Ceramide induces hepatocyte cell death through disruption of mitochondrial function in the rat

Exploring the impact of lipid droplets on the evolution and progress of hepatocarcinoma

Over the past 10 years, the biological role of lipid droplets (LDs) has gained significant attention in the context of both physiological and pathological conditions. Considerable progress has been made in elucidating key aspects of these organelles, yet much remains to be accomplished to fully comprehend the myriad functions they serve in the progression of hepatic tumors. Our current perception is that LDs are complex and active structures managed by a distinct set of cellular processes. This understanding represents a significant paradigm shift from earlier perspectives. In this review, we aim to recapitulate the function of LDs within the liver, highlighting their pivotal role in the pathogenesis of metabolic dysfunction-associated steatotic liver disease (MASLD) (Hsu and Loomba, 2024) and their contribution to the progression towards more advanced pathological stages up to hepatocellular carcinoma (HC) (Farese and Walther, 2009). We are aware of the molecular complexity and changes occurring in the neoplastic evolution of the liver. Our attempt, however, is to summarize the most important and recent roles of LDs across both healthy and all pathological liver states, up to hepatocarcinoma. For more detailed insights, we direct readers to some of the many excellent reviews already available in the literature (Gluchowski et al., 2017; Hu et al., 2020; Seebacher et al., 2020; Paul et al., 2022).

Prosaposin maintains adult neural stem cells in a state associated with deep quiescence

In most vertebrates, adult neural stem cells (NSCs) continuously give rise to neurons in discrete brain regions. A critical process for maintaining NSC pools over long periods of time in the adult brain is NSC quiescence, a reversible and tightly regulated state of cell-cycle arrest. Recently, lysosomes were identified to regulate the NSC quiescence-proliferation balance. However, it remains controversial whether lysosomal activity promotes NSC proliferation or quiescence, and a finer influence of lysosomal activity on NSC quiescence duration or depth remains unexplored. Using RNA sequencing and pharmacological manipulations, we show that lysosomes are necessary for NSC quiescence maintenance. In addition, we reveal that expression of psap, encoding the lysosomal regulator Prosaposin, is enriched in quiescent NSCs (qNSCs) that reside upstream in the NSC lineage and display a deep/long quiescence phase in the adult zebrafish telencephalon. We show that shRNA-mediated psap knockdown increases the proportion of activated NSCs (aNSCs) as well as NSCs that reside in shallower quiescence states (signed by ascl1a and deltaA expression). Collectively, our results identify the lysosomal protein Psap as a (direct or indirect) quiescence regulator and unfold the interplay between lysosomal function and NSC quiescence heterogeneities.

Acid ceramidase regulates innate immune memory

Innate immune memory, also called "trained immunity," is a functional state of myeloid cells enabling enhanced immune responses. This phenomenon is important for host defense, but also plays a role in various immune-mediated conditions. We show that exogenously administered sphingolipids and inhibition of sphingolipid metabolizing enzymes modulate trained immunity. In particular, we reveal that acid ceramidase, an enzyme that converts ceramide to sphingosine, is a potent regulator of trained immunity. We show that acid ceramidase regulates the transcription of histone-modifying enzymes, resulting in profound changes in histone 3 lysine 27 acetylation and histone 3 lysine 4 trimethylation. We confirm our findings by identifying single-nucleotide polymorphisms in the region of ASAH1, the gene encoding acid ceramidase, that are associated with the trained immunity cytokine response. Our findings reveal an immunomodulatory effect of sphingolipids and identify acid ceramidase as a relevant therapeutic target to modulate trained immunity responses in innate immune-driven disorders.

Sphingolipids in mitochondria-from function to disease

Sphingolipids are not only structural components of cellular membranes but also play vital roles in cell signaling and modulation of cellular processes. Within mitochondria, sphingolipids exert diverse effects on mitochondrial dynamics, energy metabolism, oxidative stress, and cell death pathways. In this review, we summarize literature addressing the crucial role of sphingolipids in mitochondria, highlighting their impact on mitochondrial dynamics, cellular bioenergetics, and important cell processes including apoptosis and mitophagy.

Pathogenesis and treatment of non-alcoholic steatohepatitis and its fibrosis

The initial presentation of non-alcoholic steatohepatitis (NASH) is hepatic steatosis. The dysfunction of lipid metabolism within hepatocytes caused by genetic factors, diet, and insulin resistance causes lipid accumulation. Lipotoxicity, oxidative stress, mitochondrial dysfunction, and endoplasmic reticulum stress would further contribute to hepatocyte injury and death, leading to inflammation and immune dysfunction in the liver. During the healing process, the accumulation of an excessive amount of fibrosis might occur while healing. During the development of NASH and liver fibrosis, the gut-liver axis, adipose-liver axis, and renin-angiotensin system (RAS) may be dysregulated and impaired. Translocation of bacteria or its end-products entering the liver could activate hepatocytes, Kupffer cells, and hepatic stellate cells, exacerbating hepatic steatosis, inflammation, and fibrosis. Bile acids regulate glucose and lipid metabolism through Farnesoid X receptors in the liver and intestine. Increased adipose tissue-derived non-esterified fatty acids would aggravate hepatic steatosis. Increased leptin also plays a role in hepatic fibrogenesis, and decreased adiponectin may contribute to hepatic insulin resistance. Moreover, dysregulation of peroxisome proliferator-activated receptors in the liver, adipose, and muscle tissues may impair lipid metabolism. In addition, the RAS may contribute to hepatic fatty acid metabolism, inflammation, and fibrosis. The treatment includes lifestyle modification, pharmacological therapy, and non-pharmacological therapy. Currently, weight reduction by lifestyle modification or surgery is the most effective therapy. However, vitamin E, pioglitazone, and obeticholic acid have also been suggested. In this review, we will introduce some new clinical trials and experimental therapies for the treatment of NASH and related fibrosis.

Depletion of ALMS1 affects TGF-β signalling pathway and downstream processes such as cell migration and adhesion capacity

Background: ALMS1 is a ubiquitous gene associated with Alström syndrome (ALMS). The main symptoms of ALMS affect multiple organs and tissues, generating at last, multi-organic fibrosis in the lungs, kidneys and liver. TGF-β is one of the main pathways implicated in fibrosis, controlling the cell cycle, apoptosis, cell migration, cell adhesion and epithelial-mesenchymal transition (EMT). Nevertheless, the role of ALMS1 gene in fibrosis generation and other implicated processes such as cell migration or cell adhesion via the TGF- β pathway has not been elucidated yet. Methods: Initially, we evaluated how depletion of ALMS1 affects different processes like apoptosis, cell cycle and mitochondrial activity in HeLa cells. Then, we performed proteomic profiling with TGF-β stimuli in HeLa ALMS1 -/- cells and validated the results by examining different EMT biomarkers using qPCR. The expression of these EMT biomarkers were also studied in hTERT-BJ-5ta ALMS1 -/-. Finally, we evaluated the SMAD3 and SMAD2 phosphorylation and cell migration capacity in both models. Results: Depletion of ALMS1 generated apoptosis resistance to thapsigargin (THAP) and C2-Ceramide (C2-C), and G2/M cell cycle arrest in HeLa cells. For mitochondrial activity, results did not show significant differences between ALMS1 +/+ and ALMS1 -/-. Proteomic results showed inhibition of downstream pathways regulated by TGF-β. The protein-coding genes (PCG) were associated with processes like focal adhesion or cell-substrate adherens junction in HeLa. SNAI1 showed an opposite pattern to what would be expected when activating the EMT in HeLa and BJ-5ta. Finally, in BJ-5ta model a reduced activation of SMAD3 but not SMAD2 were also observed. In HeLa model no alterations in the canonical TGF-β pathway were observed but both cell lines showed a reduction in migration capacity. Conclusion: ALMS1 has a role in controlling the cell cycle and the apoptosis processes. Moreover, the depletion of ALMS1 affects the signal transduction through the TGF-β and other processes like the cell migration and adhesion capacity.

