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Vol. 57, Issue 4, 700-708, April 2000
Department of Biology and the Biotechnology Research Institute,
Hong Kong University of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, China
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
C
(PLC) via the G protein 
-dimers. Herein, we explored the
possibility that augmentation by -dimers requires preactivation
of PLC. COS-7 cells were transiently cotransfected with cDNAs
encoding various combinations of receptors and G protein subunits. The
Gi-coupled 
- and 
-opioid receptors could not
stimulate PLC unless they were coexpressed with G
16.
The opioid-induced response was dose-dependent and partially inhibited
by pertussis toxin or coexpression with transducin, indicating the
involvement of -subunits released from the Gi
proteins. When PLC was preactivated by constitutively active mutants
of G16, Gq, or G14,
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 PLC
in the presence of a constitutively active mutant of Gq.
Coactivation of endogenous or exogenous Gq-coupled
receptors with the -opioid receptor produced strong stimulations of
PLC 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 PLC
and is mediated via the -dimer.
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