Polyphosphoinositides Produced by Phosphatidylinositol 3-Kinase Are Poor Substrates for Phospholipases C from Rat Liver and Bovine Brain

Crystal structure of a mammalian phosphoinositide-specific phospholipase C delta

Mammalian phosphoinositide-specific phospholipase C enzymes (PI-PLC) act as signal transducers that generate two second messengers, inositol-1,4,5-trisphosphate and diacylglycerol. The 2.4-A structure of phospholipase C delta 1 reveals a multidomain protein incorporating modules shared by many signalling proteins. The structure suggests a mechanism for membrane attachment and Ca2+-dependent hydrolysis of second-messenger precursors. The regulation and reversible membrane association of PI-PLC may serve as a model for understanding other multidomain enzymes involved in phospholipid signalling.

Purification and biochemical characterization of a mammalian phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase

Characterization of the enzymes involved in metabolism of 3-phosphorylated inositol lipids and their subcellular localization revealed that in vitro a 5-phosphatase activity was responsible for the degradation of phosphatidylinositol 3,4,5-trisphosphate, whereas a 3-phosphatase activity hydrolyzed phosphatidylinositol 3-phosphate and/or phosphatidylinositol 3,4-bisphosphate. All these activities were localized in the cytosol. No phospholipase activities were detected. The cytosolic phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase activity was purified to near homogeneity using ion exchange, affinity, and size exclusion chromatography. Characterization of the purified phosphatase revealed that it is a magnesium-dependent 5-phosphatase that is able to hydrolyze phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. The enzyme is only partially inhibited by neomycin and vanadate but is strongly inhibited by phosphatidylinositol 4,5-bisphosphate and to a slightly lesser extent by phosphatidylinositol 4-phosphate.

Phosphorylation of the platelet p47 phosphoprotein is mediated by the lipid products of phosphoinositide 3-kinase

Platelet stimulation by thrombin or the thrombin receptor activating peptide (TRAP) results in the activation of phosphoinositide 3-kinase and the production of the novel polyphosphoinositides phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3). We have shown previously that these lipids activate calcium-independent protein kinase C (PKC) isoforms in vitro (Toker, A., Meyer, M., Reddy, K. K., Falck, J. R., Aneja, R., Aneja, S., Parra, A., Burns, D. J., Ballas, L. M. and Cantley, L. C. (1994) J. Biol. Chem. 269, 32358-32367). Activation of platelet PKC in response to TRAP is detected by the phosphorylation of the major PKC substrate in platelets, the p47 phosphoprotein, also known as pleckstrin. Here we provide evidence for two phases of pleckstrin phosphorylation in response to TRAP. A rapid phase of pleckstrin phosphorylation (< 1 min) precedes the peak of PtdIns-3,4-P2 production and is unaffected by concentrations of wortmannin (10-100 nM) that block production of this lipid. However prolonged phosphorylation of pleckstrin (> 2 min) is inhibited by wortmannin concentrations that block PtdIns-3,4-P2 production. Phorbol ester-mediated pleckstrin phosphorylation was not affected by wortmannin and wortmannin had no effect on purified platelet PKC activity. Phosphorylation of pleckstrin could be induced using permeabilized platelets supplied with exogenous gamma-32P[ATP] and synthetic dipalmitoyl PtdIns-3,4,5-P3 and dipalmitoyl PtdIns-3,4-P2 micelles, but not with dipalmitoyl phosphatidylinositol 3-phosphate or phosphatidylinositol 4,5-bisphosphate. These results suggest two modes of stimulating pleckstrin phosphorylation: a rapid activation of PKC (via diacylglycerol and calcium) followed by a slower activation of calcium-independent PKCs via PtdIns-3,4-P2.

A family of phosphoinositide 3-kinases in Drosophila identifies a new mediator of signal transduction

BACKGROUND

Mammalian phosphoinositide 3-kinases (PI 3-kinases) are involved in receptor-mediated signal transduction and have been implicated in processes such as transformation and mitogenesis through their role in elevating cellular phosphatidylinositol (3,4,5)-trisphosphate. Additionally, a PI 3-kinase activity which generates phosphatidylinositol 3-phosphate has been shown to be required for protein trafficking in yeast.

