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Originally published In Press as doi:10.1074/jbc.M302881200 on August 11, 2003

J. Biol. Chem., Vol. 278, Issue 46, 45577-45585, November 14, 2003
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Leukotriene D4 Mediates Survival and Proliferation via Separate but Parallel Pathways in the Human Intestinal Epithelial Cell Line Int 407*

Sailaja Paruchuri and Anita Sjölander{ddagger}

From the Division of Experimental Pathology, Department of Laboratory Medicine, Lund University, University Hospital Malmö, Malmö SE-205 02, Sweden

Received for publication, March 20, 2003 , and in revised form, August 7, 2003.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
We demonstrated previously that leukotriene D4 (LTD4) regulates proliferation of intestinal epithelial cells through a CysLT receptor by protein kinase C (PKC){epsilon}-dependent stimulation of the mitogen-activated protein kinase ERK1/2. Our current study provides the first evidence that LTD4 can activate 90-kDa ribosomal S6 kinase (p90RSK) and cAMP-responsive element-binding protein (CREB) via pertussis-toxin-sensitive Gi protein pathways. Transfection and inhibitor experiments revealed that activation of p90RSK, but not CREB, is a PKC{epsilon}/Raf-1/ERK1/2-dependent process. LTD4-mediated CREB activation was not affected by expression of kinase-dead p90RSK but was abolished by transfection with the regulatory domain of PKC{alpha} (a specific dominant-inhibitor of PKC{alpha}). Kinase-negative mutants of p90RSK and CREB (K-p90RSK and K-CREB) blocked the LTD4-induced increase in cell number and DNA synthesis (thymidine incorporation). Compatible with these results, flow cytometry showed that LTD4 caused transition from the G0/G1 to the S+G2/M cell cycle phase, indicating increased proliferation. Similar treatment of cells transfected with K-p90RSK resulted in cell cycle arrest in the G0/G1 phase, consistent with a role of p90RSK in LTD4-induced proliferation. On the other hand, expression of K-CREB caused a substantial buildup in the sub-G0/G1 phase, suggesting a role for CREB in mediating LTD4-mediated survival in intestinal epithelial cells. Our results show that LTD4 regulates proliferation and survival via distinct intracellular signaling pathways in intestinal epithelial cells.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Cysteinyl leukotrienes (LT(s)1 (LTC4, LTD4, and LTE4) belong to an important group of pro-inflammatory mediators that are derived from arachidonic acid via the 5-lipoxygenase pathway that have a number of pathophysiological functions in the inflammatory process, such as induction of proliferation and contraction of smooth muscle cells, promotion of eosinophil migration, and amplification of vascular permeability (1–3). LTD4 is the most potent of the cysteinyl leukotrienes, and its effects are mediated through a seven-transmembrane G protein-coupled receptor type. Two such receptors have been cloned and characterized as CysLT1 and CysLT2 (4–6). In binding studies, the CysLT1 receptor has been shown to have the highest affinity for LTD4 and is generally believed to be the most important receptor in mediating the functional effects of this cysteinyl leukotriene (4). The CysLT2 receptor, on the other hand, exhibits equal binding affinities for LTD4 and LTC4 (7). Some of the G proteins to which CysLT receptors are linked are sensitive to pertussis toxin (PTX), whereas others are not. This indicates that different signal transduction pathways can be initiated by CysLT receptors, thus enabling this receptor type to mediate a variety of functional activities in the same cell (8, 9).

Sheng et al. (10) have suggested that there is a connection between inflammation and the development of cancer, which is also supported by results showing that ulcerative colitis is associated with an increased incidence of neoplastic transformation (11) and the observation that colon cancer is underrepresented in populations treated with nonsteroidal anti-inflammatory drugs (12). Furthermore, in previous experiments using nontransformed intestinal epithelial cells (13), we found that prolonged exposure to LTD4 resulted in up-regulation of several proteins associated with colon carcinogenesis, among others cyclooxygenase-2, {beta}-catenin, and the cell survival protein Bcl-2 (13, 14). We have also recently shown that the CysLT1 receptor is up-regulated in colon cancer tissues and that LTD4 signaling facilitates survival of colon cancer cells (15) and nontransformed epithelial cells (13). These findings are interesting because both cyclooxygenase-2 and CysLT1 are accessible targets for drug therapy. In addition to its effect on survival, LTD4 can induce a proliferative response in intestinal epithelial cells via a signaling pathway that includes activation of a PTX-sensitive G protein, PKC{epsilon}, Raf-1, and the mitogen-activated protein kinase (MAPK) ERK1/2 (16). These data indicate that the inflammatory mediator LTD4 can contribute to increased survival and growth of intestinal epithelial cells in pathological inflammatory conditions, although the effectors situated downstream of ERK1/2 have not been identified.

Considering the ways in which ERK1/2 can mediate its effect on proliferation and survival in intestinal epithelial cells, two major downstream targets/substrates are of particular interest: the p90-kDa ribosomal S6 kinase (p90RSK) and the cAMP-responsive element-binding protein (CREB). Experiments performed in vitro and in vivo have revealed that ERK1/2 can interact and directly phosphorylate p90RSK on several different serine and threonine residues (17, 18). The protein p90RSK is unique among serine/threonine kinases in that it contains two functional kinase domains (19), and it has been suggested that regulation of the N-terminal catalytic domain of p90RSK is mediated by activation of the C-terminal catalytic domain that occurs through MAPK-induced phosphorylation (17). The other potential target/substrate of ERK1/2 which might mediate the effect of this protein on intestinal cell proliferation is CREB. The C terminus of CREB contains a basic DNA binding domain and an adjacent leucine zipper domain, the latter of which is required for dimerization of CREB (20). CREB also has a transactivation domain that contains several independent regions, including one identified as the kinase-inducible domain, which comprises consensus phosphorylation sites for several kinases, among them protein kinase A (PKA) (21). Kinase-induced phosphorylation of Ser133 on CREB facilitates attachment of this protein to the 256-kDa CREB-binding protein (CBP). The CREB·CBP complex can, in turn, interact with and activate the basal transcription machinery (21). Previous studies have demonstrated that multiple signaling pathways can mediate the phosphorylation and activation of CREB in different cell lines. Transcriptional activation of CREB, through phosphorylation of the Ser133 residue, can obviously be induced by the catalytic subunit of the PKA (21). However, additional serine/threonine kinases, such as PKC (22), p38 MAPK (23), Ca2+/calmodulin-dependent protein kinases (CAMK; 24), a Ras-dependent p105 kinase (25), and, interestingly enough, also p90RSK (22), have been shown to phosphorylate and activate CREB. In some cases, several of these kinases can participate in the same agonist-induced signaling pathway; for example, engagement of the T cell receptor leads to a rapid phosphorylation and activation of CREB via a signaling cascade that involves stimulation of the tyrosine kinase p56lck, PKC, Ras, Raf-1, ERK1/2, and p90RSK2 (26).

