Genomic stability at the coding regions of the multidrug transporter gene ABCB1: insights into the development of alternative drug resistance mechanisms in human leukemia cells.

AIM: Despite considerable efforts to reverse clinical multidrug resistance (MDR), targeting the predominant multidrug transporter ABCB1/P-glycoprotein (P-gp) using small molecule inhibitors has been unsuccessful, possibly due to the emergence of alternative drug resistance mechanisms. However, the non-specific P-gp inhibitor cyclosporine (CsA) showed significant clinical benefits in patients with acute myeloid leukemia (AML), which likely represents the only proof-of-principle clinical trial using several generations of MDR inhibitors. Nevertheless, the mutational mechanisms that may underlie unsuccessful MDR modulation by CsA are not elucidated because of the absence of CsA-relevant cellular models. In this study, our aims were to establish CsA-resistant leukemia models and to examine the presence or absence of ABCB1 exonic mutations in these models as well as in diverse types of human cancer samples including AMLs. METHODS: Drug-resistant lines were established by stepwise drug co-selection and characterized by drug sensitivity assay, rhodamine-123 accumulation, [3H]-labeled drug export, ABCB1 cDNA sequencing, and RNase protection assay. The genomic stability of the ABCB1 coding regions was evaluated by exome sequencing analysis of variant allele frequencies in human populations. Moreover, the mutational spectrum of ABCB1 was further assessed in diverse types of cancer samples including AMLs in the Cancer Genome Atlas (TCGA) at the National Cancer Institute. RESULTS: We report the development of two erythroleukemia variants, RVC and RDC, which were derived by stepwise co-selection of K562/R7 drug-resistant leukemia cells with the etoposide-CsA and doxorubicin-CsA drug combinations, respectively. Interestingly, both RVC and RDC cell lines, which retained P-gp expression, showed altered multidrug-resistant phenotypes that were resistant to CsA modulation. Strikingly, no mutations were found in the ABCB1 coding regions in these variant cells even under long-term stringent drug selection. Genomically, ABCB1 displayed relatively low variant allele frequencies in human populations when compared with several ABC superfamily members. Moreover, ABCB1 also exhibited a very low mutational frequency in AMLs compared with all types of human cancer. In addition, we found that CsA played a role in undermining the selection of highly drug-resistant cells via induction of low-level and unstable drug resistance. CONCLUSION: Our data indicate that ABCB1 coding regions are genomically stable and relatively resistant to drug-induced mutations. Non-ABCB1 mutational mechanisms are responsible for the drug-resistant phenotypes in both RVC and RDC cell lines, which are also prevalent in clinical AML patients. Accordingly, we propose several relevant models that account for the development of alternative drug resistance mechanisms in the absence of ABCB1 mutations.

PKCzeta protects against UV-C-induced apoptosis by inhibiting acid sphingomyelinase-dependent ceramide production.

In a recent study, we described that UV-C irradiation resulted in redox-dependent activation and relocalization of A-SMase (acid sphingomyelinase) to the external surface of raft membrane microdomains, hydrolysis of SM (sphingomyelin) associated with the plasma membrane outer leaflet, ceramide generation and apoptosis. In the present study, we have investigated the influence of PKCzeta (protein kinase Czeta), an atypical form of PKC on this pathway. This study shows that PKCzeta overexpression resulted in the abrogation of UV-C-induced A-SMase translocation and activation into the raft microdomains, lack of ceramide generation and apoptosis inhibition. Moreover, PKCzeta overexpression resulted in a decrease in UV-C-induced ROS (reactive oxygen species) production, which correlated with increased gene expression level of various antioxidant enzymes, including TRx (thioredoxin), TR (thioredoxin reductase) 1, TR2 and peroxiredoxin 1/TPx2 (thioredoxin peroxidase 2). Importantly, enforced TPx2 gene expression inhibited UV-C-induced A-SMase translocation. Finally, PKCzeta inhibition led to a significant reduction in TPx2 protein expression. Altogether, these results suggest that PKCzeta interferes with the UV-activated sphingolipid signalling pathway by regulating the TRx system. These findings may have important consequences for UV-induced carcinogenesis and resistance to phototherapy.

