Bile acid stimulates hepatocyte polarization through a cAMP-Epac-MEK-LKB1-AMPK pathway

Lithocholic acid phenocopies anti-ageing effects of calorie restriction

Calorie restriction (CR) is a dietary intervention used to promote health and longevity1,2. CR causes various metabolic changes in both the production and the circulation of metabolites1; however, it remains unclear which altered metabolites account for the physiological benefits of CR. Here we use metabolomics to analyse metabolites that exhibit changes in abundance during CR and perform subsequent functional validation. We show that lithocholic acid (LCA) is one of the metabolites that alone can recapitulate the effects of CR in mice. These effects include activation of AMP-activated protein kinase (AMPK), enhancement of muscle regeneration and rejuvenation of grip strength and running capacity. LCA also activates AMPK and induces life-extending and health-extending effects in Caenorhabditis elegans and Drosophila melanogaster. As C. elegans and D. melanogaster are not able to synthesize LCA, these results indicate that these animals are able to transmit the signalling effects of LCA once administered. Knockout of AMPK abrogates LCA-induced phenotypes in all the three animal models. Together, we identify that administration of the CR-mediated upregulated metabolite LCA alone can confer anti-ageing benefits to metazoans in an AMPK-dependent manner.

Taurocholic acid represents an earlier and more sensitive biomarker and promotes cholestatic hepatotoxicity in ANIT-treated rats

Bile acid homeostasis is crucial for the normal physiological functioning of the liver. Disruptions in bile acid profiles are closely linked to the occurrence of cholestatic liver injury. As part of our diagnostic and therapeutic approach, we aimed to investigate the disturbance in bile acid profiles during cholestasis and its correlation with cholestatic liver injury. Before the occurrence of liver injury, alterations in bile acid profiles were detected in both plasma and liver between 8 and 16 h, persisting up to 96 h. TCA, TCDCA, and TUDCA in the plasma, as well as TCA, TUDCA, TCDCA, TDCA, TLCA, and THDCA in the liver, emerged as early sensitive and potential markers for diagnosing ANIT-induced cholestasis at 8-16 h. The distinguishing features of ANIT-induced liver injury were as follows: T-BAs exceeding G-BAs and serum biochemical indicators surpassing free bile acids. Notably, plasma T-BAs, particularly TCA, exhibited higher sensitivity to cholestatic hepatotoxicity compared with serum enzyme activity and liver histopathology. Further investigation revealed that TCA exacerbated ANIT-induced liver injury by elevating liver function enzyme activity, inflammation, and bile duct proliferation and promoting the migration of bile duct epithelial cell. Nevertheless, no morphological changes or alterations in transaminase activity indicative of liver damage were observed in the rats treated with TCA alone. Additionally, there were no changes in bile acid profiles or inflammatory responses under physiological conditions with maintained bile acid homeostasis. In summary, our findings suggest that taurine-conjugated bile acids in both plasma and liver, particularly TCA, can serve as early and sensitive markers for predicting intrahepatic cholestatic drugs and can act as potent exacerbators of cholestatic liver injury progression. However, exogenous TCA does not induce liver injury under physiological conditions where bile acid homeostasis is maintained.

Comparative analysis of bile acid composition and metabolism in the liver of Bufo gargarizans aquatic larvae and terrestrial adults

Bile acids are crucial for lipid metabolism and their composition and metabolism differ among species. However, there have been no data on the differences in the composition and metabolism of bile acids between aquatic larvae and terrestrial adults of amphibians. This study explored the differences in composition and metabolism of bile acid between Bufo gargarizans larvae and adults. The results demonstrated that adult liver had a lower total bile acid level and a higher conjugated/total bile acid ratio than larval liver. Meanwhile, histological analysis revealed that the larvae showed a larger cross-sectional area of bile canaliculi lumen compared with the adults. The transcriptomic analysis showed that B. gargarizans larvae synthesized bile acids through both the alternative and the 24-hydroxylase pathway, while adults only synthesized bile acids through the 24-hydroxylase pathway. Moreover, bile acid regulator-related genes FXR and RXRα were highly expressed in adult, whereas genes involved in bile acid synthesis (CYP27A1 and CYP46A1) were highly expressed in larvae. The present study will provide valuable insights into understanding metabolic disorders and exploring novel bile acid-based therapeutics.

Formation of functional, extended bile canaliculi, and increased bile acid production in sandwich-cultured human cryopreserved hepatocytes using commercially available culture medium

Drug-induced cholestasis results in drug discontinuation and market withdrawal, and the prediction of cholestasis risk is critical in the early stages of drug development. Animal tests and membrane vesicle assay are currently being conducted to assess the risk of cholestasis in the preclinical stage. However, these methods have drawbacks, such as species differences with humans and difficulties in evaluating the effects of drug metabolism and other transporters, implying the need for a cholestasis risk assessment system using human hepatocytes. However, human hepatocytes hardly form functional, extended bile canaliculi, a requirement for cholestasis risk assessment. We previously established a culture protocol for functional, extended bile canaliculi formation in human iPSC-derived hepatocytes. In this study, we modified this culture protocol to support the formation of functional, extended bile canaliculi in human cryopreserved hepatocytes (cryoheps). The production of bile acids, which induces bile canaliculi extension, increased time-dependently during bile canaliculi formation using this protocol, suggesting that increased bile acid production may be involved in the extended bile canaliculi formation. We have also shown that our culture protocol can be applied to cryoheps from multiple donors and that bile canaliculi can be formed stably among different culture batches. Furthermore, this protocol enables long-term maintenance of bile canaliculi and scaling down to culture in 96-well plates. We expect our culture protocol to be a breakthrough for in vitro cholestasis risk assessment.

Therapeutic perspectives on PDE4B inhibition in adipose tissue dysfunction and chronic liver injury

INTRODUCTION

Chronic liver disease (CLD) is a complex disease associated with profound dysfunction. Despite an incredible burden, the first and only pharmacotherapy for metabolic-associated steatohepatitis was only approved in March of this year, indicating a gap in the translation of preclinical studies. There is a body of preclinical work on the application of phosphodiesterase 4 inhibitors in CLD, none of these molecules have been successfully translated into clinical use.

AREAS COVERED

To design therapies to combat CLD, it is essential to consider the dysregulation of other tissues that contribute to its development and progression. As such, proper therapies must combat this throughout the body rather than focusing only on the liver. To detail this, literature characterizing the pathogenesis of CLD was pulled from PubMed, with a particular focus placed on the role of PDE4 in inflammation and metabolism. Then, the focus is shifted to detailing the available information on existing PDE4 inhibitors.

EXPERT OPINION

This review gives a brief overview of some of the pathologies of organ systems that are distinct from the liver but contribute to disease progression. The demonstrated efficacy of PDE4 inhibitors in other human inflammatory diseases should earn them further examination for the treatment of CLD.

Generation of in vivo-like multicellular liver organoids by mimicking developmental processes: A review

Liver is involved in metabolic reactions, ammonia detoxification, and immunity. Multicellular liver tissue cultures are more desirable for drug screening, disease modeling, and researching transplantation therapy, than hepatocytes monocultures. Hepatocytes monocultures are not stable for long. Further, hepatocyte-like cells induced from pluripotent stem cells and in vivo hepatocytes are functionally dissimilar. Organoid technology circumvents these issues by generating functional ex vivo liver tissue from intrinsic liver progenitor cells and extrinsic stem cells, including pluripotent stem cells. To function as in vivo liver tissue, the liver organoid cells must be arranged precisely in the 3-dimensional space, closely mimicking in vivo liver tissue. Moreover, for long term functioning, liver organoids must be appropriately vascularized and in contact with neighboring epithelial tissues (e.g., bile canaliculi and intrahepatic bile duct, or intrahepatic and extrahepatic bile ducts). Recent discoveries in liver developmental biology allows one to successfully induce liver component cells and generate organoids. Thus, here, in this review, we summarize the current state of knowledge on liver development with a focus on its application in generating different liver organoids. We also cover the future prospects in creating (functionally and structurally) in vivo-like liver organoids using the current knowledge on liver development.

