Evidence for ecto-protein kinase activity on the surface of human neutrophils

Phosphorylation mapping of laminin α1-chain: Kinases in association with active sites

Laminin-111 is a trimeric glycoprotein of the extracellular matrix (ECM) that holds a significant role in cell adhesion, migration and differentiation. Laminin-111 is the most studied laminin isoform, composed of three chains; α1, β1 and γ1. Phosphorylation is the most common eukaryotic post - translational modification and has regulatory effect on protein function. Using bioinformatic tools we computationally predicted all the possible phosphorylation sites on human laminin α1-chain sequence (LAMA1) according to kinases binding motifs. Thus, we predicted, for the first time, the possibly responsible kinases for fifteen of the nineteen already published experimentally observed phosphorylated residues in LAMA1. Searching the literature extensively, we recorded all the known functional sites (active sites) in LAMA1. We combined the experimentally observed and predicted phosphorylated residues as well as the active sites in LAMA1, generating an analytic phosphorylation map of human laminin α1-chain, which is useful for further analysis. Our results indicated fourteen kinases that might be important for the phosphorylation of human laminin α1-chain, out of which three kinases with reported ecto-phosphorylation activity (PKA, PKC and CKII) were suggested to have a more significant role. Six cancer associated-active sites were correlated with kinases, three out which were correlated with only the above ecto - kinases.

Increase of Fas-induced apoptosis by inhibition of extracellular phosphorylation of Fas receptor in Jurkat cell line

Apoptosis signalling through the Fas pathway requires several steps of aggregation of the Fas receptor in the membrane, including aggregation that may occur in the absence of Fas ligand. Association of Fas domains is determinant to signal transmission following Fas ligand binding to a specific domain. The domains involved in Fas aggregation are located in its extracellular region and contain three potential protein kinase C-binding motifs. We therefore studied the possibility that phosphorylation of the extracellular region of Fas might be implicated in the regulation of Fas-mediated apoptosis. Inhibition experiments of extracellular phosphorylation were performed in human Jurkat T leukemia cells with K252b, an impermeant protein-kinase inhibitor. Extracellular phosphorylation of Fas receptor was related to ecto-kinase, as assessed by the [gamma-(32)P] ATP labelling of Fas-116 kDa aggregates, suppressed by K252b inhibitor which significantly increased the sensitivity to Fas-mediated apoptosis. Ecto-PKC involvement was demonstrated by bisindolylmaleimide VIII, a selective inhibitor of protein kinase C which significantly increased both Fas aggregation in the membrane and Fas-mediated apoptosis and by the addition of the PKC pseudo-substrate 19-36 which inhibited the phosphorylation of 116 kDa Fas aggregates. These data support a role for Fas phosphorylation in the decreased sensitivity to apoptosis in the Jurkat T leukemia cell line.

Laminin-1 is phosphorylated by ecto-protein kinases of monocytes

Monocytes encounter basement membranes and interact with laminins while crossing the vascular barrier. It is known that these cells possess ecto-protein kinase activity on their surface. Several proteins of the extracellular matrix can be phosphorylated by ectokinases. Therefore, it has been hypothesized that monocyte ectokinases could phosphorylate laminins and influence their biological properties. In order to test the above hypothesis, we used intact human monocytes and adenosine triphosphate labeled with radioactive phosphate at the third phosphate ([gamma-32P]-ATP) to phosphorylate laminin-1. Autoradiography after sodium dodecyl sulphate polyacrylamyde gel electrophoresis (SDS-PAGE) electrophoresis indicated phosphorylation of laminin-1 on the beta and/or gamma chains. After phosphorylation, phosphoserine could be detected on Western blots by a specific monoclonal antibody. Phosphorylation was not detected when monocytes were pre-treated with trypsin and was inhibited by a specific ecto-protein kinase inhibitor (K252b). Laminin phosphorylation was also inhibited by heparin, a known inhibitor of casein kinase II and by pretreatment of monocytes by a monoclonal anti-casein kinase II antibody. Heparin binding, cell attachment and proliferation, and monocyte migration were enhanced on the phosphorylated laminin-1 as compared to the non-phosphorylated controls. These data indicate that laminin-1 can be phosphorylated by monocyte casein kinase II type ectokinase. This phosphorylation influences important functions of laminin and therefore could provide an additional means for the interaction of monocytes with basement membranes.

