The Bile Acid Receptor GPBAR1 Modulates CCL2/CCR2 Signaling at the Liver Sinusoidal/Macrophage Interface and Reverses Acetaminophen-Induced Liver Toxicity

Hyperactivation of succinate dehydrogenase promotes pyroptosis of macrophage via ROS-induced GSDMD oligomerization in acute liver failure

Acute liver failure (ALF) is a life-threatening disease with high mortality. Given excessive inflammation is one of the major pathogenesis of ALF, candidates targeting inflammation could be beneficial in the condition. Now the effect of hyperactivated succinate dehydrogenase (SDH) on promoting inflammation in lipopolysaccharide (LPS)-treated macrophages has been studied. However, its role and mechanism in ALF is not well understood. Here intraperitoneal injection of D-galactosamine and LPS was conducted in male C57BL/6 J mice to induce the ALF model. Dimethyl malonate (DMM), which inhibited SDH activity, was injected intraperitoneally 30 min before ALF induction. Macrophage pyroptosis was induced by LPS plus adenosine triphosphate (ATP). Pyroptosis-related molecules and proteins including GSDMD oligomer were examined by ELISA and western blot techniques, respectively. ROS production was assessed by fluorescence staining. The study demonstrated SDH activity was increased in liver macrophages from ALF mice. Importantly, DMM administration inhibited ROS, IL-1β, and pyroptosis-associated proteins levels (NLRP3, cleaved caspase-1, GSDMD-N, and GSDMD oligomers) both in the ALF model and in macrophages stimulated with LPS plus ATP. In vitro, ROS promoted pyroptosis by facilitating GSDMD oligomerization. Additionally, when ROS levels were increased through the addition of H2O2 to the DMM group, the levels of GSDMD oligomers were reverted. In conclusion, SDH hyperactivation promotes macrophage pyroptosis by ROS-mediated GSDMD oligomerization, suggesting that targeting this pathway holds promise as a strategy for treating ALF and other inflammatory diseases.

The role of bile acid receptor TGR5 in regulating inflammatory signalling

Takeda G protein-coupled receptor 5 (TGR5) is a bile acid receptor, and its role in regulating metabolism after binding with bile acids has been established. Since the immune response depends on metabolism to provide biomolecules and energy to cope with challenging conditions, emerging evidence reveals the regulatory effects of TGR5 on the immune response. An in-depth understanding of the effect of TGR5 on immune regulation can help us disentangle the interaction of metabolism and immune response, accelerating the development of TGR5 as a therapeutic target. Herein, we reviewed more than 200 articles published in the last 20 years in PubMed, to discuss the roles of TGR5 in regulating inflammatory response, the molecular mechanism, as well as existing problems. Particularly, its anti-inflammation effect is emphasized.

Defective Bile Acid Signaling Promotes Vascular Dysfunction, Supporting a Role for G-Protein Bile Acid Receptor 1/Farnesoid X Receptor Agonism and Statins in the Treatment of Nonalcoholic Fatty Liver Disease

BACKGROUND

Patients with nonalcoholic fatty liver disease are at increased risk to develop atherosclerotic cardiovascular diseases. FXR and GPBAR1 are 2 bile acid-activated receptors exploited in the treatment of nonalcoholic fatty liver disease: whether dual GPBAR1/FXR agonists synergize with statins in the treatment of the liver and cardiovascular components of nonalcoholic fatty liver disease is unknown.

METHODS AND RESULTS

Investigations of human aortic samples obtained from patients who underwent surgery for aortic aneurysms and Gpbar1-/-, Fxr-/-, and dual Gpbar1-/-Fxr-/- mice demonstrated that GPBAR1 and FXR are expressed in the aortic wall and regulate endothelial cell/macrophage interactions. The expression of GPBAR1 in the human endothelium correlated with the expression of inflammatory biomarkers. Mice lacking Fxr and Gpbar1-/-/Fxr-/- display hypotension and aortic inflammation, along with altered intestinal permeability that deteriorates with age, and severe dysbiosis, along with dysregulated bile acid synthesis. Vasomotor activities of aortic rings were altered by Gpbar1 and Fxr gene ablation. In apolipoprotein E-/- and wild-type mice, BAR502, a dual GPBAR1/FXR agonist, alone or in combination with atorvastatin, reduced cholesterol and low-density lipoprotein plasma levels, mitigated the development of liver steatosis and aortic plaque formation, and shifted the polarization of circulating leukocytes toward an anti-inflammatory phenotype. BAR502/atorvastatin reversed intestinal dysbiosis and dysregulated bile acid synthesis, promoting a shift of bile acid pool composition toward FXR antagonists and GPBAR1 agonists.

