The Noxa/Mcl-1 Axis Regulates Susceptibility to Apoptosis under Glucose Limitation in Dividing T Cells

Akt requires glucose metabolism to suppress puma expression and prevent apoptosis of leukemic T cells

The PI3K/Akt pathway is activated in stimulated cells and in many cancers to promote glucose metabolism and prevent cell death. Although inhibition of Akt-mediated cell survival may provide a means to eliminate cancer cells, this survival pathway remains incompletely understood. In particular, unlike anti-apoptotic Bcl-2 family proteins that prevent apoptosis independent of glucose, Akt requires glucose metabolism to inhibit cell death. This glucose dependence may occur in part through metabolic regulation of pro-apoptotic Bcl-2 family proteins. Here, we show that activated Akt relies on glycolysis to inhibit induction of Puma, which was uniquely sensitive to metabolic status among pro-apoptotic Bcl-2 family members and was rapidly up-regulated in glucose-deficient conditions. Importantly, preventing Puma expression was critical for Akt-mediated cell survival, as Puma deficiency protected cells from glucose deprivation and Akt could not readily block Puma-mediated apoptosis. In contrast, the pro-apoptotic Bcl-2 family protein Bim was induced normally even when constitutively active Akt was expressed, yet Akt could provide protection from Bim cytotoxicity. Up-regulation of Puma appeared mediated by decreased availability of mitochondrial metabolites rather than glycolysis itself, as alternative mitochondrial fuels could suppress Puma induction and apoptosis upon glucose deprivation. Metabolic regulation of Puma was mediated through combined p53-dependent transcriptional induction and control of Puma protein stability, with Puma degraded in nutrient-replete conditions and long lived in nutrient deficiency. Together, these data identify a key role for Bcl-2 family proteins in Akt-mediated cell survival that may be critical in normal immunity and in cancer through Akt-dependent stimulation of glycolysis to suppress Puma expression.

Mechanism of hypoxia-inducible factor 1 alpha-mediated Mcl1 regulation in Helicobacter pylori-infected human gastric epithelium

Hypoxia-inducible factor 1 (HIF1) consists of a hypoxia-inducible α subunit and a constitutively expressed β subunit. Reactive oxygen species (ROS) induced by Helicobacter pylori stabilize HIF1α in the human gastric epithelium in normoxia. HIF1α plays crucial role in carcinogenesis and has been associated with malignant progression of gastric cancer. Several genes contain functional hypoxia-response elements (HREs) in their promoters including Bcl2 family member, Mcl1. Cellular ratios of antiapoptotic oncogenic protein, Mcl1, and tumor suppressor proapoptotic protein, Noxa, determine cell fate by regulating normal cellular growth, cell death and oncogenic processes. The aim of the present study was to examine the mechanism of HIF1α induction in the H. pylori-infected gastric epithelium to better understand disease pathogenesis by H. pylori relevant to gastric carcinogenesis. Our data showed that the dose-dependent increase in HIF1α in H. pylori-infected gastric epithelia is mediated by induction of a ROS-inducible protein, apurinic/apyrimidinic endonuclease 1 (APE1), and an enhanced interaction of APE1 with the transcriptional coactivator p300. Surprisingly, with accumulation of HIF1α, further transcriptional activation of mcl1 was not observed. We identified a HIF-binding site (HBS) in the hif1α promoter and showed that increased HIF1α expression, whether H. pylori-induced or hypoxia-mimetic agent, CoCl(2)-induced, resulted in enhanced HIF1α binding to its own promoter. This resulted in a transcriptionally inactive hif1α promoter since hif1α HBS lacks HIF ancillary sequence (HAS) required for HIF1 transcriptional activity. We conclude that enhanced binding of "nonfunctional" HIF1α to hif1α promoter and limiting availability of p300 in the cell serves as checkpoints for uncontrolled HIF1α activity.

The metabolic life and times of a T-cell

SUMMARY

The regulation of lymphocyte homeostasis is critical for the development and formation of productive immune responses. Cell numbers must be maintained to allow sufficient numbers of lymphocytes to combat foreign pathogens but prevent the accumulation of excess lymphocytes that may increase the risk of developing autoimmunity or neoplasia. Cell extrinsic growth factors are essential to maintain homeostasis and cell survival, and it has become increasingly apparent that a key mechanism of this control is through regulation of cell metabolism. The metabolic state of T cells can have profound influences on cell growth and survival and even differentiation. In particular, resting T cells utilize an energy efficient oxidative metabolism but shift to a highly glycolytic metabolism when stimulated to grow and proliferate by pathogen encounter. After antigen clearance, T cells must return to a more quiescent oxidative metabolism to support T-cell memory. This review highlights how these metabolic changes may be intricately involved with both T-cell growth and death in the control of homeostasis and immunity.

The engine driving the ship: metabolic steering of cell proliferation and death

Metabolic activity is a crucial determinant of a cell's decision to proliferate or die. Although it is not fully understood how metabolic pathways such as glycolysis and the pentose phosphate pathway communicate to cell cycle and apoptotic effectors, it is clear that a complex network of signalling molecules is required to integrate metabolic inputs. D-type cyclins, cyclin-dependent kinases, the anaphase-promoting complex, p53, caspase 2 and B cell lymphoma 2 proteins, among others, have been shown to be regulated by metabolic crosstalk. Elucidating these pathways is of great importance, as metabolic aberrations and their downstream effects are known to contribute to the aetiology of cancer and degenerative disorders.

Sugar-free approaches to cancer cell killing

Tumors show an increased rate of glucose uptake and utilization. For this reason, glucose analogs are used to visualize tumors by the positron emission tomography technique, and inhibitors of glycolytic metabolism are being tested in clinical trials. Upregulation of glycolysis confers several advantages to tumor cells: it promotes tumor growth and has also been shown to interfere with cell death at multiple levels. Enforcement of glycolysis inhibits apoptosis induced by cytokine deprivation. Conversely, antiglycolytic agents enhance cell death induced by radio- and chemotherapy. Synergistic effects are likely due to regulation of the apoptotic machinery, as glucose regulates activation and levels of proapoptotic BH3-only proteins such as Bim, Bad, Puma and Noxa, as well as the antiapoptotic Bcl-2 family of proteins. Moreover, inhibition of glucose metabolism sensitizes cells to death ligands. Glucose deprivation and antiglycolytic drugs induce tumor cell death, which can proceed through necrosis or through mitochondrial or caspase-8-mediated apoptosis. We will discuss how oncogenic pathways involved in metabolic stress signaling, such as p53, AMPK (adenosine monophosphate-activated protein kinase) and Akt/mTOR (mammalian target of rapamycin), influence sensitivity to inhibition of glucose metabolism. Finally, we will analyze the rationale for the use of antiglycolytic inhibitors in the clinic, either as single agents or as a part of combination therapies.

PI3K inhibitors prime neuroblastoma cells for chemotherapy by shifting the balance towards pro-apoptotic Bcl-2 proteins and enhanced mitochondrial apoptosis

We recently identified activation of phosphatidylinositol 3'-kinase (PI3K)/Akt as a novel predictor of poor outcome in neuroblastoma. Here, we investigated the effect of small-molecule PI3K inhibitors on chemosensitivity. We provide first evidence that PI3K inhibitors, for example PI103, synergize with various chemotherapeutics (Doxorubicin, Etoposide, Topotecan, Cisplatin, Vincristine and Taxol) to trigger apoptosis in neuroblastoma cells (combination index: high synergy). Mechanistic studies reveal that PI103 cooperates with Doxorubicin to reduce Mcl-1 expression and Bim(EL) phosphorylation and to upregulate Noxa and Bim(EL) levels. This shifted ratio of pro- and antiapoptotic Bcl-2 proteins results in increased Bax/Bak conformational change, loss of mitochondrial membrane potential, cytochrome c release, caspase activation and caspase-dependent apoptosis. Although Mcl-1 knockdown enhances Doxorubicin- and PI103-induced apoptosis, silencing of Noxa, Bax/Bak or p53 reduces apoptosis, underscoring the functional relevance of the Doxorubicin- and PI103-mediated modulation of these proteins for chemosensitization. Bcl-2 overexpression inhibits Bax activation, mitochondrial perturbations, cleavage of caspases and Bid, and apoptosis, confirming the central role of the mitochondrial pathway for chemosensitization. Interestingly, the broad-range caspase inhibitor zVAD.fmk does not interfere with Bax activation or mitochondrial outer membrane permeabilization, whereas it blocks caspase activation and apoptosis, thus placing mitochondrial events upstream of caspase activation. Importantly, PI103 and Doxorubicin cooperate to induce apoptosis and to suppress tumor growth in patients' derived primary neuroblastoma cells and in an in vivo neuroblastoma model, underlining the clinical relevance of the results. Thus, targeting PI3K presents a novel and promising strategy to sensitize neuroblastoma cells for chemotherapy-induced apoptosis, which has important implications for the development of targeted therapies for neuroblastoma.

