From the
Dipartimento di Biochimica e Biotecnologie Mediche, Universitá di Napoli Federico II, Via Pansini 5 and ¶Dipartimento di Farmacologia Sperimentale, Universitá Federico II, Via Domenico Montesano 49, Naples 80131, Italy
Received for publication, November 20, 2002 , and in revised form, March 10, 2003.
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ABSTRACT |
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INTRODUCTION |
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Most of the mentioned experiments have been performed by exposing cells to different concentrations of H2O2, which is not a reactive oxygen species but is rapidly transformed in the hydroxyl radical (HO), the most harmful ROS. It is still unclear which of the one or more ROS function as second messenger(s) in the cell, and it cannot be excluded that different types and/or concentrations of ROS could yield different effects on different targets within specific cell types. Furthermore, exogenous administration of H2O2, often at concentrations significantly higher than those even reached within the cell in physiological conditions, is probably a procedure not so accurate to mimic what normally occurs in the cell, considering the low diffusion of many ROS as well as the possible compartmentalization of ROS-generating systems.
We previously developed a procedure to induce redox modification in intact cells (19): this is based on the exposure of cells to diethylmaleate (DEM), a GSH depleting agent (20). This experimental system allows the alteration of the concentrations of endogenously generated ROS by inactivating the scavenging systems based on GSH, through the formation of GSH-DEM conjugates. In this report a comparison between the well known consequences of H2O2 treatment and the effects of DEM-induced GSH depletion is presented. The results demonstrate that the activation of ERKs and AKT in cells exposed to DEM is independent from tyrosine phosphorylation of TKRs.
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EXPERIMENTAL PROCEDURES |
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Rat-2 fibroblasts and MCF10A were from ATCC. HEK293 cells stably transfected with eNOS were gently provided by W. C. Sessa (21). Rat-2 fibroblasts and HEK293 were grown in Dulbecco's modified Eagle's medium (DMEM) supplemented with 5 and 10% fetal calf serum (FCS, HyClone), respectively, 100 units/ml penicillin, and 100 µg/ml streptomycin, in a humidified atmosphere of 5% CO2/95% air at 37 °C. MCF10A were grown in a solution containing 50% DMEM and 50% Hams' nutrient mixture F-12 (Euroclone), supplemented with 5% FCS (HyClone), 0.24 unit/ml insulin, 0.5 µg/ml hydrocortisone, 10 ng/ml EGF, 100 units/ml penicillin, and 100 µg/ml streptomycin, in a humidified atmosphere of 5% CO2/95% air at 37 °C.
Cell Culture Conditions and TreatmentsCells were grown until they reached 90% confluence and then were starved in DMEM containing 0.2% FCS (Rat-2) or 1 mg/ml bovine serum albumin (MCF10A). After 48 h for Rat-2 and 84 h for MCF10A, cells were stimulated with FCS to a final concentration of 20% or with purified growth factors (80 ng/ml) for 5 min. The oxidizing treatments as well as treatments with tyrosine kinase inhibitors were performed before mitogenic stimulation.
Western Blot and Immunoprecipitation AnalysesGrowth-arrested fibroblasts were treated with or without 20% FCS in the presence or absence of the appropriate amounts of DEM and H2O2 with or without tyrosine kinase inhibitors (see legends of figures for details) for the indicated times at 37 °C. Cells were rinsed with phosphate-buffered saline buffer (150 mM NaCl, 0.1 M phosphate, pH 7.5) and harvested in the same buffer. Cell lysates and Western blot analysis were performed as previously described (22). Antigen·antibodies complexes were detected with a chemiluminescence reagent kit (Amersham Biosciences). For the immunoprecipitation experiments, after the appropriate treatments, the cells were washed with cold PBS and solubilized at 4 °C for 30 min in 800 µl of lysis buffer containing 10 mM Tris-HCl, pH 7.5, 150 mM NaCl, 0.1 mM EDTA, 1 mM Na3VO4,50mM NaF, 0.5% Nonidet P-40, 1 mM phenylmethylsulfonyl fluoride, and 10 µg/ml each of aprotinin, leupeptin, and pepstatin. The cell lysates were cleared by centrifugation at 4000 rpm for 10 min at 4 °C. 1 mg of proteins was immunoprecipitated by incubating with 1 µg of anti-PDGF-R or anti-EGF-R antibodies for 90 min at 4 °C followed by the addition of 30 µl of 50% (w/v) protein A-SepharoseTM CL-4B (Amersham Biosciences) for 30 min at 4 °C. The beads were washed three times with lysis buffer and solubilized in 15 µlof4x Laemmli sample buffer. The samples were heated at 95 °C for 5 min and analyzed by SDS-gel electrophoresis on 8% acrylamide gel. Phosphorylation and/or nitration of PDGF-R and EGF-R were analyzed by Western blotting using 1 µg/ml anti-phosphotyrosine mouse monoclonal or anti-nitrotyrosine rabbit polyclonal IgG antibodies. Src activity was assayed in Src immunoprecipitates from oxidanttreated and untreated cells, using enolase as substrate (23).
