Leukotriene D4-induced activation of protein kinase C in rat basophilic leukemia cells

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.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

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

We investigated the potential roles of specific isoforms of protein kinase C (PKC) in the regulation of leukotriene D(4)-induced Ca(2+) signaling in the intestinal epithelial cell line Int 407. RT-PCR and Western blot analysis revealed that these cells express the PKC isoforms alpha, betaII, delta, epsilon, zeta, and mu, but not betaI, gamma, eta, or theta;. The inflammatory mediator leukotriene D(4) (LTD(4)) caused the TPA-sensitive PKC isoforms alpha, delta, and epsilon, but not betaII, to rapidly translocate to a membrane-enriched fraction. The PKC inhibitor GF109203X at 30 microM but not 2 microM significantly impaired the LTD(4)-induced Ca(2+) signal, indicating that the response involves a novel PKC isoform, such as delta or epsilon, but not alpha. LTD(4)-induced Ca(2+) signaling was significantly suppressed in cells pretreated with TPA for 15 min and was abolished when the pretreatment was prolonged to 2 h. Immunoblot analysis revealed that the reduction in the LTD(4)-induced calcium signal coincided with a reduction in the cellular content of PKCepsilon and, to a limited extent, PKCdelta. LTD(4)-induced Ca(2+) signaling was also markedly suppressed by microinjection of antibodies against PKCepsilon but not PKCdelta. These data suggest that PKCepsilon plays a unique role in regulation of the LTD(4)-dependent Ca(2+) signal in intestinal epithelial cells.

Modulation of alveolar macrophage phagocytosis by leukotrienes is Fc receptor-mediated and protein kinase C-dependent

We have previously established an important role for leukotrienes (LTs) in augmenting rat alveolar macrophage (AM) phagocytosis of Klebsiella pneumoniae opsonized with complete immune serum (IS), which contains the two well-known opsonins, immunoglobulin (Ig) G and complement (C). In this report, the specific opsonin requirements for LT modulation of AM phagocytosis and the dependence of this response on protein kinase (PK) C activity were investigated. Phagocytosis of K. pneumoniae opsonized with IS, non-immune serum, or heat-inactivated immune serum and of inert targets (IgG-opsonized fluorescent microspheres or C-opsonized sheep red blood cells) was examined. Inhibition of endogenous LT synthesis or action attenuated, whereas the addition of exogenous LTs augmented, phagocytosis only of targets opsonized with IgG. LTs had no effect on phagocytosis of C-opsonized or unopsonized targets. LTs did not affect adherence of IgG-opsonized targets, implying instead an enhancement of internalization. Macrophage internalization of phagocytic targets has previously been shown to require PKC activity. Pretreatment of AMs with the PKC inhibitors staurosporine or calphostin C, or with phorbol 12-myristate 13-acetate to deplete PKC, completely inhibited the ability of LTB(4) and largely inhibited the ability of LTC(4) to augment phagocytosis of IgG-opsonized microspheres. These results demonstrate that LT enhancement is confined to Fc receptor (FcR)-mediated phagocytosis. Moreover, PKC activation represents an important mechanism by which LTs promote FcR-mediated phagocytosis.

Lipoxygenase inhibitors counteract protein kinase C mediated events in human T lymphocyte proliferation

Four structurally unrelated inhibitors of lipoxygenase (LO), i.e. nordihydroguaiaretic acid (NDGA), Esculetin, AA861 and 5,8,11,14-eicosatetraynoic acid (ETYA) suppressed mitogen induced proliferation of human peripheral blood lymphocytes in a dose-dependent manner. The degree of suppression was influenced by the type of the mitogenic stimulus. Receptor mediated stimulation, i.e. through phytohemagglutinin or the anti-CD3 antibody OKT3, was overall less susceptible, whereas proliferation initiated by direct activation of protein kinase C (PKC), i.e. through phorbol myristate acetate or indolactam V, was profoundly suppressed (up to 90%). The effect of the LO inhibitors was not due to non-specific interference with intracellular radical intermediates, because AA861 and ETYA showed no radical scavenging activity. Two PKC inhibitors, H-7 and H-8, similarly suppressed lymphocyte proliferation and showed essentially the same suppressive pattern as LO inhibitors. The results clearly indicate that LO product(s) participate in signal transduction mechanisms in T lymphocytes, possibly via stimulation of PKC activity and cell proliferation.