The CK2 phosphorylation of vitronectin. Promotion of cell adhesion via the alpha(v)beta 3-phosphatidylinositol 3-kinase pathway

Phosphorylation of vitronectin (Vn) by casein kinase II was previously shown to occur at Thr50 and Thr57 and to augment a major physiological function of vitronectin-cell adhesion and spreading. Here we show that this phosphorylation increases cell adhesion via the alpha(v)beta3 (not via the alpha(v)beta5 integrin), suggesting that alpha(v)beta3 differs from alpha(v)beta5 in its biorecognition profile. Although both the phospho (CK2-PVn) and non-phospho (Vn) analogs of vitronectin (simulated by mutants Vn(T50E,T57E), and Vn(T50A,T57A), respectively) trigger the alpha(v)beta3 as well as the alpha(v)beta5 integrins, and equally activate the ERK pathway, these two forms are different in their activation of the focal adhesion kinase/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) pathway. Specifically, we show (i) that, upon exposure of cells to Vn/CK2-PVn, their PKB activation depends on the availability of the alpha(v)beta3 integrin on their surface; (ii) that upon adhesion of the beta3-transfected cells onto the CK2-PVn, the extent of PKB activation coincides with the enhanced adhesion of these cells, and (iii) that both the PKB activation and the elevation in the adhesion of these cells is PI3K-dependent. The occurrence of a cell surface receptor that specifically distinguishes between a phosphorylated and a non-phosphorylated analog of Vn, together with the fact that it preferentially activates a distinct intra-cellular signaling pathway, suggest that extra-cellular CK2 phosphorylation may play an important role in the regulation of cell adhesion and migration.

Identification of ecto-PKC on surface of human platelets: role in maintenance of latent fibrinogen receptors

Human platelets express a protein phosphorylation system on their surface. A specific protein kinase C (PKC) antibody, monoclonal antibody (MAb) 1.9, which binds to the catalytic domain of PKC and inhibits its activity, causes the aggregation of intact platelets while inhibiting the phosphorylation of platelet surface proteins. Photoaffinity labeling with 100 nM 8-azido-[alpha(32)P]ATP identified this ecto-PKC as a single surface protein of 43 kDa sensitive to proteolysis by extracellular 0.0005% trypsin. Inhibition of the binding of 8-azido-[alpha(32)P]ATP to the 43-kDa surface protein by MAb 1.9 identified this site as the active domain of ecto-PKC. Covalent binding of the azido-ATP molecule to the 43-kDa surface protein inhibited the phosphorylative activity of the platelet ecto-PKC. Furthermore, PKC pseudosubstrate inhibitory peptides directly induced the aggregation of platelets and inhibited azido-ATP binding to the 43-kDa protein. Platelet aggregation induced by MAb 1.9 and by PKC inhibitory peptides required the presence of fibrinogen and resulted in an increase in the level of intracellular free calcium concentration. This increase in intracellular free calcium concentration induced by MAb 1.9 was found to be dependent on the binding of fibrinogen to activated GPIIb/IIIa integrins, suggesting that MAb 1.9 causes Ca(2+) flux through the fibrinogen receptor complex. We conclude that a decrease in the state of phosphorylation of platelet surface proteins caused by inhibition of ecto-PKC results in membrane rearrangements that can induce the activation of latent fibrinogen receptors, leading to platelet aggregation. Accordingly, the maintenance of a physiological steady state of phosphorylation of proteins on the platelet surface by ecto-PKC activity appears to be one of the homeostatic mechanisms that maintain fibrinogen receptors of circulating platelets in a latent state that cannot bind fibrinogen.

