, Volume 20, Issue 3, pp 305-317

Functional Coupling of G Proteins to Endothelin Receptors Is Ligand and Receptor Subtype Specific

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Abstract

1. The aims of the present study were (a) to determine the identity of the G proteins with which the endothelin receptor interacts and whether this interaction is subtype specific and (b) to determine whether agonist exposure can result in specific coupling between the endothelin receptor and G proteins.

2. Coupling between endothelin A (ETA) or endothelin B (ETB) receptors and G proteins was assessed in two fibroblast cell lines, each expressing one receptor subtype. Four ligands, ET-1, ET-3, SRTXb, and SRTXc, were used for receptor stimulation. The G protein α-subunit coupled to the receptor was identified by immunoprecipitation with an antibody against the endothelin receptor and immunoblotting with specific antibodies against different G protein α-subunits.

3. Unstimulated ETA and ETB receptors (ETAR and ETBR, respectively) were barely coupled to Goα. The unstimulated ETAR coimmunoprecipitated with Gi3α, whereas the unstimulated ETBR was much less strongly coupled to Gi3α. The coupling of ETBR to Gi1Gi2 α-subunits was much stronger than the coupling of ETAR to these α-subunits. Stimulation with the different ET agonists also resulted in differential coupling of G proteins to the receptor subtypes. All four ligands caused a strong increase in coupling of the ETBR to Gi3α, whereas coupling of the ETAR to this subunit was not affected by ET-1 and was even decreased by SRTXc. On the other hand, all four ligands caused a much greater increase in the coupling of ETAR to Gqα/G11α than in the coupling of ETBR to these α-subunits. Ligand-induced coupling between the receptors and the Gi1 and Gi2 α-subunits is similar for the two receptor subtypes. The same was true for ligand-induced coupling of the receptors to Goα, except that ET-3 increased the coupling of this α-subunit to ETBR and decreased the coupling to ETAR. Taken together, the results of this study show that coupling between ET receptors and G proteins is ligand and receptor subtype specific.

4. It remains to be established whether this diversity of receptor–G protein coupling is of relevance for the various endothelin signaling pathways and/or pathological states.