Activation of Proteinase-Activated Receptor-2 by Human Kallikrein-Related Peptidases

Proteinase-activated receptor-2 (PAR 2 ) is a seven transmembrane spanning, G-protein-coupled receptor, present on the membrane of many cell types including keratinocytes. In skin, PAR 2 is suggested to play a regulatory role during inflammation, epidermal barrier function, and pruritus. PAR 2 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 PAR 2 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 PAR 2 in vitro . PAR 2 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 PAR 2 signalling. We conclude that certain, but not all, epidermal KLKs are capable of activating PAR 2 . We could also show the coexpression of KLK14 and PAR 2 receptor in inflammatory skin disorders. These in vitro results suggest that KLKs may take part in PAR 2 activation in the epidermis and thereby in PAR 2 -mediated inflammatory responses, including epidermal barrier repair and pruritus. The role of KLKs in PAR 2 activation in vivo remains to be elucidated.