Cell cycle and apoptosis: common pathways to life and death

Functional discontinuities in prothymosin alpha caused by caspase cleavage in apoptotic cells

Our study examines the effect of apoptosis on prothymosin alpha, an abundant, nuclear protein intimately involved with proliferation of all mammalian cells. When HeLa cells were treated with actinomycin D, with etoposide, or with staurosporine following synchronization with hydroxyurea, they underwent apoptosis based on several specific criteria, including fragmentation of DNA and activation of specific caspases. Similarly treated NIH3T3 cells arrested and displayed no indicators of apoptosis. In HeLa, but not in NIH3T3 cells, prothymosin alpha levels declined precipitously and a truncated version of the protein was formed. The following observations implicate caspase activity: (1) The truncated polypeptide arose only in the treated HeLa cell cultures. (2) The appearance of the truncated polypeptide coincided with the activation of caspase 3 and the cleavage of poly(ADP-ribose) polymerase, a known caspase substrate. (3) Carbobenzoxy-DEVD-fluoromethylketone, a cell-permeable caspase 3 inhibitor, blocked cleavage and degradation of prothymosin alpha. (4) The same inhibitor, when added to mixed extracts of apoptotic and normal cells, prevented cleavage of intact prothymosin alpha. (5) Recombinant caspase 3 and, to a much lesser extent, caspase 7 truncated purified prothymosin alpha. (6) In HeLa cells, cleavage occurred at three overlapping caspase 3-like sites with the consensus sequence D-X-X-D and released 10 to 14 residues from the carboxyl terminus, including the core nuclear localization signal. Two immediate consequences of the cleavage were observed: truncated prothymosin alpha was no longer confined to the nucleus and it was deficient in phosphate. These data suggest that the disabling of prothymosin alpha is a significant event in apoptosis. J. Cell. Physiol. 182:256-268, 2000. Published 2000 Wiley-Liss, Inc.

Autocrine nerve growth factor in human keratinocytes

Biologically active nerve growth factor (NGF) is synthesised and released by proliferating normal human keratinocytes. NGF up-regulates the expression of NGF mRNA in keratinocytes. Keratinocytes express both the low (p75)- and the high-affinity (TrkA) NGF-receptors, which are located in the basal layer of the epidermis. K252, a specific inhibitor of trk phosphorylation, blocks NGF-induced keratinocyte proliferation, in absence of exogenous NGF. Normal keratinocytes over-expressing TrkA proliferate better than control transfectants, while the NGF mimicking anti-Trk antibody induces an increased keratinocyte proliferation in Trk over-expressing cells as compared to mock transfected keratinocytes. In addition, NGF over-expressing keratinocytes proliferate better than mock transfected cells. K252, by blocking TrkA phosphorylation, induces apoptosis in normal keratinocytes, but not in keratinocytes over-expressing bcl-2. Furthermore, NGF transfected keratinocytes are protected from UV-B-induced keratinocyte apoptosis, by maintaining constant levels of Bcl-2 and Bcl-xL . Taken together these results support the concept of an autocrine survival system sustained by NGF and its high-affinity receptor in human keratinocytes. Because NGF and Trk levels are highly expressed in psoriasis. one could speculate that NGF autocrine system plays a role in the mechanisms associated with this and other hyperproliferative skin conditions, including cancer.

Influence of age on hypoxia/reoxygenation-induced DNA fragmentation and bcl-2, bcl-xl, bax and fas in the rat heart and brain

To test the hypothesis that the aging mammalian heart and brain might have increased vulnerability to acute stress, DNA fragmentation was studied after hypoxia-reoygenation in young adult (6 months) and old (22-24 months) F344 rats. Heart and brain tissue were examined at the following time points: 30, 60, or 90 min of hypoxia (H, 5% O2, 95% N2) plus 2 h of reoxygenation (R, room air, 21% O2). With increasing duration of hypoxia preceding the reoxygenation, the extent of DNA fragmentation (in situ terminal dUTP nick end labeling, TUNEL, positive cells) was progressively higher in both age groups, greater in the old compared to that of the young adult rat. The levels of the anti-apoptotic proteins bcl-2 and bcl-xL, were similar in young and old at baseline and tended to increase in both age groups after hypoxia/reoxygenation. The pro-apoptotic protein, bax, was higher at baseline in the old; it rose after hypoxia/reoxygenation in the young adult heart and brain, but was unchanged in the old heart and was decreased in the old brain. The ratios of bcl-2/bax and of bcl-xL/bax were higher in the old heart and brain compared to that in the young adult after hypoxia/reoxygenation. Thus, compared to that of the young adult, the heart and brain of the old rat have lower thresholds and are more vulnerable to injury induced by hypoxia/reoxygenation, despite rapid and heightened expression of the anti-apoptotic proteins bcl-2 and bcl-xl. This could be due partly to the age-associated increase in the basal expression of the pro-apoptotic protein bax, as well as possibly other factors.

Adenovirus-mediated E2F-1 gene transfer efficiently induces apoptosis in melanoma cells

BACKGROUND

E2F-1 is a transcription factor that stimulates cellular proliferation and cell cycle progression from G(1) to S-phase. Somewhat paradoxically, E2F-1 also has the properties of a tumor suppressor. Overexpression of E2F-1 has been shown to induce apoptosis in some cancer cells. In the current study, the effect of adenovirus-mediated E2F-1 gene transfer on human melanoma cell growth was investigated.

METHODS

Two human melanoma cell lines, SK-MEL-28 (wild-type p53) and SK-MEL-2 (mutant p53), were treated by mock infection, infection with a control vector expressing the beta-galactosidase gene (Ad5CMV-LacZ), or infection with a vector expressing E2F-1 (Ad5CMV-E2F-1) at a multiplicity of infection of 100. Cell proliferation and viability were determined by WST-1 assay and trypan blue exclusion, respectively. Apoptosis was assessed by cell flow cytometry and confirmed by cell morphology, in situ terminal deoxynucleotidyl nick end labeling assay, and poly(ADP-ribose) polymerase cleavage assay.

RESULTS

Marked overexpression of E2F-1 was evident in both cell lines 24 hours after infection with Ad5CMVE2F-1 by Western blot analysis. E2F-1 overexpression resulted in growth inhibition and rapid loss of cell viability. Overexpression of E2F-1 also resulted in premature S-phase entry and G(2) arrest at 24 hours followed by apoptotic cell death at 48 hours. After Ad5CMVE2F-1 infection, expression of Bax and Bak was unchanged, whereas Mcl-1 levels decreased markedly. In SK-MEL-28 cells, Bcl-XL levels also declined after E2F-1 expression. Bcl-2 was undetectable in SK-MEL-28 cells but was increased in SK-MEL-2 cells in response to E2F-1 overexpression.

CONCLUSIONS

Adenovirus-mediated E2F-1 gene transfer efficiently induces widespread apoptosis in human melanoma cells. E2F-1 overexpression induced apoptosis in cell lines containing wild-type and mutant p53, suggesting that this effect does not require wild-type p53 function. Anti-apoptotic proteins of the Bcl-2 family, notably Mcl-1 and Bcl-XL, may be involved in mediating the response to E2F-1. These data suggest that adenovirus-mediated E2F-1 gene therapy may be effective in the treatment of melanoma.