Sphingolipids as modulators of cancer cell death: potential therapeutic targets.

Through modifications in the fine membrane structure, cell-cell or cell-matrix interactions, and/or modulation of intracellular signaling pathways, sphingolipids can affect the tumorigenic potential of numerous cell types. Whereas ceramide and its metabolites have been described as regulators of cell growth and apoptosis, these lipids as well as other sphingolipid molecules can modulate the ability of malignant cells to grow and resist anticancer treatments, and their susceptibility to non-apoptotic cell deaths. This review summarizes our current knowledge on the properties of sphingolipids in the regulation of cancer cell death and tumor development. It also provides an update on the potential perspectives of manipulating sphingolipid metabolism and using sphingolipid analogues in anticancer therapy.

UV-C light induces raft-associated acid sphingomyelinase and JNK activation and translocation independently on a nuclear signal.

The initiation of UV light-induced signaling in mammalian cells is largely considered to be subsequent to DNA damage. Several studies have also described ceramide (CER), a lipid second messenger, as a major contributor in mediating UV light-induced c-Jun N-terminal kinase (JNK) activation and cell death. It is demonstrated here that UV-C light irradiation of U937 cells results in the activation and translocation of a Zn2+-independent acid sphingomyelinase, leading to CER accumulation in raft microdomains. These CER-enriched rafts aggregate and play a functional role in JNK activation. The observation that UV-C light also induced CER generation and the externalization of acid sphingomyelinase and JNK in human platelets conclusively rules out the involvement of a nuclear signal generated by DNA damage in the initiation of a UV light response, which is generated at the plasma membrane.

Resistance to microtubule-targeted cytotoxins in a K562 leukemia cell variant associated with altered tubulin expression and polymerization.

A vinblastine resistant cell line, KCVB2, was established by co-selecting the parental erythroleukemic cell line K562 with step-wise increased concentrations of vinblastine (Velban) in the presence of the cyclosporin D analogue PSC 833 (2 microM), a potent modulator of the multidrug resistance phenotype. KCVB2 cells are 8-fold resistant to the selecting agent, vinblastine, but also exhibit significant resistance to other vinca alkaloids, including 14-fold resistance to vinorelbine, as well as 6-fold cross-resistance to paclitaxel. Doubling time and morphology were similar to the parental K562 cells. Rt-PCR analysis revealed no alterations in the expression of the mdr1 and MRP genes. Intracellular vinblastine accumulation was unchanged. Disruption of the mitotic spindles and multiple mitotic asters occurred in both cell lines but required higher concentrations of vinblastine in KCVB2 cells than in K562 cells. Significant differences were observed in the tubulin content of KCVB2 cells: reduction of total tubulin content, increased polymerized fraction of total tubulin, and overexpression of class III beta-tubulin which is expressed at very low levels in the parental K562 cells. K562 cells transfected with murine class III beta-tubulin did not display the resistance pattern observed in KCVB2 cells. Revertants of KCVB2 manifested reversion to parental drug sensitivity, an increase in total tubulin level, and a decrease in polymerized tubulin. In conclusion, the KCVB2 cell line displays a novel mechanism of resistance to both depolymerizing and stabilizing microtubule-targeted cytotoxins which does not involve altered cellular drug accumulation, but corresponds to alterations in the total tubulin content and polymerization status, and may involve an effect on microtubule dynamics.

Ceramide : A New Target in Anticancer Research?

An astute observer of chemotherapeutic progress concedes that anticancer research is, in effect, often more empirical that rational, and there are those who maintain that little gain has been made in the therapeutic outcome of cancer patients in the past 60 years, thus, questioning the wisdom of continued expenditure of funds for research in cancer chemotherapy. Half a decade ago few would have denied the importance of regarding the intimate drug-target interactions in cell death or in resistance thereof. However, the heretofore-parochial approach to drug screening and rationalization of structure-activity based on specific drug-target interactions has met with limited success. This standpoint has forced investigators to explore new and certainly provocative interpretations, which necessitate radical departure from accepted but perhaps outworn concepts of the action of drugs. The recent identification of a ceramide mediated apoptotic signaling pathway triggered by antitumor agents may offer new insights into mechanism of action of chemo- and radio-therapy. This review is intended to initiate a debate on the role of ceramide in apoptosis signaling: is it a consequence of cell damage, or part of an independent cytotoxic pathway ?

