Inhibition of Protein Kinase C by Staurosporine Promotes Elevated Accumulations of Inositol Trisphosphates and Tetrakisphosphate in Human Platelets Exposed to Thrombin

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

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Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

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P2X receptors do not contribute to the generation of Ca²⁺ oscillations

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Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

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Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

A Tale of two receptors

Calcium (Ca²⁺) oscillations in hepatocytes have a wide dynamic range. In particular, recent experimental evidence shows that agonist stimulation of the P2Y family of receptors leads to qualitatively diverse Ca²⁺ oscillations. We present a new model of Ca²⁺ oscillations in hepatocytes based on these experiments to investigate the mechanisms controlling P2Y-activated Ca²⁺ oscillations. The model accounts for Ca²⁺ regulation of the IP₃ receptor (IP₃R), the positive feedback from Ca²⁺ on phospholipase C (PLC) and the P2Y receptor phosphorylation by protein kinase C (PKC). Furthermore, PKC is shown to control multiple cellular substrates. Utilising the model, we suggest the activity and intensity of PLC and PKC necessary to explain the qualitatively diverse Ca²⁺ oscillations in response to P2Y receptor activation.

Conventional protein kinase C isoforms differentially regulate ADP- and thrombin-evoked Ca²⁺ signalling in human platelets

Rises in cytosolic Ca(2+) concentration ([Ca(2+)]cyt) are central in platelet activation, yet many aspects of the underlying mechanisms are poorly understood. Most studies examine how experimental manipulations affect agonist-evoked rises in [Ca(2+)]cyt, but these only monitor the net effect of manipulations on the processes controlling [Ca(2+)]cyt (Ca(2+) buffering, sequestration, release, entry and removal), and cannot resolve the source of the Ca(2+) or the transporters or channels affected. To investigate the effects of protein kinase C (PKC) on platelet Ca(2+) signalling, we here monitor Ca(2+) flux around the platelet by measuring net Ca(2+) fluxes to or from the extracellular space and the intracellular Ca(2+) stores, which act as the major sources and sinks for Ca(2+) influx into and efflux from the cytosol, as well as monitoring the cytosolic Na(+) concentration ([Na(+)]cyt), which influences platelet Ca(2+) fluxes via Na(+)/Ca(2+) exchange. The intracellular store Ca(2+) concentration ([Ca(2+)]st) was monitored using Fluo-5N, the extracellular Ca(2+) concentration ([Ca(2+)]ext) was monitored using Fluo-4 whilst [Ca(2+)]cyt and [Na(+)]cyt were monitored using Fura-2 and SFBI, respectively. PKC inhibition using Ro-31-8220 or bisindolylmaleimide I potentiated ADP- and thrombin-evoked rises in [Ca(2+)]cyt in the absence of extracellular Ca(2+). PKC inhibition potentiated ADP-evoked but reduced thrombin-evoked intracellular Ca(2+) release and Ca(2+) removal into the extracellular medium. SERCA inhibition using thapsigargin and 2,5-di(tert-butyl) l,4-benzohydroquinone abolished the effect of PKC inhibitors on ADP-evoked changes in [Ca(2+)]cyt but only reduced the effect on thrombin-evoked responses. Thrombin evokes substantial rises in [Na(+)]cyt which would be expected to reduce Ca(2+) removal via the Na(+)/Ca(2+) exchanger (NCX). Thrombin-evoked rises in [Na(+)]cyt were potentiated by PKC inhibition, an effect which was not due to altered changes in non-selective cation permeability of the plasma membrane as assessed by Mn(2+) quench of Fura-2 fluorescence. PKC inhibition was without effect on thrombin-evoked rises in [Ca(2+)]cyt following SERCA inhibition and either removal of extracellular Na(+) or inhibition of Na(+)/K(+)-ATPase activity by removal of extracellular K(+) or treatment with digoxin. These data suggest that PKC limits ADP-evoked rises in [Ca(2+)]cyt by acceleration of SERCA activity, whilst rises in [Ca(2+)]cyt evoked by the stronger platelet activator thrombin are limited by PKC through acceleration of both SERCA and Na(+)/K(+)-ATPase activity, with the latter limiting the effect of thrombin on rises in [Na(+)]cyt and so forward mode NCX activity. The use of selective PKC inhibitors indicated that conventional and not novel PKC isoforms are responsible for the inhibition of agonist-evoked Ca(2+) signalling.

Inositol phosphate metabolism and platelet activation

Platelet activation by thrombin and most other agonists appears to require two second messenger systems that are both initiated by phospholipase C-catalysed cleavage of phosphatidylinositol phosphates leading to: 1. formation of inositol phosphates with a subsequent rise in intracellular calcium from intracellular stores and from outside the cell; 2. formation of diacylglycerol with subsequent activation of protein kinase C. This review examines inositol phosphate metabolism in platelets and its involvement in calcium metabolism.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Endogenous prostaglandin F2α concentrations in bovine whole semen, seminal plasma, and extended semen

A series of experiments were conducted to quantify PGF2α in bovine semen, seminal plasma, and extended semen, and to determine if PGF2α was synthesized or released during extension of bovine semen. Concentrations of PGF2α were measured in paired samples of whole and extended semen from beef and dairy bulls. Concentrations of PGF2α did not differ between beef and dairy (mean±SEM, 273.8±42.8 vs. 210.3±18.5 pg/mL, respectively; P=0.12), but tended (P=0.08) to be greater for whole compared with extended semen (255.5±29.8 vs. 194.5±17.0 pg/mL). Whole semen was extended at eight dilution rates (regardless of initial sperm concentration), using a diluent consisting of two fractions. Samples collected after each dilution step resulted in four subsamples. Concentrations of PGF2α in subsamples decreased (P<0.001) at higher dilution rates and later steps of extension. Subsequently, whole semen and seminal plasma were extended at three dilution rates. Initial PGF2α concentration was greater (P<0.001) for whole semen compared with seminal plasma. During extension, PGF2α synthesis or release resulted in less disparity, but the amount synthesized or released was greater (P=0.03) for semen compared with seminal plasma. We concluded that synthesis or release of PGF2α during extension resulted in concentrations similar to whole semen.

PKC inhibition markedly enhances Ca2+ signaling and phosphatidylserine exposure downstream of protease-activated receptor-1 but not protease-activated receptor-4 in human platelets

BACKGROUND

Cytosolic calcium concentration is a critical regulator of platelet activation, and so platelet Ca(2+) signaling must be tightly controlled. Thrombin-induced Ca(2+) signaling is enhanced by inhibitors of protein kinase C (PKC), suggesting that PKC negatively regulates the Ca(2+) signal, although the mechanisms by which this occurs and its physiological relevance are still unclear.