Toxin injury-dependent switched expression between EF-1 alpha and its sister, S1, in rat skeletal muscle

Elongation factor-1 alpha, (EF-1 alpha), a translation factor involved in peptide chain elongation, is found ubiquitously in all cells. Previously, we identified a highly homologous EF-1 alpha sister gene, S1, whose transcript is found in only three tissues: brain, heart, and muscle, where the tissue-specific expression of S1 is caused by its exclusive presence in cells such as neurons and myocytes. Using sequence-specific synthetic peptides, we have recently produced polyclonal antibodies that can distinguish the protein product of EF-1 alpha from that of its sister, S1. Results of Western blotting show that these two proteins appear in S1-positive muscle tissue in inverse relationship, i.e., when S1 protein is in abundance, EF-1 alpha protein is in contrast in low quantity, and vice versa. During early embryonic stages, EF-1 alpha is the predominant protein species, whereas S1 is hardly detectable. This high EF-1 alpha versus low S1 protein presence undergoes a switch in that by postnatal day 14, EF-1 alpha is scarce whereas S1 is abundant; thus, there is a development-dependent shift of EF-1 alpha/S1 ratio from high to low, and the low EF-1 alpha/S1 ratio is maintained in adulthood. In this report, we describe the reversal of the EF-1 alpha/S1 ratio from low to high during muscle injury (experimentally induced by Marcaine injection), and a return to the original low ratio once the injury is repaired by regeneration. In this injury condition, EF-1 alpha is rapidly upregulated immediately after the Marcaine treatment, possibly reflecting an injury-dependent response of regeneration. The increase of EF-1 alpha corresponds with a decrease of S1 protein presence, thus resulting in a change of EF-1 alpha/S1 ratio from low to high. However, the high EF-1 alpha/S1 ratio eventually reverts to low, when regeneration-associated proliferation ceases, and fully differentiated myotubes are reestablished in the injured cells. This result shows that: (1) a high EF-1 alpha/S1 ratio is an early molecular diagnostic marker for injury-elicited regeneration; and (2) when injury repair is accomplished, there is a reversion to the low EF-1 alpha/S1 ratio, reflecting the restoration of the muscle fiber to the preinjury functional status. Results presented here not only show that a high EF-1 alpha/S1 ratio is a molecular marker for injured muscle, but also reveal the underpinning translational regulation in muscle during injury.

Molecular cloning of a cell cycle regulation gene cyclin H from ischemic rat brain: expression in neurons after global cerebral ischemia

Gene expression plays an important role in determining the fate of neurons after ischemia. To identify additional genes that promote survival or execute programmed cell death in ischemic neurons, a subtractive cDNA library was constructed from hippocampus of rats subjected to global ischemia. With use of a differential screening technique, a cDNA was identified that was up-regulated after ischemia. The cDNA was found to have high homology with human cyclin H at both the nucleotide level (89%) and the amino acid level (93%). Northern blotting detected cyclin H mRNA in nonischemic and ischemic brains. In situ hybridization studies revealed that cyclin H message was found in hippocampal neurons in nonischemic brain. After ischemia, expression was increased primarily in the dentate gyrus and CA3 regions of hippocampus. Expression of cyclin H protein, detected by western blotting of hippocampal tissue, was increased after global ischemia, but expression of cyclins B1 and D1 and other related cell cycle genes (Cdk7 and Cdc2) was not increased. Cyclin H immunoreactivity was found exclusively within neurons. After ischemia, there was increased immunoreactivity within neurons in dentate gyrus, CA3, and cortex. Thus, cyclin H is expressed in normal postmitotic neurons and expression is increased in neurons that are ischemic yet survive. These results suggest that cyclin H may have functions in neurons other than cell cycle regulation, including other known functions such as DNA repair.

Possible interactions between the NS-1 protein and tumor necrosis factor alpha pathways in erythroid cell apoptosis induced by human parvovirus B19

Human erythroid progenitor cells are the main target cells of the human parvovirus B19 (B19), and B19 infection induces a transient erythroid aplastic crisis. Several authors have reported that the nonstructural protein 1 (NS-1) encoded by this virus has a cytotoxic effect, but the underlying mechanism of NS-1-induced primary erythroid cell death is still not clear. In human erythroid progenitor cells, we investigated the molecular mechanisms leading to apoptosis after natural infection of these cells by the B19 virus. The cytotoxicity of NS-1 was concomitantly evaluated in transfected erythroid cells. B19 infection and NS-1 expression induced DNA fragmentation characteristic of apoptosis, and the commitment of erythroid cells to undergo apoptosis was combined with their accumulation in the G(2) phase of the cell cycle. Since B19- and NS-1-induced apoptosis was inhibited by caspase 3, 6, and 8 inhibitors, and substantial caspase 3, 6, and 8 activities were induced by NS-1 expression, there may have been interactions between NS-1 and the apoptotic pathways of the death receptors tumor necrosis factor receptor 1 and Fas. Our results suggest that Fas-FasL interaction was not involved in NS-1- or B19-induced apoptosis in erythroid cells. In contrast, these cells were sensitized to tumor necrosis factor alpha (TNF-alpha)-induced apoptosis. Moreover, the ceramide level was enhanced by B19 infection and NS-1 expression. Therefore, our results suggest that there may be a connection between the respective apoptotic pathways activated by TNF-alpha and NS-1 in human erythroid cells.

Human endothelial cell life extension by telomerase expression

Normal human endothelial cells, like other somatic cells in culture, divide a limited number of times before entering a nondividing state called replicative senescence. Expression of the catalytic component of human telomerase, human telomerase reverse transcriptase (hTERT), extends the life span of human fibroblasts and retinal pigment epithelial cells beyond senescence without causing neoplastic transformation (Bodnar, A. G., Ouellette, M., Frolkis, M., Holt, S. E., Chiu, C. P., Morin, G. B., Harley, C. B., Shay, J. W., Lichtsteiner, S., and Wright, W. E. (1998) Science 279, 349-352; Jiang, X., Jimenez, G., Chang, E., Frolkis, M., Kusler, B., Sage, M., Beeche, M., Bodnar, A., Wahl, G., Tlsty, T., and Chiu, C.-P. (1999) Nat. Genet. 21, 111-114). Here, we show that both human large vessel and microvascular endothelial cells also bypass replicative senescence after introduction of hTERT. For the first time, we report that hTERT expression in these life-extended vascular cells does not affect their differentiated and functional phenotype and that these cells maintain their angiogenic potential in vitro. Furthermore, hTERT(+) microvascular endothelial cells have normal karyotype, and hTERT(+) endothelial cell strains do not exhibit a transformed phenotype. Relative to parental cells at senescence, hTERT-expressing endothelial cells exhibit resistance to induction of apoptosis by a variety of different conditions. Such characteristics are highly desirable for designing vascular transplantation and gene therapy delivery systems in vivo.

