Fas induces cytoplasmic apoptotic responses and activation of the MKK7-JNK/SAPK and MKK6-p38 pathways independent of CPP32-like proteases

Sodium nitroprusside-induced mitochondrial apoptotic events in insulin-secreting RINm5F cells are associated with MAP kinases activation

Exposure of insulin-secreting RINm5F cells to the chemical nitric oxide donor sodium nitroprusside (SNP) resulted in apoptotic cell death, as detected by cytochrome c release from mitochondria and caspase 3 activation. SNP exposure also leads to phosphorylation and activation of enzymes involved in cellular response to stress such as signal-regulated kinase 2 (ERK2) and c-Jun NH(2)-terminal kinase 46 (JNK46). Both cytochrome c release and caspase 3 activation were abrogated in cells exposed to MEK and p38 inhibitors. Treatment of cells with the NO donors SNP, DETA-NO, GEA 5024, and SNAP resulted in phosphorylation of the antiapoptotic protein Bcl-2, which was resistant to blockade of MEK, p38, and JNK pathways and sensitive to phosphoinositide 3-kinase (PI3K) inhibition. In addition, transient transfection of cells with the wild-type PI3K gamma gene mimics the increased rate of Bcl-2 phosphorylation detected in NO-treated cells. The generation of phosphoinositides seems to participate in the process since Bcl-2 phosphorylation was not observed in cells overexpressing lipid-kinase-deficient PI3Kgamma. The potential of SNP toxicity directly from NO was supported by our finding that the NO scavenger carboxy-PTIO prevented cell death. We found no evidence to support the contention that oxygen radicals generated during cellular SNP metabolism mediate cell toxicity in RINm5F cells, since neither addition of catalase/superoxide dismutase nor transfection with superoxide dismutase prevented SNP-induced cell death. Thus, we propose that exposure to apoptotic concentrations of NO triggers ERK- and p38-dependent cytochrome c release, caspase 3 activation, and PI3K-dependent Bcl-2 phosphorylation.

Activation of MKK6, an upstream activator of p38, in Alzheimer's disease

Mitogen-activated protein kinase (MAPK) p38 has been implicated in the pathogenesis of Alzheimer's disease, but the upstream cascade leading to p38 activation has not been elucidated in the disease. In the present study, we focused on mitogen-activated protein kinase kinase 6 (MKK6), one of the upstream activators of p38 MAPK. We found that MKK6 was not only increased but also specifically associated with granular structures in the susceptible neurons in the hippocampus and cortex of Alzheimer's disease patients, but was only weakly diffuse in the cytoplasm in neurons in control cases. Immunoblot analysis demonstrated a significant increase of MKK6 level in Alzheimer's disease compared with age-matched controls. In this regard, in hippocampal and cortical regions of individuals with Alzheimer's disease, the activated phospho-MKK6 was localized exclusively in association with pathological alterations including neurofibrillary tangles, senile plaques, neuropil threads and granular structures, overlapping with activated p38 MAPK suggesting both a functional and mechanic link. By immunoblot analysis, phospho-MKK6 is also significantly increased in AD compared with control cases. Together, these findings lend further credence to the notion that the p38 MAPK pathway is dysregulated in Alzheimer's disease and also indicates an active role for this pathway in disease pathogenesis.

The involvement of a stress-activated pathway in equine influenza virus-mediated apoptosis

We have shown elsewhere that equine-2 influenza virus (EIV; subtype H3N8) induced pronounced cell death in infected cells through apoptosis as demonstrated by DNA fragmentation assay and a combined TUNEL and immunostaining scheme. In this study, we investigated the mechanism of EIV-mediated cytotoxicity on a permissive mammalian epithelial cell line, Madin-Darby canine kidney (MDCK) cells. EIV infection increased the cellular levels of oxidative stress and c-Jun/AP-1 protein (which is known to be affected by oxidative stress), as well as its DNA binding activity. Increased production of TGF-beta1, an inducer of c-Jun N-terminal kinase or stress-activated protein kinase (JNK/SAPK) activation, was also detected in EIV-infected MDCK cells. It has been reported that TGF-beta may initiate a signaling cascade leading to JNK/SAPK activation. Addition of c-Jun antisense oligodeoxynucleotide, antioxidant N-acetyl-cysteine (NAC), JNK/SAPK inhibitor carvedilol, or TGF-beta-neutralizing antibody effectively blocked c-Jun/AP-1 upregulation and TGF-beta1 production mediated by EIV infection. These treatments also attenuated EIV-induced cytopathogenic effects (CPE) and apoptosis. Our results suggest that a stress-activated pathway is involved in apoptosis mediated by EIV infection. It is likely that EIV infection turns on the JNK/SAPK cascade, which modulates the activity of apoptosis-promoting regulatory factor c-Jun/AP-1 and epithelial growth inhibitory cytokine TGF-beta.

Antiapoptotic signaling generated by caspase-induced cleavage of RasGAP

Activation of caspases 3 and 9 is thought to commit a cell irreversibly to apoptosis. There are, however, several documented situations (e.g., during erythroblast differentiation) in which caspases are activated and caspase substrates are cleaved with no associated apoptotic response. Why the cleavage of caspase substrates leads to cell death in certain cases but not in others is unclear. One possibility is that some caspase substrates generate antiapoptotic signals when cleaved. Here we show that RasGAP is one such protein. Caspases cleave RasGAP into a C-terminal fragment (fragment C) and an N-terminal fragment (fragment N). Fragment C expressed alone induces apoptosis, but this effect could be totally blocked by fragment N. Fragment N could also block apoptosis induced by low levels of caspase 9. As caspase activity increases, fragment N is further cleaved into fragments N1 and N2. Apoptosis induced by high levels of caspase 9 or by cisplatin was strongly potentiated by fragment N1 or N2 but not by fragment N. The present study supports a model in which RasGAP functions as a sensor of caspase activity to determine whether or not a cell should survive. When caspases are mildly activated, the partial cleavage of RasGAP protects cells from apoptosis. When caspase activity reaches levels that allow completion of RasGAP cleavage, the resulting RasGAP fragments turn into potent proapoptotic molecules.

CD95-induced JNK activation signals are transmitted by the death-inducing signaling complex (DISC), but not by Daxx

Here we investigated CD95-mediated JNK activation pathways and their physiological relevance by employing a variety of cell lines with deficiencies in individual signal transmitting proteins. JNK activation was completely dependent on the activation of caspases in type I and type II cells, as revealed by the inhibitory effects of the caspase inhibitors zVAD-fmk or the cowpoxvirus-encoded CrmA protein. Jurkat cells deficient in caspase-8 or expressing a dominant negative (DN) form of FADD were unable to induce JNK in response to CD95 ligation, indicating that these death-inducing signaling complex (DISC) proteins are required for signal transmission. Activation of caspases, JNK and apoptosis occurred with a markedly slower kinetics in cells expressing a DN version of ASK1, revealing an important contribution of ASK1 for these processes. A C-terminally truncated version of Daxx impaired CD95-mediated apoptosis without affecting the JNK signal. DN forms of FADD, MKK4 and MKK7 completely inhibited CD95-mediated JNK activation but remained without impact on cell killing, indicating that JNK activation is not required for the execution process of CD95-mediated cell killing.

Vitamin D(3)-induced apoptosis of murine squamous cell carcinoma cells. Selective induction of caspase-dependent MEK cleavage and up-regulation of MEKK-1

Vitamin D(3) inhibits cell growth and induces apoptosis in several human cancer lines in vitro and in vivo. However, little is known about the molecular events involved in vitamin D(3)-induced apoptosis. Here, we demonstrate that the growth-promoting/pro-survival signaling molecule mitogen-activated protein kinase kinase (MEK) is cleaved in a caspase-dependent manner in murine squamous cell carcinoma (SCC) cells induced to undergo apoptosis by treatment with vitamin D(3). Cleavage resulted in nearly complete loss of full-length MEK and ERK1/2 phosphorylation. ERK1/2 expression was affected only slightly. The phosphorylation and expression of Akt, a kinase regulating a second cell survival pathway, was also inhibited after treatment with vitamin D(3). However, the pro-apoptotic signaling molecule MEKK-1 was up-regulated in both apoptotic and non-apoptotic cells with greater induction and partial N-terminal proteolysis of MEKK-1 observed in apoptotic cells. In contrast to vitamin D(3), cisplatin and etoposide down-regulated Akt levels only modestly, did not promote significant loss of MEK expression, and did not up-regulate MEKK-1. We propose that vitamin D(3) induces apoptosis in SCC cells by a unique mechanism involving selective caspase-dependent MEK cleavage and up-regulation of MEKK-1. Additional evidence is provided that vitamin D(3)-induced apoptosis may be mediated via p38 MAPK.

