IKKβ stabilizes Mitofusin 2 and suppresses doxorubicin cardiomyopathy

The mitochondrial dynamics protein Mitofusin 2 (MFN2) coordinates critical cellular processes including mitochondrial bioenergetics, quality control, and cell viability. The NF-κB kinase IKKβ suppresses mitochondrial injury in doxorubicin cardiomyopathy, but the underlying mechanism is undefined.

Herein, we identify a novel signalling axis that functionally connects IKKβ and doxorubicin cardiomyopathy to a mechanism that impinges upon the proteasomal stabilization of MFN2. In contrast to vehicle-treated cells, MFN2 was highly ubiquitinated and rapidly degraded by the proteasomal-regulated pathway in cardiac myocytes treated with doxorubicin. The loss of MFN2 activity resulted in mitochondrial perturbations, including increased reactive oxygen species (ROS) production, impaired respiration, and necrotic cell death. Interestingly, doxorubicin-induced degradation of MFN2 and mitochondrial-regulated cell death were contingent upon IKKβ kinase activity. Notably, immunoprecipitation and proximity ligation assays revealed that IKKβ interacted with MFN2 suggesting that MFN2 may be a phosphorylation target of IKKβ. To explore this possibility, mass spectrometry analysis identified a novel MFN2 phospho-acceptor site at serine 53 that was phosphorylated by wild-type IKKβ but not by a kinase-inactive mutant IKKβK-M. Based on these findings, we reasoned that IKKβ-mediated phosphorylation of serine 53 may influence MFN2 protein stability. Consistent with this view, an IKKβ-phosphomimetic MFN2 (MFN2S53D) was resistant to proteasomal degradation induced by doxorubicin whereas wild-type MFN2 and IKKβ-phosphorylation defective MFN2 mutant (MFNS53A) were readily degraded in cardiac myocytes treated with doxorubicin. Concordantly, gain of function of IKKβ or MFN2S53D suppressed doxorubicin-induced mitochondrial injury and cell death.

The findings of this study reveal a novel survival pathway for IKKβ that is mutually dependent upon and obligatory linked to the phosphorylation and stabilization of the mitochondrial dynamics protein MFN2.