Rituximab antiproliferative effect in B-lymphoma cells is associated with acid-sphingomyelinase activation in raft microdomains.

Rituximab is a chimeric human immunoglobulin G1 (IgG1) anti-CD20 monoclonal antibody with significant activity against CD20+ malignant B cells. Rituximab is currently used with success in the treatment of B-cell-derived lymphoid neoplasias either alone or in combination with chemotherapy. However, the predominant mechanism by which rituximab exerts its antitumor properties in vivo remains unknown. In the present study, we demonstrate that in Daudi and RL B-lymphoma cells, rituximab (without cross-linking) used at the saturating dose of 10 microg/mL induced moderate accumulation in G1 phase, growth inhibition, and significant loss in clonogenic potential. However, in these cells, rituximab induced no apoptosis. Furthermore, we observed that treatment with rituximab resulted in a rapid and transient increase in acid-sphingomyelinase (A-SMase) activity and concomitant cellular ceramide (CER) generation in raft microdomains. We also observed that rituximab-treated cells externalized both A-SMase and CER that colocalized with the CD20 receptor. Finally, we present evidence that rituximab-induced growth inhibition may be mediated through a CER-triggered signaling pathway, leading to the induction of cell cycle-dependent kinase inhibitors such as p27Kip1 through a mitogen-activated protein kinase (MAPK)-dependent mechanism.

Cytoprotective effect of glucosylceramide synthase inhibition against daunorubicin-induced apoptosis in human leukemic cell lines.

Several studies have shown that ceramide (CER) glucosylation contributes to drug resistance in multidrug-resistant cells and that inhibition of glucosylceramide synthase sensitizes cells to various drug treatments. However, the role of glucosylceramide synthase has not been studied in drug-sensitive cancer cells. We have demonstrated previously that the anthracycline daunorubicin (DNR) rapidly induces interphasic apoptosis through neutral sphingomyelinase-mediated CER generation in human leukemic cell lines. We now report that inhibition of glucosylceramide synthase using d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) or 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) protected U937 and HL-60 cells from DNR-induced apoptosis. Moreover, blocking CER glucosylation did not lead to increased CER levels but to increased CER galactosylation. We also observed that pretreating cells with galactosylceramide (GalCER) significantly inhibited DNR-induced apoptosis. Finally, we show that GalCER-enriched lymphoblast cells (Krabbe's disease) were significantly more resistant to DNR- and cytosine arabinoside-induced apoptosis as compared with normal lymphoblasts, whereas glucosylceramide-enriched cells (Gaucher's disease) were more sensitive. In conclusion, this study suggests that sphingomyelin-derived CER in itself is not a second messenger but rather a precursor of both an apoptosis second messenger (GD3) and an apoptosis "protector" (GalCER).

Con A activates an Akt/PKB dependent survival mechanism to modulate TCR induced cell death in double positive thymocytes.

While low avidity ligation of the T cell receptor (TCR) leads to positive selection and further maturation of developing thymocytes providing the immune system with mature CD4(+) and CD8(+) (single positive) T cells, high avidity ligation triggers negative selection by apoptotic cell death and therefore the TCR repertoire is purged of autoreactive T cells. On peripheral T cells, however, high avidity ligation of the TCR triggers activation and survival not death. In the present study we used concanavalin A (Con A) and alpha-CD3 epsilon antibody to investigate a possible survival mechanism in connection with TCR ligation. Con A and alpha-CD3 epsilon were used in the study for the following reasons: (1) they both mimic the effects of high avidity TCR ligation by activating peripheral T cells, and (2) they trigger distinctively different physiological changes in developing thymocytes. While Con A supports events associated with cellular survival, alpha-CD3 epsilon induces apoptotic cell death. In our experimental system the TCR was cross-linked by Con A and alpha-CD3 epsilon in thymocytes of major histocompatibility complex (MHC) deficient thymus organ cultures, where signals from the TCR can be triggered on zero background signal level. We have found that TCR cross-linking by Con A and not by alpha-CD3 epsilon decreases the gene and protein expression of the pro-apoptotic molecule, Bad; and that Con A is capable of the activation of the survival signalling pathway including protein kinase B (Akt/PKB) independently of phosphatidyl inositol kinase (PI3K).

