Immunochemical comparison of mutant glucocorticoid receptors and wild type receptor fragments produced by neutrophil elastase and chymotrypsin

Elevated glutathione S-transferase gene expression is an early event during steroid-induced lymphocyte apoptosis

Based on the finding that glutathione S-transferase Yb1 (GST) gene expression is elevated in the regressing prostate of androgen-ablated rats, we analyzed GST transcript levels during steroid-induced lymphocyte cell death. It was found that GST gene expression was induced in steroid-sensitive cells within 4 hr of dexamethasone treatment, required functional glucocorticoid receptor, and was dose-dependent with regard to hormone. GST expression was not induced in an apoptosis-defective variant that contained normal levels of functional receptor, indicating that GST up-regulation was the result of secondary events that occur during steroid-mediated apoptosis. Using the calcium ionophore A23817 to induce lymphocyte cell death, GST RNA levels were increased in both steroid-sensitive and steroid-resistant cell lines, supporting the conclusion that elevated GST expression was the result of cellular processes associated with apoptosis, rather than a direct consequence of steroid-mediated transcriptional control. The cells were also treated with dibutyryl cAMP to cause cell death; however, this mode of killing did not result in GST up-regulation. Taken together, these results suggest that GST induction in dexamethasone-treated T-lymphocytes occurs early in the steroid-regulated apoptotic pathway and that this may be a marker of calcium-stimulated cell death. Based on the known function of GST as an antioxidant defense enzyme and its transcriptional regulation by reactive oxygen intermediates, we propose that the gene product of a primary GR target gene(s) directly or indirectly effects the redox state of the cell. Thus activation of GST gene expression in apoptotic lymphocytes is likely a indicator of oxidative stress, rather than a required step in the pathway.

Transcriptional transactivation functions localized to the glucocorticoid receptor N terminus are necessary for steroid induction of lymphocyte apoptosis

Genetic studies have suggested that transcriptional regulation of specific target genes (by either induction or repression) is the molecular basis of glucocorticoid-mediated lymphocyte apoptosis. To examine the role of transcriptional regulation more directly, we developed a complementation assay utilizing stable transfection of wild-type (wt) and mutant (nti) glucocorticoid receptor (GR) cDNA constructs into a GR-deficient S49 murine cell line (7r). Our data confirm that the level of functional GR is rate limiting for S49 apoptosis and moreover that the GR amino terminus (N terminus), which as been deleted from the nti GR, is absolutely required for complementation in this system. Surprisingly, we found that at physiological levels of receptor, expression of the nti GR in cells containing wt GR results in enhanced dexamethasone sensitivity rather than a dominant negative phenotype. One interpretation of these data is that DNA binding by wt-nti heterodimers may be functionally similar to that of wt-wt homodimers, indicating that GRE occupancy by at least one transactivation domain may be sufficient to induce the hormonal response. To determine whether acidic activating sequences such as those localized to the GR N terminus are important in the induction of lymphocyte apoptosis, we tested the activity of a chimeric receptor in which we replaced the entire GR N terminus with sequences from the herpes simplex virus VP16 protein. Our results demonstrate that 7r cells expressing VP-GR fusions are indeed steroid sensitive, strongly supporting the idea that S49 apoptosis is dependent on transcriptional regulation of specific genes which respond to acidic activating domains, implying that induction, rather than repression, may be the critical initiating event.

Transcriptional transactivation functions localized to the glucocorticoid receptor N terminus are necessary for steroid induction of lymphocyte apoptosis

Genetic studies have suggested that transcriptional regulation of specific target genes (by either induction or repression) is the molecular basis of glucocorticoid-mediated lymphocyte apoptosis. To examine the role of transcriptional regulation more directly, we developed a complementation assay utilizing stable transfection of wild-type (wt) and mutant (nti) glucocorticoid receptor (GR) cDNA constructs into a GR-deficient S49 murine cell line (7r). Our data confirm that the level of functional GR is rate limiting for S49 apoptosis and moreover that the GR amino terminus (N terminus), which as been deleted from the nti GR, is absolutely required for complementation in this system. Surprisingly, we found that at physiological levels of receptor, expression of the nti GR in cells containing wt GR results in enhanced dexamethasone sensitivity rather than a dominant negative phenotype. One interpretation of these data is that DNA binding by wt-nti heterodimers may be functionally similar to that of wt-wt homodimers, indicating that GRE occupancy by at least one transactivation domain may be sufficient to induce the hormonal response. To determine whether acidic activating sequences such as those localized to the GR N terminus are important in the induction of lymphocyte apoptosis, we tested the activity of a chimeric receptor in which we replaced the entire GR N terminus with sequences from the herpes simplex virus VP16 protein. Our results demonstrate that 7r cells expressing VP-GR fusions are indeed steroid sensitive, strongly supporting the idea that S49 apoptosis is dependent on transcriptional regulation of specific genes which respond to acidic activating domains, implying that induction, rather than repression, may be the critical initiating event.

