Agonist receptors and G proteins as mediators of platelet activation

Cinchonine: A Versatile Pharmacological Agent Derived from Natural Cinchona Alkaloids

BACKGROUND

Cinchonine is one of the Cinchona alkaloids that is commercially extracted from the Peruvian bark of Cinchona officinalis L. (Family: Rubiaceae). It is also obtained in much lower quantities from other species of Cinchona, such as Cinchona calisaya, Cinchona succirubra, and Cinchona pubescens, and in some other plants, such as Remijia peruviana. Cinchonine has been historically used as an anti-malarial agent. It also has a wide range of other biological properties, including anti-cancer, anti-obesity, anti-inflammatory, anti-parasitic, antimicrobial, anti-platelet aggregation, and anti-osteoclast differentiation.

AIM AND OBJECTIVE

This review discusses the pharmacological activity of cinchonine under different experimental conditions, including in silico, in vitro, and in vivo. It also covers the compound's physicochemical properties, toxicological aspects, and pharmacokinetics.

METHODOLOGY

A comprehensive literature search was conducted on multiple online databases, such as PubMed, Scopus, and Google Scholar. The aim was to retrieve a wide range of review/research papers and bibliographic sources. The process involved applying exclusion and inclusion criteria to ensure the selection of relevant and high-quality papers.

RESULTS

Cinchonine has numerous pharmacological properties, making it a promising compound for various therapeutic applications. It induces anti-cancer activity by activating caspase-3 and PARP-1, and triggers the endoplasmic reticulum stress response. It up-regulates GRP78 and promotes the phosphorylation of PERK and ETIF-2α. Cinchonine also inhibits osteoclastogenesis, inhibiting TAK1 activation and suppressing NFATc1 expression by regulating AP-1 and NF-κB. Its potential anti-inflammatory effects reduce the impact of high-fat diets, making it suitable for targeting obesity-related diseases. However, research on cinchonine is limited, and further studies are needed to fully understand its therapeutic potential. Further investigation is needed to ensure its safety and efficacy in clinical applications.

CONCLUSION

Overall, this review article explains the pharmacological activity of cinchonine, its synthesis, and physicochemical properties, toxicological aspects, and pharmacokinetics.

The G protein α subunit variant XLαs promotes inositol 1,4,5-trisphosphate signaling and mediates the renal actions of parathyroid hormone in vivo

GNAS, which encodes the stimulatory G protein (heterotrimeric guanine nucleotide-binding protein) α subunit (Gαs), also encodes a large variant of Gαs termed extra-large α subunit (XLαs), and alterations in XLαs abundance or activity are implicated in various human disorders. Although XLαs, like Gαs, stimulates generation of the second messenger cyclic adenosine monophosphate (cAMP), evidence suggests that XLαs and Gαs have opposing effects in vivo. We investigated the role of XLαs in mediating signaling by parathyroid hormone (PTH), which activates a G protein-coupled receptor (GPCR) that stimulates both Gαs and Gαq/11 in renal proximal tubules to maintain phosphate and vitamin D homeostasis. At postnatal day 2 (P2), XLαs knockout (XLKO) mice exhibited hyperphosphatemia, hypocalcemia, and increased serum concentrations of PTH and 1,25-dihydroxyvitamin D. The ability of PTH to reduce serum phosphate concentrations was impaired, and the abundance of the sodium phosphate cotransporter Npt2a in renal brush border membranes was reduced in XLKO mice, whereas PTH-induced cAMP excretion in the urine was modestly increased. Basal and PTH-stimulated production of inositol 1,4,5-trisphosphate (IP3), which is the second messenger produced by Gαq/11 signaling, was repressed in renal proximal tubules from XLKO mice. Crossing of XLKO mice with mice overexpressing XLαs specifically in renal proximal tubules rescued the phenotype of the XLKO mice. Overexpression of XLαs in HEK 293 cells enhanced IP3 generation in unstimulated cells and in cells stimulated with PTH or thrombin, which acts through a Gq/11-coupled receptor. Together, our findings suggest that XLαs enhances Gq/11 signaling to mediate the renal actions of PTH during early postnatal development.

