The cochaperone SGTA (small glutamine-rich tetratricopeptide repeat-containing protein alpha) demonstrates regulatory specificity for the androgen, glucocorticoid, and progesterone receptors

Steroid hormone receptors are ligand-dependent transcription factors that require the ordered assembly of multichaperone complexes for transcriptional activity. Although heat shock protein (Hsp) 90 and Hsp70 are key players in this process, multiple Hsp70- and Hsp90-associated cochaperones associate with receptor-chaperone complexes to regulate receptor folding and activation. Small glutamine-rich tetratricopeptide repeat-containing protein alpha (SGTA) was recently characterized as an Hsp70 and Hsp90-associated cochaperone that specifically regulates androgen receptor activity. However, the specificity of SGTA for additional members of the steroid hormone receptor superfamily and the mechanism by which SGTA regulates receptor activity remain unclear. Here we report that SGTA associates with and specifically regulates the androgen, glucocorticoid, and progesterone receptors and has no effect on the mineralocorticoid and estrogen receptors in both yeast and mammalian cell-based reporter assays. In both systems, SGTA knockdown/deletion enhances receptor activity, whereas SGTA overexpression suppresses receptor activity. We demonstrate that SGTA binds directly to Hsp70 and Hsp90 in vitro with similar affinities yet predominately precipitates with Hsp70 from cell lysates, suggesting a role for SGTA in early, Hsp70-mediated folding. Furthermore, SGTA expression completely abrogates the regulation of receptor function by FKBP52 (52-kDa FK506-binding protein), which acts at a later stage of the chaperone cycle. Taken together, our data suggest a role for SGTA at distinct steps in the chaperone-dependent modulation of androgen, glucocorticoid, and progesterone receptor activity.

Coregulator control of androgen receptor action by a novel nuclear receptor-binding motif

The androgen receptor (AR) is a ligand-activated transcription factor that is essential for prostate cancer development. It is activated by androgens through its ligand-binding domain (LBD), which consists predominantly of 11 α-helices. Upon ligand binding, the last helix is reorganized to an agonist conformation termed activator function-2 (AF-2) for coactivator binding. Several coactivators bind to the AF-2 pocket through conserved LXXLL or FXXLF sequences to enhance the activity of the receptor. Recently, a small compound-binding surface adjacent to AF-2 has been identified as an allosteric modulator of the AF-2 activity and is termed binding function-3 (BF-3). However, the role of BF-3 in vivo is currently unknown, and little is understood about what proteins can bind to it. Here we demonstrate that a duplicated GARRPR motif at the N terminus of the cochaperone Bag-1L functions through the BF-3 pocket. These findings are supported by the fact that a selective BF-3 inhibitor or mutations within the BF-3 pocket abolish the interaction between the GARRPR motif(s) and the BF-3. Conversely, amino acid exchanges in the two GARRPR motifs of Bag-1L can impair the interaction between Bag-1L and AR without altering the ability of Bag-1L to bind to chromatin. Furthermore, the mutant Bag-1L increases androgen-dependent activation of a subset of AR targets in a genome-wide transcriptome analysis, demonstrating a repressive function of the GARRPR/BF-3 interaction. We have therefore identified GARRPR as a novel BF-3 regulatory sequence important for fine-tuning the activity of the AR.

Quantification of a New Anti-Cancer Molecule MJC13 Using a Rapid, Sensitive, and Reliable Liquid Chromatography-Tandem Mass Spectrometry Method

MJC13 is a novel molecule that has potential use for the treatment of hormone refractory prostate cancer (HRPC). The purpose of this work was to develop a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantification of MJC13. Itraconazole was used as the internal standard (IS). Acetonitrile was used to extract MJC13 from rat plasma and urine samples. A LC system equipped with a Waters XTerra MS C₁₈ column (125Å, 3.5 µm, 4.6×150 mm) was used for chromatographic separation with acetonitrile-water as mobile phase. The API 3200 QTRAP triple quadrupole mass spectrometer was used for chromatographic analysis by multiple reaction monitoring (MRM) at positive mode with the transitions m/z 272→m/z 162 for MJC13, and m/z 705→m/z 392 for IS. The retention times for MJC13 and IS were 4.98 min and 4.42 min, respectively. The standard curves of MJC13 in solution, rat plasma, and rat urine were linear in the concentration range of 1 - 1000, 1 - 1000 and 1 - 200 ng/mL, respectively. The intra- and inter-day accuracy (relative error) ranged from 1.99 - 4.20% and 1.83 - 4.39%, respectively. The intra- and inter-day precision (coefficient of variation) ranged from 2.27 - 3.88% and 2.80 - 4.79%, respectively. The extraction recovery rates of rat plasma and urine samples were 95.3% and 96.2%, respectively. No measurable matrix effect interfered with MJC13 identification and quantification in rat plasma and urine. In summary, a rapid, specific, sensitive, and reproducible LC-MS/MS method was developed and validated to quantify MJC13 in solution, rat plasma, and rat urine.