Involvement of protein kinase Cepsilon (PKCepsilon) in thyroid cell death. A truncated chimeric PKCepsilon cloned from a thyroid cancer cell line protects thyroid cells from apoptosis

The protein kinase C (PKC) family has been implicated in the regulation of apoptosis. However, the contribution of individual PKC isozymes to this process is not well understood. We reported amplification of the chromosome 2p21 locus in 28% of thyroid neoplasms, and in the WRO thyroid carcinoma cell line. By positional cloning we identified a rearrangement and amplification of the PKCepsilon gene, that maps to 2p21, in WRO cells. This resulted in the overexpression of a chimeric/truncated PKCepsilon (Tr-PKCepsilon) mRNA, coding for N-terminal amino acids 1-116 of the isozyme fused to an unrelated sequence. Expression of the Tr-PKCepsilon protein in PCCL3 cells inhibited activation-induced translocation of endogenous PKCepsilon, but its kinase activity was unaffected, consistent with a dominant negative effect of the mutant protein on activation-induced translocation of wild-type PKCepsilon and/or displacement of the isozyme to an aberrant subcellular location. Cell lines expressing Tr-PKCepsilon grew to a higher saturation density than controls. Moreover, cells expressing Tr-PKCepsilon were resistant to apoptosis, which was associated with higher Bcl-2 levels, a marked impairment in p53 stabilization, and dampened expression of Bax. These findings point to a role for PKCepsilon in apoptosis-signaling pathways in thyroid cells, and indicate that a naturally occurring PKCepsilon mutant that functions as a dominant negative can block cell death triggered by a variety of stimuli.

p53-independent apoptosis induced by muscle differentiation stimuli in polyomavirus large T-expressing myoblasts

Abnormal proliferation signals, driven by cellular or viral oncogenes, can result in the induction of apoptosis under sub-optimal cell growth conditions. The tumor suppressor p53 plays a central role in mediating oncogene-induced apoptosis, therefore transformed cells lacking p53 are generally resistant to apoptosis-promoting treatments. In a previous work we have reported that the expression of polyomavirus large T antigen causes apoptosis in differentiating myoblasts and that this phenomenon is dependent on the onset of muscle differentiation in the absence of a correct cell cycle arrest. Here we report that polyomavirus large T increases the levels and activity of p53, but these alterations are not involved in the apoptotic mechanism. Apoptosis in polyomavirus large T-expressing myoblasts is not prevented by the expression of a p53 dominant-negative mutant nor it is increased by p53 over-expression. Moreover, forced differentiation induced through the over-expression of the muscle regulatory factor MyoD, leads to apoptosis without altering p53 function and, more significantly, even in a p53-null background. Our results indicate that apoptosis induced by the activation of muscle differentiation pathways in oncogene-expressing cells can occur in a p53-independent manner.

Age-dependent atrophy and microgravity travel: what do they have in common?

Space travel and extending human lifespan are two of the many advances of the twentieth century. However, both of these scientific wonders exact a price for their gains; i.e. deleterious effects on normal physiological processes. For example, both old age and prolonged microgravity travel are associated with atrophy in heart, muscle, and bone. The underlying signal transduction pathways, the control mechanisms for the processes of proliferation, differentiation, and apoptosis, may prove to be similarly altered in both old age and microgravity travel. We suggest that the mechanical events involved in space travel provide a telescopic compression of lifespan changes in these tissues; if so, space travel provides an excellent opportunity to investigate how long-term degeneration occurs on Earth. With the aid of biochip technology for multi-factorial analysis, a platform can be generated to create therapeutic modalities to contain, retard, reduce, or prevent this tissue atrophy, either in space or on Earth.

Aberrations of cell cycle and cell death in normal development of the chick embryo growth plate

The epiphyses of femurs from 7.5-15 day chicken embryos were studied by electron microscopy. Several forms of aberrant cell cycles were present: (1) in the perichondrium, polyploid metaphases, segmentating large (giant) cells, and mitotic catastrophe (midway between mitosis and apoptosis) were observed; (2) in the resting zone, premature chromosome condensation was found; (3) in the proliferative zone, approximately 5% of divisions were aberrant, representing most often mitosis restitution from metaphase and more seldom from the anaphase; (4) in all layers, 'dark chondrocytes' representing a premortal form of hypersecretory cells undergoing often a-mitotic nuclear segmentation were present. Many of the aberrations of cell cycle were combined with cell death. These deviations omitting or adapting the cell cycle check-points represent evidently the normal epigenetic mechanisms of development and repair. At the same time, by origin and appearances they seem very close to the loss of the growth control displayed by malignant tumours. This connection is briefly analysed in view of some current concepts of carcinogenesis.

Growth arrest of melanoma cells is differentially regulated by contact inhibition and serum deprivation

Both growth-factor deprivation and contact inhibition suppress cell growth; however, the mechanisms by which they inhibit cell proliferation may not be identical. The function of antiproliferative genes and the induction of programmed cell death are among the potential differences between these growth-arrest mechanisms. Specifically, an inverse relation between the expression of cyclin-dependent kinase inhibitors (CDKIs) and the susceptibility to apoptosis has been reported. To test this relation, we examined the features of growth arrest in a canine melanoma cell line, TLM1. Both contact inhibition and serum deprivation halted cell-cycle progression of TLM1 cells in the G1 phase. Prolonged growth arrest of the cells without restimulation resulted in apoptosis; conversely, the cells reentered the cell cycle after release from contact inhibition or on restimulation with serum. Cell-to-cell contact, but not serum deprivation, led to the expression of p53 and p21/Waf-1. The expression of p21/Waf-1 did not prevent apoptosis. Moreover, the ectopic overexpression of CDKIs increased apoptosis. These results support the premise that growth arrest induced by contact inhibition and serum deprivation are mediated through distinct mechanisms. Furthermore, CDKIs are not universal inhibitors of apoptosis, and in some cases, they may initiate or enhance the apoptotic program.

Apoptosis in developing retinal tissue

The mechanisms of apoptosis are strongly dependent on cell-cell interactions typical of organized tissues. Experimental studies of apoptosis using a histotypical preparation of retinal explants are reported in the present article. We found that various characteristics of apoptosis are selectively associated with retinal cell death depending on cell type, stage of maturation, and means of induction of apoptosis. Among these were: (1) the requirements of protein synthesis; (2) the role of cAMP; (3) the expression of certain apoptosis-associated proteins; and (4) the sensitivity to excitotoxicity, modulation of protein phosphatases and calcium mobilization. Dividing cells undergo apoptosis in response to several inducers in specific phases of the cell cycle, and in distinct regions within their pathway of interkinetic nuclear migration. Recent post-mitotic cells are selectively sensitive to apoptosis induced by blockade of protein synthesis, while both proliferating and differentiated cells are more resistant. We also studied the association of several proteins, some of which play critical roles in the cell cycle, with both differentiation and apoptosis in the retinal tissue. Detection of cell cycle markers did not support the hypothesis that retinal cells re-enter the cell cycle on their pathway to apoptosis, although some proteins associated with cell proliferation re-appeared in degenerating cells. The transcription factors c-Jun, c-Fos and c-Myc were found associated with apoptosis in retinal cells, but their sub-cellular location in apoptotic bodies is not consistent with their canonical functions in the control of gene expression. The bifunctional redox factor/AP endonuclease Ref-1 and the transcription factor Max are associated with progressive cell differentiation, and both are down-regulated during cell death in the retina. The data suggest that Ref-1 and Max may normally function as negative modulators of retinal apoptosis. The results indicate that nuclear exclusion of transcription factors and other important control proteins is a hallmark of retinal apoptosis. Histotypical explants may be a choice preparation for the experimental analysis of the mechanisms of apoptosis, in the context both of cell-cell interactions and of the dynamic behavior of developing cells within the organized retinal tissue.

