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UPRESA Centre National de la Recherche Scientifique 6032, Universite de la Mediterranée, Faculty of Pharmacy, 13005 Marseille, France [N. A., D. B., A. G., S. G., C. B.]; Department of Proteins, Bioenergetics, and Engineering, Centre National de la Recherche Scientifique, 13402 Marseille Cedex 20, France [G. B., D. L-M.]; Department of Pediatric Oncology, Childrens Hospital of La Timone,13005 Marseille, France [N. A.]; and Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, California 93106-9610 [M. A. J.]
| ABSTRACT |
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| Introduction |
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| Materials and Methods |
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Yeast and Cell Culture.
Yeast strain KM-91 was cultured as described previously
(17)
. Human neuroblastoma SK-N-SH cells were cultured in
RPMI 1640 (16)
.
Isolation of Mitochondria.
Yeast mitochondria were prepared as described previously
(17)
. Mitochondria were isolated from neuroblastoma cells
as follows (18)
. Briefly, 3 x 107 SK-N-SH cells were trypsinized and washed
with ice-cold PBS. The cell pellet was suspended in buffer A (250
mM sucrose, 20 mM HEPES, 1 mM DTT,
10 mM KCl, 1 mM EDTA, 1 mM EGTA,
1.5 mM MgCl2, phenylmethylsulfonyl
fluoride, and protease inhibitors) at 4°C. Cells were
homogenized with 45 strokes in a glass homogenizer (Kontes, Vineland,
NJ) and centrifuged at 800 x g for 10 min to
remove unbroken cells and nuclei. Mitochondria were then pelleted by
centrifugation at 15,000 x g for 10 min at
4°C. The mitochondrial pellet was rinsed twice in buffer A and
immediately resuspended, aliquoted, and incubated with drugs as
indicated.
Measurement of RR in Yeast Mitochondria.
KM-91 yeast mitochondria (1.5 mg) were suspended in respiration buffer
with a range of concentrations of paclitaxel (15225 µM)
and incubated for 40120 min at 25°C. NADH (3 mM) and
succinate (50 mM) were used as substrates to initiate
respiration, and the rate was measured using a Clark oxygen electrode
(17)
. The RR equals the slope of oxygen consumption over
time.
Incubation of Isolated Mitochondria with Drugs.
Neuroblastoma cell mitochondria were incubated for 2 h in buffer A
at 37°C with CsA (2 µM), a range of concentrations of
paclitaxel (10, 50, and 100 µM), doxorubicin (50
µM), vinorelbine (20 µM), and paclitaxel
(50 µM) or vinorelbine (20 µM) after a
2-min preincubation with CsA (2 µM). Isolated yeast
mitochondria were incubated (0.54 h) with a range of paclitaxel
concentrations (20100 µM) at temperatures ranging from
25°C to 30°C. Equivalent amounts of DMSO were added to control
samples.
Western Blot Analysis of Cytochrome c from
Mitochondria Isolated from Neuroblastoma Cells.
After centrifugation (10 min, 15,000 x g) of
treated mitochondria, supernatants were removed carefully. Pellets were
lysed in buffer B [2 mM EDTA, 100
mM NaCl, 1 mM
orthovanadate, 1% Triton X-100, and 50 mM Tris
(pH 7.5)], loaded on a 15% SDS polyacrylamide gel, transferred to a
nitrocellulose membrane, and incubated with cytochrome c
monoclonal antibody (7h82C12; PharMingen, San Diego, CA) and an
antimouse monoclonal antibody conjugated with peroxidase. Visualization
and densitometric quantitation were performed using enhanced
chemiluminescence (Amersham, Aylesbury, United Kingdom) and Traitima,
an in-house densitometric software in Visual BASIC for Windows 95
(19)
. Briefly, the gel images were digitized, and the gray
level of the pixels was used to calculate the area of each spot and to
plot the intensity curves. A monoclonal antibody against VDAC (529534;
Calbiochem, La Jolla, CA), a major component of mitochondrial membranes
(20)
, was used to ensure that equal amounts of
mitochondrial proteins were loaded onto the gel and that no
mitochondrial protein remained in the supernatant.
-Tubulin antibody
(N356; Amersham) was used to demonstrate the presence of tubulin in
mitochondrial membranes and the absence of contamination of
mitochondrial preparations.
