14-3-3 proteins and survival kinases cooperate to inactivate BAD by BH3 domain phosphorylation

Gating of the mitochondrial permeability transition pore by long chain fatty acyl analogs in vivo

The role played by long chain fatty acids (LCFA) in promoting energy expenditure is confounded by their dual function as substrates for oxidation and as putative classic uncouplers of mitochondrial oxidative phosphorylation. LCFA analogs of the MEDICA (MEthyl-substituted DICarboxylic Acids) series are neither esterified into lipids nor beta-oxidized and may thus simulate the uncoupling activity of natural LCFA in vivo, independently of their substrate role. Treatment of rats or cell lines with MEDICA analogs results in low conductance gating of the mitochondrial permeability transition pore (PTP), with 10-40% decrease in the inner mitochondrial membrane potential. PTP gating by MEDICA analogs is accounted for by inhibition of Raf1 expression and kinase activity, resulting in suppression of the MAPK/RSK1 and the adenylate cyclase/PKA transduction pathways. Suppression of RSK1 and PKA results in a decrease in phosphorylation of their respective downstream targets, Bad(Ser-112) and Bad(Ser-155). Decrease in Bad(Ser-112, Ser-155) phosphorylation results in increased binding of Bad to mitochondrial Bcl2 with concomitant displacement of Bax, followed by PTP gating induced by free mitochondrial Bax. Low conductance PTP gating by LCFA/MEDICA may account for their thyromimetic calorigenic activity in vivo.

Heat acclimation provides sustained improvement in functional recovery and attenuates apoptosis after traumatic brain injury

Heat acclimation (HA) offers functional neuroprotection in mice after traumatic brain injury (TBI). This study further characterizes endogenous neuroprotection acquired by HA (34+/-1 degrees C, 30 d) after TBI. We establish here the ability of HA to induce sustained functional benefits and to reduce activation of apoptotic pathways. Neurobehavioral recovery, assessed by the Neurological Severity Score, was greater in HA mice up to 8 days after injury as compared with normothermic controls (P<0.05) and lesion volume was also smaller in the HA group (P<0.05). Reduced apoptotic cell death in HA mice was confirmed using caspase-3 activity measurements and immunohistochemistry. To investigate the underlying molecular pathways, expression levels of intrinsic apoptotic pathway-related proteins were examined. HA mice displayed higher mitochondrial levels of antiapoptotic Bcl-xL, accompanied by lower proapoptotic Bad levels and decreased cytochrome c release, suggesting a higher apoptotic threshold. Taken together with our previous reports, indicating increased Akt phosphorylation and antioxidative capacity, alongside with reduced tumor necrosis alpha levels after TBI in HA animals, the current results support the involvement of an antiapoptotic effect in HA-induced neuroprotection. Current results warrant further study as TBI-induced apoptosis may persist over weeks after injury, possibly providing a target for belated therapeutic intervention.

Protection from rapamycin-induced apoptosis by insulin-like growth factor-I is partially dependent on protein kinase C signaling

Rapamycin-induced apoptosis in sarcoma cells is inhibited by insulin-like growth factor-I (IGF-I) through a signaling pathway independent of Ras-extracellular signal-regulated kinase 1/2 and Akt. IGF-I induces Bad phosphorylation (Ser112, Ser136, and Ser155) in a pathway involving phosphoinositide 3' kinase (PI3K) and protein kinase C (PKC; mu, epsilon, or theta) resulting in sequestering Bad from mitochondria and subsequently interacting with 14-3-3gamma in the cytosol. Gene knockdown of Bad, Bid, Akt1, Akt2, PKC-mu, PKC-epsilon, or PKC-theta was achieved by transient transfection using small interfering RNAs. Results indicate that IGF-I signaling to Bad requires activation of PI3K and PKC (mu, theta, epsilon) but not mTOR, Ras-extracellular signal-regulated kinase 1/2, protein kinase A, or p90(RSK). Wortmannin blocked the phosphorylation of PKC-mu (Ser744/Ser748), suggesting that PI3K is required for the activation of PKCs. PKCs phosphorylate Bad under in vitro conditions, and the association of phosphorylated Bad with PKC-mu or PKC-epsilon, as shown by immunoprecipitation, indicated direct involvement of PKCs in Bad phosphorylation. To confirm these results, cells overexpressing pEGFP-N1, wt-Bad, or Bad with a single site mutated (Ser112Ala; Ser136Ala; Ser155Ala), two sites mutated (Ser(112/136)Ala; Ser(112/155)Ala; Ser(136/155)Ala), or the triple mutant were tested. IGF-I protected completely against rapamycin-induced apoptosis in cells overexpressing wt-Bad and mutants having either one or two sites of phosphorylation mutated. Knockdown of Bid using small interfering RNA showed that Bid is not required for rapamycin-induced cell death. Collectively, these data suggest that IGF-I-induced phosphorylation of Bad at multiple sites via a pathway involving PI3K and PKCs is important for protecting sarcoma cells from rapamycin-induced apoptosis.

Cigarette smoke regulates VEGFR2-mediated survival signaling in rat lungs

Background

Vascular endothelial growth factor (VEGF) and VEGF receptor 2 (VEGFR2)-mediated survival signaling is critical to endothelial cell survival, maintenance of the vasculature and alveolar structure and regeneration of lung tissue. Reduced VEGF and VEGFR2 expression in emphysematous lungs has been linked to increased endothelial cell death and vascular regression. Previously, we have shown that CS down-regulated the VEGFR2 and its downstream signaling in mouse lungs. However, the VEGFR2-mediated survival signaling in response to oxidants/cigarette smoke (CS) is not known. We hypothesized that CS exposure leads to disruption of VEGFR2-mediated endothelial survival signaling in rat lungs.

Methods

Adult male Sprague-Dawley rats were exposed CS for 3 days, 8 weeks and 6 months to investigate the effect of CS on VEGFR2-mediated survival signaling by measuring the Akt/PI3-kinase/eNOS downstream signaling in rat lungs.

Results and Discussion

We show that CS disrupts VEGFR2/PI3-kinase association leading to decreased Akt and eNOS phosphorylation. This may further alter the phosphorylation of the pro-apoptotic protein Bad and increase the Bad/Bcl-xl association. However, this was not associated with a significant lung cell death as evidenced by active caspase-3 levels. These data suggest that although CS altered the VEGFR2-mediated survival signaling in the rat lungs, but it was not sufficient to cause lung cell death.

Conclusion

The rat lungs exposed to CS in acute, sub-chronic and chronic levels may be representative of smokers where survival signaling is altered but was not associated with lung cell death whereas emphysema is known to be associated with lung cell apoptosis.

Apigenin induces apoptosis via downregulation of S-phase kinase-associated protein 2-mediated induction of p27Kip1 in primary effusion lymphoma cells

OBJECTIVE

The mechanisms that regulate mitogenic and antiapoptotic signals in primary effusion lymphoma (PEL) are not well known. In efforts to identify novel approaches to block the proliferation of PEL cells, we assessed the effect of apigenin (4',5,7-trihydroxyflavone), a flavonoid on a panel of PEL cell lines.

MATERIALS AND METHODS

We studied the effect of apigenin on four PEL cell lines. Apoptosis was measured by annexin V/PI dual staining and DNA laddering. Protein expression was measured by immunoblotting.

RESULTS

Apigenin induced apoptosis in PEL cell lines in a dose dependent manner. Such effects of apigenin appeared to result from suppression of constitutively active kinase AKT resulting in down-regulation of SKP2, hypo-phosphorylation of Rb and accumulation of p27Kip1. Apigenin treatment of PEL cells caused dephosphorylation of p-Bad protein leading to down regulation of the anti-apoptotic protein, Bcl-2 and an increase in Bax/Bcl2 ratio. Apigenin treatment also triggered Bax conformational change and subsequently translocation from cytosole to mitochondria causing loss of mitochondrial membrane potential with subsequent release of cytochrome c. Released cytochrome c onto the cytosole activated caspase-9 and caspase-3, followed by polyadenosin-5'-diphosphate-ribose polymerase (PARP) cleavage. Finally, treatment of PEL cells with apigenin down-regulated the expression of inhibitor of apoptosis protein (IAPs).

