An apoptotic endonuclease activated either by decreasing pH or by increasing calcium

Relationship between intracellular pH, metabolic co-factors and caspase-3 activation in cancer cells during apoptosis

A complex cascade of molecular events occurs in apoptotic cells but cell-to-cell variability significantly complicates determination of the order and interconnections between different processes. For better understanding of the mechanisms of programmed cell death, dynamic simultaneous registration of several parameters is required. In this paper we used multiparameter fluorescence microscopy to analyze energy metabolism, intracellular pH and caspase-3 activation in living cancer cells in vitro during staurosporine-induced apoptosis. We performed metabolic imaging of two co-factors, NAD(P)H and FAD, and used the genetically encoded pH-indicator SypHer1 and the FRET-based sensor for caspase-3 activity, mKate2-DEVD-iRFP, to visualize these parameters by confocal fluorescence microscopy and two-photon fluorescence lifetime imaging microscopy. The correlation between energy metabolism, intracellular pH and caspase-3 activation and their dynamic changes were studied in CT26 cancer cells during apoptosis. Induction of apoptosis was accompanied by a switch to oxidative phosphorylation, cytosol acidification and caspase-3 activation. We showed that alterations in cytosolic pH and the activation of oxidative phosphorylation are relatively early events associated with the induction of apoptosis.

Confocal microscopy as a tool to examine DNA fragmentation, chromatin condensation and other apoptotic changes in Parkinson's disease

There is considerable controversy regarding the possibility that nigral dopaminergic neurons may die via apoptosis in Parkinson's disease. It is now clear that both single- and/or double-stranded DNA breaks can be generated in the apoptotic degradative process. Since these breaks may also be present in necrotic cell death, in situ end labeling cannot be used in isolation to identify apoptotic neurons. We have developed a fluorescent double-labeling method that combines in situ end labeling with the simultaneous visualization of chromatin condensation. When viewed with laser confocal scanning microscopy, the structural detail of the nucleus is provided to unequivocally identify apoptotic nuclei.

Induction of Ca2+ signal mediated apoptosis and alteration of IP3R1 and SERCA1 expression levels by stress hormone in differentiating C2C12 myoblasts

Glucocorticoid (GC) are stress hormones, whose cytotoxicity has been shown in various cells. The imbalance of calcium homeostasis is believed to be associated with the dexamethasone (DEX, a synthetic GC)-induced apoptosis. Here we show that in C2C12 myoblasts, DEX markedly up-regulated the expression of inositol 1,4,5-triphosphate receptor 1 (IP3R1) and down-regulated the expression of SERCA1 (sarcoendoplasmic reticulum Ca(2+)-ATPase 1), leading to calcium overload. Furthermore, the imbalance of calcium homeostasis increased the level of BAX, decreased the level of Bcl-2, induced cytochrome c release and activated caspase-3, leading to intranucleosomal DNA fragmentation and plasma membrane damage, eventually resulting in cell apoptosis. Taken together, by using C2C12 myoblasts as a model system, we demonstrated a novel mechanism for stress hormone-induced apoptosis: it is dependent on the induction of intracellular calcium overload via the alterations of IP3R1 and SERCA1 expressions.

Up-regulation of CD95 (Apo-1/Fas) is associated with spermatocyte apoptosis during the first round of spermatogenesis in the rat

Apoptosis plays a major role in controlling both the rate of sperm production and chromosomal abnormalities in adult male testes. However, little is known on the mechanisms controlling induction and execution of apoptosis under physiological conditions. In this work we have uncovered a major role for the cell death receptor Fas in both the extrinsic and intrinsic pathways in normal germ cell apoptosis. We show here that Fas levels increased significantly in a group of germ cell in 25 d old rats, which were identified as spermatocytes and only a few spermatogonia. In addition, we show that isolated spermatocytes expressing high levels of Fas display activation of caspase-8, -9, -3, -6 and -2, as well as increased levels of intracellular calcium and decreased pH, which coincides with stabilization of p53, and transcriptional activation of PUMA and Fas. Therefore, our data strongly suggests that transcriptional up regulation of Fas could predispose a group of spermatocytes to Fas ligand triggering apoptosis by the extrinsic and intrinsic pathway.

