Possible role of a cAMP-dependent phosphorylation in the calcium release mediated by inositol 1,4,5-trisphosphate in human platelet membrane vesicles

The receptor EP3 to PGE2: A rational target to prevent atherothrombosis without inducing bleeding

The prostanoid E2 (PGE2) is known to modulate the aggregative response of platelets to their conventional agonists such as ADP, TXA2, thrombin or collagen. Through the activation of its receptor EP3, PGE2 sensitizes platelets to their agonists but also inhibits them through its two other receptors, EP2 and EP4. In mice, the net result of these opposed actions is the EP3-mediated potentiation of platelet aggregation and the in vivo aggravation of murine atherothrombosis. Since the pathway PGE2/EP3 is not involved in murine hemostasis, we propose a "platelet EP3 paradigm" to describe this apparently paradoxical association between the facilitating impact on atherothrombosis and the unaltered hemostasis. Consistent with this paradigm, a drug blocking EP3 dramatically decreased atherothrombosis without inducing bleeding in mice. In humans, several studies did not agree on the effect of PGE2 on platelets. Reinterpreting these data with the notion of "potentiation window" and taking the platelet initial cAMP level into account reconciled these inconsistent results. Thereby, the in vitro potentiating effect of PGE2 on human platelets becomes clear. In addition, the EP3 blocking drug DG-041 abrogated the potentiating effect of PGE2 in whole human blood but did not prolong bleeding times in volunteers. Thus, the murine "platelet EP3 paradigm" would apply to humans if the aggravating role of PGE2 on atherothrombosis is shown in patients. Therefore, testing an EP3 blocker in a phase III trial would be of high interest to fulfill the unmet medical need which is to control atherothrombosis without impacting hemostasis and thus to improve the prevention of myocardial infarction.

Mechanisms of cross-talk between G-protein-coupled receptors resulting in enhanced release of intracellular Ca2+

Alteration in [Ca(2+)](i) (the intracellular concentration of Ca(2+)) is a key regulator of many cellular processes. To allow precise regulation of [Ca(2+)](i) and a diversity of signalling by this ion, cells possess many mechanisms by which they are able to control [Ca(2+)](i) both globally and at the subcellular level. Among these are many members of the superfamily of GPCRs (G-protein-coupled receptors), which are characterized by the presence of seven transmembrane domains. Typically, those receptors able to activate PLC (phospholipase C) enzymes cause release of Ca(2+) from intracellular stores and influence Ca(2+) entry across the plasma membrane. It has been well documented that Ca(2+) signalling by one type of GPCR can be influenced by stimulation of a different type of GPCR. Indeed, many studies have demonstrated heterologous desensitization between two different PLC-coupled GPCRs. This is not surprising, given our current understanding of negative-feedback regulation and the likely shared components of the signalling pathway. However, there are also many documented examples of interactions between GPCRs, often coupling preferentially to different signalling pathways, which result in a potentiation of Ca(2+) signalling. Such interactions have important implications for both the control of cell function and the interpretation of in vitro cell-based assays. However, there is currently no single mechanism that adequately accounts for all examples of this type of cross-talk. Indeed, many studies either have not addressed this issue or have been unable to determine the mechanism(s) involved. This review seeks to explore a range of possible mechanisms to convey their potential diversity and to provide a basis for further experimental investigation.

Differential effect of PKA on the Ca2+ release kinetics of the type I and III InsP3 receptors

The effects of protein kinase A (PKA) on the inositol 1,4,5-trisphosphate (InsP(3)) receptor isoforms type I and type III were studied. The effects of PKA on the extent and rate constants for InsP(3)-induced Ca(2+) release (IICR) were different for the two isoforms. The effects of PKA on the type I isoform showed a biphasic relationship dependent upon the concentration of PKA used. At low concentrations of PKA (<50U/ml), both the extent and rate constants for IICR increased, while at higher concentrations (>200U/ml) the extent and rate constants decreased. The type III isoform showed only an increase in the extent of IICR and not in the rate constants. The effects of PKA on the type I InsP(3) receptor using single channel electrophysiological studies were also investigated. The stimulatory effect of PKA is due to an increase in conductance levels and not to a change in the mean open time of the channel.

Anti-thrombotic effect of milrinone is caused by inhibition of calcium release from the dense tubular system in human platelets

AIM

Milrinone, a phosphodiesterase III inhibitor, exerts positive inotropic effects which induce an increase in the intracellular calcium concentration by raising the cyclic adenosine monophosphate level in cardiac muscle. Milrinone was also reported to inhibit platelet aggregation, however, its mechanism remains unknown. Therefore, we investigated the effects of milrinone on intracellular calcium mobilization when platelets were activated.

METHODS

Washed platelets, obtained from six healthy volunteers, were preincubated with milrinone (0.9 micro M) for 1 min and then exposed to 0.015 i micro ml-1 thrombin for 5 min. The effect of milrinone on changes in the intracellular calcium level using a fluorescent dye, fura-2, was also observed. Calcium mobilizations via plasma membrane calcium channels and the dense tubular system were assessed differentially.

