Mechanism of FGF23 processing in fibrous dysplasia

Abstract

Fibroblast growth factor-23 (FGF23) is a phosphate-and vitamin D-regulating hormone derived from osteoblasts/osteocytes that circulates in both active (intact, iFGF23) and inactive (C-terminal, cFGF23) forms. O-glycosylation by O-glycosyl transferase Nacetylgalactosaminyltransferase 3 (ppGalNAcT3) and differential cleavage by furin have been shown to be involved in regulating the ratio of active to inactive FGF23. Elevated iFGF23 levels are observed in a number of hypophosphatemic disorders, such as X-linked, autosomal recessive, and autosomal dominant hypophosphatemic rickets, whereas low iFGF23 levels are found in the hyperphosphatemic disorder familial tumoral calcinosis/hyperphosphatemic hyperostosis syndrome. Fibrous dysplasia of bone (FD) is associated with increased total FGF23 levels (cFGF23 þ iFGF23); however, classic hypophosphatemic rickets is uncommon. Our results suggest that it can be explained by increased FGF23 cleavage leading to an increase in inactive cFGF23 relative to active iFGF23. Given the fact that FD is caused by activating mutations in the small G-protein G s a that results in increased cyclic adenosine monophosphate (cAMP) levels, we postulated that there may be altered FGF23 cleavage in FD and that the mechanism may involve alterations in cAMP levels and ppGalNacT3 and furin activities. Analysis of blood specimens from patients with FD confirmed that the elevated total FGF23 levels are the result of proportionally increased cFGF23 levels, consistent with less glycosylation and enhanced cleavage by furin. Analysis of primary cell lines of normal and mutation-harboring bone marrow stromal cells (BMSCs) from patients with FD demonstrated that BMSCs harboring the causative G s a mutation had higher cAMP levels, lower ppGalNAcT3, and higher furin activity. These data support the model wherein glycosylation by ppGalNAcT3 inhibits FGF23 cleavage by furin and suggest that FGF23 processing is a regulated process that controls overall FGF23 activity in FD patients. ß

Dynamic regulation of FGF23 by Fam20C phosphorylation, GalNAc-T3 glycosylation, and furin proteolysis

“…The mechanisms by which Fam20C is functionally or expressionally regulated are unknown. Recent studies examining the production of intact and/ or C-terminal fragments of FGF23 in fibrous dysplasia and during physiological situations of low iron, such as in ADHR (36,37), support that GalNAc-T3 and furin are likely controlled by multiple factors including 3′,5′-cyclic adenosine monophosphate (38), iron or iron deficiency (39), and inorganic phosphate (40).…”

The PTH-Vitamin D-FGF23 axis

“…Further confirmation came from the discovery that elevated levels of FGF23 were responsible for tumor-induced osteomalacia (TIO) [6,7]. In rapid succession, FGF23 was implicated in other phosphatewasting syndromes including autosomal recessive h y p o p h o s p h a t e m i c r i c k e t s [ 8 -1 2 ] , X -l i n k e d hypophosphatemic rickets [13], and fibrous dysplasia [14,15]. There are currently at least 11 disorders of altered phosphate homeostasis associated with alternations in FGF23 physiology that have been well characterized (Table 1).…”

Fibroblast growth factor 23

“…117 Significantly, Bhattacharyya et al recently reported higher ratios of cFGF23 to iFGF23 in patients with FD compared to healthy controls (mean 1.7 vs. 0.99). 98 The authors also provided primary evidence of modified FGF23 processing from analysis of bone marrow stromal cells harbouring the causative activating G s a mutation (affecting the a-subunit of the stimulatory G-protein), which showed a cAMP-dependent inhibition of GALNT3 activity and increased furin activity. Conversely, cFGF23 to iFGF23 ratios in patients with renal failure were highly variable, ranging from $0.1 to $4 RU/pg.…”