Mitochondria inside acute myeloid leukemia cells hydrolyze ATP to resist chemotherapy

Despite early optimism, therapeutics targeting oxidative phosphorylation (OxPhos) have faced clinical setbacks, stemming from their inability to distinguish healthy from cancerous mitochondria. Herein, we describe an actionable bioenergetic mechanism unique to cancerous mitochondria inside acute myeloid leukemia (AML) cells. Unlike healthy cells which couple respiration to the synthesis of ATP, AML mitochondria were discovered to support inner membrane polarization by consuming ATP. Because matrix ATP consumption allows cells to survive bioenergetic stress, we hypothesized that AML cells may resist cell death induced by OxPhos damaging chemotherapy by reversing the ATP synthase reaction. In support of this, targeted inhibition of BCL-2 with venetoclax abolished OxPhos flux without impacting mitochondrial membrane potential. In surviving AML cells, sustained polarization of the mitochondrial inner membrane was dependent on matrix ATP consumption. Mitochondrial ATP consumption was further enhanced in AML cells made refractory to venetoclax, consequential to downregulations in both the proton-pumping respiratory complexes, as well as the endogenous F1-ATPase inhibitor ATP5IF1. In treatment-naive AML, ATP5IF1 knockdown was sufficient to drive venetoclax resistance, while ATP5IF1 overexpression impaired F1-ATPase activity and heightened sensitivity to venetoclax. Collectively, our data identify matrix ATP consumption as a cancer-cell intrinsic bioenergetic vulnerability actionable in the context of mitochondrial damaging chemotherapy.

Acid Ceramidase Inhibitor LCL-805 Antagonizes Akt Signaling and Promotes Iron-Dependent Cell Death in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy requiring urgent treatment advancements. Ceramide is a cell-death-promoting signaling lipid that plays a central role in therapy-induced cell death. We previously determined that acid ceramidase (AC), a ceramide-depleting enzyme, is overexpressed in AML and promotes leukemic survival and drug resistance. The ceramidase inhibitor B-13 and next-generation lysosomal-localizing derivatives termed dimethylglycine (DMG)-B-13 prodrugs have been developed but remain untested in AML. Here, we report the in vitro anti-leukemic efficacy and mechanism of DMG-B-13 prodrug LCL-805 across AML cell lines and primary patient samples. LCL-805 inhibited AC enzymatic activity, increased total ceramides, and reduced sphingosine levels. A median EC50 value of 11.7 μM was achieved for LCL-805 in cell viability assays across 32 human AML cell lines. As a single agent tested across a panel of 71 primary AML patient samples, a median EC50 value of 15.8 μM was achieved. Exogenous ceramide supplementation with C6-ceramide nanoliposomes, which is entering phase I/II clinical trial for relapsed/refractory AML, significantly enhanced LCL-805 killing. Mechanistically, LCL-805 antagonized Akt signaling and led to iron-dependent cell death distinct from canonical ferroptosis. These findings elucidated key factors involved in LCL-805 cytotoxicity and demonstrated the potency of combining AC inhibition with exogenous ceramide.

Acid Ceramidase Inhibitor LCL-805 Antagonizes Akt Signaling and Promotes Iron-Dependent Cell Death in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy requiring urgent treatment advancements. Ceramide is a cell death-promoting signaling lipid that plays a central role in therapy-induced cell death. Acid ceramidase (AC), a ceramide-depleting enzyme, is overexpressed in AML and promotes leukemic survival and drug resistance. The ceramidase inhibitor B-13 and next-generation lysosomal-localizing derivatives termed dimethylglycine (DMG)-B-13 prodrugs have been developed but remain untested in AML. Here, we report the in vitro anti-leukemic efficacy and mechanism of DMG-B-13 prodrug, LCL-805, across AML cell lines and primary patient samples. LCL-805 inhibited AC enzymatic activity, increased total ceramides, and reduced sphingosine levels. A median EC50 value of 11.7 μM was achieved for LCL-805 in cell viability assays across 32 human AML cell lines. As a single agent tested across a panel of 71 primary AML patient samples, a median EC50 value of 15.8 μM was achieved. Exogenous ceramide supplementation with C6-ceramide nanoliposomes, which is entering phase I/II clinical trial for relapsed/refractory AML, significantly enhanced LCL-805 killing. Mechanistically, LCL-805 antagonized Akt signaling and led to iron-dependent cell death distinct from canonical ferroptosis. These findings elucidated key factors involved in LCL-805 cytotoxicity and demonstrated the potency of combining AC inhibition with exogenous ceramide.

Pan-tissue mitochondrial phenotyping reveals lower OXPHOS expression and function across cancer types

Targeting mitochondrial oxidative phosphorylation (OXPHOS) to treat cancer has been hampered due to serious side-effects potentially arising from the inability to discriminate between non-cancerous and cancerous mitochondria. Herein, comprehensive mitochondrial phenotyping was leveraged to define both the composition and function of OXPHOS across various murine cancers and compared to both matched normal tissues and other organs. When compared to both matched normal tissues, as well as high OXPHOS reliant organs like heart, intrinsic expression of the OXPHOS complexes, as well as OXPHOS flux were discovered to be consistently lower across distinct cancer types. Assuming intrinsic OXPHOS expression/function predicts OXPHOS reliance in vivo, these data suggest that pharmacologic blockade of mitochondrial OXPHOS likely compromises bioenergetic homeostasis in healthy oxidative organs prior to impacting tumor mitochondrial flux in a clinically meaningful way. Although these data caution against the use of indiscriminate mitochondrial inhibitors for cancer treatment, considerable heterogeneity was observed across cancer types with respect to both mitochondrial proteome composition and substrate-specific flux, highlighting the possibility for targeting discrete mitochondrial proteins or pathways unique to a given cancer type.

Simultaneous Inhibition of Ceramide Hydrolysis and Glycosylation Synergizes to Corrupt Mitochondrial Respiration and Signal Caspase Driven Cell Death in Drug-Resistant Acute Myeloid Leukemia

Acute myelogenous leukemia (AML), the most prevalent acute and aggressive leukemia diagnosed in adults, often recurs as a difficult-to-treat, chemotherapy-resistant disease. Because chemotherapy resistance is a major obstacle to successful treatment, novel therapeutic intervention is needed. Upregulated ceramide clearance via accelerated hydrolysis and glycosylation has been shown to be an element in chemotherapy-resistant AML, a problem considering the crucial role ceramide plays in eliciting apoptosis. Herein we employed agents that block ceramide clearance to determine if such a "reset" would be of therapeutic benefit. SACLAC was utilized to limit ceramide hydrolysis, and D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-threo-PDMP) was used to block the glycosylation route. The SACLAC D-threo-PDMP inhibitor combination was synergistically cytotoxic in drug-resistant, P-glycoprotein-expressing (P-gp) AML but not in wt, P-gp-poor cells. Interestingly, P-gp antagonists that can limit ceramide glycosylation via depression of glucosylceramide transit also synergized with SACLAC, suggesting a paradoxical role for P-gp in the implementation of cell death. Mechanistically, cell death was accompanied by a complete drop in ceramide glycosylation, concomitant, striking increases in all molecular species of ceramide, diminished sphingosine 1-phosphate levels, resounding declines in mitochondrial respiratory kinetics, altered Akt, pGSK-3β, and Mcl-1 expression, and caspase activation. Although ceramide was generated in wt cells upon inhibitor exposure, mitochondrial respiration was not corrupted, suggestive of mitochondrial vulnerability in the drug-resistant phenotype, a potential therapeutic avenue. The inhibitor regimen showed efficacy in an in vivo model and in primary AML cells from patients. These results support the implementation of SL enzyme targeting to limit ceramide clearance as a therapeutic strategy in chemotherapy-resistant AML, inclusive of a novel indication for the use of P-gp antagonists.

Intrinsic adaptations in OXPHOS power output and reduced tumorigenicity characterize doxorubicin resistant ovarian cancer cells

Although the development of chemoresistance is multifactorial, active chemotherapeutic efflux driven by upregulations in ATP binding cassette (ABC) transporters are commonplace. Chemotherapeutic efflux pumps, like ABCB1, couple drug efflux to ATP hydrolysis and thus potentially elevate cellular demand for ATP resynthesis. Elevations in both mitochondrial content and cellular respiration are common phenotypes accompanying many models of cancer cell chemoresistance, including those dependent on ABCB1. The present study set out to characterize potential mitochondrial remodeling commensurate with ABCB1-dependent chemoresistance, as well as investigate the impact of ABCB1 activity on mitochondrial respiratory kinetics. To do this, comprehensive bioenergetic phenotyping was performed across ABCB1-dependent chemoresistant cell models and compared to chemosensitive controls. In doxorubicin (DOX) resistant ovarian cancer cells, the combination of both increased mitochondrial content and enhanced respiratory complex I (CI) boosted intrinsic oxidative phosphorylation (OXPHOS) power output. With respect to ABCB1, acute ABCB1 inhibition partially normalized intact basal mitochondrial respiration between chemosensitive and chemoresistant cells, suggesting that active ABCB1 contributes to mitochondrial remodeling in favor of enhanced OXPHOS. Interestingly, while enhanced OXPHOS power output supported ABCB1 drug efflux when DOX was present, in the absence of chemotherapeutic stress, enhanced OXPHOS power output was associated with reduced tumorigenicity.

Ceramide nanoliposomes augment the efficacy of venetoclax and cytarabine in models of acute myeloid leukemia

Despite several new therapeutic options for acute myeloid leukemia (AML), disease relapse remains a significant challenge. We have previously demonstrated that augmenting ceramides can counter various drug-resistance mechanisms, leading to enhanced cell death in cancer cells and extended survival in animal models. Using a nanoscale delivery system for ceramide (ceramide nanoliposomes, CNL), we investigated the effect of CNL within a standard of care venetoclax/cytarabine (Ara-C) regimen. We demonstrate that CNL augmented the efficacy of venetoclax/cytarabine in in vitro, ex vivo, and in vivo models of AML. CNL treatment induced non-apoptotic cytotoxicity, and augmented cell death induced by Ara-C and venetoclax. Mechanistically, CNL reduced both venetoclax (Mcl-1) and cytarabine (Chk1) drug-resistant signaling pathways. Moreover, venetoclax and Ara-C augmented the generation of endogenous pro-death ceramide species, which was intensified with CNL. Taken together, CNL has the potential to be utilized as an adjuvant therapy to improve outcomes, potentially extending survival, in patients with AML.

