Evidence for a glycoprotein IIb-IIIa- and aggregation-independent mechanism of phosphatidylinositol 3',4'-bisphosphate synthesis in human platelets

The platelet cytoskeleton regulates the aggregation-dependent synthesis of phosphatidylinositol 3,4-bisphosphate induced by thrombin

Pretreatment of intact platelets with cytochalasin D prevented actin polymerization and cytoskeleton reorganization induced by thrombin, but did not affect platelet aggregation. Under these conditions, synthesis of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) stimulated by thrombin was strongly inhibited, while production of phosphatidic acid was unaffected. The inhibitory effect of cytochalasin D was not observed when platelet aggregation was prevented by the RGDS peptide. We also found that cytochalasin D did not affect PtdIns(3,4)P2 synthesis induced by concanavalin A (ConA), which is known to occur through an aggregation-independent mechanism. Moreover, thrombin, but not ConA, induced the translocation of phosphatidylinositol 3-kinase to the cytoskeleton. This process was equally inhibited by both the RGDS peptide and cytochalasin D. These results demonstrate that the cytoskeleton represents a functional link between thrombin-induced aggregation and synthesis of PtdIns(3,4)P2.

A role for phosphatidylinositol 3-kinase in platelet aggregation in response to low, but not high, concentrations of PAF or thrombin

In this study we show that platelet activating factor (PAF) activates PI 3-kinase over a rapid time course that correlates closely with the aggregation response. Tyrosine kinases are involved in this response, since there is increased PI 3-kinase activity associated with tyrosine-phosphorylated proteins. PI 3-kinase inhibitors were used to probe the dependence of PAF-induced aggregation on PI 3-kinase. Both wortmannin and LY-294002 inhibited PAF-induced aggregation that correlated with PI 3-kinase inhibition only when using lower concentrations of PAF giving reversible aggregation (primary phase). Similar results were obtained with human platelets using thrombin or thrombin receptor activating peptide. The same pattern of response was observed when activation of GPIIbIIIa was assessed by flow cytometry, i.e., PI 3-kinase inhibitors blocked integrin activation only when lower concentrations of agonist were used. We suggest that PI 3-kinase is important for reversible (primary) aggregation of platelets in response to PAF or thrombin, perhaps by contributing to the 'inside-out' activation of the platelet integrin GPIIbIIIa, only when submaximal concentrations of agonists are used. The lack of effect of PI 3-kinase inhibitors, when high concentrations of agonist are used, suggests that PI 3-kinase-independent pathways contribute to aggregation under these conditions.

Phosphatidylinositol 3,4,5-trisphosphate triggers platelet aggregation by activating Ca2+ influx

Exogenous phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] stimulates the aggregation of washed rabbit platelets in a Ca2+- and dose-dependent manner. This aggregation is reversible at low PtdIns(3,4,5)P3 levels, but becomes irreversible when the concentration exceeds a threshold of about 20 microM. Other D-3 and D-4 phosphoinositides examined, including phosphatidylinositol 3, 4-bisphosphate [PtdIns(3,4)P2], phosphatidylinositol 4, 5-bisphosphate [PtdIns(4,5)P2], and phosphatidylinositol 3-monophosphate [PtdIns(3)P], fail to exert appreciable platelet activation at comparable concentrations. In addition, PtdIns(3,4, 5)P3 can reverse the inhibitory effect of wortmannin on thrombin-induced platelet aggregation. Taken together with the observation that PtdIns(3,4,5)P3 is readily incorporated into cell membranes, these findings reaffirm the second messenger role of PtdIns(3,4,5)P3 in thrombin receptor activation. The existence of a PtdIns(3,4,5)P3-dependent Ca2+ entry system on platelet membranes is supported by the partial inhibition of thrombin-induced Ca2+ influx by wortmannin. Evidence suggests that this system differs from receptor-operated nonselective Ca2+ channels. However, the mechanism by which PtdIns(3,4,5)P3 facilitates Ca2+ entry remains unclear. Although PtdIns(3,4,5)P3 has been known to stimulate phospholipase C-gamma (PLC-gamma), internal Ca2+ mobilization does not play a significant role in the cytosolic Ca2+ increase in response to PtdIns(3,4,5)P3 stimulation. Collectively, these data provide a putative link between PtdIns(3,4,5)P3 and Ca2+ signaling, which may, in part, account for the regulatory function of PtdIns(3,4,5)P3 during platelet aggregation. Moreover, this study bears out the notion that individual PI 3-kinase lipid products play distinct roles in the regulation of cellular functions.

