LDLs induce fibroblast spreading independently of the LDL receptor via activation of the p38 MAPK pathway
2003; Elsevier BV; Volume: 44; Issue: 12 Linguagem: Inglês
10.1194/jlr.m300266-jlr200
ISSN1539-7262
AutoresIveta Dobreva, Gérard Waeber, Vincent Mooser, Richard W. James, Christian Widmann,
Tópico(s)Protein Kinase Regulation and GTPase Signaling
ResumoBecause adventitial fibroblasts play an important role in the repair of blood vessels, we assessed whether elevation in LDL concentrations would affect fibroblast function and whether this depended on activation of intracellular signaling pathways. We show here that in primary human fibroblasts, LDLs induced transient activation of the p38 mitogen-activated protein kinase (MAPK) pathway, but not the c-Jun N-terminal kinase MAPK pathway. This activation did not require the recruitment of the LDL receptor (LDLR), because LDLs efficiently stimulated the p38 MAPK pathway in human and mouse fibroblasts lacking functional LDLR, and because receptor-associated protein, an LDLR family antagonist, did not block the LDL-induced p38 activation. LDL particles also induced lamellipodia formation and cell spreading. These effects were blocked by SB203580, a specific p38 inhibitor.Our data demonstrate that LDLs can regulate the shape of fibroblasts in a p38 MAPK-dependent manner, a mechanism that may participate in wound healing or vessel remodeling as in atherosclerosis. Because adventitial fibroblasts play an important role in the repair of blood vessels, we assessed whether elevation in LDL concentrations would affect fibroblast function and whether this depended on activation of intracellular signaling pathways. We show here that in primary human fibroblasts, LDLs induced transient activation of the p38 mitogen-activated protein kinase (MAPK) pathway, but not the c-Jun N-terminal kinase MAPK pathway. This activation did not require the recruitment of the LDL receptor (LDLR), because LDLs efficiently stimulated the p38 MAPK pathway in human and mouse fibroblasts lacking functional LDLR, and because receptor-associated protein, an LDLR family antagonist, did not block the LDL-induced p38 activation. LDL particles also induced lamellipodia formation and cell spreading. These effects were blocked by SB203580, a specific p38 inhibitor. Our data demonstrate that LDLs can regulate the shape of fibroblasts in a p38 MAPK-dependent manner, a mechanism that may participate in wound healing or vessel remodeling as in atherosclerosis. Complications of atherosclerosis are the most common cause of death in Western societies. In addition to aging, elevated levels of lipoproteins are the major risk factor for the development of the disease, and they are sufficient to induce atherosclerosis in the absence of other risk factors (1Glass C.K. Witztum J.L. Atherosclerosis. The road ahead.Cell. 2001; 104: 503-516Abstract Full Text Full Text PDF PubMed Scopus (2639) Google Scholar). Among all types of lipoproteins circulating in the blood, LDLs are considered to be the main atherogenic class (2Martin M.J. Hulley S.B. Browner W.S. Kuller L.H. Wentworth D. Serum cholesterol, blood pressure, and mortality: implications from a cohort of 361,662 men.Lancet. 1986; 2: 933-936Abstract PubMed Scopus (972) Google Scholar), but despite the indisputable epidemiologic association of LDLs and atherosclerosis, the mechanisms underlying the development of the disease are still poorly understood. The best-characterized function of LDLs is to deliver cholesterol to cells. They may, however, have functions in addition to transporting cholesterol. For example, they seem to produce a mitogenic effect on endothelial cells, smooth muscle cells, and fibroblasts, and induce growth-factor production, chemotaxis, cell proliferation, and cytotoxicity (3Gouni-Berthold I. Sachinidis A. Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?.FASEB J. 2002; 16: 1477-1487Crossref PubMed Scopus (22) Google Scholar). Moreover, an increase of LDL plasma concentration, which is observed during the development of atherosclerosis, can activate various mitogen-activated protein kinase (MAPK) pathways, including the two stress-activated protein kinase (SAPK) pathways, the c-Jun N-terminal kinase (JNK) and the p38 MAPK pathways (3Gouni-Berthold I. Sachinidis A. Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?.FASEB J. 2002; 16: 1477-1487Crossref PubMed Scopus (22) Google Scholar, 4Zhu Y. Liao H-L. Wang N. Friedli Jr., O. Verna L. Stemerman M.B. Low-density lipoprotein activates Jun N-terminal kinase (JNK) in human endothelial cells.Biochim. Biophys. Acta. 1999; 1436: 557-564Crossref PubMed Scopus (20) Google Scholar, 5Metzler B. Hu Y. Dietrich H. Xu Q. Increased expression and activation of stress-activated protein kinases/c-Jun NH(2)-terminal protein kinases in atherosclerotic lesions coincide with p53.Am. J. Pathol. 2000; 156: 1875-1886Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). The functions of LDLs in blood vessels have been assessed mainly through studies of their effect in endothelial and smooth-muscle cells. There is a third cell type found in the adventitia of blood vessels that has been studied very little so far, the cells of the fibroblastic lineage. Because it appears that these cells play a more important role in blood vessels than initially anticipated (6Sartore S. Chiavegato A. Faggin E. Franch R. Puato M. Ausoni S. Pauletto P. Contribution of adventitial fibroblasts to neointima formation and vascular remodeling: from innocent bystander to active participant.Circ. Res. 2001; 89: 1111-1121Crossref PubMed Scopus (419) Google Scholar), we assessed here the potential signaling properties of LDLs in a fibroblast cell model. Using LDL preparations that were thoroughly controlled and tested for their biological activity, we demonstrate that LDLs induce the activation of p38 MAPKs independently of LDL receptors (LDLRs). We also show that the activation of the p38 MAPKs is required for LDL-induced fibroblast spreading, a mechanism that could potentially participate in atherogenesis. dl-[3-14C]HMG-CoA was purchased from Amersham Pharmacia. HMG-CoA, glucose-6-phosphate dehydrogenase, and β-NADP were from Sigma. Anti-phospho-p38 (Thr180/Tyr182) [catalog no. 9211] and anti-p38 MAPK antibodies [catalog no. 9212] were purchased from Cell Signaling Technology. SB203580 [4-(4′-fluorophenyl)-2-(4′-methylsulfinylphenyl)-5-(4′-pyridyl imidazole] was from Promega (catalog no. V1161). Recombinant receptor-associated protein (RAP) was produced by Dr. Johannes Nimpf, University and Biocenter of Vienna, Austria. GM00316B and GM00488C human skin fibroblasts were purchased from Coriell Cell Repositories and maintained in DMEM supplemented with 15% nondecomplemented fetal calf serum (FCS) (Amimed) at 37°C and 5% CO2. GM00316B cells were derived from a healthy 12-year-old Caucasian male subject and GM00488C cells were derived from an 11-year-old Caucasian male subject suffering from homozygous familial hypercholesterolemia (FH). Mouse embryonic fibroblasts (MEFs) lacking the LDLR were isolated from E12 embryos of the B6.129S7-Ldlrtm1Her mouse strain (The Jackson Laboratory, stock number 002207; background strain: C57BL/6J). Control MEFs were isolated from E12 C57BL/6J embryos. Briefly, 12 days-gestant females were killed by cervical dislocation, and the embryos were then removed from the placenta and placed in a 10 cm Petri dish containing 10 ml of ice-cold PBS [116 mM NaCl, 10.4 mM Na2HPO4, 3.2 mM KH2PO4 (pH 7.25)], where their livers were dissected out and discarded. The embryos were then minced and transferred to a bottle containing 200 ml of HBSS without calcium and magnesium (5.4 mM KCl, 0.44 mM KH2PO4, 137 mM NaCl, 3.4 mM Na2HPO4, and 4.2 mM NaHCO3), 0.05% trypsin, 0.53 mM EDTA-4 Na, 5.5 mM glucose, and 28 μM phenolsulfonphtalein (phenol red) (Gibco; catalog no. 25300-062) for 2–3 h. The trypsinized cells were then collected by centrifugation, washed twice in cold PBS, resuspended in DMEM supplemented with 10% newborn calf serum, and 100 U/ml penicillin/streptomycin and plated in 10 cm Petri dishes (each dish containing the cells derived from one embryo). LDLs were isolated from human serum (HS) by sequential density ultracentrifugation as described previously (7Havel R.J. Eder H.A. Bragton J.H. The distribution and chemical composition of ultracentrifugally separated lipoproteins in human serum.J. Clin. Invest. 1955; 34: 1345-1353Crossref PubMed Scopus (6487) Google Scholar, 8James R.W. Pometta D. Immunofractionation of high density lipoprotein subclasses 2 and 3. Similarities and differences of fractions isolated from male and female populations.Atherosclerosis. 