Implication of raft microdomains in drug induced apoptosis.

DNA damaging agents such as 1-beta-D-arabinofuranosylcytosine (Ara-C) and daunorubicin (DNR) are widely used in the treatment of acute nonlymphocytic leukemia. These drugs have, of course, been the objects of intense basic research, as well as preclinical and clinical study. Although specific biochemical lesions (DNA damage) have been associated with Ara-C- and DNR-mediated cytotoxicity, the pathways leading to the induction of apoptosis remain ill defined. This standpoint has forced investigators to explore a new concept in cell response to cytotoxic stress: apoptosis signaling. The recent identification of a ceramide (CER) mediated apoptotic signaling pathway triggered by antitumor agents offers a new perspective for the treatment of neoplastic cells. Indeed, these agents have been shown to induce apoptosis through the activation of a sphingomyelinase (SMase) responsible for the hydrolysis of sphingomyelin (SM) and the generation of CER. The latter acts as a potent apoptosis mediator, triggering several downstream signaling pathways among which the stress-activated protein kinase cascade (MEKK1-SEK1-SAP/JNK) plays a critical role in apoptosis induction. However, the spacio-temporal organization of the key early signaling events is unclear. The present review delineates what appears to be a critical factor in apoptosis signaling: sphingomyelin enriched plasma membrane rafts. The apparent topological partitioning between DNA damage and apoptosis signaling (integrated into specialized plasma membrane domains) is discussed.

Phosphatidylcholine-derived phosphatidic acid and diacylglycerol are involved in the signaling pathways activated by docetaxel.

Taxanes are known to activate several cellular signals including mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-kappa B), tyrosine phosphorylation of Shc, and serine phosphorylation of Bcl-2. However, the mediators of these signaling pathways are unknown. Using U937 leukemic cells, we evaluated the effect of docetaxel on phosphatidylcholine (PC) and its metabolites, phosphatidic acid (PA) and diacylglycerol (DAG), and their impact on MAPK and NF-kappa B activation, as well as on Raf-1 and Bcl-2 phosphorylation. Metabolic labeling studies showed that docetaxel (10 nM) induced two waves of PA production (130-140%), which were detected at 1 and 10 min. Docetaxel also stimulated DAG production (130%), which followed the first PA wave. The initial PA burst was due to phospholipase D (PLD)-mediated PC hydrolysis. Subsequent DAG production was inhibited by the phosphatidate phosphohydrolase (PAP) inhibitor, propranolol. R59949, a DAG kinase inhibitor, increased DAG accumulation and blocked the second PA wave. These results suggest that docetaxel triggers a metabolic cascade consisting in PLD-mediated PC hydrolysis, PA release, PAP-dependent DAG production, and DAG kinase stimulation, leading to DAG conversion back to PA. Neither R59949 nor propranolol influenced docetaxel-induced Raf-1/ERK activation. However, R59949 abrogated both NF-kappa B activation and Bcl-2 phosphorylation, suggesting that DAG and/or DAG-derived PA contribute in regulating these events.

Protein kinase Czeta mediated Raf-1/extracellular-regulated kinase activation by daunorubicin.

