Coordinate regulation of glucocorticoid receptor and c-jun gene expression is cell type-specific and exhibits differential hormonal sensitivity for down- and up-regulation

JUN mediates glucocorticoid resistance by stabilizing HIF1a in T cell acute lymphoblastic leukemia

Dexamethasone (Dex) plays a critical role in T-ALL treatment, but the mechanisms of Dex resistance are poorly understood. Here, we demonstrated that the expression of JUN was regulated in Dex-resistant T-ALL cell lines and patient samples. JUN knockdown increased the sensitivity to Dex. Moreover, the survival data showed that high expression of JUN related to poor prognosis of T-ALL patients. Then, we generated dexamethasone-resistant clones and conducted RNA-seq and ATAC-seq. We demonstrated that the upregulation of JUN was most significant and regulated by JNK pathway in Dex-resistant cells. High-throughput screening showed that HIF1α inhibitors synergized with Dex could enhance Dex resistance cells death in vitro and in vivo. Additionally, JUN combined and stabilized HIF1α in Dex resistance cells. These results reveal a new mechanism of Dex resistance in T-ALL and provide experimental evidence for the potential therapeutic benefit of targeting the JNK-JUN-HIF1α axis for T-ALL treatment.

Acquired Glucocorticoid Resistance Due to Homologous Glucocorticoid Receptor Downregulation: A Modern Look at an Age-Old Problem

For over 70 years, the unique anti-inflammatory properties of glucocorticoids (GCs), which mediate their effects via the ligand-activated transcription factor, the glucocorticoid receptor alpha (GRα), have allowed for the use of these steroid hormones in the treatment of various autoimmune and inflammatory-linked diseases. However, aside from the onset of severe side-effects, chronic GC therapy often leads to the ligand-mediated downregulation of the GRα which, in turn, leads to a decrease in GC sensitivity, and effectively, the development of acquired GC resistance. Although the ligand-mediated downregulation of GRα is well documented, the precise factors which influence this process are not well understood and, thus, the development of an acquired GC resistance presents an ever-increasing challenge to the pharmaceutical industry. Recently, however, studies have correlated the dimerization status of the GRα with its ligand-mediated downregulation. Therefore, the current review will be discussing the major role-players in the homologous downregulation of the GRα pool, with a specific focus on previously reported GC-mediated reductions in GRα mRNA and protein levels, the molecular mechanisms through which the GRα functional pool is maintained and the possible impact of receptor conformation on GC-mediated GRα downregulation.

Disease- and treatment-associated acquired glucocorticoid resistance

The development of resistance to glucocorticoids (GCs) in therapeutic regimens poses a major threat. Generally, GC resistance is congenital or acquired over time as a result of disease progression, prolonged GC treatment or, in some cases, both. Essentially, disruptions in the function and/or pool of the glucocorticoid receptor α (GRα) underlie this resistance. Many studies have detailed how alterations in GRα function lead to diminished GC sensitivity; however, the current review highlights the wealth of data concerning reductions in the GRα pool, mediated by disease-associated and treatment-associated effects, which contribute to a significant decrease in GC sensitivity. Additionally, the current understanding of the molecular mechanisms involved in driving reductions in the GRα pool is discussed. After highlighting the importance of maintaining the level of the GRα pool to combat GC resistance, we present current strategies and argue that future strategies to prevent GC resistance should involve biased ligands with a predisposition for reduced GR dimerization, a strategy originally proposed as the SEMOGRAM-SEDIGRAM concept to reduce the side-effect profile of GCs.

Possible roles for glucocorticoid signalling in breast cancer

Our understanding of breast cancer biology, and our ability to manipulate breast cancers have grown exponentially in the last 20 years. Much of that expansion has focused on the roles of steroids in driving these neoplasms. Initially this research focused on estrogens and progesterone receptors, and more recently on androgen actions in breast cancers. This review aims to make the case for glucocorticoids as the next essential steroid subclass that contributes significantly to our understanding of steroidogenic regulation of these neoplasms. Glucocorticoids have the potential to play multiple roles in the regulation of breast cancers including their control of cellular differentiation, apoptosis and proliferation. Beyond this they also act as a master integrator of organ homeostats in relation to such as circadian rhythms and stress responses. Therefore a better understanding of glucocorticoids and breast cancer could help to explain some of the epidemiological links between circadian disruption and/or stress and breast cancer development. Finally glucocorticoids are currently used during chemotherapeutic treatment in breast cancer therapy and yet results of various studies suggest that this may have an adverse impact on treatment success. This review aims to summarise the current evidence for glucocorticoids as actors in breast cancer and then suggest future essential approaches in order to determine the roles of glucocorticoids in this disease.

