Elongation Factor-2 Kinase Regulates Autophagy in Human Glioblastoma Cells

Apoptosis and autophagy induction in mammalian cells by small interfering RNA knockdown of mRNA capping enzymes

Addition of a 5' cap to RNA polymerase II transcripts, the first step of pre-mRNA processing in eukaryotes from yeasts to mammals, is catalyzed by the sequential action of RNA triphosphatase, guanylyltransferase, and (guanine-N-7)methyltransferase. The effects of knockdown of these capping enzymes in mammalian cells were investigated using T7 RNA polymerase-synthesized small interfering RNA and also a lentivirus-based inducible, short hairpin RNA system. Decreasing either guanylyltransferase or methyltransferase resulted in caspase-3 activation and elevated terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) staining characteristic of apoptosis. Induction of apoptosis was independent of p53 tumor suppressor but dependent on BAK or BAX. In addition, levels of the BH3 family member Bim increased, while Mcl-1 and Bik levels remained unchanged during apoptosis. In contrast to capping enzyme knockdown, apoptosis induced by cycloheximide inhibition of protein synthesis required BAK but not BAX. Both Bim and Mcl-1 levels decreased in cycloheximide-induced apoptosis while Bik levels were unchanged, suggesting that apoptosis in siRNA-treated cells is not a direct consequence of loss of mRNA translation. siRNA-treated BAK(-/-) BAX(-/-) double-knockout mouse embryonic fibroblasts failed to activate capase-3 or increase TUNEL staining but instead exhibited autophagy, as demonstrated by proteolytic processing of microtubule-associated protein 1 light chain 3 (LC3) and translocation of transfected green fluorescent protein-LC3 from the nucleus to punctate cytoplasmic structures.

Amino acid regulation of autophagosome formation

Amino acids are not only substrates for various metabolic pathways, but can also serve as signaling molecules controlling signal transduction pathways. One of these signaling pathways is mTOR-dependent and is activated by amino acids (leucine in particular) in synergy with insulin. Activation of this pathway inhibits autophagy. Because activation of mTOR-mediated signaling also stimulates protein synthesis, it appears that protein synthesis and autophagic protein degradation are reciprocally controlled by the same signaling pathway. Recent developments indicate that amino acid-stimulated mTOR-dependent signaling is subject to complex regulation. The mechanism by which amino acids stimulate mTORdependent signaling (and other signaling pathways), and its molecular connection with the autophagic machinery, is still unknown.

Targeting the p27 E3 ligase SCF(Skp2) results in p27- and Skp2-mediated cell-cycle arrest and activation of autophagy

Decreased p27(Kip1) levels are a poor prognostic factor in many malignancies, and can occur through up-regulation of SCF(Skp2) E3 ligase function, resulting in enhanced p27 ubiquitination and proteasome-mediated degradation. While proteasome inhibitors stabilize p27(Kip1), agents inhibiting SCF(Skp2) may represent more directly targeted drugs with the promise of enhanced efficacy and reduced toxicity. Using high-throughput screening, we identified Compound A (CpdA), which interfered with SCF(Skp2) ligase function in vitro, and induced specific accumulation of p21 and other SCF(Skp2) substrates in cells without activating a heat-shock protein response. CpdA prevented incorporation of Skp2 into the SCF(Skp2) ligase, and induced G(1)/S cell-cycle arrest as well as SCF(Skp2)- and p27-dependent cell killing. This programmed cell death was caspase-independent, and instead occurred through activation of autophagy. In models of multiple myeloma, CpdA overcame resistance to dexamethasone, doxorubicin, and melphalan, as well as to bortezomib, and also acted synergistically with this proteasome inhibitor. Importantly, CpdA was active against patient-derived plasma cells and both myeloid and lymphoblastoid leukemia blasts, and showed preferential activity against neoplastic cells while relatively sparing other marrow components. These findings provide a rational framework for further development of SCF(Skp2) inhibitors as a novel class of antitumor agents.

