Divergent Role of Sphingosine 1-Phosphate in Liver Health and Disease

Cannabidiol and sphingolipid metabolism - an unexplored link offering a novel therapeutic approach against high-fat diet-induced hepatic insulin resistance

Despite extensive research on insulin resistance, which is associated with type 2 diabetes and obesity, there remains a lack of effective and safe methods to treat it. Thus, we hypothesized that cannabidiol (CBD), which influences lipid accumulation and inflammatory response, may interact with sphingolipid metabolism and insulin signaling. To investigate the effects of CBD, male Wistar rats were fed a standard rodent chow or high-fat diet for 7 weeks to induce IR and were treated with CBD or its vehicle administered intraperitoneally for the last two weeks of the experiment. High-Performance Liquid Chromatography (HPLC) was used to assess sphingolipid concentration in the liver, while multiplex assay and western blotting were used to investigate the level or expression of proteins in the insulin signaling pathway and sphingolipid metabolism. Our results revealed that CBD prevented ceramide deposition in the liver of high-fat-fed rats through inhibition of the ceramide de novo synthesis pathway. Moreover, the accumulation of sphingosine-1-phosphate was notably increased with impaired catabolic pathway. Observed changes in the sphingolipid pathway coincided with improved insulin signaling after CBD treatment in animals fed a high-fat diet. Considering the presented evidence, CBD exerted a beneficial effect on insulin sensitivity in a state of lipid overload through the modification of sphingolipid deposition. Our study reveals the importance of broadening IR treatment methods, especially with natural substances that lack serious side effects such as CBD.

Effect of Hepatic Lipid Overload on Accelerated Hepatocyte Proliferation Promoted by HGF Expression via the SphK1/S1PR2 Pathway in MCD-diet Mouse Partial Hepatectomy

Metabolic dysfunction-associated steatotic liver disease (MASLD) is becoming a major health problem worldwide. Liver regeneration is crucial for restoring liver function, and is regulated by extraordinary complex process, involving numerous factors under both physiologic and pathologic conditions. Sphingosine-1-phosphate (S1P), a bioactive sphingolipid synthesized by sphingosine kinase 1 (SphK1), plays an important role in liver function through S1P receptors (S1PRs)-expressing cells. In this study, we investigated the effect of lipid overload on hepatocyte proliferation in a mouse hepatic steatosis model induced by feeding a methionine- and choline-deficient (MCD) diet. After 50% partial hepatectomy (PHx), liver tissues were sampled at various timepoints and then analyzed by immunohistochemistry, oil Red-O staining, quantitative-polymerase chain reaction (qPCR), and flow cytometry. In mice fed the MCD-diet, significantly exacerbated hepatic steatosis and accelerated liver regeneration were observed. After PHx, hepatocyte proliferation peaked at 48 and 36 hr in the liver of chow- and MCD-diet fed mice, respectively. By contrast, increased expression of S1PR2 was observed in hepatic neutrophils and macrophages of MCD-diet fed mice. Flow cytometry and qPCR experiments demonstrated that levels of HGF and FGF2 released by neutrophils and macrophages were significantly higher in MCD-diet fed mice. In conclusion, hepatic lipid overload recruits Kupffer cells and neutrophils that release HGF and FGF2 via SphK1/S1PR2 activation to accelerate hepatocyte proliferation.

Exogenous S1P via S1P receptor 2 induces CTGF expression through Src-RhoA-ROCK-YAP pathway in hepatic stellate cells

BACKGROUND

Hepatic fibrosis, a prevalent chronic liver condition, involves excessive extracellular matrix production associated with aberrant wound healing. Hepatic stellate cells (HSCs) play a pivotal role in liver fibrosis, activated by inflammatory factors such as sphingosine 1-phosphate (S1P). Despite S1P's involvement in fibrosis, its specific role and downstream pathway in HSCs remain controversial.

METHODS

In this study, we investigated the regulatory role of S1P/S1P receptor (S1PR) in Hippo-YAP activation in both LX-2 cell lines and primary HSCs. Real-time PCR, western blot, pharmacological inhibitors, siRNAs, and Rho activity assays were adopted to address the molecular mechanisms of S1P mediated YAP activation.

RESULTS

Serum and exogenous S1P significantly increased the expression of YAP target genes in HSCs. Pharmacologic inhibitors and siRNA-mediated knockdowns of S1P receptors showed S1P receptor 2 (S1PR2) as the primary mediator for S1P-induced CTGF expression in HSCs. Results using siRNA-mediated knockdown, Verteporfin, and Phospho-Tag immunoblots showed that S1P-S1PR2 signaling effectively suppressed the Hippo kinases cascade, thereby activating YAP. Furthermore, S1P increased RhoA activities in cells and ROCK inhibitors effectively blocked CTGF induction. Cytoskeletal-perturbing reagents were shown to greatly modulate CTGF induction, suggesting the important role of actin cytoskeleton in S1P-induced YAP activation. Exogeneous S1P treatment was enough to increase the expression of COL1A1 and α-SMA, that were blocked by YAP specific inhibitor.

CONCLUSIONS

Our data demonstrate that S1P/S1PR2-Src-RhoA-ROCK axis leads to Hippo-YAP activation, resulting in the up-regulation of CTGF, COL1A1 and α-SMA expression in HSCs. Therefore, S1PR2 may represent a potential therapeutic target for hepatic fibrosis.

