Targeting Farnesoid X Receptor in Hepatic and Biliary Inflammatory Diseases

Abstract

Wang YD, Chen WD, Wang M, et al. (Department of Gene Regulation and Drug Discovery, Beckman Research Institute, City of Hope National Medical Center, Duarte, California). Farnesoid X receptor antagonizes nuclear factor kappaB in hepatic inflammatory response. Hepatology 2008;48:1632–1643.In chronic liver diseases, inflammation is a critical pathophysiologic step leading to cirrhosis and cancer. Inflammation is mediated by cytokines, chemokines, and enzymes produced by hematopoietic and epithelial cells. The expression of inflammation mediators is under the molecular control of the nuclear factor-κB (NF-κB). NF-κB is a heterodimeric transcription factor composed of a p65 (RelA) and a p52 subunit that is activated by proinflammatory stimuli, such as pathogen-associated molecular patterns (PAMPs). NF-κB has been shown to cross-talk with nuclear receptors (NRs) in the gut–liver axis to suppress inflammation. Marked liver inflammation is observed in farnesoid X receptor (FXR) ablated mice suggesting an interaction of NF-κB with FXR in the liver (Cancer Res 2007;67:863–867).Wang et al show that hepatocytes from FXR ablated (FXR−/−) mice display a higher proinflammatory gene induction to NF-κB activators (12-O-tetradecanoyl-phorbol-13-acetate [TPA], tumor necrosis factor [TNF]-α, and lipopolysaccharide [LPS]) than hepatocytes from wild-type (WT) mice. Conversely, they show that the pharmacologic activation of FXR lessens the induction of proinflammatory gene expression triggered by NF-κB in hepatoblastoma cells. The results presented by Wang et al suggest that activation of FXR may counteract the ability of NF-κB to transactivate proinflammatory genes, such as TNF-α, cyclo-oxygenase-2, interleukin (IL)-1 and IL-6. Moreover, FXR was shown to reduce inducible nitric oxide synthase gene expression in hepatocytes; however, in the latter case, the effect of FXR may be independent of NF-κB, as suggested by the data presented by Wang et al and previous published work (Proc Natl Acad Sci U S A 2006;103:3920–3925; Arterioscler Thromb Vasc Biol 2007;27:2606–2611).The authors further demonstrated the direct interaction of FXR with NF-κB by reporter plasmid transfection experiments, in which NF-κB activity was reduced in cells overexpressing FXR. In these experiments, FXR activation by the synthetic FXR agonist, GW4064, had no significant effect on the induction or amplification of the inhibition of NF-κB activity triggered by TPA, LPS or p65 overexpression. These results suggest that the inhibiting effect of FXR is mostly ligand independent and possibly linked to the sequestration of cytoplasmic NF-κB. However, against this assumption, the authors document an absence of influence of FXR on NF-κB nuclear translocation. Finally, the authors demonstrate that the ability of NF-κB to bind to specific DNA sequences is inhibited by FXR.FXR−/− mice challenged with LPS displayed both higher proinflammatory gene expression and liver necrosis when compared with WT mice. Conversely, infection of FXR−/− mice with a FXR expressing adenovirus diminishes the induction of proinflammatory gene expression triggered by LPS. These results therefore indicate that the cross-talk between FXR and NF-κB is observed in vivo.Beside proinflammatory genes, NF-κB also controls the expression of anti-apoptotic genes. Interestingly, FXR overexpression or activation had no consequence on the induction of anti-apoptotic genes elicited by NF-κB. The authors thus show that the cross-talk between NF-κB and FXR is restricted to the control of proinflammatory gene expression.CommentNRs are ligand-activated transcription factors that are involved in development and physiology. Because NRs are central in a wide range of metabolic pathways, they are increasingly recognized has potent therapeutic targets. In the NR receptor superfamily, FXR is attractive to the hepatologist because it is both highly expressed in the liver and involved in the adaptive response to cholestasis. FXR is activated by conjugated and unconjugated bile acids (BA) with chenodeoxycholic acid (CDCA) being the most potent natural agonist. The identification of potent synthetic ligands, such as GW4064 and 6-ethyl-CDCA, has recently opened the way for effective pharmacologic FXR targeting.FXR is the master regulator of BA homeostasis and enterohepatic