TAK1/JNK and p38 Have Opposite Effects on Rat Hepatic
Stellate Cells
BERND SCHNABL,1,2 CYNTHIA A. BRADHAM,2 BRYDON L. BENNETT,3 ANTHONY M. MANNING,3 BRANKO STEFANOVIC,2
AND DAVID A. BRENNER1,2
After liver injury, hepatic stellate cells (HSCs) undergo a
process of activation with expression of smooth muscle
␣-actin (␣-SMA), an increased proliferation rate, and a dramatic increase in synthesis of type I collagen. The intracellular signaling mechanisms of activation and perpetuation
of the activated phenotype in HSCs are largely unknown. In
this study the role of the stress-activated protein kinases,
c-Jun N-terminal kinase (JNK) and p38, were evaluated in
primary cultures of rat HSCs. The effect of JNK was assessed
by using an adenovirus expressing a dominant negative form
of transforming growth factor ␤ (TGF-␤)-activated kinase 1
(TAK1) (Ad5dnTAK1) and a new selective pharmacologic
inhibitor SP600125. The effect of p38 was assessed with the
selective pharmacologic inhibitor SB203580. These kinases
were inhibited starting either in quiescent HSCs (culture
day 1) or in activated HSCs (culture day 5). Although blocking TAK1/JNK and p38 decreased the expression of ␣-SMA
protein in early stages of HSC activation, no effect was observed when TAK1/JNK or p38 were inhibited in activated
HSCs. JNK inhibition increased and p38 inhibition decreased collagen ␣1(I) mRNA level as measured by RNase
protection assays, with maximal effects observed in early
stages of HSC activation. Furthermore, TAK1/JNK inhibition decreased HSC proliferation, whereas p38 inhibition
led to an increased proliferation rate of HSCs, independently of its activation status. These results show novel
roles for the TAK1/JNK pathway and p38 during HSC activation in culture. Despite similar activators of TAK1/JNK
and p38, their functions in HSCs are distinct and opposed.
(HEPATOLOGY 2001;34:953-963.)
Abbreviations: HSCs, hepatic stellate cells; ␣-SMA, smooth muscle ␣-actin; MAPK,
mitogen-activated protein kinase; ERK, extracellular signal-regulated kinase; JNK, c-Jun
N-terminal kinase; LPS, lipopolysaccharide endotoxin; TNF-␣, tumor necrosis factor ␣;
IL-1, interleukin-1; UV, ultraviolet; TGF-␤, transforming growth factor ␤; TAK1, transforming growth factor ␤–activated kinase 1; dnTAK1, dominant negative form of TAK1;
FCS, fetal calf serum; DMSO, dimethyl sulfoxide; GST, glutathione S-transferase; SDSPAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis.
From the Departments of 1Medicine and 2Biochemistry and Biophysics, University of
North Carolina at Chapel Hill, Chapel Hill, NC; and 3Signal Research Division of Celgene
Incorporation, San Diego, CA.
Received April 25, 2001; accepted August 16, 2001.
Supported in part by grants from Deutsche Forschungsgemeinschaft Schn 620/1-1 to
B.S., and NIH grants GM41804, DK34987, and AA11605 to D.A.B.
Address reprint requests to: David A. Brenner, M.D., Department of Medicine, Division of Digestive Diseases and Nutrition, CB# 7038, Glaxo Research Bldg. Rm. 156,
University of North Carolina at Chapel Hill, Chapel Hill, NC 27599. E-mail:
dab@med.unc.edu; fax: 919-966-7468.
Copyright © 2001 by the American Association for the Study of Liver Diseases.
0270-9139/01/3405-0013$35.00/0
doi:10.1053/jhep.2001.28790
Liver fibrosis is a common consequence of chronic liver
diseases and results from the activation of hepatic stellate cells
(HSCs). After liver tissue damage, HSCs undergo a transition
from a quiescent to an activated phenotype.1 This activation
process includes loss of vitamin A droplets,2 an increased
proliferation rate,3 a phenotypic transition to a myofibroblastlike, smooth muscle ␣-actin (␣-SMA) positive cell,4 and a
dramatic increase in the synthesis of extracellular matrix proteins.5 Activated HSCs are the major collagen type I–producing cells during hepatic fibrogenesis.6,7 Many of these phenotypic and metabolic changes during the activation process of
HSCs are also observed when HSCs are grown on plastic in
vitro.8 The intracellular signaling mechanisms of activation
and perpetuation of the activated phenotype in HSCs are under active investigation.
Mitogen-activated protein kinases (MAPKs) have a pivotal
role in the transduction of extracellular signals to the nucleus,
which results in numerous cellular responses, including proliferation, differentiation, and regulation of specific metabolic
pathways. Four groups of the mammalian MAPK-family have
been characterized: the extracellular signal–regulated kinases
(ERK), the stress-activated protein kinases (SAPK) c-Jun
N-terminal kinase (JNK) and p38, and the ERK5/big MAP
kinase1 (BMK1). MAPKs are phosphorylated and activated
by MAPK-kinases (MAPKKs or MKKs), whereas MAPKKs
are activated through phosphorylation by MAPKK-kinases
(MAPKKKs).
