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

Glucocorticoids, Sex Hormones, and Immunity

Glucocorticoid hormones regulate essential body functions in mammals, control cell metabolism, growth, differentiation, and apoptosis. Importantly, they are potent suppressors of inflammation, and multiple immune-modulatory mechanisms involving leukocyte apoptosis, differentiation, and cytokine production have been described. Due to their potent anti-inflammatory and immune-suppressive activity, synthetic glucocorticoids (GCs) are the most prescribed drugs used for treatment of autoimmune and inflammatory diseases. It is long been noted that males and females exhibit differences in the prevalence in several autoimmune diseases (AD). This can be due to the role of sexual hormones in regulation of the immune responses, acting through their endogenous nuclear receptors to mediate gene expression and generate unique gender-specific cellular environments. Given the fact that GCs are the primary physiological anti-inflammatory hormones, and that sex hormones may also exert immune-modulatory functions, the link between GCs and sex hormones may exist. Understanding the nature of this possible crosstalk is important to unravel the reason of sexual disparity in AD and to carefully prescribe these drugs for the treatment of inflammatory diseases. In this review, we discuss similarities and differences between the effects of sex hormones and GCs on the immune system, to highlight possible axes of functional interaction.

Tissue-specific actions of glucocorticoids on apoptosis: a double-edged sword

First described for their metabolic and immunosuppressive effects, glucocorticoids are widely prescribed in clinical settings of inflammation. However, glucocorticoids are also potent inducers of apoptosis in many cell types and tissues. This review will focus on the established mechanisms of glucocorticoid-induced apoptosis and outline what is known about the apoptotic response in cells and tissues of the body after exposure to glucocorticoids. Glucocorticoid-induced apoptosis affects the skeletal system, muscular system, circulatory system, nervous system, endocrine system, reproductive system, and the immune system. Interestingly, several cell types have an anti-apoptotic response to glucocorticoids that is cytoprotective. Lastly, we will discuss the pro- and anti-apoptotic effects of glucocorticoids in cancers and their clinical implications.

Ca2+-PKC-caspase 3-like protease pathway mediates DNA and nuclear fragmentation in ecdysteroid-induced programmed cell death

20-Hydroxyecdysone (20E) induces programmed cell death in the anterior silk gland of the silkworm. Here, we report the direct interaction between Ca(2+) and protein kinase C (PKC)-caspase 3-like protease pathway in the 20E-induced cell death. The calcium ionophore can mimic 20E effects in inducing DNA and nuclear fragmentation, but such mimicry is only possible in the glands precultured for 18 h with 20E. The simultaneous presence of translation inhibitor with 20E in the preculture showed that de novo protein synthesis was needed to mimic 20E effects by the calcium ionophore. Both a PKC inhibitor and a caspase 3 inhibitor inhibited the mimicking effects. After substitution of the calcium ionophore for 20E, caspase 3-like protease was fully activated 12h later, and DNA and nuclear fragmentation occurred faster than continuous 20E stimuli. The results show the presence of a Ca(2+)-PKC-caspase 3-like protease pathway in 20E signaling, and possible involvement of the pathway up to the mobilization of Ca(2+) in regulating the timing of cell death in vivo.

Psychological stress exacerbates primary vaginal herpes simplex virus type 1 (HSV-1) infection by impairing both innate and adaptive immune responses

Chronic psychological stress is generally immunosuppressive and contributes to an increase in herpes simplex virus (HSV) pathogenicity. We have previously shown that mice experiencing stress at the time of intranasal HSV infection have increased levels of infectious virus in their nasal cavity, as compared to control mice that were not subjected to stress. We have extended our studies to determine the effects of stress at another clinically-relevant mucosal site by examining the immune response to and pathogenesis of vaginal HSV infection. Mice experiencing psychological stress during vaginal HSV infection exhibited an increase in both vaginal viral titers and the pathology associated with this HSV infection. We demonstrate that these observations result from the failure of both the innate and HSV-specific adaptive immune responses. At 2 days post-infection, NK cell numbers were significantly decreased in mice experiencing restraint stress. Studies examining the adaptive immune response revealed a decrease in the number of HSV-specific CD8(+) T cells in not only the vaginal tissue itself but also the draining iliac lymph nodes (ILN). Furthermore, the number of functional cells, in terms of both their degranulation and interferon-gamma production, in the ILN of stressed mice was decreased as compared to non-stressed mice. We conclude that psychological stress, through its suppression of both innate and adaptive immune responses, may be an important factor in the ability to control vaginal HSV infection.

