Fibroblast Growth Factor 23: A Phosphatonin Regulating Phosphate Homeostasis?

Inorganic phosphate (Pi) is a critical circulating ion required to support multiple essential cellular processes dependent on phosphorus, including energy transport, nucleotide synthesis, and intracellular cell signaling. In addition, Pi is essential with calcium for the mineralization of bone matrix through the formation of hydroxyapatite crystals [Ca10(PO4)6(OH)2]. Normally, levels of Pi in the plasma are maintained in the range of approximately 2.5–4.5 mg/dl (0.26–0.47 mmol/liter). Regulation of Pi levels in plasma is important, as demonstrated by the harmful effects of low levels (phosphopenic osteomalacia/rickets, muscle weakness) or high levels (secondary hyperparathyroidism, renal and other soft tissue calcification) (1). In this issue, Larsson et al. (2) describe phosphaturia due to decreased renal reabsorption of Pi, hypophosphatemia, and osteomalacia/rickets in mice with targeted transgenic overexpression of fibroblast growth factor 23 (FGF23) in osteoblasts. This phenotype illustrates a role for FGF23 in Pi metabolism and a potential role for FGF23 in Pi homeostasis. These studies illustrate the ability of FGF23 to mimic some, but not all, of the effects of PTH on Pi homeostasis, illustrating a potential physiological role for FGF23 in Pi metabolism. It has been apparent for a number of years that circulating factors apart from PTH or the related molecule PTHrP are able to produce hyperphosphaturia and hypophosphatemia. However, the nature of these molecules and their potential physiological role in phosphate metabolism and homeostasis has only recently begun to be determined. The name “phosphatonin” has been suggested for this class of molecules, which includes FGF23 (3, 4). Significant homeostatic regulation of Pi occurs through PTH actions on renal Pi handling and through PTH and Pi regulation of 25-hydroxyvitamin D 1α hydroxylase activity. 25-Hydroxyvitamin D 1α hydroxylase acts to increase levels of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] with resultant increases in gut phosphate absorption (Fig. 1). However, the presence of additional Pi regulators (phosphatonins) have been demonstrated through the study of three rare conditions, x-linked hypophosphatemic rickets (XLH) (5, 6), autosomal dominant hypophosphatemic rickets (ADHR) (7), and tumor-induced osteomalacia (8–11). The mouse analog of XLH, the hyp mouse (12), has demonstrated most convincingly, through parabiosis and renal transplant experiments, that one or several circulating factor/s in hyp mice could transfer the phosphaturia and hypophosphatemia to a paired wild-type littermate (13) but transplant of a hyp kidney could not (14). The genetic defect in both humans and the hyp mouse was found to be an inactivating mutation in an endopeptidase called PHEX, suggesting that the function of PHEX may be to inactivate a phosphatonin and that, in the absence of active PHEX, an excess of soluble phosphatonin accumulates in the circulation (5, 6, 9). The recessive nature of this mutation in females would support the concept of an inactivating mutation. Another possibility would be that PHEX normally activates or stabilizes an inhibitor or degradative pathway for phosphatonins. ADHR is biochemically similar to XLH in that there is phosphaturia, hypophosphatemia, and low or inappropriately normal 1,25(OH)2D3. The genetic basis of ADHR has been found to be a mutation in the FGF family member FGF23 (7). The main effect of the mutation would seem to be a stabilization of this molecule through increased resistance to proteolytic breakdown (15–17), although there are also reports of increased activity (15). FGF23 has been found to have many of the attributes of a phosphatonin. Recombinant FGF23, or Chinese hamster ovary cell-secreted FGF23 induces phosphaturia and hypophosphatemia (16) (Fig. 2). In tumor-induced osteomalacia, tumor-secreted factor/s induce urinary phosphate wasting and associated hypophosphatemia and osteomalacia, which resolve if the tumor is successfully resected. Studies of differential gene expression have identified concurrent overexpression of normal FGF23 (18, 19) and also two other factors with phosphaturic action, matrix extracellular