The epsilon isoform of protein kinase C is involved in regulation of the LTD(4)-induced calcium signal in human intestinal epithelial cells

Cellular signalling of cysteinyl leukotriene type 1 receptor variants CysLT₁-G300S and CysLT₁-I206S

Cysteinyl-leukotrienes are pro-inflammatory lipid mediators, involved in allergic asthma, that bind the G-protein-coupled receptors CysLT1, CysLT2 and GPR99. A polymorphism in one of these receptors, CysLT1-G300S was strongly associated with atopy, whereas the CysLT1-I206S polymorphism was not. In the present work, our aim was to characterize these two variants by studying their cellular signalling. Cell surface expression of mutant receptors in transfected HEK-293 cells was comparable to that of the wild-type receptor. Compared to CysLT1-WT, production of inositol phosphates as well as IL-8 and IL-13 promoter transactivation in response to either LTD4 or LTC4 was significantly increased in CysLT1-G300S-transfected cells. Moreover, LTD4-induced phosphorylation of the signalling effector Erk, but not p38, p65 or c-Jun was higher in CysLT1-G300S-transfected cells. On the other hand, the variant CysLT1-I206S did not show a significant difference in its signal transduction compared to the wild-type receptor. Taken together, our results indicate that the variant CysLT1-G300S can induce a greater signal than the CysLT1-WT receptor, a feature that may be relevant to its association with atopy.

Cysteinyl leukotrienes regulate endothelial cell inflammatory and proliferative signals through CysLT₂ and CysLT₁ receptors

Cysteinyl leukotrienes (cys-LTs), LTC₄, LTD₄, LTE₄ are potent inflammatory lipid mediators that act through two distinct G-protein-coupled receptors, CysLT₁R and CysLT₂R. Although cys-LTs are shown to induce vascular leakage and atherosclerosis, the molecular mechanism by which cys-LTs modulate endothelial function is not known. Here, we show that cys-LTs (LTC₄ and LTD₄) induce robust calcium influx in human umbilical vein endothelial cells (HUVECs) through CysLT₂R, but not CysLT₁R. Further, cys-LT treatment induced endothelial cell (EC) contraction leading to monolayer disruption via CysLT₂R/Rho kinase dependent pathway. Furthermore, stimulation with cys-LTs potentiated TNFα-induced VCAM-1 expression and leukocyte recruitment to ECs through CysLT₂R. In contrast, we found that both LTC₄ and LTD₄ stimulated EC proliferation through CysLT₁R. Taken together, these results suggest that cys-LTs induce endothelial inflammation and proliferation via CysLT₂R/Rho kinase and CysLT₁R/Erk dependent pathways, respectively, which play critical role in the etiology of cardiovascular diseases such as atherosclerosis and myocardial infarction.

Differential regulation of cysteinyl leukotriene receptor signaling by protein kinase C in human mast cells

Cysteinyl leukotrienes (cys-LTs) are a group of lipid mediators that are potent bronchoconstrictors, powerful inducers of vascular leakage and potentiators of airway hyperresponsiveness. Cys-LTs play an essential role in asthma and are synthesized as well as activated in mast cells (MCs). Cys-LTs relay their effects mainly through two known GPCRs, CysLT₁R and CysLT₂R. Although protein kinase C (PKC) isoforms are implicated in the regulation of CysLT₁R function, neither the role of PKCs in cys-LT-dependent MC inflammatory signaling nor the involvement of specific isoforms in MC function are known. Here, we show that PKC inhibition augmented LTD₄ and LTE₄-induced calcium influx through CysLT₁R in MCs. In contrast, inhibition of PKCs suppressed c-fos expression as well MIP1β generation by cys-LTs. Interestingly, cys-LTs activated both PKCα and PKCε isoforms in MC. However, knockdown of PKCα augmented cys-LT mediated calcium flux, while knockdown of PKCε attenuated cys-LT induced c-fos expression and MIP1β generation. Taken together, these results demonstrate for the first time that cys-LT signaling downstream of CysLT₁R in MCs is differentially regulated by two distinct PKCs which modulate inflammatory signals that have significant pathobiologic implications in allergic reactions and asthma pathology.

