Sulfated Glycosaminoglycans Are Required for Specific and Sensitive Fibroblast Growth Factor (FGF) 19 Signaling via FGF Receptor 4 and betaKlotho

Abstract

Secreted from intestine, human fibroblast growth factor 19 (hFGF19) is an endocrine metabolic regulator that controls bile acid synthesis in the liver. Earlier studies have suggested that hFGF19 at 10 -100 nM levels signals through FGF receptor 4 (FGFR4) in the presence of a co-receptor, betaKlotho, but its activity and receptor specificity at physiological concentrations (picomolar levels) remain unclear. Here we report that hFGF19 at picomolar levels require sulfated glycosaminoglycans (sGAGs), such as heparan sulfate, heparin, and chondroitin sulfates, for its signaling via human FGFR4 in the presence of human betaKlotho. Importantly, sGAGs isolated from liver are highly active in enhancing the picomolar hFGF19 signaling. At nanomolar levels, in contrast, hFGF19 activates all types of human FGFRs, i.e. FGFR1c, FGFR2c, FGFR3c, and FGFR4 in the co-presence of betaKlotho and heparin and activates FGFR4 even in the absence of betaKlotho. These results show that sGAGs play crucial roles in specific and sensitive hFGF19 signaling via FGF receptors and suggest that hepatic sGAGs are involved in the highly potent and specific signaling of picomolar hFGF19 through FGFR4 and betaKlotho. The results further suggest that hFGF19 at pathological concentrations may evoke aberrant signaling through various FGF receptors.In both humans and mice, the fibroblast growth factor (FGF) family is composed of 22 structurally related proteins that can be divided into several subfamilies (1). The endocrine FGF subfamily, which function as metabolic regulators (2-5), is composed of FGF19, FGF21, and FGF23 in humans and FGF15, FGF21, and FGF23 in mice. Mouse (m)Fgf15 is an ortholog for human (h)FGF19 (6). Both mFGF15 and hFGF19 are expressed in the distal small intestine. Their secretion into the circulation is induced by bile acid, and after reaching the liver via the portal vein they suppress a key liver enzyme involved in bile acid biosynthesis, thereby closing a feedback loop regulating bile acid homeostasis (7). hFGF19/mFGF15 also reportedly contributes to the regulation of blood glucose levels, along with other metabolic regulators, including FGF21 and insulin (8 -11).Endocrine FGFs act via receptor tyrosine kinases but also require the presence of a co-receptor, alphaKlotho (KLA) 4 or betaKlotho (KLB) (12). In humans, FGF23 signaling via several FGF receptor (FGFR) subtypes is thought to specifically require KLA, whereas hFGF19 and FGF21 reportedly require . But other studies have shown that KLA can also act as a co-receptor for hFGF19 (15). It has thus been unclear how the receptor specificity of hFGF19 is achieved, or even what its precise receptor specificity is. Results from Fgf15 knockout mice suggest that mFGFR4 is the sole functional receptor for mFGF15 (7).Moreover, the potential applicability of hFGF19 to the treatment of metabolic diseases such as diabetes (8 -11) has prompted attempts to develop protocols for its clinical application, but these have been discouraged by the finding that transgenic mice expressing high le...

Mechanism of Action of Determining Target Cell Specificity of Endocrine FGF : Regulation by Sulfated Glycosaminoglycans

“…This study suggested that the increase in the blood hFGF19 concentrations not only relies on hFGFR4 and hKLB, but also on sGAGs (16). The composition ratio of these highly sulfated structures, including hepatic sGAGs, despite its lower concentration compared to that of HP, shows the same level of activity with HP.…”

Dynamics and Distribution of Klothoβ (KLB) and Fibroblast Growth Factor Receptor-1 (FGFR1) in Living Cells Reveal the Fibroblast Growth Factor-21 (FGF21)-induced Receptor Complex

“…It is therefore interesting to postulate that FGF21 regulation of cellular fatty acid metabolism is due to formation of a less potent 1:2 complex of KLB-FGFR1c (31). Future studies will also need to determine the aggregation state of HSPGs to determine their role in the FGF21 signaling complex (31).…”

Understanding cytokine and growth factor receptor activation mechanisms

“…Other growth factors require participation of noncovalently bound carbohydrate molecules to activate their receptor. For example, members of the fibroblast growth factor (FGF) family require heparan sulfate proteoglycan to interact productively with their receptors (Nakamura, Uehara et al 2011). …”