Commitment and effector phases of the physiological cell death pathway elucidated with respect to Bcl-2 caspase, and cyclin-dependent kinase activities

Cdk2 phosphorylation of Bcl-xL after stress converts it to a pro-apoptotic protein mimicking Bax/Bak

Apoptosis is a regulated form of cell death that proceeds by defined biochemical pathways. Most apoptosis is controlled by interactions between pro-survival and pro-apoptotic Bcl-2 family proteins in which death is often the consequence of permeabilization of the mitochondrial outer membrane. Many drugs affect this equilibrium to favor apoptosis but this process is not completely understood. We show that the chemotherapeutic drug cisplatin initiates an apoptotic pathway by phosphorylation of a pro-survival Bcl-2 family member, Bcl-xL, by cyclin-dependent kinase 2. The phosphorylation occurred at a previously unreported site and its biologic significance was demonstrated by a phosphomimetic modification of Bcl-xL that was able to induce apoptosis without addition of cisplatin. The mechanism of cell death induction was similar to that initiated by pro-apoptotic Bcl-2 family proteins, that is, phosphorylated Bcl-xL translocated to the mitochondrial membrane, and formed pores in the membrane. This initiated cytochrome c release and caspase activation that resulted in cell death.

Paclitaxel Induces Apoptosis in AIDS-Related Kaposi's Sarcoma Cells

Paclitaxel is a microtubule stabilizing drug that causes dividing cells to arrest and then undergo apoptosis. It also has antiangiogenic activity because it alters cytoskeletal structure, affecting migration and invasion. Paclitaxel is an effective treatment for AIDS-related Kaposi’s sarcoma (KS). KS is a tumor in which there is marked proliferation of endothelial cells in addition to the tumor cells, which themselves share many markers with activated (proliferating) endothelial cells.We sought to determine the mechanism by which paclitaxel exerts its anti-KS tumor effects. In vitro, KS cells are very sensitive to paclitaxel, with half-maximal growth inhibition observed at 0.8 nM. Inhibition of migration of KS cells was also observed at nanomolar concentrations of the drug. Paclitaxel induced cell cycle arrest with an accumulation of cells in sub-G1.This was accompanied in vitro by various events typical of apoptosis: phosphorylation of two anti-apoptotic proteins Bcl-2 and Bcl-_xL , release of cytochrome c into the cytoplasm, cleavage and activation of caspase-3. In vitro results were borne out by studies of KS tumor xenografts in nude mice. Paclitaxel (10 mg/kg) inhibited tumor growth by 75% over 21 days. Histological examination of the tumors revealed a decrease in proliferative index, a decrease in the number of mitotic figures and an increase in apoptotic cells compared to tumors from untreated mice.

Cell cycle and apoptosis regulatory gene expression in the bone marrow of patients with de novo myelodysplastic syndromes (MDS)

Deregulation of cell cycle and apoptosis pathways are known contributors to the pathogenesis of myelodysplastic syndromes (MDS). However, the underlying mechanisms are not fully clarified. The aim of our study was to examine mRNA expression levels of cell cycle and apoptosis regulatory genes, as well as the percentage of apoptotic and S phase cells and to correlate the findings with clinical characteristics and prognosis. Sixty patients with MDS, classified according to FAB (17 RA, five RARS, 19 RAEB, nine RAEBT, ten CMML) and WHO (ten RA, three RARS, seven RCMD, two RCMD-RS, 11 RAEBI, eight RAEBII, ten CMML, and nine AML) were included in the study. We found increased expression of anti-apoptotic bclxL and mcl1 genes and decreased expression of p21 gene in MDS patients. Moreover, we found increased expression of anti-apoptotic mcl1 gene in patients with higher than Intermediate-1 IPSS group. Multivariate analysis confirmed that combined expression of apoptotic caspases 8, 3, 6, 5, 2, 7, and Granzyme B was decreased in MDS patients. Regarding cell cycle regulatory genes expression, we demonstrated increased expression of cyclin D1 in patients with CMML Increased combined expression of cyclins B, C, D1, and D2 was found in patients with cytogenetic abnormalities. The two pathways seem to be interconnected as shown by the positive correlation between CDKs 1, 2, 4, p21 and the level of apoptosis and positive correlation between apoptotic caspase 3 expression and the percentage of S phase cells. In conclusion, our study showed altered expression of genes involved in apoptosis and cell cycle in MDS and increased expression of cyclin D1 in patients with CMML.

Post-translational modification of cyclin A1 is associated with staurosporine and TNFalpha induced apoptosis in leukemic cells

Understanding of molecular mechanisms underlying the effects of cell cycle proteins in response to the chemotherapeutic agents is of great importance for improving the efficacy of targeted therapeutics and overcoming resistance to chemotherapeutic agents. Staurosporine and tumor necrosis factor alpha (TNFalpha) are the therapeutic agents that inhibit tumor cell growth by inducing cell death. Staurosporine induces apoptosis through the intrinsic pathway, while TNFalpha trigger the cell death via the extrinsic apoptotic pathway. We have previously demonstrated that the cell cycle regulatory protein, cyclin A1 played an important role in the development of acute myeloid leukemia (AML), and cyclin A1 expression correlated with disease characteristics and patient outcome in leukemia. However, it remains unknown how cyclin A1 expression is regulated in leukemic cells treated with the therapeutic agents. Here, we demonstrate that cyclin A1 protein is regulated by proteasome-mediated ubiquitination and degradation in untreated U-937 cells. Interestingly, ubiquitination- and proteasomal-mediated degradation of cyclin A1 is prevented in cells treated with staurosporine or TNFalpha. Induction of apoptosis in U-937 cells by staurosporine or TNFalpha resulted in an increase in cyclin A1 protein expression, which correlated well with cyclin A1 protein modification and the activation of caspase-3. Blocking caspases activity by Z-VAD-FMK had no effect on the increased cyclin A1 expression, suggesting that cyclin A1 might be regulated by caspase-3 independent pathways. We further propose that CDC25C may be associated with cyclin A1 protein modification in response to staurosporine or TNFalpha treatment. Our results suggest that cyclin A1 protein is stabilized via post-transcriptional modification in response to apoptosis induced by staurosporine or TNFalpha.

Primary prevention of breast cancer by hormone-induced differentiation

Breast cancer is a fatal disease whose incidence is gradually increasing in most industrialized countries and in all ethnic groups. Primary prevention is the ultimate goal for the control of this disease. The knowledge that breast cancer risk is reduced by early full-term pregnancy and that additional pregnancies increase the rate of protection has provided novel tools for designing cancer prevention strategies. The protective effect of pregnancy has been experimentally reproduced in virgin rats by treatment with the placental hormone human chorionic gonadotropin (hCG). HCG prevents the initiation and inhibits the progression of chemically induced mammary carcinomas by inducing differentiation of the mammary gland, inhibiting cell proliferation, and increasing apoptosis. It also induces the synthesis of inhibin, a tumor suppressor factor, downregulates the level of expression of the estrogen receptor alpha (ER-alpha) by methylation of CpG islands, imprinting a permanent genomic signature that characterizes the refractory condition of the mammary gland to undergo malignant transformation. The genomic signature induced by hCG is identical to that induced by pregnancy and is specific for this hormone. Comparison of the mammary gland's genomic profile of virgin Sprague-Dawley rats treated daily with hCG for 21 days with that of rats receiving 17beta-estradiol (E2) and progesterone (Pg) (E2 + Pg) revealed that in hCG-treated rats 194 genes were significantly up-modulated (> 2.5 log2-folds) (p < 0.01) and commonly expressed, whereas these genes were not expressed in the E2 + Pg group. The genomic signature induced by hCG and pregnancy included activators or repressors of transcription genes, apoptosis, growth factors, cell division control, DNA repair, tumor suppressor, and cell-surface antigen genes. Our data indicate that hCG, like pregnancy, induces permanent genomic changes that are not reproduced by steroid hormones and in addition regulates gene expression through epigenetic mechanisms that are differentiation-dependent processes, leading us to conclude that hormonally induced differentiation offers enormous promise for the primary prevention of breast cancer.

