Mechanisms of T-cell Apoptosis Induced by Glucocorticoids

Dual Mechanisms of Metabolism and Gene Expression of the CCRF-CEM Leukemia Cells under Glucocorticoid Treatment

BACKGROUND

Glucocorticoids play an essential part in anti-leukemic therapies, but resistance is a crucial event for the prognosis of the disease. Glucocorticoids influence the metabolic properties of leukemic cells. The inherent plasticity of clinically evolving cancer cells justifies the characterization of drug-induced early oncogenic pathways, which represent a likely source of detrimental secondary effects.

AIM

The present work aims to investigate the effect of glucocorticoids in metabolic pathways in the CCRF-CEM leukemic cells. Metabolic factors and gene expression profiles were examined in order to unravel the possible mechanisms of the CCRF-CEM leukemic cell growth dynamics.

METHODS

CCRF-CEM cells were used as a model. Cells were treated with prednisolone with concentrations 0-700 μM. Cell culture supernatants were used for glucose, lactic acid, LDH, Na⁺, K⁺ and Ca⁺⁺ measurements. Cytotoxicity was determined with flow cytometry. Microarray analysis was performed using two different chips of 1.2 k and 4.8 k genes. Gene Ontology enrichment analysis was applied to find metabolism- and GC-related genes.

RESULTS

Higher prednisolone concentrations inhibited glucose uptake, without exhibiting any cytotoxic effects. Glucose consumption did not correlate with the total cell population, or the viable population, indicating that growth is not directly proportional to glucose consumption. Neither of the subpopulations, i.e., viable, necrotic, or apoptotic cells, contributed to this.

CONCLUSIONS

Different types of leukemic cells seem to exhibit different patterns of glucose metabolism. Both resistant and sensitive CCRF-CEM cells followed the aerobic pathway of glycolysis. There is probably a rapid change in membrane permeability, causing a general shutdown towards everything that is outside the cell. This could in part also explain the observed resistance. Glucocorticoids do not enter the cell passively anymore and therefore no effects are observed. Based on our observations, ion concentrations are measurable factors both in vitro and in vivo, which makes them possible markers of glucocorticoid cytotoxic action.

TALEN-mediated genetic inactivation of the glucocorticoid receptor in cytomegalovirus-specific T cells

Cytomegalovirus (CMV) infection is responsible for substantial morbidity and mortality after allogeneic hematopoietic stem cell transplant. T-cell immunity is critical for control of CMV infection, and correction of the immune deficiency induced by transplant is now clinically achievable by the adoptive transfer of donor-derived CMV-specific T cells. It is notable, however, that most clinical studies of adoptive T- cell therapy exclude patients with graft-versus-host disease (GVHD) from receiving systemic corticosteroid therapy, which impairs cellular immunity. This group of patients remains the highest clinical risk group for recurrent and problematic infections. Here, we address this unmet clinical need by genetic disruption of the glucocorticoid receptor (GR) gene using electroporation of transcription activator-like effector nuclease (TALEN) messenger RNA. We demonstrate efficient inactivation of the GR gene without off-target activity in Streptamer-selected CMV-specific CD8(+) T cells (HLA-A02/NLV peptide), conferring resistance to glucocorticoids. TALEN-modified CMV-specific T cells retained specific killing of target cells pulsed with the CMV peptide NLV in the presence of dexamethasone (DEX). Inactivation of the GR gene also conferred resistance to DEX in a xenogeneic GVHD model in sublethally irradiated NOD-scid IL2rγ(null) mice. This proof of concept provides the rationale for the development of clinical protocols for producing and administering high-purity genetically engineered virus-specific T cells that are resistant to the suppressive effects of corticosteroids.

Moderate physical training attenuates perinatal low-protein-induced spleen lymphocyte apoptosis in endotoxemic adult offspring rats

PURPOSE

To evaluate the effects of a moderate physical training (T) on the blood and splenic lymphocytes subsets and the rate of apoptosis in adult offspring submitted to perinatal low-protein (LP) diet.

METHODS

Male Wistar rats were divided according to their mother's diet: control (C, 17 % casein) and undernourished (LP, 8 % casein). At the 60th day, pups were submitted to moderate physical training (8 weeks, 5 days week(-1), 60 min day(-1), at 70 % of VO2max). After T period, pups received an injection of lipopolysaccharide (LPS). B, NK, and TCD3+ lymphocytes subsets were analyzed by flow cytometry. Spleen lymphocytes apoptosis was evaluated by DNA fragmentation, phosphatidylserine externalization (PSE), and mitochondrial transmembrane depolarization (MTD) using a flow cytometer. Plasma TNF-α concentrations were analyzed by ELISA.

