Identification of a glucocorticoid-induced nuclease in thymocytes. A potential “lysis gene” product

Accumulation of CAMP in γ-irradiated Thymocytes and Internucleosomal DNA Fragmentation

Ionizing radiation, glucocorticosteroids and chemical inducers of differentiation (CID) are cytotoxic to thymocytes, and induce internucleosomal DNA fragmentation. Tissue cAMP levels in thymi of irradiated mice were significantly elevated as early as 30 min post-irradiation. In contrast, cAMP content in the liver was not changed significantly up to 1 h post-irradiation, and then some decrease occurred. Irradiation of isolated thymocytes gave essentially the same results as after irradiation of animals, and the elevation in cAMP 30 min after the irradiation, DNA fragmentation and cell death were linearly related to the dose up to 2.5 Gy. The maximal induction of cAMP level occurs in the fractions of radiosensitive cortical thymocytes. In thymocytes all CID tested also induced the increase in cAMP level with concomitant DNA fragmentation. Unlike ionizing radiation, UVC light did not induce cAMP accumulation and DNA fragmentation in thymocytes. Treatment of UV-irradiated cells with But2 cAMP did not result in an increase in DNA fragmentation. Ionizing radiation induced DNA fragmentation and cell death can be prevented by adding the protein kinases inhibitor H-7. Theophylline was shown to reduce the cAMP response, DNA fragmentation and cell death in gamma-irradiated thymocytes, suggesting that the accumulation of cAMP may be partly related to adenosine receptor sites.

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.

Conditional expression of a glucocorticoid receptor transgene in thymocytes reveals a role for thymic-derived glucocorticoids in thymopoiesis in vivo

We and others have previously reported that thymic epithelial cells produce glucocorticoids (GCs). In vitro studies have also suggested that thymic-derived GCs play a role in the development of thymocytes. However, until now it has not yet been established whether thymic-derived GCs play a role in thymopoiesis in vivo. To investigate this, we conditionally overexpressed the GC receptor (GR) in thymocytes using transgenic mice with a tetracycline-inducible expression system. The influence of systemic GCs was excluded by adrenalectomizing the transgenic mice before the GR induction. Conditional expression of transgenic GR in the thymocytes of adrenalectomized transgenic mice led to a decrease in the thymocyte number. This was associated with increased thymocyte apoptosis. The effect of thymic-derived GCs on the thymocytes was confirmed after transgenic GR induction in a thymic organ culture system. Finally, the GR antagonist RU486 increased thymocyte number in adrenalectomized mice in vivo and prevented a reduction in thymocyte number in thymic organ culture after transgenic GR induction. These observations further confirmed a role for the thymic-derived GCs in regulating thymocyte homeostasis in vivo.

Glucocorticoid-induced thymocyte apoptosis in wall lizard Hemidactylus flaviviridis

The aim of the present study is to demonstrate the phenomenon and mechanism of GC-induced cell death of thymocytes in wall lizard Hemidactylus flaviviridis. Lizard thymocytes were treated in vitro with different concentration of corticosterone (CS) for varying durations and the genomic DNA was analyzed both by agarose gel electrophoresis and flow cytometry. Corticosterone induced DNA fragmentation in a dose-dependent manner, as the DNA laddering was more prominent at 10(-14) M than at lower concentrations. However, CS could activate the programmed cell death of thymocytes only when incubated for 48 h and not at 3, 6, 12, or 24 h. Similar results were obtained following the flow cytometric analysis. The highest apoptotic cell death was noted at 10(-14) M concentration. CS-induced programmed cell death was decreased dramatically when the thymocytes were preincubated with RU 486 (10(-9) ng /ml) for 24 h, suggesting the classical receptor-mediated genomic pathway of glucocorticoid action in inducing apoptosis of thymocytes in wall lizard H. flaviviridis.

Evolution of the thymus size in response to physiological and random events throughout life

During embryogenesis and in the early stages of life, the thymus is a crucial organ for the generation of the T cell repertoire. T cells are generated from hematopoietic stem cells already differentiated to precursor T cells in the bone marrow. These cells enter the thymus guided by chemotactic factors secreted by this organ. The complex maturation process takes place that ensures self-tolerance and homeostasis. Thymocytes that show autoreactivity do not leave the thymus, but rather die by apoptosis. The final percentage of mature T cells that survive to migrate from the thymus to the periphery is very low: at most 5%, under optimal conditions. The highest migration occurs in childhood and adulthood, at least in mice and humans; however, it declines throughout life and is minimal in the elderly. Under normal circumstances, the thymus commences involution soon after birth, and this involution correlates with the capacity to export mature T cells to the periphery. Hormones, cytokines, and neurotransmitters all play a role in this age-associated process, but the reasons for and mechanisms of this involution remain unknown. Apart from physiological conditions that change throughout life and govern age-related thymus evolution, random states and events provoked by intrinsic or extrinsic factors can induce either thymus involution, as in reversible transient thymic hypoplasias, or thymic hyperplasias. The age-associated involution, unlike transient involutions, follows a regular pattern for all individuals, though there are clear differences between the sexes. Nevertheless, even the age-associated involution seems to be reversible, raising the possibility of therapeutic strategies aimed at enhancing thymus function in the elderly.