CONCLUSIONS

FXR and GPBAR1 maintain intestinal, liver, and cardiovascular homeostasis, and their therapeutic targeting with a dual GPBAR1/FXR ligand and atorvastatin holds potential in the treatment of liver and cardiovascular components of nonalcoholic fatty liver disease.

Gut microbiota and tumor-associated macrophages: potential in tumor diagnosis and treatment

Avoiding immune destruction and polymorphic microbiomes are two key hallmarks of cancer. The tumor microenvironment (TME) is essential for the development of solid tumors, and the function of tumor-associated macrophages (TAMs) in the TME is closely linked to tumor prognosis. Therefore, research on TAMs could improve the progression and control of certain tumor patients. Additionally, the intestinal flora plays a crucial role in metabolizing substances and maintaining a symbiotic relationship with the host through a complex network of interactions. Recent experimental and clinical studies have suggested a potential link between gut microbiome and TME, particularly in regulating TAMs. Understanding this association could improve the efficacy of tumor immunotherapy. This review highlights the regulatory role of intestinal flora on TAMs, with a focus on gut microbiota and their metabolites. The implications of this association for tumor diagnosis and treatment are also discussed, providing a promising avenue for future clinical treatment strategies.

Pure total flavonoids from citrus improve nonalcoholic steatohepatitis liver inflammatory responses by regulating the CCL2/CCR2-PI3K-Akt signal transduction pathway

BACKGROUND AND AIM

Nonalcoholic steatohepatitis (NASH) is a critical stage in the prognosis of nonalcoholic fatty liver disease (NAFLD). Pure total flavonoids from circus (PTFC) play essential roles in the improvement of NASH symptoms, but the underlying regulatory mechanism remains elusive. Our previous high-throughput omics screening results indicate that the CCL2/CCR2-PI3K-Akt signaling pathway is a key pathway that regulates the liver inflammatory response. PTFC may regulate the CCL2/CCR2-PI3K-Akt signaling pathway to improve the liver inflammatory response.

METHODS

A mice model of NASH was established by a high-fat diet, and PTFC was used as treatment. Hematoxylin-eosin and oil red O staining were used to observe the pathological changes in the liver tissue. Western blotting and real-time PCR were used to measure the mRNA and protein levels in the liver. The expression of proinflammatory cytokines in the peripheral blood and liver tissues was measured by liquid suspension array. An automatic biochemical method was used to examine serum transaminases and lipids levels, as well as liver lipids.

RESULTS

Compared with the mice in the high-fat diet group, mice in the HFD + PTFC group showed significantly improved liver histopathology, and levels of serum transaminase and lipids, liver lipids and serum proinflammatory cytokines. Moreover, the mRNA and protein expression and phosphorylation levels of key signaling molecules in the CCL2/CCR2-PI3K-Akt signal transduction pathway were obviously reduced by PTFC treatment.

CONCLUSIVE REMARKS

PTFC can ameliorate NASH symptoms, and the mechanism may be related to regulating the CCL2/CCR2-PI3K-Akt signal transduction pathway to reduce the liver inflammatory response.

