The 90 kDa heat-shock protein (hsp90) modulates the binding of the oestrogen receptor to its cognate DNA

p23 and Aha1: Distinct Functions Promote Client Maturation

Hsp90 is a conserved molecular chaperone regulating the folding and activation of a diverse array of several hundreds of client proteins. The function of Hsp90 in client processing is fine-tuned by a cohort of co-chaperones that modulate client activation in a client-specific manner. They affect the Hsp90 ATPase activity and the recruitment of client proteins and can in addition affect chaperoning in an Hsp90-independent way. p23 and Aha1 are central Hsp90 co-chaperones that regulate Hsp90 in opposing ways. While p23 inhibits the Hsp90 ATPase and stabilizes a client-bound Hsp90 state, Aha1 accelerates ATP hydrolysis and competes with client binding to Hsp90. Even though both proteins have been intensively studied for decades, research of the last few years has revealed intriguing new aspects of these co-chaperones that expanded our perception of how they regulate client activation. Here, we review the progress in understanding p23 and Aha1 as promoters of client processing. We highlight the structures of Aha1 and p23, their interaction with Hsp90, and how their association with Hsp90 affects the conformational cycle of Hsp90 in the context of client maturation.

Targeting HSP90 as a Novel Therapy for Cancer: Mechanistic Insights and Translational Relevance

Heat shock protein (HSP90), a highly conserved molecular chaperon, is indispensable for the maturation of newly synthesized poly-peptides and provides a shelter for the turnover of misfolded or denatured proteins. In cancers, the client proteins of HSP90 extend to the entire process of oncogenesis that are associated with all hallmarks of cancer. Accumulating evidence has demonstrated that the client proteins are guided for proteasomal degradation when their complexes with HSP90 are disrupted. Accordingly, HSP90 and its co-chaperones have emerged as viable targets for the development of cancer therapeutics. Consequently, a number of natural products and their analogs targeting HSP90 have been identified. They have shown a strong inhibitory effect on various cancer types through different mechanisms. The inhibitors act by directly binding to either HSP90 or its co-chaperones/client proteins. Several HSP90 inhibitors—such as geldanamycin and its derivatives, gamitrinib and shepherdin—are under clinical evaluation with promising results. Here, we review the subcellular localization of HSP90, its corresponding mechanism of action in the malignant phenotypes, and the recent progress on the development of HSP90 inhibitors. Hopefully, this comprehensive review will shed light on the translational potential of HSP90 inhibitors as novel cancer therapeutics.

ERα-LBD, an isoform of estrogen receptor alpha, promotes breast cancer proliferation and endocrine resistance

Estrogen receptor alpha (ERα) drives mammary gland development and breast cancer (BC) growth through an evolutionarily conserved linkage of DNA binding and hormone activation functions. Therapeutic targeting of the hormone binding pocket is a widely utilized and successful strategy for breast cancer prevention and treatment. However, resistance to this endocrine therapy is frequently encountered and may occur through bypass or reactivation of ER-regulated transcriptional programs. We now identify the induction of an ERα isoform, ERα-LBD, that is encoded by an alternative ESR1 transcript and lacks the activation function and DNA binding domains. Despite lacking the transcriptional activity, ERα-LBD is found to promote breast cancer growth and resistance to the ERα antagonist fulvestrant. ERα-LBD is predominantly localized to the cytoplasm and mitochondria of BC cells and leads to enhanced glycolysis, respiration and stem-like features. Intriguingly, ERα-LBD expression and function does not appear to be restricted to cancers that express full length ERα but also promotes growth of triple-negative breast cancers and ERα-LBD transcript (ESR1-LBD) is also present in BC samples from both ERα(+) and ERα(-) human tumors. These findings point to ERα-LBD as a potential mediator of breast cancer progression and therapy resistance.

Effects of heat stress on the endometrial epidermal growth factor profile and fertility in dairy cows

The endometrial epidermal growth factor (EGF) profile is an indicator of uterine function and fertility in cattle. The present study aimed to investigate the effects of heat stress on the endometrial EGF profile and fertility in lactating Holstein cows. The endometrial EGF profiles of 365 cows in the Hokkaido and Kyushu regions were examined between June and September (heat stress period, n = 211) and between October and January (control period, n = 154). EGF profiles were investigated using uterine endometrial tissues obtained by biopsy 3 days after estrus (Day 3). The proportion of cows with an altered EGF profile was higher between June and September than between October and January (41.2 vs. 16.2%, P < 0.05). The effects of rectal temperature on Days 0 and 3 on the endometrial EGF profile were also assessed in cows (n = 79) between June and September in the Kyushu region. A single embryo was transferred to cow on Day 7 to evaluate fertility (n = 67). Regardless of the rectal temperature on Day 3, the proportion of cows with an altered EGF profile was higher (64.1 vs. 30.0%, P < 0.05) and the pregnancy rate after embryo transfer (ET) was lower (26.7 vs. 51.4%, P < 0.05) in cows with a rectal temperature ≥ 39.5°C on Day 0 than in cows with a rectal temperature < 39.5°C on Day 0. The present results indicate that alterations in the endometrial EGF profile induced by an elevated body temperature on Day 0 contributed to reductions in fertility in lactating dairy cows during the heat stress period.

The n-butanol fraction of the Xiao-Chai-Hu decoction alleviates the endocrine disturbance in the liver of mice exposed to lead

ETHNOPHARMACOLOGICAL RELEVANCE

Lead is a toxic heavy metal that causes health risks globally. However, the mechanism of endocrine poisoning and detoxification of lead poisoning, especially in the liver, still needs to be studied. Xiao-Chai-Hu decoction (XCHD) is regarded as an antidote and an anti-hepatotoxic traditional prescription that has been recorded in the pharmacopeia of the People's Republic of China.

AIM OF THE STUDY

The study aimed to probe the hepatoprotective activity of XCHD in the regulation of endocrine dysfunction in the liver and its molecular mechanism.

MATERIALS AND METHODS

The mice from the Institute of Cancer Research (ICR) were exposed to different concentrations of XCHD and lead. Then, serum biochemical indices and liver pathology were analyzed. The key differential genes were detected by qRT-PCR and Western blot.

RESULTS

According to the biochemical and histopathological analysis, XCHD-NBA was the most effective in attenuating lead-induced hepatotoxicity. From the transcriptome, we analyzed the key genes of XCHD-NBA in the regulation of lead toxicity, including Tubb2a, Stip1, Cyp4a12a, Cyp2c50, Ugt1a1, Cyp3a11, Cyp4a12b, Ahsa1, Cyp2c54, Tubb4b, Esr1, Hsp90aa1, Tuba1a, Tuba1c, and Hsph1. We also analyzed the main components of XCHD-NBA by LC-MS. Because of their extensive role in regulating the endocrine function, baicalin and glycyrrhizin were identified as the main active components of XCHD in regulating endocrine disorders caused by lead.

CONCLUSIONS

Lead can disturb the endocrine regulatory process of the liver, while XCHD-NBA alleviates lead-induced liver injury by regulating the endocrine regulatory process.

