Phospholipase C-gamma as a signal-transducing element

Differential involvement of Src family kinases in pervanadate-mediated responses in rat myometrial cells

We previously described that pervanadate, a potent tyrosine phosphatase inhibitor, induced contraction of rat myometrium via phospholipase (PL) C-gamma1 activation [Biol Reprod 54 (1996) 1383]. In this study, we found that pervanadate induced tyrosine phosphorylation of the platelet-derived growth factor (PDGF)-beta receptor, interaction of the phosphorylated PDGF receptor with the phosphorylated PLC-gamma1, production of inositol phosphates (InsPs), extracellular signal-regulated kinase (ERK) activation and DNA synthesis. All these responses were insensitive to PDGF receptor kinase inhibition or PDGF receptor down-regulation. We showed that Src family kinases were activated by pervanadate, and that InsPs production and phosphorylation of both PLC-gamma1 and the PDGF receptor were blocked by PP1, an Src inhibitor. In contrast, the stimulation of ERK by pervanadate was totally refractory to PP1. These results demonstrated that the activation of Src by pervanadate is involved in PLC-gamma1/InsPs signalling but does not play a major role in ERK activation.

Absence of erythrogenesis and vasculogenesis in Plcg1-deficient mice

Mice nullizygous for Plcg1 cease growing at early to mid-gestation. An examination of carefully preserved wild-type embryos shows clear evidence of erythropoiesis, but erythropoiesis is not evident in Plcg1 nullizygous embryos at the same stage. The analyses of embryonic materials demonstrate that in the absence of Plcg1, erythroid progenitors cannot be detected in the yolk sac or embryo body by three different assays, burst-forming units, colony-forming units, and analysis for the developmental marker Ter119. However, non-erythroid granulocyte/macrophage colonies are produced by Plcg1 null embryos. Further analysis of these embryos demonstrates significantly diminished vasculogenesis in Plcg1 nullizygous embryos based on the lack of expression of the endothelial marker platelet endothelial cell adhesion molecule-1. In addition, Plcg1 nullizygous embryos express a greatly reduced level of vascular endothelial growth factor receptor-2/Flk-1, consistent with significantly impaired vasculogenesis and erythropoiesis. Interestingly, these early embryos do express phospholipase C-gamma2, however, it is unable to substitute for the absence of phospholipase C-gamma1, which can be detected in its tyrosine-phosphorylated state.

Phospholipase C gamma 1 is a physiological guanine nucleotide exchange factor for the nuclear GTPase PIKE

Phospholipase C gamma 1 (PLC-gamma 1) hydrolyses phosphatidylinositol-4,5-bisphosphate to the second messengers inositol-1,4,5-trisphosphate and diacylglycerol. PLC-gamma 1 also has mitogenic activity upon growth-factor-dependent tyrosine phosphorylation; however, this activity is not dependent on the phospholipase activity of PLC-gamma 1, but requires an SH3 domain. Here, we demonstrate that PLC-gamma 1 acts as a guanine nucleotide exchange factor (GEF) for PIKE (phosphatidylinositol-3-OH kinase (PI(3)K) enhancer). PIKE is a nuclear GTPase that activates nuclear PI(3)K activity, and mediates the physiological activation by nerve growth factor (NGF) of nuclear PI(3)K activity. This enzymatic activity accounts for the mitogenic properties of PLC-gamma 1.

The B lymphocyte adaptor molecule of 32 kD (Bam32) regulates B cell antigen receptor signaling and cell survival

The B lymphocyte-associated adaptor protein 32 kD in size (Bam32) is expressed at high levels in germinal center (GC) B cells. It has an NH(2)-terminal src homology 2 (SH2) domain which binds phospholipase C (PLC)gamma 2, and a COOH-terminal pleckstrin homology (PH) domain. Thus, Bam32 may function to integrate protein tyrosine kinase (PTK) and phosphatidylinositol 3-kinase (PI3K) signaling pathways in B cells. To further define the role Bam32 plays in B cells, we generated Bam32-deficient DT40 cells. These Bam32(-/-) cells exhibited lower levels of B cell antigen receptor (BCR)-induced calcium mobilization with modest decreases in tyrosine phosphorylation of phospholipase C (PLC)gamma 2. Moreover, BCR-induced activation of extracellular signal-regulated kinase (ERK), c-jun NH2-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK) pathways was impaired in Bam32(-/-) cells but not the activation of Akt-related pathways. Activation of downstream transcription factors such as nuclear factor of activated T cells (NF-AT) and nuclear factor of kappa binding (NF-kappa B) was also impaired in Bam32(-/-) cells. Furthermore, Bam32(-/-) cells were more susceptible to BCR-induced death. Taken together, these findings suggest that Bam32 functions to regulate BCR-induced signaling and cell survival most likely in germinal centers.

