The drug adaptaquin blocks ATF4/CHOP-dependent pro-death Trib3 induction and protects in cellular and mouse models of Parkinson's disease.

Identifying disease-causing pathways and drugs that target them in Parkinson's disease (PD) has remained challenging. We uncovered a PD-relevant pathway in which the stress-regulated heterodimeric transcription complex CHOP/ATF4 induces the neuron prodeath protein Trib3 that in turn depletes the neuronal survival protein Parkin. Here we sought to determine whether the drug adaptaquin, which inhibits ATF4-dependent transcription, could suppress Trib3 induction and neuronal death in cellular and animal models of PD. Neuronal PC12 cells and ventral midbrain dopaminergic neurons were assessed in vitro for survival, transcription factor levels and Trib3 or Parkin expression after exposure to 6-hydroxydopamine or 1-methyl-4-phenylpyridinium with or without adaptaquin co-treatment. 6-hydroxydopamine injection into the medial forebrain bundle was used to examine the effects of systemic adaptaquin on signaling, substantia nigra dopaminergic neuron survival and striatal projections as well as motor behavior. In both culture and animal models, adaptaquin suppressed elevation of ATF4 and/or CHOP and induction of Trib3 in response to 1-methyl-4-phenylpyridinium and/or 6-hydroxydopamine. In culture, adaptaquin preserved Parkin levels, provided neuroprotection and preserved morphology. In the mouse model, adaptaquin treatment enhanced survival of dopaminergic neurons and substantially protected their striatal projections. It also significantly enhanced retention of nigrostriatal function. These findings define a novel pharmacological approach involving the drug adaptaquin, a selective modulator of hypoxic adaptation, for suppressing Parkin loss and neurodegeneration in toxin models of PD. As adaptaquin possesses an oxyquinoline backbone with known safety in humans, these findings provide a firm rationale for advancing it towards clinical evaluation in PD.

Activating Transcription Factor 4 (ATF4) Regulates Neuronal Activity by Controlling GABABR Trafficking.

Activating Transcription Factor 4 (ATF4) has been postulated as a key regulator of learning and memory. We previously reported that specific hippocampal ATF4 downregulation causes deficits in synaptic plasticity and memory and reduction of glutamatergic functionality. Here we extend our studies to address ATF4's role in neuronal excitability. We find that long-term ATF4 knockdown in cultured rat hippocampal neurons significantly increases the frequency of spontaneous action potentials. This effect is associated with decreased functionality of metabotropic GABAB receptors (GABABRs). Knocking down ATF4 results in significant reduction of GABABR-induced GIRK currents and increased mIPSC frequency. Furthermore, reducing ATF4 significantly decreases expression of membrane-exposed, but not total, GABABR 1a and 1b subunits, indicating that ATF4 regulates GABABR trafficking. In contrast, ATF4 knockdown has no effect on surface expression of GABABR2s, several GABABR-coupled ion channels or ß2 and γ2 GABAARs. Pharmacologic manipulations confirmed the relationship between GABABR functionality and action potential frequency in our cultures. Specifically, the effects of ATF4 downregulation cited above are fully rescued by transcriptionally active, but not by transcriptionally inactive, shRNA-resistant, ATF4. We previously reported that ATF4 promotes stabilization of the actin-regulatory protein Cdc42 by a transcription-dependent mechanism. To test the hypothesis that this action underlies the mechanism by which ATF4 loss affects neuronal firing rates and GABABR trafficking, we downregulated Cdc42 and found that this phenocopies the effects of ATF4 knockdown on these properties. In conclusion, our data favor a model in which ATF4, by regulating Cdc42 expression, affects trafficking of GABABRs, which in turn modulates the excitability properties of neurons.SIGNIFICANCE STATEMENT GABAB receptors (GABABRs), the metabotropic receptors for the inhibitory neurotransmitter GABA, have crucial roles in controlling the firing rate of neurons. Deficits in trafficking/functionality of GABABRs have been linked to a variety of neurological and psychiatric conditions, including epilepsy, anxiety, depression, schizophrenia, addiction, and pain. Here we show that GABABRs trafficking is influenced by Activating Transcription Factor 4 (ATF4), a protein that has a pivotal role in hippocampal memory processes. We found that ATF4 downregulation in hippocampal neurons reduces membrane-bound GABABR levels and thereby increases intrinsic excitability. These effects are mediated by loss of the small GTPase Cdc42 following ATF4 downregulation. These findings reveal a critical role for ATF4 in regulating the modulation of neuronal excitability by GABABRs.

