Ganglioside GD3 enhances apoptosis by suppressing the nuclear factor-kappa B-dependent survival pathway

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.

Sphingomyelinases and Liver Diseases

Sphingolipids (SLs) are critical components of membrane bilayers that play a crucial role in their physico-chemical properties. Ceramide is the prototype and most studied SL due to its role as a second messenger in the regulation of multiple signaling pathways and cellular processes. Ceramide is a heterogeneous lipid entity determined by the length of the fatty acyl chain linked to its carbon backbone sphingosine, which can be generated either by de novo synthesis from serine and palmitoyl-CoA in the endoplasmic reticulum or via sphingomyelin (SM) hydrolysis by sphingomyelinases (SMases). Unlike de novo synthesis, SMase-induced SM hydrolysis represents a rapid and transient mechanism of ceramide generation in specific intracellular sites that accounts for the diverse biological effects of ceramide. Several SMases have been described at the molecular level, which exhibit different pH requirements for activity: neutral, acid or alkaline. Among the SMases, the neutral (NSMase) and acid (ASMase) are the best characterized for their contribution to signaling pathways and role in diverse pathologies, including liver diseases. As part of a Special Issue (Phospholipases: From Structure to Biological Function), the present invited review summarizes the physiological functions of NSMase and ASMase and their role in chronic and metabolic liver diseases, of which the most relevant is nonalcoholic steatohepatitis and its progression to hepatocellular carcinoma, due to the association with the obesity and type 2 diabetes epidemic. A better understanding of the regulation and role of SMases in liver pathology may offer the opportunity for novel treatments of liver diseases.

Abnormal Sphingolipid World in Inflammation Specific for Lysosomal Storage Diseases and Skin Disorders

Research in recent years has shown that sphingolipids are essential signalling molecules for the proper biological and structural functioning of cells. Long-term studies on the metabolism of sphingolipids have provided evidence for their role in the pathogenesis of a number of diseases. As many inflammatory diseases, such as lysosomal storage disorders and some dermatologic diseases, including psoriasis, atopic dermatitis and ichthyoses, are associated with the altered composition and metabolism of sphingolipids, more studies precisely determining the responsibilities of these compounds for disease states are required to develop novel pharmacological treatment opportunities. It is worth emphasizing that knowledge from the study of inflammatory metabolic diseases and especially the possibility of their treatment may lead to insight into related metabolic pathways, including those involved in the formation of the epidermal barrier and providing new approaches towards workable therapies.

Detection of Naturally Occurring Human Antibodies Against Gangliosides by ELISA

Gangliosides are sialic acid-containing glycolipids that have been considered attractive targets for cancer immunotherapy, based on the qualitative and quantitative changes they suffer during malignant transformation and due to their importance for tumor biology. Natural antibodies against gangliosides have been detected not only in cancer patients but also in healthy donors. The presence of these antibodies can be used as diagnostic or prognostic factor. However, these responses are difficult to detect because anti-ganglioside antibodies are usually of IgM isotype and low affinity. Enzyme Linked Immunosorbent Assay (ELISA) is an immunoassay based on the specific binding of antibodies to antigens bound to a solid phase. These antigens can be glycolipids like gangliosides. An enzyme linked to the last reactant allows the detection of specific binding through the development of color after the addition of a suitable substrate. ELISA combines the specificity of antibodies with the sensitivity of enzyme reactions. The ELISA method described herein can be used to detect antibody responses against gangliosides not only related to cancer but also to autoimmune diseases and infections, both in healthy donors, and patients, untreated or receiving specific immunotherapy.

Glycosphingolipids and cell death: one aim, many ways

Glycosphingolipids (GSLs) are a family of bioactive lipids that in addition to their role in the regulation of structural properties of membrane bilayers have emerged as crucial players in many biological processes and signal transduction pathways. Rather than being uniformly distributed within membrane bilayers, GSLs are localized in selective domains called lipid rafts where many signaling platforms operate. One of the most important functions of GSLs, particularly ceramide, is their ability to regulate cell death pathways and hence cell fate. This complex role is accomplished by the ability of GSLs to act in distinct subcellular strategic centers, such as mitochondria, endoplasmic reticulum (ER) or lysosomes to mediate apoptosis, ER stress, autophagy, lysosomal membrane permeabilization and necroptosis. Hence better understanding the role of GSLs in cell death may be of relevance for a number of pathological processes and diseases, including neurodegeneration, metabolic liver diseases and cancer.

Acid sphingomyelinase-ceramide system in steatohepatitis: a novel target regulating multiple pathways

Steatohepatitis (SH) is an intermediate stage of fatty liver disease and is one of the most common causes of chronic liver disease worldwide that may progress to cirrhosis and liver cancer. SH encompasses alcoholic and non-alcoholic steatohepatitis, the latter being of particular concern as it is associated with obesity and insulin resistance and has become a major cause of liver transplantation. The molecular mechanisms governing the transition from steatosis to SH are not fully understood. Here we discuss emerging data indicating that the acid sphingomyelinase (ASMase), a specific mechanism of ceramide generation, is required for the activation of key pathways that regulate steatosis, fibrosis and lipotoxicity, including endoplasmic reticulum stress, autophagy and lysosomal membrane permeabilization. Moreover, ASMase modulates alterations of the methionine cycle and phosphatidylcholine homeostasis, two crucial events involved in SH that regulate methylation reactions, antioxidant defence and membrane integrity. These new findings suggest that targeting ASMase in combination with restoring methionine metabolism and phosphatidylcholine levels may be of utility in the treatment of SH.

Anti-ganglioside anti-idiotypic vaccination: more than molecular mimicry

Surgery, chemotherapy, and radiation therapy are standard modalities for cancer treatment, but the effectiveness of these treatments has reached a plateau. Thus, other strategies are being explored to combine with the current treatment paradigms in order to reach better clinical results. One of these approaches is the active immunotherapy based on the induction of anti-tumor responses by anti-idiotypic vaccination. This approach arose from Jerne’s idiotypic network theory, which postulates that B lymphocytes forms a functional network, with a role in the establishment of the immune repertoires, in the regulation of natural antibody production and even in the establishment of natural tolerance. Due to the large potential diversity of the immunoglobulin variable regions, the idiotypes repertoire can mimic the universe of self and foreign epitopes, even those of non-protein nature, like gangliosides. Gangliosides are sialic acid-containing glycolipids that have been considered attractive targets for cancer immunotherapy, based on the qualitative and quantitative changes they suffer during malignant transformation and due to their importance for tumor biology. Although any idiotype could be able to mimic any antigen, only those related to antigens involved in functions relevant for organism homeostasis, and that in consequence has been fixed by evolution, would be able not only to mimic, but also to activate the idiotypic cascades related with the nominal antigen. The present review updates the results, failures and hopes, obtained with ganglioside mimicking anti-idiotypic antibodies and presents evidences of the existence of a natural response against gangliosides, suggesting that these glycolipids could be idiotypically relevant antigens.

