Crosstalk among multiple signal-activated phospholipases

Attenuation of diacylglycerol signal in the primordial human placenta: role of phosphatidylcholine formation

Metabolic conversion of the synthetic DAG, dioctanoylglycerol (DOCG) into the dioctanoyl species of phosphatidic acid (PADOCG) and phosphatidylcholine (PCDOCG) in minced human primordial placenta incubated with [32P]phosphate was studied. Time-course experiments performed with 0.25 mM DOCG revealed a much higher capacity of the placenta to synthetize PCDOCG than PADOCG and indicated the rapid metabolism of PCDOCG. In addition, DOCG stimulated the labeling of PC whereas no such effects on the labeling of lyso-PC and PA were observed. This effect of DOCG is not related to the synthesis of PC de novo, because DOCG did not exhibit any stimulating effect on the synthesis of PC from [3H]glycerol or [3H]glucose. Experiments with varying concentrations of DOCG (0.025-0.5 mM) showed increasing rate of formation of PCDOCG over a concentration range from 0.05 to 0.5 mM whereas 0.5 mM DOCG was the lowest concentration where significant formation of PADOCG was found. Labeling of PC was about 30% over control at 0.125 mM as well as at 0.25 mM DOCG, and decreased below the control level at 0.5 mM DOCG. Labeling of PA and PI was not influenced by these DOCG concentrations. Labeling of PC, either stimulated by DOCG or not, was more sensitive to the inhibitory effect of 1 mM of the DAG analog: dioctanoylethyleneglycol (DOEG) than that of PA. Moreover, 1 mM DOEG inhibited the formation of PCDOCG without such an effect on the labeling of PADOCG. These findings indicate that in the primordial human placenta PC synthesis represents a more efficient attenuation pathway of DAG signal than the formation of PA by DAG kinase.

Formation of diacylglycerol, inositol phosphates, arachidonic acid and its metabolites in macrophages

Treatment of macrophages with zymosan, 4 beta-phorbol 12-myristate 13-acetate (PMA) and fluoride but not with A 23187 or arachidonic acid (delta Ach) leads to a generation of diacylglycerol (acyl2Gro). Formation of inositol phosphates is achieved with zymosan, only. An elevation of intracellular calcium is obtained with zymosan and A 23187 but not with PMA, fluoride or delta Ach. Prior treatment of the cells with phorbol ester for 3 h which has been shown recently to result in a down-regulation of protein kinase (PK) C-beta but not PKC-delta [Duyster, J., Schwende, H., Fitzke, E., Hidaka H. & Dieter P. (1993) Biochem. J. 292, 203-207] has no effect on the zymosan-induced formation of acyl2Gro or inositol phosphates but inhibits the PMA-induced generation of acyl2Gro. Down-regulation of PKC-delta by prior phorbol ester treatment for 24 h augments the zymosan-induced generation of acyl2Gro and inositol phosphates. The acyl2Gro lipase inhibitor RG 80267 inhibits the PMA-induced and fluoride-induced generation of prostaglandin (PG) E2, reduces the zymosan-induced release of PGE2 by 50% but has no effect on PGE2 formation of unstimulated, A 23187-treated or delta Ach-treated cells. Furthermore, RG 80267 enhances accumulation of delta Ach-labeled acyl2Gro in response to zymosan, PMA and fluoride. These data indicate that zymosan activates a phosphatidylinositol 4,5-bisphosphate-specific phospholipase (PL) C, that generation of acyl2Gro by PMA and fluoride occurs via hydrolysis of other phospholipids, that PKC-beta is involved in the PMA-induced generation of acyl2Gro and PKC-delta negatively modulates the zymosan-induced activation of PLC and PMA and fluoride induce a liberation of delta Ach from acyl2Gro, A 23187 activates the PLA2 pathway and zymosan stimulates both, the acyl2Gro- and PLA2-pathway.

Phosphatidylcholine hydrolysis activates NF-kappa B and increases human immunodeficiency virus replication in human monocytes and T lymphocytes

