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(Received for publication, July 18, 1997)
From INSERM U 55 and IFR 65 (Institut Fédératif de
Recherche du Centre Hospitalo-Universitaire Saint-Antoine),
Hôpital Saint-Antoine, 184 Rue du Faubourg Saint-Antoine, 75571, Paris Cedex 12, France
Many of the actions of serine/threonine kinase
receptors for the transforming growth factor- The transforming growth factor- The p3TP-Lux reporter construct (a gift from Dr.
Joan Massagué) contains three consecutive
12-O-tetradecanoylphorbol-13-acetate response elements, the
plasminogen activator inhibitor promoter, and a luciferase reporter
gene. pRK5-Flag-DPC4 and pRK5-Flag-Smad3 were kindly provided by Dr.
Rick Derynck. Expression plasmids for the dominant-negative mutant of
c-Jun (pCMVTAM67), glutathione S-transferase-Jun (GST-Jun),
the kinase-inactive MEKK1 mutant (pCMV5 MEKK1(K432A)), and the
dominant-interfering pcDNA3-Flag-MKK4(Ala) mutant have been
described previously (11). Gal4-DPC4 was a gift from Dr. Joan
Massagué. G15E1b-luc and Gal4-VP16 were kindly provided by Dr.
Roger Davis and Michael R. Green, respectively. The pEGFP vector
encoding for the green fluorescence protein (GFP) was purchased from
CLONTECH. GST-Jun-(1-79) was expressed in
Escherichia coli as described (13).
MDCK cells were transfected with expression vectors by
the LipofectAMINETM method (Life Technologies, Inc.). Cells
were subsequently incubated in the presence or absence of human TGF Assays for SAPK/JNK
activity were carried out as described (11). We analyzed MDCK cells for
apoptosis by phase-contrast microscopy 48 h after
transfection. Transfected MDCK cells were fixed in PBS containing 0.5%
glutaraldehyde for 15 min followed by washing in PBS with 5 mM MgCl2. Fixed cells were stained overnight with PBS containing 1 mg/ml 5-bromo-4-chloro-3-indolyl
DNA fragmentation (TUNEL staining) was determined using the in
situ cell death detection kit (Boehringer Mannheim). The
TUNEL-stained cells were visualized by microscopy.
To assess the apoptotic DNA damage, cells were collected, rinsed with
Ca2+- and Mg2+-free PBS, lysed in lysis buffer
(10 mM EDTA, 50 mM Tris (pH 8.0), 0.5% sodium
lauryl sarcosine, 0.5 mg of proteinase K/ml), and incubated at 50 °C
for 1 h. RNase A (0.5 mg/ml) was added, and lysates were incubated
for an additional hour. DNA was electrophoresed in a 1% agarose gel in
0.5 × TBE running buffer (4.5 mM Tris, 4.5 mM boric acid, 62.5 µM EDTA).
In comparison with the immediate and transient SAPK/JNK activation
induced by other stimuli such as tumor necrosis factor-
To investigate whether the SAPK/JNK signaling pathway participates in
TGF DPC4 and the related protein Smad3 are also regulated by the TGF
To determine the potential mechanism underlying the dominant-negative
activity of TAM67, we investigated whether this mutant may alter the
regulation of DPC4 transcriptional activity by TGF Our analysis of the regulation of DPC4 transcriptional activity by
c-Jun was consistent with the possibility that c-Jun and DPC4 could
interact and produce trans-activation of the 3TP-Lux reporter. One attractive possibility is that the dominant-interfering mutant of c-Jun (TAM67) binds and sequesters DPC4, thus removing it
from its natural target. However, we were unable to detect any
interactions between DPC4 and TAM67 using different methods, including
immunoprecipitation followed by Western blotting and in
vitro binding assays with GST-DPC4 produced in bacteria. Because association could be transient or unstable, these negative results do
not completely rule out direct interactions of DPC4 and c-Jun.
Another important biological activity mediated by the SAPK/JNK
signaling pathway is the induction of programmed cell death (24-26).
