Cell Death and Differentiation

Czabotar, P E; Colman, P M; Huang, D C S

doi: 10.1038/cdd.2009.83pmid: 19557009

The mechanism by which the cell death mediator Bax becomes activated to cause mitochondrial damage, a key step for the intrinsic pathway to apoptosis, remain highly contentious. Although some data support a role for certain BH3-only proteins, such as Bim or tBid, to directly activate Bax, others have led to the conclusion that BH3-only proteins act indirectly by antagonizing the prosurvival Bcl-2 proteins, thereby allowing Bax activation to proceed. A recent paper in Nature by Gavathiotis et al. provides the first biophysical evidence for a direct interaction between a BH3 domain, that of Bim, with Bax. Here, we review these intriguing observations and discuss their implications for our understanding of how the BH3-only proteins initiate apoptosis.

Wang, P; Qiu, W; Dudgeon, C; Liu, H; Huang, C; Zambetti, G P; Yu, J; Zhang, L

doi: 10.1038/cdd.2009.51pmid: 19444283

Tumor necrosis factor-α (TNF-α) is a cytokine that has an important role in immunity and inflammation by inducing cellular responses such as apoptosis. The transcription factor nuclear factor-κB (NF-κB) can paradoxically suppress and promote apoptosis in response to TNF-α. In this study, we found that p53 upregulated modulator of apoptosis (PUMA), a p53 downstream target and a BH3-only Bcl-2 family member, is directly regulated by NF-κB in response to TNF-α. TNF-α treatment led to increases in PUMA mRNA and protein levels in human colon cancer cells. The induction of PUMA was p53 independent, and mediated by the p65 component of NF-κB through a κB site in the PUMA promoter. The apoptotic effect of PUMA induction by TNF-α was unmasked by depleting the antiapoptotic protein Bcl-XL. In mice, PUMA was also induced by TNF-α in an NF-κB-dependent manner. TNF-α-induced apoptosis in a variety of tissues and cell types, including small intestinal epithelial cells, hepatocytes, and thymocytes, was markedly reduced in PUMA-deficient mice. Collectively, these results demonstrated that PUMA is a direct target of NF-κB and mediates TNF-α-induced apoptosis in vitro and in vivo.

Chiacchiera, F; Matrone, A; Ferrari, E; Ingravallo, G; Lo Sasso, G; Murzilli, S; Petruzzelli, M; Salvatore, L; Moschetta, A; Simone, C

doi: 10.1038/cdd.2009.36pmid: 19343039

Colorectal cancer cell (CRC) fate is governed by an intricate network of signaling pathways, some of which are the direct target of DNA mutations, whereas others are functionally deregulated. As a consequence, cells acquire the ability to grow under nutrients and oxygen shortage conditions. We earlier reported that p38α activity is necessary for proliferation and survival of CRCs in a cell type-specific manner and regardless of their phenotype and genotype. Here, we show that p38α sustains the expression of HIF1α target genes encoding for glycolytic rate-limiting enzymes, and that its inhibition causes a drastic decrease in ATP intracellular levels in CRCs. Prolonged inactivation of p38α triggers AMPK-dependent nuclear localization of FoxO3A and subsequent activation of its target genes, leading to autophagy, cell cycle arrest and cell death. In vivo, pharmacological blockade of p38α inhibits CRC growth in xenografted nude mice and azoxymethane-treated ApcMin mice, achieving both a cytostatic and cytotoxic effect, associated with high nuclear expression of FoxO3A and increased expression of its target genes p21 and PTEN. Hence, inhibition of p38α affects the aerobic glycolytic metabolism specific of cancer cells and might be taken advantage of as a therapeutic strategy targeted against CRCs.

Kaunisto, A; Kochin, V; Asaoka, T; Mikhailov, A; Poukkula, M; Meinander, A; Eriksson, J E