OBJECTIVES

To investigate the mechanisms by which PKC inhibitors enhance thrombin-induced Ca(2+) signaling, and to determine the importance of this pathway in platelet activation.

METHODS

Cytosolic Ca(2+) signaling was monitored in fura-2-loaded human platelets. Phosphatidylserine (PS) exposure, a marker of platelet procoagulant activity, was measured by annexin V binding and flow cytometry.

RESULTS

PKC inhibition by bisindolylmaleimide-I (BIM-I) enhanced α-thrombin-induced Ca(2+) signaling in a concentration-dependent manner. PAR1 signaling, activated by SFLLRN, was enhanced much more strongly than PAR4, activated by AYPGKF or γ-thrombin, which is a potent PAR4 agonist but a poor activator of PAR1. BIM-I had little effect on α-thrombin-induced signaling following treatment with the PAR1 antagonist, SCH-79797. BIM-I enhanced Ca(2+) release from intracellular stores and Ca(2+) entry, as assessed by Mn(2+) quench. However, the plasma membrane Ca(2+) ATPase inhibitor, 5(6)-carboxyeosin, did not prevent the effect of BIM-I. PKC inhibition strongly enhanced α-thrombin-induced PS exposure, which was reversed by blockade of PAR1.

CONCLUSIONS

Together, these data show that when PAR1 is stimulated, PKC negatively regulates Ca(2+) release and Ca(2+) entry, which leads to reduced platelet PS exposure.

Submaximal inhibition of protein kinase C restores ADP-induced dense granule secretion in platelets in the presence of Ca2+

Protein kinase C (PKC) is a family of serine/threonine kinases that play isoform-specific inhibitory and stimulatory roles in platelet activation. We show here that the pan-PKC inhibitor Ro31-8220 can be used to dissect these events following platelet activation by ADP. Submaximal concentrations of Ro31-8220 potentiated aggregation and dense granule secretion to ADP in plasma anticoagulated with citrate, in d-Phe-Pro-Arg-chloromethyl ketone-anticoagulated plasma, which has physiological levels of Ca²⁺, and in washed platelets. Potentiation was retained on inhibition of cyclooxygenase and was associated with an increase in intracellular Ca²⁺. Potentiation of aggregation and secretion was abolished by a maximally effective concentration of Ro31-8220, consistent with a critical role of PKC in secretion. ADP-induced secretion was potentiated in the presence of an inhibitor of PKCβ but not in the presence of available inhibitors of other PKC isoforms in human and mouse platelets. ADP-induced secretion was also potentiated in mouse platelets deficient in PKCϵ but not PKCθ. These results demonstrate that partial blockade of PKC potentiates aggregation and dense granule secretion by ADP in association with increased Ca²⁺. This provides a molecular explanation for the inability of ADP to induce secretion in plasma in the presence of physiological Ca²⁺ concentrations, and it reveals a novel role for PKC in inhibiting platelet activation by ADP in vivo. These results also demonstrate isoform-specific inhibitory effects of PKC in platelets.

Dual Role of Platelet Protein Kinase C in Thrombus Formation

Protein kinase C (PKC) isoforms regulate many platelet responses in a still incompletely understood manner. Here we investigated the roles of PKC in the platelet reactions implicated in thrombus formation as follows: secretion aggregate formation and coagulation-stimulating activity, using inhibitors with proven activity in plasma. In human and mouse platelets, PKC regulated aggregation by mediating secretion and contributing to alphaIIbbeta3 activation. Strikingly, PKC suppressed Ca(2+) signal generation and Ca(2+)-dependent exposure of procoagulant phosphatidylserine. Furthermore, under coagulant conditions, PKC suppressed the thrombin-generating capacity of platelets. In flowing human and mouse blood, PKC contributed to platelet adhesion and controlled secretion-dependent thrombus formation, whereas it down-regulated Ca(2+) signaling and procoagulant activity. In murine platelets lacking G(q)alpha, where secretion reactions were reduced in comparison with wild type mice, PKC still positively regulated platelet aggregation and down-regulated procoagulant activity. We conclude that platelet PKC isoforms have a dual controlling role in thrombus formation as follows: (i) by mediating secretion and integrin activation required for platelet aggregation under flow, and (ii) by suppressing Ca(2+)-dependent phosphatidylserine exposure, and consequently thrombin generation and coagulation. This platelet signaling protein is the first one identified to balance the pro-aggregatory and procoagulant functions of thrombi.

Species differences in platelet responses to thrombin and SFLLRN. receptor-mediated calcium mobilization and aggregation, and regulation by protein kinases

The thrombin receptor on human platelets is activated by thrombin to stimulate platelet aggregation through the tethered ligand SFLLRN. This study examined the effects of thrombin and SFLLRN on aggregation and calcium mobilization ([Ca2+]i) in rat, guinea pig, rabbit, dog, monkey, and human platelets, and the role of protein kinases in regulating these functions. Thrombin induced platelet aggregation and [Ca2+]i in all species studied; however, only guinea pig, monkey and human platelets were responsive to SFLLRN. Similar species specific effects were obtained with [Ca2+]i studies. The kinetic profile for [Ca2+]i differed among species, suggesting that regulatory mechanisms for calcium differed between agonists and among species. Staurosporine, a non-selective inhibitor of protein kinases, inhibited platelet aggregation induced by thrombin or SFLLRN in all species. Staurosporine inhibited thrombin-induced [Ca2+]i in guinea pigs, had no effect in rat, and increased [Ca2+]i in all other species. Staurosporine inhibited SFLLRN-induced [Ca2+]i in guinea pig, yet had no effect in monkey or human. Tyrphostin 23, a specific inhibitor of tyrosine protein kinases, inhibited thrombin-induced aggregation of rabbit, monkey, dog and human platelets. SFLLRN-induced aggregation was also inhibited by tyrphostin 23. Tyrphostin 23 inhibited [Ca2+]i induced by either thrombin or SFLLRN in all species. Based on the differential response to agonist stimulation, we propose that thrombin can activate platelets via SFLLRN-dependent and independent mechanisms, which could involve yet unrecognized subtypes of the thrombin receptor or distinct cellular activating mechanisms. Furthermore, differential regulation of calcium mobilization and aggregation was observed in those platelets responding to either thrombin or SFLLRN.

