Sphingosine kinase-1 inhibition protects primary rat hepatocytes against bile salt-induced apoptosis

Bile acid metabolism and signaling in health and disease: molecular mechanisms and therapeutic targets

Bile acids, once considered mere dietary surfactants, now emerge as critical modulators of macronutrient (lipid, carbohydrate, protein) metabolism and the systemic pro-inflammatory/anti-inflammatory balance. Bile acid metabolism and signaling pathways play a crucial role in protecting against, or if aberrant, inducing cardiometabolic, inflammatory, and neoplastic conditions, strongly influencing health and disease. No curative treatment exists for any bile acid influenced disease, while the most promising and well-developed bile acid therapeutic was recently rejected by the FDA. Here, we provide a bottom-up approach on bile acids, mechanistically explaining their biochemistry, physiology, and pharmacology at canonical and non-canonical receptors. Using this mechanistic model of bile acids, we explain how abnormal bile acid physiology drives disease pathogenesis, emphasizing how ceramide synthesis may serve as a unifying pathogenic feature for cardiometabolic diseases. We provide an in-depth summary on pre-existing bile acid receptor modulators, explain their shortcomings, and propose solutions for how they may be remedied. Lastly, we rationalize novel targets for further translational drug discovery and provide future perspectives. Rather than dismissing bile acid therapeutics due to recent setbacks, we believe that there is immense clinical potential and a high likelihood for the future success of bile acid therapeutics.

How Does CBG Administration Affect Sphingolipid Deposition in the Liver of Insulin-Resistant Rats?

BACKGROUND

Cannabigerol (CBG), a non-psychotropic phytocannabinoid found in Cannabis sativa plants, has been the focus of recent studies due to its potential therapeutic properties. We proposed that by focusing on sphingolipid metabolism, which plays a critical role in insulin signaling and the development of insulin resistance, CBG may provide a novel therapeutic approach for metabolic disorders, particularly insulin resistance.

METHODS

In a rat model of insulin resistance induced by a high-fat, high-sucrose diet (HFHS), we aimed to elucidate the effect of intragastrically administered CBG on hepatic sphingolipid deposition and metabolism. Moreover, we also elucidated the expression of sphingolipid transporters and changes in the sphingolipid concentration in the plasma.

RESULTS

The results, surprisingly, showed a lack of changes in de novo ceramide synthesis pathway enzymes and significant enhancement in the expression of enzymes involved in ceramide catabolism, which was confirmed by changes in hepatic sphingomyelin, sphinganine, sphingosine-1-phosphate, and sphinganine-1-phosphate concentrations.

CONCLUSIONS

The results suggest that CBG treatment may modulate sphingolipid metabolism in the liver and plasma, potentially protecting the liver against the development of metabolic disorders such as insulin resistance.

The altered lipidome of hepatocellular carcinoma

Alterations in metabolic pathways are a hallmark of cancer. A deeper understanding of the contribution of different metabolites to carcinogenesis is thus vitally important to elucidate mechanisms of tumor initiation and progression to inform therapeutic strategies. Hepatocellular carcinoma (HCC) is a leading cause of cancer-related death worldwide and its altered metabolic landscape is beginning to unfold with the advancement of technologies. In particular, characterization of the lipidome of human HCCs has accelerated, and together with biochemical analyses, are revealing recurrent patterns of alterations in glycerophospholipid, sphingolipid, cholesterol and bile acid metabolism. These widespread alterations encompass a myriad of lipid species with numerous roles affecting multiple hallmarks of cancer, including aberrant growth signaling, metastasis, evasion of cell death and immunosuppression. In this review, we summarize the current trends and findings of the altered lipidomic landscape of HCC and discuss their potential biological significance for hepatocarcinogenesis.

Cultured bear bile powder ameliorates acute liver injury in cholestatic mice via inhibition of hepatic inflammation and apoptosis

ETHNOPHARMACOLOGICAL RELEVANCE

Natural bear bile powder (NBBP) is a traditional Chinese medicine used for treating liver dysfunction. Cultured bear bile powder (CBBP), which is produced using biotransformation of chicken bile, acts as an appropriate substitute for NBBP when treating cholestatic liver injury.

AIM OF THE STUDY

To investigate the molecular mechanisms underlying the hepatoprotective effects of CBBP in an α-naphthylisothiocyanate (ANIT)-induced cholestatic mouse model.

