Both FGF23 and extracellular phosphate activate Raf/MEK/ERK pathway via FGF receptors in HEK293 cells.
Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone produced by bone and exerts its function in the target organs by binding the FGF receptor (FGFR) and Klotho. Since recent studies suggested that extracellular inorganic phosphate (Pi) itself triggers signal transduction and regulates gene expression in some cell types, we tested the notion that extracellular Pi induces signal transduction in the target cells of FGF23 also and influences its signaling, utilizing a human embryonic kidney cell line HEK293. HEK293 cells expressed low levels of klotho, and treatment with a recombinant FGF23[R179Q], a proteolysis-resistant mutant of FGF23, resulted in phosphorylation of ERK1/2 and induction of early growth response-1 (EGR1) expression. Interestingly, increased extracellular Pi resulted in activation of the Raf/MEK/ERK pathway and expression of EGR1, which involved type III sodium/phosphate (Na(+)/Pi) cotransporter PiT-1. Since the effects of an inhibitor of Na(+)/Pi cotransporter on FGF23 signaling suggested that the signaling triggered by increased extracellular Pi shares the same downstream cascade as FGF23 signaling, we further investigated their convergence point. Increasing the extracellular Pi concentration resulted in the phosphorylation of FGF receptor substrate 2α (FRS2α), as did treatment with FGF23. Knockdown of FGFR1 expression diminished the phosphorylation of both FRS2α and ERK1/2 induced by the Pi. Moreover, overexpression of FGFR1 rescued the decrease in Pi-induced phosphorylation of ERK1/2 in the cells where the expression of PiT-1 was knocked down. These results suggest that increased extracellular Pi triggers signal transduction via PiT-1 and FGFR and influences FGF23 signaling in HEK293 cells.
Yamazaki
Miwa
M
Department of Bone and Mineral Research, Osaka Medical Center and Research Institute for Maternal and Child Health, Izumi, Osaka 594-1101, Japan.
Ozono
Keiichi
K
Okada
Tomoko
T
Tachikawa
Kanako
K
Kondou
Hiroki
H
Ohata
Yasuhisa
Y
Michigami
Toshimi
T
D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D010710:Phosphates / Q000494:pharmacology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D017468:Receptors, Fibroblast Growth Factor / Q000378:metabolism*; D015398:Signal Transduction; D050611:Sodium-Phosphate Cotransporter Proteins, Type III / Q000378:metabolism*; D048490:raf Kinases / Q000378:metabolism
C000717427:FGF23 protein, human; D010710:Phosphates; D017468:Receptors, Fibroblast Growth Factor; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23; D017346:Protein Serine-Threonine Kinases; D048490:raf Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D020929:Mitogen-Activated Protein Kinase Kinases