Bile acid flux is necessary for normal liver regeneration

Molecular mechanisms in liver repair and regeneration: from physiology to therapeutics

Liver repair and regeneration are crucial physiological responses to hepatic injury and are orchestrated through intricate cellular and molecular networks. This review systematically delineates advancements in the field, emphasizing the essential roles played by diverse liver cell types. Their coordinated actions, supported by complex crosstalk within the liver microenvironment, are pivotal to enhancing regenerative outcomes. Recent molecular investigations have elucidated key signaling pathways involved in liver injury and regeneration. Viewed through the lens of metabolic reprogramming, these pathways highlight how shifts in glucose, lipid, and amino acid metabolism support the cellular functions essential for liver repair and regeneration. An analysis of regenerative variability across pathological states reveals how disease conditions influence these dynamics, guiding the development of novel therapeutic strategies and advanced techniques to enhance liver repair and regeneration. Bridging laboratory findings with practical applications, recent clinical trials highlight the potential of optimizing liver regeneration strategies. These trials offer valuable insights into the effectiveness of novel therapies and underscore significant progress in translational research. In conclusion, this review intricately links molecular insights to therapeutic frontiers, systematically charting the trajectory from fundamental physiological mechanisms to innovative clinical applications in liver repair and regeneration.

New insights into the regulation of bile acids synthesis during the early stages of liver regeneration: A human and experimental study

BACKGROUND AND AIMS

Liver regeneration is essential for the preservation of homeostasis and survival. Bile acids (BAs)-mediated signaling is necessary for liver regeneration, but BAs levels need to be carefully controlled to avoid hepatotoxicity. We studied the early response of the BAs-fibroblast growth factor 19 (FGF19) axis in healthy individuals undergoing hepatectomy for living donor liver transplant. We also evaluated BAs synthesis in mice upon partial hepatectomy (PH) and acute inflammation, focusing on the regulation of cytochrome-7A1 (CYP7A1), a key enzyme in BAs synthesis from cholesterol.

METHODS

Serum was obtained from twelve human liver donors. Mice underwent 2/3-PH or sham-operation. Acute inflammation was induced with bacterial lipopolysaccharide (LPS) in mice fed control or antoxidant-supplemented diets. BAs and 7α-hydroxy-4-cholesten-3-one (C4) levels were measured by HPLC-MS/MS; serum FGF19 by ELISA. Gene expression and protein levels were analyzed by RT-qPCR and western-blot.

RESULTS

Serum BAs levels increased after PH. In patients with more pronounced hypercholanemia, FGF19 concentrations transiently rose, while C4 levels (a readout of CYP7A1 activity) dropped 2 h post-resection in all cases. Serum BAs and C4 followed the same pattern in mice 1 h after PH, but C4 levels also dropped in sham-operated and LPS-treated animals, without marked changes in CYP7A1 protein levels. LPS-induced serum C4 decline was attenuated in mice fed an antioxidant-supplemented diet.

CONCLUSIONS

In human liver regeneration FGF19 upregulation may constitute a protective response from BAs excess during liver regeneration. Our findings suggest the existence of post-translational mechanisms regulating CYP7A1 activity, and therefore BAs synthesis, independent from CYP7A1/Cyp7a1 gene transcription.

Partial liver resection alters the bile salt-FGF19 axis in patients with perihilar cholangiocarcinoma: Implications for liver regeneration

BACKGROUND

Extended liver resection is the only treatment option for perihilar cholangiocarcinoma (pCCA). Bile salts and the gut hormone FGF19, both promoters of liver regeneration (LR), have not been investigated in patients undergoing resection for pCCA. We aimed to evaluate the bile salt-FGF19 axis perioperatively in pCCA and study its effects on LR.

METHODS

Plasma bile salts, FGF19, and C4 (bile salt synthesis marker) were assessed in patients with pCCA and controls (colorectal liver metastases), before and after resection on postoperative days (PODs) 1, 3, and 7. Hepatic bile salts were determined in intraoperative liver biopsies.

RESULTS

Partial liver resection in pCCA elicited a sharp decline in bile salt and FGF19 plasma levels on POD 1 and remained low thereafter, unlike in controls, where bile salts rose gradually. Preoperatively, suppressed C4 in pCCA normalized postoperatively to levels similar to those in the controls. The remnant liver volume and postoperative bilirubin levels were negatively associated with postoperative C4 levels. Furthermore, patients who developed postoperative liver failure had nearly undetectable C4 levels on POD 7. Hepatic bile salts strongly predicted hyperbilirubinemia on POD 7 in both groups. Finally, postoperative bile salt levels on day 7 were an independent predictor of LR.

CONCLUSIONS

Partial liver resection alters the bile salt-FGF19 axis, but its derailment is unrelated to LR in pCCA. Postoperative monitoring of circulating bile salts and their production may be useful for monitoring LR.

Combined transcriptome and metabolome analysis reveals breed-specific regulatory mechanisms in Dorper and Tan sheep

BACKGROUND

Dorper and Tan sheep are renowned for their rapid growth and exceptional meat quality, respectively. Previous research has provided evidence of the impact of gut microbiota on breed characteristics. The precise correlation between the gastrointestinal tract and peripheral organs in each breed is still unclear. Investigating the metabolic network of the intestinal organ has the potential to improve animal growth performance and enhance economic benefits through the regulation of intestinal metabolites.

RESULTS

In this study, we identified the growth advantage of Dorper sheep and the high fat content of Tan sheep. A transcriptome study of the brain, liver, skeletal muscle, and intestinal tissues of both breeds revealed 3,750 differentially expressed genes (DEGs). The genes PPARGC1A, LPL, and PHGDH were found to be highly expressed in Doper, resulting in the up-regulation of pathways related to lipid oxidation, glycerophospholipid metabolism, and amino acid anabolism. Tan sheep highly express the BSEP, LDLR, and ACHE genes, which up-regulate the pathways involved in bile transport and cholesterol homeostasis. Hindgut content analysis identified 200 differentially accumulated metabolites (DAMs). Purines, pyrimidines, bile acids, and fatty acid substances were more abundant in Dorper sheep. Based on combined gene and metabolite analyses, we have identified glycine, serine, and threonine metabolism, tryptophan metabolism, bile secretion, cholesterol metabolism, and neuroactive ligand-receptor interaction as key factors contributing to the differences among the breeds.

CONCLUSIONS

This study indicates that different breeds of sheep exhibit unique breed characteristics through various physiological regulatory methods. Dorper sheep upregulate metabolic signals related to glycine, serine, and threonine, resulting in an increase in purine and pyrimidine substances. This, in turn, promotes the synthesis of amino acids and facilitates body development, resulting in a faster rate of weight gain. Tan sheep accelerate bile transport, reduce bile accumulation in the intestine, and upregulate cholesterol homeostasis signals in skeletal muscles. This promotes the accumulation of peripheral and intramuscular fat, resulting in improved meat quality. This work adopts a joint analysis method of multi-tissue transcriptome and gut metabolome, providing a successful case for analyzing the mechanisms underlying the formation of various traits.

Post-hepatectomy liver failure: A timeline centered review

BACKGROUND

Post-hepatectomy liver failure (PHLF) is a leading cause of postoperative mortality after liver surgery. Due to its significant impact, it is imperative to understand the risk stratification and preventative strategies for PHLF. The main objective of this review is to highlight the role of these strategies in a timeline centered way around curative resection.

DATA SOURCES

This review includes studies on both humans and animals, where they addressed PHLF. A literature search was conducted across the Cochrane Library, Embase, MEDLINE/PubMed, and Web of Knowledge electronic databases for English language studies published between July 1997 and June 2020. Studies presented in other languages were equally considered. The quality of included publications was assessed using Downs and Black's checklist. The results were presented in qualitative summaries owing to the lack of studies qualifying for quantitative analysis.

RESULTS

This systematic review with 245 studies, provides insight into the current prediction, prevention, diagnosis, and management options for PHLF. This review highlighted that liver volume manipulation is the most frequently studied preventive measure against PHLF in clinical practice, with modest improvement in the treatment strategies over the past decade.

