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NEOPLASIA
From the John Wayne Cancer Institute at Saint John's
Health Center, Santa Monica, CA; Departments of Cancer Biology and
Medicine, Wake Forest University, School of Medicine, Winston-Salem,
NC; and Mayo Clinic, Division of Oncology Research, Rochester, MN.
DT388-GM-CSF, a targeted fusion toxin constructed by
conjugation of human granulocyte-macrophage colony-stimulating factor (GM-CSF) with the catalytic and translocation domains of diphtheria toxin, is presently in phase I trials for patients with resistant acute
myeloid leukemia. HL-60/VCR, a multidrug-resistant human myeloid
leukemia cell line, and wild-type HL-60 cells were used to study the
impact of DT388-GM-CSF on metabolism of ceramide, a
modulator of apoptosis. After 48 hours with DT388-GM-CSF
(10 nM), ceramide levels in HL-60/VCR cells rose 6-fold and viability fell to 10%, whereas GM-CSF alone was without influence. Similar results were obtained in HL-60 cells. Examination of the time course
revealed that protein synthesis decreased by about 50% and cellular
ceramide levels increased by about 80% between 4 and 6 hours after
addition of DT388-GM-CSF. By 6 hours this was accompanied
by activation of caspase-9, followed by activation of caspase-3,
cleavage of caspase substrates, and chromatin fragmentation. Hygromycin
B and emetine failed to elevate ceramide levels or induce apoptosis at
concentrations that inhibited protein synthesis by 50%. Exposure to
C6-ceramide inhibited protein synthesis (EC50 ~5 µM) and decreased viability (EC50 ~6 µM).
Sphingomyelinase treatment depleted sphingomyelin by about 10%, while
increasing ceramide levels and inhibiting protein synthesis. Diphtheria
toxin increased ceramide and decreased sphingomyelin in U-937 cells, a
cell line extremely sensitive to diphtheria toxin; exposure to
DT388-GM-CSF showed sensitivity at less than 1.0 pM.
Diphtheria toxin and conjugate trigger ceramide formation that
contributes to apoptosis in human leukemia cells through caspase
activation and inhibition of protein synthesis.
(Blood. 2001;98:1927-1934) Despite recent advances in chemotherapy and use of
allogeneic bone marrow transplantation, a large number of patients with acute myelogenous leukemia (AML) eventually die of this disease. Complete remissions can be induced in up to 60% to 80% of new cases
with current treatment modalities, but development of resistance precludes long-term cure.1,2
A number of drug-resistance mechanisms have been described in AML.
These include overexpression of drug efflux pumps such as
P-glycoprotein (P-gp) and lung resistance protein, increased activity
of the drug metabolizing enzyme, glutathione-S-transferase (GST Targeted toxins or fusion toxins are recombinant polypeptides that
consist of the catalytic and translocation domains of a toxin fused
with a tumor-selective ligand such as an antibody or a growth factor.
The latter is able to bind target cells and trigger receptor-mediated
endocytosis, enabling the toxin to gain access to cytosol. Genetic
engineering of new growth factors opens the door for synthesis of new
fused toxins targeting different cells, tissues, and tumors. The
combination of high specificity and toxicity makes fused toxins a
potentially important treatment modality in oncology. A number of
targeted cytotoxic conjugates have been developed over the last decade
using diphtheria toxin, which is among the most potent toxins
known.16 The catalytic domain of diphtheria toxin
catalyzes the adenosine diphosphate (ADP)-ribosylation and
inactivation of elongation factor-2 (EF-2, also known as translocase),
thereby inhibiting protein synthesis.17 It has also been
suggested, however, that diphtheria toxin triggers other events that
contribute to the induction of apoptosis.18,19
Granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors are
present on the cell surface of leukemia blasts in a majority of
patients with AML.13 In an effort to target multidrug- resistant AML blasts expressing GM-CSF receptors, a recombinant diphtheria fusion toxin has been synthesized by linking a truncated form of diphtheria toxin (DT388) to human GM-CSF
(DT388-GM-CSF).20,21 In vincristine-resistant
HL-60 cells, modulation of doxorubicin resistance by
DT388-GM-CSF correlated with reductions in membrane P-gp
levels and increased uptake of doxorubicin,13 raising the possibility that DT388-GM-CSF acts by decreasing synthesis
of drug-resistance proteins.
