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BackgroundRecent 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 FindingsWe 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.ConclusionsOur 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.

Genomic ablation of hepatocyte-specific fibroblast growth factor receptor (FGFR)4 in mice revealed a role of FGF signaling in cholesterol and bile acid metabolism and hepatolobular restoration in response to injury without effect on liver development or hepatocyte proliferation. Although the potential role of all 23 FGF polypeptides in the liver is still unclear, the most widely studied prototypes, FGF1 and FGF2, are present and have been implicated in liver cell growth and function in vitro. To determine whether FGF1 and FGF2 play a role in response to injury and fibrosis, we examined the impact of both acute and chronic exposure to carbon tetrachloride (CCl 4 ) in the livers of FGF1-and FGF2-deficient mice. After acute CCl 4 exposure, FGF1(؊/؊)FGF2(؊/؊) mice exhibited an accelerated release of serum alanine aminotransferase similar to FGFR4 deficiency, but no effect on overall hepatolobular restoration or bile acid metabolism. The fibroblast growth factors (FGFs) comprise a family of 23 reported members that have varying affinities for variants of four different FGF receptor kinases (FGFR1 to FGFR4). 1-3 FGF1 and FGF2, the first two cloned members of the FGF family, 4,5 have received the most study, are widely expressed, and thus, predicted to be involved in tissue-specific functions and associated pathologies at their site of expression. 6,7 In vitro FGF1 affects cells of multiple origin whereas activity of FGF2 appears more limited to cells derived from mesenchyme and neuroectoderm. 8 FGF1 and FGF2 have been implicated in derivation of the liver from foregut endoderm. 9 However, mice lacking FGF1, FGF2, or both FGF1 and FGF2 are viable, fertile, and grossly indistinguishable from wild type (WT) except for modest defects in cardiovascular tissue, healing of skin wounds, and neuronal tissue. 3,10,11 This suggests that FGF1 and FGF2 alone are not essential for embryonic development or are compensated by other members of the extensive FGF ligand family. Consequently, FGF1-and FGF2-deficient animals are available for study of the role of the two factors in adult tissue homeostasis. Despite their ubiquity, little has emerged except for the modest effects of ablation of FGF1 and FGF2 in the cardiovascular, skin, and nervous systems.Although the levels of mRNA transcripts are low, longlived FGF1 and FGF2 polypeptides are present in the resting liver at significant levels. 12 This suggests that a significant reservoir of both ligands is present in the resting liver in an inactive state before activation and

OBJECTIVE-Fibroblast growth factor (FGF) family signaling largely controls cellular homeostasis through short-range intercell paracrine communication. Recently FGF15/19, 21, and 23 have been implicated in endocrine control of metabolic homeostasis. The identity and location of the FGF receptor isotypes that mediate these effects are unclear. The objective was to determine the role of FGFR4, an isotype that has been proposed to mediate an ileal FGF15/19 to hepatocyte FGFR4 axis in cholesterol homeostasis, in metabolic homeostasis in vivo. RESEARCH DESIGN AND METHODS-FGFR4Ϫ/Ϫ micemice overexpressing constitutively active hepatic FGFR4 -and FGFR4 Ϫ/Ϫ with constitutively active hepatic FGFR4 restored in the liver were subjected to a normal and a chronic high-fat diet sufficient to result in obesity. Systemic and liver-specific metabolic phenotypes were then characterized. RESULTS-FGFR4-deficient mice on a normal diet exhibited features of metabolic syndrome that include increased mass of white adipose tissue, hyperlipidemia, glucose intolerance, and insulin resistance, in addition to hypercholesterolemia. Surprisingly, the FGFR4 deficiency alleviated high-fat diet-induced fatty liver in obese mice, which is also a correlate of metabolic syndrome. Restoration of FGFR4, specifically in hepatocytes of FGFR4-deficient mice, decreased plasma lipid levels and restored the high-fat diet-induced fatty liver but failed to restore glucose tolerance and sensitivity to insulin. CONCLUSIONS-FGFR4plays essential roles in systemic lipid and glucose homeostasis. FGFR4 activity in hepatocytes that normally serves to prevent systemic hyperlipidemia paradoxically underlies the fatty liver disease associated with chronic high-fat intake and obesity. Diabetes

Carbon tetrachloride (CCl 4 ) intoxification in rodents is a commonly used model of both acute and chronic liver injury. Recently, we showed that mice in which FGFR4 was ablated from the germline exhibited elevated cholesterol metabolism and bile acid synthesis coincident with unrepressed levels of cytochrome P450 7A (CYP7A), the rate-limiting enzyme in cholesterol disposal. Of the four fibroblast growth factor (FGF) receptor genes expressed in adult liver, FGFR4 is expressed specifically in mature hepatocytes. To determine whether FGFR4 plays a broader role in liver-specific metabolic functions, we examined the impact of both acute and chronic exposure to CCl 4 in FGFR4-deficient mice. Following acute CCl 4 exposure, the FGFR4-deficient mice exhibited accelerated liver injury, a significant increase in liver mass and delayed hepatolobular repair. Chronic CCl 4 exposure resulted in severe fibrosis in livers of FGFR4-deficient mice compared to normal mice. Analysis at both mRNA and protein levels indicated an 8-hour delay in FGFR4-deficient mice in the down-regulation of cytochrome P450 2E1 (CYP2E1) protein, the major enzyme whose products underlie CCl 4 -induced injury. These results show that hepatocyte FGFR4 protects against acute and chronic insult to the liver and prevents accompanying fibrosis. The results show that FGFR4 acts by promotion of processes that restore hepatolobular architecture rather than cellularity while limiting damage due to prolonged CYP2E1 activity. (Am J Pathol