Ceramide signaling in fenretinide-induced endothelial cell apoptosis

A comprehensive measure of Golgi sphingolipid flux using NBD C6-Ceramide: evaluation of sphingolipid inhibitors

Measurements of sphingolipid metabolism are most accurately performed by liquid chromatography-mass spectrometry. However, this technique is expensive, not widely accessible, and without the use of specific probes, it does not provide insight into metabolic flux through the pathway. Employing the fluorescent ceramide analogue NBD-C6-ceramide as a tracer in intact cells, we developed a comprehensive HPLC-based method that simultaneously measures the main nodes of ceramide metabolism in the Golgi. Hence, by quantifying the conversion of NBD-C6-Ceramide to NBD-C6-sphingomyelin, NBD-C6-Hexosylceramides, and NBD-C6-ceramide-1-phosphate (NBD-C1P), the activities of Golgi resident enzymes sphingomyelin synthase 1, glucosylceramide synthase, and ceramide kinase (CERK) could be measured simultaneously. Importantly, the detection of NBD-C1P allowed us to quantify CERK activity in cells, a usually difficult task. By applying this method, we evaluated the specificity of commonly used sphingolipid inhibitors and discovered that PDMP, which targets glucosylceramide synthase, and fenretinide (4HPR), an inhibitor for dihydroceramide desaturase, also suppress CERK activity. This study demonstrates the benefit of an expanded analysis of ceramide metabolism in the Golgi, and it provides a qualitative and easy-to-implement method.

Lipid-Restricted Culture Media Reveal Unexpected Cancer Cell Sensitivities

Cancer cell culture models frequently rely on fetal bovine serum as a source of protein and lipid factors that support cell survival and proliferation; however, serum-containing media imperfectly mimic the in vivo cancer environment. Recent studies suggest that typical serum-containing cell culture conditions can mask cancer dependencies, for example, on cholesterol biosynthesis enzymes, that exist in vivo and emerge when cells are cultured in media that provide more realistic levels of lipids. Here, we describe a high-throughput screen that identified fenretinide and ivermectin as small molecules whose cytotoxicity is greatly enhanced in lipid-restricted media formulations. The mechanism of action studies indicates that ivermectin-induced cell death involves oxidative stress, while fenretinide likely targets delta 4-desaturase, sphingolipid 1, a lipid desaturase necessary for ceramide synthesis, to induce cell death. Notably, both fenretinide and ivermectin have previously demonstrated in vivo anticancer efficacy despite their low cytotoxicity under typical cell culture conditions. These studies suggest ceramide synthesis as a targetable vulnerability of cancer cells cultured under lipid-restricted conditions and reveal a general screening strategy for identifying additional cancer dependencies masked by the superabundance of medium lipids.

Focus on ferroptosis, pyroptosis, apoptosis and autophagy of vascular endothelial cells to the strategic targets for the treatment of atherosclerosis

Vascular endothelial cells (VECs), which are lined up in the inner surface of blood vessels, are in direct contact with the metabolite-related endogenous danger signals in the circulatory system. Moreover, VECs death impairs vasodilation and increases endothelium-dependent permeability, which is strongly correlated with the development of atherosclerosis (AS). Among several forms of cell death, regulatory death of endothelial cells frequently occurs in AS, mainly including ferroptosis, pyroptosis, apoptosis and autophagy. In this review, we summarize regulatory factors and signaling mechanisms of regulatory death in endothelial cells, discussing their effects in the context of the atherosclerotic procession.

Ceramide and Related Molecules in Viral Infections

Ceramide is a lipid messenger at the heart of sphingolipid metabolism. In concert with its metabolizing enzymes, particularly sphingomyelinases, it has key roles in regulating the physical properties of biological membranes, including the formation of membrane microdomains. Thus, ceramide and its related molecules have been attributed significant roles in nearly all steps of the viral life cycle: they may serve directly as receptors or co-receptors for viral entry, form microdomains that cluster entry receptors and/or enable them to adopt the required conformation or regulate their cell surface expression. Sphingolipids can regulate all forms of viral uptake, often through sphingomyelinase activation, and mediate endosomal escape and intracellular trafficking. Ceramide can be key for the formation of viral replication sites. Sphingomyelinases often mediate the release of new virions from infected cells. Moreover, sphingolipids can contribute to viral-induced apoptosis and morbidity in viral diseases, as well as virus immune evasion. Alpha-galactosylceramide, in particular, also plays a significant role in immune modulation in response to viral infections. This review will discuss the roles of ceramide and its related molecules in the different steps of the viral life cycle. We will also discuss how novel strategies could exploit these for therapeutic benefit.

Heat stress impairs oocyte maturation through ceramide-mediated apoptosis in pigs

Heat stress (HS) is an emerging issue that greatly impairs the reproductive performance of animals and humans. In particular, disruption of oocyte maturation due to HS is considered a major cause of impaired reproductive performance. HS is known to induce ceramide generation, which causes reactive oxygen species (ROS) production and mitochondrial dysfunction, thereby inducing apoptosis. Therefore, we investigated whether inhibition of ceramide generation ameliorates HS-induced apoptosis in porcine cumulus-oocyte complexes (COCs) using specific inhibitors of the de novo (fumonisin B1, FB1) and hydrolytic pathways (desipramine, Des) of ceramide formation. We investigated the effects of FB1 and Des supplementation under HS conditions (41.5 °C for 44 h) on in vitro maturation (IVM) of porcine COCs. After IVM, HS significantly reduced proportion of COCs exhibiting fully expanded cumulus cells and the rate of metaphase II in oocytes. After parthenogenetic activation (PA), HS significantly reduced the rates of cleavage and blastocyst formation with a lower total cell number and a higher percentage of apoptosis in blastocysts. However, FB1 or Des supplementation under HS avoided detrimental effects of HS on expansion of cumulus cells, nuclear maturation of oocytes, and embryonic development after PA including the rates of cleavage and blastocyst formation, total cell number, and the percentage of apoptosis in blastocysts. Furthermore, FB1 or Des addition under HS, compared with HS alone, significantly decreased ceramide generation, ROS production, cytochrome C expression, and apoptosis and increased mitochondrial membrane potential in COCs, reaching levels comparable with those of the control. Taken together, our results indicate that HS impaired oocyte maturation through ceramide-mediated apoptosis.

Keep Your Friends Close, but Your Enemies Closer: Role of Acid Sphingomyelinase During Infection and Host Response

Breakdown of the inert and constitutive membrane building block sphingomyelin to the highly active lipid mediator ceramide by extracellularly active acid sphingomyelinase is tightly regulated during stress response and opens the gate for invading pathogens, triggering the immune response, development of remote organ failure, and tissue repair following severe infection. How do one enzyme and one mediator manage all of these affairs? Under physiological conditions, the enzyme is located in the lysosomes and takes part in the noiseless metabolism of sphingolipids, but following stress the protein is secreted into circulation. When secreted, acid sphingomyelinase (ASM) is able to hydrolyze sphingomyelin present at the outer leaflet of membranes to ceramide. Its generation troubles the biophysical context of cellular membranes resulting in functional assembly and reorganization of proteins and receptors, also embedded in highly conserved response mechanisms. As a consequence of cellular signaling, not only induction of cell death but also proliferation, differentiation, and fibrogenesis are affected. Here, we discuss the current state of the art on both the impact and function of the enzyme during host response and damage control. Also, the potential role of lysosomotropic agents as functional inhibitors of this upstream alarming cascade is highlighted.

Mitochondrial Ceramide Effects on the Retinal Pigment Epithelium in Diabetes

Mitochondrial damage in the cells comprising inner (retinal endothelial cells) and outer (retinal pigment epithelium (RPE)) blood–retinal barriers (BRB) is known to precede the initial BRB breakdown and further histopathological abnormalities in diabetic retinopathy (DR). We previously demonstrated that activation of acid sphingomyelinase (ASM) is an important early event in the pathogenesis of DR, and recent studies have demonstrated that there is an intricate connection between ceramide and mitochondrial function. This study aimed to determine the role of ASM-dependent mitochondrial ceramide accumulation in diabetes-induced RPE cell damage. Mitochondria isolated from streptozotocin (STZ)-induced diabetic rat retinas (7 weeks duration) showed a 1.64 ± 0.29-fold increase in the ceramide-to-sphingomyelin ratio compared to controls. Conversely, the ceramide-to-sphingomyelin ratio was decreased in the mitochondria isolated from ASM-knockout mouse retinas compared to wild-type littermates, confirming the role of ASM in mitochondrial ceramide production. Cellular ceramide was elevated 2.67 ± 1.07-fold in RPE cells derived from diabetic donors compared to control donors, and these changes correlated with increased gene expression of IL-1β, IL-6, and ASM. Treatment of RPE cells derived from control donors with high glucose resulted in elevated ASM, vascular endothelial growth factor (VEGF), and intercellular adhesion molecule 1 (ICAM-1) mRNA. RPE from diabetic donors showed fragmented mitochondria and a 2.68 ± 0.66-fold decreased respiratory control ratio (RCR). Treatment of immortalized cell in vision research (ARPE-19) cells with high glucose resulted in a 25% ± 1.6% decrease in citrate synthase activity at 72 h. Inhibition of ASM with desipramine (15 μM, 1 h daily) abolished the decreases in metabolic functional parameters. Our results are consistent with diabetes-induced increase in mitochondrial ceramide through an ASM-dependent pathway leading to impaired mitochondrial function in the RPE cells of the retina.

