Down-regulation of bcl-xs gene expression in rat thymocytes by dexamethasone

Glucocorticoids protect renal mesangial cells from apoptosis by increasing cellular sphingosine-1-phosphate

Neutral ceramidase (NCDase) and sphingosine kinases (SphKs) are key enzymes regulating cellular sphingosine-1-phosphate (S1P) levels. In this study we found that stress factor-induced apoptosis of rat renal mesangial cells was significantly reduced by dexamethasone treatment. Concomitantly, dexamethasone increased cellular S1P levels, suggesting an activation of sphingolipid-metabolizing enzymes. The cell-protective effect of glucocorticoids was reversed by a SphK inhibitor, was completely absent in SphK1-deficient cells, and was associated with upregulated mRNA and protein expression of NCDase and SphK1. Additionally, in vivo experiments in mice showed that dexamethasone also upregulated SphK1 mRNA and activity, and NCDase protein expression in the kidney. Fragments (2285, 1724, and 1126 bp) of the rat NCDase promoter linked to a luciferase reporter were transfected into rat kidney fibroblasts and mesangial cells. There was enhanced NCDase promoter activity upon glucocorticoids treatment that was abolished by the glucocorticoid receptor antagonist RU-486. Single and double mutations of the two putative glucocorticoid response element sites within the promoter reduced the dexamethasone effect, suggesting that both glucocorticoid response elements are functionally active and required for induction. Our study shows that glucocorticoids exert a protective effect on stress-induced mesangial cell apoptosis in vitro and in vivo by upregulating NCDase and SphK1 expression and activity, resulting in enhanced levels of the protective lipid second messenger S1P.

Epithelial cell expression of BCL-2 family proteins predicts mechanisms that regulate Helicobacter pylori-induced pathology in the mouse stomach

Corpus-predominant infection with Helicobacter pylori (HP) results in the activation of programmed cell death pathways in surface, parietal, and chief cells. At present, mechanisms that regulate these pathways to result in HP-associated pathology are not fully understood. Because it is not known which survival and death pathways are present in gastric epithelial cells, we used an antibody panel to evaluate the expression of BCL-2 family prosurvival proteins or multi-Bcl-2 homology (BH)-domains (group 1) or BH3-only (group-2) proapoptotic proteins in the stomachs of uninfected or HP-infected C57BL/6 mice. This strategy identified BCL-2, BAK, and BAD as the major prosurvival and proapoptotic proteins, in surface cells and BAD as the only BCL-2 family protein expressed in parietal cells. Chief cells express altogether different effectors, including BCL-X(L)/BCL-2, for survival but have no constitutively expressed proapoptotic proteins. In model chief cells, however, the group 1 proapoptotic protein BCL-X(S) was expressed after exposure to proinflammatory cytokines concomitant with reduced viability, demonstrating that chief cells can transcriptionally regulate the induction of proapoptotic proteins to execute apoptosis. During HP infection, no additional BCL-2 family proteins were expressed in epithelial cells, whereas those present either remained unchanged or were reduced as cell deletion occurred over time. Additional studies demonstrated that the posttranslational regulation of BAD in surface and parietal cells was negatively affected by HP infection, a result that may be directly related to an increase in apoptosis during infection. Thus, gastric epithelial cells express cell-specific prosurvival and proapoptotic pathways. From the results presented here, mechanisms that regulate HP-related changes in the survival and death profile of gastric epithelial cells can be predicted and then tested, with the ultimate goal of elucidating important therapeutic targets to inhibit the progression of HP-related pathology in the stomach.

Glucocorticoids inhibit the apoptotic actions of UV-C but not Fas ligand in hepatoma cells: direct evidence for a critical role of Bcl-xL

Our laboratory has shown that glucocorticoids can inhibit apoptosis in rat hepatoma cells; however, the mechanisms are incompletely understood. To address this issue we sought to determine if glucocorticoid inhibition is effective when death is induced by stimuli that more selectively activate either the intrinsic (UV-C) or extrinsic (FasL) apoptotic pathways. Using flow cytometric analysis, we show that pretreatment of HTC cells with dexamethasone (Dex) inhibits UV-C- but not FasL-induced apoptosis. This inhibition requires Dex pretreatment and can be abrogated by the glucocorticoid antagonist RU486 indicating glucocorticoid receptor-mediated action. Dex increases anti-apoptotic Bcl-x(L) at both mRNA and protein levels. The Bcl-x(L) protein level remains elevated even after apoptosis induction with either UV-C or FasL although only UV-C-induced cell death is inhibited. Repression of Bcl-x(L) protein with siRNA abrogates the anti-apoptotic effect of glucocorticoids. Together these data provide direct evidence that Bcl-x(L) mediates glucocorticoid inhibition of UV-C induced apoptosis.

