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. Author manuscript; available in PMC: 2020 Jun 1.
Published in final edited form as: Med Sci Sports Exerc. 2019 Jun;51(6):1106–1115. doi: 10.1249/MSS.0000000000001887

Exercise Training Prevents Doxorubicin-induced Mitochondrial Dysfunction of the Liver

J Matthew Hinkley 1,#, Aaron B Morton 1, Noriko Ichinoseki-Sekine 1,2, Andres Mor Huertas 1, Ashley J Smuder 3
PMCID: PMC6522307  NIHMSID: NIHMS1517970  PMID: 30629044

Abstract

PURPOSE:

Doxorubicin (DOX) is a highly effective chemotherapeutic agent used in the treatment of a broad spectrum of cancers. However, clinical use of DOX is limited by irreversible and dose-dependent hepatotoxicity. The liver is the primary organ responsible for the clearance of antineoplastic agents, and evidence indicates that hepatotoxicity occurs as a result of impaired mitochondrial efficiency during DOX metabolism. In this regard, exercise training is sufficient to improve mitochondrial function and protect against DOX-induced cytotoxicity. Therefore, the purpose of this study was to determine if short-term exercise preconditioning is sufficient to protect against DOX-induced liver mitochondrionopathy.

METHODS:

Female Sprague-Dawley rats (4–6 months old) were randomly assigned to one of four groups: 1) sedentary, treated with saline; 2) sedentary, treated with DOX; 3) exercise trained, treated with saline; 4) exercise trained, treated with DOX. Exercise trained animals underwent 5 days of treadmill running habituation followed by 10 days of running for 60 min/day (30 m/min; 0% grade). Following the last training bout, exercise-trained and sedentary animals were injected with either DOX (20 mg/kg; i.p.) or saline. Two days following drug treatment the liver was removed and mitochondria were isolated.

RESULTS:

DOX treatment induced mitochondrial dysfunction of the liver in sedentary animals, due to alterations in mitochondrial oxidative capacity, biogenesis, degradation, and protein acetylation. Furthermore, exercise preconditioning protected against DOX-mediated liver mitochondrionopathy, which was associated with maintenance of mitochondrial oxidative capacity and protein acetylation.

CONCLUSION:

These findings demonstrate that endurance exercise training protects against DOX-induced liver mitochondrial dysfunction, which was attributed to modifications in organelle oxidative capacity and mitochondrial protein acetylation.

Keywords: mitochondrionopathy, Adriamycin, hepatotoxicity, protein acetylation, sirtuin 3

INTRODUCTION

The anthracycline doxorubicin (DOX) is a highly effective chemotherapeutic agent used to treat a broad spectrum of human cancers. However, the clinical efficacy of DOX is limited as a result of cellular toxicity to healthy tissue. Specifically, the liver plays a key role in the metabolism and excretion of DOX, and its rapid accumulation prior to breakdown is hypothesized to contribute to hepatotoxicity (1). DOX-induced damage to the liver is irreversible in a dose-dependent manner, and it is estimated that 40% of patients undergoing DOX treatment will suffer from liver injury (2). DOX-induced impairments to liver function can result in serious health complications, including liver failure and death. Thus, it is of critical importance to understand how DOX negatively impacts organ function to generate targeted interventions to improve health outcomes.

Metabolic breakdown of DOX results in the formation of several reactive proteins hypothesized to contribute to DOX toxicity through the formation of reactive oxygen species and subsequent mitochondrial dysfunction (1). Specifically, DOX metabolism can result in the formation of a lipophilic aglycone with the ability to diffuse through the mitochondrial membrane (3). In addition, DOX localizes to the mitochondrial inner membrane due to a high affinity for cardiolipin. The accumulation of DOX within mitochondria elicits severe consequences to organelle function including an increase in proton leak into the mitochondrial matrix, resulting in a decreased efficiency of the mitochondria to produce energy (4). Indeed, this reduction in mitochondrial efficiency is a hallmark of DOX treatment (5–7), and mitochondrial dysfunction appears to mediate DOX-induced toxicity (7, 8). Collectively, these results suggest a critical importance in targeting mitochondrial health to ameliorate the deleterious effects of DOX on hepatotoxicity.

