Hsp90 regulates androgen receptor hormone binding affinity in vivo

Chemical ablation of androgen receptor in prostate cancer cells by the histone deacetylase inhibitor LAQ824

Androgen receptor plays a critical role in the development of primary as well as advanced hormone-refractory prostate cancer. Therefore, ablation of androgen receptor from prostate cancer cells is an interesting concept for developing a new therapy not only for androgen-dependent prostate cancer but also for metastatic hormone-refractory prostate cancer, for which there is no effective treatment available. We report here that LAQ824, a cinnamyl hydroxamatic acid histone deacetylase inhibitor currently in human clinical trials, effectively depleted androgen receptor in prostate cancer cells at nanomolar concentrations. LAQ824 seemed capable of depleting both the mutant and wild-type androgen receptors in either androgen-dependent and androgen-independent prostate cancer cells. Although LAQ824 may exert its effect through multiple mechanisms, several lines of evidence suggest that inactivation of the heat shock protein-90 (Hsp90) molecular chaperone is involved in LAQ824-induced androgen receptor depletion. Besides androgen receptor, LAQ824 reduced the level of Hsp90 client proteins HER-2 (ErbB2), Akt/PKB, and Raf-1 in LNCaP cells. Another Hsp90 inhibitor, 17-allyamino-17-demethoxygeldanamycin (17-AAG), also induced androgen receptor diminution. LAQ824 induced Hsp90 acetylation in LNCaP cells, which resulted in inhibition of its ATP-binding activity, dissociation of Hsp90-androgen receptor complex, and proteasome-mediated degradation of androgen receptor. Consequently, LAQ824 blocked androgen-induced prostate-specific antigen production in LNCaP cells. LAQ824 effectively inhibited cell proliferation and induced apoptosis of these prostate cancer cells. These results reveal that LAQ824 is a potent agent for depletion of androgen receptor and a potential new drug for prostate cancer.

Chaperoning steroid hormone signaling via reversible acetylation

Glucocorticoid receptor (GR) and related steroid hormone receptors are ligand-dependent transcription factors whose regulation is critical for both homeostasis and diseases. The structural maturation of the GR has been shown to require the Hsp90 molecular chaperone complex. Evidence indicates that Hsp90-dependent maturation is critical for GR ligand binding capacity and activity. While the role for Hsp90 in GR function is well established, the regulation of this process is not well understood. Here we discuss a recent finding that identifies reversible protein acetylation controlled by the deacetylase HDAC6 as a novel mechanism that regulates Hsp90-dependent GR maturation. We will also speculate on the implications of this finding in steroid hormone signaling, oncogenic transformation and its potential therapeutic utility.

Hyperthermia-induced proteasome inhibition and loss of androgen receptor expression in human prostate cancer cells

Prostate cancer is the second leading cause of death in men in western countries and is usually treated by surgery and/or radiotherapy. More recently, hyperthermia has been introduced into clinical trials investigating a possible effect in the first-line treatment of prostate cancer. However, the molecular mechanisms of hyperthermia are not completely understood. In this study, we investigated the effects of hyperthermia on proteasome function and its significance for signal transduction, cell death and androgen receptor (AR) expression in PC-3, LnCaP, and DU-145 human and TRAMP-C2 murine prostate cancer cells. Hyperthermia caused apoptosis and radiosensitization and decreased 26S proteasome activity in all three human cell lines to about 40% of untreated control cells. 20S proteasome activity was not affected by heat. Heat treatment inhibited constitutive and radiation-induced activation of nuclear factor kappaB caused by stabilization of IkappaB. Although stabilization of AR by proteasome inhibitors has been reported previously, AR protein levels in LnCaP cells decreased dramatically after heat. Our data suggest that inhibition of proteasome function and dependent signal transduction pathways might be a major molecular mechanisms of heat-induced apoptosis and radiosensitization. Hyperthermia abrogates AR expression in androgen-dependent cells and might thus promote malignant progression of prostate cancer.

Heat-shock protein 90 inhibitors in antineoplastic therapy: is it all wrapped up?

Heat-shock protein (Hsp)-90 belongs to the class of molecular chaperone proteins that are capable of sensing cellular stress. Although Hsp90 is essential for viability, the pharmacological inhibition of this chaperone has emerged as an attractive means to inhibit tumorigenesis. This phenomenon is due to a unique property of Hsp90; its 'client proteins' are universally involved in signal transduction pathways commonly dysregulated in, and contributing to, cancer. The natural product geldanamycin, a potent ansamycin Hsp90 inhibitor, has served as a lead compound for the development of several derivatives that are currently undergoing clinical trials. Inhibition of Hsp90 with geldanamycin simultaneously depletes Hsp90-associated clients and impairs numerous signalling cascades that depend on chaperone function. Importantly, tumour cells are exquisitely sensitive to Hsp90 inhibition, lending credence to the feasibility of selectively targeting cancer tissue via the pharmacological modulation of Hsp90 function. Even more remarkably, Hsp90 inhibitors sensitise tumour cells to the cytotoxic effects of a variety of standard therapeutics, and thus, they are likely to have broad utility in combination therapy. Although these are promising developments, much remains to be discovered about client-chaperone biology and the tumour-specific effects of Hsp90 blockade. This information is required to fully grasp the multi-faceted roles of Hsp90 in cancer biology towards the goal of optimising the use of these agents in the clinic. Elucidation of these nuances will undoubtedly lead to better targeting of relevant oncogenic pathways and translate into the development of more effective anticancer regimens.

Phase I Pharmacokinetic and Pharmacodynamic Study of 17-Allylamino, 17-Demethoxygeldanamycin in Patients With Advanced Malignancies

Purpose

To study the toxicity and pharmacokinetic-pharmacodynamic profile of 17-allylamino, 17- demethoxygeldanamycin (17-AAG) and to recommend a dose for phase II trials.

Patients and Methods

This was a phase I study examining a once-weekly dosing schedule of 17-AAG. Thirty patients with advanced malignancies were treated.

Results

The highest dose level reached was 450 mg/m2/week. The dose-limiting toxicities (DLTs) encountered were grade 3 diarrhea in three patients (one at 320 mg/m2/week and two at 450 mg/m2/week) and grade 3 to 4 hepatotoxicity (AST/ALT) in one patient at 450 mg/m2/week. Two of nine DLTs were at the highest dose level. Two patients with metastatic melanoma had stable disease and were treated for 15 and 41 months, respectively. The dose versus area under the curve-relationship for 17-AAG was linear (r2 = .71) over the dose range 10 to 450 mg/m2/week, with peak plasma concentrations of 8,998 μg/L (standard deviation, 2,881) at the highest dose level. After the demonstration of pharmacodynamic changes in peripheral blood leukocytes, pre- and 24 hours post-treatment, tumor biopsies were performed and demonstrated target inhibition (c-RAF-1 inhibition in four of six patients, CDK4 depletion in eight of nine patients and HSP70 induction in eight of nine patients) at the dose levels 320 and 450 mg/m2/week. It was not possible to reproducibly demonstrate these changes in biopsies taken 5 days after treatment.

Conclusion

It has been possible to demonstrate that 17-AAG exhibits a tolerable toxicity profile with therapeutic plasma concentrations and target inhibition for 24 hours after treatment and some indications of clinical activity at the dose level 450 mg/m2/week. We recommend this dose for phase II clinical trials.

