Hormone-dependent transactivation by the human androgen receptor is regulated by a dnaJ protein

Optimal functional levels of activation-induced deaminase specifically require the Hsp40 DnaJa1

The enzyme activation-induced deaminase (AID) deaminates deoxycytidine at the immunoglobulin genes, thereby initiating antibody affinity maturation and isotype class switching during immune responses. In contrast, off-target DNA damage caused by AID is oncogenic. Central to balancing immunity and cancer is AID regulation, including the mechanisms determining AID protein levels. We describe a specific functional interaction between AID and the Hsp40 DnaJa1, which provides insight into the function of both proteins. Although both major cytoplasmic type I Hsp40s, DnaJa1 and DnaJa2, are induced upon B-cell activation and interact with AID in vitro, only DnaJa1 overexpression increases AID levels and biological activity in cell lines. Conversely, DnaJa1, but not DnaJa2, depletion reduces AID levels, stability and isotype switching. In vivo, DnaJa1-deficient mice display compromised response to immunization, AID protein and isotype switching levels being reduced by half. Moreover, DnaJa1 farnesylation is required to maintain, and farnesyltransferase inhibition reduces, AID protein levels in B cells. Thus, DnaJa1 is a limiting factor that plays a non-redundant role in the functional stabilization of AID.

Analysis of interdomain interactions of the androgen receptor

High-affinity binding of testosterone or dihydrotestosterone to the androgen receptor (AR) triggers the androgen-dependent AR NH2- and carboxyl-terminal (N/C) interaction between the AR NH2-terminal FXXLF motif and the activation function 2 (AF2) hydrophobic binding surface in the ligand-binding domain. The functional importance of the AR N/C interaction is supported by naturally occurring loss-of-function AR AF2 mutations where AR retains high-affinity androgen binding but is defective in AR FXXLF motif binding. Ligands with agonist activity in vivo such as testosterone, dihydrotestosterone, and the synthetic anabolic steroids induce the AR N/C interaction and increase AR transcriptional activity in part by slowing the dissociation rate of bound ligand and stabilizing AR against degradation. AR ligand-binding domain competitive antagonists inhibit the agonist-dependent AR N/C interaction. Although the human AR N/C interaction is important for transcriptional activity, it has an inhibitory effect on transcriptional activity from AF2 by competing for p160 coactivator LXXLL motif binding. The primate-specific AR coregulatory protein, melanoma antigen gene protein-A11 (MAGE-A11), modulates the AR N/C interaction through a direct interaction with the AR FXXLF motif. Inhibition of AF2 transcriptional activity by the AR N/C interaction is relieved by AR FXXLF motif binding to the F-box region of MAGE-11. Described here are methods to measure the androgen-dependent AR N/C interdomain interaction and the influence of transcriptional coregulators.

Molecular mechanisms of androgen action--a historical perspective

Androgens and the androgen receptor (AR) are indispensable for expression of the male phenotype. The two most important androgens are testosterone and 5α-dihydrotestosterone. The elucidation of the mechanism of androgen action has a long history starting in the 19th century with the classical experiments by Brown-Séquard. In the 1960s the steroid hormone receptor concept was established and the AR was identified as a protein entity with a high affinity and specificity for testosterone and 5α-dihydrotestosterone. In addition, the enzyme 5α-reductase type 2 was discovered and found to catalyze the conversion of testosterone to the more active metabolite 5α-dihydrotestosterone. In the second half of the 1980s, the cDNA cloning of all steroid hormone receptors, including that of the AR, has been another milestone in the whole field of steroid hormone action. Despite two different ligands (testosterone and 5α-dihydrotestosterone), only one AR cDNA has been identified and cloned. The AR (NR3C4) is a ligand-dependent transcription factor and belongs to the family of nuclear hormone receptors which has 48 members in human. The current model for androgen action involves a multistep mechanism. Studies have provided insight into AR association with co-regulators involved in transcription initiation and on intramolecular interactions of the AR protein during activation. Knowledge about androgen action in the normal physiology and in disease states has increased tremendously after cloning of the AR cDNA. Several diseases, such as androgen insensitivity syndrome (AIS), prostate cancer and spinal bulbar muscular atrophy (SBMA), have been shown to be associated with alterations in AR function due to mutations in the AR gene or dysregulation of androgen signalling. A historical overview of androgen action and salient features of AR function in normal and disease states are provided herein.

The mammalian Hsp40 ERdj3 requires its Hsp70 interaction and substrate-binding properties to complement various yeast Hsp40-dependent functions

