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(Circulation Research. 1999;85:892.)
© 1999 American Heart Association, Inc.

Molecular Medicine

Activation of Epithelial Growth Factor Receptor Pathway by Unsaturated Fatty Acids

Nathalie Vacaresse¹, Isabelle Lajoie-Mazenc¹, Nathalie Augé, Isabelle Suc, Marie-Françoise Frisach, Robert Salvayre, Anne Nègre-Salvayre

From INSERM U-466 and Department of Biochemistry, IFR-31, CHU Rangueil, Toulouse, France.

Correspondence Dr A. Negre-Salvayre, Biochimie INSERM U-466, CHU Rangueil, avenue Jean Poulhès, 31403 Toulouse cedex 4, France. E-mail salvayre@rangueil.inserm.fr or anesalv{at}rangueil.inserm.fr

	Abstract

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Abstract—Nonesterified fatty acids (NEFAs) are acutely liberated during lipolysis and are chronically elevated in pathological conditions, such as insulin resistance, hypertension, and obesity, which are known risk factors for atherosclerosis. The purpose of this study was to investigate the effect and mechanism of action of NEFAs on the epithelial growth factor (EGF) receptor (EGFR). In the ECV-304 endothelial cell line, unsaturated fatty acids triggered a time- and dose-dependent tyrosine phosphorylation of EGFR (polyunsaturated fatty acids [PUFAs] were the most active), whereas saturated FAs were inactive. Although less potent than PUFAs, oleic acid (OA) was used because it is prominent in the South European diet and is only slightly oxidizable (thus excluding oxidation derivatives). EGFR is activated by OA independent of any autocrine secretion of EGF or other related mediators. OA-induced EGFR autophosphorylation triggered EGFR signaling pathway activation (as assessed through coimmunoprecipitation of SH2 proteins such as SHC, GRB2, and SHP-2) and subsequent p42/p44 mitogen-activated protein kinase (as shown by the use of EGFR- deficient B82L and EGFR- transduced B82LK⁺ cell lines). OA induced in vitro both autophosphorylation and activation of intrinsic tyrosine kinase of immunopurified EGFR, thus suggesting that EGFR is a primary target of OA. EGFR was also activated by mild surfactants, Tween-20 and Triton X-100, both in vitro (on immunopurified EGFR) and in intact living cells, thus indicating that EGFR is sensitive to amphiphilic molecules. These data suggest that EGFR is activated by OA and PUFAs, acts as a sensor for unsaturated fatty acids (and amphiphilic molecules), and is a potential transducer by which diet composition may influence vascular wall biology.

Key Words: fatty acid • growth factor • mitogen-activated protein kinase • EGF receptor

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion References

 
Non esterified fatty acids (NEFAs) are physiologically liberated during postprandial lipolysis and are chronically elevated in plasma in various diseases (eg, obesity, mellitus diabetes, ketoacidosis, hypertension), known risk factors for vascular diseases and atherosclerosis.^{1 2} Saturated fatty acids (FAs) are thought to be atherogenic, whereas unsaturated FAs (UFAs), such as oleic acid (OA) and n-3 polyunsaturated FA (PUFA), are considered to be antiatherogenic,³ despite recent contradictory reports.⁴ The antiatherogenic effect of UFAs may result in part from their lowering effect on LDL cholesterol,^{5 6} from their "anti-inflammatory" effect on vascular cells (eg, UFAs inhibit the expression of endothelial proinflammatory proteins⁷ ), and from their antithrombotic effects (for n-3 PUFA).⁸ The mechanisms of action of NEFAs are only in part understood and are complex because NEFAs are involved at various stages of cell biology—namely, membrane structure, cell metabolism, energy production, and cell signaling.^{9 10} UFAs are able to modulate the activity of various intracellular signaling pathways mediated by calcium, protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), and epithelial growth factor receptor (EGFR).^{9 10 11 12 13 14 15 16 17}