RESULTS

LP + LPS pups showed a higher percentage of blood B, CD4+, and NK and a reduction in TCD3+, CD8+ than C pups. The percentage of NK and CD3+ was restored in LP + T + LPS pups. In the spleen, T normalized the percentage of NK in LP + LPS pups. LP + LPS pups showed a higher percentage of cells with PSE and MTD than C + LPS pups that was attenuated by T. The concentration of TNF-α was higher in LP + LPS than C + LPS, but it was attenuated in LP + T + LPS pups.

CONCLUSION

Moderate physical training was able to revert the effects of perinatal LP diet on circulation lymphocytes subsets and attenuated splenic lymphocytes apoptosis and plasma TNF-α concentrations.

Thymic involution and corticosterone level in Sandhoff disease model mice: new aspects the pathogenesis of GM2 gangliosidosis

Sandhoff disease (SD) is a lysosomal disease caused by a mutation of the HEXB gene associated with excessive accumulation of GM2 ganglioside (GM2) in lysosomes and neurological manifestations. Production of autoantibodies against the accumulated gangliosides has been reported to be involved in the progressive pathogenesis of GM2 gangliosidosis, although the underlying mechanism has not been fully elucidated. The thymus is the key organ in the acquired immune system including the development of autoantibodies. We showed here that thymic involution and an increase in cell death in the organ occur in SD model mice at a late stage of the pathogenesis. Dramatic increases in the populations of Annexin-V(+) cells and terminal deoxynucletidyl transferase dUTP nick end labeling (TUNEL) (+) cells were observed throughout the thymuses of 15-week old SD mice. Enhanced caspase-3/7 activation, but not that of caspase-1/4, -6 ,-8, or -9, was also demonstrated. Furthermore, the serum level of corticosterone, a potent inducer of apoptosis of thymocytes, was elevated during the same period of apoptosis. Our studies suggested that an increase in endocrine corticosterone may be one of the causes that accelerate the apoptosis of thymocytes leading to thymic involution in GM2 gangliosidosis, and thus can be used as a disease marker for evaluation of the thymic condition and disease progression.

Changes in mouse circulating leukocyte numbers in C57BL/6 mice immunosuppressed with dexamethasone for Cryptosporidium parvum oocyst production

The Iowa strain of Cryptosporidium parvum will not propagate in immunocompetent mice, but will successfully infect genetically immunocompromised nude or SCID mice as well as immunocompetent mice which have been immunosuppressed with glucocorticoids. Using dexamethasone-tetracycline is one published method for immunosuppressing mice for the production of C. parvum oocysts. However, dexamethasone-induced immunosuppression is variable, because it is dependent on the total daily water consumption of each individual mouse. The changes in circulating leukocytes and other immune system associated organs before, during and after dexamethasone suppression were analyzed for comparison with a new single injection methylprednisolone acetate (MPA) suppression model. The dexamethasone-induced immunocompromised state was associated with a greater than 90% sustained drop in circulating T-lymphocytes, a greater than 700% increase in circulating mature segmented neutrophils and a severe depletion of circulating monocytes. The thymus and spleen decreased in size by over 80%. Oocyst shedding in suppressed mice started within 4 days of oocyst inoculation and persisted for 6 days post-dexamethasone treatment. Seven days after dexamethasone withdrawal, circulating neutrophils still were 549% higher than controls. Circulating CD3 and CD4 lymphocytes remained depressed by 85-90% while on dexamethasone and for 7 days after discontinuing dexamethasone. CD8 lymphocyte numbers initially decreased by 90%, but rose even while on dexamethasone and even with severe thymic involution. At day 7 post-dexamethasone treatment, the spleen was 119 mm(3), approximating the same size as controls. Fourteen days post-dexamethasone treatment, which was 8 days after oocyst shedding had ceased, the CD8 counts per 5000 events were only 1.6% below controls, while the CD3 and CD4 counts were still depressed by 66%. The thymus now was about one quarter smaller than the controls. The rise in circulating CD8 lymphocytes, when oocyst production stopped, suggests that CD8 positive lymphocytes may play a significant role in vivo in clearing the parasite. The overall pattern of immunosuppression was nearly identical to that observed with the methylprednisolone acetate immunosuppression model.

Protein kinase A, not Epac, suppresses hedgehog activity and regulates glucocorticoid sensitivity in acute lymphoblastic leukemia cells

Cyclic AMP synergizes strongly with glucocorticoids (GC) to induce apoptosis in normal or malignant lymphoid cells. We examined the individual roles that cAMP-dependent protein kinase (PKA) and Epac (exchange protein directly activated by cAMP), two intracellular cAMP receptors, play in this synergistic effect. Our studies demonstrate that PKA is responsible for the observed synergism with GC, whereas Epac exerts a weak antagonistic effect against GC-induced apoptosis. We find that endogenous PKA activity is higher in the GC-sensitive clone than in the GC-resistant clone. In the GC-sensitive clone, higher PKA activity is associated with lower Hedgehog (Hh) activity. Moreover, inhibition of Hh activity by Hh pathway-specific inhibitors leads to cell cycle arrest and apoptosis in CEM (human acute lymphoblastic leukemia, T lineage) cells, and the GC-sensitive clone is more sensitive to Hh inhibition. These results suggest that Hh activity is critical for leukemia cell growth and survival and that the level of Hh activity is in part responsible for the synergism between cAMP and GC.

