Paradoxical Roles of Elongation Factor-2 Kinase in Stem Cell Survival.

Protein synthesis inhibition is an immediate response during stress to switch the composition of protein pool in order to adapt to the new environment. It was reported that this response could be either protective or deleterious. However, how cells choose to live or die upon protein synthesis inhibition is largely unknown. Previously, we have shown that elongation factor-2 kinase (eEF2K), a protein kinase that suppresses protein synthesis during elongation phase, is a positive regulator of apoptosis both in vivo and in vitro Consistently, here we report that knock-out of eEF2K protects mice from a lethal dose of whole-body ionizing radiation at 8 Gy by reducing apoptosis levels in both bone marrow and gastrointestinal tracts. Surprisingly, similar to the loss of p53, eEF2K deficiency results in more severe damage to the gastrointestinal tract at 20 Gy with the increased mitotic cell death in small intestinal stem cells. Furthermore, using epithelial cell lines, we showed that eEF2K is required for G2/M arrest induced by radiation to prevent mitotic catastrophe in a p53-independent manner. Specifically, we observed the elevation of Akt/ERK activity as well as the reduction of p21 expression in Eef2k(-/-) cells. Therefore, eEF2K also provides a protective strategy to maintain genomic integrity by arresting cell cycle in response to stress. Our results suggest that protective versus pro-apoptotic roles of eEF2K depend on the type of cells: eEF2K is protective in highly proliferative cells, such as small intestinal stem cells and cancer cells, which are more susceptible to mitotic catastrophe.

Autophagy and transporter-based multi-drug resistance.

All the therapeutic strategies for treating cancers aim at killing the cancer cells via apoptosis (programmed cell death type I). Defective apoptosis endow tumor cells with survival. The cell can respond to such defects with autophagy. Autophagy is a cellular process by which cytoplasmic material is either degraded to maintain homeostasis or recycled for energy and nutrients in starvation. A plethora of evidence has shown that the role of autophagy in tumors is complex. A lot of effort is needed to underline the functional status of autophagy in tumor progression and treatment, and elucidate how to tweak autophagy to treat cancer. Furthermore, during the treatment of cancer, the limitation for the cure rate and survival is the phenomenon of multi drug resistance (MDR). The development of MDR is an intricate process that could be regulated by drug transporters, enzymes, anti-apoptotic genes or DNA repair mechanisms. Reports have shown that autophagy has a dual role in MDR. Furthermore, it has been reported that activation of a death pathway may overcome MDR, thus pointing the importance of other death pathways to regulate tumor cell progression and growth. Therefore, in this review we will discuss the role of autophagy in MDR tumors and a possible link amongst these phenomena.

Immortalized mouse epithelial cell models to study the role of apoptosis in cancer.

Human cancer cell lines are widely used to model cancer but also have serious limitations. As an alternate approach, we have developed immortalized mouse epithelial cell model systems that are applicable to different tissue types and involve generation of immortalized cell lines that are genetically defined. By applying these model systems to mutant mice, we have extended the powerful approach of mouse genetics to in vitro analysis. By use of this model we have generated immortal epithelial cells that are either competent or deficient for apoptosis by different gain- and loss-of-function mutations that have revealed important mechanisms of tumor progression and treatment resistance. Furthermore, we have derived immortalized, isogenic mouse kidney, mammary, prostate, and ovarian epithelial cell lines to address the issues of tissue specificity. One of the major advantages of these immortalized mouse epithelial cell lines is the ability to perform biochemical analysis, screening, and further genetic manipulations. Moreover, the ability to generate tumor allografts in mice allows the integration of in vitro and in vivo approaches to delineate the mechanistic aspects of tumorigenesis. These model systems can be used effectively to determine the molecular requirements of epithelial tumorigenesis and tumor-promoting functions. This approach provides an efficient way to study the role of apoptosis in cancer and also enables the interrogation and identification of potential chemotherapeutic targets involving this pathway. Applying this technology to other mouse models can provide insight into additional aspects of oncogenesis.

Role of the polarity determinant crumbs in suppressing mammalian epithelial tumor progression.

Most tumors are epithelial-derived, and although disruption of polarity and aberrant cellular junction formation is a poor prognosticator in human cancer, the role of polarity determinants in oncogenesis is poorly understood. Using in vivo selection, we identified a mammalian orthologue of the Drosophila polarity regulator crumbs as a gene whose loss of expression promotes tumor progression. Immortal baby mouse kidney epithelial cells selected in vivo to acquire tumorigenicity displayed dramatic repression of crumbs3 (crb3) expression associated with disruption of tight junction formation, apicobasal polarity, and contact-inhibited growth. Restoration of crb3 expression restored junctions, polarity, and contact inhibition while suppressing migration and metastasis. These findings suggest a role for mammalian polarity determinants in suppressing tumorigenesis that may be analogous to the well-studied polarity tumor suppressor mechanisms in Drosophila.

