Interactive regulation of Ah and glucocorticoid receptors in the synergistic induction of cleft palate by 2,3,7,8-tetrachlorodibenzo-p-dioxin and hydrocortisone

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a wide-spread environmental contaminant that produces adverse biological effects including carcinogenesis, reproductive toxicity, immune dysfunction, hyperkeratosis, hepatotoxicity, thymic involution and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. Glucocorticoids are endogenous steroid compounds that have an important role in development, but are teratogenic at pharmacological doses. The synthetic glycocorticoid, hydrocortisone (HC), induces cleft plate and a potent, synergistic interaction has been observed between TCDD and HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor, the Ah receptor (AhR) and the glucocorticoid receptor (GR), respectively. The morphology and etiology of TCDD- and HC-induced clefts are distinctly different, as HC clefting is due to formation of small palatal shelves, while TCDD-treated shelves fail to fuse due to effects on epithelial cell proliferation and differentiation. The present study examines the expression of AhR and GR in the embryonic palate following exposure to TCDD, HC, and HC + TCDD. C57BL/6N pregnant mice were treated with HC (25 or 100 mg/kg/day GD10-13, sc), TCDD (3 micrograms/kg/day GD10-13, or 24 micrograms/kg GD10, orally), or HC + TCDD (25 mg/kg/day sc and 3 micrograms/kg/day orally, GD10-13). Craniofacial tissues were collected from the embryos on GD14 and examined for AhR and GR expression using in situ hybridization. Northern blots, and immunohistochemistry. We found that in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC + TCDD produced increased expression of both receptors. Effects on AhR appeared to be regulated at the transcriptional level, as both protein and mRNA were altered in similar directions. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.

Ah receptor in embryonic mouse palate and effects of TCDD on receptor expression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most potent member of a family of halogenated aromatic hydrocarbons which are widespread environmental contaminants. In animals the adverse biological effects of TCDD include carcinogenesis, reproductive toxicity, immune function alteration, hyperkeratosis, hepatotoxicity, thymic involution, and teratogenesis. In the mouse embryo, TCDD induces cleft palate through a mechanism which involves altered differentiation and proliferation of the palatal cells, resulting in the failure of opposing shelves to fuse. Cleft palate induction by TCDD requires the Ah receptor. This study examines the expression of the Ah receptor in secondary palate of control and TCDD-exposed C57BL/6N embryos using in situ hybridization, Northern blots, and immunohistochemistry. Ah receptor protein expression was significantly higher in epithelial versus mesenchymal cells, and regional differences in expression within the epithelium were statistically significant. TCDD exposure was shown to downregulate Ah receptor mRNA and protein throughout the palatal shelf and this occurred at both the teratogenic dose and the dose which was not sufficient to produce cleft palate. This study represents the first demonstration of the tissue and cellular localization of the Ah receptor, raising questions about the extrapolation of results from cultured tumor cells to those observed in vivo.

Localization and characterization of the 86- and 84-kDa heat shock proteins in Hepa 1c1c7 cells

The 90-kDa heat shock protein (hsp90) is present in cells at high levels in the cytoplasm and is composed of two separate gene products, hsp86 and hsp84. Rabbit polyclonal antibodies to the murine N-terminal sequences of the 86- and 84-kDa heat shock proteins were isolated from serum by peptide affinity chromatography. Antibodies against each form of hsp90 are capable of immunoprecipitating hsp90. Each antibody preparation is specific against either hsp86 or hsp84 when tested on a protein blot of Hepa 1c1c7 cytosol. The over-all ratio of hsp84/hsp86 in Hepa 1 cytosol was estimated to be 2 to 1. Each antibody preparation was used to immunoprecipitate hsp84 or hsp86 from Hepa 1 cytosol to test whether hsp86/84 exists as a homo- and/or heterodimer. After electrophoresis, silver staining revealed that anti-hsp86 antibodies immunoprecipitated both hsp86 and hsp84. This result would suggest that hsp86 forms heterodimers with hsp84. In contrast, the anti-hsp84 antibodies immunoprecipitated almost entirely hsp84, suggesting that hsp84 exists largely as homodimers. Both anti-hsp86 and hsp84 antibodies were able to immunoprecipitate the 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dixoin-labeled Ah receptor from Hepa 1 cytosol, indicating that these antibodies are able to bind to hsp90 when it is complexed with other proteins. Both antibody preparations recognize hsp90 in mouse, rat, and human cell lines. Immunofluorescence and confocal microscopy were performed using both antibody preparations, and the results indicated that both hsp86 and hsp84 were located in the cytoplasm and nucleus of Hepa 1 cells. Hsp86 was found to localize unevenly in the cytoplasm, while hsp84 was found evenly dispersed throughout the cytoplasm. Hsp86 also appeared to be localized to a greater degree than hsp84 in the vicinity of the nuclear envelope.

