Activation of proteinase-activated receptor-2 by human kallikrein-related peptidases

Proteinase-activated receptor-2 (PAR2) is a seven transmembrane spanning, G-protein-coupled receptor, present on the membrane of many cell types including keratinocytes. In skin, PAR2 is suggested to play a regulatory role during inflammation, epidermal barrier function, and pruritus. PAR2 is activated by trypsin-like proteases by a unique mechanism where cleavage of the receptor leads to the release of a small peptide, which activates the receptor as a tethered ligand. The endogenous activators of PAR2 on keratinocytes have not been identified as of yet. Potential candidates are kallikrein-related peptidases (KLKs) expressed by epidermal cells. Therefore, the ability of four human skin-derived KLKs was examined with regard to their capacity to activate PAR2 in vitro. PAR2 cleavage was followed by immunofluorescence analysis and functional activation by measurements of changes in intracellular calcium levels. We found that KLK5 and KLK14, but neither KLK7 nor KLK8, induced PAR2 signalling. We conclude that certain, but not all, epidermal KLKs are capable of activating PAR2. We could also show the coexpression of KLK14 and PAR2 receptor in inflammatory skin disorders. These in vitro results suggest that KLKs may take part in PAR2 activation in the epidermis and thereby in PAR2-mediated inflammatory responses, including epidermal barrier repair and pruritus. The role of KLKs in PAR2 activation in vivo remains to be elucidated.