Reappraisal of the Concept of Hormone Therapy in Metastatic Prostate Cancer and Implications for Treatment

Almost all prostate cancer patients become resistant to hormonal therapy that blocks androgen-mediated cell proliferation. The key to this resistance may lie in expression of the androgen receptor itself. Alternative methods to block the AR-mediated signaling pathways appear to be critical for tumor survival. These signal transduction pathways that interact with AR may enhance the response to androgen ablation therapy. The identification of signaling pathways may be a major goal in the treatment of prostate cancer. The application of novel therapies must be preceded by the identification of the genetic and molecular tumor profiles for each patient.

Physiological Role for the Cochaperone FKBP52 in Androgen Receptor Signaling

Molecular chaperones mediate multiple aspects of steroid receptor function, but the physiological importance of most receptor-associated cochaperones has not been determined. To help fill this gap, we targeted for disruption the mouse gene for the 52-kDa FK506 binding protein, FKBP52, a 90-kDa heat shock protein (Hsp90)-binding immunophilin found in steroid receptor complexes. A mouse line lacking FKBP52 (52KO) was generated and characterized. Male 52KO mice have several defects in reproductive tissues consistent with androgen insensitivity; among these defects are ambiguous external genitalia and dysgenic prostate. FKBP52 and androgen receptor (AR) are coexpressed in prostate epithelial cells of wild-type mice. However, FKBP52 and AR are similarly coexpressed in testis even though testis morphology and spermatogenesis in 52KO males are usually normal. Molecular studies confirm that FKBP52 is a component of AR complexes, and cellular studies in yeast and human cell models demonstrate that FKBP52 can enhance AR-meditated transactivation. AR enhancement requires FKBP52 peptidylprolyl isomerase activity as well as Hsp90-binding ability, and enhancement probably relates to an affect of FKBP52 on AR-folding pathways. In the presence of FKBP52, but not other cochaperones, the function of a minimally active AR point mutant can be dramatically restored. We conclude that FKBP52 is an AR folding factor that has critically important physiological roles in some male reproductive tissues.

Emodin down-regulates androgen receptor and inhibits prostate cancer cell growth

Hormone-refractory relapse is an inevitable and lethal event for advanced prostate cancer patients after hormone deprivation. A growing body of evidence indicates that hormone deprivation may promote this aggressive prostate cancer phenotype. Notably, androgen receptor (AR) not only mediates the effect of androgen on the tumor initiation but also plays the major role in the relapse transition. This provides a strong rationale for searching new effective agents targeting the down-regulation of AR to treat or prevent advanced prostate cancer progression. Here, we show that emodin, a natural compound, can directly target AR to suppress prostate cancer cell growth in vitro and prolong the survival of C3(1)/SV40 transgenic mice in vivo. Emodin treatment resulted in repressing androgen-dependent transactivation of AR by inhibiting AR nuclear translocation. Emodin decreased the association of AR and heat shock protein 90 and increased the association of AR and MDM2, which in turn induces AR degradation through proteasome-mediated pathway in a ligand-independent manner. Our work indicates a new mechanism for the emodin-mediated anticancer effect and justifies further investigation of emodin as a therapeutic and preventive agent for prostate cancer.

Heat-shock protein 90 inhibitors as novel cancer chemotherapeutic agents

Heat-shock protein 90 (Hsp90) is a molecular chaperone whose association is required for the stability and function of multiple mutated, chimeric and overexpressed signalling proteins that promote cancer cell growth and/or survival. Hsp90 client proteins include mutated p53, Bcr-Abl, Raf-1, Akt, HER2/Neu (ErbB2) and hypoxia inducible factor-1alpha (HIF-1alpha). Through specific interaction with a single molecular target, Hsp90 inhibitors cause the destabilisation and eventual degradation of Hsp90 client proteins, and they have shown promising antitumour activity in preclinical model systems. One Hsp90 inhibitor, 17-allylamino-geldanamycin (17-AAG), is currently in Phase I clinical trials. Hsp90 inhibitors are unique in that, although they are directed towards a specific molecular target, they simultaneously inhibit multiple signalling pathways on which cancer cells depend for growth and survival. Further, because of the unique effect that Hsp90 inhibition has on cancer cells, combination of an Hsp90 inhibitor with standard chemotherapeutic agents may dramatically increase the in vivo efficacy of the standard agent.

Ku is a novel transcriptional recycling coactivator of the androgen receptor in prostate cancer cells

The androgen receptor (AR) dynamically assembles and disassembles multicomponent receptor complexes in order to respond rapidly and reversibly to fluctuations in androgen levels. We are interested in identifying the basal factors that compose the AR aporeceptor and holoreceptor complexes and impact the transcriptional process. Using tandem mass spectroscopy analysis, we identified the trimeric DNA-dependent protein kinase (DNA-PK) complex as the major AR-interacting proteins. AR directly interacts with both Ku70 and Ku80 in vivo and in vitro, as shown by co-immunoprecipitation, glutathione S-transferase pull-down, and Sf9 cell/baculovirus expression. The interaction was localized to the androgen receptor ligand binding domain and is independent of DNA interactions. Ku interacts with AR in the cytoplasm and nucleus regardless of the presence or absence of androgen. Ku acts as a coactivator of AR activity in a luciferase reporter assay employing both Ku-defective cells and Ku small interfering RNA knock-down in a prostate cancer cell line. DNA-PK catalytic subunit (DNA-PKcs) also acts as a coactivator of androgen receptor activity in a luciferase reporter assay employing DNA-PKcs defective cells. AR nuclear translocation is not affected in Ku defective cells, implying Ku functionality may be mainly nuclear. Chromatin immunoprecipitation experiments demonstrated that both Ku70 and Ku80 interact with the prostate-specific antigen promoter in an androgen-dependant manner. Finally, in vitro transcription assays demonstrated Ku involvement in transcriptional recycling with androgen dependent promoters.

Gamma synuclein, a novel heat-shock protein-associated chaperone, stimulates ligand-dependent estrogen receptor alpha signaling and mammary tumorigenesis

Synucleins are emerging as central players in the formation of pathologically insoluble deposits characteristic of neurodegenerative diseases. gamma synuclein (SNCG), previously identified as a breast cancer-specific gene (BCSG1), is also highly associated with breast or ovarian cancer progression. However, the molecular targets of SNCG aberrant expression in breast cancer have not been identified. Here, we demonstrated a chaperone activity of SNCG in the heat-shock protein (Hsp)-based multiprotein chaperone complex for stimulation of estrogen receptor (ER)-alpha signaling. As an ER-alpha-associated chaperone, SNCG participated in Hsp-ER-alpha complex, enhanced the high-affinity ligand-binding capacity of ER-alpha, and stimulated ligand-dependent activation of ER-alpha. The SNCG-mediated stimulation of ER-alpha transcriptional activity is consistent with its stimulation of mammary tumorigenesis in response to estrogen. These data indicate that SNCG is a new chaperone protein in the Hsp-based multiprotein chaperone complex for stimulation of ligand-dependent ER-alpha signaling and thus stimulates hormone-responsive mammary tumorigenesis.

