Elongation Factor-2 Kinase Regulates Autophagy in Human Glioblastoma Cells

Afferent regulation of chicken auditory brainstem neurons: rapid changes in phosphorylation of elongation factor 2

The relationships between protein synthesis and neuronal survival are poorly understood. In chicken nucleus magnocellularis (NM), significant alterations in overall protein synthesis precede neuronal death induced by deprivation of excitatory afferent activity. Previously we demonstrated an initial reduction in the overall rate of protein synthesis in all deprived NM neurons, followed by quick recovery (starting at 6 hours) in some, but not all, neurons. Neurons with recovered protein synthesis ultimately survive, whereas others become "ghost" cells (no detectable Nissl substance) at 12-24 hours and die within 48 hours. To explore the mechanisms underlying this differential influence of afferent input on protein synthesis and cell survival, the current study investigates the involvement of eukaryotic translation elongation factor 2 (eEF2), the phosphorylation of which reduces overall protein synthesis. Using immunocytochemistry for either total or phosphorylated eEF2 (p-eEF2), we found significant reductions in the level of phosphorylated, but not total, eEF2 in NM neurons as early as 0.5-1 hour following cochlea removal. Unexpectedly, neurons with low levels of p-eEF2 show reduced protein synthesis at 6 hours, indicated by a marker for active ribosomes. At 12 hours, all "ghost" cells exhibited little or no p-eEF2 staining, although not every neuron with a comparable low level of p-eEF2 was a "ghost" cell. These observations demonstrate that a reduced level of p-eEF2 is not responsible for immediate responses (including reduced overall protein synthesis) of a neuron to compromised afferent input but may impair the neuron's ability to initiate recovery signaling for survival and make the neuron more vulnerable to death.

Therapeutic targeting of autophagy in disease: biology and pharmacology

Autophagy, a process of self-digestion of the cytoplasm and organelles through which cellular components are recycled for reuse or energy production, is an evolutionarily conserved response to metabolic stress found in eukaryotes from yeast to mammals. It is noteworthy that autophagy is also associated with various pathophysiologic conditions in which this cellular process plays either a cytoprotective or cytopathic role in response to a variety of stresses such as metabolic, inflammatory, neurodegenerative, and therapeutic stress. It is now generally believed that modulating the activity of autophagy through targeting specific regulatory molecules in the autophagy machinery may impact disease processes, thus autophagy may represent a new pharmacologic target for drug development and therapeutic intervention of various human disorders. Induction or inhibition of autophagy using small molecule compounds has shown promise in the treatment of diseases such as cancer. Depending on context, induction or suppression of autophagy may exert therapeutic effects via promoting either cell survival or death, two major events targeted by therapies for various disorders. A better understanding of the biology of autophagy and the pharmacology of autophagy modulators has the potential for facilitating the development of autophagy-based therapeutic interventions for several human diseases.

The eEF2 kinase confers resistance to nutrient deprivation by blocking translation elongation

Metabolic adaptation is essential for cell survival during nutrient deprivation. We report that eukaryotic elongation factor 2 kinase (eEF2K), which is activated by AMP-kinase (AMPK), confers cell survival under acute nutrient depletion by blocking translation elongation. Tumor cells exploit this pathway to adapt to nutrient deprivation by reactivating the AMPK-eEF2K axis. Adaptation of transformed cells to nutrient withdrawal is severely compromised in cells lacking eEF2K. Moreover, eEF2K knockdown restored sensitivity to acute nutrient deprivation in highly resistant human tumor cell lines. In vivo, overexpression of eEF2K rendered murine tumors remarkably resistant to caloric restriction. Expression of eEF2K strongly correlated with overall survival in human medulloblastoma and glioblastoma multiforme. Finally, C. elegans strains deficient in efk-1, the eEF2K ortholog, were severely compromised in their response to nutrient depletion. Our data highlight a conserved role for eEF2K in protecting cells from nutrient deprivation and in conferring tumor cell adaptation to metabolic stress. PAPERCLIP:

Eukaryotic elongation factor 2 kinase regulates the development of hypertension through oxidative stress-dependent vascular inflammation

Eukaryotic elongation factor 2 kinase (eEF2K) is a Ca2+/calmodulin-dependent protein kinase. We recently demonstrated that eEF2K protein increases in mesenteric artery from spontaneously hypertensive rats (SHR). Pathogenesis of hypertension is regulated in part by vascular inflammation. We tested the hypothesis whether eEF2K mediates vascular inflammatory responses and development of hypertension. In vascular endothelial cells, small interfering RNA (siRNA) against eEF2K inhibited induction of VCAM-1 and endothelial-selectin as well as monocyte adhesion by TNF-α (10 ng/ml). eEF2K siRNA inhibited phosphorylation of JNK and NF-κB p65 as well as reactive oxygen species (ROS) production by TNF-α. In vascular smooth muscle cells, eEF2K siRNA also inhibited VCAM-1 induction and phosphorylation of JNK and NF-κB by TNF-α. In vivo, increased blood pressure in SHR and ROS production, induction of inflammatory molecules, and hypertrophy in SHR superior mesenteric artery were reduced by an eEF2K inhibitor NH125 (500 μg·kg(-1)·day(-1)). In SHR superior mesenteric artery, impairment of ACh-induced relaxation was normalized by NH125. The present results for the first time demonstrate that eEF2K mediates TNF-α-induced vascular inflammation via ROS-dependent mechanism, which is at least partly responsible for the development of hypertension in SHR.

Overcoming resistance to rapalogs in gliomas by combinatory therapies

Glioblastoma is the most common and aggressive brain tumor type, with a mean patient survival of approximately 1year. Many previous analyses of the glioma kinome have identified key deregulated pathways that converge and activate mammalian target of rapamycin (mTOR). Following the identification and characterization of mTOR-promoting activity in gliomagenesis, data from preclinical studies suggested the targeting of mTOR by rapamycin or its analogs (rapalogs) as a promising therapeutic approach. However, clinical trials with rapalogs have shown very limited efficacy on glioma due to the development of resistance mechanisms. Analysis of rapalog-insensitive glioma cells has revealed increased activity of growth and survival pathways compensating for mTOR inhibition by rapalogs that are suitable for therapeutic intervention. In addition, recently developed mTOR inhibitors show high anti-glioma activity. In this review, we recapitulate the regulation of mTOR signaling and its involvement in gliomagenesis, discuss mechanisms resulting in resistance to rapalogs, and speculate on strategies to overcome resistance. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Integrated regulation of autophagy and apoptosis by EEF2K controls cellular fate and modulates the efficacy of curcumin and velcade against tumor cells

Endoplasmic reticulum (ER) stress induces both autophagy and apoptosis yet the molecular mechanisms and pathways underlying the regulation of these two cellular processes in cells undergoing ER stress remain less clear. We report here that eukaryotic elongation factor-2 kinase (EEF2K) is a critical controller of the ER stress-induced autophagy and apoptosis in tumor cells. DDIT4, a stress-induced protein, was required for transducing the signal for activation of EEF2K under ER stress. We further showed that phosphorylation of EEF2K at Ser398 was essential for induction of autophagy, while phosphorylation of the kinase at Ser366 and Ser78 exerted an inhibitory effect on autophagy. Suppression of the ER stress-activated autophagy via silencing of EEF2K aggravated ER stress and promoted apoptotic cell death in tumor cells. Moreover, inhibiting EEF2K by either RNAi or NH125, a small molecule inhibitor of the enzyme, rendered tumor cells more sensitive to curcumin and velcade, two anticancer agents that possess ER stress-inducing action. Our study indicated that the DDIT4-EEF2K pathway was essential for inducing autophagy and for determining the fate of tumor cells under ER stress, and suggested that inhibiting the EEF2K-mediated autophagy can deteriorate ER stress and lead to a greater apoptotic response, thereby potentiating the efficacy of the ER stress-inducing agents against cancer.

