Phosphorylation of Bcl-2 protein by CDC2 kinase during G2/M phases and its role in cell cycle regulation

Nuclear accumulation of epidermal growth factor receptor and acceleration of G1/S stage by Epstein–Barr-encoded oncoprotein latent membrane protein 1

Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) is considered to be the major oncogenic protein of EBV-encoded proteins and has always been the core of the oncogenic mechanism of EBV. Advanced studies on nuclear translocation of the epidermal growth factor receptor (EGFR) family have greatly improved our knowledge of the biological function of cell surface receptors. In this study, we used the Tet-on LMP1 HNE2 cell line as a cell model, which is a dual-stable LMP1-integrated nasopharyngeal carcinoma (NPC) cell line and the expression of LMP1 which could be regulated by the Tet system. We found that LMP1 could regulate the nuclear accumulation of EGFR in a dose-dependent manner quantitatively and qualitatively. We also demonstrated that the nuclear localization sequence of EGFR played some roles in the location of the protein within the nucleus under LMP1 regulation and EGFR in the nucleus could bind to the promoters of cyclinD1 and cyclinE, respectively. We further demonstrated that EGFR is involved in the acceleration of the G1/S phase transition by LMP1 through binding to cyclinD1 and cyclinE directly. These findings provided a novel view that the acceleration of LMP1 on the G1/S transition via the nuclear accumulation of EGFR was critical in the process of nasopharyngeal carcinoma.

Norcantharidin-induced apoptosis in oral cancer cells is associated with an increase of proapoptotic to antiapoptotic protein ratio

Norcantharidin (NCTD), the demethylated analogue of cantharidin, has been used to treat human cancers in China since 1984. It was recently found to be capable of inducing apoptosis in human colon carcinoma, hepatoma and glioblastoma cells by way of an elusive mechanism. In this study, we demonstrated that NCTD also induces apoptosis in human oral cancer cell lines SAS (p53 wild-type phenotype) and Ca9-22 (p53 mutant) as evidenced by nuclear condensation, TUNEL labeling, DNA fragmentation and cleavage of PARP. Apoptosis induced by NCTD was both dose- and time-dependent. We found NCTD did not induce Fas and FasL, implying that it activated other apoptosis pathways. Our data showed that NCTD caused accumulation of cytosolic cytochrome c and activation of caspase-9, suggesting that apoptosis occurred via the mitochondria mediated pathway. NCTD enhanced the expression of Bax in SAS cells consistent with their p53 status. Moreover, we showed that NCTD downregulated the expression of Bcl-2 in Ca9-22 and Bcl-XL in SAS. Our results suggest that NCTD-induced apoptosis in oral cancer cells may be mediated by an increase in the ratios of proapoptotic to antiapoptotic proteins. Since oral cancer cells with mutant p53 or elevated Bcl-XL levels showed resistance to multiple chemotherapeutic agents, NCTD may overcome the chemoresistance of these cells and provide potential new avenues for treatment.

Characterization of vinblastine-induced Bcl-xL and Bcl-2 phosphorylation: evidence for a novel protein kinase and a coordinated phosphorylation/dephosphorylation cycle associated with apoptosis induction

Bcl-xL and Bcl-2 are phosphorylated in response to microtubule inhibitors, but the kinase(s) responsible and the functional significance have remained unclear. In this study, we investigated the characteristics of Bcl-xL and Bcl-2 phosphorylation in KB-3 carcinoma cells treated with vinblastine. In both asynchronous and synchronous cell cultures, Bcl-xL and Bcl-2 underwent a well-defined and coordinated cycle of phosphorylation and dephosphorylation, with a lengthy period of phosphorylation preceding apoptosis induction, and with dephosphorylation closely correlated with initiation of apoptosis. Internally, validated inhibitors of JNK, ERK, p38(MAPK), or CDK1 failed to inhibit vinblastine-induced phosphorylation of Bcl-xL or Bcl-2. In vitro, Bcl-xL and Bcl-2 were poor substrates relative to c-Jun and ATF2 for active recombinant JNK1. Both Bcl-xL and Bcl-2 were localized primarily to the mitochondrial fraction in both control and vinblastine-treated cells, indicating that phosphorylation did not promote subcellular redistribution. Bcl-xL kinase activity was demonstrated in mitochondrial extracts from vinblastine-treated, but not control, cells. These findings suggest that phosphorylation of these key antiapoptotic proteins may be catalysed by a novel or unsuspected kinase that is activated or induced in response to microtubule damage. Furthermore, the same kinase and phosphatase system may be operating in tandem on both proteins, and phosphorylation appears to maintain their antiapoptotic function, whereas dephosphorylation may trigger apoptosis. These results provide evidence for a novel signaling pathway connecting microtubule damage to apoptosis induction, and help to clarify some of the controversy concerning the role of Bcl-2 phosphorylation in microtubule inhibitor-induced apoptosis.

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UV-induced apoptosis in SH-SY5Y cells: Contribution to apoptosis by JNK signaling and cytochrome c

Activation of the c-Jun N-terminal kinase (JNK) pathway is suggested to be required for neuronal apoptosis. We investigated the role of JNK on phosphorylation of c-Jun, Bcl-2, and apoptotic translocation of cytochrome c (cyt c) in UV-induced apoptosis in human neuroblastoma SH-SY5Y cells. We confirm that UV irradiation induces both apoptosis and necrosis in SH-SY5Y cells and that phosphorylation of JNK at Thr183/Tyr185 in SH-SY5Y cells treated with UV is an early event preceding apoptosis. We also demonstrate that phosphorylation of c-Jun at Ser63 is an early event coinciding with JNK activation, and that the phosphorylation of c-Jun is partially prevented by the JNK inhibitor SP600125. Despite the use of SP600125, the amount of cyt c released into the cytoplasm is not diminished and SP600125 is also unable to decrease the extent of UV-induced apoptosis. These data support the hypothesis that in this system, UV-induced apoptosis is not dependent exclusively on JNK activation. Possible involvement of cyclin-dependent kinases (CDKs) in c-Jun phosphorylation at Ser63 was excluded by pretreating UV-irradiated SH-SY5Y cells with the CDK1/2/5 inhibitor roscovitine.

