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Gadd45 deficiency in rheumatoid arthritisEnhanced synovitis through JNK signaling.

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ARTHRITIS & RHEUMATISM
Vol. 60, No. 11, November 2009, pp 3229–3240
DOI 10.1002/art.24887
© 2009, American College of Rheumatology
Gadd45␤ Deficiency in Rheumatoid Arthritis
Enhanced Synovitis Through JNK Signaling
Camilla I. Svensson,1 Tomoyuki Inoue,1 Deepa Hammaker,1 Akihisa Fukushima,1
Salvatore Papa,2 Guido Franzoso,2 Georg Schett,3 Maripat Corr,1
David L. Boyle,1 and Gary S. Firestein1
Objective. JNK-mediated cell signaling plays a
critical role in matrix metalloproteinase (MMP) expression and joint destruction in rheumatoid arthritis (RA).
Gadd45␤, which is an NF-␬B–regulated gene, was recently identified as an endogenous negative regulator of
the JNK pathway, since it could block the upstream
kinase MKK-7. This study was carried out to evaluate
whether low Gadd45␤ expression in RA enhances JNK
activation and overproduction of MMPs in RA, and
whether Gadd45␤ deficiency increases arthritis severity
in passive K/BxN murine arthritis.
Methods. Activation of the NF-␬B and JNK pathways and Gadd45␤ expression were analyzed in human
synovium and fibroblast-like synoviocytes (FLS) using
quantitative polymerase chain reaction, immunoblotting, immunohistochemistry, electrophoretic mobility
shift assay, and luciferase reporter constructs.
Gadd45␤ⴚ/ⴚ and wild-type mice were evaluated in the
K/BxN serum transfer model of inflammatory arthritis,
and clinical signs of arthritis, osteoclast formation, and
bone erosion were assessed.
Results. Expression levels of the Gadd45␤ gene
and protein were unexpectedly low in human RA synovium despite abundant NF- ␬ B activity. Forced
Gadd45␤ expression in human FLS attenuated tumor
necrosis factor–induced signaling through the JNK
pathway, reduced the activation of activator protein 1,
and decreased the expression of MMP genes. Furthermore, Gadd45␤ deficiency exacerbated K/BxN serum–
induced arthritis in mice, dramatically increased signaling through the JNK pathway, elevated MMP3 and
MMP13 gene expression in the mouse joints, and increased the synovial inflammation and number of osteoclasts.
Conclusion. Deficient Gadd45␤ expression in RA
can contribute to activation of JNK, exacerbate clinical
arthritis, and augment joint destruction. This process
can be mitigated by enhancing Gadd45␤ expression or
by inhibiting the activity of JNK or its upstream regulator, MKK-7.
Dr. Svensson’s work was supported by NIH grant R21-DA021654, and by an Arthritis Foundation Postdoctoral Fellowship. Dr.
Franzoso’s work was supported by NIH grants R01-CA-084040 and
R01-CA-098583, and Cancer Research UK program grant C26587/
A8839. Dr. Schett’s work was supported by the Interdisciplinary
Center for Clinic Research Erlangen (project C6). Dr. Firestein’s work
was supported by NIH grant AR-047825.
1
Camilla I. Svensson, PhD (current address: Karolinska Institute, Stockholm, Sweden), Tomoyuki Inoue, PhD, Deepa Hammaker,
PhD, Akihisa Fukushima, MS, Maripat Corr, MD, David L. Boyle, BA,
Gary S. Firestein, MD: University of California San Diego School of
Medicine, La Jolla; 2Salvatore Papa, PhD, Guido Franzoso, MD, PhD:
Imperial College London, London, UK; 3Georg Schett, MD: University of Erlangen-Nuremberg, Erlangen, Germany.
Address correspondence and reprint requests to Gary S.
Firestein, MD, University of California San Diego School of Medicine,
Division of Rheumatology, Allergy and Immunology, 9500 Gilman
Drive, La Jolla, CA 92093. E-mail: gfirestein@ucsd.edu.
Submitted for publication January 2, 2009; accepted in revised form July 11, 2009.
Rheumatoid arthritis (RA) is a chronic inflammatory disease that is characterized by synovial lining
hyperplasia, infiltration of the synovium with immune
cells, and joint destruction (1). Matrix metalloproteinases (MMPs) are highly expressed in RA and are known
to be involved in the joint degradation and remodeling
of RA. JNK is thought to be especially important in
extracellular matrix degradation, because JNK is a key
regulator of MMP gene transcription. Furthermore, this
signaling pathway, which includes its upstream kinases,
MKK-4 and MKK-7, is activated in RA synovium, and
JNK inhibition suppresses MMP gene expression and
joint destruction in animal models of RA (1–3).
Recently, growth arrest and DNA damage–
3229
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SVENSSON ET AL
inducible gene 45␤ (Gadd45␤) was identified as a negative regulator of JNK. The Gadd45 genes, including
Gadd45␣, Gadd45␤, and Gadd45␥, encode for evolutionarily conserved 18-kd proteins. Initially, Gadd45␤,
also referred to as myeloid differentiation factor 118,
was identified as a primary response gene that is activated in murine myeloid leukemia cells by interleukin-6
(IL-6) during terminal differentiation (4). Gadd45␤ is
known to be involved in cellular stress responses, cell
cycle control, and cell survival (5,6). Gadd45␤ is induced
by NF-␬B, binds directly to the JNK-activating kinase
MKK-7, and inhibits the catalytic function of MKK-7 by
blocking access to ATP (6–8). Thus, Gadd45␤ serves as
an endogenous inhibitor of MKK-7 that can blunt
signaling through the JNK pathway (7,9). This is especially relevant in RA, since MKK-7 is expressed and
activated in rheumatoid synovium (10), and MKK-7,
rather than the other JNK-activating kinase, MKK-4, is
required for JNK activation in cytokine-activated synoviocytes (11).
Gadd45␤ deficiency has been linked to increased
disease severity in murine experimental allergic encephalomyelitis (12), hepatocellular carcinoma (13,14), hepatic regeneration (15), and diabetes (16). Based on the
MKK-7–inhibitory properties of Gadd45␤ and the loss
of protection associated with Gadd45␤ deficiency in
other models, we hypothesized that dysregulation of
Gadd45␤ expression contributes to enhancement of
MKK-7 and JNK responses and subsequent MMP production in RA synovium. To explore this hypothesis, we
examined the expression of Gadd45␤ in human synovial
tissue from patients with RA and patients with osteoarthritis (OA) and found no difference in Gadd45␤ gene
or protein expression between the groups, despite markedly higher NF-␬B activation in patients with RA. The
role of Gadd45␤ in the pathologic processes associated
with inflammatory arthritis was also assessed using the
K/BxN serum transfer model of arthritis in Gadd45␤⫺/⫺
mice. This experiment showed that deficient Gadd45␤
expression leads to enhanced JNK activity and exacerbation of the disease. These studies suggest that MKK-7
and JNK play a key role in synovial inflammation and
joint damage. Therapeutic strategies designed to increase
the expression of Gadd45␤, inhibit MKK-7, or block the
activity of JNK might have potential utility in RA.
