Activation of protein kinase C family members by the novel polyphosphoinositides PtdIns-3,4-P2 and PtdIns-3,4,5-P3

Phorbol ester inhibits the phosphorylation of the retinoblastoma protein without suppressing cyclin D-associated kinase in vascular smooth muscle cells

To elucidate the role of protein kinase C in vascular smooth muscle cell proliferation, we examined the effects of phorbol 12-myristate 13-acetate (PMA) on G1 events in human arterial cells. About 15 h after G0 cells were stimulated with fetal bovine serum and basic fibroblast growth factor, [3H]thymidine incorporation started. PMA (10 nM) inhibited the incorporation over 90% when added earlier than 3 h after stimulation, but had no effect when added 12 h or later. PMA inhibited the phosphorylation of the retinoblastoma protein (pRb), which normally began at about 9 h. PMA did not inhibit the gene expression of Cdk2, Cdk3, Cdk4, Cdk5, and cyclins G, C, and D, all of which began at 0-3 h. However, PMA reduced the expression of cyclins E and A, which usually began at 3-9 h and about 15 h, respectively. PMA inhibited the histone H1 kinase activity of Cdk2, which increased from about 9 h, whereas PMA did not inhibit the pRb kinase activities of cyclin D-associated kinase(s) and Cdk4, detectable from 0-3 h. These results suggested that the PMA-induced inhibition of pRb phosphorylation is not mediated by suppressing cyclin D-associated kinase(s) including Cdk4, but involves the suppression of Cdk2 activity that results from the reduced expression of cyclins E and A.

Phosphoinositide kinases

Recently, a number of cDNA clones with homology to the catalytic subunit of phosphoinositide 3-kinase have been identified, and the sequence of the first cDNA clone encoding a phosphatidylinositol 4-phosphate 5-kinase has been published. Use of both dominant-negative mutants of phosphoinositide 3-kinase and the inhibitors wortmannin and LY294002 has identified a number of processes in which phosphoinositide 3-kinase participates, including cell motility, the Ras pathway, vesicle trafficking and secretion, and apoptosis. Several possible biochemical targets of phosphoinositides have been found.

Association of phosphatidylinositol 3-kinase, via the SH2 domains of p85, with focal adhesion kinase in polyoma middle t-transformed fibroblasts

Focal adhesion kinase (FAK), a non-receptor protein tyrosine kinase, becomes activated and phosphorylated on tyrosine in cells transformed with v-src. By cytoimmunofluorescence a sub-fraction of the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase) localized in focal adhesion plaques. We examined the possibility that FAK associates with PI 3-kinase. In fibroblasts transformed with polyoma middle t, PI 3-kinase activity co-immunoprecipitated with pp125FAK using two different antibodies against this protein. PP125FAK from middle t-transformed cells associated with a glutathione-S-transferase fusion protein containing the 85-kDa subunit of phosphatidylinositol 3-kinase. Both of the SH2 domains and the SH3 domain of p85 also formed complexes with pp125FAK in vitro. Phosphopeptides that bind to the SH2 domains completely blocked the binding of full-length p85 to pp125FAK, while a peptide that binds to the SH3 domain was ineffective, indicating that the association between p85 and pp125FAK is mediated by the SH2 domains of p85.

In vitro activation and substrates of recombinant, baculovirus expressed human protein kinase C mu

To study enzymatic activity and activation conditions of the recently identified novel protein kinase C mu (PKC mu) subtype, epitope tagged PKC mu was propagated in the baculovirus expression system and was purified to homogeneity. PKC mu displays high affinity phorbol ester binding (Kd=7 nM) resulting in enhanced phosphatidylserine-dependent kinase activity. From various lipid second messengers known to activate PKCs only diacylglycerol and PtdIns-4,5-P2, were found to promote PKC mu kinase activity. Two peptides derived from the glycogen synthase, GS-peptide and syntide 2, were found to be phosphorylated efficiently in vitro. MARCKS (myristoylated alanine-rich C-kinase substrate) served as an in vitro substrate for PKC mu too. However, in contrast to other PKCs, a peptide derived from the MARCKS phosphorylation domain is phosphorylated only at serine 156, and not at serines 152 and 163, implicating a differential regulation by PKC mu.

Phosphoinositide 3-kinases and membrane traffic

Phosphoinositide 3-kinases (PI 3-kinases) and their 3-phosphoinositide products were identified initially as components of intracellular signalling pathways emanating from cell surface receptors. A new role for 3-phosphoinositides in the constitutive movement o f proteins from one intracellular compartment to another was proposed with the discovery of homology between the product of a yeast gene important for vacuolar sorting, Vps34p, and a mammalian PI 3-kinase. Recent studies have implicated PI 3-kinase as an essential component in membrane traffic at specific steps o f the trans-Golgi-network-endosomal pre-lysosomal system. Evidence largely emerging from the insulin-stimulated glucose transport system suggests that PI 3-kinase may also mediate the effects o f growth factors on membrane traffic events. These studies suggest a possible link between growth-factor-stimulated and constitutive membrane traffic in the endosomal system.