Exploring the potential of drug repurposing for liver diseases: A comprehensive study

Drug repurposing involves the investigation of existing drugs for new indications. It offers a great opportunity to quickly identify a new drug candidate at a lower cost than novel discovery and development. Despite the importance and potential role of drug repurposing, there is no specific definition that healthcare providers and the World Health Organization credit. Unfortunately, many similar and interchangeable concepts are being used in the literature, making it difficult to collect and analyze uniform data on repurposed drugs. This research was conducted based on understanding general criteria for drug repurposing, concentrating on liver diseases. Many drugs have been investigated for their effect on liver diseases even though they were originally approved (or on their way to being approved) for other diseases. Some of the hypotheses for drug repurposing were first captured from the literature and then processed further to test the hypothesis. Recently, with the revolution in bioinformatics techniques, scientists have started to use drug libraries and computer systems that can analyze hundreds of drugs to give a short list of candidates to be analyzed pharmacologically. However, this study revealed that drug repurposing is a potential aid that may help deal with liver diseases. It provides available or under-investigated drugs that could help treat hepatitis, liver cirrhosis, Wilson disease, liver cancer, and fatty liver. However, many further studies are needed to ensure the efficacy of these drugs on a large scale.

Critical Roles of the Sphingolipid Metabolic Pathway in Liver Regeneration, Hepatocellular Carcinoma Progression and Therapy

The sphingolipid metabolic pathway, an important signaling pathway, plays a crucial role in various physiological processes including cell proliferation, survival, apoptosis, and immune regulation. The liver has the unique ability to regenerate using bioactive lipid mediators involving multiple sphingolipids, including ceramide and sphingosine 1-phosphate (S1P). Dysregulation of the balance between sphingomyelin, ceramide, and S1P has been implicated in the regulation of liver regeneration and diseases, including liver fibrosis and hepatocellular carcinoma (HCC). Understanding and modulating this balance may have therapeutic implications for tumor proliferation, progression, and metastasis in HCC. For cancer therapy, several inhibitors and activators of sphingolipid signaling, including ABC294640, SKI-II, and FTY720, have been discussed. Here, we elucidate the critical roles of the sphingolipid pathway in the regulation of liver regeneration, fibrosis, and HCC. Regulation of sphingolipids and their corresponding enzymes may considerably influence new insights into therapies for various liver disorders and diseases.

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.

Lipidomics in pathogenesis, progression and treatment of nonalcoholic steatohepatitis (NASH): Recent advances

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease affecting up to 30% of the general adult population. NAFLD encompasses a histological spectrum ranging from pure steatosis to non-alcoholic steatohepatitis (NASH). NASH can progress to cirrhosis and is becoming the most common indication for liver transplantation, as a result of increasing disease prevalence and of the absence of approved treatments. Lipidomic readouts of liver blood and urine samples from experimental models and from NASH patients disclosed an abnormal lipid composition and metabolism. Collectively, these changes impair organelle function and promote cell damage, necro-inflammation and fibrosis, a condition termed lipotoxicity. We will discuss the lipid species and metabolic pathways leading to NASH development and progression to cirrhosis, as well as and those species that can contribute to inflammation resolution and fibrosis regression. We will also focus on emerging lipid-based therapeutic opportunities, including specialized proresolving lipid molecules and macrovesicles contributing to cell-to-cell communication and NASH pathophysiology.

Characterization of lipid signatures in the plasma and insulin-sensitive tissues of the C57BL/6J mice fed on obesogenic diets

Diet-induced obesity mouse models are widely utilized to investigate the underlying mechanisms of dyslipidemia, glucose intolerance, insulin resistance, hepatic steatosis, and type 2 diabetes mellitus (T2DM), as well as for screening potential drug compounds. However, there is limited knowledge regarding specific signature lipids that accurately reflect dietary disorders. In this study, we aimed to identify key lipid signatures using LC/MS-based untargeted lipidomics in the plasma, liver, adipose tissue (AT), and skeletal muscle tissues (SKM) of male C57BL/6J mice that were fed chow, LFD, or obesogenic diets (HFD, HFHF, and HFCD) for a duration of 20 weeks. Furthermore, we conducted a comprehensive lipid analysis to assess similarities and differences with human lipid profiles. The mice fed obesogenic diets exhibited weight gain, glucose intolerance, elevated BMI, glucose and insulin levels, and a fatty liver, resembling characteristics of T2DM and obesity in humans. In total, we identified approximately 368 lipids in plasma, 433 in the liver, 493 in AT, and 624 in SKM. Glycerolipids displayed distinct patterns across the tissues, differing from human findings. However, changes in sphingolipids, phospholipids, and the expression of inflammatory and fibrotic genes showed similarities to reported human findings. Significantly modulated pathways in the obesogenic diet-fed groups included ceramide de novo synthesis, sphingolipid remodeling, and the carboxylesterase pathway, while lipoprotein-mediated pathways were minimally affected.

Raspberry ketone improves non-alcoholic fatty liver disease induced in rats by modulating sphingosine kinase/sphingosine-1-phosphate and toll-like receptor 4 pathways

OBJECTIVES

To investigate the therapeutic role of calorie-restricted diet (CR) and raspberry ketone (RK) in non-alcoholic fatty liver disease (NAFLD) and the implication of sphingosine kinase-1 (SphK1)/sphingosine-1-phosphate (S1P) and toll-like receptor 4 (TLR4) signalling.

METHODS

NAFLD was induced by feeding rats high-fat-fructose-diet (HFFD) for 6 weeks. Rats were then randomly assigned to three groups (n = 6 each); NAFLD group continued on HFFD for another 8 weeks. CR group was switched to CR diet (25% calorie restriction) for 8 weeks and RK group was switched to normal diet and received RK (55 mg/kg/day; orally) for 8 weeks. Another six rats were used as normal control.

KEY FINDINGS

HFFD induced a state of NAFLD indicated by increased fat deposition in liver tissue along with dyslipidemia, elevated liver enzymes, oxidative stress and inflammation. Either CR diet or RK reversed these changes and decreased HFFD-induced elevation of hepatic SphK1, S1P, S1PR1 and TLR4. Of notice, RK along with a normal calorie diet was even better than CR alone in most studied parameters.

