How does fluoroaluminate activate human platelets?

The antiplatelet activity of Camel milk in healthy and aluminum chloride-intoxicated rats

This study examined the effect of camel milk on some marker of blood coagulation markers in aluminum chloride (ALCl₃)-treated rats. Rats (n = 6) were assigned as control, control + fresh camel milk (1 ml), ALCl₃ (0.5 mg/kg), and ALCl₃ + fresh camel milk (1 ml and 0.5 mg/kg, respectively). Treatments were conducted orally for 30 days and daily. Administration of camel milk to control and ALCl₃-intoxicated rats significantly increased platelet count, bleeding time, and collagen epinephrine (CEPI)-induced platelet aggregation. It also lowered plasma levels of thromboxane B2 and hepatic levels of glutathione (GSH) and the activities of antioxidant enzymes, catalase (CAT) and superoxide dismutase (SOD). While the treatment with camel milk has no effect on the liver structure, values of activated partial prothrombin time (aPPT), and levels of prothrombin time (PT) in control rats, it improved liver architectures and decreased serum levels alanine and aspartate aminotransferases (ALT and AST, respectively), and reduced values of both aPTT and PT in ALCl₃-intoxicated rats. In conclusion, camel milk inhibits platelets activity and aggregation in both control and ALCl₃-intoxicated rats.

Using water quality and isotope studies to inform research in chronic kidney disease of unknown aetiology endemic areas in Sri Lanka

Chronic Kidney Disease of unknown aetiology (CKDu) is a major public health concern in dry climatic, agricultural regions of Sri Lanka. The chemistry of groundwater (the main source of drinking water) in the area has been studied extensively, in relation to the occurrence of CKDu. This paper investigates water quality studies published in CKDu affected areas of Sri Lanka and also presents a new data set of 27 hydrochemical and isotopic samples collected from groundwater wells in selected CKDu endemic areas in Sri Lanka. The study outcomes do not provide evidence of pollutants such as heavy metals in groundwater. However, the study identifies elevated concentrations of silica which requires further investigation. Two groups of groundwater have been identified based on the isotopic results suggesting different sources or origins. The available water quality data, including the data from this study, is not sufficient to answer questions on whether the chemistry of groundwater is related to the CKDu occurrence. However, this study identifies the importance of detailed investigation into degradation products of agrochemicals, the organic matter content and the influence of elevate silica concentration in groundwater. The study also provides research directions in the form of isotopic tracers and the frequency of sampling that is needed to capture potential pollutants in future groundwater quality studies in CKDu endemic areas in Sri Lanka.

The impact of aluminum, fluoride, and aluminum-fluoride complexes in drinking water on chronic kidney disease

It is suspected that drinking water containing fluoride and aluminum results in negative health effects especially on brain, liver, and kidney. In this investigation, the effect of F, Al, and AlFx complex on chronic kidney disease (CKD) was investigated. Mice were treated either with WHO recommended or slightly higher F and Al levels in drinking water. Treatment solutions contained 0.05-10.0 mg/L of F, 0.08-10.0 mg/L of Al, or 0.07-15 mg/L of AlFx, and the treatment period was 42 weeks. Blood urea level and creatinine levels were investigated as a measure of malfunction of kidneys. Histopathological evaluations of kidney tissues were carried out to assess the extent of damage that F, Al, and AlFx complex could cause. It was demonstrated that the treated drinking water containing F and Al with par with WHO or moderately above the WHO levels or AlFx in low level (0.07-15 mg/L) does not lead to CKD in mice.

Determination of serum aluminum, platelet aggregation and lipid peroxidation in hemodialyzed patients

Aluminum (Al3+) overload is frequently associated with lipid peroxidation and neurological disorders. Aluminum accumulation is also reported to be related to renal impairment, anemia and other clinical complications in hemodialysis patients. The aim of the present study was to determine the degree of lipid peroxidation, platelet aggregation and serum aluminum in patients receiving regular hemodialytic treatment. The level of plasma lipid peroxidation was evaluated on the basis of thiobarbituric acid reactive substances (TBARS). Mean platelet peroxidation in patients undergoing hemodialysis was significantly higher than in normal controls (2.7 +/- 0.03 vs. 1.8 +/- 0.06 nmol/l, P<0.05). Platelet aggregation and serum aluminum levels were determined by a turbidimetric method and atomic absorption spectrophotometry, respectively. Serum aluminum was significantly higher in patients than in normal controls (44.5 +/- 29 vs. 10.8 +/- 2.5 microg/l, P<0.05). Human blood platelets were stimulated with collagen (2.2 microg/ml), adenosine diphosphate (6 microM) and epinephrine (6 microM) and showed reduced function with the three agonists utilized. No correlation between aluminum levels and platelet aggregation or between aluminum and peroxidation was observed in hemodialyzed patients.

