Abstract
In this study we investigated cross talk between m3-muscarinic and β2-adrenergic receptors coexpressed in Chinese hamster ovary (CHO-m3/β2) cells, focusing on two possible mechanisms of regulation. The first mechanism is based on recent in vitro studies demonstrating that G protein-coupled receptor kinase (GRK) activity, the protein kinase responsible for β2-adrenergic receptor homologous phosphorylation and desensitization, may be regulated by calcium/calmodulin and membrane phosphatidylinositol 4,5-bisphosphate. Stimulation of the phospholipase C signaling pathway via m3-muscarinic receptors in CHO-m3/β2 cells increased intracellular free calcium by ∼10 fold and membrane phosphatidylinositol 4,5-bisphosphate levels decreased by ∼74%. However, despite these changes the ability of endogenous kinases, possibly the GRKs, to phosphorylate the β2-adrenergic receptor was not altered. The second mechanism investigated involves a direct heterologous phosphorylation of the β2-adrenergic receptor after muscarinic receptor stimulation. Activation of m3-muscarinic receptors did mediate heterologous phosphorylation of β2-adrenergic receptors in a GRK-independent fashion, via protein kinase C. Heterologous β2-adrenergic receptor phosphorylation correlated with receptor desensitization as measured by a loss in guanine-nucleotide sensitive-high affinity agonist binding and reduction in maximal cAMP response. This receptor cross talk may have a profound physiological importance in a wide variety of cell types, for example smooth muscle, where these two receptors are known to be coexpressed.
Footnotes
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Send reprint requests to: Dr. Andrew B. Tobin, Department of Cell Physiology and Pharmacology, University of Leicester, P.O. Box 138, Medical Sciences Building, University Road, Leicester, LE1 9HN, United Kingdom. E-mail: TBA{at}le.ac.uk
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This work was supported by Wellcome Trust Grants 047600/96 and 16895/96.
- Abbreviations:
- CHO
- Chinese hamster ovary
- G protein-coupled receptor kinase
- Ins(1,4,5)P3, inositol 1,4,5-trisphosphate
- PIP2
- phosphatidylinositol 4,5-bisphosphate
- PKA
- protein kinase A
- PKA amide inhibitor
- myristoylated protein kinase A inhibitor (14–22) amide (N-Myr-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile-NH2)
- PKC
- protein kinase C
- PLC
- phosphoinositide-specific phospholipase C
- Received February 15, 1999.
- Accepted July 8, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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