CONCLUSIONS

Remnant liver volume manipulation is the most consistent preventive measure against PHLF.

Postoperative infectious complications are associated with diminished overall survival of patients undergoing resection for colorectal liver metastases

INTRODUCTION

Postoperative infectious complications (PIC) remain one of the most common complications after surgery. The influence of PIC on long-term survival for patients after liver surgery for colorectal liver metastases (CRLM) needs further investigation.

METHODS

Data of patients who underwent liver resection for CRLM between 2012 and 2017 at the Department of Surgery, Campus Charité Mitte and Campus Virchow-Klinikum, Charité - Universitätsmedizin Berlin were evaluated. Overall survival (OS) was stratified according to the development of PIC. Independent predictors of PIC and diminished survival were identified using regression models.

RESULTS

Of 270 patients, eighty-four (31%) patients developed PIC including intraabdominal infections (n = 51, 61%), cholangitis (n = 5, 6%), pneumonia (n = 12, 14%), wound infections (n = 28, 33%), urinary tract infections (n = 5, 6%), or central line-associated bloodstream infections (n = 4, 5%). PIC were associated with a significantly diminished five-year OS (30% vs. 43%, p = 0.008). Age >65 years (p = 0.016, hazard ratio [HR] = 2.2, 95% confidence interval [CI] = 1.2-4.0), comorbidity (p = 0.019, HR [95% CI] = 2.4 [1.2-4.9]), simultaneous resection of primary tumor (p = 0.005, HR [95% CI] = 4.3 [1.6-11.9]), biliary drainage (p < 0.001, HR [95% CI] = 4.1 [2.0-8.5]), and length of procedure ≥272 min (p = 0.012, HR [95% CI] = 2.2 [1.2-4.1]) were independent predictors for the development of PIC. Body-mass index (BMI) > 30 kg/m2 (p = 0.002, HR [95% CI] = 2.4 [1.4-4.0]), postoperative major complications (p = 0.003, HR [95% CI] = 2.2 [1.3-3.8]), and 3- or 4-MRGN bacteria (p = 0.001, HR [95% CI] = 7.7 [2.2-27.3]) were independently associated with diminished OS.

CONCLUSIONS

PIC are associated with diminished OS after resection for CRLM. Age >65 years, comorbidities, simultaneous resection of the primary tumor, and biliary drainages were identified as independent risk factors for the development of PIC. High BMI, postoperative major morbidity and 3-/4-MRGN bacteria were independently predictive of worse OS. These factors need to be considered in perioperative management for patients with CRLM.

Liver Regeneration in Acute on Chronic Liver Failure

Liver regeneration is a multifaceted process by which the organ regains its original size and histologic organization. In recent decades, substantial advances have been made in our understanding of the mechanisms underlying regeneration following loss of hepatic mass. Liver regeneration in acute liver failure possesses several classic pathways, while also exhibiting unique differences in key processes such as the roles of differentiated cells and stem cell analogs. Here we summarize these unique differences and new molecular mechanisms involving the gut-liver axis, immunomodulation, and microRNAs with an emphasis on applications to the patient population through stem cell therapies and prognostication.

Interleukin-33 facilitates liver regeneration through serotonin-involved gut-liver axis

BACKGROUND AND AIMS

Insufficient liver regeneration causes post-hepatectomy liver failure and small-for-size syndrome. Identifying therapeutic targets to enhance hepatic regenerative capacity remains urgent. Recently, increased IL-33 was observed in patients undergoing liver resection and in mice after partial hepatectomy (PHx). The present study aims to investigate the role of IL-33 in liver regeneration after PHx and to elucidate its underlying mechanisms.

APPROACH AND RESULTS

We performed PHx in IL-33 -/- , suppression of tumorigenicity 2 (ST2) -/- , and wild-type control mice, and found deficiency of IL-33 or its receptor ST2 delayed liver regeneration. The insufficient liver regeneration could be normalized in IL-33 -/- but not ST2 -/- mice by recombinant murine IL-33 administration. Furthermore, we observed an increased level of serotonin in portal blood from wild-type mice, but not IL-33 -/- or ST2 -/- mice, after PHx. ST2 deficiency specifically in enterochromaffin cells recapitulated the phenotype of delayed liver regeneration observed in ST2 -/- mice. Moreover, the impeded liver regeneration in IL-33 -/- and ST2 -/- mice was restored to normal levels by the treatment with (±)-2,5-dimethoxy-4-iodoamphetamine, which is an agonist of the 5-hydroxytrytamine receptor (HTR)2A. Notably, in vitro experiments demonstrated that serotonin/HTR2A-induced hepatocyte proliferation is dependent on p70S6K activation.

CONCLUSIONS

Our study identified that IL-33 is pro-regenerative in a noninjurious model of liver resection. The underlying mechanism involved IL-33/ST2-induced increase of serotonin release from enterochromaffin cells to portal blood and subsequent HTR2A/p70S6K activation in hepatocytes by serotonin. The findings implicate the potential of targeting the IL-33/ST2/serotonin pathway to reduce the risk of post-hepatectomy liver failure and small-for-size syndrome.

Recent Advances in the Digestive, Metabolic and Therapeutic Effects of Farnesoid X Receptor and Fibroblast Growth Factor 19: From Cholesterol to Bile Acid Signaling

Bile acids (BA) are amphiphilic molecules synthesized in the liver (primary BA) starting from cholesterol. In the small intestine, BA act as strong detergents for emulsification, solubilization and absorption of dietary fat, cholesterol, and lipid-soluble vitamins. Primary BA escaping the active ileal re-absorption undergo the microbiota-dependent biotransformation to secondary BA in the colon, and passive diffusion into the portal vein towards the liver. BA also act as signaling molecules able to play a systemic role in a variety of metabolic functions, mainly through the activation of nuclear and membrane-associated receptors in the intestine, gallbladder, and liver. BA homeostasis is tightly controlled by a complex interplay with the nuclear receptor farnesoid X receptor (FXR), the enterokine hormone fibroblast growth factor 15 (FGF15) or the human ortholog FGF19 (FGF19). Circulating FGF19 to the FGFR4/β-Klotho receptor causes smooth muscle relaxation and refilling of the gallbladder. In the liver the binding activates the FXR-small heterodimer partner (SHP) pathway. This step suppresses the unnecessary BA synthesis and promotes the continuous enterohepatic circulation of BAs. Besides BA homeostasis, the BA-FXR-FGF19 axis governs several metabolic processes, hepatic protein, and glycogen synthesis, without inducing lipogenesis. These pathways can be disrupted in cholestasis, nonalcoholic fatty liver disease, and hepatocellular carcinoma. Thus, targeting FXR activity can represent a novel therapeutic approach for the prevention and the treatment of liver and metabolic diseases.

The role of gut microbiota in liver regeneration

The liver has unique regeneration potential, which ensures the continuous dependence of the human body on hepatic functions. As the composition and function of gut microbiota has been gradually elucidated, the vital role of gut microbiota in liver regeneration through gut-liver axis has recently been accepted. In the process of liver regeneration, gut microbiota composition is changed. Moreover, gut microbiota can contribute to the regulation of the liver immune microenvironment, thereby modulating the release of inflammatory factors including IL-6, TNF-α, HGF, IFN-γ and TGF-β, which involve in different phases of liver regeneration. And previous research have demonstrated that through enterohepatic circulation, bile acids (BAs), lipopolysaccharide, short-chain fatty acids and other metabolites of gut microbiota associate with liver and may promote liver regeneration through various pathways. In this perspective, by summarizing gut microbiota-derived signaling pathways that promote liver regeneration, we unveil the role of gut microbiota in liver regeneration and provide feasible strategies to promote liver regeneration by altering gut microbiota composition.

