Project summary Doxorubicin (Dox), an anthracycline chemotherapeutic, is a well-established and highly effective chemotherapy drug commonly used to treat multiple cancer types, but its use is limited by cardiotoxicity. Patients with Dox therapy often develop delayed cardiomyopathy for which no effective therapies exist. The molecular mechanisms of this drug toxicity remain poorly understood and are likely to be multifactorial. DNA damage, mitochondrial dysfunction, p53 activation, and excessive cardiomyocyte apoptosis are major contributors to the pathogenesis of Dox-induced cardiac toxicity, but whether a pharmacological strategy can simultaneously work to reverse all of these events in patients that receive Dox treatment has yet to be uncovered. Our laboratories have a long track record in studying the role of nuclear receptor 4A (NR4A) biology in cardiovascular homeostasis and disease. In recent work, we have generated a significant body of evidence to support the novel concept that reduced cardiac expression of Nur77, a family member of NR4A receptors, plays a causal role in the pathogenesis of Dox cardiomyopathy. We have found that 1) Cardiac expression of Nur77 is significantly reduced in Dox treated cardiomyocytes and hearts and that 2) Cardiac specific overexpression of Nur77 effectively prevents Dox induced cardiac dysfunction, cardiomyocyte apoptosis, and fibrosis. In addition, we have identified DNA topoisomerase IIβ-binding protein 1 (TopBP1) as a key downstream effector of Nur77 in the heart, and have found that its expression is substantially reduced in Dox-treated hearts. More importantly, our preliminary data indicate that Nur77 prevents cardiac toxicity by inducing TopBP1-topoisomerase IIβ (Top2β) interactions, which in turn effectively shields Top2β/genomic DNA from Dox-induced damage in the heart. These data led us to hypothesize that Nur77 serves as an important cardioprotective agent and that targeted activation of Nur77 can effectively prevent Dox-induced cardiomyopathy while having minimal to no effects on its anti-cancer behaviors. We will test this hypothesis in the following three aims. Aim 1 will perform loss-of-function studies by employing Nur77 cardiac specific knockout mice (cNur77-KO) to determine whether cardiac specific deletion of Nur77 exacerbates Dox-induced cardiotoxicity through enhanced apoptosis, fibrosis, and inflammation in the heart. Aim 2 will determine the underlying mechanisms by which Nur77 exhibits its cardioprotective effects against Dox-induced cardiac injury. Cardiac specific TopBP1 transgenic and knockout mice will be used to assess the contribution of TopBP1 to the Nur77-initiated cardiac protection in mouse models of Dox cardiotoxicity. Aim 3 will study whether pharmacological activation of Nur77 holds the capacity to block Dox cardiomyopathy without compromising anti-cancer activity of the drug. Successful completion of these aims will guide us to develop new therapeutic agents to prevent se...