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.

Negative regulation of Gq-mediated pathways in platelets by G(12/13) pathways through Fyn kinase

Platelets contain high levels of Src family kinases (SFKs), but their functional role downstream of G protein pathways has not been completely understood. We found that platelet shape change induced by selective G(12/13) stimulation was potentiated by SFK inhibitors, which was abolished by intracellular calcium chelation. Platelet aggregation, secretion, and intracellular Ca(2+) mobilization mediated by low concentrations of SFLLRN or YFLLRNP were potentiated by SFK inhibitors. However, 2-methylthio-ADP-induced intracellular Ca(2+) mobilization and platelet aggregation were not affected by PP2, suggesting the contribution of SFKs downstream of G(12/13), but not G(q)/G(i), as a negative regulator to platelet activation. Moreover, PP2 potentiated YFLLRNP- and AYPGKF-induced PKC activation, indicating that SFKs downstream of G(12/13) regulate platelet responses through the negative regulation of PKC activation as well as calcium response. SFK inhibitors failed to potentiate platelet responses in the presence of G(q)-selective inhibitor YM254890 or in G(q)-deficient platelets, indicating that SFKs negatively regulate platelet responses through modulation of G(q) pathways. Importantly, AYPGKF-induced platelet aggregation and PKC activation were potentiated in Fyn-deficient but not in Lyn-deficient mice compared with wild-type littermates. We conclude that SFKs, especially Fyn, activated downstream of G(12/13) negatively regulate platelet responses by inhibiting intracellular calcium mobilization and PKC activation through G(q) pathways.

G(12/13) signaling pathways substitute for integrin αIIbβ3-signaling for thromboxane generation in platelets

BACKGROUND

We have previously shown that ADP-induced TXA(2) generation requires signaling from αIIbβ3 integrin in platelets. Here we observed that, unlike ADP, protease-activated receptor (PAR)-mediated TXA(2) generation occurs independently of αIIbβ3. PAR agonists, but not ADP, activate G(12/13) signaling pathways. Hence, we evaluated the role of these pathways in TXA(2) generation.

PRINCIPAL FINDINGS

Inhibition of ADP-induced thromboxane generation by fibrinogen receptor antagonist SC57101 was rescued by co-stimulation of G(12/13) pathways with YFLLRNP. This observation suggested an existence of a common signaling effector downstream of integrins and G(12/13) pathways. Hence, we evaluated role of three potential tyrosine kinases; c-Src, Syk and FAK (Focal Adhesion Kinase) that are known to be activated by integrins. c-Src and Syk kinase did not play a role in ADP-induced functional responses in platelets. Selective activation of G(12/13) pathways resulted in the activation of FAK, in the absence of integrin signaling. Interestingly, αIIbβ3-mediated FAK activation occurred in a Src family kinase (SFK)-independent manner whereas G(12/13) pathway caused FAK activation in a SFK and RhoA-dependent manner. A FAK selective inhibitor TAE-226, blocked TXA(2) generation. However, in comparison to WT mice, Pf4-Cre/Fak-Floxed mice did not show any difference in platelet TXA(2) generation.

CONCLUSIONS

Therefore, we conclude that differential activation of FAK occurs downstream of Integrins and G(12/13) pathways. However, the common effector molecule, possibly a tyrosine kinase downstream of integrins and G(12/13) pathways contributing to TXA(2) generation in platelets remains elusive.

Platelet function and Isoprostane biology. Should isoprostanes be the newest member of the orphan-ligand family?

While there have been many reports investigating the biological activity and signaling mechanisms of isoprostanes, their role in biology, particularly in platelets, appears to still be underestimated. Moreover, whether these lipids have their own receptors is still debated, despite multiple reports that discrete receptors for isporpstanes do exist on platelets, vascular tissues, amongst others. This paper provides a review of the important literature of isoprostanes and provides reasoning that isoprostanes should be classified as orphan ligands until their receptor(s) is/are identified.