Dexamethasone protects RAW264.7 macrophages from growth arrest and apoptosis induced by H2O2 through alteration of gene expression patterns and inhibition of nuclear factor-kappa B (NF-κB) activity

In this study, the effect of dexamethasone, a synthetic glucocorticoid, on H(2)O(2) stimulated murine RAW264.7 macrophages was investigated. It was found that dexamethasone protected the cells from apoptosis induced by H(2)O(2). A cDNA microarray, which consists of 1000 genes selected from a mouse clone set provided from NIA, was used to study the gene expression profiles involved in the protective effect. Our data show that dexamethasone exerts the anti-apoptosis function by changing the expression patterns of many genes involved inhibiting the up-regulation of apoptosis promoting genes and the down-regulation of cell cycle stimulating genes as well as keeping the up-regulation of cell survival related genes. Our study also revealed that dexamethasone protects RAW264.7 macrophages from H(2)O(2) induced apoptosis through blocking nuclear factor-kappa B (NF-kappaB) activity.

Effects of glucocorticoids on polyamine metabolism in liver and spleen of guinea pig during sensitization

Glucocorticoids are potent anti-inflammatory and immunosuppressive agents. As endogenous inhibitors of cytokine synthesis, glucocorticoids suppress immune activation and uncontrolled overproduction of cytokines, preventing tissue injury. Also, polyamine spermine is endogenous inhibitor of cytokine production (inhibiting IL-1, IL-6 and TNF synthesis). The idea of our work was to examine dexamethasone effects on the metabolism of polyamines, spermine, spermidine and putrescine and polyamine oxidase activity in liver and spleen during sensitization of guinea pigs. Sensitization was done by application of bovine serum albumin with addition of complete Freund's adjuvant. Our results indicate that polyamine amounts and polyamine oxidase activity increase during immunogenesis in liver and spleen. Dexamethasone application to sensitized and unsensitized guinea pigs causes depletion of polyamines in liver and spleen. Dexamethasone decreases polyamine oxidase activity in liver and spleen of sensitized guinea pigs, increasing at the same time PAO activity in tissues of unsensitized animals.

Suppression of alveolar macrophage apoptosis prolongs survival of rats and mice with pneumocystis pneumonia

The number of alveolar macrophages is decreased in patients or animals with Pneumocystis pneumonia (Pcp). This loss of alveolar macrophages is in part due to apoptosis caused by Pneumocystis infection. The mechanism of apoptosis induction is unknown. Cell-free bronchoalveolar lavage fluids from Pneumocystis-infected rats or mice have the ability to induce apoptosis in normal alveolar macrophages. To characterize the mechanisms by which apoptosis proceeds in alveolar macrophages during Pcp, specific caspase inhibitors are tested for their ability to suppress the apoptosis. In vitro induction of apoptosis can be inhibited by the caspase-9 inhibitor (Z-LEHD-FMK) but not by the inhibitor to caspase-8 or -10. The caspase-9 inhibitor can also inhibit apoptosis of alveolar macrophages in vivo when it is intranasally instilled into dexamethasone-immunosuppressed, Pneumocystis-infected rats or L3T4 cell-depleted, Pneumocystis-infected mice. The number of alveolar macrophages rebounds in caspase-9 inhibitor-treated Pcp animals. Phagocytic activity of alveolar macrophages in treated animals is also recovered, and organism burden in these animals is reduced. Administration of caspase-9 inhibitor also clears the exudate that normally fills the alveoli during Pcp and decreases lung inflammation. Furthermore, caspase-9-treated Pcp animals survive for the entire 70-day period of the study, whereas nontreated Pcp animals die 40-60 days after initiation of infection. Depletion of recovered alveolar macrophages by intranasal administration of clodronate-containing liposomes in caspase-9 inhibitor-treated animals abrogates the effects of the inhibitor. Together, these results indicate that immunomodulation of the host response may be an alternative to current treatments for Pcp.

Xenobiotic-induced alterations in thymocyte development

The thymus is a very sensitive target for environmental pollutants, which can affect this organ as well as thymocyte differentiation. A failure in thymocyte development can be due to the exacerbation of apoptosis, arrest of thymocyte maturation, generation of autoreactive T cells, and inhibition or stimulation of the output of recent thymic emigrants to the periphery. Recent data demonstrate that the immune system has the potential to maintain homeostasis under conditions of elevated risk, and the thymus plays a crucial role in this process. Environmental xenobiotics can exert their effects through receptor-mediated interactions or independently on receptor involvement. Under natural conditions organisms are exposed to a variety of xenobiotics. The final effect of such exposure is not related to the action of a single chemical, but to the action of a mixture of chemicals. The toxic effect of environmental xenobiotics on the generation and functions of immune cells may result in suppression or stimulation of the immune response. The most intensive studies have been done on halogenated aromatic hydrocarbons, heavy metals and various chemicals acting as endocrine disrupters. Recently, special interest has focused on the action of air particulate matter.