Staurosporine mobilizes Ca(2+) from secretory granules by inhibiting protein kinase C in rat submandibular acinar cells

Staurosporine was previously shown to mobilize Ca(2+) from the thapsigargin-insensitive Ca(2+) store in rat submandibular acinar cells. However, the nature of the store is not yet known. Therefore, in the present study, the staurosporine-releasable intracellular Ca(2+) store was characterized. Staurosporine increased the cytosolic Ca(2+) concentration ([Ca(2+)](c)) after the inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) store was depleted. Ionomycin caused only small increases in [Ca(2+)](c) after the depletion of the IP(3)-sensitive Ca(2+) store, whereas ionomycin+monensin caused large increases. However, ionomycin+monensin did not increase [Ca(2+)](c) when added after [Ca(2+)](c) was increased by staurosporine, indicating that the acidic Ca(2+) store was the main source of Ca(2+). The acidic Ca(2+) store appeared to be associated with secretory granules, since ionomycin+monensin- and staurosporine-induced [Ca(2+)](c) increases were significantly reduced when the acinar cells were degranulated. The effect of staurosporine on [Ca(2+)](c) was mimicked by other protein kinase C inhibitors. Therefore, we conclude that staurosporine mobilizes Ca(2+) from secretory granules, probably through the inhibition of protein kinase C in rat submandibular acinar cells.

Dual regulation of calcium mobilization by inositol 1,4, 5-trisphosphate in a living cell

Changes in cytosolic free calcium ([Ca(2+)](i)) often take the form of a sustained response or repetitive oscillations. The frequency and amplitude of [Ca(2+)](i) oscillations are essential for the selective stimulation of gene expression and for enzyme activation. However, the mechanism that determines whether [Ca(2+)](i) oscillates at a particular frequency or becomes a sustained response is poorly understood. We find that [Ca(2+)](i) oscillations in rat megakaryocytes, as in other cells, results from a Ca(2+)-dependent inhibition of inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release. Moreover, we find that this inhibition becomes progressively less effective with higher IP(3) concentrations. We suggest that disinhibition, by increasing IP(3) concentration, of Ca(2+)-dependent inhibition is a common mechanism for the regulation of [Ca(2+)](i) oscillations in cells containing IP(3)-sensitive Ca(2+) stores.

Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate by the oocytes of Xenopus laevis

The pathway and kinetics of inositol 1,4,5-trisphosphate (IP3) metabolism were measured in Xenopus laevis oocytes and cytoplasmic extracts of oocytes. Degradation of microinjected IP3 in intact oocytes was similar to that in the extracts containing comparable concentrations of IP3 ([IP3]). The rate and route of metabolism of IP3 depended on the [IP3] and the intracellular free Ca2+ concentration ([Ca2+]). At low [IP3] (100 nM) and high [Ca2+] (>/=1 microM), IP3 was metabolized predominantly by inositol 1,4, 5-trisphosphate 3-kinase (3-kinase) with a half-life of 60 s. As the [IP3] was increased, inositol polyphosphate 5-phosphatase (5-phosphatase) degraded progressively more IP3. At a [IP3] of 8 microM or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At low [IP3] and low [Ca2+] (both </=400 nM), the activities of the 5-phosphatase and 3-kinase were comparable. The calculated range of action of IP3 in the oocyte was approximately 300 micron suggesting that IP3 acts as a global messenger in oocytes. In contrast to IP3, inositol 1,3,4, 5-tetrakisphosphate (IP4) was metabolized very slowly. The half-life of IP4 (100 nM) was 30 min and independent of the [Ca2+]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 in Xenopus oocytes was an order of magnitude or greater than that in small mammalian cells.

Expression, purification, and regulation of two isoforms of the inositol 1,4,5-trisphosphate 3-kinase

The level of inositol 1,4,5-trisphosphate in the cytoplasm is tightly regulated by two enzymes, the inositol 1,4,5,5-phosphatase and the inositol 1,4,5-trisphosphate 3-kinase. Two isoforms of the inositol 1,4,5-trisphosphate 3-kinase have been identified, the A form and the B form. The regulatory properties of the two isoforms were compared following overexpression and purification of the proteins from a v-src transformed mammalian cell line. The highly purified, recombinant inositol 1,4,5-trisphosphate 3-kinases were differentially regulated by calcium/calmodulin and via phosphorylation by protein kinase C or the cyclic AMP-dependent protein kinase. Both enzymes had similar affinities for inositol 1,4, 5-trisphosphate (Km 2-5 mu M). Calcium/calmodulin stimulated the activity of isoform A about 2.5-fold, whereas the activity of isoform B was increased 20-fold. The cyclic AMP-dependent protein kinase phosphorylated the inositol 1,4,5-trisphosphate 3-kinase A to the extent of 0.9 mol/mol and isoform B to 1 mol/mol. Protein kinase C phosphorylated isoform A to the extent of 2 mol/mol and isoform B to 2.7 mol/mol. Phosphorylation of isoform A by the cyclic AMP-dependent protein kinase caused a 2.5-fold increase in its activity when assayed in the absence of calcium/calmodulin, whereas phosphorylation by protein kinase C decreased activity by 72%. The activity of isoform B in the absence of calcium/calmodulin was not affected by phosphorylation using either kinase. When assayed in the presence of calcium/calmodulin, phosphorylation of isoform A by the cyclic AMP-dependent protein kinase increased activity 1.5-fold, whereas phosphorylation of isoform B decreased activity by 45%. Phosphorylation of either isoform A or B by protein kinase C resulted in a 70% reduction of calcium/calmodulin-stimulated activity. Differential expression and regulation of the two inositol 1,4,5-trisphosphate 3-kinase isoforms provides multiple mechanisms for regulating the cytosolic level of inositol 1,4,5-trisphosphate in cells.

Selective inhibition of collagen-induced arachidonic acid liberation by 1-(5-iodonaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7), a myosin light chain kinase inhibitor, in washed rabbit platelets

Effects of myosin light chain (MLC) kinase inhibitor ML-7 [1-(5-iodonaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine hydrochloride] and protein kinase C inhibitor H-7 [1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydro-chloride] on collagen-induced platelet activation were investigated in washed rabbit platelets. Upon stimulation with collagen (1 microg/mL), H-7 decreased protein kinase C-mediated pleckstrin phosphorylation, but had no inhibitory effect on thromboxane (TX) A2 formation or platelet aggregation. In contrast, ML-7 produced a concentration-dependent inhibition of the collagen-induced platelet aggregation and TXA2 formation by preventing arachidonic acid (AA) liberation from membrane phospholipids. However, ML-7 had little effect on AA liberation induced by thrombin, Ca2+ ionophore A-23187 or melittin, suggesting that ML-7 may affect the signal transduction pathway specific for collagen-induced AA liberation, without direct inhibition of phospholipase A2 activity. In indomethacin-treated platelets, collagen caused MLC phosphorylation and AA liberation in the absence of a significant increase in intracellular Ca2+ concentration ([Ca2+]i) or protein tyrosine phosphorylation. ML-7 inhibited both MLC phosphorylation and AA liberation induced by collagen in indomethacin-treated platelets. These results demonstrate that MLC phosphorylation and AA liberation are early events detectable in collagen-stimulated platelets, and suggest that ML-7 inhibits these early steps of collagen-induced signal transduction pathway in rabbit platelets.