Endocrine Therapy-Resistant Breast Cancer Cells Are More Sensitive to Ceramide Kinase Inhibition and Elevated Ceramide Levels Than Therapy-Sensitive Breast Cancer Cells

Simple Summary

Endocrine therapy (ET) resistance is a major problem in estrogen receptor-positive breast cancer patients. Since there have been few lipidomic studies in ET resistance and sphingolipids are heavily implicated in multidrug-resistant and chemotherapy-resistant cancers, we aimed to investigate the sphingolipidome of tamoxifen-resistant breast cancer cells in search of a unique sphingolipid profile that can potentially be exploited therapeutically. We found that ET-resistant breast cancer cells maintain a lower level of ceramides for their survival. In order to achieve this, they are dependent on ceramide kinase (CERK), the activity of which helps maintain low endogenous ceramide levels, therefore promoting tamoxifen-resistant cell survival. Targeting CERK can therefore represent an opportunity to target therapy-resistant breast tumors and improve the patient outcome for women with ET-resistant disease.

Abstract

ET resistance is a critical problem for estrogen receptor-positive (ER+) breast cancer. In this study, we have investigated how alterations in sphingolipids promote cell survival in ET-resistant breast cancer. We have performed LC-MS-based targeted sphingolipidomics of tamoxifen-sensitive and -resistant MCF-7 breast cancer cell lines. Follow-up studies included treatments of cell lines and patient-derived xenograft organoids (PDxO) with small molecule inhibitors; cytometric analyses to measure cell death, proliferation, and apoptosis; siRNA-mediated knockdown; RT-qPCR and Western blot for gene and protein expression; targeted lipid analysis; and lipid addback experiments. We found that tamoxifen-resistant cells have lower levels of ceramides and hexosylceramides compared to their tamoxifen-sensitive counterpart. Upon perturbing the sphingolipid pathway with small molecule inhibitors of key enzymes, we identified that CERK is essential for tamoxifen-resistant breast cancer cell survival, as well as a fulvestrant-resistant PDxO. CERK inhibition induces ceramide-mediated cell death in tamoxifen-resistant cells. Ceramide-1-phosphate (C1P) partially reverses CERK inhibition-induced cell death in tamoxifen-resistant cells, likely through lowering endogenous ceramide levels. Our findings suggest that ET-resistant breast cancer cells maintain lower ceramide levels as an essential pro-survival mechanism. Consequently, ET-resistant breast cancer models have a unique dependence on CERK as its activity can inhibit de novo ceramide production.

Ceramides and Acute Kidney Injury

Altered lipid metabolism is a characteristic feature and potential driving factor of acute kidney injury (AKI). Of the lipids that accumulate in injured renal tissues, ceramides are potent regulators of metabolism and cell fate. Up-regulation of ceramide synthesis is a common feature shared across several AKI etiologies in vitro and in vivo. Furthermore, ceramide accumulation is an early event in the natural history of AKI that precedes cell death and organ dysfunction. Emerging evidence suggests that inhibition of ceramide accumulation may improve renal outcomes in several models of AKI. This review examines the landscape of ceramide metabolism and regulation in the healthy and injured kidney. Furthermore, we discuss the body of literature regarding ceramides as therapeutic targets for AKI and consider potential mechanisms by which ceramides drive kidney pathogenesis.

Lipids in Liver Failure Syndromes: A Focus on Eicosanoids, Specialized Pro-Resolving Lipid Mediators and Lysophospholipids

Lipids are organic compounds insoluble in water with a variety of metabolic and non-metabolic functions. They not only represent an efficient energy substrate but can also act as key inflammatory and anti-inflammatory molecules as part of a network of soluble mediators at the interface of metabolism and the immune system. The role of endogenous bioactive lipid mediators has been demonstrated in several inflammatory diseases (rheumatoid arthritis, inflammatory bowel disease, atherosclerosis, cancer). The liver is unique in providing balanced immunotolerance to the exposure of bacterial components from the gut transiting through the portal vein and the lymphatic system. This balance is abruptly deranged in liver failure syndromes such as acute liver failure and acute-on-chronic liver failure. In these syndromes, researchers have recently focused on bioactive lipid mediators by global metabonomic profiling and uncovered the pivotal role of these mediators in the immune dysfunction observed in liver failure syndromes explaining the high occurrence of sepsis and subsequent organ failure. Among endogenous bioactive lipids, the mechanistic actions of three classes (eicosanoids, pro-resolving lipid mediators and lysophospholipids) in the pathophysiological modulation of liver failure syndromes will be the topic of this narrative review. Furthermore, the therapeutic potential of lipid-immune pathways will be described.

Inhibition of acid ceramidase elicits mitochondrial dysfunction and oxidative stress in pancreatic cancer cells

Although the inhibition of acid ceramidase (AC) is known to induce antitumor effects in various cancers, there are few reports in pancreatic cancer, and the underlying mechanisms remain unclear. Moreover, there is currently no safe administration method of AC inhibitor. Here the effects of gene therapy using siRNA and shRNA for AC inhibition with its mechanisms for pancreatic cancer were investigated. The inhibition of AC by siRNA and shRNA using an adeno-associated virus 8 (AAV8) vector had antiproliferative effects by inducing apoptosis in pancreatic cancer cells and xenograft mouse model. Acid ceramidase inhibition elicits mitochondrial dysfunction, reactive oxygen species accumulation, and manganese superoxide dismutase suppression, resulting in apoptosis of pancreatic cancer cells accompanied by ceramide accumulation. These results elucidated the mechanisms underlying the antitumor effect of AC inhibition in pancreatic cancer cells and suggest the potential of the AAV8 vector to inhibit AC as a therapeutic strategy.

Ceramides and sphingosine-1-phosphate mediate the distinct effects of M1/M2-macrophage infusion on liver recovery after hepatectomy

Post-hepatectomy liver dysfunction is a life-threatening morbidity that lacks efficient therapy. Bioactive lipids involved in macrophage polarization crucially regulate tissue injury and regeneration. Herein, we investigate the key bioactive lipids that mediate the cytotherapeutic potential of polarized-macrophage for post-hepatectomy liver dysfunction. Untargeted lipidomics identified elevation of ceramide (CER) metabolites as signature lipid species relevant to M1/M2 polarization in mouse bone-marrow-derived-macrophages (BMDMs). M1 BMDMs expressed a CER-generation-metabolic pattern, leading to elevation of CER; M2 BMDMs expressed a CER-breakdown-metabolic pattern, resulting in upregulation of sphingosine-1-phosphate (S1P). After infusing M1- or M2-polarized BMDMs into the mouse liver after hepatectomy, we found that M1-BMDM infusion increased M1 polarization and CER accumulation, resulting in exaggeration of hepatocyte apoptosis and liver dysfunction. Conversely, M2-BMDM infusion enhanced M2 polarization and S1P generation, leading to alleviation of liver dysfunction with improved hepatocyte proliferation. Treatment of exogenous CER and S1P or inhibition CER and S1P synthesis by siRNA targeting relevant enzymes further revealed that CER induced apoptosis while S1P promoted proliferation in post-hepatectomy primary hepatocytes. In conclusion, CER and S1P are uncovered as critical lipid mediators for M1- and M2-polarized BMDMs to promote injury and regeneration in the liver after hepatectomy, respectively. Notably, the upregulation of hepatic S1P induced by M2-BMDM infusion may have therapeutic potential for post-hepatectomy liver dysfunction.

Prosaposin mediates inflammation in atherosclerosis

Macrophages play a central role in the pathogenesis of atherosclerosis. The inflammatory properties of these cells are dictated by their metabolism, of which the mechanistic target of rapamycin (mTOR) signaling pathway is a key regulator. Using myeloid cell-specific nanobiologics in apolipoprotein E-deficient (Apoe -/-) mice, we found that targeting the mTOR and ribosomal protein S6 kinase-1 (S6K1) signaling pathways rapidly diminished plaque macrophages' inflammatory activity. By investigating transcriptome modifications, we identified Psap, a gene encoding the lysosomal protein prosaposin, as closely related with mTOR signaling. Subsequent in vitro experiments revealed that Psap inhibition suppressed both glycolysis and oxidative phosphorylation. Transplantation of Psap -/- bone marrow to low-density lipoprotein receptor knockout (Ldlr -/-) mice led to a reduction in atherosclerosis development and plaque inflammation. Last, we confirmed the relationship between PSAP expression and inflammation in human carotid atherosclerotic plaques. Our findings provide mechanistic insights into the development of atherosclerosis and identify prosaposin as a potential therapeutic target.