Paclitaxel induced apoptosis in breast cancer cells requires cell cycle transit but not Cdc2 activity

PURPOSE

Paclitaxel (PTX) is a widely used chemotherapy agent and may cause cell death by apoptosis subsequent to microtubule (MT) disruption. In this paper, we have investigated whether cell cycle transit and or Cdc2 (Cdk1) activity is required for the apoptosis induced by PTX.

METHODS

Cell cycle was analyzed by flow cytometry, Cdc2 was assayed bio chemically. Cdc2 activity was decreased by siRNA and dominant negative (dn) Cdc2 expression. Cells were arrested by chemical or biological inhibitors in a G1 or S phase. Apoptosis was measured by DNA fragmentation and examination of nuclei by microscopy. JNK and AKT activations were assessed as well.

RESULTS

Cell cycle inhibition was highly effective in decreasing PTX induced apoptosis. MT morphology was not altered by these inhibitors. PTX induced JNK activity or AKT mediated BAD phosphorylation was unaffected by cell cycle inhibitors. Abrogation of Cdc 2 activity was without effect on PTX induced apoptosis.

CONCLUSIONS

While cell cycle transit is required for PTX induced apoptosis; Cdc2 activity is not required.

Analysis of cyclin B1 and CDK activity during apoptosis induced by camptothecin treatment

We have studied the role of cyclins and cyclin-dependent kinase (CDK) activity in apoptosis induced by camptothecin (CPT). In this model, 22% of the cells stain for annexin-V at 24 h and then proceed to be 93% positive by 72 h. This time window permits the analysis of cyclins in cells that are committed to apoptosis but not yet dead. We provide evidence that cyclin protein levels and then associated kinase levels increase after CPT treatment. Strikingly, cyclin B1 and cyclin E1 proteins are present at the same time in CPT treated HT29 cells. Although cyclin B1 and E1 CDK complexes are activated in CPT treated cells, only the cyclin B1 complex is required for apoptosis since reduction of cyclin B1 by RNAi or roscovitine treatment reduces the number of annexin-V-stained cells. We have detected poorly organized chromosomes and phosphorylated histone H3 epitopes at the time of maximum cyclin B1/CDK kinase activity in CPT-treated cells, which suggests that these cells enter a mitotic catastrophe. Understanding which CDKs are required for apoptosis may allow us to better adapt CDK inhibitors for use as anti-cancer compounds.

Effects of sulfur dioxide on apoptosis-related gene expressions in lungs from rats

Sulfur dioxide (SO2) is an air pollutant in densely populated areas as well as in areas polluted by coal-fired power plants, smelters, and sulfuric acid factories. In the present study, male Wistar rats were housed in exposure chambers and treated with 14.00+/-1.01, 28.00+/-1.77, and 56.00+/-3.44 mg/m3 SO2 for 6 h/day for 7 days, while control rats were exposed to filtered air in the same condition. The mRNA and protein levels of three apoptosis-related genes (p53 and bax are promoters of apoptosis, whereas bcl-2 is apoptotic suppressor) were analyzed in lungs using a real-time reverse transcription-polymerase chain reaction (real-time RT-PCR) assay and immunohistochemistry method, and caspase-3 activities were detected. The results showed that mRNA levels of p53 and bax were increased in a dose-dependent manner and at the concentrations of 28.00 and 56.00 mg/m3 SO2 the increases were significant (for p53: 1.23-fold at 28 mg/m3 and 1.39-fold at 56 mg/m3; for bax: 1.77-fold at 28 mg/m3 and 2.26-fold at 56 mg/m3, respectively), while mRNA levels of bcl-2 were decreased significantly (0.78-fold at 28 mg/m3 and 0.73-fold at 56 mg/m3) in lungs of rats exposed to SO2. Dose-dependent increase of p53 and bax proteins in the lungs was observed after SO2 inhalation, while decrease of bcl-2 protein levels was obtained using immunohistochemistry method. Caspase-3 activities were increased in lungs of rats after SO2 inhalation. These results lead to a conclusion that SO2 exposure can change the expression of apoptosis-related genes, and it suggests that SO2 can induce apoptosis in lung of rat and may have relations with some apoptosis-related diseases. Elucidating the expression patterns of those factors after SO2 inhalation may be critical to our understanding mechanisms of SO2 toxicity and helpful for the therapeutic intervention.

Inhibiting myosin light chain kinase induces apoptosis in vitro and in vivo

Previous short-term studies have correlated an increase in the phosphorylation of the 20-kDa light chain of myosin II (MLC20) with blebbing in apoptotic cells. We have found that this increase in MLC20 phosphorylation is rapidly followed by MLC20 dephosphorylation when cells are stimulated with various apoptotic agents. MLC20 dephosphorylation is not a consequence of apoptosis because MLC20 dephosphorylation precedes caspase activation when cells are stimulated with a proapoptotic agent or when myosin light chain kinase (MLCK) is inhibited pharmacologically or by microinjecting an inhibitory antibody to MLCK. Moreover, blocking caspase activation increased cell survival when MLCK is inhibited or when cells are treated with tumor necrosis factor alpha. Depolymerizing actin filaments or detaching cells, processes that destabilize the cytoskeleton, or inhibiting myosin ATPase activity also resulted in MLC20 dephosphorylation and cell death. In vivo experiments showed that inhibiting MLCK increased the number of apoptotic cells and retarded the growth of mammary cancer cells in mice. Thus, MLC20 dephosphorylation occurs during physiological cell death and prolonged MLC20 dephosphorylation can trigger apoptosis.

Cytotoxic chemotherapy upregulates pro-apoptotic Bax and Bak in the small intestine of rats and humans

AIMS

Small intestinal crypt cells rapidly undergo apoptosis in response to cytotoxic drug treatment that results in gastrointestinal toxicity. The Bcl-2 family have been implicated in both positive and negative regulation of intestinal cell apoptosis. The aim of this study was to examine the effect of cytotoxic treatment on Bcl-2 protein expression in patients and rats with tumours.

METHODS

Four pro- and four anti-apoptotic members of the Bcl-2 family, caspase-3 and p53 were examined in small intestinal crypts before and after treatment in rats and humans. Immunohistochemistry identified changes in protein expression over time, while relative RT-PCR was used to investigate mRNA expression in rat small intestine.

RESULTS

Cytotoxic treatment increased p53 and caspase-3 which coincided with elevated levels of apoptosis. Bax and Bak protein and mRNA expression also significantly increased at 6 hours following treatment in rats. Bax and Bak protein increased at day 1 after treatment in humans. Anti-apoptotic Mcl-1 protein decreased within 24hours. Other Bcl-2 family members showed only modest changes.

CONCLUSION

Increased expression of Bax and Bak but not other Bcl-2 family members is associated with apoptosis in small intestinal crypts and may amplify the sensitivity and susceptibility of crypt cells to chemotherapy-induced enteropathy.

