Extracellular adenosine increases Na +/I − symporter gene expression in rat thyroid FRTL-5 cells

Abstract

We studied the effect of extracellular adenosine on iodide (I −) transport in FRTL-5 thyroid cells. I − accumulation increases after a 48 h exposure to adenosine in a concentration-dependent manner, reaching a maximum of 7.9-fold basal levels at 72 h after the addition of 300 μM adenosine. Neither I − efflux nor intracellular cyclic adenosine monophosphate accumulation is affected by the exposure to adenosine. The stimulation of I − transport by adenosine is partly as a result of an increase in Na +/I − symporter (NIS) mRNA and protein levels. Northern blot analysis revealed that adenosine increases NIS mRNA levels at 24 h, reaching a maximum at 36 h. Western blot analysis demonstrated that adenosine increases NIS protein levels at 36 h, reaching a maximum at 72 h, in parallel with the kinetics of adenosine-induced I − transport. Adenosine increased the promoter activity of a full-length NIS promoter-luciferase chimera, suggesting that the effect of adenosine on NIS mRNA levels is transcriptional. The stimulatory effect of adenosine on NIS mRNA levels, is mimicked by N 6-( l-2-phenylisopropyl) adenosine (PIA), an A 1 adenosine receptor agonist, and inhibited by 1,3-dipropyl-8-cyclopentylxanthine, an A 1 adenosine receptor antagonist, suggesting that the effect is mediated via the A 1 adenosine receptor stimulation in FRTL-5 cells. Incubating cells with islet-activating protein inhibited the adenosine-induced NIS mRNA levels. In sum, extracellular adenosine increases NIS gene expression and stimulates I − transport via the A 1 adenosine receptor-Gi/Go protein signal transduction pathway.