Androgen receptor splice variants are resistant to inhibitors of Hsp90 and FKBP52, which alter androgen receptor activity and expression

Androgen ablation therapy is the most common treatment for advanced prostate cancer (PCa), but most patients will develop castration-resistant prostate cancer (CRPC), which has no cure. CRPC is androgen-depletion resistant but androgen receptor (AR) dependent. AR is a nuclear receptor whose transcriptional activity is regulated by hormone binding to the ligand-binding domain (LBD). Constitutively active AR splice variants that lack LBDs often are expressed in CRPC. The expression of these variants indicates that methods to inhibit AR activity that do not rely on inactivating the LBD are needed. Heat shock protein 90 (Hsp90), a potential therapeutic target in PCa, is an AR chaperone crucial for proper folding, hormone binding and transcriptional activity of AR. We generated LNCaP cell lines with regulated expression of the AR-V7 variant as well as a cell line expressing artificially truncated AR (termed AR-NTD) to characterize splice variant function. Using an Hsp90 inhibitor, Geldanamycin (GA), and an AR-Hsp90-FKBP52 specific inhibitor, MJC13, we sought to determine if the AR variants also require Hsp90 and associated co-chaperone, FKBP52, for their activity. GA inhibits AR transcriptional activity but has little effect on AR-V7 activity. Moreover, GA decreases the stability of AR protein, with no effect on AR-V7 levels. Full-length AR activity is strongly inhibited by MJC13 while AR-V7 is unaffected. Thus, the variants are resistant to inhibitors of the Hsp90-AR heterocomplex. Although Hsp90 inhibitors will continue to inhibit growth promoting kinases and signaling through activated full-length AR in CRPC, AR signaling through variants will be retained.

FKBP51 and FKBP52 in signaling and disease

FKBP51 and FKBP52 are diverse regulators of steroid hormone receptor signaling, including receptor maturation, hormone binding and nuclear translocation. Although structurally similar, they are functionally divergent, which is largely attributed to differences in the FK1 domain and the proline-rich loop. FKBP51 and FKBP52 have emerged as likely contributors to a variety of hormone-dependent diseases, including stress-related diseases, immune function, reproductive functions and a variety of cancers. In addition, recent studies have implicated FKBP51 and FKBP52 in Alzheimer's disease and other protein aggregation disorders. This review summarizes our current understanding of FKBP51 and FKBP52 interactions within the receptor-chaperone complex, their contributions to health and disease, and their potential as therapeutic targets for the treatment of these diseases.

FK506 binding protein 5 shapes stress responsiveness: modulation of neuroendocrine reactivity and coping behavior

BACKGROUND

The Hsp90 cochaperone FK506 binding protein 5 (FKBP5) is an established regulator of the glucocorticoid receptor (GR), and numerous genetic studies have linked it to stress-related diseases such as major depression or posttraumatic stress disorder. However, translational studies including genetic animal models are lacking.

METHODS

Mice deficient of FKBP5 were generated and analyzed in comparison with wildtype littermates. They were subjected to several test paradigms characterizing their emotionality, stress reactivity, and coping behavior as well as hypothalamus-pituitary-adrenal axis function and regulation. Moreover, protein expression of GR and FKBP5 was determined in different brain structures 8 days after stress exposure. The combined dexamethasone/corticotropin-releasing hormone test was performed both in mice and healthy human subjects of different FKBP5 genotypes. The GR function was evaluated by reporter gene assays.

RESULTS

Under basal conditions, deletion of FKBP5 did not change exploratory drive, locomotor activity, anxiety-related behavior, stress-coping, or depression-like behavior. After exposure to different acute stressors of sufficient intensity, however, it led to a more active coping behavior. Moreover, loss of FKBP5 decreased hypothalamus-pituitary-adrenal axis reactivity and GR expression changes in response to stressors. In mice and humans, the FKBP5 genotype also determined the outcome of the dexamethasone/corticotropin-releasing hormone test.