RESULTS

Milrinone (0.9 micro M) significantly suppressed the aggregation ratios at 5 min compared with those in controls (86+/-5%) to 75+/-8%. The increase in the intracellular calcium concentration was also significantly suppressed (controls, 915+/-293 nM vs. 405+/-240 nM) when stimulated by thrombin. Milrinone also significantly inhibited the release of calcium from the dense tubular system (controls, 284+/-111 nM vs. 158+/-51 nM). Calcium influx through the plasma membrane was suppressed by milrinone 2.4 micro M.

CONCLUSION

Milrinone (0.9 micro M) inhibited thrombin-induced platelet aggregation. This inhibitory effect was mainly mediated by suppressing calcium release from the dense tubular system.

Irregular spiking in free calcium concentration in single, human platelets. Regulation by modulation of the inositol trisphosphate receptors

Fluorescence ratio imaging indicates that immobilized, aspirin-treated platelets, loaded with Fura-2, respond to inositol 1,4,5-trisphosphate- (InsP3)-generating agonists such as thrombin by high-frequency, irregular rises in cytosolic [Ca2+]i with spikes that vary in peak level and peak-to-peak interval. This differs from the regular [Ca2+]i oscillations observed in other, larger cells. We found that the thiol-reactive compounds thimerosal (10 microm) and U73122 (10 microm) evoked similar irregular Ca2+ responses in platelets, but in this case in the absence of InsP3 generation. Thrombin-induced spiking was acutely abolished by inhibiting phospholipase C or elevating intracellular cAMP levels, while spiking with sulfhydryl reagents was only partially blocked by cAMP elevation. Confocal laser scanning microscopy using fluo-3-loaded platelets indicated that, with all agonists or conditions, the irregular spikes were almost instantaneously raised in various regions within a single platelet. When using saponin-permeabilized platelets, we found that InsP3-induced Ca2+ release from stores was stimulated by modest Ca2+ concentrations, pointing to a mechanism of InsP3-dependent Ca2+-induced Ca2+ release (CICR). This process was completely inhibitable by heparin. The Ca2+ release by InsP3, but not the CICR sensor, was negatively regulated by cAMP elevation. Thimerosal treatment did not release Ca2+ from intracellular stores, but markedly potentiated the stimulatory effect of InsP3. In contrast, U73122 caused a heparin/cAMP-insensitive Ca2+ leak from stores that differed from those used by InsP3. Taken together, these results demonstrate that InsP3 receptor channels play a crucial role in the irregular, spiking Ca2+ signal of intact platelets, even when induced by agents such as thimerosal or U73122 which do not stimulate InsP3 formation. The irregular Ca2+ release events appear to be subjected to extensive regulation by: (a) InsP3 level, (b) the potentiating effect of elevated Ca2+ on InsP3 action via CICR, (c) InsP3 channel sensitization by sulfhydryl (thimerosal) modification, (d) InsP3 channel-independent Ca2+ leak with U73122, and (e) down-regulation via cAMP elevation. The observation that individual Ca2+ peaks were generated in various parts of a platelet at similar intervals and amplitudes points to effective cooperation of the various stores in the Ca2+-release process.

Molecular characterization of the starfish inositol 1,4,5-trisphosphate receptor and its role during oocyte maturation and fertilization

The release of calcium ions (Ca(2+)) from their intracellular stores is essential for the fertilization of oocytes of various species. The calcium pools can be induced to release Ca(2+) via two main types of calcium channel receptor: the inositol 1,4,5-trisphosphate receptor (IP(3)R) and the ryanodine receptor. Starfish oocytes have often been used to study intracellular calcium mobilization during oocyte maturation and fertilization, but how the intracellular calcium channels contribute to intracellular calcium mobilization has never been understood fully, because these molecules have not been identified and no specific inhibitors of these channels have ever been found. In this study, we utilized a novel IP(3)R antagonist, the "IP(3) sponge," to investigate the role of IP(3) during fertilization of the starfish oocyte. The IP(3) sponge strongly and specifically competed with endogenous IP(3)R for binding to IP(3). By injecting IP(3) sponge into starfish oocyte, the increase in intracellular calcium and formation of the fertilization envelope were both dramatically blocked, although oocyte maturation was not blocked. To investigate the role of IP(3)R in the starfish oocyte more precisely, we cloned IP(3)R from the ovary of starfish, and the predicted amino acid sequence indicated that the starfish IP(3)R has 58-68% identity to mammalian IP(3)R types 1, 2, and 3. We then raised antibodies that recognize starfish IP(3)R, and use of the antibodies to perform immunoblot analysis revealed that the level of expression of IP(3)R remained unchanged throughout oocyte maturation. An immunocytochemical study, however, revealed that the distribution of starfish IP(3)R changes during oocyte maturation.