The regulation of p53, p38 MAPK, JNK and XBP-1s by sphingosine kinases in human embryonic kidney cells

Since inhibitors of sphingosine kinases (SK1, SK2) have been shown to induce p53-mediated cell death, we have further investigated their role in regulating p53, stress activated protein kinases and XBP-1s in HEK293T cells. Treatment of these cells with the sphingosine kinase inhibitor, SKi, which fails to induce apoptosis, promoted the conversion of p53 into two proteins with molecular masses of 63 and 90 kDa, and which was enhanced by over-expression of ubiquitin. The SKi induced conversion of p53 to p63/p90 was also enhanced by siRNA knockdown of SK1, but not SK2 or dihydroceramide desaturase (Degs1), suggesting that SK1 is a negative regulator of this process. In contrast, another sphingosine kinase inhibitor, ABC294640 only very weakly stimulated formation of p63/p90 and induced apoptosis of HEK293T cells. We have previously shown that SKi promotes the polyubiquitination of Degs1, and these forms positively regulate p38 MAPK/JNK pathways to promote HEK293T cell survival/growth. siRNA knockdown of SK1 enhanced the activation of p38 MAPK/JNK pathways in response to SKi, suggesting that SK1 functions to oppose these pro-survival pathways in HEK293T cells. SKi also enhanced the stimulatory effect of the proteasome inhibitor, MG132 on the expression of the pro-survival protein XBP-1s and this was reduced by siRNA knockdown of SK2 and increased by knockdown of p53. These findings suggest that SK1 and SK2 have opposing roles in regulating p53-dependent function in HEK293T cells.

Native and Polyubiquitinated Forms of Dihydroceramide Desaturase Are Differentially Linked to Human Embryonic Kidney Cell Survival

There is controversy concerning the role of dihydroceramide desaturase (Degs1) in regulating cell survival, with studies showing that it can both promote and protect against apoptosis. We have therefore investigated the molecular basis for these opposing roles of Degs1. Treatment of HEK293T cells with the sphingosine kinase inhibitor SKi [2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole] or fenretinide, but not the Degs1 inhibitor GT11 {N-[(1R,2S)-2-hydroxy-1-hydroxymethyl-2-(2-tridecyl-1-cyclopropenyl)ethyl]octan-amide}, induced the polyubiquitination of Degs1 (M _r = 40 to 140 kDa) via a mechanism involving oxidative stress, p38 mitogen-activated protein kinase (MAPK), and Mdm2 (E3 ligase). The polyubiquitinated forms of Degs1 exhibit "gain of function" and activate prosurvival pathways, p38 MAPK, c-Jun N-terminal kinase (JNK), and X-box protein 1s (XBP-1s). In contrast, another sphingosine kinase inhibitor, ABC294640 [3-(4-chlorophenyl)-adamantane-1-carboxylic acid (pyridin-4-ylmethyl)amide], at concentrations of 25 to 50 μM failed to induce formation of the polyubiquitinated forms of Degs1. In contrast to SKi, ABC294640 (25 μM) promotes apoptosis of HEK293T cells via a Degs1-dependent mechanism that is associated with increased de novo synthesis of ceramide. These findings are the first to demonstrate that the polyubiquitination of Degs1 appears to change its function from proapoptotic to prosurvival. Thus, polyubiquitination of Degs1 might provide an explanation for the reported opposing functions of this enzyme in cell survival/apoptosis.

Inhibitory effects of fenretinide metabolites N-[4-methoxyphenyl]retinamide (MPR) and 4-oxo-N-(4-hydroxyphenyl)retinamide (3-keto-HPR) on fenretinide molecular targets β-carotene oxygenase 1, stearoyl-CoA desaturase 1 and dihydroceramide Δ4-desaturase 1

The therapeutic capacity of fenretinide (N-[4-hydroxyphenyl] retinamide; 4-HPR) has been demonstrated for several conditions, including cancer, obesity, diabetes, and ocular disease. Yet, the mechanisms of action for its pleiotropic effects are still undefined. We hypothesized that investigation of two of the major physiological metabolites of fenretinide, N-[4-methoxyphenyl]retinamide (MPR) and 4-oxo-N-(4-hydroxyphenyl)retinamide (3-keto-HPR), might begin to resolve the multifaceted effects of this synthetic retinoid. We analyzed the effects of fenretinide, MPR, 3-keto-HPR, and the non-retinoid RBP4 ligand A1120, on the activity of known targets of fenretinide, stearoyl-CoA desaturase 1 (SCD1) and dihydroceramide Δ4-desaturase 1 (DES1) in ARPE-19 cells, and purified recombinant mouse beta-carotene oxygenase 1 (BCO1) in vitro. Lipids and retinoids were extracted and quantified by liquid chromatography-mass spectrometry and reversed phase HPLC, respectively. The data demonstrate that while fenretinide is an inhibitor of the activities of these three enzymes, that 3-keto-HPR is a more potent inhibitor of all three enzymes, potentially mediating most of the in vivo beneficial effects of fenretinide. However, while MPR does not affect SCD1 and DES1 activity, it is a potent specific inhibitor of BCO1. We conclude that a deeper understanding of the mechanisms of action of fenretinide and its metabolites provides new avenues for therapeutic specificity. For example, administration of 3-keto-HPR instead of fenretinide may be preferential if inhibition of SCD1 or DES1 activity is the goal (cancer), while MPR may be better for BCO1 modulation (carotenoid metabolism). Continued investigation of fenretinide metabolites in the context of fenretinide’s various therapeutic uses will begin to resolve the pleotropic nature of this compound.

Tumor suppressive functions of ceramide: evidence and mechanisms

Studies over the past two decades have identified ceramide as a multifunctional central molecule in the sphingolipid biosynthetic pathway. Given its diverse tumor suppressive activities, molecular understanding of ceramide action will produce fundamental insights into processes that limit tumorigenesis and may identify key molecular targets for therapeutic intervention. Ceramide can be activated by a diverse array of stresses such as heat shock, genotoxic damage, oxidative stress and anticancer drugs. Ceramide triggers a variety of tumor suppressive and anti-proliferative cellular programs such as apoptosis, autophagy, senescence, and necroptosis by activating or repressing key effector molecules. Defects in ceramide generation and metabolism in cancer contribute to tumor cell survival and resistance to chemotherapy. The potent and versatile anticancer activity profile of ceramide has motivated drug development efforts to (re-)activate ceramide in established tumors. This review focuses on our current understanding of the tumor suppressive functions of ceramide and highlights the potential downstream targets of ceramide which are involved in its tumor suppressive action.

Inhibitors of dihydroceramide desaturase 1: Therapeutic agents and pharmacological tools to decipher the role of dihydroceramides in cell biology

Dihydroceramide desaturase (Des1) is the last enzyme in the de novo synthesis of ceramides (Cer). It catalyzes the insertion of a double bond into dihydroceramides (dhCer) to convert them to Cer, both of which are further metabolized to more complex (dihydro) sphingolipids. For many years dhCer have received poor attention, mainly due to their supposed lack of biological activity. It was not until about ten years ago that the concept that dhCer might have regulatory roles in biology emerged for the first time. Since then, multiple publications have established that dhCer are implicated in a wide spectrum of biological processes. Physiological and pathophysiological functions of dhCer have been recently reviewed. In this review we will focus on the biochemical features of Des1 and on its inhibition by different compounds with presumably different modes of action.

