Exercise stimulates beneficial adaptations to diminish doxorubicin-induced cellular toxicity

Abstract

Doxorubicin (DOX) is a highly effective antitumor agent used for the treatment of a wide range of cancers. Unfortunately, DOX treatment results in cytotoxic side effects due to its accumulation within off-target tissues. DOX-induced cellular toxicity occurs as a result of increased oxidative damage, resulting in apoptosis and cell death. While there is no standard-of-care practice to prevent DOX-induced toxicity to healthy organs, exercise has been shown to prevent cellular dysfunction when combined with DOX chemotherapy. Endurance exercise stimulates numerous biochemical adaptations that promote a healthy phenotype in several vulnerable tissues without affecting the antineoplastic properties of DOX. Therefore, for the development of an effective strategy to combat the pathological effects of DOX, it is important to determine the appropriate exercise regimen to prescribe to cancer patients receiving DOX therapy and to understand the mechanisms responsible for exercise-induced protection against DOX toxicity to noncancer cells. This review summarizes the cytotoxic effects of DOX on the heart, skeletal muscle, liver, and kidneys and discusses the current understanding of the clinical benefits of regular physical activity and the potential mechanisms mediating the positive effects of exercise on each organ system.

INTRODUCTION

Doxorubicin (DOX), an anthracycline antibiotic, is used in the treatment of a broad spectrum of human cancers, including acute leukemia, lymphomas, stomach, breast, and ovarian cancers, Kaposi’s sarcoma, and bone tumors (78). Unfortunately, the clinical use of this highly efficacious anticancer drug is limited because of its toxicity. DOX administration results in dose-dependent, irreversible side effects, several of which include the development of cardiomyopathy, dyspnea, exercise intolerance, hepatotoxicity, and nephropathy (7, 21, 26, 31, 67, 89). While work has been done to identify risk factors, develop less-toxic derivatives, and detect subclinical toxicity earlier, there is no consensus on the best approach to prevent anthracycline-induced cytotoxicity (118). Therefore, further advances in the molecular basis of DOX-induced pathology are needed to generate preventative strategies.

It is well established that exercise training provides robust protection during a variety of injurious conditions, and emerging evidence demonstrates that exercise is a protective therapy for patients undergoing DOX treatment (21, 69, 103, 105). This review presents the current understanding of the effects of exercise on the health of multiple tissues affected by DOX chemotherapy and existing information regarding the current understanding of the mechanisms responsible for exercise-induced protection against DOX-induced toxicity. This review begins with a brief summary of the cellular events leading to DOX-induced pathology followed by a broad description of the current understanding of the effects of exercise training on cardiac muscle, skeletal muscle, liver, and kidney function.

DOXORUBICIN: MODE OF ACTION

DOX (clinically known as Adriamycin) is a potent broad-spectrum chemotherapeutic agent, the antineoplastic effects of which are mediated primarily through direct interactions with DNA. Specifically, DOX has been demonstrated to inhibit tumor cell proliferation by 1) intercalation into DNA and inhibition of macromolecular biosynthesis, 2) interference with topoisomerase II and prevention of DNA replication, 3) inhibition of helicase and promotion of DNA unwinding, and 4) production of free radicals, resulting in damage to DNA and cellular membranes (114). Despite the efficacy of DOX as a chemotherapeutic agent, the clinical use of DOX in patients is generally discontinued once the total administered dose exceeds 550 mg/m² body surface area, as a result of the drastic increase in the risk of irreversible cytotoxicity to healthy, noncancerous tissues (119).