circulation. In the intestine, FXR activation modulates the expression of specific BA transporters by repressing the human apical sodium BA transporter (ASBT) and inducing the basolateral organic solute transporters (OST-α and OST-β). Furthermore, activated FXR increases the expression of the fibroblast growth factor 15 (FGF-15), known as FGF-19 in humans. FGF-15, by binding to the type-4 FGF receptor, represses both ASBT in enterocytes and CYP7A1 in hepatocytes (Am J Physiol Gastrointest Liver Physiol 2008;295:G996–G1003; Cell Metab 2005;2:217–225). The transcription of the CYP7A1 gene is also repressed by hepatic FXR activation through the induction of the nuclear repressor SHP (Mol Cell 2000;6:517–526). In hepatocytes, FXR activation limits hepatic BA accumulation by negatively regulating the main basolateral BA uptake system, NTCP (Gastroenterology 2001;121:140–147), and by inducing the expression of the major canalicular efflux pump, the bile salt export pump (J Biol Chem 2001;276:28857–28865). FXR activation also increases expression of the bilirubin (MRP2) and of the phospholipid (MDR3) export pumps (J Clin Invest 2003;112:1678–1687; J Biol Chem 2002;277:2908–2915). In addition, FXR activation leads to the induction of hepatocyte-detoxifying activity through the transcriptional control of CYP3A4, SULT2A1, and UGT2B4 genes (Pharmacogenetics 2004;14:635–645; Gastroenterology 2003;124:1926–1940; J Biol Chem 2001;276:42549–42556). Alternative elimination routes aimed at reducing BA accumulation in hepatocytes, such as the basolateral efflux system Ost-α/-β are induced by FXR activation.Taken together, these data suggest that FXR agonists could be attractive therapeutic tools. Indeed, these agonists may potentiate the liver adaptive response to counteract the detrimental effect of cholestasis. This assumption may be true in the setting of hepatocellular cholestasis, such as estrogen-induced cholestasis, intrahepatic cholestasis of pregnancy, sepsis-induced cholestasis, and inflammation-induced cholestasis, as observed in the early stages of primary biliary cirrhosis (PBC) or primary sclerosing cholangitis (PSC). However, as shown in experimental models (Gastroenterology 2003;125:825–838), FXR activation may be detrimental in cholestatic diseases with an obstructive component, as in advanced PBC with ductopenia or in obliterative PSC. Indeed, FXR activation may increase the biliary pressure of the obstructed biliary tract by stimulating canalicular bile flow. This increase in biliary pressure could then lead to bile infarcts after the rupture of cholangioles.In addition to the control of BA homeostasis, several new and unexpected functions of FXR activation have been recently reported, including regulation of bacterial growth and translocation in the intestine and antimicrobial peptides production in biliary epithelial cells (Proc Natl Acad Sci U S A 2006;103:3920–3925; Gastroenterology 2009;136:1435–1443). The work of Wang et al further demonstrates that FXR activation protects against PAMPs-induced inflammation through anti–NF-κB properties.A hallmark feature of PBC and PSC is the inflammation observed in the vicinity of the bile ducts. Released inflammatory cytokines (TNF-α, IL-1β) or mediators (nitric oxide) are thought to alter hepatocellular and cholangiocyte functions and lead to cholestasis. In this context, activation of FXR by inhibiting NF-κB–mediated inflammation could help to abrogate cholestasis. Moreover, FXR activation could also counteract the deleterious effects of PAMPs and bacteria on the biliary tree by stimulating mucosal protection through induction of antimicrobial peptide expression (Gastroenterology 2009;136:1435–1443). Indeed, PAMPs accumulate in the biliary tree in PBC and PSC (Lancet 1989;2:1419–1422; J Hepatol 1998;29:409–416). In PBC, the immunoreactivity against PAMPs is blunted by UDCA and parallels the anticholestatic effects of the drug, suggesting a role of PAMPs in inducing inflammation and cholestasis (J Autoimmun 2004;22:153–158).In physiologic settings, PAMPs originating from the gut are delivered to the liver through the portal circulation. PAMPs are cleared from the liver by Kupffer cells, which facilitate their excretion in bile. Despite being the major clearance site of PAMPs, the normal biliary tract is devoid of inflammation. The tolerance of the biliary tract to the proinflammatory