Pleiotropic anti-apoptotic activity of glucocorticoids in ovarian follicular cells

Glucocorticoids (GC) such as hydrocortisone and dexamethasone (DEX) protect steroidogenic granulosa cells against apoptosis induced by serum deprivation, cAMP, tumor necrosis factor alpha stimulation or p53 activation. The protective effects were evident both in primary rat and human granulosa cells, which comprise the main population of the ovarian follicular cells, as well as in steroidogenic granulosa cell lines established in our laboratory. A correlation between the expression of Bcl-2 protein and protection against apoptosis induced by DEX was found in granulosa cell lines expressing various levels of Bcl-2. Incubation with DEX leads to development of a rigid network of actin cytoskeleton and increased incidence of adherence and gap junctions. Higher content of connexin 43 and total cadherins were found in GC stimulated cells compared to non-stimulated, suggesting that cell contact and intracellular communication contribute to the DEX induced resistance to apoptotic signals. Activation by DEX of MAPK and Akt/PKB but not p38 supported the view of a pleiotropic action of GC against apoptotic signals. Granzyme B, a protease characteristic for induction of apoptosis by T-cytotoxic lymphocytes and natural killer cells, was expressed and augmented during stimulation of apoptosis in the granulosa cells, and its synthesis and activation was blocked by DEX. It is concluded that GC exerted their anti-apoptotic effects in granulosa cells by multiple characteristic pathways. Moreover, the presence of endogenous granzyme B in granulosa cells suggest a novel intrinsic alternative apoptotic pathway that was earlier reported to be mediated uniquely by T-cytotoxic lymphocytes and natural killer cells. The anti-apoptotic effect of GC may play an important role in the healing process of the ovulatory follicle subsequent to follicular rupture and its rapid conversion to an active corpus luteum.

Controlling cellular reactive responses around neural prosthetic devices using peripheral and local intervention strategies

While chronic use of indwelling micromachined neural prosthetic devices has great potential, the development of reactive responses around them results in a decrease in electrode function over time. Since the cellular events responsible for these responses may be anti-inflammatory in nature, we have tested the effectiveness of dexamethasone and cyclosporin A as potential drugs for developing intervention strategies following insertion of single-shank micromachined silicon devices. Peripheral injection of dexamethasone was effective in attenuating increased expression of glial fibrillary acidic protein and astrocyte hyperplasia observed during both initial- and sustained-reactive responses observed at one and six weeks post insertion, respectively. Peripheral injection of cyclosporin A had no positive effect. If anything, application of this drug increased the early reactive response. Effectiveness of local release of dexamethasone in rat neocortex was tested by inserting ribbons of poly (ethyl-vinyl) acetate containing 35% (w/w) dexamethasone. Initial concentrations of dexamethasone were similar to those obtained by peripheral injection. Local drug release provided continued control of cellular reactive responses during the six-week study period. These results demonstrate that peripheral delivery of dexamethasone can be used to control reactive responses and that local drug delivery by slow-release from biocompatible polymers may be a more effective method of drug intervention. Incorporating these strategies on micromachined devices may provide an intervention strategy that will insure the chronic functioning of electrodes on intracortical neuroprosthetic devices.

Promoter choice influences alternative splicing and determines the balance of isoforms expressed from the mouse bcl-X gene

Differential splicing from the bcl-X gene generates several isoforms with opposite effects on the apoptotic response. To explore the mechanism controlling the balance between the various isoforms, we have characterized the 5' region of the mouse bcl-X gene. We identified three new promoters in addition to the two previously described (Grillot, D. A., M., G.-G., Ekhterae, D., Duan, L., Inohara, N., Ohta, S., Seldin, M. F., and Núñez, G. (1997) J. Immunol. 158, 4750-4757). These five promoters (P1-P5) would give rise to at least five mRNAs with different 5'-untranslated region, all sharing the same translation initiation site. Except for the product of the most proximal promoter (P1), the other mRNAs are generated by alternative splicing of noncoding exons to a common acceptor site located in the first translated exon. Reverse transcriptase-polymerase chain reaction, primer extension, and RNase protection assays demonstrate a tissue-specific pattern of promoter usage. P1 and P2 are active in all tissues analyzed, whereas the other three promoter show tissue-specific activities. P3 is active in spleen, liver, and kidney, P4 is active in uterus and spleen, and P5 is active in spleen, liver, brain, and thymus. We present evidence suggesting that promoter selection influences the outcome of the splice process. Transcripts from P1 generate mainly the mRNA for the long isoform Bcl-X(L), whereas transcripts from P2 generate mRNAs for the isoforms Bcl-X(L), Bcl-X(S), and Bcl-X(gamma) and transcripts from P3 yield mainly mRNAs for the isoform Bcl-X(gamma). Our results suggest a key role of promoter choice in determining alternative splicing and, thus, the balance of Bcl-X isoforms.

