Protein kinase C translocation in human blood platelets

Protein kinase C (PKC) activity and translocation in response to the phorbol ester, phorbol 12-myristate, 13-acetate (PMA), serotonin (5-HT) and thrombin was assessed in human platelets. Stimulation with PMA and 5-HT for 10 minutes or thrombin for 1 minute elicited platelet PKC translocation from cytosol to membrane. The catecholamines, norepinephrine or epinephrine at 10 microM concentrations did not induce redistribution of platelet PKC. Serotonin (0.5-100 microM) and the specific 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (10-100 microM) but not the 5-HT1A or 5-HT1B agonists, (+/-) 8-hydroxy-dipropylamino-tetralin (8-OH-DPAT) or 5-methoxy-3-3-(1,2,3,6-tetrahydro-4-pyridin) 1H-indole succinate (RU 24969) induced dose-dependent PKC translocations. Serotonin-evoked PKC translocation was blocked by selective 5-HT2 receptor antagonists, ketanserin and spiroperidol. These results suggest that, in human platelets, PMA, thrombin and 5-HT can elicit PKC translocation from cytosol to membrane. Serotonin-induced PKC translocation in platelets is mediated via 5-HT2 receptors.     

Caldesmon phosphorylation in intact human platelets by cAMP-dependent protein kinase and protein kinase C

Caldesmon is a calmodulin- and actin-binding protein present in both smooth and non-muscle tissue. The present study demonstrates that platelet caldesmon is a substrate for cAMP-dependent protein kinase (protein kinase A). Purified platelet caldesmon has an apparent molecular mass of 82 kDa on sodium dodecyl sulfate-polyacrylamide gels and can be phosphorylated in vitro by the catalytic subunit of protein kinase A to a level of 2 mol of phosphate/mol of caldesmon. Phosphorylation of caldesmon by protein kinase A results in a shift in the apparent molecular mass of the protein to 86 kDa. When caldesmon was immunoprecipitated from intact platelets treated with prostacyclin (PGI2) the same shift in apparent molecular mass of caldesmon was observed. Comparison of two-dimensional tryptic phosphopeptide maps of caldesmon phosphorylated in vitro by protein kinase A with caldesmon immunoprecipitated from intact platelets verified that protein kinase A was responsible for the observed increase in caldesmon phosphorylation in PGI2-treated platelets. The present study demonstrates that although caldesmon is basally phosphorylated in the intact platelet, activation of protein kinase A by PGI2 results in the significant incorporation of phosphate into two new sites. In addition, the effects of phorbol ester, collagen, and thrombin on caldesmon phosphorylation were also examined. Although phorbol ester treatment results in a significant increase in caldesmon phosphorylation apparently by protein kinase C, treatment of intact platelets with thrombin or collagen does not result in an increase in caldesmon phosphorylation.     

Two thrombin-activated Ca2+ channels in human platelets.

The regulation of extracellular Ca2+ entry into fura-2-loaded human platelets was examined following stimulation with thrombin. In the presence of external Ca2+, stimulation of platelets with thrombin resulted in a rapid increase, followed by a plateau, in intracellular Ca2+ concentration ([Ca2+]i). Pretreatment with wortmannin, a specific inhibitor of myosin light chain kinase, suppressed only the plateau phase and had no effect on the initial rapid increase in [Ca2+]i. In Ca(2+)-free EGTA buffer, thrombin induced a transient and relatively small increase in [Ca2+]i caused by Ca2+ release from internal stores. When Ca2+ was added subsequently to the Ca(2+)-free medium within 10 min after thrombin activation, a marked increase in [Ca2+]i was seen, reflecting thrombin-stimulated external Ca2+ entry. With the Ca(2+)-free medium, wortmannin did not affect either the Ca2+ mobilization from the internal stores or the rapid external Ca2+ entry at early time points (within 5 s) after thrombin stimulation, whereas it significantly inhibited Ca2+ entry when Ca2+ was added later (at 3 min). Wortmannin inhibition of this late Ca2+ entry and that of 20-kDa myosin light chain phosphorylation after thrombin stimulation were dose- and preincubation time-dependent and correlated well with each other. These results suggest that two different channels are responsible for Ca2+ entry in human platelets at the early and late phases of thrombin stimulation and that the channel responsible for the late phase of Ca2+ entry may be activated by a mechanism involving myosin light chain kinase.     

