The evolution of the MAP kinase pathways: coduplication of interacting proteins leads to new signaling cascades

The MAP-kinase pathways are intracellular signaling modules that are likely to exist in all eukaryotes. We provide an evolutionary model for these signaling pathways by focusing on the gene duplications that have occurred since the divergence of animals from yeast. Construction of evolutionary trees with confidence assessed by bootstrap clearly shows that the mammalian JNK and p38 pathways arose from an ancestral hyperosmolarity pathway after the split from yeast and before the split from C. elegans. These coduplications of interacting proteins at the MAPK and MEK levels have since evolved toward substrate specificity, thus giving distinct pathways. Mammalian duplications since the split from C. elegans are often associated with divergent tissue distribution but do not appear to confer detectable substrate specificity. The yeast kinase cascades have undergone similar fundamental functional changes since the split from mammals, with duplications giving rise to central signaling components of the filamentous and hypoosmolarity pathways. Experimentally defined cross-talk between yeast pheromone and hyperosmolarity pathways is mirrored with corresponding cross-talk in mammalian pathways, suggesting the existence of ancient orthologous cross-talk; our analysis of gene duplications at all levels of the cascade is consistent with this model but does not always provide significant bootstrap support. Our data also provide insights at different levels of the cascade where conflicting experimental evidence exists.

Distinct roles for p42/p44 and p38 mitogen-activated protein kinases in the induction of IL-2 by IL-1

Interleukin 1 (IL-1) activates p42/p44 and p38 mitogen-activated protein kinases (MAP kinases) in target cells. Here we have used two specific inhibitors, PD98059 which inhibits MAP kinase kinase (MEK), and SB203580 which inhibits p38 MAP kinase to explore the involvement of these kinases in the induction of IL-2 by IL-1 in the murine thymoma cell line EL4.NOB-1. Both kinase inhibitors suppressed IL-1-stimulated IL-2 production. PD98059 blocked IL-2 mRNA accumulation and the induction of a reporter gene linked to the IL-2 promoter. In contrast, SB203580 only marginally inhibited IL-2 promoter-linked reporter gene expression and had no inhibitory effect on IL-2 mRNA levels. Neither PD98059 nor SB203580 had an inhibitory effect on NFkappaB-driven reporter gene expression in response to IL-1. Surprisingly, higher concentrations of SB203580 (30 microM) potentiated the IL-1 responses. PD98059 also inhibited induction of IL-2 by phorbol 12-myristate 13-acetate (PMA), and AP1-linked reporter gene expression in response to PMA but not IL-1. These results indicate that p42/p44 MAP kinase is involved in the regulation of IL-2 gene transcription by IL-1, whilst p38 MAP kinase has a post-transcriptional target. Additional IL-1 signalling pathways can clearly compensate for the lack of p38 MAP kinase which result in potentiation of the IL-1 responses observed at high-dose SB203580.

The P38 mitogen-activated protein kinase inhibitor SB203580 antagonizes the inhibitory effects of interleukin-1beta on long-term potentiation in the rat dentate gyrus in vitro

Levels of the pro-inflammatory cytokine interleukin-1beta are known to be elevated in patients with chronic disorders such as Alzheimer's disease. We have investigated the effects of interleukin-1beta on long-term potentiation and N-methyl-D-aspartate receptor-mediated field potentials in the rat dentate gyrus in vitro utilizing field extracellular recordings obtained from the middle third of the molecular layer of the dentate gyrus. Presynaptic stimulation was applied to the commissural/association pathway at a frequency of 0.05 Hz and at a distance of 50 microm from the granule cell body layer. As previously reported, interleukin-1beta (1 ng/ml) caused an inhibition of long-term potentiation (108+/-2% of baseline 1 h following application of tetanic stimulation compared with 145+/-5% in vehicle control slices). This action of interleukin-1beta on long-term potentiation, as well as an inhibition of N-methyl-D aspartate receptor-mediated field potentials, was attenuated by pre-treatment of slices with the p38 mitogen-associated protein kinase inhibitor SB203580 (1 microM). SB203580 alone had no significant affect on long-term potentiation, but did cause an increase in baseline synaptic transmission [107+/-2% of baseline, 1 h after SB203580 (1 microM) treatment]. The p42/44 mitogen-activated protein kinase cascade inhibitor PD98059 (50 microM) did not inhibit the interleukin-1beta-induced inhibition of N-methyl-D-aspartate receptor-mediated field potentials. The cyclooxygenase inhibitor indomethacin (50 microM) was found to attenuate the interleukin-1beta-induced effects on both long-term potentiation and N-methyl-D-aspartate receptor-mediated field potentials. The lipid second messenger analogue C2 ceramide (20 microM) was found to attenuate the expression of long-term potentiation (108+/-3% of baseline 1 h following tetanic stimulation), and this effect was not blocked by pre-treatment with SB203580. To investigate a possible role for interleukin-1beta in the normal expression of long-term potentiation, the interleukin-1 receptor antagonist (25 ng/ml) was applied during the maintenance phase of long-term potentiation. This was found to depress the sustained expression of long-term potentiation (116+/-6% of baseline 1 h following tetanic stimulation). Our results indicate possible signalling mechanisms by which interleukin-1beta at pathophysiological concentrations may serve to inhibit long-term potentiation, and also suggests a role for IL-1beta in the physiological expression of synaptic plasticity in the rat dentate gyrus in vitro.

