Differential Coupling of Human Endothelin Type A Receptor to Gq/11 and G12 Proteins: the Functional Significance of Receptor Expression Level in Generating Multiple Receptor Signaling

This study examines the influence of receptor expression level on signaling pathways activated via endothelin type A receptor (ETAR) expressed in Chinese hamster ovary cells at 32,100 (ETAR-high-CHO) and 893 (ETAR-low-CHO) fmol.mg protein(-1). Endothelin-1 (ET-1) elicited a sustained increase in intracellular Ca2+ concentration ([Ca2+](i)), which was dependent on G(q/11) protein, phospholipase C (PLC), Na+/H+ exchanger (NHE), and p38 mitogen-activated protein kinase (p38MAPK) in ETAR-high-CHO, whereas the sustained [Ca2+](i) increase was negligible in ETAR-low-CHO. Functional study with Cytosensor (TM) microphysiometer showed that ET-1 evoked an NHE1-mediated increase in extracellular acidification rate (ECAR) in ETAR-high-CHO and ETAR-low-CHO. In ETAR-high-CHO, the ECAR response at 30 min after ET-1 Stimulation was insensitive to G(q/11) and PLC inhibitors, but sensitive to the p38MAPK inhibitor. In ETAR-low-CHO, the ECAR response at 30 min was sensitive to these inhibitors. Western blot analysis demonstrated that ET-1-induced p38MAPK phosphorylation in ETAR-low-CHO but not in ETAR-high-CHO was mediated via G(q/11) and PLC. The G(q/11)/PLC-independent p38MAPK phosphorylation in ETAR-high-CHO was suppressed by expression of the C terminus of G(alpha 12) protein to disrupt receptor-G(12) protein coupling. These results provide evidence for multiple signaling pathways of ETAR that were activated via at least the G(q/11)/PLC/NHE, G(12)/p38MAPK/NHE, and G(q/11)/PLC/p38MAPK/NHE cascades in an expression level-dependent manner.