MST1-JNK promotes apoptosis via caspase-dependent and independent pathways

BACKGROUND

MST1 is an upstream kinase of the JNK and p38 MAPK pathways whose expression induces apoptotic morphological changes such as nuclear condensation. During apoptosis, caspase cleavage of MST1 removes a C-terminal regulatory domain, increasing the kinase activity of the MST1 N-terminal domain. Downstream pathways of MST1 in the induction of apoptosis remain to be clarified.

RESULTS

In this study, we found that the expression of MST1 resulted in caspase-3 activation. Therefore, MST1 is not only a target of caspases but also an activator of caspases. This caspase activation and apoptotic changes occur through JNK, since the co-expression of a dominant-negative mutant of JNK inhibited MST1-induced morphological changes as well as caspase activation. In contrast, neither a dominant-negative p38 nor the p38 inhibitor SB203580 inhibited them. MST1 induced nucleosomal DNA fragmentation, which was suppressed by caspase inhibitors or ICAD (Inhibitor of Caspase-Activated DNase). Surprisingly, however, other changes such as membrane blebbing and chromatin condensation were not inhibited by caspase inhibitors.

CONCLUSION

These results suggest that MST1 most likely promotes two events through JNK activation; first, MST1 induces the activation of caspases, resulting in CAD-mediated DNA fragmentation, and second, MST1 induces chromatin condensation and membrane blebbing without utilizing downstream caspases.

Involvement of p38 in apoptosis-associated membrane blebbing and nuclear condensation

The stress-activated protein kinase p38 is often induced by cytotoxic agents, but its contribution to cell death is ill defined. In Rat-1 cells, we found a strong correlation between activation of p38 and induction of c-Myc-dependent apoptosis. In cells with deregulated c-Myc expression but not in control cells, cis-diamminedichloroplatinum induced p38 activity and typical features of apoptosis, including internucleosomal DNA degradation, induction of caspase activities, and both nuclear (nuclear condensation and fragmentation) and extranuclear (cell blebbing) morphological alterations. The pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone did not block p38 activation and the p38 inhibitor SB203580 had no detectable effect on the activation of caspases or the in vivo cleavage of several caspase substrates, suggesting that p38 and caspase activation can contribute distinct features of apoptosis. Accordingly, we found that cell blebbing was independent of caspase activity and, rather, depended on p38-sensitive changes in microfilament dynamics likely mediated by heat shock protein 27 phosphorylation. Furthermore, p38 activity contributed to both caspase-dependent and caspase-independent nuclear condensation and fragmentation, suggesting a role in an early event triggering both mechanisms of apoptosis or sensitizing the cells to the action of both types of apoptosis executioners. Inhibiting p38 also resulted in a significant enhancement in cell survival estimated by colony formation. This capacity to modulate the sensitivity to apoptosis in cells with deregulated c-Myc expression suggests an important role for p38 in tumor cell killing by chemotherapeutic agents.

p38 MAP kinase negatively regulates cyclin D1 expression in airway smooth muscle cells

We have demonstrated that platelet-derived growth factor (PDGF) stimulates p38 mitogen-activated protein (MAP) kinase activation in bovine tracheal myocytes, suggesting that p38 is involved in growth regulation. We therefore examined whether p38 regulates expression of cyclin D1, a G(1) cyclin required for cell cycle traversal. The chemical p38 inhibitors SB-202190 and SB-203580 each increased basal and PDGF-induced cyclin D1 promoter activity and protein abundance. Overexpression of a dominant negative allele of MAP kinase kinase-3 (MKK3), an upstream activator of p38alpha, had similar effects. Conversely, active MKK3 and MKK6, both of which increase p38alpha activity, each decreased transcription from the cyclin D1 promoter. Together, these data demonstrate that p38 negatively regulates cyclin D1 expression. We tested whether p38 regulates cyclin D1 expression via inhibition of extracellular signal-regulated kinase (ERK) activation. Chemical inhibitors of p38 induced modest ERK phosphorylation and activation. However, dominant negative MKK3 was insufficient to activate ERK, and active MKK3 and MKK6 did not attenuate platelet-derived growth factor-mediated ERK activation. These data are consistent with the notion that p38alpha negatively regulates cyclin D1 expression via an ERK-independent pathway.

X protein of hepatitis B virus inhibits Fas-mediated apoptosis and is associated with up-regulation of the SAPK/JNK pathway

The X protein from a chronic strain of hepatitis B virus (HBx) was determined to inhibit Fas-mediated apoptosis and promote cell survival. Fas-mediated apoptosis is the major cause of hepatocyte damage during liver disease. Experiments demonstrated that cell death caused by anti-Fas antibodies was blocked by the expression of HBx in human primary hepatocytes and mouse embryo fibroblasts. This effect was also observed in mouse erythroleukemia cells that lacked p53, indicating that protection against Fas-mediated apoptosis was independent of p53. Components of the signal transduction pathways involved in this protection were studied. The SAPK/JNK pathway has previously been suggested to be a survival pathway for some cells undergoing Fas-mediated apoptosis, and kinase assays showed that SAPK activity was highly up-regulated in cells expressing the HBx protein. Normal mouse fibroblasts expressing HBx were protected from death, whereas identical fibroblasts lacking the SEK1 component from the SAPK pathway succumbed to Fas-mediated apoptosis, whether HBx was present or not. Assays showed that caspase 3 and 8 activities and the release of cytochrome c from mitochondria were inhibited, in the presence of HBx, following stimulation with anti-Fas antibodies. Coprecipitation and confocal immunofluorescence microscopy experiments demonstrated that HBx localizes with a cytoplasmic complex containing MEKK1, SEK1, SAPK, and 14-3-3 proteins. Finally, mutational analysis of HBx demonstrated that a potential binding region for 14-3-3 proteins was essential for induction of SAPK/JNK activity and protection from Fas-mediated apoptosis.

Nuclear factor-kappa B activation by the CXC chemokine melanoma growth-stimulatory activity/growth-regulated protein involves the MEKK1/p38 mitogen-activated protein kinase pathway

Melanoma growth stimulatory activity/growth-regulated protein (MGSA/GRO), a CXC chemokine, plays an important role in inflammation, wound healing, growth regulation, angiogenesis, and tumorigenesis. Constitutive expression of MGSA/GROalpha in melanoma tumors is associated with constitutive nuclear factor (NF)-kappaB activity. We show here that either exogenous addition or continuous expression of MGSA/GROalpha in immortalized melanocytes enhances NF-kappaB activation, as well as mitogen-activated protein (MAP) kinase kinase kinase (MEKK) 1, MAP kinase kinase (MEK) 3/6, and p38 MAP kinase activation. Expression of dominant negative M-Ras (S27N), dominant negative MEKK1 (K432M), or specific chemical inhibitors for p38 MAP kinase (SB202190 and SB203580) block MGSA/GROalpha-induced NF-kappaB transactivation, demonstrating that Ras, MEKK1, and p38 are involved in the signal pathways of MGSA/GROalpha activation of NF-kappaB. Expression of dominant active Ras or dominant active MEKK1 alone can also stimulate NF-kappaB activation. The expression of dominant negative MEKK1 inhibits the Ras-induced NF-kappaB activation, suggesting that MEKK1 is a downstream target of Ras. Moreover, MGSA/GROalpha induction of NF-kappaB is independent of the MEK1/ERK cascade, because MGSA/GROalpha failed to increase ERK and ELK activation, and specific chemical inhibitors for MEK1 (PD98059) had no effect on MGSA/GROalpha-enhanced NF-kappaB activation. These data demonstrate that NF-kappaB activation is required for MGSA/GROalpha-induced melanocyte transformation through a Ras/MEKK1/p38 cascade in melanocytes.

