Endogenous nuclease. Properties and effects on transcribed genes in chromatin

Calcium signalling and the regulation of apoptosis

Apoptotic cell death is characterized by cell shrinkage, chromatin condensation and fragmentation, formation of apoptotic bodies and phagocytosis (Kerr et al., 1972). At the molecular level, activation of a family of cysteine proteases, caspases, related to interleukin-1beta-converting enzyme is believed to be a crucial event in apoptosis. This is associated with the proteolysis of nuclear and cytoskeletal proteins, cell shrinkage, glutathione efflux, exposure of phosphatidylserine on the cell surface, membrane blebbing, etc. In CD95- or TNF-mediated apoptosis, the proteolytic cascade is believed to be triggered directly by caspase binding to the activated plasma membrane receptor complex. In other forms of apoptosis, the mechanisms of activation of the proteolytic cascade are less well established but may involve imported proteases, such as granzyme B, or factors released from the mitochondria and, possibly, other organelles. Recently, the possibility that cytochrome c released from the mitochondria may serve to activate dormant caspases in the cytosol, and thereby to propagate the apoptotic process, has attracted considerable attention. A perturbation of intracellular Ca(2+) homeostasis has been found to trigger apoptosis in many experimental systems, and the apoptotic process has been related to either a sustained increase in cytosolic free Ca(2+) level or a depletion of intracellular Ca(2+) stores. Although many of the biochemical events involved in the apoptotic process are Ca(2+) dependent, the exact mechanism by which Ca(2+) triggers apoptosis remains unknown. The bcl-2 gene family, which includes both inhibitors and inducers of apoptosis, appears to regulate intracellular Ca(2+) compartmentalization. The induction of apoptosis by Ca(2+)-mobilizing agents results in caspase activation, which is similar to what is seen with other inducers of apoptosis. In addition, Ca(2+)-dependent proteases, such as calpain and a Ca(2+)-dependent nuclear scaffold-associated serine protease, are also activated by Ca(2+) signalling in some cell types where they appear to be involved in alpha-fodrin and lamin beta cleavage, respectively. Thus, a spectrum of proteases are activated during apoptosis depending on both cell type and inducer. This proteolytic cascade can involve both caspases and Ca(2+)-dependent proteases, which seem to interact during the apoptotic process.

Rapid assay for quantitative measurement of apoptosis in cultured cells and brain tissue

Analysis of apoptosis in brain tissue following ischemia, hypoxia, or oxidative stress has technical limitations. The use of counting cells displaying apoptotic morphology is time intensive, vulnerable to sampling errors, and suffers from low numbers of total recorded events. Other cell death assays such as agarose gel analysis of DNA fragmentation, TUNEL, or ELISA are time intensive, limited to a single endpoint measure, and can be technically difficult to perform or reproduce. To overcome these limitations, we set out to develop a technique using flow cytometry to measure apoptosis based on the physical properties of light scatter produced from isolated nuclei. This dye/marker free approach would bypass many of the inherent encumbrances and reproducibility problems found in other apoptosis assays. Here we demonstrate that this new technique, using flow cytometry performed on isolated nuclei, allows rapid quantitation of apoptosis in a variety of brain tissues without the need for intercalating dyes or fluorescent markers. We conclude that this technique significantly improves currently available protocols to quantify apoptosis from tissue and offers the possibility to perform additional analysis on the same population of nuclei via downstream assays.

Characterization of an Mg2+-dependent endonucleolytic activity of the rat hepatocyte nuclear matrix

Initial degradation of chromatin into high-molecular mass DNA fragments during apoptosis reflects the periodicity of chromatin organization into nuclear matrix-attached loops. In this article, we put forward the hypothesis that this pattern of DNA cleavage is also a result of the localization of an endonuclease on the nuclear matrix. Namely, we observed an endonucleolytic activity of the isolated rat hepatocyte nuclear matrix. It was Mg2+-dependent, with an optimal activity at pH 7.2 in the absence of either Na+ or K+. It was fully active in the presence of Zn2+ and capable of introducing single-strand breaks into plasmid DNA. It did not display a sequence-specific activity. A 23 kDa DNA nuclease that was principally localized on the rat hepatocyte nuclear matrix was detected. The enzyme shared the biochemical requirements with the nuclear matrix endonucleolytic activity, thus we proposed that p23 could be responsible for the endonucleolytic activity of the nuclear matrix. In view of its properties and preferential localization on the nuclear matrix, the endonuclease described herein could be a possible candidate that brings about initial DNA cleavage during apoptosis.

D-galactosamine induced hepatocyte apoptosis is inhibited in vivo and in cell culture by a calcium calmodulin antagonist, chlorpromazine, and a calcium channel blocker, verapamil

