The Integrated Stress Response Protein, eIF2α, Modulates UPRmt Signaling and Cell Death in Doxorubicin Treated Human Cardiac Cells

Doxorubicin (DOX), is an indispensable first-line chemotherapeutic. Despite this first-line indication, clinical use of DOX is limited by severe, off-target, and often irreversible cardiotoxicity. DOX induces cytotoxicity in rapidly dividing cancer cells via inhibition of Topoisomerase IIα. However, the underlying mechanisms by which DOX causes cell death in non-replicative, terminally differentiated cardiomyocytes remain poorly understood. Emerging evidence suggests that mitochondrial uptake of DOX is contributory to cardiotoxicity. Whether mitochondrial stress pathways, including the mitochondrial unfolded protein response (UPRmt), are activated and critical for mediating DOX cardiotoxicity is poorly understood. Moreover, whether phosphorylation of eukaryotic translation initiation factor 2α (eIF2α), a mediator of the Integrated Stress Response, regulates potential UPRmt signaling during DOX treatment is also unknown. Here, using human AC-16 cardiac cells, we examined the role of eIF2α phosphorylation during DOX treatment. Our data suggest that DOX triggers a transient increase in eIF2α phosphorylation, followed by a progressive decline. Further, knockdown of eIF2α decreased key transcriptional regulators of UPRmt signaling such as C/EBP Homologous Protein and ATF5, blunted the induction of UPRmt genes (AFG3L2, CLPP, HSPA9, HSPD1, LONP1, SPG7), and aggravated DOX induced cytotoxicity. Together, these findings identify eIF2α as a critical upstream regulator of UPRmt signaling, and suggest that activation of the UPRmt may confer cardio-protection against DOX-induced mitochondrial stress in human cardiac cells.