Requirement of phospholipase C-catalyzed hydrolysis of phosphatidylcholine for maturation of Xenopus laevis oocytes in response to insulin and ras p21

Signaling events required for transforming growth factor-beta stimulation of connective tissue growth factor expression by cultured human lung fibroblasts

It is possible that many of the fibrogenic effects of transforming growth factor-beta (TGF-beta) are mediated by connective tissue growth factor (CTGF). In the present work, we show that TGF-beta1 produces a 5- to 6-fold increase in CTGF expression by cultured human lung fibroblasts that is due mainly to increased transcription. The half-life of CTGF mRNA is 1.96 h, consistent with its role as a cytokine. In addition to requiring Smad activity, based upon the effects of specific inhibitors, the TGF-beta intracellular signaling pathway requires the activity of a phosphatidylcholine-specific phospholipase C, a protein kinase C, and one or more tyrosine kinases. It is also likely that the pathway requires a member of the Ras superfamily of small GTPases, but not trimeric G proteins. Pharmacologic inhibition of TGF-beta stimulation of CTGF expression may be an effective therapeutic approach to a variety of undesirable fibrotic reactions.

Phosphatidylcholine-specific phospholipase C-mediated induction of phospholipase D activity in Fas-expressing murine cells

We have previously reported that Fas cross-linking resulted in the activation of phosphatidylcholine-specific phospholipase C (PC-PLC) and the subsequent activation of protein kinase C (PKC) and phospholipase D (PLD) in A20 cells. In an attempt to correlate the existence of PC-PLC activity and activation of PLD by Fas activation among various Fas-expressing murine cell lines, we have investigated the effect of anti-Fas monoclonal antibody on PC-PLC and PLD activities in A20, P388D1 and YAC-1 cell lines. Upon treatment of anti-Fas monoclonal antibody to these three cell lines, the activation of PLD was only observed in A20 cells. When the effect of anti-Fas monoclonal antibody on PKC and PC-PLC activities in Fas-expressing clones were investigated, the activation of PKC and PC-PLC was detected only in A20 clones. Results presented here also show that exogenous addition of Bacillus cereus PC-PLC activates PC hydrolysis, PKC and PLD in all three murine cell lines. These findings suggest that the activation of PC-PLC is a necessary requirement for the activation of PLD by Fas cross-linking and cell lines devoid of functional PC-PLC activity could exhibit enhanced PLD activity by exogenous addition of PC-PLC.

TGF-beta1 stimulation of fibronectin transcription in cultured human lung fibroblasts requires active geranylgeranyl transferase I, phosphatidylcholine-specific phospholipase C, protein kinase C-delta, and p38, but not erk1/erk2

The cytokine transforming growth factor-beta (TGF-beta) has multiple effects on a variety of cell types, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. In the present work, we demonstrate that TGF-beta1 produces a fourfold increase in transcription of the fibronectin gene in cultured human fetal lung fibroblasts with only a small increase in mRNA stability resulting in a significant increase in fibronectin mRNA steady state level. A corresponding increase in production of fibronectin protein accompanied the increase in mRNA. Through the use of specific inhibitors, we demonstrate that geranylgeranylated, but not farnesylated or acylated protein(s), protein kinase C-delta, phosphatidylcholine-specific phospholipse C, tyrosine kinase activity, and stress-activated protein kinase p38 are required for this TGF-beta1 effect. Trimeric G proteins and mitogen-activated protein kinases erk1 and erk2 do not appear to be involved. While these results emphasize the complexities involved in the control of extracellular matrix synthesis by TGF-beta, they also identify reaction sites that may be amenable to pharmacologic modulation. Such modulation could be of great advantage in the treatment of a wide variety of undesirable fibrotic reactions.

