JBC Avanti Polar Lipids

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Bhunia, A. K.
Right arrow Articles by Chatterjee, S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Bhunia, A. K.
Right arrow Articles by Chatterjee, S.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati  
What's this?

Volume 271, Number 18, Issue of May 3, 1996 pp. 10660-10666
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
Lactosylceramide Stimulates Ras-GTP Loading, Kinases (MEK, Raf), p44 Mitogen-activated Protein Kinase, and c-fos Expression in Human Aortic Smooth Muscle Cells (*)

(Received for publication, June 19, 1995; and in revised form, December 29, 1995)

Anil Kumar Bhunia,  Hui Han ,  Ann Snowden,  Subroto Chatterjee (§)

From the Department of Pediatrics, Lipid Research Unit, The Johns Hopkins University, School of Medicine, Baltimore, Maryland 21287-3654

ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES

ABSTRACT

Previously, our laboratory has shown that lactosylceramide (LacCer) can serve as a mitogenic agent in the proliferation of aortic smooth muscle cells ``a hallmark in the pathogenesis of atherosclerosis'' (Chatterjee, S.(1991) Biochem. Biophys. Res. Commun. 181, 554-561). Here we report a novel aspect of LacCer-mediated signal transduction. We demonstrate that LacCer (10 µM) can stimulate the phosphorylation of mitogen-activated protein (MAP) kinase p44 to phosphorylated p44 in aortic smooth muscle cells from rabbit or human origin. Western immunoblot assays and direct measurement of activity in immunoprecipitated MAP kinase revealed that within 5 min of incubation of cells with LacCer there was a 3.5-fold increase in the activity of p44. This continued up to 10 min of incubation; thereafter, the MAP kinase activity decreased in these cells. Phosphoamino acid analysis revealed that the tyrosine and threonine moieties of p44 was phosphorylated by LacCer. Incubation of cells with ceramide and glucosylceramide did not significantly stimulate p44 activity. Preincubation with tyrphostin (20 µM; a potent and specific inhibitor of tyrosine kinase) markedly inhibited the LacCer mediated stimulation in p44 activity. Next we investigated the upstream and downstream parameters in MAP kinase signaling pathways. We found that lactosylceramide stimulated (7-fold) the loading of GTP on Ras. Concomitantly, LacCer stimulated the phosphorylation of MAP kinase kinases (MEK) and Raf within 2.5 min. Lactosylceramide specifically induced c-fos mRNA expression (3-fold) in these cells as compared to control. In summary, one of the biochemical mechanisms in LacCer mediated induction in the proliferation of aortic smooth muscle cells may involve Ras-GTP loading, activation of the kinase cascade (MEK, Raf, p44), and c-fos expression.

INTRODUCTION

Glycosphingolipids are minor, yet integral, constituents of cell membranes(1) . Glycosphingolipids and its metabolic products have been recently shown to play critical roles as bioregulators of a variety of processes such as cell proliferation(2, 3) , cell mobility(4) , and programmed cell death (apoptosis)(5) . Previously a ganglioside, GM(3), (^1)was shown to inhibit the phosphorylation of epidermal growth factor. That may in part, contribute to the inhibition of cell proliferation by this compound (3) . We found that lactosylceramide (LacCer) exhibited a time and concentration-dependent proliferation of aortic smooth muscle cells(2) . Since proliferation of smooth muscle cells is considered a hallmark in the pathogenesis in atherosclerosis, we measured the level of LacCer and other glycosphingolipids in human subjects who had this disease. We found that the level of glucosylceramide (GlcCer) and LacCer were markedly elevated in the plaque and calcified plaque compared to unaffected aorta from patients who died from atherosclerosis at The Johns Hopkins Hospital. (^2)Moreover, LacCer derived from plaque was at least 2-fold more potent in stimulating the proliferation of smooth muscle cells compared to LacCer from unaffected aorta tissues. Although our findings may suggest an important role for LacCer in cell proliferation, the mitogenic signaling events predicted by this lipid is not clear.

The best known mitogen activated protein (MAP) kinases are: p44 (extra cellular signal regulated kinase, ERK1), and p42 (ERK2). These are a group of serine/threonine protein kinases that constitute an activation process triggered by a variety of growth stimuli(6, 7, 8, 9, 10) . Such protein kinases have been suggested to phosphorylate and activate transcriptional factors such as c-myc(11, 12, 13) , c-fos(14) , and p62(15, 16) , which regulates the expression of genes essential for cell proliferation(17) . MAP kinase kinase (MAPK-K) identified as a dual-specificity kinase, are involved in the activation and phosphorylation of the tyrosine and threonine residues in ERKs (MAP kinases) within the TEY motif in the conserved domain VIII(18) . MAPK-K is a substrate for Rous sarcoma associated factor (Raf-1), a serine-threonine kinase(19) . The latter has been shown to integrate the signaling of various receptor tyrosine kinases(9) : G-protein coupled receptors(20, 21) , upstream serine/threonine kinase, for example, protein kinase C, and other kinases(9) .

In this paper, we present evidence that LacCer specifically activates the phosphorylation of p44 in cultured human aortic smooth muscle cells. Upstream activators, Ras/Raf/MEK, are involved in this signal kinase cascade. Concomitantly, LacCer specifically stimulated c-fos proto-oncogene expression. Such a combination of biochemical pathways may delineate signaling events involved in LacCer mediated induction in aortic smooth muscle cell proliferation.

MATERIALS AND METHODS

Isotopes

[-P]ATP (6000 mCi/mmol) and [alpha-P]dCTP (3000 Ci/mmol) were purchased from Amersham Life Science Inc. and [P]orthophosphoric acid (H(3)PO(4)) (carrier-free) was obtained from American Radiolabeled Chemicals, Inc. (St. Louis, MO).