Rescuing developing thymocytes from death by neglect

The major function of the thymus is to eliminate developing thymocytes that are potentially useless or autoreactive, and select only those that bear functional T cell antigen receptors (TCRs) through fastidious screening. It is believed that glucocorticoids (GCs) are at least in part responsible for cell death during death by neglect. In this review, we will mainly cover the topic of the GC-induced apoptosis of developing thymocytes. We will also discuss how thymocytes that are fated to die by GCs can be rescued from GC-induced apoptosis in response to a variety of signals with antagonizing properties for GC receptor (GR) signaling. Currently, a lot of evidence supports the notion that the decision is made as a result of the integration of the multiple signal transduction networks that are triggered by GR, TCR, and Notch. A few candidate molecules at the converging point of these multiple signaling pathyways will be discussed. We will particularly describe the role of the SRG3 protein as a potent modulator of GC-induced apoptosis in the crosstalk.

p38 Mitogen-activated protein kinase (MAPK) is a key mediator in glucocorticoid-induced apoptosis of lymphoid cells: correlation between p38 MAPK activation and site-specific phosphorylation of the human glucocorticoid receptor at serine 211

Glucocorticoids (GCs) induce apoptosis in lymphoid cells through activation of the GC receptor (GR). We have evaluated the role of p38, a MAPK, in lymphoid cell apoptosis upon treatment with the synthetic GCs dexamethasone (Dex) or deacylcortivazol (DAC). The highly conserved phosphoprotein p38 MAPK is activated by specific phosphorylation of its threonine180 and tyrosine182 residues. We show that Dex and DAC stimulate p38 MAPK phosphorylation and increase the mRNA of MAPK kinase 3, a specific immediate upstream activator of p38 MAPK. Enzymatic assays confirmed elevated activity of p38 MAPK. Pharmacological inhibition of p38 MAPK activity was protective against GC-driven apoptosis in human and mouse lymphoid cells. In contrast, inhibition of the MAPKs, ERK and cJun N-terminal kinase, enhanced apoptosis. Activated p38 MAPK phosphorylates specific downstream targets. Because phosphorylation of the GR is affected by MAPKs, we examined its phosphorylation state in our system. We found serine 211 of the human GR to be a substrate for p38 MAPK both in vitro and intracellularly. Mutation of this site to alanine greatly diminished GR-driven gene transcription and apoptosis. Our results clearly demonstrate a role for p38 MAPK signaling in the pathway of GC-induced apoptosis of lymphoid cells.

FoxO3 mediates antagonistic effects of glucocorticoids and interleukin-2 on glucocorticoid-induced leucine zipper expression

We have analyzed the promoter of human gilz (glucocorticoid-induced leucine zipper), a dexamethasone-inducible gene that is involved in regulating apoptosis, and identified six glucocorticoid (GC)-responsive elements and three Forkhead responsive elements (FHREs). Promoter deletion analysis and point mutations showed that individual mutation of the GC-responsive elements does not affect GC-induced transcription and that FHRE-1 and FHRE-3 elements contribute to the effects of GCs. Furthermore, overexpression of the Forkhead transcription factor FoxO3 enhances GC-induced gilz mRNA expression. The functional significance of the interaction between FoxO3 and GC receptor was established in T lymphocytes. Indeed, we show that GCs failed to induce GILZ expression in the presence of IL-2, a cytokine known to antagonize GC effects in T cells. Using a constitutive active mutant of protein kinase B that inactivates FoxO3 or a FoxO3 mutant that cannot be inactivated by protein kinase B, we demonstrate that IL-2 inhibitory effects on GILZ expression are mediated through inhibition of FoxO3 transcriptional activity. Therefore, FoxO3 appears to be a key factor mediating GC and IL-2 antagonism for gilz regulation in T lymphocytes. This regulation of GILZ expression was placed in a meaningful context in evaluating the effects of GILZ on GC-induced apoptosis in T lymphocytes.