Raf exists in a native heterocomplex with hsp90 and p50 that can be reconstituted in a cell-free system

Recently, we have demonstrated that the tyrosine kinase pp60v-src can undergo cell-free assembly into a heterocomplex with rabbit hsp90 and p50 when the immunoadsorbed protein is incubated with rabbit reticulocyte lysate (Hutchison, K. A., Brott, B. K., De Leon, J. H., Perdew, G. H., Jove, R., and Pratt, W. B. (1992) J. Biol. Chem 267, 2902-2908). Using a baculovirus system to express a high level of human c-Raf serine/threonine kinase in Sf9 insect cells, we show here that immunoadsorbed c-Raf undergoes similar lysate-mediated assembly into a heterocomplex with hsp90 and p50. As with pp60v-src and steroid receptors, binding of c-Raf to hsp90 occurs in an ATP-dependent and K(+)-dependent manner and the resulting heterocomplex is stabilized by molybdate. With a very rapid and gentle procedure of Sf9 cell cytosol preparation and c-Raf immunoadsorption, we show coimmunoadsorption of the insect homologue of hsp90. The same procedures permit detection of a native complex of v-Raf with rat hsp90 and p50 in stably transfected rat 3Y1 fibroblasts, and v-Raf is also assembled into a heterocomplex with rabbit hsp90 and p50 by reticulocyte lysate. Using the 22W mutant of c-Raf in which the NH2-terminal half has been deleted, we show that the catalytic domain of the kinase is sufficient for both formation of the native heterocomplex in mouse NIH 3T3 cells and cell-free reconstitution of the heterocomplex by rabbit reticulocyte lysate. Although the native Raf-heat shock protein heterocomplex is less stable than native pp60v-src and glucocorticoid receptor heterocomplexes, by analogy with these proteins its detection may have important implications regarding the mechanism of Raf trafficking through the cytoplasm.

Alterations in the Ah receptor level after staurosporine treatment

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to be mediated by the aryl hydrocarbon receptor (AhR). The ligand-receptor complex mediates the induction of CYP1A1 (cytochrome P(1)450) gene transcription through interaction with an Ah-responsive element. Staurosporine, a potent inhibitor of protein kinases inactivates the TCDD-induced CYP1A1 gene transcription. This study was initiated to study binding capacity, capability to translocate, and levels of the AhR in Hepa 1 cells after staurosporine treatment. Levels of the AhR were quantitated in Hepa 1 cells by sandwich type radioimmunochemical method using Rpt 1 (AhR monoclonal antibody) as the primary antibody and [125I]goat anti-mouse as the secondary antibody. Staurosporine treatment caused a time- and concentration-dependent decrease in the intracellular concentration of AhR. At 8 h, the EC50 for the staurosporine-dependent decrease in AhR was 75 nM. Although the receptor levels decreased significantly, it did not affect the properties of the receptor as judged by its ligand binding capacity and functional nuclear translocation of the existing AhR. The isoelectric point of the receptor was essentially unaltered after staurosporine treatment, an indirect indication that the degree of AhR phosphorylation did not change. There could be a variety of explanations for these results such as the decrease in the AhR levels may be due to either an increase in proteolytic activity caused by the imbalance of kinase/phosphatase levels or a decrease in de novo receptor synthesis.