CONCLUSIONS

Altogether, these data suggest a novel function for apigenin, acting as a suppressor of AKT/PKB pathway in PEL cells, and raise the possibility that this agent may have a future therapeutic role in PEL and possibly other malignancies with constitutive activation of the AKT/PKB pathway.

Assessing Bad sub-cellular localization under conditions associated with prevention or promotion of mitochondrial permeability transition-dependent toxicity

Cells belonging to the monocyte/macrophage lineage are in general highly resistant to peroxynitrite, a reactive nitrogen species extensively produced by these and other cell types under inflammatory conditions. Resistance is not dependent on the scavenging of peroxynitrite but is rather associated with the prompt activation of a survival signaling in response to various molecules largely available at the inflammatory sites, as arachidonic acid and products of the 5-lipoxygenase or cyclooxygenase pathways. We detected significant levels of Bad in the mitochondria of monocytes/macrophages and found that these signaling pathways converge in Bad phosphorylation, and thus in its cytosolic accumulation. Phosphorylation inhibits binding of Bad to Bcl-2, or BclXL, and promotes its translocation to the cytosol, thereby enabling Bcl-2 and BclXL to exert effects leading to prevention of mitochondrial permeability- transition (MPT). Upstream inhibition of the survival signaling indeed promotes the mitochondrial accumulation of Bad and the rapid onset of MPT-dependent toxicity. The above results contribute to the definition of the mechanism(s) whereby monocytes/macrophages survive to peroxynitrite in inflamed tissues.

Trypanosoma cruzi targets Akt in host cells as an intracellular antiapoptotic strategy

The parasite Trypanosoma cruzi, which causes Chagas' disease, differentiates in the cytosol of its host cell and then replicates and spreads infection, processes that require the long-term survival of the infected cells. Here, we show that in the cytosol, parasite-derived neurotrophic factor (PDNF), a trans-sialidase that is located on the surface of T. cruzi, is both a substrate and an activator of the serine-threonine kinase Akt, an antiapoptotic molecule. PDNF increases the expression of the gene that encodes Akt while suppressing the transcription of genes that encode proapoptotic factors. Consequently, PDNF elicits a sustained functional response that protects host cells from apoptosis induced by oxidative stress and the proinflammatory cytokines tumor necrosis factor-alpha and transforming growth factor-beta. Given that PDNF also activates Akt by binding to the neurotrophic surface receptor TrkA, we propose that this protein activates survival signaling both at the cell surface, by acting as a receptor-binding ligand, and inside cells, by acting as a scaffolding adaptor protein downstream of the receptor.

GLP-1 mediates antiapoptotic effect by phosphorylating Bad through a beta-arrestin 1-mediated ERK1/2 activation in pancreatic beta-cells

Strategies based on activating GLP-1 receptor (GLP-1R) are intensively developed for the treatment of type 2 diabetes. The exhaustive knowledge of the signaling pathways linked to activated GLP-1R within the beta-cells is of major importance. In beta-cells, GLP-1 activates the ERK1/2 cascade by diverse pathways dependent on either Galpha(s)/cAMP/cAMP-dependent protein kinase (PKA) or beta-arrestin 1, a scaffold protein. Using pharmacological inhibitors, beta-arrestin 1 small interfering RNA, and islets isolated from beta-arrestin 1 knock-out mice, we demonstrate that GLP-1 stimulates ERK1/2 by two temporally distinct pathways. The PKA-dependent pathway mediates rapid and transient ERK1/2 phosphorylation that leads to nuclear translocation of the activated kinases. In contrast, the beta-arrestin 1-dependent pathway produces a late ERK1/2 activity that is restricted to the beta-cell cytoplasm. We further observe that GLP-1 phosphorylates the cytoplasmic proapoptotic protein Bad at Ser-112 but not at Ser-155. We find that the beta-arrestin 1-dependent ERK1/2 activation engaged by GLP-1 mediates the Ser-112 phosphorylation of Bad, through p90RSK activation, allowing the association of Bad with the scaffold protein 14-3-3, leading to its inactivation. beta-Arrestin 1 is further found to mediate the antiapoptotic effect of GLP-1 in beta-cells through the ERK1/2-p90RSK-phosphorylation of Bad. This new regulatory mechanism engaged by activated GLP-1R involving a beta-arrestin 1-dependent spatiotemporal regulation of the ERK1/2-p90RSK activity is now suspected to participate in the protection of beta-cells against apoptosis. Such signaling mechanism may serve as a prototype to generate new therapeutic GLP-1R ligands.

The role of the mitochondrial permeability transition pore in heart disease

Like Dr. Jeckyll and Mr. Hyde, mitochondria possess two distinct persona. Under normal physiological conditions they synthesise ATP to meet the energy needs of the beating heart. Here calcium acts as a signal to balance the rate of ATP production with ATP demand. However, when the heart is overloaded with calcium, especially when this is accompanied by oxidative stress, mitochondria embrace their darker side, and induce necrotic cell death of the myocytes. This happens acutely in reperfusion injury and chronically in congestive heart failure. Here calcium overload, adenine nucleotide depletion and oxidative stress combine forces to induce the opening of a non-specific pore in the mitochondrial membrane, known as the mitochondrial permeability transition pore (mPTP). The molecular nature of the mPTP remains controversial but current evidence implicates a matrix protein, cyclophilin-D (CyP-D) and two inner membrane proteins, the adenine nucleotide translocase (ANT) and the phosphate carrier (PiC). Inhibition of mPTP opening can be achieved with inhibitors of each component, but targeting CyP-D with cyclosporin A (CsA) and its non-immunosuppressive analogues is the best described. In animal models, inhibition of mPTP opening by either CsA or genetic ablation of CyP-D provides strong protection from both reperfusion injury and congestive heart failure. This confirms the mPTP as a promising drug target in human cardiovascular disease. Indeed, the first clinical trials have shown CsA treatment improves recovery after treatment of a coronary thrombosis with angioplasty.

14-3-3 protein signaling in development and growth factor responses

Tyrosine and serine phosphorylation are central to cellular signaling in growth and development. 14-3-3 proteins function as dimeric phosphoserine-binding proteins with documented interactions throughout the eukaryotic proteome and are highly conserved in both the animal and plant kingdoms. Binding of 14-3-3 to a client protein can have a range of context-dependent effects, including conformational change, enzyme inhibition, a shielding effect, re-localization, and bridging between two molecules. Proteome-based strategies utilizing mass spectrometry have revealed an unprecedented central stage for 14-3-3 in signal transduction with interacting partners composing at least 0.6% of the cellular proteome. 14-3-3 has been shown to bind to the human GM-CSF, IL-3, and IL-5 receptors and is required for the transmission of cell survival. 14-3-3 is involved in survival-specific signals, acting not only at the receptor level but also at critical steps downstream of the receptor. This phosphoserine-mediated pathway works independently of tyrosine kinases, highlighting an alternative mechanism of signaling for this receptor family. Other growth factor receptors and their adaptors are also being shown to associate with 14-3-3 and/or have putative 14-3-3 interaction sequences, such as the prolactin receptor, IGF-1 receptor, and some G-protein coupled receptors. 14-3-3 proteins are remarkably conserved through eukaryotic organisms and in Drosophila are required for photoreceptor development, learning, timing of cell cycles, and maintenance of cellular polarity. These findings are elevating our initial description of biochemical interactions to a better understanding of 14-3-3 function at the level of the whole organism. Further study should explore the integration of phosphoserine and phosphotyrosine signaling by 14-3-3 proteins and the role of isoform-specific functions in higher organisms. The prevalence of functional 14-3-3 binding sites throughout the proteome, and especially among growth factor receptors and signaling molecules, reflects a global role for 14-3-3 in multiple cellular decision making.