Porcine glucocorticoid receptor (NR3C1) gene: tissue-specificity of transcriptional strength and glucocorticoid responsiveness of alternative promoters

Glucocorticoid receptor (GR) is transcribed in a tissue- and cell-specific manner with multiple exon 1 mRNA variants driven by selective promoters. We recently cloned and characterized the 5.3kb proximal promoter sequence of porcine GR gene containing 7 untranslated alternative first exons each processed by a distinct promoter. In this study, we showed tissue-specific expression of total GR and its exon 1 mRNA variants in hippocampus, muscle and liver of pigs. Total GR mRNA was most abundant in liver, followed by muscle and hippocampus in descending order. Among all the GR exon 1 mRNA variants detected, GR exon 1-9/10 and 1-4 were the most predominant variants in all the three tissues. The abundance of GR exon 1-4 mRNA was similar to that of 1-10 in muscle, but was significantly lower than 1-10 in liver and hippocampus. The activities of truncated short (S) and long (L) promoters of respective GR exon 1 mRNA variants were analyzed by luciferase reporter assay in 3 representative cell lines, SY5Y, C2C12 and HepG2. S1-10 and S1-4 demonstrated significantly higher activities than other short promoters in all the cell lines examined. Nevertheless, the strongest activity and cell specificity were detected for L1-10 promoter, which was consistent with the predominant exon 1-9/10 expression in porcine tissues. Moreover, with 3 potential nGRE binding sites, L1-10 promoter was more sensitive to dexamethasone (DEX) in HepG2. Our data provide basic knowledge of the transcriptional mechanism underlying the tissue- and cell-specific expression of porcine GR under basal or ligand-stimulated conditions.

Epigenetic modulation of glucocorticoid receptors in posttraumatic stress disorder

Some individuals suffering from posttraumatic stress disorder (PTSD) exhibit lower basal salivary cortisol and higher glucocorticoid receptor (GR) sensitivity. Recent studies suggest that epigenetic mechanisms regulate the activity of cortisol and GR. As a means to combine and cross-validate those findings, we compared cortisol, GR expression and promoter methylation levels in peripheral T lymphocytes of healthy controls versus individuals endorsing a diagnosis of lifetime PTSD. Thirty subjects with lifetime (current or remitted) PTSD and 16 subjects never exposed to trauma were recruited. Salivary cortisol was collected at six time points over the course of a single weekday and analyzed utilizing a time-resolved fluorescence immunoassay. GR expression (GRtotal, 1B, 1C, 1F and 1H) was measured by quantitative RT-PCR. DNA methylation levels in human glucocorticoid receptor (hGR) 1B and 1C variant's promoter were quantified by epityper in T lymphocytes isolated by magnetic-assisted cell sorting. Individuals with lifetime PTSD have lower morning cortisol release, higher mRNA expression of hGRtotal, 1B, and 1C and lower overall methylation levels in hGR 1B and 1C promoters. Cortisol levels were inversely correlated with hGR 1B mRNA expression. Moreover, overall and CpG site-specific methylation levels were inversely correlated with hGRtotal and 1B mRNA expression. There was no difference between current and remitted PTSD across cortisol, GR expression mRNA and DNA methylation data. Traumatic events induce DNA methylation alterations in distinct promoters of hGR with transcriptional modifications that associate with hypoactive hypothalamus-pituitary-adrenal axis in individuals with PTSD. Our results also point toward an important role of hGR 1B variant in PTSD.

Erg and AP-1 as determinants of glucocorticoid response in acute lymphoblastic leukemia

Glucocorticoids (GCs) are among the most widely prescribed medications in clinical practice. The beneficial effects of GCs in acute lymphoblastic leukemia (ALL) are based on their ability to induce apoptosis, but the underlying transcriptional mechanisms remain poorly defined. Computational modeling has enormous potential in the understanding of biological processes such as apoptosis and the discovery of novel regulatory mechanisms. We here present an integrated analysis of gene expression kinetic profiles using microarrays from GC sensitive and resistant ALL cell lines and patients, including newly generated and previously published data sets available from the Gene Expression Omnibus. By applying time-series clustering analysis in the sensitive ALL CEM-C7-14 cells, we identified 358 differentially regulated genes that we classified into 15 kinetic profiles. We identified GC response element (GRE) sequences in 33 of the upregulated known or potential GC receptor (GR) targets. Comparative study of sensitive and resistant ALL showed distinct gene expression patterns and indicated unexpected similarities between sensitivity-restored and resistant ALL. We found that activator protein 1 (AP-1), Ets related gene (Erg) and GR pathways were differentially regulated in sensitive and resistant ALL. Erg protein levels were substantially higher in CEM-C1-15-resistant cells, c-Jun was significantly induced in sensitive cells, whereas c-Fos was expressed at low levels in both. c-Jun was recruited on the AP-1 site on the Bim promoter, whereas a transient Erg occupancy on the GR promoter was detected. Inhibition of Erg and activation of GR lead to increased apoptosis in both sensitive and resistant ALL. These novel findings significantly advance our understanding of GC sensitivity and can be used to improve therapy of leukemia.