Dihydroceramides: From Bit Players to Lead Actors

Sphingolipid synthesis involves a highly conserved biosynthetic pathway that produces fundamental precursors of complex sphingolipids. The final reaction involves the insertion of a double bond into dihydroceramides to generate the more abundant ceramides, which are converted to sphingomyelins and glucosylceramides/gangliosides by the addition of polar head groups. Although ceramides have long been known to mediate cellular stress responses, the dihydroceramides that are transiently produced during de novo sphingolipid synthesis were deemed inert. Evidence published in the last few years suggests that these dihydroceramides accumulate to a far greater extent in tissues than previously thought. Moreover, they have biological functions that are distinct and non-overlapping with those of the more prevalent ceramides. Roles are being uncovered in autophagy, hypoxia, and cellular proliferation, and the lipids are now implicated in the etiology, treatment, and/or diagnosis of diabetes, cancer, ischemia/reperfusion injury, and neurodegenerative diseases. This minireview summarizes recent findings on this emerging class of bioactive lipids.

Vitamin A, cancer treatment and prevention: the new role of cellular retinol binding proteins

Retinol and vitamin A derivatives influence cell differentiation, proliferation, and apoptosis and play an important physiologic role in a wide range of biological processes. Retinol is obtained from foods of animal origin. Retinol derivatives are fundamental for vision, while retinoic acid is essential for skin and bone growth. Intracellular retinoid bioavailability is regulated by the presence of specific cytoplasmic retinol and retinoic acid binding proteins (CRBPs and CRABPs). CRBP-1, the most diffuse CRBP isoform, is a small 15 KDa cytosolic protein widely expressed and evolutionarily conserved in many tissues. CRBP-1 acts as chaperone and regulates the uptake, subsequent esterification, and bioavailability of retinol. CRBP-1 plays a major role in wound healing and arterial tissue remodelling processes. In the last years, the role of CRBP-1-related retinoid signalling during cancer progression became object of several studies. CRBP-1 downregulation associates with a more malignant phenotype in breast, ovarian, and nasopharyngeal cancers. Reexpression of CRBP-1 increased retinol sensitivity and reduced viability of ovarian cancer cells in vitro. Further studies are needed to explore new therapeutic strategies aimed at restoring CRBP-1-mediated intracellular retinol trafficking and the meaning of CRBP-1 expression in cancer patients' screening for a more personalized and efficacy retinoid therapy.

Enhanced killing of SCC17B human head and neck squamous cell carcinoma cells after photodynamic therapy plus fenretinide via the de novo sphingolipid biosynthesis pathway and apoptosis

Because photodynamic therapy (PDT) alone is not always effective as an anticancer treatment, PDT is combined with other anticancer agents for improved efficacy. The clinically-relevant fenretinide [N-(4-hydroxyphenyl) retinamide; 4HPR], was combined with the silicon phthalocyanine photosensitizer Pc4-mediated PDT to test for their potential to enhance killing of SCC17B cells, a clinically-relevant model of human head and neck squamous cell carcinoma. Because each of these treatments induces apoptosis and regulates the de novo sphingolipid (SL) biosynthesis pathway, the role of ceramide synthase, the pathway-associated enzyme, in PDT+4HPR-induced apoptotic cell death was determined using the ceramide synthase inhibitor fumonisin B1 (FB). PDT+4HPR enhanced loss of clonogenicity. zVAD-fmk, a pan-caspase inhibitor, and FB, protected cells from death post-PDT+4HPR. In contrast, the anti-apoptotic protein Bcl2 inhibitor ABT199 enhanced cell killing after PDT+4HPR. Combining PDT with 4HPR led to FB-sensitive, enhanced Bax associated with mitochondria and cytochrome c redistribution. Mass spectrometry data showed that the accumulation of C16-dihydroceramide, a precursor of ceramide in the de novo SL biosynthesis pathway, was enhanced after PDT+4HPR. Using quantitative confocal microscopy, we found that PDT+4HPR enhanced dihydroceramide/ceramide accumulation in the ER, which was inhibited by FB. The results suggest that SCC17B cells are sensitized to PDT by 4HPR via the de novo SL biosynthesis pathway and apoptosis, and imply potential clinical relevance of the combination for cancer treatment.

The bioactive lipid 4-hydroxyphenyl retinamide inhibits flavivirus replication

Dengue virus (DENV), a member of the Flaviviridae family, is a mosquito-borne pathogen and the cause of dengue fever. The increasing prevalence of DENV worldwide heightens the need for an effective vaccine and specific antivirals. Due to the dependence of DENV upon the lipid biosynthetic machinery of the host cell, lipid signaling and metabolism present unique opportunities for inhibiting viral replication. We screened a library of bioactive lipids and modulators of lipid metabolism and identified 4-hydroxyphenyl retinamide (4-HPR) (fenretinide) as an inhibitor of DENV in cell culture. 4-HPR inhibits the steady-state accumulation of viral genomic RNA and reduces viremia when orally administered in a murine model of DENV infection. The molecular target responsible for this antiviral activity is distinct from other known inhibitors of DENV but appears to affect other members of the Flaviviridae, including the West Nile, Modoc, and hepatitis C viruses. Although long-chain ceramides have been implicated in DENV replication, we demonstrate that DENV is insensitive to the perturbation of long-chain ceramides in mammalian cell culture and that the effect of 4-HPR on dihydroceramide homeostasis is separable from its antiviral activity. Likewise, the induction of reactive oxygen species by 4-HPR is not required for the inhibition of DENV. The inhibition of DENV in vivo by 4-HPR, combined with its well-established safety and tolerability in humans, suggests that it may be repurposed as a pan-Flaviviridae antiviral agent. This work also illustrates the utility of bioactive lipid screens for identifying critical interactions of DENV and other viral pathogens with host lipid biosynthesis, metabolism, and signal transduction.

Targeting and cytotoxicity of SapC-DOPS nanovesicles in pancreatic cancer

Only a small number of promising drugs target pancreatic cancer, which is the fourth leading cause of cancer deaths with a 5-year survival of less than 5%. Our goal is to develop a new biotherapeutic agent in which a lysosomal protein (saposin C, SapC) and a phospholipid (dioleoylphosphatidylserine, DOPS) are assembled into nanovesicles (SapC-DOPS) for treating pancreatic cancer. A distinguishing feature of SapC-DOPS nanovesicles is their high affinity for phosphatidylserine (PS) rich microdomains, which are abnormally exposed on the membrane surface of human pancreatic tumor cells. To evaluate the role of external cell PS, in vitro assays were used to correlate PS exposure and the cytotoxic effect of SapC-DOPS in human tumor and nontumorigenic pancreatic cells. Next, pancreatic tumor xenografts (orthotopic and subcutaneous models) were used for tumor targeting and therapeutic efficacy studies with systemic SapC-DOPS treatment. We observed that the nanovesicles selectively killed human pancreatic cancer cells in vitro by inducing apoptotic death, whereas untransformed cells remained unaffected. This in vitro cytotoxic effect correlated to the surface exposure level of PS on the tumor cells. Using xenografts, animals treated with SapC-DOPS showed clear survival benefits and their tumors shrank or disappeared. Furthermore, using a double-tracking method in live mice, we showed that the nanovesicles were specifically targeted to orthotopically-implanted, bioluminescent pancreatic tumors. These data suggest that the acidic phospholipid PS is a biomarker for pancreatic cancer that can be effectively targeted for therapy utilizing cancer-selective SapC-DOPS nanovesicles. This study provides convincing evidence in support of developing a new therapeutic approach to pancreatic cancer.

CXCR4-tropic, but not CCR5-tropic, human immunodeficiency virus infection is inhibited by the lipid raft-associated factors, acyclic retinoid analogs, and cholera toxin B subunit

Development of an effective low-cost anti-acquired immunodeficiency syndrome (AIDS) drugs is needed for treatment of AIDS patients in developing countries. Host cell lipid raft microdomains, which are enriched with cholesterol, glycolipids, ceramide, and gangliosides, are important for human immunodeficiency virus type 1 (HIV-1) entry. Retinoid analogs have been shown to modulate ceramide levels in the cell membrane, while cholera toxin B subunit (CT-B) specifically binds to the ganglioside GM1. In this study, we found that the acyclic retinoid analogs geranylgeranoic acid (GGA) and NIK-333 as well as CT-B efficiently attenuate CXCR4-tropic, but not CCR5-tropic, HIV-1 vector infection. We also found that GGA and NIK-333 suppress CXCR4-tropic HIV-1 infection by attenuating CXCR4 expression. CT-B also attenuated CXCR4-tropic HIV-1 infection, but did not suppress CXCR4 expression. These results suggest a distinct role for lipid raft microdomains in CXCR4- and CCR5-tropic HIV-1 infections and illuminate novel agents for the development of AIDS therapy.