Aberrations in translational regulation are associated with poor prognosis in hormone receptor-positive breast cancer

Introduction

Translation initiation is activated in cancer through increase in eukaryotic initiation factor 4E (eIF4E), eIF4G, phosphorylated eIF4E-binding protein (p4E-BP1) and phosphorylated ribosomal protein S6 (pS6), and decreased programmed cell death protein 4 (pdcd4), a translational inhibitor. Further, translation elongation is deregulated though alterations in eukaryotic elongation factor 2 (eEF2) and eEF2 kinase (eEF2K). We sought to determine the association of these translational aberrations with clinical-pathologic factors and survival outcomes in hormone receptor-positive breast cancer.

Methods

Primary tumors were collected from 190 patients with Stage I to III hormone receptor-positive breast cancer. Expression of eIF4E, eIF4G, 4E-BP1, p4E-BP1 T37/46, p4E-BP1 S65, p4E-BP1 T70, S6, pS6 S235/236, pS6 S240/244, pdcd4, eEF2 and eEF2K was assessed by reverse phase protein arrays. Univariable and multivariable analyses for recurrence-free survival (RFS) and overall survival (OS) were performed.

Results

High eEF2, S6, pS6 S240/244, p4E-BP1 T70, and low pdcd4 were significantly associated with node positivity. Median follow-up for living patients was 96 months.

High p4E-BP1 T36/47, p4E-BP1 S65, p4E-BP1 T70 and 4E-BP1 were associated with worse RFS. High p4E-BP1 T70 and pS6 S235/236, and low pdcd4, were associated with worse OS. In multivariable analysis, in addition to positive nodes, p4E-BP1 S65 remained a significant predictor of RFS (HR = 1.62, 95% CI = 1.13-2.31; P = 0.008). In addition to age, pS6 S235/236 (HR = 1.73, 95% CI = 1.03-2.90, P = 0.039), eEF2K (HR = 2.19, 95% CI = 1.35-3.56, P = 0.002) and pdcd4 (HR = 0.42, 95% CI = 0.25-0.70, P = 0.001) were associated with OS.

Conclusions

Increased pS6, p4E-BP1, eEF2K and decreased pdcd4 are associated with poor prognosis in hormone receptor-positive breast cancer, suggesting their role as prognostic markers and therapeutic targets.

Targeted silencing of elongation factor 2 kinase suppresses growth and sensitizes tumors to doxorubicin in an orthotopic model of breast cancer

Eukaryotic elongation factor 2 kinase (eEF-2K), through its phosphorylation of elongation factor 2 (eEF2), provides a mechanism by which cells can control the rate of the elongation phase of protein synthesis. The activity of eEF-2K is increased in rapidly proliferating malignant cells, is inhibited during mitosis, and may contribute to the promotion of autophagy in response to anti-cancer therapies. The purpose of this study was to examine the therapeutic potential of targeting eEF-2K in breast cancer tumors. Through the systemic administration of liposomal eEF-2K siRNA (twice a week, i.v. 150 µg/kg), the expression of eEF-2K was down-regulated in vivo in an orthotopic xenograft mouse model of a highly aggressive triple negative MDA-MB-231 tumor. This targeting resulted in a substantial decrease in eEF2 phosphorylation in the tumors, and led to the inhibition of tumor growth, the induction of apoptosis and the sensitization of tumors to the chemotherapy agent doxorubicin. eEF-2K down-modulation in vitro resulted in a decrease in the expression of c-Myc and cyclin D1 with a concomitant increase in the expression of p27^Kip1. A decrease in the basal activity of c-Src (phospho-Tyr-416), focal adhesion kinase (phospho-Tyr-397), and Akt (phospho-Ser-473) was also detected following eEF-2K down-regulation in MDA-MB-231 cells, as determined by Western blotting. Where tested, similar results were seen in ER-positive MCF-7 cells. These effects were also accompanied by a decrease in the observed invasive phenotype of the MDA-MB-231 cells. These data support the notion that the disruption of eEF-2K expression in breast cancer cells results in the down-regulation of signaling pathways affecting growth, survival and resistance and has potential as a therapeutic approach for the treatment of breast cancer.

Phosphorylation of elongation factor-2 kinase differentially regulates the enzyme's stability under stress conditions

Eukaryotic elongation factor-2 kinase (eEF-2K) is a Ca(2+)/calmodulin-dependent enzyme that negatively regulates protein synthesis. eEF-2K has been shown to be up-regulated in cancer, and to play an important role in cell survival through inhibition of protein synthesis. Post-translational modification of protein synthesis machinery is important for its regulation and could be critical for survival of cancer cells encountering stress. The purpose of our study was to examine the regulation of eEF-2K during stress with a focus on the roles of phosphorylation in determining the stability of eEF-2K. We found that stress conditions (nutrient deprivation and hypoxia) increase eEF-2K protein. mRNA levels are only transiently increased and shortly return to normal, while eEF-2K protein levels continue to increase after further exposure to stress. A seemingly paradoxical decrease in eEF-2K stability was found when glioma cells were subjected to stress despite increased protein expression. We further demonstrated that phosphorylation of eEF-2K differentially affects the enzyme's turnover under both normal and stress conditions, as evidenced by the different half-lives of phosphorylation-defective mutants of eEF-2K. We further found that the eEF-2K site (Ser398) phosphorylated by AMPK is pivotal to the protein's stability, as the half-life of S398A mutant increases to greater than 24h under both normal and stress conditions. These data indicate that eEF-2K is regulated at multiple levels with phosphorylation playing a critical role in the enzyme's turnover under stressful conditions. The complexity of eEF-2K phosphorylation highlights the intricacies of protein synthesis control during cellular stress.

Calcium/calmodulin stimulates the autophosphorylation of elongation factor 2 kinase on Thr-348 and Ser-500 to regulate its activity and calcium dependence

Eukaryotic elongation factor 2 kinase (eEF-2K) is an atypical protein kinase regulated by Ca(2+) and calmodulin (CaM). Its only known substrate is eukaryotic elongation factor 2 (eEF-2), whose phosphorylation by eEF-2K impedes global protein synthesis. To date, the mechanism of eEF-2K autophosphorylation has not been fully elucidated. To investigate the mechanism of autophosphorylation, human eEF-2K was coexpressed with λ-phosphatase and purified from bacteria in a three-step protocol using a CaM affinity column. Purified eEF-2K was induced to autophosphorylate by incubation with Ca(2+)/CaM in the presence of MgATP. Analyzing tryptic or chymotryptic peptides by mass spectrometry monitored the autophosphorylation over 0-180 min. The following five major autophosphorylation sites were identified: Thr-348, Thr-353, Ser-445, Ser-474, and Ser-500. In the presence of Ca(2+)/CaM, robust phosphorylation of Thr-348 occurs within seconds of addition of MgATP. Mutagenesis studies suggest that phosphorylation of Thr-348 is required for substrate (eEF-2 or a peptide substrate) phosphorylation, but not self-phosphorylation. Phosphorylation of Ser-500 lags behind the phosphorylation of Thr-348 and is associated with the Ca(2+)-independent activity of eEF-2K. Mutation of Ser-500 to Asp, but not Ala, renders eEF-2K Ca(2+)-independent. Surprisingly, this Ca(2+)-independent activity requires the presence of CaM.