Protein phosphorylation and activation of human platelets by sodium fluoride

The ability of sodium fluoride (NaF) and thrombin to stimulate aggregation and protein phosphorylation in intact human platelets was measured and compared. When platelets were stimulated by NaF, phosphorylation of the 20 KDa protein was transient and after 5-10 min returned to the same level as that of unstimulated cells. On the other hand, 47 KDa protein was slowly phosphorylated without obvious dephosphorylation. The slow activation of the 47 KDa protein phosphorylation correlated well with the time required for the aggregation and secretion. Phosphoamino acid analysis showed that the phosphorylated amino acids of the 47 KDa protein from platelets activated by NaF and thrombin were slightly different. These results suggest that different stimuli may lead to the same protein phosphorylation by different biochemical mechanisms of action.     

Protein kinase C catalyzes phosphorylation of guanylate cyclase in vitro

Protein kinase C catalyzes phosphorylation of purified rat brain guanylate cyclase. The phosphorylation is marked by concomitant increase in guanylate cyclase activity. TPA further enhances both phosphorylation and activity of guanylate cyclase. Data seem to provide clues to the molecular mechanism of one of the transformation-like responses mimicked by 12-O-tetradecanoylphorbol-13-acetate, i.e. the elevation of cyclic GMP. It is envisaged that protein kinase C may have a central role in the understanding of molecular events triggering carcinogenesis.     

Properties of protein kinase C subspecies in human platelets

Protein kinase C (PKC) from human platelets was resolved into two fractions by hydroxyapatite column chromatography. One of the enzymes was indistinguishable from the brain type III PKC having alpha-sequence in its kinetic and immunological properties. The other enzyme was kinetically different from any of the brain PKC subspecies so far isolated, although it resembled the brain type II PKC having beta-sequence. With H1 histone as substrate, this platelet enzyme was not very sensitive to Ca2+, and activated partly by phosphatidylserine plus diacylglycerol or by free arachidonic acid. Both platelet enzymes could phosphorylate the P47 protein in vitro, but the enzyme physiologically responsible for the P47 protein phosphorylation in the activated platelets remains to be identified.     

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.     

Synthesis of phosphatidylinositol 3,4-bisphosphate but not phosphatidylinositol 3,4,5-trisphosphate is closely correlated with protein-tyrosine phosphorylation in thrombin-activated human platelets.

Synthesis of D-3-phosphorylated phosphoinositides and its correlation with protein-tyrosine phosphorylation were examined using human platelets. Thrombin stimulation of platelets resulted in time- and dose-dependent production of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2), which is absent from resting platelets. In contrast, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) was detected in resting platelets, but remained unaffected by thrombin treatment. The production of PtdIns(3,4)P2 but not PtdIns(3,4,5)P3 was inhibited by pretreatment with staurosporine or dibutyryl cyclic adenosine monophosphate (dbcAMP). Protein-tyrosine phosphorylation, which is reportedly involved in generation of 3-phosphorylated phosphoinositides, was elicited in thrombin-activated platelets. The tyrosine phosphorylation was suppressed by pretreatment with staurosporine or dbcAMP. These observations suggest that synthesis of PtdIns(3,4)P2 but not PtdIns(3,4,5) P3 is closely correlated with protein-tyrosine phosphorylation in human platelets.     

