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Molecular and Cellular Biology, January 1999, p. 751-763, Vol. 19, No. 1
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Withdrawal of Survival Factors Results in Activation of the
JNK Pathway in Neuronal Cells Leading to Fas Ligand Induction and
Cell Death
Helen
Le-Niculescu,1,2
Emanuela
Bonfoco,3
Yoshitoshi
Kasuya,1
Francois-Xavier
Claret,1
Douglas R.
Green,3 and
Michael
Karin1,*
Laboratory of Gene Regulation and Signal
Transduction, Department of Pharmacology,1 and
Program in Molecular Pathology,2
University of California at San Diego, La Jolla, California 92093, and
Division of Cellular Immunology, La Jolla Institute for
Allergy and Immunology, San Diego, California 921213
Received 16 July 1998/Returned for modification 19 August
1998/Accepted 5 October 1998
 |
ABSTRACT |
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,
we used PC12 cells and primary neuronal cultures. 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). Sequestation 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 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-to-c-Jun-to-FasL pathway is an important
mediator of stress-induced neuronal apoptosis.
 |
INTRODUCTION |
Mitogen-activated protein kinases
(MAPKs) play an instrumental role in transmission of signals from cell
surface receptors and environmental cues to the transcriptional
machinery (13, 36, 41, 52). While in eukaryotes that
are amenable to genetic analysis, such as yeast (34),
nematodes (43), and fruit flies (88), the
biological functions of MAPK cascades are becoming clear, their role in
mammals are more nebulous. In addition to the difficulty of proper
genetic analysis, much of the ambiguity regarding the role of MAPK
cascades in mammals is due to cell-type-dependent variations in their
function. For example, the ERK MAPK cascade was initially characterized
by its involvement in oncogenic transformation and mitogenic signaling
(13, 52). However, in the rat pheochromocytoma PC12 cell
line, ERK activation is linked to induction of neuronal differentiation
which converts these cells to postmitotic cells similar to sympathetic
neurons (14, 81). This function is akin to the role of the
ERK pathway in formation of the vulva in Caenorhabditis
elegans (43) or the retina in Drosophila
melanogaster (88).
Recently, two additional MAPK cascades leading to activation of JNK
(17, 35, 45) and p38 (32, 47, 70) were identified in mammalian cells. Both pathways are evolutionarily conserved, and
several of their elements were identified in yeasts (16, 51)
and Drosophila (28, 67, 74). Unlike the ERKs,
which are most potently activated in response to signals originating from receptor and cytoplasmic protein tyrosine kinases (28, 67,
74), JNK and p38 are most potently activated by environmental stresses such as UV radiation and osmotic shock, as well as
by proinflammatory cytokines such as tumor necrosis factor
(TNF) and interleukin-1 (IL-1) (15, 40). Therefore, JNK and
p38 are collectively known as stress-activated protein kinases.
The JNKs were identified through their ability to phosphorylate
the N-terminal stimulatory sites of c-Jun (35). As
this phosphorylation is essential for cooperation between c-Jun
and activated Ras in transformation of rat embryo fibroblasts (4, 75) and since the JNKs are the only kinases that phosphorylate these sites (40), it follows that JNK activation contributes to Ras-mediated transformation. In addition c-jun-null
immortalized mouse embryo fibroblasts are refractory to transformation
by Ha-Ras (39, 63, 73). c-Jun is also essential for
proliferation of mouse embryo fibroblasts; in its absence, the cells
undergo very rapid senescence (39, 72). JNK activation is
also necessary for transformation of fibroblasts by the met
and bcr-abl oncogenes (65, 68).
Recently, however, JNK and p38 were implicated in induction of
apoptosis. It was shown that deliberate activation of JNK and p38 in
PC12 cells via transient expression of MEKK1, a MAPK kinase kinase
(MAPKKK) involved in their activation (57), or by nerve growth factor (NGF) withdrawal from postmitotic PC12 cells results in
apoptosis (93). That study also suggested that ERK
activation may protect PC12 cells against apoptosis. Activation of JNK
was also suggested to be involved in apoptosis in response to cell detachment (anoikis) (7, 26). Moreover, JNK activation was suggested to play a critical role in apoptosis of epithelial and lymphoid cells in response to radiation, TNF, or Fas ligand (FasL) (9, 85). The inducible cytokines TNF and FasL bind to
related cell surface receptors, TNF-R1 and Fas, respectively, whose
occupancy activates the apoptotic protease (caspase) cascade (12,
24, 58). In contrast to results obtained by expression of
catalytically inactive JNK kinase 1 (JNKK1 or SEK1), which was reported
to protect cells against TNF-induced apoptosis (85),
dissection of the signaling pathways activated by TNF-R1 indicated that
JNK activation is not linked to TNF-mediated apoptosis (49,
60). In addition, JNK activation by Fas in some cell types was
shown to be a delayed response dependent on caspase activation
(48). In other cell types, however, Fas activation may
result in more direct JNK activation through recruitment of the
signaling protein DAXX (95). In that case, JNK activation
may potentiate Fas-mediated apoptosis. However, lymphocytes deficient
in JNKK1/SEK1 are more sensitive rather than resistant to Fas-mediated
apoptosis (62). In addition, lymphocytes from
traf2
/
mice, which are defective in JNK
activation, are highly sensitive to TNF-induced apoptosis
(96).
While some of these results question the involvement of JNK (or p38) in
apoptosis in fibroblasts and lymphocytes, the participation of JNK and
c-Jun in death signaling is more firmly established in neuronal cells.
NGF withdrawal from sympathetic neurons results in c-jun
induction (20, 31, 56). NGF withdrawal from differentiated PC12 cells results in JNK activation (93), a signal that
leads to c-jun induction and potentiation of c-Jun
transcriptional activity (40). In both sympathetic neurons
and PC12 cells, expression of dominant negative c-Jun mutants or
microinjection of neutralizing c-Jun antibodies inhibits apoptosis
(20, 31, 93). Most conclusive are the results obtained with
jnk3
/
mice (94). Massive
stimulation of glutamate receptors which results in excitotoxicity
followed by apoptosis of hippocampal neurons in normal mice does not
cause neuronal cell death in mice that are deficient in the
neuron-specific JNK3 isozyme (94). None of these studies,
however, provided a mechanism by which JNK activation and/or c-Jun
phosphorylation can trigger apoptosis. The lack of mechanistic insight
is of concern because the assumption that JNK-mediated c-Jun
phosphorylation plays a causative role in apoptosis rests on the use of
dominant-negative c-Jun mutants. While these mutants act in the nucleus
(2), the caspase cascade is activated in the cytoplasm and
does not require new gene expression or protein phosphorylation
(58, 61). Thus, it is of great importance to define the
steps in pathways that lead to apoptosis at which JNK and c-Jun act.