Leukotriene receptors

The current challenge in research on leukotriene receptors is to clone these molecules. Traditional protein purification approaches have not been successful in providing sequence information. Solubilization of cys-LT1 has been achieved but results in the dissociation of G-proteins and the loss of high affinity binding (Mong et al., 1986b; Mong and Sarau, 1990), while cys-LT2 activity cannot be monitored by other than functional assays and there have not been any purification attempts. Partial purification of B-LT has been reported but has not been continued to homogeneity (Sherman et al., 1992; Votta et al., 1990; Miki et al., 1990). Nor have attempts to clone these receptors through either homology screening or expression cloning been successful. The cloning of the prostanoid receptors, described in detail elsewhere in this volume, has shown that these receptors belong to a distinct family within the G-protein-coupled receptor superfamily. It is probable, therefore, that the leukotriene receptors will also belong to a separate group within this superfamily since phylogenic comparisons have shown that receptors displaying high affinity for structurally related ligands exist as discrete families. Recently, a human cDNA encoding an orphan FMLP-related receptor cloned from HL60 cells of myeloid lineage was identified as the receptor for another eicosanoid, lipoxin A (Fiore et al., 1994). FMLP has a similar profile of biological actions to LTB4. Moreover, LTD4 showed a high degree of cross-reactivity with this receptor with an affinity only 20-fold less that of lipoxin A, although LTB4 was inactive. It remains to be determined whether the leukotriene receptors will fall into this class of receptors. The cloning of the leukotriene receptors will allow identification of the different receptor types and subtypes and potentially splice variants. Evaluation of currently developed antagonists at these receptor types could also open the way for novel therapies for inflammatory conditions.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Leukotriene D4-induced signal transduction

In a human epithelial cell line LTD4 induces a calcium signal that is dependent on both intracellular mobilization and influx of calcium. This calcium signal is generated via the activation of dual G protein pathways. Whereas the intracellular mobilization of calcium is regulated by a pertussis toxin-insensitive G protein, the subsequent influx of calcium is regulated by a pertussis toxin-sensitive G protein. Furthermore, a LTD4-induced cellular elevation of cAMP also participates in the regulation of this calcium signal. The increase in cAMP is directly related to the LTD4-induced influx of calcium, perhaps by an activation of protein kinase A and a subsequent phosphorylation of a plasma membrane channel. This model of the LTD4-induced signaling pathway in epithelial cells is outlined in Figure 2.

Characteristics of arachidonic-acid-mediated brain protein kinase C activation: evidence for concentration-dependent heterogeneity

Arachidonic acid (AA) activates brain protein kinase C (PKC) in a specific manner, and which differs from that of diacylglycerol (DG)-mediated PKC activation in cofactor Ca2+ and phosphatidylserine (PtdSer) requirements. We presently report that characteristics of AA-mediated activation are heterogenous, and are dependent upon the concentrations of AA. Highly sensitive PKC activation (HS) occurring at concentrations of 20 microM AA can be distinguished from less sensitive PKC activation (LS) requiring concentrations of at least 160 microM AA, on the basis of the effects of phorbol ester TPA or DG, phosphatidylcholine (PtdCho) and sodium deoxycholate (DOC). TPA, like DG suppressed the HS reaction whereas it enhanced the LS reaction. PtdCho, a phospholipid which does not affect DG-mediated activation, also prevented the HS reaction without affecting the LS reaction. This latter was inhibited at 100 microM DOC, a concentration which slightly stimulated the HS reaction. The substrate specificity was also different in the two reactions: the preferential substrate for PKC in HS was histone type VII-S, while it was histone type V-S in LS. Both reactions were similarly affected by PtdSer. In 0.1 mM CaCl2, PtdSer stimulated AA-mediated activation without evoking additive responses while this phospholipid prevented this activation in 0.5 mM EGTA, suggesting that AA and PtdSer bind PKC on the same or related sites. Together these results provide evidence for the existence of different modes of AA-mediated PKC activation with unique characteristics which presumably involve two different binding sites for AA on the same molecule and/or different PKC isoforms.