Roquin-1 resolves sepsis-associated acute liver injury by regulating inflammatory profiles via miRNA cargo in extracellular vesicles

Sepsis-associated acute liver injury (SALI) is an independent risk for sepsis-induced death orchestrated by innate and adaptive immune responses. Here, we found that Roquin-1 was decreased during SALI and expressed mainly in monocyte-derived macrophages. Meanwhile, Roquin-1 was correlated with the inflammatory profiles in humans and mice. Mechanically, Roquin-1 in macrophages promoted Ago2-K258-ubiquitination and inhibited Ago2-S387/S828-phosphorylation. Ago2-S387-phosphorylation inhibited Ago2-miRNA's complex location in multivesicular bodies and sorting in macrophages-derived extracellular vesicles (MDEVs), while Ago2-S828-phosphorylation modulated the binding between Ago2 and miRNAs by special miRNAs-motifs. Then, the anti-inflammatory miRNAs in MDEVs decreased TSC22D2 expression directly, upregulated Tregs-differentiation via TSC22D2-STAT3 signaling, and inhibited M1-macrophage-polarization by TSC22D2-AMPKα-mTOR pathway. Furthermore, WT MDEVs in mice alleviated SALI by increasing Tregs ratio and decreasing M1-macrophage frequency synchronously. Our study showed that Roquin-1 in macrophages increased Tregs-differentiation and decreased M1-macrophage-polarization simultaneously via miRNA in MDEVs, suggesting Roquin-1 can be used as a potential tool for SALI treatment and MDEVs engineering.

The Intersection of COVID-19 and Metabolic-Associated Fatty Liver Disease: An Overview of the Current Evidence

The global population is currently experiencing the impact of the SARS-CoV-2 coronavirus, which has caused the Coronavirus Disease 2019 (COVID-19) pandemic. With our profound comprehension of COVID-19, encompassing the involvement sequence of the respiratory tract, gastrointestinal system, and cardiovascular apparatus, the multiorgan symptoms of this infectious disease have been discerned. Metabolic-associated fatty liver disease (MAFLD), formerly known as non-alcoholic fatty liver disease (NAFLD), is a pervasive public health concern intricately linked with metabolic dysregulation and estimated to afflict one-fourth of the global adult population. The burgeoning focus on the association between COVID-19 and MAFLD is justified by the potential role of the latter as a risk factor for both SARS-CoV-2 infection and the subsequent emergence of severe COVID-19 symptoms. Investigations have suggested that changes in both innate and adaptive immune responses among MAFLD patients may play a role in determining the severity of COVID-19. The remarkable similarities observed in the cytokine pathways implicated in both diseases imply the existence of shared mechanisms governing the chronic inflammatory responses characterizing these conditions. The effect of MAFLD on the severity of COVID-19 illness remains uncertain, as indicated by conflicting results in cohort investigations.

Combinatorial targeting of G-protein-coupled bile acid receptor 1 and cysteinyl leukotriene receptor 1 reveals a mechanistic role for bile acids and leukotrienes in drug-induced liver injury

BACKGROUND AND AIM

Drug-induced liver injury (DILI) is a common disorder that involves both direct liver cell toxicity and immune activation. The bile acid receptor, G-protein-coupled bile acid receptor 1 (GPBAR1; Takeda G-protein-coupled receptor 5 [TGR5]), and cysteinyl leukotriene receptor (CYSLTR) 1 are G-protein-coupled receptors activated by bile acids and leukotrienes, exerting opposite effects on cell-to-cell adhesion, inflammation, and immune cell activation. To investigate whether GPBAR1 and CYSLTR1 mutually interact in the development of DILI, we developed an orally active small molecule, CHIN117, that functions as a GPBAR1 agonist and CYSLTR1 antagonist.

APPROACH AND RESULTS

RNA-sequencing analysis of liver explants showed that acetaminophen (APAP) intoxication positively modulates the leukotriene pathway, CYSLTR1, 5-lipoxygenase, and 5-lipoxygenase activating protein, whereas GPBAR1 gene expression was unchanged. In mice, acute liver injury induced by orally dosing APAP (500 mg/kg) was severely exacerbated by Gpbar1 gene ablation and attenuated by anti-Cysltr1 small interfering RNA pretreatment. Therapeutic dosing of wild-type mice with CHIN117 reversed the liver damage caused by APAP and modulated up to 1300 genes, including 38 chemokines and receptors, that were not shared by dosing mice with a selective GPBAR1 agonist or CYSLTR1 antagonist. Coexpression of the two receptors was detected in liver sinusoidal endothelial cells (LSECs), monocytes, and Kupffer cells, whereas combinatorial modulation of CYSLTR1 and GPBAR1 potently reversed LSEC/monocyte interactions. CHIN117 reversed liver damage and liver fibrosis in mice administered CCl₄ .

