Arachidonic acid in cell signaling

Diacylglycerol lipase activation and 5-lipoxygenase activation and translocation following TCR/CD3 triggering in T cells

Arachidonic acid (AA) release was observed following T cell receptor (TCR)/CD3 complex cross-linking in different tumor T cell lines as well as on purified peripheral T cells in vivo. Direct measurement of enzymatic activity in vitro of TCR/CD3-stimulated Jurkat cell extracts on labeled vesicle substrates showed that TCR/CD3 cross-linking resulted in AA release from sn-1,2-diacylglycerol (DAG) vesicles, as detected by TLC analysis, suggesting that DAG lipase was activated following TCR/CD3 stimulation and DAG generation. On the contrary, no phospholipase A2 activation was observed in response to TCR/CD3 stimulation, since no lyso-phospholipids were generated in vitro from either phosphatidylcholine or phosphatidylinositol-3,4-bisphosphate, or from phosphatidic acid vesicles. Moreover, the 1-DAG lipase inhibitor RHC80267 completely blocked TCR/CD3-dependent AA release in vitro and in vivo, without effect upon TCR/CD3-dependent inositol-1,4,5-trisphosphate (IP3) generation. Importantly, evidence for further metabolism of released AA was obtained, since synthesis and release of cysteinyl leukotrienes (CLT), but not of leukotriene B4 or cyclooxygenase products, could be detected by radioimmunoassay in different T cell lines and peripheral blood T cells following TCR/CD3 cross-linking. Moreover, HPLC analysis revealed an accumulation of leukotriene E4 in TCR/CD3 stimulated Jurkat cells. This was associated with translocation of 5-lipoxygenase from the cytosol to the cell membranes. Finally, TCR/CD3-mediated CLT production was blocked by MK886, a specific inhibitor of 5-LO translocation and activation. Our data help define a further level in the fate of second messengers generated after TCR/CD3 triggering and suggest that additional mediators can play a role in the context of T cell activation.

Arachidonic acid may mediate the galanin-induced hyperpolarization in parasympathetic neurons from Necturus maculosus

The effects of arachidonic acid (AA) and compounds that inhibit intracellular signalling pathways on membrane potential and galanin-induced hyperpolarizations were investigated in parasympathetic neurons from Necturus maculosus. Treatment for 10-90 min with 10-20 microM 4-bromophenacylbromide or 10 microM cyclosporin A caused a progressive decrease in the amplitude of galanin-induced hyperpolarizations without any change in resting membrane potential. The galanin-induced hyperpolarization was not altered following a 10-120 min treatment with the protein kinase inhibitor H-7. These results indicated that phospholipase A2 activation, but not protein kinase activation, may be required for the galanin-induced hyperpolarization. Arachidonic acid (20-100 microM) caused a concentration-dependent membrane hyperpolarization of the parasympathetic neurons and a decrease in the amplitude of the galanin-induced hyperpolarization. These data indicate that phospholipase A2-catalyzed liberation of AA may be involved in the galanin-induced membrane hyperpolarization observed in mudpuppy parasympathetic neurons.

Cytosolic phospholipase A2 gene expression in rat mesangial cells is regulated post-transcriptionally

Cytosolic phospholipase A2 (cPLA2) is thought to be the rate-limiting enzyme in the arachidonic acid/eicosanoid cascade. The ability of various agonists to increase steady-state cPLA2 mRNA levels has previously been reported. The current study delineates the contributions of transcriptional and post-transcriptional processes to the regulation of cPLA2 gene expression in response to a variety of agonists in cultured rat glomerular mesangial cells. Epidermal growth factor, platelet-derived growth factor, serum and phorbol myristate acetate all increase the half-life of cPLA2 mRNA transcripts, indicating a role for post-transcriptional modulation of gene expression. The presence of three ATTTA motifs in the 3' untranslated region (3'UTR) of the rat cPLA2 cDNA is ascertained. Heterologous expression of chimeric constructs with different 3'UTRs ligated into the 3' end of the luciferase coding region reveals that the presence of the cPLA2 3'UTR results in reduced luciferase activity compared with constructs without the cPLA2 3'UTR. Furthermore, the luciferase activity in the constructs with the cPLA2 3'UTR is increased in response to the same agonists which stabilize endogenous cPLA2 mRNA. A negligible effect of these agonists on transcriptional control of cPLA2 is evident using promoter-reporter constructs expressed in transient and stable transfectants. Taken together, these results indicate predominant post-transcriptional regulation of cPLA2 mRNA levels.