Production of leukotrienes in gonadotropin-releasing hormone-stimulated pituitary cells: potential role in luteinizing hormone release

Gonadotropin-releasing hormone (GnRH) stimulated the formation of two major metabolites of the 5-lipoxygenase pathway, leukotriene (LT) B4 and LTC4, as well as luteinizing hormone (LH) release in primary cultures of rat anterior pituitary cells. Several lines of evidence suggested the presence of a GnRH-dependent pituitary endocrine system in which LTs act as second messengers for LH release: (i) GnRH-dependent LT formation was observed within 1 min and immediately preceded GnRH-induced LH release, whereas exogenous LTs stimulated LH release at low concentrations; (ii) the dose responses of GnRH-induced LT production and LH release were similar and both effects required the presence of extracellular Ca2+ ions; (iii) GnRH-induced LH release was blocked by up to 45% following the administration of several LT receptor antagonists; (iv) LTE4 action on LH secretion was entirely abolished by LT receptor antagonists; and (v) an activator of protein kinase C acted synergistically with LTE4 to induce LH release. The major source of LT formation in the pituitary cell cultures appeared to be the gonadotrophs, as shown by GnRH receptor desensitization experiments. The results demonstrate the presence of a GnRH-activatable 5-lipoxygenase pathway in anterior pituitary cells and provide strong support for the hypothesis that LTs play a role in LH release in the GnRH signaling pathway.

Characterization of leukotriene C4 binding in anterior pituitary membrane preparations

Pituitary cells produce leukotrienes (LTs) and respond to exogenous administration of LTs by releasing gonadotropins. Specific high affinity leukotriene C4 (LTC4) binding has been found in membrane preparations of bovine anterior pituitaries. Unlabelled LTC4 displaced specific [3H]LTC4 binding. Other leukotrienes (LTB4, LTD4, LTE4, LTF4) did not compete with [3H]LTC4 for binding sites when administered at increasing concentrations together with a constant amount of radioligand indicating that the binding is highly specific for LTC4. Scatchard analysis of binding data obtained from saturation studies revealed a single binding site for [3H]LTC4 with a Kd of 8.95 +/- 5.53 nM and a B max of 15.44 +/- 6.93 pmol per mg of membrane protein. Glutathione S-transferase, a possible LTC4 binding site, did not display activity in the membrane fraction although the two glutathione derivates S-octylglutathione and S-decylglutathione competed with LTC4 in binding experiments. As leukotrienes are potent stimulators of gonadotropin secretion and modulators of gonadotropin-releasing hormone (GnRH)-induced gonadotropin release it is concluded that leukotrienes may be involved in the signal transduction pathway of GnRH and that they may act via a specific and high affinity receptor.

Inhibition of endothelin-mediated topoisomerase I activation by pertussis toxin

Cultured rat mesangial cells contain high affinity endothelin (ET) receptors at high densities. Exposure of these cells to ET resulted in a transient activation of topoisomerase I extractable activity, which reached its maximum value at approximately 2 min and returned to basal value after approximately 10 min of treatment. The activation of this enzyme was dependent upon the concentration of ET added. Incubation of the cells with pertussis toxin inhibited ET-induced increases in topoisomerase I activity in a concentration-dependent manner, suggesting that involvement of pertussis toxin-sensitive GTP-binding protein in ET-mediated action. Endothelin had no detectable effect upon extractable topoisomerase II activity.

Arachidonic acid and related methyl ester mediate protein kinase C activation in intact platelets through the arachidonate metabolism pathways

Unlike unsaturated fatty acids, which almost fully activated purified brain protein kinase C in a phosphatidylserine- and Ca2(+)-free reaction, related methyl esters were poorly active in vitro. In contrast, methyl arachidonate was revealed to be as potent as arachidonic acid in activating protein kinase C in intact platelets. Arachidonic acid-mediated activation peaked at 20 s while methyl arachidonate-mediated activation plateaued at 2 min when both lipids were added at 50 microM. At concentrations higher than 0.3 mM, all tested unsaturated fatty acids and related methyl esters were weak activators of the enzyme, with the exception of linolenic acid and methyl linolenate which evoked strong enzyme activation. However, inhibitors of arachidonate metabolism blocked both arachidonic-acid and methyl-arachidonate-induced responses. At 5 microM arachidonic acid and methyl arachidonate, protein kinase C activation was due to a cyclooxygenase product(s) whereas at 50 microM the lipoxygenase pathway was mostly involved in the reaction. Therefore, arachidonic acid and its methyl ester activate protein kinase C in platelets mainly through action of their metabolites and eicosanoid synthesis. It is suggested that such indirect protein kinase C activation may account for the tumor-promoting activity of unsaturated fatty acids and related methyl esters.

