Interactive Fly, Drosophila



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JNK and apoptosis: General considerations

The c-Jun NH2-terminal kinase (JNK) can cause cell death by activating the mitochondrial apoptosis pathway. However, JNK is also capable of signaling cell survival. The mechanism that accounts for the dual role of JNK in apoptosis and survival signaling has not been established. JNK-stimulated survival signaling can be mediated by JunD. The JNK/JunD pathway can collaborate with NF-kappaB to increase antiapoptotic gene expression, including cIAP-2. This observation accounts for the ability of JNK to cause either survival or apoptosis in different cellular contexts. Furthermore, these data illustrate the general principal that signal transduction pathway integration is critical for the ability of cells to mount an appropriate biological response to a specific challenge (Lamb, 2003).

The survival signaling role of JNK in the response to TNF contrasts with the effects of JNK to mediate apoptosis in response to the exposure of cells to environmental stress. How can one signal transduction pathway mediate two very different responses? There are two general mechanisms that could account for these different roles of JNK in apoptosis signaling. These mechanisms are not mutually exclusive. One mechanism is represented by the time course of JNK activation. Studies of MAP kinases indicate that the time course of activation can determine the cellular response. This may also apply to the response of cells to JNK activation. Sustained JNK activation is required for apoptotic signaling and is sufficient for apoptosis. In contrast, TNF causes transient JNK activation. These considerations indicate that transient JNK activation may be important for mediating a survival response in TNF-treated cells and that chronic JNK activation may contribute to apoptotic responses. A second mechanism that may account for the different roles of JNK in apoptosis signaling is that the biological consequence of JNK function may depend upon the activation state of other signal transduction pathways. For example, increased AKT activation can suppress the apoptotic effects of activated JNK. A plausible hypothesis is that the JNK signaling pathway may cooperate with other signaling pathways to mediate cell survival (e.g., NF-kappaB and AKT). For example, target genes that are induced by the antiapoptotic NF-kappaB pathway may contain JNK-responsive elements in their promoters (e.g., AP-1 sites). The cIAP-2 gene represents an example of this class of gene. JNK increases the expression of such genes in cells with activated NF-kappaB and thus increases cell survival. In contrast, in the absence of a survival pathway that can cooperate with JNK, sustained JNK activation may lead to apoptosis (Lamb, 2003 and references therein).

This analysis demonstrates that JNK mediates a survival response in cells treated with TNF. The role of JNK is mediated by the transcription factor JunD, which can collaborate with NF-kappaB to increase the expression of prosurvival genes, including cIAP-2. In the absence of activated NF-kappaB, the JNK pathway mediates an apoptotic response. Together, these data provide a mechanism that can account for the dual ability of JNK to cause either survival or apoptosis in different cellular contexts (Lamb, 2003).

The JNK pathway modulates AP-1 activity. While in some cells it may have proliferative and protective roles, in neuronal cells it is involved in apoptosis in response to stress or withdrawal of survival signals. To understand how JNK activation leads to apoptosis, PC12 cells and primary neuronal cultures were used. In PC12 cells, deliberate JNK activation is followed by induction of Fas ligand (FasL) expression and apoptosis. JNK activation detected by c-Jun phosphorylation and FasL induction are also observed after removal of either nerve growth factor from differentiated PC12 cells or KCl from primary cerebellar granule neurons (CGCs). Sequestration of FasL by incubation with a Fas-Fc decoy inhibits apoptosis in all three cases. CGCs derived from gld mice (defective in FasL) are less sensitive to apoptosis caused by KCl removal than are wild-type neurons. In PC12 cells, protection is also conferred by a c-Jun mutant lacking JNK phosphoacceptor sites and a small molecule inhibitor of p38 mitogen-activated protein kinase and JNK, which inhibits FasL induction. Hence, the JNK->c-Jun->FasL pathway is an important mediator of stress-induced neuronal apoptosis (Le-Niculescu, 1999)