Activation of the murine EP3 receptor for PGE2 inhibits cAMP production and promotes platelet aggregation

The importance of arachidonic acid metabolites (termed eicosanoids), particularly those derived from the COX-1 and COX-2 pathways (termed prostanoids), in platelet homeostasis has long been recognized. Thromboxane is a potent agonist, whereas prostacyclin is an inhibitor of platelet aggregation. In contrast, the effect of prostaglandin E2 (PGE2) on platelet aggregation varies significantly depending on its concentration. Low concentrations of PGE2 enhance platelet aggregation, whereas high PGE2 levels inhibit aggregation. The mechanism for this dual action of PGE2 is not clear. This study shows that among the four PGE2 receptors (EP1-EP4), activation of EP3 is sufficient to mediate the proaggregatory actions of low PGE2 concentration. In contrast, the prostacyclin receptor (IP) mediates the inhibitory effect of higher PGE2 concentrations. Furthermore, the relative activation of these two receptors, EP3 and IP, regulates the intracellular level of cAMP and in this way conditions the response of the platelet to aggregating agents. Consistent with these findings, loss of the EP3 receptor in a model of venous inflammation protects against formation of intravascular clots. Our results suggest that local production of PGE2 during an inflammatory process can modulate ensuing platelet responses.

Phosphorylation of the inositol 1,4,5-trisphosphate receptor. Cyclic GMP-dependent protein kinase mediates cAMP and cGMP dependent phosphorylation in the intact rat aorta

The effects of cyclic GMP (cGMP) and activation of cGMP-dependent protein kinase (PKG) on the phosphorylation of the inositol 1,4, 5-trisphosphate (IP3) receptor were examined in intact rat aorta using the technique of back phosphorylation. Aorta treated with the nitric oxide donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside, or the selective PKG activator, 8-(4-para-chlorophenylthio)-cGMP (8-CPT-cGMP), demonstrated increased IP3 receptor phosphorylation in situ, which was both time- and concentration-dependent with a stoichiometry of 0.5 mol of phosphate/mol of receptor above control. Treatment of aorta with the adenyl cyclase activator, forskolin, also demonstrated increased phosphorylation of the IP3 receptor on the PKG site, although the selective cAMP-dependent protein kinase activator, 8-(4-para-chlorophenylthio)-cAMP (8-CPT-cAMP), did not increase the phosphorylation of the IP3 receptor. Moreover, the PKG selective inhibitor, KT 5823, inhibited both sodium nitroprusside and forskolin-induced IP3 receptor phosphorylation more potently than the selective cAMP-dependent protein kinase inhibitor, KT 5720, suggesting that PKG mediates the increase in IP3 receptor phosphorylation by both cyclic nucleotides in intact aorta. These results provide further support for the notion that PKG is activated by both cAMP and cGMP in intact vascular smooth muscle and that PKG performs a critical role in cyclic nucleotide-dependent relaxation of blood vessels.

Prostacyclin and sodium nitroprusside inhibit the activity of the platelet inositol 1,4,5-trisphosphate receptor and promote its phosphorylation

Prostaglandin I2 (PGI2) and sodium nitroprusside (SNP) induce a rapid decay of the thrombin-promoted increase of [Ca2+]i in aspirin-treated platelets incubated in the absence of external Ca2+. The mechanism of their effect was studied with a new method which utilizes ionomycin to increase [Ca2+]i, followed by bovine serum albumin (BSA) to remove the Ca2+ ionophore. The rapid decay of [Ca2+]i after BSA is mostly due to the reuptake into the stores, since it is strongly inhibited by the endomembrane Ca2+-ATPase inhibitor thapsigargin. PGI2 and SNP are without effect on the BSA-promoted decay both with and without thapsigargin, showing that they do not affect the activity of the Ca2+-ATPases. The fast decay of [Ca2+]i after BSA is decreased by thrombin which produces the Ca2+ releaser inositol 1,4,5-trisphosphate (InsP3), thus counteracting the activity of the endomembrane Ca2+ pump. When added after thrombin, PGI2 and SNP accelerate the BSA-activated decay of [Ca2+]i. However, under the same conditions, they do not decrease the concentration of InsP3. In saponin-permeabilized platelets, cAMP and cGMP counteract the Ca2+ release induced by exogenous InsP3. Their inhibitory effect disappears at high InsP3 concentrations. This demonstrates that PGI2 and SNP potentiate Ca2+ reuptake by inhibiting the InsP3 receptor. Two bands of approximately 260 kDa are recognized by a monoclonal antibody recognizing the C-terminal region of the InsP3 receptor. Both are phosphorylated rapidly, the heavier more intensely, in the presence of PGI2 and SNP. The phosphorylation of the InsP3 receptor is fast enough to be compatible with its involvement in the inhibition of the receptor by cyclic nucleotides.

