Cathepsin G (Cat-G) is a neutrophil serine-protease found in the colonic lumen of ulcerative colitis (UC) patients. Cat-G is able to activate protease-activated receptor-4 (PAR₄) located at the apical side of enterocytes, leading to epithelial barrier disruption. However, the mechanisms through which Cat-G triggers inflammation are not fully elucidated. The aims of our study were to evaluate in vivo the effects of UC fecal supernatants and Cat-G on epithelial barrier function and inflammation, and the connection between these two parameters.

Male balb/c mice were used in this study. We evaluated the effect of a 2-hour intracolonic infusion of 1) fecal supernatants from UC patients pretreated or not with specific Cat-G inhibitor (SCGI); 2) PAR₄-activating peptide (PAR₄-AP); and 3) Cat-G on colonic myeloperoxidase (MPO) activity and paracellular permeability (CPP). The involvement of PAR₄ was assessed by pretreating animals with pepducin P4pal-10, which blocks PAR₄ signaling. We investigated the role of myosin light chain (MLC) kinase by using its inhibitor, ML-7, and we determined phosphorylated MLC (pMLC) levels in mice colonic mucosa.

UC fecal supernatants, Cat-G, and PAR₄ agonist increased both CPP and MPO activity in comparison with healthy subjects fecal supernatants. ML-7 inhibited the CPP increase triggered by Cat-G by 92.3%, and the enhanced MPO activity by 43.8%. Intracolonic infusion of UC fecal supernatant determined an increased phosphorylation level of MLC.

These observations support that luminal factors such as Cat-G play an important proinflammatory role in the pathogenesis of colitis, mainly depending on CPP increase by MLC phosphorylation. (Inflamm Bowel Dis 2011)