant. Gene amplification and amino acid substitutions in the AR are detected at a high frequency in recurrent tumors. These changes confer growth advantage to the tumor cells due to either hypersensitivity of AR to low, castrate-level androgens or a realignment of the receptor conformation, leading to altered ligand specificity that enables antiandrogens, adrenal androgens and non-androgen steroids act agonistically to increase AR activity. Persistence of signaling by the wild-type AR in therapy-resistant tumors is due to the increased receptor activity caused by cross talk of AR with multiple intracellular signaling cascades, especially the growth factor activated MAP kinase/ERK and PI3 kinase/Akt pathways. Ablation of AR function using antisense oligodeoxynucleotides, ribozymes or small interference RNAs (RNAi) holds promise as future approaches to the successful treatment of hormone-refractory, apoptosis-resistant prostate tumors.

2,3,7,8-Tetrachlorodibenzo-p-dioxin Blocks Androgen-Dependent Cell Proliferation of LNCaP Cells through Modulation of pRB Phosphorylation

Cell-cycle regulatory events associated with inhibition of androgen-dependent cell proliferation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in the human-derived LNCaP cell line. TCDD blocked the G(1) to S transition of LNCaP cells synchronized in G(0)/G(1) when these cells were induced to reinitiate cell-cycle progression by dihydrotestosterone (DHT). Western blot analyses of these cells revealed altered expression levels of G(1) regulatory proteins, including increases in hypophosphorylated retinoblastoma protein and concomitant decreases in cyclin D1. p21(WAF1/CIP1), which is involved in the assembly of cyclin D1/cyclin-dependent kinase-4 complexes, was increased by DHT or TCDD when each compound was administered singly but was reduced to background levels in cells simultaneously treated with DHT and TCDD. Reporter gene assays revealed the presence of several Ah receptor response-element motifs in the promoter and first intron of the p21(WAF1/CIP1) gene that respond to TCDD-mediated Ah receptor activation independently of p53. Transcription studies showed that activation of aryl hydrocarbon receptor blocks androgen-dependent gene induction in LNCaP cells as well as in African green monkey CV-1 cells. These data point to at least two mechanisms whereby TCDD blocks androgen receptor function: 1) by blocking androgen-induced cell proliferation through modulation of the expression and activities of regulatory proteins controlling cell-cycle progression; and 2) by squelching androgen receptor-mediated gene transcription through receptor cross-talk, possibly involving competition for coregulators or by direct protein interaction.

Androgen receptor in prostate cancer

The normal development and maintenance of the prostate is dependent on androgen acting through the androgen receptor (AR). AR remains important in the development and progression of prostate cancer. AR expression is maintained throughout prostate cancer progression, and the majority of androgen-independent or hormone refractory prostate cancers express AR. Mutation of AR, especially mutations that result in a relaxation of AR ligand specificity, may contribute to the progression of prostate cancer and the failure of endocrine therapy by allowing AR transcriptional activation in response to antiandrogens or other endogenous hormones. Similarly, alterations in the relative expression of AR coregulators have been found to occur with prostate cancer progression and may contribute to differences in AR ligand specificity or transcriptional activity. Prostate cancer progression is also associated with increased growth factor production and an altered response to growth factors by prostate cancer cells. The kinase signal transduction cascades initiated by mitogenic growth factors modulate the transcriptional activity of AR and the interaction between AR and AR coactivators. The inhibition of AR activity through mechanisms in addition to androgen ablation, such as modulation of signal transduction pathways, may delay prostate cancer progression.