Sphingolipids: regulators of crosstalk between apoptosis and autophagy

Apoptosis and autophagy are two evolutionarily conserved processes that maintain homeostasis during stress. Although the two pathways utilize fundamentally distinct machinery, apoptosis and autophagy are highly interconnected and share many key regulators. The crosstalk between apoptosis and autophagy is complex, as autophagy can function to promote cell survival or cell death under various cellular conditions. The molecular mechanisms of crosstalk are beginning to be elucidated and have critical implications for the treatment of various diseases, such as cancer. Sphingolipids are a class of bioactive lipids that mediate many key cellular processes, including apoptosis and autophagy. By targeting several of the shared regulators, sphingolipid metabolites differentially regulate the induction of apoptosis and autophagy. Importantly, individual sphingolipid species appear to "switch" autophagy toward cell survival (e.g., sphingosine-1-phosphate) or cell death (e.g., ceramide, gangliosides). This review assesses the current understanding of sphingolipid-induced apoptosis and autophagy to address how sphingolipids mediate the "switch" between the cell survival and cell death. As sphingolipid metabolism is frequently dysregulated in cancer, sphingolipid-modulating agents, or sphingomimetics, have emerged as a novel chemotherapeutic strategy. Ultimately, a greater understanding of sphingolipid-mediated crosstalk between apoptosis and autophagy may be critical for enhancing the chemotherapeutic efficacy of these agents.

Dietary ganglioside reduces proinflammatory signaling in the intestine

Gangliosides are integral to the structure and function of cell membranes. Ganglioside composition of the intestinal brush border and apical surface of the colon influences numerous cell processes including microbial attachment, cell division, differentiation, and signaling. Accelerated catabolism of ganglioside in intestinal disease results in increased proinflammatory signaling. Restoring proper structure and function to the diseased intestine can resolve inflammation, increase resistance to infection, and improve gut integrity to induce remission of conditions like necrotizing enterocolitis (NEC) and Crohn's disease (CD). Maintaining inactive state of disease may be achieved by reducing the rate that gangliosides are degraded or by increasing intake of dietary ganglioside. Collectively, the studies outlined in this paper indicate that the amount of gangliosides GM3 and GD3 in intestinal mucosa is decreased with inflammation, low level of GM3 is associated with higher production of proinflammatory signals, and ganglioside content of intestinal mucosa can be increased by dietary ganglioside.

Disialogangliosides and TNFα alter gene expression for cytokines and chemokines in primary brain cell cultures

Gangliosides have long been implicated in multiple pathologies affecting the central nervous system. Empirical studies have suggested the possibility that gangliosides, particularly GD3, work in tandem with pro-inflammatory cytokines, especially tumor necrosis factor alpha (TNFα), to initiate or facilitate cell death in the CNS. As a step toward unraveling the metabolic pathways activated in the pathogenesis of brain cell death, we have surveyed gene expression for a host of cytokines and chemokines in primary brain cell cultures exposed to GD3, GD1b, and TNFα for 24 h. An initial screen of 98 genes on a focused mini-array revealed the expression of at least 28 genes related to cell growth, death, or inflammation in our system of mixed cells cultured from neonatal rat brains. Clear evidence of a differential response to the gangliosides or TNFα was seen in 12 genes. Quantitative PCR was used to validate the response of six of these genes. We found that both GD3 and GD1b, but not TNFα, up-regulated expression of macrophage inflammatory protein 3 (MIP3A) and interleukin-1 receptor 1 (IL1R1), but down-regulated fibroblast growth factor 13 (FGF13). The expression of FGF receptor activating protein 1 (FRAG1) and interleukin-3 receptor alpha (IL3RA) was down-regulated by GD3. Exposure to TNFα resulted in a dramatic up-regulation of IL3RA and chemokine ligand 2 (CCL2), both of which have been implicated in multiple sclerosis. Our results provide strong evidence that the expression of these genes might be critical links in the metabolic cascades leading to cell degeneration and death in the brain.

Targeting the GD3 acetylation pathway selectively induces apoptosis in glioblastoma

The expression of ganglioside GD3, which plays crucial roles in normal brain development, decreases in adults but is upregulated in neoplastic cells, where it regulates tumor invasion and survival. Normally a buildup of GD3 induces apoptosis, but this does not occur in gliomas due to formation of 9-O-acetyl GD3 by the addition of an acetyl group to the terminal sialic acid of GD3; this renders GD3 unable to induce apoptosis. Using human biopsy-derived glioblastoma cell cultures, we have carried out a series of molecular manipulations targeting GD3 acetylation pathways. Using immunocytochemistry, flow cytometry, western blotting, and transwell assays, we have shown the existence of a critical ratio between GD3 and 9-O-acetyl GD3, which promotes tumor survival. Thus, we have demonstrated for the first time in primary glioblastoma that cleaving the acetyl group restores GD3, resulting in a reduction in tumor cell viability while normal astrocytes remain unaffected. Additionally, we have shown that glioblastoma viability is reduced due to the induction of mitochondrially mediated apoptosis and that this occurs after mitochondrial membrane depolarization. Three methods of cleaving the acetyl group using hemagglutinin esterase were investigated, and we have shown that the baculovirus vector transduces glioma cells as well as normal astroctyes with a relatively high efficacy. A recombinant baculovirus containing hemagglutinin esterase could be developed for the clinic as an adjuvant therapy for glioma.