Harnessing the power of sphingolipids: Prospects for acute myeloid leukemia

Acute myeloid leukemia (AML) is an aggressive, heterogenous malignancy characterized by clonal expansion of bone marrow-derived myeloid progenitor cells. While our current understanding of the molecular and genomic landscape of AML has evolved dramatically and opened avenues for molecularly targeted therapeutics to improve upon standard intensive induction chemotherapy, curative treatments are elusive, particularly in older patients. Responses to current AML treatments are transient and incomplete, necessitating the development of novel treatment strategies to improve outcomes. To this end, harnessing the power of bioactive sphingolipids to treat cancer shows great promise. Sphingolipids are involved in many hallmarks of cancer of paramount importance in AML. Leukemic blast survival is influenced by cellular levels of ceramide, a bona fide pro-death molecule, and its conversion to signaling molecules such as sphingosine-1-phosphate and glycosphingolipids. Preclinical studies demonstrate the efficacy of therapeutics that target dysregulated sphingolipid metabolism as well as their combinatorial synergy with clinically-relevant therapeutics. Thus, increased understanding of sphingolipid dysregulation may be exploited to improve AML patient care and outcomes. This review summarizes the current knowledge of dysregulated sphingolipid metabolism in AML, evaluates how pro-survival sphingolipids promote AML pathogenesis, and discusses the therapeutic potential of targeting these dysregulated sphingolipid pathways.

Alterations in sphingolipid composition and mitochondrial bioenergetics represent synergistic therapeutic vulnerabilities linked to multidrug resistance in leukemia

Modifications in sphingolipid (SL) metabolism and mitochondrial bioenergetics are key factors implicated in cancer cell response to chemotherapy, including chemotherapy resistance. In the present work, we utilized acute myeloid leukemia (AML) cell lines, selected to be refractory to various chemotherapeutics, to explore the interplay between SL metabolism and mitochondrial biology supportive of multidrug resistance (MDR). In agreement with previous findings in cytarabine or daunorubicin resistant AML cells, relative to chemosensitive wildtype controls, HL-60 cells refractory to vincristine (HL60/VCR) presented with alterations in SL enzyme expression and lipidome composition. Such changes were typified by upregulated expression of various ceramide detoxifying enzymes, as well as corresponding shifts in ceramide, glucosylceramide, and sphingomyelin (SM) molecular species. With respect to mitochondria, despite consistent increases in both basal respiration and maximal respiratory capacity, direct interrogation of the oxidative phosphorylation (OXPHOS) system revealed intrinsic deficiencies in HL60/VCR, as well as across multiple MDR model systems. Based on the apparent requirement for augmented SL and mitochondrial flux to support the MDR phenotype, we explored a combinatorial therapeutic paradigm designed to target each pathway. Remarkably, despite minimal cytotoxicity in peripheral blood mononuclear cells (PBMC), co-targeting SL metabolism, and respiratory complex I (CI) induced synergistic cytotoxicity consistently across multiple MDR leukemia models. Together, these data underscore the intimate connection between cellular sphingolipids and mitochondrial metabolism and suggest that pharmacological intervention across both pathways may represent a novel treatment strategy against MDR.

Intrinsic OXPHOS limitations underlie cellular bioenergetics in leukemia

Currently there is great interest in targeting mitochondrial oxidative phosphorylation (OXPHOS) in cancer. However, notwithstanding the targeting of mutant dehydrogenases, nearly all hopeful 'mito-therapeutics' cannot discriminate cancerous from non-cancerous OXPHOS and thus suffer from a limited therapeutic index. Using acute myeloid leukemia (AML) as a model, herein, we leveraged an in-house diagnostic biochemical workflow to identify 'actionable' bioenergetic vulnerabilities intrinsic to cancerous mitochondria. Consistent with prior reports, AML growth and proliferation was associated with a hyper-metabolic phenotype which included increases in basal and maximal respiration. However, despite having nearly 2-fold more mitochondria per cell, clonally expanding hematopoietic stem cells, leukemic blasts, as well as chemoresistant AML were all consistently hallmarked by intrinsic OXPHOS limitations. Remarkably, by performing experiments across a physiological span of ATP free energy, we provide direct evidence that leukemic mitochondria are particularly poised to consume ATP. Relevant to AML biology, acute restoration of oxidative ATP synthesis proved highly cytotoxic to leukemic blasts, suggesting that active OXPHOS repression supports aggressive disease dissemination in AML. Together, these findings argue against ATP being the primary output of leukemic mitochondria and provide proof-of-principle that restoring, rather than disrupting, OXPHOS may represent an untapped therapeutic avenue for combatting hematological malignancy and chemoresistance.

On the nature of ceramide-mitochondria interactions - Dissection using comprehensive mitochondrial phenotyping

Sphingolipids are a unique class of lipids owing to their non-glycerol-containing backbone, ceramide, that is constructed from a long-chain aliphatic amino alcohol, sphinganine, to which a fatty acid is attached via an amide bond. Ceramide plays a star role in the initiation of apoptosis by virtue of its interactions with mitochondria, a control point for a downstream array of signaling cascades culminating in apoptosis. Many pathways converge on mitochondria to elicit mitochondrial outer membrane permeabilization (MOMP), a step that corrupts bioenergetic service. Although much is known regarding ceramides interaction with mitochondria and the ensuing cell signal transduction cascades, how ceramide impacts the elements of mitochondrial bioenergetic function is poorly understood. The objective of this review is to introduce the reader to sphingolipid metabolism, present a snapshot of mitochondrial respiration, elaborate on ceramides convergence on mitochondria and the upstream players that collaborate to elicit MOMP, and introduce a mitochondrial phenotyping platform that can be of utility in dissecting the fine-points of ceramide impact on cellular bioenergetics.

A role for ceramide glycosylation in resistance to oxaliplatin in colorectal cancer

There is growing evidence to support a role for the ceramide-metabolizing enzyme, glucosylceramide synthase (GCS), in resistance to a variety of chemotherapeutic agents. Whether GCS contributes to oxaliplatin resistance in colorectal cancer (CRC) has not yet been determined. We have addressed this potentially important clinical issue by examining GCS function in two panels of oxaliplatin-resistant, isogenic CRC cell lines. Compared to parental cell lines, oxaliplatin-resistant cells have increased expression of GCS protein associated with increased levels of the pro-survival ceramide metabolite, glucosylceramide (GlcCer). Inhibition of GCS expression by RNAi-mediated gene knockdown resulted in a reduction in cellular GlcCer levels, with restored sensitivity to oxaliplatin. Furthermore, oxaliplatin-resistant CRC cells displayed lower ceramide levels both basally and after treatment with oxaliplatin, compared to parental cells. GlcCer, formed by GCS-mediated ceramide glycosylation, is the precursor to a complex array of glycosphingolipids. Differences in cellular levels and species of gangliosides, a family of glycosphingolipids, were also seen between parental and oxaliplatin-resistant CRC cells. Increased Akt activation was also observed in oxaliplatin-resistant CRC cell lines, together with increased expression of the anti-apoptotic protein survivin. Finally, this study shows that GCS protein levels are greatly increased in human CRC specimens, compared to matched, normal colonic mucosa, and that high levels of UGCG gene expression are significantly associated with decreased disease-free survival in colorectal cancer patients. These findings uncover an important cellular role for GCS in oxaliplatin chemosensitivity and may provide a novel cellular target for augmenting chemotherapeutic drug effectiveness in CRC.

Ceramide Analogue SACLAC Modulates Sphingolipid Levels and MCL-1 Splicing to Induce Apoptosis in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a disease characterized by uncontrolled proliferation of immature myeloid cells in the blood and bone marrow. The 5-year survival rate is approximately 25%, and recent therapeutic developments have yielded little survival benefit. Therefore, there is an urgent need to identify novel therapeutic targets. We previously demonstrated that acid ceramidase (ASAH1, referred to as AC) is upregulated in AML and high AC activity correlates with poor patient survival. Here, we characterized a novel AC inhibitor, SACLAC, that significantly reduced the viability of AML cells with an EC₅₀ of approximately 3 μmol/L across 30 human AML cell lines. Treatment of AML cell lines with SACLAC effectively blocked AC activity and induced a decrease in sphingosine 1-phosphate and a 2.5-fold increase in total ceramide levels. Mechanistically, we showed that SACLAC treatment led to reduced levels of splicing factor SF3B1 and alternative MCL-1 mRNA splicing in multiple human AML cell lines. This increased proapoptotic MCL-1S levels and contributed to SACLAC-induced apoptosis in AML cells. The apoptotic effects of SACLAC were attenuated by SF3B1 or MCL-1 overexpression and by selective knockdown of MCL-1S. Furthermore, AC knockdown and exogenous C16-ceramide supplementation induced similar changes in SF3B1 level and MCL-1S/L ratio. Finally, we demonstrated that SACLAC treatment leads to a 37% to 75% reduction in leukemic burden in two human AML xenograft mouse models. IMPLICATIONS: These data further emphasize AC as a therapeutic target in AML and define SACLAC as a potent inhibitor to be further optimized for future clinical development.