Partial agonist/antagonist properties of androstenedione and 4-androsten-3beta,17beta-diol

Androgens play important endocrine roles in development and physiology. Here, we characterize activities of two "Andro" prohormones, androstenedione (A-dione) and 4-androsten-3beta,17beta-diol (A-diol) in MDA-MB-453 (MDA) and LNCaP cells. A-dione and A-diol, like cyproterone acetate, were partial agonists of transfected mouse mammary tumor virus (MMTV) and endogenous prostate-specific antigen (PSA) promoters. Different from bicalutamide but similar to CPA, both are inducers of LNCaP cell proliferation with only mild suppression of 5alpha-dihydrotestosterone (DHT)-enhanced cell growth. Like bicalutamide and cyproterone acetate, A-dione and A-diol significantly antagonized DHT/R1881-induced PSA expression by up to 30% in LNCaP cells. Meanwhile, in MDA cells, EC(50)s for the MMTV promoter were between 10 and 100nM. Co-factor studies showed GRIP1 as most active for endogenous androgen receptor (AR), increasing MMTV transcription by up to five-fold, without substantially altering EC(50)s of DHT, A-dione or A-diol. Consistent with their transcriptional activities, A-dione and A-diol bound full-length endogenous AR from MDA or LNCaP cells with affinities of 30-70nM, although binding to expressed ligand-binding domain (LBD) was >20-fold weaker. In contrast, DHT, R1881, and bicalutamide bound similarly to LBD or aporeceptor. Together, these data suggest that A-dione and A-diol are ligands for AR with partial agonist/antagonist activities in cell-based transcription assays. Binding affinities for both are most accurately assessed by AR aporeceptor complex. In addition to being testosterone precursors in vivo, either may impart its own transcriptional regulation of AR.

An Androgen Receptor NH2-terminal Conserved Motif Interacts with the COOH Terminus of the Hsp70-interacting Protein (CHIP)

The NH2-terminal sequence of steroid receptors is highly variable between different receptors and in the same receptor from different species. In this study, a primary sequence homology comparison identified a 14-amino acid NH2-terminal motif of the human androgen receptor (AR) that is common to AR from all species reported, including the lower vertebrates. The evolutionarily conserved motif is unique to AR, with the exception of a partial sequence in the glucocorticoid receptor of higher species. The presence of the conserved motif in AR and the glucocorticoid receptor and its absence in other steroid receptors suggests convergent evolution. The function of the AR NH2-terminal conserved motif was suggested from a yeast two-hybrid screen that identified the COOH terminus of the Hsp70-interacting protein (CHIP) as a binding partner. We found that CHIP functions as a negative regulator of AR transcriptional activity by promoting AR degradation. In support of this, two mutations in the AR NH2-terminal conserved motif previously identified in the transgenic adenocarcinoma of mouse prostate model reduced the interaction between CHIP and AR. Our results suggest that the AR NH2-terminal domain contains an evolutionarily conserved motif that functions to limit AR transcriptional activity. Moreover, we demonstrate that the combination of comparative sequence alignment and yeast two-hybrid screening using short conserved peptides as bait provides an effective strategy to probe the structure-function relationships of steroid receptor NH2-terminal domains and other intrinsically unstructured transcriptional regulatory proteins.

Functional domain and motif analyses of androgen receptor coregulator ARA70 and its differential expression in prostate cancer

Androgen receptor (AR)-associated coregulator 70 (ARA70) was the first identified AR coregulator. However, its molecular mechanism and biological relevance to prostate cancer remain unclear. Here we show that ARA70 interacts with and promotes AR activity via the consensus FXXLF motif within the ARA70-N2 domain (amino acids 176-401). However, it does not promote AR activity via the classic LXXLL motif located at amino acids 92-96, although this classic LXXLL motif is important for ARA70 to interact with other receptors, such as PPARgamma. The molecular mechanisms by which ARA70 enhances AR transactivation involve the increase of AR expression, protein stability, and nuclear translocation. Furthermore, ARA70 protein is more frequently detected in prostate cancer specimens (91.74%) than in benign tissues (64.64%, p < 0.0001). ARA70 expression is also increased in high-grade prostate cancer tissues as well as the hormone-refractory LNCaP xenografts and prostate cancer cell lines. Because ARA70 can promote the antiandrogen hydroxyflutamide (HF)-enhanced AR transactivation, the increased ARA70 expression in hormone-refractory prostate tumors may confer the development of HF withdrawal syndrome, commonly diagnosed in patients with the later stages of prostate cancer. Because ARA70-N2 containing the AR-interacting FXXLF motif without coactivation function can suppress HF-enhanced AR transactivation in the hormone-refractory LNCaP cells, using the ARA70-N2 inhibitory peptide at the hormone refractory stage to battle the HF withdrawal syndrome may become an alternative strategy to treat prostate cancer.

Drug resistance reversal--are we getting closer?

Clinical drug resistance is a major barrier to overcome before chemotherapy can become curative for most patients presenting with metastatic cancer. Rational attempts to tackle clinical drug resistance need to be based on an understanding of the mechanisms involved; these are likely to be complex and multifactorial, and may be due to inadequate drug exposure or alterations in the cancer cell itself. This article reviews a number of strategies used to tackle drug resistance, focussing on work in our institution related to the treatment of ovarian cancer and resistance to platinum and taxane-based chemotherapy. Further progress towards drug resistance reversal will require a three-pronged approach, namely: the development of novel cytotoxics which exploit selectively expressed targets; modulation of resistance to conventional agents and, most importantly, a serious attempt to understand resistance mechanisms in tumour samples taken both pre- and post-chemotherapy.

Regulation of androgen receptor expression at the onset of functional overload in rat plantaris muscle

Skeletal muscle androgen receptor (AR) expression at the onset of functional overload (OV) has not been well described. It is also not known if overload and/or anabolic steroid differentially regulate AR expression. The purpose of this study was to examine AR gene expression at the onset of functional OV in rat plantaris muscle with and without nandrolone decanoate (ND) administration. The functional significance of AR protein induction was examined using skeletal α-actin promoter activity in transiently transfected CV-1 fibroblast cells. Male Sprague-Dawley rats (∼125 g) were functionally overloaded for 1, 3, 7, or 21 days. A subset of animals was given an ND (6 mg/kg) injection at day 0 and then overloaded for 3 days. Control animals underwent sham surgeries. AR protein concentration increased 106 and 279% after 7 and 21 days of OV, respectively. AR mRNA increased 430% after 7 days of OV. AR protein expression in C2C12 murine myotubes subjected to 1% chronic radial stretch for 18 h was elevated 101% compared with control. ND treatment increased AR protein concentration 1,300% compared with controls, and there was no additional effect when ND and OV were combined. ND with 3 days of OV treatment increased AR mRNA expression 50% compared with control. AR overexpression in transiently transfected CV-1 fibroblast cells increased -424 bp skeletal α-actin promoter activity 80 to 1,800% in a dose-dependent fashion. Co-overexpression of either serum response factor (SRF) or active RhoA with AR overexpression induced a synergistic 36- and 28-fold induction of skeletal α-actin promoter. Cotransfection of AR, SRF, and active RhoA induced 180-fold increase in skeletal α-actin promoter activity. In conclusion, AR protein expression is increased after 7 days of functional OV, and this induction is regulated pretranslationally. AR induction in conjunction with SRF and RhoA signaling may be an important regulator of gene expression during overload-induced muscle growth.