Molecular mechanisms and physiological significance of autophagy during myocardial ischemia and reperfusion

Autophagy is an intracellular bulk degradation process whereby cytoplasmic proteins and organelles are degraded and recycled through lysosomes. In the heart, autophagy plays a homeostatic role at basal levels, and the absence of autophagy causes cardiac dysfunction and the development of cardiomyopathy. Autophagy is induced during myocardial ischemia and further enhanced by reperfusion. Although induction of autophagy during the ischemic phase is protective, further enhancement of autophagy during the reperfusion phase may induce cell death and appears to be detrimental. In this review we discuss the functional significance of autophagy and the underlying signaling mechanism in the heart during ischemia/reperfusion.

A pharmacoproteomic approach implicates eukaryotic elongation factor 2 kinase in ER stress-induced cell death

Apoptosis triggered by endoplasmic reticulum (ER) stress has been implicated in many diseases but its cellular regulation remains poorly understood. Previously, we identified salubrinal (sal), a small molecule that protects cells from ER stress-induced apoptosis by selectively activating a subset of endogenous ER stress-signaling events. Here, we use sal as a probe in a proteomic approach to discover new information about the endogenous cellular response to ER stress. We show that sal induces phosphorylation of the translation elongation factor eukaryotic translation elongation factor 2 (eEF-2), an event that depends on eEF-2 kinase (eEF-2K). ER stress itself also induces eEF-2K-dependent eEF-2 phosphorylation, and this pathway promotes translational arrest and cell death in this context, identifying eEF-2K as a hitherto unknown regulator of ER stress-induced apoptosis. Finally, we use both sal and ER stress models to show that eEF-2 phosphorylation can be activated by at least two signaling mechanisms. Our work identifies eEF-2K as a new component of the ER stress response and underlines the utility of novel small molecules in discovering new cell biology.

Autophagy in cancer: good, bad, or both?

Autophagy has been recognized as an important cellular process for at least 50 years; however, it is only with the recent identification of key regulators of autophagy (Atg genes) that we have begun a mechanistic exploration of its importance in cancer. Recent studies suggest that autophagy may be important in the regulation of cancer development and progression and in determining the response of tumor cells to anticancer therapy. However, the role of autophagy in these processes is complicated and may, depending on the circumstances, have diametrically opposite consequences for the tumor. In this article, we discuss recent discoveries regarding autophagy in cancer.

The role of autophagy in mediating cell survival and death during ischemia and reperfusion in the heart

Autophagy is a major mechanism for degrading long-lived cytosolic proteins and the only known pathway for degrading organelles. Autophagy is activated by many forms of stress, including nutrient and energy starvation, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, and infections. Although autophagy recycles amino acids and fatty acids to produce energy and removes damaged organelles, thereby playing an essential role in cell survival, inappropriate activation of autophagy leads to cell death. In the heart, activation of autophagy can be observed in response to nutrient starvation, ischemia/reperfusion, and heart failure. In this review, the signaling mechanism and the functional significance of autophagy during myocardial ischemia and reperfusion are discussed.

Metabolic catastrophe as a means to cancer cell death

During tumorigenesis, normal growth mechanisms are deregulated and safeguards that eliminate abnormal cells by apoptosis are disabled. Tumor cells must also increase nutrient uptake and angiogenesis to support the upregulation of metabolism necessary for unrestricted growth. In addition, they have to rely on inefficient energy production by glycolysis. This glycolytic state can result from mutations that promote cell proliferation, the hypoxic tumor microenvironment and perhaps mitochondrial malfunction. Moreover, the very signals that enable unrestricted cell proliferation inhibit autophagy, which normally sustains cells during nutrient limitation. In tumors, inactivation of the autophagy pathway may enhance necrosis and inflammation and promote genomic instability, which can further enhance tumor growth. Thus, tumor cells cannot adapt efficiently to metabolic stress and could be induced to die by metabolic catastrophe, in which high energy demand is contrasted by insufficient energy production. Efforts to exploit this unique metabolic state clinically previously focused mainly on detecting tissue displaying increased glycolytic metabolism. The challenge now is to induce metabolic catastrophe therapeutically as an approach to killing the unkillable cells.