BNIP3 subfamily BH3-only proteins: mitochondrial stress sensors in normal and pathological functions

The BNIP3 subfamily of BH3-only proteins consists of BNIP3 and BNIP3-like (BNIP3L) proteins. These proteins form stable homodimerization complexes that localize to the outer membrane of the mitochondria after cellular stress. This promotes either apoptotic or non-apoptotic cell death such as autophagic cell death. Although the mammalian cells contain both members of this subfamily, the genome of Caenorhabditis elegans codes for a single BNIP3 ortholog, ceBNIP3, which shares homology in the transmembrane (TM) domain and in a conserved region close to the BH3 domain of mammalian BNIP3 protein. The cell death activities of BNIP3 and BNIP3L are determined by either the BH3 domain or the C-terminal TM domain. The TM domain of BNIP3 is unique, as it is capable of autonomous stable dimerization and contributes to mitochondrial localization of BNIP3. In knockout mouse models, BNIP3L was shown to be essential for normal erythrocyte differentiation and hematopoietic homeostasis, whereas BNIP3 plays a role in cellular responses to ischemia/reperfusion injury in the heart. Both BNIP3 and BNIP3L play a role in cellular responses to stress. Under hypoxia, both BNIP3 and BNIP3L expression levels are elevated and contribute to hypoxia-induced cell death. In addition, these proteins play critical roles in disease states. In heart disease, both BNIP3 and BNIP3L play a critical role in cardiomyocyte cell death following ischemic and non-ischemic injuries. In cancer, expression of BNIP3 and BNIP3L is downregulated by promoter hypermethylation or by homozygous deletion of the gene locus in certain cancers, whereas their expression was increased in other cancers. In addition, BNIP3 expression has been correlated with poor prognosis in some cancers. The results reviewed here suggest that BNIP3 and BNIP3L may be novel therapeutic targets for intervention because of their pathological roles in regulating cell death in disease states.

BAD: undertaker by night, candyman by day

The BH3-only pro-apoptotic proteins are upstream sensors of cellular damage that selectively respond to specific, proximal death and survival signals. Genetic models and biochemical studies indicate that these molecules are latent killers until activated through transcriptional or post-translational mechanisms in a tissue-restricted and signal-specific manner. The large number of BH3-only proteins, their unique subcellular localization, protein-interaction network and diverse modes of activation suggest specialization of their damage-sensing function, ensuring that the core apoptotic machinery is poised to receive input from a wide range of cellular stress signals. The apoptotic response initiated by the activation of BH3-only proteins ultimately culminates in allosteric activation of pro-apoptotic BAX and BAK, the gateway proteins to the mitochondrial pathway of apoptosis. From activation of BH3-only proteins to oligomerization of BAX and BAK and mitochondrial outer membrane permeabilization, an intricate network of interactions between the pro- and anti-apoptotic members of the BCL-2 family orchestrates the decision to undergo apoptosis. Beyond regulation of apoptosis, multiple BCL-2 proteins have recently emerged as active components of select homeostatic pathways carrying other cellular functions. This review focuses on BAD, which was the first BH3-only protein linked to proximal survival signals through phosphorylation by survival kinases. In addition to findings that delineated the physiological role of BAD in apoptosis and its dynamic regulation by phosphorylation, studies pointing to new roles for this protein in other physiological pathways, such as glucose metabolism, are highlighted. By executing its 'day' and 'night' jobs in metabolism and apoptosis, respectively, BAD helps coordinate mitochondrial fuel metabolism and the apoptotic machinery.

Bid: a Bax-like BH3 protein

Bid, a pro-apoptotic member of the Bcl-2 family, was initially discovered through binding to both pro-apoptotic Bax and anti-apoptotic Bcl-2. During apoptosis, Bid can be cleaved not only by caspase-8 during death receptor apoptotic signaling, but also by other caspases, granzyme B, calpains and cathepsins. Protease-cleaved Bid migrates to mitochondria where it induces permeabilization of the outer mitochondrial membrane that is dependent on the pro-apoptotic proteins Bax and/or Bak, and thus Bid acts as a sentinel for protease-mediated death signals. Although sequence analysis suggests that Bid belongs to the BH3-only subgroup of the Bcl-2 family, structural and phylogenetic analysis suggests that Bid may be more related to multi-BH region proteins such as pro-apoptotic Bax. Analysis of membrane binding by protease-cleaved Bid reveals mechanistic similarities with the membrane binding of Bax. For both proteins, membrane binding is characterized by relief of N-terminal inhibition of sequences promoting migration to membranes, insertion into the bilayer of the central hydrophobic hairpin helices and exposure of the BH3 region. These findings implicate Bid as a BH3-only protein that is both structurally and functionally related to multi-BH region Bcl-2 family proteins such as Bax.

Identification of novel in vivo phosphorylation sites of the human proapoptotic protein BAD: pore-forming activity of BAD is regulated by phosphorylation

BAD is a proapoptotic member of the Bcl-2 protein family that is regulated by phosphorylation in response to survival factors. Although much attention has been devoted to the identification of phosphorylation sites in murine BAD, little data are available with respect to phosphorylation of human BAD protein. Using mass spectrometry, we identified here besides the established phosphorylation sites at serines 75, 99, and 118 several novel in vivo phosphorylation sites within human BAD (serines 25, 32/34, 97, and 124). Furthermore, we investigated the quantitative contribution of BAD targeting kinases in phosphorylating serine residues 75, 99, and 118. Our results indicate that RAF kinases represent, besides protein kinase A, PAK, and Akt/protein kinase B, in vivo BAD-phosphorylating kinases. RAF-induced phosphorylation of BAD was reduced to control levels using the RAF inhibitor BAY 43-9006. This phosphorylation was not prevented by MEK inhibitors. Consistently, expression of constitutively active RAF suppressed apoptosis induced by BAD and the inhibition of colony formation caused by BAD could be prevented by RAF. In addition, using the surface plasmon resonance technique, we analyzed the direct consequences of BAD phosphorylation by RAF with respect to association with 14-3-3 and Bcl-2/Bcl-X(L) proteins. Phosphorylation of BAD by active RAF promotes 14-3-3 protein association, in which the phosphoserine 99 represented the major binding site. Finally, we show here that BAD forms channels in planar bilayer membranes in vitro. This pore-forming capacity was dependent on phosphorylation status and interaction with 14-3-3 proteins. Collectively, our findings provide new insights into the regulation of BAD function by phosphorylation.

The PKARIalpha subunit of protein kinase A modulates the activation of p90RSK1 and its function

Previously, we showed that interactions between p90(RSK1) (RSK1) and the subunits of type I protein kinase A (PKA) regulate the activity of PKA and cellular distribution of active RSK1 (Chaturvedi, D., Poppleton, H. M., Stringfield, T., Barbier, A., and Patel, T. B. (2006) Mol. Cell Biol. 26, 4586-4600). Here we examined the role of the PKARIalpha subunit of PKA in regulating RSK1 activation and cell survival. In mouse lung fibroblasts, silencing of the PKARIalpha increased the phosphorylation and activation of RSK1, but not of RSK2 and RSK3, in the absence of any stimulation. Silencing of PKARIalpha also decreased the nuclear accumulation of active RSK1 and increased its cytoplasmic content. The increased activation of RSK1 in the absence of any agonist and changes in its subcellular redistribution resulted in increased phosphorylation of its cytoplasmic substrate BAD and increased cell survival. The activity of PKA and phosphorylation of BAD (Ser-155) were also enhanced when PKARIalpha was silenced, and this, in part, contributed to increased cell survival in unstimulated cells. Furthermore, we show that RSK1, PKA subunits, D-AKAP1, and protein phosphatase 2A catalytic subunit (PP2Ac) exist in a complex, and dissociation of RSK1 from D-AKAP1 by either silencing of PKARIalpha, depletion of D-AKAP1, or by using a peptide that competes with PKARIalpha for binding to AKAPs, decreased the amount of PP2Ac in the RSK1 complex. We also demonstrate that PP2Ac is one of the phosphatases that dephosphorylates RSK, but not ERK1/2. Thus, in unstimulated cells, the increased phosphorylation and activation of RSK1 after silencing of PKARIalpha or depletion of D-AKAP1 are due to decreased association of PP2Ac in the RSK1 complex.

