optimal phosphate balance is important for many physiological functions from cell signaling to energy metabolism to skeletal mineralization .
inadequate phosphate balance disrupts a multitude of physiological processes , and can cause or exacerbate age - associated disorders , cardiovascular calcification and bone mineralization defects .
physiologic phosphate balance is maintained by interactions among the intestine , kidney , parathyroid gland and bone .
phosphate absorption takes place mostly in the small intestine , and the sodium - dependent phosphate ( napi ) co - transporter , napi-2b , facilitates such absorption . of relevance ,
the intestinal napi-2b activity is influenced by 1,25-dihydroxyvitamin d , which can increase the expression of intestinal napi-2b protein to augment intestinal phosphate uptake by the enterocytes .
likewise , renal phosphate reabsorption is mostly accomplished by the napi-2a and napi-2c co - transporters .
parathyroid hormone ( pth ) increases urinary phosphate excretion by reducing napi - dependent phosphate uptake in the proximal tubular epithelial cells .
recent studies have shown that bone - derived fibroblast growth factor 23 ( fgf23 ) and kidney - derived klotho can also directly suppress sodium phosphate co - transporter activities .
in addition to intestine and kidney , bone also plays a major role in maintaining phosphate balance . when serum phosphate levels are low , the bone releases additional phosphate to maintain the homeostatic balance by increasing resorption , a process that is mostly influenced by the activity of pth and 1,25-dihydroxyvitamin d. pth alters receptor activator of nuclear factor kappa - b ligand - osteoprotegerin ( rankl / opg ) balance , by acting on the stromal cells to stimulate rankl expression , and also by reducing the expression of opg and vitamin d. on the other hand , pth stimulates osteoclast differentiation and activity , resulting in increased bone resorption .
furthermore , pth and 1,25-dihydroxyvitamin d can induce skeletal fgf23 production to regulate serum phosphate levels and thereby can influence bone resorption .
fgf23 is a 30 kda protein that is proteolytically processed to smaller n - terminal ( 18 kda ) and c - terminal tail ( 12 kda ) fragments .
structural analysis of fgf23 protein found a fgf receptor ( fgfr)-binding domain at the n - terminal and a potential klotho - interacting site at the c - terminal tail .
recent studies have shown that the c - terminal tail determines the functionality of fgf23 protein .
for instance , when the c - terminal tail of the fgf2 protein was replaced with the c - terminal tail of fgf23 , the chimeric protein containing the n - terminal of fgf2 and c - terminal tail of fgf23 could act as a phosphatonin , and could also reduce renal 1(oh ) synthesis .
it is worth mentioning that the presence of the c - terminal tail of fgf23 in the chimeric protein paved the way for the klotho interaction .
recently , family with sequence similarity 20 , member c ( fam20c ) has shown to phosphorylate fgf23 on a ser - x - glu motif , and such phosphorylation promotes fgf23 proteolysis by furin through blocking o - glycosylation by polypeptide n - acetylgalactosaminyltransferase 3 ( galnac - t3 ) .
this observation suggests that interplay between phosphorylation and o - glycosylation of fgf23 may be a critical posttranslational mechanism by which the activity of secreted fgf23 protein is determined
. once secreted as a bioactive protein , fgf23 , in presence of klotho , can induce downstream signaling molecules , as demonstrated by the activation of early growth response element-1 ( egr-1 ) and the phosphorylation of fgf receptor substrate-2a , extracellular signal - regulated kinase ( erk ) , p38 , jun n - terminal kinase ( jnk ) , and akt .
of relevance , these signaling phosphoproteins were detected only when cells were challenged with both fgf23 and klotho , and not in cells treated with fgf23 without klotho . in accord with these in vitro observations , in vivo studies have shown that bioactive fgf23 protein could significantly reduce serum phosphate level in wild - type and fgf23 knockout mice , but failed to exert such phosphate lowering effects in fgf23/klotho double knockout mice , again suggesting that without klotho , fgf23 loses its phosphate regulating abilities .
moreover , the fgf23-induced hypophosphatemic phenotype of hyp mutant mice was reversed to hyperphosphatemia in the hyp / klotho double mutant mice , despite significantly higher serum fgf23 levels in double mutants . in a similar line of observation ,
an inactivating mutation in the human klotho gene resulted in severe hyperphosphatemia in a tumoral calcinosis patient , despite high serum fgf23 levels .
summarizing these above - mentioned observations , an indispensable role of klotho in fgf23-mediated urinary phosphate excretion is obvious .
one of the possible mechanisms of fgf23-induced urinary phosphate excretion is that it suppresses napi-2a and napi-2c co - transporters , either directly or through influencing pth activity .
pth , an 84 amino acid protein , is produced in response to low levels of serum calcium and secreted pth acts on the bone and kidney to increase serum calcium level .
low serum calcium levels reduce calcium - sensor receptor ( car ) signaling and allow active pth to be secreted , which then binds to the pth receptor 1 , a seven transmembrane g - protein coupled receptor , to activate the pka , pkc , and mapk pathways in kidney and bone .