PECAM-1 shedding during apoptosis generates a membrane-anchored truncated molecule with unique signaling characteristics

Shedding of cell surface molecules, including growth factor receptors, provides a mechanism by which cells regulate signal transduction events. Here we show that platelet-endothelial cell adhesion molecule (PECAM)-1 is shed from the endothelial cell surface during apoptosis and accumulates in the culture medium as a approximately 100 kDa soluble protein. The cleavage mediating the shedding is matrix metalloproteinase (MMP) dependent, as GM6001, a broad-spectrum MMP inhibitor, inhibits PECAM-1 accumulation in the culture medium in a dose-responsive manner. In addition to the 100 kDa soluble fragment, PECAM-1 cleavage generates the formation of a truncated (Tr.) approximately 28 kDa molecule, composed of the transmembrane and the cytoplasmic PECAM-1 domains. Transfections of the full-length (Fl) and the Tr. PECAM-1 gene constructs into endothelial and nonendothelial cells were performed. We found 1) significantly more gamma-catenin and SHP-2 bound to the truncated than to the full-length PECAM-1; 2) stable expression of the truncated PECAM-1 in SW480 colon carcinoma cells resulted in a dramatic decrease in cell proliferation, whereas expression of comparable levels of the full-length PECAM-1 had no effect; 3) the decrease observed in cell proliferation is due, in part, to an increase in programmed cell death (apoptosis) and correlated with continuous caspase 8 cleavage and p38/JNK phosphorylation. These results support the intimate involvement of PECAM-1 in signal transduction cascades and also suggest that caspase substrates (e.g., PECAM-1) may possess distinct and unique functions on cleavage.

Death receptors and apoptosis. Deadly signaling and evasive tactics

The availability of large amounts of sequence data has made it possible to identify death receptors by homology. Because the genome has not been analyzed completely, a few additional members of this family probably will be identified in the next few years. Rapid progress also has been made recently on the signaling mechanisms used by the death receptors. Considerable conservation of the intracellular signaling mechanisms is seen between different receptors suggesting that it is unlikely that new elements will be added to the molecular framework of death receptor signaling. The analysis of signaling mechanisms has exposed the complexity and multiplicity of cellular responses on death receptor activation. It is not surprising, therefore, that understanding the biological function the death receptors lags behind their characterization at the molecular level. In particular, the role of death receptors in many disease states, such as myocardial disease, remains to be elucidated. (38) This complexity in death-receptor function has constrained their potential for pharmacological manipulation. In most cases it is not sufficient to simply activate a specific death receptor. Manipulation of only one of the multiple responses induced by the receptor is desirable. Currently, no solutions to this challenge have been applied. The exception to this conundrum may be TRAIL. Injection of recombinant TRAIL has few side effects in animal studies and combination therapies that use TRAIL as a radiation sensitizer show early promise.

Identification and characterization of functional domains in a mixed lineage kinase LZK

The mixed lineage kinase (MLK) family is a recently described protein kinase family. The MLKs contain a kinase domain followed by a dual leucine zipper-like motif. We previously reported the molecular cloning of LZK (leucine zipper-bearing kinase), a novel MLK, and that LZK activated the c-Jun NH2 terminal kinase (JNK)/stress-activated protein kinase (SAPK) pathway through MKK7 in cells. Here, we reveal that LZK forms dimers/oligomers through its dual leucine zipper-like motif, and that this is necessary for activation of the JNK/SAPK pathway. We also identify the C-terminal functional region of LZK, which is indispensable for the activation of SEK1, but not that of MKK7.

The mechanism of geranylgeraniol-induced apoptosis involves activation, by a caspase-3-like protease, of a c-jun N-terminal kinase signaling cascade and differs from mechanisms of apoptosis induced by conventional chemotherapeutic drugs

In the present study, we investigated the effects of geranylgeraniol (GGO), a potent inducer of apoptosis in various lines of human tumor cells, on signal transduction cascades involved in apoptosis in human leukemia cells. GGO strongly induced the activation of c-Jun N-terminal kinase (JNK/SAPK) within 2 h in U937 and K562 cells, while neither ERK nor p38 was activated to any considerable extent during GGO-induced apoptosis. Transient expression of a constitutively active mutant form of mitogen-activated protein kinase kinase 1 (MEKK1), deltaMEKK1, or of deltaMEKK1-green fluorescent protein (GFP) in K562 cells activated JNK, but not a caspase-3-like protease, and was insufficient to induce cell death but rendered cells susceptible to GGO-induced cell death. Stable expressions of deltaMEKK1-GFP in U937 cells gave similar results. In contrast to VP-16-induced apoptosis, GGO-induced activation of JNK was almost completely inhibited by benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (Z-DEVD) and by benzyloxycarbonyl-Asp-CH2OC[O]-2,6,-dichlorobenzene (Z-Asp), indicating that the JNK-activation step is located downstream of the caspase signaling pathway in GGO-induced apoptosis. Moreover, apoptosis induced by GGO was significantly inhibited in two lines of cells with a dominant-negative deletion mutation in c-Jun, indicating a requirement for JNK signaling. In addition, unlike the effects on other inducers of apoptosis, the activation of JNK and of the caspase-3-like protease by GGO was significantly delayed by 12-O-tetradecanoylphorbol-13-acetate (TPA), suggesting that the site of inhibition by TPA might be located upstream of the protease and JNK in the GGO-induced apoptotic signaling pathway.

Inhibition of Daxx-mediated apoptosis by heat shock protein 27

Heat shock protein 27 (HSP27) confers cellular protection against a variety of cytotoxic stresses and also against physiological stresses associated with growth arrest or receptor-mediated apoptosis. Phosphorylation modulates the activity of HSP27 by causing a major change in the supramolecular organization of the protein, which shifts from oligomers to dimers. Here we show that phosphorylated dimers of HSP27 interact with Daxx, a mediator of Fas-induced apoptosis, preventing the interaction of Daxx with both Ask1 and Fas and blocking Daxx-mediated apoptosis. No such inhibition was observed with an HSP27 phosphorylation mutant that is only expressed as oligomers or when apoptosis was induced by transfection of a Daxx mutant lacking its HSP27 binding domain. HSP27 expression had no effect on Fas-induced FADD- and caspase-dependent apoptosis. However, HSP27 blocked Fas-induced translocation of Daxx from the nucleus to the cytoplasm and Fas-induced Daxx- and Ask1-dependent apoptosis. The observations revealed a new level of regulation of the Fas pathway and suggest a mechanism for the phosphorylation-dependent protective function of HSP27 during stress and differentiation.

Overexpression of acid ceramidase protects from tumor necrosis factor-induced cell death

Tumor necrosis factor (TNF) signals cell death and simultaneously induces generation of ceramide. To evaluate the contribution of ceramide to TNF-dependent cell death, we generated clones of the TNF-sensitive cell line L929 that constitutively overexpress human acid ceramidase (AC). Ceramidase, in concert with sphingosine kinase, metabolizes ceramide to sphingosine-1-phosphate (SPP), an inducer of proliferation. In response to TNF, parental L929 cells display a significant increase in intracellular ceramide correlated with an "atypical apoptosis" characterized by membrane blebbing, DNA fragmentation and degradation of poly(ADP-ribose) polymerase despite a lack of caspase activity. These features are strongly reduced or absent in AC-overexpressing cells. Pharmacological suppression of AC with N-oleoylethanolamine restored the accumulation of intracellular ceramide as well as the sensitivity of the transfectants to TNF, implying that an enhanced metabolization of intracellular ceramide by AC shifts the balance between intracellular ceramide and SPP levels towards cell survival. Correspondingly, inhibition of ceramide production by acid sphingomyelinase also increased survival of TNF-treated L929 cells.

Phosphorylation of the common neurotrophin receptor p75 by p38beta2 kinase affects NF-kappaB and AP-1 activities

The signaling pathways invoked by ligand binding to the common neurotrophin receptor p75NTR are incompletely understood. Using the yeast two-hybrid system, we identified the mitogen-activated protein (MAP) kinase p38beta2 as a specific interactor with the 5th and 6th alpha helices of the p75NTR intracytoplasmic region. The consequences of this interaction were studied, using primary cultures of Schwann cells and the 293T cell line. Phosphorylation of p75NTR by p38beta2 was induced in vitro and in vivo by MAP kinase kinases (MKK) 6 activation. This pathway demonstrated feedback in that nerve growth factor (NGF) binding increased p38beta2 activity, causing an increase of nuclear factor-kappaB (NF-kappaB) activation and a decrease of AP-1 activation. The mechanisms described explain at least in part why NGF binding to p75NTR increases cell survival in certain circumstances.