Ecto-protein kinases: ecto-domain phosphorylation as a novel target for pharmacological manipulation?

An increasing number of studies document the presence of protein kinases facing outwards at the cell surface of a diverse array of cells. These ecto-protein kinases phosphorylate cell-surface proteins and soluble extracellular substrates, and thus could affect many physiological processes involving cell-cell contacts, cellular differentiation and proliferation, ion fluxes and cellular activation. To date, only limited attention has been paid to exploring ecto-protein kinases as possible pharmacological targets. Here, the identification and physiological role of ecto-protein kinases in different biological systems is described; it is suggested that ecto-protein kinases are attractive and novel candidates for pharmacological manipulation under various (patho)physiological conditions.

Phosphorylation of vitronectin by casein kinase II. Identification of the sites and their promotion of cell adhesion and spreading

The cell adhesion protein vitronectin (Vn) was previously shown to be the major target in human blood for an extracellular protein kinase A, which is released from platelets upon their physiological stimulation with thrombin and also prevails as an ectoenzyme in several other types of blood cells. Because plasma Vn was shown to have only one protein kinase A phosphorylation site (Ser378) but to contain approximately 3 mol of covalently bound phosphate, and because human serum and blood cells were shown to contain also a casein kinase II (CKII) on their surface, we studied the phosphorylation of Vn by CKII attempting to find out whether such phosphorylation modulates Vn function, an acid test for its having a physiological relevance. Here we show (i) that the CKII phosphorylation of Vn has a Km of 0.5-2 microM (lower than the Vn concentration in blood, 3-6 microM), (ii) that it is targeted to Thr50 and Thr57, which are vicinal to the RGD site of Vn, and (iii) that the phosphorylation of Thr57 facilitates the phosphorylation of Thr50. The maximal stoichiometry of the CKII phosphorylation of plasma Vn was found to be low, which, in principle, could be due to its partial prephosphorylation in vivo. However, for the detection of a functional modulation, we needed a comparison between a fully phosphorylated Vn (at Thr57 and Thr50) and a nonphosphorylated Vn. Therefore, we expressed Vn in a baculovirus system and show (i) that the CKII phosphorylation of wt-Vn enhances the adhesion of bovine aorta endothelial cells; (ii) that the double mutant T50E/T57E (in which the neutral Thr residues are replaced by the negatively charged Glu residues considered analogs of Thr-P) has a significantly enhanced capacity to promote cell adhesion and to accelerate cell spreading when compared with either wild-type Vn or to the neutral T50A/T57A mutant; and (iii) that, at least in the case of bovine aorta endothelial cells, the T50E/T57E mutant exhibits an enhanced adhesion, which seems to be due to an increased affinity toward the alphav beta3 Vn receptors.

Evidence for cAMP-dependent platelet ectoprotein kinase activity that phosphorylates platelet glycoprotein IV (CD36)

