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| Cancer Research | Clinical Cancer Research |
| Cancer Epidemiology Biomarkers & Prevention | Molecular Cancer Therapeutics |
| Molecular Cancer Research | Cell Growth & Differentiation |
to Mitochondria in the Oxidative Stress Response1
Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115 [P. K. M., X. S., A. B., S. K., D. K.] and Lovelace Respiratory Research Institute, Albuquerque, New Mexico 87115 [N. C. M., S. S.]
| Abstract |
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(PKC
) to mitochondria. The results demonstrate that H2O2-induced activation of PKC
is necessary for translocation of PKC
from the cytoplasm to the mitochondria. The results also show that mitochondrial targeting of PKC
is associated with the loss of mitochondrial transmembrane potential and release of cytochrome c. The functional importance of this event is also supported by the demonstration that H2O2-induced apoptosis is blocked by the inhibition of PKC
activation and translocation to mitochondria. These findings indicate that mitochondrial targeting of PKC
is required, at least in part, for the apoptotic response of cells to oxidative stress. | Introduction |
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Certain insights into ROS-induced signaling have been derived from the finding that PKC
is phosphorylated on tyrosine in cells treated with H2O2 (7, 8, 9)
. Significantly, tyrosine phosphorylation of PKC
in the response to ROS confers independence from lipid cofactors for catalytic activity (7)
. Phosphorylation of PKC
on Tyr-512 and Tyr-523 has been shown to be important for H2O2-induced activation (7)
. Other studies have demonstrated that c-Abl interacts with PKC
in the response to H2O2 and that c-Abl phosphorylates PKC
on Tyr-512 but not Tyr-523 (6)
. These findings indicate that ROS induce phosphorylation of PKC
by c-Abl and at least one other tyrosine kinase.
Recent work has demonstrated that H2O2 induces the release of mitochondrial cytochrome c and, thereby, apoptosis (10)
. In the present studies, we show that treatment of cells with H2O2 is associated with the targeting of PKC
to mitochondria. The results also demonstrate that activation of PKC
is required for mitochondrial localization and for ROS-induced loss of mitochondrial transmembrane potential, cytochrome c release, and apoptosis.
| Results and Discussion |
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, human U-937 cells were treated with H2O2 and harvested at varying intervals. Cytoplasmic and mitochondrial fractions were subjected to immunoblotting with anti-PKC
. The results demonstrate that treatment with 1 mM H2O2 is associated with decreases in cytoplasmic and concomitant increases in mitochondrial PKC
(Fig. 1A)
to mitochondria indicated that this response is dose-dependent (Fig. 1A)
to H2O2 treatment, there was little if any effect of this agent on mitochondrial levels of PKC
or PKC
(Fig. 1B)
to mitochondria, cells were treated with NAC, a scavenger of reactive oxygen intermediates and precursor of glutathione (11
, 12)
. The results demonstrate that NAC inhibits H2O2-induced localization of PKC
to mitochondria (Fig. 1C)
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was visualized with a charge-coupled device camera and image analyzer. Fluorescence detection in control cells showed distinct patterns for PKC
(red signal) and a mitochondrial-selective dye (MitoTracker; green signal; Fig. 2
yellow/orange) provided additional support for translocation of PKC
to mitochondria (Fig. 2)
localizes to mitochondria in the response to oxidative stress.
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activity contributes to mitochondrial targeting of PKC
in response to H2O2, we treated U-937 cells with the selective PKC
inhibitor, rottlerin (13)
. The results demonstrate that H2O2-induced localization of PKC
to mitochondria is attenuated by rottlerin (Fig. 3A
to mitochondria (Fig. 3A
to mitochondria supported involvement of the PKC
kinase function (Fig. 3A
activation is necessary for mitochondrial translocation, we transfected 293 cells with vectors expressing GFP, GFP-PKC
or a kinase-inactive GFP-PKC
(K378R) mutant (6)
. Analysis of the mitochondrial fraction by immunoblotting with anti-GFP demonstrated H2O2-induced targeting of PKC
to mitochondria (Fig. 3B)
(K378R; Fig. 3B
, but not GFP-PKC
(K-R), was confirmed by fluorescence microscopy (Fig. 3C)
, but not GFP-vector or GFP-PKC
(K-R), signals with MitoTracker red showed localization to mitochondria (Fig. 3C)
kinase function is necessary for H2O2-induced mitochondrial localization.
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interacts with the c-Abl tyrosine kinase in the cellular response to oxidative stress (6)
. To determine whether c-Abl is necessary for H2O2-induced targeting of PKC
to mitochondria, wild type (c-Abl+/+) and c-Abl-/- MEFs were treated with H2O2. H2O2-induced activation of PKC
was similar in both cells (Fig. 4A)
in the cytosolic and mitochondrial fractions was similar in wild-type and c-Abl-/- cells (Fig. 4B)
to mitochondria was found in the H2O2 response of both cell types (Fig. 4C)
activation and localization to mitochondria.
