N-(4-hydroxyphenyl)retinamide increases ceramide and is cytotoxic to acute lymphoblastic leukemia cell lines, but not to non-malignant lymphocytes

Liquid chromatography method for quantifying D-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (D-threo-PPMP) in mouse plasma and liver

A high-performance liquid chromatography (HPLC) method was developed to measure levels of d-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (d-threo-PPMP) in mouse plasma and liver. d-threo-PPMP was measured by HPLC with a Luna Pheny-Hexyl column (5 microm, 250 mm x 4.6 mm) employing UV detection at 210 nm using a mobile phase of potassium phosphate buffer (20mM, pH 3.0)-acetonitrile in a 45:55 (v/v) ratio. d-threo-1-phenyl-2-pentadecanoylamino-3-morpholino-1-propanol (PC15MP) was employed as an internal standard (IS). The lower limit of quantitation (LLOQ) was 0.3 microg/ml. The assay was linear over a concentration range of 0.3-10 microg/ml, with acceptable precision and accuracy. Assayed in plasma, the intra- and inter-day validation for all coefficients of variation (R.S.D.%) were found less than 15%. The method was applied to samples from athymic (nu/nu) mice treated with d-threo-PPMP by intraperitoneal injection. d-threo-PPMP levels of approximately 10-20 microg/ml ( approximately 20-40 microM) in plasma and approximately 45 microg/g in liver were obtained. The present method can be used to quantify d-threo-PPMP in mice for bioavailability and dose-response studies.

Anticancer effects of fenretinide in human medulloblastoma

N-(4-hydroxyphenyl) retinamide (4-HPR, fenretinide) a synthetic retinoid is in clinical trials for the treatment of several malignancies. However, its biological effects and therapeutic value in childhood brain tumor medulloblastoma (MB) has not been investigated. In this study, we report for the first time that fenretinide (2.5-10 microM) induces apoptotic cell death in human MB cells. We observed significant inhibition of cell survival in four MB cell lines (D425MED, D458MED, D283MED and D341MED) as determined by MTT assays. These results were further supported by inhibition of anchorage-independent colony formation in soft agar. Fenretinide-induced decrease in cell viability was in part due to activation of caspase-3 dependent cell death, which was further supported by the cleavage of poly(ADP-ribose) polymerase-1 (PARP-1), a caspase-3 substrate. Cell death was partially prevented by the antioxidant, l-ascorbic acid suggesting that free radical intermediates might be involved in fenretinide effects. These results suggest that pharmacologically achievable concentrations of fenretinide are effective in killing MB cells and thus show its therapeutic potential to treat human MB.

Hypoxia-mediated fenretinide (4-HPR) resistance in childhood acute lymphoblastic leukemia cells

PURPOSES

N-(4-Hydroxyphenyl)-retinamide (4-HPR, Fenretinide) is a synthetic retinoid with cytotoxicity in acute lymphoblastic leukemia (ALL) cell lines. Since ALL is a disease of the bone marrow, a hypoxic tissue compartment, and it has been reported that there is an antagonistic effect of hypoxia on many chemotherapeutic agents, our purpose was to observe whether hypoxia is able to inhibit the effect of 4-HPR for ALL cell lines and to investigate its mechanisms of antagonism to 4-HPR.

METHODS

Cytotoxicity was measured by MTT method, and apoptosis was measured by flow cytometry. Mitochondrial membrane potential (DeltaPsim) was detected by JC1 staining and flow cytometry. Protein expression was analyzed by western blotting.

RESULTS

Hypoxia (2% O2) induced 4-HPR resistance in the tested two ALL cell lines (Molt-4 and Molt-3), with at least a 2.8-fold increase in IC50 values (P<0.01) compared with the IC50 values in normoxia (20% O2). Apoptotic detection showed that 2% O2 significantly suppressed 4-HPR-induced apoptosis and the percentages of 4-HPR-induced apoptotic cells at 12 and 24 h were 1.2 and 11.0%, respectively, compared with 12.6 and 76.3% in 20% O2. In addition, in 20% O2, but not in 2% O2, 4-HPR obviously downregulated the protein expression of procaspase-3, ERK1/2 and XIAP, and increased the cleavage of PARP. Also, a significant DeltaPsim loss in response to 4-HPR was observed in normoxia, but not in hypoxia.