Studies were conducted in C57BL/6N Crj male mice and in cultured hepatocytes to clarify the relationship between galactosamine (GaIN) induced apoptosis and [Ca2+]i kinetics. Chlorpromazine (CPZ), a Ca(2+)-calmodulin antagonist, and verapamil (VR), a Ca(2+)-channel blocker each inhibited GaIN-induced DNA fragmentation and the appearance of apoptotic bodies. The kinetics of calcium uptake were evaluated using a calcium analyzer with the acetoxymethyl ester of fura-PE3 (fura-PE3/AM, 2.5 microM) as the calcium reporter. An increase in [Ca2+]i was detected in the cultured hepatocytes within 3 hours after treatment with 20 mM GaIN; this increase was inhibited by pretreatment with either 20 microM CPZ or 30 microM VR. Ca2+ imaging by confocal laser scanning microscopy showed that increase in [Ca2+]i after treatment with GaIN was initially localized around nuclei, while [Ca2+]i signals were later diffuse and observed throughout the cytoplasm. The activities of lactate dehydrogenase (LDH) and serum glutamate-pyruvate transaminase (sGPT), used as indicators of plasma membrane damage and leakage, however, were not reduced by pretreatment with CPZ or VR. From these findings, we infer that the DNA fragmentation in GaIN-induced hepatocyte apoptosis is associated with an elevation in the perinuclear concentration of Ca2+, but GaIN-induced necrotic cell death is triggered through pathway(s) that are insensitive to blockage of Ca2+ influx and therefore appear to occur independently of elevation in [Ca2+]i. These results help to clarify the role of calcium flux in hepatocyte apoptosis and necrosis induced by exposure to hepatotoxins in vivo and in vitro.

Cytotoxic mechanism of Vibrio vulnificus cytolysin in CPAE cells

Vibrio vulnificus is an estuarian bacterium that causes septicemia and serious wound infection. The cytolysin, one of the important virulence determinants in V. vulnificus infection, has been reported to have lethal activity primarily by increasing pulmonary vascular permeability. In the present study, we investigated the cytotoxic mechanism of V. vulnificus cytolysin in cultured pulmonary artery endothelial (CPAE) cells, which are possible target cells of cytolysin in vivo. V. vulnificus cytolysin caused the CPAE cell damages with elevation of the cytosolic free Ca2+, DNA fragmentation, and decrease of the cellular NAD+ and ATP level. These cytotoxic effects of V. vulnificus cytolysin were prevented by EGTA and aminobenzamide, but were not affected by verapamil or catalase. These results indicate that the elevation of cytosolic free Ca2+ induced by V. vulnificus cytolysin causes the increase of DNA fragmentation and the damaged DNA activates nuclear poly(ADP-ribose) synthetase, which depletes the cellular NAD+ and ATP, resulting in cell death.

Calcium flux measurements in apoptosis

Early studies in apoptosis implicated an increase in cytosolic Ca2+ as a direct mediator of DNA fragmentation. However, efforts to delineate targets for this increase in Ca2+ have been slow in evolving. Several previous studies have implicated ER Ca2+ pool depletion in the initiation of apoptosis. Our own preliminary studies confirm that many (but not all) apoptotic stimuli empty the ER store via a mechanism that is blocked by BCL-2 expression. Furthermore, ER pool depletion is not affected by broad spectrum caspase inhibitors, indicating that it occurs via a caspase-independent mechanism. Finally, our data demonstrate that ER pool depletion occurs prior to release of cytochrome c from mitochondria. Given previous work demonstrating close coordination of ER and mitochondrial Ca2+ levels, we speculate that ER-dependent changes in mitochondrial Ca2+ serve as important signals for cytochrome c release. Alternative mechanisms include activation of caspase-12 and/or the JNK pathway, both of which can be directly stimulated by depletion of the ER Ca2+ pool. Although substantial improvements in intracellular Ca2+ imaging have emerged, compelling answers to many of the present questions related to the role of Ca2+ in apoptosis await future technical improvements. The development of organelle-specific, recombinant Ca2+ probes (targeted aequorins and cameleons) certainly should facilitate some of this work, although the target cell of interest must be amenable to molecular manipulation (transfection), which precludes straightforward analysis of primary cells. Pharmacological tools (i.e., thapsigargin and DBHQ) can provide conclusive data on ER pool status without requiring an overly sophisticated image analysis system. However, confocal microscopy allows for the effective analysis of Ca2+ pools as long as dye localization is homogeneous and properly controlled. However, current techniques should be considered semiquantitative at best and will remain so until specific organelle-targeted fluorescent dyes are developed and widely available.

Incorporation of sodium channel blocking and free radical scavenging activities into a single drug, AM-36, results in profound inhibition of neuronal apoptosis

AM-36 is a novel neuroprotective agent incorporating both antioxidant and Na(+) channel blocking actions. In cerebral ischaemia, loss of cellular ion homeostasis due to Na(+) channel activation, together with increased reactive oxygen species (ROS) production, are thought to contribute to neuronal death. Since neuronal death in the penumbra of the ischaemic lesion is suggested to occur by apoptosis, we investigated the ability of AM-36, antioxidants and Na(+) channel antagonists to inhibit toxicity induced by the neurotoxin, veratridine in cultured cerebellar granule cells (CGC's). Veratridine (10 - 300 microM) concentration-dependently reduced cell viability of cultured CGC's. Under the experimental conditions employed, cell death induced by veratridine (100 microM) possessed the characteristics of apoptosis as assessed by morphology, TUNEL staining and DNA laddering on agarose gels. Neurotoxicity and apoptosis induced by veratridine (100 microM) were inhibited to a maximum of 50% by the antioxidants, U74500A (0.1 - 10 microM) and U83836E (0.03 - 10 microM), and to a maximum of 30% by the Na(+) channel blocker, dibucaine (0.1 - 100 microM). In contrast, AM-36 (0.01 - 10 microM) completely inhibited veratridine-induced toxicity ( IC(50) 1.7 (1.5 - 1.9) microM, 95% confidence intervals (CI) in parentheses) and concentration-dependently inhibited apoptosis. These findings suggest veratridine-induced toxicity and apoptosis are partially mediated by generation of ROS. AM-36, which combines both Na(+) channel blocking and antioxidant activity, provided superior neuroprotection compared with agents possessing only one of these actions. This bifunctional profile of activity may underlie the potent neuroprotective effects of AM-36 recently found in a stroke model in conscious rats.