We have tested whether breakdown of phosphatidylcholine (PC) initiated by exogenous addition of a PC-specific phospholipase C (PC-PLC) from Bacillus cereus or by endogenous overexpression of PC-PLC induces functional activation of NF-kappa B and increases human immunodeficiency virus (HIV) enhancer activity. PC-PLC-activated hydrolysis of PC was found to induce bona fide p50/p65 NF-kappa B binding activity in three different cell lines of human or murine origin. No significant changes in the turnover of other cellular phospholipids were detected in PC-PLC-treated cells. Induction of NF-kappa B by PC-PLC did not depend on de novo synthesis of proteins or autocrine secretion of either tumor necrosis factor or interleukin 1. In human monocytic and lymphoblastoid T-cell lines, induction of NF-kappa B by PC-PLC resulted in clear induction of luciferase expression vectors placed under the control of synthetic kappa B enhancers or wild type, but not kappa B-mutated, HIV long terminal repeat constructs. HIV replication was increased by PC-PLC in chronically infected monocytes and T lymphocytes. NF-kappa B activation promoted by addition of exogenous PC-PLC correlated with an intense production of diacylglycerol. However, addition of a phosphatidylinositol-specific PLC from B. cereus also induced diacylglycerol but did not activate kappa B enhancer-directed vectors. PC-PLC-induced NF-kappa B activation could not be blocked by a specific inhibitor of phorbol ester-inducible protein kinases C. These results indicate that a cellular transduction pathway, dependent on specific PC breakdown, is functional in T lymphocytes and monocytes and may be used by various transmembrane receptors to activate HIV transcription through NF-kappa B-dependent induction of the HIV enhancer.

Role of phosphoinositide hydrolysis in astrocyte volume regulation

Astrocytes exposed to hypoosmotic stress swell and subsequently reduce their size to almost their original volume, a phenomenon called regulatory volume decrease (RVD). We found that during hypoosmotic swelling there was a twofold increase in phosphatidylinositol (PI) hydrolysis. This increase was inhibited by the phospholipase C inhibitor, U-73122 (10 microM). Inhibition of PI hydrolysis resulted in blockage of RVD. We also examined whether agents that stimulate PI hydrolysis would enhance RVD. These agents significantly accelerated RVD. The rank order of potency was endothelin (20 nM) > or = norepinephrine (100 microM) > endothelin-3 (7 nM) > thrombin (1 U/ml) > or = ATP (500 microM) > bradykinin (20 microM) > or = carbachol (500 microM), as indicated by RVD rate constants. The extent of PI hydrolysis induced by these agents at the beginning of RVD exhibited a logarithmic relationship with the magnitude of RVD enhancement. Also, there was a linear relationship between the rate of PI hydrolysis and RVD rate constants. Our results suggest that stimulated PI hydrolysis is involved in the regulation of cell volume in astrocytes.

Glutamate activates phospholipase D in hippocampal slices of newborn and adult rats

Phospholipase D (PLD) is activated by many neurotransmitters in a novel signal transduction pathway. In the present work, PLD activity was studied comparatively in hippocampal slices of newborn and adult rats. Basal PLD activity in adult rats was almost three times higher than in newborn rats. In newborn rats, L-glutamate and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) time- and concentration-dependently enhanced the formation of [3H]phosphatidylpropanol ([3H]PP) and of [3H]phosphatidic acid in the presence of 2% propanol. N-Methyl-D-aspartate and kainate (both 1 mM) caused small, but significant increases (approximately 50%), whereas alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (100 microM) was ineffective. Maximally effective concentrations of glutamate (1 mM) and of 1S,3R-ACPD (300 microM) increased the PLD activity to almost 300% of basal activity; the EC50 values were 199 and 47 microM, respectively. Glutamate receptor antagonists, such as DL-2-amino-3-phosphonopropionic acid (AP3), DL-2-amino-5-phosphonovaleric acid, and kynurenate (all 1 mM) did not inhibit the glutamate-evoked increase of PP formation. In slices of adult rats, the response to 1S,3R-ACPD was significant, but small, whereas glutamate was effective only in the presence of the glutamate uptake inhibitor L-aspartate-beta-hydroxamate. It is concluded that glutamate activates PLD in rat hippocampus through an AP3-resistant metabotropic receptor. This effect is subject to ontogenetic development, with one important factor being glutamate uptake.

Effects of purinergic agents on human mononuclear phagocytes are differentiation dependent. Implications for atherogenesis