Endurance exercise training evokes several adaptations that promote liver health, including improvements in mitochondrial function (9). Furthermore, exercise training initiated prior to DOX treatment appears to prevent maladaptations to various tissues, including cardiac and skeletal muscle (9–11). In regards to hepatotoxicity, it was recently shown that six weeks of exercise preconditioning is sufficient to prevent DOX-induced liver toxicity (12, 13); however, the mechanisms responsible for exercise preconditioning-induced liver protection following DOX treatment is unclear. Therefore, the purpose of the current study was to test the hypothesis that short-term exercise preconditioning can prevent DOX hepatotoxicity via eliciting mitochondrial adaptations that promote alterations to mitochondrial capacity, quality control and post-translational modifications. To test this hypothesis, sedentary and exercise-trained animals were acutely administered DOX, after which liver mitochondrial function and signaling were examined.

METHODS

Experimental Design

All experiments were approved by the University of Florida Institutional Animal Care and Use Committee. Female Sprague-Dawley rats (4–6 months old) were assigned to one of four experimental groups (n = 7–8 / group): 1) Sedentary Control, 2) Sedentary DOX, 3) Exercise Control, and 4) Exercise DOX. During the protocol, animals were housed in the University of Florida Animal Care facility on a 12: 12-hour light-dark cycle and were provided food and water ad libitum.

Exercise training and DOX administration.

Following 5 days of treadmill running habituation, animals were exercised for 10 days for 60 min/day, at 30 m/min, 0% grade. Previous work by Lawler et al. estimated the work rate for this exercise intensity to be ~70% VO2max (14). This was determined in 4-month old rats by measuring oxygen uptake during exercise. Twenty-four hours following the last training bout, exercise-trained and sedentary animals were injected with either saline (Control) or DOX (20 mg/kg body weight i.p.). The dosage of DOX used is a human clinical dose that is pharmacologically scaled for use in rats (15, 16).

Tissue collection.

Forty-eight hours following DOX or saline treatment, animals were anesthetized (sodium pentobarbital, 60 mg/kg i.p.), and after reaching a surgical plane of anesthesia, the liver was removed. A portion of the liver (~900 mg) was placed in ice-cold saline for isolation of the mitochondrial fraction. Animals were euthanized by removal of the heart. Tissue was analyzed two days after DOX administration. This time point was chosen because previous studies indicate that This time point was chosen because it is established that dox-induces hepatotoxicity within 48 hours following DOX exposure (12, 13, 17).

Mitochondrial isolation.

Mitochondrial fractions were isolated as previously described (18). Briefly, livers were weighed and minced in isolation buffer containing: 250 mM sucrose, 5 mM HEPES, and 1 mM EGTA. The minced tissue was homogenized with a Potter-Elvehjem PTFE pestle and glass tube, then centrifuged (500 x g for 10 min at 4°C). The supernatant was decanted through gauze and centrifuged (3500 x g for 10 min at 4°C). The supernatant was removed, and the pellet was resuspended in isolation buffer. Following centrifugation (3500 x g for 10 min at 4°C), the supernatant was removed and the final mitochondrial pellet was resuspended using a Dounce homogenizer in 250 μl of buffer containing: 220 mM mannitol, 70 mM sucrose, 10 mM Tris-HCl, and 1 mM EGTA (pH 7.4).

Biochemical Analyses

Mitochondrial respiration.

Respiratory rates of isolated liver mitochondria (30 μl) were examined using a Clark-type electrode (Hansatech Instruments, King’s Lynn, UK) in the presence of 2 mM pyruvate and 2 mM malate. ADP (0.25 mM) was added to examine maximal ADP-supported respiration (State 3). State 4 respiration was evaluated following the complete phosphorylation of ADP. Respiratory rates were normalized to mitochondrial protein concentration and expressed as nmoles O2 / mg protein / min. Respiratory control ratio (RCR), a measurement of mitochondrial efficiency, was calculated by dividing State 3 by State 4.

Immunoblot analysis.