Heat shock proteins of 70 kDa (Hsp70s) and their J domain-containing Hsp40 cofactors are highly conserved chaperone pairs that facilitate a large number of cellular processes. The observation that each Hsp70 partners with many J domain-containing proteins (JDPs) has led to the hypothesis that Hsp70 function is dictated by cognate JDPs. If this is true, one might expect highly divergent Hsp70-JDP pairs to be unable to function in vivo. However, we discovered that, when a yeast cytosolic JDP, Ydj1, was targeted to the mammalian endoplasmic reticulum (ER), it interacted with the ER-lumenal Hsp70, BiP, and bound to BiP substrates. Conversely, when a mammalian ER-lumenal JDP, ERdj3, was directed to the yeast cytosol, it rescued the temperature-sensitive growth phenotype of yeast-containing mutant alleles in two cytosolic JDPs, HLJ1 and YDJ1, and activated the ATP hydrolysis rate of Ssa1, the yeast cytosolic Hsp70 that partners with Hlj1 and Ydj1. Surprisingly, ERdj3 mutants that were compromised for substrate binding were unable to rescue the hlj1ydj1 growth defect even though they stimulated the ATPase activity of Ssa1. Yet, J domain mutants of ERdj3 that were defective for interaction with Ssa1 restored the growth of hlj1ydj1 yeast. Taken together, these data suggest that the substrate binding properties of certain JDPs, not simply the formation of unique Hsp70-JDP pairs, are critical to specify in vivo function.

Cytoplasmic retention of polyglutamine-expanded androgen receptor ameliorates disease via autophagy in a mouse model of spinal and bulbar muscular atrophy

The nucleus is the primary site of protein aggregation in many polyglutamine diseases, suggesting a central role in pathogenesis. In SBMA, the nucleus is further implicated by the critical role for disease of androgens, which promote the nuclear translocation of the mutant androgen receptor (AR). To clarify the importance of the nucleus in SBMA, we genetically manipulated the nuclear localization signal of the polyglutamine-expanded AR. Transgenic mice expressing this mutant AR displayed inefficient nuclear translocation and substantially improved motor function compared with SBMA mice. While we found that nuclear localization of polyglutamine-expanded AR is required for SBMA, we also discovered, using cell models of SBMA, that it is insufficient for both aggregation and toxicity and requires androgens for these disease features. Through our studies of cultured motor neurons, we further found that the autophagic pathway was able to degrade cytoplasmically retained expanded AR and represents an endogenous neuroprotective mechanism. Moreover, pharmacologic induction of autophagy rescued motor neurons from the toxic effects of even nuclear-residing mutant AR, suggesting a therapeutic role for autophagy in this nucleus-centric disease. Thus, our studies firmly establish that polyglutamine-expanded AR must reside within nuclei in the presence of its ligand to cause SBMA. They also highlight a mechanistic basis for the requirement for nuclear localization in SBMA neurotoxicity, namely the lack of mutant AR removal by the autophagic protein degradation pathway.

Methods for measuring ligand dissociation and nuclear receptor turnover in whole cells

Understanding the molecular mechanisms of steroid hormone action requires assays that measure rates of ligand dissociation and receptor degradation. Ligand dissociation is a pseudo-first order reaction of a high affinity [3H]-labeled ligand. Receptor turnover as described here is the rate of degradation of a radiolabeled receptor. The methods make use of transient expression of a nuclear receptor in cultured cells and are applicable to all nuclear receptors. Rates of ligand dissociation and receptor degradation provided the first insight into the interdomain interactions of the androgen receptor and the molecular basis for the phenotypic effects of naturally occurring androgen receptor loss-of-function germline mutations and gain-of-function somatic mutations, and for the potency differences between the biologically active androgens, testosterone, and dihydrotestosterone.

Yeast-based reporter assays for the functional characterization of cochaperone interactions with steroid hormone receptors

Steroid hormone receptor-mediated reporter assays in the budding yeast Saccharomyces cerevisiae have been an invaluable tool for the identification and functional characterization of steroid hormone receptor-associated chaperones and cochaperones. This chapter describes a hormone-inducible androgen receptor-mediated beta-galactosidase reporter assay in yeast. In addition, the immunophilin FKBP52 is used as a specific example of a receptor-associated cochaperone that acts as a positive regulator of receptor function. With the right combination of receptor and cochaperone expression plasmids, reporter plasmid, and ligand, the assay protocol described here could be used to functionally characterize a wide variety of nuclear receptor-cochaperone interactions. In addition to the functional characterization of receptor regulatory proteins, a modified version of this assay is currently being used to screen compound libraries for selective FKBP52 inhibitors that represent attractive therapeutic candidates for the treatment of steroid hormone receptor-associated diseases.

Optical probes for molecular processes in live cells

In this review, I summarize the development over the past several years of fluorescent and/or bioluminescent indicators to pinpoint cellular processes in living cells. These processes involve second messengers, protein phosphorylations, protein-protein interactions, protein-ligand interactions, nuclear receptor-coregulator interactions, nucleocytoplasmic trafficking of functional proteins, and protein localization.

The Hsp40 molecular chaperone Ydj1p, along with the protein kinase C pathway, affects cell-wall integrity in the yeast Saccharomyces cerevisiae

Molecular chaperones, such as Hsp40, regulate cellular processes by aiding in the folding, localization, and activation of multi-protein machines. To identify new targets of chaperone action, we performed a multi-copy suppressor screen for genes that improved the slow-growth defect of yeast lacking the YDJ1 chromosomal locus and expressing a defective Hsp40 chimera. Among the genes identified were MID2, which regulates cell-wall integrity, and PKC1, which encodes protein kinase C and is linked to cell-wall biogenesis. We found that ydj1delta yeast exhibit phenotypes consistent with cell-wall defects and that these phenotypes were improved by Mid2p or Pkc1p overexpression or by overexpression of activated downstream components in the PKC pathway. Yeast containing a thermosensitive allele in the gene encoding Hsp90 also exhibited cell-wall defects, and Mid2p or Pkc1p overexpression improved the growth of these cells at elevated temperatures. To determine the physiological basis for suppression of the ydj1delta growth defect, wild-type and ydj1delta yeast were examined by electron microscopy and we found that Mid2p overexpression thickened the mutant's cell wall. Together, these data provide the first direct link between cytoplasmic chaperone function and cell-wall integrity and suggest that chaperones orchestrate the complex biogenesis of this structure.