In the present study, we have tested an alternative hypothesis for the
action of DT388-GM-CSF. Over the last decade, it has become clear that the sphingomyelin cycle (reviewed in Rizzieri and
Hannun22 and Kolesnick and Kronke23) plays an
important role in cell maturation, senscence, and apoptosis. In
particular, a number of investigations have shown that ceramide and
sphingolipids are involved in chemotherapy-induced apoptotic events in
cancer cells.24-29 For example, synthesis of ceramide has
been shown to mediate daunorubicin-induced apoptosis in leukemia
cells.26,27 Likewise, etoposide has been reported to
induce apoptosis via generation of ceramide.30 Further
genetic targeting through cellular transfection with glucosylceramide
synthase, which enhances removal of ceramide by way of glycosylation,
confers doxorubicin and tumor necrosis factor- Materials
Cells
Lipid metabolism and analysis To determine the influence of DT388-GM-CSF, GM-CSF, or diphtheria toxin on ceramide and sphingomyelin metabolism, stock cell cultures were washed twice with room-temperature phosphate-buffered saline (PBS), suspended in medium at 2.5 × 105 cells/mL, and 2.0 mL seeded into 6-well plates. Vincristine was removed from HL-60/VCR medium during the course of each experiment. Cellular lipids were radiolabeled by the addition of [3H]palmitic acid, 1.0 µCi/mL medium; the agent under study was added, and incubations were continued for the indicated times at 37°C in a humidified, 5% CO2 atmosphere tissue culture incubator.Total cellular lipids were extracted by the method of Bligh and Dyer,37 modified to contain 2% acetic acid in methanol, in a manner previously described.38 After evaporation of the lipid-rich organic lower phase using a stream of nitrogen, total cellular radiolabeled lipids were resuspended in 50 µL chloroform/methanol (1:1, v/v), and equal aliquots from each sample were spotted onto the origin of TLC plates. Commercial lipid standards were spotted and cochromatographed. Ceramide was resolved from other lipids in a solvent system containing chloroform/acetic acid (90:10, v/v). Sphingomyelin was resolved using a solvent system containing chloroform/methanol/acetic acid/water (50:30:7:4, v/v). After drying, the appropriate areas on the plate were identified by iodine vapor staining of the lipids; identification was made by comparison of Rf values, and spots were scraped into plastic scintillation vials containing 0.5 mL water. After 4.5 mL EcoLume was added, radioactivity was analyzed by liquid scintillation spectroscopy.36,38 Sphingomyelinase treatment The HL-60/VCR cells (250 000/mL 5% FBS medium) were labeled with [3H]palmitic acid (1.0 µCi/mL) for 24 hours, washed by centrifugation, incubated in fresh tritium-free medium for 1 hour, and rewashed for distribution in 6-well plates. Cells (250 000/mL 5% FBS medium) were exposed to sphingomyelinase (1-5 U/mL) at 37°C for 4 hours. After harvest and lipid extraction, [3H]sphingomyelin and [3H]ceramide were analyzed by TLC as described above.Cytotoxicity assays Assays were performed as described previously.28 Briefly, cells from stock cultures were washed twice with PBS and seeded into 96-well plates at 10 000 cells/well in 0.1 mL medium containing 10% FBS. After a 2-hour acclimation period, 0.1 mL serum-free medium containing the agent under study was added and the incubation continued. Vincristine was eliminated from HL-60/VCR medium during cytotoxicity studies. Cytotoxicity was determined using the Promega 96 aqueous cell proliferation kit (Promega, Madison, WI), a tetrazolium-based colorimetric assay. Absorbance at 490 nm was recorded using a microplate reader, model FL600 (Bio-Tek, Winooski, VT).Apoptosis Apoptosis, in response to drug exposure, was quantitated by the Cell Death Detection enzyme-linked immunosorbent assay (ELISA; Boehringer Mannheim, Indianapolis, IN). After cell harvest and lysis (3000 cells/tube), mononucleosomes and oligonucleosomes in the soluble fraction were recognized by DNA-histone antibody and detected by peroxidase-coupled anti-DNA antibody according to instructions from the manufacturer. Absorbance was measured at 405 nm.Protein synthesis assay Cells from stock cultures were washed twice with PBS and resuspended at 5 × 105 cells/mL in leucine-free RPMI-1640 medium containing 2.5% FBS. Aliquots of 0.5 mL were added to wells of a 24-well plate. After 2 hours, the agents under investigation were coadministered along with 1.0 µCi [3H]leucine in a final well volume of 1.0 mL. Controls received vehicle and [3H]leucine only. At the indicated times, cells were transferred along with a PBS wash to Eppendorf spin tubes and pelleted by centrifugation at 10 000 rpm, 10 minutes, in an IEC Micromax centrifuge (International Equipment, Needham Heights, MA). Cell pellets were resuspended at 4°C in 10% trichloroacetic acid to precipitate cell protein. Pellets were washed in methanol, dried, dissolved in 0.5 mL 1% sodium dodecylsulfate/0.3 M sodium hydroxide, and analyzed by liquid scintillation spectroscopy using EcoLume. Data represent percent protein synthesis in treated cells compared to protein synthesis at each time point in untreated controls. Each increment with time in control synthesis is set at 100%.Caspase activity and cleavage of caspase substrates At the start of each experiment, nonviable HL-60 cells were removed by sedimentation at 200g for 20 minutes on Ficoll-Hypaque step gradients (density = 1.119 g/cm3). Cells harvested from the interface were diluted with complete RPMI 1640 medium, sedimented at 200g for 10 minutes, and resuspended in fresh medium. Cells were treated with 40 nM DT388-GM-CSF for 3 to 30 hours. Alternatively, cells were treated with etoposide (prepared as a 1000-fold concentrated stock in DMSO) at a concentration of 68 µM, a high but clinically sustainable concentration39,40 that has previously been shown to induce apoptosis in more than 85% of HL-60 cells within 6 hours41 through a caspase-9-dependent pathway.34,42After treatment, aliquots of cells were sedimented at 200g
for 10 minutes and washed once with ice cold RPMI 1640 containing 10 mM
HEPES (pH 7.4). Replicate aliquots were either fixed for morphologic
examination after Hoechst staining,41 solubilized in 6 M
guanidine hydrochloride under reducing conditions in preparation for
sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)
and immunoblotting,43 or subjected to subcellular fractionation followed by fluorogenic measurement of caspase-3-like activity.44 In brief, cells for subcellular fractionation
were sedimented, washed with calcium-, magnesium-free PBS, and
resuspended in buffer A (25 mM HEPES, pH 7.5 at 4°C, 5 mM
MgCl2, 1 mM EGTA supplemented immediately before use with 1 mM phenylmethylsulfonyl fluoride [PMSF], 10 µg/mL pepstatin A, and
10 µg/mL leupeptin). After a 20-minute incubation on ice, cells were
lysed with 20 to 30 strokes in a tight-fitting Dounce homogenizer and
sedimented at 800g for 10 minutes (to remove nuclei)
followed by 280 000g for 60 minutes to sediment other
membranous cellular components. The supernatant (cytosol) was frozen in
