A20 BLOCKS ENDOTHELIAL-CELL ACTIVATION THROUGH A NF-KAPPA-B-DEPENDENTMECHANISM

The A20 gene product is a novel zinc finger protein originally describ ed as a tumor necrosis factor alpha (TNF)-inducible early response gen e in human umbilical vein endothelial cells (HUVEC), Its described fun ction is to block TNF-induced apoptosis in fibroblasts and B lymphocyt es, but more recently it has also been shown to play a role in lymphoi d cell maturation. The mechanism of action of A20 is unknown, The aim of our study was to assess the effect of A20 upon endothelial cell act ivation, By transfecting bovine aortic endothelial cells (BAEC) with A 20 as well as reporter constructs consisting of the promoters of genes known to be up-regulated during endothelial cell activation, i.e. E s electin, interleukin (IL)-8, tissue factor (TF), and inhibitor of nucl ear factor kappa B alpha (I kappa B alpha), we demonstrate that A20 ex pression inhibits gene up-regulation associated with TNF, lipopolysacc haride (LPS), phorbol 12-myristate 13 acetate (PMA), and hydrogen pero xide (H2O2)-induced endothelial cell (EC) activation. The mechanism of action of A20 is in part, or totally, due to the blockade of nuclear factor kappa B (NF-kappa B), as shown by its ability to suppress the a ctivity of a NF-KB reporter, This effect is specific, as A20 does not block a noninducible, constitutively expressed reporter, Rous sarcoma virus-luciferase (RSV-LUC); nor does it block the c-Tat-inducible, NF- kappa B-independent reporter, human immunodeficiency virus-chloramphen icol acetyltransferase (HIV-CAT). How A20 blocks NF-kappa B is unclear , although we demonstrate that it does not affect p65 (RelA)-mediated gene transactivation, The inhibition of endothelial cell activation by A20 is a novel function for A20.