The participation of PKC isoforms in activation of CREB is particularly interesting because we have shown previously that the LTD4-induced proliferative response in intestinal epithelial cells is mediated via activation of PKC{epsilon} (16). PKCs are members of a family of serine/threonine kinases that have been shown to modulate diverse cellular functions, including proliferation, differentiation, and gene activation (27). There are at least 11 isoforms of PKC, which are classified as conventional ({alpha}, {beta}I, {beta}II, and {gamma}), novel ({delta}, {epsilon}, {theta}, {eta}), and atypical ({zeta}, {iota}/{lambda}), and also PKCµ. Upon activation, the individual isoforms can exert unique effects in the cell. In NIH3T3 cells, overexpression of PKC{epsilon} has been found to increase the growth rate, whereas PKC{delta} has the opposite effect (28), and PKC{alpha} has been implicated in an antiapoptotic response in COS-1 cells (29).

The present results show that LTD4 activates both CREB and p90RSK, but these effects occur via separate but parallel signaling pathways: for CREB a PKC{alpha}-dependent pathway, and for p90RSK a PKC{epsilon}-Raf-1-ERK1/2 signaling cascade. Based on our observations in cells transfected with dominant-inhibitory mutant signaling proteins and in cells treated with pharmacological inhibitors, we conclude that, in intestinal epithelial cells, LTD4-induced activation of the PKC{alpha}-CREB signaling pathway favors survival, whereas simultaneous activation of the ERK1/2-p90RSK pathway mediates a proliferative response.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Materials—Phosphospecific antibodies and total antibodies to p90RSK, CREB, p38 MAPK, and the inhibitors SB203580 and PD98059 were purchased from New England BioLabs, Inc. (Beverly, MA). CysLT2 antibody, LTB4, and LTD4 were obtained from Cayman Chemical Company (Ann Arbor, MI), and CysLT1 (N-terminal) was purchased from Innovagen (Lund, Sweden). ECL Western blot detection reagents, Hyperfilm and [methyl-3H]thymidine were from Amersham Biosciences. Wortmannin, LY294002, forskolin, KN-62, Gö6976, GF109203X, 12-O-tetradecanoylphorbol-13-acetate (TPA), Rp-cAMPs, and FTI-277 were acquired from Calbiochem. HA and Myc antibodies originated from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA), PTX was from Speywood Pharma Ltd. (Maidenhead, UK), and peroxidase-linked goat anti-rabbit and mouse IgG were from DAKO A/S (Copenhagen, Denmark). PP1 was purchased from Alexis (San Diego, CA). ZM198,615 (ICI-198,615) was a gift from Dr. R. Metcalf (Zeneca Pharmaceuticals, Macclesfield, Cheshire, UK). All other chemicals were of analytical grade and obtained from Sigma Chemical Co.

Cell Culture—Human intestinal epithelial cells (Int 407; 30) were used in all experiments. These cells, which exhibit typical epithelial morphology and growth, were cultured as a monolayer to ~80% confluence for 5 days. Cell cultures were kept at 37 °C in a humidified atmosphere of 5% CO2 and 95% air in Eagle's basal medium supplemented with 15% newborn calf serum, 55 IU/ml penicillin, and 55 µg/ml streptomycin. The cells were tested regularly to ensure the absence of mycoplasma contamination.

cDNAs and Transfections—Cells were transfected with different signaling protein constructs for 6 h and were then allowed to grow in medium supplemented with serum for another 24 or 48 h as indicated. The constructs we used were generously provided by the cited investigators: a full-length human HA-tagged dominant-negative Ras construct (N17 Ras; 31), EGFP- or Myc-tagged regulatory domains (RDs) of PKC{epsilon} or {alpha} constructs (32), or a W437 kinase-dead FLAG-tagged PKC{epsilon} construct (K-PKC{epsilon}), from Dr. Arthur Mercurio (Beth Israel Deaconess Medical Center, Boston); HA-tagged kinase inactive c-Raf construct (K-Raf-1), from Dr. Larry Karnitz (Mayo Clinic, Rochester, MN); a kinase-dead HA-tagged p90RSK (K-p90RSK), from Dr. John Blenis (Harvard Medical School, Boston); a kinase-dead CREB construct (K-CREB), from Dr. Richard H. Goodman (Oregon Health Sciences University, Portland). Transient transfections of the cells were achieved using 3.5 µl of LipofectAMINE (Invitrogen) and 1.8 µg of plasmid DNA/ml of medium and were performed in serum-free medium, essentially according to the protocol provided by the supplier. In all transfection experiments, it was routinely confirmed that the empty vector had no effect, and the efficiency of transfection was determined by control cotransfections with an empty pEGFP-N1 vector (Clontech), except for the transfections with an EGFP-tagged RD of PKC{epsilon} or {alpha}.