Tyrosine residues in phospholipase Cgamma 2 essential for the enzyme function in B-cell signaling

Phospholipase Cgamma (PLCgamma) isoforms are regulated through activation of tyrosine kinase-linked receptors. The importance of growth factor-stimulated phosphorylation of specific tyrosine residues has been documented for PLCgamma1; however, despite the critical importance of PLCgamma2 in B-cell signal transduction, neither the tyrosine kinase(s) that directly phosphorylate PLCgamma2 nor the sites in PLCgamma2 that become phosphorylated after stimulation are known. By measuring the ability of human PLCgamma2 to restore calcium responses to the B-cell receptor stimulation or oxidative stress in a B-cell line (DT40) deficient in PLCgamma2, we have demonstrated that two tyrosine residues, Tyr(753) and Tyr(759), were important for the PLCgamma2 signaling function. Furthermore, the double mutation Y753F/Y759F in PLCgamma2 resulted in a loss of tyrosine phosphorylation in stimulated DT40 cells. Of the two kinases that previously have been proposed to phosphorylate PLCgamma2, Btk, and Syk, purified Btk had much greater ability to phosphorylate recombinant PLCgamma2 in vitro, whereas Syk efficiently phosphorylated adapter protein BLNK. Using purified proteins to analyze the formation of complexes, we suggest that function of Syk is to phosphorylate BLNK, providing binding sites for PLCgamma2. Further analysis of PLCgamma2 tyrosine residues phosphorylated by Btk and several kinases from the Src family has suggested multiple sites of phosphorylation and, in the context of a peptide incorporating residues Tyr(753) and Tyr(759), shown preferential phosphorylation of Tyr(753).

Inhibition of 1,25-dihydroxyvitamin-D-induced keratinocyte differentiation by blocking the expression of phospholipase C-gamma1

Keratinocytes produce vitamin D3 and convert it to the most active form, 1,25-dihydroxyvitamin D3, which regulates keratinocyte proliferation and differentiation. Phospholipase C-gamma1 is the most abundant member of the phospholipase C family in keratinocytes and is induced by 1,25-dihydroxyvitamin D3. Therefore, phospholipase C-gamma1 might be important in the signaling pathway mediating 1,25-dihydroxyvitamin-D3-induced keratinocyte differentiation. To test this hypothesis, phospholipase C-gamma1 expression in human keratinocytes was reduced by transfecting the cells with an antisense phospholipase C-gamma1 construct and then evaluating the response of the keratinocyte differentiation markers involucrin and transglutaminase to 1,25-dihydroxyvitamin D3. The results showed that involucrin and transglutaminase protein and mRNA levels were markedly reduced in keratinocytes transfected by the antisense phospholipase C-gamma1 construct. Cotransfection of keratinocytes with the involucrin or transglutaminase promoter construct and the antisense phospholipase C-gamma1 construct showed decreased involucrin or transglutaminase promoter activity in response to 1,25-dihydroxyvitamin D3. To further investigate the mechanism by which phospholipase C-gamma1 regulates keratinocyte differentiation, the calcium and inositol triphosphate levels in keratinocytes transfected by the antisense phospholipase C-gamma1 construct were measured following 1,25-dihydroxyvitamin D3 administration. The increase in keratinocyte intracellular free calcium and inositol triphosphate levels following 1,25-dihydroxyvitamin D3 administration were markedly reduced by the transfection of the antisense phospholipase C-gamma1 construct. These studies indicate that phospholipase C-gamma1 plays a critical role in the signal transduction pathway mediating 1,25-dihydroxyvitamin-D3-induced keratinocyte differentiation at least in part by mediating the increase in inositol triphosphate production and intracellular calcium mobilization following 1,25-dihydroxyvitamin D3 administration.

Four tyrosine residues in phospholipase C-gamma 2, identified as Btk-dependent phosphorylation sites, are required for B cell antigen receptor-coupled calcium signaling

Activation of phospholipase C-gamma2 (PLCgamma2) is the critical step in B cell antigen receptor (BCR)-coupled calcium signaling. Although genetic dissection experiments on B cells have demonstrated that Bruton's tyrosine kinase (Btk) and Syk are required for activating PLCgamma2, the exact activation mechanism of PLCgamma2 by these kinases has not been established. We identify the tyrosine residues 753, 759, 1197, and 1217 in rat PLCgamma2 as Btk-dependent phosphorylation sites by using an in vitro kinase assay. To evaluate the role of these tyrosine residues in phosphorylation-dependent activation of PLCgamma2, PLCgamma2-deficient DT40 cells were reconstituted with a series of mutant PLCgamma2s in which the phenylalanine was substituted for tyrosine. Substitution of all four tyrosine residues almost completely eliminated the BCR-induced PLCgamma2 phosphorylation, indicating that these residues include the major phosphorylation sites upon BCR engagement. Cells expressing PLCgamma2 with a single substitution exhibited some extent of reduction in calcium mobilization, whereas those expressing quadruple mutant PLCgamma2 showed greatly reduced calcium response. These findings indicate that the phosphorylations of the tyrosine residues 753, 759, 1197, and 1217, which have been identified as Btk-dependent phosphorylation sites in vitro, coordinately contribute to BCR-induced activation of PLCgamma2.