The differentiation-dependent expression of purinergic receptors for metabolically stable analogues of adenosine and ATP was studied in human mononuclear phagocytes (MNPs). Ligands of these receptors are able to modulate cellular cholesterol metabolism. In addition, the intracellular signal transduction pathways of the purinergic receptor system were examined. ATP gamma S, the metabolic stable analogue of ATP, was used as a P2 ligand, and 2-p-(2-carboxyethyl)phenylethylamino-5'-N-ethylcarboxamido adenosine (CGS 21680) and 5'-(N-ethylcarboxamido)adenosine (NECA) were used as P1 ligands in binding studies. Binding of [35S]ATP gamma S to MNPs at 4 degrees C revealed saturable low-affinity binding sites with a Kd of 868 +/- 52 nmol/L and Bmax of 7.3 +/- 0.4 pmol per 10(6) cells in 1-day cultured human MNPs and a Kd of 780 +/- 30 nmol/L and Bmax of 14.0 +/- 0.8 pmol per 10(6) cells in 7-day cultured human MNPs. The characterization of the P1 receptors on 1- and 7-day cultured human MNPs showed that they are expressed only on 7-day cultured human MNPs. The specific binding curve of the adenosine A2 receptor agonist [3H]CGS 21680 was biphasic, with a Kd1 of 33 +/- 15 nmol/L and a Kd2 of 90 +/- 10 nmol/L and with Bmax1 of 0.19 +/- 0.06 pmol per 10(6) cells and Bmax2 of 0.41 +/- 0.09 pmol per 10(6) cells, whereas NECA did not exhibit specific binding. The typical agonists for probing A1 receptor subtypes did not bind to 1- and 7-day cultured human MNPs, indicating that only A2 receptors are expressed on 7-day cultured human MNPs. ATP gamma S enhanced [Ca2+]i in 1- and 7-day cultured human MNPs in a concentration-dependent manner, whereas the P1 ligands, adenosine and CGS 21680, induced Ca2+ flux only in 7-day cultured MNPs. All three drugs increased intracellular cAMP levels in 7-day cultured human MNPs at a concentration of 10(-5) mol/L, whereas no effect was observed in 1-day cultured human MNPs. The uptake of fluorescently labeled acetylated low-density lipoprotein (LDL) in 7-day cultured human MNPs was inhibited by adenosine, CGS 21680, ATP, and ATP gamma S. No significant influence of these compounds was measured on the uptake of LDL, acetylated LDL, and high-density lipoprotein, in 1-day cultured MNPs. Our investigations indicate that the expression of P2y and A2 receptors is increased during differentiation of blood monocytes to macrophages.(ABSTRACT TRUNCATED AT 400 WORDS)

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.

Distinct mechanisms of phospholipase D activation and attenuation utilized by different mitogens in NIH-3T3 fibroblasts

The activation of phospholipase D (PLD) by platelet-derived growth factor (PDGF), prostaglandin F2 alpha and 12-O-tetradecanoylphorbol 13-acetate (TPA) was studied in NIH-3T3 fibroblasts. PLD activation was determined by measuring the production of both [3H]phosphatidic acid and [3H]phosphatidylpropanol (products of the PLD-catalyzed hydrolysis and transphosphatidylation reactions, respectively), in cells that were metabolically pre-labeled with [3H]oleic acid. All mitogens caused a rapid (within 2 min) activation of PLD. Activation of PLD by prostaglandin F2 alpha and PDGF was transient and declined to near basal levels by 15 min and 55 min, respectively. In contrast, TPA-induced activation of PLD was sustained for at least 60 min of incubation. A combination of maximally effective concentrations of PDGF and TPA stimulated PLD activity in a non-additive manner, while the effect of prostaglandin F2 alpha was additional to that of either PDGF or TPA. The protein kinase inhibitor staurosporine inhibited PLD activation by PDGF or TPA with almost identical dose/response curves. In contrast, staurosporine potentiated prostaglandin-F2 alpha-induced PLD activation. The specific protein kinase C inhibitor GF109203X (a bisindolylmaleimide) inhibited PLD activation by prostaglandin F2 alpha and PDGF at concentrations higher than those required for inhibition of PLD activation induced by TPA. Depletion of cellular protein kinase C abolished PLD activation by all three mitogens without affecting in vitro activity of membrane-bound PLD. The distinct kinetics of PLD activation and its differential susceptibility to protein kinase inhibitors suggest the existence of agonist-specific activation and/or inactivation mechanisms. The results indicate also that protein kinase C participates in the mechanism of PLD activation via PDGF, while the effect of prostaglandin F2 alpha involves a pathway independent of protein kinase C.

Potential role of phospholipase A2 in HL-60 cell differentiation to macrophages induced by protein kinase C activation

2-Lysophosphatidylcholine and cis-unsaturated fatty acids such as linoleic and linolenic acids, which are the products of the hydrolysis of phosphatidylcholine catalyzed by phospholipase A2 (EC 3.1.1.4), significantly potentiate the differentiation of HL-60 cells to macrophages that is induced by either a membrane-permeant diacylglycerol or a phorbol ester. The cell differentiation was assayed by measuring the expression of CD11b, one of the cell surface markers of macrophages, and also by the appearance of phagocytic activity. Snake venom phospholipase A2 added directly to the cells is also active for this potentiation. Neither lysophosphatidylcholine, fatty acid, nor phospholipase A2 is active unless a membrane-permeant diacylglycerol or a phorbol ester is present. The results presented provide further evidence that activation of phospholipase A2 may be intimately related to the signal transduction pathway through protein kinase C.