1990; 83: 35-45Abstract Full Text PDF PubMed Scopus (21) Google Scholar) and dialyzed for 48 h against PBS containing 100 μM EDTA. Human lipoprotein-free serum was prepared by removal of lipoproteins by ultracentrifugation at a density of 1.23 g/l (8James R.W. Pometta D. Immunofractionation of high density lipoprotein subclasses 2 and 3. Similarities and differences of fractions isolated from male and female populations.Atherosclerosis. 1990; 83: 35-45Abstract Full Text PDF PubMed Scopus (21) Google Scholar). The protein concentration was measured by Bradford assay using BSA as a standard. The oxidation of the lipoproteins was measured by the lipoperoxide method (9Wolff S.P. Ferrous ion oxidation in the presence of the ferric ion indicator xylenol orange for the measure of hydroperoxides: the FOX assay.Methods Enzymol. 1994; 233: 182-189Crossref Scopus (1075) Google Scholar). Analysis of lipopolysaccharide (LPS; endotoxin) contents was performed by the ENDOTELL AG company (Allschwil, Switzerland) using their kinetic turbidimetric Limulus amoebocyte lysate. LDL preparations used in the present study had undetectable LPS levels (<26 pg/ml). These LDL preparations had the expected biological activities, as demonstrated by their capacity for inhibiting the binding of fluorescent LDLs to normal human fibroblasts and by their ability to suppress the activity of the HMG - CoA reductase (data not shown). GM00316 and GM00488 cells were starved for 4 h in DMEM without serum and then treated with 10 μg/ml fluorescent human LDLs (LDL-BODIPY; Molecular Probes, catalog no. L-3483; excitation at 515 nm and emission at 522 nm) in the presence or in the absence of 100 μg/ml unlabeled LDLs. The cells were washed once in cold PBS, and images were taken on live cells with an inverted Leica DMIRB microscope equipped for epifluorescence imaging using the Openlab software (Improvision). Quantitation of the cell-associated fluorescence was performed using a fluorescent-activated cell sorter (FACS) (Becton Dickinson FACScan). Cells were starved for 24 h in DMEM containing 2.5 mg/ml lpoprotein-free serum and incubated with 50 μg/ml LDL for 12 h. The HMG - CoA-reductase activity was measured according to Brown and Goldstein (10Brown M.S. Goldstein J.L. Familial hypercholesterolemia: defective binding of lipoproteins to cultured fibroblasts associated with impaired regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity.Proc. Natl. Acad. Sci. USA. 1974; 71: 788-792Crossref PubMed Scopus (391) Google Scholar). Briefly, the cells were scraped in 1 ml of buffer A [0.05 M Tris-HCl (pH 7.4) and 0.15 M NaCl] and collected by centrifugation at 900 g for 3 min. The pellet was dissolved in 200 μl of buffer B [50 mM potassium phosphate (pH 7.4), 5 mM EDTA, 0.2 M KCl, and 0.25% Triton X-100]. After 10 min incubation at 37°C, the lysate was precleared by centrifugation at 12,000 g for 1 min. Protein concentration was measured by Bradford assay using BSA as a standard. Aliquots of the supernatant (20–100 μg protein) were incubated 2 h at 37°C in a final volume of 200 μl of 0.1 M potassium phosphate (pH 7.4), 20 mM glucose-6-phosphate, 2.5 mM β-nicotinamide adenine dinucleotide phosphate (triphosphopyridine nucleotide) 0.7 units glucose-6-phosphate-dehydrogenase, 5 mM DTT, and 3 × 10−5 M dl-[3-14C]HMG - CoA (5 Ci/mol). The enzymatic reaction was stopped with 20 μl 5 N HCl. Product marker (20 μl unlabeled mevalonolactone 5 mg/ml) and 0.5 g Na2SO4 were added, and the mixture was extracted twice with 10 ml diethylether. The extracts were pooled and dried, and the residue was dissolved in 60 μl acetone. The mevalonolactone was separated from the mixture by thin-layer chromatography in acetone-benzene (1:1; v/v) and counted in a liquid scintillation counter (Wallac). The JNK activity was measured using a solid-phase kinase assay in which glutathione-S-transferase-c-Jun (1–79) bound to glutathione-Sepharose 4B beads was used to affinity purify JNK from cell lysates (11Widmann C. Gerwins P. Lassignal Johnson N. Jarpe M.B. Johnson G.L. MEKK1, a substrate for DEVD-directed caspases, is involved in genotoxin-induced apoptosis.Mol. Cell. Biol. 1998; 18: 2416-2429Crossref PubMed Scopus (210) Google Scholar). The activation