Cell metabolism: an essential link between cell growth and apoptosis

Growth factor-stimulated or cancerous cells require sufficient nutrients to meet the metabolic demands of cell growth and division. If nutrients are insufficient, metabolic checkpoints are triggered that lead to cell cycle arrest and the activation of the intrinsic apoptotic cascade through a process dependent on the Bcl-2 family of proteins. Given the connections between metabolism and apoptosis, the notion of targeting metabolism to induce cell death in cancer cells has recently garnered much attention. However, the signaling pathways by which metabolic stresses induce apoptosis have not as of yet been fully elucidated. Thus, the best approach to this promising therapeutic avenue remains unclear. This review will discuss the intricate links between metabolism, growth, and intrinsic apoptosis and will consider ways in which manipulation of metabolism might be exploited to promote apoptotic cell death in cancer cells. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

An ERK-dependent pathway to Noxa expression regulates apoptosis by platinum-based chemotherapeutic drugs

Cisplatin is a widely used cancer chemotherapeutic that promotes DNA damage-associated apoptosis. Although platinum compounds are known to form DNA adducts and provoke DNA damage, the molecular mechanism of cisplatin-induced cell death remains unclear. In this article, we show that the BH3-only protein Noxa is strongly transcriptionally upregulated in response to cisplatin and related platinum compounds. Cisplatin-induced Noxa expression was ERK dependent, but p53 independent, and inhibition of ERK activation markedly attenuated cisplatin-induced cell death, as well as Noxa expression. Furthermore, siRNA-mediated ablation of Noxa expression also inhibited cisplatin-induced cell death and permitted clonogenic survival. These observations reveal a novel ERK-regulated route to Noxa expression that is important for the cell killing activity of platinum-based chemotherapeutic drugs.

Only the strong survive

Whether apoptosis is relevant for interclonal competition of T cells after antigen encounter has remained uncertain. In this issue of Immunity, Wensveen et al. (2010) establish a critical role for the proapoptotic BH3-only protein Noxa in this selection process.

Apoptosis threshold set by Noxa and Mcl-1 after T cell activation regulates competitive selection of high-affinity clones

The adaptive immune system generates protective T cell responses via a poorly understood selection mechanism that favors expansion of clones with optimal affinity for antigen. Here we showed that upon T cell activation, the proapoptotic molecule Noxa (encoded by Pmaip1) and its antagonist Mcl-1 were induced. During an acute immune response against influenza or ovalbumin, Pmaip1(-/-) effector T cells displayed decreased antigen affinity and functionality. Molecular analysis of influenza-specific T cells revealed persistence of many subdominant clones in the Pmaip1(-/-) effector pool. When competing for low-affinity antigen, Pmaip1(-/-) TCR transgenic T cells had a survival advantage in vitro, resulting in increased numbers of effector cells in vivo. Mcl-1 protein stability was controlled by T cell receptor (TCR) affinity-dependent interleukin-2 signaling. These results establish a role for apoptosis early during T cell expansion, based on antigen-driven competition and survival of the fittest T cells.

Aspirin induces apoptosis in human leukemia cells independently of NF-kappaB and MAPKs through alteration of the Mcl-1/Noxa balance

Aspirin and other non-steroidal anti-inflammatory drugs induce apoptosis in most cell types. In this study we examined the mechanism of aspirin-induced apoptosis in human leukemia cells. We analyzed the role of nuclear factor-kappaB (NF-kappaB) and mitogen-activated protein kinases (MAPKs) pathways. Furthermore, we studied the changes induced by aspirin in some genes involved in the control of apoptosis at mRNA level, by performing reverse transcriptase multiplex ligation-dependent probe amplification (RT-MLPA), and at protein level by Western blot. Our results show that aspirin induced apoptosis in leukemia Jurkat T cells independently of NF-kappaB. Although aspirin induced p38 MAPK and c-Jun N-terminal kinase activation, selective inhibitors of these kinases did not inhibit aspirin-induced apoptosis. We studied the regulation of Bcl-2 family members in aspirin-induced apoptosis. Aspirin increased the mRNA levels of some pro-apoptotic members, such as BIM, NOXA, BMF or PUMA, but their protein levels did not change. In contrast, aspirin decreased the protein levels of Mcl-1. Interestingly, in the presence of aspirin the protein levels of Noxa remained high. This alteration of the Mcl-1/Noxa balance was also found in other leukemia cell lines and primary chronic lymphocytic leukemia cells (CLL). Furthermore, in CLL cells aspirin induced an increase in the protein levels of Noxa. Knockdown of Noxa or Puma significantly attenuated aspirin-induced apoptosis. These results indicate that aspirin induces apoptosis through alteration of the Mcl-1/ Noxa balance.

IL-7 is essential for homeostatic control of T cell metabolism in vivo

It has become apparent that T cells require growth signals to maintain function and viability necessary to maintain proper immune homeostasis. One means by which cell extrinsic signals may mediate these effects is by sustaining sufficient basal cell metabolism to prevent cell atrophy. The role of metabolism and the specific growth factors essential to maintain metabolism of mature T cells in vivo, however, are poorly defined. As IL-7 is a nonredundant cytokine required for T cell development and survival and can regulate T cell metabolism in vitro, we hypothesized it may be essential to sustain metabolism of resting T cells in vivo. Thus, we generated a model for conditional expression of IL-7R in mature T cells. After IL-7R deletion in a generally normal lymphoid environment, T cells had reduced responses to IL-7, including abrogated signaling and maintenance of antiapoptotic Bcl-2 family expression that corresponded to decreased survival in vitro. T cell survival in vivo was also reduced after loss of the IL-7R in a T cell-intrinsic manner. Additionally, IL-7R deletion resulted in delayed growth and proliferation following stimulation. Importantly, in vivo excision of IL-7R led to T cell atrophy that was characterized by delayed mitogenesis and reduced glycolytic flux. These data are the first to identify an in vivo requirement for a specific cell extrinsic signal to sustain lymphocyte metabolism and suggest that control of glycolysis by IL-7R may contribute to the well-described roles of IL-7 in T cell development, homeostatic proliferation, and survival.

Polycomb group gene product Ring1B regulates Th2-driven airway inflammation through the inhibition of Bim-mediated apoptosis of effector Th2 cells in the lung

Polycomb group (PcG) gene products regulate the maintenance of homeobox gene expression in Drosophila and vertebrates. In the immune system, PcG molecules control cell cycle progression of thymocytes, Th2 cell differentiation, and the generation of memory CD4 T cells. In this paper, we extended the study of PcG molecules to the regulation of in vivo Th2 responses, especially allergic airway inflammation, by using conditional Ring1B-deficient mice with a CD4 T cell-specific deletion of the Ring1B gene (Ring1B(-/-) mice). In Ring1B(-/-) mice, CD4 T cell development appeared to be normal, whereas the differentiation of Th2 cells but not Th1 cells was moderately impaired. In an Ag-induced Th2-driven allergic airway inflammation model, eosinophilic inflammation was attenuated in Ring1B(-/-) mice. Interestingly, Ring1B(-/-) effector Th2 cells were highly susceptible to apoptosis in comparison with wild-type effector Th2 cells in vivo and in vitro. The in vitro experiments revealed that the expression of Bim was increased at both the transcriptional and protein levels in Ring1B(-/-) effector Th2 cells, and the enhanced apoptosis in Ring1B(-/-) Th2 cells was rescued by the knockdown of Bim but not the other proapoptotic genes, such as Perp, Noxa, or Bax. The enhanced apoptosis detected in the transferred Ring1B(-/-) Th2 cells in the lung of the recipient mice was also rescued by knockdown of Bim. Therefore, these results indicate that Ring1B plays an important role in Th2-driven allergic airway inflammation through the control of Bim-dependent apoptosis of effector Th2 cells in vivo.