Fenretinide causes emphysema, which is prevented by sphingosine 1-phoshate

Sphingolipids play a role in the development of emphysema and ceramide levels are increased in experimental models of emphysema; however, the mechanisms of ceramide-related pulmonary emphysema are not fully understood. Here we examine mechanisms of ceramide-induced pulmonary emphysema. Male Sprague-Dawley rats were treated with fenretinide (20 mg/kg BW), a synthetic derivative of retinoic acid that causes the formation of ceramide, and we postulated that the effects of fenretinide could be offset by administering sphingosine 1-phosphate (S1P) (100 µg/kg BW). Lung tissues were analyzed and mean alveolar airspace area, total length of the alveolar perimeter and the number of caspase-3 positive cells were measured. Hypoxia-inducible factor alpha (HIF-1α), vascular endothelial growth factor (VEGF) and other related proteins were analyzed by Western blot analysis. Immunohistochemical analysis of HIF-1α was also performed. Ceramide, dihydroceramide, S1P, and dihydro-S1P were measured by mass spectrometer. Chronic intraperitoneal injection of fenretinide increased the alveolar airspace surface area and increased the number of caspase-3 positive cells in rat lungs. Fenretinide also suppressed HIF-1α and VEGF protein expression in rat lungs. Concomitant injection of S1P prevented the decrease in the expression of HIF-1α, VEGF, histone deacetylase 2 (HDAC2), and nuclear factor (erythroid-derived 2)-like 2 (Nrf2) protein expression in the lungs. S1P injection also increased phosphorylated sphingosine kinase 1. Dihydroceramide was significantly increased by fenretinide injection and S1P treatment prevented the increase in dihydroceramide levels in rat lungs. These data support the concept that increased de novo ceramide production causes alveolar septal cell apoptosis and causes emphysema via suppressing HIF-1α. Concomitant treatment with S1P normalizes the ceramide-S1P balance in the rat lungs and increases HIF-1α protein expression via activation of sphingosine kinase 1; as a consequence, S1P salvages fenretinide induced emphysema in rat lungs.

c-Abl is an upstream regulator of acid sphingomyelinase in apoptosis induced by inhibition of integrins αvβ3 and αvβ5

Inhibition of integrins αvβ3/αvβ5 by the cyclic function-blocking peptide, RGDfV (Arg-Gly-Asp-Phe-Val) can induce apoptosis in both normal cells and tumor cells. We show that RGDfV induced apoptosis in ECV-304 carcinoma cells, increased activity and mRNA expression of acid sphingomyelinase (ASM), and increased ceramides C₁₆, C_18∶0, C_24∶0 and C_24∶1 while decreasing the corresponding sphingomyelins. siRNA to ASM decreased RGDfV-induced apoptosis as measured by TUNEL, PARP cleavage, mitochondrial depolarization, and caspase-3 and caspase-8 activities, as well as by annexinV in a 3D collagen model. These findings indicate a causal role for ASM in RGDfV-induced apoptosis in ECV-304. We have shown that c-Abl, a non-receptor tyrosine kinase, also mediates RGDfV-induced apoptosis. However, c-Abl, has not been previously linked to ASM in any system. Here we show that STI-571 (imatinib, inhibitor of c-Abl) inhibited RGDfV-induced ASM activity. Furthermore, STI-571 and c-Abl-siRNA both inhibited RGDfV-induced increase in ASM mRNA, but ASM-siRNA did not affect c-Abl phosphorylation or expression, supporting that c-Abl regulates the RGDfV-induced increase in ASM expression. These studies implicate ASM as a mediator of apoptosis induced by inhibition of integrins αvβ3/αvβ5, and for the first time place c-Abl as an upstream regulator of ASM expression and activity.

Ceramide synthases at the centre of sphingolipid metabolism and biology

Sphingolipid metabolism in metazoan cells consists of a complex interconnected web of numerous enzymes, metabolites and modes of regulation. At the centre of sphingolipid metabolism reside CerSs (ceramide synthases), a group of enzymes that catalyse the formation of ceramides from sphingoid base and acyl-CoA substrates. From a metabolic perspective, these enzymes occupy a unique niche in that they simultaneously regulate de novo sphingolipid synthesis and the recycling of free sphingosine produced from the degradation of pre-formed sphingolipids (salvage pathway). Six mammalian CerSs (CerS1-CerS6) have been identified. Unique characteristics have been described for each of these enzymes, but perhaps the most notable is the ability of individual CerS isoforms to produce ceramides with characteristic acyl-chain distributions. Through this control of acyl-chain length and perhaps in a compartment-specific manner, CerSs appear to regulate multiple aspects of sphingolipid-mediated cell and organismal biology. In the present review, we discuss the function of CerSs as critical regulators of sphingolipid metabolism, highlight their unique characteristics and explore the emerging roles of CerSs in regulating programmed cell death, cancer and many other aspects of biology.

Phase II study of oral capsular 4-hydroxyphenylretinamide (4-HPR/fenretinide) in pediatric patients with refractory or recurrent neuroblastoma: a report from the Children's Oncology Group

PURPOSE

To determine the response rate to oral capsular fenretinide in children with recurrent or biopsy proven refractory high-risk neuroblastoma.

EXPERIMENTAL DESIGN

Patients received 7 days of fenretinide: 2,475 mg/m(2)/d divided TID (<18 years) or 1,800 mg/m(2)/d divided BID (≥18 years) every 21 days for a maximum of 30 courses. Patients with stable or responding disease after course 30 could request additional compassionate courses. Best response by course 8 was evaluated in stratum 1 (measurable disease on CT/MRI ± bone marrow and/or MIBG avid sites) and stratum 2 (bone marrow and/or MIBG avid sites only).

RESULTS

Sixty-two eligible patients, median age 5 years (range 0.6-19.9), were treated in stratum 1 (n = 38) and stratum 2 (n = 24). One partial response (PR) was seen in stratum 2 (n = 24 evaluable). No responses were seen in stratum 1 (n = 35 evaluable). Prolonged stable disease (SD) was seen in 7 patients in stratum 1 and 6 patients in stratum 2 for 4 to 45+ (median 15) courses. Median time to progression was 40 days (range 17-506) for stratum 1 and 48 days (range 17-892) for stratum 2. Mean 4-HPR steady-state trough plasma concentrations were 7.25 μmol/L (coefficient of variation 40-56%) at day 7 course 1. Toxicities were mild and reversible.

CONCLUSIONS

Although neither stratum met protocol criteria for efficacy, 1 PR + 13 prolonged SD occurred in 14/59 (24%) of evaluable patients. Low bioavailability may have limited fenretinide activity. Novel fenretinide formulations with improved bioavailability are currently in pediatric phase I studies.

Glutathione selectively inhibits Doxorubicin induced phosphorylation of p53Ser¹⁵, caspase dependent ceramide production and apoptosis in human leukemic cells

Glutathione (GSH) is the most abundant non-protein antioxidant in mammalian cells. It has been implicated in playing an important role in different signal transduction pathways, and its depletion is an early hallmark in the progression of apoptosis in response to a number of proapoptotic stimuli. We have selectively investigated the role of GSH in cytotoxic response of Jurkat and Molt-4 human leukemic cells to the anti-cancer drug Doxorubicin. In this study, we have shown that extracellular supplementation of GSH to human leukemic cells renders them a resistant phenotype to Doxorubicin treatment. Glutathione pre-treatment inhibits Doxorubicin-induced p53Ser(15) phosphorylation, caspase dependent ceramide (Cer) generation, Poly (ADP-ribose) polymerase (PARP) cleavage, and DNA fragmentation. Taken together, these results indicate that the major cellular antioxidant GSH influences the chemotherapeutic efficacy of Doxorubicin towards human leukemic cells.

Coupling in vitro and in vivo paradigm reveals a dose dependent inhibition of angiogenesis followed by initiation of autophagy by C6-ceramide

The activity of N-hexanoyl-D-erythro-sphingosine, a C6-ceramide against angiogenesis was tested in vitro and in vivo. The effect of ceramide in inhibiting MCF-7 cancer cells was also determined. The aim of this study was to potentiate the effect of ceramide as anti-angiogenic compound that can regulate tumor induced angiogenesis.

C6-ceramide inhibited vascular endothelial growth factor (VEGF)-induced human umbilical vein endothelial cells (HUVEC) tube formation in a dose-dependent manner within 24 hours. Ceramide at concentrations between 12.5 and 25 μM inhibited the viability of MCF-7 cells and reduced VEGF-induced cell migration in 24 hours. At 50 μM, ceramide induced MCF-7 cell death via autophagy as demonstrated by accumulation of MDC in ceramide-treated MCF-7 vacuoles. The expression of VEGF was reduced and the levels of cathepsin D in MCF-7 increased. In vivo, 50 μM ceramide caused a 40% reduction of new vessel formation in the CAM assay within 24 hours. Zebrafish exposed to 100 - 400 μM ceramide had a distinct disruption of blood vessel development at 48 hours post-fertilization. Ceramide-exposed embryos also had primary motoneurons exhibiting abnormal axonal trajectories and ectopic branching. Ceramide induced cell-death was not detected in the zebrafish assay. Collectively, these data indicate that ceramide is a potent anti-angiogenic compound and that the mechanism underlying its anti-angiogenic capabilities does not rely upon the induction of apoptosis.

