The effect of vitamin D supplementation on serum levels of fibroblast growth factor- 23: A systematic review and meta-analysis of randomized controlled trials

Non-Classical Effects of FGF23: Molecular and Clinical Features

This article reviews the role of fibroblast growth factor 23 (FGF23) protein in phosphate metabolism, highlighting its regulation of vitamin D, parathyroid hormone, and bone metabolism. Although it was traditionally thought that phosphate-calcium homeostasis was controlled exclusively by parathyroid hormone (PTH) and calcitriol, pathophysiological studies revealed the influence of FGF23. This protein, expressed mainly in bone, inhibits the renal reabsorption of phosphate and calcitriol formation, mediated by the α-klotho co-receptor. In addition to its role in phosphate metabolism, FGF23 exhibits pleiotropic effects in non-renal systems such as the cardiovascular, immune, and metabolic systems, including the regulation of gene expression and cardiac fibrosis. Although it has been proposed as a biomarker and therapeutic target, the inhibition of FGF23 poses challenges due to its potential side effects. However, the approval of drugs such as burosumab represents a milestone in the treatment of FGF23-related diseases.

Emerging concepts on the FGF23 regulation and activity

Fibroblast growth factor 23 (FGF23) discovery has provided new insights into the regulation of Pi and Ca homeostasis. It is secreted by osteoblasts and osteocytes, and acts mainly in the kidney, parathyroid, heart, and bone. The aim of this review is to highlight the current knowledge on the factors modulating the synthesis of FGF23, the canonical and non-canonical signaling pathways of the hormone, the role of FGF23 in different pathophysiological conditions, and the anti-FGF23 therapy. This is a narrative review based on the search of PubMed database in the range of years 2000-2023 using the keywords local and systemic regulators of FGF23 synthesis, FGF23 receptors, canonical and non-canonical pathways, pathophysiological conditions and FGF23, and anti-FGF23 therapy, focusing the data on the molecular mechanisms. The regulation of FGF23 synthesis is complex and multifactorial. It is regulated by local factors and systemic regulators mainly involved in bone mineralization. The excessive FGF23 production is associated with different congenital diseases and with diseases occurring with a secondary high FGF23 production such as in chronic disease kidney and tumor-induced osteomalacia (TIO). The anti-FGF23 therapy appears to be useful to treat chromosome X-linked hypophosphatemia and TIO, but there are doubts about the handle of excessive FGF23 production in CKD. FGF23 biochemistry and pathophysiology are generating a plethora of knowledge to reduce FGF23 bioactivity at many levels that might be useful for future therapeutics of diseases associated with high-serum FGF23 levels.

Effect of vitamin D supplementation on circulating fibroblast growth factor-23 concentration in adults with prediabetes

BACKGROUND

Recent meta-analyses report that vitamin D supplementation increases blood fibroblast growth factor-23 (FGF23) level.

OBJECTIVES

To determine the effect of 4000 IU/day of vitamin D₃ for 12 months on circulating FGF23 levels. We also examined the association of the achieved 25-hydroxyvitamin D level [25(OH)D] with the FGF23 level at 12 months and with 12-month changes in FGF23.

METHODS

An ancillary analysis among adults 70 years and older with prediabetes who participated in a trial comparing vitamin D₃ 4000 IU/day with placebo. Plasma intact FGF23 and serum 25(OH)D were measured at baseline and month 12 (M12).

RESULTS

Characteristics of the 52 participants (vitamin D₃ n = 28; placebo n = 24) did not differ significantly aside from more women than men in the vitamin D₃ group. Mean ± SD age was 73.8 ± 3.7 years, BMI 31.3 ± 4.2 kg/m2, and glomerular filtration rate (GFR) 76.3 ± 11.8 mL/min/1.73m² Baseline serum 25(OH)D level was 33.4 ± 10.8 ng/mL and increased at M12 to 54.9 ± 14.8 ng/mL in the vitamin D₃ group versus 33.4 ± 14.9 in the placebo (p < 0.001). At baseline, GFR was inversely associated with FGF23 (r = - 0.349, p = 0.011). Change in FGF23 level at M12 did not differ significantly between vitamin D₃ and placebo. In all participants combined, the achieved serum 25(OH)D level at M12 was not significantly associated with the M12 plasma FGF23 or the M12 change in FGF23.

CONCLUSION

In obese older adults with sufficient vitamin D status and normal renal function, vitamin D₃ 4000 IU/day for 12 months did not significantly alter plasma intact FGF23 levels.

CLINICALTRIALS

gov NCT 01,942,694, registered 9/16/2013.

Shedding Light on the Complex Regulation of FGF23

Early research has suggested a rather straightforward relation between phosphate exposure, increased serum FGF23 (Fibroblast Growth Factor 23) concentrations and clinical endpoints. Unsurprisingly, however, subsequent studies have revealed a much more complex interplay between autocrine and paracrine factors locally in bone like PHEX and DMP1, concentrations of minerals in particular calcium and phosphate, calciprotein particles, and endocrine systems like parathyroid hormone PTH and the vitamin D system. In addition to these physiological regulators, an expanding list of disease states are shown to influence FGF23 levels, usually increasing it, and as such increase the burden of disease. While some of these physiological or pathological factors, like inflammatory cytokines, may partially confound the association of FGF23 and clinical endpoints, others are in the same causal path, are targetable and hence hold the promise of future treatment options to alleviate FGF23-driven toxicity, for instance in chronic kidney disease, the FGF23-associated disease with the highest prevalence by far. These factors will be reviewed here and their relative importance described, thereby possibly opening potential means for future therapeutic strategies.