CONCLUSIONS

By genetic and pharmacological approaches, we demonstrated that GPBAR1 and CYSLTR1 mutually interact in the development of DILI. A combinatorial approach designed to activate GPBAR1 while inhibiting CYSLTR1 reverses liver injury in models of DILI.

Notch-activated mesenchymal stromal/stem cells enhance the protective effect against acetaminophen-induced acute liver injury by activating AMPK/SIRT1 pathway

Background

Notch signaling plays important roles in regulating innate immunity. However, little is known about the role of Notch in mesenchymal stromal/stem cell (MSC)-mediated immunomodulation during liver inflammatory response.

Methods

Notch activation in human umbilical cord-derived MSCs was performed by a tissue culture plate coated with Notch ligand, recombinant human Jagged1 (JAG1). Mice were given intravenous injection of Notch-activated MSCs after acetaminophen (APAP)-induced acute liver injury. Liver tissues were collected and analyzed by histology and immunohistochemistry.

Results

MSC administration reduced APAP-induced hepatocellular damage, as manifested by decreased serum ALT levels, intrahepatic macrophage/neutrophil infiltration, hepatocellular apoptosis and proinflammatory mediators. The anti-inflammatory activity and therapeutic effects of MSCs were greatly enhanced by Notch activation via its ligand JAG1. However, Notch2 disruption in MSCs markedly diminished the protective effect of MSCs against APAP-induced acute liver injury, even in the presence of JAG1 pretreatment. Strikingly, Notch-activated MSCs promoted AMP-activated protein kinase (AMPKα) phosphorylation, increased the sirtuins 1 (SIRT1) deacetylase expression, but downregulated spliced X-box-binding protein 1 (XBP1s) expression and consequently reduced NLR family pyrin domain-containing 3 (NLRP3) inflammasome activation. Furthermore, SIRT1 disruption or XBP1s overexpression in macrophages exacerbated APAP-triggered liver inflammation and augmented NLRP3/caspase-1 activity in MSC-administrated mice. Mechanistic studies further demonstrated that JAG1-pretreated MSCs activated Notch2/COX2/PGE2 signaling, which in turn induced macrophage AMPK/SIRT1 activation, leading to XBP1s deacetylation and inhibition of NLRP3 activity.

Conclusions

Activation of Notch2 is required for the ability of MSCs to reduce the severity of APAP-induced liver damage in mice. Our findings underscore a novel molecular insights into MSCs-mediated immunomodulation by activating Notch2/COX2/AMPK/SIRT1 pathway and thus provide a new strategy for the treatment of liver inflammatory diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02999-6.

Inhibition of BTK improved APAP-induced liver injury via suppressing proinflammatory macrophages activation by restoring mitochondrion function

BACKGROUND

Acetaminophen (APAP) overdose can cause severe liver injury and APAP-induced liver injury (AILI) is one of the leading causes of acute liver failure (ALF). Bruton's tyrosine kinase (BTK) is a key tyrosine kinase in immune responses, which plays an important role in many inflammatory diseases. However, its effect on AILI is still not clear. Here, we aimed to assess the effect of BTK on AILI and explore its underlying mechanism.

METHODS

In our study, western blot and immunohistochemistry were used to detect the expression of BTK in AILI. The C57BL/6 mice were used to check the protective effect of BTK inhibition on AILI and the activation of BTK was confirmed in mice macrophages treated with APAP. Immunofluorescence, immunohistochemistry, oxygen consumption rate (OCR) detection, polymerase chain reaction (PCR), flow cytometry and western blot were used to determine the role of BTK in mitochondrial dynamics and function of macrophages and the underlying mechanisms in AILI.

RESULTS

Our results showed that BTK upregulated in AILI. BTK inhibition protected mice from AILI and BTK was activated in mice macrophages in response to APAP. Mechanically, BTK inhibition promoted mitochondrial fusion and restored mitochondrial function through phospholipase C gamma 2 (PLCγ2)-reactive oxygen species (ROS)-Optic Atrophy 1(OPA1) pathway in macrophages and finally suppressed the release of proinflammatory cytokines.