One mechanism by which JNK activation may contribute to cell death is
by phosphorylating and enhancing the activity of c-Jun, which in turn
regulates the activity of cell-killing genes. Consistent with this
observation, death of serum-deprived PC12 cells does not require new
gene transcription, although blockade of c-Jun activity by
microinjection of anti-c-Jun antibodies or by ectopic expression of
dominant-negative TAM67 inhibits their apoptotic death (24). Since the
activity of DPC4 has been shown to be under the control of c-Jun, we
were interested in determining whether DPC4 and Smad3 were able to
trigger cell death. To test this hypothesis, we initially used a GFP
cotransfection assay to determine the effects of DPC4 and Smad3
overexpression on cell viability. The MDCK cells expressing DPC4, but
not Smad3, displayed morphological alterations typical of adherent
cells undergoing apoptosis, becoming rounded, condensed, and detaching
from the dish (Fig. 2). DNA staining with
Hoechst 33258 (DNA dye bisbenzimide) showed that many of the
DPC4-expressing cells had apoptotic nuclei, a feature not seen when
control vector or Smad3 expression plasmids were used (Fig.
2A). DNA fragmentation analysis with the TUNEL method
confirms that the death of MDCK cells induced by overexpression of DPC4
occurred by an apoptotic mechanism (Fig. 2B).
To further quantitate the effects of ectopic expression of DPC4 and
Smad3 on cell viability, in a second independent approach, cotransfections were performed with vector expressing Signaling by TGF We thank M. Birrer, R. Davis, R. Derynck, M. Karin, M. R. Green, J. Massagué, and D. Templeton for
providing reagents.
Volume 272, Number 40,
Issue of October 3, 1997
pp. 24731-24734
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
COMMUNICATION:
through
Stress-activated Protein Kinase/c-Jun N-terminal Kinase (SAPK/JNK)
Signaling Pathway*
§,
,
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES
(TGF
) are mediated
by DPC4, a human MAD-related protein identified as a tumor suppressor
gene in pancreatic carcinoma. Overexpression of DPC4 is sufficient to
induce the activation of gene expression and cell cycle arrest, characteristic of the TGF
response. The stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) is also one of the downstream targets required for TGF
-mediated signaling. Here we report that expression of the dominant-interfering mutant of various components of
the SAPK/JNK cascade specifically blocked both TGF
and DPC4-induced gene expression. These dominant-interfering mutants also inhibited TGF
-stimulated DPC4 transcriptional activity. Moreover, we find that
overexpression of DPC4 causes transfected cells to undergo the
morphological changes typical of apoptosis. These findings define a
mechanism whereby TGF
signals mediated by DPC4 and SAPK/JNK cascade
are integrated in the nucleus to activate gene expression and identify
a new cellular function for DPC4.
(TGF
)1 is a
multifunctional factor that regulates a variety of cellular processes
including extracellular matrix formation, cell proliferation,
differentiation, and apoptosis (1-5). Two types of single
transmembrane serine/threonine kinase receptors, the type I and type
II, have been found to mediate the cellular effects of the TGF
family ligands (4, 6, 7). The signaling pathways downstream of the
TGF
receptor complex that lead to the pleiotropic effects of TGF
are still poorly understood. However, the recent identification of DPC4
and the related protein Smad3 (mothers against decapentaplegic) have
provided initial insight into the mechanism of the TGF
receptor
signal transduction (8-10). Smad3 but not DPC4 associates with the
TGF
receptors and is directly phosphorylated by the ligand-bound
receptor complex (8). Coexpression of Smad3 along with DPC4 resulted in
the synergetic activation of TGF
-like responses suggesting that
Smad3 act in partnership with DPC4 to initiate TGF
signaling (8).
Since DPC4 does not associate with nor become phosphorylated by the
TGF
receptors, these data raise the possibility that DPC4 may
fulfill some function that is not regulated by
receptor-dependent phosphorylation. Because TGF
receptor
can also trigger stress-activated protein kinase/c-Jun N-terminal
kinase (SAPK/JNK) activation (11, 12), we investigated whether
the TGF
-regulated function of DPC4 involved the activation of
SAPK/JNK signaling pathway.
Plasmids
1
(2 ng/ml) for 12 h. The luciferase activities were normalized on
the basis of
-galactosidase expression from pCMV5.LacZ-Control
vector and protein content.