Caspase activity is not sufficient to execute cell death

Molecular studies of the physiological cell death process have focused attention on the role of effector caspases as critical common elements of the lethal mechanism. Diverse death signals act afferently via distinct signaling pathways to activate these resident proenzyme molecules post-translationally. Whether this molecular convergence represents the mechanistic point of irreversible commitment to cell death has not been established. That a number of caspase substrates are proteins that serve important roles in cellular homeostasis has led to the view that the acquisition of this activity must be the determinative step in cell death. Observations that caspases serve in a regulatory role to catalyze the appearance of new activities involved in orderly cellular dissolution challenge this model of death as a simple process of proteolytic destruction. We found previously that caspase-dependent nuclear cyclin dependent kinase 2 (Cdk2) activity appears to be necessary for cell death. Employing direct cytofluorimetric analyses of intracellular caspase activity and colony forming assays, we now show that transient blockade of caspase-dependent Cdk2 activity confers long-lived sparing from death on cells otherwise triggered to die and fully replete with caspase activity. These data demonstrate that caspases, while necessary for apoptosis, are not sufficient to exert lethality. Caspase activation per se does not represent an irreversible point of commitment to physiological cell death.

pRb2/p130 promotes radiation-induced cell death in the glioblastoma cell line HJC12 by p73 upregulation and Bcl-2 downregulation

This study shows that in the glioblastoma hamster cell line HJC12 the retinoblastoma family member pRb2/p130 enhances gamma-radiation-induced cell death. In HJC12 cells the tetracycline-regulated expression of pRb2/p130 increased the percentage of gamma-radiation-induced apoptotic cells from 27 to 47%. pRb2/p130 overexpression was associated with the downregulation of the anti-apoptotic factor Bcl-2 and the upregulation of the steady-state protein levels of the pro-apoptotic transcription factor p73. In particular, RT-PCR showed a significant increase in the expression of the p73delta isoform when pRb2/p130 was overexpressed. The ability of pRb2/p130 to modulate apoptosis was not associated with its role in mediating G0/G1 arrest during cell cycle progression. Our data suggest a role for pRb2/p130 in glioblastoma gamma-radiation-induced cell death, indicating that the antitumoral action of pRb2/p130 can regulate both inhibition of cell cycle progression and induction of cell death.

Multiparameter analyses of cell cycle regulatory proteins in human breast cancer: a key to definition of separate pathways in tumorigenesis

Breast cancer is one of the most common cancer forms affecting many women. The disease nevertheless has widely varying behavior and therefore patient outcome, and an important undertaking is to define and understand the molecular mechanisms behind these actions. Defects in the G1/S transition in the cell cycle affect both tumor proliferation and the fidelity of check points responsible for chromosomal integrity and DNA damage response and has lately been shown to represent one of a rather limited set of key aberrations in the transformation process. Many cell cycle regulatory proteins are either oncogenes or suppressor genes or are closely associated to the transformation process. The types of aberrations in the G1/S transition seem to be different in various cancers but are nevertheless often linked to clinical behaviors. In this review the role of multiparameter analyses of cell cycle regulatory proteins in breast cancer will be outlined with special attention to pattern analyses as well as the definition of two contrasting pathways in tumorigenesis defined by either cyclin D1 or cyclin F overexpression.

The effector phase of physiological cell death relies exclusively on the posttranslational activation of resident components

Inhibitors of transcription and translation can protect cells from physiological cell deaths induced by a variety of stimuli. These observations have been taken to suggest that de novo macromolecular synthesis may be an essential component of the cell death process. Paradoxically, the same inhibitors, at higher concentrations, themselves trigger the death of cells. Previously, we have mapped a conserved and ordered sequence of events that exerts physiological cell death. Diverse signals converge to activate this lethal pathway, composed of a proteolytic cascade of caspases and subsequent cyclin-dependent kinases. Here we report that inhibitors of nuclear gene expression, when they block cell death, act upstream of this lethal process to prevent its activation. In contrast, when cell death is triggered by high doses of the inhibitors, these same essential molecules are activated, despite the essentially complete blockade of macromolecular synthesis. This inhibitor-induced death response is associated with the release of cytochrome c from mitochondria and the activation of apical caspase 9 and is blocked by overexpression of Bcl-2. These data demonstrate that all essential molecules that exert lethality already are resident within cells and are activated posttranslationally upon stimulation. De novo macromolecular synthesis pertains idiosyncratically only to upstream, modulatory elements of particular death responses.

Different molecular events account for butyrate-induced apoptosis in two human colon cancer cell lines

We studied the molecular events underlying butyrate-induced apoptosis in two different colon cancer cell lines: Caco-2, a well defined cancer cell and RSB, a cell line obtained from a colonic tumor of an ulcerative colitis patient. Caco-2 and RSB cells were exposed to 2, 5 and 10 mmol/L butyrate for 48 h. Caspase-1 was cleaved in Caco-2-cells at all butyrate concentrations, whereas in RSB-cells caspase-1 expression was undetectable. In RSB cells, butyrate dose-dependently induced caspase-3 cleavage, whereas in Caco-2-cells, butyrate up-regulated expression of the caspase-3 active subunit. Caspase-3-specific activity, cytoplasmic nucleosome concentration and growth were directly correlated with butyrate doses in both cell lines; however, the response was more pronounced in Caco-2 than in RSB cells. Expression of the cleaved poly(ADP-ribose) polymerase (PARP) product was elevated in both cell lines at the highest butyrate concentration. Bak expression gradually increased as a function of butyrate concentrations in both cell lines. At 10 mmol/L butyrate, expression increased by fivefold and sevenfold in Caco-2 and RSB cells, respectively. The highest expression of Bcl-2 was observed in control Caco-2 cells, and expression decreased with increasing butyrate concentration. This effect was not observed in RSB cells. Inactivation of caspase-1 with Z-YVAD-FMK abrogated butyrate-induced apoptosis in Caco-2 but not in RSB cells. Inactivation of caspase-3 with Z-DVED-FMK completely inhibited butyrate-induced apoptosis in RSB cells whereas this effect was less pronounced in Caco-2 cells. Our data demonstrate that butyrate-induced apoptosis is activated via different apoptotic pathways in diversely stratified colon cancers.

Inhibition of ultraviolet B (UVB) induced apoptosis in A431 cells by mimosine is not dependent on cell cycle arrest

Ultraviolet (UV) radiation is a strong apoptotic trigger in many cell types. We have previously reported that a plant amino acid, mimosine (beta [N-(3-hydroxy-4-pyridone)]-alpha-aminopropionic acid), with a well-known reversible G1 cell cycle arrest activity can inhibit apoptosis induced by UV irradiation and RNA polymerase II blockage in human A431 cells. Here, apoptosis was measured with a fluorimetric caspase activation assay. Interestingly, the protective state was effective up to 24 h following removal of mimosine from the culture medium while cells were progressing in the cell cycle. Our results demonstrate that the protective effect of mimosine against UV-induced apoptosis can be dissociated from its G1 cell-cycle arrest activity.

Involvement of the mitochondrial death pathway in chemopreventive benzyl isothiocyanate-induced apoptosis

In the present study, we studied the molecular mechanism underlying cell death induced by a cancer chemoprotective compound benzyl isothiocyanate (BITC). The cytotoxic effect of BITC was examined in rat liver epithelial RL34 cells. Apoptosis was induced when the cells were treated with 20 mum BITC, characterized by the appearance of phosphatidylserine on the outer surface of the plasma membrane and caspase-3 activation, whereas no caspase activation and propidium iodide incorporation into cell were detected with 50 mum BITC that induced necrosis. The mitochondrial death pathway was suggested to be involved in BITC-induced apoptosis because the treatment of cells with BITC-induced caspase-9-dependent apoptosis and mitochondrial transmembrane potential (Delta Psi m) alteration. We demonstrated here for the first time that BITC directly modifies mitochondrial functions, including inhibition of respiration, mitochondrial swelling, and release of cytochrome c. Moreover, glutathione depletion by diethyl maleate significantly accelerated BITC-triggered apoptosis, suggesting the involvement of a redox-dependent mechanism. This was also implicated by the observations that intracellular accumulation of reactive oxygen species, including superoxide (O(2)) and hydroperoxides (HPOs), was indeed detected in the cells treated with BITC and that the intracellular HPO level was significantly attenuated by pretreatment with N-acetylcysteine. The treatment with a pharmacological scavenger of O(2), Tiron, also diminished the HPO formation by approximately 80%, suggesting that most of the HPOs were H(2)O(2) derived from the dismutation of O(2). These results suggest that BITC induces apoptosis through a mitochondrial redox-sensitive mechanism.

