DIFFERENTIAL EXPRESSION OF ADENOSINE A(2A) AND A(2B) RECEPTOR SUBTYPES ON MYELOID U937 AND THP-1 CELLS - ADENOSINE A(2B) RECEPTOR ACTIVATION SELECTIVELY STIMULATES CAMP FORMATION AND INHIBITION OF TNF-ALPHA RELEASE IN THP-1 CELLS

The ability of adenosine to function as an inhibitory modulator of inf lammatory processes has been well documented. While the activation of adenosine A(2) receptors has been implicated in the anti-inflammatory actions of adenosine, the specific mechanisms by which adenosine modul ates the function of proinflammatory cells is less well understood. Th e present studies were undertaken to further investigate the specific roles of adenosine A(2) receptor subtypes (A(2A) and A(2B)) in monocyt e function using two human myeloid cell lines of different development al lineage, U937 cells (promonocytes) and THP-1 cells (monocytes). Ade nosine receptor agonists stimulated the production of cAMP in both cel l lines; with an apparent A(2A) receptor-mediated agonist profile (CGS 21680 = NECA much greater than CPA) for U937 cells and an apparent A2 B receptor-mediated agonist profile (NECA much greater than CPA > CCS 21680) for THP-1 cells. NECA-stimulated cAMP production in both cell l ines could be inhibited with the adenosine receptor antagonists CGS 15 943 and DPCPX in a concentration dependent fashion. Using RNase protec tion analysis, it was demonstrated that the mRNA for the adenosine A(2 A) receptor was expressed in U937 cells, but not in THP- 1 cells, whil e the adenosine A(2B) receptor mRNA was detected in THP-1 cells, but n ot in U937 cells. Since THP-1 cells are widely used as a model of mono cyte function, additional experiments were conducted to characterize t he expression of adenosine receptor subtypes on these cells. Radioliga nd binding studies demonstrated the apparent lack of high affinity bin ding to A(1), A(2A), Or A(3) receptors on THP-1 cell membranes. Furthe r, the rank order potency of adenosine receptor agonists to inhibit th e LPS-stimulated release of TNF alpha from THP-1 cells was consistent with the activation of adenosine A(2B) receptors. These results sugges t the possibility that the effects of adenosine on myeloid cell functi on can be mediated by a differential expression of A(2) adenosine rece ptor subtypes during developmental maturation. Further, activation of adenosine A(2B) receptors may represent a significant mechanism by whi ch adenosine inhibits TNF alpha release from inflammatory cells. (C) 1 998 Wiley-Liss, Inc.