50-µL aliquots at Aliquots containing 50 µg cytosolic protein, estimated by the bicinchonic acid method,45 in 50 µL buffer A were diluted with 225 µL of freshly prepared buffer B (25 mM HEPES, pH 7.5, 0.1% [w/v] CHAPS, 10 mM DTT, 100 U/mL aprotinin, 1 mM PMSF) containing 100 µM DEVD-AFC and incubated for 2 hours at 37°C. Reactions were terminated by addition of 1.225 mL ice-cold buffer B, and fluorescence was measured in a Sequoia-Turner fluorometer using an excitation wavelength of 360 nm and emission wavelength of 475 nm. Reagent blanks containing 50 µL buffer A and 225 µL buffer B were incubated at 37 °C for 2 hours, then diluted with 1.225 mL ice-cold buffer B. Standards containing 0 to 1500 pmol of AFC were used to determine the amount of fluorochrome released. Preliminary studies demonstrated that (1) the Kd for the substrate was 20 µM,46 and (2) the release of product under these conditions was linear for at least 4 hours. These controls rule out the possibility that changes in the amount of AFC liberated reflect alterations in enzyme affinity or stability as opposed to increases in amount of active enzyme. Immunoblotting Whole cell lysates containing protein from 3 × 105 cells were solubilized in sample buffer consisting of 4 M urea, 2% SDS, 62.5 mM Tris-HCL (pH 6.8 at 4°C), and 1 mM EDTA, heated to 65°C and loaded on SDS-polyacrylamide gels containing a 5% to 15% (w/v) acrylamide gradient. Subsequent transfer to nitrocellulose and immunoblotting were performed as described.44,46
Previous studies have shown that DT388-GM-CSF induces apoptosis in human acute myeloid leukemia blasts and that sensitivity is dependent on GM-CSF receptors.20 Work with HL-60/VCR cells also demonstrated the utility of DT388-GM-CSF in modulating resistance to anthracyclines.13 In light of work showing that ceramide mediates anthracycline-induced26,27 and etoposide-induced30 apoptosis in leukemia cell models, we have now investigated the influence of DT388-GM-CSF on ceramide metabolism. Initial experiments, conducted with HL-60/VCR cells, showed that
DT388-GM-CSF had a profound impact on ceramide production. Figure 1 shows that a 48-hour exposure to
DT388-GM-CSF elicits ceramide generation in HL-60/VCR
cells in a dose-dependent manner. With DT388-GM-CSF
concentrations as low as 0.1 nM, ceramide increased to 3.5 times the
level in untreated cells, and at 10 nM, ceramide increased 6-fold. The
steep increase in ceramide in HL-60/VCR cells reached a plateau at
concentrations greater than 10 nM. DT388-GM-GSF also
promoted ceramide formation in HL-60 cells in a similar manner (data
not shown). In contrast, GM-CSF alone had little influence on cellular
ceramide metabolism (Figure 1).
Concomitant with ceramide formation induced by
DT388-GM-CSF, survival of HL-60/VCR cells fell sharply in
response to treatment (Figure 2). HL-60
cells were similarly susceptible to treatment (Figure 2). The decline
in cell survival (Figure 2) mirrored the increase in levels of
intracellular ceramide (Figure 1) in response to treatment.
To more clearly define the relationship of ceramide to the cytotoxic
action of DT388-GM-CSF, the time courses for inhibition of
cellular protein synthesis, ceramide production, and initiation of
apoptosis were compared. Protein synthesis was inhibited by 20% at 2.5 hours after addition of DT388-GM-GSF and 50% at 6 hours (Figure 3A). Despite this modest
reduction in protein synthesis (see below), ceramide production in
response to DT388-GM-GSF was initiated between 4 and 6 hours after addition, attaining a level about 75% above control at 6 hours. Increases in chromatin fragmentation were apparent within the
same time frame (4-6 hours) and were 230% of control values at 8 hours
(Figure 3B). A reduction in the level of cellular sphingomyelin was
evident by 8 hours, with depletion of 60% of the total cellular
sphingomyelin by 24 hours (Figure 3A).