MATERIALS AND METHODS
Synovial tissue samples. Synovial tissue and fibroblastlike synoviocytes (FLS) were obtained from patients with OA
or RA at the time of total joint replacement or synovectomy, as
previously described (17). The protocol was approved by the
Human Research Protection Program of the University of
California, San Diego. The diagnosis of RA conformed to the
1987 revised criteria of the American College of Rheumatology (formerly, the American Rheumatism Association) (18).
The samples were either processed for cell culture or snapfrozen and stored at ⫺80°C until processed for protein or gene
expression analysis.
Preparation of human FLS and Gadd45␤ⴚ/ⴚ mouse
FLS. For preparation of human FLS, synovial tissue samples
were minced and incubated for 1.5 hours at 37°C with 0.5
mg/ml of type VIII collagenase (Sigma, St. Louis, MO) in
serum-free RPMI 1640 (Mediatech, Herndon, VA). Tissues
were then filtered through a nylon strainer, washed, and
cultured in Dulbecco’s modified Eagle’s medium (DMEM)
supplemented with 10% fetal calf serum (FCS) (Gemini
Biosciences, Calabasas, CA), L-glutamine, penicillin, streptomycin, and gentamicin in a humidified chamber containing an
atmosphere of 5% CO2. After overnight culture, nonadherent
cells were removed and adherent cells were trypsinized and
split at a 1:3 ratio when the cells were 80–90% confluent. FLS
from passages 5–9 were used, since these cells comprised a
homogeneous population (⬍1% CD11b-positive, ⬍1% phagocytic, and ⬍1% Fc␥ receptor II–positive cells) (19). Mouse
FLS were derived from Gadd45␤⫺/⫺ (15) and wild-type (WT)
mouse knee and ankle joints by microdissecting the synovium
and enzymatically dispersing the cells, as previously described
(2). Mouse FLS from passages 4–5 and at 80–90% confluency
were used.
Antibodies and reagents. The antibodies and reagents
used were as follows: rabbit antibodies against MKK-7, JNK1/2, phosphorylated MKK-4 (Ser257/Thr261), phosphorylated
MKK-7 (Ser271/Thr275) (for Western blotting), phosphorylated
JNK-1/2 (Thr183/Tyr185), and phosphorylated c-Jun (Ser63/
Ser73) (Cell Signaling Technology, Beverly, MA); mouse antibodies against p65 (Santa Cruz Biotechnology, Santa Cruz,
CA), CD68 (Dako, Carpinteria, CA), ␤-actin (Sigma), and
Gadd45␤ (8) (for immunohistochemistry); and goat antibodies
against MKK-7 (for immunoprecipitation; Santa Cruz Biotechnology), Gadd45␤ (for Western blotting; Santa Cruz Biotechnology), recombinant human tumor necrosis factor (TNF) and
IL-1␤ (R&D Systems, Minneapolis, MN), and anisomycin
(Calbiochem, San Diego, CA). Glutathione S-transferase (GST)–
c-Jun was a gift from Roche Bioscience (Palo Alto, CA).
Immunohistochemistry. Serial cryosections (5 ␮m) of
synovial tissue from RA and OA patients were used for
immunohistochemistry. The tissue was fixed with 4% formalin
and endogenous peroxidase was depleted with 0.1% H2O2.
Blocking was performed for 1 hour in phosphate buffered
saline (PBS) containing 5% normal goat serum, 2.5% horse
serum, and 1% human serum albumin followed by overnight
incubation with antibodies against Gadd45␤, NF-␬B (p65), and
CD68 in blocking buffer at 4°C. Mouse IgG served as a
negative control. The primary antibodies were detected using
biotinylated horse anti-mouse secondary antibodies (Vector,
Burlingame, CA), followed by streptavidin–horseradish peroxidase (HRP) and aminoethylcarbazole substrate (Dako). Nuclei were counterstained with hematoxylin.
Mouse K/BxN serum transfer arthritis model. All
animal experiments were carried out according to protocols
approved by the Institutional Animal Care Committee of the
University of California, San Diego. To induce passive K/BxN
arthritis (20,21), serum samples were pooled from arthritic
ROLE OF GADD45␤ IN RHEUMATOID ARTHRITIS
adult K/BxN mice, and the sera were injected intraperitoneally
(IP) into Gadd45␤⫺/⫺ and WT mice. Preliminary studies were
performed to determine a protocol for submaximal dosing that
would permit detection of increased disease activity. In those
studies, it was found that a low dose of 75 ␮l, injected once on
day 0 and then again on day 2, was sufficient to cause mild
arthritis (results not shown), and therefore 150 ␮l (75 ␮l ⫻ 2)
was selected as the submaximal dose. Arthritis severity was
assessed using a semiquantitative clinical scoring system for
each paw, where a score of 0 indicates normal, while a score of
1 indicates swelling of any of the following areas: the digits,
mid–hind paw/mid–fore paw, and ankle/wrist joint. The maximum clinical score per leg was 4, and the maximum total score
per mouse was 16.
Histologic analysis. Paraffin sections of the hind paws
were stained with hematoxylin and eosin (H&E) and tartrateresistant acid phosphatase (TRAP) using a leukocyte staining
kit (Sigma). Areas of synovial inflammation and the number of
osteoclasts were quantified by histomorphometry using an
Axioskop 2 microscope (Zeiss, Oberkochen, Germany) and
OsteoMeasure Analysis software (OsteoMetrics, Decatur,
GA), as previously described (22).
Forced Gadd45␤ expression in cultured human FLS.
Using the Amaxa Human Dermal Fibroblast Nucleofector kit
(NHDF-adult; Amaxa, Gaithersburg, MD) with program
U-23, 1 ⫻ 106 cells were transfected with 5 ␮g pcDNA3.1Gadd45␤ plasmid or control (empty, mock) pcDNA3.1 plasmid, in accordance with the manufacturer’s protocol (Amaxa).