Fas ligand-induced c-Jun kinase activation in lymphoid cells requires extensive receptor aggregation but is independent of DAXX, and Fas-mediated cell death does not involve DAXX, RIP, or RAIDD

Jun kinase signaling can be elicited by death receptor activation, but the mechanism and significance of this event are still unclear. It has been reported that cross-linking Abs to Fas trigger c-Jun N-terminal kinase (JNK) signaling via caspase-mediated activation of MEKK1 (JNK kinase kinase), elevation of ceramide levels or by recruitment of death domain associated protein (DAXX) to Fas. The effect of physiological ligand for Fas on JNK signaling was never investigated, although evidence is accumulating that Fas ligand is able to induce cellular responses distinct from those evoked by Ab-mediated cross-linking of Fas. Therefore, we investigated the effect of Fas ligand on JNK signaling. Like its ability to induce cell death, Fas ligand reliably activated JNK only upon extensive aggregation of the receptor. Although this was partially dependent on caspase activation, DAXX was not required. DAXX and other death receptor-associated proteins, which have been reported to bind directly or indirectly to Fas, such as receptor interacting protein (RIP) and RIP-associated ICH-1/CED-3-homologous protein with a death domain (RAIDD), were shown to be dispensable for Fas ligand-induced apoptosis.

MAPK upstream kinase (MUK)-binding inhibitory protein, a negative regulator of MUK/dual leucine zipper-bearing kinase/leucine zipper protein kinase

Mitogen-activated protein kinase upstream kinase/dual leucine zipper-bearing kinase/leucine-zipper protein kinase (MUK/DLK/ZPK) is a MAPKKK class protein kinase that induces JNK/SAPK activation. We report here a protein named MBIP that binds to MUK/DLK/ZPK. MUK-binding inhibitory protein (MBIP) contains two tandemly orientated leucine-zipper-like motifs with a cluster of basic amino acids located between the two motifs. MBIP interacts with one of the two leucine-zipper-like motifs of MUK/DLK/ZPK and inhibits the activity of MUK/DLK/ZPK to induce JNK/SAPK activation. Notably, no similar effect was observed with another JNK/SAPK-inducing MAPKKK, COT/Tpl-2, showing the specificity of MBIP action. Furthermore, the overexpression of MBIP partially inhibits the activation of JNK by 0.3 m sorbitol in 293T cells. Taken together, these observations indicate that MBIP can function as a regulator of MUK/DLK/ZPK, a finding that may provide a clue to understanding the molecular mechanism of JNK/SAPK activation by hyperosmotic stress.

p38 mitogen-activated protein kinase regulates a novel, caspase-independent pathway for the mitochondrial cytochrome c release in ultraviolet B radiation-induced apoptosis

The mechanisms of UVB-induced apoptosis and the role of p38 mitogen-activated protein kinase (MAPK) were investigated in HaCaT cells. UVB doses that induced apoptosis also produced a sustained activation of p38 MAPK and mitochondrial cytochrome c release, leading to pro-caspase-3 activation. Late into the apoptotic process, UVB also induced a caspase-mediated cleavage of Bid. Caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone and benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone substantially blocked the UVB-induced apoptosis without preventing the release of mitochondrial cytochrome c and the p38 MAPK activation. The inhibition of p38 MAPK counteracted both apoptosis and cytochrome c release as well as the DEVD-amino-4-methylcoumarin cleavage activity without affecting the processing of pro-caspase-8. These results indicate that UVB induces multiple and independent apoptotic pathways, which culminate in pro-caspase-3 activation, and that the initial cytochrome c release is independent of caspase activity. Importantly, we show that a sustained p38 MAPK activation contributes to the UVB-induced apoptosis by mediating the release of mitochondrial cytochrome c into the cytosol.

Activation of the luteinizing hormone beta promoter by gonadotropin-releasing hormone requires c-Jun NH2-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by gonadotropin-releasing hormone (GnRH) in the gonadotrope cell line LbetaT2 was investigated. Treatment with gonadotropin-releasing hormone agonist (GnRHa) activates extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK). Activation of ERK by GnRHa occurred within 5 min, and declined thereafter, whereas activation of JNK by GnRHa occurred with a different time frame, i.e. it was detectable at 5 min, reached a plateau at 30 min, and declined thereafter. GnRHa-induced ERK activation was dependent on protein kinase C or extracellular and intracellular Ca(2+), whereas GnRHa-induced JNK activation was not dependent on protein kinase C or on extracellular or intracellular Ca(2+). To determine whether a mitogen-activated protein kinase family cascade regulates rat luteinizing hormone beta (LHbeta) promoter activity, we transfected the rat LHbeta (-156 to +7)-luciferase construct into LbetaT2 cells. GnRH activated the rat LHbeta promoter activity in a time-dependent manner. Neither treatment with a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, PD98059, nor cotransfection with a catalytically inactive form of a mitogen-activated protein kinase construct inhibited the induction of the rat LHbeta promoter by GnRH. Furthermore, cotransfection with a dominant negative Ets had no effect on the response of the rat LHbeta promoter to GnRH. On the other hand, cotransfection with either dominant negative JNK or dominant negative c-Jun significantly inhibited the induction of the rat LHbeta promoter by GnRH. In addition, GnRH did not induce either the rat LHbeta promoter activity in LbetaT2 cells transfected stably with dominant negative c-Jun. These results suggest that GnRHa differentially activates ERK and JNK, and a JNK cascade is necessary to elicit the rat LHbeta promoter activity in a c-Jun-dependent mechanism in LbetaT2 cells.

Activation of mitogen-activated protein kinases after transient forebrain ischemia in gerbil hippocampus

We investigated the expression, activation, and distribution of c-Jun N-terminal kinases (JNKs), p38 mitogen-activated protein kinases (p38s) and extracellular signal-regulated kinases (ERKs) using Western blotting and immunohistochemistry in gerbil hippocampus after transient forebrain ischemia to clarify the role of these kinases in delayed neuronal death (DND) in the CA1 subfield. Immunoblot analysis demonstrated that activities of JNK, p38, and ERK in whole hippocampus were increased after 5 min of global ischemia. We used an immunohistochemical study to elucidate the temporal and spatial expression of these kinases after transient global ischemia. The immunohistochemical study showed that active JNK and p38 immunoreactivities were enhanced at 15 min of reperfusion and then gradually reduced and disappeared in the hippocampal CA1 region. On the other hand, in CA3 neurons, active JNK and p38 immunoreactivities were enhanced at 15 min of reperfusion and peaked at 6 hr of reperfusion and then gradually reduced but was continuously detected 72 hr after ischemia. Active ERK immunoreactivity was observed transiently in CA3 fibers and dentate gyrus. Pretreatment with SB203580, a p38 inhibitor, but not with PD98059, an ERK kinase 1/2 inhibitor, reduced ischemic cell death in the CA1 region after transient global ischemia by inhibiting the activity of p38. These findings indicate that the p38 pathway may play an important role in DND during brain ischemia in gerbil. Components of the pathway are important target molecules for clarifying the mechanism of neuronal death.

Activation of mitogen-activated protein kinases after transient forebrain ischemia in gerbil hippocampus

We investigated the expression, activation, and distribution of c-Jun N-terminal kinases (JNKs), p38 mitogen-activated protein kinases (p38s) and extracellular signal-regulated kinases (ERKs) using Western blotting and immunohistochemistry in gerbil hippocampus after transient forebrain ischemia to clarify the role of these kinases in delayed neuronal death (DND) in the CA1 subfield. Immunoblot analysis demonstrated that activities of JNK, p38, and ERK in whole hippocampus were increased after 5 min of global ischemia. We used an immunohistochemical study to elucidate the temporal and spatial expression of these kinases after transient global ischemia. The immunohistochemical study showed that active JNK and p38 immunoreactivities were enhanced at 15 min of reperfusion and then gradually reduced and disappeared in the hippocampal CA1 region. On the other hand, in CA3 neurons, active JNK and p38 immunoreactivities were enhanced at 15 min of reperfusion and peaked at 6 hr of reperfusion and then gradually reduced but was continuously detected 72 hr after ischemia. Active ERK immunoreactivity was observed transiently in CA3 fibers and dentate gyrus. Pretreatment with SB203580, a p38 inhibitor, but not with PD98059, an ERK kinase 1/2 inhibitor, reduced ischemic cell death in the CA1 region after transient global ischemia by inhibiting the activity of p38. These findings indicate that the p38 pathway may play an important role in DND during brain ischemia in gerbil. Components of the pathway are important target molecules for clarifying the mechanism of neuronal death.