stimuli of PAMPs may involve the following mechanisms. First, PAMPs, such as LPS, are neutralized by alkaline phosphatase activity (Am J Physiol Gastrointest Liver Physiol 2006;290:G377–G385). Furthermore, β-defensins are secreted in response to infection through dedicated PAMP receptors, like the Toll-like receptors (J Immunol 2005;175:7447–7456). Last, but not least, FXR activation by BAs facilitates VPAC1-induced choleresis (Hepatology 2005;42:549–557), promotes α-crystallin expression (a putative defense molecule against oxidative stress; J Biol Chem 2005;280:31792–31800), and induces the expression of the antimicrobial peptide, cathelicidin (Gastroenterology 2009;136:1435–1443), in biliary epithelial cells.In pathophysiologic settings, PAMPs accumulate in the biliary tract and induce inflammation, as evidenced in PBC and PSC (Lancet 1989;2:1419–1422; J Hepatol 1998;29:409–416). Polymorphisms of genes, such as the vitamin D receptor (Hepatology 2002;35:126–131) and TNF-α (J Hepatol 1999;30:232–236; J Hepatol 1999;31:242–247), by either decreasing PAMPs processing or increasing the proinflammatory response, may account for the observed increase in PAMP-related inflammation. The impaired generation of a bicarbonate-rich choleresis may hinder the activity of antimicrobial peptides because of the high salt concentration of the bile. The absence of an alkaline pH in bile could also alter the activity of the biliary alkaline phosphatase, thus inhibiting its ability to dephosphorylate endotoxins and leading to stronger PAMPs-related inflammation. It was also suggested that monocytes from PBC patient are more responsive to Toll-like receptor activation and thus react to PAMPs by a pronounced proinflammatory response (Hepatology 2005;42:802–808; J Hepatol 2007;47:404–411). Finally, immune clearance by dedicated cells is defective in PBC (Gastroenterology 1991;101:1076–1082). Taken together, these observations may suggest that some features of PBC and PSC could arise from the inability of the liver to efficiently process PAMPs without excessive inflammation.In this context, compounds bearing the ability to (i) lessen hepatic inflammation, (ii) induce antibacterial defenses in the biliary tract, and (iii) increase bile alkalinization have evident therapeutic potential. Thus, FXR targeting appears as a promising tool in the treatment of inflammatory biliary diseases. In that regard, the FXR agonist 6-ethyl-CDCA is now being explored in Phase II clinical trial in PBC. The results are eagerly awaited. Wang YD, Chen WD, Wang M, et al. (Department of Gene Regulation and Drug Discovery, Beckman Research Institute, City of Hope National Medical Center, Duarte, California). Farnesoid X receptor antagonizes nuclear factor kappaB in hepatic inflammatory response. Hepatology 2008;48:1632–1643. In chronic liver diseases, inflammation is a critical pathophysiologic step leading to cirrhosis and cancer. Inflammation is mediated by cytokines, chemokines, and enzymes produced by hematopoietic and epithelial cells. The expression of inflammation mediators is under the molecular control of the nuclear factor-κB (NF-κB). NF-κB is a heterodimeric transcription factor composed of a p65 (RelA) and a p52 subunit that is activated by proinflammatory stimuli, such as pathogen-associated molecular patterns (PAMPs). NF-κB has been shown to cross-talk with nuclear receptors (NRs) in the gut–liver axis to suppress inflammation. Marked liver inflammation is observed in farnesoid X receptor (FXR) ablated mice suggesting an interaction of NF-κB with FXR in the liver (Cancer Res 2007;67:863–867). Wang et al show that hepatocytes from FXR ablated (FXR−/−) mice display a higher proinflammatory gene induction to NF-κB activators (12-O-tetradecanoyl-phorbol-13-acetate [TPA], tumor necrosis factor [TNF]-α, and lipopolysaccharide [LPS]) than hepatocytes from wild-type (WT) mice. Conversely, they show that the pharmacologic activation of FXR lessens the induction of proinflammatory gene expression triggered by NF-κB in hepatoblastoma cells. The results presented by Wang et al suggest that activation of FXR may counteract the ability of NF-κB to transactivate proinflammatory genes, such as TNF-α, cyclo-oxygenase-2, interleukin (IL)-1 and IL-6. Moreover, FXR was shown to reduce inducible nitric oxide synthase gene expression in hepatocytes; however, in the latter case, the effect of FXR