Better understanding of such pathways will facilitate the development
of strategies for preventing or decreasing neuronal cell death caused
by stress or trauma, as suggested by the phenotype of the
jnk3
/
mice (94).
To explore these pathways, we initially used the PC12 cell system to
investigate how activation of JNK (and p38) leads to apoptosis. In
stably transfected PC12 cell clones in which expression of MEKK1 can be
induced by dexamethasone (Dex), JNK and p38 activation are followed by
extensive apoptosis in which JNK and c-Jun appear to play a critical
role because expression of a dominant-negative c-Jun that lacks the JNK
phosphorylation sites, c-Jun(A63/73), but not wild-type (wt) c-Jun,
blocks cell death. In this system, apoptosis seems to require new gene
expression and JNK (and p38) activation are linked to induction of
FasL. Incubation of PC12 cells, in which JNK and p38 were activated,
with a chimeric Fas-Fc decoy protein (6, 77, 80) attenuates
the apoptotic response. We also find that NGF withdrawal from
postmitotic PC12 cells results in increased JNK and p38 activities and
induction of FasL along with apoptosis, which can be inhibited by
Fas-Fc. In both cases, FasL induction and apoptosis are attenuated by a
small molecule inhibitor of p38 and JNK. To more critically evaluate
the role of FasL induction in neuronal apoptosis, we established a
physiological system based on primary cultures of cerebellar granule
neurons (CGCs) in which elevated K+ serves as a survival
factor (15). Such cultures were established by using CGCs
isolated from either wt or gld C3H/HeJ mice, whose FasL gene codes for a nonfunctional protein (80).
While KCl withdrawal results in similar degrees of JNK activation and
c-Jun N-terminal phosphorylation in wt and gld CGCs, the
latter are considerably less sensitive to apoptosis caused by KCl
removal. Incubation of wt CGCs with Fas-Fc attenuates the apoptotic
response to KCl removal. These results strongly suggest that a
JNK-to-c-Jun-to-FasL signaling pathway plays an important role in
induction of neuronal cell death in response to various stresses.
 |
MATERIALS AND METHODS |
Plasmids.
MEKK1
is a truncated form of MEKK1
(46) consisting of its C-terminal catalytic domain. A cDNA
fragment encoding the C-terminal portion (1.7 kb) of mouse MEKK1 was
subcloned into the glucocorticoid-inducible pJ5
vector in which
expression is controlled by the murine mammary tumor virus (MMTV) long
terminal repeat (57). Hemagglutinin epitope (HA)-tagged wt
c-Jun and c-Jun(A63/73) (75) contain the influenza virus HA
fused to the NH2 terminus of c-Jun.
Cell culture and transfection.
PC12 cells (passages 6 to 20;
originally obtained from L. Greene) were maintained in RPMI 1640 medium
supplemented with 10% heat-inactivated horse serum, 5% fetal bovine
serum, 1 mM glutamine, and 1% penicillin-streptomycin in an atmosphere
of 5% CO2 at 37°C. PC12 cells were cultured on
poly-L-lysine (Sigma)- or poly-L-lysine/laminin (Becton Dickinson)-coated plates. Stable MEKK1
PC12 cell lines were
generated by transfection of pJ5
-MEKK1
, using Lipofectamine (GIBCO-BRL) according to the manufacturer's instructions. Stable transfectants were selected in medium containing 250 µg of G418 (Geneticin; GIBCO) per ml. After 2 to 3 weeks, single clones were isolated and induced with 10
8 M Dex for 24 h, and
MEKK1
-expressing clones were identified by immunoblotting with
anti-MEKK1 (C-22; Santa Cruz Biotechnology). CGCs were isolated from
7-day-old wt or gld C3H/HeJ mice (The Jackson Laboratory,
Bar Harbor, Maine) and housed under virus-free conditions as described
elsewhere (5). CGCs were seeded onto poly-D-lysine (30 µg/ml)-coated dishes at a density of
0.25 × 106 cells/cm2 and cultured in
Eagle's basal medium supplemented with 10% heat inactivated fetal
bovine serum, 25 mM KCl, and 0.5% (vol/vol) penicillin-streptomycin.
To prevent growth of glial cells, cytosine arabinoside (5 µM) was
added to the cultures 24 h after seeding. CGCs were shifted to 5 mM KCl medium after 7 days, when they were fully differentiated
(22) and the neuronal cell type represented more than 95%
of the total cell population (83). Fas-Fc (40 µM)
treatment was done 12 h prior to and during MEKK1
induction or
KCl and NGF withdrawal.
Immunoblot analysis.
Cell lysates (40 to 100 µg of
protein) were resolved by sodium dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE) on 8 to 12% polyacrylamide gels and
electroblotted onto Immobilon P membranes (Millipore). After blocking
with 5% milk in PBS-T (phosphate-buffered saline [PBS] with 0.05%
Tween) for 1 h, the membranes were probed with anti-MEKK1 (C-22;
Santa Cruz), anti-ERK2 (C-14; Santa Cruz), anti-c-Jun (Santa Cruz),
anti-phospho-c-Jun S*63 (KM-1; Santa Cruz), anti-JNK1 (333.8;
PharMingen), anti-phospho-JNK T*183/Y*185 (New England Biolabs),
anti-phospho-ERK T*202/Y*204 (New England Biolabs), and
anti-phospho-p38 T*182 (New England Biolabs) (asterisks mark sites of
phosphorylation). The antibody-antigen complexes were visualized by
enhanced chemiluminescence (Amersham International).
Kinase assays.
Endogenous JNK1 and ERK2 were
immunoprecipitated from cell lysates (57) with an anti-JNK1
monoclonal antibody (333.8; PharMingen) or with anti-ERK2 (C-14; Santa
Cruz), and their activities were measured by using 2 µg of
glutathione S-transferase (GST)-c-Jun(1-79) or 4 µg of
myelin basic protein (MBP; Sigma) respectively, as the substrate
(17, 57). Data for all experiments were quantitated with a
phosphorimager, and results were normalized to JNK or ERK levels
determined by immunoblotting.
Immunofluorescence staining and in situ hybridization.
