Abstract 11991: Fasting Induced Autophagy in the Murine Heart Occurs Independently of mTOR Signaling
Autophagy is a conserved and orchestrated process by which cellular organelles are degraded after being engulfed in double-membrane structures that are targeted for lysosomal degradation. Starvation is a potent inducer of autophagy in the heart, although the upstream signaling pathways that regulate this are incompletely understood. The mammalian target of rapamycin (mTOR) exists in two distinct complexes and suppresses autophagy through direct phosphorylation of Ulk1 and FOXO. However, in contrast to yeast, mTOR inhibition with rapamycin does not consistently induce autophagy in mouse tissues. To further investigate the role of mTOR in the regulation of cardiac autophagy in the fasted and fed states, we generated inducible cardiac specific mTOR deficient mice by mating compound transgenic mice harboring the reverse doxycycline (Dox) transactivator (rtTA), TetO-Cre and floxed mTOR alleles to generate rtTA::TetO-Cre::mTORfl/fl mice (iC-mTOR-KO), in which deletion of mTOR can be temporally controlled by Dox treatment. 3 weeks after Dox treatment, the iC-mTOR-KO hearts exhibited a 91% reduction of mTOR mRNA levels and an 86% reduction of mTOR protein levels. Insulin-stimulated phosphorylation of S6 on Ser235/236 and Akt on Ser473 were reduced, while phosphorylation on Akt Thr308 was increased, consistent with impaired insulin-mediated activation of mTOR and PDK2, but preserved activation of PDK1. 4 weeks after initial Dox treatment, iC-mTOR-KO hearts had normal cardiac function as measured by echocardiography; by 6 weeks the iC-mTOR-KO hearts had a 26.5% reduction in fraction shortening (FS) and by 8-10 weeks, the majority of the iC-mTOR-KO mice died. Interestingly, 3 weeks after initial Dox treatment, the autophagy marker LC3-II was unchanged and fasting increased LC3-II in iC-mTOR-KO hearts to the same extent as fasted control mice. Moreover, insulin suppressed starvation-induced autophagy in H9C2 cells in the presence of rapamycin. Finally, inducible (Tet off) overexpression of Akt, reduced autophagy in murine hearts despite mTOR inhibition by rapamycin treatment. Taken together, these data suggest that growth factor and fasting regulated autophagy in the murine heart is mediated via Akt-dependent but mTOR-independent pathways.
- © 2011 by American Heart Association, Inc.