The GPR40 novel agonist SZZ15-11 improves non-alcoholic fatty liver disease by activating the AMPK pathway and restores metabolic homeostasis in diet-induced obese mice

AIM

Non-alcoholic fatty liver is the most common cause of chronic liver disease. GPR40 is a potential therapeutic target for energy metabolic disorders. GPR40 is a potential therapeutic target for energy metabolic disorders. SZZ15-11 is a newly synthesized GPR40 agonist. In this study, we estimate the potency of SZZ15-11 in fatty liver treatment.

METHODS

In vivo, diet-induced obese (DIO) mice received SZZ15-11 (50 mg/kg) and TAK875 (50 mg/kg) for 6 weeks. Blood glucose and lipid, hepatocyte lipid and liver morphology were analysed. In vitro, HepG2 cells and GPR40-knockdown HepG2 cells induced with 0.3 mM oleic acid were treated with SZZ15-11. Triglyceride and total cholesterol of cells were measured. At the same time, the AMPK pathway regulating triglycerides and cholesterol esters synthesis was investigated via western blot and quantitative polymerase chain reaction in both liver tissue and HepG2 cells.

RESULTS

SZZ15-11 was found to not only attenuate hyperglycaemia and hyperlipidaemia but also ameliorate fatty liver disease in DIO mice. At the same time, SZZ15-11 decreased triglyceride and total cholesterol content in HepG2 cells. Whether examined in the liver of DIO mice or in HepG2 cells, SZZ15-11 upregulated AMPKα phosphorylation and then downregulated the expression of the cholesterogenic key enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase and inhibited acetyl-CoA carboxylase activity. Furthermore, SZZ15-11 promotes AMPK activity via [cAMP]i accumulation.

CONCLUSION

This study confirmed that SZZ15-11, a novel GPR40 agonist, improves hyperlipidaemia and fatty liver, partially via Gs signalling and the AMPK pathway in hepatocytes.

Non-invasive Visualization and Characterization of Bile Canaliculus Formation Using Refractive Index Tomography

The vital role of bile canaliculus (BC) in liver function is closely related to its morphology. Electron microscopy has contributed to understanding BC morphology; however, its invasiveness limits its use in living specimens. Here, we report non-invasive characterization of BC formation using refractive index (RI) tomography. First, we investigated and characterized the RI distribution of BCs in two-dimensional (2D) cultured HepG2 cells. BCs were identified based on their distinct morphology and functionality, as confirmed using a fluorescence-labeled bile acid analog. The RI distribution of BCs exhibited three common features: (1) luminal spaces with a low RI between adjacent hepatocytes; (2) luminal spaces surrounded by a membranous structure with a high RI; and (3) multiple microvillus structures with a high RI within the lumen. Second, we demonstrated the characterization of BC structures in a three-dimensional (3D) culture model, which is more relevant to the in vivo environment but more difficult to evaluate than 2D cultures. Various BC structures were identified inside HepG2 spheroids with the three features of RI distribution. Third, we conducted comparative analyses and found that the BC lumina of spheroids had higher circularity and lower RI standard deviation than 2D cultures. We also addressed comparison of BC and intracellular lumen-like structures within a HepG2 spheroid, and found that the BC lumina had higher RI and longer perimeter than intracellular lumen-like structures. Our demonstration of the non-destructive, label-free visualization and quantitative characterization of living BC structures will be a basis for various hepatological and pharmaceutical applications.

Bile Acid Overload Induced by Bile Duct and Portal Vein Ligation Improves Survival after Staged Hepatectomy in Rats

OBJECTIVE

Compared to portal vein ligation (PVL), simultaneous bile duct and portal vein ligation (BPL) can significantly enhance hypertrophy of the intact liver. This study aimed to investigate whether BPL could improve survival after extended hepatectomy independently of an increased remnant liver.

METHODS

We adopted rat models of 90% BPL or 90% PVL. To investigate the role of bile acids (BAs) the BA pools in the PVL and BPL groups were altered by the diet. Staged resection preserving 10% of the estimated liver weight was performed 3 days after BPL; PVL; or sham operation. Histology, canalicular network (CN) continuity; and hepatocyte polarity were evaluated.

RESULTS

At 3 days after BPL; PVL; or sham operation when the volumetric difference of the intended liver remained insignificant, the survival rates after extended hepatectomy were 86.7%, 47%, and 23.3%, respectively (P<0.01). BPL induced faster restoration of canalicular integrity along with an intensive but transient BA overload. Staged hepatectomy after BPL shortened the duration of the bile CN disturbance and limited BA retention. Decreasing the BA pools in the rats that underwent BPL could compromise these effects, whereas increasing the BA pools of rats that underwent PVL could induce similar effects. The changes in CN restoration were associated with activation of LKB1.

CONCLUSION

In addition to increasing the future remnant liver, BPL shortened the duration of the spatial disturbance of the CN and could significantly improve the tolerance of the hypertrophied liver to staged resection. BPL may be a safe and efficient future option for patients with an insufficient remnant liver.

Bidirectional crosstalk of the cAMP/ROS-dependent signaling pathways in inflammatory macrophage: An activation of formononetin

Bacterial lipopolysaccharide (LPS) is a toxic stimulant to macrophage inflammation. Inflammation intersects cell metabolism and often directs host immunopathogenesis stress. We aim here at pharmacological discovering of formononetin (FMN) action, to which anti-inflammatory signaling spans across immune membrane receptors and second messenger metabolites. In ANA-1 macrophage stimulated by LPS, and simultaneous treatment with FMN, results show the Toll-like receptor 4 (TLR4) and estrogen receptor (ER) signals, in concert with reactive oxygen species (ROS) and cyclic adenosine monophosphate (cAMP), respectively. LPS stimulates inactivation of the ROS-dependent nuclear factor erythroid 2-related factor 2 (Nrf2) by upregulating TLR4, but it does not affect cAMP. However, FMN treatment not only activates Nrf2 signaling by TLR4 inhibition, but also it activates cAMP-dependent protein kinase activities by upregulating ER. The cAMP activity gives rise to phosphorylation (p-) of protein kinase A, liver kinase B1 and 5'-AMP activated protein kinase (AMPK). Moreover, bidirectional signal crosstalk is amplified between p-AMPK and ROS, as FMN combinational validation with AMPK activator/inhibitor/target small-interfering RNA or ROS scavenger. The signal crosstalk is well positioned serving as the 'plug-in' knot for rather long signaling axis, and the immune-to-metabolic circuit via ER/TLR4 signal transduction. Collectively, convergence of the FMN-activated signals drives significant reduction of cyclooxygenase-2, interleukin-6 and NLR family pyrin domain-containing protein 3, in LPS-stimulated cell. Although anti-inflammatory signaling is specifically related to the immune-type macrophage, the p-AMPK antagonizing effect arises from FMN combination with ROS scavenger H-bond donors. Information of our work assists in predictive traits against macrophage inflammatory challenges, using phytoestrogen discoveries.