Regulation of human bone marrow stromal cell proliferation and differentiation capacity by glucocorticoid receptor and AP-1 crosstalk

Although marrow adipocytes and osteoblasts derive from a common bone marrow stromal cells (BMSCs), the mechanisms that underlie osteoporosis-associated bone loss and marrow adipogenesis during prolonged steroid treatment are unclear. We show in human BMSCs (hBMSCs) that glucocorticoid receptor (GR) signaling in response to high concentrations of glucocorticoid (GC) supports adipogenesis but inhibits osteogenesis by reducing c-Jun expression and hBMSC proliferation. Conversely, significantly lower concentrations of GC, which permit hBMSC proliferation, are necessary for normal bone mineralization. In contrast, platelet-derived growth factor (PDGF) signaling increases both JNK/c-Jun activity and hBMSC expansion, favoring osteogenic differentiation instead of adipogenesis. Indeed, PDGF antagonizes the proadipogenic qualities of GC/GR signaling. Thus our results reveal a novel c-Jun-centered regulatory network of signaling pathways in differentiating hBMSCs that controls the proliferation-dependent balance between osteogenesis and adipogenesis.

Glucocorticoid receptor knock down reveals a similar apoptotic threshold but differing gene regulation patterns in T-cell and pre-B-cell acute lymphoblastic leukemia

Glucocorticoids (GCs) are used in combination therapy for treating acute lymphoblastic leukemia (ALL). In T-cell (CEM-C7) and pre-B-cell (697) ALL cell lines, dexamethasone (Dex) treatment causes an auto-upregulation of glucocorticoid receptor (GR) mRNA transcripts and protein. We hypothesized that there is a threshold level of GR transcripts/protein needed for cells to respond to the apoptosis-inducing effects of hormone. GR knock down using a doxycycline-controllable shRNAmir indicated that the apoptotic response changes from sensitive to resistant with changing GR levels. Titration of the 697 cell GR to equal that of the CEM-C7 T-cell ALL line caused a shift in sensitivity to that seen in CEM-C7 cells. While the same level of GR is required to trigger apoptosis in both T-cell and pre-B-cell ALL lineages, similarities and differences were observed for the regulation of target genes in these lineages. These preliminary gene regulation patterns may lead to the development of a molecular signature for GC-sensitive and GC-resistant leukemia cells.

Glucocorticoid-induced apoptosis of healthy and malignant lymphocytes

Glucocorticoids exert a wide range of physiological effects, including the induction of apoptosis in lymphocytes. The progression of glucocorticoid-induced apoptosis is a multi-component process requiring contributions from both genomic and cytoplasmic signaling events. There is significant evidence indicating that the transactivation activity of the glucocorticoid receptor is required for the initiation of glucocorticoid-induced apoptosis. However, the rapid cytoplasmic effects of glucocorticoids may also contribute to the glucocorticoid-induced apoptosis-signaling pathway. Endogenous glucocorticoids shape the T-cell repertoire through both the induction of apoptosis by neglect during thymocyte maturation and the antagonism of T-cell receptor (TCR)-induced apoptosis during positive selection. Owing to their ability to induce apoptosis in lymphocytes, synthetic glucocorticoids are widely used in the treatment of haematological malignancies. Glucocorticoid chemotherapy is limited, however, by the emergence of glucocorticoid resistance. The development of novel therapies designed to overcome glucocorticoid resistance will dramatically improve the efficacy of glucocorticoid therapy in the treatment of haematological malignancies.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Minireview: latest perspectives on antiinflammatory actions of glucocorticoids

Taking into consideration that glucocorticoid (GC) hormones have been used clinically for over half a century and that more than 20 yr have passed since the cloning of the GC receptor (GR), it is hard to imagine that novel aspects in the molecular mechanism by which GCs mediate their antiinflammatory actions are still being unveiled today. Partly, this is because almost on a daily basis, novel insights arise from parallel fields, e.g. nuclear receptor cofactor and chromatin regulation and their concomitant impact on gene transcription events, eventually leading to a revisitation or refinement of old hypotheses. On the other hand, it does remain striking and puzzling why GCs use different mechanisms in so many different cell types and on many different target genes to elicit an antiinflammatory effect. Meanwhile, the obvious question for the clinic remains: is the separation of GR functionalities through differential ligand design the strategy of choice to avoid most GC-mediated side effects? This minireview aims to highlight some of the latest findings on aspects of the antiinflammatory working mechanisms of GCs.

