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*
Cytokine Research Section, Department of Molecular Oncology, University of Texas M. D. Anderson Cancer Center, Houston, TX 77030; and
Laboratory of Immunochemistry, Division of Transfusion Transmitted Diseases, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, MD 20892
| Abstract |
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| Introduction |
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Among these are the mitogen-activated protein
(MAP)3 kinases, which are
involved in a variety of cellular responses mediated by cytokines,
hormones, and stress-inducing reagents (13). Depending upon the set of
substrates they phosphorylate, MAP kinases are classified into at least
three groups, including extracellular response kinase (ERK), c-Jun
N-terminal kinase/stress-activated protein kinase (JNK/SAPK), and p38
MAP kinase (14). The activity of activator protein-1 (AP-1), a
transcription factor, which consists of a homodimer and heterodimers of
members of the Jun family (c-Jun, JunB, and JunD) and heterodimers of
the Jun and Fos (c-Fos, FosB, Fra1, and Fra2) families, is regulated,
at least in part, by the activation of JNK (15, 16). JNK also plays an
important role in the regulation of cellular proliferation (15).
HIV-tat has been reported to activate NF-
B, a transcription factor
involved in many inflammatory responses (17, 18). It has also been
suggested that the activation of NF-
B is regulated by some upstream
MAP kinases that also regulate JNK activation in the cells (19).
Several cellular responses of HIV-tat mimic those of TNF. Finally,
while TNF is known to activate JNK and AP-1, whether HIV-tat also
activates is not known. In the present investigation we show that
HIV-tat activates JNK and AP-1, possibly through generation of free
radicals.
| Materials and Methods |
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HIV-tat protein, neutralizing polyclonal Abs against HIV-tat, H9, Raji and Jurkat cells, and their HIV-tat gene-transfected counterparts were obtained from AIDS Research and Reference Reagent Program of the National Institute of Allergy and Infectious Diseases (Rockvile, MD). GST-Jun (179) was a kind gift from Dr. B. Su of the M. D. Anderson Cancer Center (Houston, TX). Anti-JNK1 Abs were obtained from Santa Cruz Biotechnology (Santa Cruz, CA). The phospho-specific anti-p44/42 MAP Kinase (Thr202/Tyr204) Ab was obtained from New England Biolabs (Beverly, MA).
c-Jun N-terminal kinase assay
The assay for c-jun kinase was modified from a published
protocol (20). After treatment of cells (3 x
106/ml) with HIV-tat or TNF, cell extracts were
prepared by lysing cells in buffer containing 20 mM HEPES, pH 7.4, 2 mM
EDTA, 250 mM NaCl, 0.1% Nonidet P-40, 2 µg/ml leupeptin, 2 µg/ml
aprotinin, 1 mM PMSF, 0.5 µg/ml benzamidine, and 1 mM DTT.
Cytoplasmic extracts (250 µg) were subjected to immunoprecipitation
with 0.3 µg anti-JNK (Santa Cruz) for 30 min at 4°C. Immune
complexes were collected by incubation with protein A/G-Sepharose beads
(Pierce, Rockford, IL) for 30 min at 4°C. The beads were collected by
centrifugation and washed extensively with lysis buffer (4 x 400
µl) and kinase buffer (2 x 400 µl:20 mM HEPES, pH 7.4, 1 mM
DTT, 25 mM NaCl). Kinase assays were performed for 15 min at 37°C
with 2 µg GST-Jun(179) in 20 µl containing 20 mM HEPES, pH 7.4,
10 mM MgCl2, 1 mM DTT, and 10 µCi
[
-32P]ATP. Reactions were stopped with 15 µl
SDS-sample buffer, boiled for 5 min, and subjected to SDS-PAGE.
GST-Jun(179) was visualized by staining with Coomassie blue, and the
dried gel was analyzed by a Phosphorimager (Molecular Dynamics,
Sunnyvale, CA) and quantitated by ImageQuant Software (Molecular
Dynamics).
MAP kinase kinase assay
U937 cells were treated with TNF or with different concentrations of HIV-tat protein for 30 min at 37°C. The cells were washed with PBS and extracted with lysis buffer containing 20 mM HEPES, pH 7.4, 2 mM EDTA, 250 mM NaCl, 0.1% Nonidet P-40, 2 µg/ml leupeptin, 2 µg/ml aprotinin, 1 mM PMSF, 0.5 µg/ml benzamidine, 1 mM DTT, and 1 mM sodium orthovanadate. Protein (50 µg) was resolved on 10% SDS-PAGE, electrotransferred onto nitrocellulose membrane, and probed with the phospho-specific anti-p44/42 MAP Kinase (Thr202/Tyr204) Ab (New England Biolabs) raised in rabbit (1:3000 dilution). The membrane was then incubated with peroxidase conjugated anti-rabbit IgG (1:3000 dilution), and bands were detected by chemiluminescence (ECL, Amersham, Arlington, Heights, IL).
