Mechanisms of regulation of receptors and signal transduction pathways for the peptidyl leukotrienes

Mechanisms of leukotriene D4-induced constriction in human small bronchioles

We examined the mechanisms underlying leukotriene D4- (LTD4) induced constriction of human small (300 - 500 micron i.d.) bronchioles, and the effect of LTD4 on ion currents and Ca2+ transients in smooth muscle cells (SMC) isolated from these bronchioles. LTD4 caused a concentration-dependent bronchoconstriction with an EC50=0.58+/-0.05 nM (n=7) which was not easily reversible upon washout. This bronchoconstriction was entirely dependent on extracellular Ca2+. Blockade of L-type Ca2+ channels with nifedipine (10 microM) reduced LTD4 response by 39+/-2% (n=8), whilst La3+, Gd3+ and SK&F 96,365 abolished LTD4-induced bronchoconstriction completely and reversibly, suggesting the majority of Ca2+ entry was via non-selective cation channels. Antagonists of PI-PLC (U73,122 and ET-18-OCH3), PLD (propranolol) and PKC (cheleretrine and Ro31-8220) were without any effect on LTD4-induced bronchoconstriction, whilst the PC-PLC inhibitor D609 caused complete relaxation. Inhibition of protein tyrosine kinase with tyrphostin A23 (100 microM) caused about 50% relaxation, although the inactive analogue tyrphostin A1 was without effect. In freshly isolated SMC from human small bronchioles LTD4 caused a slow increase of intracellular Ca2+ concentration, with a consequent rise of the activity of large conductance Ca2+-dependent K+ channels and the amplitude of depolarization-induced outward whole-cell current. Again, no effect of LTD4 could be observed in the absence of extracellular Ca2+. We conclude that LTD4 causes constriction of these small bronchioles primarily by activating Ca2+ entry via non-voltage gated channels, possibly by a PC-PLC mediated pathway.

Leukotriene D4-induced contraction of cat esophageal and lower esophageal sphincter circular smooth muscle

BACKGROUND & AIMS

In esophageal circular muscle, acetylcholine activates phosphatidylcholine-specific phospholipases C and D and phospholipase A2, producing diacylglycerol and arachidonic acid, which cause contraction by interacting synergistically to activate protein kinase C. In a model of acute esophagitis, leukotriene D4 (LTD4) contributes to acetylcholine-induced contraction. We examined intracellular signaling in LTD4-induced contraction.

METHODS

Esophageal and lower esophageal sphincter (LES) cells, isolated by enzymatic digestion, were contracted by LTD4 in the absence or presence of inhibitors. Permeabilization by saponin allowed use of G-protein antibodies and heparin.

RESULTS

Esophageal contraction was inhibited by pertussis toxin, Gi3 antibodies, D609 (phosphatidylcholine-specific phospholipase C inhibitor), propranolol (phospholipase D pathway inhibitor), and chelerythrine (protein kinase C antagonist) but not W7 (calmodulin antagonist). LES contraction was unaffected by pertussis toxin. It was inhibited by Gq antibodies, U-73122 (phosphatidylinositol-specific phospholipase C inhibitor), heparin (inositol 1,4,5-trisphosphate inhibitor), and W7 and reduced by D609.

CONCLUSIONS

In the esophagus, LTD4 activates a protein kinase C-dependent pathway through pertussis toxin-sensitive Gi3 proteins and phosphatidylcholine-specific phospholipase. In the LES, LTD4 activates a calmodulin-dependent pathway through pertussis toxin-insensitive Gq proteins and phosphatidylinositol-specific phospholipase C. The intracellular pathways activated by LTD4 in the esophagus and the LES are similar to those activated by acetylcholine and other agonists.

The immunosuppressive effects of a leukotriene B4 receptor antagonist on liver allotransplantation in rats

The immunosuppressive effects of a leukotriene B4 (LTB4) receptor antagonist, ONO4057, on liver allotransplantation in rats were evaluated, and the levels of prostaglandin E2 (PGE2) in the liver tissue during rejection of the allografts examined. The rats were divided into four groups: group 1: Lewis rats (LEW) given a sham operation with dimethyl sulfoxide (DMSO); group 2: LEW given syngenic orthotopic liver transplantation (OLT) from LEW, with DMSO; group 3: LEW given allogenic OLT from ACI rats (ACI), with DMSO; and group 4: LEW given allogenic OLT from ACI, with ONO4057 as 10, 30, or 100 mg/kg per day dissolved in DMSO to subgroups 4a, 4b, and 4c, respectively. Histological examinations were performed, survival times monitored, and liver tissue PGE2 levels 3, 5, 7, and 14 days after transplantation measured. The mean graft survival times in groups 4a, b, and c, at 37.5 +/- 10.4, 52.2 +/- 24.4, and 34.0 +/- 4.9 days (mean +/- SEM), respectively, were significantly longer than that in group 3 (at 13.0 +/- 3.2 days). Moreover, the levels of tissue PGE2 in the liver allografts in group 4a were significantly higher than those in group 3 on days 5 and 7. These results suggest that ONO4057 has an immunosuppressive effect on liver allotransplantation since it reduces the activities of LTB4 which augments immune responses, and also because it indirectly increases the PGE2 level.