CONCLUSIONS

Hypoxia is able to induce 4-HPR resistance in Molt-4 cells and the mechanism may be involved in the inhibition of 4HPR-induced DeltaPsim depolarization and regulation of mitochondrial pathway-related proteins associated in signaling apoptosis.

Effects of ceramide inhibition on radiation-induced apoptosis in human leukemia MOLT-4 cells

In the present study, using inhibitors of ceramide synthase (fumonisin B1), ketosphinganine synthetase (L-cycloserine), acid sphingomyelinase (D609 and desipramine) and neutral sphingomyelinase (GW4869), the role of ceramide in X-ray-induced apoptosis was investigated in MOLT-4 cells. The diacylglycerol kinase (DGK) assay showed that the intracellular concentration of ceramide increased time-dependently after X irradiation of cells, and this radiation-induced accumulation of ceramide did not occur prior to the appearance of apoptotic cells. Treatment with D609 significantly inhibited radiation-induced apoptosis, but did not inhibit the increase of intracellular ceramide. Treatment with desipramine or GW4869 prevented neither radiation-induced apoptosis nor the induced increase of ceramide. On the other hand, fumonisin B1 and L-cycloserine had no effect on the radiation-induced induction of apoptosis, in spite of significant inhibition of the radiation-induced ceramide. From these results, it was suggested that the increase of the intracellular concentration of ceramide was not essential for radiation-induced apoptosis in MOLT-4 cells.

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.

β-Sitosterol stimulates ceramide metabolism in differentiated Caco2 cells

Previous studies from our laboratory on tumor cells suggest that phytosterols stimulate ceramide production, which was associated with cell growth inhibition and stimulation of apoptosis. The objective of the present study was to examine the effect of phytosterols on ceramide metabolism in small intestinal cells that represent the first cells in contact with dietary phytosterols. Caco(2) cells, an accepted model for human intestinal epithelial cells, were used in this study. Ceramide and ceramide-containing lipids were examined by labeling the ceramide pool with (3)H-serine. Cells were supplemented with 16 microM of sterols (cholesterol, beta-sitosterol or campesterol) for 16 days postconfluence and continued to differentiate. Of the two phytosterols, beta-sitosterol, but not campesterol, induced more than double the serine labeling when compared with cholesterol. This increase was uniform in sphingomyelin (SM), ceramide and sphingosine labeling. Sterols had no effect on SM concentration in the cells. In addition, sterol had no effect on the activity of SM synthase or sphingomyelinases. There was an inhibition of ceramidases with campesterol supplementation. These data suggest that the observed increases in SM and sphingosine labeling were due to an increase in ceramide turnover. The increase in ceramide turnover with beta-sitosterol supplementation was not associated with growth inhibition but was with increases in ceramide glycosylation products such as cerebrosides and gangliosides. It was concluded that beta-sitosterol has no effect on differential Caco(2), a model of normal small intestinal cells. The increase in the glycosylated ceramide products may offer a means to protect the cells from the harmful effect of ceramide by excreting them with lipoproteins.

N-(4-Hydroxyphenyl)Retinamide Inhibits Invasion, Suppresses Osteoclastogenesis, and Potentiates Apoptosis through Down-regulation of IκBα Kinase and Nuclear Factor-κB–Regulated Gene Products

