Proliferation-dependent changes in arachidonic acid mobilization from phospholipids of 3T6 fibroblasts

Specialized pro-resolvin mediators induce cell growth and improve wound repair in intestinal epithelial Caco-2 cell cultures

Specialized pro-resolvin mediators (SPMs) are a superfamily of bioactive molecules synthesized from polyunsaturated fatty acids (arachidonic, eicosapentaenoic and docosahexaenoic acids) that include resolvins, protectins and maresins. These metabolites are important to control the resolution phase of inflammation and the epithelial repair, which is essential in restoring the mucosal barriers. Unfortunately, the effects of SPMs on intestinal epithelial cell growth remain poorly understood. Caco-2 cell were used as intestinal epithelial cell model. Cell growth/DNA synthesis, cell signalling pathways, western blot and wound repair assay were performed. Our data demonstrated that SPMs such as lipoxin LxA₄, resolvin (Rv) E1, RvD1, protectin D 1 and maresin 1 were able to enhance intestinal epithelial Caco-2 cell growth and DNA synthesis. Furthermore, our results provide evidence that these effects of RvE1 and RvD1 were associated with a pertussis toxin-sensitive G protein-coupled receptor, and that leukotriene B₄ receptor 2 could be involved, at least in part, in these effects of RvE1/RvD1. Moreover, these mitogenic effects induced by SPMs were dependent on the ERK 1/2 and p38 MAPK pathways as well as phospholipase C and protein kinase C activation. Thus, these mitogenic effects of RvE1/RvD1 on intestinal epithelial cells could be involved in this signalling circuit involved in wounded epithelium and the catabasis process.

Role of eicosanoids on intestinal epithelial homeostasis

The intestinal epithelium is a highly dynamic system that is continuously renewed by a process involving cell proliferation and differentiation. Moreover, it is the main interface with the external environment, and maintenance and regulation of the epithelial structure and epithelial barrier function are key determinants of digestive health and host well being. The tight junction, a multiprotein complex composed of transmembrane proteins associated with the cytoskeletal peri-junctional ring of actin and myosin, is an essential component of this barrier that is strictly regulated in a spatio-temporal manner by a complex signaling network. Defects in the intestinal epithelial barrier function have been observed in inflammatory bowel disease, and a classic example of the connection between inflammation and cancer is the increased risk of colorectal cancer in patients with inflammatory bowel disease. In recent years, several molecules have emerged as critical players contributing to inflammation-associated colorectal cancer. For example, eicosanoids derived from arachidonic acid are proposed as mediators involved in the regulation of epithelial structure/function. Interestingly, the tissue concentration of eicosanoids increases during mucosal inflammation and colorectal cancer development. This overview focuses on the physiological and physiopathological roles of eicosanoids in cell growth/cell differentiation/apoptosis and in the paracellular permeability of the intestinal epithelium. A better understanding of these processes will foster new ideas for the development of therapies for these chronic disorders.

GR 63799X, an EP3 receptor agonist, induced S phase arrest and 3T6 fibroblast growth inhibition

The importance of arachidonic acid metabolites on the control of cell growth, particularly those derived from cyclooxygenase pathway has long been recognized. Recently, we observed that prostaglandin E(2) (PGE(2)) interaction with EP(1) and EP(4) receptors is involved in serum-induced 3T6 fibroblast growth due to their effect at various levels of the cell cycle machinery. This study shows that prostanoid EP(3) receptor was expressed in 3T6 fibroblast. We studied the role of EP(3) receptor agonist GR 63799X in serum-induced 3T6 cell proliferation. This was concentration-dependent inhibit (IC(50) approximately 10 microM) to a complete inhibition without any cytotoxic or proapoptotic effect. The prostanoid EP(3) receptor agonist treatment decreased the G(0)/G(1) and G(2)/M populations whereas cells were accumulated in S phase. This arrest in S phase was associated with a decrease in cyclin B levels and the enhancement of p21 expression. Our data show that EP(3) agonist decreases cAMP levels in our experimental conditions. Interestingly, the S arrest caused by prostanoid EP(3) receptor agonist seems to be cAMP dependent, at least in part, because forskolin treatment allowed S-arrested cells to progress through cell cycle and consequently growth. Thus, our results suggest that PGE(2) EP(3) receptor interaction may be involved in serum-induced 3T6 fibroblast growth due to their effects on cAMP levels and on cell cycle machinery of the S phase.

