Farnesoid X Receptor: A New Player in Glucose Metabolism?
http://www.100md.com
内分泌学杂志 2005年第3期
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@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).
FIG. 1. Mechanism of transcriptional regulation by FXR. Bile acid-activated FXR heterodimerizes with the retinoid X receptor (RXR) and binds to FXRE, i.e. inverted repeat-1 (IR-1), on the promoter region of its target genes. In addition, FXR can bind to atypical FXREs as monomer. The regulation of target gene expression by FXR will affect bile acid and lipid metabolism.
The generation and characterization of FXR-deficient mice has established a critical role of FXR also in lipid metabolism, as these mice display elevated serum levels of triglycerides and high-density lipoprotein-cholesterol (3, 4). This action of FXR does not appear to be restricted to mice. In gallstone patients, serum high-density lipoprotein-cholesterol and triglyceride levels decrease upon treatment with bile acids (5), whereas interruption of the enterohepatic circulation of bile acids by sequestrants such as cholestyramine has the opposite effect in hypercholesterolemic subjects (6). These findings have identified FXR as a potential drug target for the treatment of dyslipoproteinemias.
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).
FIG. 2. Proposed mechanisms of PEPCK regulation by FXR. Hepatic glucose production in the liver is tightly controlled by hormones. Glucagon and catecholamines stimulate the cAMP pathway and cAMP response element binding protein (CREB), which activates PGC-1 gene expression. PGC-1 is then recruited to different transcription factors that bind to the promoter of gluconeogenic genes such as PEPCK. Glucocorticoids induce a specific interaction between PGC-1 and GR-activating PEPCK. Insulin represses PEPCK gene expression by inducing Akt-mediated Foxo1 phosphorylation. The activation of FXR by its natural ligands, i.e. bile acids, may both activate (blue) or repress (red) PEPCK gene expression. For more details, see the main text.
Stayrook et al. (7) suggest the regulation of PEPCK gene expression to be a crucial site of impact for bile acids on glucose metabolism. However, unresolved discrepancies exist between their results and those of available studies in the literature. Han et al. (12) recently reported that bile acid treatment directly enhances insulin signaling, leading to increased glycogen synthase activity, in rat primary hepatocytes. Enhanced insulin signaling should in theory result in PEPCK inhibition. Moreover, several studies reported that bile acid treatment, both in vitro in human hepatoma cell lines and in vivo in mice, decreases gene expression of PEPCK as well as other gluconeogenic genes such as glucose-6-phosphatase and fructose 1,6-bis phosphatase (13, 14, 15, 16). As a potential explanation, bile acids may influence these pathways in an FXR-independent manner by decreasing the activity of hepatocyte nuclear factor 4 (HNF-4), a positive regulator of PEPCK gene expression. Indeed, De Fabiani et al. (13) reported that treatment with the nonsteroidal specific FXR agonist GW4064 did not modify HNF-4 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.
References
Chiang JY 2002 Bile acid regulation of gene expression: roles of nuclear hormone receptors. Endocr Rev 23:443–463
Claudel T, Sturm E, Kuipers F, Staels B 2004 The farnesoid X receptor: a novel drug target? Expert Opin Investig Drugs 13:1135–1148
Sinal CJ, Tohkin M, Miyata M, Ward JM, Lambert G, Gonzalez FJ 2000 Targeted disruption of the nuclear receptor FXR/BAR impairs bile acid and lipid homeostasis. Cell 102:731–744
Kok T, Hulzebos CV, Wolters H, Havinga R, Agellon LB, Stellaard F, Shan B, Schwarz M, Kuipers F 2003 Enterohepatic circulation of bile salts in farnesoid X receptor-deficient mice: efficient intestinal bile salt absorption in the absence of ileal bile acid-binding protein. J Biol Chem 278:41930–41937
Leiss O, von Bergmann K 1982 Different effects of chenodeoxycholic acid and ursodeoxycholic acid on serum lipoprotein concentrations in patients with radiolucent gallstones. Scand J Gastroenterol 17:587–592
