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* This work was supported by the Health Research Board, Ireland, Enterprise Ireland, The Wellcome Trust, European Commission FP6 Grant LSHM-CT-2004-0050333, Science Foundation Ireland, and The Government of Ireland Programme for Research in Third Level Institutions. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement”in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3 and supplemental text. 1 Present address: Dept. of Pharmacology, New York University School of Medicine, New York, NY 10016.
Mesangial cell proliferation is pivotal to the pathology of glomerular injury in inflammation. We have previously reported that lipoxins, endogenously produced eicosanoids with anti-inflammatory and pro-resolution bioactions, can inhibit mesangial cell proliferation in response to several agents. This process is associated with elaborate receptor cross-talk involving modification receptor tyrosine kinase phosphorylation (McMahon, B., Mitchell, D., Shattock, R., Martin, F., Brady, H. R., and Godson, C. (2002) FASEB J. 16, 1817–1819). Here we demonstrate that the lipoxin A4 (LXA4) receptor is coupled to activation and recruitment of the SHP-2 (SH2 domain-containing tyrosine phosphatase-2) within a lipid raft microdomain. Using site-directed mutagenesis of the cytosolic domain of the platelet-derived growth factor receptor β (PDGFRβ), we report that mutation of the sites for phosphatidylinositol 3-kinase (Tyr740 and Tyr751) and SHP-2 (Tyr763 and Tyr1009) recruitment specifically inhibit the effect of LXA4 on the PDGFRβ signaling; furthermore inhibition of SHP-2 expression with short interfering RNA constructs blocked the effect of LXA4 on PDGFRβ phosphorylation. We demonstrate that association of the PDGFRβ with lipid raft microdomains renders it susceptible to LXA4-mediated dephosphorylation by possible reactivation of oxidatively inactivated SHP-2. These data further elaborate on the potential mechanisms underlying the anti-inflammatory, proresolution, and anti-fibrotic bioactions of lipoxins.
There is a growing appreciation that novel, endogenously produced lipid mediators can regulate inflammatory processes and their resolution (
). MC proliferation is a prototypic inflammatory response coupled to matrix accumulation and further production of cytokines that may culminate in the onset of many subacute and chronic forms of glomerulonephritis and eventual fibrosis (
). PDGFRβ activation leads to increased tyrosine phosphorylation and recruitment of Src homology 2 (SH2) domain-containing molecules to specific phosphotyrosine residues within the cytosolic domain of the receptor, initiating processes that promote cell proliferation and gene expression (
). Here we report that the ALXR may be coupled to reactivation of a protein-tyrosine phosphatase (PTP), SHP-2, that specifically dephosphorylates the recruitment sites of the p85 subunit of PI 3-kinase on the PDGFRβ.
Reagents, antibodies, plasmids/primers information, and cell culture techniques are described in the supplemental text.
Cell Transfection and Preparation of Lysates—Human embryonic kidney 293 (HEK293) cells and HEK293 cells stably overexpressing the HA epitope-tagged ALXR (designated ALXR cells) were transiently transfected with various plasmids using Genejuice® transfection reagent as per the manufacturer's protocol. Briefly, the cells were plated to 70% confluency, allowed to adhere for 24 h, and subsequently transfected with 1 μg of plasmid. 24 h post-transfection, the cells were serum-restricted in 0.01% FCS medium for 24 h prior to stimulation. For short interfering RNA (siRNA)-mediated knockdown of SHP-2 levels, ALXR cells were co-transfected with targeted SHP-2 PTP siRNA (sequence 5′-CCTCTGAAAGGTGGTTTCATGGACATCTCTCTGGGAAAGAAGCAGAGAAAT-3′, residues 703–753 of the SHP-2 sequence) (Qiagen) using RNAfect® transfection reagent as per the manufacturer's protocol. As a control, a nonspecific siRNA duplex with a random nucleotide sequence) (Qiagen) was used. Two additional siRNA sequences complementary to the SHP-2 gene were designed using custom SMARTPools from Dharmacon (Lafayette, CO) (Dharmacon catalogue numbers J-003947-09 and J-003947-10) and were transfected using Lipofectamine (Invitrogen) as per the manufacturer's protocol.