EGFR, now considered to be a critical crossroad of multiple receptor pathways,¹⁸ is potentially implicated in the regulation of cell migration, proliferation, or differentiation and may be involved in atherogenesis.¹⁹ EGFR is a 170- kDa transmembrane receptor tyrosine kinase that is shared by several growth factors, such as EGF, heparin-binding EGF, tumor necrosis factor-, amphiregulin, and betacellulin.^{20 21} Moreover, EGFR activation is modulated by various non specific factors, such as UV irradiation,²² H₂O₂,²³ oxidized lipoproteins,²⁴ UFAs, and their oxidation derivatives.^{16 25 26}

Ligand binding induces EGFR dimerization, stimulation of its intrinsic tyrosine kinase, and autophosphorylation of its own tyrosine residues.^{20 21} Phosphotyrosines of the C-terminal domain of EGFR are binding sites for SH2 domains of adaptors or enzymatic proteins, including phospholipase C1, GTPase-activating protein of p21ras (rasGAP), SHP2, p85 subunit of phosphatidylinositol 3-kinase, SHC, Nck, c-cbl, and GRB2-Sos.^{27 28} The activation of GRB2-Sos complex may in turn activate p21ras and the kinase cascade leading to MAPK activation.²⁸ MAPK or extracellular-regulated kinase (ERK) (p44/ERK1 and p42/ERK2) can also be activated through receptors for growth factors, hormones or cytokines, or G protein-coupled receptors or in response to stress.^{29 30}

The aim of this study was to investigate whether FAs are able to activate EGFRs and whether they are active per se or only after metabolic activation (eg, after the generation of oxidized or other bioactive derivatives).

Our data show that (1) in intact living cells, UFAs induce EGFR autophosphorylation and activation, and subsequent MAPK activation; (2) UFA activity is related to the degree of unsaturation but is, at least in part, independent of FA metabolism; (3) in vitro, UFAs elicit autophosphorylation of the immunopurified EGFR, thus suggesting that EGFR may be considered a primary target of NEFAs; and (4) EGFR acts a sensor for amphiphiles (or for membrane fluidity changes).

	Materials and Methods

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Chemicals
FCS and culture reagents were obtained from GIBCO. [¹²⁵I]EGF (150 µCi/µg) was from New England Nuclear Research Products. E nhanced chemiluminescence reagent (ECL Kit), nitrocellulose, and autoradiography films were from Amersham Corp. FAs, 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDAC), and general reagents were obtained from Sigma Chemical Co. Recombinant EGF and antibodies against EGFR (monoclonal SC-101 and rabbit polyclonal), GRB2, SHP2, ERK1/ERK2 (MK12), and SHC were from Santa Cruz/Peprotech/Tebu. Activated MAPK was from Promega. Phosphotyrosine (4G10) was from UBI/Euromedex.

Cell Culture and Cell Extracts
Human endothelial ECV-304 cells (American Type Culture Collection) were grown in RPMI-1640 containing 10% FCS. The murine B82L parental fibroblasts (EGFR deficient) and B82LK⁺ cells (transduced with wild- type EGFR),^{31 32} a generous gift from Dr M. Weber (Charlottesville, Va), and SrcK^- cells, C3H-10T1/2 fibroblasts overexpressing kinase defective c-src (clone 430c-src),³³ a generous gift from Dr S.J. Parsons (Charlottesville, Va), were grown as described by Wright et al³² and Wilson et al,³³ respectively. Before experiments, cells were starved overnight in 0.5% FCS- containing medium.

Immunoprecipitation, Western Blotting, and Dimerization
After incubation, cells were scraped off, pelleted, and solubilized in RIPA buffer for total extracts or in solubilizing buffer for immunoprecipitates, as previously used.²⁴ Immunoprecipitation was performed with anti-phosphotyrosine or anti-EGFR antibodies, and immune complexes, recovered on protein G/Sepharose, were solubilized in Laemmli’s buffer. Westerns blots were performed as previously described.²⁴

Dimerization studies were performed according to Van der Vliet et al.³⁴ After agonist treatment, cells were incubated with 10 mmol/L EDAC for 40 minutes at 37°C, immunoprecipitated by (monoclonal) anti-EGFR, resolved by SDS-PAGE (5% polyacrylamide), and probed with (polyclonal) anti-EGFR.