T Cell Receptor Signaling Inhibits Glucocorticoid-induced Apoptosis by Repressing the SRG3 Expression via Ras Activation

Activation of T cell antigen receptor (TCR) signaling inhibits glucocorticoid (GC)-induced apoptosis of T cells. However, the detailed mechanism regarding how activated T cells are protected from GC-induced apoptosis is unclear. Previously, we have shown that the expression level of SRG3, a murine homolog of BAF155 in humans, correlated well with the GC sensitivity of T cells either in vitro or in vivo. Intriguingly, the expression of SRG3 decreased upon positive selection in the thymus. Here we have shown that TCR signaling inhibits the SRG3 expression via Ras activation and thereby renders primary thymocytes and some thymoma cells resistant to GC-mediated apoptosis. By using pharmacological inhibitors, we have shown that Ras-mediated down-regulation of the SRG3 gene expression is mediated by MEK/ERK and phosphatidylinositol 3-kinase pathways. Moreover, TCR signals repressed the SRG3 transcription through the putative binding sites for E proteins and Ets family transcription factors in the proximal region of the SRG3 promoter. Introduction of mutations in these elements rendered the SRG3 promoter immune to the Ras or TCR signals. Taken together, these observations suggest that TCR signals result in GC desensitization in immature T cells by repressing SRG3 gene expression via Ras activation.

Carbohydrate supplementation and exercise-induced changes in T-lymphocyte function

Carbohydrate (CHO) ingestion during exercise has been shown to reduce perturbations in immune cell numbers and function, possibly through a reduction in the cortisol response to exercise. We have previously observed that exercise decreases T-lymphocyte responses to mitogen via an increase in cell death of both CD4 and CD8 T lymphocytes (Green KJ and Rowbottom DG. J Appl Physiol. 95: 57-63, 2003). This study tested the hypothesis that CHO ingestion rather than placebo (Pl) would result in an attenuation of the cortisol response to exercise and a reduction of the exercise-associated alterations in cell death. Six well-trained cyclists completed two exercise trials consisting of 2.5 h of cycling at 85% of individual ventilatory threshold. In a random order, trials were completed under either CHO (6% CHO solution, 3.2 g CHO/kg body wt total) or Pl conditions. Blood samples were collected before exercise, midexercise (after 60 min of exercise), immediately after exercise, and after 60 min of recovery. T-lymphocyte responses to mitogen were determined by using carboxyfluorescein diacetate succinimidyl ester fluorescent cell division tracking and expansion rates, and cell death rates were calculated for each sample as well as mitosis rates for each cell generation. Cellular expansion of T lymphocytes was decreased after exercise in Pl only. The reduction in cellular expansion was related to an increase in cell death of both CD4 and CD8 cells in culture rather than a decrease in the ability of cells to undergo mitosis. CHO ingestion compared with Pl was associated with no reductions in cellular expansion or increases in cell death. CHO ingestion during exercise acted to reduce the impairment of T-lymphocyte function by decreasing cell death within mitogen-stimulated cell cultures; however, the mechanism of action appears to be independent of cortisol.

The ubiquitin-proteasome pathway in thymocyte apoptosis: caspase-dependent processing of the deubiquitinating enzyme USP7 (HAUSP)

Programmed cell death (apoptosis) is crucial for thymocyte development. We analyzed the role of the ubiquitin (Ub)-proteasome pathway in dexamethasone-triggered and TCR-mediated apoptosis in fetal thymic organ culture (FTOC). Proteasome activity was increased in apoptotic thymocytes, as visualized by active-site labeling of proteasomal beta subunits. The activity of deubiquitinating enzymes in murine apoptotic thymocytes was likewise examined by active-site labeling. We show that the deubiquitinating enzyme USP7 (HAUSP) is proteolytically processed upon dexamethasone-, gamma-irradiation-, and antigen-induced cell death. Such processing of HAUSP does not occur in caspase 3-/- thymocytes, or upon pretreatment of wild type thymocytes with the general caspase inhibitor ZVAD-fmk. Thus, our results suggest that thymocyte apoptosis leads to modification of deubiquitinating enzymes by caspase activity and may provide an additional link between the ubiquitin-proteasome pathway and the caspase cascade during programmed cell death.

Reversible disruption of thymic function by steroid treatment

The effect of steroid treatment on the thymic output of T cells was examined in an avian model. Recent thymic emigrants in chickens transiently express the chicken T cell Ag 1 thymocyte marker, and thymic function can be monitored indirectly by measuring the levels of TCR gene rearrangement excision circles in peripheral T cells. Both parameters were used to show that intensive steroid treatment induces thymic involution and a profound reduction in the supply of naive T cells to the periphery. Conversely, resident T cells in the peripheral lymphocyte pool were relatively spared. Thymopoiesis immediately recovered following cessation of steroid treatment, concurrent with restoration of the thymic output of newly formed T cells. Repopulation of the peripheral T cell pool recapitulated the ontogenetic pattern of gamma delta T cell replenishment before alpha beta T cell reseeding, thereby indicating the complete recovery of thymic function after a course of steroid treatment.