Re-thinking of T-tube use in whole liver transplantation: an analysis on the risk of delayed graft function

The liver-gut axis has been identified as crucial mediator of liver regeneration. Thus, the use of a T-tube in liver transplantation (LT), which interrupts the enterohepatic bile circulation, may potentially have a detrimental effect on the early allograft functional recovery. We retrospectively analyzed a cohort of 261 patients transplanted with a whole liver graft, with a duct-to-duct biliary anastomosis, who did not develop any surgical complication within postoperative day 14. Early allograft dysfunction (EAD) was defined according to the criteria of Olthoff et al. (EAD-O), and graded according to the Model for Early Allograft Function (MEAF) score. EAD-O developed in 24.7% of recipients and the median MEAF score was 4.0 [interquartile range 2.9-5.5]. Both MEAF and EAD predicted 90-day post-LT mortality. A T-tube was used in 49.4% of cases (n = 129). After a propensity score matching for donor age, cold and warm ischemia time, donor risk index, balance of risk score, Child-Pugh class C, and MELD score, the T-tube group showed a significantly higher prevalence of EAD-O and value of MEAF than the no-T-tube group (EAD-O: 29 [34.1%] vs 16 [19.0%], p = 0.027; MEAF 4.5 [3.5-5.7] vs 3.7 [2.9-5.0], p = 0.014). In conclusion, T-tube use in LT may be a risk factor for EAD and higher MEAF, irrespective of graft quality and severity of pre-LT liver disease.

Systems-level analysis of transcriptome reorganization during liver regeneration

Tissue homeostasis and regeneration depend on the reversible transitions between quiescence (G0) and proliferation. The liver has a remarkable capacity to regenerate after injury or resection by cell growth and...

Liver regeneration biology: Implications for liver tumour therapies

The liver has remarkable regenerative potential, with the capacity to regenerate after 75% hepatectomy in humans and up to 90% hepatectomy in some rodent models, enabling it to meet the challenge of diverse injury types, including physical trauma, infection, inflammatory processes, direct toxicity, and immunological insults. Current understanding of liver regeneration is based largely on animal research, historically in large animals, and more recently in rodents and zebrafish, which provide powerful genetic manipulation experimental tools. Whilst immensely valuable, these models have limitations in extrapolation to the human situation. In vitro models have evolved from 2-dimensional culture to complex 3 dimensional organoids, but also have shortcomings in replicating the complex hepatic micro-anatomical and physiological milieu. The process of liver regeneration is only partially understood and characterized by layers of complexity. Liver regeneration is triggered and controlled by a multitude of mitogens acting in autocrine, paracrine, and endocrine ways, with much redundancy and cross-talk between biochemical pathways. The regenerative response is variable, involving both hypertrophy and true proliferative hyperplasia, which is itself variable, including both cellular phenotypic fidelity and cellular trans-differentiation, according to the type of injury. Complex interactions occur between parenchymal and non-parenchymal cells, and regeneration is affected by the status of the liver parenchyma, with differences between healthy and diseased liver. Finally, the process of termination of liver regeneration is even less well understood than its triggers. The complexity of liver regeneration biology combined with limited understanding has restricted specific clinical interventions to enhance liver regeneration. Moreover, manipulating the fundamental biochemical pathways involved would require cautious assessment, for fear of unintended consequences. Nevertheless, current knowledge provides guiding principles for strategies to optimise liver regeneration potential.

Hepatic pseudolesions caused by alterations in intrahepatic hemodynamics

Hepatic pseudolesion may occur in contrast-enhanced computed tomography and magnetic resonance imaging due to the unique haemodynamic characteristics of the liver. The concept of hepatic arterial buffer response (HABR) has become mainstream for the understanding of the mechanism of the reciprocal effect between hepatic arterial and portal venous flow. And HABR is thought to be significantly related to the occurrence of the abnormal imaging findings on arterial phase of contrast enhanced images, such as hepatic arterial-portal vein shunt and transient hepatic attenuation difference, which mimic hypervascular tumor and may cause clinical problems. Third inflow to the liver also cause hepatic pseudolesion, and some of the cases may show histopathologic change such as focal hyperplasia, focal fatty liver, and focal sparing of fatty liver, and called pseudotumor. To understand these phenomena might be valuable for interpreting the liver imaging findings.

The Role of Farnesoid X Receptor in Accelerated Liver Regeneration in Rats Subjected to ALPPS

Background: the role of bile acid (BA)-induced farnesoid X receptor (Fxr) signaling in liver regeneration following associating liver partition and portal vein ligation for staged hepatectomy (ALPPS) was investigated in a rat model. Methods: Male Wistar rats underwent portal vein ligation (PVL) (n = 30) or ALPPS (n = 30). Animals were sacrificed pre-operatively and at 24, 48, 72, or 168 h after intervention. Regeneration rate, Ki67 index, hemodynamic changes in the hepatic circulation, and BA levels were assessed. Transcriptome analysis of molecular regulators involved in the Fxr signaling pathway, BA transport, and BA production was performed. Results: ALLPS induced more extensive liver regeneration (p < 0.001) and elevation of systemic and portal BA levels (p < 0.05) than PVL. The mRNA levels of proteins participating in hepatic Fxr signaling were comparable between the intervention groups. More profound activation of the intestinal Fxr pathway was observed 24 h after ALPPS compared to PVL. Conclusion: Our study elaborates on a possible linkage between BA-induced Fxr signaling and accelerated liver regeneration induced by ALPPS in rats. ALPPS could trigger liver regeneration via intestinal Fxr signaling cascades instead of hepatic Fxr signaling, thereby deviating from the mechanism of BA-mediated regeneration following one-stage hepatectomy.

Pregnancy and weaning regulate human maternal liver size and function

During pregnancy, the rodent liver undergoes hepatocyte proliferation and increases in size, followed by weaning-induced involution via hepatocyte cell death and stromal remodeling, creating a prometastatic niche. These data suggest a mechanism for increased liver metastasis in breast cancer patients with recent childbirth. It is unknown whether the human liver changes in size and function during pregnancy and weaning. In this study, abdominal imaging was obtained in healthy women at early and late pregnancy and postwean. During pregnancy time points, glucose production and utilization and circulating bile acids were measured. Independently of weight gain, most women's livers increased in size with pregnancy, then returned to baseline postwean. Putative roles for bile acids in liver growth and regression were observed. Together, the data support the hypothesis that the human liver is regulated by reproductive state with growth during pregnancy and volume loss postwean. These findings have implications for sex-specific liver diseases and for breast cancer outcomes.

A modified animal model of hepatic regeneration induced by hilar bile duct ligation

Mechanisms of the proliferation of liver are mainly studied in animal model of liver regeneration after partial hepatectomy (PH). However, the PH model involves complex regeneration mechanisms, including hemodynamic factors, cytokines, growth factors, and metabolites. Among liver metabolites, bile acid (BA) is a key signaling molecule that regulates liver regeneration. This study aimed to establish a new type of rapid liver hyperplasia model induced mainly by bile acid pathway through hepatoenteral circulation with hilar bile duct ligation (HBDL). We first established the HBDL model by ligating the bile duct of all hepatic lobes but the right lateral lobe in rabbits and compared with the PVL model and sham operation group. Changes in the liver lobe and hemodynamics were observed. Liver function and the bile acid level were also analyzed. Then we verified the HBDL model in mice. Liver function and the levels of bile acids and cytokines were tested. The protein and mRNA levels of FXR, FGF15, CYP7A1 and FoxM1b in liver tissue were also analyzed. After hilar ligation of the biliary tract, the unligated liver lobes proliferated significantly. Compared with those in the sham group, the volume and weight of the unligated right lateral lobe of the liver in the HBDL group and the PVL group increased significantly (P < 0.05). Transient liver function impairment occurred both in the HBDL group and PVL group in the rabbit model as well as the mouse models. The bile acid levels in the HBDL groups of the rabbit model and mouse model increased significantly within first week after surgery (P < 0.05). The immunohistochemistry results confirmed the proliferation of hepatocytes in the unligated liver lobe. Compared with those in the sham group, the levels of FXR, FGF15 and FoxM1b in the HBDL group were significantly increased (P < 0.05), while the expression of CYP7A1 was inhibited. Compared with those in the HBDL group, the postoperative hemodynamic changes in the PVL group were significant (P < 0.05). The levels of IL-6 and TNF-α in the HBDL group were higher than those in the sham group. The HBDL model is simple to establish and exhibits good surgical tolerance. The model has definite proliferative effect and strong specificity of bile acid pathway. This is an ideal animal model to study the mechanism of liver regeneration through bile acid pathway.