Glucose deprivation induces an atypical form of apoptosis mediated by caspase-8 in Bax-, Bak-deficient cells

Apoptosis induced by most stimuli proceeds through the mitochondrial pathway. One such stimulus is nutrient deprivation. In this study we studied death induced by glucose deprivation in cells deficient in Bax and Bak. These cells cannot undergo mitochondrial outer membrane permeabilization (MOMP) during apoptosis, but they undergo necrosis when treated with MOMP-dependent apoptotic stimuli. We find in these cells that glucose deprivation, rather than inducing necrosis, triggered apoptosis. Cell death required caspase activation as inhibition of caspases with peptidic inhibitors prevented death. Glucose deprivation-induced death displayed many hallmarks of apoptosis, such as caspase cleavage and activity, phosphatidyl-serine exposure and cleavage of caspase substrates. Neither overexpression of Bcl-xL nor knockdown of caspase-9 prevented death. However, transient or stable knockdown of caspase-8 or overexpression of CrmA inhibited apoptosis. Cell death was not inhibited by preventing death receptor-ligand interactions, by overexpression of c-FLIP or by knockdown of RIPK1. Glucose deprivation induced apoptosis in the human tumor cell line HeLa, which was prevented by knockdown of caspase-8. Thus, we have found that glucose deprivation can induce a death receptor-independent, caspase-8-driven apoptosis, which is engaged to kill cells that cannot undergo MOMP.

Metabolism in T cell activation and differentiation

When naïve or memory T cells encounter foreign antigen along with proper co-stimulation they undergo rapid and extensive clonal expansion. In mammals, this type of proliferation is fairly unique to cells of the adaptive immune system and requires a considerable expenditure of energy and cellular resources. While research has often focused on the roles of cytokines, antigenic signals, and co-stimulation in guiding T cell responses, data indicate that, at a fundamental level, it is cellular metabolism that regulates T cell function and differentiation and therefore influences the final outcome of the adaptive immune response. This review will focus on some earlier fundamental observations regarding T cell bioenergetics and its role in regulating cellular function, as well as recent work that suggests that manipulating the immune response by targeting lymphocyte metabolism could prove useful in treatments against infection and cancer.

Different contribution of BH3-only proteins and caspases to doxorubicin-induced apoptosis in p53-deficient leukemia cells

Bcl-2 family proteins are key regulators of the intrinsic apoptotic pathway, either facilitating (Bax, Bak, BH3-only) or inhibiting (Bcl-2, Bcl-x(L), Mcl-1, A1) mitochondrial release of apoptogenic factors. The role of caspases in this process is a matter of controversy. We have analyzed the relative contribution of caspases and Bcl-2 family of proteins in the induction phase of apoptosis triggered by doxorubicin in two p53-deficient leukemia cell lines, Jurkat and U937. First, we have found that caspases are dispensable for the induction phase of doxorubicin-induced apoptosis in both cell lines but they are needed to speed up the execution phase in Jurkat cells, not expressing Bax. Thus, down-regulation of Bak expression by siRNA significantly prevented doxorubicin-induced apoptosis in Jurkat but not in U937 cells. Reduction of Mcl-1 protein levels with siRNA increased sensitivity to apoptosis in both cell lines. Moreover, our results indicate that the contribution of BH3-only proteins to apoptosis is cell line specific. In Jurkat cells simultaneous silencing of Bim and PUMA was necessary to reduce doxorubicin-induced apoptosis. In U937 cells silencing of Bim or Noxa reduced sensitivity to doxorubicin. Immunoprecipitation experiments discarded an interaction between Mcl-1 and Bak in both cell lines and underscored the role of Bim and PUMA as mediators of Bax/Bak activation.

The role of glucocorticoid receptor phosphorylation in Mcl-1 and NOXA gene expression

Background

The cyclin-dependent kinase (CDK) and mitogen-activated protein kinase (MAPK) mediated phosphorylation of glucocorticoid receptor (GR) exerts opposite effects on GR transcriptional activity and affects other posttranslational modifications within this protein. The major phosphorylation site of human GR targeted by MAPK family is the serine 226 and multiple kinase complexes phosphorylate receptor at the serine 211 residue. We hypothesize that GR posttranslational modifications are involved in the determination of the cellular fate in human lymphoblastic leukemia cells. We investigated whether UV signalling through alternative GR phosphorylation determined the cell type specificity of glucocorticoids (GCs) mediated apoptosis.

Results

We have identified putative Glucocorticoid Response Elements (GREs) within the promoter regulatory regions of the Bcl-2 family members NOXA and Mcl-1 indicating that they are direct GR transcriptional targets. These genes were differentially regulated in CEM-C7-14, CEM-C1-15 and A549 cells by glucocorticoids and JNK pathway. In addition, our results revealed that the S211 phosphorylation was dominant in CEM-C7-14, whereas the opposite was the case in CEM-C1-15 where prevalence of S226 GR phosphorylation was observed. Furthermore, multiple GR isoforms with cell line specific patterns were identified in CEM-C7-14 cells compared to CEM-C1-15 and A549 cell lines with the same antibodies.

Conclusions

GR phosphorylation status kinetics, and site specificity as well as isoform variability differ in CEM-C7-14, CEM-C1-15, and A549 cells. The positive or negative response to GCs induced apoptosis in these cell lines is a consequence of the variable equilibrium of NOXA and Mcl-1 gene expression potentially mediated by alternatively phosphorylated GR, as well as the balance of MAPK/CDK pathways controlling GR phosphorylation pattern. Our results provide molecular base and valuable knowledge for improving the GC based therapies of leukaemia.

Proapoptotic signalling through Toll-like receptor-3 involves TRIF-dependent activation of caspase-8 and is under the control of inhibitor of apoptosis proteins in melanoma cells

Toll-like receptor-3 (TLR3), a member of an immune recognition receptor family, is widely expressed in tumour cells and has been shown previously to have the capacity to not only activate immune signalling pathways, but also to exert proapoptotic activity in some cells. We show here that HaCaT human keratinocytes are susceptible to apoptosis induction by the TLR3 ligand poly I:C, and use these cells as a model to analyse the apoptotic signalling pathway. Although the BH3-only protein Noxa was transcriptionally induced by poly I:C and translocated to mitochondria, RNAi experiments showed that the BH3-only proteins Noxa, Bim and Puma were individually dispensable for poly I:C-induced apoptosis. Instead, poly I:C-induced activation of caspase-8 via TLR3 and its adapter TRIF was required for apoptosis. In human melanoma cell lines poly I:C failed to induce apoptosis unless protein synthesis was blocked. Significantly, sensitisation towards poly I:C-dependent caspase-8 activation and apoptosis in melanoma cells was also achieved by the synthetic Smac mimetic/inhibitor of apoptosis protein (IAP) antagonist, LBW242, or by specific downregulation of cIAP1 by siRNA. Inactivation of caspase-8 by CrmA overexpression reduced poly I:C/LBW242-induced apoptosis. These results indicate that the proapoptotic activity of TLR3/TRIF/caspase-8 in melanoma cells is under the control of IAPs, and the use of novel Smac mimetics might be a feasible approach to target melanoma.

