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

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

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.

Coordinate expression of GPEET procyclin and its membrane-associated kinase in Trypanosoma brucei procyclic forms

GPEET procyclin is a major glycosylphosphatidylinositol-anchored protein of procyclic (insect stage) trypanosomes in culture and is heavily phosphorylated in the GPEET pentapeptide repeat. The phosphorylation reaction is a late event and occurs during maturation and transport of GPEET or on the parasite surface by an ecto-protein kinase. Initial biochemical characterization of the GPEET kinase activity now shows that it depends on bivalent cations for maximal activity, is stimulated by sulfhydryl group reagents, and is specific for ATP as phosphoryl donor. No kinase activity is detected in bloodstream form trypanosomes in culture, whereas strong phosphorylation is observed in early procyclic forms. In addition, the GPEET kinase activity is absent from procyclic trypanosomes that have repressed GPEET synthesis but can be induced in these same stocks by conditions, which also induce GPEET expression. However, the presence of an active kinase does not depend on the presence of (functional) GPEET because it can be detected in parasites expressing a non-phosphorylatable GPEET mutant protein and in procyclin null mutant trypanosomes. Interestingly, the presence of the glycosylphosphatidylinositol lipid moiety seems necessary for GPEET to become phosphorylated. Together, the results demonstrate that GPEET and its kinase are expressed during the same life cycle stages and that factors that induce the expression of GPEET in vitro also induce the expression of the GPEET kinase.

Nucleotide-evoked relaxation of rat vas deferens: possible mechanisms

ATP causes relaxation of the K(+)-contracted rat vas deferens. Possible sites of action were investigated. ATP and adenosine relaxed the vas deferens precontracted with 80 mM K(+); EC(50) values and maximal relaxations averaged, respectively, 760 microM and 56% for ATP and 74 microM and 30% for adenosine. The adenosine P1 receptor antagonist 8-(para-sulfophenyl)theophylline (8-SPT) reduced relaxations caused by adenosine and low concentrations of ATP, as did the Rp-diastereomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS), an inhibitor of protein kinase A. The phosphodiesterase inhibitor 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724) augmented responses to adenosine and low concentrations of ATP. alpha,beta-Methylene ADP, an inhibitor of 5'-nucleotidase, reduced relaxations caused by ATP to a similar extent as did 8-SPT. In the presence of an almost saturating concentration of adenosine, ATP caused further relaxation. Conversely, in the presence of ATP, adenosine had little effect. Like ATP, UTP and other nucleoside triphosphates relaxed the vas deferens. The P2 receptor antagonists reactive blue 2, acid blue 25 and 4,4'-diisothiocyanotostilbene-2,2'-disulphonate (DIDS) attenuated the relaxation caused by ATP; suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulphonate (PPADS), Evans blue, trypan blue, reactive red 2 and brilliant blue G had no effect. Three non-selective inhibitors of protein kinases, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), staurosporine and (8R*,9S*,11S*)-(-)-9-hydroxy-9-carboxy-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[cde]trinden-1-one (K-252b), markedly reduced the relaxation caused by ATP. The results indicate that adenosine, derived from enzymatic dephosphorylation, contributes to the relaxant effect of ATP, presumably by activation of a smooth muscle adenosine receptor linked to the accumulation of cAMP and activation of protein kinase A. Yet, the main part of the response to ATP is mediated by a site distinct from the adenosine receptor. The pharmacological properties of this site differ from known P2 receptor subtypes. Possibly, the nucleotide-evoked relaxation is due to a phosphoryl transfer catalyzed by an ecto-protein kinase.

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.

NADH oxidase activity of soybean plasma membranes inhibited by submicromolar concentrations of ATP

The activity of an auxin-stimulated NADH oxidase activity from soybean hypocotyls was inhibited by submicromolar concentrations of ATP. Auxins are plant growth regulators that increase the rate of cell enlargement in plant stems. A synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), was used. The inhibition was half maximal at 1 nM ATP and was not observed with other nucleotides and nucleosides. The inhibition was the result of an increase in the Km for NADH from about 60 microM to > 100 microM and was noncompetitive. The decrease in Km due to ATP was enhanced by the addition of 1 microM 2,4-D. The Vmax of the plasma membrane NADH oxidase was approximately doubled (1.5-2.8-fold) by ATP and by 1 microM 2,4-D. No further increase in the Vmax was observed by the combination of 1 nM to 0.1 mM ATP in the presence of 1 microM 2,4-D. The results demonstrate a response of the NADH oxidase activity of isolated vesicles of soybean plasma membranes to ATP distinct from that observed previously with other nucleotide di- and triphosphates. The results are suggestive either of control of the cell surface NADH oxidase by phosphorylation or a direct response to ATP binding at nanomolar concentrations of ATP.