RESULTS

We have identified a family of three distinct PI 3-kinases in Drosophila, using an approach based on the polymerase chain reaction to amplify a region corresponding to the conserved catalytic domain of PI 3-kinases. One of these family members, PI3K_92D, is closely related to the prototypical PI 3-kinase, p110 alpha; PI3K_59F is homologous to Vps34p, whereas the third, PI3K_68D, is a novel PI 3-kinase which is widely expressed throughout the Drosophila life cycle. The PI3K_68D cDNA encodes a protein of 210 kDa, which lacks sequences implicated in linking p110 PI 3-kinases to p85 adaptor proteins, but contains an amino-terminal proline-rich sequence, which could bind to SH3 domains, and a carboxy-terminal C2 domain. Biochemical analyses demonstrate that PI3K_68D has a novel substrate specificity in vitro, restricted to phosphatidylinositol and phosphatidylinositol 4-phosphate, and is unable to phosphorylate phosphatidylinositol (4,5)-bisphosphate, the implied in vivo substrate for p110.

CONCLUSIONS

A family of PI 3-kinases in Drosophila, including a novel class represented by PI3K_68D, is described. PI3K_68D has the potential to bind to signalling molecules containing SH3 domains, lacks p85-adaptor-binding sequences, has a Ca(2+)-independent phospholipid-binding domain and displays a restricted in vitro substrate specificity, so it could define a novel signal transduction pathway.

Phosphatidylinositol (3,4,5)P3 interacts with SH2 domains and modulates PI 3-kinase association with tyrosine-phosphorylated proteins

Src homology 2 (SH2) domains on the regulatory subunit of phosphoinositide 3-kinase (PI 3-kinase) mediate its binding to specific tyrosine-phosphorylated proteins in stimulated cells. Using a pharmacological and genetic approach, we show that the amount of PI 3-kinase associated with tyrosine-phosphorylated proteins inversely correlates with the amount of PI 3-kinase lipid products present in the cell. An explanation for this observation is provided by our finding that phosphatidylinositol (3,4,5)trisphosphate (Ptdlns [3,4,5]P3) binds directly and selectively to the SH2 domains of the 85 kDa subunit of PI 3-kinase and thereby blocks binding to tyrosine-phosphorylated proteins. The SH2 domain of pp60C-STC also specifically bound Ptdlns (3,4,5)P3, and the binding was competed by a phosphopeptide specific for the Src SH2 domain. These results indicate that production of Ptdlns (3,4,5)P3 at the membrane disrupts the binding of PI 3-kinase to phosphoproteins. This lipid may also recruit other SH2-containing proteins to the membrane to initiate downstream signaling.

Localization of mRNA for phosphatidylinositol 3-kinase in brain of developing and mature rats

In the adult brain, the gene expression for phosphatidylinositol 3-kinase was weak in almost all neurons with relatively high levels in the olfactory bulb, hippocampus, and cerebellum. On the embryonic days high levels of the gene expression were found throughout the entire neuraxis, then decreased gradually to adult levels during postnatal development. The present finding suggests that phosphatidylinositol 3-kinase may contribute not only to neurogenesis but also to some important physiological roles in differentiating and mature neurons.

Phosphorylation of tyrosine 503 in the erythropoietin receptor (EpR) is essential for binding the P85 subunit of phosphatidylinositol (PI) 3-kinase and for EpR-associated PI 3-kinase activity

We recently reported that phosphatidylinositol (PI) 3-kinase becomes associated with the activated erythropoietin receptor (EpR), most likely through the Src homology 2 (SH2) domains within the p85 subunit of PI-3 kinase and one or more phosphorylated tyrosines within the EpR. We have now investigated this interaction in more detail and have found, based on both blotting studies with glutathione S-transferase-p85-SH2 fusion proteins and binding of these fusion proteins to SDS-denatured EpRs, that this binding is direct. Moreover, both in vitro competition studies, involving phosphorylated peptides corresponding to the amino acid sequences flanking the eight tyrosines within the intracellular domain of the EpR, and in vivo studies with mutant EpRs bearing tyrosine to phenylalanine substitutions, indicate that phosphorylation of Tyr503 within the EpR is essential for the binding of PI 3-kinase. The presence of PI 3-kinase activity in EpR immunoprecipitates from DA-3 cells infected with wild-type but not Y503F EpRs confirms this finding. Our results demonstrate that the SH2 domains of p85 can bind, in addition to their well established Tyr-Met/Val-X-Met consensus binding sequence, a Tyr-Val-Ala-Cys motif that is present in the EpR. A comparison of erythropoietin-induced tyrosine phosphorylations and proliferation of wild-type and Y503F EpR-infected DA-3 cells revealed no differences. However, the PI-3 kinase inhibitor, wortmannin, markedly inhibited the erythropoietin-induced proliferation of both cell types, suggesting that PI 3-kinase is activated in Y503F EpR expressing cells. This was confirmed by carrying out PI 3-kinase assays with anti-phosphotyrosine immunoprecipitates from erythropoietin-stimulated Y503F EpR-infected DA-3 cells and suggested that PI 3-kinase has a role in regulating erythropoietin-induced proliferation, but at a site distinct from the EpR.