Effect of Total SMS Activity on LDL Catabolism in Mice

BACKGROUND

Sphingomyelin (SM) and cholesterol are 2 key lipid partners on cell membranes and on lipoproteins. Many studies have indicated the influence of cholesterol on SM metabolism. This study examined the influence of SM biosynthesis on cholesterol metabolism.

METHODS

Inducible global Sms1 KO (knockout)/global Sms2 KO mice were prepared to evaluate the effect of whole-body SM biosynthesis deficiency on lipoprotein metabolism. Tissue cholesterol, SM, ceramide, and glucosylceramide levels were measured. Triglyceride production rate and LDL (low-density lipoprotein) catabolism were measured. Lipid rafts were isolated and LDL receptor mass and function were evaluated. Also, the effects of exogenous sphingolipids on hepatocytes were investigated.

RESULTS

We found that total SMS (SM synthase) depletion significantly reduced plasma SM levels. Also, the total deficiency significantly induced plasma cholesterol, apoB (apolipoprotein B), and apoE (apolipoprotein E) levels. Importantly, total SMS deficiency, but not SMS2 deficiency, dramatically decreased LDL receptors in the liver and attenuated LDL uptake through the receptor. Further, we found that total SMS deficiency greatly reduced LDL receptors in the lipid rafts, which contained significantly lower SM and significantly higher glucosylceramide, as well as cholesterol. Furthermore, we treated primary hepatocytes and Huh7 cells (a human hepatoma cell line) with SM, ceramide, or glucosylceramide, and we found that only SM could upregulate LDL receptor levels in a dose-dependent fashion.

CONCLUSIONS

Whole-body SM biosynthesis plays an important role in LDL cholesterol catabolism. The total SMS deficiency, but not SMS2 deficiency, reduces LDL uptake and causes LDL cholesterol accumulation in the circulation. Given the fact that serum SM level is a risk factor for cardiovascular diseases, inhibiting SMS2 but not SMS1 should be the desirable approach.

Posttranslational regulation of liver kinase B1 (LKB1) in human cancer

Liver kinase B1 (LKB1) is a serine-threonine kinase that participates in multiple cellular and biological processes, including energy metabolism, cell polarity, cell proliferation, cell migration, and many others. LKB1 is initially identified as a germline-mutated causative gene in Peutz-Jeghers syndrome (PJS) and is commonly regarded as a tumor suppressor due to frequent inactivation in a variety of cancers. LKB1 directly binds and activates its downstream kinases including the AMP-activated protein kinase (AMPK) and AMPK-related kinases by phosphorylation, which has been intensively investigated for the past decades. An increasing number of studies has uncovered the posttranslational modifications (PTMs) of LKB1 and consequent changes in its localization, activity, and interaction with substrates. The alteration in LKB1 function as a consequence of genetic mutations and aberrant upstream signaling regulation leads to tumor development and progression. Here, we review current knowledge about the mechanism of LKB1 in cancer and the contributions of PTMs, such as phosphorylation, ubiquitination, SUMOylation, acetylation, prenylation, and others, to the regulation of LKB1 function, offering new insights into the therapeutic strategies in cancer.

Hexafluoropropylene oxide trimer acid (HFPO-TA) disturbs embryonic liver and biliary system development in zebrafish

Hexafluoropropylene oxide trimer acid (HFPO-TA), a novel alternative to perfluorooctanoic acid (PFOA), has emerged as a potential environmental pollutant. Here, to investigate the toxic effects of HFPO-TA on liver and biliary system development, zebrafish embryos were exposed to 0, 50, 100, or 200 mg/L HFPO-TA from 6 to 120 h post-fertilization (hpf). Results showed that the 50 % lethal concentration (LC₅₀) of HFPO-TA was 231 mg/L at 120 hpf, lower than that of PFOA. HFPO-TA exposure decreased embryonic hatching, survival, and body length. Furthermore, HFPO-TA exerted higher toxicity at the specification stage than during the differentiation and maturation stages, leading to small-sized livers in Tg(fabp10a: DsRed) transgenic larvae and histopathological changes. Significant decreases in the mRNA expression of genes related to liver formation were observed. Alanine transaminase (ALT), aspartate aminotransferase (AST), total bilirubin (TBIL), and direct bilirubin (DBIL) levels were significantly increased. HFPO-TA decreased total cholesterol (TCHO) and triglyceride (TG) activities, disturbed lipid metabolism through the peroxisome proliferator-activated receptor (PPAR) pathway, and induced an inflammatory response. Furthermore, HFPO-TA inhibited intrahepatic biliary development in Tg(Tp1:eGFP) transgenic larvae and interfered with transcription of genes associated with biliary duct development. HFPO-TA reduced bile acid synthesis but increased bile acid transport, resulting in disruption of bile acid metabolism. Therefore, HFPO-TA influenced embryonic liver and biliary system morphogenesis, caused liver injury, and may be an unsafe alternative for PFOA.

Effect of Total Sphingomyelin Synthase Activity on Low Density Lipoprotein Catabolism in Mice

Background

Sphingomyelin (SM) and cholesterol are two key lipid partners on cell membranes and on lipoproteins. Many studies have indicated the influence of cholesterol on SM metabolism. This study examined the influence of SM biosynthesis on cholesterol metabolism.

Methods

Inducible global Sms1 KO/global Sms2 KO mice were prepared to evaluate the effect of whole-body SM biosynthesis deficiency on lipoprotein metabolism. Tissue cholesterol, SM, ceramide, and glucosylceramide levels were measured. TG production rate and LDL catabolism were measured. Lipid rafts were isolated and LDL receptor mass and function were evaluated. Also, the effects of exogenous sphingolipids on hepatocytes were investigated.

Results

We found that total SMS depletion significantly reduced plasma SM levels. Also, the total deficiency significantly induced plasma cholesterol, apoB, and apoE levels. Importantly, total SMS deficiency, but not SMS2 deficiency, dramatically decreased LDL receptors in the liver and attenuated LDL uptake through the receptor. Further, we found that total SMS deficiency greatly reduced LDL receptors in the lipid rafts which contained significantly lower SM and significantly higher glucosylceramide as well as cholesterol. Furthermore, we treated primary hepatocytes and Huh7 cells (a human hepatoma cell line) with SM, ceramide, or glucosylceramide, and we found that only SM could up-regulate LDL receptor levels in a dose-dependent fashion.

Conclusions

Whole-body SM biosynthesis plays an important role in LDL-cholesterol catabolism. The total SMS deficiency, but not SMS2 deficiency, reduces LDL uptake and causes LDL-cholesterol accumulation in the circulation. Given the fact that serum SM level is a risk factor for cardiovascular diseases, inhibiting SMS2 but not SMS1 should be the desirable approach.

Graphic Abstract

Hepatic Polarized Differentiation Promoted the Maturity and Liver Function of Human Embryonic Stem Cell-Derived Hepatocytes via Activating Hippo and AMPK Signaling Pathways

Hepatocytes exhibit a multi-polarized state under the in vivo physiological environment, however, human embryonic stem cell-derived hepatocytes (hEHs) rarely exhibit polarity features in a two-dimensional (2D) condition. Thus, we hypothesized whether the polarized differentiation might enhance the maturity and liver function of hEHs. In this study, we obtained the polarized hEHs (phEHs) by using 2D differentiation in conjunct with employing transwell-based polarized culture. Our results showed that phEHs directionally secreted albumin, urea and bile acids, and afterward, the apical membrane and blood-bile barrier (BBIB) were identified to form in phEHs. Moreover, phEHs exhibited a higher maturity and capacitity of cellular secretory and drug metabolism than those of non-phEHs. Through transcriptome analysis, it was found that the polarized differentiation induced obvious changes in gene expression profiles of cellular adhesion and membrane transport in hEHs. Our further investigation revealed that the activation of Hippo and AMPK signaling pathways made contributions to the regulation of function and cellular polarity in phEHs, further verifying that the liver function of hEHs was closely related with their polarization state. These results not only demonstrated that the polarized differentiation enhanced the maturity and liver function of hEHs, but also identified the molecular targets that regulated the polarization state of hEHs.