Phospholipase A2 activity and exocytosis of the ram sperm acrosome: regulation by bivalent cations

Previous work has shown that the sequence leading to exocytosis of the sperm acrosome involves at least three Ca(2+)-requiring processes, the first one probably represented by breakdown of the polyphosphoinositides and the final one by membrane fusion. We have investigated whether phospholipase A2 (PLA2) represents the intermediate Ca(2+)-requiring event by stimulating ram spermatozoa with the ionophore A23187 and various bivalent cations. Spermatozoa prelabelled with [14C]arachidonic acid and treated with ionophore and millimolar Ca2+ showed a considerable release of arachidonic acid; parallel sperm samples similarly treated underwent acrosomal exocytosis. Mn2+ was capable of completely substituting for Ca2+, even if residual Ca2+ in the system was chelated with EGTA: both arachidonic acid release and acrosomal exocytosis took place after treatment with A23187, EGTA and Mn2+. Neither Mg2+ nor Ba2+ promoted arachidonic acid release or exocytosis. The effects of Sr2+ were more complex and allowed us to probe the sequence of events leading to membrane fusion. Both arachidonic acid release and exocytosis occurred after treatment with A23187 and Sr2+ but none of these responses were seen if EGTA was also included. These results suggest that residual micromolar Ca2+ is either needed for Sr2+ to fully promote PLA2 activity, or that micromolar Ca2+ is needed for one or more upstream events that may in turn serve to activate PLA2. Evidence for or against the first possibility was sought by examining PLA2 activity in sperm sonicates. Enzyme activity was maximal in the presence of any bivalent cation and it was not reduced (in the case of Sr2+) or only reduced slightly (Mg2+, Mn2+, Ba2+) if residual Ca2+ was chelated with EGTA; this indicates that Sr2+ can promote PLA2 activity in the total absence of Ca2+. The second possibility was explored by treating spermatozoa with A23187 for 5 min (to allow for complete phosphoinositide breakdown; Roldan and Harrison (1989) Biochem. J. 259, 397-406), and then adding EGTA and Sr2+. This resulted in neither arachidonic acid release nor exocytosis, thus indicating that another as yet unidentified Ca(2+)-dependent event may occur before PLA2 activation.

Signal transduction by Fc receptors: the Fc epsilon RI case

The family of proteins collectively known as Fc receptors (FcR) plays a variety of roles both in the initiation of the immune response and in its consequences. During the past five years the structure of these proteins and the genes that code for them have been largely elucidated. The most unexpected finding has been their extensive diversity. Considerable efforts are now being expended to define the molecular events initiated by these various FcR and these events are the focus of our review.

Vasopressin, unlike phorbol ester, fails to synergistically interact with pituitary adenylate cyclase activating polypeptide (PACAP) in stimulating cyclic AMP formation and ACTH secretion in cultured anterior pituitary cells

In an attempt to determine if PACAP synergistically interacts with vasopressin (VP) and protein kinase C (PKC) to enhance cyclic AMP formation and adrenocorticotrophic hormone (ACTH) secretion, the effects of PACAP, either alone or together with VP and the phorbol ester phorbol 12-myristate 13-acetate (PMA) were examined in primary cultures of rat anterior pituitary cells. VP failed to potentiate the stimulatory effect of PACAP on cyclic AMP formation, while it dramatically enhanced the effect of corticotropin-releasing factor (CRF). However, activation of PKC upon exposure of cells to PMA amplified cyclic AMP production induced by both peptides, though in the case of PACAP, contrary to that of CRF, potentiation was markedly dependent on the blockade of phosphodiesterase (PDE) activity, for it was undetectable in the absence of the inhibitor Rolipram. Depletion of PKC by long-term treatment of pituitary cells with PMA abolished the synergistic influence of PMA. There was no significant effect of PACAP, either alone or together with PMA, on ACTH secretion, while PMA enhanced peptide secretion elicited by CRF. The data show that in anterior pituitary cells cyclic AMP accumulation induced by PACAP and CRF was differentially modulated by PKC and PDE activities and that the potentiation of PACAP-stimulated cyclic AMP accumulation by PMA was not reflected by parallel increment of ACTH secretion.

A novel GPI-anchored glycoprotein shows properties of a coohterminus truncated cadherin involved in cell-cell contacts of cultured human epithelial cells

We report the characterization of a cell-cell adhesion molecule identified by mAbBD31 on human epithelioid cells. This glycoprotein shows cadherin features but appears to be truncated being devoid of the transmembrane and cytoplasmic domains. It is directly membrane-anchored by a GPI moiety. We propose to call this molecule HT-cadherin.

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.