DOX-INDUCED TOXICITY TO NONCANCER CELLS

DOX chemotherapy is often administered to patients via three unique routes. Most commonly, DOX is given systemically by intravenous infusion (7). In addition, to treat ovarian cancer and peritoneal carcinomatosis, DOX is administered by intraperitoneal injection to target the abdominal cavity (27). Finally, in cases of unresectable limb tumors, soft tissue sarcoma, and melanoma, isolated limb perfusion may be used (45). Regardless of the route of administration, DOX has debilitating effects on healthy tissues as a result of nonspecific targeting of rapidly dividing cells. These cytotoxic effects are primarily responsible for the severe limitations in the use of this highly effective antineoplastic agent. Indeed, DOX has been demonstrated to have debilitating effects on a wide variety of tissues, including cardiac muscle, skeletal muscle, liver, and kidney (Fig. 1).

Fig. 1.

Potential side effects of doxorubicin chemotherapy on heart, skeletal muscle, liver, and kidneys. CHF, congestive heart failure; LV, left ventricle.

DOX-Induced Cardiotoxicity

The risk of cardiotoxicity is one of the greatest limiting factors to the clinical use of DOX. Indeed, repetitive administration of DOX is associated with the dose-dependent development of cardiomyopathy and heart failure. The effects of DOX on the heart vary between patients and can result in both acute and chronic cardiovascular events. Acute effects of DOX can develop within minutes to days after administration and normally manifest as chest pain as a result of hypotension, left ventricular failure, tachycardia, and various arrhythmias (12). The mechanisms for these immediate changes are not fully understood but are often transient and reversible in patients. Indeed, it is hypothesized that the acute effects of DOX on the heart occur as a result of reversible myocardial edema (112). It is estimated that ~11% of patients receiving DOX treatment will experience these acute symptoms of cardiac dysfunction (111, 112). The incidence of chronic cardiovascular effects after DOX administration is much lower (~1.7%) as a result of limiting the dose of DOX given to patients (119). Specifically, an analysis of the incidence of DOX cardiotoxicity estimated that the occurrence of cardiomyopathy increases from 4% to 36% when the administered dose of DOX increases from 500 to 600 mg/m² (74). Patients presenting with chronic DOX cardiotoxicity normally demonstrate symptoms within 30 days after cessation of treatment; however, cardiomyopathy has been demonstrated 6–10 yr after the last dose of DOX is administered (12). The chronic effects of DOX include the insidious onset of cardiomyopathy, which often leads to congestive heart failure. Heart failure in DOX-treated patients results in systemic complications that can affect the health of many organs. Indeed, variation in adequate organ perfusion as a result of reduced cardiac output and hemodynamic disturbances is associated with skeletal muscle atrophy, liver failure, and chronic kidney disease (CKD) in heart failure patients (49, 80, 116). Furthermore, the development of DOX-induced congestive heart failure is extremely harmful, with ~50% mortality in patients (119).

DOX-Induced Skeletal Muscle Toxicity

Fatigue affects >75% of patients undergoing cancer treatment and is the most commonly reported symptom of patients undergoing chemotherapy (110). Fatigue is reported to cause extreme distress due to its negative impact on the activities of daily living, reduced quality of life, and increased morbidity and mortality (51, 86, 117). In cancer patients, fatigue can manifest as a perceived tiredness with a lack of energy and as physical weakness with a decrease in muscle strength (41, 86, 110). These disconcerting symptoms can result in discontinued or delayed treatment and often persist for years following chemotherapy treatment (31, 41, 115).

Preclinical models demonstrating the effects of DOX on skeletal muscle have established that severe myopathy occurs after treatment, with no preferential deficits demonstrated between fiber types (85). These models have described DOX-induced skeletal muscle weakness as defined by impaired muscle relaxation and contraction, reduction in skeletal muscle force production and maximal twitch force, and muscle fiber atrophy as early as 2 days following DOX exposure (38–40, 85). These findings are corroborated in the clinic, as reports have described lower-extremity weakness and loss of functional performance and muscle mass in patients after DOX chemotherapy (31, 103, 115). Locoregional DOX therapy in the form of hyperthermic isolated limb perfusion to treat limb sarcoma tumors has also been demonstrated to result in muscle dysfunction and atrophy, potentially due to neurotoxicity and the development of neuromuscular damage (8, 95). Remarkably, DOX can also be used clinically as a chemomyectomy agent to treat facial spasms localized to the eyelids. In these conditions, DOX is injected directly into the muscle to induce permanent skeletal muscle necrosis and decreased muscle mass to relieve involuntary muscle spasms (82, 83, 124).