N-(4-hydroxyphenyl) retinamide [4-HPR], a synthetic retinoid, has been shown to inhibit tumor cell growth, invasion, and metastasis by a mechanism that is not fully understood. Because the nuclear factor-kappaB (NF-kappaB) has also been shown to regulate proliferation, invasion, and metastasis of tumor cells, we postulated that 4-HPR modulates the activity of NF-kappaB. To test this postulate, we examined the effect of this retinoid on NF-kappaB and NF-kappaB-regulated gene products. We found that 4-HPR potentiated the apoptosis induced by tumor necrosis factor (TNF) and chemotherapeutic agents, suppressed TNF-induced invasion, and inhibited RANKL-induced osteoclastogenesis, all of which are known to require NF-kappaB activation. We found that 4-HPR suppressed both inducible and constitutive NF-kappaB activation without interfering with the direct DNA binding of NF-kappaB. 4-HPR was found to be synergistic with Velcade, a proteasome inhibitor. Further studies showed that 4-HPR blocked the phosphorylation and degradation of IkappaBalpha through the inhibition of activation of IkappaBalpha kinase (IKK), and this led to suppression of the phosphorylation and nuclear translocation of p65. 4-HPR also inhibited TNF-induced Akt activation linked with IKK activation. NF-kappaB-dependent reporter gene expression was also suppressed by 4-HPR, as was NF-kappaB reporter activity induced by TNFR1, TRADD, TRAF2, NIK, and IKK but not that induced by p65 transfection. The expression of NF-kappaB-regulated gene products involved in antiapoptosis (IAP1, Bfl-1/A1, Bcl-2, cFLIP, and TRAF1), proliferation (cyclin D1 and c-Myc), and angiogenesis (vascular endothelial growth factor, cyclooxygenase-2, and matrix metalloproteinase-9) were also down-regulated by 4-HPR. This correlated with potentiation of apoptosis induced by TNF and chemotherapeutic agents.

Fenretinide Cytotoxicity for Ewing’s Sarcoma and Primitive Neuroectodermal Tumor Cell Lines Is Decreased by Hypoxia and Synergistically Enhanced by Ceramide Modulators

Patients with disseminated Ewing's family of tumors (ESFT) often experience drug-resistant relapse. We hypothesize that targeting minimal residual disease with the cytotoxic retinoid N-(4-hydroxyphenyl) retinamide (4-HPR; fenretinide) may decrease relapse. We determined the following: (a) 4-HPR cytotoxicity against 12 ESFT cell lines in vitro; (b) whether 4-HPR increased ceramide species (saturated and desaturated ceramides); (c) whether physiological hypoxia (2% O(2)) affected cytotoxicity, mitochondrial membrane potential (DeltaPsi(m)) change, or ceramide species or reactive oxygen species levels; (d) whether cytotoxicity was enhanced by l-threo-dihydrosphingosine (safingol); (e) whether physiological hypoxia increased acid ceramidase (AC) expression; and (f) the effect of the AC inhibitor N-oleoyl-ethanolamine (NOE) on cytotoxicity and ceramide species. Ceramide species were quantified by thin-layer chromatography and scintillography. Cytotoxicity was measured by a fluorescence-based assay using digital imaging microscopy (DIMSCAN). Gene expression profiling was performed by oligonucleotide array analysis. We observed, in 12 cell lines tested in normoxia (20% O(2)), that the mean 4-HPR LC(99) (the drug concentration lethal to 99% of cells) = 6.1 +/- 5.4 microm (range, 1.7-21.8 microm); safingol (1-3 microm) synergistically increased 4-HPR cytotoxicity and reduced the mean 4-HPR LC(99) to 3.2 +/- 1.7 microm (range, 2.0-8.0 microm; combination index < 1). 4-HPR increased ceramide species in the three cell lines tested (up to 9-fold; P < 0.05). Hypoxia (2% O(2)) reduced ceramide species increase, DeltaPsi(m) loss, reactive oxygen species increase (P < 0.05), and 4-HPR cytotoxicity (P = 0.05; 4-HPR LC(99), 19.7 +/- 23.9 microm; range, 2.3-91.4). However, hypoxia affected 4-HPR + safingol cytotoxicity to a lesser extent (P = 0.04; 4-HPR LC(99), 4.9 +/- 2.3 microm; range, 2.0-8.2). Hypoxia increased AC RNA expression; the AC inhibitor NOE enhanced 4-HPR-induced ceramide species increase and cytotoxicity. The antioxidant N-acetyl-l-cysteine somewhat reduced 4-HPR cytotoxicity but did not affect ceramide species increase. We conclude the following: (a) 4-HPR was active against ESFT cell lines in vitro at concentrations achievable clinically, but activity was decreased in hypoxia; and (b) combining 4-HPR with ceramide modulators synergized 4-HPR cytotoxicity in normoxia and hypoxia.