CONCLUSIONS

In conclusion, we found that BTK inhibition protected mice from AILI by restoring the mitochondrial function of macrophages through the improvement of the mitochondrial dynamic imbalance via PLCγ2-ROS-OPA1 signaling pathway, which indicated that BTK might be a potential therapeutic target of AILI.

Immunomodulatory functions of FXR

The Farnesoid-x-receptor (FXR) is a bile acids sensor activated in humans by primary bile acids. FXR is mostly expressed in liver, intestine and adrenal glands but also by cells of innate immunity, including macrophages, liver resident macrophages, the Kupffer cells, natural killer cells and dendritic cells. In normal physiology and clinical disorders, cells of innate immunity mediate communications between liver, intestine and adipose tissues. In addition to FXR, the G protein coupled receptor (GPBAR1), that is mainly activated by secondary bile acids, whose expression largely overlaps FXR, modulates chemical communications from the intestinal microbiota and the host's immune system, integrating epithelial cells and immune cells in the entero-hepatic system, providing a mechanism for development of a tolerogenic state toward the intestinal microbiota. Disruption of FXR results in generalized inflammation and disrupted bile acids metabolism. While FXR agonism in preclinical models provides counter-regulatory signals that attenuate inflammation-driven immune dysfunction in a variety of liver and intestinal disease models, the clinical relevance of these mechanisms in the setting of FXR-related disorders remain poorly defined.

Targeting innate immune responses to attenuate acetaminophen-induced hepatotoxicity

Acetaminophen (APAP) hepatotoxicity is an important cause of acute liver failure, resulting in massive deaths in many developed countries. Currently, the metabolic process of APAP in the body has been well studied. However, the underlying mechanism of APAP-induced liver injury remains elusive. Increasing clinical and experimental evidences indicate that the innate immune responses are involved in the pathogenesis of APAP-induced acute liver injury (AILI), in which immune cells have dual roles of inducing inflammation to exacerbate hepatotoxicity and removing dead cells and debris to help liver regeneration. In this review, we summarize the latest findings of innate immune cells involved in AILI, particularly emphasizing the activation of innate immune cells and their different roles during the injury and repair phases. Moreover, current available treatments are discussed according to the different roles of innate immune cells in the development of AILI. This review aims to update the knowledge about innate immune responses in the pathogenesis of AILI, and provide potential therapeutic interventions for AILI.

Discovery of a Potent and Orally Active Dual GPBAR1/CysLT1R Modulator for the Treatment of Metabolic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are two highly prevalent human diseases caused by excessive fat deposition in the liver. Although multiple approaches have been suggested, NAFLD/NASH remains an unmet clinical need. Here, we report the discovery of a novel class of hybrid molecules designed to function as cysteinyl leukotriene receptor 1 (CysLT₁R) antagonists and G protein bile acid receptor 1 (GPBAR1/TGR5) agonists for the treatment of NAFLD/NASH. The most potent of these compounds generated by harnessing the scaffold of the previously described CystLT₁R antagonists showed efficacy in reversing liver histopathology features in a preclinical model of NASH, reshaping the liver transcriptome and the lipid and energy metabolism in the liver and adipose tissues. In summary, the present study described a novel orally active dual CysLT₁R antagonist/GPBAR1 agonist that effectively protects against the development of NAFLD/NASH, showing promise for further development.

