DNA fragmentation in S49 lymphoma cells killed with glucocorticoids and other agents

Targeting Bcl-2-IP3 receptor interaction to treat cancer: A novel approach inspired by nearly a century treating cancer with adrenal corticosteroid hormones

Bcl-2 inhibits cell death by at least two different mechanisms. On the one hand, its BH3 domain binds to pro-apoptotic proteins such as Bim and prevents apoptosis induction. On the other hand, the BH4 domain of Bcl-2 binds to the inositol 1,4,5-trisphosphate receptor (IP₃R), preventing Ca²⁺ signals that mediate cell death. In normal T-cells, Bcl-2 levels increase during the immune response, protecting against cell death, and then decline as apoptosis ensues and the immune response dissipates. But in many cancers Bcl-2 is aberrantly expressed and exploited to prevent cell death by inhibiting IP₃R-mediated Ca²⁺ elevation. This review summarizes what is known about the mechanism of Bcl-2's control over IP₃R-mediated Ca²⁺ release and cell death induction. Early insights into the role of Ca²⁺ elevation in corticosteroid-mediated cell death serves as a model for how targeting IP₃R-mediated Ca²⁺ elevation can be a highly effective therapeutic approach for different types of cancer. Moreover, the successful development of ABT-199 (Venetoclax), a small molecule targeting the BH3 domain of Bcl-2 but without effects on Ca²⁺, serves as proof of principle that targeting Bcl-2 can be an effective therapeutic approach. BIRD-2, a synthetic peptide that inhibits Bcl-2-IP₃R interaction, induces cell death induction in ABT-199 (Venetoclax)-resistant cancer models, attesting to the value of developing therapeutic agents that selectively target Bcl-2-IP₃R interaction, inducing Ca²⁺-mediated cell death.

A Role for Calcium in Regulating Apoptosis in Rat Thymocytes Irradiatedin Vitro

Thymus-derived lymphocytes undergo death after gamma-irradiation via a pathway termed apoptosis, or programmed cell death. An early step in this pathway is the production of nucleosome-sized fragments of DNA. DNA fragmentation was used as the endpoint in these investigations to examine apoptosis in lymphocytes extracted from the rat thymus and irradiated in vitro. In unirradiated thymocytes the level of DNA fragmentation rose to 15% by the first hour of culture, where it remained approximately constant until the fifth hour. In contrast, thymocytes irradiated with a dose of 2.5 Gy exhibited a large and dramatic increase in DNA fragmentation beginning 2 h postirradiation. DNA fragmentation measured 6 h after irradiation was detected after as little as 0.25 Gy and reached a maximum of 90% with 10 Gy. Metabolic control of DNA fragmentation after irradiation was evidenced by the suppression of DNA fragmentation when thymocytes were incubated with cyclohexamide or actinomycin D. When gamma-irradiated thymocytes were incubated with the Ca2+ chelator EGTA, DNA fragmentation was reduced significantly. BAPTA-AM, a highly specific intracellular Ca2+ chelator, essentially eliminated DNA fragmentation in cells irradiated with 2.5 Gy and, unlike EGTA, eliminated the background level of fragmentation in unirradiated samples. Therefore, our data are consistent with the possibility that Ca2+ serves as a second messenger to induce DNA fragmentation in irradiated thymocytes, suggesting a common pathway for cells prompted to enter apoptosis from seemingly dissimilar interval events.

Cell death by bortezomib-induced mitotic catastrophe in natural killer lymphoma cells

