Ligand binding and co-activator assembly of the peroxisome proliferator-activated receptor-gamma

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a ligand-dependent transcription factor that is important in adipocyte differentiation and glucose homeostasis and which depends on interactions with co-activators, including steroid receptor co-activating factor-1 (SRC-1). Here we present the X-ray crystal structure of the human apo-PPAR-gamma ligand-binding domain (LBD), at 2.2 A resolution; this structure reveals a large binding pocket, which may explain the diversity of ligands for PPAR-gamma. We also describe the ternary complex containing the PPAR-gamma LBD, the antidiabetic ligand rosiglitazone (BRL49653), and 88 amino acids of human SRC-1 at 2.3 A resolution. Glutamate and lysine residues that are highly conserved in LBDs of nuclear receptors form a 'charge clamp' that contacts backbone atoms of the LXXLL helices of SRC-1. These results, together with the observation that two consecutive LXXLL motifs of SRC-1 make identical contacts with both subunits of a PPAR-gamma homodimer, suggest a general mechanism for the assembly of nuclear receptors with co-activators.

Differential use of CREB binding protein-coactivator complexes

CREB binding protein (CBP) functions as an essential coactivator of transcription factors that are inhibited by the adenovirus early gene product E1A. Transcriptional activation by the signal transducer and activator of transcription-1 (STAT1) protein requires the C/H3 domain in CBP, which is the primary target of E1A inhibition. Here it was found that the C/H3 domain is not required for retinoic acid receptor (RAR) function, nor is it involved in E1A inhibition. Instead, E1A inhibits RAR function by preventing the assembly of CBP-nuclear receptor coactivator complexes, revealing differences in required CBP domains for transcriptional activation by RAR and STAT1.

Transcription factor-specific requirements for coactivators and their acetyltransferase functions

Different classes of mammalian transcription factors-nuclear receptors, cyclic adenosine 3',5'-monophosphate-regulated enhancer binding protein (CREB), and signal transducer and activator of transcription-1 (STAT-1)-functionally require distinct components of the coactivator complex, including CREB-binding protein (CBP/p300), nuclear receptor coactivators (NCoAs), and p300/CBP-associated factor (p/CAF), based on their platform or assembly properties. Retinoic acid receptor, CREB, and STAT-1 also require different histone acetyltransferase (HAT) activities to activate transcription. Thus, transcription factor-specific differences in configuration and content of the coactivator complex dictate requirements for specific acetyltransferase activities, providing an explanation, at least in part, for the presence of multiple HAT components of the complex.

Peroxisome proliferator-activated receptors and retinoic acid receptors differentially control the interactions of retinoid X receptor heterodimers with ligands, coactivators, and corepressors

As the obligate member of most nuclear receptor heterodimers, retinoid X receptors (RXRs) can potentially perform two functions: cooperative binding to hormone response elements and coordinate regulation of target genes by RXR ligands. In this paper we describe allosteric interactions between RXR and two heterodimeric partners, retinoic acid receptors (RARs) and peroxisome proliferator-activated receptors (PPARs); RARs and PPARs prevent and permit activation by RXR-specific ligands, respectively. By competing for dimerization with RXR on response elements consisting of direct-repeat half-sites spaced by 1 bp (DR1 elements), the relative abundance of RAR and PPAR determines whether the RXR signaling pathway will be functional. In contrast to RAR, which prevents the binding of RXR ligands and recruits the nuclear receptor corepressor N-CoR, PPAR permits the binding of SRC-1 in response to both RXR and PPAR ligands. Overexpression of SRC-1 markedly potentiates ligand-dependent transcription by PPARgamma, suggesting that SRC-1 serves as a coactivator in vivo. Remarkably, the ability of RAR to both block the binding of ligands to RXR and interact with corepressors requires the CoR box, a structural motif residing in the N-terminal region of the RAR ligand binding domain. Mutations in the CoR box convert RAR from a nonpermissive to a permissive partner of RXR signaling on DR1 elements. We suggest that the differential recruitment of coactivators and corepressors by RAR-RXR and PPAR-RXR heterodimers provides the basis for a transcriptional switch that may be important in controlling complex programs of gene expression, such as adipocyte differentiation.

