Thrombin stimulates the production of a novel polyphosphoinositide in human platelets

Partial purification and characterization of two forms of phosphatidylinositol 4-phosphate 5-kinase from human platelet membrane

The phosphatidylinositol 4-phosphate 5-kinase (PIP kinase) was isolated from the cholate extract of human platelet membranes. Two major activity peaks (PIP kinase I and PIP kinase II) were resolved by successive chromatographies on Fast Q-Sepharose, heparin-Sepharose, Mono Q and heparin-agarose columns. The PIP kinase I appears to be distinct from the PIP kinase II with regard to Mr (51 kDa and 47 kDa as determined by SDS-PAGE). The two forms of PIP kinase showed similarity in Km for ATP and Mg2+ dependency, but some differences were observed in effects of Mn2+ and phosphatidylethanolamine on the activity.

BW755C or staurosporine inhibits collagen-stimulated phosphoinositide phosphorylation in platelets

Stimulation of platelets by collagen results in increased formation of the polyphosphoinositides, phosphatidylinositol phosphate (PtdInsP) and phosphatidylinositol bisphosphate (PtdInsP2) through stimulation of phosphoinositide kinase activities. We investigated a possible regulatory role of endogenous thromboxane formation and protein kinase C (PKC) activation in the induction of phosphoinositide phosphorylation following collagen stimulation, as well as following stimulation by the thromboxane mimetic, U-46619. Human platelets were prelabeled with [3H]inositol and stimulated with collagen (2 micrograms/mL) or U-46619 (1 microM), in the absence or presence of either the cyclo-oxygenase/lipoxygenase inhibitor, BW755C, or staurosporine, a putative inhibitor or PKC. Collagen stimulation resulted in a time-dependent increase in [3H]inositol-labeled PtdInsP and PtdInsP2 which was completely inhibited in the presence of BW755C. Addition of U-46619 to BW755C-treated, collagen-stimulated platelets restored the increased polyphosphoinositide formation. Stimulation of platelets with U-46619 alone also resulted in increased formation of [3H]PtdInsP and [3H]PtdInsP2, but this was not affected by the presence of BW755C. These results suggest that the collagen-induced activation of phosphoinositide kinases was dependent upon thromboxane formation, but that U-46619-induced phosphoinositide formation was rather independent of further thromboxane production. Pretreatment of platelets with staurosporine, prior to agonist addition, completely blocked the collagen-stimulated rise in radiolabeled PtdInsP and the U-46619-induced PtdInsP and PtdInsP2 generations, suggesting that protein kinase, possibly PKC, may play a role in the activation of phosphoinositide kinases by these agonists.

Thrombin stimulates the production of phosphatidylinositol 3,4-bisphosphate in human erythroleukemia cells

The human erythroleukemic cell line, HEL, which has numerous platelet markers, shows enhanced inositol phosphate production in response to thrombin. We investigated the production of phosphoinositides in HEL cells and showed that thrombin stimulates the turnover of several phosphoinositides including the synthesis of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). Phosphatidylinositol 3-monophosphate is also produced in HEL cells and its synthesis is not stimulated by thrombin. Pretreatment of HEL cells with the stable prostacyclin analog iloprost inhibits the thrombin-induced increase in the production of PtdIns(3,4)P2. 3-Phosphorylated phosphoinositides have been implicated in signal transduction and regulation of cell proliferation in other cells and may be involved in signal transduction in HEL cells.

The role of phospholipases and phospholipid-derived signals in cell activation

The complexity of receptor-regulated breakdown and modification of phospholipids continues to grow. New developments extend our concepts of signalling enzymes and possible messengers.

