Evidence for functionally active protease-activated receptor-4 (PAR-4) in human vascular smooth muscle cells

1 This study investigates, whether in addition to the protease-activated receptor-1 (PAR-1), PAR-4 is present in vascular smooth muscle cells (SMC) of the human saphenous vein and whether this receptor is functionally active. PAR-1 and PAR-4 are stimulated by thrombin and by the synthetic peptides SFLLRN and GYPGQV, respectively. 2 mRNAs for both, PAR-1 and PAR-4, were detected in the SMC by using reverse transcriptase polymerase chain reaction (RT-PCR). 3 Treatment of the SMC with GYPGQV (200 muM) resulted in a transient increase in free intracellular calcium. This calcium signal was completely abolished after a preceding challenge with thrombin (10 nM), indicating homologous receptor desensitization. 4 Stimulation of the SMC with 10 nM thrombin or 200 muM SFLLRN caused a time-dependent activation of the extracellular signal-regulated kinases-1/2 (ERK-1/2) with a maximum at 5 min. In contrast, 100 nM thrombin as well as 200 muM of GYPGQV induced a prolonged phosphorylation of ERK-1/2 with a maximum at 60 min. These data suggest that PAR-1 and PAR-4 are activated by thrombin at distinct concentrations and with distinct kinetics. 5 GYPGQV stimulated [H-3]-thymidine incorporation in SMC. At 500 muM, the peptide increased DNA synthesis 2.5 fold above controls. A comparable mitogenic effect was obtained after stimulation of the SMC by 10 nM thrombin or 100 muM SFLLRN, respectively. 6 These data indicate that a functionally active PAR-4 is present in SMC and, in addition to PAR-1, might contribute to thrombin-induced mitogenesis.