DOX-Induced Hepatotoxicity

The liver plays a key role in the metabolism and elimination of DOX. Indeed, while DOX is taken up by many tissues, it appears to be rapidly taken up by the liver, where it accumulates before metabolism and elimination (71, 73). While ~50% of all DOX is unmodified before removal from the body (64), it is broken down in several ways. The majority of DOX is metabolized in the liver by a two-electron reduction to the metabolite doxorubicinol; this process is facilitated primarily by the hepatic reducing enzyme carbonyl reductase 1 (67, 114). In addition, DOX can undergo a one-electron reduction by NADPH cytochrome P-450 reductase to form a semiquinone free radical (6, 114). Finally, detoxification of reactive DOX metabolites can occur via hepatic microsomal reductases to form deoxyglycones (3, 71).

The metabolism of a high volume of DOX results in toxic effects to the liver: an estimated 40% of individuals undergoing treatment incur some form of liver injury (23, 35). In patients receiving DOX, liver injury has been classified by elevated levels of aminotransferases, hyperbilirubinemia, and hepatic vascular injury (4, 23, 63); in rodent models of DOX hepatotoxicity, liver damage presents as hepatocyte degeneration, parenchymal necrosis, proliferation of the biliary duct, thrombosis of the central vein, and liver fibrosis (84, 126).

In addition to direct DOX-induced hepatotoxicity, patients with preexisting liver disease are at a greater risk for DOX-induced toxicity as a result of elevated systemic concentrations of DOX. The rate of hepatic extraction from the circulation is determined by hepatic blood flow, hepatocyte mass, and hepatic function (71). In general, liver disease is associated with hepatocyte dysfunction and delayed excretion and elevated plasma concentrations of DOX, causing increased systemic toxicity and deterioration of liver function (35, 71). Importantly, it is recommended that patients identified to have abnormal liver function (as assessed by bilirubin levels) receive reduced dosages of DOX depending on the severity of the impairment (35, 71).

DOX-Induced Nephrotoxicity

While the majority (~40–50%) of DOX and its metabolites are excreted through bile, the kidneys also play a role in DOX excretion from the body (72). Specifically, metabolized DOX accumulates in the kidneys, and ~4–8% of administered DOX is slowly excreted through urine over several days (72, 113). Although the accretion of DOX within the kidneys is necessary for its elimination, renal failure has been demonstrated in patients undergoing Adriamycin chemotherapy (10, 44, 100). Indeed, the necessity to closely monitor kidney function in patients was first established in 1977 in a case study demonstrating that DOX therapy was associated with renal failure (10). DOX can cause nephropathy in cancer patients for a variety of reasons. For example, DOX-induced tumor lysis and collection within the kidneys can result in the increased deposition of potassium, phosphate, and uric acid into the renal tubules, as well as deposition of fibrin thrombi within the glomeruli and the need for maintenance dialysis (44, 100).

In addition to the findings within the patient population, it is well established that DOX causes renal failure in experimental animals, and DOX therapy is commonly used as a rodent model of CKD because of the rapid occurrence (within days) of renal injury following drug administration (13, 72, 96). Specifically, studies demonstrate that DOX induces direct toxicity to the kidneys, resulting in damage to the glomerulus, tubulointerstitial inflammation and fibrosis, podocyte effacement and elevated serum creatinine, reduced creatinine clearance, reduced serum albumin, dyslipidemia, and proteinuria (13, 42, 46, 60, 96, 121).

