Activation of phospholipases A and C in human platelets exposed to epinephrine: role of glycoproteins IIb/IIIa and dual role of epinephrine.

Abstract

Human platelets stimulated by epinephrine undergo enhanced turnover of phosphatidylinositol 4,5-bisphosphate, accumulate inositol trisphosphate, diacylglycerol, and phosphatidic acid, and phosphorylate a 47-kDa protein.All of these phenomena indicate stimulation of phospholipase C. These responses are blocked completely by inhibitors of a2-adrenergic receptors (yohimbine), cyclooxygenase (aspirin or indomethacin), phospholipase A [2-(p-amylcinnamnoyl)amino-4-chlorobenzoic acid (ONO-RS-082)], Na+/H+ exchange [ethylisopropylamiloride (EIPA)], fibrinogen binding to glycoprotein [lb/I11a (antibody A2A9), Ca2+/Mg' binding (EDTA), or removal of fibrinogen. Epinephrine evokes (i) an increased turnover of ester-linked arachidonic acid in aspirintreated platelets that is inhibited by ONO-RS-082, EDTA, yohinbine, or the absence of fibrinogen and (ii) a rapid cytoplasmic alkalinization that is inhibited partially by blockage of cyclooxygenase activity and completely by A2A9 or EIPA. In contrast, when incubated with subaggregatory concentrations of the prostaglandin H2/thromboxane A2 analogue [(15S)-hydroxy-1 ia,9a-(epoxymethano)prosta-5,13-dienoic acid (U46619) and epinephrine, aspirin-treated platelets show a potentiation of phospholipase C activation that is unaffected by the above inhibitors. We propose that epinephrine, in promoting exposure of glycoprotein Ilb/ia sites for fibrinogen binding, leads to a cytoplasmic alkalinization, which, in conjunction with local shifts in Ca2 , promotes low-level activation of phospholipase A. The resulting free arachidonic acid is converted to cyclooxygenase products, which, potentiated by epinephrine, activate phospholipase C. This further amplifies the initial stimulatory response.Human platelets undergo aggregation and secretion of granule contents following exposure to epinephrine in the presence of fibrinogen and Ca2". Accompanying such activation is an enhanced turnover of phosphatidylinositol catalyzed by phospholipase C (PLC), indicated in earlier studies by stimulated incorporation of [3H]glycerol (1, 2), diacylglycerol accumulation, and activation of protein kinase C (3). However, human platelet adrenergic receptors have been characterized as being primarily of the a2 subclass (4, 5), which, in other tissues, is not linked with phosphatidylinositol phosphate turnover (6).A resolution of this paradox may lie in a report that epinephrine-stimulated generation of [32P]phosphatidic acid (PtdOH), an indirect indicator of PLC activation, can be inhibited by aspirin (7). This implies a possible role for the cyclooxygenase-catalyzed oxygenation of arachidonic acid in mediating the process of PLC activation in response to epinephrine. Recently, investigators have found that epinephrine promotes arachidonic acid mobilization (a precondition for cyclooxygenase action) in a manner dependent upon Na+/H+ exchange (8). This mobilization was quantitated by measuring thromboxane B2 (TXB2), the stable metabolite of thromboxane A2, which is, in turn, formed by way of cyclooxygenase. ...

Enrichment of vitronectin‐ and fibronectin‐like proteins in NaCl‐adapted plant cells and evidence for their involvement in plasma membrane–cell wall adhesion

“…The adhesion system could provide a mechanism for sensing the external osmotic environment so that the cell can respond with appropriate osmotic adjustment. lntegrins are implicated in the activation of specific genes in animal cells either directly (Du et al, 1991) or via the signal transduction pathway (Banga et a/., 1986;Werb et al, 1989). For plant cells, disturbance of the matrix-integrin complex through a turgor change could be transduced inside the cell through the cytoskeletal network and could trigger other biochemical processes that regulate cell division and growth (volume change).…”

Cytokines in context.

“…This response is mediated, in part, by a fall in cAMP (Nathan and Sanchez, 1990). In experiments on platelets that did not involve cytokines, glycoprotein IIb/IIIa (a t3 family integrin) was required for epinephrine to activate phospholipases (Banga et al, 1986) and for thrombin to induce a subset of tyrosine phosphorylation reactions (Ferrell and Martin, 1989). In each of these examples, the biochemical basis for the interdependence of two distinct receptor systems remains a mystery.…”

Rapid recruitment of p120RasGAP and its associated protein, p190RhoGAP, to the cytoskeleton during integrin mediated cell-substrate interaction

“…Integrin mediated cell-substrate interactions unleash a complex array of signals that include activation of calcium¯uxes (Miyauchi et al, 1991); Na + /H + antiport activity (Banga et al, 1986); elevation of intracellular pH (Schwartz et al, 1989) and tyrosine phosphorylation (reviewed in Burridge and Chrzanowska-Wodnicka, 1996;Clark and Brugge, 1995;Schwartz et al, 1995). The best studied of these changes, tyrosine phosphorylations, occurs on proteins, such as p125FAK (Burridge et al, 1992;Guan et al, 1991;Hanks et al, 1992;Kornberg et al, 1992;Lipfert et al, 1992), paxillin (Burridge et al, 1992), tensin (Bockholt and Burridge, 1993), PI-3-kinase (Chen and Guan, 1994a) …”