Chemotherapy selection pressure alters sphingolipid composition and mitochondrial bioenergetics in resistant HL-60 cells

The combination of daunorubicin (dnr) and cytarabine (Ara-C) is a cornerstone of treatment for acute myelogenous leukemia (AML); resistance to these drugs is a major cause of treatment failure. Ceramide, a sphingolipid (SL), plays a critical role in cancer cell apoptosis in response to chemotherapy. Here, we investigated the effects of chemotherapy selection pressure with Ara-C and dnr on SL composition and enzyme activity in the AML cell line HL-60. Resistant cells, those selected for growth in Ara-C- and dnr-containing medium (HL-60/Ara-C and HL-60/dnr, respectively), demonstrated upregulated expression and activity of glucosylceramide synthase, acid ceramidase (AC), and sphingosine kinase 1 (SPHK1); were more resistant to ceramide than parental cells; and displayed sensitivity to inhibitors of SL metabolism. Lipidomic analysis revealed a general ceramide deficit and a profound upswing in levels of sphingosine 1-phosphate (S1P) and ceramide 1-phosphate (C1P) in HL-60/dnr cells versus parental and HL-60/Ara-C cells. Both chemotherapy-selected cells also exhibited comprehensive upregulations in mitochondrial biogenesis consistent with heightened reliance on oxidative phosphorylation, a property that was partially reversed by exposure to AC and SPHK1 inhibitors and that supports a role for the phosphorylation system in resistance. In summary, dnr and Ara-C selection pressure induces acute reductions in ceramide levels and large increases in S1P and C1P, concomitant with cell resilience bolstered by enhanced mitochondrial remodeling. Thus, strategic control of ceramide metabolism and further research to define mitochondrial perturbations that accompany the drug-resistant phenotype offer new opportunities for developing therapies that regulate cancer growth.

Acid ceramidase promotes drug resistance in acute myeloid leukemia through NF-κB-dependent P-glycoprotein upregulation

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. More than half of older AML patients fail to respond to cytotoxic chemotherapy, and most responders relapse with drug-resistant disease. Failure to achieve complete remission can be partly attributed to the drug resistance advantage of AML blasts that frequently express P-glycoprotein (P-gp), an ATP-binding cassette transporter. Our previous work showed that elevated acid ceramidase (AC) levels in AML contribute to blast survival. Here, we investigated P-gp expression levels in AML relative to AC. Using parental HL-60 cells and drug-resistant derivatives as our model, we found that P-gp expression and efflux activity were highly upregulated in resistant derivatives. AC overexpression in HL-60 conferred resistance to the AML chemotherapeutic drugs, cytarabine, mitoxantrone, and daunorubicin, and was linked to P-gp upregulation. Furthermore, targeting AC through pharmacologic or genetic approaches decreased P-gp levels and increased sensitivity to chemotherapeutic drugs. Mechanistically, AC overexpression increased NF-κB activation whereas NF-kB inhibitors reduced P-gp levels, indicating that the NF-kappaB pathway contributes to AC-mediated modulation of P-gp expression. Hence, our data support an important role for AC in drug resistance as well as survival and suggest that sphingolipid targeting approaches may also impact drug resistance in AML.

Acid ceramidase is upregulated in AML and represents a novel therapeutic target

There is an urgent unmet need for new therapeutics in acute myeloid leukemia (AML) as standard therapy has not changed in the past three decades and outcome remains poor for most patients. Sphingolipid dysregulation through decreased ceramide levels and elevated sphingosine 1-phosphate (S1P) promotes cancer cell growth and survival. Acid ceramidase (AC) catalyzes ceramide breakdown to sphingosine, the precursor for S1P. We report for the first time that AC is required for AML blast survival. Transcriptome analysis and enzymatic assay show that primary AML cells have high levels of AC expression and activity. Treatment of patient samples and cell lines with AC inhibitor LCL204 reduced viability and induced apoptosis. AC overexpression increased the expression of anti-apoptotic Mcl-1, significantly increased S1P and decreased ceramide. Conversely, LCL204 induced ceramide accumulation and decreased Mcl-1 through post-translational mechanisms. LCL204 treatment significantly increased overall survival of C57BL/6 mice engrafted with leukemic C1498 cells and significantly decreased leukemic burden in NSG mice engrafted with primary human AML cells. Collectively, these studies demonstrate that AC plays a critical role in AML survival through regulation of both sphingolipid levels and Mcl-1. We propose that AC warrants further exploration as a novel therapeutic target in AML.

Role of P-glycoprotein inhibitors in ceramide-based therapeutics for treatment of cancer

The anticancer properties of ceramide, a sphingolipid with potent tumor-suppressor properties, can be dampened via glycosylation, notably in multidrug resistance wherein ceramide glycosylation is characteristically elevated. Earlier works using the ceramide analog, C6-ceramide, demonstrated that the antiestrogen tamoxifen, a first generation P-glycoprotein (P-gp) inhibitor, blocked C6-ceramide glycosylation and magnified apoptotic responses. The present investigation was undertaken with the goal of discovering non-anti-estrogenic alternatives to tamoxifen that could be employed as adjuvants for improving the efficacy of ceramide-centric therapeutics in treatment of cancer. Herein we demonstrate that the tamoxifen metabolites, desmethyltamoxifen and didesmethyltamoxifen, and specific, high-affinity P-gp inhibitors, tariquidar and zosuquidar, synergistically enhanced C6-ceramide cytotoxicity in multidrug resistant HL-60/VCR acute myelogenous leukemia (AML) cells, whereas the selective estrogen receptor antagonist, fulvestrant, was ineffective. Active C6-ceramide-adjuvant combinations elicited mitochondrial ROS production and cytochrome c release, and induced apoptosis. Cytotoxicity was mitigated by introduction of antioxidant. Effective adjuvants markedly inhibited C6-ceramide glycosylation as well as conversion to sphingomyelin. Active regimens were also effective in KG-1a cells, a leukemia stem cell-like line, and in LoVo human colorectal cancer cells, a solid tumor model. In summary, our work details discovery of the link between P-gp inhibitors and the regulation and potentiation of ceramide metabolism in a pro-apoptotic direction in cancer cells. Given the active properties of these adjuvants in synergizing with C6-ceramide, independent of drug resistance status, stemness, or cancer type, our results suggest that the C6-ceramide-containing regimens could provide alternative, promising therapeutic direction, in addition to finding novel, off-label applications for P-gp inhibitors.

Ceramide-tamoxifen regimen targets bioenergetic elements in acute myelogenous leukemia

The objective of our study was to determine the mechanism of action of the short-chain ceramide analog, C6-ceramide, and the breast cancer drug, tamoxifen, which we show coactively depress viability and induce apoptosis in human acute myelogenous leukemia cells. Exposure to the C6-ceramide-tamoxifen combination elicited decreases in mitochondrial membrane potential and complex I respiration, increases in reactive oxygen species (ROS), and release of mitochondrial proapoptotic proteins. Decreases in ATP levels, reduced glycolytic capacity, and reduced expression of inhibitors of apoptosis proteins also resulted. Cytotoxicity of the drug combination was mitigated by exposure to antioxidant. Cells metabolized C6-ceramide by glycosylation and hydrolysis, the latter leading to increases in long-chain ceramides. Tamoxifen potently blocked glycosylation of C6-ceramide and long-chain ceramides. N-desmethyltamoxifen, a poor antiestrogen and the major tamoxifen metabolite in humans, was also effective with C6-ceramide, indicating that traditional antiestrogen pathways are not involved in cellular responses. We conclude that cell death is driven by mitochondrial targeting and ROS generation and that tamoxifen enhances the ceramide effect by blocking its metabolism. As depletion of ATP and targeting the "Warburg effect" represent dynamic metabolic insult, this ceramide-containing combination may be of utility in the treatment of leukemia and other cancers.

Abstract B17: Ceramide-antiestrogen regimen targets bioenergetic elements in acute myelogenous leukemia

Acute myelogenous leukemia (AML), the most common type of leukemia in adults, is heterogeneous in nature and aggressive and only about 25% of patients that experience remission with cytotoxic chemotherapy remain disease-free. The present work demonstrates the therapeutic efficacy of a novel antiestrogen-ceramide regimen in AML. Herein we show that the antiestrogen tamoxifen (tam) magnifies the cytotoxic impact of ceramide, a tumor-suppressor sphingolipid, inducing apoptosis through mitochondrial-bioenergetic targeting, an expedient, practicable avenue for disrupting the energy-producing systems of cancer cells and a potentially fruitful therapeutic direction. C6-ceramide (C6-cer), a short-chain, hydrophilic analog of ceramide was used in these studies. Nanoliposomal formulations of C6-cer and tam were also utilized and found equally effective. In human AML cell lines and in patient-derived AML cells, the C6-cer-tam combination was far more active than single agents in depressing viability and in inducing apoptosis (Annexin V binding; DNA fragmentation). Interestingly, the major tam metabolite in humans, and the one with the longest serum half-life, N-desmethyltamoxifen (DMT), was equally effective in combination with C6-cer. As an example, in KG-1 cells, whereas C6-cer (5 μM) and DMT (5 μM) reduced viability to only 95 and 90% of control, respectively, the combination resulted in 10% viability after 72 hr exposure. In-depth investigations revealed that cytotoxic responses to the C6-cer-antiestrogen regimen were accompanied by i) time- and dose-dependent loss of mitochondrial membrane potential, ii) inhibition of mitochondrial Complex I respiration, iii) mitochondrial cytochrome c release, a key initiative step in the apoptotic process, iv) decreases in cellular ATP levels as marked by robust increases the ratio of ADP to ATP, and v) a >50% reduction in cellular glycolytic capacity, a demonstration of commitment to energy severance. As loss of mitochondrial membrane potential constitutes a critical event in cell death and depletion of ATP in conjunction with targeting the “Warburg effect” represent potent metabolic “hits”, we conclude that the C6-cer-tamoxifen regimen could be a potentially effective targeted therapy for AML, independent of the multidrug resistant phenotype, as shown by efficacy in vincristine-resistant AML cells. Although a two-drug combination, this regimen could be classified as a mitocan, an agent that targets mitochondria and exhibits anti-cancer activity. Support: NCI CA171983

Citation Format: Samy A.F. Morad, Terence E. Ryan, Brian M. Barth, David F. Claxton, Mark Kester, Thomas P. Loughran, Jr., Myles C. Cabot. Ceramide-antiestrogen regimen targets bioenergetic elements in acute myelogenous leukemia. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B17.