Investigating the kinetic mechanism of inhibition of elongation factor 2 kinase by NH125: evidence of a common in vitro artifact

Evidence that elongation factor 2 kinase (eEF-2K) has potential as a target for anticancer therapy and possibly for the treatment of depression is emerging. Here the steady-state kinetic mechanism of eEF-2K is presented using a peptide substrate and is shown to conform to an ordered sequential mechanism with ATP binding first. Substrate inhibition by the peptide was observed and revealed to be competitive with ATP, explaining the observed ordered mechanism. Several small molecules are reported to inhibit eEF-2K activity with the most notable being the histidine kinase inhibitor NH125, which has been used in a number of studies to characterize eEF-2K activity in cells. While NH125 was previously reported to inhibit eEF-2K in vitro with an IC(50) of 60 nM, its mechanism of action was not established. Using the same kinetic assay, the ability of an authentic sample of NH125 to inhibit eEF-2K was assessed over a range of substrate and inhibitor concentrations. A typical dose-response curve for the inhibition of eEF-2K by NH125 is best fit to an IC(50) of 18 ± 0.25 μM and a Hill coefficient of 3.7 ± 0.14, suggesting that NH125 is a weak inhibitor of eEF-2K under the experimental conditions of a standard in vitro kinase assay. To test the possibility that NH125 is a potent inhibitor of eEF2 phosphorylation, we assessed its ability to inhibit the phosphorylation of eEF2. Under standard kinase assay conditions, NH125 exhibits a similar weak ability to inhibit the phosphorylation of eEF2 by eEF-2K. Notably, the activity of NH125 is severely abrogated by the addition of 0.1% Triton to the kinase assay through a process that can be reversed upon dilution. These studies suggest that NH125 is a nonspecific colloidal aggregator in vitro, a notion further supported by the observation that NH125 inhibits other protein kinases, such as ERK2 and TRPM7 in a manner similar to that of eEF-2K. As NH125 is reported to inhibit eEF-2K in a cellular environment, its ability to inhibit eEF2 phosphorylation was assessed in MDA-MB-231 breast cancer, A549 lung cancer, and HEK-293T cell lines using a Western blot approach. No sign of a decrease in the level of eEF2 phosphorylation was observed up to 12 h following addition of NH125 to the media. Furthermore, contrary to the previously reported literatures, NH125 induced the phosphorylation of eEF-2.

The mTOR signalling pathway in human cancer

The conserved serine/threonine kinase mTOR (the mammalian target of rapamycin), a downstream effector of the PI3K/AKT pathway, forms two distinct multiprotein complexes: mTORC1 and mTORC2. mTORC1 is sensitive to rapamycin, activates S6K1 and 4EBP1, which are involved in mRNA translation. It is activated by diverse stimuli, such as growth factors, nutrients, energy and stress signals, and essential signalling pathways, such as PI3K, MAPK and AMPK, in order to control cell growth, proliferation and survival. mTORC2 is considered resistant to rapamycin and is generally insensitive to nutrients and energy signals. It activates PKC-α and AKT and regulates the actin cytoskeleton. Deregulation of multiple elements of the mTOR pathway (PI3K amplification/mutation, PTEN loss of function, AKT overexpression, and S6K1, 4EBP1 and eIF4E overexpression) has been reported in many types of cancers, particularly in melanoma, where alterations in major components of the mTOR pathway were reported to have significant effects on tumour progression. Therefore, mTOR is an appealing therapeutic target and mTOR inhibitors, including the rapamycin analogues deforolimus, everolimus and temsirolimus, are submitted to clinical trials for treating multiple cancers, alone or in combination with inhibitors of other pathways. Importantly, temsirolimus and everolimus were recently approved by the FDA for the treatment of renal cell carcinoma, PNET and giant cell astrocytoma. Small molecules that inhibit mTOR kinase activity and dual PI3K-mTOR inhibitors are also being developed. In this review, we aim to survey relevant research, the molecular mechanisms of signalling, including upstream activation and downstream effectors, and the role of mTOR in cancer, mainly in melanoma.

Modulation of pediatric brain tumor autophagy and chemosensitivity

Brain and spinal tumors are the second most common malignancies in childhood after leukemia, and they remain the leading cause of death from childhood cancer. Autophagy is a catabolic cellular process that is thought to regulate chemosensitivity, however its role in pediatric tumors is unknown. Here we present studies in pediatric medulloblastoma cell lines (DAOY, ONS76) and atypical teratoid/rhabdoid tumor cell lines (BT-16, BT-12) to test this role. Autophagy was inhibited using siRNA against autophagy-related genes ATG12 and ATG7 or pharmacologically induced or inhibited using rapamycin and chloroquine to test the effect of autophagy on chemosensitivity. Autophagic flux was measured using Western blot analysis of LC3-II and p62 and cell viability was determined using MTS assays and clonogenic growth. We found that when pediatric brain tumor cells under starvation stress, exposed to known autophagy inducers such as rapamycin, or treated with current chemotherapeutics (lomustine, cisplatin), all stimulate autophagy. Silencing ATG12 and ATG7 or exposure to a known autophagy inhibitor, chloroquine, could inhibit this autophagy increase; however, the effect of autophagy on tumor cell killing was small. These results may have clinical relevance in the future planning of therapeutic regimens for pediatric brain tumors.

The combination of tephrosin with 2-deoxy-D-glucose enhances the cytotoxicity via accelerating ATP depletion and blunting autophagy in human cancer cells

2-Deoxy-D-glucose (2-DG), a synthetic glucose analog that acts as a glycolytic inhibitor, is currently under clinical evaluation for targeting tumor cells. Tephrosin (TSN), a plant rotenoid, is known as an anticancer agent. In this study, we describe that the addition of TSN to 2-DG enhanced the cytotoxic activity of 2-DG against various types of cancer cells by accelerating ATP depletion and blunting autophagy. TSN increased the sensitivity of cancer cells to the cytotoxic effect of 2-DG. The combination of TSN and 2-DG induced acceleration of intracellular ATP depletion and the drastic activation of AMP-activated protein kinase (AMPK), which resulted in the inactivation of the mammalian target of rapamycin (mTOR) pathway. Of particular interest, TSN suppressed 2-DG-induced autophagy, a cell survival process in response to nutrient deprivation. We also showed that TSN inhibited 2-DG-induced activation of elongation factor-2 kinase (eEF-2K), which has been known to regulate 2-DG-induced autophagy. Inhibition of eEF-2K by RNA interference blunted 2-DG-induced autophagy and increased the sensitivity of cancer cells to the cytotoxic effect of 2-DG. The addition of TSN to 2-DG, however, did not enhance the cytotoxic activity of 2-DG by knockdown of eEF-2K, suggesting that inhibition of eEF-2K by tephrsoin could be a critical role in the potentiating effect of TSN on the cytotoxicity of 2-DG. Furthermore, we showed that the blunted autophagy and enhanced cytotoxicity of 2-DG was accompanied by the augmentation of apoptosis. These results show that TSN may be valuable for augmenting the therapeutic efficacy of 2-DG.