The Pivotal Mediating Role of Adenosine Monophosphate-Activated Protein Kinase (AMPK) in Liver Tight Junctions and Liver Regeneration of a Partial-Hepatectomy Mouse Model

PURPOSE

This study aims to explore the pivotal mediating role of adenosine monophosphate-activated protein kinase (AMPK) in liver tight junctions and liver regeneration of a partial hepatectomy (PH) mouse model.

METHODS

A 70% PH mouse model was used. Firstly, mice were randomly divided into sham, 70% PH, AMPK-activated, and AMPK-inhibited groups. Then serum levels of alanine aminotransferase, aspartate transaminase, total bilirubin, direct bilirubin, albumin, and prealbumin were tested on postoperative days 1, 2 and 3. Furthermore, the expression of tight junction proteins like occludin, claudin-3, and ZO-1, together with bile salt export pump (BSEP), which reflects liver function, and AMPK were measured by Western blot and quantitative real-time polymerase chain reaction. Moreover, the expression of tight junction proteins, BSEP, and Ki-67 were examined by immunohistochemistry.

RESULTS

After 70% PH, without intervention, the changes in expression of hepatic tight junction proteins (occludin, claudin-3, and ZO-1) were consistent with that of BSEP, which could reflect liver function. After treatment with AMPK activator, the high expression status of tight junction proteins occurred in advance and was maintained stably and for a longer time. It was beneficial to liver function and liver regeneration was promoted at early periods and enhanced continuously after PH.

CONCLUSIONS

Activation of AMPK could effectively enhance the expression of hepatic tight junction proteins after PH. Therefore, it could speed up the recovery of liver function and promote liver regeneration especially early after PH.

Harnessing conserved signaling and metabolic pathways to enhance the maturation of functional engineered tissues

The development of induced-pluripotent stem cell (iPSC)-derived cell types offers promise for basic science, drug testing, disease modeling, personalized medicine, and translatable cell therapies across many tissue types. However, in practice many iPSC-derived cells have presented as immature in physiological function, and despite efforts to recapitulate adult maturity, most have yet to meet the necessary benchmarks for the intended tissues. Here, we summarize the available state of knowledge surrounding the physiological mechanisms underlying cell maturation in several key tissues. Common signaling consolidators, as well as potential synergies between critical signaling pathways are explored. Finally, current practices in physiologically relevant tissue engineering and experimental design are critically examined, with the goal of integrating greater decision paradigms and frameworks towards achieving efficient maturation strategies, which in turn may produce higher-valued iPSC-derived tissues.

LKB1 acts as a critical brake for the glucagon-mediated fasting response

As important as the fasting response is for survival, an inability to shut it down once nutrients become available can lead to exacerbated disease and severe wasting. The liver is central to transitions between feeding and fasting states, with glucagon being a key initiator of the hepatic fasting response. However, the precise mechanisms controlling fasting are not well defined. One potential mediator of these transitions is liver kinase B1 (LKB1), given its role in nutrient sensing. Here, we show LKB1 knockout mice have a severe wasting and prolonged fasting phenotype despite increased food intake. By applying RNA sequencing and intravital microscopy, we show that loss of LKB1 leads to a dramatic reprogramming of the hepatic lobule through robust up-regulation of periportal genes and functions. This is likely mediated through the opposing effect that LKB1 has on glucagon pathways and gene expression. Conclusion: Our findings show that LKB1 acts as a brake to the glucagon-mediated fasting response, resulting in "periportalization" of the hepatic lobule and whole-body metabolic inefficiency. These findings reveal a mechanism by which hepatic metabolic compartmentalization is regulated by nutrient-sensing.

The liver in sepsis: molecular mechanism of liver failure and their potential for clinical translation

Liver failure is a life-threatening complication of infections restricting the host's response to infection. The pivotal role of the liver in metabolic, synthetic, and immunological pathways enforces limits the host's ability to control the immune response appropriately, making it vulnerable to ineffective pathogen resistance and tissue damage. Deregulated networks of liver diseases are gradually uncovered by high-throughput, single-cell resolved OMICS technologies visualizing an astonishing diversity of cell types and regulatory interaction driving tolerogenic signaling in health and inflammation in disease. Therefore, this review elucidates the effects of the dysregulated host response on the liver, consequences for the immune response, and possible avenues for personalized therapeutics.

Quantitative understanding of HepaRG cells during drug-induced intrahepatic cholestasis through changes in bile canaliculi dynamics

An understanding of the quantitative relationship between bile canaliculus (BC) dynamics and the disruption of tight junctions (TJs) during drug-induced intrahepatic cholestasis may lead to new strategies aimed at drug development and toxicity testing. To investigate the relationship between BC dynamics and TJ disruption, we retrospectively analyzed the extent of TJ disruption in response to changes in the dynamics of BCs cultured with entacapone (ENT). Three hours after adding ENT, the ZO-1-negative BC surface area ratio became significantly higher (4.1-fold) than those of ZO-1-positive BCs. Based on these data, we calculated slopes of surface area changes, m, of each ZO-1-positive and ZO-1-negative BC. BCs with m ≤ 15 that fell within the 95% confidence interval of ZO-1-positive BCs were defined as ZO-1-positive. To validate this method, we compared the frequency of ZO-1-positive BCs, F_Z , with that of BCs with m ≤ 15, F_T , in culture using drugs that regulate TJ, or induce intrahepatic cholestasis. F_T values were correlated with F_Z under all culture conditions (R²  = .99). Our results indicate that the magnitude of BC surface area changes is a factor affecting TJ disruption, suggesting that maintaining TJ integrity by slowing BC dilation inhibits cell death.

The metabolomic profiling identifies N, N-dimethylglycine as a facilitator of dorsal root ganglia neuron axon regeneration after injury

Identifying novel molecules involved in axon regeneration of neurons in the peripheral nervous system (PNS) will be of benefit in obtaining a therapeutic strategy for repairing axon damage both in the PNS and the central nervous system (CNS). Metabolism and axon regeneration are tightly connected. However, the overall metabolic processes and the landscape of the metabolites in axon regeneration of PNS neurons are uncovered. Here, we used an ultra high performance liquid tandem chromatography quadrupole time of flight mass spectrometry (UHPLC-QTOFMS)-based untargeted metabolomics to analyze dorsal root ganglia (DRG) metabolic characteristics at different time points post sciatic nerve injury and acquired hundreds of differentially changed metabolites. In addition, the results reveal that several metabolic pathways were significantly altered, such as 'Histidine metabolism', 'Glycine serine and threonine metabolism', 'Arginine and proline metabolism', 'taurine and hypotaurine metabolism' and so on. Given metabolite could alter a cell's or an organism's phenotype, further investigation demonstrated that N, N-dimethylglycine (DMG) has a promoting effect on the regenerative ability post injury. Overall, our data may serve as a resource useful for further understanding how metabolites contribute to axon regeneration in DRG during sciatic nerve regeneration and suggest DMG may be a candidate drug to repair nerve injury.