Psychological stress impairs the local CD8+ T cell response to mucosal HSV-1 infection and allows for increased pathogenicity via a glucocorticoid receptor-mediated mechanism

Psychological stress and its associated increases in corticosterone are generally immunosuppressive and contribute to increased herpes simplex virus (HSV)-associated pathogenicity. However, the impact of stress on local control of the initial mucosal-based HSV infection has not been elucidated, nor have the ramifications of such failures of the immune response in terms of viral spread. To address these gaps in knowledge, the studies described herein sought to determine how psychological stress and associated increases in corticosterone may increase susceptibility to HSV encephalitis by allowing for increased viral titers at the site of initial infection. We have shown that in mice intranasally infected with HSV-1, a cell-mediated immune response occurs in the nasopharyngeal-associated lymphoid tissue (NALT), mediastinal lymph nodes (MLN), and superficial cervical lymph nodes (CLN). However, psychological stress induced by restraint decreased the number of lymphocytes in these tissues in HSV-infected mice. Surprisingly, the effects of this restraint stress on HSV-specific CTL function varied by immune tissue. Increased viral titers were found in the nasal cavity of stressed mice, an observation which correlated with an increased CD8+ cell response in the CLN. These findings led us to extend our studies to also determine the ramifications of decreased numbers of locally derived lymphocytes on viral titers following infection. Using an approach in which the NALT was surgically removed prior to infection, we confirmed that decreased numbers of NALT-derived lymphocytes at the time of infection allows for increased viral replication. We conclude that the increased viral titers observed in mice experiencing psychological stress are the consequence of a glucocorticoid-mediated reduction in the numbers of lymphocytes responsible for resolving the initial infection.

Cellular and molecular themes in apoptosis

Apoptosis, an active mechanism of cell death, is of central importance in many biological scenarios. Research in this area has the potential to contribute to our understanding of many diseases and raises several potential therapeutic opportunities. Given this potential and the speed with which our understanding of this field has advanced over recent years, it is timely to introduce the clinician to the background on which the clinical implications of this research will be built. This review begins with contrasting apoptosis with the other mechanism of cell death, necrosis, and then outlines the features by which apoptosis may be recognised. With a view to understanding the level at which this process may be involved in disease and therapeutics, it is important to be aware of the basic mechanistic features of the induction and execution of apoptosis. In this, surface molecules such as CD95 (Fas) and the cascade of intracellular enzymes involved at many levels in apoptosis, the caspases, are of central importance. In all this, the mitochondrion is crucial to the induction of apoptosis and the regulation of the whole process. In the last part of this review, we attempt to draw out the clinical relevance of all this information. It is clear that apoptosis has an important role in the pathophysiology of malignancy, particularly with respect to haematological cancers, but also other oncological diseases. Apoptosis is also very important in autoimmune disease and viral infection. Finally, it is clear that apoptosis may be manipulated therapeutically to the benefit of patients in various scenarios. This is clearly an exciting area for future development, but one which clearly depends on a thorough mechanistic understanding.

Nuclear translocation of a leukocyte elastase Inhibitor/Elastase complex during staurosporine-induced apoptosis: role in the generation of nuclear L-DNase II activity

Using L1210 murine leukemia cells, we have previously shown that in response to treatment with drugs having different targets, apoptotic cell death occurs through at least two different signaling pathways. Here, we present evidence that nuclear extracts from staurosporine-treated cells elicit DNase II activity that is not detected in nuclear extracts from cisplatin-treated cells. This activity correlates with the accumulation of two nuclear proteins (70 and 30 kDa) which are detected by an anti-L-DNase II antibody. Partial purification of this DNase II activity suggests that the 30-kDa protein could be the nuclease responsible for staurosporine-induced DNA fragmentation. The 70-kDa protein is also recognized by an anti-elastase antibody, suggesting that it carries residues belonging to both L-DNase II and elastase. Since previous findings showed that L-DNase II was generated from the leukocyte inhibitor of elastase, we propose that the 70-kDa protein results from an SDS-stable association between these two proteins and is translocated from the cytoplasm to the nucleus during staurosporine-induced apoptosis.

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.

