Crosstalk among multiple signal-activated phospholipases

Disentangling the Molecular Mechanisms of the Antidepressant Activity of Omega-3 Polyunsaturated Fatty Acid: A Comprehensive Review of the Literature

Major depressive disorders (MDDs) are often associated with a deficiency in long-chain omega-3 polyunsaturated fatty acids (ω-3 PUFAs), as well as signs of low-grade inflammation. Epidemiological and dietary studies suggest that a high intake of fish, the major source of ω-3 PUFAs, is associated with lower rates of MDDs. Meta-analyses of randomized placebo-controlled ω-3 PUFAs intervention-trials suggest that primarily eicosapentaenoic acid (EPA), but not docosahexaenoic acid (DHA), is responsible for the proposed antidepressant effect. In this review, we dissect the current biological knowledge on EPA and DHA and their bioactive lipid metabolites to search for a pharmacological explanation of this, to date, unexplained clinical observation. Through enzymatic conversion by cyclooxygenase (COX), lipoxygenase (ALOX), and cytochrome P-450 monooxygenase (CYP), EPA and DHA are metabolized to major anti-inflammatory and pro-resolving lipid mediators. In addition, both ω-3 PUFAs are precursors for endocannabinoids, with known effects on immunomodulation, neuroinflammation, food intake and mood. Finally, both ω-3 PUFAs are crucial for the structure and organization of membranes and lipid rafts. While most biological effects are shared by these two ω-3 PUFAs, some distinct features could be identified: (1) The preferential CYP monooxygenase pathway for EPA and EPA derived eicosanoids; (2) The high CB2 receptor affinities of EPA-derived EPEA and its epoxy-metabolite 17,18-EEQ-EA, while the DHA-derived endocannabinoids lack such receptor affinities; (3) The competition of EPA but not DHA with arachidonic acid (AA) for particular glycerophospholipids. EPA and AA are preferentially incorporated into phosphatidylinositols, while DHA is mainly incorporated into phosphatidyl-ethanolamine, -serine and -choline. We propose that these distinct features may explain the superior antidepressant activity of EPA rich ω-3 PUFAs and that these are potential novel targets for future antidepressant drugs.

Multiplicity of Glucocorticoid Action in Inhibiting Allograft Rejection

Glucocorticoids (GCs) are used as immunosuppressive and antiinflammatory agents in organ transplantation and in treating autoimmune diseases and inflammatory disorders. GCs were shown to exert their antiproliferative effects directly through blockade of certain elements of an early membrane-associated signal transduction pathway, modulation of the expression of select adhesion molecules, and by suppression of cytokine synthesis and action. GCs may act indirectly by inducing lipocortin synthesis, which in turn, inhibits arachidonic acid release from membrane-bound stores, and also by inducing transforming growth factor (TGF)-β expression that subsequently blocks cytokine synthesis and T cell activation. Furthermore, by preferentially inhibiting the production of Th1 cytokines, GCs may enhance Th2 cell activity and, hence, precipitate a long-lasting state of tolerance through a preferential promotion of a Th2 cytokine-secreting profile. In exerting their antiproliferative effects, GCs influence both transcriptional and posttranscriptional events by binding their cytosolic receptor (GR), which subsequently binds the promoter region of cytokine genes on select DNA sites compatible with the GCs responsible elements (GRE) motif. In addition to direct DNA binding, GCs may also directly bind to, and hence antagonize, nuclear factors required for efficient gene expression, thereby markedly reducing transcriptional rate. The pleiotrophy of the GCs action, coupled with the diverse experimental conditions employed in assessing the GCs effects, indicate that GCs may utilize more than one mechanism in inhibiting T cell activation, and warrant careful scrutiny in assigning a mechanism by which GCs exert their antiproliferative effects. © 1998 Elsevier Science Inc.

Arachidonate 12/15-lipoxygenase-induced inflammation and oxidative stress are involved in the development of diabetic cardiomyopathy

Diabetes affects cardiac structure and function, and it has been suggested that diabetes leads to cardiomyopathy. Arachidonate 12/15-lipoxygenase (LOX) has been suggested to play an important role in atherogenesis and heart failure. However, the role of 12/15-LOX in diabetic cardiomyopathy has not been examined. In this study, we investigated the effects of cardiac 12/15-LOX on diabetic cardiomyopathy. We created streptozotocin (STZ)-induced diabetic mice and compared them with Alox15-deficient mice. Expression of 12/15-LOX and inflammatory cytokines such as tumor necrosis factor (TNF)-α and nuclear factor (NF)-κB were upregulated in STZ-induced diabetic hearts. Disruption of 12/15-LOX significantly improved STZ-induced cardiac dysfunction and fibrosis. Moreover, deletion of 12/15-LOX inhibited the increases of TNF-α and NF-κB as well as the production of STZ-induced reactive oxygen species in the heart. Administration of N-acetylcysteine in diabetic mice prevented STZ-induced cardiac fibrosis. Neonatal cultured cardiomyocytes exposed to high glucose conditions induced the expression of 12/15-LOX as well as TNF-α, NF-κB, and collagen markers. These increases were inhibited by treatment of the 12/15-LOX inhibitor. Our results suggest that cardiac 12/15-LOX-induced inflammation and oxidative stress are involved in the development of diabetic cardiomyopathy and that inhibition of 12/15-LOX could be a novel treatment for this condition.

Analysis of the IFN-gamma-induced signal transduction pathway in fetal rejection

The placenta, one of the most important fetal tissues during gestation, ensures nutrition, development and protection of the fetus. Although placenta lacks expression of class II MHC antigens, they can be induced either by interferon-gamma (IFN-γ) on the spongiotrophoblast zone, or by 5-azacytidine (5-azaC) on the labyrinthine trophoblast zone, two agents actively participating in a plethora of immunological and inflammatory reactions. This induction is correlated with fetal abortion and fetal developmental abnormalities. In this work the in vitro and in vivo signal transduction pathways followed by IFN-γ or 5-azaC to induce class H antigen expression on placental cells by using specific pathway inhibitors has been studied. It is shown that at least three intracellular pathways are implicated in the Ia induction, p21^ras is the first protein activated by the two agents while further signalling requires Ca²⁺ mobilization and PKC activations. When the in vitro results are transferred to live animals using the same inducing agents and pathway inhibitors, it is found that theophylline (Ca²⁺/CaM inhibitor) and anti-p21^ras are the most potent suppressors of the IFN-γ- and 5-azaC-induced side effects during pregnancy. The data presented here point to novel directions not only as to the intracellular signalling, but also to the use of pathway inhibitors in vivo to treat aberrant antigen expression associated with fetal loss.