Molecular chaperones throughout the life cycle of the androgen receptor

Aberrant signaling by the androgen receptor contributes to the initiation and progression of prostate cancer. The involvement of molecular chaperones in the processes of folding, activation, trafficking, and transcriptional activity of the androgen receptor provide different points along the signaling axis where regulation of androgen receptor activity can be hijacked to provide growth signals for clonal selection in cancer progression. Evidence exists of abnormal chaperone expression that could contribute to the upregulation of AR activity in prostate tumors. Regardless of whether chaperones are involved in the causation of prostate carcinogenesis, molecular chaperones provide therapeutic targets for the treatment of prostate cancer.

Chaperoning of glucocorticoid receptors

A multiprotein hsp90/hsp70-based chaperone machinery functions as a 'cradle-to-grave' system for regulating the steroid binding, trafficking and turnover of the glucocorticoid receptor (GR). In an ATP-dependent process where hsp70 and hsp90 act as essential chaperones and Hop, hsp40, and p23 act as nonessential co-chaperones, the machinery assembles complexes between the ligand binding domain of the GR and hsp90. During GR-hsp90 heterocomplex assembly, the hydrophobic ligand-binding cleft is opened to access by steroid, and subsequent binding of steroid within the cleft triggers a transformation of the receptor such that it engages in more dynamic cycles of assembly/disassembly with hsp90 that are required for rapid dynein-dependent translocation to the nucleus. Within the nucleus, the hsp90 chaperone machinery plays a critical role both in GR movement to transcription regulatory sites and in the disassembly of regulatory complexes as the hormone level declines. The chaperone machinery also plays a critical role in stabilization of the GR to ubiquitylation and proteasomal degradation. The initial GR interaction with hsp70 appears to be critical for the triage between hsp90 heterocomplex assembly and preservation of receptor function vs CHIP-dependent ubiquitylation and proteasomal degradation. The hsp90 chaperone machinery is ubiquitous and functionally conserved among eukaryotes, and it is possible that all physiologically significant actions of hsp90 require the hsp70-dependent assembly of client protein-hsp90 heterocomplexes.

Tetratricopeptide repeat cochaperones in steroid receptor complexes

The molecular chaperone machinery contains multiple protein components that have 1 or more structural domains composed of tetratricopeptide repeat (TPR) motifs. Many other proteins of separate or unknown function also have TPR domains, so this motif is not exclusive to molecular chaperones. A general function of TPR domains is to bind other polypeptides, but this otherwise prosaic function has been exploited in an assortment of ways that link chaperones and other protein systems into cooperative networks. Among the best-characterized TPR proteins are several cochaperones that participate in assembly and regulation of steroid receptor complexes. Steroid receptors, members of the nuclear receptor subfamily, are hormone-dependent transcription factors that regulate many vertebrate pathways of homeostasis, growth, differentiation, reproduction, and pathology and, as such, have been of great interest to biologists and clinicians. Moreover, the steroid receptors are among the first recognized native clients for chaperones and have been widely studied models for complex chaperone interactions. To provide a coherent, representative minireview of TPR protein function, the scope of this article has been narrowed down primarily to functions of steroid receptor-associated TPR cochaperones.

The type I Hsp40 zinc finger-like region is required for Hsp70 to capture non-native polypeptides from Ydj1

The cytosolic yeast Hsp40 Ydj1 contains a conserved zinc finger-like region (ZFLR), which has two zinc-binding domains (ZBD), that helps regulate and specify Hsp70 function. To investigate the mechanism for Ydj1 ZFLR action, ZBDI and ZBDII mutants were constructed and characterized. ZBDII mutants exhibited temperature-sensitive growth defects, but yeast tolerated mutation of ZBDI. However, ZBDI and ZBDII mutants were defective at facilitating androgen receptor (AR) folding. Defective AR folding was associated with the accumulation of complexes between AR and Ydj1 ZFLR mutants and a reduction in Hsp70.AR complex formation. Purified Ydj1 ZBDI and ZBDII mutants could bind non-native polypeptides but could not deliver luciferase to Hsp70 and were defective at luciferase refolding. Interestingly, the ability of Ydj1 to synergize with Hsp70 to suppress thermally induced protein aggregation was blocked by mutation of ZBDII, but not ZBDI. Hence, ZBDII is required for yeast to survive heat stress because it is essential for Ydj1 to cooperate with Hsp70 to suppress protein aggregation. On the other hand, protein folding is dependent upon the action of both ZBDI and ZBDII because each is required for Hsp70 to capture non-native polypeptides from Ydj1.