Lucigenin AssaysTo measure intracellular superoxide anion production we used lucigenin-enhanced chemiluminescence (10, 24) with slight modifications: exponentially growing Rat-2 cells were grown as above described and trypsinized; cell suspension was exposed to DEM and H2O2 for the times indicated in Fig. 4, centrifuged and suspended in Hanks' balanced salt solution (5.5 mM D-glucose, 5.4 mM KCl, 0.44 mM KH2PO4, 136 mM NaCl, 4.2 mM NaHCO3, 0.34 mM Na2HPO4) containing 1 mM lucigenin. Chemiluminescence was recorded using a Berthold LB 9505 luminometer, and consecutive readings were taken at various intervals (5 min). For the determination of a single value, the average of four readings over a 2- to 4-min period was taken, and the mean ± S.D. of four experiments was used to plot the data.
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Measurement of NitritesAfter the oxidizing treatments, as described in the figure legends, the culture medium of Rat-2 and HEK293/eNOS cells was collected for the measurement of nitrites. Nitrites were assayed fluorometrically in microtiter plates using a standard curve of sodium nitrite (26).
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RESULTS |
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Again, both serum and H2O2 treatments induced EGF-R Tyr phosphorylation. On the contrary, in cells exposed to DEM the Tyr phosphorylation content of PDGF-R- was undistinguishable from that of starved control cells. Furthermore, PDGF-R Tyr phosphorylation was unchanged when DEM treatment was performed simultaneously to serum stimulation (see Fig. 2B). Similar results were obtained with EGF-R, as shown in Fig. 2D.
These results demonstrate that H2O2 and DEM exert different effects on the Tyr phosphorylation of PDGF and EGF receptors: the exposure of cells to H2O2 mimics the effects of growth factors even in the absence of mitogenic stimulation, whereas treatment with DEM is unable to change phospho-Tyr content of these two proteins, both in the presence or in the absence of growth factor stimulation.
Protein kinase B (AKT) is another target of ROS downstream the TKR (27). We therefore studied the effects of DEM treatment on the activation of this Ser-Thr protein kinase and observed a further difference by comparing the effects of H2O2 and DEM on the phosphorylation of AKT. Fig. 3 shows a Western blot of total cell lysates from starved Rat-2 fibroblasts stimulated with serum (panels A and B) or with PDGF-BB (panels C and D). An increased phosphorylation of AKT can be observed using antibodies specific for the phosphorylated Ser-473 of the kinase. A similar activation can be induced by the treatment with H2O2 as well as by the two concomitant treatments (mitogenic and oxidant). However, the effects of DEM on the activation of AKT appear to be different: as shown in Fig. 3B, the phosphorylation of this kinase in cells exposed only to DEM is very low, much lower in comparison to H2O2 treatment, but when serum and DEM are added simultaneously, these two treatments yield a strong synergistic effect on AKT phosphorylation at Ser-473. This result is less pronounced when PDGF-BB, instead of serum, is used to stimulate starved fibroblasts (Fig. 3D): the explanation for this could probably be based on the higher extent of AKT activation by the purified growth factor.
