Direct control of mitochondrial function by mTOR

Abstract

mTOR is a central regulator of cellular growth and metabolism. Using metabolic profiling and numerous small-molecule probes, we investigated whether mTOR affects immediate control over cellular metabolism by posttranslational mechanisms. Inhibiting the FKBP12/rapamycin-sensitive subset of mTOR functions in leukemic cells enhanced aerobic glycolysis and decreased uncoupled mitochondrial respiration within 25 min. mTOR is in a complex with the mitochondrial outer-membrane protein Bcl-xl and VDAC1. Bcl-xl, but not VDAC1, is a kinase substrate for mTOR in vitro, and mTOR regulates the association of Bcl-xl with mTOR. Inhibition of mTOR not only enhances aerobic glycolysis, but also induces a state of increased dependence on aerobic glycolysis in leukemic cells, as shown by the synergy between the glycolytic inhibitor 2-deoxyglucose and rapamycin in decreasing cell viability.metabolomics ͉ mitochondria ͉ chemical biology m TOR functions as a multichannel processor in a cellular nutrient-sensing network by receiving multiple inputs derived from distinct environmental cues and directing different outputs to appropriate downstream effectors. Upstream regulators of mTOR are primarily mitogens, nutrients, and energy (1). mTOR exists in 2 separate protein complexes: the mitogen-, nutrient-, and rapamycin-sensitive mTOR complex 1 (mTORC1) and the mitogen-sensitive and nutrient-and rapamycin-insensitive mTORC2 (2). mTOR regulates key cellular processes, including mRNA translation, ribosome biogenesis, autophagy, and metabolism. The most extensively studied targets of mTOR are the translation regulators S6K1 and 4E-BP1 (3). Yeast cells treated with rapamycin mount an immediate (within 20 min) and widespread transcriptional response that results in metabolic reprogramming characteristic of the diauxic shift (4). Mammalian cells do not display an immediate and widespread transcriptional response to mTOR inhibition by rapamycin (5). Inhibition of mTORC1 using either rapamycin or RNA-mediated interference of proteins involved in the signaling network decreases mitochondrial respiration and levels of fully uncoupled respiration, independent of S6K1 and 4EBP1 (6). Recently, rapamycin was reported to decrease the transcription of genes involved in mitochondrial oxidative function by disrupting a complex involving TORC1, YY1, and PGC-1␣, thereby preventing the coactivation of YY1 by PGC-1␣ (7). The authors suggested this transcriptional mechanism as the means by which mTOR controls mitochondrial function; however, treatment with rapamycin does not result in decreased mitochondrial content (6) even after 8 h.Here we describe an immediate change in mitochondrial function following the inhibition of mTOR. Using global physiological profiling, we show that inhibition of mTOR has immediate effects on carbon and mitochondrial metabolism in a leukemic cell line. Rapamycin-treated leukemic cells display reduced mitochondrial function, resulting in energy production via enhanced aerobic glycolysis in preference over mitochondrial respiration....

mTOR-Dependent Cell Survival Mechanisms

“…In fact, two recent proteomic studies identifying Grb10 as an mTORC1 substrate further suggest that the insulin-stimulated phosphorylation cascade is largely mTOR-dependent (Hsu et al 2011;Yu et al 2011). A role for mTORC1 in regulating PGC1a activity and mitochondrial function has also been described (Cunningham et al 2007;Ramanathan and Schreiber 2009). …”

TOR and ageing: a complex pathway for a complex process

“…mTOR has been shown to physically interact with the transcription factor yin-yang 1 (YY1) [52], which regulates mitochondrial gene expression, as well as the mitochondrial outermembrane proteins Bcl-xl and VDAC1 [95]. Inhibition of mTORC1 activities by RNAi knockdown of genes coding components of the mTORC1 complex or rapamycin results in the reduction of oxygen consumption and decreased mitochondrial respiration in vitro [94]. The difference in outcomes between in vivo and in vitro studies points to the need to further elucidate the relationship between TOR signalling and respiration in mammals.…”

Glutamine at focus: versatile roles in cancer

“…It is evident from many reports that the PI3K/Akt/mTOR pathway regulates glucose metabolism. Improved mitochondrial activity through the downstream control of mTORC1 [77][78][79][80] requires amino acids to promote protein synthesis, cell growth, and proliferation [8,81]. Early evidence has suggested that the uptake of exogenous L-glutamine and its efflux out cells serves to maintain intracellular levels of essential amino acids (EAA) such as leucine, leading to the activation of the mTOR-S6K1 pathway.…”