Protein kinase C induces phosphorylation and desensitization of the human 5-HT1A receptor

The effects of short-term phorbol ester treatment of CHO cells that stably express 900 fmol of recombinant human serotonin 5-HT1A receptor/mg of protein on coupling to the inhibition of adenylyl cyclase and on phosphorylation of the receptor were studied. Pretreatment of cell monolayers with phorbol 12-myristate 13-acetate (PMA) caused a dose- and time-dependent shift of the half-maximal dose of serotonin (5-HT) required to inhibit membrane adenylyl cyclase (from IC50 approximately 100 nM to approximately 400 nM). This desensitization (shift in IC50) was rapid, occurring with 5 min of pretreatment and being maximal by 10-15 min; it was also dose-dependent, being half-maximal at approximately 300 nM PMA. Desensitization was also induced by sn-dioctanoylglycerol (DiC8) and blocked by the protein kinase C (PKC) inhibitors sphingosine and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7). In detached permeabilized cells, PMA pretreatment caused a rapid phosphorylation of immunoprecipitated 5-HT1A receptors, with an approximately 3-4-fold increase that was maximal after 15 min and persisted for 90 min. The phosphorylation occurred at a similar dose of PMA as that which induced desensitization (half-maximal at approximately 300 nM, maximal at 500 nM to 1 microM), could be reproduced by pretreatment with the PKC activators DiC8 or phorbol 12,13-dibutyrate (PDBu), and could be blocked by the PKC inhibitors sphingosine or H-7. The stoichiometry of the phosphorylation was approximately 2 mol of [32P]ATP/mol of receptor, suggesting the involvement at least two of three putative PKC sites within the 5-HT1A receptor. The close concordance between the PKC-induced desensitization and phosphorylation suggests a potential causative link between these two effects of PKC on the human 5-HT1A receptor.     

Protein kinase C activation in human ventricular myocardium

We investigated the effects of 12-deoxyphorbol 13 isobutyrate 20 acetate (DPBA) on contractile function and intracellular calcium handling in normal human ventricular myocardium. The activation of protein kinase C by DPBA resulted in a decrease in sarcoplasmic reticulum calcium release and a reduction in isometric twitch. Force-Calcium relationships were obtained by tetanizing intact muscles or by chemically skinning muscle fibers. These relationships were fitted to a modified Hill function. In intact preparations, DPBA shifted the force-calcium relationship towards higher intracellular calcium concentrations by 0.12 microM (n = 5) and maximal force production was decreased 45.5 +/- 6.1%. These experiments show that protein kinase C activation affects intracellular calcium availability and myofibrillar calcium responsiveness.     

Protein kinase C mediates human neutrophil cytotoxicity

Human neutrophils stimulated with phorbol myristate acetate were able to damage human erythroleukemic K562 cells, in the absence of specific antibody, as assessed by a two hour 51Cr release assay. Neutrophils treated with formyl-peptide fMet-Leu-Phe did not display tumoricidal response, but the addition of diacylglycerol kinase inhibitor R59022 together with formyl-peptide induced the cytotoxic capacity against tumor target cells. Phorbol ester is a potent activator of certain functions of neutrophils because of its ability to directly and irreversibly stimulate protein kinase C; formyl-peptide, on the contrary, activates protein kinase C by inducing a rapid and transient production of diacylglycerol, that is quickly metabolized. The addition of an inhibitor of diacylglycerol kinase, R59022, however potentiated the action of formyl-peptide. These results indicate that protein kinase C is involved in the tumoricidal activity of neutrophils against K562 cells, and that maximal activation of the enzyme is required to achieve the cytotoxic response.     