Phosphatidylinositol 3' kinase: one of the effectors of Ras

Ras proteins are proto-oncogene products that are critical components of signalling pathways leading from cell surface receptors to control of cellular proliferation, morphology and differentiation. the ability of Ras to activate the MAP kinase pathway through interaction with the serine/threonine kinase Raf is now well established. However, recent work has shown that Ras can also interact directly with the catalytic subunit of phosphatidylinositol 3' kinase and is involved in control of the lipid kinase in intact cells. A model is presented in which both tyrosine phosphoprotein interaction with the regulatory p85 subunit and Ras. GTP interaction with the catalytic p110 subunit is required to achieve optimal activation of phosphatidylinositol 3'kinase in response to extracellular stimuli. The ability of Ras to regulate phosphatidylinositol 3' kinase may be important both in Ras control of cellular morphology through the actin cytoskeleton and also in Ras control of DNA synthesis.

Inhibition of phosphatidylinositol 3-kinase blocks T cell antigen receptor/CD3-induced activation of the mitogen-activated kinase Erk2

The production of 3-phosphorylated inositol phospholipids is implicated in regulation of cell growth and transformation. To explore the role of these lipids in T cell antigen receptor (TCR)/CD3-induced signaling, we have examined the effects of a specific phosphatidylinositol 3-kinase (PtdIns3K) inhibitor, wortmannin, and overexpression of two PtdIns3K constructs on the activation of down-stream effectors in anti-CD3 treated T cells. We report that treatment of cells with wortmannin blocked anti-CD3-induced activation of the mitogen-activation kinase Erk2 while not affecting phorbol-ester-induced Erk2 activation. An inactive analog of wortmannin, WM12, did not affect TCR/CD3-induced Erk2 activation, and wortmannin had no effect on the activity of Erk2 when added directly to the in vitro assays. Expression of a disruptive PtdIns3K construct also reduced Erk2 activation, while a construct that stimulates PtdIns3K enhanced the activation of Erk2. Receptor-induced activation of other Ser/Thr kinases, such as c-Raf, B-Raf, Mek1, Mek2, Mekk, was not affected by wortmannin. Our results suggest that the production of 3-phosphorylated inositol phospholipids is involved in the activation of Erk2, but does not regulate the enzymes that are thought to be upstream of Erk2.

Insulin regulation of phosphoenolpyruvate carboxykinase gene expression does not require activation of the Ras/mitogen-activated protein kinase signaling pathway

Expression of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting step in hepatic gluconeogenesis, is primarily regulated at the level of gene transcription. Insulin and phorbol esters inhibit basal PEPCK transcription and antagonize the induction of PEPCK gene expression by glucocorticoids and glucagon (or its second messenger cAMP). Insulin activates a signaling cascade involving Ras --> Raf --> p42/p44 mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (ERK 1 and 2). Recent reports suggest that activation of this Ras/MAP kinase pathway is critical for the effects of insulin on mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and Glut-4-mediated glucose transport. We have used three distinct approaches to examine the role of the Ras/MAP kinase pathway in the regulation of PEPCK transcription by insulin in H4IIE-derived liver cells: (i) chemical inhibition of Ras farnesylation, (ii) infection of cells with an adenovirus vector encoding a dominant-negative mutant of Ras, and (iii) use of a chemical inhibitor of MEK. Although each of these methods blocks insulin activation of MAP kinase, none alters insulin antagonism of cAMP- and glucocorticoid-stimulated PEPCK transcription. Although phorbol esters activate MAP kinase and mimic the effects of insulin on PEPCK gene transcription, inhibition of MEK has no effect on phorbol ester inhibition of PEPCK gene transcription. Using the structurally and mechanistically distinct phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors, wortmannin and LY 294002, we provide further evidence supporting a role for PI 3-kinase activation in the regulation of PEPCK gene transcription by insulin. We conclude that neither insulin nor phorbol ester regulation of PEPCK gene transcription requires activation of the Ras/MAP kinase pathway and that insulin signaling to the PEPCK promoter is dependent on PI 3-kinase activation.