Stage-specific effect of N-(4-hydroxyphenyl)retinamide on cell growth in squamous cell carcinogenesis

Squamous cell carcinoma (SCC) is the most prevalent form of epithelial cancer. SCC results when normal epithelial cells undergo multiple neoplastic changes that culminate in the evolution of an invasive cancer. Retinoids are commonly used as chemopreventive and treatment agents in skin cancer; however, SCC progression is accompanied by a gradual loss of retinoid responsiveness. The synthetic retinoid N-(4-hydroxyphenyl)retinamide (HPR) has shown promising anti-neoplastic activity in a variety of tumor cells, including those that are resistant to all-trans retinoic acid (t-RA). We investigated the effect of HPR on growth and apoptosis of squamous cells at different stages of carcinogenesis. We then determined if retinoic acid receptor (RAR) overexpression affected the outcome of HPR treatment. To model SCC malignant progression, we used a panel of murine keratinocytes representing different stages of squamous cell carcinogenesis. This panel consisted of primary keratinocytes, SP1 and 308 papilloma cell lines, the PAM-212 squamous carcinoma cell line, and the spindle I7 cell line. With the exception of the primary keratinocytes, all cells were unresponsive to t-RA treatment. Pharmacological concentrations of HPR were non-cytotoxic to all keratinocytes tested and HPR sensitivity was stage-dependent, with the papilloma cell lines being the most sensitive, and the spindle cells being the most resistant. Overexpression of RARgamma in SP1 papilloma cells enhanced growth suppression and apoptosis induction by HPR. HPR-induced growth suppression was accompanied by a simultaneous block in the G(1) phase of the cell cycle in RAR-transduced and control SP1 cells and differential regulation of cell cycle and apoptotic mediators.

Overexpression of glucosylceramide synthase and P-glycoprotein in cancer cells selected for resistance to natural product chemotherapy

Resistance to natural product chemotherapy drugs is a major obstacle to successful cancer treatment. This type of resistance is often acquired in response to drug exposure; however, the mechanisms of this adverse reaction are complex and elusive. Here, we have studied acquired resistance to Adriamycin, Vinca alkaloids, and etoposide in MCF-7 breast cancer cells, KB-3-1 epidermoid carcinoma cells, and other cancer cell lines to determine if there is an association between expression of glucosylceramide synthase, the enzyme catalyzing ceramide glycosylation to glucosylceramide, and the multidrug-resistant (MDR) phenotype. This work shows that glucosylceramide levels increase concomitantly with increased drug resistance in the KB-3-1 vinblastine-resistant sublines KB-V.01, KB-V.1, and KB-V1 (listed in order of increasing MDR). The levels of glucosylceramide synthase mRNA, glucosylceramide synthase protein, and P-glycoprotein (P-gp) also increased in parallel. Increased glucosylceramide levels were also present in Adriamycin-resistant KB-3-1 sublines KB-A.05 and KB-A1. In breast cancer, detailed analysis of MCF-7 wild-type and MCF-7-AdrR cells (Adriamycin-resistant) demonstrated enhanced glucosylceramide synthase message and protein, P-gp message and protein, and high levels of glucosylceramide in resistant cells. Similar results were seen in vincristine-resistant leukemia, etoposide-resistant melanoma, and Adriamycin-resistant colon cancer cell lines. Cell-free glucosylceramide synthase activity was higher in lysates obtained from drug-resistant cells. Lastly, glucosylceramide synthase promoter activity was 15-fold higher in MCF-7-AdrR compared with MCF-7 cells. We conclude that selection pressure for resistance to natural product chemotherapy drugs selects for enhanced ceramide metabolism through glucosylceramide synthase in addition to enhanced P-gp expression. A possible connection between glucosylceramide synthase and P-gp in drug resistance biology is suggested.