An increase in nitric oxide produced by rat peritoneal neutrophils is not involved in cell apoptosis

Polymorphonuclear neutrophils (PMN) obtained from carrageenin-stimulated peritoneal cavities of rats, but not blood PMN, spontaneously produced nitric oxide (NO) when incubated in vitro. Incubation of the cells with the NO synthase inhibitors, L-imino-ethyl-L-ornithine (L-NIO) or N^G-monomethyl-L-arginine (L-NMMA), inhibited NO production. This inhibition could be reversed by L-arginine. Incubation of PMN with lipopolysaccharide (LPS) failed to enhance NO production. Pretreatment of the rats with dexamethasone (DEXA) prior to carrageenin injection or incubation of PMN with the glucocorticoid in vitro partially inhibited the spontaneous release of NO. On the other hand, when PMN obtained from DEXA pretreated rats were incubated in vitro with DEXA, NO synthase activity and hence NO generation were almost abolished. A similar inhibition was also observed following the addition of L-NIO or cycloheximide to cultures of carrageenin-elicited PMN. The NO production by PMN did not appear to be related to cell viability or apoptosis. Indeed, neither the blockade of NO generation by L-NIO nor the incubation of the neutrophils with a NO donor, S-nitroso-acetylpenicillamine (SNAP) modified the pattern of LDH release or DNA fragmentation. In summary, it appears that PMN migration triggers a continuous NO synthesis, and that NO produced by these cells is not related to their apoptosis.

SRSF1 regulates the alternative splicing of caspase 9 via a novel intronic splicing enhancer affecting the chemotherapeutic sensitivity of non-small cell lung cancer cells

Increasing evidence points to the functional importance of alternative splice variations in cancer pathophysiology with the alternative pre-mRNA processing of caspase 9 as one example. In this study, we delve into the underlying molecular mechanisms that regulate the alternative splicing of caspase 9. Specifically, the pre-mRNA sequence of caspase 9 was analyzed for RNA cis-elements known to interact with SRSF1, a required enhancer for caspase 9 RNA splicing. This analysis revealed 13 possible RNA cis-elements for interaction with SRSF1 with mutagenesis of these RNA cis-elements identifying a strong intronic splicing enhancer located in intron 6 (C9-I6/ISE). SRSF1 specifically interacted with this sequence, which was required for SRSF1 to act as a splicing enhancer of the inclusion of the 4 exon cassette. To further determine the biological importance of this mechanism, we employed RNA oligonucleotides to redirect caspase 9 pre-mRNA splicing in favor of caspase 9b expression, which resulted in an increase in the IC(50) of non-small cell lung cancer (NSCLC) cells to daunorubicin, cisplatinum, and paclitaxel. In contrast, downregulation of caspase 9b induced a decrease in the IC(50) of these chemotherapeutic drugs. Finally, these studies showed that caspase 9 RNA splicing was a major mechanism for the synergistic effects of combination therapy with daunorubicin and erlotinib. Overall, we have identified a novel intronic splicing enhancer that regulates caspase 9 RNA splicing and specifically interacts with SRSF1. Furthermore, we showed that the alternative splicing of caspase 9 is an important molecular mechanism with therapeutic relevance to NSCLCs.

Regulation of HERG (KCNH2) potassium channel surface expression by diacylglycerol

The HERG (KCNH2) channel is a voltage-sensitive potassium channel mainly expressed in cardiac tissue, but has also been identified in other tissues like neuronal and smooth muscle tissue, and in various tumours and tumour cell lines. The function of HERG has been extensively studied, but it is still not clear what mechanisms regulate the surface expression of the channel. In the present report, using human embryonic kidney cells stably expressing HERG, we show that diacylglycerol potently inhibits the HERG current. This is mediated by a protein kinase C-evoked endocytosis of the channel protein, and is dependent on the dynein-dynamin complex. The HERG protein was found to be located only in early endosomes and not lysosomes. Thus, diacylglycerol is an important lipid participating in the regulation of HERG surface expression and function.

Protein kinase C-dependent control of Bcl-x alternative splicing

The alternative splicing of Bcl-x generates the proapoptotic Bcl-x(S) protein and the antiapoptotic isoform Bcl-x(L). Bcl-x splicing is coupled to signal transduction, since ceramide, hormones, and growth factors alter the ratio of the Bcl-x isoforms in different cell lines. Here we report that the protein kinase C (PKC) inhibitor and apoptotic inducer staurosporine switches the production of Bcl-x towards the x(S) mRNA isoform in 293 cells. The increase in Bcl-x(S) elicited by staurosporine likely involves signaling events that affect splicing decisions, because it requires active transcription and no new protein synthesis and is independent of caspase activation. Moreover, the increase in Bcl-x(S) is reproduced with more specific inhibitors of PKC. Alternative splicing of the receptor tyrosine kinase gene Axl is similarly affected by staurosporine in 293 cells. In contrast to the case for 293 cells, PKC inhibitors do not influence the alternative splicing of Bcl-x and Axl in cancer cell lines, suggesting that these cells have sustained alterations that uncouple splicing decisions from PKC-dependent signaling. Using minigenes, we show that an exonic region located upstream of the Bcl-x(S) 5' splice site is important to mediate the staurosporine shift in Bcl-x splicing. When transplanted to other alternative splicing units, portions of this region confer splicing modulation and responsiveness to staurosporine, suggesting the existence of factors that couple splicing decisions with PKC signaling.

SAP155 Binds to ceramide-responsive RNA cis-element 1 and regulates the alternative 5' splice site selection of Bcl-x pre-mRNA

Two splice variants are derived from the BCL-x gene, proapoptotic Bcl-x(s) and antiapoptotic Bcl-x(L), via alternative 5' splice site selection. In previous studies, our laboratory identified an RNA cis-element within exon 2 of Bcl-x pre-mRNA that is a ceramide responsive termed CRCE 1. In this study, mass spectrometric analysis identified the splicing factor SAP155, as an RNA trans-acting factor binding to the purine-rich CRCE 1. The interaction of SAP155 with CRCE 1 was confirmed by the addition of an anti-SAP155 antibody (Ab) to EMSA decreasing the mobility of a protein:CRCE 1 complex (SuperShift). Furthermore, the down-regulation of SAP155 in A549 cells by RNA interference (RNAi) technology resulted in the loss of a 155 kDa protein complexed with CRCE 1. Moreover, this down-regulation of SAP155 induced an increase in the Bcl-x(s) with a concomitant decrease in the Bcl-x(L) splice variants and immunoreactive protein levels, thereby decreasing the Bcl-x(L)/Bcl-x(s) ratio. Specific down-regulation of SAP155 also inhibited the ability of exogenous ceramide treatment to further induce the activation of the Bcl-x(s) 5' splice site. Additionally, the specific down-regulation of SAP155 sensitized cells to undergo apoptosis in response to daunorubicin in a manner similar to ceramide. Therefore, we have identified SAP155 as an RNA trans-acting factor that binds to CRCE 1, functions to regulate the alternative 5' splice site selection of Bcl-x pre-mRNA, and is required for ceramide to induce the activation of the Bcl-x(s) 5' splice site. Furthermore, we have demonstrated that activation of the Bcl-x(s) 5' splice site can increase the effectiveness of chemotherapeutic drug treatment, thus establishing a role for the alternative splicing mechanism of Bcl-x in chemotherapeutic sensitivity.