MATERIALS AND METHODS

Cholestatic mice were pretreated with CBBP or NBBP via oral gavage once a day for two weeks. Their blood biochemistry and liver histopathology were then evaluated using standard protocols. Western blot analyses, real-time polymerase chain reaction, and immunohistochemistry were used to evaluate changes in the protein levels and gene expression profiles of factors associated with hepatic inflammation and apoptosis in cholestatic mice.

RESULTS

CBBP significantly decreased the serum indices of liver injury, and ameliorated neutrophil infiltration and hepatocyte necrosis within liver tissue of cholestatic mice. Expression of the inflammatory factors, such as tumor necrosis factor-α, interleukin-1β (IL-1β), IL-6, monocyte chemoattractant protein-1, and intercellular adhesion molecule 1, was significantly reduced in CBBP-treated cholestatic mice. Moreover, proteins involved in the toll-like receptor 4/myeloid differentiation factor 88/nuclear factor-kappa B (TLR4/Myd88/NF-κB) signaling pathway, such as CD14, TLR4, Myd88, and NF-κB, that were increased in cholestatic mice, were downregulated by CBBP. Meanwhile, increased expression of the apoptosis-related proteins, caspase-3 and Bax, in cholestatic mice was reversed by CBBP treatment.

CONCLUSION

CBBP treatment alleviates ANIT-induced cholestasis and liver injury by reducing hepatocyte inflammation and apoptosis.

Redox-Dependent Effects in the Physiopathological Role of Bile Acids

Bile acids (BA) are recognized by their role in nutrient absorption. However, there is growing evidence that BA also have endocrine and metabolic functions. Besides, the steroidal-derived structure gives BA a toxic potential over the biological membrane. Thus, cholestatic disorders, characterized by elevated BA on the liver and serum, are a significant cause of liver transplant and extrahepatic complications, such as skeletal muscle, central nervous system (CNS), heart, and placenta. Further, the BA have an essential role in cellular damage, mediating processes such as membrane disruption, mitochondrial dysfunction, and the generation of reactive oxygen species (ROS) and oxidative stress. The purpose of this review is to describe the BA and their role on hepatic and extrahepatic complications in cholestatic diseases, focusing on the association between BA and the generation of oxidative stress that mediates tissue damage.

Network pharmacology-based mechanism prediction and pharmacological validation of Xiaoyan Lidan formula on attenuating alpha-naphthylisothiocyanate induced cholestatic hepatic injury in rats

ETHNOPHARMACOLOGICAL RELEVANCE

The well-known Chinese prescription, Xiaoyan Lidan Formula (XYLDF), possesses efficiency of heat-clearing, dampness-eliminating and jaundice-removing. It has long been used clinically for the treatment of hepatobiliary diseases due to intrahepatic cholestasis (IHC). However, the mechanism of XYLDF for its therapeutic effects remains elusive.

AIM OF THE STUDY

The study aimed to explore the potential targets for liver protective mechanism of XYLDF based on network pharmacology and experimental assays in ANIT-induced cholestatic hepatic injury (CHI) in rats.

MATERIALS AND METHODS

On the basis of the 29 serum migrant compounds of XYLDF elucidated by UPLC-TOF-MS/MS, a network pharmacology approach was applied for the mechanism prediction. Systematic networks were constructed to identify potential molecular targets, biological processes, and signaling pathways. And the interactions between significantly potential targets and active compounds were simulated by molecular docking. For the mechanism validation, an ANIT-induced rat model was used to evaluate the effects of XYLDF on CHI according to serum biochemistry, bile flow rates, histopathological examination, and the gene and protein expression including enzymes related to synthesis, export, and import of bile acid in liver and ileum, and those of inflammatory cytokines, analyzed by RT-qPCR and WB.