A novel mode of chaperone action: heme activation of Hap1 by enhanced association of Hsp90 with the repressed Hsp70-Hap1 complex

Molecular chaperones Hsp90 and Hsp70 control many signal transducers, including cyclin-dependent kinases and steroid receptors. The yeast heme-responsive transcriptional activator Hap1 is a native substrate of both Hsp90 and Hsp70. Hsp90 and Hsp70 are critical for the precise regulation of Hap1 activity by heme. Here, to decipher the molecular events underlying the actions of Hsp90 and Hsp70 in heme regulation, we purified various multichaperone-Hap1 complexes and characterized the complexes linked to Hap1 repression and activation by two-dimensional electrophoresis analysis. Notably, we found that in vitro Hap1 is associated continuously with Ssa and its co-chaperones, and this association is not weakened by heme. Heme enhances the interaction between Hap1 and Hsp90. In vivo, defective Ssa, Ydj1, or Sro9 function causes Hap1 derepression in the absence of heme, whereas defective Hsp90 function causes reduced Hap1 activity at high heme concentrations. These results show that continuous association of Hap1 with Ssa, Ydj1, and Sro9 confers Hap1 repression, whereas enhanced association of Hsp90 with the repressed Hap1-Ssa-Ydj1-Sro9 complex by heme causes Hap1 activation. This novel mechanism of chaperone action may operate to control the activity of other important signal transducers.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

The Hsp90-binding peptidylprolyl isomerase FKBP52 potentiates glucocorticoid signaling in vivo

Hsp90 is required for the normal activity of steroid receptors, and in steroid receptor complexes it is typically bound to one of the immunophilin-related co-chaperones: the peptidylprolyl isomerases FKBP51, FKBP52 or CyP40, or the protein phosphatase PP5. The physiological roles of the immunophilins in regulating steroid receptor function have not been well defined, and so we examined in vivo the influences of immunophilins on hormone-dependent gene activation in the Saccharomyces cerevisiae model for glucocorticoid receptor (GR) function. FKBP52 selectively potentiates hormone-dependent reporter gene activation by as much as 20-fold at limiting hormone concentrations, and this potentiation is readily blocked by co-expression of the closely related FKBP51. The mechanism for potentiation is an increase in GR hormone-binding affinity that requires both the Hsp90-binding ability and the prolyl isomerase activity of FKBP52.

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.

Influence of ageing, heat shock treatment and in vivo total antioxidant status on gene-expression profile and protein synthesis in human peripheral lymphocytes

Ageing results in a progressive, intrinsic and generalised imbalance of the control of regulatory systems. A key manifestation of this complex biological process includes the attenuation of the universal stress response. Here we provide the first global assessment of the ageing process as it affects the heat shock response, utilising human peripheral lymphocytes and cDNA microarray analysis. The genomic approach employed in our preliminary study was supplemented with a proteomic approach. In addition, the current study correlates the in vivo total antioxidant status with the age-related differential gene expression as well as the translational kinetics of heat shock proteins (hsps). Most of the genes encoding stress response proteins on the 4224 element microarray used in this study were significantly elevated after heat shock treatment of lymphocytes obtained from both young and old individuals albeit to a greater extent in the young. Cell signaling and signal transduction genes as well as some oxidoreductases showed varied response. Results from translational kinetics of induction of major hsps, from 0 to 24 h recovery period were broadly consistent with the differential expression of HSC 70 and HSP 40 genes. Total antioxidant levels in plasma from old individuals were found to be significantly lower by comparison with young, in agreement with the widely acknowledged role of oxidant homeostasis in the ageing process.

The Cdc37 protein kinase-binding domain is sufficient for protein kinase activity and cell viability

Cdc37 is a molecular chaperone required for folding of protein kinases. It functions in association with Hsp90, although little is known of its mechanism of action or where it fits into a folding pathway involving other Hsp90 cochaperones. Using a genetic approach with Saccharomyces cerevisiae, we show that CDC37 overexpression suppressed a defect in v-Src folding in yeast deleted for STI1, which recruits Hsp90 to misfolded clients. Expression of CDC37 truncation mutants that were deleted for the Hsp90-binding site stabilized v-Src and led to some folding in both sti1Delta and hsc82Delta strains. The protein kinase-binding domain of Cdc37 was sufficient for yeast cell viability and permitted efficient signaling through the yeast MAP kinase-signaling pathway. We propose a model in which Cdc37 can function independently of Hsp90, although its ability to do so is restricted by its normally low expression levels. This may be a form of regulation by which cells restrict access to Cdc37 until it has passed through a triage involving other chaperones such as Hsp70 and Hsp90.

Inhibition of orthotopic growth and metastasis of androgen-sensitive human prostate tumors in mice by bioactive soybean components

BACKGROUND

Systematic analysis of the influence of diet on the initiation and progression of prostate cancer is often difficult in human populations, for which dietary variables overlap a diversity of genetic backgrounds and social behaviors. Animal models that emulate human prostate cancer allow experimental analysis of the mechanisms of action of nutritional agents that show anti-prostate cancer activity.

METHODS

We have used an orthotopic implant model to characterize the in vivo response of androgen-sensitive LNCaP prostate tumors to three well-characterized soy dietary supplements: isoflavone depleted soy protein, soy phytochemical concentrate (SPC), and genistin.