The dephosphorylating enzyme alkaline phosphatase, by removing phosphate groups from the external platelet membrane proteins, modulates platelet activation (Hatmi, M., Haye, B., Gavaret, J. M., Vargaftig, B. B., and Jacquemin, C. (1991) Br. J. Pharmacol. 104, 554-558). This observation, together with findings reported by others (Ehrlich, Y. H., Davis, T. B., Bock, E., Kornecki, E., and Lenox, R. H. (1986) Nature 320, 67-70; Dusenbery, K. E., Mendiola, J. R., and Skubitz, K. M. (1988) Biochem. Biophys. Res. Commun. 153, 7-13), indicate the existence of ectoprotein kinase activity on the blood platelet surface. In this study, we demonstrate that washed human platelets phosphorylate the synthetic heptapeptide kemptide in a cAMP-dependent mode. The intensity of the phosphorylation was concentration-dependent for kemptide. In addition, incubation of platelets with [gamma-32P]ATP resulted in a rapid incorporation of [32P] phosphate into proteins at the outer membrane surface that was sensitive to alkaline phosphatase treatment. When cAMP was added to the medium, major phosphorylation of an 88-kDa ectoprotein occurred. Its isoelectric point determined by isoelectric focusing SDS-polyacrylamide gel electrophoresis was around pH 6.2. Phosphorylations of this 88-kDa polypeptide and of the exogenous kemptide substrate were both prevented by the specific protein kinase A inhibitor peptide. When platelets were preincubated with [32P]inorganic phosphate to label intracellular proteins, the protein phosphorylation pattern was different from that obtained with [gamma-32P]ATP, indicating that the latter occurred at the outer surface of the cells. Prostacyclin, which induces the increase of intracellular cAMP levels and, consequently, its liberation into the extracellular medium, increased phosphorylation of both kemptide and platelet 88-kDa polypeptide. The major protein of 88-kDa, which was phosphorylated in the presence of cAMP and external [gamma-32P]ATP, was identified by immunoprecipitation to GPIV (CD36), one of thrombospondin and collagen binding sites on platelets. The phosphorylation of CD36 also occurred in platelet-rich plasma, suggesting a physiological role for this ectoenzyme. In the present study, we clearly demonstrate the presence of an ectoprotein kinase A activity at the surface of intact human platelets, and we revealed its principal endogenous substrate as being CD36.

Ectoprotein kinase activities on non-differentiated and differentiated U-937 cells

Incubation of intact U-937 cells with 1 micron [gamma-32P] ATP resulted in rapid (10 min) incorporation of radioactivity into phosvitin, kemptide and protein kinase C (PKC)-peptide. The amount of incorporation was dependent on substrate type and concentration, and on incubation time. Staurosporine, H-7 and Mg(2+)-exclusion abolished phosphorylation of kemptide and PKC-peptide but not phosvitin. Cyclic AMP and phorbol ester enhanced kemptide and PKC-peptide phosphorylation. Protein kinase inhibitor (PKI) inhibits only kemptide phosphorylation. Cell differentiation enhanced 2-fold the phosphorylation of phosvitin and PKC-peptide without significant effect on kemptide phosphorylation. ATP concentrations sufficient to trigger changes in intracellular Ca2+ were sufficient to support extracellular phosphorylation reactions. The results suggest the presence of at least three ectokinase activities on U-937 cells that may play important roles in regulating membrane associated specific functions of developing and mature monocytes.

Ecto-phosphorylation on aortic endothelial cells. Exquisite sensitivity to staurosporine

One- and two-dimensional gel electrophoresis of proteins from bovine aortic endothelial cells (BAEC) incubated with [gamma-32P]ATP revealed the preferential labelling of a cell-associated 21 kDa substrate. The labelling of this band was detectable within 30 s, increased up to 30 min and was stable for at least 3 h following the wash-out of the ATP. This protein was also labelled after incubation of the cells with [gamma-35S]ATP. Incorporation of radioactivity into the 21 kDa band did not occur if the endothelial cells were treated with low concentrations of trypsin (0.01%) before or after the labelling period. The pattern of BAEC protein phosphorylation by [gamma-32P]ATP was completely different from that of the fetal calf serum used for the cell culture. The presence of serum during the incubation of BAEC with [gamma-32P]ATP did not modify qualitatively the labelling pattern and, in particular, did not enhance the phosphorylation of the 21 kDa substrate; this suggests that neither the kinase nor the 21 kDa substrate are adsorbed serum proteins. Staurosporine, a protein kinase inhibitor with low specificity, decreased the labelling of the 21 kDa protein with an IC50 of 2 nM. In contrast, at 100 nM, staurosporine did not decrease the accumulation of inositol phosphates induced by ATP via the activation of P2y receptors. These data indicate the presence of aortic endothelial cells of an ecto-kinase which uses extracellular ATP to produce the selective and long-lived phosphorylation of a 21 kDa endothelial substrate. Ecto-phosphorylation of this protein might play a role in the modulation of endothelial cell functions by ATP, in addition to the P2y receptors [Boeynaems & Pearson (1990) Trends Pharmacol. Sci. 11, 34-37]. The exquisite sensitivity of ecto-phosphorylation to inhibition by staurosporine and its specific inhibition by some isoquinolinesulphonamide compounds provide potential pharmacological tools to investigate this hypothesis.