Longitudinal profiling of plasma and urine metabolites during liver regeneration in living liver donors

BACKGROUND

Knowledge of metabolic processes affected by major hepatectomy (MHx), and the metabolic pathways involved in liver regeneration and recovery of function, is limited and mainly derived from animal models. Assessment of restoration of hepatic function is essential in human living liver donors (LD).

METHODS

We used a targeted metabolomic approach to longitudinally quantify changes in plasma and urine biomarkers from healthy LD. The biomarkers were analyzed before MHx and at scheduled intervals up to 12 months thereafter.

RESULTS

Marked changes were found in the concentration of 15 primary and secondary plasma bile acids. Most significant changes occurred 2 days after MHx and persisted for up to 3 months. In addition, there were significant changes in acylcarnitine, phospholipid, and amino acid metabolism. The sum of aromatic amino acids and the Fischer ratio, both metabolic markers of liver damage, and the symmetrically demethylated arginine to arginine ratio, a marker of kidney function, were affected.

CONCLUSIONS

This is the first comprehensive longitudinal study investigating metabolic processes during recovery of liver function after MHx in LD. It provides further evidence of full restoration of metabolic processes 3 months after MHx and supports future investigation to understand how metabolic changes affect donors' hepatic function.

Bile Salt and FGF19 Signaling in the Early Phase of Human Liver Regeneration

The involvement of bile salt-fibroblast growth factor 19 (FGF19) signaling in human liver regeneration (LR) is not well studied. Therefore, we studied aspects of bile salt-FGF19 signaling shortly after liver resection in patients. We compared plasma bile salt and FGF19 levels in arterial, portal and hepatic venous blood, calculated venous-arterial differences (ΔVA), and determined hepatic transcript levels on two intra-operative time points: before (< 1 hour) and immediately after (> 2-3 hours) liver resection (i.e., following surgery). Postoperative bile salt and FGF19 levels were assessed on days 1, 2, and 3. LR was studied by computed tomography (CT)-liver volumetry. Following surgery, the liver, arterial, and portal bile salt levels were elevated (P < 0.05). Furthermore, an increased amount of bile salts was released in portal blood and extracted by the remnant liver (P < 0.05). Postoperatively, bile salt levels were elevated from day 1 onward (P < 0.001). For FGF19, intra-operative or postoperative changes of ΔVA or plasma levels were not observed. The bile salt-homeostatic regulator farnesoid X receptor (FXR) was markedly up-regulated following surgery (P < 0.001). Cell-cycle re-entry priming factors (interleukin 6 [IL-6], signal transducer and activator of transcription 3 [STAT3], and cJUN) were up-regulated following surgery and were positively correlated with FXR expression (P < 0.05). Postoperative hyperbilirubinemia was preceded by postsurgery low FXR and high Na+/Taurocholate cotransporting polypeptide (NTCP) expression in the remnant liver coupled with higher liver bile salt content (P < 0.05). Finally, bile salt levels on postoperative day 1 were an independent predictor of LR (P < 0.05). Conclusion: Systemic, portal, and liver bile salt levels are rapidly elevated after liver resection. Postoperative bile salts were positively associated with liver volume gain. In the studied time frame, FGF19 levels remained unaltered, suggesting that FGF19 plays a minor role in human LR. These findings indicate a more relevant role of bile salts in human LR.

Role of FGF15 in Hepatic Surgery in the Presence of Tumorigenesis: Dr. Jekyll or Mr. Hyde?

The pro-tumorigenic activity of fibroblast growth factor (FGF) 19 (FGF15 in its rodent orthologue) in hepatocellular carcinoma (HCC), as well as the unsolved problem that ischemia-reperfusion (IR) injury supposes in liver surgeries, are well known. However, it has been shown that FGF15 administration protects against liver damage and regenerative failure in liver transplantation (LT) from brain-dead donors without tumor signals, providing a benefit in avoiding IR injury. The protection provided by FGF15/19 is due to its anti-apoptotic and pro-regenerative properties, which make this molecule a potentially beneficial or harmful factor, depending on the disease. In the present review, we describe the preclinical models currently available to understand the signaling pathways responsible for the apparent controversial effects of FGF15/19 in the liver (to repair a damaged liver or to promote tumorigenesis). As well, we study the potential pharmacological use that has the activation or inhibition of FGF15/19 pathways depending on the disease to be treated. We also discuss whether FGF15/19 non-pro-tumorigenic variants, which have been developed for the treatment of liver diseases, might be promising approaches in the surgery of hepatic resections and LT using healthy livers and livers from extended-criteria donors.

Loss of Claudin-3 Impairs Hepatic Metabolism, Biliary Barrier Function, and Cell Proliferation in the Murine Liver

BACKGROUND & AIMS

Tight junctions in the liver are essential to maintain the blood-biliary barrier, however, the functional contribution of individual tight junction proteins to barrier and metabolic homeostasis remains largely unexplored. Here, we describe the cell type-specific expression of tight junction genes in the murine liver, and explore the regulation and functional importance of the transmembrane protein claudin-3 in liver metabolism, barrier function, and cell proliferation.

METHODS

The cell type-specific expression of hepatic tight junction genes is described using our mouse liver single-cell sequencing data set. Differential gene expression in Cldn3-/- and Cldn3+/+ livers was assessed in young and aged mice by RNA sequencing (RNA-seq), and hepatic tissue was analyzed for lipid content and bile acid composition. A surgical model of partial hepatectomy was used to induce liver cell proliferation.

RESULTS

Claudin-3 is a highly expressed tight junction protein found in the liver and is expressed predominantly in hepatocytes and cholangiocytes. The histology of Cldn3-/- livers showed no overt phenotype, and the canalicular tight junctions appeared intact. Nevertheless, by RNA-seq we detected a down-regulation of metabolic pathways in the livers of Cldn3-/- young and aged mice, as well as a decrease in lipid content and a weakened biliary barrier for primary bile acids, such as taurocholic acid, taurochenodeoxycholic acid, and taurine-conjugated muricholic acid. Coinciding with defects in the biliary barrier and lower lipid metabolism, there was a diminished hepatocyte proliferative response in Cldn3-/- mice after partial hepatectomy.

CONCLUSIONS

Our data show that, in the liver, claudin-3 is necessary to maintain metabolic homeostasis, retention of bile acids, and optimal hepatocyte proliferation during liver regeneration. The RNA-seq data set can be accessed at: https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE159914.

Bile Acids and GPBAR-1: Dynamic Interaction Involving Genes, Environment and Gut Microbiome

Bile acids (BA) are amphiphilic molecules synthesized in the liver from cholesterol. BA undergo continuous enterohepatic recycling through intestinal biotransformation by gut microbiome and reabsorption into the portal tract for uptake by hepatocytes. BA are detergent molecules aiding the digestion and absorption of dietary fat and fat-soluble vitamins, but also act as important signaling molecules via the nuclear receptor, farnesoid X receptor (FXR), and the membrane-associated G protein-coupled bile acid receptor 1 (GPBAR-1) in the distal intestine, liver and extra hepatic tissues. The hydrophilic-hydrophobic balance of the BA pool is finely regulated to prevent BA overload and liver injury. By contrast, hydrophilic BA can be hepatoprotective. The ultimate effects of BA-mediated activation of GPBAR-1 is poorly understood, but this receptor may play a role in protecting the remnant liver and in maintaining biliary homeostasis. In addition, GPBAR-1 acts on pathways involved in inflammation, biliary epithelial barrier permeability, BA pool hydrophobicity, and sinusoidal blood flow. Recent evidence suggests that environmental factors influence GPBAR-1 gene expression. Thus, targeting GPBAR-1 might improve liver protection, facilitating beneficial metabolic effects through primary prevention measures. Here, we discuss the complex pathways linked to BA effects, signaling properties of the GPBAR-1, mechanisms of liver damage, gene-environment interactions, and therapeutic aspects.