The proteasome inhibitor bortezomib (PS-341/Velcade) is used for the treatment of relapsed and refractory multiple myeloma and mantle-cell lymphoma. We recently reported its therapeutic potential against natural killer (NK)-cell neoplasms. Here, we investigated the molecular mechanisms of bortezomib-induced cell death in NK lymphoma cells. NK lymphoma cell lines (SNK-6 and NK-YS) and primary cultures of NK lymphomas treated with bortezomib were examined for alterations in cell viability, apoptosis, cellular senescence, and cell cycle status. Bortezomib primarily induced mitochondrial apoptosis in NK-YS cells and in primary lymphoma cells at the same concentration as reported in myeloma cells. Unexpectedly, SNK-6 cells required a significantly higher median inhibitory concentration of bortezomib (23 nmol/L) than NK-YS and primary lymphoma cells (6-13 nmol/L). Apoptosis was limited in SNK-6 cells due to the extensively delayed turnover of Bcl-2 family members. These cells were killed by bortezomib, albeit at higher pharmacologic concentrations, via mitotic catastrophe-a mitotic cell death associated with M-phase arrest, cyclin B1 accumulation, and increased CDC2/CDK1 activity. Our results suggest that, in addition to cell death by apoptosis at lower bortezomib concentrations, NK lymphoma cells resistant to bortezomib-induced apoptosis can be killed via mitotic catastrophe, an alternative cell death mechanism, at higher pharmacologic concentrations of bortezomib. Hence, activating mitotic catastrophe by bortezomib may provide a novel therapeutic approach for treating apoptosis-resistant NK-cell malignancies and other cancers.

Flow cytometric discrimination between viable neutrophils, apoptotic neutrophils and eosinophils by double labelling of permeabilized blood granulocytes

Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) is frequently used to detect apoptotic cells in tissues, cytospins and suspensions. Here we show that TUNEL staining of freshly isolated granulocytes results in non-specific positivity of a distinct population, which can be observed in the presence or absence of TdT. The morphological features of the false-positive cells examined in fluorescence microscopy suggest that the non-specifically stained cells are eosinophilic granulocytes. Granules of eosinophilic granulocytes were brightly stained by non-specific TUNEL reaction independent of TdT. This staining does not, therefore, indicate apoptosis and most likely reflects 'stickiness' of the permeabilized eosinophils. Immunofluorescence with phycoerythrin (PE)-labelled CD16 antibodies performed simultaneously with conventional TUNEL staining confirmed that the false-positive cells in TUNEL staining were CD16-negative eosinophils. In this report we describe a new procedure that allows: (i) the differentiation of neutrophilic and eosinophilic granulocytes in forward scatter versus log side scatter histograms after permeabilisation; (ii) the reliable discrimination between viable neutrophils, apoptotic neutrophils and eosinophilic granulocytes in cytofluorimetry.

Dexamethasone enhances serum deprivation-induced necrotic death of rat C6 glioma cells through activation of glucocorticoid receptors

Glucocorticoids have been shown to be neurotoxic and appear to play a role in neuronal cell loss during aging and following neuropathological insults. However, very little is known about the effects of these steroid hormones on glial cells. The effect of the synthetic glucocorticoid dexamethasone (DEX) on glial cell viability was therefore examined by measuring neutral red uptake into rat C6 glioma cells. Serum deprivation markedly reduced cell viability, and this effect was significantly enhanced by DEX. Electrophoretic analysis showed that the cell damage induced by either serum deprivation alone or in combination with DEX was not accompanied by the degradation of DNA into nucleosomic fragments. Electron microscopic studies confirmed that serum deprivation and glucocorticoid treatment caused necrotic cell death. Furthermore, the effect of DEX on cell viability could be mimicked by the glucocorticoid receptor agonist RU28362, and completely prevented by the glucocorticoid receptor antagonist RU38486. These results indicate that dexamethasone can enhance the necrotic death of glioma cells induced by serum deprivation, suggesting that glucocorticoids may be involved in the chronic alteration of brain function arising from neuropathological damage to glial cells.

Increased Activity of Oleate-Dependent Type Phospholipase D During Actinomycin D-Induced Apoptosis in Jurkat T Cells