CONCLUSIONS

SphK1/S1P and TLR4 are interconnected and related to the establishment of HFFD-induced NAFLD and can be modulated by RK. Supplementation of RK without calorie restriction to patients with NAFLD unable to follow CR diet to achieve their treatment goals would be a promising therapeutic modality.

Modulating sphingosine 1-phosphate receptor signaling skews intrahepatic leukocytes and attenuates murine nonalcoholic steatohepatitis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid associated with nonalcoholic steatohepatitis (NASH). Immune cell-driven inflammation is a key determinant of NASH progression. Macrophages, monocytes, NK cells, T cells, NKT cells, and B cells variably express S1P receptors from a repertoire of 5 receptors termed S1P1 - S1P5. We have previously demonstrated that non-specific S1P receptor antagonism ameliorates NASH and attenuates hepatic macrophage accumulation. However, the effect of S1P receptor antagonism on additional immune cell populations in NASH remains unknown. We hypothesized that S1P receptor specific modulation may ameliorate NASH by altering leukocyte recruitment. A murine NASH model was established by dietary feeding of C57BL/6 male mice with a diet high in fructose, saturated fat, and cholesterol (FFC) for 24 weeks. In the last 4 weeks of dietary feeding, the mice received the S1P1,4,5 modulator Etrasimod or the S1P1 modulator Amiselimod, daily by oral gavage. Liver injury and inflammation were determined by histological and gene expression analyses. Intrahepatic leukocyte populations were analyzed by flow cytometry, immunohistochemistry, and mRNA expression. Alanine aminotransferase, a sensitive circulating marker for liver injury, was reduced in response to Etrasimod and Amiselimod treatment. Liver histology showed a reduction in inflammatory foci in Etrasimod-treated mice. Etrasimod treatment substantially altered the intrahepatic leukocyte populations through a reduction in the frequency of T cells, B cells, and NKT cells and a proportional increase in CD11b+ myeloid cells, polymorphonuclear cells, and double negative T cells in FFC-fed and control standard chow diet (CD)-fed mice. In contrast, FFC-fed Amiselimod-treated mice showed no changes in the frequencies of intrahepatic leukocytes. Consistent with the improvement in liver injury and inflammation, hepatic macrophage accumulation and the gene expression of proinflammatory markers such as Lgals3 and Mcp-1 were decreased in Etrasimod-treated FFC-fed mice. Etrasimod treated mouse livers demonstrated an increase in non-inflammatory (Marco) and lipid associated (Trem2) macrophage markers. Thus, S1P1,4,5 modulation by Etrasimod is more effective than S1P1 antagonism by Amiselimod, at the dose tested, in ameliorating NASH, likely due to the alteration of leukocyte trafficking and recruitment. Etrasimod treatment results in a substantial attenuation of liver injury and inflammation in murine NASH.

The impact of subchronic ozone exposure on serum metabolome and the mechanisms of abnormal bile acid and arachidonic acid metabolisms in the liver

Ambient ozone (O₃) pollution can induce respiratory and cardiovascular toxicity. However, its impact on the metabolome and the underlying mechanisms remain unclear. This study first investigated the serum metabolite changes in rats exposed to 0.5 ppm O₃ for 3 months using untargeted metabolomic approach. Results showed chronic ozone exposure significantly altered the serum levels of 34 metabolites with potential increased risk of digestive, respiratory and cardiovascular disease. Moreover, bile acid synthesis and secretion, and arachidonic acid (AA) metabolism became the most prominent affected metabolic pathways after O₃ exposure. Further studies on the mechanisms found that the elevated serum toxic bile acid was not due to the increased biosynthesis in the liver, but the reduced reuptake from the portal vein to hepatocytes owing to repressed Ntcp and Oatp1a1, and the decreased bile acid efflux in hepatocytes as a results of inhibited Bsep, Ostalpha and Ostbeta. Meanwhile, decreased expressions of detoxification enzyme of SULT2A1 and the important regulators of FXR, PXR and HNF4α also contributed to the abnormal bile acids. In addition, O₃ promoted the conversion of AA into thromboxane A2 (TXA2) and 20-hydroxyarachidonic acid (20-HETE) in the liver by up-regulation of Fads2, Cyp4a and Tbxas1 which resulting in decreased AA and linoleic acid (LA), and increased thromboxane B2 (TXB2) and 20-HETE in the serum. Furthermore, apparent hepatic chronic inflammation, fibrosis and abnormal function were found in ozone-exposed rats. These results indicated chronic ozone exposure could alter serum metabolites by interfering their metabolism in the liver, and inducing liver injury to aggravate metabolic disorders.

Dietary L-theanine supplementation improves lipid metabolism and antioxidant capacity in weaning piglets

The aim of this study was to explore the effects of dietary L-theanine (LT) supplementation on lipid metabolism and antioxidant capacity in weaned piglets. Twenty-one castrated DLY weaning piglets were randomly divided into three groups: a basal diet, a basal diet supplemented with 0.05% and 0.1% LT, respectively. Our data showed that dietary LT supplementation decreased T-CHO, TG, LDL-C and apoB levels and increased apoA and HDL-C levels in serum, but decreased the hepatic TG and T-CHO contents. Dietary LT supplementation increased the antioxidant capacity in serum and liver, and significantly increased the Nrf2 mRNA level and the nucleus Nrf2 protein level, but decreased the mRNA level of keap1 in the liver. In addition, dietary LT supplementation significantly increased HSL enzyme activity and the levels of CPT1 and TBA, while decreasing the enzyme activities of LPL and FAS in the liver. Furthermore, the mRNA levels HMG-CoAR, CPT-1a and PPARα and the protein levels of phosphorylated-AMPK and PGC-1α were increased by LT. Together, our data provide the first evidence that dietary supplementation of LT could improve lipid metabolism and antioxidant capacity in the liver of weaned piglets, and the effect might be mediated by activation of AMPK and Nrf2 signaling, respectively.