Liver Regeneration after Hepatectomy and Partial Liver Transplantation

The liver is a unique organ with an abundant regenerative capacity. Therefore, partial hepatectomy (PHx) or partial liver transplantation (PLTx) can be safely performed. Liver regeneration involves a complex network of numerous hepatotropic factors, cytokines, pathways, and transcriptional factors. Compared with liver regeneration after a viral- or drug-induced liver injury, that of post-PHx or -PLTx has several distinct features, such as hemodynamic changes in portal venous flow or pressure, tissue ischemia/hypoxia, and hemostasis/platelet activation. Although some of these changes also occur during liver regeneration after a viral- or drug-induced liver injury, they are more abrupt and drastic following PHx or PLTx, and can thus be the main trigger and driving force of liver regeneration. In this review, we first provide an overview of the molecular biology of liver regeneration post-PHx and -PLTx. Subsequently, we summarize some clinical conditions that negatively, or sometimes positively, interfere with liver regeneration after PHx or PLTx, such as marginal livers including aged or fatty liver and the influence of immunosuppression.

Enhanced alcoholic liver disease in mice with intestine-specific farnesoid X receptor deficiency

Alcoholic fatty liver disease (AFLD) is one of the major causes of liver morbidity and mortality worldwide. We have previously shown that whole-body, but not hepatocyte-specific, deficiency of farnesoid X receptor (FXR) in mice worsens AFLD, suggesting that extrahepatic FXR deficiency is critical for AFLD development. Intestinal FXR is critical in suppressing hepatic bile acid (BA) synthesis by inducing fibroblast growth factor 15 (FGF15) in mice and FGF19 in humans. We hypothesized that intestinal FXR is critical for reducing AFLD development in mice. To test this hypothesis, we compared the AFLD severity in wild type (WT) and intestine-specific Fxr knockout (FXR^Int-/-) mice following treatment with control or ethanol-containing diet. We found that FXR^Int-/- mice were more susceptible to ethanol-induced liver steatosis and inflammation, compared with WT mice. Ethanol treatment altered the expression of hepatic genes involved in lipid and BA homeostasis, and ethanol detoxification. Gut FXR deficiency increased intestinal permeability, likely due to reduced mucosal integrity, as revealed by decreased secretion of Mucin 2 protein and lower levels of E-cadherin protein. In summary, intestinal FXR may protect AFLD development by maintaining gut integrity.

Molecular Physiology of Bile Acid Signaling in Health, Disease, and Aging

Over the past two decades, bile acids (BAs) have become established as important signaling molecules that enable fine-tuned inter-tissue communication from the liver, their site of production, over the intestine, where they are modified by the gut microbiota, to virtually any organ, where they exert their pleiotropic physiological effects. The chemical variety of BAs, to a large extent determined by the gut microbiome, also allows for a complex fine-tuning of adaptive responses in our body. This review provides an overview of the mechanisms by which BA receptors coordinate several aspects of physiology and highlights new therapeutic strategies for diseases underlying pathological BA signaling.

Biliary Drainage Before and After Liver Resection for Perihilar Cholangiocarcinoma

Perihilar cholangiocarcinoma (PHC) is a rare tumor that requires surgical resection for a potential cure. The role of preoperative biliary drainage has long been debated, given its treatment of biliary sepsis and decompression of the future liver remnant (FLR), but high procedure-specific morbidity. The indications, methods, and outcomes for preoperative biliary drainage are discussed to serve as a guide for perioperative management of patients with resectable PHC. Multiple studies from the literature related to perihilar cholangiocarcinoma, biliary drainage, and management of the FLR were reviewed. Commonly employed preoperative biliary drainage includes endoscopic biliary stenting and percutaneous transhepatic biliary drainage. Drainage of the FLR remains controversial, with most experts recommending drainage of the only in patients with an FLR <50%. Biliary drainage for resectable PHC requires a patient-specific approach with careful determination of the FLR and balancing of potential morbidity with the benefits of drainage.

Different Forms of Ursolic Acid and Their Effect on Liver Regeneration

The aim of this study was to determine the effect of natural and encapsulated sources of ursolic acid on liver regeneration. Four ursolate sources were tested. Two forms of ursolic acid encapsulates were combined with cyclodextrins, i.e., gamma-CD (gCD) and beta-CD, and two natural sources were adjusted by homogenization (HAP) and micronization of apple peel using Jonagold apples. All ursolate forms were applied intragastrically in daily doses of 20 mg for 7 days. Laboratory rats were fed with standard laboratory diet. Further, gCD and MAP were also tested with a high-fat diet (6 weeks). Partial hepatectomy (PH) was performed 24 hours before the end of the experiment. The concentration of plasma hepatocyte growth factor (HGF) was determined with an immunoassay; simultaneously, the expression of HGF and CYP7A1 in the liver was quantified through qPCR. HGF expression and plasma levels were significantly increased 24 hours after PH in both the HAP (p=0.038) and HFgCD groups (p=0.036), respectively. The correlation between HGF expression and plasma values was significant (p=0.04). The positive effects on liver regeneration were found in both the gCD and HAP forms of ursolic acid, whose effects were confirmed through the upregulation of HGF.

Hepatoprotective impact of the bile acid receptor TGR5

During liver repair after injury, bile secretion has to be tightly modulated in order to preserve liver parenchyma from bile acid (BA)-induced injury. The mechanisms allowing the liver to maintain biliary homeostasis during repair after injury are not completely understood. Besides their historical role in lipid digestion, bile acids (BA) and their receptors constitute a signalling network with multiple impacts on liver repair, both stimulating regeneration and protecting the liver from BA overload. BA signal through nuclear (mainly Farnesoid X Receptor, FXR) and membrane (mainly G Protein-coupled BA Receptor 1, GPBAR-1 or TGR5) receptors to elicit a wide array of biological responses. While a great number of studies have been dedicated to the hepato-protective impact of FXR signalling, TGR5 is by far less explored in this context. Because the liver has to face massive and potentially harmful BA overload after partial ablation or destruction, BA-induced protective responses crucially contribute to spare liver repair capacities. Based on the available literature, the TGR5 BA receptor protects the remnant liver and maintains biliary homeostasis, mainly through the control of inflammation, biliary epithelial barrier permeability, BA pool hydrophobicity and sinusoidal blood flow. Mouse experimental models of liver injury reveal that in the lack of TGR5, excessive inflammation, leaky biliary epithelium and hydrophobic BA overload result in parenchymal insult and compromise optimal restoration of a functional liver mass. Translational perspectives are thus opened to target TGR5 with the aim of protecting the liver in the context of injury and BA overload.

Bile canaliculi remodeling activates YAP via the actin cytoskeleton during liver regeneration

The mechanisms of organ size control remain poorly understood. A key question is how cells collectively sense the overall status of a tissue. We addressed this problem focusing on mouse liver regeneration. Using digital tissue reconstruction and quantitative image analysis, we found that the apical surface of hepatocytes forming the bile canalicular network expands concomitant with an increase in F-actin and phospho-myosin, to compensate an overload of bile acids. These changes are sensed by the Hippo transcriptional co-activator YAP, which localizes to apical F-actin-rich regions and translocates to the nucleus in dependence of the integrity of the actin cytoskeleton. This mechanism tolerates moderate bile acid fluctuations under tissue homeostasis, but activates YAP in response to sustained bile acid overload. Using an integrated biophysical-biochemical model of bile pressure and Hippo signaling, we explained this behavior by the existence of a mechano-sensory mechanism that activates YAP in a switch-like manner. We propose that the apical surface of hepatocytes acts as a self-regulatory mechano-sensory system that responds to critical levels of bile acids as readout of tissue status.

Variation in the gut microbial community is associated with the progression of liver regeneration

AIM

To highlight a potential dynamic interaction between intestinal bacteria (IB) and metabolites that might contribute to liver regeneration (LR).

METHODS

Male Sprague-Dawley rats were subjected to surgical removal of two-thirds of the liver and samples were collected over a 14-day period. Intestinal community and metabolic profiles were characterized to establish their potential interactions during liver regeneration.

