Doxorubicin (DOX) is an anthracycline antibiotic used in the treatment of a broad spectrum of human cancers, including acute leukemia, lymphomas, stomach, breast and ovarian cancers. Unfortunately, the clinical use of this highly efficacious anticancer drug is limited due to the development of respiratory and diaphragm muscle dysfunction in patients. Doxorubicin-induced ventilatory impairment is a debilitating condition that promotes the onset dyspnea, fatigue and exercise intolerance. While the mechanisms responsible for DOX-induced respiratory insufficiency are unclear, previous work demonstrates that the incidence of ventilatory dysfunction greatly correlates to the concentration of DOX taken up by the diaphragm. DOX accumulates rapidly within the diaphragm muscle following exposure, where it preferentially localizes to the mitochondria and promotes free radical production. Elevated free radical production in the mitochondria can lead to severe damaging events resulting in cell death, and evidence suggests that prevention of mitochondrial dysfunction is sufficient to attenuate the toxic effects of DOX on the diaphragm. Therefore, elucidating ways in which the mitochondrial accumulation of DOX can be reduced could result in the development of a therapeutic approach to mitigate the myotoxic effects of DOX. In this regard, we recently discovered that endurance exercise training prior to DOX treatment is sufficient to reduce the mitochondrial accumulation of DOX and preserve diaphragm and ventilatory function. While the mechanisms responsible for the exercise-induced reduction in the levels of diaphragm mitochondrial DOX are unknown, we hypothesize that activity-induced increases in the expression of xenobiotic transport proteins are required. Specifically, the ATP-binding cassette (ABC) transporters are a class of proteins with the capability of facilitating the efflux of chemotherapeutics from the diaphragm. Moreover, four mitochondria-localized ABC transporters are expressed in the diaphragm (i.e. ABCB6, ABCB7, ABCB8 and ABCB10), all of which are upregulated with exercise. Therefore, the goal of this proposal is to establish the effects of these transport proteins in mediating the exercise-induced extrusion of DOX from the diaphragm, and to determine their therapeutic potential to prevent DOX-induced respiratory dysfunction. We will accomplish this by testing the following specific aims:
Specific Aim 1) will determine if exercise-mediated protection against DOX-induced respiratory dysfunction is dependent on increased levels of mitochondria-localized ABC transport proteins;
and Specific Aim 2) will determine if overexpression of mitochondrial ABC transport proteins in the diaphragm is sufficient to reduce DOX accumulation and prevent DOX-induced respiratory dysfunction.
Although doxorubicin (DOX) is a highly efficacious chemotherapeutic agent used in the treatment of a wide variety of cancers, its clinical use is limited by the development of respiratory failure. The long-term goal of this project is to identify biological targets that will assist in the development of a therapeutic strategy to reduce the risk of respiratory dysfunction in cancer patients. The results from this study will advance the understanding of the molecular basis of DOX-induced toxicity to respiratory muscles and serve as the foundation for future translational studies to generate preventative strategies.