PC12
cells were cultured on poly-L-lysine-coated Permanox
chamber slides (Labtek II; Nunc) and treated as indicated below. The
slides were washed once with PBS, fixed for 40 min with 4% paraformaldehyde, washed three times, permeabilized with 0.1% Triton
X-100 for 10 min, and washed four times with 0.1% bovine serum albumin
(BSA)-PBS. Slides were blocked in 1% BSA or 0.5% goat serum for
1 h at room temperature and then incubated for 1 h with 0.1%
BSA-PBS containing anti-HA (12CA5; Boehringer Mannheim) monoclonal
antibody. Following four washes, fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse (1:200 dilution; Jackson
ImmunoResearch Laboratories Inc.) in 0.1% BSA-PBS was added;
incubation for 1 h at room temperature was followed by three
subsequent washes. Hoechst dye H33258 (Sigma), 2.5 µg/ml in PBS, was
added along with the secondary antibody. Cells were examined and
photographed with a Zeiss Axioskop microscope equipped for
epifluorescence with the appropriate filters.
For in situ hybridization, FasL (5'-GAGGATCTGGTGCTAATGGA-3')
and c-Jun (5'-CGCGGATCCCTATGACTGCAAAGATG-3') oligonucleotide probes
were labeled with biotin-16-dUTP by random priming (Biotin-high Prime;
GIBCO-BRL). CGCs cultured on poly-D-lysine-coated Labtek II
chamber slides were fixed in 100% methanol for 15 min and washed in
PBS. The slides were incubated with prehybridization solution (50%
deionized formamide, 10% dextran sulfate, 2× SSC [1× SSC is 0.15 M
NaCl plus 0.015 M sodium citrate], 0.12 mM EDTA, 0.33 mg of sheared
salmon sperm DNA per ml) for 4 h at 42°C. After decanting of the
prehybridization solution, the hybridization solution (prehybridization
solution with 1 ng of biotin-labeled probe per ml) prewarmed to 42°C
was added, and the slides were left in a humid chamber overnight at
42°C. Slides were washed twice in 2× SSC for 10 min and twice in
0.1× SSC for 10 min at 42°C. Biotin-labeled cells were detected with
a Dako kit (Dako Inc.), using alkaline phosphatase as a secondary
detection system, and visualized with a Nikon light microscope with a
blue filter. As a control for specificity, cells were hybridized to the
probes in the presence of 10-fold-excess unlabeled competitor
(91).
RT-PCR and DNA analysis.
Total RNA was isolated from CGCs or
PC12 cells by using TRIazol reagent (GIBCO-BRL). First-strand cDNA was
prepared by reverse transcription (RT) of 1 µg of RNA in a 20-µl
reaction volume containing 10× PCR buffer, 1 mM dithiothreitol, 200 µM deoxynucleoside triphosphates, and 5 µM random primers
(Superscript reverse transcriptase; Life Technologies). PCR
amplifications were performed in a 50-µl reaction volume containing 1 µg of cDNA, with a master mix composed of 10× PCR buffer, 1 mM MgCl,
200 µM deoxynucleoside triphosphates, Taq polymerase, and
20 nM each primer. Primers used were as follows: mouse FasL sense
(5'-CAGCAGTGCCACTTCATCTTGG-3') and antisense (5'-TTCACTCCAGAGATCAGAGCGG-3');
-actin sense
(5'-TGGATTCCTGTCGCATCCATGAAAG-3') and antisense
(5'-TTAAACGCAGCTCAGTAACAGTCCG-3'); rat FasL sense (5'-CAGCCCCTGAATTACCCATGTC-3') and antisense
(5'-CACTCCAGAGATCAAAGCAGTTC-3'); rat Fas sense
(5'-CAAGGGACTGATAGCATCTTTGAGG-3') and antisense (5'-TCCAGATTCAGGGTCACAGGTTG-3'); rat glyceraldehyde
3-phosphate dehydrogenase (GAPDH) sense
(5'-ACCACAGTCCATGCCATCAC-3') and antisense (5'-TCCACCACCCTGTTGCTGTA-3'). The intactness of total RNA
used for the RT reaction was tested by electrophoresis on a 1.1%
agarose gel. The RT-PCR assays were first performed on identical
amounts of RNA for each sample. Next, a dilution series (1:10, 1:100, and 1:1,000) of the cDNA was performed with GAPDH or actin primers. After normalizing and adjusting the cDNA to ensure that each sample yielded equal amount of GAPDH or actin signal, the actual PCR assays
were performed with Fas and FasL primers. We also checked the linearity
of the reactions to determine the optimal number of cycles for each PCR
product. The PCR products, 507, 568, and 452 bases corresponding to
nucleotides 80 to 586 of rat FasL cDNA, 1 to 568 of Fas cDNA, and 550 to 1001 of GAPDH cDNA, respectively (44, 78, 82), were
separated by 1.5% agarose gel electrophoresis and stained with
ethidium bromide. To confirm their identities, the FasL and Fas PCR
products were purified and sequenced. The nucleotide sequences were in
full agreement with those expected for the corresponding fragments of
the rat Fas and FasL cDNAs (data not shown). Standard procedures were
used for Southern blot analysis (71).
Apoptosis assays.
For detection of apoptosis, PC12 cells
were cultured on poly-L-lysine-coated Labtek II chamber slides.
Apoptotic cells were detected with an in situ cell death detection kit
incorporating FITC labeling and TUNEL) (in situ terminal
deoxynucleotidyltransferase-mediated dUTP nick-end labeling) as
instructed by the manufacturer (Boehringer Mannheim). The fraction of
apoptotic cells was also determined by the acridine orange-ethidium
bromide dye uptake method (55). At least 200 cells were
counted for each time point. Apoptotic cells were also detected by
nuclear staining with Hoechst 33258 (2.5 µg/ml) in PBS after fixation
in 4% paraformaldehyde. Cells were examined and photographed in a
Zeiss Axioskop microscope as described above. CGCs cultured on
poly-D-lysine coated Labtek II chamber slides (Nunc)
treated as described, fixed in 100% methanol for 15 min, washed in
PBS, and subsequently stained with hematoxylin (Sigma) for 10 min.
Coverslips were mounted onto a glass slide in glycerol-PBS (1:1) and
examined with a 100× objective of a Nikon light microscope. The number
of apoptotic cells was expressed as a percentage of total neuronal
cells visible in each field. These experiments were done five times in triplicate.
 |
RESULTS |
MEKK1
expression in PC12 cells results in JNK and p38 activation
and apoptosis.