Mitochondrial Dysfunction is a Key Pathway that Links Saturated Fat Intake to the Development and Progression of NAFLD

Non-Alcoholic fatty liver disease (NAFLD) is the most common form of liver disease and is characterized by fat accumulation in the liver. Hypercaloric diets generally increase hepatic fat accumulation, whereas hypocaloric diets decrease liver fat content. In addition, there is evidence to suggest that moderate amounts of unsaturated fatty acids seems to be protective for the development of a fatty liver, while consumption of saturated fatty acids (SFA) appears to predispose toward hepatic steatosis. Recent studies highlight a key role for mitochondrial dysfunction in the development and progression of NAFLD. It is proposed that changes in mitochondrial structure and function are key mechanisms by which SFA lead to the development and progression of NAFLD. In this review, it is described how SFA intake is associated with liver steatosis and decreases the efficiency of the respiratory transport chain. This results in the production of reactive oxygen species and damage to nearby structures, eventually leading to inflammation, apoptosis, and scarring of the liver. Furthermore, studies demonstrating that SFA intake affects the composition of mitochondrial membranes are presented, and this process accelerates the progression of NAFLD. It is likely that events are intertwined and reinforce each other, leading to a constant deterioration in health.

Cross-talk between lipid homeostasis and endoplasmic reticulum stress in neurodegeneration: Insights for HIV-1 associated neurocognitive disorders (HAND)

The dysregulation of lipid homeostasis is emerging as a hallmark of many CNS diseases. As aberrant protein regulation is suggested to be a shared pathological feature amongst many neurodegenerative conditions, such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD), disruptions in neuronal lipid processing may contribute to disease progression in the CNS. Specifically, given the endoplasmic reticulum (ER) dual role in lipid homeostasis as well as protein quality control (PQC) via unfolded protein response (UPR), lipid dysregulation in the CNS may converge on ER functioning and constitute a crucial mechanism underlying aberrant protein aggregation. In the current review, we discuss the diverse roles of lipid species as essential components of the CNS. Moreover, given the importance of both lipid dysregulation and protein aggregation in pathology of CNS diseases, we attempt to assess the potential downstream cross-talk between lipid dysregulation and ER dependent PQC mechanisms, with special focus on HIV-associated neurodegenerative disorders (HAND).

Too Much of a Good Thing? An Evolutionary Theory to Explain the Role of Ceramides in NAFLD

Non-alcoholic fatty liver disease (NAFLD), which ranges from the relatively benign and reversible fatty liver (NAFL) to the more advanced and deadly steatohepatitis (NASH), affects a remarkably high percentage of adults in the population. Depending upon severity, NAFLD can increase one's risk for diabetes, cardiovascular disease, and hepatocellular carcinoma. Though the dominant histological feature of all forms of the disease is the accumulation of liver triglycerides, these molecules are likely not pathogenic, but rather serve to protect the liver from the damaging consequences of overnutrition. We propose herein that the less abundant ceramides, through evolutionarily-conserved actions intended to help organisms adapt to nutrient excess, drive the cellular events that define NAFL/NASH. In early stages of the disease process, they promote lipid uptake and storage, whilst inhibiting utilization of glucose. In later stages, they stimulate hepatocyte apoptosis and fibrosis. In rodents, blocking ceramide synthesis ameliorates all stages of NAFLD. In humans, serum and liver ceramides correlate with the severity of NAFLD and its comorbidities diabetes and heart disease. These studies identify key roles for ceramides in these hepatic manifestations of the metabolic syndrome.

Loss of Neurological Disease HSAN-I-Associated Gene SPTLC2 Impairs CD8+ T Cell Responses to Infection by Inhibiting T Cell Metabolic Fitness

Patients with the neurological disorder HSAN-I suffer frequent infections, attributed to a lack of pain sensation and failure to seek care for minor injuries. Whether protective CD8⁺ T cells are affected in HSAN-I patients remains unknown. Here, we report that HSAN-I-associated mutations in serine palmitoyltransferase subunit SPTLC2 dampened human T cell responses. Antigen stimulation and inflammation induced SPTLC2 expression, and murine T-cell-specific ablation of Sptlc2 impaired antiviral-T-cell expansion and effector function. Sptlc2 deficiency reduced sphingolipid biosynthetic flux and led to prolonged activation of the mechanistic target of rapamycin complex 1 (mTORC1), endoplasmic reticulum (ER) stress, and CD8⁺ T cell death. Protective CD8⁺ T cell responses in HSAN-I patient PBMCs and Sptlc2-deficient mice were restored by supplementing with sphingolipids and pharmacologically inhibiting ER stress-induced cell death. Therefore, SPTLC2 underpins protective immunity by translating extracellular stimuli into intracellular anabolic signals and antagonizes ER stress to promote T cell metabolic fitness.

Acid sphingomyelinase deficiency protects mitochondria and improves function recovery after brain injury

Traumatic brain injury (TBI) is one of the leading causes of disability worldwide and a prominent risk factor for neurodegenerative diseases. The expansion of nervous tissue damage after the initial trauma involves a multifactorial cascade of events, including excitotoxicity, oxidative stress, inflammation, and deregulation of sphingolipid metabolism that further mitochondrial dysfunction and secondary brain damage. Here, we show that a posttranscriptional activation of an acid sphingomyelinase (ASM), a key enzyme of the sphingolipid recycling pathway, resulted in a selective increase of sphingosine in mitochondria during the first week post-TBI that was accompanied by reduced activity of mitochondrial cytochrome oxidase and activation of the Nod-like receptor protein 3 inflammasome. TBI-induced mitochondrial abnormalities were rescued in the brains of ASM KO mice, which demonstrated improved behavioral deficit recovery compared with WT mice. Furthermore, an elevated autophagy in an ASM-deficient brain at the baseline and during the development of secondary brain injury seems to foster the preservation of mitochondria and brain function after TBI. Of note, ASM deficiency attenuated the early stages of reactive astrogliosis progression in an injured brain. These findings highlight the crucial role of ASM in governing mitochondrial dysfunction and brain-function impairment, emphasizing the importance of sphingolipids in the neuroinflammatory response to TBI.

Lipotoxic very-long-chain ceramides cause mitochondrial dysfunction, oxidative stress, and cell death in cardiomyocytes

Accumulating data support a role for bioactive lipids as mediators of lipotixicity in cardiomyocytes. One class of these, the ceramides, constitutes a family of molecules that differ in structure and are synthesized by distinct enzymes, ceramide synthase (CerS)1-CerS6. Data support that specific ceramides and the enzymes that catalyze their formation play distinct roles in cell function. In a mouse model of diabetic cardiomyopathy, sphingolipid profiling revealed increases in not only the CerS5-derived ceramides but also in very long chain (VLC) ceramides derived from CerS2. Overexpression of CerS2 elevated VLC ceramides caused insulin resistance, oxidative stress, mitochondrial dysfunction, and mitophagy. Palmitate induced CerS2 and oxidative stress, mitophagy, and apoptosis, which were prevented by depletion of CerS2. Neither overexpression nor knockdown of CerS5 had any function in these processes, suggesting a chain-length dependent impact of ceramides on mitochondrial function. This concept was also supported by the observation that synthetic mitochondria-targeted ceramides led to mitophagy in a manner proportional to N-acyl chain length. Finally, blocking mitophagy exacerbated cell death. Taken together, our results support a model by which CerS2 and VLC ceramides have a distinct role in lipotoxicity, leading to mitochondrial damage, which results in subsequent adaptive mitophagy. Our data reveal a novel lipotoxic pathway through CerS2.-Law, B. A., Liao, X., Moore, K. S., Southard, A., Roddy, P., Ji, R., Szulc, Z., Bielawska, A., Schulze, P. C., Cowart, L. A. Lipotoxic very-long-chain ceramides cause mitochondrial dysfunction, oxidative stress, and cell death in cardiomyocytes.

