Received for publication June 1, 2005.
Revised September 25, 2005.
Accepted for publication September 27, 2005.

Activation of adenylyl cyclase by endogenous G_s-coupled receptors in HEK293 cells is attenuated by 5-HT₇ receptor expression

Abstract

Human 5-HT₇ receptors display characteristics shared with receptors thought to form a tight physical coupling with G protein in the absence of ligand. Some receptors apparently preassociated with G_i/o and G_q/11 are reported to inhibit signaling of other similarly coupled G protein-coupled receptors by limiting access to activate a common G protein pool. Therefore, we determined whether 5-HT₇ receptor expression was sufficient to limit signaling of endogenously expressed G_s-coupled receptors in HEK293 cells. Using the ecdysone-inducible expression system, which allows for titration of increasing receptor density in the same clonal cell line, we compared the effects of 5-HT_4(b) and 5-HT_7(a,b,d) receptor expression on adenylyl cyclase (AC) stimulation by the endogenous G_s-coupled -adrenergic (AR) and prostanoid EP receptors. AR- and EP-stimulated AC activity was attenuated by 5-HT₇ receptor expression in both membrane preparations and intact HEK293 cells. AR- and EP-stimulated AC activity was unaffected by expression of the G_s-coupled 5-HT₄ receptor. The mechanism of this heterologous desensitization seems independent of PKA activation; nor does it occur at the level of G protein activation since 1) AR- and EP-stimulated AC activity was not restored to control values when G_s was overexpressed and 2) ₁- and ₂AR activation of G_s was unaffected by expression of 5-HT₇ receptors. In addition, overexpression of AC isoforms was unable to rescue AR- and EP-stimulated AC activity. Therefore, 5-HT₇ receptors likely limit access and/or impede activation of AC by AR and EP receptors. Although the 5-HT₇ receptor may preassociate with G-protein and/or AC, the mechanism of this heterologous desensitization remains elusive.

Key words: Adrenergic, Prostanoid, Serotonin, Gs family, Adenylyl cyclases, Desensitization/uncoupling