Taurodeoxycholic acid and valine reverse obesity-associated augmented alloimmune responses and prolong allograft survival

Obesity initiates a chronic inflammatory network linked to perioperative complications and increased acute rejection rates in organ transplantation. Bariatric surgery is the most effective treatment of obesity recommended for morbidly obese transplant recipients. Here, we delineated the effects of obesity and bariatric surgery on alloimmunity and transplant outcomes in diet-induced obese (DIO) mice. Allograft survival was significantly shorter in DIO-mice. When performing sleeve gastrectomies (SGx) prior to transplantation, we found attenuated T cell-derived alloimmune responses resulting in prolonged allograft survival. Administering taurodeoxycholic acid (TDCA) and valine, metabolites depleted in DIO-mice and restored through SGx, prolonged graft survival in DIO-mice comparable with SGx an dampened Th1 and Th17 alloimmune responses while Treg frequencies and CD4+ T cell-derived IL-10 production were augmented. Moreover, in recipient animals treated with TDCA/valine, levels of donor-specific antibodies had been reduced. Mechanistically, TDCA/valine restrained inflammatory M1-macrophage polarization through TGR5 that compromised cAMP signaling and inhibited macrophage-derived T cell activation. Consistently, administering a TGR5 agonist to DIO-mice prolonged allograft survival. Overall, we provide novel insights into obesity-induced inflammation and its impact on alloimmunity. Furthermore, we introduce TDCA/valine as a noninvasive alternative treatment for obese transplant patients.

The neonatal liver: Normal development and response to injury and disease

The liver emerges from the ventral foregut endoderm around 3 weeks in human and 1 week in mice after fertilization. The fetal liver works as a hematopoietic organ and then develops functions required for performing various metabolic reactions in late fetal and neonatal periods. In parallel with functional differentiation, the liver establishes three dimensional tissue structures. In particular, establishment of the bile excretion system consisting of bile canaliculi of hepatocytes and bile ducts of cholangiocytes is critical to maintain healthy tissue status. This is because hepatocytes produce bile as they functionally mature, and if allowed to remain within the liver tissue can lead to cytotoxicity. In this review, we focus on epithelial tissue morphogenesis in the perinatal period and cholestatic liver diseases caused by abnormal development of the biliary system.

Emerging Role of cAMP/AMPK Signaling

The 5′-Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a natural energy sensor in mammalian cells that plays a key role in cellular and systemic energy homeostasis. At the cellular level, AMPK supports numerous processes required for energy and redox homeostasis, including mitochondrial biogenesis, autophagy, and glucose and lipid metabolism. Thus, understanding the pathways regulating AMPK activity is crucial for developing strategies to treat metabolic disorders. Mounting evidence suggests the presence of a link between cyclic AMP (cAMP) and AMPK signaling. cAMP signaling is known to be activated in circumstances of physiological and metabolic stress due to the release of stress hormones, such as adrenaline and glucagon, which is followed by activation of membrane-bound adenylyl cyclase and elevation of cellular cAMP. Because the majority of physiological stresses are associated with elevated energy consumption, it is not surprising that activation of cAMP signaling may promote AMPK activity. Aside from the physiological role of the cAMP/AMPK axis, numerous reports have suggested its role in several pathologies, including inflammation, ischemia, diabetes, obesity, and aging. Furthermore, novel reports have provided more mechanistic insight into the regulation of the cAMP/AMPK axis. In particular, the role of distinct cAMP microdomains generated by soluble adenylyl cyclase in regulating basal and induced AMPK activity has recently been demonstrated. In the present review, we discuss current advances in the understanding of the regulation of the cAMP/AMPK axis and its role in cellular homeostasis and explore some translational aspects.

Preparation of Functional Human Hepatocytes Ex Vivo

Hepatocytes are liver parenchymal cells involved in performing various metabolic reactions. During the development of therapeutic drugs, toxicological assays are conducted using hepatocyte cultures before clinical trials. However, since primary hepatocytes cannot proliferate and rapidly lose their functions in vitro, many efforts have been put into modifying culture conditions to expand primary hepatocytes and induce hepatocyte functions in intrinsic and extrinsic stem/progenitor cells. In this chapter, we summarize recent advances in preparing hepatocyte cultures and induction of hepatocytes from various cellular sources.

Collagen Sandwich Culture of Primary Hepatocytes for Image-Based Investigations

Collagen sandwich culture of hepatocytes retains many in vivo-like properties and is used for many investigations in liver cell biology, and hepatic pharmacology and toxicology. This chapter describes the method of establishing collagen sandwich culture of hepatocytes in a glass bottom dish for image-based studies.

Hepatocyte polarity establishment and apical lumen formation are organized by Par3, Cdc42, and aPKC in conjunction with Lgl

Hepatocytes differ from columnar epithelial cells by their multipolar organization, which follows the initial formation of central lumen-sharing clusters of polarized cells as observed during liver development and regeneration. The molecular mechanism for hepatocyte polarity establishment, however, has been comparatively less studied than those for other epithelial cell types. Here, we show that the tight junction protein Par3 organizes hepatocyte polarization via cooperating with the small GTPase Cdc42 to target atypical protein kinase C (aPKC) to a cortical site near the center of cell-cell contacts. In 3D Matrigel culture of human hepatocytic HepG2 cells, which mimics a process of liver development and regeneration, depletion of Par3, Cdc42, or aPKC results in an impaired establishment of apicobasolateral polarity and a loss of subsequent apical lumen formation. The aPKC activity is also required for bile canalicular (apical) elongation in mouse primary hepatocytes. The lateral membrane-associated proteins Lgl1 and Lgl2, major substrates of aPKC, seem to be dispensable for hepatocyte polarity establishment because Lgl-depleted HepG2 cells are able to form a single apical lumen in 3D culture. On the other hand, Lgl depletion leads to lateral invasion of aPKC, and overexpression of Lgl1 or Lgl2 prevents apical lumen formation, indicating that they maintain proper lateral integrity. Thus, hepatocyte polarity establishment and apical lumen formation are organized by Par3, Cdc42, and aPKC; Par3 cooperates with Cdc42 to recruit aPKC, which plays a crucial role in apical membrane development and regulation of the lateral maintainer Lgl.

Rab35 controls formation of luminal projections required for bile canalicular morphogenesis

Hepatocytes display a unique biaxial polarity with shared apical luminal connections between adjacent hepatocytes that merge into a network of bile canaliculi. Belicova et al. (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202103003) discovered that hepatocyte apical membranes generate Rab35-dependent extensions that traverse the lumen and are essential for bile canalicular formation and maintenance.

Anisotropic expansion of hepatocyte lumina enforced by apical bulkheads

Lumen morphogenesis results from the interplay between molecular pathways and mechanical forces. In several organs, epithelial cells share their apical surfaces to form a tubular lumen. In the liver, however, hepatocytes share the apical surface only between adjacent cells and form narrow lumina that grow anisotropically, generating a 3D network of bile canaliculi (BC). Here, by studying lumenogenesis in differentiating mouse hepatoblasts in vitro, we discovered that adjacent hepatocytes assemble a pattern of specific extensions of the apical membrane traversing the lumen and ensuring its anisotropic expansion. These previously unrecognized structures form a pattern, reminiscent of the bulkheads of boats, also present in the developing and adult liver. Silencing of Rab35 resulted in loss of apical bulkheads and lumen anisotropy, leading to cyst formation. Strikingly, we could reengineer hepatocyte polarity in embryonic liver tissue, converting BC into epithelial tubes. Our results suggest that apical bulkheads are cell-intrinsic anisotropic mechanical elements that determine the elongation of BC during liver tissue morphogenesis.