A role of the Ca2+/Mg2+-dependent endonuclease in apoptosis and its inhibition by Poly(ADP-ribose) polymerase

Apoptosis is characterized by various cell morphological and biochemical features, one of which is the internucleosomal degradation of genomic DNA. The role of the human chromatin-bound Ca(2+)- and Mg(2+)-dependent endonuclease (CME) DNAS1L3 and its inhibition by poly(ADP-ribosyl)ation in the DNA degradation that accompanies apoptosis was investigated. The nuclear localization of this endonuclease is the unique feature that distinguishes it from other suggested apoptotic nucleases. Purified recombinant DNAS1L3 was shown to cleave nuclear DNA into both high molecular weight and oligonucleosomal fragments in vitro. Furthermore, exposure of mouse skin fibroblasts expressing DNAS1L3 to inducers of apoptosis resulted in oligonucleosomal DNA fragmentation, an effect not observed in cells not expressing this CME, as well as in a decrease in cell viability greater than that apparent in the control cells. Recombinant DNAS1L3 was modified by recombinant human poly(ADP-ribose) polymerase (PARP) in vitro, resulting in a loss of nuclease activity. The DNAS1L3 protein also underwent poly(ADP-ribosyl)ation in transfected mouse skin fibroblasts in response to inducers of apoptosis. The cleavage and inactivation of PARP by a caspase-3-like enzyme late in apoptosis were associated with a decrease in the extent of DNAS1L3 poly(ADP-ribosyl)ation, which likely releases DNAS1L3 from inhibition and allows it to catalyze the degradation of genomic DNA.

Cyclothiazide and GYKI 52466 modulate AMPA receptor-mediated apoptosis in cortical neuronal cultures

In neocortical neuronal cultures, (S)-AMPA caused neurotoxicity which was concentration-dependent, receptor-mediated, slow and apoptotic in nature. (S)-AMPA (3-600 microM) failed to produce rapid neuronal swelling, but morphological observations and monitoring of viability at 24-72 h revealed 50% cell death consistent with apoptosis. (S)-AMPA induced cell shrinkage, neurite blebbing and nuclear condensation. Cyclothiazide (50 and 100 microM), which blocks AMPA receptor desensitization potentiated excitotoxicity with 75% of neurones undergoing slow death. The AMPA-selective antagonist GYKI 52466 (10-50 microM), attenuated (S)-AMPA-mediated neurotoxicity. DNA condensation, a hallmark of apoptosis, was found by labelling neurones with the DNA binding dye 4,6-diamidino-2-phenylindole HCl (DAPI). Gel electrophoresis revealed DNA fragmentation, which was increased by cyclothiazide and reduced by GYKI 52466 and cycloheximide. Overstimulation of the AMPA receptor produces a novel form of neuronal death, which is apoptotic, very slow in nature, and which could contribute to various neuropathologies.

Role of DNAS1L3 in Ca2+- and Mg2+-dependent cleavage of DNA into oligonucleosomal and high molecular mass fragments

Ca2+- and Mg2+-dependent endonucleases have been implicated in DNA fragmentation during apoptosis. We have demonstrated that particular nucleases of this type are inhibited by poly(ADP-ribosyl)ation and suggested that subsequent cleavage of PARP by caspase-3 might release these nucleases from poly(ADP-ribosyl)ation-induced inhibition. Hence, we purified and partially sequenced such a nuclease isolated from bovine seminal plasma and identified human, rat and mouse homologs of this enzyme. The extent of sequence homology among these nucleases indicates that these four proteins are orthologous members of the family of DNase I-related enzymes. We demonstrate that the activation of the human homolog previously specified as DNAS1L3 can induce Ca2+- and Mg2+-dependent DNA fragmentation in vitro and in vivo. RT-PCR analysis failed to detect DNAS1L3 mRNA in HeLa cells and nuclei isolated from these cells did not exhibit internucleosomal DNA fragmentation when incubated in the presence of Ca2+and Mg2+. However, nuclei isolated from HeLa cells that had been stably transfected with DNAS1L3 cDNA underwent such DNA fragmentation in the presence of both ions. The Ca2+ionophore ionomycin also induced internucleosomal DNA degradation in transfected but not in control HeLa cells. Transverse alternating field electrophoresis revealed that in nuclei from transfected HeLa cells, but not in those from control cells, DNA was cleaved into fragments of >1000 kb in the presence of Mg2+; addition of Ca2+in the presence of Mg2+resulted in processing of the >1000 kb fragments into 50 kb and oligonucleosomal fragments. These results demonstrate that DNAS1L3 is necessary for Ca2+- and Mg2+-dependent cleavage of DNA into both oligonucleosomal and high molecular mass fragments in specific cell types.

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.

Biochemical determinants of apoptosis and necrosis

Although apoptosis and necrosis were originally thought to be entirely distinct mechanisms of cell death, recent work has shown that the processes are regulated by many of the same biochemical intermediates, most notably the levels of cellular ATP, Ca2+, reactive oxygen species, and thiol antioxidants. Beyond a certain threshold, it appears that stress-induced changes in these modulators 'switches' the cell death mechanism from apoptosis to necrosis. Importantly, even when this occurs, cell death can be attenuated by bcl-2 and caspase inhibitors, which are known for their abilities to block apoptosis. This review will summarize these observations within the context of what is currently known about the effector machinery for apoptotic cell death, and possible mechanistic explanations for the switch between apoptosis and necrosis will be provided.