A balance between inductive signals and endogenous anti-apoptotic mechanisms determines whether or not programmed cell death occurs. Widely expressed members of the inhibitor of apoptosis gene family include three closely related mammalian proteins: c-IAP1, c-IAP2, and hILP. The anti-apoptotic properties of these proteins have been linked to caspase inhibition. One member of this group, hILP, inhibits interleukin-1beta-converting enzyme-induced apoptosis via a mechanism dependent on the selective activation of c-Jun N-terminal kinase 1. These data demonstrate that apoptosis can be inhibited by an endogenous cellular protein; the mechanism requires the activation of a single member of the mitogen-activating protein kinase family. JNK has been implicated in the induction of programmed cell death in serum-deprived PC12 cells, and in ceramide- or TNF-induced apoptosis in human monoblastic leukemic U937 cells, but has been reported to be insufficient for TNF-induced apoptosis of MCF7 and L929 cells. Activation of the JNK substrate, c-Jun, has also been shown to promote apoptosis in sympathetic neurons and NIH 3T3 fibroblasts. In addition, Fas-mediated JNK activation via the protein Daxx, leading to cell death in 293 and L929 cells, has been reported. JNK activation has recently been shown to antagonize the anti-apoptotic action of Bcl-2 (Drosophila homolog: death executioner Bcl-2 homologue) in N18TG neuroglioma cells. However, in many of these previous studies JNK activation is also accompanied by activation of other members of the MAPK family so that a specific role for JNK cannot be assigned. The current study supports the contention that selective activation of JNK1 is necessary for the anti-apoptotic activity of hILP. Other results suggesting an anti-apoptotic role for JNK can be found in studies showing that B cell survival after CD40 activation is accompanied by potent stimulation of JNK. More recently, Sek1/MKK4 has been suggested to protect thymocytes from Fas- and CD3-mediated apoptosis. Thus, the different activities of the JNK protein suggest a dual role for JNK in apoptosis that may be cell type specific and may depend on upstream signaling events, including whether or not JNK1 is selectively activated to the exclusion of other MAPK family members (Sanna, 1998 and references).

Based on the expression pattern of JNK3 restricted to neurons and the established role for JNK in cell death pathways under stress, neuron-specific functions for JNK3 beyond the general effects of c-Jun phosphorylation have been hypothesized. The yeast two-hybrid system was used to search for novel proteins that interact with JNK3. One such protein proves to be a splice variant of the mitogen-activated kinase activating death domain protein (MADD - See Drosophila Reaper for information on death domain proteins). This variant, which is differentially expressed in neoplastic vs. normal cells and is thus termed DENN, reveals a relationship between JNK3 activation and the neuronal stresses of hypoxia/ischemia and the inflammatory response in the human central nervous system (CNS). The discovery that DENN is a substrate for c-Jun N-terminal kinase 3 is the first demonstration of such an activity for this brain expressed stress-activated kinase. Differential effects on DENN/MADD are observed in a stressed vs. basal environment. Using in situ hybridization, both the substrate and the kinase have been located to large pyramidal neurons in the human hippocampus. In four of four patients with neuropathologically confirmed acute hypoxic changes, a unique translocation of DENN/MADD to the nucleolus is observed. These changes are apparent only in neurons sensitive to hypoxia. Moreover, in those cells, translocation of the substrate is accompanied by nuclear translocation of JNK3. These findings place DENN/MADD and JNK in important hypoxia insult-induced intracellular signaling pathways. These conclusions are important for future studies that seek to understand these stress-activated mechanisms. DENN/MADD homologs have been cloned from rat and C. elegans as GDP/GTP exchange proteins specific for the Rab3 subfamily members that function to regulate the exocytosis of neurotransmitters. Consistent with such a function in vesicle release, expression of DENN/MADD is observed in the neuronal processes in the cerebellum and hippocampus. In the latter site, Rab3 is essential for long term potentiation. It is yet unknown whether phosphorylation alters the GTP/GDP exchange activity of DENN/MADD (Zhang, 1998).