The phosphatidylinositol 4-phosphate 5-kinase family

The existence of a PIP5K family of enzymes has been suggested by Western blotting and purification of numerous PIP5Ks from various tissues and cell types. The erythrocyte has at least two PIP5Ks, named PIP5KI and PIP5KII, while the brain appears to have even more isoforms. The cloning of the first PIP5K, the PIP5KII alpha, is just the beginning of the molecular classification of this protein family. The PIP5KII alpha sequence has shown that these enzymes lack obvious homology to protein, sugar and other lipid kinases. The identification of two S. cerevisiae homologues, Mss4p and Fab1p, confirms that this family of kinases is widely distributed in eukaryotes. Not surprisingly, cloning experiments have identified additional isoforms. By cloning additional isoforms, insights into the structure and functions of this family of enzymes will be gained. One reason for a large family of PIP5Ks is that many forms of regulation and cellular functions have been ascribed to PIP5Ks, as summarized in Figure 10. Some of these functional links result from PtdIns[4,5]P2 being required for a given process, but the direct involvement of specific PIP5Ks is not well defined. Which PIP5K isoforms are regulated by a specific mechanism or are involved in a cellular process often is not clear. For example, which PIP5Ks produce PtdIns[4,5]P2 that is hydrolyzed by PLC or phosphorylated by the PI 3-kinase is not known. A few exceptions are PIP5KII not being able to phosphorylate PtdIns[4,5]P2 in native membranes, and PIP5KIs being stimulated by PtdA, required for secretion, and possibly regulated by G proteins of the Rho subfamily. The multiplicity of regulation and functions of each PIP5K isoform remains to be elucidated. Another factor governing the number of isoforms may be presence of multiple pools of polyphosphoinositides and the localizing of PIP5K function within cells. The polyphosphoinositides appear to be compartmentalized within cells and each pool appears to be sensitive to specific signals. These polyphosphoinositide pools may include those in the plasma membrane that are used by PLC, nuclear pools that appear to turn over separately from cytoplasmic pools and a small pool at sites of vesicle fusion with the plasma membrane. Each pool may be controlled by a specific PIP5K isoform. This would explain the diversity of PIP5K cellular roles. Another possibility is that the PIP5Ks are localized to certain areas of the cell by being part of a protein or proteolipid complex. Furthermore, the presence of PITP or PLC in the complex would potentially impart specificity and speed on the use of PtdIns[4]P and PtdIns[4,5]P2 because these lipids could be channeled quickly from one enzyme to the next. The concept of localized complexes containing particular PIP5K isoforms that control the composition of different polyphosphoinositide pools will likely be important as the family of PIP5K isoforms grows.

Protein kinase C-zeta mediates NF-kappa B activation in human immunodeficiency virus-infected monocytes

The molecular mechanisms regulating human immunodeficiency virus (HIV) persistence in a major cell reservoir such as the macrophage remain unknown. NF-kappa B is a transcription factor involved in the regulation of the HIV long terminal repeat and is selectively activated following HIV infection of human macrophages. Although little information as to what signal transduction pathways mediate NF-kappa B activation in monocytes-macrophages is available, our previous work indicated that classical protein kinase C (PKC) isoenzymes were not involved in the HIV-mediated NF-kappa B activation. In this study, we have focused on atypical PKC isoenzymes. PKC-zeta belongs to this family and is known to be an important step in NF-kappa B activation in other cell systems. Immunoblotting experiments with U937 cells demonstrate that PKC-zeta is present in these cells, and its expression can be downmodulated by antisense oligonucleotides (AO). The HIV-mediated NF-kappa B activation is selectively reduced by AO to PKC-zeta. In addition, cotransfection of a negative dominant molecule of PKC-zeta (PKC-zeta mut) with NF-kappa B-dependent reporter genes selectively inhibits the HIV- but not phorbol myristate acetate- or lipopolysaccharide-mediated activation of NF-kappa B. That PKC-zeta is specific in regulating NF-kappa B is concluded from the inability of PKC-zeta(mut) to interfere with the basal or phorbol myristate acetate-inducible CREB- or AP1-dependent transcriptional activity. Lastly, we demonstrate a selective inhibition of p24 production by HIV-infected human macrophages when treated with AO to PKC-zeta. Altogether, these results suggest that atypical PKC isoenzymes, including PKC-zeta, participate in the signal transduction pathways by which HIV infection results in the activation of NF-kappa B in human monocytic cells and macrophages.

Cell cycle arrest in the G2 phase induced by phorbol ester and diacylglycerol in vascular endothelial cells

The role of protein kinase C (PKC) in vascular endothelial cell proliferation was investigated using human umbilical vein endothelial cells released from the G1/S border. Phorbol 12-myristate 13-acetate (PMA) caused G2 arrest because 1) when added to G2 cells, PMA inhibited subsequent cell division; 2) these growth-arrested cells did not show morphological features of mitotic cells; and 3) PMA did not interrupt mitosis in cells released from nocodazole-induced M phase arrest. 1-Oleoyl-2-acetyl-sn-glycerol (OAG) added repeatedly from G2 also inhibited mitosis. The activation of cdc2 kinase around the G2/M transition was suppressed by PMA and OAG. Although cdc2 was expressed in the presence of PMA, dephosphorylation of its tyrosine residue was inhibited by PMA. In parallel, the expression of cdc25B was suppressed by PMA. The total and the cdc2-associated amount of cyclin B were both reduced by PMA. These data suggested that the PKC pathway negatively regulates the G2/M transition and that the inhibition of cdc2 kinase by the reduction in the levels of cdc25B and cyclin B may contribute to this effect.