MECHANISMS OF DOX-INDUCED CYTOTOXICITY

While the mechanisms responsible for DOX-induced toxicity to nontumor cells are not well understood, evidence suggests that the primary mediator of DOX toxicity is the induction of oxidative stress (30). There are several ways in which DOX can produce reactive oxygen species (ROS) (Fig. 2). First, a one-electron reduction in the DOX quinone structure leads to the formation of a semiquinone free radical intermediate. The unpaired electron of the semiquinone moiety can be donated to oxygen to form superoxide radicals (24). Second, metabolism of DOX via cleavage of the sugar residue and reduction in the carbonyl group at C-13 to produce doxorubicinol also results in ROS generation through redox cycling and increased levels of free iron (16). Third, DOX can generate ROS through a direct interaction with iron or other metal ions. Specifically, DOX can form a complex with iron that is capable of reacting with O₂ or H₂O₂ to produce free radicals (48, 76). Superoxide and its decomposition products can initiate lipid peroxidation and cause injury to healthy tissue.

Fig. 2.

Primary mechanisms of doxorubicin (DOX)-induced reactive oxygen species production: 1-electron reduction in the DOX quinone structure (1), metabolic breakdown of DOX (2), and direct interaction of DOX with iron and/or other metal ions (3). IMM and OMM, inner and outer membranes of mitochondria, respectively.

While multiple potential sources of ROS exist, evidence indicates that the mitochondria are a significant source of oxidant production following DOX administration (120). Specifically, it is well established that DOX accumulates in the mitochondria because of its high affinity for cardiolipin, a phospholipid expressed on the inner mitochondrial membrane (65, 120). The complex that DOX forms with cardiolipin places it in close proximity to the electron transport chain, and redox cycling of DOX is mediated through its interaction with NADH dehydrogenase (24, 30). Elevated free radical production can lead to several damaging events in the mitochondria that, taken as a whole, can drastically diminish mitochondrial function and result in cell death (120).

In addition to the ability of DOX to damage mitochondria as a result of futile redox cycling, emerging evidence suggests that DOX-induced toxicity may be mediated by an interaction between DOX and topoisomerase IIβ (127). Specifically, it was demonstrated that genetic deletion prevented DOX cytotoxicity. Mechanistically, this protection occurred as a result of inhibition of DOX-induced mitochondrial dysfunction that occurs as a result of topoisomerase IIβ-induced activation of apoptosis. This was sufficient to elicit alterations in the transcriptome that increases mitochondrial ROS production and reduces the expression of genes necessary for mitochondrial biogenesis (127).

There are no clinically approved therapeutics to combat DOX-induced toxicity. However, as a nonpharmacological treatment, exercise is recommended at all points following cancer diagnosis (104). Indeed, exercise training has been demonstrated to have cytoprotective effects against DOX toxicity and may be a vital tool in the development of countermeasures to protect against the toxic effects.

EXERCISE PROTECTS AGAINST DOX TOXICITY

In 1979 Combs and colleagues provided the first evidence that exercise training elicits therapeutic effects that counteract the toxicity of DOX (20). Specifically, it was reported that an acute 30-min bout of swimming was sufficient to increase the survival rate of mice treated with Adriamycin (20). This finding was significant at the time, because it opposed the popular belief that exercise training would exacerbate the toxic effects of DOX as a result of increased ROS production and mitochondrial dysfunction. This initial finding has led to an abundance of work dedicated to determining the precise mechanisms by which exercise can reduce the off-target effects of DOX and to several clinical trials aimed at assessing the safety and efficacy of exercise as an adjuvant therapy for cancer patients receiving DOX chemotherapy. The following sections highlight some exciting findings within the patient population and detail what is currently known about the mechanisms for exercise protection.