To examine the consequences of JNK (and p38)
activation in the absence of ERK activity on PC12 cells, we took
advantage of the finding that moderate MEKK1 overexpression results in
JNK and p38 activation while having no stimulatory effect on ERK
(57). We stably transfected PC12 cells with a
pJ5
-MEKK1
construct, in which expression of the MEKK1 catalytic
domain is controlled by the Dex-inducible MMTV promoter. Individual
clones were screened by immunoblotting with anti-MEKK1 (data not shown)
and by immunocomplex kinase assay for the ability of Dex to activate
JNK (Fig. 1A). Much of the following work
was done with clone 21 because of its high JNK activation ratio
(16-fold). However, similar results were obtained with clones 18 and 22 (data not shown). Treatment of these cells with Dex results in
dose-dependent MEKK1
induction, JNK and p38 activation, and c-Jun
N-terminal phosphorylation (Fig. 1B). As previously found
(57), MEKK1
expression in these cells did not activate
ERK. Moreover, MEKK1
induction inhibited activation of ERK by either
NGF (Fig. 1C), epidermal growth factor, or tetradecanoyl phorbol
acetate (data not shown). No JNK or p38 activation or inhibition of ERK
activity was observed in mock-transfected cells treated with Dex (Fig.
1D). Efficient MEKK1
induction and JNK activation required
approximately 6 h of incubation with Dex and lasted for 3 to 5 days (data not shown). Inhibition of ERK activation required more than
6 h of incubation with Dex and therefore is not an immediate
response to MEKK1
induction or JNK activation (data not shown).

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FIG. 1.
JNK and p38 activation and ERK inhibition following
MEKK1 expression in PC12 cells. (A) Individual clones of PC12 cells
stably cotransfected with pJ5 -MEKK1 and RSV-Neo were examined for
induction of JNK activity by Dex. After 24 h of incubation in the
absence or presence of 10 8 M Dex, cell lysates were
prepared and JNK activity was measured by immunocomplex kinase assay
with GST-c-Jun(1-79) as a substrate. (B) Clone 21 cells were incubated
with the indicated concentrations of Dex for 24 h, at which time
they were lysed and the contents of MEKK1 , activated JNK, JNK1,
activated p38, c-Jun phosphorylated at S63, and total c-Jun were
determined by immunoblotting with specific antibodies. (C) Clone 21 cells were incubated in the absence or presence (10 8 and
10 7 M) of Dex for 24 h and then treated with or
without NGF (50 ng/ml) for 15 and 30 min as indicated. Cell lysates
were prepared, and the levels of MEKK1 , activated ERK, and total
ERK2 were determined by immunoblotting. ERK enzymatic activity was
determined by immunocomplex kinase assay with MBP as a substrate. (D)
MMTV plasmid mock-transfected cells were incubated in the absence or
presence of 10 7 M Dex for 24 h and then treated with
or without NGF (50 ng/ml) for 15 and 30 min as indicated. Cell lysates
were prepared, and the levels of MEKK1 , activated ERK and total ERK2
were determined by immunoblotting. ERK enzymatic activity was
determined by immunocomplex kinase assay with MBP as a substrate.
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Microscopic examination of cells in which MEKK1

expression was
induced revealed morphological changes characteristic of apoptosis,
such as nuclear condensation and fragmentation and membrane blebbing.
To detect DNA fragmentation, we used TUNEL; the karyophylic dye
H33258
was used to visualize changes in nuclear morphology. MEKK1
expression resulted in induction of apoptosis, while no apoptotic
cells
were detected following Dex treatment of cells stably transfected
with
a mock expression vector (Fig.
2A and B).
In addition, we
determined the number of dead cells
by acridine orange-ethidium
bromide staining. Close to 50% of the
cells underwent apoptosis
after 3 days of Dex treatment, and
approximately 60% of the cells
were apoptotic after 5 days (Fig.
2C).

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FIG. 2.
Induction of MEKK1 expression causes apoptotic cell
death. (A) Clone 21 cells (2 × 104 per well) were
cultured in the absence or presence of Dex (10 7 M) for
the indicated times. DNA fragmentation was examined at the time
indicated, using an in situ cell death detection assay (TUNEL). Nuclei
were stained with Hoechst dye H33258. The cells were examined in a
Zeiss Axioskop microscope with the appropriate filters. EtoH, ethanol.
(B) PC12 cells stably transfected with empty pJ5 expression vector
were incubated with ethanol or Dex (10 7 M) for 5 days and
examined as described above. (C) Clone 21 cells were incubated with
10 7 M Dex for the indicated duration, at which point they
were stained with a mixture of acridine orange and ethidium bromide and
the percentage of apoptotic cells was determined. The results shown are
averages of three experiments. (D) Clone 21 cells were cultured in the
absence or presence of Dex (10 7 M). At the indicated
times (in days) the induction medium was removed, and the cells were
washed twice with PBS and then cultured in normal growth medium until day 7, at which time they were stained
with acridine orange-ethidium bromide. A minimum of 200 cells were
placed in a hemocytometer, and the relative numbers of apoptotic cells
were determined. The results shown are averages of two experiments.
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Efficient induction of apoptosis requires prolonged MEKK1

expression. When cells were incubated with Dex for only 1 day and
then washed and placed in Dex-free medium for 6 more days, little
cell
death occurred (Fig.
2D). However, if cells were incubated
with Dex for
3 days and then washed and placed in Dex-free medium
for 4 more days,
the extent of cell death was nearly as high as
in cells that were
subjected to continuous incubation with Dex
for 5 to 7
days.
MEKK1
-induced cell death is dependent on c-Jun N-terminal
phosphorylation and JNK activity.
The delay between MEKK1
induction and JNK (or p38) activation and the onset of apoptosis, as
well as the requirement for prolonged MEKK1
expression,
suggested that new gene expression may be required to activate the
apoptotic program. It was shown that inhibition of RNA or protein
synthesis blocks apoptosis induced by NGF deprivation (54).
However, as MEKK1
induction depends on RNA and protein synthesis, we
were unable to use general inhibitors to examine the requirement for
gene expression in this system. Because the c-Jun transcription factor
is an important and specific target for JNK (40) and was
suggested to be involved in apoptosis triggered by NGF withdrawal
(20, 31), we examined whether a c-Jun mutant that is not
phosphorylated by JNK could inhibit apoptosis of PC12 cells expressing
MEKK1
. While transient expression of wt c-Jun in clone 21 cells did
not affect the apoptotic response to MEKK1
, expression of
c-Jun(A63/73) markedly decreased cell death (Fig.
3A and B). This protective effect was
restricted to c-Jun(A63/73)-expressing cells; induction of cell death
in surrounding cells not expressing this protein was not affected.

