Vascular Biology |
B
BIndependent, RSKMediated Phosphorylation of p65
From the Departments of Internal Medicine (L.Z., Y.M., J.Z., J.C., J.D.) and Human Biological Chemistry and Genetics (J.D.), The University of Texas Medical Branch, Galveston.
Correspondence to Jie Du, Department of Internal Medicine, 9.138 Medical Research Building, 301 University Blvd, University of Texas Medical Branch, Galveston, TX 77555-1064. E-mail jidu{at}utmb.edu
| Abstract |
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B (NF-
B)mediated transcription of proinflammatory genes such as interleukin-6 (IL-6). We examined the signaling mechanism whereby Ang II activates NF-
B in vascular smooth muscle cells (VSMCs).
Methods and Results Ang II treatment did not increase phosphorylation of inhibitor of
B
(I
B
) or I
Bß or decrease their levels. In contrast, mitogen-activated protein kinase kinase-1 (MEK1) inhibition (dominant-negative MEK1 adenovirus or inhibitor U0126) suppressed Ang IIinduced NF-
B promoter activity, NF-
B DNA-binding activity, p65 phosphorylation, and led to 70% reduction in IL-6 transcription/production. The mechanism involved Ang II activation of Ras and MEK1. Signaling distal to MEK1 involved extracellular signal-regulated kinase (ERK) because inhibition of MEK1 suppressed the Ang IIinduced activation of ribosomal S6 kinase (RSK), a substrate of ERK. Downregulation of RSK by small interfering RNA (SiRNA) in VSMCs was found to suppress Ang IIinduced activation of NF-
B and p65 phosphorylation. Immunopurified RSK from Ang IItreated VSMCs phosphorylated recombinant glutathione S-transferasep65 in vitro.
Conclusion We uncovered a nonclassical signaling pathway (Ras/MEK1/ERK/RSK) from Ang II to activation of NF-
B, a mechanism by which Ang II stimulates RSK-mediated phosphorylation of p65 to participate in vascular inflammation.
We examined an intracellular mechanism whereby Ang II activates NF-
B. Ang II caused no significant increase in I
B phosphorylation or protein levels. Instead, we found a nonclassical signaling pathway, Ras/MEK1/ERK/RSK, that activated NF-
B via phosphorylation of p65, leading to IL-6 production and suggesting a mechanism by which Ang II causes vascular inflammation.
Key Words: angiotensin II signaling pathway NF-
B MAP kinase inflammation
| Introduction |
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B (NF-
B), a key transcription factor that stimulates the transcription of chemokines, as well as other genes. Several groups have demonstrated that Ang II activates NF-
B in vivo and in vitro after binding to the Ang II type 1 (AT1) or AT2 receptor.68 Moreover, in an experimental model of atherosclerosis exhibiting increased inflammation, NF-
B activity is increased in vascular tissue; this response was diminished by an angiotensin-converting enzyme inhibitor.9
The mechanism by which Ang II activates NF-
B is unclear. The classical NF-
B activation model involves sequestration of NF-
B in the cytoplasm by inhibitor of
B (I
B), preventing NF-
B from migrating to the nucleus. When I
B is phosphorylated, it is degraded, permitting NF-
B to translocate to the nucleus. However, several reports suggest that Ang II can stimulate degradation of I
B, but the change in the level of I
B was small.7,8,10 A nonclassical model suggests that Ang II induces tyrosine phosphorylation of I
B
to promote its physical dissociation from NF-
B.11 In fact, there are observations suggesting that this or other nonclassical pathways are active. For example, the induction of NF-
B by UV light does not require phosphorylation of I
B
.12 There are also reports that NF-
B can be activated by phosphorylation of the NF-
B p65 subunit.13,14 A study by Bohuslav et al demonstrated that p65 can be phosphorylated in the nucleus. This change reduces the ability of I
B to bind to either a p65 homodimer or the p65/p50 heterodimer.15 Consequently, phosphorylated p65 can bind to NF-
B consensus sequences in a promoter.
In exploring how Ang II regulates the transcriptional activity of NF-
B, we found that inhibition of mitogen-activated protein kinase kinase-1 (MEK1) suppressed the transcriptional activity of NF-
B. We explored the cellular signaling pathway linking Ang II to MEK and then to stimulation of NF-
B transcriptional activity. This pathway involves Ras/MEK1/extracellular signal-regulated kinase (ERK)/ribosomal S6 kinase (RSK) and activates NF-
B in vascular smooth muscle cells (VSMCs) by phosphorylating the p65 subunit of NF-
B. Biologically, activation of this pathway contributes to Ang IIinduced interleukin-6 (IL-6) expression that has been implicated in vascular inflammation.
| Methods |
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-actin (Sigma-Aldrich).