of p38 MAPKs was assessed by Western blot analysis using an antibody specific for the active form of p38 MAPK (Cell Signaling Technology, catalog no. 9211). Total levels of p38 MAPKs were visualized using a p38 MAPK-specific antibody from Cell Signaling Technology (catalog no. 9212). Western blotting was performed as described previously (12Yang J-Y. Widmann C. Antiapoptotic signaling generated by caspase-induced cleavage of RasGAP.Mol. Cell. Biol. 2001; 21: 5346-5358Crossref PubMed Scopus (104) Google Scholar). Human fibroblasts were plated on uncoated 12 mm-diameter glass coverslips (VWR International; catalog no. 6012401) in 10 cm dishes supplemented with DMEM containing 15% FCS. On the following day, the medium was replaced with DMEM containing 2.5 mg/ml lipoprotein-free serum, and the cells were starved for 24 h. The cells were then pretreated or not with 10 μM SB203580 for 30 min and incubated for 24 h with 200 μg/ml LDL. After various periods of time, cells on coverslips were washed in PBS and fixed in 4% paraformaldehyde/PBS for 30 min. The coverslips were then mounted in Vectashield mounting medium (Vector Lab.; catalog no. H-1000), and pictures were taken with a Zeiss Axioplan 2 microscope. Cell area was measured on digital pictures of the cells using the Object-Image 2.08 software (Norbert Vischer, University of Amsterdam; http://simon. bio.uva.nl). Comparisons between groups were made by homoscedastic two-tailed t-test analysis using the Excel 2000 software. To determine whether LDLs could activate the p38 MAPK pathway in fibroblasts, the human cell line GM00316C was incubated with 200 μg/ml LDL for up to 60 min. Western blot detection of the active form of p38 MAPKs demonstrated that the LDL treatment resulted in a transient p38 activation peaking between 5 min and 15 min of stimulation (Fig. 1A). This p38 MAPK activation was not a consequence of increased protein expression, because the levels of total p38 MAPK remained constant during the LDL incubation (Fig. 1A). In contrast to the p38 MAPKs, the JNK MAPKs were not activated by LDLs (Fig. 1B). The absence of JNK activation was not due to a nonfunctional JNK MAPK pathway in GM00316C cells, because anisomycin, a known SAPK activator (13Aplin A.E. Hogan B.P. Tomeu J. Juliano R.L. Cell adhesion differentially regulates the nucleocytoplasmic distribution of active MAP kinases.J. Cell Sci. 2002; 115: 2781-2790Crossref PubMed Google Scholar), potently stimulated the JNKs in these cells (Fig. 1B). To assess at which LDL concentrations human fibroblasts activated the p38 MAPK pathway, GM00316C cells were stimulated with increasing concentrations of LDLs for 10 min. Figures 1C and D show that p38 MAPK activation started to be detected when cells were incubated with 50–100 μg/ml of LDLs. The maximal stimulation was observed when LDL concentration reached 200 μg/ml and higher. Because the extravascular LDL concentration is ∼100 μg/ml (14Busso N. Dudler J. Salvi R. Peclat V. Lenain V. Marcovina S. Darioli R. Nicod P. So A.K. Mooser V. Plasma apolipoprotein(a) co-deposits with fibrin in inflammatory arthritic joints.Am. J. Pathol. 2001; 159: 1445-1453Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar), these results indicate that the p38 MAPK pathway can be activated by physiological concentrations of LDLs in fibroblasts. The maximal p38 MAPK response is, however, observed at higher, pathological, extravascular concentrations of these lipoproteins. A series of experiments were performed to rule out that the p38 activation induced by LDLs was due to serum contaminants. First, sera or LDLs were dialyzed against membranes with a 100 kDa cutoff. This procedure removes molecules with molecular mass lower than 100 kDa, including hormones such as insulin, epidermal growth factor, and estrogens, known to potently activate the p38 MAPK pathway in various cell types (15Ouwens D.M. de Ruiter N.D. van der Zon G.C. Carter A.P. Schouten J. van der Burgt B.C. Kooistra K. Bos J.L. Maassen J.A. van Dam H. Growth factors can activate ATF2 via a two-step mechanism: phosphorylation of Thr71 through the Ras-MEK-ERK pathway and of Thr69 through RalGDS-Src-p38.EMBO J. 2002; 21: 3782-3793Crossref PubMed Scopus (186) Google Scholar, 16Lee H. Bai W. Regulation of estrogen receptor nuclear export by ligand-induced