These data indicates that JNK phosphorylates DENN/MADD with significantly higher activity than does ERK. In addition to JNK, the C terminus of DENN/MADD also interacts with TNFR1 as part of the TNFR signaling complex. In COS cells, overexpression of a partial cDNA starting at F1269, but not the full length DENN/MADD, strongly induces JNK activity, independent of TNF. In contrast, ERK activity is augmented by full length DENN/MADD and TNF has an additive effect. Concomitant activation of ERK and JNK has been documented to occur with TNFalpha acting through either TNFR1 or TNFR2, although ERK and JNK appear to have opposing effects on cellular growth and death. For example, ERK activation counteracts Fas- or nerve growth factor deprivation-induced/JNK-mediated apoptosis. In this scenario, ERK is the principal effector of the TNF-TNFR1-DENN/MADD pathway. Subsequently, TNFalpha stimulates JNK, probably by other pathways, which, in turn, phosphorylates DENN/MADD. How, then, does phosphorylation of DENN/MADD impact on its activation of the mitogen-activated kinases? It is speculated that phosphorylated DENN/MADD may suppress the ERK pathway when the JNK pathway is active. TNF also induces a mitogen-activated protein kinase kinase kinase, ASK1, with apoptosis as the consequence. It remains to be examined whether DENN/MADD is involved in the action of ASK1 as well as the physiological consequences of TNF induction of JNK (Zhang, 1999).

Certain cell types undergo apoptosis when they lose integrin-mediated contacts with the extracellular matrix ("anoikis"). The Jun N-terminal kinase (JNK) pathway is activated in and promotes anoikis. This activation requires caspase activity. A DEVD motif-specific caspase that specifically cleaves MEKK-1 is activated when cells lose matrix contact. Cleavage activates MEKK-1. The full length MEKK-1 is found in both attached and suspended cultured cells as a series of bands ranging from 160 kDa to 200 kDa, a hyperphosphorylated form. The C-terminal 80 kDa cleavage product increases significantly after suspension of the cells. Cleavage is required in order to activate the kinase. When overexpressed, the MEKK-1 cleavage product stimulates apoptosis; the wild-type, full-length MEKK-1 sensitizes cells to anoikis; a cleavage-resistant mutant of MEKK-1 partially protects cells against anoikis. The cleavage-resistant or kinase-inactive mutants also prevent caspase-7 from being activated completely. Thus, caspases can induce apoptosis by activating MEKK-1, which in turn activates more caspase activity, comprising a positive feedback loop (Cardone, 1997).

Activated MEKK induces cell death in cultured cells involving cytoplasmic shrinkage, nuclear condensation, and DNA fragmentation characteristic of apoptosis. Expression of activated MEKK enhances the apoptotic response to UV, indicating that MEKK-regulated pathways sensitize cells to apoptotic stimuli. Inducible expression of activated MEKK stimulates the transactivation of c-Myc and Elk-1. Activated Raf, the serine-threonine protein kinase that activates the ERK members of the MAPK family, stimulated Elk-1 transactivation but not c-Myc; expression of activated Raf does not induce any of the cellular changes associated with MEKK-mediated cell death. Thus, MEKK selectively regulates signal transduction pathways that contribute to the apoptotic response (Johnson, 1996).

Ligation of major histocompatability complex class I (MHC-I) molecules expressed on T cells leads to both growth arrest and apoptosis. The current study investigated the intracellular signal pathways that mediate these effects. MHC-I ligation of human Jurkat T cells induces a morphologically distinct form of apoptosis within 6 h. A specific caspase inhibitor, which inhibits Fas-induced apoptosis, does not affect apoptosis induced by MHC-I ligation. MHC-I-induced apoptosis does not involve cleavage and activation of the poly(ADP- ribose) polymerase (PARP) endonuclease or degradation of genomic DNA into the typical fragmentation ladder, both prominent events of Fas-induced apoptosis. These results suggest that MHC-I ligation of Jurkat T cells induces apoptosis through a signal pathway distinct from the Fas molecule. In a search for other signal pathways leading to apoptosis, the regulatory 85-kD subunit of the phosphoinositide-3 kinase (PI-3) kinase was found to be tyrosine phosphorylated after ligation of MHC-I and the PI-3 kinase inhibitor wortmannin was found to selectively block MHC-I-, but not Fas-induced, apoptosis. Since the c-Jun NH2-terminal kinase (JNK) can be activated by PI-3 kinase activity, and has been shown to be involved in the apoptosis of lymphocytes, JNK activation was examined after MHC-I ligation. Strong JNK activity is observed after MHC-I ligation; the activity is completely blocked by wortmannin. Inhibition of JNK activity, by transfecting cells with a dominant-negative JNKK- MKK4 construct, leads to a strong reduction of apoptosis after MHC-I ligation. These results suggest that in apoptosis induced by MHC-I ligation there is a critical engagement of PI-3 kinase-induced JNK activity (Skov, 1997).