Preferential involvement of both ROS and ceramide in fenretinide-induced apoptosis of HL60 rather than NB4 and U937 cells

Leukemic cells responding to apoptosis-inducing drugs can be varied in terms of the mechanisms of action. Fenretinide, a synthetic retinoid, is worth of study as a promising candidate for apoptosis-based therapy of leukemia. Yet, it remains unclear whether this drug exerts the similar mechanisms on different leukemic cells. Here, we report a comparative analysis of fenretinide-induced apoptosis in three acute myeloid leukemic (AML) cell lines including HL60, NB4 and U937. Through a series of antagonist assays, we revealed similarities and differences of mechanisms involved in these three cell lines. Antioxidant vitamin C completely abrogated fenretinide-induced apoptosis in all cell lines, demonstrating that ROS is an essential and common mediator. However, the apoptotic effects of fenretinide could be blocked by ceramide synthase inhibitor fumonisin B1 only in HL60 rather than the other two. Moreover, fumonisin B1 was unable to inhibit the generation of ROS in fenretinide-treated HL60 cells, indicating that ROS may function as upstream stimulus of ceramide-mediated apoptosis. These comparative results strongly suggest that the apoptotic response induced by fenretinide in HL60 involves both ROS and ceramide, whereas drug-induced apoptosis in NB4 and U937 requires ROS but is independent of ceramide. Differentiated modes of action exerting on AML may guide the use of this apoptosis-inducing drug, and hence advance our knowledge about the nature of cancer-specific responses to this drug.

Ceramide in cystic fibrosis: a potential new target for therapeutic intervention

Patients with cystic fibrosis (CF) are afflicted with many symptoms but the greatest challenge is the fight against chronic bacterial infections, leading to decreased lung function and ultimately death. Our group has recently found reduced levels of ceramides in CF patients and mice. Ceramides are sphingolipids involved in the structure of cell membranes but also participate in the inflammatory response, in cell signalling through membrane microdomains (lipid rafts), and in apoptosis. These characteristics of ceramides make them strong candidates for therapeutic intervention in CF. As more studies have come to evaluate the role of ceramide in CF, conflicting results have been described. This paper discusses various views regarding the potential role of ceramide in CF, summarizes methods of ceramide detection and their role in the regulation of cellular and molecular processes.

Sphingolipidomics of A2780 human ovarian carcinoma cells treated with synthetic retinoids

The dihydroceramide, ceramide, sphingomyelin, lactosylceramide, and ganglioside species of A2780 human ovarian carcinoma cells treated with the synthetic retinoids N-(4-hydroxyphenyl)retinamide (fenretinide, 4-HPR) and 4-oxo-N-(4-hydroxyphenyl)retinamide (4-oxo-4-HPR) in culture were characterized by ESI-MS. We characterized 32 species of ceramide and dihydroceramide, 15 of sphingomyelin, 12 of lactosylceramide, 9 of ganglioside GM2, and 6 of ganglioside GM3 differing for the long-chain base and fatty acid structures. Our results indicated that treatment with both 4-HPR and 4-oxo-4-HPR led to a marked increase in dihydroceramide species, while only 4-oxo-4-HPR led to a minor increase of ceramide species. Dihydroceramides generated in A2780 cells in response to 4-HPR or 4-oxo-4-HPR differed for their fatty acid content, suggesting that the two drugs differentially affect the early steps of sphingolipid synthesis. Dihydroceramides produced upon treatments with the drugs were further used for the synthesis of complex dihydrosphingolipids, whose levels dramatically increased in drug-treated cells.

c-Abl mediates endothelial apoptosis induced by inhibition of integrins alphavbeta3 and alphavbeta5 and by disruption of actin

Inhibition of integrins alphavbeta3 and alphavbeta5 in human brain microvascular endothelial cells (HBMECs) by the function-blocking peptide RGDfV induces loss of spreading on vitronectin, cell detachment, and apoptosis. We demonstrate that cell detachment is not required for apoptosis because plating on bovine serum albumin-blocked poly-L-lysine (allows attachment, but not integrin ligation and cell spreading) also induced apoptosis. Latrunculin B (LatB), which inhibits F-actin polymerization, induced transient loss of HBMEC spreading on vitronectin, but not their detachment, and induced apoptosis despite recovery of cell spreading. However, LatB did not cause apoptosis in 5 tumor cell lines. In HBMECs, both LatB and RGDfV induced transient Y412 and Y245 phosphorylation of endogenous c-Abl, a nonreceptor tyrosine kinase that reciprocally regulates F-actin. LatB also induced nuclear translocation of c-Abl in HBMECs. STI-571 (imatinib), a targeted therapy for BCR-ABL1(+) leukemias and inhibitor of c-Abl, platelet-derived growth factor receptor, and c-Kit, decreased endothelial apoptosis. LatB-induced HBMEC apoptosis, and its inhibition by STI-571 also occurred in a 3-dimensional collagen model, supporting physiologic relevance. Last, siRNA to c-Abl (but not nonspecific siRNA) also inhibited RGDfV- and LatB-induced apoptosis. Thus, endogenous c-Abl mediates endothelial apoptosis induced by inhibition of integrins alphavbeta3/alphavbeta5 or by LatB-induced disruption of F-actin.

Cancer treatment strategies targeting sphingolipid metabolism

Ceramide and sphingosine-1-phosphate are related sphingolipid metabolites that can be generated through a de novo biosynthetic route or derived from the recycling of membrane sphingomyelin. Both these lipids regulate cellular responses to stress, with generally opposing effects. Sphingosine-1-phosphate functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms. A growing body of evidence has implicated ceramide, sphingosine-1-phosphate and the genes involved in their synthesis, catabolism and signaling in various aspects of oncogenesis, cancer progression and drug- and radiation resistance. This may be explained in part by the finding that both lipids impinge upon the PI3K/ AKT pathway, which represses apoptosis and autophagy. In addition, sphingolipids influence cell cycle progression, telomerase function, cell migration and stem cell biology. Considering the central role of ceramide in mediating physiological as well as pharmacologically stimulated apoptosis, ceramide can be considered a tumor-suppressor lipid. In contrast, sphingosine-1-phosphate can be considered a tumor-promoting lipid, and the enzyme responsible for its synthesis functions as an oncogene. Not surprisingly, genetic mutations that result in reduced ceramide generation, increased sphingosine-1-phosphate synthesis or which reduce steady state ceramide levels and increase sphingosine-1-phosphate levels have been identified as mechanisms of tumor progression and drug resistance in cancer cells. Pharmacological tools for modulating sphingolipid pathways are being developed and represent novel therapeutic strategies for the treatment of cancer.

Cancer-selective targeting and cytotoxicity by liposomal-coupled lysosomal saposin C protein

PURPOSE

Saposin C is a multifunctional protein known to activate lysosomal enzymes and induce membrane fusion in an acidic environment. Excessive accumulation of lipid-coupled saposin C in lysosomes is cytotoxic. Because neoplasms generate an acidic microenvironment, caused by leakage of lysosomal enzymes and hypoxia, we hypothesized that saposin C may be an effective anticancer agent. We investigated the antitumor efficacy and systemic biodistribution of nanovesicles comprised of saposin C coupled with dioleoylphosphatidylserine in preclinical cancer models.

EXPERIMENTAL DESIGN

Neuroblastoma, malignant peripheral nerve sheath tumor and, breast cancer cells were treated with saposin C-dioleoylphosphatidylserine nanovesicles and assessed for cell viability, ceramide elevation, caspase activation, and apoptosis. Fluorescently labeled saposin C-dioleoylphosphatidylserine was i.v. injected to determine in vivo tumor-targeting specificity. Antitumor activity and toxicity profile of saposin C-dioleoylphosphatidylserine were evaluated in xenograft models.