Incubations and Lysis of the Cells—Cells in tissue culture flasks were preincubated with one of the following: the CysLT1 receptor antagonist ZM198,615 (50 µM, 15 min); the Gi/o protein inhibitor PTX (500 ng/ml, 2 h); the MEK inhibitor PD98059 (50 µM, 30 min); the p38 MAPK inhibitor SB203580 (20 µM, 30 min); the phosphoinositide-3 kinase (PI3K) inhibitor LY294002 (50 µM, 30 min); and then the farnesyltransferase inhibitor FTI-277 (20 µM, 48 h); the CAMKII inhibitor KN-62 (10 µM, 30 min); the two PKC inhibitors GF109203X and Gö6976 (both 2 µM, 30 min). The cells in each of the flasks were subsequently stimulated with 80 nM LTD4 or 100 nM TPA in the absence or presence of the respective inhibitor for the indicated periods of time. The stimulation was terminated by adding ice-cold lysis buffer (50 mM Tris, pH 7.5, 1 mM EDTA, 1 mM EGTA, 1 mM Na3VO4, 1% Triton X-100, 50 mM NaF, 5 mM sodium pyrophosphate, 10 mM sodium glycerophosphate, 4 µg/ml leupeptin, and 30 µg/ml phenylmethanesulfonyl fluoride). Thereafter, the cells were kept on ice for 30 min in the lysis buffer, and the remaining cell debris was scraped loose from the bottom of the flasks into the buffer. The lysates were homogenized 10 times on ice with a glass tissue grinder (Dounce) and then centrifuged at 10,000 x g for 15 min. The supernatants were collected, and the protein content was measured and compensated for and further processed for protein separation by gel electrophoresis.

Gel Electrophoresis—Cell lysates were solubilized by boiling at 100 °C for 5 min in a sample buffer (final concentrations of the components: 62 mM Tris, pH 6.8, 1.0% SDS, 10% glycerol, 15 mg/ml dithiothreitol, and 0.05% bromphenol blue), and the solubilized proteins were separated by electrophoresis on a 10–12% homogeneous polyacrylamide gel in the presence of SDS.

Immunoblotting—The separated proteins were electrophoretically transferred to polyvinylidene difluoride membranes. The membranes were blocked for 1 h with 5% non-fat dried milk at room temperature and then incubated with a primary antibody (diluted 1:1,000) for 1 h at room temperature or overnight at 4 °C. The membranes were subsequently washed extensively and incubated with a horseradish peroxidase-linked goat anti-rabbit, anti-sheep, or anti-mouse antibody (1: 5,000) for 1 h at room temperature. Thereafter, the membranes were washed extensively, incubated with ECL Western blot detection reagents, and finally exposed to Hyperfilm-ECL to visualize immunoreactive proteins. The phospho-p90RSK and phospho-CREB blots were routinely stripped and reprobed to determine the total amounts of p90RSK and CREB.

Immunofluorescence—The cells were seeded on glass coverslips and grown for 5 days. During the last 24 h of the incubation, the cells were cotransfected with dominant-negative K-p90RSK and EGFP vector or transfected with EGFP-tagged PKC{alpha}-RD as described above. Thereafter, the cells were serum starved for 2 h and stimulated with LTD4. The stimulation was terminated by fixing for 10 min at room temperature in a 3.7% paraformaldehyde and PBS solution, after which the cells were permeabilized in a 0.5% Triton X-100 and PBS solution for 15 min. The coverslips were subsequently washed twice in PBS and incubated at room temperature in a 3% bovine serum albumin and PBS solution for 15 min, and the cells were stained for 1 h with a phosphospecific antibody against CREB. The coverslips were then washed six times in PBS and incubated with Alexa Fluor 568 goat anti-rabbit secondary antibody (diluted 1:200 in blocking buffer). The coverslips were thereafter washed six times in PBS and mounted in fluorescent mounting medium (DAKO A/S). Samples were examined and photographed with a Nikon Eclipse 800 microscope, using a Plan-Apo 60x objective. Images were recorded with a scientific grade, charge-coupled device camera (Hamamatsu, Japan) and subsequently analyzed with HazeBuster deconvolution software (Vay Tek, Inc., Fairland, CT).

Cell Counting and Thymidine Incorporation—The cells were cotransfected with dominant-negative K-p90RSK or dominant-negative K-CREB and EGFP vector or transfected with EGFP vector alone and allowed to grow for another 24 h in medium supplemented with serum as described above. Thereafter, the cells were serum starved and stimulated with LTD4 and or inhibitors as indicated in Fig. 6A. During the course of the experiment, fresh media, LTD4, and inhibitors were added every 24 h. To determine the number of viable cells, counting was done in the presence of 0.2% trypan blue after treatment for the periods of time indicated in Fig. 6A. For thymidine incorporation, cells were cultured in 24-well plates and cotransfected as described above or incubated with the indicated inhibitors. In the case of transfected samples, cells were either transfected with empty vector or with the vector indicated for 6 h and were then allowed to grow for an additional 24 h in medium supplemented with serum. Cells were then stimulated with LTD4 for 48 h, and cellular DNA synthesis was assayed by adding 0.5 µCi of 3H-labeled thymidine during the last 24 h of stimulation. Cells were washed twice with PBS, treated with 10% trichloroacetic acid for 30 min, and then lysed in 1 M NaOH. The level of radioactivity, indicating the incorporation of [3H]thymidine into DNA, was measured using a {beta}-liquid scintillation counter (LKB RackBeta, Wallace).



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  		FIG. 6.
LTD4 mediates both the proliferation and survival of intestinal epithelial cells. All cells were either preincubated in the absence or presence of 2 µM Gö6976 or 50 µM PD98059 for 30 min, or they were transfected with either empty (tr), K-p90RSK,or K-CREB vectors. The cells in A were subsequently incubated in the absence or presence of 80 nM LTD4, the specified inhibitors or vectors, as indicated in A, for up to 5 days. The proliferative responses were measured each day by counting the cells in the presence of trypan blue. B, thymidine uptake analysis of cells pretreated as in A but incubated in the absence or presence of 80 nM LTD4 for 48 h during which 0.5 µCi/well [methyl-3H]thymidine was added for the last 24 h. Then, duplicates of each lysate were mixed with scintillation liquid, and their radioactivities were measured in a LKB Wallace, 1209 RackBeta counter. The results shown in B are based on four independent experiments and are given as the means ± S.D. C, a representative flow cytometric analysis of cells that were pretreated as in A and then labeled with propidium iodide after incubation in the absence or presence of 80 nM LTD4 for 48 h. D, statistical analysis of the flow cytometric data outlined in C. Values given represent percentages of cells in sub-Go/G1, Go/G1, and S+G2/M phases, and they are means ± S.D. of four experiments.

