Differential Specificity of Endocrine FGF19 and FGF21 to FGFR1 and FGFR4 in Complex with KLB Chaofeng Yang 1 , Chengliu Jin1, Xiaokun Li3, Fen Wang 1 , Wallace L. McKeehan1,2, Yongde Luo1,2* 1 Center for Cancer and Stem Cell Biology, Institute of Biosciences and Technology, Texas A&M Health Science Center, Houston, Texas, United States of America, 2 IBT Proteomics and Nanotechnology Laboratory, Institute of Biosciences and Technology, Texas A&M Health Science Center, Houston, Texas, United States of America, 3 School of Pharmacy, Wenzhou Medical College, Wenzhou, China Abstract Background: Recent studies suggest that betaKlotho (KLB) and endocrine FGF19 and FGF21 redirect FGFR signaling to regulation of metabolic homeostasis and suppression of obesity and diabetes. However, the identity of the predominant metabolic tissue in which a major FGFR-KLB resides that critically mediates the differential actions and metabolism effects of FGF19 and FGF21 remain unclear. Methodology/Principal Findings: We determined the receptor and tissue specificity of FGF21 in comparison to FGF19 by using direct, sensitive and quantitative binding kinetics, and downstream signal transduction and expression of early response gene upon administration of FGF19 and FGF21 in mice. We found that FGF21 binds FGFR1 with much higher affinity than FGFR4 in presence of KLB; while FGF19 binds both FGFR1 and FGFR4 in presence of KLB with comparable affinity. The interaction of FGF21 with FGFR4-KLB is very weak even at high concentration and could be negligible at physiological concentration. Both FGF19 and FGF21 but not FGF1 exhibit binding affinity to KLB. The binding of FGF1 is dependent on where FGFRs are present. Both FGF19 and FGF21 are unable to displace the FGF1 binding, and conversely FGF1 cannot displace FGF19 and FGF21 binding. These results indicate that KLB is an indispensable mediator for the binding of FGF19 and FGF21 to FGFRs that is not required for FGF1. Although FGF19 can predominantly activate the responses of the liver and to a less extent the adipose tissue, FGF21 can do so significantly only in the adipose tissue and adipocytes. Among several metabolic and endocrine tissues, the response of adipose tissue to FGF21 is predominant, and can be blunted by the ablation of KLB or FGFR1. Conclusions: Our results indicate that unlike FGF19, FGF21 is unable to bind FGFR4-KLB complex with affinity comparable to FGFR1-KLB, and therefore, at physiological concentration less likely to directly and significantly target the liver where FGFR4- KLB predominantly resides. However, both FGF21 and FGF19 have the potential to activate responses of primarily the adipose tissue where FGFR1-KLB resides. Citation: Yang C, Jin C, Li X, Wang F, McKeehan WL, et al. (2012) Differential Specificity of Endocrine FGF19 and FGF21 to FGFR1 and FGFR4 in Complex with KLB. PLoS ONE 7(3): e33870. doi:10.1371/journal.pone.0033870 Editor: Vincent Laudet, Ecole Normale Supe´rieure de Lyon, France Received August 1, 2011; Accepted February 19, 2012; Published March 19, 2012 Copyright: 2012 Yang et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by U.S. Public Health Service grants DK56338 (Texas Medical Center Digestive Diseases Center) and an Enhancement Grant of the Texas A&M Health Science Center to YL, P50 CA140388 to WLM, the Susan Komen Breast Cancer grant, and the John S. Dunn Research Foundations and the Alliance of NanoHealth to YL and WLM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: yluo@ibt.tamhsc.edu Introduction The diverse FGF homologues constitute a large family of 22 distinct proteins in human that sense environmental cues to regulate cellular and metabolic homeostasis [1,2,3,4,5]. Their transmembrane receptor tyrosine kinase FGFR family encoded by 4 genes contains a plethora of isoforms through combinatorial alternative splicing of their gene products [5,6], among which are the principal IIIb and IIIc isoforms for FGFR1–3 but not FGFR4. With heparan sulfate glycan motifs as the cofactor, the canonic FGFs including FGF1–10, 16–18, 20 and 22 are predominant for cell growth, survival, differentiation and migration that when aberrant may drive cancer progression [5,7]. With the assistance from transmembrane cofactors Klotho (KL) and betaKlotho (KLB), the endocrine FGF19 subfamily including FGF19, 21 and 23 play important roles in control of bile acid synthesis and systemic lipid, glucose, energy and minerals metabolic homeostasis [8,9,10,11]. Unlike the canonic FGFs that possess high affinity for matrix heparan sulfate (HS) motifs [5,7,12,13,14,15] and thus act only locally as autocrine and paracrine factors, the FGF19 subfamily has extremely low affinity for HS, which permits their circulation as endocrine hormones to distal tissues/organs where the FGFR and KL or KLB are co-expressed [16,17]. KL and KLB are single transmembrane domain proteins containing tandem glycosidase domains of unknown functions in the extracellular region and short intracellular tails [18,19,20]. KLB is expressed in liver, adipose tissue, pancreas and muscle, and KL in kidney and intestine [18,21,22,23]. It has been thought that the endocrine effects of these hormonal FGFs are determined by expression PLoS ONE | www.plosone.org 1 March 2012 | Volume 7 | Issue 3 | e33870