Induction of apoptosis by diterpenes from the soft coral Xenia elongata.

Four new diterpenes ( 1- 4) were isolated from the soft coral Xenia elongata using a novel cell-based screen for apoptosis-inducing, potential anticancer compounds. The molecular structures of the diterpenes were determined using a combination of NMR and mass spectrometry. The bioactivities were confirmed using a specific apoptosis induction assay based on genetically engineered mammalian lines with differential, defined capacities for apoptosis. The diterpenes induce apoptosis in micromolar concentrations. This is the first report of apoptosis induction by marine diterpenes in xenicane skeletons.

ERK1/2-dependent phosphorylation of BimEL promotes its rapid dissociation from Mcl-1 and Bcl-xL.

The proapoptotic protein Bim is expressed de novo following withdrawal of serum survival factors. Here, we show that Bim-/- fibroblasts and epithelial cells exhibit reduced cell death following serum withdrawal in comparison with their wild-type counterparts. In viable cells, Bax associates with Bcl-2, Bcl-x(L) and Mcl-1. Upon serum withdrawal, newly expressed Bim(EL) associates with Bcl-x(L) and Mcl-1, coinciding with the dissociation of Bax from these proteins. Survival factors can prevent association of Bim with pro-survival proteins by preventing Bim expression. However, we now show that even preformed Bim(EL)/Mcl-1 and Bim(EL)/Bcl-x(L) complexes can be rapidly dissociated following activation of ERK1/2 by survival factors. The dissociation of Bim from Mcl-1 is specific for Bim(EL) and requires ERK1/2-dependent phosphorylation of Bim(EL) at Ser(65). Finally, ERK1/2-dependent dissociation of Bim(EL) from Mcl-1 and Bcl-x(L) may play a role in regulating Bim(EL) degradation, since mutations in the Bim(EL) BH3 domain that disrupt binding to Mcl-1 cause increased turnover of Bim(EL). These results provide new insights into the role of Bim in cell death and its regulation by the ERK1/2 survival pathway.

Autophagy suppresses tumor progression by limiting chromosomal instability.

Autophagy is a bulk degradation process that promotes survival under metabolic stress, but it can also be a means of cell death if executed to completion. Monoallelic loss of the essential autophagy gene beclin1 causes susceptibility to metabolic stress, but also promotes tumorigenesis. This raises the paradox that the loss of a survival pathway enhances tumor growth, where the exact mechanism is not known. Here, we show that compromised autophagy promoted chromosome instability. Failure to sustain metabolism through autophagy was associated with increased DNA damage, gene amplification, and aneuploidy, and this genomic instability may promote tumorigenesis. Thus, autophagy maintains metabolism and survival during metabolic stress that serves to protect the genome, providing an explanation for how the loss of a survival pathway leads to tumor progression. Identification of this novel role of autophagy may be important for rational chemotherapy and therapeutic exploitation of autophagy inducers as potential chemopreventive agents.

NBK/BIK antagonizes MCL-1 and BCL-XL and activates BAK-mediated apoptosis in response to protein synthesis inhibition.

Ribonucleases, antibiotics, bacterial toxins, and viruses inhibit protein synthesis, which results in apoptosis in mammalian cells. How the BCL-2 family of proteins regulates apoptosis in response to the shutoff of protein synthesis is not known. Here we demonstrate that an Escherichia coli toxin, MazF, inhibited protein synthesis by cleavage of cellular mRNA and induced apoptosis in mammalian cells. MazF-induced apoptosis required proapoptotic BAK and its upstream regulator, the proapoptotic BH3-only protein NBK/BIK, but not BIM, PUMA, or NOXA. Interestingly, in response to MazF induction, NBK/BIK activated BAK by displacing it from anti-apoptotic proteins MCL-1 and BCL-X(L) that sequester BAK. Furthermore, NBK/BIK- or BAK-deficient cells were resistant to cell death induced by pharmacologic inhibition of translation and by virus-mediated shutoff of protein synthesis. Thus, the BH3-only protein NBK/BIK is the apical regulator of a BAK-dependent apoptotic pathway in response to shutoff of protein synthesis that functions to displace BAK from sequestration by MCL1 and BCL-X(L). Although NBK/BIK is dispensable for development, it is the BH3-only protein targeted for inactivation by viruses, suggesting that it plays a role in pathogen/toxin response through apoptosis activation.