 
Flow Cytometry—The cells were cotransfected with dominant-negative K-p90RSK or dominant-negative K-CREB and EGFP vector or transfected with EGFP vector alone and allowed to grow for another 24 h in medium supplemented with serum as described above. Thereafter, the cells were serum starved and stimulated with 80 nM LTD4 for 48 h in the absence or presence of the indicated inhibitors. During the course of the experiment, fresh media, LTD4, and inhibitors were added after 24 h. Cells that had spontaneously detached from the monolayer and were floating in the medium were collected by centrifugation and mixed with cells that had been detached from the same monolayer by the addition of trypsin-EDTA. These cells were slowly fixed by adding ice-cold ethanol while gently vortexing the sample. The DNA content of each sample (~2 x 105 cells) was estimated by staining with propidium iodide using the Vindelöv reagent (33) and then filtering (40 µm) the samples and quantifying the fluorescence by use of a Cell Quest FAC-Scan flow cytometer (BD Biosciences). Each measurement was based on analysis of 10,000 cells (events).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
LTD4 Induces Phosphorylations of p90RSK and CREB in a Time-dependent Manner—To examine the ability of LTD4 to activate p90RSK and/or CREB, we studied the effects of this leukotriene on the phosphorylation status of these two proteins. We found that LTD4 significantly increased the level of phosphorylation of both p90RSK and CREB. For p90RSK, the phosphorylation was rapid and transient (Fig. 1A), reaching a maximum (3.5-fold increase) after only 5 min and approaching the initial basal level after 90 min. By comparison, the LTD4-induced phosphorylation of CREB was also transient but had a slower onset, reaching a maximum level after 30 min and starting to decline after 90 min of exposure to the leukotriene (Fig. 1B). In our investigation of the intracellular signaling pathways involved in LTD4-induced activation of p90RSK and CREB, our first objective was to confirm the participation of a PTX-sensitive G protein in the initial signaling of LTD4. We incubated cells with 500 ng/ml PTX for 2 h and then stimulated them with LTD4 for 5 min (p90RSK) or 30 min (CREB). These periods of time were chosen because they represent the time required to reach maximum levels in p90RSK and CREB activities (Fig. 1, A and B, respectively) and were subsequently used throughout this study when testing the effects of transfections and inhibitors on their activities. PTX inhibited the LTD4-provoked activation of both p90RSK and CREB (Fig. 1C), which agrees well with our previous observation that the LTD4-induced proliferative response in intestinal epithelial cells is mediated via a PTX-sensitive G protein (16). Western blot analyses of cell lysates show expression of both CysLT1 and CysLT2 receptors in these cells (Fig. 1D). We therefore tested the sensitivity of the LTD4-induced p90RSK and CREB activity for the CysLT1 receptor antagonist ZM198,615. Our data show that the LTD4-induced activation of both p90RSK and CREB could be blocked by this antagonist, although at a relatively high concentration, 50 µM (Fig. 1E). Consequently, we cannot in the present study conclude whether the LTD4 effects are mediated via either or both the CysLT1 and the CysLT2 receptor. For comparison, we also examined the ability of LTB4 to activate p90RSK and CREB. It is cleat that LTB4 induces phosphorylation of both p90RSK and CREB to a degree similar to that of LTD4 (Fig. 1F). This finding is in good agreement with our previous observation that LTB4 also triggers a proliferative response in intestinal epithelial cells (34). The total levels of p90RSK and CREB during the LTB4- and LTD4-induced stimulations were unchanged (Fig. 1, A–E).



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  		FIG. 1.
Time course of LTD4-induced activation of p90RSK and CREB. Int 407 cells were incubated in the absence or presence of 80 nM LTD4 for the indicated periods of time and then lysed. The proteins in the lysates were separated by SDS-PAGE and immunoblotted with antibodies specific for phosphorylated p90RSK or CREB. Thereafter, the blots were stripped and reprobed for total p90RSK or CREB. Representative blots and the accumulated results of densitometric analysis of LTD4-induced p90RSK phosphorylation (A) and CREB phosphorylation (B) are shown. In C, cells were preincubated for 2 h in the absence (left and center lanes) or presence of 500 ng/ml PTX (right lane) and subsequently stimulated with 80 nM LTD4 (center and right lanes) for 5 min (p90RSK) or 30 min (CREB), and thereafter cell samples were analyzed as in A and B. In D, cells were lysed, and two samples were taken from the same lysate and run in parallel until the membrane was cut to enable incubation with antibodies against the CysLT1 and CysLT2 protein, respectively. The blots were developed together on the same film. In E, cells were preincubated in the absence (left and center lanes) or presence of 50 µM ZM198,615 (right lane) for 15 min and thereafter stimulated and analyzed as in C. In F, cells were incubated in the absence (left lane) or presence of either 80 nM LTB4 (center lane) or LTD4 (right lane) and thereafter analyzed as in C. The phosphorylation values were calculated as percentages of unstimulated cells, and they are the means ± S.D. of four separate experiments. The illustrated blots are representative of at least four separate experiments.

 
LTD4-induced Activation of p90RSK Is Mediated by an ERK1/2 Signaling Pathway—We previously have found that in addition to and downstream of G protein activity, the LTD4-induced proliferative response requires the sequential activation of PKC{epsilon}, Raf-1, and ERK1/2 (16). To ascertain whether the LTD4-elicited activation of p90RSK is a downstream target/substrate of this signaling pathway, we first examined the ability of this leukotriene to activate p90RSK in cells transfected with K-PKC{epsilon}. The results showed that the expression of K-PKC{epsilon} completely blocked LTD4-induced stimulation of p90RSK (Fig. 2A). In subsequent experiments, we also found that such activation of p90RSK was inhibited in cells transfected with an HA-tagged kinase-dead Raf-1-expressing vector (K-Raf-1) (Fig. 2B). Thereafter, we preincubated nontransfected cells with PD98059, which is a MEK inhibitor that prevents the activation of ERK1/2, and observed that this treatment completely blocked the LTD4-mediated activation of p90RSK (Fig. 2C). Our data indicate that p90RSK is a downstream substrate/target of the LTD4-induced PKC{epsilon}/Raf-1/ERK1/2 signaling cascade.