Programmed cell death in procyclic form Trypanosoma brucei rhodesiense --identification of differentially expressed genes during con A induced death

Trypanosoma brucei rhodesiense can be induced to undergo apoptosis after stimulation with Con A. As cell death in these parasites is associated with de novo gene expression we have applied a differential display technique, Randomly Amplified Differential Expressed Sequence-Polymerase Chain Reaction (RADES-PCR) to the study of gene expression during Con A induced cell death in these organisms. Twenty-two differentially displayed products have been cloned and sequenced. These represent the first endogenous genes to be identified as implicated in cellular death in trypanosomatids (the most primitive eukaryote in which apoptosis has been described). Evidence for an ancestral death machinery, 'proto-apoptosis' in single celled organisms is discussed.

Caspase-mediated cleavage of p21Waf1/Cip1 converts cancer cells from growth arrest to undergoing apoptosis

The cyclin-dependent kinase inhibitor p21waf1/Cip1 is a downstream effector of the p53-dependent cell growth arrest. We report herein that p21 was cleaved by caspase-3/CPP32 at the site of DHVD112L during the DNA damage-induced apoptosis of cancer cells. The cleaved p21 fragment could no more arrest the cells in G1 phase nor suppress the cells undergoing apoptosis because it failed to bind to the proliferating cell nuclear antigen (PCNA) and lost its capability to localize in the nucleus. Thus, caspase-3-mediated cleavage and inactivation of p21 protein may convert cancer cells from growth arrest to undergoing apoptosis, leading to the acceleration of chemotherapy-induced apoptotic process in cancer cells.

Non-steroidal anti-inflammatory drug-induced apoptosis in gastric cancer cells is blocked by protein kinase C activation through inhibition of c-myc

Apoptosis plays a major role in gastrointestinal epithelial cell turnover, ulcerogenesis and tumorigenesis. We have examined apoptosis induction by non-steroidal anti-inflammatory drugs (NSAIDs) in human gastric (AGS) cancer cells and the role of protein kinase C (PKC) and apoptosis-related oncogenes. After treatment with aspirin or indomethacin, cell growth was quantified by MTT assay, and apoptosis was determined by acridine orange staining, DNA fragmentation and flow cytometry. The mRNA and protein of p53, p21waf1/cip1 and c-myc was detected by Northern and Western blotting respectively. The influence of PKC on indomethacin-induced apoptosis was determined by co-incubation of 12-O-tetradecanoylphorbol 13-acetate (TPA). The role of c-myc was determined using its antisense oligonucleotides. The results showed that both aspirin and indomethacin inhibited cell growth and induced apoptosis of AGS cells in a dose- and time-dependent manner, without altering the cell cycle. Indomethacin increased c-myc mRNA and protein, whereas p53 and p21wafl/cip1 were unchanged. Down-regulation of c-myc by its antisense oligonucleotides reduced apoptosis induction by indomethacin. TPA could inhibit indomethacin-induced apoptosis and accumulate cells in G2/M. Overexpression of c-myc was inhibited by TPA and p21waf1/cip1 mRNA increased. In conclusion, NSAIDs induce apoptosis in gastric cancer cells which may be mediated by up-regulation of c-myc proto-oncogene. PKC activation can abrogate the effects of NSAIDs by decreasing c-myc expression.

Inhibition of caspases rescues brown adipocytes from apoptosis downregulating BCL-XS and upregulating BCL-2 gene expression

Serum deprivation of the immortalized brown adipocyte cell line resulted in growth arrest in G0/G1 phases of the cell cycle and apoptosis, as detected by DNA laddering, nuclei condensation and fragmentation, and an increase in the percentage of hypodiploid cells. In addition, apoptosis in these cells is accompanied by an induction of the expression of the apoptotic form of the Bcl-x gene, the isoform Bcl-xS, and by a decrease of Bcl-2 expression, Bcl-xL remaining almost undetectable. The loss of mitochondrial membrane potential was associated with apoptosis. Z-VAD, a cell-permeable inhibitor of caspases, but not cycloheximide, precludes DNA laddering under serum deprivation. Moreover, Z-VAD rescues serum-deprived brown adipocytes from apoptosis, decreasing the percentage of hypodiploid cells, the percentage of apoptotic cells under Tunnel assay, and the external display of phosphatidylserine. More importantly, Z-VAD survival effects on immortalized brown adipocytes concur with a downregulation of Bcl-xS mRNA/protein and an upregulation of Bcl-2 protein content. Ultimately, Z-VAD prevents the loss of mitochondrial membrane potential.

Reovirus-induced apoptosis is preceded by increased cellular calpain activity and is blocked by calpain inhibitors

The cellular pathways of apoptosis have not been fully characterized; however, calpain, a cytosolic calcium-activated cysteine protease, has been implicated in several forms of programmed cell death. Reoviruses induce apoptosis both in vitro and in vivo and serve as a model for studying virus-induced cell death. We investigated the potential role of calpain in reovirus-induced apoptosis in vitro by measuring calpain activity as well as evaluating the effects of calpain inhibitors. L929 cells were infected with reovirus type 3 Abney (T3A), and calpain activity, measured as cleavage of the fluorogenic calpain substrate Suc-Leu-Leu-Val-Tyr-AMC, was monitored. There was a 1.6-fold increase in calpain activity in T3A-infected cells compared to mock-infected cells; this increase was completely inhibited by preincubation with calpain inhibitor I (N-acetyl-leucyl-leucyl-norleucinal [aLLN]), an active-site inhibitor. Both aLLN and PD150606, a specific calpain inhibitor that interacts with the calcium-binding site, inhibited reovirus-induced apoptosis in L929 cells by 54 to 93%. Apoptosis induced by UV-inactivated reovirus was also reduced 65 to 69% by aLLN, indicating that inhibition of apoptosis by calpain inhibitors is independent of effects on viral replication. We conclude that calpain activation is a component of the regulatory cascade in reovirus-induced apoptosis.

DEFT, a novel death effector domain-containing molecule predominantly expressed in testicular germ cells

Apoptosis is a physiological process by which multicellular organisms eliminate unwanted cells. Death factors such as Fas ligand induce apoptosis by triggering a series of intracellular protein-protein interactions mediated by defined motifs found in the signaling molecules. One of these motifs is the death effector domain (DED), a stretch of about 80 amino acids that is shared by adaptors, regulators, and executors of the death factor pathway. We have identified the human and rat complementary DNAs encoding a novel protein termed DEFT (Death EFfector domain-containing Testicular molecule). The N-terminus of DEFT shows a high degree of homology to the DEDs found in FADD (an adaptor molecule) as well as procaspase-8/FLICE and procaspase-10/Mch4 (executors of the death program). Northern blot hybridization experiments have shown that the DEFT messenger RNA (mRNA) is expressed in a variety of human and rat tissues, with particularly abundant expression in the testis. In situ hybridization analysis further indicated the expression of DEFT mRNA in meiotic male germ cells. In a model of germ cell apoptosis induction, an increase in testis DEFT mRNA was found in immature rats after 2 days of treatment with a GnRH antagonist. Unlike FADD and procaspase-8/FLICE, overexpression of DEFT did not induce apoptosis in Chinese hamster ovary cells. Although cotransfection studies indicated that DEFT is incapable of modulating apoptosis effected by FADD and procaspase-8/FLICE, interactions between DEFT and uncharacterized DED-containing molecules in the testis remain to be studied in the future. In conclusion, we have identified a novel DED-containing protein with high expression in testis germ cells. This protein may be important in the regulation of death factor-induced apoptosis in the testis and other tissues.