Tamoxifen regulation of sphingolipid metabolism--Therapeutic implications

Tamoxifen, a triphenylethylene antiestrogen and one of the first-line endocrine therapies used to treat estrogen receptor-positive breast cancer, has a number of interesting, off-target effects, and among these is the inhibition of sphingolipid metabolism. More specifically, tamoxifen inhibits ceramide glycosylation, and enzymatic step that can adventitiously support the influential tumor-suppressor properties of ceramide, the aliphatic backbone of sphingolipids. Additionally, tamoxifen and metabolites N-desmethyltamoxifen and 4-hydroxytamoxifen, have been shown to inhibit ceramide hydrolysis by the enzyme acid ceramidase. This particular intervention slows ceramide destruction and thereby depresses formation of sphingosine 1-phosphate, a mitogenic sphingolipid with cancer growth-promoting properties. As ceramide-centric therapies are becoming appealing clinical interventions in the treatment of cancer, agents like tamoxifen that can retard the generation of mitogenic sphingolipids and buffer ceramide clearance via inhibition of glycosylation, take on new importance. In this review, we present an abridged, lay introduction to sphingolipid metabolism, briefly chronicle tamoxifen's history in the clinic, examine studies that demonstrate the impact of triphenylethylenes on sphingolipid metabolism in cancer cells, and canvass works relevant to the use of tamoxifen as adjuvant to drive ceramide-centric therapies in cancer treatment. The objective is to inform the readership of what could be a novel, off-label indication of tamoxifen and structurally-related triphenylethylenes, an indication divorced from estrogen receptor status and one with application in drug resistance.

Modification of sphingolipid metabolism by tamoxifen and N-desmethyltamoxifen in acute myelogenous leukemia--Impact on enzyme activity and response to cytotoxics

The triphenylethylene antiestrogen, tamoxifen, can be an effective inhibitor of sphingolipid metabolism. This off-target activity makes tamoxifen an interesting ancillary for boosting the apoptosis-inducing properties of ceramide, a sphingolipid with valuable tumor censoring activity. Here we show for the first time that tamoxifen and metabolite, N-desmethyltamoxifen (DMT), block ceramide glycosylation and inhibit ceramide hydrolysis (by acid ceramidase, AC) in human acute myelogenous leukemia (AML) cell lines and in AML cells derived from patients. Tamoxifen (1-10 μM) inhibition of AC in AML cells was accompanied by decreases in AC protein expression. Tamoxifen also depressed expression and activity of sphingosine kinase 1 (SphK1), the enzyme-catalyzing production of mitogenic sphingosine 1-phosphate (S1-P). Results from mass spectroscopy showed that tamoxifen and DMT (i) increased the levels of endogenous C16:0 and C24:1 ceramide molecular species, (ii) nearly totally halted production of respective glucosylceramide (GC) molecular species, (iii) drastically reduced levels of sphingosine (to 9% of control), and (iv) reduced levels of S1-P by 85%, in vincristine-resistant HL-60/VCR cells. The co-administration of tamoxifen with either N-(4-hydroxyphenyl)retinamide (4-HPR), a ceramide-generating retinoid, or a cell-deliverable form of ceramide, C6-ceramide, resulted in marked decreases in HL-60/VCR cell viability that far exceeded single agent potency. Combination treatments resulted in synergistic apoptotic cell death as gauged by increased Annexin V binding and DNA fragmentation and activation of caspase-3. These results show the versatility of adjuvant triphenylethylene with ceramide-centric therapies for magnifying therapeutic potential in AML. Such drug regimens could serve as effective strategies, even in the multidrug-resistant setting.

Abstract A31: Revisited use of tamoxifen for improving melanoma therapy

The rapid increase in the incidence of malignant melanoma, a highly aggressive, drug-resistant cancer, has not been accompanied by significant therapeutic breakthroughs for decades, despite the introduction of biochemotherapies. The Dartmouth regimen, of which tamoxifen is a standard component, has also had little impact on outcome. Therefore, examination of alternative melanoma vulnerabilities is essential. Sphingolipid (SL) metabolism is an area of cancer science that has recently risen to prominence. This is because ceramide, the aliphatic backbone of SL's, can act as a powerful tumor censor. However, cancer cells rapidly inactivate ceramide via two major metabolic routes: glycosylation to form glucosylceramide and hydrolysis, which generates sphingosine 1-phosphate, a mitogenic SL. These actions severely blunt ceramide anticancer properties, thwarting potential utility in treatment. Our previous work has shown that tamoxifen is a potent inhibitor of ceramide metabolism at glycosylation and hydrolysis. Herein, we exploit these unique actions and pair tamoxifen with a cell-permeable ceramide, C6-ceramide, and demonstrate that this novel drug duo, unlike the single agents, was highly effective in inhibiting melanoma cell (A375 and cell lines established from patients) proliferation, inducing caspase-dependent apoptosis, promoting cell cycle arrest, diminishing cellular adhesion and migration, reducing cell surface expression of integrins, and downregulating expression of the main source of drug resistance in melanoma, survivin, all essential elements of a drug regimen useful for limiting melanoma growth. Because integrin expression often correlates with conversion of melanoma growth from radial to vertical, we propose that tamoxifen can serve as an effective adjuvant to C6-ceramide for countering metastatic potential. Whereas the role of tamoxifen in the Dartmouth regimen has always remained elusive, its role as a ceramide sensitizer is clear and presents novel inroads for improving melanoma therapy.

Citation Format: Samy A.F Morad, Lance C. Bridges, Myles C. Cabot. Revisited use of tamoxifen for improving melanoma therapy. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr A31.

Engraftment of Human Primary Acute Myeloid Leukemia Defined by Integrated Genetic Profiling in NOD/SCID/IL2rγnull Mice for Preclinical Ceramide-Based Therapeutic Evaluation

Acute Myeloid Leukemia (AML) is a highly heterogeneous and poor prognosis disease with few available therapeutic options. Novel advances are urgently needed, however effective models to test experimental therapeutics have been lacking. Recently, NOD/SCID/IL2rγnull (NSG) mice were shown to engraft primary human AML in a manner that recapitulated the natural disease and its progression. Additionally, integrated genomic profiling was used to refine risk stratification of AML. In this study, we demonstrated the engraftment of molecularly defined primary AML in NSG mice. We showed that AML that express DNMT3A mutations, which predict for adverse outcome, engrafted with exceptional efficacy. Lastly, we demonstrated that human AML-engrafted NSG mice can be effectively used to study novel ceramide-based therapeutics. Ceramide is a bioactive sphingolipid that has been implicated as an inducer of apoptosis. Elevation in cancer cell ceramide levels either via exogenous delivery or by provoking intracellular ceramide generation is the goal of ceramide-based therapeutics. In this study, we used the human AML-engrafted NSG mouse model to evaluate nanoliposomal short-chain C6-ceramide and a nanoliposomal formulation of the ceramide-inducer tamoxifen. Altogether, the NSG model is likely to prove invaluable in the study of novel agents, sushc as ceramide-based therapeutics, with the ability to define therapeutic activity against specific molecularly defined and risk stratified AML.

Novel off-target effect of tamoxifen--inhibition of acid ceramidase activity in cancer cells

Acid ceramidase (AC), EC 3.5.1.23, a lysosomal enzyme, catalyzes the hydrolysis of ceramide to constituent sphingoid base, sphingosine, and fatty acid. Because AC regulates the levels of pro-apoptotic ceramide and mitogenic sphingosine-1-phosphate, it is considered an apt target in cancer therapy. The present study reveals, for the first time, that the prominent antiestrogen, tamoxifen, is a pan-effective AC inhibitor in the low, single digit micromolar range, as demonstrated in a wide spectrum of cancer cell types, prostate, pancreatic, colorectal, and breast. Prostate cancer cells were chosen for the detailed investigations. Treatment of intact PC-3 cells with tamoxifen produced time- and dose-dependent inhibition of AC activity. Tamoxifen did not impact cell viability nor did it inhibit AC activity in cell-free assays. In pursuit of mechanism of action, we demonstrate that tamoxifen induced time-, as early as 5min, and dose-dependent, as low as 5μM, increases in lysosomal membrane permeability (LMP), and time- and dose-dependent downregulation of AC protein expression. Assessing various protease inhibitors revealed that a cathepsin B inhibitor blocked tamoxifen-elicited downregulation of AC protein; however, this action failed to restore AC activity unless assayed in a cell-free system at pH4.5. In addition, pretreatment with tamoxifen inhibited PC-3 cell migration. Toremifene, an antiestrogen structurally similar to tamoxifen, was also a potent inhibitor of AC activity. This study reveals a new, off-target action of tamoxifen that may be of benefit to enhance anticancer therapies that either incorporate ceramide or target ceramide metabolism.

Abstract 2027: Tamoxifen amplifies antitumor impact of nanoformulated ceramide- pan efficacy in a myriad of tumor cell types

Although ceramide has potent tumor suppressor properties, its use in cancer therapeutics has been limited; this is mainly due to rapid metabolism of ceramide by cancer cells, the inability to attain therapeutic levels when ceramide-generating agents are employed, and the inherent insolubility of ceramide as a therapeutic. Our approach circumvents these disadvantages and provides advantages over existing methodologies. For example, rather than promoting intracellular ceramide generation by application of exogenous drugs that activate de novo and sphingomyelinase pathways, we have chosen to administer ceramide exogenously in the form of short-chain C6-ceramide nanoliposomes; this overcomes solubility issues and improves uptake. In addition, nanoliposomes are efficient platforms for delivering other hydrophobic chemotherapeutics. The antiestrogen tamoxifen was selected for our studies because of its unusual “off-target” actions, which include inhibition of ceramide metabolism at glycosylation and hydrolysis. Blocking these junctures increases C6-ceramide intracellular residence time and diminishes production of sphingosine 1-phosphate, a mitogenic lipid derived via the action of ceramidase and sphingosine kinase. Hence, C6-ceramide and tamoxifen make up a versatile drug duo. Using in vitro models of human colon and breast cancer, acute mylogenous leukemia (AML), and melanoma, we show that simultaneous administration of nanoliposomal C6-ceramide and tamoxifen potently and synergistically decreased cell viability, compared to the single agents. The mix induced caspase-dependent apoptosis, cell cycle arrest at G1 and G2, upregulation of JNK, p38, and p53, and downregulation of Akt and survivin. In triple negative, hormone-insensitive breast cancer cells, C6-ceramide-tamoxifen induced cell cycle arrest, caspase-dependent apoptosis, and lysosomal and mitochondrial membrane permeability. Evaluation of single agents showed that tamoxifen elicited lysosomal membrane permeability and inhibited acid ceramidase. We have obtained similar results in in vitro models of human melanoma and AML. In an in vivo retroviral transduced murine AML model, the combination lowered leukemia burden in spleen and marrow. The combination also perpetuated synergistic increases in long-chain ceramides, likely compounding the overall ceramide effect. Of interest regarding in vivo utility, the major metabolites of tamoxifen, N-desmethyltamoxifen and 4-hydroxytamoxifen, were effective in combination with C6-ceramide. Our findings show that tamoxifen magnifies the antiproliferative effects of C6-ceramide via aggregate mechanisms. This proficient, ceramide-based nanoliposomal drug regimen may have potential as an effective anticancer therapeutic.