At the crossroads of homoeostasis and disease: roles of the PACS proteins in membrane traffic and apoptosis

The endomembrane system in mammalian cells has evolved over the past two billion years from a simple endocytic pathway in a single-celled primordial ancestor to complex networks supporting multicellular structures that form metazoan tissue and organ systems. The increased organellar complexity of metazoan cells requires additional trafficking machinery absent in yeast or other unicellular organisms to maintain organ homoeostasis and to process the signals that control proliferation, differentiation or the execution of cell death programmes. The PACS (phosphofurin acidic cluster sorting) proteins are one such family of multifunctional membrane traffic regulators that mediate organ homoeostasis and have important roles in diverse pathologies and disease states. This review summarizes our current knowledge of the PACS proteins, including their structure and regulation in cargo binding, their genetics, their roles in secretory and endocytic pathway traffic, interorganellar communication and how cell-death signals reprogramme the PACS proteins to regulate apoptosis. We also summarize our current understanding of how PACS genes are dysregulated in cancer and how viral pathogens ranging from HIV-1 to herpesviruses have evolved to usurp the PACS sorting machinery to promote virus assembly, viral spread and immunoevasion.

Molecular profiles of Quadriceps muscle in myostatin-null mice reveal PI3K and apoptotic pathways as myostatin targets

Background

Myostatin (MSTN), a member of the TGF-β superfamily, has been identified as a negative regulator of skeletal muscle mass. Inactivating mutations in the MSTN gene are responsible for the development of a hypermuscular phenotype. In this study, we performed transcriptomic and proteomic analyses to detect altered expression/abundance of genes and proteins. These differentially expressed genes and proteins may represent new molecular targets of MSTN and could be involved in the regulation of skeletal muscle mass.

Results

Transcriptomic analysis of the Quadriceps muscles of 5-week-old MSTN-null mice (n = 4) and their controls (n = 4) was carried out using microarray (human and murine oligonucleotide sequences) of 6,473 genes expressed in muscle. Proteomic profiles were analysed using two-dimensional gel electrophoresis coupled with mass spectrometry. Comparison of the transcriptomic profiles revealed 192 up- and 245 down- regulated genes. Genes involved in the PI3K pathway, insulin/IGF pathway, carbohydrate metabolism and apoptosis regulation were up-regulated. Genes belonging to canonical Wnt, calcium signalling pathways and cytokine-receptor cytokine interaction were down-regulated. Comparison of the protein profiles revealed 20 up- and 18 down-regulated proteins spots. Knockout of the MSTN gene was associated with up-regulation of proteins involved in glycolytic shift of the muscles and down-regulation of proteins involved in oxidative energy metabolism. In addition, an increased abundance of survival/anti-apoptotic factors were observed.

Conclusion

All together, these results showed a differential expression of genes and proteins related to the muscle energy metabolism and cell survival/anti-apoptotic pathway (e.g. DJ-1, PINK1, 14-3-3ε protein, TCTP/GSK-3β). They revealed the PI3K and apoptotic pathways as MSTN targets and are in favour of a role of MSTN as a modulator of cell survival in vivo.

Role of protein kinase C-iota in transformed non-malignant RWPE-1 cells and androgen-independent prostate carcinoma DU-145 cells

Prostate cancer is one of the leading causes of death among men in the USA.

OBJECTIVE

In this study, we investigated the role of atypical protein kinase C-iota (PKC-iota) in androgen independent prostate DU-145 carcinoma cellscompared to transformed non-malignant prostate RWPE-1 cells.

MATERIALS AND METHODS

Western blotting and immunoprecipitations demonstrated that PKC-iotaisassociated with cyclin-dependent kinase activating kinase (CAK/Cdk7) in RWPE-1 cells, but not in DU-145 cells.

RESULTS

Treatment of prostate RWPE-1 cells with PKC-iota silencing RNA (siRNA) decreased cell viability,cell-cycle accumulation at G2/M phase, and phosphorylation of Cdk7 and Cdk2. In addition, PKC-iota siRNA treatment caused less phosphorylation ofBad at ser-155, ser-136, and greater Bad/Bcl-xL heterodimerization, leading to apoptosis. In DU-145 cells, PKC-iota was anti-apoptotic and was required for cell survival. Treatment with PKC-iota siRNA blocked increase in cell number, and inhibited G1/S transition by accumulation of cells in G0/G1phase. In addition to cell-cycle arrest, both RWPE-1 and DU-145 cells underwent apoptosis due to mitochondrial dysfunction and apoptosis cascades, such as release of cytochrome c,activation of caspase-7, and poly (ADP-ribose)polymerase (PARP) cleavage.

CONCLUSION

Our results suggest that PKC-iota is required for cell survival in both transformed non-malignant prostate RWPE-1 cells and androgen-independent malignant prostate DU-145 cells, whereas suppressing PKC-iota lead to apoptosis in DU-145 prostate cells.

Helicobacter pylori regulates cellular migration and apoptosis by activation of phosphatidylinositol 3-kinase signaling

Helicobacter pylori is the strongest identified risk factor for gastric adenocarcinoma. One H. pylori virulence constituent that augments cancer risk is the cag secretion system, which translocates CagA and peptidoglycan into host cells, eventuating in activation of signal transduction pathways. AKT is a target of phosphatidylinositol 3-kinase (PI3K) and is activated in gastric cancer, but the relationship between PI3K-AKT and H. pylori-induced cellular responses with carcinogenic potential remains unclear. We defined the molecular pathways mediating H. pylori-stimulated AKT activation and the biological consequences of these events in gastric epithelial cells. H. pylori enhanced PI3K-AKT signaling in a Src- and epidermal growth factor receptor-dependent manner, which was also mediated by a functional cag secretion system and peptidoglycan. PI3K activation attenuated apoptosis in response to infection and was required for H. pylori-induced cell migration. These results indicate that PI3K-AKT signaling regulates pathophysiologic responses to H. pylori that may lower the threshold for carcinogenesis.

Bad targets the permeability transition pore independent of Bax or Bak to switch between Ca2+-dependent cell survival and death

Calcium oscillations exert physiological control on mitochondrial energy metabolism and can also lead to mitochondrial membrane permeabilization and cell death. The outcome of the mitochondrial calcium signaling is altered by stress factors such as ceramide or staurosporine. However, the mechanism of this proapoptotic switch remains unclear. Using genetic, biochemical, pharmacological, and functional approaches, we here show that ceramide and staurosporine target PP2A and protein kinases A and C, respectively, in a mitochondria-associated signaling complex to induce dephosphorylation of the BH3-only protein Bad. Dephosphorylated Bad sensitizes the mitochondrial permeability transition pore (PTP) to Ca2+ through a Bcl-xL-sensitive and VDAC-mediated process. Furthermore, the Bad-induced sensitization of the PTP to Ca2+ does not require Bax or Bak. Thus, phospho-regulatory mechanisms converge on Bad to switch between the survival and apoptotic functions of mitochondrial calcium signaling by activating a mechanism whereby a BH3-only protein bypasses Bax/Bak and engages the PTP.