Is there a causal association between genotoxicity and the imposex effect?

There is a growing body of evidence that indicates common environmental pollutants are capable of disrupting reproductive and developmental processes by interfering with the actions of endogenous hormones. Many reports of endocrine disruption describe changes in the normal development of organs and tissues that are consistent with genetic damage, and recent studies confirm that many chemicals classified to have hormone-modulating effects also possess carcinogenic and mutagenic potential. To date, however, there have been no conclusive examples linking genetic damage with perturbation of endocrine function and adverse effects in vivo. Here, we provide the first evidence of DNA damage associated with the development of imposex (the masculinization of female gastropods considered to be the result of alterations to endocrine-mediated pathways) in the dog-whelk Nucella lapillus. Animals (n = 257) that displayed various stages of tributyltin (TBT) -induced imposex were collected from sites in southwest England, and their imposex status was determined by physical examination. Linear regression analysis revealed a very strong relationship (correlation coefficient of 0.935, p < 0.0001) between the degree of imposex and the extent of DNA damage (micronucleus formation) in hemocytes. Moreover, histological examination of a larger number of dog-whelks collected from sites throughout Europe confirmed the presence of hyperplastic growths, primarily on the vas deferens and penis in both TBT-exposed male snails and in females that exhibited imposex. A strong association was found between TBT body burden and the prevalence of abnormal growths, thereby providing compelling evidence to support the hypothesis that environmental chemicals that affect reproductive processes do so partly through DNA damage pathways.

Toxicity of tributyltin in the marine mollusc Mytilus edulis

Our previous studies have demonstrated that tributyltin (TBT) is genotoxic to the early life stages of marine mussels and worms. Here, the toxicity of TBT to adult organisms was determined using a suite of biomarkers designed to detect cytotoxic, immunotoxic and genotoxic effects. Exposure of adult mussels, Mytilus edulis, to environmentally realistic concentrations of TBTO for 7 days resulted in a statistically significant decrease in cell viability at concentrations of 0.5 microg/l and above. TBT had no effect on phagocytic activity or antioxidant capacity (FRAP assay). There was a statistically significant increase in DNA damage detected using the comet and micronucleus assays between the controls and 0.5, 1 and 5 microg/l of TBTO (P > 0.0005). Furthermore there was a strong correlation between DNA strand breaks (comet assay) and formation of micronuclei (P = 0.0005; R2 = 61.5%). Possible mechanisms by which TBT could damage DNA either directly or indirectly are discussed including the possibility that TBT is genotoxic due to its ability to disrupt calcium homeostasis.

Alterations of intracellular pH homeostasis in apoptosis: origins and roles

Intracellular pH (pHi) has an important role in the maintenance of normal cell function, and hence this parameter has to be tightly controlled within a narrow range, largely through the activity of transporters located at the plasma membrane. These transporters can be modulated by endogenous or exogenous molecules as well as, in some pathological situations, leading to pHi changes that have been implicated in both cell proliferation and cell death. Whereas intracellular alkalinization seems to be a common feature of proliferative processes, the precise role of pHi in apoptosis is still unclear. The present review gathers the most recent advances along with previous data on both the origin and the role of pHi alterations in apoptosis and highlights the major concerns that merit further research in the future. Special attention is given to the possible role played by pHi-regulating transporters.

Decreased apoptosis of β2‐ integrin‐deficient bovine neutrophils

Stimulant-induced viability of neutrophils, nuclear-fragmentation, increase in intracellular calcium ([Ca2+]i), expression of annexin V on neutrophils and proteolysis of a fluorogenic peptide substrate Ac-DEVD-MCA (acetyl Asp-Glu-Val-Asp alpha-[4-methyl-coumaryl-7-amide]) by neutrophil lysates from five normal calves and three calves with leucocyte adhesion deficiency were determined to evaluate the apoptosis of normal and CD18-deficient neutrophils. Viability was markedly decreased in control neutrophils stimulated with opsonized zymosan (OPZ), compared to CD18-deficient neutrophils at 37 degrees C after incubation periods of 6 and 24 hours. The rate of apoptosis of control neutrophils stimulated with OPZ increased significantly depending on the incubation time, whereas no apparent increase in apoptosis was found in CD18-deficient neutrophils under the same conditions. Aggregated bovine (Agg) IgG-induced apoptosis of control neutrophils was not significantly different from that of CD18-deficient neutrophils. The expression of annexin V on OPZ-stimulated control neutrophils was greater than that of unstimulated ones 6 h after stimulation. No apparent increase in annexin V expression on CD18-deficient neutrophils was found with OPZ stimulation. A delay in apoptosis was demonstrated in CD18-deficient bovine neutrophils and this appeared to be closely associated with lowered signalling via [Ca2+]i, diminished annexin V expression on the cell surface, and decreased caspase 3 activity in lysates.