Cannabinoid receptor agonist protects cultured dopaminergic neurons from the death by the proteasomal dysfunction

Cannabinoids have been proposed to possess neuroprotective properties; though their mechanism of action remains contentious, they are posited to prevent neurodegenerative disorders, including Parkinson's disease, the pathogenesis of which has not been established. Recent studies have demonstrated that induction of proteasomal dysfunction in animal models results in a phenotype similar to Parkinson's disease. Here, we investigated the neuroprotective function of a synthetic cannabinoid-receptor agonist (WIN55.212.2) in dopaminergic neuronal death induced by a proteasomal synthase inhibitor (PSI), additionally testing the hypothesis that WIN55.212.2 modulates cytoplasmic accumulation of parkin and α-synuclein, a key feature of proteasomal dysfunction in Parkinson's. WIN55.212.2 protects PC12 cells from PSI-induced cytotoxicity, concomitantly inhibiting PSI-induced polyADP ribose polymerase expression and activation of caspase-3. While PSI induces cytoplasmic accumulation of α-synuclein and parkin, WIN55.212.2 counters these effects. Interestingly, however, while PSI induces the activation and nuclear translocalization of nuclear factor κB, WIN55.212.2 potentiates this effect. These data are suggestive that WIN55.212.2 might confer a neuroprotective benefit in PSI-induced proteasomal dysfunction, and could further protect against neuronal degeneration stemming from cytoplasmic accumulation of α-synuclein and parkin. These results indicate that WIN55.212.2 may be a candidate for treatment of neurodegenerative diseases, including Parkinson's disease.

GD3 accumulation in cell surface lipid rafts prior to mitochondrial targeting contributes to amyloid-β-induced apoptosis

Neuronal apoptosis induced by amyloid β-peptide (Aβ) plays an important role in the pathophysiology of Alzheimer's disease (AD). However, the molecular mechanism underlying Aβ-induced apoptosis remains undetermined. The disialoganglioside GD3 involves ceramide-, Fas- and TNF-α-mediated apoptosis in lymphoid cells and hepatocytes. Although the implication of GD3 has been suggested, the precise role of GD3 in Aβ-induced apoptosis is still unclear. Here, we investigated the changes of GD3 metabolism and characterized the distribution and trafficking of GD3 during Aβ-induced apoptosis using human brain-derived TE671 cells. Extracellular Aβ-induced apoptosis in a mitochondrial-dependent manner. GD3 level was negligible in the basal condition. However, in response to extracellular Aβ, both the expression of GD3 synthase mRNA and the intracellular GD3 level were dramatically increased. Neosynthesized GD3 rapidly accumulated in cell surface lipid microdomains, and was then translocated to mitochondria to execute the apoptosis. Disruption of membrane lipid microdomains with methyl-β-cyclodextrin significantly prevented both GD3 accumulation in cell surface and Aβ-induced apoptosis. Our data suggest that rapidly accumulated GD3 in plasma membrane lipid microdomains prior to mitochondrial translocation is one of the key events in Aβ-induced apoptosis.

Redox control of liver function in health and disease

Reactive oxygen species (ROS), a heterogeneous population of biologically active intermediates, are generated as by-products of the aerobic metabolism and exhibit a dual role in biology. When produced in controlled conditions and in limited quantities, ROS may function as signaling intermediates, contributing to critical cellular functions such as proliferation, differentiation, and cell survival. However, ROS overgeneration and, particularly, the formation of specific reactive species, inflicts cell death and tissue damage by targeting vital cellular components such as DNA, lipids, and proteins, thus arising as key players in disease pathogenesis. Given the predominant role of hepatocytes in biotransformation and metabolism of xenobiotics, ROS production constitutes an important burden in liver physiology and pathophysiology and hence in the progression of liver diseases. Despite the recognized role of ROS in disease pathogenesis, the efficacy of antioxidants as therapeutics has been limited. A better understanding of the mechanisms, nature, and location of ROS generation, as well as the optimization of cellular defense strategies, may pave the way for a brighter future for antioxidants and ROS scavengers in the therapy of liver diseases.

Mitochondria, cholesterol and amyloid beta peptide: a dangerous trio in Alzheimer disease

The molecular mechanisms of Alzheimer's disease (AD) are not fully understood. Extensive evidence from experimental models has involved the overgeneration and accumulation of toxic amyloid beta peptides (Abeta) in the onset and progression of the disease. The amyloidogenic processing of amyloid precursor protein into pathogenic Abeta fragments is thought to occur in specific domains of the plasma membrane and favored by cholesterol enrichment. Intracellular Abeta accumulation is known to induce oxidative stress, predominantly via mitochondria targeting of toxic Abeta. Recent evidence using mouse models of cholesterol loading has demonstrated that the specific mitochondrial cholesterol pool sensitizes neurons to Abeta-induced oxidant cell death and caspase-independent apoptosis due to selective mitochondrial GSH (mGSH) depletion induced by cholesterol-mediated perturbation of mitochondrial membrane dynamics. mGSH replenishment by permeable precursors such as glutathione ethyl ester protected against Abeta-mediated neurotoxicity and inflammation. Thus, these novel data expand the pathogenic role of cholesterol in AD indicating that in addition to fostering Abeta generation, mitochondrial cholesterol determines Abeta neurotoxicity via mGSH regulation.

GD3 synthase overexpression sensitizes hepatocarcinoma cells to hypoxia and reduces tumor growth by suppressing the cSrc/NF-kappaB survival pathway

Background

Hypoxia-mediated HIF-1α stabilization and NF-κB activation play a key role in carcinogenesis by fostering cancer cell survival, angiogenesis and tumor invasion. Gangliosides are integral components of biological membranes with an increasingly recognized role as signaling intermediates. In particular, ganglioside GD3 has been characterized as a proapoptotic lipid effector by promoting cell death signaling and suppression of survival pathways. Thus, our aim was to analyze the role of GD3 in hypoxia susceptibility of hepatocarcinoma cells and in vivo tumor growth.

Methodology/Principal Findings

We generated and characterized a human hepatocarcinoma cell line stably expressing GD3 synthase (Hep3B-GD3), which catalyzes the synthesis of GD3 from GM3. Despite increased GD3 levels (2–3 fold), no significant changes in cell morphology or growth were observed in Hep3B-GD3 cells compared to wild type Hep3B cells under normoxia. However, exposure of Hep3B-GD3 cells to hypoxia (2% O₂) enhanced reactive oxygen species (ROS) generation, resulting in decreased cell survival, with similar findings observed in Hep3B cells exposed to increasing doses of exogenous GD3. In addition, hypoxia-induced c-Src phosphorylation at tyrosine residues, NF-κB activation and subsequent expression of Mn-SOD were observed in Hep3B cells but not in Hep3B-GD3 cells. Moreover, MnTBAP, an antioxidant with predominant SOD mimetic activity, reduced ROS generation, protecting Hep3B-GD3 cells from hypoxia-induced death. Finally, lower tumor growth, higher cell death and reduced Mn-SOD expression were observed in Hep3B-GD3 compared to Hep3B tumor xenografts.