JNK and p38 are activated in response to similar stimuli and
agents, like lipopolysaccharide endotoxin (LPS), proinflammatory cytokines, such as tumor necrosis factor ␣ (TNF-␣)
and interleukin-1 (IL-1), ultraviolet (UV) and ionizing irradiation, heat shock, hyperosmolarity, and inhibitors of translation, such as cycloheximide or anisomycin.9-12 Activated JNK
translocates to the nucleus, phosphorylates, and activates
transcription factors, including the AP-1 component c-Jun,
and activating transcription factor 2 (ATF-2).13 c-Jun activation has been implicated in a wide range of cellular events,
including cell proliferation, neoplastic transformation, apoptosis, and anti-apoptosis.14 MKK4 (also called JNKK1) and
MKK7 (also called JNKK2) phosphorylate JNK15-18 and
p38.19-21
Transforming growth factor ␤ (TGF-␤)–activated kinase 1
(TAK1), a MAPKKK family member originally identified as a
mediator in the TGF-␤–signaling pathway,22 can be activated
in response to several stress stimuli, like UV radiation and
hyperosmolarity, and agonists, including IL-1, TNF-␣, ceramide, and LPS.23 A dominant negative form of TAK1 suppresses ceramide-induced activation of JNK.23 In vitro, MKK3
and MKK6, both activators of p38, are activated when coex-
953
954 SCHNABL ET AL.
pressed with TAK1 in cultured cells.24 Coexpression of TAK1
with p38 in vitro resulted in activation of p38.24
p38 phosphorylates MAPKAP kinase-2, which in turn
phosphorylates heat shock protein hsp25/hsp27.25 Other than
MAPKAP kinase-2, potential substrates for p38 include several nuclear transcription factors, notably ATF2,12 TCF/Elk1,26 myocyte-enhancing factor 2C (MEF2C),27 and CHOP
(GADD153).28 p38 activation requires dual phosphorylation
by specific upstream MAPK kinases. MKK3 and MKK6 selectively activate p38.20,24,29,30
Although the role of ERK in cell growth and proliferation
has been shown in HSCs,31,32 the role of JNK, p38, and TAK1
in the activation process of HSCs is unclear. Fibronectin and
cytokines (IL-1 and TNF-␣) increase JNK and AP-1 activity in
rat HSCs.33 4-Hydroxy-2,3-nonenal, an aldehydic end product of hepatic lipid peroxidation, interacts directly with JNK
isoforms in human HSCs.34 A recent study showed that basal
JNK and especially basal p38 kinase activity are higher in
activated than quiescent HSCs, emphasizing the potential importance of these kinases in the activation process of HSCs.35
The aim of this study was to evaluate the effect of continuously blocking TAK1/JNK or p38 on proliferation and expression of ␣-SMA and type I collagen mRNA steady state levels in
quiescent (day 1) and activated (day 5) HSCs. Because there
are no established agents to inhibit JNK activity, we inhibited
JNK activity with an adenoviral vector expressing a dominant
negative form of TAK1 (dnTAK1) and then corroborated
our findings with a new pharmacologic JNK inhibitor
SP600125.36 p38 activity was blocked with the well-validated
pharmacologic inhibitor SB203580. We found that both JNK
and p38 are required for ␣-SMA expression in early stages of
HSC activation. Collagen ␣1(I) mRNA expression was increased by p38 activity, but decreased by JNK activity, with
maximal effects in early HSC cultures. TAK1/JNK stimulate
HSC proliferation, whereas p38 inhibits the proliferation rate
of HSCs. Thus, despite similar activation pathways for TAK1/
JNK and p38, their functions in HSCs are distinct and opposite.
MATERIALS AND METHODS
HSC Isolation and Culture. HSCs were isolated from adult male
Sprague-Dawley rats (⬎400 g) by in situ perfusion of the livers with
collagenase and pronase, followed by arabinogalactan gradient ultracentrifugation, as previously described.37 HSC purity (as estimated
by the autofluorescence of the cells by UV-excited fluorescence microscopy) was between 90% and 95%. Cells were seeded on uncoated
plastic tissue culture dishes and cultured in Dulbecco’s modified
Eagle medium (GibcoBRL, Grand Island, NY) supplemented with
10% fetal calf serum (FCS), 2 mmol/L L-glutamine and standard
antibiotics in 95% air, 5% CO2 humidified atmosphere at 37°C.
Growth medium was changed daily. All animal procedures were
performed under the guidelines set by The University of North Carolina Institutional Animal Care and Use Committee and are in accordance with those set by the National Institutes of Health.
Experimental Design. To analyze the effects of inhibiting kinase
activity in early stages of HSC activation and in activated HSCs, 2
experimental approaches were used. For the first approach, quiescent HSCs were either infected with adenoviruses 24 hours after
isolation or treated with 10 ␮mol/L SB203580 (Calbiochem, La Jolla,
CA) starting 12 hours after isolation. For the second approach, HSCs
were permitted to activate 4 days in culture and were then
trypsinized and seeded. On day 5 activated HSCs were either infected
with adenoviruses or incubated with 10 ␮mol/L SB203580. HSCs
were followed until day 8 in culture and then harvested. Similarly,
quiescent (day 1) or activated (day 6) HSCs were treated with differ-
HEPATOLOGY November 2001
ent concentrations of SP60012536 (Celgene Inc., San Diego, CA) and
harvested 48 hours later. No toxicity, as evaluated by trypan blue
staining, was observed during a 48-hour incubation period with
SP600125 in medium containing 10% FCS.
To ensure a continuous inhibition of kinase activity, regular
growth medium containing SB203580 or SP600125 was replaced
daily until cells were harvested. As controls, HSCs were treated in the
same way with an equivalent amount of vehicle (dimethyl sulfoxide;
DMSO).
Adenoviruses and Adenoviral Infection of HSCs. The hemagglutenintagged adenovirus Ad5dnTAK1 has been described.38 AdLuc, which
expresses luciferase driven by the cytomegalovirus promoter,39 was
used as a control virus in cell proliferation assays. Ad5LacZ, which
contains the Escherichia coli ␤-galactosidase gene driven by the cytomegalovirus promoter,40 was used as a control virus in all other
experiments. Amplification of adenoviruses was performed in replication-permissive 293 cells, followed by purification on CsCl gradients.41 After dialyzing against phosphate-buffered saline containing
1 mmol/L MgCl2 to remove CsCl, adenoviruses were stored in 10%
glycerol at ⫺20°C. Viral titers were determined as described.41 HSCs
were infected at a multiplicity of infection of 500 for 12 hours in
Dulbecco’s modifed Eagle medium containing 10% FCS. After infection cells were washed once with phosphate-buffered saline and then
incubated with fresh growth medium. A multiplicity of infection of
500 resulted in a 100% transduction rate of HSCs.42 No toxicity was
observed after adenoviral transfections by using trypan blue staining.