Pathway for the formation of D-3 phosphate containing inositol phospholipids in PDGF stimulated NIH 3T3 fibroblasts

PtdIns (3, 4, 5) P3 is formed rapidly in NIH-3T3 cells stimulated with platelet derived growth factor (PDGF). We have determined the pathway of formation of this lipid in these cells. Cells were labeled briefly with 32PO4 and then stimulated with PDGF under conditions where the specific activity of each phosphate group determines the order of its addition. The D-5 phosphate of this lipid contained approximately 42% of the total radioactivity present in the molecule, while approximately 32% was in the D-4 position, 25% in the D-3 position, and approximately 2% in the D-1 position. This indicates that PtdIns (3, 4) P2 and not PtdIns (4, 5)P2 is the immediate precursor of PtdIns (3, 4, 5) P3, and defines the pathway of formation of these lipids to be PtdIns (3) P----PtdIns (3, 4) P2----PtdIns (3, 4, 5) P3. This pathway is the same as that in thrombin-stimulated platelets and infers that the pathways are not different in non-growing and proliferating cells.

The production of phosphatidylinositol trisphosphate is stimulated by thrombin in human platelets

Untreated human platelets labeled to equilibrium with 32Pi contained undetectable levels of 3-phosphorylated phosphoinositides. Stimulation of platelets with thrombin for 5 min resulted in an enormous increase in the amount of phosphatidylinositol 3,4-bisphosphate (Nolan and Lapetina, J. Biol. Chem. 265, 2441-2445; 1990). We now report that the levels of phosphatidylinositol 3,4,5-trisphosphate are greatly elevated within 90 s of treatment of platelets with thrombin. Phosphatidylinositol 3,4,5-phosphate might have an important role in platelet aggregation.

GTP binding proteins and growth factor signal transduction

There is a large body of evidence supporting a role for GTP-binding proteins in signal transduction by growth factors. In certain cells, ligands which activate or inhibit the production of cAMP via heterotrimeric G proteins promote replication of the target cell. These mechanisms play an important role in a limited number of tumours. Ligands which activate PI hydrolysis through heterotrimeric G proteins may also promote growth in certain systems, but the precise role for PI hydrolysis remains to be determined. Receptors with intrinsic tyrosine kinases may also interact with the heterotrimeric G proteins, but it is not known if these interactions represent side reactions, or whether they are central in the responses of certain cell types. Lastly, p21ras and other small molecular weight G proteins appear to be profoundly important in growth control. The tyrosine kinase growth factor receptors may interact indirectly with these GTP binding proteins via GAP proteins. The molecular detail of this process is emerging rapidly and is likely to be worked out in the near future.

Chemotactic peptides but not macrophage colony stimulating factor effect the synthesis of inositol lipids in macrophages

Normal bone marrow derived macrophages display a wide variety of biological responses to a number of distinct agonists, for example, Macrophage Colony Stimulating Factor (M-CSF) and chemotactic peptides (such as FMLP). FMLP stimulates reactive oxygen intermediate production in these cells, whilst M-CSF stimulates DNA synthesis. We have compared the effects of these two agents on the production of novel inositol lipids in macrophages. Evidence is presented that FMLP, but not M-CSF elevate the levels of a lipid putatively identified as phosphatidylinositol-3,4-bisphosphate. The implications of this observation on proposed role of novel inositol lipids in macrophage proliferation are discussed.

Phosphoinositide 3-kinase: a new effector in signal transduction?

Interest in phosphoinositide 3-kinase (PI 3-kinase) has been fuelled by its identification as a major phosphotyrosyl protein detected in cells following growth factor stimulation and oncogenic transformation. It is found complexed with activated growth factor receptors and non-receptor tyrosine kinases, thus suggesting that it participates in the signal transduction pathways initiated by the activation of tyrosine kinases. PI 3-kinase phosphorylates the 3-position in the inositol ring of the well known inositol phospholipids in vitro giving phosphatidylinositol 3-phosphate, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate [PtdIns3P, PtdIns(3,4)P2 and PtdIns(3,4,5)P3], respectively. The cellular levels of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 rapidly increase in circumstances where PI 3-kinase becomes complexed with tyrosine kinases. Accumulation of the same lipids also occurs in platelets and neutrophils following stimulation of G-protein linked alpha-thrombin and chemotactic peptide receptors, respectively, leading to speculation that one or both of these lipids is a new second messenger whose function is not yet known. This review brings together recent information on the isolation, characterization and regulation of PI 3-kinase, the cellular occurrence of 3-phosphorylated inositol phospholipids and possible functions of the PI 3-kinase pathway in cell signalling.