Mitochondrial fractions were lysed by three freeze-thaw cycles. Protein concentration was determined by the Bradford method (Sigma). Equal amounts of mitochondrial fractions were separated by SDS-PAGE and transferred to PVDF membranes. After transfer membranes were cut at appropriate molecular weights to allow for assessment of multiple proteins of different molecular weights. When necessary membranes were stripped (LI-COR Biosciences) and reprobed with an antibody raised against another species. Acetylated lysine and OXPHOS blots were stripped and probed for the loading control. Membranes were incubated with the following antibodies: acetylated lysine (#9441) (Cell Signaling, Danvers, MA), citrate synthase (ab96600) (Abcam, Cambridge, MA), mitochondrial heat shock protein 70 (ab2799) (mtHSP70) (Abcam), mitochondrial transcription factor A (TFAM) (DR1071) (Calbiochem, Darmstadt, Germany), nuclear respiratory factor 1 (Nrf1) (200-401-869) (Rockland Immunochemicals, Limerick, PA), p62 (ab56416) (Abcam), Parkin (ab77924) (Abcam), PTEN-induced putative kinase 1 (PINK1) (ab23707) (Abcam), sirtuin 3 (Sirt3) (sc-365175) (Santa Cruz Biotechnology, Dallas, TX) and total OXPHOS (ab110413) (Abcam). The total OXPHOS antibody contains 5 monoclonal antibodies against the complex I subunit NDUFB8, complex II-30kDa, complex III-Core protein 2, complex IV subunit I and the complex V alpha subunit. Each subunit was analyzed independently. The acetylated lysine antibody detects proteins post-translationally modified by acetylation. All detected proteins in each lane were analyzed to determine changes in protein acetylation status. Protein bands were identified using IRDye secondary antibodies (LI-COR Biosciences) and band intensities quantified using Odyssey software (LI-COR Biosciences). Voltage-dependent anion channel (VDAC) (sc-32063) (Santa Cruz Biotechnology) was used to verify equal loading. Importantly, there were no changes in protein abundance with either DOX or exercise treatments.

Data Analyses

Data are presented as means ± SE. Comparisons between groups were evaluated by a one-way ANOVA. When appropriate, Tukey’s honestly significant differences tests were performed post hoc. Correlations were performed to examine the relationship between mitochondrial function and abundance of key mitochondrial proteins. Significance was established at P < 0.05.

RESULTS

Mitochondrial Function and Oxidative Capacity

Previous reports suggest liver toxicity following DOX treatment is related to mitochondrial dysfunction (19). Specifically, isolated liver mitochondria from animals treated with DOX reveal a reduction in mitochondrial efficiency following DOX treatment (19). Similarly, we observed a significant reduction in RCR in isolated liver mitochondria from sedentary animals treated with DOX (Figure 1c). Moreover, maximal ADP-supported respiration (State 3) was unaltered between groups (Figure 1a), while State 4 respiration was significantly increased in sedentary animals treated with DOX (Figure 1b), suggesting an increase in proton leak in liver mitochondria. In line with its proposed hepatoprotective effects, we observed an exercise-mediated prevention of both DOX-induced increases in State 4 respiration (Figure 1b), and the reduction in mitochondrial efficiency (Figure 1c) in isolated liver mitochondria from exercise-trained animals treated with DOX.

Figure 1.

Figure 1.

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Combined effects of DOX treatment and exercise training on liver mitochondrial function and oxidative capacity. a) State 3 respiration; b) State 4 respiration; c) Respiratory Control Ratio (RCR), d) citrate synthase protein content, and f) respiratory chain protein content (complexes I, II, III, and V) in isolated liver mitochondria from Sedentary Control, Sedentary DOX, Exercise Control, and Exercise DOX treated animals. Values are mean ± SEM. *, P < 0.05 vs. Sedentary Control. e) Relationship between mitochondrial efficiency (RCR) and mitochondrial citrate synthase content from Sedentary Control (black circle), Sedentary DOX (gray circles), Exercise Control (black triangles) and Exercise DOX (gray triangles) animals. g) Representative immunoblot images for citrate synthase, respiratory chain (OXPHOS) complexes, and loading control VDAC.

Furthermore, these data show that the preservation of mitochondrial function following DOX treatment in exercise-trained animals may be related to the preservation of mitochondrial oxidative capacity. Specifically, mitochondrial citrate synthase protein content, a surrogate of oxidative capacity of the organelle, was reduced in sedentary animals treated with DOX, which was prevented by exercise training (Figure 1d). Additionally, oxidative capacity appears to play a critical role in liver mitochondrial efficiency, as a positive correlation was evident between RCR and citrate synthase protein content (Figure 1e). Finally, similar to previous findings in cardiac muscle mitochondria (6), DOX treatment did not alter mitochondrial respiratory chain content in sedentary animals (Figure 1f).