Tyrosine phosphorylation and relocation of SHIP are integrin-mediated in thrombin-stimulated human blood platelets

The SH2 domain-containing inositol 5-phosphatase, SHIP, known to dephosphorylate inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate has recently been shown to be expressed in a variety of hemopoietic cells. This 145-kDa protein is induced to associate with Shc by multiple cytokines and may play an important role in the negative regulation of immunocompetent cells mediated by FcgammaRIIB receptor. We report here that SHIP is present in human blood platelets and may be involved in platelet activation evoked by thrombin. Platelet SHIP was identified by Western blotting as a single 145-kDa protein. Both phosphatidylinositol 3,4,5-trisphosphate and inositol 1,3,4, 5-tetrakisphosphate 5-phosphatase activities could be demonstrated in anti-SHIP immunoprecipitates of platelet lysate. Thrombin stimulation induced a tyrosine phosphorylation of SHIP, this effect being prevented if platelets were not shaken or if RGD-containing peptides were present, indicating an aggregation-dependent, integrin-mediated event. Moreover, although the intrinsic phosphatase activity of SHIP did not appear to be significantly increased, tyrosine-phosphorylated SHIP was relocated to the actin cytoskeleton upon activation in an aggregation- and integrin engagement-dependent manner. Finally, the striking correlation observed between phosphatidylinositol 3,4-bisphosphate production and the tyrosine phosphorylation of SHIP, as well as its relocation to the cytoskeleton upon thrombin stimulation, suggest a role for SHIP in the aggregation-dependent and GpIIb-IIIa-mediated accumulation of this important phosphoinositide.

Feedback regulation of extracellular ATP-stimulated phosphoinositide hydrolysis by protein kinase C-alpha in bovine glomerular endothelial cells

In glomerular endothelial cells, extracellular ATP stimulates a phospholipase C with subsequent hydrolysis of polyphosphoinositides and an increase in cytosolic free Ca2+ concentration ([Ca2+]i). Short-term (30 min) pretreatment of endothelial cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent activator of protein kinase C (PKC), decreases the ATP-stimulated phosphoinositide degradation and Ca2+ mobilization. However, this inhibition was lost after incubating the cells for four hours with TPA. Longer-term pretreatment (10 to 48 hr) even potentiated ATP-induced phosphoinositide breakdown and Ca2+ mobilization. In addition, pretreating the cells for 30 minutes with the specific PKC inhibitor Ro 31-8220 dose-dependently increased ATP-stimulated phosphoinositide hydrolysis, thus clearly indicating a regulatory role for PKC in the inositol lipid signaling pathway in glomerular endothelial cells. By using specific antibodies recognizing the different PKC isoenzymes, it is observed that glomerular endothelial cells express five isoenzymes: PKC-alpha, -delta, -epsilon, -zeta and -theta. No PKC-beta, -gamma, -eta and -mu isoenzymes were detected. On exposure to TPA, a complete depletion of PKC-alpha is observed within four hours. In contrast, PKC-epsilon was more resistant to phorbol ester, and even after 48 hours of TPA treatment, only 60% of PKC-epsilon was down-regulated. PKC-theta decreased very slowly from the cytosol (47% left after 24 hr of phorbol ester treatment) and translocated to the Triton X100-insoluble fraction. Moreover, PKC-delta and PKC-zeta were not significantly affected by 48 hours of phorbol ester incubation. Thus, only PKC-alpha is depleted with a kinetic that corresponds to the loss of feedback inhibition of ATP-stimulated phosphoinositide turnover. In the next step, [Ca2+]i changes were measured in single cells loaded with Fura-2 after microinjection of neutralizing PKC isoenzyme-specific antibodies. Injection of antibodies specific for PKC-alpha potently increased Ca2+ mobilization in response to ATP stimulation when compared to cells injected with buffer only or antibodies specific for PKC-epsilon. These results provide evidence that PKC-alpha mediates feedback inhibition of ATP-stimulated phosphoinositide hydrolysis in glomerular endothelial cells.

Post-translational modification of human brain type I inositol-1,4,5-trisphosphate 5-phosphatase by farnesylation

In brain, type I inositol-1,4,5-trisphosphate 5-phosphatase (InsP3 5-phosphatase) is the major isoenzyme hydrolyzing the calcium-mobilizing second messenger InsP3. Activity of this enzyme could be measured in both soluble and particulate fractions of tissue homogenates. The protein sequence showed a putative C-terminal isoprenylation site (CVVQ). In this study, two mutants have been generated. The first mutant (C409S) has a serine replacing a cysteine at position 409 of the wild-type enzyme. The second mutant (K407D1) is a deletion mutant that lacks the last five C-terminal amino acids. These constructs were individually expressed by transfection in COS-7 cells. Western blot analysis of wild-type transfected cells indicated that both soluble and particulate fractions had a 43-kDa immunoreactive band, with a higher proportion of the original homogenate associated with the particulate part. On the contrary, when the two mutated constructs were transfected in COS-7 cells, the phosphatase was predominantly soluble. Confocal immunofluorescence studies showed the wild-type enzyme to be present on the cell surface of transfected COS-7 cells and in subcellular compartments around the nucleus. This was not observed for the two mutants, where uniform immunofluorescence labeling was observed throughout the cytosol. Recombinant type I InsP3 5-phosphatase expressed in Escherichia coli was a substrate of purified farnesyltransferase. Altogether, the data therefore suggest a direct participation of Cys-409 in a C-terminally anchored InsP3 5-phosphatase by farnesylation.