In control experiments, infection was confirmed by immunohistochemistry using a hemagglutenin monoclonal antibody (Babco,
Berkeley, CA) for Ad5dnTAK1, by ␤-galactosidase staining for
Ad5LacZ and by measuring the luciferase activity for AdLuc (data
not shown).
Kinase Assays. For determination of JNK activity, HSCs were serum-starved in medium containing 0.5% FCS for 24 hours and stimulated with recombinant mouse TNF-␣ (R&D Systems, Minneapolis, MN; 30 ng/mL) or recombinant human IL-1␤ (R&D Systems; 2.5
ng/mL) for 30 minutes. Whole cell extracts were prepared, and JNK
assays were performed.43 Whole cell extracts (25 ␮g) were incubated
with recombinant substrate protein glutathione S-transferase (GST)c-Jun (1-79).9 This GST fusion protein contains the activation domain of c-Jun. After 3 washes of the complexes, kinase reactions were
performed using ␥-32P adenosine triphosphate as a phosphate donor
for 30 minutes at 30°C. The substrate protein was resolved on a 12%
sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE), and phosphate incorporation was determined by autoradiography and quantitated by phosphoimager analysis (Molecular Dynamics, Sunnyvale, CA). Equal loading of the substrate protein was
shown by Coomassie staining.
To assess the effect of SB203580 on p38 activity, the activity of
MAPKAP kinase-2, a downstream target of p38, was assessed. HSCs
were serum-starved in medium containing 0.1% FCS for 48 hours,
then stimulated with TNF-␣ (30 ng/mL) for 30 minutes. MAPKAP
kinase-2 activity was determined by using a MAPKAP kinase-2 immunoprecipitation assay (Upstate Biotechnology, Lake Placid, NY)
according to the manufacturer’s protocol. Briefly, MAPKAP kinase-2
was immunoprecipitated using a sheep polyclonal anti-MAPKAP kinase-2 antibody, then reacted with its substrate peptide KKLNRTLSVA in kinase reactions containing ␥-32P adenosine triphosphate
as phosphate donor. The substrate was then transferred to phosphocellulose paper and washed, and bound radioactivity was measured
by liquid scintillation counting.
Western Blot Analysis. Whole cell extracts were prepared from
HSCs by using Dignam C buffer44 containing protease and phosphatase inhibitors as described.45 Protein concentration was determined
using the Bio-Rad Protein Assay (Bio-Rad Laboratories, Hercules,
CA). Proteins were separated on a 10% or 12% SDS-PAGE and then
electroblotted onto a nitrocellulose membrane (Schleicher and
Schuell, Keene, NH). After blocking with 5% nonfat milk in Trisbuffered saline (20 mmol/L Tris, pH 7.5, 150 mmol/L NaCl) containing 0.1% Tween 20 (TBS-T) for 1 hour, membranes were incubated
HEPATOLOGY Vol. 34, No. 5, 2001
with the primary antibody, mouse anti-human ␣-SMA (DAKO,
Carpinteria, CA), mouse anti-human JNK1 (Pharmingen, San Diego,
CA), goat anti-human JNK2 (Santa Cruz, Santa Cruz, CA), mouse
anti-human phospho-c-Jun (Santa Cruz), rabbit anti-phospho-p38
(New England Biolabs, Beverly, MA), and mouse anti-actin (ICNBiomedical, Costa Mesa, CA) (all diluted 1:1,000 in 5% nonfat milk)
for 1 hour, and then washed 3 times in TBS-T. The secondary antibodies, horseradish peroxidase– conjugated anti-mouse IgG, antirabbit IgG, or anti-goat IgG (Santa Cruz) were incubated with the
membrane at a dilution of 1:1,000 in 5% nonfat milk for 30 minutes.
After washing the filter 4 times in TBS-T the antibody complexes
were detected by using the Amersham ECL system (Amersham Pharmacia Biotech, Piscataway, NJ).
Cell Proliferation Assay. Quiescent HSCs were seeded directly after
isolation in 35-mm dishes, infected with adenoviruses 24 hours after
isolation, or treated with SB203580 starting 12 hours after isolation.
HSCs were trypsinized and counted with a hemacytometer in duplicate. The cell number of HSCs was determined daily from days 3 to
10 in culture. Activated HSCs were plated after 4 days in culture in
35-mm dishes, infected with adenoviruses on day 5 or treated with
SB203580 starting on day 5. Daily cell counts were performed in
duplicate from days 6 to 10 in culture.
3H-Thymidine Incorporation Assay. Activated HSCs were plated 5
days after isolation at a density of 1.25 ⫻ 104/well in a 24-well plate
in standard medium containing 10% FCS. HSCs were starved in
serum-free medium for 24 hours. The cells were incubated with
DMSO or SP600125 for 2 hours before HSCs were either left unstimulated or were stimulated with 10% FCS for 24 hours. A total of
1 ␮Ci/mL 3H-thymidine was present during the last 24 hours. At the
end of the incubation on day 8, cells were washed 3 times with cold
phosphate-buffered saline, and twice with 10% trichloric acetic acid,
solubilized in 0.2 N NaOH, and then radioactive incorporation was
measured by using a scintillation counter. Experiments were performed in triplicate.