p300 is a component of an estrogen receptor coactivator complex

The estrogen receptor (ER) is a ligand-dependent transcription factor that regulates expression of target genes in response to estrogen in concert with other cellular signaling pathways. This suggests that the mechanism by which ER transmits an activating signal to the general transcription machinery may include factors that integrate these diverse signals. We have previously characterized the estrogen receptor-associated protein, ERAP160, as a factor that complexes with ER in an agonist-dependent manner. We have now found that the transcriptional coactivator p300 associates with agonist bound ER and augments ligand-dependent activation by ER. Our studies show that an ER coactivator complex involves a direct hormone-dependent interaction between ER and ERAP160, resulting in the recruitment of p300. In addition, antibodies directed against the cloned steroid receptor coactivator 1 (SRC1) recognize ERAP160. The known role of p300 in multiple signal transduction pathways, including those involving the second messenger cAMP, suggests p300 functions as a point of integration between ER and these other pathways.

Genetic analysis of the sam mutations, which induce sexual development with no requirement for nutritional starvation in fission yeast

The cAMP pathway and the Ras pathway are the two major pathways to sexual development in the fission yeast Schizosaccharomyces pombe. To understand the cAMP pathway or the related pathway, we analyzed mutants that display a phenotype similar to cyr1-, that is, hyper-sporulation. Nine mutants termed sam (sporulation abnormal mutant), which are highly inclined to sexual development despite the presence of nitrogen sources, were partially characterized. Cyclic AMP was detected in all nine sam mutant cells, and over-expression of the adenylyl cyclase gene (cyr1) failed to suppress the hyper-sporulation phenotype of these sam mutants, suggesting that none of the sam mutants were likely to be allelic to cyr1. Epistatic tests of sam mutants showed that they were divided into two dominant and seven recessive mutants. Dominants were able to make spores in sam/sam+ heterodiploid cells upon abundant nutrients. Both two dominant mutants bypassed the inability to make spores in ras1 deficient diploid cells, suppressed the deficiency to execute sporulation in byr2 deficient diploid cells, but failed to suppress the byr1 deficiency. Two dominant mutations seem not to occur within the byr2 gene.

A CBP integrator complex mediates transcriptional activation and AP-1 inhibition by nuclear receptors

Nuclear receptors regulate gene expression by direct activation of target genes and inhibition of AP-1. Here we report that, unexpectedly, activation by nuclear receptors requires the actions of CREB-binding protein (CBP) and that inhibition of AP-1 activity is the apparent result of competition for limiting amounts of CBP/p300 in cells. Utilizing distinct domains, CBP directly interacts with the ligand-binding domain of multiple nuclear receptors and with the p160 nuclear receptor coactivators, which upon cloning have proven to be variants of the SRC-1 protein. Because CBP represents a common factor, required in addition to distinct coactivators for function of nuclear receptors, CREB, and AP-1, we suggest that CBP/p300 serves as an integrator of multiple signal transduction pathways within the nucleus.

Ligand-independent repression by the thyroid hormone receptor mediated by a nuclear receptor co-repressor

Thyroid-hormone and retinoic-acid receptors exert their regulatory functions by acting as both activators and repressors of gene expression. A nuclear receptor co-repressor (N-CoR) of relative molecular mass 270K has been identified which mediates ligand-independent inhibition of gene transcription by these receptors, suggesting that the molecular mechanisms of repression by thyroid-hormone and retinoic-acid receptors are analogous to the co-repressor-dependent transcriptional inhibitory mechanisms of yeast and Drosophila.

Polarity-specific activities of retinoic acid receptors determined by a co-repressor

Retinoic acid receptors (RARs) and retinoid-X receptors (RXRs) activate or repress transcription by binding as heterodimers to DNA-response elements that generally consist of two direct repeat half-sites of consensus sequence AGGTCA. On response elements consisting of direct repeats spaced by five base pairs (DR + 5 elements), RAR/RXR heterodimers activate transcription in response to RAR-specific ligands, such as all-trans-retinoic acid (RA). In contrast, on elements consisting of direct repeats spaced by one base pair (DR + 1 elements), RAR/RXR heterodimers exhibit little or no response to activating ligands and repress RXR-dependent transcription. Here we show that ligand-dependent transactivation by RAR on DR + 5 elements requires the dissociation of a new nuclear receptor co-repressor, N-CoR, and recruitment of the putative co-activators p140 and p160. Surprisingly, on DR + 1 elements, N-CoR remains associated with RAR/RXR heterodimers even in the presence of RAR ligands, resulting in constitutive repression. These observations indicate that DNA-response elements can allosterically regulate RAR-co-repressor interactions to determine positive or negative regulation of gene expression.