Stimulation of protein kinase C redistribution and inhibition of leukotriene B4-induced inositol 1,4,5-trisphosphate generation in human neutrophils by lipoxin A4

1. To test the hypothesis that protein kinase C (PKC) is involved in the inhibitory actions of lipoxin A4 (LXA4) on second messenger generation, we studied the effects of LXA4 on PKC in human neutrophils and on leukotriene B4 (LTB4)-stimulated inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) generation. 2. LXA4, 1 microM, caused a fall in cytosolic PKC-dependent histone phosphorylating activity to 23.5% of basal levels. 3. LXA4, caused an increase in particulate PKC-dependent histone phosphorylating activity with a bell-shaped dose-response fashion; maximal stimulation was observed at 10 nM LXA4. 4. Western blot analysis with affinity-purified antibodies to alpha- and beta-PKC showed that only the beta-PKC isotype was translocated by LXA4. 5. LXA4 inhibited LTB4-stimulated Ins(1,4,5)P3 generation in a bell-shaped fashion with maximal inhibition at 1 nM LXA4. The observed inhibition was dose-dependently removed by pre-incubation with a PKC inhibitor (Ro-31-8220). 6. These results show that LXA4 activates PKC in whole cells and supports a role for PKC activation in the inhibitory action of LXA4 on LTB4-induced Ins(1,4,5)P3 generation. 7. LXA4 (1-1000 nM) pre-incubation did not affect specific binding of [3H]-LTB4 to neutrophils. Thus, the inhibitory effect of LXA4 on LTB4-stimulated Ins(1,4,5)P3 generation could not be attributed to an effect on LTB4 receptors.

C6-ceramide maintains elevated cytosolic calcium levels in activated platelets

The effects of cell-permeable C2 and C6-ceramides on human platelet responses were investigated. In thrombin-activated platelets, C6(5-30 microM) potentiated Ca2+ mobilization and Ca2+ influx, and decreased the rate of removal of Ca2+ from cytosol. The effect of C2 was not significant. Phorbol ester or calyculin A inhibition of thrombin-induced rises in platelet [Ca2+]i was attenuated by C6. Assays show that C6 either prolonged the generation, or retarded the metabolism of inositol trisphosphates. Previous studies indicate that protein kinase C (PKC) acts in a negative feedback manner by inhibiting phosphatidylinositol breakdown, accelerating inositol trisphosphate metabolism, and increasing Ca2+ pump activity. C6 may counter these PKC effects indirectly. The synthetic ceramides inhibited platelet aggregation weakly and had no effect on pleckstrin (p47) phosphorylation. Recently we reported that C2 but not C6 inhibits superoxide generation and store-regulated Ca2+ influx in neutrophils at similar concentrations. Cellular differences in ceramide metabolism or ceramide-sensitive enzymes and their substrates may account for the disparate results.

Production of recombinant human brain type I inositol-1,4,5-trisphosphate 5-phosphatase in Escherichia coli. Lack of phosphorylation by protein kinase C

The dephosphorylation of inositol 1,4,5-trisphosphate (InsP3) to inositol 1,4-bisphosphate is catalyzed by InsP3 5-phosphatase. The coding region of human brain type I InsP3 5-phosphatase was expressed as a fusion protein with the maltose-binding protein (MBP) in Escherichia coli, using the pMAL-cR1 vector. The relative molecular mass of the purified fusion protein (MBP-InsP3-5-phosphatase) was approximately M(r) 85,000 as analysed by SDS/PAGE. The yield was about 10 mg fusion protein/l lysate. After cleavage from MBP with factor Xa, the specific activity of recombinant 5-phosphatase was 120-250 mumol.mg-1.min-1. The molecular mass of purified protein by SDS/PAGE was M(r) 43,000. The activity was inactivated by p-hydroxymercuribenzoate. The possibility that protein kinase C might phosphorylate InsP3 5-phosphatase was tested on the purified 43,000 M(r) protein. In this study, we show that recombinant 5-phosphatase is not a substrate of protein kinase C.

Staurosporine inhibits inositol phosphate formation in bovine adrenal medullary cells

The effect of protein kinase C activators and inhibitors on histamine-stimulated phospholipase C in bovine adrenal medullary cells has been investigated. The protein kinase C activators, phorbol 12,13-dibutyrate (PDB) or sn-1,2-dioctanoylglycerol (DOG), inhibited histamine-stimulation of phospholipase C. This inhibition was prevented by the protein kinase C-selective inhibitor Ro 31-8220 (3-[1-[3-(2-isothioureido) propyl]indol-3-yl]-4-(1-methylindol-3-yl)-3-pyrrolin-2,5-dio ne) but not the broad spectrum protein kinase inhibitor staurosporine. Indeed staurosporine on its own inhibited both the histamine-stimulated response and, in permeabilized cells, phospholipase C activated by Ca2+. Staurosporine inhibition of phospholipase C is unlikely to be mediated via protein kinase A or Ca2+/calmodulin-dependent protein kinase because it was not reproduced by selective inhibition of these kinases. Staurosporine treatment, however, reduced inositol phospholipid levels in stimulated cells. Thus staurosporine and Ro 31-8220, two widely used protein kinase C inhibitors, have quite different effects on phospholipase C activation. Furthermore, staurosporine may cause this inhibition through a reduction in the level of phospholipase C substrate.

Role of protein kinase C in the regulation of inositol phosphate production and Ca2+ mobilization evoked by ATP and acetylcholine in rat lacrimal acini

Stimulation of rat lacrimal acinar cells with ATP and acetylcholine (ACh) induced a rapid accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and its degradation products, resulting in an initial release of Ca2+ from intracellular stores. However, after pretreating the acini with U73122 no increase in the intracellular free Ca2+ concentration ([Ca2+]i) or Ins(1,4,5)P3 production was observed. A short pretreatment with the phorbol ester 4-beta-phorbol-12-beta-myristate-13-alpha-acetate (PMA) significantly attenuated the ATP- and ACh-induced increase in [Ca2+]i and overall inositol phosphate production. In contrast, staurosporine enhanced Ins(1,4,5)P3 and inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] production and [Ca2+]i above control values in ATP- and ACh-stimulated cells. Stimulation of phospholipase C by ionomycin-evoked changes in [Ca2+]i were unaltered by pretreatment with staurosporine and PMA. The data show that a change in protein kinase C activity during cell stimulation affects the inositol phosphate metabolism and thereby the cellular Ca2+ signalling processes in lacrimal acinar cells.