Nutlin-3 protects kidney cells during cisplatin therapy by suppressing Bax/Bak activation.

Nutlins, the newly developed small molecule antagonists of MDM2, activate p53 and induce apoptosis in cancer cells, offering a novel strategy of chemotherapy. Recent studies have further suggested synergistic effects of nutlins with other chemotherapeutic drugs. However, it is unclear whether nutlins increase or decrease the side effects of these drugs in normal non-malignant cells or tissues. Cisplatin is a widely used chemotherapy drug, which has a major side effect of kidney injury. Here we show that Nutlin-3 protected kidney cells against cisplatin-induced apoptosis. The cytoprotective effects of Nutlin-3 were not related to its regulation of p53 or consequent gene expression during cisplatin treatment. Moreover, the protective effects were shown in MDM2-, MDM4-, or p53-deficient cells. On the other hand, Nutlin-3 suppressed mitochondrial events of apoptosis during cisplatin incubation, including Bax activation and cytochrome c release. Nutlin-3 attenuated cisplatin-induced oligomerization of Bax and Bak but not their interactions with Bcl-XL. In isolated mitochondria, Nutlin-3 inhibited cytochrome c release induced by Ca2+, Bim peptide, and recombinant tBid. Importantly, it blocked both Bax and Bak oligomerization under these conditions. Together, the results have uncovered a new pharmacological function of nutlins, i.e. suppression of Bax and Bak, two critical mediators of apoptosis.

A mouse model system to genetically dissect the molecular mechanisms regulating tumorigenesis.

The vast majority of human tumors are of epithelial origin and result from the accumulation of mutations that alter the function of pathways that control critical cellular processes, including proliferation, checkpoint regulation, and apoptosis. Authentically replicating these events in animal models is critical to understanding the biology of cancer and for testing the feasibility of novel therapies. We developed a mouse model that recapitulates the steps of epithelial tumor progression of multiple tissue types (kidney, breast, ovarian surface, and prostate epithelia), which takes advantage of the power of mouse genetics, and that allows for biochemical analysis, genetic selection, and screening. Moreover, this model enables functional interrogation of far more complex tumor genotypes, both of the tumor cells themselves, and of the cells in the tumor microenvironment. This is a crucial advantage, as human tumors result from multiple compound mutations, most of which are difficult to achieve through standard mutant mouse technology. We have applied this model to establish the role of apoptosis in epithelial solid tumor progression and in treatment response, which has provided novel opportunities for cancer therapies in humans.

Autophagy promotes tumor cell survival and restricts necrosis, inflammation, and tumorigenesis.

Defective apoptosis renders immortalized epithelial cells highly tumorigenic, but how this is impacted by other common tumor mutations is not known. In apoptosis-defective cells, inhibition of autophagy by AKT activation or by allelic disruption of beclin1 confers sensitivity to metabolic stress by inhibiting an autophagy-dependent survival pathway. While autophagy acts to buffer metabolic stress, the combined impairment of apoptosis and autophagy promotes necrotic cell death in vitro and in vivo. Thus, inhibiting autophagy under conditions of nutrient limitation can restore cell death to apoptosis-refractory tumors, but this necrosis is associated with inflammation and accelerated tumor growth. Thus, autophagy may function in tumor suppression by mitigating metabolic stress and, in concert with apoptosis, by preventing death by necrosis.

Grm5 expression is not required for the oncogenic role of Grm1 in melanocytes.

Melanoma is the aberrant proliferation of melanocytes, the cells in the skin responsible for pigment (melanin) production. In its early stages, melanoma can be surgically removed with great success, however, advanced stages of melanoma have a high mortality rate due to the lack of responsiveness to currently available therapies. We have previously characterized a mouse melanoma model, TG-3, which has implicated the ectopic expression of metabotropic glutamate receptor 1 (Grm1, formerly mGluR1), in melanomagenesis and metastasis [Pollock et al., 2003. Melanoma mouse model implicates metabotropic glutamate signaling in melanocytic neoplasia. Nat Genet. 34, 108-112.]. Here we report the characterization of several in vitro cell lines derived from independent mouse melanoma tumors. These cell lines show characteristic phenotypes of transformed melanocytes, and express Grm1, and Grm5 (another metabotropic glutamate receptor), as well as melanocyte-specific protein markers. To investigate the possible role of Grm5 in vivo during melanoma development in our mice, we have crossed Grm5 null mice with TG-3, generating a new line of transgenic mice, TGM. TGMs, which are homozygote knockouts for Grm5 and carry the TG transgene, develop tumors with onset, progression, and metastasis very similar to that described for TG-3. Taken together, these results indicate that Grm1 can act as an oncogene in melanocytes independently of Grm5 expression.