may be independent of NF-κB, as suggested by the data presented by Wang et al and previous published work (Proc Natl Acad Sci U S A 2006;103:3920–3925; Arterioscler Thromb Vasc Biol 2007;27:2606–2611). The authors further demonstrated the direct interaction of FXR with NF-κB by reporter plasmid transfection experiments, in which NF-κB activity was reduced in cells overexpressing FXR. In these experiments, FXR activation by the synthetic FXR agonist, GW4064, had no significant effect on the induction or amplification of the inhibition of NF-κB activity triggered by TPA, LPS or p65 overexpression. These results suggest that the inhibiting effect of FXR is mostly ligand independent and possibly linked to the sequestration of cytoplasmic NF-κB. However, against this assumption, the authors document an absence of influence of FXR on NF-κB nuclear translocation. Finally, the authors demonstrate that the ability of NF-κB to bind to specific DNA sequences is inhibited by FXR. FXR−/− mice challenged with LPS displayed both higher proinflammatory gene expression and liver necrosis when compared with WT mice. Conversely, infection of FXR−/− mice with a FXR expressing adenovirus diminishes the induction of proinflammatory gene expression triggered by LPS. These results therefore indicate that the cross-talk between FXR and NF-κB is observed in vivo. Beside proinflammatory genes, NF-κB also controls the expression of anti-apoptotic genes. Interestingly, FXR overexpression or activation had no consequence on the induction of anti-apoptotic genes elicited by NF-κB. The authors thus show that the cross-talk between NF-κB and FXR is restricted to the control of proinflammatory gene expression. CommentNRs are ligand-activated transcription factors that are involved in development and physiology. Because NRs are central in a wide range of metabolic pathways, they are increasingly recognized has potent therapeutic targets. In the NR receptor superfamily, FXR is attractive to the hepatologist because it is both highly expressed in the liver and involved in the adaptive response to cholestasis. FXR is activated by conjugated and unconjugated bile acids (BA) with chenodeoxycholic acid (CDCA) being the most potent natural agonist. The identification of potent synthetic ligands, such as GW4064 and 6-ethyl-CDCA, has recently opened the way for effective pharmacologic FXR targeting.FXR is the master regulator of BA homeostasis and enterohepatic circulation. In the intestine, FXR activation modulates the expression of specific BA transporters by repressing the human apical sodium BA transporter (ASBT) and inducing the basolateral organic solute transporters (OST-α and OST-β). Furthermore, activated FXR increases the expression of the fibroblast growth factor 15 (FGF-15), known as FGF-19 in humans. FGF-15, by binding to the type-4 FGF receptor, represses both ASBT in enterocytes and CYP7A1 in hepatocytes (Am J Physiol Gastrointest Liver Physiol 2008;295:G996–G1003; Cell Metab 2005;2:217–225). The transcription of the CYP7A1 gene is also repressed by hepatic FXR activation through the induction of the nuclear repressor SHP (Mol Cell 2000;6:517–526). In hepatocytes, FXR activation limits hepatic BA accumulation by negatively regulating the main basolateral BA uptake system, NTCP (Gastroenterology 2001;121:140–147), and by inducing the expression of the major canalicular efflux pump, the bile salt export pump (J Biol Chem 2001;276:28857–28865). FXR activation also increases expression of the bilirubin (MRP2) and of the phospholipid (MDR3) export pumps (J Clin Invest 2003;112:1678–1687; J Biol Chem 2002;277:2908–2915). In addition, FXR activation leads to the induction of hepatocyte-detoxifying activity through the transcriptional control of CYP3A4, SULT2A1, and UGT2B4 genes (Pharmacogenetics 2004;14:635–645; Gastroenterology 2003;124:1926–1940; J Biol Chem 2001;276:42549–42556). Alternative elimination routes aimed at reducing BA accumulation in hepatocytes, such as the basolateral efflux system Ost-α/-β are induced by FXR activation.Taken together, these data suggest that FXR agonists could be attractive therapeutic tools. Indeed, these agonists may potentiate the liver adaptive response to counteract the detrimental effect of cholestasis. This assumption may be true in the setting of hepatocellular cholestasis, such as estrogen-induced cholestasis, intrahepatic cholestasis