Browsing by Subject "Fibroblast Growth Factor"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item The Liver-Derived Endocrine Hormone FGF21 Alters Metabolism and Diurnal Behavior via the Nervous System(2012-07-09) Bookout, Angie Lynn; Mangelsdorf, David J.Fuel acquisition is essential to survival. During privation, the body protects glucose concentrations acutely by glycogenolysis, and later by gluconeogenesis and ketogenesis. Additionally, animals alter daily behavioral patterns to seek food, but eventually reduce energetically costly activities (growth, reproduction, locomotion). Little is known about the mechanisms that orchestrate and coordinate these physiological and behavioral responses to starvation. The liver-derived endocrine hormone fibroblast growth factor 21 (FGF21) is induced in chronic fasting and acts as a global starvation signal. Previous studies focusing on FGF21 as an anti-diabetic drug indicate that FGF21 coordinates whole-body fat utilization and energy expenditure. However, its basic physiological role is underexplored. // Acute injection of recombinant FGF21 quickly elicits a coordinated program between tissues resulting in reduced plasma insulin and gluconeogenic and thermogenic gene expression programs in liver and brown adipose, effects that require an intact animal. Mice with chronic FGF21 overexpression (FGF21tg) are smaller in size, females are infertile, and if fasted, they undergo torpor, an energy-conserving process. Taken together, these data suggest that FGF21 may exert some effects through the nervous system. // To explore this idea, I utilized anatomically-guided laser capture microdissection followed by quantitative, real-time PCR to profile expression of the FGF receptor/co-receptor family in specific hypothalamic nuclei of mice. Surprisingly, the FGFR1-IIIc/βKlotho complex is found in the suprachiasmatic nucleus (SCN), area postrema (AP), nucleus tractus solitarii (NTS), and nodose ganglion (cell body of vagus nerve), implicating roles in circadian and metabolic regulation. // Results of surgical, pharmacological, and genetic strategies indicate the vagus senses circulating FGF21, resulting in adrenergic efferent responses that reduce insulin secretion, while a different adrenergic site modulates liver and brown adipose gene expression. //Analyses of the effects of FGF21 on the SCN, the body’s master clock, using running wheels show FGF21tg mice have dramatically altered circadian activity, likely as a consequence of inhibiting SCN output functions. Deletion of βKlotho specifically from the SCN rescues this behavior in addition to growth defects of FGF21tg mice. To date, this is the first description of a liver-derived endocrine hormone that affects such diverse aspects of the starvation response by acting on the nervous system. [Keywords: FGF21. Circadian, metabolism, starvation, hypothalamus, PPAR, diabetes, fasting]Item Metabolic Regulation by Fibroblast Growth Factor 21(2011-12-12) Dutchak, Paul Anthony; Kilewer, Steven A.Fibroblast growth factor 21 (FGF21) is a secreted hormone that can beneficially regulate glucose and lipid homeostasis. Through a reverse endocrinology approach, we uncovered that FGF21 expression is transcriptionally regulated by the peroxisome proliferator activated-receptor alpha (PPARa) in liver. PPARa is a member of the nuclear hormone receptor superfamily that is physiologically activated by increased fatty acid mobilization to liver during fasting, and regulates the genetic program whereby lipids are converted to ketone bodies through a process known as ketogenesis. Here, I show the effects of FGF21 as a fasting hormone that is expressed in liver and contributes to the regulation of adipose tissue and hepatic ketogenesis during the fasted state. Using in vitro and in vivo methods to investigate the effects of FGF21, a model whereby FGF21 stimulates lipolysis in adipose tissue was generated. Intriguingly, using our FGF21 transgenic mice, I observed the expression of many genes involved in lipogenesis was highly induced in adipose tissue in an FGF21-dependent manner. Moreover, many of these lipogenic genes were found to be down-regulated in adipose of the FGF21 knockout mouse. The inhibition of lipogenic genes in adipose tissue was associated with increased SUMOylation of PPARg protein in this tissue. Using a feeding-fasting paradigm, I found that FGF21 expression in the liver and adipose tissue was rhythmic, peaking in liver prior to feeding and peaking in the adipose after feeding. Furthermore, the induction of FGF21 by PPARg ligands suggested a unique function for this protein in adipose, independent from its role in the fasted state. To assess the contribution of FGF21 to the anti-diabetic properties of PPARg agonists (ie. thiazolidinediones), diet-induced obese wild type and Fgf21-/- mice were treated with the TZD rosiglitazone. Rosiglitazone produced a significant increase in adipose FGF21 expression, but decreased hepatic FGF21 mRNA and circulating FGF21 protein. These data suggest that FGF21 functions as an autocrine factor within adipose tissue. Moreover, the therapeutic effects of rosiglitazone as an insulin sensitizer were lost in the Fgf21-/- mouse, as assessed by glucose and insulin tolerance tests. Several other effects of rosiglitazone were lost in the Fgf21-/- mice, including increased adipose mass, edema, and PPARg target gene expression in the adipose. These data indicated that PPARg can control the expression of FGF21, which functions as a feed-forward mechanism to stimulate PPARg target genes and PPARg dependent physiology. Since PPARg can be modified by SUMO on two different sites on the protein, in vitro experiments were performed to show that PPARg is SUMOylated at Lysine-107, a previously identified negative regulator of its transcriptional activity. Importantly, I found that treatment of Fgf21-/- adipocytes with FGF21 reduced the amount of SUMOylated PPARg, thereby allowing it to be it an active state. Collectively, these data reveal that FGF21 has two independent roles in regulating metabolism in vivo: as a hepatic