Mitochondrial Biogenesis

Mitochondrial quality is dependent on numerous factors, including the production of new healthy mitochondria (e.g., mitochondrial biogenesis). Previous reports suggest mitochondrial biogenesis is impaired following DOX treatment, resulting in the stimulation of apoptosis (20). Mitochondrial biogenesis is stimulated by several transcription factors that bind to mitochondrial DNA to induce gene expression. In this regard, protein content of TFAM in the liver was unaltered between treatment groups (Figure 2a) and no correlation between its expression and RCR existed (Figure 2b). In contrast, Nrf1 protein expression was reduced in sedentary animals treated with DOX (Figure 2c). Moreover, mitochondrial Nrf1 protein content appears to play a role in organelle function, as a positive correlation was observed between RCR and Nrf1 content (Figure 2d). However, exercise training was insufficient in preventing the decrease in mitochondrial Nrf1 content in DOX treated animals (Figure 2c).

Figure 2.

Figure 2.

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Alterations in liver mitochondrial biogenesis markers in sedentary and exercise trained animals treated with DOX. a) Mitochondrial transcription factor A (TFAM) and c) nuclear response factor 1 (Nrf1) were examined as markers of mitochondrial biogenesis. Proteins were examined in isolated liver mitochondria. Values are mean ± SEM. *, P < 0.05 vs. Sedentary Control. Correlative relationships between mitochondrial efficiency (respiratory control ratio) and mitochondrial TFAM (b) and Nrf1 (d) were examined in isolated mitochondria from Sedentary Control (black circle), Sedentary DOX (gray circles), Exercise Control (black triangles) and Exercise DOX (gray triangles) animals. e) Representative immunoblot images for TFAM, Nrf1, and loading control VDAC.

Mitophagy

The proper removal of damaged organelles is crucial for maintaining a quality pool of mitochondria. Autophagy is the process of removing damaged organelles by lysosomal proteases, with mitophagy being the mitochondria-specific process. We have previously shown that markers of autophagy (11, 21) and mitophagy (6) are upregulated following DOX treatment in cardiac and skeletal muscle. The current study corroborates these findings as liver mitochondria also showed an increase in markers of mitophagy. Specifically, Pink1, which acts as an initiatory signal for mitophagy and p62, which is required to bind ubiquinated proteins to the autophagosome for degradation, were increased in liver mitochondria from both sedentary and exercise trained animals treated with DOX (Figure 3a and b). Interestingly, there was a negative correlation between mitochondrial efficiency and Pink1 content (Figure 3d), suggesting increased translocation of Pink1, and subsequent mitophagy, results in mitochondrial inefficiency. Additionally, there were no changes in Parkin following DOX treatment in either sedentary or exercised groups (Figure 3c).

Figure 3.

Figure 3.

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Effects of DOX treatment and exercise training on markers of mitophagy in the liver. a) Pink1, b) p62, and c) Parkin protein content were examined as markers of mitophagy in isolated liver mitochondria from Sedentary Control, Sedentary DOX, Exercise Control, and Exercise DOX treated animals. Values are mean ± SEM. *, P < 0.05 vs. Sedentary Control. Correlative relationships between mitochondrial efficiency (respiratory control ratio) and Pink1 (d), p62 (e) and Parkin (f) were examined in isolated mitochondria from Sedentary Control (black circle), Sedentary DOX (gray circles), Exercise Control (black triangles) and Exercise DOX (gray triangles) animals. g) Representative immunoblot images for Pink1, p62, Parkin, and loading control VDAC.

Mitochondrial HSP70

Within the mitochondria, HSP70 plays a critical role in protein stability by chaperoning proteins synthesized in the cytosol into the mitochondrial compartment and aiding in the proper folding of proteins. In the liver, mitochondrial HSP70 was decreased in Sedentary DOX animals compared to Sedentary Control animals (Figure 4a). Furthermore, exercise training prevented the DOX-induced decrease in the liver (Figure 4a), suggesting a potential protective role of mitochondrial HSP70 in preventing DOX-induced liver mitochondrial dysfunction. However, mitochondrial efficiency (RCR) was not correlated to mitochondrial HSP70 protein content (Figure 4b).