The detection of enzyme activity following sodium dodecyl sulfate-polyacrylamide gel electrophoresis

More than a hundred different enzymes impinging on aspects of cell function ranging from carbohydrate and lipid metabolism to signal transduction and gene expression to biomolecule degradation have been detected by the assay of their enzymatic activities following SDS-PAGE. The strategies by which this has been accomplished are as varied as the enzymes themselves and offer testimony to the creativeness and ingenuity of life scientists. Assay of enzyme activity following SDS-PAGE is well adapted to identifying the source of catalytic activity in a heterogeneous protein mixture or a heterooligomeric protein (20), or determining if multiple catalytic activities reside in a single polypeptide (60). The alliance of versatile enzyme assay techniques with the molecular resolution of SDS-PAGE offers a powerful means for meeting the increasing demand for the high-throughput screening arising from protein engineering, combinatorial chemistry, and functional genomics.

Reactive oxygen species and intracellular Ca2+, common signals for apoptosis induced by gallic acid

Gallic acid (3,4,5-trihydroxybenzoic acid), a naturally occurring plant phenol, induces cell death in apparently different manners, depending on cell lines. Flow cytometric analysis and agarose gel electrophoresis indicated that internucleosomal breakdown of chromatin DNA was observed in HL-60RG cells but not in dRLh-84, HeLa, and PLC/PRF/5 cells, and that the action of gallic acid was independent of cell cycle. A detailed study of signal transduction revealed that the gallic acid-induced cell death of all cells tested in this study was prevented by treatment with the intracellular thiol antioxidant N-acetyl-L-cysteine, catalase, and the intracellular calcium chelator bis-(o-aminophenoxy)-N,N,N,N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM). However, the effects of ascorbic acid, superoxide dismutase, EGTA, the endonuclease inhibitor zinc sulfate, the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and the NADPH oxidase inhibitor diphenyleneiodonium chloride on cell death were different depending on the cell type, suggesting that the death signal induced by gallic acid was diverse among different cell types, although the production of reactive oxygen species, such as H2O2, and the elevation of intracellular calcium concentration were required as common signals.

Glucocorticoid resistance and the immune function in the immunodeficiency syndrome

Glucocorticoids, the final product of HPA axis, and their receptors (GRs) on mononuclear cells are crucial mediators in the endocrine-immune interaction. An alteration in GRs involving a lower receptor affinity (Kd) for glucocorticoids has been found in a group of advanced AIDS patients, who developed Addisonian symptoms (weakness, weight loss, hypotension, hyponatremia, and intense mucocutaneous melanosis) in spite of hypercortisolism and normal or slightly elevated values of ACTH (AIDS-GR). In these patients, data for the suppression test showed decreased cortisol and ACTH suppression in response to exogenous dexamethasone. The inhibitory effect of dexamethasone on radiolabeled-thymidine incorporation in mononuclear cells from these patients was also reduced. Monocytes of AIDS-GR patients had a receptor Kd of 10.5 +/- 4.2 nmol/l that was higher than that of other AIDS patients (AIDS-C) (2.9 +/- 0.8 nmol/l) and normal subjects (2.0 +/- 0.8 nmol/l: p < 0.01). Correlations were found between plasmatic IFN-alpha and receptor Kd on monocytes of AIDS-GR (r = 0.77). Poly (i)-poly (c)-induced IFN-alpha production by monocytes was inhibited by glucocorticoids in the AIDS-C group and controls (approx. 80% in both groups): The effect was reversed by the receptor antagonist RU-486. By contrast, glucocorticoid did not inhibit IFN-alpha production in AIDS-GR group. In conclusion, levels of plasmatic IFN-alpha, a cytokine with antiviral properties, may be increased several times, and dexamethasone fails to inhibit monocytes' IFN-alpha production only in AIDS with cortisol resistance, a disturbance that confirms an important immunoregulatory role of glucocorticoids in HIV disease.

Androgen ablation leads to an upregulation and intranuclear accumulation of deoxyribonuclease I in rat prostate epithelial cells paralleling their apoptotic elimination