Conditions that lithium inhibits or potentiates vasopressin V1-receptor-mediated platelet aggregation and [Ca++]i mobilization

1. Besides clinical use, there are many explanations for the mechanism of action of lithium. Although it is shown that lithium may reduce the supply of inositol that is required to sustain phosphoinositide synthesis, evidence exists concerning the potentiating effect of lithium on this pathway. We therefore decided to evaluate conditions in which lithium inhibits or potentiates platelet aggregation and calcium response induced by vasopressin. 2. Platelet aggregation was measured by the photometric method, and changes in intracellular free calcium were measured using fura-2/AM. 3. We show an inhibitory action of neomycin on vasopressin-induced platelet aggregation. Lithium, according to the preincubation time, could both potentiate or inhibit platelet aggregation and calcium responses induced by vasopressin. The inhibitory effect of lithium on platelet aggregation is dependent on concentrations of both lithium and vasopressin and also the presence of indomethacin, for example, in the absence of indomethacin there was no clear inhibitory action of lithium on vasopressin-induced platelet aggregation. 4. These results show the importance of arachidonate metabolites concerning lithium effects on platelet V1-receptor signaling. In conclusion, because the arachidonate metabolites are responsible for the release of other active substances from platelets' granules, the aggregatory responses in the absence of indomethacin may be amplified, and this subsequently may change the net inhibitory action of lithium.

Ca2+ influx in platelets: activation by thrombin and by the depletion of the stores. Effect of cyclic nucleotides

In aspirin-treated platelets the thrombin-induced increase of cytosolic Ca2+ ([Ca2+]i) associated with the release from the intracellular stores is followed by a decrease to the baseline which is largely dependent on the re-uptake into the stores. This is shown by the further increase of [Ca2+]i upon inhibition of the endomembrane Ca(2+)-ATPase with thapsigargin. The re-uptake of Ca2+ into the stores is accelerated by sodium nitroprusside (SNP) or prostacyclin (PGI2). In all cases, after store depletion with thapsigargin the influx of external Ca2+ is maximal. After a thrombin-induced cycle of Ca(2+)-release re-uptake the stores are partly full: in these conditions the addition of external Ca2+ elicits a significant increment of [Ca2+]i and a further filling of the stores. Both are strongly reduced if Ca2+ addition is preceded by SNP or PGI2. Similar results are obtained also if (by supplementing and then cheleting Ca2+) the stores are as full as in native platelets at the moment of adding Ca2+. The thrombin-activated Ca2+ influx is reversed by hirudin. A PGI2- and SNP-sensitive Mn2+ influx is observed if Mn2+ is added in place of Ca2+. It is concluded that thrombin activates a cyclic nucleotide-sensitive Ca2+ (and Mn2+) influx pathway dependent on the occupancy of the thrombin receptor and independent of the filling state of the stores. In the absence of thrombin, thapsigargin releases Ca2+ relatively rapidly from a fraction of the stores; the remaining deposits are discharged much more slowly. This may indicate that platelets contain two distinct classes of agonist-sensitive stores. The addition of external Ca2+ (or Mn2+) at short or long incubation times with thapsigargin monitors the influx of Ca2+ activated by the depletion of one or both types of stores. The depletion of each type of store activates Ca2+ (Mn2+) influx. This type of cation influx is not inhibited by the cyclic nucleotides.

Multiple mechanisms by which protein kinase A potentiates inositol 1,4,5-trisphosphate-induced Ca2+ mobilization in permeabilized hepatocytes

The mobilization of Ca2+ from intracellular stores by Ins(1,4,5)P3 in suspensions of permeabilized rat hepatocytes was potentiated by preincubating intact cells with adenosine 3':5'-cyclic phosphorothioate (cpt-cAMP), or by addition of the catalytic subunit of cyclic-AMP-dependent protein kinase (PKA) after cell permeabilization. This action of PKA involved both an enhancement in Ins(1,4,5)P3 sensitivity and an increase in the size of the Ins(1,4,5)P3-releasable Ca2+ pool. Inclusion of the protein phosphatase inhibitor okadaic acid in the permeabilization medium augmented the effects of PKA. Treatment with PKA catalytic subunit also increased the rate of ATP-dependent Ca2+ sequestration. To determine whether the effects of PKA on the Ca(2+)-release mechanism were secondary to alterations in the Ca2+ load of the Ins(1,4,5)P3-sensitive stores, a method was developed using Mn2+ as a Ca2+ surrogate to examine the permeability properties of the Ins(1,4,5)P3-gated channels independent of Ca2+ fluxes. This approach utilized the ability of Mn2+ to quench the fluorescence of fura-2 compartmentalized within intracellular Ca2+ stores in an Ins(1,4,5)P3-dependent manner, with thapsigargin added to block the ATP-activated Ca2+ pump and to ensure that the Ca2+ stores were fully depleted of Ca2+. The initial rate and extent of Mn2+ quenching of compartmentalized fura-2 was increased in a dose-dependent manner by Ins(1,4,5)P3. PKA activation increased both the initial rate and the extent of Mn2+ quenching at sub-maximal Ins(1,4,5)P3 doses, but there was no effect on the quench rate in the presence of saturating Ins(1,4,5)P3. However, the amount of compartmentalized fura-2 that could be quenched by Mn2+ in the presence of maximal Ins(1,4,5)P3 was increased by PKA. These data suggest two distinct actions of PKA on the Ins(1,4,5)P3-sensitive Ca2+ stores. (1) Modification of the ion-permeability properties of the Ins(1,4,5)P3 receptor/channel through an increase in the sensitivity to Ins(1,4,5)P3 for channel opening. (2) A recruitment of Ca2+ stores from the Ins(1,4,5)P3-insensitive pool. Both actions were independent of the Ca(2+)-loading state of the stores. Imaging studies of single permeabilized hepatocytes showed that the Ins(1,4,5)P3-sensitive stores were distributed throughout the cell and PKA enhanced the rate of Ins(1,4,5)P3-stimulated Mn2+ quench in individual cells, without modifying the subcellular distribution of Ins(1,4,5)P3-sensitive stores.