Immunoprecipitation and Immunoblotting—MCs were serum-restricted in 0.2% FCS MCDB131 for 48 h, whereas HEK293 and ALXR cells were serum-restricted in 0.01% FCS modified Eagle's medium for 24 h prior to exposure to various agents. The lysates were harvested in RIPA lysis buffer (20 mm Tris-HCl, pH 7.4, 50 mm NaCl, 5 mm ethylene diaminetetraacetic acid, 1% Nonidet P-40, 0.5% sodium deoxycholate, 0.1% SDS, 5 mm NaF, 1 mm phenylmethylsulfonyl fluoride, 1 mm Na3VO4,1 μm leupeptin, 0.3 μm aprotinin). The lysates were clarified by centrifugation at 14,000 rpm for 20 min, and the samples were normalized for total protein. Thereafter, the lysates were precleared with Protein G-Plus Agarose beads for 1 h at 4 °C. For MC lysates, PDGFRβ or epidermal growth factor receptor (EGFR) was immunopurified from 250 μg of precleared lysate with anti-PDGFRβ or anti-EGFR (both 1:100) overnight with constant rocking at 4 °C. For HEK293 or ALXR cell lysates, PDGFRβ (1:100), HA-tag (1:300), SHP-1, or SHP-2 (1:100) were immunopurified from 50–70 μg of precleared lysate overnight at 4 °C with constant rocking. Mouse IgG (1:100) was used as an antibody control. 10 μl of protein G-agarose beads were added to the protein antibody mixture, and the samples were rocked for a further 2 h at 4°C. Precipitated immunocomplexes were washed three times in fresh lysis buffer and boiled in sample buffer. The samples were subsequently resolved by SDS-PAGE, transferred onto polyvinylidene difluoride membrane, and probed for phosphotyrosine antigenicity (1:1000), total PDGFRβ/EGFR (1:1000), SHP-2 (1:2500), or HA tag ALXR (1:2000).
RTK Activity Assay—MCs were serum-restricted and exposed to various agents for 0–60 min. The cells were lysed, and PDGFRβ was immunoprecipitated as previously described. Thereafter, immunocomplexes were washed thrice in fresh RIPA buffer and once in kinase assay buffer (50 mm Tris-HCl, pH 7.5, 10 mm MgCl2, 1 mm dithiothreitol, 1 mm benzamidine, and 0.5 mm phenylmethylsulfonyl fluoride) before resuspending in 25 μl of kinase assay buffer. 10 μCi of [γ-32P]ATP was subsequently added, and the reactions were incubated for 30 min at 37 °C. Immunocomplexes were then denatured in sample buffer and were resolved by SDS/PAGE. After electrophoresis, the gels were dried and subsequently subjected to autoradiography. Autoradiograms were scanned using Chemidoc Quantity One software (Bio-Rad). Normalization of data were achieved by an anti-PDGFRβ immunoblot using a small amount of each sample collected after the immunoprecipitation stage.
Analysis of SHP-2 Activity—SHP-2 activity was measured in SHP-2 immunoprecipitates using a Malachite Green PTP activity assay (Upstate Biotechnology Inc.) according to the manufacturer's instructions. Briefly, SHP-2 was immunoprecipitated from 200–300 μg of MC lysate using a specific monoclonal SHP-2 antibody (BD Transduction Laboratories) for 3 h at 4 °C. Protein G-agarose beads were subsequently added to each immunoprecipitate and incubated for further 2 h at 4°C with constant rocking. Immunocomplexes were washed three times in fresh lysis buffer and then twice in the phosphatase assay buffer (Tris-HCl, 50 mm, pH 7.0, 0.1% bovine serum albumin) before finally resuspending in 100 μl of assay buffer. To each sample, phosphopeptide substrate was added, and the samples were incubated for 30 min at 37 °C. The reactions were terminated by the addition of 100 μl of malachite green solution, allowing 15 min for color development followed by spectrophotometric analysis at 650 nm. The results were expressed as pmol of phosphate released per microgram of protein.
