Previously postulated "ligand-independent" signaling of GPR4 is mediated through proton-sensing mechanisms.
GPR4 was initially identified as a receptor for sphingosylphosphorylcholine and lysophosphatidylcholine; however, lipid actions have not always been confirmed. Instead, ligand-independent actions have sometimes been observed in GPR4- and other OGR1 family receptor-expressing cells. Here, we examined the possible involvement of extracellular protons, which have recently been proposed as another ligand for GPR4. At pH 7.4, the epidermal growth factor-induced extracellular signal-regulated kinase activity was lower in GPR4-transfected RH7777 cells, in association with increased cAMP accumulation, than in vector-transfected cells. The serum response element (SRE)-driven transcriptional activity was also clearly higher in GPR4-expressing HEK293 cells than in vector-transfected cells at pH 7.4. These apparent ligand-independent actions were very small at alkalinic 7.8. The SRE activity was further increased by extracellular acidification in a manner dependent on the G13 protein/Rho signaling pathway in HEK293 cells expressing GPR4 or other OGR1 receptor family members. GPR4-expressing cells also showed a calcineurin-dependent nuclear factor of activated T cell (NFAT) promoter activation at pH 7.4, and this activity was further increased by pH below 7.2 in association with inositol phosphate production. In contrast to the cAMP and SRE responses, however, alkalinization to pH 7.8 hardly affected the high basal activity. Finally, the expression of GPR4 hardly modulated the sphingosylphosphorylcholine- or lysophosphatidylcholine-induced action. These results suggest that an extracellular proton play a role as a ligand in some of previously postulated ligand-independent actions through GPR4 receptors. Moreover, GPR4 may be a multi-functional receptor coupling to Gs, G13, and Gq/11 proteins in response to extracellular acidification.
Tobo
Masayuki
M
Laboratory of Signal Transduction, Institute for Molecular and Cellular Regulation, Gunma University, 3-39-15 Showa-machi, Maebashi 371-8512, Japan.
Tomura
Hideaki
H
Mogi
Chihiro
C
Wang
Ju-Qiang
JQ
Liu
Jin-Peng
JP
Komachi
Mayumi
M
Damirin
Alatangaole
A
Kimura
Takao
T
Murata
Naoya
N
Kurose
Hitoshi
H
Sato
Koichi
K
Okajima
Fumikazu
F
D002460:Cell Line; D045744:Cell Line, Tumor; D000242:Cyclic AMP / Q000096:biosynthesis; D066246:ErbB Receptors / Q000235:genetics / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008244:Lysophosphatidylcholines / Q000378:metabolism; D050778:NFATC Transcription Factors / Q000235:genetics / Q000378:metabolism; D010767:Phosphorylcholine / Q000031:analogs & derivatives / Q000378:metabolism; D011522:Protons; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*; D027602:Serum Response Element / Q000235:genetics; D015398:Signal Transduction; D013110:Sphingosine / Q000031:analogs & derivatives / Q000378:metabolism
C091670:GPR4 protein, human; D008024:Ligands; D008244:Lysophosphatidylcholines; D050778:NFATC Transcription Factors; D011522:Protons; D043562:Receptors, G-Protein-Coupled; C005356:sphingosine phosphorylcholine; D010767:Phosphorylcholine; D000242:Cyclic AMP; C512478:EGFR protein, human; D066246:ErbB Receptors; D013110:Sphingosine