Glycolysis inhibition sensitizes tumor cells to death receptors-induced apoptosis by AMP kinase activation leading to Mcl-1 block in translation

Most cancer cells exhibit increased glycolysis for generation of their energy supply. This specificity could be used to preferentially kill these cells. In this study, we identified the signaling pathway initiated by glycolysis inhibition that results in sensitization to death receptor (DR)-induced apoptosis. We showed, in several human cancer cell lines (such as Jurkat, HeLa, U937), that glucose removal or the use of nonmetabolizable form of glucose (2-deoxyglucose) dramatically enhances apoptosis induced by Fas or by tumor necrosis factor-related apoptosis-inducing ligand. This sensitization is controlled through the adenosine monophosphate (AMP)-activated protein kinase (AMPK), which is the central energy-sensing system of the cell. We established the fact that AMPK is activated upon glycolysis block resulting in mammalian target of rapamycin (mTOR) inhibition leading to Mcl-1 decrease, but no other Bcl-2 anti-apoptotic members. Interestingly, we determined that, upon glycolysis inhibition, the AMPK-mTOR pathway controlled Mcl-1 levels neither through transcriptional nor through posttranslational mechanism but rather by controlling its translation. Therefore, our results show a novel mechanism for the sensitization to DR-induced apoptosis linking glucose metabolism to Mcl-1 downexpression. In addition, this study provides a rationale for the combined use of DR ligands with AMPK activators or mTOR inhibitors in the treatment of human cancers.

Interleukin-7 regulates Bim proapoptotic activity in peripheral T-cell survival

Interleukin-7 (IL-7) is critical for T-cell development and peripheral T-cell homeostasis. The survival of pro-T cells and mature T cells requires IL-7. The survival function of IL-7 is accomplished partly through induction of the antiapoptotic protein Bcl-2 and inhibition of proapoptotic proteins Bax and Bad. We show here that the proapoptotic protein Bim, a BH3-only protein belonging to the Bcl-2 family, also plays a role in peripheral T-cell survival. Deletion of Bim partially protected an IL-7-dependent T-cell line and peripheral T cells, especially cells with an effector memory phenotype, from IL-7 deprivation. However, T-cell development in the thymus was not restored in IL-7(-/-) Rag2(-/-) mice reconstituted with Bim(-/-) bone marrow. IL-7 withdrawal altered neither the intracellular location of Bim, which was constitutively mitochondrial, nor its association with Bcl-2; however, a reduction in its association with the prosurvival protein Mcl-1 was observed. IL-7 withdrawal did not increase Bim mRNA or protein expression but did induce changes in the isoelectric point of Bim(EL) and its reactivity with an antiphosphoserine antibody. Our findings suggest that the maintenance of peripheral T cells by IL-7 occurs partly through inhibition of Bim activity at the posttranslational level.

Bim is the key mediator of glucocorticoid-induced apoptosis and of its potentiation by rapamycin in human myeloma cells

Glucocorticoids are widely used in anti-myeloma therapy and their action is potentiated by rapamycin, a mTOR inhibitor. However, the molecular mechanisms underlying these effects remain poorly characterized. We show here that dexamethasone (Dex)-induced apoptosis in MM.1S and OPM-2 cells is characterized by Bax and Bak conformational changes, DeltaPsi(m) loss, cytochrome c release and caspase-3 activation. Rapamycin, which had minimal cytotoxic effect by itself, strongly potentiated Dex-induced apoptosis. Apoptotic gene expression profiling showed an increase in mRNA levels of Bim in MM.1S cells after Dex treatment and further increases in both cell lines when co-treated with rapamycin. Western blot analysis revealed a moderate increase in Bim protein levels in both MM.1S and OPM-2 cells. Immunoprecipitation experiments revealed that most Bim was complexed to Mcl-1 in untreated cells. Upon treatment with Dex, and specially Dex plus rapamycin, Bim-Mcl-1 complex was disrupted and Bim was found associated to a CHAPS-insoluble fraction. Overexpression of Mcl-1 stabilized Bim-Mcl-1 complexes upon treatment with Dex or Dex+rapamycin and fully prevented apoptosis. Gene silencing of Bim inhibited for the most part Dex-induced apoptosis and, to a large extent, apoptosis induced by Dex plus rapamycin. These results, taken together, indicate that Bim protein is the key mediator of apoptosis induced by Dex and also responsible for the potentiating effect of rapamycin, providing molecular criteria for the use of glucocorticoids combined with mTOR inhibitors in myeloma therapy.

Noxa: at the tip of the balance between life and death

Among all Bcl2 homology domain 3 (BH3)-only proteins known to date, APR/PMAIP1/Noxa, albeit showing weak proapoptotic potential on its own, appears to be crucial in fine-tuning cell death decisions by targeting the prosurvival molecule Mcl1 for proteasomal degradation. This event appears critical for cell death induction along the mitochondrial Bcl2-regulated apoptosis pathway in response to factor deprivation or DNA damage, presumably by sensitizing the cell toward the action of additional BH3-only protein family members. This review aims to summarize the function of Noxa in normal physiology, stress-induced cell death and tumorigenesis.

Differential effects of anti-apoptotic Bcl-2 family members Mcl-1, Bcl-2, and Bcl-xL on celecoxib-induced apoptosis

The cyclooxygenase-2 inhibitor Celecoxib is a potent inducer of apoptosis in tumor cells. In most cellular systems Celecoxib induces apoptosis via an intrinsic, mitochondrial apoptosis pathway. We recently showed that in Bax-negative Jurkat cells expression of pro-apoptotic Bak is essential for Celecoxib-induced mitochondrial damage and apoptosis induction. Aim of the present study was to identify specific pro- and anti-apoptotic members of the Bcl-2 family involved in the regulation of Bak activation, and subsequent apoptosis upon treatment with Celecoxib in the Jurkat cell model. Our results show that apoptosis in response to Celecoxib required the presence of Noxa and downregulation of the anti-apoptotic protein Mcl-1. Celecoxib-induced Bak activation and subsequent apoptosis could be inhibited by overexpression of Bcl-xL but not by the very similar Bcl-2. In Bcl-xL-overexpressing cells neutralization of both, Mcl-1 and Bcl-xL, was prerequisite for an efficient induction of apoptosis. Our data reveal an important role of the Mcl-1/Noxa axis for Celecoxib-induced apoptosis and suggest that Celecoxib may be of value for treatment of tumors addicted to Mcl-1 and for combined treatment approaches targeting anti-apoptotic Bcl-2 family members.

The presumed hyporesponsive behavior of rheumatoid arthritis T lymphocytes can be attributed to spontaneous ex vivo apoptosis rather than defects in T cell receptor signaling

Genetic associations and the clinical success of compounds targeting TCR costimulatory proteins suggest an active role for TCR signaling in the initiation and perpetuation of rheumatoid arthritis (RA). Paradoxically, T cells isolated from affected joints in RA show impaired proliferative and cytokine responses following stimulation with mitogens and recall Ags attributed in part to chronic T cell exposure to oxidative stress and inflammatory cytokines. Therefore, it is uncertain how local autoreactive TCR signaling contributes to pathology in established RA. Using single-cell analysis, we show that in contrast to results obtained in bulk culture assays, T cells from the synovial fluid of RA patients proliferate and produce cytokines (IL-2, TNF-alpha, and IFN-gamma) as efficiently, if not more so, than T cells isolated from healthy donors and RA patient peripheral blood following TCR/CD28 stimulation. RA synovial fluid T cell hyporesponsiveness observed in bulk cultures can be attributed to spontaneous apoptosis ex vivo, which is associated with altered ratios of proapoptotic Noxa and anti-apoptotic Mcl-1 expression. The absence of RA synovial T cell proliferation and cytokine production in situ, despite the capacity of these cells to support productive TCR signaling, suggests that T cells contribute to local pathology in established RA by TCR-independent mechanisms.