After androgen ablation by castration, the epithelial cells of the rat ventral prostate are eliminated by apoptosis. The number of cells showing apoptotic chromatin degradation increases with time up to day 3 after castration as verified by in situ end labeling of fragmented DNA. Apoptotic chromatin degradation is catalyzed by a Ca2+, Mg2+-dependent endonuclease. Recently, evidence has been presented that suggests deoxyribonuclease I (DNase I) is identical or very closely related to the apoptotic endonuclease (Peitsch, M.C., B. Polzar, H. Stephan, T. Crompton, H.R. MacDonald, H.G. Mannherz, and J. Tschopp. 1993. EMBO [Eur. Mol. Biol. Organ.] J. 12:371-377). Therefore, the expression of DNase I in the ventral prostate of the rat was analyzed before and after androgen ablation at the level of protein, enzymatic activity, and gene transcripts using immunohistochemical and biochemical techniques. DNase I immunoreactivity was detected only in a few single epithelial cells before androgen ablation. After castration, a time-dependent increase in DNase I immunoreactivity was observed within the epithelial cells. It first appeared after about 12 h in the apical region of a large number of epithelial cells. Up to day 3 after castration, the intracellular DNase I antigenicity continuously increased, and the cell nuclei gradually became DNase I positive. At day 5, almost all nuclei of the epithelium were stained by anti-DNase I. DNase I immunoreactivity was particularly concentrated in cells showing morphological signs of apoptosis, like nuclear fragmentation, and in many cases was found to persist in apoptotic bodies. DNase I gene transcripts were detected in control animals using dot and Northern blotting as well as RNase protection assay. After androgen ablation, the amount of DNase I gene transcripts in total extractable RNA was found unchanged or only slightly decreased up to day 5. Their exclusive localization within the epithelial cells was verified by in situ hybridization. Before castration, the DNase I gene transcripts were homogeneously distributed in all epithelial cells. At day 3, DNase I-specific mRNA was found to be highly concentrated in cells of apoptotic morphology. Using the zymogram technique, a single endonucleolytic activity of about 32 kD was detected in tissue homogenates before castration. After androgen ablation, the endonucleolytic activity increased about four- to sevenfold up to day 3. At day 5, however, it had dropped to its original level. At day 1, three new endonucleolytic variants of higher molecular mass were expressed. At day 3, the predominant endonucleolytic activity exhibited an apparent molecular mass of 32 kD. Enzymatic analysis of the endonucleases present in prostate homogenates before and after castration demonstrated properties identical to DNase I. They were inhibited by chelators of divalent cations, Zn2+ ions and monomeric actin. Immunodepletion was achieved by immobilized antibodies specific for rat parotid DNase I. A polyclonal antibody raised against denatured DNase I was shown by Western blotting to stain a 32-kD band after enrichment of the endonuclease from day 0 and 3 homogenates by preparative gel electrophoresis. The data thus indicate that androgen ablation leads to translational upregulation of an endonucleolytic activity with properties identical to DNase I in rat ventral prostate, followed by its intracellular retention and final nuclear translocation in those epithelial cells that are destined to apoptotic elimination.

Use of two-dimensional gel electrophoresis to characterize protein synthesis during neuronal death in cerebellar culture

Two-dimensional gel electrophoresis was performed to further investigate the biochemical changes in protein synthesis observed in two neuronal death models, induced respectively by cytosine arabinoside and glutamate. These drugs induced, respectively, apoptotic and necrotic types of cell death in cerebellar cultures, as previously reported. Most of the proteins showed decreased labeling after toxic exposure, as expected, but some polypeptides showed increased labeling or appeared to be newly synthesized. The identification of these polypeptides and their implication in neuronal death are discussed.

Endo-exonuclease of human leukaemic cells: evidence for a role in apoptosis

Inactive forms of endo-exonuclease, activated in vitro by treatment with trypsin, have been identified in human leukaemic CEM and MOLT-4 cells. They comprise over 95% of the total single-strand DNase activity in nuclei and are mainly bound to chromatin and the nuclear matrix. The activated enzyme had Mg2+(Mn2+)-dependent, Ca(2+)-stimulated activities with single- and double-strand DNAs and RNA (polyriboadenylic acid) and other properties characteristic of endo-exonucleases previously described. At least twice as much inactive endo-exonuclease has also been localised in extranuclear compartments of CEM and MOLT-4 cells, 85% bound to the membranes of the endoplasmic reticulum and 15% free in the cytosol. The soluble cytosolic trypsin-activatable endo-exonuclease was immunoprecipitated by antibodies raised independently to both Neurospora and monkey CV-1 cell endo-exonucleases. The free and bound enzymes of both nuclear and extranuclear compartments also cross-reacted on immunoblots with the antibody raised to Neurospora endo-exonuclease to reveal multiple polypeptides ranging in size from 18 to 145 kDa, many of which exhibited activity on DNA gels. The major species bound to the chromatin/matrix were in the 55–63 kDa range. Limited proteolysis of the large polypeptides to those of 18 to 46 kDa accompanied spontaneous chromatin DNA fragmentation to form DNA “ladders' in an isolated nuclei/cytosol system. When the leukaemic cells were treated in culture with either etoposide or podophyllotoxin to induce apoptosis, the largest polypeptides disappeared and smaller endo-exonuclease-related polypeptides of 18 to 46 kDa were detected in the nuclear extracts. The appearance of these polypeptides also correlated with extensive chromatin DNA fragmentation. In addition, there were correlations between the depletion of the major 55–63 kDa species bound to the membranes of the endoplasmic reticulum, depletion of the extranuclear trypsin-activatable activity and the onset and extent of chromatin DNA fragmentation in both cell lines. The extranuclear 55–63 kDa species may be precursors of the chromatin/matrix bound endo-exonuclease. The results indicate that endo-exonuclease plays a role in chromatin DNA degradation in mammalian cells during apoptosis.