Sucrose Flotation Gradients—ALXR cells seeded in 150-mm plates were washed twice with ice-cold phosphate-buffered saline and scraped into 1 ml of phosphate-buffered saline. Thereafter, the cells were harvested and lysed with 1 ml of TNE buffer (50 mm Tris-HCl, pH 7.4, 150 mm NaCl, 5 mm EDTA, 1%Triton X-100, 1× Complete™ protease inhibitor mixture) for 30 min at 4 °C. The lysates were subsequently homogenized with 15 strokes of a Dounce homogenizer, transferred to a microcentrifuge tube, and centrifuged at 4 °C for 10 min at 830 × g. 1 ml of the resultant cell supernatant was mixed with an equal volume of 80% (w/v) sucrose in TNE buffer, transferred to a 5-ml ultracentrifuge tube, and then carefully overlaid with 2 ml of 30% (w/v) sucrose in TNE buffer and finally with 1 ml of 5% (w/v) sucrose in TNE buffer. The tube was centrifuged at 4 °C for 17 h at 134,400 × g. Eight fractions (600 μl) were collected from the top to the bottom of the gradient. Protein from each fraction was precipitated using a standard methanolchloroform protocol. The resulting precipitates were resuspended in reducing sample buffer, heated to 90 °C for 5 min, and resolved on an SDS/PAGE gel or stored at -80 °C until required.
Detection of Reactive Oxygen Species—Reactive oxygen species (ROS) were detected in MCs stimulated with various agents using an Image-iT™ LIVE Green ROS detection kit. Briefly, subconfluent cultures of MC were plated on 8-well chamber slides and serum-restricted in 0.2% FCS for 48 h. Thereafter, MCs were treated with vehicle (30 min), PDGF-BB (10 ng/ml, 30 min), LXA4 (10 nm, 30 min), or with LXA4 (10 nm, 30 min) followed by PDGF-BB (10 ng/ml, 30 min). The cells were washed once with warm Dulbecco's phosphate buffered saline (DPBS) solution. Carboxy-H2DCFDA (10 μmol/liter) was added to each well, and the cells were further incubated for 30 min at 37 °C. In addition, the nuclei were countered stained with a Hoechst 33342 nuclei stain (1 μm) for 5 min at 37 °C. Subsequently, the slides were gently immersed and washed three times in a warm DPBS solution, after which one drop of Slowfade® Light Antifade solution was added before placing a coverslip over the wells. The fluorescence of the oxidized form of carboxy-H2DCFDA was visualized using a M.1 Zeiss microscope, and the images were captured using an Axiocam system and Axiovision Rel 4.4 software (Carl Zeiss).
Data Analysis—Statistical analysis was carried out using the unpaired Student's t test. All of the values are expressed as either the means ± S.D. or the means ± S.E. p values of less than or equal to 0.05 were considered to indicate a statistically significant difference.
Treatment of MCs with LXA4 resulted in a concentration-dependent (100 pm to 100 nm) decrease in activated PDGFRβ upon stimulation with PDGF-BB. (The EC50 was ∼10 nm LXA4; data not shown). Similarly, the amount of immunodetected PDGFRβ in phosphotyrosine immunoprecipitates was decreased by 52 ± 8% following preincubation with LXA4 (10 nm) prior to stimulation with PDGF (Fig. 1A, inset). To investigate whether altered receptor phosphorylation reflected decreased autokinase activity or possible phosphatase activation, the effect of LXA4 on RTK activation was investigated. As shown in Fig. 1B, LXA4 did not impact receptor tyrosine kinase activity.
). Interestingly, we have found that Ac2-26 mimics the effect of LXA4 on PDGFR activation (Fig. 1C). In addition, we investigated whether LXA4 could impact EGFR activation, another important pathologic event in renal dysfunction (
). As shown in Fig. 1D, preincubation of MC with LXA4 (10 nm) caused a decrease in the EGF-activated tyrosine phosphorylation of the EGFR. Similarly, specific PDGFR and EGFR protein-tyrosine kinase inhibitors AG1296 and AG1478 inhibited PDGF- and EGF-induced activation of the PDGFRβ and EGFR, respectively (Fig. 1, C and D).
