Nuclear Bile Acid Receptor Farnesoid X ReceptorMeets Nuclear Factor-�B: New Insights

into Hepatic Inflammation

See Article on Page 1621

When the orphan nuclear hormone receptor far-nesoid X receptor (FXR, NR1H4) wasadopted in 1999 through identification of bile

acids as its naturally activating ligands, nobody wouldhave predicted the central role FXR is now recognized toplay for a wide range of key metabolic and regenerativepathways in liver and extrahepatic tissues.1 Although ithas been known for decades that bile acids can modulatebile secretory function, lipid and glucose metabolism,mucosal protection against bacteria in the gut, inflamma-tion and immune function, as well as cellular proliferationand cancer formation,2 the underlying transcriptionalpathways involving FXR have not been unravelled untilrecently.1 FXR, which is predominantly expressed in liver,intestine, kidney and adrenal glands is an intranuclear bileacid sensing receptor and binds as heterodimer with thecommon heterodimerization partner retinoid X receptor(RXR) to distinct FXR response elements in target genepromoter regions and initiates gene transcription.1 In ad-dition to direct activation of gene transcription, FXR canalso indirectly inhibit gene expression via induction of thetranscriptional repressor short heterodimer partner(SHP).1

FXR protects liver integrity through control of bileacid homeostasis1 and hepatic regeneration.3 Mice lack-ing FXR (FXR�/�) show increased levels of inflammationand develop hepatocellular carcinomas (HCC), suggest-ing that FXR may also play a key role in the protectionagainst hepatic inflammation and inflammation-driven

carcinogenesis,4,5 although the underlying molecularmechanisms have so far remained unresolved. Moreover,FXR may protect the liver against gut-derived toxins andinflammation by maintaining gut integrity through in-duction of antibacterial factors such as angiogenin, induc-ible nitric oxide synthase (iNOS) and interleukin (IL)-18.6 This may explain the detrimental effects of biliaryobstruction with lack of bile acids in the gut, leading tointestinal bacterial overgrowth, mucosal injury followedby bacterial translocation across the injured mucosal bar-rier and a systemic inflammatory response.6

The work by Wang and coworkers in this issue ofHEPATOLOGY7 now expands our understanding of therole of FXR for hepatic inflammation and development ofinflammation-driven HCC by demonstrating that FXR isable to interact with nuclear factor-�B (NF-�B), thusadding FXR to the list of nuclear receptors reciprocallyinteracting with NF-�B. NF-�B is a key transcriptionalregulator of the inflammatory response and cell prolifer-ation, which is rapidly activated in response to proinflam-matory stimuli such as bacterial lipopolysaccharide (LPS)and cytokines under various inflammatory conditions.8,9

Interactions of NF-�B with other nuclear receptors suchas the common nuclear receptor dimerization partnerRXR and the xenobiotic receptor pregnane X receptor(PXR) may explain reduced cytochrome p450-mediateddrug metabolism observed in inflammatory and infec-tious diseases.10-12 Conversely, activated PXR inhibits theNF-�B pathway which may contribute to immunosup-pressive effects of drug metabolism-inducing xenobiotics/drugs, such as the antibiotic rifampicin.12 In addition,osteoporosis and periarticular bone loss in inflammatoryarthritis have been linked to NF-�B mediated inhibitionof the vitamin D receptor.13 With the current experimen-tal work Wang et al. now demonstrate, that (1) livers fromFXR�/� mice are more susceptible to inflammatory (e.g.,LPS-induced) stress than wildtype animals, (2) FXR ago-nists, conversely, inhibit classical pro-inflammatoryNF-�B target genes (such as tumor necrosis factor �(TNF�), iNOS and cyclooxygenase-2) in hepatocytes,(3) FXR activation inhibits hepatocellular NF-�B activa-tion, and (4) inflammation-induced NF-�B activation re-ciprocally antagonizes FXR activity and FXR target geneexpression.7 Interestingly, FXR has already been linked tothe NF-�B pathway in cells other than hepatocytes. As

Abbreviations: FXR, farnesoid X receptor; IKK, inhibitor of kappa B kinase;iNOS, inducible nitric oxide synthase; IR, insulin resistance; NAFLD, nonalcoholicliver disease; NASH, nonalcoholic steatohepatitis; NF-�B, nuclear factor-kappa B;TNF�, tumor necrosis factor �.

Supported by grants P18613-B05 P19118-B05 and F3008-B05 (to M.T.)from the Austrian Science Foundation.