Enhancement of p53 activity and inhibition of neural cell proliferation by glucocorticoid receptor activation

In analyzing the molecular mechanisms underlying glucocorticoid-induced apoptosis in neural cells, we observed that dexamethasone, by activating glucocorticoid receptors, causes arrest of HT-22 cells in the G1 phase of the cell cycle; upon withdrawal of the agonist, cells resume proliferation. Our investigations revealed that glucocorticoid treatment, although having no effects on endogenous p53 protein stability, induces rapid translocation of p53 to the nucleus and enhances its transcriptional activity. Consistently, transfection studies with p53-responsive promoters revealed a substantial stimulation of the trans-activation potential of exogenous p53 by dexamethasone. Cells arrested in G1 failed to show signs of apoptosis even after overexpression of p53. Although dexamethasone induced transcription of the proapoptotic gene bax, there was no increase of Bax protein levels. We conclude that glucocorticoid receptor-induced neural cell cycle arrest is associated with an increase in nuclear translocation and transcriptional activity of p53, and suggest that potentiation of p53 may serve as a brake on cell proliferation and may prime cells for differentiation or death induced by other signals.

Effects of altered glucocorticoid sensitivity in the T cell lineage on thymocyte and T cell homeostasis

The homeostatic regulation that controls total thymocyte and peripheral T-cell numbers is not clearly understood. We describe here a direct hormonal influence of endogenous levels of glucocorticoids (GCs) on thymocyte and peripheral T-cell homeostasis independent of indirect systemic effects of GCs. The results were obtained by generating transgenic mice with an altered GC sensitivity targeted to thymocytes and peripheral T cells by increasing or decreasing glucocorticoid receptor (GR) expression specifically in thymocytes and peripheral T cells. A twofold increase in GC sensitivity resulted in a major decrease in thymocyte number, affecting all subpopulations, although single-positive CD8+ cells were less influenced. In the thymus, this was due to increased apoptosis in the organ, whereas proliferation of thymocyte populations was unaffected. In the periphery, a pronounced reduction in T-cell number was seen, demonstrating an effect of endogenous GCs also on T-cell homeostasis. The effects were confirmed in transgenic mice with reduced GR expression, which showed increased thymocyte and T-cell numbers. Thus, our data demonstrate that physiological GC levels are directly involved in controlling the size of both thymocyte and T-cell pools.

Thymic epithelial cells mediate a Bcl-2-independent protection of single-positive thymocytes from dexamethasone-induced apoptosis

Intrathymic maturation of thymocytes is essential for the proper formation of T-cell repertoire. This process involves two major biochemical pathways, one initiated by the recognition of MHC/peptide by the T-cell receptor and the other mediated by glucocorticoids. These hormones seem to affect thymocyte maturation by increasing the threshold of TCR-mediated positive and negative selection, and by inducing apoptosis of nonselected thymocytes. We have previously reported that an SV40-immortalized murine thymic epithelial cell line, namely 2BH4, was able to protect thymocytes from dexamethasone-induced apoptosis. Here we show that this protection is independent of cell-to-cell contact and does not seem to involve a Bcl-2-mediated resistance, since incubation of thymocytes with 2BH4 cells or its supernatant does not interfere with the levels of this antiapoptotic molecule. The protection conferred by 2BH4 cells, or by a primary culture of thymic stromal cells, is specific for the CD4(+)CD8(-) and CD4(-)CD8(+) single-positive thymocytes, whereas the broad-spectrum caspase inhibitor z-VAD-fmk blocks apoptosis induced by dexamethasone in all thymocyte subpopulations. Our results suggest that positively selected single-positive thymocytes are still susceptible to glucocorticoid-induced apoptosis but are protected from it through the action of a heat-stable protein(s) released by thymic stromal cells.

Glucocorticoids and polyamine inhibitors synergize to kill human leukemic CEM cells

Glucocorticoids are well-known apoptotic agents in certain classes of lymphoid cell malignancies. Reduction of intracellular polyamine levels by use of inhibitors that block polyamine synthesis slows or inhibits growth of many cells in vitro. Several such inhibitors have shown efficacy in clinical trials, though the toxicity of some compounds has limited their usefulness. We have tested the effects of combinations of the glucocorticoid dexamethasone (Dex) and two polyamine inhibitors, difluoromethylornithine (DFMO) and methyl glyoxal bis guanylhydrazone (MGBG), on the clonal line of human acute lymphoblastic leukemia cells, CEM-C7-14. Dex alone kills these cells, though only after a delay of at least 24 hours. We also evaluated a partially glucocorticoid-resistant c-Myc-expressing CEM-C7-14 clone. We show that Dex downregulates ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine synthesis. Pretreatment with the ODC inhibitor DFMO, followed by addition of Dex, enhances steroid-evoked kill slightly. The combination of pretreatment with sublethal concentrations of both DFMO and the inhibitor of S-adenosylmethionine decarboxylase, MGBG, followed by addition of Dex, results in strong synergistic cell kill. Both the rapidity and extent of cell kill are enhanced compared to the effects of Dex alone. These results suggest that use of such combinations in vivo may result in apoptosis of malignant cells with lower overall toxicity.