Conclusion

These findings underscore a role for GD3 in hypoxia susceptibility by disabling the c-Src/NF-κB survival pathway resulting in lower Mn-SOD expression, which may be of relevance in hepatocellular carcinoma therapy.

GD3-7-aldehyde is an apoptosis inducer and interacts with adenine nucleotide translocase

We prepared GD3-7-aldehyde (GD3-7) and determined its apoptotic potential. GD3-7 proved to be more efficient to induce pro-apoptotic mitochondrial alterations than GD3 when tested on mouse liver mitochondria. GD3-7-induced mitochondrial swelling and depolarization was blocked by cyclosporin A (CsA) supporting a critical role of the permeability transition pore complex (PTPC) during GD3-7-mediated apoptosis. In contrast to GD3, GD3-7 was able to induce channel formation in proteoliposomes containing adenine nucleotide translocase (ANT). This suggests that ANT is the molecular target of GD3-7. Using a specific antiserum, GD3-7 was detected in the lipid extract of the myeloid tumor cell line HL-60 after apoptosis induction, but not in living cells. Therefore, GD3-7 might be a novel mediator of PTPC-dependent apoptosis in cancer cells.

Ceramide and mitochondria in ischemia/reperfusion

A hallmark of tissue injury in various models of ischemia/reperfusion (IR) is mitochondrial dysfunction and the release of mitochondrial proapoptotic proteins leading to cell death. Although IR-induced mitochondrial injury has been extensively studied and key mitochondrial functions affected by IR are chiefly characterized, the nature of the molecule that causes loss of mitochondrial integrity and function remains obscure. It has become increasingly clear that ceramide, a membrane sphingolipid and a key mediator of cell stress responses, could play a critical role in IR-induced mitochondrial damage. Emerging data point to excessive ceramide accumulation in tissue and, specifically, in mitochondria after IR. Exogenously added to isolated mitochondria, ceramide could mimic some of the mitochondrial dysfunctions occurring in IR. The recent identification and characterization of major enzymes in ceramide synthesis is expected to contribute to the understanding of molecular mechanisms of ceramide involvement in mitochondrial damage in IR. This review will examine the experimental evidence supporting the important role of ceramide in mitochondrial dysfunction in IR to highlight potential targets for pharmacological manipulation of ceramide levels.

GD3, an overexpressed tumor-derived ganglioside, mediates the apoptosis of activated but not resting T cells

We previously elucidated an important role for gangliosides in renal cell carcinoma-mediated T lymphocyte apoptosis, although the mechanism by which they mediated lymphocyte death remained unclear. Here, we show that when added in purified form, GD3 is internalized by activated T cells, initiating a series of proapoptotic events, including the induction of reactive oxygen species (ROS), an enhancement of p53 and Bax accumulation, an increase in mitochondrial permeability, cytochrome c release, and the activation of caspase-9. GD3-induced apoptosis of activated T cells was dose dependent and inhibitable by pretreating the lymphocytes with N-acetylcysteine, cyclosporin A, or bongkrekic acid, emphasizing the essential role of ROS and mitochondrial permeability to the process. Ganglioside-induced T-cell killing was associated with the caspase-dependent degradation of nuclear factor-kappaB-inducible, antiapoptotic proteins, including RelA; this suggests that their loss is initiated only after the cascade is activated and that their disappearance amplifies but not triggers GD3 susceptibility. Resting T cells did not internalize appreciable levels of GD3 and did not undergo any of the proapoptotic changes that characterize activated T lymphocytes exposed to the ganglioside. RelA overexpression endows Jurkat cells with resistance to GD3-mediated apoptosis, verifying the role of the intact transcription factor in mediating protection from the ganglioside.

Sphingolipids and cell death

Sphingolipids (SLs) have been considered for many years as predominant building blocks of biological membranes with key structural functions and little relevance in cellular signaling. However, this view has changed dramatically in recent years with the recognition that certain SLs such as ceramide, sphingosine 1-phosphate and gangliosides, participate actively in signal transduction pathways, regulating many different cell functions such as proliferation, differentiation, adhesion and cell death. In particular, ceramide has attracted considerable attention in cell biology and biophysics due to its key role in the modulation of membrane physical properties, signaling and cell death regulation. This latter function is largely exerted by the ability of ceramide to activate the major pathways governing cell death such as the endoplasmic reticulum and mitochondria. Overall, the evidence so far indicates a key function of SLs in disease pathogenesis and hence their regulation may be of potential therapeutic relevance in different pathologies including liver diseases, neurodegeneration and cancer biology and therapy.

Do mitochondria act as "cargo boats" in the journey of GD3 to the nucleus during apoptosis?

Plasma membrane lipid rafts have been considered as a sort of "chamber", where several subcellular activities, including CD95/Fas-mediated pro-apoptotic signaling, can take place. Recently, we demonstrated that, after CD95/Fas triggering, raft-like microdomains could be detected in mitochondrial membranes. The mitochondrion appears as a dynamic and subcompartmentalized organelle in which microdomains might act as controllers of apoptosis-associated fission that results in the release of apoptogenic factors. Here, we hypothesize that some "small" mitochondria, possibly derived from their fission process, can reach the nuclear envelope and strictly interact with this. Mitochondria could act as a signaling "device" contributing to molecular trafficking of molecules, including raft-like components, during apoptosis.

Sphingolipid signalling and liver diseases

Sphingolipids (SLs) comprise a class of lipids with important structural functions and increasing relevance in cellular signalling. In particular, ceramide has attracted considerable attention owing to its role as a second messenger modulating several cell functions such as proliferation, gene expression, differentiation, cell cycle arrest and cell death. Increasing evidence documents the role of SLs in stress and death ligand-induced hepatocellular death, which contributes to the progression of several liver diseases including steatohepatitis, ischaemia-reperfusion liver injury or hepatocarcinogenesis. Furthermore, recent data indicate that the accumulation of SLs in specific cell subcompartments, characteristic of many sphingolipidoses, contributes to the hepatic dysfunctions that accompany these inherited diseases. Hence, the regulation of the cell biology and metabolism of SLs may open up a novel therapeutic avenue in the treatment of liver diseases.

Gangliosides as immunomodulators

Gangliosides are glycosphingolipids expressed at the outer leaflet of the plasmatic membrane of cells from vertebrate organisms. These molecules exert diverse biological functions including modulation of the immune system responses. Aberrant expression of gangliosides has been demonstrated on malignant cells. Besides expression on tumor cell membranes, gangliosides are also shed in the tumor microenvironment and eventually circulate in patients blood. Gangliosides derived from tumors posses the capability to affect the immune system responses by altering the function of lymphocytes and antigen-presenting cells and promoting tumor growth. These molecules can be considered as tumor weapons directed to attack and destroy immunosurveillance mechanisms devoted to control cancer progression.