Address correspondence to: Michael Trauner, M.D., Professor of Medicine andMolecular Hepatology, Laboratory of Experimental and Molecular Hepatology,Division of Gastroenterology and Hepatology, Dept. of Internal Medicine, MedicalUniversity of Graz, Auenbruggerplatz 15, A-8036 Graz, Austria. E-mail:michael.trauner@meduni-graz.at; fax: (43) 316-385-3062.

Copyright © 2008 by the American Association for the Study of Liver Diseases.Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/hep.22668Potential conflict of interest: Nothing to report.

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such, FXR has previously been shown to inhibit NF-�Bactivation in endothelial cells and vascular smooth musclecells, potentially via SHP-dependent mechanisms14

which could contribute to an anti-inflammatory net effectof FXR in liver. It is attractive to speculate, that SHP mayalso be involved in FXR-mediated inhibition of NF-�B inhepatocytes. Other possibilities such as physical protein–protein interactions and SUMOylation pathways, whichwere previously described for the inhibitory effects of theglucocorticoid receptor, the peroxisome-proliferator acti-vating receptor � and the liver X receptor on NF-�Bactivation, also remain to be explored.

Apart from the potential therapeutic relevance for FXRas novel anti-inflammatory therapeutic strategy, the find-ings of the current study7 may broaden our understandingof the multiple biological functions of bile acids — thenatural ligands of FXR.2 Several in part conflicting studieshave shown both pro- and anti-inflammatory propertiesof bile acids in hepatocytes and nonparenchymal cells ofthe liver. While previous studies reported pro-inflamma-tory effects of bile acids in macrophages, more convincingevidence now indicates that bile acids may in fact inhibitLPS-induced release of pro-inflammatory cytokines bymacrophages,15 thus contributing to the well-known im-munesuppressive effects of bile acid.5,16,17 In cells whichdo not express FXR (e.g., Kupffer cells/macrophages)these anti-inflammatory properties of bile acids may bemediated via the membrane-bound bile acid receptorTGR5.15 Notably, hepatocytes (expressing FXR) are notonly targets of cytokine action, but can also produceproinflammatory cytokines18-20 making them active play-ers in the inflammatory response. Hepatocellular accu-mulation of bile acids in cholestasis is clearly cytotoxicand thus contributes to inflammation.2 The current workby Wang et al.7 now raises the intriguing questionwhether lower (noncytotoxic) concentrations of bile acidscould in fact exert anti-inflammatory effects in hepato-cytes via FXR mediated inhibition of NF-�B.7 The inter-pretation of increased inflammation in FXR�/� mice is,however, complicated by the fact that these animals alsohave a severely disturbed hepatocellular bile acid ho-meostasis which per se could contribute to hepatocellulardamage and thereby inflammation. Of note, the findingsof this study by Wang and coworkers7stand in some con-trast to previous reports of bile acid-induced intracellularadhesion molecule-1 (ICAM-1) expression in humanhepatocytes via FXR.21 From a therapeutic perspective,this Gordian knot may be overcome by pharmacologicFXR agonists lacking the potential cytotoxicity of endog-enous bile acids.

What can we learn from this study by Wang et al.7

regarding the pathogenesis and treatment of inflamma-

tory liver diseases and inflammation-associated cancer?NF-�B activation in hepatocytes and nonparenchymalliver cells is a central event in various forms of liver injuryinduced by alcohol, fatty liver, cholestasis, hepatitis B andC virus infections9,22 Generally, activation of NF-�B isconsidered a protective response, which may limit cell lossby apoptosis and stimulates hepatocyte proliferation/re-generation by providing a cellular survival signal.9 In linewith the protective role of NF-�B, total inhibition ofNF-�B exclusively in hepatocytes leads to increased hepa-tocyte apoptosis, which triggers hepatic inflammation viaNF-�B expressing Kupffer cells, and finally results in thedevelopment of steatohepatitis and HCC.23 On the otherhand, persistently activated NF-�B in hepatocytes con-tributes to a chronic inflammatory condition19 and con-stitutive activation of NF-�B is considered as one of theearly key events involved in liver carcinogenesis.22 In linewith its disease aggravating role, partial inhibition ofNF-�B activation results in reduced liver injury after cy-tokine challenge and ischemia/reperfusion injury.24

These studies therefore indicate that NF-�B has a multi-tude of functions in the diseased liver and that the cellularsource and degree of NF-�B inhibition may critically de-termine the final outcome of liver damage. Of note,NF-�B regulated anti-apoptotic genes were not repressedby FXR activation in the current study by Wang et al.,7

suggesting that FXR selectively inhibits only the effects ofNF-�B on inflammation, although the molecular mech-anisms underlying this selectivity require further studies.Lack of FXR-mediated repression of NF-�B-dependentanti-apoptotic genes may have important implications forthe potential therapeutic use and safety of FXR agonists ininflammatory liver disorders, since major side effects oftotal NF-�B blockage may derive from deregulated pro-grammed cell death.23 Notably, FXR ligands rescueHepG2 cells from serum deprivation-induced apoptosis25

and the FXR antagonist guggulsterone enhances apopto-sis in Barrett’s esophagus cells26 raising the possibility thatFXR may directly inhibit apoptosis under other condi-tions.