Survival and death of endoplasmic-reticulum-stressed cells: Role of autophagy

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) results in ER stress, which subsequently activates the unfolded protein response that induces a transcriptional program to alleviate the stress. Another cellular process that is activated during ER stress is autophagy, a mechanism of enclosing intracellular components in a double-membrane autophagosome, and then delivering it to the lysosome for degradation. Here, we discuss the role of autophagy in cellular response to ER stress, the signaling pathways linking ER stress to autophagy, and the possible implication of modulating autophagy in treatment of diseases such as cancer.

1-Benzyl-3-cetyl-2-methylimidazolium iodide (NH125) induces phosphorylation of eukaryotic elongation factor-2 (eEF2): a cautionary note on the anticancer mechanism of an eEF2 kinase inhibitor

Eukaryotic elongation factor-2 kinase (eEF2K) relays growth and stress signals to protein synthesis through phosphorylation and inactivation of eukaryotic elongation factor 2 (eEF2). 1-Benzyl-3-cetyl-2-methylimidazolium iodide (NH125) is a widely accepted inhibitor of mammalian eEF2K and an efficacious anti-proliferation agent against different cancer cells. It implied that eEF2K could be an efficacious anticancer target. However, eEF2K siRNA was ineffective against cancer cells including those sensitive to NH125. To test if pharmacological intervention differs from siRNA interference, we identified a highly selective small molecule eEF2K inhibitor A-484954. Like siRNA, A-484954 had little effect on cancer cell growth. We carefully examined the effect of NH125 and A-484954 on phosphorylation of eEF2, the known cellular substrate of eEF2K. Surprisingly, NH125 increased eEF2 phosphorylation, whereas A-484954 inhibited the phosphorylation as expected for an eEF2K inhibitor. Both A-484954 and eEF2K siRNA inhibited eEF2K and reduced eEF2 phosphorylation with little effect on cancer cell growth. These data demonstrated clearly that the anticancer activity of NH125 was more correlated with induction of eEF2 phosphorylation than inhibition of eEF2K. Actually, induction of eEF2 phosphorylation was reported to correlate with inhibition of cancer cell growth. We compared several known inducers of eEF2 phosphorylation including AMPK activators and an mTOR inhibitor. Interestingly, stronger induction of eEF2 phosphorylation correlated with more effective growth inhibition. We also explored signal transduction pathways leading to NH125-induced eEF2 phosphorylation. Preliminary data suggested that NH125-induced eEF2 phosphorylation was likely mediated through multiple pathways. These observations identified an opportunity for a new multipathway approach to anticancer therapies.

Inhibition of eEF-2 kinase sensitizes human glioma cells to TRAIL and down-regulates Bcl-xL expression

Elongation factor-2 kinase (eEF-2 kinase, also known as calmodulin-dependent protein kinase III), is a unique calcium/calmodulin-dependent enzyme that inhibits protein synthesis by phosphorylating and inactivating elongation factor-2 (eEF-2). We previously reported that expression/activity of eEF-2 kinase was up-regulated in several types of malignancies including Gliomas, and was associated with response of tumor cells to certain therapeutic stress. In the current study, we sought to determine whether eEF-2 kinase expression affected sensitivity of glioma cells to treatment with tumor the necrosis factor-related apoptosis-inducing ligand (TRAIL), a targeted therapy able to induce apoptosis in cancer cells but causes no toxicity in most normal cells. We found that inhibition of eEF-2 kinase by RNA interference (RNAi) or by a pharmacological inhibitor (NH125) enhanced TRAIL-induced apoptosis in the human glioma cells, as evidenced by an increase in apoptosis in the tumor cells treated with eEF-2 kinase siRNA or the eEF-2 kinase inhibitor. We further demonstrated that sensitization of tumor cells to TRAIL was accompanied by a down-regulation of the anti-apoptotic protein, Bcl-xL, and that overexpression of Bcl-xL could abrogate the sensitizing effect of inhibiting eEF-2 kinase on TRAIL. The results of this study may help devise a new therapeutic strategy for enhancing the efficacy of TRAIL against malignant glioma by targeting eEF-2 kinase.

WT1/EGR1-mediated control of STIM1 expression and function in cancer cells

There have been numerous publications linking Ca(2+) signaling and cancer, however, a clear explanation for this link has remained elusive. We recently identified the oncogenes/tumor suppressors Wilms Tumor Suppressor 1 (WT1) and Early Growth Response 1 (EGR1) as regulators of the expression of STIM1, an essential regulator of Ca(2+) entry in non-excitable cells. The current review focuses on the literature defining both differential Ca(2+) signaling and WT1/EGR1 expression patterns in 6 specific cancer subtypes: Acute Myeloid Leukemia, Wilms Tumor, breast cancer, ovarian cancer, glioblastoma and prostate cancer. For each tumor-type, we have assessed how specific changes in WT1 and EGR1 expression might contribute to aberrant Ca(2+) homeostasis as well as the therapeutic potential of these observations.

Purification and characterization of tagless recombinant human elongation factor 2 kinase (eEF-2K) expressed in Escherichia coli

The eukaryotic elongation factor 2 kinase (eEF-2K) modulates the rate of protein synthesis by impeding the elongation phase of translation by inactivating the eukaryotic elongation factor 2 (eEF-2) via phosphorylation. eEF-2K is known to be activated by calcium and calmodulin, whereas the mTOR and MAPK pathways are suggested to negatively regulate kinase activity. Despite its pivotal role in translation regulation and potential role in tumor survival, the structure, function, and regulation of eEF-2K have not been described in detail. This deficiency may result from the difficulty of obtaining the recombinant kinase in a form suitable for biochemical analysis. Here we report the purification and characterization of recombinant human eEF-2K expressed in the Escherichia coli strain Rosetta-gami 2(DE3). Successive chromatography steps utilizing Ni-NTA affinity, anion-exchange, and gel filtration columns accomplished purification. Cleavage of the thioredoxin-His(6)-tag from the N-terminus of the expressed kinase with TEV protease yielded 9 mg of recombinant (G-D-I)-eEF-2K per liter of culture. Light scattering shows that eEF-2K is a monomer of ∼85 kDa. In vitro kinetic analysis confirmed that recombinant human eEF-2K is able to phosphorylate wheat germ eEF-2 with kinetic parameters comparable to the mammalian enzyme.

