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 TNFα release in THP-1 cells

The ability of adenosine to function as an inhibitory modulator of inflammatory processes has been well documented. While the activation of adenosine A₂ receptors has been implicated in the anti-inflammatory actions of adenosine, the specific mechanisms by which adenosine modulates the function of proinflammatory cells is less well understood. The present studies were undertaken to further investigate the specific roles of adenosine A₂ receptor subtypes (A(2A) and A(2B)) in monocyte function using two human myeloid cell lines of different developmental lineage, U937 cells (promonocytes) and THP-1 cells (monocytes). Adenosine receptor agonists stimulated the production of cAMP in both cell lines; with an apparent A(2A) receptor-mediated agonist profile (CGS 21680 = NECA >> CPA) for U937 cells and an apparent A(2A) receptor-mediated agonist profile (NECA >> CPA > CGS 21680) for THP-1 cells. NECA-stimulated cAMP production in both cell lines could be inhibited with the adenosine receptor antagonists CGS 15943 and DPCPX in a concentration-dependent fashion. Using RNase protection analysis, it was demonstrated that the mRNA for the adenosine A(2A) receptor was expressed in U937 cells, but not in THP-1 cells, while the adenosine A(2B) receptor mRNA was detected in TH-1 cells, but not in U937 cells. Since THP-1 cells are widely used as a model of monocyte function, additional experiments were conducted to characterize the expression of adenosine receptor subtypes on these cells. Radioligand binding studies demonstrated the apparent lack of high affinity binding to A₁, A(2A), or A₃ receptors on THP-1 cell membranes. Further, the rank order potency of adenosine receptor agonists to inhibit the LPS-stimulated release of TNFα from THP-1 cells was consistent with the activation of adenosine A(2B) receptors. These results suggest the possibility that the effects of adenosine on myeloid cell function can be mediated by a differential expression of A₂ adenosine receptor subtypes during developmental maturation. Further, activation of adenosine A(2B) receptors may represent a significant mechanism by which adenosine inhibits TNFα release from inflammatory cells.