The phosphorylation status and anti-apoptotic activity of Bcl-2 are regulated by ERK and protein phosphatase 2A on the mitochondria

Bcl-2 protein play important roles in the regulation of apoptosis. We previously reported that the phosphorylation of Bcl-2 was augmented by treatment with protein phosphatase 2A (PP2A) inhibitor; however, the kinase responsible for Bcl-2 phosphorylation had not yet been identified. In this study, we identified extracellular-signal-regulated kinase (ERK) as the responsible kinase for the phosphorylation of Bcl-2. We also found that the transmembrane region (TM) deleted form of Bcl-2 (Bcl-2DeltaTM), which was unable to localize on the mitochondria was constitutively phosphorylated, whereas wild-type Bcl-2 that localized on the mitochondria, was present in its hypophosphorylated form. The phosphorylation of Bcl-2DeltaTM was retarded by treatment with MAP kinase ERK kinase (MEK) inhibitor and PP2A did not bind to Bcl-2DeltaTM. These observations suggest that Bcl-2DeltaTM is constitutively phosphorylated by ERK, but is not dephosphorylated by PP2A in human tumor cell lines. The phosphorylation of Bcl-2 resulted in a reduction in anti-apoptotic function, implying that dephosphorylation promoted the anti-apoptotic activity of Bcl-2 protein in human tumor cell lines. Thus, the present findings suggest that ERK and PP2A are physiological regulators of Bcl-2 phosphorylation, and these enzymes exert an influence on the anti-apoptotic function of Bcl-2.

MCL1 is phosphorylated in the PEST region and stabilized upon ERK activation in viable cells, and at additional sites with cytotoxic okadaic acid or taxol

BCL2 family members are subject to regulation at multiple levels, providing checks on their ability to contribute to tumorigenesis. However, findings on post-translational BCL2 phosphorylation in different systems have been difficult to integrate. Another antiapoptotic family member, MCL1, exhibits a difference in electrophoretic mobility upon phosphorylation induced by an activator of PKC (12-O-tetradecanoylphorbol 13-acetate; TPA) versus agents that act on microtubules or protein phosphatases 1/2A. A multiple pathway model is now presented, which demonstrates that MCL1 can undergo distinct phosphorylation events - mediated through separate signaling processes and involving different target sites - in cells that remain viable in the presence of TPA versus cells destined to die upon exposure to taxol or okadaic acid. Specifically, TPA induces phosphorylation at a conserved extracellular signal-regulated kinase (ERK) site in the PEST region (Thr 163) and slows turnover of the normally rapidly degraded MCL1 protein; however, okadaic acid and taxol induce ERK-independent MCL1 phosphorylation at additional discrete sites. These findings add a new dimension to our understanding of the complex regulation of antiapoptotic BCL2 family members by demonstrating that, in addition to transcriptional and post-transcriptional regulation, MCL1 is subject to multiple, separate, post-translational phosphorylation events, produced in living versus dying cells at ERK-inducible versus ERK-independent sites.

Genistein inversely affects tubulin-binding agent-induced apoptosis in human breast cancer cells

Genistein, a natural isoflavone phytoestrogen present in soybeans, has been extensively studied as a chemopreventive or therapeutic agent in several types of cancer. The traditional Asian diet is rich in soy products may explain in part why the incidence of breast cancer in Asian women is relatively low. To improve therapeutic benefits, we investigated the combination of genistein with chemotherapeutic agents in phenotypically dissimilar human breast cancer cells, MCF-7 and MDA-MB-231, in which estrogen receptor expression is positive and negative, respectively. In the present study, genistein significantly decreased cell apoptosis induced by tubulin-binding agents, paclitaxel and vincristine. FACScan analysis revealed that genistein also diminished the accumulation of the G2/M phase in the cell cycle caused by tubulin-binding agents. In situ staining of microtubules revealed that genistein could decrease paclitaxel-induced tubulin polymerization. However, in vivo tubulin polymerization assay revealed that simultaneous treatment of genistein did not change the tubulin/microtubule dynamic. Genistein reduced Bcl-2 phosphorylation triggered by paclitaxel and vincristine without changing Bax protein expression. p53 and p21 expression, monitored by Western blotting, was not altered by genistein. However, the expression of cyclin B1 and CDC2 kinase was markedly decreased in combination with genistein. In conclusion, genistein inversely affected tubulin-binding agent-induced apoptosis via down-regulation of cyclin B1/CDC2 kinase expression resulting in reduced Bcl-2 phosphorylation.

Control of proliferation by Bcl-2 family members

The anti-proliferative effect of Bcl-2 acts mainly at the level of the G0/G1 phase of the cell cycle. Deletions and point mutations in the bcl-2 gene show that the anti-proliferative activity of Bcl-2, can in some cases, be dissociated from its anti-apoptotic function. This indicates that the effect of Bcl-2 on cell cycle progression can be a direct effect and not only a consequence of its anti-apoptotic activity. Bcl-2 appears to mediate its anti-proliferative effect by acting on both signal transduction pathways (NFAT, ERK) and on specific cell cycle regulators (p27, p130).

