Regulation of inositol phospholipid-specific phospholipase C isozymes

Age-related loss of calcitriol stimulation of phosphoinositide hydrolysis in rat skeletal muscle

We have examined the effects in vitro of calcitriol [1,25(OH)2D3], the hormonal form of vitamin D3, on the breakdown of membrane phosphoinositides in skeletal muscle from young (3 months) and aged (24 months) rats. Calcitriol (10(-9) M) induced a rapid and transient release of IP3/inositol phosphates and diacylglycerol (DAG) from muscle slices/membranes prelabeled with [3H]myo-inositol and [3H]arachidonate, respectively. Inositol phosphate release was maximal at 15 s and then declined. The effects of hormone specificity exhibited as the closely related derivatives of vitamin D3, 25OHD3, 1alphaOHD3 and 24,25(OH)2D3 did not alter muscle inositol phosphate levels. The stimulation of DAG was biphasic, the early phase (15 s) being abolished by neomycin (0.5 mM), an inhibitor of phosphoinositide hydrolysis, similar to IP3 formation and consistent with a role of phospholipase C (PLC) in intracellular signal generation. Neomycin had no effect on the second DAG peak (2 min) induced by calcitriol, suggesting that the late phase of DAG formation is independent from the hydrolysis of phosphoinositides. Higher basal inositol phosphate and DAG levels were detected in muscle from aged rats thereby reducing the effects of the hormone on second messenger generation ( -80 and -60% for IP3 and DAG, respectively). Calcitriol stimulation of PLC was mimicked, in both young and old rats, by GTPgammaS, a non-hydrolyzable analogue of GTP, while GDPbetaS, a G protein inhibitor, suppressed the effect of the hormone. The early effects of calcitriol and GTPgammaS were not additive. Bordetella pertussis toxin abolished by 85% the effects of calcitriol on inositol phosphate release in young rats but was without effect in aged animals. These results demonstrate that calcitriol activates phosphoinositide-PLC in rat skeletal muscle by a mechanism which involves a pertussis-sensitive G protein and that the effects of the hormone are altered with ageing.

Genetic characterization of a phospholipase C gene from Candida albicans: presence of homologous sequences in Candida species other than Candida albicans

Phospholipase C (PLC) enzymes are essential in regulating several important cellular functions in eukaryotes, including yeasts. In this study, PCR was used to identify a gene encoding PLC activity in Candida albicans, using oligonucleotide primers complementary to sequences encoding highly conserved amino acid regions within the X domains of previously characterized eukaryotic phospholipase C genes. The nucleotide sequence of the C. albicans gene, CAPLC1 (2997 bp), was determined from a recombinant clone containing C. albicans 132A genomic DNA; it encoded a polypeptide of 1099 amino acids with a predicted molecular mass of 124.6 kDa. The deduced amino acid sequence of this polypeptide (CAPLC1) exhibited many of the features common to previously characterized PLCs, including specific X and Y catalytic domains. The CAPLC1 protein also exhibited several unique features, including a novel stretch of 18-19 amino acid residues within the X domain and an unusually long N-terminus which did not contain a recognizable EF-hand Ca(2+)-binding domain. An overall amino acid homology of more than 27% with PLCs previously characterized from Saccharomyces cerevisiae and Schizosaccharomyces pombe suggested that the CAPLC1 protein is a delta-form of phosphoinositide-specific PLC (PI-PLC). PLC activity was detected in cell-free extracts of both yeast and hyphal forms of C. albicans 132A following 7 h and 24 h growth using the PLC-specific substrate p-nitrophenylphosphorylcholine (p-NPPC). In addition, CAPLC1 mRNA was detected by reverse transcriptase PCR in both yeast and hyphal forms of C. albicans 132A at the same time intervals. Expression of CAPLC1 activity was also detected in extracts of Escherichia coli DH5 alpha harbouring plasmids which contained portions of the CAPLC1 gene lacking sequences encoding part of the N-terminus. Southern hybridization and PCR analyses revealed that all C. albicans and Candida dubliniensis isolates examined possessed sequences homologous to CAPLC1. Sequences related to CAPLC1 were detected in some but not all isolates of Candida tropicalis, Candida glabrata and Candida parapsilosis tested, but not in the isolates of Candida krusei, Candida kefyr, Candida guillermondii and Candida lusitaniae examined. This paper reports the first description of the cloning and sequencing of a PLC gene from a pathogenic yeast species.

Homologous and heterologous desensitisation of somatostatin-induced increases in intracellular Ca2+ and inositol 1,4,5-trisphosphate in CHO-K1 cells expressing human recombinant somatostatin sst5 receptors

The mechanisms responsible for somatostatin (SRIF)-induced increases in intracellular Ca2+ concentration ([Ca2+]i) and subsequent desensitisation were studied in CHO-K1 cells expressing human sst5 receptors (CHOsst5 cells). To study the nature of the desensitisation, interactions with uridine triphosphate (UTP) were examined. SRIF (pEC50 7.10) and UTP (pEC50) 5.14) caused concentration-dependent increases in [Ca2+]i but the SRIF maximum was about 40% of that to UTP. SRIF-, but not UTP-, induced increases in [Ca2+]i were transient and abolished by pertussis toxin. SRIF and UTP caused sustained increases in Ins(1,4,5)P3 but the SRIF maximum was about 30% of that to UTP. Removal of [Ca2+]e attenuated the SRIF-induced peak rise in [Ca2+]i but had no effect on the peak increases in Ins(1,4,5)P3. UTP-induced increases in [Ca2+]i and Ins(1,4,5)P3 were attenuated in the absence of [Ca2+]e. Following pre-exposure to SRIF (1 microM) or UTP (100 microM) for 5 min, subsequent SRIF responses were desensitised. Similar results were obtained in the absence of [Ca2+]e. Pre-exposure to SRIF had no effect on subsequent responses to UTP but in the absence of [Ca2+]e, responses to UTP were attenuated. The results suggest that SRIF but not UTP-induced increases in [Ca2+]i in CHOsst5 cells are mediated by pertussis toxin sensitive G proteins and are caused by an entry of extracellular Ca2+ and release from an Ins(1,4,5)P3 sensitive Ca2+ store. Homologous or heterologous desensitisation of agonist-induced increases in [Ca2+]i could be demonstrated in the presence or absence of extracellular Ca2+ respectively, and the latter appeared to involve depletion of a common intracellular Ca2+ store.

Differences in the stimulation of the phosphoinositide cycle by amine neurotransmitters in cultured rat forebrain neurones and astrocytes

In this study, we compared the stimulation by carbachol (CCh), noradrenaline (NA), and histamine (HA) of phosphoinositide hydrolysis in rat forebrain neuronal and glial cultures. When Ca2+ was omitted from the stimulation buffer (low microM extracellular Ca2+), amine-induced [3H]inositol phosphate accumulation was reduced to a higher extent in astrocytes (70-80% for CCh and NA and 100% for HA) than in neurones (around 50-60% for all the amines). Furthermore, guanosine 5'-[gamma-thio]trisphosphate (GTP[S]) stimulation of phosphoinositidase C (PIC) in membranes was 5-fold higher in neurones than in astrocytes. These results indicate differences in the mechanism of PIC stimulation in the two cell types. After 30 min stimulation in the presence of 10 mM Li+, a higher accumulation of [3H]inositol 4-monophosphate and [3H]inositol 1,4-bisphosphate than of [3H]inositol 1/3-monophosphate occurred for all agonists in neurones, whereas the opposite was observed in astrocytes. Moreover, in these cells stimulation for 5 min in the absence of Li+ produced a 2-3-fold accumulation of all metabolites of the 3-kinase pathway of inositol-1,4,5-trisphosphate metabolism but not of those of the 5-phosphatase pathway. Thus, regardless of the amine receptor stimulated, the 3-kinase route appeared to prevail in astrocytes and the 5-phosphatase pathway in neurones. The histamine response in neurones differed from that of the other agonists in that it rapidly declined. Taken together these results indicate that the heterogeneity in amine stimulation of the phosphoinositide cycle previously observed in brain slices could arise to a great extent from the cellular diversity of this preparation and be related to the differential contribution of the amine receptors located in neurones and astrocytes.