Supported by NIGMS 77391; ABC's, Los Angeles; Fashion Footwear Association of New York Charitable Foundation.

Citation Format: Samy A.F. Morad, Jonathan C. Levin, Noha Abdelmageed, Mark Kester, Sriram S. Shanmugavdlandy, Robert F. Paulson, Daniel W. Rosenberg, James P. Madigan, Myles C. Cabot. Tamoxifen amplifies antitumor impact of nanoformulated ceramide- pan efficacy in a myriad of tumor cell types. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2027. doi:10.1158/1538-7445.AM2013-2027

Tamoxifen magnifies therapeutic impact of ceramide in human colorectal cancer cells independent of p53

Poor prognosis in patients with later stage colorectal cancer (CRC) necessitates the search for new treatment strategies. Ceramide, because of its role in orchestrating death cascades in cancer cells, is a versatile alternative. Ceramide can be generated by exposure to chemotherapy or ionizing radiation, or it can be administered in the form of short-chain analogs (C6-ceramide). Because intracellular P-glycoprotein (P-gp) plays a role in catalyzing the conversion of ceramide to higher sphingolipids, we hypothesized that administration of P-gp antagonists with C6-ceramide would magnify cell death cascades. Human CRC cell lines were employed, HCT-15, HT-29, and LoVo. The addition of either tamoxifen, VX-710, verapamil, or cyclosporin A, antagonists of P-gp, enhanced C6-ceramide cytotoxicity in all cell lines. In depth studies with C6-ceramide and tamoxifen in LoVo cells showed the regimen induced PARP cleavage, caspase-dependent apoptosis, mitochondrial membrane permeabilization (MMP), and cell cycle arrest at G1 and G2. At the molecular level, the regimen, but not single agents, induced time-dependent upregulation of tumor suppressor protein p53; however, introduction of a p53 inhibitor staved neither MMP nor apoptosis. Nanoliposomal formulations of C6-ceramide and tamoxifen were also effective, yielding synergistic cell kill. We conclude that tamoxifen is a favorable adjuvant for enhancing C6-ceramide cytotoxicity in CRC, and demonstrates uniquely integrated effects. The high frequency of expression of P-gp in CRC presents an adventitious target for complementing ceramide-based therapies, a strategy that could hold promise for treatment of resistant disease.

Gaucher's disease and cancer: a sphingolipid perspective

Gaucher's disease is a sphingolipidosis characterized by a specific deficiency in an acidic glucocerebrosidase, which results in aberrant accumulation of glucosylceramide primarily within the lysosome. Gaucher's disease has been correlated with cases of myeloma, leukemia, glioblastoma, lung cancer, and hepatocellular carcinoma, although the reasons for the correlation are currently being debated. Some suggest that the effects of Gaucher's disease may be linked to cancer, while others implicate the therapies used to treat Gaucher's disease. This debate is not entirely surprising, as the speculations linking Gaucher's disease with cancer fail to address the roles of ceramide and glucosylceramide in cancer biology. In this review, we will discuss, in the context of cancer biology, ceramide metabolism to glucosylceramide, the roles of glucosylceramide in multidrug-resistance, and the role of ceramide as an anticancer lipid. This review should reveal that it is most practical to associate elevated glucosylceramide, which accompanies Gaucher's disease, with the progression of cancer. Furthermore, this review proposes that the therapies used to treat Gaucher's disease, which augment ceramide accumulation, are likely not linked to correlations with cancer.

Ceramide-orchestrated signalling in cancer cells

One crucial barrier to progress in the treatment of cancer has been the inability to control the balance between cell proliferation and apoptosis: enter ceramide. Discoveries over the past 15 years have elevated this sphingolipid to the lofty position of a regulator of cell fate. Ceramide, it turns out, is a powerful tumour suppressor, potentiating signalling events that drive apoptosis, autophagic responses and cell cycle arrest. However, defects in ceramide generation and metabolism in cancer cells contribute to tumour cell survival and resistance to chemotherapy. This Review focuses on ceramide signalling and the targeting of specific metabolic junctures to amplify the tumour suppressive activities of ceramide. The potential of ceramide-based therapeutics in the treatment of cancer is also discussed.

Targeting glucosylceramide synthase synergizes with C6-ceramide nanoliposomes to induce apoptosis in natural killer cell leukemia

Abstract Natural killer (NK) cell leukemia is characterized by clonal expansion of CD3 - NK cells and comprises both chronic and aggressive forms. Currently no effective treatment exists, thus providing a need for identification of novel therapeutics. Lipidomic studies revealed a dysregulated sphingolipid metabolism as evidenced by decreased levels of overall ceramide species and increased levels of cerebrosides in leukemic NK cells, concomitant with increased glucosylceramide synthase (GCS) expression. GCS, a key enzyme of this pathway, neutralizes pro-apoptotic ceramide by transfer of a uridine diphosphate (UDP)-glucose. Thus, we treated both rat and human leukemic NK cells in combination with: (1) exogenous C6-ceramide nanoliposomes in order to target mitochondria and increase physiological pro-apoptotic levels of long chain ceramide, and (2) 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP), an inhibitor of GCS. Co-administration of C6-ceramide nanoliposomes and PPMP elicited an increase in endogenous long-chain ceramide species, which led to cellular apoptosis in a synergistic manner via the mitochondrial intrinsic cell death pathway in leukemic NK cells.

Potential role of acid ceramidase in conversion of cytostatic to cytotoxic end-point in pancreatic cancer cells

PURPOSE

Acid ceramidase (AC) occupies an important place in the control of cancer cell proliferation. We tested the influence of AC inhibition on the effects of PSC 833, a P-glycoprotein antagonist with potent ceramide-generating capacity, to determine whether AC could be a therapeutic target in pancreatic cancer.

METHODS

Ceramide metabolism was followed using (3)H-palmitate, and molecular species were determined by mass spectroscopy. Apoptosis was measured by DNA fragmentation, autophagy by acridine orange staining, and cell cycle was assessed by flow cytometry and RB phosphorylation. AC was measured in intact cells using fluorescent substrate.

RESULTS

Exposure of human PANC-1 or MIA-PaCa-2 cells to PSC 833 promoted increases in de novo (dihydro)ceramides, (dihydro)glucosylceramides, and (dihydro)sphingomyelins, demarking ceramide generation and robust metabolism. Despite the multifold increases in (dihydro)ceramide levels, cells were refractory to PSC 833. However, PSC 833 produced a dose-dependent decrease in DNA synthesis and dose- and time-dependent decreases in RB phosphorylation, consistent with cell cycle arrest as demonstrated at G1. Cytostatic effects of PSC 833 were converted to cytotoxic end-point by acid ceramidase inhibition. Cytotoxicity was accompanied by formation of acridine orange-stained acidic vesicles and an increase in LC3 expression, indicative of autophagic response. Cell death was not reversed by preexposure to myriocin, which blocks PSC 833-induced ceramide generation.

CONCLUSION

Although the role of ceramide in end-point cytotoxicity is unclear, our results suggest that acid ceramidase is a viable target in pancreatic cancer. We propose that AC inhibition will be effective in concert with other anticancer therapies.

Ceramide--antiestrogen nanoliposomal combinations--novel impact of hormonal therapy in hormone-insensitive breast cancer

Although the sphingolipid ceramide exhibits potent tumor suppressor effects, efforts to harness this have been hampered by poor solubility, uptake, bioavailability, and metabolic conversion. Therefore, identification of avenues to improve efficacy is necessary for development of ceramide-based therapies. In this study, we used mutant p53, triple-negative breast cancer (TNBC) cells, a type of breast cancer highly refractory to treatment, and cell-permeable nanoliposomal C6-ceramide in conjunction with the antiestrogen tamoxifen, which has been shown to be an effective modulator of ceramide metabolism. We show for the first time that nanoliposomal tamoxifen enhances nanoliposomal C6-ceramide cytotoxicity in cultured TNBC cells, a response that was accompanied by induction of cell-cycle arrest at G(1) and G(2), caspase-dependent induction of DNA fragmentation, and enhanced mitochondrial and lysosomal membrane permeability at 18 and 2 hours, respectively. Tamoxifen metabolites were also effective. Only tamoxifen promoted lysosomal membrane permeability. In addition, we show for the first time that tamoxifen inhibits acid ceramidase, as measured in intact cell assays; this effect was irreversible. Together, our findings show that tamoxifen magnifies the antiproliferative effects of C6-ceramide via combined targeting of cell-cycle traverse and lysosomal and mitochondrial integrity. We adduce that C6-ceramide-induced apoptosis is amplified by tamoxifen's impact on lysosomes and perhaps accompanying inhibition of acid ceramidase, which could result in decreased levels of sphingosine 1-phosphate. This drug regimen could serve as a promising therapy for chemoresistant and triple-negative types of breast cancer, and thus represents an indication for tamoxifen, irrespective of estrogen receptor status.

Ceramide-based therapeutics for the treatment of cancer

The bioactive sphingolipid, ceramide, has garnered major interest as a principle regulator of cellular stress, proliferation, senescence, and death. Of particular interest to cancer biologists and clinical oncologist, dysregulated ceramide metabolism has been documented in both solid and non-solid malignancies. Moreover, most anticancer chemotherapeutics stimulate ceramide accumulation through increased ceramide synthesis or through the inhibition of ceramide catabolism. In fact, neutralization of ceramide via glycosylation or phosphorylation in malignant cells has been linked to multidrug chemoresistance. New therapeutic strategies to overcome chemoresistance focus on increasing endogenous ceramide levels by stimulating ceramide synthesis, by inhibiting ceramide neutralization, or by the direct delivery of exogenous ceramide. This review will discuss new therapeutic strategies designed specifically to modulate ceramide metabolism, as well as nanoscale delivery systems engineered to selectively deliver ceramide to cancerous cells and tissues.