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FIG. 3.
MEKK1 -induced cell death is blocked by
phosphorylation-defective c-Jun mutant and SB202190. (A) Clone 21 cells
(5 × 104) were cultured on chamber slides and
transiently transfected with HA-tagged wt c-Jun or c-Jun(A63/73)
expression vector. After 16 h, the cells were washed twice with
PBS and incubated in the presence of 10 7 M Dex. After 3 more days, the cells were fixed with 4% paraformaldehyde and
permeabilized with 0.1% Triton X-100. Expression of FITC-labeled
HA-c-Jun proteins was detected by indirect immunofluorescence using a
monoclonal HA antibody. Nuclear morphology was determined by staining
with Hoechst dye H33258. The arrows indicate cells with apoptotic
morphology, which in the case of wt c-Jun are also positive for
HA-c-Jun expression. However, in the case of c-Jun(A63/73), only 5% of
the HA-positive cells have apoptotic morphology. (B) The results of the
experiments shown above were quantitated by counting six independently
transfected cultures, 80 to 90 cells each. The total numbers of
transfected cells that were counted are indicated above each column.
(C) Clone 21 cells were induced with or without 10 7 M Dex
in the absence or presence of SB202190 (SB; 3 or 30 µM) for 6 h,
followed by two washes to remove the drug. Dex-containing medium was
added for an additional 18 h; then the cells were stained with
acridine orange-ethidium bromide and the percentage of apoptotic cells
was determined. The results are averages of three experiments.
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Another way to examine the involvement of p38 or JNK in a
biological process is to use specific inhibitors of these
MAPKs.
The pyridinyl imidazole compound (SB202190) was
reported to be
a specific p38 inhibitor (
47). However, when
applied to cells
at 30 µM, SB202190 or related compounds also inhibit
JNK activity
(
38,
89) and possibly other protein kinases. We
incubated
clone 21 cells with or without Dex and in the absence or
presence
of SB202190. While 3 µM SB202190 yielded a partial
protective
effect, incubation with 30 µM SB202190 resulted in
complete protection
against MEKK1

-induced apoptosis (Fig.
3C).
At 3 µM, SB202190
did not inhibit JNK activity, detected by
c-Jun phosphorylation
at serine 63, while at 30 µM it fully inhibited
JNK activity (Fig.
4B).

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FIG. 4.
Expression of MEKK1 causes induction
of FasL and thereby leads to apoptosis. (A) Clone 21 cells were
incubated with 10 7 M Dex, and total cellular RNA was
extracted at the indicated times. Expression of Fas,
FasL, and GAPDH mRNAs was determined by
quantitative RT-PCR using specific primers. To confirm the PCR product
as a fragment of FasL cDNA, the reaction products were
blotted and detected with a rat FasL cDNA probe (bottom
panel). In addition, the nucleotide sequence of this fragment was
determined and found to correspond to that of rat FasL cDNA
(data not shown). d, days. (B) Clone 21 cells were incubated with or
without 10 7 M Dex in the absence or presence of SB202190
(SB; 3 or 30 µM) for 6 h, followed by two washes to remove the
drug. Dex-containing medium was added for an additional 18 h, at
which time the cells were collected and lysed. After separation by
SDS-PAGE, c-Jun phosphorylation was examined by immunoblotting with
antibody against c-Jun phosphorylated at S63. Expression of
FasL and GAPDH mRNAs was determined by RT-PCR
(bottom two panels). (C) To determine the role of FasL in
MEKK1 -induced apoptosis, clone 21 cells were cultured for the
indicated times with Dex (10 7 M) in the presence of
chimeric Fas-Fc protein (20 µg/ml) or purified IgG (20 µg/ml). At
the indicated times, the cells were stained with acridine
orange-ethidium bromide and the percentage of apoptotic cells was
determined. The results shown are averages of two experiments done in
duplicate.
|
|
MEKK1
or NGF withdrawal lead to FasL induction.
A possible
explanation for the protective effect of c-Jun(A63/73) is interference
with the transcriptional activity of c-Jun or another transcription
factor that requires JNK-mediated phosphorylation. One inducible
protein that plays a critical role in induction of apoptosis during
various pathological conditions is FasL (10, 27, 30, 80). We
therefore examined whether induction of MEKK1
results in
upregulation of FasL expression. Incubation of clone 21 cells with Dex
resulted in induction of FasL mRNA (Fig. 4A), with kinetics that were
similar to the kinetics of apoptotic cell death (Fig. 2C). By contrast,
expression of Fas mRNA was constitutive. Incubation of Dex-treated
clone 21 cells with 30 µM SB202190 interfered with induction of FasL
mRNA (Fig. 4B). At 3 µM, SB202190 did not inhibit FasL induction.
To determine whether FasL induction played a causal role in the
apoptotic response, we incubated the cells with a chimeric
Fas-Fc
protein. This protein protects cells against FasL-triggered
apoptosis,
most likely by sequestering it and thus preventing
its binding to
functional receptors (Fas) on the cell surface
(
6,
77).
Incubation of clone 21 cells with chimeric Fas-Fc
but not with
control immunoglobulin (immunoglobulin G [IgG]) conferred
considerable protection (approximately threefold decrease in cell
death) against Dex-induced apoptosis (Fig.
4C).
We tested whether a more physiological paradigm, such as NGF
withdrawal, which causes apoptosis of PC12 cells and primary
sympathetic neurons, leads to induction of FasL. PC12 cells were
incubated in the presence of NGF for 11 days, at which point extensive
neuronal differentiation occurred (data not shown). Removal of
NGF and
incubation with NGF neutralizing antibody resulted in
increased JNK and
p38 activities peaking after 1 to 2 days (Fig.
5A) and extensive apoptosis (data not
shown). JNK activity and
c-Jun phosphorylation after NGF withdrawal
were severalfold higher
than in cells kept in NGF for 11 days, and the
latter had higher
activity than nontreated PC12 cells. Stimulation of
JNK activity
in response to NGF withdrawal was followed by induction of
FasL
mRNA and protein (Fig.
5A). Despite prolonged JNK activation,
no
FasL expression or apoptosis was detected in cells that were
continuously incubated with NGF (Fig.
5A, lane 11d).

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|
FIG. 5.