To characterize molecular mechanisms that regulate neuronal apoptosis, the contributions to cell death of MAP kinase family members, including ERK (extracellular signal-regulated kinase), JNK (c-JUN NH2-terminal protein kinase), and p38 have been examined after withdrawal of nerve growth factor (NGF) from rat PC-12 pheochromocytoma cells. NGF withdrawal leads to sustained activation of the JNK and p38 enzymes, and inhibition of ERKs. The effects of dominant-interfering or constitutively activated forms of various components of the JNK-p38 and ERK signaling pathways demonstrate that activation of JNK and p38 and concurrent inhibition of ERK are critical for induction of apoptosis in these cells. Therefore, the dynamic balance between growth factor-activated ERK and stress-activated JNK-p38 pathways may be important in determining whether a cell survives or undergoes apoptosis (Xia, 1995).

The c-Jun N-terminal kinases (JNK) are activated by various stimuli, including UV light, interleukin-1, tumor necrosis factor-alpha (TNF-alpha), and CD28 costimulation. Induction of JNK by TNF-alpha, a strong apoptosis inducer, implies a possible role for JNK in the regulation of programmed cell death. Present studies show that lethal doses of gamma radiation (GR) induce JNK activities at the early phase of apoptosis in Jurkat T-cells. JNK1 is activated in a number of different ways: T-cell activation signals, anti-CD28 monoclonal antibody plus phorbol 12-myristate 13-acetate (PMA), or the apoptosis-inducing treatment, GR; however, the induction patterns are different. In contrast to the rapid and transient JNK1 activation caused by CD28 signaling plus PMA, GR induces a delayed and persistent JNK1 activation. This implies a distinct regulatory mechanism and specific function of JNK1 in irradiated cells. The nuclear and cytosolic JNK1 activities are simultaneously increased in the irradiated cells without an evident change in the protein levels. The abilities of GR to induce JNK1 activation has been correlated with DNA fragmentation. Peripheral blood lymphocytes are more sensitive to GR than Jurkat cells in JNK1 induction. The responsiveness of JNK1 to GR suggests the involvement of JNK1 in the initiation of the apoptosis process (Chen, 1996).

Activation of c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) has been implicated in the induction of apoptosis in a variety of systems. BAF3 cells are pre-B cells that undergo apoptosis following either IL-3 withdrawal or ceramide treatment. JNK/SAPK in BAF3 cells is stimulated by ceramide and also during cell proliferation in response to IL-3, but its role in the apoptotic response is not clear. A method was devised for selectively inhibiting JNK/SAPK activity using a dual-specificity threonine/tyrosine phosphatase, M3/6. Expression of this phosphatase in BAF3 cells prevents ceramide stimulation of JNK/SAPK activity but does not affect apoptosis following IL-3 withdrawal or ceramide treatment. IL-3-stimulated proliferation of BAF3 cells expressing the phosphatase is, however, inhibited. Hence JNK/SAPK activation is likely to be involved in the proliferative response of these cells but is not required for apoptosis. Selective ablation by dual-specificity phosphatases should be a general method for determining the functions of specific mitogen-activated kinase pathways (Smith, 1997).

