Molecular regulation of mitochondrial dynamics in Anthracycline cardiotoxicity

Abstract

Doxorubicin (Dox) is a highly effective anti-tumor agent that is widely used to treat a variety of malignancies. However, its use is limited by its cardio-toxic effects which can induce heart failure. The underlying mechanism of Dox induced cardiotoxicity is not fully understood. The IKKβ -NF-κB signaling pathway regulates a variety of processes including inflammation, differentiation, and cell survival. Previous work from our lab established a critical survival role for IKKβ -NF-κB signaling in cardiomyocytes through suppressing mitochondrial perturbations induced by hypoxia. However, despite these findings, it remained undetermined whether the survival properties of IKKβ were restricted to hypoxia or extended more broadly to other cardiac abnormalities such as Dox cardiotoxicity. In this study we explore the role of IKKβ -NF-κB pathway in cardiac myocytes treated with Dox. Dox treatment caused critical mitochondrial defects including mPTP opening, loss of membrane potential (∆Ψm), and ROS production. Levels of the mitochondrial fusion protein Mitofusin 2 (MFN2) were decreased in cardiac myocytes treated with DOX resulting in increased mitochondrial fission, loss of mitochondrial networks and impaired respiration. Interestingly, we identified that MFN2 degradation was through an autophagic process. Knockdown of ATG7 (a key regulator of autophagy) suppressed autophagy-mediated loss of MFN2 in cells treated with DOX. Markedly, wild type (IKKβ) but not a kinase inactive mutant (IKKβk/m) suppressed autophagy mediated degradation of MFN2, and mitochondrial perturbation induced by Dox. Notably, restoration of mitochondrial fusion, oxidative respiration and cell viability by IKKβ was contingent upon the GTPase activity of MFN2. Importantly, we show that IKKβ directly forms a novel protein-protein complex with the outer mitochondrial membrane protein MFN2 which is critical for IKKβ mediated cardioprotection against doxorubicin cardiotoxicity. Hence, the findings of the present study reveal a novel signaling axis that functionally couples innate signaling through IKKβ and MFN2 to mitochondrial fission and necrotic cell death during DOX cardiomyopathy