JNK and p38 activation and FasL induction in response to
NGF withdrawal. (A) PC12 cells cultured on
poly-L-lysine/laminin-coated plates were incubated with or
without (lane C) NGF (50 ng/ml) for 11 days, at which time the cells
were carefully washed with NGF-free medium and incubated for 6 h
(6hr), 1 day (1d), 2 days (2d), and 3 days (3d) with neutralizing NGF
(2.5S) antibodies (1:4,000). As an additional control, PC12 cells were
incubated with NGF for 11 days before harvesting (lane NGF, 11d). The
cells were collected and lysed, and the level of JNK activity was
determined by immunocomplex kinase assays with GST-c-Jun(1-79) as a
substrate. The levels of activated p38, c-Jun phosphorylated at S63,
JNK1, and ERK2 and of FasL expression were determined by immunoblotting
with specific antibodies (top six panels). The levels of
FasL and GAPDH mRNAs were determined by RT-PCR
using extracts from cultures treated in parallel to those used for
protein analysis (bottom two panels). (B) PC12 cells were subjected to
the same treatment as for panel A. The lane labeled Control represents
PC12 cells cultured without NGF. One hour prior to NGF withdrawal, the
cells were incubated with or without SB202190 (SB; 3 or 30 µM) for
6 h. The cells were then washed to remove the drug, and NGF-free
medium was added for an additional 18 h. After 1 day of NGF
withdrawal, the cells were stained with acridine orange-ethidium
bromide and the percentage of apoptotic cells was determined. The level
of c-Jun phosphorylated at S63 was determined by immunoblotting, and
FasL and GAPDH mRNA levels were determined by
RT-PCR. The results in the top panel are averages of three experiments.
(C) PC12 cells were treated as described for panel A. Purified IgG or
chimeric Fas-Fc protein (20 µg/ml) was added 12 h prior to and
during the time of NGF withdrawal or to cells that were continuously
incubated with NGF. The lane labeled Con represents PC12 cells cultured
without NGF. After 1 day, the cells were stained with acridine
orange-ethidium bromide and the percentage of apoptotic cells was
determined. The levels of c-Jun phosphorylated at S63, FasL,
and GAPDH mRNAs were examined as described above. The
results in the top panel are averages of three experiments.
|
|
To test whether JNK activation was linked to FasL induction in
this system, we incubated postmitotic PC12 cells with SB202190
prior to
NGF removal. When used at 30 µM, SB202190 inhibited FasL
induction and c-Jun phosphorylation (Fig.
5B). In addition, incubation
with 30 µM SB202190 partially protected PC12 cells against apoptosis
induced by NGF withdrawal (Fig.
5B). Incubation of postmitotic
PC12
cells following NGF removal with Fas-Fc (40 µM) also resulted
in a
considerable reduction in cell death (Fig.
5C). Due to a
nonspecific
cytotoxic effect (
48a), the cells could not be incubated
with SB202190 for longer than 6 to 8 h. Thus, we could not
determine
whether longer incubation with this compound would result in
more
extensive
protection.
gld CGCs are less sensitive to induction of apoptosis
after survival factor withdrawal.
The results described above
suggest that FasL is an important mediator of neuronal apoptosis caused
by withdrawal of survival factors. To examine this point more
critically, we used primary cultures of CGCs derived from wt or
gld C3H/HeJ mice. The latter harbor a loss-of-function
mutation in the FasL gene (gld) which renders its
product nonfunctional because it can no longer bind to Fas
(80). Survival of CGCs requires culturing in the presence of
25 mM KCl, which causes membrane depolarization and provides a survival
signal (19). Once the K+ concentration is
reduced to 5 mM, mature differentiated CGCs undergo apoptosis
(19). CGCs isolated from wt and gld mice were cultured in the presence of 25 mM KCl. After 7 days, approximately 90%
of the CGCs were alive and differentiated. These cells were either kept
in 25 mM KCl or placed in medium containing 5 mM KCl. Incubation in the
presence of 5 mM KCl caused wt CGCs to undergo extensive apoptosis such
that within 24 h approximately 70% of the cells exhibited
apoptotic morphology (Fig. 6A and B). By
contrast, only 20% of the CGCs isolated from gld mice were
apoptotic after 24 h in the presence of 5 mM KCl, whereas the
basal level of apoptosis in either wt or gld CGCs maintained
in 25 mM KCl was very similar, approximately 10% (Fig. 6B).

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FIG. 6.
Neurons from gld mice are less sensitive to
induction of apoptosis following KCl withdrawal. (A) Detection of
apoptotic nuclei by hematoxylin staining 12 h after KCl
withdrawal. CGCs of wt type and gld C3H/HeJ mice were
cultured in 25 mM KCl for 7 days, at which time the KCl concentration
was lowered to 5 mM. (B) Time course of appearance of apoptotic nuclei
in wt and gld CGC cultures after KCl deprivation, with and
without addition of Fas-Fc (40 µM). Apoptotic nuclei were detected by
hematoxylin staining, and their frequency as a percentage of the total
neuronal population was determined. Five fields were counted for each
experiment; all experiments were repeated five times in triplicate. (C)
RT-PCR analysis of FasL mRNA expression in wt and
gld CGC cultures under control conditions (25 mM KCl) and
12 h after KCl deprivation (5 mM KCl). The number of PCR cycles
was determined as described in Materials and Methods, and
-actin mRNA was used as a control. (D) Parallel samples
of CGCs treated as described above were collected and lysed, and the
levels of c-Jun phosphorylation at S63, activated JNK, and total c-Jun
were determined by immunoblotting.
|
|
It is also important to note that
gld CGCs were not entirely
resistant to induction of apoptosis and that after 48 h in 5
mM
KCl approximately 50% of the cells exhibited apoptotic morphology,
whereas 80% of the wt CGCs were apoptotic at this time point.
Incubation of wt CGCs with Fas-Fc produced similar results: a
clear
delay in the onset of apoptosis in response to KCl withdrawal
and a
decrease in its frequency, especially at the earlier time
points (12 and 24 h). Only 25% of wt CGCs incubated in 5 mM KCl
in the
presence of Fas-Fc were apoptotic after 24 h, in comparison
to
70% of the CGCs that were not treated with Fas-Fc. After 48
h,
approximately 40% of the wt CGCs kept in 5 mM KCl and Fas-Fc
were
apoptotic, in comparison to 80% of the CGCs incubated without
Fas-Fc
(Fig.
6B).
As found in PC12 cells, incubation of CGCs in 5 mM KCl resulted in
induction of FasL expression which was detected within
12 h (Fig.
6C). In this case no obvious differences were found
between wt and
gld CGCs, indicating that the signaling pathway
leading to
FasL induction is functional in these cells. In support
of this
conclusion, we find that culture of either wt or
gld CGCs
in
5 mM KCl results in similar levels of JNK activation and c-Jun
N-terminal phosphorylation (Fig.