Trib3 Is Elevated in Parkinson's Disease and Mediates Death in Parkinson's Disease Models.

Parkinson's disease (PD) is characterized by the progressive loss of select neuronal populations, but the prodeath genes mediating the neurodegenerative processes remain to be fully elucidated. Trib3 (tribbles pseudokinase 3) is a stress-induced gene with proapoptotic activity that was previously described as highly activated at the transcriptional level in a 6-hydroxydopamine (6-OHDA) cellular model of PD. Here, we report that Trib3 immunostaining is elevated in dopaminergic neurons of the substantia nigra pars compacta (SNpc) of human PD patients. Trib3 protein is also upregulated in cellular models of PD, including neuronal PC12 cells and rat dopaminergic ventral midbrain neurons treated with 6-OHDA, 1-methyl-4-phenylpyridinium (MPP+), or α-synuclein fibrils (αSYN). In the toxin models, Trib3 induction is substantially mediated by the transcription factors CHOP and ATF4. Trib3 overexpression is sufficient to promote neuronal death; conversely, Trib3 knockdown protects neuronal PC12 cells as well as ventral midbrain dopaminergic neurons from 6-OHDA, MPP+, or αSYN. Mechanism studies revealed that Trib3 physically interacts with Parkin, a prosurvival protein whose loss of function is associated with PD. Elevated Trib3 reduces Parkin expression in cultured cells; and in the SNpc of PD patients, Parkin levels are reduced in a subset of dopaminergic neurons expressing high levels of Trib3. Loss of Parkin at least partially mediates the prodeath actions of Trib3 in that Parkin knockdown in cellular PD models abolishes the protective effect of Trib3 downregulation. Together, these findings identify Trib3 and its regulatory pathways as potential targets to suppress the progression of neuron death and degeneration in PD. SIGNIFICANCE STATEMENT: Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Current treatments ameliorate symptoms, but not the underlying neuronal death. Understanding the core neurodegenerative processes in PD is a prerequisite for identifying new therapeutic targets and, ultimately, curing this disease. Here, we describe a novel pathway involving the proapoptotic protein Trib3 in neuronal death associated with PD. These findings are supported by data from multiple cellular models of PD and by immunostaining of postmortem PD brains. Upstream, Trib3 is induced by the transcription factors ATF4 and CHOP; and downstream, Trib3 interferes with the PD-associated prosurvival protein Parkin to mediate death. These findings establish this new pathway as a potential and promising therapeutic target for treatment of PD.

ATF4 protects against neuronal death in cellular Parkinson's disease models by maintaining levels of parkin.

Parkinson's disease (PD) is a common neurodegenerative disorder, for which there are no effective disease-modifying therapies. The transcription factor ATF4 (activating transcription factor 4) is induced by multiple PD-relevant stressors, such as endoplasmic reticulum stress and oxidative damage. ATF4 may exert either protective or deleterious effects on cell survival, depending on the paradigm. However, the role of ATF4 in the pathogenesis of PD has not been explored. We find that ATF4 levels are increased in neuromelanin-positive neurons in the substantia nigra of a subset of PD patients relative to controls. ATF4 levels are also upregulated in neuronal PC12 cells treated with the dopaminergic neuronal toxins 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium (MPP+). To explore the role of ATF4 in cell survival in PD-relevant contexts, we either silenced or overexpressed ATF4 in cellular models of PD. In neuronal PC12 cells, silencing of ATF4 enhanced cell death in response to either 6-OHDA or MPP+. Conversely, overexpression of ATF4 reduced cell death caused by dopaminergic neuronal toxins. ATF4 was also protective against 6-OHDA-induced death of cultured mouse ventral midbrain dopaminergic neurons. We further show that parkin, a gene associated with autosomal recessive PD, plays a critical role in ATF4-mediated protection. After treatment with 6-OHDA or MPP+, parkin protein levels fall, despite an increase in mRNA levels. ATF4 silencing exacerbates the toxin-induced reduction of parkin, whereas ATF4 overexpression partially preserves parkin levels. Finally, parkin silencing blocked the protective capacity of ATF4. These results indicate that ATF4 plays a protective role in PD through the regulation of parkin.