Apoptotic pathways of oxidative damage to renal tubular epithelial cells

Toxic renal failure induced by gentamicin, glycerol, or cisplatin, as well as ischemic renal failure in vivo and hypoxia/reoxygenation of tubular epithelial cells in vitro, induces the production of reactive oxygen metabolites (ROM). Generation of ROM is responsible for the induction of tubular epithelial cell death, which is mediated by caspases and/or endonucleases. Scavenging of ROM protects tubular epithelium from caspase and endonuclease activation and from cell death. Thus, the inhibition of ROM production combined with the pharmacological control of caspase and endonuclease pathways may provide future modalities in the prevention or treatment of acute renal failure in humans.

CRE-mediated transcriptional activation is involved in cAMP protection of T-cell receptor-induced apoptosis but not in cAMP potentiation of glucocorticoid-mediated programmed cell death

Apoptosis of thymic cells induced by glucocorticoids (GC) and T-cell receptor (TCR) engagement are mutually antagonistic. We demonstrate that cAMP enhances GC and antagonizes TCR (anti-CD3) apoptosis on the same cell (DO-11.10 and 2B4.11 T-cell hybridomas). We analyzed the activity of several transcription factors in this cAMP dual, stimulus-dependent, regulatory action. Anti-CD3 increases kB-activity which is inhibited by CPTcAMP or dexamethasone (DEX), supporting the proapoptotic role of NFkB on TCR-induced apoptosis. Anti-CD3 not only increases kB- but diminishes GC response element (GRE)-activity induced by DEX, suggesting that TCR-mediated blockade of GC-induced apoptosis involves not only the proposed antiapoptotic action of NF-kB on GC, but also the inhibition of GRE-regulated proapoptotic genes. To test the involvement of CRE-driven transcription in the cAMP dual apoptotic regulation, cells were transfected with a CRE decoy DNA oligomer. Blockade of CRE transactivation with decoy targeting of CRE completely blocked the protection of TCR-induced apoptosis by cAMP, while it did not modify the enhancement by cAMP on GC-induced apoptosis. We show that CRE-binding factors have a definite role in T-cell apoptosis: they are involved in cAMP protection of TCR- but not in cAMP potentiation of GC-induced apoptosis.

Wogonin and fisetin induce apoptosis in human promyeloleukemic cells, accompanied by a decrease of reactive oxygen species, and activation of caspase 3 and Ca(2+)-dependent endonuclease

Seven structurally related flavonoids including luteolin, nobiletin, wogonin, baicalein, apigenin, myricetin and fisetin were used to study their biological activities on the human leukemia cell line, HL-60. On MTT assay, wogonin, baicalein, apigenin, myricetin and fisetin showed obvious cytotoxic effects on HL-60 cells, with wogonin and fisetin being the most-potent apoptotic inducers among them. The cytotoxic effects of wogonin and fisetin were accompanied by the dose- and time-dependent appearance of characteristics of apoptosis including DNA fragmentation, apoptotic bodies and the sub-G1 ratio. Treatment with an apoptosis-inducing concentration of wogonin or fisetin causes rapid and transient induction of caspase 3/CPP32 activity, but not caspase 1 activity. Further, cleavage of poly(ADP-ribose) polymerase (PARP) and decrease of pro-caspase 3 protein were detected in wogonin- and fisetin-treated HL-60 cells. An increase in the pro-apoptotic protein, bax, and a decrease in the anti-apoptotic protein, Mcl-1, were detected in fisetin- and wogonin-treated HL-60 cells. However, Bcl-2, Bcl-XL, and Bad all remained unchanged in wogonin- and fisetin-treated HL-60 cells. In vitro chromatin digestion revealed that endonuclease activity was profoundly enhanced in wogonin- and fisetin-treated HL-60 cells, and the addition of ethylenediaminetetraacetic acid (EDTA) or ethyleneglycoltetraacetic acid (EGTA) into the reaction blocked endonuclease activation and at an optimum pH of 7.5. The caspase 3 inhibitor, Ac-DEVD-CHO, but not the caspase 1 inhibitor, Ac-YVAD-CHO, attenuated wogonin- and fisetin-induced DNA ladders, PARP cleavage, and endonuclease activation. Pretreatment of HL-60 cells with N-acetyl-cysteine or catalase efficiently inhibited H(2)O(2) (200 microM)-induced apoptosis, but showed no inhibitory effect on wogonin- and fisetin-induced DNA ladders, caspase 3 activation, or bax protein induction. Decrease in endogenous ROS production was detected in wogonin- and fisetin-treated HL-60 cells by DCHF-DA assay. In conclusion, our experiments indicate that a decrease in intracellular peroxide level was involved in wogonin- and fisetin-induced apoptosis; activation of caspase 3 and endonuclease, induction of bax protein and suppression of Mcl-1 protein were detected in the process.

Granulocyte-colony stimulating factor induced intranuclear endonuclease in murine leukemia cell line

We have reported that murine leukemia cell line (C2M-A5) induced apoptosis by G-CSF. To clarify the mechanism, mRNA expression of apoptosis-related genes was studied. It revealed transient over-expression of c-myc, H-ras and p53 and down-expression of bcl-2. These changes were known as triggers of endonuclease induction. After 96 h culture with G-CSF, apoptosis was occurred simultaneously with endonuclease (37 kd) activation. This endonuclease induced the digestion of double-strand DNA and might be associated with caspase3. Although G-CSF accelerates cell growth and prevents apoptosis in general, it is a contradictory effect. We concluded that G-CSF induced endogenous endonuclease activity in C2M-A5.

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.