Isolation and identification of L-dopa decarboxylase as a protein that binds to and enhances transcriptional activity of the androgen receptor using the repressed transactivator yeast two-hybrid system

The AR (androgen receptor) is a ligand-regulated transcription factor, which belongs to the steroid receptor family and plays an essential role in growth and development of the prostate. Transcriptional activity of steroid receptors is modulated by interaction with co-regulator proteins and yeast two-hybrid analysis is commonly used to identify these steroid receptor-interacting proteins. However, a limitation of conventional two-hybrid systems for detecting AR protein partners has been that they only allow for analysis of the ligand- and DNA-binding domains of the receptor, as its NTD (N-terminal domain) possesses intrinsic transactivation activity. To identify AR N-terminus-interacting proteins, its NTD was used in the RTA (repressed transactivator) system, which is specifically designed for transactivator bait proteins and was shown to be suitable for two-hybrid analysis with the AR NTD. DDC (L-dopa decarboxylase) was detected multiple times as a novel AR-interacting protein, which was subsequently confirmed in vitro and in vivo. Furthermore, transient transfection of DDC in prostate cancer cells strongly enhanced ligand-dependent AR transcriptional activity, an effect that was antagonized using high concentrations of the anti-androgen bicalutamide. Glucocorticoid receptor activity was also strongly enhanced with DDC co-transfection, while oestrogen receptor activity was only mildly affected. Together, our data demonstrate that DDC interacts with AR to enhance steroid receptor transactivation, which may have important implications in prostate cancer progression.

P23 enhances the formation of the aryl hydrocarbon receptor-DNA complex

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that requires heterodimerization with its partner, the Ah receptor nuclear translocator (Arnt), for activation of transcription. The heterodimer specifically recognizes the dioxin response element (DRE), which contains a core sequence (5'-TNGCGTG-3'). This AhR/Arnt/DRE complex has been well characterized and can be observed readily by the gel shift assay. Human AhR and Arnt with a C-terminal histidine tag have been expressed functionally using a baculovirus expression system. However, after purification of these proteins using the metal resin, they are not able to bind the response element in a ligand-dependent manner unless crude extracts, such as the rabbit reticulocyte lysate (RRL), are reconstituted with these proteins. Proteins in the RRL are responsible for this restoration of the gel shift complex because the activity is sensitive to both heat and proteolytic treatments. We have examined whether hsp90 and p23 are among the protein factors in the RRL that are responsible for this activity. By performing fractionation studies using filtration devices and immunodepletion studies, we have selectively fractionated these proteins. Among all the fractions, the centricon-10 retentate, which contains 100% of p23 but no hsp90, possessed the most enriched activity. Purified bacterial-expressed p23 restored the gel shift complex; the mechanism was mediated at the heterodimerization step and was hsp90-dependent.

Environmental factors affecting transcription of the human L1 retrotransposon. II. Stressors

Retrotransposons have clearly molded the structure of the human genome. The reverse transcriptase coded for by long interspersed nuclear elements (LINEs) accounts for 35% of the human genome, with 8-9 x 10(5) copies of the most common human LINE element, L1Hs. Retrotransposons cycle through an RNA intermediate with transcription as the rate limiting step. Because various retrotransposons have been demonstrated to be induced by environmental stimuli, we investigated the response of the L1Hs promoter to various agents. L1Hs promoter activity was analyzed by transfecting an L1Hs-expressing cell line with plasmids containing one of two L1Hs promoters fused to the LacZ reporter gene. L1Hs promoter activity was then monitored with a beta-galactosidase assay. Treatment with UV light and heat shock resulted in a small increase in beta-galactosidase activity from one promoter, while treatment with tetradecanoylphorbol 13-acetate resulted in small increases in beta-galactosidase activity from both promoters. No increase in beta-galactosidase activity was observed after exposure to X-rays or hydrogen peroxide.

Androgen-independent prostate cancer: potential role of androgen and ErbB receptor signal transduction crosstalk

In prostate cancer (PC), increasing evidence suggests that androgen receptor (AR) signalling is functional under conditions of maximal androgen blockade. PC cells survive and proliferate in the altered hormonal environment possibly by interactions between growth factor-activated pathways and AR signalling. The present review article summarizes the current evidence of this crosstalk and focuses on the interactions among the ErbB receptor network, its downstream pathways, and the AR. The potential role of this crosstalk in the development of androgen independence and in relation to antiandrogen therapy is discussed. Such interactions provide insight into possible complementary or additional strategies in the management of PC.

Gene expression is differentially regulated in the epididymis after orchidectomy

The epididymis is the site for the transport, maturation, and storage of spermatozoa. Regulation of epididymal structure and function is highly dependent on the ipsilateral testis. At the molecular level, however, few studies have been undertaken to determine which genes are expressed in the epididymis under testicular regulation. The goal of this study was to identify genes for which expression is regulated after orchidectomy, both throughout the epididymis and in a segment-specific manner. Microarrays spotted with 474 rat cDNAs were used to examine gene expression changes over the first 7 d post orchidectomy in the initial segment, caput, corpus, and cauda epididymidis of the adult Brown Norway rat. Using k-means cluster analysis, we show that four patterns of gene expression are activated in each epididymal segment over the first week following orchidectomy. Transient up-regulation of gene expression in the epididymis after orchidectomy is described for the first time. Potential androgen-repressed genes, including Gpx-1, show increased expression in the epididymis after orchidectomy. Several glutathione-S-transferases and calcium-binding proteins decline throughout the epididymis after orchidectomy, indicating that these may be novel androgen-regulated epididymal genes. Other genes coding for metabolism-associated proteins, transporters, and alpha-1 acid glycoprotein show segment-specific regulation in the epididymis after orchidectomy. Finally, we describe the expression of the previously uncharacterized heat shock proteins, and apoptosis-associated genes in the epididymis after orchidectomy. Thus, gene expression in the epididymis is differentially affected over time after orchidectomy. These results provide novel insight into androgen-dependent and segment-specific epididymal function.

Regulation of androgen receptor activity by the nuclear receptor corepressor SMRT

Androgen receptor (AR) is a hormone-regulated transcription factor that mediates a wide array of biological processes including sexual differentiation, spermatogenesis, and prostate cancer progression. The transcriptional activity of AR and other members of the nuclear receptor superfamily are modulated by coregulatory proteins. In this study, we have investigated the regulation of AR transcriptional activity by the silencing mediator for retinoid and thyroid hormone receptors (SMRT). We found that AR possesses an intrinsic transcriptional repression activity, and AR interacts directly with SMRT. One interacting surface on AR is mapped to the ligand-binding domain, and the presence of a DNA binding/hinge region enhances this interaction. The binding surface on SMRT is mapped to the C-terminal ID2 region, and mutation in the ID2 corepressor motif inhibits the interaction. Overexpression of SMRT inhibits dihydrotestosterone-dependent transactivation by AR and further suppresses the antiandrogen flutamide-mediated inhibition of AR activity. We provide evidence to suggest that the mechanisms of SMRT-mediated inhibition of AR activity involves inhibition of AR N/C interaction and competition with the p160 coactivator. Our data establish a significant role of SMRT in modulating AR transcriptional activity.