Alterations in the Noxa/Mcl-1 axis determine sensitivity of small cell lung cancer to the BH3 mimetic ABT-737

To understand the molecular basis for variable sensitivity to the BH3 mimetic drug ABT-737, the abundance of Bcl-2 family members was assayed in a panel of small cell lung cancer cell lines whose sensitivity varied over a 2-log range. Elevated Noxa and Bcl-2 levels directly correlated with sensitivity to ABT-737, whereas Mcl-1 levels were similar in all cell lines tested regardless of sensitivity. Transgenically enforced expression of Noxa but not Bcl-2 resulted in increased sensitivity to ABT-737 in multiple cell lines. This increase was especially pronounced in the H209 cell line in which expression of Noxa resulted in a proportionate decline in Mcl-1 expression. Although overexpression of Noxa enhanced sensitivity of the H526 and H82 cell lines to ABT-737, it did not result in altered Mcl-1 levels. Similarly, small interfering RNA-mediated knockdown of Noxa expression in the H146 cell line, which increased resistance to ABT-737, did not result in altered Mcl-1 levels. Therefore, three of four cell lines studied failed to show Noxa-mediated regulation of Mcl-1 expression. However, despite failure to regulate Mcl-1 levels, Noxa blocked binding of Bim to Mcl-1 following its release from Bcl-2 by ABT-737. Finally, we observed that a 24-hour incubation of the H526 and WBA cell lines with ABT-737 resulted in increased Noxa expression, suggesting that Noxa may play a direct role in ABT-737-mediated apoptosis. These results indicate that Noxa expression is the critical determinant of ABT-737 sensitivity and loss of Noxa-mediated regulation of Mcl-1 expression may be an important feature of small cell lung cancer biology.

The urokinase receptor (uPAR) facilitates clearance of Borrelia burgdorferi

The causative agent of Lyme borreliosis, the spirochete Borrelia burgdorferi, has been shown to induce expression of the urokinase receptor (uPAR); however, the role of uPAR in the immune response against Borrelia has never been investigated. uPAR not only acts as a proteinase receptor, but can also, dependently or independently of ligation to uPA, directly affect leukocyte function. We here demonstrate that uPAR is upregulated on murine and human leukocytes upon exposure to B. burgdorferi both in vitro as well as in vivo. Notably, B. burgdorferi-inoculated C57BL/6 uPAR knock-out mice harbored significantly higher Borrelia numbers compared to WT controls. This was associated with impaired phagocytotic capacity of B. burgdorferi by uPAR knock-out leukocytes in vitro. B. burgdorferi numbers in vivo, and phagocytotic capacity in vitro, were unaltered in uPA, tPA (low fibrinolytic activity) and PAI-1 (high fibrinolytic activity) knock-out mice compared to WT controls. Strikingly, in uPAR knock-out mice partially backcrossed to a B. burgdorferi susceptible C3H/HeN background, higher B. burgdorferi numbers were associated with more severe carditis and increased local TLR2 and IL-1β mRNA expression. In conclusion, in B. burgdorferi infection, uPAR is required for phagocytosis and adequate eradication of the spirochete from the heart by a mechanism that is independent of binding of uPAR to uPA or its role in the fibrinolytic system.

Lyme borreliosis is caused by the spirochete Borrelia burgdorferi and is transmitted through ticks. Since its discovery approximately 30 years ago it has become the most important vector-borne disease in the Western world. The pathogenesis of this complex zoonosis is still not entirely understood. We here demonstrate that the urokinase receptor (uPAR) is upregulated in mice and humans upon exposure to B. burgdorferi in vitro and in vivo. Importantly, we describe the function of uPAR in the immune response against the spirochete; using uPAR knock-out mice, we show that uPAR plays an important role in phagocytosis of B. burgdorferi by leukocytes both in vitro as well as in vivo. In addition, we show that the mechanism by which uPAR is involved in the phagocytosis of B. burgdorferi is independent of ligation to its natural ligand uPA or uPAR's role in fibrinolysis. Our study contributes to the understanding of the pathogenesis of Lyme borreliosis and might contribute to the development of innovative novel treatment strategies for Lyme borreliosis.

Loss of the pro-apoptotic BH3-only Bcl-2 family member Bim sustains B lymphopoiesis in the absence of IL-7

IL-7 is pivotal for B cell development. Proteins of the Bcl-2 family are essential regulators of lymphocyte survival. Particularly, the pro-apoptotic BH3-only members Bim and Puma mediate lymphocyte apoptosis provoked by cytokine deprivation. Herein, we addressed whether the absence of Bim or Puma within the hematopoietic compartment could bypass the requirement for IL-7-driven B cell development in adult mice. We found that deficiency of Bim, but not Puma, partially rescued B cell development in the absence of IL-7. The numbers of both sIgM(-) and sIgM(+) B cells were markedly increased in the bone marrow of recipients lacking IL-7 upon reconstitution with Bim-deficient hematopoietic progenitors, compared with their control or Puma-deficient counterparts. The augmentation of B cell lymphopoiesis in the absence of Bim was reflected in the mature peripheral compartment by an increase in both the number of immature and mature B cells in the spleen and in the circulating IgM levels. Bim-deficient B cells were also increased in IL-7-sufficient recipients suggesting that peripheral B cells homeostasis is governed by a Bim-dependent and IL-7-independent mechanism. Our data highlight the role of Bim as a key regulator of cell survival during B lymphocyte development in vivo.

Glucose metabolism attenuates p53 and Puma-dependent cell death upon growth factor deprivation

Growth factor stimulation and oncogenic transformation lead to increased glucose metabolism that may provide resistance to cell death. We have previously demonstrated that elevated glucose metabolism characteristic of stimulated or cancerous cells can stabilize the anti-apoptotic Bcl-2 family protein Mcl-1 through inhibition of GSK-3. Here we show that the pro-apoptotic Bcl-2 family protein, Puma, is also metabolically regulated. Growth factor deprivation led to the loss of glucose uptake and induction of Puma. Maintenance of glucose uptake after growth factor withdrawal by expression of the glucose transporter, Glut1, however, suppressed Puma up-regulation and attenuated growth factor withdrawal-induced activation of Bax, DNA fragmentation, and cell death. Conversely, glucose deprivation led to Puma induction even in the presence of growth factor. This regulation of Puma expression was a central component in cell death as a consequence of growth factor or glucose deprivation because Puma deficiency suppressed both of these cell death pathways. Puma induction in growth factor or glucose withdrawal was dependent on p53 in cell lines and in activated primary T lymphocytes because p53 deficiency suppressed Puma induction and delayed Bax and caspase activation, DNA fragmentation, and loss of clonogenic survival. Importantly, although p53 levels did not change or were slightly reduced, p53 activity was suppressed by elevated glucose metabolism to inhibit Puma induction after growth factor withdrawal. These data show that p53 is metabolically regulated and that glucose metabolism initiates a signaling mechanism to inhibit p53 activation and suppress Puma induction, thus promoting an anti-apoptotic balance to Bcl-2 family protein expression that supports cell survival.

Induction of Noxa sensitizes human colorectal cancer cells expressing Mcl-1 to the small-molecule Bcl-2/Bcl-xL inhibitor, ABT-737

PURPOSE

The intrinsic drug resistance of colorectal cancers is related in part to overexpression of prosurvival Bcl-2 family proteins. We determined the effects of ABT-737, a small-molecule inhibitor of Bcl-2/Bcl-xL but not Mcl-1, on apoptosis induction alone and in combination with CPT-11 and explored mechanisms underlying their cooperativity.

EXPERIMENTAL DESIGN

Human colorectal carcinoma cell lines (HCT116 wild-type and Bax(-/-), HT-29, and RKO) were incubated with ABT-737 alone and combined with CPT-11 or bortezomib, and cell viability, caspase cleavage, and Annexin V labeling were measured. In drug-treated cell lines, protein-protein interactions were analyzed by immunoprecipitation. Lentiviral short hairpin RNA was used to knockdown Noxa expression.