Half-life of aryl hydrocarbon receptor in Hepa 1 cells: evidence for ligand-dependent alterations in cytosolic receptor levels

The rate of turnover of the Ah receptor (AhR) was determined using the density shift method in Hepa 1 and in a Hepa 1 mutant line, c4, which fails to accumulate AhR complexes in the nucleus. The half-life of the AhR was found to be 7.7 and 9.7 h in Hepa 1 and c4 cells, respectively. The effect of AhR occupation with either an agonist, beta-naphthoflavone (beta NF), or a partial antagonist, alpha-naphthoflavone (alpha NF), on AhR half-life and concentration in the cytosolic fraction was examined. In Hepa 1 cells, a 12-h exposure to beta NF resulted in a 62% decrease in AhR concentration. The same treatment, using alpha NF as the ligand, resulted in a 14% decrease. The half-life of the AhR increased from 7.7 to 9.3 h during beta NF treatment and was essentially the same during alpha NF treatment in Hepa 1 cells. In c4 cells, a 12-h exposure to beta NF resulted in a 44% decrease in AhR concentrations, whereas exposure to alpha NF resulted in an 8% decrease. The half-life of the AhR in c4 cells during beta NF exposure increased from 9.7 to 14.6 h, and alpha NF exposure increased half-life to 17.6 h. These results indicate: (a) cytosolic AhR concentrations are modulated by ligand occupation, (b) exposure to AhR ligands, after an initial decrease in AhR levels, resulted in an increase in AhR half-life, and (c) similar results were obtained in Hepa 1 and c4 cells, this would indicate that AhR occupation with ligand and subsequent AhR-ligand nuclear translocation does not appear to play a significant role in regulation of AhR half-life in Hepa 1 cells.

Reconstitution of the multiprotein complex of pp60src, hsp90, and p50 in a cell-free system

A rabbit reticulocyte lysate system that has been used to reconstitute functional complexes between steroid receptors and the 90-kDa heat shock protein (hsp90) has been used here to form complexes between the pp60src tyrosine kinase and hsp90. Reticulocyte lysate forms complexes between hsp90 and a temperature-sensitive mutant of Rous sarcoma virus pp60v-src, which is normally present in cytosol virtually entirely in the multiprotein complex form. In addition, hsp90 in the lysate complexes with wild-type pp60v-src, of which only a small portion is normally recovered in cytosol in the native multiprotein complex, and with the cellular homolog, pp60c-src, which has never been recovered in cytosol in the form of a native multiprotein complex with hsp90. Moreover, the reticulocyte lysate-reconstituted complex also contains the 50-kDa phosphoprotein component of the native pp60v-src multiprotein complex. The native and reconstituted pp60src-hsp90 complexes have similar thermal stability and, like steroid receptor heterocomplexes, they are stabilized by molybdate. As previously shown with reticulocyte lysate-reconstituted steroid receptor heteroprotein complexes, the reconstituted pp60src multiprotein complex contains hsp70, which is a major candidate for providing the protein unfoldase activity required for hsp90 association.

Chemical cross-linking of the cytosolic and nuclear forms of the Ah receptor in hepatoma cell line 1c1c7

Both cytosolic and high salt nuclear extracts were isolated from Hepa 1c1c7 cells incubated with 2-azido-3[125I]iodo-7,8-dibromo-dibenzo-p-dioxin ([125I]N3Br2DpD). The [125I]N3Br2DpD-labeled cytosolic fraction was subjected to chemical cross-linking with dimethyl pimelimidate and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Chemical cross-linking of the cytosolic form of the AhR revealed monomeric (97 kDa), dimeric (185 kDa), trimeric (281 kDa), and tetrameric (327 kDa) complexes. In a time course of exposure to the cross-linking reagent, the largest form given above became the predominant AhR form observed in the cytosolic extracts. The 327 kDa cytosolic species apparently consists of a 97 kDa AhR, an approximately 88 kDa protein, an approximately 96 kDa protein, and an approximately 46 kDa protein. Nuclear extracts from [125I]N3Br2DpD-labeled Hepa 1c1c7 cells were applied to sucrose density gradients. The 6 S nuclear receptor peak fractions were pooled and subjected to chemical cross-linking. Analysis by SDS-PAGE revealed a monomeric (97 kDa) ligand binding protein and a dimeric (182 kDa) complex. This would suggest that the nuclear 6 S AhR consists of a 97 kDa AhR and an approximately 85 kDa protein. These findings would indicate that the AhR exists in cytosol as a tetrameric species, while in the nucleus the AhR exists as a heterodimer.