Animals
Three-month-old male Sprague-Dawley rats were implanted with osmotic pumps (model 2001; Alza Corp). Animals in the Ang IItreated group received a pump filled with Ang II and delivered at a dosage of 500 ng/kg per minute in Ringers solution (0.01 mol/L acetic acid in saline). Another Ang IItreated group received antihypertensive therapy hydralazine (10 mg/kg per day) in drinking water. Control animals received a pump with Ringers solution without Ang II. Systolic blood pressure (SBP) was measured by tail-cuff plethysmography method (Visitech Systems Inc). SBP values were derived from an average of 6 to 8 measurements per animal at each time point. Each group of 6 rats was pair fed for 7 days and perfuse-fixed with 4% paraformaldehyde. The thoracic aorta was embedded in paraffin and examined.
NF-
B Luciferase Reporter Assay
NF-
B transcriptional activity was evaluated by using an adenovirus NF-
B-Luc luciferase reporter (Ad.NF-
B-Luc). Briefly, cells were infected with Ad.NF-
B-Luc at a multiplicity of infection of 5 for 24 hours before 100 nmol/L Ang II treatment; from each sample, 5 µg of total protein was assayed for luciferase activity using the manufacturer protocol (Promega).
Electrophoretic Mobility Shift Analysis
Nuclear extracts were prepared as described16 and incubated with an NF-
B oligonucleotide (Promega). The gel shift analysis was performed at least 3x.
Agarose Oligonucleotide Pull-Down Assay
The NF-
B sequence from -192/-172 of intercellular adhesion molecule (ICAM) with 3-terminal biotinylation (5' GTTCCACGGCACCCCCTG3') and its complementary strand (5' CCTGCAGGGGGTGCCGTG 3') were synthesized (BioSource International). After annealing, the double-stranded oligonucleotide was incubated with streptavidin-conjugated agarose beads (Pierce) for 1 hour at 4°C and then washed twice with capsid lysis buffer (CLB) buffer (20 mmol/L Tris-HCI, pH 7.5, 2 mmol/L EDTA, 2 mmol/L EGTA, 100 µg/mL aprotinin, 10 mmol/L benzamidine, 5 mmol/L dithioerythritol [DTT], 1 mmol/L phenylmethylsulfonyl fluoride (PMSF), 100 µg/mL leupeptin, 50 mmol/L NaF, 5 mmol/L Na4P2O7, 1 mmol/L Na3VO4, and 1% Nonidet P-40). Nuclear protein extract (20 µg per sample) was suspended in 300 µL of CLB after being precleared with agarose beads for 1 hour at 4°C to remove nonspecific binding to the beads. Lysates were incubated with the -192/-172 ICAM beads for 1 hour at 4°C, and beads were washed 3x with CLB buffer and eluted by boiling in Laemmli buffer for 10 minutes. Proteins were separated using a sodium dodecyl sulfate (SDS)polyacrylamide gel, transferred to a polyvinylidene fluoride membrane, and immunoblotted with anti-p65 antibody (1:1000).
Ras Activation
Ras activity was determined by measuring binding of only the activated form of Ras to the protein binding domain (PBD) of Raf (Upstate). A total of 200 µg of cell lysates from control or Ang II (100 nmol/L)treated VSMCs were incubated with glutathione S-transferase (GST)Raf-1PBD agarose beads to precipitate activated Ras. After separation using SDS-PAGE, activation of Ras by Ang II was detected by Western blotting using an anti-Ras monoclonal antibody (Upstate).
RSK Kinase Assay
VSMCs were cultured to 95% confluence, serum starved for 24 hours, and then treated for 5 minutes by adding 100 nmol/L Ang II. RSKs were immunoprecipitated by phospho-RSK antibody (Cell Signaling), and its kinase activity in this immune complex was measured by using recombinant p65 as substrate. Briefly, kinase activity was determined in 50 µL by incubating RSK immune complex with GST-p65 for 30 minutes at 30°C in a mixture (20 mmol/L 3-(N-morpholino)propanesulfonic acid, pH 7.2, 25 mmol/L ß-glycerol phosphate, 5 mmol/L EGTA, 1 mmol/L Na3VO4, 1 mmol/L DTT, 2 µmol/L protein kinase A inhibitor peptide, 1 µmol/L microcystin-LR, 15 mmol/L MgCl2, 100 µmol/L cold ATP, and 10 µmol/L
-32P ATP). The reaction was stopped by adding 2x loading buffer and immediately boiled before being separated on a 10% SDS-PAGE.