Oscillations of the p53-Akt network: implications on cell survival and death

Intracellular protein levels of p53 and MDM2 have been shown to oscillate in response to ionizing radiation (IR), but the physiological significance of these oscillations remains unclear. The p53-MDM2 negative feedback loop – the putative cause of the oscillations – is embedded in a network involving a mutual antagonism (or positive feedback loop) between p53 and AKT. We have shown earlier that this p53-AKT network predicts an all-or-none switching behavior between a pro-survival cellular state (low p53 and high AKT levels) and a pro-apoptotic state (high p53 and low AKT levels). Here, we show that upon exposure to IR, the p53-AKT network can also reproduce the experimentally observed p53 and MDM2 oscillations. The present work is based on the hypothesis that the physiological significance of the experimentally observed oscillations could be found in their role in regulating the switching behavior of the p53-AKT network between pro-survival and pro-apoptotic states. It is shown here that these oscillations are associated with a significant decrease in the threshold level of IR at which switching from a pro-survival to a pro-apoptotic state occurs. Moreover, oscillations in p53 protein levels induce higher levels of expression of p53-target genes compared to non-oscillatory p53, and thus influence cell-fate decisions between cell cycle arrest/DNA damage repair versus apoptosis.

C-jun N-terminal kinase regulates the interaction between 14-3-3 and Bad in ethanol-induced cell death

Activation of the c-jun N-terminal kinase (JNK) is known to be an important step during ethanol-induced cell death, but it has yet to be identified how JNK regulates apoptosis. Therefore, we investigated the mechanism by which JNK induces cell death following ethanol treatment. Ethanol (6 g/kg, 20% in saline) was administered subcutaneously to postnatal 7 day rat pups. Twelve hours after the first ethanol administration, rat pups were decapitated, and extracts of total protein from cerebral cortices were prepared. Ethanol exposure induced phosphorylation of JNK but did not affect the expression levels of pro- and antiapoptotic proteins. Furthermore, interactions of phospho-JNK (p-JNK) with 14-3-3 as well as with Bad were enhanced in the cerebral cortices of ethanol-treated rats. Pretreatment with JNK inhibitor (SP600125) of SH-SY5Y cells inhibited JNK phosphorylation and interaction between p-JNK and 14-3-3 resulting from ethanol. Furthermore, 14-3-3 interaction with Bad was diminished in the cerebral cortices of ethanol-treated rats. These findings suggest that JNK induces Bad release from 14-3-3 by inhibiting their interaction. After this event, Bad binds to Bcl-xL, releasing Bax from Bcl-xL and leading to cell death. We hypothesize that JNK may play an important role during ethanol-induced cell death via the inhibition of antiapoptotic function of 14-3-3 as well as activation of proapoptotic function of Bad.

{beta}-Arrestin-2 Mediates Anti-apoptotic Signaling through Regulation of BAD Phosphorylation

β-Arrestins, originally discovered as terminators of G protein-coupled receptor signaling, have more recently been appreciated to also function as signal transducers in their own right, although the consequences for cellular physiology have not been well understood. Here we demonstrate that β-arrestin-2 mediates anti-apoptotic cytoprotective signaling stimulated by a typical 7-transmembrane receptor the angiotensin ATII 1A receptor, expressed endogenously in rat vascular smooth muscle cells or by transfection in HEK-293 cells. Receptor stimulation leads to concerted activation of two pathways, ERK/p90RSK and PI3K/AKT, which converge to phosphorylate and inactivate the pro-apoptotic protein BAD. Anti-apoptotic effects as well as pathway activities can be stimulated by an angiotensin analog (SII), which has been previously shown to activate β-arrestin but not G protein-dependent signaling, and are abrogated by β-arrestin-2 small interfering RNA. These findings establish a key role for β-arrestin-2 in mediating cellular cytoprotective functions by a 7-transmembrane receptor and define the biochemical pathways involved.

A novel resveratrol derivative, HS1793, overcomes the resistance conferred by Bcl-2 in human leukemic U937 cells

The chemopreventive and chemotherapeutic properties associated with resveratrol offer promise for the design of new chemotherapeutic agents. However, resveratrol is not a potent cytotoxic compound when compared with other chemotherapeutic drugs. Thus, several studies were undertaken to obtain synthetic analogues of resveratrol with potent activity. The present study was undertaken to examine whether four resveratrol analogues (HS-1784, -1792, -1791 and -1793) that we had designed and synthesized show antitumor activity. Here, we observed that all of these resveratrol analogues displayed stronger antitumor effects than resveratrol in most cancer cells tested. We further examined whether HS-1793, showing potent antitumor effects in most cancer cells tested, overcomes the resistance conferred by Bcl-2, since overcoming the resistance conferred by Bcl-2 represents an attractive therapeutic strategy against cancer. Our viability assay showed that HS-1793 overcomes the resistance conferred by Bcl-2 in human leukemic U937 cells. Various apoptosis assessment assays demonstrated that HS-1793 overcomes the resistance conferred by Bcl-2 in human leukemic U937 cells by inducing apoptosis. Noticeably, we elucidated the marked downregulation of 14-3-3 protein by HS-1793, indicating that HS-1793 overcomes the resistance conferred by Bcl-2 in U937 cells via 14-3-3. We also observed that HS-1793 exerts its antitumor activity via Bad. However, overall data obtained from methylation specific PCR, RT-PCR and real-time PCR suggest that HS-1793 plays a role in the downregulation of 14-3-3 at a post-transcriptional level. Further understanding exactly how HS-1793 overcomes the resistance conferred by Bcl-2 via 14-3-3 may guide the development of future anticancer agents.

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

The role of Akt/protein kinase B subtypes in retinal ischemic preconditioning

Potent endogenous protection from ischemia can be induced in the retina by ischemic preconditioning (IPC). Protein kinase B/Akt is a cellular survival factor. We hypothesized that Akt was integral to IPC based upon differential effects of Akt subtypes. Rats were subjected to retinal ischemia after IPC or IPC-mimicking by the opening of mitochondrial KATP (mKATP) channels. The effects of blocking Akt using wortmannin, API-2, or small interfering RNA (siRNA) were examined. Electroretinography assessed functional recovery after ischemia, and TUNEL examined retinal ganglion cell apoptosis. We studied the relationship between Akt activation and known initiators of IPC, including adenosine receptor stimulation and the opening of mKATP channels. The PI-3 kinase inhibitor wortmannin 1 or 4 mg/kg (i.p.), the specific Akt inhibitor API-2, 5-500 microM in the vitreous, or intravitreal siRNA directed against Akt2 or -3, but not Akt1, significantly attenuated the neuroprotective effect of IPC. Interfering RNA against any of the three Akt subtypes significantly but time-dependently attenuated mKATP channel opening to mimic IPC. Adenosine A1 receptor blockade (DPCPX), A2a blockade (CSC), or the mKATP channel blocker 5-hydroxydecanoic acid significantly attenuated Akt activation after IPC. Interfering RNA directed against Akt subtypes prevented the ameliorative effect of IPC on post-ischemic apoptosis. All three Akt subtypes are involved in functional retinal neuroprotection by IPC or IPC-mimicking. Akt is downstream of adenosine A1 and A2a receptors and mKATP channel opening. The results indicate the presence in the retina of robust and redundant endogenous neuroprotection based upon subtypes of Akt.