Exposure to 2.45 GHz radiofrequency modulates calcitonin-dependent activity and HSP-90 protein in parafollicular cells of rat thyroid gland

In this study we analyzed the response of parafollicular cells in rat thyroid gland after exposure to radiofrequency at 2.45 GHz using a subthermal experimental diathermy model. Forty-two Sprague Dawley rats, divided into two groups of 21 rats each, were individually exposed at 0 (control), 3 or 12 W in a Gigahertz Transverse Electro-Magnetic (GTEM) chamber for 30 min. After radiation, we used simple or fluorescence immunohistochemistry to measure calcitonin cells or cellular stress levels, indicated by the presence hyperplasia of parafollicular cells, heat shock protein (HSP) 90. Immunomarking of calcitonin-positive cells was statistically significant higher in the thyroid tissue of rats exposed to 2.45 GHz radiofrequency and cell hyperplasia appeared 90 min after radiation at the SAR levels studied. At the same time, co-localized expression of HSP-90 and calcitonin in parafollicular cells was statistically significant attenuated 90 min after radiation and remained statistically significantly low 24 h after radiation, even though parafollicular cell levels normalized. These facts indicate that subthermal radiofrequency (RF) at 2.45 GHz constitutes a negative external stress stimulus that alters the activity and homeostasis of parafollicular cells in the rat thyroid gland. However, further research is needed to determine if there is toxic action in human C cells.

The Nuclear and DNA-Associated Molecular Chaperone Network

Maintenance of a healthy and functional proteome in all cellular compartments is critical to cell and organismal homeostasis. Yet, our understanding of the proteostasis process within the nucleus is limited. Here, we discuss the identified roles of the major molecular chaperones Hsp90, Hsp70, and Hsp60 with client proteins working in diverse DNA-associated pathways. The unique challenges facing proteins in the nucleus are considered as well as the conserved features of the molecular chaperone system in facilitating DNA-linked processes. As nuclear protein inclusions are a common feature of protein-aggregation diseases (e.g., neurodegeneration), a better understanding of nuclear proteostasis is warranted.

Daily heat treatment maintains mitochondrial function and attenuates atrophy in human skeletal muscle subjected to immobilization

Skeletal muscle immobilization leads to atrophy, decreased metabolic health, and substantial losses in function. Animal models suggest that heat stress can provide protection against atrophy in skeletal muscle. This study investigated the effects of daily heat therapy on human skeletal muscle subjected to 10 days of immobilization. Muscle biopsies were collected, and MRIs were analyzed from the vastus lateralis of 23 healthy volunteers (11 women, 12 men) before and after either 10 days of immobilization with a daily sham treatment (Imm) or with a targeted, daily 2-h heat treatment using pulsed shortwave diathermy (Imm + H). Diathermy increased intramuscular temperature 4.2 ± 0.29°C (P < 0.0001), with no change during sham treatment. As a result, heat shock protein (HSP)70 and HSP90 increased (P < 0.05) following Imm + H (25 ± 6.6 and 20 ± 7.4%, respectively) but were unaltered with Imm only. Heat treatment prevented the immobilization-induced loss of coupled (-27 ± 5.2% vs. -8 ± 6.0%, P = 0.0041) and uncoupled (-25 ± 7.0% vs. -10 ± 3.9%, P = 0.0302) myofiber respiratory capacity. Likewise, heat treatment prevented the immobilization-induced loss of proteins associated with all five mitochondrial respiratory complexes (P < 0.05). Furthermore, decreases in muscle cross-sectional area following Imm were greater than Imm + H at both the level of the whole muscle (-7.6 ± 0.96% vs. -4.5 ± 1.09%, P = 0.0374) and myofiber (-10.8 ± 1.52% vs. -5.8 ± 1.49%, P = 0.0322). Our findings demonstrate that daily heat treatments, applied during 10 days of immobilization, prevent the loss of mitochondrial function and attenuate atrophy in human skeletal muscle. NEW & NOTEWORTHY Limb immobilization results in substantial decreases in skeletal muscle size, function, and metabolic capacity. To date, there are few, if any, interventions to prevent the deleterious effects of limb immobilization on skeletal muscle health. Heat stress has been shown to elicit a stress response, resulting in increased heat shock protein expression and improved mitochondrial function. We show that during 10 days of lower-limb immobilization in humans, daily exposure to heat stress maintains mitochondrial respiratory capacity and attenuates atrophy in skeletal muscle. Our findings suggest that heat stress may serve as an effective therapeutic strategy to attenuate the decreases of muscle mass and metabolic function that accompany periods of disuse.

Control of steroid receptor dynamics and function by genomic actions of the cochaperones p23 and Bag-1L

Molecular chaperones encompass a group of unrelated proteins that facilitate the correct assembly and disassembly of other macromolecular structures, which they themselves do not remain a part of. They associate with a large and diverse set of coregulators termed cochaperones that regulate their function and specificity. Amongst others, chaperones and cochaperones regulate the activity of several signaling molecules including steroid receptors, which upon ligand binding interact with discrete nucleotide sequences within the nucleus to control the expression of diverse physiological and developmental genes. Molecular chaperones and cochaperones are typically known to provide the correct conformation for ligand binding by the steroid receptors. While this contribution is widely accepted, recent studies have reported that they further modulate steroid receptor action outside ligand binding. They are thought to contribute to receptor turnover, transport of the receptor to different subcellular localizations, recycling of the receptor on chromatin and even stabilization of the DNA-binding properties of the receptor. In addition to these combined effects with molecular chaperones, cochaperones are reported to have additional functions that are independent of molecular chaperones. Some of these functions also impact on steroid receptor action. Two well-studied examples are the cochaperones p23 and Bag-1L, which have been identified as modulators of steroid receptor activity in nuclei. Understanding details of their regulatory action will provide new therapeutic opportunities of controlling steroid receptor action independent of the widespread effects of molecular chaperones.

Combination use of ferulic acid, ligustrazine and tetrahydropalmatine inhibits the growth of ectopic endometrial tissue: a multi-target therapy for endometriosis rats

ETHNOPHARMACOLOGICAL RELEVANCE

Ferulic acid (FA), ligustrazine (LZ) and tetrahydropalmatine (THP) are separately isolated from Chinese Angelica, Szechwan Lovage Rhizome and Rhizoma in the Jiawei-Foshou-San formula, a popular traditional Chinese medicine for irregular menses. It has been reported that the combination use of FA+LZ+THP has similar effect on endometriosis, but the underlying mechanisms are unclear. This study was to investigate the combination effects and mechanisms of FA+LZ+THP on endometriosis rats.

MATERIALS AND METHODS

Fifty endometriosis rats were intragastricly treated with FA+LZ+THP for 4 wk. The volume of ectopic endometrial tissue was measured to evaluate the effects. Then the changes in hypothalamic-pituitary-ovarian axis and ERE pathway were indicated by the levels of E2, GnRH, FSH and LH, and the expressions of ER, HSP90 and COX-2, respectively. In addition, peritoneal macrophages of each rat were cultured in vitro and treated with (FA+LZ+THP)-medicated serum for 24h. The proliferation and phagocytosis abilities, the levels of IL-1β and TNF-α, and the expression of IκBα were then measured for the changes of peritoneal macrophage activities.