Apoptosis is an active form of cell death that can be induced by a wide variety of agents and conditions. In response to actinomycin D, hydrogen peroxide (H2O2), or TNF-α, Jurkat T cells underwent typical apoptosis. Phospholipase D (PLD) activity in intact cells determined by phosphatidylbutanol generation was up-regulated by these agents. The PLD activation was in a time-dependent manner during apoptosis. It was also shown that the PLD activity measured by using exogenous substrate in the lysate from apoptotic cells was higher than that in the lysate from control untreated cells. The PLD activity in lysate from control untreated cells was stimulated by unsaturated fatty acids (UFA), but not by guanosine 5′-O-(3-thiotriphosphate). However, the PLD activity in the apoptotic cell lysate was no longer enhanced by the addition of oleate, suggesting that the increased PLD activity during apoptosis was attributed to the PLD of UFA-dependent type, but not the small G protein-dependent one. In fact, the release of free UFA was increased during apoptosis. The caspase inhibitors, z-DEVD and z-VAD, effectively suppressed PLD activation and apoptosis, but UFA release was unaffected. These results suggest the possibility that UFA-dependent type PLD may be implicated in apoptotic process in Jurkat T cells. This is the first demonstration that the PLD of UFA-dependent type would be involved in cellular responses.

Cytotoxicity and apoptosis produced by arachidonic acid in Hep G2 cells overexpressing human cytochrome P4502E1

The goal of the current study was to evaluate the effects of arachidonic acid, as a representative polyunsaturated fatty acid, on the viability of a Hep G2 cell line, which has been transduced to express human cytochrome P4502E1 (CYP2E1). Arachidonic acid produced a concentration- and time-dependent toxicity to Hep G2-MV2E1-9 cells, which express CYP2E1, but little or no toxicity was found with control Hep G2-MV-5 cells, which were infected with retrovirus lacking human CYP2E1 cDNA. In contrast to arachidonic acid, oleic acid was not toxic to the Hep G2-MV2E1-9 cells. The cytotoxicity of arachidonic acid appeared to involve a lipid peroxidation type of mechanism since toxicity was enhanced after depletion of cellular glutathione; formation of malondialdehyde and 4-hydroxy-2-nonenal was markedly elevated in the cells expressing CYP2E1, and toxicity was prevented by antioxidants such as alpha-tocopherol phosphate, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox), propylgallate, ascorbate, and diphenylphenylenediamine, and the iron chelator desferrioxamine. Transfection of the Hep G2-MV2E1-9 cells with plasmid containing CYP2E1 in the sense orientation enhanced the arachidonic acid toxicity, whereas transfection with plasmid containing CYP2E1 in the antisense orientation decreased toxicity. The CYP2E1-dependent arachidonic acid toxicity appeared to involve apoptosis, as demonstrated by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling and DNA laddering experiments. Trolox, which prevented toxicity of arachidonic acid, also prevented the apoptosis. Transfection with a plasmid containing bcl-2 resulted in complete protection against the CYP2E1-dependent arachidonic acid toxicity. It is proposed that elevated production of reactive oxygen intermediates by cells expressing CYP2E1 can cause lipid peroxidation, which subsequently promotes apoptosis and cell toxicity when the cells are enriched with polyunsaturated fatty acids such as arachidonic acid. The Hep G2-MV2E1-9 cells appear to be a valuable model to study interaction between CYP2E1, polyunsaturated fatty acids, reactive radicals, and the consequence of these interactions on cell viability and to reproduce several of the key features associated with ethanol hepatotoxicity in the intragastric infusion model of ethanol treatment.

Variation of heat shock protein 70 through the cell cycle in HL-60 cells and its relationship to apoptosis

Cells respond to a heat shock by synthesizing heat shock proteins, which help to protect the cells from further heating. Recent results indicate that heat shock protein 70 (hsp70) may help to protect cells from apoptosis. We have used flow cytometry to investigate the relationship between constitutive and inducible hsp70 and apoptosis through the cell cycle in HL-60 cells. Specific antibodies were used to measure hsp70 in cells costained with propidium iodide. In separate samples apoptosis was measured using the TdT assay. The apoptotic cells have a subdiploid DNA content, which allows them to be identified also in the bivariate histograms of heat shock protein vs DNA content. After HL-60 cells were heated at 45.0 degrees C for 7.5 min and incubated for various times at 37 degrees C, many of them underwent apoptosis. The level of inducible hsp70 (hsp72) was lower in the apoptotic cells than in the nonapoptotic population, but constitutive hsp70 (hsp73) was the same in both populations. Pretreatment with sodium vanadate increased the fraction of apoptotic cells twofold, slightly increased the level of hsp72 in the nonapoptotic cells, but did not affect hsp73. These results suggest that hsp72, but not hsp73, is involved in the development or prevention of apoptosis.