Hepatic lymphatic vascular system in health and disease

In recent years, significant advances have been made in the study of lymphatic vessels with the identification of their specific markers and the development of research tools that have accelerated our understanding of their role in tissue homeostasis and disease pathogenesis in many organs. Compared to other organs, the lymphatic system in the liver is understudied despite its obvious importance for hepatic physiology and pathophysiology. In this review, we describe fundamental aspects of the hepatic lymphatic system and its role in a range of liver-related pathological conditions such as portal hypertension, ascites formation, malignant tumours, liver transplantation, congenital liver diseases, non-alcoholic fatty liver disease, and hepatic encephalopathy. The article concludes with a discussion regarding the modulation of lymphangiogenesis as a potential therapeutic strategy for liver diseases.

Sphingosine 1-Phosphate Receptor 5 (S1P5) Knockout Ameliorates Adenine-Induced Nephropathy

S1P and its receptors have been reported to play important roles in the development of renal fibrosis. Although S1P5 has barely been investigated so far, there are indications that it can influence inflammatory and fibrotic processes. Here, we report the role of S1P5 in renal inflammation and fibrosis. Male S1P5 knockout mice and wild-type mice on a C57BL/6J background were fed with an adenine-rich diet for 7 days or 14 days to induce tubulointerstitial fibrosis. The kidneys of untreated mice served as respective controls. Kidney damage, fibrosis, and inflammation in kidney tissues were analyzed by real-time PCR, Western blot, and histological staining. Renal function was assessed by plasma creatinine ELISA. The S1P5 knockout mice had better renal function and showed less kidney damage, less proinflammatory cytokine release, and less fibrosis after 7 days and 14 days of an adenine-rich diet compared to wild-type mice. S1P5 knockout ameliorates tubular damage and tubulointerstitial fibrosis in a model of adenine-induced nephropathy in mice. Thus, targeting S1P5 might be a promising goal for the pharmacological treatment of kidney diseases.

To Be or Not to Be: The Divergent Action and Metabolism of Sphingosine-1 Phosphate in Pancreatic Beta-Cells in Response to Cytokines and Fatty Acids

Sphingosine-1 phosphate (S1P) is a bioactive sphingolipid with multiple functions conveyed by the activation of cell surface receptors and/or intracellular mediators. A growing body of evidence indicates its important role in pancreatic insulin-secreting beta-cells that are necessary for maintenance of glucose homeostasis. The dysfunction and/or death of beta-cells lead to diabetes development. Diabetes is a serious public health burden with incidence growing rapidly in recent decades. The two major types of diabetes are the autoimmune-mediated type 1 diabetes (T1DM) and the metabolic stress-related type 2 diabetes (T2DM). Despite many differences in the development, both types of diabetes are characterized by chronic hyperglycemia and inflammation. The inflammatory component of diabetes remains under-characterized. Recent years have brought new insights into the possible mechanism involved in the increased inflammatory response, suggesting that environmental factors such as a westernized diet may participate in this process. Dietary lipids, particularly palmitate, are substrates for the biosynthesis of bioactive sphingolipids. Disturbed serum sphingolipid profiles were observed in both T1DM and T2DM patients. Many polymorphisms were identified in genes encoding enzymes of the sphingolipid pathway, including sphingosine kinase 2 (SK2), the S1P generating enzyme which is highly expressed in beta-cells. Proinflammatory cytokines and free fatty acids have been shown to modulate the expression and activity of S1P-generating and S1P-catabolizing enzymes. In this review, the similarities and differences in the action of extracellular and intracellular S1P in beta-cells exposed to cytokines or free fatty acids will be identified and the outlook for future research will be discussed.

Alcoholic and Non-Alcoholic Liver Diseases: Promising Molecular Drug Targets and their Clinical Development

Alcoholic and non-alcoholic fatty liver diseases have become a serious concern worldwide. Both these liver diseases have an identical pathology, starting from simple steatosis to cirrhosis and, ultimately to hepatocellular carcinoma. Treatment options for alcoholic liver disease (ALD) are still the same as they were 50 years ago which include corticosteroids, pentoxifylline, antioxidants, nutritional support and abstinence; and for non-alcoholic fatty liver disease (NAFLD), weight loss, insulin sensitizers, lipid-lowering agents and anti-oxidants are the only treatment options. Despite broad research in understanding the disease pathophysiology, limited treatments are available for clinical use. Some therapeutic strategies based on targeting a specific molecule have been developed to lessen the consequences of disease and are under clinical investigation. Therefore, focus on multiple molecular targets will help develop an efficient therapeutic strategy. This review comprises a brief overview of the pathogenesis of ALD and NAFLD; recent molecular drug targets explored for ALD and NAFLD that may prove to be effective for multiple therapeutic regimens and also the clinical status of these promising drug targets for liver diseases.

Deletion or inhibition of SphK1 mitigates fulminant hepatic failure by suppressing TNFα-dependent inflammation and apoptosis

Acute liver failure (ALF) causes severe liver dysfunction that can lead to multi-organ failure and death. Previous studies suggest that sphingosine kinase 1 (SphK1) protects against hepatocyte injury, yet not much is still known about its involvement in ALF. This study examines the role of SphK1 in D-galactosamine (GalN)/lipopolysaccharide (LPS)-induced ALF, which is a well-established experimental mouse model that mimics the fulminant hepatitis. Here we report that deletion of SphK1, but not SphK2, dramatically decreased GalN/LPS-induced liver damage, hepatic apoptosis, serum alanine aminotransferase levels, and mortality rate compared to wild-type mice. Whereas GalN/LPS treatment-induced hepatic activation of NF-κB and JNK in wild-type and SphK2-/- mice, these signaling pathways were reduced in SphK1-/- mice. Moreover, repression of ALF in SphK1-/- mice correlated with decreased expression of the pro-inflammatory cytokine TNFα. Adoptive transfer experiments indicated that SphK1 in bone marrow-derived infiltrating immune cells but not in host liver-resident cells, contribute to the development of ALF. Interestingly, LPS-induced TNFα production was drastically suppressed in SphK1-deleted macrophages, whereas IL-10 expression was markedly enhanced, suggesting a switch to the anti-inflammatory phenotype. Finally, treatment with a specific SphK1 inhibitor ameliorated inflammation and protected mice from ALF. Our findings suggest that SphK1 regulates TNFα secretion from macrophages and inhibition or deletion of SphK1 mitigated ALF. Thus, a potent inhibitor of SphK1 could potentially be a therapeutic agent for fulminant hepatitis.