Pathogenesis of Nonalcoholic Steatohepatitis and Hormone-Based Therapeutic Approaches

Non-alcoholic fatty liver disease (NAFLD) is an emerging global health problem and a potential risk factor for type 2 diabetes, cardiovascular disease, and chronic kidney disease. Nonalcoholic steatohepatitis (NASH), an advanced form of NAFLD, is a predisposing factor for development of cirrhosis and hepatocellular carcinoma. The increasing prevalence of NASH emphasizes the need for novel therapeutic approaches. Although therapeutic drugs against NASH are not yet available, fundamental insights into the pathogenesis of NASH have been made during the past few decades. Multiple therapeutic strategies have been developed and are currently being explored in clinical trials or preclinical testing. The pathogenesis of NASH involves multiple intracellular/extracellular events in various cell types in the liver or crosstalk events between the liver and other organs. Here, we review current findings and knowledge regarding the pathogenesis of NASH, focusing on the most recent advances. We also highlight hormone-based therapeutic approaches for treatment of NASH.

Chemical Compensation of Mitochondrial Phospholipid Depletion in Yeast and Animal Models of Parkinson's Disease

We have been investigating the role that phosphatidylethanolamine (PE) and phosphatidylcholine (PC) content plays in modulating the solubility of the Parkinson’s disease protein alpha-synuclein (α-syn) using Saccharomyces cerevisiae and Caenorhabditis elegans. One enzyme that synthesizes PE is the conserved enzyme phosphatidylserine decarboxylase (Psd1/yeast; PSD-1/worms), which is lodged in the inner mitochondrial membrane. We previously found that decreasing the level of PE due to knockdown of Psd1/psd-1 affects the homeostasis of α-syn in vivo. In S. cerevisiae, the co-occurrence of low PE and α-syn in psd1Δ cells triggers mitochondrial defects, stress in the endoplasmic reticulum, misprocessing of glycosylphosphatidylinositol-anchored proteins, and a 3-fold increase in the level of α-syn. The goal of this study was to identify drugs that rescue this phenotype. We screened the Prestwick library of 1121 Food and Drug Administration-approved drugs using psd1Δ + α-syn cells and identified cyclosporin A, meclofenoxate hydrochloride, and sulfaphenazole as putative protective compounds. The protective activity of these drugs was corroborated using C. elegans in which α-syn is expressed specifically in the dopaminergic neurons, with psd-1 depleted by RNAi. Worm populations were examined for dopaminergic neuron survival following psd-1 knockdown. Exposure to cyclosporine, meclofenoxate, and sulfaphenazole significantly enhanced survival at day 7 in α-syn-expressing worm populations whereby 50–55% of the populations displayed normal neurons, compared to only 10–15% of untreated animals. We also found that all three drugs rescued worms expressing α-syn in dopaminergic neurons that were deficient in the phospholipid cardiolipin following cardiolipin synthase (crls-1) depletion by RNAi. We discuss how these drugs might block α-syn pathology in dopaminergic neurons.

Myristic acid potentiates palmitic acid-induced lipotoxicity and steatohepatitis associated with lipodystrophy by sustaning de novo ceramide synthesis

Palmitic acid (PA) induces hepatocyte apoptosis and fuels de novo ceramide synthesis in the endoplasmic reticulum (ER). Myristic acid (MA), a free fatty acid highly abundant in copra/palmist oils, is a predictor of nonalcoholic steatohepatitis (NASH) and stimulates ceramide synthesis. Here we investigated the synergism between MA and PA in ceramide synthesis, ER stress, lipotoxicity and NASH. Unlike PA, MA is not lipotoxic but potentiated PA-mediated lipoapoptosis, ER stress, caspase-3 activation and cytochrome c release in primary mouse hepatocytes (PMH). Moreover, MA kinetically sustained PA-induced total ceramide content by stimulating dehydroceramide desaturase and switched the ceramide profile from decreased to increased ceramide 14:0/ceramide16:0, without changing medium and long-chain ceramide species. PMH were more sensitive to equimolar ceramide14:0/ceramide16:0 exposure, which mimics the outcome of PA plus MA treatment on ceramide homeostasis, than to either ceramide alone. Treatment with myriocin to inhibit ceramide synthesis and tauroursodeoxycholic acid to prevent ER stress ameliorated PA plus MA induced apoptosis, similar to the protection afforded by the antioxidant BHA, the pan-caspase inhibitor z-VAD-Fmk and JNK inhibition. Moreover, ruthenium red protected PMH against PA and MA-induced cell death. Recapitulating in vitro findings, mice fed a diet enriched in PA plus MA exhibited lipodystrophy, hepatosplenomegaly, increased liver ceramide content and cholesterol levels, ER stress, liver damage, inflammation and fibrosis compared to mice fed diets enriched in PA or MA alone. The deleterious effects of PA plus MA-enriched diet were largely prevented by in vivo myriocin treatment. These findings indicate a causal link between ceramide synthesis and ER stress in lipotoxicity, and imply that the consumption of diets enriched in MA and PA can cause NASH associated with lipodystrophy.

Cytotoxic effects of psychotropic benzofuran derivatives, N-methyl-5-(2-aminopropyl)benzofuran and its N-demethylated derivative, on isolated rat hepatocytes

The novel psychoactive compounds derived from amphetamine have been illegally abused as recreational drugs, some of which are known to be hepatotoxic in humans and experimental animals. The cytotoxic effects and mechanisms of 5-(2-aminopropyl)benzofuran (5-APB) and N-methyl-5-(2-aminopropyl)benzofuran (5-MAPB), both of which are benzofuran analogues of amphetamine, and 3,4-methylenedioxy-N-methamphetamine (MDMA) were studied in freshly isolated rat hepatocytes. 5-MAPB caused not only concentration-dependent (0-4.0 mm) and time-dependent (0-3 h) cell death accompanied by the depletion of cellular ATP and reduced glutathione and protein thiol levels, but also accumulation of oxidized glutathione. Of the other analogues examined at a concentration of 4 mm, 5-MAPB/5-APB-induced cytotoxicity with the production of reactive oxygen species and loss of mitochondrial membrane potential was greater than that induced by MDMA. In isolated rat liver mitochondria, the benzofurans resulted in a greater increase in the rate of state 4 oxygen consumption than did MDMA, with a decrease in the rate of state 3 oxygen consumption. Furthermore, the benzofurans caused more of a rapid mitochondrial swelling dependent on the mitochondrial permeability transition than MDMA. 5-MAPB at a weakly toxic level (1 mm) was metabolized slowly: levels of 5-MAPB and 5-APB were approximately 0.9 mm and 50 μm, respectively, after 3 h incubation. Taken collectively, these results indicate that mitochondria are target organelles for the benzofuran analogues and MDMA, which elicit cytotoxicity through mitochondrial failure, and the onset of cytotoxicity may depend on the initial and/or residual concentrations of 5-MAPB rather than on those of its metabolite 5-APB. Copyright © 2016 John Wiley & Sons, Ltd.

Sphingolipids in liver injury, repair and regeneration

Sphingolipids are not only essential components of cellular membranes but also function as intracellular and extracellular mediators that regulate important physiological cellular processes including cell survival, proliferation, apoptosis, differentiation, migration and immune responses. The liver possesses the unique ability to regenerate after injury in a complex manner that involves numerous mediators, including sphingolipids such as ceramide and sphingosine 1-phosphate. Here we present the current understanding of the involvement of the sphingolipid pathway and the role this pathway plays in regulating liver injury, repair and regeneration. The regulation of sphingolipids and their enzymes may have a great impact in the development of novel therapeutic modalities for a variety of liver injuries and diseases.