Protein kinase C regulates the phosphorylation and cellular localization of occludin

Occludin is an integral membrane phosphoprotein specifically associated with tight junctions, contributing to the structure and function of this intercellular seal. Occludin function is thought to be regulated by phosphorylation, but no information is available on the molecular pathways involved. In the present study, the involvement of the protein kinase C pathway in the regulation of the phosphorylation and cellular distribution of occludin has been investigated. Phorbol 12-myristate 13-acetate and 1,2-dioctanoylglycerol induced the rapid phosphorylation of occludin in Madin-Darby canine kidney cells cultured in low extracellular calcium medium with a concomitant translocation of occludin to the regions of cell-cell contact. The extent of occludin phosphorylation as well as its incorporation into tight junctions induced by protein kinase C activators or calcium switch were markedly decreased by the protein kinase C inhibitor GF-109203X. In addition, in vitro experiments showed that the recombinant COOH-terminal domain of murine occludin could be phosphorylated by purified protein kinase C. Ser(338) of occludin was identified as an in vitro protein kinase C phosphorylation site using peptide mass fingerprint analysis and electrospray ionization tandem mass spectroscopy. These findings indicate that protein kinase C is involved in the regulation of occludin function at tight junctions.     

Evidence for coupling of phospatidic acid formation and calcium influx in thrombin-activated human platelets

The time-sequential relationship between Ca²⁺ flux, phospholipid metabolism and platelet activation have been examined. Thrombin-activation caused a marked enhancement in ⁴⁵Ca²⁺ influx and a decrease in extracellular Ca²⁺ concentration measured by murexide dye, which occurred in parallel with the conversion of 1,2-diacylglycerol (DG) to phosphatidic acid (PA). The incorporated ⁴⁵Ca²⁺ was located mainly in cytosolic fraction. The influx of Ca²⁺ was observed to commence prior to the onset of lysophospholipids formation and subsequent liberation of arachidonic acid. These data provide evidence which indicates a coupling between the rapid PI-turnover and the active Ca²⁺ influx, in which phosphatidic acid (PA) may serve as a Ca²⁺ ionophore.     

Diglyceride kinase in human platelets

Human platelets contain diglyceride kinase, an enzyme that catalyzes the phosphorylation of diacylglycerol by adenosine 5'-triphosphate to yield phosphatidic acid. The majority of the platelet enzyme is particulate-bound, and membrane fractions of platelet homogenates have a higher specific activity than granule fractions. Both deoxycholate and magnesium are necessary for optimal enzyme activity. The K(m) of the enzyme for adenosine 5'-triphosphate is 1.3 mm, and the apparent K(m) for diacylglycerol is 0.4 mm. The pH optimum is 6.6-6.8 in imidazole-HCl or maleate-NaOH buffer. The enzyme activity of platelets from normal subjects was similar to the activity from patients with renal and hepatic failure.     

Protein kinase C enhances Ca2+ mobilization in human platelets by activating Na+/H+ exchange

Control of cytoplasmic pH (pHi) by a Na+/H+ antiport appears a general property of most eukaryotic cells. In human platelets activation of the Na+/H+ exchanger enhances Ca2+ mobilization and aggregation induced by low concentrations of thrombin (Siffert, W., and Akkerman, J. W. N. (1987) Nature 325, 456-458). Several observations indicate that the exchanger is regulated by protein kinase C. (i) Inhibitors of protein kinase C (trifluoperazine, sphingosine) inhibit the increase in pHi seen during thrombin stimulation as well as Ca2+ mobilization; artificially increasing pHi by monensin or NH4Cl then restores Ca2+ mobilization. (ii) Direct activation of protein kinase C by 1-oleoyl-2-acetylglycerol initiates an increase in pHi that depends on the presence of extracellular Na+ and is sensitive to inhibition by ethylisopropylamiloride. The pHi sensitivity of thrombin-induced Ca2+ mobilization is particularly evident in the range between pH 6.8 and 7.4 and at low thrombin concentrations, whereas thrombin concentrations of more than 0.2 unit/ml bypass the pH sensitivity. In the absence of thrombin an increase in pHi, either induced artificially (by addition of the ionophores nigericin or monensin) or via activation of protein kinase C (by addition of 1-oleoyl-2-acetylglycerol), does not induce Ca2+ mobilization. We conclude that activation of protein kinase C is essential for Ca2+ mobilization in platelets stimulated by low concentrations of thrombin and that protein kinase C exerts this effect via activation of the Na+/H+ exchanger.     