Estrogen-induced cholestasis results in a dramatic increase of b-series gangliosides in the rat liver

Hepatic ganglioside composition was investigated in normal and cholestatic Wistar rats. Cholestasis was induced by 17alpha-ethinylestradiol (EE; 5 mg/kg body weight s.c. for 18 days). As compared with controls, the EE administration resulted in severe cholestasis, as indicated by biochemical as well as morphological signs. Gangliosides isolated from the liver tissue were separated by TLC, with resorcinol-HCl detection and densitometric evaluation. As compared with controls, the total hepatic lipid sialic acid content in cholestatic rats was increased almost 2-fold (44.3 +/- 15.2 vs 79.1 +/- 9.0 nmol/g wet weight of liver tissue, p < 0.01). This increase was primarily due to the increase of ganglioside GD1a (3.6 +/- 1.0 vs 11.8 +/- 3.0 nmol/g wet weight of liver tissue, p = 0.001), as well as to the enormous up-regulation of b-series gangliosides GD3 (0.08 +/- 0.03 vs 2.0 +/- 1.2 nmol/g wet weight of liver tissue, p = 0.002), GD1b (0.1 +/- 0.06 vs 5.4 +/- 1.6 nmol/g wet weight of liver tissue, p = 0.002) and GT1b (0.06 +/- 0.03 vs 6.4 +/- 2.6 nmol/g wet weight of liver tissue, p = 0.002). As the majority of gangliosides are concentrated in cell membranes, our findings suggest that dramatic increase of b-series gangliosides might contribute to the protection of hepatocytes against the deleterious effects of cholestasis.

Training Depletes Muscle Glutathione in Patients with Chronic Obstructive Pulmonary Disease and Low Body Mass Index

BACKGROUND

A physiological increase in muscle glutathione after training is not seen in patients with chronic obstructive pulmonary disease (COPD), indicating abnormal peripheral muscle adaptations to exercise.

OBJECTIVE

We hypothesized that oxidative stress is primarily associated with low body mass index (BMI).

METHODS

Eleven patients with preserved BMI (BMI(N): 28.2 +/- 1.2 kg.m(-2)), 9 patients with low BMI (BMI(L): 19.7 +/- 0.60 kg.m(-2)) and 5 age-matched controls (26.5 +/- 0.9 kg.m(-2)) were studied before and after 8 weeks of high-intensity endurance training. Reduced glutathione (GSH) and gamma-glutamyl cysteine synthase heavy-subunit chain mRNA expression (gammaGCS-HS mRNA) were measured in the vastus lateralis.

RESULTS

After training, exercise capacity increased (DeltaVO(2)PEAK, 13 +/- 5.2%; 10 +/- 5.6% and 15 +/- 4.3% in BMI(L), BMI(N) and controls, respectively; p < 0.05 each). GSH levels decreased in BMI(L) (from 5.2 +/- 0.7 to 3.7 +/- 0.8 nmol/mg protein, DeltaGSH -1.5 +/- 0.7 nmol/mg protein, p < 0.05); no changes were seen in BMI(N) (from 5.4 +/- 0.7 to 6.7 +/- 0.9 nmol/mg protein, DeltaGSH 1.3 +/- 0.9 nmol/mg protein), whereas GSH markedly increased in controls (from 4.6 +/- 1 to 8.7 +/- 0.4 nmol/mg protein, DeltaGSH 4.1 +/- 1 nmol/mg protein, p < 0.01). DeltaGSH in BMI(L) was different from DeltaGSH in BMI(N) and controls (p < 0.05, each). Consistent changes were observed in gammaGCS-HS mRNA expression.

CONCLUSIONS

GSH depletion after training in BMI(L) may suggest that oxidative stress plays a key role in muscle wasting in COPD patients.

Mitochondria: more than just a powerhouse

Pioneering biochemical studies have long forged the concept that the mitochondria are the 'energy powerhouse of the cell'. These studies, combined with the unique evolutionary origin of the mitochondria, led the way to decades of research focusing on the organelle as an essential, yet independent, functional component of the cell. Recently, however, our conceptual view of this isolated organelle has been profoundly altered with the discovery that mitochondria function within an integrated reticulum that is continually remodeled by both fusion and fission events. The identification of a number of proteins that regulate these activities is beginning to provide mechanistic details of mitochondrial membrane remodeling. However, the broader question remains regarding the underlying purpose of mitochondrial dynamics and the translation of these morphological transitions into altered functional output. One hypothesis has been that mitochondrial respiration and metabolism may be spatially and temporally regulated by the architecture and positioning of the organelle. Recent evidence supports and expands this idea by demonstrating that mitochondria are an integral part of multiple cell signaling cascades. Interestingly, proteins such as GTPases, kinases and phosphatases are involved in bi-directional communication between the mitochondrial reticulum and the rest of the cell. These proteins link mitochondrial function and dynamics to the regulation of metabolism, cell-cycle control, development, antiviral responses and cell death. In this review we will highlight the emerging evidence that provides molecular definition to mitochondria as a central platform in the execution of diverse cellular events.

Pharmacological inhibition or small interfering RNA targeting acid ceramidase sensitizes hepatoma cells to chemotherapy and reduces tumor growth in vivo

Ceramidases (CDases) play a key role in cancer therapy through enhanced conversion of ceramide into sphingosine 1-phosphate (S1P), but their involvement in hepatocarcinogenesis is unknown. Here, we report that daunorubicin (DNR) activated acid CDase post-transcriptionally in established human (HepG2 cells) or mouse (Hepa1c1c7) hepatoma cell lines as well as in primary cells from murine liver tumors, but not in cultured mouse hepatocytes. Acid CDase silencing by small interfering RNA (siRNA) or pharmacological inhibition with N-oleoylethanolamine (NOE) enhanced the ceramide to S1P balance compared to DNR alone, sensitizing hepatoma cells (HepG2, Hep-3B, SK-Hep and Hepa1c1c7) to DNR-induced cell death. DNR plus NOE or acid CDase siRNA-induced cell death was preceded by ultrastructural changes in mitochondria, stimulation of reactive oxygen species generation, release of Smac/DIABLO and cytochrome c and caspase-3 activation. In addition, in vivo siRNA treatment targeting acid CDase reduced tumor growth in liver tumor xenografts of HepG2 cells and enhanced DNR therapy. Thus, acid CDase promotes hepatocarcinogenesis and its antagonism may be a promising strategy in the treatment of liver cancer.

