D609-phosphatidylcholine-specific phospholipase C inhibitor attenuates thapsigargin-induced sodium influx in human lymphocytes

Tricyclodecan-9-yl-Xanthogenate (D609): Mechanism of Action and Pharmacological Applications

Tricyclodecan-9-yl xanthogenate (D609) is a synthetic tricyclic compound possessing a xanthate group. This xanthogenate compound is known for its diverse pharmacological properties. Over the last three decades, many studies have reported the biological activities of D609, including antioxidant, antiapoptotic, anticholinergic, anti-tumor, anti-inflammatory, anti-viral, anti-proliferative, and neuroprotective activities. Its mechanism of action is extensively attributed to its ability to cause the competitive inhibition of phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) and sphingomyelin synthase (SMS). The inhibition of PCPLC or SMS affects secondary messengers with a lipidic nature, i.e., 1,2-diacylglycerol (DAG) and ceramide. Various in vitro/in vivo studies suggest that PCPLC and SMS inhibition regulate the cell cycle, block cellular proliferation, and induce differentiation. D609 acts as a pro-inflammatory cytokine antagonist and diminishes Aβ-stimulated toxicity. PCPLC enzymatic activity essentially requires Zn2+, and D609 might act as a potential chelator of Zn2+, thereby blocking PCPLC enzymatic activity. D609 also demonstrates promising results in reducing atherosclerotic plaque formation, post-stroke cerebral infarction, and cancer progression. The present compilation provides a comprehensive mechanistic insight into D609, including its chemistry, mechanism of action, and regulation of various pharmacological activities.

The plant non-specific phospholipase C gene family. Novel competitors in lipid signalling

Non-specific phospholipases C (NPCs) were discovered as a novel type of plant phospholipid-cleaving enzyme homologous to bacterial phosphatidylcholine-specific phospholipases C and responsible for lipid conversion during phosphate-limiting conditions. The six-gene family was established in Arabidopsis, and growing evidence suggests the involvement of two articles NPCs in biotic and abiotic stress responses as well as phytohormone actions. In addition, the diacylglycerol produced via NPCs is postulated to participate in membrane remodelling, general lipid metabolism and cross-talk with other phospholipid signalling systems in plants. This review summarises information concerning this new plant protein family and focusses on its sequence analysis, biochemical properties, cellular and tissue distribution and physiological functions. Possible modes of action are also discussed.

Effect of lead on apoptosis in cultured rat primary osteoblasts

To investigate the effect of lead exposure on apoptosis of cultured rat primary osteoblasts (ROBs), which were derived from newborn calvariae of Sprague Dawley rat. They were identified by the staining of alkaline phosphatase and mineralized matrix. The ROBs were received at 0, 20, 40 and 80 μM Pb2+ of lead acetate solution for 24 h, respectively, before being doubly marked by Annexin V-fluorescein isothiocyanate/propidium iodide. The intracellular concentration of calcium ([Ca2+]i) was detected under the laser scan confocal microscope. The activities of phosphatidylcholine-specific phospholipase C (PC-PLC) were measured and the effect of lead exposure on the expression of PC-PLC was observed by immunoblotting assay. The results showed that when compared with that of the control group, lead exposure induced an increase of [Ca2+]i of lead-treated ROBs, resulting in a significant development in apoptosis. In the meantime, a significant decline in protein level and enzymatic activities of PC-PLC were observed in a dose-dependent manner. It was concluded that lead can induce apoptosis in ROBs, and one of the mechanisms of lead-induced apoptosis may be that activating intracellular calcium stores by decreasing protein levels and enzymatic activities of PC-PLC can increase the [Ca2+]i, and consequently, the apoptotic signal pathway can be induced.

Calcium signaling in non-excitable cells: Ca2+ release and influx are independent events linked to two plasma membrane Ca2+ entry channels