Protein kinase C delta is required for p47phox phosphorylation and translocation in activated human monocytes

Our laboratory is interested in understanding the regulation of NADPH oxidase activity in human monocyte/macrophages. Protein kinase C (PKC) is reported to be involved in regulating the phosphorylation of NADPH oxidase components in human neutrophils; however, the regulatory roles of specific isoforms of PKC in phosphorylating particular oxidase components have not been determined. In this study calphostin C, an inhibitor for both novel PKC (including PKCdelta, -epsilon, -theta;, and -eta) and conventional PKC (including PKCalpha and -beta), inhibited both phosphorylation and translocation of p47phox, an essential component of the monocyte NADPH oxidase. In contrast, GF109203X, a selective inhibitor of classical PKC and PKCepsilon, did not affect the phosphorylation or translocation of p47phox, suggesting that PKCdelta, -theta;, or -eta is required. Furthermore, rottlerin (at doses that inhibit PKCdelta activity) inhibited the phosphorylation and translocation of p47phox. Rottlerin also inhibited O2 production at similar doses. In addition to pharmacological inhibitors, PKCdelta-specific antisense oligodeoxyribonucleotides were used. PKCdelta antisense oligodeoxyribonucleotides inhibited the phosphorylation and translocation of p47phox in activated human monocytes. We also show, using the recombinant p47phox-GST fusion protein, that p47phox can serve as a substrate for PKCdelta in vitro. Furthermore, lysate-derived PKCdelta from activated monocytes phosphorylated p47phox in a rottlerin-sensitive manner. Together, these data suggest that PKCdelta plays a pivotal role in stimulating monocyte NADPH oxidase activity through its regulation of the phosphorylation and translocation of p47phox.     

Protein kinase C-related kinase 2 regulates hepatitis C virus RNA polymerase function by phosphorylation

The hepatitis C virus (HCV) NS5B protein is the viral RNA-dependent RNA polymerase required for replication of the HCV RNA genome. We have identified a peptide that most closely resembles a short region of the protein kinase C-related kinase 2 (PRK2) by screening of a random 12-mer peptide library displayed on the surface of the M13 bacteriophage with NS5B proteins immobilized on microwell plates. Competitive phage enzyme-linked immunosorbent assay with a synthetic peptide showed that the phage clone displaying this peptide could bind HCV RNA polymerase with a high affinity. Coimmunoprecipitation and colocalization studies demonstrated in vivo interaction of NS5B with PRK2. In vitro kinase assays demonstrated that PRK2 specifically phosphorylates NS5B by interaction with the N-terminal finger domain of NS5B (amino acids 1-187). Consistent with the in vitro NS5B-phosphorylating activity of PRK2, we detected the phosphorylated form of NS5B by metabolic cell labeling. Furthermore, HCV NS5B immunoprecipitated from HCV subgenomic replicon cells was specifically recognized by an antiphosphoserine antibody. Knock-down of the endogenous PRK2 expression using a PRK2-specific small interfering RNA inhibited HCV RNA replication. In contrast, PRK2 overexpression, which was accompanied by an increase of in the level of its active form, dramatically enhanced HCV RNA replication. Altogether, our results indicate that HCV RNA replication is regulated by NS5B phosphorylation by PRK2.     

Alterations in C3 activation and binding caused by phosphorylation by a casein kinase released from activated human platelets.