C-terminal Hsp-interacting protein slows androgen receptor synthesis and reduces its rate of degradation

The androgen receptor (AR) is a member of the nuclear receptor superfamily that requires the action of molecular chaperones for folding and hormone binding. C-terminal Hsp-interacting protein (Chip) is a cochaperone that interacts with Hsp70 and Hsp90 molecular chaperones via a tetratricopeptide domain and inhibits chaperone-dependent protein folding in vitro. Chip also stimulates protein degradation by acting as an E3 ubiquitin ligase via a modified ring finger domain called a U box. We analyzed whether Chip affected AR levels using a transient transfection strategy. Chip overexpression led to a large decrease in AR steady state levels and increased levels of AR ubiquitinylation. However, Chip effects were not fully reversed by proteasome inhibitors, suggesting that mechanisms alternative to or in addition to proteasome-mediated degradation were involved. This hypothesis was supported by the finding that Chip overexpression reduced the rate of AR degradation, consistent with an effect on AR folding, perhaps leading to aggregation. The possibility that Chip affected AR folding was further supported by the finding that the effects of exogenous Chip were reproduced by a mutant lacking the U box. These results are discussed in terms of the role played by molecular chaperones in AR biogenesis.

Regulation of signaling protein function and trafficking by the hsp90/hsp70-based chaperone machinery

Nearly 100 proteins are known to be regulated by hsp90. Most of these substrates or "client proteins" are involved in signal transduction, and they are brought into complex with hsp90 by a multiprotein hsp90/hsp70-based chaperone machinery. In addition to binding substrate proteins at the chaperone site(s), hsp90 binds cofactors at other sites that are part of the heterocomplex assembly machinery as well as immunophilins that connect assembled substrate*hsp90 complexes to protein-trafficking systems. In the 5 years since we last reviewed this subject, much has been learned about hsp90 structure, nucleotide-binding, and cochaperone interactions; the most important concept is that ATP hydrolysis by an intrinsic ATPase activity results in a conformational change in hsp90 that is required to induce conformational change in a substrate protein. The conformational change induced in steroid receptors is an opening of the steroid-binding cleft so that it can be accessed by steroid. We have now developed a minimal system of five purified proteins-hsp90, hsp70, Hop, hsp40, and p23- that assembles stable receptor*hsp90 heterocomplexes. An hsp90*Hop*hsp70*hsp40 complex opens the cleft in an ATP-dependent process to produce a receptor*hsp90 heterocomplex with hsp90 in its ATP-bound conformation, and p23 then interacts with the hsp90 to stabilize the complex. Stepwise assembly experiments have shown that hsp70 and hsp40 first interact with the receptor in an ATP-dependent reaction to produce a receptor*hsp70*hsp40 complex that is "primed" to be activated to the steroid-binding state in a second ATP-dependent step with hsp90, Hop, and p23. Successful use of the five-protein system with other substrates indicates that it can assemble signal protein*hsp90 heterocomplexes whether the substrate is a receptor, a protein kinase, or a transcription factor. This purified system should facilitate understanding of how eukaryotic hsp70 and hsp90 work together as essential components of a process that alters the conformations of substrate proteins to states that respond in signal transduction.

A novel androgen receptor mutant, A748T, exhibits hormone concentration-dependent defects in nuclear accumulation and activity despite normal hormone-binding affinity

Functional analysis of androgen receptor (AR) gene mutations isolated from prostate cancer has led to the identification of residues that play important roles in the structure and function of the receptor. Here we report the characteristics of a novel AR mutation A748T located in helix 5 of the ligand-binding domain, which was identified in metastatic prostate cancer. Despite a normal hormone-binding affinity, A748T causes hormone concentration-dependent defects in nuclear accumulation and transcriptional activation. Moreover, when equivalent amounts of DNA are transfected, the mutant is expressed at much lower levels than the wild-type AR (ARWT). Treatment with geldanamycin to disrupt receptor-heat shock protein complexes rapidly decreases the levels of ARWT but not A748T, suggesting that the lower expression and rapid degradation rate of A748T is due to weaker interactions with heat shock proteins. Further analysis revealed that hormone dissociates from A748T five times faster than from ARWT. Loss of the ability to form stable amino/carboxyl-terminal interactions causes accelerated dissociation rates in some AR mutants. However, A748T exhibits normal amino/carboxyl-terminal interactions at high hormone concentrations, suggesting that the mutation alters interactions with ligand. Consistent with this conclusion, our structural model predicts that A748T disrupts crucial contact points with ligand, thereby altering the conformation of the ligand-binding domain.

Hsp90 inhibitors as novel cancer chemotherapeutic agents

Heat shock protein 90 (Hsp90) is a molecular chaperone whose association is required for the stability and function of multiple mutated, chimeric and over-expressed signaling proteins that promote the growth and/or survival of cancer cells. Hsp90 client proteins include mutated p53, Bcr-Abl, Raf-1, Akt, ErbB2 and hypoxia-inducible factor 1 alpha (HIF-1 alpha). Hsp90 inhibitors, by interacting specifically with a single molecular target, cause the destabilization and eventual degradation of Hsp90 client proteins, and they have shown promising antitumor activity in preclinical model systems. One Hsp90 inhibitor, 17-allylaminogeldanamycin (17AAG), is currently in phase I clinical trial. Because of the chemoprotective activity of several proteins that are Hsp90 clients, the combination of an Hsp90 inhibitor with a standard chemotherapeutic agent could dramatically increase the in vivo efficacy of the therapeutic agent.

Overexpression of yeast Hsp110 homolog Sse1p suppresses ydj1-151 thermosensitivity and restores Hsp90-dependent activity

The Saccharomyces cerevisiae heat-shock protein (Hsp)40, Ydj1p, is involved in a variety of cellular activities that control polypeptide fate, such as folding and translocation across intracellular membranes. To elucidate the mechanism of Ydj1p action, and to identify functional partners, we screened for multicopy suppressors of the temperature-sensitive ydj1-151 mutant and identified a yeast Hsp110, SSE1. Overexpression of Sse1p also suppressed the folding defect of v-Src kinase in the ydj1-151 mutant and partially reversed the alpha-factor translocation defect. SSE1-dependent suppression of ydj1-151 thermosensitivity required the wild-type ATP-binding domain of Sse1p. However, the Sse1p mutants maintained heat-denatured firefly luciferase in a folding-competent state in vitro and restored human androgen receptor folding in sse1 mutant cells. Because the folding of both v-Src kinase and human androgen receptor in yeast requires the Hsp90 complex, these data suggest that Ydj1p and Sse1p are interacting cochaperones in the Hsp90 complex and facilitate Hsp90-dependent activity.

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.

The molecular chaperone Hsp90 mediates heme activation of the yeast transcriptional activator Hap1

Hsp90 plays critical roles in the proper functioning of a wide array of eukaryotic signal transducers such as steroid receptors and tyrosine kinases. Hap1 is a naturally occurring substrate of Hsp90 in Saccharomyces cerevisiae. Hap1 transcriptional activity is precisely and stringently controlled by heme. Previous biochemical studies suggest that in the absence of heme, Hap1 is bound to Hsp90 and other proteins, forming a higher order complex termed HMC (high molecular weight complex), and is repressed. Heme promotes the disruption of the HMC and activates Hap1, permitting Hap1 to bind to DNA with high affinity and to stimulate transcription. By lowering the expression levels of wild-type Hsp90, using a highly specific Hsp90 inhibitor, and by examining the effects of various Hsp90 mutants on Hap1, we show that Hsp90 is critical for Hap1 activation by heme. Furthermore, we show that many Hsp90 mutants exert differential effects on Hap1 and steroid receptors. Notably, mutant G313N weakens Hsp90 steroid receptor interaction but strongly enhances Hsp90-Hap1 interaction and increases Hap1 resistance to protease digestion. Additionally, we found that a heme-independent Hap1 mutant still depends on Hsp90 for high activity. These experiments together suggest that Hsp90 promotes Hap1 activation by inducing or maintaining Hap1 in a transcriptionally active conformation.