Effect of pentoxifylline on arachidonic acid metabolism, neutral lipid synthesis and accumulation during induction of the lipocyte phenotype by retinol in murine hepatic stellate cell

In liver fibrosis, the quiescent hepatic stellate cells (HSC) are activated to proliferate and express the activated myofibroblast phenotype, losing fat droplets and the stored vitamin A, and depositing more extracellular matrix. Therapeutic strategies for liver fibrosis are focused on HSC. Pentoxifylline (PTF), an analog of the methylxanthine, prevents the biochemical and histological changes associated with animal liver fibrosis. The aim of the present study was to investigate the phenotypic change of myofibroblasts into quiescent lipocytes by PTF and/or retinol, using a permanent cell line GRX that represents murine HSC. We studied the action of both drugs on the synthesis of neutral lipids, activity of phospholipase A2 (PLA2), release of arachidonic acid (AA) and prostaglandins synthesis. Accumulation and synthesis of neutral lipids was dependent upon association of retinol with PTF. PTF (0.5 mg/mL) alone did not induce lipid accumulation and synthesis, but in cells induced by physiologic concentration of retinol (1-2.5 microM), it increased the quantity of stored lipids. Retinol and PTF (5 microM and 0.1 mg/mL, respectively) had a synergistic effect on neutral lipid synthesis and accumulation. In higher PTF concentrations (0.5 and 0.7 mg/ml), the synthesis was stimulated but accumulation decreased. Membrane-associated PLA2 activity decreased after PTF treatment, which increased the AA release 8 fold, and significantly increased the production of PGE2, but not of PGF2. However, when in presence of retinol, we observed a slightly higher increase in PGE2 and PGF2a production. In conclusion, PTF treatment generated an excess of free AA. We propose that retinol counteracts the action of PTF on the AA release and PGs production, even though both drugs stimulated the lipocyte induction in the HSC.

Cyclooxygenase and cytochrome P-450 pathways induced by fetal calf serum regulate wound closure in 3T6 fibroblast cultures through the effect of prostaglandin E2 and 12 and 20 hydroxyeicosatetraenoic acids

Wound-induced injury of 3T6 fibroblast cultures initiated a repair process stimulated by fetal calf serum (FCS) that restored the integrity of cell cultures. In these experimental conditions, FCS induced arachidonic acid (AA) release and eicosanoid production. Our results show that the inhibition of the cyclooxygenase (COX) and/or cytochrome P-450 pathways significantly decreases the wound closure, whereas that of the lipoxygenase pathway does not modify the wound repair process. Both EP(1) and EP(4) receptors of prostaglandin E(2) (PGE(2)) mediate PGE(2) stimulated 3T6 fibroblast wound closure. Our data suggest that calcium and cAMP are involved in the signaling event induced by PGE(2) during the 3T6 fibroblast wound repair process. On the other hand, we show that ketoconazole, a cytochrome P-450 inhibitor, hinders the wound closure induced by FCS in wounded 3T6 fibroblast cultures. 12 and 20 Hydroxyeicosatetraenoic acids (HETEs), which are key AA metabolites synthesized by cytochrome P-450, partially revert the effects of ketoconazole on the wound repair process. Thus, the COX and cytochrome P-450 pathways of the arachidonate cascade are involved in 3T6 fibroblast wound closure.