Structural requirements for binding of adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) to human neutrophils

Human polymorphonuclear leukocytes (PMN) respond via pertussis toxin-sensitive pathways to extracellular nucleotides with an elevation in intracellular calcium ([Ca2+]i) and enhancement of the O2- generation induced by the chemotactic peptide N'-formyl-Met-Leu-Phe (fMLP). Binding studies with adenosine 5'-O-(3-thio[35S]triphosphate) ([35S]ATP gamma S) have recently identified at least two classes of receptors on intact human neutrophils. In this study, we further characterize nucleotide binding to this receptor with respect to its specific structural requirements. Utilizing [35S]ATP gamma S as the primary ligand and various nucleotides and ATP analogues, competitive binding studies demonstrate that: (1) the triphosphate tail is essential for maximal receptor binding; (2) chemical modifications of the phosphate tail have profound effects on binding efficacy; (3) the base ring is recognized by the receptor, with purines being preferentially recognized; and (4) except for a spacing function, the ribose ring of nucleotides does not appear to be important for nucleotide binding. In addition, we demonstrate that the presence of divalent cations inhibits [35S]ATP gamma S binding, suggesting that the tetraanionic form of ATP (ATP4-) is the nucleotide species reactive with the receptor.

Nucleotide-, chemotactic peptide- and phorbol ester-induced exocytosis in HL-60 leukemic cells

Undifferentiated and differentiated HL-60 leukemic cells possess nucleotide receptors which functionally couple to phospholipase C via pertussis toxin-sensitive guanine nucleotide-binding proteins (G-proteins). We investigated the role of extracellular nucleotides in the regulation of beta-glucuronidase release in HL-60 cells. In dibutyryl cyclic AMP (Bt2cAMP)-differentiated HL-60 cells, the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe), the phosphorothioate analogue of ATP, adenosine 5'-O-[3-thio]triphosphate (ATP[gamma S]), and UTP increased cytosolic Ca2+ from 100 nM up to 1.2 microM with EC50 values of 4 nM, 1 microM and 100 nM, respectively. In these cells, ATP[gamma S] induced exocytosis with an EC50 of 4 microM and an effectiveness amounting to 50-70% of that of fMet-Leu-Phe. ATP, ITP, UTP, CTP, and uridine 5'-O-[2-thio]diphosphate activated exocytosis as well. Phorbol myristate acetate (PMA) induced exocytosis with an EC50 of 115 ng/ml and an effectiveness similar to that of ATP[gamma S]. Cytochalasin B (CB) differently potentiated exocytosis induced by ATP[gamma S], fMet-Leu-Phe and PMA. Treatment of Bt2cAMP-differentiated HL-60 cells with pertussis toxin (500 ng/ml) for 24 h resulted in ADP-ribosylation of more than 97.5% of the G-proteins. Under these conditions, pertussis toxin almost completely inhibited the increase in cytosolic Ca2+ and beta-glucuronidase release induced by fMet-Leu-Phe but only partially inhibited the effects of ATP[gamma S] and UTP. fMet-Leu-Phe at a non-stimulatory concentration (1 nM) potentiated ATP[gamma S]-induced beta-glucuronidase release in the presence but not in the absence of CB. In contrast, ATP[gamma S] and fMet-Leu-Phe synergistically activated superoxide formation in the absence of CB. PMA potentiated superoxide formation induced by ATP[gamma S] or fMet-Leu-Phe and did not affect exocytosis induced by ATP[gamma S] or fMet-Leu-Phe. In undifferentiated HL-60 cells, fMet-Leu-Phe, ATP[gamma S], UTP and PMA did not induce beta-glucuronidase release. fMet-Leu-Phe did not increase cytosolic Ca2+ in undifferentiated HL-60 cells, whereas ATP[gamma S] and UTP were similarly potent and effective as in Bt2cAMP-differentiated cells. In differentiated HL-60 cells, fMet-Leu-Phe induced aggregation, and ATP[gamma S] induced a transient shape change. Our results show (I) that exocytosis in HL-60 cells does not obligatorily depend on CB. (II) Purine and pyrimidine nucleotides activate exocytosis via pertussis toxin-sensitive and -insensitive signal transduction pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemical parameters of initiation and regulation of sperm motility