Collapse of the inner mitochondrial transmembrane potential is not required for apoptosis of HL60 cells

Apoptotic cell death involves a series of morphological and biochemical changes orchestrated by activated proteases belonging to the caspase family. Recent studies have suggested that the activation of this process of execution is dependent upon events associated with the loss of mitochondrial inner transmembrane potential (Deltapsi(m)), as a consequence of the formation of the permeability transition (PT) pore. This has led to the proposal that mitochondrial depolarization represents a central irreversible checkpoint in the apoptotic program. Here, we present evidence that HL-60 cells undergo apoptosis in response to the cytotoxic insults of actinomycin-D, etoposide, and staurosporine without showing significant changes in Deltapsi(m). Instead, the loss of Deltapsi(m) could be detected only later in the cell death pathway. In addition, the uncoupling agent CCCP produced an early mitochondrial depolarization in HL-60s but these cells showed few signs of apoptosis up to 8 h after the insult. Furthermore, examination of these cells in response to staurosporine revealed the release of mitochondrial cytochrome c into the cytosol over time, corresponding to caspase activation irrespective of mitochondrial depolarization. In summary, our data suggest that the collapse of Deltapsi(m) as a consequence of PT is not a universal early marker for apoptosis and, moreover, it is not part of the central apoptotic machinery.

Purification, characterization, and role of nucleases and serine proteases in Streptomyces differentiation. Analogies with the biochemical processes described in late steps of eukaryotic apoptosis

Two exocellular nucleases with molecular masses of 18 and 34 kDa, which are nutritionally regulated and reach their maximum activity during aerial mycelium formation and sporulation, have been detected in Streptomyces antibioticus. Their function appears to be DNA degradation in the substrate mycelium, and in agreement with this proposed role the two nucleases cooperate efficiently with a periplasmic nuclease previously described in Streptomyces antibioticus to completely hydrolyze DNA. The nucleases cut DNA nonspecifically, leaving 5'-phosphate mononucleotides as the predominant products. Both proteins require Mg2+, and the additional presence of Ca2+ notably stimulates their activities. The two nucleases are inhibited by Zn2+ and aurin tricarboxylic acid. The 18-kDa nuclease from Streptomyces is reminiscent of NUC-18, a thymocyte nuclease proposed to have a key role in glucocorticoid-stimulated apoptosis. The 18-kDa nuclease was shown, by amino-terminal protein sequencing, to be a member of the cyclophilin family and also to possess peptidylprolyl cis-trans-isomerase activity. NUC-18 has also been shown to be a cyclophilin, and "native" cyclophilins are capable of DNA degradation. The S. antibioticus 18-kDa nuclease is produced by a proteolytic processing from a less active protein precursor. The protease responsible has been identified as a serine protease that is inhibited by Nalpha-p-tosyl-L-lysine chloromethyl ketone and leupeptin. Inhibition of both of the nucleases or the protease impairs aerial mycelium development in S. antibioticus. The biochemical features of cellular DNA degradation during Streptomyces development show significant analogies with the late steps of apoptosis of eukaryotic cells.

Panax ginseng blocks morphine-induced thymic apoptosis by lowering plasma corticosterone level

The effects of Panax ginseng on morphine-induced immune suppression were studied. Morphine (20 mg/kg, SC, 4 days) decreased body weight increment rate and caused atrophy of thymus and spleen. These changes were partly reversed by concomitant administration of ginseng total saponin (GTS, 100 mg/kg, oral, 9 days). Morphine elevated the serum corticosterone level and caused the DNA fragmentation of thymocytes. These sequential events were completely blocked by a concomitant administration of GTS. Flow cytometry analysis showed that GTS specifically blocked morphine-induced apoptosis of thymocytes.

Isolation and characterization of NUC70, a cytoplasmic, hematopoietic apoptotic endonuclease

Endonucleolytic DNA fragmentation is the common end point and the prevailing indicator of apoptosis. We have identified a 70-kDa endonuclease (NUC70) that is activated in drug-induced apoptosis of human hematopoietic cells. We purified NUC70 to homogeneity and generated a rabbit polyclonal antibody to distinguish it from previously identified nucleases. Biochemical characterization of isolated NUC70 demonstrates that it is Ca2+/Mg2+-dependent and active over a pH range of 6-8. When incubated with isolated HeLa nuclei, NUC70 was capable of generating internucleosomal DNA fragmentation. This endonucleolytic activity was inhibited by Zn2+, aurintricarboxylic acid, N-ethylmaleimide, spermine, and iodoacetamide. Western immunoblots using the anti-NUC70 antibody and DNA-SDS-polyacrylamide gel electrophoresis assays indicate that NUC70 expression and activity is restricted to human hematopoietic cells. No such activity was detected in human epithelial cell lines or murine hematopoietic cells. We also observed no difference in levels of NUC70 expression between apoptotic and nonapoptotic cells, suggesting that activation of NUC70 may be by posttranslational modification. We demonstrate that NUC70 activity is diminished in cells pretreated with the caspase inhibitors z-DEVD-fmk, z-VAD-fmk, and Z-CH2-Asp-DCB. Time course studies of cytoplasmic and nuclear endonuclease activities during apoptosis show that NUC70 is a cytoplasmic endonuclease that is translocated to the nucleus after the initiation of apoptosis. We confirmed this with immunostaining studies using anti-NUC70 antibody. These results demonstrate that NUC70 is an endogenous cytoplasmic endonuclease that is activated during apoptosis in a caspase-dependent mechanism.

Tumor cell apoptosis present at diagnosis may predict treatment outcome for patients with medulloblastoma

PURPOSE

To determine if the degree of tumor cell apoptosis at diagnosis predicts outcome, tissue sections of medulloblastoma were examined and the amount of apoptosis and progression-free survival were correlated.