Following transfection, FLS were seeded in 6-well dishes and
cultured in DMEM with 10% FCS at 37°C for 24 hours. The
cells were incubated in fresh medium for 24 hours, and were
subsequently synchronized (0.1% FCS/DMEM) for 48 hours
and then stimulated with TNF (50 ng/ml) or anisomycin (1
␮g/ml) for 15 minutes for studies of phosphorylation of
MKK-4, MKK-7, and JNK, or with TNF (50 ng/ml) for 60
minutes (for electrophoretic mobility shift assay [EMSA]), 7
hours (for luciferase reporter assays), or 24 hours (for gene
expression analysis).
Western blot analysis. Protein was extracted from
human synovial tissue, human FLS, and mouse ankle joints
using lysis buffer (150 mM NaCl, 50 mM Tris, 0.5% Triton
X-100, 3% sodium dodecyl sulfate, 1 mM EDTA, protease
inhibitor cocktail [Sigma], and phosphatase inhibitor cocktail I
and II [Sigma]) followed by sonication. The homogenates were
centrifuged at 14,000 revolutions per minute for 15 minutes,
and the supernatant was fractionated by NuPAGE 4–12%
Bis-Tris gel electrophoresis (Invitrogen) and then transferred
to nitrocellulose membranes (Bio-Rad, Hercules, CA). After
blocking nonspecific binding sites with 5% nonfat milk in 0.1%
Tween 20/Tris buffered saline (TBS-T) for 1 hour at room
temperature, the membranes were incubated with antibodies
in 5% bovine serum albumin in TBS-T overnight at 4°C. After
washing the membranes with TBS-T, the antibody–protein
complexes were probed with appropriate HRP-conjugated
secondary antibodies in 5% nonfat milk in TBS-T for 1 hour at
room temperature. The immunoreactive proteins were detected with chemiluminescent reagents (Pico and Femto SuperSignal; Pierce, Rockford, IL). The nitrocellulose membranes were stripped with a Re-Blot Western blot recycling kit
(Chemicon, Temecula, CA) and reblotted with different antibodies. Densitometry analysis was done using ImageQuant
(Molecular Dynamics, Sunnyvale, CA) (23), and immunoposi-
3231
tive bands were normalized relative to the bands for ␤-actin or
GAPDH.
In vitro kinase assays. Kinase assays were performed
according to previously described methods (11), with modifications. Following transfection, 3 ⫻ 106 FLS were seeded in
10-cm dishes and cultured in DMEM with 10% FCS at 37°C
for 24 hours. The cells were incubated in fresh medium for 24
hours, and were subsequently synchronized (0.1% FCS/
DMEM) for 48 hours and then stimulated with TNF (50 ng/ml)
with PBS or TNF alone (50 ng/ml) for 15 minutes. Cells were
washed with cold PBS and scraped directly into lysis buffer (50
mM HEPES, pH 7.4, 150 mM NaCl, 25 mM MgCl2, 1 mM
EDTA, 10% glycerol, 1% Triton X-100, 20 mM ␤ glycerophosphate, 10 mM sodium fluoride, 1 mM Na3VO4,
protease inhibitor cocktail [Complete Mini; Roche Applied
Sciences, Indianapolis, IN], 1 mM dithiothreitol, and 5 mM
p-nitrophenyl phosphate). After 30 minutes of incubation on
ice, the homogenate was centrifuged at 14,000g and the
supernatant was retained for immunoprecipitation. Samples
containing the same amount of protein were incubated with
specific goat anti–MKK-4, anti–MKK-7, or control IgG for 3
hours at 4°C on a rotating wheel, after which protein
A–Sepharose CL-4B (Oncogene Research Products, Cambridge, MA) was added and incubation was continued overnight. After centrifugation, the immune complexes were
washed with lysis buffer and then with kinase buffer (25 mM
HEPES, pH 7.4, 25 mM MgCl2, 20 mM ␤-glycerophosphate,
0.1 mM Na3VO4, protease inhibitor cocktail [Roche Applied
Sciences], 2 mM dithiothreitol, 10 mM p-nitrophenyl phosphate, and 20 ␮M ATP). The kinase reaction was started by
adding 30 ␮l of the kinase buffer, with GST–c-Jun as substrate
at 8 ␮g per reaction, and 2 ␮Ci of 32P-ATP, and then
incubating for 30 minutes at 37°C. Samples were heated for 5
minutes at 95°C, separated on NuPAGE 4–12% Bis-Tris gels
(Invitrogen), and visualized by autoradiography.
EMSA. Following transfection, FLS were seeded in
10-cm dishes and cultured in DMEM with 10% FCS at 37°C
for 24 hours. The cells were incubated in fresh medium for
24 hours and subsequently synchronized (0.1% FCS/DMEM)
for 48 hours (for NF-␬B) or 72 hours (for activator protein 1
[AP-1]) and then stimulated with TNF (50 ng/ml) for 60
minutes. Nuclear extracts were isolated using a nuclear protein extraction kit (Chemicon), in accordance with the manufacturer’s instructions. Nuclear extracts (5 ␮g) were incubated
with ␥-32P–ATP–labeled or unlabeled AP-1 (5⬘-CGCTTGATGAGTCAGCCGGAA-3⬘) and NF-␬B (5⬘-AGTTGAGGGGACTTTCCCAGGC-3⬘) oligonucleotides (Promega, Madison, WI) for 30 minutes at room temperature, and resolved on
6% DNA retention gels (Invitrogen). The DNA binding was
visualized by autoradiography, and bands were quantified
using ImageQuant (Molecular Dynamics).
Luciferase assays. The Amaxa program U-23 was also
used for transfection of luciferase reporter constructs. FLS
(5 ⫻ 105 cells) were transfected with 2.8 ␮g of Gadd45␤ or
control (empty, mock) pcDNA3.1 plasmid, 2 ␮g AP-1–
luciferase (Luc) or NF-␬B–Luc vector, and 0.2 ␮g SV-40–Luc
(Renilla) reporter control vector (generously provided by Dr.