Caloric restriction mimetic 2-deoxyglucose antagonizes doxorubicin-induced cardiomyocyte death by multiple mechanisms

Caloric restriction (CR) is a dietary intervention known to enhance cardiovascular health. The glucose analog 2-deoxy-D-glucose (2-DG) mimics CR effects in several animal models. However, whether 2-DG is beneficial to the heart remains obscure. Here, we tested the ability of 2-DG to reduce cardiomyocyte death triggered by doxorubicin (DOX, 1 μm), an antitumor drug that can cause heart failure. Treatment of neonatal rat cardiomyocytes with 0.5 mm 2-DG dramatically suppressed DOX cytotoxicity as indicated by a decreased number of cells that stained positive for propidium iodide and reduced apoptotic markers. 2-DG decreased intracellular ATP levels by 17.9%, but it prevented DOX-induced severe depletion of ATP, which may contribute to 2-DG-mediated cytoprotection. Also, 2-DG increased the activity of AMP-activated protein kinase (AMPK). Blocking AMPK signaling with compound C or small interfering RNA-mediated knockdown of the catalytic subunit markedly attenuated the protective effects of 2-DG. Conversely, AMPK activation by pharmacological or genetic approach reduced DOX cardiotoxicity but did not produce additive effects when used together with 2-DG. In addition, 2-DG induced autophagy, a cellular degradation pathway whose activation could be either protective or detrimental depending on the context. Paradoxically, despite its ability to activate autophagy, 2-DG prevented DOX-induced detrimental autophagy. Together, these results suggest that the CR mimetic 2-DG can antagonize DOX-induced cardiomyocyte death, which is mediated through multiple mechanisms, including the preservation of ATP content, the activation of AMPK, and the inhibition of autophagy.

Targeting autophagy during cancer therapy to improve clinical outcomes

Autophagy is a catabolic process that turns over long-lived proteins and organelles and contributes to cell and organism survival in times of stress. Current cancer therapies including chemotherapy and radiation are known to induce autophagy within tumor cells. This is therefore an attractive process to target during cancer therapy as there are safe, clinically available drugs known to both inhibit and stimulate autophagy. However, there are conflicting positive and negative effects of autophagy and no current consensus on how to manipulate autophagy to improve clinical outcomes. Careful and rigorous evaluation of autophagy with a focus on how to translate laboratory findings into relevant clinical therapies remains an important aspect of improving clinical outcomes in patients with malignant disease.

Eukaryotic elongation factor-2 (eEF2): its regulation and peptide chain elongation

Regulation at the level of translation in eukaryotes is feasible because of the longer lifetime of eukaryotic mRNAs in the cell. The elongation stage of mRNA translation requires a substantial amount of energy and also eukaryotic elongation factors (eEFs). The important component of eEFs, i.e. eEF2 promotes the GTP-dependent translocation of the nascent protein chain from the A-site to the P-site of the ribosome. Mostly the eEF2 is regulated by phosphorylation and dephosphorylation by a specific kinase known as eEF2 kinase, which itself is up-regulated by various mechanisms in the eukaryotic cell. The activity of this kinase is dependent on calcium ions and calmodulin. Recently it has been shown that the activity of eEF2 kinase is regulated by MAP kinase signalling and mTOR signalling pathway. There are also various stimuli that control the peptide chain elongation in eukaryotic cell; some stimuli inhibit and some activate eEF2. These reports provide the mechanisms by which cells likely serve to slow down protein synthesis and conserve energy under nutrient deprived conditions via regulation of eEF2. The regulation via eEF2 has also been seen in mammary tissue of lactating cows, suggesting that eEF2 may be a limiting factor in milk protein synthesis. Regulation at this level provides the molecular understanding about the control of protein translocation reactions in eukaryotes, which is critical for numerous biological phenomenons. Further the elongation factors could be potential targets for regulation of protein synthesis like milk protein synthesis and hence probably its foreseeable application to synthetic biology.

Expression of elongation factor-2 kinase contributes to anoikis resistance and invasion of human glioma cells

AIM

To determine whether elongation factor-2 kinase (eEF-2 kinase) contributes to the malignant phenotype of glioblastoma multiforme by promoting the migration and invasion of glioma cells. The mechanism involved was also explored.

METHODS

Human glioma cell lines T98G and LN-229 were used. The expression of eEF-2 kinase was silenced using siRNA, and the invasive potential of tumor cells was assessed using a wound-healing assay and a Matrigel invasion assay. Apoptosis was determined using propidium iodide (PI) staining and Western blot analysis of cleaved caspase-3.

RESULTS

Silencing the expression of eEF-2 kinase by siRNA significantly suppressed both the migration and invasion of human glioma cells. Silencing eEF-2 kinase expression also sensitized glioma cells to anoikis, thereby decreasing tumor cell viability in the absence of attachment. Treatment of tumor cells with the caspase inhibitor z-VAD-fmk down-regulated Bim accumulation and abolished glioma cell sensitivity to anoikis.

CONCLUSION

The results suggest that the expression of eEF-2 kinase contributes to migration and invasion of human glioma cells by protecting them from anoikis. eEF-2 kinase expression may serve as a prognostic marker and a novel target for cancer therapy.

eEF-2 kinase dictates cross-talk between autophagy and apoptosis induced by Akt Inhibition, thereby modulating cytotoxicity of novel Akt inhibitor MK-2206

Inhibition of the survival kinase Akt can trigger apoptosis, and also has been found to activate autophagy, which may confound tumor attack. In this study, we investigated regulatory mechanisms through which apoptosis and autophagy were modulated in tumor cells subjected to Akt inhibition by MK-2206, the first allosteric small molecule inhibitor of Akt to enter clinical development. In human glioma cells, Akt inhibition by MK-2206 or siRNA-mediated attenuation strongly activated autophagy, whereas silencing of eukaryotic elongation factor-2 (eEF-2) kinase, a protein synthesis regulator, blunted this autophagic response. Suppression of MK-2206-induced autophagy by eEF-2 silencing was accompanied by a promotion of apoptotic cell death. Similarly, siRNA-mediated inhibition of eEF-2 kinase potentiated the efficacy of MK-2206 against glioma cells. Together, these results showed that blunting autophagy and augmenting apoptosis by inhibition of eEF-2 kinase could modulate the sensitivity of glioma cells to Akt inhibition. Our findings suggest that targeting eEF-2 kinase may reinforce the antitumor efficacy of Akt inhibitors such as MK-2206.

mTORC1 signaling: what we still don't know

The mammalian target of rapamycin (mTOR) is a protein kinase that plays key roles in cellular regulation. It forms complexes with additional proteins. The best-understood one is mTOR complex 1 (mTORC1). The regulation and cellular functions of mTORC1 have been the subjects of intense study; despite this, many questions remain to be answered. They include questions about the actual mechanisms by which mTORC1 signaling is stimulated by hormones and growth factors, which involves the small GTPase Rheb, and by amino acids, which involves other GTPase proteins. The control of Rheb and the mechanism by which it activates mTORC1 remain incompletely understood. Although it has been known for many years that rapamycin interferes with some functions of mTORC1, it is not known how it does this, or why only some functions of mTORC1 are affected. mTORC1 regulates diverse cellular functions. Several mTORC1 substrates are now known, although in several cases their physiological roles are poorly or incompletely understood. In the case of several processes, although it is clear that they are regulated by mTORC1, it is not known how mTORC1 does this. Lastly, mTORC1 is implicated in ageing, but again it is unclear what mechanisms account for this. Given the importance of mTORC1 signaling both for cellular functions and in human disease, it is a high priority to gain further insights into the control of mTORC1 signaling and the mechanisms by which it controls cellular functions and animal physiology.