Protein kinase C activity does not mediate the inhibitory effect of carbachol on chloride secretion by T84 cells

Carbachol induces calcium-dependent chloride secretion and activates protein kinase C in T84 cells. However, prolonged stimulation with carbachol or direct activation of protein kinase C inhibits subsequent calcium-dependent chloride secretion. Furthermore, the ability of carbachol to elevate inositol tetrakisphosphate levels may be linked to inhibition of chloride secretion. Here we demonstrate that protein kinase C activation increases levels of inositol tetrakisphosphates (1,3,4,6- and 3,4,5,6-isomers) in T84 colonic epithelia. Furthermore, this corresponds to an inhibition of chloride secretion. However, protein kinase C is unlikely to mediate the analogous effects of carbachol. Neither the ability of carbachol to inhibit calcium-dependent chloride secretion nor its effects on inositol 3,4,5,6-tetrakisphosphate levels were reversed by staurosporine. Carbachol also has quantitatively and qualitatively different effects on inositol tetrakisphosphate isomers than protein kinase C activators. Thus protein kinase C activity can increase levels of various inositol tetrakisphosphate isomers within T84 cells but does not mediate carbachol-induced increases in these putative messengers. These data further support the hypothesis that inositol 3,4,5,6-tetrakisphosphate is a negative second messenger, uncoupling epithelial chloride secretion from changes in intracellular calcium.

Staurosporine induces hydrolysis of phosphatidyl inositol 4,5-bisphosphate in human platelets

Staurosporine in the micromolar range raised inositol trisphosphate in intact human platelets to levels comparable to that mediated by thrombin. This response was inhibited by neomycin, a phospholipase C antagonist. Staurosporine alone induced a weak, transient rise in cytosolic free calcium levels ([Ca2+]i) from release of internal Ca2+ stores but potentiated the effect induced by thrombin. Therefore, it is unlikely that this alkaloid suppressed inositol trisphosphate mobilization of Ca2+. Additional studies show that staurosporine, 0.5-5 microM, stimulated GTPase activity in platelet membranes while 2 microM K252a and 20 microM H7 were inactive. Present results suggest that staurosporine may activate platelet phospholipase C at the level of G proteins or receptors.

Phosphoinositide metabolism in airway smooth muscle

Agonist-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, which generates inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol, is thought to be one of the major mechanisms underlying pharmacomechanical coupling in airway smooth muscle. This article is a review of the currently available information on phosphoinositide and inositol 1,4,5-trisphosphate metabolism in this tissue and includes data on inositol 1,4,5-trisphosphate-induced Ca2+ release and the receptor mediating this effect. The final section outlines the potential mechanisms underlying physiological regulation of phosphoinositide metabolism by other second-messenger pathways operative in this tissue.

Differential regulation of inositol 1,4,5-trisphosphate by co-existing P2Y-purinoceptors and nucleotide receptors on bovine aortic endothelial cells

1. We have examined the inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) responses in bovine aortic endothelial (BAE) cells to purines (ATP, ADP and analogues) and the pyrimidine, uridine triphosphate (UTP). 2. Exchange of medium on BAE cells in the absence of agonist was found to be a stimulus for Ins(1,4,5)P3 generation. BAE cells stimulated with 100 microM ATP, 30 microM 2MeSATP (an agonist at P2Y-purinoceptors but not nucleotide receptors) or 100 microM UTP (an agonist at nucleotide receptors but not P2Y-purinoceptors) gave Ins(1,4,5)P3 responses above that caused by exchange of medium. The time course was rapid, with peak response within the first 5 s and levels returning close to basal after 30 s of stimulation. 3. Significant differences in Ins(1,4,5)P3 responses to 100 microM UTP and 30 microM 2MeSATP stimulation were observed. The response to UTP was reproducibly more sustained than that to 2MeSATP. 4. Stimulation of BAE cells with 100 microM UTP plus 30 microM 2MeSATP produced a response statistically indistinguishable from that predicted by addition of the responses to the two agonists in isolation. 5. The Ins(1,4,5)P3 response to UTP was attenuated to 25% of control by pretreatment of BAE cells with pertussis toxin. Responses to 2MeSATP and ADP were essentially unaffected. ATP stimulation was reduced to 65% of control. 6. Activation of protein kinase C with tetradecanoyl phorbol acetate (TPA) profoundly inhibited Ins(1,4,5)P3 responses to 2MeSATP and ADP but had no effect on UTP stimulation. The protein kinase C inhibitor, Ro 31-8220, enhanced responses to 2MeSATP, ADP and ATP but no effect was observed on UTP stimulation. 7. These observations show that nucleotide and P2Y-receptors mobilise the second messenger Ins(1,4,5)P3 by separate routes resulting in different patterns of generation and suggest that while ATP activates both receptors, ADP principally influences these cells by interacting with the P2Y-purinoceptors.

Two different spatiotemporal patterns for Ca2+ oscillations in pancreatic acinar cells: evidence of a role for protein kinase C in Ins(1,4,5)P3-mediated Ca2+ signalling

The oscillations in cytosolic Ca2+ evoked in pancreatic exocrine acinar cells by submaximal concentrations of the two phosphoinositidase-coupled agonists acetylcholine (ACh) and cholecystokinin octapeptide (CCK-8) have very different temporal patterns. In the present study we use digital video imaging of Fura-2 fluorescence to map the spatial distribution of Ca2+ during the oscillating responses to these two agonists. The spatial patterns induced are very different for each of these agonists. ACh oscillations are sinusoidal and initiated at the secretory pole of these morphologically and functionally polarized cells. As they spread across the cell, pronounced gradients in Ca2+ develop that persist throughout the oscillating response. CCK-8 induces a series of discrete Ca2+ transients of longer duration and lower frequency. These elevations in Ca2+ arise slowly, throughout the cells and without any detectable gradients in Ca2+. We consider that the different spatiotemporal patterns can be explained on the basis of a physiologically relevant interaction between Ins(1,4,5)P3 and protein kinase C in second messenger-mediated Ca2+ signalling.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Phospholipase D activation regulates endothelin-1 stimulation of phosphoinositide-specific phospholipase C in SK-N-MC cells

Endothelin-1 (ET-1) is known to stimulate phospholipase C (PLC) activity in SK-N-MC human neuroblastoma/epithelioma cells: here we show that phospholipase D (PLD) is also stimulated. The generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) by ET-1-stimulated PLC was attenuated by protein kinase C (PKC) activation and enhanced by PKC inhibition. An enhancement of ET-1-stimulated Ins(1,4,5)P3 accumulation was also seen when the product of PLD activity was either diverted into phosphatidyl butanol in the presence of butanol, or phosphatidate phosphohydrolase (PPH) activity was inhibited by DL-propranolol. We conclude that there is an inhibitory, PKC-mediated, feedback loop in these cells which is dependent, in part, on the activation of PKC by product(s) of the PLD/PPH pathway. This provides a novel role for agonist-stimulated PLD activation.