Michael David, San Diego, CA). Following transfection, FLS
were seeded in 12-well dishes and cultured in DMEM with
10% FCS at 37°C for 24 hours. The cells were incubated in
fresh medium for 24 hours and subsequently synchronized
(0.1% FCS/DMEM) for 48 hours, and then stimulated with
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SVENSSON ET AL
Figure 1. NF-␬B activity and Gadd45␤ gene and protein expression in rheumatoid arthritis (RA) and osteoarthritis (OA) synovium. A, Analysis of
NF-␬B activity in 2 RA and 2 OA synovial samples by electrophoretic mobility shift assay (top), showing high NF-␬B nuclear binding in RA
compared with OA synovium, and quantitative results determined by densitometry (bottom), expressed as the relative band density (optical density
[OD]) of NF-␬B DNA binding. B and C, Gene expression of cyclooxygenase 2 (COX-2) (B) and Gadd45␤ (C) in RA and OA synovial tissue extracts,
as determined by quantitative polymerase chain reaction. D, Gadd45␤ protein expression in RA and OA synovia, as assessed by Western blot
analysis. These results indicate that NF-␬B DNA binding and products of NF-␬B activity are increased in RA synovium as compared with OA
synovium, and yet there is no difference in Gadd45␤ gene or protein expression between the 2 groups. Bars in A–D show the mean and SEM results
for 7 samples per group. E–H, Immunohistochemical analyses of RA and OA synovium, showing Gadd45␤ protein expression in RA synovial tissue
(E) and OA synovial tissue (G), as well as staining for NF-␬B in consecutive RA and OA sections (F and H, respectively). Representative results
are shown. I and J, Gadd45␤ protein expression in RA synovium (I), with a consecutive section stained for CD68, a macrophage marker (J). Original
magnification ⫻ 400 in E, F, G, and H; ⫻ 200 in I and J. Bars ⫽ 50 ␮m.
TNF (50 ng/ml) for 7 hours. Luciferase activity was measured
in cell lysates using a luminometer (MGM Instruments, Hamden, CT) and the Dual Luciferase Reporter Assay System
(Promega) according to the manufacturer’s instructions. Relative AP-1 activity and NF-␬B activity were determined by
normalization to the values for SV-40–Renilla Luc activity.
Quantitative real-time polymerase chain reaction
(PCR). Messenger RNA (mRNA) in frozen mouse and human
tissues and cultured FLS was isolated using RNA Stat (TelTest, Friendswood, TX), as described previously (24). Complementary DNA (cDNA) was prepared, and quantitative
real-time PCR was performed with TaqMan Gene Expression
Assays (Applied Biosystems, Foster City, CA), both according
to the manufacturer’s instructions, to determine the relative
mRNA levels using the GeneAmp 7300 Sequence Detection
system (Applied Biosystems). Predeveloped specific primers
were used to detect Gadd45␤ (assay no. Hs01063373), MMP3
(assay no. Hs00233962, Mm00440295), MMP13 (assay no.
Mm00439490), and cyclooxygenase 2 (COX-2) (assay no.
Hs00153133) (all from Applied Biosystems). Threshold cycle
values in each sample were used to calculate the number of cell
equivalents in the test samples. The data were normalized to
the values for GAPDH expression (for human samples) (cat-
alog no. 402869; Applied Biosystems) and Hprt1 expression
(for mouse samples) (assay no. Mm00446968; Applied Biosystems) to obtain the relative cell equivalents, with results
expressed as relative expression units (REU).
Statistical analysis. Results are expressed as the
mean ⫾ SEM. Differences were assessed by one-way and
two-way repeated-measures analysis of variance (ANOVA),
followed by the Bonferroni post hoc test for comparison of
multiple groups or a Student’s t-test for comparison of 2
groups. The criterion for significance was set at P values less
than 0.05.
RESULTS
NF-␬B activity and Gadd45␤ expression in human RA and OA synovial tissue. Gadd45␤ is expressed
at very low levels in most tissues, but its expression is
induced by NF-␬B (4,6). Therefore, we expected
Gadd45␤ levels to be high in RA synovial tissue relative
to OA synovial tissue due to the marked activation of
NK-␬B. To study the link between NF-␬B activation,
ROLE OF GADD45␤ IN RHEUMATOID ARTHRITIS
3233
Figure 2. Gadd45␤ gene expression, as assessed by quantitative real-time polymerase chain reaction, in rheumatoid arthritis (RA)
fibroblast-like synoviocytes (FLS) and HS68 human dermal fibroblasts at various time points following stimulation with tumor
necrosis factor (TNF) (50 ng/ml) (A and D), interleukin-1␤ (IL-1␤) (2 ng/ml) (B), or anisomycin (1 ␮g/ml) (C and E). Bars show
the mean and SEM of 6–8 separate RA FLS lines in A–C and 3 HS68 dermal fibroblasts in D and E. ⴱ ⫽ P ⬍ 0.05 versus untreated
control (C).
Gadd45␤ expression, and JNK activity in human synovial tissue, we first assessed NF-␬B DNA binding by
EMSA in synovia from RA and OA patients. In agreement with previous studies (3), we found higher NF-␬B
DNA binding in rheumatoid synovium as compared with
OA synovium (mean ⫾ SEM 4.7 ⫾ 0.6 REU versus
1.4 ⫾ 0.3 REU [n ⫽ 7 each]; P ⫽ 0.003) (Figure 1A).
Moreover, expression of an NF-␬B–regulated gene,
COX-2, was elevated in the RA group as compared with
the OA group (17.5 ⫾ 6.3 REU versus 3.2 ⫾ 1.0 REU
[n ⫽ 7 each]; P ⫽ 0.04) (Figure 1B). These data confirm
that RA synovial tissue exhibits higher NF-␬B activity
and increased expression of NF-␬B–regulated genes
compared with OA synovial tissue.
Surprisingly, when the relative expression levels
of the Gadd45␤ gene and protein were assessed in the
same synovial tissue samples, we found that Gadd45␤
mRNA and protein levels were similar between RA and
OA (n ⫽ 7 each; P ⬎ 0.05) (Figures 1C and D).
The cellular distribution of Gadd45␤ in synovial
tissue was then determined by immunohistochemistry in
serial sections from RA and OA synovia. Gadd45␤ was
expressed predominantly in the intimal lining (Figures
1E, G, and I) and was mainly present in CD68⫹
macrophages rather than intimal lining FLS (Figures 1I
and J). The Gadd45␤ detected in the synovial sublining
was mainly present in macrophages rather than T cells.
Gadd45␤ and NF-␬B (p65) immunoreactivity showed
similar patterns of expression in RA and OA, with no
apparent difference in Gadd45␤ immunoreactivity between the groups (Figures 1E and G), despite stronger
immunoreactivity for NF-␬B (p65) in the RA group
(Figures 1F and H).