Targeted Metabolomics Analysis of Bile Acids in Patients with Idiosyncratic Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is rare but clinically important due to a high rate of mortality. However, specific biomarkers for diagnosing and predicting the severity and prognosis of DILI are lacking. Here, we used targeted metabolomics to identify and quantify specific types of bile acids that can predict the severity of DILI. A total of 161 DILI patients were enrolled in this prospective cohort study, as well as 31 health controls. A targeted metabolomics method was used to identify 24 types of bile acids. DILI patients were divided into mild, moderate, and severe groups according to disease severity. A multivariate analysis was performed to identify characteristic bile acids. Then the patients were divided into severe and non-severe groups, and logistic regression was used to identify bile acids that could predict DILI severity. Among the enrolled DILI patients, 32 were in the mild group, 90 were in the moderate group, and 39 were in the severe group. Orthogonal partial least squares-discriminant analysis (OPLS-DA) modeling clearly discriminated among the different groups. Among the four groups, glycochenodeoxycholate (GCDCA), taurochenodeoxycholate (TCDCA), deoxycholic acid (DCA), Nor Cholic acid (NorCA), glycocholic acid (GCA), and taurocholic acid (TCA) showed significant differences in concentration between at least two groups. NorCA, GCDCA, and TCDCA were all independent risk factors that differentiated severe DILI patients from the other groups. The area under the receiver operating characteristic curve (AUROC) of GCDCA, TCDCA, and NorCA was 0.856, 0.792, and 0.753, respectively. Together, these three bile acids had an AUROC of 0.895 for predicting severe DILI patients. DILI patients with different disease severities have specific bile acid metabolomics. NorCA, GCDTA, and TCDCA were independent risk factors for differentiating severe DILI patients from less-severe patients and have the potential to predict DILI severity.

Structural Basis for Developing Multitarget Compounds Acting on Cysteinyl Leukotriene Receptor 1 and G-Protein-Coupled Bile Acid Receptor 1

G-protein-coupled receptors (GPCRs) are the molecular target of 40% of marketed drugs and the most investigated structures to develop novel therapeutics. Different members of the GPCRs superfamily can modulate the same cellular process acting on diverse pathways, thus representing an attractive opportunity to achieve multitarget drugs with synergic pharmacological effects. Here, we present a series of compounds with dual activity toward cysteinyl leukotriene receptor 1 (CysLT₁R) and G-protein-coupled bile acid receptor 1 (GPBAR1). They are derivatives of REV5901─the first reported dual compound─with therapeutic potential in the treatment of colitis and other inflammatory processes. We report the binding mode of the most active compounds in the two GPCRs, revealing unprecedented structural basis for future drug design studies, including the presence of a polar group opportunely spaced from an aromatic ring in the ligand to interact with Arg79^2.60 of CysLT₁R and achieve dual activity.

Foresight regarding drug candidates acting on the succinate-GPR91 signalling pathway for non-alcoholic steatohepatitis (NASH) treatment

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and it is a liver manifestation of metabolic syndrome, with a histological spectrum from simple steatosis to non-alcoholic steatohepatitis (NASH). NASH can evolve into progressive liver fibrosis and eventually lead to liver cirrhosis. The pathological mechanism of NASH is multifactorial, involving a series of metabolic disorders and changes that trigger low-level inflammation in the liver and other organs. In the pathogenesis of NASH, the signal transduction pathway involving succinate and the succinate receptor (G-protein-coupled receptor 91, GPR91) regulates inflammatory cell activation and liver fibrosis. This review describes the mechanism of the succinate-GPR91 signalling pathway in NASH and summarizes the drugs that act on this pathway, with the aim of providing a new approach to NASH treatment.

Critical roles of bile acids in regulating intestinal mucosal immune responses

Bile acids are a class of cholesterol derivatives that have been known for a long time for their critical roles in facilitating the digestion and absorption of lipid from the daily diet. The transformation of primary bile acids produced by the liver to secondary bile acids appears under the action of microbiota in the intestine, greatly expanding the molecular diversity of the intestinal environment. With the discovery of several new receptors of bile acids and signaling pathways, bile acids are considered as a family of important metabolites that play pleiotropic roles in regulating many aspects of human overall health, especially in the maintenance of the microbiota homeostasis and the balance of the mucosal immune system in the intestine. Accordingly, disruption of the process involved in the metabolism or circulation of bile acids is implicated in many disorders that mainly affect the intestine, such as inflammatory bowel disease and colon cancer. In this review, we discuss the different metabolism profiles in diseases associated with the intestinal mucosa and the diverse roles of bile acids in regulating the intestinal immune system. Furthermore, we also summarize recent advances in the field of new drugs that target bile acid signaling and highlight the importance of bile acids as a new target for disease intervention.