Calcium-independent phospholipase A2 through arachidonic acid mobilization is involved in Caco-2 cell growth

Several studies indicate that phospholipase A(2) (PLA(2)) expression and/or activation account for the high levels of arachidonic acid (AA) detected in cancer and, together with the elevated expression of cyclooxygenase-2, lead to cell proliferation and tumor formation. Using Caco-2 cells, a human colorectal carcinoma cell, we studied the role of high-molecular-weight PLA(2)s, cytosolic PLA(2) (cPLA(2)), and calcium-independent PLA(2) (iPLA(2)) in the AA cascade and in cell growth. Treatment with an antisense oligonucleotide against cPLA(2)alpha decreased [(3)H]AA release induced by ionophore A23187 or by a phorbol ester but did not affect the release of [(3)H]AA, [(3)H]thymidine incorporation, or Caco-2 growth induced by fetal calf serum (FCS). However, these parameters were significantly modified by iPLA(2) inhibitors and by an antisense oligonucleotide against iPLA(2)beta. Our results show that iPLA(2) was involved in AA release and the subsequent prostaglandin production induced by serum. Moreover, these data indicate that iPLA(2) may be involved in the signaling pathways involved in the control of Caco-2 proliferation.

Role of phospholipases A(2) in growth-dependent changes in prostaglandin release from 3T6 fibroblasts

Previously, we reported a growth-dependent change in prostaglandin production as a consequence of a marked growth-dependent alteration in arachidonic acid (AA) mobilization from phospholipids. Our present results show that fetal calf serum (FCS) and 4 beta-phorbol-12-myristate acetate (PMA) caused an enhancement of phospholipase A(2) (PLA(2)) activity in the membrane fraction of non-confluent cells allowing PLA(2) access to its substrate and the release of AA. Western blot analysis has shown that FCS and PMA increased secreted PLA(2) (sPLA(2)) expression in non-confluent 3T6 fibroblast cultures. Moreover, FCS and PMA induced dithiothreitol-sensitive and bromoenol lactone-sensitive PLA(2) activities in cytosol and membrane fraction. However, these stimuli did not modify significantly the PLA(2) activity in both fractions when 3T6 fibroblasts reached a high cell density. This could be associated with the impairment of AA mobilization in these cell culture conditions. On the other hand, we observed that FCS and PMA induced the same prostaglandin H synthase-2 induction in non-confluent and confluent culture conditions. Moreover, the prostaglandin E(2) levels reached in cell culture supernatants were independent of the degree of confluence when AA was added exogenously. These results suggest that the changes of intracellular distribution of PLA(2) activity of sPLA(2) and iPLA(2) stimulated by exogenous stimuli may be controlled by cell density conditions which constitute an important mechanism in the regulation of prostaglandin release.

Role of Ca2+-independent phospholipase A2 on arachidonic acid release induced by reactive oxygen species

Previous studies have shown that reactive oxygen species (ROS) enhance arachidonic acid (AA) release and the subsequent AA metabolism in macrophages. The purpose of this study was determined the implication of phospholipases A2 (PLA2s) in these events. Our results show that oxidative stress induced by exogenous adding of hydrogen peroxide or superoxide anion in macrophage RAW 264.7 and mouse peritoneal macrophage cultures caused a marked enhancement of calcium-independent PLA2 (iPLA2) activity,whereas the increment of secreted PLA2 (sPLA2) and calcium-dependent cytosolic PLA2 (cPLA2) activities were slight. This increase of iPLA2 activity by ROS was rapid and dose-dependent. ROS also induced a significant [3H] arachidonic acid (AA) release. The iPLA2 selective inhibitor, bromoenol lactone, almost completely suppressed the mobilization of [3H]AA induced by ROS whereas antisense oligonucleotide against cPLA2 did not have any appreciable effect. Thus, our data show that iPLA2 activity is involved in the mechanism by which ROS increases the availability of free AA in macrophages RAW 264.7. Moreover, the protein kinase C (PKC) inhibitor, calphostin C, and calcium chelators had no effect on the [3H]AA release induced by ROS, suggesting this is a regulatory role of iPLA2.