To establish the molecular basis of ALXR-PDGFRβ cross-talk, we generated a HEK293 cell line stably overexpressing the ALXR (ALXR cells) in a PDGFRβ null background. Thereafter, we examined the effect of LXA4 on PDGFRβ phosphorylation in ALXR and control cells that were transiently transfected with PDGFRβ. Consistent with data generated in MCs, LXA4 significantly reduced PDGF-BB stimulated PDGFRβ phosphorylation (Fig. 2A). In contrast, whereas stimulation of HEK293 cells transiently transfected with PDGFRβ resulted in PDGFRβ phosphorylation, preincubation of those cells with LXA4 had no significant effect on receptor phosphorylation (Fig. 2B). These data indicate that the observed effects of LXA4 are mediated via the ALXR.
To define the locus/loci at which LXA4 modulates growth factor receptor tyrosine phosphorylation and subsequent mitogenic events, a series of PDGFRβ mutants were generated whereby specific tyrosines were substituted for phenylalanine at positions 579, 771, 740/751, and 763/1009. Although stimulation of ALXR cells transiently expressing Y579F and Y771F mutants displayed a clear increase in receptor phosphorylation when stimulated with PDGF-BB, preincubation with LXA4 reduced the extent of phosphorylation similar to that of vehicle (Fig. 2, C and D). These residues are specific for recruitment of Src and Ras GTPase-activating protein, respectively (
), and therefore we concluded that LXA4 was not acting at these loci. In contrast, mutation of the PDGFRβ binding sites for p85 recruitment (Y740F/Y751F) blocked the effect of LXA4 on receptor phosphorylation (Fig. 2E). Interestingly, the effect of LXA4 in cells expressing the Y740F alone was maintained, whereas in cells expressing Y751F, the effect of LXA4 was partially reduced (supplemental data). These data suggest that the high affinity binding site for the recruitment of p85 (Tyr751) is the site most likely to be modulated by LXA4, although both sites must be mutated before a complete block in the effect of LXA4 is observed. These data are particularly noteworthy given that p85 is a component of the PI 3-kinase complex, and our previous data have demonstrated that LXA4 inhibits MC PI 3-kinase activity in response to leukotriene D4 and PDGF, respectively (
). Consistent with our observations in ALXR cells expressing the Y740F/Y751F mutant, PDGF-induced phosphorylation of Tyr751 was significantly inhibited by preincubation with LXA4 in primary MC as assessed by immunoblot analysis using an antibody specific against PDGFRβ-Y751 phosphorylation (supplemental data).
The phosphorylation status and signaling activity of RTKs is determined not only by the kinase activity of the RTK but also by the activities of PTPs (
). Herein, the attenuation of RTK tyrosine phosphorylation levels, without alteration in RTK activity (Fig. 1B), suggests the involvement of PTPs in LXA4 ability to modulate PDGFRβ phosphorylation. Consistent with this hypothesis, the observed reduction in phosphorylation of the PDGFRβ in MC in response to preincubation with LXA4 was abolished by the broad spectrum, nonspecific PTP inhibitors vanadate and phenylarsine oxide (Fig. 3A). Interestingly, incubation of MC with the inhibitor mixture alone resulted in a 4-fold maximal increase in basal activation of the PDGFRβ (data not shown), suggesting a tonic inhibition of RTK activation by PTPs in quiescent MC. To test whether SHP-2 contributes to LXA4 modulation of PDGF-induced PDGFRβ phosphorylation, ALXR cells were transfected with a double mutant for the recruitment sites for SHP-2 PTP (Y763F/Y1009F). Consistent with the non-specific PTP inhibitor data, LXA4 (10 nm, 30 min) had no effect on PDGF-induced receptor phosphorylation (Fig. 3B), suggesting that LXA4 may require the recruitment of SHP-2 PTP to the PDGFRβ to exert its effect.