RESULTS

Combination use of FA+LZ+THP diminished the volume of the ectopic endometrial tissues (P<0.05 or P<0.01). It also decreased the E2 level, suppressed the expression of GnRH, FSH and LH, and decreased the protein expression of ER, HSP90 and COX-2 (all P<0.05 or P<0.01). The phagocytosis ability of peritoneal macrophage was enhanced by (FA+LZ+THP)-medicated serum (P<0.05) with no change of proliferation (P>0.05). Moreover, IL-1β and TNF-α were downregulated (both P<0.05 or P<0.01) and IκBα was upregulated by the (FA+LZ+THP)-medicated serum (P<0.01).

CONCLUSIONS

The combination use of FA, LZ and THP could inhibit the growth of ectopic endometrial tissue in endometriosis rats. It might be related to the down-regulation of hypothalamic-pituitary-ovarian axis, the amelioration in ERE pathway and the improvement of peritoneal macrophage activities.

Bisphenol-A affects the developmental progression and expression of heat-shock protein genes in the moth Sesamia nonagrioides

The effects of bisphenol A (BPA) on the endocrine system of vertebrates have been demonstrated in several studies. Here, we report the impact of BPA on the developmental progression and expression of heat shock protein genes on the terrestrial insect Sesamia nonagrioides (Lepidoptera: Noctuidae). S. nonagrioides 1st instar larvae were exposed until the end of 6th (last) instar to selected concentrations of BPA (1 μg/L, 10 μg/L, 100 μg/L, 1 mg/L and 10 mg/L) applied in their artificial diets. The lower doses of BPA (1-10 μg/L) were found to decrease larvae's weight while the 100 μg/L dose increased it. The higher doses of BPA were found to induce various abnormal phenotypes during 5th instar larval molting, larval-pupal transformation and metamorphosis. The developmental and metamorphosis endpoints presented here may indicate the possible impact of BPA on terrestrial insects. Additionally, 6th instar larvae were injected with several concentrations of BPA. Semi-quantitative and Real-Time PCR assays were used to identify the effects of BPA in the transcriptional regulation of five heat shock protein genes (SnoHsp19.5, SnoHsp20.8, SnoHsp70, SnoHsc70 and SnoHsp83). Application of BPA by feeding or by injection induced the synthesis of the SnoHsp19.5 and SnoHsp20.8 mRNAs. The expression levels of SnoHsp70 were not affected. In contrast, SnoHsc70 and SnoHsp83, which play a pivotal role in vertebrate sex steroid signal transduction, were elevated by BPA. Our results suggest that SnoHsp19.5, SnoHsp20.8, SnoHsp83 and SnoHsc70 genes can be modulated by BPA.

Hepatic protein expression networks associated with masculinization in the female fathead minnow (Pimephales promelas)

Endocrine disruptors that act via the androgen receptor (AR) are less well studied than environmental estrogens, and there is evidence that treatment with AR agonists can result in masculinization of female fish. In this study, female fathead minnows (FHM) were exposed to the model nonaromatizable androgen 5-alpha dihydrotestosterone (DHT) (100 μg/L), the ureic-based herbicide linuron (LIN) (100 μg/L), and a mixture of DHT and LIN (100 μg/L each) to better characterize androgen action in females. LIN was used because of reports that this chemical has an antiandrogenic mode of action in fish. After 21d, DHT and LIN treatments resulted in a significant depression of plasma vitellogenin (Vtg) and DHT and DHT+LIN increased the prevalence of nuptial tubercles in female FHMs indicating masculinization. Using iTRAQ and an LTQ Orbitrap Velos, ∼2000 proteins were identified in the FHM liver and the number of proteins quantified after exposures was >1200. Proteins that significantly and consistently changed in abundance across biological replicates included prostaglandin E synthase 3, programmed cell death 4a, glutathione S transferases, canopy, selenoprotein U, and ribosomal proteins. Subnetwork enrichment analysis identified that interferon and epidermal growth factor signaling were regulated by DHT and LIN, suggesting that these signaling pathways are correlated to depressed plasma vitellogenin. These data provide novel insight into hepatic protein networks that are associated with the process of masculinization in teleosts.

Comparison of expression levels of candidate genes in endometrium of dairy heifers and lactating dairy cows

Pretheeban, T., Gordon, M. B., Singh, R. and Rajamahendran, R. 2011. Comparison of expression levels of candidate genes in endometrium of dairy heifers and lactating dairy cows. Can. J. Anim. Sci. 91: 255–264. Pregnancy rates (PR) in high-producing lactating dairy cows have declined drastically over the past several decades, but those of heifers have remained constant. Reduced PR could be due to multiple causes, and the underlying pathophysiological mechanisms are still unclear. A compromised maternal uterine environment could be one of factors that could affect the PR. This study was performed to compare the nature of the uterine environment in dairy heifers and lactating dairy cows (2nd/3rd parity) by analyzing the expression levels of selected endometrial genes. Estrus was synchronized in heifers (n=5) and lactating dairy cows (n=5) and endometrial biopsies were performed during the mid luteal phase (day 11) of the estrous cycle. Real-time polymerase chain reaction (Q-RT PCR) and immunohistochemistry were performed to analyse the mRNA and protein levels of genes respectively. Relative abundance of BCL2, HSPA1A, IL1A, TNF, IGF1, FGF2 and SERPINA14 transcripts and the protein expression of IL1A, TNF and FGF2 were significantly higher in heifers in comparison with lactating dairy cows. Our findings suggest an altered endometrial environment in lactating dairy cows compared with heifers. However, whether these differences play a role in pregnancy outcomes should be further investigated.

Influence of selective estrogen receptor modulators on interleukin-6 expression in human retinal pigment epithelial cells (ARPE-19)

Since estrogen and selective estrogen receptor modulators can inhibit inflammatory responses, we studied the regulatory role of several selective estrogen receptor modulators on interleukin-6 (IL-6) expression in human retinal pigment epithelial cells (ARPE-19). ARPE-19 cells were exposed to lipopolysaccharide with simultaneous exposure to different selective estrogen receptor modulators with the secretion of IL-6 cytokine being analyzed by enzyme-linked immunosorbent assay (ELISA). We demonstrate that 17beta-estradiol and HM-D, a novel selective estrogen receptor modulator compound, clearly reduced the IL-6 expression levels after lipopolysaccharide exposure in ARPE-19 cells. Molecular effects of selective estrogen receptor modulators and estrogen on the estrogen response element-mediated transcription were studied using MCF-7 and ARPE-19 cell lines carrying the estrogen response element-luciferase reporter gene. Estrogen and HM-D stimulated the activity of estrogen response element-reporter gene in MCF-7 cells but did not affect the activity in ARPE-19 cells. In addition, HM-D did not activate estrogen receptor alpha when studied by nuclear receptor peptide estrogen receptor alpha ELISA in ARPE-19 cells. These results indicate that estrogen and HM-D can suppress the lipopolysaccharide-induced inflammatory response but signalling is not mediated through estrogen response element transcription in human retinal pigment epithelial cells.