Interaction of an approximately 40 kDa protein from regenerating rat liver with the -148 to -124 region of c-jun complexed with RLjunRP coincides with enhanced c-jun expression in proliferating rat liver

The c-jun belongs to the family of proto-oncogenes and encodes for the protein Jun, a component of transcription factor AP-1 involved in regulation of the expression of genes indispensable for cell proliferation and differentiation. While the role of c-jun in the regulation of such genes has been well examined, the regulation of c-jun in proliferating cells is not fully understood. We have earlier reported that the -148 to -124 region of c-jun is involved in the positive regulation of c-jun transcription, and interacts with a positive regulatory factor (rat liver jun regulatory protein; RLjunRP) present in rat liver. In this investigation, we report that this region is differentially recognized in proliferating liver as evidenced by the formation of a complex, different from that observed with normal liver extract. The new complex appears as early as 2 h after partial hepatectomy and its appearance coincides with the rise in c-jun mRNA levels after partial hepatectomy. In regenerating rat liver nuclear extract, an additional protein of approximately 40 kDa (rRLjunRP) interacts with a pre-existing dimer of RLjunRP complexed with the -148 to -124 region of c-jun to form a slow-migrating complex. rRLjunRP appears to pre-exist in the cytosol and translocate to the nucleus as indicated by the continued presence of the retarded complex in nuclear extract prepared from partially hepatectomized rats treated with cycloheximide. UV crosslinking studies, South-Western blot analysis, SDS/PAGE of affinity-purified factor(s), and 2D-PAGE analysis clearly demonstrate that the additional factor induced in response to growth stimulus is an approximately 40 kDa, that binds with the dimer of RLjunRP and enhances the c-jun transcription.

Expression of the glucocorticoid receptor from the 1A promoter correlates with T lymphocyte sensitivity to glucocorticoid-induced cell death

Glucocorticoid (GC) hormones cause pronounced T cell apoptosis, particularly in immature thymic T cells. This is possibly due to tissue-specific regulation of the glucocorticoid receptor (GR) gene. In mice the GR gene is transcribed from five separate promoters designated: 1A, 1B, 1C, 1D, and 1E. Nearly all cells express GR from promoters 1B-1E, but the activity of the 1A promoter has only been reported in the whole thymus or lymphocyte cell lines. To directly assess the role of GR promoter use in sensitivity to glucocorticoid-induced cell death, we have compared the activity of the GR 1A promoter with GC sensitivity in different mouse lymphocyte populations. We report that GR 1A promoter activity is restricted to thymocyte and peripheral lymphocyte populations and the cortex of the brain. The relative level of expression of the 1A promoter to the 1B-1E promoters within a lymphocyte population was found to directly correlate with susceptibility to GC-induced cell death, with the extremely GC-sensitive CD4+CD8+ thymocytes having the highest levels of GR 1A promoter activity, and the relatively GC-resistant alphabetaTCR+CD24(int/low) thymocytes and peripheral T cells having the lowest levels. DNA sequencing of the mouse GR 1A promoter revealed a putative glucocorticoid-response element. Furthermore, GR 1A promoter use and GR protein levels were increased by GC treatment in thymocytes, but not in splenocytes. These data suggest that tissue-specific differences in GR promoter use determine T cell sensitivity to glucocorticoid-induced cell death.

Mineralo- and glucocorticoid receptor mrnas are differently regulated by corticosterone in the rat hippocampus and anterior pituitary

In most cell lines and animal tissues, glucocorticoid receptors undergo downregulation after exposure to corticosterone. However, corticosterone treatment has not shown a consistent effect on mineralocorticoid (MR) and glucocorticoid receptors (GR) in the hippocampus, and it has been rarely assessed in the anterior pituitary. In this study we investigated dose-dependent effects of corticosterone on MR and GR mRNAs in the hippocampus and anterior pituitary. Adrenalectomized rats substituted with corticosterone in drinking fluid were injected subcutaneously with vehicle or 1, 10, 50, 100, or 200 mg of corticosterone, and sacrificed 4 h later. In the hippocampus we found a progressive decrease in MR and GR mRNAs with increasing doses of corticosterone. This was significant with 50 and 100 mg corticosterone for MR mRNA and with 10-200 mg corticosterone for GR mRNA at plasma corticosterone levels above 30 microg/dl. The anterior pituitary did not show significant changes at any dose. A time-course with 2 mg of corticosterone (non-response dose range at 4 h) revealed a significant decrease in MR and GR mRNAs in the hippocampus 8 h after the subcutaneous injection. In the anterior pituitary both mRNAs showed an increase that was significant 24 h after injection for MR and from 8 to 24 h for GR. In the hippocampus, adrenalectomy (absence of corticosterone) induced a significant increase in MR and GR mRNAs on day 3, but not on days 1, 8 and 21 after adrenalectomy. In the anterior pituitary there were no significant changes at any time after adrenalectomy. In summary, we have found an in vivo corticosterone dose- and time-dependent downregulation of MR and GR mRNAs in the hippocampus, whereas anterior pituitary MRs and GRs seem relatively insensitive to the excess or the absence of corticosterone, suggesting the lack of an autoregulatory effect in this tissue. Significant mRNA changes appearing later in time could suggest a secondary response via a glucocorticoid-induced gene product. Corticosteroid receptor downregulation in the hippocampus could prevent overstimulation or tissue damage when plasma corticosterone is high, while increased corticosteroid receptors in the anterior pituitary could buffer the excessive brain drive on the pituitary during chronic stress or pathological conditions associated with increased plasma glucocorticoids, such as depression.