Regulation of retinoid signalling by receptor polarity and allosteric control of ligand binding

Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) regulate transcription by binding to response elements in target genes that generally consist of two direct repeat half-sites of consensus sequence AGGTCA (ref. 1). RAR/RXR heterodimers activate transcription in response to all-trans or 9-cis retinoic acid by binding to direct repeats spaced by five base pairs (DR5 elements), such that RAR occupies the downstream half-site. RXR homodimers activate transcription in response to 9-cis retinoic acid by binding to direct repeats spaced by one base pair (DR1 elements). Although RXR/RAR heterodimers bind to DR1 elements with higher affinity than RXR homodimers, in most contexts they are unable to activate transcription in response to either all-trans or 9-cis retinoic acid. As a result, RARs inhibit RXR-dependent transcription from these sites. We report that the switching of the RAR from an activator to an inhibitor of retinoid-dependent transcription requires that it be bound to the upstream half-site of DR1 elements and that it allosterically block the binding of ligand to the RXR.

Peripolar cells in guinea pigs under experimental hyperplasia of juxtaglomerular cells induced by long-term, low-dose calcium condition

The peripolar cell was described in the glomeruli of sheep by Ryan et al. in 1979 and these cells have subsequently been detected in many species (Ryan et al. 1982; Gall et al. 1986; Hanner et al. 1980). The peripolar cells are located at the junction between the podocytes of the glomerular capillaries and the epithelial lining of Bowmann's capsule, encircling the hilar region of the glomerular tuft. Functionally, the peripolar cells have been considered to be a part of the juxtaglomerular apparatus but the precise nature of the cells has not been identified (Gardiner et al. 1985). Recently, it has been found that an antibody against rat urinary kallikrein reacts positively with sheep peripolar cell (Gall et al. 1984). This finding has led to the suggestion that the peripolar cells may influence the renin secretion through the kallikrein-kinin system. In our experiments with long-term low-calcium condition accompanied with hyperplasia of juxtaglomerular cells the peripolar cells were easily detected. The results suggests that the increase in the number of peripolar cells is closely related to the hyperplasia of juxtaglomerular cells.

Necklace-like detachment of endothelial cell layer from arterial wall under low-calcium condition

The diversified morphological manifestations in various tissues and organs obtained by administration of differing amounts of calcium chelating agents were reported in previous papers (Yamaguchi et al 1981 a & b; 1982; 1990; 1993). In our recent research described here, administration of a moderate dose of Na2EDTA over the short term demonstrated necklace-like detachment from the arterial wall without disruption of the endothelial cell chain. Intercellular spaces in the media just beneath the detached endothelial cell layer was stained strongly with colloidal iron staining. Electron microscopic observation revealed that the detached endothelial cells showed a lot of elongated anchoring villi from the basal surface, usually seen at the luminal surface, adhered to the degenerative and thin flattened internal elastic lamellae. The alteration of the colloidal iron staining of the vascular wall under the low-calcium condition is suggested to be induced by loosening of the molecular structure of glycosaminoglycans (GAGs) as well as glycoproteins (GPs), comprising the important component of the intercellular matrix and elastic lamellae, which would induce a change in their pasty or viscous character. This would be an accelerative factor for detachment of endothelial cells. Moreover, the lack of the waving of the internal elastic lamellae, trapping of endothelial cytoplasmic processes among them, would play the decisive role in the total detachment of the endothelial cell layer. On the other hand, the low-calcium condition did not adversely influence the joining of endothelial cells. The pathognomatic mechanism will be discussed, with a comparison made to the angiolytic changes provoked by a large amount of Na2EDTA.

Experimental idiopathic dilated cardiomyopathy under low-calcium condition

Administration of long-term, low-dose Na2EDTA leads to moderate or severe thinning of the right ventricular wall with or without rupture. These morphological manifestations are known to be site-dependent on the constriction of pulmonary arteries (Yamaguchi et al. 1993a). They sometimes ensue from aneurysmal dilatation and/or plexiform-like-lesion in a pulmonary artery (Yamaguchi et al. 1993b). The present contribution reports that experimental animals who survived for a longer period, maximum for 6 months, showed dilatation of the left ventricle with a mode-rately thin ventricular wall as well as right ventricular changes, which are similar to the morphological manifestations in idiopathic dilated cardiomyopathy.