Potentiating and inhibitory effects of periodate-oxidized ATP analogs on contractions of vas deferens to ATP

Previous studies have shown that treatment of guinea-pig isolated vas deferens with the affinity label periodate-oxidized ATP (2',3'-dialdehyde ATP), results in two irreversible effects on biphasic contractile responses to ATP, i.e., potentiation of the P2X purinoceptor-mediated first phase and inhibition of the ecto-kinase-mediated second phase. The present experiments were designed to evaluate whether periodate-oxidized ADP, periodate-oxidized AMP, and periodate-oxidized adenosine, produce similar effects. Periodate-oxidized ATP and periodate-oxidized ADP (10(-2) M) elicited contraction of the vas deferens (periodate-oxidized ATP > periodate-oxidized ADP; periodate-oxidized AMP and periodate-oxidized adenosine had no agonist activity. After incubation of the preparations for 5 min with 10(-2) M periodate-oxidized ATP, periodate-oxidized ADP, periodate-oxidized AMP or periodate-oxidized adenosine, the first phase of contraction to submaximal ATP concentrations was potentiated. Simultaneously, periodate-oxidized ATP, periodate-oxidized ADP and periodate-oxidized AMP inhibited the second contractile phase, whereas periodate-oxidized adenosine did not. The results indicate that the requirement for 5'-phosphate to produce potentiation and inhibition is different: 5'-phosphate is not needed to potentiate the first phase of contraction to ATP, but at least one 5'-phosphate is required to inhibit the second phase of contraction.

Binding of [35S]adenosine 5'-O-(2-thiodiphosphate) to endothelial cells in culture

We have investigated the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADP beta S) to intact cultured bovine aortic endothelial cells which have been previously shown to co-express P2y and P2u purinoceptors and to bovine adrenal medulla endothelial cells which solely possess P2u purinoceptors. ADP beta S has been shown to stimulate phospholipase C activity in these cells via the P2y purinoceptor and does not interact with the P2u purinoceptor. We describe a simple equilibrium binding procedure designed for the study of low affinity agonists and compare these results with those obtained by separation of bound and free by filtration. Saturation analysis of equilibrium binding data revealed two sites for ADP beta S binding; one with KD = 3.3 x 10(-8) M, Bmax = 32 pmol/mg protein; and the other with KD = 4.3 x 10(-6) and Bmax = 2155 pmol/mg protein. Use of filtration did not significantly alter the KD of either of these sites, nor the Bmax of the high affinity site, but reduced the Bmax of the low affinity site by more than 95%. The rank order of agonist potency for competing for [35S]ADP beta S binding indicated that most of this was to non-P2y purinoceptor sites as beta,gamma-methylene ATP, a P2x purinoceptor agonist, was more potent than 2-methylthio ATP, a P2y purinoceptor agonist. Binding was also carried out in the presence of beta,gamma-methylene ATP, in an attempt to reduce non-P2y purinoceptor binding and produced similar results. Specific [35S]ADP beta S binding sites were also found in bovine adrenal medulla endothelial cells which do not possess P2y purinoceptors. These results indicate that [35S]ADP beta S was able to bind to endothelial cells from different parts of the vasculature but that the ligand can only be considered suitable for investigation of P2y purinoceptors on mammalian cells when specific conditions are designed to reduce the large amount of non-receptor binding.

Involvement of multiple receptors in the actions of extracellular ATP: the example of vascular endothelial cells

The role of ATP and ADP as intercellular mediators is now well established. The presence of the nucleotides in extracellular fluids can result from several mechanisms: cell lysis, selective permeabilization of the plasma membrane and exocytosis of secretory vesicles, such as platelet dense bodies. Extracellular adenine nucleotides are rapidly degraded by ectonucleotidases expressed inter alia on the surface of endothelial cells. They act on cells via the family of P2 receptors which encompasses more than 5 subtypes, some of which have been cloned recently. The P2T, P2U and P2Y receptors belong to the superfamily of receptors coupled to G proteins, whereas the P2X receptor is a cation channel and the P2Z receptor a non-selective pore. ATP and ADP stimulate the endothelial production of prostacyclin (PGI2) and nitric oxide (NO), two vasodilators and inhibitors of platelet aggregation, via an increase in cytosolic Ca2+. This action of adenine nucleotides is believed to limit the extent of intravascular platelet aggregation and to help localize thrombus formation to areas of endothelial damage. The endothelial response to nucleotides is mediated by at least two distinct subtypes of P2 receptors, P2Y and P2U, both coupled to phospholipase C.