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One of the mechanisms responsible for the observed effects could be related to possible qualitative and/or quantitative differences in the ROS produced by the two oxidants: to this aim, we exposed Rat-2 fibroblasts to DEM and to H2O2 and measured the generation of superoxide ion using the lucigenin assay (10, 24). As shown in Fig. 4, when Rat-2 fibroblasts are exposed to DEM, a peak of superoxide anion production is observed 30 min after DEM treatment (see Fig. 4A), which follows the DEM-induced GSH depletion (data not shown), whereas, upon treatment of cells with H2O2, the production of superoxide ion is much faster, reaching the maximal value after 1 min, and the amount of ROS generated by H2O2 treatment is at least 10 times higher (Fig. 4A). The differences between DEM and H2O2 suggest the hypothesis that also the activation of MAPK and AKT by these two oxidizing agents could be different. To evaluate this phenomenon we performed time course experiments of MAPK and AKT activation during the treatment with both oxidizing agents: as shown in Fig. 4B, the peak of ERKs and AKT phosphorylation following the treatment with H2O2 is observed already 15 min after the treatment, whereas a longer time (30 min) is required for DEM. However, the delay of DEM-induced ERKs activation is not sufficient to explain the absence of TKR phosphorylation, because, even at a longer time of DEM exposure (up to 3 h), the phosphorylation of PDGF-R was undistinguishable from that of starved cells (Fig. 3C). One possible explanation of the observed differences between H2O2 and DEM effects could be the different levels of ROS generation following the exposure of cells to the two agents. In the attempt to analyze H2O2 and DEM effects in comparable experimental conditions, we measured Tyr phosphorylation of PDGF-R following the treatment of cells with various concentrations of H2O2 and DEM. At the same concentration the extent of DNA damage provoked by ROS was measured by using the comet assay (24). As shown in Fig. 5, even at a comparable degree of DNA damage provoked by DEM and H2O2, as for example 0.05 mM H2O2 and 1 mM DEM (see Fig. 5A), the PDGF-R Tyr phosphorylation is induced only by the treatment with H2O2 (see Fig. 5B).
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Activation of a Non-receptor Tyrosine Kinase Is Involved in DEM-induced EffectsTo investigate on possible mechanisms involved in the activation of mitogenic cascade in the absence of the activated tyrosine kinase receptor, we exposed Rat-2 fibroblasts to genistein, a competitive inhibitor of ATP binding to the catalytic domain of tyrosine kinase and inhibitor of tyrosine kinase activity of both growth factor receptor and non-receptor Tyr kinases. As shown in Fig. 6A, the activation of AKT by DEM is blocked by genistein pretreatment. To test the involvement of a non-receptor tyrosine kinase in the activation of AKT following DEM treatment, we focused our attention on Src, a tyrosine kinase activated by H2O2 (28). To this aim we exposed Rat-2 cells to DEM and H2O2 and assayed Src tyrosine kinase activity on a heterologous substrate: Fig. 6B shows an increased Src activity, measured through the phosphorylation of the enolase, in Src immunoprecipitates from DEM- and H2O2-treated cells. The involvement of Src in the activation of AKT was confirmed by exposing the cells to PP2, a selective Src inhibitor, and to PP3, a structurally inactive analogue. Fig. 6C shows a specific inhibition of DEM- and H2O2-induced AKT activation in the presence of this Src inhibitor. On the contrary, PP2 treatment was ineffective in the abolishment of MAPK activation by DEM (Fig. 6C); this last finding is in agreement with previously described results demonstrating that H2O2-induced ERK1 and p38 activities in endothelial cells are Src-independent (18). These results are consistent with a Src-mediated activation of AKT upon oxidizing treatments.
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DEM-mediated Effects on AKT and ERKs Are Independent of Tyrosine Kinase Receptor PhosphorylationFig. 7A illustrates a Western blot of total cell lysates from starved Rat-2 fibroblasts exposed to PDGF-BB, DEM, or H2O2, with or without AG1296, a specific inhibitor of the tyrosine kinase activity of PDGF-R. The effects of DEM on AKT do not require PDGF-R phosphorylation: in fact, Fig. 7A shows that, although the activation of AKT by PDGF-BB is highly decreased in Rat-2 cells pretreated with the inhibitor, the same treatment does not yield any effect in cells exposed to DEM. Surprisingly, in cells pretreated with the PDGF-R inhibitor and then exposed simultaneously to the growth factor and to DEM, the effect of DEM is reinforced. Similar results were obtained for ERKs. As a control, the analysis of Tyr phosphorylation of PDGF-R is shown to exclude direct inactivation of AG1296 inhibitor by the oxidizing treatments (Fig. 7A). These results confirm that the activation of downstream targets following DEM treatment does not require tyrosine autophosphorylation of PDGF-R but suggest that PDGF·PDGF-R