SRp30a (ASF/SF2) regulates the alternative splicing of caspase-9 pre-mRNA and is required for ceramide-responsiveness

Two splice variants are derived from the caspase-9 gene, proapoptotic caspase-9a and antiapoptotic caspase-9b, by either the inclusion or exclusion of an exon 3, 4, 5, and 6 cassette. Previous studies from our laboratory have shown that the alternative splicing of caspase-9 and the phosphorylation status of SR proteins, a conserved family of splicing factors, are regulated by chemotherapy and ceramide via the action of protein phosphatase-1. In this study, a link between ceramide, SR proteins, and the alternative splicing of caspase-9 was established. The downregulation of SRp30a in A549 cells by RNA interference technology resulted in an increase in the caspase-9b splice variant, with a concomitant decrease in the caspase-9a splice variant, thereby significantly decreasing the caspase-9a/9b ratio from 1.67 +/- 0.11 to 0.56 +/- 0.08 (P < 0.005). The specific downregulation of SRp30a also inhibited the ability of exogenous ceramide treatment to induce the inclusion of the exon 3, 4, 5, and 6 cassette. Therefore, we have identified SRp30a as an RNA trans-acting factor that functions as a major regulator of caspase-9 pre-mRNA processing and is required for ceramide to mediate the alternative splicing of caspase-9.

Acute ozone-induced differential gene expression profiles in rat lung

Ozone is an oxidant gas that can directly induce lung injury. Knowledge of the initial molecular events of the acute O3 response would be useful in developing biomarkers of exposure or response. Toward this goal, we exposed rats to toxic concentrations of O3 (2 and 5 ppm) for 2 hr and the molecular changes were assessed in lung tissue 2 hr postexposure using a rat cDNA expression array containing 588 characterized genes. Gene array analysis indicated differential expression in almost equal numbers of genes for the two exposure groups: 62 at 2 ppm and 57 at 5 ppm. Most of these genes were common to both exposure groups, suggesting common roles in the initial toxicity response. However, we also identified the induction of nine genes specific to 2-ppm (thyroid hormone-beta receptor c-erb-A-beta; and glutathione reductase) or 5-ppm exposure groups (c-jun, induced nitric oxide synthase, macrophage inflammatory protein-2, and heat shock protein 27). Injury markers in bronchoalveolar lavage fluid (BALF) were used to assess immediate toxicity and inflammation in rats similarly exposed. At 2 ppm, injury was marked by significant increases in BALF total protein, N-acetylglucosaminidase, and lavageable ciliated cells. Because infiltration of neutrophils was observed only at the higher 5 ppm concentration, the distinctive genes suggested a potential amplification role for inflammation in the gene profile. Although the specific gene interactions remain unclear, this is the first report indicating a dose-dependent direct and immediate induction of gene expression that may be separate from those genes involved in inflammation after acute O3 exposure.

Phosphatidylcholine-specific phospholipase C in mitogen-stimulated fibroblasts

To investigate expression, subcellular localization and mechanisms of translocation of phosphatidylcholine-specific phospholipase C (PC-PLC) during the cell proliferative response, biochemical, immunoblotting, and immunofluorescence analyses were performed on quiescent and mitogen-stimulated NIH-3T3 fibroblasts. Platelet-derived growth factor (PDGF), insulin and 12-O-tetradecanoylphorbol-13-acetate induced, in 10-60 min, PC-PLC translocation from a perinuclear cytoplasmic area to the plasma membrane. Following cell exposure to PDGF (60 min), the overall PC-PLC expression increased up to 2-3x, while the enzyme activity increased 5x in total cell lysates, 2x in the plasma membrane, and 4x in the nucleus; moreover, confocal laser scanning microscopy showed a progressive externalization of PC-PLC on the outer plasma membrane surface and its accumulation in the nuclear matrix. Pre-incubation of cells with the PC-PLC inhibitor tricyclodecan-9-yl potassium xanthate (D609), before PDGF-stimulation, not only reduced the enzyme activity in total cell lysates as well as in plasma membrane and nuclear fractions, but also blocked the mechanisms of PC-PLC subcellular redistribution. These effects were associated with a D609-induced long-lasting cell cycle block in Go.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Angiotensin II-stimulated cortisol secretion is mediated by phospholipase D

Angiotensin II (Ang-II) regulates a variety of cellular functions including cortisol secretion. In the present report, we demonstrate that Ang-II activates phospholipase D (PLD) in zona fasciculata (ZF) cells of bovine adrenal glands, and that this effect is associated to the stimulation of cortisol secretion by this hormone. PLD activation was dependent upon extracellular Ca2+, and was blocked by inhibition of protein kinase C (PKC). Using the reverse transcription-polymerase chain reaction technique, we demonstrated that ZF cells express both PLD-1 and PLD-2 isozymes. Primary alcohols, which attenuate the formation of phosphatidate (the product of PLD), and cell-permeable ceramides, which inhibit PLD potently, blocked Ang-II-stimulated cortisol secretion. Furthermore, propranolol or chlorpromazine, which are potent inhibitors of phosphatidate phosphohydrolase (PAP) (the enzyme that produces diacylglycerol from phosphatidate), also blocked cortisol secretion. These data suggest that the PLD/PAP pathway plays an important role in the regulation of cortisol secretion by Ang-II in ZF cells.

Lipid inflammatory mediators in diabetic vascular disease

Type 2 diabetes is associated with significantly accelerated rates of macrovascular complications such as atherosclerosis. Emerging evidence now indicates that atherosclerosis is an inflammatory disease and that certain inflammatory markers may be key predictors of diabetic atherosclerosis. Proinflammatory cytokines and cellular adhesion molecules expressed by vascular and blood cells during stimulation by growth factors and cytokines seem to play major roles in the pathophysiology of atherosclerosis and diabetic vascular complications. However, more recently, data suggest that inflammatory responses can also be elicited by smaller oxidized lipids that are components of atherogenic oxidized low-density lipoprotein or products of phospholipase activation and arachidonic acid metabolism. These include oxidized lipids of the lipoxygenase and cyclooxygenase pathways of arachidonic acid and linoleic acid metabolism. These lipids have potent growth, vasoactive, chemotactic, oxidative, and proinflammatory properties in vascular smooth muscle cells, endothelial cells, and monocytes. Cellular and animal models indicate that these enzymes are induced under diabetic conditions, have proatherogenic effects, and also mediate the actions of growth factors and cytokines. This review highlights the roles of the inflammatory cyclooxygenase and 12/15-lipoxygenase pathways in the pathogenesis of diabetic vascular disease. Evidence suggests that inflammatory responses in the vasculature can be elicited by small oxidized lipids that are components of oxidized low-density lipoprotein or products of the lipoxygenase and cyclooxygenase pathways of arachidonic and linoleic acid metabolism. This review evaluates these inflammatory and proatherogenic pathways in the pathogenesis of diabetic vascular disease.