On the mechanism of the spermine-exerted inhibition on alpha-thrombin-induced platelet activation

Previous reports have shown that various amines inhibited platelet activation, but no definitive conclusions on their action mechanism were drawn. We have further investigated the action of spermine on platelet responses evoked by alpha-thrombin and other agonists. Spermine inhibited in a concentration-dependent manner (1-10 mM), and more efficiently than spermidine and putrescine, the alpha-thrombin-induced (1.5 nM) platelet activation. Spermine added at a concentration that inhibited completely aggregation only partially affected the thrombin-induced increase in cytosolic Ca(2+) concentration, protein phosphorylation, and ATP secretion. The polyamine had little effect on the morphology of resting platelets, as measured by electron microscopy, thrombin hydrolytic activity, and fibrinogen clotting capacity but decreased the thrombin binding to platelets and isolated glycocalicin. Spermine partially inhibited the aggregation elicited by ADP, vasopressin, platelet-activating factor, thrombin receptor-activating peptide, fluoroaluminate, ionomycin, and dioctanoylglycerol but did not affect the cytosolic Ca(2+) increase induced by these agonists. The polyamine bound to both glycocalicin and platelets, and it inhibited the fibrinogen binding to stimulated platelets. The amount of 14C-spermine bound to resting cells decreased in the presence of the glycoprotein GPIb-antibody LJIB1, whereas the polyamine bound to activated platelets, which was higher than that tied to resting cells, was markedly reduced by LJCP8 or decorsin, a GPIIb/IIIa antibody and antagonist-peptide, respectively. These results indicate that spermine specifically inhibits the thrombin binding to GPIb of resting platelets and the fibrinogen binding to GPIIb/IIIa (integrin alpha(IIb)beta(3)) of activated platelets.

Aluminum induces lipid peroxidation and aggregation of human blood platelets

Aluminum (Al3+) intoxication is thought to play a major role in the development of Alzheimer's disease and in certain pathologic manifestations arising from long-term hemodialysis. Although the metal does not present redox capacity, it can stimulate tissue lipid peroxidation in animal models. Furthermore, in vitro studies have revealed that the fluoroaluminate complex induces diacylglycerol formation, 43-kDa protein phosphorylation and aggregation. Based on these observations, we postulated that Al(3+) -induced blood platelet aggregation was mediated by lipid peroxidation. Using chemiluminescence (CL) of luminol as an index of total lipid peroxidation capacity, we established a correlation between lipid peroxidation capacity and platelet aggregation. Al3+ (20-100 microM) stimulated CL production by human blood platelets as well as their aggregation. Incubation of the platelets with the antioxidants nor-dihydroguaiaretic acid (NDGA) (100 microM) and n-propyl gallate (NPG) (100 microM), inhibitors of the lipoxygenase pathway, completely prevented CL and platelet aggregation. Acetyl salicylic acid (ASA) (100 microM), an inhibitor of the cyclooxygenase pathway, was a weaker inhibitor of both events. These findings suggest that Al3+ stimulates lipid peroxidation and the lipoxygenase pathway in human blood platelets thereby causing their aggregation.

Protein kinase C inhibitors enhance G-protein induced phospholipase A2 activation in intact human platelets

Washed intact human platelets were prelabelled with [3H]arachidonic acid ([3H]AA) and stimulated with thrombin or with AlF4-, a known unspecific activator of G-proteins. Both stimuli induced the liberation of [3H]AA, the release of beta-thromboglobulin (beta-TG) and platelet aggregation. PMA did not induce liberation of [3H]AA although it induced beta-TG release and aggregation; preincubation with PMA did not modify significantly the amounts of [3H]AA and beta-TG released by thrombin or AlF4-. Different inhibitors of PKC (staurosporine, H-7 and calphostin C) increased the release of [3H]AA and inhibited beta-TG release and aggregation induced by AlF4- but they had no effect when platelets were stimulated with thrombin (0.5 U/ml). Calphostin C was able to release [3H]AA by itself without inducing aggregation of beta-TG release. Okadaic acid (a serine/threonine phosphoprotein phosphatase inhibitor) greatly inhibited the release of [3H]AA, beta-TG and aggregation in AlF4--stimulated platelets. These results indicate the presence of a G-protein mediated mechanism for the activation of a platelet phospholipase A2 which is negatively affected by a protein kinase, sensible to putative inhibitors of protein kinase C, and it is activated by a protein phosphatase, sensible to okadaic acid.