Tumour phospholipid metabolism

Following the impetus of early clinical and experimental investigations, in vivo and in vitro MRS studies of tumours pointed in the eighties to the possible significance of signals arising from phospholipid (PL) precursors and catabolites as novel biochemical indicators of in vivo tumour progression and response to therapy. In the present decade, MRS analyses of individual components contributing to the 31P PME (phosphomonoester) and PDE (phosphodiester) resonances, as well as to the 1H 'choline peak', have reinforced some of these expectations. Moreover, the absolute quantification of these signals provided the basis for addressing more specific (although still open) questions on the biochemical mechanisms responsible for the formation of intracellular pools of PL derivatives in tumours, under different conditions of cell proliferative status and/or malignancy level. This article is aimed at providing an overview on: (a) quantitative MRS measurements on the contents of phosphocholine (PCho), phosphoethanolamine (PEtn) and their glycerol derivatives ģlycerol 3-phosphocholine (GPC) and glycerol 3-phosphoethanolamine (GPE)[ in human tumours and cells (with particular attention to breast and brain cancer and lymphomas), as well as in normal mammalian tissues (including developing organs and rapidly proliferating tissues); (b) possible correlations of MRS parameters like PEtn/PCho and PCho/GPC ratios with in vitro cell growth status and/or cell tumorigenicity; and (c) current and new hypotheses on the role and interplay of biosynthetic and catabolic pathways of the choline and ethanolamine cycles in modulating the intracellular sizes of PCho and PEtn pools, either in response to mitogenic stimuli or in relation to malignant transformation.

Signaling pathway by which TGF-beta1 increases expression of latent TGF-beta binding protein-2 at the transcriptional level

The cytokine transforming growth factor-beta has multiple effects on a wide variety of cell types. These effects include modulation of growth and regulation of gene transcription. In the present work, we demonstrate that TGF-beta1 increases transcription of the latent transforming growth factor-beta binding protein-2 ( LTBP-2) gene in cultured human fetal lung fibroblasts leading to a significant increase in LTBP-2 mRNA steady state level. The stability of LTBP-2 mRNA was not appreciably altered. A corresponding increase in production of LTBP-2 protein accompanied the increase in mRNA. Through the use of specific inhibitors, we demonstrate that a member of the Ras super family and a protein kinase C, probably of the atypical (non-diacylglycerol, non-Ca++ dependent) class are likely to be components in the signaling pathway. However, phospholipases, G proteins and extracellular-signal regulated kinases do not appear to be involved. These results combined with previous findings on elastin regulation by TGF-beta1 (Kucich et al. (1997). Am. J. Respir. Cell Mol. Biol., 17: 10-16) demonstrate that TGF-beta1 can coordinately increase the steady state levels of mRNAs encoding components of the elastic fiber, but through diverse mechanisms. In contrast to LTBP-2, increased elastin expression is achieved by message stabilization. Furthermore, the TGF-beta1 signaling pathways differ and while the pathway leading to increased LTBP-2 transcription shares components with those modulating transcription of other genes, it is unlikely to be precisely congruent with any other previously described one.

Requirement for geranylgeranyl transferase I and acyl transferase in the TGF-beta-stimulated pathway leading to elastin mRNA stabilization

The TGF-betas are multipotent in their biological activity, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. Most of these activities involve modulation of gene transcription. However, TGF-beta1 has been shown previously to substantially increase the expression of elastin by stabilization of tropoelastin mRNA through a signaling pathway which involves a phosphatidylcholine-specific phospholipase and a protein kinase C. The present results, through the use of specific inhibitors of geranylgeranyl transferase I, farnesyl transferase, and acyl transferase, demonstrate that geranylgeranylated and acylated, but not farnesyslated protein(s) is required for this TGF-beta1 effect. In addition, the general tyrosine kinase inhibitor genistein completely blocked this TGF-beta1 effect. The results suggest that the TGF-beta1 signaling pathway requires not only receptor ser/thr kinase activity, but also tyrosine kinase and small GTPase activities.

Interfacial catalysis by phospholipases at conjugated lipid vesicles: colorimetric detection and NMR spectroscopy

BACKGROUND

Self-assembled conjugated polymers are rapidly finding biological and biotechnological applications. This work describes a synthetic membrane system based on self-assembled polydiacetylenes, which are responsive to the enzymatic activity of phospholipases - a ubiquitous class of enzymes that catalyze the hydrolysis of phospholipid molecules embedded in cell membranes.

RESULTS

We show that phospholipases are active at bilayer vesicles composed of the natural enzyme substrate, dimyristoylphosphatidylcholine (DMPC), and a synthetic pi-conjugated polymerized lipid based on polydiacetylene (PDA). In addition, the enzymatic reaction induces an optical transition in the surrounding PDA matrix, visible to the naked eye. Nuclear magnetic resonance spectroscopy confirms the occurrence of enzymatic catalysis and reveals the fate of the cleavage products.