Identification of two RNA cis-elements that function to regulate the 5' splice site selection of Bcl-x pre-mRNA in response to ceramide

Two splice variants derived from the BCL-x gene, proapoptotic Bcl-x(s) and anti-apoptotic Bcl-x(L), are produced via alternative 5' splice site selection within exon 2 of Bcl-x pre-mRNA. In previous studies, our laboratory demonstrated that ceramide regulated this 5' splice site selection, inducing the production of Bcl-x(s) mRNA with a concomitant decrease in Bcl-x(L) correlating with sensitization to chemotherapy (Chalfant, C. E., Rathman, K., Pinkerman, R. L., Wood, R. E., Obeid, L. M., Ogretmen, B., and Hannun, Y. A. (2002) J. Biol. Chem. 277, 12587-12595). We have now identified several possible RNA cis-elements within exon 2 of Bcl-x pre-mRNA by sequence analysis. To study the possible roles of these RNA cis-elements in regulating the alternative 5' splice site selection of Bcl-x pre-mRNA, we developed a BCL-x minigene construct which conferred the same ratio of Bcl-x(L)/Bcl-x(s) mRNA as the endogenous Bcl-x and was responsive to ceramide treatment. Mutagenesis of either a purine-rich splicing enhancer or a pyrimidine tract element within exon 2 induced a change in the ratio of Bcl-x(L)/Bcl-x(s) mRNA from 7 to 1 and 0.23, thereby diminishing the selection of the Bcl-x(L) 5' splice site with a concomitant increase in Bcl-x(s) 5' splice site selection. Furthermore, mutagenesis of these cis-elements abolished the ability of ceramide to affect the 5' splice site selection. In vitro binding assays coupled with competitor studies demonstrated specific binding of RNA trans-activating proteins to these regions. SDS-PAGE analysis of cross-linked RNA trans-activating factors with these RNA cis-elements revealed the binding of 215-, 120-, and 30-kDa proteins to the purine-rich element and 120- and 76-kDa proteins to the pyrimidine tract element. In addition, exogenous treatment of A549 cells with ceramide increased the formation of protein complexes with these RNA cis-elements. Therefore, we have identified two ceramide-responsive RNA cis-elements within exon 2 of Bcl-x pre-mRNA, and this is the first report of an RNA cis-element responsive to a bioactive lipid.

Transcriptional repression of cyclin-dependent kinase inhibitor p21 gene by phospholipase D1 and D2

Phospholipase D (PLD) is known to stimulate cell cycle progression and to transform murine fibroblast cells into tumorigenic forms, although the precise mechanisms are not elucidated. In this report, we demonstrated that both PLD1 and PLD2 repressed expression of cyclin-dependent kinase inhibitor p21 gene in an additive manner. The phospholipase activity of PLDs was important for the effect. PLD1 repressed the p21 promoter by decreasing the level of p53, whereas PLD2 via a p53-independent pathway through modulating Sp1 activity. Taken together, we suggest that PLD isozymes stimulate cell growth by repressing expression of p21 gene, which may ultimately lead to carcinogenesis.

Mechanism of persistent protein kinase D1 translocation and activation

The specificity of many signal transduction pathways relies on the spatiotemporal features of each signaling step. G protein-coupled receptor-mediated activation of protein kinases leads to diverse cellular effects. Upon receptor activation, PKD1 and several C-type protein kinases (PKCs), translocate to the plasma membrane and become catalytically active. Here we show that, unlike PKCs, PKD1 remains active at the membrane for hours. The two DAG binding C1 domains of PKD1 have distinct functional roles in targeting and maintaining PKD1 at the plasma membrane. C1A achieves fast, maximal, and reversible translocation, while C1B translocates partially, but persistently, to the plasma membrane. The persistent localization requires the C1B domain of PKD1, which binds Galphaq. We incorporate the kinetics of PKD1 translocation into a three-state model that suggests how PKD1 binding to DAG and Galphaq uniquely encodes frequency-dependent PKD1 signaling.

The second phase activation of protein kinase C delta at late G1 is required for DNA synthesis in serum-induced cell cycle progression

BACKGROUND

Cell lines that stably over-express protein kinase C (PKC) delta frequently show a decrease in growth rate and saturation density, leading to the hypothesis that PKC delta has a negative effect on cell proliferation. However, the mode of PKC delta activation, the cell cycle stage requiring PKC delta activity, and the exact role of PKC delta at that stage remains unknown.

RESULTS

Here we show that the treatment of quiescent fibroblasts with serum activates PKC delta at two distinct time points, within 10 min after serum treatment, and for a longer duration between 6 and 10 h. This biphasic activation correlates with the phosphorylation of Thr-505 at the activation loop of PKC delta. Importantly, an inhibitor of PKC delta, rottlerin, suppresses the biphasic activation of PKC delta, and suppression of the second phase of PKC delta activation is sufficient for the suppression of DNA synthesis. Consistent with this, the transient over-expression of PKC delta mutant molecules lacking kinase activity suppresses serum-induced DNA synthesis. These results imply that PKC delta plays a positive role in cell cycle progression. While the over-expression of PKC delta enhances serum-induced DNA synthesis, this was not observed for PKC epsilon. Similar experiments using a series of PKCdelta/ epsilon chimeras showed that the carboxyl-terminal 51 amino acids of PKC delta are responsible for the stimulatory effect. On the other hand, the over-expression of PKC delta suppresses cell entry into M-phase, being consistent with the previous studies based on stable over-expressors.

CONCLUSIONS

We conclude that PKC delta plays a role in the late-G1 phase through the positive regulation of cell-cycle progression, in addition to negative regulation of the entry into M-phase.

Mechanism of angiotensin II-induced phospholipase D activation in bovine adrenal glomerulosa cells

Based on previous data demonstrating activation of phospholipase D (PLD) in response to angiotensin II (AngII), we have hypothesized a role for PLD in mediating aldosterone secretion from bovine adrenal glomerulosa cells. In this study we demonstrate that a PLD-generated signal(s) is required for the AngII-elicited secretory response, since interfering with lipid second messenger formation using a primary alcohol inhibited AngII-induced aldosterone secretion, but not that elicited by incubation with a hydrophilic cholesterol analog, 22(R)-hydroxycholesterol, which bypasses signaling pathways. Three mechanisms for hormonal activation of PLD have been described in other systems: direct receptor coupling, activation through protein kinase C (PKC) and a combination of these two mechanisms. Our results indicate that the PKC activator, phorbol 12-myristic 13-acetate (PMA), is able to activate PLD, and that receptor engagement is apparently not necessary for PLD activation in response to this agent. Maximal doses of AngII and PMA produced no additive effect on PLD activation, suggesting that these two agents function through a common PKC pathway. This interpretation was confirmed by the ability of a PKC inhibitor, Gö 6976, to inhibit partially AngII-induced PLD activation. Finally, treatment with the calcium ionophores A23187 or ionomycin or the calcium channel agonist BAY K8644 had no effect on PLD activity. Likewise, inhibiting calcium influx with high-dose nitrendipine affected neither basal PLD activity nor that stimulated by AngII. Thus, our results suggest a role for PKC, independent of calcium influx, in mediating AngII-induced PLD activation in glomerulosa cells.

Ketamine suppresses norepinephrine-induced inositol 1,4,5-trisphosphate formation via pathways involving protein kinase C

Inositol 1,4,5-trisphosphate (IP(3)) is not only involved in the physiologic regulation of excitation-contraction coupling, but could also play a role in cardiac pathophysiology. We investigated the mechanism of ketamine modulation of norepinephrine (NE)-induced IP(3) formation in neonatal rat cardiomyocytes. Ketamine 1 and 10 microM significantly decreased the IP(3) response to 1 microM NE by 27% and 43%, respectively. One micromolar TMB-8 (an intracellular calcium antagonist) produced 42% more decreases in IP(3) production than produced by ketamine alone. One hundred micromolar anthranilic acid (a phospholipase A(2) inhibitor) significantly decreased NE (1 microM)-induced IP(3) formation, and the inhibition was further enhanced by ketamine. Ten micromolar U 73122 (a phospholipase C inhibitor) did not significantly affect NE-induced IP(3) in the presence or absence of ketamine. One micromolar ketamine significantly inhibited staurosporine (a nonselective protein kinase C antagonist)-, bisindolylmaleimide (a selective protein kinase C antagonist)-, and wortmannin (a phosphatidylinositide 3-kinase antagonist)-stimulated IP(3) formation. In conclusion, ketamine suppresses NE-induced IP(3) production, and the inhibition is caused through pathways including protein kinase C and a decrease in intracellular Ca(2+) concentrations.