Molgaard J, von Schenck H, Olsson AG 1989 Comparative effects of simvastatin and cholestyramine in treatment of patients with hypercholesterolaemia. Eur J Clin Pharmacol 36:455–460
Stayrook KR, Bramlett KS, Savkur RS, Ficorilli J, Cook T, Christe ME, Michael LF, Burris TP 2005 Regulation of carbohydrate metabolism by the farnesoid X receptor. Endocrinology 146:984–991
Hanson RW, Reshef L 1997 Regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression. Annu Rev Biochem 66:581–611
Magnusson I, Rothman DL, Katz LD, Shulman RG, Shulman GI 1992 Increased rate of gluconeogenesis in type II diabetes mellitus. A 13C nuclear magnetic resonance study. J Clin Invest 90:1323–1327
Garg A, Grundy SM 1994 Cholestyramine therapy for dyslipidemia in non-insulin-dependent diabetes mellitus. A short-term, double-blind, crossover trial. Ann Intern Med 121:416–422
Duran-Sandoval D, Mautino G, Martin G, Percevault F, Barbier O, Fruchart JC, Kuipers F, Staels B 2004 Glucose regulates the expression of the farnesoid X receptor in liver. Diabetes 53:890–898
Han SI, Studer E, Gupta S, Fang Y, Qiao L, Li W, Grant S, Hylemon PB, Dent P 2004 Bile acids enhance the activity of the insulin receptor and glycogen synthase in primary rodent hepatocytes. Hepatology 39:456–463
De Fabiani E, Mitro N, Gilardi F, Caruso D, Galli G, Crestani M 2003 Coordinated control of cholesterol catabolism to bile acids and of gluconeogenesis via a novel mechanism of transcription regulation linked to the fasted-to-fed cycle. J Biol Chem 278:39124–39132
Borgius LJ, Steffensen KR, Gustafsson JA, Treuter E 2002 Glucocorticoid signaling is perturbed by the atypical orphan receptor and corepressor SHP. J Biol Chem 277:49761–49766
Yamagata K, Daitoku H, Shimamoto Y, Matsuzaki H, Hirota K, Ishida J, Fukamizu A 2004 Bile acids regulate gluconeogenic gene expression via small heterodimer partner-mediated repression of hepatocyte nuclear factor 4 and Foxo1. J Biol Chem 279:23158–23165
Kim JY, Kim HJ, Kim KT, Park YY, Seong HA, Park KC, Lee IK, Ha H, Shong M, Park SC, Choi HS 2004 Orphan nuclear receptor small heterodimer partner represses hepatocyte nuclear factor 3/Foxa transactivation via inhibition of its DNA binding. Mol Endocrinol 18:2880–2894
Puigserver P, Spiegelman BM 2003 Peroxisome proliferator-activated receptor-gamma coactivator 1 (PGC-1): transcriptional coactivator and metabolic regulator. Endocr Rev 24:78–90
Zhang Y, Castellani LW, Sinal CJ, Gonzalez FJ, Edwards PA 2004 Peroxisome proliferator-activated receptor-gamma coactivator 1 (PGC-1) regulates triglyceride metabolism by activation of the nuclear receptor FXR. Genes Dev 18:157–169
Savkur RS, Thomas JS, Bramlett KS, Gao Y, Michael LF, Burris TP2004 Ligand-dependent coactivation of the human bile acid receptor, FXR, by the peroxisome proliferator-activated receptor coactivator-1 (PGC-1). J Pharmacol Exp Ther 312:170–178
Kanaya E, Shiraki T, Jingami H 2004 The nuclear bile acid receptor FXR is activated by PGC-1 in a ligand-dependent manner. Biochem J 382:913–921
She P, Burgess SC, Shiota M, Flakoll P, Donahue EP, Malloy CR, Sherry AD, Magnuson MA 2003 Mechanisms by which liver-specific PEPCK knockout mice preserve euglycemia during starvation. Diabetes 52:1649–1654
Nakae J, Park BC, Accili D 1999 Insulin stimulates phosphorylation of the forkhead transcription factor FKHR on serine 253 through a Wortmannin-sensitive pathway. J Biol Chem 274:15982–15985
Koo SH, Satoh H, Herzig S, Lee CH, Hedrick S, Kulkarni R, Evans RM, Olefsky J, Montminy M 2004 PGC-1 promotes insulin resistance in liver through PPAR--dependent induction of TRB-3. Nat Med 10:530–534
Pineda Torra I, Claudel T, Duval C, Kosykh V, Fruchart JC, Staels B 2003 Bile acids induce the expression of the human peroxisome proliferator-activated receptor gene via activation of the farnesoid X receptor. Mol Endocrinol 17:259–272
Iynedjian PB 7 October 2004 Lack of evidence for a role of TRB3/NIPK as inhibitor of PKB-mediated insulin signaling in primary hepatocytes. Biochem J 10.142/BJ0041425(Bertrand Cariou, Daniel D)
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@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).
