The protease-activated receptor-2-specific agonists 2-aminothiazol-4-yl-LIGRL-NH2 and 6-aminonicotinyl-LIGRL-NH2 stimulate multiple signaling pathways to induce physiological responses in vitro and in vivo.

Protease-activated receptor-2 (PAR(2)) is one of four protease-activated G-protein-coupled receptors. PAR(2) is expressed on multiple cell types where it contributes to cellular responses to endogenous and exogenous proteases. Proteolytic cleavage of PAR(2) reveals a tethered ligand that activates PAR(2) and two major downstream signaling pathways: mitogen-activated protein kinase (MAPK) and intracellular Ca(2+) signaling. Peptides or peptidomimetics can mimic binding of the tethered ligand to stimulate signaling without the nonspecific effects of proteases. The most commonly used peptide activators of PAR(2) (e.g. SLIGRL-NH(2) and SLIGKV-NH(2)) lack potency at the receptor. However, although the potency of 2-furoyl-LIGRLO-NH(2) (2-f-LIGRLO-NH(2)) underscores the use of peptidomimetic PAR(2) ligands as a mechanism to enhance pharmacological action at PAR(2), 2-f-LIGRLO-NH(2) has not been thoroughly evaluated. We evaluated the known agonist 2-f-LIGRLO-NH(2) and two recently described pentapeptidomimetic PAR(2)-specific agonists, 2-aminothiazol-4-yl-LIGRL-NH(2) (2-at-LIGRL-NH(2)) and 6-aminonicotinyl-LIGRL-NH(2) (6-an-LIGRL-NH(2)). All peptidomimetic agonists stimulated PAR(2)-dependent in vitro physiological responses, MAPK signaling, and Ca(2+) signaling with an overall rank order of potency of 2-f-LIGRLO-NH(2) ≈ 2-at-LIGRL-NH(2) > 6-an-LIGRL-NH(2) ≫ SLIGRL-NH(2). Because PAR(2) plays a major role in pathological pain conditions and to test potency of the peptidomimetic agonists in vivo, we evaluated these agonists in models relevant to nociception. All three agonists activated Ca(2+) signaling in nociceptors in vitro, and both 2-at-LIGRL-NH(2) and 2-f-LIGRLO-NH(2) stimulated PAR(2)-dependent thermal hyperalgesia in vivo. We have characterized three high potency ligands that can be used to explore the physiological role of PAR(2) in a variety of systems and pathologies.