To further investigate the role of SHP-2 on LXA4 modulation of PDGF-induced PDGFRβ phosphorylation, specific SHP-2 activity was measured in SHP-2 immunoprecipitates from differentially treated MC. As seen in Fig. 3C, LXA4 induced significant increases in SHP-2 activity. Consistent with previous reports (
), PDGF significantly increased SHP-2 activity; however, the kinetics at which it did so were considerably different to LXA4. Maximal activation in response to PDGF was observed at 5 min, after which no significant activation was detected. In contrast, LXA4 stimulated a prolonged activation of SHP-2 (Fig. 3C). To evaluate the role of the ALXR in the activation of SHP-2 in response to LXA4, MCs were treated with an ALXR antagonist, Boc-2 (
) prior to treatment with LXA4. Pretreatment with Boc-2 inhibited the SHP-2 activation in response to LXA4 (Fig. 3D).
Collectively, our data suggest that LXA4 regulation of PDGFRβ activation is mediated via ALXR activation of SHP-2. To explore this hypothesis directly, we investigated whether inhibition of SHP-2 expression could block the effect of LXA4 on PDGFRβ activation in ALXR cells. To do so, we used SHP-2-specific siRNA to silence the expression of SHP-2 by RNA interference. Treatment of ALXR cells with siRNA for 0–96 h resulted in strong reduction of SHP-2 levels relative to non-treated cells or cell treated with scrambled siRNA (Fig. 4A), with maximal inhibition at 50 nm occurring after 48 h. Thereafter, we compared the ability of LXA4 to modulate PDGFRβ activation in cells transfected with SHP-2 siRNA and in cells transfected with scrambled siRNA (Fig. 4C). Importantly these effects on inhibition of SHP-2 expression were also seen with other SHP-2 siRNA constructs (supplemental Fig. S3). Consistent with Y740F/Y751F mutant data, LXA4 had no effect on PDGF-induced receptor phosphorylation in SHP-2 knock-out ALXR cells, whereas the effect of LXA4 was maintained in ALXR cells transfected with scrambled siRNA (Fig. 4C and supplemental data).
Several RTKs and GPCRs as well as their downstream effector proteins have been reported to be present in or recruit to lipid rafts following activation. Indeed, there is a growing body of evidence that caveolae-containing lipid rafts may provide platforms for specific signaling molecules (
). To investigate whether such microdomains are implicated in ALXR-PDGFRβ cross-talk, we used a cholesterol-binding agent, methyl-β-cyclodextrin, to sequester cholesterol and cause disassembly of cholesterol-rich rafts and caveolae. Incubation of MC with 10 mm methyl-β-cyclodextrin for 30 min did not alter cell morphology or integrity as measured by trypan blue exclusion (data not shown). Fig. 5A shows that LXA4-dependent dephosphorylation of the PDGFRβ was significantly reduced and approached nontreated, PDGF-stimulated levels after depletion of plasma membrane cholesterol. In contrast, methyl-β-cyclodextrin did not significantly decrease PDGF-BB-stimulated PDGFRβ phosphorylation (Fig. 5A), implying that it is the transinactivation process that requires cholesterol-rich microdomains. To examine whether the apparent importance of cholesterol for LXA4 activity reflects the association of the ALXR with lipid microdomains, we used sucrose gradient centrifugation to separate detergent-resistant rafts, which include caveolae, from ALXR cell lysates. Immunoblotting of the eight different fractions obtained revealed that fraction 3 was highly enriched in both caveolin-1 and flotillin-1 (Fig. 5B), whereas proteins such as clathrin and β-actin, which are thought not to reside within caveolae in situ, localized to fractions 4–8 (bulk plasma membrane). Importantly, fraction 3 contained the highest levels of the ALXR, whereas significant levels of transfected PDGFRβ and SHP-2 are also shown to be present (Fig. 5B).