Experimental study on the hyperplasia of juxtaglomerular cells under low-dose, long-term administration of calcium chelating agents. Similar to the morphological manifestation of Bartter's syndrome

Bartter's syndrome is a genetic disorder which has very rarely been clinically encountered. However, it is of specific interest with respect to the hormonal layer of the kidney, including renin, angiotensin and aldosterone as well as biological alteration of the various electrolytes. The fundamental morphological manifestation of this disorder is known to be hyperplasia of juxtaglomerular cells, although, until now, no experimental studies on this condition have been reported. Demonstrated in this initial study is the remarkable hyperplasia of the juxtaglomerular cells in all terminal portions of the afferent glomerular arterioles situated near the hilum of the glomerular tuft, using a low-dose, long-term administration of calcium chelating agents. This hyperplasia reported in this paper may lead to a new procedure in the analysis of this syndrome.

Co-expression of epidermal growth factor-receptor and c-erb B-2 proto-oncogene product in human salivary-gland adenocarcinoma cell line HSG and the implications for HSG cell autocrine growth

The autonomous proliferation of HSG cells is mediated by an autocrine growth factor, a 46K epidermal growth factor (EGF)-like molecule. The receptor for this molecule was investigated. Immunoprecipitation and immunoblotting revealed the expression of two possible receptor molecules, EGF-R and p185erbB-2, in HSG cells. Northern blotting also revealed the co-expression of 5.6-kb EGF-R mRNA and 4.6-kb c-erb B-2 mRNA. When the purified EGF-like molecule was added to the cultures, EGF-R but not p185erbB-2 was autophosphorylated. These results suggest that, although both EGF-R and p185erbB-2 are co-expressed in HSG cells, the EGF-R is the genuine receptor for the EGF-like molecule. However, there is a possibility that p185erB-2 is involved in the signal transduction system. This possibility was examined by using specific antibodies to human EGF-R (hEGF-R), p185erbB-2, and EGF to inhibit the functions of these molecules. Addition of these three antibodies to the cultures inhibited the growth of HSG cells. The antibodies to EGF-R and p185erbB-2 also caused morphological changes such as disturbances of the plasma membrane, and some cell death. Surprisingly, the effect of the anti-p185erbB-2 antibody on growth inhibition and morphology was stronger than that of the anti-hEGF-R antibody. Thus, p185erB-2 expressed in HSG cells has an important function in the signal transduction of HSG cell growth.

Delineation of three different thyroid hormone-response elements in promoter of rat sarcoplasmic reticulum Ca2+ATPase gene. Demonstration that retinoid X receptor binds 5' to thyroid hormone receptor in response element 1

Thyroid hormone (3,5,3'-triiodothyronine) positively regulates transcription of the sarcoplasmic reticulum Ca2+ATPase gene in rat heart, and sequences within 559 nucleotides upstream from the transcription start site confer thyroid hormone responsiveness upon a reporter gene. In the present study, three thyroid hormone-response elements (TREs) are identified between nucleotides -485 and -190. Each TRE is active in transient transfection assays and specifically binds 3,5,3'-triiodothyronine receptors (TRs) alpha 1 and beta 1 alone and in combination with retinoid X receptors (RXRs) alpha and beta. TRE 1 is a direct repeat of two half-sites separated by four nucleotides; TREs 2 and 3 are inverted palindromes of two half-sites separated by four and six nucleotides, respectively. Methylation interference analysis of TRE 1 showed binding of a TR alpha 1 monomer to the 3' half-site, whereas the heterodimer contacts both half-sites. Subsequent studies employed TR beta and RXR alpha mutants in which their P-boxes were replaced with the P-box of the glucocorticoid receptor. Bandshifts of wild type and mutant proteins with either wild type TRE 1 or a mutant version, in which the 5' half-site was converted to a glucocorticoid response element half-site, demonstrated preferential binding of RXR to the 5' half-site and of TR to the 3' half-site of TRE 1.

Differential orientations of the DNA-binding domain and carboxy-terminal dimerization interface regulate binding site selection by nuclear receptor heterodimers