Repression of glucocorticoid receptor gene transcription by c-Jun

The regulation of glucocorticoid receptor gene expression by members of the AP-1 family was examined in glucocorticoid-free NIH3T3 cells transfected with the human glucocorticoid receptor gene promoter driving expression of a CAT reporter gene. c-Jun inhibited the promoter activity by 80% and JunB by 30%, whereas c-Fos and JunD had no inhibitory effect. Electrophoretic mobility shift assays showed that c-Jun is unable to efficiently interact with the AP-1-like site present in the human glucocorticoid receptor promoter. Moreover, c-Jun was still able to repress promoter mutants in which the region containing the AP-1-like site was deleted. NIH3T3 cell clones overexpressing c-Jun exhibited lower glucocorticoid receptor mRNA levels, which suggests that the murine glucocorticoid receptor gene can also be regulated by AP-1. These results provide a new mechanism for cross-talk between the glucocorticoid receptor and the AP-1 family of transcription factors in the absence of glucocorticoid ligands.

Effect of glucocorticoid therapy on glucocorticoid receptors in children with autoimmune diseases

Low-dose glucocorticoids (GC) achieve their action completely by classical genomic effects, mediated by the glucocorticoid receptor (GCR). In high doses of GC, nongenomic effects have also been found, but it is still unclear to what extent they contribute to a beneficial outcome. In this study, we present a determination of the number of lymphocyte GCR sites and the binding affinity in healthy children and children with autoimmune diseases. We further assess the effect of GC administration, especially of high-dose pulse therapy on the number of binding sites. The number of GCR sites per cell was analyzed with [(3)H]-dexamethasone radioligand binding assay and binding affinity (Kd given in nM) in peripheral blood mononuclear cells isolated from 48 healthy children and 35 patients. The patients were divided into three groups based on GC treatment: 0 mg/kg (group 1), 0.01-0.3 mg/kg orally (group 2), and 10-15 mg/kg i.v. pulse therapy (group 3) of prednisolone equivalent per day. Gender- and age-independent normal values of 4338 +/- 1687 sites/lymphocytes and Kd 6.7 +/- 2.2 nM were found. At 3463 +/- 1574, the number of receptor sites in patients without GC (group 1) was significantly lower than that of healthy volunteers (p < 0.05). In patients receiving GC treatment, this value was reduced to 2952 +/- 512 (group 2). Significant down-regulation to a minimum of 479 +/- 168 (group 3) was found after pulse therapy compared with untreated patients (p < 0.01). In pulse therapy, GC lead to a fast and dramatic receptor down-regulation. We suppose that the increase in therapeutic success of pulse-therapy may partly be mediated through additional nongenomic effects.

The delayed induction of c-jun in apoptotic human leukemic lymphoblasts is primarily transcriptional

Because of their ability to induce lymphoid cell apoptosis, glucocorticoids have been used for decades to treat certain human leukemias and lymphomas. Studies presented in this paper complement our previous work demonstrating that sustained induction of the proto-oncogene c-jun plays a crucial role in the glucocorticoid-induced apoptotic pathway in CEM cells, human leukemic lymphoblasts. Results from measurements of c-jun mRNA half-life with RNase protection assays and of transcription by nuclear run-on assays indicate that, in the dexamethasone-sensitive cloned CEM-C7 cells, c-jun is induced at the transcriptional level. Consideration of time-course, however, suggested that this might be a secondary or possibly a delayed primary response. Use of cycloheximide to block protein synthesis strongly induced c-jun mRNA, suggesting that there had been relief from a labile protein repressor of transcription. Comparing the level of induction by cycloheximide with that of dexamethasone indicated that the two did not induce by an identical mechanism. The high induction by cycloheximide obscured simple interpretation of elevated c-jun mRNA levels after concomitant administration of cycloheximide and dexamethasone. This was resolved by nuclear run-on experiments, which showed that the dexamethasone induction of c-jun mRNA in this system does require protein synthesis.

