Roles of cyclic nucleotides in platelet function

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

12(S)-HETrE, a 12-Lipoxygenase Oxylipin of Dihomo-γ-Linolenic Acid, Inhibits Thrombosis via Gαs Signaling in Platelets

OBJECTIVE

Dietary supplementation with polyunsaturated fatty acids has been widely used for primary and secondary prevention of cardiovascular disease in individuals at risk; however, the cardioprotective benefits of polyunsaturated fatty acids remain controversial because of lack of mechanistic and in vivo evidence. We present direct evidence that an omega-6 polyunsaturated fatty acid, dihomo-γ-linolenic acid (DGLA), exhibits in vivo cardioprotection through 12-lipoxygenase (12-LOX) oxidation of DGLA to its reduced oxidized lipid form, 12(S)-hydroxy-8Z,10E,14Z-eicosatrienoic acid (12(S)-HETrE), inhibiting platelet activation and thrombosis.

APPROACH AND RESULTS

DGLA inhibited ex vivo platelet aggregation and Rap1 activation in wild-type mice, but not in mice lacking 12-LOX expression (12-LOX(-/-)). Similarly, wild-type mice treated with DGLA were able to reduce thrombus growth (platelet and fibrin accumulation) after laser-induced injury of the arteriole of the cremaster muscle, but not 12-LOX(-/-) mice, supporting a 12-LOX requirement for mediating the inhibitory effects of DGLA on platelet-mediated thrombus formation. Platelet activation and thrombus formation were also suppressed when directly treated with 12(S)-HETrE. Importantly, 2 hemostatic models, tail bleeding and arteriole rupture of the cremaster muscle, showed no alteration in hemostasis after 12(S)-HETrE treatment. Finally, the mechanism for 12(S)-HETrE protection was shown to be mediated via a Gαs-linked G-protein-coupled receptor pathway in human platelets.

CONCLUSIONS

This study provides the direct evidence that an omega-6 polyunsaturated fatty acid, DGLA, inhibits injury-induced thrombosis through its 12-LOX oxylipin, 12(S)-HETrE, which strongly supports the potential cardioprotective benefits of DGLA supplementation through its regulation of platelet function. Furthermore, this is the first evidence of a 12-LOX oxylipin regulating platelet function in a Gs α subunit-linked G-protein-coupled receptor-dependent manner.

Loss of high-affinity prostacyclin receptors in platelets and the lack of prostaglandin-induced inhibition of platelet-stimulated thrombin generation in subjects with spinal cord injury

Coronary artery disease is a leading cause of death in individuals with chronic spinal cord injury (SCI). However, platelets of those with SCI (n = 30) showed neither increased aggregation nor resistance to the antiaggregatory effects of prostacyclin when compared with normal controls (n = 30). Prostanoid-induced cAMP synthesis was similar in both groups. In contrast, prostacyclin, which completely inhibited the platelet-stimulated thrombin generation in normal controls, failed to do so in those with SCI. Scatchard analysis of the binding of [3H]prostaglandin E1, used as a prostacyclin receptor probe, showed the presence of one high-affinity (Kd1 = 8.11 +/- 2.80 nM; n1 = 172 +/- 32 sites per cell) and one low-affinity (Kd2 = 1.01 +/- 0.3 microM; n2 = 1772 +/- 226 sites per cell) prostacyclin receptor in normal platelets. In contrast, the same analysis in subjects with SCI showed significant loss (P < 0.001) of high-affinity receptor sites (Kd1 = 6.34 +/- 1.91 nM; n1 = 43 +/- 10 sites per cell) with no significant change in the low affinity-receptors (Kd2 = 1.22 +/- 0.23; n2 = 1820 +/- 421). Treatment of these platelets with insulin, which has been demonstrated to restore both of the high- and low-affinity prostaglandin receptor numbers to within normal ranges in coronary artery disease, increased high-affinity receptor numbers and restored the prostacyclin effect on thrombin generation. These results demonstrate that the loss of the inhibitory effect of prostacyclin on the stimulation of thrombin generation was due to the loss of platelet high-affinity prostanoid receptors, which may contribute to atherogenesis in individuals with chronic SCI.

