Fibroblast growth factor 7: an inhibitor of phosphate transport derived from oncogenic osteomalacia-causing tumors.

Oncogenic osteomalacia (OO), a tumor-associated phosphate-wasting syndrome, provides an opportunity to identify regulators of renal phosphate homeostasis. We established cultures from OO-associated tumors. Conditioned medium from these cultures inhibited phosphate uptake in renal tubular epithelial cells. We then compared RNA from tumor-derived cultures expressing inhibitory activity with RNA from tumor-derived cultures in which inhibitory activity was not evident and identified candidate mRNAs specifically expressed by cultures inhibiting renal phosphate transport. Testing of identified candidates revealed that one protein, fibroblast growth factor 7 (FGF7), was a potent and direct inhibitor of phosphate uptake in vitro. A neutralizing monoclonal antibody to FGF7 reversed FGF7-dependent phosphate transport inhibition and inhibitory activity in conditioned medium from tumor cell cultures. Immunoassay revealed abundant FGF7 in inhibitory conditioned medium and minimal amounts in nonconditioned medium or conditioned medium with no phosphate transport inhibitory activity. Furthermore, only small amounts of FGF23 were present in inhibitory conditioned medium, comparable to concentrations found in conditioned medium with no phosphate transport inhibitory activity. Thus, FGF7 was specifically identified when selecting for in vitro phosphate transport inhibitory activity of tumor-derived cultures and was confirmed as a potent inhibitor of phosphate transport. Finally, FGF7 message was confirmed in PCR products of mRNA extracted from fragments of each tumor. Members of the FGF family (other than FGF23) are expressed by OO-associated tumors and may play a role in mediating this syndrome.

Effect of paricalcitol on circulating parathyroid hormone in X-linked hypophosphatemia: a randomized, double-blind, placebo-controlled study.

CONTEXT: Hyperparathyroidism occurs frequently in X-linked hypophosphatemia (XLH) and may exacerbate phosphaturia, potentially affecting skeletal abnormalities. OBJECTIVE: The objective of the study was to suppress elevated PTH levels in XLH patients. DESIGN: This was a prospective, randomized, placebo-controlled, double-blind, 1-year trial of paricalcitol, with outcomes measured at entry and 1 year later. SETTING: PATIENTS were recruited from the investigators' clinics or referred from throughout the United States. Data were collected in an in-patient hospital research unit. PATIENTS: Subjects with a clinical diagnosis of XLH and hyperparathyroidism were offered participation and were eligible if they were 9 years old or older and not pregnant, and their serum calcium level was less than 10.7 mg/dL, their 25-hydroxyvitamin D level was 20 ng/mL or greater, and their creatinine level was 1.5 mg/dL or less. INTERVENTION: The intervention for this study was the use of paricalcitol or placebo for 1 year. MAIN OUTCOME MEASURES: Determined prior to trial onset was the change in PTH area under the curve. Secondary outcomes included renal phosphate threshold per glomerular filtration rate, serum phosphorus, serum alkaline phosphatase activity, and (99m)Tc-methylenediphosphonate bone scans. RESULTS: PTH area under the curve decreased 17% with paricalcitol, differing (P = .007) from the 20% increase with placebo. The renal phosphate threshold per glomerular filtration rate increased 17% with paricalcitol and decreased 21% with placebo (P = .05). Serum phosphorus increased 12% with paricalcitol but did not differ from placebo. Paricalcitol decreased alkaline phosphatase activity in adults by 21% (no change with placebo, P = .04). Bone scans improved in 6 of 17 paricalcitol subjects, whereas no placebo-treated subject improved. Hypercalciuria developed in six paricalcitol subjects and persisted from baseline in one placebo subject. CONCLUSIONS: Suppression of PTH may be a useful strategy for skeletal improvement in XLH patients with hyperparathyroidism, and paricalcitol appears to be an effective adjunct to standard therapy in this setting. Although paricalcitol was well tolerated, urinary calcium and serum calcium and creatinine should be monitored closely with its use.