Lymphocyte apoptosis: mediation by increased type 3 inositol 1,4,5-trisphosphate receptor

B and T lymphocytes undergoing apoptosis in response to anti-immunoglobulin M antibodies and dexamethasone, respectively, were found to have increased amounts of messenger RNA for the inositol 1,4,5-trisphosphate receptor (IP3R) and increased amounts of IP3R protein. Immunohistochemical analysis revealed that the augmented receptor population was localized to the plasma membrane. Type 3 IP3R (IP3R3) was selectively increased during apoptosis, with no enhancement of type 1 IP3R (IP3R1). Expression of IP3R3 antisense constructs in S49 T cells blocked dexamethasone-induced apoptosis, whereas IP3R3 sense, IP3R1 sense, or IP3R1 antisense control constructs did not block cell death. Thus, the increases in IP3R3 may be causally related to apoptosis.

Characteristics of cancer cell death after exposure to cytotoxic drugs in vitro

The characteristics of cell death were investigated after exposure of CCRF-CEM.f2 cells to five drugs over a broad concentration range; these were the glucocorticoid dexamethasone (DXM), the mitotic inhibitor vincristine (VIN) and three antimetabolites, methotrexate (MTX), 5'-fluoro-2'-deoxyuridine (FUdR) and 5'-fluorouracil (5-FU). Drug-treated cells were monitored for cell death mechanisms at different times by examining the pattern of DNA degradation, cell morphology and flow cytometric profile, together with effects on cell growth over 72 h. At growth-inhibitory drug concentrations, the first changes were cell cycle perturbations detectable after 4-6 h of drug exposure. The appearance of features characteristic of apoptotic cell death was noted after all drug treatments in the CCRF-CEM.f2 cell line, but the pattern and kinetics varied considerably. VIN induced apoptotic changes by 12 h, while DXM treatment caused apoptosis only after 48 h. Both MTX and FUdR induced morphological changes characteristic of apoptosis at least 24 h before internucleosomal DNA cleavage, which was detectable only after 48 h. In contrast, 5-FU did not cause internucleosomal DNA cleavage by 48 h at any concentration, despite the presence of morphologically apoptotic cells 24 h earlier. These data suggest that disruption of the cell cycle caused by drug treatment may be the common trigger initiating the drug-specific apoptotic sequence of dying cells.

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

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

Evidence that glucocorticoid- and cyclic AMP-induced apoptotic pathways in lymphocytes share distal events

WEHI7.2 murine lymphocytes undergo apoptotic death when exposed to glucocorticoids or elevated levels of intracellular cyclic AMP (cAMP), and these pathways are initiated by the glucocorticoid receptor (GR) and protein kinase A, respectively. We report the isolation and characterization of a novel WEHI7.2 variant cell line, WR256, which was selected in a single step for growth in the presence of dexamethasone and arose at a frequency of approximately 10(-10). The defect was not GR-related, as WR256 expressed functional GR and underwent GR-dependent events associated with apoptosis, such as hormone-dependent gene transcription and inhibition of cell proliferation. Moreover, the glucocorticoid-resistant phenotype was stable in culture and did not revert after treatment with 5-azacytidine or upon stable expression of GR cDNA. In addition, WR256 did not exhibit the diminished mitochondrial activity commonly associated with apoptosis. Interestingly, WR256 was also found to be resistant to 8-bromo-cAMP and forskolin despite having normal levels of protein kinase A activity and the ability to induce cAMP-dependent transcription. We examined the steady-state transcript levels of bcl-2, a gene whose protein product acts dominantly to inhibit thymocyte apoptosis, to determine whether elevated bcl-2 expression could account for the resistant phenotype. Our data showed that bcl-2 RNA levels were similar in the two cell lines and not altered by either dexamethasone or 8-bromo-cAMP treatment. These results suggest that WR256 exhibits a "deathless" phenotype and has a unique defect in a step of the apoptotic cascade that may be common to the glucocorticoid- and cAMP-mediated cell death pathways.