Chemicals

All standard cultured reagents were supplied by Life Technologies, Inc. Tyrphostin AG-126 was obtained from Calbiochem (San Diego, CA). Myelin basic protein substrate peptide (APR TPGG RR), specific for MAP kinase, anti-MAP kinase (ERK-T) polyclonal antibody, specific for p44 and p42 (for mouse, rat, and human systems), anti-Raf-1, anti-MEK, and anti- p21 antibody were obtained from Upstate Biotechnology Inc. (Lake Placid, NY). Glycosphingolipids and other standard reagents were obtained from Sigma. cDNAs for c-fos and c-jun were a generous gift from Prof. Daniel Nathans and c-myc was a gift from Dr. Chi Van Dang at The Johns Hopkins University. cDNA for MAP kinase phosphatase-I, 3CH134 was a gift from Dr. Jennifer L. Duff (University of Washington, Seattle, WA). cDNA for GAPDH was a gift from Dr. D. Dewitt, Dept. of Biochemistry, Michigan State University. Human aortic smooth muscle cells were a gift from Dr. Cecila Giachelli. The polyethyleneimine TLC plates were purchased from E.M. Separations, Gibbstown, NJ.

Cells

Human aortic smooth muscle cells (A-SMC) were prepared and cultured in minimum essential medium supplemented with 10% fetal calf serum/penicillin/streptomycin, 100 units/ml, and 50 µg/ml glutamine according to the procedure of Ross(22) .

Incubation of Cells with LacCer and Other Lipids

Cells (times10^5) were seeded in 100-mm^2 plastic Petri dishes in the above growth medium. Fresh medium was added every 3 days. On the seventh day of cell growth when cells were confluent, the medium was replaced and washed with sterile phosphate-buffered saline. Next, 8 ml of Ham's F-10 medium was added to each plate. After priming the cells for 2 h in this medium, various agonists/antagonists were added. Vehicle alone was added to control dishes. After incubation for a certain time (described separately in individual experiments), cells were washed and harvested in sterile phosphate-buffered saline containing 1 mM sodium vanadate (Na(3)VO(4)) to inhibit phosphatase activity, and lysed in RIPA lysis buffer. The cell lysate was immunoprecipitated with anti-MAP kinase antibody conjugated with protein A-agarose.

Incubation of Cells with Tyrphostin

Confluent culture of cells were incubated with 20 µM tryphostin for 30 min. Next, LacCer was added and incubation continued for various time periods and cells were harvested (detailed in legends to Figures).

Immunoprecipitation of MAP Kinase

A-SMC were lysed in 100 µl of modified RIPA buffer containing 150 mM NaCl, 5 mM EGTA, 5 mM EDTA, 10 mM sodium fluoride, 1 mM Na(3)VO(4), 1 mM phenylmethylsulfonyl fluoride, 10 µg/ml aprotinin, 10 µg/ml leupeptin, 1 mM pepstatin, 25 mM Tris-HCl, pH 7.4, 1% Triton X-100, and 0.5% Nonidet P-40(23) . The lysate was centrifuged and the supernatant was incubated with 4 µg of anti-MAP kinase antibody conjugated with protein A-agarose overnight at 4 °C. The immunoprecipitates were washed twice with 25 mM Tris buffer, pH 7.4, containing 2 mM EDTA and 150 mM NaCl. A portion of the immunoprecipitate was boiled for 5 min in Laemmli sample buffer, rapidly sedimented, and the supernatant was analyzed by SDS-PAGE. The immunocomplex was used for MAP kinase activity assays.

Measurement of MAP Kinase Activity

MAP kinase activity was determined by the phosphorylation of MAP kinase-specific myelin basic protein (peptide APRTPGGRR) as described previously(24) . The assay was performed with 2-3 µg of protein in a final volume of 25 µl containing 1 mg/ml myelin basic protein, 50 µM [-P]ATP (1800 cpm/pmol), 0.5 mM adenosine 3`-5`-cyclic monophosphate-dependent protein kinase inhibitor, and assay dilution buffer containing 30 mM beta-glycerophosphate, 20 mM MOPS, pH 7.2, 20 mM MgCl(2), 5 mM EGTA, 1 mM dithiothreitol, and 0.5 mM Na(3)VO(4). The kinase reaction was initiated upon the addition of [-P]ATP for 15 min at 30 °C. The reaction was terminated with the addition of 10 µl of ice-cold 40% trichloroacetic acid and spotted onto a 2.4-cm^2 piece of Whatman P-81 phosphocellulose paper. Free -P was removed by five washes (5 min each) with 1% phosphoric acid and one wash in 95% ethanol. Radioactivity was measured by liquid scintillation counting. The activity of MAPK was expressed as picomole/min/mg protein. Protein was determined according to the method of Lowry et al.(25) with the use of bovine serum albumin as standard.

Western Blot Analysis

Following incubation ± agonists/antagonists, A-SMC were washed in situ with ice-cold phosphate-buffered saline containing 1 mM Na(3)VO(4), then lysed in RIPA lysis buffer as described earlier. The detergent insoluble material was separated by centrifugation (10,000 times g, 15 min, 4 °C), and the soluble supernatant fraction was used as enzyme source. The enzyme preparation (40 µg) was subjected to 12.5% SDS-PAGE. The protein was then transferred electrophoretically onto a polyvinylidine difluoride membrane for 1 h at 50 V and 52 mA/gel at room temperature(26) . Next, the membrane was blocked for 1 h with 3% non-fat dry milk in TBS-T (10 mM Tris-HCl and 50 mM NaCl with 0.005% Tween 20, pH 8.0), and then incubated overnight at 4 °C with the appropriate primary antibody. Next, the membrane was incubated for 2 h with the secondary antibody (alkaline phosphatase-conjugated anti-rabbit IgG) after washing four times with TBS-T. Membrane was then washed and incubated in the dark for 3-6 min with the substrate 5-bromo-4-chloro-4-indolylphosphate AP buffer (100 mM Tris-HCl, 100 mM NaCl, 5 mM MgCl(2), pH 9.5), and nitro blue tetrazolium. The reaction was terminated by rinsing the membrane with distilled water.