Regulation of ER alpha signaling pathway in neuronal HN10 cells: role of protein acetylation and Hsp90

Estrogen has a variety of neuroprotective effects but the molecular basis of its function is still mainly unclear. Estrogen receptor (ER) signaling is highly dependent on posttranslational modifications and the assembly of coactivator and corepressor complexes. Several proteins involved in ER alpha signaling have recently been found to be acetylated, including ER alpha itself and Hsp90, a key chaperone in the functional regulation of ER alpha. ER alpha complexes also contain histone deacetylases (HDAC) which repress transactivation. Our purpose was to clarify the role of protein acetylation and Hsp90 function in the ERE-mediated ER alpha signaling in neuronal HN10 cells. We observed that increasing protein/histone acetylation status with trichostatin A, a potent HDAC inhibitor, increased the 17beta-estradiol (E2)-induced transactivation of ERE-driven luciferase in non-transfected cells, and even more extensively in pER alpha-transfected cells. E2-induced ERE-driven transactivation was blocked by ICI 182.780. Several ER antagonists, such as raloxifene and tamoxifen, were unresponsive. Valproate, an antiepileptic drug which is recently characterized as a HDAC inhibitor, was also able to potentiate the E2-induced ERE-transactivation. Inhibition of the function of Hsp90 chaperone with geldanamycin strongly inhibited the E2-induced ERE-transactivation. Overexpression of SIRT2 protein deacetylase did not inhibit the acetylation-potentiated ERE-driven transactivation indicating that SIRT2 deacetylase is not involved in ER alpha signaling. Our results reveal that ER alpha signaling is dependent on protein acetylation and epigenetic regulation.

Heat shock protein 90alpha recruits FLIPS to the death-inducing signaling complex and contributes to TRAIL resistance in human glioma

Heat shock protein 90 (HSP90) is a molecular chaperone that contributes to the proper folding and stability of target proteins. Because HSP90 has been suggested to interact with FLIP(S), the key regulator of tumor necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in glioma cells, we examined the role HSP90 played in controlling TRAIL response. HSP90alpha was found to associate with FLIP(S) in resting cells in a manner dependent on the ATP-binding NH2-terminal domain of HSP90alpha. Following TRAIL exposure, HSP90alpha and the client FLIP(S) protein were recruited to the death-inducing signaling complex (DISC). Short interfering RNA-mediated suppression of HSP90alpha did not alter the total cellular levels of FLIP(S), but rather inhibited the recruitment of FLIP(S) and other antiapoptotic proteins such as RIP and FLIP(L) to the DISC, and sensitized otherwise resistant glioma cells to TRAIL-induced apoptosis. These results show that HSP90alpha, by localizing FLIP(S) to the DISC, plays a key role in the resistance of tumor cells to TRAIL, and perhaps other proapoptotic agents. The results also define a novel means of apoptotic control by a HSP90alpha that may in turn help explain the global antiapoptotic effects of this protein.

Are heat shock proteins therapeutic target for Parkinson's disease?

Heat shock proteins (HSPs), known as molecular chaperone to assist protein folding, have recently become a research focus in Parkinson's disease (PD) because the pathogenesis of this disease is highlighted by the intracellular protein misfolding and inclusion body formation. The present review will focus on the functions of different HSPs and their protective roles in PD. It is postulated that HSPs may serve as protein folding machinery and work together with ubiquitin-proteasome system (UPS) to assist in decomposing aberrant proteins. Failure of UPS is thought to play a key role in the pathogenesis of PD. In addition, HSPs may possess anti-apoptotic effects and keep the homeostasis of dopaminergic neurons against stress conditions. The critical role of HSPs and recent discovery of some novel HSPs inducers suggest that HSPs may be potential therapeutic targets for PD and other neurodegenerative disorders.

Bisphenol A in artificial indoor streams: II. Stress response and gonad histology in Gammarus fossarum (Amphipoda)

The effects of the world wide-distributed chemical bisphenol A (BPA) on the endocrine system of vertebrates have been demonstrated in several studies. Here, we report on the impact of BPA (0, 5, 50 and 500 microg/l nominally, deduced effective concentrations 0, 0.24, 2.4, and 24.1 microg/l, respectively, all at 15 degrees C) on the 70 kD stress protein family (hsp70), the 90 kD stress protein family (hsp90), and gonad histology of the crustacean Gammarus fossarum exposed in artificial indoor streams. The animals were exposed for a maximum of 103 days and samples were taken at the beginning and at days 34, 69 and 103 of the experiment. Exposure to BPA resulted in accelerated maturation of oocytes in females and in a decline in the number and size of early vitellogenic oocytes. The level of hsp90, which plays a pivotal role in vertebrate sex steroid signal transduction, was significantly reduced by BPA. In all five streams, measured parameters did not indicate any captivity stress for a period of 69 days. Beyond this time, the mortality rate and proteotoxic effects, the latter measured by hsp70 expression, were found to be elevated.

Antagonist-induced, activation function-2-independent estrogen receptor alpha phosphorylation

Estrogen receptor alpha (ERalpha) serine 118 (Ser118) phosphorylation modulates activation function-1 (AF1) function. Correct positioning of helix 12 promotes agonist-dependent recruitment of cyclin-dependent kinase-7 to catalyze this event. In this study we show robust cyclin-dependent kinase-7-independent, AF2 antagonist-induced Ser118 phosphorylation. Estradiol (E2) and ICI-182,780 (ICI-780) induce Ser118 phosphorylation of wild-type ERalpha and either of two helix 12 mutants, suggesting AF2-independent action, probably via shedding of 90-kDa heat shock protein. With E2 treatment, the predominantly nuclear, phosphorylated ERalpha in COS-1 cells is detergent soluble. Although levels of ICI-780-induced phosphorylation are profound, Ser118-phosphorylated ERalpha is aggregated over the nucleus or in the cytoplasm, fractionating with the cell debris and making detection in cleared lysates improbable. Selective ER modulators (SERMs) elicit a mixed response with phosphorylated ERalpha in both detergent-soluble and -insoluble compartments. Apparent ligand-induced loss of ERalpha protein from cleared lysates is thus due to ligand-induced redistribution into the pellet, not degradation. The COS-1 response to ICI-780 can be mimicked in MCF-7 cells treated with a proteasome inhibitor to block authentic ligand-induced degradation. With SERMs and antagonists, the magnitude of Ser118-phosphorylated receptor redistribution into the insoluble fraction of COS-1 cells correlates with the magnitude of authentic ERalpha degradation in MCF-7 cells. A strong inverse correlation with ligand-induced uterotropism in vivo (P < 0.0001) and direct correlation with AF2-independent transrepression of the matrix metalloprotease-1 promoter in endometrial cells in vitro are seen. These data suggest that ligand-induced Ser118 phosphorylation of ERalpha can be AF2 independent. Furthermore, they identify translocation of Ser118-phosphorylated ERalpha out of the nucleus, leading to cytoplasmic aggregation, as an antagonist pathway that may precede receptor degradation.

Biological roles of alpha-fetoprotein during pregnancy and perinatal development

The use of alpha-fetoprotein (AFP) as a serum marker in cancer actually predates its employment in the detection of congenital defects; however, the latter use of AFP as a fetal defect marker has propelled its clinical utilization. Although the serum-marker capacity of AFP has long been exploited, less is known of the biological activities of this oncofetal protein during fetal and perinatal development. In the present review, the biological activities of AFP are discussed in light of this glycoprotein's presence in various biological fluid compartments: embryonic and fetal tissues, serum, urine, and reproductive fluids. After a review of the histochemical detection of AFP in various cells and tissues during development, AFP concentrations within various biological fluids were discussed in the context of gestational age and anatomic location. Discussion follows concerning the relationships and roles of AFP in developmental events such as erthyropoiesis, histogenesis/organogenesis, and ligand binding and in developmental disorders such as hypothyroidism, folate deficiencies, and acquired immunodeficiency disorder (AIDS). Based on its association with so many types of birth defects, malformations, and congenital anomalies, AFP can be viewed as a molecular "troubleshooter" until signal transduction pathways are established during pregnancy and prenatal development. The review concludes with a discussion of the place of AFP in the rapidly expanding field of proteomics.

