NITRIC OXIDE-MEDIATED EXPRESSION OF Bcl-2 AND Bcl-xl AND PROTECTION FROM TUMOR NECROSIS FACTOR-??-MEDIATED APOPTOSIS IN PORCINE ENDOTHELIAL CELLS AFTER EXPOSURE TO LOW CONCENTRATIONS OF XENOREACTIVE NATURAL ANTIBODY1

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Abstract
Cardiac and renal allo- and xenografts can acquire a natural resistance to vascular rejection. This "accommodation" involves endothelial cell (EC) expression of "survival genes" such as Bcl family members and hemoxygenase 1. Understanding what initiates this protective process would have profound implications; our hypothesis is that low concentrations of antigraft antibodies may mediate these changes. In vitro cultured primary and immortalized porcine EC were incubated with polyclonal human IgG for 6 days and then examined for phenotype changes. The cells acquired resistance to tumor necrosis factor-alpha-mediated apoptosis (50-100% reduction at 6 hr) and up-regulated expression of Bcl-2 and Bcl-xl; sustained expression was accompanied by inducible nitric oxide (NO) synthase expression and by enhanced production of NO by EC. Two observations suggested that NO was actively involved in the process of Bcl-2 and Bcl-xl induction. First, (z)-1-2-[2-aminoethyl)-N- (2-ammonioethyl)amino]diazen-1-ium-1,2-diolate, an NO donor, was able to induce similar changes in porcine EC to those induced by anti-pig antibodies. Second, an NO synthase inhibitor NG-monomethyl-L-arginine.monoacetate was able to specifically inhibit the anti-pig antibody-mediated expression of Bcl-2 or Bcl-xl. These data strongly support the hypothesis that Bcl-2 and Bcl-xl expression and protection from apoptosis in EC may result from antibody-mediated NO production through the neoexpression of inducible NO synthase.