Retinoic acid, thyroid hormone, and vitamin D receptors preferentially activate target genes through response elements that consist of direct repeat arrangements of a core recognition motif of consensus sequence AGGTCA. We present evidence that the preference for direct repeat elements arises from two fundamental differences from steroid hormone receptors. First, retinoic acid, thyroid hormone, and vitamin D receptors are demonstrated to preferentially form heterodimers with the retinoid X receptors. These interactions are mediated by the carboxy-terminal dimerization interface, with heterodimer preference specified by actions of the DNA-binding domain. Second, the DNA-binding domains of heterodimeric receptors appear to be rotationally flexible with respect to the carboxy-terminal dimerization interface. Several independent lines of evidence suggest that, relative to the retinoid X and steroid hormone receptors, the DNA-binding domain of the thyroid hormone receptor is preferentially rotated by approximately 180 degrees with respect to its carboxy-terminal dimerization interface. As a result, solution interactions between the carboxy-terminal dimerization interfaces of the retinoid X and thyroid hormone receptors are predicted to lead to the preferential alignment of their respective DNA-binding domains in a direct repeat configuration. This alignment would position the retinoid X receptor over the upstream recognition motif of direct repeat response elements. Differential orientations of the DNA-binding domain, which contribute to the polarity of heterodimer binding, are regulated by a short sequence (the A box) that is located between the conserved DNA-binding and carboxy-terminal dimerization domains.

Retinoic acid receptor in subclone of human salivary gland adenocarcinoma cell line HSG and effect of retinoic acid on cellular growth

Retinoic acid (RA) binding has been detected in the nuclei of a subclone (CL-1) of human submandibular adenocarcinoma cell line HSG conditioned to grow in a serum-free defined medium. Competition assay confirmed the specificity of the RA binding. Scatchard analysis showed the binding molecule to have a high affinity and low capacity. From the analyses by gel-filtration and glycerol density gradient centrifugation, the nuclear binding molecule appears to be distinct from cellular RA binding protein (CRABP) in terms of molecular weight. Furthermore, immunoblotting analysis revealed a band (Mr 47,000) reactive with specific antibody to RA receptor (RAR) alpha in the gel containing the nuclear fraction of CL-1 cells. Northern blotting analysis with specific cDNA probes revealed the expression of RAR alpha and RAR gamma in CL-1 cells. These results indicate that CL-1 cells express two types of RAR subtype, suggesting that these receptor molecules may mediate biological effects of RA. Treatment of CL-1 cells with RA resulted in an increase in the incorporation of [3H]thymidine into TCA-insoluble materials. The maximal increase was observed at 10(-6) M around 48 h. Previously, we demonstrated the autocrine growth of HSG cells mediated by epidermal growth factor (EGF) receptors and EGF-like molecules (Kurokawa et al. (1989) Cancer Res. 49, 5136-5142) and showed that RA had no significant effect on the secretion of the EGF-like molecule. RA induced an increase in [125I]EGF binding to CL-1 cells. The increase in the EGF binding was maximal at 24 h at 10(-6) M RA. RA also increased the amount of [3H]leucine-labeled EGF receptor dose-dependently. No significant change was observed in total protein synthesis of CL-1 cells by treatment with RA. These results suggest that RA stimulates the growth of CL-1 cells by increasing EGF receptor levels.

Effect of glucocorticoid on epidermal growth factor receptor in human salivary gland adenocarcinoma cell line HSG

Human salivary gland adenocarcinoma (HSG) cells treated with 10(-6) M triamcinolone acetonide for 48 h exhibited a 1.7- to 2.0-fold increase in [125I]human epidermal growth factor (hEGF) binding capacity as compared with untreated HSG cells. Scatchard analysis of [125I]EGF binding data revealed that the number of binding sites was 83,700 (+/- 29,200) receptors/cell in untreated cells and 160,500 (+/- 35,500) receptors/cell in treated cells. No substantial change in receptor affinity was detected. The dissociation constant of the EGF receptor was 0.78 (+/- 0.26).10(-9) M for untreated cells, whereas it was 0.93 (+/- 0.31).10(-9)M for treated cells. The triamcinolone acetonide-induced increase in [125I]EGF binding capacity was dose-dependent between 10(-9) and 10(-6)M, and maximal binding was observed at 10(-6)M. EGF receptors on HSG cells were affinity-labeled with [125I]EGF by use of the cross-linking reagent disuccinimidyl suberate (DSS). The cross-linked [125I]EGF was 3-4% of the total [125I]EGF bound to HSG cells. The affinity-labeled EGF receptor was detected as a specific 170 kDa band in the autoradiograph after SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Densitometric analysis revealed that triamcinolone acetonide amplified the intensity of this band 2.0-fold over that of the band of untreated cells. EGF receptor synthesis was also measured by immunoprecipitation of [3H]leucine-labeled EGF receptor protein with anti-hEGF receptor monoclonal antibody. Receptor synthesis was increased 1.7- to 1.8-fold when HSG cells were treated with 10(-8)-10(-6)M triamcinolone acetonide for 48 h. When the immunoprecipitated, [35S]methionine-pulse-labeled EGF receptor was analyzed by SDS-PAGE and fluorography, the newly synthesized EGF receptor was detected at the position of 170 kDa; and treatment of HSG cells with triamcinolone acetonide resulted in a 2.0-fold amplification of this 170 kDa band. There was no significant difference in turnover rate of EGF receptor between treated and untreated HSG cells. These results demonstrate that the triamcinolone acetonide-induced increase in [125I]EGF binding capacity is due to the increased synthesis of EGF receptor protein in HSG cells.