Apoptosis: molecular regulation of cell death

The field of apoptosis is unusual in several respects. Firstly, its general importance has been widely recognised only in the past few years and its surprising significance is still being evaluated in a number of areas of biology. Secondly, although apoptosis is now accepted as a critical element in the repertoire of potential cellular responses, the picture of the intra-cellular processes involved is probably still incomplete, not just in its details, but also in the basic outline of the process as a whole. It is therefore a very interesting and active area at present and is likely to progress rapidly in the next two or three years. This review emphasises recent work on the molecular mechanisms of apoptosis and, in particular, on the intracellular interactions which control this process. This latter area is of crucial importance since dysfunction of the normal control machinery is likely to have serious pathological consequences, probably including oncogenesis, autoimmunity and degenerative disease. The genetic analysis of programmed cell death during the development of the nematode Caenorhabditis elegans has proved very useful in identifying important events in the cell death programme. Recently defined genetic connections between C. elegans cell death and mammalian apoptosis have emphasised the value of this system as a model for cell death in mammalian cells, which, inevitably, is more complex. The signals inducing apoptosis are very varied and the same signals can induce differentiation and proliferation in other situations. However, some pathways appear to be of particular significance in the control of cell death; recent analysis of the apoptosis induced through the cell-surface Fas receptor has been especially important for immunology. Two gene families are dealt with in particular detail because of their likely importance in apoptosis control. These are, first, the genes encoding the interleukin-1 beta-converting enzyme family of cysteine proteases and, second, those related to the proto-oncogene bcl-2. Both of these families are homologous to cell death genes in C. elegans. In mammalian cells the number of members of both families which have been identified is growing rapidly and considerable effort is being directed towards establishing the roles played by each member and the ways in which they interact to regulate apoptosis. Other genes with established roles in the regulation of proliferation and differentiation are also important in controlling apoptosis. Several of these are known proto-oncogenes, e.g. c-myc, or tumour suppressors, e.g. p53, an observation which is consistent with the importance of defective apoptosis in the development of cancer. Viral manipulation of the apoptosis of host cells frequently involves interactions with these cellular proteins. Finally, the biochemistry of the closely controlled cellular self-destruction which ensues when the apoptosis programme has been engaged is also very important. The biochemical changes involved in inducing phagocytosis of the apoptotic cell, for example, allow the process to be neatly integrated within the tissues, under physiological conditions. Molecular defects in this area too may have important pathological consequences.

NF-M (chicken C/EBP beta) induces eosinophilic differentiation and apoptosis in a hematopoietic progenitor cell line

CAAT/enhancer binding proteins (C/EBPs) are transcriptional activators implicated in the differentiation processes of various cell lineages. We have shown earlier that NF-M, the chicken homolog of C/EBP beta, is specifically expressed in myelomonocytic and eosinophilic cells of the hematopoietic system. To investigate the role of NF-M in hematopoietic cell lineage commitment, we constructed a conditional form of the protein by fusing it to the hormone binding domain of the human estrogen receptor. This construct was stably expressed in a multipotent progenitor cell line transformed by the Myb-Ets oncoprotein. We report here that both NF-M-dependent promoter constructs and resident genes could be activated by addition of beta-estradiol to the NF-M-estrogen receptor expressing progenitors. At the same time, we observed a down-regulation of progenitor-specific surface markers and the up-regulation of differentiation markers restricted to the eosinophil and myeloid lineages. In addition to the onset of differentiation, cell death was induced with typical apoptotic features. Our results suggest that NF-M plays an important role in commitment along the eosinophil lineage and in the induction of apoptosis.