Evidence that glucocorticoid- and cyclic AMP-induced apoptotic pathways in lymphocytes share distal events

WEHI7.2 murine lymphocytes undergo apoptotic death when exposed to glucocorticoids or elevated levels of intracellular cyclic AMP (cAMP), and these pathways are initiated by the glucocorticoid receptor (GR) and protein kinase A, respectively. We report the isolation and characterization of a novel WEHI7.2 variant cell line, WR256, which was selected in a single step for growth in the presence of dexamethasone and arose at a frequency of approximately 10(-10). The defect was not GR-related, as WR256 expressed functional GR and underwent GR-dependent events associated with apoptosis, such as hormone-dependent gene transcription and inhibition of cell proliferation. Moreover, the glucocorticoid-resistant phenotype was stable in culture and did not revert after treatment with 5-azacytidine or upon stable expression of GR cDNA. In addition, WR256 did not exhibit the diminished mitochondrial activity commonly associated with apoptosis. Interestingly, WR256 was also found to be resistant to 8-bromo-cAMP and forskolin despite having normal levels of protein kinase A activity and the ability to induce cAMP-dependent transcription. We examined the steady-state transcript levels of bcl-2, a gene whose protein product acts dominantly to inhibit thymocyte apoptosis, to determine whether elevated bcl-2 expression could account for the resistant phenotype. Our data showed that bcl-2 RNA levels were similar in the two cell lines and not altered by either dexamethasone or 8-bromo-cAMP treatment. These results suggest that WR256 exhibits a "deathless" phenotype and has a unique defect in a step of the apoptotic cascade that may be common to the glucocorticoid- and cAMP-mediated cell death pathways.

Development of an apoptosis endonuclease assay

A biochemical hallmark of cells undergoing programmed cell death, or apopotosis, is the endonucleolytic cleavage of genomic DNA at internucleosomal sites. To study further the nuclease involved in this process, an assay system was developed to measure internucleosomal DNA degradation. Micrococcal nuclease (MNase), a bacterial enzyme that cleaves chromatin at internucleosomal intervals, was used to validate the assay procedure. Thymocyte nuclear proteins obtained from glucocorticoid-treated chickens, a source of internucleosomal DNA-degrading activity, were incubated with chicken red blood cell nuclei, and genomic DNA was subsequently extracted and analyzed by agarose gel electrophoresis. Generation of internucleosomal DNA degradation products by the thymocyte protein extract required ATP and was both time and protein concentration dependent. This nuclease activity could be inhibited by EDTA, EGTA, alkylating agents, or heat denaturation. Addition of purified proteinases, RNases, or other types of nucleases to the assay failed to generate discrete internucleosomal lengths of DNA, thus confirming the nuclease specificity of this assay. On the basis of these data, we believe that this assay system will be instrumental in isolating and characterizing the nuclease(s) associated with apoptosis.

Activation of thymocyte deoxyribonucleic acid degradation by endogenous glucocorticoids

Immature lymphocytes in the thymus gland are killed by treatment with exogenous glucocorticoids. This steroid-mediated lymphocytolysis is preceded by numerous alterations in lymphocyte metabolism, including a DNA-degrading process in which the genome is cleaved at internucleosomal intervals. To date, this process has only been characterized by treating lymphocytes in vitro with glucocorticoids or by exogenous treatment of whole animals with adrenal steroids. To determine whether thymocyte DNA degradation could be activated by endogenous glucocorticoids, 4-wk-old chicks were treated with porcine adrenocorticotropic hormone (ACTH). This procedure elevated serum corticosterone levels approximately 80-fold within 2 h of hormone treatment. Following ACTH administration, thymocyte DNA was isolated and analyzed by agarose gel electrophoresis. The ACTH activated a DNA-degrading process that generated internucleosomal fragments of DNA identical in size to those observed following exogenous treatment with synthetic or naturally occurring glucocorticoids. Furthermore, this response could be inhibited by the glucocorticoid antagonist RU486 (17 beta-hydroxy-11 beta, 4-dimethylaminophenyl-17 alpha-propynl-estra-4,9,diene-3-one), indicating that adrenal steroids activate this process via the glucocorticoid receptor. These results demonstrate that lymphocyte DNA degradation does not result solely from exogenous glucocorticoid treatment; moreover, endogenous glucocorticoids can mediate this process and may thereby play an important role in thymic gland function.