9-O-acetyl GD3 protects tumor cells from apoptosis

The ganglioside GD3 (Neu5Ac alpha8Neu5Ac alpha3Gal beta4GlcCer) is an intracellular lipid messenger that induces apoptosis by targeting mitochondria in various cell types. GD3 can also promote apoptosis when externally added to cells. Previous studies showed that the proapoptotic effects of GD3 can be counteracted by 9-O-acetylation. To determine whether 9-O-acetyl GD3 (acGD3) has a general antiapoptotic potential, the apoptosis-sensitive Jurkat cell line and an apoptosis-sensitive variant of the cell line Molt-4 were preincubated with micromolar concentrations of acGD3 and then treated with inducers of apoptosis. A reduced apoptotic index and an increased cell viability were observed. On the other hand, when the Jurkat cells were treated with GD3 for extended periods of time, a population was selected that was resistant to apoptosis induction by N-acetyl sphingosine as well as by the anti-leukemic drug daunorubicin. Comparative analysis of gangliosides revealed the formation of acGD3 in the resistant Jurkat cells that was not found in the apoptosis-sensitive cells. Conversely, exposing the acGD3 positive and apoptosis-resistant cell line Molt-4 to the O-deacetylating activity of salicylate resulted in a complete disappearance of acGD3 and an enhanced sensitivity to N-acetyl sphingosine-mediated apoptosis. Formation of acGD3 might thus represent a new mechanism how tumor cells can escape apoptosis.

The Ganglioside GD3 as the Greek Goddess Hecate: Several Faces Turned Towards as Many Directions

The disialoganglioside GD3 can mediate biological functions as diverse as proliferation, differentiation, and apoptosis. Since intracellular level of GD3 is crucial for the cell, understanding the mechanisms by which GD3 metabolism is tightly regulated seems of particular importance. GD3 can be enlisted among the most potent natural inducers of mitochondrial damage and apoptosis. However, some cell types resist GD3-mediated mitochondrial damage through complex mechanisms which are beginning to be unveiled.

Milk-Derived GM3 and GD3 Differentially Inhibit Dendritic Cell Maturation and Effector Functionalities

Gangliosides are complex glycosphingolipids, which exert immune-modulating effects on various cell types. Ganglioside GD(3) and GM(3) are the predominant gangliosides of human breast milk but during the early phase of lactation, the content of GD(3) decreases while GM(3) increases. The biological value of gangliosides in breast milk has yet to be elucidated but when milk is ingested, dietary gangliosides might conceptually affect immune cells, such as dendritic cells (DCs). In this study, we address the in vitro effect of GD(3) and GM(3) on DC effector functionalities. Treatment of bone marrow-derived DCs with GD(3) before lipopolysaccharide-induced maturation decreased the production of interleukin-6 (IL-6), IL-10, IL-12 and tumor necrosis factor-alpha as well as reduced the alloreactivity in mixed leucocyte reaction (MLR). In contrast, only IL-10 and IL-12 productions were significantly inhibited by GM(3,) and the potency of DCs to activate CD4(+) cells in MLR was unaffected by GM(3). However, both gangliosides suppressed expression of CD40, CD80, CD86 and major histocompatibility complex class II on DCs. Because GD(3) overall inhibits DC functionalities more than GM(3), the immune modulating effect of the ganglioside fraction of breast milk might be more prominent in the commencement of lactation during which the milk contains the most GD(3).

Glycosphingolipids and mitochondria: role in apoptosis and disease

Glycosphingolipids (GSLs) comprise a class of lipids with important structural and signaling functions. Synthesized from ceramide in the Golgi, they are subsequently distributed to different compartments, most predominantly in the plasma membrane where they integrate signaling platforms. A recently characterized trafficking of ganglioside GD3 (GD3), a GSLs with two sialic-acid residues, to mitochondria has revealed a novel function of this lipid as a death effector. In addition to the interaction of GD3 with mitochondria recruiting these organelles to apoptotic pathways, GD3 disables survival paths dependent on NF-kappaB, thus favoring the balance towards cell death. The present review gathers the evidence documenting this emerging function of GSLs in cell death and their involvement in pathological states.

Activation of Fas receptor is required for the increased formation of the disialoganglioside GD3 in cultured cerebellar granule cells committed to apoptotic death

Apoptosis was induced in cultured cerebellar granule cells by lowering extracellular K+ concentrations (usually from 25 to 10 mM). The apoptotic phenotype was preceded by an early and transient increase in the intracellular levels of the disialoganglioside, GD3, which behaves as a putative pro-apoptotic factor. We examined whether activation of Fas receptor mediates the increase in GD3 formation in granule cells committed to die. Degenerating granule cells showed increased expression of both Fas receptor and its ligand (Fas-L), at times that coincided with the increase in GD3 levels and the induction of GD3 synthase mRNA. Addition of neutralizing anti-Fas-L antibodies reduced the extent of 'low-K+'-induced apoptosis and abolished the increase in GD3 levels and GD3 synthase mRNA. Similar reductions were observed in cultures prepared from gld or lpr mice, which harbor loss-of-function mutations of Fas-L and Fas receptor, respectively. In addition, exogenous application of soluble Fas-L further enhanced both the increase in GD3 formation and cell death in cultured granule cells switched from 25 into 10 mM K+. We conclude that activation of Fas receptor is entirely responsible for the increase in GD3 levels and contributes to the development of apoptosis by trophic deprivation in cultured cerebellar granule cells.

Overexpression of GD3 synthase induces apoptosis of vascular endothelial ECV304 cells through downregulation of Bcl-2

The disialoganglioside GD3 plays a major role in proliferation, differentiation, and apoptosis. It has been reported that ganglioside GD3 can induce apoptosis through bcl-2 mediated mitochondrial pathway. However, the relationship between ganglioside GD3 and B-cell/CLL lymphoma 2 (Bcl-2) is not fully understood. In this study, we have demonstrated that the downregulation of Bcl-2 by overexpression of CMP-NeuAc:GM3 alpha-2,8-sialyltransferase (GD3 synthase) results in an accelerated apoptosis in vascular endothelial cells (ECV304), as evidenced by DNA fragmentation and caspase-3 activation. In addition, phosphorylation of AKT and cyclic-AMP responsive element binding protein (CREB) was reduced by GD3 synthase overexpression. Moreover, the activation of CREB as a transcriptional factor was also inhibited, as evidenced by electrophoretic mobility shift assay. Therefore, we conclude that GD3 synthase has an apoptotic effect on ECV304 cells through downregulation of Bcl-2 expression via dephosphorylation of AKT and CREB.