To determine if increased ceramide formation is a response common to
the action of protein synthesis inhibitors, the influence of emetine
and hygromycin B were investigated. As shown in Figure 4, these compounds, at concentrations
that inhibit protein synthesis by 50%, did not significantly alter
ceramide metabolism in HL-60/VCR cells. After 6 hours of exposure to
either emetine or hygromycin B, cellular ceramide levels were only 5%
below and 7% above untreated control levels, respectively. GM-CSF in
combination with protein synthesis inhibitors also failed to increase
ceramide, whereas the diphtheria toxin-GM-CSF conjugate,
DT388-GM-CSF, at 6 hours, elicited a 75% increase in
ceramide levels (Figure 4).
The HL-60 wild-type cells demonstrate ceramide production and are
extremely sensitive when challenged with DT388-GM-CSF
(Figure 2). Because recent studies have documented the participation of ceramide in etoposide-induced apoptosis30,47 and because
caspase activation is a hallmark of the cell death cascade, we studied the effects of DT388-GM-CSF on cellular caspase activity.
Etoposide was used in these studies as a positive control. Treatment
with DT388-GM-CSF caused time-dependent caspase
activation, as evidenced by DEVD-AFC cleavage (Figure
5), which was readily detectable at 6 hours. Consistent with these results, active species of caspase-9 and
caspase-3 were also detectable at this time point (Figure 6, right), with proteolytic fragments of
caspase-8 becoming detectable later. Cleavage of the caspase substrates
PARP, lamin B1, and protein kinase C
To determine whether diphtheria toxin alone would activate ceramide
generation, we used U-937 myeloid leukemia cells, which are extremely
sensitive to diphtheria toxin.18 As illustrated in Figure
7, diphtheria toxin promoted formation of
ceramide with a simultaneous decline in sphingomyelin content in U-937
cells. Thus, the pattern of lipid changes is very similar to the impact of DT388-GM-CSF on lipid metabolism in HL-60/VCR cells
(Figure 3A). Because U-937 cells express high levels of GM-CSF
receptor, it was of interest to determine the sensitivity of cells to
conjugated toxin. The data of Figure 8
clearly show that U-937 cells are exceptionally responsive to
DT388-GM-CSF with an EC50 (amount of drug
eliciting 50% kill) of approximately 0.2 pM. This represents a 3-log
increase in sensitivity, compared to HL-60 cells (Figure 2).
To more firmly support the premise that ceramide generation by
diphtheria toxin contributes to cytotoxicity, we tested the impact of
exogenous ceramide exposure and sphingomyelinase treatment on cell
response. HL-60 and HL-60/VCR cells were sensitive to supplements of
cell-permeable C6-ceramide (EC50 ~6.5 µM,
data not shown). C6-Ceramide also strongly inhibited
protein synthesis after only 4 hours of exposure (EC50 ~5
µM, data not shown). Another route to increase ceramide levels,
sphingomyelinase treatment of cells, also resulted in inhibition of
protein synthesis. As shown in Figure 9,
enzyme treatment of HL-60/VCR cells caused an approximate 10%
depletion in sphingomyelin, a 2.5-fold increase in endogenous ceramide,
and inhibition of protein synthesis. After 4 hours in the presence of 2 U enzyme/mL culture medium, cellular ceramide levels doubled and
protein synthesis was inhibited 50% (Figure 9B,C).