Increased calcium release from sarcoplasmic reticulum stimulated by inositol trisphosphate in spontaneously hypertensive rat heart cells

It is known that inositol (1, 4, 5)-trisphosphate (IP3) stimulates Ca2+ release from sarcoplasmic reticulum (SR) in several tissues, but in cardiac myocytes this phenomenon has not been confirmed. The purpose of the present study was to confirm the effect of (1, 4, 5)-IP3 on Ca2+ release from SR in cardiac myocytes. The effect of IP3 on Ca2+ release from SR in hypertrophic cardiac cells was also determined. We examined the effects of IP3 on Ca2+ release from cardiac myocyte SR by the digital-image method in a single cell. We also determined the effect of IP3 on calcium release from isolated SR. SR was prepared from spontaneous hypertensive rat hearts and Wistar kyoto rat hearts. The SR was prelabeled with 45Ca2+, and then incubated with the indicated concentrations of IP3 for 1 min at 37 degrees C. In cardiac myocytes treated with saponin, Ca2+ release stimulated by 10 microM (1, 4, 5)-IP3 was detected by fura-2. In 45Ca2+ prelabeled SR, the maximal Ca2+ release was achieved at 10 microM IP3 incubated for 1 min. The release of Ca2+ was higher in SR of SHR than in the SR of WKY. IP3 stimulates Ca2-release from cardiac SR, and this release is greater in SHR than in WKY. However, it is uncertain whether this phenomenon plays a role in cardiac hypertrophy.

Changes in the mechanism of Ca2(+) mobilization during the differentiation of BC3H1 muscle cells

Ca2+ sequestration and release in BC3H1 muscle cells is strongly dependent on the stage of differentiation. In proliferating cells, more than 90% of the sequestered Ca2+ was Ins(1,4,5)P3-sensitive and 25% was caffeine-sensitive. In differentiated cells, the Ca2+ accumulation was 5-fold higher and was InsP3-insensitive, but about 60% of the sequestered Ca2+ was caffeine-sensitive. These changes were reversible upon addition of growth stimuli. Similarly, by measuring the intracellular Ca2+ concentration in single intact BC3H1 cells, it was found that the number of histamine-responsive cells decreased and the number of caffeine-responsive cells increased during muscle cell differentiation. These data indicate that the development of the muscle phenotype in BC3H1 myoblasts induces a major rearrangement of the mechanisms for Ca2+ mobilization.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release. II. Effect of cAMP-dependent protein kinase

The effect of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) on Ca2+ loading, inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release, and [3H]IP3 binding of canine cerebellar membrane fractions was investigated. PKA in the presence of cAMP and the catalytic subunit of PKA did not change Ca2+ loading yet increased the extent of IP3-induced Ca2+ release by approximately 35%. Hill plot analysis indicated that the catalytic subunit of PKA increased the apparent Michaelis constant of IP3-induced Ca2+ release twofold, from 0.3 to 0.7 microM IP3. The protein kinase inhibitor reversed these changes. cAMP affected neither Ca2+ loading nor IP3-induced Ca2+ release. The catalytic subunit of PKA did not appreciably affect the maximum binding and dissociation constant of [3H]IP3 binding, as judged by Scatchard analysis. Thus the catalytic subunit of PKA influences the opening of Ca2+ channels by IP3 without interfering with the binding of IP3 to its receptor sites.

Activation of platelets induced by mAb P256 specific for glycoprotein IIb-IIIa. Possible evidence for a role for IIb-IIIa in membrane signal transduction

Monoclonal antibody P256, which is specific for glycoprotein IIb-IIIa complex, was found to induce aggregation of normal platelets in plasma. The mechanism of platelet activation induced by this monoclonal antibody was thoroughly studied. The divalent binding to the IIb-IIIa molecule was necessary for triggering aggregation since Fab' fragments did not induce aggregation as did IgG and F(ab')2 fragments; however, F(ab')2 did not induce the release as did the whole IgG. P256-induced aggregation was accompanied by release of all three granule constituents, namely dense granules, alpha-granules and lysosomes, with parallel kinetics showing half-maximum release 50 s after addition of P256. Thromboxane synthesis was initiated at the same time. Using 32P-prelabeled platelets, no variation in level of [32P]phosphatidylinositol 4,5-bisphosphate could be detected in the first minute after P256 addition, indicating no activation of the calcium-independent phospholipase C specific for polyphosphoinositol phospholipid. P256 induced a calcium mobilization as measured by Indo-1 fluorescence of about the third of that measured in the presence of a thrombin concentration giving the same intensity of aggregation. P256 induced phosphorylation of the myosin light chain p20 and of the main substrate of protein kinase C, p43. Addition of aspirin inhibited almost totally calcium mobilization and partially aggregation, release and protein phosphorylations. By contrast, in the absence of external calcium, although no aggregation could occur, the release reaction was only partially reduced. In this activation, the glycoprotein IIb-IIIa complex thus appears to play a role in modulating platelet response, not only via calcium fluxes but also in activating protein kinase C responsible for p43 phosphorylation.