Disassembly of genome of higher eukaryotes: pulsed-field gel electrophoretic study of initial stages of chromatin and DNA degradation in rat liver and thymus nuclei by VM-26 and selected proteases

Nuclei were treated with VM-26, topoisomerase II inhibitor, or exogenous proteases. Initial stages of multi-step chromatin degradation were analyzed by pulsed-field gel (PFG) electrophoresis. VM-26 induced the total genome breakdown into discrete chromatin fragments containing 0.3 Mbp DNA. Treatment of nuclei with recombinant cathepsin B and chymotrypsin also revealed 0.3-Mbp DNA fragments which were mediated by endonucleolysis. Longer incubation of nuclei with chymotrypsin exhibited the appearance of 0.05-Mbp DNA fragments and their oligomers. Proteases might trigger the detachment of chromatin from nuclear matrix and contribute to the release or activation of the endonucleolytic activity leading to the initial degradation of the DNA into high-molecular weight fragments. The fragments observed correspond in size to the suggested chromatin higher-order structures. The presented data imply that the initial stages of chromatin degradation represents in reality the genome disassembly into two basic units of genome topology--0.3-Mbp DNA chromatin loop-domains which are presumably the bits of biological information, and fundamental 0.05-Mbp DNA loopsize units which might generate the functional loops of different sizes.

Trace elements and apoptosis

It is known that apoptosis is considered to be responsible for selective deletion of cells during embryogenesis, the homeostasis of cell populations in continuously renewing tissues (i.e., serving as a counterbalance to mitosis), and tissue involution in response to chemical or physical stimuli. There are many publications on these questions. On the other hand, the intracellular processes that contribute to apoptosis are incompletely understood. Therefore, the role of apoptosis in the intracellular accumulation and outflow of minerals is of considerable importance in light of both their essential functions and toxic effects.

Intracellular calcium distribution in apoptosis of HL-60 cells induced by harringtonine: intranuclear accumulation and regionalization

Harringtonine (HT), an anticancer drug with high chemotherapeutic efficiency to human chronic granulocytic/myelomonocytic leukemia, has been reported to rapidly induce apoptosis in HL-60 cells in a wide scope/range of dosage by investigators from our lab and others. In the present studies, by using video enhancement contrast (VEC) microscopy, we dynamically analyzed changes in intracellular calcium distribution in a single HL-60 cell over the period from the initiation of apoptosis to the obvious appearance of chromatin condensation. The results from this paper demonstrated the striking distinction of intracellular calcium distribution at different time points after treatment with HT. Before treatment in normal HL-60 cells the highest [Ca2+]i accumulation was observed in the peri-nuclear area and the lowest was observed in the nucleus; after treatment with 1 microg/ml HT for 30 min intracellular calcium diffused all over the cell compartments, while intranuclear calcium increased comparatively and significantly. The phenomenon of intranuclear calcium accumulation was further confirmed by using laser scanning confocal microscopy (LSCM). In addition, co-localization of the highest calcium region with condensed chromatin in apoptotic HL-60 cells was also observed by LSCM. Our results suggest that two sequential alterations of intracellular calcium distribution occurred in apoptotic HL-60 cells induced by HT, i.e. (a) accumulation of calcium in the nucleus and (b) regionalization in a specific nuclear region.

The role of calcium in the regulation of apoptosis

Apoptosis (programmed cell death) has gained widespread attention due to its roles in a variety of physiological and pathological processes, yet precisely how apoptosis is regulated by external and internal cues remains unclear. Work from our laboratories and others has implicated alterations in intracellular Ca2+ in apoptosis, and more recent work has defined particular biochemical processes that are targeted by Ca2+ in apoptotic cells. This review will summarize the role of Ca2+ in apoptosis within the context of what is known about the core components of the effector machinery for apoptosis.

Calcium-dependent, interleukin 1-converting enzyme inhibitor-insensitive degradation of lamin B1 and DNA fragmentation in isolated thymocyte nuclei

Recent work suggests that the proteolytic degradation of the nuclear lamins is a common event in apoptosis, although the nature of the proteases involved is still not clear. Our previous work showed that the degradation of lamin B1 in glucocorticoid-treated thymocytes occurs via a Ca2+-sensitive mechanism and that exogenous Ca2+ promotes lamin degradation in isolated thymocyte nuclei from untreated cells. Here we demonstrate that peptide-based inhibitors of the interleukin 1beta-converting enzyme family of cysteine proteases (Tyr-Val-Ala-Asp fluoromethyl ketone) and of the nuclear scaffold multicatalytic proteinase (Ala-Pro-Phe chloromethyl ketone) block the degradation of lamin B1 to a 21-kDa fragment in thymocytes treated with glucocorticoid, the Ca2+-mobilizing agent thapsigargin, or antibodies to the T cell receptor. However, among a panel of inhibitors specific for several different proteases implicated in apoptosis, only tosylphenylalanyl chloromethyl ketone and the nuclear scaffold protease inhibitor block lamin degradation, histone H1 cleavage, and DNA fragmentation in isolated thymocyte nuclei incubated with Ca2+. Overexpression of human BCL-2 in nuclei by stable transfection resulted in an inhibition of Ca2+-stimulated lamin degradation and DNA fragmentation, suggesting that endogenous nuclear BCL-2 regulates activation of the nuclear scaffold protease. The results demonstrate the existence of an alternative pathway of lamin degradation and DNA fragmentation mediated by a resident Ca2+-stimulated nuclear protease that is not directly dependent upon activation of the interleukin 1beta-converting enzyme family of cell death regulators.