LTD4 increases cytosolic free calcium and inositol phosphates in human neutrophils: inhibition by the novel LTD4 receptor antagonist, SR2640, and possible relation to modulation of chemotaxis

LTD4 increased the level of free intracellular calcium ([Ca++]i) and stimulated the production of inositol phosphates (IP) in human polymorphonuclear neutrophils (PMN). Calcium was predominantly mobilized from intracellular pools. After a single stimulus, the cells were refractory to a second challenge with the same concentration of LTD4, but the calcium response to LTB4 was normal. The rise in [Ca++]i as well as the stimulated production of IP was inhibited by the novel LTD4 antagonist, SR2640. SR2640 also abolished the attenuation by LTD4 of LTB4-directed PMN chemotaxis. The results suggest that human PMN contain specific LTD4 receptor that trigger phosphatidyl inositol hydrolysis by activation of phospholipase C, leading to intracellular calcium mobilization, which may be involved in modulation of chemotaxis.

Rapid and serial determination of protein kinase C activity and of the associated [3H]PDBu binding using a 96-well microtiter plate and a cell harvester

We propose a serial assay of both protein kinase C activity and the related [3H]phorbol 12,13-dibutyrate binding, each carried out in 96-multiwell dishes, started and stopped row by row using a multipipet. Protein kinase C activity is observed through the transfer of the gamma-phosphoryl group of radioactive ATP onto histone H1 type III-S. Enzymatic reactions are started by adding enzyme extracts and stopped by adding trichloroacetic acid. Acidic precipitates of each row are simultaneously collected on glass fiber paper using a cell harvester. The addition of bovine serum albumin and cold ATP at the end of the reaction and the addition of trichloroacetic acid in the washing fluid lead to a high recovery of protein kinase C activity and reproducible results. Measurement of [3H]phorbol 12,13-dibutyrate binding to protein kinase C was carried out in a mixed micellar solution as described elsewhere (Y. Hannun and R. M. Bell (1987) in Methods in Enzymology, Vol. 141, pp. 287-293). The quaternary complex formed from protein kinase C, phosphatidylserine, calcium, and [3H]phorbol 12,13-dibutyrate was then bound to a beaded anionic exchanger which was automatically separated from the free phorbol 12,13-dibutyrate by microfiltration using a cell harvester. The binding reaction was highly calcium- and phosphatidylserine-dependent and calcium had to be added to washing fluid for optimal recovery. Determination of protein kinase C activity and phorbol 12,13-dibutyrate binding gave results similar to those of other published methods and the signal/noise ratio was greatly increased. Using a semi-automated cell harvester, the system is partially automated and provides accurate and reproducible results.

Transient activation of topoisomerase I in leukotriene D4 signal transduction in human cells

U937 human monoblast cells incubated with leukotriene D4 (LTD4) rapidly released arachidonic acid metabolites into the culture medium. Release was suppressed by the high-affinity LTD4 receptor antagonist SK&F 104353. Arachidonic acid release induced by LTD4 has been linked to a rapid induction of gene expression, and the propagation of the receptor binding signal is probably associated with enzymes that regulate gene expression. We have studied the participation of DNA topoisomerase I in LTD4 signal transduction. LTD4-specific release of arachidonic acid metabolites was inhibited (60-80%) by the topoisomerase I inhibitor camptothecin. LTD4 increased protein-linked DNA strand breakage induced by camptothecin in U937 cells; this enhancement was prevented by coincubation of the cells with LTD4 plus the receptor antagonist SK&F 104353. In addition, LTD4 produced a rapid transient increase in extractable topoisomerase I activity, which was maximum within the first 10 min after addition of LTD4 to the culture medium. Incubation of cultures for greater than 10 min with LTD4 before the addition of camptothecin resulted in no enhancement of camptothecin-induced DNA strand breakage, consistent with a reversal of topoisomerase I activation. Staurosporine, an inhibitor of protein kinase C, blocked LTD4-induced arachidonic acid release and attenuated the effect of LTD4 on camptothecin-induced DNA strand breakage. These results are consistent with the view that the regulation of topoisomerase I activity is involved in the propagation of LTD4-mediated signals in U937 cells.

