Bile acids are amphiphilic signaling molecules involved in a number of pathophysiological processes including digestion and adsorption of fats, inflammation, cell proliferation, and cancer.1Li T. Apte U. Bile acid metabolism and signaling in cholestasis, inflammation, and cancer.Adv Pharmacol. 2015; 74: 263-302Google Scholar, 2Zhou H. Hylemon P.B. Bile acids are nutrient signaling hormones.Steroids. 2014; 86: 62-68Google Scholar, 3Hylemon P.B. Zhou H. Pandak W.M. Ren S. Gil G. Dent P. Bile acids as regulatory molecules.J Lipid Res. 2009; 50: 1509-1520Google Scholar Whereas an increase in the intracellular levels of bile acid is generally associated with pathogenic outcomes, bile acids are critical for multiple nondigestive physiological processes at normal levels. One such process is liver regeneration, where bile acid plays a critical regulatory role.4Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Bile acids, FGF15/19 and liver regeneration: from mechanisms to clinical applications.Biochim Biophys Acta Mol Basis Dis. 2018; 1864: 1326-1334Google Scholar, 5Merlen G. Ursic-Bedoya J. Jourdainne V. Kahale N. Glenisson M. Doignon I. Rainteau D. Tordjmann T. Bile acids and their receptors during liver regeneration: "dangerous protectors".Mol Aspects Med. 2017; 56: 25-33Google Scholar, 6Bhushan B. Apte U. Liver regeneration after acetaminophen hepatotoxicity: mechanisms and therapeutic opportunities.Am J Pathol. 2019; 189: 719-729Google Scholar, 7Bhushan B. Borude P. Edwards G. Walesky C. Cleveland J. Li F. Ma X. Apte U. Role of bile acids in liver injury and regeneration following acetaminophen overdose.Am J Pathol. 2013; 183: 1518-1526Google Scholar The liver is known for its remarkable capacity to regenerate after surgical resection or injury induced by viruses and chemicals.6Bhushan B. Apte U. Liver regeneration after acetaminophen hepatotoxicity: mechanisms and therapeutic opportunities.Am J Pathol. 2019; 189: 719-729Google Scholar,8Michalopoulos G.K. Bhushan B. Liver regeneration: biological and pathological mechanisms and implications.Nat Rev Gastroenterol Hepatol. 2021; 18: 40-55Google Scholar Having a robust hepatic regeneration response is critical for maintaining liver health and overall animal survival. Although the role of bile acid in stimulating liver regeneration has been known since late 1960s, detailed mechanistic studies in the past 3 decades have revealed involvement of various bile acid receptors and the role of gut-liver crosstalk in the regulation of liver regeneration by bile acid. These studies conclusively indicate that bile acids at physiological levels are involved in stimulating liver regeneration. Bile acids are derived from cholesterol in a CYP7A1-dependent process exclusively in the hepatocytes.9Chiang J.Y. Bile acids: regulation of synthesis.J Lipid Res. 2009; 50: 1955-1966Google Scholar They are secreted by hepatocytes into the biliary system in the form of bile, which is stored in the gallbladder. Upon stimulation by cholecystokinin, bile is released and enters the duodenum where it is required for the digestion and absorption of fats and fat-soluble nutrients. Further down the gut in the ileum, bile acids are reabsorbed and recycled to the liver via the portal circulation. Bile acid stimulates farnesoid X receptor (FXR)-dependent fibroblast growth factor (FGF)-15/19 production in the ileum, which plays an important role in regulation of total bile acid pool via FGF receptor 4–mediated suppression of cytochrome P450 family 7 subfamily A member 1 (CYP7A1).4Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Bile acids, FGF15/19 and liver regeneration: from mechanisms to clinical applications.Biochim Biophys Acta Mol Basis Dis. 2018; 1864: 1326-1334Google Scholar Because of this enterohepatic circulation, bile acid and bile acid–stimulated FGF-15/19 form the crux of the gut-liver signaling axis (Figure 1). One of the most common models to study liver regeneration is partial hepatectomy in rodents, where approximately two-thirds of liver is surgically removed, and the remnant liver is allowed to grow back.8Michalopoulos G.K. Bhushan B. Liver regeneration: biological and pathological mechanisms and implications.Nat Rev Gastroenterol Hepatol. 