Vitamin D binding protein is a key determinant of 25-hydroxyvitamin D levels in infants and toddlers.

Circulating 25-hydroxyvitamin D (25-OHD) levels vary among human populations. Only limited information regarding determinants of these measures is available for infants and children, particularly in minority groups at greatest risk for vitamin D deficiency. We identified demographic determinants of circulating 25-OHD in a large cohort of minority children, and now extend our studies to examine potential roles of vitamin D binding protein (DBP) as a determinant of 25-OHD levels. Serum DBP level and common single nucleotide polymorphisms (SNPs) at positions 432 and 436 in the GC gene, encoding DBP, were examined. We confirmed self-reported ancestry using ancestry informative markers (AIMs), and included quantitative AIMs scores in the analysis. The multivariate model incorporated previously identified demographic and nutritional determinants of 25-OHD in this cohort, as well as GC SNPs and circulating DBP. Genetic variants in GC differed by self-reported ancestry. The 1f allele (D432/T436) was enriched in African Americans, occurring in 71%. Homozygosity for the 1f allele (DDTT) occurred in 53% of African Americans but only 6% of Caucasians and 13% of Hispanics. Circulating DBP was significantly correlated with 25-OHD. GC SNPs were associated with both circulating DBP and 25-OHD. It appears that progressive substitution of lysine for threonine at the 436 position results in lower circulating 25-OHD. Multivariate analysis revealed that genetic variance in GC significantly contributes to circulating DBP as well as 25-OHD. Moreover, the effect of GC SNPs on 25-OHD are evident after adjusting for their effects on circulating DBP. Thus in young children genetic variance of the common GC T436K SNP affects circulating levels of the DBP protein, which in turn affects circulating 25-OHD. However, the GC genotype also affects circulating 25-OHD independently of its effect on circulating DBP. These findings provide data that may be important in the interpretation of vitamin D status in children of varying ancestral backgrounds.

Demographic, dietary, and biochemical determinants of vitamin D status in inner-city children.

BACKGROUND: Reports of clinical rickets are particularly evident in minority infants and children, but only limited analyses of vitamin D are available in this demographic group. OBJECTIVE: We sought to characterize circulating 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)(2)D], and their determinants, including circulating parathyroid hormone (PTH), total alkaline phosphatase activity (ALP), calcium, and phosphorus, in minority infants and children. DESIGN: We obtained demographic information and blood samples for measurement of PTH, ALP, 25(OH)D, and 1,25(OH)(2)D in >750 6-mo- to 3-y-old children. Dietary intake data were obtained and analyzed. RESULTS: The mean (±SD) 25(OH)D concentration was 66 ± 22 nmol/L (26.3 ± 8.7 ng/dL). A total of 15% of children had 25(OH)D concentrations less than the recommended target threshold of 50 nmol/L. Combined elevations of PTH and ALP occurred in only 2.5% of children. Determinants of 25(OH)D included vitamin D intake, age (decreasing with age), skin type (greater concentrations in lighter-skinned children than in darker-skinned children), formula use (higher intakes), season (greater concentrations in the summer and fall than in the winter and spring), and, inversely, PTH. The mean 1,25(OH)(2)D concentration was 158 ± 58 pmol/L (60.6 ± 22.5 pg/mL), which was consistent with a reference range of 41-274 pmol/L or 15.7-105.5 pg/mL. Determinants for 1,25(OH)(2)D were age (decreasing with age), sex (greater concentrations in girls than in boys), skin type (greater concentrations in lighter-skinned children than in darker-skinned children), and, inversely, serum calcium and phosphorus. CONCLUSIONS: Although 15% of subjects were vitamin D insufficient, only 2.5% of subjects had elevations of both PTH and ALP. The greater 25(OH)D concentrations observed with formula use confirm that dietary vitamin D fortification is effective in this demographic group. Circulating 1,25(OH)(2)D is higher in infants than in older children and adults and, in contrast to 25(OH)D, is not directly correlated with nutrient intakes.