and p38-mediated receptor phosphorylation.Mol. Cell. Biol. 2002; 22: 5835-5845Crossref PubMed Scopus (172) Google Scholar). This procedure will not eliminate LDLs weighing ∼3 million Da. The dialyzed sera or LDL preparations had similar abilities to activate the p38 MAPK pathway compared with the nondialyzed preparations (Fig. 1E), indicating that the observed response is not due to small-to-medium molecular weight contaminants. Second, LPS, which is a possible contaminant known to induce the p38 MAPK pathway (17Kim S.O. Ono K. Han J. Apoptosis by pan-caspase inhibitors in lipopolysaccharide-activated macrophages.Am. J. Physiol. Lung Cell. Mol. Physiol. 2001; 281: L1095-L1105Crossref PubMed Google Scholar), was not detected in our LDL preparations (see Experimental Procedures). Finally, while normal HS efficiently stimulated the p38 MAPK pathway in human fibroblasts, the serum depleted from lipoproteins was less potent in activating the p38 MAPK pathway (Fig. 1F). This indicates that among the various proteins and particles found in blood, lipoproteins are major stimulators of the p38 MAPK pathway. Oxidation may be important for the signaling properties of LDLs (5Metzler B. Hu Y. Dietrich H. Xu Q. Increased expression and activation of stress-activated protein kinases/c-Jun NH(2)-terminal protein kinases in atherosclerotic lesions coincide with p53.Am. J. Pathol. 2000; 156: 1875-1886Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar, 18Jing Q. Xin S.M. Cheng Z.J. Zhang W.B. Zhang R. Qin Y.W. Pei G. Activation of p38 mitogen-activated protein kinase by oxidized LDL in vascular smooth muscle cells: mediation via pertussis toxin-sensitive G proteins and association with oxidized LDL-induced cytotoxicity.Circ. Res. 1999; 84: 831-839Crossref PubMed Scopus (82) Google Scholar). Using the lipoperoxide method, we found that no oxidation could be detected in our LDL preparations (see Experimental Procedures). Moreover, incubating LDLs with cells for the time required to induce the p38 MAPK pathway (10 min) did not lead to detectable modifications of the migration pattern of the LDLs in SDS-PAGE, such as those observed for mildly and strongly oxidized LDLs (Fig. 1G). However, mildly and strongly oxidized LDLs had a tendency to stimulate the p38 MAPKs in fibroblasts more strongly than did native LDLs (Fig. 1H). These results indicate that oxidation of LDLs, even though it is not a prerequisite for the stimulation of the p38 MAPK pathway, may enhance this response. We next assessed whether the LDL-induced p38 activation requires the recruitment of the LDLR. Fibroblasts from a control subject or fibroblasts from an FH patient lacking functional LDLRs were stimulated with 200 μg/ml LDL. As shown in Fig. 2A and B, this resulted in transient p38 activation in both types of fibroblasts. There was a tendency for the FH-derived fibroblasts to generate a more sustained p38 MAPK activation in response to LDLs, compared with control cells, but this did not reach statistical significance (Fig. 2B). Similarly, MEFs derived from wild-type mice or from LDLR knockout mice both activated the p38 MAPK pathway in response to LDLs (Fig. 2C). Sterol-regulatory element binding proteins (SREBPs) mediate the transcription of both the LDLR and the HMG - CoA reductase genes (19Edwards P.A. Tabor D. Kast H.R. Venkateswaran A. Regulation of gene expression by SREBP and SCAP.Biochim. Biophys. Acta. 2000; 1529: 103-113Crossref PubMed Scopus (264) Google Scholar). When cells are cultured in lipoprotein-containing medium (e.g., serum-containing medium), SREBPs remain membrane associated and do not translocate to the nucleus; hence, the LDLR gene and the HMG - CoA reductase gene are not transcribed, and therefore their cellular levels decrease. Conversely, in lipoprotein-free medium, LDLR levels and HMG - CoA reductase activity increase [(19Edwards P.A. Tabor D. Kast H.R. Venkateswaran A. Regulation of gene expression by SREBP and SCAP.Biochim. Biophys. Acta. 2000; 1529: 103-113Crossref PubMed Scopus (264) Google Scholar) and data not shown]. Figure 2D shows that LDLs stimulated the p38 MAPK pathway to a similar extent in cells cultured in serum-containing medium or in starved cells. This indicates that variations in HMG - CoA reductase activities and LDLR levels do not