CONCLUSIONS

The results indicate that the structural and color changes of the PDA matrix are directly related to interfacial catalysis by phospholipase. This novel biocatalytic method of inducing optical transitions in conjugated polymers might lead to new approaches towards rapidly screening new enzyme inhibitor compounds.

Engineering of the nonspecific phospholipase C from Bacillus cereus: replacement of glutamic acid-4 by alanine results in loss of interfacial catalysis and enhanced phosphomonoesterase activity

The nonspecific phospholipase C from Bacillus cereus is a zinc metalloenzyme that catalyzes the hydrolysis of phospholipids to yield diacylglycerol and a phosphate monoester. Glu-4 has been proposed as a potential candidate for the general base in the hydrolysis reaction and was shown to interact with the substrate headgroup. Site-specific mutagenesis studies suggest that Glu-4 is important for substrate binding but not for catalysis. This residue is also critical for the enzyme's preference for a phosphodiester substrate. PA, both monomeric and micellar, is shown to be a poor substrate and inhibitor of wild-type PLC. When Glu-4 was mutated to an alanine, a significant increase in PA hydrolysis and a decrease in PC hydrolysis were observed. Unlike the wild type, kinetic studies suggest that the Glu-4-->Ala mutant does not exhibit interfacial activation and processive catalysis. Glu-4 is part of a highly flexible loop flanking the entrance to the active site, suggesting that this loop might constitute an interfacial binding recognition site. This is the first evidence for the presence of an interfacial binding site distinct from the active site in the nonspecific PLC.

Ethanolamine, but not phosphoethanolamine, potentiates the effects of insulin, phosphocholine, and ATP on DNA synthesis in NIH 3T3 cells--role of mitogen-activated protein-kinase-dependent and protein-kinase-independent mechanisms

NIH 3T3 fibroblasts express a phospholipase D activity hydrolyzing phosphatidylethanolamine (PtdEtn) which produces ethanolamine (Etn) in response to a variety of growth regulating agents. The main objective of this work was to evaluate the effects of Etn on mitogenesis and to determine whether these effects require its metabolism to phosphoethanolamine (PEtn) or PtdEtn. To increase conversion of Etn to PEtn, an Etn-specific kinase derived from Drosophila was highly expressed in NIH 3T3 cells. Overexpression of this Etn kinase resulted in large (10-12.5-fold) increases in PEtn formation, but only in modest (1.2-1.7-fold) increases in PtdEtn synthesis. In both vector control and Etn kinase overexpressor cells, Etn had biphasic effects on insulin-induced DNA synthesis with maximal (approximately 2-fold) potentiating effects being observed at 0.5-1 mM concentrations, followed by an inhibitory phase at higher Etn concentrations. In the Etn kinase overexpressor lines, the inhibitory phase was elicited by lower Etn concentrations and it was partially blocked by 5 mM choline due to decreased formation of PEtn. In both vector control and Etn kinase overexpressor cells, phosphocholine (PCho) and insulin synergistically stimulated DNA synthesis; their effects were further enhanced by physiologically relevant (5-60 microM) concentrations of Etn by a mechanism independent of mitogen-activated protein (MAP) kinase. Concentrations of Etn >50 microM also enhanced the effects of both PCho and the synergistic effects of PCho plus ATP; however, in the latter case 20 microM Etn was inhibitory. The magnitude of both the potentiating and inhibitory effects of Etn on PCho-induced as well as PCho + ATP-induced DNA synthesis were similar in the vector control and Etn kinase overexpressor cells; they were associated with stimulation and inhibition, respectively, of p42 MAP kinase activity. The results indicate that in NIH 3T3 cells Etn exerts significant effects on DNA synthesis which, except inhibition of insulin-induced DNA synthesis by higher concentrations of Etn, do not correlate with the metabolism of Etn to PEtn or PtdEtn.