Regulation of mammalian autophagy in physiology and pathophysiology

(Macro)autophagy is a bulk degradation process that mediates the clearance of long-lived proteins and organelles. Autophagy is initiated by double-membraned structures, which engulf portions of cytoplasm. The resulting autophagosomes ultimately fuse with lysosomes, where their contents are degraded. Although the term autophagy was first used in 1963, the field has witnessed dramatic growth in the last 5 years, partly as a consequence of the discovery of key components of its cellular machinery. In this review we focus on mammalian autophagy, and we give an overview of the understanding of its machinery and the signaling cascades that regulate it. As recent studies have also shown that autophagy is critical in a range of normal human physiological processes, and defective autophagy is associated with diverse diseases, including neurodegeneration, lysosomal storage diseases, cancers, and Crohn's disease, we discuss the roles of autophagy in health and disease, while trying to critically evaluate if the coincidence between autophagy and these conditions is causal or an epiphenomenon. Finally, we consider the possibility of autophagy upregulation as a therapeutic approach for various conditions.

Autophagic cell death of human hepatoma cells induced by endostar, a recombinant human endostatin

AIM

The aim of this study was to explore the contribution of different doses of Endostar (endostatin, YH-16) on autophagic death in human hepatocellular carcinoma cells.

METHODS

Exponentially growing HepG2 cells were treated by different doses of Endostar. Cell viability was measured by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. The existence of autophagosomes in hepatocellular carcinoma cells was observed using transmission electron microscopy (TEM) and the quantities of autophagosomes were assessed using a confocal laser microscopy. Further, the expression level of microtubule-associated protein light chain 3-II(LC3-II) was tested and quantitated by Western blotting.

RESULTS

Autophagosomes of HepG2 induced by Endostar were observed at 24 hours after treatment, and the amount of autophagosomes increased along with concentration. Meanwhile, Western blotting showed LC3-II protein, indicating the occurrence of autophagy, was expressed to correlate with the dose of Endostar. Meanwhile, the result of the MTT assay showed that the proliferation of cells was inhibited by Endostar in a dose-dependent manner; when the concentration was as high as 400 microg/mL, the inhibition of cells proliferation was 55.0%.

CONCLUSIONS

Taken as a whole, the results demonstrate that Endostar suppressed proliferation and triggered cell death in HepG2 cells by autophagy induction that was dose-dependent. These findings provide mechanistic insight into Endostar action.

Targeting the prodeath and prosurvival functions of autophagy as novel therapeutic strategies in cancer

Autophagy is an evolutionarily conserved lysosomal pathway for degrading cytoplasmic proteins, macromolecules, and organelles. While autophagy has become one of the most attractive topics in cancer research, the current autophagy literature is often viewed as confusing, because of its association with apparently contradictory roles, such as survival and cell death. Autophagy can serve as a tumor suppressor, as a partial reduction in autophagic capacity or defective autophagy (e.g., heterozygous knockdown BECN1 (+/-) in mice) provides an oncogenic stimulus, causing malignant transformation and spontaneous tumors. In addition, autophagy seems to function as a protective cell survival mechanism against environmental and cellular stress (e.g., nutrient deprivation, hypoxia and therapeutic stress) and causes resistance to antineoplastic therapies. Recent studies have demonstrated that the inhibition of autophagy in cancer cells may be therapeutically beneficial in some circumstances, as it can sensitize cancer cells to different therapies, including DNA-damaging agents, antihormone therapies (e.g., tamoxifen), and radiation therapy. This supports the hypothesis that inhibiting autophagy can negatively influence cancer cell survival and increase cell death when combined with anticancer agents, providing a therapeutic advantage against cancer. On the other hand, the induction of autophagy by the inhibition of anti-autophagic proteins, such as Bcl-2, PKCdelta, and tissue transglutaminase 2 (TG2), may lead to autophagic cell death in some apoptosis-resistant cancers (i.e., breast and pancreatic cancers), indicating that the induction of autophagy alone may also be used as a potential therapy. Overall, the data suggest that, depending on the cellular features, either the induction or the inhibition of autophagy can provide therapeutic benefits to patients and that the design and synthesis of the first-generation modulators of autophagy may provide the tools for proof of concept experiments and the impetus for translational studies that may ultimately lead to new therapeutic strategies in cancer.

Cytoprotective effect of the elongation factor-2 kinase-mediated autophagy in breast cancer cells subjected to growth factor inhibition

Background

Autophagy is a highly conserved and regulated cellular process employed by living cells to degrade proteins and organelles as a response to metabolic stress. We have previously reported that eukaryotic elongation factor-2 kinase (eEF-2 kinase, also known as Ca²⁺/calmodulin-dependent protein kinase III) can positively modulate autophagy and negatively regulate protein synthesis. The purpose of the current study was to determine the role of the eEF-2 kinase-regulated autophagy in the response of breast cancer cells to inhibitors of growth factor signaling.

Methodology/Principal Findings

We found that nutrient depletion or growth factor inhibitors activated autophagy in human breast cancer cells, and the increased activity of autophagy was associated with a decrease in cellular ATP and an increase in activities of AMP kinase and eEF-2 kinase. Silencing of eEF-2 kinase relieved the inhibition of protein synthesis, led to a greater reduction of cellular ATP, and blunted autophagic response. We further showed that suppression of eEF-2 kinase-regulated autophagy impeded cell growth in serum/nutrient-deprived cultures and handicapped cell survival, and enhanced the efficacy of the growth factor inhibitors such as trastuzumab, gefitinib, and lapatinib.

Conclusion/Significance

The results of this study provide new evidence that activation of eEF-2 kinase-mediated autophagy plays a protective role for cancer cells under metabolic stress conditions, and that targeting autophagic survival may represent a novel approach to enhancing the effectiveness of growth factor inhibitors.

The alpha-kinase family: an exceptional branch on the protein kinase tree

The alpha-kinase family represents a class of atypical protein kinases that display little sequence similarity to conventional protein kinases. Early studies on myosin heavy chain kinases in Dictyostelium discoideum revealed their unusual propensity to phosphorylate serine and threonine residues in the context of an alpha-helix. Although recent studies show that some members of this family can also phosphorylate residues in non-helical regions, the name alpha-kinase has remained. During evolution, the alpha-kinase domains combined with many different functional subdomains such as von Willebrand factor-like motifs (vWKa) and even cation channels (TRPM6 and TRPM7). As a result, these kinases are implicated in a large variety of cellular processes such as protein translation, Mg(2+) homeostasis, intracellular transport, cell migration, adhesion, and proliferation. Here, we review the current state of knowledge on different members of this kinase family and discuss the potential use of alpha-kinases as drug targets in diseases such as cancer.