RESULTS

Partial hepatectomy caused fluctuating changes in the gut microbiome, which paralleled the biological processes of LR. Briefly, the enhanced cell proliferation occurring within 30-48 h was associated with a decreased ratio of Firmicutes to Bacteroidetes reflected by a reduction in Ruminococcaceae and Lachnospiraceae, and an increase in Bacteroidaceae, Rikenellaceae, and Porphyromonadaceae, which was indicative of a lean phenotype. The microbiota derived from rats at 12-24 h and 3-14 days were characterized by elevated F/B ratios, suggesting the differing energy extract behaviors of microbiota during the course of LR. Functional changes of the shifted microbiota revealed by PICRUSt software confirmed the pyrosequencing results. The microbiome derived from hour 12 rats showed overpresentation of metabolism-related modules. In contrast, the microbiome derived from day 2 rats was functionally unique in "replication and repair", "amino acid metabolism," and "nucleoid metabolism." Upon examining the dynamic pattern of metabolic response, the specific pathways, including glycerophospholipid metabolism, taurine, and hypotaurine metabolism, were identified to be attributable to the systemic alterations in LR-related metabolism. Moreover, our data indicated that several key functional bacteria were strongly related to perturbations of the above pathways.

CONCLUSION

Gut flora could play a central role in manipulating metabolic responses in LR.

FXR agonist obeticholic acid induces liver growth but exacerbates biliary injury in rats with obstructive cholestasis

Cholestasis impairs liver regeneration following partial liver resection (PHx). Bile acid receptor farnesoid X-receptor (FXR) is a key mediator of liver regeneration. The effects of FXR agonist obeticholic acid (OCA) on liver (re)growth were therefore studied in cholestatic rats. Animals underwent sham surgery or reversible bile duct ligation (rBDL). PHx with concurrent internal biliary drainage was performed 7 days after rBDL. Animals were untreated or received OCA (10 mg/kg/day) per oral gavage from rBDL until sacrifice. After 7 days of OCA treatment, dry liver weight increased in the rBDL + OCA group, indicating OCA-mediated liver growth. Enhanced proliferation in the rBDL + OCA group prior to PHx concurred with a rise in Ki67-positive hepatocytes, elevated hepatic Ccnd1 and Cdc25b expression, and an induction of intestinal fibroblast growth factor 15 expression. Liver regrowth after PHx was initially stagnant in the rBDL + OCA group, possibly due to hepatomegaly prior to PHx. OCA increased hepatobiliary injury markers during BDL, which was accompanied by upregulation of the bile salt export pump. There were no differences in histological liver injury. In conclusion, OCA induces liver growth in cholestatic rats prior to PHx but exacerbates biliary injury during cholestasis, likely by forced pumping of bile acids into an obstructed biliary tree.

Fibroblast Growth Factor 15-Dependent and Bile Acid-Independent Promotion of Liver Regeneration in Mice

The role of intestine-derived factors in promoting liver regeneration after partial hepatectomy (PHx) are not entirely known, but bile acids (BAs) and fibroblast growth factor 15 (Fgf15) that is highly expressed in the mouse ileum could promote hepatocyte proliferation. Fgf15 strongly suppresses the synthesis of BAs, and emerging evidence indicates that Fgf15 is important for liver regeneration. The mechanisms by which Fgf15 promotes liver regeneration are unclear, but Fgf15 may do so indirectly by reducing BA levels and/or directly by promoting cell proliferation. However, it remains undetermined whether these two mechanisms are independent or integrated. In this study, we aimed to clarify these relationships by generating Fgf15 Tet-Off, transgenic mice (Fgf15 Tg) that had very low BA levels as a result from overexpressed Fgf15-mediated suppression of BA synthesis. Compared with wild-type mice, the Fgf15 Tg mice showed increased hepatocyte proliferation even without surgery, and a further induction of the genes in cell-cycle progression after PHx. Moreover, overexpression of Fgf15 by adeno-associated virus (AAV)-Fgf15 transduction or treatment with the recombinant Fgf15 protein led to increased cell proliferation in vivo. Furthermore, Fgf15 Tg mice exhibited an earlier and greater activation of mitogen-activated protein kinase, signal transducer and activator of transcription 3, and NF-κB signaling pathways in the priming stage, and a disruption of the hippo signaling pathway in the termination stage of liver regeneration. Conclusion: Direct in vivo evidence demonstrates that Fgf15 is critical in stimulating the phases of priming and termination of liver regeneration that are critical for cell survival and liver-size determination, independent of BA levels. (Hepatology 2018; 00:000-000).

Bile acid homeostasis in gastrointestinal and metabolic complications of cystic fibrosis

With the improved treatment of the pulmonary complications of cystic fibrosis (CF), gastrointestinal problems have become more important in the morbidity in CF. A hallmark of the gastrointestinal phenotype of CF, apart from pancreatic insufficiency, is a disruption of bile acid homeostasis. Bile acid homeostasis is important for many gastrointestinal processes including fat absorption, inflammation, microbial composition, as well as regulation of whole body energy metabolism. This review describes the impairment of bile acid homeostasis in CF, its possible consequences for gastrointestinal and metabolic complications and its potential as a target for therapy.

Liver Regeneration in the Acute Liver Failure Patient

Liver regeneration after simple resection represents a unique process in which the organ returns to its original size and histologic structure. Over the past 30 years, there has been significant progress in elucidating the mechanisms associated with regeneration after loss of hepatic mass. Liver regeneration after acute liver failure shares several of these classical pathways. It differs, however, in key processes, including the role of both differentiated and stemlike cells. This article outlines these differences in addition to new molecular mechanisms, including immunomodulation, microRNAs, and the gut-liver axis. In addition, applications to the patient population, including prognostication and stem cell therapies, are explored.

Deciphering the role of hydrophobic and hydrophilic bile acids in angiogenesis using in vitro and in vivo model systems

Bile acids have emerged as strong signaling molecules capable of influencing various biological processes like inflammation, apoptosis, cancer progression and atherosclerosis depending on their chemistry. In the present study, we investigated the effect of major hydrophobic bile acids lithocholic acid (LCA) and deoxycholic acid (DCA) and hydrophilic bile acids cholic acid (CA) and chenodeoxycholic acid (CDCA) on angiogenesis. We employed human umbilical vein endothelial cells (HUVECs) and zebrafish embryos as model systems for studying the role of bile acids in angiogenesis. Our studies revealed that the hydrophilic CDCA enhanced ectopic vessel formation as observed by the increase in the number of sub-intestinal vessels (SIVs) in the zebrafish embryos. The pro-angiogenic role of CDCA was further corroborated by in vitro vessel formation studies performed with human umbilical vein endothelial cells (HUVECs), whereas the hydrophobic LCA reduced tubulogenesis and was toxic to the zebrafish embryos. We validated that CDCA enhances angiogenesis by increasing the expression of vascular growth factor receptors (VEGFR1 and VEGFR2) and matrix metalloproteinases (MMP9) and by decreasing the expression of adhesion protein vascular endothelial cadherin (VE-cadherin). Our work implicates that the nature of bile acids plays a critical role in dictating their biological functions and in regulating angiogenesis.

Bile acids, FGF15/19 and liver regeneration: From mechanisms to clinical applications

The liver has an extraordinary regenerative capacity rapidly triggered upon injury or resection. This response is intrinsically adjusted in its initiation and termination, a property termed the "hepatostat". Several molecules have been involved in liver regeneration, and among them bile acids may play a central role. Intrahepatic levels of bile acids rapidly increase after resection. Through the activation of farnesoid X receptor (FXR), bile acids regulate their hepatic metabolism and also promote hepatocellular proliferation. FXR is also expressed in enterocytes, where bile acids stimulate the expression of fibroblast growth factor 15/19 (FGF15/19), which is released to the portal blood. Through the activation of FGFR4 on hepatocytes FGF15/19 regulates bile acids synthesis and finely tunes liver regeneration as part of the "hepatostat". Here we review the experimental evidences supporting the relevance of the FXR-FGF15/19-FGFR4 axis in liver regeneration and discuss potential therapeutic applications of FGF15/19 in the prevention of liver failure. This article is part of a Special Issue entitled: Cholangiocytes in Health and Disease edited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Hepatostat: Liver regeneration and normal liver tissue maintenance

In contrast to all other organs, liver-to-body-weight ratio needs to be maintained always at 100% of what is required for body homeostasis. Adjustment of liver size to 100% of what is required for homeostasis has been called "hepatostat." Removal of a portion of any other organ is followed with local regeneration of a limited degree, but it never attempts to reach 100% of the original size. The complex mechanisms involved in this uniquely hepatic process encompass a variety of regenerative pathways that are specific to different types of injury. The most studied form of liver regeneration (LR) is that occurring after loss of hepatocytes in a single acute injury, such as rodent LR after two-thirds partial hepatectomy or administration of damaging chemicals (CCl4 , acetaminophen, etc.). Alternative regenerative pathways become activated when normal regeneration is thwarted and trigger the appearance of "progenitor" cells. Chronic loss of hepatocytes is associated with regenerative efforts characterized by continual hepatocyte proliferation and often has adverse consequences (development of cirrhosis or liver cancer). Even though a very few hepatocytes proliferate at any given time in normal liver, the mechanisms involved in the maintenance of liver weight by this slow process in the absence of liver injury are not as well understood. (Hepatology 2017;65:1384-1392).