[¹²⁵I] EGF Binding Assays
Competition between OA and [¹²⁵I]EGF binding was performed according to Marikovsky et al.³⁵ Cells were incubated with [¹²⁵I]EGF (70 000 cpm/mL, 30 pmol/L) and with or without 50 µmol/L OA and then washed in PBS containing 0.5% BSA, and the cell-associated radioactivity was determined (Minaxi; Packard). Nonspecific binding was determined on the basis of excess unlabeled EGF (10 nmol/L).

In Vitro EGFR Autophosphorylation and Tyrosine Kinase Activity
EGFR autophosphorylation and EGFR kinase, assayed on immunoprecipitated EGFR with the use of poly-GluTyr and [-³³P]ATP as substrates, were evaluated as described by Suc et al.²⁴

MAPK Activity
MAPK activity of cells stimulated by OA was determined through myelin basic protein (MBP) phosphorylation with [-³³P]ATP (0.2 µCi/assay), as previously described.³⁶ Proteins were determined according to the bicinchoninic method. Statistical analysis was performed with the use of the Student t test.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
OA and PUFA Induce Tyrosine Phosphorylation and Dimerization of EGFR and Recruitment of SH2-Containing Proteins
Human ECV-304 endothelial cells were used as a model because (1) endothelium is in direct contact with circulating UFAs and (2) this cell line expresses EGFR.

The incubation of ECV-304 cells with OA induced tyrosine phosphorylation of a 170-kDa membrane protein (Figure 1) that was identified as EGFR through immunoprecipitation and immunoblotting. The OA-induced EGFR phosphorylation began rapidly (2 minutes) and was sustained for 45 minutes (Figure 1A). EGFR phosphorylation increased progressively with OA concentration, apparently without saturation up to 100 µmol/L (Figure 1B). In the presence of BSA (50 µmol/L), the OA-induced EGFR phosphorylation was clearly visible with a molar ratio (OA/BSA) of 1:1 and was more intense when the ratio was >2 (Figure 1C). This suggests that the activity may result from unbound OA.

View larger version (42K): [in this window] [in a new window]   Figure 1. OA and PUFA induce EGFR activation in human endothelial ECV-304 cells. Cells were starved for 16 hours in RPMI-1640 containing 0.5% FCS and were incubated with FA or EGF, under indicated conditions. Cell lysates were analyzed by Western blotting (SDS-PAGE on 10% polyacrylamide gels) and probed with anti-phosphotyrosine (P-tyr) or anti-EGFR. A, Time course of EGFR tyrosine phosphorylation in cells incubated with OA (50 µmol/L) or EGF (10 nmol/L, used as positive control) for indicated time (minutes). B and C, Dose-response of EGFR tyrosine phosphorylation in cells incubated for 15 minutes with OA (concentrations expressed as µmol/L) without (B) or with (C) BSA (50 µmol/L). EGF (10 nmol/L) as positive control. D and E, Influence of chain length, unsaturation, or both of FA on EGFR tyrosine phosphorylation induced by FA (50 µmol/L, 15 minutes). (PUFAs: 18:1, oleic; 18:2, linoleic; 20:4, arachidonic; 20:5, n-3 eicosapentaenoic; 22:6, n-3 docosahexaenoic acids). F, Comparison of EGFR tyrosine phosphorylation induced by OA or AA (50 µmol/L each) and increasing concentrations of EGF (as pmol/L).