The cysteinyl leukotriene 2 receptor contributes to all-trans retinoic acid-induced differentiation of colon cancer cells

Background

Cysteinyl leukotrienes (CysLTs) are potent pro-inflammatory mediators that are increased in samples from patients with inflammatory bowel diseases (IBDs). Individuals with IBDs have enhanced susceptibility to colon carcinogenesis. In colorectal cancer, the balance between the pro-mitogenic cysteinyl leukotriene 1 receptor (CysLT₁R) and the differentiation-promoting cysteinyl leukotriene 2 receptor (CysLT₂R) is lost. Further, our previous data indicate that patients with high CysLT₁R and low CysLT₂R expression have a poor prognosis. In this study, we examined whether the balance between CysLT₁R and CysLT₂R could be restored by treatment with the cancer chemopreventive agent all-trans retinoic acid (ATRA).

Methods

To determine the effect of ATRA on CysLT₂R promoter activation, mRNA level, and protein level, we performed luciferase gene reporter assays, real-time polymerase chain reactions, and Western blots in colon cancer cell lines under various conditions.

Results

ATRA treatment induces CysLT₂R mRNA and protein expression without affecting CysLT₁R levels. Experiments using siRNA and mutant cell lines indicate that the up-regulation is retinoic acid receptor (RAR) dependent. Interestingly, ATRA also up-regulates mRNA expression of leukotriene C₄ synthase, the enzyme responsible for the production of the ligand for CysLT₂R. Importantly, ATRA-induced differentiation of colorectal cancer cells as shown by increased expression of MUC-2 and production of alkaline phosphatase, both of which could be reduced by a CysLT₂R-specific inhibitor.

Conclusions

This study identifies a novel mechanism of action for ATRA in colorectal cancer cell differentiation and demonstrates that retinoids can have anti-tumorigenic effects through their action on the cysteinyl leukotriene pathway.

Regulatory characteristics of the Vibrio vulnificus rtxHCA operon encoding a MARTX toxin

Vibrio vulnificus MARTX encoded by rtxA, an open reading frame of the rtxHCA operon, is essential for virulence in vitro and in mice. In this study, a primer extension analysis revealed that transcription of the rtxHCA operon begins at a single site, and is under the direction of a single promoter, P( rtxHCA ). P( rtxHCA ) activity appeared at the beginning of growth and reached a maximum in mid-exponential phase. P( rtxHCA ) activity was induced by exposure to INT-407 cells, and the membrane fraction of INT-407 cells was the most effective for the induction.

PKCα mediates acetylcholine-induced activation of TRPV4-dependent calcium influx in endothelial cells

Transient receptor potential vanilloid channel 4 (TRPV4) is a polymodally activated nonselective cationic channel implicated in the regulation of vasodilation and hypertension. We and others have recently shown that cyclic stretch and shear stress activate TRPV4-mediated calcium influx in endothelial cells (EC). In addition to the mechanical forces, acetylcholine (ACh) was shown to activate TRPV4-mediated calcium influx in endothelial cells, which is important for nitric oxide-dependent vasodilation. However, the molecular mechanism through which ACh activates TRPV4 is not known. Here, we show that ACh-induced calcium influx and endothelial nitric oxide synthase (eNOS) phosphorylation but not calcium release from intracellular stores is inhibited by a specific TRPV4 antagonist, AB-159908. Importantly, activation of store-operated calcium influx was not altered in the TRPV4 null EC, suggesting that TRPV4-dependent calcium influx is mediated through a receptor-operated pathway. Furthermore, we found that ACh treatment activated protein kinase C (PKC) α, and inhibition of PKCα activity by the specific inhibitor Go-6976, or expression of a kinase-dead mutant of PKCα but not PKCε or downregulation of PKCα expression by chronic 12-O-tetradecanoylphorbol-13-acetate treatment, completely abolished ACh-induced calcium influx. Finally, we found that ACh-induced vasodilation was inhibited by the PKCα inhibitor Go-6976 in small mesenteric arteries from wild-type mice, but not in TRPV4 null mice. Taken together, these findings demonstrate, for the first time, that a specific isoform of PKC, PKCα, mediates agonist-induced receptor-mediated TRPV4 activation in endothelial cells.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Role of dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 in protein kinase A- and protein kinase C-mediated regulation of the steroidogenic acute regulatory protein expression in mouse Leydig tumor cells: mechanism of action

Dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome, gene 1 (DAX-1) is an orphan nuclear receptor that has been demonstrated to be instrumental to the expression of the steroidogenic acute regulatory (StAR) protein that regulates steroid biosynthesis in steroidogenic cells. However, its mechanism of action remains obscure. The present investigation was aimed at exploring the molecular involvement of DAX-1 in protein kinase A (PKA)- and protein kinase C (PKC)-mediated regulation of StAR expression and its concomitant impact on steroid synthesis using MA-10 mouse Leydig tumor cells. We demonstrate that activation of the PKA and PKC pathways, by a cAMP analog dibutyryl (Bu)2cAMP [(Bu)2cAMP] and phorbol 12-myristate 13-acetate (PMA), respectively, markedly decreased DAX-1 expression, an event that was inversely correlated with StAR protein, StAR mRNA, and progesterone levels. Notably, the suppression of DAX-1 requires de novo transcription and translation, suggesting that the effect of DAX-1 in regulating StAR expression is dynamic. Chromatin immunoprecipitation studies revealed the association of DAX-1 with the proximal but not the distal region of the StAR promoter, and both (Bu)2cAMP and PMA decreased in vivo DAX-1-DNA interactions. EMSA and reporter gene analyses demonstrated the functional integrity of this interaction by showing that DAX-1 binds to a DNA hairpin at position -44/-20 bp of the mouse StAR promoter and that the binding of DAX-1 to this region decreases progesterone synthesis by impairing transcription of the StAR gene. In support of this, targeted silencing of endogenous DAX-1 elevated basal, (Bu)2cAMP-, and PMA-stimulated StAR expression and progesterone synthesis. Transrepression of the StAR gene by DAX-1 was tightly associated with expression of the nuclear receptors Nur77 and steroidogenic factor-1, demonstrating these factors negatively modulate the steroidogenic response. These findings provide insight into the molecular events by which DAX-1 influences the PKA and PKC signaling pathways involved in the regulation of the StAR protein and steroidogenesis in mouse Leydig tumor cells.

Alpha2beta1 integrin signalling enhances cyclooxygenase-2 expression in intestinal epithelial cells

Inflammatory bowel diseases (IBD) are linked to an increased risk of developing colon cancer, by inflammatory mediators and alterations to the extracellular matrix (ECM). The events induced by inflammatory mediators lead to dysregulated activation and induction of inflammatory genes such as cyclooxygenase-2 (COX-2). COX-2 is involved in the conversion of arachidonic acid to biologically active prostanoids and is highly upregulated in colon cancer. Since inflammation-induced changes to the extracellular matrix could affect integrin activities, we here investigated the effect of integrin signalling on the level of COX-2 expression in the non-transformed intestinal epithelial cell lines, Int 407 and IEC-6. Adhesion of these cells to a collagen I- or IV-coated surface, increased surface expression of alpha2beta1 integrin. Activation of integrins with collagen caused an increased cox-2 promoter activity, with a subsequent increase in COX-2 expression. The signalling cascade leading to this increased expression and promoter activity of cox-2, involves PKCalpha, the small GTPase Ras and NFkappaB but not Erk1/2 or Src activity. The integrin-induced increase in cellular COX-2 activity is responsible for an elevated generation of reactive oxygen species (ROS) and increased cell migration. This signalling pathway suggests a mechanism whereby inflammation-induced modulations of the ECM, can promote cancer transformation in the intestinal epithelial cells.