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

DNA fragmentation in isolated nuclei from the murine IL3-dependent bone marrow cell line BAF3 could be stimulated either by decreasing pH below 6.5 or by adding microM calcium at neutral pH. An endonuclease which could also be stimulated either by a decrease in pH, to 6.5, or by the presence of microM calcium at neutral pH, was purified 10(4)-fold from nuclei of BAF3 cells. Digestion of DNA with the purified enzyme resulted in 5′-terminal hydroxyl and 3′-terminal phosphate ends. These characteristics are distinct from those described for other mammalian endonucleases. The possible role of this enzyme in genome digestion during apoptosis is discussed.

Apoptosis: a programmed cell death involved in ovarian and uterine physiology

Apoptosis is a form of programmed cell death which occurs through the activation of a cell-intrinsic suicide machinery. The biochemical machinery responsible for apoptosis is expressed in most, if not all, cells. Contrary to necrosis, an accidental form of cell death, apoptosis does not induce inflammatory reaction noxious for the vicinity. Apoptosis is primarily a physiologic process necessary to remove individual cells that are no longer needed or that function abnormally. Apoptosis plays a major role during development, homeostasis. Many stimuli can trigger apoptotic cell death, but expression of genes can modulate the sensibility of the cell. The aim of this review is to summarise current knowledge of the molecular mechanisms of apoptosis and its roles in human endometrium and ovary physiology.

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

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

Regulation of transforming growth factor-beta 1-mediated apoptosis in head and neck squamous cell carcinoma

The apoptotic cell death in Cal-27 cells induced by exposure to transforming growth factor-beta 1 was inhibited by the endonuclease inhibitor aurintricarboxylic acid (ATA) in a concentration-dependent fashion. In vitro studies of cytotoxicity, DNA fragmentation, and protein synthesis by Cal-27 cell lines were performed. Inhibition of cytotoxicity as well as endonucleolytic DNA cleavage was detected. ATA did not inhibit cytotoxicity either via transforming growth factor cell-surface-receptor alteration or by inhibition of macromolecular synthesis. ATA-sensitive events occurred late during treatment. These data suggest that endonucleolytic DNA cleavage is a mandatory event leading to cell death in this system.

Apoptosis--molecular mechanisms and biomedical implications

Apoptosis is a distinct form of cell death of importance in tissue development and homeostasis and in several diseases. This review summarizes current knowledge about the regulation and molecular mechanisms of apoptosis and discusses the potential role of disregulated apoptosis in several major diseases. Finally, we speculate that modulation of apoptosis may be a target in future drug therapy.

Interfering with apoptosis: Ca(2+)-binding protein ALG-2 and Alzheimer's disease gene ALG-3

Two apoptosis-linked genes, named ALG-2 and ALG-3, were identified by means of a functional selection strategy. ALG-2 codes for a Ca(2+)-binding protein required for T cell receptor-, Fas-, and glucocorticoid-induced cell death. ALG-3, a partial complementary DNA that is homologous to the familial Alzheimer's disease gene STM2, rescues a T cell hybridoma from T cell receptor- and Fas-induced apoptosis. These findings suggest that ALG-2 may mediate Ca(2+)-regulated signals along the death pathway and that cell death may play a role in Alzheimer's disease.

Glucocorticoid-induced changes in liver: inhibition of nuclear Ca2+, Mg(2+)-dependent endonuclease activity in response to dexamethasone administration

The endogenous Ca2+, Mg(2+)-dependent endonuclease activity in nuclei from livers of rats receiving daily injections of the synthetic glucocorticoid dexamethasone was examined with respect to the production of both single and double strand breaks in chromatin DNA. The ability to form single strand breaks was measured by means of a nick translation assay and double strand breaks by following the appearance of nucleosomal ladders. A fall in the activity causing double strand breaks to approximately 50 per cent of the control value was apparent at 12 h after the first injection of the steroid. A fall of 25-30 per cent was also observed in the nicking activity but this was not apparent until 24 h after the first steroid injection. Both endonuclease activities remained at these lower levels for the remainder of the period of treatment. Nuclear extracts from dexamethasone-treated rats also showed a reduced ability to produce nucleosomal ladders when incubated with rat muscle nuclei, indicating that the inhibition observed in intact nuclei from treated animals was independent of any changes in chromatin structure. On the other hand the nick translation activity of the two extracts was the same when calf thymus DNA was used as the substrate suggesting that steroid-induced alterations in chromatin structure may be a critical factor in the reduced level of this activity observed in intact nuclei.