Identification and characterization of ART-27, a novel coactivator for the androgen receptor N terminus

The androgen receptor (AR) is a ligand-regulated transcription factor that stimulates cell growth and differentiation in androgen-responsive tissues. The AR N terminus contains two activation functions (AF-1a and AF-1b) that are necessary for maximal transcriptional enhancement by the receptor; however, the mechanisms and components regulating AR transcriptional activation are not fully understood. We sought to identify novel factors that interact with the AR N terminus from an androgen-stimulated human prostate cancer cell library using a yeast two-hybrid approach designed to identify proteins that interact with transcriptional activation domains. A 157-amino acid protein termed ART-27 was cloned and shown to interact predominantly with the AR(153-336), containing AF-1a and a part of AF-1b, localize to the nucleus and increase the transcriptional activity of AR when overexpressed in cultured mammalian cells. ART-27 also enhanced the transcriptional activation by AR(153-336) fused to the LexA DNA-binding domain but not other AR N-terminal subdomains, suggesting that ART-27 exerts its effect via an interaction with a defined region of the AR N terminus. ART-27 interacts with AR in nuclear extracts from LNCaP cells in a ligand-independent manner. Interestingly, velocity gradient sedimentation of HeLa nuclear extracts suggests that native ART-27 is part of a multiprotein complex. ART-27 is expressed in a variety of human tissues, including sites of androgen action such as prostate and skeletal muscle, and is conserved throughout evolution. Thus, ART-27 is a novel cofactor that interacts with the AR N terminus and plays a role in facilitating receptor-induced transcriptional activation.

Effect of geldanamycin on androgen receptor function and stability

In the ligand-binding inactive state, the steroid receptor heterocomplex contains Hsp90, Hsp70, high-molecular weight immunophilins, and other proteins. Hsp90 acts in association with co-chaperones to maintain the native state of the receptor within the cells. It was reported earlier that Hsp90 might not be as important for the androgen receptor (AR) activity as for the glucocorticoid receptor (GR) and the progesterone receptor (PR) activities. We used the Hsp90 inhibitor geldanamycin (GA) to explore the role of Hsp90 in the function of the AR heterocomplex. GA selectively binds to Hsp90 and inhibits its activity, leading to the loss of steroid receptor activity, and frequently, its degradation. In our study, LNCaP prostate cancer cells were treated with GA for 30 minutes or 24 hours, in the presence of mibolerone, a synthetic androgen. GA reduced the androgen-induced AR protein levels to 15% after 24 hours of treatment. Several androgen up-regulated genes, including immunophilin FKBP51 and prostate specific antigen (PSA), were reduced by GA treatment. In cells treated with GA after transfection with a PSA promoter or an androgen response element-driven reporter gene, AR-mediated transactivation of reporter gene expression was reversibly inhibited by GA. Loss of androgen-binding ability and AR levels was attributed to reduced transcription of AR-regulated gene expression. Degradation rate of 35S-labeled AR was significantly increased by GA in the presence or absence of mibolerone. GA induced the degradation of AR through the proteasomal pathway. AR in cells treated with proteasomal inhibitor lactacystin, was insoluble in Nonidet P-40 (NP40)-based buffer and could not restore the androgen-binding ability. We report here that GA treatment disrupted both hormone-binding activity and receptor protein stability, resulting in a dramatic loss of androgen-induced gene activation. These results show that Hsp90 activity is important for both the chaperone-mediated folding of the AR into a high-affinity ligand-binding conformation and the functional activity of the AR.

HSP90 as a new therapeutic target for cancer therapy: the story unfolds

Current anticancer drug development strategies involve identifying novel molecular targets which are crucial for tumourigenesis. The molecular chaperone heat shock protein (HSP) 90 is of interest as an anticancer drug target because of its importance in maintaining the conformation, stability and function of key oncogenic client proteins involved in signal transduction pathways leading to proliferation, cell cycle progression and apoptosis, as well as other features of the malignant phenotype such as invasion, angiogenesis and metastasis. The natural product HSP90 inhibitors geldanamycin and radicicol exert their antitumour effect by inhibiting the intrinsic ATPase activity of HSP90, resulting in degradation of HSP90 client proteins via the ubiquitin proteosome pathway. Anticancer selectivity may derive from the simultaneous combinatorial effects of HSP90 inhibitors on multiple cancer targets and pathways. 17-allylamino, 17-demethoxygeldanamycin (17AAG), a geldanamycin derivative, showed good activity and cancer selectivity in preclinical models and has now progressed to Phase I clinical trial in cancer patients with encouraging initial results. Phase II trials including combination studies with cytotoxic agents are now being planned and these should allow the therapeutic activity of 17AAG to be determined. Second generation HSP90 inhibitors may be designed to overcome some of the drawbacks of 17AAG, including limited oral bioavailability and solubility. They could also be engineered to target specific functions of HSP90, which may not only provide greater molecular selectivity and clinical benefit but may also increase understanding of the complex functions of this molecular chaperone. HSP90 inhibitors provide proof of concept for drugs directed at HSP90 and protein folding and this principle may be applicable to other medical conditions involving protein aggregation and stability.

Bakers yeast rises to the challenge: reconstitution of mammalian steroid receptor signalling in S. cerevisiae

Steroid hormones are an important class of signalling molecule, regulating a diverse range of processes in metazoan eukaryotes. The actions of these hormones are mediated by intracellular receptor proteins that act as ligand-activated transcription factors. The ability to reconstitute steroid receptor signalling in the budding yeast, Saccharomyces cerevisiae, provides a genetically tractable model system in which to investigate steroid receptor structure and function. Through targeted disruption and genetic screening, an increasing number of genes have been identified that are likely to have a role in steroid receptor action.

Androgen receptor signalling: comparative analysis of androgen response elements and implication of heat-shock protein 90 and 14-3-3eta

Androgen receptor (AR) signalling was analysed using as models the cysteine-rich secretory protein-1 (CRISP-1) and CRISP-3 gene promoters, which are differentially regulated by androgen in vivo and contain multiple potential androgen response elements. Using electrophoretic mobility shift assay, we identified several elements with differing affinities for the AR at positions -3706, -1270, -1253 and -350 of the CRISP-1 promoter and at positions -369 and -349 of the CRISP-3 promoter. The strongest binding was observed for the -1253 element of CRISP-1. In transactivation assays using a PC-3 cell line stably transfected with the AR (PC-3/AR), the -1253 element placed as two or four copies upstream of the TK minimal promoter yielded a strong induction of luciferase reporter gene activity in the presence of the androgen methyltrienolone (R1881). In the context of the CRISP promoters a 2-fold induction by R1881 was measured for the CRISP-3 upstream region whereas only limited effects were noted for the CRISP-1 upstream region. The androgenic stimulation of the p(-1253 ARE)(4x)-TK-luciferase reporter construct was dose-dependently inhibited by geldanamycin and radicicol, two compounds that selectively interact with the chaperone protein, heat-shock protein 90. Cotransfection with an expression vector for the 14-3-3eta protein markedly enhanced the androgen-dependent stimulation. These results emphasize the influence of promoter context on androgen regulation and the importance of AR-associated proteins.