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  		FIG. 2.
Characterization of the signals involved in LTD4-induced activation of p90RSK. A, the cells were transfected with either empty vectors (left and center lanes) or K-PKC{epsilon} vectors (right lane) and thereafter incubated in the absence (left lane) or presence of 80 nM LTD4 for 5 min (center and right lanes). B, the cells were transfected with empty vectors (left and center lanes) or K-Raf-1 vectors (right lane) and thereafter incubated in the absence (left lane) or presence of 80 nM LTD4 for 5 min (center and right lanes). C, the cells were incubated in the absence (left and center lanes) or presence of 50 µM PD98059 (right lane) for 30 min and were then incubated with vehicle alone (left lane) or with 80 nM LTD4 (center and right lanes) for 5 min. After all incubations, the cells were lysed, the proteins were separated by SDS-PAGE and immunoblotted with an antibody specific for phosphorylated p90RSK, and the blots were subsequently reprobed with an antibody specific for total p90RSK. The blots shown are representative of four separate experiments.

 
LTD4-induced Activation of CREB Is Not Mediated by PKA, p90RSK, p38 MAPK, PI3K, Ras, or CAMK Signaling Pathways—An intracellular increase in cAMP is know to cause activation of CREB via an activation of PKA (21). In agreement with this, our control experiments revealed that forskolin-induced activation of adenylate cyclase caused significant activation of CREB also in intestinal epithelial cells (Fig. 3A). However, the PKA inhibitor Rp-cAMPs did not block the LTD4-mediated phosphorylation of CREB (Fig. 3A). G protein-coupled receptors have been shown to propagate membrane receptor signals to the nucleus through ERK1/2-induced activation of CREB (22). Consequently, we next performed experiments to determine whether the signaling cascade by which LTD4 induces activation of p90RSK (Fig. 2) also leads to stimulation of CREB. However, neither transfection of the RD of PKC{epsilon} nor use of the K-PKC{epsilon} had any effect on the LTD4-induced activation of CREB (Fig. 3B). It has been shown previously that RDs can act as specific dominant-negative inhibitors of their PKC isoforms (16, 35). Furthermore, we noted that neither transfection of cells with kinase-dead Raf-1 nor inhibition of ERK1/2 activation had any effect on the LTD4-mediated phosphorylation of CREB (Fig. 3, B and C). Likewise, activation of CREB by LTD4 was not influenced by transfection of cells with HA-tagged K-p90RSK (Fig. 3D, third and fourth lanes). Together these data indicate that LTD4-induced stimulation of CREB does not involve the PKC{epsilon}/Raf-1/ERK1/2 signaling cascade or activation of p90RSK, which suggests that this leukotriene activates p90RSK and CREB via independent but parallel signaling pathways.



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  		FIG. 3.
Characterization of the signals involved in LTD4-induced activation of CREB. A, Int 407 cells were pretreated with 50 µM Rp-cAMPs for 30 min (third lane) or were not (first and second lanes) or 100 µM forskolin for 15 min (fourth lane). In B, the cells were transfected with either empty vectors (first and second lanes), PKC{epsilon}-RD vectors (third lane), K-PKC{epsilon} vectors (fourth lane), or K-Raf-1 vectors (fifth lane) and then incubated in the absence (first lane) or presence (second through fifth lanes) of 80 nM LTD4 for 30 min. C, the cells were preincubated in the absence (left and center lanes) or presence (right lane) of 50 µM PD98059 for 30 min and then incubated in the absence (left lane) or presence (center and right lanes) of 80 nM LTD4 for 30 min. D, the cells were transfected with either empty vectors (first two lanes) or K-p90RSK vectors (third and fourth lanes) and then incubated with 80 nM LTD4 for 30 min (second and fourth lanes). E–H, the cells were or were not (as indicated in the figure) pretreated with 20 µM SB203580 for 30 min (E), 50 µM LY294002 for 30 min (F), 20 µM FTI-277 for 48 h (G), or 10 µM KN-62 for 30 min (H). The cells analyzed in the third lane in G were transfected with dominant-negative N17 Ras vectors. After the pretreatments, all cells were, as indicated in the figure, incubated in the absence or presence of 80 nM LTD4 for 30 min, and they were subsequently lysed. The proteins were separated by SDS-PAGE and immunoblotted, as indicated in the figure, with a specific anti-phospho-CREB antibody (all except E) or with a specific anti-phospho-p38 MAPK antibody (E). The blots were then stripped and reprobed, as indicated in the figure, with either an anti-total-CREB (A–D and F–H) or an anti-total p38 MAPK (E) antibody, and in addition with an anti-HA antibody (D) for the detection of HA-tagged K-p90RSK. The illustrated blots are representative of at least three separate experiments.

 
In an attempt to identify signaling molecules that participate in LTD4-induced activation of CREB, we studied regulatory proteins that have previously been found to be involved in stimulation of CREB, among others the p38 MAPK (23). We found that LTD4 caused a significant (3-fold) activation of p38 MAPK (Fig. 3E). However, SB203580 had no effect on the LTD4-generated phosphorylation of CREB, although it inhibited the activation of p38 MAPK (Fig. 3E). In light of our earlier results showing that LTD4 also activates Ras (16) and PI3K (36) in intestinal epithelial cells and also a report implicating a Ras-dependent kinase in the activation of CREB (25), we conducted further work to investigate the possible participation of a Ras and/or PI3K signaling pathway(s) in LTD4-mediated phosphorylation of CREB. We found that activation of CREB was not prevented by the following: preincubation with the PI3K inhibitor LY294002 (Fig. 3F), exposure to the farnesyltransferase inhibitor FTI-277, or transfection with the dominant-negative mutant N17 Ras (Fig. 3G). Even though inhibition of PI3K and Ras did not reduce the CREB phosphorylation, our data might instead suggest that these signals negatively regulate CREB (Fig. 3, F and G). But a great deal of work is obviously needed to confirm such roles for these signaling molecules. Also of interest in this context, CAMKs, especially CAMKII, have been reported to be involved in phosphorylation of CREB in other types of cells (24), but we found that the CAMK inhibitor KN-62 had no effect on LTD4-stimulated CREB phosphorylation (Fig. 3H).