Gadd45␤ gene expression in human FLS. The
discoordinate expression of Gadd45␤ and NF-␬B in RA
synovium and the relatively low expression of Gadd45␤
in synovial lining fibroblast-like cells led us to explore
the expression and regulation of Gadd45␤ in cultured
FLS. To determine whether RA FLS express Gadd45␤
in response to cytokines, the cells were stimulated with
TNF (50 ng/ml) or IL-1␤ (2 ng/ml), and Gadd45␤ gene
expression was examined by quantitative PCR. Stimula-
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SVENSSON ET AL
Figure 3. Effect of forced Gadd45␤ expression on the JNK signaling pathway in rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS). A and
B, Quantitative results (top) and representative Western blots (bottom) for the extent of phosphorylation of MKK-7 (A) and MKK-4 (B) in tumor
necrosis factor (TNF)–stimulated RA FLS transfected with Gadd45␤ cDNA plasmid or empty pcDNA3.1 (mock) plasmid. C, Representative
autoradiographs of MKK-4 and MKK-7 activity, as determined from in vitro kinase assays in which control and Gadd45␤-overexpressing FLS were
stimulated with TNF. These experiments were repeated 4 times, with similar results obtained each time. D and E, Quantitative results (top) and
representative Western blots (bottom) for the extent of JNK phosphorylation in TNF- or anisomycin (Aniso)–stimulated RA FLS transfected with
Gadd45␤ cDNA plasmid or empty pcDNA3.1 (mock) plasmid. Bars in A, B, D, and E show the mean and SEM percent change in expression in
Gadd45␤-overexpressing RA FLS relative to the TNF- or anisomycin-stimulated mock control cultures (set at 100%). F–H, Activator protein 1
(AP-1) luciferase reporter activity (F), NF-␬B luciferase reporter activity (G), and matrix metalloproteinase 3 (MMP3) gene expression (H) in mock
control and Gadd45␤-overexpressing RA FLS stimulated with TNF. Bars in F–H show the mean and SEM results in TNF-stimulated cultures relative
to the phosphate buffered saline (PBS)–treated control cultures. MMP3 expression values were normalized to the values for GAPDH. In each
experiment, PBS was used as the unstimulated control, and 5–7 separate RA FLS lines were studied.
tion of RA FLS with TNF or IL-1␤ under conditions
that optimized the activation of NF-␬B (see below and
ref. 25) induced only low-level, transient expression of
Gadd45␤ (Figures 2A and B). The cells were also
stimulated with anisomycin, which is a potent and nonselective activator of kinases (including the MKK-4/
MKK-7/JNK pathway), and thus was used as a positive
control for comparison with cytokine receptor–mediated
JNK signaling. Stimulation of the cells with anisomycin
(1 ␮g/ml) evoked massive and persistent Gadd45␤ expression in human FLS (Figure 2C), indicating that
synoviocytes are able to express abundant Gadd45␤
under some circumstances. Similar results were observed in OA FLS (results not shown) and in HS68 cells,
a human dermal fibroblast cell line (Figures 2D and E),
suggesting that this is a general property of cultured FLS
as well as fibroblast-lineage cells.
Gadd45␤ regulation of MKK-4 and MKK-7 activity in FLS. To assess the effect of Gadd45␤ expression
on JNK-mediated signaling, we used human RA FLS
transfected with Gadd45␤ cDNA containing pcDNA3.1
plasmid or empty pcDNA3.1 control plasmid. The effect
of forced Gadd45␤ protein expression on the TNFinduced phosphorylation and activity of MKK-4 and
MKK-7 was examined. Using Western blotting, phosphorylation of MKK-4 and MKK-7 was readily detected
in RA FLS stimulated with TNF (50 ng/ml for 15
minutes), with no difference observed between Gadd45␤
pcDNA3.1–transfected cells and empty pcDNA3.1–
transfected cells (n ⫽ 5 each; P ⬎ 0.05) (Figures 3A
and B).
Because Gadd45 ␤ is thought to inactivate
MKK-7 through binding within the kinase ATP pocket,
rather than via the prevention of phosphorylation (7),
the functional activity of MKK-7 and MKK-4 was measured using an in vitro protein kinase activity assay, with
GST–c-Jun as a substrate. JNK forms a tight complex
with c-Jun, and therefore GST–c-Jun may serve as a
substrate in MKK-4/MKK-7 kinase activity assays (26).
As shown in Figure 3C, forced expression of Gadd45␤ in
FLS blocked the TNF-induced function of MKK-7 without affecting the function of MKK-4.
ROLE OF GADD45␤ IN RHEUMATOID ARTHRITIS
Gadd45␤ regulation of JNK phosphorylation in
FLS. After observing that forced Gadd45␤ expression in
FLS attenuates MKK-7 activity, we assessed the effect of
Gadd45␤ on JNK phosphorylation. RA FLS were transfected with the Gadd45␤ plasmid or control plasmid and
then stimulated with TNF (50 ng/ml for 15 minutes).
Forced Gadd45␤ overexpression significantly reduced
JNK phosphorylation compared with that in the TNFstimulated mock control cultures (mean ⫾ SEM phosphorylation of JNK 0.31 ⫾ 0.05 relative to TNFstimulated mock control cultures [n ⫽ 7 each]; P ⬍
0.0001) (Figure 3D). JNK phosphorylation was normal
in Gadd45␤-transfected cells that had been stimulated
with anisomycin (1.26 ⫾ 0.14–fold increase relative to
TNF-stimulated mock control cultures [n ⫽ 5 each]; P ⬎
0.05) (Figure 3E). This finding is consistent with the
observations in previous studies showing that cytokineinduced JNK phosphorylation is strictly MKK-7 dependent, whereas anisomycin-induced JNK activation can
utilize either MKK-4 or MKK-7 (11). Thus, Gadd45␤
modulation of MKK-7/JNK signaling suppresses
cytokine-mediated FLS activation while leaving other
stress responses through MKK-4 intact.
Gadd45␤ regulation of JNK function in FLS.
The consequences of increased Gadd45␤ expression for
JNK function were then assessed by examining the
transcriptional activity of AP-1 and NF-␬B. Cultured
RA FLS were cotransfected with Gadd45␤ plasmid or
mock pcDNA3.1 plasmid and with an AP-1 or NF-␬B
promoter luciferase construct. Stimulation of the cells
with TNF (50 ng/ml for 7 hours) increased the transcriptional activity of AP-1 and NF-␬B in the pcDNA3.1
mock–transfected cultured FLS. Forced Gadd45␤ overexpression blocked AP-1 promoter activity (2.8 ⫾ 0.3–
fold change in TNF-stimulated mock cultures versus
0.9 ⫾ 3–fold change in TNF-stimulated Gadd45␤ cultures [n ⫽ 4 each, relative to PBS cultures]; P ⫽ 0.004),
but had no effect on NF-␬B promoter activity (7.3 ⫾
1.7–fold change in TNF-stimulated mock cultures versus
5.5 ⫾ 0.6–fold change in TNF-stimulated Gadd45␤
cultures [n ⫽ 4 each, relative to PBS cultures]; P ⫽ 0.34)
(Figures 3F and G).