Bile formation and secretion: An update

Bile formation is a fundamental physiological process that is vital to the survival of all vertebrates. However, little was known about the mechanisms of this secretion until after World War II. Initial studies involved classic physiologic studies in animal models and humans, which progressed to include studies in isolated cells and membrane vesicles. The advent of molecular biology then led to the identification of specific transport systems that are the determinants of this secretion. Progress in this field was reviewed in the American Physiologic Society's series on "Comprehensive Physiology" in 2013. Herein, we provide an in-depth update of progress since that time.

Tight junction stabilization prevents HepaRG cell death in drug-induced intrahepatic cholestasis

Entacapone (ENT), a catechol-O-methyltransferase inhibitor, causes liver injury by inducing bile canaliculi (BC) dilation through inhibition of the myosin light kinase pathway. Loss of tight junctions (TJs) induces hepatocyte depolarization, which causes bile secretory failure, leading to liver damage. To understand the influence of TJ structural changes as a consequence of BC dynamics, we compared the datasets of time-lapse and immunofluorescence images for TJ protein ZO-1 in hepatocytes cultured with ENT, forskolin (FOR), ENT/FOR, and those cultured without any drugs. Retrospective analysis revealed that the drastic change in BC behaviors caused TJ disruption and apoptosis in cells cultured with ENT. Exposure to FOR or sodium taurocholate facilitated TJ formation in the cells cultured with ENT and suppressed BC dynamic changes, leading to the inhibition of TJ disruption and apoptosis. Our findings clarify that hepatocyte TJ stabilization protects against cell death induced by BC disruption.

Metformin impairs bile acid homeostasis in ethinylestradiol-induced cholestasis in mice

Metformin, an oral antidiabetic drug, recently demonstrated a reducing effect on bile acids (BA) plasma concentrations in one patient with intrahepatic cholestasis of pregnancy (ICP) by unknown mechanism. Therefore, the aim of the present study was to examine the effect of metformin on BA homeostasis and related molecular pathways in the liver and intestine using a mouse model of ICP. The cholestasis was induced in female C57BL/6 mice by repeated administration of ethinylestradiol (10 mg/kg BW s.c.) and/or metformin (150 mg/kg BW orally) over 5 consecutive days with subsequent bile collection and molecular analysis of samples. We demonstrated that metformin significantly increased the rate of bile secretion in control mice. This increase was BA dependent and was produced both by increased liver BA synthesis via induced cholesterol 7α-hydroxylase (Cyp7a1) and by increased BA reabsorption in the ileum via induction of the apical sodium-dependent BA transporter (Asbt). In contrast, metformin further worsened ethinylestradiol-induced impairment of bile secretion. This reduction was also BA dependent and corresponded with significant downregulation of Bsep, and Ntcp, major excretory and uptake transporters for BA in hepatocytes, respectively. The plasma concentrations of BA were consequently significantly increased in the metformin-treated mice. Altogether, our data indicate positive stimulation of bile secretion by metformin in the intact liver, but this drug also induces serious impairment of BA biliary secretion, with a marked increase in plasma concentrations in estrogen-induced cholestasis. Our results imply that metformin should be used with caution in situations with hormone-dependent cholestasis, such as ICP.

The Bile Salt Export Pump: Molecular Structure, Study Models and Small-Molecule Drugs for the Treatment of Inherited BSEP Deficiencies

The bile salt export pump (BSEP/ABCB11) is responsible for the transport of bile salts from hepatocytes into bile canaliculi. Malfunction of this transporter results in progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2) and intrahepatic cholestasis of pregnancy (ICP). Over the past few years, several small molecular weight compounds have been identified, which hold the potential to treat these genetic diseases (chaperones and potentiators). As the treatment response is mutation-specific, genetic analysis of the patients and their families is required. Furthermore, some of the mutations are refractory to therapy, with the only remaining treatment option being liver transplantation. In this review, we will focus on the molecular structure of ABCB11, reported mutations involved in cholestasis and current treatment options for inherited BSEP deficiencies.

SEW2871 attenuates ANIT-induced hepatotoxicity by protecting liver barrier function via sphingosine 1-phosphate receptor-1-mediated AMPK signaling pathway

Cholestatic liver injury, a group of diseases characterized with dysregulated bile acid (BA) homeostasis, was partly resulted from BA circulation disorders, which is commonly associated with the damage of hepatocyte barrier function. However, the underlying hepatocyte barrier-protective molecular mechanisms of cholestatic liver injury remain poorly understood. Interestingly, recent studies have shown that sphingosine-1-phosphate (S1P) participated in the process of cholestasis by activating its G protein-coupled receptors S1PRs, regaining the integrity of hepatocyte tight junctions (TJs). Here, we showed that SEW2871, a selective agonist of sphingosine-1-phosphate receptor 1(S1PR1), alleviated ANIT-induced TJs damage in 3D-cultured mice primary hepatocytes. Molecular mechanism studies indicated that AMPK signaling pathways was involved in TJs protection of SEW2871 in ANIT-induced hepatobiliary barrier function deficiency. AMPK antagonist compound C (CC) and agonist AICAR were all used to further identify the important role of AMPK signaling pathway in SEW2871's TJs protection of ANIT-treated mice primary hepatocytes. The in vivo data showed that SEW2871 ameliorated ANIT-induced cholestatic hepatotoxicity. Further protection mechanism research demonstrated that SEW2871 not only regained hepatocyte TJs by the upregulated S1PR1 via AMPK signaling pathway, but also recovered hepatobiliary barrier function deficiency, which was verified by the restored BA homeostasis by using of high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS). These results revealed that the increased expression of S1PR1 induced by SEW2871 could ameliorate ANIT-induced cholestatic liver injury through improving liver barrier function via AMPK signaling and subsequently reversed the disrupted BA homeostasis. Our study provided strong evidence that S1PR1 may be a promising therapeutic approach for treating intrahepatic cholestatic liver injury. Graphical abstract.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Picroside II alleviates liver injury induced by alpha-naphthylisothiocyanate through AMPK-FXR pathway

Alpha-naphthylisothiocyanate (ANIT) is a typical hepatotoxicant that causes cholestasis, which causes toxic bile acid accumulation in the liver and leads to liver injury. Picroside II (PIC), one of the dominant effective components extracted from Picrorhiza scrophulariiflora Pennell, exhibits many pharmacological effects. However, the role of AMP-activated protein kinase (AMPK)-Farnesoid X receptor (FXR) pathway in the hepatoprotective effect of PIC against ANIT-induced cholestasis remains largely unknown. This study aimed to investigate the mechanisms of PIC on ANIT-induced cholestasis in vivo and in vitro. Our results showed that PIC protected against ANIT-induced liver injury in primary mouse hepatocytes, and decreased serum biochemical markers and lessened histological injuries in mice. ANIT inhibited FXR and its target genes of bile acid synthesis enzymes sterol-12α-hydroxylase (CYP8B1), and increase bile acid uptake transporter Na + -dependent taurocholate transporter (NTCP), efflux transporter bile salt export pump (BSEP) and bile acid metabolizing enzymes UDP-glucuronosyltransferase 1a1 (UGT1A1) expressions. PIC prevented its downregulation of FXR, NTCP, BSEP and UGT1A1, and further reduced CYP8B1 by ANIT. Furthermore, ANIT activated AMPK via ERK1/2-LKB1 pathway. PIC inhibited ERK1/2, LKB1 and AMPK phosphorylation in ANIT-induced cholestasis in vivo and in vitro. AICAR, an AMPK agonist, blocked PIC-mediated changes in FXR, CYP8B1 and BSEP expression in vitro. Meanwhile, U0126, an ERK1/2 inhibitor, further repressed ERK1/2-LKB1-AMPK pathway phosphorylation. In conclusion, PIC regulated bile acid-related transporters and enzymes to protect against ANIT-induced liver injury, which related to ERK1/2-LKB1-AMPK pathway. Thus, this study extends the understanding of the anti-cholestasis effect of PIC and provides new therapeutic targets for cholestasis treatment.