RNase Protection Assay
Expression of IL-6 mRNA was investigated by RNase protection assay (RPA) by using a rat cytokine probe set (rCK-1; Pharmingen). A total of 5 µg of total RNA, prepared by Tri Reagent (Sigma-Aldrich), was hybridized with 5x105 cpm of
-P32labeled probe.17
Statistical Analysis
Statistical comparisons were made using 2-tailed Student t test. Experimental values were reported as means±SE. Differences in mean values were considered significant at P<0.05.
| Results |
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B,10,18 after mutating the IL-6 promoter, and thus eliminating an NF-
B site in an IL-6 promoter luciferase construct, Ang II no longer stimulates IL-6 luciferase reporter activity.10 We also found that Ang II can increase IL-6 mRNA expression 3-fold above control within 1 hour in VSMCs, measured by RPA (Figure 1A). To ask whether Ang II regulates IL-6 in vivo, we infused Ang II into rats via osmotic minipumps for 7 days. Infusion of Ang II (500 ng/kg per minute) increased the SBPs compared with sham infusion at day 3 (153±9 mm Hg versus 135±7 mm Hg; n=6; P<0.05) and day 7 (178±14 mm Hg versus 132±8 mm Hg; n=6; P<0.01). At day 7, the thoracic aorta were perfuse-fixed and processed for immumohistochemical staining of IL-6. The immunostaining indicated that IL-6 expression in aorta is increased (Figure IA, available online at http://atvb.ahajournals.org) compared with control rats (Figure IB). We then tested whether the increased IL-6 expression is caused by hypertension. Ang IIinfused rats were treated with the antihypertensive drug hydralazine, and blood pressure dropped from 178±14 mm Hg (Ang II infusion) to 138±9 mm Hg (n=6; P<0.01; Ang II with hydralazine) at day 7, but the increase in IL-6 expression in the aorta was essentially unchanged (Figure IC). Thus, Ang II elicits an inflammatory response in vascular cells and artery.
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Ang II Induces NF-
B Transcriptional Activity
Several reports suggest that Ang IIinduced IL-6 expression is mediated by activation of NF-
B.10,18 We examined this pathway and found that Ang II increases NF-
B luciferase activity in VSMCs that were infected with an NF-
BLuc reporter minigene (Figure 1B). Luciferase activity increased in an Ang II dose-dependent fashion (Figure 1C), but the ability of Ang II to stimulate NF-
Bdependent transcription was less than that of tumor necrosis factor-
(TNF-
; Figure 1B). Analysis of NF-
B binding activity by electrophoretic mobility-shift assay (EMSA) using nuclear extracts from VSMCs that had been treated with Ang II for 10 to 30 minutes revealed a significant increase in NF-
B binding activity versus the basal activity in nonstimulated cells (Figure 1D). The specificity of NF-
B binding activity was confirmed because either competition with a 100-fold molar excess of unlabeled probe or use of a mutated probe eliminated binding (data not shown).
Ang IIInduced Activation of NF-
B Does Not Associate With Significant Downregulation of I
B
The classical pathway for NF-
B activation involves phosphorylation and proteolytic degradation of the I
B isoforms I
B
and I
Bß. We found that incubating VSMCs with 100 nmol/L Ang II caused minimal phosphorylation and degradation of I
B
, and we also detected very little decrease in I
Bß level (Figure 2A). In sharp contrast, incubation with TNF-
(10 ng/mL) caused significant I
B
phosphorylation at ser32 and decreased the I
B
protein level (Figure 2B). These results indicate that the classical pathway of phosphorylation and degradation of I
B
is not a major pathway by which Ang II activates NF-
B.
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Ang II Induces p65 Binding to NF-
B Consensus Site and p65 Phosphorylation at Ser536
NF-
B binding to its consensus sequence stimulated by Ang II could arise from increased nuclear translocation of NF-
B or from reduced binding of I
B with NF-
B. Treating of VSMCs with Ang II caused a minimal increase in nuclear translocation of p65 (Figure II, available online at http://atvb.ahajournals.org) compared with TNF-
stimulation; this finding is consistent with a minimal decrease in cytoplasmic I
B (Figure 2A). However, we did find a significant increase in p65 from nuclear proteins that bound to an NF-
B consensus site (Figure 3A). Others have demonstrated that phosphorylation of p65 reduces the ability of I
B to bind a p65 homodimer or the p65/p50 heterodimer in cell nuclei with enhanced binding of p65 to NF-
B consensus sequences.15 As shown in Figure 3B, Ang II increased phosphorylation of p65 at ser536 in VSMC nuclei; this phosphorylation was time- and Ang II concentrationdependent (Figure 3B and 3C). These results suggest that Ang II causes phosphorylation of p65 that increase NF-
B transcriptional activity.
