Effects of etidronate-mediated suppression of bone remodeling on aluminum-induced de novo bone formation

Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease

FGF23 is a bone-derived hormone that regulates systemic phosphate homeostasis, vitamin D metabolism and α-Klotho expression through a novel bone-kidney axis. FGF23 inhibits renal tubular reabsorption of phosphate through mechanisms independent of PTH as well as reduces circulating 1, 25(OH)(2)D through its dual effects to suppress Cyp27b1 production and to stimulate Cyp24 catabolism of 1,25(OH)(2)D. 1,25(OH)(2)D and other factors regulating bone remodeling/mineralization are the major physiological regulators of FGF23 expression. FGF23 also suppresses the gene transcription of α-klotho by the kidney, which exists as a membrane and soluble protein. Membrane Klotho acts as a coreceptor for and dictates organ specificity of FGF23, whereas soluble Klotho act as an endocrine factor that regulates activity of cell surface glycoproteins and receptors in multiple tissues. Elevated FGF23 levels are responsible for several hereditary and acquired hypophosphatemic rickets disorders. FGF23 and Klotho deficiency have similar phenotypes characterized by hyperphosphatemia, elevated 1,25(OH)(2)D and tumoral calcinosis. FGF23 levels progressively increase during chronic kidney disease (CKD). FGF23 has been proposed to be the initial adaptive response leading to reductions in 1,25(OH)(2)D and secondary hyperparathyroidism (HPT) in CKD. The overall biological effect of this initial step may be to orchestrate a coordinated adaptation to protect the organism from the adverse effects of excess phosphate retention. The second step involves the effects of PTH on bone remodeling that further stimulates FGF23 production through both direct and indirect mechanisms related to alterations in extracellular matrix factors. PTH further amplifies FGF23 expression in later stages of CKD to compensate for the increased phosphate efflux from bone caused by excessive bone turnover. While many aspects of the regulation and functions of FGF23 remain to be established, the idea that FGF23 hormone is the initial adaptive hormonal response in CKD that suppresses 1,25(OH)(2)D, reduces gastrointestinal calcium and phosphate absorption and leads to a secondary HPT represents a paradigm shift in the conceptualization of the pathogenesis of secondary hyperparathyroidism. In addition, the prevalent thought that CKD is a functional "vitamin D deficient state" requiring therapy with 1,25(OH)(2)D analogs is challenged by effects of FGF23 to potentially lower both 25(OH)D and 1,25(OH)D by induction of Cyp24-mediated degradation. Finally, increments in FGF23 are associated with increased cardiovascular mortality in CKD. Whether these effects represent direct effects of FGF23 or represent a marker of other abnormalities in CKD remains to be determined.

Distinct roles for intrinsic osteocyte abnormalities and systemic factors in regulation of FGF23 and bone mineralization in Hyp mice

X-linked hypophosphatemia (XLH) is characterized by hypophosphatemia and impaired mineralization caused by mutations of the PHEX endopeptidase (phosphate-regulating gene with homologies to endopeptidases on the X chromosome), which leads to the overproduction of the phosphaturic fibroblast growth factor 23 (FGF23) in osteocytes. The mechanism whereby PHEX mutations increase FGF23 expression and impair mineralization is uncertain. Either an intrinsic osteocyte abnormality or unidentified PHEX substrates could stimulate FGF23 in XLH. Similarly, impaired mineralization in XLH could result solely from hypophosphatemia or from a concomitant PHEX-dependent intrinsic osteocyte abnormality. To distinguish between these possibilities, we assessed FGF23 expression and mineralization after reciprocal bone cross-transplantations between wild-type (WT) mice and the Hyp mouse model of XLH. We found that increased FGF23 expression in Hyp bone results from a local effect of PHEX deficiency, since FGF23 was increased in Hyp osteocytes before and after explantation into WT mice but was not increased in WT osteocytes after explantation into Hyp mice. WT bone explanted into Hyp mice developed rickets and osteomalacia, but Hyp bone explanted into WT mice displayed persistent osteomalacia and abnormalities in the primary spongiosa, indicating that both phosphate and PHEX independently regulate extracellular matrix mineralization. Unexpectedly, we observed a paradoxical suppression of FGF23 in juvenile Hyp bone explanted into adult Hyp mice, indicating the presence of an age-dependent systemic inhibitor of FGF23. Thus PHEX functions in bone to coordinate bone mineralization and systemic phosphate homeostasis by directly regulating the mineralization process and producing FGF23. In addition, systemic counterregulatory factors that attenuate the upregulation of FGF23 expression in Hyp mouse osteocytes are present in older mice.

Osteoblast calcium-sensing receptor has characteristics of ANF/7TM receptors

There is evidence for a functionally important extracellular calcium-sensing receptor in osteoblasts, but there is disagreement regarding its identity. Candidates are CASR and a putative novel calcium-sensing receptor, called Ob.CASR. To further characterize Ob.CASR and to distinguish it from CASR, we examined the extracellular cation-sensing response in MC3T3-E1 osteoblasts and in osteoblasts derived from CASR null mice. We found that extracellular cations activate ERK and serum response element (SRE)-luciferase reporter activity in osteoblasts lacking CASR. Amino acids, but not the calcimimetic NPS-R568, an allosteric modulator of CASR, also stimulate Ob.CASR-dependent SRE-luciferase activation in MC3T3-E1 osteoblasts. In addition, we found that the dominant negative Galphaq(305-359) construct inhibited cation-stimulated ERK activation, consistent with Ob.CASR coupling to Galphaq-dependent pathways. Ob.CASR is also a target for classical GPCR desensitization mechanisms, since beta-arrestins, which bind to and uncouple GRK phosphorylated GPCRs, attenuated cation-stimulated SRE-luciferase activity in CASR deficient osteoblasts. Finally, we found that Ob.CASR and CASR couple to SRE through distinct signaling pathways. Ob.CASR does not activate RhoA and C3 toxin fails to block Ob.CASR-induced SRE-luciferase activity. Mutational analysis of the serum response factor (SRF) and ternary complex factor (TCF) elements in SRE demonstrates that Ob.CASR predominantly activates TCF-dependent mechanisms, whereas CASR activates SRE-luciferase mainly through a RhoA and SRF-dependent mechanism. The ability of Ob.CASR to sense cations and amino acids and function like a G-protein coupled receptor suggests that it may belong to the family of receptors characterized by an evolutionarily conserved amino acid sensing motif (ANF) linked to an intramembranous 7 transmembrane loop region (7TM).