Substrate binding and catalytic mechanism in phospholipase C from Bacillus cereus: a molecular mechanics and molecular dynamics study

For the first time a consistent catalytic mechanism of phospholipase C from Bacillus cereus is reported based on molecular mechanics calculations. We have identified the position of the nucleophilic water molecule, which is directly involved in the hydrolysis of the natural substrate phosphatidylcholine, in phospholipase C. This catalytically essential water molecule, after being activated by an acidic residue (Asp55), performs the nucleophilic attack on the phosphorus atom in the substrate, leading to a trigonal bipyramidal pentacoordinated intermediate (and structurally similar transition state). The subsequent collapse of the intermediate, regeneration of the enzyme, and release of the products has to involve a not yet identified second water molecule. The catalytic mechanism reported here is based on a series of molecular mechanics calculations. First, the x-ray structure of phospholipase C from B cereus including a docked substrate molecule was subjected to a stepwise molecular mechanics energy minimization. Second, the location of the nucleophilic water molecule in the active site of the fully relaxed enzyme-substrate complex was determined by evaluation of nonbonded interaction energies between the complex and a water molecule. The nucleophilic water molecule is positioned at a distance (3.8 A) from the phosphorus atom in the substrate, which is in good agreement with experimentally observed distances. Finally, the stability of the complex between phospholipase C, the substrate, and the nucleophilic water molecule was verified during a 100 ps molecular dynamics simulation. During the simulation the substrate undergoes a conformational change, but retains its localization in the active site. The contacts between the enzyme, the substrate, and the nucleophilic water molecule display some fluctuations, but remain within reasonable limits, thereby confirming the stability of the enzyme-substrate-water complex. The protocol developed for energy minimization of phospholipase C containing three zinc ions located closely together at the bottom of the active site cleft is reported in detail. In order to handle the strong electrostatic interactions in the active site realistically during energy minimization, delocalization of the charges from the three zinc ions was considered. Therefore, quantum mechanics calculations on the zinc ions and the zinc-coordinating residues were carried out prior to the molecular mechanics calculations, and two different sets of partial atomic charges (MNDO-Mulliken and AMI-ESP) were applied. After careful assignment of partial atomic charges, a complete energy minimization of the protein was carried out by a stepwise procedure without explicit solvent molecules. Energy minimization with either set of charges yielded structures, which were very similar both to the x-ray structure and to each other, although using AMI-ESP partial atomic charges and a dielectric constant of 4, yielded the best protein structure.

ATP-dependent choline phosphate-induced mitogenesis in fibroblasts involves activation of pp70 S6 kinase and phosphatidylinositol 3'-kinase through an extracellular site. Synergistic mitogenic effects of choline phosphate and sphingosine 1-phosphate

In serum-starved NIH 3T3 clone 7 fibroblasts, choline phosphate (ChoP) (0.5-1 mM) and insulin synergistically stimulate DNA synthesis. Here we report that ATP also greatly enhanced the mitogenic effects of ChoP (0.1-1 mM) both in the absence and presence of insulin; maximal potentiating effects required 50-100 microM ATP. The co-mitogenic effects of ATP were mimicked by adenosine 5'-O-(3-thiotriphosphate), adenosine 5'-O-(2-thiodiphosphate), ADP, and UTP, but not by AMP or adenosine, indicating the mediatory role of a purinergic P2 receptor. Externally added ChoP acted on DNA synthesis without its detectable uptake into fibroblasts, indicating that ChoP can be a mitogen only if it is released from cells. Extracellular ATP (10-100 microM) induced extensive release of ChoP from fibroblasts. ChoP had negligible effects, even in the presence of ATP or insulin, on the activity state of p42/p44 mitogen-activated protein kinases, while in combination these agents stimulated the activity of phosphatidylinositol 3'-kinase (PI 3'-kinase). Expression of a dominant negative mutant of the p85 subunit of PI 3'-kinase or treatments with the PI 3'-kinase inhibitor wortmannin only partially (approximately 40-50%) reduced the combined effects of ChoP, ATP, and insulin on DNA synthesis; in contrast, the pp70 S6 kinase inhibitor rapamycin almost completely inhibited these effects. ATP and insulin also potentiated, while rapamycin strongly inhibited, the mitogenic effects of sphingosine 1-phosphate (S1P). Furthermore, even maximally effective concentrations of ChoP and S1P synergistically stimulated DNA synthesis. The results indicate that in the presence of extracellular ATP and/or S1P, ChoP induces mitogenesis through an extracellular site by mechanisms involving the activation of pp70 S6 kinase and, to a lesser extent, PI 3'-kinase.