Transgenic Bcl-2 expressed in photoreceptor cells confers both death-sparing and death-inducing effects

To examine its potential role within the retina as a modulator of cell death and photoreceptor degeneration, bcl-2 expression was targeted to the photoreceptors of transgenic mice by the human IRBP promoter. Three transgenic families were established, with levels of transgene expression between 0.2 and two-fold relative to that of endogenous bcl-2. The effect of bcl-2 expression on genetically programmed photoreceptor degeneration was evaluated by crossing these transgenic mice with mice that develop a rapid degeneration of rod photoreceptors due to expression of a distinct transgene, SV40 T antigen (Tag). Transgenic Bcl-2 was localized to photoreceptor inner segments and was capable of abrogating the activation of caspase activity and the resulting cell death associated with ectopic expression of Tag. However, Bcl-2 itself ultimately caused photoreceptor cell death and retinal degeneration. Several proteins not expressed normally in Tag or other transgenic retinas undergoing photoreceptor degeneration were induced in the Bcl-2 transgenic retinas. Analysis by mass spectroscopy identified one of these proteins as alphaA-crystallin, a member of a protein family that associates with cellular stress. Since Bcl-2 can promote as well as spare cell death in the same photoreceptor population, its potential utility in ameliorating photoreceptor death in human hereditary blinding disorders is compromised.

c-Myc is necessary for DNA damage-induced apoptosis in the G(2) phase of the cell cycle

The c-myc proto-oncogene encodes a transcription factor that participates in the regulation of cellular proliferation, differentiation, and apoptosis. Ectopic overexpression of c-Myc has been shown to sensitize cells to apoptosis. We report here that cells lacking c-Myc activity due to disruption of the c-myc gene by targeted homologous recombination are defective in DNA damage-initiated apoptosis in the G(2) phase of the cell cycle. The downstream effector of c-Myc is cyclin A, whose ectopic expression in c-myc(-/-) cells rescues the apoptosis defect. The kinetics of the G(2) response indicate that the induction of cyclin A and the concomitant activation of Cdk2 represent an early step during commitment to apoptosis. In contrast, expression of cyclins E and D1 does not rescue the apoptosis defect, and apoptotic processes in G(1) phase are not affected in c-myc(-/-) cells. These observations link DNA damage-induced apoptosis with cell cycle progression and implicate c-Myc in the functioning of a subset of these pathways.

Membrane-targeted green fluorescent protein reliably and uniquely marks cells through apoptotic death

BACKGROUND

An understanding of the molecular processes that comprise the program of physiological cell death demands analytical techniques for the assessment of death events on the level of the individual cell, especially among transfectants and within heterogeneous populations. The utility of available transfection markers is limited by the variability of marker retention and discrimination as cells die. For example, soluble green fluorescent protein (GFP) leaks from dying cells and is not useful when fixation is required; conversely, transfected beta-galactosidase can be visualized only after fixation and staining.

METHODS

We have tested a GFP variant as a marker for the direct identification and visualization of transfected cells. We have explored the utility of this membrane-targeted GFP, the genetic fusion of the enhanced GFP and the farnesylation sequence of p21(Ras) (EGFP-F), in a variety of cell death assays.

RESULTS

EGFP-F is retained reliably in unfixed dying cells, permitting numerous events of the cell death process to be analyzed in real time in marked cells. Moreover, the cell rounding and shrinkage associated with the loss of adhesion during cell death result in a characteristic condensed EGFP-F signal.

CONCLUSIONS

EGFP-F serves to identify transfectants consistently, independent of their ultimate fate. Cellular condensation of EGFP-F provides a specific and quantitative measure of physiological cell death.

Cyclin A/cdk2 activation is involved in hypoxia-induced apoptosis in cardiomyocytes

Cardiomyocytes are terminally differentiated cells characterized as withdrawal cell-cycle machinery, but nonetheless they are known to express cell-cycle regulators. Because many proteins related to the cell cycle induce apoptosis in proliferating cells, we examined the involvement of these proteins in the apoptosis pathway in cardiomyocytes. Primary rat cardiomyocytes were exposed to a severe hypoxic condition to induce apoptosis. The apoptosis rate of cardiomyocytes increased to approximately 40% under 24 hours of hypoxia as evaluated by the TUNEL method. The cyclin A protein level assessed by immunoblot analysis accumulated in a time-dependent manner in cardiomyocytes, but there was no increase in nonmyocytes. Hypoxia increased the activity of cyclin A-associated kinase but not the activity of cyclin E-associated kinase, and the apoptosis was inhibited by infection of dominant-negative cdk2 adenovirus, suggesting that cyclin A and its associated kinase play significant roles in the apoptosis of cardiomyocytes. To investigate the cyclin A-mediated apoptosis, we infected cultured cells with cyclin A adenovirus. Apoptosis was induced in 63+/-12% of the infected cardiomyocytes in contrast to only 12+/-3% of the LacZ-infected control cells. In addition, the cells in the hypoxic condition showed an increase in caspase-3 activity and a subsequent decrease in p21(cip1/waf1) protein, which is partly cleaved by caspase-3. These findings confirm that cyclin A-associated kinase mediates hypoxia-induced apoptosis in cardiomyocytes, and they also suggest that additional elements of the cell-cycle-dependent machinery participate in this mechanism.

Molecular mapping of the physiological cell death process. Mitochondrial events may be disordered

The mitochondrion plays a central role in Bcl-2-inhibitable physiological cell deaths. The detailed order of mitochondrial and other events during cell death in vivo remains ambiguous, however. As part of an effort to explore this issue, we have asked whether mitochondrial dissolution during physiological cell death occurs in an orderly and concerted process. Here, we describe the characterization of two elements of mitochondrial disintegration on the level of individual cells. Using a novel cytofluorimetric approach, we have assessed simultaneously the release of cytochrome c (specifically a fluorescently tagged transfected construct) from mitochondria and the dissipation of mitochondrial membrane potential. Our results indicate that mitochondrial disintegration does not follow a strictly ordered process and is not concerted. We are extending these studies to further characterize mitochondrial events in the context of Bcl-2 family members and place them definitively within the context of the caspase cascade.

IL-4 induces apoptosis of endothelial cells through the caspase-3-dependent pathway

The present study was designed to investigate the hypothesis that interleukin-4 (IL-4) can induce apoptosis of human endothelial cells and to study regulatory pathways of this process. Indeed, DNA ladder assay and flow cytometry study showed that IL-4 can induce apoptosis of endothelial cells in a time- and dose-dependent manner. In addition, IL-4 markedly increased activity of caspase-3, and inhibition of this enzyme suppressed IL-4-induced apoptosis in a dose-dependent manner. These results provide the first evidence that IL-4 can induce apoptosis of human endothelial cells. In addition, the data indicate that the caspase-3-dependent pathway is critically involved in this process.