2021; 18: 40-55Google Scholar This is an excellent model to study liver regeneration because it is highly reproducible, is not complicated by toxicant-induced tissue injury, and is a good surrogate for liver regeneration after partial liver resection performed in humans. Studies using rats show that draining bile before partial hepatectomy surgery inhibits liver regeneration after partial hepatectomy. This is also observed in humans where external biliary drainage in patients undergoing partial hepatic resection results in decreased postoperative regeneration.10Naugler W.E. Bile acid flux is necessary for normal liver regeneration.PLoS One. 2014; 9e97426Google Scholar In addition, moderate bile acid supplementation stimulates liver regeneration and bile acid sequestration, and depletion by using the exchange resin cholestyramine results in attenuated liver regeneration after partial hepatectomy.11Huang W. Ma K. Zhang J. Qatanani M. Cuvillier J. Liu J. Dong B. Huang X. Moore D.D. Nuclear receptor-dependent bile acid signaling is required for normal liver regeneration.Science. 2006; 312: 233-236Google Scholar Finally, deletion or inhibition of transporters and enzymes involved in bile acid uptake and synthesis results in delayed liver regeneration. These data clearly demonstrate that bile acids are critical stimulators of liver regeneration.4Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Bile acids, FGF15/19 and liver regeneration: from mechanisms to clinical applications.Biochim Biophys Acta Mol Basis Dis. 2018; 1864: 1326-1334Google Scholar Bile acids signal via at least 4 nuclear receptors (FXR, VDR, CAR, and PXR) and 3 membrane receptors (TGR5, S1PR2, and CHRM2). Of these, bile acid signaling via FXR and TGR5 has been conclusively shown to be involved in liver regeneration.4Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Bile acids, FGF15/19 and liver regeneration: from mechanisms to clinical applications.Biochim Biophys Acta Mol Basis Dis. 2018; 1864: 1326-1334Google Scholar,5Merlen G. Ursic-Bedoya J. Jourdainne V. Kahale N. Glenisson M. Doignon I. Rainteau D. Tordjmann T. Bile acids and their receptors during liver regeneration: "dangerous protectors".Mol Aspects Med. 2017; 56: 25-33Google Scholar TGR5 is expressed on nonparenchymal cells in the liver including Kupffer cells, endothelial cells, and cholangiocytes. Deletion of TGR5 in the setting of partial hepatectomy results in increased hepatic necrosis because of the inability to compensate for increased bile acid load and subsequent induction of proinflammatory cytokines such as interleukin 6 in the regenerating liver. Furthermore, the TRG5 null mice show delayed liver regeneration.12Pean N. Doignon I. Garcin I. Besnard A. Julien B. Liu B. Branchereau S. Spraul A. Guettier C. Humbert L. Schoonjans K. Rainteau T. Tordjmann T. The receptor TGR5 protects the liver from bile acid overload during liver regeneration in mice.Hepatology. 2013; 58: 1451-1460Google Scholar FXR is expressed in both the hepatocytes in the liver and enterocytes in the ileum. The role of FXR in liver regeneration after partial hepatectomy has been studied by using both the whole-body and tissue-specific gene deletion mice. The global FXR null mice show severe impairment in liver regeneration leading to significant mortality.11Huang W. Ma K. Zhang J. Qatanani M. Cuvillier J. Liu J. Dong B. Huang X. Moore D.D. Nuclear receptor-dependent bile acid signaling is required for normal liver regeneration.Science. 2006; 312: 233-236Google Scholar Interestingly, hepatocyte-specific deletion of FXR produced only a moderate delay in liver regeneration without any mortality.13Borude P. Edwards G. Walesky C. Li F. Ma X. Kong B. Guo G.L. Apte U. Hepatocyte-specific deletion of farnesoid X receptor delays but does not inhibit liver regeneration after partial hepatectomy in mice.Hepatology. 2012; 56: 2344-2352Google Scholar A similar but smaller delay in liver regeneration was observed in mice with intestine-specific deletion of FXR.14Zhang L. Wang Y.D. Chen W.D. Wang X. Lou G. Liu N. Lin M. Forman B.M. Huang W. Promotion of liver regeneration/repair by farnesoid X receptor in both liver and intestine.Hepatology. 2012; 56: 2336-2343Google Scholar Hepatic FXR deletion resulted in decreased expression of the cell cycle transcription factor FoxM1B, whereas intestine-specific deletion resulted in decreased FGF15 expression after partial hepatectomy. Ectopic expression of FGF15 in intestine-specific FXR null mice restored liver regeneration after partial hepatectomy. These data revealed a role for FGF15 in liver regeneration beyond its role in regulation of bile acids. This was confirmed in further studies using FGF15 knockout mice, which showed extensive liver injury, delayed regeneration, and mortality similar to global FXR knockout mice.15Kong B. Huang J. Zhu Y. Li G. Williams J. Shen S. Aleksunes L.M. Richardson J.R. Apte U. Rudnick D.A. Guo G.L. Fibroblast growth factor 15 deficiency impairs liver regeneration in mice.Am J Physiol Gastrointest Liver Physiol. 