Protease-activated Receptor 2 in Colon Cancer

Abstract

Several lines of evidence suggest that tumor-derived trypsin contributes to the growth and invasion of cancer cells. We have recently shown that trypsin is a potent growth factor for colon cancer cells through activation of the G protein-coupled receptor protease-activated receptor 2 (PAR2). Here, we analyzed the signaling pathways downstream of PAR2 activation that lead to colon cancer cell proliferation in HT-29 cells. Our data are consistent with the following cascade of events upon activation of PAR2 by the serine protease trypsin or the specific PAR2-activating peptide (AP2): (i) a matrix metalloproteinasedependent release of transforming growth factor (TGF)-␣, as demonstrated with TGF-␣-blocking antibodies and measurement of TGF-␣ in culture medium; (ii) TGF-␣-mediated activation of epidermal growth factor receptor (EGF-R) and subsequent EGF-R phosphorylation; and (iii) activation of ERK1/2 and subsequent cell proliferation. The links between these events are demonstrated by the fact that stimulation of cell proliferation and ERK1/2 upon activation of PAR2 is reversed by the metalloproteinase inhibitor batimastat, TGF-␣-neutralizing antibodies, EGF-R ligand binding domain-blocking antibodies, and the EGF-R tyrosine kinase inhibitors AG1478 and PD168393. Therefore, transactivation of EGF-R appears to be a major mechanism whereby activation of PAR2 results in colon cancer cell growth. By using the Src tyrosine kinase inhibitor PP2, we further showed that Src plays a permissive role for PAR2-mediated ERK1/2 activation and cell proliferation, probably acting downstream of the EGF-R. These data explain how trypsin exerts robust trophic action on colon cancer cells and underline the critical role of EGF-R transactivation.Proteases have been increasingly recognized as important factors in pathophysiology of tumor diseases. Besides their contribution to cancer progression by the degradation of extracellular matrix proteins, there is now substantial evidence that certain proteases serve as signal molecules controlling cell functions through specific membrane receptors, the proteaseactivated receptors. PARs 1 are seven transmembrane-spanning domain G protein-coupled receptors comprising four receptors named PAR1, PAR2, PAR3, and PAR4 (1, 2). Thrombin is the physiological activator of PAR1, PAR3, and PAR4, whereas PAR2 is activated by multiple trypsin-like enzymes including trypsin and mast cell tryptase but not thrombin. The mechanism of activation of PARs was initially established for PAR1 (3) and seems to be a paradigm for the other PARs (1, 2, 4). They are irreversibly activated by a proteolytic mechanism in which the protease binds to and cleaves the amino-terminal exodomain of the receptor. This cleavage generates a new amino-terminal sequence that binds intramolecularly to the core receptor and serves as a tethered ligand. Synthetic activating peptides that mimic the tethered ligand domains of PAR1, PAR2, and PAR4 have been developed. The activation of PARs by these synthetic peptides APs is independent of rec...

Commutators of PAR-1 signaling in cancer cell invasion reveal an essential role of the Rho–Rho kinase axis and tumor microenvironment

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