Characterization of CD44 induction by IL-1: a critical role for Egr-1

The adhesion molecule CD44 is a multifunctional, ubiquitously expressed glycoprotein that participates in the process of leukocyte recruitment to sites of inflammation and to their migration through lymphatic tissues. In this study, we have investigated the effect of the proinflammatory cytokine IL-1alpha on CD44 gene expression in the human immortalized endothelial cell line ECV304. Immunoblotting of cell extracts showed constitutive expression of a 85-kDa protein corresponding to the standard form of CD44, which was potently up-regulated following IL-1alpha treatment. Furthermore, IL-1alpha induced expression of v3- and v6-containing isoforms of CD44, which migrated at 110 and 140-180 kDa, respectively. The effect of IL-1alpha on CD44 standard, v3- and v6-containing isoforms was dose and time dependent and was inhibited in the presence of IL-1 receptor antagonist. To elucidate the molecular mechanisms regulating CD44 expression in response to IL-1alpha, we investigated the effect of IL-1alpha on CD44 mRNA expression. Reverse-transcriptase PCR and Northern analysis demonstrated an increase in CD44 mRNA expression indicating a transcriptional mechanism of control by IL-1alpha. Furthermore, IL-1alpha increased expression of a reporter gene under the control of the CD44 promoter (up to -1.75 kb). The effect of IL-1alpha was critically dependent on the site spanning -151 to -701 of the promoter. This effect required the presence of an Egr-1 motif at position -301 within the CD44 promoter since mutation of this site abolished responsiveness. IL-1alpha also induced Egr-1 expression in these cells. These studies therefore identify Egr-1 as a critical transcription factor involved in CD44 induction by IL-1alpha.

Ceramide activates NFkappaB by inducing the processing of p105

The role of ceramide as a second messenger in tumor necrosis factor (TNF)-mediated signal transduction has been much debated. It is supported by recent reports describing an expanding number of potential targets for this lipid, but is opposed by those describing how ceramide is not necessary for many TNF-mediated cellular events. In this paper, we directly compare the effects of the cell-permeable ceramide analogue, N-acetylsphingosine (C2-ceramide), with TNF, on NFkappaB function, a transcription factor whose activation is central to many TNF-mediated effects. We describe how C2-ceramide failed to drive kappaB-linked chloramphenicol acetyltransferase gene expression in either HL60 promyelocytic or Jurkat T lymphoma cells. Furthermore, it had no effect on TNF-mediated transcription of this reporter gene. However, electrophoretic mobility shift analysis following cell stimulation with this ceramide analogue revealed a dose-responsive activation of NFkappaB, which was not apparent following cell treatment with the inactive dihydro form. Activated complexes from treated cells were shown to contain predominantly the p50 subunit, in contrast to complexes from TNF-treated cells, where both p50 and p65/RelA subunits were present. The specific activation of p50 homodimeric complexes by C2-ceramide, which are known to lack trans-activating activity, was strongly suggested from these data. Further investigations revealed that C2-ceramide had only a marginal effect on IkappaBalpha degradation but strongly promoted the processing of p105 to its p50 product as revealed by immunoblot analysis. The increase in p50 arising from the processing of its p105 precursor was further established from p105/p50 ratios obtained by scanning densitometric analysis of bands from immunoblots. TNF, on the other hand, stimulated both IkappaBalpha degradation and p105 processing, in accordance with previous findings. Furthermore, the effect of TNF on NFkappaB activation was rapid, whereas C2-ceramide required an optimal treatment time of 1 h. Interestingly, TNF was found to increase ceramide in cells but only after a 1-h contact time. Our data therefore suggest that ceramide promotes the activation of NFkappaB complexes that lack transactivating activity by enhanced processing of p105.