Figure 4.

Figure 4.

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Role of mitochondrial heat shock protein 70 (HSP70) in mediating alterations in liver mitochondrial efficiency following DOX treatment. a) Mitochondrial HSP70 protein content in isolated liver mitochondria from Sedentary Control, Sedentary DOX, Exercise Control and Exercise DOX treated animals. Values are mean ± SEM. *, P < 0.05 vs. Sedentary Control. b) Relationship between mitochondrial efficiency (respiratory control ratio) and mitochondrial HSP70 content from Sedentary Control (black circle), Sedentary DOX (gray circles), Exercise Control (black triangles) and Exercise DOX (gray triangles) animals. c) Representative immunoblot images for mitochondrial HSP70 and loading control VDAC.

Mitochondrial Protein Acetylation

Beyond content and quality, mitochondrial function is controlled by several post-translational modifications, including protein acetylation. An increase in mitochondrial protein acetylation has been observed in metabolic disturbances that result in dysfunctional mitochondria (22). Interestingly, we observed an increase in liver mitochondrial protein acetylation in sedentary animals treated with DOX, which was attenuated in exercise-trained animals (Figure 5a). Furthermore, the acetylation status of mitochondrial proteins appears to be related to alterations in mitochondrial efficiency, as there was a negative correlation with liver mitochondrial RCR and mitochondrial protein acetylation (Figure 5b). In line with these findings, expression of the mitochondrial deacetylase Sirt3 was reduced in sedentary animals treated with DOX, which was prevented in exercise-trained animals (Figure 5c). Additionally, there was a positive correlation between mitochondrial Sirt3 content and RCR (Figure 5d). However, there was no correlation between Sirt3 and acetylated lysine protein expression (Pearson r = −0.172; p-value = 0.482). Collectively, these results suggest that DOX treatment reduces the abundance of Sirt3, resulting in an increase in protein acetylation and reduced mitochondrial function, which is prevented by prior exercise training.

Figure 5.

Figure 5.

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Mitochondrial protein acetylation mediates mitochondrial efficiency following DOX treatment in isolated liver mitochondria. a) Mitochondrial protein acetylation in isolated liver mitochondria from Sedentary Control, Sedentary DOX, Exercise Control and Exercise DOX treated animals. b) Relationship between mitochondrial efficiency (respiratory control ratio) and mitochondrial protein acetylation from Sedentary Control (black circle), Sedentary DOX (gray circles), Exercise Control (black triangles) and Exercise DOX (gray triangles) animals. c) Sirtuin 3 (Sirt3) protein content in isolated liver mitochondria from Sedentary Control, Sedentary DOX, Exercise Control and Exercise DOX treated animals. Values are mean ± SEM. *, P < 0.05 vs. all groups. d) Relationship between mitochondrial efficiency (respiratory control ratio) and mitochondrial Sirt3 protein content from Sedentary Control (black circle), Sedentary DOX (gray circles), Exercise Control (black triangles) and Exercise DOX (gray triangles) animals. e) Representative immunoblot images for acetylated lysine, Sirt3, and loading control VDAC.

DISCUSSION

Toxicity to non-cancerous tissue is a severe consequence of DOX treatment, which limits its use clinically. Upon administration, DOX is distributed systemically and accumulates in tissues to varying capacities, with liver tissue demonstrating a relatively high capacity to bind DOX (19, 23). The accumulation of DOX within the liver causes dose-dependent hepatotoxicity that is mediated by the development of oxidative stress and impairments in mitochondrial function (1, 19). In this regard, endurance exercise training has been demonstrated to reduce DOX-induced oxidative damage to the liver (12). However, the effects of exercise training on DOX-induced liver mitochondrial function are unknown. We hypothesize that exercise preconditioning is an effective strategy to alleviate mitochondrial dysfunction after acute DOX treatment. A brief overview of our findings follows.