The regulatory mechanism of Ca2+ influx into the cytosol from the extracellular space in non-excitable cells is not clear. The "capacitative calcium entry" (CCE) hypothesis suggested that Ca2+ influx is triggered by the IP(3)-mediated emptying of the intracellular Ca2+ stores. However, there is no clear evidence for CCE and its mechanism remains elusive. In the present work, we have provided the reported evidences to show that inhibition of IP(3)-dependent Ca2+ release does not affect Ca2+ influx, and the experimental protocols used to demonstrate CCE can stimulate Ca2+ influx by means other than emptying of the Ca2+ stores. In addition, we have presented the reports showing that IP(3)-mediated Ca2+ release is linked to a Ca2+ entry from the extracellular space, which does not increase cytosolic [Ca2+] prior to Ca2+ release. Based on these and other reports, we have provided a model of Ca2+ signaling in non-excitable cells, in which IP(3)-mediated emptying of the intracellular Ca2+ store triggers entry of Ca2+ directly into the store, through a plasma membrane TRPC channel. Thus, emptying and direct refilling of the Ca2+ stores are repeated in the presence of IP(3), giving rise to the transient phase of oscillatory Ca2+ release. Direct Ca2+ entry into the store is regulated by its filling status in a negative and positive manner through a Ca2+ -binding protein and Stim1/Orai complex, respectively. The sustained phase of Ca2+ influx is triggered by diacylglycerol (DAG) through the activation of another TRPC channel, independent of Ca2+ release. The plasma membrane IP(3) receptor (IP(3)R) plays an essential role in Ca2+ influx, by interacting with the DAG-activated TRPC, without the requirement of binding to IP(3).

Involvement of phosphatidylinositol-phospholipase C in immune response to Salmonella lipopolysacharide in chicken macrophage cells (HD11)

The activation of phospholipases is one of the earliest key events in receptor-mediated cellular responses to a number of extracellular signaling molecules. Lipopolysaccharide (LPS) is a principle component of the outer membrane of Gram-negative bacteria and a prime target for recognition by the innate immune system. In the present study, we evaluated the role of specific phospholipase in the activation of a chicken macrophage cell line HD11 by LPS. Activation of HD11 cells by LPS results in induction of nitric oxide (NO). Using selective inhibitors, we have identified that phosphatidylinositol (PI)-phospholipase C (PI-PLC), but not phosphatidylcholine (PC)-phospholipase C (PC-PLC) nor PC-phospholipase D (PC-PLD), was required for LPS-induced NO production. Preincubation with PI-PLC selective inhibitors (U-73122 and ET-18-OCH3) abrogated LPS-induced NO production in HD11 cells, whereas PC-PLC inhibitor (D609), phosphatide phosphohydrolase inhibitor (propranolol), and PC-PLD inhibitor (n-butanol) had no inhibitory effects. We also showed that inhibition of protein kinase C (PKC) by selective inhibitors Ro 31-8220 and calphostin C and chelating intracellular Ca2+ by BAPTA-AM significantly reduced NO production in LPS-stimulated HD11 cells. Our results demonstrate that PI-PLC plays a critical role, most likely through activation of PKC pathway, in TLR4 mediated immune responses of avian macrophage cells to LPS.

A store-operated nonselective cation channel in human lymphocytes

1. Agonist interaction with phospholipase C-linked receptors at the plasma membrane can elicit both Ca2+ and Na+ influxes in lymphocytes. While Ca2+ influx is mediated by Ca2+ release-activated Ca2+ (CRAC) channels, the pathway responsible for Na+ influx is largely unknown. 2. We show that thapsigargin, ionomycin, ADP-ribose and IP3 activated a nonselective cation channel in lymphocytes that had a slightly outwardly rectifying I-V relationship, and a single channel conductance of 23.1 pS. We termed this channel a Ca2+ release-activated nonselective cation (CRANC) channel. 3. On activation in cell-attached configuration, switching to an inside-out configuration abolished CRANC channel activity. 4. Transfection of Jurkat T cells with antisense oligonucleotides for LTRPC2 reduced capacitative Ca2+ entry. 5. These results suggest that CRANC channels are responsible for the Na+ influx as well as a portion of the Ca2+ influx in lymphocytes induced by store depletion, that sustained activation of CRANC channels requires some property of the environment of a cell depleted of its Ca2+ stores; and that LTRPC2 protein is a likely component of the CRANC channel.

Keratinocytes Rapidly Readjust Ceramide–Sphingomyelin Homeostasis and Contain a Phosphatidylcholine–Sphingomyelin Transacylase