New site-directed polyclonal antibody maps N-terminus of occluded region of the non-transformed glucocorticoid receptor oligomer to within BUGR epitope

Using a 17-mer synthetic peptide for immunization, a polyclonal antibody (WS933) directed against amino acid residues 395-411 of the mouse glucocorticoid receptor (GCR) has been raised and used to probe the significance of this region in forming the receptor oligomer and to localize the truncation site of the mutant GCR of the P1798 lymphosarcoma. This region of the receptor, which encompasses the BUGR epitope, is amino-terminal of and immediately adjacent to the DNA-binding domain. The polyclonal antibody WS933 reacted with both native and denatured forms of the wild-type mouse GCR as judged by its ability to shift the transformed receptor peak on Sephacryl S300 columns, to immunoadsorb the receptor to protein A Sepharose, and by immunoblot analysis where it identified the 98 kDa receptor protein in the cortisol-sensitive line of the P1798 mouse lymphosarcoma. WS933 also reacted with rat and rabbit GCR, but not human GCR. These characteristics were shared by the BUGR-2 monoclonal antibody. Unexpectedly, there were two highly significant differences between WS933 and BUGR-2. The first was the ability of WS933 to bind to the mutant 45 kDa GCR of the cortisol-resistant P1798 lymphosarcoma as judged by its capability of shifting the receptor peak on Sephacryl S300 columns. BUGR-2, in contrast, was unable to shift this mutant receptor peak. Secondly, WS933 was unable to react with the non-DNA-binding form of the wild-type (or mutant) GCR, whereas BUGR-2 could react with the non-DNA-binding form of the wild-type GCR. The first observation suggests that the truncation site of the mutant receptor may lie within a portion of the BUGR domain. Additionally, the second observation implies that at least part of the region lying within amino acid residues 395-411 of the mouse GCR is occluded in the receptor oligomer and that this site only becomes available upon transformation of the GCR to the DNA-binding form. This data provides the first mapping of the amino-terminus of the occluded region of the non-transformed receptor, and suggests that WS933 will be a useful probe for characterizing mutant as well as wild type glucocorticoid receptors.

nti glucocorticoid receptor transcripts lack sequences encoding the amino-terminal transcriptional modulatory domain

Glucocorticoid induction of cell death (apoptosis) in mouse lymphoma S49 cells has long been studied as a molecular genetic model of steroid hormone action. To better understand the transcriptional control of glucocorticoid-induced S49 cell death, we isolated and characterized glucocorticoid receptor (GR) cDNA from two steroid-resistant nti S49 mutant cell lines (S49.55R and S49.143R) and the wild-type parental line (S49.A2). Our data reveal that nti GR transcripts encode intact steroid- and DNA-binding domains but lack 404 amino-terminal residues as a result of aberrant RNA splicing between exons 1 and 3. Results from transient cotransfection experiments into CV1 cells using nti receptor expression plasmids and a glucocorticoid-responsive reporter gene demonstrated that the truncated nti receptor exhibits a reduced transcriptional regulatory activity. Gene fusions containing portions of both the wild-type and the nti GR-coding sequences were constructed and used to functionally map the nti receptor mutation. We found that the loss of the modulatory domain alone is sufficient to cause the observed defect in nti transcriptional transactivation. These results support the proposal that glucocorticoid-induced S49 cell death requires GR sequences which have previously been shown to be required for transcriptional regulation, suggesting that steroid-regulated apoptosis is controlled at the level of gene expression.

nti glucocorticoid receptor transcripts lack sequences encoding the amino-terminal transcriptional modulatory domain

