Increased arginase activity and endothelial dysfunction in human inflammatory bowel disease

Nitric oxide (center dot NO) generation from conversion of L- arginine to citrulline by nitric oxide synthase isoforms plays a critical role in vascular homeostasis. Loss of center dot NO is linked to vascular pathophysiology and is decreased in chronically inflamed gut blood vessels in inflammatory bowel disease ( IBD; Crohn's disease and ulcerative colitis). Mechanisms underlying decreased center dot NO production in IBD gut microvessels are not fully characterized. Loss of center dot NO generation may result from increased arginase ( AR) activity, which enzymatically competes with nitric oxide synthase for the common substrate L- arginine. We characterized AR expression in IBD microvessels and endothelial cells and its contribution to decreased center dot NO production. AR expression was assessed in resected gut tissues and human intestinal microvascular endothelial cells (HIMEC). AR expression significantly increased in both ulcerative colitis and Crohn's disease microvessels and submucosal tissues compared with normal. TNF-alpha/ lipopolysaccharide increased AR activity, mRNA and protein expression in HIMEC in a time-dependent fashion. RhoA/ ROCK pathway, a negative regulator of center dot NO generation in endothelial cells, was examined. The RhoA inhibitor C3 exoenzyme and the ROCK inhibitor Y-27632 both attenuated TNF-alpha/lipopolysaccharideinduced MAPK activation and blocked AR expression in HIMEC. A significantly higher AR activity and increased RhoA activity were observed in IBD submucosal tissues surrounding microvessels compared with normal control gut tissue. Functionally, inhibition of AR activity decreased leukocyte binding to HIMEC in an adhesion assay. Loss of center dot NO production in IBD microvessels is linked to enhanced levels of AR in intestinal endothelial cells exposed to chronic inflammation in vivo.