6D). The kinetics of JNK activation
and c-Jun N-terminal phosphorylation correlate with the kinetics
of
apoptosis induction in wt CGCs (compare Fig.
6B and D). The
CGCs also
express Fas mRNA, but unlike FasL, these transcripts
are expressed
constitutively, as in PC12 cells (Fig.
4A and data
not shown). These
results indicate that FasL upregulation plays
an important role in
induction of apoptosis in wt CGC cultured
in 5 mM KCl and that the
pathway triggered by KCl withdrawal exhibits
the same hallmarks as the
pathway triggered by NGF withdrawal
from differentiated postmitotic
PC12
cells.
Microscopic examination of the primary cultures indicates that
approximately 95% of the cells are typical CGCs, while the
remainder
appear to be glial cells (data not shown). In situ hybridization
with
FasL and c-
jun probes confirmed that the major
cell type
in which these transcripts were induced after incubation in 5
mM KCl was neuronal (Fig.
7). Although
the increase in c-
jun mRNA
is quite high (Fig.
7D and E),
the increase in the total amount
of c-Jun protein is not as high (Fig.
6D). Such discordance between
induction of c-
jun mRNA and
c-Jun protein was described previously
(
1). It should be
noted, however, that at early time points
(6 to 8 h) after
placement of the cultures in 5 mM KCl, the first
cells to react
positively with the
FasL probe have glial rather
than
neuronal morphology (data not shown). While these cells may
contribute
to the initiation of the apoptotic response (see Discussion),
our
results, nevertheless, demonstrate that neuronal cells such
as CGCs
have the capacity to express FasL and respond to it.

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FIG. 7.
FasL and c-Jun mRNA are upregulated upon KCl withdrawal.
In situ hybridization using probes for FasL (A to C) and
c-jun (D to F) mRNAs was performed on CGCs cultured in the
presence of 25 mM KCl (A and D) or on CGCs incubated for 12 h in
medium containing only 5 mM KCl (B, C, E, and F). As a specificity
control, the cells in panels C and F were hybridized to the probes in
the presence of a 10-fold excess of unlabeled competitor.
|
|
 |
DISCUSSION |
The results described above chart a signaling pathway that
explains how activation of JNK following stress or elimination of
survival factors can lead to neuronal apoptosis. Previous studies have
indicated that induction of c-jun transcription and elevated c-Jun expression are associated with apoptosis in sympathetic neurons,
differentiated PC12 cells (which closely resemble sympathetic neurons),
and central neurons. In addition to a correlation between c-Jun
induction and neuronal cell death, interference with c-Jun function
either by expression of dominant-negative c-Jun mutants or
microinjection of neutralizing c-Jun antibodies protect sympathetic neurons or postmitotic PC12 cells from apoptosis induced by NGF withdrawal or MEKK expression (20, 31, 93). The most
conclusive evidence for the involvement of JNK in neuronal apoptosis is
provided by the phenotype of mice that are deficient in the
neuron-specific JNK3 isoform (94). While these mice are
apparently normal in their gross and neurologic anatomies, they are
resistant to induction of hippocampal neuron apoptosis by injection of
kainic acid (94), a potent glutamate agonist (3).
These studies, however, did not explain how JNK activation (which can
be elicited by kainic acid administration or removal of survival
signals) can trigger apoptosis. The molecular mechanism responsible for
execution of the apoptotic program appears to be universally conserved
(58, 59). A key step is activation of the caspase cascade,
which occurs in the cytoplasm (61). Thus, it was not clear
how interference with c-Jun function in the nucleus can prevent caspase
activation. One possible explanation for this quandary is that
elevated c-Jun expression and transcriptional activity (through
JNK-mediated phosphorylation) (40) serve to induce the
transcription of a gene(s) whose product triggers the apoptotic
process. Indeed, it was previously shown that inhibitors of RNA and
protein synthesis protect sympathetic neurons from death caused by NGF
withdrawal (54). The experiments described above demonstrate
that an important mediator of cell death either in PC12 cells or CGCs
undergoing apoptosis in response to either direct activation of the JNK
cascade or withdrawal of survival signals (KCl or NGF) is FasL.
Interference with binding of FasL to its receptor, Fas, through
incubation with the chimeric Fas-Fc protein, which acts as a molecular
decoy (6, 77), results in significant but incomplete
protection of both cell types from apoptosis. Binding of FasL to Fas
rapidly activates the apoptotic machinery through a series of
protein-protein interactions (11, 58, 87). The time delay
required for maximal FasL induction provides an attractive explanation
for why stress-induced neuronal apoptosis is a rather slow process in
comparison to apoptosis induced by direct Fas activation. The strongest
evidence for the involvement of FasL in neuronal apoptosis comes from
comparing the responses of CGCs derived from wt and gld mice
after KCl withdrawal. The survival of such neurons depends on
cultivation in the presence of 25 mM KCl (a depolarizing K+
level), and once placed in 5 mM KCl they undergo apoptosis within 12 to
24 h (19). Both the onset and the rate of apoptosis are significantly decreased in CGCs derived from gld mice, which
express a nonfunctional FasL protein (80). Furthermore,
apoptosis in wt CGC caused by KCl withdrawal is attenuated upon
incubation in the presence of Fas-Fc (Fig. 6A and B).
Expression of FasL, like other members of the TNF family to which it
belongs (76), is inducible (58, 59, 78). Recent results indicate that, similar to expression of TNF (66,
79), expression of FasL is transcriptionally regulated and that
transcription factors NF-
B and AP-1 play an important role in this
induction process (21, 42). These results also suggest that
activation of the JNK pathway is required for induction of FasL
promoter activity in response to various genotoxic stimuli (21,
42). We find that in both CGCs and PC12 cells, JNK activation in
response to elimination of survival signals, as diverse as 25 mM KCl
and NGF, occurs with kinetics that are consistent with a causal role in
FasL induction. Treatment of PC12 cells with an inhibitor of p38 and
JNK (and possibly other protein kinases), SB202190, at a dose that
inhibits both JNK and p38 but not at a dose that inhibits only p38
results in inhibition of both JNK activity measured by c-Jun N-terminal
phosphorylation and FasL induction in response to either MEKK1
expression (Fig. 4B) or NGF withdrawal (Fig. 5B). A likely mediator of
FasL induction in this case is the N-terminally phosphorylated c-Jun
protein, a component of transcription factor AP-1 (40). This
possibility is supported by the strong correlation between c-Jun
N-terminal phosphorylation in the three experimental systems that we
examined, FasL induction, and the onset of apoptosis. In addition, we
find that expression of a nonphosphorylatable c-Jun mutant,
c-Jun(A63/73), protects PC12 cells from apoptosis caused by MEKK1
activation (Fig. 3A and B). These results suggest that an important
downstream mediator leading from JNK activation to FasL induction is
the N-terminally phosphorylated c-Jun transcription factor. A clear
demonstration of the role of JNK in AP-1 activation is provided by the
jnk3
/
mice, which exhibit a large decrease
in induction of an AP-1-dependent reporter in response to kainic acid
in comparison to their wt counterparts (94).