IMPLICATIONS

Ketamine inhibits norepinephrine-induced inositol 1,4,5-triphosphate formation in a dose-dependent manner via pathways that involve protein kinase C and a decrease in intracellular Ca(2+) concentrations.

Transcriptional and post-transcriptional mechanisms of glucocorticoid antiproliferative effects

Glucocorticoids (GCs) are used as immunosuppressive and anti-inflammatory agents in treating organ transplantation rejection, autoimmune diseases, (hematological) cancers, and inflammatory disorders. GCs exert their effects through a multitude of mechanisms, the most significant of which is inhibition of cytokine production, and for some cytokines their effects on target cells. Paradoxically, GCs also upregulate the expression of (pro-inflammatory) high-affinity cytokine receptors on target cells in the face of lost ligand (cytokine) stimulation. GC inhibition of cytokine expression occurs at both transcriptional and post-transcriptional levels. GCs acted transcriptionally by binding their cytosolic receptor (GR), thereby facilitating its nuclear translocation and subsequent binding to the promoter region of cytokine genes on sites compatible with GC response element (GRE) motifs, which in turn directly or indirectly regulated gene expression. In addition to direct DNA binding, GCs acted post-transcriptionally by: (1) antagonism of nuclear factors required for efficient gene expression either directly or through induction of the expression of specific transcription factor antagonists, (2) altered Th lineage development by favouring the generation of (anti-inflammatory) Th2 cells and suppressing the induction or the activity of established (pro-inflammatory) Th1 cells, and (3) stimulating the expression of transforming growth factor (TGF)-beta, an immunosuppressive cytokine which inhibited cytokine production. However, these mechanisms are not mutually exclusive, since GCs may utilize more than one mechanism in exerting their anti-proliferative effect.

FAS activation induces dephosphorylation of SR proteins; dependence on the de novo generation of ceramide and activation of protein phosphatase 1

The search for potential targets for ceramide action led to the identification of ceramide-activated protein phosphatases (CAPP). To date, two serine/threonine protein phosphatases, protein phosphatase 2A (PP2A) and protein phosphatase 1 (PP1), have been demonstrated to function as ceramide-activated protein phosphatases. In this study, we show that treatment with either anti-FAS IgM (CH-11) (150 ng/ml) or exogenous d-(e)-C(6-)ceramide (20 microm) induces the dephosphorylation of the PP1 substrates, serine/arginine-rich (SR) proteins, in Jurkat acute leukemia T-cells. The serine/threonine protein phosphatase inhibitor, calyculin A, but not the PP2A-specific inhibitor, okadaic acid, inhibited both FAS- and ceramide-induced dephosphorylation of SR proteins. Anti-FAS IgM treatment of Jurkat cells led to a significant increase in levels of endogenous ceramide beginning at 2 h with a maximal increase of 10-fold after 7 h. A 2-h pretreatment of Jurkat cells with fumonisin B(1) (100 microm), a specific inhibitor of CoA-dependent ceramide synthase, blocked 80% of the ceramide generated and completely inhibited the dephosphorylation of SR proteins in response to anti-FAS IgM. Moreover, pretreatment of Jurkat cells with myriocin, a specific inhibitor of serine-palmitoyl transferase (the first step in de novo synthesis of ceramide), also blocked FAS-induced SR protein dephosphorylation, thus demonstrating a role for de novo ceramide. These results were further supported using A549 lung adenocarcinoma cells treated with d-(e)-C(6-)ceramide. Dephosphorylation of SR proteins was inhibited by fumonisin B(1) and by overexpression of glucosylceramide synthase; again implicating endogenous ceramide generated de novo in regulating the dephosphorylation of SR proteins in response to FAS activation. These results establish a specific intracellular pathway involving both de novo ceramide generation and activation of PP1 to mediate the effects of FAS activation on SR proteins.

Protein expression of phospholipase C in pregnant and nonpregnant rat uterine tissue

OBJECTIVE

The phosphatidylinositol signaling pathway appears to play a significant role in the intracellular events leading to agonist-stimulated phasic myometrial contractions. The studies described in this report were performed to characterize phospholipase C isoform expression at the protein level and to confirm histologic localization of these proteins within the myometrial smooth muscle layers of the uterus.

METHODS

For these studies, uterine tissue was obtained from timed- pregnant and spontaneously cycling adult female Sprague-Dawley rats. After isolation of myometrial cell membranes and cytosolic proteins, Western blots were performed by using phospholipase C isoform-specific antibodies. Tissue cross-sections of near-term pregnant rat uterus were used with the phospholipase C isoform-specific antibodies for immunohistochemical studies.

RESULTS

The Western blot studies confirmed expression of the phospholipase C-beta3, -gamma1, -gamma2, and -delta1 proteins in both the membrane and cytosolic fractions of rat myometrium; in contrast, only trace amounts of the phospholipase C-beta1 protein was observed in this tissue. The immunohistochemical studies demonstrated localization of the phospholipase C-beta3, -gamma1, -gamma2, -delta1 and to a lesser degree phospholipase C-beta1 isoforms within the longitudinal and circular smooth muscle layers of the near-term pregnant rat uterus.

CONCLUSION

These studies have confirmed the simultaneous expression of several phospholipase C proteins within the smooth muscle cells of the pregnant and nonpregnant rat uterus, thereby providing support for the possible redundant role of these signal transduction enzymes during the generation of cytosolic calcium oscillations and phasic myometrial contractions.

Cellular localization and functional role of phosphatidylcholine-specific phospholipase C in NK cells

Although several classes of phospholipases have been implicated in NK cell-mediated cytotoxicity, no evidence has been reported to date on involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) in NK activation by lymphokines and/or in lytic granule exocytosis. This study demonstrated the expression of two PC-PLC isoforms (M(r) 40 and 66 kDa) and their IL-2-dependent distribution between cytoplasm and ectoplasmic membrane surface in human NK cells. Following cell activation by IL-2, cytoplasmic PC-PLC translocated from the microtubule-organizing center toward cell periphery, essentially by kinesin-supported transport along microtubules, while PC-PLC exposed on the outer cell surface increased 2-fold. Preincubation of NK cells with a PC-PLC inhibitor, tricyclodecan-9-yl-xanthogenate, strongly reduced NK-mediated cytotoxicity. In IL-2-activated cells, this loss of cytotoxicity was associated with a decrease of PC-PLC exposed on the cell surface, and accumulation of cytoplasmic PC-PLC in the Golgi region. Massive colocalization of PC-PLC-rich particles with perforin-containing granules was found in the cytoplasm of NK-activated (but not NK-resting) cells; both organelles clustered at the intercellular contact region of effector-target cell conjugates. These newly detected mechanisms of PC-PLC translocation and function support an essential role of this enzyme in regulated granule exocytosis and NK-mediated cytotoxicity.