RESULTS

In male SCID mice orthotopically implanted with the androgen-sensitive human prostate cell line LNCaP, dietary supplements of soy protein, genistin, and SPC reduced primary tumor weight by 42% (P = 0.07), 57% (P < 0.05) and 70% (P < 0.005), respectively. All three soy supplements significantly increased tumor apoptosis and decrease microvessel density, with no significant change in tumor proliferation. Each supplement produced a distinct serum androgen response, with genistin producing the greatest decrease in total serum testosterone and dihydrotestosterone (DHT) (P < 0.05) and the greatest increase in testosterone to DHT ratio (P < 0.05) and soy protein the greatest decrease in bioactive androgen (P < 0.05). Only SPC significantly inhibited metastases to lymph nodes and lungs, and only SPC produced a significant increase in tumor p53 expression.

CONCLUSION

Taken together, these data suggest that the anti-prostate cancer activity of dietary soy protein, soy phytochemicals, and genistin use different molecular pathways. In addition, we have demonstrated that this animal model can be used in the design of dietary strategies for prostate cancer prevention and therapy.

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.

Chaperoning signaling pathways: molecular chaperones as stress-sensing`heat shock' proteins

Heat shock proteins interact with multiple key components of signaling pathways that regulate growth and development. The molecular relationships between heat shock proteins, various signaling proteins and partner proteins appear to be critical for the normal function of signal transduction pathways. The relative levels of these proteins may be important, as too little or too much Hsp70 or Hsp90 can result in aberrant growth control, developmental malformations and cell death. Although the functions of heat shock proteins as molecular chaperones have been well characterized, their complementary role as a `stress-induced' proteins to monitor changes and alter the biochemical environment of the cell remains elusive. Genetic and molecular interactions between heat shock proteins, their co-chaperones and components of signaling pathways suggest that crosstalk between these proteins can regulate proliferation and development by preventing or enhancing cell growth and cell death as the levels of heat shock proteins vary in response to environmental stress or disease.

HSP40 binding is the first step in the HSP90 chaperoning pathway for the progesterone receptor

The progesterone receptor (PR) can be isolated in its native conformation able to bind hormone, yet its ligand-binding domain rapidly loses its activity at elevated temperature. However, an in vitro chaperoning system consisting of five proteins (HSP40, HSP70, HOP, HSP90, and p23) with ATP is capable of restoring this function. The first step of this chaperoning mechanism is usually thought to be the binding of HSP70 to PR. Our findings here show that the binding of HSP40 to PR is, instead, the first step. HSP40 binding occurred rapidly and was not dependent on ATP or other proteins. The stoichiometry of HSP40 to native PR in these complexes was approximately 1:1. HSP40 bound specifically and with a high affinity to native PR (K(d) = 77 nm). The binding of HSP40 to PR was sustained and did not interact in the highly dynamic fashion that has been observed previously for HSP90 in this system. The HSP40 small middle dotPR complex could be isolated as a functional unit that could, after the addition of the other chaperones, progress to a PR complex capable of hormone binding. These results indicate that HSP40 initiates the entry of PR into the HSP90 pathway.

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 FXXLF motif mediates androgen receptor-specific interactions with coregulators

The androgen receptor (AR) activation function 2 region of the ligand binding domain binds the LXXLL motifs of p160 coactivators weakly, engaging instead in an androgen-dependent, interdomain interaction with an FXXLF motif in the AR NH(2) terminus. Here we show that FXXLF motifs are present in previously reported AR coactivators ARA70/RFG, ARA55/Hic-5, and ARA54, which account for their selection in yeast two-hybrid screens. Mammalian two-hybrid assays, ligand dissociation rate studies, and glutathione S-transferase adsorption assays indicate androgen-dependent selective interactions of these FXXLF motifs with the AR ligand binding domain. Mutagenesis of residues within activation function 2 indicates distinct but overlapping binding sites where specificity depends on sequences within and flanking the FXXLF motif. Mutagenesis of the FXXLF motifs eliminated interaction with the ligand binding domain but only modestly reduced AR coactivation in transcription assays. The studies indicate that the FXXLF binding motif is specific for the AR and mediates interactions both within the AR and with coregulatory proteins.

A novel method for the analysis of the androgen receptor

The role of the androgen receptor in the regulation of prostate cancer development and progression has been a focus of intense research. Until recent years, the level of expression of the androgen receptor protein was described qualitatively. Immunohistochemical parameters have been established that show a linear relationship between androgen receptor expression and immunostaining. Intensity of immunostaining can be accurately measured using computer-assisted color video image analysis. Studies of progression and treatment of prostate cancer will benefit from the ability to quantitatively measure androgen receptor expression.