Mammalian caspases: structure, activation, substrates, and functions during apoptosis

Apoptosis is a genetically programmed, morphologically distinct form of cell death that can be triggered by a variety of physiological and pathological stimuli. Studies performed over the past 10 years have demonstrated that proteases play critical roles in initiation and execution of this process. The caspases, a family of cysteine-dependent aspartate-directed proteases, are prominent among the death proteases. Caspases are synthesized as relatively inactive zymogens that become activated by scaffold-mediated transactivation or by cleavage via upstream proteases in an intracellular cascade. Regulation of caspase activation and activity occurs at several different levels: (a) Zymogen gene transcription is regulated; (b) antiapoptotic members of the Bcl-2 family and other cellular polypeptides block proximity-induced activation of certain procaspases; and (c) certain cellular inhibitor of apoptosis proteins (cIAPs) can bind to and inhibit active caspases. Once activated, caspases cleave a variety of intracellular polypeptides, including major structural elements of the cytoplasm and nucleus, components of the DNA repair machinery, and a number of protein kinases. Collectively, these scissions disrupt survival pathways and disassemble important architectural components of the cell, contributing to the stereotypic morphological and biochemical changes that characterize apoptotic cell death.

Cell cycle and apoptosis

In multicellular organisms, cell proliferation and death must be regulated to maintain tissue homeostasis. Many observations suggest that this regulation may be achieved, in part, by coupling the process of cell cycle progression and programmed cell death by using and controlling a shared set of factors. An argument in favor of a link between the cell cycle and apoptosis arises from the accumulated evidence that manipulation of the cell cycle may either prevent or induce an apoptotic response. This linkage has been recognized for tumor suppressor genes such as p53 and RB, the dominant oncogene, c-Myc, and several cyclin-dependent kinases (Cdks) and their regulators. These proteins that function in proliferative pathways may also act to sensitize cells to apoptosis. Indeed, unregulated cell proliferation can result in pathologic conditions including neoplasias if it is not countered by the appropriate cell death. Translating the knowledge gained by studying the connection between cell death and cell proliferation may aid in identifying novel therapies to circumvent disease progression or improve clinical outcome.

Paclitaxel-induced cell death: where the cell cycle and apoptosis come together

BACKGROUND

Compelling evidence indicates that paclitaxel kills cancer cells through the induction of apoptosis. Paclitaxel binds microtubules and causes kinetic suppression (stabilization) of microtubule dynamics. The consequent arrest of the cell cycle at mitotic phase has been considered to be the cause of paclitaxel-induced cytotoxicity. However, the biochemical events, downstream from paclitaxel's binding to microtubules, that lead to apoptosis are not well understood.

METHODS

The authors examined recent scientific literature about the mechanisms by which paclitaxel exerts cytotoxicity.

RESULTS

In addition to an arrest of the cell cycle at the mitotic phase in paclitaxel-treated cells, recent discoveries of activation of signaling molecules by paclitaxel and paclitaxel-induced transcriptional activation of various genes indicate that paclitaxel initiates apoptosis through multiple mechanisms. The checkpoint of mitotic spindle assembly, aberrant activation of cyclin-dependent kinases, and the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) are shown to be involved in paclitaxel-induced apoptosis. Consistent with observations that microtubules of different status (e.g., cytoskeletal microtubules vs. mitotic spindles) have different sensitivity to paclitaxel, the concentration of paclitaxel appears to be the major determinant of its apoptogenic mechanisms.

CONCLUSIONS

Advances in research of the cell cycle and apoptosis have extended our understanding of the mechanisms of paclitaxel-induced cell death. Further elucidation of resistance and enhancement of paclitaxel-induced apoptosis should expedite the development of better paclitaxel-based regimens for cancer therapy.

Functional role of cyclin A on induction of fibroblast apoptosis due to ligation of CD44 matrix receptor by anti-CD44 antibody

Ligation of cell surface matrix adhesion receptors such as integrins can increase expression of specific cell cycle regulatory proteins such as cyclin A, thereby regulating cell cycle progression. Disruption of cell surface matrix receptor interaction with the extracellular matrix can trigger apoptosis. Induction of apoptosis has been linked to unscheduled up-regulation of cyclin A and activation of cyclin-A-associated dependent kinase 2 activity due to cleavage of cyclin-dependent kinase inhibitors by caspases. We have found that ligation of the cell surface matrix adhesion receptor CD44 by anti-CD44 antibody induces cell detachment and triggers apoptosis. In this report we show that ligation of CD44 by anti-CD44 antibody increases the expression of cyclin A protein prior to activation of caspase-3-like activity and morphological changes of apoptosis. Down-regulation of cyclin A protein levels by cyclin A antisense oligonucleotides dramatically decreased fibroblast apoptosis in response to anti-CD44 antibody. These data identify an important functional role of cyclin A in the induction of fibroblast apoptosis due to the ligation of the cell surface adhesion receptor CD44 by anti-CD44 antibody.

Induction of apoptosis by all-trans-retinoic acid and C2-ceramide treatment in rat stromal-vascular cultures

Apoptosis of preadipocytes and adipocytes contributes to the balance of adipose tissue mass by reducing adipocyte number. To address this phenomenon, we treated cultured rat S-V cells with all-trans-retinoic acid (RA) (10 microM) or C2-ceramide (50 microM) during adipogenesis. Gel electrophoresis of DNA from treated cells cultured in serum-free medium showed that 10 microM RA or 50 microM ceramide induced a distinct laddering pattern of DNA fragments. Cellular caspase 3 activity, another marker of apoptosis, was increased by RA (10 microM) (P < 0.05), but not by 50 microm C2-ceramide. RT-PCR results showed that RA (10 microM) decreased the expression of Bcl-2 mRNA. These results suggest that fat cell loss by apoptosis can be regulated, in part, by RA (10 microM) which increases caspase 3 activity and decreases Bcl-2 expression in rat S-V cells. C2-ceramide apparently works through a different cellular mechanism to induce apoptosis.

DNA repair is activated in early stages of p53-induced apoptosis

p53 is a complex molecule involved in apoptosis, cell cycle arrest, and DNA repair. Since apoptosis may play an important role in deletion of neoplastic cells, an understanding of the mechanism of p53-induced apoptosis may be critical for possible future therapeutic interventions. Recent evidence suggests that p53-induced apoptosis may involve members of the nucleotide excision repair (NER) family, linking these two cellular events. Our work using a temperature-sensitive p53 construct further analyzes p53-induced apoptosis in cultured murine mammary epithelial cells and also suggests that DNA repair plays a role in that process. Although p21 is induced in our system, apoptosis occurs without a detectable preceding G1 cell cycle arrest and independent of cellular alterations brought on by the temperature shift. In addition, clonogenic assays suggest that early stages of p53-induced apoptosis may be reversible upon removal of the apoptosis stimulus. As a possible explanation for this reversibility, our results show that general DNA repair activity increases early in p53-induced apoptosis. We also show that caspase-3 is activated at a timepoint when colony formation begins to drop, suggesting a possible mechanism for the point of no return in p53-induced apoptosis.

Glycogen synthase kinase-3beta facilitates staurosporine- and heat shock-induced apoptosis. Protection by lithium

The potential role of glycogen synthase kinase-3beta in modulating apoptosis was examined in human SH-SY5Y neuroblastoma cells. Staurosporine treatment caused time- and concentration-dependent increases in the activities of caspase-3 and caspase-9 but not caspase-1, increased proteolysis of poly(ADP-ribose) polymerase, and induced morphological changes consistent with apoptosis. Overexpression of glycogen synthase kinase-3beta to levels 3.5 times that in control cells did not alter basal indices of apoptosis but potentiated staurosporine-induced activation of caspase-3, caspase-9, proteolysis of poly(ADP-ribose) polymerase, and morphological changes indicative of apoptosis. Inhibition of glycogen synthase kinase-3beta by lithium attenuated the enhanced staurosporine-induced activation of caspase-3 in cells overexpressing glycogen synthase kinase-3beta. In cells subjected to heat shock, caspase-3 activity was more than three times greater in glycogen synthase kinase-3beta-transfected than control cells, and this potentiated response was inhibited by lithium treatment. Thus, glycogen synthase kinase-3beta facilitated apoptosis induced by two experimental paradigms. These findings indicate that glycogen synthase kinase-3beta may contribute to pro-apoptotic-signaling activity, that inhibition of glycogen synthase kinase-3beta can contribute to anti-apoptotic-signaling mechanisms, and that the neuroprotective actions of lithium may be due in part to its inhibitory modulation of glycogen synthase kinase-3beta.