affect the LDL-induced p38 MAPK response. Figure 2D also shows that fibroblasts cultured in 15% FCS-containing medium that contains LDLs do not have a sustained p38 MAPK activation (first lane of Fig. 2D). This is likely a consequence of the fact that LDL-induced p38 MAPK activation is quickly desensitized (Fig. 1A). As exogenous addition of 200 μg/ml human LDLs in 15% FCS-containing medium increases by about eight times the LDL concentration in the cell culture medium (data not shown), this may be sufficient, however, to override the desensitization and induce p38 MAPK activation (Fig. 2D, left part). Altogether, the experiments presented in Fig. 2A–D demonstrate that LDL-induced p38 MAPK activation does not involve the LDLR. To assess whether other members of the LDLR family are required for the LDL-induced p38 MAPK activation, we investigated whether the 39 kDa RAP that functions as a general LDLR family antagonist could block the activation of p38 in human fibroblasts. RAP did not block the LDL-induced p38 MAPK activation but rather had a marginal potentiation effect on the LDL-induced p38 phosphorylation that did not, however, reach statistical significance (Fig. 2E, F). RAP was functional in these experiments because it could efficiently reduce the internalization of fluorescent LDLs (LDL-BODIPY), as measured by FACS analysis (Fig. 2G). Therefore, antagonizing LDLR family members did not inhibit the ability of LDLs to stimulate the p38 MAPKs. Activation of the p38 MAPK pathway can regulate the dynamic of actin polymerization and therefore cell morphology (20Rousseau S. Houle F. Landry J. Huot J. p38 MAP kinase activation by vascular endothelial growth factor mediates actin reorganization and cell migration in human endothelial cells.Oncogene. 1997; 15: 2169-2177Crossref PubMed Scopus (729) Google Scholar). Because LDLs can stimulate p38 MAPKs, we tested whether LDLs could modulate cell shape. Human fibroblasts were incubated in lipoprotein-free medium. Under these conditions, the fibroblasts remained elongated and developed few lamellipodias (Fig. 3A, left panel). Twenty-four hours later, 200 μg/ml of LDLs were added. This treatment favored the development of numerous lamellipodias, which resulted in a dramatic cell spreading (Fig. 3A, right panel; see also the movie in the supplemental data). Quantitation of the cell area prior to and after the LDL stimulation indicated that LDLs induced a 2- to 3-fold increase in cell surface (Fig. 3B). To assess whether LDL-induced cell increase depended on the activation of the p38 MAPK pathway, cells were pretreated or not with the specific p38 MAPK inhibitor SB203580 before being subjected to LDL stimulation. The SB compound efficiently blocked the increase in cell size stimulated by the LDLs (Fig. 4). Altogether, our results indicate that LDLs induce cell spreading via activation of the p38 MAPK pathway in an LDLR-independent manner.Fig. 4LDL-induced cell spreading requires p38 activation. Human fibroblasts plated on coverslips were starved as above, pretreated or not with 10 μM SB203580 for 30 min, and incubated for 24 h with 200 μg/ml LDL. Cell surface was then measured. The results correspond to the mean ± SE. This experiment was performed three times with similar results.View Large Image Figure ViewerDownload Hi-res image Download (PPT) The p38 MAPK pathway can be stimulated by cytokines via G protein-coupled receptors or in response to various stress conditions, such as increase in osmolarity (21Widmann C. Gibson S. Jarpe M.B. Johnson G.L. MAPK pathways: conservation of a three-kinase module from yeast to man.Physiol. Rev. 1999; 79: 143-180Crossref PubMed Scopus (2274) Google Scholar) or, as shown here in fibroblasts and by others in other cell types, by LDLs (18Jing Q. Xin S.M. Cheng Z.J. Zhang W.B. Zhang R. Qin Y.W. Pei G. Activation of p38 mitogen-activated protein kinase by oxidized LDL in vascular smooth muscle cells: mediation via pertussis toxin-sensitive G proteins and association with oxidized LDL-induced cytotoxicity.Circ. Res. 1999; 84: 831-839Crossref PubMed Scopus (82) Google Scholar, 22Hackeng C.M. Relou I.A. Pladet M.W. Gorter G. van Rijn H.J. Akkerman J.W. Early platelet activation by low density lipoprotein via p38MAP kinase.Thromb. Haemost. 