Fibroblast growth factor-23 (FGF23) is a circulating factor that decreases serum levels of inorganic phosphate (Pi) as well as 1,25-dihydroxyvitamin D 3 . Recent studies also suggest a correlation between serum levels of FGF23 and parathyroid hormone (PTH) in patients with chronic kidney disease. It is, however, unknown whether FGF23 directly modulates PTH expression, or whether the correlation is secondary to abnormalities in Pi and vitamin D metabolism. The objective of the current study was therefore to elucidate possible direct effects of FGF23 on bovine parathyroid cells in vitro. Treatment of parathyroid cells with a stabilized form of recombinant FGF23 (FGF23(R176Q)) induced a rise in early response gene-1 mRNA transcripts, a marker of FGF23 signaling. FGF23(R176Q) potently and dose-dependently decreased the PTH mRNA level within 12 h. In agreement, FGF23(R176Q) also decreased PTH secretion into conditioned media. In contrast, FGF23(R176Q) dose-dependently increased 1a-hydroxylase expression within 3 h. FGF23 (R176Q) did not affect cell viability nor induce apoptosis, whereas a small but significant increase in cell proliferation was found. We conclude that FGF23 is a negative regulator of PTH mRNA expression and secretion in vitro. Our data suggest that FGF23 may be a physiologically relevant regulator of PTH. This defines a novel function of FGF23 in addition to the previously established roles in controlling vitamin D and Pi metabolism.

Current studies suggest that short-term exposure of parathyroid glands to fibroblast growth factor 23 (FGF23) reduces parathyroid hormone secretion. However, patients with chronic kidney disease (CKD) develop secondary hyperparathyroidism despite high levels of serum FGF23, indicating a parathyroid FGF23 'resistance'. Here we analyzed the expression of the FGF23 receptors Klotho and FGF receptor 1 (FGFR1) in 88 hyperplastic parathyroid glands from 31 patients with CKD (including 21 renal allograft recipients), and their regulation in isolated bovine and human hyperplastic parathyroid cells. Glandular expression was variable, yet the Klotho and FGFR1 mRNA levels declined in parallel with the decreasing glomerular filtration rate, significantly decreasing over CKD stages. We found no association between the expression of Klotho, FGFR1, and the proliferation marker Ki67. In vitro treatment of bovine cells with FGF23 or calcium reduced the Klotho level, whereas active vitamin D(3) compounds increased its expression. Phosphate and parathyroid hormone had no effect. Treatment had less impact on Klotho in cultured human cells than in the bovine healthy cell model, whereas FGFR1 expression was induced in the hyperplastic cells. Thus parathyroid Klotho and FGFR1 decrease with declining renal function, possibly because of alterations in mineral metabolism related to the failing kidney. This could explain the observed parathyroid resistance to FGF23 in late CKD.

FGF23 transgenic mice display differentially expressed transcript levels of several genes essential in renal Pi regulation. These findings may lead to further understanding of how FGF23 mediates its actions on renal Pi regulation.

Objective: Fibroblast growth factor-23 (FGF23) is a circulating factor involved in phosphate (Pi) and vitamin D metabolism. Serum FGF23 is increased at later stages of chronic kidney disease due to chronic hyperphosphatemia and decreased renal clearance. Recent studies also indicate that FGF23 may directly regulate the expression of parathyroid hormone (PTH) in vitro. Therefore, the objective of the current study was to determine the relationship between FGF23, PTH, and other biochemistries in vivo in subjects with no history of renal disease. Design: Serum biochemistries were measured in a subsample of the population-based Swedish part of the MrOS study. In total, 1000 Caucasian men aged 70-80 years were randomly selected from the population. Methods: Intact FGF23, Pi, calcium, albumin, estimated glomerular filtration rate (eGFR, calculated from cystatin C), PTH, and 25(OH)D 3 were measured. Association studies were performed using linear univariate and multivariate regression analyses. Results: The median FGF23 level was 36.6 pg/ml, ranging from 0.63 to 957 pg/ml. There was a significant correlation between log FGF23 and eGFR (rZK0.21; P!0.00001) and log PTH (rZ0.13; P!0.001). These variables remained as independent predictors of FGF23 in multivariate analysis. In addition, log PTH (bZ0.082; P!0.05) and eGFR (bZK0.090; P!0.05) were associated with log FGF23 in subjects with eGFRO60 ml/min. Only eGFR (bZK0.35; P!0.0001) remained as a predictor of log FGF23 in subjects with eGFR!60 ml/min. Conclusions: Serum FGF23 and PTH are associated in vivo, supporting recent findings that FGF23 directly regulates PTH expression in vitro. Additionally, eGFR is associated with FGF23 in subjects with normal or mildly impaired renal function, indicating that GFR may modulate FGF23 levels independent of serum Pi.