Degradation of NF-kappa B in T cells by gangliosides expressed on renal cell carcinomas

T cells from cancer patients are often functionally impaired, which imposes a barrier to effective immunotherapy. Most pronounced are the alterations characterizing tumor-infiltrating T cells, which in renal cell carcinomas includes defective NF-kappaB activation and a heightened sensitivity to apoptosis. Coculture experiments revealed that renal tumor cell lines induced a time-dependent decrease in RelA(p65) and p50 protein levels within both Jurkat T cells and peripheral blood T lymphocytes that coincided with the onset of apoptosis. The degradation of RelA/p50 is critical for SK-RC-45-induced apoptosis because overexpression of RelA in Jurkat cells protects against cell death. The loss of RelA/p50 coincided with a decrease in expression of the NF-kappaB regulated antiapoptotic protein Bcl-xL at both the protein and mRNA level. The disappearance of RelA/p50 protein was mediated by a caspase-dependent pathway because pretreatment of T lymphocytes with a pan caspase inhibitor before coculture with SK-RC-45 blocked RelA and p50 degradation. SK-RC-45 gangliosides appear to mediate this degradative pathway, as blocking ganglioside synthesis in SK-RC-45 cells with the glucosylceramide synthase inhibitor, PPPP, protected T cells from tumor cell-induced RelA degradation and apoptosis. The ability of the Bcl-2 transgene to protect Jurkat cells from RelA degradation, caspase activation, and apoptosis implicates the mitochondria in these SK-RC-45 ganglioside-mediated effects.

Mitochondrial permeability transition induced by reactive oxygen species is independent of cholesterol-regulated membrane fluidity

Cholesterol enrichment of rat liver mitochondria (CHM) impairs atractyloside-induced mitochondrial permeability transition (MPT) due to decreased membrane fluidity. In this study we addressed the effect of cholesterol enrichment on MPT induced by reactive oxygen species (ROS). Superoxide anion generated by xanthine plus xanthine oxidase triggered mitochondrial swelling and cytochrome c release in CHM, which was prevented by butylated hydroxytoluene, an anti-voltage-dependent anion channel antibody, or cyclosporin A. Furthermore, hydrogen peroxide generated by the combination of ganglioside GD3 and mitochondrial GSH depletion elicited mitochondrial swelling and release of cytochrome c, Smac/Diablo and apoptosis-inducing factor in control mitochondria and CHM. Thus, ROS induce MPT and apoptosome activation regardless of decreased mitochondrial membrane dynamics due to cholesterol enrichment.

Expansion of vitiligo lesions is associated with reduced epidermal CDw60 expression and increased expression of HLA-DR in perilesional skin

BACKGROUND

Detection of CDw60 in skin is representative of ganglioside D3 expression. This ganglioside is expressed primarily by melanocytes, and is of interest as a membrane antigen targeted by immunotherapy for melanoma patients. Expression of CDw60 by keratinocytes is defined by the presence of T-helper cell (Th)1 vs. Th2 cytokines, and can serve as a sentinel molecule to characterize an ongoing skin immune response.

OBJECTIVES

These immunobiological characteristics have provided the incentive to study the expression of CDw60 in the context of progressive vitiligo.

METHODS

Frozen sections were obtained from control skin and from vitiligo lesions and immunostained to show CDw60. Cells were cultured, their CDw60 expression studied and ribonuclease protection assays run to detect cytokine mRNA.

RESULTS

Resistance to cytokine-mediated regulation of CDw60 expression was demonstrated in vitro by melanocytes, which appeared capable of generating autocrine and paracrine regulatory molecules supporting CDw60 expression. Induction of CDw60 expression was inhibited by antibodies to interleukin (IL)-4, suggesting that this cytokine was responsible, at least in part, for melanocyte-induced CDw60 expression. Marginal skin from patients with progressive generalized vitiligo consistently showed a reduction in epidermal CDw60 expression alongside elevated human leucocyte associated antigen (HLA)-DR expression at the margin. It thus appears that inflammatory infiltrates present in marginal skin generate type 1 rather than type 2 cytokines, supportive of a cell-mediated autoimmune response.

CONCLUSIONS

These results support an active role of melanocytes within the skin immune system, and associate their loss in generalized vitiligo with a cell-mediated immune response mediated by type 1 cytokines.

Redox regulation and signaling lipids in mitochondrial apoptosis

Apoptosis can be regulated at multiple levels. A number of proteins with regulatory function in cell death are sensitive to cellular redox environment. The antioxidant glutathione (GSH) and redox-sensitive proteins, thioredoxin and glutathione S-transferase, thus regulate cell death pathways by modulating the redox state of specific thiol residues of target proteins including stress kinases, transcription factors, and caspases. GSH in mitochondria plays an important role in the integrity of mitochondrial proteins and lipids known to play a vital role in the permeabilization of mitochondrial membranes and release of proapoptotic factors. The regulation of mitochondrial GSH (mGSH) is determined by its uptake from the cytosol which is dependent on appropriate membrane dynamics. The deposition of cholesterol in mitochondria induced by alcohol intake impairs this translocation, resulting in severe depletion of mGSH and in sensitization to apoptosis stimuli. Although the interaction of proapoptotic proteins with mitochondria initiates apoptotic pathways, recent data indicate that the mitochondrial trafficking of glycosphingolipids, e.g., ganglioside GD3, induced by apoptotic stimuli is a key event that sets off mitochondrial-dependent apoptotic cascades.