Classical PKC Isoform(s) Are Involved in LTD4-induced Activation of CREB—Other investigators have described PKC-mediated activation of CREB in different kinds of cells (22, 24), whereas we found that neither kinase-dead PKC{epsilon} nor the RD of PKC{epsilon} had an impact on LTD4-induced activation of CREB (Fig. 3B). However, because several isoforms of PKC are expressed in intestinal epithelial cells, and LTD4 causes activation not only of PKC{epsilon} but also of the {alpha} and {delta} isoforms, we examined the influence of TPA on CREB phosphorylation. TPA (100 nM, 15 min) treatment led to an unambiguous activation of CREB which was reduced by 2 µM Gö6976 (30 min; Fig. 4A). The LTD4-induced (80 nM, 30 min) phosphorylation of CREB was blocked by both 2 µM GF109203X (30 min) and Gö6976 (Fig. 4B). Notably, we used these two inhibitors at concentrations that are known to prevent activation of the classical isoforms of PKC, which suggests that such PKCs play a role in the LTD4-mediated activation of CREB. Under similar conditions, these classical PKC inhibitors had no effect on the LTD4-mediated p90RSK activation (80 nM LTD4 for 5 min; Fig. 4B). We have shown previously that (16, 37) different PKC isoforms vary in regard to their sensitivity to prolonged TPA-induced down-regulation. Cells exposed to 1 µM TPA for 16 h completely down-regulated their expression of PKC{alpha}, whereas their expression of another classical PKC isoform, {beta}II, was not affected (Fig. 4C). That finding, together with our observation that pretreatment with TPA significantly decreased a subsequent LTD4-mediated phosphorylation of CREB, suggests that PKC{alpha} is involved in activation of CREB (80 nM LTD4 for 30 min; Fig. 4D). Accordingly, we transfected the cells with the Myc-tagged RD of PKC{alpha} and found that expression of PKC{alpha}-RD blocked LTD4-induced phosphorlylation of CREB (Fig. 4E, third and fourth lanes). These data confirm the signaling function of PKC{alpha} in LTD4-induced activation of CREB.



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  		FIG. 4.
Identification of a possible PKC isoform(s) involved in LTD4-induced activation of CREB. A, the cells were pretreated in the absence (left and center lanes) or presence of 2 µM Gö6976 (right lane) for 30 min and were then incubated in the absence (left lane) or presence of 100 nM TPA (center and right lanes) for 15 min. B, the cells were pretreated in the absence (first and second lanes) or presence of 2 µM GF109203X (third lane) or 2 µM Gö6976 (fourth lane) for 30 min. The cells were subsequently incubated in the absence (first lane) or presence (second through fourth lanes) of 80 nM LTD4 for either 30 min (CREB) or 5 min (p90RSK). C, the cells were preincubated in the absence (left lane) or presence (right lane) of 1 µM TPA (down-regulation) for 16 h, after which whole cell lysates were analyzed by immunoblotting for the presence of the PKC isoforms {alpha} and {beta}II, as indicated in the figure. D, the cells were preincubated in the absence (first two lanes) or presence (third lane) of 1 µM TPA (down-regulation) for 16 h and then incubated in the absence (first lane) or presence (second and third lanes) of 80 nM LTD4 for 30 min. E, the cells were transfected with either empty vectors (first two lanes) or PKC{alpha}-RD vectors (third and fourth lanes) and then incubated in the absence (first and third lanes) or presence (second and fourth lanes) of 80 nM LTD4 for 30 min. The stimulations in A, B, D, and E were terminated by lysis of the cells. The proteins were separated by SDS-PAGE and immunoblotted with an anti-phospho-CREB (A, B, D, and E) and in B also with an anti-p90RSK antibody, after which the blots were stripped and reprobed with an anti-total CREB (A, B, D, and E), an anti-total p90RSK (B), or an anti-Myc (E) antibody for the detection of Myc-tagged PKC{alpha}-RD. In C, the membranes were immunoblotted with either an anti-PKC{alpha} or an anti-PKC{beta}II antibody. The illustrated blots are representative of at least three separate experiments.

 
To gain further evidence that PKC{alpha} is involved in LTD4-induced activation of CREB, we used deconvolution microscopy to examine cells transfected with EGFP-tagged PKC{alpha}-RD or K-p90RSK. These cells were stimulated with LTD4 and then immunostained with a phospho-CREB antibody (Fig. 5). The antibody staining was weak in untreated control cells but was strong in the nuclei of the LTD4-stimulated cells (Fig. 5A). This LTD4-induced CREB phosphorylation was not impaired by expression of K-p90RSK, which again indicates that such phosphorylation does not require an activation of p90RSK (Fig. 5A). On the other hand, expression of an EGFP-tagged RD of PKC{alpha} completely abolished the LTD4-mediated phosphorylation of CREB (Fig. 5B), which further implies that PKC{alpha} is involved in the LTD4-induced signal that leads to CREB phosphorylation.



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  		FIG. 5.
Fluorescence microscopy of the effects of K-p90RSK and PKC{alpha}-RD on LTD4-induced phosphorylation of CREB. The cells were cotransfected with dominant-negative K-p90RSK vectors along with empty EGFP vectors (A) or with EGFP-tagged PKC{alpha}-RD vectors (B), and they were subsequently incubated in the absence or presence of 80 nM LTD4 for 30 min, as indicated in the figure. Thereafter, the cells were fixed, permeabilized, and stained with a specific phospho-CREB antibody as described under "Experimental Procedures." In both A and B, the top three images show (from left to right) EGFP expression, phospho-CREB staining, and an overlay image of nonstimulated control cells; the bottom three images depict the same analysis of cells stimulated with LTD4. The illustrated results are representative of three separate experiments.

 
We have observed previously that LTD4 increases the survival (13) and proliferation (16) of intestinal epithelial cells, and in the current study we therefore evaluated the roles of LTD4-induced activation of p90RSK and CREB in those processes. The growth rate of the intestinal epithelial cells was elevated in the presence of LTD4; the results suggest that the LTD4-induced increase in growth rate was limited to the first 2 days (Fig. 6A). This conclusion is also supported by the LTD4-induced increase in thymidine incorporation, evaluated between 24 and 48 h (Fig. 6B). In cells expressing either K-p90RSK or K-CREB and subsequently stimulated with LTD4, the growth rate was even less than that exhibited by unstimulated control cells (Fig. 6A). Inasmuch as the transfection efficiency was not 100% in those experiments, we also tested inhibitors of ERK1/2 (PD98059) and PKC{alpha} (Gö6976), which, respectively, blocked the LTD4-induced activation of p90RSK and CREB (Fig. 6A). As expected, these inhibitors counteracted the LTD4-induced increase in growth rate even more than that obtained by transfection with K-p90RSK or K-CREB (Fig. 6A). Similar results were obtained when cell growth was assayed by thymidine incorporation. Cells stimulated with 80 nM LTD4 for 48 h) exhibited a 164% increased uptake of thymidine that was reduced to 78 and 68% by Gö6976 and PD98059, respectively (Fig. 6B). In cells transfected with an empty vector, LTD4 stimulation caused a 159% increased uptake of thymidine which was reduced to 107 and 100% when the cells were transfected with K-p90RSK and K-CREB, respectively (Fig. 6B).