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

Src tyrosine kinase is the trigger but not the mediator of ischemic preconditioning

The signal cascade that triggers and mediates ischemic preconditioning (IPC) remains unclear. The present study investigated the role of the Src family of tyrosine kinases in IPC. Isolated and buffer-perfused rat hearts underwent IPC with three cycles of 5-min ischemia and 5-min reperfusion, followed by 30-min ischemia and 120-min reperfusion. The Src tyrosine kinase family-selective inhibitor PP1 was administered between 45 and 30 min before ischemia (early PP1 treatment) or for 15 min before IPC [early PP1-preconditioning (PC) treatment]. PP1 was also administered for 5 min before the sustained ischemia (late PP1 treatment) or after IPC (late PP1-PC treatment). Src kinase was activated after 30 min of ischemia in both the membrane and cytosolic fractions. Src kinase was also activated by IPC but was attenuated after the sustained ischemia. Early and late PP1 treatment inhibited Src activation after the sustained ischemia and reduced infarct size. Early PP1-PC inhibited Src activation after IPC but not after the sustained ischemia and blocked cardioprotection afforded by IPC. Late PP1-PC treatment abrogated IPC-induced activation of Src and protein kinase C (PKC)-epsilon in the membrane but not in the cytosolic fraction. This treatment modality abrogated Src activation after the sustained ischemia and failed to block cardioprotection afforded by IPC. These results suggest that Src kinase activation mediates ischemic injury but triggers IPC in the position either upstream of or parallel to membrane-associated PKC-epsilon.

A repertoire library that allows the selection of synthetic SH2s with altered binding specificities

Tyrosine phosphorylation is one of the major mechanisms involved in the intracellular propagation of external signals. Strategies aimed at interfering with this process might allow the control of several cellular phenotypes. SH2 domains mediate protein-protein interactions by recognizing phosphotyrosine (pY) residues in the context of specific phosphopeptides. We created an SH2-scaffolded repertoire library by randomly mutagenizing five critical amino acid positions in the specificity-determining region of the PLCgamma C-terminal SH2 domain. Synthetic SH2 domains were selected from the library using biotinylated phosphopeptides derived from a natural PLCgamma-SH2 ligand as well as unrelated SH2 ligands. The isolated SH2s displayed high binding affinity constants for the selecting peptides and were capable of interacting with the corresponding proteins.

Evidence that phospholipase C from the sperm is not responsible for initiating Ca(2+) release at fertilization in mouse eggs

Release of Ca(2+) from intracellular stores at fertilization of mammalian eggs is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by which the sperm initiates IP3 production is not yet understood. We tested the hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm-egg fusion is responsible for causing Ca(2+) release. We demonstrated that microinjecting purified, recombinant PLCgamma1 protein into mouse eggs caused Ca(2+) oscillations like those seen at fertilization. However, the PLC activity in the minimum amount of purified PLCgamma1 protein needed to elicit Ca(2+) release when injected into eggs was approximately 500-900 times the PLC activity contained in a single sperm. This indicates that a single mouse sperm does not contain enough PLC activity to be responsible for causing Ca(2+) release at fertilization. We also examined whether phosphatidylinositol 3-kinase (PI3K) could have a role in this process, and found that several inhibitors of PI3K-mediated signaling had no effect on Ca(2+) release at fertilization.

Identification of a phospholipase C-gamma1 (PLC-gamma1) SH3 domain-binding site in SLP-76 required for T-cell receptor-mediated activation of PLC-gamma1 and NFAT

SLP-76 is an adapter protein required for T-cell receptor (TCR) signaling. In particular, TCR-induced tyrosine phosphorylation and activation of phospholipase C-gamma1 (PLC-gamma1), and the resultant TCR-inducible gene expression, depend on SLP-76. Nonetheless, the mechanisms by which SLP-76 mediates PLC-gamma1 activation are not well understood. We now demonstrate that SLP-76 directly interacts with the Src homology 3 (SH3) domain of PLC-gamma1. Structure-function analysis of SLP-76 revealed that each of the previously defined protein-protein interaction domains can be individually deleted without completely disrupting SLP-76 function. Additional deletion mutations revealed a new, 67-amino-acid functional domain within the proline-rich region of SLP-76, which we have termed the P-1 domain. The P-1 domain mediates a constitutive interaction of SLP-76 with the SH3 domain of PLC-gamma1 and is required for TCR-mediated activation of Erk, PLC-gamma1, and NFAT (nuclear factor of activated T cells). The adjacent Gads-binding domain of SLP-76, also within the proline-rich region, mediates inducible recruitment of SLP-76 to a PLC-gamma1-containing complex via the recruitment of both PLC-gamma1 and Gads to another cell-type-specific adapter, LAT. Thus, TCR-induced activation of PLC-gamma1 entails the binding of PLC-gamma1 to both LAT and SLP-76, a finding that may underlie the requirement for both LAT and SLP-76 to mediate the optimal activation of PLC-gamma1.