EGFR- phosphorylating activity of FAs was dependent on chain length and unsaturation. Short chain FAs were inactive, and PUFAs were more effective (C20:5, or eicosapentaenoic acid, and C22:6, or docosahexaenoic acid, were the most effective) (Figure 1, D and E). In the present study, we used pre-ferentially OA (although less potent than PUFA) because (1) OA is the major circulating UFA and (2) we sought to understand whether UFAs may be active per se or only through their oxidation derivatives (eg, 13(S)-hydroperoxyoctadecadienoic and epoxyeicosatrienoic acids, which are able to activate EGFR).^{25 26} OA is only slightly oxidizable and does not generate these oxidized lipids.

The EGFR- phosphorylating activity by 50 µmol/L OA or arachidonic acid (AA) was grossly equivalent to 50 and 200 pmol/L EGF, respectively (Figure 1F). When OA was removed from the culture medium (cells were later incubated in delipidated medium), EGFR autophosphorylation persisted for 15 to 20 minutes after washout (data not shown). Similar to EGF, OA and AA were also able to induce EGFR dimerization, concomitant with autophosphorylation of the receptor (Figure 2A). As shown in Figure 2B, OA was effective in activating the EGFR pathway, as assessed by the recruitment of SH2-containing proteins SHP-2, SHC, and GRB2.

View larger version (52K): [in this window] [in a new window]   Figure 2. Dimerization of EGFR and recruitment of substrates to EGFR activated by FAs. Cells, treated as described in legend to Figure 1, were immunoprecipitated and analyzed by Western blotting probed with indicated antibodies. A, Dimerization of EGFR in murine (EGFR-transduced) fibroblasts B82K+ cells, either unstimulated (Co) or stimulated for 15 minutes by OA or AA (50 µmol/L), Tween-20 (TW, 100 ng/mL), or EGF (10 pmol/L or 10 nmol/L). Then cells were incubated with EDAC, immunoprecipitated by (monoclonal) anti-EGFR, resolved by SDS-PAGE (5% polyacrylamide gels), and probed with (polyclonal) anti-EGFR. B, Coimmunoprecipitation of EGFR and of SH2-containing proteins in ECV-304 endothelial cells incubated with OA (50 µmol/L) or EGF (10 nmol/L) for 15 minutes. Immunoprecipitation was performed using monoclonal anti-EGFR under previously described conditions,24 and immune complexes were recovered, solubilized, and resolved by SDS-PAGE (10% polyacrylamide gels), blotted by indicated antibodies.

OA-Induced EGFR Activation Elicits MAPK Activation
The OA-induced activation of the EGFR signaling pathway was associated with MAPK activation, as shown by tyrosine phosphorylation of p42/p44 MAPK (Figure 3A) and activation of MBP kinase activity of immunoprecipitated MAPK (Figure 3B). To investigate whether EGFR and MAPK activations were causally related or independent events, similar experiments were performed on genetically engineered B82L-derived cell lines expressing or not expressing EGFR (parental B82L cells were EGFR deficient and transduced B82LK⁺ cells overexpressed EGFR)^{31 32} (Figure 3C). As shown in Figure 3, D and E, OA (50 µmol/L, 15 minutes) induced no significant MAPK activation in cells lacking EGFR (parental B82L cells), whereas it elicited both EGFR autophosphorylation and MAPK activation in B82LK⁺ cells (expressing EGFR). Conversely, OA-induced MAPK activation was inhibited when EGFR autophosphorylation was inhibited with genistein (Figure 4). These events were not or were slightly influenced by the PKC inhibitor bisindolylmaleimide or phorbol-12-myristate-13-acetate–mediated down regulation of PKC (Figure 4). Taken together, these data suggest that the moderate OA-induced activation of the EGFR signaling pathway is effective for the inducement of MAPK activation (whereas classic PKC is apparently dispensable).