Comparison of the nuclear and cytosolic forms of the Ah receptor from Hepa 1c1c7 cells: charge heterogeneity and ATP binding properties

2-[125I]iodo-7,8-dibromo-p-dioxin ([125I]Br2DpD) and 2-[125I]iodo-3-azido-7,8-dibromo-p-dioxin ([125I]N3Br2-DpD) are both capable of binding to the Ah receptor (AhR) with a high degree of specificity in cultured Hepa 1c1c7 cells. After incubation with either [125I]N3Br2DpD or [125I]Br2DpD Hepa 1c1c7 cytosolic and high salt nuclear extracts were analyzed by sucrose density gradient analysis with the following results: (i) With both radioligands an approximately 9 S form of the AhR was observed in cytosolic extracts. (ii) Nuclear extracts labeled with [125I]N3Br2DpD revealed both approximately 6 S and approximately 9 S forms of the AhR. (iii) In contrast, analysis of nuclear extracts labeled with [125I]Br2DpD revealed only an approximately 6 S form of the AhR. The approximately 9 S [125I]N3Br2DpD-labeled AhR was preferentially extracted with 100 mM KCl from a nuclear fraction and mixed with monoclonal antibody 8D3, an anti-90-kDa heat shock protein antibody. Monoclonal antibody 8D3 was able to bind to the approximately 9 S nuclear form of the AhR and caused the receptor to sediment as a heavier complex on sucrose density gradients. This would indicate that the AhR can reside in the nucleus bound to 90-kDa heat shock protein. The [125I]N3Br2DpD-labeled approximately 6 S peak fractions were collected and subjected to denaturing two-dimensional gel electrophoresis. A comparison of [125I]N3Br2DpD-labeled cytosolic (9 S) AhR preparations with the nuclear (6 S) AhR by 2-D gel electrophoresis was performed. The cytosolic form of the AhR was present in the apparent pI range of 5.2-5.7; the nuclear form focused between 5.5 and 6.2. The [125I]N3Br2DpD-labeled nuclear extracts were incubated with ATP-agarose and 43% of the photoaffinity-labeled AhR bound to the affinity gel. In contrast, approximately threefold lower binding of [125I]N3Br2DpD-labeled receptor was obtained when GTP-, AMP-, or ADP-agarose was used. Only 2% of the [125I]N3Br2DpD-labeled cytosolic AhR was able to bind to ATP-agarose. These results suggest that after the AhR translocates into the nucleus the following biochemical changes occur: (i) The sedimentation value for the AhR changes from an approximately 9 S to an approximately 6 S species. (ii) The AhR attains the ability to bind with specificity to ATP. (iii) The AhR undergoes a shift to a more basic pI.

A 50-kDa cytosolic protein complexed with the 90-kDa heat shock protein (hsp90) is the same protein complexed with pp60v-src hsp90 in cells transformed by the Rous sarcoma virus

We have previously shown that a 50-kDa protein is one component of a heteromeric complex immunoprecipitated by the 90-kDa heat shock protein (hsp90) monoclonal antibodies 8D3 and 3G3 (Perdew, G. H., and Whitelaw, M. L. (1991) J. Biol. Chem. 266, 6708-6713). In this report, we compare the 50-kDa protein with that found in pp60v-src-hsp90-p50 complexes immunoprecipitated from Rous sarcoma virus-transformed cells with antibodies to pp60v-src. 35S- and 32P-labeled p50 proteins from each system were identical in their mobilities by sodium dodecyl sulfate-polyacryl-amide gel electrophoresis. The profile of N-chlorosuccinimide cleavage products derived from each 32P-labeled p50 protein were also identical when resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We have developed a mouse monoclonal antibody, 3M/1B5p50, capable of detecting p50 on Western blots. This antibody detected the 50-kDa protein which co-purified with the pa104 pp60v-src mutant of the avian sarcoma virus oncoprotein in 44A rat fibroblasts. We did not detect p50 in association with native glucocorticoid receptor in L cells or with the overexpressed glucocorticoid receptor in Chinese hamster ovary cells. Two experiments utilizing immunochemical staining implied that essentially all cytosolic p50 is associated with hsp90. Firstly, immunoprecipitating hsp90 from Hepa 1 cytosol with monoclonal antibody 3G3 left the cytosol depleted of p50. Secondly, cytosol fractionated by sucrose gradient revealed that p50 cosedimented with hsp90, confirming the existence of p50 only in association with hsp90.

