Endocrine fibroblast growth factors as regulators of metabolic homeostasis

Endocrine fibroblast growth factors (FGFs) function as hormones that maintain specific metabolic states by controlling homeostasis of bile acid, glucose, fatty acid, phosphate, and vitamin D. Endocrine FGFs exert their biological activity through a common design of coreceptor system consisting of the Klotho gene family of transmembrane proteins and cognate FGF receptors. Moreover, expression of endocrine FGFs is regulated by nuclear receptors whose lipophilic ligands are generated under the control of these hormones in the target organs. Thus, novel endocrine axes have emerged that regulate diverse metabolic processes through feedback loops composed of the FGF, Klotho, FGF receptor, and nuclear receptor gene families. This review summarizes the role of Klotho family proteins in the regulation of metabolic activity and expression of the endocrine FGFs.