Diacylglycerol metabolism in phospholipase C-treated mammalian cells

Bacterial phospholipases C with dual activity: phosphatidylcholinesterase and sphingomyelinase

Bacterial phospholipases and sphingomyelinases are lipolytic esterases that are structurally and evolutionarily heterogeneous. These enzymes play crucial roles as virulence factors in several human and animal infectious diseases. Some bacterial phospholipases C (PLCs) have both phosphatidylcholinesterase and sphingomyelinase C activities. Among them, Listeria  monocytogenes PlcB, Clostridium perfringens PLC, and Pseudomonas aeruginosa PlcH are the most deeply understood. In silico predictions of substrates docking with these three bacterial enzymes provide evidence that they interact with different substrates at the same active site. This review discusses structural aspects, substrate specificity, and the mechanism of action of those bacterial enzymes on target cells and animal infection models to shed light on their roles in pathogenesis.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Control of the CDPethanolamine pathway in mammalian cells: effect of CTP:phosphoethanolamine cytidylyltransferase overexpression and the amount of intracellular diacylglycerol

For an insight regarding the control of PtdEtn (phosphatidylethanolamine) synthesis via the CDPethanolamine pathway, rat liver cDNA encoding ECT (CTP:phosphoethanolamine cytidylyltransferase) was transiently or stably transfected in Chinese-hamster ovary cells and a rat liver-derived cell line (McA-RH7777), resulting in a maximum of 26- and 4-fold increase in specific activity of ECT respectively. However, no effect of ECT overexpression on the rate of [3H]ethanolamine incorporation into PtdEtn was detected in both cell lines. This was explored further in cells overexpressing four times ECT activity (McA-ECT1). The rate of PtdEtn breakdown and PtdEtn mass were not changed in McA-ECT1 cells in comparison with control-transfected cells. Instead, an accumulation of CDPethanolamine (label and mass) was observed, suggesting that in McA-ECT1 cells the ethanolaminephosphotransferase-catalysed reaction became rate-limiting. However, overexpression of the human choline/ethanolaminephosphotransferase in McA-ECT1 and control-transfected cells had no effect on PtdEtn synthesis. To investigate whether the availability of DAG (diacylglycerol) limited PtdEtn synthesis in these cells, intracellular DAG levels were increased using PMA or phospholipase C. Exposure of cells to PMA or phospholipase C stimulated PtdEtn synthesis and this effect was much more pronounced in McA-ECT1 than in control-transfected cells. In line with this, the DAG produced after PMA exposure was consumed more rapidly in McA-ECT1 cells and the CDPethanolamine level decreased accordingly. In conclusion, our results suggest that the supply of CDPethanolamine, via the expression level of ECT, is an important factor governing the rate of PtdEtn biosynthesis in mammalian cells, under the condition that the amount of DAG is not limiting.

Effects of Clostridium perfringens phospholipase C in mammalian cells

Clostridium perfringens phospholipase C (Cp-PLC), the major virulence factor in the pathogenesis of gas gangrene, is a Zn(2+) metalloenzyme with lecithinase and sphingomyelinase activities. Its structure shows an N-terminal domain containing the active site, and a C-terminal Ca(2+) binding domain required for membrane interaction. Although the knowledge of the structure of Cp-PLC and its interaction with aggregated phospholipids has advanced significantly, an understanding of the effects of Cp-PLC in mammalian cells is still incomplete. Cp-PLC binds to artificial bilayers containing cholesterol and sphingomyelin or phosphatidylcholine (PC) and degrades them, but glycoconjugates present in biological membranes influence its binding or positioning toward its substrates. Studies with Cp-PLC variants harboring single amino-acid substitutions have revealed that the active site, the Ca(2+) binding region, and the membrane interacting surface are required for cytotoxic and haemolytic activity. Cp-PLC causes plasma membrane disruption at high concentrations, whereas at low concentrations it perturbs phospholipid metabolism, induces DAG generation, PKC activation, Ca(2+) mobilization, and activates arachidonic acid metabolism. The cellular susceptibility to Cp-PLC depends on the composition of the plasma membrane and the capacity to up-regulate PC synthesis. The composition of the plasma membrane determines whether Cp-PLC can bind and acquire its active conformation, and thus the extent of phospholipid degradation. The capacity of PC synthesis and the availability of precursors determine whether the cell can replace the degraded phospholipids. Whether the perturbations of signal transduction processes caused by Cp-PLC play a role in cytotoxicity is not clear. However, these perturbations in endothelial cells, platelets and neutrophils lead to the uncontrolled production of intercellular mediators and adhesion molecules, which inhibits bacterial clearance and induces thrombotic events, thus favouring bacterial growth and spread in the host tissues.

