Platelet-neutrophil interaction and superoxide anion generation: involvement of purine nucleotides

Apocynin prevents cyclooxygenase 2 expression in human monocytes through NADPH oxidase and glutathione redox-dependent mechanisms

In the present study we report the preventive effect of apocynin, an active constituent of the Himalayan herb Picrorhiza kurrooa, on cyclooxygenase-2 (Cox-2) synthesis and activity in human adherent monocytes exposed to serum treated zymosan (STZ) and phorbol myristate acetate (PMA). Apocynin markedly decreases the intracellular reduced/oxidized glutathione ratio (GSH/GSSG) and prevents nuclear factor-kappaB (NF-kappaB) activation in stimulated monocytes. Moreover, it reduces intracellular reactive oxygen species (ROS) generation, NADPH oxidase activity in monocyte homogenates and translocation of p47phox subunit in monocyte membranes. p47phox levels are also reduced in lysates of apocynin-treated monocytes. The inhibition of Cox-2 by apocynin is completely abrogated by GSH provision. Results from this study indicate that apocynin inhibits Cox-2 synthesis and activity induced in monocytes by an increased oxidative tone and provide an explanation for the protective effect exerted by this compound in numerous cell and animal models of inflammation. Attenuation of NADPH oxidase derived ROS coupled with GSH/GSSG reduction and suppression of NF-kappaB activation are highlighted as the molecular mechanisms responsible for Cox-2 inhibition.

Enhanced leukocyte-platelet cross-talk in Type 1 diabetes mellitus: relationship to microangiopathy

BACKGROUND

Platelets and leukocytes may influence each others' function, i.e. platelet-leukocyte cross-talk. Diabetes mellitus (DM) is associated with platelet and leukocyte dysfunction.

OBJECTIVE

To evaluate platelet-leukocyte cross-talk, and if this might contribute to platelet and leukocyte dysfunction and microangiopathy in DM patients.

PATIENTS AND METHODS

We evaluated platelet and leukocyte function, and cross-talk between these cells in Type 1 DM patients without (n = 19) and with (n = 20) microangiopathy, and healthy subjects (n = 27), using whole blood flow cytometry. Platelet-leukocyte cross-talk was studied in hirudinized whole blood incubated at 37 degrees C with stirring.

RESULTS

Basal single platelet P-selectin and leukocyte CD11b expression were similar in DM patients and healthy subjects, whilst circulating platelet-leukocyte aggregates and plasma elastase levels were elevated in DM patients. The thromboxane A2 analog U46619 (3 x 10(-7) m) induced more marked increases of platelet P-selectin expression and platelet-leukocyte aggregation in DM patients than in healthy subjects. The leukocyte-specific agonist N-formyl-methionyl-leucyl-phenylalanine (fMLP) (10(-7) m) induced more marked CD11b expression in DM patients with microangiopathy, compared with healthy subjects. Platelet-leukocyte cross-talk induced by U46619 (10(-6) m) showed no difference between DM patients and healthy subjects. fMLP (10(-6) m) evoked marked leukocyte activation, which subsequently caused mild platelet P-selectin expression. This leukocyte-platelet cross-talk was more pronounced in DM patients than in healthy subjects. Furthermore, enhanced leukocyte-platelet cross-talk was correlated to platelet hyperreactivity among DM patients with microangiopathy only.

CONCLUSIONS

Type 1 DM is associated with platelet and leukocyte hyperactivity, and enhanced leukocyte-platelet cross-talk, which may contribute to platelet hyperactivity and the microvascular complications seen in Type 1 DM.

Platelets abrogate leukotriene B(4) generation by human blood neutrophils stimulated with monosodium urate monohydrate or f-Met-Leu-Phe in vitro

Neutrophils are physiologically associated with platelets in whole blood. Inflammatory reactions can be modulated by the presence of platelets. To investigate the influence of platelets on neutrophil activity, we studied the 5-lipoxygenase (5-LOX) metabolic pathway in normal human blood neutrophils stimulated with f-Met-Leu-Phe (fMLP) or monosodium urate monohydrate (MSUM) in the presence of autologous platelets. Platelets inhibited by more than 90% the synthesis of leukotriene B(4) and 5-HETE in neutrophils activated with fMLP or MSUM. The addition of exogenous arachidonic acid did not reverse the inhibitory effect of platelets on 5-LOX-generated metabolites in fMLP- or MSUM-activated neutrophils. Preincubation of neutrophils with adenosine deaminase reversed the inhibitory effect of platelets in fMLP-treated neutrophils, indicating that adenosine was responsible for the platelet inhibition of leukotriene B(4) and 5-HETE formation. In contrast, adenosine deaminase had no influence on the inhibitory effects of platelets in MSUM-stimulated cells. These results suggest that platelets can inhibit the synthesis of 5-LOX products (a). by acting mainly downstream to phospholipase A(2) in cells stimulated by fMLP or MSUM, (b). through adenosine when neutrophils are activated with fMLP, and (c). by an adenosine-independent mechanism in MSUM-activated neutrophils by an as-yet-unidentified mediator.