Mechanism of activation and functional role of protein kinase Ceta in human platelets

The novel class of protein kinase C (nPKC) isoform eta is expressed in platelets, but not much is known about its activation and function. In this study, we investigated the mechanism of activation and functional implications of nPKCeta using pharmacological and gene knock-out approaches. nPKCeta was phosphorylated (at Thr-512) in a time- and concentration-dependent manner by 2MeSADP. Pretreatment of platelets with MRS-2179, a P2Y1 receptor antagonist, or YM-254890, a G(q) blocker, abolished 2MeSADP-induced phosphorylation of nPKCeta. Similarly, ADP failed to activate nPKCeta in platelets isolated from P2Y1 and G(q) knock-out mice. However, pretreatment of platelets with P2Y12 receptor antagonist, AR-C69331MX did not interfere with ADP-induced nPKCeta phosphorylation. In addition, when platelets were activated with 2MeSADP under stirring conditions, although nPKCeta was phosphorylated within 30 s by ADP receptors, it was also dephosphorylated by activated integrin alpha(IIb)beta3 mediated outside-in signaling. Moreover, in the presence of SC-57101, a alpha(IIb)beta3 receptor antagonist, nPKCeta dephosphorylation was inhibited. Furthermore, in murine platelets lacking PP1cgamma, a catalytic subunit of serine/threonine phosphatase, alpha(IIb)beta3 failed to dephosphorylate nPKCeta. Thus, we conclude that ADP activates nPKCeta via P2Y1 receptor and is subsequently dephosphorylated by PP1gamma phosphatase activated by alpha(IIb)beta3 integrin. In addition, pretreatment of platelets with eta-RACK antagonistic peptides, a specific inhibitor of nPKCeta, inhibited ADP-induced thromboxane generation. However, these peptides had no affect on ADP-induced aggregation when thromboxane generation was blocked. In summary, nPKCeta positively regulates agonist-induced thromboxane generation with no effects on platelet aggregation.

Role for phosphoinositide 3-kinase in Fc gamma RIIA-induced platelet shape change

Platelets transform from disks to irregular spheres, grow filopodia, form ruffles, and spread on surfaces coated with anti-Fc gamma RIIA antibody. Fc gamma RIIA cross-linking leads to a tenfold increase in actin filament barbed end exposure and robust actin assembly. Activation of the small GTPases Rac and Cdc42 follows Fc gamma RIIA cross-linking. Shape change, actin filament barbed end exposure, and quantifiable actin assembly require phosphoinositide 3-kinase (PI3-kinase) activity and a rise in intracellular calcium. PI3-kinase inhibition blocks activation of Rac, but not of Cdc42, and diminishes the association of Arp2/3 complex and CapZ with polymerized actin. Furthermore, addition of constitutively active D-3 phosphorylated polyphosphoinositides or recombinant PI3-kinase subunits to octylglucoside-permeabilized platelets elicits actin filament barbed end exposure by releasing gelsolin and CapZ from the cytoskeleton. Our findings place PI3-kinase activity upstream of Rac, gelsolin, and Arp2/3 complex activation induced by Fc gamma RIIA and clearly distinguish the Fc gamma RIIA signaling pathway to actin filament assembly from the thrombin receptor protease-activated receptor (PAR)-1 pathway.

The assembly of the factor X-activating complex on activated human platelets

Platelet membranes provide procoagulant surfaces for the assembly and expression of the factor X-activating complex and promote the proteolytic activation and assembly of the prothrombinase complex resulting in normal hemostasis. Recent studies from our laboratory and others indicate that platelets possess specific, high-affinity, saturable, receptors for factors XI, XIa, IX, IXa, X, VIII, VIIIa, V, Va and Xa, prothrombin, and thrombin. Studies described in this review support the hypothesis that the factor X-activating complex on the platelet surface consists of three receptors (for the enzyme, factor IXa; the substrate, factor X; and the cofactor, factor VIIIa), the colocalization of which results in a 24 million-fold acceleration of the rate of factor X activation. Whether the procoagulant surface of platelets is defined exclusively by procoagulant phospholipids, or whether specific protein receptors exist for the coagulant factors and proteases, is currently unresolved. The interaction between coagulation proteins and platelets is critical to the maintenance of normal hemostasis and is pathogenetically important in human disease.

