Anabolic effects of a G protein–coupled receptor kinase inhibitor expressed in osteoblasts
Robert F. Spurney, … , Farshid Guilak, L. Darryl Quarles
Robert F. Spurney, … , Farshid Guilak, L. Darryl Quarles
Published May 15, 2002
Citation Information: J Clin Invest. 2002;109(10):1361-1371. https://doi.org/10.1172/JCI14663.
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Article Endocrinology

Anabolic effects of a G protein–coupled receptor kinase inhibitor expressed in osteoblasts

Abstract

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.

Authors

Robert F. Spurney, Patrick J. Flannery, Sanford C. Garner, Krairerk Athirakul, Shiguang Liu, Farshid Guilak, L. Darryl Quarles

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Effect of the transgene on urinary excretion of DPD and expression OPG a...
Effect of the transgene on urinary excretion of DPD and expression OPG and OPGL mRNA in mouse calvaria. (a) Excretion of DPD in urine was measured as an index of osteoclast-mediated bone resorption in vivo (40). Excretion of urinary DPD was significantly enhanced in transgenic (TG) mice compared with nontransgenic (Non-TG) littermate controls. (b–d) Total cellular RNA was prepared from mouse calvaria prior to performing semiquantitative RT-PCR as described in Methods. Molecular size is indicated in base pairs. (b) The OPG PCR product was decreased in transgenic mice compared with nontransgenic littermate controls. (c) The OPGL PCR product was increased in transgenic animals compared with control animals. The GAPDH control PCR reaction shown in c confirmed that the RT reaction was successful in the animals studied. *P < 0.025 vs. nontransgenic littermate controls.