View larger version (26K): [in this window] [in a new window]   Figure 3. OA-induced EGFR activation leads to MAPK activation. A and B, ECV-304 endothelial cells, untreated (Co) or stimulated by OA (50 µmol/L, 15 minutes) or EGF (10 nmol/L, 15 minutes), were immunoprecipitated by anti-phosphotyrosine (P-tyr) antibody. Immunoprecipitates were immunoblotted by anti-EGFR or anti-ERK1/ERK2 antibodies (tot MAPK) (A) or used for assaying MAPK activity (by MBP phosphorylation). C through E, OA- induced MAPK activation is dependent on EGFR activation (and expression) in B82L (EGFR deficient) and B82LK+ (EGFR-transduced) fibroblasts. Cells were incubated for 15 minutes without (Co) or with 50 µmol/L OA or 10 nmol/L EGF. C, Cells were immunoprecipitated by anti-EGFR antibody, and Western blots were probed with anti-phosphotyrosine or anti-EGFR antibodies. D, Cells lysated were resolved by Western blots and probed by anti-activated MAPK (act MAPK) or tot MAPK (anti-ERK1/ERK2 antibodies). E, Cells lysates were used for MAPK determination (MBP phosphorylation), as in B. *P<0.01 compared with control.

View larger version (50K): [in this window] [in a new window]   Figure 4. Effect of tyrosine kinase and PK C inhibitors on EGFR phosphorylation and MAPK activation induced by OA. After preincubation with genistein (Gen) (10 µmol/L, 1 hour), bisindolylmaleimide (BIM) (10 µmol/L, 1 hour), or phorbol-12-myristate-13-acetate (200 nmol/L, 24 hours), ECV-304 endothelial cells were incubated with OA (50 µmol/L) for an additional 15 minutes. Co indicates untreated control; EGF, positive control (10 nmol/L EGF, 15 minutes). Western blotting was performed with antibodies raised against phosphotyrosine (P-tyr), EGFR, activated (act) MAPK, and total (tot) MAPK.

The next experiments were designed to understand the mechanism of the OA-induced EGFR. It was hypothesized that OA may (1) trigger an autocrine secretion of EGF or (2) interact (more or less directly) with and activate EGFR.

EGFR Autophosphorylation and Activation by OA Is Independent of Any Autocrine Effect
In our experimental system, a role for autocrine secretion of EGF or other diffusible mediators very likely will be excluded because OA-induced activations of EGFR and MAPK were not inhibited by anti-EGF antibody (in contrast to that elicited by added exogenous EGF) (Figure 5, A and B) and because the transfer of preconditioned medium (from cells pretreated with OA) triggered neither EGFR phosphorylation nor MAPK activation (Figure 5C). This conclusion (no requirement of autocrine EGF) was consistent with the very rapid response (2 minutes) (Figure 1) and was confirmed by the lack of inhibition with cycloheximide and with phenylmethylsulfonyl fluoride (inhibitors of proteins synthesis and of pro-EGF processing, respectively)³⁷ (data not shown).

View larger version (29K): [in this window] [in a new window]   Figure 5. Activation of EGFR and MAPK by OA is independent of any autocrine effect. A and B, B82LK+ (EGFR-transduced) fibroblasts were stimulated by OA (50 µmol/L, 10 minutes) or EGF (1 nmol/L, 5 minutes) in presence or absence of 10 µmol/L neutralizing anti-EGF antibody. A, Western blot probed by anti-phosphotyrosine and anti-activated (act) MAPK antibodies. B, EGFR kinase activity of EGFR immunoprecipitates determined by poly-GluTyr phosphorylation. Values are mean±SEM of 3 experiments. *P<0.01. C, Transfer of preconditioned medium. After preincubation without (Co) or with OA (50 µmol/L) or EGF (10 nmol/L) for 15 minutes, B82LK+ cells were washed once in PBS and incubated in fresh medium (MEM containing 0.5% FCS) for an additional 15 minutes (preconditioned medium). This preconditioned medium was transferred onto unstimulated (reporter) B82LK+ cells for a 15- minute incubation. Then, extracts from reporter cells were used for Western blotting and probed with anti-phosphotyrosine and anti-activated (act) MAPK antibodies (EGF-treated cells as positive control).

