Metabolic remodeling precedes mTORC1-mediated cardiac hypertrophy.

RATIONALE: The nutrient sensing mechanistic target of rapamycin complex 1 (mTORC1) and its primary inhibitor, tuberin (TSC2), are cues for the development of cardiac hypertrophy. The phenotype of mTORC1 induced hypertrophy is unknown. OBJECTIVE: To examine the impact of sustained mTORC1 activation on metabolism, function, and structure of the adult heart. METHODS AND RESULTS: We developed a mouse model of inducible, cardiac-specific sustained mTORC1 activation (mTORC1iSA) through deletion of Tsc2. Prior to hypertrophy, rates of glucose uptake and oxidation, as well as protein and enzymatic activity of glucose 6-phosphate isomerase (GPI) were decreased, while intracellular levels of glucose 6-phosphate (G6P) were increased. Subsequently, hypertrophy developed. Transcript levels of the fetal gene program and pathways of exercise-induced hypertrophy increased, while hypertrophy did not progress to heart failure. We therefore examined the hearts of wild-type mice subjected to voluntary physical activity and observed early changes in GPI, followed by hypertrophy. Rapamycin prevented these changes in both models. CONCLUSION: Activation of mTORC1 in the adult heart triggers the development of a non-specific form of hypertrophy which is preceded by changes in cardiac glucose metabolism.

Autophagic signaling promotes systems-wide remodeling in skeletal muscle upon oncometabolic stress by D2-HG.

OBJECTIVES: Cachexia is a metabolic disorder and comorbidity with cancer and heart failure. The syndrome impacts more than thirty million people worldwide, accounting for 20% of all cancer deaths. In acute myeloid leukemia, somatic mutations of the metabolic enzyme isocitrate dehydrogenase 1 and 2 cause the production of the oncometabolite D2-hydroxyglutarate (D2-HG). Increased production of D2-HG is associated with heart and skeletal muscle atrophy, but the mechanistic links between metabolic and proteomic remodeling remain poorly understood. Therefore, we assessed how oncometabolic stress by D2-HG activates autophagy and drives skeletal muscle loss. METHODS: We quantified genomic, metabolomic, and proteomic changes in cultured skeletal muscle cells and mouse models of IDH-mutant leukemia using RNA sequencing, mass spectrometry, and computational modeling. RESULTS: D2-HG impairs NADH redox homeostasis in myotubes. Increased NAD+ levels drive activation of nuclear deacetylase Sirt1, which causes deacetylation and activation of LC3, a key regulator of autophagy. Using LC3 mutants, we confirm that deacetylation of LC3 by Sirt1 shifts its distribution from the nucleus into the cytosol, where it can undergo lipidation at pre-autophagic membranes. Sirt1 silencing or p300 overexpression attenuated autophagy activation in myotubes. In vivo, we identified increased muscle atrophy and reduced grip strength in response to D2-HG in male vs. female mice. In male mice, glycolytic intermediates accumulated, and protein expression of oxidative phosphorylation machinery was reduced. In contrast, female animals upregulated the same proteins, attenuating the phenotype in vivo. Network modeling and machine learning algorithms allowed us to identify candidate proteins essential for regulating oncometabolic adaptation in mouse skeletal muscle. CONCLUSIONS: Our multi-omics approach exposes new metabolic vulnerabilities in response to D2-HG in skeletal muscle and provides a conceptual framework for identifying therapeutic targets in cachexia.

Induction of antioxidant gene expression in a mouse model of ischemic cardiomyopathy is dependent on reactive oxygen species.

Ischemia and reperfusion (I/R) are characterized by oxidative stress as well as changes in the antioxidant enzymes of the heart. However, little is known about the transcriptional regulation of myocardial antioxidant enzymes in repetitive I/R and hibernating myocardium. In a mouse model of ischemic cardiomyopathy induced by repetitive I/R, we postulated that induction of antioxidant gene expression was dependent on reactive oxygen species (ROS). Repetitive closed-chest I/R (15 min) was performed daily in C57/BL6 mice and in mice overexpressing extracellular superoxide dismutase (EC-SOD). Antioxidant enzyme expression was measured at 3, 5, 7, and 28 days of repetitive I/R as well as 15 and 30 days after discontinuation of I/R. In order to determine whether ROS directly modulates antioxidant gene expression, transcript levels were measured in cardiomyocytes exposed to hydrogen peroxide. Repetitive I/R caused an early and sustained increase in glutathione peroxidase (GPX) transcript levels, while heme oxygenase-1 (HO-1) expression increased only after 7 days of repetitive I/R. Overexpression of EC-SOD prevented the upregulation of GPX and HO-1 transcript levels by repetitive I/R, suggesting that both genes are regulated by ROS. However, while HO-1 transcript levels increased in cardiomyocytes exposed to hydrogen peroxide, oxidative stress failed to induce the expression of GPX implying that ROS regulates GPX transcript levels only indirectly in repetitive I/R. In conclusion, repetitive I/R was associated with an early upregulation of GPX expression as well as a delayed increase of HO-1 transcript levels in the heart. The induction of both antioxidant genes was dependent on ROS, suggesting that alterations in redox balance mediate not only tissue injury but also components of "programmed cell survival" in hibernating myocardium.