The Arabidopsis thaliana genome has multiple divergent forms of phosphoinositol-specific phospholipase C1

Highly degenerate primers to conserved regions of the eukaryotic phosphoinositol-specific phospholipase C (PLC) were used to amplify fragments of plant PLCs from Arabidopsis thaliana genomic DNA. Eight completely different fragment sequences that showed high homology to PLCs of both animals and plants were isolated. The variation between these putative PLCs was high and suggests that, like animals, plants have multiple isoforms of PLC. Using one of the PCR clones, we isolated a corresponding full-length Arabidopsis PLC gene (ATHATPLC1G), and sequence analysis indicated that it was most like a delta-type PLC. This gene is 2.5 kb and contains seven introns, all but one of which has intron/exon border sequences that conform to the Arabidopsis consensus. The structural complexity of the gene is relatively simple compared to mammalian beta-type PLCs that can be 15 kb long with up to 30 introns. The plant gene is a single copy and was mapped to four Arabidopsis YACs, one located on chromosome 2. The promoter region contained two TATA-like elements at -43 and -185 and other putative regulatory elements that suggest that this PLC is hormonally regulated. This is the first plant PLC gene and the first delta type-PLC gene from a higher organism to be sequenced.

Polymorphonuclear leukocytes from asthmatics release more calcium from intracellular stores and have enhanced calcium increase after stimulation with N-formyl-methionyl-leucyl-phenylalanine

Polymorphonuclear leukocytes isolated from peripheral blood of asthmatics appear to be primed to release more reactive oxygen species than cells of healthy subjects. The enhanced agonist-induced rise in the intracellular free calcium concentration may be responsible for this increased respiratory burst. To test this hypothesis we studied the N-formyl-methionyl-leucyl-phenylalanine- and cyclopiazonic acid--(an inhibitor of Ca(2+)-ATPase of intracellular calcium stores) induced calcium increase in the polymorphonuclear leukocytes of 28 subjects (16 with moderate asthma, 69.6% +/- 8.3% predicted normal peak expiratory flow and 12 normal controls) using a fluorescent probe Fura-2AM at 100 nM and 1 mM extracellular calcium concentrations. In 1 mN calcium, the N-formyl-methionyl-leucyl-phenylalanine-induced calcium increase was 1.7-fold higher in asthmatics than in healthy subjects. Similarly, the contribution of calcium from intracellular stores to the calcium response to N-formyl-methionyl-leucyl-phenylalanine was higher in asthmatics (55% +/- 14% vs. 39% +/- 14%, P < 0.01). The pool of calcium released from intracellular stores by N-formyl-methinoyl-leucyl-phenylalanine and cyclopiazonic acid was 2.3- and 2.2-fold larger than in control cells. There was a correlation between maximal intracellular calcium concentration related to N-formyl-methionyl-leucyl-phenylalanine-induced calcium release from intracellular stores and forced expiratory volume in 1 s expressed as percentage predicted and reversibility in asthmatics (r = 0.63, r = -0.53, P < 0.05). In conclusion, polymorphonuclear leukocytes of asthmatics exhibit an altered calcium response that is mainly dependent on increased calcium release from intracellular stores.

Association of INAD with NORPA is essential for controlled activation and deactivation of Drosophila phototransduction in vivo

Visual transduction in Drosophila is a G protein-coupled phospholipase C-mediated process that leads to depolarization via activation of the transient receptor potential (TRP) calcium channel. Inactivation-no-afterpotential D (INAD) is an adaptor protein containing PDZ domains known to interact with TRP. Immunoprecipitation studies indicate that INAD also binds to eye-specific protein kinase C and the phospholipase C, no-receptor-potential A (NORPA). By overlay assay and site-directed mutagenesis we have defined the essential elements of the NORPA-INAD association and identified three critical residues in the C-terminal tail of NORPA that are required for the interaction. These residues, Phe-Cys-Ala, constitute a novel binding motif distinct from the sequences recognized by the PDZ domain in INAD. To evaluate the functional significance of the INAD-NORPA association in vivo, we generated transgenic flies expressing a modified NORPA, NORPAC1094S, that lacks the INAD interaction. The transgenic animals display a unique electroretinogram phenotype characterized by slow activation and prolonged deactivation. Double mutant analysis suggests a possible inaccessibility of eye-specific protein kinase C to NORPAC1094S, undermining the observed defective deactivation, and that delayed activation may similarly result from NORPAC1094S being unable to localize in close proximity to the TRP channel. We conclude that INAD acts as a scaffold protein that facilitates NORPA-TRP interactions required for gating of the TRP channel in photoreceptor cells.

Measurement of phospholipase D activity

Phosphodiesteric cleavage of phosphatidylcholine by members of a growing family of phospholipases D produces choline and phosphatidic acid. These enzymes can also catalyse a transphosphatidylation reaction in which the aliphatic chain of a primary alcohol is transferred to the phosphatidyl moiety of the phosphatidic acid product. PLD enzymes are found in a variety of organisms including bacteria, yeast, plants, and vertebrates. In mammalian systems, biochemical and cell biological approaches have identified phosphatidic acid as a mediator (or progenitor of mediators) that play important roles in the transduction of extracellular signals. Phosphatidic acid or its metabolites may be regulators of key cellular processes such as the control of intracellular protein trafficking, secretion, and alterations in cell morphology and motility. This review discusses methods for the determination of PLD activity both in vitro and in intact cells.

Calcium release at fertilization in starfish eggs is mediated by phospholipase Cgamma

Although inositol trisphosphate (IP3) functions in releasing Ca2+ in eggs at fertilization, it is not known how fertilization activates the phospholipase C that produces IP3. To distinguish between a role for PLCgamma, which is activated when its two src homology-2 (SH2) domains bind to an activated tyrosine kinase, and PLCbeta, which is activated by a G protein, we injected starfish eggs with a PLCgamma SH2 domain fusion protein that inhibits activation of PLCgamma. In these eggs, Ca2+ release at fertilization was delayed, or with a high concentration of protein and a low concentration of sperm, completely inhibited. The PLCgammaSH2 protein is a specific inhibitor of PLCgamma in the egg, since it did not inhibit PLCbeta activation of Ca2+ release initiated by the serotonin 2c receptor, or activation of Ca2+ release by IP3 injection. Furthermore, injection of a PLCgamma SH2 domain protein mutated at its phosphotyrosine binding site, or the SH2 domains of another protein (the phosphatase SHP2), did not inhibit Ca2+ release at fertilization. These results indicate that during fertilization of starfish eggs, activation of phospholipase Cgamma by an SH2 domain-mediated process stimulates the production of IP3 that causes intracellular Ca2+ release.

The 5'-upstream region of the rat phospholipase C-beta 3 gene contains two critical Sp1 sites and an HIV Inr-like element

The 5'-upstream region of the rat phospholipase C-beta 3 gene (PLC-beta 3) has been cloned and characterized. Sequence analysis of the 5'-upstream region showed that it contains a GC-rich region (-166 to +1: 79%) and multiple binding sites for the transcription factors Sp1, AP-1 and AP-2, but does not contain a canonical TATA box. Primer extension analysis of total RNA isolated from rat glial cell C6Bul revealed that single transcription start point (tsp) is located at an initiator (Inr) element similar to that found in the HIV promoter. Gel mobility shift and competitive mobility shift assays indicated that this Inr element forms a DNA-protein complex with the HIV Inr-binding protein, LBP-1/CP2 or a homologue. In order to localize functional elements of the 5'-upstream region of the rat PLC-beta 3 gene, 5'-deletion fragments were cloned into a chloramphenicol acetyltransferase (CAT) reporter vector. Transient transfection analyses of the 5'-deletion mutants identified a crucial promoter element located at -128 to -14. Supershift mobility assays, site-directed mutagenesis and DNase I footprints indicated that Sp1 binds to three GC boxes within the sequence between -128 and -14 of the PLC-beta 3 promoter. Transient transfection analyses of promoter constructs containing site-specific mutation(s) of these three GC boxes demonstrated that two GC boxes, located proximal to the tsp, are important elements for normal promoter activity.