Factors affecting flow cytometric detection of apoptotic nuclei by DNA analysis

Apoptotic thymocyte nuclei normally appear on a flow cytometric DNA histogram as a subdiploid peak. We observed that addition of a specific RNase A preparation to the detergent-based lysing buffer increased the fluorescence of toxicant-induced apoptotic nuclei to the level of untreated diploid nuclei. The chelating agent EDTA partially inhibited the RNase effect, suggesting contaminating divalent cations may have been involved. Moreover, spectrofluorometric analysis revealed that addition of RNase or divalent cations decreased the amount of DNA present in the lysate. This suggested that the upscale fluorescence shift was due to a decrease in the ability of the lysing buffer to extract DNA, possibly as a result of cation-induced chromatin condensation, rather than increased accessibility of fluorochrome binding sites due to apoptotic degeneration. Moreover, during a 16-h culture, we observed a similar, but time-dependent, upscale shift in the fluorescence of thymocytes undergoing apoptosis either spontaneously or as a result of exposure to 1 microM tributyltin methoxide (TBT), 2% ethanol, 2% methanol, or 1 microM dexamethasone phosphate (DEX). This commonality of effect suggests that a similar magnitude of chromatin reorganization occurs in apoptotic cells in prolonged culture regardless of the method of apoptotic induction. These findings should alert investigators to potential inaccuracies in the flow cytometric quantitation of apoptosis in in vitro systems employing prolonged toxicant exposures or complex lysing cocktails that may contain active contaminants.

Dimethylnitrosamine-induced DNA damage and toxic cell death in cultured mouse hepatocytes

Chronic exposure to dimethylnitrosamine produces hepatic tumors through recurrent DNA alkylation, whereas acute exposure can cause liver necrosis through mechanisms that remain largely unknown. Our laboratory recently demonstrated that DNA fragmentation occurs early on and may be a causal event in dimethylnitrosamine-induced necrosis in liver. A challenge to interpreting these results is that up to 30% of liver cells are non-parenchymal and could account for the observed DNA fragmentation. In the present study, we have examined whether dimethylnitrosamine induces early genomic DNA fragmentation in cultured mouse hepatocytes. Hepatic parenchymal cells isolated from male ICR mice were cultured in Williams E medium. DNA damage was assessed quantitatively as a fragmented fraction that was not sedimented at 27,000 x g, and qualitatively from agarose gel electrophoresis. Cellular response to DNA damage was assessed by measuring induction of the DNA repair enzyme DNA ligase. Toxic cell death was estimated from release of lactate dehydrogenase (LDH) or adenine nucleotides from cells prelabeled with [3H]adenine. Dimethylnitrosamine produced a twofold increase in [3H]adenine release by 6 h and LDH release at 36 h. DNA fragmentation and DNA ligase activity increased by as early as 1 h. The Ca(2+)-endonuclease inhibitor aurintricarboxylic acid and the Ca2+ chelator ethylenediamine tetraacetic acid (EDTA) prevented DNA fragmentation through 6 h and virtually abolished cytotoxicity through 30 h. DNA ligase induction was strongly associated with DNA fragmentation. Early increases in DNA fragmentation and DNA ligase were highly correlated with later toxic cell death. Such results strongly suggest that dimethylnitrosamine-induced fragmentation of DNA in target parenchymal cells is a causal factor in the toxic death of these liver cells.

Apoptosis: molecular mechanisms and implications for human disease

Apoptosis is a highly regulated process of cell death with characteristic morphological changes that are distinct from necrosis. The biochemical machinery responsible for apoptotic cell death appears to be constitutively expressed in most, if not all, cells and can be triggered by a variety of signals, including sustained increases in the intracellular Ca2+ level. Apoptosis is the main mechanism of cell deletion during development, normal cell turnover, hormone-induced tissue atrophy, and pathological processes such as T-cell depletion in HIV/AIDS and neurodegenerative disease. The aim of this review is to briefly summarize current knowledge of the molecular mechanisms of apoptosis and its role in human disease.

Comparative study of induction of endogenous DNA degradation in rat liver nuclei and intact thymocytes

A common strongly ordered multi-step-pattern of endogenous DNA degradation was induced in rat liver nuclei and intact thymocytes, prepared in the presence of chelating agents and incubated in the presence of CaCl2 and/or MgCl2. It consisted of sequential generation of 0.3 Mbp, then 0.05 Mbp DNA fragments and finally of oligo- and mononucleosomal DNA. Oligonucleosomal DNA was generated when the genome had already been disintegrated into 0.05 Mbp DNA fragments. ZnCl2 completely inhibited advanced genome cleavage to oligo- and mononucleosomal DNA without affecting the initial generation of large DNA fragments. Therefore, the endonucleolytic activity which produce large DNA fragments is different from Ca2+/Mg2+ endonuclease. The similar pattern of DNA degradation was observed in thymocytes treated with dexamethasone and with the topoisomerase II inhibitor VM-26, the agents known to induce apoptosis. The effect of VM-26 strongly suggests the involvement of topoisomerase II in generation of large DNA fragments. Multi-level organization and regulation of the chromatin structure determine the stepwise process of genome degradation. Detachment of chromatin from the nuclear matrix attachment regions may be one of the possible mechanisms of switching off the genome function and triggering the multi-step process of endogenous chromatin degradation thus leading to cell death in terminal differentiation or stress-induced apoptosis.

Histochemical demonstration of DNA double strand breaks by in situ 3'-tailing reaction in apoptotic endometrium

A new histochemical technique, called in situ 3'-tailing reaction (ISTR), to detect DNA double strand breaks (DSB) was developed and applied to tissue sections of apoptotic endometrium. To demonstrate DSB, biotin-labeled and unlabeled dATPs with terminal deoxynucleotidyl transferase (TdT) were added to the many 3-hydroxyl termini of DNA fragments generated in the apoptotic cells. For an efficient 3'-end labeling, it was necessary to treat the sections with lambda-exonuclease (lambdaEx) prior to the TdT reaction to generate 3'-protruding ends. The lambdaEx-TdT reaction specifically labeled nuclear fragments in the apoptotic cells in paraformaldehyde fixed frozen sections. In paraffin sections, pretreatment with proteinase K was effective for 3'-tailing reaction. ISTR should be a useful tool for detecting dying cells in both physiological and pathological states.