Effects of protein kinase C activation on norepinephrine-induced phosphatidylinositide hydrolysis in intact rat aorta

The purpose of this study was to investigate the role of protein kinase C in the regulation of alpha 1-adrenoceptor-mediated phosphatidylinositide hydrolysis in intact vascular smooth muscle. Phorbol myristate acetate (0.1 and 1 microM) and staurosporine inhibited and potentiated, respectively, norepinephrine-induced inositol phosphate formation in intact rat aorta. In contrast, 30 microM prostaglandin F2 alpha, which activated protein kinase C to a similar magnitude as 1 microM phorbol myristate acetate, was without effect on norepinephrine-induced inositol phosphate formation. These results suggest that protein kinase C activated in response to physiologic agonists, but not in response to phorbol esters, may be compartmentalized within the smooth muscle cell.

Protein kinase C inhibitors potentiate angiotensin II-induced phosphoinositide hydrolysis and intracellular Ca2+ mobilization in renal mesangial cells

Stimulation of mesangial cells with angiotensin II leads to rapid phosphoinositide hydrolysis and subsequent mobilization of intracellular Ca2+. Previous studies indicated that activation of protein kinase C (PKC) triggers a negative-feedback signal, which limits phosphoinositide turnover. By comparing the relative susceptibility of PKC isoenzymes to phorbol ester-induced down-regulation with the down-regulation of the functional cell response, i.e. feedback inhibition of inositol trisphosphate production, we inferred that PKC-alpha and PKC-delta are candidates for regulating phosphoinositide hydrolysis in mesangial cells. To test this hypothesis further, we examined the effects of inhibitors of PKC, that are reportedly not active on PKC-delta, on angiotensin II-stimulated phosphoinositide degradation and Ca2+ mobilization. Pretreatment of mesangial cells with the PKC inhibitors staurosporine and K252a potently augmented inositol trisphosphate and 1,2-diacylglycerol formation as well as Ca2+ mobilization in response to angiotensin II. These results suggest that PKC-alpha, but not PKC-delta, is the most likely candidate mediating feedback inhibition of angiotensin II-stimulated phosphoinositide turnover in mesangial cells.

The role of protein kinase C in the initial events of platelet activation by thrombin assessed with a selective inhibitor

The role of protein kinase C (PKC) in platelet activation by thrombin was assessed using a PKC inhibitor Ro 31-7549/001 (R2) which, in vitro, shows more selectivity for PKC than other kinase inhibitors. During early (1.5 s) thrombin-induced platelet activation, when phosphorylation of 47 kDa protein (pleckstrin) and myosin light chain by PKC and myosin light chain kinase, respectively, are most readily differentiated, R2 suppressed phosphorylation of pleckstrin more effectively than myosin light chain. R2-inhibited dense granule secretion (measured 0-10 s using quenched-flow techniques) with a dose dependency similar to that for inhibition of pleckstrin phosphorylation, supporting a role for PKC in this process. R2, at 0.5 microM inhibited 47 kDa protein phosphorylation by more than 60%, but had only minimal effects on the kinetics (0-3s) of ADP-induced primary aggregation. At this same concentration, R2 potentiated the thrombin-induced rise in cytosolic calcium during early (0-15 s) activation as measured in the presence or absence of external calcium. These data support the hypothesis that activation of PKC during early platelet function helps regulate cytosolic calcium levels by limiting calcium release into the cytosol.

Role of protein kinase C in the regulation of histamine and bradykinin stimulated inositol polyphosphate turnover in adrenal chromaffin cells

1. The possibility that bradykinin- or histamine-stimulated inositol polyphosphate accumulation may be regulated by protein kinase C (PKC) in bovine adrenal chromaffin cells has been addressed. 2. Initial experiments confirmed that the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) dramatically inhibited agonist-stimulated [3H]-inositol phosphate accumulations in [3H]-inositol prelabelled cells. In contrast, the PKC inhibitor, Ro 31-8220, did not affect this response. 3. Histamine (100 microM) or bradykinin (100 nM) evoked rapid increases in inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) mass accumulations (maximal accumulations within 10 s and 30 s, respectively) which declined towards basal values over a 10 min incubation period. TPA (1 microM) significantly attenuated the peak Ins(1,4,5)P3 response to bradykinin and histamine by 30% and 70% respectively. In contrast, TPA did not significantly affect agonist-stimulated Ins(1,3,4,5)P4 responses. 4. Ro 31-8220 (10 microM) significantly enhanced the maximal Ins(1,4,5)P3 accumulations elicited by both bradykinin and histamine. 5. The results indicate that the initial Ins(1,4,5)P3 response to either bradykinin or histamine in bovine adrenal chromaffin cells can be attenuated by PKC activation by phorbol ester and enhanced by PKC inhibition by Ro 31-8220. In contrast, agonist-stimulated Ins(1,3,4,5)P4 accumulation does not appear to be affected by these manipulations of PKC activity. Possible bases for differential modulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 are discussed.

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.

Influence of staurosporine, a more selective derivative CGP 41,251 and an inactive analogue CGP 42,700 on histamine release from isolated rat mast cells

The influence of staurosporine, a potent but nonselective inhibitor of protein kinase C, on rat mast cell histamine release, was compared with that of two derivatives, CGP 41,251 with a high degree of selectivity for protein kinase C and the related CGP 42,700 which is without activity. Staurosporine was a more potent inhibitor of mast cell responses than CGP 41,251, in accordance with their reported potencies. CGP 42,700 was investigated in the same concentration range as CGP 41,251 and served as a control for unspecific effects. Antigen induced histamine release was more effectively inhibited by staurosporine than by CGP 41,251, and responses to compound 48/80 were only modestly affected by both drugs. Responses to the ionophore A23187 were unaffected by staurosporine whereas CGP 41,251 was an effective inhibitor at suboptimal ionophore concentrations. In contrast, responses to combinations of the phorbol ester TPA and subthreshold concentrations of the ionophore could be potently inhibited by staurosporine but were under certain conditions moderately enhanced by lower concentrations of the drug, whereas CGP 41,251 was only inhibitory. Except for a slight inhibition of ionophore responses CGP 42,700 was without effect. The results demonstrate that the actions of staurosporine cannot be ascribed solely to inhibition of protein kinase C, whereas the influence of CGP 41,251 appears to be consistent with an inhibition of this kinase.