Ceramide synthesis and metabolism as a target for cancer therapy

Sphingolipids, which include ceramides and sphingosine, are essential structural components of cell membranes that also have messenger functions that regulate the proliferation, survival, and death of cells. Exogenous application of ceramide is cytotoxic, and exposure of cells to radiation or chemotherapy is associated with increased ceramide levels due to enhanced de novo synthesis, catabolism of sphingomyelin, or both. Ceramide can be metabolized to less toxic forms by glycosylation, acylation, or by catabolism to sphingosine, which is then phosphorylated to the anti-apoptotic sphingosine 1-phosphate. Glucosylceramide synthase overexpression has been shown to enhance resistance to doxorubicin, suggesting that inhibition of ceramide metabolism or catabolism might enhance cancer chemotherapy. Several anticancer agents, including the cytotoxic retinoid, fenretinide (4-HPR), have been shown to act, at least in part, by increasing tumor cell ceramide via de novo synthesis. Combinations of 4-HPR and modulators of ceramide action and/or metabolism demonstrated increased anti-tumor activity in pre-clinical models with minimal toxicity for non-malignant cells, and were effective in a p53-independent manner against tumor cell lines resistant to standard cytotoxic agents. Phase I trials of ceramide metabolism inhibitors in combination with 4-HPR and with other cytotoxic agents are in development. Thus, pharmacological manipulation of sphingolipid metabolism to enhance tumor cell ceramide is being realized and offers a novel approach to cancer chemotherapy.

N-(4-hydroxyphenyl)retinamide induces growth arrest and apoptosis in HTLV-I-transformed cells

N-(4-hydroxyphenyl)retinamide (HPR) is a synthetic retinoid that inhibits growth and induces apoptosis in many human cell lines. We explored the effects of HPR on human T-cell lymphotropic virus type I (HTLV-I)-positive and HTLV-I-negative malignant T-cell lines, most of which are resistant to all-trans retinoic acid. Clinically achievable concentrations of HPR caused a dramatic inhibition of cell proliferation, G(0)/G(1) arrest, and massive apoptosis in all tested malignant T cells, while no effect was observed on resting or activated normal lymphocytes. Interestingly, HTLV-I-negative cell lines were significantly more sensitive to HPR compared to HTLV-I-positive and Tax-transfected cells. In HTLV-I-negative cells only, HPR-induced apoptosis was associated with ceramide accumulation, sharp decrease in mitochondrial membrane potential, and activation of caspases 8, 9 and 3, and could be partially reverted by the caspase inhibitor z-VAD suggesting that Tax protects infected cells from ceramide accumulation and caspase-mediated apoptosis. In HTLV-I-positive cells, HPR treatment rapidly induced proteasomal-mediated degradation of p21, downregulated cyclin D(1), and upregulated bax protein levels. These findings support a potential therapeutic role for HPR in both HTLV-I-associated adult T-cell leukemia/lymphoma (ATL) and HTLV-I-negative peripheral T-cell lymphomas.

N-(4-Hydroxyphenyl)retinamide (4-HPR) induces leukemia cell death via generation of reactive oxygen species