CONCLUSIONS

This study in mice and humans presents FKBP5 as a decisive factor for the physiological stress response, shaping neuroendocrine reactivity as well as coping behavior. This lends strong support to the concept emerging from human studies of FKBP5 as important factor governing gene-environment interactions relevant for the etiology of affective disorders.

A new anti-depressive strategy for the elderly: ablation of FKBP5/FKBP51

The gene FKBP5 codes for FKBP51, a co-chaperone protein of the Hsp90 complex that increases with age. Through its association with Hsp90, FKBP51 regulates the glucocorticoid receptor (GR). Single nucleotide polymorphisms (SNPs) in the FKBP5 gene associate with increased recurrence of depressive episodes, increased susceptibility to post-traumatic stress disorder, bipolar disorder, attempt of suicide, and major depressive disorder in HIV patients. Variation in one of these SNPs correlates with increased levels of FKBP51. FKBP51 is also increased in HIV patients. Moreover, increases in FKBP51 in the amygdala produce an anxiety phenotype in mice. Therefore, we tested the behavioral consequences of FKBP5 deletion in aged mice. Similar to that of naïve animals treated with classical antidepressants FKBP5−/− mice showed antidepressant behavior without affecting cognition and other basic motor functions. Reduced corticosterone levels following stress accompanied these observed effects on depression. Age-dependent anxiety was also modulated by FKBP5 deletion. Therefore, drug discovery efforts focused on depleting FKBP51 levels may yield novel antidepressant therapies.

The anti-estrogenic activity of sediments from agriculturally intense watersheds: assessment using in vivo and in vitro assays

The goal of the current study was to determine whether sediments from agriculturally intense watersheds can act as a potential source of anti-estrogenic endocrine-disrupting compounds. The specific objectives of the current study were to determine (1) whether female fathead minnows (Pimephales promelas) experience alterations in endocrine function when exposed to sediments collected from agriculturally intense watersheds and (2) if these sediments display anti-estrogenic activity in an in vitro assay. In addition, sediment samples were analyzed for the presence of steroid hormones and pesticides associated with local agricultural practices. To accomplish this, sediments and water were collected from three sites within two agriculturally intense Nebraska watersheds (Bow Creek and the Elkhorn River). In 2009, minnows were exposed to sediment and/or water collected from the two Bow Creek sites (East Bow Creek and the Confluence) in the laboratory, while in 2010, minnows were exposed to sediment and/or water from East Bow Creek, the Confluence and the Elkhorn River. Following the 7-day exposure period, the hepatic mRNA expression of two-estrogen responsive genes, estrogen receptor α (ERα) and vitellogenin (Vtg) was determined. In 2009, females exposed to Confluence sediments, in the presence of laboratory water or Confluence water, experienced significant reductions in ERα expression relative to unexposed and Confluence water-exposed females. The defeminization of these females suggests the presence of a biologically available anti-estrogenic compound in sediments collected from this site. In 2010, sediments were assessed for anti-estrogenic activity on days 0 and 7 of the exposure period using a 4-h yeast estrogen screen. Lipophilic extracts (LEs) of day 0 sediments collected from the Confluence and the Elkhorn River induced significant reductions in the estrogenic reporter activity of treated yeast cultures suggesting the presence of a lipophilic anti-estrogenic compound in these extracts. Chemical analysis revealed the presence of a variety of steroid hormones, including those associated with the production of beef cattle (i.e. β-trenbolone, α-zearalanol and α-zearalenol), in sediments indicating that compounds utilized by local beef cattle operations are capable of entering nearby watersheds. Overall, the results of this study indicate that an environmentally relevant anti-estrogenic compound is present in sediments from agriculturally intense watersheds and that this compound is bioavailable to fish. Furthermore, the presence of steroid hormones in sediments from these watersheds provides evidence indicating that steroids are capable of sorbing to sediments.