Phosphatidylinositol (3,4,5)-trisphosphate stimulates phosphorylation of pleckstrin in human platelets

We have reported that platelets exposed to thrombin or thrombin receptor-directed ligand activate phospholipase C and rapidly accumulate phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) and phosphatidylinositol (3,4)-bisphosphate (PtdIns(3,4)P2) as a function of the activation of phosphoinositide (PI) 3-kinases in a GTP-binding protein-dependent manner. In such platelets, serine- and threonine-directed phosphorylation of pleckstrin also occurs and has been attributed to protein kinase C activation. We now report that the phosphorylation of pleckstrin is partially dependent upon PI 3-kinase. Pleckstrin phosphorylation in response to thrombin receptor stimulation is progressively susceptible to inhibition by wortmannin, a potent and specific inhibitor of platelet PI 3-kinases. PI 3-kinase thus seems to play a gradually increasing role in promoting pleckstrin phosphorylation. The IC50 for wortmannin in inhibiting SFLLRN-stimulated 3-phosphorylated phosphoinositide accumulation is 10 nM, and that (i.e. 50% of maximum inhibition) for inhibiting pleckstrin phosphorylation is 15 nM. Synthetic PtdIns(3,4,5)P3, when added to saponin-permeabilized (but not intact) platelets, causes wortmannin-insensitive phosphorylation of pleckstrin. PtdIns(3,4,5)P3 also overcomes the inhibition by wortmannin of thrombin- or guanosine 5'-3-O-(thio)trisphosphate-stimulated pleckstrin phosphorylation. In contrast, PtdIns(4,5)P2 or inositol (1,3,4,5)-tetrakisphosphate are ineffective in these respects. The pattern of phosphorylation of pleckstrin activated by PtdIns(3,4,5)P3 is not distinguishable from that of pleckstrin phosphorylated in intact platelets exposed to protein kinase C-activating beta-phorbol myristate acetate, mimicking diacylglycerol. Activation of protein kinase(s) by PtdIns(3,4,5)P3 thus offers a route for pleckstrin phosphorylation in vivo that is an alternative to activation of phospholipase C-->diacylglycerol-->protein kinase C.

Quantitative chromatographic analysis of inositol phospholipids and related compounds

The metabolism of phospholipids and the mobilization of second messengers such as inositol-1,4,5-trisphosphate, 1,2-diacylglycerol (DAG) and arachidonic acid (AA) from phospholipids is commonly studied by radiolabelling phospholipids with [3H]myo-inositol or [32P]ATP and measuring the incorporation of radioactivity in different phospholipids or their hydrolysis products. However, for the radiolabelling method to accurately reflect changes in the compound's mass, it is essential that the tissue is labelled to isotopic equilibrium which is difficult to achieve. To circumvent the disadvantages of the radiolabelling method, several analytical procedures have been developed for the mass analysis of phospholipids and inositolphosphates (IPs). Quantitation of the mass or the radiolabelling of phospholipids is a complex multi-step procedure that involves quantitative isolation of phospholipids, fractionation of individual phospholipids and either determination of radioactivity in each component or the measurement of their mass. Phospholipids, DAG and AA are extracted from tissue sample with organic solvents such as chloroform-methanol (2:1) containing HCl or formic acid. The extract is separated by TLC, cartridge-column chromatography or HPLC on a reversed-phase column. Phospholipids are quantitated by measuring inorganic phosphate, absorption at 200 nm or mass spectrometry. Inositol phosphates are extracted with perchloric acid or trichloroacetic acid and separated by ion-exchange cartridge-column or HPLC with an ion-exchange column. IPs are quantitated by measuring inorganic phosphate or by using enzymatic reaction, metal-dye coupling, NMR or mass spectrometry.

The isolation and characterization of cDNA encoding human and rat brain inositol polyphosphate 4-phosphatase

Inositol polyphosphate 4-phosphatase, an enzyme of the inositol phosphate signaling pathway, catalyzes the hydrolysis of the 4-position phosphate of inositol 3,4-bisphosphate, inositol 1,3,4-trisphosphate, and phosphatidylinositol 3,4-bisphosphate. The amino acid sequences of tryptic and CNBr peptides of the enzyme isolated from rat brain were determined. Degenerate oligonucleotide primers based on this sequence were used to amplify a 74-base pair polymerase chain reaction product. This product was used to isolate a 5607-base pair composite cDNA, which had an open reading frame encoding a protein with 939 amino acids with a predicted molecular mass of 105,588 Da. The rat brain polymerase chain reaction product was used as a probe to isolate a human brain cDNA that predicts a protein with 938 amino acids and a molecular mass of 105,710 Da. Remarkably, the human and rat proteins were 97% identical. Recombinant rat protein expressed in Escherichia coli catalyzed the hydrolysis of all three substrates of the 4-phosphatase. Northern blot hybridization indicates that the 4-phosphatase is widely expressed in rat tissues with the highest levels of expression occurring in brain, heart, and skeletal muscle. Polyclonal antiserum directed against the carboxyl terminus of the 4-phosphatase immunoprecipitated > 95% of the 4-phosphatase activity in crude homogenates of rat brain, heart, skeletal muscle, and spleen, suggesting that this enzyme accounts for the 4-phosphate activity present in rat tissues. This antiserum also immunoprecipitated the 4-phosphatase from human platelet sonicates.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