EGF-dependent translocation of green fluorescent protein-tagged PLC-gamma1 to the plasma membrane and endosomes

Growth factor-dependent translocation of phospholipase C-gamma1 (PLC-gamma1) was investigated using a green fluorescent protein-tagged PLC-gamma1 (PLC-gamma1-GFP) expressed in human epidermoid carcinoma A-431 cells. In the absence of growth factors, PLC-gamma1-GFP was present throughout the cytoplasm of A-431 cells. Treatment of the cells with epidermal growth factor (EGF) produced a very rapid redistribution of PLC-gamma1-GFP to the plasma membrane in a nonuniform manner. This translocation to the plasma membrane was insensitive to an inhibitor of phosphatidylinositol 3-kinase and was independent of cell adhesion. However, the translocation was disrupted by an agent which depolymerizes the actin cytoskeleton. At later times following the addition of EGF, PLC-gamma1-GFP appeared associated with intracellular vesicles. Stimulation of A-431 cells by Texas red-conjugated EGF for more than 10 min resulted in punctate intracellular PLC-gamma1-GFP distribution that colocalized with Texas red-conjugated EGF. This suggests that PLC-gamma1 is translocated to endosomes after EGF treatment, probably by associating with the internalized and autophosphorylated EGF receptor. Fractionation studies demonstrated that the EGF-induced plasma membrane-localized PLC-gamma1 is concentrated in caveolae microdomains. Disruption of caveolae with methyl-beta-cyclodextrin resulted in the ablation of EGF-induced, but not bradykinin-induced, mobilization of intracellular Ca(2+). This treatment, however, only partially decreased PLC-gamma1 membrane translocation.

Regulation of NK cell-mediated cytotoxicity by the adaptor protein 3BP2

Stimulation of lymphocytes through multichain immune recognition receptors activates multiple signaling pathways. Adaptor proteins play an important role in integrating these pathways by their ability to simultaneously bind multiple signaling components. Recently, the 3BP2 adaptor protein has been shown to positively regulate the transcriptional activity of T cells. However, the mechanisms by which signaling components are involved in this regulation remain unclear, as does a potential role for 3BP2 in the regulation of other cellular functions. Here we describe a positive regulatory role for 3BP2 in NK cell-mediated cytotoxicity. We also identify p95(vav) and phospholipase C-gamma isoforms as binding partners of 3BP2. Our results show that tyrosine-183 of 3BP2 is specifically involved in this interaction and that this residue critically influences 3BP2-dependent function. Therefore, 3BP2 regulates NK cell-mediated cytotoxicity by mobilizing key downstream signaling effectors.

Oxidative stress-induced phospholipase C-gamma 1 activation enhances cell survival

Phospholipase C-gamma1 (PLC-gamma1) is rapidly activated in response to growth factor stimulation and plays an important role in regulating cell proliferation and differentiation through the generation of the second messengers diacylglycerol and inositol 1,4,5-trisphosphate, leading to the activation of protein kinase C (PKC) and increased levels of intracellular calcium, respectively. Given the existing overlap between signaling pathways that are activated in response to oxidant injury and those involved in responding to proliferative stimuli, we investigated the role of PLC-gamma1 during the cellular response to oxidative stress. Treatment of normal mouse embryonic fibroblasts (MEF) with H2O2 resulted in time- and concentration-dependent tyrosine phosphorylation of PLC-gamma1. Phosphorylation could be blocked by pharmacological inhibitors of Src family tyrosine kinases or the epidermal growth factor receptor tyrosine kinase, but not by inhibitors of the platelet-derived growth factor receptor or phosphatidylinositol 3-kinase. To investigate the physiologic relevance of H2O2-induced tyrosine phosphorylation of PLC-gamma1, we compared survival of normal MEF and PLC-gamma1-deficient MEF following exposure to H2O2. Treatment of PLC-gamma1-deficient MEF with H2O2 resulted in rapid cell death, whereas normal MEF were resistant to the stress. Pretreatment of normal MEF with a selective pharmacological inhibitor of PLC-gamma1, or inhibitors of inositol trisphosphate receptors and PKC, increased their sensitivity to H2O2, whereas treatment of PLC-gamma1-deficient MEF with agents capable of directly activating PKC and enhancing calcium mobilization significantly improved their survival. Finally, reconstitution of PLC-gamma1 protein expression in PLC-gamma1-deficient MEF restored cell survival following H2O2 treatment. These findings suggest an important protective function for PLC-gamma1 activation during the cellular response to oxidative stress.