The involvement of FK506-binding protein 51 (FKBP5) in the behavioral and neuroendocrine effects of chronic social defeat stress

Chronic stress is increasingly considered to be a main risk factor for the development of a variety of psychiatric diseases such as depression. This is further supported by an impaired negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis, which has been observed in the majority of depressed patients. The effects of glucocorticoids, the main hormonal endpoint of the HPA axis, are mediated via the glucocorticoid receptor (GR) and the mineralocorticoid receptor. The FK506-binding protein 51 (FKBP5), a co-chaperone of the Hsp90 and component of the chaperone-receptor heterocomplex, has been shown to reduce ligand sensitivity of the GR. This study aimed to investigate the function of FKBP5 as a possible mediator of the stress response system and its potential role in the development of stress-related diseases. Therefore, we assessed whether mice lacking the gene encoding FKBP5 (51KO mice) were less vulnerable to the adverse effects of three weeks of chronic social defeat stress. Mice were subsequently analyzed with regards to physiological, neuroendocrine, behavioral and mRNA expression alterations. Our results show a less vulnerable phenotype of 51KO mice with respect to physiological and neuroendocrine parameters compared to wild-type animals. 51KO mice demonstrated lower adrenal weights and basal corticosterone levels, a diminished response to a novel acute stimulus and an enhanced recovery, as well as more active stress-coping behavior. These results suggest an enhanced negative glucocorticoid feedback within the HPA axis of 51KO mice, possibly modulated by an increased sensitivity of the GR. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Regulation of steroid hormone receptor function by the 52-kDa FK506-binding protein (FKBP52)

The large FK506-binding protein FKBP52 has been characterized as an important positive regulator of androgen, glucocorticoid and progesterone receptor signaling pathways. FKBP52 associates with receptor-Hsp90 complexes and is proposed to have roles in both receptor hormone binding and receptor subcellular localization. Data from biochemical and cellular studies have been corroborated in whole animal models as fkbp52-deficient male and female mice display characteristics of androgen, glucocorticoid and/or progesterone insensitivity. FKBP52 receptor specificity and the specific phenotypes displayed by the fkbp52-deficient mice have firmly established FKBP52 as a promising target for the treatment of a variety of hormone-dependent diseases. Recent studies demonstrated that the FKBP52 FK1 domain and the proline-rich loop within this domain are functionally important for FKBP52 regulation of receptor function. Based on these data, efforts are currently underway to target the FKBP52 FK1 domain and the proline-rich loop with small molecule inhibitors.

The role of p23, Hop, immunophilins, and other co-chaperones in regulating Hsp90 function

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes, such as protein translation, translocation, and disassembly of protein complexes (1). Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

High-yield expression and purification of the Hsp90-associated p23, FKBP52, HOP and SGTα proteins

Hsp90 is a ubiquitous molecular chaperone that plays a key role in the malignant development of hormone-dependent pathologies such as cancer. An important role for Hsp90 is to facilitate the stable binding of steroid hormones to their respective receptors enabling the ligand-based signal to be carried to the nucleus and ultimately resulting in the up-regulation of gene expression. Along with Hsp90, this dynamic and transient process also involves the recruitment of additional proteins and co-chaperones that add further stability to the mature receptor-chaperone complex. In the work presented here, we describe four new protocols for the bacterial over-expression and column chromatographic purification of the human p23, FKBP52, HOP and SGTα proteins. Each of these proteins plays a distinct role in the steroid hormone receptor regulatory cycle. Affinity, ion-exchange and size-exclusion techniques were used to produce target yields greater than 50mg/L of cultured media, with each purified sample reaching near absolute sample homogeneity. These results reveal a reliable system for the production of p23, FKBP52, HOP and SGTα substrate proteins for use in the investigation of the Hsp90-associated protein interactions of the steroid hormone receptor cycle.

Alterations in the steroid hormone receptor co-chaperone FKBPL are associated with male infertility: a case-control study

BACKGROUND

Male infertility is a common cause of reproductive failure in humans. In mice, targeted deletions of the genes coding for FKBP6 or FKBP52, members of the FK506 binding protein family, can result in male infertility. In the case of FKBP52, this reflects an important role in potentiating Androgen Receptor (AR) signalling in the prostate and accessory glands, but not the testis. In infertile men, no mutations of FKBP52 or FKBP6 have been found so far, but the gene for FKBP-like (FKBPL) maps to chromosome 6p21.3, an area linked to azoospermia in a group of Japanese patients.

METHODS

To determine whether mutations in FKBPL could contribute to the azoospermic phenotype, we examined expression in mouse and human tissues by RNA array blot, RT-PCR and immunohistochemistry and sequenced the complete gene from two azoospermic patient cohorts and matching control groups. FKBPL-AR interaction was assayed using reporter constructs in vitro.