An Hsp27-related, dominant-negative-acting intracellular estradiol-binding protein

New World primates (NWPs) exhibit a compensated form of resistance to gonadal steroid hormones. We demonstrated recently that estrogen resistance in NWP cells was associated with the overexpression of two proteins, a nonreceptor-related, dominant-negative-acting estrogen response element (ERE)-binding protein (ERE-BP) and an intracellular estradiol-binding protein (IEBP). Based on the N-terminal sequences of tryptic fragments of IEBP isolated from a 17beta-estradiol (E2) affinity column we cloned a full-length cDNA for IEBP from the estrogen-resistant NWP cell line, B95-8. Subsequent sequence analysis revealed 87% sequence identity between the deduced peptide for IEBP and human Hsp27. When hormone-responsive, wild-type Old World primate (OWP) cells were transiently transfected with IEBP cDNA, E2-directed ERE reporter luciferase activity was reduced by 50% compared with vector only-transfected OWP cells (p < 0.0018). When IEBP and ERE-BP were cotransfected, ERE promoter-reporter activity was reduced by a further 60% (p < 0.0001). Electrophoresis mobility shift analyses showed that IEBP neither bound to ERE nor competed with the estrogen receptor (ER) for binding to ERE. However, there was evidence of protein-protein interaction of IEBP and ERalpha; IEBP was coimmunoprecipitated with anti-ERalpha antibody in wild-type cells stably transfected with IEBP. A specific interaction between ERalpha and IEBP was confirmed in glutathione S-transferase pull-down and yeast two-hybrid assays. Data indicate that the Hsp27-related IEBP interacts with the ligand binding domain of the ERalpha. In summary, by inhibiting the ERalpha-E2 interaction, IEBP acts to squelch ERalpha-directed ERE-regulated transactivation and promote estrogen resistance in NWP cells.

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.

Novel activation step required for transcriptional competence of progesterone receptor on chromatin templates

To elucidate the earliest molecular steps in the activation of transcription by the progesterone receptor (PR), we investigated its activity in a cell-free transcription system utilizing chromatin templates. PR prepared as a ligand-free, recombinant protein failed to induce transcription on chromatin templates. However, transcriptional competence could be restored by coincubation with rabbit reticulocyte lysate (RRL). The interaction of PR with chaperones results in a receptor conformation competent to bind ligand and RRL contains abundant chaperone-mediated protein folding activity. Blocking this activity with the specific inhibitor geldanamycin inhibited receptor-dependent transcriptional activity. However, recombinant chaperones could not replace RRL in the restoration of transcriptional activity on chromatin templates, suggesting the presence of an additional activity in the lysate. Under chromatin assembly conditions, PR could bind naked DNA and RRL did not increase that binding. In contrast, PR bound to a chromatin template only poorly. Interestingly, RRL stimulated sequence-specific binding by PR to target sites in chromatin and the concomitant recruitment of the steroid receptor coactivator 1 to the promoter. Thus, our results indicate that a novel protein-mediated activity in RRL is involved in an additional, heretofore unrecognized, activation step required for PR to become transcriptionally competent on chromatin templates.

Overexpressed glucocorticoid receptor negatively regulates gene expression under conditions that favour accumulation of non-hormone-binding forms of the receptor

Previous reports have suggested that the native hormone-responsive glucocorticoid receptor is a heterocomplex with hsp90 and that the receptor constantly cycles between the hormone-responsive and an inactive state, with complex assembly and turnover being driven by hsp70 and hsp90, respectively. Since hsp70 appears to be titrated in cells that transiently overexpress the receptor, assembly intermediates may accumulate when more receptor is produced than can be assembled to hormone-responsive complex. Comparison of receptor protein and hormone-binding levels in extracts from transiently transfected COS-7 cells revealed the presence of non-hormone-binding receptor forms in addition to the native heterocomplex. The receptor was predominantly nuclear in the majority of the transfected cells even in the absence of hormone, with the DNA-binding domain (DBD) being necessary for nuclear localisation. Moreover, the unliganded receptor exhibited constitutive DNA-binding activity and reactivity towards antibodies against the hinge region where NLS1 is known to reside. By comparing fluorography to immunoblotting of two-dimensional SDS-PAGE of cross-linked [3H]dexamethasone-mesylate-labelled receptor, we detected non-hormone-binding receptor species capable of binding DNA in vitro. In addition, using a constitutively active receptor mutant, we found that the overexpressed wild-type receptor was capable of repressing mutant-activated transcription of transiently and stably transfected reporter genes alike in a DBD-dependent manner.

Kinetic analysis of estrogen receptor homo- and heterodimerization in vitro

The coexistence of ERalpha and ERbeta suggests that active receptor complexes are present as homo- or heterodimers. In addition each of three forms of active receptors may trigger different cellular responses. A real-time biosensor based on surface plasmon resonance was used as instrument to determine binding kinetics of homo- and heterodimerization of estrogen receptor alpha and beta. Partially purified full-length estrogen receptor alpha was expressed intracellularly as a C-terminal fusion to a hexa-histidine tag using the baculovirus-expression system. Purified estrogen receptor alpha and beta without tags were used as partners in the dimerization process. An association rate constant of 3.6 x 10(3) to 1.5 x 10(4)M(-1)s(-1) for the homodimer formation of ERalpha and 5.7 x 10(3) to 1.5 x 10(4)M(-1)s(-1) for the heterodimer formation was found assuming a pseudo first-order reaction kinetic. The equilibrium dissociation constant for homodimerization of ERalpha was 2.2 x 10(-8) to 5.4 x 10(-8) and 1.8 x 10(-8) to 2.6 x 10(-8)M for the heterodimer formation. The homo- and heterodimer formation was characterized by a slow association kinetics and kinetic rate constants were within the same range.

Increases in mouse uterine heat shock protein levels are a sensitive and specific response to uterotrophic agents

There is increasing consensus that the uterotrophic estrogenicity assay should be coupled with other morphometric or molecular end points that might enhance its sensitivity. We have previously shown that bisphenol A (BPA), similarly to 17ss-estradiol (E2), increases levels of uterine heat shock proteins (hsps), mainly hsp90alpha and glucose-regulated protein (grp) 94. In this study we investigated whether increases in uterine hsp levels are a specific response of estrogens or estrogen mimics. We therefore examined the ability of a) E2, diethylstilbestrol (DES), and tamoxifen (TAM); b) the xenoestrogens coumestrol (CM), methoxychlor (MXC), BPA, and dibutyl phthalate (DBP); c) the progestin medroxyprogesterone (MED); d) the glucocorticoid dexamethasone (DEX); and e) phytol (PHY), a precursor to a retinoid X and peroxisome proliferator-activating receptor agonist, to increase uterine weights and alter uterine morphology and hsp levels. We showed that DES, TAM, CM, MXC, and BPA significantly increased uterine weights and uterine hsp90alpha and grp94 levels. Even though the doses of CM, MXC, and BPA used were much higher than the E2 dose, those treatments resulted in lower increases in uterine weight. On the other hand, increases in grp94 levels were equal to those induced by E2 treatment. Treatments with MED, DEX, DBP, or PHY did not significantly alter uterine weight or morphology and had no significant effects on uterine hsp levels. The results of this study suggest that only the estrogens increase uterine hsp90alpha and grp94 levels, and that this hsp effect is a more sensitive uterotrophic response than uterine weight increase.