Ultrastructural observations on effects of different concentrations of calcium and thyroxine in vitro on larval epidermal cells of Rana catesbeiana tadpoles

During anuran metamorphosis dramatic changes in morphogenesis and differentiation of epidermis occur under the influence of thyroid hormones. Modification of ionic calcium concentration also markedly alters the pattern of proliferation and differentiation in amphibian epidermal cells in vitro. The present study was designed to determine the direct effect of low (0.05 mM) and high (0.5 mM) calcium (Ca2+) in the absence or presence of thyroxine (10(-7) M) on epidermal cells of the body and tail tissue in vitro. When tail fin and body skin explants were maintained in low (0.05 mM) calcium for 48 h, normal ultrastructural morphology and integrity of the cells was observed in both the tissue types. When tissues were exposed to high levels of calcium (0.5 mM) in culture medium, tail epidermis showed stratification, and skein cells exhibited apoptosis, both in the presence or absence of thyroid hormones. Under high calcium conditions, the body epidermis showed keratinization of apical cells, apoptosis of skein cells, and increased desmosome formation. These results suggest that (1) optimal Ca2+ concentration for larval epidermal cells is quite low (0.05 mM), (2) high Ca2+ leads to keratinization only in body epidermis, and (3) apoptosis occurred in skein cells of both the tissues at high Ca2+ concentrations (0.5 mM). The present study therefore suggests that the extracellular calcium concentration regulates the process of cell death and differentiation in Rana catesbeiana larval epidermis, and this effect may be similar to the effect of calcium on mammalian epidermal cells.

The C. elegans apoptotic nuclease NUC-1 is related in sequence and activity to mammalian DNase II

The Caenorhabditis elegans nuc-1 gene has previously been implicated in programmed cell death due to the presence of persistent undegraded apoptotic DNA in nuc-1 mutant animals. In this report, we describe the cloning and characterization of nuc-1, which encodes an acidic nuclease with significant sequence similarity to mammalian DNase II. Database searches performed with human DNase II protein sequence revealed a significant similarity with the predicted C. elegans C07B5.5 ORF. Subsequent analysis of crude C. elegans protein extracts revealed that wild-type animals contained a potent endonuclease activity with a cleavage preference similar to DNase II, while nuc-1 mutant worms demonstrated a marked reduction in this nuclease activity. Sequence analysis of C07B5.5 DNA and mRNA also revealed that nuc-1(e1392), but not wild-type animals contained a nonsense mutation within the CO7B5.5 coding region. Furthermore, nuc-1 transgenic lines carrying the wild-type C07B5.5 locus demonstrated a complete complementation of the nuc-1 mutant phenotype. Our results therefore provide compelling evidence that the C07B5.5 gene encodes the NUC-1 apoptotic nuclease and that this nuclease is related in sequence and activity to DNase II.

Microenvironment-induced cancer metastasis

PURPOSE

To present and evaluate clinical data suggesting that cancer metastasis may be induced by the microenvironment of the primary tumour and to discuss possible mechanisms of microenvironment-induced metastasis, based on a critical review of relevant data from studies of experimental tumours and cells in culture.

