RESUMO
Gh is a GTP binding protein that couples to the thromboxane receptor (TP), but also functions as tissue transglutaminase II (tTG). A transgenic mouse model was generated in which Gh was overexpressed (GhOE) in ventricular myocytes under the control of the alpha-myosin heavy chain promoter. Heart rate was elevated and both blood pressure and left ventricular ejection fraction were depressed in GhOEs. Left ventricular mass was increased, consistent with genetic and ultrastructural evidence of hypertrophy. Fibrosis and apoptosis were also augmented. Survival declined disproportionately in older GhOEs. Cardiomyocyte expression of COX-2, thromboxane synthase (TxS), and the receptors for TxA2 (the TP), PGF2alpha (the FP), and PGI2 (the IP) were upregulated and urinary 8,12-iso-iPF2alpha-VI,2,3-dinor-6-keto-PGF1alpha and 2,3-dinor-thromboxane B2 were increased in GhOEs, reflecting increased lipid peroxidation and cyclooxygenase (COX) activation. Selective COX-2 inhibition, TP antagonism, and suppression of lipid peroxidation each rescued the cardiac phenotype. Infusion of an FP agonist exacerbated the phenotype, whereas administration of an IP agonist improved cardiac function. Directed cardiac overexpression of Gh/tTG causes both TG activation and increased TP/Gh-dependent signaling. The COX-2-dependent increase in TxA2 generation augments cardiac hypertrophy, whereas formation of PGI2 by the same isozyme ameliorates the phenotype. Oxidant stress may contribute, via regulation of COX-2 expression and/or ligation of the TP and the FP by isoprostanes. Gh/tTG activation regulates expression of COX-2 and its products may differentially modulate cardiomyocyte commitment to cell death or survival.
Assuntos
Proteínas de Ligação ao GTP/biossíntese , Insuficiência Cardíaca/enzimologia , Isoenzimas/metabolismo , Prostaglandina-Endoperóxido Sintases/metabolismo , Transglutaminases/biossíntese , Animais , Apoptose/genética , Pressão Sanguínea/genética , Ciclo-Oxigenase 2 , Modelos Animais de Doenças , Progressão da Doença , Fibrose/genética , Fibrose/patologia , Proteínas de Ligação ao GTP/genética , Insuficiência Cardíaca/genética , Insuficiência Cardíaca/patologia , Frequência Cardíaca/efeitos dos fármacos , Frequência Cardíaca/genética , Isoenzimas/genética , Peroxidação de Lipídeos/genética , Camundongos , Camundongos Transgênicos , Miocárdio/metabolismo , Miocárdio/patologia , Especificidade de Órgãos , Fenótipo , Regiões Promotoras Genéticas , Prostaglandina-Endoperóxido Sintases/genética , Prostaglandinas/biossíntese , Prostaglandinas/farmacologia , Prostaglandinas/urina , Proteína 2 Glutamina gama-Glutamiltransferase , RNA Mensageiro/biossíntese , Receptores de Superfície Celular/biossíntese , Volume Sistólico/efeitos dos fármacos , Volume Sistólico/genética , Taxa de Sobrevida , Tromboxano-A Sintase/biossíntese , Transglutaminases/genética , Miosinas Ventriculares/genéticaRESUMO
Arachidonic acid (AA) is a potent inducer of platelet aggregation in vitro; this activity is due to its conversion to biologically active metabolites, prostaglandin (PG) endoperoxides and thromboxane A2 (TxA2). PG endoperoxides and TxA, are thought to act on the same receptor; however, at least two isoforms of this receptor have been identified. The aim of our work was to clarify whether endoperoxides and TxA2 activate the same or different receptor subtypes to induce aggregation and calcium movements in human platelets. AA-induced aggregation and calcium rises were still detectable in platelets preincubated with thromboxane synthase inhibitors, which suppress TxA2 formation and induce PGH2 accumulation, suggesting that PG endoperoxides can activate platelets. Exogenously added PGH2 was able to induce aggregation and calcium rises. Pretreatment of platelets with GR32191B or platelet activating factor, which desensitize one of the two receptor subtypes identified in platelets, did not prevent calcium rises induced by endogenously generated or by exogenouly added PGH2, indicating that TxA2 and PG endoperoxides share the same receptor subtype(s) to activate platelets. HEK-293 cells overexpressing either of the two thromboxane receptor isoforms cloned to date (TPalpha and TPbeta) and identified in human platelets, stimulated with PGH2, or with the stable endoperoxide analog U46619, formed inositol phosphates. These data show that endoperoxides and TXA2 mediate their effects on platelets acting on both, and the same, receptor isoform(s).