RESULTS

The results of network pharmacology research indicated TNF (TNF-α), RELA (NF-κB), NR1H4 (FXR), and ICAM1 (ICAM-1) to be the important potential targets of XYLDF for cholestatic liver injury, which are related to bile metabolism and NF-κB-mediated inflammatory signaling. And the molecular docking had pre-validated the prediction of network pharmacology, as the core active compounds of XYLDF had shown strong simulation binding affinity with FXR, followed by NF-κB, TNF-α, and ICAM-1. Meanwhile, the effects of XYLDF after oral administration on ANIT-induced CHI in rats exhibited the decreased levels of transaminases (ALT and AST), TBA, and TBIL in serum, raised bile flow rates, and markedly improved hepatic histopathology. Furthermore, consistent to the above targets prediction and molecular docking, XYLDF significantly up-regulated the expression of FXR, SHP, BSEP, and MRP2, and down-regulated CYP7A1 and NTCP in liver, and promoted expression of IBABP and OSTα/β in ileum, suggesting the activation of FXR-mediated pathway referring to bile acid synthesis, transportation, and reabsorption. Moreover, the lower levels of TNF-α in plasma and liver, as well as the reduced hepatic gene and protein expression of NF-κB, TNF-α, and ICAM-1 after XYLDF treatment revealed the suppression of NF-κB-mediated inflammatory signaling pathway, as evidenced by the inhibition of nuclear translocation of NF-κB.

CONCLUSIONS

XYLDF exhibited an ameliorative liver protective effect on ANIT-induced cholestatic hepatic injury. The present study has confirmed its mechanism as activating the FXR-regulated bile acid pathway and inhibiting inflammation via the NF-κB signaling pathway.

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.

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

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

The role of sphingosine kinase 2 in alcoholic liver disease

Alcoholic liver disease (ALD) is one of the most common liver diseases worldwide. However, the exact mechanisms underlying ALD remain unclear. Previous studies reported that sphingosine kinase 2 (SphK2) plays an essential role in regulating hepatic lipid metabolism. In the current study, we demonstrate that compared to wild-type (WT) mice, SphK2 deficient (SphK2^-/-) mice exhibited a greater degree of liver injury and hepatic lipid accumulation after feeding with an alcohol diet for 60 days. This is accompanied by a down-regulation of steroid 7-alpha-hydroxylase (Cyp7b1) and an up-regulation of pro-inflammatory mediators (Tnfα, F4/80, Il-1β). In vitro experiments showed that alcohol induced SphK2 expression in mouse primary hepatocytes and cultured mouse macrophages. Furthermore, alcohol feeding induced a more severe intestinal barrier dysfunction in SphK2^-/- mice than WT mice. Deficiency of SphK2 impaired the growth of intestinal organoids. Finally, SphK2 expression levels were down-regulated in the livers of human patients with alcoholic cirrhosis and hepatocellular carcinoma compared to healthy controls. In summary, these findings suggest that SphK2 is a crucial regulator of hepatic lipid metabolism and that modulating the SphK2-mediated signaling pathway may represent a novel therapeutic strategy for the treatment of ALD and other metabolic liver diseases.

Sphingosine-1-phosphate signaling and the gut-liver axis in liver diseases

The liver is the central organ involved in lipid metabolism and the gastrointestinal (GI) tract is responsible for nutrient absorption and partitioning. Obesity, dyslipidemia and metabolic disorders are of increasing public health concern worldwide, and novel therapeutics that target both the liver and the GI tract (gut-liver axis) are much needed. In addition to aiding fat digestion, bile acids act as important signaling molecules that regulate lipid, glucose and energy metabolism via activating nuclear receptor, G protein-coupled receptors (GPCRs), Takeda G protein receptor 5 (TGR5) and sphingosine-1-phosphate receptor 2 (S1PR2). Sphingosine-1-phosphate (S1P) is synthesized by two sphingosine kinase isoforms and is a potent signaling molecule that plays a critical role in various diseases such as fatty liver, inflammatory bowel disease (IBD) and colorectal cancer. In this review, we will focus on recent findings related to the role of S1P-mediated signaling pathways in the gut-liver axis.

Bile Acid-Activated Receptors: GPBAR1 (TGR5) and Other G Protein-Coupled Receptors