Dephosphorylation of Bcl-2 by protein phosphatase 2A results in apoptosis resistance

The anti-apoptotic protein, Bcl-2 was phosphorylated at the Ser-87 residue in normal human blood cells, while it was not phosphorylated in tumor cells. We identified protein phosphatase 2A (PP2A) as a Bcl-2-associated phosphatase that is responsible for dephosphorylation of Bcl-2 in tumor cell lines. Treatment of the tumor cells with a PP2A inhibitor resulted in the appearance of Bcl-2 phosphorylation at Ser-87. This observation suggests that Bcl-2 is constitutively phosphorylated, but is immediately dephosphorylated by PP2A in tumors. Phosphorylation of Bcl-2 protein at the Ser-87 residue resulted in a reduction in anti-apoptotic function in human tumor cell lines. Thus, not only the expression level, but also the dephosphorylation status may have important implications for the oncogenic activity of Bcl-2.

Nerve growth factor-dependent survival of CESS B cell line is mediated by increased expression and decreased degradation of MAPK phosphatase 1

The sIgG(+) lymphoblastoid B cell line CESS spontaneously produces a high amount of nerve growth factor (NGF) and expresses both high affinity (p140(Trk-A)) and low affinity (p75(NTR)) NGF receptors. Autocrine production of NGF maintains the survival of CESS cells through the continuous deactivation of p38 MAPK, an enzyme able to induce Bcl-2 phosphorylation and subsequent cytochrome c release and caspase activation. In this paper, we show that NGF induces transcriptional activation and synthesis of MAPK phosphatase 1 (MKP-1), a dual specificity phosphatase that dephosphorylates p38 MAPK, thus preventing Bcl-2 phosphorylation. Furthermore, NGF increases MKP-1 protein stability by preventing its degradation through the proteasome pathway. Following NGF stimulation, MKP-1 protein mainly localizes on mitochondria, suggesting an interaction with p38 MAPK in this compartment. Incubation of CESS cells with MKP-1-specific antisense oligonucleotides induces cell death, which was not prevented by exogenous NGF. By contrast, overexpression of native MKP-1, but not of its catalytically impaired form, inhibits apoptosis induced by NGF neutralization in CESS cells. Thus, the molecular mechanisms underlying the survival function of NGF in CESS B cell line predominantly consist in maintaining elevated levels of MKP-1 protein, which controls p38 MAPK activation.

Inhibition of cell proliferation and cell cycle progression by specific inhibition of basal JNK activity: evidence that mitotic Bcl-2 phosphorylation is JNK-independent

The c-Jun NH(2)-terminal kinase (JNK) subgroup of mitogen-activated protein kinases has been implicated largely in stress responses, but an increasing body of evidence has suggested that JNK also plays a role in cell proliferation and survival. We examined the effect of JNK inhibition, using either SP600125 or specific antisense oligonucleotides, on cell proliferation and cell cycle progression. SP600125 was selective for JNK in vitro and in vivo versus other kinases tested including ERK, p38, cyclin-dependent protein kinase 1 (CDK1), and CDK2. SP600125 inhibited JNK activity and KB-3 cell proliferation with the same dose dependence, suggesting that inhibition of proliferation was a direct consequence of JNK inhibition. Inhibition of proliferation by SP600125 was associated with an increase in the G(2)-M and apoptotic fractions of cells but was not associated with p53 or p21 induction. Antisense oligonucleotides to JNK2 but not JNK1 caused highly significant inhibition of cell proliferation. Wild-type mouse fibroblasts responded similarly with proliferation inhibition and apoptosis induction, whereas c-jun(-/-) fibroblasts were refractory to the effects of SP600125, suggesting that JNK signaling to c-Jun is required for cell proliferation. Studies in synchronized KB-3 cells indicated that SP600125 delayed transit time through S and G(2)-M phases. Correspondingly, JNK activity increased in late S phase and peaked in late G(2) phase. During synchronous mitotic progression, cyclin B levels increased concomitant with phosphorylation of c-Jun, H1 histone, and Bcl-2. In the presence of SP600125, mitotic progression was prolonged, and c-Jun phosphorylation was inhibited, but neither H1 nor Bcl-2 phosphorylation was inhibited. However, the CDK inhibitor roscovitine inhibited mitotic Bcl-2 phosphorylation. These results indicate that JNK, and more specifically the JNK2 isoform, plays a key role in cell proliferation and cell cycle progression. In addition, conclusive evidence is presented that a kinase other than JNK, most likely CDK1 or a CDK1-regulated kinase, is responsible for mitotic Bcl-2 phosphorylation.

Apoptotic rate in patients with myelodisplastic syndrome treated with modulatory compounds of pro-apoptotic cytokines

Excessive apoptosis has a central role in ineffective hematopoiesis in myelodysplastic syndrome (MDS). The aim of the study was to quantify apoptosis and Bcl-2 expression in patients with MDS and to use these parameters in the evaluation of treatment efficacy with compounds modulating proapoptotic cytokines. Bone marrow (BM) samples from eight MDS patients were studied: four with refractory anemia and four with refractory anemia with ringed sideroblasts. Two patients with Hodgkin disease without BM determination were studied for control. Therapy consisted in administration of pentoxyphylline, dexamethasone and ciprofloxacin. Biochemical assay of apoptosis and Bcl-2 was performed using annexin V-biotin conjugate antibody and anti-human Bcl-2 antibody respectively, followed by streptavidine-peroxidase conjugate, and peroxidase substrate. Ultrastructural investigation of BM samples was performed with standard electron microscopy techniques. Most of BM hematopoietic cells in the MDS patients had ultrastructural features of various stages of apoptosis including chromatin condensation and margination, cytoplasm condensation and budding of nuclear and plasma membranes to produce apoptotic bodies. Bcl-2 expression showed an inverse correlation with the rate of the apoptotic process. Periodic evaluation of these two parameters has shown an increase of Bcl-2 expression and a decrease of apoptotic rate in patients who had responded to the treatment. Response to the treatment was appreciated in accordance with their transfusion needs. Treatment efficiency diminished in time. The rate of apoptosis was inversely correlated with the level of Bcl-2 expression. These results confirm the importance of the apoptotic process evaluation in monitoring MDS treatment.