Metabolomics discloses donor liver biomarkers associated with early allograft dysfunction

BACKGROUND & AIMS

Early allograft dysfunction (EAD) dramatically influences graft and patient outcome after orthotopic liver transplantation and its incidence is strongly determined by donor liver quality. Nevertheless, objective biomarkers, which can assess graft quality and anticipate organ function, are still lacking. This study aims to investigate whether there is a preoperative donor liver metabolomic biosignature associated with EAD.

METHODS

A comprehensive metabolomic profiling of 124 donor liver biopsies collected before transplantation was performed by mass spectrometry coupled to liquid chromatography. Donor liver grafts were classified into two groups: showing EAD and immediate graft function (IGF). Multivariate data analysis was used to search for the relationship between the metabolomic profiles present in donor livers before transplantation and their function in recipients.

RESULTS

A set of liver graft dysfunction-associated biomarkers was identified. Key changes include significantly increased levels of bile acids, lysophospholipids, phospholipids, sphingomyelins and histidine metabolism products, all suggestive of disrupted lipid homeostasis and altered histidine pathway. Based on these biomarkers, a predictive EAD model was built and further evaluated by assessing 24 independent donor livers, yielding 91% sensitivity and 82% specificity. The model was also successfully challenged by evaluating donor livers showing primary non-function (n=4).

CONCLUSIONS

A metabolomic biosignature that accurately differentiates donor livers, which later showed EAD or IGF, has been deciphered. The remarkable metabolomic differences between donor livers before transplant can relate to their different quality. The proposed metabolomic approach may become a clinical tool for donor liver quality assessment and for anticipating graft function before transplant.

Plasma sphingolipids as potential indicators of hepatic necroinflammation in patients with chronic hepatitis C and normal alanine aminotransferase level

Accurate estimation of hepatic necroinflammation caused by chronic hepatitis C (CHC) is crucial for prediction of prognosis and design of therapeutic strategy, which is particularly true for CHC patients with normal alanine aminotransferase (ALT) level. Recent studies have shown that sphingolipids have a close relationship with hepatitis C virus infection. The present study aimed to identify plasma sphingolipids related to hepatic necroinflammation. We included 120 treatment-naïve CHC patients and 64/120 had normal ALT levels (<40 U/L). CHC patients who underwent liver biopsies were subjected to Scheuer scoring analysis for scope of hepatic inflammation. Plasma sphingolipids were detected by high-performance liquid chromatography tandem mass spectrometry. Our results showed 44 plasma sphingolipids were detected altogether. Of all detected sphingolipids, hexosylceramide (HexCer) (d18∶1/22∶0) and HexCer (d18∶1/24∶0) showed a significant difference among G0/G1, G2, and G3/G4 (P<0.05). For identifying hepatic necroinflammation (G≥2), after adjusting other factors, the odds ratio (OR) of HexCer (d18∶1/22∶0) reached 1.01 (95% confidence interval [CI]: 1.00–1.02). Furthermore, the area under the curve (AUC) of HexCer (d18∶1/22∶0) was 0.7 (P = 0.01) and approached that of ALT (AUC = 0.78). However, in CHC patients with normal ALT, HexCer (d18∶1/22∶0) was an independent factor (OR: 1.02, 95% CI: 1.01–1.03) to identify the hepatic necroinflammation (G≥2). HexCer (d18∶1/22∶0) not only showed the largest AUC (0.78, P = 0.001), but also exhibited the highest specificity of all indicators. These results indicate that plasma HexCer (d18∶1/22∶0) is a potential indicator to distinguish hepatic necroinflammation in CHC patients. For CHC with normal ALT, the ability of HexCer (d18∶1/22∶0) to distinguish hepatic necroinflammation might be superior to conventional serum indicators.

Rapid chromatographic method to decipher distinct alterations in lipid classes in NAFLD/NASH

AIM: To establish a simple method to quantify lipid classes in liver diseases and to decipher the lipid profile in p62/IMP2-2/IGF2BP2-2 transgenic mice.

METHODS: Liver-specific overexpression of the insulin-like growth factor 2 mRNA binding protein p62/IMP2-2/IGF2BP2-2 was used as a model for steatosis. Steatohepatitis was induced by feeding a methionine-choline deficient diet. Steatosis was assessed histologically. For thin layer chromatographic analysis, lipids were extracted from freeze-dried tissues by hexane/2-propanol, dried, redissolved, and chromatographically separated by a two-solvent system. Dilution series of lipid standards were chromatographed, detected, and quantified. The detection was performed by either 2’,7’-dichlorofluoresceine or a sulfuric acid/ethanol mixture.

RESULTS: Histological analyses confirmed steatosis and steatohepatitis development. The extraction, chromatographic, and detection method showed high inter-assay reproducibility and allowed quantification of the different lipid classes. The analyses confirmed an increase of triglycerides and phosphatidylethanolamine and a decrease in phosphatidylcholine in the methionine-choline deficient diet. The method was used for the first time to asses the lipid classes induced in the p62-overexpressing mouse model and showed a significant increase in all detected lipid species with a prominent increase of triglycerides by 2-fold. Interestingly, the ratio of phosphatidylcholine to phosphatidylethanolamine was decreased, as previously suggested as a marker in the progression from steatosis to steatohepatitis.

CONCLUSION: The thin layer chromatography analysis allows a reliable quantification of lipid classes and provides detailed insight into the lipogenic effect of p62.

Decoding cell death signals in liver inflammation

Inflammation can be either beneficial or detrimental to the liver, depending on multiple factors. Mild (i.e., limited in intensity and destined to resolve) inflammatory responses have indeed been shown to exert consistent hepatoprotective effects, contributing to tissue repair and promoting the re-establishment of homeostasis. Conversely, excessive (i.e., disproportionate in intensity and permanent) inflammation may induce a massive loss of hepatocytes and hence exacerbate the severity of various hepatic conditions, including ischemia-reperfusion injury, systemic metabolic alterations (e.g., obesity, diabetes, non-alcoholic fatty liver disorders), alcoholic hepatitis, intoxication by xenobiotics and infection, de facto being associated with irreversible liver damage, fibrosis, and carcinogenesis. Both liver-resident cells (e.g., Kupffer cells, hepatic stellate cells, sinusoidal endothelial cells) and cells that are recruited in response to injury (e.g., monocytes, macrophages, dendritic cells, natural killer cells) emit pro-inflammatory signals including - but not limited to - cytokines, chemokines, lipid messengers, and reactive oxygen species that contribute to the apoptotic or necrotic demise of hepatocytes. In turn, dying hepatocytes release damage-associated molecular patterns that-upon binding to evolutionary conserved pattern recognition receptors-activate cells of the innate immune system to further stimulate inflammatory responses, hence establishing a highly hepatotoxic feedforward cycle of inflammation and cell death. In this review, we discuss the cellular and molecular mechanisms that account for the most deleterious effect of hepatic inflammation at the cellular level, that is, the initiation of a massive cell death response among hepatocytes.

Ceramide and the mitochondrial respiratory chain

Ceramide is a group of sphingolipids found in cell membranes, composed of a sphingoid base linked to a fatty acid of varying chain length. Initially regarded as purely structural components, this group of molecules is now recognized as a key signaling and regulatory elements in cell biology. Ceramide species differing in acyl chain length, with distinct biophysical properties, execute distinct functions and effects. Some of these modulate mitochondrial function and oxidative phosphorylation (OXPHOS). Certain ceramides were associated with decreased mitochondrial respiratory chain (MRC) activity, increased reactive oxygen species (ROS) production and oxidative stress, mitochondrial outer membrane permeabilization (MOMP), reduced mitochondrial membrane potential, mitophagy and apoptosis. In this review we aim to summarize the most relevant findings linking ceramide to mitochondria. The physiological significance of synthetic short and naturally occurring long chain ceramides in modulating mitochondrial function with emphasis on the MRC will be discussed.