RESULTS

FKBPL is strongly expressed in mouse testis, with expression upregulated at puberty. The protein is expressed in human testis in a pattern similar to FKBP52 and also enhanced AR transcriptional activity in reporter assays. We examined sixty patients from the Japanese patient group and found one inactivating mutation and one coding change, as well as a number of non-coding changes, all absent in fifty-six controls. A second, Irish patient cohort of thirty showed another two coding changes not present in thirty proven fertile controls.

CONCLUSIONS

Our results describe the first alterations in the gene for FKBPL in azoospermic patients and indicate a potential role in AR-mediated signalling in the testis.

A four-hour yeast bioassay for the direct measure of estrogenic activity in wastewater without sample extraction, concentration, or sterilization

The assay described here represents an improved yeast bioassay that provides a rapid yet sensitive screening method for EDCs with very little hands-on time and without the need for sample preparation. Traditional receptor-mediated reporter assays in yeast were performed twelve to twenty four hours after ligand addition, used colorimetric substrates, and, in many cases, required high, non-physiological concentrations of ligand. With the advent of new chemiluminescent substrates a ligand-induced signal can be detected within thirty minutes using high picomolar to low nanomolar concentrations of estrogen. As a result of the sensitivity (EC(50) for estradiol is approximately 0.7nM) and the very short assay time (2-4h) environmental water samples can typically be assayed directly without sterilization, extraction, and concentration. Thus, these assays represent rapid and sensitive approaches for determining the presence of contaminants in environmental samples. As proof of principle, we directly assayed wastewater influent and effluent taken from a wastewater treatment plant in the El Paso, TX area for the presence of estrogenic activity. The data obtained in the four-hour yeast bioassay directly correlated with GC-mass spectrometry analysis of these same water samples.

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.

FK506-binding protein 52 phosphorylation: a potential mechanism for regulating steroid hormone receptor activity

Functional maturation of steroid hormone receptors requires ordered assembly into a large multichaperone complex consisting of receptor monomer, an Hsp90 dimer, the p23 cochaperone, and an FK506-binding protein (FKBP) family member or alternate peptidylprolyl isomerase-related cochaperone. Previous cellular studies demonstrated that FKBP52 can potentiate receptor function. These results have been confirmed in fkbp4 gene knockout mice in which males are partially androgen insensitive and females display characteristics of progesterone insensitivity. Conversely, FKBP51, which has a high degree of similarity to FKBP52, antagonizes FKBP52-mediated potentiation. Both proteins consist of three domains: two FKBP12-like domains termed FK1 and FK2 and a tetratricopeptide repeat domain that targets binding to Hsp90. To help understand why the two FKBPs behave differently and to gain insight into FKBP52 potentiation activity, we have analyzed the loop structure that links FK1 and FK2. Within the FK linker of FKBP52 is the sequence TEEED, which forms a consensus casein kinase II phosphorylation site; the corresponding sequence in FKBP51 is FED. We demonstrate that the distinct FK linker sequences per se do not account for lack of potentiation activity by FKBP51. However, phosphorylation of the FK linker appears to be an important regulatory determinant of FKBP52-mediated potentiation of steroid receptor activity.

Cochaperone immunophilin FKBP52 is critical to uterine receptivity for embryo implantation

Embryo implantation in the uterus is a critical step in mammalian reproduction, requiring preparation of the uterus receptive to blastocyst implantation. Uterine receptivity, also known as the window of implantation, lasts for a limited period, and it is during this period blastocysts normally implant. Ovarian steroid hormones estrogen and progesterone (P(4)) are the primary regulators of this process. The immunophilin FKBP52 serves as a cochaperone for steroid hormone nuclear receptors to govern appropriate hormone action in target tissues. Here we show a critical role for FKBP52 in mouse implantation. This immunophilin has unique spatiotemporal expression in the uterus during implantation, and females missing the Fkbp52 gene have complete implantation failure due to lack of attainment of uterine receptivity. The overlapping uterine expression of FKBP52 with nuclear progesterone receptor (PR) in wild-type mice together with reduced P(4) binding to PR, attenuated PR transcriptional activity and down-regulation of several P(4)-regulated genes in uteri of Fkbp52(-/-) mice, establishes this cochaperone as a critical regulator of uterine P(4) function. Interestingly, ovulation, another P(4)-mediated event, remains normal. Collectively, the present investigation provides evidence for an in vivo role for this cochaperone in regulating tissue-specific hormone action and its critical role in uterine receptivity for implantation.