CYTOPROTECTIVE EFFECTS OF EXERCISE

Heart

In 1983 reports detailing the first evidence that DOX enhances free radical damage to the heart in a dose-dependent manner led to subsequent investigation into the effects of exercise training on cardiac muscle morphology (24, 66). Kanter et al. exposed mice to a 21-wk training period and a 40 mg/kg cumulative dose of DOX (66). Assessment of cardiac muscle morphology following completion of the training period revealed significantly less cardiac damage in the exercise-trained than the drug-treated sedentary animals, leading to the conclusion that chronic aerobic exercise in combination with DOX chemotherapy elicits positive cardiac adaptations (66). Future studies support this work and have elucidated numerous preclinical endurance exercise paradigms that preserve cardiac function following DOX treatment. Specifically, both acute (75, 125) and chronic (17, 19, 55, 56, 75, 125) exercise preconditioning have been shown to ameliorate DOX-induced cardiac dysfunction, while further work has demonstrated that aerobic exercise initiated simultaneously with DOX treatment can also preserve cardiac function (18, 29, 54).

The majority of reports have focused on the beneficial effects of endurance exercise training to mitigate DOX cardiomyopathy. However, Pfannenstiel et al. (94) recently demonstrated that 12 wk of progressive resistance training before DOX treatment reduced oxidative stress in the heart and preserved cardiac function in a preclinical model. This finding is significant, because resistance exercise training is an important component of rehabilitation for patients with coronary artery disease to increase muscle strength and reduce perceived exertion (81), and recently it was demonstrated that resistance training may be equally beneficial in cancer rehabilitation (107, 123). Moreover, breast cancer patients who participated in supervised resistance training during chemotherapy reported improvements in quality of life compared with those who participated in endurance training and sedentary interventions (101).

Based on the numerous preclinical reports demonstrating exercise-induced cardioprotection against DOX toxicity, clinical trials investigating the effects of exercise on cardiac health in patients receiving DOX treatment are ongoing to determine safety, efficacy, and appropriate exercise prescriptions for cancer patients during treatment and rehabilitation following treatment. In 2003 Courneya et al. (21) completed a randomized controlled trial to determine the effects of a 15-wk exercise-training protocol following surgery, radiotherapy, and/or chemotherapy on postmenopausal breast cancer survivors, some of whom received DOX as part of their chemotherapy regimen. This study demonstrated improved cardiopulmonary function in the exercise-trained group compared with the nontrained group, which correlated to improvements in perceived quality of life (21). Subsequently, both a case study and a phase II randomized trial demonstrated that aerobic training in breast cancer patients is safe and well tolerated at appropriate levels (25, 52). Importantly, both studies also revealed a significant improvement in peak O₂ uptake, which is significant because reduced fitness capacity is a predictor of anthracycline-induced left ventricular dysfunction and cardiovascular disease risk in breast cancer patients (25, 52). Furthermore, exercise-based rehabilitation in a group of 15 cancer survivors was shown to support the current physical activity guidelines for cancer survivors, as a 10-wk exercise intervention reduced fatigue and systemic oxidative stress (98). Finally, studies are ongoing and continue to evaluate the feasibility of exercise interventions in the prevention of DOX-induced cardiotoxicity (69).

Skeletal Muscle

Growing evidence supports the notion that participation in regular bouts of physical activity improves muscular strength, reduces fatigue, and improves quality of life in cancer survivors (104). Importantly, exercise programs initiated at any point following diagnosis can provide important benefits to cancer patients, and current exercise prescriptions exist to promote the safety and efficacy of exercise independent of the disease stage (104). Unfortunately, it has been reported that 80% of cancer care providers are unaware of the exercise guidelines for patients and, thus, do not implement them or provide necessary information to patients (88, 104). In fact, advice from oncology caregivers often suggests rest following treatment, which is now believed to promote muscle weakness and reduced functional capacity (102).

While past studies have assessed the efficacy of regular physical activity to reduce fatigue and improve quality of life following adjuvant therapy for breast cancer, the majority of which included DOX chemotherapy, Schwartz et al. (105) were the first to directly measure muscle strength. Patients from this randomized clinical trial were assigned to aerobic exercise, resistance exercise, or usual care, with exercise testing at the beginning of chemotherapy and again in 6 mo. Results from this trial demonstrated a significant increase in 12-min walk distance, greater muscle strength for single repetitions of the seated row and leg extension, and increased aerobic capacity in the aerobic exercise group compared with the other groups (105).