2014; 306: G893-G902Google Scholar Whole body deletion of FXR results in complete disruption of the bile acid regulation system, leading to massive injury, lack of regeneration, and cancer. However, deletion only in either the liver or intestine maintains partial function by maintaining elements of the regulatory loop. These data indicate that bile acid signaling via FXR in the liver and intestine both plays a coordinated role in regulation of liver regeneration. The role of bile acid in the regulation of liver regeneration and cytoprotection has been also demonstrated in drug-induced liver injury models. The majority of these studies have been performed in conjunction with the partial hepatectomy studies mentioned above using CCl4 as an injury-inducing toxicant. In addition, a pro-regenerative role of bile acid has been also shown in the more clinically relevant model of acetaminophen overdose, the major cause of acute liver failure in the Western world.6Bhushan B. Apte U. Liver regeneration after acetaminophen hepatotoxicity: mechanisms and therapeutic opportunities.Am J Pathol. 2019; 189: 719-729Google Scholar,7Bhushan B. Borude P. Edwards G. Walesky C. Cleveland J. Li F. Ma X. Apte U. Role of bile acids in liver injury and regeneration following acetaminophen overdose.Am J Pathol. 2013; 183: 1518-1526Google Scholar Bile acids play a dual role in acetaminophen-induced acute liver failure such that they prevent development of injury and also stimulate liver regeneration. Depletion of endogenous bile acid by administration of the bile acid sequestrant cholestyramine markedly aggravates acetaminophen-induced liver injury in mice.7Bhushan B. Borude P. Edwards G. Walesky C. Cleveland J. Li F. Ma X. Apte U. Role of bile acids in liver injury and regeneration following acetaminophen overdose.Am J Pathol. 2013; 183: 1518-1526Google Scholar,16Wang Y. Li J. Matye D. Zhang Y. Dennis K. Ding W.X. Li T. Bile acids regulate cysteine catabolism and glutathione regeneration to modulate hepatic sensitivity to oxidative injury.JCI Insight. 2018; 3Google Scholar In addition, supplementation of cholic acid in the diet has a reverse effect by causing delayed development of liver injury after acetaminophen overdose in mice. The protective effects of endogenous bile acids during acetaminophen-induced acute liver injury are possibly due to their role in regulation of hepatic glutathione replenishment.7Bhushan B. Borude P. Edwards G. Walesky C. Cleveland J. Li F. Ma X. Apte U. Role of bile acids in liver injury and regeneration following acetaminophen overdose.Am J Pathol. 2013; 183: 1518-1526Google Scholar,16Wang Y. Li J. Matye D. Zhang Y. Dennis K. Ding W.X. Li T. Bile acids regulate cysteine catabolism and glutathione regeneration to modulate hepatic sensitivity to oxidative injury.JCI Insight. 2018; 3Google Scholar Glutathione is an important antioxidant molecule that is depleted during acetaminophen-induced acute liver injury, leading to massive oxidative damage caused by acetaminophen toxic metabolites. Bile acid depletion via cholestyramine feeding in mice impairs hepatic glutathione regenerative capacity, which can be fully prevented by administration of a glutathione replenishing agent, N-acetylcysteine. Bile acids also might govern glutathione homeostasis by inhibiting the cysteine dioxygenase type-1–mediated cysteine catabolic pathway via an FXR–dependent mechanism because cysteine is a rate-limiting precursor in glutathione synthesis.16Wang Y. Li J. Matye D. Zhang Y. Dennis K. Ding W.X. Li T. Bile acids regulate cysteine catabolism and glutathione regeneration to modulate hepatic sensitivity to oxidative injury.JCI Insight. 2018; 3Google Scholar The protective role of bile acid in acetaminophen-induced acute liver failure may be, at least in part, due to FGF15 signaling. The FGF15 null mice display higher susceptibility to acetaminophen-induced liver injury.17Huang M. Williams J. Kong B. Zhu Y. Li G. Zhu Z. Guo G.L. Fibroblast growth factor 15 deficiency increases susceptibility but does not improve repair to acetaminophen-induced liver injury in mice.Dig Liver Dis. 2018; 50: 175-180Google Scholar Furthermore, treatment with engineered FGF19, which is the human homologue of FGF15, can drastically decrease liver injury after both sub-lethal and lethal overdose of acetaminophen in mice.18Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Corrales F.J. Baulies A. García-Ruiz C. Fernandez-Checa J.C. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Engineered fibroblast growth factor 19 protects from acetaminophen-induced liver injury and stimulates aged liver regeneration in mice.Cell Death Dis. 2017; 8e3083Google Scholar In fact, engineered FGF19 decreases liver injury even after late administration after acetaminophen overdose in mice, when N-acetylcysteine (the standard therapy for acetaminophen intoxication) is known to be ineffective, which demonstrates its potential therapeutic applicability. FGF15/19 might play a role in bile acid–driven regulation of glutathione homeostasis during acetaminophen-induced liver injury because treatment with engineered FGF19 can accelerate recovery of hepatic glutathione levels after acetaminophen overdose in mice.18Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Corrales F.J. Baulies A. García-Ruiz C. Fernandez-Checa J.C. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Engineered fibroblast growth factor 19 protects from acetaminophen-induced liver injury and stimulates aged liver regeneration in mice.Cell Death Dis. 2017; 8e3083Google Scholar Apart from the direct protection from development of acute liver injury, bile acids are also involved in promoting recovery after liver injury by inducing liver regeneration. This is evident from our previous study showing that cholic acid feeding in mice can result in faster and stronger regenerative response to acetaminophen-induced hepatotoxicity, leading to early regression of injury and recovery.7Bhushan B. Borude P. Edwards G. Walesky C. Cleveland J. Li F. Ma X. Apte U. Role of bile acids in liver injury and regeneration following acetaminophen overdose.Am J Pathol. 2013; 183: 1518-1526Google Scholar The bile acid–driven higher proliferative response in hepatocytes in the acetaminophen model can be attributed to its regulation of cyclin D1, which is a critical regulator that governs entry into cell cycle. Whereas cholic acid feeding causes rapid and strong induction of cyclin D1 after acetaminophen overdose in mice, cholestyramine-driven bile acid depletion causes delayed and inhibited induction of cyclin D1.7Bhushan B. Borude P. Edwards G. Walesky C. Cleveland J. Li F. Ma X. Apte U. Role of bile acids in liver injury and regeneration following acetaminophen overdose.Am J Pathol. 2013; 183: 1518-1526Google Scholar FGF15 also contributes to bile acid–driven improved liver regeneration after acetaminophen overdose in mice. Treatment with engineered FGF19 not only attenuates liver injury but also boosts liver regeneration in the murine acetaminophen overdose model.18Alvarez-Sola G. Uriarte I. Latasa M.U. Jimenez M. Barcena-Varela M. Santamaria E. Urtasun R. Rodriguez-Ortigosa C. Prieto J. Corrales F.J. Baulies A. García-Ruiz C. Fernandez-Checa J.C. Berraondo P. Fernandez-Barrena M.G. Berasain C. Avila M.A. Engineered fibroblast growth factor 19 protects from acetaminophen-induced liver injury and stimulates aged liver regeneration in mice.Cell Death Dis. 2017; 8e3083Google Scholar In fact, engineered FGF19 treatment improves liver regeneration and survival even after severe acetaminophen overdose that normally results in failed spontaneous regeneration and high mortality. However, it is not clear whether this is due to a direct pro-regenerative effect of FGF19 or secondary to attenuated liver injury because FGF15 null mice do not show any alteration of liver regeneration compared with wild-type mice after treatment with equitoxic doses of acetaminophen.17Huang M. Williams J. Kong B. Zhu Y. Li G. Zhu Z. Guo G.L. Fibroblast growth factor 15 deficiency increases susceptibility but does not improve repair to acetaminophen-induced liver injury in mice.Dig Liver Dis. 2018; 50: 175-180Google Scholar In summary, the evidence for pro-regenerative role of bile acid in liver regeneration and recovery is strong and compelling. The data indicate that maintaining normal levels of bile acid and activity of bile acid signaling pathways is critical for proper liver regeneration. Depletion of bile acid below physiological levels seems to remove the protection against development of injury they afford and completely eliminates pro-mitogenic activity. Although the protective role of bile acids in acute drug-induced liver injury and regeneration is clear, the mechanisms by which bile acids regulate liver regeneration needs further investigations. This is of high significance considering strategies involving bile acids have significant therapeutic implications for the treatment of acute liver failure. Rebuttal to: The Benevolent Bile: Bile Acids as Stimulants of Liver RegenerationCellular and Molecular Gastroenterology and HepatologyPreviewBhushan and Apte highlight the beneficial effects of bile acids (BAs), such that increasing BA levels stimulate regeneration and recovery after partial hepatectomy (PH) or drug-induced acute liver injury. Although we agree with the protective role of BAs in liver regeneration, increasing evidence points to the fact that the composition of the BA pool is crucial for downstream signaling. Full-Text PDF Open Access