Aluminum-induced DNA synthesis in osteoblasts: mediation by a G-protein coupled cation sensing mechanism

Aluminum (Al3+) stimulates de novo bone formation in dogs and is a potent stimulus for DNA synthesis in non-transformed osteoblasts in vitro. The recent identification of a G-protein coupled cation-sensing receptor (BoPCaR), which is activated by polyvalent agonists [e.g., gadolinium (Gd3+) > neomycin > calcium (Ca2+)], suggests that a similar physiologically important cation sensing receptor may be present in osteoblasts and pharmacologically activated by Al3+. To evaluate that possibility, we assessed whether known BoPCaR agonists stimulate DNA synthesis in MC3T3-E1 osteoblasts and examined the additive effects of Al3+ and BoPCaR agonists on DNA synthesis in MC3T3-E1 osteoblast-like cells. We found that Al3+, Gd3+, neomycin, and Ca2+ stimulated DNA synthesis in a dose-dependent fashion, achieving 50% effective extracellular concentrations (EC50) of 10 microM, 30 microM, 60 microM, and 2.5 mM, respectively. Al3+ displayed non-additive effects on DNA synthesis with the BoPCaR agonists as well as an unrelated G-protein coupled receptor agonist, PGF2 alpha, suggesting shared mechanisms of action. In contrast, the receptor tyrosine kinase agonist, IGF-I (10 eta g/ml), displayed additive proliferative effects when combined with AlCl3, indicating distinct signalling pathways. AlCl3 (25 microM) induced DAG levels 2-fold and the phosphorylation of the myristoylated alanine-rich C kinase (MARCKS) substrate 4-fold, but did not increase intracellular calcium concentrations. Down-regulation of PKC by pre-treatment with phorbol 12-myristate 13-acetate as well as PKC inhibition by H-7 and staurosporine blocked Al(3+)-induced DNA synthesis. Finally, Al3+, Gd3+, neomycin, and Ca2+ activated G-proteins in osteoblast membranes as evidenced by increased covalent binding of [32P]-GTP-azidoanilide to putative G alpha subunits. Our findings suggest that Al3+ stimulates DNA synthesis in osteoblasts through a cation sensing mechanism coupled to G-protein activation and signalling cascades involving DAG and PKC-dependent pathways.

Activation of phospholipase A2 and beta-thromboglobulin release in human platelets: comparative effects of thrombin and fluoroaluminate stimulation

Several reports have suggested that the activity of platelet phospholipase A2 is modulated by GTP-binding protein(s) whose nature and properties need to be defined. Fluoroaluminate is known to activate G-proteins and this leads to a number of cellular responses including the activation of phospholipases. This paper demonstrates that human platelets, prelabelled with [3H]arachidonic acid, produce free arachidonic acid when stimulated with fluoroaluminate and this effect is time- and dose-dependent. The production of arachidonic acid is not inhibited by neomycin, a PI-cycle inhibitor, but is completely abolished by mepacrine, an inhibitor of both phospholipase A2 and C. At low concentration of fluoroaluminate (10 mM NaF) phospholipase A2 but not phospholipase C is activated. In addition, fluoroaluminate treatment releases beta-thromboglobulin (beta-TG) and this effect is not inhibited by acetylsalicylic acid. Under identical conditions both neomycin and mepacrine suppress the release of arachidonic acid and beta-TG induced by thrombin. Sodium nitroprusside, which increases cGMP levels in platelets, inhibits arachidonic acid liberation and beta-TG release in thrombin-stimulated platelets but has no effect in fluoroaluminate-treated platelets; cGMP was reported to suppress phospholipase C activation. These results are consistent with the hypothesis that, in thrombin-stimulated platelets, the liberation of arachidonic acid and beta-TG are strictly dependent on the activation of phospholipase C. We have also provided evidence for the existence of a phospholipase A2 activated by a G-protein which is independent from the degradation of phosphoinositides and, contrary to phospholipase C, it is not down regulated by cGMP.