of pregnancy, sepsis-induced cholestasis, and inflammation-induced cholestasis, as observed in the early stages of primary biliary cirrhosis (PBC) or primary sclerosing cholangitis (PSC). However, as shown in experimental models (Gastroenterology 2003;125:825–838), FXR activation may be detrimental in cholestatic diseases with an obstructive component, as in advanced PBC with ductopenia or in obliterative PSC. Indeed, FXR activation may increase the biliary pressure of the obstructed biliary tract by stimulating canalicular bile flow. This increase in biliary pressure could then lead to bile infarcts after the rupture of cholangioles.In addition to the control of BA homeostasis, several new and unexpected functions of FXR activation have been recently reported, including regulation of bacterial growth and translocation in the intestine and antimicrobial peptides production in biliary epithelial cells (Proc Natl Acad Sci U S A 2006;103:3920–3925; Gastroenterology 2009;136:1435–1443). The work of Wang et al further demonstrates that FXR activation protects against PAMPs-induced inflammation through anti–NF-κB properties.A hallmark feature of PBC and PSC is the inflammation observed in the vicinity of the bile ducts. Released inflammatory cytokines (TNF-α, IL-1β) or mediators (nitric oxide) are thought to alter hepatocellular and cholangiocyte functions and lead to cholestasis. In this context, activation of FXR by inhibiting NF-κB–mediated inflammation could help to abrogate cholestasis. Moreover, FXR activation could also counteract the deleterious effects of PAMPs and bacteria on the biliary tree by stimulating mucosal protection through induction of antimicrobial peptide expression (Gastroenterology 2009;136:1435–1443). Indeed, PAMPs accumulate in the biliary tree in PBC and PSC (Lancet 1989;2:1419–1422; J Hepatol 1998;29:409–416). In PBC, the immunoreactivity against PAMPs is blunted by UDCA and parallels the anticholestatic effects of the drug, suggesting a role of PAMPs in inducing inflammation and cholestasis (J Autoimmun 2004;22:153–158).In physiologic settings, PAMPs originating from the gut are delivered to the liver through the portal circulation. PAMPs are cleared from the liver by Kupffer cells, which facilitate their excretion in bile. Despite being the major clearance site of PAMPs, the normal biliary tract is devoid of inflammation. The tolerance of the biliary tract to the proinflammatory stimuli of PAMPs may involve the following mechanisms. First, PAMPs, such as LPS, are neutralized by alkaline phosphatase activity (Am J Physiol Gastrointest Liver Physiol 2006;290:G377–G385). Furthermore, β-defensins are secreted in response to infection through dedicated PAMP receptors, like the Toll-like receptors (J Immunol 2005;175:7447–7456). Last, but not least, FXR activation by BAs facilitates VPAC1-induced choleresis (Hepatology 2005;42:549–557), promotes α-crystallin expression (a putative defense molecule against oxidative stress; J Biol Chem 2005;280:31792–31800), and induces the expression of the antimicrobial peptide, cathelicidin (Gastroenterology 2009;136:1435–1443), in biliary epithelial cells.In pathophysiologic settings, PAMPs accumulate in the biliary tract and induce inflammation, as evidenced in PBC and PSC (Lancet 1989;2:1419–1422; J Hepatol 1998;29:409–416). Polymorphisms of genes, such as the vitamin D receptor (Hepatology 2002;35:126–131) and TNF-α (J Hepatol 1999;30:232–236; J Hepatol 1999;31:242–247), by either decreasing PAMPs processing or increasing the proinflammatory response, may account for the observed increase in PAMP-related inflammation. The impaired generation of a bicarbonate-rich choleresis may hinder the activity of antimicrobial peptides because of the high salt concentration of the bile. The absence of an alkaline pH in bile could also alter the activity of the biliary alkaline phosphatase, thus inhibiting its ability to dephosphorylate endotoxins and leading to stronger PAMPs-related inflammation. It was also suggested that monocytes from PBC patient are more responsive to Toll-like receptor activation and thus react to PAMPs by a pronounced proinflammatory response (Hepatology 2005;42:802–808; J Hepatol 2007;47:404–411). Finally, immune clearance by dedicated cells is defective in PBC (Gastroenterology 1991;101:1076–1082). Taken together, these observations may suggest that some features