c-Jun is a major component of the heterodimeric transcription factor AP-1 and is essential for embryonic development, since fetuses lacking Jun die at mid-gestation with impaired hepatogenesis and primary Jun-/- fibroblasts have a severe proliferation defect and undergo premature senescence in vitro. c-Jun and AP-1 activities are regulated by c-Jun N-terminal phosphorylation (JNP) at serines 63 and 73 through Jun N-terminal kinases (JNKs). JNP is thought to be required for the anti-apoptotic function of c-Jun during hepatogenesis, because mice lacking the JNK kinase SEK1 exhibit liver defects similar to those seen in Jun-/- fetuses. To investigate the physiological relevance of JNP, endogenous Jun was replaced by a mutant Jun allele with serines 63 and 73 mutated to alanines [Jun(tm1wag); hereafter referred to as JunAA]. Primary JunAA fibroblasts have proliferation- and stress-induced apoptotic defects, accompanied by reduced AP-1 activity. JunAA mice are viable and fertile, smaller than controls and resistant to epileptic seizures and neuronal apoptosis induced by the excitatory amino acid kainate. Primary mutant neurons are also protected from apoptosis and exhibit unaltered JNK activity. These results provide evidence that JNP is dispensable for mouse development, and identify c-Jun as the essential substrate of JNK signaling during kainate-induced neuronal apoptosis (Behrens, 1999).

The c-Jun NH2-terminal kinase (Jnk) family is implicated in apoptosis, but its function in brain development is unclear. This issue has been addressed using mutant mice that lack different members of the family (Jnk1, Jnk2, and Jnk3). Mice deficient in Jnk1, Jnk2, Jnk3, and Jnk1/Jnk3 or Jnk2/Jnk3 double mutants all survive normally. Compound mutants lacking Jnk1 and Jnk2 genes are embryonic lethal and have severe dysregulation of apoptosis in the brain. Specifically, there is a reduction of cell death in the lateral edges of hindbrain prior to neural tube closure. In contrast, increased apoptosis and caspase activation are found in the mutant forebrain, leading to precocious degeneration. These results suggest that Jnk1 and Jnk2 regulate region-specific apoptosis during early brain development (Kuan, 1999).

14-3-3 family members are dimeric phosphoserine-binding proteins that participate in signal transduction and checkpoint control pathways. In this work, dominant-negative mutant forms of 14-3-3 were used to disrupt 14-3-3 function in cultured cells and in transgenic animals. Transfection of cultured fibroblasts with the R56A and R60A double mutant form of 14-3-3zeta (DN-14-3-3zeta) inhibits serum-stimulated ERK MAPK activation, but increases the basal activation of JNK1 and p38 MAPK. Fibroblasts transfected with DN-14-3-3zeta exhibit markedly increased apoptosis in response to UVC irradiation that is blocked by pre-treatment with a p38 MAPK inhibitor, SB202190. Targeted expression of DN-14-3-3eta to murine postnatal cardiac tissue increases the basal activation of JNK1 and p38 MAPK, and affects the ability of mice to compensate for pressure overload, which results in increased mortality, dilated cardiomyopathy and massive cardiomyocyte apoptosis. These results demonstrate that a primary function of mammalian 14-3-3 proteins is to inhibit apoptosis (Xing, 2000).

During inflammation, NF-kappaB transcription factors antagonize apoptosis induced by tumor necrosis factor TNFalpha. This antiapoptotic activity of NF-kappaB involves suppressing the accumulation of reactive oxygen species (ROS) and controlling the activation of the c-Jun N-terminal kinase (JNK) cascade. However, the mechanism(s) by which NF-kappaB inhibits ROS accumulation is unclear. Ferritin heavy chain (FHC) -- the primary iron storage factor -- is an essential mediator of the antioxidant and protective activities of NF-kappaB. FHC is induced downstream of NF-kappaB and is required to prevent sustained JNK activation and, thereby, apoptosis triggered by TNFalpha. FHC-mediated inhibition of JNK signaling depends on suppressing ROS accumulation and is achieved through iron sequestration. These findings establish a basis for the NF-kappaB-mediated control of ROS induction and identify a mechanism by which NF-kappaB suppresses proapoptotic JNK signaling. These results suggest modulation of FHC or, more broadly, of iron metabolism as a potential approach for anti-inflammatory therapy (Pham, 2004).