Bile acids and their receptors during liver regeneration: "Dangerous protectors"

Tissue repair is orchestrated by a finely tuned interplay between processes of regeneration, inflammation and cell protection, allowing organisms to restore their integrity after partial loss of cells or organs. An important, although largely unexplored feature is that after injury and during liver repair, liver functions have to be maintained to fulfill the peripheral demand. This is particularly critical for bile secretion, which has to be finely modulated in order to preserve liver parenchyma from bile-induced injury. However, mechanisms allowing the liver to maintain biliary homeostasis during repair after injury are not completely understood. Besides cytokines and growth factors, bile acids (BA) and their receptors constitute an insufficiently explored signaling network during liver regeneration and repair. BA signal through both nuclear (mainly Farnesoid X Receptor, FXR) and membrane (mainly G Protein-coupled BA Receptor 1, GPBAR-1 or TGR5) receptors which distributions are large in the organism, and which activation elicits a wide array of biological responses. While a number of studies have been dedicated to FXR signaling in liver repair processes, TGR5 remains poorly explored in this context. Because of the massive and potentially harmful BA overload that faces the remnant liver after partial ablation or destruction, both BA-induced adaptive and proliferative responses may stand in a central position to contribute to the regenerative response. Based on the available literature, both BA receptors may act in synergy during the regeneration process, in order to protect the remnant liver and maintain biliary homeostasis, otherwise potentially toxic BA overload would result in parenchymal insult and compromise optimal restoration of a functional liver mass.

Obeticholic acid reduces bacterial translocation and inhibits intestinal inflammation in cirrhotic rats

BACKGROUND & AIMS

In advanced cirrhosis, gut bacterial translocation is the consequence of intestinal barrier disruption and leads to bacterial infection. Bile acid abnormalities in cirrhosis could play a role in the integrity of the intestinal barrier and the control of microbiota, mainly through the farnesoid X receptor. We investigated the long-term effects of the farnesoid X receptor agonist, obeticholic acid, on gut bacterial translocation, intestinal microbiota composition, barrier integrity and inflammation in rats with CCl4-induced cirrhosis with ascites.

METHODS

Cirrhotic rats received a 2-week course of obeticholic acid or vehicle starting once ascites developed. We then determined: bacterial translocation by mesenteric lymph node culture, ileum expression of antimicrobial peptides and tight junction proteins by qPCR, fecal albumin loss, enteric bacterial load and microbiota composition by qPCR and pyrosequencing of ileum mucosa-attached contents, and intestinal inflammation by cytometry of the inflammatory infiltrate.

RESULTS

Obeticholic acid reduced bacterial translocation from 78.3% to 33.3% (p<0.01) and upregulated the expression of the farnesoid X receptor-associated gene small heterodimer partner. Treatment improved ileum expression of antimicrobial peptides, angiogenin-1 and alpha-5-defensin, tight junction proteins zonulin-1 and occludin, and reduced fecal albumin loss and liver fibrosis. Enteric bacterial load normalized, and the distinctive mucosal microbiota of cirrhosis was reduced. Gut immune cell infiltration was reduced and inflammatory cytokine and Toll-like receptor 4 expression normalized.

CONCLUSIONS

In ascitic cirrhotic rats, obeticholic acid reduces gut bacterial translocation via several complementary mechanisms at the intestinal level. This agent could be used as an alternative to antibiotics to prevent bacterial infection in cirrhosis.

The role of bile salts in liver regeneration

A growing body of evidence has demonstrated that bile salts are important for liver regeneration following partial hepatectomy. The relative bile salt overload after partial liver resection causes activation of bile salt receptors in non-parenchymal (viz. the plasma membrane receptor TGR5) and parenchymal (viz. the intracellular receptor FXR) cells in the liver, thus, providing signals to the regenerative process. Impaired bile salt signaling in mice with genetic deficiency of Tgr5 or Fxr results in delayed liver regeneration after partial hepatectomy, and is accompanied by mortality in case of Fxr knock-out mice. Conversely, compensatory liver re-growth in hepatectomized mice can be stimulated by feeding of bile salts or alisol B 23-acetate, a natural triterpenoid agonist of Fxr. A large number of animal studies underscore the importance of strict maintenance of bile salt homeostasis for proper progression of liver regeneration. Both ileal and hepatic Fxr play a key role in regulation of bile salt homeostasis and, thus, preventing hepatotoxicity caused by excessive levels of bile salts. They further contribute to liver regeneration by induction of mitogenic factors. Agents that target bile salt receptors hold promise as drugs to stimulate liver regeneration in selected patients.

External biliary drainage following major liver resection for perihilar cholangiocarcinoma: impact on development of liver failure and biliary leakage

BACKGROUND

Preoperative biliary drainage is considered essential in perihilar cholangiocarcinoma (PHC) requiring major hepatectomy with biliary-enteric reconstruction. However, evidence for postoperative biliary drainage as to protect the anastomosis is currently lacking. This study investigated the impact of postoperative external biliary drainage on the development of post-hepatectomy biliary leakage and liver failure (PHLF).

METHODS

All patients who underwent major liver resection for suspected PHC between 2000 and 2015 were retrospectively analyzed. Biliary leakage and PHLF was defined as grade B or higher according to the International Study Group of Liver Surgery (ISGLS) criteria.

RESULTS

Eighty-nine out of 125 (71%) patients had postoperative external biliary drainage. PHLF was more prevalent in the drain group (29% versus 6%; P = 0.004). There was no difference in the incidence of biliary leakage (32% versus 36%). On multivariable analysis, postoperative external biliary drainage was identified as an independent risk factor for PHLF (Odds-ratio 10.3, 95% confidence interval 2.1-50.4; P = 0.004).

CONCLUSIONS

External biliary drainage following major hepatectomy for PHC was associated with an increased incidence of PHLF. It is therefore not recommended to routinely use postoperative external biliary drainage, especially as there is no evidence that this decreases the risk of biliary anastomotic leakage.

Mice Abundant in Muricholic Bile Acids Show Resistance to Dietary Induced Steatosis, Weight Gain, and to Impaired Glucose Metabolism

High endogenous production of, or treatment with muricholic bile acids, strongly reduces the absorption of cholesterol. Mice abundant in muricholic bile acids may therefore display an increased resistance against dietary induced weight gain, steatosis, and glucose intolerance due to an anticipated general reduction in lipid absorption. To test this hypothesis, mice deficient in steroid 12-alpha hydroxylase (Cyp8b1-/-) and therefore abundant in muricholic acids were monitored for 11 weeks while fed a high fat diet. Food intake and body and liver weights were determined, and lipids in liver, serum and feces were measured. Further, responses during oral glucose and intraperitoneal insulin tolerance tests were evaluated. On the high fat diet, Cyp8b1-/- mice displayed less weight gain compared to wildtype littermates (Cyp8b1+/+). In addition, liver enlargement with steatosis and increases in serum LDL-cholesterol were strongly attenuated in Cyp8b1-/- mice on high fat diet. Fecal excretion of cholesterol was increased and there was a strong trend for doubled fecal excretion of free fatty acids, while excretion of triglycerides was unaltered, indicating dampened lipid absorption. On high fat diet, Cyp8b1-/- mice also presented lower serum glucose levels in response to oral glucose gavage or to intraperitoneal insulin injection compared to Cyp8b1+/+. In conclusion, following exposure to a high fat diet, Cyp8b1-/- mice are more resistant against weight gain, steatosis, and to glucose intolerance than Cyp8b1+/+ mice. Reduced lipid absorption may in part explain these findings. Overall, the results suggest that muricholic bile acids may be beneficial against the metabolic syndrome.