G protein-independent stimulation of human myocardial phospholipase C by mastoparan

1 Phosphoinositide-specific phospholipase C (PLC) is involved in the regulation of many cellular functions. In the myocardium, PLC-generated second messengers play a role in the regulation of contractile function and in the pathophysiology of myocardial hypertrophy. 2 In the present study, the effect of mastoparan, a tetradecapeptide which is capable of activating heterotrimeric G proteins by mimicking the action of an activated receptor, on membrane-bound human myocardial PLC, was investigated in a cell-free assay with exogenous phospholipids as a substrate. 3 Mastoparan stimulated human myocardial PLC approximately two fold with a half-maximal effect at approximately 2 microM and a maximal effect at 10 microM. The peptide did not alter the dependence of PLC on free calcium ions. In order to exclude non-specific effects of mastoparan due to its amphiphilic properties, different mastoparan derivatives were used as positive and negative controls. Mas17, an inactive mastoparan analogue with physical properties very similar to mastoparan, did not induce substantial PLC stimulation in human myocardial membranes. In contrast, Mas7, the most active mastoparan derivative known, caused a more pronounced PLC activation compared with the mother compound indicating that the effect was sequence-specific. Human myocardial PLC stimulation was pertussis toxin-insensitive and could not be abolished by addition of excess alpha-subunits from purified retinal transducin or by excess GDP or GDP/beta S. In order to investigate whether mastoparan stimulate PLC via pertussis toxin-insensitive alpha q, a deletion mutant of PLC beta 2 deficient of the site of interaction with alpha q-subunits was expressed in COS-1 cells. Both wild-type and mutant PLC beta 2 were similarly sensitive to stimulation by mastoparan. It is concluded that mastoparan stimulates human myocardial PLC by a mechanism distinct from heterotrimeric G proteins.

Modulation of the dihydropyridine-sensitive calcium channels in Drosophila by a phospholipase C-mediated pathway

Disruption of phospholipase C-beta (PLC) by the norpA mutations of Drosophila renders flies blind by affecting the light-evoked photoreceptor potential. We report here that the norpA-coded PLC modulates the 1,4-dihydropyridine (DHP)-sensitive Ca2+ channels in larval muscles. The DHP-sensitive current was reduced in the norpA mutants. Application of 1 microM phorbol 12-myristate 13-acetate (TPA) and 1 microM phorbol 12,13-didecanoate (PDD), activators of protein kinase C (PKC), rescued the current in the mutant fibers without significantly affecting the normal current. 4Alpha-phorbol 12,13-didecanoate (4alphaPDD), an inactive analog of PDD, did not affect either the normal or the mutant current. One micromolar bisindolylmaleimide (BIM), an inhibitor of PKC, reduced the current in the normal fibers without affecting the mutant current. 300 microM sn-1,2-dioctanoyl-glycerol (DOG), an analog of diacylglycerol (DAG), increased the current in the mutant fibers. These experiments suggest that the DHP-sensitive Ca2+ channels in Drosophila may be modulated by the PLC-DAG-PKC pathway, and that the same PLC isozyme which is involved in phototransduction in the adult flies may also modulate muscle Ca2+ channels in the larval stage of development.

Suppression of thyrotropin receptor-G protein-phospholipase C coupling by activation of protein kinase C in thyroid carcinoma cells

In human thyroid follicular cells TSH exerts its action on growth and function at least via two distinct pathways, the adenylate cyclase cascade and the phospholipase Cbeta (PLCbeta)-mediated inositol phosphate generation. We investigated the effect of TSH on activation of phosphoinositide hydrolysis and inositol phosphate generation by PLCbeta in HTh74 thyroid carcinoma cells that express functional TSH receptors and in HTC-TSHr thyroid carcinoma cells that are devoid of endogenous TSH receptors but express recombinant human TSH receptors. In both cell lines, TSH up to concentrations of 300 mU/ml failed to stimulate myo-inositol 1,4,5-trisphosphate and myo-inositol-tetrakisphosphate generation, but led to a decrease in these compounds within 1 min of stimulation. However, ATP and bradykinin increased concentrations of inositol phosphates in both thyroid carcinoma cell lines. In contrast, in differentiated FRTL5 thyroid cell line and CHO-TSHr cell line expressing recombinant human TSH receptors, TSH elicited a significant increase in myo-inositol 1,4,5-trisphosphate and its metabolic derivatives. However, when HTC-TSHr cells were pretreated with calphostin C or staurosporine, inhibitors of protein kinase C, a TSH concentration of 20 mU/ml enhanced generation of inositol phosphates in these cells. From our data we conclude that in HTC-TSHr and HTh74 thyroid carcinoma cells, the coupling within the TSH receptor-Gq protein-PLCbeta signaling pathway is impaired compared to that in nontransformed cells. It is conceivable that this is at least in part dependent on the level of protein kinase C activation in these cells.

Cell-matrix interactions induce tyrosine phosphorylation of MAP kinases ERK1 and ERK2 and PLCgamma-1 in two-dimensional and three-dimensional cultures of human fibroblasts

Using immunoprecipitation and phosphotyrosine detection by Western blotting, intracellular signaling intermediates were analyzed in human primary dermal fibroblasts, either seeded as monolayers on collagen I coats (2D) or seeded within three-dimensional collagen I lattices (3D). Previous results demonstrated that integrin activation in these systems resulted in a cascade of protein tyrosine phosphorylation, including focal adhesion kinase (D. Roeckel and T. Krieg, 1994, Exp. Cell Res. 211, 42-48). Further downstream signaling events are now shown to include coordinate activation of ERK1 and ERK2 at 2 h after cell-collagen contact, irrespective of 2D or 3D culture conditions. Applying U-73122, an inhibitor of PLC, inhibits collagen lattice contraction in a dose-dependent fashion. Immunoprecipitation identified the isoform PLCgamma-1 as playing a role as signaling intermediate in fibroblast-collagen interactions. PLCgamma-1 becomes phosphorylated within 10 min after culture initiation and declines after 2 h. So far, no qualitative differences in signaling intermediates between 2D and 3D cultures have been identified.

Regulation of effectors by G-protein alpha- and beta gamma-subunits. Recent insights from studies of the phospholipase c-beta isoenzymes

Both the alpha- and beta gamma-subunits of heterotrimeric guanine nucleotide-dependent regulatory proteins (G-proteins) couple members of the heptahelical class of cell-surface receptors to a diverse range of signal-generating effectors including retinal cyclic GMP phosphodiesterase, ion channels, adenylylcyclases, phosphoinositide 3-kinase, and members of the beta-class of inositol lipid-specific phospholipases C. Although the molecular details of the G-protein-regulated phospholipase C system were elucidated comparatively recently, these enzymes have become an important model for investigations of the process of G-protein effector coupling. A combination of molecular biological, biochemical, and structural studies using the phospholipase C-beta enzymes has provided some important insights into the interplay between G-proteins and their effectors and promises to reveal the mechanisms by which G-protein alpha- and beta gamma-subunits selectively associate with and activate effectors.