Swelling-activated ion channels: functional regulation in cell-swelling, proliferation and apoptosis

Cell volume regulation is one of the most fundamental homeostatic mechanisms and essential for normal cellular function. At the same time, however, many physiological mechanisms are associated with regulatory changes in cell size meaning that the set point for cell volume regulation is under physiological control. Thus, cell volume is under a tight and dynamic control and abnormal cell volume regulation will ultimately lead to severe cellular dysfunction, including alterations in cell proliferation and cell death. This review describes the different swelling-activated ion channels that participate as key players in the maintenance of normal steady-state cell volume, with particular emphasis on the intracellular signalling pathways responsible for their regulation during hypotonic stress, cell proliferation and apoptosis.

Role of PKC isoforms in the Fc(gamma)R-mediated inhibition of LPS-stimulated IL-12 secretion by macrophages

Ligation of Fc receptors for immunoglobulin G (FcgammaRs) inhibits lipopolysaccharide (LPS)-stimulated secretion of interleukin (IL)-12 by macrophages. FcgammaR activation of protein kinase C (PKC) contributes to several functions of this receptor including phagocytosis, activation of the reduced nicotinamide adenine dinucleotide phosphate oxidase, and secretion of certain cytokines. Therefore, we tested the hypothesis that PKC mediates the FcgammaR inhibition of IL-12 secretion by macrophages. In murine macrophages, FcgammaR ligation augmented LPS-stimulated activation of PKC-alpha and PKC-delta but reduced IL-12p40 secretion. Similarly, activation of PKC with phorbol 12-myristate 13-acetate (PMA) depressed LPS-stimulated IL-12p40 secretion, and depletion of PKC augmented LPS-stimulated IL-12p40 secretion. Antisense down-regulation of PKC-delta increased LPS-stimulated IL-12p40 secretion and fully prevented the effects of FcgammaR ligation or PMA on IL-12p40 secretion. In contrast, down-regulation of PKC-epsilon blocked LPS-stimulated secretion of IL-12p40. Down-regulation of PKC-alpha had no effect on LPS-stimulated IL-12p40 secretion. The results suggest a negative role for PKC-delta and a positive role for PKC-epsilon in the regulation of LPS-stimulated IL-12p40 secretion.

Activation of cPLA 2 is required for leukotriene D 4 -induced proliferation in colon cancer cells

It is well documented that prolonged inflammatory conditions, particularly those relating to the colon, have been shown to induce cancer. We have previously demonstrated that the pro-inflammatory mediator leukotriene D(4) (LTD(4)) induces survival and proliferation in intestinal cells and that its receptor, CysLT(1), is upregulated in human colon cancer tissue. Here we demonstrate, for the first time that in both Int 407 (a non-transformed human intestinal epithelial cell line) and Caco-2 cells (a human colorectal carcinoma cell line), cytosolic phospholipase A(2)alpha (cPLA(2)alpha) is activated and translocates to the nucleus upon LTD(4) stimulation via a calcium-dependent mechanism that involves activation of protein kinase C (PKC), and the mitogen-activated protein kinases ERK1/2 and p38. We also show with a cPLA(2)alpha promoter luciferase assay, that LTD(4) induces an increase in the transcriptional activity of cPLA(2)alpha via activation of cPLA(2)alpha and the transcription factor NFkappaB. Interestingly we demonstrate here that both the basal and the LTD(4)-induced cPLA(2)alpha activity is elevated approximately 3-fold in Caco-2 colon cancer cells compared with Int 407 cells. The difference in basal activity was confirmed in human colon tumor samples by the finding of a similar increase in cPLA(2)alpha activity when compared with normal colon tissue. A functional role of the increased cPLA(2)alpha activity in tumor cells was revealed by our findings that inhibition of this enzyme reduced both basal and LTD(4)-induced proliferation, the effects being most pronounced in Caco-2 tumor cells. The present data reveal that cPLA(2)alpha, an important intracellular signal activated by inflammatory mediators, is an important regulator of colon tumor growth.

