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Unité de Recherche 545 (B.C., D.D.-S., B.S.), Institut National de la Santé et de la Recherche Médicale, Département d‘Athérosclérose, Institut Pasteur de Lille & Faculté de Pharmacie, Université de Lille 2, BP245-59019 Lille, France; and Center for Liver, Digestive and Metabolic Diseases (F.K.), Laboratory of Pediatrics, University Hospital, 9713GZ Groningen, The Netherlands
Address all correspondence and requests for reprints to: Bart Staels, Unité de Recherche 545, Institut National de la Santé et de la Recherche Médicale-Institut Pasteur de Lille, 1, rue du Professeur Calmette BP245-59019 Lille, France. E-mail: bart.staels{at}pasteur-lille.fr.
Bile acids are liver-synthesized cholesterol derivatives that are postprandially released into the small intestine, where they act as detergents to promote fat absorption. Conversion to bile acids also represents the major route for removal of excess cholesterol from the body. Moreover, it has now been clearly demonstrated that bile acids also exert signaling activities and regulate gene expression in a variety of tissues including liver and intestine. A major breakthrough came with the finding that bile acids are endogenous ligands of the farnesoid X receptor (FXR), a member of the nuclear receptor superfamily of ligand-activated transcription factors (1). Bile acids cross the plasma membrane by passive diffusion or facilitated transport according to their solubility properties. The hydrophobic bile acid chenodeoxycholic acid is the most effective activator of FXR, whereas hydrophilic ursodeoxycholic and muricholic acids are inactive. Although bile acids can also influence gene expression via FXR-independent pathways, it is now well established that FXR activation by bile acids results in the regulation of several genes controlling bile acid metabolism. FXR binds either as monomer or as heterodimer with the retinoid X receptor to DNA sequence motifs called FXREs (Fig. 1
). A major physiological role of FXR is to protect liver cells from the deleterious effects of bile acid overload by decreasing their endogenous production and by accelerating bile acid biotransformation and cellular excretion (2).
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In this issue of Endocrinology, Stayrook et al. (7) report data pointing to a novel function of FXR in the control of glucose metabolism. These authors found that FXR activation by bile acids or synthetic FXR-specific agonists led to an increased expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene in vitro in rat and human hepatocytes, as well as in vivo in mouse liver. PEPCK is considered to be a rate-controlling enzyme of gluconeogenesis. Its expression is regulated at the transcriptional level by hormones controlling glucose homeostasis: glucagon and glucocorticoids, which have strong gluconeogenic actions, induce PEPCK, whereas insulin, which suppresses hepatic gluconeogenesis, represses PEPCK (8). Because type 2 diabetes is characterized by an increased hepatic glucose output, which contributes to fasting hyperglycemia (9), pharmacological inhibition of PEPCK gene expression appears to be an attractive possibility for type 2 diabetes treatment.
It is noteworthy that several previous reports already indicated a possible cross-talk between bile acids and/or FXR and glucose metabolism. In a randomized, double-blind, crossover trial, administration of cholestyramine, an intestinal bile acid sequestrant, improved glycemic control in patients with type 2 diabetes and dyslipidemia (10). In addition, the observation that hepatic FXR gene expression is decreased in several rodent models of diabetes as well as the fact that FXR gene expression is stimulated by glucose and repressed by insulin in vitro in rat primary hepatocytes suggested a link between FXR, bile acid metabolism, and diabetes (11). The findings by Stayrook et al. (7) that bile acids induce PEPCK expression takes this hypothesis a step further. Although all these data argue in favor of a role of bile acids in hepatic carbohydrate metabolism, the exact molecular mechanisms as well as the role of FXR therein are still unclear and undoubtedly are very complex (Fig. 2).
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activity, suggesting a mechanism independent of FXR. By contrast, other data indicate that FXR may decrease PEPCK expression via induction of the small heterodimer partner (SHP) (14, 15, 16). SHP is an atypical orphan nuclear receptor that lacks a DNA-binding domain and acts as a transcriptional corepressor. SHP has been shown to inhibit the transactivation of the PEPCK promoter by the glucocorticoid receptor (GR) (14) and HNF-4 (15) and of the glucose-6-phosphatase promoter by HNF-3 (16) and Foxo1 (15). The reasons for these discrepant findings concerning the actions of bile acids are currently unclear and may be related to the distinct responses of the studied in vitro model systems.
On the other hand, several arguments are in favor of a role for FXR in the induction of gluconeogenic gene expression. Fasting markedly induces the hepatic expression of peroxisome proliferator-activated receptor (PPAR)-
coactivator-1 (PGC-1), which subsequently stimulates the entire program of genes involved in hepatic gluconeogenesis by acting as a coactivator for GR and HNF-4 (17). Interestingly, a recent study reported that FXR mRNA levels are induced in response to fasting (18). In addition, PGC-1 coactivates FXR-target gene promoter activity in vitro (18, 19, 20). Thus, it is plausible that FXR activation participates in the induction of gluconeogenesis. In accordance with the findings of Stayrook et al. (7), De Fabiani et al. (13) also reported that GW4064 increased PEPCK mRNA levels, but to a lesser extent than cAMP, the second messenger in the glucagon pathway (1.6-fold vs. 19-fold, respectively). Moreover, we found that GW4064 does not modify the induction of PEPCK expression by a combination of dexamethasone and cAMP, nor is its inhibition modified in response to insulin in primary rat hepatocytes (our submitted manuscript). This suggests that FXR primarily regulates basal rather than fasting-induced PEPCK expression. Although all these findings indicate a role for FXR in the control of hepatic glucose metabolism, it will be important to clearly establish its physiological relevance in vivo.