Ceramide as central second messenger of the apoptosis-related sphingomyelin signaling pathway is a potential target for the control of cancer. A complex metabolizing network defines cell type and stage-specific final ceramide concentrations. Successful therapeutic control of ceramide levels requires a knowledge of multiple related turnover rates. The metabolism of ceramide and sphingomyelin was studied in keratinocytes under the condition of an unstimulated sphingomyelin signaling pathway. Preparations enriched in plasma membranes contain a neutral Mg(2+)-dependent sphingomyelinase and a Mg(2+)-independent sphingomyelin synthase that vigorously preserve balanced ceramide and sphingomyelin levels. Ceramide regulates neutral sphingomyelinase. Inhibition of sphingomyelin synthase by D609 treatment results in temporary loss of intercelluar contacts and in cellular shrinking. It is ineffective for sustained elevation of ceramide levels. Ceramide phosphorylating and deacylating activities are insignificant. Recently, fatty-acid remodeling in sphingomyelin was reported as likely to counteract the membrane-rigidifying effects of cholesterol. Keratinocytes transfer fluorescence labeled acyl-chains between phosphatidylcholine and sphingomyelin. A transferase of that kind would allow rapid adjustment of local lipid composition in response to acutely changed conditions. In addition, this transferase might have a function in the formation of the epidermal permeability barrier.

Non-voltage-gated L-type Ca2+ channels in human T cells: pharmacology and molecular characterization of the major alpha pore-forming and auxiliary beta-subunits

In T lymphocytes, engagement of the antigen receptor leads to a biphasic Ca2+ flux consisting of a mobilization of Ca2+ from intracellular stores followed by a lower but sustained elevation that is dependent on extracellular Ca2+. The prolonged Ca2+ flux is required for activation of transcription factors and for subsequent activation of the T cell. Ca2+ influx requires as yet unidentified Ca2+ channels, which potentially play a role in T cell activation. Here we present evidence that human T cells express a non-voltage-gated Ca2+ channel related to L-type voltage-gated Ca2+ channels. Drugs that block classical L-type channels inhibited the initial phase of the antigen receptor-induced Ca2+ flux and could also inhibit the sustained phase of the Ca2+ signal suggesting a role for the L-type Ca2+ channel in antigen receptor signaling. T cells expressed transcripts for the alpha(1) 1.2 and alpha(1) 1.3 pore-forming subunits of L-type voltage-gated Ca2+ channels and transcripts for all four known beta-subunits including several potential new splice variants. Jurkat T leukemia cells expressed a small amount of full-length alpha(1)1.2 protein but the dominant form was a truncated protein identical in size to a truncated alpha(1) 1.2 protein known to be expressed in B lymphocytes. They further expressed a truncated form of the alpha(1) 1.3 subunit and auxiliary beta1- and beta3-subunit proteins. Our data strongly suggest that functional but non-voltage-gated L-type Ca2+ channels are expressed at the plasma membrane in T cells and play a role in the antigen receptor-mediated Ca2+ flux in these cells.

The Effect of Eicosapentaenoic Acid on Rat Lymphocyte Proliferation Depends Upon Its Position in Dietary Triacylglycerols

Animal and human studies have shown that greatly increasing the amount of fish oil [rich in long-chain (n-3) PUFA] in the diet can decrease lymphocyte functions. The effects of a more modest provision of long-chain (n-3) PUFA and whether eicosapentaenoic acid (20:5) and docosahexaenoic acid (22:6) have the same effects as one another are unclear. Whether the position of 20:5 or 22:6 in dietary triacylglycerols (TAG) influences their incorporation into immune cells and their subsequent functional effects is not known. In this study, male weanling rats were fed for 6 wk one of 9 diets that contained 178 g lipid/kg and that differed in the type of (n-3) PUFA and in the position of these in dietary TAG. The control diet contained 4.4 g alpha-linolenic acid (18:3)/100 g total fatty acids. In the other diets, 20:5 or 22:6 replaced a portion (50 or 100%) of 18:3, and were in the sn-2 or the sn-1(3) position of dietary TAG. There were significant dose-dependent increases in the proportion of 20:5 or 22:6 in spleen mononuclear cell phospholipids when 20:5 or 22:6 was fed. These increases were at the expense of arachidonic acid and were largely independent of the position of 20:5 or 22:6 in dietary TAG. Spleen lymphocyte proliferation increased dose dependently when 20:5 was fed in the sn-1(3) position of dietary TAG. There were no significant differences in interleukin-2, interferon-gamma or interleukin-10 production among spleen cells from rats fed the different diets. Prostaglandin E(2) production by spleen mononuclear cells was decreased by inclusion of either 20:5 or 22:6 in the diet in the sn-1(3) position. Thus, incorporation of 20:5 or 22:6 into spleen mononuclear cell phospholipids is not influenced by the position in dietary TAG. However, the pattern of incorporation may be influenced, and there are some differential functional effects of the position of long-chain (n-3) PUFA in dietary TAG. A moderate increase in the intake of 20:5 at the sn-1(3) position of dietary TAG increases lymphocyte proliferation.