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is functional in inducing release of cytochrome c, U-937 cells were pretreated with NAC or rotterlin. The results demonstrate that NAC blocks H2O2-induced release of cytochrome c (Fig. 5B)
to mitochondria support the involvement of PKC
in H2O2-induced release of cytochrome c.
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consists of an NH2-terminal RD and a COOH-terminal catalytic fragment (14)
. To further assess the role of PKC
in H2O2-induced apoptosis, we studied MCF-7 cells that stably express the empty neo vector (MCF-7/neo) or the Mr 35,000 RD (MCF-7/PKC
RD) (15)
. In contrast to MCF-7/neo cells, translocation of PKC
to mitochondria was attenuated in H2O2-treated MCF-7 cells stably expressing PKC
RD (Fig. 6A)
m; 16
). To determine whether PKC
is functional in inducing both loss of 
m and cytochrome c release, H2O2-treated MCF-7/neo and MCF-7/PKC
RD cells were incubated with rhodamine 123. Analysis by flow cytometry demonstrated that H2O2-induced loss of 
m is attenuated in MCF-7/PKC
RD as compared with MCF-7/neo, cells (Fig. 6B)
was also attenuated in MCF-7/PKC
RD cells (Fig. 6C)
RD cells (Fig. 6D)
to mitochondria contributes to H2O2-induced loss of 
m, cytochrome c release, and apoptosis.
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is phosphorylated on tyrosine in the cellular response to H2O2 treatment (7, 8, 9)
. Other studies have shown that c-Abl interacts with PKC
and is in part responsible for tyrosine phosphorylation of PKC
in the response to H2O2 (6)
. The available findings indicate that PKC
is activated by ROS and that PKC
phosphorylates and activates c-Abl (6
, 10)
. In a potential auto-catalytic loop, c-Abl phosphorylates and further activates PKC
(6)
. The present studies demonstrate that ROS induce targeting of PKC
to mitochondria, and that this response is dependent on activation of the PKC
kinase function. These findings are in concert with recent reports showing that phorbol ester-induced activation of PKC
is associated with translocation of PKC
to mitochondria (15
, 20)
. However, in contrast to the present results obtained with H2O2, phorbol ester-induced activation of PKC
is not dependent on ROS generation or phosphorylation of PKC
on tyrosine (data not shown). The results also demonstrate that ROS-induced targeting of PKC
to mitochondria occurs in c-Abl-/- cells. These findings indicate that, while c-Abl activation is dependent on PKC
(10)
, activation and translocation of PKC
to mitochondria in the response to H2O2 is independent of the c-Abl kinase.
Release of cytochrome c from mitochondria triggers the activation of caspases and the induction of apoptosis (21)
. Recent work has demonstrated that the response of cells to oxidative stress includes loss of mitochondrial transmembrane potential and release of cytochrome c (10
, 16)
. The available findings also indicate that these effects of ROS on mitochondria are mediated in part by a c-Abl-dependent mechanism (10
, 16)
. The results of the present studies show that ROS-induced cytochrome c release is also regulated by activation and translocation of PKC
to mitochondria. Taken together with the demonstration that c-Abl functions in the apoptotic response to oxidative stress (10)
, these findings indicate that signaling by both PKC
and c-Abl is needed for ROS-induced loss of mitochondrial transmembrane potential and release of cytochrome c. Indeed, although treatment of c-Abl-/- cells with H2O2 is associated with PKC
activation and localization to mitochondria, these cells failed to respond to oxidative stress with release of cytochrome c and the induction of apoptosis (10)
. Finally, PKC
is also activated by PDK1-mediated phosphorylation in the cellular response to serum stimulation (22)
. Thus, PKC
seems to be functional in both pro- and antiapoptotic pathways, and therefore it could represent a switch that determines cell fate.
| Materials and Methods |
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RD (15)
breast cancer cells, 293T cells, and wild-type and c-Abl-/- MEFs (23)
were grown in Dulbeccos modified Eagles medium containing 10% fetal bovine serum and antibiotics. Cells were treated with 1 mM H2O2 (Sigma Chemical Co.), 10 µM rottlerin (Sigma Chemical Co.), and 30 mM NAC (Calbiochem). Transfections were performed with Superfect (Qiagen).
Isolation of the Cytosolic Fraction.
Cells were suspended in ice-cold 20 mM HEPES (pH 7.5), 1.5 mM MgCl2, 10 mM KCl, 1 mM EDTA, 1 mM EGTA, 1 mM DTT, 0.1 mM phenylmethylsulphonyl fluoride, 10 µg/ml leupeptin, 10 µg/ml aprotinin, 10 µg/ml pepstatin A, and 250 mM sucrose. The cells were disrupted by Douce homogenization. After centrifugation at 1500 x g for 5 min at 4°C, the supernatants were centrifuged at 105,000 x g for 30 min at 4°C. The resulting supernatant was used as the soluble cytoplasmic fraction.