In addition to the work done in the patient population, preclinical animal studies focusing on DOX-induced skeletal muscle dysfunction have demonstrated that both short-term (2 wk) and long-term (10 wk) endurance exercise preconditioning programs and resistance exercise preconditioning are sufficient to protect against skeletal muscle atrophy, contractile dysfunction, and muscular fatigue in animals receiving a bolus dose of DOX (9, 87). However, while DOX-induced skeletal muscle dysfunction occurs independent of muscle fiber type composition, exercise appears to preferentially protect slow-twitch (soleus) and mixed (diaphragm) muscles compared with fast-twitch (extensor digitorum longus) muscles (9, 87). Finally, a recent study demonstrated that interval training throughout a chronic DOX dosing protocol was also sufficient to prevent DOX-induced atrophy of the soleus muscle (28). Therefore, exercise can protect against skeletal muscle atrophy and weakness when initiated before or at the start of DOX treatment.

Liver

Although evidence indicates that exercise has no detrimental effects on patients with liver disease, physical activity is often restricted because of the proposed risk of increased portal pressures, bleeding, or hepatic encephalopathy (5, 37, 77, 99). However, a recent systematic literature review elucidating the efficacy of exercise interventions on patients with cirrhosis concluded that exercise in liver failure patients should be considered safe and potentially beneficial (77). In addition, exercise training may also prevent liver disease in populations at risk for the development of fatty liver disease (108). Currently, the potential benefits of exercise on DOX-induced liver toxicity remain to be elucidated in the patient population.

Significantly, preclinical studies have demonstrated that aerobic exercise training can modify DOX-induced hepatotoxicity and prevent DOX-induced maladaptations to the liver. Specifically, preconditioning protocols utilizing treadmill exercise ranging from 6 wk to 5 days before DOX administration provided valuable modifications to hepatocyte signaling (1, 2, 50, 79, 128). These studies demonstrate improvements in oxidative stress signaling, mitochondrial function, iron dysregulation, and insulin-like growth factor signaling and support the inclusion of exercise training as an advantageous therapy to preclude DOX hepatotoxicity (1, 2, 50, 79, 128).

Kidneys

Pathological changes within the kidneys following DOX administration strongly resemble those observed in patients with chronic renal disease (89), and guidelines for management of CKD now recommend regular physical activity (59, 122). The physiological gains from exercise in patients with CKD extend beyond improvements in kidney function; they also include decreased systemic inflammation, increased muscular strength, and improved cardiovascular health (70, 122). In addition to improving the symptoms of kidney disease, participation in regular physical activity is associated with a reduced risk of developing CKD. Indeed, in a cohort of 17,979 patients, fitness level was inversely related to the risk of developing CKD (26). This finding is confirmed by several studies demonstrating that lack of regular exercise is a factor associated with the highest risk for proteinuria (36, 90). Therefore, exercise has the potential to reduce the incidence of CKD, reduce complications associated with chronic kidney dysfunction, and improve quality of life.

Although exercise is a protective strategy for the prevention and management of progressive renal dysfunction, few studies have investigated the effects of exercise on DOX-induced nephropathy. Only preclinical animal studies of DOX nephrotoxicity have reported on the effects of exercise on kidney health, with results from these studies predominantly demonstrating beneficial outcomes. Specifically, Robert Peng and colleagues demonstrated duration-dependent effects of swimming and treadmill exercise training on kidney function (14, 15, 92, 93). Indeed, DOX (7.5 mg/kg) administered before initiation of a 9-wk swim-training program (3 days/wk for 30 or 60 min/day) showed reductions in renal edema and inflammation, with the 60-min/day program providing improved restoration of kidney morphology compared with the 30-min/day program (92). In a follow-up to this study, 60 min/day of swimming exercise for 9 wk effectively improved glomerular filtration rate, reduced blood urea nitrogen, and increased urine creatinine clearance rate (14). Similar results were demonstrated between 30 and 60 min of treadmill exercise (3 days/wk for 11 wk) (14, 15, 93): the 60-min/day regimen elicited greater protective effects against DOX-induced CKD. Finally, reports have also demonstrated a protective effect against kidney dysfunction when exercise is performed before DOX administration (2.5 mg/kg) (32) and when it is initiated 2 wk following a 7-wk dosing protocol (2 mg·kg⁻¹·wk⁻¹) (11).