A novel class of microbial phosphocholine-specific phospholipases C

In this report we describe the 1,500-fold purification and characterization of the haemolytic phospholipase C (PLC) of Pseudomonas aeruginosa, the paradigm member of a novel PLC/phosphatase superfamily. Members include proteins from Mycobacterium tuberculosis, Bordetella spp., Francisella tularensis and Burkholderia pseudomallei. Purification involved overexpression of the plcHR1,2 operon, ion exchange chromatography and native preparative polyacrylamide gel electrophoresis. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry confirmed the presence of two proteins in the purified sample with sizes of 17,117.2 Da (PlcR2) and 78,417 Da (PlcH). Additionally, liquid chromatography electrospray mass spectrometry (LCMS) revealed that PlcH and PlcR2 are at a stoichiometry of 1 : 1. Western blot analysis demonstrated that the enzyme purifies as a heterodimeric complex, PlcHR2. PlcHR2 is only active on choline-containing phospholipids. It is equally active on phosphatidylcholine (PC) and sphingomyelin (SM) and is able to hydrolyse plasmenylcholine phospholipids (plasmalogens). Neither PlcHR2 nor the M. tuberculosis homologues are inhibited by D609 a widely used, competitive inhibitor of the Bacillus cereus PLC. PlcH, PlcR2, and the PlcHR2 complex bind calcium. While calcium has no detectable effect on enzymatic activity, it inhibits the haemolytic activity of PlcHR2. In addition to being required for the secretion of PlcH, the chaperone PlcR2 affects both the enzymatic and haemolytic properties of PlcH. Inclusive in these data is the conclusion that the members of this PC-PLC and phosphatase family possess a novel mechanism for the recognition and hydrolysis of their respective substrates.

Ceramide regulates lipopolysaccharide-induced phosphatidylinositol 3-kinase and Akt activity in human alveolar macrophages

The phosphatidylinositol (PI) 3-kinase pathway is an important regulator of cell survival. In human alveolar macrophages, we found that LPS activates PI 3-kinase and its downstream effector, Akt. LPS exposure of alveolar macrophages also results in the generation of ceramide. Because ceramide exposure induces apoptosis in other cell types and the PI 3-kinase pathway is known to inhibit apoptosis, we determined the relationship between LPS-induced ceramide and PI 3-kinase activation in alveolar macrophages. We found that ceramide exposure activated PI 3-kinase and Akt. When we blocked LPS-induced ceramide with the inhibitor D609, we blocked LPS-induced PI 3-kinase and Akt activation. Evaluating cell survival after ceramide or LPS exposure, we found that blocking PI 3-kinase induced a significant increase in cell death. Because these effects of PI 3-kinase inhibition were more pronounced in ceramide- vs LPS-treated alveolar macrophages, we also evaluated NF-kappaB, which has also been linked to cell survival. We found that LPS, to a greater degree than ceramide, induced NF-kappaB translocation to the nucleus. As a composite, these studies suggest that the effects of ceramide exposure in alveolar macrophages may be very different from the effects described for other cell types. We believe that LPS induction of ceramide results in PI 3-kinase activation and represents a novel effector mechanism that promotes survival of human alveolar macrophages in the setting of pulmonary sepsis.

A store-operated nonselective cation channel in lymphocytes is activated directly by Ca(2+) influx factor and diacylglycerol

Agonist-receptor interactions at the plasma membrane often lead to activation of store-operated channels (SOCs) in the plasma membrane, allowing for sustained Ca(2+) influx. While Ca(2+) influx is important for many biological processes, little is known about the types of SOCs, the nature of the depletion signal, or how the SOCs are activated. We recently showed that in addition to the Ca(2+) release-activated Ca(2+) (CRAC) channel, both Jurkat T cells and human peripheral blood mononuclear cells express novel store-operated nonselective cation channels that we termed Ca(2+) release-activated nonselective cation (CRANC) channels. Here we demonstrate that activation of both CRAC and CRANC channels is accelerated by a soluble Ca(2+) influx factor (CIF). In addition, CRANC channels in inside-out plasma membrane patches are directly activated upon exposure of their cytoplasmic side to highly purified CIF preparations. Furthermore, CRANC channels are also directly activated by diacylglycerol. These results strongly suggest that the Ca(2+) store-depletion signal is a diffusible molecule and that at least some SOCs may have dual activation mechanisms.