Effect of staurosporine on fMet-Leu-Phe-stimulated human neutrophils: dissociated release of inositol 1,4,5-trisphosphate, diacylglycerol and intracellular calcium

Staurosporine, a microbial alkaloid, enhances inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DG) production rapidly and dose-dependently in fMet-Leu-Phe (FMLP)-stimulated human neutrophils showing maximal effects at 1 microM concentration. The IP3 increase was specific for staurosporine as three other putative protein kinase C (PKC) inhibitors, H7, sphingosine and palmitoylcarnitine were unable to enhance the IP3 generation in FMLP-stimulated human neutrophils. Staurosporine, at concentrations 0.3-1.0 microM, did not affect the initial mobilization of FMLP-induced intracellular Ca2+ (Ca2+i), although a sustained elevation of cytosolic Ca2+ level was observed within 5 min. This effect could not be suppressed, even by 1 microM phorbol-myristate 12,13-acetate (PMA). Whereas lower concentrations of staurosporine (less than or equal to 100 nM) were unable to affect FMLP-induced IP3 production, DG accumulation and Ca2+i, the PMA-inhibited initial Ca2+i signal and IP3 formation triggered by FMLP were almost completely restored. At higher concentrations (greater than or equal to 300 nM) staurosporine reversed the inhibitory effect of other protein kinases, distinct from the PMA-inducible one, which may be responsible for the phosphatidyl inositol 4,5-bisphosphate (PIP2) breakdown, thus causing accumulation of IP3 and DG and an elevation of C2+i level. Whereas IP3 declined to basal level within 5 min, the DG level remained elevated during the same period. This phenomenon is attributed to phospholipase D (PLD) stimulation by staurosporine, which augments the DG synthesis, in part through PA degradation via phosphatidic acid (PA) phosphohydrolase.

The metabolism of microinjected inositol trisphosphate in Xenopus oocytes

Microinjection of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) into Xenopus oocytes evokes a complex physiological response composed of a transient and a slow depolarizing chloride current. We investigated the relationship between intracellular levels of Ins(1,4,5)P3 and the kinetics of the physiological response. Microinjected Ins(1,4,5)P3 was slowly degraded following first order kinetics of disappearance (t1/2 = 10 min). The degradation products were inositol bisphosphate (InsP2), inositol monophosphate (InsP) and inositol, as well as inositol tetrakisphosphate (InsP4). The rate of degradation of injected 3[H]-Ins(1,4)P2 was much greater (t1/2 = 3 min), indicating that the conversion of InsP3 to InsP2 may be the rate-limiting step in the degradation process. The slow degradation of 3[H]-Ins(1,4,5)P3 was not a result of its conversion to Ins(1,3,4)P3 since no accumulation of InsP3 was observed within 10 min of microinjection of 3[H]-Ins(1,3,4,5)P4. Activation of protein kinase C (PK-C) with a phorbol ester transiently increased the rate of conversion of 3[H]-Ins(1,4,5)P3 to InsP2. This, however, did not significantly affect the overall kinetics of 3[H]-Ins(1,4,5)P3 disappearance. Our results indicate that the kinetics of Ins(1,4,5)P3 degradation do not correlate well with the termination of both the rapid and the slow components of the physiological response. The termination of the slow component of the response, however, may be related to the decay of Ins(1,4,5)P3-induced 45Ca efflux, which lasted about 10 min.

Human platelets deficient in dense granules contain normal amounts of pp60c-src

We have determined that pp60c-src, a protein tyrosine kinase abundant in normal platelets, is present at comparable levels in platelets that are deficient in dense granules (Hermansky-Pudlak syndrome). Relative quantitation of pp60c-src was performed by immunoblot analysis after protein separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our data suggest that human platelet dense granules, unlike chromaffin cell secretory granules, are not the major intracellular site of localization of pp60c-src.

Characterization of glucocorticoid inhibition of antigen-induced inositolphosphate formation by rat basophilic leukemia cells: possible involvement of phosphatases

The suppressive effect of glucocorticoids (GC) upon antigen-induced phosphatidylinositol phospholipase C (PI-PLC) activity and inositol phosphate formation by rat basophilic leukemia cells (RBL-2H3) has been characterized. Addition of antigen for a period of 1-30 min enhanced production of [3H]inositol monophosphate (IP1), inositol 1,4-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3) by about 5-10-fold. Pretreatment with hydrocortisone (HC) reduced formation of the various inositol phosphates (IPs) and degradation of phosphatidylinositol 4,5-bisphosphate (PIP2) by an average of 50%. Maximal inhibition of hydrolysis of PIP2 and reduction in stimulation of IP3 formation was reached after 4 h of preincubation with 2.10(-6) M of HC. Cycloheximide and RU486, a GC receptor antagonist, completely prevented the inhibitory effect of HC on IP formation. Other GC, dexamethasone (DEX) and triamcinolone (each at 2.10(-7) M) markedly suppressed antigen induced IP3 production, while aldosterone and sex steroids such as estradiol and progesterone (each at 2.10(-6) M) were virtually inactive. Antigen-stimulated phosphorylation of a 18 kDa and other proteins was inhibited by about 60% following pretreatment with the GC. This inhibition was in turn prevented by cycloheximide. DEX also doubled the activity of cellular acid phosphatase activity. The results suggest that the inhibitory effect of GC is specific, receptor-mediated, dependent on protein synthesis and possibly mediated by protein phosphatase activity.

Staurosporine, a protein kinase inhibitor, attenuates intracellular Ca(2+)-dependent contractions of strips of rabbit aorta

The protein kinase inhibitor staurosporine was found to be a potent relaxant of rabbit aortic strips contracted by various agonists. This relaxing effect was slow, long-lasting and in a non-competitive fashion against various agonists. The effect of staurosporine on the KCl-induced contraction was not altered by atropine, propranolol, theophylline or indomethacin. Staurosporine (5-200 nM) inhibited the contractile responses to prostaglandin F2 alpha (3 microM), endothelin (0.1 microM), phenylephrine (3 microM) and KCl (30 mM), with ED50 values of 10.8 +/- 0.6, 24.6 +/- 8.7, 48.8 +/- 7.0, 54.0 +/- 12.7 nM, respectively. Even in Ca(2+)-free physiological salt solution, staurosporine potently antagonized the contractile responses elicited by 10 microM prostaglandin F2 alpha and 0.1 microM endothelin; in fact it was more effective than it was in normal Ca2+ solution: ED50 of 11.3 +/- 3.8 vs. 21.0 +/- 4.4 nM (P less than 0.02), and of 10.6 +/- 3.6 vs. 24.6 +/- 4.3 nM (P less than 0.01), respectively. Thus, staurosporine has an equipotent inhibitory action on intracellular and extracellular calcium-dependent contractions of aortic tissues.