INHIBITION OF TNF-ALPHA EXPRESSION BY ADENOSINE - ROLE OF A3 ADENOSINE RECEPTORS

Adenosine agonists inhibit TNF-alpha production in macrophage and mono cytes, but the mechanism is unknown. Therefore, we studied the human m acrophage cell line U937 to determine the adenosine receptor subtypes responsible and the intracellular signaling mechanisms involved, The A 1/A3 agonist N-6-(4-amino-3-iodobenzyl)adenosine (I-ABA) decreased LPS -stimulated TNF-alpha protein production by 79 +/- 5% (p = 0.003). The mechanism was pretranslational, as adenosine receptor stimulation cau sed a marked decrease in TNF-alpha mRNA. IL-1 beta, IL-6, and IL-8 mRN A were not changed by adenosine agonists, The rank order of agonists a s TNF-alpha inhibitors suggested that the A3 receptor might be involve d nzyl)-9-[5-(methylcarbamoyl)-beta-D-ribofuranosyl] adenosine > 2-chl oroadenosine greater than or equal to I-ABA > N-6-benzyl 5'-N-ethylcar boxamidoadenosine (NECA) > NECA > CGS21680 > N-6-cyclohexyladenosine), and this was supported by the fact that a mixed A1/A3 antagonist (xan thine amine congener) reversed the effect, whereas A1-specific (1,3-di propyl-8-cyclopentylxanthine) and A2-specific (3,7-dimethyl-1-propargy lxanthine) antagonists did not. Receptor signaling did not involve cAM P or protein kinase A, nor did it alter the activation and binding cha racteristics of the transcription factor NF-kappa B, However, the comp osition of the AP-1 transcription complex was altered by I-ABA, These data suggest that stimulation of the A3 adenosine receptor can alter t he cytokine milieu by decreasing TNF-alpha. Adenosine agonists or aden osine regulating agents have potential therapeutic uses in acute and c hronic inflammatory diseases.