To investigate whether the LXA4-driven dephosphorylation of the PDGFRβ is localized to the lipid raft fraction, we isolated the soluble (fraction 7) and lipid raft fractions (fraction 3) from the sucrose gradient and immunoblotted for the aforementioned proteins. The PDGFRβ in both fractions was phosphorylated in response to PDGF-BB treatment, but only the PDGFRβ in the lipid raft fraction was dephosphorylated in response to preincubation with LXA4 (Fig. 5C). This reduction in PDGFRβ phosphorylation in response to LXA4 was accompanied by concomitant increases in levels of SHP-2 within the lipid raft fraction. Interestingly, SHP-2 levels in the soluble fraction were increased in response to PDGF-BB (Fig. 5C), suggesting that the different ligands activate different subpopulations of the PTP.
Previous reports have suggested that the interaction between PTPs and GPCRs/RTKs might be controlled by regulated distribution of PTPs to lipid rafts (
). To investigate possible recruitment of SHP-2 to the lipid raft-bound ALXR, we immunoprecipitated the HA-tagged ALXR from the lipid raft (Fraction 3) and soluble (Fraction 7) fractions of ALXR cells and immunoblotted for SHP-2. As Fig. 5D demonstrates, SHP-2 was constitutively associated with the ALXR in the lipid raft fraction (fraction 3) of quiescent ALXR cells, whereas no association was seen within the soluble (fraction 7) fraction or with the mock-transfected HEK293 cell lysate control. Moreover, the SHP-2 association seen in the lipid raft fraction was increased upon activation of the receptor by LXA4 and returned to basal levels 30 min post-stimulation. In contrast, no SHP-1 recruitment to the ALXR was observed in either the lipid raft fraction (fraction 3) or in the soluble fraction (fraction 7) (data not shown).
To understand the molecular mechanism by which PDGF signaling is down-regulated by ALXR activation of SHP-2 within lipid microdomains, we analyzed endogenous H2O2 production, which has been shown to mediate site-selective amplification of PDGFR phosphorylation (
). Recent studies have implicated the involvement of a cellular protease, 2-Cys peroxiredoxin type II (Prx II), in the down-regulation of PDGF signaling via a decrease in localized endogenous H2O2 production (
). To assess the role of LXA4 within this signaling system, we examined the induction of ROS in MC. Upon PDGF stimulation, LXA4-pretreated cells exhibited a significant reduction in the amount of ROS released compared with MC treated with PDGF alone, as measured by reduced 5-(and-6)-carboxyl 2′,7′dichlorodihydrofluorescein diacetate oxidation (Fig. 6A). Whereas MC exhibited significant PDGF-induced Prx II protein expression, pretreatment of cells with LXA4 prior to PDGF stimulation reduced Prx II expression to levels that were not significantly different from those generated by nontreated cells (Fig. 6B).
The mechanisms underlying the resolution of glomerular inflammation remain enigmatic. However, it has been proposed that regulation of MC proliferation by LX may play a key role (
We have previously demonstrated that the anti-proliferative effects of LXA4 are mediated through activation of the ALXR and result in modulation of growth factor (PDGF and EGF) receptor phosphorylation leading to down-regulation of the PI 3-kinase signaling pathway (
). Here, we report that the ability of LXA4 to reduce the tyrosine phosphorylation level of the PDGFRβ is independent of inhibition of RTK activity but dependent on SHP-2 PTP activation. Moreover, we demonstrate that the effect of LXA4 is replicated by a low affinity agonist of the ALXR, Ac2-26, (Kd for Ac2-26 is 0.9 μmversus 1.7 nm for LXA4), a peptide derivative of annexin 1, and that these effects are not restricted to the PDGFR signaling pathway but encompass other pleiotropic RTK such as the EGFR.