Studies of in vitro models demonstrate that a forward motility protein (FMP) is required for the initiation of forward motility in the immature epididymal spermatozoa. FMP is a heat-stable glycoprotein derived from epididymal plasma. During the epididymal maturation of spermatozoa in vivo, there is a marked increase of intrasperm pH and level of cyclic adenosine monophosphate (cAMP). Several studies suggest that exogenous FMP in concert with elevated intrasperm pH and level of cAMP initiates flagellar motility during the epididymal transit of sperm. cAMP activates sperm cytosolic cAMP-dependent protein kinases, which in turn phosphorylate multiple intrasperm phosphoproteins that may regulate flagellar motility. Exogenous calcium ion activates intact sperm motility, although it inhibits motility of demembranated cells on reactivation. Occurrence of cAMP-dependent type I and II protein kinases, a novel cAMP-independent protein kinase, and a phosphoprotein phosphatase has been demonstrated on the external surface of spermatozoa. The sperm surface has a coupled-enzyme system: ecto-cAMP-independent protein kinase and phosphoprotein phosphatase that regulate the phosphorylation and dephosphorylation of endogenous sperm ectophosphoproteins. The specific activities of these ecto-enzymes increase markedly during forward progression, suggesting that they may have a role in regulating flagellar motility.

Differential effects of heparin, fibronectin, and laminin on the phosphorylation of basic fibroblast growth factor by protein kinase C and the catalytic subunit of protein kinase A

Basic fibroblast growth factor (FGF) is synthesized as a phosphoprotein by both bovine capillary endothelial and human hepatoma cells in culture. Because basic FGF is characterized by its high affinity for heparin and its association in vivo with the extracellular matrix, we examined the possibility that the phosphorylation of this growth factor by purified protein kinase C (PK-C) and the catalytic subunit of cAMP-dependent protein kinase-A (PK-A) can be modulated by components of the extracellular matrix. Heparin and other glycosaminoglycans (GAGs) inhibit the ability of PK-C to phosphorylate basic FGF. In contrast, heparin can directly increase the phosphorylation of basic FGF by PK-A. While fibronectin, laminin, and collagen IV have no effect on the ability of PK-C to phosphorylate basic FGF, they all can inhibit the effects of PK-A. Thus, there is a differential effect of extracellular matrix-derived proteins and GAGs on the phosphorylation of basic FGF. The enhanced phosphorylation of basic FGF that is mediated by heparin is associated with a change in the kinetics of the reaction and the identity of the amino acid targeted by this enzyme. The amino acids that are targeted by PK-C and PK-A have been identified by phosphopeptide analyses as Ser64 and Thr112, respectively. In the presence of heparin, basic FGF is no longer phosphorylated by PK-A at the usual PK-A consensus site (Thr112), but instead is phosphorylated at the canonical PK-C site (Ser64). Accordingly, heparin inhibits the phosphorylation of basic FGF by PK-C presumably by masking the PK-C dependent consensus sequence surrounding Ser64. Thus, when basic FGF is no longer phosphorylated by PK-A in the receptor binding domain (Thr112), it loses the increased receptor binding ability that characterizes PK-A phosphorylated basic FGF. The results presented here demonstrate three novel features of basic FGF. First, they identify a functional effect of the binding of heparin to basic FGF. Second, they establish that the binding of heparin to basic FGF can induce structural changes that alter the substrate specificity of protein kinases. Third, and perhaps most important, the results demonstrate the existence of a novel interaction between basic FGF, fibronectin, and laminin. Although the physiological significance of this phosphorylation is not known, these results clearly suggest that the biological activities of basic FGF are regulated by a complex array of biochemical interactions with the proteins, proteoglycans, and glycosaminoglycans present in the extracellular milieu and the cytoplasm.