Characterization of a protein which binds phosphatidylinositol 3,4,5-trisphosphate and 4,5-bisphosphate

Protein(s) which bind polyphosphatidylinositol phosphates (PI 3,4,5-P3 and PI 4,5-P2) were identified in the wheat-germ agglutinin bound fraction of cells and tissues. The binding of this protein(s) to the phospholipid could be demonstrated in two ways, either by a shift in the migration of the lipid by size exclusion column chromatography or directly by binding to the protein after capture on wheat-germ agglutinin-coupled beads. Of the rat tissues tested (muscle, spleen, brain, heart, kidney and liver), the activity was highest in liver. The protein(s) was purified more than 5000-fold by sequential chromatography on columns of wheat-germ agglutinin, phosphocellulose, Blue-Sepharose, Mono Q and Superose 6. The peak of activity appeared to have a molecular weight on this latter column of approx. 240,000. The protein(s) bound PI 3,4,5-P3, PI 3,4-P2, and PI 3-P in the ratio of 4:2:1. The binding of 3-phosphorylated PI phosphates to the protein(s) was not significantly inhibited by 36 micrograms/ml of either phosphatidylinositol or phosphatidylcholine, but was inhibited 10% and 65% by 36 micrograms/ml of PI 4-P and PI 4,5-P2, respectively. Since these results suggested that the binding protein(s) could also bind PI 4,5-P2, binding of this lipid was directly tested and found to be comparable to that of PI 3,4,5-P3. These results suggest that this protein(s) could be involved in the signaling mechanism elicited by these polyphosphoinositides.

Lysophosphoinositide-specific phospholipase C in rat brain synaptic plasma membranes

A membrane preparation from rat brain catalyzed the hydrolysis of [2-3H]glycerol-labeled lysophosphatidylinositol (lysoPI) to yield monoacylglycerol (MG) and inositolphosphates. This phospholipase C activity had an optimal pH of 8.2. The membrane preparation did not require the addition of Ca2+ for its maximum activity, but the activity was inhibited by addition of 0.1 mM EDTA to the assay mixture and was restored by simultaneous addition of 0.2 mM Ca2+. The activity was found to be localized in synaptic plasma membranes prepared by Ficoll and Percoll density gradients. The phospholipase C was highly specific for lysoPI; diacylglycerol formation from phosphatidylinositol, and MG formation from lysophosphatidylcholine, lysophosphatidylethanolamine, and lysophosphatidylserine were below 5% of that observed with lysoPI under the conditions used. We concluded that there is a pathway for phosphatidylinositol metabolism in brain synaptic membranes which is different from the well-characterized phosphoinositide-specific phospholipase C pathway.

Phosphoinositide metabolism in airway smooth muscle

Agonist-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, which generates inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol, is thought to be one of the major mechanisms underlying pharmacomechanical coupling in airway smooth muscle. This article is a review of the currently available information on phosphoinositide and inositol 1,4,5-trisphosphate metabolism in this tissue and includes data on inositol 1,4,5-trisphosphate-induced Ca2+ release and the receptor mediating this effect. The final section outlines the potential mechanisms underlying physiological regulation of phosphoinositide metabolism by other second-messenger pathways operative in this tissue.

Characterization of cytosolic phospholipases C from porcine aortic endothelial cells