Activation of SAPK/JNK by camptothecin sensitizes androgen-independent prostate cancer cells to Fas-induced apoptosis

We have previously shown that the androgen-independent prostate cancer cells DU145, despite expressing Fas and FasL, were resistant to anti-Fas-induced apoptosis, and that this resistance could be overcome by pretreating the cells with sublethal doses of camptothecin. Here, we provide evidence that SAPK/JNK activity is required for camptothecin sensitization to anti-Fas-induced apoptosis. Camptothecin, but not Fas ligation, was shown to activate SAPK/JNK in a time-dependent manner, and to induce c-Jun expression. The effects were more prominent in cells treated with both camptothecin and anti-Fas. The expression levels of MKP-1, a phosphatase which regulates SAPK/JNK and which has been implicated in prostate cancer resistance to apoptosis, remained unchanged. Inhibition of caspases had no effect on the SAPK/JNK activation, suggesting that this activation is an upstream event in the Fas-signalling pathway, and is independent of caspase activity. Antisense oligonucleotides targeted to JNK1 and JNK2 reversed the effect of camptothecin. These results suggest that stress kinase activation can significantly influence the fate of androgen-independent prostate cancer cells following Fas receptor ligation.

JNK (c-Jun N-terminal kinase) and p38 activation in receptor-mediated and chemically-induced apoptosis of T-cells: differential requirements for caspase activation

Activation of the stress-activated mitogen-activated protein kinases (MAP kinases), c-Jun N-terminal kinase (JNK) and p38, is necessary for the induction of apoptosis in neuronal cells; however, in other cell types their involvement may be stimulus-dependent. In the present study we investigate the activation of JNK and p38 in a single non-neuronal cell type, undergoing receptor-mediated (tumour necrosis factor-related apoptosis-inducing ligand and CD95) or chemically-induced (lactacystin) apoptosis. In Jurkat T-cells, receptor-mediated and chemically-induced apoptosis resulted in a time-dependent activation of the initiator caspases-8 and -9, respectively. Both types of stimuli resulted in a significant activation of JNK and p38, which closely paralleled the time-dependent induction of apoptosis. The caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone (z-VAD.FMK) inhibited receptor-mediated apoptosis and suppressed JNK and p38 activation. In contrast, inhibition of lactacystin-induced apoptosis with z-VAD.FMK, as assessed by phosphatidylserine exposure and poly(ADP-ribose) polymerase cleavage, did not inhibit activation of JNK or p38, demonstrating that during chemically-induced apoptosis, activation of JNK and p38 is independent of effector caspases. The role of p38 in apoptosis was assessed using the specific p38 inhibitor, SB203580. No effect on the induction of apoptosis or caspase activation was observed, although activation of mitogen-activated protein kinase-activated protein kinase-2 (MAPKAPK-2), an immediate downstream target of p38, was inhibited. Therefore neither p38 activation nor activation of MAPKAPK-2 is critical for induction of either receptor- or chemically-induced apoptosis. Thus, within a single cell type, (1) the mechanism of p38 and JNK activation during apoptosis is stimulus-dependent and (2) activation of the p38 pathway is not required for caspase activation or apoptosis, assessed by phosphatidylserine exposure, but may still be required to elicit other features of the apoptotic phenotype.

Caspase-3-dependent and -independent degradation of 28 S ribosomal RNA may be involved in the inhibition of protein synthesis during apoptosis initiated by death receptor engagement

Activation of death receptors initiates intrinsic apoptosis programs in various parts of the cell. To explore the possibility that ribosomal RNA (rRNA), essential for translation in ribosomes, is a target of pro-apoptotic proteins, rRNA was analyzed by electrophoresis in two apoptosis systems: human Jurkat cells treated with anti-Fas antibody and human U937 cells treated with tumor necrosis factor-alpha. In both systems, bands in addition to those of unmodified rRNA were detected a few hours after death receptor engagement. In both systems, the primary additional band was identical and comprised the 3'-terminal region of 28 S rRNA. The degradation of 28 S rRNA was simultaneous with protein synthesis inhibition in both systems. The caspase-3 inhibitor Z-DEVD-FMK suppressed rRNA degradation and protein synthesis inhibition in Jurkat cells but not in U937 cells. Together, our data suggest that different pathways are activated in the two systems we studied, and the final steps in these pathways use very similar or identical ribonucleases to cleave 28 S rRNA. These data suggest a physiological link between rRNA degradation and inhibition of protein synthesis. In general, apoptosis execution initiated by death receptor engagement is promoted by protein synthesis inhibition. Triggered by rRNA degradation, malfunction of the protein synthesis machinery may prompt death execution.

CD95/Fas signaling in T lymphocytes induces the cell cycle control protein p21cip-1/WAF-1, which promotes apoptosis

Ligation of CD95 on T lymphocytes resulted in the up-regulation of a cell cycle control protein, p21cip-1/WAF-1, an inhibitor of cyclin-dependent kinases. This up-regulation was completely blocked by the cysteine protease inhibitor Z-VAD-fmk (benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone), whereas DEVD-CHO (succinyl-Asp-Glu-Val-Asp-aldehyde), a caspase 3 inhibitor, had no effect. In Faslpr-cg mice, a point mutation in the death domain of CD95 results in failure to recruit FADD (Fas-associated death domain), and in the present study this mutation prevented both CD95-mediated apoptosis and p21cip-1/WAF-1 induction. During apoptotic cell death due to irradiation, p21cip-1/WAF-1 is up-regulated by a p53-dependent pathway that responds to DNA damage. However, CD95-induced up-regulation of p21cip-1/WAF-1 in T cells was p53-independent. T cells deficient in p21cip-1/WAF-1 were less susceptible to CD95-induced apoptosis. We conclude that in T cells, ligation of CD95 and activation of caspases cause the induction of p21cip-1/WAF-1, which acts to promote cell death.

Regulation of fas ligand expression during activation-induced cell death in T cells by p38 mitogen-activated protein kinase and c-Jun NH2-terminal kinase

Activation-induced cell death (AICD) is a mechanism of peripheral T cell tolerance that depends upon an interaction between Fas and Fas ligand (FasL). Although c-Jun NH2-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) may be involved in apoptosis in various cell types, the mode of regulation of FasL expression during AICD in T cells by these two MAPKs is incompletely understood. To investigate the regulatory roles of these two MAPKs, we analyzed the kinetics of TCR-induced p38 MAPK and JNK activity and their regulation of FasL expression and AICD. We report that both JNK and p38 MAPK regulate AICD in T cells. Our data suggest a novel model of T cell AICD in which p38 MAPK acts early to initiate FasL expression and the Fas-mediated activation of caspases. Subsequently, caspases stimulate JNK to further upregulate FasL expression. Thus, p38 MAPK and downstream JNK converge to regulate FasL expression at different times after T cell receptor stimulation to elicit maximum AICD.

Inhibition of homodimerization of poly(ADP-ribose) polymerase by its C-terminal cleavage products produced during apoptosis

The biochemical role of the C-terminal fragment of poly(ADP-ribose) polymerase (PARP) was investigated in HeLa cells undergoing UV-mediated apoptosis. During the course of apoptosis, the C-terminal cleavage product of PARP interacted with intact PARP and down-regulated PARP activity by blocking the homodimerization of PARP. The basic leucine zipper motif in the auto-modification domain of the C-terminal fragment of PARP represented the site of association, and Leu(405) was critical to the ability of the basic leucine zipper motif to associate with intact PARP. The expression of the C-terminal fragment of PARP stimulated UV-mediated apoptosis. These results suggest that the C-terminal cleavage product of PARP produced during apoptosis blocks the homodimerization of PARP and inhibits the cellular PARP activity. The inhibition of the cellular PARP activity might prevent cellular NAD(+) depletion and stimulate apoptosis by maintaining the basal cellular energy level required for the completion of apoptosis.