UDP-glucose deficiency causes hypersensitivity to the cytotoxic effect of Clostridium perfringens phospholipase C

A Chinese hamster cell line with a mutation in the UDP-glucose pyrophosphorylase (UDPG:PP) gene leading to UDP-glucose deficiency as well as a revertant cell were previously isolated. We now show that the mutant cell is 10(5) times more sensitive to the cytotoxic effect of Clostridium perfringens phospholipase C (PLC) than the revertant cell. To clarify whether there is a connection between the UDP-glucose deficiency and the hypersensitivity to C. perfringens PLC, stable transfectant cells were prepared using a wild type UDPG:PP cDNA. Clones of the mutant transfected with a construct having the insert in the sense orientation had increased their UDP-glucose level, whereas those of the revertant transfected with a UDPG:PP antisense had reduced their level of UDP-glucose compared with control clones transfected with the vector. Exposure of these two types of transfectant clones to C. perfringens PLC demonstrated that a cellular UDP-glucose deficiency causes hypersensitivity to the cytotoxic effect of this phospholipase. Further experiments with genetically engineered C. perfringens PLC variants showed that the sphingomyelinase activity and the C-domain are required for its cytotoxic effect in UDP-glucose-deficient cells.

Signal transduction through epidermal growth factor receptor is altered in HeLa monolayer cells during mitosis

Epidermal growth factor (EGF)-induced signalling was studied separately in the mitosis and G2-phases of HeLa monolayer cells presynchronized (1) by amethopterin inhibition and thymidine release or (2) by nocodazole. For comparison, cells were treated with the phorbol ester phorbol 12-myristate 13-acetate (PMA). In contrast with the observed responses effected by PMA, which seem to be independent of cell cycle and synchronization conditions, those induced by EGF are greatly influenced by both criteria. Synchronization with nocodazole abolished the EGF-induced stimulation of phosphoinositide hydrolysis in G2 as well as in mitotic cells although tyrosine phosphorylation of the EGF receptor and phospholipase Cgamma1 could be shown to occur, especially in G2 cells. Synchronization with amethopterin/thymidine showed that, in contrast with G2 cells, mitotic cells were not able to react to EGF with an increase in phosphoinositide hydrolysis although a certain degree of EGF receptor dimerization and autophosphorylation as well as tyrosine phosphorylation of phospholipase Cgamma1 could still be shown to occur in mitosis. The results seem to indicate that the EGF pathway leading to a stimulation of phosphoinositide hydrolysis is attenuated at different levels and requires a cytoskeletal condition that is not present either after treatment (24 h) with nocodazole or during normal mitosis of a monolayer cell.

Regulation and differential expression of neutral endopeptidase 24.11 in human endothelial cells

Neutral endopeptidase 24.11, a membrane-bound metallopeptidase, cleaves, and degrades vasoactive peptides such as atrial natriuretic peptide, endothelin, angiotensin I, substance P, and bradykinin. Therefore, the presence of this metallopeptidase may contribute to the regulation of vascular tone and local inflammatory responses in the vascular endothelium and elsewhere. We determined neutral endopeptidase in cultured human endothelial cells from different vascular beds and studied its regulation by protein kinase C. Neutral endopeptidase was detected in all cultured endothelial cell types. Lowest concentrations were measured in human endothelial cells from umbilical veins (360 +/- 14 pg/mg protein), followed by pulmonary and coronary arteries; higher concentrations were found in endothelial cells from the cardiac microcirculation (1099 +/- 73 pg/mg protein). Neutral endopeptidase content increased during cell growth but was not affected by endothelial cell growth factor or modifications of the growth medium. Stimulation of protein kinase C with 1-oleoyl-2-acetyl-rac-glycerol (0.1 to 1 mumol/L) and phorbol 12-myristate 13-acetate (0.01 to 0.1 mumol/L) induced a time- and concentration-dependent increase of endothelial cells that was inhibited by cycloheximide (5 mumol/L), an inhibitor of protein synthesis. Incubation with phospholipase C (1 mumol/L) and thrombin (10 IU/mL) induced upregulation of neutral endopeptidase, resulting in 158 +/- 26% and 150 +/- 22% increases, respectively, compared with controls. The thrombin effect was inhibited by calphostin C (1 mumol/L), an inhibitor of protein kinase C. Endothelial neutral endopeptidase is constitutively expressed in endothelial cells from different origins and is inducible by thrombin via activation of the protein kinase C pathway.