Protein kinase C mediation of Ca(2+)-independent contractions of vascular smooth muscle

Tumour-promoting phorbol esters induce slow, sustained contractions of vascular smooth muscle, suggesting that protein kinase C (PKC) may play a role in the regulation of smooth muscle contractility. In some cases, e.g., ferret aortic smooth muscle, phorbol ester induced contractions occur without a change in [Ca2+]i or myosin phosphorylation. Direct evidence for the involvement of PKC came from the use of single saponin-permeabilized ferret aortic cells. A constitutively active catalytic fragment of PKC induced a slow, sustained contraction similar to that triggered by phenylephrine. Both responses were abolished by a peptide inhibitor of PKC. Contractions of similar magnitude occurred even when the [Ca2+] was reduced to close to zero, implicating a Ca(2+)-independent isoenzyme of PKC. Of the two Ca(2+)-independent PKC isoenzymes, epsilon and zeta, identified in ferret aorta, PKC epsilon is more likely to mediate the contractile response because (i) PKC epsilon, but not PKC zeta, is responsive to phorbol esters; (ii) upon stimulation with phenylephrine, PKC epsilon translocates from the sarcoplasm to the sarcolemma, whereas PKC zeta, translocates from a perinuclear localization to the interior of the nucleus; and (iii) when added to permeabilized single cells of the ferret aorta at pCa 9, PKC epsilon, but not PKC zeta, induced a contractile response similar to that induced by phenylephrine. A possible substrate of PKC epsilon is the smooth muscle specific, thin filament associated protein, calponin. Calponin is phosphorylated in intact smooth muscle strips in response to carbachol, endothelin-1, phorbol esters, or okadaic acid. Phosphorylation of calponin in vitro by PKC (a mixture of alpha, beta, and gamma isoenzymes) dramatically reduces its affinity for F-actin and alleviates its inhibition of the cross-bridge cycling rate. Calponin is phosphorylated in vitro by PKC epsilon but is a very poor substrate of PKC zeta. A signal transduction pathway is proposed to explain Ca(2+)-independent contraction of ferret aorta whereby extracellular signals trigger diacylglycerol production without a Ca2+ transient. The consequent activation of PKC epsilon would result in calponin phosphorylation, its release from the thin filaments, and alleviation of inhibition of cross-bridge cycling. Slow, sustained contraction then results from a slow rate of cross-bridge cycling because of the basal level of myosin light chain phosphorylation (approximately 0.1 mol Pi/mol light chain). We also suggest that signal transduction through PKC epsilon is a component of contractile responses triggered by agonists that activate phosphoinositide turnover; this may explain why smooth muscles often develop more force in response, e.g., to alpha 1-adrenergic agonists than to K+.

Wortmannin inhibits the activation of MAP kinase following vasopressin V1 receptor stimulation

Treatment of rat 3Y1 fibroblasts with vasopressin (AVP) results in a transient activation of MAP kinase as potent as with EGF and serum. An antagonist of vasopressin receptor V1, but not an antagonist of V2, inhibited the AVP-induced activation of MAP kinases, indicating that AVP activates MAP kinases through V1 receptor. Prolonged TPA treatment of cells resulted in partial MAP kinase activation, indicating the presence of PKC-independent pathway. The pathway was inhibited by wortmannin, an inhibitor of PI3-kinase. The results suggest that wortmannin-sensitive molecules such as PI3-kinase, are involved in the V1 receptor-mediated activation of the MAP kinase pathway independent of TPA-sensitive PKC.

Overexpression of epsilon-protein kinase C enhances nerve growth factor-induced phosphorylation of mitogen-activated protein kinases and neurite outgrowth

Protein kinase C (PKC) activation enhances neurite outgrowth in several cell lines and primary neurons. The PKC isozymes that mediate this response are unknown. One clue to their identity has come from studies using PC12 cells treated with ethanol. In these cells, ethanol increases levels of delta-PKC and epsilon-PKC and markedly enhances nerve growth factor (NGF)-induced neurite outgrowth and activation of mitogen-activated protein (MAP) kinases by a PKC-dependent mechanism. Since these findings suggest that delta-PKC or epsilon-PKC can promote neural differentiation, we studied neurite outgrowth in stably transfected PC12 cell lines that overexpress these isozymes. Overexpression of epsilon-PKC markedly increased NGF-induced neurite outgrowth. This effect was blocked by down-regulating PKC or by treating cells with the PKC inhibitor GF 109203X. In addition, overexpression of epsilon-PKC enhanced NGF-induced phosphorylation of MAP kinases. In contrast, overexpression of delta-PKC did not alter responses to NGF. These results demonstrate that epsilon-PKC promotes NGF-induced neurite outgrowth by enhancing NGF signal transduction. These findings suggest a role for epsilon-PKC in neural differentiation and plasticity.