Functional interaction of human Cdc37 with the androgen receptor but not with the glucocorticoid receptor

Cdc37 is a molecular chaperone closely associated with the folding of protein kinases. Results from studies using a yeast model system showed that it was also important for activation of the human androgen receptor (AR). Based on results from the yeast model system (Fliss, A. E., Fang, Y., Boschelli, F., and Caplan, A. J. (1997) Mol. Biol. Cell 8, 2501-2509), we initiated studies to address whether AR and Cdc37 interact with each other in animal cell systems. Our results show that Cdc37 binds to AR but not to glucocorticoid receptors (GR) synthesized in rabbit reticulocyte lysates. This binding occurs via the ligand-binding domain of the AR in a manner that is partially dependent on Hsp90 and the presence of hormone. Further studies using the yeast system showed that Cdc37 is not interchangeable with Hsp90, suggesting that it functions at a distinct step in the activation pathway. Expression of a dominant negative form of Cdc37 in animal cells down-regulates full-length AR but has very little effect on an AR truncation lacking the ligand-binding domain or full-length GR. These results reveal differences in the mechanisms by which AR and GR become active transcription factors and strengthen the notion that Cdc37 has a wider range of polypeptide clients than was realized previously.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

Heat-shock factor-1, steroid hormones, and regulation of heat-shock protein expression in the heart

Heat-shock proteins (HSPs) are an important family of endogenous, protective proteins. Overexpression of HSPs is protective against cardiac injury. Previously, we observed that dexamethasone activated heat-shock factor-1 (HSF-1) and induced a 60% increase in HSP72 in adult cardiac myocytes. The mechanism responsible for this effect of dexamethasone is unknown. Because HSP90 is known to bind the intracellular hormone receptors, we postulated that the interaction between HSP90, the receptors, and HSF was an important element in activation of HSF-1 by hormones. We hypothesized that there is an equilibrium between HSP90 and the various receptors/enzymes that it binds and that alteration in levels of certain hormones will alter the intracellular distribution of HSP90 and activate HSF-1. We report that, in adult cardiac myocytes, HSF-1 coimmunoprecipitates with HSP90. HSP90 redistributes in cardiac myocytes after treatment with 17beta-estradiol or progesterone. Estrogen and progesterone activate HSF-1 in adult male isolated cardiac myocytes, and this is followed by an increase in HSP72 protein. Testosterone had no effect on HSP levels; however, no androgen receptor was found in cardiac myocytes; therefore, testosterone would not be expected to effect binding of HSP90 to HSF. Geldanamycin, which inactivates HSP90 and prevents it from binding to receptors, activates HSF-1 and stimulates HSP72 synthesis. Activation of HSF-1 by steroid hormones, resulting from a change in the interaction of HSP90 and HSF-1, represents a novel pathway for regulating expression of HSPs. These findings may explain some of the gender differences in cardiovascular disease.

Inhibiting proteasomes in human HepG2 and LNCaP cells increases endogenous androgen receptor levels

Treating HepG2 cells with MG132 for 4 h to inhibit proteasomal activity increased androgen receptor immunoreactivity in two major bands with molecular weights of 102 and 110 kDa by 77% each (P < 0. 05). MG132 treatment also increased the overall level of polyubiquitinated proteins between 66 and 220 kDa by 140% (P < 0.05). Antiubiquitin immunoreactivity comigrating with the androgen receptor bands was also increased by MG132 treatment. Two other proteasome inhibitors, lactacystin and epoxomycin, caused similar increases in the androgen receptor in HepG2 cells. Proteosome-inhibition studies conducted in LNCaP cells also showed that the two major androgen receptor bands with molecular weights of 102 and 110 kDa were increased by 85 and 115%, respectively (P < 0. 05 for both) by MG132 treatment. Overall levels of polyubiquitinated proteins with molecular weights between 66 and 220 kDa increased 365%. Ubiquitin immunoreactivity comigrating with the androgen receptor bands was also significantly increased. Thus inhibiting proteasomes in two human androgen-responsive cell lines increases endogenous androgen receptor levels as well as androgen receptor-associated ubiquitin-modified immunoreactivity. The regulation of steady-state levels of endogenous androgen receptor by proteasomal degradation could be involved in its rapid turnover in the absence of ligand and would provide a mechanism for limiting androgen responses. A PEST sequence similar to one in the vitamin D receptor is present in the hinge region of all known mammalian androgen receptors, suggesting that it may function in proteasome-mediated androgen receptor turnover.

Synthesis and evaluation of geldanamycin-testosterone hybrids

Geldanamycin (GDM) binds to the Hsp90 chaperone protein resulting in the degradation of several important signaling proteins. A series of GDM-testosterone linked hybrids has been synthesized and evaluated for activity against prostate cancer cell lines. The hybrid with the greatest activity exhibits potent and selective cytotoxicity against prostate cancer cells containing the androgen receptor.

Control of estrogen receptor ligand binding by Hsp90

The molecular chaperone Hsp90 interacts with unliganded steroid hormone receptors and regulates their activity. We have analyzed the function of yeast and mammalian Hsp90 in regulating the ability of the human estrogen receptor (ER) to bind ligands in vivo and in vitro. Using the yeast system, we show that the ER expressed in several different hsp82 mutant strains binds reduced amounts of the synthetic estrogen diethylstilbestrol compared to the wild type. This defect in hormone binding occurs without any significant change in the steady state levels of ER protein. To analyze the role of mammalian Hsp90, we synthesized the human ER in rabbit reticulocyte lysates containing geldanamycin, an Hsp90 inhibitor. At low concentrations of geldanamycin we observed reduced levels of hormone binding by the ER. At higher concentrations, we found reduced synthesis of the receptor. These data indicate that Hsp90 functions to maintain the ER in a high affinity hormone-binding conformation.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

Domain requirements of DnaJ-like (Hsp40) molecular chaperones in the activation of a steroid hormone receptor

DnaJ-like proteins function in association with Hsp70 molecular chaperones to facilitate protein folding. We previously demonstrated that a yeast DnaJ-like protein, Ydj1p, was important for activation of heterologously expressed steroid hormone receptors (Caplan, A. J., Langley, E., Wilson, E. M., and Vidal, J. (1995) J. Biol. Chem. 270, 5251-5257). In the present study, we analyzed Ydj1p function by assaying hormone binding to the human androgen receptor (AR) heterologously expressed in yeast. We analyzed hormone binding in strains that were wild type or deleted for the YDJ1 gene. In the deletion mutant, the AR did not bind hormone to the same extent as the wild type. Introduction of mutant forms of Ydj1p to the deletion strain revealed that the J-domain is necessary but not sufficient for Ydj1p action, and that other domains of the protein are also functionally important. Of three human DnaJ-like proteins introduced into the deletion mutant, only Hdj2, which displays full domain conservation with Ydj1p, suppressed the hormone binding defect of the deletion mutant. By comparison of the domains shared by these three human proteins, and with mutants of Ydj1p that were functional, it was deduced that the cysteine-rich zinc binding domain is important for Hdj2/Ydj1p action in hormone receptor function. A model for the mechanism of DnaJ-like protein action is discussed.

