Regulation of C-terminal and intact FGF-23 by dietary phosphate in men and women.

UNLABELLED: FGF-23 is a novel regulator of phosphate metabolism. We studied the regulation of FGF-23 by dietary phosphate in 66 men and women using two assays. Dietary phosphate restriction decreased FGF-23 and loading increased FGF-23 significantly. An assay that measured intact FGF-23 showed the effects of dietary phosphate much more clearly than an assay that also measures presumed biologically inactive fragments. Dietary phosphate is a key regulator of circulating FGF-23; choice of assay is critical when studying FGF-23 physiology. INTRODUCTION: Fibroblast growth factor 23 (FGF-23) is a novel phosphaturic factor discovered through genetic studies of patients with renal phosphate wasting disorders. Ablation of the FGF-23 gene in mice reduces renal phosphate excretion and increases serum phosphate, suggesting that FGF-23 is critical for normal phosphate homeostasis. We examined the role of dietary phosphate in the regulation of FGF-23 in humans. MATERIALS AND METHODS: Sixty-six healthy males and females were randomized to either phosphate-depleted or -loaded diets for 5 days, after a 4-day run-in diet. FGF-23 was measured using an "intact" assay that only detects intact FGF-23 peptide and with a "C-terminal" assay that measures both intact FGF-23 peptide and presumed biologically inactive carboxyl terminal fragments. The main outcome was the within group change in FGF-23 with either phosphate depletion or loading. RESULTS: Using the intact FGF-23 assay, mean FGF-23 area under the curve (AUC) decreased by 9 +/- 16% with phosphate depletion (p = 0.0041) and increased by 35 +/- 29% with loading (p < 0.0001). Using the C-terminal FGF-23 assay, mean FGF-23 AUC decreased by 8 +/- 12% with phosphate depletion (p = 0.0003) and increased by 13 +/- 20% with loading (p = 0.0016). Increases in FGF-23 with phosphate loading were greater with the intact assay than with the C-terminal assay (p = 0.0003). Using the intact assay only, FGF-23 was significantly associated with serum phosphate (r = 0.39, p < 0.01), 24-h urinary phosphate (r = 0.47, p < 0.01), fractional excretion of phosphate (r = 0.29, p < 0.01), and 1,25-dihydroxyvitamin D (r = -0.30, p < 0.01). The association between the assays was weak (r = 0.26, p < 0.01). CONCLUSIONS: Dietary phosphate is a key regulator of circulating FGF-23 levels in humans. Additionally, choice of assay is critical when performing physiologic investigations of FGF-23.

Effects of teriparatide, alendronate, or both on bone turnover in osteoporotic men.

CONTEXT: We have previously demonstrated that alendronate reduces the ability of teriparatide to increase bone mineral density (BMD) in osteoporotic men. The underlying basis for this observation is poorly understood. OBJECTIVE: The primary aim of this study was to determine whether teriparatide increases osteoblast activity when the ability of teriparatide to increase osteoclast activity is suppressed by alendronate. DESIGN: This was a nonblinded, randomized, controlled trial. SETTING: The study was conducted at the General Clinical Research Center of a teaching hospital. PATIENTS: We studied 63 men, age 46-85, with low spine and/or hip BMD. INTERVENTIONS: Subjects received alendronate 10 mg daily (group 1), teriparatide 37 microg sc daily (group 2), or both (group 3) for 30 months. Teriparatide was begun at month 6. MAIN OUTCOME MEASURES: The primary endpoint was the change in serum N-telopeptide, osteocalcin, and amino-terminal propeptide of type 1 procollagen. RESULTS: In men receiving teriparatide monotherapy (group 2), levels of all bone turnover markers increased markedly during the first 6 months of teriparatide administration and then declined toward baseline during the next 18 months. In men who received combination therapy (group 3), bone turnover marker levels declined in the first 6 months (while receiving alendronate alone) and then returned to baseline levels (N-telopeptide) or above (osteocalcin and amino-terminal propeptide of type 1 procollagen) after teriparatide was added. Changes in each marker were significantly different between groups 1 and 2 (all P values < 0.001), groups 1 and 3 (all P values < 0.001), and groups 2 and 3 (all P values < 0.03). CONCLUSIONS: As with BMD, alendronate impairs the action of teriparatide to increase bone turnover in men.

Comparison of weekly treatment of postmenopausal osteoporosis with alendronate versus risedronate over two years.

OBJECTIVE: A 1-yr extension of the Fosamax Actonel Comparison Trial was completed to compare changes in bone mineral density (BMD), bone turnover, and upper gastrointestinal tolerability over 2 yr of treatment. DESIGN: This was a randomized, double-blind extension conducted at 72 U.S. sites. PATIENTS AND METHODS: Of the 1053 women who completed yr 1, 833 postmenopausal women with low BMD entered the extension, continuing their same treatment allocation [once-weekly (OW) alendronate 70 mg or OW risedronate 35 mg]. Changes in BMD at the hip trochanter, total hip, femoral neck, and lumbar spine and in markers of bone turnover were compared at 24 months. Tolerability was assessed by adverse experience reporting. RESULTS: Alendronate produced greater increases from baseline in BMD at 24 months than did risedronate at the trochanter (alendronate, 4.6%; risedronate, 2.5%, P < 0.001) as well as at all other BMD sites. Significantly more alendronate than risedronate patients had measured BMD increases of 0% or more and 3% or more at all BMD sites (P < 0.001), and fewer alendronate patients had measured decreases of 3% or more at all BMD sites. Significantly greater reductions in all biochemical markers of bone turnover occurred with alendronate, compared with risedronate. No differences were seen in occurrence or discontinuations due to upper gastrointestinal adverse experiences. CONCLUSIONS: Patients receiving 70 mg OW alendronate had greater gains in BMD, were more likely to maintain or gain BMD, and had greater reductions in bone turnover markers than patients receiving 35 mg OW risedronate after 24 months, with no differences in upper gastrointestinal tolerability.