Abstract
Pharmacological studies suggest that A2B adenosine receptors mediate proinflammatory effects of adenosine. This concept was recently challenged by the finding that A2B adenosine receptor knockout (A2BKO) mice had moderate inflammation due to elevated basal plasma tumor necrosis factor (TNF)-α and an exaggerated response to lipopolysaccharide (LPS) challenge. However, it is unclear whether this phenomenon actually reflects the loss of putative taming of proinflammatory cytokine production via activation of A2B receptors by endogenous adenosine. In this report, we examined adenosine receptor-dependent regulation of interleukin (IL)-6 and TNF-α blood plasma levels in A2BKO and wild-type mice in vivo and their release from peritoneal macrophages ex vivo. Stimulation of adenosine receptors with 5′-N-ethylcarboxamidoadenosine (NECA) up-regulated IL-6 and suppressed LPS-induced TNF-α in wild-type mice. The selective A2B antagonists 3-isobutyl-8-pyrrolidinoxanthine and 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine (MRS 1754) inhibited NECA-induced IL-6 release but not the suppression of LPS-induced TNF-α secretion from macrophages. Genetic ablation of A2B receptors abrogated NECA-induced increases in IL-6 release from mouse peritoneal macrophages and dramatically reduced the ability of NECA to raise IL-6 plasma levels in vivo. In contrast, the absence of A2B adenosine receptors did not affect NECA-induced suppression of LPS-activated TNF-α release in macrophages, nor did it reduce the ability of NECA to suppress LPS-induced increase in TNF-α plasma levels in vivo. Thus, our results indicate that stimulation of A2B receptors up-regulates the proinflammatory cytokine IL-6 and argue against the recently suggested anti-inflammatory role of A2B receptors in suppression of LPS-stimulated TNF-α production by adenosine.
Footnotes
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This work was supported by National Institutes of Health Grant R01 HL076306 and by American Heart Association Southeastern Affiliate Grantin-aid 0755221B.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.107.131540.
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ABBREVIATIONS: IL, interleukin; A2BKO, A2B adenosine receptor knockout; TNF, tumor necrosis factor; LPS, lipopolysaccharide; NECA, 5′-N-ethylcarboxamidoadenosine; MRS 1754, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine; IPDX, 3-isobutyl-8-pyrrolidinoxanthine; DMSO, dimethyl sulfoxide; WT, wild type; RT-PCR, reverse transcription-polymerase chain reaction; PBS, phosphate-buffered saline; ANOVA, analysis of variance; A2AKO, A2A adenosine receptor knockout.
- Received September 11, 2007.
- Accepted October 25, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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