Transmission Electron Microscopy.
Aliquots of isolated mitochondria were treated with 100
µM paclitaxel or 0.2% DMSO for 2 h at 37°C, fixed
in 4% glutaraldehyde, dehydrated in ethanol, embedded in Epon, and cut
into thin sections (19)
. The samples were imaged by
a transmission electron microscope (JEOL 100C). The longest diameter of
all mitochondria in a given area was measured on printed photographs.
Statistical analysis was performed using Students t test.
| Results |
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15 µM) induced a significant increase
(18 ± 9%) in the RR of isolated yeast mitochondria
(P < 0.001; Fig. 1A
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Paclitaxel Induced the Release of Cytochrome c from
Mitochondria Isolated from Neuroblastoma Cells.
Mitochondria were successfully isolated from SK-N-SH cells as measured
by Western blots of VDAC, an outer mitochondrial membrane protein, and
tubulin (Fig. 2A)
. VDAC was present at the same level in the mitochondrial
pellets but was absent in the supernatants of all of the samples. The
mitochondrial pellets contained a significant amount of tubulin (Fig. 2
A right panel) that appears to be in the mitochondria and
not simply a contaminant of the pellets. Pelleted tubulin did not
appear to be a contaminant for several reasons. First, it did not
appear in the supernatants and thus is not soluble. Secondly,
microtubules were not pelleted under the conditions for isolating
mitochondria (4°C, 15,000 x g, 10 min)
because no microtubules were observed by transmission electron
microscopy of the paclitaxel-treated samples (data not shown). Thus,
immunodetection of tubulin in mitochondrial pellets confirms the
presence of tubulin in mitochondria (21)
.
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To examine how cytochrome c was released, we pretreated
mitochondria isolated from neuroblastoma cells with CsA, a compound
known to inhibit PTP opening. After preincubation with CsA, the release
of cytochrome c was inhibited (Fig. 2A)
. Thus,
paclitaxel induced the release of cytochrome c from
mitochondria isolated from SK-N-SH neuroblastoma cells in a
CsA-sensitive manner, suggesting the involvement of PTP.
Effects of Doxorubicin and Vinorelbine on Cytochrome
c Release from Mitochondria Isolated from Neuroblastoma
Cells.
Because paclitaxel induced translocation of cytochrome c by
acting directly on mitochondria, we investigated whether this novel
property was shared by other antimicrotubule agents or by doxorubicin,
a drug whose mechanism of action is independent of microtubules.
Interestingly, doxorubicin (50 µM) had no
effect on the cytochrome c release from neuroblastoma
mitochondria, but vinorelbine (20 µM), an
antimicrotubule agent that depolymerizes microtubules, did induce
cytochrome c release from mitochondria in a CsA-sensitive
way (Fig. 2C)
.
Mitochondrial Swelling after Paclitaxel Treatment in
Vitro.
Swelling is an important feature of apoptotic mitochondria
(5)
. Thus, we wanted to know whether the PTP-dependent
release of cytochrome c from neuroblastoma mitochondria
induced by paclitaxel was associated with mitochondrial swelling.
Mitochondria isolated from neuroblastoma cells were treated with
paclitaxel under the conditions that induced cytochrome c
release and were examined by transmission electron microscopy (Fig. 3, A and B)
. The mean diameter of
paclitaxel-treated mitochondria (1.2 ± 0.05 µm) was
significantly greater than that of control mitochondria (1.0 ± 0.04 µm; P < 0.05; Fig. 3C
), suggesting that the PTP opening resulted in significant
swelling and thus was sustained and not merely transient.
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| Discussion |
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Paclitaxel concentrations required to induce the release of cytochrome c from isolated mitochondria are higher than those usually used to induce apoptosis in cultured cells. However, paclitaxel accumulates in cells. For example, Jordan et al. (22) found that addition of 10 or 100 nM paclitaxel to HeLa cells resulted in an intracellular paclitaxel concentration of 4.8 and 40.5 µM, respectively. Thus, the intracellular concentration of paclitaxel can reach 500 times the extracellular concentration. Because 1 µM paclitaxel is necessary to induce apoptosis in SK-N-SH cells (16) , the concentrations that we used with isolated mitochondria may reflect the intracellular concentrations that induce apoptosis.