Mitochondrial dysfunction and liver damage

Maintenance of mitochondrial health is integral to sustaining overall liver function, with mitochondrial functional impairments often preceding liver damage (24). The origin of mitochondrial damage appears multifactorial; however, a role for impaired cellular respiration seems to be critical (6, 7). An increase in electron flow through the respiratory chain without a concomitant increase in ATP production results in greater proton leak into the mitochondrial matrix (state 4 respiration). Importantly, increased state 4 respiration can potentiate oxidant production within the mitochondrial matrix, which stimulates several maladaptations including apoptosis, mitochondrial DNA damage, and further deficiencies in cellular respiration (6, 7). In regards to DOX-induced hepatotoxicity, there is considerable evidence that mitochondrial damage is a hallmark of DOX treatment (19), with the current findings confirming that DOX administration increases mitochondrial proton leak and impairs mitochondrial coupling.

Endurance exercise training is an effective therapeutic tool demonstrated to attenuate DOX-induced hepatotoxicity in several pre-clinical studies (12, 13, 25). While the mechanisms responsible for the exercise-induced reduction in liver toxicity remain unknown, evidence in other tissue types (i.e. cardiac and skeletal muscle) suggest that exercise-induced cytoprotection occurs as a result of mitochondrial phenotypic adaptations that elicit resistance to the toxic effects of DOX exposure (5, 10). Our data corroborate these findings in the liver, as two weeks of endurance exercise preconditioning was sufficient to prevent DOX-induced liver mitochondrial dysfunction.

Impaired mitochondrial efficiency in the liver following DOX treatment and the effects of exercise

Several mechanisms have been proposed for impaired mitochondrial function following DOX treatment. Recently, Kavazis et al identified various signaling events within isolated cardiac muscle mitochondria that were altered during DOX treatment, these include enhanced mitophagy, reduced mitochondrial oxidative capacity and reduced mitochondrial biogenesis (6). Similar to the heart, Dirks-Naylor et al demonstrated a reduction in the expression of proteins required for mitophagy and mitochondrial oxidative capacity in the liver (26). Our results are in line with several of these observations made in whole-liver homogenates. However, protein expression of PINK1 and p62 were elevated in the mitochondrial fraction and remained elevated with exercise training. This finding may indicate impaired mitophagy flux and reduced degradation of ubiquitinated proteins. However, increased PINK1 has also been demonstrated to localize to the mitochondria where it may play a role in maintaining mitochondrial function and promoting cell survival. Specifically, PINK1 has been demonstrated to modulate complex I activity, activate the mitochondrial chaperone of HSP90 (TNF receptor-associated protein 1 (TRAP1)) through direct phosphorylation and promote the preservation of mitochondrial function by extraction of damaged proteins via mitochondria-derived vesicles (27, 28). Therefore, further work is needed to elucidate the role of increased PINK1 protein expression following DOX exposure and exercise training.

During various conditions, exercise training can act as a preventive measure against mitochondrial dysfunction and tissue damage of the liver following acute and chronic perturbations (25, 29). In cardiac and skeletal muscle tissue exposed to DOX, it is established that endurance exercise training elicits beneficial adaptations to markers of mitophagy, mitochondrial biogenesis and oxidative capacity which are hypothesized to protect against DOX-induced muscle mitochondrial dysfunction (30). Our results reveal that the mechanisms by which exercise facilitates protection against mitochondrial dysfunction may differ between tissues as our data suggest that exercise-induced mitochondrial hepatoprotection is dependent on changes in the oxidative capacity of the organelle, while maintenance of mitophagy and mitochondrial biogenesis is not a requirement for exercise-induced protection against DOX toxicity.

Mitochondrial HSP70 expression and exercise

HSP70 is considered a major mitochondrial control mechanism associated with various protective mechanisms, including maintenance in oxidative capacity (31). The current investigation demonstrated that DOX treatment is sufficient to induce a reduction in mitochondrial HSP70 protein content, which is ameliorated in exercise trained animals. This corresponds to a previous evaluation of total hepatic HSP70 protein that demonstrated significantly reduced levels of total HSP70 in DOX treated animals compared to exercise preconditioned animals treated with DOX (25). While the precise role of exercise-induced increases in HSP70 protein expression in the liver are unknown, mitochondrial protein content was not associated with changes in RCR, suggesting that mitochondrial HSP70 is not required to mediate mitochondrial efficiency. Similar findings were demonstrated in cardiac muscle, where exercise preconditioning maintained mitochondrial function in DOX treated animals even in the absence of increased HSP70 protein expression (5).