PATIENTS AND METHODS

The study cohort consisted of 43 children in whom medulloblastoma was diagnosed between 1984 and 1995: 29 patients at high risk (HR) treated with radiation and chemotherapy, and 14 children at low risk (LR) treated with radiation alone. A terminal deoxynucleotidyl transferase (TdT) end-labeling assay was used to detect apoptosis in paraffin-embedded tissue sections prepared at diagnosis.

RESULTS

Progression-free survival was examined in cohorts of children whose tumors were divided into quartiles based on the apoptotic index (AI) of their pretreatment tumor specimens. A comparison of these four groups of children revealed an association between AI and outcome (p = 0.03); patients with tumors in the highest AI quartile had substantially improved outcome compared to all other patients combined (p = 0.02). In this cohort of patients treated with different therapies, assignment at the time of diagnosis to LR and HR groups based on widely-accepted clinical criteria was not closely associated with outcome (p = 0.47).

CONCLUSION

AI is a strong indicator of treatment outcome for children with medulloblastoma after treatment with cytotoxic therapy, independent of risk group. Because HR and LR patients included in this study received different modalities of cytotoxic therapy, it is possible that AI predicts outcome independent of the precise antineoplastic therapy a patient receives.

Glucocorticoid-induced thymocyte apoptosis: protease-dependent activation of cell shrinkage and DNA degradation

Glucocorticoids are well known to stimulate apoptosis in immature thymocytes. Apoptosis in this and other cells is characterized by cell shrinkage, DNA fragmentation and activation of a class of proteases named caspases. We have utilized the flow cytometer to evaluate the coordinate regulation of cell shrinkage and DNA fragmentation in glucocorticoid-treated rat thymocytes and explore the role of caspases upstream of both changes. The results indicate that the activation of apoptosis by glucocorticoids in a cell population is an asynchronous event with only a percentage of the cells displaying apoptotic characteristics at any given time. Both cell shrinkage and chromatin degradation are tightly coupled with similar proportions of the cells displaying each characteristic. The coordinate appearance of these characteristics may suggest a similar mechanism of regulation. Incubation of thymocytes with the general caspase inhibitor Z-VAD-FMK completely blocked both cell shrinkage and DNA fragmentation in spontaneous and glucocorticoid-induced thymocyte apoptosis, implicating an early upstream role for proteases in the activation of thymocyte apoptosis.

Apoptosis in the chick wing bud and the permanence of FGF-2 rescue

Two regions of programmed cell death that occur in the mesoderm of developing chick wing buds were studied in vitro. The opaque patch (OP) and posterior necrotic zone (PNZ) were examined for the presence of internucleosomal DNA degradation and for rescue by protein synthesis inhibition, two defining characteristics of apoptosis. Agarose gel electrophoresis showed that DNA from OP and PNZ tissue was cleaved into nucleosome size pieces and this cleavage was prevented by inhibition of protein synthesis with cycloheximide. Both regions showed rescue with cycloheximide as determined by the chromium release assay and examination of electron micrographs. Also, the permanence of basic fibroblast growth factor (EGF-2) rescue in the OP and NPZ was examined using the chromium release assay. While rescue in the OP was found to be permanent, rescue in the PNZ only delayed death while FGF-2 was present in the culture medium. This research shows that death in the OP and PNZ exhibits internucleosomal DNA fragmentation and is prevented by inhibition of protein synthesis with cycloheximide, biochemically characterizing this death as apoptosis. It also suggests that in vitro FGF-2 rescue is permanent in the OP but is merely a delay of cell death in the PNZ.

Restraint stress-induced thymic involution and cell apoptosis are dependent on endogenous glucocorticoids

The aim of this study was to investigate the specific role of endogenous glucocorticoids (GC) following restraint stress on thymic involution and apoptosis. Restraint stress has been reported to alter physiological and behavioral responses in experimental animals. Exposure of mice to restraint stress led to involution of the thymus, to a decrease of the CD4+ 8+ thymocyte subset, and to fragmentation of thymic DNA. The role of endogenous GC in restraint stress-induced changes in the thymus was studied by three experimental approaches: surgical adrenalectomy, chemical adrenalectomy, and blocking of GC receptors by a specific type II receptor antagonist. In surgically-Adx mice, which lack endogenous GC, the effects of restraint on the thymus were wholly abrogated. Pretreatment of restrained mice with metyrapone (an 11beta hydroxylase inhibitor that specifically inhibits GC biosynthesis) had the same consequence, and blockage of GC receptors with the specific GC type II receptor antagonist RU-38486 attenuated the effects of the stressor. These findings indicate that GC are involved in the restraint-induced effects on the thymus.

Intracellular K+ suppresses the activation of apoptosis in lymphocytes

Little is known about the mechanisms of suppression of apoptosis. We have addressed the novel possibility that the level of intracellular K+ regulates the apoptotic process by controlling the activity of death enzymes. We show that K+, at normal intracellular levels, inhibits both apoptotic DNA fragmentation and caspase-3(CPP32)-like protease activation, suggesting that intracellular K+ loss must occur early during apoptosis. Direct measurement of K+ by inductively coupled plasma/mass spectrometry and flow cytometry indicates a major decrease in intracellular K+ concentration in the apoptotic cell. Flow cytometric analysis revealed that caspase and nuclease activity were restricted to the subpopulation of cells with reduced K+. Disruption of the natural K+ electrochemical gradient suppressed the activity of both caspase and nuclease independent of the mode of activation of the apoptotic inducing agent, demonstrating that a decrease in intracellular K+ concentration is a necessary, early event in programmed cell death.