Evidence for direct coupling of primary agonist-receptor interaction to the exposure of functional IIb-IIIa complexes in human blood platelets. Results from studies with the antiplatelet compound ajoene

Ajoene, (E,Z)-4,5,9-trithiadodeca-1,6,11-triene 9-oxide, is a potent antiplatelet compound isolated from alcoholic extracts of garlic. In vitro, ajoene reversibly inhibits platelet aggregation as well as the release reaction induced by all known agonists. In this paper we show that ajoene has a unique locus of action, that is not shared by any other known antiplatelet compound. For example, ajoene inhibits agonist-induced exposure of fibrinogen receptors, as well as intracellular responses such as activation of protein kinase C and the increase in cytoplasmic free calcium induced by receptor-dependent agonists (collagen, ADP, PAF, low-dose thrombin). On the other hand, with agonists that can by-pass (at least partially) the receptor-transductor-effector sequence, such as high-dose thrombin, PMA, NaF, only the exposure of fibrinogen receptors is blocked by ajoene. Binding of fibrinogen to chymotrypsin-treated platelets is only slightly inhibited by ajoene. The results reported here also show that: (a) ajoene does not act as a calcium chelator, does not impair the initial agonist-receptor interaction and does not influence the basal levels of intracellular inhibitors of platelet activation such as cyclic GMP; (b) the locus of action of ajoene is a yet unknown molecular step that links, in the case of physiological agonists, specific agonist-receptor complexes to the sequence of the signal transduction system on the plasma membrane of platelets. In the case of non-physiological, receptor-independent agonists (PMA, NaF), we can only speculate on the hypothesis that they somehow mimic the effect of the agonist-receptor complexes on the signal transduction system; and (c) the exposure of fibrinogen receptors is not a direct consequence of other intracellular processes. These observations clearly show, for the first time, that the exposure of fibrinogen receptors is a membrane event proximally and obligatorily coupled to the occupancy of other membrane receptors by their agonists without any intervention by the cytoplasmic biochemical processes. Additional results support the involvement of G-proteins in these early events of platelet activation. Furthermore, a role of the beta tau subunits of G-proteins in the exposure of fibrinogen receptors is proposed.

Mechanisms involved in platelet activation induced by a monoclonal antibody anti glycoprotein IIb-IIIa: inositol phosphate production is not the primary event

The mechanisms involved in platelet aggregation by a monoclonal antibody (mAb) P256 specific for the GPIIb-IIIa complex was investigated following metabolic 32P labelling of platelets. When compared with thrombin, inositol phosphates (InsP) production during P256-induced activation was delayed and no apparent peak, but a small and sustained production of [32P]-Ins(1,4,5)P3 and [32P]-Ins(1,3,4,5)P4, was observed between 20 and 90 s. [32P]-Ins(1,3,4)P3 was also produced with a maximum after 90 s. Addition of the ADP scavenger creatinine phosphate/creatine phosphokinase (CP/CPK) and of the cyclooxygenase inhibitor aspirin together with P256 almost totally abolished InsP formation, whereas platelet aggregation and protein phosphorylation were partially inhibited. F(ab')2 fragments of P256 also aggregated platelets but to a smaller extent than IgG, and without any measurable InsPs. To characterize further P256-induced activation, the phosphorylation of p43, the main substrate of protein kinase C (PKC) and the phosphorylation of tyrosine protein (P-Tyr) was also studied. PKC activation was smaller with P256-IgG than with thrombin but both thrombin and P265-IgG induced a similar profile of P-Tyr involving seven major bands, whereas P256-F(ab')2 only occasionally activated PKC but always significantly phosphorylated a 64,000 molecular weight P-Tyr. The data indicate that the binding of P256 to GPIIb-IIIa, in contrast with thrombin, does not initially lead directly to the activation of the phosphoinositide phospholipase C to produce InsP's but rather involves the activation of protein kinases and also both fragments F(ab')2 and Fc play a specific role in the platelet responses to the mAb. Only the crosstalk between the two pathways evoked by F(ab')2 and Fc respectively allows the activation of all platelet activation systems.