Lyso-phospholipids exert a major injurious effect on lung cell membranes during Acute Respiratory Distress Syndrome (ARDS), but the mechanisms leading to their in vivo generation are still unknown. Intratracheal administration of LPS to guinea pigs induced the secretion of type II secretory phospholipase A2 (sPLA2-II) accompanied by a marked increase in fatty acid and lyso-phosphatidylcholine (lyso-PC) levels in the bronchoalveolar lavage fluid (BALF). Administration of LY311727, a specific sPLA2-II inhibitor, reduced by 60% the mass of free fatty acid and lyso-PC content in BALF. Gas chromatography/mass spectrometry analysis revealed that palmitic acid and palmitoyl-2-lyso-PC were the predominant lipid derivatives released in BALF. A similar pattern was observed after the intratracheal administration of recombinant guinea pig (r-GP) sPLA2-II and was accompanied by a 50-60% loss of surfactant phospholipid content, suggesting that surfactant is a major lung target of sPLA2-II. In confirmation, r-GP sPLA2-II was able to hydrolyze surfactant phospholipids in vitro. This hydrolysis was inhibited by surfactant protein A (SP-A) through a direct and selective protein-protein interaction between SP-A and sPLA2-II. Hence, our study reports an in vivo direct causal relationship between sPLA2-II and early surfactant degradation and a new process of regulation for sPLA2-II activity. Anti-sPLA2-II strategy may represent a novel therapeutic approach in lung injury, such as ARDS. (

Several Gi‐linked neurotransmitter receptors, including dopamine D2 receptors, act synergistically with Ca2+‐mobilizing stimuli to potentiate release of arachidonic acid (AA) from membrane phospholipids. In brain, AA and its metabolites are thought to act as intracellular second messengers, suggesting that receptor‐dependent potentiation of AA release may participate in neuronal transmembrane signaling. To study the molecular mechanisms underlying this modulatory response, we have now used Chinese hamster ovary cells transfected with rat D2‐receptor cDNA, CHO(D2). Two antisense oligodeoxynucleotides corresponding to distinct cDNA sequences of cytosolic, AA‐specific phospholipase A2 (cPLA2) were synthesized and added to cultures of CHO(D2) cells. Incubation with antisense oligodeoxynucleotides inhibited D2 receptor‐dependent release of AA but had no effect on D2‐receptor binding or D2 inhibition of cyclic AMP accumulation. In addition, pharmacological experiments showed that D2 receptor‐dependent AA release was prevented by nonselective phospholipase inhibitors (such as mepacrine) but not by inhibitors of membrane‐bound, non‐AA‐specific PLA2 (such as p‐bromophenacyl bromide). cPLA2 is expressed in brain tissue. The results, showing that cPLA2 participates in receptor‐dependent potentiation of AA release in CHO(D2) cells, suggest that this phospholipase may serve a similar signaling function in brain.

Background: Regulation of integrin activation has important implications for tumor cell invasion and metastasis. Results: EGF activates ERK/p90RSK and Rho/Rho kinase signaling in A431 and DiFi colon cancer cells, leading to phosphorylation of filamin A (FLNa) and inactivation of the ␣5␤1 integrin receptor. Conclusion: EGF promotes ␣5␤1 inactivation through the p90RSK-dependent phosphorylation of FLNa. Significance: We have identified a novel EGF-dependent mechanism controlling the ␣5␤1 integrin activation state.

SummaryPAI1 stimulates assembly of the fibronectin matrix in osteosarcoma cells through crosstalk between the αvβ5 and α5β1 integrins

We have shown previously that guinea pig alveolar macrophages (AM) synthesize a secretory phospholipase A2 (PLA2) during in vitro incubation. Here, we report the molecular cloning of this enzyme and show that it has structural features closely related to all known mammalian type-II PLA2. The mRNA and PLA2 activity were undetectable in freshly collected AM, but their levels increased dramatically to reach maximal values after 16 h of culture. Thereafter, the PLA2 activity remained constant with a parallel secretion in the medium, in contrast to mRNA level which returned to near basal values after 32 h. Incubation of AM for 16 h with the inflammatory secretagogue peptide f-Met-Leu-Phe (fMLP) markedly reduced the PLA2 activity and mRNA levels. This inhibition was prevented by preexposure of AM to pertussis toxin, an inhibitor of G-protein. In contrast, when AM were first cultured for 16 h and then incubated with fMLP, no significant change was observed in their PLA2 activity. In conditions where the type-II PLA2 was completely abrogated by fMLP, the latter did not alter the lipopolysaccharide-induced accumulation of tumor necrosis factor alpha mRNA or the release of arachidonic acid induced by the subsequent addition of the calcium ionophore A23187. These studies show that the inflammatory peptide fMLP down-regulates the expression of the type-II PLA2 by AM through a process mediated by G-protein. A possible negative control of the type-II PLA2 expression during AM activation is suggested.