RNA Isolation and RNase Protection Assay. After viral infection or
treatment with SB203580 or SP600125, total RNA was isolated from
confluent HSCs in culture by using an RNA extraction kit (Qiagen,
Valencia, CA) according to the manufacturer’s protocol. Riboprobes
were derived from the 375 base pair PstI-AvaI fragment from rat
collagen ␣1(I) complementary DNA (cDNA), cloned into HincIIPstI sites of the pGEM 3zf⫹ vector46 and from the plasmid pTRIGAPDH-rat (Ambion, Austin, TX). RNase protection assays were
performed as described.47 Total RNA (5 ␮g) was hybridized with 105
cpm of each riboprobe. The protected riboprobes were visualized by
autoradiography and quantitated by phosphoimager analysis (Molecular Dynamics).
Statistics. The results were analyzed for statistical significance according to the Mann-Whitney U-statistic test. For 3H-thymidine incorporation, the groups were compared by using the F-test from a
general linear model while controlling for the effect of HSC preparation. Statistical values of P ⱕ .05 were considered to be significant.
RESULTS
Ad5dnTAK1 Inhibits JNK Activity. An adenoviral vector expressing a hemagglutenin-tagged kinase-inactive dominant
negative form of TAK1, TAK1 K63W,22 has been described.38
Because TAK1 acts as a mediator for ceramide activation of
JNK,23 the effect of Ad5dnTAK1 on both basal JNK activity
and JNK activity in response to cytokine stimulation in HSCs
was investigated. Quiescent HSCs were infected with
Ad5dnTAK1 and cultured for 8 days. After stimulation with
TNF-␣ for 30 minutes, JNK activity was measured. TNF-␣
stimulation increased JNK activity both in uninfected (3-fold)
and in Ad5LacZ control infected HSCs (1.7-fold) (Fig. 1A and
B), as previously reported for other cells11 and for HSCs.33,35,42
Ad5dnTAK1 significantly decreased both basal and TNF-␣–
stimulated JNK activity by 48% and 44%, respectively, as com-
SCHNABL ET AL.
955
pared with Ad5LacZ-infected HSCs. Although TNF-␣ increased JNK activity by 1.8-fold in Ad5dnTAK1 infected
HSCs, this induced activity was still lower than the basal JNK
activity in Ad5LacZ infected HSCs. Similar to TNF-␣,
dnTAK1 inhibited IL-1␤–induced JNK activity in activated
HSCs (Fig. 1C). These data show that Ad5dnTAK1 is inhibiting basal and cytokine-induced JNK activation in HSCs.
Since it has been reported that dnTAK1 inhibits p38 activity
upon TGF-␤1 stimulation,48 phosphorylation of p38 was assessed by Western blotting by using a phospho-p38 –specific
antibody. The same membrane was immunoblotted by using
anti-actin antibody to ensure equal protein loading. After normalization to actin protein, dnTAK1 did not affect basal or
TNF-␣–stimulated phosphorylation of p38 (Fig. 1D). In vitro
JNK assays do not discriminate between JNK isoforms; therefore, Western blot analysis was performed to determine which
isoforms of JNK are present in HSCs. Both, JNK1 and JNK2 are
expressed in activated HSCs (Fig. 1E). The two bands at 46 kd
and 55 kd of JNK1 represent alternatively spliced transcripts
of the same gene.
SP600125 Inhibits JNK Activity. Recently, a novel selective
JNK inhibitor SP600125 has been developed, which reversibly
competes with endogenous ATP for the ATP binding site in
JNK. Phosphorylation of GST-c-Jun by recombinant JNK2
was inhibited with an IC50 of 100 nmol/L, whereas the activities of ERK, p38, PKA and IKK2 were not affected (IC50 ⬎ 30
␮mol/L).36 To show the inhibitory effect of SP600125 on JNK
activity, c-Jun serine 63 phosphorylation was assessed by
Western blotting using a phospho-specific antibody. Activated HSCs were incubated 2 hours with DMSO or different
concentrations of SP600125 and subsequently stimulated
with TNF-␣ for 30 minutes. Phospho-c-Jun was detected in
unstimulated control and vehicle treated HSCs (Fig. 2A). cJun phosphorylation was induced by TNF-␣ stimulation.
SP600125 inhibited basal and TNF-␣–stimulated phosphorylation of c-Jun, in a dose-dependent manner (Fig. 2A). The
induction of c-Jun phosporylation by IL-1␤ or LPS was also
blocked by SP600125 (Fig. 2B).
MAPKAP Kinase-2 Activity Is Inhibited by SB203580. SB203580,
a member of the pyridinyl imidazole class, is a highly specific
inhibitor of p38␣ and p38␤.49,50 To assess the inhibitory effect
of SB203580 on p38, the basal and cytokine-stimulated activity of MAPKAP kinase-2, a downstream target of p38,12 was
measured using an in vitro kinase assay. HSCs were treated
with 10 ␮mol/L SB203580 for 8 days, then stimulated with
TNF-␣ for 30 minutes. TNF-␣ stimulation increased MAPKAP kinase-2 activity both in untreated (1.4-fold) and in vehicle (DMSO)-treated HSCs (1.6-fold) (Fig. 3). SB203580 decreased the basal and the TNF-␣–stimulated MAPKAP
kinase-2 activity by 25% and 49%, respectively, as compared
with HSCs incubated with DMSO (Fig. 3). These data show
that SB203580 inhibits basal and TNF-␣–stimulated p38 activity in HSCs.