Transcriptional activation by the oestrogen receptor alpha is modulated through inhibition of cyclin-dependent kinases

We have investigated the interaction between the expression of p21(WAF1/CIP1/SDI1), a stoichiometric inhibitor of Cdk, and the transcriptional activity of the oestrogen receptor alpha (ER(alpha). Transient transfection experiments demonstrated that the expression of p21(WAF1/CIP1/SDI1) amplified the transcriptional activation by ER(alpha). A dominant negative mutant of Cdk2 also enhanced the ER(alpha) transcriptional activity, indicating that the underlying mechanism relies on the inhibition of Cdk2 activity and cell cycle arrest. In agreement with this conclusion, experiments with p21(WAF1/CIP1/SDI1) mutants demonstrated that the domain involved in the binding of p21(WAF1/CIP1/SDI1) to Cdks was indispensable for the modulation of ER(alpha) activity. In addition, we show that expression of p21(WAF1/CIP1/SDI1) alleviates the block on CBP function mediated by Cdk2 and in turn stimulates transcriptional activation by ER(alpha) in a CBP-histone acetyltransferase (HAT)-dependent manner. These results suggest a novel mechanism by which p21(WAF1/CIP1/SDI1) functions as an enhancer of ER(alpha) activity through the modulation of CBP function.

The amount of estrogen receptor alpha increases after heat shock in a cholinergic cell line from the basal forebrain

Estrogen exerts a neuroprotective action in response to a variety of cell stresses. However, to what extent intracellular estrogen receptors are involved in these functions remains to be determined. We have found that SN56 cells, a neuronal-derived cholinergic cell line which produces luteinizing hormone-releasing hormone and contains the mRNAs encoding estrogen and progesterone receptors, also express estrogen receptor alpha, as well as the heat shock protein 90. Exposure of these cells to drastic temperature elevation for 1 h immediately increased the intracellular levels of these two proteins, whereas it rapidly reduced the content of estrogen receptor alpha mRNA. In addition, the amount of estrogen receptor alpha-heat shock protein 90 complexes was increased in response to thermal stress. Pre-treatment with geldanamycin, a potent inhibitor of heat shock protein 90, decreased the amount of estrogen receptor alpha, suggesting that its elevation after the heat insult may be related to its association with heat shock protein 90. In contrast, exposure of heat-shocked cells to 17beta-estradiol reduced the number of estrogen receptor alpha-heat shock protein 90 complexes, suggesting that the receptor conserves the affinity for its cognate ligand under these conditions. Therefore, the interaction of the estrogen receptor with heat shock protein 90 may serve to prevent its degradation during the thermal insult, as well as to maintain it in a high-affinity hormone-binding conformation. Since neuroprotective estrogen effects have been described in a variety of cytotoxic situations, these findings may be suggestive of an integrated neuronal response to injury, which includes the protection of available estrogen receptors through their association with heat shock protein 90.

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

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

The Hsp70-Ydj1 molecular chaperone represses the activity of the heme activator protein Hap1 in the absence of heme

In Saccharomyces cerevisiae, heme directly mediates the effects of oxygen on transcription through the heme activator protein Hap1. In the absence of heme, Hap1 is bound by at least four cellular proteins, including Hsp90 and Ydj1, forming a higher-order complex, termed HMC, and its activity is repressed. Here we purified the HMC and showed by mass spectrometry that two previously unidentified major components of the HMC are the Ssa-type Hsp70 molecular chaperone and Sro9 proteins. In vivo functional analysis, combined with biochemical analysis, strongly suggests that Ssa proteins are critical for Hap1 repression in the absence of heme. Ssa may repress the activities of both Hap1 DNA-binding and activation domains. The Ssa cochaperones Ydj1 and Sro9 appear to assist Ssa in Hap1 repression, and only Ydj1 residues 1 to 172 containing the J domain are required for Hap1 repression. Our results suggest that Ssa-Ydj1 and Sro9 act together to mediate Hap1 repression in the absence of heme and that molecular chaperones promote heme regulation of Hap1 by a mechanism distinct from the mechanism of steroid signaling.

The N-terminal adenosine triphosphate binding domain of Hsp90 is necessary and sufficient for interaction with estrogen receptor

To understand how the molecular chaperone Hsp90 participates in conformational maturation of the estrogen receptor (ER), we analyzed the interaction of immobilized purified avian Hsp90 with mammalian cytosolic ER. Hsp90 was either immunoadsorbed to BF4 antibody-Sepharose or GST-Hsp90 fusion protein (GST.90) was adsorbed to glutathione-Sepharose. GST.90 was able to retain specifically ER, similarly to immunoadsorbed Hsp90. When cells were treated with estradiol and the hormone treatment was maintained during cell homogenization, binding, and washing steps, GST.90 still interacted efficiently with ER, suggesting that ER may form complexes with Hsp90 even after its activation by hormone and salt extraction from nuclei. The GST.90-ER interaction was consistently reduced in the presence of increasing concentrations of potassium chloride or when cytosolic ER-Hsp90 complexes were previously stabilized by molybdate, indicating that GST.90-ER complexes behave like cytosolic Hsp90-ER complexes. A purified thioredoxin-ER fusion protein was also able to form complexes with GST.90, suggesting that the presence of other chaperones is not required. ER was retained only by GST.90 deletion mutants bearing an intact Hsp90 N-terminal region (1-224), the interaction being more efficient when the charged region A was present in the mutant (1-334). The N-terminal fragment 1-334, devoid of the dimeric GST moiety, was also able to interact with ER, pointing to the monomeric N-terminal adenosine triphosphate binding region of Hsp90 (1-224) as the region necessary and sufficient for interaction. These results contribute to understand the Hsp90-dependent process responsible for conformational competence of ER.

Identification of interaction partners for two closely-related members of the ETS protein family, FLI and ERG

Fli and erg are two members of the ETS gene family that encodes transcription factors related to the c-ets-1 proto-oncogene. The products of the ETS genes act as transcriptional effectors in cell proliferation, differentiation, and oncogenic transformation. FLI and ERG, two closely-related proteins, bind, as do all the ETS proteins characterized so far, to DNA sequences with an invariable central GGA core flanked by preferred nucleotides. Nevertheless, promoter-specific responses to FLI or ERG may be driven by mechanisms involving multicomponent complexes. Using a yeast two-hybrid screen, we have identified several proteins that physically interact with either FLI or ERG proteins used as bait. The Xenopus developmentally implicated Xvent-2 and Xvent-2B proteins, and the Xenopus splicing factor RNP-C/U1C physically interact with Xl-FLI and Xl-ERG, both in the yeast two-hybrid system and in vitro. We also report the potential interaction of FLI and ERG with Sox-D, a stabilizing protein that may modulate their transcriptional activity. Furthermore, the possible involvement of the transcriptional effectors FLI and ERG in mRNA processing, hematopoiesis or in the control of angiogenesis is suggested through possible interactions with, respectively, RNA binding proteins and hnRNPs, a repressor of the hematopoietic pathway (SAP18), and the HAF protein.