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.

Functional specificity of co-chaperone interactions with Hsp90 client proteins

A wide array of proteins in signal transduction pathways depend on Hsp90 and other chaperone components for functional maturation, regulation, and stability. Among these Hsp90 client proteins are steroid receptors, members from other classes of transcription factors, and representatives of both serine/threonine and tyrosine kinase families. Typically, dynamic complexes form on the client protein, and these consist of Hsp90- plus bound co-chaperones that often have enzymatic activities. In addition to its direct influence on client folding, Hsp90 locally concentrates co-chaperone activity within the client complex, and dynamic exchange of co-chaperones on Hsp90 facilitates sampling of co-chaperone activities that may, or may not, act on the client protein. We are just beginning to understand the nature of biochemical and molecular interactions between co-chaperone and Hsp90-bound client. This review focuses on the differential effects of Hsp90 co-chaperones toward client protein function and on the specificity that allows co-chaperones to discriminate between even closely related clients.

Cooperation of heat shock protein 90 and p23 in aryl hydrocarbon receptor signaling

Aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by the binding of xenobiotic and endogenous ligands. AhR interacts with heat shock protein (Hsp) 90 complexes and can be used as a functional substrate to detect chaperone-dependent processes. Yeast Hsp90 (hsp82) mutants that variably affected AhR signaling were identified using reporter gene assays. Some mutated alleles resided in the p23/adenosine triphosphate (ATP)-binding pocket of Hsp90, so the relationship between the cochaperone Sba1 (yeast p23) and adenosine triphosphatase (ATPase) activity was investigated. Deletion of the p23 gene in the hsp82G170D mutant background had a greater effect on AhR signaling than the individual mutations, suggesting that these 2 mutations have separate actions on AhR signaling. In contrast, p23 overexpression suppressed temperature sensitivity and AhR signaling defects in the hsp82G170D mutant strain, suggesting that there is a relationship between these 2 proteins. The mutated hsp82G170D protein lacked detectable ATPase activity and p23 binding in vitro, which may relate to the weakened AhR signaling observed in mutant cells. Sba1 (p23) suppressed Hsp82 ATPase activity in vitro. These studies implicate the p23 protein and the G170 region of Hsp90 as being important, but not essential, for AhR signaling. Our results are consistent with a model in which p23 inhibits Hsp90 ATPase activity, thereby stabilizing ATP-Hsp90-client protein complexes.

Pharmacological and genetic analysis of 90-kDa heat shock isoprotein-aryl hydrocarbon receptor complexes

The 90-kDa heat shock protein (Hsp90) is an abundant chaperone that regulates a diverse set of intracellular signaling proteins. Drugs that inhibit Hsp90 activity have been useful in the identification of novel Hsp90-dependent signaling pathways. One class of inhibitory compounds disrupts Hsp90-dependent processes by binding to the N-terminal ATPase/p23-binding domain of Hsp90, whereas a second inhibitor class binds within the C-terminal domain. We used signaling by aryl hydrocarbon receptor (AhR), an Hsp90-dependent transcription factor, as a functional probe to study the effects of Hsp90 inhibitors in yeast strains with deletion mutations of individual Hsp90 and p23 cochaperone genes. The more abundant and constitutively expressed Hsp90 isoform, Hsc82, functioned best in supporting AhR signaling. Deletion of the more inducible isoform, Hsp82, had no effect on signaling. AhR complexes containing Hsc82 were preferentially sensitive to the effects of low concentrations of the N-terminal inhibitors radicicol and herbimycin A. However, both Hsp90 isoforms were equally sensitive to the AhR-specific effects of novobiocin, which binds to the C terminus. Hsp90 inhibitors had no preferential effects on AhR signaling in strains that lacked p23, suggesting that the inhibitors exert their effects through a p23-independent mechanism. In contrast, overexpression of p23 buffered the effects of radicicol and herbimycin A, but not novobiocin, on AhR signaling. The data collectively suggest preferential use or function of the Hsc82 isoprotein in AhR signaling and provide new insight into the effects of three structurally unrelated Hsp90 inhibitors.