PT  - JOURNAL ARTICLE
AU  - Chan, Joy S. C.
AU  - Lee, Jonathan W. M.
AU  - Ho, Maurice K. C.
AU  - Wong, Yung H.
TI  - Preactivation Permits Subsequent Stimulation of Phospholipase C by G<sub>i</sub>-Coupled Receptors
AID  - 10.1124/mol.57.4.700
DP  - 2000 Apr 01
TA  - Molecular Pharmacology
PG  - 700--708
VI  - 57
IP  - 4
4099  - http://molpharm.aspetjournals.org/content/57/4/700.short
4100  - http://molpharm.aspetjournals.org/content/57/4/700.full
SO  - Mol Pharmacol2000 Apr 01; 57
AB  - In the complex signal transduction networks involving G protein-coupled receptors there are numerous examples where Gi -linked receptors augment Gq-dependent signals. The mechanistic basis of such occurrences is thought to entail signal convergence at phospholipase Cβ (PLCβ) via the G protein βγ-dimers. Herein, we explored the possibility that augmentation by βγ-dimers requires preactivation of PLCβ. COS-7 cells were transiently cotransfected with cDNAs encoding various combinations of receptors and G protein subunits. The Gi-coupled δ- and κ-opioid receptors could not stimulate PLCβ unless they were coexpressed with Gα16. The opioid-induced response was dose-dependent and partially inhibited by pertussis toxin or coexpression with transducin, indicating the involvement of βγ-subunits released from the Giproteins. When PLCβ was preactivated by constitutively active mutants of Gα16, Gαq, or Gα14, opioids enhanced the activity by 80 to 300% and such responses were mostly pertussis toxin-sensitive. The opioid-induced enhancement was dose-dependent and could not be blocked by staurosporin, a protein kinase C inhibitor. Other Gi-coupled receptors that were ineffective on their own also acquired the ability to stimulate PLCβ in the presence of a constitutively active mutant of Gαq. Coactivation of endogenous or exogenous Gq-coupled receptors with the δ-opioid receptor produced strong stimulations of PLCβ and such responses could be partially blocked by pertussis toxin. These results show that enhancement of Gq-dependent signals by Gi-coupled receptors requires activated PLCβ and is mediated via the βγ-dimer.