Contribution of N- and C-terminal domains to the function of Hsp90 in Saccharomyces cerevisiae

The molecular chaperone Hsp90 is a regulatory component of some key signalling proteins in the cytosol of eukaryotic cells. For some of these functions, its interaction with co-chaperones is required. Limited proteolysis defined stable folded units of Hsp90. Both an N-terminal (N210) and a C-terminal (262C) fragment interact with non-native substrate proteins in vitro, but with different specificity and ATP dependence. Here, we analysed the functional properties of these Hsp90 fragments in vivo and in vitro. We determined their influence on the general viability and cell growth of Saccharomyces cerevisiae. Expression of N210 or 262C resulted in a dominant-negative phenotype in several yeast strains tested. Their expression was not toxic, but inhibited cell growth. Further, both were unable to restore viability to Hsp90-depleted cells. In addition, N210 and 262C influence the maturation of Hsp90 substrates, such as the glucocorticoid receptor and pp60v-Src kinase. Specifically, 262C forms partially active chaperone complexes, leading to an arrest of the chaperoned substrate at a certain stage of its maturation cycle. This demonstrates the requirement of a sophisticated and cofactor-regulated interplay between N- and C-terminal activities for Hsp90 function in vivo.

Ubc9 interacts with the androgen receptor and activates receptor-dependent transcription

Ubc9, a homologue of the class E2 ubiquitin-conjugating enzymes, has recently been shown to catalyze conjugation of a small ubiquitin-like molecule-1 (SUMO-1) to a variety of target proteins. SUMO-1 modifications have been implicated in the targeting of proteins to the nuclear envelope and certain intranuclear structures and in converting proteins resistant to ubiquitin-mediated degradation. In the present work, we find that Ubc9 interacts with the androgen receptor (AR), a member of the steroid receptor family of ligand-activated transcription factors. In transiently transfected COS-1 cells, AR-dependent but not basal transcription is enhanced by the coexpression of Ubc9. The N-terminal half of the AR hinge region containing the C-terminal part of the bipartite nuclear localization signal is essential for the interaction with Ubc9. Deletion of this part of the nuclear localization signal, which does not completely prevent the transfer of AR to the nucleus, abolishes the AR-Ubc9 interaction and attenuates the transcriptional response to cotransfected Ubc9. The C93S substitution of Ubc9, which prevents SUMO-1 conjugation by abrogating the formation of a thiolester bond between SUMO-1 and Ubc9, does not influence the capability of Ubc9 to stimulate AR-dependent transactivation, implying that Ubc9 is able to act as an AR coregulator in a fashion independent of its ability to catalyze SUMO-1 conjugation.

Hsp90's secrets unfold: new insights from structural and functional studies

Hsp90 is a molecular chaperone associated with the folding of signal-transducing proteins, such as steroid hormone receptors and protein kinases. Results from recent studies have shed light on the structure of Hsp90 and have demonstrated that it can bind to and hydrolyse ATP. Hsp90 forms several discrete subcomplexes, each containing distinct groups of co-chaperones that function in folding pathways. Although Hsp90 is not generally involved in the folding of nascent polypeptide chains, there is a growing list of proteins whose activity depends on its function, including heat-shock factor. This review addresses recent developments in our understanding of the structure and function of Hsp90.

Investigation of steroid receptor function in the budding yeast Saccharomyces cerevisiae

Steroid hormones are small lipophilic molecules that control a wide range of responses in both the developing and adult organism. The actions of these molecules are mediated by soluble receptor proteins that function as hormone-activated transcription factors. The first steroid receptors were expressed in the yeast Saccharomyces cerevisiae over 10 years ago, and to date virtually all the classical steroid receptors, together with a number of non-steroid members of the nuclear receptor superfamily, have been expressed in yeast. The ability to reconstitute steroid receptor signalling in yeast cells by co-expression of the receptor protein and a reporter gene driven by the appropriate hormone response element has presented researchers with a powerful model system for investigating receptor action. In this review, the use of yeast-based steroid receptor transactivation assays to investigate the roles of molecular chaperones, the mechanisms of DNA binding and gene activation, and the functional properties of hormone mimics will be discussed.

Characterization of two nuclear androgen receptors in Atlantic croaker: comparison of their biochemical properties and binding specificities

Two distinct androgen receptors (ARs) with different characteristics were identified in the brain and ovary of Atlantic croaker, Micropogonias undulatus. A nuclear AR, AR1, was identified in the brain that had high affinity binding sites for testosterone (T; Kd, 1.1 +/- 0.15 nM; binding capacity, 1.4 +/- 0.14 pmol/g tissue; n = 16). A second nuclear AR, AR2, was found in the ovary that had high affinity binding sites for 5alpha-dihydrotestosterone (DHT; Kd, 0.62 +/- 0.1 nM; binding capacity, 0.38 +/- 0.06 pmol/g tissue; n = 14). AR2 has physiochemical properties similar to those of other vertebrate ARs. AR2 has high affinity binding for a broad spectrum of natural and synthetic androgens, including 17alpha-methyl-5alpha-dihydrotestosterone, which has a relative binding affinity of DHT = 100% > T > mibolerone > 11-ketotestosterone = 16%, a rapid association (t1/2, 44 min) and a slow dissociation (t1/2, 45 h) rate, as well as specific binding to purified DNA. The cytosolic AR2 interacts with heat shock proteins in a manner similar to other steroid receptors, as sodium molybdate stabilizes the receptor, and it has a 7.4-7.8S sedimentation coefficient in a 5-20% sucrose gradient. In contrast, AR1 is highly specific for only a few androgens, with T = 100% relative binding affinity >> DHT >> 11-ketotestosterone > mibolerone > 17alpha-methyl-5alpha-dihydrotestosterone = 0, has rapid association (t1/2, 15 min) and dissociation (t1/2, 2.6 +/- 0.7 h) rates, and has specific binding to purified DNA upon heat activation. The cytosolic binding component sediments at 5.6-5.7S in a 5-20% sucrose gradient and is not affected by sodium molybdate, which suggests that AR1 does not interact with heat shock proteins in the usual manner. This is the first report of the presence of two different nuclear ARs displaying markedly different steroid binding specificities within a single vertebrate species.

Synergistic activation of yeast-expressed rat androgen receptor by modulators of protein kinase-A

We have employed a yeast (Saccharomyces cerevisiae) based rat androgen receptor expression system to examine the cross-talk between different signalling pathways. We report here the synergistic modulation of androgen regulated transcriptional activation of beta-galactosidase reporter activity by the activators of protein kinase-A, like forskolin and 8-bromo-cyclic AMP. A similar ligand-dependent enhancement of reporter activity compared to a DHT treated control has been noticed with okadaic acid, which is a potent inhibitor of protein phosphatase. The activation could be blocked by protein kinase-A/C inhibitor, H7. Forskolin treatment neither altered levels of receptor mRNA nor [3H]R1881 binding to the receptor. Although it promotes binding of receptor to an androgen response element, forskolin was unable to activate subsequent interaction with the transcription machinery in the absence of androgen. Additionally, the synergistic actions of these activators were independent of the degree of androgen response element occupancy. Anti-androgens, cyproterone acetate and flutamide, which failed to exhibit antagonistic behaviour with yeast expressed receptor, were able to antagonize only the forskolin mediated augmentation of reporter activity. Finally, analyses of mutants established the role of DNA and steroid binding domains of receptor for this synergism.