title: Involvement of the G-protein-coupled receptor 4 in RANKL expression by osteoblasts in an acidic environment.
abstract: Osteoclast activity is enhanced in acidic environments following systemic or local inflammation. However, the regulatory mechanism of receptor activator of NF-κB ligand (RANKL) expression in osteoblasts under acidic conditions is not fully understood. In the present paper, we detected the mRNA expression of the G-protein-coupled receptor (GPR) proton sensors GPR4 and GPR65 (T-cell death-associated gene 8, TDAG8), in osteoblasts. RANKL expression and the cyclic AMP (cAMP) level in osteoblasts were up-regulated under acidic culture conditions. Acidosis-induced up-regulation of RANKL was abolished by the protein kinase A inhibitor H89. To clarify the role of GPR4 in RANKL expression, GPR4 gain and loss of function experiments were performed. Gene knockdown and forced expression of GPR4 caused reduction and induction of RANKL expression, respectively. These results suggested that, at least in part, RANKL expression by osteoblasts in an acidic environment was mediated by cAMP/PKA signaling resulting from GPR4 activation. A comprehensive microarray analysis of gene expression of osteoblasts revealed that, under acidic conditions, the phenotype of osteoblasts was that of an osteoclast supporting cell rather than that of a mineralizing cell. These findings will contribute to a molecular understanding of bone disruption in an acidic environment.
authors: Department of Cellular Physiological Chemistry, Tokyo Medical and Dental University, Tokyo, Japan; Department of Orthodontic Science, Tokyo Medical and Dental University, Tokyo, Japan.
authors: Asuka
authors: A
authors: Okito
authors: Department of Cellular Physiological Chemistry, Tokyo Medical and Dental University, Tokyo, Japan. Electronic address: nakacell@tmd.ac.jp.
authors: Ken-Ichi
authors: K
authors: Nakahama
authors: Department of Cellular Physiological Chemistry, Tokyo Medical and Dental University, Tokyo, Japan.
authors: Masako
authors: M
authors: Akiyama
authors: Department of Orthodontic Science, Tokyo Medical and Dental University, Tokyo, Japan.
authors: Takashi
authors: T
authors: Ono
authors: Department of Cellular Physiological Chemistry, Tokyo Medical and Dental University, Tokyo, Japan.
authors: Ikuo
authors: I
authors: Morita
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D006863:Hydrogen-Ion Concentration; D051379:Mice; D010006:Osteoblasts / Q000737:chemistry* / Q000166:cytology / Q000378:metabolism*; D053245:RANK Ligand / Q000378:metabolism*; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*
chemical_list: C517811:GPR4 protein, mouse; D053245:RANK Ligand; D043562:Receptors, G-Protein-Coupled; C505689:Tnfsf11 protein, mouse
keywords: Acidosis; Cyclic AMP; GPR4; RANKL