1999; 82: 1749-1756Crossref PubMed Scopus (55) Google Scholar, 23Zhao M. Liu Y. Wang X. New L. Han J. Brunk U.T. Activation of the p38 MAP kinase pathway is required for foam cell formation from macrophages exposed to oxidized LDL.APMIS. 2002; 110: 458-468Crossref PubMed Scopus (80) Google Scholar, 24Gouni-Berthold I. Berthold H.K. Weber A.A. Seul C. Vetter H. Sachinidis A. Troglitazone and rosiglitazone inhibit the low density lipoprotein-induced vascular smooth muscle cell growth.Exp. Clin. Endocrinol. Diabetes. 2001; 109: 203-209Crossref PubMed Scopus (18) Google Scholar, 25Zhu Y. Liao H. Wang N. Ma K.S. Verna L.K. Shyy J.Y. Chien S. Stemerman M.B. LDL-activated p38 in endothelial cells is mediated by Ras.Arterioscler. Thromb. Vasc. Biol. 2001; 21: 1159-1164Crossref PubMed Scopus (24) Google Scholar). How LDLs activate p38 MAPKs is, however, unclear. The LDLR is not involved because the p38 MAPK pathway is activated by LDLs, both in normal and LDLR-deficient human and mouse fibroblasts, and because RAP, an LDLR antagonist, does not block LDL-induced p38 activation (Fig. 2). The LDLR seems also not to be involved in situations in which LDLs stimulate the extracellular signal-regulated kinase or the JNK MAPK pathways (5Metzler B. Hu Y. Dietrich H. Xu Q. Increased expression and activation of stress-activated protein kinases/c-Jun NH(2)-terminal protein kinases in atherosclerotic lesions coincide with p53.Am. J. 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Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?.FASEB J. 2002; 16: 1477-1487Crossref PubMed Scopus (22) Google Scholar). Because the first two of these compounds are, in principle, only found in oxidized LDLs (3Gouni-Berthold I. Sachinidis A. Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?.FASEB J. 2002; 16: 1477-1487Crossref PubMed Scopus (22) Google Scholar) and because oxidation seems not to be a prerequisite for LDL stimulation of p38 MAPKs (Fig. 1), it is unlikely that they mediate the activation of the p38 MAPK in fibroblasts in response to native LDLs. However, we cannot rule out that incubation with fibroblasts could have caused a low level of oxidation of LDLs, as reported by previous studies (27Ezaki M. Witztum J.L. Steinberg D. Lipoperoxides in LDL incubated with fibroblasts that overexpress 15-lipoxygenase.J. Lipid Res. 1995; 36: 1996-2004Abstract Full Text PDF PubMed Google Scholar). These studies revealed, however, that LDLs thus modified did not display the pathological characteristics of "minimally modified LDL" (that is, no proinflammatory activity such as monocyte activation, migration, and adhesion) (28Sigari F. Lee C. Witztum J.L. Reaven P.D. Fibroblasts that overexpress 15-lipoxygenase generate bioactive and minimally modified LDL.Arterioscler. Thromb. Vasc. Biol. 1997; 17: 3639-3645Crossref PubMed Scopus (72) Google Scholar). In contrast, fibroblasts that were transfected to overexpress 15-lipoxygenase, leading to increased seeding of LDLs with peroxidized lipids, converted LDLs to proinflammatory particles with properties typical of minimally modified LDLs. This suggests that normal fibroblasts should not modify LDLs to the extent that they adopt pathological characteristics. Phospholipids in LDLs may be involved in the activation of the p38 MAPK. For example, lysophosphatidylcholine that is present in native LDLs (3Gouni-Berthold I. Sachinidis A. Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?.FASEB J. 2002; 16: 1477-1487Crossref PubMed Scopus (22) Google Scholar) has been shown to activate the p38 MAPK pathway (29Jing Q. Xin S.M. Zhang W.B. Wang P. Qin Y.W. Pei G. Lysophosphatidylcholine activates p38 and p42/44 mitogen-activated protein kinases in monocytic THP-1 cells, but only p38 activation is involved in its stimulated chemotaxis.Circ. Res. 2000; 87: 52-59Crossref PubMed Scopus (73) Google Scholar, 30Takahashi M. Okazaki H. Ogata Y. Takeuchi K. Ikeda U. Shimada K. Lysophosphatidylcholine induces apoptosis in human endothelial cells through a p38-mitogen-activated protein kinase-dependent mechanism.Atherosclerosis. 