JDD1, a novel member of the DnaJ family, is expressed in the germinal zone of the rat brain

We identified a novel gene encoding a new member of the DnaJ family, JDD1 (J domain of DnaJ-like-protein 1), from the rat. The cloned JDD1 cDNA is 1689 bp in size and its deduced amino acid sequence consists of 259 amino acid residues. Immunoblot analysis revealed that JDD1 protein is approximately 30 kDa in size. JDD1 has a J domain that is unique to the DnaJ family but lacks the G/F region (a region that is rich in the amino acids glycine and phenylalanine) and the zinc finger region (also known as the cysteine-rich region)-both characteristic to the DnaJ. JDD1 mRNA is expressed heterogeneously in vivo. In the central nervous system, JDD1 mRNA expression is confined to the germinal (ventricular and subventricular) zone where, except for cells situated deepest in the ventricular zone, neurons and glias are generated and then differentiate during the embryonic period. Expression of JDD1 mRNA in the subventricular zone persists after birth. In addition to the brain, its robust expression is notable in the liver, lung, cortex of the kidney, and several other tissues in the embryo.

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.

The role of heat shock protein 70 in vitamin D receptor function

We previously demonstrated that the 1alpha,25-dihydroxyvitamin D(3) receptor (VDR) interacts with the constitutive heat shock protein, hsc70 in vitro, and with DnaK (Biochem. Biophys. Res. Commun. 260, 446-452, 1999). The biological significance of VDR-heat shock protein interactions, however, is unknown. To examine the role of such interactions in eukaryotic cells, we heterologously expressed VDR and RXRalpha together with a vitamin D-responsive reporter system in Saccharomyces cerevisiae and examined the consequences of heat shock protein 70 gene (SSA) deletion in these cells. We show that heterologously expressed VDR associates with the yeast cytosolic hsp70 protein, Ssa1p. Deletion of the SSA2, SSA3, and SSA4 genes and reduction of Ssa1p activity, reduces the intracellular concentrations of the VDR and its heterodimeric partner, RXRalpha and reduces the activity of a vitamin D-dependent gene. Hsp70-like chaperone proteins play a role in controlling concentrations of the VDR within the cell.

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.

High level expression of full-length estrogen receptor in Escherichia coli is facilitated by the uncoupler of oxidative phosphorylation, CCCP

The expression of high levels of full-length human estrogen receptor alpha (hERalpha) in Escherichia coli has proven difficult. We found that expression of the ER DNA binding domain is highly toxic to E. coli, resulting in rapid loss of the expression plasmid. Using a tightly regulated arabinose expression system and the antibiotic Timentin, we were able to overcome ER toxicity and express substantial levels of ER. The expressed ER exhibited protease cleavage at a single site near the N-terminus of the hinge region. Of the many measures we tested to eliminate ER cleavage, only addition of carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), an uncoupler of oxidative phosphorylation, completely blocked intracellular proteolysis of the ER. Using CCCP and our expression methods, full-length FLAG epitope-tagged hERalpha (fER) was expressed in E. coli at approximately 1 mg/l. The fER was purified to homogeneity in a single step by immunoaffinity chromatography with anti-FLAG monoclonal antibody. Purified full-length bacterial fER binds 17beta-estradiol with the same affinity as hER expressed in human cells (K(D) approximately 0.5 nM). At high concentrations of fER (20 nM), a bell-shaped estrogen binding curve with a Hill coefficient of 1.7 was seen. Bacterially-expressed fER exhibits a reduced affinity for the estrogen response element (ERE). Anti-FLAG antibody restores high affinity binding of the fER to the ERE, suggesting that impaired dimerization may be responsible for the reduced affinity of bacterially-expressed fER for the ERE. The use of Timentin and CCCP may provide a general method for high level bacterial expression of steroid/nuclear receptors and other proteins important in hormone action.

The N-terminal domain of thyroid hormone receptor-alpha is required for its biological activities

Thyroid hormone (T3) receptors (T3Rs) are ligand-modulated transcription factors that belong to the nuclear receptor superfamily. Whereas the well-conserved DNA-binding domain and the relatively well-conserved ligand-binding domain in T3Rs have been characterized in detail, limited information is available on the contribution of the variable N terminus to the transcriptional properties of T3Rs. To gain greater insight into the function of the N terminus, we generated a deletion mutant of T3Ralpha, T3Ralpha-deltaN1, that lacks amino acids 7-45 and assessed the effect of this deletion on all known transcriptional activities of T3Ralpha. Despite the fact that T3Ralpha-deltaN1 was expressed and bound T3 with an affinity similar to that of wildtype T3Ralpha, all of its common transcriptional activities were lost. That is, T3Ralpha-deltaN1 did not activate transcription from a positive or negative T3 response element, and it could not interfere with AP-1 transcriptional activity. Surprisingly, T3Ralpha-deltaN1 lost its ability to bind DNA, which can account for its deficiencies as a transcriptional activator. In contrast, the ability of T3Ralpha-deltaN1 to interact with putative coactivators or corepressors was not significantly altered from that of wildtype T3Ralpha. However, overall folding of T3Ralpha-deltaN1 was altered, as indicated by differential sensitivity to limited protease digestion. These data document that the N terminus of T3Ralpha, albeit relatively short and representing a variable and unconserved region when compared with other nuclear receptors, has a critical role in proper folding of the DNA-binding domain and is required for the biological activities of full-length T3Ralpha.