ADP receptor antagonists inhibit platelet aggregation induced by the chemokines SDF-1, MDC and TARC

The ability of the chemokines SDF-1, MDC and TARC to induce platelet aggregation depends strongly on low levels of ADP. The ADP receptors involved have now been characterized using the P2Y(1) and P2T(AC) receptor antagonists, A2P5P and AR-C69931MX. Stimulation of aggregation by the chemokines at 10 s was not blocked by AR-C69931MX, but was strongly inhibited by A2P5P. Pertussis toxin abolished the chemokine-stimulated aggregation. We conclude that the P2Y(1) ADP receptor plays a critical role in the initial phases of SDF-1-, MDC- and TARC-induced platelet aggregation, which involve a pertussis toxin-sensitive G protein.

LAT is required for tyrosine phosphorylation of phospholipase cgamma2 and platelet activation by the collagen receptor GPVI

In the present study, we have addressed the role of the linker for activation of T cells (LAT) in the regulation of phospholipase Cgamma2 (PLCgamma2) by the platelet collagen receptor glycoprotein VI (GPVI). LAT is tyrosine phosphorylated in human platelets heavily in response to collagen, collagen-related peptide (CRP), and FcgammaRIIA cross-linking but only weakly in response to the G-protein-receptor-coupled agonist thrombin. LAT tyrosine phosphorylation is abolished in CRP-stimulated Syk-deficient mouse platelets, whereas it is not altered in SLP-76-deficient mice or Btk-deficient X-linked agammaglobulinemia (XLA) human platelets. Using mice engineered to lack the adapter LAT, we showed that tyrosine phosphorylation of Syk and Btk in response to CRP was maintained in LAT-deficient platelets whereas phosphorylation of SLP-76 was slightly impaired. In contrast, tyrosine phosphorylation of PLCgamma2 was substantially reduced in LAT-deficient platelets but was not completely inhibited. The reduction in phosphorylation of PLCgamma2 was associated with marked inhibition of formation of phosphatidic acid, a metabolite of 1,2-diacylglycerol, phosphorylation of pleckstrin, a substrate of protein kinase C, and expression of P-selectin in response to CRP, whereas these parameters were not altered in response to thrombin. Activation of the fibrinogen receptor integrin alpha(IIb)beta(3) in response to CRP was also reduced in LAT-deficient platelets but was not completely inhibited. These results demonstrate that LAT tyrosine phosphorylation occurs downstream of Syk and is independent of the adapter SLP-76, and they establish a major role for LAT in the phosphorylation and activation of PLCgamma2, leading to downstream responses such as alpha-granule secretion and activation of integrin alpha(IIb)beta(3). The results further demonstrate that the major pathway of tyrosine phosphorylation of SLP-76 is independent of LAT and that there is a minor, LAT-independent pathway of tyrosine phosphorylation of PLCgamma2. We propose a model in which LAT and SLP-76 are required for PLCgamma2 phosphorylation but are regulated through independent pathways downstream of Syk.

p-Aminobenzoic acid and its metabolite p-acetamidobenzoic acid inhibit agonist-induced aggregation and arachidonic acid-induced [Ca2+]i transients in human platelets

We have previously found that the naturally occurring amine p-aminobenzoic acid (PABA) inhibits the thrombin-induced thromboxane B2 production in human platelets. In this report we show that PABA and its acetylated metabolite p-acetamidobenzoic acid (PACBA) inhibit platelet aggregation induced by agonists such as adenosine diphosphate (ADP) and arachidonic acid (AA). Both substances were equipotent to acetylsalicylic acid regarding inhibition of ADP-induced aggregation and approximately 50% as potent as acetylsalicylic acid regarding arachidonic acid-induced aggregation. Although not significantly inhibiting collagen aggregation, PABA and PACBA reduced the concomitant adenosine triphosphate (ATP) secretion by approximately 30 and 20%, respectively. The antiaggregatory effect does not seem to be mediated through cyclic adenosine monophosphate (cAMP) increase because in our experiments PABA and PACBA did not significantly affect cAMP levels. However, we have found that PABA and PACBA inhibit the intracellular aequorin indicated Ca2+ transient upon arachidonic acid stimulation. Our results describe a hitherto unknown effect of PABA and PACBA on platelet aggregation.