The role of reactive oxygen species (ROS) in the cytotoxicity of N-(4-hydroxyphenyl)retinamide (4-HPR) was studied with use of the B-precursor lymphoblastic leukemia cell line YCUB-2. The increase in intracellular ROS measured with 2'-7'-dichlorodihydrofluorescein diacetate after 3 hours' incubation was 3.7-fold with 1 microM 4-HPR and 5.8-fold with 5 microM 4-HPR. The rate of apoptosis after 48 hours' incubation was 9.8% and 56.4% in comparison with untreated cells. Hydroethidine, which is a more specific indicator of superoxide anion radical level, did not effectively detect 4-HPR-induced ROS. The antioxidant 3-methyl-1-phenyl-2-pyrazolin-5-one suppressed 4-HPR-induced ROS production and apoptosis. The cytotoxicity of 4-HPR was analyzed in 4 other leukemia/lymphoma lines (CCRF-HSB2, Molt-4, KG-1, HL-60). We found that the cytotoxicity of 4-HPR correlated with the amount of ROS produced in cell lines, except in HL-60 cells. The intracellular glutathione level varied among the 5 cell lines, the highest levels occurring in Molt-4 and KG-1, which were less sensitive to 4-HPR. Suppression of glutathione by buthionine sulfoximine enhanced the level of 4-HPR-induced ROS production and apoptosis in Molt-4. Our findings suggest that ROS play a significant role in the antileukemia effect of 4-HPR and that the glutathione level in leukemias may be associated the sensitivity of the cells to 4-HPR.

Hydrolysis of 4-HPR to atRA occurs in vivo but is not required for retinamide-induced apoptosis

The retinamide, N-(4-hydroxyphenyl)retinamide (4-HPR), has shown promising anti-tumor activity, but it is unclear whether this compound is hydrolyzed to all-trans retinoic acid (atRA) and if so, whether this plays any role in its chemotherapeutic activity. To address this issue, the ability of 4-hydroxybenzylretinone (4-HBR), a carbon-linked analog of 4-HPR, to support growth in vitamin A-deficient (VAD) animals and to activate an atRA-responsive gene in vivo was compared to 4-HPR and atRA. Further, the non-hydrolyzable 4-HBR analog was used to determine whether the presence of the labile amide linkage in 4-HPR is essential for the induction of apoptosis in cultured MCF-7 breast cancer cells. Studies in VAD rats showed that 4-HPR, like atRA, supports animal growth and induces CYP26B1 mRNA expression in lung whereas 4-HBR does not. Analysis of plasma from 4-HPR- and atRA-treated VAD animals revealed the presence of atRA whereas it was not detected in plasma from animals given 4-HBR. To determine whether hydrolysis to atRA is necessary for apoptosis induced by 4-HPR in MCF-7 breast cancer cells, morphological and biochemical assays for apoptosis were performed. 4-HBR, like 4-HPR, induced apoptosis in MCF-7 cells. Apoptosis was not induced even at high concentrations of atRA, showing that 4-HPR and 4-HBR act in cells via a distinct signaling pathway. These results show that although limited hydrolysis of 4-HPR occurs in vivo, the ability to liberate atRA is not required for these 4-hydroxyphenyl retinoids to induce apoptosis in MCF-7 breast cancer cells. Thus the non-hydrolyzable analog, 4-HBR, may have significant therapeutic advantage over 4-HPR because it does not liberate atRA that can contribute to the adverse side effects of drug administration in vivo.

Killing tumours by ceramide-induced apoptosis: a critique of available drugs

Over 1000 research papers have described the production of programmed cell death (apoptosis) by interventions that elevate the cell content of ceramide (Cer). Other interventions, which lower cellular Cer, have been found to interfere with apoptosis induced by other agents. Some studies have shown that slowing the formation of proliferation-stimulating sphingolipids also induces apoptosis. These relationships are due to the two different aspects of Cer: Cer itself produces apoptosis, but metabolic conversion of Cer into either sphingosine 1-phosphate or glucosphingolipids leads to cell proliferation. The balance between these two aspects is missing in cancer cells, and yet intervention by stimulating or blocking only one or two of the pathways in Cer metabolism is very likely to fail. This results from two properties of cancer cells: their high mutation rate and the preferential survival of the most malignant cells. Tumours treated with only one or two drugs that elevate Cer can adjust the uncontrolled processes to either maintain or to 'aggravate' the excessive growth, angiogenesis and metastasis characteristics of tumours. These treatments might simply elevate the production of growth factors, receptors and other substances that reduce the effectiveness of Cer. Tumour cells that do not adapt in this way undergo apoptosis, leaving the adapted cells free to grow and, ultimately, to 'subdue' their host. Thus it is important to kill every type of cancer cell present in the tumour rapidly and simultaneously, using as many different agents to control as many pathways as possible. To aid this approach, this article catalogues many of the drugs that act on different aspects of Cer metabolism. The techniques described here may lead to the development of practical chemotherapy for cancer and other diseases of excess proliferation.