Developmental Sensitivity of the Bovine Corpus Luteum to Prostaglandin F2α (PGF2α) and Endothelin-1 (ET-1): Is ET-1 a Mediator of the Luteolytic Actions of PGF2α or a Tonic Inhibitor of Progesterone Secretion?1

We examined the responsiveness of large luteal cells (LLC), small luteal cells (SLC), and endothelial cells of the Day 4 and Day 10 bovine corpus luteum (CL) to prostaglandin (PG) F2alpha and endothelin (ET)-1. Using a single-cell approach, we tested the ability of each agonist to increase the cytoplasmic concentration of calcium ions ([Ca2+]i) as function of luteal development. All tested concentrations of agonists significantly (P = 0.05) increased [Ca2+]i in all cell populations isolated from Day 4 and Day 10 CL. Day 10 steroidogenic cells were more responsive than Day 4 cells to PGF2alpha and ET-1. Response amplitudes and number of responding cells were affected significantly by agonist concentration, luteal development, and cell type. Response amplitudes were greater in LLC than in SLC; responses of maximal amplitude were elicited with lower agonist concentrations in Day 10 cells than in Day 4 cells. Furthermore, on Day 10, as the concentration of PGF2alpha increased, larger percentages of SLC responded. Endothelial cells responded maximally, regardless of agonist concentration and luteal development. In experiment 2, we tested the developmental responsiveness of total dispersed and steroidogenic-enriched cells to the inhibitory actions of PGF2alpha and ET-1 on basal and LH-stimulated progesterone accumulation. The potency of PGF2alpha steroidogenic-enriched cells on Day 4 was lower than on Day 10; in contrast, the potency of ET-1 was not different. Therefore, ET-1 was a tonic inhibitor of progesterone accumulation rather than a mediator of PGF2alpha action. The lower efficacy of PGF2alpha in the early CL more likely is related to signal transduction differences associated with its receptor at these two developmental stages than to the inability of PGF2alpha to up-regulate ET-1.

A Novel Localization of the G-Protein-Coupled CysLT1 Receptor in the Nucleus of Colorectal Adenocarcinoma Cells

Searching for a link between inflammation and colon cancer, we have found that the inflammatory mediator leukotriene D4 (LTD4), via its receptor CysLT1, induces cyclooxygenase-2 expression, survival, and proliferation in intestinal epithelial cells. In conjunction with our previous observation that CysLT1 receptor expression is increased in colorectal adenocarcinomas, we here found an increased nuclear localization of the CysLT1 receptor in colorectal adenocarcinomas. This novel discovery of CysLT1 receptors in the nucleus was further analyzed. It was found to be located in the outer nuclear membrane in colon cancer cells and in the nontransformed epithelial cell line Int 407 cells by Western blot and electron microscopy. Cancer cells displayed higher amounts of the nuclear CysLT1 receptor, but prolonged LTD4 exposure induced its nuclear translocation in nontransformed cells. Truncation of a nuclear localization sequence abrogated this translocation as well as the LTD4-induced proliferative response. In accordance, nuclear CysLT1 receptors exhibited proliferative extracellular signal-regulated kinase 1/2 signaling. The significance of these experimental findings is supported by the observed correlation between the proliferative marker Ki-67 and nuclear CysLT1 receptor localization in colorectal adenocarcinomas. The present findings indicate that LTD4 cannot only be synthesized but also signal proliferation through nuclear CysLT1 receptors, stressing the importance of leukotrienes in inflammation-induced colon carcinogenesis.

Leukotriene D4 activates distinct G-proteins in intestinal epithelial cells to regulate stress fibre formation and to generate intracellular Ca2+ mobilisation and ERK1/2 activation

We have shown that the pro-inflammatory mediator LTD4, via its G-protein-coupled receptor CysLT1, signals through both pertussis-toxin-sensitive and -insensitive G-proteins to induce various cellular responses. To further characterise the initial step of the different signalling pathways emanating from the CysLT1 receptor, we transfected intestinal epithelial cells, Int 407, with different mini vectors that each express a specific inhibitory peptide directed against a unique alpha subunit of a specific heterotrimeric G-protein. Our results revealed that LTD4-induced stress fibre formation is inhibited approximately 80% by a vector expressing an inhibitory peptide against the pertussis-toxin-insensitive Galpha12-protein in intestinal epithelial Int 407 cells. Control experiments revealed that the LPA-induced stress fibre formation, mediated via the Galpha12-protein in other cell types, was blocked by the same peptide in intestinal Int 407 cells. Furthermore, the CysLT1-receptor-mediated calcium signal and activation of the proliferative ERK1/2 kinase are blocked in cells transfected with a vector expressing an inhibitory peptide against the Galphai3-protein, whereas in cells transfected with an empty ECFP-vector or vectors expressing inhibitory peptides against the Galphai1-2-, Galpha12-, GalphaR-proteins these signals are not significantly affected. Consequently, the CysLT1 receptor has the capacity to activate at least two distinctly different heterotrimeric G-proteins that transduce activation of unique downstream cellular events.