Growth-Stimulating Pathways in Lung Cancer: Implications for Targets of Therapy

Growth-stimulating pathways activated independently of their normal tissue environment are critical to the carcinogenesis and progression of lung cancer. These pathways are comprised of extracellular growth factors; their specific receptors on the cellular membrane; signal transduction cascades in the cytosol; and target molecules, including cytoskeletal proteins, metabolic regulators, and transcription factors in the nucleus. Growth factors can be divided into two groups based on their receptors: G-protein-coupled receptors and receptor tyrosine kinases. Growth factors induce clonal expansion of lung cancer cells by autocrine and/or paracrine mechanisms. Signal transduction cascades form an extremely large and complicated network with cross-talk connections. Ras, phosphatidylinositol-3-OH kinase, and phospholipase C are three key regulators involved in the network. Recent progress in our understanding of the oncoproteins functioning in the pathways has led to the development of novel therapeutic agents. Some of the most exciting results have been obtained with inhibitors of receptor tyrosine kinases. Phase I studies of epidermal growth factor-receptor inhibitors demonstrate objective responses without severe toxicity as single agents in patients with non-small-cell lung cancer refractory to conventional chemotherapy. This new strategy might lead to breakthroughs in the treatment of lung cancer with distant metastases not curable by conventional chemotherapy alone.

Signaling properties of VEGF receptor-1 and -2 homo- and heterodimers

Vascular endothelial growth factor (VEGF-A) exerts its effects through receptor tyrosine kinases VEGF receptor-1 (VEGFR-1) and VEGFR-2, which are expressed on most endothelial cell types in vitro and in vivo. We have examined VEGF-A-induced signal transduction in porcine aortic endothelial (PAE) cells individually expressing VEGFR-1 or VEGFR-2, and cells co-expressing both receptor types. We show that VEGF-A-stimulated PAE cells co-expressing VEGFR-1 and -2 contain receptor heterodimers. VEGF-A-stimulation of all three cell lines (expressing VEGFR-1, -2 and -1/2) resulted in signal transduction with different efficiencies. Thus, tyrosine phosphorylation of phospholipase Cgamma, and accumulation of inositol polyphosphates were efficiently transduced in the VEGFR-1/2 cells whereas cells expressing VEGFR-1 responded poorly in these assays. In contrast, VEGF-A-induced activation of phosphoinositide 3-kinase and induction of Ca2+ fluxes were transduced well by VEGFR-1 and VEGFR-2 homo- and heterodimers. The pattern of Ca2+ fluxes was unique for each type of VEGF receptor dimer. Our data show that signal transduction induced by VEGF-A is transduced in distinct manners by homo- and heterodimers of VEGF receptors.

Sustained signaling by phospholipase C-gamma mediates nerve growth factor-triggered gene expression

In contrast to conventional signaling by growth factors that requires their continual presence, a 1-min pulse of nerve growth factor (NGF) is sufficient to induce electrical excitability in PC12 cells due to induction of the peripheral nerve type 1 (PN1) sodium channel gene. We have investigated the mechanism for this triggered signaling pathway by NGF in PC12 cells. Mutation of TrkA at key autophosphorylation sites indicates an essential role for the phospholipase C-gamma (PLC-gamma) binding site, but not the Shc binding site, for NGF-triggered induction of PN1. In concordance with results with Trk mutants, drug-mediated inhibition of PLC-gamma activity also blocks PN1 induction by NGF. Examination of the kinetics of TrkA autophosphorylation indicates that triggered signaling does not result from sustained activation and autophosphorylation of the TrkA receptor kinase, whose phosphorylation state declines rapidly after NGF removal. Rather, TrkA triggers an unexpectedly prolonged phosphorylation and activation of PLC-gamma signaling that is sustained for up to 2 h. Prevention of the elevation of intracellular Ca2+ levels using BAPTA-AM results in a block of PN1 induction by NGF. Sustained signaling by PLC-gamma provides a means for differential neuronal gene induction after transient exposure to NGF.

Calmodulin and profilin coregulate axon outgrowth in Drosophila

Coordinated regulation of actin cytoskeletal dynamics is critical to growth cone movement. The intracellular molecules calmodulin and profilin actively regulate actin-based motility and participate in the signaling pathways used to steer growth cones. Here we show that in the developing Drosophila embryo, calmodulin and profilin convey complimentary information that is necessary for appropriate growth cone advance. Reducing calmodulin activity by expression of a dominant inhibitor (KA) stalls axon extension of pioneer neurons within the CNS, while a partial loss of profilin function decreases extension of motor axons in the periphery. Yet, surprisingly, when calmodulin and profilin are simultaneously reduced, the ability of both CNS pioneer axons and motor axons to extend beyond the choice points is restored. In the CNS, at the time when growth cones must decide whether to cross or not to cross the midline, a reduction in calmodulin and/or roundabout signaling causes axons to cross the midline inappropriately. These inappropriate crossings are suppressed when profilin activity is simultaneously reduced. Interestingly, the mutual suppression of calmodulin and profilin activity requires a minimal level of profilin. In KA combinations with profilin null alleles, defects in axon extension and midline guidance are synergistically enhanced rather than suppressed. Together, our data indicate that the growth cone must coordinate the activity of both calmodulin and profilin in order to advance past selected choice points, including those dictating midline crossovers.