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.

The Influence of Coumestrol on Sphingolipid Signaling Pathway and Insulin Resistance Development in Primary Rat Hepatocytes

Coumestrol is a phytoestrogen widely known for its anti-diabetic, anti-oxidant, and anti-inflammatory properties. Thus, it gets a lot of attention as a potential agent in the nutritional therapy of diseases such as obesity and type 2 diabetes. In our study, we evaluated whether coumestrol affects insulin resistance development via the sphingolipid signaling pathway in primary rat hepatocytes. The cells were isolated from the male Wistar rat's liver with the use of collagenase perfusion. Next, we incubated the cells with the presence or absence of palmitic acid and/or coumestrol. Additionally, some groups were incubated with insulin. The sphingolipid concentrations were assessed by HPLC whereas the expression of all the proteins was evaluated by Western blot. Coumestrol markedly reduced the accumulation of sphingolipids, namely, ceramide and sphinganine through noticeable inhibition of the ceramide de novo synthesis pathway in insulin-resistant hepatocytes. Moreover, coumestrol augmented the expression of fatty acid transport proteins, especially FATP5 and FAT/CD36, which also were responsible for excessive sphingolipid accumulation. Furthermore, coumestrol altered the sphingolipid salvage pathway, which was observed as the excessive deposition of the sphingosine-1-phosphate and sphingosine. Our study clearly showed that coumestrol ameliorated hepatic insulin resistance in primary rat hepatocytes. Thus, we believe that our study may contribute to the discovery of novel preventive and therapeutic methods for metabolic disorders.

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.

The role of the sphingosine axis in immune regulation: A dichotomy in the anti-inflammatory effects between sphingosine kinase 1 and sphingosine kinase 2-dependent pathways

Background: Sphingosine kinase has been identified as playing a central role in the immune cascade, being a common mediator in the cellular response to a variety of signals. The different effects of sphingosine kinase 1 and 2 (SphK1 and SphK2, respectively) activity have not been completely characterized. Aim: To determine the different roles played by SphK1 and SphK2 in the regulation of immune-mediated disorders. Methods: Nine groups of mice were studied. Concanavalin A (ConA) injection was used to induce immune-mediated hepatitis. Mice were treated with SphK1 inhibitor (termed SphK-I) and SphK2 inhibitor (termed ABC294640), prior to ConA injection, and effects of treatment on liver enzymes, subsets of T lymphocytes, and serum levels of cytokines were observed. Results: While liver enzyme elevation was ameliorated by administration of SphK1 inhibitor, SphK2 inhibitor-treated mice did not show this tendency. A marked decrease in expression of CD25+ T-cells and Foxp+ T-cells was observed in mice treated with a high dose of SphK1 inhibitor. Alleviation of liver damage was associated with a statistically significant reduction of serum IFNγ levels in mice treated with SphK1 inhibitor and not in those treated with SphK2 inhibitor. Conclusions: Early administration of SphK1 inhibitor in a murine model of immune-mediated hepatitis alleviated liver damage and inflammation with a statistically significant reduction in IFN-γ levels. The data support a dichotomy in the anti-inflammatory effects of SphK1 and SphK2, and suggests that isoenzyme-directed therapies can improve the effect of targeting these pathways.

Ceramides and Sphingosino-1-Phosphate in Obesity

Obesity is a growing worldwide problem, especially in developed countries. This disease adversely affects the quality of life and notably contributes to the development of type 2 diabetes, metabolic syndrome, and cardiovascular disorders. It is characterised by excessive lipids accumulation in the subcutaneous and visceral adipose tissue. Considering the secretory function of adipose tissue, this leads to impaired adipokines and cytokines release. Changes in adipose tissue metabolism result in chronic inflammation, pancreatic islets dysfunction and peripheral insulin resistance. In addition to saturating various adipocytes, excess lipids are deposited into non-adipose peripheral tissues, which disturbs cell metabolism and causes a harmful effect known as lipotoxicity. Fatty acids are metabolised into bioactive lipids such as ceramides, from which sphingolipids are formed. Ceramides and sphingosine-1-phosphate (S1P) are involved in intracellular signalling, cell proliferation, migration, and apoptosis. Studies demonstrate that bioactive lipids have a crucial role in regulating insulin signalling pathways, glucose homeostasis and β cell death. Data suggests that ceramides may have an opposite cellular effect than S1P; however, the role of S1P remains controversial. This review summarises the available data on ceramide and sphingolipid metabolism and their role in obesity.

Toxicant-Induced Metabolic Alterations in Lipid and Amino Acid Pathways Are Predictive of Acute Liver Toxicity in Rats

Liver disease and disorders associated with aberrant hepatocyte metabolism can be initiated via drug and environmental toxicant exposures. In this study, we tested the hypothesis that gene and metabolic profiling can reveal commonalities in liver response to different toxicants and provide the capability to identify early signatures of acute liver toxicity. We used Sprague Dawley rats and three classical hepatotoxicants: acetaminophen (2 g/kg), bromobenzene (0.4 g/kg), and carbon tetrachloride (0.3 g/kg), to identify early perturbations in liver metabolism after a single acute exposure dose. We measured changes in liver genes and plasma metabolites at two time points (5 and 10 h) and used genome-scale metabolic models to identify commonalities in liver responses across the three toxicants. We found strong correlations for gene and metabolic profiles between the toxicants, indicative of similarities in the liver response to toxicity. We identified several injury-specific pathways in lipid and amino acid metabolism that changed similarly across the three toxicants. Our findings suggest that several plasma metabolites in lipid and amino acid metabolism are strongly associated with the progression of liver toxicity, and as such, could be targeted and clinically assessed for their potential as early predictors of acute liver toxicity.