Detection of the Ah receptor in rainbow trout: use of 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin in cell culture

The Ah receptor was detected in RTG-2 cells (rainbow trout embryonic gonad cells) following the addition of the photoaffinity ligand, [125I]2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, to cells in culture. Cytosolic and nuclear extracts were prepared and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and revealed one radiolabeled band. Very little non-specific binding was observed under the conditions employed when compared to photoaffinity labeling RTG-2 cytosolic extracts in vitro. The photoaffinity-labeled Ah receptor in RTG-2 cytosol was analyzed by sucrose density centrifugation. The cytosolic form was observed to sediment at approximately 9.8S and the high salt nuclear extract form at approximately 7.5S. The relative molecular weight of the Ah receptor was determined to be 145 kDa under denaturing conditions and is considerably larger than the Ah receptor from mammalian sources. Inhibition of photoaffinity ligand binding to the RTG-2 cytosolic Ah receptor by competing ligands revealed the same rank order of ligand affinity as that previously demonstrated for the mouse Ah receptor.

Retinoic acid receptor belongs to a subclass of nuclear receptors that do not form "docking" complexes with hsp90

We have recently reported that, in contrast to the glucocorticoid receptor, the thyroid hormone receptor does not bind to hsp90 when the receptor is translated in rabbit reticulocyte lysate [Dalman, F. C., Koenig, R. J., Perdew, G. H., Massa, E., & Pratt, W. B. (1990) J. Biol. Chem. 265, 3615-3618]. All of the steroid receptors that are known to bind hsp90 are recovered in the cytosolic fraction when hormone-free cells are ruptured in hypotonic buffer. In contrast, unliganded thyroid hormone receptors and retinoic acid receptors are tightly associated with nuclear components. In this paper, we translated the human estrogen receptor and the human retinoic acid receptor in reticulocyte lysate and then immunoadsorbed the [35S]methionine-labeled translation products with the 8D3 monoclonal antibody against hsp90. The estrogen receptor is bound to hsp90, as indicated by coimmunoadsorption, but the retinoic acid receptor is not. Translation and immunoadsorption of chimeric proteins containing the DNA binding domain of one receptor and the N-terminal and COOH-terminal segments of the other show that the DNA binding finger region of the estrogen receptor is neither necessary nor sufficient for hsp90 binding. These observations suggest that there are two classes within the steroid receptor family. In one class (e.g., glucocorticoid, mineralocorticoid, sex hormone, and dioxin receptors), the receptors bind to hsp90 and remain in some kind of inactive "docking" mode until hormone-triggered release of hsp90 occurs. In the retinoic acid/thyroid hormone class, the unligated receptors do not bind to hsp90, and the receptors appear to proceed directly to their high-affinity nuclear acceptor sites without entering the "docking" state.

Evidence that the 90-kDa heat shock protein (HSP90) exists in cytosol in heteromeric complexes containing HSP70 and three other proteins with Mr of 63,000, 56,000, and 50,000

Monoclonal antibody (mAb) 8D3 and 3G3 are unique antibodies capable of precipitating both free 90-kDa heat shock protein (HSP90) and HSP90-protein complexes. Immunoprecipitation of [35S]methionine-labeled Hepa 1c1c7 cytosolic extracts were performed using mAb 8D3 or 3G3. The resulting immunoprecipitates can be dissociated from the mAb with a 500 mM NaCl wash. These washes were subjected to both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis. Five major protein spots were detected in addition to HSP90 with the following relative molecular weights: 68,000, 63,000, 56,000, 50,000, and 188,000. On Western blots mAb 3G3 was capable of specifically binding to HSP90. Each of these proteins was localized on two-dimensional gels. Using one-dimensional gel electrophoresis and immunochemical localization on Western blots, the p68 spot was identified as HSP70, and the p56 spot was found to cross-react with polyclonal antibody JP-1 raised against a 59-kDa protein. This 59-kDa protein has been found previously to be associated with several steroid hormone receptors in rabbit uterine cytosol. Immunoprecipitation of [32P]orthophosphate-labeled Hepa 1c1c7 cytosol with mAb 8D3 or 3G3 revealed two major phosphorylated proteins with relative molecular weights of 90,000 and 50,000. The identities of p63 and p188 are currently unknown. This is the first report examining the major proteins that are complexed with HSP90 in mammalian cells.