Cell cycle and apoptosis

Apoptosis and proliferation are intimately coupled. Some cell cycle regulators can influence both cell division and programmed cell death. The linkage of cell cycle and apoptosis has been recognized for c-Myc, p53, pRb, Ras, PKA, PKC, Bcl-2, NF-kappa B, CDK, cyclins and CKI. This review summarizes the different functions of the proteins presently known to control both apoptosis and cell cycle progression. These proteins can influence apoptosis or proliferation but different variables, including cell type, cellular environment and genetic background, make it difficult to predict the outcome of cell proliferation, cell cycle arrest or cell death. These important decisions of cell proliferation or cell death are likely to be controlled by more than one signal and are necessary to ensure a proper cellular response.

A1 is a growth-permissive antiapoptotic factor mediating postactivation survival in T cells

The regulation of cell death in activated naive T cells is not well understood. We examined the expression of A1, an antiapoptotic member of the Bcl-2 family, following activation of naive mouse splenocytes. A1 gene expression was strongly but transiently induced during the first day of activation, with a peak at 2 to 6 hours, whereas Bcl-2 mRNA was simultaneously transiently down-regulated. Transgenic (Tg) overexpression of A1-a in T cells via the lck distal promoter resulted in decreased apoptosis following activation either with concanavalin A or with antibodies to CD3 and CD28 and led to a doubling of T-cell yield by 5 days. Tg A1-a also partially protected thymocytes from several proapoptotic stimuli but did not protect T-cell blasts from cell death induced by reactivation via the T-cell receptor. Tg Bcl-2 and Tg A1-a showed a similar ability to reduce apoptosis in both resting and activated T cells. However, in activated splenocyte cultures, the increase in 5-day T-cell yield observed with Tg Bcl-2 was only half that produced by Tg A1-a. This difference could be attributed at least in part to the fact that A1, unlike Bcl-2, did not inhibit S-phase entry of activated cells. The A1 protein may represent an adaptation of the Bcl-2 gene family to the need for survival regulation in the context of a proliferative stimulus.

Sequential expression patterns of apoptosis- and cell cycle-related proteins in neuronal response to severe or mild transient hypoxia

Severe hypoxia was shown to induce apoptotic death in developing brain neurons, whereas mild hypoxia was demonstrated to stimulate neurogenesis. Since the apoptotic process may share common pathways with mitosis, expression profiles of proteins involved in apoptosis or the cell cycle were analyzed by immunohistochemistry and/or western blotting, in relation with cell outcome of cultured neurons from fetal rat forebrain subjected to either lethal (6 h) or non-lethal (3 h) hypoxia (95% N(2)/5% CO(2)). Hypoxia for 6 h led to apoptosis that was inhibited by the cell cycle blocker olomoucine. Transient overexpression of proliferating cell nuclear antigen was followed by increasing expression of p53, p21, Bax and caspases, whereas Bcl-2 and heat shock proteins were progressively repressed. Conversely, a 3-h hypoxic insult initiated neuronal mitosis, with increased thymidine incorporation. In these conditions, levels of proliferating cell nuclear antigen, Rb, Bcl-2 and heat shock proteins were persistently elevated, while expression of p53, p21, Bax and caspases gradually decreased. These data confirm that hypoxia promotes cell cycle activation, whatever the stress intensity. This process is then aborted following apoptosis-inducing hypoxia, whereas sublethal insult would trigger neurogenesis, at least in developing brain neurons in vitro, by stimulating timed expression of neurogenic and survival-associated proteins.

Genome-wide examination of myoblast cell cycle withdrawal during differentiation

Skeletal and cardiac myocytes cease division within weeks of birth. Although skeletal muscle retains limited capacity for regeneration through recruitment of satellite cells, resident populations of adult myocardial stem cells have not been identified. Because cell cycle withdrawal accompanies myocyte differentiation, we hypothesized that C2C12 cells, a mouse myoblast cell line previously used to characterize myocyte differentiation, also would provide a model for studying cell cycle withdrawal during differentiation. C2C12 cells were differentiated in culture medium containing horse serum and harvested at various time points to characterize the expression profiles of known cell cycle and myogenic regulatory factors by immunoblot analysis. BrdU incorporation decreased dramatically in confluent cultures 48 hr after addition of horse serum, as cells started to form myotubes. This finding was preceded by up-regulation of MyoD, followed by myogenin, and activation of Bcl-2. Cyclin D1 was expressed in proliferating cultures and became undetectable in cultures containing 40% fused myotubes, as levels of p21(WAF1/Cip1) increased and alpha-actin became detectable. Because C2C12 myoblasts withdraw from the cell cycle during myocyte differentiation following a course that recapitulates this process in vivo, we performed a genome-wide screen to identify other gene products involved in this process. Using microarrays containing approximately 10,000 minimally redundant mouse sequences that map to the UniGene database of the National Center for Biotechnology Information, we compared gene expression profiles between proliferating, differentiating, and differentiated C2C12 cells and verified candidate genes demonstrating differential expression by RT-PCR. Cluster analysis of differentially expressed genes revealed groups of gene products involved in cell cycle withdrawal, muscle differentiation, and apoptosis. In addition, we identified several genes, including DDAH2 and Ly-6A, whose expression specifically was up-regulated during cell cycle withdrawal coincident with early myoblast differentiation.