Membrane unpacking and the rapid disposal of apoptotic cells

There is evidence that macrophages can recognize phosphatidylserine (PS) on the surface of apoptotic thymocytes, where PS exposure relates to looser packing ('unpacking') of the polar headgroups of the outer lamina, detectable by lipophilic dyes (Fadok, V.A., Voelker, D.R., Campbell, P.A., Cohen, J.J., Bratton, D.L. and Henson, P.M. (1992) J. Immunol. 148, 2207). We have shown that membrane unpacking also occurs in B cells, where this event actually precedes DNA cleavage (Mower, D.A. Jr., Peckham, D.W., Illera, V.A., Fishbaugh, J.K., Stunz, L.L. and Ashman, R.F. (1994) J. Immunol. 152, 4832). This paper demonstrates that the time interval between membrane unpacking (detected as merocyanine 540 binding) and DNA cleavage (detected by flow cytometry of propidium iodide stained nuclei) also occurs in both T cells and thymocytes. The tight coupling of these two apparently distinct events is emphasized by their co-regulation by a variety of agents which accelerate or inhibit apoptosis. One hypothesis to explain the very low numbers of free apoptotic cells seen in vivo is that macrophages can recognize cells with unpacked membranes and destroy them before they cleave their DNA. In support of this hypothesis, we demonstrated that parenteral cycloheximide triggers a wave of apoptosis in the spleen detected by merocyanine 540 as well as by hypodiploid nuclei. Significantly, both parameters returned from peak values at 2 h virtually to normal by 4 h, testifying to the existence of a rapid disposal mechanism in vivo for cells with unpacked membranes as well as hypodiploid nuclei.

Apoptosis in lactating and involuting mouse mammary tissue demonstrated by nick-end DNA labelling

Mammary involution after cessation of milk removal is associated with extensive loss of secretory epithelial cells. Ultrastructural changes and the appearance of oligonucleosomal DNA laddering in ethidium bromide-stained gels indicates that cell loss during involution occurs by apoptosis. In this study, a technique for nick end-labelling of genomic DNA with radiolabelled deoxynucleotide has been used to monitor the induction of programmed cell death in mice after litter removal at peak lactation. This technique proved more sensitive than conventional ethidium bromide staining, and results suggested that apoptosis was induced rapidly by milk stasis, before extensive tissue re-modelling had begun. Oligonucleosomal DNA laddering on agarose gels was detected within 24 h of milk stasis, and increased progressively for at least 4 days. Nick-end labelling also detected laddering before litter removal, suggesting that programmed cell death is a normal feature of the lactating tissue. The DNA end-labelling technique was also adapted for in situ visualisation of apoptotic cells in tissue sections. By this criterion, apoptotic cells were identified in both the secretory epithelium lining the alveoli of the gland and, increasingly with prolonged milk stasis, amongst those sloughed into the alveolar lumen. The results demonstrate the utility of these techniques for study of mammary cell death and suggest that, whilst apoptosis is rapidly induced by milk stasis, it is also a normal physiological event in the lactating mammary gland.

Enhancement of Ca(2+)-dependent endonuclease activity in L1210 cells during apoptosis induced by 1-beta-D-arabinofuranosylcytosine: possible involvement of activating factor(s)

Internucleosomal DNA fragmentation and morphological changes in nuclei typical of apoptosis were observed in L1210 cells incubated with 1.0 micrograms/ml of 1-beta-D-arabinofuranosylcytosine (ara-C). To investigate the mechanisms involved, we examined the activities of endogenous endonucleases in nuclei and cytoplasm. Both fractions of control cells contained Ca(2+)-dependent endonuclease which was capable of mediating internucleosomal DNA fragmentation. The assay system using two kinds of target substrates, i.e., nuclear chromatin of CCRF-CEM cells and naked DNA purified from the same cells, revealed that the activity of Ca(2+)-dependent endonuclease was enhanced in the crude nuclear extracts of cells treated with 1.0 microgram/ml of ara-C for 24 h or 48 h. The activity was extracted more easily from ara-C-treated cells than control cells without sonication of the nuclear fraction. On the other hand, in the cytoplasmic fraction of the cells, the activity towards naked DNA was unchanged, whereas that towards nuclear chromatin was clearly enhanced. These results suggest that internucleosomal DNA fragmentation induced by ara-C treatment is associated with enhancement and activation of constitutively expressed Ca(2+)-dependent endonuclease in L1210 cells.