Bile Acids Activated Receptors in Inflammatory Bowel Disease

Once known exclusively for their role in nutrients absorption, bile acids have emerged as signaling molecules, generated from cholesterol breakdown, acting on several immune cells by activating a variety of receptors including the G protein-coupled bile acid receptor 1 (GPABR1 or TGR5), the Farnesoid-X-receptor (FXR) and, as recently discovered, the retinoid-related orphan receptors (ROR)γt. GPBAR1, FXR, and RORγt are highly expressed in cells of the innate and adaptive immune system (i.e., dendritic cells (DCs), macrophages, innate lymphoid 3 cells (ILC3s), and T helper 17 (Th17) lymphocytes) and plays an important role in regulating intestinal and liver immunity, highlighting a role for various bile acid species in regulating immune responses to intestinal microbial antigens. While primary bile acids are generated from the cholesterol breakdown secondary bile acids, the GPBAR1 ligands, and oxo-bile acids derivatives, the RORγt ligands, are generated by the intestinal microbiota, highlighting the potential of these bile acids in mediating the chemical communication between the intestinal microbiota and the host. Changes in intestinal microbiota, dysbiosis, alter the composition of the bile acid pool, promoting the activation of the immune system and development of chronic inflammation. In this review, we focus on the molecular mechanisms by which an altered bile acid signaling promotes intestinal inflammation.

Therapeutic effects of myocardin-related transcription factor A (MRTF-A) knockout on experimental mice with nonalcoholic steatohepatitis induced by high-fat diet

OBJECTIVE

To explore the effects of myocardin-related transcription factor A (MRTF-A) knockout on mice with nonalcoholic steatohepatitis (NASH) induced by high-fat diet (HFD).

METHODS

Normal-fat diet (NFD) or HFD was fed to MRTF-A-knockout (MRTF-A^-/-) and wild-type (WT) mice for 16 weeks. Liver histopathological status was observed using Hematoxylin and Eosin (HE) staining, Oil Red O staining, Sirius Red staining, and Immunohistochemical staining. The mRNA and protein levels in liver tissues were measured through quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and Western blot.

RESULTS

Compared with WT + HFD group, mice in MRTF-A^-/- + HFD group were decreased in body weight, blood glucose, plasma insulin, liver TG and NAFLD activity score (NAS), with liver function recovery. Besides, compared with HFD-fed WT mice, HFD-fed MRTF-A^-/- mice were improved in hepatic fibrosis, accompanied by decreased collagen content (%) and down-regulated expressions of α-SMA, COL1A2, TGFβ1, and SMAD3. In mice fed with HFD, the expression of MCP-1, CCR2, F4/80 and CD68 declined in liver tissues of MRTF-A^-/- mice as compared with WT mice. Besides, in hepatic macrophages isolated from HFD-fed mice, the observed increased expression of TNF-α, IL-1β, MCP-1, as well as decreased expression of CCR2. Compared with WT + HFD group, MRTF-A^-/- + HFD group mice were decreased regarding NF-κB p65 in liver tissues.

CONCLUSION

MRTF-A knockout reduced macrophage infiltration, down-regulated NF-κB p65 expression, and ameliorated inflammation and fibrosis of liver tissues in mice, thereby becoming a potential therapeutic target for NASH treatment.

Expanding the drug discovery space with predicted metabolite-target interactions

Metabolites produced in the human gut are known modulators of host immunity. However, large-scale identification of metabolite-host receptor interactions remains a daunting challenge. Here, we employed computational approaches to identify 983 potential metabolite-target interactions using the Inflammatory Bowel Disease (IBD) cohort dataset of the Human Microbiome Project 2 (HMP2). Using a consensus of multiple machine learning methods, we ranked metabolites based on importance to IBD, followed by virtual ligand-based screening to identify possible human targets and adding evidence from compound assay, differential gene expression, pathway enrichment, and genome-wide association studies. We confirmed known metabolite-target pairs such as nicotinic acid-GPR109a or linoleoyl ethanolamide-GPR119 and inferred interactions of interest including oleanolic acid-GABRG2 and alpha-CEHC-THRB. Eleven metabolites were tested for bioactivity in vitro using human primary cell-types. By expanding the universe of possible microbial metabolite-host protein interactions, we provide multiple drug targets for potential immune-therapies.