Mitotic phosphorylation of Bcl-2 during normal cell cycle progression and Taxol-induced growth arrest

There is increasing evidence that prolonged mitotic arrest initiates apoptosis; however, little is known about the signaling pathways involved. Several studies have associated deregulated Cdc2 activity with apoptosis. Herein, we report that the anti-apoptotic protein, Bcl-2, undergoes cell cycle-dependent phosphorylation during mitosis when there is elevated Cdc2 activity. We found that paclitaxel (Taxol(R)) treatment of epithelial tumor cells induced a prolonged mitotic arrest, elevated levels of mitotic kinase activity, hyperphosphorylation of Bcl-2, and subsequent cell death. The Taxol-induced Bcl-2 phosphorylation was dose-dependent. Furthermore, phosphorylated Bcl-2 remained complexed with Bax in Taxol-treated cells undergoing apoptosis. Immunoprecipitation experiments revealed a Bcl-2-associated kinase capable of phosphorylating histone H1 in vitro. However, the kinase was likely not cyclin B1/Cdc2, since cyclin B1/Cdc2 was not detectable in Bcl-2 immunoprecipitates, nor was recombinant Bcl-2 phosphorylated in vitro by cyclin B1/Cdc2. The results of this study further define a link between mitotic kinase activation and the apoptotic machinery in the cell. However, the role, if any, of prolonged Bcl-2 phosphorylation in Taxol-mediated apoptosis awaits further definition of Bcl-2 mechanism of action. Taxol may increase cellular susceptibility to apoptosis by amplifying the normal downstream events associated with mitotic kinase activation.

Antizyme prevents ornithine decarboxylase-mediated cell death in human fibroblasts

Ornithine decarboxylase (ODC) is a key enzyme of polyamine biosynthesis essential for growth-related cellular functions. Apart from its physiological role in cell proliferation, ODC also contributes to the induction of apoptosis under certain conditions, e.g. following growth factor withdrawal. The rate of cell death is a function of its enzyme activity, ODC activity is inhibited by a regulatory protein antizyme, also known to suppress polyamine uptake. We report that forced expression of antizyme prevents ODC-mediated cell death in human gingival fibroblasts under very low serum conditions. These data suggest an important antiapoptotic role for antizyme in cell survival.

H-7 stimulates desmosome formation and inhibits growth in KB oral carcinoma cells

Protein kinase inhibitor H-7 was reported to stimulate desmosome formation in normal keratinocytes and to inhibit proliferation of neural cell lines. In the present study, the effects of this inhibitor on adhesion and growth of KB human oral carcinoma cells were investigated. H-7 was found to enhance desmosome assembly, as evidenced by an increased punctate labeling for the major desmosomal markers. Immunogold labeling confirmed the formation of desmosomes both at the cell surface and in the cytoplasm. In order to assess cell proliferation and possible correlation with adhesion, confluent cultures were treated and both adherert and detached cell fractions were counted. Under serum-free conditions, H-7 significantly reduced cell detachment. In contrast, EGF stimulated cell detachment, and this effect was abolished when cells were simultaneously treated with both EGF and H-7. Total cell counts were also significantly reduced by H-7, both in the presence and absence of EGF. Using the TUNEL technique, labeled cells were increased after H-7 treatment, thus implicating protein kinase inhibition in cell death. These results indicate that H-7 inhibits growth and stimulates adhesion of KB carcinoma cells.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Epidermal cell kinetics of pig skin in vivo following UVB irradiation: apoptosis induced by UVB is enhanced in hyperproliferative skin condition

We investigated the effects of ultraviolet B (UVB) irradiation on pig epidermal sunburn cell (apoptotic cell) formation. Expression of p53 tumor suppressor gene product, p21 (WAF1/CIP1), and proliferating cell nuclear antigen (PCNA) was also determined immunohistochemically. Apoptotic cells appeared at 12 h and reached a peak at 48 h following 2 MED-UVB irradiation. The formation of sunburn cells was confirmed by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling (TUNEL) method. p53-positive cells, and p21-positive cells appeared at 6 h, and 12 h, respectively, following the UVB-irradiation. The peak of p53-positive cells was observed at 24 h, and that of p21-positive cells was at 48 h. No expression of TUNEL-, p53-, or p21-positive cells was detected in non-irradiated epidermis. The increase in PCNA-positive cells was observed at 24 h and reached its peak at 96 h following the UVB-irradiation. Flow cytometric analyses indicated a decrease in S-phase cells at 24 h, that was followed by their increase at 96 h. Cells in G2/M phase were also considerably decreased at 6 h and 48 h following the UVB-irradiation, and was followed by their increase thereafter. The [3H]thymidine uptake and mitotic counts remained low up until 48 h, and then both parameters increased reaching their peaks at 72 96 h. Effects of UVB irradiation were also determined in tape stripping-induced hyperproliferative epidermis. The numbers of UVB-induced apoptotic cells and PCNA positive cells were markedly enhanced in the tape stripping-treated epidermis, while the numbers of p53- and p21-positive cells were not significantly altered. No induction of apoptosis, p53, or p21 was observed by tape stripping alone. Our results indicate that UVB irradiation induces G1 arrest, prolonged S, and G2/M block of epidermal keratinocytes as well as apoptosis. These processes provide a G1 check point and the elimination of possibly hazardous cells carrying DNA damage, respectively. Our results also indicate that the UVB-induced apoptotic process is enhanced in hyperproliferative skin condition suggesting that apoptosis is closely associated with cell cycle progression.

Induction of p21 during ceramide-mediated apoptosis in human hepatocarcinoma cells

Ceramide acts as a mediator of programmed cell death in various cell types, but its molecular mechanisms linked to the cell cycle are poorly understood. In this study, we investigated the expression of the p21 gene and its relationship to apoptosis induced by ceramide. In SK-HEP-1 cells, the addition of C6-ceramide resulted in a dose- and time-dependent growth suppression and DNA fragmentation characteristics of apoptosis. p21 protein was induced during that process, while the protein level of p53, known as a transcriptional activator of p21, was not elevated under the same condition. This apoptotic cell death with p21 induction was also observed in the Hep 3B cells lacking functional p53 after exposure to C6-ceramide. These findings suggest that ceramide-induced apoptosis is associated with the upregulation of p21 mRNA and protein in a p53-independent pathway.

Tissue remodelling in the adrenal gland

Adaptation of the adrenal gland to the demands of the organism is regulated functionally and structurally. Three common hypotheses on zonation in the adrenal gland, the migrational, zonal, and transformation field theories, try independently to reconcile the findings on structure, proliferation, and cell death. The classical theories on zonation are revisited in the light of recent data on cell death and renewal. In accordance with data on cell death as immunoreactivity against FAS(CD 95), an apoptosis-inducing receptor, in situ end labelling of fragmented DNA, and ultrastructural analyses, programmed cell death (PCD) occurs throughout the whole organ. The angiotensin II receptor subtypes described in the adrenal allow an additional regulation of tissue homeostasis by proliferative and even by the antiproliferative effects of the angiotensin II type 2 receptor. Proto-oncogenes are involved in the regulation of cell cycle and PCD, and adrenocorticotropin asserts its tissue integrating and differentiating effects by regulating proto-oncogenes such as c-jun, c-fos, jun-B and c-myc. Polypeptides involved in proliferation and DNA repair, such as proliferating cell nuclear antigen and Ki-67, have been found within zones of expected cell senescence. The expression of the class II major histocompatibility complex on normal adrenocortical cells allows cell-to-cell communication with the immune system and may trigger the Fas/Fas-ligand system to permit tissue regression and decreasing activity in both systems. In summary, new data allow us to reappraise and to reconcile the classical theories. Apoptosis is a physiological process in the adrenal gland. There is a differential regulation of apoptosis in the different zones. An investigation of this process may elucidate the basic mechanisms of adrenal zonation.