of PBC and PSC could arise from the inability of the liver to efficiently process PAMPs without excessive inflammation.In this context, compounds bearing the ability to (i) lessen hepatic inflammation, (ii) induce antibacterial defenses in the biliary tract, and (iii) increase bile alkalinization have evident therapeutic potential. Thus, FXR targeting appears as a promising tool in the treatment of inflammatory biliary diseases. In that regard, the FXR agonist 6-ethyl-CDCA is now being explored in Phase II clinical trial in PBC. The results are eagerly awaited. NRs are ligand-activated transcription factors that are involved in development and physiology. Because NRs are central in a wide range of metabolic pathways, they are increasingly recognized has potent therapeutic targets. In the NR receptor superfamily, FXR is attractive to the hepatologist because it is both highly expressed in the liver and involved in the adaptive response to cholestasis. FXR is activated by conjugated and unconjugated bile acids (BA) with chenodeoxycholic acid (CDCA) being the most potent natural agonist. The identification of potent synthetic ligands, such as GW4064 and 6-ethyl-CDCA, has recently opened the way for effective pharmacologic FXR targeting. FXR is the master regulator of BA homeostasis and enterohepatic circulation. In the intestine, FXR activation modulates the expression of specific BA transporters by repressing the human apical sodium BA transporter (ASBT) and inducing the basolateral organic solute transporters (OST-α and OST-β). Furthermore, activated FXR increases the expression of the fibroblast growth factor 15 (FGF-15), known as FGF-19 in humans. FGF-15, by binding to the type-4 FGF receptor, represses both ASBT in enterocytes and CYP7A1 in hepatocytes (Am J Physiol Gastrointest Liver Physiol 2008;295:G996–G1003; Cell Metab 2005;2:217–225). The transcription of the CYP7A1 gene is also repressed by hepatic FXR activation through the induction of the nuclear repressor SHP (Mol Cell 2000;6:517–526). In hepatocytes, FXR activation limits hepatic BA accumulation by negatively regulating the main basolateral BA uptake system, NTCP (Gastroenterology 2001;121:140–147), and by inducing the expression of the major canalicular efflux pump, the bile salt export pump (J Biol Chem 2001;276:28857–28865). FXR activation also increases expression of the bilirubin (MRP2) and of the phospholipid (MDR3) export pumps (J Clin Invest 2003;112:1678–1687; J Biol Chem 2002;277:2908–2915). In addition, FXR activation leads to the induction of hepatocyte-detoxifying activity through the transcriptional control of CYP3A4, SULT2A1, and UGT2B4 genes (Pharmacogenetics 2004;14:635–645; Gastroenterology 2003;124:1926–1940; J Biol Chem 2001;276:42549–42556). Alternative elimination routes aimed at reducing BA accumulation in hepatocytes, such as the basolateral efflux system Ost-α/-β are induced by FXR activation. Taken together, these data suggest that FXR agonists could be attractive therapeutic tools. Indeed, these agonists may potentiate the liver adaptive response to counteract the detrimental effect of cholestasis. This assumption may be true in the setting of hepatocellular cholestasis, such as estrogen-induced cholestasis, intrahepatic cholestasis of pregnancy, sepsis-induced cholestasis, and inflammation-induced cholestasis, as observed in the early stages of primary biliary cirrhosis (PBC) or primary sclerosing cholangitis (PSC). However, as shown in experimental models (Gastroenterology 2003;125:825–838), FXR activation may be detrimental in cholestatic diseases with an obstructive component, as in advanced PBC with ductopenia or in obliterative PSC. Indeed, FXR activation may increase the biliary pressure of the obstructed biliary tract by stimulating canalicular bile flow. This increase in biliary pressure could then lead to bile infarcts after the rupture of cholangioles. In addition to the control of BA homeostasis, several new and unexpected functions of FXR activation have been recently reported, including regulation of bacterial growth and translocation in the intestine and antimicrobial peptides production in biliary epithelial cells (Proc Natl Acad Sci U S A 2006;103:3920–3925; Gastroenterology 2009;136:1435–1443). The work of Wang et al further demonstrates that FXR activation protects against PAMPs-induced inflammation through anti–NF-κB properties. A