Expression of the Death Gene Bik/Nbk Promotes Sensitivity to Drug-Induced Apoptosis in Corticosteroid-Resistant T-Cell Lymphoma and Prevents Tumor Growth in Severe Combined Immunodeficient Mice

Members of the Bcl-2 gene family have been implicated in the regulation of cell death induced by cytostatic drugs. In some malignancies such as B-cell lymphoma, there is evidence that high expression of Bcl-2 is an independent negative prognostic marker and the overexpression of Bcl-2 has been shown to confer resistance to cytotoxic drugs by preventing drug-induced apoptosis. This function of Bcl-2 can be antagonized by apoptosis-promoting members of the Bcl-2 family. We previously showed that overexpression of Bax restores the chemosensitivity of Bax-deficient breast cancer cell lines. Therefore, we investigated whether the death-promoting Bcl-2 homologue Bik/Nbk can enhance cytostatic drug-induced apoptosis. As a model, we used the T-cell leukemia H9 (CD3+ and CD4+CD8−), which is resistant to corticosteroid-induced cell death and does not express endogenous Bik/Nbk. Sensitivity for drug-induced apoptosis was increased 10- to 39-fold in cells transfected with the full-length coding sequence of Bik/Nbk. In addition, apoptosis induced via CD95/Fas or heat shock was increased to a similar extent. These data show that Bik/Nbk, which, unlike Bax, carries only a BH3 but no BH1 or BH2 domain may be a target to enhance chemosensitivity. The complete suppression of tumor growth in a severe combined immunodeficient mouse xenotransplant model suggests that, in analogy to Bax, Bik/Nbk may function as a tumor suppressor gene.

Aldosterone action: induction of p21(ras) and fra-2 and transcription-independent decrease in myc, jun, and fos

Adrenal steroids induce an increase in transcellular Na+ reabsorption across Xenopus laevis A6 cell epithelia that requires the action of transcriptionally regulated gene products. In a previous study we identified K-ras2 as an aldosterone-upregulated mRNA in A6 epithelia. Here, we show that in vivo injection of aldosterone in Xenopus (2.5 h) increases K-ras2 mRNA specifically in the kidney (2.5-fold) and that in A6 epithelia aldosterone (2.5 h) increases Ras protein synthesis ( approximately 6-fold). Xl-ras, another ras mRNA expressed at a low level in A6 cells, was also induced (2-fold). Aldosterone was shown to regulate the mRNA levels of several transcription factors as well. After 2 h of aldosterone treatment, fra-2 mRNA was upregulated by 130%, whereas c-myc, c-jun, c-fos, and glucocorticoid receptor mRNAs were downregulated by 23-43%. After 16 h, c-fos and GR mRNAs were further decreased, whereas levels of fra-2, c-jun, and c-myc began to return to control levels. Interestingly, the downregulation of the protooncogene mRNAs was independent of transcription. These results support the view that aldosterone exerts complex pleiotropic transcriptional and nontranscriptional actions that involve the regulation of signaling cascade elements (i.e., K-Ras2) as well as that of transcription factors.

The novel progesterone receptor antagonists RTI 3021-012 and RTI 3021-022 exhibit complex glucocorticoid receptor antagonist activities: implications for the development of dissociated antiprogestins

We have identified two novel compounds (RTI 3021-012 and RTI 3021-022) that demonstrate similar affinities for human progesterone receptor (PR) and display equivalent antiprogestenic activity. As with most antiprogestins, such as RU486, RTI 3021-012, and RTI 3021-022 also bind to the glucocorticoid receptor (GR) with high affinity. Unexpectedly, when compared with RU486, the RTI antagonists manifest significantly less GR antagonist activity. This finding indicates that, with respect to antiglucocorticoid function, receptor binding affinity is not a good predictor of biological activity. We have determined that the lack of a clear correlation between the GR binding affinity of the RTI compounds and their antagonist activity reflects the unique manner in which they modulate GR signaling. Previously, we proposed a two step "active inhibition" model to explain steroid receptor antagonism: 1) competitive inhibition of agonist binding; and 2) competition of the antagonist bound receptor with that activated by agonists for DNA response elements within target gene promoters. Accordingly, we observed that RU486, RTI 3021-012, and RTI 3021-022, when assayed for PR antagonist activity, accomplished both of these steps. Thus, all three compounds are "active antagonists" of PR function. When assayed on GR, however, RU486 alone functioned as an active antagonist. RTI 3021-012 and RTI 3021-022, on the other hand, functioned solely as "competitive antagonists" since they were capable of high affinity GR binding, but the resulting ligand receptor complex was unable to bind DNA. These results have important pharmaceutical implications supporting the use of mechanism based approaches to identify nuclear receptor modulators. Of equal importance, RTI 3021-012 and RTI 3021-022 are two new antiprogestins that may have clinical utility and are likely to be useful as research reagents with which to separate the effects of antiprogestins and antiglucocorticoids in physiological systems.