MECHANISM OF EXERCISE-INDUCED PROTECTION AGAINST DOX TOXICITY

While it is well established that aerobic exercise training promotes cellular adaptations necessary to preclude DOX-induced toxicity, the mechanisms responsible for this cytoprotection remain unclear. Numerous signaling pathways have been hypothesized to promote the exercise-induced reduction in noncancerous tissue toxicity (Fig. 3).

Fig. 3.

Potential mechanisms responsible for exercise-induced cytoprotection against doxorubicin cellular toxicity include increased endogenous antioxidant enzyme expression, increased heat shock protein 72 (HSP72) expression, and/or increased multidrug resistance protein (MRP) expression. GPX, glutathione peroxidase; SOD, superoxide dismutase; ABCB, ATP-binding cassette transporter subfamily B.

Endogenous Antioxidants

The discovery that DOX can undergo redox cycling within the mitochondria to induce free radical production led to the first investigation into the effects of exercise training on endogenous antioxidant enzyme expression in DOX-treated animals (66). The study hypothesized that exercise could prevent DOX toxicity by inducing an increase in the activity of endogenous antioxidant enzymes, which would augment the oxidant-buffering capacity. This notion was supported by the finding that swim training in mice was sufficient to increase antioxidant enzyme activity in the heart and liver, which was maintained in swim-trained animals treated with DOX (66). Interestingly, further evaluation of endogenous antioxidant enzyme expression in the heart and liver following exercise and DOX administration has yielded mixed results. In the heart, several reports have revealed improved cardiac function following exercise independent of exercise-induced enhancement of antioxidant protein expression, leading to the possibility that increased antioxidant expression is not required for exercise-induced cardioprotection (17, 18, 62, 125). In contrast, exercise training induces an increase in antioxidant enzyme expression in the liver regardless of the exercise program (1, 128). Evidence for the requirement of exercise-induced antioxidant expression to prevent skeletal muscle dysfunction is mixed, as conflicting reports exist as to whether exercise induces antioxidant expression in skeletal muscle and, to date, reports that have evaluated skeletal muscle functional properties have not assessed antioxidant enzyme level (53, 109). Finally, increased antioxidant expression appears to facilitate the exercise-induced reduction in renal dysfunction, as long-term endurance exercise training was shown to protect against kidney dysfunction as a result of increased antioxidant expression (14).

Heat Shock Protein 72

Induction of heat shock protein (HSP) expression is a well-documented effect of endurance exercise training that can assist in the folding of damaged proteins, act as protein chaperones, and reduce cellular proteolysis (33, 106). HSP72, the highly conserved 72-kDa family member, has been shown to be upregulated following exercise in multiple organs, including the liver, kidneys, skeletal muscle, and heart (34, 47). The effects of combining DOX treatment with exercise training on HSP72 expression in the kidneys are unknown. However, significant increases in HSP72 expression have been reported in the liver, skeletal muscle, and cardiac muscle when exercise is combined with DOX treatment (1, 18, 68, 109).

