Inhibition of ErbB2 causes mitochondrial dysfunction in cardiomyocytes
Implications for herceptin-induced cardiomyopathy
Luanda P. Grazette, MD, MPH*, ,
Wolfgang Boecker, MD*,
Takashi Matsui, MD, PhD*,,
Marc Semigran, MD,
Thomas L. Force, MD ,
Roger J. Hajjar, MD*, and
Anthony Rosenzweig, MD*,,*
* Program in Cardiovascular Gene Therapy, Cardiovascular Research Center, Boston, Massachusetts
Cardiology Division, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
Tufts New England Medical Center, Boston, Massachusetts
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Figure 1 Effects of erbB2 inhibition on Bcl-xL and -xS expression. (a) ErbB2 inhibition was associated with an increase in expression of Bcl-xS and decreased expression of Bcl-xL in a time-dependent manner. Western blot shown is representative of five independent experiments. (b) A 21-base-pair double-stranded ribonucleic acid sequence corresponding to a section within the open reading frame of the rodent erbB2 gene (Gralb) was solubilized in TransIT-TKO (Mirus Research Solutions, Houston, Texas) for transfection of cardiomyocytes. Reduced expression of erbB2 and altered Bcl-x expression were seen 48 h after transfection. Western blots representative of four independent experiments.
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Figure 2 Activation of mitochondrial death agonists. Neonatal rat ventricular cardiomyocytes were exposed to anti-erbB2 antibody (erbB2 Ab) for the specified times. Lysates were collected and separated by electrophoresis under denaturing conditions with protein detection by immunoblotting following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. (a) Time-dependent increase in expression of bcl-associated protein. (b) Treated cells were subjected to subcellular fractionation to yield a mitochondria rich heavy membrane fraction (HM). The HM was incubated for 30 min with the irreversible crosslinker BMH. Immunoblot demonstrates crosslinked BAX multimers. (c) (Upper panel) Depletion of cytochrome c from the mitochondrial fraction, beginning at 8 hours after anti-erbB2 treatment. (Lower panel) Cytochrome oxidase used for control of mitochondrial protein loading. (d) Anti-erbB2 induced a time-dependent increase in cleavage and activation of caspase 9 beginning at 16 h. (e) Increase cleavage and activation of caspase 3 were also observed beginning as early as 8 h after treatment. (Bottom panel) Actin immunoblotting to control for cytosolic protein loading. All Western blots shown are representative of three or more independent experiments.
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Figure 3 Anti-erbB2 effects in adult rat ventricular myocytes. Adult rat ventricular myocytes were exposed to anti-erbB2 antibody (Ab) for the specified times. Lysates were collected, separated by electrophoresis, and immunoblotted. (a) ErbB2 inhibition was associated with an increase in expression of Bcl-xS and Bcl-xL. (b) Cytochrome c release into the cytosol. Bax = bcl-associated protein.
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Figure 4 Anti-erbB2 reduces mitochondrial membrane potential adenosine triphosphate (ATP) concentration and redox capacity in neonatal rat ventricular cardiomyocytes. Neonatal rat ventricular cardiomyocytes were stained with a fluorescent probe selective for metabolically active mitochondria Mitotracker red. (a) Results of flow cytometry of cells stained with Mitotracker red dye. Anti-erbB2 treatment resulted in decreased red fluorescence (76.85 ± 2.4 vs. 51.1 ± 0.072, *p < 0.05, n = 4 independent experiments). (b) ATP content was measured using luciferin-luciferase bioluminescence. There was a 35% decrease in ATP content of anti-erbB2-treated cells at 36 h compared to untreated cells. Results, expressed as relative light units, are from three independent experiments with six replicates/condition (1,297 ± 112 vs. 847 ± 75, *p < 0.01). (c) Reduction of the tetrazolium salt mitochondrial reductase activity (MTT) to a corresponding formazan is an indicator of redox capacity. Redox capacity was quantified using a spectrophotometer at a wavelength of 590. Reported results represent pooled data from nine independent experiments with three or more replicates, for each condition reported, in each experiment (1.0 vs. 0.804, *p < 0.01: normalized data pooled from nine independent experiments, three replicates/condition). Ab = antibody; M1% = percent positive of gated cells.
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Figure 5 Transduction of TAT-BH4. To assess the functional significance of reduced Bcl-xL expression, cardiomyocytes were pretreated with a cell permeable Bcl-xL peptide (TAT-BH4). TAT-BH4 completely prevented the decline in (a) mitochondrial membrane potential (70 ± 0.57 vs. 51.2 ± 1.26, *p < 0.01, n = 4 independent experiments), (b) intracellular adenosine triphosphate (5867 ± 166 vs. 4946 ± 272 relative light units, *p < 0.01, n = 5 independent experiments), and (c) MTT activity (1.070 ± 0.02 vs. 0.9744 ± 0.03, *p < 0.05, n = 5 independent experiments). Abbreviations as in Figure 4.
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Figure 6 Apoptosis. (a) Anti-erbB2 increased apoptosis as measured by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) (6.5 ± 0.7% vs. 3.1 ± 0.4%, *p < 0.05, n = 5 independent experiments). (b) Propidium iodide flow cytometry. Control, anti-erbB2, and control antibody (control Ab)-treated cells were fixed in ethanol and stained with PI. Anti-erbB2 increased the percentage of apoptotic cells. (8.3 ± 0.9% vs. 4.5 ± 0.6%, *p < 0.01, n = 8 independent experiments). (c) Deoxyribonucleic acid (DNA) laddering. DNA was collected from untreated, anti-erbB2, and doxorubicin (positive control)-treated cells, radiolabeled as described in Methods, and subjected to 1.8% agarose gel electrophoresis and autoradiography.
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