Glucocorticoid-induced apoptosis and regulation of NF-kappaB activity in human leukemic T cells

Glucocorticoid-induced apoptosis was investigated in glucocorticoid-sensitive 6TG1.1 and resistant ICR27TK.3 human leukemic T cells. Following glucocorticoid treatment of 6TG1.1 cells, chromatin fragmentation was observed after a delay of 24 h. Fragmentation was not observed in ICR27TK.3 cells containing mutant glucocorticoid receptors (L753F) that are activation-deficient but retain the ability to repress AP-1 activity. Nor was fragmentation observed after treatment with RU38486, indicating that repression of AP-1 activity is not involved. As described in other systems, fragmentation required ongoing protein synthesis. However, inhibition of protein synthesis with cycloheximide anytime during the first 18 h of steroid treatment was as effective in blocking chromatin fragmentation as inhibition for the entire period, suggesting that synthesis of a component with a rapid turnover rate is required. Dexamethasone treatment completely blocked 12-O-tetradecanoylphorbol 13-acetate induction of nuclear factor-kappaB (NF-kappaB) activity and elicited an increase in the amount of immunoreactive IkappaB alpha in sensitive 6TG1.1 cells but not in resistant ICR27TK.3 cells. In addition, mild detergent treatment of cell extracts indicated that a substantial amount of cytoplasmic NF-kappaB is complexed with IkappaB alpha or some other inhibitory factor. These results suggest that induction of a labile inhibitory factor such as IkappaB alpha may contribute to glucocorticoid-induced apoptosis.

Glucocorticoid receptor mRNA and protein in fetal rat lung in vivo: modulation by glucocorticoid and androgen

Pulmonary glucocorticoid receptor (GR) is essential to timely preparation for the onset of breathing air at birth. We have previously used primary culture of late-gestation fetal rat lung cells to demonstrate differential regulation of GR by glucocorticoid depending on cell type. In this study, we hypothesized that the action of glucocorticoid on GR mRNA expression and protein elaboration in lung cells might be modulated by interactions present in vivo but not in primary culture. Given that male sex hormone (androgen) has an inhibitory effect on antenatal lung development, we also postulated that androgen would decrease antenatal lung GR. We report that antenatal maternal injection of the glucocorticoid dexamethasone (1 mg/kg) enhanced fetal lung cellular levels of GR mRNA and protein as assessed by in situ hybridization and immunocytochemistry (ICC), respectively. ICC was performed using polyclonal rabbit anti-human antibody that reacts with rat GR whether bound to ligand or not and does not interfere with GR binding to DNA. Levels of GR mRNA and protein were enhanced in cells throughout all areas of the lung tissue, suggesting that interactions occurring in intact tissue may override the previously reported direct inhibition by glucocorticoid of GR protein elaboration in isolated fetal rat lung epithelial cells. Furthermore, antenatal administration of the androgen 5alpha-dihydrotestosterone (0.2 mg/kg) reduced tissue levels of GR mRNA and protein, consistent with androgenic inhibition of antenatal lung development by decreasing GR. We conclude that glucocorticoids and androgens exert opposite effects on fetal lung GR.

Three independent lines of evidence suggest retinoids as causal to schizophrenia

Retinoid dysregulation may be an important factor in the etiology of schizophrenia. This hypothesis is supported by three independent lines of evidence that triangulate on retinoid involvement in schizophrenia: (i) congenital anomalies similar to those caused by retinoid dysfunction are found in schizophrenics and their relatives; (ii) those loci that have been suggestively linked to schizophrenia are also the loci of the genes of the retinoid cascade (convergent loci); and (iii) the transcriptional activation of the dopamine D2 receptor and numerous schizophrenia candidate genes is regulated by retinoic acid. These findings suggest a close causal relationship between retinoids and the underlying pathophysiological defects in schizophrenia. This leads to specific strategies for linkage analyses in schizophrenia. In view of the heterodimeric nature of the retinoid nuclear receptor transcription factors, e.g., retinoid X receptor beta at chromosome 6p21.3 and retinoic acid receptor beta at 3p24.3, two-locus linkage models incorporating genes of the retinoid cascade and their heterodimeric partners, e.g., peroxisome proliferator-activated receptor alpha at chromosome 22q12-q13 or nuclear-related receptor 1 at chromosome 2q22-q23, are proposed. New treatment modalities using retinoid analogs to alter the downstream expression of the dopamine receptors and other genes that are targets of retinoid regulation, and that are thought to be involved in schizophrenia, are suggested.

