Reversal of angiogenesis in vitro, induction of apoptosis, and inhibition of Akt phosphorylation in endothelial cells by thromboxane A(2)

Thromboxane A(2) (TxA(2)) causes platelet aggregation, vasoconstriction, an d inhibition of endothelial cell (EC) migration and prevents vascular tube formation via its specific receptors (TP), of which there are two isoforms (TP alpha and TP beta), both expressed in human ECs. In this study, we demo nstrate that the TxA(2) mimetic IBOP increases apoptosis of human ECs and i nhibits the phosphorylation of Akt kinase, an intracellular mediator requir ed for cell survival. Treatment with IBOP destroyed EC networks formed on a basement membrane matrix in vitro. To distinguish the role of the TP isofo rms, each isoform was expressed in TP-null ECs to create TP alpha and TP be ta ECs. IBOP induced apoptosis and inhibited phosphorylation of Akt kinase in both TP alpha and TP beta. IBOP increased cAMP levels in TP alpha but no t in TP beta. Apoptosis induced by IBOP in TP alpha was not affected by eit her the adenylyl cyclase activator forskolin or the protein kinase A inhibi tor 14-22 amide or H-89, whereas that in TP beta was suppressed by forskoli n and enhanced by the protein kinase A inhibitor 14-22 amide or H-89, sugge sting that the TP isoforms differ in their signal pathways in mediating apo ptosis. In conclusion, apoptosis may be the mechanism by which TxA(2)-media ted destruction of vascular structures in ECs occurs; although both TP isof orms induce apoptosis, possibly via inhibiting Akt phosphorylation, the sig naling differs in each isoform, in that activation of the adenylyl cyclase pathway prevents apoptosis caused by TP beta, but not by TP alpha, stimulat ion.