A role for cell cycle-regulated phosphorylation in Groucho-mediated transcriptional repression

Transcriptional corepressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family are involved in a variety of cell differentiation mechanisms in both invertebrates and vertebrates. They become recruited to specific promoter regions by forming complexes with a number of different DNA-binding proteins thereby contributing to the regulation of multiple genes. To understand how the functions of Gro/TLE proteins are regulated, it was asked whether their ability to mediate transcriptional repression might be controlled by cell cycle-dependent phosphorylation events. It is shown here that activation of p34(cdc2) kinase (cdc2) with okadaic acid is correlated with hyperphosphorylation of Gro/TLEs. Moreover, pharmacological inhibition of cdc2 activity results in Gro/TLE dephosphorylation. In agreement with these findings, a purified cdc2-cyclin B complex can directly phosphorylate Gro/TLEs in vitro. Two separate Gro/TLE domains, the CcN and SP regions, contain sequences that are phosphorylated by cdc2. Deletion of these sequences is correlated with loss of Gro/TLE phosphorylation by cdc2 in vitro and okadaic acid-induced Gro/TLE hyperphosphorylation in vivo. In addition, Gro/TLEs are phosphorylated during the G(2)/M phase of the cell cycle, and this is correlated with a decreased nuclear interaction. Finally, the transcription repression ability of Gro/TLEs is enhanced by pharmacological inhibition of cdc2. Taken together, these results demonstrate that Gro/TLE proteins are phosphorylated as a function of the cell cycle and implicate phosphorylation events occurring during mitosis in the negative regulation of Gro/TLE activity.

Phosphorylation of Bcl-2 in G2/M phase-arrested cells following photodynamic therapy with hypericin involves a CDK1-mediated signal and delays the onset of apoptosis

The role of Bcl-2 in photodynamic therapy (PDT) is controversial, and some photosensitizers have been shown to induce Bcl-2 degradation with loss of its protective function. Hypericin is a naturally occurring photosensitizer with promising properties for the PDT of cancer. Here we show that, in HeLa cells, photoactivated hypericin does not cause Bcl-2 degradation but induces Bcl-2 phosphorylation in a dose- and time-dependent manner. Bcl-2 phosphorylation is induced by sublethal PDT doses; increasing the photodynamic stress promptly leads to apoptosis, during which Bcl-2 is neither phosphorylated nor degraded. Bcl-2 phosphorylation involves mitochondrial Bcl-2 and correlates with the kinetics of a G(2)/M cell cycle arrest, preceding apoptosis. The co-localization of hypericin with alpha-tubulin and the aberrant mitotic spindles observed following sublethal PDT doses suggest that photodamage to the microtubule network provokes the G(2)/M phase arrest. PDT-induced Bcl-2 phosphorylation is not altered by either the overexpression or inhibition of p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun NH(2)-terminal protein kinase 1 (JNK1) nor by inhibiting the extracellular signal-regulated kinases (ERKs) or protein kinase C. By contrast, Bcl-2 phosphorylation is selectively suppressed by the cyclin-dependent protein kinase (CDK)-inhibitor roscovitine, completely blocked by the protein synthesis inhibitor cycloheximide and enhanced by the overexpression of CDK1, suggesting a role for this pathway. However, in an in vitro kinase assay, active CDK1/cyclin B1 complex failed to phosphorylate immunoprecipitated Bcl-2, suggesting that this protein kinase may not directly modify Bcl-2. Mutation of serine-70 to alanine in Bcl-2 abolishes PDT-induced phosphorylation and restores the caspase-3 activation to the same levels of the vector-transfected cells, indicating that Bcl-2 phosphorylation may be a signal to delay apoptosis in G(2)/M phase-arrested cells.

Schedule-dependent synergism and antagonism between methotrexate and cytarabine against human leukemia cell lines in vitro

Methotrexate (MTX) and cytarabine have been widely used for the treatment of acute leukemias and lymphomas for over 30 years. However, the optimal schedule of this combination is yet to be determined and a variety of schedules of the combination has been used. We studied the cytotoxic effects of MTX and cytarabine in combination against human leukemia cell lines at various schedules in vitro. The effects of the combinations at the concentration of drug that produced 80% cell growth inhibition (IC(80)) were analyzed using the isobologram method of Steel and Peckham. Simultaneous exposure to MTX and cytarabine for 3 days produced antagonistic effects in human T cell leukemia, MOLT-3 and CCRF-CEM, B cell leukemia, BALL-1, Burkitt's lymphoma, Daudi, promyelocytic leukemia, HL-60 and Philadelphia chromosome-positive leukemia, K-562 cells. Simultaneous exposure to MTX and cytarabine for 24 h produced antagonistic effects, sequential exposure to MTX for 24 h followed by cytarabine for 24 h produced synergistic effects, and the reverse sequence produced additive effects in both CCRF-CEM and HL-60 cells. Sequential exposure to MTX for 24 h followed by cytarabine for 3 days also produced synergistic effects in MOLT-3 cells. Cell cycle analysis supported these observations. Our findings suggest that the simultaneous administration of MTX and cytarabine is not appropriate and the sequential administration of MTX followed by cytarabine may be the optimal schedule of this combination.

Functional cloning of SPIN-2, a nuclear anti-apoptotic protein with roles in cell cycle progression

The balance between hematopoietic cell viability and apoptosis is regulated by exogenous growth factors, however, the molecular mechanisms by which these trophic factors exert their effects remain obscure. A functional retroviral cDNA library-based screen was employed to identify genes that prevent growth factor withdrawal-mediated apoptosis in the myeloid progenitor cell 32Dcl3. This approach identified three classes of genes: those with known roles in apoptosis (bcl-X(L) and ornithine decarboxylase); genes previously identified but not linked directly to apoptotic signaling (O-linked N-acetylglucosamine transferase); and a previously uncharacterized gene we termed SPIN-2. In 32Dcl3 cells, expression of exogenous SPIN-2 provides 25% protection from apoptosis following growth factor withdrawal compared to controls which show approximately 1-2% survival. SPIN-2 overexpression slows cell growth rates and increases the percentage of cells in G(2)/M (32% vs control cells at 12%). Immunolocalization studies indicate that myc-epitope tagged SPIN-2 proteins, which retain their anti-apoptotic function, reside in the nucleus, whereas a C-terminal deletion mutant that loses its anti-apoptotic activity is located in the cytoplasm. These studies suggest that SPIN-2 is a novel nuclear protein that functions to regulate cell cycle progression and this activity is related to the inhibition of apoptosis following the removal of essential growth factors.