in addition to serum calcium , vitamin d can also suppress pth expression and parathyroid hyperplasia .
it is believed that fgf23 and pth mutually regulate each other in a negative feedback loop , where pth stimulates fgf23 production and fgf23 in turn suppresses pth synthesis .
when pth was genetically ablated from fgf23 knockout mice , serum calcium levels were normalized in double mutant ( fgf23/pth ) mice , despite high serum 1,25-dihydroxyvitamin d and high serum phosphate levels , suggesting that some of the biochemical changes documented in fgf23 mice are mediated by pth .
it is a well - known fact that continuous administration of pth for a prolonged period can reduce bone mass .
however , when fgf23 mice were challenged with constant pth infusion through osmotic minipumps for 3 weeks , pth - induced bone loss was more severe , suggesting that fgf23 might exert a protective effect against the long - term catabolic effects of pth on bone . in a mouse model of hyperparathyroidism , fgf23 levels positively correlated with those of pth , but inversely with serum phosphate levels .
presence of both klotho and fgf receptors in the parathyroid glands raised the possibility that parathyroid gland might be a target organ for fgf23 activities . using bovine parathyroid cells ,
on the other hand , other studies have claimed that the activation of the pth receptor in bone via the pka signaling pathway suppresses the wnt inhibitor sclerostin , thereby allowing wnt signaling to increase fgf23 synthesis .
it is , however , worth mentioning that reducing wnt signaling in vivo , by genetically inactivating its co - receptor , low - density lipoprotein receptor - related protein 6 ( lrp6 ) , did not affect fgf23-induced hypophosphatemia in hyp mice , as shown in hyp / lrp6 double mutant mice .
furthermore , injection of bioactive fgf23 protein into lrp6 mutant mice reduced serum phosphate levels to a similar degree as fgf23 injection into wild - type mice , providing a genetic and pharmacological evidence for a wnt - independent function of fgf23 in the regulation of phosphate homeostasis .
( this issue ) claimed that pth , by activating nuclear orphan receptor ( nurr1 ) , can increase the transcription of fgf23 in bone cells .
structural analysis has found that nurr1 protein is lacking a ligand - binding cavity , and therefore may act as a ligand - independent transcription factor . in the fgf23 promoter region ,
the presence of nurr1 response elements raises the possibility of its role in fgf23 synthesis . in a cell - based system , through over - expression and knock down of nurr1 , an association between pth and fgf23 is suggested .
moreover , in a rat model of chronic kidney disease ( ckd ) , increased nurr1 mrna and protein levels were associated with increased fgf23 mrna expression , calcimimetic treatment of these ckd animals reduced pth and fgf23 levels , along with decreased calvarial nurr1 mrna and protein expression . despite the presence of nurr1 responsive elements in fgf23 promoter regions , the functionality of nurr1 responsive elements in fgf23 synthesis is not yet defined , and without mutagenesis studies , whether increased expression of nurr1 and fgf23 is a mutual regulation or merely an epiphenomenon , could not be established . moreover , to provide in vivo direct evidence , further studies will be needed to show that inactivating pth signaling , by targeting its receptors , can block pth induced nurr1 and fgf23 expression in long bones .
it is also worth mentioning that pth induced fgf23 synthesis is a cell - line specific phenomenon .
for instance , while pth can induce fgf23 in umr106 cell lines , no such response of pth on fgf23 is noted in ros16/2.8 cells . despite a better understanding of fgf23 biology in systemic regulation of phosphate turnover , factors inducing its skeletal expression are not yet fully documented .
1,25-dihydroxyvitamin d , phosphate , calcium , iron , leptin , acidosis , secreted klotho and pth are the factors currently known to induce fgf23 production ( fig .
1 ) . it is a well - accepted fact that pth can induce the synthesis of 1,25-dihydroxyvitamin d in the kidney , and that , in turn , 1,25-dihydroxyvitamin d can inhibit pth secretion by the parathyroid glands .
recent studies have claimed that 1,25-dihydroxyvitamin d can stimulate fgf23 production in bone and that fgf23 can suppress 1,25-dihydroxyvitamin d production and pth secretion .
the ability of pth to directly stimulate fgf23 expression forms an endocrine regulatory feedback loop to control mineral ion metabolism .
studies , as the one highlighted here , will help us to understand molecular regulation of fgf23 synthesis and identify its yet to be documented functions .
 bone - derived fibroblast growth factor-23 ( fgf23 ) plays an important role in systemic phosphate turnover . 
 increased fgf23 activity results in hypophosphatemic , while reduced activity is linked to hyperphosphatemic disorders . 
 fgf23 , together with klotho as co - factor , can activate fgf - receptors in its target tissues to exert its functions 
 . however , molecular regulation of fgf23 synthesis is not clearly defined , and recent studies have found that pth can activate the nuclear receptor - associated protein-1 ( nurr1 ) to induce fgf23 transcription in bone cells . 