In addition, cultured RA FLS transfected with
Gadd45␤ plasmid or mock pcDNA3.1 plasmid and
stimulated with TNF (50 ng/ml for 1 hour) were also
assessed by EMSA for AP-1 and NF-␬B DNA binding.
In agreement with the findings from the luciferase
reporter construct assays described above, Gadd45␤
overexpression attenuated AP-1 DNA binding, but had
no effect on NF-␬B DNA binding (results not shown).
The MMP3 gene contains key AP-1 binding sites
in its promoter. Thus, the presence of JNK-activated
3235
Figure 4. Gadd45␤ gene expression in the ankle joints of wild-type
mice with K/BxN serum transfer arthritis. K/BxN serum was injected at
the standard dose (150 ␮l intraperitoneally on day 0), which caused
severe arthritis, and Gadd45␤ gene expression in the knee joints was
then assessed by quantitative reverse transcription–polymerase chain
reaction. Gadd45␤ gene expression was modestly increased on day 4
and day 8, confirming that this protein is expressed in response to
innate immune challenges in normal mice. Bars show the mean and
SEM results in 7 samples. Gadd45␤ gene expression values were
normalized to the values for Hprt1. ⴱ ⫽ P ⬍ 0.05 versus day 0.
AP-1 is required for transcription of this MMP gene, and
increased Gadd45␤ expression should suppress this
pathway. As shown in Figure 3H, TNF-stimulated (50
ng/ml for 24 hours) expression of MMP3 mRNA was
reduced in cells transfected with the Gadd45␤ plasmid
compared with that in TNF-stimulated mock control–
transfected FLS (n ⫽ 6 each; P ⬍ 0.0001). Previous
studies have demonstrated that the expression levels of
MMP3 mRNA regulated by MKK-7 and JNK in cultured
FLS closely parallel the protein levels in culture supernatants (11).
Gadd45␤ expression and function in K/BxN serum transfer arthritis. Initial studies were performed to
determine the time course of Gadd45␤ expression in the
passive K/BxN arthritis model (20,21). Serum (150 ␮l)
was injected IP on day 0, and the joints were harvested
on days 0, 1, 4, 8, and 12. Quantitative PCR showed that
articular Gadd45␤ gene expression could be detected in
naive WT mice, similar to that seen in human RA and
OA synovium. The levels of Gadd45␤ mRNA increased
modestly after serum injection, and were similar in
magnitude to the changes observed in cytokinestimulated human FLS (Figure 4).
The regulatory role of Gadd45␤ in this murine
model was then evaluated using Gadd45␤⫺/⫺ mice. To
permit detection of disease exacerbation, a modified
3236
SVENSSON ET AL
Figure 5. Development of K/BxN serum–induced arthritis, synovial inflammation, and bone destruction in Gadd45␤⫺/⫺ mice. A, To determine the
arthritis severity in Gadd45␤⫺/⫺ and wild-type (WT) mice over time after the induction of K/BxN serum transfer arthritis, clinical scores were
assessed showing that inflammatory arthritis was exacerbated in Gadd45␤⫺/⫺ mice compared with WT mice. Bars show the mean ⫾ SEM results
in 8 mice per group, representative of 1 of 3 separate experiments. ⴱ ⫽ P ⬍ 0.01. B and C, The ankles from Gadd45␤⫺/⫺ and WT mice were harvested
on day 6 and prepared for histologic evaluation by hematoxylin and eosin (H&E) and tartrate-resistant acid phosphatase (TRAP) staining, to assess
synovial inflammation (B) and bone destruction (C). In B, results are expressed as the area of inflammation (in mm2) on H&E-stained sections, as
determined using image analysis. ⴱ ⫽ P ⬍ 0.002. In C, results are expressed as the number of osteoclasts per bone perimeter, as assessed by TRAP
staining. ⴱ ⫽ P ⬍ 0.05. Bars in B and C show the mean and SEM results in 10 mice per group. D and E, Representative photomicrographs of the
WT mouse ankle joints (D) and Gadd45␤⫺/⫺ mouse ankle joints (E), as analyzed by TRAP staining 6 days after the induction of K/BxN passive
transfer arthritis (original magnification ⫻ 20). The results show greater local bone erosions and osteoclast formation in the Gadd45␤⫺/⫺ mice
(indicated by purple staining and arrows).
protocol with a submaximal dose of K/BxN serum was
used (see Materials and Methods). Mild arthritis was
observed in the WT group, but disease severity was significantly greater in the Gadd45␤⫺/⫺ group (P ⬍ 0.01 by
2-way ANOVA) (representative results are shown in
Figure 5A).
Synovial inflammation and osteoclast generation
in K/BxN serum transfer arthritis. Consistent with the
increased clinical score of arthritis in the Gadd45␤⫺/⫺
group, histologic evaluation of the joints at the peak of
K/BxN serum–induced arthritis (day 6) demonstrated
significantly greater synovial inflammation in
Gadd45␤⫺/⫺ mice compared with WT mice (area of
H&E-stained sections, 0.47 ⫾ 0.09 mm2 in Gadd45␤⫺/⫺
mice versus 0.15 ⫾ 0.05 mm2 in WT mice [n ⫽ 10 each];
P ⬍ 0.002) (Figure 5B).
Because NF-␬B and JNK participate in osteoclast
differentiation and bone erosion (27,28), we assessed the
number of osteoclasts in the mouse joints. Using TRAP
staining, we found that the number of osteoclasts was
higher in Gadd45␤⫺/⫺ mice compared with that in the
WT group (5.00 ⫾ 1.09 per mm in Gadd45␤⫺/⫺ mice
versus 2.10 ⫾ 0.46 per mm in WT mice [n ⫽ 10 each];
P ⬍ 0.05) (Figures 5C–E).
JNK phosphorylation in Gadd45␤ⴚ/ⴚ mice. In
separate experiments, the joints were collected at the
peak of K/BxN serum–induced arthritis (day 6) and
evaluated by Western blot analysis to determine the
phosphorylation state of JNK. Whereas only a modest
increase in the levels of phosphorylated JNK was observed in the WT group, a marked increase in the levels
of phosphorylated JNK was observed in the Gadd45␤⫺/⫺
mice (16.7 ⫾ 0.4–fold increase in Gadd45␤⫺/⫺ mice
versus 3.6 ⫾ 0.6–fold increase in WT mice relative to
ROLE OF GADD45␤ IN RHEUMATOID ARTHRITIS
untreated WT controls [n ⫽ 7 each]; P ⬍ 0.05 versus
untreated WT controls) (Figures 6A and B).