Abstract 3833: Ceramide/tamoxifen nanoliposomal combination - A promising strategy for treatment of chemoresistant breast cancer

Drug resistance in breast and other cancers results from multiple gene interactions. Ceramide, a key intermediate in the sphingolipid pathway, can act as a powerful tumor suppressor. The employ of ceramide-based agents as opposed to the administration of ceramide generators such as daunorubicin, could be a smart strategy for cancer therapy. Polychemotherapy remains the best option for treatment of breast cancer at the aggressive, metastatic stage. The aim of our study was to investigate the cytotoxic and antiproliferative effects of ceramide (short-chain C6-ceramide) administered in combination with tamoxifen, used here as an inhibitor of ceramide glycosylation. We previously demonstrated the cytotoxicity of this combination in MDA-MB-231 cells; however, little regarding mechanism was established. The present study employs nanoliposomal formulations of both agents, a strategy to enhance efficacy. Four triple-negative chemoresistant breast cancer cell lines, MDA-MB-231, MDA-MB-468, BT-20 and Hs578T were used. Cytotoxicity assays revealed that C6-ceramide/tamoxifen synergistically reduced viability, compared to single agents in all four cell lines. For example, in MDA-MB-468 cells, nanoliposomal C6-ceramide (2.5 µM), tamoxifen (5 µM), and the combination reduced cell viability to 70, 85, and 10% of control, respectively (96 hr). A hallmark of cancer, implicated in tumor growth as well as metastasis, is resistance to induction of apoptosis. Assessment of cellular apoptosis showed that C6-ceramide/tamoxifen treatment promoted DNA fragmentation in a dose-dependant manner, attaining a value that was 36% over control at 24 hr. Introduction of the pan-caspase inhibitor, z-VAD-fmk, completely reversed DNA fragmentation, showing that apoptosis was mediated by caspase activation. The aggressive and metastatic behavior of any tumor is dependent on the ability of the cells to proliferate. Interestingly, the lower concentrations of our drug combination (2.5 µM) induced cell cycle arrest at G1, the first growth phase. In summary, this approach has potential to circumvent limitations of ceramide-based therapies, such as rapid ceramide clearance due to metabolism, the inability to attain therapeutic levels when ceramide-generating agents are employed, and inherent insolubility that is encountered. These findings demonstrate the in vitro efficacy of combination C6-ceramide/tamoxifen nanoliposomal formulations for suppressing growth of breast cancer cells, and suggest that tamoxifen may be an effective adjuvant for enhancing ceramide-driven cell death cascades. Supported by NIGMS 77391 and ABC's, Los Angeles.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3833. doi:1538-7445.AM2012-3833

Combinatorial therapies improve the therapeutic efficacy of nanoliposomal ceramide for pancreatic cancer

Poor prognosis cancers, such as pancreatic cancer, represent inherent challenges for ceramide-based nanotherapeutics due to metabolic pathways, which neutralize ceramide to less toxic or pro-oncogenic metabolites. We have recently developed a novel 80 nanometer diameter liposomal formulation that incorporates 30 molar percent C6-ceramide, a bioactive lipid that is pro-apoptotic to many cancer cells, but not to normal cells. In this manuscript, we evaluated the efficacy of combining nanoliposomal C6-ceramide (Lip-C6) with either gemcitabine or an inhibitor of glucosylceramide synthase. We first assessed the biological effect of Lip-C6 in PANC-1 cells, a gemcitabine-resistant human pancreatic cancer cell line, and found that low doses alone did not induce cell toxicity. However, cytotoxicity was achieved by combining Lip-C6 with either non-toxic sub-therapeutic concentrations of gemcitabine or with the glucosylceramide synthase inhibitor D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP). Furthermore, these combinations with Lip-C6 cooperatively inhibited PANC-1 tumor growth in vivo. Mechanistically, Lip-C6 inhibited pro-survival Akt and Erk signaling, whereas the nucleoside analog gemcitabine did not. Furthermore, by including PDMP within the nanoliposomes, which halted ceramide neutralization as evidenced by LC-MS3, the cytotoxic effects of Lip-C6 were enhanced. Collectively, we have demonstrated that nanoliposomal ceramide can be an effective anti-pancreatic cancer therapeutic in combination with gemcitabine or an inhibitor of ceramide neutralization.

C6-ceramide and targeted inhibition of acid ceramidase induce synergistic decreases in breast cancer cell growth

The sphingolipid ceramide is known to play a central role in chemo- and radiation-induced cell death. Acid ceramidase (AC) hydrolyzes ceramide, and thus reduces intracellular levels of this proapoptotic lipid. The role of AC as a putative anticancer target is supported by reports of upregulation in prostate cancer and in some breast tumors. In this study, we determined whether the introduction of an AC inhibitor would enhance the apoptosis-inducing effects of C6-ceramide (C6-cer) in breast cancer cells. Cultured breast cancer cells were treated with DM102 [(2R,3Z)-N-(1-hydroxyoctadec-3-en-2-yl)pivalamide, C6-cer, or the combination. Cell viability and cytotoxic synergy were assessed. Activation of apoptotic pathways, generation of reactive oxygen species, and mitochondrial transmembrane potential were determined. DM102 was a more effective AC inhibitor than N-oleoylethanolamine (NOE) and (1R,2R)-2-N-(tetradecanoylamino)-1-(4'-nitrophenyl)-1,3-propandiol (B-13) in MDA-MB-231, MCF-7, and BT-474 cells. As single agents, C6-cer (IC(50) 5-10 μM) and DM102 (IC(50) 20 μM) were only moderately cytotoxic in MDA-MB-231, MCF-7, and SK-BR-3 cells. Co-administration, however, produced synergistic decreases in viability (combination index <0.5) in all cell lines. Apoptosis was confirmed in MDA-MB-231 cells by detection of caspase 3 cleavage and a >3-fold increase in caspase 3/7 activation, PARP cleavage, and a >70% increase in Annexin-V positive cells. C6-cer/DM102 increased ROS levels 4-fold in MDA-MB-231 cells, shifted the ratio of Bax:Bcl-2 to >9-fold that of control cells, and resulted in mitochondrial membrane depolarization. DM102 also increased the synthesis of (3)H-palmitate-labeled long-chain ceramides by 2-fold when C6-cer was present. These data support the effectiveness of targeting AC in combination with exogenous short-chain ceramide as an anticancer strategy, and warrant continued investigation into the utility of the C6-cer/DM102 drug duo in human breast cancer.

Inhibition of acid ceramidase by a 2-substituted aminoethanol amide synergistically sensitizes prostate cancer cells to N-(4-hydroxyphenyl) retinamide

BACKGROUND

The purpose of this study was to determine whether the therapeutic efficacy of fenretinide (4-HPR), a ceramide-generating anticancer agent, could be enhanced in prostate cancer cells by inclusion of a novel synthetic acid ceramidase (AC) inhibitor, DM102, a pivaloylamide of a 2-substituted aminoethanol. In prostate cancer, AC plays a role in progression and resistance to chemotherapy.

METHODS

PC-3 and DU 145 hormone-refractory human prostate cancer cell lines were used. Cells were exposed to 4-HPR, DM102, and combinations; viability, apoptosis, cell migration, ceramide metabolism, and levels of reactive oxygen species (ROS) were assessed.

RESULTS

Single agent 4-HPR and DM102 (2.5-10 µM) were weakly cytotoxic; however, combinations synergistically decreased cell viably to as low as 1.5% of control. N-oleoylethanolamine (NOE), a frequently employed AC inhibitor, was not effective in producing synergy. The 4-HPR/DM102 regimen enhanced caspase activity and increased [(3) H](dihydro)ceramide and ROS levels 6- and 30-fold over control, respectively. The antioxidant vitamin E, but not the de novo ceramide synthesis inhibitor myriocin, partially rescued cells from 4-HPR/DM102 cytotoxicity. The 4-HPR/DM102 combination also elicited synergistic cytotoxicity in DU 145 cells, another human hormone-refractory prostate cancer cell line.

CONCLUSION

This study shows that 4-HPR cytotoxicity is enhanced in a synergistic fashion by inclusion of the AC inhibitor DM102, by a mechanism that enlists generation of ROS, and thus provides a system to raise 4-HPR therapeutic potential. The role of ceramide however in the cytotoxic response is not clear, as blocking ceramide generation failed to rescue PC-3 cells from 4-HPR/DM102 cytotoxicity.

Glucosylceramide synthase, a factor in modulating drug resistance, is overexpressed in metastatic breast carcinoma

Drug resistance causes treatment failure in approximately 50% of breast cancer patients with chemotherapy. Overexpression of glucosylceramide synthase (GCS) confers drug resistance in cancer cells, and suppression of GCS sensitizes cancers to chemotherapy in preclinical studies. Thus, GCS becomes a potential target to reverse drug resistance; however, little is known about GCS expression levels in normal tissues and whether GCS overexpression is associated with metastatic cancers. Herewith, we report our studies in GCS expression levels and breast cancer from patients. GCS levels were analyzed using cancer profiling arrays, breast cancer histo-arrays and quantitative RT-PCR in tumor tissues. We found that breast (18 exp. index) and other hormone-dependent organs (testis, cervix, ovary, prostate) displayed the lowest levels of GCS mRNA, whereas liver (52 exp. index) and other organs (kidney, bladder, stomach) displayed the highest levels of GCS. GCS mRNA levels were significantly elevated in tumors of breast, cervix, rectum and small intestine, as compared to each paired normal tissue. In mammary tissue, GCS overexpression was detected in breast cancers with metastasis, but not in benign fibroadenoma or primary tumors. GCS overexpression was coincident with HER2 expression (γ2=0.84) in ER-negative breast adenocarcinoma. In tumor specimens, GCS mRNA was elevated by 4-fold and significantly associated with stage III (5/7), lymph node-positive (7/8) and estrogen receptor-positive breast cancers (7/9). GCS expression was significantly and selectively elevated in breast cancer, in particular in metastatic disease. GCS overexpression was highly associated with ER-positive and HER2-positive breast cancer with metastasis. Although a small study, these data suggest that GCS may be a prognostic indicator and potential target for the treatment of chemotherapy-refractory breast cancer.