-D-galactopyranoside, 5 mM potassium
ferricyanide, 5 mM potassium ferrocyanide, 2 mM MgCl2, 0.02% Nonidet P-40. The number of blue-staining
cells was determined microscopically. For the GFP, cells were fixed in
PBS containing 4% paraformaldehyde, stained with
Hoechst (2 µg/ml), and mounted.
(5 min) and
anisomycin (13, 14), treatment of MDCK cells with human TGF
1 induces
a delayed and persistent increase in SAPK/JNK activity that peaks at
8 h and reaches a maximal value of about 3-5-fold over the basal
activity (11). Therefore, we used
conditioned medium from cells treated with TGF
under conditions in
which SAPK/JNK activation was readily observed (8 h) to address the question of whether TGF
-induced SAPK/JNK activation occurs via a
paracrine mechanism. This possibility seems more unlikely, since conditioned medium stimulation did not significantly induce the activation of SAPK/JNK at any time up to 30 min, although the addition
of anisomycin to these cells led to the increase in SAPK/JNK activity
under these experimental conditions (data not shown). In contrast with
SAPK/JNK, TGF
failed to activate p38 kinase under the same
experimental conditions (data not shown). Thus, the increase in
SAPK/JNK activity most likely resulted from phosphorylation and
activation by an upstream kinase such as mitogen-activated protein
kinase (MAPK) kinase 4 (MKK4, also termed Sek1 or JNKK), a dual
specificity protein kinase, which is structurally related to MKK3, the
kinase that phosphorylates and promotes activation of p38 MAP kinase
(15-18). The activation of SAPK/JNK indicated by our observations
agrees with recent studies showing that TGF
activates a novel MAPK
kinase kinase (MAPKKK), known as TAK1, that may be involved in signal
transduction by members of the TGF
superfamily (15, 19). Activated
TAK1 phosphorylates and promotes activation of MKK4, the kinase that
controls activation of SAPK/JNK (19). Whether TAK1 kinase is downstream
of the TGF
receptor in the biochemical route to SAPK/JNK warrants
further investigation.
Fig. 1.
Relationship between SAPK/JNK cascade and
Smad proteins. A, effects of dominant-negative mutants of
MEKK1 and c-Jun on DPC4-induced gene expression. MDCK cells were
transfected with 1 µg of p3TP-Lux together with the indicated
expression vector or empty vector (1 µg per plate in each case).
Cells were then treated with TGF
(11), and cell lysates were assayed
for luciferase activity. B, inhibition of TGF
-stimulated
DPC4 transcriptional activity by dominant-negative c-Jun. Wild-type or
the potent acidic activating region of the herpes simplex virus VP16
protein was fused to the yeast Gal4 binding site (22, 23). DPC4 or VP16 transcriptional activity was examined with a reporter plasmid driven by
the Gal4 binding site (G15E1b-luc; see Ref. 24). MDCK cells were
transfected with 1 µg of G15E1b-luc together with the indicated
expression vector or empty vector (1 µg per plate in each case).
Where indicated, cells were treated with TGF
(2 ng/ml) (11), and
cell lysates were assayed for luciferase activity. Values represent
luciferase activity relative to vector-transfected cells and are shown
as mean ± S.D. for representative experiments performed at least
three times, with triplicates.
[View Larger Version of this Image (19K GIF file)]
signaling, we examined the effect of dominant-negative mutants
of MEKK1, MKK4, and c-Jun expression on transcriptional activation by
TGF
. We made use of p3TP-Lux, a reporter construct that directs
luciferase expression in response to TGF
(6). Expression of
dominant-negative mutants MEKK1(K432A), MKK4(Ala), and c-Jun(TAM67)
suppressed the TGF
-induced reporter gene activity; similar results
were obtained with all of the SAPK/JNK signaling pathway components in
HepG2 and CHO cells ((11) and data not shown). Further evidence for the
specificity of JNK signaling pathways in mediating these processes is
provided by the inability of the dominant-interfering mutant of
MKK3(Ala) to inhibit reporter gene activation by TGF
(data not
shown).