Defective TNF-alpha-mediated hepatocellular apoptosis and liver damage in acidic sphingomyelinase knockout mice

This study addressed the contribution of acidic sphingomyelinase (ASMase) in TNF-alpha-mediated hepatocellular apoptosis. Cultured hepatocytes depleted of mitochondrial glutathione (mGSH) became sensitive to TNF-alpha, undergoing a time-dependent apoptotic cell death preceded by mitochondrial membrane depolarization, cytochrome c release, and caspase activation. Cyclosporin A treatment rescued mGSH-depleted hepatocytes from TNF-alpha-induced cell death. In contrast, mGSH-depleted hepatocytes deficient in ASMase were resistant to TNF-alpha-mediated cell death but sensitive to exogenous ASMase. Furthermore, although in vivo administration of TNF-alpha or LPS to galactosamine-pretreated ASMase(+/+) mice caused liver damage, ASMase(-/-) mice exhibited minimal hepatocellular injury. To analyze the requirement of ASMase, we assessed the effect of glucosylceramide synthetase inhibition on TNF-alpha-mediated apoptosis. This approach, which blunted glycosphingolipid generation by TNF-alpha, protected mGSH-depleted ASMase(+/+) hepatocytes from TNF-alpha despite enhancement of TNF-alpha-stimulated ceramide formation. To further test the involvement of glycosphingolipids, we focused on ganglioside GD3 (GD3) because of its emerging role in apoptosis through interaction with mitochondria. Analysis of the cellular redistribution of GD3 by laser scanning confocal microscopy revealed the targeting of GD3 to mitochondria in ASMase(+/+) but not in ASMase(-/-) hepatocytes. However, treatment of ASMase(-/-) hepatocytes with exogenous ASMase induced the colocalization of GD3 and mitochondria. Thus, ASMase contributes to TNF-alpha-induced hepatocellular apoptosis by promoting the mitochondrial targeting of glycosphingolipids.

GD3 ganglioside and apoptosis

Lipid and glycolipid mediators are important messengers of the adaptive responses to stress, including apoptosis. In mammalian cells, the intracellular accumulation of ganglioside GD3, an acidic glycosphingolipid, contributes to mitochondrial damage, a crucial event during the apoptopic program. GD3 is a minor ganglioside in most normal tissues. Its expression increases during development and in pathological conditions such as cancer and neurodegenerative disorders. Intriguingly, GD3 can mediate additional biological events such as cell proliferation and differentiation. These diverse and opposing effects indicate that tightly regulated mechanisms, including 9-O-acetylation, control GD3 function, by affecting intracellular levels, localization and structure of GD3, and eventually dictate biological outcomes and cell fate decisions.

Emerging concepts in inflammation and fibrosis

Hepatic and pancreatic response to several insults commonly includes similar pathways of inflammation, fibrogenesis/regeneration, which may occur simultaneously and without appropriate coordination, resulting in chronic inflammation, scarring, and organ dysfunction. This review highlights the opinion of experts gathered for the Mexican Digestive Disease Week (2001) to analyze these molecular events with emphasis on identifying possible therapeutic opportunities. Inflammatory response encompasses leukocyte infiltration, favored by adhesion molecules of the selectin family, chemokines, integrins, and activated stellate cells (SC). Quiescent SC undergo activation mediated by mechanical stress and expression of cytokines, oxidative stress products, and growth factors and play a significant role in fibrosis and in reparation toward synthesis of extracellular matrix components. Also, hepatocytes and acinar cells contribute to the inflammatory and fibrotic response. Molecules that down-regulate this response are overexpressed. Therapeutic strategies with targeting to such mechanisms underlying chronic hepatic and pancreatic injury are an emerging reality.

Disialoganglioside GD3 is released by microglia and induces oligodendrocyte apoptosis

Increased brain ganglioside levels are a hallmark of various neuroinflammatory pathologies. Here, we provide evidence that murine microglia can secrete disialoganglioside GD3 upon exposure to inflammatory stimuli. Comparison of different neural cell types revealed a particular and specific sensitivity of oligodendrocytes towards exogenous GD3. Oligodendrocyte death triggered by GD3 was preceded by degeneration of cellular processes, and associated with typical features of apoptosis, such as chromatin condensation, exposure of phosphatidylserine, release of cytochrome c from mitochondria, and loss of mitochondrial membrane potential, followed by the loss of plasma membrane integrity and detachment of disintegrated oligodendrocytes. Overexpression of bcl-2 partially protected oligodendrocytes from death. In contrast, treatment with the pan-caspase inhibitor zVAD-fmk did not prevent phosphatidylserine exposure, chromatin margination at the nuclear periphery, and death, although caspase-3 was blocked. Thus, GD3 produced by microglia under neuroinflammatory conditions may function as a novel mediator triggering mitochondria-mediated, but caspase-independent, apoptosis-like death of oligodendrocytes.

An early increase in the disialoganglioside GD3 contributes to the development of neuronal apoptosis in culture

We induced apoptosis in primary cultures of cerebellar granule neurons by switching the growing medium into a medium containing lower concentrations of K(+) (5 or 10 mM instead of 25 mM) or, alternatively, by addition of staurosporine. The apoptotic phenotype was always preceded by an early increase in the intracellular levels of the disialoganglioside GD3, which peaked at 2-6 h and returned back to normal at 12 h. GD3 synthase, the enzyme that forms GD3 from the monosialoganglioside GM3, was also induced at early times after the induction of apoptosis in granule cells. Immunofluorescent staining showed that GD3 increased in neuronal cell bodies and neurites, but was never localized in cell nuclei. In cultures switched into a low K(+)-containing medium, exogenously applied GD3, but not the disialoganglioside GD1a, accelerated the development of neuronal apoptosis. In contrast, the antisense-induced knock-down of GD3 synthase was protective against granule cell death induced by lowering extracellular K(+) from 25 to 10 - but not 5 - mM. These results demonstrate that an early and transient increase in GD3 synthesis is one of the factors that contribute to the induction of neuronal apoptosis in culture.

Beta-amyloid-induced synthesis of the ganglioside GD3 is a requisite for cell cycle reactivation and apoptosis in neurons

We have shown that cortical neurons challenged with toxic concentrations of beta-amyloid peptide (betaAP) enter the S phase of the cell cycle before apoptotic death. Searching for a signaling molecule that lies at the border between cell proliferation and apoptotic death, we focused on the disialoganglioside GD3. Exposure of rat cultured cortical neurons to 25 microm betaAP(25-35) induced a substantial increase in the intracellular levels of GD3 after 4 hr, a time that precedes neuronal entry into S phase. GD3 levels decreased but still remained higher than in the control cultures after 16 hr of exposure to betaAP(25-35). Confocal microscopy analysis showed that the GD3 synthesized in response to betaAP colocalized with nuclear chromatin. The increase in GD3 was associated with a reduction of sphingomyelin (the main source of the ganglioside precursor ceramide) and with the induction of alpha-2,8-sialyltransferase (GD3 synthase), the enzyme that forms GD3 from the monosialoganglioside GM3. A causal relationship between GD3, cell-cycle activation, and apoptosis was demonstrated by treating the cultures with antisense oligonucleotides directed against GD3 synthase. This treatment, which reduced betaAP(25-35)-stimulated GD3 formation by approximately 50%, abolished the neuronal entry into the S phase and was protective against betaAP(25-35)-induced apoptosis.