trans-Resveratrol inhibits calcium influx in thrombin-stimulated human platelets

1. The phytoestrogenic compound trans-resveratrol (trans-3,5, 4'-trihydroxystilbene) is found in appreciable quantities in grape skins and wine. It has been shown that both products rich in trans-resveratrol and pure trans-resveratrol inhibit platelet aggregation both in vivo and in vitro. However the mechanism of this action still remains unknown. 2. An essential component of the aggregation process in platelets is an increase in intracellular free Ca2+ ([Ca2+]i). Ca2+ must enter the cell from the external media through specific and tightly regulated Ca2+ channels in the plasma membrane. The objective of this study was to characterize what effect trans-resveratrol had on the Ca2+ channels in thrombin stimulated platelets. 3. In this study we showed that trans-resveratrol immediately inhibited Ca2+ influx in thrombin-stimulated platelets with an IC50 of 0.5 microM. trans-Resveratrol at 0.1, 1.0 and 10.0 microM produced 20+/-6, 37+/-6 and 57+/-4% inhibition respectively of the effect of thrombin (0.01 u ml(-1)) to increase [Ca2+]i. 4. trans-Resveratrol also inhibited spontaneous Ba2+ entry into Fura-2 loaded platelets, with 0.1, 1.0 and 10.0 microM trans-resveratrol producing 10+/-5, 30+/-5 and 50+/-7% inhibition respectively. This indicated that trans-resveratrol directly inhibited Ca2+ channel activity in the platelets in the absence of agonist stimulation. 5. trans-Resveratrol also inhibited thapsigargin-mediated Ca2+ influx into platelets. This suggests that the store-operated Ca2+ channels are one of the possible targets of trans-resveratrol. These channels rely on the emptying of the internal Ca2+ stores to initiate influx of Ca2+ into the cell. 6. The phytoestrogens genistein, daidzein, apigenin and genistein-glucoside (genistin) produced inhibitory effects against thrombin similar to those seen with trans-resveratrol. 7. We conclude that trans-resveratrol is an inhibitor of store-operated Ca2+ channels in human platelets. This accounts for the ability of trans-resveratrol to inhibit platelet aggregation induced by thrombin.

The inhibitory guanine nucleotide-binding protein Gi2alpha induces and potentiates adipocyte differentiation

The present study further elucidates the involvement of the alpha-subunit of the GTP-binding protein Gi2 in the differentiation of murine 3T3-L1 cells. Control and vector-transfected cells attained a fully differentiated adipocyte phenotype showing ample lipid droplets. Cells expressing wild type (WT)-Gi2alpha or the constitutively active R179E-Gi2alpha, however, became enlarged, less confluent, and produced large amounts of lipids. Differentiation consistently increased the triglyceride (TAG) content in control cells. In both WT-Gi2alpha and R179E-Gi2alpha clones, a marked increase in TAG could be detected even prior to insulin/dexamethasone/isobutyl methylxanthine exposure. The activity of palmitoyl-CoA synthetase (PCS) and glycerophosphate acyltransferase (GPAT) also increased upon differentiation. WT-Gi2alpha and R179E-Gi2alpha overexpression also enhanced PCS and GPAT activities even before differentiation medium was added. The total amount of phospholipids (PL) generally increased upon differentiation; however, pre- and postdifferentiation values were insignificantly different in cells expressing WT-Gi2alpha and R179E-Gi2alpha. Differentiation altered the PL profile with a relative shift from phosphatidylcholine and phosphatidylethanolamine to phosphatidylinositol (PI) in differentiated cells. Finally, differentiation yielded a general increase in the activity of basal PI-phospholipase-C activity. Again, cells expressing WT-Gi2alpha and R179E-Gi2alpha demonstrated elevated enzyme activity and enhanced second messenger accumulation subsequent to differentiation. In summary, cells with the R179E-mutants of Gi2alpha exhibited stimulated lipid turnover and accumulation in both undifferentiated and differentiated cells.