The effect of high molecular phospholipase A2 inhibitors on 3T6 fibroblast proliferation

Recently, we suggested that arachidonic acid and/or its cyclooxygenase pathway metabolites may be involved in regulating 3T6 fibroblast proliferation. In the present study we evaluate the role of high-molecular phospholipase A2 (PLA2) enzymes in the 3T6 fibroblast growth. Our results demonstrate that the cytosolic PLA2 inhibitor, arachidonyl trifluoromethylketone and the cytosolic calcium-independent PLA2 (iPLA2) inhibitor, bromoenol lactone, decrease arachidonic acid release and prostaglandin E2 production in 3T6 fibroblast cultures stimulated by fetal calf serum. These effects were correlated with the impairment of 3T6 fibroblast proliferation and DNA synthesis at the S/G2 boundary, which prolongs the S phase. These data suggest a role of iPLA2 in the control of 3T6 fibroblast growth.

Arachidonic acid release and prostaglandin E2 synthesis as irritant index of surfactants in 3T6 fibroblast cultures

Surface-active agents are components of many drugs and cosmetics. [3H]arachidonic acid ([3H]AA) is rapidly incorporated into cell membrane phospholipids. As a result of membrane disintegration or enzymatic catalysis, it is released to the medium and [3H]AA metabolites can be measured by the scintillation technique. In order to validate the use of this test to predict the irritant and toxic effects of surfactants in vitro, various surfactants were examined using this methodology and the neutral red test. Benzalkonium chloride and sodium dodecylsulfate were more irritant and cytotoxic than amphoteric and non-ionic surfactants such as cocoamidopropylbetaine and Tween 80, respectively. The rank order of the toxicity/irritancy of these agents was similar to that given by other tests in vivo and in vitro. Moreover, our results confirm that the measurement of [3H]AA release in 3T6 fibroblast cultures is a sensitive assay of membrane damage.

Ketoprofen S(+) enantiomer inhibits prostaglandin production and cell growth in 3T6 fibroblast cultures

The ketoprofen S(+) enantiomer inhibits with great stereoselectivity both prostaglandin H synthase isoenzymes. Thus, the biological effects of ketoprofen on inflammation are due almost entirely to the S(+) isomer. Here, we report that the S(+) enantiomer, at doses that inhibit prostaglandin synthesis, is effective in reducing DNA synthesis and 3T6 fibroblast growth. Our data suggest that prostaglandins are involved in the control of 3T6 fibroblast growth and that the effect of the ketoprofen S(+) enantiomer on 3T6 proliferation is correlated with its effects on prostaglandin H synthase and prostaglandin production.

Expression of COX-2 by normal and reactive astrocytes in the adult rat central nervous system

We have used a previously characterized antiserum against cycloxygenase-2 (COX-2) together with cold methanol fixation to immunohistochemically locate the protein in astrocytes in rat brain. Although in cerebral cortex most enzyme was located in neuronal perikarya as previously described, a number of glial fibrillary acidic protein (GFAP)-positive astrocytes were also labeled. No COX-2-positive neurons were seen in the cerebellum, but here also a subset of GFAP+ astrocytes was present which contained the enzyme. The number of COX-2-positive astrocytes increased considerably after injection of the neurotoxin kainate into the cerebellum. These immunohistochemical data were supported by semiquantitative RT-PCR results, which were used to assess the levels of COX-2 mRNA relative to the housekeeping gene hypoxanthine phosphoribosyl transferase. PGE2 levels were measured in contralateral and lesioned cerebellum to correlate changes in COX-2 immunoreactivity and mRNA with physiological events. PGE2 levels increased by 230% in the lesioned cerebellar hemispheres in comparison to the contralateral ones. We discuss the possibility that the targets for astrocytic prostaglandins might include both autocrine effects and paracrine responses of neurons, lymphocytes and capillary endothelial cells.

Induction by interleukin-1beta peptide of prostaglandin E2 formation via enhanced prostaglandin H synthase-2 expression in 3T6 fibroblasts

Several synthetic interleukin-1 (IL-1) peptides were tested in vivo for pyrogenic activity and in vivo for their ability to stimulate prostaglandin production. Only the IL-1beta fragment (208-240) enhanced body temperature, although both IL-1beta (208-240) and IL-1alpha (223-250) stimulated prostaglandin E2 (PGE2) production in vitro. We report here that the IL-1beta fragment (208-240) did not have the capacity to induce arachidonic acid (AA) mobilization by 3T6 fibroblasts. However, this peptide was able to increase the expression of the inducible prostaglandin H synthase isoform (PGHS-2; EC 1.14.99.1.), which is related to its ability to stimulate prostaglandin E2 synthesis.