The effects of inhibition of p90RSK and CREB on LTD4-induced cell growth were examined further by flow cytometry (Fig. 6, C and D). In these experiments, the cells were stained with propidium iodide, which becomes fluorescent when it binds to DNA, thus the level of fluorescence is directly proportional to the amount of cellular DNA. Cells containing an unreplicated complement of DNA are in G0/G1 phase, and those that have a fully replicated complement of DNA are in G2/M phase. Compared with unstimulated control cells, cells stimulated with LTD4 exhibited a significantly lower G0/G1 peak but a higher S+G2/M peak, which indicates elevated synthesis of DNA (Fig. 6, C and D). Cells transfected with K-CREB or treated with the PKC inhibitor Gö6976 and then exposed to LTD4 accumulated in the sub-G0/G1 phase (Fig. 6, C and D), which suggests an elevated level of cell death. In contrast, examining cells transfected with K-p90RSK or treated with the ERK1/2 inhibitor PD98059, we found that exposure to LTD4 caused a significant arrest in the G0/G1 phase, a totally reverted S+G2/M transition phase, and only a slight change on the sub-G0/G1 peak (Fig. 6, C and D), which indicates that p90RSK is involved in progression of the cell cycle.

Our data are summarized in the schematic model outlined in Fig. 7. This model reveals our present knowledge of the LTD4-induced signaling pathways involved in the activation of CREB and p90RSK and how these kinases affect the cellular regulation of survival and proliferation.



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  		FIG. 7.
A schematic model of the LTD4-induced signaling pathways involved in the activations of CREB and p90RSK, respectively.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
We have found previously that the proinflammatory mediator LTD4 induces proliferation of intestinal epithelial cells, Int 407, by Gi protein-initiated dual pathways: the Ras-independent PKC{epsilon}/Raf-1/ERK1/2 cascade and the Ras-dependent pathway (16). In the present study, we again used Int 407 cells and investigated the signaling routes responsible for activation of two downstream proteins implicated in cell proliferation, p90RSK and CREB. Our results are the first to show that LTD4 mediates the phosphorylation and thus activation of p90RSK and CREB in a transient and time-dependent manner. Moreover, LTB4 and LTD4 had almost identical effects on p90RSK and CREB phosphorylation, suggesting that several inflammatory mediators could have stimulatory effects on survival and proliferation of intestinal epithelial cells.

It is well known that p90RSK is a substrate of ERK1/2, and several reports have suggested that when activated, both ERK1/2 and p90RSK are translocated to the nucleus, where p90RSK phosphorylates CREB at Ser133 (38, 39). In accordance, we found that activation of these two proteins by LTD4 is stimulated via a Gi pathway and that LTD4 induced phosphorylation of CREB in the nuclei of intestinal cells. We found that expression of kinase-dead K-Raf-1 and K-PKC{epsilon} constructs blocked LTD4-mediated phosphorylation of both ERK1/2 and p90RSK as did preincubation with the MEK inhibitor PD98059. Furthermore, our observation that p90RSK was activated more rapidly than CREB by LTD4, and earlier investigations demonstrating that CREB can be phosphorylated via an ERK signaling pathway (18, 22), make it plausible to assume that LTD4 regulates activation of CREB through p90RSK phosphorylation. However, both overexpression of K-PKC{epsilon}/PKC{epsilon}-RD constructs and preincubation with PD98059 blocked LTD4-mediated activations of ERK1/2 and p90RSK but did not affect the phosphorylation of CREB induced by this leukotriene. In addition, transfection of cells with K-p90RSK constructs had no effect on CREB phosphorylation. These results indicate that LTD4 activates p90RSK and CREB by different mechanisms.

The members of the CREB family have been extensively characterized as nuclear substrates of PKA. We found that a strong activation of the PKA signaling pathway in intestinal epithelial cells by forskolin caused phosphorylation of CREB. However, the present LTD4-mediated CREB phosphorylation was not mediated by such a PKA signal. We could also rule out the participation of other potential signals previously suggested to mediate CREB phosphorylation such as p38 MAPK (23), PI3K (40), and Ras (25) in the present LTD4-induced CREB phosphorylation.

In certain situations CREB proteins have also been shown to be phosphorylated by PKCs at multiple sites, including Ser133 (22). A role for PKCs is further implied by observations demonstrating that the phorbol ester and PKC activator TPA stimulates CREB in several cell systems (22), which agrees with our finding that TPA caused substantial phosphorylation of CREB in Int 407 cells. Experiments on the human erythroleukemia cell line TF-1 have shown that PKC{epsilon} stimulates a CREB-dependent transcription in response to {beta}cR cytokines (41). However, our experiments with PKC inhibitors suggested that a classical PKC(s) is involved in the LTD4-induced activation of CREB. In accordance, down-regulation of PKC{alpha} followed by exposure to LTD4 completely blocked the activation of CREB. We have observed previously that treatment with TPA for 16 h also down-regulates PKC{epsilon} (16). However, LTD4-mediated CREB phosphorylation was not affected by transfection with either K-PKC{epsilon} or PKC{epsilon}-RD in our present experiments, thus PKC{epsilon} cannot mediate this phosphorylation. Finally, transfection of cells with PKC{alpha}-RD inhibited the LTD4-induced CREB phosphorylation, which also indicates that the PKC{alpha} isoform mediates the LTD4-induced activation of CREB. Additional work is needed to determine whether LTD4 activates CREB through direct phosphorylation by PKCs, or whether other kinases are involved.