Induction of Androgen Receptor-Dependent Apoptosis in Prostate Cancer Cells by the Retinoblastoma Protein

Re-expression of a tumor suppressor in tumor cells that lack it is an effective way to study its functional activities. However, because tumor cells contain multiple mutations, tumor suppressor functions that are dependent on (an)other regulators are unlikely to be identified by its re-expression alone if the other regulators are also mutated. In this study, we show that re-expression of retinoblastoma (RB) together with the androgen receptor (AR) in RB- and AR-deficient prostate cancer DU-145 cells resulted in an apoptotic activity, acting through the mitochondria damage-initiated caspase activation pathway, which was not present when RB, or the AR, was re-expressed alone. The ability of RB + AR to induce mitochondria damage was dependent on the proapoptotic proteins Bax and Bak and could be blocked by the antiapoptotic protein Bcl-x(L). Coexpressed AR did not detectably change RB's regulation of E2F and cell cycle progression in culture. On the other hand, coexpressed RB could activate the transactivation activity of the AR in an androgen-depleted media. Although androgen induced greater AR transactivation activity in this condition, it did not induce apoptosis in the absence of coexpressed RB. Analysis of mutants of RB and the AR indicated that intact pocket function of RB and the transactivation activity of the AR were required for RB + AR-induced apoptosis. These results provide direct functional data for an AR-dependent apoptosis-inducing activity of RB and highlight the importance of cell type-specific regulators in obtaining a more complete understanding of RB.

TRANSACTIVATION OF GLUCOCORTICOID-INDUCIBLE RAT ARYL SULFOTRANSFERASE (SULT1A1) GENE TRANSCRIPTION

The purpose of the current study was to establish the role of the glucocorticoid receptor (GR) and androgen receptor (AR) transcription factors in the transactivation of rat aryl sulfotransferase (SULT1A1) gene transcription and to identify the functional hormone-responsive element(s) in the SULT1A1 gene. A cis-acting inverted repeat with three intervening bases (IR3) was identified in the 5'-flanking of the SULT1A1 gene that mediates the transactivation of SULT1A1 gene transcription by both the GR and AR. CV-1 cells were cotransfected with SULT1A1-luciferase reporter plasmids and either wild-type or mutant GR or AR expression constructs. In cotransfectants expressing the wild-type GR, treatment with triamcinolone acetonide produced an approximately 4- to 6-fold induction of luciferase activity in IR3-containing SULT1A1 reporter plasmids. IR3-containing SULT1A1 reporter constructs were also activated by treatment with the synthetic androgen R1881 in cells cotransfected with wild-type but not mutant AR. In primary cultured rat hepatocytes, androgen-inducible expression of IR3-containing SULT1A1 reporter plasmids required cotransfection with AR expression plasmid. Targeted disruption of the SULT1A1 IR3 by mutation of a conserved GT sequence in the 3' half-site of the element ablated GR and AR responsiveness. These results indicate that a proximal IR3 element in the 5'-flanking region of the rat SULT1A1 gene is sufficient for the transactivation of SULT1A1 gene transcription by the GR and AR, and that relative to the GR, functional AR activity is reduced in primary cultured rat hepatocytes.

Alterations of cell cycle-regulatory genes in prostate cancer

Deregulated proliferation is one of the main events in neoplastic transformation, and this has prompted increased attention being given to the understanding of the mechanisms involved in cell cycle regulation and its alterations. The 'retinoblastoma pathway', a key effector controlling G1-S phase transition, includes several oncogenes and tumour suppressor genes which display a wide range of abnormalities with potential usefulness as markers of evolution or treatment response in prostate cancer. Among these, the existence of p53 mutations seems to predict resistance to radiotherapy or systemic treatment, and p16 overexpression or p27 downregulation seems to serve as markers of poor evolution. The well-established existence of a critical hormonal role in prostate carcinogenesis coupled with the relationship of androgenic activity and regulation of several cell cycle modulators forces cell cycle control in the prostate to be envisioned as a highly complex steroid-influenced system, which will undoubtedly have critical implications in the future management of prostate cancer patients.

The role of retinoblastoma protein family in the control of germ cell proliferation, differentiation and survival

Retinoblastoma family proteins pRb, p107 and p130 are differentially expressed in the rat testis. They function in specific cell types during testicular development and spermatogenesis, participating in the control of proliferation, differentiation, and survival. Their expression levels and phosphorylation status are modulated during germ cell cycle progression and apoptosis. Hyperphosphorylated states and elevated levels of p107 are correlated with cell cycle progression, whereas hypophosphorylated states and reduced levels are associated with suppression of proliferation and apoptosis in germ cells and Leydig cells. These proteins may also serve as markers of cell cycle status of germ cells during spermatogenesis.