Accumulation of cyclic AMP elicited by vasoactive intestinal peptide is potentiated by noradrenaline, histamine, adenosine, baclofen, phorbol esters, and ouabain in mouse cerebral cortical slices: studies on the role of arachidonic acid metabolites and protein kinase C

In mouse cerebral cortical slices, noradrenaline (NA) potentiates cyclic AMP (cAMP) accumulation elicited by vasoactive intestinal peptide (VIP) through alpha 1-adrenergic receptors. This synergism is inhibited by indomethacin, and the prostaglandins E2 and F2 alpha mimic the effect of NA. In the present study, we observed that the synergism between VIP and NA is not inhibited by the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) or the diacylglycerol-lipase inhibitor RHC 80267, thus further stressing the role of phospholipase A2 activation. Various neuroactive agents that potentiate the stimulatory effect of VIP on cAMP formation were also examined. As with NA, the potentiation by histamine and adenosine is inhibited by indomethacin. In contrast to NA, histamine, and adenosine, the synergistic interaction between phorbol esters and VIP on cAMP formation is abolished by H-7 but not by indomethacin. The potentiation by baclofen, a gamma-aminobutyric acidB receptor agonist, is partially inhibited by the 5-lipoxygenase inhibitor nafazatrom. The synergism between ouabain and VIP is reduced by H-7 but not by indomethacin and nafazatrom. These data indicate that the stimulation of cAMP formation elicited by VIP is under the modulation of various neuroactive agents that trigger diverse intracellular mechanisms to potentiate the effect of the peptide.

Leukotriene D4-induced proliferation of glomerular epithelial cells: PKC- and Na+-H+ exchanger-mediated response

The growth-promoting effect of leukotriene D4 (LTD4) has been observed in a variety of cells, including human glomerular epithelial cells. The purpose of this study was to determine the mechanisms underlying this process. LTD4 induction of [3H]thymidine uptake in human glomerular epithelial cells was blocked by the LTD4 receptor antagonist L648,051 when added in a 50-fold excess and by pertussis toxin. Neither drug affected basal DNA synthesis. These results suggest that the LTD4-mediated signal transduction implies activation of a GTP-binding protein that is coupled to a specific receptor. The possible role of protein kinase C (PKC) activation was also studied. In the presence of the PKC inhibitor H-7 or after downregulation of PKC levels by chronic treatment with phorbol ester, stimulation of [3H]thymidine uptake by LTD4 was greatly inhibited. Moreover, treatment of the cells by LTD4 resulted in a time-dependent increase of cytosolic PKC activity, whereas addition of phorbol 12-myristate 13-acetate reduced this activity. Therefore PKC-dependent mechanisms are likely to mediate the growth-promoting effect of LTD4. Finally, three approaches were used to determine the potential role of the Na+-H+ exchanger. First, progressive removal of extracellular Na+ using N-methyl-D-glucamine+ as a substitute inhibited LTD4-induced [3H]thymidine uptake with a 50% inhibitory concentration (IC50) of 85 mM. Second, addition of amiloride reduced the LTD4 growth effect with an IC50 of 6.5 microM, whereas three amiloride analogues exhibited lower IC50 values in accordance with their greater affinity for the Na+-H+ exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)

The signal transduction system of the leukotriene D4 receptor

During the past several years, substantial progress in understanding the receptors and signal transduction processes for peptidyl leukotrienes has been reported. Receptors have been identified and characterized, the major steps in the signal transduction pathway have been described, and the genetic and epigenetic regulatory processes have been characterized. Very recent studies have defined the mechanisms by which LTE4 acts as a partial agonist at the LTD4 receptor. The cloning of the genes for the proteins involved in the major steps of the signalling process has also been initiated. Stanley Crooke and co-authors summarize this recent progress and present their current notions about the LTD4 receptor signalling process.