Binding of a thrombin receptor tethered ligand analogue to human platelet thrombin receptor

A thrombin receptor-radioligand binding assay was developed using [3H]A(pF-F)R(ChA)(hR)Y-NH2 ([3H]haTRAP), a high affinity thrombin receptor-activating peptide (TRAP), and human platelet membranes. Scatchard analysis of saturation binding data indicated that [3H]haTRAP bound to platelet membranes with a Kd of 15 nM and a Bmax of 5.2 pmol/mg of protein. The binding was reduced by GPPNHP, a nonmetabolizable GTP analogue. Various TRAPs and a TRAP antagonist, but not other receptor agonists, displaced [3H]haTRAP from the binding sites. SFLLRN-NH2, a thrombin receptor-tethered ligand analogue, and [3H]haTRAP exhibited competitive binding for the same binding sites. The relative affinity of these peptides for the binding site paralleled their EC50 or IC50 values for platelet aggregation. These data indicate that [3H]haTRAP binds specifically and saturably to the functioning G protein-linked thrombin (tethered ligand) receptor in human platelet membranes.

Importance of photo activation of rose bengal for platelet activation in experimental models of photochemically induced thrombosis

Generation of oxygen free radicals in the lumen of a vessel leads to NO inactivation, modification of lipid components of platelets and endothelial cell membranes, and platelet activation. On this basis, many experimental models of thrombosis have been developed, where the formation of a platelet rich thrombus follows the illumination of a vessel with an appropriately filtered light after intravenous administration of Rose Bengal or another sensitizing dye. However the detailed mechanism of thrombus formation remains poorly known. This work appreciates the contribution of platelet activation directly induced by oxygen free radicals in formation of the platelet rich thrombus, from a study of human platelet aggregation in presence of photo-activated Rose Bengal. The results demonstrate that direct activation of platelets by free radicals generated by Rose Bengal is of low or no importance in formation of the thrombus. Therefore, the main trigger of platelet aggregation and thrombosis in these models is primary endothelial cells injury.

Defective thrombin-induced calcium changes and aggregation of Bernard-Soulier platelets are not associated with deficient moderate-affinity receptors

Cloning of the moderate-affinity, serpentine thrombin receptor has helped clarify the mechanism of thrombin-induced platelet activation. Proteolytic cleavage by thrombin generates a new amino terminal that autostimulates the receptor, leading to activation of multiple signaling pathways and the platelet response. The function of other thrombin receptors, such as high-affinity glycoprotein Ib (GPIb), on platelets and their relationships to the moderate-affinity receptor remain unclear. The present study examined the role of the moderate-affinity thrombin receptor in Bernard-Soulier syndrome (BSS) platelets, which contain low amounts of GPIb. Platelets from four BSS subjects displayed normal aggregation profiles and cytosolic calcium changes in response to moderate or high concentrations of thrombin. In contrast, the BSS platelet aggregation response was delayed and calcium changes were absent in response to low thrombin concentrations. Platelets from an asymptomatic BSS heterozygote displayed an activation profile similar to those of control individuals. Specific activation of the moderate-affinity receptor by a synthetic peptide caused similar aggregation in platelets from all individuals. The synthetic peptide also elicited calcium responses in BSS platelets. Platelets from the BSS subjects and from an individual with the May-Hegglin anomaly showed increased expression of the moderate-affinity thrombin receptor by flow-cytometric analyses. These results suggest that BSS platelets possess high levels of a functional moderate-affinity thrombin receptor, probably due to large platelet size, and provide indirect evidence that a high-affinity thrombin receptor is associated with GPIb.