Phospholipases C (PLCs) are ubiquitous enzymes which play key roles in the response of cells to extracellular agonists. Endothelial cells are involved in myriad normal and pathophysiologic functions. Although it is known that agonists activate PLCs in endothelial cells, second messengers form, and cellular responses ensue, more knowledge is needed about the specific types of PLCs in these cells. To this end, cytosolic PLCs from porcine aortic endothelial cells were partially purified by ammonium sulfate fractionation and column chromatography on DEAE-Sepharose CL-6B and heparin-agarose. Three PLC isozymes immunologically similar to bovine brain PLC-beta, PLC-gamma, and PLC-delta were identified. The relative levels of PLC activities in the cytosol were: PLC-beta, 50%; PLC-gamma, 44%; PLC-delta, 6%. The level of PLC-beta activity in porcine endothelial cells appeared higher than the levels reported for several established cell lines, suggesting that this enzyme may play a specific role in endothelial cell function. Elution profiles of PLC activity with phosphatidylinositol 4,5-bisphosphate (Ptdlns(4,5)P2) as substrate were similar to those with phosphatidylinositol (Ptdlns) as substrate, indicating that cytosolic PLCs hydrolyze both Ptdlns and Ptdlns(4,5)P2 and no Ptdlns(4,5)P2-specific PLC was present in the cytosol. The catalytic properties of the partially purified PLC isozymes from porcine endothelial cells were similar to their counterparts from bovine brain. These include the dependence of hydrolysis of Ptdlns on Ca2+, the optimal Ca2+ concentrations for the hydrolysis of Ptdlns and Ptdlns(4,5)P2, the pH optima, and the stimulatory effects of deoxycholate.

Phosphoinositide 3-phosphatase segregates from phosphatidylinositol 3-kinase in EGF-stimulated A431 cells and fails to in vitro hydrolyse phosphatidylinositol(3,4,5)trisphosphate

Beside 4- and 5-phosphatases playing a role in the interconversion between the D-3 phosphorylated polyphosphoinositides, the only enzyme described so far to be responsible for a phosphomonesterasic activity on the D-3 position of inositol lipids is a specific 3-phosphatase that hydrolyzes PtdIns(3)P in NIH 3T3 cells. We report here the presence of a potent 3-phosphatase activity in different cell types. This activity is detected both in cytosol and membranes of A431 cells and is inhibited by VO4(-3) and Zn2+. Interestingly, the cytosolic activity from A431 cells selectively hydrolyzes in vitro PtdIns(3)P and PtdIns(3,4)P2, whereas PtdIns(3,4,5)P3 remains a very poor substrate under the same conditions. Finally, assays of phosphatidylinositol 3-kinase and 3-phosphatase activities in the pool of phosphotyrosine-containing proteins isolated from EGF-stimulated A431 cells suggest a compartmentation of these two antagonistic activities during cell activation.

Cellular pharmacology of D-3-azido-3-deoxy-myo-inositol, an inhibitor of phosphatidylinositol signaling having antiproliferative activity

D-3-Azido-3-deoxy-myo-inositol (3AMI) is an inhibitor of the growth of v-sis-transformed NIH 3T3 cells but not of wild-type NIH 3T3 cells, whose effects may be mediated through the phosphatidylinositol-3'-kinase pathway. We studied some properties of the cellular pharmacology of 3AMI using high-specific-activity [3H]-3AMI. The uptake of [3H]-3AMI by wild-type NIH 3T3 and v-sis NIH 3T3 cells was similar. [3H]-3AMI was a substrate for phosphatidylinositol synthetase, with the maximal velocity (Vmax) being 1.0 nmol min-1 mg-1 and the Michaelis constant (Km) being 23 mM. Corresponding values obtained for [3H]-myo-inositol as a substrate were 5.5 nmol min-1 mg-1 and 3.2 mM. [3H]-3AMI was incorporated into the cellular inositol lipids of v-sis NIH 3T3 cells to a similar extent as that observed for [3H]-myo-inositol but was not incorporated into the inositol lipids of wild-type NIH 3T3 cells. The [3H]-3AMI incorporated by the v-sis NIH 3T3 cells was present in the phosphatidylinositol and phosphatidylinositol phosphate fractions but not in bisphosphorylated phosphatidylinositol. myo-Inositol antagonized the growth-inhibitory effects of 3AMI. The v-sis NIH 3T3 cells were found to be more sensitive than the wild-type NIH 3T3 cells to growth inhibition (without 3AMI) caused by the removal of myo-inositol from the medium. The results of the study suggest that 3AMI is an antimetabolite of myo-inositol. The relative sensitivity of v-sis NIH 3T3 and some other cells to 3AMI may be a reflection of increased myo-inositol requirements for the growth of these cells as compared with wild-type NIH 3T3 cells.