CONCLUSIONS

Low oxygen tension in the primary tumour is associated with metastasis in soft tissue sarcoma, cervix carcinoma and carcinoma of the head and neck. Multiple mechanisms may be involved in hypoxia-induced metastasis. Thus, hypoxia followed by reoxygenation may induce point mutations and DNA strand breakage leading to deletions, amplifications and genomic instability. Hypoxia may also provide a physiological pressure in tumours selecting for metastatic cell phenotypes. Moreover, hypoxia may induce a temporary increase in the expression of gene products involved in the metastatic cascade, either through gene amplifications or through normal physiological processes by activating oxygen sensors, hypoxia signal transduction pathways and DNA transcription factors. Low glucose concentration, high lactate concentration and low extracellular pH may induce metastasis by similar mechanisms as hypoxia. Tumour reoxygenation during radiation therapy may promote microenvironment-induced metastasis by rescuing hypoxic or nutritionally deprived metastatic cells from dying. Ionizing radiation can elicit a stress response in tumour cells similar to that elicited by hypoxia. Radiation therapy may therefore adversely affect the rate of metastasis in patients who do not achieve control of the primary tumour by enhancing the expression of gene products of importance in metastasis.

Both necrosis and apoptosis contribute to HIV-1-induced killing of CD4 cells

BACKGROUND

Data currently available on HIV-1-induced cytopathology is unclear regarding the mechanism of cell killing.

OBJECTIVE

To clarify the extent to which apoptosis or necrosis is involved in HIV-1-induced cell death in view of conflicting existing data.

METHODS

T lymphoblastoid cells or peripheral blood mononuclear cells were infected by various strains of HIV-1 and the numbers of apoptotic or necrotic cells were quantified at various times after infection using video-image analysis techniques; the results were compared with the amount of fragmented DNA using a quantitative method. Measurement of mitochondrial transmembrane potential (deltapsi(m)) and intracellular calcium concentrations [Ca2+]i was performed with fluorescent probes and fluorescence concentration analysis (FCA).

RESULTS

Although lymphoblastoid and monocytoid cells acutely infected by HIV-1 had increased levels of fragmented DNA, a marker of apoptotic cell death, few (<12%) had condensed chromatin and fragmented nuclei, the morphological features of apoptosis. The predominant alterations in acutely infected cells were distended endoplasmic reticulum and abnormal mitochondria; these ultrastructural changes are consistent with necrosis, although some infected cells simultaneously displayed features of both necrosis and apoptosis. Viability of cells persistently infected by HIV-1 was only minimally reduced from that of uninfected cells. This reduction was accounted for by an increased propensity of the persistently infected cells to die by apoptosis. Alterations in [Ca2+]i and deltapsi(m) occurred in both acutely and persistently infected cells.

CONCLUSION

Both necrosis and apoptosis contribute to HIV-1-induced killing of CD4 cells.

An acidic environment leads to p53 dependent induction of apoptosis in human adenoma and carcinoma cell lines: implications for clonal selection during colorectal carcinogenesis

As tumours are known to acidify their microenvironment and fluctuations in lumenal pH have been reported in a number of colonic disease conditions, we investigated whether loss of p53 function, commonly associated with the adenoma to carcinoma transition in human colorectal epithelium, was implicated in the cellular response to changes in extracellular pH. Human colonic adenoma and carcinoma derived cell lines were incubated at an initial pH range of 5.5-8.0 and the attached cell yield and apoptotic cell yield determined after 4 days. Exposure of all cell lines to an acidic growth environment was associated with a G1 arrest, down regulation of the retinoblastoma protein (pRb) protein and switch to the hypophosphorylated form of the protein, and increased expression of the p21 protein. However, induction of apoptosis, associated with increased p53 protein expression but not with changes in Bcl-2 expression, was only detected in the adenoma derived BH/C1 and AA/C1 cell lines which express wild type p53 activity. Furthermore, this induction of apoptosis was inhibited in the transfected cell line AA/273p53/B, in which the wild type p53 function has been abrogated. These results suggest that acidification of the microenvironment would provide a selective growth advantage for cells that have lost wild type p53 function, leading to clonal expansion of aberrant cell populations.