LCL124, a cationic analog of ceramide, selectively induces pancreatic cancer cell death by accumulating in mitochondria

Treatment of pancreatic cancer that cannot be surgically resected currently relies on minimally beneficial cytotoxic chemotherapy with gemcitabine. As the fourth leading cause of cancer-related death in the United States with dismal survival statistics, pancreatic cancer demands new and more effective treatment approaches. Resistance to gemcitabine is nearly universal and appears to involve defects in the intrinsic/mitochondrial apoptotic pathway. The bioactive sphingolipid ceramide is a critical mediator of apoptosis initiated by a number of therapeutic modalities. It is noteworthy that insufficient ceramide accumulation has been linked to gemcitabine resistance in multiple cancer types, including pancreatic cancer. Taking advantage of the fact that cancer cells frequently have more negatively charged mitochondria, we investigated a means to circumvent resistance to gemcitabine by targeting delivery of a cationic ceramide (l-t-C6-CCPS [LCL124: ((2S,3S,4E)-2-N-[6'-(1″-pyridinium)-hexanoyl-sphingosine bromide)]) to cancer cell mitochondria. LCL124 was effective in initiating apoptosis by causing mitochondrial depolarization in pancreatic cancer cells but demonstrated significantly less activity against nonmalignant pancreatic ductal epithelial cells. Furthermore, we demonstrate that the mitochondrial membrane potentials of the cancer cells were more negative than nonmalignant cells and that dissipation of this potential abrogated cell killing by LCL124, establishing that the effectiveness of this compound is potential-dependent. LCL124 selectively accumulated in and inhibited the growth of xenografts in vivo, confirming the tumor selectivity and therapeutic potential of cationic ceramides in pancreatic cancer. It is noteworthy that gemcitabine-resistant pancreatic cancer cells became more sensitive to subsequent treatment with LCL124, suggesting that this compound may be a uniquely suited to overcome gemcitabine resistance in pancreatic cancer.

Cholesterol: recapitulation of its active role during liver regeneration

Liver regeneration is a compensatory hyperplasia produced by several stimuli that promotes proliferation in order to provide recovery of the liver mass and architecture. This process involves complex signalling cascades that receive feedback from autocrine and paracrine pathways, recognized by parenchymal as well as non-parenchymal cells. Nowadays the dynamic role of lipids in biological processes is widely recognized; however, a systematic analysis of their importance during liver regeneration is still missing. Therefore, in this review we address the role of lipids including the bioactive ones such as sphingolipids, but with special emphasis on cholesterol. Cholesterol is not only considered as a structural component but also as a relevant lipid involved in the control of the intermediate metabolism of different liver cell types such as hepatocytes, hepatic stellate cells and Kupffer cells. Cholesterol plays a significant role at the level of specific membrane domains, as well as modulating the expression of sterol-dependent proteins. Moreover, several enzymes related to the catabolism of cholesterol and whose activity is down regulated are related to the protection of liver tissue from toxicity during the process of regeneration. This review puts in perspective the necessity to study and understand the basic mechanisms involving lipids during the process of liver regeneration. On the other hand, the knowledge acquired in this area in the past years, can be considered invaluable in order to provide further insights into processes such as general organogenesis and several liver-related pathologies, including steatosis and fibrosis.

Mitochondria and redox signaling in steatohepatitis

Alcoholic and nonalcoholic fatty liver diseases are potentially pathological conditions that can progress to steatohepatitis, fibrosis, and cirrhosis. These conditions affect millions of people throughout the world in part through poor lifestyle choices of excess alcohol consumption, overnutrition, and lack of regular physical activity. Abnormal mitochondrial and cellular redox homeostasis has been documented in steatohepatitis and results in alterations of multiple redox-sensitive signaling cascades. Ultimately, these changes in signaling lead to altered enzyme function and transcriptional activities of proteins critical to mitochondrial and cellular function. In this article, we review the current hypotheses linking mitochondrial redox state to the overall pathophysiology of alcoholic and nonalcoholic steatohepatitis and briefly discuss the current therapeutic options under investigation.

Novel pathway of ceramide production in mitochondria: thioesterase and neutral ceramidase produce ceramide from sphingosine and acyl-CoA

Reports suggest that excessive ceramide accumulation in mitochondria is required to initiate the intrinsic apoptotic pathway and subsequent cell death, but how ceramide accumulates is unclear. Here we report that liver mitochondria exhibit ceramide formation from sphingosine and palmitoyl-CoA and from sphingosine and palmitate. Importantly, this activity was markedly decreased in liver from neutral ceramidase (NCDase)-deficient mice. Moreover, the levels of ceramide were dissimilar in liver mitochondria of WT and NCDase KO mice. These results suggest that NCDase is a key participant of ceramide formation in liver mitochondria. We also report that highly purified liver mitochondria have ceramidase, reverse ceramidase, and thioesterase activities. Increased accessibility of palmitoyl-CoA to the mitochondrial matrix with the pore-forming peptide zervamicin IIB resulted in 2-fold increases in palmitoyl-CoA hydrolysis by thioesterase. This increased hydrolysis was accompanied by an increase in ceramide formation, demonstrating that both outer membrane and matrix localized thioesterases can regulate ceramide formation. Also, ceramide formation might occur both in the outer mitochondrial membrane and in the mitochondrial matrix, suggesting the existence of distinct ceramide pools. Taken together, these results suggest that the reverse activity of NCDase contributes to sphingolipid homeostasis in this organelle in vivo.

Cytotoxic effects of hydroxylated fullerenes on isolated rat hepatocytes via mitochondrial dysfunction

The cytotoxic effects of hydroxylated fullerenes, also termed fullerenols or fullerols [C(60)(OH)( n )], which are known nanomaterials and water-soluble fullerene derivatives, were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to C(60)(OH)(24) caused not only concentration (0-0.25 mM)- and time (0-3 h)-dependent cell death accompanied by the formation of cell blebs, loss of cellular ATP, reduced glutathione (GSH), and protein thiol levels, but also the accumulation of glutathione disulfide and malondialdehyde, indicating lipid peroxidation. Of the other analogues examined, the cytotoxic effects of C(60)(OH)(12) and fullerene C(60) at a concentration of 0.125 mM were less than those of C(60)(OH)(24). The loss of mitochondrial membrane potential and generation of oxygen radical species in hepatocytes incubated with C(60)(OH)(24) were greater than those with C(60)(OH)(12) and fullerene C(60). In the oxygen consumption of mitochondria isolated from rat liver, the ratios of state-3/state-4 respiration were more markedly decreased by C(60)(OH)(24) and C(60)(OH)(12) compared with C(60). In addition, C(60)(OH)(24) and C(60)(OH)(12) resulted in the induction of the mitochondrial permeability transition (MPT), and the effects of C(60)(OH)(12) were less than those of C(60)(OH)(24). Taken collectively, these results indicate that (a) mitochondria are target organelles for fullerenols, which elicit cytotoxicity through mitochondrial failure related to the induction of the MPT, mitochondrial depolarization, and inhibition of ATP synthesis in the early stage and subsequently oxidation of GSH and protein thiols, and lipid peroxidation through oxidative stress at a later stage; and (b) the toxic effects of fullerenols may depend on the number of hydroxyl groups participating in fullerene in rat hepatocytes.

Ceramide-induced activation of cytosolic NADH/cytochrome c electron transport pathway: An additional source of energy for apoptosis

We have investigated whether increase in the oxidation rate of exogenous cytochrome c (cyto-c), induced by long-chain ceramides, might be due to an increased rate of cytosolic NADH/cyto-c electron transport pathway. This process was identified in isolated liver mitochondria and has been studied in our laboratory for many years. Data from highly specific test of sulfite oxidase prove that exogenous cyto-c both in the absence and presence of ceramide cannot permeate through the mitochondrial outer membrane. However, the oxidation of added NADH, mediated by exogenous cyto-c and coupled to the generation of a membrane potential supporting the ATP synthesis, can also be stimulated by ceramide. The results obtained suggest that ceramide molecules, by increasing mitochondrial permeability, with the generation of either raft-like platforms or channels, may have a dual function. They can promote the release of endogenous cyto-c and activate, with an energy conserving process, the oxidation of cytosolic NADH either inducing the formation of new respiratory contact sites or increasing the frequency of the pre-existing porin contact sites. In agreement with the data in the literature, an increase of mitochondrial ceramide molecules level may represent an efficient strategy to activate and support the correct execution of apoptotic program.

