Characterization of fMet-Leu-Phe-stimulated phospholipase C in streptolysin-O-permeabilised cells

Phosphatidylinositol transfer protein dictates the rate of inositol trisphosphate production by promoting the synthesis of PIP2

BACKGROUND

Phosphatidylinositol transfer protein (PI-TP), which has the ability to transfer phosphatidylinositol (PI) from one membrane compartment to another, is required in the inositol lipid signalling pathway through phospholipase C-beta (PLC-beta) that is regulated by GTP-binding protein(s) in response to extracellular signals. Here, we test the hypothesis that the principal role of PI-TP is to couple sites of lipid hydrolysis to sites of synthesis, and so to replenish depleted substrate for PLC-beta.

RESULTS

We have designed an experimental protocol that takes advantage of the different rates of release of endogenous PI-TP and PLC-beta from HL60 cells permeabilized with streptolysin O. We have examined the kinetics of stimulated inositol lipid hydrolysis in cells depleted of PI-TP, but not of endogenous PLC-beta, in the presence and absence of exogenous PI-TP. Linear time-courses were observed in the absence of any added protein, and the rate was accelerated by PI-TP using either guanosine 5'[gamma-thio]-triphosphate (GTP gamma S) or the receptor-directed agonist fMetLeuPhe as activators. In addition, depletion from the cells of both PI-TP and PLC-beta isoforms by extended permeabilization (40 minutes) allowed us to control the levels of PLC-beta present in the cells. Once again, PI-TP increased the rates of reactions. To identify whether the role of PI-TP was to make available the substrate phosphatidylinositol bisphosphate (PIP2) for the PLC, we examined the synthesis of PIP2 in cells depleted of PI-TP. We found that PI-TP was essential for the synthesis of PIP2.

CONCLUSIONS

The predicted function of PI-TP in inositol lipid signalling is the provision of substrate for PLC-beta from intracellular sites where PI is synthesized. We propose that PI-TP is in fact a co-factor in inositol lipid signalling and acts by interacting with the inositol lipid kinases. We hypothesize that the preferred substrate for PLC-beta is not the lipid that is resident in the membrane but that provided through PI-TP.

An essential role for phosphatidylinositol transfer protein in phospholipase C-mediated inositol lipid signaling

Transmembrane signaling by the phospholipase C-beta (PLC-beta) pathway is known to require at least three components: the receptor, the G protein, and the PLC. Recent studies have indicated that if the cytosol is allowed to leak out of HL60 cells, then G protein-stimulated PLC activity is greatly diminished, indicating an essential role for a cytosolic component(s). We now report the complete purification of one component based on its ability to reconstitute GTP gamma S-mediated PLC activity and identify it as the phosphatidylinositol transfer protein (PI-TP). Based on the in vitro effects of PI-TP, we surmise that it is involved in transporting PI from intracellular compartments for conversion to PI bisphosphate (PIP2) prior to hydrolysis by PLC-beta 2/PLC-beta 3, the endogenous PLC isoforms present in these cells.

Correlation between secretion and phospholipase D activation in differentiated HL60 cells

Receptor-directed agonists including N-formylmethionyl-leucyl-phenylalanine (fMetLeuPhe), C5a, ATP and UTP all activate phospholipase D (PLD), which is accompanied by secretion in differentiated HL60 cells. Interference in the production of phosphatidase (PA) by the PLD pathway by diverting it towards the production of phosphatidylethanol (PEt) in the presence of ethanol leads to near-total inhibition of the secretion evoked by ATP and UTP and a partial inhibition of that evoked by fMetLeuPhe and C5a. In streptolysin-O-permeabilized cells, fMetLeuPhe is able to activate PLD, and this is dependent on the presence of a low concentration of guanosine 5'-[gamma-thio]-triphosphate (GTP[S]). Ca2+ (10 microM) and GTP[S] individually or in combination are also able to activate PLD and secretion. The stimulation of secretion in permeabilized cells stimulated by Ca2+ alone or fMetLeuPhe or GTP[S] is also abrogated when the production of PA is diverted to PEt by the presence of ethanol. Activation of PLD by GTP[S] or fMetLeuPhe is decreased if the cells are permeabilized first and GTP[S] or fMetLeuPhe is added subsequently. This corresponds well with the loss of the secretory response. We conclude that the ability of GTP[S] or fMetLeuPhe to stimulate secretion from permeabilized cells is dependent on a prior activation of the PLD signalling pathway. PA, generated as a consequence of PLD activation, acts as second messenger that can provide an initiating signal for secretion and is not required for exocytosis itself.