RESULTS

ABT-737 induced apoptosis in a dose-dependent manner and its coadministration with the topoisomerase I inhibitor, CPT-11, resulted in a synergistic cytotoxic effect. Apoptosis induction by the drug combination was associated with enhanced caspase-8, caspase-9, and caspase-3 activation and poly(ADP-ribose) polymerase cleavage that were completely abrogated in Bax knockout cells. ABT-737 unsequestered the BH3-only protein Bim from its complex with Bcl-xL or Bcl-2 and disrupted the interaction of Bcl-xL with Bak. CPT-11 treatment up-regulated Noxa expression, as did bortezomib, and enhanced Noxa/Mcl-1 complexes. CPT-11 also disrupted the Mcl-1/Bak interaction. Knockdown of Noxa using short hairpin RNA lentiviral constructs was shown to significantly attenuate the cytotoxic effect of CPT-11 or bortezomib combined with ABT-737 and inhibited caspase-3 cleavage.

CONCLUSIONS

Induction of Noxa by CPT-11 or bortezomib can sensitize colorectal cancer cells expressing Mcl-1 to ABT-737. Up-regulation of Noxa may therefore represent an important strategy to enhance the therapeutic efficacy of ABT-737 against colorectal cancer and other solid tumors.

A global transcriptional view of apoptosis in human T-cell activation

Background

T-cell activation is an essential step of immune response. The process of proper T-cell activation is strictly monitored and regulated by apoptosis signaling. Yet, regulation of apoptosis, an integral and crucial facet during the process of T-cell activation, is not well understood.

Methods

In this study, a Gene-Ontology driven global gene expression analysis coupled with protein abundance and activity assays identified genes and pathways associated with regulation of apoptosis in primary human CD3+ T cells and separately CD4+ and CD8+ T cells.

Results

We identified significantly regulated apoptotic genes in several protein families, such as BCL2 proteins, CASPASE proteins, and TNF receptors, and detailed their transcriptional kinetics during the T-cell activation process. Transcriptional patterns of a few select genes (BCL2A1, BBC3 and CASP3) were validated at the protein level. Many of these apoptotic genes are involved in NF-κB signaling pathway, including TNFRSF10A, TNFRSF10B, TRAF4, TRAF1, TRAF3, and TRAF6. Upregulation of NF-κB and IκB family genes (REL, RELA, and RELB, NFKBIA, NFKBIE and NFKB1) at 48 to 96 hours, supported by the increase of phosphorylated RELA (p65), suggests that the involvement of the NF-κB complex in the process of T-cell proliferation is not only regulated at the protein level but also at the transcriptional level. Examination of genes involved in MAP kinase signalling pathway, important in apoptosis, suggests an induction of p38 and ERK1 cascades in T-cell proliferation (at 48 to 96 hours), which was explored using phosphorylation assays for p38 (MAPK14) and ERK1 (MAPK3). An immediate and short-lived increase of AP-1 activity measured by DNA-binding activity suggests a rapid and transient activation of p38 and/or JNK cascades upon T-cell activation.

Conclusion

This comparative genome-scale, transcriptional analysis of T-cell activation in the CD4+ and CD8+ subsets and the mixed CD3+ population identified many apoptosis genes not previously identified in the context of T-cell activation. Furthermore, it provided a comprehensive temporal analysis of the transcriptional program of apoptosis associated with T-cell activation.

LXR signaling couples sterol metabolism to proliferation in the acquired immune response

Cholesterol is essential for membrane synthesis; however, the mechanisms that link cellular lipid metabolism to proliferation are incompletely understood. We demonstrate here that cellular cholesterol levels in dividing T cells are maintained in part through reciprocal regulation of the LXR and SREBP transcriptional programs. T cell activation triggers induction of the oxysterol-metabolizing enzyme SULT2B1, consequent suppression of the LXR pathway for cholesterol transport, and promotion of the SREBP pathway for cholesterol synthesis. Ligation of LXR during T cell activation inhibits mitogen-driven expansion, whereas loss of LXRbeta confers a proliferative advantage. Inactivation of the sterol transporter ABCG1 uncouples LXR signaling from proliferation, directly linking sterol homeostasis to the antiproliferative action of LXR. Mice lacking LXRbeta exhibit lymphoid hyperplasia and enhanced responses to antigenic challenge, indicating that proper regulation of LXR-dependent sterol metabolism is important for immune responses. These results implicate LXR signaling in a metabolic checkpoint that modulates cell proliferation and immunity.

Mechanisms of regulatory diversity within the p53 transcriptional network

p53 is arguably the most intensively studied protein to date, yet there is much that we ignore about its function as a transcription factor. The p53-dependent transcriptional program is remarkably flexible, as it varies with the nature of p53-activating stimuli, the cell type and the duration of the activation signal. This flexibility may allow cells to mount alternative responses to p53 activation, such as cell cycle arrest or apoptosis. Here, I organize the available data into two alternative models to explain how this regulatory diversity is achieved.

Differential regulation of human IL-7 receptor alpha expression by IL-7 and TCR signaling

IL-7Ralpha is essential for the development and homeostatic maintenance of mature T cells. Studies in humans and mice have shown that IL-7Ralpha expression is reduced by its cognate cytokine, IL-7, and Ag, suggesting that active regulation of IL-7 responsiveness is necessary to balance T cell numbers. We show that IL-7- or TCR/CD28-mediated signaling induced a rapid down-regulation of IL-7Ralpha expression on naive T cells on the mRNA and protein level, with a mild (10-fold) or strong (50-fold) gene suppression, respectively. In both situations, the down-regulation of IL-7Ralpha was blocked by cyclohexamide and actinomycin D, indicating the involvement of an active mechanism dependent on new transcription and protein synthesis. Upon IL-7 withdrawal, IL-7Ralpha mRNA and surface protein reappeared in a transcription-dependent manner within 7 h. Yet, IL-7Ralpha was hardly re-expressed during the same period after TCR/CD28-activation. Likewise, T cells that were activated through CMV in vivo did not re-express IL-7Ralpha after in vitro culture. Functionally, IL-7-induced down-regulation of IL-7Ralpha did not hinder the responsiveness of naive T cells to IL-7. Conversely, down-regulation of IL-7Ralpha on TCR/CD28-activated cells limited IL-7 responsiveness. Strikingly, ectopic expression of IL-7Ralpha cells on TCR/CD28-activated cells conferred a selective advantage in the response to IL-7. In conclusion, our data show that IL-7- and TCR/CD28-mediated signaling differentially regulate IL-7Ralpha expression on human T cells with a transient and chronic effect, respectively. The stringent and active regulation of IL-7Ralpha may constitute a homeostatic mechanism to curtail unwarranted T cell expansion.

Increased glucose availability activates chicken thymocyte metabolism and survival

Glucose metabolism in mammalian thymocytes is coupled to their development and selection in the thymus. In chickens, thymocytes develop in a low glucose concentration in ovo and a high glucose concentration posthatch. To determine the effect of glucose concentration on thymocyte glucose metabolism, embryonic thymic lobes were cultured in media containing varying glucose concentrations and thymocytes were isolated for analysis. Glucose transporter-1 (Glut-1) and Glut-3 mRNA abundance was at least 2-fold higher in thymocytes incubated with 10 and 20 mmol/L glucose than in those incubated with 0 mmol/L glucose (P < 0.05) and glucose uptake was greatest in thymocytes incubated with 20 mmol/L glucose (P < 0.05). Thymocytes incubated with 0 and 20 mmol/L glucose had 185% greater hexokinase activity than in those incubated with 10 mmol/L glucose (P < 0.05). Consequently, thymocyte glucose utilization was dependent upon glucose availability. Increased glucose utilization resulted in a higher mitochondrial membrane potential in thymocytes incubated with 15 mmol/L glucose than in those incubated with 5 mmol/L glucose (P < 0.05), indicating enhanced thymocyte energy status in response to high glucose concentrations. Additionally, thymocyte viability was lower in thymocytes incubated with 5 mmol/L glucose than in those incubated with 10 and 15 mmol/L glucose (P < 0.05) and rates of thymocyte apoptosis were higher in thymocytes incubated with 5 mmol/L glucose than in those incubated with 15 mmol/L glucose (P < 0.05). Glucose availability induced metabolic changes in thymocytes that altered their energy status and survival. Consequently, these data indicate that glucose availability may influence the development of naïve T cells in the chicken thymus.