The BA-responsive GPCRs S1PR2 and TGR5 are almost ubiquitously expressed in human and rodent tissues. In the liver, S1PR2 is expressed in all cell types, while TGR5 is predominately found in non-parenchymal cells. In contrast to S1PR2, which is mainly activated by conjugated bile acids (BAs), all BAs serve as ligands for TGR5 irrespective of their conjugation state and substitution pattern.Mice with targeted deletion of either S1PR2 or TGR5 are viable and develop no overt phenotype. In liver injury models, S1PR2 exerts pro-inflammatory and pro-fibrotic effects and thus aggravates liver damage, while TGR5 mediates anti-inflammatory, anti-cholestatic, and anti-fibrotic effects. Thus, inhibitors of S1PR2 signaling and agonists for TGR5 have been employed to attenuate liver injury in rodent models for cholestasis, nonalcoholic steatohepatitis, and fibrosis/cirrhosis.In biliary epithelial cells, both receptors activate a similar signaling cascade resulting in ERK1/2 phosphorylation and cell proliferation. Overexpression of both S1PR2 and TGR5 was found in human cholangiocarcinoma tissue as well as in CCA cell lines, where stimulation of both GPCRs resulted in transactivation of the epidermal growth factor receptor and triggered cell proliferation as well as increased cell migration and invasiveness.This chapter will focus on the function of S1PR2 and TGR5 in different liver cell types and summarizes current knowledge on the role of these receptors in liver disease models.

Sphingosine 1-phosphate (S1P) reduces hepatocyte growth factor-induced migration of hepatocellular carcinoma cells via S1P receptor 2

A bioactive lipid, sphingosine 1-phosphate (S1P), acts extracellularly as a potent mediator, and is implicated in the progression of various cancers including hepatocellular carcinoma (HCC). S1P exerts its functions by binding to five types of specific receptors, S1P receptor 1 (S1PR1), S1PR2, S1PR3, S1PR4 and S1PR5 on the plasma membrane. However, the exact roles of S1P and each S1PR in HCC cells remain to be clarified. In the present study, we investigated the effect of S1P on the hepatocyte growth factor (HGF)-induced migration of human HCC-derived HuH7 cells, and the involvement of each S1PR. S1P dose-dependently reduced the HGF-induced migration of HuH7 cells. We found that all S1PRs exist in the HuH7 cells. Among each selective agonist for five S1PRs, CYM5520, a selective S1PR2 agonist, significantly suppressed the HGF-induced HuH7 cell migration whereas selective agonists for S1PR1, S1PR3, S1PR4 or S1PR5 failed to affect the migration. The reduction of the HGF-induced migration by S1P was markedly reversed by treatment of JTE013, a selective antagonist for S1PR2, and S1PR2- siRNA. These results strongly suggest that S1P reduces the HGF-induced HCC cell migration via S1PR2. Our findings may provide a novel potential of S1PR2 to therapeutic strategy for metastasis of HCC.

Early indications of ANIT-induced cholestatic liver injury: Alteration of hepatocyte polarization and bile acid homeostasis

Hepatocyte polarization is essential for biliary secretion, and loss of polarity causes bile secretory failure and hepatotoxicity. Here, we showed that alpha-naphthyl isothiocyanate (ANIT)-induced liver injury was accompanied by the dynamic interruption of bile acid homeostasis in rat plasma, liver and bile, which was characterized by the redistribution of bile acids in plasma and bile and a small range of fluctuations in the liver. Molecular mechanism studies indicated that these factors are dynamically mediated by the disruption of bile acid transporters and hepatic tight junctions. Dynamic changes in tight junction (TJ) permeability were observed by hepatobiliary barrier function assessment. Hepatocyte polarization was disrupted by ANIT before the development of cholestatic hepatotoxicity and alteration of bile acid metabolic profiles, which were assayed by high-performance liquid chromatography-tandem mass spectrometry, further verifying TJ deficiency. S1PR1 activation with SEW2871 reduced ANIT-induced liver injury by reducing the total serum bile acid concentration, liver functional enzyme activity and inflammation. Our data suggest that hepatocyte polarization plays an important role in maintaining bile acid homeostasis before the development of cholestatic hepatotoxicity and that TJs were more prominent in the early stage of cholestasis. S1PR1 may be a potential target for the prevention of drug-induced cholestatic liver injury.

Sphingosine Kinases/Sphingosine 1-Phosphate Signaling in Hepatic Lipid Metabolism

The ever-increasing prevalence of metabolic diseases such as dyslipidemia and diabetes in the western world continues to be of great public health concern. Biologically active sphingolipids, such as sphingosine 1-phosphate (S1P) and ceramide, are important regulators of lipid metabolism. S1P not only directly functions as an active intracellular mediator, but also activates multiple signaling pathways via five transmembrane G-protein coupled receptors (GPCRs), S1PR1-5. S1P is exclusively formed by sphingosine kinases (SphKs). Two isoforms of SphKs, SphK1 and SphK2, have been identified. Recent identification of the conjugated bile acid-induced activation of S1PR2 as a key regulator of SphK2 opened new directions for both the sphingolipid and bile acid research fields. The role of SphKs/S1P-mediated signaling pathways in health and various human diseases has been extensively reviewed elsewhere. This review focuses on recent findings related to SphKs/S1P-medaited signaling pathways in regulating hepatic lipid metabolism.