Quantitative analysis of phospholipids and gangliosides in bone marrow progenitors of lymphocytes, thymocytes and mature lymphocytes in tumor-bearing animals

The phospholipid and ganglioside composition in bone marrow progenitors of lymphocytes, thymocytes and mature lymphocytes of intact rats and rats with sarcoma 45 were studied. The lymphocytes and their progenitors were isolated by Ficoll-Paque density centrifugation. The phospholipids and gangliosides were separated by thin-layer chromatography following standard chloroform:methanol extraction from the cells. Alterations in the lipid spectrum (both phospholipids and gangliosides) were shown to take place during lymphocyte differentiation. The rate of ganglioside sialylation diminished, which was expressed as an increase in mono- and di-, and a decrease in tri- and tetrasialoganglioside levels. Tumor-induced alterations in lymphocyte lipid composition involve all stages of lymphocyte differentiation. These shifts are believed to be connected with a disturbance of the antineoplastic function of lymphocytes and, consequently, the immune response of the tumor-bearing organism.

Phosphatidylcholine-specific phospholipase C and D in stimulation of RAW264.7 mouse macrophage-like cells by lipopolysaccharide

The purpose of these studies was to identify the role of phospholipases in the activation of macrophages by lipopolysaccharide (LPS). Tricyclodecan-9-yl-xanthogenate (D609), an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC); butanol, an inhibitor of phosphatidylcholine phospholipase D (PC-PLD); and propranolol, an inhibitor of phosphatidate phosphohydrolase, were used in the study. Treatment of RAW264.7 murine macrophage-like cells with LPS resulted in expression of inducible nitric oxide synthase and tumor necrosis factor-alpha. The expression was partially inhibited by D609, butanol, or propranolol and was completely blocked by the combination of D609 and butanol. RAW264.7 cells constitutively produced low basal levels of diacylglycerol and phosphatidic acid; production of both was significantly increased after stimulation with LPS, reaching a peak in 2-3 min and remaining elevated after 30 min. In LPS-induced RAW264.7 cells, diacylglycerol was suppressed by each of the three inhibitors alone and almost abolished by D609 plus butanol or D609 plus propranolol. Phosphatidic acid was reduced to basal level by butanol after LPS stimulation for 2.5 min and by butanol plus D609 after LPS stimulation for 2.5 or 10 min. Taken together, these data indicate that activation of RAW264.7 cells by LPS can be mediated by the activities of both PC-PLC and PC-PLD.

1,25-Dihydroxyvitamin D(3), phospholipase D and protein kinase C in keratinocyte differentiation

1,25-Dihydroxyvitamin D(3), thought to be a physiological regulator of epidermal keratinocyte growth and differentiation, also elicits the complete differentiative program in vitro, with expression of various genes/proteins characteristic of both early and late differentiation. 1,25-Dihydroxyvitamin D(3) functions by interacting with an intracellular receptor that binds to DNA at vitamin D response elements (VDRE) thereby affecting transcription. 1,25-Dihydroxyvitamin D(3) has been demonstrated to alter the expression of several enzymes involved in signal transduction, and presumably this is the mechanism through which the hormone regulates differentiation. It has recently been shown that 1,25-dihydroxyvitamin D(3) specifically increases the expression/activity of phospholipase D-1, an enzyme that hydrolyzes phospholipids to generate lipid messengers, such as diacylglycerol (DAG). DAG, in turn, is known to activate several members of the protein kinase C (PKC) family. It has been proposed that this signaling pathway mediates late differentiation events in epidermal keratinocytes. In this article the data supporting a role for PKC and phospholipase D in keratinocyte differentiation, as well as in the pathogenesis of skin diseases, are reviewed and a model is proposed for the signaling pathways that regulate this process upon exposure to 1,25-dihydroxyvitamin D(3).

Phospholipase D activation by norepinephrine is mediated by 12(s)-, 15(s)-, and 20-hydroxyeicosatetraenoic acids generated by stimulation of cytosolic phospholipase a2. tyrosine phosphorylation of phospholipase d2 in response to norepinephrine

Norepinephrine (NE) stimulates phospholipase D (PLD) through a Ras/MAPK pathway in rabbit vascular smooth muscle cells (VSMC). NE also activates calcium influx and calmodulin (CaM)-dependent protein kinase II-dependent cytosolic phospholipase A(2) (cPLA(2)). Arachidonic acid (AA) released by cPLA(2)-catalyzed phospholipid hydrolysis is then metabolized into hydroxyeicosatetraenoic acids (HETEs) through lipoxygenase and cytochrome P450 4A (CYP4A) pathways. HETEs, in turn, have been shown to stimulate Ras translocation and to increase MAPK activity in VSMC. This study was conducted to determine the contribution of cPLA(2)-derived AA and its metabolites (HETEs) to the activation of PLD. NE-induced PLD activation was reduced by two structurally distinct CaM antagonists, W-7 and calmidazolium, and by CaM-dependent protein kinase II inhibition. Blockade of cPLA(2) activity or protein depletion with selective cPLA(2) antisense oligonucleotides abolished NE-induced PLD activation. The increase in PLD activity elicited by NE was also blocked by inhibitors of lipoxygenases (baicalein) and CYP4A (17-octadecynoic acid), but not of cyclooxygenase (indomethacin). AA and its metabolites (12(S)-, 15(S)-, and 20-HETEs) increased PLD activity. PLD activation by AA and HETEs was reduced by inhibitors of Ras farnesyltransferase (farnesyl protein transferase III and BMS-191563) and MEK (U0126 and PD98059). These data suggest that HETEs are the mediators of cPLA(2)-dependent PLD activation by NE in VSMC. In addition to cPLA(2), PLD was also found to contribute to AA release for prostacyclin production via the phosphatidate phosphohydrolase/diacylglycerol lipase pathway. Finally, a catalytically inactive PLD(2) (but not PLD(1)) mutant inhibited NE-induced PLD activity, and PLD(2) was tyrosine-phosphorylated in response to NE by a MAPK-dependent pathway. We conclude that NE stimulates cPLA(2)-dependent PLD(2) through lipoxygenase- and CYP4A-derived HETEs via the Ras/ERK pathway by a mechanism involving tyrosine phosphorylation of PLD(2) in rabbit VSMC.

Ribozyme-mediated inhibition of rat leukocyte-type 12-lipoxygenase prevents intimal hyperplasia in balloon-injured rat carotid arteries

BACKGROUND

12-Lipoxygenase (12-LO) products of arachidonate metabolism have growth and chemotactic effects in vascular smooth muscle cells. We have also recently demonstrated increased 12-LO mRNA and protein expression in the neointima of balloon-injured rat carotid arteries. In this study, we evaluated whether 12-LO activation plays a role in neointimal thickening in this rat model by using a specific ribozyme (Rz) directed to rat 12-LO.

METHODS AND RESULTS

We designed a chimeric DNA-RNA hammerhead Rz to cleave rat leukocyte-type 12-LO mRNA. This Rz dose-dependently cleaved a 166-nucleotide target 12-LO mRNA substrate in vitro and reduced 12-LO mRNA and protein expression in rat vascular smooth muscle cells. A control mutant Rz (MRz) with a point mutation in the catalytic site was inactive. To test the in vivo efficacy of the 12-LO Rz, the left common carotid arteries of rats were injured with a balloon catheter. The distal half of the injured arteries was treated with Rz or MRz mixed with lipofectin. The proximal half received only lipofectin. Twelve days after injury, intima-to-media ratios were significantly lower in the Rz-treated sections than in untreated sections from the same rat (0.742+/-0.16 versus 1.749+/-0.12, P:<0.001). In contrast, the MRz had no significant effect.