Identification and characterization of glucocorticoid-regulated nuclease(s) in lymphoid cells undergoing apoptosis

Apoptosis is a physiological process by which selected cells are deleted from a population in response to specific regulatory signals. A hallmark of apoptosis is the internucleosomal degradation of DNA prior to cell death. We are studying glucocorticoid-induced lymphocytolysis as a model system for apoptosis within the immune system. In rat thymocytes, the internucleosomal DNA cleavage which occurs following glucocorticoid treatment is both time- and dose-dependent, and is blocked by the glucocorticoid antagonist RU 486, indicating that this effect is mediated by the glucocorticoid receptor. Similar experiments using glucocorticoid-responsive (wt) and glucocorticoid-resistant (nt-) S49.1 lymphoma cell lines confirm that internucleosomal DNA degradation and cell death are glucocorticoid receptor-mediated events and thus reflect the direct effects of glucocorticoids on lymphocytes. In an effort to identify the nuclease(s) responsible for the DNA degradation, we have developed two assays to detect nucleases whose activity is altered by glucocorticoid treatment. The first assay involves electrophoresing extracts of nuclear protein from control and glucocorticoid-treated lymphoid cells into SDS-polyacrylamide gels containing [32P]DNA within the gel matrix. This assay is used to estimate the molecular mass of the nuclease, based on the observed in situ nuclease activity. The second assay uses HeLa nuclei as a substrate to detect internucleosomal cleavage activity present in nuclear extracts of control and glucocorticoid-treated lymphoid cells. Using these assays we have identified a novel Ca2+, Mg(2+)-dependent nuclease with an apparent molecular weight of 18 kDa in both S49 wt cells and rat thymocytes treated with glucocorticoids. Furthermore, nuclear extracts of glucocorticoid-treated, but not control, rat thymocytes and S49 wt cells were capable of cleaving HeLa chromatin at internucleosomal sites. In an effort to determine the identity of the nuclease capable of internucleosomal cleavage of DNA, nuclear extracts from dex-treated rat thymocytes were fractionated by gel filtration chromatography under non-denaturing conditions, and the fractions were analyzed using the [32P]DNA SDS-PAGE and HeLa nuclei assays. When analyzed under native conditions, the 18 kDa nuclease described previously appears to exist as a congruent to 25 kDa protein which may be part of a high molecular weight complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of programmed cell death (apoptosis) by cisplatin, other anticancer drugs, toxins and hyperthermia

Cell death induced by cisplatin was studied in Chinese hamster ovary cell lines, one proficient and the other deficient (100-fold sensitive) in DNA excision repair. Previous experiments demonstrated that cells progressed to and arrested in the G2 phase of the cell cycle before dying. DNA double-strand breaks were detected following G2 arrest and prior to loss of membrane integrity. These DNA breaks have been studied in more detail. DNA fragments were observed consisting of multimers of approximately 180 base pairs. These fragments are consistent with internucleosomal cleavage of chromatin by an endonuclease. At LC90 concentrations, DNA digestion began 48 hr cisplatin treatment followed by loss of membrane integrity and cell shrinkage 24 hr later. High concentrations of cisplatin (170 logs of kill) induced DNA digestion 12 hr after drug treatment but loss of membrane integrity occurred 12 hr later. Both cell death and DNA fragmentation were inhibited by cycloheximide, suggesting the requirement for new protein synthesis. Cells incubated with many other agents demonstrated the same characteristic pattern of DNA degradation. At 90% lethal conditions, DNA digestion was induced within 30 min by hyperthermia, 18 hr by methotrexate, and 48-72 hr by all other agents tested. DNA digestion always preceded loss of membrane integrity and cell shrinkage. These observations are consistent with cell death occurring by the process of apoptosis, or prorammed cell death, and demonstrate the importance of DNA digestion as an early and presumably essential step in cell death. The results suggest that, irrespective of the primary site of action of a drug, cell death by most pharmacologic agents is mediated by activation of the signal transduction pathway for apoptosis. The results also suggest two signal pathways for apoptosis, one directly associated with drug action and a second that requires cell cycle-related events.