Lysosomal-mitochondrial cross-talk during cell death

Lysosomes are membrane-bound organelles, which contain an arsenal of different hydrolases, enabling them to act as the terminal degradative compartment of the endocytotic, phagocytic and autophagic pathways. During the last decade, it was convincingly shown that destabilization of lysosomal membrane and release of lysosomal content into the cytosol can initiate the lysosomal apoptotic pathway, which is dependent on mitochondria destabilization. The cleavage of BID to t-BID and degradation of anti-apoptotic BCL-2 proteins by lysosomal cysteine cathepsins were identified as links to the mitochondrial cytochrome c release, which eventually leads to caspase activation. There have also been reports about the involvement of lysosome destabilization and lysosomal proteases in the extrinsic apoptotic pathway, although the molecular mechanism is still under debate. In the present article, we discuss the cross-talk between lysosomes and mitochondria during apoptosis and its consequences for the fate of the cell.

BEX2 regulates mitochondrial apoptosis and G1 cell cycle in breast cancer

We have recently demonstrated that BEX2 is differentially expressed in primary breast tumors and BEX2 expression is required for the Nerve Growth factor inhibition of ceramide-induced apoptosis in breast cancer. In this study we investigate the functional role of BEX2 in the survival and growth of breast cancer cells. We demonstrate that BEX2 downregulation induces mitochondrial apoptosis and sensitizes breast cancer cells to the pro-apoptotic effects of ceramide, doxorubicin and staurosporine. In addition, BEX2 overexpression protects the breast cancer cells against mitochondrial apoptosis. We show that this effect of BEX2 is mediated through the modulation of Bcl-2 protein family, which involves the positive regulation of anti-apoptotic member Bcl-2 and the negative regulation of pro-apoptotic members BAD, BAK1 and PUMA. Moreover, our data suggests that BEX2 expression is required for the normal cell cycle progression during G1 in breast cancer cells through the regulation of cyclin D1 and p21. To further support the significance of BEX2 in the pathogenesis of breast cancer we demonstrate that BEX2 overexpression is associated with a higher activation of the Bcl-2/NF-kappaB pathway in primary breast tumors. Furthermore, we show that BEX2 downregulation results in a higher expression and activity of protein phosphatase 2A. The modulation of protein phosphatase 2A, which is also known to mediate the cellular response to ceramide, provides a possible mechanism to explain the BEX2-mediated cellular effects. This study demonstrates that BEX2 has a significant role in the regulation of mitochondrial apoptosis and G1 cell cycle in breast cancer.

Regulation of ceramide channels by Bcl-2 family proteins

Mitochondrial outer membrane permeabilization to proteins, an irreversible step in apoptosis by which critical proteins are released, is tightly regulated by Bcl-2 family proteins. The exact nature of the release pathway is still undefined. Ceramide is an important sphingolipid, involved in various cellular processes including apoptosis. Here we describe the structural properties of ceramide channels and their regulation by the anti-apoptotic and pro-apoptotic proteins of the Bcl-2 family. The evolutionarily conserved regulation of ceramide channels by Bcl-2 family proteins, consistent with their role in apoptosis, lends credibility to the notion that ceramide channels constitute the protein release pathway.

Redox regulation of tumor necrosis factor signaling

Tumor necrosis factor-alpha (TNF) is a key cytokine that has been shown to play important physiologic (e.g., inflammation) and pathophysiologic (e.g., various liver pathologies) roles. In liver and other tissues, TNF treatment results in the simultaneous activation of an apoptotic pathway (i.e., TRADD, RIP, JNK) and a survival pathway mediated by NF-kappaB transcription of survival genes (i.e., GADD45beta, Mn-SOD, cFLIP). The cellular response (e.g., proliferation versus apoptosis) to TNF is determined by the balance between the apoptotic signaling pathway and the NF-kappaB survival pathway stimulated by TNF. Reactive oxygen species (ROS) are important modulators of signaling pathways and can regulate both apoptotic signaling and NF-kappaB transcription triggered by TNF. ROS are important in mediating the sustained activation of JNK, to help mediate apoptosis after TNF treatment. In some cells, ROS are second messengers that mediate apoptosis after TNF stimulation. Conversely, ROS can cause redox modifications that inhibit NF-kappaB activation, which can lead to cell death triggered by TNF. Consequently, the redox status of cells can determine the biologic response that TNF will induce in cells. In many liver pathologies, ROS generated extrinsically (e.g., inflammation) or intrinsically (i.e., drugs, toxins) may act in concert with TNF to promote hepatocyte death and liver injury through redox inhibition of NF-kappaB.

Long-chain ceramide is a potent inhibitor of the mitochondrial permeability transition pore

The sphingolipid ceramide has been implicated in mediating cell death that is accompanied by mitochondrial functional alterations. Moreover, ceramide has been shown to accumulate in mitochondria upon induction of apoptotic processes. In this study, we sought to evaluate the effects of natural, highly hydrophobic long-chain ceramides on mitochondrial function in vitro. Ceramide in a dodecane/ethanol delivery system inhibited the opening of the mitochondrial permeability transition pore (PTP) induced by either oxidative stress, SH group cross-linking, or high Ca2+ load, suggesting that the inhibitory point is at a level at which major PTP regulatory pathways converge. Moreover, ceramide had no effect on well known mitochondrial components that modulate PTP activity, such as cyclophilin D, voltage-dependent anion channel, adenine nucleotide transporter, and ATP synthase. The inhibitory effect of ceramide on PTP was not stereospecific, nor was there a preference for ceramide over dihydroceramide. However, the effect of ceramide on PTP was significantly influenced by the fatty acid moiety chain length. These studies are the first to show that long-chain ceramide can influence PTP at physiologically relevant concentrations, suggesting that it is the only known potent natural inhibitor of PTP. These results suggest a novel mechanism of ceramide regulation of mitochondrial function.

Effects of flavonoids on sphingolipid turnover in the toxin-damaged liver and liver cells

BACKGROUND

The ceramide generation is an early event in the apoptotic response to numerous stimuli including the oxidative stress and ceramide analogs mimic the stress effect and induce apoptosis. Flavonoids of German chamomile are reported to exhibit the hepatoprotective effect. Flavonoids affect sphingolipid metabolism and reduce the elevated ceramide level in the aged liver. In the present paper, the ceramide content and production in the CCl4- and ethanol-treated liver and hepatocytes as well as the correction of sphingolipid metabolism in the damaged liver using the mixture of German chamomile flavonoids (chamiloflan) or apigenin-7-glucoside (AP7Glu) have been investigated.

RESULTS

The experiments were performed in either the rat liver or hepatocytes of normal, CCl4- and ethanol-treated or flavonoid- and toxin plus flavonoid-treated animals. [14C]palmitic acid and [methyl-14C-phosphorylcholine]sphingomyelin were used to investigate the sphingolipid turnover. Addition of the CCl4 or ethanol to isolated hepatocyte suspensions caused loss of cell viability and increased the lactate dehydrogenase release from the cells into supernatant and ceramide level in the cells. CCl4 administration to the rats enlarged ceramide mass as well as neutral sphingomyelinase (SMase) activity and reduced ceramide degradation by the neutral ceramidase. Pretreatment of isolated hepatocytes with flavonoids abrogated the CCl4 effects on the cell membrane integrity and normalized the ceramide content. Flavonoid administration to the rats normalized the elevated ceramide content in the damaged liver via neutral SMase inhibition and ceramidase activation.

CONCLUSION

The data obtained have demonstrated that flavonoids affect sphingolipid metabolism in the CCl4- and ethanol-damaged liver and liver cells. Flavonoids normalized activities of key enzymes of sphingolipid turnover (neutral SMase and ceramidase) and ceramide contents in the damaged liver and liver cells, and stabilized the hepatocyte membranes.