Tyrosine phosphorylation and bradykinin-induced signaling in endothelial cells

It is generally accepted that in endothelial cells the occupation of bradykinin B2 receptors, which are linked to the guanine nucleotide-dependent regulatory proteins, Gi and Gq, results in the activation of phospholipase C-beta1 (PLC-beta1), followed by a transient increase in the formation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. The PLC-beta1 isoform, in contrast to the gamma1 isoform, is present only at a low level in cultured endothelial cells, implying that PLC-gamma1 activation may play an important role in endothelial signaling pathways. In cultured human endothelial cells, bradykinin induced a rapid increase in the tyrosine phosphorylation of several Triton-soluble proteins. Immunoprecipitation of tyrosine-phosphorylated proteins from bradykinin-stimulated cells followed by Western blotting using the respective antibodies facilitated the identification of a 77 kiloDalton (kDa) protein as paxillin, a 130 kDa protein as PLC-gamma1, and a 42/44 kDa doublet as mitogen-activated protein (MAP) kinase. The bradykinin-induced tyrosine phosphorylation of PLC-gamma1 was relatively transient and was associated with an increase in intracellular levels of IP3. Bradykinin also induced the rapid and transient activation of phosphotyrosine phosphatases localized mainly in the Triton X-100-soluble cell fraction; this tyrosine phosphatase activity was apparently initiated after the release of Ca2+ from intracellular stores.

Structural and mechanistic features of phospholipases C: effectors of inositol phospholipid-mediated signal transduction

The production of the intracellular second messengers inositol (1,4,5)-trisphosphate (InsP3) and sn 1,2-diacylglycerol (DG) in response to a wide variety of extracellular primary messengers is achieved by an extended family of inositol phospholipid phosphodiesterases termed phospholipases C (PLC, E.C. 3.1.4.11). This family has been the subject of extensive research and it is clear that the different isoenzymes exhibit some common characteristics (e.g., interactions with substrates) and other distinctive features (e.g., modes of regulation). The recent description of the X-ray crystal structure of a mammalian PLC has served to clarify much about the behaviour of the PLCs, emphasising the "modular" structure of these enzymes. The main focus of this review will concern the specific adaptations of PLC molecules which make them efficient lipid-metabolising enzymes. We also describe what is known about how these enzymes interact with their lipid substrates, which will serve as a basis for considering how PLCs may be activated.

The mechanisms of action of disease-modifying antirheumatic drugs: a review with emphasis on macrophage signal transduction and the induction of proinflammatory cytokines

1. Rheumatoid arthritis (RA) is probably the most common source of treatable disability. A major problem in modern rheumatology is that the mechanism(s) of action of the currently used disease-modifying antirheumatic drugs (DMARDs) remain unclear. Many of these drugs entered rheumatology mainly through clinical intuition and have been used for decades. 2. The former T-cell-centered paradigm of rheumatoid inflammation has given way to a model of inflammation highlighting the macrophage and its proinflammatory cytokines. In particular, tumor necrosis factor alpha (TNF-alpha) has gained prominence as a central proinflammatory mediator in RA, and antibodies against TNF-alpha have been successfully used in patients with RA. 3. This review will summarize the recent advances in determining the mechanisms of action of the currently used DMARDs, with particular emphasis on their effects on the induction of TNF-alpha and interleukin 1 (IL-1) in mononuclear phagocytes. Although some DMARDs, such as auranofin, antimalarials and tenidap, act as inhibitors of the induction of these cytokines in monocytes or macrophages or both, other drugs, such as methotrexate, D-penicillamine and aurothiomalate, do not seem to affect either TNF-alpha or IL-1. 4. The drugs' effects on proinflammatory cytokine induction are correlated to those on other macrophage responses.

Roles of phospholipase C beta2 in chemoattractant-elicited responses

The physiological roles of phospholipase C (PLC) beta2 in hematopoiesis, leukocyte function, and host defense against infection were investigated using a mouse line that lacks PLC beta2. PLC beta2 deficiency did not affect hematopoiesis, but it blocked chemoattractant-induced Ca2+ release, superoxide production, and MAC-1 up-regulation in neutrophils. In view of these effects, it was surprising that the absence of PLC beta2 enhanced chemotaxis of different leukocyte populations and sensitized the in vivo response of the PLC beta2-deficient mice to bacteria, viruses, and immune complexes. These data raise questions about the roles that PLC beta2 may play in signal transduction induced by chemoattractants in leukocytes.

The spasmogenic effects of vanadate in human isolated bronchus

1. Inhalation of vanadium compounds, particularly vanadate, is a cause of occupational bronchial asthma. We have now studied the action of vanadate on human isolated bronchus. Vanadate (0.1 microM-3 mM) produced concentration-dependent, well-sustained contraction. Its -logEC50 was 3.74 +/- 0.05 (mean +/- s.e.mean) and its maximal effect was equivalent to 97.5 +/- 4.2% of the response to acetylcholine (ACh, 1 mM). 2. Vanadate (200 microM)-induced contraction of human bronchus was epithelium-independent and was not inhibited by indomethacin (2.8 microM), zileuton (10 microM), a mixture of atropine, mepyramine and phentolamine (each at 1 microM), or by mast cell degranulation with compound 48/80. 3. Vanadate (200 microM)-induced contraction was unaltered by tissue exposure to verapamil or nifedipine (each 1 microM) or to a Ca2+-free, EGTA (0.1 mM)-containing physiological salt solution (PSS). However, tissue incubation with ryanodine (10 microM) in Ca2+-free, EGTA (0.1 mM)-containing PSS reduced vanadate-induced contraction. A series of vanadate challenges was made in tissues exposed to Ca2+-free EGTA (0.1 mM)-containing PSS with the object of depleting intracellular Ca2+ stores. In such tissues cyclopiazonic acid (CPA; 10 microM) prevented Ca2+-induced recovery of vanadate-induced contraction. 4. Tissue incubation in K+-rich (80 mM) PSS, K+-free PSS, or PSS containing ouabain (10 microM) did not alter vanadate (200 microM)-induced contraction. Ouabain (10 microM) abolished the K+-induced relaxation of human bronchus bathed in K+-free PSS. This action was not shared by vanadate (200 microM). The tissue content of Na+ was increased and the tissue content of K+ was decreased by ouabain (10 microM). In contrast, vanadate (200 microM) did not alter the tissue content of these ions. Tissue incubation in a Na+-deficient (25 mM) PSS or in PSS containing amiloride (0.1 mM) markedly inhibited the spasmogenic effect of vanadate (200 microM). 5. Vanadate (200 microM)-induced contractions were markedly reduced by tissue treatment with each of the protein kinase C (PKC) inhibitors H-7 (10 microM), staurosporine (1 microM) and calphostin C (1 microM). Genistein (100 microM), an inhibitor of protein tyrosine kinase, also reduced the response to vanadate. 6 Vanadate (0.1-3 mM) and ACh (1 microM- 3 mM) each increased inositol phosphate accumulation in bronchus. Such responses were unaffected by a Ca2+-free medium either alone or in combination with ryanodine (10 microM). 7. In human cultured tracheal smooth muscle cells, histamine (100 microM) and vanadate (200 microM) each produced a transient increase in intracellular Ca2+ concentration ([Ca2+]i). 8. Intracellular microelectrode recording showed that the contractile effect of vanadate (200 microM) in human bronchus was associated with cellular depolarization. 9. It is concluded that vanadate acts directly on human bronchial smooth muscle, promoting the release of Ca2+ from an intracellular store. The Ca2+ release mechanism involves both the production of inositol phosphate second messengers and inhibition of Ca-ATPase. The activation of PKC plays an important role in mediating vanadate-induced contraction at values of [Ca2+]i that are close to basal.

Stabilization of elastin mRNA by TGF-beta: initial characterization of signaling pathway

The cytokine transforming growth factor-beta (TGF-beta) has multiple effects on a wide variety of cell types. These effects include modulation of growth and regulation of gene transcription. In a few instances, TGF-beta has also been shown to regulate gene expression posttranscriptionally by altering message stability, but the pathway by which this activity is executed remains largely unknown. In the present work, we demonstrate that TGF-beta 1 has no effect on transcription of the elastin gene in cultured human fetal lung fibroblasts, but does stabilize elastin messenger RNA (mRNA), leading to a dramatic increase in the steady-state level of elastin mRNA. A corresponding increase in production of tropoelastin accompanies the increase in elastin mRNA. Through the use of specific inhibitors, we demonstrate that phosphatidylcholine (PC)-specific phospholipase C (PLC) and protein kinase C (PKC) are involved in mediating the elastin message stabilization. In contrast, G proteins and extracellularly regulated kinases do not appear to be involved. These results suggest that although the TGF-beta signaling pathway leading to message stabilization shares components with that modulating transcription, the message-stabilization pathway also contains diverse other elements.