Key roles of BIM-driven apoptosis in epithelial tumors and rational chemotherapy.

Defective apoptosis not only promotes tumorigenesis, but also can confound chemotherapeutic response. Here we demonstrate that the proapoptotic BH3-only protein BIM is a tumor suppressor in epithelial solid tumors and also is a determinant in paclitaxel sensitivity in vivo. Furthermore, the H-ras/mitogen-activated protein kinase (MAPK) pathway conferred resistance to paclitaxel that was dependent on functional inactivation of BIM. Whereas paclitaxel induced BIM accumulation and BIM-dependent apoptosis in vitro and in tumors in vivo, the H-ras/MAPK pathway suppressed this BIM induction by phosphorylating BIM and targeting BIM for degradation in proteasomes. The proteasome inhibitor Velcade (P-341, Bortezomib) restored BIM induction, abrogated H-ras-dependent paclitaxel resistance, and promoted BIM-dependent tumor regression, suggesting the potential benefits of combinatorial chemotherapy of Velcade and paclitaxel.

Hypoxia and defective apoptosis drive genomic instability and tumorigenesis.

Genomic instability is a hallmark of cancer development and progression, and characterizing the stresses that create and the mechanisms by which cells respond to genomic perturbations is essential. Here we demonstrate that antiapoptotic BCL-2 family proteins promoted tumor formation of transformed baby mouse kidney (BMK) epithelial cells by antagonizing BAX- and BAK-dependent apoptosis. Cell death in vivo correlated with hypoxia and induction of PUMA (p53 up-regulated modulator of apoptosis). Strikingly, carcinomas formed by transformed BMK cells in which apoptosis was blocked by aberrant BCL-2 family protein function displayed prevalent, highly polyploid, tumor giant cells. Examination of the transformed BMK cells in vivo revealed aberrant metaphases and ploidy changes in tumors as early as 9 d after implantation, which progressed in magnitude during the tumorigenic process. An in vitro ischemia system mimicked the tumor microenvironment, and gain of BCL-2 or loss of BAX and BAK was sufficient to confer resistance to apoptosis and to allow for accumulation of polyploid cells in vitro. These data suggest that in vivo, even in cells in which p53 function is compromised, apoptosis is an essential response to hypoxia and ischemia in the tumor microenvironment and that abrogation of this response allows the survival of cells with abnormal genomes and promotes tumorigenesis.

ASAP, a novel protein complex involved in RNA processing and apoptosis.

Different isoforms of a protein complex termed the apoptosis- and splicing-associated protein (ASAP) were isolated from HeLa cell extract. ASAP complexes are composed of the polypeptides SAP18 and RNPS1 and different isoforms of the Acinus protein. While Acinus had previously been implicated in apoptosis and was recently identified as a component of the spliceosome, RNPS1 has been described as a general activator of RNA processing. Addition of ASAP isoforms to in vitro splicing reactions inhibits RNA processing mediated by ASF/SF2, by SC35, or by RNPS1. Additionally, microinjection of ASAP complexes into mammalian cells resulted in acceleration of cell death. Importantly, after induction of apoptosis the ASAP complex disassembles. Taken together, our results suggest an important role for the ASAP complexes in linking RNA processing and apoptosis.

Association of Bax and Bak homo-oligomers in mitochondria. Bax requirement for Bak reorganization and cytochrome c release.