Abstract 5484: Targeted inhibition of acid ceramidase and C6-ceramide exposure induces synergistic killing in cancer cells

Introduction. In this study, we evaluated the anticancer activity of DM102 [(2R,3Z)-N-(1-hydroxyoctadec-3-en-2-yl)pivalamide], a novel inhibitor of the enzyme acid ceramidase (AC), a key regulator of ceramide metabolism, in combination with C6-ceramide (C6-cer) in human breast cancer cells. Ceramide, a bioactive sphingolipid and potent enhancer of apoptosis, is known to play a role in chemo- and radiation-induced cell death. AC tempers ceramide-induced apoptosis by hydrolysis, producing sphingosine and free fatty acid, thus reducing intracellular ceramide levels and providing substrate for sphingosine kinase, which converts sphingosine to sphingosine 1-phosphate. AC has emerged as a putative anticancer target following reports of upregulation in prostate cancer and in some breast tumors. In the present work, we determined whether the introduction of an AC inhibitor would enhance the apoptosis-inducing effects of C6-cer in breast cancer cells.

Methods. Cultured breast cancer cells were pretreated with 10 μM DM102 before exposure to 2.5-10 μM C6-cer. Cell viability was assessed at 72 hr by 96-well colorimetric assays. Cytotoxic synergy was assessed by the Chou-Talalay method of combination index (CI) determined using CalcuSyn® software. Apoptosis was measured by Annexin V/propidium iodide staining using flow cytometry, and by caspase 3/7 activity. Generation of reactive oxygen species (ROS) was measured by flow cytometry. Autophagy was assessed by Western blot for LC3II activation. Lipid metabolic studies were performed using [3H]palmitic acid.

Results. Treatment with DM102 (10 μM) for 24 hr increased levels of 3H-palmitic acid-labeled ceramide by 2-fold, compared to controls, in MDA-MB 231 breast cancer cells. As single agents in viability studies, both C6-cer (IC50 5-10 μM) and DM102 (IC50 20 μM) were only moderately cytotoxic in MDA-MB 231, MCF-7, and SKBr3 cells. Co-administration however, produced synergistic decreases in viability (CI &lt;0.5) in all cell lines, at doses that were half the IC50 of either agent alone. Similar cytotoxic profiles were observed with C6-cer and the AC inhibitor B13, (1R,2R)-2-N-(tetradecanoylamino)-1-(4′-nitrophenyl)-1,3-propiandiol, suggesting that blocking AC activity was key to the synergistic effect. At 24 hr, combination C6-cer/DM102 increased ROS levels &gt; 2-fold in MDA-MB 231 cells. This was accompanied by activation of the autophagy marker, LC3II and a decrease in phospho-Akt, a promoter of cell survival and negative regulator of autophagy. At 48 hr, caspase 3/7 activity increased &gt; 2-fold, and by 72 hr there was a &gt;70% increase in Annexin-V positive cells, compared to controls.

Conclusions. These data support the efficacy of targeting AC in combination with exogenous short-chain ceramide as an anticancer strategy, and warrant further investigation into the mechanism of action of the C6 ceramide/DM102 drug duo. (Support: NIGMS77391)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5484. doi:10.1158/1538-7445.AM2011-5484

P-glycoprotein antagonists confer synergistic sensitivity to short-chain ceramide in human multidrug-resistant cancer cells

P-glycoprotein (P-gp) antagonists inhibit ceramide metabolism at the juncture of glycosylation. The purpose of this study was to test whether targeting P-gp would be a viable alternative to targeting glucosylceramide synthase (GCS) for enhancing ceramide cytotoxicity. A2780 wild-type, and multidrug-resistant 2780AD and NCI/ADR-RES human ovarian cancer cell lines and the cell-permeable ceramide analog, C6-ceramide (C6-cer), were employed. Compared to P-gp-poor A2780 cells, P-gp-rich 2780AD cells converted 3.7-fold more C6-cer to nontoxic C6-glucosylceramide (C6-GC), whereas cell-free GCS activities were equal. 2780AD cells displayed resistance to C6-cer (10 μM) that was reversed by inclusion of the P-gp antagonist tamoxifen (5 μM) but not by inclusion of a GCS inhibitor. Co-administration of C6-cer and P-gp antagonists was also effective in NCI/ADR-RES cells. For example, C6-cer, VX-710 (Biricodar), and cyclosporin A (cyc A) exposure resulted in viabilities of ~90% of control; however, C6-cer/VX-710 and C6-cer/cyc A additions were synergistic and resulted in viabilities of 22% and 17%, respectively. Further, whereas C6-ceramide and cyc A imparted 1.5- and 0-fold increases in caspase 3/7 activity, the combination produced a 3.5-fold increase. Although the upstream elements of cell death have not been elucidated, the novel C6-ceramide/P-gp antagonist combination merits further study and assessment of clinical translational potential.

Expression of P-glycoprotein in HeLa cells confers resistance to ceramide cytotoxicity

The role of glucosylceramide synthase (GCS) in regulating ceramide-induced apoptosis has been widely studied. The purpose of this investigation was to evaluate the role of P-glycoprotein (P-gp) in regulating ceramide cytotoxicity by using C6-ceramide. To accomplish this, we employed HeLa cells with conditional expression of the multidrug resistance gene 1/P-gp. HeLa cells expressing P-gp (P-gp/on cells) challenged with [14C]C6-ceramide (6 µM), synthesized 4.5-fold the amount of C6-glucosylceramide (GC) compared to HeLa cells with suppressed expression of P-gp (P-gp/off cells), whereas the generated levels of C6-sphingomyelin were almost equal (33 and 29% of intracellular 14C, respectively). Tamoxifen, a P-gp antagonist, decreased the C6-GC levels from 3.5-1.0% in the P-gp/off and from 17-2.8% of the total lipid 14C levels in the P-gp/on cells. Tamoxifen did not inhibit cell-free C6-GC synthesis in the P-gp/off or P-gp/on homogenates. However, a specific GCS inhibitor, ethylenedioxy-1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol (ethylenedioxy-P4), blocked synthesis by 90%. In the cytotoxicity assays, the P-gp/off cells were sensitive to C6-ceramide and the P-gp/on cells were resistant. Resistance to C6-ceramide in the P-gp/on cells was reversed by tamoxifen but not by ethylenedioxy-P4. Experiments in another cervical cancer model showed that multidrug-resistant P-gp-rich KB-V1 cells synthesized 3-fold more C6-GC from C6-ceramide than the parental, P-gp-poor KB-3-1 cells, and whereas tamoxifen had no effect on the C6-GC synthesis in the KB-3-1 cells, it inhibited synthesis by 70% in the KB-V1 cells. This study demonstrates that P-gp potentiates C6-ceramide glycosylation and if antagonized augments C6-ceramide sensitivity, both features previously ascribed to GCS. We propose that P-gp can be an effective target for enhancing short-chain ceramide cytotoxicity in the treatment of drug-resistant cancer.

Inhibition of NADPH oxidase by glucosylceramide confers chemoresistance

The bioactive sphingolipid ceramide induces oxidative stress by disrupting mitochondrial function and stimulating NADPH oxidase (NOX) activity, both implicated in cell death mechanisms. Many anticancer chemotherapeutics (anthracyclines, Vinca alkaloids, paclitaxel, and fenretinide), as well as physiological stimuli such as tumor necrosis factor α (TNFα), stimulate ceramide accumulation and increase oxidative stress in malignant cells. Consequently, ceramide metabolism in malignant cells and, in particular the up-regulation of glucosylceramide synthase (GCS), has gained considerable interest in contributing to chemoresistance. We hypothesized that increases in GCS activity and thus glucosylceramide, the product of GCS activity, represents an important resistance mechanism in glioblastoma. In our study, we determined that increased GCS activity effectively blocked reactive oxygen species formation by NOX. We further showed, in both glioblastoma and neuroblastoma cells that glucosylceramide directly interfered with NOX assembly, hence delineating a direct resistance mechanism. Collectively, our findings indicated that pharmacological or molecular targeting of GCS, using non-toxic nanoliposome delivery systems, successfully augmented NOX activity, and improved the efficacy of known chemotherapeutic agents.

Glucosylceramide synthase upregulates MDR1 expression in the regulation of cancer drug resistance through cSrc and beta-catenin signaling

Background

Drug resistance is the outcome of multiple-gene interactions in cancer cells under stress of anticancer agents. MDR1 overexpression is most commonly detected in drug-resistant cancers and accompanied with other gene alterations including enhanced glucosylceramide synthase (GCS). MDR1 encodes for P-glycoprotein that extrudes anticancer drugs. Polymorphisms of MDR1 disrupt the effects of P-glycoprotein antagonists and limit the success of drug resistance reversal in clinical trials. GCS converts ceramide to glucosylceramide, reducing the impact of ceramide-induced apoptosis and increasing glycosphingolipid (GSL) synthesis. Understanding the molecular mechanisms underlying MDR1 overexpression and how it interacts with GCS may find effective approaches to reverse drug resistance.

Results

MDR1 and GCS were coincidently overexpressed in drug-resistant breast, ovary, cervical and colon cancer cells; silencing GCS using a novel mixed-backbone oligonucleotide (MBO-asGCS) sensitized these four drug-resistant cell lines to doxorubicin. This sensitization was correlated with the decreased MDR1 expression and the increased doxorubicin accumulation. Doxorubicin treatment induced GCS and MDR1 expression in tumors, but MBO-asGCS treatment eliminated "in-vivo" growth of drug-resistant tumor (NCI/ADR-RES). MBO-asGCS suppressed the expression of MDR1 with GCS and sensitized NCI/ADR-RES tumor to doxorubicin. The expression of P-glycoprotein and the function of its drug efflux of tumors were decreased by 4 and 8 times after MBO-asGCS treatment, even though this treatment did not have a significant effect on P-glycoprotein in normal small intestine. GCS transient transfection induced MDR1 overexpression and increased P-glycoprotein efflux in dose-dependent fashion in OVCAR-8 cancer cells. GSL profiling, silencing of globotriaosylceramide synthase and assessment of signaling pathway indicated that GCS transfection significantly increased globo series GSLs (globotriaosylceramide Gb3, globotetraosylceramide Gb4) on GSL-enriched microdomain (GEM), activated cSrc kinase, decreased β-catenin phosphorylation, and increased nuclear β-catenin. These consequently increased MDR1 promoter activation and its expression. Conversely, MBO-asGCS treatments decreased globo series GSLs (Gb3, Gb4), cSrc kinase and nuclear β-catenin, and suppressed MDR-1 expression in dose-dependent pattern.

