Doxorubicin and its derivatives are among the most potent anticancer drugs known. Unfortunately their therapeutic efficacy is severely restricted by a dose-accumulated cardiotoxicity. Our preliminary results using adult rat cardiomyocytes as a working model, have shown evidence for the mechanism which could explain how reactive oxygen species generated by doxorubicin provoke myofibrillar degeneration while not causing severe oxidative damage. Specifically, 1) we visualized directly doxorubicin accumulation in cardiac mitochondria and subsequent increase in intracellular oxidation in living cardiomyocytes, 2) observed that doxorubicin administration is associated with redistribution of the protein kinase C epsilon isoform from cytosol to myofibrils, and 3) detected disruption of the periodicity of actin staining after repetitive exposure of myocytes to clinically relevant drug concentrations. Based on these data we propose the following sequence of events: doxorubicin rapidly accumulates in cardiac mitochondria; it then initiates lipid peroxidation via formation of superoxide and drug complexes with transition metals; and although the degree of lipid peroxidation is small and no significant membrane damage occurs, it leads to the activation of phospholipases; lipase thereupon release several second messengers, including arachidonic acid, activating a specific protein kinase C isoform; the kinase then initiates myofibrillar degeneration. To support the above hypothesis we aim to 1) establish a causal relationship between observed doxorubicin-induced kinase translocation and increased free radical formation; 2) to determine whether doxorubicin-induced free radicals and/or protein kinase C translocation are prerequisites for changes in myocyte myofibrillar organization and cell contractility; 3) to uncover role of phospholipase A2 in doxorubicin-induced activation of kinase and ensuing changes in myofibrillar organization and contractility. The proposed experiments aim to reveal explicit pathways through which reactive oxygen species are involved in anthracycline- induced cardiomyopathy. The studies will also provide new information about kinases involvement in myofilament degeneration and interaction between reactive oxygen species and signal transduction pathways.