The dominant negative effects of H-Ras harboring a Gly to Ala mutation at position 60

v-H-Ras harboring the Gly-60 to Ala mutation (G60A) lacks the ability to induce germinal vesicle breakdown in Xenopus oocytes. Moreover, this mutant is capable of inhibiting the activity of v-H-Ras to induce oocyte germinal vesicle breakdown when co-injected. The duration and the extent of inhibition depends on the molar ratio of v-H-Ras(G60A) to v-H-Ras. The inhibition is not due to a general toxicity of v-H-Ras(G60A) to oocytes because oocytes injected with v-H-Ras(G60A) can be readily induced to mature by other mitogenic agents, such as insulin, insulin-like growth factor 1, insulin-like growth factor 2, and phosphatidylcholine-specific phospholipase C. The dominant negative effect of v-H-Ras(G60A) requires proper membrane attachment of v-H-Ras(G60A). By using a competition assay, it was concluded that the dominant negative phenotype of v-H-Ras(G60A) resulted from sequestering H-Ras downstream effector(s). Raf-1 was identified as one of the sequestered targets.

Analysis of R59022 actions in Xenopus laevis oocytes

R59022, a diacylglycerol (DAG) kinase inhibitor, stimulated meiotic maturation of Xenopus laevis oocytes when applied extracellularly. The time course of R59022-induced oocyte maturation was proportional to the concentration of R59022 in the low micromolor range, and the 30 microM-induced response was a fast or faster than progesterone-induced maturation. Dose-response analysis yielded an apparent EC50 for R59022-induced oocyte maturation of approximately 15 microM. An increase in total oocyte DAG levels was observed following treatment with 10 microM R59022. Treatment of oocytes with R59022 also resulted in a significant increase in intracellular pH similar to the increase observed with progesterone. When various phosphodiesterase (PDE) inhibitors were tested for their effects on R59022-induced oocyte maturation, papaverine (a potent nonselective inhibitor of PDE) and CI-930 (a selective PDE III inhibitor) were observed to significantly inhibit the R59022-stimulated response. The sensitivity of R59022-induced oocyte maturation to inhibition by papaverine was intermediate between the sensitivities of the IGF-1- or progesterone-induced responses. Treatment of oocytes with R59022 did not significantly affect the level of oocyte PDE activity measured in vivo, suggesting that elevated levels of DAG may parallel observed increases in PDE but do not directly lead to a stimulation of PDE. The t ime course for stimulation of ribosomal S6 kinase activity by R59022 followed the pattern for stimulation of ribosomal S6 kinase activity by R59022 followed the pattern for stimulation by progesterone rather than IGF-1. Treatment of isolated membranes with R59022 resulted in inhibition of membrane-associated adenyl cyclas e activity that was not mimicked by DAG analogs. Thus, in addition to elevating oocyte levels of DAG, R59022 also has steroid-like actions.

Alterations in levels of different protein kinase C isotypes and their influence on behavior of squamous cell carcinoma of the oral cavity: epsilon PKC, a novel prognostic factor for relapse and survival

BACKGROUND

Recent results suggest that some PKC isotypes, when overexposed, confer to cultured fibroblasts certain proliferative advantages, and enhanced tumorigenicity in nude mice, suggesting their participation in carcinogenic process. These findings need to be validated through the investigation of potential alterations of these kinases in common forms of human cancers.

MATERIAL AND METHODS

In this prospective study we determined levels of different PKC isozymes by Western blot in tissue extracts from 29 human primary squamous cell carcinomas of the oral cavity, and their respective controls. These expressions were correlated with behavior of tumor and histologic characteristics.

RESULTS

Dramatic alterations in different PKC isotypes were found. Thus, increased levels of isotypes alpha, beta, or gamma, and zeta were found in most of the patients, as well as significant correlations between levels of the isotype epsilon and survival-relapse rate and classical PKC isotypes with irregular morphology of tumoral interphase.

CONCLUSIONS

These results suggest participation of some PKC isotypes (alpha, beta, gamma, and zeta) in the genesis and behavior (epsilon) of oral cancers. Levels of epsilon PKC could be used as prognostic marker.