Production of Ah receptor ligands in rat fecal suspensions containing tryptophan or indole-3-carbinol

An assay system was developed to test whether bacteria in the gastrointestinal tract are capable of metabolizing tryptophan to compounds that are able to bind to the aryl hydrocarbon (Ah) receptor. Tryptophan (1 mM) was added to feces diluted 1:1,000 in phosphate-buffered saline and incubated at 37 degrees C overnight. The suspensions were extracted with chloroform to obtain the hydrophobic compounds. To test for the presence of Ah receptor ligands, a competition binding assay using [2-125I]iodo-7, 8-dibromodibenzo-p-dioxin and Hepa 1c1c7 cytosol was employed; it was capable of detecting picogram levels of a competing ligand with similar affinity. Fecal suspensions in the presence of 1 mM tryptophan and oxygen are capable of producing greater than 60% inhibition of radioligand binding per 10 micrograms of feces. In contrast, oxygen-equilibrated fecal suspensions without tryptophan and argon-equilibrated fecal suspensions with tryptophan exhibited 10% inhibition of radioligand binding per 10 micrograms of feces in the competition binding assay. Other indolylic compounds and amino acids were similarly tested. Histidine, tyrosine, phenylalanine, glycine, indole-3-acetic acid, and tryptamine were all negative in this assay. Indole-3-carbinol was capable of forming compounds that bind to the Ah receptor under a variety of conditions: in fecal suspensions with or without oxygen, in 50 mM HCl for 80 minutes, and in neutral pH buffer overnight at 37 degrees C. Addition of oxygenated tryptophan-fecal incubation extracts to Hepa 1 and Hepa c4 mutant (defective Ah receptor) cell cultures resulted in the induction of ethoxyresorufin O-deethylase activity in Hepa 1 cells, but no induction was observed in Hepa c4 cells. These results suggest that bacteria in the gastrointestinal tract under the proper conditions are able to metabolize tryptophan to compounds that are Ah receptor ligands.

Analysis of photoaffinity-labeled aryl hydrocarbon receptor heterogeneity by two-dimensional gel electrophoresis

The level of charge heterogeneity in the aryl hydrocarbon receptor (AhR) was examined by high-resolution denaturing two-dimensional (2D) gel electrophoresis. Hepa 1c1c7 cell cytosolic fraction was photoaffinity-labeled with 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin and applied to isoelectric focusing (IEF) tube gels. After optimization of focusing conditions a broad peak of radioactivity was detected in the apparent pI range of 5.2-5.7. IEF tube gels were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by visualization of the radiolabeled AhR by autoradiography; three distinct isoforms were detected. The same 2D electrophoretic isoform pattern was obtained when the AhR from Hepa 1c1c7 was photoaffinity-labeled in cell culture. BPrCl cells, a mutant line derived from Hepa 1c1c7 cells, contain an AhR that is unable to bind to DNA. Photoaffinity-labeled BPrCl cytosolic fractions were subjected to 2D gel electrophoretic analysis resulting in essentially the same molecular weight and isoform pattern as seen in Hepa 1c1c7 cytosol. This result would suggest that if a mutation is present in the BPrCl AhR it has not caused a significant change in its IEF pattern, although a small shift in the pI values was observed. Two-dimensional gel electrophoresis of photoaffinity-labeled cytosolic fractions from HeLa cells, the rat liver tumor cell line McA-RH7777, and buffalo rat thymus revealed three isoforms, essentially the same isoform pattern as in Hepa 1c1c7 cells. This would indicate that despite the considerable molecular weight polymorphism between species the level of charge heterogeneity is highly conserved.

In contrast to the glucocorticoid receptor, the thyroid hormone receptor is translated in the DNA binding state and is not associated with hsp90