OA Triggers In Vitro EGFR Phosphorylation and EGFR Kinase Activation
Because immunopurified EGFR can be activated in vitro by EGF and by nonspecific "agonists," such as oxidized lipids and hydroxynonenal (4-HNE),²⁴ we investigated whether OA was also able to stimulate in vitro autophosphorylation of immunopurified EGFR.

As shown in Figure 6, OA (50 µmol/L) incubated in vitro with EGFR (immunopurified from B82LK⁺ cells) induced both EGFR tyrosine phosphorylation (Figure 6A) and activation of EGFR intrinsic tyrosine kinase (Figure 6B).

View larger version (23K): [in this window] [in a new window]   Figure 6. OA triggers in vitro tyrosine phosphorylation and activation of immunopurified EGFR. EGFR, immunopurified from unstimulated B82LK+ (EGFR transduced) fibroblasts, was incubated in vitro without (Co) or with OA (50 µmol/L) or with EGF (1 nmol/L) for 10 minutes. A, Assay was performed in phosphorylation buffer24 and Western blot was probed with anti-phosphotyrosine (P-tyr) or anti-EGFR antibodies. B, EGFR kinase activity determined by poly-GluTyr phosphorylation, as in Figure 5B. Values are mean±SEM of 3 experiments. *P<0.01 compared with control.

These data suggest that OA may interact with EGFR, thereby activating it. This led us to investigate 2 possible mechanisms of the interaction between OA and EGFR: (1) through specific interaction at the binding site of EGF and (2) through nonspecific interaction involving the amphiphilic properties of OA.

Study of Mechanism of EGFR Activation by OA: Analogy With Mild Surfactants
As shown in Figure 7, [¹²⁵I]EGF (30 pmol/L) binding was not altered by OA (50 µmol/L) (these EGF and OA concentration induced a grossly similar EGFR autophosphorylation). This suggests that OA does not interfere with the EGF-binding site of EGFR.

View larger version (24K): [in this window] [in a new window]   Figure 7. OA does not compete with [125I]EGF binding to EGFR. Cells were incubated with a tracer amount of [125I]EGF for variable periods of time (up to 60 minutes) in absence (•) or presence of OA (50 µmol/L), added to incubation medium either simultaneously () or 30 minutes before [125I]EGF ().

Because OA is an amphiphilic compound with mild surfactant properties (under the study conditions), we examined whether other mild surfactants were also able to activate EGFR. The two mild surfactants Triton X-100 and Tween-20 were able to induce in vitro tyrosine phosphorylation of immunopurified EGFR (Figure 8, A and B), thus suggesting that EGFR interaction with amphiphilic compounds induces activation of its intrinsic tyrosine kinase (probably by eliciting conformation changes). Similar to UFAs, these mild surfactants were also able to trigger EGFR activation in situ (under non lytic conditions, as assessed through trypan blue exclusion) (Figure 8, C and D). These data suggest that EGFR may act as a sensor for amphiphilic compounds, namely UFAs, under physiological conditions (and other mild surfactants under experimental conditions).

View larger version (30K): [in this window] [in a new window]   Figure 8. EGFR activation by surfactants Tween-20 and Triton X-100. A and B, In vitro activation of EGFR by surfactants. A, Immunopurified EGFR (from B82LK+ cells, prepared in absence of Triton X-100) was incubated for 20 minutes in phosphorylation buffer without or with Triton X-100 (TX, 10 ng/mL), Tween-20 (TW, 100 ng/mL), OA (30 µmol/L), or EGF (1 nmol/L). Then, Western blots were probed with anti-phosphotyrosine (P-tyr) and anti-EGFR antibodies. B, Same as in A but in presence of [33]ATP. After spotting onto phosphocellulose membrane, EGFR radioactivity was determined. *P<0.01 compared with control. C and D, ECV-304 cells were incubated for 15 minutes in the presence of increasing concentrations of Triton X-100 or Tween-20, and cell extracts were immunoblotted with anti-phosphotyrosine (P-tyr) or anti-EGFR antibodies.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
AA and oxidized PUFAs induce EGFR activation,^{17 25 26} but the mechanism of action is largely unknown, except for oxidized PUFAs, which inhibit protein tyrosine phosphatases (PTPases) and thereby increase tyrosine phosphorylation of cell proteins.²⁵ In the present work, we investigated whether other major UFAs were able to activate EGFR and sought to identify their mechanism of action.

