During the past decade a lot of work has been done to better understand the roles of fibroblast growth factor-23 (FGF-23); a relatively newly discovered endocrine hormone, in multiple organ systems in the body. This review focuses on expressions of FGF-23, coexpressions of α-Klotho and FGF receptors, and FGF-23 mediated end-organ effects in the physiological and pathological conditions. We also discuss the controversial reports regarding α-Klotho-dependent and α-Klotho-independent functions of FGF-23.
EXPRESSION OF FGF-23, α-KLOTHO, AND FGF RECEPTORS
FGF-23FGF-23 is a family member of 22 fibroblast growth factors (FGFs) including FGF-1-FGF-23, all of which are not only structurally but also evolutionarily related proteins. These FGFs have been classified as being paracrine (15 FGFs), endocrine (3 FGFs), or intracrine (4 FGFs) 1 and using a mammalian (murine) model the endocrine FGFs, especially FGF-23 an approximately 32 kD protein, 2 have been thoroughly investigated in recent years.FGF-23 is expressed in multiple cells in the body. It has been conclusively shown to be secreted in bone by cells of the osteoblast lineage and osteocytes, as a part of the lacunacanalicular system, under the influence of phosphate.3 Local factors derived from bone itself also regulate its expression as seen in cases of inactivating mutations of PHEX which results in increased transcription and circulating levels of It is also expressed in the pericytelike endothelial cells surrounding the venous sinusoids in bone marrow 5 and in the thymus.2 FGF-23 also plays a part in the body's immune response since its expression is induced in activated dendritic cells and macrophages in response to inoculation with E. coli and S. aureus via nuclear factor-kappa B (NF-κB) signaling. 6 FGF-23 is found to be expressed in the venterolateral (VL) thalamic nucleus as well. In addition, FGF-23 is also expressed in heart by cardiac interstitial fibroblasts. 7,8 Apart from these tissues, there is also FGF-23 expression seen in the muscle, spleen, skin, lung, testes, kidney, and liver to a much lesser extent. α-Klotho α-Klotho is a unique molecule that establishes a regulatory system of calcium homeostasis by affecting transepithelial transport of calcium, parathyroid hormone secretion, and FGF-23 signal transduction.9 Evolutionarily the FGF-23-α-Klotho system is part of a major milestone in vertebrate evolution that started in the ocean when the early piscine ancestors acquired the bony endoskeleton.10 α-Klotho has been demonstrated to enhance FGF-23 activity over 10-fold 11 and has also been identified as a necessary co-receptor for FGF-23 binding due to the phenotypic similarity observed in α-Klotho and FGF-23 knockout mice, i.e.; hyperphosphatemia and hypercalcemia. 11 α-Klotho is expressed in high levels in kidney, parathyroidgland (PTG), testis, ovary, brain, pituitary, and apical plasma membrane of ependymal cells in the choroid plexus but