Biochemistry and genetics of inositol phosphate metabolism in Dictyostelium

Biochemical and genetic data on the metabolism of inositol phosphates in the microorganism Dictyostelium are combined in a scheme composed of in five subroutes. The first subroute is the inositol cycle as found in other organisms: inositol is incorporated into phospholipids that are hydrolysed by PLC producing Ins(1,4,5)P3 which is dephosphorylated to inositol. The second subroute is the sequential phosphorylation of inositol to InsP6; the Ins(3,4,6)P3 intermediate does not release Ca2+. The third subroute is the sequential phosphorylation of Ins(1,4,5)P3 to InsP6 in a nucleus associated fraction, whereas the fourth subroute is the dephosphorylation of Ins(1,3,4,5,6)P5 to Ins(1,4,5)P3 at the plasma membrane. This last route mediates Ins(1,4,5)P3 formation in cells with a disruption of the single PLC gene. Finally, we recognize the formation of InsP7 and InsP8 as the fifth subroute.

beta-Adrenergic stimulation induces activation of protein kinase C and inositol 1,4,5-trisphosphate increase in epidermis

Epidermal keratinocytes express beta 2-adrenergic receptors on the cell membrane. The binding of the agonists to the beta 2-adrenergic receptors regulates activation of adenylate cyclase. This transmembrane signaling system has been regarded to be one of the important pathways for the functions of keratinocytes. We previously reported that beta-adrenergic stimulation induced a transient increase of intracellular Ca2+ in normal human epidermal keratinocytes. Thus we investigated the effects of epinephrine on another transmembrane signaling system, the phosphatidyl-inositol signal transduction pathway in pig epidermis. Treatment of pig pure epidermis with epinephrine resulted in a transient increase in inositol 1,4,5-trisphosphate with a peak at 30 s. Epinephrine induced translocation of protein kinase C from cytosol to the membrane fraction. The activation of protein kinase C, translocation of protein kinase C from cytosol to the membrane fraction, was confirmed using the beta-adrenergic agonist isoproterenol. Moreover, the effect of epinephrine on the activation of protein kinase C was inhibited by preincubation with propranolol, a beta-adrenergic antagonist. The increase in inositol 1,4,5-trisphosphate and translocation of protein kinase C by epinephrine are consistent with the view that beta-adrenergic stimulation induces turnover of inositol phospholipid in pig epidermis.

Modulation of cyclic AMP levels in a clonal neural cell line by inhibitors of tyrosine phosphorylation

The convergence of tyrosine kinase and cyclic AMP (cAMP) signal transduction pathways was investigated in the HT4.7 neural cell line with inhibitors of tyrosine kinases and tyrosine phosphatases. The protein tyrosine kinase inhibitor genistein inhibited isoproterenol-stimulated cAMP production by 40-60% in whole cells, with no effect on basal cAMP levels. In both whole cells and membranes, genistein also inhibited cAMP produced in response to direct stimulation of adenylyl cyclase with forskolin. However, in the absence of phosphodiesterase inhibitors, genistein presentation resulted in an increase in cAMP levels. Genistein inhibited phosphodiesterase activity by 80-85%, indicating that tyrosine phosphorylation stimulates both cAMP synthesis and degradation. The decrease in cAMP levels by genistein was not merely competitive inhibition of adenylyl cyclase with respect to ATP, since the Km of adenylyl cyclase for ATP remained essentially the same in either the presence or the absence of genistein. Another tyrosine kinase inhibitor, herbimycin A, which inhibits by a different mechanism than genistein, also decreased forskolin-stimulated cAMP in whole cells. As would be expected for the involvement of tyrosine phosphorylation in the control of cAMP production, inhibition of tyrosine phosphatases by vandate increased forskolin-stimulated cAMP production. These results suggest that cAMP production can be regulated by tyrosine phosphorylation, and the simultaneous activation of both cAMP synthesis and degradation may serve to alter the duration of cAMP elevation.

Phospholipase C beta and membrane action of calcitriol and estradiol

We have shown that estrogens and calcitriol, the hormonally active form of vitamin D, increase the concentration of intracellular calcium ([Ca2+]i) within 5 s by mobilizing calcium from the endoplasmic reticulum and the formation of inositol 1,4, 5-trisphosphate and diacylglycerol. Because the activation of effectors as phospholipase C (PLC) coupled to G-proteins is the early event in the signal transduction pathway leading to the inositol 1,4,5-trisphosphate formation and to [Ca2+]i increase, we described different PLC isoforms (beta1, beta2, gamma1, and gamma2, but not beta4) in female rat osteoblasts using Western immunoblotting. The data showed that phospholipase C beta was involved in the mobilization of Ca2+ from the endoplasmic reticulum of Fura-2-loaded confluent osteoblasts by calcitriol and 17beta estradiol, and PLC gamma was ineffective. The data also showed that only a PLC beta1 linked to a Pertussis toxin-insensitive G-protein and a PLC beta2 coupled to a Pertussis toxin-sensitive G-protein are involved in the effects of calcitriol and 17beta estradiol on the mobilization of Ca2+ from intracellular Ca2+ stores. In conclusion, these results may be an important step toward understanding membrane effects of these steroids and may be an additional argument in favor of membrane receptors to steroid hormones.

Phospholipase C-mediated signaling is altered during HaCaT cell proliferation and differentiation

To elucidate the signaling mechanisms associated with keratinocyte differentiation, we studied in vitro phospholipase C-mediated signal transduction, which results in the generation of inositol phosphates, comparing proliferating versus differentiated HaCaT cells, a human keratinocyte line. Bradykinin- or A23187-induced formation of inositol 1,4,5-trisphosphate, inositol 1,4-bisphosphate, and inositol monophosphates, as determined by anion exchange high performance liquid chromatography, were found to be highest in the early logarithmic growth phase of the cells. In more highly differentiated HaCaT cells, which expressed maximal amounts of the differentiation marker involucrin, inositol phosphate formation was reduced to about one third of that in proliferating cells. Thin layer chromatography of membrane phosphatidylinositol phosphates revealed that this reduction was associated with a steady decrease in phospholipase C substrates. Immunoblot analysis of phospholipase C isozymes, however, and of expression of Gq alpha, the G protein subunit that activates phospholipase C beta, revealed no decrease during the differentiation phase. The results suggest that the inositol-phospholipid signal transduction pathway is involved in keratinocyte proliferation and in the induction of differentiation, with attenuated signal transduction activity via phospholipase C-coupled receptors in more differentiated keratinocytes.

Signal transduction in gastrointestinal smooth muscle

Signal transduction in gastric and intestinal smooth muscle is mediated by receptors coupled via distinct G proteins to various effector enzymes, including PI-specific PLC-beta 1 and PLC-beta 3, and phosphatidylcholine (PC)-specific PLC, PLD and PLA2. Activation of these enzymes is different in circular and longitudinal muscle cells, generating Ca(2+)-mobilizing (IP3, AA, cADPR) and other (DAG) messengers responsible for the initial and sustained phases of contraction, respectively. IP3-dependent Ca2+ release occurs only in circular muscle. Ca2+ mobilization in longitudinal muscle involves a cascade initiated by agonist-induced transient activation of PLA2 and formation of AA, AA-dependent depolarization of the plasma membrane and opening of voltage-sensitive Ca2+ channels. The influx of Ca2+ induces Ca2+ release by activating sarcoplasmic ryanodine receptor/Ca2+ channel and stimulates cADPR formation which enhances Ca(2+)-induced Ca2+ release. The initial [Ca2+]i transient in both muscle cell types results in Ca2+/calmodulin-dependent activation of MLC kinase, phosphorylation of MLC20 and interaction of actin and myosin. The sustained phase is mediated by a Ca(2+)-independent isoform of PKC, PKC-epsilon DAG for this process is generated by PLC- and PLD-mediated hydrolysis of PC. Relaxation is mediated by cAMP-and/or cGMP-dependent protein kinase which inhibit the initial [Ca2+]i transient and reduce the sensitivity of MLC kinase to [Ca2+]i. Relaxation induced by the main neurotransmitters, VIP and PACAP, involves two cascades, one of which reflects activation of adenylyl cyclase. A distinct cascade involves G-protein-dependent stimulation of Ca2+ influx leading to Ca2+/calmodulin-dependent activation of a constitutive eNOS in muscle cells; the generation of NO activates soluble guanylyl cyclase. The resultant activation of PKA and PKG is jointly responsible for muscle relaxation.