Dietary regulation and localization of apoptosis cascade proteins in the colonic crypt

This study was designed primarily to assess the localization of apoptosis cascade proteins along the rat colonic crypt and secondarily to test whether the activity and/or localization of these proteins are affected by the enrichment of the diet with the soluble fiber pectin. Expression of apoptosis cascade proteins was assessed in isolated colonocytes harvested from the luminal and basal crypt colonocyte populations. Two different dietary regimens were tested: a standard diet (diet A), and a diet enriched in pectin (diet B), a soluble fiber that undergoes fermentation in the cecum and produces high concentrations of intracolonic short-chain fatty acids. Caspase-1 expression was maximal in luminal colonocytes of rats fed diet B, as evidenced by Western blot and immunohistological analyses. Expression of the cleaved poly(ADP-ribose) polymerase product was elevated in both the luminal and basal colonocytes of the pectin-fed group, whereas in rats fed diet A, the expression was lower, especially in basal crypt colonocytes. The highest expression of the antiapoptotic protein Bcl-2 was observed in the lower compartments of the colonic crypt tissue and was maximal in the rat group fed a standard diet. The apoptotic index in colonocytes of rats fed diet B was higher than that measured in rats fed diet A. Cumulatively, our results indicate that apoptosis cascade proteins are differentially localized along the lumen-crypt axis, and their expression and activity may be controlled by dietary components. These results may, at least partially, account for the documented protective effect of butyrogenic fibers on colorectal cancer.

Caspase-dependent Cdk activity is a requisite effector of apoptotic death events

The caspase-dependent activation of cyclin-dependent kinases (Cdks) in varied cell types in response to disparate suicidal stimuli has prompted our examination of the role of Cdks in cell death. We have tested the functional role of Cdk activity in cell death genetically, with the expression of dominant negative Cdk mutants (DN-Cdks) and Cdk inhibitory genes. Here we demonstrate that Cdk2 activity is necessary for death-associated chromatin condensation and other manifestations of apoptotic death, including cell shrinkage and the loss of adhesion to substrate. Susceptibility to the induction of the cell death pathway, including the activation of the caspase cascade, is unimpaired in cells in which Cdk2 activity is inhibited. The direct visualization of active caspase activity in these cells confirms that death-associated Cdk2 acts downstream of the caspase cascade. Cdk inhibition also does not prevent the loss of mitochondrial membrane potential and membrane phospholipid asymmetry, which may be direct consequences of caspase activity, and dissociates these events from apoptotic condensation. Our data suggest that caspase activity is necessary, but not sufficient, for the full physiological cell death program and that a requisite function of the proteolytic caspase cascade is the activation of effector Cdks.

Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis

Hereditary tyrosinemia type I is the most severe metabolic disease of the tyrosine catabolic pathway mainly affecting the liver. It is caused by deficiency of fumarylacetoacetate hydrolase, which prevents degradation of the toxic metabolite fumarylacetoacetate (FAA). We report here that FAA induces common effects (i.e., cell cycle arrest and apoptosis) in both human (HepG2) and rodent (Chinese hamster V79) cells, effects that seem to be temporally related. Both the antiproliferative and apoptosis-inducing activities of FAA are dose dependent and enhanced by glutathione (GSH) depletion with L-buthionine-(S,R)-sulfoximine (BSO). Short treatment (2 h) with 35 microM FAA/+BSO or 100 microM FAA/-BSO induced a transient cell cycle arrest at the G2/M transition (20% and 37%, respectively) 24 h post-treatment. In cells treated with 100 microM FAA/-BSO, an inactivation, followed by a rapid over-induction of cyclin B-dependent kinase occurred, which peaked 24 h post-treatment. Maximum levels of caspase-1 and caspase-3 activation were detected at 3 h and 32 h, respectively, whereas release of mitochondrial cytochrome c was maximal at 24-32 h post-treatment. The G2/M peak declined 24 h later, concomitantly with the appearance of a sub-G1, apoptotic population showing typical nucleosomal-sized DNA fragmentation and reduced mitochondrial transmembrane potential (Deltapsi(m)). These events were prevented by the general caspase inhibitor z-VAD-fmk, whereas G2/M arrest and subsequent apoptosis were abolished by GSH-monoethylester or N-acetylcysteine. Other tyrosine metabolites, maleylacetoacetate and succinylacetone, had no antiproliferative effects and induced only very low levels of apoptosis. These results suggest a modulator role of GSH in FAA-induced cell cycle disturbance and apoptosis where activation of cyclin B-dependent kinase and caspase-1 are early events preceding mitochondrial cytochrome c release, caspase-3 activation, and Deltapsi(m) loss. -Jorquera, R., Tanguay, R. M. Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis.

Cdc2 and Cdk2 kinase activated by transforming growth factor-beta1 trigger apoptosis through the phosphorylation of retinoblastoma protein in FaO hepatoma cells

The signaling pathway leading to TGF-beta1-induced apoptosis was investigated using a TGF-beta1-sensitive hepatoma cell line, FaO. Cell cycle analysis demonstrated that the accumulation of apoptotic cells was preceded by a progressive decrease of the cell population in the G(1) phase concomitant with a slight increase of the cell population in the G(2)/M phase in response to TGF-beta1. TGF-beta1 induced a transient increase in the expression of Cdc2, cyclin A, cyclin B, and cyclin D1 at an early phase of apoptosis. During TGF-beta1-induced apoptosis, the transient increase in cyclin-dependent kinase (Cdk) activities coincides with a dramatic increase in the hyperphosphorylated forms of RB. Treatment with roscovitine or olomoucine, inhibitors of Cdc2 and Cdk2, blocked TGF-beta1-induced apoptosis by inhibiting RB phosphorylation. Overexpression of Bcl-2 or adenovirus E1B 19K suppressed TGF-beta1-induced apoptosis by blocking the induction of Cdc2 mRNA and the subsequent activation of Cdc2 kinase, whereas activation of Cdk2 was not affected, suggesting that Cdc2 plays a more critical role in TGF-beta1-induced apoptosis. In conclusion, we present the evidence that Cdc2 and Cdk2 kinase activity transiently induced by TGF-beta1 phosphorylates RB as a physiological target in FaO cells and that RB hyperphosphorylation may trigger abrupt cell cycle progression, leading to irreversible cell death.

Multiparametric assessment of bursal lymphocyte apoptosis

When bursal lymphocytes are placed in cell culture, they undergo an apoptotic form of cell death that can be inhibited by phorbol esters and protein synthesis inhibitors. The goal of the current study was to evaluate the time course of this process and the inhibition of this process using several different assays to detect apoptosis: (1) terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) of lymphocyte DNA strand breaks with dUTP-FITC; (2) propidium iodide (PI) staining of lymphocyte chromatin; (3) chloromethyl-x-rosamine (CMX-Ros) binding to lymphocyte mitochondria; (4) merocyanine-540 (MC-540) binding to the lymphocyte plasma membrane; (5) flow cytometric analysis of light scatter from lymphocytes; (6) analysis of genomic DNA from lymphocytes by agarose gel electrophoresis; and (7) cellular caspase-3 activity of lymphocytes. When bursal lymphocyte apoptosis was analyzed as a function of time, or inhibited by phorbol esters or cycloheximide, all of these assays corroborated the apoptotic process. However, treatment of lymphocytes with a cytotoxic level of the proteinase inhibitor, n-ethylmaleimide (NEM) resulted in a putative, necrotic form of cell death that revealed discrepancies among the various assays in the detection of apoptotic cells. Specifically, the CMX-Ros and MC-540 assays erroneously detected the necrotic cells as being apoptotic cells following NEM treatment. These findings indicate the need for additional assays and appropriate treatment controls to verify the apoptotic process when using the CMX-Ros and MC-540 assays.