Autophagy and cell death

Autophagy is a physiological and evolutionarily conserved phenomenon maintaining homeostatic functions like protein degradation and organelle turnover. It is rapidly upregulated under conditions leading to cellular stress, such as nutrient or growth factor deprivation, providing an alternative source of intracellular building blocks and substrates for energy generation to enable continuous cell survival. Yet accumulating data provide evidence that the autophagic machinery can be also recruited to kill cells under certain conditions generating a caspase-independent form of programed cell death (PCD), named autophagic cell death. Due to increasing interest in nonapoptotic PCD forms and the development of mammalian genetic tools to study autophagy, autophagic cell death has achieved major prominence, and is recognized now as a legitimate alternative death pathway to apoptosis. This chapter aims at summarizing the recent data in the field of autophagy signaling and autophagic cell death.

When translation meets transformation: the mTOR story

There is currently a high level of interest in signalling through the mammalian target of rapamycin (mTOR). This reflects both its key role in many cell functions and its involvement in disease states such as cancers. The best understood targets for mTOR signalling are proteins involved in controlling the translational machinery, including the ribosomal protein S6 kinases and proteins that regulate the initiation and elongation phases of translation. Indeed, there is compelling evidence that at least one of these targets of mTOR (eukaryotic initiation factor eIF4E) plays a key role in tumorigenesis. It is regulated through the mTOR-dependent phosphorylation of inhibitory proteins such as eIF4E-binding protein 1. Thus, targeting mTOR signalling may be an effective anticancer strategy, in at least a significant subset of tumours. Not all effects of mTOR are sensitive to the classical anti-mTOR drug rapamycin, and this compound also interferes with other processes besides eIF4E function. Developing new approaches to targeting mTOR for cancer therapy requires more detailed knowledge of signalling downstream of mTOR. Such advances are likely to come from further work to understand the regulation of mTOR targets such as components of the translational apparatus.

mTOR and cancer therapy

Proteins regulating the mammalian target of rapamycin (mTOR), as well as some of the targets of the mTOR kinase, are overexpressed or mutated in cancer. Rapamycin, the naturally occurring inhibitor of mTOR, along with a number of recently developed rapamycin analogs (rapalogs) consisting of synthetically derived compounds containing minor chemical modifications to the parent structure, inhibit the growth of cell lines derived from multiple tumor types in vitro, and tumor models in vivo. Results from clinical trials indicate that the rapalogs may be useful for the treatment of subsets of certain types of cancer. The sporadic responses from the initial clinical trials, based on the hypothesis of general translation inhibition of cancer cells are now beginning to be understood owing to a more complete understanding of the dynamics of mTOR regulation and the function of mTOR in the tumor microenvironment. This review will summarize the preclinical and clinical data and recent discoveries of the function of mTOR in cancer and growth regulation.

Signalling and autophagy regulation in health, aging and disease

It has become clear in recent years that autophagy not only serves to produce amino acids for ongoing protein synthesis and to produce substrates for energy production when cells become starved but autophagy is also able to eliminate defective cell structures and for this reason the process may be implicated in several diseased states. Autophagy is controlled by complex signalling pathways, including that used by insulin. In these pathways, phosphatidylinositol 3-kinases and the protein kinase mTOR play important roles.

AMP-activated protein kinase and the regulation of autophagic proteolysis

Interruption of mTOR-dependent signaling by rapamycin is known to stimulate autophagy, both in mammalian cells and in yeast. Because activation of AMPK also inhibits mTOR-dependent signaling one would expect stimulation of autophagy by AMPK activation. According to the literature, this is true for yeast but, unexpectedly, not for mammalian cells on the basis of the use of AICAR, a pharmacological activator of AMPK. In the present study, carried out with hepatocytes, HT-29 cells, and HeLa cells, we have reexamined the possible role of AMPK in the control of mammalian autophagy. Inhibition of AMPK activity by compound C or by transfection with a dominant negative form of AMPK almost completely inhibited autophagy. These results suggest that the inhibition of autophagy by AICAR is not related to its ability to activate AMPK. We conclude that in mammalian cells, as in yeast, AMPK is required for autophagy.