Phosphoamino Acid Analysis of p44

A-SMC were metabolically labeled with [P]orthophosphoric acid (H(3)PO(4)) as described previously(27) . Briefly, cells were incubated for 90 min in phosphate-free Dulbecco's minimal essential medium, and subsequently labeled by incubation for 16 h with 3 mCi/ml carrier-free O-[P]phosphate. The labeled cells were stimulated by incubation with LacCer (10 µM) for 5 min. Cell lysis and immunoprecipitation of p44 were performed as described above. P-Labeled p44 immunoprecipitates were digested with 6 N HCl at 110 °C under vacuum for 1 h. Phosphoamino acids were separated by two-dimensional thin layer cellulose chromatography together with unlabeled DL-O-phosphoserine, DL-O-phosphothreonine, and DL-O-phosphotyrosine (1 mg/ml). The solvents used for the first and second dimensions were butanol:pyridine:acetic acid:water (13:10:2:8) (v/v), and butanol:pyridine:acetic acid:water (15:10:3:12) (v/v), respectively(28) . Following development, the chromatograms were stained with ninhydrin reagent and subjected to autoradiography at -70 °C for 6 days to identify the labeled phosphoamino acid.

Raf-1 and MEK Assay

A-SMC were incubated with 10 µM LacCer and harvested at different time periods. Whole cell lysates were prepared as described above. Fourty micrograms of total cell protein were separated by 10% SDS-PAGE and electrophoretically transferred onto polyvinylidine difluoride membrane. Next, the membrane was incubated with Raf-1 antibody overnight at 4 °C. For the MEK assay, total cell proteins were separated by 12.5% SDS-PAGE and blotted with MEK antibody overnight at 4 °C.

Ras Activation Assay

A-SMC were labeled with [P]orthophosphate in phosphate-free media for 16 h as described and incubated with 10 µM LacCer. At various time points, cells were lysed in RIPA lysis buffer(23) . The whole cell lysates were immunoprecipitated with anti-human p21 antibody. Immunoprecipitates were washed with 8 times 1 ml of 50 mM HEPES, pH 7.4, 500 mM NaCl, 5 mM MgCl(2), 0.1% Triton X-100, and 0.005% SDS. Nucleotide (GTP and GDP) associated with ras were eluted with 2 mM EDTA, 2 mM dithiothreitol, 0.2% SDS, 0.5 mM GTP, 0.5 mM GDP at 68 °C for 20 min(29) . The eluted nucleotides were separated on polyethyleneimine TLC plates using 0.75 M KH(2)PO(4) (pH 3.4) as a solvent and exposed to x-ray film.

Northern Blot Analysis of c-fos, c-jun, and c-myc Gene Expression

A-SMC were grown in a p100 times 20-mm dish in 8 ml of complete Dulbecco's minimum essential medium supplemented with 10% serum with antibiotics. When the cells were sparsely confluent (80%) fresh medium containing 0.5% serum was added and incubation continued for 72 h. Then the fresh F-10 medium was added and 2 h later cells were incubated for 1 h with and without linoleic acid (20 µM) and LacCer (10 µM). The total RNA was isolated by the modified acid guanidinium thiocycanate-phenol-choroform extraction method (30) and 20 µg of total RNA were separated by electrophoresis on 1% formaldehyde gel (25 mM MOPS, pH 7.8, 1 mM EDTA, 1% (w/v) formaldehyde), transferred to Zeta Probe blotting membrane (Bio-Rad) by alkaline blotting in transfer buffer (pH 12.0) containing 3 M NaCl, 8 mM NaOH, 2 mM sodium lauryl sarcosine overnight. 1.25 Kilobases of HindII and EcoRI fragment of human c-fos cDNA, 2.6 kilobases of EcoRI fragment of mouse c-jun cDNA, and 1.8 kilobases of EcoRI fragment of human c-myc cDNAs were labeled with [alpha-P]dCTP using a random labeling method according to the instructions provided by the manufacturer (Bethesda Research Laboratories). The membranes were prehybridized in prehybridization buffer (50% formamide, 5 times SSC, 5 times Denhardt's reagent, 50 mM sodium phosphate, 250 µg/ml salmon sperm DNA) at 42 °C for 2 h and hybridized in hybridization buffer (prehybridization buffer with 10% (w/v) dextran sulfate and appropriate probe, 2 times 10^6 cpm/ml) overnight at 42 °C. The blots were washed with wash buffer I (2 times SSC, 0.2% SDS) twice at room temperature, 15 min each time. Then the blots were washed with buffer-II (0.1 [times ]SSC, 0.1% SDS) twice at 65 °C, 20 min each time prior to autoradiography. As a control, the blot was stripped off and reprobed with labeled cDNA for GAPDH and photographed.

RESULTS

Effects of Concentration of LacCer, GlcCer, and Ceramide on the Activity of p44

LacCer exerted a concentration-dependent stimulation in the phosphorylation/activation of p44 (Fig. 1). Maximum stimulation 3.5-fold in the activity of MAPK occurred with 10 µM LacCer. At a higher concentration of LacCer (50 µM) the induction of phosphorylation of MAPK was on the order of 2-fold compared to control. In contrast, incubation of cells with similar concentrations of either GlcCer or Cer did not appreciably stimulate the phosphorylation of p44 (Fig. 1).

Figure 1: Effects of lactosylceramide, glucosylceramide, and ceramide on MAP kinase activity in cultured human aortic smooth muscle cells. Cells were incubated with various concentrations (0-50 µM) of LacCer, bullet; GlcCer, circle; and ceramide, , for 5 min and lysed in lysis buffer. Immunocomplex assay of the MAP kinase activity was measured as described under ``Materials and Methods.'' Each point is the mean ± S.D. of three individual experiments.