Generation of sphingosine-1-phosphate by sphingosine kinase 1 protects nonalcoholic fatty liver from ischemia/reperfusion injury through alleviating reactive oxygen species production in hepatocytes

BACKGROUND

Nonalcoholic fatty liver (NAFL) is emerging as a leading risk factor of hepatic ischemia/reperfusion (I/R) injury lacking of effective therapy. Lipid dyshomeostasis has been implicated in the hepatopathy of NAFL. Herein, we investigate the bioactive lipids that critically regulate I/R injury in NAFL.

METHODS

Lipidomics were performed to identify dysregulated lipids in mouse and human NAFL with I/R injury. The alteration of corresponding lipid-metabolizing genes was examined. The effects of the dysregulated lipid metabolism on I/R injury in NAFL were evaluated in mice and primary hepatocytes.

RESULTS

Sphingolipid metabolic pathways responsible for the generation of sphingosine-1-phosphate (S1P) were uncovered to be substantially activated by I/R in mouse NAFL. Sphingosine kinase 1 (Sphk1) was found to be essential for hepatic S1P generation in response to I/R in hepatocytes of NAFL mice. Sphk1 knockdown inhibited the hepatic S1P rise while accumulating ceramides in hepatocytes of NAFL mice, leading to aggressive hepatic I/R injury with upregulation of oxidative stress and increase of reactive oxygen species (ROS). In contrast, administration of exogenous S1P protected hepatocytes of NAFL mice from hepatic I/R injury. Clinical study revealed a significant activation of S1P generation by I/R in liver specimens of NAFL patients. In vitro studies on the L02 human hepatocytes consolidated that inhibiting the generation of S1P by knocking down SPHK1 exaggerated I/R-induced damage and oxidative stress in human hepatocytes of NAFL.

CONCLUSIONS

Generation of S1P by SPHK1 is important for protecting NAFL from I/R injury, which may serve as therapeutic targets for hepatic I/R injury in NAFL.

Sphingosine-1-Phosphate Metabolism in the Regulation of Obesity/Type 2 Diabetes

Obesity is a pathophysiological condition where excess free fatty acids (FFA) target and promote the dysfunctioning of insulin sensitive tissues and of pancreatic β cells. This leads to the dysregulation of glucose homeostasis, which culminates in the onset of type 2 diabetes (T2D). FFA, which accumulate in these tissues, are metabolized as lipid derivatives such as ceramide, and the ectopic accumulation of the latter has been shown to lead to lipotoxicity. Ceramide is an active lipid that inhibits the insulin signaling pathway as well as inducing pancreatic β cell death. In mammals, ceramide is a key lipid intermediate for sphingolipid metabolism as is sphingosine-1-phosphate (S1P). S1P levels have also been associated with the development of obesity and T2D. In this review, the current knowledge on S1P metabolism in regulating insulin signaling in pancreatic β cell fate and in the regulation of feeding by the hypothalamus in the context of obesity and T2D is summarized. It demonstrates that S1P can display opposite effects on insulin sensitive tissues and pancreatic β cells, which depends on its origin or its degradation pathway.

The role of cytokine and immune responses in intestinal fibrosis

The rapidly increasing incidence of inflammatory bowel disease (IBD) in South America, eastern Europe, Asia, and Africa has resulted in a global public health challenge. Intestinal fibrosis is a common complication in patients with long-term IBD, which may develop into stenosis and subsequent obstruction. Hitherto, the origin of IBD is unclear and several factors may be involved, including genetic, immune, environmental and microbial influences. Little is known about how the recurrent inflammation in patients with IBD develops into intestinal fibrosis and currently, there is no suitable treatment to reverse intestinal fibrosis in these patients. Here, we review the role of immune components in the pathogenesis of IBD and intestinal fibrosis, including cytokine networks, host-microbiome interactions, and immune cell trafficking.

Reduction of Sphingosine Kinase 1 Phosphorylation and Activity in Plasmodium-Infected Erythrocytes

Sphingosine-1-phosphate (S1P), a bioactive lipid mediator is involved in an array of biological processes and linked to pathological manifestations. Erythrocyte is known as the major reservoir for S1P as they lack S1P-degrading enzymes (S1P lyase and S1P phosphohydrolase) and harbor sphingosine kinase-1 (SphK-1) essential for sphingosine conversion to S1P. Reduced S1P concentration in serum was correlated with disease severity in patients with Plasmodium falciparum and Plasmodium vivax infections. Herein, we aimed to identify the underlying mechanism and contribution of host erythrocytes toward depleted S1P levels in Plasmodium-infected patients vs. healthy individuals. The level and activity of SphK-1 were measured in vitro in both uninfected and cultured P. falciparum-infected erythrocytes. Infected erythrocytes demonstrated a significant decrease in SphK-1 level in a time-dependent manner. We found that 10–42 h post invasion (hpi), SphK1 level was predominantly reduced to ∼50% in rings, trophozoites, and schizonts compared to uninfected erythrocytes. We next analyzed the phosphorylation status of SphK-1, a modification responsible for its activity and S1P production, in both uninfected control and Plasmodium-infected erythrocytes. Almost ∼50% decrease in phosphorylation of SphK-1 was observed that could be corroborated with significant reduction in the production and release of S1P in infected erythrocytes. Serum S1P levels were studied in parallel in P. falciparum (N = 15), P. vivax (N = 36)-infected patients, and healthy controls (N = 6). The findings revealed that S1P concentration was significantly depleted in uncomplicated malaria cases and was found to be lowest in complicated malaria and thrombocytopenia in both P. falciparum and P. vivax-infected groups (^∗∗p < 0.01). The lower serum S1P level could be correlated with the reduced platelet count defining the role of S1P level in platelet formation. In conclusion, erythrocyte SphK-1 and S1P levels were studied in Plasmodium-infected individuals and erythrocytes that helped in characterizing the complications associated with malaria and thrombocytopenia, providing insights into the contribution of host erythrocyte biology in malaria pathogenesis. Finally, this study proposes the use of S1P and its analog as a novel adjunct therapy for malaria complications.