Dexamethasone inhibits TNF-alpha-induced apoptosis and IAP protein downregulation in MCF-7 cells

Exposure of human mammary carcinoma cell line MCF-7 to TNF-alpha leads to apoptotic cell death within 24 h. In search for apoptosis-preventing signals, we identified glucocorticoids as potent death-preventing compounds. Ten nM dexamethasone provided a significant protective effect whereas 100 nM dexamethasone roughly blocked 80 - 90% of TNF-alpha-induced apoptosis. Surprisingly, dexamethasone exerted a protective effect even when supplied several hours after TNF-alpha. This points to a powerful inhibition of even advanced apoptotic processes by dexamethasone. To further pinpoint the anti-apoptotic glucocorticoid action, we investigated the expression levels of several members of the inhibitors of apoptosis (IAPs) family of proteins in response to TNF-alpha and dexamethasone. IAP proteins directly block caspase protease activities including caspase-3, caspase-7, and caspase-9. Exposure of MCF-7 cells to TNF caused an extensive downregulation of cIAP1, cIAP2, and XIAP protein levels. The decline of the IAP protein levels temporally paralleled the appearance of apoptotic DNA fragments which started 12 - 14 h following TNF-alpha addition and maximal effects were seen within 24 h. Coincubation of cells with TNF-alpha and dexamethasone potently blocked cIAP1, cIAP2, and XIAP downregulation. TNF-alpha-mediated IAP protein downregulation was not affected by proteasome inhibitors like lactacystin, ALLN or ALLM, whereas it was blocked by the broad-spectrum caspase inhibitor Z-VAD-fmk which also prevented TNF-alpha-induced apoptotic cell death. These data suggest that inhibition of IAP downregulation mediated by a caspase proteolytic activity constitutes the anti-apoptotic action of glucocorticoids in MCF-7 carcinoma cells.

Suppression of apoptosis by glucocorticoids in glomerular endothelial cells: effects on proapoptotic pathways

Tumour necrosis factor-alpha (TNF-alpha)- and lipopolysaccharide (LPS)-induced apoptosis of bovine glomerular endothelial cells is now recognized as an important part in the pathogenesis of glomerulonephritis characterized by early mitochondrial cytochrome c release, mitochondrial permeability transition, Bak protein upregulation, Bcl-X(L) protein downregulation and caspase-3 activation. Co-treatment of cells with 10 nM dexamethasone and TNF-alpha or LPS blocked roughly 90% of apoptotic cell death in glomerular endothelial cells. The action of glucocorticoids could be documented in that they prevented all apoptotic markers such as DNA laddering, DNA fragmentation measured by the diphenylamine assay as well as morphological alterations. To mechanistically elucidate the action of glucocorticoids we evaluated whether glucocorticoids elicit a time-dependent effect. For dexamethasone, to maximally inhibit DNA fragmentation a preincubation period was not required. Even if dexamethasone was supplemented 6 h following TNF-alpha or LPS we observed a maximal inhibitory effect. Concerning its influence on TNF-alpha and LPS signal transduction, we found that dexamethasone only partially prevented cytochrome-c-release as a first sign of apoptotic cell death but efficiently blocked mitochondrial permeability transition. Moreover, TNF-alpha- and LPS-induced Bak upregulation, Bcl-X(L)-downregulation, and the activation of caspase-3-like proteases, measured fluorometrically using DEVD-AMC and PARP cleavage, were efficiently blocked by dexamethasone. We postulate that glucocorticoids exert their inhibitory action upstream of the terminal death pathways but downstream of primary receptor mediated signals by blocking pro-apoptotic signals pre- and/or post cytochrome c release and mitochondrial signalling.