FIG. 1. Mechanism of transcriptional regulation by FXR. Bile acid-activated FXR heterodimerizes with the retinoid X receptor (RXR) and binds to FXRE, i.e. inverted repeat-1 (IR-1), on the promoter region of its target genes. In addition, FXR can bind to atypical FXREs as monomer. The regulation of target gene expression by FXR will affect bile acid and lipid metabolism.
The generation and characterization of FXR-deficient mice has established a critical role of FXR also in lipid metabolism, as these mice display elevated serum levels of triglycerides and high-density lipoprotein-cholesterol (3, 4). This action of FXR does not appear to be restricted to mice. In gallstone patients, serum high-density lipoprotein-cholesterol and triglyceride levels decrease upon treatment with bile acids (5), whereas interruption of the enterohepatic circulation of bile acids by sequestrants such as cholestyramine has the opposite effect in hypercholesterolemic subjects (6). These findings have identified FXR as a potential drug target for the treatment of dyslipoproteinemias.
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).
FIG. 2. Proposed mechanisms of PEPCK regulation by FXR. Hepatic glucose production in the liver is tightly controlled by hormones. Glucagon and catecholamines stimulate the cAMP pathway and cAMP response element binding protein (CREB), which activates PGC-1 gene expression. PGC-1 is then recruited to different transcription factors that bind to the promoter of gluconeogenic genes such as PEPCK. Glucocorticoids induce a specific interaction between PGC-1 and GR-activating PEPCK. Insulin represses PEPCK gene expression by inducing Akt-mediated Foxo1 phosphorylation. The activation of FXR by its natural ligands, i.e. bile acids, may both activate (blue) or repress (red) PEPCK gene expression. For more details, see the main text.
Stayrook et al. (7) suggest the regulation of PEPCK gene expression to be a crucial site of impact for bile acids on glucose metabolism. However, unresolved discrepancies exist between their results and those of available studies in the literature. Han et al. (12) recently reported that bile acid treatment directly enhances insulin signaling, leading to increased glycogen synthase activity, in rat primary hepatocytes. Enhanced insulin signaling should in theory result in PEPCK inhibition. Moreover, several studies reported that bile acid treatment, both in vitro in human hepatoma cell lines and in vivo in mice, decreases gene expression of PEPCK as well as other gluconeogenic genes such as glucose-6-phosphatase and fructose 1,6-bis phosphatase (13, 14, 15, 16). As a potential explanation, bile acids may influence these pathways in an FXR-independent manner by decreasing the activity of hepatocyte nuclear factor 4 (HNF-4), a positive regulator of PEPCK gene expression. Indeed, De Fabiani et al. (13) reported that treatment with the nonsteroidal specific FXR agonist GW4064 did not modify HNF-4 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.
References
Chiang JY 2002 Bile acid regulation of gene expression: roles of nuclear hormone receptors. Endocr Rev 23:443–463
Claudel T, Sturm E, Kuipers F, Staels B 2004 The farnesoid X receptor: a novel drug target? Expert Opin Investig Drugs 13:1135–1148
Sinal CJ, Tohkin M, Miyata M, Ward JM, Lambert G, Gonzalez FJ 2000 Targeted disruption of the nuclear receptor FXR/BAR impairs bile acid and lipid homeostasis. Cell 102:731–744
Kok T, Hulzebos CV, Wolters H, Havinga R, Agellon LB, Stellaard F, Shan B, Schwarz M, Kuipers F 2003 Enterohepatic circulation of bile salts in farnesoid X receptor-deficient mice: efficient intestinal bile salt absorption in the absence of ileal bile acid-binding protein. J Biol Chem 278:41930–41937
Leiss O, von Bergmann K 1982 Different effects of chenodeoxycholic acid and ursodeoxycholic acid on serum lipoprotein concentrations in patients with radiolucent gallstones. Scand J Gastroenterol 17:587–592