Protein tyrosine phosphatase CD148-mediated inhibition of T-cell receptor signal transduction is associated with reduced LAT and phospholipase Cgamma1 phosphorylation

In this study, we investigate the role of the receptor-like protein tyrosine phosphatase CD148 in T-cell activation. Overexpression of CD148 in the Jurkat T-cell line inhibited activation of the transcription factor nuclear factor of activated T cells following T-cell receptor (TCR) stimulation but not following stimulation through a heterologously expressed G protein-coupled receptor, the human muscarinic receptor subtype 1. Using a tetracycline-inducible expression system, we show that the TCR-mediated activation of both the Ras and calcium pathways was inhibited by expression of CD148 at levels that approximate those found in activated primary T cells. These effects were dependent on the phosphatase activity of CD148. Analysis of TCR-induced protein tyrosine phosphorylation demonstrated that most phosphoproteins were unaffected by CD148 expression. However, phospholipase Cgamma1 (PLCgamma1) and LAT were strikingly hypophosphorylated in CD148-expressing cells following TCR stimulation, whereas the phosphorylation levels of Slp-76 and Itk were modestly reduced. Based on these results, we propose that CD148 negatively regulates TCR signaling by interfering with the phosphorylation and function of PLCgamma1 and LAT.

Rap1 is activated by erythropoietin or interleukin-3 and is involved in regulation of beta1 integrin-mediated hematopoietic cell adhesion

The CrkL adaptor protein is involved in signaling from the receptor for erythropoietin (Epo) as well as interleukin (IL)-3 and activates beta(1) integrin-mediated hematopoietic cell adhesion through its interaction with C3G, a guanine nucleotide exchange factor for Rap1. We demonstrate here that Epo as well as IL-3 activates Rap1 in an IL-3-dependent hematopoietic cell line, 32D, expressing the Epo receptor. The cytokine-induced activation of Rap1 was augmented in cells that inducibly overexpress CrkL or C3G. The CrkL-mediated enhancement of cell adhesion was inhibited by expression of a dominant negative mutant of Rap1, Rap1A-17N, whereas an activated mutant of Rap1, Rap1A-63E, activated beta(1) integrin-dependent adhesion of hematopoietic cells. In 32D cells, Rap1 was also activated by phorbol 12-myristate 13-acetate and ionomycin, which also enhanced cell adhesion to fibronectin, whereas, an inhibitor of phospholipase C, inhibited both cytokine-induced activation of Rap1 and cell adhesion. It was also demonstrated that Rap1 as well as CrkL is involved in signaling from the EpoR endogenously expressed in a human leukemic cell line, UT-7. These results suggest that Epo and IL-3 activate Rap1 at least partly through the CrkL-C3G complex as well as through additional pathways most likely involving phospholipase Cgamma and strongly implicate Rap1 in regulation of beta(1) integrin-mediated hematopoietic cell adhesion.

Phospholipase C-gamma1 is required for the induction of immediate early genes by platelet-derived growth factor

To explore the functional role of phospholipase C-gamma1 (PLC-gamma1) in the induction of immediate early genes (IEGs), we have examined the influence of Plcg1 gene disruption on the expression of 14 IEG mRNAs induced by platelet-derived growth factor (PDGF). Plcg1-null embryos were used to produce immortalized fibroblasts genetically deficient in PLC-gamma1 (Null cells), and retroviral infection of those cells was used to derive PLC-gamma1 re-expressing cells (Null+ cells). In terms of PDGF activation of PDGF receptor tyrosine phosphorylation as well as the mitogen-activated protein kinases Erk1 and Erk2, Null and Null+ cells responded equivalently. However, the PDGF-dependent expression of all IEG mRNAs was diminished in cells lacking PLC-gamma1. The expression of FIC, COX-2, KC, JE, and c-fos mRNAs were most strongly compromised, as the stimulation of these genes was reduced by more than 90% in cells lacking PLC-gamma1. The combination of PMA and ionomycin, downstream analogs of PLC activation, did provoke expression of mRNAs for these IEGs in the Null cells. We conclude that PLC-gamma1 is necessary for the maximal expression of many PDGF-induced IEGs and is essential for significant induction of at least five IEGs.