Bmi1 regulates memory CD4 T cell survival via repression of the Noxa gene

The maintenance of memory T cells is central to the establishment of immunological memory, although molecular details of the process are poorly understood. In the absence of the polycomb group (PcG) gene Bmi1, the number of memory CD4⁺ T helper (Th)1/Th2 cells was reduced significantly. Enhanced cell death of Bmi1^−/− memory Th2 cells was observed both in vivo and in vitro. Among various proapoptotic genes that are regulated by Bmi1, the expression of proapoptotic BH3-only protein Noxa was increased in Bmi1^−/− effector Th1/Th2 cells. The generation of memory Th2 cells was restored by the deletion of Noxa, but not by Ink4a and Arf. Direct binding of Bmi1 to the Noxa gene locus was accompanied by histone H3-K27 methylation. The recruitment of other PcG gene products and Dnmt1 to the Noxa gene was highly dependent on the expression of Bmi1. In addition, Bmi1 was required for DNA CpG methylation of the Noxa gene. Moreover, memory Th2-dependent airway inflammation was attenuated substantially in the absence of Bmi1. Thus, Bmi1 controls memory CD4⁺ Th1/Th2 cell survival and function through the direct repression of the Noxa gene.

Glucose uptake is limiting in T cell activation and requires CD28-mediated Akt-dependent and independent pathways

T cell activation potently stimulates cellular metabolism to support the elevated energetic and biosynthetic demands of growth, proliferation, and effector function. We show that glucose uptake is limiting in T cell activation and that CD28 costimulation is required to allow maximal glucose uptake following TCR stimulation by up-regulating expression and promoting the cell surface trafficking of the glucose transporter Glut1. Regulation of T cell glucose uptake and Glut1 was critical, as low glucose prevented appropriate T cell responses. Additionally, transgenic expression of Glut1 augmented T cell activation, and led to accumulation of readily activated memory-phenotype T cells with signs of autoimmunity in aged mice. To further examine the regulation of glucose uptake, we analyzed CD28 activation of Akt, which appeared necessary for maximal glucose uptake of stimulated cells and which we have shown can promote Glut1 cell surface trafficking. Consistent with a role for Akt in Glut1 trafficking, transgenic expression of constitutively active myristoylated Akt increased glucose uptake of resting T cells, but did not alter Glut1 protein levels. Therefore, CD28 appeared to promote Akt-independent up-regulation of Glut1 and Akt-dependent Glut1 cell surface trafficking. In support of this model, coexpression of Glut1 and myristoylated Akt transgenes resulted in a synergistic increase in glucose uptake and accumulation of activated T cells in vivo that were largely independent of CD28. Induction of Glut1 protein and Akt regulation of Glut1 trafficking are therefore separable functions of CD28 costimulation that cooperate to promote glucose metabolism for T cell activation and proliferation.

Mechanisms and methods in glucose metabolism and cell death

Glucose metabolism represents a critical physiological program that not only provides energy to support cell proliferation, but also directly modulates signaling pathways of cell death. With the growing recognition of regulation of cell death by glucose metabolism, many techniques that can be applied in the study have been developed. This chapter discusses several protocols that aid in the analysis of glucose metabolism and cell death and the principles in practicing them under different conditions.

Noxa/Mcl-1 balance regulates susceptibility of cells to camptothecin-induced apoptosis

Although camptothecin (CPT) has been reported to induce apoptosis in various cancer cells, the molecular details of this regulation remain largely unknown. In this study, we demonstrate that BH3-only protein Noxa is upregulated during CPT-induced apoptosis, which is independent of p53. In addition, we show that phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway is responsible for Noxa's induction. Luciferase assay and cAMP response element binding protein (CREB) knockdown experiments further demonstrate that CREB is involved in the transcriptional upregulation of Noxa. Moreover, blocking Noxa expression using specific small interfering ribonucleic acid (siRNA) significantly reduces the apoptosis in response to CPT, indicating that Noxa is an essential mediator for CPT-induced apoptosis. Interestingly, antiapoptotic Mcl-1 was also upregulated through PI3K/Akt signaling pathway upon CPT treatment. Using immunoprecipitation assay, Noxa was found to interact with Mcl-1 in the presence or absence of CPT. Knockdown of Mcl-1 expression by short hairpin ribonucleic acid (shRNA) was shown to potentiate CPT-induced apoptosis. Consistently, ectopic overexpression of Mcl-1 rescued cells from apoptosis induced by CPT. Cells coexpressing Noxa and Mcl-1 at different ratio correlates well with the extent of apoptosis, suggesting that the balance between Noxa and Mcl-1 may determine the susceptibility of HeLa cells to CPT-induced apoptosis.

Actinomycin D upregulates proapoptotic protein Puma and downregulates Bcl-2 mRNA in normal peripheral blood lymphocytes

We have examined the ability of actinomycin D to induce apoptosis in human peripheral blood lymphocytes. Run-On assays were performed to specify the primary molecular damage, reverse transcription-PCR, Western blots and flow cytometry studies were performed to ascertain which proteins of the apoptosis machinery were affected to cause actinomycin D-induced cell death. Expression of 23 apoptosis-related genes was investigated. The down-regulation of ribosomal RNA synthesis caused by actinomycin D induced a mitochondria-dependent apoptosis. Although the expression of the majority of examined genes remained indifferent against actinomycin D activity, the cellular level of p53 protein increased, subsequently upregulating both Puma mRNA and protein. Puma-mediated mitochondrial apoptosis was accompanied by nucleolin cleavage and Bcl-2 mRNA destabilization. The stability of the cellular level of Bcl-2 protein independent of a mRNA decrease suggests that protection of Bcl-2 protein against proteasomal degradation can moderate the apoptotic process. In peripheral blood lymphocytes cultured in vitro, the apoptosis induced by a low concentration of actinomycin D (10 nmol/l) is dependent on p53 and Puma activation. This apoptotic pathway is demonstrated in peripheral blood lymphocytes for the first time. A different apoptotic pathway induced in peripheral blood lymphocytes using this drug has, however, been previously revealed by other authors. The combination of cell specificity and dose-dependent effects can likely play a decisive role in apoptosis observed in peripheral blood lymphocytes after genotoxic drug application.

Crosstalk among Bcl-2 family members in B-CLL: seliciclib acts via the Mcl-1/Noxa axis and gradual exhaustion of Bcl-2 protection

Seliciclib (R-roscovitine) is a cyclin-dependent kinase inhibitor in clinical development. It triggers apoptosis by inhibiting de novo transcription of the short-lived Mcl-1 protein, but it is unknown how this leads to Bax/Bak activation that is required for most forms of cell death. Here, we studied the effects of seliciclib in B-cell chronic lymphocytic leukemia (B-CLL), a malignancy with aberrant expression of apoptosis regulators. Although seliciclib-induced Mcl-1 degradation within 4 h, Bax/Bak activation occurred between 16 and 20 h. During this period, no transcriptional changes in apoptosis-related genes occurred. In untreated cells, prosurvival Mcl-1 was engaged by the proapoptotic proteins Noxa and Bim. Upon drug treatment, Bim was quickly released. The contribution of Noxa and Bim as a specific mediator of seliciclib-induced apoptosis was demonstrated via RNAi. Significantly, 16 h after seliciclib treatment, there was accumulation of Bcl-2, Bim and Bax in the 'mitochondria-rich' insoluble fraction of the cell. This suggests that after Mcl-1 degradation, the remaining apoptosis neutralizing capacity of Bcl-2 is gradually overwhelmed, until Bax forms large multimeric pores in the mitochondria. These data demonstrate in primary leukemic cells hierarchical binding and crosstalk among Bcl-2 members, and suggest that their functional interdependence can be exploited therapeutically.