Glucocorticoid-mediated activation of DNA degradation in avian lymphocytes

Little information is known about the molecular mechanism of programmed cell death in the avian species. In the current study we have analyzed this process in chickens using a glucocorticoid-lymphocyte model system. Three-week-old male broiler chicks were treated in vivo with the synthetic glucocorticoid, dexamethasone. Following this treatment genomic DNA was isolated from thymocytes and analyzed by agarose gel electrophoresis. Dexamethasone activated a DNA degrading process in which the genome was specifically cleaved at internucleosomal intervals. This steroid-induced response occurred prior to thymocyte cell death and was time and glucocorticoid dose dependent, as well as tissue and steroid specific. Only the glucocorticoid class of steroid hormones could elicit this response and DNA degradation was only detectable in lymphoid tissues that contained immature lymphocytes. Internucleosomal DNA degradation could also be elicited via administration of adrenocorticotrophic hormone, a treatment that elevates endogenous glucocorticoids. Based on these data, glucocorticoid-activated DNA degradation of the avian thymocyte genome appears to be a steroid receptor-mediated process which involves the activation of an endogenous nuclease that cleaves the genome at internucleosomal sites. Degradation of the thymocyte genome occurs prior to cell death and may represent an initial event in a cascade of hormone-mediated processes that culminate in a type of cellular suicide referred to as programmed cell death.

Glucocorticoid activation of deoxyribonucleic acid degradation in bursal lymphocytes

Treatment of animals with exogenous adrenal steroids or elevation of endogenous glucocorticoids results in a profound involution of lymphoid tissue. In rodent species, this lymphoinvolution is accompanied by lymphocyte cell death and extensive degradation of the genome prior to lymphocytolysis. In the present study, this process was investigated in the bursa of Fabricius of domestic fowl. Four-wk-old chicks were treated with a single injection of dexamethasone, and bursal regression and cell viability were monitored over a 72-h period. Following hormone treatment, DNA was extracted from bursal lymphocytes and analyzed by agarose gel electrophoresis. Dexamethasone treatment resulted in a rapid regression of bursal tissue that could be detected as soon as 6 h posttreatment, but lymphocyte viability was not altered until 24 h afterward. The DNA isolated from bursal lymphocytes of glucocorticoid-treated birds appeared to be degraded at internucleosomal sites and generated a "ladder" of discrete DNA fragments when analyzed by agarose gel electrophoresis. This form of hormone-induced cell death, referred to as programmed cell death, may play a key role in glucocorticoid-mediated immunosuppression.

Isolation and characterization of glucocorticoid- and cyclic AMP-induced genes in T lymphocytes

Glucocorticoids and cyclic AMP exert dramatic effects on the proliferation and viability of murine T lymphocytes through unknown mechanisms. To identify gene products which might be involved in glucocorticoid-induced responses in lymphoid cells, we constructed a lambda cDNA library prepared from murine thymoma WEHI-7TG cells treated for 5 h with glucocorticoids and forskolin. The library was screened with a subtracted cDNA probe enriched for sequences induced by the two drugs, and cDNA clones representing 11 different inducible genes were isolated. The pattern of expression in BALB/c mouse tissues was examined for each cDNA clone. We have identified two clones that hybridized to mRNAs detected exclusively in the thymus. Other clones were identified that demonstrated tissue-specific gene expression in heart, brain, brain and thymus, or lymphoid tissue (spleen and thymus). The kinetics of induction by dexamethasone and forskolin were examined for each gene. The majority of the cDNA clones hybridized to mRNAs that were regulated by glucocorticoids and forskolin, two were regulated only by glucocorticoids, and three hybridized to mRNAs that required both drugs for induction. Inhibition of protein synthesis by cycloheximide resulted in the induction of all mRNAs that were inducible by glucocorticoids. Preliminary sequence analysis of four of the 11 cDNAs suggests that two cDNAs represent previously undescribed genes while two others correspond to the mouse VL30 retrovirus-like element and the mouse homolog of chondroitin sulfate proteoglycan core protein.