Glucocorticoid induction of cell death (apoptosis) in mouse lymphoma S49 cells has long been studied as a molecular genetic model of steroid hormone action. To better understand the transcriptional control of glucocorticoid-induced S49 cell death, we isolated and characterized glucocorticoid receptor (GR) cDNA from two steroid-resistant nti S49 mutant cell lines (S49.55R and S49.143R) and the wild-type parental line (S49.A2). Our data reveal that nti GR transcripts encode intact steroid- and DNA-binding domains but lack 404 amino-terminal residues as a result of aberrant RNA splicing between exons 1 and 3. Results from transient cotransfection experiments into CV1 cells using nti receptor expression plasmids and a glucocorticoid-responsive reporter gene demonstrated that the truncated nti receptor exhibits a reduced transcriptional regulatory activity. Gene fusions containing portions of both the wild-type and the nti GR-coding sequences were constructed and used to functionally map the nti receptor mutation. We found that the loss of the modulatory domain alone is sufficient to cause the observed defect in nti transcriptional transactivation. These results support the proposal that glucocorticoid-induced S49 cell death requires GR sequences which have previously been shown to be required for transcriptional regulation, suggesting that steroid-regulated apoptosis is controlled at the level of gene expression.

Glucocorticoid receptor binding to calf thymus DNA. 1. Identification and characterization of a macromolecular factor involved in receptor-steroid complex binding to DNA

Activation of receptor-steroid complexes to a form with high affinity for DNA is a poorly understood process involving multiple components in addition to the holoreceptor. Employing rat HTC cells as the source of glucocorticoid receptor, we show that maximal receptor binding to calf thymus DNA is mediated by a previously unknown small molecular weight factor. This factor can be removed from cytosolic preparations of receptor by gel filtration chromatography. Salt extraction of crude nuclear pellets afforded much larger amounts of a similar DNA-binding activity factor. The cytoplasmic factor and the more abundant nuclear factor were identical on the basis of their similar physical properties. The factor was precipitable in the crude state with (NH4)2SO4 and stable to heat as well as freezing and thawing. Chromatography on DNA-cellulose revealed that the factor itself did not bind to DNA. The factor could be filtered through a Centricon C-3 microconcentrator (molecular weight cutoff approximately 3000) but was excluded from Sephadex G-10 columns. These parameters enable us to determine an apparent molecular weight of 700-3000 for this factor. The presence of large amounts of this factor in nuclei accounts for the previously unexplained observation that, following size exclusion chromatography, more activated complexes bind to nuclei than to DNA. These data indicate that some, but not all, of the activated complexes require factor to be able to bind to DNA. The predominantly nuclear localization of this factor, coupled with its ability to increase DNA binding, attests to the biological relevance of this factor in the whole cell action of receptor-glucocorticoid complexes.

Isolation and characterization of cortisol-sensitive and -resistant P1798 mouse lymphosarcoma cell lines

Three cell lines have been isolated and characterized from the P1798 mouse lymphosarcoma. One line, derived from a glucocorticoid-resistant tumor, was glucocorticoid-resistant in vitro. The other two cell lines, derived from glucocorticoid-sensitive and -resistant parental tumors, respectively, were shown to be glucocorticoid-sensitive in vitro. The glucocorticoid receptor from all three cell lines bound glucocorticoid with similar affinity and capacity. However, based on Sephacryl S-300 gel filtration, the glucocorticoid receptor from the resistant cell line was smaller than that of the two sensitive cell lines. Moreover, the glucocorticoid receptor from the resistant cell line accumulated to a greater extent in the nucleus. This resistant cell line thus resembles the nti variant of the S49 lymphoma cell line. All three cell lines were tumorigenic and metastatic when reimplanted into mice, contained the normal mouse diploid complement of 40 chromosomes and exhibited the same responsiveness to cortisol in vivo as they did in vitro. It is concluded that the ready passage of these cell lines in vitro or in vivo and the presence of the small receptor in the resistant line should make them excellent model systems for the study of glucocorticoid resistance.

Effect of the 90 kDa heat shock protein, HSP90, on glucocorticoid receptor binding to DNA-cellulose

Glucocorticoid receptors in the IM-9 human lymphoblastoid cell line were affinity labeled with [3H]dexamethasone 21-mesylate and activated to a DNA-binding form by filtration through a Bio-Gel A-1.5m column. The 90 kDa heat shock protein, HSP90, was identified by labeling IM-9 cells with 35S-methionine at both 37 degrees C and 42 degrees C and purified to near homogeneity by sequential chromatography through DE52 and hydroxyapatite. Addition of purified HSP90 to activated, affinity labeled glucocorticoid receptors in a molecular ratio of 16 to 1 inhibited the binding of the receptors to DNA-cellulose. HSP90 did not affect the binding of other proteins to DNA-cellulose, indicating that the inhibitory effect of HSP90 was specific for the glucocorticoid receptor. These results suggest that HSP90 may associate with the glucocorticoid receptor, masking its DNA-binding site and thereby inhibiting receptor interaction with DNA.