Role of the aryl hydrocarbon receptor in cell cycle regulation

Traditionally, the aryl hydrocarbon receptor (AHR) is considered to be a ligand-activated receptor and transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was neatly established long before the first report of an AHR cDNA sequence was published. Only recently, other functions of this protein have begun to be recognized. This review addresses novel findings relating to AHR functions that have resulted from experimental approaches markedly outside traditional receptor analyses. Here we examine the aspects of AHR biology relevant to its role in cell cycle regulation, from the activation of mitogen-activated protein kinases to the cross-talk between AHR and the RAS pathway and the functional significance of the interaction between AHR and the retinoblastoma protein. We have attempted to provide the reader with a balanced interpretation of the evidence, highlighting areas of consensus as well as areas still being contested.

Loss of androgen receptor transcriptional activity at the G(1)/S transition

Androgens are essential for the differentiation, growth, and maintenance of male-specific organs. The effects of androgens in cells are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily of transcription factors. Recently, transient transfection studies have shown that overexpression of cell cycle regulatory proteins affects the transcriptional activity of the AR. In this report, we characterize the transcriptional activity of endogenous AR through the cell cycle. We demonstrate that in G0, AR enhances transcription from an integrated steroid-responsive mouse mammary tumor virus promoter and also from an integrated androgen-specific probasin promoter. This activity is strongly reduced or abolished at the G(1)/S boundary. In S phase, the receptor regains activity, indicating that there is a transient regulatory event that inactivates the AR at the G(1)/S transition. This regulation is specific for the AR, since the related glucocorticoid receptor is transcriptionally active at the G(1)/S boundary. Not all of the effects of androgens are blocked, however, since androgens retain the ability to increase AR protein levels. The transcriptional inactivity of the AR at the G(1)/S junction coincides with a decrease in AR protein level, although activity can be partly rescued without an increase in receptor. Inhibition of histone deacetylases brings about this partial restoration of AR activity at the G(1)/S boundary, demonstrating the involvement of acetylation pathways in the cell cycle regulation of AR transcriptional activity. Finally, a model is proposed that explains the inactivity of the AR at the G(1)/S transition by integrating receptor levels, the action of cell cycle regulators, and the contribution of histone acetyltransferase-containing coactivators.

Androgen receptor (AR) coregulators: an overview

The biological action of androgens is mediated through the androgen receptor (AR). Androgen-bound AR functions as a transcription factor to regulate genes involved in an array of physiological processes, most notably male sexual differentiation and maturation, and the maintenance of spermatogenesis. The transcriptional activity of AR is affected by coregulators that influence a number of functional properties of AR, including ligand selectivity and DNA binding capacity. As the promoter of target genes, coregulators participate in DNA modification, either directly through modification of histones or indirectly by the recruitment of chromatin-modifying complexes, as well as functioning in the recruitment of the basal transcriptional machinery. Aberrant coregulator activity due to mutation or altered expression levels may be a contributing factor in the progression of diseases related to AR activity, such as prostate cancer. AR demonstrates distinct differences in its interaction with coregulators from other steroid receptors due to differences in the functional interaction between AR domains, possibly resulting in alterations in the dynamic interactions between coregulator complexes.