Because FAs are liberated and transported in the blood flow, they are in contact with vascular wall cells and may alter their physiology. This led us to use an endothelial cell line (ECV-304) that exhibits a stable phenotype, does not require exogenous growth factors, and expresses sufficient EGFR to perform Western blotting. Similar results were obtained with vascular smooth muscle cells and other cell types when EGFR was expressed (these data are not reported here to avoid redundant data).

The data reported here suggest that (1) EGFR is a primary target for UFAs, (2) UFAs activate the EGFR signaling pathway, (3) this UFA activity is correlated to their unsaturation degree and does not require FA oxidation, and (4) EGFR may act as a sensor of amphiphiles and of membrane fluidity changes. This sensitivity of EGFR to its microenvironment is not a general property of all membrane receptor tyrosine kinases, because OA triggered no significant activation of platelet-derived growth factor and insulin receptors (data not shown).

The OA-induced EGFR autophosphorylation is moderate but is effective in activation of the EGFR signaling pathway (ie, recruitment of SH2-containing substrates) and MAPK. In genetically engineered B82L cells, EGFR expression is necessary for OA-induced MAPK activation (no MAPK activation in EGFR- deficient cells B82L), but PKC activation is not required (because OA-induced MAPK activation is not inhibited by PKC inhibitors). This is consistent with the conclusions of Casabiell et al,³⁸ but it cannot be excluded that the OA-induced PKC activation may be effective in other cell types.^{12 13 14}

To investigate the molecular mechanism of the OA-induced EGFR activation, several mechanistic hypotheses were considered: (1) autocrine secretion of EGF (or other mediators able to activate EGFR), (2) oxidative stress, lipid oxidation, or both, which in turn may induce EGFR activation, and (3) direct EGFR activation (with EGFR being a primary target).

An autocrine secretion of EGF was not involved in the OA-induced EGFR activation because OA-induced EGFR autophosphorylation is very rapid (2 minutes), is not blocked by cycloheximide (thus excluding de novo synthesis of EGF), is not inhibited by phenylmethylsulfonyl fluoride or leupeptin (two inhibitors of pro-EGF processing)³⁷ (data not shown), is not blocked by anti-EGF antibody, and is not induced by the transfer of preconditioned medium.

Oxidative stress (induced by UV-C irradiation²² or H₂O₂²³ ) and oxidized lipids^{16 25} or 4-HNE²⁴ may activate EGFR either directly²⁴ or indirectly through PTPase inhibition.³⁹ The short-term OA-induced EGFR activation is probably not mediated through reactive oxygen species (ROS) generation or PTPase inhibition because (1) OA did not generate intracellular ROS, (2) antioxidants (probucol, tocopherol, trolox) did not inhibit the short-term OA-induced EGFR autophosphorylation, (3) OA-induced EGFR autophosphorylation occurs in vitro on immunopurified EGFR independent of any cellular generation of ROS, and (4) all of the in vitro assays on immunopurified EGFR contained Na₃VO₄, an inhibitor of PTPase (thus excluding a role for active PTPase in vitro). Moreover, OA-induced EGFR activation is probably not mediated via oxidation products of OA because OA is relatively resistant to autoxidation,⁴⁰ is not a substrate for lipoxygenases, and did not induce the formation of 4-HNE/protein adducts. However, it is not excluded that the relatively sustained phase of EGFR activation (30 to 45 minutes) may involve ROS generation, because EGFR activation induces H₂O₂ generation, which plays a role in EGFR pathway activation.⁴¹

