Studies on the molecular mechanism of cellular signal transduction via adenosine receptors

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We first screened cDNA libraries from mouse brains in order to find a new subtype adenosine receptor. However, no such clones were detected in our screening. We then examined if there is an adenosine binding protein which shows a unique adenosine ligand specificity on rat brain membrane fractions using [H^3N]NECA as a tracer, which is a non-selective adenosine receptor agonist. After several trials, we could detect a distinct adenosine binding activity which is neither A1 nor A2 adenosine receptor. This binding site has been highly purified by several chromatographical steps including adenine nucleotide-coupled agarose affinity chromatography. Interestingly, the purified adenosine binding site shows high affinity to adenine nucleotides such as AMP or ADP in addition to NECA and adenosine. The order of the affinity for various adenosine receptor ligands of the purified adenosine binding site was defferent from any known purinergic receptors. Further studies including complete purificati … More on and cDNA cloning are now in progress.Gene expression of adenosine receptor was also investigated by studying the effect of glucocorticoid or adenosine receptor agonist treatment of cultured DDT_1MF-2 or PC12 cells on mRNA expression. Dexamethasone induced (〕SY.apprxeq.〔) 50% increase in the steady-state level of A1 adenosine receptor mRNA accompanied with a significant activation of A1 adenosine receptor binding activity. A2 adenosine receptor expression was also modulated by A2 adenosine receptor agonists. NECA or CGS-21680 caused a transient increase followed by a decrease of A2 adenosine receptor mRNA in PC12 cells. Forskolin also induced similar changes in the level of A2 mRNA.These changes were blocked by an adenosine receptor antagonist. These results suggest that A2 adenosine receptor gene expression in PC12 cells is regulated by an activation of A2 adenosine receptor via a mechanism involving the second messenger, cAMP.Further analysis of the cAMP or other response elements in the receptor gene is now under investigation. Less