RESULTS

Saposin C-dioleoylphosphatidylserine nanovesicles, with a mean diameter of approximately 190 nm, showed specific tumor-targeting activity shown through in vivo imaging. Following i.v. administration, saposin C-dioleoylphosphatidylserine nanovesicles preferentially accumulated in tumor vessels and cells in tumor-bearing mice. Saposin C-dioleoylphosphatidylserine induced apoptosis in multiple cancer cell types while sparing normal cells and tissues. The mechanism of saposin C-dioleoylphosphatidylserine induction of apoptosis was determined to be in part through elevation of intracellular ceramides, followed by caspase activation. In in vivo models, saposin C-dioleoylphosphatidylserine nanovesicles significantly inhibited growth of preclinical xenografts of neuroblastoma and malignant peripheral nerve sheath tumor. I.v. dosing of saposin C-dioleoylphosphatidylserine showed no toxic effects in nontumor tissues.

CONCLUSIONS

Saposin C-dioleoylphosphatidylserine nanovesicles offer promise as a novel, nontoxic, cancer-targeted, antitumor agent for treating a broad range of cancers.

AMPK activator, AICAR, inhibits palmitate-induced apoptosis in osteoblast

Osteoblast apoptosis reduces bone mineral density. Apoptosis can be induced in a variety of cells by palmitate, which is one of the most common saturated fatty acids in dietary fat. The AMPK activator, AICAR, has been shown to inhibit palmitate-induced apoptosis. However, the role of palmitate in osteoblast apoptosis is currently unknown. This study examined whether palmitate could induce apoptosis in osteoblasts, and if so, whether AICAR could alleviate palmitate-induced apoptosis. Palmitate reduced cell survival and induced apoptosis in a dose- and time-dependent manner in human fetal osteoblasts (hFOB) 1.19. While the long-chain acyl-CoA synthetase inhibitor, triacsin C, inhibited palmitate-induced apoptosis, anti-oxidants and ceramide synthesis inhibitors did not attenuate the apoptosis. AICAR prevented palmitate-induced apoptosis and the inhibition of AICAR-mediated increase in fatty acid oxidation by etomoxir did not affect the prevention of apoptosis by AICAR. Constitutively-active AMPK also inhibited palmitate-induced apoptosis. Treatment with an AMPK inhibitor (compound C) and a dominant-negative AMPK adenovirus suppressed the inhibitory effect of AICAR on apoptosis. Palmitate impaired the activation of ERK by fetal bovine serum, which was blocked by AICAR. Moreover, AICAR increased ERK activation, and ERK inhibitors, PD98059 and U0126, as well as a dominant-negative MEK1, abolished the inhibitory effect of AICAR on palmitate-induced apoptosis. AICAR also inhibited palmitate-induced apoptosis in osteoblastic differentiated cells from human bone marrow, which was accompanied by recovered ERK activity. These results suggest that palmitate induces apoptosis in osteoblasts through the impaired activation of ERK, and the activation of AMPK inhibits palmitate-induced apoptosis by activating ERK.

The marine lipopeptide somocystinamide A triggers apoptosis via caspase 8

Screening for novel anticancer drugs in chemical libraries isolated from marine organisms, we identified the lipopeptide somocystinamide A (ScA) as a pluripotent inhibitor of angiogenesis and tumor cell proliferation. The antiproliferative activity was largely attributable to induction of programmed cell death. Sensitivity to ScA was significantly increased among cells expressing caspase 8, whereas siRNA knockdown of caspase 8 increased survival after exposure to ScA. ScA rapidly and efficiently partitioned into liposomes while retaining full antiproliferative activity. Consistent with the induction of apoptosis via the lipid compartment, we noted accumulation and aggregation of ceramide in treated cells and subsequent colocalization with caspase 8. Angiogenic endothelial cells were extremely sensitive to ScA. Picomolar concentrations of ScA disrupted proliferation and endothelial tubule formation in vitro. Systemic treatment of zebrafish or local treatment of the chick chorioallantoic membrane with ScA resulted in dose-dependent inhibition of angiogenesis, whereas topical treatment blocked tumor growth among caspase-8-expressing tumors. Together, the results reveal an unexpected mechanism of action for this unique lipopeptide and suggest future development of this and similar agents as antiangiogenesis and anticancer drugs.

Fenretinide Corrects Newly Found Ceramide Deficiency in Cystic Fibrosis

Chronic and persistent lung infections cause the majority of morbidity and mortality in patients with cystic fibrosis (CF). Galactosyl ceramide has been previously shown to be involved in Pseudomonas internalization. Therefore, we assessed ceramide levels in the plasma of patients with CF and compared them to healthy volunteers using high-performance liquid chromatography followed by mass spectrometry. Our results demonstrate that patients with CF display significantly lower levels of several ceramide sphingolipid species, specifically C14:0, C20:1, C22:0, C22:1, and C24:0 ceramides, and dihydroxy ceramide (DHC16:0). We report that Cftr-knockout mice display diminished ceramide levels in CF-related organs (lung, pancreas, ileum, and plasma) compared with their littermate controls. Since it has been previously reported that in vitro treatment with fenretinide induced ceramide in neuroblastoma cell lines, we decided to test this drug in vivo using our Cftr-knockout mice in an attempt to correct this newly identified defect in ceramide levels. We demonstrate that treatment with fenretinide is able to increase ceramide concentrations in CF-related organs. We further assessed the biological effect of fenretinide on the ability of Cftr-knockout mice to combat lung infection with P. aeruginosa. Our data show dramatic improvement in the ability of Cftr-knockout mice to control P. aeruginosa infection. Overall, these findings not only document a novel deficiency in several ceramide species in patients with CF, but also demonstrate a pharmacologic means to correct this defect in Cftr-knockout mice. Our data provide a strong rationale for clinical intervention that may benefit patients with CF suffering from CF lung disease.

Apoptosis effector mechanisms: a requiem performed in different keys

Apoptosis is the regulated form of cell death utilized by metazoans to remove unneeded, damaged, or potentially deleterious cells. Certain manifestations of apoptosis may be associated with the proteolytic activity of caspases. These changes are often held as hallmarks of apoptosis in dying cells. Consequently, many regard caspases as the central effectors or executioners of apoptosis. However, this "caspase-centric" paradigm of apoptotic cell death does not appear to be as universal as once believed. In fact, during apoptosis the efficacy of caspases may be highly dependent on the cytotoxic stimulus as well as genetic and epigenetic factors. An ever-increasing number of studies strongly suggest that there are effectors in addition to caspases, which are important in generating apoptotic signatures in dying cells. These seemingly caspase-independent effectors may represent evolutionarily redundant or failsafe mechanisms for apoptotic cell elimination. In this review, we will discuss the molecular regulation of caspases and various caspase-independent effectors of apoptosis, describe the potential context and/or limitations of these mechanisms, and explore why the understanding of these processes may have relevance in cancer where treatment is believed to engage apoptosis to destroy tumor cells.

Diphenyleneiodonium suppresses apoptosis in cerulein-stimulated pancreatic acinar cells

NADPH oxidase has been considered a major source of reactive oxygen species in phagocytic and non-phagocytic cells. Apoptosis linked to oxidative stress has been implicated in pancreatitis. Recently, we demonstrated that NADPH oxidase subunits Nox1, p27phox, p47phox, and p67phox are constitutively expressed in pancreatic acinar cells, which are activated by cerulein, a cholecystokinin analogue. Cerulein induces an acute and edematous form of pancreatitis. We investigated whether inhibition of NADPH oxidase by diphenyleneiodonium suppresses the production of reactive oxygen species and apoptosis by determining viable cell numbers, DNA fragmentation, TUNEL staining, caspase-3 activity, and the expression of apoptosis-inducing factor in pancreatic acinar AR42J cells stimulated with cerulein. Inhibition on NADPH oxidase by diphenyleneiodonium was assessed by the alterations in NADPH oxidase activity and translocation of the cytosolic subunits p67phox and p47phox to the membrane. Intracellular Ca2+ level was monitored to investigate the relationship between NADPH oxidase and Ca2+ in cells stimulated with cerulein. As a result, cerulein induced the activation of NADPH, increased production of reactive oxygen species, and apoptotic indices determined by the expression of apoptosis-inducing factor, caspase-3 activation, TUNEL staining, DNA fragmentation, and cell viability. Treatment with DPI inhibited cerulein-induced activation of NADPH oxidase, the production of reactive oxygen species, and apoptosis, but not the increase of intracellular Ca2+ levels in pancreatic acinar cells. These results demonstrate that the cerulein-induced increase in intracellular Ca2+ level may be an upstream event of NADPH oxidase activation. Diphenyleneiodonium, an NADPH oxidase inhibitor, inhibits the expression of apoptosis-inducing factor and caspase-3 activation, and thus apoptosis in pancreatic acinar cells.