CONCLUSIONS

These results indicate the important role of the leukocyte-type 12-LO pathway in restenosis in response to injury.

Vascular β-adrenoceptor function in hypertension and in ageing

Functional β-adrenoceptors (β-AR) have been identified and characterized in blood vessels under in vivo conditions as well as in vascular smooth muscle cells (SMC) grown in culture. Agonist occupancy of β-AR activates adenylyl cyclase (AC) via the stimulatory guanine nucleotide-binding protein (Gs) and leads to elevations in intracellular adenosine 3',5'-cyclic monophosphate levels (cAMP). Increased cAMP activates the cAMP-dependent protein kinase (PKA), with subsequent phosphorylation of various target proteins. This β-AR pathway interacts with several other intracellular signalling pathways via cross-talk, so that activation by β-AR agonists may also modulate other second messengers and protein kinases. SMC β-AR play an important role in SMC function. In intact blood vessels they mediate SMC relaxation by various intracellular mechanisms, ultimately causing a decrease in intracellular Ca2+ levels. In cultured SMC, activation of the β-AR pathway results in inhibition of cellular proliferation, the development of SMC polyploidy, and SMC apoptosis. Blood vessels from hypertensive animals are characterized by an increase in SMC cell mass, a greater incidence of SMC polyploidy in the aorta, and an impairment in the β-agonist-mediated SMC relaxation. Some of these changes may result from an attenuation of β-AR function due to agonist-induced receptor desensitization caused by the uncoupling of receptors from the Gs-AC system. The phosphorylated β-AR may in turn trigger new signals and activate different intracellular pathways. However, the details of these mechanisms are still unresolved. Since functional β-AR play such a prominent and multi-faceted role in SMC function, it is important to understand how these diverse physiological effects are mediated by this receptor system, and how they contribute to the development of hypertension. With ageing, a decrease in β-AR-Gs-AC coupling is observed, and this is implicated in the reduced responsiveness of SMC. The similarities in SMC β-AR functional changes in hypertension and in ageing suggest that the underlying mechanisms are also analogous.Key words: smooth muscle, β-adrenoceptors, cyclic AMP, protein kinase A, cell proliferation, polyploidy, relaxation, apoptosis, hypertension, ageing.

An alternate mechanism of glucocorticoid anti-proliferative effect: promotion of a Th2 cytokine-secreting profile

Glucocorticoids (GCs) are used as immunosuppressive and anti-inflammatory agents in organ transplantation and in treating autoimmune diseases and inflammatory disorders and they exert their effects by several mechanisms, the most significant of which is inhibition of cytokine production and action. Recent reports suggested that GCs inhibit cytokine expression indirectly through promotion of a T helper cell type 2 (Th2) cytokine-secreting profile, thereby resulting in preferential blockade of pro-inflammatory monokine and T helper cell type 1 (Th1) cytokine expression. The target of GCs appeared to be monocytes macrophages, whereby altered regulation of interleukin (IL)-1/IL-1 receptor antagonist (IL-1ra), coupled with profound blockade of IL-12 synthesis and inhibition of interferon (IFN)-gamma-induced major histocompatibility complex (MHC) class II expression, lead to a preferential cognate stimulation of Th2 cells at the expense of Th1 cells. It is possible that this may have involved the expansion of a Th2-cell pool or, in addition, frank stimulation of uncommitted naive CD4 + T cells toward the Th2 lineage. In addition, GCs may have blocked Th1 cytokine expression, thereby inhibiting ongoing Th1 cytokine secretion, and consequently provided for the unimpeded production of Th2 cytokines. Collectively, this indicates that, in exerting their anti-proliferative effects, GCs act indirectly by altering Th1/Th2 cytokine balance, blocking the (pro-inflammatory) Th1 program and favoring the (anti-inflammatory) Th2 program.

Long chain ceramides activate protein phosphatase-1 and protein phosphatase-2A. Activation is stereospecific and regulated by phosphatidic acid

The search for potential targets for ceramide action led to the identification of ceramide-activated protein phosphatases, which include protein phosphatase-2A (PP2A) and protein phosphatase-1 (PP1) with roles in regulating apoptosis and cell growth. Thus far, in vitro studies on ceramide-activated protein phosphatases have been restricted to the use of short chain ceramides, limiting the extent of mechanistic insight. In this study, we show that the long chain D-erythro-C18-ceramide activated PP2A (AB'C trimer), PP2Ac (catalytic subunit of PP2A), and PP1gammac and -alphac (catalytic subunits of PP1gamma and -1alpha isoforms, respectively) 2-6-fold in the presence of dodecane, a lipid-solubilizing agent, with 50% maximal activation achieved at approximately 10 microM D-erythro-C18-ceramide. The diastereoisomers of D-erythroC18-ceramide, D-threo-, and L-threo-C18-ceramide, as well as the enantiomeric L-erythro-C18-ceramide, did not activate PP1 or PP2A, but they inhibited PP1 and PP2A activity. The addition of phosphatidic acid decreased the basal activity of PP1c but also increased the stimulation by D-erythro-C18-ceramide from 1.8- to 2. 8-fold and decreased the EC50 of D-erythro-C18-ceramide to 4.45 microM. The addition of 150 mM KCl decreased the basal activity of PP1 and the dose of D-erythro-C18-ceramide necessary to activate PP1c (EC50 = 6.25 microM) and increased the ceramide responsiveness up to 10-17-fold. These studies disclose stereospecific activation of PP1 and PP2A by long chain natural ceramides under near physiologic ionic strengths in vitro. The implications of these studies for mechanisms of ceramide action are discussed.

Amphitropic proteins: regulation by reversible membrane interactions (review)

What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

Sustained phospholipase D activation is associated with keratinocyte differentiation

Our previous results and data in the literature have suggested a potential role for phospholipase D (PLD) in the regulation of epidermal keratinocyte growth and differentiation. Therefore, we investigated the effect of agents reported to modulate keratinocyte growth and differentiation on PLD activation. The purported protein kinase C (PKC) 'inhibitor', staurosporine (Stsp), has been reported to activate PKC in keratinocytes, eliciting many of the same effects as active tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate (TPA). Stsp also induces a programmed pattern of differentiation similar to that seen in keratinocytes in vivo; TPA, on the other hand, appears to preferentially elicit markers consistent with late (granular) differentiation. In contrast, bradykinin is reported to stimulate keratinocyte proliferation. We found that these three agents had different effects on PLD activation in primary mouse epidermal keratinocytes. TPA increased PLD activity acutely and in a sustained fashion. In contrast, Stsp did not acutely activate PLD and inhibited acute TPA-induced activation of PLD. However, treatment of keratinocytes with Stsp for longer time periods (3-5 h) induced sustained PLD activation and this long-term effect was additive with that of TPA. Bradykinin activated PLD acutely but transiently. Both TPA and Stsp increased transglutaminase activity, a marker of late differentiation, whereas bradykinin had little or no effect on either cell proliferation or transglutaminase activity. These results suggest that a sustained activation of PLD is associated with the induction of keratinocyte differentiation. We hypothesize that PLD activity mediates late keratinocyte differentiation through generation of diacylglycerol and activation of specific PKC isoforms. Furthermore, we propose that the profound and immediate TPA-induced stimulation of PLD activity 'drives' the keratinocytes to late differentiation steps. However, the less efficacious (and more gradual) sustained activation of PLD by Stsp may allow a patterned differentiation more like that observed in skin.