Hydrophobic bile acid apoptosis is regulated by sphingosine-1-phosphate receptor 2 in rat hepatocytes and human hepatocellular carcinoma cells

The hepatotoxic bile acid glycochenodeoxycholate (GCDC) modulates hepatocyte cell death through activation of JNK, Akt, and Erk. The nonhepatotoxic bile acid taurocholate activates Akt and Erk through the sphingosine-1-phosphate receptor 2 (S1PR2). The role of the S1PR2 in GCDC-mediated apoptosis and kinase activation is unknown. Studies were done in rat hepatocytes, HUH7 cells, and HUH7 cells stably transfected with rat Ntcp (HUH7-Ntcp). Cells were treated with GCDC and apoptosis was monitored morphologically by Hoechst staining and biochemically by immunoblotting for the active cleaved fragment of caspase 3. Kinase activation was determined by immunoblotting with phospho-specific antibodies. JTE-013, an inhibitor of S1PR2, significantly attenuated morphological evidence of GCDC-induced apoptosis and prevented caspase 3 cleavage in rat hepatocytes and HUH7-Ntcp cells. In hepatocytes, JTE-013 mildly suppressed, augmented, and had no effect on GCDC-induced JNK, Akt, and Erk phosphorylation, respectively. Similar results were seen in HUH7-Ntcp cells except for mild suppression of JNK and Erk phosphorylation. Knockdown of S1PR2 in HUH7-Ntcp augmented Akt, inhibited JNK, and had no effect on Erk phosphorylation. GCDC failed to induce apoptosis or kinase activation in HUH7 cells. In conclusion, SIPR2 inhibition attenuates GCDC-induced apoptosis and inhibits and augments GCDC-induced JNK and Akt phosphorylation, respectively. In addition, GCDC must enter hepatocytes to mediate cell death or activate kinases. These results suggest that SIPR2 activation is proapoptotic in GCDC-induced cell death but that this effect is not due to direct ligation of the S1PR2 by the bile acid.

Hormesis in Cholestatic Liver Disease; Preconditioning with Low Bile Acid Concentrations Protects against Bile Acid-Induced Toxicity

Introduction

Cholestasis is characterized by accumulation of bile acids and inflammation, causing hepatocellular damage. Still, liver damage markers are highest in acute cholestasis and drop when this condition becomes chronic, indicating that hepatocytes adapt towards the hostile environment. This may be explained by a hormetic response in hepatocytes that limits cell death during cholestasis.

Aim

To investigate the mechanisms that underlie the hormetic response that protect hepatocytes against experimental cholestatic conditions.

Methods

HepG2.rNtcp cells were preconditioned (24 h) with sub-apoptotic concentrations (0.1–50 μM) of various bile acids, the superoxide donor menadione, TNF-α or the Farsenoid X Receptor agonist GW4064, followed by a challenge with the apoptosis-inducing bile acid glycochenodeoxycholic acid (GCDCA; 200 μM for 4 h), menadione (50 μM, 6 h) or cytokine mixture (CM; 6 h). Levels of apoptotic and necrotic cell death, mRNA expression of the bile salt export pump (ABCB11) and bile acid sensors, as well as intracellular GCDCA levels were analyzed.

Results

Preconditioning with the pro-apoptotic bile acids GCDCA, taurocholic acid, or the protective bile acids (tauro)ursodeoxycholic acid reduced GCDCA-induced caspase-3/7 activity in HepG2.rNtcp cells. Bile acid preconditioning did not induce significant levels of necrosis in GCDCA-challenged HepG2.rNtcp cells. In contrast, preconditioning with cholic acid, menadione or TNF-α potentiated GCDCA-induced apoptosis. GCDCA preconditioning specifically reduced GCDCA-induced cell death and not CM- or menadione-induced apoptosis. The hormetic effect of GCDCA preconditioning was concentration- and time-dependent. GCDCA-, CDCA- and GW4064- preconditioning enhanced ABCB11 mRNA levels, but in contrast to the bile acids, GW4064 did not significantly reduce GCDCA-induced caspase-3/7 activity. The GCDCA challenge strongly increased intracellular levels of this bile acid, which was not lowered by GCDCA-preconditioning.