Differential regulation of discrete apoptotic pathways by Ras

The products of the ras genes are known to regulate cell proliferation and differentiation; recently, they have been found to play a role in apoptosis. The expression of oncogenic p21(ras) in a number of cell types, including Jurkat (a human T lymphoblastoid cell line) and murine fibroblasts, makes the cells susceptible to apoptosis following suppression of protein kinase C (PKC) activity (PKC/Ras-mediated apoptosis). Engagement of Fas antigen, a potent effector of apoptosis, activates cellular p21(ras), which may be required for completion of the cell death program. To further investigate the role of p21(ras) in the regulation of apoptosis, the cellular mechanisms employed in these two apoptotic processes in which Ras activity is involved (PKC/Ras-related and Fas-triggered apoptosis), was explored. Increasing p21(ras) activity by expressing v-ras or by treatment with an antisense oligonucleotide to the GTPase-activating protein was found to accelerate the Fas-mediated apoptotic process in Jurkat and mouse LF cells. PKC/Ras-related apoptosis was associated with, and required, cell cycle progression, accompanied by the expression of the G1/S cyclins. In contrast, Fas engagement, although inducing a vigorous and PKC-independent activation of endogenous p21(ras), did not alter cell cycle progression, nor did it require such progression for apoptosis. Both the protein synthesis inhibitor cycloheximide and cyclin E antisense oligonucleotides partially abolished PKC/Ras-mediated apoptosis but had only a moderate effect on Fas-induced apoptosis. In contrast, the CED-3/interleukin-1beta-converting enzyme (ICE) protease inhibitor Z-VADfmk efficiently suppressed Fas-induced apoptosis and only marginally inhibited PKC/Ras-mediated apoptosis. Induction of both pathways resulted in activation of the Jun NH2-terminal kinase/JUN signaling system. These results suggest that different cell death programs, such as PKC/Ras-mediated and Fas-mediated apoptosis, may be interconnected via p21(ras) and perhaps Jun NH2-terminal kinase/JUN. In response to various death stimuli, p21(ras) may act as a common intermediate regulator in the transduction of apoptotic signals.

The endogenous and cell cycle-dependent phosphorylation of tau protein in living cells: implications for Alzheimer's disease

In Alzheimer's disease the neuronal microtubule-associated protein tau becomes highly phosphorylated, loses its binding properties, and aggregates into paired helical filaments. There is increasing evidence that the events leading to this hyperphosphorylation are related to mitotic mechanisms. Hence, we have analyzed the physiological phosphorylation of endogenous tau protein in metabolically labeled human neuroblastoma cells and in Chinese hamster ovary cells stably transfected with tau. In nonsynchronized cultures the phosphorylation pattern was remarkably similar in both cell lines, suggesting a similar balance of kinases and phosphatases with respect to tau. Using phosphopeptide mapping and sequencing we identified 17 phosphorylation sites comprising 80-90% of the total phosphate incorporated. Most of these are in SP or TP motifs, except S214 and S262. Since phosphorylation of microtubule-associated proteins increases during mitosis, concomitant with increased microtubule dynamics, we analyzed cells mitotically arrested with nocodazole. This revealed that S214 is a prominent phosphorylation site in metaphase, but not in interphase. Phosphorylation of this residue strongly decreases the tau-microtubule interaction in vitro, suppresses microtubule assembly, and may be a key factor in the observed detachment of tau from microtubules during mitosis. Since S214 is also phosphorylated in Alzheimer's disease tau, our results support the view that reactivation of the cell cycle machinery is involved in tau hyperphosphorylation.

Distinctive inhibitory activity of docosahexaenoic acid against sphingosine-induced apoptosis

The effect of supplementation of docosahexaenoic acid (DHA) on the apoptosis of HL60 cells was examined using N-acetyl sphingosine (C2-ceramide) and sphingosine as apoptosis-inducing agents. Although C2-ceramide-induced apoptosis was not affected by DHA supplementation, sphingosine-induced apoptosis was reduced almost to the background level by preincubation with 10 microM DHA for 24 h. Among the fatty acids, only DHA appeared to be endowed with the ability to reduce sphingosine-induced apoptosis, whereas, other unsaturated fatty acids, such as arachidonic acid (AA) and eicosapentaenoic acid (EPA), did not show this activity. Incubation of HL60 with DHA within 6 h did not affect the apoptosis, suggesting that DHA probably expressed the inhibitory activity after modulation of the membrane fatty acid composition. DHA also attenuated the apoptosis induced by dimethylsphingosine and H-7, but not by calphostin C, indicating that enrichment of DHA in membranous phospholipid does not necessarily prevent all of the apoptosis associated with the inhibition of protein kinase C. The mechanism of the inhibition against sphingosine-induced apoptosis by DHA remains to be further explored. However, the inhibition of cytosolic phospholipase A2 (cPLA2) may be involved in the mechanism, because distinctive inhibitory activity of DHA against cPLA2 has been demonstrated [M. Shikano, Y. Masuzawa, K. Yazawa, K. Takayama, I. Kudo, K. Inoue, Biochim. Biophys. Acta, 1212, 1994, 211-216], and arachidonyl trifluoromethylketone, a specific inhibitor of cPLA2, attenuated the apoptosis induced by sphingosine.

Bcl-2 slows in vitro breast cancer growth despite its antiapoptotic effect

BACKGROUND

Although the Bcl-2 protein promotes tumor cell survival by blocking programmed cell death (apoptosis), Bcl-2 expression has been associated with favorable prognostic indicators in breast cancer. We hypothesize that despite its antiapoptotic effects, Bcl-2 slows tumor cell proliferation.

MATERIALS AND METHODS

Bcl-2-negative breast cancer cells (SKBr3) were transfected with the bcl-2 gene (Bcl2-1 clone, low expression; Bcl2-2 clone, high expression) or plasmid control (Neo). Cell cycle distribution and kinetics were analyzed using bivariate flow cytometry (PI staining and pulse BrdU uptake). Cells were treated for 72 h with doxorubicin (100 ng/ml) or vehicle (0.01% DMSO) and assayed for cytosolic DNA with diphenylamine to measure apoptosis.

RESULTS

Cell counting showed increased doubling time in the Bcl-2-expressing clones Bcl2-1 and Bcl2-2 (Bcl-2(+)) relative to the Bcl-2-nonexpressing lines SKBr3 and Neo (Bcl-2(-)). Cell cycle analysis showed a decreased S phase fraction in Bcl-2(+) cells. Pulse BrdU uptake showed an increased G1/G0 fraction in Bcl-2(+) cells. Doxorubicin-induced apoptosis occurred in Bcl-2(-) but not in Bcl-2(+) cell lines.