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.

The 70 kDa S6 kinase: regulation of a kinase with multiple roles in mitogenic signalling

The activation of the 70kDa S6 kinase, pp70S6k, is a well documented mitogenic response, yet until recently little was known of how pp70S6k is activated, or of the identities of its crucial targets. The past year has revealed the complexity of pp70S6k regulation, with the overriding theme being that enzymes which have proven or putative roles in phospholipid metabolism mediate its activation. Studies also indicate that pp70S6k may regulate many more pathways than previously recognized.

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.

Arachidonoyl-diacylglycerol kinase. Specific in vitro inhibition by polyphosphoinositides suggests a mechanism for regulation of phosphatidylinositol biosynthesis

We previously described the purification of a membrane-bound diacylglycerol kinase highly selective for sn-1-acyl-2-arachidonoyl diacylglycerols (Walsh, J. P., Suen, R., Lemaitre, R. N., and Glomset, J. A. (1994) J. Biol. Chem. 269, 21155-21164). This enzyme appears to be responsible for the rapid clearance of the arachidonate-rich pool of diacylglycerols generated during stimulus-induced phosphoinositide turnover. We have now shown phosphatidylinositol 4,5-bisphosphate to be a potent and specific inhibitor of arachidonoyl-diacylglycerol kinase. Kinetic analyses indicated a Ki for phosphatidylinositol 4,5-bisphosphate of 0.04 mol %. Phosphatidic acid also was an inhibitor with a Ki of 0.7 mol %. Other phospholipids had only small effects at these concentrations. A series of multiply phosphorylated lipid analogs also inhibited the enzyme, indicating that the head group phosphomonoesters are the primary determinants of the polyphosphoinositide effect. However, these compounds were not as potent as phosphatidylinositol 4,5-bisphosphate, indicating some specificity for the polyphosphoinositide additional to its total charge. Five other diacylglycerol kinases were activated to varying degrees by phosphatidylinositol 4,5-bisphosphate and phosphatidic acid, suggesting that inhibition by acidic lipids may be specific for the arachidonoyl-DAG kinase isoform. Given the presumed role of arachidonoyl-diacylglycerol kinase in the phosphoinositide cycle, this inhibition may represent a mechanism for polyphosphoinositides to regulate their own synthesis.

Phosphoinositide 3-kinase binds constitutively to alpha/beta-tubulin and binds to gamma-tubulin in response to insulin

Recently we reported the localization of phosphoinositide 3-kinase (PI 3-kinase) by immunofluorescence to microtubule bundles and the centrosome (Kapeller, R., Chakrabarti, R., Cantley, L., Fay, F., and Corvera, S. (1993) Mol. Cell. Biol. 13, 6052-6063). In complementary experiments we used the recombinant p85 subunit of PI 3-kinase to identify proteins that associate with phosphoinositide 3-kinase and found that phosphoinositide 3-kinase associates with alpha/beta-tubulin. The association occurs in vivo but was not significantly affected by growth factor stimulation. We localized the region of p85 that interacts with alpha/beta-tubulin to the inter-SH2 domain. These results support the immunofluorescence data and show that p85 directly associates with alpha/beta-tubulin. We then determined whether phosphoinositide 3-kinase associates with gamma-tubulin. We found a dramatic growth factor-dependent association of phosphoinositide 3-kinase with gamma-tubulin. Phosphoinositide 3-kinase associates with gamma-tubulin in response to insulin and, to a lesser extent, in response to platelet-derived growth factor. Neither epidermal growth factor nor nerve growth factor treatment of cells results in association of phosphoinositide 3-kinase and gamma-tubulin. Phosphoinositide 3-kinase is also immunoprecipitated with antibodies to pericentrin in response to insulin, indicating that phosphoinositide 3-kinase is recruited to the centrosome. Neither phosphoinositide 3-kinase activity, nor intact microtubules are necessary for the association. Treatment of cells with 0.5 M NaCl dissociates gamma-tubulin from the centrosome and disrupts the association of phosphoinositide 3-kinase with pericentrin, but not gamma-tubulin. Recombinant p85 binds to gamma-tubulin from both insulin stimulated and quiescent cells. These results suggest that the association of phosphoinositide 3-kinase with gamma-tubulin is direct. These data suggest that phosphoinositide 3-kinase may be involved in regulating microtubule responses to insulin and platelet-derived growth factor.

