The evolutionarily conserved checkpoint protein kinase, TOR (target of rapamycin), has emerged as a major effector of cell growth and proliferation via the regulation of protein synthesis. Work in the last decade clearly demonstrates that TOR controls protein synthesis through a stunning number of downstream targets. Some of the targets are phosphorylated directly by TOR, but many are phosphorylated indirectly. In this review, we summarize some recent developments in this fast-evolving field. We describe both the upstream components of the signaling pathway(s) that activates mammalian TOR (mTOR) and the downstream targets that affect protein synthesis. We also summarize the roles of mTOR in the control of cell growth and proliferation, as well as its relevance to cancer and synaptic plasticity.

Overcoming metabolic stress is a critical step for solid tumour growth1,2. However, the underlying mechanisms of cell death and survival under metabolic stress are not well understood. A key signalling pathway involved in metabolic adaptation is the liver kinase B1 (LKB1)–AMP-activated protein kinase (AMPK) pathway2,3. Energy stress conditions that decrease intracellular ATP levels below a certain level promote AMPK activation by LKB1. Previous studies showed that LKB1-deficient or AMPK-deficient cells are resistant to oncogenic transformation and tumorigenesis4–6, possibly because of the function of AMPK in metabolic adaptation. However, the mechanisms by which AMPK promotes metabolic adaptation in tumour cells are not fully understood. Here we show that AMPK activation, during energy stress, prolongs cell survival by redox regulation. Under these conditions, NADPH generation by the pentose phosphate pathway is impaired, but AMPK induces alternative routes to maintain NADPH and inhibit cell death. The inhibition of the acetyl-CoA carboxylases ACC1 and ACC2 by AMPK maintains NADPH levels by decreasing NADPH consumption in fatty-acid synthesis and increasing NADPH generation by means of fatty-acid oxidation. Knockdown of either ACC1 or ACC2 compensates for AMPK activation and facilitates anchorage-independent growth and solid tumour formation in vivo, whereas the activation of ACC1 or ACC2 attenuates these processes. Thus AMPK, in addition to its function in ATP homeostasis, has a key function in NADPH maintenance, which is critical for cancer cell survival under energy stress conditions, such as glucose limitations, anchorage-independent growth and solid tumour formation in vivo.

The pentose phosphate pathway (PPP), which branches from glycolysis at the first committed step of glucose metabolism, is required for the synthesis of ribonucleotides and is a major source of NADPH. NADPH is required for and consumed during fatty acid synthesis and the scavenging of reactive oxygen species. Therefore, the PPP plays a pivotal role in helping glycolytic cancer cells to meet their anabolic demands and combat oxidative stress. Recently, several neoplastic lesions were shown to have evolved to facilitate the flux of glucose into the pentose phosphate pathway. This review summarizes the fundamental functions of the PPP, its regulation in cancer cells, and its importance in cancer cell metabolism and survival.

Serum and certain growth factors have the ability to inhibit programmed cell death (apoptosis) and promote survival. The mechanism by which growth factors deliver an anti-apoptotic signal and the mechanism by which this survival signal is uncoupled from mitogenesis are not clear. We studied five downstream effectors of growth factor receptors-Ras, Raf, Src, phosphoinositide 3-kinase (PI 3-kinase), and Akt (PKB)-for their abilities to block apoptosis. Activated forms of Ras, Raf, and Src, although transforming, were not sufficient to deliver a survival signal upon serum withdrawal. In contrast, inhibition of PI 3-kinase accelerated apoptosis, and an activated form of the serine/threonine kinase Akt, a downstream effector of PI 3-kinase, blocked apoptosis. The ability of Akt to promote survival was dependent on and proportional to its kinase activity. In Ratla fibroblasts, activated Akt did not alter Bcl-2 or Bcl-X^ expression but inhibited Ced3/ICE-like activity. Thus, the PI 3-kinase/Akt (PKB) signaling pathway transduces a survival signal that ultimately blocks Ced3/ICE-like activity. These results suggest that uncoupling of survival and mitogenesis can be explained by differing abilities of distinct mitogens to efficiently induce the PI 3-kinase/Akt signaling pathway.

The serine/threonine kinase Akt/PKB is a major downstream effector of growth factor-mediated cell survival. Activated Akt, like Bcl-2 and Bcl-xL, prevents closure of a PT pore component, the voltage-dependent anion channel (VDAC); intracellular acidification; mitochondrial hyperpolarization; and the decline in oxidative phosphorylation that precedes cytochrome c release. However, unlike Bcl-2 and Bcl-xL, the ability of activated Akt to preserve mitochondrial integrity, and thereby inhibit apoptosis, requires glucose availability and is coupled to its metabolism. Hexokinases are known to bind to VDAC and directly couple intramitochondrial ATP synthesis to glucose metabolism. We provide evidence that such coupling serves as a downstream effector function for Akt. First, Akt increases mitochondria-associated hexokinase activity. Second, the antiapoptotic activity of Akt requires only the first committed step of glucose metabolism catalyzed by hexokinase. Finally, ectopic hexokinase expression mimics the ability of Akt to inhibit cytochrome c release and apoptosis. We therefore propose that Akt increases coupling of glucose metabolism to oxidative phosphorylation and regulates PT pore opening via the promotion of hexokinase-VDAC interaction at the outer mitochondrial membrane. Extrinsic signals emanating from cell-surface growth factor and cytokine receptors are major determinants of mammalian cell survival. The transduction of these signals oppose both basal intrinsic proapoptotic activity, as well as external proapoptotic stimuli (Raff 1992;Raff et al. 1993). Analysis of downstream signaling pathways has shown that activation of the PI-3 kinase/Akt(PKB) pathway plays a major role in cell survival induced by cell surface receptors. Following the initial demonstration that activation of the serine/threonine kinase Akt promotes cell survival (Dudek et al. 1997; KauffmannZeh et al. 1997;Kennedy et al. 1997), mounting reports established Akt as a major determinant of cell survival. Akt has been reported to mediate cell survival by various growth factors and cytokines in a variety of cell types and blocks apoptosis induced by multiple apoptotic stimuli (for review, see Datta et al. 1999;Kandel and Hay 1999). Various specific targets of Akt have been proposed to mediate the antiapoptotic activity of Akt (for review, see Datta et al. 1999;Kandel and Hay 1999). However, because growth factors promote cell survival via maintenance of the metabolic function of mitochondria , it is likely that Akt exerts its effect through similar mechanisms, which may be more fundamental and generally conserved.In mammalian cells, apoptosis has been described as a multistep process that can be initiated by a variety of stimuli. Mitochondria play a major role in this process through the release of cytochrome c and other proapoptotic proteins that normally reside in the intermembrane space between the inner and outer mitochondrial membranes (for review, see Gross et al. 1999;Desagher and Martinou 2000). Cytochrome c release is considered an ea...

The serine/threonine kinase Akt has been implicated in the control of cell survival and metabolism. Here we report the disruption of the most ubiquitously expressed member of the akt family of genes, akt1, in the mouse.

The regulatory circuits that control the activities of the two distinct target of rapamycin (TOR) complexes, TORC1 and TORC2, and of Akt have been a focus of intense research in recent years. It has become increasingly evident that these regulatory circuits control some of the most fundamental aspects of metabolism, cell growth, proliferation, survival, and differentiation at both the cellular and organismal levels. As such, they also play a pivotal role in the genesis of diseases including cancer, diabetes, aging, and degenerative diseases. This review highlights recent developments aimed at deciphering the interplay between Akt and mTORCs as well as their role in embryonic development and in cancer.

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