In light of the emerging concept of a protective function of the mitogen-activated protein kinase (MAPK) pathway under stress conditions, we investigated the influence of the anthracycline daunorubicin (DNR) on MAPK signaling and its possible contribution to DNR-induced cytotoxicity. We show that DNR increased phosphorylation of extracellular-regulated kinases (ERKs) and stimulated activities of both Raf-1 and extracellular-regulated kinase 1 (ERK1) within 10 to 30 minutes in U937 cells. ERK1 stimulation was completely blocked by either the mitogen-induced extracellular kinase (MEK) inhibitor PD98059 or the Raf-1 inhibitor 8-bromo-cAMP (cyclic adenosine monophosphate). However, only partial inhibition of Raf-1 and ERK1 stimulation was observed with the antioxidant N-acetylcysteine (N-Ac). Moreover, the xanthogenate compound D609 that inhibits DNR-induced phosphatidylcholine (PC) hydrolysis and subsequent diacylglycerol (DAG) production, as well as wortmannin that blocks phosphoinositide-3 kinase (PI3K) stimulation, only partially inhibited Raf-1 and ERK1 stimulation. We also observed that DNR stimulated protein kinase C zeta (PKCzeta), an atypical PKC isoform, and that both D609 and wortmannin significantly inhibited DNR-triggered PKCzeta activation. Finally, we found that the expression of PKCzeta kinase-defective mutant resulted in the abrogation of DNR-induced ERK phosphorylation. Altogether, these results demonstrate that DNR activates the classical Raf-1/MEK/ERK pathway and that Raf-1 activation is mediated through complex signaling pathways that involve at least 2 contributors: PC-derived DAG and PI3K products that converge toward PKCzeta. Moreover, we show that both Raf-1 and MEK inhibitors, as well as PKCzeta inhibition, sensitized cells to DNR-induced cytotoxicity.

Overexpression of protein kinase Czeta confers protection against antileukemic drugs by inhibiting the redox-dependent sphingomyelinase activation.

Induction of apoptosis by chemotherapeutic drugs involves the sphingomyelin-ceramide (SM-CER) pathway. This signaling is critically dependent on reactive oxygen species (ROS) generation and p53/p56 Lyn activation. In this study, we have investigated the influence of protein kinase C (PKC) zeta overexpression on the SM-CER pathway in U937 human leukemia cell line. We show that PKCzeta overexpression resulted in delayed apoptosis and significant resistance to both 1-beta-D-arabinofuranosylcytosine (ara-C) and daunorubicin (DNR), but there was no significant protection against cell-permeant C(6)-CER. Moreover, PKCzeta overexpression abrogated drug-induced neutral sphingomyelinase stimulation and CER generation by inhibiting ROS production. We further investigated p53/p56 Lyn activation in PKCzeta-overexpressing U937 cells treated with ara-C or DNR. We demonstrate that PKCzeta inhibited p53/p56 Lyn phosphorylation and stimulation in drug- or H(2)O(2)-treated cells, suggesting that p53/p56 Lyn redox regulation is altered in PKCzeta-overexpressing cells. Finally, we show that PKCzeta-overexpressing U937 cells displayed accelerated H(2)O(2) detoxification. Altogether, our study provides evidence for the role of PKCzeta in the negative regulation of drug-induced SM-CER pathway.

Daunorubicin- and Ara-C-induced interphasic apoptosis of human type II leukemia cells is caspase-8-independent.

Drug-induced interphasic apoptosis in human leukemia cells is mediated through intracellular signaling pathways, of which the most proximal (initiating) event remains unclear. Indeed, both early ceramide generation and procaspase-8 cleavage have been individually identified as the initial apoptotic signaling events which precede the mitochondrial control of the apoptotic execution phase in Type II cells. In order to evaluate whether or not procaspase-8 cleavage is requisite for initial ceramide generation and rapid interphasic apoptosis, we investigated the chronological ordering of early ceramide generation and caspase-8 cleavage induced by daunorubicin (DNR) and 1-beta-D-arabinofuranosylcytosine (Ara-C) in U937 cells. We further evaluated the impact of these two drugs on initial ceramide generation and apoptosis in wild-type Jurkat cells and Jurkat clones mutated for caspase-8 and Fas-associated death domain. We show that while both DNR and Ara-C similarly induced early ceramide generation (within 5-20 min) and interphasic apoptosis in all cell models, caspase-8 cleavage was only observed farther downstream (4.5 h) and only in DNR-treated cells. Furthermore, neither DNR or Ara-C induced caspase-8 activation. These results demonstrate that caspase-8 cleavage is not requisite for the drug-induced activation of the ceramide-mediated interphasic apoptotic pathway in human Type II leukemic cells.