One of the leading causes for liver inflammation andHCC development in adults are chronic viral infectionswith hepatitis B and C virus and both viruses are knownto activate hepatocellular NF-�B.22 One might speculatethat treatment with FXR ligands in these diseases mightreduce hepatic inflammation and, thus, the risk for devel-oping liver cirrhosis and cancer. However, it has to bekept in mind, that FXR activation by bile acids increasesvirus replication (at least hepatitis C genotype 1) and highbile acid levels are a negative predictor for the therapeuticoutcome.27 Nonalcoholic fatty liver disease (NAFLD) isthe most common cause of chronic liver disease in the

1384 WAGNER, ZOLLNER, AND TRAUNER HEPATOLOGY, November 2008

Western world and a rising cause of HCC. The key patho-physiological feature of NAFLD is insulin resistance (IR)and a hallmark in the progression from bland steatosis toclinical relevant inflammation in nonalcoholic steato-hepatitis (NASH) is the activation of NF-�B.28 SinceFXR is critically involved in insulin sensitivity, lipid andglucose homeostasis, it is also a potential key player inNAFLD.1 Recent reports have linked NF-�B to IR. Micewith constitutive hepatocellular overexpression of inhibi-tor of kappa B kinase 2 (IKK2), which activates NF-�Bvia the classical/canonical pathway develop chronic in-flammation leading to profound hepatocyte and moder-ate systemic IR.19 Conversely, deletion of IKK2 reversesIR, making NF-�B inhibition via FXR agonists also apossible strategy for the treatment of NASH.18 Pilot stud-ies using a pharmacological pan-IKK2 inhibitor in obeseand IR-mice were able to block liver steatosis, IR, andNASH.29 Collectively, these studies suggest that the useof FXR agonists for the treatment of NASH could have adual beneficial effect through direct metabolic effects ofFXR target genes and inhibiting chronic inflammation viablocking the NF-�B pathway. However, one has to keepin mind that targeting NF-�B via FXR agonists will limitthe effects to FXR-expressing cells (i.e., hepatocytes,cholangiocytes, sinusoidal endothelial cells) and will notaffect other cells involved in inflammation and fibrosis(Kupffer cells, myofibroblasts, stellate cells).

Finally, FXR may not only be an active player ininflammation but is also a target of the inflammatoryresponse itself. LPS, TNF� and IL1� decrease FXRexpression, DNA binding and FXR target gene expres-sion during the acute phase response.30 This could fi-nally end up in a vicious circle where reduced FXRexpression results in less NF-�B inhibition, which aug-ments the initial inflammatory signal, inhibits apopto-sis and leads to HCC formation. Moreover, the currentstudy by Wang et al. now shows that NF-�B itselfinhibits FXR, which would further contribute to FXRreduction in inflammatory conditions and wouldtherefore enhance NF-�B activity.7 In humans, geneticdefects or variants of FXR contributing to developmentof HCC were so far not observed, but FXR is reducedin chronic (cholestatic) liver diseases.31

In summary, the study by Wang et al.7 has certainlyexpanded our understanding of FXR as key regulator ofhepatic (patho)physiology and potential pharmacologictarget. Apart from liver, these findings may also be rele-vant for extrahepatic tissues and organs linked to liverdisease (e.g., inflammatory bowel disease, atherosclerosis)(Fig. 1). The biggest challenge will be to translate thesefindings into clinical practice. Of note, inhibition ofNF-�B in cells expressing FXR may explain some of the

anti-inflammatory effects of FXR observed in experimen-tal studies in the liver and beyond (e.g., intestine) andcould also contribute to clinical effects of FXR agonistscurrently investigated in ongoing clinical trials.

MARTIN WAGNER

GERNOT ZOLLNER

MICHAEL TRAUNER

Laboratory of Experimental and Molecular Hepatology,Division of Gastroenterology and Hepatology,Department of Internal Medicine, Medical University ofGraz, Austria

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