Nuclear receptors in cell life and death

The balance between cell proliferation and programmed cell death (apoptosis) determines body patterns during animal development and controls compartment sizes, tissue architecture and remodeling. The removal of primordial structures by apoptosis allows the organism to develop sex specifically and to adapt for novel functions at later stages; apoptosis also limits the size of evolving structures. It is a ubiquitous function that is essential for all cells. Although inappropriate regulation or execution of apoptosis leads to disease, such as cancer, there is now evidence for its great therapeutic potential. This would be particularly true if apoptosis could be targeted at defined cell compartments, rather than acting ubiquitously like chemotherapy. Here, we discuss the potential of nuclear receptor ligands, many of which act through their cognate receptors in defined body compartments as modulators of cell life and death, with special emphasis on the molecular pathways by which these receptors affect cell-cycle progression, survival and apoptosis.

c-Rel is required for the protection of B cells from antigen receptor-mediated, but not Fas-mediated, apoptosis

The NF-kappaB/Rel transcription factor family has been shown to protect many cell types from apoptotic signals. However, it is not known whether NF-kappaB is required for all survival pathways and whether each NF-kappaB member plays a unique or a redundant role. Here we describe the results of studies on the role of c-Rel in survival. Mature B cells from c-Rel(-/-) mice exhibit defects in survival, including sensitivity to Ag receptor-mediated apoptosis as well as increased sensitivity to ionizing radiation and glucocorticoids. Transgene expression of Bcl-x(L), a c-Rel target gene, rescues c-Rel(-/-) B cells from their survival defects. Thus, c-Rel-dependent survival pathways are crucial for protection from apoptotic signals that target the mitochondrial pathway. Despite a lack of Bcl-x(L), c-Rel(-/-) B cells can still be rescued from Fas-mediated apoptosis via B cell receptor signaling. The Fas apoptosis inhibitor molecule and FLICE inhibitory protein (c-FLIP) proteins are up-regulated normally in c-Rel(-/-) B cells, and these two molecules may play a more physiological role in the Fas pathway. Furthermore, unlike the TNF sensitivity of RelA(-/-) fibroblasts, c-Rel-deficient fibroblasts are refractory to TNF-mediated cell death. Thus, c-Rel is dispensable for protection against death receptor-mediated apoptosis. Taken together, our data suggest that distinct NF-kappaB/Rel members are required for protecting cells from different types of apoptotic signals.

Flow cytometric analysis of spontaneous and dexamethasone-induced apoptosis in thymocytes from severely malnourished rats

Severe malnutrition is widely distributed throughout the world, showing a high prevalence in developing countries. Experimental animal models have been useful to study the effects of malnutrition at different levels and ages. Apoptosis is a well recognised process of cell death occurring under several physiological and pathological conditions. It represents the principal mechanism involved in cell selection in the thymus. Thymocyte apoptosis induction by dexamethasone is one of the best characterised experimental models of programmed cell death. The aim of the present study was to determine whether severe malnutrition increased spontaneous and/or dexamethasone-induced apoptosis in vivo in thymocytes of experimentally malnourished rats during lactation. Thymocytes were obtained from malnourished rats at weaning (21d of age). Apoptosis frequency was estimated by the terminal transferase-mediated dUTP nick end labelling assay. Spontaneous apoptosis was 1.9 (sd 1.0) % in well nourished rats in contrast to 13.3 (sd 3.8) % in malnourished animals; this is seven times greater (P<0.001). Interestingly, the frequency of dexamethasone-induced apoptosis was similar in both groups of animals (47.9 (sd 10.1) % in well nourished rats and 53.8 (sd 8.0) % in malnourished rats). The results obtained in the present study indicate that malnutrition is associated with a significant increase of spontaneously apoptotic cells. In addition, the data showed that the fraction of thymocytes susceptible to dexamethasone-induced apoptosis was similar in well nourished and malnourished animals. The greater levels of spontaneously apoptotic cells associated with malnutrition could be related to alterations of the microenvironment of the thymus and/or to an obstruction of early thymocyte maturation.

Constitutive expression of ectopic c-Myc delays glucocorticoid-evoked apoptosis of human leukemic CEM-C7 cells

Sensitivity to glucocorticoid (GC)-evoked apoptosis in lymphoid cell lines correlates closely with GC-mediated suppression of c-Myc expression. To establish a functional role for c-Myc in GC-mediated apoptosis, we have stably expressed MycER(TM), the human c-Myc protein fused to the modified ligand-binding domain of the murine estrogen receptor alpha, in GC-sensitive CEM-C7-14 cells. In CEM-C7-14 cells, MycER(TM) constitutively imparts c-Myc functions. Cells expressing MycER(TM) (C7-MycER(TM)) exhibited a marked reduction in cell death after 72 h in 100 nM dexamethasone (Dex), with 10-20-fold more viable cells when compared to the parental CEM-C7-14 clone. General GC responsiveness was not compromised, as evidenced by Dex-mediated suppression of endogenous c-Myc and cyclin D3, and induction of c-Jun and the glucocorticoid receptor. MycER(TM) also blunted Dex-mediated upregulation of p27(kipI) and suppression of the Myc target p53. In comparison to parental CEM-C7-14 cells, Dex-evoked DNA strand breaks were negligible and caspase activation was delayed, but the extent of G1 cell cycle arrest was similar in C7-MycER(TM) cells. Myc-ER(TM) did not result in permanent, complete resistance to GC however, and the GC-treated cells eventually died, indicative of redundant or interactive mechanisms in the GC-evoked lytic response of lymphoid cells. Our results emphasize the importance of c-Myc suppression in GC-evoked apoptosis of CEM-C7-14 cells.

