Kallikrein-related peptidases: proteolysis and signaling in cancer, the new frontier.

The exact mechanism(s) by which kallikrein-related peptidases (KLKs) function, their levels of activity and their potential endogenous targets in vivo have only recently begun to be revealed. Our group and others have shown that KLKs can have hormonal properties by signaling via proteinase-activated receptors (PARs), a family of G-protein-coupled receptors. Signals by PAR(1), PAR(2), and PAR(4) can regulate calcium release or mitogen-activated protein kinase activation and lead to platelet aggregation, vascular relaxation, cell proliferation, cytokine release, and inflammation. We have further documented the presence of active KLK6 and 10 (by activity-based ELISA or proteomics) and the presence of proteinase inhibitors, such as alpha(1)-antitrypsin, in cancer-derived fluids. We suggest that tumors and inflamed tissues can release active KLKs, which are under tight regulation by proteinase inhibitors. These enzymes can potentially control cell/tissue behavior by regulating PAR activation in specific settings and disease stages.