MicroRNA-184 antagonizes microRNA-205 to maintain SHIP2 levels in epithelia

Despite their potential to regulate approximately one-third of the whole genome, relatively few microRNA (miRNA) targets have been experimentally validated, particularly in stratified squamous epithelia. Here we demonstrate not only that the lipid phosphatase SHIP2 is a target of miRNA-205 (miR-205) in epithelial cells, but, more importantly, that the corneal epithelial-specific miR-184 can interfere with the ability of miR-205 to suppress SHIP2 levels. This is the first example of a miRNA negatively regulating another to maintain levels of a target protein. Interfering with miR-205 function by using a synthetic antagomir, or by the ectopic expression of miR-184, leads to a coordinated damping of the Akt signaling pathway via SHIP2 induction. This was associated with a marked increase in keratinocyte apoptosis and cell death. Aggressive squamous cell carcinoma (SCC) cells exhibited elevated levels of miR-205. This was associated with a concomitant reduction in SHIP2 levels. Partial knockdown of endogenous miR-205 in SCCs markedly decreased phosphorylated Akt and phosphorylated BAD levels and increased apoptosis. We were able to increase SHIP2 levels in SCC cells after inhibition of miR-205. Therefore, miR-205 might have diagnostic value in determining the aggressivity of SCCs. Blockage of miR-205 activity with an antagomir or via ectopic expression of miR-184 could be novel therapeutic approaches for treating aggressive SCCs.

PI3K/Akt: getting it right matters

The Akt serine/threonine kinase (also called protein kinase B) has emerged as a critical signaling molecule within eukaryotic cells. Significant progress has been made in clarifying its regulation by upstream kinases and identifying downstream mechanisms that mediate its effects in cells and contribute to signaling specificity. Here, we provide an overview of present advances in the field regarding the function of Akt in physiological and pathological cell function within a more generalized framework of Akt signal transduction. An emphasis is placed on the involvement of Akt in human diseases ranging from cancer to metabolic dysfunction and mental disease.

PGE2 potentiates tonicity-induced COX-2 expression in renal medullary cells in a positive feedback loop involving EP2-cAMP-PKA signaling

Cyooxygenase-2 (COX-2)-derived PGE2 is critical for the integrity and function of renal medullary cells during antidiuresis. The present study extended our previous finding that tonicity-induced COX-2 expression is further stimulated by the major COX-2 product PGE2 and investigated the underlying signaling pathways and the functional relevance of this phenomenon. Hyperosmolality stimulated COX-2 expression and activity in Madin-Darby canine kidney (MDCK) cells, a response that was further increased by PGE2-cAMP signaling, suggesting the existence of a positive feedback loop. This effect was diminished by AH-6809, an EP2 antagonist, and by the PKA inhibitor H-89, but not by AH-23848, an EP4 antagonist. The effect of PGE2 was mimicked by forskolin and dibutyryl-cAMP, suggesting that the stimulatory effect of PGE2 on COX-2 is mediated by a cAMP-PKA-dependent mechanism. Accordingly, cAMP-responsive element (CRE)-driven reporter activity paralleled the effects of PGE2, AH-6809, AH-23848, H-89, forskolin, and dibutyryl-cAMP on COX-2 expression. In addition, the stimulatory effect of PGE2 on tonicity-induced COX-2 expression was blunted in cells transfected with dominant-negative CRE binding (CREB) protein, as was the case in a COX-2 promoter reporter construct in which a putative CRE was deleted. Furthermore, PGE2 resulted in PKA-dependent phosphorylation of the pro-apoptotic protein Bad at Ser155, a mechanism that is known to inactivate Bad, which coincided with reduced caspase-3 activity during osmotic stress. Conversely, pharmacological interruption of the PGE2-EP2-cAMP-PKA pathway abolished Ser155 phosphorylation of Bad and blunted the protective effect of PGE2 on cell survival during osmotic stress. These observations indicate the existence of a positive feedback loop of PGE2 on COX-2 expression during osmotic stress, an effect that apparently is mediated by EP2-cAMP-PKA signaling, and that contributes to cell survival under hypertonic conditions.

Therapeutic modulation of apoptosis: targeting the BCL-2 family at the interface of the mitochondrial membrane

A vast portion of human disease results when the process of apoptosis is defective. Disorders resulting from inappropriate cell death range from autoimmune and neurodegenerative conditions to heart disease. Conversely, prevention of apoptosis is the hallmark of cancer and confounds the efficacy of cancer therapeutics. In the search for optimal targets that would enable the control of apoptosis, members of the BCL-2 family of anti- and pro-apoptotic factors have figured prominently. Development of BCL-2 antisense approaches, small molecules, and BH3 peptidomimetics has met with both success and failure. Success-because BCL-2 proteins play essential roles in apoptosis. Failure-because single targets for drug development have limited scope. By examining the activity of the BCL-2 proteins in relation to the mitochondrial landscape and drawing attention to the significant mitochondrial membrane alterations that ensue during apoptosis, we demonstrate the need for a broader based multi-disciplinary approach for the design of novel apoptosis-modulating compounds in the treatment of human disease.

Lipid mediators in innate immunity against tuberculosis: opposing roles of PGE2 and LXA4 in the induction of macrophage death

Virulent Mycobacterium tuberculosis (Mtb) induces a maladaptive cytolytic death modality, necrosis, which is advantageous for the pathogen. We report that necrosis of macrophages infected with the virulent Mtb strains H37Rv and Erdmann depends on predominant LXA(4) production that is part of the antiinflammatory and inflammation-resolving action induced by Mtb. Infection of macrophages with the avirulent H37Ra triggers production of high levels of the prostanoid PGE(2), which promotes protection against mitochondrial inner membrane perturbation and necrosis. In contrast to H37Ra infection, PGE(2) production is significantly reduced in H37Rv-infected macrophages. PGE(2) acts by engaging the PGE(2) receptor EP2, which induces cyclic AMP production and protein kinase A activation. To verify a role for PGE(2) in control of bacterial growth, we show that infection of prostaglandin E synthase (PGES)(-/-) macrophages in vitro with H37Rv resulted in significantly higher bacterial burden compared with wild-type macrophages. More importantly, PGES(-/-) mice harbor significantly higher Mtb lung burden 5 wk after low-dose aerosol infection with virulent Mtb. These in vitro and in vivo data indicate that PGE(2) plays a critical role in inhibition of Mtb replication.

Repeated electroconvulsive seizure induces c-Myc down-regulation and Bad inactivation in the rat frontal cortex

Repeated electroconvulsive seizure (ECS), a model for electroconvulsive therapy (ECT), exerts neuroprotective and proliferative effects in the brain. This trophic action of ECS requires inhibition of apoptotic activity, in addition to activation of survival signals. c-Myc plays an important role in apoptosis of neurons, in cooperation with the Bcl-2 family proteins, and its activity and stability are regulated by phosphorylation and ubiquitination. We examined c-Myc and related proteins responsible for apoptosis after repeated ECS. In the rat frontal cortex, repeated ECS for 10 days reduced the total amount of c-Myc, while increasing phosphorylation of c-Myc at Thr58, which reportedly induces degradation of c-Myc. As expected, ubiquitination of both phosphorylated and total c-Myc increased after 10 days ECS, suggesting that ECS may reduce c-Myc protein level via ubiquitination-proteasomal degradation. Bcl-2 family proteins, caspase, and poly(ADP-ribose) polymerase (PARP) were investigated to determine the consequence of down-regulating c-Myc. Protein levels of Bcl-2, Bcl-X(L), Bax, and Bad showed no change, and cleavage of caspase-3 and PARP were not induced. However, phosphorylation of Bad at Ser-155 and binding of Bad to 14-3-3 increased without binding to Bcl-X(L) after repeated ECS, implying that repeated ECS sequesters apoptotic Bad and frees pro-survival Bcl-XL. Taken together, c-Myc down-regulation via ubiquitination-proteasomal degradation and Bad inactivation by binding to 14-3-3 may be anti-apoptotic mechanisms elicited by repeated ECS in the rat frontal cortex. This finding further supports the trophic effect of ECS blocking apoptosis as a possible therapeutic effect of ECT.