Parathyroid Klotho expression is decreased or undetectable in pHPT. We provide evidence that 1) serum calcium is strongly associated with parathyroid Klotho expression in pHPT; and 2) abnormal PTH secretion in hypercalcemic pHPT subjects is mediated by Klotho-independent mechanisms.

Objective: Hyperostosis-hyperphosphataemia syndrome (HHS) is a rare hereditary disorder characterized by hyperphosphataemia, inappropriately normal or elevated 1,25-dihydroxyvitamin D 3 and localized painful cortical hyperostosis. HHS was shown to be caused by inactivating mutations in GALNT3, encoding UDP-N-acetyl-a-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase 3 (GalNAc-transferase; GALNT3). Herein, we sought to identify the genetic cause of hyperphosphataemia and tibial hyperostosis in a 19-year-old girl of Colombian origin. Methods: Genomic DNA was extracted and sequencing analysis of the GALNT3 and fibroblast growth factor 23 (FGF23) genes performed. Serum levels of intact and C-terminal FGF23 were measured using two different ELISA methods. Results: Mutational analysis identified a novel homozygous missense mutation in exon 6 of GALNT3 (1584 GOA), leading to an amino acid shift from Arg to His at residue 438 (R438H). The mutation was not found in over 200 control alleles or in any single nucleotide polymorphism databases. The R438 residue is highly conserved throughout species and in all known GalNAc-transferase family members. Modelling predicted the substitution deleterious for protein structure. Importantly, the phosphaturic factor FGF23 was differentially processed, as reflected by low intact (15 pg/ml) but high C-terminal (839 RU/ml) serum FGF23 levels. Conclusions: We report on the first missense mutation in GALNT3 giving rise to HHS, since previous GALNT3 mutations in HHS caused aberrant splicing or premature truncation of the protein. The R438H substitution likely abrogates GALNT3 activity, in turn causing enhanced FGF23 degradation and subsequent hyperostosis/hyperphosphataemia. European Journal of Endocrinology 158 929-934

Background: The proneural subtype of glioblastoma (GBM) is characterized by PDGFRα alterations. In a retroviral insertional mutagenesis screen (Johansson et al., PNAS, 2004) the transcription factor Sox5 was identified as a candidate brain tumor locus and the mode of insertions indicated an overexpression of the short version of Sox5 (s-Sox5). This led to an investigation of the role of s-Sox5 in glioma development, and we subsequently found that opposite to our hypothesis, s-Sox5 could inhibit glioma development in mice, reduce cell proliferation and clone formation ability of human glioma cell lines, and induce acute cellular senescence in primary mouse glial cells through regulation of AKT and p27Kip1 (Tchougounova et al., Oncogene, 2009). Aims: To investigate if s-SOX5 alterations are associated with a certain subtype of human GBM, and to identify the mechanism behind the anti-tumoral properties of s-SOX5 in newly established human glioma cell cultures (HGCCs). Methods: The TCGA dataset for GBM was analyzed via the cBio Cancer Genomics Portal (http://cbioportal.org) for alterations in SOX5 + other SOX genes and glioma genes. Endogenous SOX5 + expression of other SOX proteins were analyzed with immunostainings in six newly established HGCCs maintained under stem cell conditions. These HGCCs were also transfected with pcDNA-V5-s-Sox5, and analyzed by flow cytometry for EdU incorporation, cleaved Caspase-3 and V5. Results: We found that the majority of SOX5 alterations were confined to the proneural subtype (10/56 cases) with 90% of the alterations being upregulations. In fact, all upregulations of SOX5 were found in the proneural group. Only a few alterations (3/56; all downregulations) were found in the mesenchymal subtype, and none in the classical or neural subgroups. The upregulated SOX5 expression in the proneural subtype could be connected to a slight increase in patient survival. Co-occurring alterations in s-SOX5, PDGFRα, SOX6 and SOX10 were found in the proneural subtype. These findings are only suggestive and need to be validated in larger set of tumor samples. Immunostainings showed that all HGCCs expressed SOX2, SOX6, SOX9 and SOX10. For SOX5, two were completely negative while the remaining four showed varying expression. The transfection efficiency was very low (10%), but when specifically analyzing the V5-s-SOX5 positive cells, we found in all HGCCs a reduced number of EdU positive cells compared to the uninfected cell population of the same sample (statistically significant in three HGCCs). Conclusion: Our preliminary data suggest that upregulation of s-SOX5 could be a defining alteration in the proneural subgroup of GBM, and that s-SOX5 suppresses proliferation of human glioma cell cultures. Further studies are underway to solidify our findings and clarify the mechanism behind s-SOX5/Sox5 inhibition of glioma cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3343. doi:1538-7445.AM2012-3343