GPCRs get fatty: the role of G protein-coupled receptor signaling in the development and progression of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), characterized by the abnormal deposition of lipids within the liver not due to alcohol consumption, is a growing epidemic affecting over 30% of the United States population. Both simple fatty liver and its more severe counterpart, nonalcoholic steatohepatitis, represent one of the most common forms of liver disease. Recently, several G protein-coupled receptors have emerged as targets for therapeutic intervention for these disorders. These include those with known hepatic function as well as those involved in global metabolic regulation. In this review, we highlight these emerging therapeutic targets, focusing on several common themes including their activation by microbial metabolites, stimulatory effect on insulin and incretin secretion, and contribution to glucose tolerance. The overlap in ligands, localization, and downstream effects of activation indicate the interdependent nature of these receptors and highlight the importance of this signaling family in the development and prevention of NAFLD.

G protein-coupled receptors as potential targets for nonalcoholic fatty liver disease treatment

Nonalcoholic fatty liver disease (NAFLD) is a broad-spectrum disease, ranging from simple hepatic steatosis to nonalcoholic steatohepatitis, which can progress to cirrhosis and liver cancer. Abnormal hepatic lipid accumulation is the major manifestation of this disease, and lipotoxicity promotes NAFLD progression. In addition, intermediate metabolites such as succinate can stimulate the activation of hepatic stellate cells to produce extracellular matrix proteins, resulting in progression of NAFLD to fibrosis and even cirrhosis. G protein-coupled receptors (GPCRs) have been shown to play essential roles in metabolic disorders, such as NAFLD and obesity, through their function as receptors for bile acids and free fatty acids. In addition, GPCRs link gut microbiota-mediated connections in a variety of diseases, such as intestinal diseases, hepatic steatosis, diabetes, and cardiovascular diseases. The latest findings show that gut microbiota-derived acetate contributes to liver lipogenesis by converting dietary fructose into hepatic acetyl-CoA and fatty acids. GPCR agonists, including peptides and natural products like docosahexaenoic acid, have been applied to investigate their role in liver diseases. Therapies such as probiotics and GPCR agonists may be applied to modulate GPCR function to ameliorate liver metabolism syndrome. This review summarizes the current findings regarding the role of GPCRs in the development and progression of NAFLD and describes some preclinical and clinical studies of GPCR-mediated treatment. Overall, understanding GPCR-mediated signaling in liver disease may provide new therapeutic options for NAFLD.

Bile Acid Signaling in Inflammatory Bowel Diseases

Bile acids are a group of chemically different steroids generated at the host/microbial interface. Indeed, while primary bile acids are the end-product of cholesterol breakdown in the host liver, secondary bile acids are the products of microbial metabolism. Primary and secondary bile acids along with their oxo derivatives have been identified as signaling molecules acting on a family of cell membrane and nuclear receptors collectively known as "bile acid-activated receptors." Members of this group of receptors are highly expressed throughout the gastrointestinal tract and mediate the bilateral communications of the intestinal microbiota with the host immune system. The expression and function of bile acid-activated receptors FXR, GPBAR1, PXR, VDR, and RORγt are highly dependent on the structure of the intestinal microbiota and negatively regulated by intestinal inflammation. Studies from gene ablated mice have demonstrated that FXR and GPBAR1 are essential to maintain a tolerogenic phenotype in the intestine, and their ablation promotes the polarization of intestinal T cells and macrophages toward a pro-inflammatory phenotype. RORγt inhibition by oxo-bile acids is essential to constrain Th17 polarization of intestinal lymphocytes. Gene-wide association studies and functional characterizations suggest a potential role for impaired bile acid signaling in development inflammatory bowel diseases (IBD). In this review, we will focus on how bile acids and their receptors mediate communications of intestinal microbiota with the intestinal immune system, describing dynamic changes of bile acid metabolism in IBD and the potential therapeutic application of targeting bile acid signaling in these disorders.