interaction could have some role in the activation of downstream cascades, even in the absence of receptor phosphorylation. We therefore hypothesized that modifications of Tyr residues other than phosphorylation could be involved in the activation of the downstream cascade: to this aim we focused our attention on tyrosine nitration, a modification that can occur in proteins and is involved in the modulation of signaling processes starting from receptors of tyrosine kinases to downstream signaling cascades (29, 30). Fig. 7B shows a Western blot of cell lysates from starved Rat-2 fibroblasts exposed to serum, DEM, or spermidine NOnoate, a positive control of intracellular NO generation. After these treatments cells were immunoprecipitated with anti-PDGF-R antibodies and then analyzed by immunoblot with anti-nitrotyrosine antibody: the amount of PDGF-R tyrosine nitration in DEM-treated cells was comparable to that present in the positive control, whereas it seemed to be absent in serum-treated cells. To investigate how DEM treatment induces the nitrosylation of PDGF-R, we measured the amount of NO produced by Rat-2 cells after DEM treatment. A total nitrites assay in the culture medium demonstrated that a significant nitrite accumulation was present after DEM exposure. Conversely, in the same experimental conditions, treatment with H2O2 was ineffective (Fig. 8A). Furthermore, we have also demonstrated that DEM-induced NO accumulation is due to eNOS activation: this specific matter was demonstrated by using HEK293 cells stably transfected with eNOS and exposed to the two oxidizing agents (Fig. 8A). Interestingly, by using antibodies specific for the phosphorylated Ser-1177 of the endothelial nitric-oxide synthase, we demonstrated that DEM treatment promotes eNOS phosphorylation, which indicates eNOS activation (Fig. 8B).
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DISCUSSION |
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This different behavior between DEM and H2O2 could be due to qualitative and/or quantitative differences in the ROS generated by these two agents. In fact, the timing and the extent of superoxide anion production are very different, being much higher, at least 10 times compared with DEM, and faster in the cells treated with H2O2. Furthermore, we found that DEM induces milder redox modifications, as demonstrated by the lower extent of DNA damage in cells exposed to this compound, compared to that observed in H2O2-treated cells. The timing and the quantitative differences of ROS production could explain the different timing of MAPK and AKT activation by DEM and H2O2; in fact, both kinases are rapidly activated by H2O2 (within 1 min), whereas longer exposures to DEM are required. However, even by using concentrations of H2O2 and DEM leading to similar intracellular redox modifications, Tyr phosphorylation of GF-R could be observed only with H2O2 and not with DEM.
Therefore, one possible explanation of this difference is that the intracellular compartments in which ROS are produced respectively by the two oxidants is different. In fact H2O2 generates ROS both outside and inside the cell, whereas DEM, acting through the enzymatic depletion of GSH and the consequent accumulation of endogenous "unscavenged" ROS, is expected to affect mainly intracellular redox conditions. The main finding of this report is, then, that ERK and AKT pathways can be activated by ROS also through mechanisms different from the GF-R Tyr phosphorylation. At least in the case of AKT, the observed activation is, however, dependent on Tyr kinase activity, given the abolishment of the phenomenon in cells treated with genistein.
The non-receptor Tyr kinase Src is activated by both DEM and H2O2, and treatment of cells with a specific inhibitor of this kinase, PP2, completely blocks AKT activation by both prooxidant molecules. These results support the hypothesis that intracellular ROS accumulation leads to the activation of nonreceptor TKs, without a contemporary phosphorylation of GF-R. This phenomenon triggers the induction of AKT survival pathway and has no effect on the machinery of ERKs. This last result is in agreement with the data of Yoshizumi et al. (18) demonstrating that neither ERK or p38 are activated by H2O2 through an Src-dependent mechanism.
To explain these observations a direct effect of ROS on Src TK activity could be hypothesized, although it is quite surprising that ROS selectively activate Src and not GF-R. As mentioned above, one possibility is that ROS generated by DEM are restricted to the intracellular compartments where they are normally generated (e.g. mitochondria); it is possible that in these areas they are only challenged with non-receptor TK as Src and not with membrane bound GF-R. The mechanisms through which ERKs are activated by DEM in the absence of both GF-R and non-receptor TK remain to be established. Previous results demonstrated that this activation of ERKs is blocked by dominant negative mutants of ras (17), and this result contributes to the positioning of ROS targets on ras itself or upstream of this molecule.