Metabolic instability of plasmid DNA in the cytosol: a potential barrier to gene transfer

Inefficient nuclear delivery of plasmid DNA is thought to be one of the daunting hurdles to gene transfer, utilizing a nonviral delivery system such as polycation-DNA complex. Following its internalization by endocytosis, plasmid DNA has to be released into the cytosol before its nuclear entry can occur. However, the stability of plasmid DNA in the cytoplasm, that may play a determinant role in the transfection efficiency, is not known. The turnover of plasmid DNA, delivered by microinjection into the cytosol, was determined by fluorescence in situ hybridization (FISH) and quantitative single-cell fluorescence video-image analysis. Both single- and double-stranded circular plasmid DNA disappeared with an apparent half-life of 50-90 min from the cytoplasm of HeLa and COS cells, while the amount of co-injected dextran (MW 70,000) remained unaltered. We propose that cytosolic nuclease(s) are responsible for the rapid-degradation of plasmid DNA, since (1) elimination of plasmid DNA cannot be attributed to cell division or to the activity of apoptotic and lysosomal nucleases; (2) disposal of microinjected plasmid DNA was inhibited in cytosol-depleted cells or following the encapsulation of DNA in phospholipid vesicles; (3) generation and subsequent elimination of free 3'-OH ends could be detected by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay (TUNEL), reflecting the fragmentation of the injected DNA; and finally (4) isolated cytosol, obtained by selective permeabilization of the plasma membrane, exhibits divalent cation-dependent, thermolabile nuclease activity, determined by Southern blotting and 32P-release from end-labeled DNA. Collectively, these findings suggest that the metabolic instability of plasmid DNA, caused by cytosolic nuclease, may constitute a previously unrecognized impediment for DNA translocation into the nucleus and a possible target to enhance the efficiency of gene delivery.

Metabotropic glutamate receptors prevent programmed cell death through the modulation of neuronal endonuclease activity and intracellular pH

Metabotropic glutamate receptor (mGluR) activation prevents neurodegeneration against nitric oxide (NO)-induced programmed cell death (PCD). We therefore investigated whether specific neuronal endogenous deoxyribonucleases, enzymes recently identified to be responsible for the maintenance of DNA integrity, mediated mGluR protection against NO. In rat primary hippocampal neurons, injury was assessed by using a 0.4% trypan blue dye exclusion method and TUNEL assay 24 h following treatment with the NO generators sodium nitroprusside (300 microM) or SIN-1 (300 microM). DNA digestion studies using neuronal cell extracts were employed to assess specific endonuclease activity. Individual application of aurintricarboxylic acid (ATA) (10 microM), an endonuclease inhibitor, or the mGluR agonists 1S,3R-ACPD (750 microM), DHPG (750 microM), L-CCG-I (750 microM), or L-AP4 (750 microM) prior to NO exposure significantly increased neuronal survival. Yet, combination therapy with ATA (10 microM) and the mGluR agonists did not synergistically improve neuronal survival, suggesting a common pathway of protection for ATA and the mGluRs that is dependent upon the modulation of neuronal endonuclease activity. In further support of this premise, protection by the mGluR agonists 1S,3R-ACPD, DHPG, L-CCG-I, and L-AP4 was significantly decreased during enhancement of endonuclease activity with the zinc chelator, N,N,N',N',-tetrakis (2-pyridylmethyl) ethylenediamine. Antagonism of the mGluR system was ineffective against endonuclease induced DNA destruction. Further assessment with DNA digestion assays identified two distinct mechanisms to maintain DNA integrity, a Ca2+/Mg2+-dependent endonuclease inhibited by L-AP4 and a magnesium dependent endonuclease inhibited by 1S,3R-ACPD. These neuroprotective mechanisms during activation of the mGluR system were also intricately linked to the active reversal of the biphasic intracellular pH changes induced by NO. Further investigation into the molecular pathways modulated by mGluRs may identify specific mechanisms that can maintain DNA integrity during adverse cellular environments.

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.