Activation of extracellular-regulated kinases by normal and mutant EGF receptors

Glioblastoma cells express a mutant EGF receptor (EGFRvIII) that has constitutive tyrosine kinase activity and enhances their tumorigenicity. Here we show that EGFRvIII promotes constitutive phosphorylation of extracellular regulated kinases (ERKs) in glioblastoma cells in the absence of EGF. EGFRvIII also promoted constitutive activation of phosphoinositide 3-kinase in these cells, as assessed by phosphorylation of protein kinase B/akt. As expected, phosphorylation of protein kinase B/akt was blocked by the phosphoinositide 3-kinase inhibitors wortmannin and LY294002. Less expectedly, we found that this treatment also blocked EGFRvIII-induced phosphorylation of ERKs. In contrast, ERK phosphorylation induced by EGF-activated normal EGF receptor in the same cells was largely unaffected by treatment with phosphoinositide 3-kinase inhibitors. This difference in behavior between the normal receptor and EGFRvIII was not due to differences in the levels of activated EGFRvIII and wild-type EGF receptor, as the two types of receptor were tyrosine phosphorylated to a similar extent under the experimental conditions used. EGFRvIII activation of ERKs was also sensitive to the phospholipase C inhibitor U73122, whereas ERK activation by normal EGF receptor was not. These results show that EGFRvIII and wild-type EGF receptor preferentially use different signaling pathways to induce ERK phosphorylation. The different mechanisms of ERK activation used by normal and mutant EGF receptors may be important in understanding the potent tumorigenic activity of EGFRvIII.

NF-kappaB activation by N-CAM and cytokines in astrocytes is regulated by multiple protein kinases and redox modulation

Interaction of the neural cell adhesion molecule (N-CAM) with astrocytes activates a transcription factor, NF-kappaB, that mediates inflammatory responses after neural injury. Here we describe intracellular signaling events that link N-CAM binding to NF-kappaB-mediated transcription. Addition of the third immunoglobulin domain of N-CAM (Ig III), which mimics the activity of intact N-CAM, or of cytokines (interleukin-1beta or tumor necrosis factor-alpha), increased transcription from an NF-kappaB-responsive luciferase reporter gene construct that had been transiently transfected into neonatal rat forebrain astrocytes. NF-kappaB activity induced by Ig III or cytokines was decreased by inhibition of nonreceptor protein tyrosine kinases (PTKs), phospholipase C, protein kinase C (PKC), calcium/calmodulin-dependent protein kinase II (CaMKII), or oxidative stress. Inhibition of PKC blocked nuclear translocation of NF-kappaB protein while binding of NF-kappaB to DNA was decreased by modulation of redox homeostasis. In contrast, inhibition of CaMKII and nonreceptor PTKs altered neither nuclear translocation nor DNA binding, suggesting that these kinases affect NF-kappaB transactivation. A number of agents that inhibit NF-kappaB activation in other cell types did not affect activation in astrocytes. These findings suggest that activation of NF-kappaB by N-CAM and cytokines in astrocytes involves multiple signals that differentially affect NF-kappaB nuclear translocation, DNA binding, and transactivation.

Salivary cellular signaling and gene regulation

Protein tyrosine kinase and protein serine kinase activation has been implicated in the regulation of salivary cell proliferation and differentiation. Aberrant expression and alterations of certain tyrosine or serine kinases, such as Raf or erbB2, are known to trigger salivary tumor development (Li et al., 1997; Cho et al., 1999). It has been estimated that there are about 1000 to 2000 protein kinases in the mammalian genome, with 100 to 200 of them (i.e., 10%) being tyrosine kinase (Hanks and Hunter, 1995). At present, there are approximately 85 different tyrosine kinases identified in the GenBank database. Based on the relatively slow rate of discovery in the past few years, 100 is a better approximation of the total number of tyrosine kinases encoded by each mammalian genome. It is reasonable to assume that there are about 30 to 50 tyrosine kinases expressed in a given cell at a given differentiation/proliferation stage. This number is large enough to provide a characteristic tissue-specific tyrosine kinase expression profile, but small enough to be identified in a simple screening. The hope for tyrosine kinases as differentiation or proliferation markers rests with the possibility for the identification and characterization of a differentiation/proliferation stage-specific expression pattern in salivary cells. Several ligands that transmit signal through receptor tyrosine kinases and/or Ras/Raf/ERK kinases have been extensively studied in salivary cells. This review focuses mainly on the signaling pathways activated by Raf and Etk.

The invasion protein InIB from Listeria monocytogenes activates PLC-gamma1 downstream from PI 3-kinase

Entry of the bacterial pathogen Listeria monocytogenes into non-phagocytic mammalian cells is mainly mediated by the InlB protein. Here we show that in the human epithelial cell line HEp-2, the invasion protein InlB activates sequentially a p85beta-p110 class I(A) PI 3-kinase and the phospholipase C-gamma1 (PLC-gamma1) without detectable tyrosine phosphorylation of PLC-gamma1. Purified InlB stimulates association of PLC-gamma1 with one or more tyrosine-phosphorylated proteins, followed by a transient increase in intracellular inositol 1,4,5-trisphosphate (IP3) levels and a release of intracellular Ca2+ in a PI 3-kinase-dependent manner. Infection of HEp-2 cells with wild-type L. monocytogenes bacteria also induces association of PLC-gamma1 with phosphotyrosyl proteins. This interaction is undetectable upon infection with a deltainlB mutant revealing an InlB specific signal. Interestingly, pharmacological or genetic inactivation of PLC-gamma1 does not significantly affect InlB-mediated bacterial uptake, suggesting that InlB-mediated PLC-gamma1 activation and calcium mobilization are involved in post-internalization steps.