Implications of microbiota and bile acid in liver injury and regeneration

Studies examining the mechanisms by which the liver incurs injury and then regenerates usually focus on factors and pathways directly within the liver, neglecting the signaling derived from the gut-liver axis. The intestinal content is rich in microorganisms as well as metabolites generated from both the host and colonizing bacteria. Through the gut-liver axis, this complex "soup" exerts an immense impact on liver integrity and function. This review article summarizes data published in the past 30 years demonstrating the signaling derived from the gut-liver axis in relation to liver injury and regeneration. Due to the intricate networks of implicated pathways as well as scarcity of available mechanistic data, it seems that nutrigenomic, metabolomics, and microbiota profiling approaches are warranted to provide a better understanding regarding the interplay and impact between nutrition, bacteria, and host response in influencing liver function and healing. Therefore elucidating the possible molecular mechanisms that link microbiota alteration to host physiological response and vice versa.

Protective Mechanisms of Hypothermia in Liver Surgery and Transplantation

Hepatic ischemia/reperfusion (I/R) injury is a side effect of major liver surgery that often cannot be avoided. Prolonged periods of ischemia put a metabolic strain on hepatocytes and limit the tolerable ischemia and preservation times during liver resection and transplantation, respectively. In both surgical settings, temporarily lowering the metabolic demand of the organ by reducing organ temperature effectively counteracts the negative consequences of an ischemic insult. Despite its routine use, the application of liver cooling is predicated on an incomplete understanding of the underlying protective mechanisms, which has limited a uniform and widespread implementation of liver-cooling techniques. This review therefore addresses how hypothermia-induced hypometabolism modulates hepatocyte metabolism during ischemia and thereby reduces hepatic I/R injury. The mechanisms underlying hypothermia-mediated reduction in energy expenditure during ischemia and the attenuation of mitochondrial production of reactive oxygen species during early reperfusion are described. It is further addressed how hypothermia suppresses the sterile hepatic I/R immune response and preserves the metabolic functionality of hepatocytes. Lastly, a summary of the clinical status quo of the use of liver cooling for liver resection and transplantation is provided.

Fibroblast Growth Factor Signaling Controls Liver Size in Mice With Humanized Livers

BACKGROUND & AIMS

The ratio of liver size to body weight (hepatostat) is tightly controlled, but little is known about how the physiologic functions of the liver help determine its size. Livers of mice repopulated with human hepatocytes (humanized livers) grow to larger than normal; the human hepatocytes do not recognize the fibroblast growth factor (FGF)-15 produced by mouse intestine. This results in up-regulation of bile acid synthesis in the human hepatocytes and enlargement of the bile acid pool. We investigated whether abnormal bile acid signaling affects the hepatostat in mice.

METHODS

We crossed Fah(-/-), Rag2(-/-), Il2r(-/-) mice with nonobese diabetic mice to create FRGN mice, whose livers can be fully repopulated with human hepatocytes. We inserted the gene for human FGF19 (ortholog to mouse Fgf15), including regulatory sequences, into the FRGN mice to create FRGN19(+) mice. Livers of FRGN19(+) mice and their FRGN littermates were fully repopulated with human hepatocytes. Liver tissues were collected and bile acid pool sizes and RNA sequences were analyzed and compared with those of mice without humanized livers (controls).

RESULTS

Livers were larger in FRGN mice with humanized livers (13% of body weight), compared with control FRGN mice; they also had much larger bile acid pools and aberrant bile acid signaling. Livers from FRGN19(+) normalized to 7.8% of body weight, and their bile acid pool and signaling more closely resembled that of control FRGN19(+) mice. RNA sequence analysis showed activation of the Hippo pathway, and immunohistochemical and transcription analyses revealed increased hepatocyte proliferation, but not apoptosis, in the enlarged humanized livers of FRGN mice. Cell sorting experiments showed that although healthy human liver does not produce FGF19, nonparenchymal cells from cholestatic livers produce FGF19.

CONCLUSIONS

In mice with humanized livers, expression of an FGF19 transgene corrects bile acid signaling defects, resulting in normalization of bile acid synthesis, the bile acid pool, and liver size. These findings indicate that liver size is, in part, regulated by the size of the bile acid pool that the liver must circulate.

A novel, dynamic pattern-based analysis of NF-κB binding during the priming phase of liver regeneration reveals switch-like functional regulation of target genes

Following partial hepatectomy, a coordinated series of molecular events occurs to regulate hepatocyte entry into the cell cycle to recover lost mass. In rats during the first 6 h following resection, hepatocytes are primed by a tightly controlled cytokine response to prepare hepatocytes to begin replication. Although it appears to be a critical element driving regeneration, the cytokine response to resection has not yet been fully characterized. Specifically, the role of one of the key response elements to cytokine signaling (NF-κB) remains incompletely characterized. In this study, we present a novel, genome-wide, pattern-based analysis characterizing NF-κB binding during the priming phase of liver regeneration. We interrogated the dynamic regulation of priming by NF-κB through categorizing NF-κB binding in different temporal profiles: immediate sustained response, early transient response, and delayed response to partial hepatectomy. We then identified functional regulation of NF-κB binding by relating the temporal response profile to differential gene expression. We found that NF-κB bound genes govern negative regulation of cell growth and inflammatory response immediately following hepatectomy. NF-κB also transiently regulates genes responsible for lipid biosynthesis and transport as well as induction of apoptosis following hepatectomy. By the end of the priming phase, NF-κB regulation of genes involved in inflammatory response, negative regulation of cell death, and extracellular structure organization became prominent. These results suggest that NF-κB regulates target genes through binding and unbinding in immediate, transient, and delayed patterns. Such dynamic switch-like patterns of NF-κB binding may govern different functional transitions that drive the onset of regeneration.

The Bile Acid Receptor TGR5 and Liver Regeneration

BACKGROUND

Most of the literature on the bile acid (BA) membrane receptor TGR5 is dedicated to its potential role in the metabolic syndrome, through its regulatory impact on energy expenditure, insulin and GLP-1 secretion, and inflammatory processes. While the receptor was cloned in 2002, very little data are available on TGR5 functions in the normal and diseased liver. However, TGR5 is highly expressed in Kupffer cells and liver endothelial cells, and is particularly enriched in the biliary tract [cholangiocytes and gallbladder (GB) smooth muscle cells]. We recently demonstrated that TGR5 has a crucial protective impact on the liver in case of BA overload, including after partial hepatectomy.

KEY MESSAGES

TGR5-KO mice after PH exhibited periportal bile infarcts, excessive hepatic inflammation and defective adaptation of biliary composition (bicarbonate and chloride). Most importantly, TGR5-KO mice had a more hydrophobic BA pool, with more secondary BA than WT animals, suggesting that TGR5-KO bile may be harmful for the liver, mainly in situations of BA overload. As GB is both the tissue displaying the highest level of TGR5 expression and a crucial physiological site for the regulation of BA pool hydrophobicity by reducing secondary BA, we investigated whether TGR5 may control BA pool composition through an impact on GB. Preliminary data suggest that in the absence of TGR5, reduced GB filling dampens the cholecystohepatic shunt, resulting in more secondary BA, more hydrophobic BA pool and extensive liver injury in case of BA overload.

CONCLUSIONS

In the setting of BA overload, TGR5 is protective of the liver through the regulation of not only secretory and inflammatory processes, but also through the control of BA pool composition, at least in part by targeting the GB. Thereby, TGR5 appears to be crucial for protecting the regenerating liver from BA overload.