Macroautophagy signaling and regulation

Macroautophagy is a vacuolar degradation pathway that terminates in the lysosomal compartment. Macroautophagy is a multistep process involving: (1) signaling events that occur upstream of the molecular machinery of autophagy; (2) molecular machinery involved in the formation of the autophagosome, the initial multimembrane-bound compartment formed in the autophagic pathway; and (3) maturation of autophagosomes, which acquire acidic and degradative capacities. In this chapter we summarize what is known about the regulation of the different steps involved in autophagy, and we also discuss how macroautophagy can be manipulated using drugs or genetic approaches that affect macroautophagy signaling, and the subsequent formation and maturation of the autophagosomes. Modulating autophagy offers a promising new therapeutic approach to human diseases that involve macroautophagy.

Regulation mechanisms and signaling pathways of autophagy

Autophagy is a process of self-degradation of cellular components in which double-membrane autophagosomes sequester organelles or portions of cytosol and fuse with lysosomes or vacuoles for breakdown by resident hydrolases. Autophagy is upregulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation, ER stress, and pathogen infection. Defective autophagy plays a significant role in human pathologies, including cancer, neurodegeneration, and infectious diseases. We present our current knowledge on the key genes composing the autophagy machinery in eukaryotes from yeast to mammalian cells and the signaling pathways that sense the status of different types of stress and induce autophagy for cell survival and homeostasis. We also review the recent advances on the molecular mechanisms that regulate the autophagy machinery at various levels, from transcriptional activation to post-translational protein modification.

Regulation mechanisms and signaling pathways of autophagy

Autophagy is a process of self-degradation of cellular components in which double-membrane autophagosomes sequester organelles or portions of cytosol and fuse with lysosomes or vacuoles for breakdown by resident hydrolases. Autophagy is upregulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation, ER stress, and pathogen infection. Defective autophagy plays a significant role in human pathologies, including cancer, neurodegeneration, and infectious diseases. We present our current knowledge on the key genes composing the autophagy machinery in eukaryotes from yeast to mammalian cells and the signaling pathways that sense the status of different types of stress and induce autophagy for cell survival and homeostasis. We also review the recent advances on the molecular mechanisms that regulate the autophagy machinery at various levels, from transcriptional activation to post-translational protein modification.

Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation

Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.

Expanding roles for AMP-activated protein kinase in neuronal survival and autophagy

AMP-activated protein kinase (AMPK) is an evolutionarily conserved cellular switch that activates catabolic pathways and turns off anabolic processes. In this way, AMPK activation can restore the perturbation of cellular energy levels. In physiological situations, AMPK senses energy deficiency (in the form of an increased AMP/ATP ratio), but it is also activated by metabolic insults, such as glucose or oxygen deprivation. Metformin, one of the most widely prescribed anti-diabetic drugs, exerts its actions by AMPK activation. However, while the functions of AMPK as a metabolic regulator are fairly well understood, its actions in neuronal cells only recently gained attention. This review will discuss newly emerged functions of AMPK in neuroprotection and neurodegeneration. Additionally, recent views on the role of AMPK in autophagy, an important catabolic process that is also involved in neurodegeneration and cancer, will be highlighted.

Inhibition of autophagy induced by overexpression of mda-7/interleukin-24 strongly augments the antileukemia activity in vitro and in vivo

Melanoma differentiation-associated gene-7/interleukin-24 (mda-7/IL-24) is a novel candidate of tumor suppressor that can selectively induce apoptosis experimentally in a spectrum of human cancer cells including leukemia cells. However, a recent study suggests that mda-7/IL-24 promotes the survival of chronic lymphocytic leukemia B-cells. In this study, we showed that mda-7/IL-24 was constitutively expressed in leukemia cell lines and primary acute myeloid leukemia samples. Using a conditionally replicating adenovirus expressing mda-7/IL-24 (ZD55-IL-24), we showed that enforced expression of mda-7/IL-24 in leukemia cells induced autophagy, which was triggered by the upregulation of Beclin-1. Immunofluorescence and coimmunoprecipitation studies suggested that mda-7/IL-24 protein interacts with Beclin-1. Class III PI3K/Beclin-1 complex was shown involved in the mda-7/IL-24-induced autophagy. Moreover, autophagy inhibition by phosphatidylinositol 3-kinase inhibitor, wortmannin, resulted in a reduced Beclin-1 expression and autophagosome formation associated with significantly enhanced cell death. Importantly, the combination of ZD55-IL-24 with wortmannin elicited a strongly enhanced antileukemia efficacy in established leukemia xenografts. These results suggest that mda-7/IL-24-induced autophagy in leukemia cells may provide survival advantage and mda-7/IL-24 combined with agents that disrupt autophagy is a promising new strategy for the treatment of leukemia.

The double-edged sword of autophagy modulation in cancer

Macroautophagy (autophagy) is a lysosomal degradation pathway for the breakdown of intracellular proteins and organelles. Although constitutive autophagy is a homeostatic mechanism for intracellular recycling and metabolic regulation, autophagy is also stress responsive, in which it is important for the removal of damaged proteins and organelles. Autophagy thereby confers stress tolerance, limits damage, and sustains viability under adverse conditions. Autophagy is a tumor-suppression mechanism, yet it enables tumor cell survival in stress. Reconciling how loss of a prosurvival function can promote tumorigenesis, emerging evidence suggests that preservation of cellular fitness by autophagy may be key to tumor suppression. As autophagy is such a fundamental process, establishing how the functional status of autophagy influences tumorigenesis and treatment response is important. This is especially critical as many current cancer therapeutics activate autophagy. Therefore, efforts to understand and modulate the autophagy pathway will provide new approaches to cancer therapy and prevention.

Regulation of autophagy by a beclin 1-targeted microRNA, miR-30a, in cancer cells

beclin 1, the mammalian homologue of the yeast Atg6, is a key autophagy-promoting gene that plays a critical role in the regulation of cell death and survival of various types of cells. However, recent studies have observed that the expression of beclin 1 is altered in certain diseases including cancers. The causes underlying the aberrant expression of beclin 1 remain largely unknown. We report here that microRNAs (miRNAs), a class of endogenous, 22-24 nucleotide noncoding RNA molecules able to affect stability and translation of mRNA, may represent a previously unrecognized mechanism for regulating beclin 1 expression and autophagy. We demonstrated that beclin 1 is a potential target for miRNA miR-30a, and this miRNA could negatively regulate beclin 1 expression resulting in decreased autophagic activity. Treatment of tumor cells with the miR-30a mimic decreased, and with the antagomir increased, the expression of beclin 1 mRNA and protein. Dual luciferase reporter assay confirmed that the miR-30a binding sequences in the 3'-UTR of beclin 1 contribute to the modulation of beclin 1 expression by miR-30a. Furthermore, inhibition of beclin 1 expression by the miR-30a mimic blunted activation of autophagy induced by rapamycin. Our study of the role of miR-30a in regulating beclin 1 expression and autophagy reveals a novel function for miRNA in a critical cellular event with significant impacts in cancer development, progression and treatment, and in other diseases.