Autocrine growth factor in defined serum-free medium of human salivary gland adenocarcinoma cell line HSG

Human salivary gland adenocarcinoma cell line HSG secretes an epidermal growth factor (EGF)-like molecule and contains EGF receptors. Growth of HSG cells is inhibited by glucocorticoid. We have identified that the growth inhibition by glucocorticoid is induced by the reduced secretion of the EGF-like molecule and that addition of anti-human EGF antibody to the culture specifically inhibits the growth of HSG cells, suggesting that autocrine secretion is involved in the growth of HSG cells. To prove that autocrine secretion functions in glucocorticoid-regulated growth of the HSG cell line, we purified the EGF-like molecule from serum-free, defined medium conditioned by the HSG cells and examined the growth-stimulatory effect of the purified molecule. The cultivation of HSG cells in serum-free defined medium, which contains insulin (10 micrograms/ml) and transferrin (10 micrograms/ml) only as proteinaceous components, resulted in establishment of a new cell line (HSG-SF) which had different morphological features from the parental HSG cell line. HSG-SF cells were found to have basically the same responsiveness to glucocorticoid as parental HSG cells. Parental HSG cells secreted high molecular weight EGF-like molecules (Mr 46,000 and 57,000), which were recognized by specific antibody to low molecular weight human EGF (Mr 6,201). From conditioned, serum-free medium of HSG-SF cells, an EGF-like molecule (Mr 46,000) was purified by using an anti-human EGF antibody-coupled Sepharose CL-4B column. This EGF-like molecule induced a maximal increase (36%) in incorporation of [3H]thymidine into DNA of parental HSG cells as well as low molecular weight human EGF. These observations demonstrate that growth of the HSG cell line is regulated by autocrine secretion.

Nonactivated and activated glucocorticoid receptor complexes from human salivary gland adenocarcinoma cell line

[3H]Triamcinolone acetonide glucocorticoid receptor complexes from human salivary gland adenocarcinoma cells (HSG cells) were shown to be activated with an accompanying decrease in molecular weight in intact cells, as analyzed by gel filtration, DEAE chromatography, the mini-column method and glycerol gradient centrifugation. Glucocorticoid receptor complexes consist of steroid-binding protein (or glucocorticoid receptor) and non-steroid-binding factors such as the heat-shock protein of molecular weight 90,000. To determine whether the steroid-binding protein decreases in molecular weight upon activation, affinity labeling of glucocorticoid receptor in intact cells by incubation with [3H]dexamethasone 21-mesylate, which forms a covalent complex with glucocorticoid receptor, was performed. Analysis by gel filtration and a mini-column method indicated that [3H]dexamethasone 21-mesylate-labeled receptor complexes can be activated under culture conditions at 37 degrees C. SDS-polyacrylamide gel electrophoresis of [3H]dexamethasone 21-mesylate-labeled steroid-binding protein resolved only one specific 92 kDa form. Furthermore, only one specific band at 92 kDa was detected in the nuclear fraction which was extracted from the cells incubated at 37 degrees C. These results suggest that there is no change in the molecular weight of steroid-binding protein of HSG cell glucocorticoid receptor complexes upon activation and that the molecular weight of nuclear-binding receptor does not change, although the molecular weight of activated glucocorticoid receptor complexes does decrease. Triamcinolone acetonide induced an inhibitory effect on DNA synthesis in HSG cells. Dexamethasone 21-mesylate exerted no such effect and blocked the action of triamcinolone acetonide on DNA synthesis. These results suggests that dexamethasone 21-mesylate acts as antagonist of glucocorticoid in HSG cells. The fact that dexamethasone 21-mesylate-labeled receptor complexes could be activated and could bind to DNA or nuclei as well as triamcinolone acetonide-labeled complexes suggests that dexamethasone 21-mesylate-labeled complexes can not induce specific gene expression after their binding to DNA.