JNK or p38 Inhibition Decreases ␣-SMA in Quiescent but not in
Activated HSCs. To evaluate the effect of inhibiting JNK or p38
on ␣-SMA expression in quiescent HSCs, quiescent HSCs
were virally infected or treated with SB203580 and were then
followed until day 8 in culture. HSCs, infected with
Ad5dnTAK1 or treated with SB203580, showed decreased
levels of ␣-SMA expression compared with HSCs infected
with control Ad5LacZ (Fig. 4A) or vehicle (DMSO)-treated
cells (Fig. 4B), respectively. Similarly, quiescent HSCs (day 1)
956 SCHNABL ET AL.
HEPATOLOGY November 2001
FIG. 1. Dominant negative TAK1 inhibits JNK activity in HSCs after 7 days in culture. (A, C, and D) Quiescent HSCs were infected with Ad5dnTAK1 (lanes
5 and 6), Ad5LacZ (lanes 3 and 4) or left as uninfected controls (lanes 1 and 2). On day 8 after 24 hours of serum starvation in medium containing 0.5% FCS,
cells were either stimulated with TNF-␣ (30 ng/mL) (A and D) or IL-1␤ (2.5 ng/mL) (C) for 30 minutes (lanes 2, 4, and 6) or left untreated (lanes 1, 3, and
5), and whole cell extracts were prepared. (A) GST-c-Jun as substrate was incubated with 25 ␮g of cell extract in an in vitro JNK assay. JNK-mediated
phosphorylation of GST-c-Jun was visualized after SDS-PAGE by autoradiography. The single band represents GST-c-Jun. Shown is a representative experiment. (B) Incorporation of ␥-32P adenosine triphosphate was assessed by phosphoimager analysis. Data represent the mean ⫾ SD of 4 independent experiments
using different HSC isolations each time. The results are expressed relative to unstimulated, Ad5LacZ infected HSCs. ‡P ⫽ .0286; §P ⫽ .0571; *P ⫽ .0286 when
compared with unstimulated HSCs infected with Ad5LacZ, P ⫽ .05 when compared with unstimulated uninfected HSCs; #P ⫽ .0286 when compared with
TNF-␣–stimulated HSCs infected with Ad5LacZ or left uninfected. (C) An in vitro kinase assay for JNK was performed. (D) Cell lysates (50 ␮g) were loaded
on a 12% SDS-PAGE. After electrophoresis and transfer on a nitrocellulose membrane, immunoblotting was performed with a phoshpo-p38 specific antibody.
The same membrane was immunoblotted using anti-actin antibody. The level of phospho-p38 protein was normalized to the level of actin protein. (E) Total
cellular protein (50 ␮g) was obtained from untreated activated HSCs, separated on a 12% SDS-PAGE, and transferred to a nitrocellulose membrane. Western
blot analysis was performed by using anti-JNK1 antibody and anti-JNK2 antibody.
incubated for 48 hours with SP600125 did not show any detectable ␣-SMA protein expression as compared with untreated control HSCs and HSCs incubated with vehicle DMSO
(Fig. 4C).
To investigate the effect of JNK or p38 inhibition on already
activated HSCs, HSCs were allowed to activate until day 5 or
day 6 in culture, and then were infected or incubated with
SB203580 or SP600125, and followed until day 8. Infection of
HEPATOLOGY Vol. 34, No. 5, 2001
SCHNABL ET AL.
957
This effect of kinase inhibition was less profound in activated
HSCs.
JNK and p38 Inhibition Have Opposite Effects on HSC Proliferation. To determine whether inhibition of JNK or p38 affects
FIG. 2. SP600125 inhibits JNK in a dose-dependent manner. HSCs were
plated on day 7 after isolation and starved in medium containing 0.5% FCS for
24 hours. HSCs were either left untreated as control, or preincubated with
DMSO or different concentrations of SP600125 for 2 hours, before stimulation with TNF-␣ (30 ng/mL) (A), IL-1␤ (2.5 ng/mL) or LPS (E. coli strain
0127; Sigma, St. Louis, MO; 100 ng/mL) (B) for 30 minutes. Whole cell
extracts (25-50 ␮g) were assessed for phospho-c-Jun expression using Western blotting.
activated HSCs with Ad5dnTAK1 or treatment with SB203580
did not change the ␣-SMA protein expression as compared
with Ad5LacZ-infected cells (Fig. 4D) or to DMSO-treated
HSCs (Fig. 4E), respectively. Similarly, incubation of activated HSCs with SP600125 for 48 hours did not alter the
expression of ␣-SMA as compared with untreated and vehicletreated HSCs (Fig. 4F).
Thus, JNK or p38 inhibition in quiescent HSCs (1 day culture) decreased the expression of ␣-SMA, whereas inhibition
of JNK or p38 did not affect activated HSCs, suggesting that
JNK and p38 are required for ␣-SMA expression in early stages
of HSC activation.
their proliferation rate, quiescent HSCs were either infected
with Ad5dnTAK1 or incubated with SB203580. Daily cell
counts were performed from day 3 until day 10 in culture.
Ad5dnTAK1-infected HSCs had lower cell counts than control cells, which were either not infected or infected with a
control virus AdLuc. On day 10 the number of AdLuc infected
cells was 1.9-fold higher than the number of Ad5dnTAK1
infected HSCs (Fig. 6A). In contrast, incubation of quiescent
HSCs with SB203580 stimulated the cell number of HSCs
2.6-fold compared with cells not treated or treated with
DMSO (Fig. 6B).
Similar results were observed using activated HSCs, which
were infected with Ad5dnTAK1 or incubated with 10 ␮mol/L
SB203580. Cells were counted daily from day 6 to day 10 in
culture. The number of AdLuc-infected HSCs was 2.8-fold
higher compared with the number of Ad5dnTAK1-infected
cells (Fig. 6C). The number of SB203580-treated HSCs was
1.8-fold higher than the number of DMSO-treated cells on day
10 (Fig. 6D).