Rat brain cytosol contains a factor which reconstitutes guanine-nucleotide-binding-protein-regulated phospholipase-D activation in HL60 cells previously permeabilized with streptolysin O

We report that guanosine 5'-[gamma-thio]triphosphate (GTP[S]) can stimulate phospholipase D (PLD) in HL60 cells acutely permeabilized with streptolysin O. The ability of GTP[S] to stimulate PLD is impaired if the cells are previously permeabilized such that the majority of the cytosol has leaked out. Rat brain and HL60 cytosols were both found to restore GTP[S]-stimulated PLD activity in a reconstitution assay consisting of previously permeabilized HL60 cells. Rat brain cytosol was fractionated on heparin agarose and assayed for reconstitution of GTP[S]-stimulated PLD activity. The active fractions were pooled, concentrated and chromatographed on gel filtration to assess its molecular mass. The molecular mass of the reconstituting factor was found to be 16 kDa. Reconstitution by the cytosolic factor was dependent on GTP[S]. Ca2+ (pCa 5), MgATP and MgCl2 enhanced GTP[S]-dependent reconstitution of PLD activity in the previously permeabilized HL60 cells. These results demonstrate the presence in rat brain cytosol of a factor which is an activator of GTP[S]-stimulated PLD.

G-proteins are not directly involved in the CD3-antigen-mediated production of inositol phosphates in HPB-ALL T-leukaemia cells expressing phospholipase C isoforms gamma 1 and beta 3

The possible involvement of G-proteins in T cell antigen-receptor complex (TCR)-mediated inositol phosphate production was investigated in HPB-ALL T-cells, which were found to express the phospholipase C gamma 1 and beta 3 isoforms. Cross-linking the CD3 antigen on streptolysin-O-permeabilized cells stimulated a dose-dependent increase in inositol phosphate production, as did addition of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) or vanadate, a phosphotyrosine phosphatase inhibitor. It was possible, therefore, that the CD3-antigen-mediated production of inositol phosphates was either via a G-protein-dependent mechanism or by stimulation of protein tyrosine phosphorylation. The CD3-induced inositol phosphate production was potentiated by addition of vanadate, but not by addition of GTP[S]. Guanosine 5'-[beta-thio]diphosphate (GDP[S]) inhibited the rise in inositol phosphates induced by GTP[S], vanadate or cross-linking the CD3 antigen. The increase in protein tyrosine phosphorylation stimulated by vanadate or the OKT3 monoclonal antibody was not observed in the presence of GDP[S], showing that in permeabilized HPB-ALL cells, GDP[S] inhibits the actions of tyrosine kinases as well as G-protein function. Addition of either ADP[S] or phenylarsine oxide inhibited CD3- and vanadate-mediated increases in both tyrosine phosphorylation and inositol phosphate production, but did not inhibit GTP[S]-stimulated inositol phosphate production. On the other hand, pretreatment of cells with phorbol 12,13-dibutyrate inhibited subsequent GTP[S]-stimulated inositol phosphate production but did not inhibit significantly inositol phosphate production stimulated by either OKT3 F(ab')2 fragments or vanadate. Our results are consistent with the CD3 antigen stimulating inositol phosphate production by increasing the level of protein tyrosine phosphorylation, but not by activating a G-protein.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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The superoxide-generating oxidase of phagocytic cells. Physiological, molecular and pathological aspects

Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O2-. The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were diassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O2- upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.