Release of intracellularly stored Ca2+ by inositol 1,4,5-trisphosphate--an overview

1. Inositol 1,4,5-trisphosphate (I(1,4,5)P3) releases Ca2+ from ATP-dependent Ca2+ stores in permeabilized cells and in microsomal fractions. 2. Various factors affect the amount of Ca2+ released by I(1,4,5)P3. 3. The molecular mechanism involved in the I(1,4,5)P3-induced Ca2+ release is now being investigated and I(1,4,5)P3-specific receptors and/or specific release channels are being given special attention. 4. While the I(1,4,5)P3-sensitive Ca2+ stores are presumed to locate at the endoplasmic reticulum, the relation between the I(1,4,5)P3- and the agonist-sensitive Ca2+ stores remains to be elucidated.

Cyclic AMP-dependent protein kinase does not increase calcium transport in platelet microsomes

Cyclic AMP inhibits platelet activation, at least in part, by reducing intracellular levels of ionic calcium. Previous studies using platelet microsomal fractions have suggested that one mechanism for this effect is stimulation by cyclic AMP and its protein kinase of calcium uptake into microsomal storage sites. In the present study, the effect of cyclic AMP and its protein kinase on calcium uptake by microsomal membranes has been re-examined using the active catalytic subunit of cyclic AMP-dependent protein kinase. The catalytic subunit increased calcium uptake two-fold, but this effect was not inhibited by boiling the catalytic subunit or by recombination with the regulatory subunit of cyclic AMP-dependent protein kinase, conditions that inhibited catalytic subunit activity. Conversely, dialysis of the catalytic subunit preparation against low phosphate buffer, which did not inhibit catalytic subunit activity, inhibited the stimulation of calcium uptake by the catalytic subunit preparation. Finally, the addition of high phosphate buffer, similar in phosphate concentration to that of the catalytic subunit preparation, stimulated calcium uptake. We conclude that the catalytic subunit does not directly stimulate calcium uptake by platelet microsomes.

Further characterization of the plasma membrane- and intracellular membrane-associated platelet Ca2+ transport systems

Biochemical characterization of the Ca2+-ATPases isolated from human platelet intracellular and plasma membranes is reported. A comparative study of the previously partly described plasma membrane Ca2+-ATPase [Enouf, Bredoux, Bourdeau & Levy-Toledano (1987) J. Biol. Chem. 261, 9293-9297] and the intracellular membrane Ca2+-ATPase obtained simultaneously shows differences in the following parameters: (1) different kinetics of the two enzymes; (2) similar apparent affinity towards Ca2+ (10(-7) M), though the intracellular membrane enzyme was inhibited at Ca2+ concentrations above 10(-6) M; (3) different pH dependence with an activity maximum at pH 7 for the intracellular membrane Ca2+-ATPase and no detectable pH maximum for the plasma membrane Ca2+-ATPase; (4) a 10-fold difference in the ATP requirement of the two Ca2+-ATPases; (5) different patterns of inhibition by vanadate. Finally, the possible regulation of the Ca2+-ATPases was examined by studying the effect of chlorpromazine on the two Ca2+-ATPase activities, with only the plasma membrane enzyme being inhibited. It is concluded that the two platelet Ca2+ transport systems show biochemical differences in spite of the previously shown similarity in the molecular masses of their Ca2+-ATPases, thus conferring a definite specificity to the platelet system.

Inhibitory action of cyclic AMP on inositol 1,4,5-trisphosphate-induced Ca2+ release in saponin-permeabilized platelets

Ca2+ release triggered by inositol 1,4,5-trisphosphate (IP3) has been measured in saponin-permeabilized human platelets with quin2 or 45Ca2+. Ca2+ was sequestered by intracellular organelles in the presence of ATP, and IP3 released half of the sequestered Ca2+. The addition of cyclic AMP (cAMP) to permeabilized platelets transiently accelerated Ca2+ sequestration, but did not alter the steady-state level. In contrast, IP3-induced Ca2+ release was greatly inhibited by cAMP. Phorbol myristate acetate, an activator of protein kinase C did not affect IP3-induced Ca2+ release. These results indicate that cAMP may be involved in the regulation of IP3-induced Ca2+ release in human platelets.

Calcium sequestration in human platelets: is it stimulated by protein kinase C?