Delayed neuronal death after global incomplete ischemia in dogs is accompanied by changes in phospholipase C protein expression

Activation of phospholipase C (PLC) increases intracellular Ca2+ and may play a role in delayed neuronal death after ischemia. Because changes in intracellular Ca2+ are believed to participate in ischemic neuronal injury, we tested the hypothesis that PLC beta protein levels are temporally altered in brain regions that undergo neurodegeneration after global incomplete ischemia. Dogs (n = 12) were subjected to 20 minutes of global incomplete ischemia followed by recovery of either 1 (n = 5) or 7 days (n = 7). Six sham-operated animals were used as nonischemic controls. In hematoxylin and eosin-stained brain sections, neuronal density at 1 day after ischemia was unchanged relative to nonischemic controls in hippocampus CA1, caudate, and cerebellar cortex (anterior lobule). However, at 7 days after ischemia, neuronal densities were decreased to 56 +/- 15% (mean +/- SD) and 75 +/- 17% of control in CA1 and caudate, respectively. At 1 and 7 days after ischemia, the percentage of neurons showing ischemic injury increased from 13 +/- 10 to 40 +/- 35% in CA1, 24 +/- 25 to 59 +/- 16% in cerebellum, and 4 +/- 2 to 18 +/- 12% in caudate. Densitometric analysis of immunocytochemically stained brain sections from controls (n = 3). 1 day after ischemia (n = 3), and 7 days after ischemia (n = 5) revealed that PLC beta immunoreactivity was increased in cerebellum at 1 day (0.274 +/- 0.013 v 0.295 +/- 0.005 optical density units [OD] in control and 1 day, respectively) and 7 days (0.108 +/- 0.009 v 0.116 +/- 0.005 O.D. in control and 7 days, respectively). PLC beta immunoreactivity was unchanged after ischemia in caudate and hippocampus. Western blot analysis of PLC beta immunoreactivity in the cerebellar cortex and hippocampus in the control (n = 3), 1 day (n = 2), and 7 days after ischemia (n = 2) groups showed that PLC beta levels were increased after ischemia in cerebellum 266% and 227% above control at 1 and 7 days, respectively. However, in hippocampus, PLC expression after ischemia was unchanged at 97% and 84% of control at 1 and 7 days, respectively. These results show that delayed neuronal degeneration after global incomplete ischemia is accompanied by regional abnormalities in PLC levels. Elevated PLC levels early may represent an aberrant signal transduction mechanism resulting in delayed cell damage, whereas decreased PLC levels later after ischemia may reflect ongoing neurodegeneration.

Angiotensin II signal transduction in vascular smooth muscle: role of tyrosine kinases

In this review, the role of tyrosine kinases in angiotensin II-mediated signal transduction pathways in vascular smooth muscle is discussed. Angiotensin II was isolated by virtue of its vasoconstrictor abilities and has long been thought to play a critical role in hypertension. However, recent studies indicate important roles for angiotensin II in inflammation, atherosclerosis, and congestive heart failure. The expanding role of angiotensin II indicates that multiple signal transduction pathways are likely to be activated in a tissue-specific manner. Exciting recent data show that angiotensin II directly stimulates tyrosine kinases, including pp60(c-src) kinase (c-Src), focal adhesion kinase (FAK), and Janus kinases (JAK2 and TYK2). Angiotensin II may activate receptor tyrosine kinases, such as Axl and platelet-derived growth factor, by as-yet-undefined autocrine mechanisms. Finally, unknown tyrosine kinases may mediate tyrosine phosphorylation of Shc, Raf, and phospholipase C-gamma after angiotensin II stimulation. These angiotensin II-regulated tyrosine kinases appear to be required for angiotensin II effects, such as vasoconstriction, proto-oncogene expression, and protein synthesis, on the basis of studies with tyrosine kinase inhibitors. Thus, understanding angiotensin II-stimulated signaling events, especially those related to tyrosine kinase activity, may form the basis for the development of new therapies for cardiovascular diseases.

Impairment of phosphatidylinositol signaling in acetylshikonin-treated neutrophils

In rat neutrophils, formylmethionyl-leucyl-phenylalanine (fMLP)-induced inositol phosphate formation was concentration-dependently inhibited by acetylshikonin as well as by a putative phospholipase C (PLC) inhibitor [6-[[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-p yrrole-2,5-dione (U73122). The IC50 value of acetylshikonin for the inhibition of inositol trisphosphate (IP3) formation was estimated to be 16.1 +/- 1.5 microM. The reduction of inositol phosphate levels appeared to reflect inhibition of PLC activity because the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) catalyzed by a soluble fraction from neutrophils was also inhibited by acetylshikonin (IC50 value 21.4 +/- 6.1 microM) over the same range of concentrations. Although acetylshikonin alone evoked Ca2+ and Mn2+ influx into neutrophils in Ca2+-containing medium, acetylshikonin, like U73122, inhibited Ca2+ release (IC50 value approximately 5.3 +/- 0.4 microM) from internal stores in Ca2+-free medium. These results indicate that acetylshikonin inhibits phosphatidylinositol signaling in neutrophils.

Fluid shear stress-mediated signal transduction: how do endothelial cells transduce mechanical force into biological responses?

We propose a model for signaling events induced by fluid shear stress that incorporates many of the features discussed in this paper (FIG. 4). First, heterotrimeric G-proteins, as well as a small G-proteins, are activated by flow. Indeed, a G protein appears to be required for ERK1/2 activation by flow because ERK1/2 activation is completely inhibited by GDP-beta S. Then, flow activates phospholipase C and generates IP3 and diacylglycerol (DG). IP3 releases Ca2+ from internal Ca2+ stores via IP3 receptor and DG activates PKC. Nollert and colleagues have shown that flow activates PLC and increases IP3. It is possible that several different PKC isozymes are activated by flow including both Ca(2+)-dependent and Ca(2+)-independent isozymes. These different isozymes may have specific downstream substrates. For example, PKC-epsilon may be involved in activation of ERK1/2, while the PKC isozyme responsible for activation of JNK remains unknown. It is also possible that these PKC isozymes may be important in gene transcription events. For example, PKC-zeta has been suggested to be involved in NF-kappa B-mediated gene transcription. Longer term changes in endothelial cell morphology and structure are likely to involve separate kinases. Important candidates for these changes include members of the c-Src and FAK families. c-Src is now considered to be a component of the focal adhesion complex and regulate focal adhesion formation and/or cytoskeletal rearrangement. Recently, stretch, another mechanostress, has been shown to activate c-Src in fetal rat lung cells. It has been clarified that ERK1/2 and JNK are regulated by the small G-proteins, Ras and Rac/Cdc42H, respectively, and their effectors in parallel with each other. Rac and Rho are also thought to be involved in membrane ruffling and/or cytoskeletal rearrangement. Fluid shear stress causes stress fiber formation and focal adhesion rearrangement. Recent study by Malek and Izumo suggested the importance of microtubules in shear stress-induced morphological change and actin stress fiber formation. It is clear that the focal adhesion complex plays an important role in shear stress-induced signal and it is interesting to speculate that shear stress-induced signaling has cross-talk with signaling induced by integrins. As a general model we propose that the integration between the rapid events stimulated by shear stress and the longer term events is mediated by tyrosine kinases that serve to regulate these multiple signal transduction pathways.