Paclitaxel had direct effects on both yeast and neuroblastoma mitochondria, but the responses were different. In yeast mitochondria, paclitaxel triggered an increase of respiration but no translocation of cytochrome c. This may be due in part to differences between the structure of the PTP in mammalian cells and the unselective channel in yeast mitochondria. Indeed, the unselective channel of yeast mitochondria is not a clear counterpart to PTP because it is neither induced by Ca2+ nor inhibited by CsA (23) . Previous studies have shown that paclitaxel could induce changes in respiration (24) and in the PTP conformation (25) , although the release of cytochrome c was not investigated. In contrast to the results reported here, the study by Manzano et al. (24) found a decrease in respiration in mitochondria isolated from rat liver. This difference may have at least two possible explanations. The first is that only one very high paclitaxel concentration (250 µM) was used, and the second is the differing sources of the mitochondria (rat liver as opposed to human cancer cells). The quantity of Bcl-2 family proteins in the mitochondrial membranes may differ, depending on the tissue and the species origin of the mitochondria. Finally, mitochondria in malignant cells may differ from mitochondria in nontumor cells and thus may respond differently to the same apoptotic signal.
Two additional antitumor drugs were tested, vinorelbine and doxorubicin. Interestingly, vinorelbine, a depolymerizing antimicrotubule agent, also induced the release of cytochrome c in a CsA-sensitive manner, whereas doxorubicin, whose mechanism of action is independent of microtubules, had no effect at a concentration of 50 µM. Doxorubicin did not release cytochrome c even at concentrations 100 times higher than the extracellular concentration inducing apoptosis in SK-N-SH cells (16) . Paclitaxel and vinorelbine both bind to tubulin in microtubules but bind to different sites on the tubulin molecule (8) . Thus the direct mitochondrial effect of paclitaxel and vinorelbine may be a phenomenon shared by many antimicrotubule agents. Moreover, evidence suggests that lonidamine and arsenic, compounds that can induce apoptosis by acting directly on mitochondria, may also act on the microtubule network (26 , 27) . Thus, antimicrotubule and antimitochondrial agents may be able to perturb mitochondrial and microtubule physiology as a result of a common target or a common intermediate actor and thus cooperate to induce apoptosis. Bim, a BH3-only protein proapoptotic member of Bcl-2 family (28) , calcium (29) , and tubulin (25) are possible candidates for effectors of the microtubule-mitochondria interrelationship during apoptosis.
How paclitaxel induces the opening of PTP is not known. Our results confirm that tubulin is present on mitochondrial membranes as reported previously (21) , but it is not yet known whether the tubulin is polymerized (which would enhance the binding of paclitaxel to mitochondria). Nevertheless, when present in the mitochondrial membranes, tubulin could adopt a conformation similar to its conformation in the microtubule. Paclitaxel may also interact with Bcl-2 (30) to mediate changes in PTP. The real impact of the direct mitochondrial effect of paclitaxel on apoptosis must be further investigated. The release of cytochrome c in a cell-free system as a result of a direct effect of paclitaxel on mitochondria might explain how paclitaxel could induce apoptosis without mitosis and phosphorylation of Bcl-2 (9) .
Finally, we have demonstrated that antimicrotubule agents are the first widely used family of anticancer agents that act directly on mitochondria. Such an effect has also been reported with some new anticancer agents (lonidamine, arsenite compounds, retinoic acids, and betulinic acid; Refs. 12, 13, 14, 15 ). Altogether, these data suggest that (a) mitochondria may be a new target for anticancer agents, and thus antimitochondrial agents may represent a new class of anticancer agents; and (b) antimicrotubule agents may work at least in part because of their antimitochondrial effect.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 M. A. J. was supported by NIH Grant CA
57291. ![]()
2 To whom requests for reprints should be
addressed, at UPRESA Centre National de la Recherche
Scientifique 6032, Universite de la Mediterranée, Faculty of
Pharmacy, 27 Boulevard Jean Moulin, 13005 Marseille, France.
Phone: 33491835635; Fax: 33491782024; E-mail: diane.braguer{at}pharmacie.univ-mrs.fr ![]()
3 The abbreviations used are: PTP, permeability
transition pore; CsA, cyclosporin A; RR, respiration rate; VDAC,
voltage-dependent anion channel. ![]()
Received 4/24/00. Accepted 8/16/00.
| REFERENCES |
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