Prolactin-releasing peptide activation of the prolactin promoter is differentially mediated by extracellular signal-regulated protein kinase and c-Jun N-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by prolactin-releasing peptide (PrRP) in both GH3 rat pituitary tumor cells and primary cultures of rat anterior pituitary cells was investigated. PrRP rapidly and transiently activated extracellular signal-regulated protein kinase (ERK) in both types of cells. Both pertussis toxin, which inactivates G(i)/G(o) proteins, and exogenous expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase I, which specifically blocks signaling mediated by the betagamma subunits of G proteins, completely blocked the PrRP-induced ERK activation, suggesting the involvement of G(i)/G(o) proteins in the PrRP-induced ERK activation. Down-regulation of cellular protein kinase C did not significantly inhibit the PrRP-induced ERK activation, suggesting that a protein kinase C-independent pathway is mainly involved. PrRP-induced ERK activation was not dependent on either extracellular Ca(2+) or intracellular Ca(2+). However, the ERK cascade was not the only route by which PrRP communicated with the nucleus. JNK was also shown to be significantly activated in response to PrRP. JNK activation in response to PrRP was slower than ERK activation. Moreover, to determine whether a MAPK family cascade regulates rat prolactin (rPRL) promoter activity, we transfected the intact rPRL promoter ligated to the firefly luciferase reporter gene into GH3 cells. PrRP activated the rPRL promoter activity in a time-dependent manner. Co-transfection with a catalytically inactive form of a MAPK construct or a dominant negative JNK, partially but significantly inhibited the induction of the rPRL promoter by PrRP. Furthermore, co-transfection with a dominant negative Ets completely abolished the response of the rPRL promoter to PrRP. These results suggest that PrRP differentially activates ERK and JNK, and both cascades are necessary to elicit rPRL promoter activity in an Ets-dependent mechanism.

Active killing of neurons during development and following stress: a role for p75(NTR) and Fas?

Evidence for active triggering of neuronal death continues to accumulate. The transmembrane receptors p75(NTR) and Fas can trigger (and in some cases are required for) programmed cell death of the neurons that express them, through signalling pathways that are regulated by a variety of cytoplasmic effectors. Neuronal death induced by trophic deprivation often requires Fas signalling, further blurring the boundaries between naturally occurring and stress-induced neuronal death.

Alternatively spliced products CC3 and TC3 have opposing effects on apoptosis

The human gene CC3 is a metastasis suppressor for small cell lung carcinoma (SCLC) in vivo. The ability of CC3 to impair the apoptotic resistance of tumor cells is likely to contribute to metastasis suppression. We describe here an alternatively spliced RNA of CC3, designated TC3, that encodes an unstable protein with antiapoptotic activity. TC3 and CC3 proteins share amino-terminal sequences, but TC3 has a unique short hydrophobic carboxyl terminus. Overexpression of CC3 results in massive death of rodent fibroblasts, but TC3 protects cells from CC3-induced death and from other death stimuli such as treatment with tumor necrosis factor or overexpression of Bax protein. The death-inducing activity of CC3 resides within its amino-terminal domain, which is conserved in TC3. The carboxyl terminus of TC3 is responsible for the antiapoptotic function of TC3; mutations in this domain abolish the ability of TC3 to protect cells from apoptosis. TC3 protein is short-lived due to its rapid degradation by proteasome, and it forms complexes with a regulatory subunit of proteasome known as s5alpha. The signal for the rapid degradation of TC3 resides within its carboxyl terminus, which is capable of conferring instability on a heterologous protein. The proapoptotic activity of CC3 in SCLC cells is induced by a wide variety of signals and involves disruption of the mitochondrial membrane potential (Deltapsim). The CC3 protein has sequence similarity to bacterial short-chain dehydrogenases/reductases and might represent a phylogenetically old effector of cell death similar to the recently identified apoptosis-inducing factor. CC3 and TC3 have opposing functions in apoptosis and represent a novel dual regulator of cell death.

Advances in the signal transduction of ceramide and related sphingolipids

Recently, the sphingolipid metabolites ceramide, sphingosine, ceramide 1-P, and sphingosine 1-P have been implicated as second messengers involved in many different cellular functions. Publications on this topic are appearing at a rapidly increasing rate and new developments in this field are also appearing rapidly. It is thus important to summarize the results obtained from many different laboratories and from different fields of research to obtain a clearer picture of the importance of sphingolipid metabolites. This article reviews the studies from the last few years and includes the effects of a variety of extracellular agents on sphingolipid signal transduction pathways in different tissues and cells and on the mechanisms of regulation. Sphingomyelin exists in a number of functionally distinct pools and is composed of distinct molecular species. Sphingomyelin metabolites may be formed by many different pathways. For example, the generation of ceramide from sphingomyelin can be catalyzed by at least five different sphingomyelinases. A large variety of stimuli can induce the generation of ceramide, leading to activation or inhibition of various cellular events such as proliferation, differentiation, apoptosis, and inflammation. The effect of ceramide on these physiological processes is due to its many different downstream targets. It can activate ceramide-activated protein kinases and ceramide-activated protein phosphatases. It also activates or inhibits PKCs, PLD, PLA2, PC-PLC, nitric oxide synthase, and the ERK and SAPK/JNK signaling cascades. Ceramide activates or inhibits transcription factors, modulates calcium homeostasis and interacts with the retinoblastoma protein to regulate cell cycle progression. Most of the work in this field has involved the study of ceramide effects, but the roles of the other three sphingomyelin metabolites is now attracting much attention. The complex interactions between signaling components and ceramide and the controls regulating these interactions are now being identified and are presented in this review.

Herbimycin A and geldanamycin inhibit okadaic acid-induced apoptosis and p38 activation in NRK-52E renal epithelial cells

It is important to understand the mechanisms by which phosphorylation-dependent events play a role in regulation of apoptosis in toxicant-metabolizing organs such as the kidney. Our previous work demonstrated that the toxicant and phosphatase inhibitor okadaic acid induces apoptosis of renal epithelial cells via a mechanism that appears to involve the modulation of c-raf-1, p38 kinase, and extracellular regulatory kinase (ERK) cascades. Using the benzoquinone ansamycins and tyrosine kinase inhibitors geldanamycin and herbimycin A, we examined the contribution of tyrosine phosphorylation and c-raf-1 activities to okadaic acid-induced apoptosis. In this report we show that both geldanamycin and herbimycin A protected NRK-52E cells from okadaic acid-induced apoptosis, abrogated the overall okadaic acid-induced kinase activation, and specifically inhibited activation of p38 kinase by okadaic acid. Herbimycin A and geldanamycin also abrogated okadaic-acid induced morphologic changes such as cell rounding and cell membrane blebbing. Herbimycin A and geldanamycin caused pronounced cell spreading, cell flattening, and a decrease in okadaic acid-induced loss of actin filaments. Interestingly, herbimycin A showed more potent inhibitory effect than geldanamycin, and herbimycin A alone inhibited okadaic acid-induced movement of p38 kinase into the cytosol. These results imply that decreased p38 activity and its cytosolic translocation together with cellular resistance to cytoskeletal disorganization may play a significant role in resistance to phosphorylation-dependent apoptosis. Furthermore, the results imply that changes in cell shape may partially modulate the observed alterations in signal transduction induced by okadaic acid.

Bax membrane insertion during Fas(CD95)-induced apoptosis precedes cytochrome c release and is inhibited by Bcl-2

Ligation of the Fas cell surface receptor leads to activation of caspases and subsequent apoptosis. Members of the Bcl-2 family of proteins control the cellular commitment to apoptosis, although their role in Fas-induced apoptosis is ill-defined. In this report we demonstrate that the pro-apoptotic protein, Bax, translocates from the cytosol specifically to the mitochondria following Fas ligation in MCF10A1 breast epithelial cells. Bax translocation was dependent on caspase activation, and preceded the release of cytochrome c and loss of mitochondrial respiratory activity. Bax translocation occurred in concert with activation of downstream caspases as determined by cleavage of a synthetic substrate, proteolysis of poly(ADP-ribose) polymerase, and processing of procaspase-3 and -7. Overexpression of the anti-apoptotic protein, Bcl-2, prevented Bax insertion, cytochrome c release, complete processing of caspase-3 and -7, and full activation of DEVD-specific cleavage activity. These data establish a role for Bax mitochondrial insertion during Fas-mediated apoptosis, and support a model in which Bax insertion amplifies the Fas apoptotic cascade through cytochrome c release and complete processing of caspases-3 and -7. In addition, our findings indicate that prevention of Bax insertion into the mitochondria represents a novel mechanism by which Bcl-2 inhibits Fas-induced apoptosis.