The effect of a prostaglandin DP-receptor partial agonist (192C86) on platelet aggregation and the cardiovascular system in healthy volunteers

1. PGD2 (DP)-receptors mediate inhibition of platelet aggregation and vasodilatation. If receptor reserve were greater on platelets it might be possible to separate these effects. To determine whether such a difference in receptor reserve exists, we have examined the effects of a highly selective DP-receptor partial agonist 192C86 on platelet aggregation and the cardiovascular system in healthy volunteers. 2. Using an open, dose-escalating study design, four male volunteers received constant rate intravenous infusions of 192C86 for up to 60 min. Ex vivo platelet aggregation to ADP and collagen in platelet-rich plasma (PRP) and whole blood (WB) was studied at baseline, after 15, 30 and 60 min of each infusion and at 180 min post-infusion. Heart rate (HR), systolic and diastolic (DBP) blood pressure were measured at frequent intervals. Adverse experiences were monitored by checklist. Facial flushing was assessed by the volunteer using a visual analogue scale, by an observer using a numerical scale and by full-face colour photographs. Blood was taken for assay of plasma 192C86 concentrations by radio-immunoassay (r.i.a.). 3. 192C86 (0.007-0.058 micrograms kg-1 min-1) inhibited platelet aggregation to ADP and collagen both in PRP and WB in a dose-dependent manner. However, this was always accompanied by a decrease in DBP, increase in HR and facial flushing. Plasma concentrations of 192C86 were at or below the limits of sensitivity of the r.i.a. (0.5 ng ml-1). 4. The highest infusion rate was stopped after 20 min due to symptomatic hypotension on standing.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of prostaglandin D2 with prostacyclin, carbacyclin and the hydantoin prostaglandin, BW245C, in guinea-pig platelets

The anti-aggregating actions of prostaglandin D2 (PGD2) have been compared to prostacyclin (PGI2), its stable analogue carbacyclin and a hydantoin prostaglandin, BW245C, in guinea-pig PGI2, carbacyclin and BW245C were potent inhibitors of ADP-induced aggregation in guinea-pig platelets, with ID50 values comparable to those obtained in human platelet-rich-plasma. In contrast, PGD2 acted as a weak and partial inhibitor in guinea-pig platelet aggregation, producing a bell-shaped dose-response relationship. PGD2 induced a dose-related antagonism of the inhibitory actions of BW245C, prostacyclin and carbacyclin on guinea-pig platelets. However, PGD2 did not antagonize the inhibitory actions of either forskolin or dibutyryl cyclic AMP on this platelet preparation. The results suggest a non-specific interaction of PGD2 with these prostanoid binding sites on guinea-pig platelets.

The effect of low-density lipoproteins on the synthesis of cyclic nucleotides induced by prostacyclin in isolated platelets

Isolated platelets are strongly sensitized by the presence of low-density lipoproteins (LDL) so that they aggregate with very low concentrations of other agonists or exhibit spontaneous aggregation. Prostacyclin (PGI2) is a potent inhibitor of aggregation, but its action was reversed by LDL. This effect of LDL was accompanied by a decrease in the synthesis of cyclic AMP induced by PGI2, but its efficacy depended on the relative concentrations of LDL and PGI2. PGI2 also enhanced the synthesis of cyclic GMP, but this was completely reversed by the presence of LDL. LDL did not remove inhibitory prostaglandins, e.g. E1, from their receptor sites. The lipoproteins also decreased cyclic AMP synthesis induced by forskolin, which has its effect on the GTP-sensitive protein or the catalytic unit of the adenylate cyclase enzyme complex. It is proposed that LDL may act on the enzyme catalytic unit via an inhibitory GTP-sensitive protein or by a separate mechanism which indirectly impedes the production of cyclic AMP.

Effects of alpha 2-adrenoceptor agonists and of related compounds on aggregation of, and on adenylate cyclase activity in, human platelets

A range of 2-(,5-dihydroimidazolyl)-benzene, -quinoline, and -quinoxaline derivatives and 2-morpholino-4-catechol have been characterized as agonists or partial agonists for human platelet aggregation; and for inhibition of adenylate cyclase by measurement of their effect on platelet [cyclic-3',5'-AMP]. Antagonist activity for these compounds versus adrenaline as agonist has also been assessed for these two responses. The compounds can be divided into 4 groups. Group I contains compounds that are agonists for both responses; group II, compounds that are agonists for inhibition of adenylate cyclase but antagonists for the aggregatory response; group III, compounds that are agonists for the aggregatory response but are antagonists for inhibition of adenylate cyclase by adrenaline; and group IV, compounds that are antagonists for both responses. In group I the EC50 values for induction of aggregation are not significantly different from the EC50 values for inhibition of adenylate cyclase except for 2-morpholino-4-catechol which is significantly more potent as an inhibitor of adenylate cyclase. In group IV a linear correlation is observed between the K1 values for the two responses for 8 compounds but 2 other compounds do not conform to this correlation. The data are not consistent with a model in which a single chi 2-adrenoceptor mediates both the aggregatory response and inhibition of adenylate cyclase and hence support a model in which unique chi 2-adrenoceptors mediate these two responses.