MMP3 and MMP13 gene expression in
Gadd45␤ⴚ/ⴚ mice and cultured murine FLS. The relationship between elevated JNK activity in the mouse
joints and changes in MMP gene expression was then
evaluated by quantitative PCR. On day 6 after serum
administration, the expression levels of the MMP3 and
MMP13 genes were significantly greater in Gadd45␤⫺/⫺
mice compared with WT mice (for MMP3, mean ⫾ SEM
3.6 ⫾ 0.8 REU versus 1.3 ⫾ 0.3 REU [n ⫽ 7 each] [P ⫽
0.02]; for MMP13, 2.2 ⫾ 0.3 REU versus 1.2 ⫾ 0.1 REU
[n ⫽ 7 each] [P ⫽ 0.03]) (Figure 6C). These findings link
Gadd45␤ not only to a protective role in the progression
of inflammatory arthritis but also to critical regulatory
mechanisms in relation to the activities and function of
MKK-7 and JNK.
FLS are thought to be a primary source of MMPs
in inflammatory arthritis (29). Therefore, we determined whether MMP gene expression is altered in
murine FLS from Gadd45␤-deficient mice. As shown in
Figure 6D, Gadd45␤⫺/⫺ FLS expressed significantly
more MMP3 and MMP13 mRNA than did WT mouse
FLS after TNF stimulation (for TNF-stimulated MMP3,
mean ⫾ SEM 179 ⫾ 11 REU versus 82 ⫾ 4 REU [n ⫽
3 each] [P ⬍ 0.001]; for TNF-stimulated MMP13, 19.3 ⫾
1.8 REU versus 9.7 ⫾ 2.4 REU [n ⫽ 3 each] [P ⬍ 0.05]).
DISCUSSION
RA is a chronic autoimmune disease that is
marked by synovial inflammation and joint destruction
(1). Degradation of articular extracellular matrix in the
RA joint is, in part, mediated by MMPs (29,30). The
production of these enzymes is regulated by cytokines,
such as TNF and IL-1, most notably through activation
of the MAPKs. Activation of a key MAPK, namely JNK,
is especially important in this process, since it phosphorylates transcription factors, such as AP-1, that are required for MMP transcription (2,31). JNK activity is
regulated by 2 upstream kinases, MKK-4 and MKK-7.
Of these 2 kinases, MKK-7 is particularly important in
RA, since only MKK-7 is required for JNK activation in
FLS after cytokine stimulation (10,11). The kinases
in the JNK pathway are highly activated in RA synovium
and contribute to the production of cytokines and
MMPs. Although this could be a response to proinflammatory cytokines, we considered whether Gadd45␤ deficiency might also be a contributor to overactivation of
JNK in rheumatoid synovitis.
Our initial studies showed that Gadd45␤ expression in RA synovium is similar to that in OA synovium,
3237
despite the activation of NF-␬B in the intimal lining. In
contrast, other NF-␬B–driven genes are readily detected
in higher amounts in RA than in OA samples, which
indicates that the NF-␬B activity is sufficient for induction of other NF-␬B–regulated genes in the synovial
environment. Similarly, cultured synoviocytes had only
minimal and transient Gadd45␤ induction, despite stimulation with cytokines that maximally increase NF-␬B
translocation. The mechanism for this deficient transcriptional response to NF-␬B activation has not yet
been defined. However, it is stimulus specific, because
anisomycin induced abundant Gadd45␤ expression. It
appears to be a common feature of fibroblasts, rather
than being disease specific, because OA FLS and HS68
dermal fibroblasts also had limited TNF-induced
Gadd45␤ gene expression as compared with that after
stimulation with anisomycin. Therefore, the relative lack
of Gadd45␤ is not specifically associated with RA.
Preprogrammed deficient Gadd45␤ responses in the
intimal lining would not cause RA but could amplify the
innate immune processes that lead to joint damage.
Uncoupled NF-␬B activation and Gadd45␤ in the
presence of proinflammatory cytokines is a potential
mechanism that could explain why the JNK pathway is
highly responsive in an inflammatory disease state such
as RA. Thus, MKK-7 could be phosphorylated but
would not be restrained by Gadd45␤. Because MKK-7 is
the pivotal upstream kinase that regulates JNK in FLS,
it could readily phosphorylate JNK and increase MMP
production. The end result would be enhanced extracellular matrix destruction (31). This hypothesis is supported by our studies demonstrating that Gadd45␤
overexpression in cultured FLS reverses this process by
suppressing AP-1 binding, AP-1–mediated transcription,
and MMP expression. Other negative regulators of JNK
activity could also be important and may contribute to
the regulation of cytokines and MMPs. The relative
contribution of one such factor, X-linked inhibitor of
apoptosis protein, has not been studied in relation to
Gadd45␤, but it has been associated with deficient
synoviocyte apoptosis in RA (32).
Injection of K/BxN serum increased synovial
Gadd45␤ expression transiently, up to 2–3-fold, which
was similar to that observed in TNF-stimulated cultured
FLS. Direct comparisons between RA and normal tissue
with respect to Gadd45␤ expression, and how this
correlates with observations in the mouse model, are
difficult because matched normal human samples are
rarely available. Thus, the passive model needs to be
interpreted with some caution. However, the experiments allowed us to evaluate the role of Gadd45␤ in
inflammation and the consequences of a deficient
3238
SVENSSON ET AL
Figure 6. JNK phosphorylation and matrix metalloproteinase (MMP) gene expression in the joints of Gadd45␤⫺/⫺ and wild-type (WT) mice. A,
JNK phosphorylation was evaluated by Western blotting in the ankle joints of Gadd45␤⫺/⫺ and WT mice on day 6 after induction of K/BxN serum
transfer arthritis as compared with naive WT controls. Representative results are shown. ␤-actin was used as the reference. tot ⫽ total. B,
Phosphorylation of JNK was assessed quantitatively in Gadd45␤⫺/⫺ and WT mice with K/BxN serum–induced arthritis, expressed as the percent
change relative to naive WT controls. C, Gene expression levels of MMP3 and MMP13 in the mouse joints were determined on day 6 by quantitative
reverse transcription–polymerase chain reaction. Bars show the mean and SEM results in 7 mice per group. ⴱ ⫽ P ⬍ 0.05 versus naive WT; # ⫽
P ⬍ 0.05 versus WT group that received K/BxN serum. D, Fibroblast-like synoviocytes (FLS) were established from the knee joints of Gadd45␤⫺/⫺
and WT mice, and the relative gene expression of MMP3 and MMP13 in response to tumor necrosis factor (TNF) (50 ng/ml for 24 hours) was
determined in both FLS groups. Bars show the mean and SEM results in 3 mice per group. Gene expression values were normalized to the values
for Hprt1. ⴱ ⫽ P ⬍ 0.05 versus phosphate buffered saline (PBS)–treated WT; # ⫽ P ⬍ 0.05 versus TNF-treated WT.