Caspase-dependent and -independent death of camptothecin-treated embryonic cortical neurons

This study investigates the mechanisms underlying death of cultured embryonic cortical neurons exposed to the DNA-damaging agent camptothecin and in particular the interdependence of the roles of cyclin-dependent kinases (Cdks), caspases, and mitochondrial function. Camptothecin evokes rapid neuronal death that exhibits nuclear features of apoptosis. This death is accompanied by loss of cytochrome c and mitochondrial transmembrane potential as well as by induction of caspase-3-like activity and caspase-2 processing. The Cdk inhibitor flavopiridol provides long-term rescue from death and prevents loss of cytochrome c and mitochondrial transmembrane potential as well as caspase activation and processing. General caspase inhibitors rescue neurons from this rapid apoptotic death but do not prevent them from undergoing delayed death in which nuclear features of apoptosis are absent. Moreover, the caspase inhibitors do not affect early cytochrome c release and delay but do not prevent the loss of transmembrane potential. Agents that directly disrupt mitochondrial function without inducing cytochrome c release lead to a caspase-independent death. These observations favor a model in which (1) DNA damage leads to Cdk activation, which lies upstream of release of cytochrome c and caspase activation; (2) cytochrome c release is caspase-independent and may occur upstream of caspase activation; (3) early apoptotic death requires caspases; and (4) delayed nonapoptotic death that occurs in the presence of caspase inhibitors is a consequence of prolonged loss of mitochondrial function. These findings shed light on the mechanisms by which DNA damage kills neurons and raise questions regarding the general utility of caspase inhibitors as neurotherapeutic agents.

Caspase-dependent and -independent death of camptothecin-treated embryonic cortical neurons

This study investigates the mechanisms underlying death of cultured embryonic cortical neurons exposed to the DNA-damaging agent camptothecin and in particular the interdependence of the roles of cyclin-dependent kinases (Cdks), caspases, and mitochondrial function. Camptothecin evokes rapid neuronal death that exhibits nuclear features of apoptosis. This death is accompanied by loss of cytochrome c and mitochondrial transmembrane potential as well as by induction of caspase-3-like activity and caspase-2 processing. The Cdk inhibitor flavopiridol provides long-term rescue from death and prevents loss of cytochrome c and mitochondrial transmembrane potential as well as caspase activation and processing. General caspase inhibitors rescue neurons from this rapid apoptotic death but do not prevent them from undergoing delayed death in which nuclear features of apoptosis are absent. Moreover, the caspase inhibitors do not affect early cytochrome c release and delay but do not prevent the loss of transmembrane potential. Agents that directly disrupt mitochondrial function without inducing cytochrome c release lead to a caspase-independent death. These observations favor a model in which (1) DNA damage leads to Cdk activation, which lies upstream of release of cytochrome c and caspase activation; (2) cytochrome c release is caspase-independent and may occur upstream of caspase activation; (3) early apoptotic death requires caspases; and (4) delayed nonapoptotic death that occurs in the presence of caspase inhibitors is a consequence of prolonged loss of mitochondrial function. These findings shed light on the mechanisms by which DNA damage kills neurons and raise questions regarding the general utility of caspase inhibitors as neurotherapeutic agents.

Distinct glucocorticoid receptor transcriptional regulatory surfaces mediate the cytotoxic and cytostatic effects of glucocorticoids

Glucocorticoids act through the glucocorticoid receptor (GR), which can function as a transcriptional activator or repressor, to elicit cytostatic and cytotoxic effects in a variety of cells. The molecular mechanisms regulating these events and the target genes affected by the activated receptor remain largely undefined. Using cultured human osteosarcoma cells as a model for the GR antiproliferative effect, we demonstrate that in U20S cells, GR activation leads to irreversible growth inhibition, apoptosis, and repression of Bcl2. This cytotoxic effect is mediated by GR's transcriptional repression function, since transactivation-deficient mutants and ligands still bring about apoptosis and Bcl2 down-regulation. In contrast, the antiproliferative effect of GR in SAOS2 cells is reversible, does not result in apoptosis or repression of Bcl2, and is a function of the receptor's ability to stimulate transcription. Thus, the cytotoxic versus cytostatic outcome of glucocorticoid treatment is cell context dependent. Interestingly, the cytostatic effect of glucocorticoids in SAOS2 cells involves multiple GR activation surfaces. GR mutants and ligands that disrupt individual transcriptional activation functions (activation function 1 [AF-1] and AF-2) or receptor dimerization fail to fully inhibit cellular proliferation and, remarkably, discriminate between the targets of GR's cytostatic action, the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1). Induction of p21(Cip1) is agonist dependent and requires AF-2 but not AF-1 or GR dimerization. In contrast, induction of p27(Kip1) is agonist independent, does not require AF-2 or AF-1, but depends on GR dimerization. Our findings indicate that multiple GR transcriptional regulatory mechanisms that employ distinct receptor surfaces are used to evoke either the cytostatic or cytotoxic response to glucocorticoids.

CDK-inhibitor olomoucine inhibits cell death after exposure of cell lines to cytosine-arabinoside

Signal transduction for apoptosis or programmed cell death, after DNA damage in mammalian cells, is believed to involve activation of cyclin-dependent kinases (CDKs), especially CDK-1 (cdc2) and CDK-2. We used CDK-inhibitor olomoucine, a purine analogue to evaluate the role CDK inhibition on cytosine-arabinoside (Ara-C)-induced cell death. The two drugs showed an antagonistic effect, suggesting that apoptosis after exposure to Ara-C is inhibited by olomoucine. DNA-electrophoresis showed a clear inhibition of the apoptotic pattern when olomoucine was added to Ara-C. We conclude that CDK-inhibitor olomoucine inhibits cell death induced by Ara-C.

Bcl-xL blocks activation of related adhesion focal tyrosine kinase/proline-rich tyrosine kinase 2 and stress-activated protein kinase/c-Jun N-terminal protein kinase in the cellular response to methylmethane sulfonate

The stress-activated protein kinase/c-Jun N-terminal protein kinase (JNK) is induced in response to ionizing radiation and other DNA-damaging agents. Recent studies indicate that activation of JNK is necessary for induction of apoptosis in response to diverse agents. Here we demonstrate that methylmethane sulfonate (MMS)-induced activation of JNK is inhibited by overexpression of the anti-apoptotic protein Bcl-xL, but not by caspase inhibitors CrmA and p35. By contrast, UV-induced JNK activity is insensitive to Bcl-xL. The results demonstrate that treatment with MMS is associated with an increase in tyrosine phosphorylation of related adhesion focal tyrosine kinase (RAFTK)/proline-rich tyrosine kinase 2 (PYK2), an upstream effector of JNK and that this phosphorylation is inhibited by overexpression of Bcl-xL. Furthermore, overexpression of a dominant-negative mutant of RAFTK (RAFTK K-M) inhibits MMS-induced JNK activation. The results indicate that inhibition of RAFTK phosphorylation by MMS in Bcl-xL cells is attributed to an increase in tyrosine phosphatase activity in these cells. Hence, treatment of Bcl-xL cells with sodium vanadate, a tyrosine phosphatase inhibitor, restores MMS-induced activation of RAFTK and JNK. These findings indicate that RAFTK-dependent induction of JNK in response to MMS is sensitive to Bcl-xL, but not to CrmA and p35, by a mechanism that inhibits tyrosine phosphorylation and thereby activation of RAFTK. Taken together, these findings support a novel role for Bcl-xL that is independent of the caspase cascade.