ATP depletion induced by hypoxia or mitochondrial inhibitors results in Bax translocation from cytosol to mitochondria and release of cytochrome c from mitochondria into cytosol in cultured rat proximal tubule cells. Translocated Bax undergoes further conformational changes to oligomerize into high molecular weight complexes (Mikhailov, V., Mikhailova, M., Pulkrabek, D. J., Dong, Z., Venkatachalam, M. A., and Saikumar, P. (2001) J. Biol. Chem. 276, 18361-18374). Here we report that following Bax translocation in ATP-depleted rat proximal tubule cells, Bak, a proapoptotic molecule that normally resides in mitochondria, also reorganizes to form homo-oligomers. Oligomerization of both Bax and Bak occurred independently of Bid cleavage and/or translocation. Western blots of chemically cross-linked membrane extracts showed nonoverlapping "ladders" of Bax and Bak complexes in multiples of approximately 21 and approximately 23 kDa, respectively, consistent with molecular homogeneity within each ladder. This indicated that Bax and Bak complexes were homo-oligomeric. Nevertheless, each oligomer could be co-immunoprecipitated with the other, suggesting a degree of affinity between Bax and Bak that permitted co-precipitation but not cross-linking. Furthermore, dissociation of cross-linked complexes by SDS and renaturation prior to immunoprecipitation did not prevent reassociation of the two oligomeric species. Notably, expression of Bcl-2 prevented not only the oligomerization of Bax and Bak, but also the association between these two proteins in energy-deprived cells. Using Bax-deficient HCT116 and BMK cells, we show that there is stringent Bax requirement for Bak homo-oligomerization and for cytochrome c release during energy deprivation. Using Bak-deficient BMK cells we further show that Bak deficiency is associated with delayed kinetics of Bax translocation but does not affect either the oligomerization of translocated Bax or the leakage of cytochrome c. These results suggest a degree of functional cooperation between Bax and Bak in this form of cell injury, but also demonstrate an absolute requirement of Bax for mitochondrial permeabilization.

BAX and BAK mediate p53-independent suppression of tumorigenesis.

BAX and BAK are essential regulators of proapoptotic signaling, and the disruption of apoptosis is linked to the development of cancer. To investigate the role of BAX and BAK in tumorigenesis, primary baby mouse kidney epithelial cells (BMKs) from wild-type, BAX-, BAK-, or BAK- and BAK-deficient mice were transformed by adenovirus E1A and dominant-negative p53 (p53DD). In wild-type BMKs, the expression of E1A and inactivation of p53 was sufficient for transformation but not tumorigenesis. In contrast, E1A- and p53DD-transformed BAX- and BAK-deficient BMKs formed highly invasive carcinomas. Transformed BMKs deficient for either BAX or BAK were also tumorigenic, but only when heterozygous for the remaining bax or bak allele, the expression of which was lost in most resulting tumors. Thus, BAX and BAK function to suppress tumorigenesis, and their deficiency was selected for in vivo.

Bak and Bax function to limit adenovirus replication through apoptosis induction.

Adenovirus infection and expression of E1A induces both proliferation and apoptosis, the latter of which is blocked by the adenovirus Bcl-2 homologue E1B 19K. The mechanism of apoptosis induction and the role that it plays in productive infection are not known. Unlike apoptosis mediated by death receptors, infection with proapoptotic E1B 19K mutant viruses did not induce cleavage of Bid but nonetheless induced changes in Bak and Bax conformation, Bak-Bax interaction, caspase 9 and 3 activation, and apoptosis. In wild-type-adenovirus-infected cells, in which E1B 19K inhibits apoptosis, E1B 19K was bound to Bak, precluding Bak-Bax interaction and changes in Bax conformation. Infection with E1B 19K mutant viruses induced apoptosis in wild-type and Bax- or Bak-deficient baby mouse kidney cells but not in those deficient for both Bax and Bak. Furthermore, Bax and Bak deficiency dramatically increased E1A expression and virus replication. Thus, Bax- and Bak-mediated apoptosis severely limits adenoviral replication, demonstrating that Bax and Bak function as an antiviral response at the cellular level.

Bax and Bak independently promote cytochrome C release from mitochondria.

Pro-apoptotic Bax and Bak have been implicated in the regulation of p53-dependent apoptosis. We assessed the ability of primary baby mouse kidney (BMK) epithelial cells from bax(-/-), bak(-/-), and bax(-/-) bak(-/-) mice to be transformed by E1A alone or in conjunction with dominant-negative p53 (p53DD). Although E1A alone transformed BMK cells from p53-deficient mice, E1A alone did not transform BMK cells from bax(-/-), bak(-/-), or bax(-/-) bak(-/-) mice. Thus, the loss of both Bax and Bak was not sufficient to relieve p53-dependent suppression of transformation in epithelial cells. To test the requirement for Bax and Bak in other death signaling pathways, stable E1A plus p53DD-transformed BMK cell lines were derived from the bax(-/-), bak(-/-), and bax(-/-) bak(-/-) mice and characterized for their response to tumor necrosis factor-alpha (TNF-alpha)-mediated apoptosis. The loss of both Bax and Bak severely impaired TNF-alpha-mediated apoptosis, but the presence of either Bax or Bak alone was sufficient for cell death. Cytochrome c was released from mitochondria, and caspase-9 was activated in Bax- or Bak-deficient cells in response to TNF-alpha but not in cells deficient in both. Thus, either Bax or Bak is required for death signaling through mitochondria in response to TNF-alpha, but both are dispensable for p53-dependent transformation inhibition.