Protease-activated receptor-2 turnover stimulated independently of receptor activation in porcine coronary endothelial cells.

1. Protease-activated receptors (PARs) are activated by an irreversible proteolytic mechanism which renders cleaved receptors unresponsive to subsequent challenges with activating enzymes. Non-specific proteolysis of PARs downstream of the activation site also prevents subsequent enzymic activation. Therefore, we investigated the effects of non-activating amino-terminal proteolysis with the bacterial protease thermolysin on PAR-mediated relaxation of porcine coronary artery ring preparations contracted with the thromboxane A2 mimetic U46619 (1-10 nM). 2. Treatment of contracted artery ring segments with thermolysin (0.01-1 u ml-1, 20 min) caused no response, but abolished endothelium-dependent relaxations induced by the enzymic activators of PAR-1, and PAR-2, thrombin (0.01-0.3 u ml-1) and trypsin (0.003-0.1 u ml-1) respectively. The same treatment, however, did not affect similar responses to the proteolysis-independent PAR-1 and PAR-2 activating peptides, SFLLRN-NH2 and SLIGRL-NH2 respectively (0.1-10 microM). 3. The inhibition of responsiveness to trypsin after thermolysin treatment recovered in a time-dependent manner, with maximal recovery (77.3 +/- 8.0% of time controls) occurring 150 min after thermolysin treatment. No recovery of responsiveness to thrombin after thermolysin treatment was observed within this time, however, the thrombin response returned to control levels after 20 h. 4. The recovery of responsiveness to trypsin was inhibited by the translation inhibitor cycloheximide (100 microM; 17.3 +/- 4.7%) and the protein trafficking inhibitor brefeldin A (10 microM; 12.1 +/- 4.8%) but was unaffected by the transcription inhibitor actinomycin D (2 microM; 65.1 +/- 3.6%), which did, however, abolish upregulation of B1-kinin receptors in this preparation. 5. In conclusion, our findings indicate that activation-independent amino-terminal proteolysis of PARs stimulates selective recovery of endothelial cell PAR-2 responsiveness, which appears to be regulated by translation. Such a novel mechanism for the maintenance of responsiveness to enzymic PAR-2 activators may imply that these receptors play important roles in vascular homeostasis.