Sequestration of calcium into an intracellular storage site is an important mechanism in helping to maintain a low cytoplasmic Ca2+ level in many cells. In platelets, increasing cytoplasmic cAMP lowers the free calcium level in correlation with the phosphorylation of a 22 kD protein. This protein has been thought to enhance uptake of calcium into a platelet membrane bound storage site by activating a calcium-ATPase activity by analogy with phospholamban in cardiac muscle. The evidence for an analogue of phospholamban in platelets is unclear. A pathway involving cAMP dependent kinase also seems unlikely to account for the transience of the calcium signal following agonists in platelets, some of which inhibit the cAMP dependent kinase. Here we discuss the issue of whether activation of protein kinase C, which follows agonist action, leads to enhanced calcium sequestration in platelets and if so, what indications there are for a mechanism. The evidence from our experiments with phorbol myristate acetate treated platelets shows that such an enhancement can be produced by activating protein kinase C. Phosphorylation studies suggest the involvement of a polypeptide or polypeptides distinct from the 22 kD polypeptide. Further work to test this idea is necessary. A brief overview of research on the role of phosphoproteins in calcium regulation in platelets and comparison with their role in cardiac muscle is also presented.

A ras-related protein is phosphorylated and translocated by agonists that increase cAMP levels in human platelets

The antigenicity of platelet proteins was assayed against various monoclonal antibodies (mAbs) that recognize specific epitopes of the ras-encoded p21 protein. mAb M90, which detects the region of p21 protein within amino acids 107-130 and inhibits its GTP-binding activity, strongly reacted with a 22-kDa protein present in the particulate fraction of human platelets. Other mAbs against ras-encoded proteins, including Y13-259, which efficiently detects ras proteins from a variety of organisms, did not recognize the platelet 22-kDa protein. Transfer of the platelet 22-kDa protein to nitrocellulose paper showed that the protein binds [alpha-32P]GTP. Moreover, preincubation of the transferred protein with mAb M90 drastically reduced its GTP-binding activity. Treatment of platelets with iloprost, a prostacyclin analog, caused (i) a time-dependent increase of a 24-kDa protein that is recognized by mAb M90 in particulate and cytosolic fractions and (ii) the gradual decrease of the 22-kDa protein from the particulate fraction. When platelets were labeled with 32P and then treated with iloprost, the 24-kDa protein was found to be phosphorylated. The 32P-labeled 24-kDa protein was specifically immunoprecipitated by mAb M90. These results suggest that appearance of the 24-kDa protein results from phosphorylation of the 22-kDa protein, which shifts its mobility to a higher molecular mass area.

Release of Ca2+ by inositol 1,4,5-trisphosphate in platelet membrane vesicles is not dependent on cyclic AMP-dependent protein kinase

In contrast with previous reports, it was found that membrane-protein phosphorylation by the catalytic subunit (CS) of cyclic AMP-dependent protein kinase had no effect on Ca2+ uptake into platelet membrane vesicles or on subsequent Ca2+ release by inositol 1,4,5-trisphosphate (IP3). Furthermore, IP-20, a highly potent synthetic peptide inhibitor of CS, which totally abolished membrane protein phosphorylation by endogenous or exogenous CS, also had no effect on either Ca2+ uptake or release by IP3. Commercial preparations of protein kinase inhibitor protein (PKI) usually had no effect, but one preparation partially inhibited Ca2+ uptake, which is attributable to the gross impurity of the commercial PKI preparation. IP3-induced release of Ca2+ was also unaffected by the absence of ATP from the medium, supporting the conclusion that Ca2+ release by IP3 does not require the phosphorylation of membrane protein.

Coordinate interactions of cyclic nucleotide and phospholipid metabolizing pathways in calcium-dependent cellular processes

It is hoped that his review enables the reader to appreciate the complexities implicit in the interactions among Ca2+, cyclic nucleotides, and phospholipid-metabolizing pathways in cell signal transduction. The interactions are varied and intricate, often involving several levels of cell amplification mechanisms. Upsetting the balance of fatty acids in membrane phospholipids can have detrimental effects on adenylate cyclase. Thus, n - 3 fatty acid enrichment of phospholipids suppresses adenylate cyclase activity. The effects of significant alterations in dietary fatty acids, such as might occur with the current vogue for n - 3 eicosapentaenoic acid and docosahexaenoic acid (fish oil) dietary enrichment regimens, will need to be assessed more fully with regard to stimulus-induced changes in cyclic nucleotide production in various tissues. Since the n - 3 fatty acids have not been demonstrated to affect guanylate cyclase activity, dietary changes in certain of these fatty acids would not be expected to contribute to changes in cGMP generation as much as in cAMP production. Moreover, the ingestion of large quantities of these n - 3 fatty acids can alter the profile of cyclooxygenase and lipoxygenase products produced in cells. According to the paradigm developed in this article, changes in the metabolism of fatty acids are amplified by alterations in cyclic nucleotide production and phospholipase activities, with the eventual physiological impact predicated on the tissue type and the specific stimulus response. There appears to be a rather clear distinction between the regulatory properties of eicosanoids regarding adenylate and guanylate cyclase activities. Whereas prostaglandins often stimulate adenylate cyclase activity, they have little effect on guanylate cyclase activity. On the other hand, the HETE compounds seem to play an important role in guanylate cyclase regulation in certain cells. Moreover, arachidonic acid affects adenylate cyclase activity without prior peroxidation, whereas endoperoxides and hydroperoxides are more effective than arachidonic acid with regard to guanylate cyclase stimulation. However, in the intact cell there is a strong implication that the dual stimulation of guanylate cyclase by Ca2+ and fatty acid evokes optimal enzyme activity. An advantage of multidimensional response mechanisms in cells includes the ability to recognize different stimuli and to respond with specific, coordinated responses modulated in their intensity and/or duration by messenger interaction. Few cell types respond to receptor stimulation in an all-or-none fashion, and the "milieu interior" depends on specific, graded responses to the autonomic nervous system and endocrine stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

