INOS EXPRESSION IN HUMAN INTESTINAL MICROVASCULAR ENDOTHELIAL-CELLS INHIBITS LEUKOCYTE ADHESION

Increased nitric oxide (NO) production by inducible nitric oxide synth ase (iNOS) has been associated with intestinal inflammation, including human inflammatory bowel disease. However, NO can downregulate endoth elial activation and leukocyte adhesion, critical steps in the inflamm atory response, Using primary cultures of human intestinal microvascul ar endothelial cells (HIMEC), we determined the role of NO in the regu lation of HIMEC activation and interaction with leukocytes. Both nonse lective (NG-monomethyl-L-arginine) and specific (N-iminoethyl-L-lysine ) competitive inhibitors of iNOS significantly increased binding of le ukocytes by HIMEC activated with cytokines and lipopolysaccharide. Inc reased adhesion was reversible with the NOS substrate L-arginine and w as not observed in human umbilical vein endothelial cells (HUVEC). Act ivation of HIMEC significantly upregulated HIMEC iNOS expression and N O production. NOS inhibitors did not augment cell adhesion molecule le vels in activated HIMEC but did result in sustained increases in intra cellular reactive oxygen species. In addition, antioxidant compounds r eversed the effect of NOS inhibitors on HIMEC-leukocyte interaction. T aken together, these data suggest that after HIMEC activation, iNOS-de rived NO is an endogenous antioxidant, downregulating leukocyte bindin g and potentially downregulating intestinal inflammation.Increased nit ric oxide (NO) production by inducible nitric oxide synthase (iNOS) ha s been associated with intestinal inflammation, including human inflam matory bowel disease. However, NO can downregulate endothelial activat ion and leukocyte adhesion, critical steps in the inflammatory respons e, Using primary cultures of human intestinal microvascular endothelia l cells (HIMEC), we determined the role of NO in the regulation of HIM EC activation and interaction with leukocytes. Both nonselective (N-G- monomethyl-L-arginine) and specific (N-iminoethyl-L-lysine) competitiv e inhibitors of iNOS significantly increased binding of leukocytes by HIMEC activated with cytokines and lipopolysaccharide. Increased adhes ion was reversible with the NOS substrate L-arginine and was not obser ved in human umbilical vein endothelial cells (HUVEC). Activation of H IMEC significantly upregulated HIMEC iNOS expression and NO production . NOS inhibitors did not augment cell adhesion molecule levels in acti vated HIMEC but did result in sustained increases in intracellular rea ctive oxygen species. In addition, antioxidant compounds reversed the effect of NOS inhibitors on HIMEC-leukocyte interaction. Taken togethe r, these data suggest that after HIMEC activation, iNOS-derived NO is an endogenous antioxidant, downregulating leukocyte binding and potent ially downregulating intestinal inflammation.