Signal transduction pathways activated by the IL-1 receptor family: ancient signaling machinery in mammals, insects, and plants

Interleukin-1 (IL-1) is a central regulator of the immune and inflammatory responses. Recently, significant advances have been made in the area of IL-1 receptors and IL-1 signal transduction. A family of proteins has been described that share significant homology in their signaling domains with the Type I IL-1 receptor (IL-1RI). These include the IL-1 receptor accessory protein (IL-1AcP), which does not bind IL-1 but is essential for IL-1 signaling; a Drosophila protein Toll; a number of human Toll-like receptors (hTLRs); the putative IL-18/IL-1-gamma receptor IL-1Rrp (IL-1 receptor-related protein); and a number of plant proteins. All appear to be involved in host responses to injury and infection. These homologies also extend to novel signaling proteins implicated in IL-1 action. Two IL-1 receptor-associated kinases, IRAK-1 and IRAK-2, which have homologs in Drosophila (Pelle) and plants (Pto), have been implicated in the activation of the transcription factor, nuclear factor kappaB (NF-kappaB). IRAK-1 has also been implicated in AP1 induction, Jun amino-terminal kinase (JNK) activation, and IL-2 induction. It recruits the adapter protein TRAF6 to the IL-1 receptor complex via an interaction with IL-1AcP. TRAF6 then relays the signal via NF-kappaB-inducing kinase (NIK) to two I-kappaB kinases (IKK-1 and -2), leading to NF-kappaB activation. Progress has also been made on other IL-1-responsive kinases, including JNK and p38 MAP kinase, with the latter having a role in multiple responses to IL-1. The remarkable conservation between diverse species indicates that the IL-1 system represents an ancient signaling machine critical for responses to environmental stresses and attack by pathogens.

Inhibition of nuclear factor kappaB by direct modification in whole cells--mechanism of action of nordihydroguaiaritic acid, curcumin and thiol modifiers

This study was set up to investigate the mechanism of four inhibitors of interleukin-1(IL-1)-alpha and tumor necrosis factor-(TNF)alpha activated nuclear factor kappaB (NFkappaB) in whole cells. The compounds fall into two classes: the first comprised two chain-breaking antioxidants, curcumin (diferulolylmethane) and nordihydroguaiaritic acid. The second class were two thiol-modifying agents, N-ethylmaleimide (NEM) and 2-chloro-1,3dinitrobenzene (CDNB). Both sets of compounds were found to inhibit NFkappaB in tumour necrosis factor-activated Jurkat T lymphoma cells and interleukin 1-activated EL4.NOB-1 thymoma cells as determined by electrophoretic mobility shift assay using a specific NFkappaB DNA probe. In unstimulated cells the compounds were found to modify NFkappaB prior to chemical dissociation with sodium deoxycholate. They also inhibited DNA binding by NFkappaB when added to nuclear extracts from stimulated cells. Both of these effects occurred over a concentration range comparable to that which inhibited cytokine-activated NFkappaB in intact cells. All four agents were found to react directly with the p50 subunit of NFkappaB. However, only the antioxidants, curcumin and nordihydroguaiaritic acid (NDGA) were found to inhibit IkappaBalpha degradation activated by tumour necrosis factor-alpha. These results suggest that NFkappaB itself is susceptible to direct inhibition by a range of pharmacological agents. Furthermore, curcumin and nordihydroguaiaritic acid inhibit NFkappaB by interfering with IkappaBalpha degradation and reacting with p50 in the NFkappaB complex. These findings are likely to be useful in the attempt to develop agents which inhibit NFkappaB-dependent gene transcription.