Bcl-2 phosphorylation and proteasome-dependent degradation induced by paclitaxel treatment: consequences on sensitivity of isolated mitochondria to Bid

Several studies have suggested that Bcl-2 phosphorylation, which occurs during mitotic arrest induced by paclitaxel, inhibits its antiapoptotic function. In the present study, we demonstrated that the level of phosphorylated Bcl-2 was threefold higher in mitochondria than in the nuclear membrane or endoplasmic reticulum. Our results show, in isolated mitochondria, that phosphorylation of Bcl-2 in mitosis does not modify either its integration into the mitochondrial membrane or the ability to release cytochrome c in response to Bid, a cytochrome c releasing agent. In HeLa cells, in which paclitaxel induces apoptosis, the nonphosphorylated form of Bcl-2 is degraded by a proteasome-dependent degradation pathway, whereas the phosphorylated forms of mitochondrial Bcl-2 appear to be resistant to proteasome-induced degradation. We found that low concentrations of recombinant Bid triggered a greater release of cytochrome c from mitochondria isolated from paclitaxel-treated HeLa cells than from mitochondria isolated from control HeLa cells. Taken together, these results show that Bcl-2 phosphorylation does not inhibit its function. On the contrary, Bcl-2 phosphorylation indirectly regulated its antiapoptotic action via protection against degradation. Indeed, in response to paclitaxel treatment, the level of Bcl-2 expression in mitochondria rather than its phosphorylation state could regulate the sensitivity of mitochondria to cytochrome c releasing agents in vitro.

PP1 phosphatase is involved in Bcl-2 dephosphorylation after prolonged mitotic arrest induced by paclitaxel

During mitotic arrest induced by paclitaxel, most of the mitochondrial Bcl-2 is phosphorylated. This mitotic arrest is transient; exit from mitosis, due to mitotic slippage, occurs and Bcl-2 is rapidly dephosphorylated. In the present study, we characterized PP1 as the cytosolic phosphatase involved in Bcl-2 dephosphorylation. When mitochondria and cytosol prepared from mitotic arrested cells were incubated in vitro, the proportion of phosphorylated forms of Bcl-2 in mitochondria remained unchanged. In contrast, cytosol prepared from cells during mitotic slippage led to a dose-dependent loss of phosphorylated forms of Bcl-2. Depletion of these cytosol extracts by microcystin-Sepharose maintained Bcl-2 phosphorylated forms, indicating that this cytosol possessed phosphatase activity. Furthermore, the dephosphorylation of Bcl-2 by cytosol prepared from cells exiting mitotic block was inhibited by okadaic acid, at a dose known to inhibit PP1, and by inhibitor 2, a specific inhibitor of PP1 and by immunodepletion of PP1. Finally, we showed that PP1 is associated with mitochondrial Bcl-2 in vivo. Taken together, these results demonstrate that PP1 is directly involved in Bcl-2 dephosphorylation during mitotic slippage.

Variation in the kinetics of caspase-3 activation, Bcl-2 phosphorylation and apoptotic morphology in unselected human ovarian cancer cell lines as a response to docetaxel

Paclitaxel is able to cause cell death through the induction of apoptosis. Cell death characteristics for docetaxel have not yet been described in detail. We investigated four unselected human ovarian cancer cell lines for the sensitivity to a 1hr exposure to docetaxel and calculated the concentrations inhibiting 50% (IC(50)) and 90% (IC(90)) of cell growth. Of the cell lines A2780, H134, IGROV-1 (all wild-type p53) and OVCAR-3 (mutant, mt p53) A2780 was most sensitive and OVCAR-3 least sensitive. Equitoxic drug concentrations representing IC(90) values (25-510nM) were applied for 1hr to measure cell cycle distribution, DNA degradation, and to count apoptotic cell bodies and cells with multifragmented nuclei at various time-points after drug exposure. H134, IGROV-1 and OVCAR-3 showed a continued mitotic block up to at least 72hr and prolonged presence of cells with multifragmented nuclei. High percentages of apoptosis were calculated at 48hr and at later time-points. In contrast, A2780 cells accumulated in the S-phase of the cell cycle and apoptosis was hardly present. The changes in the expression levels of p53, p21/WAF1, Bax and Bcl-2, were not predictive for docetaxel-induced apoptosis. Caspase-3 activation occurred only in cells with accumulation in the G2/M phase starting as early as 8hr in OVCAR-3. Prolonged Bcl-2 phosphorylation was evident in OVCAR-3, visible at 24hr in H134 and IGROV-1, while this phenomenon did not occur in A2780. The mitogen-activated protein kinase pathway (JNKs/SAPKs or c-Jun N-terminal kinases/stress-activated protein kinases, JNK1/2; extracellular response kinase, ERK1/2; p38) did not seem to be directly involved in Bcl-2 phosphorylation or apoptosis. We conclude that docetaxel is able to activate caspase-3, induce Bcl-2 phosphorylation and apoptosis in cells that show a prolonged G2/M arrest, but cells may also die by a caspase-3-independent cell death mechanism.

Differential effects of sex steroids on T and B cells: modulation of cell cycle phase distribution, apoptosis and bcl-2 protein levels

Sex steroids have dramatic and differential effects on classic endocrine organ proliferation and apoptosis. In this investigation we sought to delineate similar effects of sex steroids on proliferation, cell cycle phase and apoptosis in lymphocyte cell lines as models for T and B cells. Estrogen and testosterone inhibited T cell line proliferation, induced accumulation of cells in S/G(2)M phases of the cell cycle, and increased apoptosis in a concentration- and time-dependent manner. There was a more modest effect of estrogen and testosterone on cell cycling and apoptosis in B lymphocyte cell lines, suggesting that estrogen and testosterone are inhibitory to T but not B cell lines. In comparison, progesterone induced cytostasis and modestly increased apoptosis in both T and B cell lines. Estrogen and testosterone were not antagonistic or synergistic to each other in their effects on cell cycle phase distribution, and only minimally synergistic for apoptosis. In contrast, progesterone antagonized cell cycle and apoptotic effects of estrogen in T cells. Estrogen-induced cell cycle and apoptotic effects in T cell lines were associated with suppression of bcl-2 protein levels, which were unaffected in Raji B cells. Progesterone also antagonized the estrogen-induced changes in T cell bcl-2 protein levels. These results suggest that there may be significant and differential sex steroid effects on T and B lymphocytes that may be important to sexual dichotomies in immune and autoimmune responses.