Time-course of the murine lymphoid tissue involution during and following stressor exposure

Groups of 35-day-old male C57BL/6 mice were stressed 1 hour per day by immobilization for 1, 2, 3, 5, 8, 11 or 14 consecutive days. Control groups were left undisturbed. The animals were then killed and body weight and the weights of the thymus, spleen and axillary lymph nodes determined. Chronic immobilization stress caused involution of the thymus, spleen and lymph nodes to an extent depending on the number of days of stress. The thymus showed the fastest response: thymus weight was significantly lower in stressed animals than in controls by the third day of stress while significant effects on spleen and lymph node weight were not observed until day 5. Fast recovery of lymphoid organ weight was observed after the stress period. The thymus recovered most quickly: control values were re-attained approximately 8 days after cessation of stress, and indeed by day 20 thymus weight was about 12% higher than in normal animals. The spleen and lymph nodes recuperated weight more slowly, re-attaining control values after about 20 days.

Relationship of dehydroepiandrosterone and cortisol in disease

Does dehydroepiandrosterone act as an adrenal hormone in humans to maintain cortisol homeostasis by serving as a cortisol antagonist? If so, dehydroepiandrosterone might block the development of the diverse pathological processes potentiated by prolonged cortisol hyperactivity. And the plasma concentrations of total dehydroepiandrosterone and total cortisol, expressed as a C/D ratio, would have an important influence on the development of age-related pathology in diseases exacerbated by cortisol hyperactivity. Several major age-related diseases, designated as cortisol-potentiated diseases, belong in this category. The C/D concept predicts, other factors being equal, that the risk of initiation and progression of these diseases at all ages is directly related to the C/D ratio, individuals with elevated C/D ratios being at high risk.

Heteronuclear ribonucleoproteins C1 and C2, components of the spliceosome, are specific targets of interleukin 1beta-converting enzyme-like proteases in apoptosis

Apoptosis induced by a variety of agents results in the proteolytic cleavage of a number of cellular substrates by enzymes related to interleukin 1beta-converting enzyme (ICE). A small number of substrates for these enzymes have been identified to date, including enzymes involved in DNA repair processes: poly(ADP-ribose) polymerase and DNA-dependent protein kinase. We describe here for the first time the specific cleavage of the heteronuclear ribonucleoproteins (hnRNPs) C1 and C2 in apoptotic cells induced to undergo apoptosis by a variety of stimuli, including ionizing radiation, etoposide, and ceramide. No cleavage was observed in cells that are resistant to apoptosis induced by ionizing radiation. Protease inhibitor data implicate the involvement of an ICE-like protease in the cleavage of hnRNP C. Using recombinant ICE-like proteases and purified hnRNP C proteins in vitro, we show that the C proteins are cleaved by Mch3alpha and CPP32 and, to a lesser extent, by Mch2alpha, but not by ICE, Nedd2, Tx, or the cytotoxic T-cell protease granzyme B. The results described here demonstrate that the hnRNP C proteins, abundant nuclear proteins thought to be involved in RNA splicing, belong to a critical set of protein substrates that are cleaved by ICE-like proteases during apoptosis.

Cellular catabolism in apoptosis: DNA degradation and endonuclease activation

Recent research has focused on identifying the biochemical events associated with the apoptotic process. These include specific degradation of the chromatin which was described by Wyllie in 1980 [1], with the report of the appearance of discretely sized DNA fragments from apoptotic rat thymocytes. The fragments corresponded in size to strands of DNA that were cleaved at internucleosomal regions and create a 'ladder pattern' when electrophoresed on an agarose gel. Because of its near universality, internucleosomal DNA degradation is considered a diagnostic hallmark of cells undergoing apoptosis. It is of great interest to identify the enzymes involved, and some of the candidates will be discussed.

Regulation of a physiological apoptosis: mouse mammary involution

Continuous milk production during lactation is dependent on a complex interplay of lactogenic hormones and the suckling stimulus exerted by the young. Involution can be initiated in the mouse mammary gland at any stage of lactation by removing the pups; involution then remains reversible for about 30 to 36 h. Involution in the mouse mammary gland is characterized by a massive loss of secretory epithelial cells from programmed cell death. The nuclear activation of protein kinase A and transcription factor activator protein 1 precede the irreversible phase of involution that is characterized by internucleosomal DNA fragmentation. Activation of activator protein 1 and fragmentation of chromosomal DNA can be prevented by lactogenic hormone treatment in explant cultures derived from mammary tissue at lactation. The elevation in activator protein 1 coincides with the epithelial expression of sulfated glycoprotein 2, a potential target gene of activator protein 1. Programmed cell death in the mammary gland is associated with the expression of the growth arrest gene, gas-1, and the integrin-associated protein gene, IAP, which codes for a putative Ca2+ channel that is dependent on integrin. Their potential roles during involution are discussed.