Cyclin D1: mechanism and consequence of androgen receptor co-repressor activity

Androgen receptor regulation is pivotal for prostate growth and development. Activation of the receptor is dictated by association with androgen (ligand) and through interaction with co-activators and co-repressors. We have shown previously that cyclin D1 functions as a co-repressor to inhibit ligand-dependent androgen receptor activation. We demonstrate that cyclin D1 directly binds the N terminus of the androgen receptor and that this interaction is independent of ligand. Furthermore, we show that the interaction occurs in the nucleus and does not require the LXXLL motif of cyclin D1. Although two distinct transactivation domains exist in the N terminus (AF-1 and AF-5), the data shown support the hypothesis that cyclin D1 targets the AF-1 transactivation function. The constitutively active AF-5 domain was refractory to cyclin D1 inhibition. By contrast, cyclin D1 completely abolished androgen receptor activity, even in the presence of potent androgen receptor co-activators. This action of cyclin D1 at least partially required de-acetylase activity. Finally, we show that transient, ectopic expression of cyclin D1 results in reduced cell cycle progression in androgen-dependent LNCaP cells independent of CDK4 association. Collectively, our data support a model wherein cyclin D1 has a mitogenic (CDK4-dependent) function and an anti-mitogenic function (dependent on regulation of the AF-1 domain) that can collectively control the rate of androgen-dependent cellular proliferation. These findings provide insight into the non-cell cycle functions of cyclin D1 and provide the impetus to study its pleiotropic effects in androgen-dependent cells, especially prostatic adenocarcinomas.

Prostatic intraepithelial neoplasia in mice expressing an androgen receptor transgene in prostate epithelium

Prostate cancer (PCa) is an androgen dependent disease that can be treated by androgen ablation therapy, and clinical trials are under way to prevent PCa through the reduction of androgen receptor (AR) activity. However, there are no animal models of AR-mediated prostatic neoplasia, and it remains unclear whether the AR is a positive or negative regulator of cell growth in normal prostate secretory epithelium. To assess the direct effects of the AR in prostate epithelium, a murine AR transgene regulated by the rat probasin promoter (Pb) was used to generate transgenic mice expressing increased levels of AR protein in prostate secretory epithelium. The prostates in younger (<1 year) Pb-mAR transgenic mice were histologically normal, but Ki-67 immunostaining revealed marked increases in epithelial proliferation in ventral prostate and dorsolateral prostate. Older (>1 year) transgenic mice developed focal areas of intraepithelial neoplasia strongly resembling human high-grade prostatic intraepithelial neoplasia (PIN), a precursor to PCa. These results demonstrate that the AR is a positive regulator of cell growth in normal prostate epithelium and provide a model system of AR-stimulated PIN that can be used for assessing preventative hormonal therapies and for identifying secondary transforming events relevant to human PCa.

Retinoblastoma protein is functionally distinct from its homologues in affecting glucocorticoid receptor-mediated transcription and apoptosis

The cell cycle regulator, retinoblastoma protein, is known to potentiate glucocorticoid receptor-activated transcription through the interaction of its pocket domain with the transcription coactivator, hBRM. We now show that glucocorticoid receptor-induced apoptosis is also dependent on both the retinoblastoma protein and hBRM. p107 and p130, which share extensive sequence homology with the pocket domain of the retinoblastoma protein but not its N-terminal region, also interact with hBRM but do not support either glucocorticoid receptor-dependent activity. This difference arises from the divergent N-terminal domain of the retinoblastoma protein, which, when fused to the pocket domains, confers upon p107 and p130 the ability to influence glucocorticoid receptor activities. This effect probably results from the promotion of glucocorticoid receptor-targeted chromatin remodeling by the hBRM-containing SWI/SNF complex because the N-terminal domain of the retinoblastoma protein enhances glucocorticoid receptor-hBRM interactions. These results highlight that, besides the interaction between hBRM and the pocket domain of RB, the N-terminal region of the retinoblastoma protein is also essential for glucocorticoid receptor-induced apoptosis and the potentiation of glucocorticoid receptor-mediated transcription and provide a basis for functional distinction between the retinoblastoma protein and its homologues.

