Abstract—As thrombin binding to the G protein–coupled proteinase activated receptor-1 (PAR-1) induces endothelial adhesivity to leukocytes through NF-&kgr;B activation and intercellular adhesion molecule-1 (ICAM-1) expression, we determined the signaling pathways mediating the response. Studies showed that the heterotrimeric G proteins, G&agr;q, and the G&bgr;&ggr; dimer were key determinants of the PAR-1 agonist peptide (TFLLRNPNDK)-induced NF-&kgr;B activation and ICAM-1 expression in endothelial cells. Cotransfection of RGS3T, a regulator of G-protein signaling that inhibits G&agr;q, or &agr;-transducin (G&agr;t), a scavenger of the G&bgr;&ggr;, markedly decreased NF-&kgr;B activity induced by PAR-1 activation. We determined the downstream signaling targets activated by G&agr;qand G&bgr;&ggr; that mediate NF-&kgr;B activation. Expression of the kinase-defective protein kinase C (PKC)-&dgr; mutant inhibited NF-&kgr;B activation induced by the constitutively active G&agr;qmutant, but had no effect on NF-&kgr;B activity induced by G&bgr;1&ggr;2. In related experiments, NF-&kgr;B as well as ICAM-1 promoter activation induced by G&bgr;1&ggr;2were inhibited by the expression of the dominant-negative mutant of 85-kDa regulatory subunit of PI 3-kinase; however, the expression of this mutant had no effect on the response induced by activated G&agr;q. Cotransfection of the catalytically inactive Akt mutant inhibited the NF-&kgr;B activation induced by the constitutively active PI 3-kinase mutant as well as that by the activated forms of G&agr;qand PKC-&dgr;. These results support a model in which ligation of PAR-1 induces NF-&kgr;B activation and ICAM-1 transcription by the engagement of parallel G&agr;q/PKC-&dgr; and G&bgr;&ggr;/PI3-kinase pathways that converge at Akt.