The production of a reactive oxygen intermediate during the induction of apoptosis by cytotoxic insult

We report the protective effects of two novel antioxidant compounds, 3 beta-doxyl-5 alpha cholestane and 2,2,6,6-tetramethyl-piperdinoxyl, in HL-60 and U937 leukemic cells subjected to a number of cytotoxic insults. In addition, the rapid production of peroxide is demonstrated as a general response to cytotoxic agents in these leukemic cell lines, indicating that changes in the redox status of a leukemic cell may contribute to the ultimate death of these cells. Closer examination of this peroxide production has identified enzymic production and/or disruption of resident antioxidants as possible sources. However, in contrast to recent reports from other model systems, mitochondrial transmembrane depolarization did not appear to be required for the production of peroxide in these cells. Finally, we demonstrated the activation of the redox-sensitive transcription factor, NF-kappa B, in response to these cytotoxic insults.

Lipid peroxidation is involved in the activation of NF-kappaB by tumor necrosis factor but not interleukin-1 in the human endothelial cell line ECV304. Lack of involvement of H2O2 in NF-kappaB activation by either cytokine in both primary and transformed endothelial cells

It has been proposed that reactive oxygen species, and in particular H2O2, may be involved in the activation of NF-kappaB by diverse stimuli in different cell types. Here we have investigated the effect of a range of putative antioxidants on NF-kappaB activation by interleukin-1 and tumor necrosis factor as well as the ability of H2O2 to activate NF-kappaB in primary human umbilical vein endothelial cells and the transformed human endothelial cell line ECV304. Activation of NF-kappaB and stimulation of IkappaBalpha degradation by H2O2 was only evident in the transformed cells and required much longer contact times than that observed with interleukin-1 or tumor necrosis factor. Furthermore, only H2O2 was sensitive to N-acetyl-L-cysteine, and no increase in H2O2 was detected in response to either cytokine. Pyrrolidine dithiocarbamate has been purported to be a specific antioxidant inhibitor of NF-kappaB that acts independently of activating agent or cell type. However, we found that tumor necrosis factor- but not interleukin-1-driven NF-kappaB activation and IkappaBalpha degradation were sensitive to pyrrolidine dithiocarbamate in transformed cells, while neither pathway was inhibited in primary cells. Phorbol ester-mediated activation was sensitive in both transformed and primary cells. Other antioxidants failed to inhibit either cytokine, while the iron chelators desferrioxamine and 2,2,6, 6-tetramethylpiperidine-1-oxyl mimicked the pattern of inhibition seen for the dithiocarbamate. This suggested that pyrrolidine dithiocarbamate was inhibiting NF-kappaB activation in endothelial cells primarily through its iron-chelating properties. Tumor necrosis factor, but not interleukin-1, was found to induce lipid peroxidation in ECV304 cells. This was inhibited by pyrrolidine dithiocarbamate and desferrioxamine. t-Butyl hydroperoxide, which induces lipid peroxidation, activated NF-kappaB. Finally, butylated hydroxyanisole, which inhibits lipid peroxidation but has no iron-chelating properties, inhibited NF-kappaB activation by tumor necrosis factor but not interleukin-1. Taken together, the results argue against a role for H2O2 in NF-kappaB activation by cytokines in endothelial cells. Furthermore, tumor necrosis factor and interleukin-1 activate NF-kappaB through different mechanisms in ECV304 cells, with the tumor necrosis factor pathway involving iron-catalyzed lipid peroxidation.

Staurosporine, but not Ro 31-8220, induces interleukin 2 production and synergizes with interleukin 1alpha in EL4 thymoma cells

Protein kinase C (PKC) has been implicated in interleukin 1 (IL1) signal transduction in a number of cellular systems, either as a key event in IL1 action or as a negative regulator. Here we have examined the effects of two PKC inhibitors, staurosporine and the more selective agent Ro 31-8220, on IL1 responses in the murine thymoma line EL4.NOB-1. A 1 h pulse of staurosporine was found to strongly potentiate the induction of IL2 by IL1alpha in these cells. In contrast, neither a pulse nor prolonged incubation with Ro 31-8220 affected the response to IL1alpha. Both agents blocked the response to PMA, however. A 1 h pulse of staurosporine was also found to induce IL2 production on its own, activate the transcription factor nuclear factor kappaB (NFkappaB) and increase the expression of a NFkappaB-linked reporter gene. It synergized with IL1alpha in all of these responses. Ro 31-8220 was again without effect, although both staurosporine and Ro 31-8220 blocked the activation of NFkappaB by PMA. Finally, staurosporine caused the translocation of PKC-alpha and -epsilon, and to a lesser extent PKC-beta, but not PKC-θ or -zeta, from the cytosol to the membrane, although a similar effect was observed with Ro 31-8220. The results suggest that PKC is not involved in IL1alpha signalling in EL4 cells. Furthermore, the potentiating effect of staurosporine on IL1alpha action does not involve PKC inhibition, and is likely to be at the level of NFkappaB activation.