Role of protein acetylation and Sirt3 content in mediating DOX- and exercise-induced mitochondrial adaptations

Acetylation of mitochondrial proteins is one of the most abundant post-translational modifications controlling organelle function, with ~50% of mitochondrial proteins acetylated on lysine residues, affecting numerous functions including energy metabolism (32). Protein acetylation can have severe implications on mitochondrial function, and several disease states associated with mitochondrial dysfunction (i.e., obesity, diabetes, and heart failure) present hyperacetylation of mitochondrial proteins (32). Protein acetylation is controlled by an increase flux of acetyl groups within the mitochondria and/or reduced activity of deacetylases. Sirt3 is considered the major deacetylase within the mitochondrial matrix, and controls several metabolic functions, including lipid metabolism, respiration, and oxidant production (32). Deacetylation by Sirt3 leads to enhanced mitochondrial oxidative metabolism and stimulates isocitrate dehydrogenase and superoxide dismutase 2 activity (33). Conversely, loss of Sirt3 function is linked to increased ROS production and altered oxidative metabolism (34).

Previous studies in cardiomyocytes revealed that DOX induces acetylation of mitochondrial proteins, which resulted in mitochondrial dysfunction (35). Conversely, in the liver, reports looking at whole tissue homogenate have shown a decrease in liver lysine acetylation and no change in Sirt3 expression following DOX exposure (36). While our results demonstrate both an increase in acetylated lysine and a reduction in Sirt3 protein levels, the disparity between these studies could be accounted for by the differences in cellular compartment assessed (cytosolic versus mitochondrial), as well as differences in experimental design between these studies.

Evidence indicates that endurance exercise training is sufficient to increase Sirt3 expression (37). This is important because Sirt3 expression is sufficient to protect against mitochondrial DNA damage and rescue mitochondrial respiration in the hearts of DOX treated animals (35). In the liver, Sirt3 overexpression can enhance anti-oxidative capacity, attenuate mitochondrial fragmentation and improve mitochondrial function (38). Similarly, the exercise-induced rescue of mitochondrial Sirt3 expression in DOX treated animals was positively related to RCR. Additionally, control of the mitochondrial acetylation proteome appears critical for mitochondrial efficiency, as lysine acetylation was negatively associated with RCR. As mitochondrial efficiency plays a critical role in liver health, this finding suggests Sirt3 may play an integral role in mediating the protective effects of exercise against DOX hepatotoxicity. Together, these data suggest liver mitochondrial protein acetylation is an integral post-translational modification that impacts organelle function following DOX exposure, and that the exercise-induced elevation in Sirt3 expression may be critical in the maintenance of mitochondrial efficiency after DOX treatment.

CONCLUSION

Dox treatment induces multi-tissue damage to mitochondria, resulting in impaired efficiency of the organelle (6, 7). Our results further solidify these previous reports, as liver mitochondria were dysfunctional in sedentary animals following DOX administration. Detriments in liver mitochondrial function appeared to be related to alterations in oxidative capacity, mitophagy, biogenesis, and regulation of protein acetylation. Importantly, exercise preconditioning was sufficient to prevent DOX-induced mitochondrial dysfunction, which correlated to maintenance in oxidative capacity and protein acetylation status. Collectively, these data suggest that exercise preconditioning induces a mitochondrial phenotype that is resistant to mitochondrial dysfunction in the liver following DOX treatment. These findings identify similar mechanisms for exercise-induced protection in liver and cardiac muscle (5). This is important because clinical reports have demonstrated that exercise conditioning is feasible in patients undergoing DOX chemotherapy (39), and it may be sufficient to prevent DOX-induced cardiotoxicity (40). Therefore, in patients receiving DOX, exercise prescriptions may also confer protection against hepatotoxicity. Future studies are required to further examine the mechanisms by which exercise prevents DOX-induced liver mitochondrionopathy and its effect on liver function.

ACKNOWLEDGEMENTS

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. ABM was supported by NIH T32 HD043730.

Footnotes

CONFLICT OF INTEREST

No conflicts of interest, financial or otherwise, are declared by the authors. The results of the present study do not constitute endorsement by ACSM, and are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation

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