Effects of Time of Incubation with LacCer on MAPKs

Western immunoblot assays (Fig. 2A) followed by densitometric analysis (Fig. 2B) revealed that incubation of cells with LacCer (10 µM) exerted a nearly 3-fold increase in the phosphorylation of p44. In contrast, LacCer did not phosphorylate p42. Immunocomplex kinase assay showed that within 2.5 min of incubation of cells with LacCer (10 µM) a significant increase (2.5-fold) in the activation of p44 occurred (Fig. 2C). Maximum stimulation, 3.5-fold compared to control, occurred 5 min after the incubation of cells with LacCer. Thereafter, phosphorylation decreased continually up to 60 min when it was near normal levels.

Figure 2: Effect of time of incubation with LacCer on MAP kinases. Cells were incubated with 10 µM LacCer for various time intervals (0-60 min) as indicated, harvested, and lysed. Whole cell lysates (40 µg of protein/lane) were subjected to (A) Western immunoblot assays after separation on 12.5% SDS-PAGE, transferred to nitrocellulose paper, and blotted with anti-MAP kinase antibody and photographed. B, densitometric scan of phosphorylated p44 (upper band). C, MAP kinase activity was assayed after immunoprecipitation as described under ``Materials and Methods.''


Phosphoamino Acid Analysis of p44

To determine which amino acids in p44 were phosphorylated as a consequence of incubation of cells with LacCer, we pursued the experiments in the presence of [P]orthophosphate. Following immunoprecipitaton with MAPK antibodies and washing, the immunoprecipitates were digested with acid, neutralized, and subjected to two-dimensional TLC. We found that LacCer markedly increased the phosphorylation of tyrosine and threonine residues in p44, but not the serine residue (Fig. 3). The ratio of phosphorylation of tryrosine versus threonine residues in p44 in control was 1.6:1. Whereas, the labeling ratio of tyrosine versus threonine residues in p44 in LacCer treated cells was on the order of 1.8:1. Moreover, we observed a 8.5-fold and 7.5-fold increase in the phosphorylation in tyrosine and threonine residues, respectively, in p44 in LacCer-treated cells as compared to control.

Figure 3: Phosphoamino acid analysis of p44 kinase by thin layer chromatography. LacCer stimulated P-labeled cells were lysed and immunoprecipitated with the use of p44 antibody. The immunoprecipitates were digested with 6 N HCl along with unlabeled phosphoserine, phosphotyrosine, and phosphothreonine and separated by two-dimensional TLC. The solvents in the first and second dimensions were: butanol:pyridine:acetic acid:water (13:10:2:8, v/v) and butanol:pyridine:acetic acid:water (15:10:3:12, v/v), respectively. Nonradioactive phosphoamino acid standards were detected by ninhydrin staining. P-Y, phosphotyrosine; P-T, phosphothreonine; and P-S, phosphoserine.


Effects of LacCer on MAPK Phosphatase

To ascertain that the increase in the phosphorylation of p44 in cells incubated with LacCer was not due to the inhibition of MAPK phosphatase, we pursued Northern assays in cells incubated with and without LacCer. The level of mRNA for MAPK phosphatase remained unchanged in cells incubated with or without LacCer (data not shown).

Effects of Tyrphostin on MAPK Activity

When cells were incubated with tyrphostin, a potent inhibitor of tyrosine kinase upstream of MAPK, it inhibited the LacCer (10-20 µM) induced phosphorylation of MAPK (Fig. 4). Such findings suggest that tyrosine kinase is involved in LacCer-mediated signal transduction cascade.

Figure 4: Effect of protein tyrosine kinase inhibitor, tyrphostin, on MAP kinase activity. Cells were treated with 20 µM tyrphostin AG 126 for 30 min prior to the addition of 10-25 µM LacCer for 5 min. Next, the cells were harvested and lysed. The MAP kinase activity was assayed after immunoprecipitation of cell lysates as described in legend to Fig. 1. Tyr, tyrphostin.


Effects of LacCer on Ras-GTP Loading

Cells prelabeled with P were incubated with LacCer as described earlier. At various time points, the cell lysates were prepared and immunoprecipitated with p21 antibody. The nucleotides eluted from immunoprecipitates were subjected to TLC analysis (Fig. 5A). We found that within 1 min there was a 6-fold increase in Ras-GTP loading that decreased to 3-fold in 2.5 min as compared to control (Fig. 5B). After 30 min of incubation of cells with LacCer, significant GTP radioactivity was still associated with Ras.

Figure 5: Effects of LacCer on p21activation in aortic smooth muscle cells. P-Labeled cells were stimulated with LacCer (10 µM) for different times as indicated. The cell lysate was immunoprecipitated with p21 antibody. The bound nucleotides (both GTP and GDP) were eluted and separated on polyethyleneimine TLC plate using 0.75 M KH(2)PO(4), pH 3.4, as solvent and subjected to autoradiography (A) and densitometric scanning (B).


Effects of LacCer on the Phosphorylation of Raf

The effects of time of incubation of cells, with LacCer on the phosphorylation of Raf-1 was assessed by Western immunoblot assay. We observed that 2.5 min after incubation of cells with LacCer, there was an appearance of a band with reduced mobility (presumably the phosphorylated form of Raf) compared to Raf (Fig. 6). A similar observation was made at 5 min after incubation of cells with LacCer thereafter, the phosphorylated form of Raf was not observed.

Figure 6: The electrophoretic mobility of Raf-1 in aortic smooth muscle cells treated with LacCer. Whole cell lysates were prepared as described under ``Materials and Methods'' and 40 µg of protein/lane were separated by 10% SDS-PAGE and transferred onto polyvinylidine difluoride membrane and detected with anti-Raf-1 antibody.