Evidence for a bifurcation of the mitogenic signaling pathway activated by Ras and phosphatidylcholine-hydrolyzing phospholipase C

NIH 3T3 cells stably transfected with the gene encoding phosphatidylcholine-hydrolyzing phospholipase C (PC-PLC) from Bacillus cereus display a chronic elevation of intracellular diacylglycerol levels and a transformed phenotype. We have used such PC-PLC-transformed cells to evaluate the roles of the cytoplasmic serine/threonine kinases Raf-1, zeta protein kinase C (zeta PKC) and protein kinase A (PKA) in oncogenesis and mitogenic signal transduction elicited by phosphatidylcholine hydrolysis. We demonstrate here that stable expression of dominant negative mutants of both zeta PKC and Raf-1 lead to reversion of PC-PLC-transformed cells. Interestingly, expression of kinase defective zeta PKC also reverted NIH 3T3 cells transformed by the v-Ha-ras oncogene. Activation of PKA in response to elevation of cAMP levels also lead to reversion of PC-PLC-induced transformation, implicating PKA as a negative regulator acting downstream of PC-PLC. On the other hand, inhibition or depletion of phorbol ester responsive PKCs attenuated but did not block the ability of PC-PLC-transformed cells to induce DNA synthesis in the absence of growth factors. These results clearly implicate both Raf-1 and zeta PKC as necessary downstream components for transduction of the mitogenic/oncogenic signal generated by PLC-mediated hydrolysis of phosphatidylcholine and suggest, together with other recent evidence, a bifurcation in the signaling pathway downstream of PC-PLC.

Effect of insulin on SN-1,2-diacylglycerol species and de novo synthesis in rat skeletal muscle

Insulin treatment increases the SN-1,2-diacylglycerol (DAG) concentration in skeletal muscle. Because DAG may participate in transmission or modulation of the insulin receptor signal, we examined the effect of insulin on total DAG and on different DAG species in isolated rat hemidiaphragms incubated with 5 mmol/L glucose. Five DAG species (16:0-18:1 omega 9, 16:0-18:1 omega 7, 18:0-18:1 omega 9, 18:0-18:2 omega 6, and 18:1-18:2) were identified and quantified. After a 5-minute incubation with 60 nmol/L insulin, neither total DAG nor a DAG species increased; exposure to insulin for 10 or 20 minutes increased the concentration of total DAG and of several DAG species. Insulin did not increase DAG in muscles incubated without glucose. Two sources for the insulin-mediated DAG increase were considered: phosphatidylcholine (PC) hydrolysis and de novo DAG synthesis from glucose. Concentrations of choline and phosphocholine in muscle were not increased after 10-minute incubations with insulin. However, insulin increased 14C incorporation from [U-14C]glucose into DAG, triacylglycerol (TAG), and total lipids approximately threefold. Okadaic acid (OKA), an inhibitor of phosphoprotein phosphatases 1 and 2A, increased muscle DAG content and synthesis from glucose, similar to the effect of insulin. Doses of OKA or insulin that increased DAG mass greatly exceeded those required for stimulation of glucose transport. The insulin-mediated, relatively slow increase in muscle DAG observed here likely reflects primarily de novo synthesis from glucose. This effect would be downstream of insulin stimulation of glucose transport. However, a possible insulin-mediated, rapid transient increase in muscle DAG content and PC hydrolysis cannot be ruled out by our studies.

Second messenger signalling during hormone-induced Xenopus oocyte maturation

Although much information about such processes as cell cycle control, second messenger systems, protein kinases and steroid hormone action has been collected from studies of Xenopus oocyte maturation, we still have very little idea about how the steroid hormone, progesterone, signals the resumption of meiosis from the oocyte plasma membrane. In this review we re-examine the data on second messenger systems in Xenopus oocytes and discuss some of the unresolved questions about hormone signal transduction during maturation. We outline some reasons for the contradictions in the literature and offer some suggestions for avenues of future research.

Purification and properties of zinc-metallophospholipase C from Pseudomonas fluorescens

Phospholipase C produced by Pseudomonas fluorescens, isolated as a laboratory contaminant, has been purified to apparent homogeneity by ammonium sulphate fractionation, anion-exchange and size-exclusion chromatographies. The apparent molecular mass of the purified polypeptide was 39.5 kDa. Purified preparations of phospholipase C were used to characterize its enzymic properties and to obtain amino acid sequence of the N-terminus of the molecule. The P. fluorescens phospholipase C hydrolysed PtdEtn, PtdCho and PtdSer (PtdEtn > PtdCho >> PtdSer) and was relatively thermostable. The enzyme was inactivated in the presence of chelating agent o-phenanthroline and the activity restored after addition of zinc. Properties of this enzyme and in particular the requirement for zinc ions for the activity, revealed similarity with the well characterised Bacillus cereus phospholipase C. Similarities with other bacterial and mammalian enzymes reported to be related to the B. cereus type are discussed.