Microtubule-targeting drugs induce bcl-2 phosphorylation and association with Pin1

Bcl-2 is a critical suppressor of apoptosis that is overproduced in many types of cancer. Phosphorylation of the Bcl-2 protein is induced on serine residues in tumor cells arrested by microtubule-targeting drugs (paclitaxel, vincristine, nocodazole) and has been associated with inactivation of antiapoptotic function through an unknown mechanism. Comparison of a variety of pharmacological inhibitors of serine/threonine-specific protein kinases demonstrated that the cyclin-dependent kinase inhibitor, flavopiridol, selectively blocks Bcl-2 phosphorylation induced by antimicrotubule drugs. Bcl-2 could also be coimmunoprecipitated with the kinase Cdc2 in M-phase-arrested cells, suggesting that Cdc2 may be responsible for phosphorylation of Bcl-2 in cells treated with microtubule-targeting drugs. Examination of several serine-->alanine substitution mutants of Bcl-2 suggested that serine 70 and serine 87 represent major sites of Bcl-2 phosphorylation induced in response to microtubule-targeting drugs. Both these serines are within sequence contexts suitable for proline-directed kinases such as Cdc2. Phosphorylated Bcl-2 protein was discovered to associate in M-phase-arrested cells with Pin1, a mitotic peptidyl prolyl isomerase (PPIase) known to interact with substrates of Cdc2 during mitosis. In contrast, phosphorylation of Bcl-2 induced by microtubule-targeting drugs did not alter its ability to associate with Bcl-2 (homodimerization), Bax, BAG1, or other Bcl-2-binding proteins. Since the region in Bcl-2 containing serine 70 and serine 87 represents a proline-rich loop that has been associated with autorepression of its antiapoptotic activity, the discovery of Pin1 interactions with phosphorylated Bcl-2 raises the possibility that Pin1 alters the conformation of Bcl-2 and thereby modulates its function in cells arrested with antimicrotubule drugs.

Apoptosis is associated with modifications of bcl-2 mRNA AU-binding proteins

The expression of genes requiring finely tuned control is regulated by a posttranscriptional mechanism involving mRNA A + U-rich elements (AREs) cooperating with ARE-binding proteins (AUBPs) in modulation of mRNA stability. We reported previously that an ARE in the bcl-2 mRNA 3'-untranslated region (3'-UTR) had destabilizing activity and was involved in bcl-2 downregulation during apoptosis in vitro. Here we demonstrate that the bcl-2 ARE complexes with a number of specific AUBPs, whose pattern undergoes changes following application of apoptotic stimuli. The caspase inhibitor Z-VAD-fmk strongly attenuates both bcl-2 mRNA decay and bcl-2 AUBP pattern changes elicited by apoptotic stimuli, indicating the involvement of bcl-2 AUBPs in bcl-2 mRNA stability control.

Unwinding the loop of Bcl-2 phosphorylation

Recent evidence indicates that anti-apoptotic functions of BcI-2 can be regulated by its phosphorylation. According to the 'mitotic arrest-induced' model, multi-site phosphorylation of the BcI-2 loop domain is followed by cell death. In contrast, in cytokine-dependent cell lines, cytokines mediate phosphorylation of BcI-2 on S70, preventing apoptosis. As discussed in this review, these models are not mutually exclusive but reflect different cellular contexts. During mitotic arrest, signal transduction is unique and is fundamentally different from classical mitogenic signaling, since the nucleus membrane is dissolved, gene expression is reduced, and numerous kinases and regulatory proteins are hyperphosphorylated. Hyperphosphorylation of BcI-2 mediated by paclitaxel and other microtubule-active drugs is strictly dependent on targeting microtubules that in turn cause mitotic arrest. In addition to serine-70 (S70), microtubule-active agents promote phosphorylation of S87 and threonine-69 (T69), inactivating BcI-2. A major obstacle for identification of the mitotic BcI-2 kinase(s) is that inhibition of putative kinase(s) by any means (dominant-negative mutants, antisense oligonucleotides, pharmacological agents) may arrest cycle, preventing mitosis and BcI-2 phosphorylation. The role of BcI-2 phosphorylation in cell death is discussed.

Coordinate involvement of cell cycle arrest and apoptosis strengthen the effect of FTY720

A novel reagent, FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]-1,3-propanediol hydrochloride), has been shown to induce a significant decrease of lymphocytes and lymphoma cells and is expected to be a potent immunosuppressant and anti-tumor drug. The decrease in lymphocytes and lymphoma cells is mainly the result of FTY720-induced apoptosis. FTY720 directly affects mitochondria and induces cell death. Moreover, FTY720 activates protein phosphatase (PP) 2A and affects anti-apoptotic intracellular signal transduction proteins to attenuate the anti-apoptotic effect. In this study, we examined the relationship between FTY720-induced apoptosis and cell cycle regulation. FTY720 induced apoptosis significantly at the G0 / G1 phase and caused G0 / G1 cell cycle arrest of the human lymphoma cell lines HL-60RG and Jurkat. Simultaneously, retinoblastoma protein (pRB) was dephosphorylated, suggesting that dephosphorylation of pRB was related to FTY720-induced G0 / G1 cell cycle arrest. Because this dephosphorylation was completely blocked by a specific PP1 / 2A inhibitor, okadaic acid, it appears that FTY720-activated PP2A is essential for FTY720-induced cell cycle arrest. FTY720-induced apoptosis was inhibited by Bcl-2 overexpression in Jurkat cells, but this did not prevent FTY720-induced cell cycle arrest, suggesting that the mechanism of FTY720-induced cell cycle arrest is independent of the mechanism of FTY720-induced apoptosis. These two independent pathways strengthen the effect of FTY720.