hallmark feature of PBC and PSC is the inflammation observed in the vicinity of the bile ducts. Released inflammatory cytokines (TNF-α, IL-1β) or mediators (nitric oxide) are thought to alter hepatocellular and cholangiocyte functions and lead to cholestasis. In this context, activation of FXR by inhibiting NF-κB–mediated inflammation could help to abrogate cholestasis. Moreover, FXR activation could also counteract the deleterious effects of PAMPs and bacteria on the biliary tree by stimulating mucosal protection through induction of antimicrobial peptide expression (Gastroenterology 2009;136:1435–1443). Indeed, PAMPs accumulate in the biliary tree in PBC and PSC (Lancet 1989;2:1419–1422; J Hepatol 1998;29:409–416). In PBC, the immunoreactivity against PAMPs is blunted by UDCA and parallels the anticholestatic effects of the drug, suggesting a role of PAMPs in inducing inflammation and cholestasis (J Autoimmun 2004;22:153–158). In physiologic settings, PAMPs originating from the gut are delivered to the liver through the portal circulation. PAMPs are cleared from the liver by Kupffer cells, which facilitate their excretion in bile. Despite being the major clearance site of PAMPs, the normal biliary tract is devoid of inflammation. The tolerance of the biliary tract to the proinflammatory stimuli of PAMPs may involve the following mechanisms. First, PAMPs, such as LPS, are neutralized by alkaline phosphatase activity (Am J Physiol Gastrointest Liver Physiol 2006;290:G377–G385). Furthermore, β-defensins are secreted in response to infection through dedicated PAMP receptors, like the Toll-like receptors (J Immunol 2005;175:7447–7456). Last, but not least, FXR activation by BAs facilitates VPAC1-induced choleresis (Hepatology 2005;42:549–557), promotes α-crystallin expression (a putative defense molecule against oxidative stress; J Biol Chem 2005;280:31792–31800), and induces the expression of the antimicrobial peptide, cathelicidin (Gastroenterology 2009;136:1435–1443), in biliary epithelial cells. In pathophysiologic settings, PAMPs accumulate in the biliary tract and induce inflammation, as evidenced in PBC and PSC (Lancet 1989;2:1419–1422; J Hepatol 1998;29:409–416). Polymorphisms of genes, such as the vitamin D receptor (Hepatology 2002;35:126–131) and TNF-α (J Hepatol 1999;30:232–236; J Hepatol 1999;31:242–247), by either decreasing PAMPs processing or increasing the proinflammatory response, may account for the observed increase in PAMP-related inflammation. The impaired generation of a bicarbonate-rich choleresis may hinder the activity of antimicrobial peptides because of the high salt concentration of the bile. The absence of an alkaline pH in bile could also alter the activity of the biliary alkaline phosphatase, thus inhibiting its ability to dephosphorylate endotoxins and leading to stronger PAMPs-related inflammation. It was also suggested that monocytes from PBC patient are more responsive to Toll-like receptor activation and thus react to PAMPs by a pronounced proinflammatory response (Hepatology 2005;42:802–808; J Hepatol 2007;47:404–411). Finally, immune clearance by dedicated cells is defective in PBC (Gastroenterology 1991;101:1076–1082). Taken together, these observations may suggest that some features of PBC and PSC could arise from the inability of the liver to efficiently process PAMPs without excessive inflammation. In this context, compounds bearing the ability to (i) lessen hepatic inflammation, (ii) induce antibacterial defenses in the biliary tract, and (iii) increase bile alkalinization have evident therapeutic potential. Thus, FXR targeting appears as a promising tool in the treatment of inflammatory biliary diseases. In that regard, the FXR agonist 6-ethyl-CDCA is now being explored in Phase II clinical trial in PBC. The results are eagerly awaited. ReplyGastroenterologyVol. 137Issue 2PreviewWe agree with the commentary by Drs Chignard and Poupon, particularly with respect to the point that FXR activation suppresses inducible nitric oxide synthase (iNOS) mRNA level induced by lipopolysaccharide (LPS) probably in an nuclear factor (NF)-κB–independent manner. LPS activates not only NF-κB, but also other transcriptional factors, such as AP-1. iNOS is a target gene of both NF-κB and AP-1, so it is possible that FXR activation may suppress iNOS expression induced by LPS in an AP-1 dependent manner. Full-Text PDF