Phospholipase C gamma activation, phosphotidylinositol hydrolysis, and calcium mobilization are not required for FGF receptor-mediated chemotaxis

Basic fibroblast growth factor (FGF) is a potent angiogenic factor that stimulates several cell types to migrate along a chemotactic gradient. Most chemoattractant receptors appear to share a common mechanism that involves activation of phospholipase C (PLC), hydrolysis of phosphotidylinositol, and mobilization of intracellular calcium. We transfected two different cell lines with either human FGF receptor-1 cDNA or chimeric FGF receptor cDNA. Ligand stimulation induced chemotaxis, activation of PLC gamma, phosphotidylinositol hydrolysis, and calcium mobilization in both wild-type receptor cell lines. No such response was elicited in control cells. Mutation of the two fibroblast growth factor receptors at residue 766, replacing tyrosine with phenylalanine, made the receptors incapable of associating with and activating PLC gamma following ligand stimulation. These mutant receptors also failed to mediate phosphotidylinositol hydrolysis and calcium mobilization. However, cells transfected with the mutant fibroblast growth factor receptors were as chemotactically responsive to the appropriate ligand as were cells transfected with the wild-type receptors. These findings demonstrate that the ability of the fibroblast growth factor receptor to promote chemotaxis is not dependent on increased activation of PLC gamma, increased hydrolysis of phosphotidylinositol, or increased global mobilization of calcium.

Haematopoietic colony stimulating factors CSF-1 and GM-CSF increase phosphatidylinositol 3-kinase activity in murine bone marrow-derived macrophages

The activity of phosphatidylinositol (PI) 3-kinase was examined in murine bone marrow-derived macrophages (BMM) stimulated with the haematopoietic growth factors colony stimulating factor-1 (CSF-1) and granulocyte/macrophage-CSF (GM-CSF). PI 3-kinase was immunoprecipitated from cell lysates using anti-phosphotyrosine antibody or an antibody directed against the 85K subunit of PI 3-kinase, and the activity assayed by the phosphorylation of PI in the presence of [gamma 32P]-ATP. The results demonstrate that CSF-1 increases the activity of PI 3-kinase, as compared to the non-stimulated control, in murine macrophages. Maximum activity was seen after 10 min of stimulation with CSF-1 at 3000-5000 U/ml. The dose-response of CSF-1 is consistent with other biochemical effects of CSF-1 seen in the BMM. GM-CSF also stimulated PI 3-kinase activity although to a lesser extent than CSF-1, correlating well with their degree of mitogenic activity on the BMM. Non-mitogenic macrophage activating agents, such as the phorbol myristate acetate, lipopolysaccharide, concanavalin A and formyl-methionyl-leucyl-phenylalanine, did not significantly increase the PI 3-kinase activity. Furthermore, CSF-1 failed to stimulate PI 3-kinase activity in resident peritoneal macrophages, a population of macrophages with poor proliferative capacity. These results suggest that the PI 3-kinase activity may be involved in the haemopoietic growth factor signalling pathways regulating macrophage growth.

Ligation of CD28 receptor by B7 induces formation of D-3 phosphoinositides in T lymphocytes independently of T cell receptor/CD3 activation

The co-stimulatory role of B7/CD28 interactions is important in promoting T cell activation. Very little is known about the intracellular events that follow CD28 engagement although recent evidence has implicated coupling of CD28 to a protein tyrosine kinase signal transduction pathway. In this study we have investigated the putative role of D-3 phosphoinositides as mediators of CD28 receptor signaling, since phosphoinositide (PI) 3-kinase, the enzyme responsible for D-3 phosphoinositide formation, is a known substrate for protein tyrosine kinases associated with certain T cell surface receptors such as CD4 and interleukin-2 receptor. The lipid products of PI 3-kinase activity have been suggested to play a role in mitogenic signaling and growth regulation in other cells. Chinese hamster ovary cells (CHO) previously transfected with B7 cDNA, induced time-dependent elevation above basal levels of phosphatidylinositol(3,4)-bisphosphate (PtdIns(3,4)P2) and PtdIns(3,4,5)P3, while parental CHO cells that did not express B7 had no effect on these lipids. Moreover, the elevation of these same lipids by CD3 ligation was potentiated in an additive manner by CHO-B7+ but not by CHO-B7- cells. CHO-B7+ and CHO-B7- cells did not activate phospholipase C as evidenced by their inability to modulate basal or CD3-induced changes in the levels of phosphatidic acid or D-4 and D-5 phosphoinositides. These data imply that PI 3-kinase but not phospholipase C, may be an important signal transduction molecule with respect to CD28-mediated co-stimulation and T cell activation following ligation by B7.

Intracellular signalling mediated by protein-tyrosine kinases: networking through phospholipid metabolism

In recent years, it has become apparent that receptor-mediated intracellular signals are not linear cascades beginning at the plasma membrane and terminating with the production of a needed metabolite or the induction of gene expression. Instead, complex networks of interactive intracellular signals are activated in response to extracellular stimuli. Many responses to extracellular stimuli are mediated by protein-tyrosine kinases (PTKs). Activating PTKs leads to the recruitment of a variety of intracellular signalling molecules that execute a complex set of instructions. The response to PTK activity is dependent upon which PTK is activated and the cellular context in which the PTK exists. Several signalling molecules recruited by PTKs are involved in the metabolism of phospholipids. In this Mini Review, intracellular signalling networks activated by PTKs are discussed with an emphasis on the potential for generating highly specific and sophisticated responses to PTK activity through phospholipid metabolism.