2002; 161: 387-394Abstract Full Text Full Text PDF PubMed Scopus (138) Google Scholar). It could therefore be a critical LDL component mediating the activation of the p38 MAPK. Consistent with this possibility is the observation that oxidation, which is known to increase the content of lysophosphatidylcholine in LDL particles, seems to enhance the ability of LDLs to stimulate the p38 MAPK pathway (Fig. 1). LDLs function as cargo particles for delivering cholesterol to cells (31Goldstein J.L. Brown M.S. Molecular medicine. The cholesterol quartet.Science. 2001; 292: 1310-1312Crossref PubMed Scopus (202) Google Scholar), but they also participate in the development of atherosclerosis when their blood levels increase (1Glass C.K. Witztum J.L. Atherosclerosis. The road ahead.Cell. 2001; 104: 503-516Abstract Full Text Full Text PDF PubMed Scopus (2639) Google Scholar). It is not known whether the capacity of LDL to promote the development of atherogenic plaques is related to their cargo function or to their ability to transduce intracellular signals such as the activation of the p38 MAPK pathway. The observation that the thickening of blood vessels can be inhibited by p38 MAPK inhibitors (32Ohashi N. Matsumori A. Furukawa Y. Ono K. Okada M. Iwasaki A. Miyamoto T. Nakano A. Sasayama S. Role of p38 mitogen-activated protein kinase in neointimal hyperplasia after vascular injury.Arterioscler. Thromb. Vasc. Biol. 2000; 20: 2521-2526Crossref PubMed Scopus (87) Google Scholar) supports, however, the notion that activation of the p38 MAPK pathway by LDLs could participate, possibly by promoting lamellipodia formation and cell size increase, in the thickening of the walls of blood vessels observed during atherogenesis. In the human fibroblasts used in this study, the p38 MAPK pathway was activated at LDL concentrations of 100 μg/ml and above. In healthy subjects, the plasmatic concentration of LDL cholesterol is ∼2–4 mM, corresponding to a protein concentration of ∼350–700 μg/ml. The extravascular LDL concentration is much lower, however. Measured in human synovial fluid samples, the extravascular LDL concentration was found to be about five times lower than the blood concentration (14Busso N. Dudler J. Salvi R. Peclat V. Lenain V. Marcovina S. Darioli R. Nicod P. So A.K. Mooser V. Plasma apolipoprotein(a) co-deposits with fibrin in inflammatory arthritic joints.Am. J. Pathol. 2001; 159: 1445-1453Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). Therefore, leakage of LDLs from the vascular space (as a result of blood vessel microhemorrhages for example) would induce a local increase in the LDL concentration in the connective tissue surrounding the blood vessels. If this increase reaches the 100 μg/ml threshold, fibroblasts in the connective tissue would activate their p38 MAPK pathway. The activation of the p38 MAPK pathway in these fibroblasts would then promote an increase in their cell surface, which could prevent excessive LDL leakage from the blood vessel and facilitate closure of the wound. In this case, activation of the p38 MAPK pathway in the fibroblasts found in the conjunctive tissue of blood vessels would have beneficial effects. On the other hand, if there is a continuous activation of this pathway, caused by an increase in LDL blood levels for example, chronic hyperplasia of cells that could ensue may participate in thickening of the blood vessels and contribute to the atherogenic process (see above). Activation of the p38 MAPK pathway by LDLs could therefore induce a beneficial or detrimental response, depending on whether vascular lipoprotein concentrations are elevated. This work was supported by the Botnar Foundation (Lausanne, Switzerland) and grants from the Leenaards Foundation to C.W. and R.W.J. (Lausanne, Switzerland) and from the Swiss National Science Foundation (grant 32-66892.01) to G.W. The authors thank Dr. Fulvio Magara for help in preparing embryos from LDLR knockout mice. The authors are grateful to Dr. Pedro Romero for granting access to his FACS apparatus. The authors thank Dr. Rudi Kraftsik and Daniel Huber for their help in the measurement of the cell surface and Marc-Estienne Roehrich for helpful discussions. fluorescent-activated cell sorter fetal calf serum familial hypercholesterolemia human serum c-Jun N-terminal kinase LDL receptor lipopolysaccharide mitogen-activated protein kinase mouse embryonic fibroblast receptor-associated protein stress-activated protein kinase sterol-regulatory element binding protein
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