Maximal aryl hydrocarbon receptor activity depends on an interaction with the retinoblastoma protein

The aryl hydrocarbon receptor (AhR) belongs to the basic helix-loop-helix/periodicity/AhR nuclear translocator/simple-minded (Per-Arnt-Sim) family of transcription factors that regulate critical functions during development and tissue homeostasis. Within this family, the AhR is the only member conditionally activated in response to ligand binding, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We recently demonstrated that the AhR interacts with the retinoblastoma protein (pRb). This report presents evidence that a LXCXE motif in the AhR protein confers pRb binding, which is necessary for maximal TCDD induced G(1) arrest in rat 5L hepatoma cells. The data support a mechanism whereby pRb seems to regulate G(1) cell cycle progression distinct from the direct repression of E2F-mediated transcription. Furthermore, the results indicate that the AhR-pRb interaction regulates TCDD induction of CYP1A1, suggesting that pRb may be a general AhR coactivator.

Differential expression and regulation of the retinoblastoma family of proteins during testicular development and spermatogenesis: roles in the control of germ cell proliferation, differentiation and apoptosis

Normal spermatogenesis is highly dependent on well-balanced germ cell proliferation, differentiation, and apoptosis. However, the molecular mechanisms that govern these processes are largely unknown. Retinoblastoma family proteins (pRb, p107 and p130) are potentially important regulators of cell growth, differentiation and apoptosis. pRb has been shown to be expressed in the rat testis and involved in the regulation of spermatogenesis. In the present study, the expression and localization of the other two pRb family members, p107 and p130, were analysed at both mRNA and protein levels during testicular development and spermatogenesis using Northern, Western blotting, immunohistochemistry, and in situ hybridization. Furthermore, changes of levels and phosphorylation status of pRb family proteins in response to growth suppression and/or apoptosis induction were investigated using a seminiferous tubule culture system and three animal models. Our data suggest that: (1) pRb family proteins are differentially expressed in the rat testis and they function in a cell-type-specific manner during testicular development and spermatogenesis; (2) they participate in the control of germ cell cycle and act in a cell cycle-phase-specific fashion during germ cell proliferation, and (3) they are also involved in the regulation of apoptosis of germ cells and Leydig cells.

The androgen receptor (AR) amino-terminus imposes androgen-specific regulation of AR gene expression via an exonic enhancer

Androgen and glucocorticoid receptor (AR, GR), two closely related members of the nuclear receptor superfamily, can recognize a similar cis-acting DNA sequence, or hormone response element (HRE). Despite this apparent commonality, these receptors regulate distinct target genes in vivo. The AR gene itself is regulated by AR but not GR in a variety of cell types, including osteoblast-like cells, as shown here. To understand this specificity, we first identified the DNA sequences responsible for androgen-mediated up-regulation of AR messenger RNA. A 6.5-kb region encompassing exon D, intron 4, and exon E of the AR gene contains four exonic HREs and exhibits cell type-specific, AR-mediated transcriptional enhancement when placed upstream of a heterologous promoter and reporter gene. A 350-bp fragment consisting of just exons D and E exhibits the same cell- and androgen-specificity as the 6.5-kb region, as well as the native AR gene. Consistent with a role for the exonic HREs, androgen regulation via this intragenic enhancer requires the HREs as well as a functional receptor DNA binding domain. A panel of AR/GR chimeric receptors was used to test which AR domains (amino-terminal, DNA binding or ligand binding) confer androgen-specific regulation of the 350-bp enhancer. Only chimeric receptors containing the amino-terminus of AR induced reporter gene activity from the AR gene enhancer. Further, a constitutively active AR consisting of only the AR amino-terminus and DNA binding domain (AA phi) retained the capacity to activate the internal responsive region, unlike a constitutively active chimera harboring the GR amino-terminus and AR DNA binding domain (GA phi). Thus, the AR amino terminus is the sole determinant for androgen-specific regulation of the AR gene internal enhancer. These findings support a model in which the amino termini of ARs bound to HREs within the AR gene interact with an exclusive auxiliary factor(s) to elicit androgen-specific regulation of AR messenger RNA. This is the first example of androgen-specific response in which the necessary and sufficient distinguishing capacity resides within the AR amino terminus.