The c-Jun NH2-terminal kinase (JNK) has been implicated in both cell death and survival responses to different stimuli. This study reexamines the function of JNK in tumor necrosis factor (TNF)-stimulated cell death using fibroblasts isolated from wild-type, Mkk4-/- Mkk7-/-, and Jnk1-/- Jnk2-/- mice. JNK can act to suppress TNF-stimulated apoptosis. However, JNK can also potentiate TNF-stimulated necrosis by increasing the production of reactive oxygen species (ROS). Together, these data indicate that JNK can shift the balance of TNF-stimulated cell death from apoptosis to necrosis. Increased necrosis may represent a contributing factor in stress-induced inflammatory responses mediated by JNK (Ventura, 2004).

JNK and apoptosis: The receptor Fas and its targets

Fas receptor ligation activates the small G-protein Rac1, Jun N-terminal kinase (JNK)/p38 kinases (p38-K), and the transcription factor GADD153. Cellular treatment with synthetic C6-ceramide results in the phosphorylation of these same proteins. A signaling cascade from the Fas receptor via ceramide, Ras, Rac1, and JNK/p38-K to GADD153 has been demonstrated, employing either transfection of transdominant inhibitory N17Ras, N17Rac1, c-Jun, or treatment with a specific p38-K inhibitor. The critical function of this signaling cascade is indicated by prevention of Fas- or C6-ceramide-induced apoptosis after inhibition of Ras, Rac1, or JNK/p38-K (Brenner, 1997).

The TNFR1 signaling complex leads to activation of at least three distinct effector functions: JNK activation, NF-kappaB (Drosophila homolog: Dorsal) activation, and induction of apoptosis. In the induction of apoptosis, TNFR1 recruits TNFR1-associated death domain protein (TRADD) which in turn interacts directly with two other proteins, TNFR-1 associated protein 1 (TRAF1) and Fas-associated protein with death domain (FADD) (Hsu, 1996). TRADD is required for induction of apoptosis by TNFR1; expression of both TRADD and RIP, another death domain protein, is sufficient to activate this process. Apoptosis induction by TNFR1 also appears to require FADD, but unlike TRADD and RIP, this activity is mediated by the FADD N-terminal domain rather than its death domain. In fact, the FADD death domain blocks TNF-induced apoptosis. The N-terminal death effector domain of FADD interacts with an ICE-like protease that appears to be a direct activator of the apoptotic protease cascade. As far as the two other effector functions of TNFR1, recruitment of FADD to the TNFR1 does not activate NF-kappaB or JNK. Two other signal transducers, RIP and TRAF2, mediate both JNK and NF-kappaB activation. These two responses diverge downstream of TRAF2. JNK activation is not involved in induction of apoptosis but may be a protective response. Likewise, NF-kappaB activation may also be a protective response. The NF-kappaB activated antiapoptotic genes remain to be identified, but one likely candidate is manganese superoxide dismutase (Liu, 1996 and references).

The Fas cell surface receptor induces apoptosis upon receptor oligomerization. The Fas-FADD-FLICE connection is currently the best understood model for apoptotic signal transduction, but its inability to explain Fas-inducd JNK activation suggests that the current model is at best incomplete. Fas can robustly activate the JNK pathway. JNK activation is unlikely to take place secondary to apoptosis because it is not inhibited by the blocking of apoptotic caspases. The fact that FADD induces cell death but not JNK activation provides powerful evidence that Fas must engage additional signaling molecules to activate JNK. While RIP and TRAF2 are recruited by TNFR1 to activate JNK, until now no known Fas effector has been able to account for JNK activation. A novel signaling protein is described, termed Daxx, that binds specifically to the Fas death domain. Overexpression of Daxx enhances Fas-mediated apoptosis and activates the Jun N-terminal kinase (JNK) pathway. A C-terminal portion of Daxx interacts with the Fas death domain, while a different region activates both JNK and apoptosis. The Fas-binding domain of Daxx is a dominant-negative inhibitor of both Fas-induced apoptosis and JNK activation, while the FADD death domain partially inhibits death but not JNK activation. The Daxx apoptotic pathway is sensitive to both Bcl-2 and dominant-negative JNK pathway components and acts cooperatively with the FADD pathway. Thus, Daxx and FADD define two distinct apoptotic pathways downstream of Fas. The second pathway via FADD is Bcl-2 insensitive (Yang, 1997).