doi: 10.1038/cdd.2009.35pmid: 19343040

Cellular FLICE-inhibitory protein (c-FLIP) proteins are crucial regulators of the death-inducing signaling complex (DISC) and caspase-8 activation. To date, three c-FLIP isoforms with distinct functions and regulation have been identified. Our previous studies have shown that the stability of c-FLIP proteins is subject to isoform-specific regulation, but the underlying molecular mechanisms have not been known. Here, we identify serine 193 as a novel in vivo phosphorylation site of all c-FLIP proteins and demonstrate that S193 phosphorylation selectively influences the stability of the short c-FLIP isoforms, as S193D mutation inhibits the ubiquitylation and selectively prolongs the half-lives of c-FLIP short (c-FLIPS) and c-FLIP Raji (c-FLIPR). S193 phosphorylation also decreases the ubiquitylation of c-FLIP long (c-FLIPL) but, surprisingly, does not affect its stability, indicating that S193 phosphorylation has a different function in c-FLIPL. The phosphorylation of this residue is operated by the protein kinase C (PKC), as S193 phosphorylation is markedly increased by treatment with 12-O-tetradecanoylphorbol-13-acetate and decreased by inhibition of PKCα and PKCβ. S193 mutations do not affect the ability of c-FLIP to bind to the DISC, although S193 phosphorylation is increased by death receptor stimulation. Instead, S193 phosphorylation affects the intracellular level of c-FLIPS, which then determines the sensitivity to death-receptor-mediated apoptosis. These results reveal that the differential stability of c-FLIP proteins is regulated in an isoform-specific manner by PKC-mediated phosphorylation.

Audo, R; Combe, B; Coulet, B; Morel, J; Hahne, M

doi: 10.1038/cdd.2009.38pmid: 19407827

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) has gained much attention as a possible therapeutic reagent for the treatment of tumors, as TRAIL was originally described to induce apoptosis specifically in cancer cells, but not in normal cells. Fibroblast-like synoviocytes (FLS) in rheumatoid arthritis (RA) patients exhibit tumor-like features and we have described earlier that TRAIL induces apoptosis only in a subset of RA FLS, but an induction of proliferation in the surviving cells. This observation corresponds to the pleiotropic effects of TRAIL observed on primary human tumor cells. Here, we describe that the PI3 kinase/Akt-signaling pathway, but not that of the MAP kinases ERK and p38, protects RA FLS from TRAIL-induced apoptosis by modulating the expression of the cell survival regulators p21, XIAP, Mcl-1 and RIP. Moreover, we found that not only TRAIL-induced apoptosis, but also TRAIL-triggered proliferation in RA FLS is mediated by caspases with a crucial role for caspase 8. TRAIL was found to induce degradation of p21 and p27 that was caspase-dependent, but independent of the ERK, p38 and PI3 kinase/Akt-signaling pathways. The finding that TRAIL-triggered proliferation and apoptosis share intracellular routes has to be taken in consideration in defining therapeutic strategies on the basis of the administration of TRAIL.

Sebastià, J; Kieran, D; Breen, B; King, M A; Netteland, D F; Joyce, D; Fitzpatrick, S F; Taylor, C T; Prehn, J H M

doi: 10.1038/cdd.2009.52pmid: 19444281

Cells can adapt to hypoxia through the activation of hypoxia-inducible factor-1 (HIF-1), which in turn regulates the expression of hypoxia-responsive genes. Defects in hypoxic signaling have been suggested to underlie the degeneration of motoneurons in amyotrophic lateral sclerosis (ALS). We have recently identified mutations in the hypoxia-responsive gene, angiogenin (ANG), in ALS patients, and have shown that ANG is constitutively expressed in motoneurons. Here, we show that HIF-1α is sufficient and required to activate ANG in cultured motoneurons exposed to hypoxia, although ANG expression does not change in a transgenic ALS mouse model or in sporadic ALS patients. Administration of recombinant ANG or expression of wild-type ANG protected motoneurons against hypoxic injury, whereas gene silencing of ang1 significantly increased hypoxia-induced cell death. The previously reported ALS-associated ANG mutations (Q12L, K17I, R31K, C39W, K40I, I46V) all showed a reduced neuroprotective activity against hypoxic injury. Our data show that ANG plays an important role in endogenous protective pathways of motoneurons exposed to hypoxia, and suggest that loss of function rather than loss of expression of ANG is associated with ALS.

Andina, N; Conus, S; Schneider, E M; Fey, M F; Simon, H U

doi: 10.1038/cdd.2009.50pmid: 19407828

Under inflammatory conditions, neutrophil apoptosis is delayed due to survival-factor exposure, a mechanism that prevents the resolution of inflammation. One important proinflammatory cytokine involved in the regulation of neutrophil survival/activation is granulocyte–macrophage colony-stimulating factor (GM-CSF). Although GM-CSF mediates antiapoptotic effects in neutrophils, it does not prevent apoptosis, and the survival effect is both time dependent and limited. Here, we identified the proapoptotic Bcl-2 family member Bim as an important lifespan limiting molecule in neutrophils, particularly under conditions of survival factor exposure. Strikingly, GM-CSF induced Bim expression in both human and mouse neutrophils that was blocked by pharmacological inhibition of phosphatidylinositol-3 kinase (PI3K). Increased Bim expression was also seen in human immature bone marrow neutrophils as well as in blood neutrophils from septic shock patients; both cell populations are known to be exposed to GM-CSF under in vivo conditions. The functional role of Bim was investigated using Bim-deficient mouse neutrophils in the presence and absence of the survival cytokines interleukin (IL)-3 and GM-CSF. Lack of Bim expression resulted in a much higher efficacy of the survival cytokines to block neutrophil apoptosis. Taken together, these data demonstrate a functional role for Bim in the regulation of neutrophil apoptosis and suggest that GM-CSF and other neutrophil hematopoietins initiate a proapoptotic counterregulation that involves upregulation of Bim.