Anti-apoptotic Bcl-2 Family Proteins Disassemble Ceramide Channels

Early in mitochondria-mediated apoptosis, the mitochondrial outer membrane becomes permeable to proteins that, when released into the cytosol, initiate the execution phase of apoptosis. Proteins in the Bcl-2 family regulate this permeabilization, but the molecular composition of the mitochondrial outer membrane pore is under debate. We reported previously that at physiologically relevant levels, ceramides form stable channels in mitochondrial outer membranes capable of passing the largest proteins known to exit mitochondria during apoptosis (Siskind, L. J., Kolesnick, R. N., and Colombini, M. (2006) Mitochondrion 6, 118-125). Here we show that Bcl-2 proteins are not required for ceramide to form protein-permeable channels in mitochondrial outer membranes. However, both recombinant human Bcl-x(L) and CED-9, the Caenorhabditis elegans Bcl-2 homologue, disassemble ceramide channels in the mitochondrial outer membranes of isolated mitochondria from rat liver and yeast. Importantly, Bcl-x L and CED-9 disassemble ceramide channels in the defined system of solvent-free planar phospholipid membranes. Thus, ceramide channel disassembly likely results from direct interaction with these anti-apoptotic proteins. Mutants of Bcl-x L act on ceramide channels as expected from their ability to be anti-apoptotic. Thus, ceramide channels may be one mechanism for releasing pro-apoptotic proteins from mitochondria during the induction phase of apoptosis.

Increase of ceramide in the liver and plasma after carbon tetrachloride intoxication in the rat

In fulminant hepatic failure, various toxins causing multi-organ failure increase in plasma. As a novel toxin, levels of ceramide, a well-studied lipid mediator of apoptosis, were determined by LC-MS/MS in the liver and plasma of carbon tetrachloride (CCl4)-intoxicated rats. After 6 h of oral administration of CCl4 (4 mL/kg body weight as a 1:1 mixture of CCl4 and mineral oil) to rats, extensive hepatic failure occurred as evidenced by a severe elevation in plasma AST and ALT. The liver concentration of major ceramide components (C16:0, C24:0, C24:1, C18:0, C22:0, and C24:2 in decreasing order), and the sum of these ceramides increased significantly 2 h after CCl4 intoxication compared to that in the control group given mineral oil. The total ceramide concentration in the plasma was also increased to 4.1 times that in the control 24 h after administration of CCl4. In conclusion, the early increase in liver ceramides may contribute to hepatic cell death and the increase in plasma ceramides during fulminant hepatic failure may cause damage in other organs including the brain and kidney.

Bezafibrate decreases growth stimulatory action of the sphingomyelin signaling pathway in regenerating rat liver

Liver regeneration after partial hepatectomy (PH) is achieved through proliferation of hepatocytes and non-parenchymal cells. The nuclear peroxisome proliferator-activated receptor alpha (PPARalpha) is involved in regulation of lipid metabolism and proliferation of hepatic cells. The sphingomyelin signal transduction pathway is involved in the regulation of the cell cycle in eukaryotic organisms. Sphingosine-1-phosphate (S1P) and ceramide (CER)-- the intermediates of the pathway--are known to stimulate and to inhibit cellular proliferation. The aim of the present study was to investigate the effect of PPARalpha activation by bezafibrate on the sphingomyelin signaling pathway during the first 24h of liver regeneration after PH in the rat. The content of sphingomyelin, ceramide, sphingosine, sphinganine, sphingosine-1-phosphate and the activity of sphingomyelinases and ceramidases were determined at various time points after PH. It has been found that the activity of neutral Mg(2+)-dependent sphingomyelinase (nSMase) increased, whereas the activity of acidic sphingomyelinase (aSMase) decreased in the regenerating liver. Activation of PPARalpha by bezafibrate lower the activity of nSMase and increased the activity of aSMase in the regenerating rat liver. The content of ceramide was higher in bezafibrate-treated rats, whereas the content of sphingosine-1-phosphate was markedly lower as compared to the untreated rats. Therefore, it is concluded that activation of PPARalpha by bezafibrate decreases the growth-stimulatory activity of the sphingomyelin pathway in regenerating rat liver.

Sphingolipid metabolite ceramide causes metabolic perturbation contributing to HERG K+ channel dysfunction

Ceramide, a sphingolipid metabolite, has emerged as a key second messenger molecule that mediates multiple cellular functions. Its de nova synthesis and accumulation in ischemic myocardium, congestive heart failure and diabetic cardiomyopathy is associated with the abnormalities such as abnormal QT prolongation and increased risk of arrhythmias. To investigate how ceramide is involved in modulating cardiac repolarization, we performed whole-cell patch-clamp studies on HERG current (I(HERG)), a critical determinant of cardiac repolarization, expressed in HEK293 cells. Acute application (superfusion for 25 min) of membrane permeable ceramide (C2, 5 microM) did not alter I(HERG). Prolonged incubation with C2 for 10 hrs caused pronounced I(HERG) inhibition in a concentration-dependent and voltage-independent fashion and positive shift of voltage-dependent HERG activation. The IC(50) for I(HERG) suppression was 19.5 microM. C2 did not affect the inactivation property and time-dependent kinetics of I(HERG). Similar effects were observed with production of endogenous ceramide catalyzed by sphingomyelinase. Tyrosine kinase inhibitors failed to reverse C2-induced suppression of HERG function, and PKA and PKC inhibitors only slightly reversed the I(HERG) depression. Western blotting and immunocytochemical analyses indicate that C2 does not alter HERG protein expression on the cytoplasmic membrane. The inhibitory effect of C2 on I(HERG) was reversed by antioxidants vitamin E or MnTBAP. C2 caused considerable production of intracellular reactive oxygen species (ROS), which was prevented by vitamin E or MnTBAP. We conclude that ceramide depresses I(HERG) mainly via ROS overproduction and ceramide-induced I(HERG) impairment may contribute to QT prolongation in prolonged myocardial ischemia, heart failure and diabetic cardiomyopathy.

Granulocyte differentiation inducer, hexapeptide HLDF-6, decreases cytotoxic effect of tumor necrosis factor on HL-60 cell line

The effect of hexapeptide HLDF-6, the granulocytic differentiation inducer, on the tumor necrosis factor alpha (TNF-alpha)-induced differentiation and apoptosis of human promyelocytic leukemia HL-60 cells has been investigated. Costimulation of HL-60 cells with HLDF-6 and TNF-alpha enhanced granulocyte differentiation, whereas the level of monocyte differentiation remained unchanged; however, the cytotoxic action of TNF-alpha on these cells decreased. The protective effect of HLDF-6 peptide did not depend on activation of NF-kappaB (nuclear transcription factor). Since HLDF-6 peptide decreases the number of cells entering apoptosis caused by C(2)-ceramide, a mediator of TNF-induced apoptosis, and also reduces TNF-alpha-mediated activation of caspase-3, we have proposed the hypothesis that HLDF-6 increases resistance of HL-60 cells to the TNF-alpha cytotoxic effect due to inhibition of some stages of mitochondria-dependent apoptotic signaling.

Sphingosine, a product of ceramide hydrolysis, influences the formation of ceramide channels

Ceramides are known to have a regulatory function in apoptosis, including the release of cytochrome c and other proapoptotic factors from the mitochondrial intermembrane space. Ceramides can form large, stable channels in the outer mitochondrial membrane, leading to the proposal that ceramide channels are the pathway through which these proteins are released. Here, we report that sphingosine, a product of ceramide hydrolysis by ceramidase, is capable of destabilizing ceramide channels, leading to their disassembly. Sphingosine is directly responsible for the disassembly of ceramide channels in planar membrane experiments and markedly reduces the ability of ceramide to induce the release of intermembrane space proteins from mitochondria in vitro. Low concentrations of both L and D sphingosine potentiate the release of intermembrane space proteins by long-chain ceramide and channel formation in liposomes. These results provide evidence for a mechanism by which the disassembly of ceramide channels, as initiated by ceramidase, could be accelerated by the direct interaction of the hydrolysis product with the ceramide channels themselves. This mechanism therefore could form a positive feedback loop for rapid shut-down of ceramide channels. However, potentiation of ceramide channel formation is also possible and thus both effects could influence the propensity for mitochondria-mediated apoptosis.