The effects of agonist stimulation and β 2-adrenergic receptor level on cellular distribution of Gs α protein

Abstract

This study examines the effects of adrenergic ligands, cholera toxin, forskolin, and varying levels of β 2 adrenergic receptors (β 2AR) on the cellular distribution of Gs α subunits in CHO cells. Localization of Gs α was evaluated by confocal microscopy and β 2AR-mediated signalling was assessed by adenylyl cyclase (AC) activity. In cells expressing 0.2 pmol/mg protein β 2ARs (WT18), the localization of Gs α subunit was restricted to the plasma membrane region. Isoproterenol (ISO), cholera toxin or forskolin elicited redistribution of cellular Gs α so that Gs α appeared as intense spots throughout the plasma membrane as well as the cytoplasm. Exposure to a neutral β 2AR antagonist, alprenolol, prevented the ISO-stimulated Gs α translocation from peripheral to inner cytoplasm. In cells expressing high level of β 2ARs (8.2 pmol/mg) (WT4), basal and ISO-stimulated AC activities were significantly elevated when compared to the values detected in WT18 clone, suggesting a positive correlation between receptor expression and receptor-mediated signalling. Basal Gs α distribution in this group was similar to that observed in ISO-, cholera toxin-, or forskolin-stimulated WT18 clone. ISO, cholera toxin, or forskolin did not change the distribution of Gs α significantly when tested in WT4 clone. No difference in the cellular level of Gs α protein between WT18 and WT4 clones was detected. Alprenolol did not affect the distribution of Gs α in WT4 clone. ICI 118,551, a negative β 2AR antagonist, altered Gs α distribution from a dispersed basal pattern to a membrane-confined pattern. The latter appearance was similar to that observed in unstimulated WT18 clone. Taken together, these data suggest that: (1) enhanced β 2AR–Gs α coupling induced by agonist stimulation or by increased expression of β 2ARs remodel the cellular distribution of Gs α; (2) the alteration in Gs α distribution induced by β 2AR overexpression provides evidence for agonist-independent interaction of β 2AR and Gs α, that can be inhibited by a negative antagonist but not by a neutral antagonist; and (3) forskolin influences the activity state of Gs α that displays a Gs α distribution pattern comparable to that observed when Gs α is activated via β 2AR stimulation or directly by cholera toxin.