The PDGFRβ is activated by binding of PDGF and undergoes autophosphorylation at multiple tyrosine residues. The tyrosine-phosphorylated receptor associates with numerous SH2 domain-containing signal transduction molecules that include phospholipase Cγ1, the GTPase-activating protein of Ras, the p85 subunit of PI 3-kinase, the phosphotyrosine phosphatase SHP-2, and several other proteins (
). Indeed, individual mutation of these specific tyrosine residues prevents association of its cognate effectors without altering the binding of other enzymes to the receptor. Thus, this phenomenon has afforded us a convenient method to assess the role of LXA4 transinhibition on the receptor-associated effector-binding sites of the PDGFRβ in HEK293 cells overexpressing both the ALXR and wild type/mutant PDGFRβ receptors. Our results suggest that, among the sites for binding of effector molecules (i.e. PI 3-kinase, Ras GTPase-activating protein, Src kinase, and SHP-2), only mutation of the sites for PI-3-kinase and SHP-2 affected the dephosphorylation of the PDGFRβ by LXA4 pretreatment. Our data indicate that the observed LXA4-mediated transinhibition of PDGFRβ does not reflect a diminution in receptor autokinase activity but a specific effect on particular phosphotyrosines residues.
SHP-2, a ubiquitously expressed PTP, is a critical intracellular regulator of cytokine and growth factor-induced cell survival, proliferation, and differentiation (
). SHP-2 binds to Tyr(P)763 and Tyr(P)1009 on activated PDGFRβ and in turn may dephosphorylate selective docking sites that partake in the down-regulation of receptor signaling, including Tyr(P)751 and Tyr(P)740 (p85 recruitment sites) (
). However, the effect of SHP-2 on the PDGFRβ is contradictory. Depending on the cell type and the nature of the agonist, SHP-2 either augments or antagonizes the signaling pathway initiated by activation of the PDGFRβ. For example, SHP-2 can act as a positive regulator for PDGF signaling by acting as an adapter between the PDGF receptor and the Grb2-Sos complex, leading to activation of the Ras signaling pathway (
). From our results, activation of SHP-2 PTP by LXA4 via the ALXR leading to reduced PDGFRβ phosphorylation is supported by multiple lines of evidence. First, the effect of LXA4 on PDGFRβ phosphorylation in MC was diminished in the presence of PTP inhibitors vanadate and phenylarsine oxide. Second, we detected sustained elevation of SHP-2 PTP activity in immunoprecipitates from LXA4-treated MC. This effect was mediated via the ALXR given that inclusion of the nonspecific ALXR antagonist, Boc-2, blocked LXA4-induced SHP-2 activation. Third, we demonstrate that mutation of the SHP-2 recruitment sites on the PDGFRβ results in an inhibition of the LXA4 effect and therefore suggests that SHP-2 is recruited to the PDGFRβ in response to activation by LXA4. Furthermore, siRNA-directed modulation of SHP-2 expression resulted in inhibition of the LXA4 effect on the PDGFRβ. Importantly, the EGFR, like the PDGFRβ, also contains tyrosine residues for the recruitment of SHP-2 (
), suggesting that multiple phosphorylated growth factor receptors may act as substrates for SHP-2 and in turn may be modulated by LXA4.
The temporal requirements of exposure to LXA4 for significant inhibition of phosphorylation of RTK (30–60 min) suggested that the effect may reflect RTK trafficking from the cell membrane (combined with PTP activation). Receptor down-regulation appears to be an important element in the control of cellular proliferation, because mutations that impair the ability of some receptors to internalize and degrade cause cell transformation (
). We have investigated the involvement of cholesterol-rich lipid microdomains in the LXA4 effect on the PDGFRβ. These microdomains participate in endocytosis and are thought to facilitate recruitment and organization of downstream effector molecules into specialized signaling complexes (
). We have employed a cholesterol-depleting agent to disrupt lipid rafts and demonstrate that intact lipid rafts are necessary for LXA4 transinhibition of the PDGFRβ. From our data, we identify that caveolae-enriched fractions of HEK293 cells contain both the ALXR and PDGFRβ, and we conclude that the ALXR requires association with lipid rafts to initiate both the recruitment of SHP-2 and the dephosphorylation of the PDGFRβ. The targeting of GPCRs and RTKs to lipid rafts following activation can occur through different mechanisms. The PDGFRβ is known to be recruited to lipid raft fractions upon ligand activation (
). Both the ALXR and PDGFRβ were found to reside in the lipid raft fraction in ALXR cells. However, we found no enrichment of either receptor in the lipid raft fraction in response to ligand activation. A previous study has identified distinct mechanisms by which SHP-2 is recruited to the cell membrane in a multiprotein complex that is distributed differently inside and outside lipid rafts (
). In our study we found increased levels of SHP-2 in the lipid raft fraction of ALXR cells in response to LXA4 treatment, whereas PDGF stimulated an increase of SHP-2 levels in the soluble fraction, suggesting that both ligands target different cellular pools of the PTP. Interestingly, only the lipid raft-associated PDGFRβ was found to be dephosphorylated in response to LXA4 pretreatment, indicating a specific deactivation process involving the lipid raft population of SHP-2.