While the necessity for exercise-induced HSP72 expression in the liver and skeletal muscle have not been elucidated, reports by Chicco et al. and Kavazis et al. revealed that HSP72 expression is not required for exercise-induced cardioprotection against DOX toxicity (17, 68). Indeed, using a training protocol designed to mimic walking exercise shown to benefit cancer patients during treatment, Chicco et al. (17) showed that low-intensity treadmill exercise during chronic DOX administration is sufficient to attenuate left ventricular dysfunction. However, this exercise-training protocol did not induce an increase in cardiac expression of HSP72. In addition, Kavazis et al. (68) discovered that inhibiting the exercise-induced expression of HSP72 in the heart by training animals in a cold environment abolished the protective effects of exercise against DOX-induced oxidative stress and mitochondrial dysfunction. Therefore, HSP72 expression does not appear to be required for exercise-induced cardioprotection against DOX toxicity. However, further work is needed to elucidate the role of HSP72 in the skeletal muscle, liver, and kidneys.

Multidrug Resistance Proteins

DOX administration results in nonspecific tissue accumulation, causing its buildup in noncancerous cells (65). After DOX enters the cell, its retention is mediated by the expression of drug efflux proteins on the cellular membrane (22). While the effects of exercise on liver and kidney accumulation of DOX are unknown, recent evidence revealed that exercise training is sufficient to reduce the cardiac and skeletal muscle accumulation of DOX (43, 61, 87, 91, 97). The ATP-binding cassette (ABC) proteins are a class of transporters that are capable of extruding xenobiotics from the cell, and previous reports have demonstrated that increased activity is sufficient to increase the expression of specific family members within the heart and skeletal muscle (87, 91, 97). Specifically, Parry and Hayward (91) demonstrated that 12 wk of voluntary wheel running resulted in a significant induction of the cardiac protein expression of the ABC transporters multidrug resistance proteins (MRP)-1 and MRP-2, which coincided with a significant reduction in left ventricle DOX accumulation. Similar to this finding, Quinn et al. (97) showed a significant reduction in DOX accumulation in the left ventricle following 10 wk of voluntary wheel running. However, in this study, protein expression of MRP-1, MRP-2, and MRP-7 was elevated in both the sedentary DOX-treated animals and the wheel-running animals, suggesting that exercise-induced increases in these proteins are not responsible for the exercise-induced reduction in DOX accumulation in the left ventricle (97). Additionally, this study also reported a significant reduction in DOX accumulation within the soleus muscle, and this decline was not associated with any change in ABC transporter protein expression (97).

A recent report by Morton et al. (87) showed that DOX accumulation is reduced not only within the cardiac and skeletal muscle but, specifically, from within the mitochondrial fraction of each of these tissues. In addition, the cardiac and diaphragm expression of the four mitochondria-localized ABC transporters was increased with exercise training (87). Therefore, the exercise-induced reduction in DOX accumulation in muscle may be due to increased expression of ABCB6, ABCB7, ABCB8, and/or ABCB10. Significantly, independent of exercise, in cardiac muscle, Ichikawa et al. demonstrated that overexpression of ABCB8 protein is sufficient to protect against cardiac DOX accumulation and cardiac dysfunction (57, 58). Therefore, these mitochondrial ABC transporters may play an important role in mediating exercise-induced cytoprotection against DOX toxicity. Further work is needed to elucidate the contribution of each mitochondrial ABC transporter to exercise-induced protection.

CONCLUSIONS

Given the effectiveness of DOX against a wide spectrum of solid tumors and hematological malignancies, it is important to find a countermeasure to protect against DOX-induced cytotoxicity. While it is established that DOX can induce cellular toxicity through the generation of mitochondrial ROS production, there are no suitable therapeutic approaches to combat DOX toxicity. Given the abundance of reports indicating that exercise can protect against DOX toxicity and that exercise training produces beneficial adaptations to multiple organ systems, it is necessary to elucidate a safe and effective exercise protocol for patients receiving DOX chemotherapy. In addition, continuing work is needed to elucidate the mechanisms responsible for exercise-induced cytoprotection to help develop pharmacological therapeutics to combat the toxic effects of DOX.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

A.J.S. drafted manuscript; A.J.S. edited and revised manuscript; A.J.S. approved final version of manuscript.