Apoptosis of ileal Peyer's patch B cells is increased by glucocorticoids or anti-immunoglobulin antibodies

The ileal Peyer's patch (PP) in sheep plays a central role in the development and production of B cells. Associated with a tremendous amount of B cell proliferation in this site is the extensive diversification of the Ig repertoire by somatic hypermutation. Very few (< 5%) of the B cells produced in the ileal PP differentiate and emigrate; instead, the vast majority of these cells soon die, and we have previously shown that death is associated with apoptosis. When placed in culture, ileal PP B cells die rapidly by apoptosis, such that after 24 h, 60 +/- 1% of DNA is fragmented. Here, we show that the extent of this spontaneous B cell apoptosis in culture, as quantitated by DNA fragmentation, was significantly increased in a dose-dependent manner by the glucocorticoids hydrocortisone or dexamethasone. Furthermore, treatment of lambs with 2-2.5 mg/kg of dexamethasone resulted in a marked increase in the number of apoptotic cells in the ileal PP and an increase in ileal PP B cell DNA fragmentation to 20 +/- 6%, compared with 2.4 +/- 0.1% in untreated lambs. Anti-immunoglobulin (Ig) antibodies also increased the extent of DNA fragmentation in cultured ileal PP B cells. After 24 or 48 h of culture with anti-Ig (PIg47A), DNA fragmentation was 74 +/- 2% and 75 +/- 3%, respectively. Ileal PP B cells are rescued from apoptosis by agents that activate protein kinase C and increase cytosolic Ca2+, and here we show that this treatment also results in apoptotic rescue in the presence of dexamethasone or anti-Ig. We speculate that the apoptosis of ileal PP B cells in situ may be modulated by glucocorticoids and by the cross-linking of surface Ig. Apoptosis, induced by a signal through surface Ig, may be an important mechanism in the deletion of self-reactive B cells during the expansion of the Ig repertoire in the ileal PP.

Regulation of apoptosis by steroid hormones

Steroid hormones play major roles in regulation in growth, development, homeostasis, and cell death. Together with other hormones and growth factors, steroids regulate both the function and cellular composition of organs throughout the body. In this article we will discuss the mechanisms of steroid hormone regulated apoptosis. Emphasis will be placed on the effect of glucocorticoids on lymphoid cells.

Analysis of internucleosomal DNA fragmentation in apoptotic thymocytes by dynamic sieving capillary electrophoresis

The analysis, by slab gel electrophoresis, of internucleosomal DNA cleavage or laddering, characteristic of apoptosis in many cell systems, is labour intensive, difficult to automate and at best only semi-quantitative. In this report we show that CE, using dilute solutions of hydroxyethylcellulose as a replaceable sieving matrix, can be applied to the relatively rapid analysis of DNA laddering in whole digests of apoptotic rat thymocytes. Also, using the sensitivity of laser-induced fluorescence detection and the highly sensitive nuclei acid stain YO-PRO-1, the CE method reported here can use 1000-2000 fold fewer cells than needed for traditional slab gel methods.

The role of DNA fragmentation in apoptosis

The formation of distinct DNA fragments of oligonucleosomal size (180-200 bp lengths) is a biochemical hallmark of apoptosis in many cells. Recent observations also suggest large DNA fragments and even single-strand cleavage events occur during cell death. These observations have raised many questions. What are the types of DNA cleavage observed during apoptosis? What are the nucleases involved? And what is the role of these nucleolytic events in apoptosis?

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

Glucocorticoid receptors and resistance to glucocorticoids in hematologic malignancies

Glucocorticoids are highly effective in inducing the cytolysis of cells of lymphocytic origin. This property has resulted in their incorporation into chemotherapy regimens used in the treatment of hematologic malignancies. Studies at the molecular and cellular levels have demonstrated that the hormone-induced cytolytic response is mediated through a highly specific cytoplasmic glucocorticoid receptor (GR). The GR has been cloned and sequenced and found to be organized into a discrete series of domains which mediate the receptor functions of hormone binding, nuclear translocation, DNA binding and transcriptional modulation. Thus, the binding of glucocorticoids by the GR induces a series of cellular events which result in the activation or repression of a network of glucocorticoid responsive genes and produces a specific cellular response. Prolonged exposure to glucocorticoids ultimately causes resistance to develop; thereby limiting the usefulness of this class of drugs. Studies addressing the mechanism of resistance have shown that the GR is the primary target of genetic alterations that lead to resistance to cytolysis. Using mouse and human cell lines as model systems, it has been shown that the vast majority of glucocorticoid resistant mutants express low levels or altered forms of the GR. Similarly, in vivo studies on patients have suggested that low GR levels are associated with a poor response to glucocorticoid based therapies. Recently, aberrant GR isolated from a patient with multiple myeloma resistant to glucocorticoids were found to harbor deletions in their hormone binding domains. Sequencing of the receptors suggested that each arose as a result of alternate splicing events. In both cases, the latter event produces a receptor unable to bind hormone leading to the speculation that alternate splicing may serve as a mechanism by which a cell evades the effects of glucocorticoids. The therapeutic implications for patients expressing aberrant receptors is discussed.

Formation of high molecular mass DNA fragments is a marker of apoptosis in the human leukaemic cell line, U937

Inhibitors of macromolecular synthesis and topoisomerases induce apoptosis in the human leukaemic cell line, U937. In this study, U937 cells were treated with the RNA synthesis inhibitor, actinomycin D (1 microM), the protein synthesis inhibitors, emetine (1 microM) and cycloheximide (100 microM), the topoisomerase II inhibitor, teniposide (5 microM), or the topoisomerase I inhibitor, camptothecin (1 microM). Apoptotic cell death was assessed both by flow cytometry and agarose gel electrophoresis, and was correlated to the appearance of large (20 to &gt; or = 580 kilobase pairs) DNA fragments, as assessed by field inversion gel electrophoresis. In all cases, the appearance of DNA fragments of 20–50 kilobase pairs accompanied the appearance of an apoptotic population and of internucleosomal cleavage. However, teniposide additionally induced a marked increase in fragmentation to &gt; or = 580 kilobase pairs. The cotreatment of cells with zinc (1 mM) inhibited the formation of all large DNA fragments, internucleosomal cleavage and the appearance of an apoptotic population. We conclude that the generation of large DNA fragments is characteristic of apoptosis induced by various stimuli in U937, as has been found previously in rat thymocytes. However, unlike what occurs in rat thymocytes, zinc treatment does not dissociate the formation of large fragments from conventional markers of apoptosis.