A number of fusion toxins have been engineered with use of the diphtheria toxin enzymatic domain. Diphtheria toxin conjugates constructed with interleukin (IL)-2 are being investigated for treatment of lymphoma48-50 and human immunodeficiency virus.51,52 Other diphtheria fusion toxins targeting brain tumors have been described.53 Diphtheria toxin fused to IL-3 is toxic to blasts from patients with myeloid leukemias,54 and DT388-GM-CSF, the object of the present study, is being evaluated in a clinical trial for patients with relapsed AML.55 Despite this widespread interest in toxin molecules, there is relatively limited information about how they actually kill target cells. Here for the first time we have identified changes in sphingomyelin metabolism leading to ceramide generation in response to diphtheria toxin and the targeted conjugate DT388-GM-CSF. Moreover, we have placed these changes in ceramide metabolism upstream of caspase-9 activation. These results have potentially important implications for current understanding of how toxin molecules kill cells. In both U-937 and K-562 cells, diphtheria toxin induces
internucleosomal DNA fragmentation, a characteristic of apoptosis, whereas other protein synthesis inhibitors were without influence at
similar levels of protein synthesis inhibition.18,56 Thus, cytolysis initiated by diphtheria toxin does not appear to be a simple
consequence of translation inhibition, but also has been hypothesized
to involve a second pathway of cytotoxicity.56 Consistent
with this hypothesis, when we exposed HL-60/VCR cells to either
hygromycin B or emetine at concentrations that inhibited protein
synthesis by 50%, neither agent caused a remarkable change in cellular
ceramide levels, and further, both agents were relatively nontoxic.
This strengthens the argument that ceramide generation contributes to
diphtheria toxin toxicity. Because diphtheria toxin also potentiates
the cytotoxic effect of TNF- We propose that ceramide constitutes an element in the death-signaling cascade initiated by diphtheria toxin and DT388-GM-CSF. Several observations support this view. First, the coincidence of the dose-response curves for ceramide elevation (Figure 1) and decreased cell survival (Figure 2) suggest that these are linked processes after treatment with DT388-GM-CSF. Second, the time course experiments show that ceramide elevation becomes detectable 4 to 6 hours after DT388-GM-CSF addition (Figure 3), with detectable activation of caspase-9 and caspase-3 (Figure 6, right) as well as internucleosomal cleavage (Figure 3B) by 6 hours. These results not only place ceramide elevation temporally upstream of other apoptotic changes, but also suggest that ceramide might be triggering the mitochondrial pathway of caspase activation. Third, in the absence of diphtheria toxin, we have shown that ceramide potently inhibits cellular protein synthesis. Moreover, either exogenous C6-ceramide, or endogenous ceramide, generated by sphingomyelinase, inhibits cell growth. These data provide direct evidence that ceramide is cytotoxic and corroborate recent studies showing that ceramide inhibits protein synthesis.62 The data also indicate that inhibition of protein synthesis by DT388-GM-CSF is not responsible for the elevation in ceramide. Thus, diphtheria toxin appears to signal not only caspase activation but also inhibition of protein synthesis through ceramide. We have not determined if ceramide inactivates EF-2. This is obviously a complex situation warranting further evaluation. Our observations raise the possibility that diphtheria toxin-induced ADP-ribosylation of another polypeptide lies upstream of ceramide elevation. This event then appears to lead to ceramide generation through sphingomyelin hydrolysis (Figures 3 and 7). Consistent with this hypothesis, 2 agents that block de novo ceramide formation (fumonisin B1 and L-cycloserine), failed to alter the apoptotic index of HL-60/VCR cells treated with DT388-GM-CSF. The influence of diphtheria toxin on sphingomyelinase activity has yet to be determined. In addition, diphtheria toxin activated sphingomyelinase activity in the human myeloid leukemia cell line, U-937. The extreme sensitivity of U-937 cells to the conjugate, DT388-GM-CSF, approximately 1000-fold greater than HL-60 cells, may be related to GM-CSF receptor number; however, the literature is not clear on this point. Current understanding suggests that there are at least 2 major pathways for drug-induced apoptosis, one triggered by ligation of the death receptor Fas, and another triggered by mitochondrial release of cytotochrome c (reviewed in Kaufmann and Earnshaw61). Several observations suggest that DT388-GM-CSF is activating the mitochondrial pathway rather than the Fas pathway. First, the HL-60 subline used in Figures 5 and 6 fails to express detectable levels of the Fas receptor and fails to die in response to agonistic anti-Fas antibodies.63 Second, active caspase-8 is detectable in cells treated with DT388-GM-CSF only at late time points (Figure 6), a result consistent with the activation of caspase-8 downstream of caspase-364 rather than upstream. A body of work supports a role for ceramide metabolism in cancer
chemotherapy. For example, the accumulation of ceramide in cancer cells
is involved in the induction of apoptosis,26,27,38,65,66 whereas glycosylation of ceramide is associated with resistance to
anthracyclines and vinblastine28,29,67-69 as well as
TNF-
Submitted August 29, 2000; accepted May 17, 2001.