To assess the effect of JNK inhibition by SP600125 on the
proliferation rate of activated HSCs, 3H-thymidine incorporation assays were performed. Stimulation of DMSO-treated
HSCs with 10% FCS for 24 hours resulted in a 2.4-fold increase of 3H-thymidine incorporation. SP600125 had a
marked and dose-dependent inhibitory effect on 3H-thymidine incorporation into the DNA (Fig. 6E). Taken together,
these results suggest that while JNK stimulates proliferation,
JNK and p38 Inhibition Have Opposite Effects on Collagen ␣1(I)
mRNA Levels. To study the effect of JNK and p38 inhibition on
endogenous collagen ␣1(I) gene expression, quiescent HSCs
infected with Ad5dnTAK1 or treated with 10 ␮mol/L
SB203580 were cultured for 8 days. Inhibition of JNK activity
caused an approximate 2.3-fold increase in endogenous
steady state mRNA levels of collagen ␣1(I) (Fig. 5A). In contrast, incubation of quiescent HSCs with 10 ␮mol/L SB203580
until day 8 in culture resulted in a 2.4-fold decrease of collagen ␣1(I) mRNA (Fig. 5B).
Next, we investigated whether already activated HSCs respond to kinase inhibition in the same way as quiescent HSCs.
Similar levels of type I collagen mRNA were found after infection of activated HSCs with Ad5dnTAK1 or treatment with
SB203580 as compared with Ad5LacZ-infected HSCs (Fig.
5C) or to DMSO-treated HSCs (Fig. 5D), respectively.
To assess the effect of pharmacologically blocking JNK,
quiescent and activated HSCs were incubated with no treatment, DMSO, or different concentrations of SP600125 for 48
hours. HSCs treated with 10 ␮mol/L and 20 ␮mol/L of
SP600125 starting in a quiescent state 1 day after isolation
showed up-regulation of collagen ␣1(I) mRNA of 2-fold and
2.8-fold, respectively, as compared with DMSO-treated cells
(Fig. 5E). Similar levels of collagen ␣1(I) mRNA were observed in control HSCs and HSCs, which were incubated with
2 different concentrations of SP600125 starting on day 6 after
isolation (data not shown). These data show that in quiescent
HSCs inhibition of TAK1/JNK increases, whereas inhibition
of p38 decreases steady state levels of collagen ␣1(I) mRNA.
FIG. 3. SB203580 inhibits p38 activity of HSCs. Quiescent HSCs were
incubated with 10 ␮mol/L SB203580 or vehicle (DMSO), or were left untreated. After serum starvation in medium containing 0.1% FCS for 48 hours,
HSCs were either stimulated with TNF-␣ (30 ng/mL) for 30 minutes, or not
treated. Whole cell extracts were prepared on day 8. Anti-MAPKAP kinase-2
antibody was used to immunoprecipitate MAPKAP kinase-2 from 50 ␮g of
cell extract. After washing, an in vitro kinase assay was performed by using the
peptide KKLNRTLSVA as substrate. After transfer onto a phosphocellulose
paper, MAPKAP kinase-2–mediated phosphorylation of this peptide was
measured by liquid scintillation counting. Data represent the mean ⫾ SD of 3
experiments and are expressed relative to the kinase activity of unstimulated,
DMSO-treated HSCs. The Mann-Whitney U statistic test with a one-sided P
value showed ‡P ⫽ .05 and §P ⫽ .025. The Mann-Whitney U statistic test
with a two-sided P value showed *P ⫽ .05 when compared with unstimulated
HSCs incubated with DMSO or left untreated, #P ⫽ .05 when compared with
TNF-␣–stimulated HSCs incubated with DMSO or left untreated.
958 SCHNABL ET AL.
HEPATOLOGY November 2001
FIG. 4. Dominant negative TAK1
and SB203580 decrease expression
of ␣-SMA in quiescent, but not in
activated, HSCs. Quiescent HSCs were
either infected with Ad5dnTAK1,
Ad5LacZ, or left uninfected (A), or
incubated with 10 ␮mol/L SB203580,
DMSO, or left untreated (B). Activated
HSCs were infected with Ad5dnTAK1
or Ad5LacZ, or left uninfected (D).
To inhibit p38 kinase activity, activated HSCs were incubated with 10
␮mol/L SB203580. Untreated and vehicle (DMSO)-treated cells served as
controls (E). HSCs on day 6 in culture were treated either with DMSO,
10 ␮mol/L or 20 ␮mol/L SP600125
for 48 hours, or were left untreated
(F). On day 8, whole cellular proteins (10 ␮g) were separated on a
12% SDS-PAGE, then transferred to
nitrocellulose membranes. Immunoblots were probed with monoclonal
anti–␣-SMA antibody. (C) Quiescent HSCs (day 1) were incubated
with DMSO, 10 ␮mol/L or 20
␮mol/L SP600125 for 48 hours, or
were left untreated. Western blotting
of cell lysates (50 ␮g) for ␣-SMA
protein was performed. Results shown
represent a typical experiment.
p38 decreases proliferation of HSCs independent of the activation stage.
DISCUSSION
After a fibrogenic stimulus, HSCs convert into proliferating, ␣-SMA– expressing, and collagen-producing cells. The
response of cells to extracellular stimuli is in part mediated by
intracellular kinases. We found that despite a similar pattern
of activators for the SAPKs, JNK and p38, including proinflammatory cytokines and environmental stress, their functions in HSCs are distinct.12 JNK and p38 were required for
␣-SMA expression in early stages of HSC activation, but not in
activated HSCs. JNK inhibition increased, whereas p38 inhibition decreased, collagen type I mRNA expression with maximal effects in early stages of HSC activation. Furthermore,
JNK inhibition decreased HSC proliferation, whereas p38 inhibition increased the proliferation rate in HSCs.