Effects of selective protein kinase c isozymes in prostaglandin2alpha-induced Ca2+ signaling and luteinizing hormone-induced progesterone accumulation in the mid-phase bovine corpus luteum

A single-cell approach for measuring the concentration of cytoplasmic calcium ions ([Ca(2+)](i)) and a protein kinase C-epsilon (PKCepsilon)-specific inhibitor were used to investigate the developmental role of PKCepsilon in the prostaglandin F(2alpha)(PGF(2alpha))-induced rise in [Ca(2+)](i) and the induced decline in progesterone accumulation in cultures of cells isolated from the bovine corpus luteum. PGF(2alpha) increased [Ca(2+)](i) in Day 4 large luteal cells (LLCs), but the response was significantly lower than in Day 10 LLCs (4.3 +/- 0.6, n = 116 vs. 21.3 +/- 2.3, n = 110). Similarly, the fold increase in the PGF(2alpha)-induced rise in [Ca(2+)](i) in Day 4 small luteal cells (SLCs) was lower than in Day 10 SLCs (1.6 +/- 0.2, n = 198 vs. 2.7 +/- 0.1, n = 95). A PKCepsilon inhibitor reduced the PGF(2alpha)-elicited calcium responses in both Day 10 LLCs and SLCs to 3.5 +/- 0.3 (n = 217) and 1.3 +/- 0.1 (n = 205), respectively. PGF(2alpha) inhibited LH-stimulated progesterone (P(4)) accumulation only in the incubation medium of Day 10 luteal cells. Both conventional and PKCepsilon-specific inhibitors reversed the ability of PGF(2alpha) to decrease LH-stimulated P(4) accumulation, and the PKCepsilon inhibitor was more effective at this than the conventional PKC inhibitor. In conclusion, the evidence indicates that PKCepsilon, an isozyme expressed in corpora lutea with acquired PGF(2alpha) luteolytic capacity, has a regulatory role in the PGF(2alpha)-induced Ca(2+) signaling in luteal steroidogenic cells, and that this in turn may have consequences (at least in part) on the ability of PGF(2alpha) to inhibit LH-stimulated P(4) synthesis at this developmental stage.

Co-expression of mCysLT1 receptors and IK channels in Xenopus laevis oocytes elicits LTD4-stimulated IK current, independent of an increase in [Ca2+]i

Addition of LTD4 (10 nM) to Xenopus laevis oocytes expressing the mCysLT1 receptor together with hBK or hIK channels resulted in the activation of both channels secondary to an LTD4-induced increase in [Ca2+]i. In addition, the hIK channel is activated by low concentrations of LTD4 (<0.1 nM), which did not result in any increase in [Ca2+]i. Even though activation of hIK by low concentrations of LTD4 was independent of an increase in [Ca2+]i, a certain "permissive" level of [Ca2+]i was required for its activation, since buffering of intracellular Ca2+ by EGTA completely abolished the response to LTD4. Neither hTBAK1 nor hTASK2 was activated following stimulations with LTD4 (0.1 and 100 nM).

Leukotriene D4 mediates survival and proliferation via separate but parallel pathways in the human intestinal epithelial cell line Int 407

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 PKCepsilon/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 PKCalpha (a specific dominant-inhibitor of PKCalpha). 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.

