Role of mTOR, a Component of the Akt Pathway, in Regulating Cardiac Function

University Of California, San Diego, La Jolla CA

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Abstract

PROJECT SUMMARY The overall aim of this project is to uncover the role of the mTOR (mammalian target of rapamycin) pathway and its relationship to p53 during normal and heart failure (HF) conditions. Myocardial mTOR activity, while increased in the early phases of pressure overload hypertrophy, eventually decreases when compensated hypertrophy switches to decompensation, ending in frank HF. We hypothesized therefore that mTOR and its downstream substrates are central in this process. For proving this, we therefore generated an inducible and cardiac-specific mouse model defective in mTOR (mTOR-cKO). We found that upon loss of mTOR, mice developed dilated heart phenotype that progressed rapidly to death. Markers of autophagy and apoptosis increased significantly; importantly, the tumor suppressor gene p53 significantly accumulated in the heart of mTOR KO mice; moreover, there was a remarkable myocardial accumulation of active 4E-BP1, a downstream member of the mTOR pathway which negatively controls mRNA translation in its dephosphorylated form and a major substrate of TORC-1, one of the two mTOR kinase multiprotein complexes. We found that acumulation of dephosphorylated 4E-BP1 is a feature of HF of diverse etiologies, and thus might represent a common mechanism underlying this state. We also found that deleting 4E-BP1 in mice determines a very significant improvement of cardiac function and survival in the context of cardiac mTOR KO. In other cellular systems, 4E-BPs were shown to regulate p53 protein levels. The specific aims of this project therefore are 1) to determine the relative role of apoptosis and autophagy in mTOR-defective HF and in particular the role of the key gene p53 and 2) determine the role of 4E-BPs in mTOR-dependent HF as well as the reciprocal control between 4E-BPs and p53. These tasks will be accomplished through cross-breeding experiments of mTOR-cKO mice with strains in which Atg5, a gene critical for autophagy, or Nix, a gene critical for apoptosis, are deleted. Also, the role of p53 will be assessed by crossing our model with mice with p53-floxed alleles. Gene expression and proteomic studies will be conducted with the aim to identify genes critical for mTOR-cKO HF. The involvement of 4E-BPs in regulating cardiac function as well as p53 levels will be addressed by crossing mTOR-cKO with double 4E-BP1/4E-BP2 KO mice and by studying the role of p53 in regulating 4E-BP1 expression. Results of our research will assess the relative and reciprocal role of mTOR/4E-BP1 and p53 in controlling cardiac function in the normal and diseased heart, opening the possibility to interfere with these molecules for therapeutic purposes.