Modulation of hormone-dependent glucocorticoid receptor function using a tetracycline-regulated expression system

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor capable of stimulating and inhibiting the expression of target genes. To better understand the biological action of glucocorticoids and the function of GR, we have utilized the tetracycline (Tc)-regulated mammalian expression system to develop a novel cell line, E8.2/GR3, derived from GR null mouse L929 fibroblasts, that exhibits conditional expression of rat GR. The intracellular concentration of rGR in E8.2/GR3 cells--from undetectable levels to levels more than 10-fold greater than that observed in wild-type L929 cells--could be manipulated by varying the Tc concentration in the culture media. Similarly, dexamethasone (DEX)-dependent transactivation of the mouse mammary tumor virus long terminal repeat and transrepression of the cadmium-induced activity of the mouse heme oxygenase-1 gene enhancer, SX2, were strictly dependent on the presence of rGR, and the levels of these activities could be modulated by Tc. Similar levels of Tc, and thus rGR, were required for half-maximal transactivation and transrepression whereas a 6-fold lower concentration of DEX was required for half-maximal transrepression than for transactivation. RU486 inhibited both DEX-dependent transactivation and transrepression. DEX decreased the steady-state level of rGR mRNA and protein in a Tc dependent manner. DEX also induced morphological changes in E8.2/GR3 cells that were dependent on rGR as no alterations were observed in the presence of Tc. These cells provide a powerful system for examining the various activities of GR, particularly as a function of different intracellular receptor concentrations.

A DNA-binding element for a steroid receptor-binding factor is flanked by dual nuclear matrix DNA attachment sites in the c-myc gene promoter

The receptor-binding factor (RBF) for the avian oviduct progesterone (Pg) receptor (PR) has previously been shown to be a unique 10-kDa nuclear matrix protein that generates high affinity PR-binding sites on avian DNA. This paper describes the use of Southwestern blot and DNA gel shift analyses with RBF protein to identify a minimal 54-base pair RBF-binding element in the matrix-associated region (MAR) of the Pg-regulated c-myc gene promoter. This element contains a 5'-GC-rich domain and a 3'-AT-rich domain, the latter of which has a homopurine/homopyrimidine structure. The gel shift assays required the generation of an RBF-maltose fusion protein (RBF-MBP), which specifically binds this element and is supershifted when the anti-RBF polyclonal antibody is added. Computer analysis of the full-length amino acid sequence for RBF predicts a DNA-binding motif involving a beta-sheet structure at the N-terminal domain. Southern blot analyses using nuclear matrix DNA suggests that there are dual MAR sites in the c-myc promoter, which flank an intervening domain containing the RBF element. The co-transfection of this MAR sequence, containing the RBF element and cloned into a luciferase reporter vector, together with an RBF expression vector construct, into steroid treated human MCF-7 cells, results in a decrease of the c-myc promoter activity relative to control transfections containing only the parent vector of the RBF expression construct. These data suggest that a unique chromatin/nuclear matrix structure, composed of the RBF-DNA element complex which is flanked by nuclear matrix attachment sites, serves to bind the PR and repress the c-myc promoter.

Glucocorticoid inhibition of fibroblast proliferation and regulation of the cyclin kinase inhibitor p21Cip1

Glucocorticoids inhibit the proliferation of fibroblastic cells in vivo and in culture; however, the molecular mechanism that accounts for this effect has remained obscure. We have undertaken to elucidate the mechanism whereby glucocorticoids decrease the rate of proliferation of mouse L929 fibroblastic cells. Addition of dexamethasone to mid-log phase fibroblasts prolongs G1 phase. This increase in the G1 interval is associated with, and probably due to, inhibition of phosphorylation of the product of the Rb-1 tumor suppressor gene, pRb. Inhibition of pRb phosphorylation by cyclin D-dependent kinases can be demonstrated in vitro. Nevertheless, there is no detectable change in the expression of cyclin D1, cyclin D2, or cyclin D3. Cyclin-dependent kinase-4 (Cdk4) and Cdk6 are not down-regulated in L929 cells after addition of glucocorticoids, and the abundance of cyclin D/Cdk4 complexes does not change. Inhibition of pRb kinase activity is associated with an increase in the abundance of one of the Cdk inhibitors, p21Cip1. The abundance of another cyclin kinase inhibitor, p27Kip1, remains constant. The amount of Cdk4 that is bound to p21Cip1 increases rapidly after addition of dexamethasone, and the activity of Cdk4-pRb kinase decreases in parallel. These results indicate that glucocorticoid inhibition of fibroblast proliferation is due to induction of p21Cip1, which binds to and inactivates cyclinD/Cdk4 complexes. The abundance of p21 mRNA increases about 5-fold within 2 h after addition of dexamethasone. This effect does not obtain in L929 mutants that are null for the glucocorticoid receptor, and a variant that expresses the glucocorticoid receptor from a tetracycline-repressible expression vector demonstrates induction of p21 mRNA only in the absence of tetracycline. Cycloheximide does not block induction of p21 mRNA, and dexamethasone has no detectable effect on the apparent rate of degradation of p21 mRNA. Nuclear run-on transcription of the Cip1 gene increases within 2 h after addition of dexamethasone. This effect can be blocked by tetracycline-mediated repression of the glucocorticoid receptor.