The Klotho gene family as a regulator of endocrine fibroblast growth factors

The Klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends lifespan when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates phosphate and vitamin D homeostasis. It has been shown that Klotho-deficient mice and Fgf23 knockout mice exhibit identical phenotypes. This observation led to the identification of Klotho as a cofactor essential for interactions between FGF23 and FGF receptors. In addition to the Klotho–FGF23 axis, recent studies has shown that βKlotho, a Klotho family protein, also functions as a cofactor required for FGF19 and FGF21 signaling and determines the tissue-specific metabolic activities of FGF19 and FGF21. This review summarizes recent progress in understanding of Klotho and βKlotho function in the regulation of tissue-specific metabolic activity of the endocrine fibroblast growth factors (FGF19, FGF21, and FGF23).