Phosphate homeostasis is maintained by a counterbalance between efflux from the kidney and influx from intestine and bone. FGF23 is a bone-derived phosphaturic hormone that acts on the kidney to increase phosphate excretion and suppress biosynthesis of vitamin D. FGF23 signals with highest efficacy through several FGF receptors (FGFRs) bound by the transmembrane protein Klotho as a coreceptor. Since most tissues express FGFR, expression of Klotho determines FGF23 target organs. Here we identify the parathyroid as a target organ for FGF23 in rats. We show that the parathyroid gland expressed Klotho and 2 FGFRs. The administration of recombinant FGF23 led to an increase in parathyroid Klotho levels. In addition, FGF23 activated the MAPK pathway in the parathyroid through ERK1/2 phosphorylation and increased early growth response 1 mRNA levels. Using both rats and in vitro rat parathyroid cultures, we show that FGF23 suppressed both parathyroid hormone (PTH) secretion and PTH gene expression. The FGF23-induced decrease in PTH secretion was prevented by a MAPK inhibitor. These data indicate that FGF23 acts directly on the parathyroid through the MAPK pathway to decrease serum PTH. This bone-parathyroid endocrine axis adds a new dimension to the understanding of mineral homeostasis.

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Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) target the kidney to cause a phosphaturia. FGF23 also acts on the parathyroid to decrease PTH expression, but in chronic kidney disease (CKD) there are high-serum PTH and FGF23 levels and resistance of the parathyroid to FGF23. We now report that PTH acts on bone to increase FGF23 expression and characterize the signal transduction pathway whereby PTH increases FGF23 expression. Remarkably, we show that PTH is necessary for the high-FGF23 levels of early kidney failure due to an adenine high-phosphorus diet. Parathyroidectomy before the diet totally prevented the fivefold increase in FGF23 levels in kidney failure rats. Moreover, parathyroidectomy of early kidney failure rats corrected their high-FGF23 levels. Therefore, in early kidney failure, the high-FGF23 levels are dependent on the high-PTH levels. PTH infusion for 3 days to mice with normal renal function increased serum FGF23 and calvaria FGF23 mRNA levels. To demonstrate a direct effect of PTH on FGF23, we added PTH to rat osteoblast-like UMR106 cells. PTH increased FGF23 mRNA levels (4-fold) and this effect was mimicked by a PKA activator, forskolin. PTH also decreased SOST mRNA levels (3-fold). SOST codes for sclerostin, a Wnt pathway inhibitor, which is a PTH receptor (PTH1R) target. The effect of PTH was prevented by added sclerostin. Therefore, PTH increases FGF23 expression which involves the PKA and Wnt pathways. The effect of PTH on FGF23 completes a bone-parathyroid endocrine feedback loop. Importantly, secondary hyperparathyroidism is essential for the high-FGF23 levels in early CKD.

Parathyroid hormone (PTH) regulates serum calcium and phosphate levels, which, in turn, regulate PTH secretion and mRNA levels. PTH mRNA levels are markedly increased in rats fed low calcium diets and decreased after low phosphate diets, and this effect is posttranscriptional. Protein-PTH mRNA binding studies, with parathyroid cytosolic proteins, showed three protein-RNA bands. This binding was to the 3-untranslated region (UTR) of the PTH mRNA and was dependent upon the terminal 60 nucleotides. Parathyroid proteins from hypocalcemic rats showed increased binding, and proteins from hypophosphatemic rats decreased binding, correlating with PTH mRNA levels. There is no parathyroid cell line; however, a functional role was provided by an in vitro degradation assay. Parathyroid proteins from control rats incubated with a PTH mRNA probe led to an intact transcript for 40 min; the transcript was intact with hypocalcemic proteins for 180 min and with hypophosphatemic proteins only for 5 min. A PTH mRNA probe without the 3-UTR, or just the terminal 60 nucleotides, incubated with hypophosphatemic proteins, showed no degradation at all, indicating that the sequences in the 3-UTR determine PTH mRNA degradation. Hypocalcemia and hypophosphatemia regulate PTH gene expression post-transcriptionally. This correlates with binding of proteins to the PTH mRNA 3-UTR, which determines its stability. PTH1 is the major hormone that regulates calcium homeostasis and also has an important role in regulating bone strength. In turn, the synthesis and secretion of PTH is finely regulated by the serum calcium concentration, with hypocalcemia resulting in a marked increase in serum PTH, PTH mRNA levels, and parathyroid cell number (1, 2). Changes in extracellular calcium are sensed by the PT cell by a cell membrane sensor, which then leads to a change in intracellular calcium and inositol triphosphate concentrations (3). How these factors then determine the levels of PTH secretion, gene expression, and PT cell proliferation is not clear.A negative calcium regulatory element in the atrial natriuretic peptide gene, with a homologous sequence in the PTH gene (4) has been shown to bind a protein (ref1), which was known to activate several transcription factors (5). Because no parathyroid cell line is available, these studies were performed in nonparathyroid cell lines, so their relevance to physiologic PTH gene regulation remains to be established. A post-transcriptional effect of calcium on PTH gene expression in primary cultures of bovine parathyroid cells has been demonstrated (6), which correlated with binding of parathyroid proteins to the 5Ј-and 3Ј-untranslated regions (UTRs) of bovine PTH mRNA probes (7).PTH leads to a decrease in serum phosphate by increasing renal phosphate excretion, and in turn, serum phosphate has a direct effect to increase PTH secretion and PTH mRNA levels (8 -11). Dietary induced hypophosphatemia leads to a dramatic decrease in PTH gene expression, and this effect is posttranscriptional (8).We have now studied th...

In vitro 1,25-dihydroxycholecalciferol (1,25(0H)2D3) decreased levels of preproparathyroid(preproPTH) hormone mRNA. We have now pursued these studies in vivo in the rat. Rats were administered vitamin D metabolites i.p. and the levels of preproPTH mRNA were determined in excised parathyroidthyroid glands by blot hybridization. PreproPTH mRNA levels were <4% of basal at 48 h after 100 pmol 1,25(OH)2D3, with no increase in serum calcium. Gel blots showed that 1,25(OH)2D3 decreased preproPTH mRNA levels without any change in its size (833 basepair). Microdissected parathyroids after 1,25(0H)2D3 (100 pmol) showed mRNA levels for preproPTH were 40±8% of controls, but for f-actin were 100% of controls.The relative potencies of vitamin D metabolites were: 1,25(0H)2D3> 24,25(0H1)D3> 25(OH)D3 > vitamin D3. In vitro nuclear transcription showed that 1,25(0H)2D3-treated (100 pmol) rats' PTH transcription was 10% of control, while j5-actin was 100%. These results show that 1,25(0H1)D3 regulates PTH gene transcription. PTH stimulates 1,25(OH)2D3 synthesis, which then inhibits PTH synthesis, thus completing an endocrinological feedback loop.