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Interactions of purified bovine brain A1-adenosine receptors with G-proteins. Reciprocal modulation of agonist and antagonist binding.

ABSTRACT: The bovine brain A1-adenosine receptor was purified 8000-fold by affinity chromatography on xanthine-amine-congener (XAC)-Sepharose. Addition of a 120-fold molar excess of a purified bovine brain G-protein preparation (Go,i a mixture of Go and Gi, containing predominantly Go) decreases the Bmax of the binding of the antagonist radioligand [3H]XAC to the receptor. This decrease is observed not only after insertion into phospholipid vesicles but also in detergent solution, and is reversed by GTP analogues. In the presence of Go,i, about 20 and 40% of the receptors display guanine-nucleotide-sensitive high-affinity binding of the agonist radioligand (-)-N6-3-([125I]iodo-4-hydroxyphenylisopropyl)adenosine after reconstitution into lipid vesicles and in detergent solution, respectively. The ability of Go,i to enhance agonist binding and decrease antagonist binding is concentration-dependent, with a half-maximal effect occurring at approximately 10-fold molar excess of G-proteins over A1-adenosine receptors. In the presence of the receptor, the rate of guanosine 5'-[gamma-[35S]thio]triphosphate (GTP[35S]) binding to Go,i is accelerated. This rate is further enhanced if the receptor is activated by the agonist (-)(R)-N6-phenylisopropyladenosine, whereas the antagonist XAC decreases the association rate of GTP[35S] to levels observed in the absence of receptor. These results show (1) that detergent removal is not a prerequisite for the observation of coupling between the A1-adenosine receptor and Go,i, and (2) that the regulatory effect of G-proteins on antagonist binding to the A1-adenosine receptor can be reconstituted by using purified components.

SUBMITTER: Freissmuth M

PROVIDER: S-EPMC1150104 | biostudies-other | 1991 May

REPOSITORIES: biostudies-other

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Project description:Background and purposeAdenosine may be generated by hydrolysis of extracellular nucleotides by ectonucleotidases, including ectonucleoside triphosphate diphosphohydrolase 1 (CD39), ecto-5'-nucleotidase (CD73), nucleotide pyrophosphatase phosphodiesterase 1 (NPP-1) and tissue non-specific alkaline phosphatase (TNAP). Previous work from our laboratory has uncovered a critical role for adenosine A1 receptors (A1 R) in osteoclastogenesis; blockade or deletion of these receptors diminishes osteoclast differentiation. Interestingly, selective A1 R agonists neither affect basal osteoclastogenesis nor do they reverse A1 R antagonist-mediated inhibition of osteoclastogenesis. In this study, we determined whether ectonucleotidase-mediated adenosine production was required for A1 R antagonist-mediated inhibition, and, when we saw no effect, determined whether A1 R was constitutively activated and the antagonist was acting as an inverse agonist to diminish osteoclast differentiation.Experimental approachOsteoclast formation derived from wild-type, CD39 knockout (KO), CD73 KO, NPP-1 KO and TNAP KO mice was examined by tartrate-resistant acid phosphatase staining of receptor activator of NF-κB ligand-macrophage colony-stimulating factor-stimulated osteoclasts and osteoclast gene expression (Ctsk, Acp5, MMP-9 and NFATc1). Intracellular cAMP concentration was determined by elisa.Key resultsRolofylline inhibited osteoclast formation in a dose-dependent manner (IC50 = 20-70 nM) in mice lacking all four of these phosphatases, although baseline osteoclast formation was significantly less in precursors from CD73 KO mice. Rolofylline (1 μM) stimulates cAMP production in bone marrow macrophages by 10.23 ± 0.89-fold.Conclusions and implicationsBased on these findings, we hypothesize that the A1 R is constitutively activated in osteoclast precursors, thereby diminishing basal AC activity, and that A1 R antagonists act as inverse agonists to release A1 R-mediated inhibition of basal AC activity and permit osteoclast differentiation. The constitutive activity of A1 R promotes osteoclast formation and down-regulation of this activity blocks osteoclast formation.

Project description:The diuretic drug amiloride and its analogues were found previously to be allosteric modulators of antagonist binding to A(2A) adenosine receptors. In this study, the possibility of the allosteric modulation by amiloride analogues of antagonist binding at A(1) and A(3) receptors, as well as agonist binding at A(1), A(2A), and A(3) receptors, was explored. Amiloride analogues increased the dissociation rates of two antagonist radioligands, [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) and [3H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one ([3H]PSB-11), from A(1) and A(3) receptors, respectively. Amiloride and 5-(N,N-dimethyl)amiloride (DMA) were more potent at A(1) receptors than at A(3) receptors, while 5-(N,N-hexamethylene)amiloride (HMA) was more potent at A(3) receptors. Thus, amiloride analogues are allosteric inhibitors of antagonist binding at A(1), A(2A), and A(3) adenosine receptor subtypes. In contrast to their effects on antagonist-occupied receptors, amiloride analogues did not affect the dissociation rates of the A(1) agonist [3H]N(6)-[(R)-phenylisopropyl]adenosine ([3H]R-PIA) from A(1) receptors or the A(2A) agonist [3H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine ([3H]CGS21680) from A(2A) receptors. The dissociation rate of the A(3) agonist radioligand [125I]N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]I-AB-MECA) from A(3) receptors was decreased significantly by amiloride analogues. The binding modes of amiloride analogues at agonist-occupied and antagonist-occupied receptors differed markedly, which was demonstrated in all three subtypes of adenosine receptors tested in this study. The effects of the amiloride analogues on the action of the A(3) receptor agonist were explored further using a cyclic AMP functional assay in intact CHO cells expressing the human A(3) receptor. Both binding and functional assays support the allosteric interactions of amiloride analogues with A(3) receptors.

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Interactions of purified bovine brain A1-adenosine receptors with G-proteins. Reciprocal modulation of agonist and antagonist binding.

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