Regulation mechanisms and signaling pathways of autophagy

Autophagy is a process of self-degradation of cellular components in which double-membrane autophagosomes sequester organelles or portions of cytosol and fuse with lysosomes or vacuoles for breakdown by resident hydrolases. Autophagy is upregulated in response to extra- or intracellular stress and signals such as starvation, growth factor deprivation, ER stress, and pathogen infection. Defective autophagy plays a significant role in human pathologies, including cancer, neurodegeneration, and infectious diseases. We present our current knowledge on the key genes composing the autophagy machinery in eukaryotes from yeast to mammalian cells and the signaling pathways that sense the status of different types of stress and induce autophagy for cell survival and homeostasis. We also review the recent advances on the molecular mechanisms that regulate the autophagy machinery at various levels, from transcriptional activation to post-translational protein modification.

The role of protein synthesis in cell cycling and cancer

Cell cycling and protein synthesis are both key physiological tasks for cancer cells. Here we present a model for how the elongation phase of protein synthesis, governed by elongation factor 2 and elongation factor 2 kinase, both modulates and responds to cell cycling. Within this framework we also discuss survivin, a protein with both pro-mitotic and anti-apoptotic roles whose persistence in the cell is tied to protein synthesis due to its short half-life. Finally, we provide a brief overview of efforts of cancer researchers to target EF2 and EF2 kinase.

A critical role of eEF-2K in mediating autophagy in response to multiple cellular stresses

The phosphorylation of the subunit alpha of eukaryotic translation initiation factor 2 (eIF2alpha), a critical regulatory event in controlling protein translation, has recently been found to mediate the induction of autophagy. However, the mediators of autophagy downstream of eIF2alpha remain unknown. Here, we provide evidence that eIF2alpha phosphorylation is required for phosphorylation of eukaryotic elongation factor 2 (eEF-2) during nutrient starvation. In addition, we show that eukaryotic elongation factor 2 kinase (eEF-2K) is also required for autophagy signaling during ER stress, suggesting that phosphorylation of eEF-2 may serve as an integrator of various cell stresses for autophagy signaling. On the other hand, although the activation of eEF-2K in response to starvation requires the phosphorylation of eIF2alpha, additional pathways relying partly on Ca(2+) flux may control eEF-2K activity during ER stress, as eIF2alpha phosphorylation is dispensable for both eEF-2 phosphorylation and autophagy in this context.

Silencing of elongation factor-2 kinase potentiates the effect of 2-deoxy-D-glucose against human glioma cells through blunting of autophagy

2-Deoxy-d-glucose (2-DG), a synthetic glucose analogue that acts as a glycolytic inhibitor, is currently being evaluated in the clinic as an anticancer agent. In this study, we observed that treatment of human glioma cells with 2-DG activated autophagy, a highly conserved cellular response to metabolic stress and a catabolic process of self-digestion of intracellular organelles for energy use and survival in stressed cells. The induction of autophagy by 2-DG was associated with activation of elongation factor-2 kinase (eEF-2 kinase), a structurally and functionally unique enzyme that phosphorylates eEF-2, leading to loss of affinity of this elongation factor for the ribosome and to termination of protein elongation. We also showed that inhibition of eEF-2 kinase by RNA interference blunted the 2-DG-induced autophagic response, resulted in a greater reduction of cellular ATP contents, and increased the sensitivity of tumor cells to the cytotoxic effect of 2-DG. Furthermore, the blunted autophagy and enhanced 2-DG cytotoxicity were accompanied by augmentation of apoptosis in cells in which eEF-2 kinase expression was knocked down. The results of this study indicate that the energy stress and cytotoxicity caused by 2-DG can be accelerated by inhibition of eEF-2 kinase, and suggest that targeting eEF-2 kinase-regulated autophagic survival pathway may represent a novel approach to sensitizing cancer cells to glycolytic inhibitors.

Productive Chlamydia trachomatis lymphogranuloma venereum 434 infection in cells with augmented or inactivated autophagic activities

Autophagy, a eukaryotic cellular activity leading to the degradation of cellular components, serves as a defense mechanism against facultative intracellular bacteria as well as a growth niche for the obligate intracellular bacterium Coxiella burnetii. We here demonstrate that the obligate intracellular bacterial pathogen Chlamydia trachomatis lymphogranuloma venereum strongly induced autophagy in the middle of the chlamydial developmental cycle (24 h after infection), a time point with maximal level of chlamydial replication, but not during the early stages with low overall chlamydial metabolism (before 8 h). No autophagy induction was evident in cells exposed to heat- and UV-inactivated elementary bodies (EBs, the infectious form of Chlamydia) or to inocula from which EBs had been removed before inoculation. Blocking chlamydial development with chloramphenicol also prevented autophagy induction in cells infected with infectious EBs. It appears that autophagy is activated primarily in response to the metabolic stress consequent to chlamydial replication. However, autophagy-defective ATG5(-/-) cells supported chlamydial development as efficiently as autophagy-proficient ATG5(+/+) cells.

Autophagy regulates selective HMGB1 release in tumor cells that are destined to die

Macroautophagy (hereafter referred to as autophagy) can increase or decrease the amount of cell death in response to various stimuli. To test if autophagy also controls the characteristics associated with dying cells, we studied tumor cell killing by Epidermal Growth Factor Receptor (EGFR)-targeted diphtheria toxin (DT-EGF). DT-EGF kills epithelial and glioblastoma tumor cells with similar efficiency but by different mechanisms that depend on whether the cells activate autophagy when treated with the drug. Dying cells in which autophagy is induced selectively release the immune modulator HMGB1 without causing lysis of the cell membrane and classical necrosis. Conversely, cells that are killed by DT-EGF where autophagy is blocked, activate caspases but retain HMGB1. These data suggest that it may be feasible to manipulate the immunogenicity of dying cells by increasing or decreasing autophagy.

Discovery, identification, and characterization of candidate pharmacodynamic markers of methionine aminopeptidase-2 inhibition

The catalytic activity of methionine aminopeptidase-2 (MetAP2) has been pharmacologically linked to cell growth, angiogenesis, and tumor progression, making this an attractive target for cancer therapy. An assay for monitoring specific protein changes in response to MetAP2 inhibition, allowing pharmacokinetic (PK)/pharmacodynamic (PD) models to be established, could dramatically improve clinical decision-making. Candidate MetAP2-specific protein substrates were discovered from undigested cell culture-derived proteomes by MALDI-/SELDI-MS profiling and a biochemical method using (35)S-Met labeled protein lysates. Substrates were identified either as intact proteins by FT-ICR-MS or applying in-gel protease digestions followed by LC-MS/MS. The combination of these approaches led to the discovery of novel MetAP2-specific substrates including thioredoxin-1 (Trx-1), SH3 binding glutamic acid rich-like protein (SH3BGRL), and eukaryotic elongation factor-2 (eEF2). These studies also confirmed glyceraldehye 3-phosphate dehydrogenase (GAPDH) and cyclophillin A (CypA) as MetAP2 substrates. Additional data in support of these proteins as MetAP2-specific substrates were provided by in vitro MetAP1/MetAP2 enzyme assays with the corresponding N-terminal derived peptides and 1D/2D Western analyses of cellular and tissue lysates. FT-ICR-MS characterization of all intact species of the 18 kDa substrate, CypA, enabled a SELDI-MS cell-based assay to be developed for correlating N-terminal processing and inhibition of proliferation. The MetAP2-specific protein substrates discovered in this study have diverse properties that should facilitate the development of reagents for testing in preclinical and clinical environments.