Isolation of the Mitochondrial Fraction.
Cells were suspended in ice-cold 5 mM HEPES (pH 7.5), 210 mM mannitol, 1 mM EGTA, 70 mM sucrose, and 110 µg/ml digitonin. The cells were disrupted in a glass homogenizer (Pyrex No. 7727-07) and centrifuged at 2,000 x g for 20 min at 4°C. The pellets were resuspended in the same buffer, homogenized again (Pyrex No. 7726), and centrifuged at 2000 x g for 5 min at 4°C. The supernatants (S1) were collected. The pellets were rehomogenized, centrifuged at 2,000 x g for 5 min, and the resultant supernatants (S2) collected. Supernatants S1 and S2 were pooled and centrifuged at 11,000 x g for 10 min. The mitochondrial pellets were resuspended in lysis buffer [50 mM Tris-HCl (pH 7.4), 150 mM NaCl, 1% Nonidet P-40, 1 mM sodium vanadate, 1 mM phenylmethylsulphonyl fluoride, 1 mM DTT, 10 µg/ml leupeptin, and 10 µg/ml aprotinin] for 30 min on ice and then centrifuged at 15,000 x g for 20 min. The supernatant was used as the soluble mitochondrial fraction. Protein concentration was determined by the BioRad protein estimation kit.
Immunoblot Analysis.
Soluble proteins were subjected to immunoblot analysis with anti-PKC
(Santa Cruz Biotechnology), anti-ß-actin (Sigma), anti-Hsp60 (Stressgen), anti-I
B
(Santa Cruz), anti-PKC
(Santa Cruz), anti-PKC
(Santa Cruz), anti-GFP (Clontech), and anti-cytochrome c (24)
. The immune complexes were detected with antirabbit or antimouse IgG peroxidase conjugate (Amersham) and visualized by enhanced chemiluminescence (Amersham Pharmacia).
Immunofluorescence Microscopy.
Cells were plated onto poly-D-lysine-coated glass coverslips. After 24 h, the cells were treated with 1 mM H2O2 for 1 h and then fixed with 3.7% formaldehyde in PBS (pH 7.4) for 10 min. Cells were washed with PBS, permeabilized with 0.2% Triton X-100 for 10 min, washed again, and incubated for 30 min in complete medium. The coverslips were incubated with 5 µg/ml anti-PKC
for 1 h and then Texas Red-goat antirabbit Ig (heavy and light chains) conjugate (Molecular Probes, Eugene, OR). Mitochondria were stained with 100 nM MitoTracker Green FM for experiments with GFP; mitochondria were stained with 100 nM MitoTracker Red CMXRos (Molecular Probes). Coverslips were mounted onto slides with 0.1 M Tris (pH 7.0) in 50% glycerol. The cells were visualized by digital confocal immunofluorescence, and images were captured with a charge-coupled device camera mounted on a Zeiss Axiioplan 2 microscope. Images were deconvolved using Slidebook software (Intelligent Imaging Innovations, Inc., Denver, CO).
Assessment of PKC
Activity.
PKC
activity was assayed by incubating anti-PKC
immunoprecipitates in PKC kinase buffer containing 20 mM Tris-HCl (pH 7.5), 10 mM MgCl2, 20 µM ATP, 2.5 µCi [
-32P]ATP, and 200 µg/ml histone H1 (7)
for 5 min at 30°C. The reaction products were analyzed by SDS-PAGE and autoradiography.
Analysis of Mitochondrial Membrane Potential.
Cells were treated with 1 mM H2O2 and incubated with 50 ng/ml Rhodamine 123 (Molecular Probes) for 30 min at 37°C. After washing with PBS, cells were analyzed by flowcytometry using 488 nm excitation and a 575/26 ethidium bandpass filter.
Assessment of Apoptosis.
Cells were fixed with 80% ethanol, washed, and incubated with 2.5 µg/ml propidium iodide and 50 µg/ml RNase. Cells with sub-G1 DNA were determined by FACScan (Becton Dickinson).
| Acknowledgments |
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| Footnotes |
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1 Supported by Public Health Service Grant CA42802, awarded by the National Cancer Institute, NIH, and the Department of Health and Human Services and by the Office of Health and Biological Research, U.S. Department of Energy, Cooperative Agreement DE-FCO4-96AL76406. ![]()
2 To whom requests for reprints should be addressed, at Dana-Farber Cancer Institute, Harvard Medical School, 44 Binney Street, Boston, MA 02115. Phone: (617) 632-3141; Fax: (617) 632-2934. ![]()
3 The abbreviations used are: ROS, reactive oxygen species; PKC, protein kinase C; H2O2, hydrogen peroxide; NAC, N-acetyl-L-cysteine; MEF, mouse embryo fibroblast; GFP, green fluorescence protein; RD, regulatory domain. ![]()
Received for publication 2/ 8/01. Revision received 6/21/01. Accepted for publication 6/21/01.
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