The promoter activity of the phospholipase C-gamma2 gene is regulated by a cell-type-specific control element

We have cloned and characterized a genomic DNA spanning the 5'-flanking region, the first and second exons, and the first intron of the human PLC-gamma2 gene. The proximal upstream region is highly GC-rich and lacks a TATA box, whereas the distal region contains several AT-rich tracts. Multiple transcription initiation sites were identified by primer extension analysis. Based on the transient transfection assays, the major transcriptional activation element was identified between -183 and +43 (G2SE) and a transcriptional repressive element was found between -303 and -184 (G2RE). The expression of PLC-gamma2 in various cell lines was examined using monoclonal anti-PLC-gamma2 antibody. PLC-gamma2 was highly expressed in B-cell lines such as Daudi, SP2, and Ramos cells, whereas it existed at very low levels in Jurkat, 3T3-L1, NBL-7, and C6Bu-1 cells. Moderate levels of PLC-gamma2 were also detected in C2C12, P19, U937, HL60, A431, and PC12 cells. The 4-kb genomic fragment upstream of -1,654 was able to activate transcription from the PLC-gamma2 promoter in Daudi and C2C12 cells, but not in Jurkat cells, which is consistent with the PLC-gamma2 protein expression levels in those cell lines. These results suggest that the cell-type-specific expression of PLC-gamma2 might be attributed to the transcriptional regulation by the upstream cis-element.

Role of intracellular calcium in the angiotensin II-mediated tyrosine phosphorylation and dephosphorylation of PLC-gamma 1

Angiotensin II induces the rapid temporal tyrosine phosphorylation and activation of phospholipase C-gamma 1 (PLC-gamma 1) and the elevation of intracellular calcium levels. To investigate the relationship of these intracellular signaling events, rat aortic smooth muscle cells were treated with the calcium chelator BAPTA-AM, the calcium channel blocker verapamil, the intracellular calcium antagonist TMB-8, and the calcium ionophore ionomycin. The effects of these agents on PLC-gamma 1 tyrosine phosphorylation were then measured. We found that treatment of these cells with the calcium inhibitors augmented the basal level of PLC-gamma 1 tyrosine phosphorylation, without changing the peak level of tyrosine phosphorylation induced by angiotensin II. The rapid dephosphorylation of PLC-gamma 1 that follows angiotensin II stimulation was prevented by these calcium antagonists. In contrast, angiotensin II-induced tyrosine phosphorylation of PLC-gamma 1 was inhibited by ionomycin. These results suggest that the angiotensin II-induced tyrosine phosphorylation of PLC-gamma 1 is calcium-independent, while the dephosphorylation is calcium-dependent.

Increased G alpha q/11 immunoreactivity in postmortem occipital cortex from patients with bipolar affective disorder

As disturbances in guanine nucleotide binding (G) protein-coupled phosphoinositide second messenger systems have been implicated in bipolar disorder, we examined whether the abundance of G alpha q/11 and phospholipase C (PLC)-beta 1 two key transducing proteins in this signaling pathway, are altered in this disorder. Compared with the controls, immunoreactive levels of G alpha q/11 were significantly elevated by 62% (p = .047) in occipital cortex of bipolar subjects. A similar increase (52%) in the PLC-beta 1 immunolabeling was also found in the occipital cortex of the bipolar subjects, but only reached marginal statistical significance (p = .07). In contrast, frontal and temporal cortex G alpha q/11 or PLC-beta 1 immunolabeling did not differ between bipolar and control subjects. Cerebral cortical immunoreactive levels of G beta 1 or G beta 2, included as a negative control, were not different between comparison groups. These findings support and extend earlier observations suggesting that disturbances in G protein-coupled second messenger signaling pathways may play an important role in the pathophysiology of bipolar affective disorder.

Tubulin, Gq, and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cbeta1 signaling

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein alpha subunits, Galphas or Galphai1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Galpha conformation. To study the possibility of tubulin involvement in Galphaq-mediated phospholipase Cbeta1 (PLCbeta1) signaling, the m1 muscarinic receptor, Galphaq, and PLCbeta1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCbeta1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCbeta1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Galphaq and PLCbeta1, accompanied by guanine nucleotide transfer from tubulin to Galphaq, is suggested as a mechanism for the enzyme activation. The PLCbeta1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCbeta1 by tubulin, since high tubulin concentrations might prevent the access of PLCbeta1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Galphaq for tubulin. Involvement of tubulin in the articulation between PLCbeta1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events.

Identification of components of a phosphoinositide signaling pathway in retinal rod outer segments

Phototransduction in retinal rods involves a G protein-coupled signaling cascade that leads to cGMP hydrolysis and the closure of cGMP-gated cation channels that are open in darkness, producing a membrane hyperpolarization as the light response. For many years there have also been reports of the presence of a phosphoinositide pathway in the rod outer segment, though its functions and the molecular identities of its components are still unclear. Using immunocytochemistry with antibodies against various phosphoinositide-specific phospholipase C (PLC) isozymes (beta1-4, gamma1-2, and delta1-2), we have found PLCbeta4-like immunoreactivity in rod outer segments. Similar experiments with antibodies against the alpha-subunits of the G(q) family of G proteins, which are known to activate PLCbeta4, have also demonstrated G(alpha11)-like immunoreactivity in this location. Immunoblots of total proteins from whole retina or partially purified rod outer segments with anti-PLCbeta4 and anti-G(alpha11) antibodies gave, respectively, a single protein band of the expected molecular mass, suggesting specific labelings. The retinal locations of the two proteins were also supported by in situ hybridization experiments on mouse retina with probes specific for the corresponding mouse genes. These two proteins, or immunologically identical isoforms, therefore likely mediate the phosphoinositide signaling pathway in the rod outer segment. At present, G(alpha11) or a G(alpha11)-like protein represents the only G protein besides transducin (which mediates phototransduction) identified so far in the rod outer segment. Although absent in the outer segment layer, other PLC isoforms as well as G(alpha q) (another G(q) family member), are present elsewhere in the retina.

Phospholipase C isoforms delta 1 and delta 3 from human fibroblasts. High-yield expression in Escherichia coli, simple purification, and properties

Phospholipase C isoforms delta 1 and delta 3 (PLC delta 1 and delta 3) were expressed in Escherichia coli using the cDNA sequences from human fibroblasts. The enzymes were also expressed with the sequence Met-Gly-His6-Ser-Gly-Leu-Phe-Lys-Arg, a hexahistidine sequence followed by a Kex2 protease cleavage site, denoted as "-H6K2," attached to their amino termini. PLC delta 1, PLC delta 1-H6K2, PLC delta 3, PLC delta 3-H6K2 all expressed in highly active form. The H6K2-bearing isoforms were each purified to homogeneity in a single step, with yields of 90-100%, using agarose-iminodiacetic acid-Ni columns and imidazole buffer as eluting agent. Yields in terms of activity increased as the temperature of expression was decreased. Expression at 16 degrees C for 72 h yielded 33 mg of pure PLC delta 1-H6K2 and 13 mg of pure PLC delta 3-H6K2 per liter of culture. Removal of H6K2 from both isoforms with Kex2 protease resulted in little or no loss of activity. Expression of PLC isoforms bearing -H6K2 at the amino terminus resulted in about 12 times more activity than expression of the isoforms lacking -H6K2. PLC delta 3 is much less stable than PLC delta1. Successful purification and storage of PLC delta 3 depends on a suitable stabilizing medium. Both isoforms require 0.3 microM calcium ion for half-maximum activity. The specific activities of the isoforms expressed with and without -H6K2 are the same, as are their calcium saturation curves.