Effect of the activation of phosphatidylinositol 3-kinase by a thiophosphotyrosine peptide on glucose transport in 3T3-L1 adipocytes

Insulin causes the activation of phosphatidylinositol 3-kinase (PI 3-kinase) through complexation of tyrosine-phosphorylated YMXM motifs on insulin receptor substrate 1 with the Src homology 2 domains of PI 3-kinase. Previous studies with inhibitors have indicated that activation of PI 3-kinase is necessary for the stimulation of glucose transport in adipocytes. Here, we investigate whether this activation is sufficient for this effect. Short peptides containing two tyrosine-phosphorylated or thiophosphorylated YMXM motifs potently activated PI 3-kinase in the cytosol from 3T3-L1 adipocytes. Introduction of the phosphatase-resistant thiophosphorylated peptide into 3T3-L1 adipocytes through permeabilization with Staphylococcus aureus alpha-toxin stimulated PI 3-kinase as strongly as insulin. However, under the same conditions the peptide increased glucose transport into the permeabilized cells only 20% as well as insulin. Determination of the distribution of the glucose transporter isotype GLUT4 by confocal immunofluorescence showed that GLUT4 translocation to the plasma membrane can account for the effect of the peptide. These results suggest that one or more other insulin-triggered signaling pathways, besides the PI 3-kinase one, participate in the stimulation of glucose transport.

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.

Activation of PRK1 by phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. A comparison with protein kinase C isotypes

As potential targets for polyphosphoinositides, activation of protein kinase C (PKC) isotypes (beta 1, epsilon, zeta, nu) and a member of the PKC-related kinase (PRK) family, PRK1, has been compared in vitro. PRK1 is shown to be activated by both phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P2) as well as phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3) either as pure sonicated lipids or in detergent mixed micelles. When presented as sonicated lipids, PtdIns-4,5-P2 and PtdIns-3,4,5-P3 were equipotent in activating PRK1, and, furthermore, sonicated phosphatidylinositol (PtdIns) and phosphatidylserine (PtdSer) were equally effective. In detergent mixed micelles, PtdIns-4,5-P2 and PtdIns-3,4,5-P3 also showed a similar potency, but PtdIns and PtdSer were 10-fold less effective in this assay. Similarly, PKC-beta 1, -epsilon, and -nu were all activated by PtdIns-4,5-P2 and PtdIns-3,4,5-P3 in detergent mixed micelles. The activation constants for PtdIns-4,5-P2 and PtdIns-3,4,5-P3 were essentially the same for all the kinases tested, implying no specificity in this in vitro analysis. Consistent with this conclusion, the effects of PtdIns-4,5-P2 and PtdIns-3,4,5-P3 were found to be inhibited at 10 mM Mg2+ and mimicked by high concentrations of inositol hexaphosphate and inositol hexasulfate. The similar responses of these two classes of lipid-activated protein kinase to these phosphoinositides are discussed in light of their potential roles as second messengers.

New frontiers in insulin receptor substrate signaling

Although most tyrosine kinase growth factor receptors directly bind Src homology 2 (SH2) proteins, the insulin receptor, and a select group of other hormone receptors-including an emerging group of cytokine receptors-phosphorylate intracellular insulin receptor substrate (IRS) proteins, which subsequently bind SH2 proteins. There are currently two members of the IRS family (IRS-1 and IRS-2); these IRS proteins contain elements of substantial similarity, but may also play divergent roles in mammalian physiology. The engagement of IRS proteins by other receptors suggests that IRS proteins mediate diverse biological signals.

Vitamin E prevents diabetes-induced abnormal retinal blood flow via the diacylglycerol-protein kinase C pathway

We have characterized effects of d-alpha-tocopherol (vitamin E) on activation of protein kinase C (PKC) and diacylglycerol (DAG) levels in retinal tissues of diabetic rats and correlated its effects to diabetes-induced changes in retinal hemodynamics. Membrane PKC specific activities were increased by 71% in streptozocin-induced diabetic rats compared with controls (P < 0.05). Western blot analysis showed that membrane PKC-beta II was increased by 133 +/- 5% (P < 0.05). Injection of d-alpha-tocopherol (40 mg/kg ip) every other day prevented the increases in membrane PKC specific activity and PKC-beta II protein by immunoblots. Diabetes-induced increases in DAG levels were also normalized by d-alpha-tocopherol treatment of 2 wk duration. Physiologically, angiographic abnormalities of retinal hemodynamics based on computerized video-based fluorescein angiography and associated with increases of DAG and membranous PKC levels were also prevented by d-alpha-tocopherol treatment in diabetic rats. The effect of d-alpha-tocopherol on retinal vascular cells was also studied. Exposure of retinal endothelial cells to 22 mM glucose for 3 days increased total DAG and [3H]palmitate-labeled DAG levels by 35 +/- 8 and 50 +/- 8% (P < 0.05), respectively, compared with exposure to 5.5 mM glucose. The presence of d-alpha-tocopherol (50 micrograms/ml) prevented the increases in total DAG and [3H]palmitate-labeled DAG levels in cells exposed to 22 mM glucose. These findings suggested that treatment with d-alpha-tocopherol can prevent diabetes-induced abnormalities in rat retinal blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.

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.