Electrophoresis mobility shift assay
The electrophoresis mobility shift assay (EMSA) was performed by incubating 4 to 5 µg of nuclear extract with 16 fmol of 32P-end-labeled AP-1 consensus oligonucleotide 5'-CGCTTGATGACTCAGCCGGAA-3' (Santa Cruz) for 15 min at 37°C, as described earlier (21). The specificity of binding was examined by competition with unlabeled oligonucleotide. Visualization and quantitation of radioactive bands was conducted by phosphorimager (Molecular Dynamics) using Imagequant software.
| Results and Discussion |
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-irradiation, and the cancer
chemopreventive agent phenethyl isothiocyanate (23, 24), cause a
persistent activation of JNK, whereas other agents, such as TNF,
produce a rapid transient activation (22). In the next part of this
study, a time kinetics of HIV-tat-induced activation of JNK was
studied. Cells were treated for different time intervals with 100 ng/ml
of HIV-tat protein. As shown in Figure 2
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In contrast to exogenous treatment, whether endogenously produced
HIV-tat can activate JNK was also examined. We checked the activation
of JNK in cells transfected with the HIV-tat gene. As shown
in Figure 3
A, the transfection
of H9 cells with HIV-tat gene resulted in an increase in JNK
activity. On treatment with exogenous HIV-tat protein, no further
increase in JNK activity was observed in H9 cells transfected with the
HIV-tat gene. The JNK activity of normal H9 cells was
augmented on treatment with exogenous HIV-tat protein. H9 cells
transfected with HIV-tat gene have been previously shown by
Western blot analysis to produce HIV-tat protein in the culture
supernatant (26), similar to HIV-infected cells.
|
The physiologic significance of activation of JNK by HIV-tat
protein is not understood. JNK has been shown to increase the
transcriptional activity of c-Jun, ATF-2 (activating transcription
factor 2), and Elk1 upon their phosphorylation. These transcription
factors are the components of AP-1, and so the activation of JNK leads
to the activation of AP-1-dependent gene expression (15, 16). We have
recently shown that HIV-tat treatment of the cells leads to the
increased production of metalloproteinase-9 (MMP-9) and the expression
of adhesion molecules (10, 27). The promotor region of both MMP-9 and
adhesion molecules contains AP-1 binding sites (28). In the present
study we found that HIV-tat caused a time- and dose-dependent
activation of the DNA-binding of AP-1 (Fig. 4
). The activation of AP-1 by HIV-tat
thus suggests that one mechanism by which HIV-tat could lead to the
increased expression of MMP-9 and adhesion molecules is through the
activation of AP-1 in the JNK cascade. HIV-tat has indeed been reported
to regulate the expression of a wide variety of other cytokines and
their receptors (4, 5, 6, 7, 8, 9), some of which contain AP-1-binding sequence in
their promotor region.
|
Although HIV-tat has been reported to interact with such cell surface
molecules as CD26 and integrin
5ß1 (33, 34), the molecular mechanisms of HIV-tat-mediated cell signaling are
not understood. One effector may be oxygen free radicals, whose role in
signal transduction of TNF and other cytokines has been extensively
described in the literature. Some recent reports indicate that
H2O2 plays an important role as a second
messenger in signal transduction pathways that regulate the activity of
transcription factors such as NF-
B and AP-1 (35, 36).
To determine whether HIV-tat-mediated activation of JNK involves a
reactive oxygen intermediate (ROI), we examined the effects of NAC, a
free radical scavenger, on the HIV-tat-mediated activation of JNK.
Pretreatment of U937 cells with NAC inhibited the HIV-tat-mediated
activation of JNK in a dose-dependent manner (Fig. 5
A), suggesting the
involvement of oxygen free radicals. The effect of NAC was specific
since mannitol, a hydroxyl radical quencher, or pyrrolidine
dithiocarbamate (PDTC), an iron chelator, had no effect on
HIV-tat-induced JNK activation (Fig. 5
B). Both PDTC
and NAC have been previously shown to block NF-
B activation and
transcription of HIV (37, 38). There are other reports, however, that
show that PDTC and NAC differentially regulate NF-
B activation (39).
That PDTC and NAC display different effects is in agreement with our
results, which show that NAC but not PDTC can block JNK activation. The
use of NAC as a novel approach for anti-HIV therapy has also been
suggested (40). Thus, overall, our results suggest that HIV-tat
activates JNK and AP-1 through an ROI-dependent pathway and that this
may play a role in production of cytokines and growth modulation
induced by HIV-tat (Fig. 6
).
|
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| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Bharat B. Aggarwal, Cytokine Research Section, Department of Molecular Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030. E-mail address: ![]()
3 Abbreviations used in this paper: MAP, mitogen-activated protein; JNK, c-Jun N-terminal kinase; AP-1, activator protein-1; NAC, N-acetylcysteine; ROI, reactive oxygen intermediates; PDTC, pyrrolidine dithiocarbamate; MEK, MAP kinase kinase; GST, glutathione S-transferase; MMP-9, matrix metalloproteinase-9. ![]()
Received for publication December 12, 1997. Accepted for publication March 10, 1998.
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B. Proc. Natl. Acad. Sci. USA 93:9090.
B and AP-1 in response to reactive oxygen intermediates. Methods 11:301.[Medline]
B and AP-1 in lymphocytes. Biochem. Pharmacol. 50:735.[Medline]
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