While the results described above strongly support a role for FasL as
an important mediator of neuronal apoptosis caused by stress or
elimination of survival factors, it is important to realize that the
protection conferred by Fas-Fc or the gld mutation is
incomplete. It is very likely that in addition to FasL, JNK activation
may result in induction of several other death mediators that belong to
the TNF family, including TNF itself (96), TRAIL (53,
90), or TRANCE (92). A role for AP-1 in TNF induction has been established (66, 79).
It is also important to note that both jnk3
/
(94) and gld (80) mice do not exhibit
any obvious behavioral or neuroanatomical abnormalities. Therefore, it
is unlikely that either JNK, FasL, or probably N-terminally
phosphorylated c-Jun is involved in the apoptotic cell death that
occurs during development of the central nervous system (CNS)
(64). Most likely, the JNK-to-c-Jun-to-FasL pathway is used
strictly to activate the apoptotic program in response to stress
signals. One such form of stress is caused by the massive activation of
glutamate receptors after kainic acid injection leading to death of
hippocampal neurons (3). The
jnk3
/
mice appear to be completely resistant
to glutamate excitotoxicity (94). Massive activation of
glutamate receptors may also be caused by cerebral ischemia-reperfusion
(69). Indeed, it was recently found that cerebral
ischemia-reperfusion results in JNK activation, c-Jun N-terminal
phosphorylation, induction of FasL, and apoptosis, all within the same
group of neurons affected by this lesion (33). Thus, it is
quite possible that the JNK-to-c-Jun-to-FasL pathway plays an important
role in triggering neuronal apoptosis in response to a variety of
lesions and insults. Consistent with this hypothesis, gld
mice were shown to be relatively resistant to development of
experimental autoimmune encephalitis, not because they do not mount an
immune response to the encephalitogenic peptide but because of
reduced apoptosis in their CNS (86).
It has also been suggested that another cytokine, IL-1, is
involved in neuronal cell death (25, 37, 50, 84). The major source of IL-1 and other proinflammatory cytokines in the CNS are
microglial cells (37). Although glial cells are a minor contaminant in our primary CGC cultures, they appear to be the first
cell type to express FasL mRNA after incubation in 5 mM KCl (data not
shown). While they may be a minor contributor to the apoptotic response
seen in this culture system, they are likely to be important players in
the apoptotic response to CNS injuries due to their ability to produce
IL-1. Although IL-1 does not bind and activate a death receptor, it is
a potent JNK and NF-
B activator (18, 29) and known to be
capable of inducing members of the TNF family (8). The exact
source of FasL and other death mediators produced in response to
various neuronal injuries remains to be determined. It is also not yet
clear whether FasL and related factors act in an autocrine or a
paracrine manner in the experimental systems that we have used.
Nevertheless, our experiments show that neuronal cells can produce FasL
and respond to it. The description of the JNK to c-Jun to FasL neuronal
cell death pathway and its further exploration are likely to have
important practical implications, as it appears that interference with
at least two components of this pathway can prevent or decrease the
extent of several cases of stress-induced neuronal apoptosis.
Finally, as a word of caution, it is important to realize that not
every situation that leads to JNK activation or c-Jun N-terminal phosphorylation results in FasL induction and apoptosis. For instance, we find that treatment of PC12 cells with NGF also leads to JNK activation, but in this case no FasL induction or apoptosis ensues (Fig. 5A and data not shown). Most likely, NGF activates protective pathways such as the one mediated by the protein kinase AKT
(23). Alternatively, NGF either fails to activate additional
signals that could be required along with JNK activation for FasL
induction or may generate signals that interfere with the ability of
activated JNK to induce FasL. Like other MAPKs, the biological effects
of JNK activation are cell type dependent. In other cell types,
therefore, these protein kinases may be involved in cell proliferation
or even protection from apoptosis.
 |
ACKNOWLEDGMENTS |
The first two authors contributed equally to the work.
We thank A. Minden for the pJ5
-MEKK1
construct and S. Nagata for
the rat FasL cDNA and Fas-Fc construct.
This work was supported by National Institutes of Health grants ES04151
and ES06376 (to M.K.) and CA69381 (to D.R.G.). H.L. was supported by
the Lucille Markey Foundation and Molecular Endocrinology training
grants. E.B. was supported by NIH grant GM52735 (to D.R.G.). Y.K. was
supported by the Ministry of Education, Japanese Government, and
F.-X.C. was supported by the French and Swiss Leagues against Cancer.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Laboratory
of Gene Regulation and Signal Transduction, Department of Pharmacology, University of California at San Diego, 9500 Gilman Dr., La Jolla, CA
92093. Phone: (619) 534-1361. Fax: (619) 534-8158. E-mail: Karinoffice{at}ucsd.edu.
 |
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Desagher, S., Severac, D., Lipkin, A., Bernis, C., Ritchie, W., Le Digarcher, A., Journot, L.
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Collett, G. P., Campbell, F. C.
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Bharti, A. C., Takada, Y., Shishodia, S., Aggarwal, B. B.
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Kuan, C.-Y., Whitmarsh, A. J., Yang, D. D., Liao, G., Schloemer, A. J., Dong, C., Bao, J., Banasiak, K. J., Haddad, G. G., Flavell, R. A., Davis, R. J., Rakic, P.
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Giuliano, P., de Cristofaro, T., Affaitati, A., Pizzulo, G. M., Feliciello, A., Criscuolo, C., De Michele, G., Filla, A., Avvedimento, E. V., Varrone, S.
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Sah, N. K., Munshi, A., Kurland, J. F., McDonnell, T. J., Su, B., Meyn, R. E.
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Choudhury, J. A., Russell, C. L., Randhawa, S., Young, L. S., Adams, D. H., Afford, S. C.
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Konishi, Y., Bonni, A.
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Brown, J., O'Prey, J., Harrison, P.R.
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Major, C. D., Wolf, B. A.
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