Joseph, B; Andersson, E R; Vlachos, P; Södersten, E; Liu, L; Teixeira, A I; Hermanson, O

doi: 10.1038/cdd.2009.72pmid: 19590511

Mammalian central nervous system (CNS) development is a highly organized process involving the precise and coordinated timing of cell-cycle exit, differentiation, survival, and migration. These events require proper expression of pro-neuronal genes but also repression of alternative cell fates and restriction of cell-type-specific gene expression. Here, we show that the cyclin-dependent kinase (CDK) inhibitor p57Kip2 interacted with pro-neuronal basic helix-loop-helix (bHLH) factors such as Mash1, NeuroD, and Nex/Math2. Increased levels of p57Kip2 inhibited Mash1 transcriptional activity independently of CDK interactions and acted as a direct repressor in transcriptional assays. Proliferating telencephalic neural progenitors co-expressed basal levels of Mash1 and p57Kip2, and endogenous p57Kip2 accumulated transiently in the nuclei of neural stem cells (NSCs) during early stages of astrocyte differentiation mediated by ciliary neurotrophic factor (CNTF), independent of cell-cycle exit and at times when Mash1 expression was still prominent. In accordance with these observations, gain- and loss-of-function studies showed that p57Kip2 repressed neuronal differentiation after mitogen withdrawal, but exerted little or no effect on CNTF-mediated astroglial differentiation of NSCs. Our data suggest a novel role for p57Kip2 as a context-dependent repressor of neurogenic transcription factors and telencephalic neuronal differentiation.

López-Jiménez, M E; Bartolomé-Martín, D; Sánchez-Prieto, J; Torres, M

doi: 10.1038/cdd.2009.57pmid: 19461654

The increased expression of different soluble guanylyl cyclase (sGC) subunits during development is consistent with these proteins participating in the formation and establishment of interneuronal contacts. Functional sGC is generated by the dimerization of an α-subunit (sGCα1/2) with the β1-subunit (sGCβ1), and both depletion of the sGCβ1 subunit and inhibiting sGC activity impair neurite outgrowth. Similarly, impairing sGC activity diminishes the amount of growth-associated protein (GAP-43) and synapsin I, two proteins that participate in axon elongation and synaptogenesis, suggesting a role for sGC in these processes. Indeed, fewer synapses form when sGC is inhibited, as witnessed by FM1-43 imaging and synapsin I immunostaining, and the majority of synapses that do form remain functionally immature. These findings highlight the importance of sGC in the regulation of neurite outgrowth and synapse formation, and in the functional maturation of cerebellar granule cells in vitro.

Legarda-Addison, D; Hase, H; O'Donnell, M A; Ting, A T

doi: 10.1038/cdd.2009.41pmid: 19373245

TNF receptor 1 (TNFR1) ligation can result in cell survival or cell death. What determines which of the two opposing responses is triggered is not fully understood. The current model suggests that it is the activation of the NF-κB pathway and its induction of prosurvival genes, or the lack thereof, which determines the outcome. NF-κB essential modifier (NEMO)/IκB kinase-γ (IKKγ)-deficient cells are highly sensitive to apoptosis, and as NEMO is essential for NF-κB activation, it has been assumed that this is due to the lack of NF-κB. This study demonstrates that this assumption was incorrect and that NEMO has another antiapoptotic function that is independent of its role in the NF-κB pathway. NEMO prevents receptor interacting protein-1 (RIP1) from engaging CASPASE-8 before NF-κB-mediated induction of antiapoptotic genes. Without NEMO, RIP1 associates with CASPASE-8 resulting in rapid tumor necrosis factor (TNF)-induced apoptosis. These results suggest that there are two cell-death checkpoints following TNF stimulation: an early transcription-independent checkpoint whereby NEMO restrains RIP1 from activating the caspase cascade, followed by a later checkpoint dependent on NF-κB-mediated transcription of prosurvival genes.