0452 : MTOR inactivation during early postnatal development of mice myocardium leads to severe dilated cardiomyopathy due to altered translational efficiency and hypoxia-induced apoptosis

Abstract

Mechanistic target of rapamycin (mTOR) is a central regulator of cell growth, proliferation, survival and metabolism. mTOR inhibition is increasingly used in antitumoral therapies and mTOR inhibition with rapamycin was shown to be cardioprotective during aging and cardiac stress. Studies in genetic mice models have shown that mTOR is essential for heart development and cardiac function in adult. However, mTOR functions during postnatal cardiac development are not fully elucidated. We have therefore generated a cardiac-specific mTOR knockout mouse using α-MHC-Cre mice leading to mTOR inactivation in early postnatal mouse myocardium. The mutant mice develop a severe lethal dilated cardiomyopathy due to defects in cardiomyocyte growth, survival and subsequent fibrosis. In contrast to adult myocardium, both mTORC1 and mTORC2 activities are impaired in juvenile heart, as shown by hypophosphorylation of the translation inhibitor 4E-BP1 and loss of the cardioprotective AKTS473 phosphorylation. We find that translation initiation defects and altered ribosome biogenesis both contribute to impaired cardiomyocyte growth. In addition, we show that increased apoptosis is associated with activation of JNK kinase and p53 accumulation. Moreover mTORcmKO hearts display a strong decreased expression of the primary oxygen carrier, myoglobin, and HIF1α accumulation suggesting hypoxia. However, mTORcmKO hearts do not display HIF1 hypoxic response consistently with mTOR being essential for HIF1-dependant trancriptionnal activity. These observations indicate that hypoxia-induced apoptosis likely contribute to DCM in mTORcmKO mice. Altogether, our results demonstrate that mTOR is a key regulator of cardiomyocyte growth, viability and oxygen supply in early postnatal myocardium. Our findings highlight potential cardiotoxicity of new mTOR inhibitors and the importance to set up optimal treatments in cardiology to both target mTOR hypertrophic functions and maintain adequate oxygen supply.