ras-p21 activates phospholipase D and A2, but not phospholipase C or PKC, in Xenopus laevis oocytes

Xenopus laevis oocytes are a powerful tool for the characterization of signal transduction pathways leading to the induction of DNA synthesis. Since activation of PLA2, PLC, or PLD has been postulated as a mediator of ras function, we have used the oocyte system to study the putative functional relationship between ras-p21 and these phospholipases. A rapid generation of PA and DAG was observed after ras-p21 microinjection, suggesting the activation of both PLC and PLD enzymes. However, production of DAG was sensitive to inhibition of the PA-hydrolase by propranolol, indicating that PLD is the enzyme responsible for the generation of both PA and DAG. Microinjection of PLD or ras-p21 induced the late production of lysophosphatidylcholine on a p42MAPK-dependent manner, an indication of the activation of a PLA2. Inhibition of this enzyme by quinacrine does not inhibit PLD- or ras-induced GVBD, suggesting that PLA2 activation is not needed for ras or PLD function. Contrary to 3T3 fibroblasts, where ras-p21 is functionally dependent for its mitogenic activity on TPA- and staurosporine-sensitive PKC isoforms, in Xenopus oocytes, induction of GVBD by ras-p21 was independent of PKC, while PLC-induced GVBD was sensitive to PKC inhibition. Thus, our results demonstrate the activation of PLD and PLA2 by ras-p21 proteins, while no effect on PLC was observed.

Protein kinase C zeta isoform is critical for mitogenic signal transduction

The requirement of protein kinase C zeta (zeta PKC) for maturation of X. laevis oocytes in response to insulin, p21ras, and phosphatidylcholine-hydrolyzing phospholipase C has recently been shown. Here we present experimental evidence demonstrating that activation of zeta PKC is not only necessary but also sufficient by itself to activate maturation in oocytes and to produce deregulation of growth control in mouse fibroblasts. Furthermore, by using a dominant kinase-defective mutant of zeta PKC, we confirm that this kinase is required for mitogenic activation in oocytes and fibroblasts. These results permit us to propose zeta PKC as a critical step downstream of p21ras in mitogenic signal transduction.

Phospholipase-induced maturation of Xenopus laevis oocytes: mitogenic activity of generated metabolites

Signal transduction induced by generation of second messengers from membrane phospholipids is considered a major regulatory mechanism in control of cell proliferation. We report here that in the Xenopus laevis oocytes model, microinjection of the three most relevant types of phospholipases acting on membrane phospholipids (A2, C, and D) are capable of inducing oocyte maturation with similar efficiencies. This effect is mediated by the generation of known second messengers such as lyso-phospholipids, arachidonic acid, diacylglycerol, and phosphatidic acid. Specific inhibitors of protein kinase C made it possible to identify alternative independent signalling pathways for induction of oocyte maturation. Our results indicate that while phospholipase C seems to be dependent on protein kinase C (PKC), phospholipase A2, and phospholipase D are completely independent of protein kinase C function. Thus, the oocyte system is a powerful tool for the analysis of the potential mitogenic activity of lipid metabolites. It is also an excellent tool for unravelling the different routes involved in the regulation of cell growth.

Hepatic insulin resistance in KKA(y) mice and its amelioration by pioglitazone do not involve alterations in phospholipase C activity

It has been proposed that an abnormality in the regulation of cytosolic-free Ca2+ may be the cause of some forms of insulin resistance. In support of this proposition, it was reported that phospholipase C-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by liver plasma membranes from obese patients with non-insulin-dependent diabetes mellitus (NIDDM) was abnormally augmented (Thakker et al., J. Biol. Chem. 264, 7169-7175). The objective of this investigation was to determine if a novel antidiabetic agent, pioglitazone, ameliorated hepatic insulin resistance in KKA(y) mice and to identify any alterations in PIP2-phospholipase C activity of liver plasma membranes that may accompany changes in insulin sensitivity. Treatment of KKA(y) mice for 4 days with pioglitazone (20 mg/kg per day) decreased blood glucose and insulin and improved a variety of indices of hepatic insulin resistance, but did not alter the rate of PIP2 hydrolysis by liver plasma membranes. Acute treatment of isolated liver plasma membranes with pioglitazone (1-100 microM) also failed to alter PIP2-phospholipase C activity. Furthermore, the specific activity, Ca(2+)-requirement, pH-dependence and sensitivity to guanosine 5'-thiotriphosphate of the PIP2-phospholipase C in KKA(y) liver membranes were indistinguishable from those of C57BL/6J (normal) mice. Among C57BL/6J and KKA(y) mice fed either a control or pioglitazone-supplemented diet, there was no correlation between PIP2-phospholipase C activity in isolated liver membranes and either glucose or insulin concentrations in the circulation. These data indicate that an alteration in PIP2-phospholipase C activity of liver plasma membranes is neither a cause nor an obligatory consequence of insulin resistance in KKAy mice or its amelioration by pioglitazone. Alterations of liver membrane phospholipase C activity in NIDDM, therefore, may reflect diabetic pathology other than the insulin resistance associated with this disease.