signal transduction pathway and may function to transmit TGF
signals
from the cytoplasm to the nucleus (4, 8, 9). Consistent with previous
studies, overexpression of wild-type DPC4, but not Smad3, into MDCK
cells strongly increased the basal level of p3TP-Lux expression and
potentiated the ability of TGF
to induce reporter gene activity (8,
9). Coexpression of Smad3 along with DPC4 resulted in the synergetic
activation of gene reporter expression, and this effect was enhanced by
TGF
addition (Fig. 1A). The
ability of the dominant-interfering mutant of c-Jun (TAM67) to inhibit
TGF
-induced transcriptional activation allows us to address the
functional role of SAPK/JNK pathways in Smad3 and DPC4-mediated
signaling events. TAM67 acts as a dominant-interfering mutant because
of a deletion in the N-terminal transactivation domain of c-Jun that
includes the binding site for SAPK/JNK (20, 21). Expression of TAM67 is
sufficient to block the superinduction of the gene reporter activity by
TGF
in cells coexpressing wild-type Smad3 and DPC4. Immunoblotting
with anti-Flag antibody confirms that expression of TAM67 did not alter
expression of Flag-Smad3 and Flag-DPC4 proteins (data not shown).
Similar results were obtained with the dominant-negative mutant
MEKK1(K432A) (Fig. 1A), which is consistent with the
hypothesis that SAPK/JNK function is required for transcriptional
activation by Smad3 and DPC4 proteins.
Fig. 2.
Induction of apoptosis by DPC4. A,
MDCK cells were transiently transfected with 1 µg of a green
fluorescence protein expression vector (pEGFP) in the presence or
absence of 1 µg of the constructs encoding DPC4 or Smad3. Forty-eight
hours after transfection, the cells were fixed, and alterations of
chromatin were evaluated by Hoechst staining. B, MDCK cells
were transiently transfected with 2 µg of each plasmid. Forty-eight
hours after transfection, the cells were fixed, and apoptosis was
examined using the TUNEL assay.
[View Larger Version of this Image (89K GIF file)]
. When fused to
the DNA binding domain of the yeast transactivator Gal4-(1-147), the
C-terminal domain of DPC4 activated transcription from a promoter
containing Gal4-binding sites (22), thus raising the interesting
possibility that DPC4 may act as a transcriptional activator to
initiate TGF
responses. In support of this hypothesis, we show that
a Gal4 fusion protein containing the full-length DPC4 failed to
stimulate transcription in this assay, but a strong increase of
Gal4-DPC4 transcriptional activity was detected in MDCK cells
expressing both Gal4-DPC4 and full-length Smad3 in the presence of
TGF
(Fig. 1B). Similar to its effect on p3TP-Lux promoter
activity, expression of TAM67 inhibits both basal and TGF
-induced
Gal4-DPC4 transcriptional activity (Fig. 1B). As a control
(Fig. 1B), transfection of TAM67 produces little or no
effect on a Gal4 fusion containing the potent acidic activating region
of the herpes simplex virus VP16 protein (23). From these results, it
is becoming evident that TAM67 acts specifically as a dominant-negative
inhibitor in TGF
signaling by blocking the function of endogenous
DPC4 and Smad3 proteins and that inhibition of DPC4 transcriptional
activity occurs at a level downstream of the SAPK/JNK signaling
pathway.
Fig. 3.
Smad3 synergizes with DPC4 to trigger cell
death. A, MDCK cells were transiently transfected with
1 µg of a
-galactosidase expression vector (pCMV5-LacZ) in the
presence or absence of 1 µg of the indicated expression constructs
encoding DPC4 or Smad3, respectively. Data (mean ± S.D.) shown are the
percentage of apoptotic cells among the total number of cells counted.
B, MDCK cells were transiently transfected with 2 µg of
either control plasmid or expression vectors for Smad3, DPC4, or Smad3
plus DPC4. Forty-eight h after transfection, the cells were fixed, and
apoptosis in attached MDCK cells was examined using the TUNEL assay.
The number of TUNEL-positive cells per plate was determined by counting
five different fields. C, overexpression of Smad proteins
induces chromatin cleavage. MDCK cells were transiently transfected
with 2 µg of either control plasmid or expression vectors for Smad3,
DPC4, or Smad3 plus DPC4. Transfection mixture was normalized to 6 µg
of total plasmid DNA. Laddered electrophoretic patterns of
oligonucleosomal DNA fragments were resolved 48 h post-transfection by
electrophoresis.