Involvement of pyrimidinoceptors in the regulation of cell functions by uridine and by uracil nucleotides

Uridine and uracil nucleotides are involved in the regulation of various cell functions. Here, Roland Seifert and Günter Schultz review the evidence that, rather than by binding to purinoceptors, pyrimidine nucleotides exert their effects by binding to distinct pyrimidinoceptors, which are coupled to pertussis toxin-sensitive G proteins in human phagocytes. However, many questions remain to be answered: no antagonists for these pyrimidinoceptors are available, and binding studies have not been carried out; the receptor proteins and subtypes have not been characterized; and little is known about the G proteins and effector systems involved, or the regulation of storage and release of pyrimidine nucleotides.

Phosphorylation of p56lck by external ATP in intact cells

Recent studies have suggested a role for extracellular ATP. In this report we show that extracellular labelled ATP crosses the plasma membrane of intact lymphoma cells and peripheral blood lymphocytes and phosphorylates p56lck a tyrosine protein kinase specific of lymphoid cells. Two other phosphoproteins of 92Kd and 35Kd become detectable on alkali treated gels. Phosphorylation occurs within minutes following addition of ATP. ATP, GTP, ADP and an ATP analog prevent phosphorylation but not AMP nor Pi; trypsinization of cells abolishes labelling. The possible involvement of P2 purinergic receptors is discussed.

Origin, metabolism and function of extracellular adenine nucleotides in the blood

In previous views the role of adenine nucleotides was thought to be confined to the intracellular space of the cell. However, research of the last decades has revealed that nucleotides also occur in the extracellular space. This survey deals with the sources, metabolism and the role in blood of the extracellular adenine mononucleotides ATP, ADP, AMP and the dinucleotides diadenosine tetraphosphate (Ap4A) and diadenosine triphosphate (Ap3A). The latter two are novel compounds, which have recently been discovered in human platelets. The mononucleotides originate from damaged tissues, from red blood cells during haemolysis, from activated platelets, the working muscle and from the nervous system, whereas the dinucleotides are exclusively released from stimulated platelets. Both the adenine mono- and the dinucleotides act as signal molecules on blood cells as well as on cells of the vascular wall, thereby modulating physiological processes such as platelet aggregation, histamine release from mast cells, regulation of vascular tone and white cell functions. In order to limit the signal effects of extracellular nucleotides, blood cells, plasma and the interior of the vessel walls are provided with nucleotide splitting enzymes: ATP, ADP and AMP are mainly degraded by ectoenzymes present on blood cells, endothelial and on smooth muscle cells, whereas dinucleotides are primarily metabolized by plasma enzymes. This review closes with the presentation of the clinical utility of Ap3A and Ap4A as tools for the diagnosis of platelet storage pool defects.

Protein kinases in neutrophils: a review

In chemotactic factor-stimulated neutrophils, rapid increases of intracellular levels of cyclic AMP, calcium, and diacylglycerol have been observed and may be linked to protein kinase activation. The study of the physiological role and regulation of protein kinases in the neutrophil and the identification of their substrates has provided valuable information on the molecular mechanism of neutrophil activation. The focus of this review is on those aspects of protein kinases that are relevant to neutrophil activation and on the substrate proteins for these protein kinases. The possible role of protein phosphorylation in neutrophil function is also discussed.