Anabolic effects of a G protein-coupled receptor kinase inhibitor expressed in osteoblasts.
G protein-coupled receptors (GPCRs) play a key role in regulating bone remodeling. Whether GPCRs exert anabolic or catabolic osseous effects may be determined by the rate of receptor desensitization in osteoblasts. Receptor desensitization is largely mediated by direct phosphorylation of GPCR proteins by a family of enzymes termed GPCR kinases (GRKs). We have selectively manipulated GRK activity in osteoblasts in vitro and in vivo by overexpressing a GRK inhibitor. We found that expression of a GRK inhibitor enhanced parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor-stimulated cAMP generation and inhibited agonist-induced phosphorylation of this receptor in cell culture systems, consistent with attenuation of receptor desensitization. To determine the effect of GRK inhibition on bone formation in vivo, we targeted the expression of a GRK inhibitor to mature osteoblasts using the mouse osteocalcin gene 2 (OG2) promoter. Transgenic mice demonstrated enhanced bone remodeling as well as enhanced urinary excretion of the osteoclastic activity marker dexoypyridinoline. Both osteoprotegrin and OPG ligand mRNA levels were altered in calvaria of transgenic mice in a pattern that would promote osteoclast activation. The predominant effect of the transgene, however, was anabolic, as evidenced by an increase in bone density and trabecular bone volume in the transgenic mice compared with nontransgenic littermate controls.
Spurney
Robert F
RF
Division of Nephrology, Department of Medicine, Duke University and Durham Veterans Administration Medical Centers, North Carolina 27710, USA. spurn002@mc.duke.edu
Flannery
Patrick J
PJ
Garner
Sanford C
SC
Athirakul
Krairerk
K
Liu
Shiguang
S
Guilak
Farshid
F
Quarles
L Darryl
LD
D000818:Animals; D016723:Bone Remodeling; D002460:Cell Line; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D054770:G-Protein-Coupled Receptor Kinase 3; D015971:Gene Expression Regulation, Enzymologic; D006801:Humans; D051379:Mice; D010006:Osteoblasts / Q000201:enzymology / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D017434:Protein Structure, Tertiary / Q000235:genetics / Q000502:physiology; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D019076:Transgenes; D051552:beta-Adrenergic Receptor Kinases
D017346:Protein Serine-Threonine Kinases; D017868:Cyclic AMP-Dependent Protein Kinases; D054770:G-Protein-Coupled Receptor Kinase 3; C517848:GRK3 protein, human; C517850:GRK3 protein, mouse; C517851:Grk3 protein, rat; D051552:beta-Adrenergic Receptor Kinases