Conclusions

Sub-toxic concentrations of bile acids in the range that occur under normal physiological conditions protect HepG2.rNtcp cells against GCDCA-induced apoptosis, which is independent of FXR-controlled changes in bile acid transport.

SphK1 promotes tumor cell migration and invasion in colorectal cancer

Colorectal cancer (CRC) is one of the most common cancers worldwide. Sphingosine kinase 1 (SphK1), which phosphorylates sphingosine to sphingosine-1-phosphate (S1P), is overexpressed in various types of cancers and may act as an oncogene in tumorigenesis. However, little is known about the role of SphK1 in CRC patients. We studied the expression of SphK1 in 85 cases of CRC tissues by immunohistochemistry, qRT-PCR, and western blot. We also evaluated the effect of SphK1 on cell proliferation and invasion by MTT and transwell invasion assay. SphK1 is overexpressed in CRC tissues and cell lines, and upregulation of SphK1 correlated significantly with the following parameters: lymph node metastasis, liver metastasis, and advanced TNM stage. SphK1 knockdown results in inhibition of cancer cell proliferation. Inhibition of CRC cell migration and invasion is also evident through reversal of EMT by increases in E-cadherin expression and decreases in vimentin expression. In conclusion, SphK1 is associated with the proliferation and invasiveness of CRC cells and the SphK1 gene may contribute to a novel therapeutic approach against CRC.

Therapeutic targets for cholestatic liver injury

INTRODUCTION

Cholestasis is a reduction in bile flow that occurs during numerous pathologies. Blockage of the biliary tracts results in hepatic accumulation of bile acids or their conjugate bile salts. The molecular mechanisms behind liver injury associated with cholestasis are extensively studied, but not well understood. Multiple models of obstructive cholestasis result in a significant inflammatory infiltrate at the sites of necrosis that characterize the injury.

AREAS COVERED

This review will focus on direct bile acid toxicity during cholestasis, bile acid signaling processes and on the development and continuation of inflammation during cholestasis, with a focus on novel proposed molecular mediators of neutrophil recruitment. While significant progress has been made on these molecular mechanisms, a continued focus on how cholestasis and the innate immune system interact is necessary to discover targetable therapeutics that might protect the liver while leaving global immunity intact.

EXPERT OPINION

While bile acid toxicity likely occurs in humans and other mammals when toxic bile acids accumulate, persistent inflammation is likely responsible for continued liver injury during obstructive cholestasis. Targeting molecular mediators of inflammation may help prevent liver injury during acute cholestasis both in murine models and human patients.

SphK1 mediates hepatic inflammation in a mouse model of NASH induced by high saturated fat feeding and initiates proinflammatory signaling in hepatocytes

Steatohepatitis occurs in up to 20% of patients with fatty liver disease and leads to its primary disease outcomes, including fibrosis, cirrhosis, and increased risk of hepatocellular carcinoma. Mechanisms that mediate this inflammation are of major interest. We previously showed that overload of saturated fatty acids, such as that which occurs with metabolic syndrome, induced sphingosine kinase 1 (SphK1), an enzyme that generates sphingosine-1-phosphate (S1P). While data suggest beneficial roles for S1P in some contexts, we hypothesized that it may promote hepatic inflammation in the context of obesity. Consistent with this, we observed 2-fold elevation of this enzyme in livers from humans with nonalcoholic fatty liver disease and also in mice with high saturated fat feeding, which recapitulated the human disease. Mice exhibited activation of NFκB, elevated cytokine production, and immune cell infiltration. Importantly, SphK1-null mice were protected from these outcomes. Studies in cultured cells demonstrated saturated fatty acid induction of SphK1 message, protein, and activity, and also a requirement of the enzyme for NFκB signaling and increased mRNA encoding TNFα and MCP1. Moreover, saturated fat-induced NFκB signaling and elevation of TNFα and MCP1 mRNA in HepG2 cells was blocked by targeted knockdown of S1P receptor 1, supporting a role for this lipid signaling pathway in inflammation in nonalcoholic fatty liver disease.

