1.Functionally biased modulation of A(3) adenosine receptor agonist efficacy and potency by imidazoquinolinamine allosteric enhancers.
Gao ZG;Verzijl D;Zweemer A;Ye K;Göblyös A;Ijzerman AP;Jacobson KA Biochem Pharmacol. 2011 Sep 15;82(6):658-68. doi: 10.1016/j.bcp.2011.06.017. Epub 2011 Jun 21.
Allosteric modulators for the G(i)-coupled A(3) adenosine receptor (AR) are of considerable interest as therapeutic agents and as pharmacological tools to probe various signaling pathways. In this study, we initially characterized the effects of several imidazoquinolinamine allosteric modulators (LUF5999, LUF6000 and LUF6001) on the human A(3) AR stably expressed in CHO cells using a cyclic AMP functional assay. These modulators were found to affect efficacy and potency of the agonist Cl-IB-MECA differently. LUF5999 (2-cyclobutyl derivative) enhanced efficacy but decreased potency. LUF6000 (2-cyclohexyl derivative) enhanced efficacy without affecting potency. LUF6001 (2-H derivative) decreased both efficacy and potency. We further compared the agonist enhancing effects of LUF6000 in several other A(3) AR-mediated events. It was shown that although LUF6000 behaved somewhat differently in various signaling pathways, it was more effective in enhancing the effects of low-efficacy than of high-efficacy agonists. In an assay of cyclic AMP accumulation, LUF6000 enhanced the efficacy of all agonists examined, but in the membrane hyperpolarization assay, it only enhanced the efficacy of partial agonists.
2.Adenosine triggers preconditioning through MEK/ERK1/2 signalling pathway during hypoxia/reoxygenation in neonatal rat cardiomyocytes.
Germack R;Dickenson JM J Mol Cell Cardiol. 2005 Sep;39(3):429-42.
Three subtypes of adenosine receptors (A(1), A(2A) and A(3) ARs) are functionally expressed in cardiomyocytes. Adenosine released during ischemia and ischemia/reperfusion plays a major role in cardioprotection. Phosphatidylinositol 3-kinase (PI-3K)/protein kinase B (PKB) and MEK/ERK1/2 pathways are involved in cell survival. Since the role of these pathways in AR-mediated preconditioning is poorly understood, we have investigated whether PI-3K/PKB and/or MEK1/ERK1/2 pathways are involved in AR-induced cardioprotection in neonatal rat cardiomyocytes. Cells were pre-treated (15 min) with adenosine (non-selective), CPA (A(1)), CGS 21680 (A(2A)) or Cl-IB-MECA (A(3)) before 4 h hypoxia (0.5% O(2)) and 18 h reoxygenation (HX4/R). HX4/R-induced increase in LDH release was significantly reduced by adenosine (70%), CPA (59%) and Cl-IB-MECA (46%). The MEK1 inhibitor PD 98059 suppressed the effects of adenosine, CPA, and Cl-IB-MECA on LDH release, whereas the PI-3K inhibitor wortmannin did not reverse this cardioprotection. Western blotting of phosphorylated ERK1/2 and PKB during HX4/R supported the involvement of ERK1/2 and not PKB in A(1) and A(3) agonist-mediated cardioprotection. In addition, adenosine, CPA and Cl-IB-MECA inhibited HX4/R-induced caspase 3 activity by 75%, 70% and 59%, respectively, and this inhibition was abolished by PD 98059.
3.Characterization of ERK1/2 signalling pathways induced by adenosine receptor subtypes in newborn rat cardiomyocytes.
Germack R;Dickenson JM Br J Pharmacol. 2004 Jan;141(2):329-39.
1. Adenosine A(1), A(2A), and A(3) receptors (ARs) and extracellular signal-regulated kinase 1/2 (ERK1/2) play a major role in myocardium protection from ischaemic injury. In this study, we have characterized the adenosine receptor subtypes involved in ERK1/2 activation in newborn rat cardiomyocytes. 2. Adenosine (nonselective agonist), CPA (A(1)), CGS 21680 (A(2A)) or Cl-IB-MECA (A(3)), all increased ERK1/2 phosphorylation in a time- and dose-dependent manner. The combined maximal response of the selective agonists was similar to adenosine alone. Theophylline (nonselective antagonist) inhibited completely adenosine-mediated ERK1/2 activation, whereas a partial inhibition was obtained with DPCPX (A(1)), ZM 241385 (A(2A)), and MRS 1220 (A(3)). 3. PD 98059 (MEK1; ERK kinase inhibitor) abolished all agonist-mediated ERK1/2 phosphorylation. Pertussis toxin (PTX, G(i/o) blocker) inhibited completely CPA- and partially adenosine- and Cl-IB-MECA-induced ERK1/2 activation. Genistein (tyrosine kinase inhibitor) and Ro 318220 (protein kinase C, PKC inhibitor) partially reduced adenosine, CPA and Cl-IB-MECA responses, without any effect on CGS 21680-induced ERK1/2 phosphorylation. H89 (protein kinase A, PKA inhibitor) abolished completely CGS 21680 and partially adenosine and Cl-IB-MECA responses, without any effect on CPA response.