Reciprocal regulation of platelet responses to P2Y and thromboxane receptor activation

Abstract

Summary. Background: Thromboxane A 2 and ADP are two major platelet agonists that stimulate two sets of G proteincoupled receptors to activate platelets. Although aggregation responses to ADP and thromboxane desensitize, there are no reports currently addressing whether activation by one agonist may heterologously desensitize responses to the other. Objectives: To demonstrate whether responses to ADP or U46619 may be modulated by prior treatment of platelets with the alternate agonist, revealing a level of cross-desensitization between receptor systems. Results: Here we show that pretreatment of platelets with either agonist substantially desensitizes aggregation responses to the other agonist. Calcium responses to thromboxane receptor activation are desensitized by preactivation of P2Y 1 but not P2Y 12 receptors. This heterologous desensitization is mediated by a protein kinase C (PKC)-independent mechanism. Reciprocally, calcium responses to ADP are desensitized by pretreatment of platelets with the thromboxane analogue, U46619, and P2Y 12 -mediated inhibition of adenylate cyclase is also desensitized by pretreatment with U46619. In this direction, desensitization is comprised of two components, a true heterologous component that is PKC-independent, and a homologous component that is mediated through stimulated release of dense granule ADP. Conclusions: This study reveals cross-desensitization between ADP and thromboxane receptor signaling in human platelets. Cross-desensitization is mediated by protein kinases, involving PKC-dependent and independent pathways, and indicates that alterations in the activation state of one receptor may have effects upon the sensitivity of the other receptor system.

Selective Impairment of P2Y Signaling by Prostaglandin E2 in Macrophages: Implications for Ca2+-Dependent Responses

“…Local elevated concentrations of ATP are thought to mediate most of its effects through binding to the P2X7 receptor, whereas UTP and UDP can act through P2Y 2 / P2Y 4 and P2Y 6 , respectively (15,(45)(46)(47). Although the P2Y 2 receptor appears to be the dominant target for UTP in resident macrophages (30), our data suggest that P2Y 4 receptor could be also involved in UTP-dependent calcium responses in peritoneal macrophages, as supported by the high expression of both P2Y 4 and P2Y 6 receptors and the sensitivity to PPADS antagonism. Even the contribution of P2Y 2 /P2Y 4 or P2Y 4 /P2Y 6 heteromers can be considered (48).…”

“…1B). The response to UTP was inhibited by PPADS, which has been reported to depress responses mediated by metabotropic (P2Y 1 , P2Y 4 , and P2Y 13 ) and ionotropic (P2X1-3, P2X5, and P2X7) receptors in a reversible manner (Fig. 1C).…”

“…In macrophages, it has been described that UTP potentiates PGE 2 release, which is involved in the enhancement of NO synthase 2 induction by LPS (14). In platelets, a cross-desensitization between ADP and thromboxane receptor signaling has been reported previously (4), and there is mounting evidence supporting that these interactions play important roles in several inflammatory and degenerative disorders, such as multiple sclerosis, amyotrophic lateral sclerosis, or Alzheimer's disease (15)(16)(17)(18). In these pathologies, extracellular ATP exerts proinflammatory actions provoking cytokine release and PGs production (19).…”

“…In addition to PGs, nucleotides can be released during the course of inflammatory responses and activate immune cells, such as monocytes, macrophages, and lymphocytes (3). These extracellular nucleotides exert several effects on immune cells and are involved in cytoskeleton reorganization, cell migration, phagocytosis, or exocytosis (3)(4)(5)(6). Extracellular nucleotides can also play tissue-specific roles.…”

“…These actions of nucleotides are mediated by specific receptors, namely P2 receptors, which are classified in two major families: G protein-coupled P2Y receptors and ionotropic P2X receptors (10)(11)(12). Currently, eight subtypes of P2Y (P2Y 1,2,4,6,11,12,13,14 ) and seven subtypes of P2X (P2X1-7) have been cloned and characterized (for a review, see Ref. 13).…”