Suppression of epidermal growth factor-induced phospholipase C activation associated with actin rearrangement in rat hepatocytes in primary culture

Hepatocytes maintained in primary culture for periods of 1 to 24 hours exhibited a rapid decline in epidermal growth factor (EGF)-induced activation of phospholipase C (PLC), as was evident in a loss of EGF-induced inositol 1,4,5-trisphosphate (IP(3)) formation and mobilization of Ca(2+) from intracellular Ca(2+) stores. The loss of PLC activation was not the result of a decrease in EGF receptor or phospholipase C-gamma1 (PLCgamma1) protein levels, nor the result of a loss of tyrosine phosphorylation of these proteins, but was associated with a decrease in EGF-induced translocation of PLCgamma1 to the Triton-insoluble fraction, presumably reflecting binding to the actin cytoskeleton. Disruption of F-actin by treatment of cultured hepatocytes with cytochalasin D recovered the EGF-induced IP(3) formation and Ca(2+) mobilization to the same level and with the same dose-response relationship as was obtained in freshly isolated cells. Analysis of PLCgamma1 colocalization with F-actin by confocal microscopy showed that PLCgamma1 was mostly distributed diffusely in the cytosol, both in freshly plated cells and in cells in culture for 24 hours, despite marked differences in actin structures. EGF stimulation caused a modest redistribution of PLCgamma1 and a detectable increase in colocalization with cortical actin structures in freshly plated cells or in cytochalasin D-treated cells, but in cells that had been maintained and spread in culture only a limited PLCgamma1 relocation was detected to specific actin-structure associated with lamellipodia and membrane ruffles. We conclude that actin cytoskeletal structures can exert negative control over PLCgamma1 activity in hepatocytes and the interaction of the enzyme with specific actin structures dissociates PLCgamma1 tyrosine phosphorylation from activation of its enzymatic activity.

Effects of a constitutively active form of calcineurin on T cell activation and thymic selection

Calcineurin is a calcium/calmodulin-dependent phosphatase whose activity is required for the induction of T cell lymphokine production and proliferation. Although its specific role in T cell development is less well defined, studies with the immunosuppressive drugs cyclosporin A and FK-506 suggest that it is involved in both positive and negative selection of immature thymocytes. To more completely characterize a role for calcineurin in T cell development in vivo, we have generated transgenic mice that express an activated form of this enzyme in thymocytes and peripheral T cells. We find that the transgene causes a block in early thymic development, resulting in a reduction in the steady-state number of CD4 and CD8 double positives, but not on the number of mature T cells. We also find that thymocytes and mature T cells expressing this transgene are more sensitive to signals through their TCR. In thymocytes this sensitivity difference is manifested as an increase in positive selection, although negative selection seems to remain unaffected. Therefore, these studies confirm and extend past reports that suggested a role for calcineurin in thymic development and selection.

Signaling specificities of fibroblast growth factor receptors in early Xenopus embryo

Formation of mesoderm and posterior structures in early Xenopus embryos is dependent on fibroblast growth factor (FGF) signaling. Although several FGF receptors (FGFRs) are expressed in the early embryo, their respective role in these processes remains poorly understood. We provide evidence that FGFR-1 and FGFR-4 signals elicit distinct responses both in naive and neuralized ectodermal cells. We show that naive ectodermal cells expressing a constitutively active chimeric torso-FGFR-1 (t-R1) are converted into mesoderm in a Ras-dependent manner, while those expressing torso-FGFR-4 (t-R4) differentiate into epidermis without significant activation of Erk-1. In neuralized ectoderm, expression of t-R4 causes the up-regulation of the midbrain markers En-2 and Wnt-1, but not of the hindbrain nor the spinal cord markers Krox20 and Hoxb9. Mutation of tyr(776) in the phospholipase C-(gamma) binding consensus sequence YLDL of t-R4 completely abolishes En-2 and Wnt-1 induction. In contrast to t-R4, platelet derived growth factor (PDGF)-dependent FGFR-1 activation in neuralized ectodermal cells expressing a chimeric PDGFR-FGFR-1 receptor results in the expression of Krox20 and Hoxb9. A similar effect is observed when an inducible form of oncogenic Raf is expressed, therefore implicating FGFR-1 and Raf in the transduction of FGF-caudalizing signals in neural tissue. Our results suggest that FGFR-1 and FGFR-4 transduce distinct signals in embryonic cells, and mainly differ in their ability to activate the Ras/MAPK pathway.