NF-kappa B: a crucial transcription factor for glial and neuronal cell function

Transcription factors provide the link between early membrane-proximal signalling events and changes in gene expression. NF-kappa B is one of the best-characterized transcription factors. It is expressed ubiquitously and regulates the expression of many genes, most of which encode proteins that play an important and often determining role in the processes of immunity and inflammation. Apart from its role in these events, evidence has begun to accumulate that NF-kappa B is involved in brain function, particularly following injury and in neurodegenerative conditions such as Alzheimer's disease. NF-kappa B might also be important for viral replication in the CNS. An involvement of NF-kappa B in neuronal development is suggested from studies that demonstrate its activation in neurones in certain regions of the brain during neurogenesis. Brain-specific activators of NF-kappa B include glutamate (via both AMPA/KA and NMDA receptors) and neurotrophins, pointing to an involvement in synaptic plasticity. NF-kappa B can therefore be considered as one of the most important transcription factors characterized in brain to date and it might be as crucial for neuronal and glial cell function as it is for immune cells.

Daunorubicin activates NFkappaB and induces kappaB-dependent gene expression in HL-60 promyelocytic and Jurkat T lymphoma cells

The anthracycline antibiotic, daunorubicin, can induce programmed cell death (apoptosis) in cells. Recent work suggests that this event is mediated by ceramide via enhanced ceramide synthase activity. Since the generation of ceramide has been directly linked with the activation of the transcription factor, NFkappaB, this was investigated as a novel target for the action of daunorubicin. Here we describe how treatment of HL-60 promyelocytes and Jurkat T lymphoma cells with daunorubicin results in the activation of the transcription factor NFkappaB. The effect of daunorubicin was evident following 1-2 h treatment, which was in contrast to the time course of activation obtained with the cytokine, tumor necrosis factor, where NFkappaB activation was detected within minutes of cellular stimulation. Activated complexes were shown to contain predominantly p50 and p65/RelA subunit components. Daunorubicin also induced IkappaB degradation and increased the expression of an NFkappaB-linked reporter gene. In addition, the drug was found to strongly potentiate the ability of tumor necrosis factor to induce an NFkappaB-linked reporter gene, suggesting a synergy between these two agents in this response. These events were sensitive to the iron chelator, deferoxamine mesylate (desferal), and the anti-oxidant and metal chelator pyrrolidine dithiocarbamate. A structurally related compound, mitoxantrone, which, unlike daunorubicin, is unable to undergo redox cycling in cells, also activated NFkappaB in a pyrrolidine dithiocarbamate-sensitive manner. A specific inhibitor of ceramide synthase, fumonisin B1, had no effect on daunorubicin induced NFkappaB activation at a range of concentrations previously reported to block apoptosis induced by this drug. However, this agent could inhibit increases in ceramide induced by daunorubicin, in addition to blocking ceramide synthase activity from HL-60 cells which was activated in response to daunorubicin treatment. These data therefore suggest that the effect of daunorubicin on NFkappaB is unlikely to involve ceramide, but may involve reactive oxygen species generated as a result of endogenous cellular processes rather than reductive metabolism of the drug. As NFkappaB may be involved in apoptosis, this effect may be an important aspect of the cellular responses to this agent.