Interleukin 15-mediated survival of natural killer cells is determined by interactions among Bim, Noxa and Mcl-1

Interleukin 15 (IL-15) promotes the survival of natural killer (NK) cells by preventing apoptosis through mechanisms unknown at present. Here we identify Bim, Noxa and Mcl-1 as key regulators of IL-15-dependent survival of NK cells. IL-15 suppressed apoptosis by limiting Bim expression through the kinases Erk1 and Erk2 and mechanisms dependent on the transcription factor Foxo3a, while promoting expression of Mcl-1, which was necessary and sufficient for the survival of NK cells. Withdrawal of IL-15 led to upregulation of Bim and, accordingly, both Bim-deficient and Foxo3a-/- NK cells were resistant to cytokine deprivation. Finally, IL-15-mediated inactivation of Foxo3a and cell survival were dependent on phosphotidylinositol-3-OH kinase. Thus, IL-15 regulates the survival of NK cells at multiple steps, with Bim and Noxa being key antagonists of Mcl-1, the critical survivor factor in this process.

Effects of intravenous triacylglycerol emulsions on lymphocyte responses to mitogens in fasted dairy cows undergoing intense lipomobilization

The objective of the study was to assess the effects of intravenous infusion of triacylglycerol (TAG) emulsions derived from different lipid sources on responses to mitogens of peripheral blood mononuclear cells (PBMC) isolated from fasted dairy cows. Six multiparous, non-pregnant, non-lactating Holstein cows were used in a replicated 3×3 Latin Square design. For 4 d, cows were fasted and infused intravenously with a 20% TAG emulsions derived from tallow (TA), linseed oil (LO) or fish oil (FO). Fasting was employed to induce energy deficit and lipid mobilization. Emulsions were administered for 20 to 30 min every 4 h throughout the 4 d fast at a rate of 0·54 g TAG/kg BW/d. Blood samples were taken before the first infusion, and then every 24 h during the fast. Blood was utilized to assess DNA synthesis, IgM and interferon-gamma (IFN-γ) secretion by PBMC stimulated with mitogens. In TA infused cows there was a decline of PBMC ability to respond to mitogens, which was significant 48 h after initiation of the infusion period for DNA synthesis and IFN-γ secretion. In LO or FO infused cows, PBMC responses to mitogens were not altered during the infusion period, and in some cases PBMC responses to mitogen was improved at 72 and 96 h after initiation of treatments. Effects of TAG infusion on PBMC responses to mitogens depended on the lipid source suggesting that LO or FO can attenuate the negative effects of fasting on immune functions.

Therapeutic targets in the mitochondrial apoptotic pathway

Every cell in the human body has most of the components of the apoptotic apparatus and is thus principally equipped to die by apoptosis. Situations of increased or decreased apoptosis contribute to many forms of human disease, making this pathway an attractive target of therapeutic intervention. The past few years have seen an enormous refinement in the understanding how apoptosis works on a molecular level and the role of mitochondria as a central element in apoptotic signal transduction has become obvious. Here, the authors consider the events that are critical in this mitochondrial pathway, in particular at mitochondria but also upstream and downstream. The authors' opinion is presented on the merits and feasibility of approaches that aim at treating disease by interfering with the mitochondrial apoptotic pathway.

Myeloid cell leukemia-1 as a therapeutic target

Apoptosis, or programmed cell death, is essential for normal development and homeostasis. Insufficient apoptosis may contribute to the pathogenesis of malignancy and acute and chronic inflammation. Apoptosis may be induced by the death receptor or the mitochondrial pathways. Myeloid cell leukemia (Mcl)-1 is a member of the Bcl-2 family that contributes to the control of mitochondrial integrity, which is critical for maintaining cell viability. Mcl-1 has been shown to be essential for the development and survival of a variety of cell types. This review characterizes the role of Mcl-1 in the regulation of apoptosis and the promotion of disease, and defines novel strategies that have been identified to target this molecule.

Chronic lymphocytic leukemia requires BCL2 to sequester prodeath BIM, explaining sensitivity to BCL2 antagonist ABT-737

Antiapoptotic B cell leukemia/lymphoma 2 (BCL2) family proteins are expressed in many cancers, but the circumstances under which these proteins are necessary for tumor maintenance are poorly understood. We exploited a novel functional assay that uses BCL2 homology domain 3 (BH3) peptides to predict dependence on antiapoptotic proteins, a strategy we call BH3 profiling. BH3 profiling accurately predicts sensitivity to BCL2 antagonist ABT-737 in primary chronic lymphocytic leukemia (CLL) cells. BH3 profiling also accurately distinguishes myeloid cell leukemia sequence 1 (MCL1) from BCL2 dependence in myeloma cell lines. We show that the special sensitivity of CLL cells to BCL2 antagonism arises from the requirement that BCL2 tonically sequester proapoptotic BIM in CLL. ABT-737 displaced BIM from BCL2's BH3-binding pocket, allowing BIM to activate BAX, induce mitochondrial permeabilization, and rapidly commit the CLL cell to death. Our experiments demonstrate that BCL2 expression alone does not dictate sensitivity to ABT-737. Instead, BCL2 complexed to BIM is the critical target for ABT-737 in CLL. An important implication is that in cancer, BCL2 may not effectively buffer chemotherapy death signals if it is already sequestering proapoptotic BH3-only proteins. Indeed, activator BH3-only occupation of BCL2 may prime cancer cells for death, offering a potential explanation for the marked chemosensitivity of certain cancers that express abundant BCL2, such as CLL and follicular lymphoma.

Extended longevity mechanisms in short-lived progeroid mice: identification of a preservative stress response associated with successful aging

Semantic distinctions between "normal" aging, "pathological" aging (or age-related disease) and "premature" aging (otherwise known as segmental progeria) potentially confound important insights into the nature of each of the complex processes. Here we review a recent, unexpected discovery: the presence of longevity-associated characteristics typical of long-lived endocrine-mutant and dietary-restricted animals in short-lived progeroid mice. These data suggest that a subset of symptoms observed in premature aging, and possibly normal aging as well, may be indirect manifestations of a beneficial adaptive stress response to endogenous oxidative damage, rather than a detrimental result of the damage itself.

Withdrawal symptoms on display: Bcl-2 members under investigation

The balance between survival and apoptosis of lymphocytes is considered to be regulated by specific signals delivered through cell surface receptors binding either antigen (fragments) or homeostatic cytokines. Expanding lymphocytes must also compete for nutrients. For growth factors and nutrients, recent data indicate how these generic environmental signals couple to members of the apoptosis-regulating Bcl-2 family. The prosurvival molecule Mcl-1 is engaged by lethal BH3-only proteins Puma and Noxa under these circumstances. We propose that Puma and Noxa have specific roles in tipping the balance towards apoptosis after growth factor withdrawal and nutrient shortage, respectively. These complementary mechanisms tune survival in the various niches when lymphocytes compete for resources during selection and expansion.

Regulation of the proapoptotic BH3-only protein BIM by glucocorticoids, survival signals and proteasome in chronic lymphocytic leukemia cells

Glucocorticoids induce apoptosis in chronic lymphocytic leukemia (CLL) cells through a caspase-dependent mechanism. However, their mechanism of action remains unknown. We have studied the regulation of the proapoptotic BH3-only Bcl-2 interacting mediator of cell death (BIM) in CLL cells. We demonstrate that glucocorticoids upregulate BIM at protein and mRNA levels. We have investigated the ability of different survival signals, such as 12-O-tetradecanoylphorbol 13-acetate (TPA), stromal cell-derived factor-1alpha (SDF-1alpha), interleukin 4 (IL-4) and B-cell receptor (BCR) activation, to influence the levels of BIM and its induction by glucocorticoids. TPA downregulates BIM(EL) by extracellular signal-regulated kinase (ERK)-mediated BIM phosphorylation and further proteasome-mediated degradation. However, SDF-1alpha and BCR activation induce transient BIM phosphorylation, without protein degradation. Proteasome inhibitors do not modify the levels of BIM with respect to untreated cells. However, they induce apoptosis and inhibit TPA-induced BIM(EL) degradation, leading to its accumulation. In conclusion, the results implicate BIM in glucocorticoid-induced apoptosis in CLL cells. BIM(EL) phosphorylation through the ERK pathway targets the protein for proteasomal degradation.