Protein kinase C inhibitors stimulate arachidonic and docosahexaenoic acids release from uterine stromal cells through a Ca2+-independent pathway

The mechanisms underlying arachidonic acid (AA) release by uterine stromal (U(III)) cells were studied. Stimulation of AA release by calcium ionophore and PMA are inhibited by various PKC inhibitors and by calcium deprivation. These results suggest the involvement of an AA-specific cPLA2 as the release of docosahexaenoic acid (DHA) from prelabelled cells is much lower than the release of AA. The results also show a more original stimulation of AA and DHA release induced by PKC inhibitors, which is insensitive to calcium deprivation. This stimulation is not due to acyltransferase inhibition, suggesting the participation of a Ca2+-independent PLA2 (iPLA2). However, iPLA2 activity measured in U(III) cells is inhibited by the specific iPLA2 inhibitor, BEL, and is not stimulated by PKC inhibitors, in contrast with the AA and DHA release. It seems therefore that this iPLA2 cannot be involved in this mechanism. The participation of another iPLA2, BEL-insensitive, is discussed.

Role of prostaglandin H synthase-2-mediated conversion of arachidonic acid in controlling 3T6 fibroblast growth

The specific role(s) of arachidonic acid (AA) and its metabolites in the signaling pathways that regulated fibroblast growth was studied. A Western blot analysis demonstrated that prostaglandin H synthase-2 (PGHS-2) was expressed by 3T6 fibroblast cultures in RPMI 1640 supplemented with fetal calf serum (10%). Dexamethasone, which inhibits AA release and PGHS-2 expression, significantly reduced cell proliferation. Ketoprofen, a dual cyclooxygenase inhibitor, and CGP-28238, a specific PGHS-2 inhibitor, reduced fibroblast proliferation in a dose-dependent manner. These drugs also reduced [3H]thymidine incorporation into the DNA of fibroblasts. These effects were correlated with a decrease in prostaglandin (PG) E2 levels in the cell medium. However, piroxicam at doses that selectively inhibit PGHS-1 did not have a significant effect on fibroblast proliferation. Finally, we showed that the antiproliferative effect of dexamethasone and PGHS-2 inhibitors was significantly antagonized when PGE2 was added to the culture medium. Our results suggest that PGHS-2 and prostaglandins such as PGE2 might play an important role in the regulation of 3T6 fibroblast growth stimulated by growth factors of serum.

Arachidonic acid mediates calcium influx induced by basic fibroblast growth factor in Balb-c 3T3 fibroblasts

Basic fibroblast growth factor (bFGF), a peptide acting as a mitogen in different cell types, is able to induce a long lasting non capacitative calcium influx from the extracellular medium in Balb-c 3T3 mouse fibroblasts. This effect is mediated by the tyrosine kinase activity of bFGF receptors and the opening of voltage independent, agonist activated calcium channels. In this paper we investigate the signal transduction steps involved in this process using single cell calcium fluorimetry and electrophysiological techniques. One of the pathways initiated by the binding of growth factors to their tyrosine kinase receptors is the activation of cytosolic phospholipase A2 (cPLA2) and the release of arachidonic acid (AA) from the plasma membrane with the subsequent production of eicosanoids. We show here that, in our preparation, this pathway is involved in the opening of the bFGF-activated calcium permeable channels, through the activation of mitogen activated protein kinase (MAPK) and cPLA2. Evidence for direct involvement of AA is given by the finding that: (i) bFGF induces AA release from Balb-c 3T3 cells; (ii) blockers of AA metabolism are not effective; and (iii) the application of either arachidonic acid or its non metabolizable analogue 5,8,11,14-eicosatetraynoic acid (ETYA) reproduces the responses described for bFGF. Finally, single channel analysis indicates that bFGF, AA and ETYA can activate the same calcium permeable channel.