Altered sphingolipid metabolism in N-(4-hydroxyphenyl)-retinamide-resistant A2780 human ovarian carcinoma cells

In the present work, we studied the effects of fenretinide (N-(4-hydroxyphenyl)retinamide (HPR)), a hydroxyphenyl derivative of all-trans-retinoic acid, on sphingolipid metabolism and expression in human ovarian carcinoma A2780 cells. A2780 cells, which are sensitive to a pharmacologically achievable HPR concentration, become 10-fold more resistant after exposure to increasing HPR concentrations. Our results showed that HPR was able to induce a dose- and time-dependent increase in cellular ceramide levels in sensitive but not in resistant cells. This form of resistance in A2780 cells was not accompanied by the overexpression of multidrug resistance-specific proteins MDR1 P-glycoprotein and multidrug resistance-associated protein, whose mRNA levels did not differ in sensitive and resistant A2780 cells. HPR-resistant cells were characterized by an overall altered sphingolipid metabolism. The overall content in glycosphingolipids was similar in both cell types, but the expression of specific glycosphingolipids was different. Specifically, our findings indicated that glucosylceramide levels were similar in sensitive and resistant cells, but resistant cells were characterized by a 6-fold lower expression of lactosylceramide levels and by a 6-fold higher expression of ganglioside levels than sensitive cells. The main gangliosides from resistant A2780 cells were identified as GM3 and GM2. The possible metabolic mechanisms leading to this difference were investigated. Interestingly, the mRNA levels of glucosylceramide and lactosylceramide synthases were similar in sensitive and resistant cells, whereas GM3 synthase mRNA level and GM3 synthase activity were remarkably higher in resistant cells.

Ceramide signaling in fenretinide-induced endothelial cell apoptosis

Stress stimuli can mediate apoptosis by generation of the lipid second messenger, ceramide. Herein we investigate the molecular mechanism of ceramide signaling in endothelial apoptosis induced by fenretinide (N-(4-hydroxyphenyl)retinamide (4-HPR)). 4-HPR, a synthetic derivative of retinoic acid that induces ceramide in tumor cell lines, has been shown to have antiangiogenic effects, but the molecular mechanism of these is largely unknown. We report that 4-HPR was cytotoxic to endothelial cells (50% cytotoxicity at 2.4 microm, 90% at 5.36 microm) and induced a caspase-dependent endothelial apoptosis. 4-HPR (5 microm) increased ceramide levels in endothelial cells 5.3-fold, and the increase in ceramide was required to achieve the apoptotic effect of 4-HPR. The 4-HPR-induced increase in ceramide was suppressed by inhibitors of ceramide synthesis, fumonisin B(1), myriocin, and l-cycloserine, and 4-HPR transiently activated serine palmitoyltransferase, demonstrating that 4-HPR induced de novo ceramide synthesis. Sphingomyelin levels were not altered by 4-HPR, and desipramine had no effect on ceramide level, suggesting that sphingomyelinase did not contribute to the 4-HPR-induced ceramide increase. Finally, the pancaspase inhibitor, t-butyloxycarbonyl-aspartyl[O-methyl]-fluoromethyl ketone, suppressed 4-HPR-mediated apoptosis but not ceramide accumulation, suggesting that ceramide is upstream of caspases. Our results provide the first evidence that increased ceramide biosynthesis is required for 4-HPR-induced endothelial apoptosis and present a molecular mechanism for its antiangiogenic effects.