Phosphatidylinositol 3-kinase, but not p70/p85 ribosomal S6 protein kinase, is required for the regulation of phosphoenolpyruvate carboxykinase (PEPCK) gene expression by insulin. Dissociation of signaling pathways for insulin and phorbol ester regulation of PEPCK gene expression

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the rate-limiting step in hepatic gluconeogenesis. Glucagon (via the second messenger cAMP) and glucocorticoids stimulate the transcription of the PEPCK gene, whereas insulin and phorbol esters inhibit, in a dominant fashion, these effects. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, prevents the stimulation of glycogen synthesis, glucose transport, mitogen-activated protein kinase, and p70/p85 ribosomal S6 protein kinase by insulin. We now show that wortmannin can also block the inhibition of glucocorticoid- and cAMP-stimulated PEPCK gene expression by insulin. PEPCK-chloramphenicol acetyltransferase fusion gene experiments demonstrate that wortmannin blocks an activity that is required for insulin signaling to elements within the PEPCK promoter. Phorbol esters mimic the action of insulin on the regulation of PEPCK gene expression, but wortmannin does not block the effect of these agents. Thus, phosphatidylinositol 3-kinase is required for the regulation of PEPCK gene expression by insulin, but not by phorbol esters. The immunosuppressant rapamycin, a potent inhibitor of insulin or phorbol ester stimulation of p70/p85 ribosomal S6 protein kinase, has no significant effect on the regulation of PEPCK gene expression by insulin or phorbol esters. Thus, p70/p85 ribosomal S6 protein kinase does not have a role in signaling to the PEPCK promoter by insulin or phorbol esters.

Carbachol, substance P, and phorbol ester promote the tyrosine phosphorylation of protein kinase C delta in salivary gland epithelial cells

The initiation of saliva formation by parotid acinar cells, which comprise the majority of cells in this salivary gland, is initiated by the release of neurotransmitters (acetylcholine, substance P) from parasympathetic nerves. In response to substance P and the muscarinic agonist carbachol, two ligands that activate phospholipase C-linked receptors, which stimulate fluid secretion, PKC delta was phosphorylated on tyrosine residues. The maximal agonist-dependent tyrosine phosphorylation occurred within seconds of the addition of either agonist and then returned rapidly to a smaller increased level. Phorbol ester also caused a rapid increase in tyrosine phosphorylation, which reached a maximal level 5 min after the addition of phorbol 12-myristate 13-acetate. The increase in tyrosine phosphorylation of PKC delta was blocked by tyrosine kinase inhibitors genistein and staurosporine. Ionophore-mediated elevation of [Ca2+]i or activation of the beta-adrenergic receptor, epidermal growth factor receptor, or insulin receptor did not promote the tyrosine phosphorylation of PKC delta. These results indicate that tyrosine phosphorylation plays a role in early signal transduction events promoted by the activation of muscarinic and substance P receptors and suggests that the tyrosine phosphorylation of PKC delta has a role in the activation of fluid secretion by neurotransmitters binding to phospholipase C-linked receptors.

Antagonists of phosphatidylinositol 3-kinase block activation of several novel protein kinases in neutrophils

Several novel protein kinases are known to be rapidly activated in neutrophils stimulated with the chemoattractant fMet-Leu-Phe (fMLP). These kinases include a histone H4 protein kinase and several renaturable kinases with molecular masses of about 69, 63, 49, and 40 kDa. The renaturable kinases can catalyze the phosphorylation of a peptide that corresponds to residues 297-331 of the 47-kDa subunit of the NADPH-oxidase system (p47-phox). Previous studies have indicated that the activation of all of these protein kinases involves an uncharacterized stimulatory pathway and/or novel second messenger. The studies reported herein were undertaken to determine if phosphatidylinositol 3-kinase (PI3-K) is a component of this pathway. We report that certain chromosome derivatives (e.g. 2-(4-morpholinyl)-8-phenylchromone (LY294002)) and wortmannin, which inhibit PI3-K by distinct mechanisms, blocked activation of all of these novel kinases. These antagonists also inhibited the phosphorylation of p47-phox (about 50%) and O2.- release (about 80%) in cells stimulated with fMLP, but not with 4 beta-phorbol 12-myristate 13-acetate. A strong correlation exists between the amounts of these antagonists required to produce 50% inhibition of PI3-K in vitro and O2.- release in vivo. In contrast, a single atom substitution of LY294002 produced a compound (LY303511) that did not inhibit PI3-K. Compound LY303511 did not appreciably inhibit the activation of the novel protein kinases or O2.- generation. These data strongly suggest that PI3-K is involved in the activation of several novel protein kinases in neutrophils, one or more of which may be involved in O2.- release.