Sphingosine Kinase 1 Protects Hepatocytes from Lipotoxicity via Down-regulation of IRE1α Protein Expression

Aberrant deposition of fat including free fatty acids in the liver often causes damage to hepatocytes, namely lipotoxicity, which is a key pathogenic event in the development and progression of fatty liver diseases. This study demonstrates a pivotal role of sphingosine kinase 1 (SphK1) in protecting hepatocytes from lipotoxicity. Exposure of primary murine hepatocytes to palmitate resulted in dose-dependent cell death, which was enhanced significantly in Sphk1-deficient cells. In keeping with this, expression of dominant-negative mutant SphK1 also markedly promoted palmitate-induced cell death. In contrast, overexpression of wild-type SphK1 profoundly protected hepatocytes from lipotoxicity. Mechanistically, the protective effect of SphK1 is attributable to suppression of ER stress-mediated pro-apoptotic pathways, as reflected in the inhibition of IRE1α activation, XBP1 splicing, JNK phosphorylation, and CHOP induction. Of note, SphK1 inhibited the IRE1α pathway by reducing IRE1α expression at the transcriptional level. Moreover, S1P mimicked the effect of SphK1, suppressing IRE1α expression in a receptor-dependent manner. Furthermore, enforced overexpression of IRE1α significantly blocked the protective effect of SphK1 against lipotoxicity. Therefore, this study provides new insights into the role of SphK1 in hepatocyte survival and uncovers a novel mechanism for protection against ER stress-mediated cell death.

Bile acids and sphingosine-1-phosphate receptor 2 in hepatic lipid metabolism

The liver is the central organ involved in lipid metabolism. Dyslipidemia and its related disorders, including non-alcoholic fatty liver disease (NAFLD), obesity and other metabolic diseases, are of increasing public health concern due to their increasing prevalence in the population. Besides their well-characterized functions in cholesterol homoeostasis and nutrient absorption, bile acids are also important metabolic regulators and function as signaling hormones by activating specific nuclear receptors, G-protein coupled receptors, and multiple signaling pathways. Recent studies identified a new signaling pathway by which conjugated bile acids (CBA) activate the extracellular regulated protein kinases (ERK1/2) and protein kinase B (AKT) signaling pathway via sphingosine-1-phosphate receptor 2 (S1PR2). CBA-induced activation of S1PR2 is a key regulator of sphingosine kinase 2 (SphK2) and hepatic gene expression. This review focuses on recent findings related to the role of bile acids/S1PR2-mediated signaling pathways in regulating hepatic lipid metabolism.

Conjugated bile acids promote cholangiocarcinoma cell invasive growth through activation of sphingosine 1-phosphate receptor 2

Cholangiocarcinoma (CCA) is an often fatal primary malignancy of the intra- and extrahepatic biliary tract that is commonly associated with chronic cholestasis and significantly elevated levels of primary and conjugated bile acids (CBAs), which are correlated with bile duct obstruction (BDO). BDO has also recently been shown to promote CCA progression. However, whereas there is increasing evidence linking chronic cholestasis and abnormal bile acid profiles to CCA development and progression, the specific mechanisms by which bile acids may be acting to promote cholangiocarcinogenesis and invasive biliary tumor growth have not been fully established. Recent studies have shown that CBAs, but not free bile acids, stimulate CCA cell growth, and that an imbalance in the ratio of free to CBAs may play an important role in the tumorigenesis of CCA. Also, CBAs are able to activate extracellular signal-regulated kinase (ERK)1/2- and phosphatidylinositol-3-kinase/protein kinase B (AKT)-signaling pathways through sphingosine 1-phosphate receptor 2 (S1PR2) in rodent hepatocytes. In the current study, we demonstrate S1PR2 to be highly expressed in rat and human CCA cells, as well as in human CCA tissues. We further show that CBAs activate the ERK1/2- and AKT-signaling pathways and significantly stimulate CCA cell growth and invasion in vitro. Taurocholate (TCA)-mediated CCA cell proliferation, migration, and invasion were significantly inhibited by JTE-013, a chemical antagonist of S1PR2, or by lentiviral short hairpin RNA silencing of S1PR2. In a novel organotypic rat CCA coculture model, TCA was further found to significantly increase the growth of CCA cell spheroidal/"duct-like" structures, which was blocked by treatment with JTE-013.

CONCLUSION

Our collective data support the hypothesis that CBAs promote CCA cell-invasive growth through S1PR2.