Molgaard J, von Schenck H, Olsson AG 1989 Comparative effects of simvastatin and cholestyramine in treatment of patients with hypercholesterolaemia. Eur J Clin Pharmacol 36:455–460
Stayrook KR, Bramlett KS, Savkur RS, Ficorilli J, Cook T, Christe ME, Michael LF, Burris TP 2005 Regulation of carbohydrate metabolism by the farnesoid X receptor. Endocrinology 146:984–991
Hanson RW, Reshef L 1997 Regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression. Annu Rev Biochem 66:581–611
Magnusson I, Rothman DL, Katz LD, Shulman RG, Shulman GI 1992 Increased rate of gluconeogenesis in type II diabetes mellitus. A 13C nuclear magnetic resonance study. J Clin Invest 90:1323–1327
Garg A, Grundy SM 1994 Cholestyramine therapy for dyslipidemia in non-insulin-dependent diabetes mellitus. A short-term, double-blind, crossover trial. Ann Intern Med 121:416–422
Duran-Sandoval D, Mautino G, Martin G, Percevault F, Barbier O, Fruchart JC, Kuipers F, Staels B 2004 Glucose regulates the expression of the farnesoid X receptor in liver. Diabetes 53:890–898
Han SI, Studer E, Gupta S, Fang Y, Qiao L, Li W, Grant S, Hylemon PB, Dent P 2004 Bile acids enhance the activity of the insulin receptor and glycogen synthase in primary rodent hepatocytes. Hepatology 39:456–463
De Fabiani E, Mitro N, Gilardi F, Caruso D, Galli G, Crestani M 2003 Coordinated control of cholesterol catabolism to bile acids and of gluconeogenesis via a novel mechanism of transcription regulation linked to the fasted-to-fed cycle. J Biol Chem 278:39124–39132
Borgius LJ, Steffensen KR, Gustafsson JA, Treuter E 2002 Glucocorticoid signaling is perturbed by the atypical orphan receptor and corepressor SHP. J Biol Chem 277:49761–49766
Yamagata K, Daitoku H, Shimamoto Y, Matsuzaki H, Hirota K, Ishida J, Fukamizu A 2004 Bile acids regulate gluconeogenic gene expression via small heterodimer partner-mediated repression of hepatocyte nuclear factor 4 and Foxo1. J Biol Chem 279:23158–23165
Kim JY, Kim HJ, Kim KT, Park YY, Seong HA, Park KC, Lee IK, Ha H, Shong M, Park SC, Choi HS 2004 Orphan nuclear receptor small heterodimer partner represses hepatocyte nuclear factor 3/Foxa transactivation via inhibition of its DNA binding. Mol Endocrinol 18:2880–2894
Puigserver P, Spiegelman BM 2003 Peroxisome proliferator-activated receptor-gamma coactivator 1 (PGC-1): transcriptional coactivator and metabolic regulator. Endocr Rev 24:78–90
Zhang Y, Castellani LW, Sinal CJ, Gonzalez FJ, Edwards PA 2004 Peroxisome proliferator-activated receptor-gamma coactivator 1 (PGC-1) regulates triglyceride metabolism by activation of the nuclear receptor FXR. Genes Dev 18:157–169
Savkur RS, Thomas JS, Bramlett KS, Gao Y, Michael LF, Burris TP2004 Ligand-dependent coactivation of the human bile acid receptor, FXR, by the peroxisome proliferator-activated receptor coactivator-1 (PGC-1). J Pharmacol Exp Ther 312:170–178
Kanaya E, Shiraki T, Jingami H 2004 The nuclear bile acid receptor FXR is activated by PGC-1 in a ligand-dependent manner. Biochem J 382:913–921
She P, Burgess SC, Shiota M, Flakoll P, Donahue EP, Malloy CR, Sherry AD, Magnuson MA 2003 Mechanisms by which liver-specific PEPCK knockout mice preserve euglycemia during starvation. Diabetes 52:1649–1654
Nakae J, Park BC, Accili D 1999 Insulin stimulates phosphorylation of the forkhead transcription factor FKHR on serine 253 through a Wortmannin-sensitive pathway. J Biol Chem 274:15982–15985
Koo SH, Satoh H, Herzig S, Lee CH, Hedrick S, Kulkarni R, Evans RM, Olefsky J, Montminy M 2004 PGC-1 promotes insulin resistance in liver through PPAR--dependent induction of TRB-3. Nat Med 10:530–534
Pineda Torra I, Claudel T, Duval C, Kosykh V, Fruchart JC, Staels B 2003 Bile acids induce the expression of the human peroxisome proliferator-activated receptor gene via activation of the farnesoid X receptor. Mol Endocrinol 17:259–272
Iynedjian PB 7 October 2004 Lack of evidence for a role of TRB3/NIPK as inhibitor of PKB-mediated insulin signaling in primary hepatocytes. Biochem J 10.142/BJ0041425(Bertrand Cariou, Daniel D)