Antidepressant drug sertraline modulates AMPK-MTOR signaling-mediated autophagy via targeting mitochondrial VDAC1 protein

Macroautophagy/autophagy (hereafter autophagy), the process of mass degradation of unnecessary elements within the cell, is often dysregulated in many diseases such as cancer, atherosclerosis, and neurodegenerative diseases. Hence, autophagy modulating agents have a great potential to be therapeutic agents for the autophagy-related diseases. Here we report that an anti-depressant drug sertraline (Sert) is an autophagy-inducing agent. Mechanistically, Sert potentially binds to and antagonizes the mitochondrial VDAC1 (voltage dependent anion channel 1), resulting in reduced cellular ATP (adenosine triphosphate) level, activation of AMP-activated protein kinase (AMPK) and inhibition of its downstream, MTOR (mechanistic target of rapamycin kinase)-RPS6KB1 (ribosomal protein S6 kinase B1) signaling pathway. Cells lacking VDAC1 expression completely abrogate the modulatory effect of Sert on AMPK-MTOR pathway and autophagy-inducing activity. We further show that Sert suppresses tauopathy by promoting the autophagic degradation of MAPT (microtubule associated protein tau) protein via inducing autophagy. Our study demonstrates the potential of Sert as a novel small molecule autophagy-inducing agent and provides a new drug candidate to treat autophagy related diseases by targeting VDAC1.

Abbreviations: AMP: adenosine monophosphate; AMPK: AMP-activated protein kinase; ATP: adenosine triphosphate; Baf: bafilomycin A₁; BiFC: biomolecular fluorescence complementation; CAMKK2/CAMKKB: calcium/calmodulin dependent protein kinase kinase 2; CC: compound C; DARTS: drug affinity responsive target stability; HUVECs: human umbilical vein endothelial cells; Inda: indatraline; STK11/LKB1: serine/threonine kinase 11; MAPT: microtubule associated protein tau; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; 3-MA: 3-methyladenine; MEFs: mouse embryonic fibroblasts; MTOR: mechanistic target of rapamycin kinase; PI3K: phosphoinositide 3-kinase; Rapa: rapamycin; Sert: sertraline; RPS6KB1: ribosomal protein S6 kinase B1; SQSTM1/p62: sequestosome 1; SLC6A4/SERT1: solute carrier family 6 member 4; TFEB: transcription factor EB; VDAC1: voltage dependent anion channel 1; WT: wild-type; WM: wortmannin.

The ABCG2/BCRP transporter and its variants - from structure to pathology

The ABCG2 protein has a key role in the transport of a wide range of structurally dissimilar endo- and xenobiotics in the human body, especially in the tissue barriers and the metabolizing or secreting organs. The human ABCG2 gene harbors a high number of polymorphisms and mutations, which may significantly modulate its expression and function. Recent high-resolution structural data, complemented with molecular dynamic simulations, may significantly help to understand intramolecular movements and substrate handling, as well as the effects of mutations on the membrane transporter function of ABCG2. As reviewed here, structural alterations may result not only in direct alterations in drug binding and transporter activity, but also in improper folding or problems in the carefully regulated process of trafficking, including vesicular transport, endocytosis, recycling, and degradation. Here, we also review the clinical importance of altered ABCG2 expression and function in general drug metabolism, cancer multidrug resistance, and impaired uric acid excretion, leading to gout.

Liver Buds and Liver Organoids: New Tools for Liver Development, Disease and Medical Application

The current understanding and effective treatment of liver disease is far from satisfactory. Liver organoids and liver buds (LBs) transforming cell culture from two dimensions(2D) to three dimensions(3D) has provided infinite possibilities for stem cells to use in clinic. Recent technological advances in the 3D culture have shown the potentiality of liver organoids and LBs as the promising tool to model in vitro liver diseases. The induced LBs and liver organoids provide a platform for cell-based therapy, liver disease models, liver organogenesis and drugs screening. And its genetic heterogeneity supplies a way for the realization of precision medicine.

PKA-independent vasopressin signaling in renal collecting duct

Vasopressin regulates renal water excretion by binding to a Gα s-coupled receptor (V2R) in collecting duct cells, resulting in increased water permeability through regulation of the aquaporin-2 (AQP2) water channel. This action is widely accepted to be associated with cAMP-mediated activation of protein kinase A (PKA). Here, we use phosphoproteomics in collecting duct cells in which PKA has been deleted (CRISPR-Cas9) to identify PKA-independent responses to vasopressin. The results show that V2R-mediated vasopressin signaling is predominantly, but not entirely, PKA-dependent. Upregulated sites in PKA-null cells include Ser256 of AQP2, which is critical to regulation of AQP2 trafficking. In addition, phosphorylation changes in the protein kinases Stk39 (SPAK) and Prkci (an atypical PKC) are consistent with PKA-independent regulation of these protein kinases. Target motif analysis of the phosphopeptides increased in PKA-null cells indicates that vasopressin activates one or more members of the AMPK/SNF1-subfamily of basophilic protein kinases. In vitro phosphorylation assays using recombinant, purified SNF1-subfamily kinases confirmed postulated target specificities. Of interest, measured IBMX-dependent cAMP levels were an order of magnitude higher in PKA-null than in PKA-intact cells, indicative of a PKA-dependent feedback mechanism. Overall, the findings support the conclusion that V2-receptor mediated signaling in collecting duct cells is in part PKA-independent.

The importance of transporters and cell polarization for the evaluation of human stem cell-derived hepatic cells

One of the most promising applications of human pluripotent stem cells is their utilization for human-based pharmacological models. Despite the fact that membrane transporters expressed in the liver play pivotal role in various hepatic functions, thus far only little attention was devoted to the membrane transporter composition of the stem cell-derived liver models. In the present work, we have differentiated HUES9, a human embryonic stem cell line, toward the hepatic lineage, and monitored the expression levels of numerous differentiation marker and liver transporter genes with special focus on ABC transporters. In addition, the effect of bile acid treatment and polarizing culturing conditions on hepatic maturation has been assessed. We found that most transporter genes crucial for hepatic functions are markedly induced during hepatic differentiation; however, as regards the transporter composition the end-stage cells still exhibited dual, hepatocyte and cholangiocyte character. Although the bile acid treatment and sandwich culturing only slightly influenced the gene expressions, the stimulated cell polarization resulted in formation of bile canaliculi and proper localization of transporters. Our results point to the importance of membrane transporters in human stem cell-derived hepatic models and demonstrate the relevance of cell polarization in generation of applicable cellular models with correctly localized transporters. On the basis of our observations we suggest that conventional criteria for the evaluation of the quality of stem cell-derived hepatocyte-like cells ought to be augmented with additional elements, such as polarized and functional expression of hepatic transporters.