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Mitogen-Activated Protein Kinase Participates in the Ang IIInduced NF-
B Activity and Increases IL-6 mRNA Expression
What is the signaling system by which Ang II induces NF-
B transcriptional activity? Ang II treatment induced phosphorylation of ERK in <5 minutes (Figure IIIA, available online at http://atvb.ahajournals.org). Inhibition of MEK1 with 20 µmol/L U0126 blocked Ang IImediated activation of NF-
B reporter activity by 60.9±9.05% (n=4; P<0.01; Figure 4A). U0126 also suppressed Ang IIinduced p65 phosphorylation (Figure 4B). To examine the specificity of MEK1 in this process, we used an adenovirus vector containing a dominant-negative MEK1 to block MEK1 activity in VSMCs. Under these conditions, NF-
B transcriptional activity stimulated by Ang II was reduced by 54±7.15% (n=4; P<0.001) versus infection with Ad.ß gal (empty vector control; Figure IIIB). Inhibition of MEK1 also blocked binding of NF-
B to DNA, nuclear p65 binding (data not shown), and p65 phosphorylation (Figure IIIC). Finally, blocking MEK1 with U0126 blocked 70% of Ang IIinduced IL-6 mRNA in VSMCs (Figure 1A). These data indicate that the MEK1 pathway is involved in Ang IIinduced NF-
B activation and IL-6 transcription.
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Ras Plays a Role in Ang IIInduced NF-
B Activity
To link Ang II to MEK1 activation, we examined the role of Ras, which can activate MEK1. First, we evaluated Ang IIinduced Ras activation by a nonradioactive assay that measures binding of activated Ras to the PBD domains of Raf. Compared with values from nonstimulated VSMCs, Ang II stimulated activation of Ras as early as 1 minute (Figure 5A). We also infected VSMCs with a recombinant adenovirus that encodes a dominant-negative form of Ras (Ad.Ras Y57). As expected, infection with Ad.Ras Y57 resulted in overexpression of a mutant Ras that blocked the ability of Ang II to activate the downstream target of Ras: ERK (Figure 5B). It also blocked Ang IIstimulated NF-
B promoter activity (Figure 5C) and serine phosphorylation of p65 (Figure 5B).
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RSK Mediates NF-
B Activation in Response to Ang II Treatment
To identify the kinase that is downstream of the MEK1 pathway, we examined the downstream target of ERK: RSK. RSK has been shown to activate NF-
B after phorbol ester treatment.19 Ang II treatment induced phosphorylation of RSK at ser380 (Figure IVA, available online at http://atvb.ahajournals.org) in a dose-dependent manner (Figure IVB). Inhibition of MEK1 activity by U0126 blocked phosphorylation of RSK and p65 (Figure 6A). Next, we knocked down 80% of RSK by transfecting VSMCs with 4 small interfering RNA (SiRNA) vectors for RSK1 and RSK220 (Figure 6B). There was a 63.9±3.3% (n=5; P<0.01) reduction of NF-
B activation after Ang II treatment and an almost complete inhibition of p65 serine phosphorylation (Figure 6C). This result indicates that RSK activation participates in the phosphorylation of p65 and ultimately increased NF-
B activity. To determine whether RSK can directly phosphorylate p65, we immunoprecipitated RSK from Ang IItreated or untreated VSMCs and incubated it with a wild-type GST-p65 fusion protein or an ser536-mutated GST-p65 protein. As depicted in Figure 6D, RSK from Ang IItreated cells caused a 3- to 4-fold induction of p65 phosphorylation over control values. In contrast, the serine536-mutated p65 was not phosphorylated in response to Ang II.
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| Discussion |
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B in VSMCs.21 Because inflammation plays a role in the development of atherosclerosis, we decided to study mechanisms that increase activation of NF-
B and the expression of proinflammatory cytokines. We chose to study the proinflammatory cytokine IL-6 because results from in vivo and in vitro studies have demonstrated that IL-6 is present in atherosclerotic plaques22 and that Ang II, a mediator of atherosclerosis, can increase IL-6 expression through activation of NF-
B.10,18 Our results show that Ang IIinduced NF-
B transcriptional activity is mediated by a nonclassical pathway. NF-
Bmediated transcription rises in response to stimulation of the Ras/MEK1/ERK/RSK pathway. RSK phosphorylates p65 of NF-
B in nuclei of Ang IItreated VSMCs, leading to increased binding of p65 to NF-
B binding sequence.