Supported by grants from the National Institutes of Health (CA77632 to M.C.C.; CA 69008 to S.H.K.; CA76178 to A.E.F.), Strauss Foundation, Los Angeles (Sandra Krause, Trustee), Associates for Breast and Prostate Cancer Studies, Los Angeles, and the Leukemia Lymphoma Society (6114 to A.E.F.).
This work was presented at the 92nd Annual Meeting, American Association for Cancer Research, March 24-28, 2001, New Orleans, LA.
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: Myles C. Cabot, John Wayne Cancer Institute, 2200 Santa Monica Blvd, Santa Monica, CA 90404; e-mail: cabot{at}jwci.org.
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  J. M. Carton, D. J. Uhlinger, A. D. Batheja, C. Derian, G. Ho, D. Argenteri, and M. R. D'Andrea Enhanced Serine Palmitoyltransferase Expression in Proliferating Fibroblasts, Transformed Cell Lines, and Human Tumors J. Histochem. Cytochem., June 1, 2003; 51(6): 715 - 726. [Abstract] [Full Text] [PDF]
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Top
Abstract
Introduction
),
and alterations in apoptotic regulators such as Bcl-2 and
p53.
(TNF-
70°C in 5-µL aliquots, and diluted into
medium before use. All lipids were purchased from Avanti Polar Lipids
(Alabaster, AL). [9,10-3H] Palmitic acid (56.5 Ci/mmol)
was from DuPont/NEN (Boston, MA) and [4,5-3H(N)]
L-leucine was from American Radiolabeled Chemicals (St
Louis, MO). RPMI-1640 and leucine-free RPMI-1640 medium were from Gibco BRL (Gaithersburg, MD). Fetal bovine serum (FBS) was from Hyclone (Logan, UT). Thin-layer chromatography (TLC) plates, Silica Gel G, 0.25 mm thick, were purchased from Analtech (Newark, DE). Sodium azide, 2-deoxyglucose, nicotinamide, sphingomyelinase (human placenta) 100 U/mg protein, and N-acetyl-5-farnesyl-L-cysteine (AFC) were purchased from Sigma (St Louis, MO). Plastic tissue cultureware was from Corning-Costar (Cambridge, MA). Etoposide and
N-acetyl-Asp-Glu-Val-Asp-AFC (7-amino-4-trifluoromethyl coumarin
(DEVD-AFC) were from Biomol (Plymouth Meeting, PA). Enhanced
chemiluminescence (ECL) reagents were from Amersham (Newark,
NJ). Murine monoclonal anti-caspase-2 and anti-caspase-3 were
obtained from Transduction Laboratories (Lexington, KY). Rabbit
anti-protein kinase C
was from Santa Cruz Biotechnology (Santa
Cruz, CA), and peroxidase-labeled secondary antibodies were from KPL
(Gaithersburg, MD). Reagents that recognize poly(ADP-ribose) polymerase
(PARP), procaspase-9, and procaspase-8 were kindly provided by Guy
Poirier (Laval University, Ste-Foy, Quebec, Canada), Yuri Lazebnik
(Cold Spring Harbor Laboratory, NY), and John Reed and Stan Krajewski
(Burnham Institute, La Jolla, CA), respectively. Chicken serum that
reacts with lamin B1 and rabbit sera that recognize the
large subunit of caspase-3, the PEPD epitope at the C-terminus of the
caspase-9 large subunit, and the VETD epitope at the C-terminus of the
caspase-8 large subunit were raised as previously
described.
a strategy for overcoming drug resistance.
J Natl Cancer Inst.
2001;93:347-357