Differential inhibition of polymorphonuclear leucocyte functions by cloricromene

Leucocytes play an essential role in the pathogenesis of ischaemia and reperfusion injury and inhibition of their adhesion and of mediator release can reduce vascular and tissue damage. Previous studies have shown that cloricromene modifies several granulocyte as well as monocyte/macrophage functions and it has been shown that cloricromene administration exerts a clear protective action in several experimental models of ischaemia. The present work describes new data on polymorphonuclear leukocyte (PMN) inhibition exerted by cloricromene and compares these observations with earlier ones. Human washed PMN and human whole blood (HWB) were studied in vitro upon stimulation with f-MLP in the presence of cytochalasin B, with opsonized zymosan and with a phorbol ester (PMA). Amongst free radicals, superoxide anions were chosen as index of oxidative burst. Phagocytosis and beta-glucuronidase, as lysosomal release indicators, were measured to characterize PMN function: cloricromene inhibited concentration-dependently all the parameters upon stimulation by each activator tested. Experiments performed in rabbit whole blood (RWB) showed that cloricromene inhibited free radical generation with IC50 values similar to those obtained in human whole blood. Comparing the action of cloricromene on human cells in different tests, we found that some parameters were more sensitive than others, even when the same stimulus was used. In particular, free radical generation was inhibited by cloricromene with IC50 values below 36 microM, while other functions, like lysosomal release and phagocytosis were inhibited with IC50 values over 100 microM. These data confirm that cloricromene exerts a notable inhibitory effect on PMN and may explain the activity of the compound, observed in vivo in several experimental models of ischaemia-reperfusion and shock.

Purine metabolites and malondialdehyde in platelets of diabetic patients

The concentration of some of the purine nucleotides and their metabolites together with that of malondialdehyde (MDA) have been measured in resting and stimulated platelets of type 1 and type 2 diabetic patients. While control platelets show a net decrease of guanosine triphosphate (GTP) (3.1 vs. 2.3 nmol per 10(9) platelets) and guanosine diphosphate (GDP) (3.0 vs. 2.0 nmol per 10(9) platelets) and a significant increase of adenosine (0.04 vs. 0.55 nmol per 10(9) platelets) with platelet stimulation, platelets of type 1 and type 2 diabetic patients have a lesser change of these metabolites (GTP, 2.6 vs. 2.4; GDP, 2.3 vs. 2.4; adenosine, 0.04 vs. 0.30 (P < 0.05 vs. control) nmol per 10(9) platelets in type 1 diabetics; GTP, 2.4 vs. 2.7; GDP, 2.4 vs. 2.1; adenosine, 0.08 vs. 0.32 (P < 0.05 vs. control) nmol per 10(9) platelets in type 2 diabetics). These results indicate that the change (stimulated minus resting) of GTP, GDP and adenosine in diabetic platelets is significantly different from that of controls (P < 0.001). Moreover, the amount of MDA produced during platelet activation seems to be lower than controls only in type 2 diabetes (1.81 vs. 2.86 nmol per 10(9) platelets, P < 0.05). These results seem to indicate that a difference in the pattern of platelet nucleotides could be an important feature even in well-controlled diabetes, while MDA is probably modified only in association with the late vascular complications of diabetes.

Ischemia/reperfusion injury in human kidney transplantation: an immunohistochemical analysis of changes after reperfusion

Organs used for transplantation undergo varying degrees of cold ischemia and reperfusion injury after transplantation. In renal transplantation, prolonged cold ischemia is strongly associated with delayed graft function, an event that contributes to inferior graft survival. At present, the pathophysiological changes associated with ischemia/reperfusion injury in clinical renal transplantation are poorly understood. We have performed an immunohistochemical analysis of pre- and postreperfusion biopsies obtained from cadaver (n = 55) and living/related donor (LRD) (n = 11) renal allografts using antibodies to adhesion molecules and leukocyte markers to investigate the intragraft changes after cold preservation and reperfusion. Neutrophil infiltration and P-selectin expression were detected after reperfusion in 29 of 55 (53%) and 24 of 55 (44%) cadaver renal allografts, respectively. In marked contrast, neutrophil infiltration was not observed in LRD allografts, and only 1 of 11 (9%) had an increased level of P-selectin after reperfusion. Immunofluorescent double-staining demonstrated that P-selectin expression resulted from platelet deposition and not from endothelial activation. No statistically significant association was observed between neutrophil infiltration and P-selectin expression in the glomeruli or intertubular capillaries despite the large number of cadaver renal allografts with postreperfusion changes. Neutrophil infiltration into the glomeruli was significantly associated with long cold ischemia times and delayed graft function. Elevated serum creatinine levels at 3 and 6 months after transplantation were also associated with the presence of neutrophils and platelets after reperfusion. Our results suggest that graft function may be influenced by early inflammatory events after reperfusion, which can be targeted for future therapeutic intervention.

Platelet activation In mice and human Helicobacter pylori infection

Extracts of Helicobacter pylori (HP) have been shown to induce leukocyte adhesion in mesenteric venules, but the effects of HP infection on gastric microvessels are unknown. Inflammatory cell interactions in the gastric microcirculation were studied by intravital videomicroscopy in mice inoculated with either saline or fresh isolates of HP. Platelet aggregates were detected and quantified in murine portal blood, while endothelial P-selectin expression was determined using the dual radiolabeled mAb technique. Platelet activation and aggregation were studied in HP-infected patients and controls by measuring the platelet-aggregate ratio and platelet P-selectin expression. HP infection induced a marked increase in the flux of rolling leukocytes and the appearance of platelet and leukocyte- platelet aggregates in murine gastric venules. The HP-induced rolling and platelet aggregate formation was abrogated by mAbs against L- or P-, but not E- selectin. Endothelial cell expression of P-selectin was not altered, but platelet P-selectin expression was enhanced in HP-infected mice. Circulating platelet aggregates and activated platelets were also detected in HP-infected patients. These findings indicate that platelet activation and aggregation contribute to the microvascular dysfunction and inflammatory cell recruitment associated with HP infections.