Apoptosis induced by DNA topoisomerase I and II inhibitors in human leukemic HL-60 cells

The induction of apoptosis following topoisomerase inhibitors proceeds in at least three distinct steps: (1) induction of cleavable complexes (potentially lethal damage), (2) topoisomerase-induced DNA damage, and (3) a presently unknown sequence of events that must either lead to cell cycle arrest (G2-block, differentiation) or apoptosis. DNA degradation provides a convenient way to quantify apoptosis in HL-60 cells. Extensive apoptosis can be induced rapidly in undifferentiated HL-60 cells without prevention by cycloheximide or actinomycin D. Therefore, HL-60 cells appear to express constitutively the apoptotic machinery that may be kept under control of a yet unknown repressor. The absence of the tumor suppressor p53 and the presence of bcl-2 are in contrast with the sensitivity of these cells to apoptosis. Agents that modify chromatin structure (zinc, poly[ADPribose] inhibitors, spermine) can block DNA fragmentation without affecting cell survival. By contrast macrophage-like differentiation by phorbol esters suppresses apoptosis without affecting topoisomerase-induced DNA damage. Better understanding of the apoptotic regulation in the widely used and characterized HL-60 cell line should allow the identification of new mechanisms and parameters of cellular sensitivity and resistance to the cytotoxic activity of anticancer agents.

Thymocyte apoptosis induced by elevated endogenous corticosterone levels

A well-known model of apoptosis is induction in thymocytes by injection of pharmacological doses of exogenous steroids. The aim of this study was to investigate whether this process also occurs under physiological conditions, i.e. by stimulation of endogenous glucocorticoid release, using the chicken as an experimental model. Endogenous glucocorticoid levels can be elevated by immunization with exogenous antigens or by injection of conditioned medium, e.g. supernatant of mitogen-stimulated spleen cells. This effect is mediated by so-called glucocorticoid-increasing factors, and is considered to act as an immunoregulatory principle. Thymocyte DNA of so treated birds showed a typical "ladder" pattern after electrophoresis in a 1.8% agarose gel, and degradation could be prevented by RU 38,486. This provides evidence that apoptosis can be induced by elevating endogenous corticosterone levels in vivo. By means of in situ nick translation (ISNT) and simultaneous immunofluorescence tests, it was possible to analyze various thymic subpopulations during apoptosis after treatment with exogenous glucocorticoids. Additionally, using confocal microscopical techniques, apoptosis of the same cells as analyzed by ISNT is shown. The possible role of elevated concentrations of endogenous glucocorticoids in regulating thymocyte cell death and autoimmune diseases is discussed.

The apoptosis endonucleases: cleaning up after cell death?

The term apoptosis describes the predictable structural changes associated with many forms of programmed cell death. One of the first visible events of apoptosis is the collapse of the nucleus. Nuclear degradation is manifested by digestion of chromatin into nucleosome-sized fragments or multiples of these. This digestion of DNA is enzymatic, and several attempts have been made to characterize apoptosis-specific endodeoxyribonucleases. Although there are strong candidates for such enzymes, direct evidence for their role in apoptosis is yet to be provided.

Increased production of atrial natriuretic peptide in the rat thymus after irradiation

The rat thymus involutes when exposed to dexamethasone, and this phenomenon is accompanied by a striking increase of ANP localized in thymic macrophages. This study was designed to investigate whether a stimulus, that is X-rays, well known to also cause involution of the thymus, is followed by an elevation of ANP-expression. Rats were irradiated with 4, 6 and 8 Gy, and after 4 days we found a dose-dependent increase (3-, 30- and 40-fold) of mRNA coding for ANP. The corresponding ANP precursor (ANP1-126) has been found elevated 4-, 10-, and 13-fold, respectively. Immunohistochemical analysis of irradiated (6 Gy) tissue revealed heavily IR-ANP positive cells in the cortex, which, due to their morphological and enzyme-histochemical characteristics, were identified as highly phagocytising macrophages. We suggest, thus, that regardless of the stimulus applied (DEX or X-rays) causing the vast cell death in the thymus, the increased ANP-expression is linked to the highly activated phagocytic system of the thymus under these conditions.

Programmed cell death in avian thymocytes: role of the apoptotic endonuclease

Apoptosis, or programmed cell death, is a physiological process whereby a target cell dies in response to a specific signal. A prominent model system used to study this process is the glucocorticoid-mediated killing of immature thymocytes. Following glucocorticoid treatment, apoptotic thymocytes undergo a series of distinct morphological alteration including cellular shrinkage, blebbing of the cytoplasmic membrane, and chromatin condensation. The chromatin condensation that occurs during apoptosis is associated with a characteristic endonuclease activity that degrades the genome at internucleosomal sites. To study this characteristic endonuclease activity further, nuclear extracts were prepared from thymocytes of glucocorticoid-treated chicks and nuclease activity present in the protein extracts was analyzed using chicken red blood cell nuclei as a substrate. Using this in vitro assay system, it was demonstrated that the avian endonuclease activity degrades chromatin at internucleosomal sites and can be inhibited by EDTA and zinc ions. Current efforts are focused on purifying the avian apoptotic endonuclease and further characterizing this nuclease activity.