To evaluate the effects of JNK inhibition, an adenoviral
vector expressing dnTAK1 was used. Ad5dnTAK1 efficiently
inhibited basal and cytokine-induced JNK activity even 7 days
after infection. In contrast, kinase-deficient JNK or activation
domain–mutated JNK are not effective inhibitors of JNK activity (unpublished observations). dnTAK1 did not affect
basal and TNF-␣–induced phospho-p38 levels in activated
HSCs. We cannot ensure that dnTAK1 is a specific JNK inhibitor and does not affect other kinases. But the fact that the
effects of JNK inhibition using either dnTAK1 or the selective
pharmacologic inhibitor SP600125 are very similar corrobo-
rates that JNK is indeed the major downstream kinase of
TAK1.
Consistent with the findings of others, we found that JNK
and p38 were weakly stimulated by TNF-␣ in activated HSCs.
Others have observed increased basal activation of both kinases in activated (day 15) as compared with quiescent (day
2) HSCs.35 Furthermore, TNF-␣ stimulated JNK and p38 activity 4-fold and 12-fold, respectively, in quiescent (day 2)
HSCs, whereas TNF-␣ led only to a minimal stimulation of
JNK and p38 activity in activated HSCs,35 suggesting that in
activated HSCs, both kinases are close to maximally activated,
and further supporting the potential importance of both kinases in the mechanism of HSC activation.
Except for proliferative effects, quiescent and activated
HSCs had different responses to kinase inhibition. This suggests that kinase activity is crucial during the activation process, whereas the proliferation pathways apparently need continuous kinase activity. One possible explanation for the
differential effect of the treatment on quiescent and activated
HSCs is that cells adapt to these stimuli. Under physiologic
conditions activation of MAPKs is transient. However, under
pathophysiologic conditions, such as liver fibrosis or during
culture on plastic, stress stimuli are prolonged and sustained,
which might change the response of the cells. Another possible explanation lies in the activation process of HSCs. In an
initiating phase of activation quiescent HSCs produce less
cytokines and growth factors and have less cell membrane
receptors for these factors. Once activated, HSCs show en-
HEPATOLOGY Vol. 34, No. 5, 2001
FIG. 5. Steady state mRNA levels
of collagen ␣1(I) in HSCs are increased by JNK inhibition and decreased by p38 inhibition. Quiescent
HSCs were either infected with
Ad5dnTAK1 or Ad5LacZ, or left uninfected (A), or treated with 10
␮mol/L SB203580 or DMSO (vehicle), or left untreated (B). Activated
HSCs were either infected with
Ad5dnTAK1 or Ad5LacZ, or left uninfected (C), or incubated with 10
␮mol/L SB203580 or DMSO, or left
untreated (D). Shown are the ratios
between collagen ␣1(I) mRNA and
GAPDH mRNA level, quantitated by
phosphoimager analysis. Data represent the mean ⫾ SD of 2 to 3 independent experiments using different
HSC isolations each time. The results are expressed relative to
Ad5LacZ-infected or DMSO-treated
HSCs. *P ⫽ .05 when compared with
uninfected or Ad5LacZ-infected
HSCs. (E) HSCs on day 1 in culture
were incubated with DMSO, 10
␮mol/L or 20 ␮mol/L SP600125, or
were left untreated. Total RNA was
extracted 48 hours later and RNase
protection assay was performed. The
ratios between collagen ␣1(I) mRNA
and GAPDH mRNA levels are indicated, and the results are expressed
relative to DMSO-treated HSCs. Data
represent the mean ⫾ SEM of 3 independent experiments using different HSC isolations each time. *P ⫽
.05 when compared with vehicle
DMSO-treated HSCs.
SCHNABL ET AL.
959
960 SCHNABL ET AL.
HEPATOLOGY November 2001
FIG. 6. Inhibition of JNK decreases HSC proliferation, and inhibition of
p38 stimulates HSC proliferation. Quiescent HSCs were infected with
Ad5dnTAK1 or AdLuc, or left uninfected (A). Quiescent HSCs were treated
with 10 ␮mol/L SB203580 or DMSO (vehicle), or left untreated (B). Cell
numbers were determined in duplicate daily from day 3 to day 10 by using a
hemacytometer. Activated HSCs were infected with Ad5dnTAK1 or AdLuc,
or left uninfected (C), or they were incubated with 10 ␮mol/L SB203580 or
DMSO (vehicle), or left untreated (D). Daily cell counts were performed in
duplicate from day 6 to day 10 in culture by using a hemacytometer. Results
shown represent a typical experiment, which was reproduced twice. (E) HSCs
were grown for 7 days and then serum starved for 24 hours in serum-free
medium. HSCs were preincubated with DMSO, or different concentrations of
SP600125 for 2 hours, then stimulated with 10% FCS for 24 hours in medium
containing 1 ␮Ci/mL 3H-thymidine. HSCs maintained in 0% FCS served as
controls. Experiments were performed 3 times in triplicate. Data represent
the mean ⫾ SEM. #P ⫽ .003 when serum-starved and DMSO-treated HSCs
were compared with HSCs that were preincubated with DMSO and stimulated with
10% FCS to show appropriate proliferation, *P ⬍ .0001 when compared with HSCs
that were preincubated with DMSO and stimulated with 10% FCS.
hanced cytokine expression, secretion, and responsiveness,
thereby acting in an autocrine and paracrine manner. Cells
might react in a different way if they are exposed to this pronounced alteration of extracellular stimuli. A recent study
described such a modulation of the response to TGF-␤ in
HSCs upon passage in culture.51 Inhibition of proliferation
and stimulation of collagen ␣2(I) mRNA with TGF-␤1 was
achieved in nonpassaged 3-day-old rat HSCs, but not in passaged myofibroblast-like rat HSCs. A suggested mechanism
for this unresponsiveness to TGF-␤ treatment is that despite
the expression of TGF-␤ receptors type I and II on the cell
surface of myofibroblast-like HSCs, there is strongly reduced
TGF-␤ ligand-binding activity and no DNA binding of intracellular Smad proteins.51 The exact mechanisms for the different response of quiescent and activated rat HSCs to kinase
inhibition in our study are unknown.
