Doxorubicin may damage cardiomyocytes more than cardiac progenitor cells

Long term survival of childhood cancers is now more than 70%. Unfortunately, adult survivors of childhood cancer are at risk for treatment‐related adverse health outcomes. Survivors with a median time from diagnosis of 25 years found 56% had cardiac dysfunction. Analysis of heart transplantation patients found doxorubicin as the underlying cause in 2–3% of all cases. Doxorubicin is a topoisomerase poison that accumulates in the nucleus and the mitochondria. Doxorubicin impacts heavily on the mitochondria as significant mtDNA depletion and mtDNA rearrangements have been reported. Unfortunately nearly all preclinical studies have only determined short effects. Very few studies have examined the long term effects of doxorubicin or used doses that were comparable to clinical treatment protocols. We have developed a murine model for late onset doxorubicin induced heart failure in which the onset was delayed and mimicked that seen clinically (~18 human years). Doxorubicin was injected (1–45 mg/kg, i.p. ) at 3 weeks of age. We observed no change in cardiovascular function 2–4 months post‐injection, but a significant decrease in ejection shortening was observed after 6–8 months post‐injection. Low doses of doxorubicin (1–15 mg/kg) did not significantly alter cardiovascular function even out to 16 months post injection. Using flow cytometry we examined cell populations from the left ventricle, circulating non‐RBC cells, and bone marrow. Despite degradation of cardiovascular function we observed significant increases (p<.05) in c‐kit + , SSEA1 + , and Nkx2.5 + cells within the heart. In contrast, significant decreases of the c‐kit + , SSEA1 + cells were found in the circulating cells, while no changes were observed in the cell populations from the bone marrow compartment. We assessed mtDNA damage and observed significant increases in mtDNA damage in the ventricular cells and but not ventricular c‐kit + cells. In cultured neonatal cardiomyocytes, doxorubicin increased mtDNA damage and promoted cell death below the threshold that significantly increase intramitochondrial superoxide levels. Measuring mitochondrial topoisomerase function we observed that H 2 O 2 significantly increased DNA relaxation of supercoiled DNA suggesting that low levels of ROS alter mitochondrial topoisomerase function to promote DNA strand breakage. These findings suggest the heart attempted to regenerate by increased replacement of healthy cardiac progenitor cells but that degradation cardiomyocytes may have been the underlying cause of cardiovascular dysfunction. This suggests that the ability of the heart to direct self repair may be limited and that although the stem cell compartment attempted to compensate for cardiomyocyte loss, the degradation of cardiac function indicated that it was not successful. Support or Funding Information Supported in part by NIH RO3HD065551