Protein kinase C-dependent stimulation of phospholipase D in phospholipase C-treated fibroblasts

Treatment of [14C]choline- or [14C]ethanolamine-labeled NIH 3T3 fibroblasts with Bacillus cereus phosphatidyl-choline-specific phospholipase C (PLC) enhanced phospholipase D (PLD)-mediated hydrolysis of the respective 14C-labeled phospholipids. PLD activity was stimulated by 1.5 U/mL of PLC and by 100 nM of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) to similar extents. Treatment of [14C]palmitic acid-labeled fibroblasts with PLC in the presence of ethanol also enhanced PLD-mediated formation of phosphatidylethanol; the effects of PLC and PMA were nonadditive. PLC had no effect on PLD activity in fibroblasts in which PKC was down-regulated by prolonged (24 h) treatment with 300 nM PMA. These data indicate that treatment of fibroblasts with exogenous PLC results in PKC-dependent activation of PLD.

Involvement of G proteins, cytoplasmic calcium, phospholipases, phospholipid-derived second messengers, and protein kinases in signal transduction from mitogenic cell surface receptors

Some putative mitogenic signal transduction mechanisms involving G proteins, calcium, phospholipases, and protein kinases have been discussed. Several elements in this signal transduction scheme are not yet well understood and require further experimental investigation. With regard to the heptahelix receptors, exactly how do they activate PLA2? Is PLA2 activation linked to mitogenic pathways? Is this via stimulation of protein kinase C or perhaps another mechanism? How do heptahelix receptors activate tyrosine phosphorylation, and is it important in their ability to stimulate cell growth? With regard to the various phospholipases that are thought to be regulated by receptor-mediated stimuli, only PI-PLC beta and PI-PLC gamma are well characterized. PLA2, PC-PLD, and PC-PLC require further study in regard to determination of molecular structure and elucidation of mechanisms of phospholipase activation (e.g., what are the molecular mechanisms whereby tyrosine kinases and Ras affect PC-PLC?). The protein kinase C dependent and protein kinase C independent mechanisms that enable mitogenic stimuli to activate the Erk/MAP kinase are enigmatic at this time. How Raf-1 activates SRE-containing gene promoters (such as the fos promoter) is also not known. However, given the current rapid rate of progress in this field, it is likely that a much more complete understanding of the mitogenic signal transduction process will soon be obtained.

Crosstalk among multiple signal-activated phospholipases

Transduction of extracellular signals across the plasma membrane often involves activation of several phospholipases that generate multiple, sometimes interconvertible, lipid-derived messengers. Coordination and integration of these signal-activated phospholipases may require crosstalk between both the messengers and target protein constituents of these pathways.

Hepatocyte growth factor (HGF) mediates the sustained formation of 1,2-diacylglycerol via phosphatidylcholine-phospholipase C in cultured rat hepatocytes

The addition of hepatocyte growth factor (HGF) to rat hepatocytes in primary culture resulted in the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and 1,2-diacylglycerol (DG) by a phosphoinositide-specific phospholipase C (PI-PLC). DG showed a biphasic increase; the first phase, corresponding with the peak of Ins(1,4,5)P3 and a second larger and prolonged phase. The HGF stimulates the phosphatidylcholine (PC)-derived prolonged DG formation by a phospholipase C pathway (PC-PLC) but not by a phospholipase D pathway. HGF also was found to elicit [Ca2+] oscillations which may be associated with the prolonged DG production from PC via the PC-PLC phospholipase C pathway.