Insulin stimulates phosphatidylinositol 3-kinase activity in rat neuronal primary cultures

We investigated the effect of insulin on phosphatidylinositol (PtdIns) 3-kinase (PtdIns 3-kinase) activity in neuronal cultures to determine if this enzyme is involved with the neurotrophic actions of insulin. Insulin caused a concentration-dependent increase in PtdIns 3-kinase activity in anti-phosphotyrosine immunoprecipitates. The kinase activity was able to phosphorylate PtdIns, PtdIns 4-phosphate, and PtdIns 4,5-bisphosphate. In intact neurons, a 10-min 1 mM insulin treatment in the presence of [32P]orthophosphate increased the levels of both 3-[32P]PtdIns phosphate and 3,4-[32P]PtdIns bisphosphate by 55 and 193%, respectively. This increase was associated with an increase in neurite outgrowth mediated by insulin. Our results indicate that insulin treatment of neuronal cells in primary culture increases PtdIns 3-kinase activity and the formation of the unique D-3-phosphorylated phosphoinositides, suggesting that growth factor-mediated neuronal growth may include the formation of novel phosphoinositide 3-phosphate phospholipids.

Rapid phosphorylation of phospholipase C gamma 1 by brain-derived neurotrophic factor and neurotrophin-3 in cultures of embryonic rat cortical neurons

Phospholipase C gamma 1 (PLC-gamma 1) is involved at an early step in signal transduction of many hormones and growth factors and catalyzes the hydrolysis of phosphatidylinositol (PI) 4,5-bisphosphate to diacylglycerol and inositol trisphosphate, two potent intracellular second messenger molecules. The transformation of PC12 cells into neuron-like cells induced by nerve growth factor is preceded by a rapid stimulation of PLC-gamma 1 phosphorylation and PI hydrolysis. The present study analyzed the effects of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) on phosphorylation of PLC-gamma 1 in primary cultures of embryonic rat brain cells. BDNF and NT-3 stimulated the phosphorylation of PLC-gamma 1, followed by hydrolysis of PI. The stimulation of PLC-gamma 1 phosphorylation occurred within 20 s after addition of BDNF or NT-3 and lasted up to 30 min, with a peak after 4 min. ED50 values were similar for BDNF and NT-3, with approximately 25 ng/ml. Phosphorylation of PLC-gamma 1 by BDNF and NT-3 was found in cultures from all major brain areas. K-252b, a compound known to inhibit selectively neutrophin actions by interfering with the phosphorylation of trk-type neutrophin receptors, prevented the BDNF- and NT-3-stimulated phosphorylation of PLC-gamma 1. Receptors of the trk type were coprecipitated with anti-PLC-gamma 1 antibodies. The presence of trkB mRNA in the cultures was substantiated by northern blot analysis. The action of BDNF and NT-3 seems to be neuron specific because no phosphorylation of PLC-gamma 1 was observed in cultures of nonneuronal brain cells. The results provide evidence that developing neurons of the cerebral cortex and other brain areas are responsive to BDNF and NT-3, and they indicate that the transduction mechanism of BDNF and NT-3 in the brain involves rapid phosphorylation of PLC-gamma 1 followed by PI hydrolysis.

Target of rapamycin in yeast, TOR2, is an essential phosphatidylinositol kinase homolog required for G1 progression

The yeast TOR2 gene encodes an essential 282 kd phosphatidylinositol (PI) 3-kinase homolog. TOR2 is related to the catalytic subunit of bovine PI 3-kinase and to yeast VPS34, a vacuolar sorting protein also shown to have PI 3-kinase activity. The immunosuppressant rapamycin most likely acts by inhibiting PI kinase activity because TOR2 mutations confer resistance to rapamycin and because a TOR1 TOR2 double disruption (TOR1 is a nonessential TOR2 homolog) confers G1 arrest, as does rapamycin. Our results further suggest that 3-phosphorylated phosphoinositides, whose physiological significance has not been determined, are an important signal in cell cycle activation. In yeast, this signal may act in a signal transduction pathway similar to the interleukin-2 signal transduction pathway in T cells.