CONCLUSIONS

Despite antiapoptotic effects favoring tumor survival, Bcl-2 prolongs cell cycle. Decreased tumor proliferation may account for the association of Bcl-2 expression with a favorable outcome in breast cancer, even though Bcl-2 may mediate chemoresistance in some patients.

Neuronal cyclin expression in the hippocampus in temporal lobe epilepsy

This study used immunocytochemistry to explore the expression of cyclins A, B, D, and E and the apoptosis-associated Bax protein in hippocampal subfields of 35 lobectomy specimens with medial temporal lobe sclerosis removed for the treatment of temporal lobe epilepsy (TLE), 2 age-matched controls, and 2 elderly patients suffering from drug-responsive epilepsy. Cyclins A and D were not detected at all in neuronal nuclei. Cyclin E was only rarely detected in neuronal nuclei in drug-controlled and TLE groups and in controls. Cyclin B was expressed in significantly more neuronal nuclei in the hippocampi in TLE than in the other groups studied. The nuclear expression of these proteins suggested that neurons had reentered the cell division cycle and reached the G2 phase. The nuclear expression of cyclin B in the hippocampus from these patients was accompanied by neuronal cytoplasmic expression of the death-related Bax protein. We interpret these neuronal findings as evidence of cell cycle disturbances and a possible apoptotic mechanism of hippocampal neuronal cell death in TLE.

Accumulation of cyclin B1, activation of cyclin B1-dependent kinase and induction of programmed cell death in human epidermoid carcinoma KB cells treated with taxol

Cyclin B1 plays a critical role in regulating cell-cycle progression from G2 through M phase (including exit from M phase). In this study, we investigated the relationship between taxol-induced M-phase arrest, disruption of the cyclin B1-regulation pathway and apoptosis in KB cells. Continuous exposure of KB cells to 0.5 microg/ml taxol caused mitotic arrest and >90% cell death at 48 hr. Mitotic blockade peaked at 24 hr, with 68% of cells in mitosis at that time compared with 3% at baseline, and decreased thereafter. Apoptosis assessed by morphological changes and DNA ladder fragmentation was a later event, peaking at 48 hr (later time points were not studied). Taxol also caused an increase in cyclin B1 accumulation, as assessed by Western blot analysis, and stimulated cyclin B1-dependent kinase. Cyclin B1 accumulation and kinase stimulation peaked at 12 and 24 hr, respectively, at which times they were 5-fold and 90-fold higher than in control untreated cells. These effects decreased thereafter. All taxol-induced cellular effects were abrogated by the protein and RNA synthesis inhibitors cycloheximide and actinomycin D. In contrast, the endonuclease inhibitors aurintricarboxilic acid and zinc markedly inhibited taxol-induced DNA ladder fragmentation without altering taxol-induced cell-cycle arrest, cyclin B1 accumulation, activation of cyclin B1 kinase activity and cytotoxicity. We conclude that taxol-induced stimulation of cyclin B1-dependent kinase activity parallels mitotic arrest, is more pronounced than mitotic arrest and precedes the induction of programmed cell death.

Cleavage of p21Cip1/Waf1 and p27Kip1 mediates apoptosis in endothelial cells through activation of Cdk2: role of a caspase cascade

Apoptosis of human endothelial cells after growth factor deprivation is associated with rapid and dramatic up-regulation of cyclin A-associated cyclin-dependent kinase 2(cdk2) activity. In apoptotic cells, the C termini of the cdk inhibitors p21Cip1/Waf1 and p27Kip1 are truncated by specific cleavage. The enzyme involved in this cleavage is CPP32 and/or a CPP32-like caspase. After cleavage, p21Cip1/Waf1 loses its nuclear localization sequence and exits the nucleus. Cleavage of p21Cip1/Waf1 and p27Kip1 results in a substantial reduction in their association with nuclear cyclin-cdk2 complexes, leading to a dramatic induction of cdk2 activity. Dominant-negative cdk2, as well as a mutant of p21Cip1/Waf1 resistant to caspase cleavage, partially suppress apoptosis. These data suggest that cdk2 activation, through caspase-mediated cleavage of cdk inhibitors, may be instrumental in the execution of apoptosis following caspase activation.

Upregulation of apoptosis with dietary restriction: implications for carcinogenesis and aging

The maintenance of cell number homeostasis in normal tissues reflects a highly regulated balance between the rates of cell proliferation and cell death. Under pathologic conditions such as exposure to cytotoxic, genotoxic, or nongenotoxic agents, an imbalance in these rates may indicate subsequent risk of carcinogenesis. Apoptotic cell death, as opposed to necrotic cell death, provides a protective mechanism by selective elimination of senescent, preneoplastic, or superfluous cells that could negatively affect normal function and/or promote cell transformation. The relative efficiency or dysfunction of the cell death program could therefore have a direct impact on the risk of degenerative or neoplastic disease. Dietary restriction of rodents is a noninvasive intervention that has been reproducibly shown to retard tumor development and most physiologic indices of aging relative to ad libitum-fed animals. As such, it provides a powerful model in which to study common mechanistic processes associated with both aging and cancer. In a recent study we established that chronic dietary restriction (DR) induces an increase in spontaneous apoptotic rate and a decrease in cell proliferation rate in hepatocytes of 12-month-old B6C3F1 DR mice relative to ad libitum (AL)-fed mice. This diet-induced shift in cell death/proliferation rates was associated with a marked reduction in subsequent development of spontaneous hepatoma and a marked increase in disease-free life span in DR relative to AL-fed mice. These results suggest that total caloric intake may modulate the rates of cell death and proliferation in a direction consistent with a cancer-protective effect in DR mice and a cancer-promoting effect in AL mice. To determine whether the increase in spontaneous apoptotic rate was maintained over the life span of DR mice, apoptotic rates were quantified in 12-, 18-, 24- and 30-month-old DR and AL mice. The rate of apoptosis was elevated with age in both diet groups; however, the rate of apoptosis was significantly and consistently higher in DR mice regardless of age. In double-labeling experiments, an age-associated increase in the glutathione S-transferase-II expression in putative preneoplastic hepatocytes in AL mice was rapidly reduced by apoptosis upon initiation of DR. Thus, intervention that promote a low-level increase in apoptotic cell death may be expected to protect genotypic and phenotypic stability with age. If during tumor promotion an adaptive increase in apoptosis effectively balances the dysregulated increase proliferation, the risk of permanent genetic error and carcinogenesis would be minimized.

Programmed cell death in plant disease: the purpose and promise of cellular suicide

The interaction of pathogens with plants leads to a disruption in cellular homeostasis, often leading to cell death, in both compatible and incompatible relationships. The mechanistic basis of this cellular disruption and consequent death is complex and poorly characterized, but it is established that host responses to pathogens are dependent on gene expression, involve signal transduction, and require energy. Recent data suggest that in animals, a genetically regulated, signal transduction-dependent programmed cell death process, commonly referred to as apoptosis, is conserved over a wide range of phyla. The basic function of apoptosis is to direct the selective elimination of certain cells during development, but it also is a master template that is involved in host responses to many pathogens. Programmed cell death in plants, while widely observed, has not been studied extensively at either the biochemical or genetic level. Current data suggest that activation or suppression of programmed cell death may underlie diseases in plants as it does in animals. This review describes some of the fundamental characteristics of apoptosis in animals and points to a number of connections to programmed cell death in plants that may lead to both a better understanding of disease processes and novel strategies for engineering disease resistance in plants.

