Reversal of Angiogenesis In Vitro, Induction of Apoptosis, and Inhibition of Akt Phosphorylation in Endothelial Cells by Thromboxane A 2

Abstract

Thromboxane A(2) (TxA(2)) causes platelet aggregation, vasoconstriction, and inhibition of endothelial cell (EC) migration and prevents vascular tube formation via its specific receptors (TP), of which there are two isoforms (TPalpha and TPbeta), both expressed in human ECs. In this study, we demonstrate that the TxA(2) mimetic IBOP increases apoptosis of human ECs and inhibits the phosphorylation of Akt kinase, an intracellular mediator required for cell survival. Treatment with IBOP destroyed EC networks formed on a basement membrane matrix in vitro. To distinguish the role of the TP isoforms, each isoform was expressed in TP-null ECs to create TPalpha and TPbeta ECs. IBOP induced apoptosis and inhibited phosphorylation of Akt kinase in both TPalpha and TPbeta. IBOP increased cAMP levels in TPalpha but not in TPbeta. Apoptosis induced by IBOP in TPalpha was not affected by either the adenylyl cyclase activator forskolin or the protein kinase A inhibitor 14-22 amide or H-89, whereas that in TPbeta was suppressed by forskolin and enhanced by the protein kinase A inhibitor 14-22 amide or H-89, suggesting that the TP isoforms differ in their signal pathways in mediating apoptosis. In conclusion, apoptosis may be the mechanism by which TxA(2)-mediated destruction of vascular structures in ECs occurs; although both TP isoforms induce apoptosis, possibly via inhibiting Akt phosphorylation, the signaling differs in each isoform, in that activation of the adenylyl cyclase pathway prevents apoptosis caused by TPbeta, but not by TPalpha, stimulation.