Novel cell death pathways induced by N-(4-hydroxyphenyl)retinamide: therapeutic implications

We previously reported that N-(4-hydroxyphenyl)retinamide (4HPR) inhibits retinoblastoma tumor growth in a murine model in vivo and kills Y79 retinoblastoma cells in vitro. In this work, we assayed different cell death-related parameters, including mitochondrial damage and caspase activation, in Y79 cells exposed to 4HPR. 4HPR induced cytochrome c release from mitochondria, caspase-3 activation, and oligonucleosomal DNA fragmentation. However, pharmacologic inactivation of caspases by the pan-caspase inhibitor BOC-D-fmk, or specific caspase-3 inhibition by Z-DEVD-fmk, was not sufficient to prevent cell death, as assessed by loss of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, lactate dehydrogenase release, disruption of mitochondrial transmembrane potential (Deltapsi(m)), and ATP depletion. We found that 4HPR causes lysosomal membrane permeabilization and cytosolic relocation of cathepsin D. Pepstatin A partially rescued cell viability and reduced DNA fragmentation and cytosolic cytochrome c. The antioxidant N-acetylcysteine attenuated cathepsin D relocation into the cytosol, suggesting that lysosomal destabilization is dependent on elevation of reactive oxygen species and precedes mitochondrial dysfunction. Activation of AKT, which regulates energy level in the cell, by the retinal survival facto]r insulin-like growth factor I was impaired and insulin-like growth factor I was ineffective against ATP and Deltapsi(m) loss in the presence of 4HPR. Lysosomal destabilization, associated with mitochondrial dysfunction, was induced by 4HPR also in other cancer cell lines, including PC3 prostate adenocarcinoma and the vascular tumor Kaposi sarcoma KS-Imm cells. The novel finding of a lysosome-mediated cell death pathway activated by 4HPR could have implications at clinical level for the development of combination chemoprevention and therapy of cancer.

Mechanisms of fenretinide-induced apoptosis

Fenretinide, a synthetic retinoid, has emerged as a promising anticancer agent based on numerous in vitro and animal studies, as well as chemoprevention clinical trials. In vitro observations suggest that the anticancer activity of fenretinide may arise from its ability to induce apoptosis in tumor cells. Diverse signaling molecules including reactive oxygen species, ceramide, and ganglioside GD3 can mediate apoptosis induction by fenretinide in transformed, premalignant, and malignant cells. In many cell types, these signaling intermediates appear to be induced by mechanisms that are independent of retinoic acid receptor activation, and ultimately initiate the intrinsic or mitochondrial-mediated pathway of cell elimination. Numerous investigations conducted during the past 10 years have discovered a great deal about the apoptogenic activity of fenretinide. In this review we explore the mechanisms associated with fenretinide-induced apoptosis and highlight certain mechanistic underpinnings of fenretinide-induced cell death that remain poorly understood and thus warrant further characterization.

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

Phase I trial of oral fenretinide in children with high-risk solid tumors: a report from the Children's Oncology Group (CCG 09709)

PURPOSE

To determine the maximal tolerated dosage (MTD) of oral fenretinide given as intact capsules for 7 days, repeated every 21 days, in children with high-risk solid tumors.

METHODS

Children 21 years of age or younger received daily doses from 350 mg/m2 to 3,300 mg/m2 (divided into two or three doses), with pharmacokinetics during course one. The MTD was defined as zero to one of six patients with dose-limiting toxicity (DLT), with at least two of three or two of six DLT at next higher dose.

RESULTS

Fifty-four patients, age 2 years to 20 years (median, 9 years), were treated: neuroblastoma (n = 39), Ewing sarcoma (n = 5), and other (n = 10). Prior therapy included autologous stem cell transplantation (n = 42), 13-cis-RA (n = 35), and 9-cis-RA (n = 1). One of four patients at 1,050 mg/m2 with prior liver transplant had grade 3 ALT/abdominal pain/nausea/dehydration and grade 4 AST/emesis. At 1,860 mg/m2, one of seven patients had grade 3 hypoalbuminemia/hypophosphatemia. At 2,475 mg/m2, one of eight patients had grade 3 alkaline phosphatase; three of five patients had DLT at 3,300 mg/m2: grade 3 AST/ALT (n = 1), grade 4 bilirubin/grade 3 AST/ALT (n = 1), pseudotumor cerebri (n = 1). Pseudotumor cerebri also occurred at 600 mg/m2 and 800 mg/m2. There was one complete response and 13 patients with stable disease (SD) for 8 or more courses in 30 assessable neuroblastoma patients. SD for 8 or more courses was seen in one of five Ewing sarcoma patients and one melanoma patient. Mean N-4-hydroxyphenyl retinamide plasma level (day 7, steady-state concentration) was 9.9 mumol/L at MTD.

CONCLUSION

The pediatric MTD of oral capsular fenretinide was 2,475 mg/m2 per day, which achieved levels active against neuroblastoma in vitro with minimal toxicity. Response data support a phase II trial in neuroblastoma.

New insights on the use of desipramine as an inhibitor for acid ceramidase

Treatment of different cancer cell lines with desipramine induced a time- and dose-dependent downregulation of acid ceramidase. Desipramine's effect on acid ceramidase appeared specific for amphiphilic agents (desipramine, chlorpromazine, and chloroquine) but not other lysomotropic agents such as ammonium chloride and bafilomycin A1, and was not transcriptionally regulated. The cathepsin B/L inhibitor, CA074ME, but not the cathepsin D inhibitor, pepstatin A, blocked desipramine's effect on acid ceramidase. Desipramine led to a more pronounced downregulation of sphingosine compared to ceramide suggesting acid ceramidase inhibition is important to desipramine's mechanism of action. This study reveals a new mechanism of action for desipramine.

Inhibition of IL-8 release from CFTR-deficient lung epithelial cells following pre-treatment with fenretinide

Cystic fibrosis (CF) is characterized by a biochemical abnormality in the cystic fibrosis transmembrane conductance regulator (CFTR) channel. CFTR-deficient lung epithelial cells may have high constitutive glutathione (GSH) levels that could decrease the intracellular content of the sphingolipid second messenger, ceramide. Altered ceramide levels in CF cells could, in turn, lead to their resistance to apoptosis and an immune hyper-responsiveness. As fenretinide is a ceramide up-regulating drug that inhibits the activation of the pro-inflammatory transcriptional factor, nuclear factor (NF)-kappaB, the impact of fenretinide on unstimulated and tumor necrosis factor (TNF)-alpha stimulated production of NF-kappaB-dependent interleukin (IL)-8 was studied in immortalized wild-type (non-CF; 9HTEo-) and mutant DeltaF508 CFTR (CF; CFTE29o-) tracheal epithelial cells. Despite higher constitutive levels of GSH in CF cells, their intracellular ceramide content showed a greater enhancement following fenretinide and TNF-alpha treatment than non-CF cells. Clinically relevant concentrations of fenretinide (1.25, 2.5 and 5 microM) inhibited TNF-alpha-induced IL-8 production of CF cells by up to 73% but had no effect or increased the IL-8 production in non-CF cells. Although fenretinide treatment was associated with a higher intracellular ceramide content in the mutant DeltaF508 CFTR cells, the fenretinide-mediated decrease in IL-8 secretion was not consistently explained by changes in the intracellular content of this sphingolipid. Fenretinide was ineffective in increasing the susceptibility to apoptosis in CF cells whereas non-CF cells were sensitive to the apoptosis induced by both fenretinide and cisplatin exposure. The fenretinide mediated decrease in IL-8 release in CF cells under TNF-alpha stimulated conditions presents the possibility that the lung inflammation in CF could be attenuated via low dose fenretinide treatment.

Targeting cyclin D1, a downstream effector of INI1/hSNF5, in rhabdoid tumors

Rhabdoid tumors (RTs) are aggressive and currently incurable pediatric malignancies. INI1/hSNF5 is a tumor suppressor biallelically inactivated in RTs. Our previous studies have indicated that cyclin D1 is a key downstream target of INI1/hSNF5 and genesis and/or survival of RTs in vivo is critically dependent on the presence of cyclin D1. In this report, we have tested the hypothesis that therapeutic targeting of cyclin D1 is an effective means of treating RTs. We found that RNA interference of cyclin D1 in rhabdoid cells was sufficient to induce G1 arrest and apoptosis. Furthermore, we found that pharmacological intervention with low micromolar concentrations of N-(4-hydroxyphenyl)retinamide (4-HPR), which downmodulates cyclin D1, induced G1 arrest and apoptosis in rhabdoid cell lines. 4-HPR in combination with 4-hydroxy-tamoxifen (4OH-Tam), synergistically inhibited survival as well as anchorage-dependent and -independent growth of rhabdoid cells and caused synergistic induction of cell cycle arrest and apoptosis. 4-HPR and tamoxifen exhibited synergistic growth inhibition of RTs in xenograft models in vivo. The effects of combination of drugs were correlated to the depletion of cyclin D1 levels both in in vitro and in vivo tumor models. These results demonstrate that 4-HPR and tamoxifen are effective chemotherapeutic agents for RTs. We propose that downmodulation of cyclin D1 is a novel and effective therapeutic strategy for RTs.