Chen et al²⁶ recently reported that epoxyeicosatrienoic acid activates Src kinase (SrcK), which initiates a tyrosine kinase cascade (involving EGFR). Although OA is poorly oxidizable and cannot lead to epoxyeicosatrienoic acid formation, we investigated whether SrcK may be involved with the use of SrcK⁺ and SrcK^- (overexpressing a negative dominant SrcK^-) cell lines.³³ OA induced EGFR autophosphorylation in both SrcK⁺ and SrcK^- cells, but the basal and OA-stimulated EGFR activations were higher in SrcK⁺ (data not shown). This suggests that c-src is not strictly required for the OA-induced EGFR activation, which is in agreement with the in vitro OA-induced activation of immunopurified EGFR. Therefore, non oxidized OA may directly activate EGFR, via a mechanism different from that of epoxyeicosatrienoic acid,³⁴ but it is not excluded that in vivo, EGFR activation induced by OA may be potentiated by c-src.⁴²

Finally, in vitro experiments suggest that OA interacts (probably directly) with an EGFR domain (different from the EGF binding site), thereby activating it. Because of its amphiphilic properties, OA may interact with hydrophobic domains, such as with the transmembrane domain either directly (in vitro) or after insertion in membrane lipid bilayer, where it elicits changes in the membrane fluidity.⁴³ This hypothetical mechanism was supported by EGFR activation induced by mild detergents (Tween-20 and Triton X-100), which is in agreement with the results of Igarashi et al.⁴⁴ Amphiphiles may alter the membrane fluidity, thereby inducing conformational changes and activation of EGFR. This hypothesis is consistent with the data of Miloso et al,⁴⁵ who report that point mutation in the EGFR transmembrane domain induce (probably through conformational change) a mild constitutive activation of EGFR. Finally, the reported data suggest that EGFR may act as a sensor for amphiphiles and membrane fluidity changes. In vivo, it cannot be excluded that OA may also modulate the activity of an EGFR ligand.

From a pathophysiological point of view, a local NEFA concentration effective in the activation of EGFR may be reached acutely during intravascular lipolysis of chylomicrons at the endothelial surface; during triglyceride lipolysis of adipocytes occurring during fasting, ketoacidosis, or other conditions associated with increased lipolysis; or during phospholipolysis by phospholipases during inflammation. Because the level of UFA (or the ratio of unsaturated to saturated FA) is largely dependent on diet composition, the reported data point out a new nutritional mechanism that regulates EGFR activity and subsequent cell functions. The EGFR pathway may have have interplay with the other NEFA-activated signaling pathways,^{9 10 11 12 13 14 15 16 17} which participate in the regulation of major intracellular events, such as cell proliferation, migration and adhesion, gene expression, glucose transport, and cellular metabolism. EGFR plays a role in wound healing and may be involved in repair processes, remodeling, and fibrosis of the vascular wall in response to injury in normal and atherosclerotic areas (where EGFR is highly expressed in proliferating cells).⁴⁶ This may help to stabilize the plaque and may therefore in part account for the rather favorable effect of OA intake. The effects of PUFA are more complex because they are oxidizable and also lead to eicosanoid formation, which exhibits various potent effects on the vascular wall and the hemostasis equilibrium.

In conclusion, the reported data provide novel insight into the mechanism of cis-UFAs as mediators triggering cell signaling via EGFR, which apparently is a novel primary target of UFAs, acts as a sensor for amphilic agents, and may participate in vascular wall biology regulation.

	Acknowledgments

 
This work was supported by INSERM, University Toulouse-3, and European Community (Biomed-2 BMH4-CT98-3191). Drs Vacaresse and Lajoie-Mazenc were recipients of fellowships from the Association de Recherche contre le Cancer, Ligue contre le Cancer, and Association Française contre les Myopathies). We thank Dr M. Weber for the B82LK⁺ cells and Dr S.J. Parsons for the SrcK^- cells and for fruitful discussions.

	Footnotes

 
¹ Both authors contributed equally to this study.

Received July 12, 1999; accepted August 31, 1999.

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