Reversible translocation of phosphoinositide 3-kinase to the cytoskeleton of ADP-aggregated human platelets occurs independently of Rho A and without synthesis of phosphatidylinositol (3,4)-bisphosphate

The aim of our study was to evaluate the effect of ADP and the role of cytoskeleton reorganization during reversible and irreversible platelet aggregation induced by ADP and thrombin, respectively, on the heterodimeric (p85alpha-p110) phosphoinositide 3-kinase translocation to the cytoskeleton and its activation. Reversible ADP-induced aggregation was accompanied by a reversible reorganization of the cytoskeleton and an increase in levels of the regulatory subunit p85alpha in this cytoskeleton similar to the increase observed in thrombin-activated platelets. This translocation followed a course parallel to the amplitude of aggregation. No increase in levels of both phosphatidylinositol (3, 4)-bisphosphate (PtdIns(3,4)P2) and phosphatidylinositol-(3,4,5)P3 could, however, be detected even at the maximum aggregation and PI 3-kinase alpha translocation. Moreover, in contrast to the situation for thrombin stimulation, the GTP-binding protein RhoA was hardly translocated to the cytoskeleton when platelets were stimulated with ADP, whereas translocation of pp60(c-)src and focal adhesion kinase did occur. These results suggest (i) translocation of signaling enzymes does not necessarily imply their activation, (ii) the reversibility of ADP-induced platelet aggregation may be the cause or the result of a lack of PI 3-kinase activation and hence of PtdIns(3,4)P2 production, and (iii) RhoA does not seem to be involved in the ADP activation pathway of platelets. Whether PtdIns(3,4)P2 or RhoA may contribute to the stabilization of platelet aggregates remains to be established.

Transduction of cytotoxic signals in natural killer cells: a general model of fine tuning between activatory and inhibitory pathways in lymphocytes

NK-cells are large granular lymphocytes, which are capable of exerting two major types of effector function, cell cytotoxicity and lymphokine secretion. NK-cells can exert cell cytotoxicity in one of two ways. First, NK-cells are able to recognize and to induce the lysis of antibody-coated target cells during antibody-dependent cell cytotoxicity (ADCC). Second, during natural cytotoxicity NK-cells are also able to recognize and to induce the lysis of a variety of target cells, including primarily virus-infected cells as well as tumor cells. Recently, a novel mechanism has been elucidated which controls NK-cell-activation programs and which is based on the cell surface expression of killer-cell inhibitory receptors (KIR). We will review here the molecular dissection of this inhibitory signalling pathway which utilizes immunoreceptor tyrosine-based inhibition motifs (ITIM) expressed in KIR intracytoplasmic domain. We will also show that this strategy used by NK-cells to regulate their effector functions is a general decision mechanism which exists not only in T- and B-lymphocytes, but also in a variety of other hematopoietic cells.

Total inhibition of phospholipase C and phosphatidylinositol 3-kinase by okadaic acid in thrombin-stimulated platelets

The strong inhibition of thrombin-induced platelet functions induced by okadaic acid is not correlated with the partial modification of pleckstrin phosphorylation, which remains still phosphorylated two min after stimulation, indicating that protein kinase C is not affected by okadaic acid. We then investigated the effect of okadaic acid on platelet lipid metabolism. Our data indicate that inhibition indeed strongly affects phosphatidic acid as well as phosphatidylinositol 3,4-bisphosphate synthesis at low concentrations of okadaic acid, and phosphatidylinositol 4,5-bisphosphate at higher concentrations. Since thrombin-induced tyrosine phosphorylations were completely inhibited in the presence of okadaic acid, as a consequence, phosphatidylinositol 3-kinase was no longer detected in antiphosphotyrosine immunoprecipitates, thus explaining the absence of phosphatidylinositol, 3,4-bisphosphate synthesis. Finally, okadaic acid inhibited thrombin-induced fibrinogen binding, indicating that serine/threonine phosphatases may affect the inside-out signalling which regulates the alpha 11bb3 integrin, downstream protein kinase C activation.