Human T-cell lymphotropic virus type I-transformed T-cells have a partial defect in ceramide synthesis in response to N-(4-hydroxyphenyl)retinamide

Treatment with the synthetic retinoid HPR [N-(4-hydroxyphenyl)-retinamide] causes growth arrest and apoptosis in HTLV-I (human T-cell lymphotropic virus type-I)-positive and HTLV-I-negative malignant T-cells. It was observed that HPR-mediated growth inhibition was associated with ceramide accumulation only in HTLV-I-negative cells. The aim of the present study was to investigate the mechanism by which HPR differentially regulates ceramide metabolism in HTLV-I-negative and HTLV-I-positive malignant T-cells. Clinically achievable concentrations of HPR caused early dose-dependent increases in ceramide levels only in HTLV-I-negative cells and preceded HPR-induced growth suppression. HPR induced de novo synthesis of ceramide in HTLV-I-negative, but not in HTLV-I-positive, cells. Blocking ceramide glucosylation in HTLV-I-positive cells, which leads to accumulation of endogenous ceramide, rendered these cells more sensitive to HPR. Exogenous cell-permeant ceramides that function partially by generating endogenous ceramide induced growth suppression in all tested malignant lymphocytes, were consistently found to be less effective in HTLV-I-positive cells confirming their defect in de novo ceramide synthesis. Owing to its multipotent activities, the HTLV-I-encoded Tax protein was suspected to inhibit ceramide synthesis. Tax-transfected Molt-4 and HELA cells were less sensitive to HPR and C6-ceramide mediated growth inhibition respectively and produced lower levels of endogenous ceramide. Together, these results indicate that HTLV-I-positive cells are defective in de novo synthesis of ceramide and that therapeutic modalities that bypass this defect are more likely to be successful.

Wood smoke extract induces oxidative stress-mediated caspase-independent apoptosis in human lung endothelial cells: role of AIF and EndoG

Although a link between toxic smoke and oxidant lung vascular injury has been indicated, the cellular mechanisms of smoke-induced injury to lung endothelial cells are unknown. We investigated oxidative stress and apoptosis induced by wood smoke extract (SE) in human pulmonary artery endothelial cells (HPAECs) and delineated their relationship. We found that SE increased intracellular reactive oxygen species (ROS), depleted intracellular glutathione, and upregulated Cu/Zn superoxide dismutase and heme oxygenase-1 (2 antioxidant enzymes), but it failed to alter the expression of catalase and glutathione peroxidase. In addition, SE promoted apoptosis as indicated by the external exposure of membrane phosphatidylserine, the loss of mitochondrial membrane potential, an increase in the level of Bax (a proapoptotic protein), and enhanced DNA fragmentation. This apoptosis was associated with mitochondrial-to-nuclear translocation of apoptosis-inducing factor (AIF) and endonuclease G (EndoG) (2 apoptogenic proteins) but was independent of caspase cascade activation. Whereas N-acetylcysteine (an antioxidant) effectively reversed the SE-induced increase in ROS and depletion of glutathione, it also suppressed SE-induced nuclear translocation of either AIF or EndoG and prevented the enhanced DNA fragmentation that would have resulted from this. We conclude that 1) although SE upregulates Cu/Zn superoxide dismutase and heme oxygenase-1, it nevertheless increases intracellular oxidative stress in HPAECs, and 2) SE promotes oxidative stress-mediated caspase-independent HPAEC apoptosis that involves mitochondrial-to-nuclear translocation of AIF and EndoG. Thus modulations of the expression of antioxidant enzymes and the caspase-independent apoptotic pathway are possible target choices for potential therapeutic regimes to treat smoke-induced lung injury.

Inhibitory effects of N-(4-hydrophenyl) retinamide on liver cancer and malignant melanoma cells

AIM: To investigate the effect of N-(4-hydrophenyl) retinamide (4-HPR), the derivative of retinoic acid, on inhibition of migration, invasion, cell growth, and induction of apoptosis in hepatocellular carcinoma cells (HCCs) and malignant melanoma cells.

METHODS: 4-HPR was chemically synthesized. Cellular migration and invasion were assayed by Borden chamber experiment. Cell growth was assayed by MTT chromometry. Apoptosis effect was measured using Hoechst 32258 staining and flow cytometry. Gene transfection was performed with lipofectamine.

RESULTS: We observed that the migration of HCC and melanoma cells was significantly suppressed by 4-HPR and the migration cells were reduced to 585.03 (control 20127.2, P < 0.05, n = 4) in SMMC 7721-k3 HCC, and to 25425.04 (control 30230.1, P < 0.05, n = 4) in melanoma cells after 6-h incubation with 4-HPR. The invasion through reconstituted basement membrane was also significantly reduced by 4-HPR treatment to 11.23.3 in SMMC 7721-k3 HCC (control 2713.1), and to 24.33.2 in melanoma cells (control 67.510.1, P < 0.05, n = 3). Cell growth, especially in melanoma cells, was also significantly inhibited. Furthermore, 3 mmol/L of 4-HPR induced apoptosis in B16 melanoma cells (37.110.94%) more significantly than all-trans retinoic acid (P < 0.05), but it failed to induce apoptosis in SMMC 7721-k3 HCC. The mechanism for 4-HPR-induced apoptosis was not clear, but we observed that 4-HPR could regulate p27kip1, and overexpression of cerebroside sulfotransferase (CST) diminished the apoptosis induced by 4-HPR in melanoma cells.

CONCLUSION: 4-HPR is a potent inhibitor of HCC migration and inducer of melanoma cell apoptosis. CST and p27kip1expression might be associated with 4-HPR-induced apoptosis.

The role of gangliosides in fenretinide-induced apoptosis of neuroblastoma

Fenretinide is thought to induce apoptosis via increases in ceramide levels but the mechanisms of ceramide generation and the link between ceramide and subsequent apoptosis in neuroblastoma cells is unclear. In SH-SY5Y neuroblastoma cells, evidence suggests that acid sphingomyelinase activity is essential for the induction of ceramide and apoptosis in response to fenretinide. Downstream of ceramide, apoptosis in response to fenretinide is mediated by increased glucosylceramide synthase activity resulting in increased levels of gangliosides GD3 and GD2 via GD3 synthase. GD3 is a key signalling intermediate leading to apoptosis via the activation of 12-Lipoxygenase, and the parallel induction of GD2 suggests that fenretinide might enhance the response of neuroblastoma to therapy with anti-GD2 antibodies.

Systemic delivery of liposomal short-chain ceramide limits solid tumor growth in murine models of breast adenocarcinoma

In vitro tumor cell culture models have illuminated the potential therapeutic utility of elevating the intracellular concentration of the antimitogenic and proapoptotic sphingolipid, ceramide. However, although cell-permeable, short-chain ceramide is an effective apoptotic agent in vitro, its use as an in vivo, systemically delivered therapeutic is limited by its inherent lipid hydrophobicity and physicochemical properties. Here, we report that the systemic i.v. delivery of C6-ceramide (C6) in a pegylated liposomal formulation significantly limited the growth of solid tumors in a syngeneic BALB/c mouse tumor model of breast adenocarcinoma. Over a 3-week treatment period, a well-tolerated dose of 36 mg/kg liposomal-C6 elicited a >6-fold reduction in tumor size compared with empty ghost liposomes. Histologic analyses of solid tumors from liposomal-C6-treated mice showed a marked increase in the presence of apoptotic cells, with a coincident decrease in cellular proliferation and in the development of a microvessel network. Liposomal-C6 accumulated within caveolae and mitochondria, suggesting putative mechanisms by which ceramide induces selective cancer cell cytotoxicity. A pharmacokinetic analysis of systemic liposomal-C6 delivery showed that the pegylated liposomal formulation follows first-order kinetics in the blood and achieves a steady-state concentration in tumor tissue. Confirming the therapeutic utility of i.v. liposomal-C6 administration, we also shown diminution of solid tumor growth in a human xenograft model of breast cancer. Together, these results indicate that bioactive ceramide analogues can be incorporated into pegylated liposomal vehicles for improved solubility, drug delivery, and antineoplastic efficacy.