A casein kinase released from activated human platelets phosphorylates a number of plasma proteins extracellularly, and that activation of platelets in systemic lupus erythematosus patients parallels an increase in the phosphate content of plasma proteins, including C3. The present study was undertaken to characterize this platelet protein kinase and to further elucidate the effect(s) on C3 function of phosphorylation by platelet casein kinase. The phosphate content of human plasma C3 was increased from 0.15 to 0.60 mol phosphate/mol of C3 after platelet activation in whole blood or platelet-rich plasma. The platelet casein kinase was distinct from other casein kinases in terms of its dependence on cations, inhibition by specific protein kinase inhibitors, and immunological reactivity. C3 that had been phosphorylated with platelet casein kinase was tested for its susceptibility to cleavage by trypsin or the classical and alternative pathway convertases and its binding to EAC and IgG. Phosphorylation did not affect the cleavage of C3 into C3a and C3b, but the binding of fragments from phosphorylated C3 to EAC14oxy2 cells and to IgG in purified systems and in serum was increased by 1.6-4.5 times over that of unphosphorylated C3. A covariation was seen between the enhanced binding of C3 fragments to IgG after phosphorylation and an increased ratio of glycerol/glycine binding, from 2.0 for unphosphorylated C3 to 4.9 for phosphorylated C3. The present study suggests that an overall effect of phosphorylation of C3 by platelet casein kinase is to enhance the opsonization of immune complexes.     

Protein kinase C mediates platelet-derived growth factor-induced tyrosine phosphorylation of p42

One of the early events after stimulation of Swiss 3T3 cells with either platelet-derived growth factor (PDGF), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), diacylglycerol, or several other mitogens is the near stoichiometric phosphorylation at tyrosine and serine of a scarce cytoplasmic protein (p42). TPA and diacylglycerol are known to directly stimulate the activity of a protein-serine/threonine kinase, protein kinase C (PKC). PDGF and several other mitogens stimulate tyrosine kinases directly and PKC indirectly. We have therefore examined the involvement of PKC in p42 tyrosine phosphorylation in Swiss 3T3 cells. Firstly, six agents which stimulated phosphorylation of p42 also stimulated phosphorylation of a known PKC substrate, an 80,000-Mr protein (p80). Secondly, in PKC-deficient cells (cells in which PKC activity was reduced to undetectable levels by prolonged exposure to TPA), PDGF-induced p42 phosphorylation was reduced three- to fourfold. Phosphoamino acid analysis of phosphorylated p42 from PDGF-stimulated PKC-deficient cells revealed primarily phosphoserine and only a trace of phosphotyrosine, suggesting that the reduction in PDGF-stimulated tyrosine phosphorylation of p42 resulting from PKC deficiency is greater than three- to fourfold. Finally, comparison of antiphosphotyrosine immunoprecipitates of PKC-deficient versus naive cells revealed that most other PDGF-induced tyrosine phosphorylation events were quite similar. These data suggest that mitogens such as PDGF, which directly stimulate phosphorylation of some proteins at tyrosine, induce p42 tyrosine phosphorylation via a cascade of events involving PKC.     

Activators of protein kinase C decrease serotonin transport in human platelets.

Treatment of human platelets with activators of protein kinase C (PKC) for 5-20 min resulted in substantial reductions in the rate of platelet serotonin (5-HT) transport. The mean Vmax observed after 5 min treatment with 1 microM 4-beta-12-tetradecanoylphorbol 13-acetate (beta-TPA) was 66% (n = 16, P = 0.0001) of the control value. 5 min of treatment with 1 microM mezerein reduced uptake to 78% (n = 3, P = 0.01) of control. Both beta-TPA and mezerein had little effect on the Km of transport and had EC50 values of approx. 100 mM when a 20-min treatment period was used. The maximum effects of both were reached at approx. 20 min and could be blocked with staurospine. The beta-TPA effect was stereospecific, as alpha-TPA did not alter platelet 5-HT uptake. Although the PKC activators may have altered transmembrane ion-gradients for Na+ and Cl-, which are co-transported with 5-HT, minimizing ion-gradient changes had little effect on the observed reductions in transport. The PKC activators also had little or no effect on platelet 5-HT release or on the number (Bmax) of 5-HT transporters expressed at the platelet surface. The data indicate that PKC activation may down-regulate the activity of the 5-HT transporter in platelets. Apparently, most of this effect is mediated through mechanisms other than changes in ion-gradients, reductions in the number of available transporters, or increased 5-HT release. The apparent regulation of 5-HT transport by PKC may have important implications in platelet and neuronal functioning.