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.

Role of Ca2+ in alloxan-induced pancreatic beta-cell damage

Pretreatment of rats with verapamil, a Ca(2+)-antagonist, completely prevented alloxan-induced hyperglycemia. Verapamil also abolished the inhibition of insulin secretion by alloxan and H2O2 in isolated rat pancreatic islets. H2O2 generation from alloxan was not affected by verapamil, but alloxan- and H2O2-induced DNA strand breaks were completely prevented. Treatment of beta-cells with alloxan and H2O2 caused elevation of cytosolic free Ca2+, and this increase of Ca2+ was also abolished by verapamil. These results suggest that alloxan-derived oxygen radicals may disturb intracellular Ca2+ homeostasis by increasing Ca2+ influx, which results in secondary reactions ultimately leading to DNA strand breaks and cytotoxicity of beta-cells.

Protein kinase C modulation in apoptotic rat thymocytes: an ultrastructural analysis

Numerous events in the cell, such as gene expression, cell growth and metabolism are regulated by signal transduction pathways involving protein kinase C (PKC). Recent data indicate that a PKC-dependent mechanism also underlies the apoptotic death of cells induced by glucocorticoid hormones. In this report we have analysed the changes of PKC during dexamethasone-induced apoptosis in thymocytes by means of immunocytochemical and immunochemical analysis. The data obtained show an increase and intracellular movement of protein kinase C, which is translocated to the nucleus and linked to the nuclear matrix during the apoptotic process.

Effect of extracellular ATP on breast tumor cell growth, implication of intracellular calcium

We studied the effects of purine nucleotides and particularly adenosine triphosphate (ATP) in two (one hormonosensitive, MCF7 and one hormonoinsensitive, MDA-MB 231) human breast tumor cell lines. As described in other cells, we observed that purine nucleotides produced transient elevations in intracellular calcium ions, [Ca2+]i, in both types of cells as determined from Indo-1 fluorescence of loaded cells. In the absence of external calcium the [Ca2+]i transients consisted of single narrow peaks while an extension of peak duration along with a biphasic appearance were observed in the presence of external calcium. The potency of different purine nucleotides in elevating [Ca2+]i was ATP > ADP >> AMP > adenosine (which was inefficient) proving the presence of P2 purinergic receptor subtypes. Suramin, a compound known to compete with ATP for its binding sites, nearly abolished the effect of ATP on [Ca2+]i increase. while verapamil, a calcium channel blocker, was unable to abolish such an an ATP-induced [Ca2+]i increase. The concentrations of ATP required to increase [Ca2%bdi ranged from 10(-7) M to 10(-3) M, the maximal effect being obtained with 10(-4) M ATP. At this latter concentration, ATP induced cell growth inhibition which was dose-independent as triggered only when maximal elevation of [Ca2+]i was attained. This ATP concentration also induced maximal apoptotic features in both types of cells. Together, our results highlighted an 'all or none' effect of ATP on breast tumor cell growth mediated by its effect on [Ca2+]i liberation from intracellular stores, the first rise of [Ca2+]i being further amplified by an influx of calcium from extracellular space. The attainment of sufficient [Ca2+]i level then triggers cellular events.

Calcium chelators induce apoptosis--evidence that raised intracellular ionised calcium is not essential for apoptosis

A moderate sustained rise in intracellular ionised calcium has been observed to be associated with apoptosis occurring in many experimental systems. The application of extracellular and intracellular chelators of calcium has been reported to produce a decrease in apoptosis, while the addition of calcium ionophores often increases apoptosis. These findings, together with the observation of calcium-induced internucleosomal DNA cleavage in isolated nuclei, have suggested that DNA cleavage (and apoptosis) is a calcium-dependent process. However, a number of studies have shown that apoptosis is not always associated with a rise in the level of intracellular ionised calcium. In the present study, calcium chelators were found to induce apoptosis in cultured cells, concomitant with a decrease in both intracellular ionised calcium and total cell calcium content. Decreased intracellular ionised magnesium was also induced by extracellular chelators. These findings provide further evidence that a raised intracellular ionised calcium is not universally present during the induction of apoptosis. It is proposed that loss of calcium homeostasis, rather than a sustained rise in cytosolic calcium, is a determining factor in cell death by apoptosis.

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 molecular chaperones HSP28, GRP78, endoplasmin, and calnexin exhibit strikingly different levels in quiescent keratinocytes as compared to their proliferating normal and transformed counterparts: cDNA cloning and expression of calnexin