Vascular smooth muscle cells, exposed to the vasoconstrictor arginine vasopressin, increase ␣-SMA expression through
activation of the ␣-SMA promoter. Inhibition of JNK or p38
decreased arginine vasopressin–stimulated ␣-SMA promoter
activity, whereas inhibition of ERK had no effect.52,53 Furthermore, transient transfection studies showed that expression of
constitutively active MKK7/JNK fusion plasmid or a constitutively active form of MKK6, a specific p38 MAPKK, increased
␣-SMA promoter activity in the absence of vasoconstrictor
stimulation.52 Direct evidence for the involvement of p38 in
␣-SMA expression in cultured HSCs results from a study
showing that pharmacologic inhibition of p38 for 48 hours
reduces ␣-SMA expression in rat HSCs on day 5.35 Our study
showed that inhibition of JNK or p38 in early stages of HSC
activation suppresses ␣-SMA protein expression. Thus, regulation of ␣-SMA protein expression seems to be multifactorial,
involving the TAK1/JNK pathway and p38, and requiring kinase activity during but not after HSC activation.
Studies assessing the role of the JNK pathway in collagen
type I gene expression have reported conflicting results. UV
irradiation-activated JNK used a distal GC box in the 5⬘ UPS of
the gene to up-regulate collagen ␣1(I) reporter gene expres-
HEPATOLOGY Vol. 34, No. 5, 2001
sion.54 In addition, the protein BTEB (basic transcription element binding protein) binds to the same GC box and mediates
acetaldehyde-induced collagen ␣1(I) gene expression in
HSCs via a JNK-dependent pathway.55 Since a specific biochemical inhibitor for JNK was not available, the findings of
these studies are based on transient transfections. However, in
human mesangial cells, the major extracellular matrix–producing cells in renal fibrosis, transient transfections of a dominant negative mutant of JNK had no effect on a human collagen ␣1(I) reporter gene upon TGF-␤1 treatment.56 Another
study showed that exposure to UV irradiation significantly
reduces type I and type III procollagen mRNA levels in human
skin.57 This inhibition of type I collagen is mediated in part by
c-Jun, which is induced by UV irradiation and interferes with
procollagen transcription.57 Earlier studies showed that phorbol esters, which activate JNK and c-Jun transcriptional activity, inhibit collagen ␣1(I) mRNA expression.58,59 In our study,
JNK inhibition increased collagen ␣1(I) mRNA expression
with maximal effects in early stages of HSC activation. In
contrast, p38 inhibition decreased collagen type I mRNA expression, again with maximal effects in early stages of HSC
activation. Because p38 has been involved in stabilizing the
unstable mRNAs of several cytokines,60 the effect of SB203580
on the stability of collagen ␣1(I) mRNA was investigated, but
no effect was detected (data not shown). We conclude that
activated JNK inhibits collagen ␣1(I) mRNA levels in HSCs.
c-Jun has been implicated as a positive regulator of cell
proliferation and of G1 to S-phase progression.61 Several studies suggest that JNK also has a proproliferative role.62,63 Fibroblasts with only mutant alleles of c-Jun containing serineto-alanine substitutions at 63 and 73 have a proliferation
defect, but grow faster than c-Jun null cells.64 Further evidence for a connection between c-Jun phosphorylation by
JNK and the proliferative role of c-Jun has been suggested
from a study showing that SEK1 (MKK4)-deficient fibroblasts
grow slower than wild-type cells.65 In contrast, the requirement for c-Jun during progression through the G1 phase in
fibroblasts is independent of phosphorylation of serines 63
and 73.66 Mutant mice lacking JNK1 show increased T-cell
proliferation and defective T-cell differentiation.67 Thus, it
appears that the role of the JNK pathway and of c-Jun in
proliferation is cell type specific. Our study clearly shows that
blocking JNK resulted in an inhibition of HSC proliferation,
independent of the activation state of HSCs. Consistent with
our findings, endothelin-1 inhibits the proliferation of activated human HSCs via endothelin B receptors, through a
prostaglandin/cAMP pathway that leads to inhibition of
JNK.68 Furthermore, C-type natriuretic peptide activates natriuretic peptide receptors-B and inhibits growth of human
myofibroblastic HSCs, which is associated with elevation of
cGMP and with inhibition of JNK.69 Thus, JNK inhibition is
growth inhibitory in HSCs.
Several studies suggest that p38 negatively regulates cell
cycle progression in fibroblasts. p38 was shown to have an
inhibitory effect on cyclin D1 transcription and protein synthesis, which is essential for progression through G1.70 Conditionally activated, stably expressed MEKK3 in fibroblasts
causes a profound growth arrest in early G1 via suppression of
cyclin D1 protein levels, which might be mediated by p38.71
Similarly, we found that inhibition of p38 in HSCs increases
SCHNABL ET AL.
961
the proliferation rate independently of the activation stage of
HSCs.
Taken together, our results indicate that the role of p38 is
anti-proliferative and pro-differentiation in HSCs. Because
withdrawal from the cell cycle is coupled to a more differentiated phenotype in many cell types, p38 might have a major
role in the process of differentiation of HSCs and other cells.
In contrast, the TAK1/JNK pathway in HSCs is pro-proliferative and anti-differentiation. Because the myofibroblast-activated state of HSCs is accompanied by increased proliferation,
a balance of activation of both kinases may be needed to maintain this unusual combination of myofibroblast differentiation
and proliferation. Ultimately, specific inhibition of these kinases in a liver fibrosis model in vivo will be necessary to fully
delineate their individual contributions to the disease state.
Acknowledgment: The authors thank Richard A. Rippe for
experimental assistance and Joe Galenko for statistical analysis.
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