Leukotriene D4-induced adhesion of Caco-2 cells is mediated by prostaglandin E2 and upregulation of α2β1-integrin

Cell-cell and extracellular matrix adhesions play important roles in the progression of cancer. We investigated the involvement of the inflammatory mediator leukotriene D4 (LTD4) in the regulation of cell-matrix adhesion of colon cancer (Caco-2) cells. We observed that LTD4 acted via its CysLT1 receptor in these cells to induce increased adhesion to collagen I. LTD4 also enhanced the activation and expression of alpha2beta1-integrins on the cell surface, which we found to be responsible for mediating the increased adhesion to collagen I. LTD4 simultaneously augmented expression of the prostaglandin-generating enzyme cyclooxygenase-2 (COX-2) and increased prostaglandin E2 (PGE2) production in Caco-2 cells. The adhesive capacity of the Caco-2 cells was reduced by specific inhibition of COX-2 and was subsequently restored by PGE2, but not by LTD4. A selective PGE2 receptor antagonist abolished the increased adhesion and the augmented alpha2beta1-integrin expression induced by both PGE2 and LTD4. Summarizing, the inflammatory mediator LTD4 regulates the adhesive properties and migration of the Caco-2 cell line by upregulating COX-2 and stimulating PGE2-induced expression of alpha2beta1-integrins. This suggests that inflammatory mediators such as LTD4 can be involved in the dissemination and survival of colon cancer cells.

pp60c-Src Kinase mediates growth effects of the full-length precursor progastrin1-80 peptide on rat intestinal epithelial cells, in vitro

Growth factor effects of precursor forms of gastrins have become evident in recent years. However, intracellular pathways that mediate growth effects of the precursor molecules are not known. In previous studies, we reported an increase in Tyr phosphorylation of pp60(c-Src) in intestinal epithelial cells (IEC) in response to the fully processed form of gastrin [gastrin(1-17) (G17)]. We have now examined whether c-Src kinase is similarly phosphorylated and activated in response to the full-length precursor molecule, progastrin (PG)(1-80), (recombinant human PG) in IEC cells. We found a significant increase in pp60(c-Src) kinase activity in response to both G17 and PG (0.1-1.0 nM), suggesting that growth effects of both the precursor and fully processed gastrin molecules may be mediated via similar pathways. On the other hand, pp62(c-Yes) was not phosphorylated or activated in response to either G17 or PG. To examine whether c-Src kinase mediates proliferative effects of PG, IEC cells were microinjected with anti-Src-IgG and (3)H-thymidine ((3)H-Tdr) uptake of the cells measured. Control cells received nonimmune IgG. The (3)H-Tdr uptake of cells stimulated with 1.0 nM PG was significantly reduced in cells microinjected with anti-c-Src-IgG; control IgG had no effect. In cells stimulated with 1.0% fetal calf serum, microinjection with c-Src-IgG had no effect on (3)H-Tdr uptake. The specificity of the effect was further confirmed by blocking the inhibitory effect of anti-c-Src-IgG with antigenic Src peptide. These results suggest that activation of c-Src kinase likely represents a critical step in mediating proliferative effects of both the precursor and fully processed forms of gastrins on IEC.

Leukotriene D4 induces stress-fibre formation in intestinal epithelial cells via activation of RhoA and PKCδ

The intestinal epithelial barrier, which is regulated by the actin cytoskeleton, exhibits permeability changes during inflammation. Here we show that activation of the CysLT1 receptor by the inflammatory mediator leukotriene D4 (LTD4) causes a rapid increase in stress-fibre formation in intestinal epithelial cells. This effect was mimicked by cytotoxic necrotising factor-1 (CNF-1)-induced activation of RhoA,overexpression of constitutively active RhoA (L63-RhoA) and phorbol-ester-induced activation of protein kinase C (PKC). In accordance,inhibition of RhoA, by C3 exoenzyme or by dominant-negative RhoA (N19-RhoA),as well as GF109203X-induced inhibition of PKC, suppressed the LTD4-induced stress-fibre formation. Introduction of the dominant-negative regulatory domain of PKCδ, but not the corresponding structures from PKCα, βII or ϵ, blocked the LTD4-induced stress-fibre formation. Evaluating the relationship between PKCδ and RhoA in LTD4-induced stress-fibre formation,we found that C3 exoenzyme inhibited the rapid LTD4-elicited translocation of PKCδ to the plasma membrane. Furthermore, CNF-1-induced stress-fibre formation was blocked by GF109203X and by overexpression of the regulatory domain of PKC-δ, whereas PKC-induced stress-fibre production was not affected by N19-RhoA. We conclude that PKC-δ is located downstream of RhoA and that active RhoA and PKCδ are both necessary for LTD4-induced stress-fibre formation.