The heat shock cognate protein hsc73 assembles with A(1) adenosine receptors to form functional modules in the cell membrane

A(1) adenosine receptors (A(1)Rs) are G protein-coupled heptaspanning receptors that interact at the outer face of the plasma membrane with cell surface ecto-adenosine deaminase (ecto-ADA). By affinity chromatography the heat shock cognate protein hsc73 was identified as a cytosolic component able to interact with the third intracellular loop of the receptor. As demonstrated by surface plasmon resonance, purified A(1)Rs interact specifically with hsc73 with a dissociation constant in the nanomolar range (0.5 +/- 0.1 nM). The interaction between hsc73 and A(1)R led to a marked reduction in the binding of the ligands and prevented activation of G proteins, as deduced from (35)S-labeled guanosine-5'-O-(3-thio)triphosphate binding assays. Interestingly this effect was stronger than that exerted by guanine nucleotide analogs, which uncouple receptors from G proteins, and was completely prevented by ADA. As assessed by immunoprecipitation a high percentage of A(1)Rs in cell lysates are coupled to hsc73. A relatively high level of colocalization between A(1)R and hsc73 was detected in DDT(1)MF-2 cells by means of confocal microscopy, and no similar results were obtained for other G protein-coupled receptors. Colocalization between hsc73 and A(1)R was detected in specific regions of rat cerebellum and in the body of cortical neurons but not in dendrites or synapses. Remarkably, agonist-induced receptor internalization leads to the endocytosis of A(1)Rs by two qualitatively different vesicle types, one in which A(1)R and hsc73 colocalize and another in which hsc73 is absent. These results open the interesting possibility that signaling via G protein-coupled receptors may be regulated by heat shock proteins.

A role for the Hsp40 Ydj1 in repression of basal steroid receptor activity in yeast

In addition to its roles in translocation of preproteins across membranes, Ydj1 facilitates the maturation of Hsp90 substrates, including mammalian steroid receptors, which activate transcription in yeast in a hormone-dependent manner. To better understand Ydj1's function, we have constructed and analyzed an array of Ydj1 mutants in vivo. Both the glucocorticoid receptor and the estrogen receptor exhibited elevated activity in the absence of hormone in all ydj1 mutant strains, indicating a strict requirement for Ydj1 activity in hormonal control. Glucocorticoid receptor containing a mutation in the carboxy-terminal transcriptional activation domain, AF-2, retained elevated basal activity, while mutation of the amino-terminal transactivation domain, AF-1, eliminated the elevated basal activity observed in ydj1 mutant strains. This result indicates that the source of activity is AF-1, which is normally repressed by the carboxy-terminal hormone binding domain in the absence of hormone. Chimeric proteins containing the hormone binding domain of the estrogen or glucocorticoid receptor fused to heterologous activation and DNA binding domains also exhibited elevated activity in the absence of hormone. Thus, Ydj1 mutants appear to increase basal receptor activity by altering the ability of the hormone binding domain of the receptor to repress nearby activation domains. We propose that Ydj1 functions to present steroid receptors to the Hsp90 pathway for folding and hormonal control. In the presence of Ydj1 mutants that fail to bind substrate efficiently, some receptor escapes the Hsp90 pathway, resulting in constitutive activity.

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.

Differential regulation of testosterone vs. 5alpha-dihydrotestosterone by selective androgen response elements

There are two major physiological androgens, testosterone (T), and 5alpha-dihydrotestosterone (DHT), which induce different responses in mammals. These androgens regulate the target gene transcription via binding to and activating the same androgen receptor (AR). The molecular mechanisms that differ between these two very close androgens through the same AR protein to target the distinct genomic responses remain unknown. Using yeast genetic selection, we identified two kinds of androgen response elements (ARE), which could respond differentially to T vs. DHT. These two AREs also show different T- vs. DHT-induced AR transactivation in mammalian Chinese hamster ovary (CHO) cells in terms of copy number and comparisons with the classic mouse mammary tumor virus ARE. Together, our results suggest that the selective ARE sequence may play an important role in the differential T- vs. DHT-induced AR transactivation.

The p23 molecular chaperones act at a late step in intracellular receptor action to differentially affect ligand efficacies

Multiple molecular chaperones, including Hsp90 and p23, interact with members of the intracellular receptor (IR) family. To investigate p23 function, we compared the effects of three p23 proteins on IR activities, yeast p23 (sba1p) and the two human p23 homologs, p23 and tsp23. We found that Sba1p was indistinguishable from human p23 in assays of seven IR activities in both animal cells and in yeast; in contrast, certain effects of tsp23 were specific to that homolog. Transcriptional activation by two IRs was increased by expression of any of the p23 species, whereas activation by five other IRs was decreased by Sba1p or p23, and unaffected by tsp23. p23 was expressed in all tissues examined except striated and cardiac muscle, whereas tsp23 accumulated in a complementary pattern; hence, p23 proteins might contribute to tissue-specific differences in IR activities. Unlike Hsp90, which acts on IR aporeceptors to stimulate ligand potency (i.e., hormone-binding affinity), p23 proteins acted on IR holoreceptors to alter ligand efficiencies (i.e., transcriptional activation activity). Moreover, the p23 effects developed slowly, requiring prolonged exposure to hormone. In vitro, p23 interacted preferentially with hormone–receptor–response element ternary complexes, and stimulated receptor–DNA dissociation. The dissociation was reversed by addition of a fragment of the GRIP1 coactivator, suggesting that the two reactions may be in competition in vivo. Our findings suggest that p23 functions at one or more late steps in IR-mediated signal transduction, perhaps including receptor recycling and/or reversal of the response.