G protein coupled receptor signaled apoptosis is associated with activation of a cation insensitive acidic endonuclease and intracellular acidification

Apoptosis associated oligonucleosomal fragmentation of DNA can result from the activation of endonucleases that exhibit different pH optima and are either sensitive or insensitive to divalent cations. DNA fragmentation due to activation of cation sensitive endonucleases occurs in the absence of a change in intracellular pH whereas intracellular acidification is a feature of apoptosis characterized by activation of cation insensitive acidic endonuclease. We have reported earlier that somatostatin (SST) induced DNA fragmentation and apoptosis is signaled in a receptor subtype selective manner uniquely via human somatostatin receptor subtype 3 (hSSTR3). In the present study we investigated the pH dependence and cation sensitivity of endonuclease induced in hSSTR3 expressing CHO-K1 cells by the SST agonist octreotide (OCT) and its effect on intracellular pH. We show that OCT induced apoptosis is associated with selective stimulation of a divalent cation insensitive acidic endonuclease. The intracellular pH of of cells undergoing OCT induced apoptosis was 0.9 pH units lower than that of control cells. The effect of OCT on endonuclease and pH was inhibited by orthovanadate as well as by pretreatment with pertussis toxin, suggesting that hSSTR3 initiated cytotoxic signaling is protein tyrosine phosphatase mediated and is G protein dependent. These findings suggest that intracellular acidification and activation of acidic endonuclease mediate wild type p53 associated apoptosis signaled by hormones acting via G protein coupled receptors.

A cytofluorometric assay of nuclear apoptosis induced in a cell-free system: application to ceramide-induced apoptosis

Purified nuclei exposed to apoptogenic factors in vitro undergo morphological and biochemical changes in chromatin organization. Most cell-free models of nuclear apoptosis are based on the quantitation of endonuclease-mediated DNA fragmentation on agarose gels or on the changes of nuclear morphology revealed by the DNA-intercalating fluorochrome 4'-6-diamidino-2-phenylindole dihydrochloride. In this work we develop a cytofluorometric system for the accurate quantitation of nuclear DNA loss. This system has been used to determine the conditions of nuclear apoptosis induced by apoptosis-inducing factor (AIF) contained in the supernatant of mitochondria induced to undergo permeability transition. AIF can provoke significant nuclear DNA loss in < or = 5 min, acts over a wide pH range (pH 6 to 9), and resists cysteine protease inhibitors such as iodoacetamide and N-ethylmaleimide. Moreover, we applied this system to the question of how the proapoptotic second messenger ceramide would induce apoptosis in vitro: via a direct effect on nuclei, a direct effect on mitochondria, or via indirect mechanisms? Our data indicate that ceramide has to activate yet unknown cytosolic effectors that, in the presence of mitochondria, can induce nuclear apoptosis in vitro.

Gene expression induced by cerebral ischemia: an apoptotic perspective

The flow of new information on gene expression related to apoptosis has been relentless in the last several years. This has also been the case with respect to gene expression after cerebral ischemia. Many of genes associated with an apoptotic mode of cell death have now been studied in the context of experimental cerebral ischemia from the immediate early genes through modulating genes such as bcl-2 to genes in the final execution phase such as interleukin-1β converting enzyme (ICE)-related proteases. It was impossible to adequately cite all primary reports on these subjects. However, many excellent reviews have appeared in the last year, which together, cover all these areas of interest. In this review, we have elected to cite only reports published since January 1996 and use an extensive collection of reviews (indicated in italics) to guide the reader to the earlier literature. Our intent is to provide the reader with a timely and useful analysis of the current state of the art. It is hoped that this approach does not cause offense with our colleagues whose contributions before 1996 laid the foundation for much of this work.

Intracellular acidification induces apoptosis by stimulating ICE-like protease activity

ICE-like protease activation and DNA fragmentation are preceded by a decrease in intracellular pH (pHi) during apoptosis in the IL-3 dependent cell line BAF3. Acidification occurs after 7 hours in cells deprived of IL-3 and after 4 hours when cells are treated with etoposide, close to the time of detection of ICE-like protease activity. Increasing extracellular pH reduces ICE-like protease activation and DNA fragmentation. Bcl-2 over-expression both delays acidification and inhibits ICE-like protease activation. Generation of a rapid intracellular pH decrease, using the ionophore nigericin, induces ICE-like protease activation and apoptosis. ZVAD, a cell permeable inhibitor of ICE-like proteases, does not affect acidification but inhibits apoptosis induced by IL-3 removal or nigericin treatment. These data suggest that intracellular acidification triggers apoptosis by directly or indirectly activating ICE-like proteases.