Abstract 239: p53 Protects Cardiac Stem Cells From the Toxic Effects of Chemotherapy and Controls their Fate

Abstract

Doxorubicin (DOXO) causes dilated cardiomyopathy and heart failure. We have documented previously that DOXO-mediated cardiotoxicity is dictated by functional alterations of cardiac stem cells (CSCs). DOXO-induced myopathy was coupled with a reduction in CSC number due to increased death, inhibition of proliferation, and senescence. We raised the possibility that survival and growth of CSCs following DOXO treatment may be enhanced by modulating the intracellular level of p53, which plays a critical role in the determination of stem cell fate. For this purpose, transgenic mice carrying an additional p53 allele (Sp53) were studied. With respect to wild-type mice (WT), CSCs isolated from Sp53 mice (Sp53-CSCs) showed increased apoptosis with accumulation of the pro-apoptotic p53 targets BAX, PUMA and Pidd. Conversely, the expression of p21Cip1, a cell cycle inhibitor and inducer of cell senescence, was lower in Sp53-CSCs than WT cells. Upon DOXO treatment, Sp53-CSCs exhibited accelerated onset of apoptosis. However, viable Sp53-CSCs showed enhanced formation of DNA damage response foci, indicative of a very efficient DNA repair mechanism. Following removal of DOXO, Sp53-CSCs re-entered the cell cycle and divided, while WT cells continued to die by apoptosis or became senescent. The response of WT-CSCs to DOXO involved the pro-apoptotic Bcl2 family member Noxa and the senescence-associated protein p16INK4a. In contrast, exposure of Sp53-CSCs to DOXO provoked pulses of p53 expression, which favored sustained upregulation of Mdm2. Mdm2 antagonized the inhibitory effect of p53 on cell growth and prevented apoptosis. Ultimately, Sp53-CSCs showed accumulation of PCNA, which is required for DNA repair and synthesis. Importantly, IGF-1 release was higher in Sp53-CSCs, promoting their replication through an autocrine-paracrine mechanism. Collectively, our data demonstrate that changes in the pattern of p53 expression have beneficial effects on CSCs by amplifying the DNA repair response, facilitating the clearance of cells with non-repairable DNA damage, and enabling the proliferation of cells in which DNA lesions are effectively removed. Thus, targeting p53 expression in CSCs may protect the heart from the toxic effects of chemotherapy.