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.

Localization of deoxyribonuclease I gene transcripts and protein in rat tissues and its correlation with apoptotic cell elimination

The expression and distribution of deoxyribonuclease I (DNase I) in rat parotid gland, kidney, small intestine and keratinized epithelium was further analysed at the level of its mRNA by in situ hybridization and correlated to immunohistochemical results using polyclonal anti-DNase I antibodies. High amounts of DNase I-specific mRNA and immunoreactivity were detected in the parotid gland, kidney and small intestine in agreement with previous immunohistochemical results. In the parotid gland, both the DNase I-specific mRNA and antigenicity were detected within the secretory cells. In the kidney, DNase I gene transcripts were localized in distal tubules and the collecting duct system. In this organ an identical localization of DNase I antigenicity was obtained. In the small intestine only the enterocytes covering the villi were shown to express DNase I-specific mRNA; the highest level being detected within the enterocytes along the lower third of the villi. In contrast, the highest level of immunoreactivity was found in enterocytes covering the middle and upper thirds of the villi. Within the stratified epithelium of the tongue, DNase I gene transcription and protein expression started in lower parts of the stratum spinosum and reached into the stratum granulosum. However, the gradient of DNase I gene transcript expression appeared to be shifted to lower layers of the stratum spinosum when compared to DNase I immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

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?

Evolutionary conservation of function among mammalian, avian, and viral homologs of the Bcl-2 oncoprotein

The bcl-2 gene was originally cloned because of its involvement in B-cell lymphomas and encodes a 25-kD integral membrane protein that has been shown to inhibit programmed cell death (also termed apoptosis) in a wide variety of circumstances. The Epstein-Barr Virus (EBV) also has been implicated in B-cell malignancies and interestingly contains an open reading frame (BHRF-1) predicting a 19-kD protein with 22% homology to Bcl-2. To compare the functions of p26-Bcl-2 and p19-BHRF-1, we stably introduced expression plasmids encoding these proteins into a murine interleukin-3 (IL-3)-dependent hemopoietic cell line, 32D. Removal of IL-3 from cultures of control-transfected 32D cells resulted in internucleosomal DNA cleavage (a hallmark of programmed cell death) and loss of cell survival. In contrast, 32D cells containing high levels of p26-Bcl-2 or p19-BHRF-2 proteins exhibited prolonged survival and markedly delayed DNA degradation under the same conditions of IL-3 deprivation. As a first attempt to determine the functional importance of amino acid sequences that are conserved between the Bcl-2 and BHRF-1 proteins, we used site-specific mutagenesis to replace two conserved cysteine residues with alanines (positions 158 and 219) in the human Bcl-2 protein. Comparisons of the wild-type and cysteine-minus human Bcl-2 proteins in S49 lymphoma cells revealed equivalent ability to block glucocorticoid-induced cell death and DNA fragmentation, indicating that these two conserved cysteines are not critical for Bcl-2 oncoprotein function. Investigations in 32D cells of an avian homolog of Bcl-2 cloned from the chicken also revealed conservation of function with the human Bcl-2 protein, despite the presence of a 48-amino-acid region of divergent sequence. Taken together, these data demonstrate that despite marked differences in their predicted amino-acid sequences, the human, chicken, and EBV versions of Bcl-2 have retained the structural characteristics necessary to interface with pathways involved in the regulation of programmed cell death in murine cells. The findings thus contribute to the mapping of functional domains in Bcl-2 proteins, and raise the possibility that the EBV-encoded p19-BHRF-1 protein may be able to substitute for p26-Bcl-2 in the development of some types of cancer.