Interaction of FADD with the p55 tumor necrosis factor receptor 1 (TNF-R1)-associated signal transducer TRADD acts to signal apoptosis, whereas the TNF receptor-associated factor 2 protein (TRAF2) is required for activation of the nuclear transcription factor nuclear factor kappa B. TNF-induced activation of the stress-activated protein kinase (SAPK) occurs through a noncytotoxic TRAF2-dependent pathway. TRAF2 is both sufficient and necessary for activation of SAPK by TNF-R1; conversely, expression of a dominant-negative FADD mutant, which blocks apoptosis, does not interfere with SAPK activation. Therefore, SAPK activation occurs through a pathway that is not required for TNF-R1-induced apoptosis (Natoli, 1997).

Cross-linking of Fas (CD95) induces apoptosis, a response that has been reported to depend upon the Ras activation pathway. Since many examples of apoptosis have been reported to involve AP-1 and/or the AP-1-activating enzyme Jun kinase (JNK), downstream effectors of Ras (See Drosophila Ras) or Ras-like small GTP-binding proteins, the role of these molecules in Fas-mediated apoptosis was evaluated. Although cross-linking of Fas on Jurkat T cells does result in JNK activation, increased activity is observed relatively late, being detectable only after 60 min of stimulation. Expression of a dominant negative form of SEK1 that blocks Fas-mediated induction of JNK activity has no effect on Fas-mediated apoptosis. Furthermore, maximally effective concentrations of anti-Fas does not cause JNK activation if apoptosis is blocked by a cysteine protease inhibitor, suggesting that under these conditions, activation of JNK may be secondary to the stress of apoptosis rather than a direct result of Fas engagement. Despite the activation of JNK, there is no induction of AP-1 activity. The lack of a requirement for AP-1 induction in Fas-mediated death is further substantiated with Jurkat cells, stably transfected with a dominant negative cJun, TAM-67. While TAM-67 effectively prevents AP-1-dependent transcription of both the interleukin-2 and cJun genes, it has no effect on Fas-induced cell death, even at limiting levels of Fas signaling. Thus, induction of JNK activity in Jurkat cells by ligation of Fas at levels sufficient to cause cell death is likely a result, rather than a cause, of the apoptotic response, and AP-1 function is not required for Fas-induced apoptosis (Lenczowski, 1997).

Tumor necrosis factor (TNF)-induced activation of the c-jun N-terminal kinase (JNK, also known as SAPK; stress-activated protein kinase) requires TNF receptor-associated factor 2 (TRAF2). The apoptosis signal-regulating kinase 1 (ASK1) is activated by TNF and stimulates JNK activation. ASK1 is shown to interact with members of the TRAF family and is activated by TRAF2, TRAF5, and TRAF6 overexpression. A truncated derivative of TRAF2, which inhibits JNK activation by TNF, blocks TNF-induced ASK1 activation. A catalytically inactive mutant of ASK1 is a dominant-negative inhibitor of TNF- and TRAF2-induced JNK activation. In untransfected mammalian cells, ASK1 rapidly associates with TRAF2 in a TNF-dependent manner. Thus, ASK1 is a mediator of TRAF2-induced JNK activation (Nishitoh, 1998).

Table of contents

basket/JNK: Biological Overview | Regulation | Protein Interactions | Developmental Biology | Effects of Mutation | References

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