In further studies, we uncovered a constitutive association of SHP-2 with the lipid raft-bound ALXR in HEK293 cells stably expressing the ALXR. This association was increased in response to ligand activation and returned to basal levels in a time-dependent manner. In this context, it is noteworthy that SHP-2 itself has been shown to be directly recruited and subsequently activated by the immunoreceptor tyrosine-based inhibitor motif (
) in the ALXR (supplemental data), located at the interface of the N-terminal part of the second transmembrane domain and the first intracellular domain. Previous studies have demonstrated that occupation of the SH2 domain of SHP-2 by a ligand may stimulate both subcellular relocalization of the enzyme to the vicinity of substrates or may also stimulate the phosphatase activity (
). Thus, we envision that SHP-2 is recruited to the ALXR in lipid microdomains via Tyr(P)-SH2 interaction, and subsequent ALXR dephosphorylation eventually leads to dephosphorylation of the SH2-binding domain-binding sites and disruption of the complex.
Numerous studies suggest that ROS such as O2. and H2O2 function as mitogenic mediators of activated growth factor receptor signaling such as the PDGFR (
) report that the mammalian Prx II allows oxidatively inactivated protein-tyrosine phosphatases to be reactivated by removing endogenous H2O2, resulting in decreased phosphorylation of key tyrosine residues in both PDGFRβ and the downstream signaling molecule phospholipase Cγ1 (
). In an effort to elucidate the molecular mechanism by which PDGF signaling is down-regulated by ALXR activation of SHP-2, we examined the influence of LXA4 on ROS and Prx II levels produced in response to PDGF in MC. LXA4 attenuated PDGF-stimulated ROS production, consistent with previous data from experiments carried out in human polymorphonuclear leukocytes (
). Interestingly, although we have shown that LXA4 results in the prolonged activation of SHP-2, LXA4 abolished PDGF-induced Prx II protein expression. This leads us to suggest that LXA4 attenuation of ROS/H2O2 production and hence activation of SHP-2 may be independent of Prx II. Conversely, Prx II down-regulation is a direct result of LXA4 attenuation of ROS production (
). Therefore, we propose that LXA4 possibly mimics Prx II in that it potentially reactivates oxidatively inactivated SHP-2 PTP by removing endogenous H2O2.
In summary, based on our general understanding of PDGFR signaling, we now propose a model for SHP-2 regulation of the PDGFRβ in response to LXA4 (Fig. 7). LXA4 binding to the lipid raft-bound ALXR induces a recruitment to and reactivation of oxidatively inactivated SHP-2 in the lipid raft region. In a time-dependent manner, recruited SHP-2 is released from the ALXR whereby it is free to bind to and dephosphorylate the lipid raft population of the PDGFRβ and hence diminish its subsequent mitogenic activities. Interestingly, SHP-2 has been shown to exert site-selective dephosphorylating activity, showing preference for the PI 3-kinase-binding sites (
). Our results support this hypothesis as mutation of the PDGFRβ-binding sites for p85 recruitment (Y740F/Y751F) blocked the effect of LXA4 on receptor phosphorylation. Taken together, these data further elaborate on the potential mechanisms underlying the anti-inflammatory, pro-resolution, and anti-fibrotic bioactions of LX.
We thank Prof. Finian Martin for critical review of the manuscript; Dr. Hugh Brady for continued support and advice, Prof. Mauro Perretti, William Harvey Research Institute, London, for provision of Ac2-26; and Dr. Susan K. Logan, New York University School of Medicine, for support.