Evidence indicates that a single membrane receptor subtype may be responsible for the generation of multiple intracellular signals, but mechanisms allowing for the selection of a specific effector pathway have not yet been documented. In neurons and other cells, the stimulation of dopamine D2 receptors produces, via G-protein activation, a spectrum of intracellular responses including inhibition of adenylyl cyclase activity, modulation of K+ currents, and potentiation of Ca(2+)-evoked arachidonic acid (AA) release. In this study, we report that, in Chinese hamster ovary cells, stimulation of protein kinase C (PKC) directs the preferential coupling of transfected D2 receptors from inhibition of adenylyl cyclase to potentiation of AA release, two responses mediated by Gi. The switch between these two signaling systems is accompanied by marked changes in their GTP sensitivities, indicating that it may result from the phosphorylation of component(s) of the receptor-Gi-protein complex. Brain PKC activity is enhanced by neurotransmitters and by neuronal depolarization. Thus, the ability of this protein kinase to remodel signaling pathways at the D2 receptor may regulate these Gi-mediated responses in an activity- dependent manner, and represent a novel form of synaptic plasticity.

We have demonstrated previously that isolated guinea-pig alveolar macrophages (AM) synthesize type-II phospholipase A2 (PLA2-II) through a tumour necrosis factor-alpha (TNF-alpha)-dependent process. This synthesis is enhanced by lipopolysaccharide (LPS) and accompanied by a release of prostaglandin E2 (PGE2) into the medium. Because agents elevating intracellular cAMP, such as PGE2, have been shown to stimulate PLA2-II expression in various cell types, we investigated the modulation of PLA2-II synthesis by cAMP in AM. Surprisingly, incubation of AM with PGE2, dibutyryl-cAMP, cholera toxin or rolipram (an inhibitor of specific cAMP-phosphodiesterase) inhibited both basal and LPS-stimulated PLA2-II expression. The inhibitory effect of PGE2 was observed at concentrations similar to those released by AM. Moreover, treatment of AM with either aspirin or neutralizing PGE2 monoclonal antibody stimulated PLA2-II synthesis. These effects were closely correlated with the ability of these agents to modulate TNF-alpha release, which was decreased by dibutyryl-cAMP and exogenous PGE2, whereas neutralizing PGE2 antibody markedly increased this release. Hence, in contrast to other cell systems, we report that: (i) agents elevating intracellular cAMP levels down-regulate both basal and LPS-induced PLA2-II synthesis, (ii) prostaglandins exert a negative feedback effect on this synthesis, probably through an elevation of intracellular cAMP levels, and (iii) inhibition of TNF-alpha release may account, at least in part, for the down-regulation of PLA2-II expression by endogenously produced prostaglandins and cAMP-elevating agents.

We have recently shown that modified natural pulmonary surfactant Curosurf inhibits the synthesis of type II phospholipase A2 (sPLA2-II) by cultured guinea-pig alveolar macrophages (AM). The goal of the present study was to identify the surfactant components and the mechanisms involved in this process. We show that protein-free artificial surfactant (AS) mimicked the inhibitory effect of Curosurf, suggesting that phospholipid components of surfactant play a role in the inhibition of sPLA2-II expression. Among surfactant phospholipids, dioleylphosphatidylglycerol (DOPG) was the most effective in inhibiting the synthesis of sPLA2-II. By contrast, the concentrations of platelet-activating factor (PAF)-acetylhydrolase and lysophospholipase activities remained unchanged, indicating that inhibition of sPLA2-II synthesis was caused by a specific effect of surfactant. The effect of DOPG on sPLA2-II synthesis was concentration-dependent and was accompanied by a rapid and time-dependent uptake of DOPG by AM whereas dipalmitoylphosphatidylcholine (DPPC) was only marginally taken up. Curosurf, AS, and DOPG inhibited tumor necrosis factor-alpha (TNF-alpha) secretion, a key step in the induction of sPLA2-II synthesis by AM, in contrast to DPPC which had only a marginal effect. We conclude that phospholipid components, especially DOPG, play a major role in the inhibition of sPLA2-II synthesis by surfactant and that this effect can be explained, at least in part, by an impairment of TNF-alpha secretion.