Isolation and characterization of glucocorticoid- and cyclic AMP-induced genes in T lymphocytes

Glucocorticoids and cyclic AMP exert dramatic effects on the proliferation and viability of murine T lymphocytes through unknown mechanisms. To identify gene products which might be involved in glucocorticoid-induced responses in lymphoid cells, we constructed a lambda cDNA library prepared from murine thymoma WEHI-7TG cells treated for 5 h with glucocorticoids and forskolin. The library was screened with a subtracted cDNA probe enriched for sequences induced by the two drugs, and cDNA clones representing 11 different inducible genes were isolated. The pattern of expression in BALB/c mouse tissues was examined for each cDNA clone. We have identified two clones that hybridized to mRNAs detected exclusively in the thymus. Other clones were identified that demonstrated tissue-specific gene expression in heart, brain, brain and thymus, or lymphoid tissue (spleen and thymus). The kinetics of induction by dexamethasone and forskolin were examined for each gene. The majority of the cDNA clones hybridized to mRNAs that were regulated by glucocorticoids and forskolin, two were regulated only by glucocorticoids, and three hybridized to mRNAs that required both drugs for induction. Inhibition of protein synthesis by cycloheximide resulted in the induction of all mRNAs that were inducible by glucocorticoids. Preliminary sequence analysis of four of the 11 cDNAs suggests that two cDNAs represent previously undescribed genes while two others correspond to the mouse VL30 retrovirus-like element and the mouse homolog of chondroitin sulfate proteoglycan core protein.

Glucocorticoid action on the immune system

Glucocorticoids have profound effects on immune function that are mediated, in part, by steroid-induced cell death. Our studies have been aimed at identifying the mechanism of this lymphocytolytic process using the rat thymocyte as a model system. Administration of glucocorticoids in vivo resulted in internucleosomal cleavage of the lymphocyte genome that was detectable within 2 h of treatment and increased with time after hormone administration. Six h after steroid treatment greater than 50% of the genome was degraded, yet cell viability remained greater than 90% indicating that this event preceded cell death. Furthermore, this process appeared to be mediated by the glucocorticoid receptor since the antagonist RU 486 blocked glucocorticoid-mediated DNA degradation. To further characterize this lymphocytolysis we have analyzed glucocorticoid-treated thymocytes for nucleases. Two families of nuclear proteins have been identified, a 30-32 kDa doublet and a series of 3-4 proteins that are 12-19 kDa, both of which are induced by glucocorticoid treatment (137 +/- 6% and 342 +/- 24%, respectively) and have prominent nuclease activity. These nucleases can also be induced in vitro indicating that glucocorticoids act directly on thymocytes to mediate this response. Moreover, this nuclease induction, like glucocorticoid-mediated DNA degradation, could be blocked by RU 486. Based on these findings we propose a working model of glucocorticoid-mediated lymphocytolysis in which these steroids, acting via a receptor mediated process, induce the expression of a lysis gene product (nuclease) which degrades the genome and results in cell death.

Glucocorticoid-induced lymphoma cell death: the good and the evil

Glucocorticoid hormones and their synthetic derivatives are widely used in therapy due to their strong anti-inflammatory and immunosuppressive potential. While the molecular basis of the anti-inflammatory action is to date at least partially understood, knowledge regarding the mechanism underlying glucocorticoid effects on the immune system is rather fragmentary. The immunosuppression could be attributed to at least two distinct processes: inhibition of the production of growth mediators and glucocorticoid-induced cell death. The mechanism of glucocorticoid-induced cell death can be divided into two steps, a reversible growth inhibition and cell lysis. The first step is characterized by many metabolic alterations typical of the catabolic potential of corticosteroids. After a delay of several hours activation of an endonuclease appears to initiate the lytic phase. By the action of this endonuclease the DNA is fragmented. In opposition to the chromatin damage, poly(ADP-ribosyl)ation is activated in order to stabilize the chromatin structure until the antagonistic potential is exhausted and the cells die. Therefore it can be speculated that the lethal event in glucocorticoid-induced cell death is a destruction of the regular chromatin structure.