Continuous recycling: a mechanism for modulatory signal transduction

Modulatory signal transduction commonly requires efficient "on demand" assembly of specific multicomponent cellular machines that convert signals to cellular actions. This article suggests that for these signaling machines to detect and respond to fluctuations in signal strength, they must be continuously disassembled in an energy-dependent process that probably involves molecular chaperones.

Steroid receptor and molecular chaperone encounters in the nucleus

Steroid hormone receptors interact with several different molecular chaperones. DeFranco and Csermely discuss how the molecular chaperones p23 and Hsp90 may serve to regulate the activity of the ligand-bound steroid receptors within the nucleus. The authors hypothesize that these chaperone proteins may have a proactive role in promoting recycling of receptors once they have interacted with chromatin and in allowing rebinding of ligand once the receptors have been recycled.

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.

Role of molecular chaperones in subnuclear trafficking of glucocorticoid receptors

The delivery of activated steroid receptors to high-affinity genomic sites must be efficient enough to account for the rapidity and selectivity of many transcriptional responses to steroid hormones. Thus, the signal transduction capacity of steroid hormone receptors will be influenced by the efficiency of receptor trafficking both between different subcellular compartments (that is, the cytoplasm and nucleus) and within a specific compartment (that is, the nucleus). Molecular chaperones, such as heat shock proteins, have long been recognized to play important roles in the management of protein folding in both stressed and nonstressed cells. In recent years, the participation of these proteins in various signal transduction pathways (for example, steroid hormone responses) has also been recognized. In this review, recent results that implicate a role for distinct heat shock proteins in subnuclear trafficking of glucocorticoid receptors are discussed. These studies also highlight the importance of mobilizing the cellular chaperone machinery for managing steroid receptor folding within the nucleus.

Induction of heat-shock (stress) protein gene expression by selected natural and anthropogenic disturbances in the octocoral Dendronephthya klunzingeri

Previously it was found that the expression of selected heat-shock proteins is upregulated in corals after exposure to elevated temperature. We published that HSPs are suitable markers in sponges to monitor the degree of environmental stress on these animals. In the present study the heat-shock proteins (HSPs) with a molecular weight of 90 kDa have been selected to prove their potential usefulness as biomarkers under controlled laboratory conditions and in the field. The studies have been performed with the octocoral Dendronephthya klunzingeri4.5-fold higher steady-state level of the respective mRNA. Also animals taken from stressed locations in the field showed an increased expression. The amount of HSP90 protein in D. klunzingeri was found to be strongly increased under thermal stress, or exposure to polychlorinated biphenyl (congener 118), but not after treatment with cadmium. Field studies revealed that samples taken from a nonstressed area have a low level of HSP90, but those collected from locations at which the corals are under physical stress (sedimentation through landfilling) show a high expression of HSP90. It is concluded that the chaperone HSP90 might become a suitable biomarker to monitor environmental stress on corals.

Use of digitonin-permeabilized cells in studies of steroid receptor subnuclear trafficking

The application of a cell permeabilization technique to the analysis of nuclear import has led to many major breakthroughs in our understanding of this trafficking pathway. Digitonin permeabilization maintains the nucleus in a state competent for faithful, signal-dependent translocation through the nuclear pore complex. This system has also been used to probe the mechanism of hormone-regulated nuclear import through the use of glucocorticoid receptors (GR) as a model substrate. In this report we provide detailed descriptions of the digitonin-permeabilized cell system for use in studies of GR nuclear import. In addition, we present several novel applications that expand the utility of this system to probe for mechanisms of nuclear protein export and subnuclear trafficking.

Role of HSP90 in mediating cross-talk between the estrogen receptor and the Ah receptor signal transduction pathways

Tetrachlorodibenzo-p-dioxin (TCDD)-mediated gene transactivation via the Ah receptor (AhR) has been shown to be dependent upon estrogen receptor (ER) expression in human breast cancer cells. We have investigated the 90-kDa heat shock protein (HSP90) as a mediator of cross-talk between the AhR and the ER signal transduction pathways. The effect of HSP90 overexpression on receptor activity was determined by transient transfection assays using a HSP90 expression vector. Ligand-inducible gene expression was inhibited when the HSP90 expression vector was cotransfected with a TCDD-responsive reporter plasmid. However, overexpression of HSP90 did not block induction of an estrogen-responsive reporter plasmid. To determine whether ER facilitates AhR signaling through its ability to squelch HSP90, two vectors expressing protein products that bind HSP90 were transfected into MDA-MB-231 cells. Introduction of (i) He11, an ER deletion mutant that does not bind DNA, and (ii) the ligand-binding domain of human AhR, both led to increased basal and TCDD-inducible CYP1A1 expression. Finally, the subcellular distribution of HSP90 was investigated in human breast cancer cell lines. These studies showed HSP90 to be primarily cytoplasmic in ER-positive cell lines, whereas in matched ER-negative cell lines HSP90 was distributed equally between the cytoplasm and nucleus. Taken together, these results demonstrate that HSP90 can regulate AhR activity in vivo, and that Ah-responsiveness is dependent upon cellular ER content through a mechanism that involves HSP90.

Estrogen induction of the cyclin D1 promoter: involvement of a cAMP response-like element

Estrogens induce cell proliferation in target tissues by stimulating progression through the G(1) phase of the cell cycle. Induction of cyclin D1 expression is a critical feature of the mitogenic action of estrogen. We have determined a region between -96 and -29 in the cyclin D1 promoter that confers regulation by estrogens in the human mammary carcinoma cells MCF-7. This region encompasses a unique known transcription factor binding site with a sequence of a potential cAMP response element (CRE-D1). The induction is strictly hormone dependent and requires the DNA binding domain as well as both AF-1 and AF-2 domains of the estrogen receptor (ER) alpha. Destruction of the CRE-D1 motif caused complete loss of estrogen responsiveness. Both c-Jun and ATF-2 transactivated the cyclin D1 promoter in transient transfection experiments, and a clear additional increase was detected when ER was cotransfected with either c-Jun or with c-Jun and ATF-2 but not with ATF-2 alone. Furthermore, the expression of a dominant negative variant of c-Jun, TAM67, completely abolished the induction of the cyclin D1 promoter both in the absence and presence of ER. We show that ATF-2 homodimers and ATF-2/c-Jun heterodimers, but not c-Jun homodimers, were able to bind the CRE of the cyclin D1 promoter. To interpret these results, we propose a mechanism in which ATF-2/c-Jun heterodimers bind to the CRE-D1 element and mediate the activation of cyclin D1 promoter by the ER. This mechanism represents a pathway by which estrogens control the proliferation of target cells.