Ara-C- and daunorubicin-induced recruitment of Lyn in sphingomyelinase-enriched membrane rafts.

Induction of apoptosis by DNA-damaging agents such as 1-beta-D-arabinofuranosylcytosine (Ara-C) includes the activation of Lyn protein tyrosine kinase. We have previously established that Ara-C-induced activation of Lyn results in its binding to a neutral sphingomyelinase (SMase) and is requisite for its stimulation and the induction of apoptosis in U937 cells. However, the spacio-temporal organization of these events is unclear. This study demonstrates that part of the total cellular SMase activity is sequestered in sphingomyelin-enriched plasma membrane microdomains (rafts). Under Ara-C and daunorubicin (DNR) treatment, Lyn is rapidly activated and translocated into rafts. The compartmentalization of Lyn (as well as neutral SMase activation and apoptosis) induced by these drugs was blocked by the tyrosine kinase inhibitor herbimycin A and raft disruption. In conclusion, this study establishes that DNA-damaging agents such as Ara-C and DNR rapidly induce Lyn activation and its translocation into membrane rafts. This, in turn leads to neutral SMase activation and raft-associated sphingomyelin hydrolysis with the concomitant generation of the proapoptotic lipid second messenger, ceramide. The apparent topological partitioning between DNA damage and apoptosis signaling (integrated into specialized plasma membrane domains) is discussed.

Kit signaling inhibits the sphingomyelin-ceramide pathway through PLC gamma 1: implication in stem cell factor radioprotective effect.

Previous studies demonstrated that Kit activation confers radioprotection. However, the mechanism by which Kit signaling interferes with cellular response to ionizing radiation (IR) has not been firmly established. Based on the role of the sphingomyelin (SM) cycle apoptotic pathway in IR-induced apoptosis, we hypothesized that one of the Kit signaling components might inhibit IR-induced ceramide production or ceramide-induced apoptosis. Results show that, in both Ba/F3 and 32D murine cell lines transfected with wild-type c-kit, stem cell factor (SCF) stimulation resulted in a significant reduction of IR-induced apoptosis and cytotoxicity, whereas DNA repair remained unaffected. Moreover, SCF stimulation inhibited IR-induced neutral sphingomyelinase (N-SMase) stimulation and ceramide production. The SCF inhibitory effect on SM cycle was not influenced by wortmannin, a phosphoinositide-3 kinase (PI3K) inhibitor. The SCF protective effect was maintained in 32D-KitYF719 cells in which the PI3K/Akt signaling pathway is abolished due to mutation in Kit docking site for PI3K. In contrast, phospholipase C gamma (PLC gamma) inhibition by U73122 totally restored IR-induced N-SMase stimulation, ceramide production, and apoptosis in Kit-activated cells. Moreover, SCF did not protect 32D-KitYF728 cells (lacking a functional docking site for PLC gamma 1), from IR-induced SM cycle. Finally, SCF-induced radioprotection of human CD34(+) bone marrow cells was also inhibited by U73122. Altogether, these results suggest that SCF radioprotection is due to PLC gamma 1-dependent negative regulation of IR-induced N-SMase stimulation. Beyond the scope of Kit-expressing cells, it suggests that PLC gamma 1 status could greatly influence the post-DNA damage cellular response to IR, and perhaps, to other genotoxic agents.

Sphingomyelin content conditions insertion of daunorubicin within phosphatidylcholine monolayers.

Cell death induced by the chemotherapeutic drug daunorubicin (DNR) implicates an apoptotic pathway originating at the plasma membrane and characterized by sphingomyelin (SM) hydrolysis and ceramide generation. The mechanisms by which such a drug (hypothetically passively diffusing across a structural membrane) can trigger SM hydrolysis is unknown, but raises the question of the precise interaction between DNR and membrane lipid constituents. In this initial study, using alternative current polarography together with voltammetry, we report that after a first step of adsorption, insertion of DNR within a condensed phosphatidylcholine monolayer was significantly facilitated by SM content.