cAMP-dependent protein phosphorylation of membrane proteins in the parotid gland, platelets and liver. Comparison of a 22-kDa phosphoprotein from rat parotid microsomes (protein III) with phosphoproteins of similar molecular size from platelet and liver membranes

Stimulation of secretion in exocrine secretory glands leads to the phosphorylation of a 22-kDa membrane protein (protein III) whose function is still unknown [Jahn et al. (1980) Eur. J. Biochem. 112, 345-352; Jahn & Söling (1980) Proc. Natl Acad. Sci. USA 78, 6903-6906]. This report describes the comparison of this protein with phosphorylated membrane proteins of similar molecular mass in platelets and liver. Incubation of platelets with agents which raise the intracellular cAMP concentration results in the phosphorylation of a 22-kDa protein which is also phosphorylated in membrane preparations by endogenous kinases or by exogenous cAMP-dependent protein kinase. It is shown that this protein is distinct from protein III although both proteins have the same molecular mass and are substrates of cAMP-dependent protein kinase. In contrast to platelets, protein III could be demonstrated in liver microsomes. This indicates that the function of protein III is not exclusively linked to the stimulus-secretion coupling in exocrine cells.

Partial purification and characterization of thrombolamban, a 22,000 dalton cAMP-dependent protein kinase substrate in platelets

In preparations of human platelet microsomes, cyclic AMP-dependent protein kinase induced the rapid phosphorylation of a single protein that was electrophoretically identical to the 22,000 dalton protein (P22) phosphorylated by cAMP in intact platelets. Phosphorylation of the microsomal protein was maximal at one minute and was followed by slow dephosphorylation. Although the protein was associated with a microsomal fraction, it could be separated from the membrane by 2 M NaCl indicating that it was a peripheral protein. Molecular weight was estimated by NaDodSO4-PAGE and by gel filtration chromatography. The molecular weight estimated by NaDodSO4-PAGE was 22,400 daltons and was somewhat larger than the 16,000 molecular weight estimated by gel filtration in the presence of NaDodSO4. In the absence of NaDodSO4, the protein chromatographed as a 36,000 dalton form. The presence of the 36,000 dalton form was not dependent on the phosphorylation state of the protein. The partially purified protein contained phosphoserine, but no phosphothreonine or phosphotyrosine. Two dimensional NaDodSO4-PAGE and isoelectric focusing of the phosphorylated protein revealed isomers with pl values of 5.9 and 6.3. These studies indicate that the 22 kDa microsomal protein and P22 in intact platelets are the same protein and that the 22 kDa protein is tightly bound to the microsomal membrane although the nature of this binding and the microsomal component(s) to which it is bound remain to be determined. We conclude that the 22 kDa protein in platelet microsomes is structurally distinct from, but functionally similar to, phospholamban, the cAMP-dependent protein kinase substrate in muscle, and may play a similar role in calcium transport. Based on this similarity, it is proposed that the 22 kDa protein in platelets be called thrombolamban.

Relationship between cAMP and Ca2+ fluxes in human platelet membranes

The effect of cAMP (which involved a 23 kDa protein phosphorylation) has been studied on the Ca2+ uptake and Ca2+ release from a human platelet membrane vesicle fraction. It was tested in the presence of the catalytic subunit of the cAMP-dependent protein kinase (C Sub). The addition of C Sub increased the steady state level of the Ca2+ uptake into the membrane vesicles. The effect was enhanced when tested in the absence of Ca2+ precipitating agent. The response was proportional to the dose of C Sub. Moreover, the effect varied with the Ca2+ concentration. The effect of C Sub has been tested on the inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release. A phosphorylated state of the 23 kDa protein appeared to be necessary. Indeed, a phosphorylation inhibition prevented the IP3 effect and the addition of C Sub increased the percentage of released Ca2+ (without modification of the time course). However, the C Sub dose-dependent response was not linear. The effect of cAMP on the two functions (Ca2+ uptake and Ca2+ release) appears to be different. Therefore, these results led us to suggest a more complex role of cAMP in the regulation of platelet Ca2+ concentration.