Inhibition of mitogen-activated kinase signaling sensitizes HeLa cells to Fas receptor-mediated apoptosis

The Fas receptor (FasR) is an important physiological mediator of apoptosis in various tissues and cells. However, there are also many FasR-expressing cell types that are normally resistant to apoptotic signaling through this receptor. The mitogen-activated protein kinase (MAPK) signaling cascade has, apart from being a growth-stimulating factor, lately received attention as an inhibitory factor in apoptosis. In this study, we examined whether MAPK signaling could be involved in protecting FasR-insensitive cells. To this end, we used different approaches to inhibit MAPK signaling in HeLa cells, including treatment with the MAPK kinase inhibitor PD 98059, serum withdrawal, and expression of dominant-interfering MAPK kinase mutant protein. All of these treatments were effective in sensitizing the cells to FasR-induced apoptosis, demonstrating that MAPK indeed is involved in the control of FasR responses. The MAPK-mediated control seemed to occur at or upstream of caspase 8, the initiator caspase in apoptotic FasR responses. Transfection with the constitutively active MAPK kinase abrogated FasR-induced apoptosis also in the presence of cycloheximide, indicating that the MAPK-generated suppression of FasR-mediated apoptotic signaling is protein synthesis independent. In cells insensitive to FasR-induced apoptosis, stimulation of the FasR with an agonistic antibody resulted in significant MAPK activation, which was inhibited by PD 98059. When different cell types were compared, the FasR-mediated MAPK activation seemed proportional to the degree of FasR insensitivity. These results suggest that the FasR insensitivity is likely to be a consequence of FasR-induced MAPK activation, which in turn interferes with caspase activation.

Role of caspases and possible involvement of retinoblastoma protein during TGFbeta-mediated apoptosis of human B lymphocytes

In this study, we investigated the involvement of caspases in TGFbeta-induced apoptosis in human B cells. Our results show that TGFbeta-mediated nuclear fragmentation, observed in the Epstein-Barr virus-negative Burkitt's Lymphoma cell line BL41, was abolished in the presence of the tripeptide caspase inhibitor zVAD-fmk or the specific caspase-3 inhibitor DEVD-fmk. Other apoptotic manifestations such as cell shrinkage, surface phosphatidylserine expression and chromatin condensation were strongly inhibited by zVAD-fmk but only partially by DEVD-fmk. This suggests that other caspases in addition to caspase-3 control these apoptotis-associated features. Specific activation of caspase-3 during TGFbeta-induced apoptosis was demonstrated by the DEVD-fmk-sensitive expression of the active p17 subunit of caspase-3 and by in vivo cleavage of PARP. In addition, TGFbeta treatment of BL41 promoted the expression of both dephosphorylated and truncated forms of the retinoblastoma protein. Inhibition of caspase-3 activity abolished both nuclear fragmentation and expression of the truncated retinoblastoma protein, without modifying the G1 cell cycle arrest induced by TGFbeta. Our data thus demonstrate that TGFbeta-induced apoptosis of lymphoma B lymphocytes is dependent on caspase activation and involves caspase-dependent cleavage of the retinoblastoma protein.

Mature T lymphocyte apoptosis--immune regulation in a dynamic and unpredictable antigenic environment

Apoptosis of mature T lymphocytes preserves peripheral homeostasis and tolerance by countering the profound changes in the number and types of T cells stimulated by diverse antigens. T cell apoptosis occurs in at least two major forms: antigen-driven and lymphokine withdrawal. These forms of death are controlled in response to local levels of IL-2 and antigen in a feedback mechanism termed propriocidal regulation. Active antigen-driven death is mediated by the expression of death cytokines such as FasL and TNF. These death cytokines engage specific receptors that assemble caspase-activating protein complexes. These signaling complexes tightly regulate cell death but are vulnerable to inherited defects. Passive lymphokine withdrawal death may result from the cytoplasmic activation of caspases that is regulated by mitochondria and the Bcl-2 protein. The human disease, Autoimmune Lymphoproliferative Syndrome (ALPS) is due to dominant-interfering mutations in the Fas/APO-1/CD95 receptor and other components of the death pathway. The study of ALPS patients reveals the necessity of apoptosis for preventing autoimmunity and allows the genetic investigation of apoptosis in humans. Immunological, cellular, and molecular evidence indicates that throughout the life of a T cell, apoptosis may be evoked in excessive, harmful, or useless clonotypes to preserve a healthy and balanced immune system.

Tumor necrosis factor receptor and Fas signaling mechanisms

Four members of the tumor necrosis factor (TNF) ligand family, TNF-alpha, LT-alpha, LT-beta, and LIGHT, interact with four receptors of the TNF/nerve growth factor family, the p55 TNF receptor (CD120a), the p75 TNF receptor (CD120b), the lymphotoxin beta receptor (LT beta R), and herpes virus entry mediator (HVEM) to control a wide range of innate and adaptive immune response functions. Of these, the most thoroughly studied are cell death induction and regulation of the inflammatory process. Fas/Apo1 (CD95), a receptor of the TNF receptor family activated by a distinct ligand, induces death in cells through mechanisms shared with CD120a. The last four years have seen a proliferation in knowledge of the proteins participating in the signaling by the TNF system and CD95. The downstream signaling molecules identified so far--caspases, phospholipases, the three known mitogen activated protein (MAP) kinase pathways, and the NF-kappa B activation cascade--mediate the effects of other inducers as well. However, the molecules that initiate these signaling events, including the death domain- and TNF receptor associated factor (TRAF) domain-containing adapter proteins and the signaling enzymes associated with them, are largely unique to the TNF/nerve growth factor receptor family.

A predominant apoptotic death pathway of neuronal PC12 cells induced by activated microglia is displaced by a non-apoptotic death pathway following blockage of caspase-3-dependent cascade

Activated microglia have been implicated in the regulation of neuronal cell death. However, the biochemical mechanism for neuronal death triggered by activated microglia is still unclear. When treated with activated microglia, neuronal PC12 cells undergo apoptosis accompanied by caspase-3-like protease activation and DNA fragmentation. Apoptotic bodies formed were subsequently phagocytosed by neighboring activated microglia. Pretreatment of the cells with the caspase-3-like protease inhibitor N-acetyl-Asp-Glu-Val-Asp-aldehyde did not reverse this cell death. Although Bcl-2 overexpression in the cells caused the inhibition of caspase-3-like protease activity and DNA fragmentation and the effective interference of apoptosis induced by deprivation of trophic factors, it could not suppress the activated microglia-induced neuronal death. At the electron microscopic level, degenerating cells with high levels of Bcl-2 were characterized by slightly condensed chromatins forming irregular-shaped masses, severely disintegrated perikarya, and marked vacuolation. Various protease inhibitors tested did not inhibit this cell death, whereas the radical oxygen species scavenger N-acetyl-L-cysteine significantly suppressed this death. Altogether, our study provides an alternative death pathway for the activated microglia-induced neuronal death by blockage of the caspase-3 protease cascade.