Gadd45␤ response. A submaximal dose of serum in
Gadd45␤⫺/⫺ mice led to rapid onset of arthritis, higher
clinical scores, increased inflammation, and increased
numbers of osteoclasts in the joints compared with the
findings in serum-treated WT mice. However, it is not
known whether the differences in osteoclast differentiation in vivo are due to an inherent property of
Gadd45␤⫺/⫺ precursors or are a reflection of an altered
cytokine milieu. These findings demonstrate that the net
effect of Gadd45␤ deficiency is disease exacerbation.
Gadd45␤ deficiency was accompanied by increased JNK
phosphorylation and elevated MMP3 and MMP13 gene
expression in the joints of Gadd45␤⫺/⫺ mice, as com-
pared with the arthritic controls. Of note, the K/BxN
serum transfer model represents an inflammatory arthritis that is independent of adaptive immune responses
(33), and thus our studies are the first demonstration
that Gadd45␤ influences diseases that are strictly dependent on innate immunity.
The expression and role of Gadd45␤ in arthritis
were not clearly delineated in previous studies. For
instance, a previous report suggested that Gadd45␤
expression in synovial fluid T cells in RA might suppress
apoptosis (34). In that case, Gadd45␤ deficiency could
enhance lymphocyte death and ameliorate disease.
However, this distribution of Gadd45␤ was not observed
ROLE OF GADD45␤ IN RHEUMATOID ARTHRITIS
in our studies of synovial tissue, since we observed that
Gadd45␤ expression was localized to the lining synoviocytes and sublining macrophages rather than T cells. In
addition to focusing on the role of Gadd45␤ in human
FLS in this study, we also assessed the role of Gadd45␤
in murine macrophages. Preliminary studies have shown
that MMP13 gene expression in Gadd45␤-deficient peritoneal macrophages is higher than that in WT cells in
response to TNF stimulation (results not shown). Therefore, Gadd45␤ might be an important factor in synovial
FLS and macrophages and warrants further studies to
examine how Gadd45␤ may regulate JNK activity in RA
synovial macrophages in situ.
In contrast to FLS, chondrocytes from normal
individuals and patients with early OA (35) constitutively express Gadd45␤ mRNA. The role of Gadd45␤ in
chondrocytes is complex and age dependent. At the
embryonic stage, Gadd45␤ is critical for terminal differentiation of mouse chondrocytes in the growth plate.
However, at this stage, Gadd45␤ activates, rather than
inhibits, JNK through interactions with MTK-1/
MEKK-4, upstream activators of JNK (36). As a consequence, Gadd45␤⫺/⫺ mouse embryos display a decreased MMP13 gene expression, which leads to
defective mineralization and decreased bone growth. In
adult cartilage, chondrocytes are quiescent, and cell
survival is essential for maintenance of the avascular
tissue. In late-stage OA, a reduction in Gadd45␤ expression by chondrocytes is associated with increased TNFinduced cell death (35,36). Thus, endogenous Gadd45␤
might provide a protective function in adult chondrocytes by promoting cell survival. On the other hand,
Gadd45␤ decreases collagen production by chondrocytes, which could have a negative effect on cartilage
composition and integrity.
Gadd45␤ might play an important role in many
other diseases by mediating a delicate balance between
different cell survival and cell death pathways. For
example, inadequate expression of Gadd45␤ is thought
to be a key feature in insulin-secreting beta cells undergoing apoptosis in response to IL-1␤ (16). In contrast,
down-regulation of Gadd45␤ has been associated with
cell survival, since it was strongly correlated with the
degree of malignancy in hepatocellular carcinoma
(13,14). Gadd45␤ is also thought to play an especially
important role in adaptive immunity (12). Liu et al
showed that Gadd45␤ limits the proliferation of CD4⫹
T cells in response to T cell receptor signaling and
cytokines, and that T cells lacking Gadd45␤ proliferated
faster than WT controls and were more resistant to
activation-induced cell death (12). In addition, the study
3239
by Liu et al showed that deletion of Gadd45␤ exacerbates
murine experimental allergic encephalomyelitis (12).
While Gadd45␤ is a complex molecule that can
potentially affect many cellular functions, its role in the
JNK pathway is especially noteworthy in arthritis. Previous studies have implicated JNK activity in RA (34),
and recent in vitro studies with cultured FLS suggested
that MKK-7 is the dominant upstream kinase for
cytokine-mediated processes (10,11). However, MKK-4
does play a role in some circumstances, such as in
Toll-like receptor 3–mediated signaling or anisomycin
stimulation (37). One potential therapeutic approach in
RA is to block a subset of relevant JNK functions, which
might be safer than inhibiting all JNK activity. This
could be accomplished by suppressing MKK-7 while
leaving MKK-4 intact. Gadd45␤ accomplishes this goal
by binding to MKK-7 and preventing it from activating
JNK (8).
Whereas the role of JNK in RA has been well
documented, our present findings in Gadd45␤⫺/⫺ mice
suggest that MKK-7 is a pivotal kinase for synovial JNK
regulation. The beneficial effects of Gadd45␤ provide
support for a therapeutic strategy that targets the JNK
pathway for diseases involving innate immunity, either
by directly inhibiting JNK or MKK-7 or by enhancing
Gadd45␤ expression. This approach could potentially
suppress osteoclast development, expression of MMPs,
and synovial inflammation in RA.
AUTHOR CONTRIBUTIONS
All authors were involved in drafting the article or revising it
critically for important intellectual content, and all authors approved
the final version to be published. Dr. Firestein had full access to all of
the data in the study and takes responsibility for the integrity of the
data and the accuracy of the data analysis.
Study conception and design. Svensson, Schett, Corr, Boyle, Firestein.
Acquisition of data. Svensson, Inoue, Hammaker, Fukushima, Papa,
Franzoso.
Analysis and interpretation of data. Svensson, Boyle, Firestein.
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