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Stereochemistry of an agonist determines coupling preference of beta2-adrenoceptor to different G proteins in cardiomyocytes.

ABSTRACT: A fundamental question regarding receptor-G protein interaction is whether different agonists can lead a receptor to different intracellular signaling pathways. Our previous studies have demonstrated that although most beta(2)-adrenoceptor agonists activate both G(s) and G(i) proteins, fenoterol, a full agonist of beta(2)-adrenoceptor, selectively activates G(s) protein. Fenoterol contains two chiral centers and may exist as four stereoisomers. We have synthesized a series of stereoisomers of fenoterol and its derivatives and characterized their receptor binding and pharmacological properties. We tested the hypothesis that the stereochemistry of an agonist determines selectivity of receptor coupling to different G protein(s). We found that the R,R isomers of fenoterol and methoxyfenoterol exhibited more potent effects to increase cardiomyocyte contraction than their S,R isomers. It is noteworthy that although (R,R)-fenoterol and (R,R)-methoxyfenoterol preferentially activate G(s) signaling, their S,R isomers were able to activate both G(s) and G(i) proteins as evidenced by the robust pertussis toxin sensitivities of their effects on cardiomyocyte contraction and on phosphorylation of extracellular signal-regulated kinase 1/2. The differential G protein selectivities of the fenoterol stereoisomers were further confirmed by photoaffinity labeling studies on G(s),G(i2), and G(i3) proteins. The inefficient G(i) signaling with the R,R isomers is not caused by the inability of the R,R isomers to trigger the protein kinase A (PKA)-mediated phosphorylation of the beta(2)-adrenoceptor, because the R,R isomers also markedly increased phosphorylation of the receptor at serine 262 by PKA. We conclude that in addition to receptor subtype and phosphorylation status, the stereochemistry of a given agonist plays an important role in determining receptor-G protein selectivity and downstream signaling events.

SUBMITTER: Woo AY

PROVIDER: S-EPMC2654765 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:1 The principal aim of the present study was to determine whether long-term treatment of human lung mast cells (HLMC) with the clinically-relevant beta(2)-adrenoceptor agonists, salbutamol and terbutaline, leads to desensitization of beta(2)-adrenoceptor-mediated responses in these cells. 2 The non-selective beta-adrenoceptor agonist, isoprenaline, and the selective beta(2)-adrenoceptor agonists, salbutamol and terbutaline, inhibited the IgE-mediated release of histamine from HLMC. Salbutamol (pD(2); 7.7+/-0.3) and terbutaline (pD(2); 7.3+/-0.2) were roughly equipotent as inhibitors of histamine release although both agonists were less potent than isoprenaline (pD(2); 8.6+/-0.2). 3 Isoprenaline (10(-5) M), salbutamol (10(-5) M) and terbutaline (10(-5) M) enhanced total cell cAMP levels in HLMC over basal by 361+/-90, 150+/-38 and 165+/-35%, respectively. 4 Long-term exposure (24 h) of HLMC to either salbutamol (10(-7) M) or terbutaline (10(-7) M) led to a subsequent reduction in the effectiveness of salbutamol and terbutaline (both 10(-9)-10(-4) M) to inhibit histamine release. However, salbutamol was significantly (P<0.05) more effective than terbutaline at promoting the functional desensitization. 5 Radioligand binding studies, using iodinated cyanopindolol, were performed to determine beta(2)-adrenoceptor density in cell membranes after pretreatment (24 h) of cells with either salbutamol (10(-6) M) or terbutaline (10(-6) M). Both agonists reduced beta(2)-adrenoceptor density in membranes to about the same extent (approximately 25% reduction) but these changes in receptor density were not statistically significant (P>0.05). 6 These data indicate that long-term exposure of mast cells to salbutamol causes greater levels of desensitization to beta(2)-adrenoceptor-mediated responses in HLMC than terbutaline. These findings may have wider clinical significance in the context of asthma treatment as compromised mast cell inhibition could result following long-term exposure of mast cells to short-acting bronchodilators.

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Stereochemistry of an agonist determines coupling preference of beta2-adrenoceptor to different G proteins in cardiomyocytes.

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