Effects of LacCer on the Phosphorylation of MEK

Western immunoblot assays employing MEK2 antibody revealed that LacCer exerted a time-dependent increase in the phosphorylation of MEK2 (Fig. 7). This was evidenced by a shift in the mobility of a MEK2 antibody recognizable band (Fig. 7A). Densitometric analysis of the gel revealed that after 2.5 min of incubation of cells with LacCer an increase in MEK2 phosphorylation on the order of 3-fold occurred. Maximum increase in MEK2 (4-fold) occurred 5 min after incubation of cells with LacCer as compared to control (Fig. 7B). Ten minutes after incubation of cells with LacCer there was 2-fold higher activity of MEK kinase as compared to control

Figure 7: Gel shift assay of MEK (MAP kinase kinase) in aortic smooth muscle cells incubated with LacCer. Whole cell lysate protein (40 µg of protein/lane) was separated by 12.5% SDS-PAGE and (A) subjected to Western immunoblot assay employing MEK2 antibody (slower migrating upper bands represent the phosphorylated form of MEK2, MEK(P)). B, densitometric scan MEK(P) (upper band).


Effects of LacCer on the Expression of Proto-oncogenes: c-fos, c-myc, and c-jun

Northern assays revealed that after 1 h of incubation of cells with 10 µM LacCer, the level of c-fos but not c-myc and c-jun was increased by 3-fold (Fig. 8B) as compared to control. In contrast, preincubation of cells with linoleic acid (20 µM) (Fig. 8) increased the level of c-fos, c-jun, and c-myc on the order of 6-fold, 4-fold, and 1.5-fold, respectively, as compared to control. The level of GAPDH remained unchanged upon incubation of cells with LacCer or linoleic acid.

Figure 8: Northern blot analysis of the expression of proto-oncogene mRNAs in aortic smooth muscle cells stimulated by LacCer. Aortic smooth muscle cells were incubated with 10 µM LacCer for 1 h and total cellular RNA was isolated. Twenty micrograms of total RNA were analyzed by Northern blotting (A) followed by densitometric scanning (B) for c-fos, c-jun, c-myc, and GAPDH transcripts using respective P-labeled cDNA probes. Analysis of total cellular RNA for each condition from three separate experiments provided similar results. Lane 1, control; lane 2, 20 µM linoleic acid; lane 3, 10 µM LacCer.


DISCUSSION

Our studies generated several novel findings. First, we found that LacCer specifically stimulated the phosphorylation of p44, whereas GlcCer and Cer did not. Second, tyrphostin (tyrosine kinase inhibitor) markedly abrogated LacCer mediated induction in the phosphorylation of p44. Third, LacCer stimulated Ras-GTP loading and the phosphorylation of MEK2 and Raf. Fourth, LacCer specifically stimulated the mRNA level of c-fos proto-oncogene in A-SMC.

To delineate the signal transduction events in LacCer-mediated proliferation in human A-SMC, we examined its effects on various parameters in the signal transduction cascade under carefully controlled yet, varying conditions. The cells were grown to confluence, washed, and incubated with Ham's F-10 medium without serum for 2 h. Next, various agonists and antagonists were added. We were concerned that contamination of any reagents or cell cultures with bacteria or lipopolysaccharide may also effect the phosphorylation of MAPK(31) . Accordingly, appropriate control experiments were pursued to demonstrate that lipopolysaccharide (10 µg/ml) did not alter the phosphorylation of either p44 or p42 in cultured human A-SMC (data not shown). Next, we analyzed the effects of LacCer and other glycosphingolipids simultaneously employing two methods, these were: 1) the measurement of MAPK activity of immunoprecipitated MAPK employing myelin basic protein fragment (APRTPGGRR) as substrate and [P]ATP as the phosphate donor; and 2) Western immunoblot assay followed by densitometric scanning of individual gel bands. As shown in various figures, both of these approaches yielded similar results confirming the validity of our experimental techniques and the interpretation of results.

Our studies clearly revealed that LacCer induced the phosphorylation of tyrosine and threonine residues in p44. This phenomenon was abrogated by an inhibitor of tyrosine kinase, e.g. tyrphostin(32) . However, staurosporine, an inhibitor of protein kinase C, failed to impair this process (^3)(data not shown). These findings reveal that LacCer mediated p44 phosphorylation and cell proliferation are independent of protein kinase C. This is in contrast to a previous study in which GlcCer mediated proliferation in a continuous renal cell line, Madin-Darby canine kidney, was shown to be abrogated by inhibitors of protein kinase C(33) .

Recent studies from several laboratories reveal that the induction of MAPKs may also be due to inactivation of MAPK phosphatases(34) . In particular, generation of free oxygen radicals and hydrogen peroxide were shown to inhibit MAPK phosphatase and consequently increase the level of p44 in neutrophils(35) . To rule out this possibility, we pursued Northern blot assays in cells incubated ± LacCer and found that the mRNA level of MAPK phosphatase was similar (data not shown). Thus, our findings indicate that LacCer mediated induction of p44 is not due to the inhibition of phosphatase activity.

The catabolic products of LacCer are GlcCer, Cer, sphingosine, and stearic acid. In order to determine the specificity of LacCer mediated stimulation of p44 phosphorylation, we also investigated the effects of GlcCer and Cer that are highly enriched in atherosclerotic plaque intima and media, but not in unaffected intima and media.^2 We found that GlcCer and Cer did not alter p44 phosphorylation.

The effects of LacCer on upstream regulators, for example, Ras, Raf, and MEK of MAPK, were investigated. The proto-oncogene p21 has been identified as a key molecular switch involved in regulating cell activation triggered by various mitogens(29, 36, 37) . In its resting state, p21 is in a GDP-bound state. Upon in vivo activation p21 releases GDP and binds GTP(38, 39) . Immunoprecipitation of p21 from stimulated and nonstimulated cells followed by nucleotide (both GTP and GDP) elution revealed that a substantial increase in GTP bound p21 occurred upon LacCer treatment. The maximal effect was observed at 1 min after LacCer addition. Thereafter, GTP bound p21 was decreased. This time dependent increase of p21 GTP loading suggests that LacCer activates p44 via the activation of p21. We speculate that the relatively short duration of LacCer induced p21 GTP loading may be due to the interaction of the latter with its effector or GTPase activating protein. An investigation is required whether the LacCer mediated activation occurs by direct interaction with p21 or indirectly through secondary factors.