[View Larger Version of this Image (25K GIF file)]
-galactosidase instead of GFP. In this assay, MDCK cells which die by apoptosis round
up and detach from the dishes, making the relative number of blue cells
a good indicator of DPC4 and Smad3 function (27, 28). By 48 h
after transfection, over 57% of blue cells arising from cotransfection
of DPC4 and the
-galactosidase showed morphological changes
consistent with apoptosis, whereas only 10-20% of the blue cells that
had been transfected with
-galactosidase plasmid or in combination
with Smad3 exhibited such a phenotype (Fig. 3A). However,
coexpression of Smad3 along with DPC4 resulted in a dramatic decrease
in blue cell number, suggesting that Smad3 may act in partnership with
DPC4 to trigger cell death (Fig. 3A). A similar conclusion
could be drawn when the experiments were performed using the TUNEL
detection method (Fig. 3B). In addition to morphological
changes, the effects of DPC4 and Smad3 on apoptosis were associated
with a marked increase in nuclear DNA fragmentation, because apoptosis
is frequently accompanied by the cleavage of DNA at internucleosomal
sites, which results in a distinct laddering pattern when analyzed by
agarose gel electrophoresis (Fig. 3B). Collectively, these
data provide the first demonstration that DPC4 and Smad3 together may
function in a cooperative way to induce cell death.
family members is now relatively well understood at
the level of receptor activation, and the diverse biological responses
are becoming increasingly well characterized in terms of gene targets
that are regulated by these signaling pathways (1-5, 11, 29). The
recent identifications of the Smad protein family and SAPK/JNK
signaling pathways provided an important clue as to how members of the
TGF
receptor superfamily signal downstream responses (4, 8, 9, 11,
12, 30, 31). The findings outlined in the present study provide
molecular evidence that the activation of SAPK/JNK signaling pathways
is required for the function of Smad proteins. The molecular framework
that we proposed gives rise to a number of areas for future study,
including the identification of downstream targets of DPC4 and Smad3
and to determine how these targets are regulated to initiate TGF
responses and apoptotic cell death.
*
This work was supported by INSERM.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
These authors contributed equally to this work.
§
Recipient of a grant and fellowship from "la Ligue contre le
Cancer" (1997).
¶
To whom correspondence should be addressed. Tel.:
33-1-43-45-34-77; Fax: 33-1-49-28-46-94.
1
The abbreviations used are: TGF
, transforming
growth factor-
; Smad, mothers against decapentaplegic; SAPK,
stress-activated protein kinase; JNK, c-Jun N-terminal kinases; GFP,
green fluorescent protein; GST, glutathione S-transferase;
MAPK, mitogen-activated protein kinase; MEKK1, MAPK kinase kinase 1;
MKK4, MAPK kinase 4; MDCK, Madin-Darby canine kidney; PBS,
phosphate-buffered saline.
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
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E. A. Ariazi, Y. Satomi, M. J. Ellis, J. D. Haag, W. Shi, C. A. Sattler, and M. N. Gould Activation of the Transforming Growth Factor {beta} Signaling Pathway and Induction of Cytostasis and Apoptosis in Mammary Carcinomas Treated with the Anticancer Agent Perillyl Alcohol Cancer Res., April 1, 1999; 59(8): 1917 - 1928. [Abstract] [Full Text] [PDF] |
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J. D. Brown, M. R. DiChiara, K. R. Anderson, M. A. Gimbrone Jr., and J. N. Topper MEKK-1, a Component of the Stress (Stress-activated Protein Kinase/c-Jun N-terminal Kinase) Pathway, Can Selectively Activate Smad2-mediated Transcriptional Activation in Endothelial Cells J. Biol. Chem., March 26, 1999; 274(13): 8797 - 8805. [Abstract] [Full Text] [PDF] |
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T. Adachi-Yamada, M. Nakamura, K. Irie, Y. Tomoyasu, Y. Sano, E. Mori, S. Goto, N. Ueno, Y. Nishida, and K. Matsumoto p38 Mitogen-Activated Protein Kinase Can Be Involved in Transforming Growth Factor beta Superfamily Signal Transduction in Drosophila Wing Morphogenesis Mol. Cell. Biol., March 1, 1999; 19(3): 2322 - 2329. [Abstract] [Full Text] [PDF] |
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