Bacterial phospholipases C

A variety of pathogenic bacteria produce phospholipases C, and since the discovery in 1944 that a bacterial toxin (Clostridium perfringens alpha-toxin) possessed an enzymatic activity, there has been considerable interest in this class of proteins. Initial speculation that all phospholipases C would have lethal properties has not been substantiated. Most of the characterized enzymes fall into one of four groups of structurally related proteins: the zinc-metallophospholipases C, the sphingomyelinases, the phosphatidylinositol-hydrolyzing enzymes, and the pseudomonad phospholipases C. The zinc-metallophospholipases C have been most intensively studied, and lethal toxins within this group possess an additional domain. The toxic phospholipases C can interact with eukaryotic cell membranes and hydrolyze phosphatidylcholine and sphingomyelin, leading to cell lysis. However, measurement of the cytolytic potential or lethality of phospholipases C may not accurately indicate their roles in the pathogenesis of disease. Subcytolytic concentrations of phospholipase C can perturb host cells by activating the arachidonic acid cascade or protein kinase C. Nonlethal phospholipases C, such as the Listeria monocytogenes PLC-A, appear to enhance the release of the organism from the host cell phagosome. Since some phospholipases C play important roles in the pathogenesis of disease, they could form components of vaccines. A greater understanding of the modes of action and structure-function relationships of phospholipases C will facilitate the interpretation of studies in which these enzymes are used as membrane probes and will enhance the use of these proteins as models for eukaryotic phospholipases C.

Role of protein kinase C and cyclic adenosine monophosphate in the regulation of tissue-type plasminogen activator, plasminogen activator inhibitor-1, and platelet-derived growth factor mRNA levels in human endothelial cells. Possible involvement of proto-oncogenes c-jun and c-fos

Activation of protein kinase C leads to a strong induction of tissue-type plasminogen activator (t-PA) expression in endothelial cells. Using endothelial cells from human umbilical vein (HUVECs) and human aorta (HAECs), we have studied this regulation of t-PA and its inhibitor, plasminogen activator inhibitor-1 (PAI-1), at the mRNA level and have compared their induction with the expression of platelet-derived growth factors A and B (PDGF-A and PDGF-B) and the proto-oncogenes c-jun and c-fos. Treatment of HUVECs with exogenous bacterial phospholipase C or the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol led to a threefold and a twofold increase, respectively, in t-PA concentrations in 24-hour-conditioned medium. Similarly, the more stable protein kinase C activator 4 beta-phorbol-12-myristate-13-acetate (PMA) caused about a 10-fold increase in t-PA antigen levels. This effect of PMA is maximal between 8 and 16 hours at a concentration of 10 nM and is fully accounted for by parallel increases in t-PA mRNA levels. An increase in intracellular cyclic adenosine monophosphate levels by forskolin (10 microM) slightly diminished t-PA expression but further enhanced the PMA-induced increases in t-PA synthesis and mRNA levels by at least twofold. PMA also enhanced the mRNA levels of two other important endothelium-expressed genes, PDGF-A and PDGF-B, with a time profile similar to that of t-PA, with peak values about fivefold higher than control values. Forskolin did not further stimulate this PMA-induced PDGF expression in HUVECs, which suggests a regulatory mechanism different from that of t-PA. Qualitatively very similar induction patterns of t-PA, PDGF-A, and PDGF-B were seen with HAECs. In contrast to t-PA and PDGF, PAI-1 mRNA and antigen levels increased only slightly after PMA treatment of HUVECs or HAECs; forskolin alone or in combination with PMA diminished the expression of PAI-1. The induction of t-PA mRNA by PMA was dependent on protein synthesis and was preceded by a strong transient increase in c-jun and c-fos mRNA levels; the induction of c-fos but not of c-jun was potentiated by forskolin. Because the products of these two proto-oncogenes form dimeric complexes for which specific binding sites are present in the t-PA promoter region, they may mediate the protein kinase C-dependent increase in t-PA gene expression, including the stimulating action of cyclic adenosine monophosphate.

Characterization of phospholipase C from Pseudomonas aeruginosa as a potent inflammatory agent

Phospholipase C (PLC) from Pseudomonas aeruginosa induced a marked inflammatory response when injected intraperitoneally in C3H/HeJ mice. This inflammation was characterized by the accumulation of inflammatory cells and plasma protein and the release of arachidonic acid metabolites (6-trans-12-epi-leukotriene B4 [LTB4], 6-trans-LTB4, LTB4, 5-HETE (5-hydroxyeicosatetraenoic acid), LTC4, LTD4, LTE4, prostaglandin E2 [PGE2], PGF2-alpha, and thromboxane B2 [TxB2]) in the peritoneal cavity of the mice. Heat-inactivated PLC did not evoke any of these effects, suggesting that enzyme activity is necessary for PLC-induced inflammation. When human granulocytes were incubated with PLC in vitro, 6-trans-12-epi-LTB4, 6-trans-LTB4, LTB4, 5-HETE, and PGE2 were generated. Mouse peritoneal cells stimulated with PLC released 6-trans-LTB4, LTB4, PGE2, PGF2-alpha, and TxB2. Both human granulocytes and mouse peritoneal cells stimulated with PLC generated significantly increased levels of arachidonic acid metabolites as compared with cells incubated with heat-inactivated PLC. Leukotriene production by both populations of cells was inhibited when the cells were preincubated with nordihydroguaiaretic acid and subsequently stimulated with PLC. Similarly, both cell types released significantly lower amounts of cyclooxygenase pathway products when they were preincubated with indomethacin and subsequently stimulated with PLC.