Conclusion

This study demonstrates, for the first time, that GCS upregulates MDR1 expression modulating drug resistance of cancer. GSLs, in particular globo series GSLs mediate gene expression of MDR1 through cSrc and β-catenin signaling pathway.

Abstract 1658: Fenretinide induces lethal autophagy in pancreatic cancer cells

Background: Recent studies show that the synthetic vitamin A analog fenretinide (4-HPR) is cytotoxic in a number of human tumor cell lines and is a chemopreventative agent that is well-tolerated clinically. 4-HPR has been shown to augment intracellular levels of reactive oxygen species (ROS) and ceramides, and to activate the MAPK family of kinases. Pancreatic cancer cells are sensitive to increased levels of ROS, ceramides, and MAPK phosphorylation, suggesting 4-HPR will be an effective agent against this disease.

Results: 4-HPR exposure markedly decreased viability of MIA PaCa-2 and PANC-1 cells, as shown by a dose-dependent decrease in absorbance from an MTS assay and a 3-fold increase in propidium iodide staining. A 50% reduction in [3H]thymidine incorporation showed that cell proliferation was halted as well. 4-HPR treatment caused a 3-fold increase in de novo ceramide production at 24 h and a time- and dose-dependent generation of ROS. 4-HPR led to a minor increase in caspase 3/7 activity and in FITC-Annexin V-positive cells. However, other measures of apoptosis were lacking, suggesting apoptosis was not the major death pathway elicited by 4-HPR. Further investigation of the mechanism of cell death suggests that the dominant pathway triggered by 4-HPR is lethal autophagy. After 4-HPR treatment, microscopic analysis of cells exposed to acridine orange showed bright red autophagic vesicles, and Western blot showed a robust increase in LC3 II protein expression. Pretreatment with the autophagy inhibitor 3-methyladenine (3-MA) or the ROS scavenger vitamin E (VE) abrogated the increase in LC3 II expression caused by 4-HPR exposure, and both 3-MA and VE partially preserved viability. A sustained, dose-dependent increase in phosphorylation of the stress kinases JNK and p38 was observed following incubation with 4-HPR, whereas phosphorylation of the mitogenic kinase ERK 1/2 decreased. Pretreatment with myriocin (an inhibitor of de novo ceramide production), 3-MA, or VE attenuated the increase in JNK and p38 phosphorylation caused by 4-HPR. Additionally, VE pretreatment blocked the decrease in ERK 1/2 phosphorylation elicited by 4-HPR.

Conclusions: These data suggest that 4-HPR induces lethal autophagy in human pancreatic cancer cells by a mechanism that involves generation of ROS and engagement of the MAPK and sphingolipid signaling pathways. 4-HPR used alone or in combination with current therapies or signaling pathway modulators may be a promising avenue for future pancreatic cancer therapies. [Support: NIH/NIGMS grant no. GM077391]

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1658.

Metabolism of short-chain ceramide by human cancer cells--implications for therapeutic approaches

Due to recent use of short-chain ceramides in preclinical studies, we characterized C6-ceramide metabolism in cancer cell lines and assessed metabolic junctures for enhancing efficacy. MDA-MB-231 breast cancer cells decreased the amount of C6-ceramide metabolized to C6-sphingomyelin (C6-SM) and increased the amount metabolized to C6-glucosylceramide (C6-GC) in response to increasing concentrations. A similar trend was seen in DU-145 (prostate cancer), PANC-1 (pancreatic cancer), and LoVo (colorectal cancer) cells. KG-1 leukemia cells favored C6-SM synthesis at low (0.6muM) and high-dose (12muM) C6-ceramide. Partnering C6-ceramide with tamoxifen, a P-glycoprotein antagonist that impedes ceramide glycosylation, was an effective regimen for enhancing cytotoxicity in cells. Experiments to assess the mechanism of cell death using KG-1 cells showed that tamoxifen inhibited synthesis of C6-GC and C6-SM from C6-ceramide by 80% and 50%, respectively, which was accompanied by enhanced apoptosis. Radiolabeling of KG-1 cells with [(3)H]palmitic acid produced a 2-fold increase in (3)H-long-chain ceramides when unlabeled C6-ceramide was added and a 9-fold increase when C6-ceramide and tamoxifen were added. The increase in (3)H-palmitate radiolabeling of long-chain ceramides was blocked by inclusion of a ceramide synthase inhibitor; however, inhibiting synthesis of long-chain ceramide did not rescue cells. These studies show that tamoxifen enhances the apoptotic effects of C6-ceramide. The proposed mechanism involves blocking short-chain ceramide anabolism to favor hydrolysis and generation of sphingosine. We propose that use of tamoxifen and other P-glycoprotein antagonists can be an effective means for enhancing cytotoxic potential of short-chain ceramides in the treatment of cancer.

Direct quantitative determination of ceramide glycosylation in vivo: a new approach to evaluate cellular enzyme activity of glucosylceramide synthase

Glucosylceramide synthase (GCS or GlcT-1), converting ceramide to glucosylceramide, is a key enzyme for the synthesis of glycosphingolipids. Due to its diverse roles in physiology and diseases, GCS may be a disease marker and drug target. Current assays for enzymes including GCS are based on reactions conducted in a test tube using enzyme preparations. Measurement of enzyme activity in laboratory-made conditions cannot directly evaluate the role of GCS in cells. Here, we introduce a new approach to determine GCS cellular activity using fluorescent NBD C6-ceramide in vivo. Cellular GCS transfers UDP-glucose to NBD C6-ceramide and produces NBD C6-glucosylceramide. C6-glucosylceramide is then separated from C6-ceramide by thin-layer chromatography and both are then quantitated by spectrophotometer. This cell-based method is able to quantitate glucosylceramide in pmol range, produced by approximately 50,000 cells or 1.0 mg tissue. This method has been used successfully to evaluate the degrees of GCS enzyme in cells and in tumors subjected to gene manipulation and chemical inhibition. These data indicate that this cell-based fluorescent method is direct, reproducible, and simple for assessing ceramide glycosylation. It is applicable to validate GCS activity in drug-resistant cancers and in other disorders.

N-(4-Hydroxyphenyl)retinamide increases dihydroceramide and synergizes with dimethylsphingosine to enhance cancer cell killing

Fenretinide [N-(4-hydroxyphenyl)retinamide (4-HPR)] is cytotoxic in many cancer cell types. Studies have shown that elevation of ceramide species plays a role in 4-HPR cytotoxicity. To determine 4-HPR activity in a multidrug-resistant cancer cell line as well as to study ceramide metabolism, MCF-7/AdrR cells (redesignated NCI/ADR-RES) were treated with 4-HPR and sphingolipids were analyzed. TLC analysis of cells radiolabeled with [3H]palmitic acid showed that 4-HPR elicited a dose-responsive increase in radioactivity migrating in the ceramide region of the chromatogram and a decrease in cell viability. Results from liquid chromatography/electrospray tandem mass spectrometry revealed large elevations in dihydroceramides (N-acylsphinganines), but not desaturated ceramides, and large increases in complex dihydrosphingolipids (dihydrosphingomyelins, monohexosyldihydroceramides), sphinganine, and sphinganine 1-phosphate. To test the hypothesis that elevation of sphinganine participates in the cytotoxicity of 4-HPR, cells were treated with the sphingosine kinase inhibitor d-erythro-N,N-dimethylsphingosine (DMS), with and without 4-HPR. After 24 h, the 4-HPR/DMS combination caused a 9-fold increase in sphinganine that was sustained through +48 hours, decreased sphinganine 1-phosphate, and increased cytotoxicity. Increased dihydrosphingolipids and sphinganine were also found in HL-60 leukemia cells and HT-29 colon cancer cells treated with 4-HPR. The 4-HPR/DMS combination elicited increased apoptosis in all three cell lines. We propose that a mechanism of 4-HPR-induced cytotoxicity involves increases in dihydrosphingolipids, and that the synergy between 4-HPR and DMS is associated with large increases in cellular sphinganine. These studies suggest that enhanced clinical efficacy of 4-HPR may be realized through regimens containing agents that modulate sphingoid base metabolism.

A role for ceramide in driving cancer cell resistance to doxorubicin

Advanced cancers acquire resistance to chemotherapy, and this results in treatment failure. The cellular mechanisms of chemotherapy resistance are not well understood. Here, for the first time, we show that ceramide contributes to cellular resistance to doxorubicin through up-regulating the gene expression of glucosylceramide synthase (GCS). Ceramide, a cellular lipid messenger, modulates doxorubicin-induced cell death. GCS catalyzes ceramide glycosylation, converting ceramide to glucosylceramide; this process hastens ceramide clearance and limits ceramide-induced apoptosis. In the present study, we evaluated the role of the GCS gene in doxorubicin resistance using several paired wild-type and drug-resistant (doxorubicin-selected) cancer cell lines, including breast, ovary, cervical, and colon. GCS was overexpressed in all drug-resistant counterparts, and suppressing GCS overexpression using antisense oligonucleotide restored doxorubicin sensitivity. Characterizing the effect mechanism showed that doxorubicin exposure increased ceramide levels, enhanced GCS expression, and imparted cellular resistance. Exogenous C(6)-ceramide and sphingomyelinase treatments mimicked the influence of doxorubicin on GCS, activating the GCS promoter and up-regulating GCS gene expression. Fumonisin B(1), an inhibitor of ceramide synthesis, significantly suppressed doxorubicin-up-regulated GCS expression. Promoter truncation, point mutation, gel-shift, and protein-DNA ELISA analysis showed that transcription factor Sp1 was essential for ceramide-induced GCS up-regulation. These data indicate that ceramide-governed GCS gene expression drives cellular resistance to doxorubicin.