Endonuclease activation following focal ischemic injury in the rat brain

The structural changes which occur in chromatin DNA after ischemic brain injury are poorly understood. This study examined the appearance of double-strand DNA breaks and the temporal profile of DNA degradation following focal ischemic injury in rat brain. Focal cerebral ischemia was produced by tandem occlusion of the common carotid and proximal middle cerebral arteries. The effects of decapitation ischemia were also studied by DNA analysis. DNA was extracted by standard methods from the ischemic brain tissues and electrophoresed on a 1.5% agarose gel. With decapitation ischemia, random DNA cleavage was observed as a dense "smear" on the gel electrophoresis beginning 6 h after the ischemic insult, and increasing in amount thereafter. Focal ischemia provided DNA fragmentation, which is specific DNA cleavage at the internucleosomal linker regions, particularly in the caudoputamen. Coexisting random degradation and specific fragmentation of DNA was observed in the cortex following focal ischemia. To determine whether an endonuclease responsible for DNA fragmentation was present, nuclear proteins were extracted from normal brain nuclei and the endonuclease activity was determined using plasmid DNA and a nuclear incubation system. This demonstrated that brain nuclear proteins have Ca(2+)-dependent endonuclease activity which is related to DNA fragmentation. Ischemic injury causes both random and specific DNA cleavage in the brain, which is probably mediated by Ca(2+)-dependent endonuclease.

Inhibition of DNA synthesis and DNA fragmentation in stimulated splenocytes by the concerted action of topoisomerase I and II poisons

Stimulated splenocytes were used as a model system to investigate the effects of topoisomerase inhibitors on normal, non-transformed, non-tumoral proliferating cells. The concerted action of camptothecin (a poison of topoisomerase I) and etoposide (a poison of topoisomerase II) lead to nearly complete inhibition of DNA synthesis in concanavalin A-stimulated splenocytes. Analysis of replicated cellular DNA after a short treatment with both drugs revealed a DNA cleavage to medium size fragments. This effect was additive, suggesting that cleavable complexes were formed independently by both topoisomerases on their respective DNA sites. In contrast, prolonged contact with both drugs was followed by degradation of the bulk cellular DNA to nucleosome size fragments, indicating that apoptosis took place in these cells. Combination of camptothecin and etoposide enhanced this phenomenon, consistent with the fact that degradation was the result of secondary events which may amplify the signal. Thus, aphidicolin, an inhibitor of eukaryotic replicases which blocks replication, also triggered DNA degradation in proliferating splenocytes.

Internucleosomal chromatin degradation in myeloma and B-hybridoma cell cultures

The activity of Ca/Mg-dependent endonuclease (CME) is strongly inhibited in myeloma X-63.Ag8.653 and B-hybridoma MLC-1c as compared with mouse splenocytes. Nevertheless, pronounced internucleosomal chromatin degradation occurs in both cell lines during long-term cultivation without passing. In isolated cell nuclei of X-63 the activation of CME, which precedes chromatin fragmentation in vivo and loss of cell viability, is revealed. The time-course of CME activation is opposite to cell proliferation and is not accompanied by alterations in enzyme quantity. The results suggest that cell death of X-63 and MLC-1c occurs via apoptosis, and involves the mechanisms controlling the activation and/or interaction of CME with chromatin.

A biochemical hallmark of apoptosis: internucleosomal degradation of the genome

Apoptosis, or programmed cell death, is an endogenous cellular process whereby an external signal activates a metabolic pathway that results in cell death. This form of cell death appears to be a common feature in many biological processes where cell deletion is a mechanism for altering tissue structure and function. Historically, apoptosis has been studied using histological techniques; however, more recent interest has focused on analyzing this process at the biochemical level. A biochemical hallmark of apoptosis is a characteristic form of DNA degradation in which the genome is cleaved at internucleosomal sites, generating a 'ladder' of DNA fragments when analyzed by agarose gel electrophoresis. A number of assay systems have been developed to study this nuclease activity. For example, nuclease activity has been analyzed by measuring the release of endogenous DNA from apoptotic cells, by flow cytometric analysis of apoptotic cells and by analyzing in situ apoptotic nuclease activity in polyacrylamide gels containing DNA. Use of these assay systems has enabled investigators to study the signal transduction pathways that mediate apoptosis and to characterize the endonuclease itself. Future biochemical studies in this field will focus on isolating the genes and gene products that mediate apoptosis.

Apoptosis induced by anticancer drugs

Most of the cytotoxic anticancer drugs in current use have been shown to induce apoptosis in susceptible cells. The fact that disparate agents, which interact with different targets, induce cell death with some common features (endonucleolytic cleavage of DNA, changes in chromatin condensation) suggests that cytotoxicity is determined by the ability of the cell to engage this so-called 'programmed' cell death. The mechanism of the coupling of a stimulus (drug-target interaction) to a response (cell death) is not known, but modulation of this coupling may affect the outcome of drug treatment. This review surveys the recent evidence which supports the idea that the drug-target interaction per se is not the sole determinant of cellular sensitivity of cytotoxic drugs. Studies of the signals which might engage apoptosis, the genes which modulate it and the biochemical process of drug-induced apoptosis itself are described, where possible, for glucocorticoids, topoisomerase inhibitors, alkylating agents, antimetabolites and antihormones. It is suggested that identification of the gene products which couple the stimulus to the response, and so determine intrinsic cellular sensitivity (and resistance), will be important targets for new types of drugs. These might then allow responses to occur in the major cancers of man, which are chemoresistant.