Divergent mechanisms for loss of Ah-responsiveness in benzo[a]pyrene- and adriamycinR-resistant MCF-7 cells

The intracellular aryl hydrocarbon receptor (AhR) mediates signal transduction by environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene by functioning as a ligand-activated transcription factor. We have investigated AhR signaling in sublines of the human breast cancer cell line MCF-7 selected for resistance to AdriamycinR (AdrR) and benzo[a]pyrene (BP(R)). Previously we reported that AdrR cells have a loss of estrogen receptor (ER) expression and are Ah-nonresponsive. Here we show that AhR mRNA and protein are expressed at normal levels in AdrR cells, and the activated AhR complex is functionally capable of binding a xenobiotic responsive element. In MCF-7 cells AhR was depleted to 15% of normal levels after 4 hr TCDD treatment; however, 45% of AhR remained in AdrR cells during this time course. In BP(R) cells AhR mRNA levels were found to be decreased relative to wild-type cells, which led to decreased AhR protein levels and DNA-binding activity. Cellular ER content has been shown to correlate with Ah-responsiveness in human breast cancer cell lines. BP(R) cells were found to be ER-positive, although chronic (BP(R) cells) and acute (24 hr) exposure to benzo[a]pyrene led to significantly lower ER protein levels in MCF-7 cells. We conclude that loss of Ah-responsiveness occurs by different mechanisms in xenobiotic-resistant MCF-7 sublines: AhR mRNA is down-regulated in BP(R) cells, whereas AdrR cells are deficient in AhR signaling by a mechanism unrelated to AhR expression and activity.

Chromatin recycling of glucocorticoid receptors: implications for multiple roles of heat shock protein 90

Unliganded glucocorticoid receptors (GRs) released from chromatin after hormone withdrawal remain associated with the nucleus within a novel subnuclear compartment that serves as a nuclear export staging area. We set out to examine whether unliganded nuclear receptors cycle between distinct subnuclear compartments or require cytoplasmic transit to regain hormone and chromatin-binding capacity. Hormone-withdrawn rat GrH2 hepatoma cells were permeabilized with digitonin to deplete cytoplasmic factors, and then hormone-binding and chromatin-binding properties of the recycled nuclear GRs were measured. We found that recycled nuclear GRs do not require cytosolic factors or ATP to rebind hormone. Nuclear GRs that rebind hormone in permeabilized cells target to high-affinity chromatin-binding sites at 30 C, but not 0 C, in the presence of ATP. Since geldanamycin, a heat shock protein-90 (hsp90)-binding drug, inhibits hormone binding to recycled nuclear GRs, hsp90 may be required to reassemble the receptor into a form capable of productive interactions with hormone. Geldanamycin also inhibits GR release from chromatin during hormone withdrawal, suggesting that hsp90 chaperone function may play multiple roles to facilitate chromatin recycling of GR.

The molecular chaperone Hsp90 can negatively regulate the activity of a glucocorticosteroid-dependent promoter

Hsp90, a molecular chaperone required for the functioning of glucocorticosteroid receptor (GR), ensures, by direct interaction, the conformational competence of the steroid-binding pocket. In addition to having this positive function, Hsp90 maintains steroid receptors in an inactive form in the absence of hormone. However, neither the participation of Hsp90 once the pathway has been activated by the ligand nor the importance of increased Hsp90 levels in determining the amplitude of the response has ever been assessed directly. Here, by increasing the Hsp90/GR ratio in the nuclear compartment, we found an attenuation of the response to glucocorticosteroids that was not due to a nonspecific or toxic effect of the Hsp90 modified by nuclear targeting. Since this negative effect was more pronounced at high levels of hormone, when receptor and Hsp90 are maximally dissociated, the possibility of an interaction between Hsp90 and GR, already activated to a DNA-binding form, was directly investigated. Indeed GR, after in vivo activation by ligand, was still able to reassociate with Hsp90, suggesting that this interaction plays a role in vivo, possibly in receptor recycling. Moreover, the GR binding to its DNA response element was inhibited by an excess of Hsp90, pointing to a function of Hsp90 in the nuclear compartment. It is thus proposed that an increased Hsp90/GR ratio influences the responsiveness to ligand at a step that is after receptor activation. This increased ratio may be of pathophysiological relevance in the different circumstances that lead to an elevated level of nuclear Hsp90.

Heat-induced oligomerization of the molecular chaperone Hsp90. Inhibition by ATP and geldanamycin and activation by transition metal oxyanions

It has been previously reported that heat shock protein 90 (Hsp90) oligomerizes at high temperatures and displays concomitantly a novel chaperone activity (Yonehara, M., Minami, Y., Kawata, Y., Nagai, J., and Yahara, I. (1996) J. Biol. Chem., 271, 2641-2645). In order to better define these oligomerization properties at high temperatures and to know whether they are influenced by modulators of Hsp90 function, heat-induced oligomerization of highly purified dimeric Hsp90 has been investigated over a wide range of temperature and protein concentrations by native polyacrylamide gel electrophoresis and size exclusion chromatography. Whereas below 50 degreesC, the dimeric form is maintained over a large range of concentrations, at the critical temperature of 50 degreesC, a sharp transition from dimeric to higher order oligomeric species takes place within minutes, in a highly ordered process, suggesting that a conformational change, leading to the appearance of a new oligomerization site, occurs in Hsp90 dimer. Moreover, at and above the critical temperature, the extent of oligomerization increases with Hsp90 concentration. Formation of high order oligomers at high temperatures is sensitive to modulators of Hsp90 function. ATP and geldanamycin, both known to bind to the same pocket of Hsp90, are inhibitors of this process, whereas molybdate, vanadate, and Nonidet P-40, which are thought to increase surface hydrophobicity of the protein, are activators. Thus, oligomerization of Hsp90 at high temperatures may be mediated through hydrophobic interactions that are hindered by ligands and favored by transition metal oxyanions. The fact that the heat-induced oligomerization of Hsp90 is affected by specific ligands that modulate its properties also suggests that this process may be involved in cell protection during heat shock.

Oestrogen receptor facilitates the formation of preinitiation complex assembly: involvement of the general transcription factor TFIIB

The action of oestrogen hormones is mediated through the oestrogen receptor (ER), a member of a large superfamily of nuclear receptors that function as ligand-activated transcription factors. Sequence-specific transcription factors, including the nuclear receptor superfamily, are thought to interact either directly or indirectly with general transcription factors to regulate transcription. Although numerous studies have focused on the identification of potential co-activators interacting with isolated trans-activation domains of ER, few have investigated the mechanisms by which ER transmits its signal to the basal transcription machinery. We show that ER does not stabilize the binding of the TATA-box binding protein (TBP) of the TFIID complex, or of TFIIB to the promoter, although a stable ER-TBP-TFIIB-promoter complex was detected, suggesting that ER, TBP and TFIIB might interact with each other to form a complex to the promoter. We also demonstrate that ER binds specifically to TFIIB, a key component of the preinitiation complex. Affinity chromatography with immobilized deletion mutants of ER maps a TFIIB interaction region that encompasses the DNA-binding domain. The addition of excess TFIIB to transcription reactions in vitro did not, however, affect the magnitude of transcriptional activation by ER. These results indicate that, in contrast with current models, ER does not activate transcription by increasing the rate of assembly of TFIIB into the transcription complex. An increased concentration of TFIIB was unable, by itself, to overcome the requirement for ER. By using an immobilized promoter-template assay employing nuclear extract from HeLa cells, recombinant human ER increased the stable association of subsequent components of the transcription machinery (TFIIE and TFIIF), in correlation with ER-induced transcription. Our results suggest that ER acts, in an early step, during or immediately after the formation of template-committed complexes containing TFIIB, favouring the recruitment of one or more components of the basic transcription machinery as well as co-activators.