Cross-talking between lymphocytes and platelets and its regulation by nitric oxide and peroxynitrite in physiological condition and endotoxemia.

AIMS: Cross-talk between platelets and lymphocytes may play a role in different pathological conditions like sepsis. This study aimed to investigate the effect of lymphocytes on platelet aggregation in lipopolysaccharide (LPS)-stimulated and non-stimulated cells. MAIN METHODS: Lymphocytes and platelet-rich plasma (PRP) were obtained from rat arterial blood. Platelets (1.2×108platelets/ml) were incubated with lymphocytes (0.8×106cells/ml) in the presence or not of LPS (100µg/ml), after which ADP (5µM)-induced platelet aggregation was carried out. KEY FINDINGS: Lymphocytes inhibited by 51% the platelet aggregation, which was significantly prevented by the non-selective NO inhibitor l-NAME (300µM) or the selective iNOS inhibitor 1400W (100µM), as well as by the soluble guanylyl cyclase (sGC) inhibitor ODQ (10µM). The platelet inhibition by lymphocytes was accompanied by 2-fold increase of intraplatelet cGMP levels. Next, lymphocytes and platelets were co-incubated with LPS for 6h. In LPS-treated cells, lymphocytes produced a larger inhibition of platelet aggregation (62%), despite the same elevation of cGMP levels (2.2-fold increase). This inhibitory effect was prevented by l-NAME and 1400W, but rather unaffected by ODQ. The peroxynitrite (ONOO-) scavenger -(-)epigallocatechin gallate (ECG, 100µM) abolished the inhibition by lymphocytes on platelet aggregation in LPS-treated cells, but not in non-treated cells. SIGNIFICANCE: Our results show that lymphocytes act to inhibit platelet aggregation via iNOS-derived NO release and cGMP generation. In presence of LPS, ONOO- production accounts for the platelet inhibition.

PKC and AKT Modulate cGMP/PKG Signaling Pathway on Platelet Aggregation in Experimental Sepsis.

Sepsis severity has been positively correlated with platelet dysfunction, which may be due to elevations in nitric oxide (NO) and cGMP levels. Protein kinase C, Src kinases, PI3K and AKT modulate platelet activity in physiological conditions, but no studies evaluated the role of these enzymes in platelet aggregation in sepsis. In the present study we tested the hypothesis that in sepsis these enzymes positively modulate upstream the NO-cGMP pathway resulting in platelet inhibition. Rats were injected with lipopolysaccharide (LPS, 1 mg/kg, i.p.) and blood was collected after 6 h. Platelet aggregation was induced by ADP (10 µM). Western blotting assays were carried out to analyze c-Src and AKT activation in platelets. Intraplatelet cGMP levels were determined by enzyme immunoassay kit. Phosphorylation of c-SRC at Tyr416 was the same magnitude in platelets of control and LPS group. Incubation of the non-selective Src inhibitor PP2 (10 µM) had no effect on platelet aggregation of LPS-treated rats. LPS increased intraplatelet cGMP levels by 5-fold compared with control group, which was accompanied by 76% of reduction in ADP-induced platelet aggregation. The guanylyl cyclase inhibitor ODQ (25 µM) and the PKG inhibitor Rp-8-Br-PET-cGMPS (25 µM) fully reversed the inhibitory effect of LPS on platelet aggregation. Likewise, the PKC inhibitor GF109203X (10 µM) reversed the inhibition by LPS of platelet aggregation and decreased cGMP levels in platelets. AKT phosphorylation at Thr308 was significantly higher in platelets of LPS compared with control group, which was not reduced by PI3K inhibition. The AKT inhibitor API-1 (20 µM) significantly increased aggregation and reduced cGMP levels in platelets of LPS group. However, the PI3K inhibitor wortmannin and LY29004 had no effect on platelet aggregation of LPS-treated rats. Therefore, inhibition of ADP-induced platelet aggregation after LPS injection is mediated by cGMP/PKG-dependent mechanisms, and PKC and AKT act upstream upregulating this pathway.