Leukotriene C(4) biosynthesis in isolated August rat peritoneal leukocytes

The mixed leukocyte population obtained from the peritoneum of the August rat is a potentially important experimental model of inherent eosinophilia that has not been well characterized. In the present study, isolated cell preparations generated a concentration-dependent release of leukotriene (LT) C(4) when exposed to the Ca(2+) ionophore A23187, reaching maximal stimulation at 5.0 muM. This response was inhibited by the 5-lipoxygenase activating protein antagonist MK-886 (0.1 muM), nominally Ca(2+) and Mg(2+)-free incubation media and by activation of protein kinase C via phorbol 12-myristate 13-acetate (50 nM). These findings establish a model system for investigating LTC(4) profiles contingent with innate peritoneal eosinophilia and are consistent with the hypothesis that cellular LTC(4) biosynthesis is phosphoregulated.

Characterization of CoA-independent transacylase activity in U937 cells

Coenzyme A-independent transacylase (CoA-IT) mediates the transfer of polyunsaturated fatty acids from the sn-2 position of a donor phospholipid to the sn-2 position of an acceptor lyso-phospholipid. We have characterized this activity in U937 cells, a human monocytic cell line. The microsomes of these cells contained CoA-IT activity which demonstrated a fatty acid preference for transferring arachidonic acid into exogenously added 1-alkyl-2-lyso-GPC. This enzymatic activity was optimum between pH 6.5 and 9, was heat labile and displayed an apparent Km for 1-alkyl-2-lyso-GPC of 0.4 microM. This activity was not dependent on Ca2+, Mg2+, CoA or ATP, was not inhibited by 2-mercaptoethanol nor by addition of product, 1-alkyl-2-acyl-GPC. The activity of this enzyme was not altered by differentiation of U937 cells towards the macrophage with Me2SO. Treatment of U937 cells with dexamethasone had no effect on transacylase activity. The activity of this enzyme was decreased by the serine esterase inhibitors phenylmethyl-sulfonyl fluoride and N-tosyl-L-phenylalanine chloromethyl ketone and by the histidine modifier diethyl pyrocarbonate, suggesting that CoA-IT may belong to a family of acyltransferase enzymes typified by LCAT. CoA-IT activity was not affected by compounds that affect PLA2 activity, such as quinacrine, aristolochic acid and arachidonic acid, suggesting a mechanism of action for CoA-IT different from classical, low molecular weight PLA2 enzymes. In conclusion, U937 cells contain CoA-IT activity and this study extends our previous knowledge of this enzyme by demonstrating the differences between CoA-IT and PLA2 enzymes and suggesting similarities between CoA-IT and LCAT.

Leukotriene C4 decreases the activity of respiratory cilia in vitro

In the present study we assessed the effect of leukotriene C4 on ciliary beat frequency in vitro. Chicken tracheas were sliced into thin rings and ciliary activity was viewed microscopically. Ciliary beat frequency was measured through an optical fibre by a fast Fourier transformer analyser and recorded on an oscilloscope. Ciliary beat frequency was measured at one fixed point during 30 min at room temperature in RPMI-HEPES medium alone and in medium containing leukotriene C4 at a range of concentrations (10(-6)-10(-9) M). Our results demonstrate that leukotriene C4 (10(-7) and 10(-8) M) caused a significant decrease in ciliary beat frequency in all tracheal rings tested (mean decrease of 24%). The specificity and Ca2(+)-dependency of leukotriene C4 activity was elucidated by the ability of FPL 55712 (SRS-A specific antagonist), Ca2(+)-free medium, calcium channel blockers (nifedipine and verapamil) and the calmodulin inhibitor trifluroperazine to abrogate its effect on ciliary beating.

Lipid modulators of cell function

Lipids have surfaced as potent and diverse modulators of cell functions, as determinants of membrane structure, as ligands for cell-surface receptors, as anchors for membrane-associated proteins, and as "second messengers." Some of these functions involve the complex lipids directly, as exemplified by the alteration of receptor behavior by gangliosides. However, many other functions entail cleavage of membrane lipids to yield (as examples): unsaturated fatty acids, which are converted to prostaglandins, prostacyclins, thromboxanes, and other compounds; diacylglycerols, which activate protein kinase C; inositol phosphates, which stimulate release of calcium from intracellular stores; and lysoalkylphosphatidylcholine, which is converted to platelet-activating factor. New roles for membrane lipids are constantly appearing, such as the inhibition of protein kinase C by sphingosine and the release of phosphatidylinositol-linked proteins in response to hormones. Dietary modification of these lipid systems could have important implications for normal cell function and disease.

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.

Phorbol 12-myristate 13-acetate inhibition of leukotriene D4-induced signal transduction was rapidly reversed by staurosporine

Activation of leukotriene D4 receptors results in phospholipase C-mediated breakdown of phosphatidylinositol and increases in intracellular Ca2+ in U-937 cells. Treatment (10 min) with phorbol 12-myristate 13-acetate blocked leukotriene D4-induced phosphatidylinositol metabolism and Ca2+ mobilization (IC50 = 0.2 nM). Treatment with 10 nM phorbol 12-myristate 13-acetate produced blockade which was complete within 1 min and no recovery was observed over 7 days. Addition of the protein kinase C inhibitor staurosporine (100 nM) to U-937 cells pretreated with phorbol 12-myristate 13-acetate for 5 min or 24 hr resulted in a rapid reappearance of leukotriene D4-induced Ca2+ mobilization. Half of the response recovered within 2 min, with complete recovery in 20 min. Staurosporine produced a concentration-related recovery of signal transduction, with an EC50 of 30 nM. These data describe cells which have a novel response to phorbol 12-myristate 13-acetate in that the inhibition of leukotriene D4 signal transduction is persistent and yet rapidly reversed by staurosporine.