Dexamethasone protection of rat intestinal epithelial cells against oxidant injury is mediated by induction of heat shock protein 72

Although the therapeutic actions of glucocorticoids are largely attributed to their anti-inflammatory and immunosuppressive effects, they have been implicated in enhancing tissue and cellular protection. In this study, we demonstrate that dexamethasone significantly enhances viability of IEC-18 rat small intestinal cells against oxidant-induced stress in a dose-dependent fashion. This protective action is mediated by induction of hsp72, the major inducible heat shock protein in intestinal epithelial cells. Dexamethasone stimulates a time- and dose-dependent response in hsp72 protein expression that parallels its effects on cell viability. Furthermore, the induction of hsp72 is tissue dependent, as nonintestinal epithelioid HeLa cells show differential induction of hsp72 expression in response to the same dexamethasone treatment. Antisense hsp72 cDNA transfection of IEC-18 cells abolishes the dexamethasone-induced hsp72 response, without significantly affecting constitutive expression of its homologue, hsc73. Dexamethasone treatment also significantly induces hsp72 protein expression in rat intestinal mucosal cells in vivo. These data demonstrate that glucocorticoids protect intestinal epithelial cells against oxidant-induced stress by inducing hsp72.

Dexamethasone suppresses apoptosis in a human gastric cancer cell line through modulation of bcl-x gene expression

Treatment of human gastric cancer TMK-1 cells with transcription and translation inhibitors rapidly triggered cell apoptosis. Along with cell apoptosis, the Bcl-xS level was markedly upregulated suggesting a crucial role of this protein in promoting the apoptotic process. In the presence of dexamethasone, however, cell apoptosis was greatly attenuated as demonstrated by DNA histogram shift and DNA fragmentation. Studies using the glucocorticoid receptor antagonist RU486 indicated that attenuation of apoptosis was mediated through glucocorticoid receptors. Dexamethasone not only suppressed the apoptosis-associated upregulation of Bcl-xS but also enhanced the basal level of Bcl-xL in the cells. In addition, bcl-x mRNA stability was significantly extended in the presence of dexamethasone. These results indicate that dexamethasone exerted a protective effect and delayed apoptosis of TMK-1 cells by modulating bcl-x gene expression.

[Implication of type II glucocorticoid receptors in aldosterone induced apoptosis of rat thymocytes]

The thymolytic action of dexamethasone (DEXA) and aldosterone (ALDO) has been studied in vitro and in vivo. In vitro, in the presence of DEXA, the number of apoptotic cells increased with time. After 6 hours of incubation, 55 and 86% of thymocytes are dead with 10(-7) and 10(-5) M of DEXA, respectively. Whereas, in the presence of equivalent concentrations of ALDO, the rate of mortality of cells was only 30-40%. In vivo study confirmed these results and showed that apoptotic action of ALDO remained less potent than that of DEXA. On the other hand, addition of the potent glucocorticoid antagonist, RU 38486 prevents not only the dexamethasone but also the aldosterone-stimulated cell death. We conclude that the thymolytic action of the endogenous mineralocorticoid hormone is not mediated by its specific receptor but paradoxically by type II glucocorticoid receptor.

Glucocorticoid receptors in the pig intestinal tract and muscle tissue

Due to the known differences of protein turnover in skeletal muscle (slow turnover) and intestinal tract (rapid turnover), these tissues were analysed for the free glucocorticoid receptor (GR) concentrations. Cytosol was prepared from different parts of pig intestinal tract and two different muscles (longissimus thoracis: ML; trapezius: MT) under either energy restriction (5.8 MJ ME; 4 kg/d) or sufficient supply (14.0 MJ ME; 4 kg/d) for 4 days before sacrification. Care was taken that all samples were obtained within 15 min post-mortem and that premortem treatment did not elevate circulating cortisol. A Scatchard plot was performed both for muscle and gut tissue and the KD (0.4 +/- 0.04 nM; 0.7 +/- 0.02 nM) as well as Bmax (0.2 +/- 0.01 nM; 1.2 +/- 0.02 nM) pointed to specific binding. GR were generally high in intestinal tract irrespective of the anatomical site, but were significantly (P < or = 0.001) higher under energy restriction compared to adequate feeding (103.2 vs. 76.5 fmol/mg protein). GR was much lower in muscle tissues and tended to be slightly higher in MT (with high proportion of red fibres) compared to ML (white fibres) (12.4 vs. 9.1 fmol/mg protein). GR in muscles was not significantly influenced by energy. The tissue specific differences in GR provide the explanation for the known differences in the protein turnover rate.