Association of Plasmodium berghei With the Apical Domain of Hepatocytes Is Necessary for the Parasite's Liver Stage Development

Plasmodium parasites undergo a dramatic transformation during the liver stage of their life cycle, amplifying over 10,000-fold inside infected hepatocytes within a few days. Such a rapid growth requires large-scale interactions with, and manipulations of, host cell functions. Whereas hepatocyte polarity is well-known to be critical for liver function, little is presently known about its involvement during the liver stage of Plasmodium development. Apical domains of hepatocytes are critical components of their polarity machinery and constitute the bile canalicular network, which is central to liver function. Here, we employed high resolution 3-D imaging and advanced image analysis of Plasmodium-infected liver tissues to show that the parasite associates preferentially with the apical domain of hepatocytes and induces alterations in the organization of these regions, resulting in localized changes in the bile canalicular architecture in the liver tissue. Pharmacological perturbation of the bile canalicular network by modulation of AMPK activity reduces the parasite's association with bile canaliculi and arrests the parasite development. Our findings using Plasmodium-infected liver tissues reveal a host-Plasmodium interaction at the level of liver tissue organization. We demonstrate for the first time a role for bile canaliculi, a central component of the hepatocyte polarity machinery, during the liver stage of Plasmodium development.

Role of PI3K/Akt and MEK/ERK Signalling in cAMP/Epac-Mediated Endothelial Barrier Stabilisation

Background and Aims

Activation of the cAMP/Epac signalling stabilises endothelial barrier function. Moreover, its activation is accompanied by an activation of PI3K/Akt and MEK/ERK signalling in diverse cell types but their impact on endothelial barrier function is largely unknown. Here the role of PI3K/Akt and MEK/ERK signalling in cAMP/Epac-mediated endothelial barrier stabilisation was analysed.

Methods

Endothelial barrier function was analysed in cultured human umbilical vein endothelial cells (HUVECs) by measuring flux of albumin. A modified cAMP analogue 8-pCPT-2′-O-Me-cAMP (Epac agonist) was used to specifically activate cAMP/Epac signalling.

Results

Epac agonist reduces the basal and attenuates thrombin-induced endothelial hyperpermeability accompanied by an activation of PI3K/Akt and MEK/ERK signalling. The qPCR data demonstrate HUVECs express PI3Kα, PI3Kβ, and PI3Kγ but not PI3Kδ isoforms. The western blot data demonstrate Epac agonist activates PI3Kα and PI3Kβ isoforms. Inhibition of MEK/ERK but not PI3K/Akt pathway potentiates the endothelial barrier protective effects of cAMP/Epac signalling. Inhibition of MEK/ERK signalling in the presence of Epac agonist induces a reorganisation of actin cytoskeleton to the cell periphery, enhanced VE-cadherin localisation at cell-cell junctions, and dephosphorylation of myosin light chains (MLC) but not inhibition of RhoA/Rock signalling. Moreover, Epac agonist promotes endothelial cell (EC) survival via reduction in activities of pro-apoptotic caspases in a PI3K/Akt and MEK/ERK signalling-dependent manner.

Conclusion

Our data demonstrate that the Epac agonist simultaneously activates diverse signalling pathways in ECs, which may have differential effects on endothelial barrier function. It activates PI3K/Akt and MEK/ERK signalling which mainly govern its pro-survival effects on ECs. Inhibition of MEK/ERK but not PI3K/Akt signalling enhances barrier stabilising and barrier protective effects of cAMP/Epac activation.

Chemical Compounds Used In This Study

8-pCPT-2′-O-Me-cAMP (PubChem CID: 9913268); Akt inhibitor VIII (PubChem CID: 10196499); AS-252424 (PubChem CID: 11630874); IC-87114 (PubChem CID: 9908783); PD 98059 (PubChem CID: 4713); PIK-75 (PubChem CID: 10275789); TGX-221 (PubChem CID: 9907093); Thrombin (PubChem CID: 90470996); U0126 (PubChem CID: 3006531); Wortmannin (PubChem CID: 312145).

Mice depleted for Exchange Proteins Directly Activated by cAMP (Epac) exhibit irregular liver regeneration in response to partial hepatectomy

The exchange proteins directly activated by cAMP 1 and 2 (Epac1 and Epac2) are expressed in a cell specific manner in the liver, but their biological functions in this tissue are poorly understood. The current study was undertaken to begin to determine the potential roles of Epac1 and Epac2 in liver physiology and disease. Male C57BL/6J mice in which expression of Epac1 and/or Epac2 are deleted, were subjected to partial hepatectomy and the regenerating liver was analyzed with regard to lipid accumulation, cell replication and protein expression. In response to partial hepatectomy, deletion of Epac1 and/or Epac2 led to increased hepatocyte proliferation 36 h post surgery, and the transient steatosis observed in wild type mice was virtually absent in mice lacking both Epac1 and Epac2. The expression of the protein cytochrome P4504a14, which is implicated in hepatic steatosis and fibrosis, was substantially reduced upon deletion of Epac1/2, while a number of factors involved in lipid metabolism were significantly decreased. Moreover, the number of Küpffer cells was affected, and Epac2 expression was increased in the liver of wild type mice in response to partial hepatectomy, further supporting a role for these proteins in liver function. This study establishes hepatic phenotypic abnormalities in mice deleted for Epac1/2 for the first time, and introduces Epac1/2 as regulators of hepatocyte proliferation and lipid accumulation in the regenerative process.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

From a common progenitor to distinct liver epithelial phenotypes

The vertebrate liver presents a fascinating case study for how cell form is optimized for function. To execute its duties the liver assembles two distinct lumen-forming epithelial phenotypes: Firstly, cords with a branched, capillary-like luminal network formed between hepatocytes (bile canaliculi); and secondly, tubular ducts formed by biliary epithelial cells arranged around a central cavity and connected to the bile canaliculi. How these remarkably different epithelial polarity phenotypes are generated and joined into a contiguous luminal network are major unresolved questions. Recent studies have characterized the divergence of the two epithelial lineages from common progenitors, described the coordination of bile canaliculi formation with bile duct branching during biliary tree morphogenesis and implicated RhoA-dependent E-cadherin adhesion in the decision to polarize with hepatocytic or biliary phenotype.

The emerging role of AMP-activated protein kinase in cholestatic liver diseases

AMP-activated protein kinase (AMPK), recognized as an energy sensor with three heterotrimeric subunits (α, β and γ), not only maintains basal intracellular adenosine triphosphate levels but also regulates energy-intensive pathological responses, such as neurodegenerative and metabolic diseases, through multiple signaling pathways. Recent studies open a new direction for AMPK research and demonstrate that AMPK is a critical player in the pathogenesis of cholestatic liver injury and plays paradoxical roles in the regulation of different pathological processes, including the disruption of bile acid homeostasis and the regulation of hepatic polarity, inflammation and fibrosis. In the present review, we summarize recent findings that implicate AMPK-mediated signaling pathways in the pathogenesis of cholestatic liver injury. These findings provide novel insight regarding the potential use of AMPK as a therapeutic target for the treatment of cholestatic liver injury.

Epithelial Morphogenesis during Liver Development

Tissue stem/progenitor cells supply multiple types of epithelial cells that eventually acquire specialized functions during organ development. In addition, three-dimensional (3D) tissue structures need to be established for organs to perform their physiological functions. The liver contains two types of epithelial cells, namely, hepatocytes and cholangiocytes, which are derived from hepatoblasts, fetal liver stem/progenitor cells (LPCs), in mid-gestation. Hepatocytes performing many metabolic reactions form cord-like structures, whereas cholangiocytes, biliary epithelial cells, form tubular structures called intrahepatic bile ducts. Analyses for human genetic diseases and mutant mice have identified crucial molecules for liver organogenesis. Functions of those molecules can be examined in in vitro culture systems where LPCs are induced to differentiate into hepatocytes or cholangiocytes. Recent technical advances have revealed 3D epithelial morphogenesis during liver organogenesis. Therefore, the liver is a good model to understand how tissue stem/progenitor cells differentiate and establish 3D tissue architectures during organ development.