Phospholipase C-gamma 1 and phosphatidylinositol 3 kinase are the downstream mediators of the PDGF receptor's mitogenic signal

Upon ligand-induced tyrosine phosphorylation, the platelet-derived growth factor (PDGF) receptor (PDGFR) beta subunit associates with PLC-gamma 1, RasGAP, P13K, and a 64 kd protein. To determine the relative role of each of these associated proteins in PDGFR signaling, we constructed a PDGFR mutant (F5) unable to bind any of them and a panel of "add-back" mutants that could bind only one of the receptor-associated proteins. F5 PDGFR failed to activate PLC-gamma 1, P13K, or Ras and was unable to trigger DNA synthesis. Permitting association of F5 PDGFR with either PLC-gamma 1 or P13K restored Ras activation and a mitogenic response. Surprisingly, even though binding of the 64 kd protein almost fully restored Ras activation, it did not rescue the receptor's ability to trigger DNA synthesis. Thus Ras activation is insufficient to trigger PDGF-dependent DNA synthesis, and PLC-gamma 1 and P13K are independent downstream mediators of PDGF's mitogenic signal.

Phosphatidylinositol 3-kinase encoded by yeast VPS34 gene essential for protein sorting

The VPS34 gene product (Vps34p) is required for protein sorting to the lysosome-like vacuole of the yeast Saccharomyces cerevisiae. Vps34p shares significant sequence similarity with the catalytic subunit of bovine phosphatidylinositol (PI) 3-kinase [the 110-kilodalton (p110) subunit of PI 3-kinase], which is known to interact with activated cell surface receptor tyrosine kinases. Yeast strains deleted for the VPS34 gene or carrying vps34 point mutations lacked detectable PI 3-kinase activity and exhibited severe defects in vacuolar protein sorting. Overexpression of Vps34p resulted in an increase in PI 3-kinase activity, and this activity was specifically precipitated with antisera to Vps34p. VPS34 encodes a yeast PI 3-kinase, and this enzyme appears to regulate intracellular protein trafficking decisions.

Growth factor second messenger systems: oncogenes and the heterotrimeric GTP-binding protein connection

We feel that there is now compelling evidence that the GTP-binding proteins play more than just a coordinating role in the actions of both tyrosine kinase and nontyrosine kinase receptor signal transduction. These similarities appear to represent just a small component of the convergence in the signaling pathways for structurally dissimilar receptor subsets. Future years will see further understanding of the intricacies of these G-protein-proto-oncogene interactions, and the extension into the potential role in growth factor action played by the expanding number of known members of this G-protein family.

Biochemistry of B lymphocyte activation

The activation of B lymphocytes from resting cells proceeds from the events of early activation to clonal proliferation to final differentiation into either an antibody-secreting plasma cell or a memory B cell. This is a complex activation process marked by several alternative pathways, depending on the nature of the initial antigenic stimulus. Over the past 5-10 years, there has been an explosion of studies examining the biochemical nature of various steps in these pathways. Some of that progress is reviewed here. In particular, we have described in detail what is known about the structure and function of the AgR, as this molecule plays a pivotal role in B cell responses of various types. We have also reviewed recent progress in understanding the mechanism of action of contact-dependent T cell help and of the cytokine receptors, particularly the receptors for IL-2, IL-4, and IL-6. Clearly, all of these areas represent active areas of investigation and great progress can be anticipated in the next few years.

Phosphoinositide-specific phospholipase C-delta 1: effect of monolayer surface pressure and electrostatic surface potentials on activity

We added phospholipase C-delta 1 (PLC-delta) to the aqueous subphase beneath monolayers formed from mixtures of phosphatidylinositol 4,5-bisphosphate (2% PIP2), phosphatidylserine (33% PS), and phosphatidylcholine (65% PC) and then measured the initial rate of hydrolysis of PIP2 after addition of 10 microM free calcium. Increasing the surface pressure of the monolayer, pi, from 20 to 40 mN/m decreased the rate of hydrolysis 200-fold. The rate of hydrolysis depends exponentially on the surface pressure: rate alpha exp(-pi Ap/kT) where k is the Boltzmann constant, T is the temperature, and Ap congruent to 1 nm2. Similar results were obtained with different (1 and 100 microM) free [Ca2+] and with different mole fractions of PIP2. The results are consistent with a model in which PLC-delta binds to PIP2 with high affinity (Ka = 10(6) M-1) in the absence of calcium ions [Rebecchi, M.J., Peterson, A., & McLaughlin, S. (1993) Biochemistry (preceding paper in this issue)], and a portion of PLC-delta of area Ap inserts into the monolayer doing work = pi Ap prior to hydrolysis of PIP2. Removing the monovalent acidic lipid PS from the monolayer decreases the activity of PLC-delta 4-fold, this effect of PS on activity is similar to the effect of monovalent acidic lipids on the binding of PLC-delta to PIP2 in bilayer vesicles.