Raf-1, the product of the c-Raf 1 proto-oncogene is pivotal in transmitting signals from Ras in the plasma membrane to cytosolic nuclear compartments of the cells(40, 41) . The stimulation of Raf-1 activity depends on the activation of the small G-protein p21(42) . Active GTP-ras binds to the NH(2)-terminal domain of Raf-1 and recruits Raf-1 to the plasma membranes, which is sufficient to cause its activation(40, 41, 43) . We investigated whether Raf-1 is phosphorylated/activated after stimulation of p21 (measured by p21bulletGTP loading) by LacCer. Significant phosphorylation of Raf-1 was observed within 2.5-5 min of incubation of cells with LacCer. Thus the activation of Raf-1 by LacCer was transient in A-SMC. Since Raf-1 activity is known to participate in the G(0)/G(1) transition of cells(42) , we may speculate that LacCer mediated Raf-1 activation may similarly help in the transition of A-SMC from the G(0) phase to the G(1) phase of the cells. Next, we measured the phosphorylation of MEK by LacCer to assess whether the Raf-1, a serine-threonine kinase, transmits proliferative signals to downstream effectors, e.g. p44 via the activation of putative MAPK activator MEK. MEK2 phosphorylation was stimulated within 2.5-5 min after the addition of 10 µM LacCer, and then it was dissipated at 10 min. Taken together, these findings lead us to speculate that LacCer induced activation of p44 may be mediated by a kinase cascade that includes Ras, Raf, and MEK2 in a descending order.

Induced expression of proto-oncogene c-fos, c-jun, and c-myc mRNA is an early response to various growth stimuli(44, 45, 46) . These proto-oncogenes encode for nuclear binding transcriptional factors (44, 45) and play a crucial role in mitogen-induced cell proliferation(47) . To determine whether these proto-oncogenes are also involved in LacCer induced A-SMC proliferation, subsequent to p44 activation, we measured mRNA levels of these proto-oncogenes. Among the three proto-oncogenes tested, only the c-fos mRNA level increased after 1 h of LacCer incubation as compared to control. In contrast, cellular levels of c-jun or c-myc mRNA in LacCer treated and control cells were similar. Equal amounts of RNA loading was confirmed by GAPDH analysis. This data suggested that LacCer specifically induced c-fos proto-oncogene expression via activation of p44.

Why LacCer specifically induced the phosphorylation of p44 and c-fos mRNA expression is not clearly understood from our studies. Previous studies with smooth muscle cells have revealed that fatty acids, e.g. linoleic acid and arachidonic acid, stimulate the phosphorylation of both p42 and p44(48) . In addition, linoleic acid stimulated the mRNA levels of c-fos, c-myc, and c-jun in rat vascular smooth muscle cells (49) and also in our studies with human A-SMC. Furthermore, recent studies have shown that tumor necrosis factor-alpha specifically stimulate MEK1, that in turn induce the activation of p42(50) . Our preliminary studies indicate that LacCer did not stimulate MEK1, instead LacCer stimulated MEK2. Thus there may be a clear dichotomy in regard to agonist specific induction of MEK2 and activation/phosphorylation of p44 (ERK1). Since other studies have shown that the ERK group of MAP kinases phosphorylate Elk-1 and increases ternary complex formation(15, 16) and activation of c-fos downstream in the signaling pathway, it is possible that LacCer, like other growth factors, specifically mediates this process. This may ultimately increase transcriptional activity and DNA synthesis, subsequently cell proliferation. Further work in this area is warranted to explain this phenomena to understand the pathophysiology of LacCer mediated A-SMC proliferation in atherosclerosis.

FOOTNOTES

*
This work was supported by National Institutes of Health Grants RO1-DK31722 1-P50 HL47212. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§
To whom all correspondence should be addressed. Tel.: 410-955-3197; Fax: 410-955-1276.

(^1)
The abbreviations used are: GM(3), monosialoganglioside; LacCer, lactosylceramide; GlcCer, glucosylceramide; Raf, Rous sarcoma associated factor; MOPS, 3-(N-morpholino)propanesulfonic acid; ERK, extracellular signal regulated kinase; MAPK, mitogen-activated protein kinase; p44, phosphorylated p44; p42, phosphorylated p42; GAPDH, glyceraldehyde-3-phosphodehydrogenase; A-SMC, aortic smooth cells; PAGE, polyacrylamide gel electrophoresis.

(^2)
S. Chatterjee, S. Dey, W. Y. Shi, K. Thomas, and G. Hutchins, personal communication.

(^3)
S. Chatterjee and A. Snowden, unpublished observations.

ACKNOWLEDGEMENTS

We acknowledge Tammy DeMoss for assistance with typing and Dr. Dmitry Mukhin for assistance with computer analyses of our data.