In this regard, Stayrook et al. report that FXR activation increases glucose output by primary rat hepatocytes in vitro (7). In mice in vivo, FXR agonist treatment did not modify blood glucose levels, while it increased hepatic PEPCK expression. In accordance with this latter finding, we also found that FXR-deficient mice have similar blood glucose concentrations as wild-type mice after 24 h of fasting (our submitted manuscript). This lack of systemic effect of FXR-regulated PEPCK expression on glucose homeostasis may be due to compensatory effects on peripheral insulin-sensitive tissues. On the other hand, liver-specific PEPCK-deficient mice maintain normal fasting blood glucose concentration despite impaired gluconeogenesis (21). Thus, the physiological relevance of the regulation of PEPCK by FXR agonists as well as the effects of FXR on whole-body glucose homeostasis require further assessment in vivo. Studies using FXR-deficient mice are therefore warranted.
Based on expression profiling upon FXR activation, Stayrook et al. (7) propose a model in which FXR increases PPAR expression, which in turn induces the expression of the serine/threonine kinase Akt/protein kinase B inhibitor TRB3 (protein tribbles 3). Because Akt activity is crucial for the phosphorylation and subsequent nuclear exclusion of Foxo1 (22, 23), FXR-induction of TRB3 may therefore contribute to the induction of PEPCK gene expression (Fig. 2
), but this attractive scheme raises some questions. Although the authors observed an induction of PPAR expression upon treatment with a FXR agonist, previous studies reported a species-specific regulation of PPAR by bile acids. Whereas PPAR regulation by FXR was observed in human hepatocytes (24), neither FXR deficiency nor bile acid treatment influenced its expression in mice (3). The decrease of Akt activity upon FXR activation is contradictory with the fact that bile acid treatment may enhance insulin signaling (12). Finally, the role of TRB3 in regulating insulin-dependent effects has been questioned recently (25). Additional mechanistic studies appear therefore necessary.
In conclusion, the study of Stayrook et al. provides additional evidence for a link between bile acid and glucose metabolism, possibly via FXR. These findings substantiate a novel physiological function of the enterohepatic cycle of bile acids and may open new attractive pharmacological approaches for the treatment of metabolic disorders, such as type 2 diabetes. However, before considering such approaches, several questions remain to be resolved, among which are the following: What is the contribution of FXR to the effects of bile acids on glucose metabolism? What is the real physiological relevance of the regulation of PEPCK expression by bile acids? Although additional studies are obviously needed to unravel the precise molecular mechanism(s), the findings of Stayrook et al. add another piece to the puzzle connecting bile acid metabolism to glucose homeostasis, which clearly represents an exciting new area of research.
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Footnotes |
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, PPAR- coactivator 1; PPAR, peroxisome proliferator-activated receptor; SHP, small heterodimer partner; TRB3, protein tribbles 3. Received December 8, 2004.
Accepted for publication December 20, 2004.
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(PGC-1): transcriptional coactivator and metabolic regulator. Endocr Rev 24:78–90 (PGC-1) regulates triglyceride metabolism by activation of the nuclear receptor FXR. Genes Dev 18:157–169 coactivator-1 (PGC-1). J Pharmacol Exp Ther 312:170–178
in a ligand-dependent manner. Biochem J 382:913–921[CrossRef][Medline]
-dependent induction of TRB-3. Nat Med 10:530–534[CrossRef][Medline]
gene via activation of the farnesoid X receptor. Mol Endocrinol 17:259–272This article has been cited by other articles:
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  R. B. Goldberg, V. A. Fonseca, K. E. Truitt, and M. R. Jones Efficacy and Safety of Colesevelam in Patients With Type 2 Diabetes Mellitus and Inadequate Glycemic Control Receiving Insulin-Based Therapy Arch Intern Med, July 28, 2008; 168(14): 1531 - 1540. [Abstract] [Full Text] [PDF]
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B. Cariou, K. van Harmelen, D. Duran-Sandoval, T. H. van Dijk, A. Grefhorst, M. Abdelkarim, S. Caron, G. Torpier, J.-C. Fruchart, F. J Gonzalez, et al. The Farnesoid X Receptor Modulates Adiposity and Peripheral Insulin Sensitivity in Mice J. Biol. Chem., April 21, 2006; 281(16): 11039 - 11049. [Abstract] [Full Text] [PDF]
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 T. Claudel, B. Staels, and F. Kuipers The Farnesoid X Receptor: A Molecular Link Between Bile Acid and Lipid and Glucose Metabolism Arterioscler. Thromb. Vasc. Biol., October 1, 2005; 25(10): 2020 - 2030. [Abstract] [Full Text] [PDF]
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