We have identified nine molecular chaperones in human keratinocytes by one or a combination of three methods: (i) reaction with antibodies raised against the purified proteins, (ii) microsequencing of two-dimensional (2-D) gel purified proteins, or (iii), by cloning of the cDNA and expression of its encoded protein in transformed human amnion cells using the vaccinia virus expression system. The expression levels of each of the molecular chaperones were analyzed in quiescent, normal proliferating, and simian virus SV40 transformed K14 keratinocytes by cutting the corresponding protein spots from dried 2-D gels and counting the radioactivity by liquid scintillation. The most striking observation was the strong up-regulation (936%) of the small heat shock protein HSP28 in the quiescent keratinocytes, a fact that is in line with recent data indicating that the murine homologue (HSP25) may act as a growth inhibitor. Several chaperones that localize to the endoplasmic reticulum and that are involved in the secretory pathway (GRP78, GRP78v, endoplasmin, and calnexin) were expressed at approximately similar levels in normal proliferating and K14 keratinocytes but were down-regulated by 50% or more in the quiescent cells, implying that these cells may possess an impaired ability to secrete certain proteins. Both GRP78 and endoplasmin genes have similar sequences in the promoter regions, suggesting that they may be partly co-regulated at the transcriptional level (McCauliffe et al., J. Biol. Chem. 1992, 267, 2557-2562).(ABSTRACT TRUNCATED AT 250 WORDS)

Increased membrane permeability of apoptotic thymocytes: a flow cytometric study

We have recently developed a method for the separation and quantification of viable apoptotic cells without the need for permeabilisation or fixation of the cells. The method is based on the observation that apoptotic rat thymocytes fluoresce more brightly than normal cells after a brief incubation with the DNA binding dye, Hoechst 33342. In order to understand these differences, we have investigated the uptake of Hoechst 33342 by normal and apoptotic thymocytes. By staining with fluorescein diacetate, we have shown that the efflux of fluorescein from apoptotic cells is more rapid than that from normal thymocytes. This result demonstrated an increase in the permeability of the plasma membrane of the apoptotic thymocytes and it is this change which probably results in the more rapid uptake of Hoechst 33342. The data also revealed two populations of apoptotic thymocytes.

Condensation of the chromatin at the membrane of an apoptotic nucleus is not associated with activation of an endonuclease

A current hypothesis holds that chromatin fragmentation into oligonucleosomal patterns is an early event during apoptosis. In contrast, induction of apoptosis in cultured hepatocytes by TGF-beta 1 was not associated with DNA fragmentation into oligonucleosomes in hepatocyte monolayers and apoptotic fragments. For a more rigorous test of the hypothesis we performed a number of experiments. We compared nuclear changes resulting from TGF-beta 1 with those induced by Ca2+, a known activator of endonuclease. The morphology of apoptotic and Ca(2+)-treated nuclei was different as judged by DNA staining with Hoechst 33258. Likewise, electron microscopy of apoptotic nuclei showed characteristic condensation of the chromatin as well as dissolution of the nucleolar structure and nuclear fragmentation, changes not seen after Ca2+ treatment, after three hours of incubation. Analysis of DNA fluorescence of nuclei by FACS revealed that treatment with Ca2+ reduced the signal by 20%. In contrast, nuclei from TGF-beta 1-treated hepatocytes did not exhibit a reduced signal and after sorting by FACS, apoptotic nuclei remained in the 2N and 4N fractions. The absence of detectable DNA fragmentation in apoptotic nuclei was further verified by in situ nick translation, not only in hepatocytes but also in a mouse lymphoma cell line. From these findings we conclude that activation of an endonuclease is not an early event on the pathway to morphologically recognizable apoptosis.

C-myc gene chromatin of estrogen receptor positive and negative breast cancer cells

Expression of the c-myc protooncogene is estrogen regulated in estrogen receptor (ER) positive, hormone-dependent human breast cancer cells, but it is constitutively active in ER negative, hormone-independent breast cancer cells. To determine whether these differences are reflected in c-myc chromatin, DNase I hypersensitive sites (DHS) were mapped. Six DHS were detected in all cell lines studied, with DHS 3(2) being more prominent than DHS 3(1). The accessibility of DHS 2 was markedly greater in ER negative cells than in ER positive cells, and this relative accessibility remained unchanged when cells were grown in estrogen free medium. DHS 2, 3(1) and 3(2) map near the P0, P1 and P2 promoters, respectively. An analysis of promoter usage demonstrated that P2 was the preferred promoter. Thus, the differences in the accessibility of DHS 2 in c-myc chromatin of ER positive and negative cells likely reflects alterations in DNA-protein interactions in this region.

Induction of apoptosis in cultured hepatocytes and in the regressing liver by transforming growth factor-beta 1 occurs without activation of an endonuclease

In previous studies in vivo apoptotic liver cells were found to be positive for transforming growth factor-beta 1 (TGF-beta 1). In hepatocyte cultures TGF-beta 1 induced rounding up and fragmentation of the cells into multiple vesicles. As revealed by the DNA specific stain H33258 the chromatin of these cells condensed and segregated into masses at the nuclear membrane, followed by nuclear fragmentation. Ultrastructurally the cytoplasm was well preserved as demonstrated by the presence of intact cell organelles. These features strongly suggest that occurrence of apoptosis. Furthermore we administered TGF-beta 1 in vivo using an experimental model in which regression of the liver was initiated by a short preceding treatment with the hepatomitogen cyproterone acetate (CPA). Two doses of 1 nM TGF-beta 1/kg each augmented the incidence of apoptotic hepatocytes 5-fold. These studies strongly suggest that TGF-beta 1 is involved in the initiation of apoptosis in the liver In TGF-beta 1 treated hepatocytes both from the liver and monolayer culture no DNA fragmentation into oligosomes could be detected. Comparison of nuclei in which endonuclease was activated by Ca2+ with apoptotic nuclei revealed no obvious similarities, as demonstrated by FACS analysis, H33258 staining and electron microscopy. Thus, apoptosis induced by a growth inhibitor obviously occurs without activation of an endonuclease.

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.