Downregulation of hepatoma-derived growth factor activates the Bad-mediated apoptotic pathway in human cancer cells

Hepatoma-derived growth factor (HDGF) is highly expressed in human cancer and its expression is correlated with poor prognosis of cancer. The growth factor is known to stimulate cell growth while the underlying mechanism is however not clear. Transfection with HDGF cDNA stimulated while its specific antisense oligonucleotides repressed the growth of human hepatocellular carcinoma HepG2 cells. Furthermore, knock-down of HDGF by antisense oligos also induced apoptosis in HepG2 cells and in other human cancer cells, e.g. human squamous carcinoma A431 cells. HDGF knock-down was found to induce the expression of the pro-apoptotic protein Bad and also inactivate ERK and Akt, which in turn led to dephosphorylation of Bad at Ser-112, Ser-136, and activation of the intrinsic apoptotic pathway, i.e. depolarization of the mitochondrial membrane, release of mitochondrial cytochrome c, increase in the processing of caspase 9 and 3. As HDGF knock-down not only suppresses the growth but also induces apoptosis in human cancer cells, HDGF may therefore serve as a survival factor for human cancer cells and a potential target for cancer therapy.

Fish oil targets PTEN to regulate NFkappaB for downregulation of anti-apoptotic genes in breast tumor growth

The molecular mechanism for the beneficial effect of fish oil on breast tumor growth is largely undefined. Using the xenograft model in nude mice, we for the first time report that the fish oil diet significantly increased the level of PTEN protein in the breast tumors. In addition, the fish oil diet attenuated the PI 3 kinase and Akt kinase activity in the tumors leading to significant inhibition of NFkappaB activation. Fish oil diet also prevented the expression of anti-apoptotic proteins Bcl-2 and Bcl-XL in the breast tumors with concomitant increase in caspase 3 activity. To extend these findings we tested the functional effects of DHA and EPA, the two active omega-3 fatty acids of fish oil, on cultured MDA MB-231 cells. In agreement with our in vivo data, DHA and EPA treatment increased PTEN mRNA and protein expression and inhibited the phosphorylation of p65 subunit of NFkappaB in MDA MB-231 cells. Furthermore, DHA and EPA reduced expression of Bcl-2 and Bcl-XL. NFkappaB DNA binding activity and NFkappaB-dependent transcription of Bcl-2 and Bcl-XL genes were also prevented by DHA and EPA treatment. Finally, we showed that PTEN expression significantly inhibited NFkappaB-dependent transcription of Bcl-2 and Bcl-XL genes. Taken together, our data reveals a novel signaling pathway linking the fish oil diet to increased PTEN expression that attenuates the growth promoting signals and augments the apoptotic signals, resulting in breast tumor regression.

Tumour cell survival signalling by the ERK1/2 pathway

Several advances in recent years have focused increasing attention on the role of the RAF-MEK-ERK1/2 pathway in promoting cell survival. The demonstration that BRAF is a human oncogene mutated at high frequency in melanoma, thyroid and colon cancer has provided a pathophysiological context, whilst the description of potent and highly selective inhibitors of BRAF or MEK has allowed a more informed and rational intervention in both normal and tumour cells. In addition, separate studies have uncovered new mechanisms by which the ERK1/2 pathway can control the activity or abundance of members of the BCL-2 protein family to promote cell survival. It is now apparent that various oncogenes co-opt ERK1/2 signalling to de-regulate these BCL-2 proteins and this contributes to, and even underpins, survival signalling in some tumours. New oncogene-targeted therapies allow direct or indirect inhibition of ERK1/2 signalling and can cause quite striking tumour cell death. In other cases, inhibition of the ERK1/2 pathway may be more effective in combination with other conventional and novel therapeutics. Here, we review recent advances in our understanding of how the ERK1/2 pathway regulates BCL-2 proteins to promote survival, how this is de-regulated in tumour cells and the opportunities this might afford with the use of new targeted therapies.

Regulation of caspase-3 activity by insulin in skeletal muscle cells involves both PI3-kinase and MEK-1/2

A hallmark of skeletal muscle atrophy is increased activities of several proteolytic systems, including caspase-3. We have previously shown that conditions involving insulin deficiency or insulin resistance increase both overall protein degradation and caspase-3-mediated actin cleavage. In the present experiments, we examined how insulin regulates caspase-3 activity in L6 myotubes. Reducing the serum concentration in the culture media from 2 to 0.5% overnight increased caspase-3 activity and actin cleavage. Addition of insulin to proteolytically active cells attenuated both responses within 4 h. Individually, inhibitors of either phosphatidylinositide 3-kinase (PI3K) or MEK1/2 partially blocked the insulin-induced reduction in caspase-3 activity; in combination, the inhibitors completely prevented insulin from attenuating caspase-3 activity. Insulin suppressed caspase-3 activity by a complex mechanism that included direct inhibition due to an increased interaction between caspase-3 and cellular inhibitor of apoptosis-1 and indirect inhibition via phosphorylation (i.e., inactivation) of the proapoptotic protein Bad, which participates in the intrinsic (i.e., mitochondrial) apoptosis activation cascade. Unlike other cell types, the phosphorylation of Bad Ser112 was mediated by the PI3K/Akt pathway rather than the MEK/ERK/ribosomal S6 protein kinase pathway. In summary, our findings indicate that insulin regulates caspase-3 activity by a multistep process that is unique to skeletal muscle, thus providing insights about the muscle-specific nature of the atrophy process.

Cooperative effect of p21Cip1/WAF-1 and 14-3-3sigma on cell cycle arrest and apoptosis induction by p14ARF

P14(ARF) (p19(ARF) in the mouse) plays a central role in the regulation of cellular proliferation. Although the capacity of p14(ARF) to induce a cell cycle arrest in G1 phase depends on a functional p53/p21-signaling axis, the G2 arrest triggered by p14(ARF) is p53/p21-independent. Using isogeneic HCT116 cells either wild-type or homozygously deleted for p21, 14-3-3sigma or both, we further investigated the cooperative effect of p21 and 14-3-3sigma on cell cycle regulation and apoptosis induction by p14(ARF). In contrast to DNA damage, which induces mitotic catastrophe in 14-3-3sigma-deficient cells, we show here that the expression of p14(ARF) triggers apoptotic cell death, as evidenced by nuclear DNA fragmentation and induction of pan-caspase activities, irrespective of the presence or absence of 14-3-3sigma. The activation of the intrinsic mitochondrial apoptosis pathway by p14(ARF) was confirmed by cytochrome c release from mitochondria and induction of caspase-9- (LEHDase) and caspase-3/7-like (DEVDase) activities. Moreover, 14-3-3sigma/p21 double-deficient cells were exceedingly sensitive to apoptosis induction by p14(ARF) as compared to wild-type cells or cells lacking either gene alone. Notably, p14(ARF)-induced apoptosis was preceded by an arrest in the G2 phase of cell cycle, which coincided with downregulation of cdc2 (cdk1) protein expression and lack of its nuclear localization. This indicates that p14(ARF) impairs mitotic entry by targeting the distal DNA damage-signaling pathway and induces apoptotic cell death, rather than mitotic catastrophe, out of a transient G2 arrest. Furthermore, our data delineate that the disruption of G2/M cell cycle checkpoint control critically determines the sensitivity of the cell toward p14(ARF)-induced mitochondrial apoptosis.

Purified extract from Clematis mandshurica prevents adenoviral-TRAIL induced apoptosis on rat articular chondrocytes

Since TNF-related apoptosis inducing ligand (TRAIL) is one of several apoptotic stimuli on articular chondrocytes, the modulation of the mechanism mediated by TRAIL could be considered as a novel strategy for the treatment of osteoarthritis (OA). Previous studies demonstrated that Clematis mandshurica prevents staurosporin-induced apoptosis in articular chondrocytes. This study was undertaken to examine whether Clematis mandshurica could prevent TRAIL-induced apoptosis in articular chondrocytes. Our data show that Clematis mandshurica prevents adenoviral TRAIL (Ad-TRAIL)-induced apoptosis in primary cultured articular chondrocytes. Clematis mandshurica prevents Ad-TRAIL-induced down-regulation of 14-3-3 and phosphorylated Akt. In addition, Clematis mandshurica treatment prevents the Ad-TRAIL-induced reduction of the interactions between 14-3-3 with phospho-ser112-Bad and phospho-ser136-Bad, and BcL-xL with phospho-ser155-Bad. A better understanding of the mechanism underlying inhibition of apoptosis in OA chondrocytes by Clematis mandshurica might lead to the development of a new therapeutic strategy for OA.