Evidence for a role of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase in thapsigargin and Bcl-2 induced changes in Xenopus laevis oocyte maturation

Thapsigargin (Tg), a selective inhibitor of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA), causes depletion of intracellular Ca(2+) stores, hence activation of capacitative Ca(2+) entry (CCE). Incubation of Xenopus laevis oocytes with Tg resulted in an increased rate of progesterone-induced meiotic maturation. Non-mitochondrial (45)Ca(2+) uptake by SERCA-containing microsomes prepared from control wild-type oocytes microinjected with sterile water was inhibited essentially 100% by Tg. However, overexpression of Bcl-2, an oncogene known to protect against Tg-induced apoptosis in certain cell types, resulted in only 40% inhibition of microsomal (45)Ca(2+) uptake by Tg while non-inhibited (45)Ca(2+) uptake remained unchanged. Moreover Bcl-2 overexpression also protected against inhibition of CCE. I(Cl(Ca)) was similar in Bcl-2-overexpressing and control oocytes when intracellular Ca(2+) store depletion was induced by microinjection of inositol 1,4,5-trisphosphate (InsP(3)) and other means and when CCE was induced by means independent of SERCA inhibition. Our data indicate that Bcl-2 affects neither the InsP(3) receptor nor Ca(2+) entry itself. At the end of a 24-h period after progesterone addition to the medium, only 25% of Bcl-2-overexpressing oocytes had matured compared to 85% of control oocytes. Our data suggest that SERCA participates in Xenopus oocyte maturation by controlling cytosolic Ca(2+) and/or intracellular Ca(2+) stores, hence CCE. An observed progesterone-dependent protein kinase-catalysed phosphorylation of SERCA is further indication of its role in oocyte maturation.

Microtubule-targeting drugs induce Bcl-2 phosphorylation and association with Pin1

Bcl-2 is a critical suppressor of apoptosis that is overproduced in many types of cancer. Phosphorylation of the Bcl-2 protein is induced on serine residues in tumor cells arrested by microtubule-targeting drugs (paclitaxel, vincristine, nocodazole) and has been associated with inactivation of antiapoptotic function through an unknown mechanism. Comparison of a variety of pharmacological inhibitors of serine/threonine-specific protein kinases demonstrated that the cyclin-dependent kinase inhibitor, flavopiridol, selectively blocks Bcl-2 phosphorylation induced by antimicrotubule drugs. Bcl-2 could also be coimmunoprecipitated with the kinase Cdc2 in M-phase-arrested cells, suggesting that a Cdc2 may be responsible for phosphorylation of Bcl-2 in cells treated with microtubule-targeting drugs. Examination of several serine-->alanine substitution mutants of Bcl-2 suggested that serine 70 and serine 87 represent major sites of Bcl-2 phosphorylation induced in response to microtubule-targeting drugs. Both these serines are within sequence contexts suitable for proline-directed kinases such as Cdc2. Phosphorylated Bcl-2 protein was discovered to associate in M-phase-arrested cells with Pin1, a mitotic peptidyl prolyl isomerase (PPIase) known to interact with substrates of Cdc2 during mitosis. In contrast, phosphorylation of Bcl-2 induced by microtubule-targeting drugs did not alter its ability to associate with Bcl-2 (homodimerization), Bax, BAG1, or other Bcl-2-binding proteins. Since the region in Bcl-2 containing serine 70 and serine 87 represents a proline-rich loop that has been associated with autorepression of its antiapoptotic activity, the discovery of Pin1 interactions with phosphorylated Bcl-2 raises the possibility that Pin1 alters the conformation of Bcl-2 and thereby modulates its function in cells arrested with antimicrotubule drugs.

p53 mediates bcl-2 phosphorylation and apoptosis via activation of the Cdc42/JNK1 pathway

A member of the small G protein family, cdc42, was isolated from a screen undertaken to identify p53-inducible genes during apoptosis in primary baby rat kidney (BRK) cells transformed with E1A and a temperature-sensitive mutant p53 using a PCR-based subtractive hybridization method. Cdc42 is a GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. In response to external stimuli, Cdc42 is known to transduce signals to regulate the organization of the actin cytoskeleton, induce DNA synthesis in quiescent fibroblasts, and promote apoptosis in neuronal and immune cells. In this study, we have demonstrated that cdc42 mRNA and protein were up-regulated in the presence of wild-type p53 in BRK cells, followed by cytoplasmic to plasma membrane translocation of Cdc42. Overexpression of Cdc42 in the presence of a dominant-negative mutant p53 induced apoptosis rapidly, indicating that Cdc42 functions downstream of p53. Furthermore, stable expression of a dominant-negative mutant of Cdc42 partially inhibited p53-mediated apoptosis. The Bcl-2 family members Bcl-xL, and the adenovirus protein E1B 19K, inhibited Cdc42-mediated apoptosis, whereas Bcl-2 did not. We provide evidence that PAK1 and JNK1 may play a role downstream of Cdc42 to transduce its apoptotic signal. Cdc42/PAK1 activates JNK1-induced phosphorylation of Bcl-2, thereby inactivating its function, and that a phosphorylation resistant mutant (Bcl-2S70,87A,T56,74A) gains the ability to inhibit Cdc42- and p53-mediated apoptosis. Thus, one mechanism by which p53 promotes apoptosis is through activation of Cdc42 and inactivation of Bcl-2.