Apoptosis. Its significance in cancer and cancer therapy

Apoptosis is a distinct mode of cell death that is responsible for deletion of cells in normal tissues; it also occurs in specific pathologic contexts. Morphologically, it involves rapid condensation and budding of the cell, with the formation of membrane-enclosed apoptotic bodies containing well-preserved organelles, which are phagocytosed and digested by nearby resident cells. There is no associated inflammation. A characteristic biochemical feature of the process is double-strand cleavage of nuclear DNA at the linker regions between nucleosomes leading to the production of oligonucleosomal fragments. In many, although not all of the circumstances in which apoptosis occurs, it is suppressed by inhibitors of messenger RNA and protein synthesis. Apoptosis occurs spontaneously in malignant tumors, often markedly retarding their growth, and it is increased in tumors responding to irradiation, cytotoxic chemotherapy, heating and hormone ablation. However, much of the current interest in the process stems from the discovery that it can be regulated by certain proto-oncogenes and the p53 tumor suppressor gene. Thus, c-myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl-2 has emerged as a new type of proto-oncogene that inhibits apoptosis, rather than stimulating mitosis. In p53-negative tumor-derived cell lines transfected with wild-type p53, induction of the gene has, in rare cases, been found to cause extensive apoptosis, instead of growth arrest. Finally, the demonstration that antibodies against a cell-surface protein designated APO-1 or Fas can enhance apoptosis in some human lymphoid cell lines may have therapeutic implications.

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.

DNA fragmentation during apoptosis is caused by frequent single-strand cuts

One of the hallmarks of apoptosis is the digestion of genomic DNA by an endonuclease, generating a ladder of small fragments of double-stranded DNA. We have examined the nature of the DNA breaks produced in mouse thymocytes triggered to undergo apoptosis by steroids or by stimulation of the T cell receptor. Whereas the typical ladder pattern of oligonucleosomal fragments was observed after agarose gel electrophoresis, numerous single-strand cuts were detected after electrophoresis under denaturing conditions. Single-strand nicks were found to be very frequent in the internucleosomal regions, but also to occur in the core particle-associated DNA. An identical pattern of single-strand nicks was obtained when chromatin DNA was exposed to the single-strand cleaving deoxyribonuclease I. The nicked DNA fragments, extracted from apoptotic thymocytes, were sensitive to the action of S1-nuclease. We propose that DNA fragmentation induced during apoptosis is not due to a double-strand cutting enzyme as previously postulated, but rather is the result of single-strand breaks. This ensures the dissociation of the DNA molecule at sites where cuts are found within close proximity.

Apoptotic death in epithelial cells: cleavage of DNA to 300 and/or 50 kb fragments prior to or in the absence of internucleosomal fragmentation

To date, apoptosis has been characterized biochemically by the production of 180-200 bp internucleosomal DNA fragments resulting from the activation of an endonuclease(s). The principal morphological feature of apoptosis is the condensation of chromatin and it has been assumed that this may reflect the oligonucleosomal fragmentation pattern. We have re-examined this dogma by comparing the biochemical and morphological features of cell death in several epithelial cell types (HT-29-I1 colon adenocarcinoma, CC164 mink lung, DU-145 human prostatic carcinoma and MCF-7 human breast adenocarcinoma) and one mesenchymal cell line (H11ras-R3 ras-transformed rat fibroblasts). Cell death was induced either by serum deprivation, TGF-beta 1 or etoposide, or by leaving cells to reach confluence. Cell death was assessed with respect to detachment from monolayers, morphological changes and DNA integrity. The DNA-binding fluorophore Hoechst 33258 revealed chromatin condensation patterns consistent with apoptotic cell death in all cell types except MCF-7 cells. Using field inversion gel electrophoresis in conjunction with conventional 2% agarose gel electrophoresis, cleavage of DNA to 50 kbp fragments was observed in all cases except MCF-7 cells. This preceded the appearance of oligonucleosomal fragments in HT-29-I1, CC164 and H11ras-R3 cells. Although the DNA of DU-145 cells fragmented into 50 kbp units, and although the cells exhibited classical apoptotic morphology, no subsequent internucleosomal cleavage was observed. These results suggest that changes in the integrity of DNA indicative of the release of chromatin loop domains occur before cleavage at internucleosomal sites is initiated and that the latter is not an essential step in the apoptotic process.

Thymocyte apoptosis: a model of programmed cell death

Recently there has been widespread appreciation for the role of apoptosis, or programmed cell death, in the maintenance of tissue structure and function. Studies in several model systems have revealed that apoptosis is profoundly regulated by a number of diverse hormones, including steroids. The killing of immature thymic lymphocytes by glucocorticoids has emerged as an important model to define the biochemical mechanisms that mediate the programmed cell death process. Using this model, we, and others, have shown that lymphocytes degrade their DNA in response to glucocorticoids. The onset of DNA degradation precedes cell death and is the probable cause of apoptosis. This unique response to endocrine signal transduction will undoubtedly promise new insights into the mechanism of hormone action.