Protease-activated receptor-2 (PAR-2): structure-function study of receptor activation by diverse peptides related to tethered-ligand epitopes

Protease-activated receptor-2 (PAR-2) is a tethered-ligand, G-protein-coupled receptor that is activated by proteolytic cleavage or by small peptides derived from its cleaved N-terminal sequence, such as SLIGRL-NH2. To assess specific PAR activity, we developed an immortalized murine PAR-1 (-/-) cell line transfected with either human PAR-2 or PAR-1. A "directed" library of more than 100 PAR agonist peptide analogues was synthesized and evaluated for PAR-2 and PAR-1 activity to establish an in-depth structure-function profile for specific action on PAR-2. The most potent agonist peptides (EC50 = 2-4 microM) had Lys at position 6, Ala at position 4, and pFPhe at position 2; however, these also exhibited potent PAR-1 activity (EC50 = 0.05-0.35 microM). We identified SLIARK-NH2 and SL-Cha-ARL-NH2 as relatively potent, highly selective PAR-2 agonists with EC50 values of 4 microM. Position 1 did not tolerate basic, acidic, or large hydrophobic amino acids. N-Terminal capping by acetyl eliminated PAR-2 activity, although removal of the amino group reduced potency by just 4-fold. At position 2, substitution of Leu by Cha or Phe gave equivalent PAR-2 potency, but this modification also activated PAR-1, whereas Ala, Asp, Lys, or Gln abolished PAR-2 activity; at position 3, Ile and Cha were optimal, although various amino acids were tolerated; at position 4, Ala or Cha increased PAR-2 potency 2-fold, although Cha introduced PAR-1 activity; at position 5, Arg or Lys could be replaced successfully by large hydrophobic amino acids. These results with hexapeptide C-terminal amides that mimic the native PAR-2 ligand indicate structural modes for obtaining optimal PAR-2 activity, which could be useful for the design of PAR-2 antagonists.