Possible involvement of arachidonic acid and eicosanoids in metamorphic events in Hydractinia echinata (Coelenterata; Hydrozoa)

Upon induction of metamorphosis, larvae of the marine hydroid Hydractinia echinata release [14C]-arachidonic acid from previously labeled endogenous sources. The lipoxygenase inhibitors nordihydroguaiaretic acid and 5,8,11,14-eicosatetraynoic acid inhibited metamorphosis induced by Cs+ and 1,2-sn-dioctanoylglycerol, whereas the inhibitors of cyclooxygenase, indomethacin, and acetylsalicylic acid were ineffective, suggesting a role for lipoxygenase metabolites of arachidonic acid in induction of metamorphosis. Lipoxygenase products in Hydractinia echinata were isolated and identified by gas chromatography/mass spectrometry. 8- and 12-HETE were the most abundant metabolites. In cytosolic fractions from larvae activity of an arachidonic acid metabolizing enzyme, presumably a lipoxygenase, was found. The metabolic product was identified by 1H-NMR and chiral phase HPLC as 8(R)-HETE. Its production was strongly inhibited by NDGA, but not by indomethacin.

Characterisation of the second messenger pathway underlying neurite outgrowth stimulated by FGF

Cerebellar neurons, cultured on monolayers of 3T3 fibroblasts or on a polylysine/laminin-coated substratum, responded to recombinant basic FGF by extending longer neurites. The response was biphasic reaching a maximum at 5 ng/ml FGF, but desensitising at 100–200 ng/ml FGF. The response to FGF could be inhibited by a tyrosine kinase inhibitor (the erbstatin analogue), by a diacylglycerol lipase inhibitor (RHC-80267) and by a combination of N- and L-type calcium channel antagonists or other agents that negate the effects of calcium influx into neurons. The response to FGF could be fully mimicked by arachidonic acid added directly to the cultures, or generated via activation of phospholipase A2 with melittin. The response to melittin, but not to FGF or arachidonic acid, was inhibited by 4-bromophenacyl bromide, a phospholipase A2 inhibitor. The response to arachidonic acid was also biphasic and high concentrations of this agent could cross-desensitise the FGF response and vice versa. The response to arachidonic acid could be fully inhibited by the agents that block or negate the effects of calcium influx into neurons, but was not inhibited by the tyrosine kinase or diacylglycerol lipase inhibitors. These data suggest that FGF stimulates neurite outgrowth by activating a cascade that involves activation of phospholipase C gamma to produce diacylglycerol, conversion of diacylglycerol to arachidonic acid by diacylglycerol lipase and the activation of voltage-gated calcium channels by arachidonic acid.

Annexin-I inhibits phospholipase A2 by specific interaction, not by substrate depletion

Annexin-I is a calcium dependent phospholipid binding and phospholipase A2 (PLA2) inhibitory protein. A 'substrate depletion' model has been proposed for the mechanism of PLA2 inhibition by annexin-I in studies with 14 to 18 kDa PLA2s. Herein, we have studied the inhibition mechanism using 100 kDa cytosolic PLA2 from porcine spleen. The inhibition has been measured at various substrate and calcium ion concentrations. The pattern of PLA2 inhibition by annexin-I was consistent with a 'specific interaction' mechanism rather than the 'substrate depletion' model. Apparent contraction with previous studies can be explained by the calcium-dependent binding of annexin-I to the substrate.

Isolation of promoter for cytosolic phospholipase A2 (cPLA2)

Cytosolic phospholipase A2 (cPLA2) releases arachidonic acid from membrane phospholipids and is believed to be the rate-limiting enzyme in the arachidonic acid pathway. We report herein the isolation of a 3 kb fragment of rodent genomic DNA containing part of the first intron, the first exon and 5'-flanking sequence. The start site of transcription was mapped by 5'-rapid amplification of cDNA ends and corroborated by ribonuclease protection assay. The gene has a TATAless promoter with no classical Sp1 binding sites or initiator element. A microsatellite series of CA repeats was noted in the 5'-flanking region of both the rodent and human promoters. Deletion constructs have been analysed for luciferase activity and confirmed promoter activity.

The production of arachidonic acid can account for calcium channel activation in the second messenger pathway underlying neurite outgrowth stimulated by NCAM, N-cadherin, and L1

We have used monolayers of control 3T3 fibroblasts and 3T3 fibroblasts expressing transfected cell adhesion molecules (CAMs)--NCAM, N-cadherin, and L1--as a culture substrate for cerebellar neurones. The transfected CAMs promote neurite outgrowth by activating a second messenger pathway that culminates in calcium influx into neurones through N- and L-type calcium channels. We show that the same neurite outgrowth response can be directly induced by arachidonic acid (10 microM) and that this response can be inhibited by N- and L-type calcium channel antagonists. In cells, arachidonic acid can be generated by phospholipase A2 or by the sequential activities of a phospholipase C (to generate diacylglycerol) and diacylglycerol lipase. In the present study we show the neurite outgrowth stimulated by CAMs (but not by various other agents) can be abolished by an inhibitor of diacylglycerol lipase acting at a site upstream from calcium channel activation. The results suggest that arachidonic acid and/or one of its metabolites is the second messenger that activates calcium channels in the CAM signalling pathway leading to axonal growth, and this is supported by recent evidence that shows the same concentrations of arachidonic acid can increase voltage-dependent calcium currents in cardiac myocytes.