LTD4 induced significantly an increase in cell proliferation compared with untreated controls, as we had also seen in an earlier study (16). Expression of the kinase-negative mutants of both p90RSK and CREB, in particular the former, significantly inhibited the LTD4-mediated proliferative response, which supports the notion that the ERK/p90RSK and PKC{alpha}/CREB pathways play an active role in the proliferative response of Int 407 cells. The effects of the p90RSK and CREB signaling pathways, induced by LTD4, on cell proliferation were confirmed by three different methods (cell counting, thymidine uptake, and flow cytometry). Furthermore, we have complemented our experimental approach to transfect the cells with the use of relevant and specific inhibitors because in the latter case all cells are affected. Interestingly enough, a number of studies have shown that CREB regulates the transcription and expression of several proteins involved in cell proliferation, including cyclin A and D1, proliferating cell nuclear antigen, and cyclooxygenase-2 (42–44). In good agreement, we have shown previously that LTD4 triggers an increased transcription and expression of cyclooxygenase-2 in intestinal epithelial cells through a ERK1/2-dependent pathway, as revealed by a cyclooxygenase-2 reporter gene assay and by Western blotting (13–15). The clinical relevance of these experimental findings is underlined by our recent finding of an increased expression of both cyclooxygenase-2 and the CysLT1 receptor in colon cancer tissue (15).

To determine whether activation of p90RSK and CREB is involved in cell survival or regulation of the cell cycle, we used these dominant-negative mutants to analyze these two processes in the presence of LTD4. Interestingly, we found with flow cytometry that cells treated with LTD4 exhibited a significantly reduced G0/G1 phase, and they also showed a buildup in the S+G2/M phases, which indicates proliferation. Furthermore, the sub-G0/G1 phase was lowered to some extent in LTD4-treated cells compared with untreated control cells. An increase in the sub-G0/G1 phase, indicating cell death, occurred after inhibition of PKC{alpha}-induced CREB activation or overexpression of K-CREB and subsequent exposure to LTD4. These data indicate that CREB is related to LTD4-mediated cell survival. In support of such a concept, expression of dominant-negative PKCs induced apoptosis in COS cells. However, coexpression of wild-type PKC{alpha} rescued these cells from apoptosis (29). Furthermore, studies in different cell types have also shown that CREB promotes cell survival and protects against apoptosis provoked by diverse stimuli (45–47), whereas expression of dominant-negative CREB leads to increased susceptibility to apoptosis (46). Notably, induction of Bcl-2 has been reported to occur in a CREB-dependent manner (48, 49). In prostate cancer cell lines expression of CREB repressed inhibition of Bcl-2 promoter activity caused by the tumor suppressor gene PTEN (50). These investigators also noted that expression of Bcl-2 saved cells from PTEN-induced death but did not stop the cycle in the G1 phase, suggesting that CREB and Bcl-2 play an active role in survival rather than cell cycle transition in prostate cancer cell lines. Moreover, earlier investigations in our laboratory revealed that treatment of Int 407 and Caco-2 cells with LTD4 leads to significant increased expression of Bcl-2 protein (13, 14). Nonetheless, further research is needed to evaluate whether LTD4-induced expression of Bcl-2 in these cells is actually mediated by CREB and also to determine whether the survival effect of CREB is transduced via Bcl-2 protein.

In contrast, we found that the arrest in G0/G1 phase and the increase in S phase caused by LTD4 were inhibited significantly in cells in which the activation of p90RSK was impaired. In addition, such treatments also caused a relatively low rate of dead cells. It is well known that ERK mediates cell cycle progression by activating the c-Fos transcription factor through phosphorylation of Elk-1 (51). In addition, ERK has also been shown to phosphorylate several other nuclear transcription factors such as Jun, Myc, p62TCF, and Fos, and also regulate the expression of their downstream genes (38, 51, 52). In studies of the human myeloerythroid CD34+ leukemia cell line it was observed that ionomycin-induced MEK-dependent biphasic activation of ERK1/2 was sufficient to cause the G0/G1 to S/M phase transformation (53). Involvement of LTD4 in proliferation has been established in several cell systems (54, 55), but the actual LTD4 signaling pathways that regulate cell proliferation have not been elucidated thoroughly. Our results show that LTD4 increases proliferation by a PKC{epsilon}/Raf-1/ERK1/2 signaling cascade that leads to the activation of p90RSK.

Our present findings are the first to show that LTD4 can activate p90RSK and CREB by two distinct signaling pathways and that the CREB pathway is more important for cell survival, whereas the ERK/p90RSK pathway play a significant role in cell proliferation.


   FOOTNOTES
 
* This work was supported by grants from the Swedish Medical Research Council, the Magnus Bergvall Foundation, the Crafoord Foundation, the Foundations at Malmö University Hospital, the Kock Foundation, and the Zoega Fondation (to A. S.) and by grants from the Royal Physiographic Society in Lund (to S. P.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

{ddagger} To whom correspondence should be addressed. Tel.: 46-40-337223; Fax: 46-40-337353; E-mail: anita.sjolander{at}exppat.mas.lu.se.

1 The abbreviations used are: LT(s), leukotriene(s); CAMK, Ca2+/calmodulin-dependent protein kinase; CBP, CREB-binding protein; CREB, cAMP-responsive element-binding protein; EGFP, enhanced green fluorescent protein; ERK1/2, extracellular signal-regulated kinase 1 and 2; HA, hemagglutinin; K-p90RSK and K-CREB, kinase-negative mutants of p90RSK and CREB, respectively; MAPK, mitogen-activated protein kinase; MEK, mitogen-activated protein kinase/extracellular signal-regulated kinase kinase; p90RSK, 90-kDa ribosomal S6 kinase; PBS, phosphate-buffered saline; PI3K, phosphoinositide-3 kinase; PKA, protein kinase A; PKC, protein kinase C; PTX, pertussis toxin; RD, regulatory domain; TPA, 12-O-tetradecanoylphorbol-13-acetate. Back


   ACKNOWLEDGMENTS
 
We are grateful to Patricia Ödman for linguistic revision of the manuscript and to Dr. R. Metcalf, Zeneca Pharmaceuticals, Macclesfield, United Kingdom, for providing ZM198,615.



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