Leukotriene D(4) activates MAPK through a Ras-independent but PKCepsilon-dependent pathway in intestinal epithelial cells

We have recently shown that leukotriene D(4) (LTD(4)) increases cell survival in intestinal epithelial cells. Here we report and explore the complementary finding that LTD(4) also enhances proliferation in these cells. This proliferative response was approximately half of that induced by epidermal growth factor (EGF) and its required activation of protein kinase C (PKC), Ras and the mitogen-activated protein kinase (MAPK) Erk-1/2. EGF also activated Erk-1/2 in these cells; however the EGF-receptor inhibitor PD153035 did not affect the LTD(4)-induced activation of Erk-1/2. In addition, LTD(4) did not induce phosphorylation of the EGF receptor, nor did pertussis toxin (PTX) block EGF-induced activation of Erk-1/2, thus refuting a possible crosstalk between the receptors. Furthermore, LTD(4)-induced, but not EGF-induced, activation of Erk-1/2 was sensitive to PTX, PKC inhibitors and downregulation of PKCepsilon. A definite role for PKCepsilon in LTD(4)-induced stimulation of Erk-1/2 was documented by the inability of LTD(4) to activate Erk-1/2 in cells transfected with either the regulatory domain of PKCepsilon (an isoform specific dominant-negative inhibitor) or a kinase-dead PKCepsilon. Although Ras and Raf-1 were both transiently activated by LTD(4), only Raf-1 activation was abolished by abrogation of the PKC signal. Furthermore, the LTD(4)-induced activation of Erk-1/2 was unaffected by transfection with dominant-negative N17 Ras but blocked by transfection with kinase-dead Raf-1. Consequently, LTD(4) regulates the proliferative response by a distinct Ras-independent, PKCepsilon-dependent activation of Erk-1/2 and a parallel Ras-dependent signaling pathway.

Leukotriene D(4) affects localisation of vinculin in intestinal epithelial cells via distinct tyrosine kinase and protein kinase C controlled events

Local inflammatory reactions affect the integrity of intestinal epithelial cells, such as E-cadherin-mediated cell-cell interactions. To elucidate this event, we investigated the effects of an inflammatory mediator, leukotriene D(4)(LTD(4)), on the phosphorylation status and properties of vinculin, a multi-binding protein known to interact with both the E-cadherin-catenin complex and the cytoskeleton. Treatment of an intestinal epithelial cell line with LTD(4)induced rapid tyrosine phosphorylation of vinculin, which was blocked by the Src family tyrosine kinase inhibitor PP1. Simultaneously, LTD(4) caused an increased association between vinculin and actin, and that association was decreased by PP1. LTD(4) also induced dissociation of vinculin from (α)-catenin without affecting the catenin complex itself. This dissociation was not blocked by PP1 but was mimicked by the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol 13-acetate (TPA). Also, the PKC inhibitor GF109203X abolished both the LTD(4)- and the TPA-induced dissociation of vinculin from (α)-catenin. Furthermore, LTD(4) caused a colocalisation of vinculin with PKC-(α) in focal adhesions. This accumulation of vinculin was blocked by transfection with a dominant negative inhibitor of PKC (PKC regulatory domain) and also by preincubation with either GF109203X or PP1. Thus, various LTD(4)-induced phosphorylations of vinculin affect the release of this protein from catenin complexes and its association with actin, two events that are necessary for accumulation of vinculin in focal adhesions. Functionally this LTD(4)-induced redistribution of vinculin was accompanied by a PKC-dependent upregulation of active (β)1 integrins on the cell surface and an enhanced (β)1 integrin-dependent adhesion of the cells to collagen IV.