INTERPRETACIÓN DE LA NUBE DE VARIABLES

1 .3 .1 Identification o f arachidonic ac id as a second m essenger

When neutrophils or HL60 cells are stimulated with fMetLeuPhe, arachidonic acid (AA) is released from phospholipids catalysed by cytosolic phospholipase A2 (29, 126, 127). The role of AA had, up until recently, been considered to be solely a precursor of the eicosenoids. Now, AA is considered a novel intracellular second messenger.

Very broadly, the PLA2 enzymes can be divided into tw o groups, the "secreted" and the "cytosolic" forms. They differ in their location, molecular weight and substrate preference. Neutrophils secrete these PLA2S during degranulation in response to chemotactic and phagocytic stimuli. The secreted PLA2S have a molecular weight of 13-18 KDa (128). The secretory PLA2S do not demonstrate any selectivity among the fatty acids in the sn-2 position of phospholipids.

In contrast, cytosolic PLA2 specifically cleaves AA from the sn-2 position of a glycerophospholipid releasing the corresponding lyso-phospholipid (129). Phosphatidylinositol (29), phosphatidylcholine, phosphatidylethanolamine and triglycerides are all potential substrates (130). The molecular weight of PLA2 is in the range of 8 5 -1 1 0 KDa and has been identified in many cell types e.g. U 937 (131), RAW 2 6 4 .7 cell

in HL60 cells (135). In quiescent cells PLAg is present in the cytosol but in response to elevated ( > 3 0 0 nM) translocates to membranes by virtue of a 45 amino acid Ca^+-dependent phospholipid binding motif. PLAg shares this property with PKC, GTPase activating protein and PLC7I (131).

1 .3 .2 Regulation o f PLA2

FMetLeuPhe-stimuiated AA release is G-protein-dependent. The evidence for this is that a) GTP7S and NaF are potent activators of AA release in permeabilized neutrophils (136), HL60 cells (135) and platelets (137) b) GTP7S and fMetLeuPhe are synergistic in activating PLA 2 in acutely permeabilized neutrophils (138) and c) pertussis toxin treatment results in the inhibition of fmetLeuPhe-mediated release of AA in guinea pig human neutrophils (29, 127), rod outer segments (139) and HL60 cells (29).

Since PI-PLC is also activated via a G-protein thus elevating CaZ+ and activating PKC, the question arises whether PLAg activation is a consequence of the products of PLC activation or if it is activated by a distinct G-protein. There is substantial evidence for the latter. Under conditions were the CaZ+ concentration is controlled with EGTA buffers and PLC activity is inhibited by the removal of MgATP from the assay, PLAg activation is still possible with fMetLeuPhe and GTP7S in human neutrophils(138) and differentiated HL60 cells (135). In addition, treating cells with neomycin prevented inositol phosphate but not AA production in both thyroid cells and human platelet membranes(137, 140). Furthermore, AA release but not PLC activity was inhibited by pertussis toxin in FRTL5 thyroid cells (140).

Assuming PLAg is directly activated by a G-protein it remains to be established whether it is the a-subunit or the B7-complex that is the transducer of PLAg activation in neutrophils. In rod outer segments the activation of PLAg is mediated by 137-subunits of transducin. The a- subunit of transducin slightly stimulated PLAg activity alone and inhibited the R7 complex-induced increase in AA presumably by reassociating with it (139). The fact that PLAg activation is inhibited by pertussis toxin in many cell types (4) could be interpreted that Gi/Go is the G-protein linking the receptor to PLAg.

Over the last few years the role of PLAg phosphorylation has come under scrutiny. Most recently, Lin et al (141) have demonstrated that PLAg is a substrate for MAP kinase and MAP kinase-mediated phosphorylation is essential for PLAg activity. First, they showed that purified PLAg can be stoichiometrically phosphorylated by MAP kinase and this enhanced the activity of the enzyme. Second, PLAg containing a mutation at the consensus site for MAP kinase (ser-505) failed to be phosphorylated. Third, phosphorylation of PLAg isolated from phorbol-treated cells by MAP kinase was dramatically reduced as compared with that from untreated cells. With this information thay have proposed a model whereby a ligand binds to its cognate receptor and activates PLC causing an increase in the intracellular concentration of Ca^*. Increased CaZ+ causes PLAg to translocate from the cytosol to the membrane. At the same time DAG activates PKC, giving rise to the activation of MAP kinase. Activated MAP kinase can then phosphorylate PLAg at serine-505, causing an increase in PLCAg activity. Both the Ca^^-dependent membrane association of PLAg and phosphorylation of PLA2 are required for the full activation of PLAg (141).

1 .3 .3 Function o f arachidonic acid

Studies with intact and permeabilized neutrophils and HL60 cells show a strong correlation between the ability of agonists to induce AA release and exocytosis (29, 138). Secretion is independent of cyclooxygenase and lipooxygenase pathways indicating that AA is acting as an intracellular second messenger.

In addition, there is evidence that AA has a modulatory role in in the activation and maintenance of 0*2 generation by NADPH oxidase (142). The precise site of action of of AA mediating the respiratory burst within neutrophils remains to be positively identified but activation of the PKC isoform PKC7 has been suggested (143). On the other hand, Dana et al (144) have demonstrated that while PKC and PLAg are both necessary for NADPH oxidase activation in human neutrophils, PLAg operates downstream from PKC.

Cleavage of phosphatidic acid by PLAg will generate both AA and lyso- phosphatidic acid (LPA). LPA is emerging as an important intercellular messenger. The best documented example of LPA production and release

thrombin-stimulated platelets, predominantly through the PLAg attack of newly formed PA in the inner leaflet of the plasma membrane. Since LPA is quite water soluble most of it is released extracellularly via secretion or diffusion from leaky platelets. LPA has specific G-protein coupled receptors on a variety of cell types but not HL60 cells. Activation of LPA receptors can evoke a wide range of biological effects including Caz+ release from intracellular stores, stimulation of fibroblast proliferation, platelet aggregation, cellular motility and tumour cell invasiveness (145).