Hepatic insulin resistance induced by mitochondrial oxidative stress can be ameliorated by sphingosine 1-phosphate

The bioactive lipid mediator sphingosine 1-phosphate (S1P) is considered to be involved in the development of insulin resistance (IR) via effects on oxidative stress; the mechanism however is not yet fully revealed. To this end, we investigated the role and mechanism of S1P on hepatic IR. We found that treatment of the normal human liver cell LO2 with 1000 nM insulin for 48 h reduced glucose uptake and increased serine phosphorylation of insulin receptor substrate-1, indicating a reduction in insulin receptor signaling. Moreover, the same concentration of insulin caused accumulation of reactive oxygen species (ROS) in the cytosol and mitochondria, and enhanced expression of the antioxidant transcription factor (Nrf2) and upregulated Nrf2 nuclear translocation. Using known inhibitors and donors of ROS (H₂O₂, ·O₂-, ·OH), the results demonstrated the differential roles for the specific ROS in regulating IR in LO2 cells, with H₂O₂ having a more significant inhibitory role compared with ·O₂^- and ·OH. Cell treatment with S1P at 0.1-5.0 μM reversed the effects of high insulin concentrations on ROS generation, glucose uptake, and insulin signaling. H₂O₂ also reversed the beneficial effects of S1P in alleviating IR. These results show that H₂O₂ signaling plays a key determinant in hepatic IR induced by insulin. S1P can ameliorate hepatic IR by reducing mitochondrial ROS generation, and the possible anti-IR effect mechanism may be involved in H₂O₂ signaling.

Sphingolipids in Obesity and Correlated Co-Morbidities: The Contribution of Gender, Age and Environment

This paper reviews our present knowledge on the contribution of ceramide (Cer), sphingomyelin (SM), dihydroceramide (DhCer) and sphingosine-1-phosphate (S1P) in obesity and related co-morbidities. Specifically, in this paper, we address the role of acyl chain composition in bodily fluids for monitoring obesity in males and females, in aging persons and in situations of environmental hypoxia adaptation. After a brief introduction on sphingolipid synthesis and compartmentalization, the node of detection methods has been critically revised as the node of the use of animal models. The latter do not recapitulate the human condition, making it difficult to compare levels of sphingolipids found in animal tissues and human bodily fluids, and thus, to find definitive conclusions. In human subjects, the search for putative biomarkers has to be performed on easily accessible material, such as serum. The serum “sphingolipidome” profile indicates that attention should be focused on specific acyl chains associated with obesity, per se, since total Cer and SM levels coupled with dyslipidemia and vitamin D deficiency can be confounding factors. Furthermore, exposure to hypoxia indicates a relationship between dyslipidemia, obesity, oxygen level and aerobic/anaerobic metabolism, thus, opening new research avenues in the role of sphingolipids.

Extracellular vesicles: Future diagnostic and therapeutic tools for liver disease and regeneration

Extracellular vesicles are membrane fragments that can be produced by all cell types. Interactions between extracellular vesicles and various liver cells constitute an emerging field in hepatology and recent evidences have established a role for extracellular vesicles in various liver diseases and physiological processes. Extracellular vesicles originating from liver cells are implicated in intercellular communication and fluctuations of specific circulating extracellular vesicles could constitute new diagnostic tools. In contrast, extracellular vesicles derived from progenitor cells interact with hepatocytes or non-parenchymal cells, thereby protecting the liver from various injuries and promoting liver regeneration. Our review focuses on recent developments investigating the role of various types of extracellular vesicles in acute and chronic liver diseases as well as their potential use as biomarkers and therapeutic tools.

Deciphering the Link Between Hyperhomocysteinemia and Ceramide Metabolism in Alzheimer-Type Neurodegeneration

Aging is one of the strongest risk factor for Alzheimer's disease (AD). However, several data suggest that dyslipidemia can either contribute or serve as co-factors in AD appearance. AD could be examined as a metabolic disorder mediated by peripheral insulin resistance. Insulin resistance is associated with dyslipidemia, which results in increased hepatic ceramide generation. Hepatic steatosis induces pro-inflammatory cytokine activation which is mediated by the increased ceramides production. Ceramides levels increased in cells due to perturbation in sphingolipid metabolism and upregulated expression of enzymes involved in ceramide synthesis. Cytotoxic ceramides and related molecules generated in liver promote insulin resistance, traffic through the circulation due to injury or cell death caused by local liver inflammation, and because of their hydrophobic nature, they can cross the blood-brain barrier and thereby exert neurotoxic responses as reducing insulin signaling and increasing pro-inflammatory cytokines. These abnormalities propagate a cascade of neurodegeneration associated with oxidative stress and ceramide generation, which potentiate brain insulin resistance, apoptosis, myelin degeneration, and neuro-inflammation. Therefore, excess of toxic lipids generated in liver can cause neurodegeneration. Elevated homocysteine level is also a risk factor for AD pathology and is narrowly associated with metabolic diseases and non-alcoholic fatty liver disease. The existence of a homocysteine/ceramides signaling pathway suggests that homocysteine toxicity could be partly mediated by intracellular ceramide accumulation due to stimulation of ceramide synthase. In this article, we briefly examined the role of homocysteine and ceramide metabolism linking metabolic diseases and non-alcoholic fatty liver disease to AD. We therefore analyzed the expression of mainly enzymes implicated in ceramide and sphingolipid metabolism and demonstrated deregulation of de novo ceramide biosynthesis and S1P metabolism in liver and brain of hyperhomocysteinemic mice.