Anoikis: a necessary death program for anchorage-dependent cells

Cell to matrix adhesion is a key factor for cellular homeostasis and disruption of such interaction has adverse effects on cell survival. It leads to a specific type of apoptosis known as "anoikis" in most non-transformed cell types. This kind of apoptosis following loss of cell anchorage is important for development, tissue homeostasis and several diseases. Integrins sense mechanical forces arising from the matrix, thereby converting these stimuli to downstream signals modulating cell viability. Anchorage-independent growth is a crucial step during tumorigenesis and in particular during the metastatic spreading of cancer cells. The disruption of the tight control leading an "homeless" cell to death is therefore able to violate the cell defences against transformation. This review analyses the recent investigations into the molecular mechanisms governing anoikis, discussing the different ways in which adhesion can influence this process and addressing the relevance of this unique apoptosis mode in the development of metastatic cancers, as well as in other diseases.

Apoptosis induced by ardipusilloside III through BAD dephosphorylation and cleavage in human glioblastoma U251MG cells

Ardipusilloside III is a saponin newly isolated from Ardisia pusilla A.DC. Since saponins have exhibited broad anti-cancer and pro-apoptotic activity, we investigated the ability of ardipusilloside III to induce apoptosis in human glioblastoma U251MG cells, as well as the involvement of apoptotic signaling pathways. Ardipusilloside III markedly suppressed proliferation of U251MG cells in a time- and dose-dependent manner (P < 0.05, IC50 = 8.2 microg/ml), but did not affect the growth of primary cultures of human astrocytes. Ardipusilloside III-treated U251MG cells underwent typical apoptotic changes. Exposure to a low dose of ardipusilloside III provoked G2/M-phase cell cycle arrest, which preceded apoptosis characterized by the appearance of cells with sub-G1 DNA content. However, a higher dose of ardipusilloside III induced apoptosis without first causing cell cycle arrest. In addition, ardipusilloside III exposure resulted in time-dependent BAD dephosphorylation and cleavage as well as activation of caspase-8 and caspase-3. Therefore, both the intrinsic pathway of apoptosis, mediated by BAD dephosphorylation and cleavage, and the extrinisic pathway of apoptosis, mediated by caspase-8 and caspase-3 activation, were involved in ardipusilloside III-induced apoptosis. These data suggest that ardipusilloside III is a reliable candidate for chemotherapeutic treatment of human glioblastomas, and should be investigated further.

Depletion of 14-3-3 zeta elicits endoplasmic reticulum stress and cell death, and increases vulnerability to kainate-induced injury in mouse hippocampal cultures

14-3-3 proteins are ubiquitous signalling molecules that regulate development and survival pathways in brain. Altered expression and cellular localization of 14-3-3 proteins has been implicated in neurodegenerative diseases and in neuronal death after acute neurological insults, including seizures. Presently, we examined expression and function of 14-3-3 isoforms in vitro using mouse organotypic hippocampal cultures. Treatment of cultures with the endoplasmic reticulum (ER) stressor tunicamycin caused an increase in levels of 14-3-3 zeta within the ER-containing microsomal fraction, along with up-regulation of Lys-Asp-Glu-Leu-containing proteins and calnexin, and the selective death of dentate granule cells. Depletion of 14-3-3 zeta levels using small interfering RNA induced both ER stress proteins and death of granule cells. Treatment of hippocampal cultures with the excitotoxin kainic acid increased levels of Lys-Asp-Glu-Leu-containing proteins and microsomal 14-3-3 zeta levels and caused cell death within the CA1, CA3 and dentate gyrus of the hippocampus. Kainic acid-induced damage was significantly increased in each hippocampal subfield of cultures treated with small interfering RNA targeting 14-3-3 zeta. The present data indicate a role for 14-3-3 zeta in survival responses following ER stress and possibly protection against seizure injury to the hippocampus.

Protein phosphatase-1 inhibitor-3 is an in vivo target of caspase-3 and participates in the apoptotic response

Inh3 (inhibitor-3) is a potent inhibitor of protein phosphatase-1 that selectively associates with PP1gamma1 and PP1alpha but not the PP1beta isoform. We demonstrate that Inh3 is a novel substrate for caspase-3 and is degraded in vivo during apoptosis induced by actinomycin D. Inh3 was not degraded in apoptotic MCF-7 cells, which lack caspase-3. These experiments establish that Inh3 is a novel physiological substrate of caspase-3. Electroporation of the caspase-3-resistant Inh3-D49A mutant into HL-60 cells resulted in a significant attenuation of apoptosis induced by actinomycin D. These results show that Inh3 degradation contributes to the apoptotic process. Immunofluorescence based examination of the subcellular localizations of Inh3 and PP1gamma1 revealed a major relocalization of the cellular pool of PP1gamma1 from the nucleolus to the nucleus and then to the cytoplasm during actinomycin D-induced apoptosis. A similar redistribution of PP1alpha from the nucleus to the cytoplasm occurred. These results are consistent with an unexpected discovery that significant fractions of the cellular pools of PP1gamma1 and PP1alpha are associated with Inh3 in HL-60 cells. Thus, Inh3 is a major factor in the cellular economy of PP1gamma1 and PP1alpha subunits. The unscheduled relocalization of this large a pool of PP1 subunits and their release from a potent inhibitor could deregulate a diverse range of essential cellular processes and signaling pathways. We discuss the significance of these findings in relation to working hypotheses whereby Inh3 destruction could contribute to the apoptotic process.

Gonadotropin-regulated testicular helicase (DDX25), an essential regulator of spermatogenesis, prevents testicular germ cell apoptosis

Gonadotropin-regulated testicular helicase (GRTH)/DDX25 is an essential post-transcriptional regulator of spermatogenesis. In GRTH null mice severe apoptosis was observed in spermatocytes entering the metaphase of meiosis. Pro- and anti-apoptotic factors were found to be under GRTH regulation in comparative studies of spermatocytes from wild type and GRTH(-/-) knock-out (KO) mice. KO mice displayed decreased levels of Bcl-2 and Bcl-xL (anti-apoptotic factors), an increase in Bid, Bak, and Bad (pro-apoptotic), reduced phospho-Bad, and release of cytochrome c. Also, an increase on Smac, a competitor of inhibitor apoptotic proteins that release caspases, was observed. These changes caused an increase in cleavage of caspases 9 and 3, activation of caspase 3 and increases in cleavage products of PARP. The half-life of caspase 3 transcripts was markedly increased in KO, indicating that GRTH had a negative role on its mRNA stability. IkappaBalpha, which sequesters NF-kappaB from its transcriptional activation of pro-apoptotic genes, was highly elevated in KO, and its phospho-form, which promotes its dissociation, was reduced. The increase of HDAC1 and abolition of p300 expression in KO indicated a nuclear action of GRTH on the NF-kappaB-mediated transcription of anti-apoptotic genes. It also regulates the associated death domain pathway and caspase 8-mediated events. GRTH-mediated apoptotic regulation was further indicated by its selective binding to pro- and anti-apoptotic mRNAs. These studies have demonstrated that GRTH, as a component of mRNP particles, acts as a negative regulator of the tumor necrosis factor receptor 1 and caspase pathways and promotes NF-kappaB function to control apoptosis in spermatocytes of adult mice.