Microtubule dysfunction induced by paclitaxel initiates apoptosis through both c-Jun N-terminal kinase (JNK)-dependent and -independent pathways in ovarian cancer cells

The antineoplastic agent paclitaxel (TaxolTM), a microtubule stabilizing agent, is known to arrest cells at the G2/M phase of the cell cycle and induce apoptosis. We and others have recently demonstrated that paclitaxel also activates the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) signal transduction pathway in various human cell types, however, no clear role has been established for JNK/SAPK in paclitaxel-induced apoptosis. To further examine the role of JNK/SAPK signaling cascades in apoptosis resulting from microtubular dysfunction induced by paclitaxel, we have coexpressed dominant negative (dn) mutants of signaling proteins of the JNK/SAPK pathway (Ras, ASK1, Rac, JNKK, and JNK) in human ovarian cancer cells with a selectable marker to analyze the apoptotic characteristics of cells expressing dn vectors following exposure to paclitaxel. Expression of these dn signaling proteins had no effect on Bcl-2 phosphorylation, yet inhibited apoptotic changes induced by paclitaxel up to 16 h after treatment. Coexpression of these dn signaling proteins had no protective effect after 48 h of paclitaxel treatment. Our data indicate that: (i) activated JNK/SAPK acts upstream of membrane changes and caspase-3 activation in paclitaxel-initiated apoptotic pathways, independently of cell cycle stage, (ii) activated JNK/SAPK is not responsible for paclitaxel-induced phosphorylation of Bcl-2, and (iii) apoptosis resulting from microtubule damage may comprise multiple mechanisms, including a JNK/SAPK-dependent early phase and a JNK/SAPK-independent late phase.

Modulation of apoptosis by the cyclin-dependent kinase inhibitor p27(Kip1)

Proliferation and apoptosis are increased in many types of inflammatory diseases. A role for the cyclin kinase inhibitor p27(Kip1) (p27) in limiting proliferation has been shown. In this study, we show that p27(-/-) mesangial cells and fibroblasts have strikingly elevated rates of apoptosis, not proliferation, when deprived of growth factors. Apoptosis was rescued by restoration of p27 expression. Cyclin A-cyclin-dependent kinase 2 (CDK2) activity, but not cyclin E-CDK2 activity, was increased in serum-starved p27(-/-) cells, and decreasing CDK2 activity, either pharmacologically (Roscovitine) or by a dominant-negative mutant, inhibited apoptosis. Our results show that a new biological function for the CDK inhibitor p27 is protection of cells from apoptosis by constraining CDK2 activity. These results suggest that CDK inhibitors are necessary for coordinating the cell cycle and cell-death programs so that cell viability is maintained during exit from the cell cycle.

Hydrogen peroxide-induced apoptosis in HL-60 cells requires caspase-3 activation

Apoptosis has been associated with oxidative stress in biological systems. Caspases have been considered to play a pivotal role in the execution phase of apoptosis. However, which caspases function as executioners in reactive oxygen species (ROS)-induced apoptosis is not known. The present study was performed to identify the major caspases acting in ROS-induced apoptosis. Treatment of HL-60 cells with 50 microM hydrogen peroxide (H2O2) for 4 h induced the morphological changes such as condensed and/or fragmented nuclei, increase in caspase-3 subfamily protease activities, reduction of the procaspase-3 and a DNA fragmentation. To determine the role of caspases in H2O2-induced apoptosis, caspase inhibitors, acetyl-Tyr-Val-Ala-Asp-chloromethyl ketone (Ac-YVAD-cmk), acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO) and acetyl-Val-Glu-Ile-Asp-aldehyde (Ac-VEID-CHO), selective for caspase-1 subfamily, caspase-3 subfamily and caspase-6, respectively, were loaded into the cells using an osmotic lysis of pinosomes method. Of these caspase inhibitors, only Ac-DEVD-CHO completely blocked morphological changes, caspase-3 subfamily protease activation and DNA ladder formation in H2O2-treated HL-60 cells. This inhibitory effect was dose-dependent. These results suggest that caspase-3, but not caspase-1 is required for commitment to ROS-triggered apoptosis.

Caspase-2 (Nedd-2) processing and death of trophic factor-deprived PC12 cells and sympathetic neurons occur independently of caspase-3 (CPP32)-like activity

We have previously shown that caspase-2 (Nedd-2) is required for apoptosis induced by withdrawal of trophic support from PC12 cells and sympathetic neurons. Here, we examine the relationship of caspase-2 processing and cell death to induction of caspase-3 (CPP32)-like activity in PC12 cells. Caspase-2 processing, at a site tentatively identified as D333, led to the formation of an N-terminal 37 kDa product. This processing correlated temporally with induction of caspase-3-like activity. Agents previously shown to inhibit caspase-3-like activation, such as bcl-2 and the Cdk inhibitor flavopiridol, also acted upstream of caspase-2 processing. The general caspase inhibitors BAF and zVAD-FMK inhibited N-terminal caspase-2 processing. In contrast, the more selective caspase inhibitor DEVD-FMK inhibited the induction of caspase-3-like activity but did not affect caspase-2 processing or significantly suppress death in PC12 cells or sympathetic neurons. This indicates that caspase-3-like activity is not required for either caspase-2 processing or apoptosis in this paradigm. An antisense oligonucleotide to caspase-2 inhibited cell death but did not affect caspase-3-like activity, indicating that caspase-2 is not upstream of this activity and that activation of caspase-3-like caspases is not sufficient for death. Thus, in our paradigm, caspase-2 processing and caspase-3-like activity are induced independently of each other. Moreover, although death requires caspase-2, caspase-3-like activity is neither necessary nor sufficient for death.

Caspase-2 (Nedd-2) processing and death of trophic factor-deprived PC12 cells and sympathetic neurons occur independently of caspase-3 (CPP32)-like activity

We have previously shown that caspase-2 (Nedd-2) is required for apoptosis induced by withdrawal of trophic support from PC12 cells and sympathetic neurons. Here, we examine the relationship of caspase-2 processing and cell death to induction of caspase-3 (CPP32)-like activity in PC12 cells. Caspase-2 processing, at a site tentatively identified as D333, led to the formation of an N-terminal 37 kDa product. This processing correlated temporally with induction of caspase-3-like activity. Agents previously shown to inhibit caspase-3-like activation, such as bcl-2 and the Cdk inhibitor flavopiridol, also acted upstream of caspase-2 processing. The general caspase inhibitors BAF and zVAD-FMK inhibited N-terminal caspase-2 processing. In contrast, the more selective caspase inhibitor DEVD-FMK inhibited the induction of caspase-3-like activity but did not affect caspase-2 processing or significantly suppress death in PC12 cells or sympathetic neurons. This indicates that caspase-3-like activity is not required for either caspase-2 processing or apoptosis in this paradigm. An antisense oligonucleotide to caspase-2 inhibited cell death but did not affect caspase-3-like activity, indicating that caspase-2 is not upstream of this activity and that activation of caspase-3-like caspases is not sufficient for death. Thus, in our paradigm, caspase-2 processing and caspase-3-like activity are induced independently of each other. Moreover, although death requires caspase-2, caspase-3-like activity is neither necessary nor sufficient for death.

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.