Fibroblast Growth Factor-23 (FGF23) is a systemic phosphaturic hormone that inhibits osteoblastic mineralization, blocks renal 25-OH-vitamin D3 bioactivation to 1,25(OH)2D3 (D3) and elicits hypophosphatemia by repressing the renal type IIa sodium-phosphate cotransporter. It has been shown that FGF23 causes a reduction in serum phosphate and D3 concentrations that appear to be caused by a preceding decrease and increase in the expression of 1αOHase and 24OHase genes, respectively. Also, serum FGF23 increases after treatment with D3, suggesting that D3 negatively feedback controls its circulating concentrations by inducing FGF23. We thus undertook this investigation to determine the molecular mechanism by which D3 increases the circulating level of FGF23. To this end we examined by Real-Time PCR the effect of D3 on FGF23 mRNA expression in the rat osteoblastic cell line, UMR-106, as well as in mouse calvaria. The results reveal a dose- and time-dependent stimulation of endogenous FGF23 mRNA level in UMR osteoblasts in response to D3. The maximal increase was 117-fold at 10-6M D3, although for further experiments we chose a 10-7 M concentration of D3, to represent a more physiological concentration of the hormone. The increase of FGF23 mRNA was 55.8-fold after 24 hrs treatment, but statistically significant differences were observed as early as after 4-8 hrs. In addition, using cotreatment with actinomycin D or cyclohexamide, we observed that D3 regulates FGF23 expression at the transcriptional level, likely via the synthesis of an intermediary transfactor. To evaluate the biological significance of vitamin D regulation of FGF23 mRNA expression in vivo, we administered D3 (6μg/kg B.W.) to 4-5 week old C57BL/6 mice by IP injection. Calvaria were collected at 48 hrs after single injection and FGF23 mRNA levels were measured. The expression of FGF23 mRNA in this bone tissue was significantly up-regulated by D3. These results suggest that FGF23 and D3 are reciprocal phosphate regulatory hormones, with D3 initially acting to enhance blood phosphate levels via intestinal absorption and renal reabsorption, followed by a negative feedback effect of D3 to reduce phosphate by inducing FGF23 both diminishing D3 through 1;gaOHase repression and eliciting phosphaturia.
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