REFERENCES

  1. Hakomori, S. (1990) J. Biol. Chem. 265, 18713-18716 [Abstract/Free Full Text]
  2. Chatterjee, S. (1991) Biochem. Biophys. Res. Commun. 181, 554-561 [CrossRef][Medline] [Order article via Infotrieve]
  3. Bremer, E., Schlessinger, J., and Hakomori, S. (1986) J. Biol. Chem. 261, 2434-2440 [Abstract/Free Full Text]
  4. Sadahira, Y., Ruan, F., Hakomori, S. I., and Igarashi, Y. (1994) Proc. Natl. Acad. Sci. U. S. A. 89, 9686-9690 [Abstract/Free Full Text]
  5. Obeid, L. M., Linardic, C. M., Karolak, L. A., and Hannun, Y. A. (1993) Science 259, 1769-1771 [Abstract/Free Full Text]
  6. Nishida, E., and Gotoh, Y. (1992) Int. Rev. Cytol. 138, 211-238 [Medline] [Order article via Infotrieve]
  7. Boulton, T. G., Nye, S. H., Robbins, D. J., Ip, N. Y., Radziejewska, E., Morgenbesser, S. D., DePinho, R. A., Panayotatos, N., Cobb, M. H., and Yancopoulos, G. D. (1991) Cell 65, 663-675 [CrossRef][Medline] [Order article via Infotrieve]
  8. Davis, R. J. (1994) Trends Biochem. Sci. 19, 470-473 [CrossRef][Medline] [Order article via Infotrieve]
  9. Cano, E., and Mahadevan, L. C. (1995) Trends Biochem. Sci. 20, 117-122 [CrossRef][Medline] [Order article via Infotrieve]
  10. Rossomando, A. J., Sanghera, J. S., Marsden, L. A., Weber, M. J., Pelech, S. L., and Sturgill, T. W. (1992) J. Biol. Chem. 266, 20270-20275 [Abstract/Free Full Text]
  11. Traverse, S., Gomez, N., Peterson, H., Marshall, C., and Cohen, P. (1992) Biochem. J. 288, 351-355
  12. Seth, A., Alvarez, E., Gupta, S., and Davis, R. J. (1991) J. Biol. Chem. 266, 23521-23524 [Abstract/Free Full Text]
  13. Seth, A., Gonzalez, F. A., Gupta, S., Raden, D. L., and Davis, R. J. (1992) J. Biol. Chem. 267, 24796-24804 [Abstract/Free Full Text]
  14. Deug, T., and Karin, M. (1994) Nature 371, 171-175 [CrossRef][Medline] [Order article via Infotrieve]
  15. Gille, H., Sharrocks, A. D., and Shaw, P. E. (1992) Nature 358, 414-417 [CrossRef][Medline] [Order article via Infotrieve]
  16. Marais, R., Wynne, J., and Treisman, R. (1993) Cell 73, 381-393 [CrossRef][Medline] [Order article via Infotrieve]
  17. Davis, R. J. (1993) J. Biol. Chem. 268, 14553-14556 [Free Full Text]
  18. Ahn, N. G., and Krebs, E. G. (1992) Current Opin. Cell Biol. 4, 992-999 [CrossRef][Medline] [Order article via Infotrieve]
  19. Kyriakis, J. M., App, H., Zhang, X., Banerjee, P., Brautigan, D. Z., Rapp, U. R., and Avruch, J. (1992) Nature 358, 417-421 [CrossRef][Medline] [Order article via Infotrieve]
  20. Blenis, J. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 5889-5892 [Abstract/Free Full Text]
  21. Blummer, K. J., and Johnson, G. L. (1994) Trends. Biochem. Sci. 19, 236-240 [CrossRef][Medline] [Order article via Infotrieve]
  22. Ross, R. (1971) J. Cell Biol. 50, 172-186 [Abstract/Free Full Text]
  23. Laird, A. D., Taylor, S. J., Oberst, M., and Shalloway, D. (1995) J. Biol. Chem. 270, 26742-26745 [Abstract/Free Full Text]
  24. Clark-Lewis, I., Sanghera, J. S., and Pelech, S. L. (1991) J. Biol. Chem. 266, 15180-15184 [Abstract/Free Full Text]
  25. Lowry, O. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951) J. Biol. Chem. 193, 256-275
  26. Chatterjee, S., and Ghosh, N. (1989) J. Biol. Chem. 264, 12554-12561 [Abstract/Free Full Text]
  27. Sengupta, A., Liu, W. K., Yeung, Y. G., Yeung, D. C. Y., Frackelton, A. R., and Stanley E. R. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 8062-8066 [Abstract/Free Full Text]
  28. Boyle, W. J., Geer, P. V. D., and Hunter, T. (1991) Methods Enzymol. 201, 110-152 [Medline] [Order article via Infotrieve]
  29. Downward, J., Graves, J. D., Warne, P. H., Rayter, S., and Cantrell, D. A. (1990) Nature 346, 719-723 [CrossRef][Medline] [Order article via Infotrieve]
  30. Chomczynski, P., and Sacchi, N. (1987) Anal. Biochem. 162, 156-159 [Medline] [Order article via Infotrieve]
  31. Weinstein, S. L., Sanghera, J. S., Lemke, K., DeFranco, A. L., and Pelech, S. L. (1992) J. Biol. Chem. 267, 14955-14962 [Abstract/Free Full Text]
  32. Novogrodsky, A, Vanichkin, A., Patya, M., Gazit, A., Osherov, N., and Levitzki, A. (1994) Science 264, 1319-1322 [Abstract/Free Full Text]
  33. Shayman, J. A., Deshmukh, G. D., Mahdiyoun, S., Thomas, T. P., Wu, D., Barcelon, F. S., and Radin, N. S. (1991) J. Biol. Chem. 266, 22968-22974 [Abstract/Free Full Text]
  34. Ward Y., Gupta, S., Jensen, P., Wartmann, M., Davis, R. J., and Kelly, K. (1994) Nature 367, 651-654 [CrossRef][Medline] [Order article via Infotrieve]
  35. Fialkov, L., Chan, C. K., Rotin, D., Grinstein, S., and Downey, G. P. (1994) J. Biol. Chem. 269, 31234-31242 [Abstract/Free Full Text]
  36. Johnson, G. L., and Langer-Carter, C. A. (1994) Science 265, 1458-1461 [Abstract/Free Full Text]
  37. Lander, H. M., Ogiste, J. S., Pearce, S. F. A., Levi, R., and Novogrodsky, A. (1995) J. Biol. Chem. 270, 7017-7020 [Abstract/Free Full Text]
  38. Boguski, M. S., and Macormick, F. (1993) Nature 366, 643-654 [CrossRef][Medline] [Order article via Infotrieve]
  39. Bokoch, G. M., and Der, C. J. (1993) FASEB J. 7, 750-759 [Abstract]
  40. Warne, P. H., Viciana, P. R., and Downward, J. (1993) Nature 364, 352-355 [CrossRef][Medline] [Order article via Infotrieve]
  41. Vojtek, A. B., Hollenberg, S. M., and Cooper, J. A. (1993) Cell 74, 205-214 [CrossRef][Medline] [Order article via Infotrieve]
  42. Kikuchi, A., and Williams L. T. (1994) J. Biol. Chem. 269, 20054-20059 [Abstract/Free Full Text]
  43. Ghosh, S., Xie, W. Q., Quest, A. F. G., Mabrouk, G. M., Strum, J. C., and Bell, R. M. (1994) J. Biol. Chem. 269, 10000-10007 [Abstract/Free Full Text]
  44. Lau, L. F., and Nathans, D. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 1182-1186 [Abstract/Free Full Text]
  45. Curran, T., and Franza, B. R. J. (1988) Cell 55, 395-397 [CrossRef][Medline] [Order article via Infotrieve]
  46. Angel, P., and Karin, M. (1991) Biochim. Biophys. Acta 1072, 129-137 [Medline] [Order article via Infotrieve]
  47. Holt, J. T., Gopal, T. V., Moulton, A. D., and Nienhuis, A. W. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 4794-4798 [Abstract/Free Full Text]
  48. Rao, G. N., Baas, A. S., Glasgow, W. C., Eling, T. E., Runge, M. S., and Alexander, R. W. (1994) J. Biol. Chem. 269, 32586-32591 [Abstract/Free Full Text]
  49. Rao, G. N., Alexander, R. W., and Runge, M. S. (1995) J. Clin. Invest. 96, 842-847
  50. Winston, B. W., Remigio, L. K., and Riches, D. W. H. (1995) J. Biol. Chem. 270, 27391-27394 [Abstract/Free Full Text]
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.


Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati    What's this?


This article has been cited by other articles:


Home page
 J. Biol. Chem.Home page
E. N. Glaros, W. S. Kim, C. M. Quinn, J. Wong, I. Gelissen, W. Jessup, and B. Garner
Glycosphingolipid Accumulation Inhibits Cholesterol Efflux via the ABCA1/Apolipoprotein A-I Pathway: 1-PHENYL-2-DECANOYLAMINO-3-MORPHOLINO-1-PROPANOL IS A NOVEL CHOLESTEROL EFFLUX ACCELERATOR
J. Biol. Chem., July 1, 2005; 280(26): 24515 - 24523.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurosci.Home page
R. Pannu, J.-S. Won, M. Khan, A. K. Singh, and I. Singh
A Novel Role of Lactosylceramide in the Regulation of Lipopolysaccharide/Interferon-{gamma}-Mediated Inducible Nitric Oxide Synthase Gene Expression: Implications for Neuroinflammatory Diseases
J. Neurosci., June 30, 2004; 24(26): 5942 - 5954.
[Abstract] [Full Text] [PDF]

Home page
 Mol. Cell. Biol.Home page
L.-F. Belanger, S. Roy, M. Tremblay, B. Brott, A.-M. Steff, W. Mourad, P. Hugo, R. Erikson, and J. Charron
Mek2 Is Dispensable for Mouse Growth and Development
Mol. Cell. Biol., July 15, 2003; 23(14): 4778 - 4787.
[Abstract] [Full Text] [PDF]

Home page
 FASEB J.Home page
I. GOUNI-BERTHOLD and A. SACHINIDIS
Does the coronary risk factor low density lipoprotein alter growth and signaling in vascular smooth muscle cells?
FASEB J, October 1, 2002; 16(12): 1477 - 1487.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
A. K. Bhunia, G. Schwarzmann, and S. Chatterjee
GD3 Recruits Reactive Oxygen Species to Induce Cell Proliferation and Apoptosis in Human Aortic Smooth Muscle Cells
J. Biol. Chem., May 3, 2002; 277(19): 16396 - 16402.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
T. Iwamoto, S. Fukumoto, K. Kanaoka, E. Sakai, M. Shibata, E. Fukumoto, J.-i. Inokuchi, K. Takamiya, K. Furukawa, K. Furukawa, et al.
Lactosylceramide Is Essential for the Osteoclastogenesis Mediated by Macrophage-Colony-stimulating Factor and Receptor Activator of Nuclear Factor-kappa B Ligand
J. Biol. Chem., November 30, 2001; 276(49): 46031 - 46038.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
A. K. Bhunia, T. Arai, G. Bulkley, and S. Chatterjee
Lactosylceramide Mediates Tumor Necrosis Factor-alpha -induced Intercellular Adhesion Molecule-1 (ICAM-1) Expression and the Adhesion of Neutrophil in Human Umbilical Vein Endothelial Cells
J. Biol. Chem., December 18, 1998; 273(51): 34349 - 34357.
[Abstract] [Full Text] [PDF]

Home page
 Arterioscler. Thromb. Vasc. Bio.Home page
S. Chatterjee
Sphingolipids in Atherosclerosis and Vascular Biology
Arterioscler. Thromb. Vasc. Biol., October 1, 1998; 18(10): 1523 - 1533.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
J. W. Zimmerman, J. Lindermuth, P. A. Fish, G. P. Palace, T. T. Stevenson, and D. E. DeMong
A Novel Carbohydrate-Glycosphingolipid Interaction between a beta -(1-3)-Glucan Immunomodulator, PGG-glucan, and Lactosylceramide of Human Leukocytes
J. Biol. Chem., August 21, 1998; 273(34): 22014 - 22020.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
N. Auge, I. Escargueil-Blanc, I. Lajoie-Mazenc, I. Suc, N. Andrieu-Abadie, M.-T. Pieraggi, M. Chatelut, J.-C. Thiers, J.-P. Jaffrezou, G. Laurent, et al.
Potential Role for Ceramide in Mitogen-activated Protein Kinase Activation and Proliferation of Vascular Smooth Muscle Cells Induced by Oxidized Low Density Lipoprotein
J. Biol. Chem., May 22, 1998; 273(21): 12893 - 12900.
[Abstract] [Full Text] [PDF]

Home page
 Circ. Res.Home page
T. Arai, A. K. Bhunia, S. Chatterjee, and G. B. Bulkley
Lactosylceramide Stimulates Human Neutrophils to Upregulate Mac-1, Adhere to Endothelium, and Generate Reactive Oxygen Metabolites In Vitro