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.

Hepatocyte-Derived Lipotoxic Extracellular Vesicle Sphingosine 1-Phosphate Induces Macrophage Chemotaxis

Background: The pathophysiology of non-alcoholic steatohepatitis involves hepatocyte lipotoxicity due to excess saturated free fatty acids and concomitant proinflammatory macrophage effector responses. These include the infiltration of macrophages into hepatic cords in response to incompletely understood stimuli. Stressed hepatocytes release an increased number of extracellular vesicles (EVs), which are known to participate in intercellular signaling and coordination of the behavior of immune cell populations via their cargo. We hypothesized that hepatocyte-derived lipotoxic EVs that are enriched in sphingosine 1-phosphate (S1P) are effectors of macrophage infiltration in the hepatic microenvironment. Methods: Lipotoxic EVs were isolated from palmitate treated immortalized mouse hepatocytes and characterized by nanoparticle tracking analysis. Lipotoxic EV sphingolipids were quantified using tandem mass spectrometry. Wildtype and S1P1 receptor knockout bone marrow-derived macrophages were exposed to lipotoxic EV gradients in a microfluidic gradient generator. Macrophage migration toward EV gradients was captured by time-lapse microscopy and analyzed to determine directional migration. Fluorescence-activated cell sorting along with quantitative PCR and immunohistochemistry were utilized to characterize the cell surface expression of S1P1 receptor on intrahepatic leukocytes and hepatic expression of S1P1 receptor, respectively. Results: Palmitate treatment induced the release of EVs. These EVs were enriched in S1P. Palmitate-induced S1P enriched EVs were chemoattractive to macrophages. EV S1P enrichment depended on the activity of sphingosine kinases 1 and 2, such that, pharmacological inhibition of sphingosine kinases 1 and 2 resulted in a significant reduction in EV S1P cargo without affecting the number of EVs released. When exposed to EVs derived from cells treated with palmitate in the presence of a pharmacologic inhibitor of sphingosine kinases 1 and 2, macrophages displayed diminished chemotactic behavior. To determine receptor-ligand specificity, we tested the migration responses of macrophages genetically deleted in the S1P1 receptor toward lipotoxic EVs. S1P1 receptor knockout macrophages displayed a marked reduction in their chemotactic responses toward lipotoxic palmitate-induced EVs. Conclusions:Palmitate-induced lipotoxic EVs are enriched in S1P through sphingosine kinases 1 and 2. S1P-enriched EVs activate persistent and directional macrophage chemotaxis mediated by the S1P1 receptor, a potential signaling axis for macrophage infiltration during hepatic lipotoxicity, and a potential therapeutic target for non-alcoholic steatohepatitis.

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.

Different Roles of Sphingosine Kinase 1 and 2 in Pancreatic Cancer Progression

BACKGROUND

Pancreatic cancer is a disease with poor prognosis, and development of new treatments is necessary. Sphingosine-1-phosphate (S1P), a bioactive lipid mediator produced by sphingosine kinases (SphK1 and SphK2), plays a critical role in progression of many types of cancer. However, little is known about the role of sphingosine kinases in pancreatic cancer. This study investigated the roles of sphingosine kinases in pancreatic cancer progression.

MATERIALS AND METHODS

S1P levels in pancreatic cancer and noncancerous pancreatic tissue were measured in 10 patients. We generated PAN02 murine pancreatic cancer cell lines with a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system genes 9 (Cas9)-mediated deletion of SphK1 or SphK2 and assessed cell growth and migration. In an animal model, we assessed the survival of mice injected with PAN02 cells intraperitoneally.

RESULTS

S1P levels in the pancreatic cancer tissue were significantly higher than those in noncancerous tissue. SphK1 knockout (KO) cells showed greater proliferation and migration than wild type (WT) cells, and SphK2 KO cells showed less proliferation and migration than WT cells. Animal experiments showed that the survival of mice injected with SphK1 KO cells was significantly shorter than those injected with WT cells, and the survival of mice injected with SphK2 KO cells was longer than those injected with WT cells. Surprisingly, cytotoxic assay using gemcitabine showed that SphK1 KO cells survived less than WT cells, and SphK2 KO cells survived more than WT cells.

CONCLUSIONS

S1P produced by SphK1 and SphK2 may have different functions in pancreatic cancer cells. Targeting both SphK1 and SphK2 may be a potential strategy for pancreatic cancer treatment.

Non-alcoholic fatty liver disease: Insights from sphingolipidomics

Non-alcoholic fatty liver disease (NAFLD) is a major clinical concern and its treatment consumes abundant resources. While accumulation of lipids in hepatocytes initiates the disease, this in itself is not necessarily harmful; rather, initiation of inflammation and subsequent fibrosis and cirrhosis are critical steps in NAFLD pathology. Mechanisms linking lipid overload to downstream disease progression are not fully understood; however, bioactive lipid metabolism may underlie instigation of proinflammatory signaling. With the advent of high-throughput, sensitive, and quantitative mass spectrometry-based methods for assessing lipid profiles in NAFLD, several trends have emerged, including that increases in specific sphingolipids correlate with the transition from the relatively benign condition of simple fatty liver to the much more concerning inflamed state. Continued studies that implement sphingolipid profiling will enable the extrapolations of candidate enzymes and pathways involved in NAFLD, either in biopsies or plasma from human samples, and also in animal models, from which data are much more abundant. While most data thus far are derived from targeted lipidomics approaches, unbiased, semi-quantitative approaches hold additional promise for furthering our understanding of sphingolipids as markers of and players in NAFLD.