The results obtained with the selective PDGF-R inhibitor AG1296, besides being of support for a PDGF-R phospho-Tyr-independent activation of AKT and ERKs upon DEM treatment, also suggest an interesting hypothesis. In fact, even though PDGF-R is not phosphorylated following DEM treatment, in the presence of the specific PDGF-R inhibitor AG1296 and the growth factor, the effects of DEM are significantly reinforced. These results suggest that the PDGF·PDGF-R complex could have some role in the activation of downstream cascades, even in the absence of receptor autophosphorylation, e.g. PDGF·PDGF-R could act as a scaffold protein, facilitating the transduction of the signals independently from its Tyr kinase activity. Other proteins interacting with tyrosine kinase receptors not through TKR tyrosine phosphorylation include the Enigma protein whose binding to Ret/ptc2, the constitutively active, oncogenic form of the c-Ret receptor tyrosine kinase, is required for mitogenic signaling by this TKR (31), and RALT, an ErbB-2-interacting protein, whose binding does not require the receptor autophosphorylation (32).
A mechanism that could contribute to the activation of the mitogenic pathway in the absence of Tyr phosphorylation includes Tyr modifications different from phosphorylation such as, for example, nitration. It is well known that the nitration of tyrosine residues in proteins occurs through the action of ROS and nitrogen species and is considered a marker of oxidative stress in physiological conditions (33, 34). It was recently suggested that nitration of tyrosine residues may have some role in the modulation of signaling processes: a nitrotyrosine-containing peptide could induce lyn kinase activation through a displacement of the phosphotyrosine, present in lyn C-terminal tail, from its SH2 binding site (35, 36). Furthermore, there are results demonstrating that nitration of Tyr-15 of p34cdc2 kinase prevents the phosphorylation of this residue catalyzed by the lck kinase (37). Our results suggest that Tyr nitration in PDGF-R occurs following DEM treatment, whereas it is completely absent upon serum stimulation. Interestingly, our results strongly support the hypothesis that PDGF-R nitrosylation upon DEM treatment is most likely linked to eNOS activation. On this basis it could be hypothesized that DEM treatment does not induce Tyr phosphorylation of PDGF-R because Tyr residues are already modified (by nitration) in these experimental conditions. Finally, the synergistic activation of AKT that we observe when the cells are simultaneously exposed to serum and DEM could be the result of two independent mechanisms: one mediated by the phosphorylation of some Tyr residues in PDGF-R, which follows serum stimulation, and the other by the nitration of some other Tyr residues, upon DEM treatment; both processes can converge on and contribute to AKT activation.
In conclusion, our results demonstrate that ROS can activate signaling cascades, even in the absence of growth factor receptor phosphorylation, and suggest the possibility of a new mechanism for the activation of transduction pathways based on Tyr nitration of TKRs. From a regulatory point of view it should be ascertained that, due to the existence of specific "denitrases" (38), the nitration of Tyr residues is a reversible process.
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FOOTNOTES |
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* This work was supported by grants from the Italian National Research Council PF Biotecnologie and PSt/74 Determinanti di salute e invecchiamento della popolazione and from Ministero dell'Universitá e della Ricerca Scientifica e Tecnologica (Prin 2000 and Prin 2002 and Piano Biomedicina-Progetto 1, Cluster 04). 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.
|| Present address: Dipartimento di Biologia Cellulare e dello Sviluppo, Sez. Biochimica, Universitá degli Studi di Palermo, Via del Vespro 127, Palermo 90128, Italy.
To whom correspondence should be addressed. Tel.: 39-81-746-3145; Fax: 39-81-746-4359 or 39-81-746-3650; E-mail: espositof{at}dbbm.unina.it.
1 The abbreviations used are: ROS, reactive oxygen species; GF, growth factor; AKT, protein kinase B; MAPK, mitogen-activated protein kinase; JNK, c-Jun NH2-terminal kinase; DEM, diethylmaleate; GSH, reduced glutathione; GF-R, growth factor receptor; PDGF-R, platelet-derived growth factor receptor; EGF, epidermal growth factor; ERK, extracellular signal-regulated kinase; DMEM, Dulbecco's modified Eagle's medium; FCS, fetal calf serum; PBS, phosphate-buffered saline; TK, tyrosine kinase; TKR, TK receptor; PDGF-BB, plateletderived growth factor BB; PP2, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazole[3,4-d]pyrimidine; PP3, 4-amino-7-phenylpyrazole[3,4-d]pyrimidine; eNOS, endothelial nitric-oxide synthase.
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ACKNOWLEDGMENTS |
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REFERENCES |
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