Organic phosphate but not inorganic phosphate regulates Fgf23 expression through MAPK and TGF-? signaling

One of the main regulators of phosphate homeostasis is fibroblast growth factor 23 (FGF23), secreted by osteocytes. The effects of organic versus inorganic dietary phosphate on this homeostasis is unclear. This study used MC3T3-E1 osteocyte-like cells to examine the transcriptomic responses to these phosphates. Most importantly, the expression and secretion of FGF23 was only increased in response to organic phosphate. Gene ontology terms related to a response to environmental change were only enriched in osteocytes treated with organic phosphate while osteocytes treated with inorganic phosphate were enriched for terms associated with regulation of cellular phosphate metabolism. Inhibition of MAPK signaling diminished the response of Fgf23 to organic phosphate, suggesting it activates FGF23. TGF-ß signaling inhibition increased Fgf23 expression after the addition of organic phosphate, while the negative TGF-ß regulator Skil decreased this response. In summary, the observed differential response of osteocytes to phosphate types may have consequences for phosphate homeostasis. Overall design: RNA sequencing was performed on MC3T3-E1 (day 28) after 16 hour treament with 4 mM BGP or 4 mM NaHPO4