Annexin V Delays Apoptosis While Exerting an External Constraint Preventing the Release of CD4+ and PrPc+ Membrane Particles in a Human T Lymphocyte Model

Phosphatidylserine exposure in the exoplasmic leaflet of the plasma membrane is one of the early hallmarks of cells undergoing apoptosis. The shedding of membrane particles carrying Ags testifying to their tissue origin is another characteristic feature. Annexin V, a protein of as yet unknown specific physiologic function, presents a high Ca2+-dependent affinity for phosphatidylserine and forms two-dimensional arrays at the membrane surface. In this study, we report the delaying action of annexin V on apoptosis in the CEM human T cell line expressing CD4 and the normal cellular prion protein (PrPc), two Ags of particular relevance to cell degeneration and with different attachments to the membrane. The effect of annexin V was additive to that of z-Val-Ala-Asp-fluoromethyl ketone, a potent caspase inhibitor. Annexin V significantly reduced the degree of proteolytic activation of caspase-3, and totally blocked the release of CD4+ and PrPc+ membrane particles. z-Val-Ala-Asp-fluoromethyl ketone was a more powerful antagonist of caspase-3 processing, but prevented the shedding of CD4+ vesicles only partially and had no effect on that of PrPc+ ones. These results suggest that an external membrane constraint, such as that exerted by annexin V, has important consequences on the course of programmed cell death and on the dissemination of particular Ags. In vivo, annexin V had a significant protective effect against spleen weight loss in mice treated by an alkylating agent previously shown to induce lymphocyte apoptosis.

Uncouplers of oxidative phosphorylation can enhance a Fas death signal

Recent work suggests a participation of mitochondria in apoptotic cell death. This role includes the release of apoptogenic molecules into the cytosol preceding or after a loss of mitochondrial membrane potential DeltaPsim. The two uncouplers of oxidative phosphorylation carbonyl cyanide m-chlorophenylhydrazone (CCCP) and 2, 4-dinitrophenol (DNP) reduce DeltaPsim by direct attack of the proton gradient across the inner mitochondrial membrane. Here we show that both compounds enhance the apoptosis-inducing capacity of Fas/APO-1/CD95 signaling in Jurkat and CEM cells without causing apoptotic changes on their own account. This amplification occurred upstream or at the level of caspases and was not inhibited by Bcl-2. The effect could be blocked by the cowpox protein CrmA and is thus likely to require caspase 8 activity. Apoptosis induction by staurosporine in Jurkat cells as well as by Fas in SKW6 cells was unaffected by CCCP and DNP. The role of cytochrome c during Fas-DNP signaling was investigated. No early cytochrome c release from mitochondria was detected by Western blotting. Functional assays with cytoplasmic preparations from Fas-DNP-treated cells also indicated that there was no major contribution by cytochrome c or caspase 9 to the activation of effector caspases. Furthermore, an increase of rhodamine-123 uptake into intact cells, which has been explained by mitochondrial swelling, occurred considerably later than the caspase activation and was blocked by Z-VAD-fmk. These data show that uncouplers of oxidative phosphorylation can presensitize some but not all cells for a Fas death signal and provide information about the existence of separate pathways in the induction of apoptosis.

Tiam1 is involved in the regulation of bufalin-induced apoptosis in human leukemia cells

Bufalin, a component of the Chinese medicine chan'su, induces apoptosis in various lines of human tumor cells, such as leukemia HL60 and U937 cells, by altering the expression of apoptosis-related genes, for example, bcl-2 and c-myc. In this study, we characterized a gene that is involved in bufalin-induced apoptosis by the differential display (DD) technique. The partial nucleotide sequence of one of the differentially expressed clones obtained after treatment with bufalin was identical to that of the human gene for Tiam1. When U937 cells were treated with 10(-7) M bufalin, expression of both Tiam1 mRNA and the protein was induced 1 h after the start of the treatment. The increase of Tiam1 mRNA was transient but the level of Tiam1 protein continued to increase at least for 6 h. In addition, the activities of Rac1 and p21-activated kinase (PAK) were also stimulated by bufalin treatment. To evaluate the role of Tiam1 in the apoptotic process, we examined the effects of the expression of sense and antisense RNA for Tiam1 in U937 cells. Apoptosis was strongly induced by bufalin in cells that expressed sense RNA for Tiam1 as compared to apoptosis in control cells treated with bufalin only. Cells expressing antisense RNA for Tiaml were significantly more resistant than the control bufalin-treated cells to induction of DNA fragmentation in response to bufalin. Moreover, sense transformants had elevated activities of PAK and c-Jun NH2-terminal kinase (JNK). These results suggest that Tiaml might play a critical role in bufalin-induced apoptosis through the activation of Rac1, PAK, and JNK pathway.

Caspase-mediated cleavage of cytoskeletal actin plays a positive role in the process of morphological apoptosis

Tumors result from the imbalance between cell growth and apoptosis. One of the characteristic changes in cancers is the abnormality in cytoskeleton, which suggests some roles of cytoskeletal proteins in tumorigenesis or the maintenance of tumor cells. Previously we showed that cytoskeletal actin is the substrate of caspases, the proteases responsible for apoptosis, while the role of actin cleavage in apoptosis remained unknown. To examine the cleavage of actin in vivo, we extensively performed immunoblot analysis using actin fragment-specific antibody. Here, we showed that, in some solid tumor cells, induction of apoptosis was accompanied by caspase-dependent actin-cleavage to 15 and 31 kDa fragments in vivo. To elucidate the role of actin-cleavage further, we introduced actin cleaved-fragments. We found that ectopic expression of an actin 15 kDa fragment induces morphological changes resembling those of apoptotic cells. The expression of the actin fragment induced a dramatic change of cellular actin localization, as visualized by enhanced green fluorescent protein (EGFP)-tagged actin, while the actin fragment expression did not cause caspase activation nor the cleavage of a marker substrate protein, poly (ADP-ribose) polymerase. These results indicate that actin cleavage could play a positive role in the morphological changes of apoptosis downstream of caspase activation.

p38 Mitogen-Activated Protein Kinase Mediates Signal Integration of TCR/CD28 Costimulation in Primary Murine T Cells

Optimal T cell activation requires two signals, one generated by TCR and another by the CD28 costimulatory receptor. In this study, we investigated the regulation of costimulation-induced mitogen-activated protein kinase (MAPK) activation in primary mouse T cells. In contrast to that reported for human Jurkat T cells, we found that p38 MAPK, but not Jun NH2-terminal kinase (JNK), is weakly activated upon stimulation with either anti-CD3 or anti-CD28 in murine thymocytes and splenic T cells. However, p38 MAPK is activated strongly and synergistically by either CD3/CD28 coligation or PMA/Ca2+ ionophore stimulation, which mimics TCR-CD3/CD28-mediated signaling. Activation of p38 MAPK correlates closely with the stimulation of T cell proliferation. In contrast, PMA-induced JNK activation is inhibited by Ca2+ ionophore. T cell proliferation and production of IL-2, IL-4, and IFN-γ induced by both CD3 and CD3/CD28 ligation and the nuclear expression of the c-Jun and ATF-2 proteins are each blocked by the p38 MAPK inhibitor SB203580. Our findings demonstrate that p38 MAPK 1) plays an important role in signal integration during costimulation of primary mouse T cells, 2) may be involved in the induction of c-Jun activation and augmentation of AP-1 transcriptional activity, and 3) regulates whether T cells enter a state of functional unresponsiveness.

Dynamic expression of SEK1 suggests multiple roles of the gene during embryogenesis and in adult brain of mice

Stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), a member of the MAP kinase (MAPK) superfamily, plays a key role in a variety of cellular processes. It is well established that SAPK/JNK activation is controlled by SEK1/MKK4, an up-stream MAP kinase kinase. To gain insight into the role of SEK1 during embryonic development and in adult life, we examined the temporal and spatial patterns of sek1 expression in mice by using in situ hybridization and immunohistochemical study. Dynamic changes of sek1 expression were observed during embryogenesis. Strong sek1 expression was detected in most of the central nervous system and in liver and thymus during early stages of development. While the sek1 expression in nervous system increases over time, expression in fetal liver and thymus gradually decreases as embryogenesis proceeds. High level of the sek1 expression in the central nervous system was persisted throughout postnatal development and remained at a stable level in adult brain. These observations provide an anatomical basis for the vital role(s) of SEK1 in development, for example, in hepatogenesis and/or neurogenesis. Although SEK1 was widely expressed in adult brain, more strong expression of the sek1 was observed at layers 2 and 6 in cerebral cortex, in Purkinje cells of cerebellum, and also in hypothalamic nuclei. The strongest expression of the sek1 was found in the CA3 region of hippocampus, the region being highly vulnerable to exitotoxicity-induced apoptosis in kainate-treated animal models. Interestingly, SEK1 was localized not only in cytoplasm but in dendrites and/or in nucleus of neurons depending on the regions of adult mouse brain. Taken together, these results suggest multiple roles of the SEK1 during embryogenesis and in adult brain.