A novel interaction between the juxtamembrane region of the p55 tumor necrosis factor receptor and phosphatidylinositol-4-phosphate 5-kinase

Tumor necrosis factor-alpha (TNF-alpha) binding to its receptors leads to a diversity of biological responses. The actions of TNF are the result of the interaction of cytoplasmic proteins that bind directly to the intracellular domains of the two TNF receptors, p55 and p75. Here we report a novel interaction between the juxtamembrane region of the p55 TNF receptor and a newly discovered 47-kDa isoform of phosphatidylinositol-4-phosphate 5-kinase (PIP5K), a member of the enzyme family that generates the key signaling messenger, phosphatidylinositol 4,5-bisphosphate. The interaction was found to be specific for the p55 TNF receptor and was not observed with the p75 TNF receptor, the Fas antigen, or the p75 neurotrophin receptor, which are other members of the TNF receptor superfamily. In vitro experiments using recombinant fusion proteins verify the authenticity of the interaction between the p55 receptor and PIP5KIIbeta, a new isoform of PIP5K, but not the previously identified 53-kDa PIP5KIIalpha. Treatment of HeLa cells with TNF-alpha resulted in an increased PIP5K activity. These results indicate that phosphatidylinositol turnover may be linked to stimulation of the p55 TNF receptor and suggest that a subset of TNF responses may result from the direct association of PIP5KIIbeta with the p55 TNF receptor.

Protein kinase C and c-myc gene activation pathways in thrombopoietin signal transduction

Thrombopoietin (TPO) is the major regulator of the proliferation and differentiation of megakaryocyte precursors through interaction with its receptor encoded by the c-mpl protooncogene. We established the human TPO-dependent leukemia cell line, UT-7/TPO (Blood 87, 4552, 1996). In these cells, TPO activated protein kinase C (PKC) in a time dependent manner. Subsequently, the c-myc gene was transiently induced to a maximal level 60-90 minutes after TPO exposure. In addition, we found that stimulating UT-7/TPO cells with TPO rapidly induces the significant accumulation of inositol 1, 4, 5-trisphosphate (Ins-P3), leading to the mobilization of calcium from intracellular stores. Taken together, the activation of PKC and subsequent c-myc gene induction are involved in the TPO-induced cellular response(s), presumably through the activation of PLC.

A role for tyrosine phosphorylation in generation of inositol phosphates and prostacyclin production in endothelial cells

We have examined the effects of the protein tyrosine phosphatase inhibitor pervanadate on activation of signal transduction in human umbilical vein endothelial cells. Endothelial cells responded to pervanadate treatment by increasing tyrosine phosphorylation of cellular proteins, including phospholipase C (PLC) gamma 1, generating inositol phosphates (IPs), releasing arachidonic acid, and producing prostacyclin (prostaglandin [PG] I2). The dose and time responses for these events were similar. Tyrosine phosphorylation and formation of IPs in response to pervanadate were reduced by both staurosporine and genistein. Short-term incubation with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, which inhibits thrombin-induced IP generation, did not affect the IP response to pervanadate. To investigate the possible involvement of tyrosine phosphorylation in thrombin or histamine-induced IP generation and PGI2 production, we examined the effects of costimulation with pervanadate and either thrombin or histamine. These responses proved to be different. While the tyrosine phosphorylation of PLC gamma 1 was enhanced after cotreatment with thrombin and pervanadate compared with pervanadate alone, costimulation with pervanadate and histamine resulted in no more tyrosine phosphorylation of PLC gamma 1 than after pervanadate alone. Similarly, while cotreatment with pervanadate and thrombin caused synergistic increase in IP generation, costimulation with pervanadate and histamine resulted in an additive response. However, PGI2 responses to costimulation of pervanadate with either thrombin or histamine were both synergistic. Furthermore, stimulation with histamine, thrombin, or pervanadate all caused tyrosine phosphorylation of a mitogen-activated protein kinase (ERK1/p44). The results suggest that a tyrosine phosphorylation-dependent mechanism has a role in the phosphoinositide signal transduction pathway of human endothelial cells. Moreover, thrombin- but not histamine-induced generation of IPs appears to be partly caused by tyrosine phosphorylation of PLC gamma 1.

A caffeine/ryanodine-sensitive Ca2+ pool is involved in triggering spontaneous variations of Ca2+ in Jurkat T lymphocytes by a Ca(2+)-induced Ca2+ release (CICR) mechanism

Caffeine and ryanodine triggered an increase in [Ca2+]i (73 +/- 22 and 61 +/- 18 nM, respectively) in Jurkat cell populations that was independent of external Ca2+. In individual cells, caffeine and ryanodine induced Ca2+ spikes. Jurkat cell populations initially exposed to caffeine did not respond further to ryanodine and vice versa, suggesting an overlap of the Ca2+ pool that was contained within the thapsigargin-sensitive Ca2+ reserve. [3H]ryanodine bound to a single class of sites of Jurkat microsomes (KD, 18.4 +/- 5.7 nM; Bmax, 24.3 +/- 7.7 fmol/mg protein). Photolytic release (Nitr5) of caged Ca2+ induced a time-dependent increase of Ca2+ in individual Jurkat cells. The profile of the release of Ca2+ was characterized, 1) by a kinetic (0.55 +/- 0.07 nM s-1) slower than the Ca2+ response to caffeine (3.93 +/- 0.66 nM s-1) or to ryanodine (3.96 +/- 0.94 nM s-1), 2) by a release of Ca2+ (131 +/- 43 nM) that slowly returned to baseline and during which low amplitude oscillations were present (room temperature) or Ca2+ spikes (37 degrees C) and, 3) by a lack of dependency on an influx of Ca2+. Inhibitors of CICR (ruthenium red and 1-octanol) prevented the photolysis-dependent increase in [Ca2+]i but not the InsP3-dependent Ca2+ response. Our data suggest that Jurkat T cells possess at least two Ca2+ pools, one that is sensitive to InsP3 and one that is insensitive. These two Ca2+ pools may be involved in a CICR that generates spontaneous Ca2+ spikes and oscillations in these cells.

Reconstitution of receptors and GTP-binding regulatory proteins (G proteins) in Sf9 cells. A direct evaluation of selectivity in receptor.G protein coupling

The selectivity in coupling of various receptors to GTP-binding regulatory proteins (G proteins) was examined directly by a novel assay entailing the use of proteins overexpressed in Spodoptera frugiperda (Sf9) cells. Activation of G proteins was monitored in membranes prepared from Sf9 cells co-expressing selected pairs of receptors and G proteins (i.e. alpha, beta1, and gamma2 subunits). Membranes were incubated with [35S]guanosine 5'-(3-O-thio)triphosphate (GTPgammaS) +/- an agonist, and the amount of radiolabel bound to the alpha subunit was quantitated following immunoprecipitation. When expressed without receptor (but with beta1gamma2), the G protein subunits alphaz, alpha12, and alpha13 did not bind appreciable levels of [35S]GTPgammaS, consistent with a minimal level of GDP/[35S]GTPgammaS exchange. In contrast, the subunits alphas and alphaq bound measurable levels of the nucleotide. Co-expression of the 5-hydroxytryptamine1A (5-HT1A) receptor promoted binding of [35S]GTPgammaS to alphaz but not to alpha12, alpha13, or alphas. Binding to alphaz was enhanced by inclusion of serotonin in the assay. Agonist activation of both thrombin and neurokinin-1 receptors promoted a modest increase in [35S]GTPgammaS binding to alphaz and more robust increases in binding to alphaq, alpha12, and alpha13. Binding of [35S]GTPgammaS to alphas was strongly enhanced only by the activated beta1-adrenergic receptor. Our data identify interactions of receptors and G proteins directly, without resort to measurements of effector activity, confirm the coupling of the 5-HT1A receptor to Gz and extend the list of receptors that interact with this unique G protein to the receptors for thrombin and substance P, imply constitutive activity for the 5-HT1A receptor, and demonstrate for the first time that the cloned receptors for thrombin and substance P activate G12 and G13.