We have recently reported that the glucocorticoid receptor (GR) becomes bound to the 90-kDa heat shock protein (hsp90) at or near the end of receptor translation in vitro (Dalman, F. C., Bresnick, E. H., Patel, P. D., Perdew, G. H., Watson, S. J., Jr., and Pratt, W. B. (1989) J. Biol. Chem. 264, 19815-19821). In this paper we compare the hsp90 binding and DNA binding activities of the thyroid hormone receptor (TR) to those of the GR after cell-free translation of the two receptors in rabbit reticulocyte lysate. In contrast to the newly translated GR, which is bound to hsp90 and must be transformed to the DNA binding state, the TR is not bound to hsp90 and is translated in its DNA binding form without any requirement for transformation. When the GR is translated in wheat germ extract, which does not contain hsp90, it is translated in its DNA binding form in the same manner as the TR synthesized in reticulocyte lysate. These observations provide direct evidence that binding of GR to hsp90 is associated with repression of its DNA binding function. The fact that the TR does not bind to hsp90 and is translated in its DNA binding form is consistent with the different behavior of this receptor with respect to classic steroid receptors in the intact cell. We propose that binding to hsp90 may account for the fact that most of the steroid receptors are recovered in the cytosolic fraction after lysis of hormone-free cells in low salt buffer whereas the hormone-free TR is recovered in tight association with the nucleus.

Direct evidence that the glucocorticoid receptor binds to hsp90 at or near the termination of receptor translation in vitro

We have translated the rat glucocorticoid receptor in both reticulocyte lysate and in wheat germ extract. Receptor synthesized in the reticulocyte lysate is immunoadsorbed by the 8D3 monoclonal antibody directed against the 90-kDa heat shock protein (hsp90) and it has a normal ability to bind glucocorticoid in a high affinity manner. Although the wheat germ extract synthesizes the full length receptor, the receptor is not immunoadsorbed by 8D3 and we cannot demonstrate high affinity steroid binding. Receptor synthesized by the reticulocyte lysate can be immunoadsorbed by antibody directed against hsp90 as soon as the translation product is full length, suggesting that the receptor becomes associated with hsp90 late during translation or immediately at the termination of translation. When newly synthesized receptor is bound with steroid and incubated at 25 degrees C, it is converted to a form that binds to DNA. This study provides direct evidence that association of hsp90 with the glucocorticoid receptor is a very early event and that the newly formed heteromeric receptor-hsp90 complex is fully competent to undergo transformation.

Association of the Ah receptor with the 90-kDa heat shock protein

Partially purified Ah receptor preparations were used to produce a monoclonal antibody, designated as 8D3, that is capable of immunoprecipitating the Ah receptor. Hepa 1c1c7 cytosol was photoaffinity-labeled with [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin followed by immunoprecipitation, and the resulting precipitate was applied to a sodium dodecyl sulfate-polyacrylamide electrophoretic gel. These gels were stained with Coomassie Blue and revealed the presence of a major immunoprecipitated 90-kDa protein, and after autoradiography a radiolabeled 95-kDa protein (Ah receptor) was detected. The 90-kDa protein was determined to be the 90-kDa heat shock protein (HSP90) by western blot analysis using an antibody (AC88) previously shown to be specific for HSP90. An increase in the sedimentation of the Ah receptor on sucrose density gradients was seen upon addition of monoclonal antibody 8D3 to Hepa 1c1c7 cytosol. Monoclonal antibody 8D3 immunoprecipitates the Ah receptor from Hepa 1 cells (murine), HeLa cells (human), and rat liver cytosolic extracts, indicating that the Ah receptor is complexed with HSP90 in several mammalian species tested. These results illustrate another physicochemical property that the supergene family of soluble steroid receptors and the Ah receptor have in common.

Purification of the Ah receptor from C57BL/6J mouse liver

The photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, previously has been shown to selectively label two peptides in the cytosol fraction of C57BL/6J mouse liver: a 95-kDa peptide, the ligand binding moiety of the Ah receptor, and a 70-kDa proteolytic fragment formed from the larger peptide (Poland, A., Glover, E., Ebetino, F. H., and Kende, A.S. (1986) J. Biol. Chem. 261, 6352-6365). These two peptides were partially purified to an approximately 20,000-fold enrichment with a 15-20% yield by the following scheme: 1) photoaffinity labeling of the 35-55% ammonium sulfate fraction of liver cytosol; 2) chromatography on polyethyleneimine-Sepharose coupled at low charge density and heparin/Mn2+ precipitation of the dilute column eluate; 3) DEAE-Sepharose chromatography to remove heparin; 4) chromatography on heparin-Sepharose; 5) preparative sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis followed by electroelution of the protein and ion pair extraction to remove sodium dodecyl sulfate; and 6) high performance liquid chromatography on a reverse-phase C-4 column. Following initial chromatography on polyethyleneimine Sepharose, it was found that substantial subsequent purification could only be achieved under denaturing conditions.