Molecular/cellular biology of the heat stress response and its role in agent-induced teratogenesis

Available data indicate that heat shock proteins act as chaperones under non-stress conditions by assisting in: (1) the folding of newly synthesized proteins, (2) the intracellular translocation of proteins, and (3) the function of other proteins. As we gain additional information concerning cellular physiology, we may find that heat shock proteins play a key role in many additional cellular functions. When cells experience thermal or chemical stress, heat shock proteins take on a new role, conserved from bacteria to humans, of protecting cells from the detrimental effects of stress. This latter role takes on added significance for the embryo in which the developmental program must be read linearly, with little opportunity to cycle backward to complete a missed segment of the program. Although circumstantial evidence clearly implicates heat shock proteins in protecting embryos from thermal stress, definitive evidence is still lacking. The challenge for the future is to obtain such definitive data. Ideally, such information will lead to new therapeutic paradigms that will afford protection to the human embryo/fetus exposed to thermal/chemical stress.

The promyelocytic leukemia protein PML has a pro-apoptotic activity mediated through its RING domain

The promyelocytic leukemia protein PML is known to form nuclear multiprotein complexes which are compromised in several pathogenic conditions including acute promyelocytic leukemia. We show that in cells infected with a single stranded RNA virus, which relocates PML bodies to the cytoplasm, the infected cells are more resistant to serum starvation induced apoptosis than their uninfected counterparts. Antisense PML oligonucleotides increase cell survival under serum deprivation conditions indicating that PML is directly involved in the apoptotic activity. Transient transfection studies have indicated that this pro-apoptotic activity of PML is mediated through the zinc binding region known as the RING finger. Viral attack of PML nuclear bodies appears to allow the virus to deregulate host cell apoptotic machinery in order to establish chronic infection.

Inhibition of proliferation and apoptosis of human and rat T lymphocytes by curcumin, a curry pigment

Curcumin (diferuoylmethane), the yellow pigment in the rhizome of tumeric (Curcuma longa), an ingredient of curry spice, is known to exhibit a variety of pharmacological effects including antitumor, antiinflammatory, and antiinfectious activities. Although its precise mode of action remains elusive, curcumin has been shown to suppress the activity of the AP-1 transcription factor in cells stimulated to proliferate. In this study, we observed that curcumin (50 microM) inhibited proliferation of rat thymocytes stimulated with concanavalin A (Con A) as well as that of human Jurkat lymphoblastoid cells in the logarithmic growth phase. The pigment also inhibited apoptosis in dexamethasone-treated rat thymocytes and in UV-irradiated Jurkat cells as judged by DNA ladder formation, cellular morphological changes, and flow cytometry analysis. The inhibition of apoptosis by curcumin in rat thymocytes was accompanied by partial suppression of AP-1 activity. Complete suppression of AP-1 activity was observed in Con A-treated, proliferating thymocytes. The capacity of curcumin to inhibit both cell growth and death strongly implies that these two biological processes share a common pathway at some point and that curcumin affects a common step, presumably involving a modulation of the AP-1 transcription factor.

Zygotic transcription is required to block a maternal program of apoptosis in Xenopus embryos

At the midblastula transition during Xenopus development, the cell cycle is remodeled, and zygotic transcription is initiated. Additionally, cyclin E1 is degraded at the midblastula transition independently of protein synthesis, the number of cell cycles, and the nuclear-to-cytoplasmic ratio. In the studies reported here, cell cycles were delayed by transient inhibition of protein synthesis with cycloheximide (100 microg/ml) prior to the midblastula transition. Even after reaccumulation of mitotic cyclins and resumption of cell divisions, cycloheximide-treated embryos did not resume DNA synthesis, failed to initiate transcription, and synchronously became apoptotic before the gastrula stage. These results were independent of the stage at which embryos were treated or the duration of treatment. Inhibition of zygotic transcription with alpha-amanitin also induced apoptosis. These data suggest that a developmental checkpoint at the midblastula transition is maternally regulated and can trigger apoptosis. Apoptosis induced by cycloheximide or alpha-amanitin was blocked by injection of RNA encoding Xenopus Bcl-2, suggesting that this maternal program is normally blocked by expression of an apoptotic inhibitor. Embryos pulsed with lower doses of cycloheximide (10 microg/ml) delayed development prior to the midblastula transition but resumed DNA synthesis, initiated transcription, and gastrulated normally. This indicates that the apoptotic response is initiated only when delayed embryos are unable to support initiation of zygotic transcription.

Cell cycle regulation of membrane glucocorticoid receptor in CCRF-CEM human ALL cells: correlation to apoptosis

The human leukemic cell line (CCRF-CEM) and a subline enriched for the plasma membrane-resident glucocorticoid receptor (mGR) were studied for the influence of the cell cycle on the expression and function of mGR. Three synchronization procedures (double thymidine, colcemid, and combined thymidine-colcemid blocks) were used. Fluorescent microscopy and flow cytometry simultaneously assessed antibody-tagged mGR and DNA. In addition, mGR was quantitated and characterized by immunoprecipitation and immunoblotting. Apoptosis was assayed by DNA fragmentation (TUNEL assay) and by cell survival (trypan blue exclusion). All synchronization procedures demonstrated that progression from DNA replication (S) to the second growth phase and mitosis (G2/M) leads to cells having the highest levels of mGR expression and being highly glucocorticoid sensitive in the apoptosis assays: 32 and 80% sensitivity of wild type and mGR-enriched cells, respectively, compared with 12 and 30% sensitivity in asynchronous cells. Therefore, mGR expression appears to be cell cycle regulated, with its highest expression at late S-G2/M, when the cells are most sensitive to the lymphocytolytic effects of glucocorticoids.

Apoptosis in colorectal carcinoma occurring in patients aged 45 years and under: relationship to prognosis, mitosis, and immunohistochemical demonstration of p53, c-myc and bcl-2 protein products

The aim of this study was to ascertain whether apoptotic counts have prognostic significance in colorectal cancer and if such counts are related to the expression of proteins implicated in cell cycle regulation. Material from a cohort of patients aged 45 years or less with colorectal carcinoma was re-examined to determine apoptotic and mitotic counts by light microscopy, in addition to assessing p53, c-myc, and bcl-2 protein status by immunohistochemistry. The apoptotic index in the 74 patients who were alive or who had died of colorectal carcinoma ranged from 1.2 per cent to 12.3 per cent and exhibited independent prognostic significance, with high counts predicting better survival (P = 0.02). Mitotic counts were not related to survival, despite a close correlation with apoptosis (r = 0.85). Tumours regarded as not staining with the CM1 antibody for p53 protein demonstrated higher apoptotic counts, compared with those that stained (medians 5.2 and 4.0 per cent, respectively; P = 0.03), but p53 expression was found not to be related to survival. The 68 tumours which stained for c-myc appeared to exhibit higher mitotic counts than those that did not. bcl-2 was detected in only four tumours. The latter two proteins exhibited no apparent relationship to the apoptotic index or survival. Although these results indicate a potential role for apoptotic counting in prognostic prediction in colorectal tumours, this is an uncommon group of patients who exhibited some atypical features. The likelihood of a proportion of cases arising within hereditary non-polyposis colorectal cancer syndrome may limit the application of the findings to a more general population with cancer of the colon and rectum. Further work is required, including critical measurement of reproducibility and assessment of the relative impact of this parameter compared with 'traditional' prognostic markers.