Chaperones in progesterone receptor complexes

Studying the components, pathways, and dynamics of progesterone receptor (PR) assembly with chaperones has provided a highly valuable model system for understanding the coordinate actions of chaperones. Chaperones are primarily adapted to facilitate protein folding processes, and the actions of chaperones toward PR and other steroid receptors probably remain within this general functional boundary. Unlike a typical misfolded protein substrate, PR's folding is effectively arrested prior to hormone binding, thus extending indefinitely the chaperone-interaction phase that normally would be transitory during progressive protein folding. While one could consider this a limitation in PR's ability to fold properly, perhaps a more accurate view is that PR is specially adapted to remain 'misfolded', and thus extend chaperone interactions that function efficiently in repressing PR's transcriptional activity while the receptor awaits an activating signal.

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.

Mutations in the yeast Hsp40 chaperone protein Ydj1 cause defects in Axl1 biogenesis and pro-a-factor processing

The heat shock protein (Hsp) 70/Hsp40 chaperone system plays an essential role in cell physiology, but few of its in vivo functions are known. We report that biogenesis of Axl1p, an insulinase-like endoprotease from yeast, is dependent upon the cytosolic Hsp40 protein Ydj1p. Axl1 is responsible for cleavage of the P2 processing intermediate of pro-a-factor, a mating pheromone, to its mature form. Mutant ydj1 strains exhibited a severe mating defect, which correlated with a 90% reduction in a-factor secretion. Reduced levels of a-factor export were caused by defects in the endoproteolytic processing of P2, which led to its intracellular accumulation. Defective P2 processing correlated with the reduction in the steady state level of active Axl1p. Two mechanisms were uncovered to explain why Axl1p activity was diminished in ydj1 strains. First, AXL1 mRNA levels were reduced ydj1 strains. Second, the half-life of newly synthesized Axl1p was greatly diminished in ydj1 strains. Collectively, these data indicate Ydj1p functions to promote AXL1 mRNA accumulation and in addition appears to facilitate the proper folding of nascent Axl1p. This study is the first to suggest a role for Ydj1p in RNA metabolism and identifies Axl1p as an in vivo substrate of the Hsp70/Ydj1p chaperone system.

Reporter gene technology: the future looks bright

Reporter gene technology is widely used to monitor the cellular events associated with signal transduction and gene expression. Based upon the splicing of transcriptional control elements to a variety of reporter genes (with easily measurable phenotypes), it "reports" the effects of a cascade of signalling events on gene expression inside cells. The principal advantage of these assays is their high sensitivity, reliability, convenience, and adaptability to large-scale measurements. This review summarises the current status of reporter gene technology including its role in monitoring gene transfer and expression and its development as a biological screen. With the advances in this technology and in detection methods, it is likely that luciferase and green fluorescent protein will become increasingly popular for the non-invasive monitoring of gene expression in living tissues and cells. Such techniques will be important in defining the molecular events associated with gene transcription, which has implications for our understanding of the molecular basis of disease and will influence our approach to gene therapy and drug development.

Update of the androgen receptor gene mutations database

The current version of the androgen receptor (AR) gene mutations database is described. The total number of reported mutations has risen from 309 to 374 during the past year. We have expanded the database by adding information on AR-interacting proteins; and we have improved the database by identifying those mutation entries that have been updated. Mutations of unknown significance have now been reported in both the 5' and 3' untranslated regions of the AR gene, and in individuals who are somatic mosaics constitutionally. In addition, single nucleotide polymorphisms, including silent mutations, have been discovered in normal individuals and in individuals with male infertility. A mutation hotspot associated with prostatic cancer has been identified in exon 5. The database is available on the internet (http://www.mcgill.ca/androgendb/), from EMBL-European Bioinformatics Institute (ftp.ebi.ac.uk/pub/databases/androgen), or as a Macintosh FilemakerPro or Word file (MC33@musica.mcgill.ca).

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.

Polyglutamine-expanded androgen receptors form aggregates that sequester heat shock proteins, proteasome components and SRC-1, and are suppressed by the HDJ-2 chaperone

Spinal bulbar muscular atrophy is a neurodegenerative disorder caused by a polyglutamine expansion in the androgen receptor (AR). We show in transiently transfected HeLa cells that an AR containing 48 glutamines (ARQ48) accumulates in a hormone-dependent manner in both cytoplasmic and nuclear aggregates. Electron microscopy reveals both types of aggregates to have a similar ultrastructure. ARQ48 aggregates sequester mitochondria and steroid receptor coactivator 1 and stain positively for NEDD8, Hsp70, Hsp90 and HDJ-2/HSDJ. Co-expression of HDJ-2/HSDJ significantly represses aggregate formation. ARQ48 aggregates also label with antibodies recognizing the PA700 proteasome caps but not 20S core particles. These results suggest that ARQ48 accumulates due to protein misfolding and a breakdown in proteolytic processing. Furthermore, the homeostatic disturbances associated with aggregate formation may affect normal cell function.