Gene dose effect reveals no G sub(s)-coupled A sub(2A) adenosine receptor reserve in murine T-lymphocytes: studies of cells from A sub(2A)- receptor-gene-deficient mice

• Published in: Biochemical journal 2001-02, Vol.354 (1), p.123-130
• Main Authors: Armstrong, J M, Chen, Jiang Fan, Schwarzschild, MA, Apasov, S, Smith, P T, Caldwell, C, Chen, P, Figler, H, Sullivan, G, Fink, S, Linden, J, Sitkovsky, M
• Format: Article
• Language: English
• Subjects: adenosine A2a receptors
• ISSN: 0264-6021

Agonist binding to extracellular A sub(2A) adenosine receptors (A sub(2A)RS) inhibits the activation of virtually all tested functions of T-cells and can induce apoptosis in thymocytes. The evaluation of levels of expression of these immunosuppressive receptors is expected to clarify whether the absence of spare A sub(2A)Rs (no 'receptor reserve') might be one of the mechanisms of attenuation of the effects of extracellular adenosine on T-cells. A sub(2A) transcript is found in T-cells and functional receptors can be demonstrated, but the density of receptor on T-cells is too low to be detected by radioligand binding. Studies of direct radioligand binding to murine brain with the selective A sub(2A)R agonist [ super(3)H]CGS21680 (2-{4-[(2-carboxyethyl)-phenyl]ethylamino}-5'-N-ethylcarboxamidoad enosine ) established that striata levels of A sub(2A)R are virtually absent from A sub(2A) knock-out mice. Mice that are heterozygous (A sub(2A)R super(+/-)) for the A sub(2A)R express significantly decreased levels of A sub(2A)R. To test for the presence of spare receptors in T-cells we took advantage of this gene dose effect and examined whether the decrease in the number of receptors in thymocytes from A sub(2A)R super(+/-) mice was proportionately reflected in a decrease in the functional cAMP response of T-cells to adenosine. cAMP accumulation and apoptosis induced by adenosine and by A sub(2A)R agonist are of a lower magnitude in T-cells from A sub(2A)R super(+/-) heterozygous mice than in T-cells from A sub(2A)R super(+/+) littermate control mice. These results indicate that there is no A sub(2A)R reserve in murine T-cells. Strongly decreased adenosine-triggered cAMP increases were detected in thymocytes from A sub(2A)R super(-/-) mice, suggesting that A sub(2B) adenosine receptors cannot fully compensate for the loss of A sub(2A)Rs in murine T-cells. We conclude that the number of A sub(2A)Rs is the limiting factor in determining the maximal cAMP response of T-lymphocytes to extracellular adenosine, thereby minimizing the immunosuppressive effects of extracellular adenosine.