phosphoglycoprotein (8, 20) and FRIZZLED-related protein-4 (9). The concurrent expression of multiple phosphatonin molecules is suggestive of some linkage in regulation of expression of these genes and that phenotype is dependent on interactions between these and perhaps other factors. The recent report of the effects of gene deletion of FGF23 (21) is also interesting because these mice have an opposite phenotype with hyperphosphatemia and markedly elevated 1,25(OH)2D3 levels. These effects of gene deletion add additional strong evidence that FGF23 has an integral role in Pi metabolism. Intriguingly, these mice also have an osteomalacic phenotype. Whether this reflects a direct role of FGF23 in bone mineralization or an indirect effect through the hyperphosphatemia, high vitamin D levels, or another mechanism remains to be determined. It would be intellectually tempting to link PHEX and the phosphatonins to a common pathway; however, current data are not sufficient to do this definitively (1). Full-length FGF23 has not been definitively identified as a substrate for PHEX (22), although smaller fragments are cut by this enzyme (23) and matrix extracellular phosphoglyocoprotein may be stabilized by PHEX (24). Whereas FGF23 levels are frequently elevated in tumor-induced osteomalacia, XLH, and ADHR (10), there is considerable overlap with normal populations or populations with other disorders (25–28). It remains unclear whether FGF23, or the other phosphatonin candidates, have a role in physiological phosphate homeostasis or whether their role is in the regulation of local or tissue-specific aspects of phosphate handling. To unravel this question, it will be important to determine the relevant tissue or cell sources of these molecules, to determine how they are regulated, and to identify local vs. systemic effects. A very recent publication addresses the systemic actions of FGF23 by use of tissue nonspecific transgenic overexpression under a chicken β-actin promotor (29). Larsson et al., in this issue, refine this approach to begin to address the tissue-specific effects for FGF23 through bone-specific overexpression of FGF23 in osteoblasts under the α1(I) collagen promoter. In both of these models, mice have high circulating levels of FGF23 and the phenotype of hypophosphatemic rickets/osteomalacia. In these mice, hypophosphatemia results from hyperphosphaturia associated with reduced renal expression of the type IIa sodium-phosphate cotransporter, and inappropriately low levels 1,25(OH)2D3 levels. The high circulating level of FGF23 in the transgenic mice produced by the models of Larsson et al. (2) does not allow clear differentiation of local from systemic effects, although FGF23 levels would be expected to be highest in the bone microenvironment. However, organ cultures and primary cell cultures from the bone-specific transgenic mouse will provide useful in vitro tools to address local effects and interactions at the tissue levels with other phosphatonin candidates. The generation of mice with tissue-specific but lower or regulated expression will also enable this issue to be further examined. Regulation of plasma Pi levels by PTH and 1,25(OH)2D3. Elevated plasma Pi levels induce PTH secretion through the reciprocal relationship between Pi and calcium levels. PTH decreases urinary Pi reabsorption, thus increasing excretion of Pi. Elevated Pi levels suppress 1,25(OH)2D3 production, causing a reduction in gut Pi absorption. The concerted effects lower plasma Pi levels. Low Pi exerts the opposite effects, resulting decreased Pi excretion and increased Pi gut absorption. Regulation of plasma Pi by FGF23 and other phosphatonins. FGF23 acts like PTH to decrease phosphate reabsorption and thus produce hyperphosphaturia. Unlike PTH, which acts to increase 1,25(OH)2D3 synthesis, FGF23 acts to inhibit 1,25(OH)2D3 synthesis (30) and to block the normal 1,25(OH)2D3 response to hypophosphatemia. Phosphaturia coupled with reduced Pi absorption from the gut leads to hypophosphatemia, which results in the defective mineralization of bone matrix that characterizes rickets. Autosomal dominant hypophosphatemic rickets; 25(OH)2D3, 1,25-dihydroxyvitamin D3; fibroblast growth factor 23; inorganic phosphate; x-linked hypophosphatemic rickets.