Expression profiling of glucocorticoid-treated T-ALL cell lines: rapid repression of multiple genes involved in RNA-, protein- and nucleotide synthesis

To arrive at a better understanding of the effects of the glucocorticoid component of chemotherapy protocols on lymphocytic leukemia cells, we analysed early responses of T-lymphocytic leukemia cell lines Jurkat and CEM-C7, both of which undergo apoptosis in response to dexamethasone, via gene chips. Among genes identified as repressed, a notable cluster seemed to be of importance for the processes of transcription, mRNA splicing and protein synthesis. Consequently, we assessed time-resolved uptake of uridine and methionine to monitor RNA and protein synthesis, along with parameters quantifying apoptosis. Repression of uptake to about 65% of that in untreated cells preceded the first sign of apoptosis by several hours in both cell lines. In addition to this general repression of RNA and protein synthesis, several genes were found to be regulated that may contribute to synergistic action of glucocorticoids with other components of frequently used chemotherapy protocols such as antimetabolites, methotrexate and alkylating agents.

Cloning and sequencing of caspase 6 in rainbow trout, Oncorhynchus mykiss, and analysis of its expression under conditions known to induce apoptosis

The rainbow trout caspase 6 gene has been cloned and sequenced. The open reading frame consisted of 906bp, which translated into a protein of 302 amino acids, containing the caspase active site pentapeptide (QACRG) and the caspase family signature (HADADCFVCVFLSHG). Amino acids involved in catalysis and those known to form the P1 carbohydrate binding pocket were conserved. Phylogenetic tree analysis showed a tight grouping with other known caspase 6 genes. Conserved aspartic acid residues at positions 33, 191 and 202 suggested that this molecule is produced as a proenzyme that is subsequently cleaved to release active subunits, with the region between Asp-191 and Ala-203 acting as a linker that is cleaved out. RT-PCR analysis revealed that the trout caspase 6 gene was expressed in brain, blood, gill, liver, head kidney and spleen. Addition of LPS or cortisol to head kidney leucocyte cultures had no effect upon caspase 6 expression. However, addition of LPS after preincubation with cortisol increased expression relative to control cultures. Incubation with RU486 abrogated this effect, confirming it was mediated via glucocorticoid receptors. Lastly, a confinement stress in vivo increased caspase 6 expression. The data are discussed with respect to the immunoregulatory role of apoptosis in fish immune responses.

Nongenomic membrane actions of glucocorticoids in vertebrates

For decades, it was widely assumed that glucocorticoids (GCs) work solely through changes in gene expression to exert their physiological actions, a process that normally takes several hours to occur. However, recent evidence indicates that GCs might also act at the membrane through specific receptors to exert multiple rapid effects on various tissues and cells. GCs modulate hormone secretion, neuronal excitability, behavior, cell morphology, carbohydrate metabolism and other processes within seconds or minutes. These early actions occur independent of the genome and are transduced by the same biochemical effector pathways responsible for mediating rapid responses to neurotransmitters. The biological significance of most rapid GC effects are not well understood, but many might be related to the important functions that this hormone plays in modulating stress responses.

Hormonal regulation of physiological cell turnover and apoptosis

Physiological cell turnover plays an important role in maintaining normal tissue function and architecture. This is achieved by the dynamic balance of cellular regeneration and elimination, occurring periodically in tissues such as the uterus and mammary gland, or at constant rates in tissues such as the gastrointestinal tract and adipose tissue. Apoptosis has been identified as the prevalent mode of physiological cell loss in most tissues. Cell turnover is precisely regulated by the interplay of various endocrine and paracrine factors, which modulate tissue and cell-specific responses on proliferation and apoptosis, either directly, or by altering expression and function of key cell proliferative and/or death genes. Although recent studies have provided significant information on specific tissue systems, a clearly defined pathway that mediates cell turnover has not yet emerged for any tissue. Several similarities exist among the various tissues with regard to the intermediates that regulate tissue homeostatis, enabling a better understanding of the general mechanisms involved in the process. Here we review the mechanisms by which hormonal and cytokine factors mediate cell turnover in various tissues, emphasizing common themes and tissue-specific differences.