Phosphoinositide 3-kinase inhibition spares actin assembly in activating platelets but reverses platelet aggregation

Platelet stimulation by thrombin leads to the activation of phosphoinositide 3-kinase (PI 3K) and to the production of the D3 phosphoinositides, phosphatidylinositol 3,4-bisphosphate (PdtIns-3,4P2) and 3,4,5-trisphosphate (PdtIns-3,4,5-P3). Because changes in the levels of these phosphoinositides correlate with the kinetics of actin assembly, they have been proposed to mediate actin assembly, causing cell shape changes. Wortmannin and LY294002, two unrelated inhibitors of PI 3-K, were used to investigate the role of PI 3-K in platelet actin assembly and aggregation. Both PI 3-K inhibitors abrogated the production of PdtIns-3,4-P2 and PdtIns-3,4,5-P3 in thrombin receptor-activating peptide (TRAP)-stimulated cells. However, neither wortmannin nor LY294002 altered the kinetics of actin assembly or the exposure of nucleation sites in TRAP-stimulated cells. In contrast, PI 3-K inhibitors showed a specific inhibitory pattern of cell aggregation, characterized by a primary phase of aggregation followed by progressive disaggregation. Flow cytometry analysis with the PAC1 monoclonal antibody or with FITC-labeled fibrinogen indicated that wortmannin inhibited the maintenance of the platelet integrin GPIIb-IIIa in its active state. Wortmannin also inhibited, in a dose-dependent manner, platelet aggregation induced by the binding of the monoclonal antibodies P256 and LIBS-6 to GPIIb-IIIa. LIBS Fab-induced aggregation also led to the production of PdtIns-3,4-P2. Platelet secretion, as evidenced by the release of preloaded 14C-5-hydroxy-tryptamine secretion or P-selectin up-regulation, was not affected by PI 3-K inhibition. These results demonstrate that the generation of D3 phosphoinositides is not required for actin assembly in TRAP-activated platelets. However, PI 3-K stimulation is necessary for prolonged GPIIb-IIIa activation and irreversible platelet aggregation. PI 3-K stimulation downstream of GPIIb-IIIa engagement may provide positive feedback required to sustain active GPIIb-IIIa.

Type 2 phosphatidylinositol 4-kinase is recruited to CD4 in response to CD4 cross-linking

CD4 serves as a cell-cell adhesion molecule, with specific affinity for class II MHC molecules, and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein. Phosphoinositide (PI)-3-kinase and 1-phosphatidylinositol (PtdIns)-4-kinase activities were previously found to associate with the CD4:p56lck complex, but the protein responsible for PtdIns 4-kinase activity was not identified. Here we demonstrate that the 53 kDa type 2 PtdIns 4-kinase associates with CD4 using a monoclonal antibody specific for this enzyme. We also show that an increase in PtdIns 4-kinase activity is due to recruitment of the type 2 PtdIns 4-kinase protein to the CD4:p56lck complex after cross-linking with anti-CD4.

Phosphatidylinositol 4,5-bisphosphate synthesis is required for activation of phospholipase D in U937 cells

Phospholipase D (PLD) has been implicated in signal transduction and membrane traffic. We have previously shown that phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) stimulates in vitro partially purified brain membrane PLD activity, defining a novel function of PtdIns-4,5-P2 as a PLD cofactor. In the present study we extend these observations to permeabilized U937 cells. In these cells, the activation of PLD by guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) is greatly potentiated by MgATP. We have utilized this experimental system to test the hypothesis that MgATP potentiates PLD activation by G proteins because it is required for PtdIns-4,5-P2 synthesis by phosphoinositide kinases. As expected, MgATP was absolutely required for maintaining elevated phosphatidylinositol 4-phosphate (PtdIns-4-P) and PtdIns-4,5-P2 levels in the permeabilized cells. In the presence of MgATP, GTP gamma S further elevated the levels of the phosphoinositides. The importance of PtdIns-4,5-P2 for PLD activation was examined by utilizing a specific inhibitory antibody directed against phosphatidylinositol 4-kinase (PtdIns 4-kinase), the enzyme responsible for the first step in the synthesis of PtdIns-4,5-P2. Anti-PtdIns 4-kinase completely inhibited PtdIns 4-kinase activity in vitro and reduced by 75-80% PtdIns-4-P and PtdIns-4,5-P2 levels in the permeabilized cells. In parallel, the anti-PtdIns 4-kinase fully inhibited the activation of PLD by GTP gamma S and caused a 60% inhibition of PLD activation by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, indicating that elevated PtdIns-4,5-P2 levels are required for PLD activation. This conclusion is supported by the fact that neomycin, a high affinity ligand of PtdIns-4,5-P2, also blocked PLD activation. Furthermore, the activity of PLD in U937 cell lysate was stimulated by PtdIns-4,5-P2 in a dose-dependent manner. The current results indicate that PtdIns-4,5-P2 synthesis is required for PLD activation in permeabilized U937 cells and strongly support the proposed function of PtdIns-4,5-P2 as a cofactor for PLD. In addition, the results further establish PtdIns-4,5-P2 as a key component in the generation of second messengers via multiple pathways including phosphoinositide-phospholipase C, phosphoinositide 3-kinase and PLD.