endocrine hormone that is induced during the fasting response through PPARa, and as an adipose autocrine/paracrine factor that is induced in a feed-forward loop to stimulate PPARg activity.Item Regulation of Liver Metabolism by Fibroblast Growth Factor 19(2012-07-20) Kir, Serkan; Mangelsdorf, David J.Fibroblast Growth Factor (FGF) 19 is a postprandial enterokine up-regulated by bile acid receptor FXR upon bile acid uptake into the ileum. FGF19 inhibits hepatic bile acid synthesis through transcriptional repression of cholesterol 7 alpha-hydroxylase (CYP7A1) via a mechanism involving nuclear receptor Small Heterodimer Partner (SHP). Here, I show that two other nuclear receptors, Hepatocyte Nuclear Factor 4 alpha (HNF4 alpha) and Liver Receptor Homolog-1 (LRH-1), enable SHP binding to the Cyp7a1 promoter and therefore are important for negative feedback regulation of Cyp7a1. HNF4 alpha and LRH-1 are also crucial activators of Cyp7a1 transcription. They maintain active transcription histone marks on the Cyp7a1 promoter, whereas FGF19 down-regulates these marks in a SHP-dependent way. Secondly, I show that the MEK/ERK signaling pathway is an integral regulator of bile acid metabolism. ERK activity is necessary to maintain hepatic Shp and Cyp7a1 transcription at their physiologic levels. Inhibition of this pathway causes loss of Shp transcription by disrupting HNF4 alpha and LRH-1 binding to the Shp promoter. Independent from the effects on Shp, MEK/ERK inhibition induces Cyp7a1 transcription. Unexpectedly, the MEK/ERK pathway is not required for repression of Cyp7a1 by FGF19. Although this pathway is activated by FGF19 in livers of Fgf receptor 4 (Fgfr4)-deficient mice probably via other FGFRs, Cyp7a1 repression is largely impaired. Thus, I propose that a signaling mechanism uniquely regulated by FGFR4 must be responsible for FGF19-dependent repression of bile acid synthesis. In addition to its roles in bile acid metabolism, I also show that FGF19 stimulates hepatic protein and glycogen synthesis, but does not induce lipogenesis. The effects of FGF19 are independent of the activity of either insulin or the protein kinase Akt, and instead are mediated through a mitogen-activated protein kinase signaling pathway that activates components of the protein translation machinery and stimulates glycogen synthase activity. Mice lacking FGF15 (the mouse FGF19 ortholog) fail to properly regulate blood glucose and fail to maintain normal postprandial amounts of liver glycogen. FGF19 treatment restored the loss of glycogen in diabetic animals lacking insulin. Thus, FGF19 activates a physiologically important, insulin-independent endocrine pathway that regulates hepatic protein and glycogen metabolism. [Keywords: bile acids, FGF19, CYP7A1, HNF4 alpha, LRH-1, SHP, protein synthesis, glycogen synthesis, ERK, GSK3]Item Understanding the Molecular Basis for FGF15/19 and FGF21 Actions on Energy Homeostasis(2012-07-09) Boney-Montoya, Jamie; Kliewer, Steven A.Insulin and glucagon have long been known to play essential roles in controlling energy balance during the fed and fasted states, respectively. Recently, additional metabolic hormones have been discovered within a subfamily of the fibroblast growth factor superfamily. The FGF15/19 subfamily is composed of atypical FGFs lacking the heparin-binding domain, which enables them to act in an endocrine fashion by diffusing away from their tissues of origin. They signal through cell-surface receptors complexed with beta-Klotho, a membrane-spanning protein, to mediate signaling cascades that lead to physiological responses. One member, FGF19, causes reduced glucose and insulin levels with enhanced insulin sensitivity when expressed in transgenic mice. Another member, FGF21, has been shown to act as an insulin sensitizer pharmacologically by improving glucose tolerance and reducing insulin. The prevalence of metabolic disorders (e.g. type 2 diabetes) in today’s society has led to the investigation of these two endocrine FGFs for use in a clinical setting. However, the mechanisms underlying these responses have not been characterized. // To elucidate the mechanisms utilized by FGF15/19, we used several animal models to show a role for FGF15/19 in regulating hepatic glucose production. Like insulin, FGF15/19 represses gluconeogenesis. Specifically, FGF15/19 inhibits expression of the transcriptional coactivator PGC1-alpha, a key regulator of gluconeogenic gene expression. The repressive effect of FGF15/19 on gluconeogenic gene expression is lost when PGC1-alpha is overexpressed. FGF15/19 causes the dephosphorylation and inactivation of the transcription factor CREB, thereby blunting its ability to bind and induce the PGC1-alpha promoter. The results demonstrated that FGF15/19 works subsequent to insulin as a postprandial regulator of gluconeogenesis through inhibition of the CREB/ PGC1-alpha pathway. // To fully understand the effects of FGF21, we began studying the downstream kinase signaling cascades and the protein substrates affected by this hormone. Utilizing stable isotope labeling of amino acids in cell culture (SILAC), an unbiased phosphoproteomic profile was obtained of potential FGF21 targets in rat H4IIE hepatoma cells. One of the most highly regulated targets was FetuinA, which was dephosphorylated by FGF21 treatment. FetuinA is an inhibitor of insulin receptor signaling and the FetuinA knockout mouse exhibits aberrant glucose homeostatsis. Our in vitro data suggested a relationship between FGF21 and FetuinA in regulating insulin sensitivity but further exploration lead to the conclusion that FGF21 was not directly regulating FetuinA in vivo. // Taken together, the important role of FGF15/19 and FGF21 in regulating carbohydrate metabolism as well as their pharmacological actions makes them attractive drug candidates for metabolic diseases. However, further study will be required to determine their molecular mechanisms more completely and their long-term efficacy in the clinic. [Keywords: FGF15/19, FGF21, gluconeogenesis, CREB, PGCI-alpha, Fetuin-A, phosphorylation, beta-klotho]