Metabolic aspects of the secretion of stored compounds from blood platelets. The effect of NaF at different pH on nucleotide metabolism and function of washed platelets

The purpose of this study was to investigate the response of human blood platelets to fluoride at different pH. The results were as follows. (1) Fluoride induced secretion faster and at a lower concentration when pH was lowered. (2) Platelets exposed to 2 mM-fluoride at 0 degrees C at pH 5.3 underwent secretion when first pH and then temperature was raised, although no secretion was seen at 2 mM-fluoride concentration in the absence of the preincubation at low pH. (3) The concentration of [14C]ATP in platelets decreased steeply in response to fluoride before induction of secretion. Addition of antimycin blocked or partly inhibited secretion. Fluoride thus exerts an inhibitory effect on platelet glycolysis before induction of secretion. (4) Fluoride accumulated in the platelet pellet by a time course that preceded secretion. The accumulation was faster and greater at pH 6 than at 7.4. These four points are taken as indirect evidence that fluoride has to penetrate to the interior of the platelet to induce secretion. The activation takes place over a wide range of acid pH in contrast with induction of platelet function via the outside of the plasma membrane. In addition evidence is presented that the salvage pathway may under special circumstances play an important role in the re-synthesis of platelet adenine nucleotides.

Interaction of selective alpha-adrenoceptor agonists and antagonists with human and rabbit blood platelets

1 The selectivity of alpha-adrenoceptors mediating the pro-aggregatory response of human and rabbit platelets to adrenaline and the conditions required to permit expression of an aggregatory response to partial agonists at these alpha-adrenoceptors have been studied.2 Yohimbine causes effective blockade of the pro-aggregatory responses whereas indoramin and prazosin are ineffective.3 The clonidine analogue, UK-14304, is nearly as effective as adrenaline in inducing an aggregatory response in human platelets and a pro-aggregatory response in rabbit platelets. Cross-tachyphylaxis between adrenaline and UK-14304 has been demonstrated.4 Clonidine is a weak agonist for the pro-aggregatory response of rabbit platelets and in some donors for the aggregatory response of human platelets.5 Methoxamine induces a pro-aggregatory response in human platelets which is blocked by indoramin or prazosin but not by yohimbine. No such response to methoxamine is observed in rabbit platelets.6 The divalent cation ionophore, A-23187, induces an aggregatory response to clonidine (in platelets from a non-responsive donor), phenylephrine and methoxamine in human platelets and to adrenaline, UK-14304 and clonidine in rabbit platelets. A secretory response to clonidine is also induced by A-23187 in human platelets.7 The adenylate cyclase inhibitor, SQ-22536, is ineffective in either inducing a response to the alpha-agonists or potentiating the effect of A-23187.8 The aggregatory responses to adrenaline and UK-14304 in rabbit platelets and to clonidine in human and rabbit platelets, which can be induced by A-23187, are blocked by yohimbine but not by prazosin or indoramin.9 From these studies we conclude that the pro-aggregatory responses of human and rabbit platelets to adrenaline are mediated primarily by alpha(2)-adrenoceptors. The presence of alpha(1)-adrenoceptors on human platelets is confirmed but these receptors do not appear to be present on rabbit platelets. The conditions required for expression of an aggregatory response to partial agonists at the human and rabbit platelet alpha-adrenoceptors implicate an increase in cytosolic Ca(2+) concentration as a key event in stimulus-response coupling but do not indicate such a role for depression of cyclic adenosine-3',5'-monophosphate concentration.

Inhibition of adenosine deaminase and of platelet aggregation by 2-azidoadenosine, a photolysable analogue of adenosine

Aggregation of human blood platelets induced by ADP was strongly inhibited by 2-azidoadenosine, a photolysable analogue of adenosine. The deamination of adenosine by adenosine deaminase (E. C. 3.5.4.4) was competitively inhibited by 2-azidoadenosine. In the presence of 2-azidoadenosine the enzyme was inactivated by u. v. radiation. The inactivation was diminished in the presence of a protein scavanger.