Anti-phosphotyrosine immunoprecipitation of phosphatidylinositol 3' kinase activity in different cell types after exposure to epidermal growth factor

In previous studies from this laboratory, it was shown that mouse epidermal growth factor (mEGF) or insulin increased the labeling of phosphaditylinositol-3,4-bisphosphate (PI-3,4-P2) in MA-10 cells prelabeled with different radioactive precursors (Pignataro, O.P., and Ascoli, M. (1990) J. Biol. Chem. 265, 1718-1723 and Mol. Endocrinol. (1990) 4, 758-765). In order to further characterize this phenomenon we sought to determine if we could use anti-phosphotyrosine antibodies to immunoprecipitate a phosphatidylinositol (PI) kinase activity from MA-10 cells treated with mEGF or insulin. Our data indicate that this is indeed the case, and that the PI kinase precipitated is a PI-3' kinase. A second cell type, A431 cells, in which we were unable to detect an increase in PI-3,4-P2 labeling when stimulated with mEGF or insulin, was also studied. It was found that, as in MA-10 cells, A431 cells also contain an immunoprecipitable PI-3' kinase activity that is increased in response to mEGF or insulin.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.

The signal transduction induced by thrombin in human platelets

The stimulation of human platelets by thrombin leads to the activation of phospholipases C and A2, protein kinases, formation of 3-inositol phospholipids and mobilization of Ca2+. These biochemical reactions closely parallel platelet shape change, granular secretion and aggregation. The membrane-bound transducers for the thrombin receptor seem to be the heterotrimeric G protein Gi2 and the ras-related G protein rap 1-b. Phosphorylation of rap 1-b by the action of the cyclic AMP-dependent protein kinase seems to uncouple the thrombin receptor from phospholipases. This causes inhibition of the formation of second messenger molecules and the onset of physiological responses.

Effect of genistein, a tyrosine kinase inhibitor, on U46619-induced phosphoinositide phosphorylation in human platelets

Recent evidence suggests that the agonist-induced formation of phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) via PI and PIP kinases may play an important role in transmembrane signalling. In the present work, the effect of genistein, a specific inhibitor of protein-tyrosine kinase, on phosphoinositide phosphorylation was studied in human platelets stimulated with the endoperoxide analogue, U46619. At 100 microM concentration, genistein, but not the related compounds, flavone and biochanin A, which possess only weak anti-protein-tyrosine kinase activity, significantly inhibited the U46619-induced accumulation of [3H]PIP (by 71%) and [3H]PIP2. These data suggest that phosphoinositide phosphorylation may be regulated, in part, by tyrosine phosphorylation in U46619-stimulated platelets.

Protein kinase C-mediated formation of phosphatidylinositol 3,4-bisphosphate in human platelets

The effect of phorbol 12,13-dibutyrate on the formation of phosphatidylinositol 3,4-bisphosphate in washed human platelets was studied. Platelets labelled with [32P]Pi were stimulated with phorbol 12,13-dibutyrate or thrombin in the presence or absence of staurosporine. Lipids were extracted, and deacylated, and the glycerophosphoinositol derivatives were analyzed by high performance liquid chromatography. Phorbol 12,13-dibutyrate increased formation of phosphatidylinositol 4-monophosphate and phosphatidylinositol 3,4-bisphosphate in a dose- and time-dependent manner. Thrombin also increased formation of phosphatidylinositol 3,4-bisphosphate. Staurosporine completely inhibited phorbol 12,13-dibutyrate or thrombin-stimulated production of phosphatidylinositol 3,4-bisphosphate. These data indicate that production of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4-monophosphate is mediated by protein kinase C. It is widely recognized that production of phosphatidylinositol 3,4-bisphosphate is caused by the tyrosine kinase-mediated activation of phosphatidylinositol 3-kinase. However, in platelets, production of phosphatidylinositol 3,4-bisphosphate might be related to stimulation of phosphatidylinositol 4-kinase, which is activated by protein kinase C.