Phorbol ester enhances activation of adenylate cyclase in bovine aortic endothelial cells

Endothelial cells possess beta-adrenoceptors linked to adenylate cyclase which may regulate several aspects of endothelial cell function. The potential for this second messenger system to be modulated by protein kinase C activity was investigated. Bovine aortic endothelial cells (BAECs) were cultured in the absence or presence of phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C. Basal and forskolin-, sodium fluoride (NaF)-, and isoproterenol-stimulated adenylate cyclase activity was measured in homogenates from BAECs. beta-adrenoceptor density on membranes from BAECs was measured by 125I-iodocyanopindolol binding. Sodium dodecylsulfate-polyacrylamide gel electrophoresis of immunoprecipitated proteins was used to identify phosphorylated proteins. Pretreatment of BAECs with 100 nM PMA for 30 min increased basal adenylate cyclase activity above control levels, and also increased enzyme activity stimulated by forskolin, NaF, or isoproterenol. Pretreatment of BAECs for 60 min with 100 nM staurosporine, an inhibitor of protein kinase C, prevented the enhancement of adenylate cyclase activity caused by PMA. Treatment of BAECs with PMA did not trigger phosphorylation of the inhibitory guanine nucleotide-binding protein, and there was no change in BAEC beta-adrenoceptor density following PMA pretreatment. Exposure of BAECs to ATP or bradykinin did not mimic the effects of phorbol ester. In conclusion, activation of protein kinase C by PMA enhanced adenylate cyclase activity in BAECs. However, ATP and bradykinin which activate endothelial cell surface receptors linked to phospholipase C did not mimic this effect.

The biology of radioresistance: similarities, differences and interactions with drug resistance

Cells and tissues have developed a variety of ways of responding to a hostile environment, be it from drugs (toxins) or radiation (summarized in Fig. 1). Three categories of radiation damage limitation are: (i) DNA repair (ii) changes in cellular metabolism (iii) changes in cell interaction (cell contact or tissue-based resistance; whole organism based resistance). DNA repair has been evaluated predominantly by the study of repair-deficient mutants. The function of the repair genes they lack is not fully understood, but some of their important interactions are now characterized. For example, the interaction of transcription factors with nucleotide excision repair is made clear by the genetic syndromes of xeroderma-pigmentosum groups B, D and G. These diseases demonstrate ultraviolet light sensitivity and general impairment of transcription: they are linked by impaired unwinding of the DNA required for both transcription and repair. The transfer of DNA into cells is sometimes accompanied by a change in sensitivity to radiation, and this is of special interest when this is the same genetic change seen in tumors. DNA repair has a close relationship with the cell cycle and cell cycle arrest in response to damage may determine sensitivity to that damage. DNA repair mechanisms in response to a variety of drugs and types of radiation can be difficult to study because of the inability to target the damage to defined sequences in vivo and the lack of a satisfactory substrate for in vitro studies. Changes in cellular metabolism as a result of ionizing radiation can impart radiation resistance, which is usually transient in vitro, but may be more significant in vivo for tissues or tumors. The mechanisms by which damage is sensed by cells is unknown. The detection of free radicals is thought likely, but distortion to DNA structure or strand breakage and a direct effect on membranes are other possibilities for which there is evidence. Changes in extracellular ATP occur in response to damage, and this could be a direct membrane effect. External purinergic receptors can then be involved in signal transduction pathways resulting in altered levels of thiol protection or triggering apoptosis. Changes in the functional level of proteins as a consequence of ionizing radiation include transcription factors, for example c-jun and c-fos; cell cycle arrest proteins such as GADD (growth arrest and DNA damage inducible proteins) and p53; growth factors such as FGF, PDGF; and other proteins leading to radioresistance.(ABSTRACT TRUNCATED AT 400 WORDS)