We explored this nonclassical pathway because we found that Ang IIinduced activation of NF-
B transcription in VSMCs could not be attributed to phosphorylation and degradation of I
B (Figure 2A). Others have reported that there is weak degradation of I
B in response to Ang II,7,8,10 and there are reports that other stimuli such as UV light, hypoxia, and reoxygenation will activate NF-
B via nonclassical pathways.12,23 For example, induction of NF-
B by UV light does not require activation of I
B kinases (IKKs) or phosphorylation of I
B
on serines 32 and 36;12 hypoxia, reoxygenation, or treatment with the tyrosine phosphatase inhibitor pervanadate causes tyrosine phosphorylation of I
B
, which promotes its physical dissociation from NF-
B and consequent activation of NF-
B.23
An interesting result was the requirement for involvement of the Ras/MEK1/ERK signaling pathway in the induction of NF-
B transcriptional activity by Ang II. For example, when MEK1 was inhibited, there was a significant reduction in NF-
B transcriptional activity and expression of IL-6 in VSMCs treated with Ang II (Figures 1A and 4A
). Presumably, a similar pathway is activated in airway inflammation, where stimulation of MAP kinase plays an important role in activation of NF-
B transcriptional activity.24
Although the pathway from MEK1 to ERK is defined, it was not clear initially how ERK could activate NF-
B without phosphorylating I
B. We found that Ang II increases the kinase activity of RSK, leading to direct phosphorylation of p65 and increased NF-
B transcriptional activity. In support of this sequence of events, we found that RSK phosphorylates the p65 component of NF-
B in an in vitro biochemical assay (using recombinant GST-p65; Figure 6D). Phosphorylation of the p65 subunit occurs in other settings. For example, p53-induced, RSK1-mediated phosphorylation of p65 reduces the affinity of p65 for the NF-
B inhibitor I
B; it also decreases I
B-mediated export of shuttling forms of NF-
B from the nucleus, thereby promoting binding of NF-
B to cognate
B enhancers.15 Indeed, our data show that Ang II does not increase translocation (Figure II) but increases p65 phosphorylation and p65 binding to NF-
B consensus sequence in the nuclei (Figure 3A and 3B)
Our evidence for a nonclassical Ras/MEK1/ERK pathway that is activated by Ang II includes a suppression of RSK and NF-
B transcriptional activity when we inhibited components of the pathway (Ras and MEK1). Specifically, inhibition of components of the Ras/MEK1/ERK pathway led to a decrease in RSK phosphorylation, p65 phosphorylation, and NF-
B luciferase reporter activity, plus suppressed expression of IL-6. The importance of p65 phosphorylation was also demonstrated when we downregulated RSK in VSMCs by SiRNAs; the serine phosphorylation of p65 and Ang IIactivated NF-
B activity were reduced (Figure 6B and 6C).
Notably, suppression of MEK1/RSK inhibited Ang IIinduced NF-
B activation and expression of IL-6 by only
70%. The remaining 30% of Ang IIinduced NF-
B activity may involve another pathway, such as production of reactive oxygen species (ROS) or other kinases. This possibility is raised because Pueyo et al found that antioxidants will diminish Ang IIinduced NF-
B activation,25 and we have found that Ang II could activate IKK but did not phosphorylate I
B by the classic pathway (Zhang et al, unpublished data, 2005). The contribution of ROS to activation of p65 and NF-
B is presently unknown. Previous studies have shown that AT1 and AT2 receptors are involved in Ang II activation of NF-
B.6,7 The role of these receptors in Ras/MEK1/RSK-mediated phosphorylation of p65 remains to be determined.
In summary, we have uncovered a new pathway by which Ang II activates NF-
B in VSMCs. It is I
B independent and proceeds through a MEK1-dependent RSK1-mediated p65 phosphorylation. This signaling pathway differs from the pathways stimulated by proinflammatory cytokines such as TNF-
and, hence, elucidates a mechanism for Ang IIinduced inflammatory responses. The actions of MEK1/RSK could provide a potential therapeutic target for cardiovascular diseases.
| Acknowledgments |
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Received January 10, 2005; accepted March 21, 2005.
| References |
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