TNF-mediated cytotoxicity of L929 cells: role of staurosporine in enhancement of cytotoxicity and translocation of protein kinase C isozymes

The role of protein kinase C (PKC) in tumour necrosis factor (TNF)-mediated cytotoxicity was investigated, using the L929 cell line. The PKC activator phorbol-12-myristate 13-acetate (PMA) increased proliferation and inhibited TNF-induced cytotoxicity of L929 cells. A range of specific PKC inhibitors had no effect on TNF-mediated killing, suggesting that PKC does not play a direct role in this response. However, staurosporine enhanced cytotoxicity by TNF in the presence of actinomycin D, a protein synthesis inhibitor. In view of this finding, the authors investigated the role of specific PKC isozymes in both TNF-mediated cytotoxicity and staurosporine-induced sensitization to killing. PKC-alpha, beta, epsilon and zeta were detected in L929 cells. PKC-beta was only weakly detected in the cytoplasm, PKC alpha, epsilon and zeta were all found in resting cytoplasm and membrane. Stimulation with PMA caused a strong translocation of PKC-alpha but not zeta to the membrane. TNF had no effect on these isozymes but interestingly caused a translocation of PKC-epsilon, which also occurred in response to PMA. Staurosporine caused a translocation of PKC-zeta to the plasma membrane and a loss of PKC-epsilon from the cytosol. Although TNF induced PKC-epsilon translocation, this is unlikely to be involved in cytotoxicity as this effect was also induced by PMA which protected against TNF-mediated cytotoxicity. Staurosporine also induced translocation of PKC-zeta, an isozyme whose activity was previously found to be resistant to inhibition by staurosporine. These findings suggest the possibility that the mechanism by which staurosporine potentiates TNF action does not involve inhibition of PKC, but in contrast may involve modulation of PKC-zeta.

2-mercaptoethanol restores the ability of nuclear factor kappa B (NF kappa B) to bind DNA in nuclear extracts from interleukin 1-treated cells incubated with pyrollidine dithiocarbamate (PDTC). Evidence for oxidation of glutathione in the mechanism of inhibition of NF kappa B by PDTC

The metal chelator and anti-oxidant pyrollidine dithiocarbamate (PDTC) has been used extensively in studies implicating reactive oxygen intermediates in the activation of nuclear factor kappa B (NF kappa B). In agreement with other studies, we have shown that PDTC inhibits NF kappa B activation in response to the pro-inflammatory cytokines interleukin 1 (IL1) and tumour necrosis factor (TNF). However, we have found that the inhibition was reversed by treatment of inhibited nuclear extracts with the reducing agent 2-mercaptoethanol. This was observed in extracts prepared from IL1-treated EL4.NOB-1 thymoma cells and TNF-treated Jurkat E6.1 lymphoma cells. These results suggested that the inhibition was caused by oxidation of NF kappa B on a sensitive thiol, possibly on the p50 subunit (which was detected in NF kappa B complexes in both cell types), and not by inhibition of the activation pathway. The possibility that PDTC was acting as a pro-oxidant was therefore investigated. PDTC caused an increase in oxidized glutathione, suggesting that it acts as an oxidizing agent in the cells tested rather than as an anti-oxidant. Similar results were obtained with diamide, a compound designed to oxidize glutathione. Finally, an increase in the ratio of oxidized to reduced glutathione was shown to inhibit NF kappa B-DNA binding in vitro. On the basis of these results we suggest that, while NF kappa B activation is unaffected by PDTC, DNA binding is inhibited through a mechanism involving a shift towards oxidizing conditions, and that this is the mechanism of action of both PDTC and diamide in the cells tested here.

Endothelin-1 increases arachidonic acid release in C6 glioma cells through a potassium-modulated influx of calcium

Endothelin-1 (Et-1) but not a range of other receptor agonists stimulated the release of arachidonic acid (AA) in C6 glioma. Et-1 activation was concentration dependent and was inhibited by chelation of extracellular calcium. The calcium ionophores A23187 and ionomycin could also stimulate release of AA. Et-1 caused an early increase in intracellular Ca2+ concentration ([Ca2+]i) followed by a sustained but lower plateau level. The sensitivity of the response to quinacrine, its dependence on Ca2+, and the demonstration of an increase in phospholipase A2 (PLA2) activity that was insensitive to dithiothreitol suggested that the release of AA was due to activation of cytosolic PLA2 in the cells. Staurosporine, a protein kinase C (PKC) inhibitor, had no effect on Et-1-induced AA release but abolished that by phorbol 12-myristate 13-acetate, demonstrating that the Et-1 response was PKC independent. Raised levels of extracellular KCl inhibited both AA release and the increase in [Ca2+]i triggered by Et-1, whereas valinomycin, which causes K+ efflux, not only caused a rapid rise in [Ca2+]i but also caused AA mobilisation. The results therefore suggest that Et-1 activation of PLA2 in this cell type requires calcium influx dependent on K+ efflux.