Mycoplasma penetrans infection of Molt-3 lymphocytes induces changes in the lipid composition of host cells

The AIDS-associated Mycoplasma penetrans is capable of inducing its own uptake by non-phagocytic cells. The ability of M. penetrans to both adhere to and invade Molt-3 lymphocytes was markedly increased in the presence of polyethylene glycol 8000 (PEG). The effect of PEG was more pronounced in the more alkaline pH range, where the binding kinetics were much faster and almost unaffected by temperature (4-37 degrees C). Incubation of [14C]oleic-acid-labelled Molt-3 cells with viable M. penetrans resulted in a substantial release of radioactive fatty acids, whereas treating the host cells with heat-inactivated mycoplasmas, isolated M. penetrans membrane preparations, or M. penetrans growth medium, had no effect. Total lipid analysis of Molt-3 lymphocytes infected by M. penetrans revealed an augmented level of the neutral lipid fraction that was associated with a decrease in the relative amounts of polar lipids, mainly a decrease in the amount of phosphatidylserine and diphosphatidylglycerol. Analysis of the neutral lipid fraction in the infected Molt-3 cells revealed a fivefold increase in the relative amount of diacylglycerol and a marked increase in the free fatty acid (FFA) fraction. The profile of the FFAs released was dominated by a relatively high concentration of the polyunsaturated fatty acid docosahexaenoic acid. The release of lipid intermediates suggests that the degradation of Molt-3 cell phospholipids induced by M. penetrans may initiate a signal transmission cascade in the host cell.

Nuclear diacylglycerol produced by phosphoinositide-specific phospholipase C is responsible for nuclear translocation of protein kinase C-alpha

It is well established that an independent inositide cycle is present within the nucleus, where it is involved in the control of cell proliferation and differentiation. Previous results have shown that when Swiss 3T3 cells are treated with insulin-like growth factor-I (IGF-I) a rapid and sustained increase in mass of diacylglycerol (DAG) occurs within the nuclei, accompanied by a decrease in the levels of both phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. However, it is unclear whether or not other lipids could contribute to this prolonged rise in DAG levels. We now report that the IGF-I-dependent increase in nuclear DAG production can be inhibited by the specific phosphatidylinositol phospholipase C inhibitor 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine or by neomycin sulfate but not by the purported phosphatidylcholine-phospholipase C specific inhibitor D609 or by inhibitors of phospholipase D-mediated DAG generation. Treatment of cells with 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine or neomycin sulfate inhibited translocation of protein kinase C-alpha to the nucleus. Moreover, exposure of cells to 1-O-octadeyl-2-O-methyl-sn-glycero-3-phosphocholine, but not to D609, dramatically reduced the number of cells entering S-phase upon stimulation with IGF-I. These results suggest that the only phospholipase responsible for generation of nuclear DAG after IGF-I stimulation of 3T3 cells is PI-PLC. When this activity is inhibited, neither DAG rise is seen nor PKC-alpha translocation to the nucleus occurs. Furthermore, this PI-PLC activity appears to be essential for the G0/G1 to S-phase transition.

Induction of signalling anergy via the T-cell receptor in cultured Jurkat T cells by pre-exposure to a filarial nematode secreted product

Filarial nematodes constitute major causes of morbidity in the Tropics. The worms have a life-span exceeding five years, a longevity which is considered to reflect at least in part, their ability to interfere with host lymphocyte responsiveness. To date the molecular mechanisms underlying this ability have not been defined but we now demonstrate that ES-62, a phosphorylcholine (PC)-containing glycoprotein released by the rodent filarial parasite Acanthocheilonema viteae, is able to render Jurkat T cells anergic to intracellular signalling via the antigen receptor (TCR). In particular, ES-62 acts by modulating activation of the tyrosine kinases Fyn, Lck and ZAP-70 leading to selective disruption of TCR coupling to the phospholipase D, protein kinase C, phosphoinositide-3-kinase and RasMAPkinase signalling cascades. These cascades are key elements in the transduction of transcriptional and proliferative signals following ligation of TCR. As PC-containing secreted products (PC-ES) are also released by human filarial parasites, our data suggest that PC-ES may play a role in the induction of T lymphocyte hyporesponsiveness observed during filarial infections.

PLA2 activity regulates Ca2+ storage-dependent cellular proliferation

The objective of this study is to determine the role of arachidonic acid (AA) in cell proliferation by inhibiting AA synthetic enzyme phospholipase A2 (PLA2) and to determine its involvement in the role of the second messenger intracellular calcium (Ca2+). Methods used to determine the effects on proliferation of cell cultures of primary meningioma and astrocytoma U373-MG included treatment with micromolar concentrations of PLA2 inhibitors 4-bromophenacylbromide and quinacrine. Effects of these drugs on proliferation were further investigated by the application of concentrations that inhibit growth by 50% while antagonizing these agents with AA replacement. Free cytosolic Ca2+ was measured with the use of fluorescent dye Fura-2 during PLA2 agonist/antagonist studies. These Ca2+ measurements were performed in the absence of extracellular Ca2+ to identify the contribution of intracellular Ca2+ sources. PLA2 inhibition resulted in decreased growth of cultured astrocytoma and meningioma cells in a dose-dependent manner in the micromolar range. This inhibitory effect was antagonized by the addition of AA. PLA2 inhibition caused an elevation of basal-cytosolic-free [Ca2+] while depleting internal Ca2+ stores. These Ca2+ changes were also antagonized by the addition of AA. In conclusion, these results demonstrate that AA, a PLA2 enzyme product, is involved in regulating the growth rate of these cell types. The PLA2 pathway also regulates the maintenance of the internal Ca2+ stores. Ca2+ is known to be a growth-related intracellular second messenger. These results suggest that the growth regulatory functions of AA are mediated by Ca2+-dependent mechanisms.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

The sevenfold way of PKC regulation

Protein kinase C (PKC) is a family of enzymes that are physiologically activated by 1,2-diacylglycerol (DAG) and other lipids. To date, 11 different isozymes, alpha, betaI, betaII, gamma, delta, epsilon, nu, lambda(iota), mu, theta and zeta, have been identified. On the basis of their structure and activators, they can be divided into three groups, two of which are activated by DAG or its surrogate, phorbol 12-myristate 13-acetate (PMA). PKC isozymes are remarkably different in number and prevalence in different cell lines and tissues. When activated, the isozymes bind to membrane phospholipids or to receptors that are located in and anchor the enzymes in a subcellular compartment. Some PKCs may also be activated in their soluble form. These enzymes phosphorylate serine and threonine residues on protein substrates, perhaps the best known of which are the myristoylated, alanine-rich C kinase substrate and nuclear lamins A, B and C. The enzymes clearly play a role in signal transduction, and, because of the importance of PMA as a tumor promoter, they are thought to affect some aspect of cell cycling. How PKC takes part in the regulation of cell transformation, growth, differentiation, ruffling, vesicle trafficking and gene expression, however, is largely unknown.