Caspase-2 is essential for c-Jun transcriptional activation and Bim induction in neuron death.

Neuronal apoptotic death generally requires de novo transcription, and activation of the transcription factor c-Jun has been shown to be necessary in multiple neuronal death paradigms. Caspase-2 has been implicated in death of neuronal and non-neuronal cells, but its relationship to transcriptional activation has not been clearly elucidated. In the present study, using two different neuronal apoptotic paradigms, ß-amyloid treatment and NGF (nerve growth factor) withdrawal, we examined the hierarchical role of caspase-2 activation in the transcriptional control of neuron death. Both paradigms induce rapid activation of caspase-2 as well as activation of the transcription factor c-Jun and subsequent induction of the pro-apoptotic BH3 (Bcl-homology domain 3)-only protein Bim (Bcl-2-interacting mediator of cell death). Caspase-2 activation is dependent on the adaptor protein RAIDD {RIP (receptor-interacting protein)-associated ICH-1 [ICE (interleukin-1ß-converting enzyme)/CED-3 (cell-death determining 3) homologue 1] protein with a death domain}, and both caspase-2 and RAIDD are required for c-Jun activation and Bim induction. The present study thus shows that rapid caspase-2 activation is essential for c-Jun activation and Bim induction in neurons subjected to apoptotic stimuli. This places caspase-2 at an apical position in the apoptotic cascade and demonstrates for the first time that caspase-2 can regulate transcription.

Revolutionizing Brain Tumor Care: Emerging Technologies and Strategies.

Glioblastoma multiforme (GBM) is one of the most aggressive forms of brain tumor, characterized by a daunting prognosis with a life expectancy hovering around 12-16 months. Despite a century of relentless research, only a select few drugs have received approval for brain tumor treatment, largely due to the formidable barrier posed by the blood-brain barrier. The current standard of care involves a multifaceted approach combining surgery, irradiation, and chemotherapy. However, recurrence often occurs within months despite these interventions. The formidable challenges of drug delivery to the brain and overcoming therapeutic resistance have become focal points in the treatment of brain tumors and are deemed essential to overcoming tumor recurrence. In recent years, a promising wave of advanced treatments has emerged, offering a glimpse of hope to overcome the limitations of existing therapies. This review aims to highlight cutting-edge technologies in the current and ongoing stages of development, providing patients with valuable insights to guide their choices in brain tumor treatment.

DPEP Inhibits Cancer Cell Glucose Uptake, Glycolysis and Survival by Upregulating Tumor Suppressor TXNIP.

We have designed cell-penetrating peptides that target the leucine zipper transcription factors ATF5, CEBPB and CEBPD and that promote apoptotic death of a wide range of cancer cell types, but not normal cells, in vitro and in vivo. Though such peptides have the potential for clinical application, their mechanisms of action are not fully understood. Here, we show that one such peptide, Dpep, compromises glucose uptake and glycolysis in a cell context-dependent manner (in about two-thirds of cancer lines assessed). These actions are dependent on induction of tumor suppressor TXNIP (thioredoxin-interacting protein) mRNA and protein. Knockdown studies show that TXNIP significantly contributes to apoptotic death in those cancer cells in which it is induced by Dpep. The metabolic actions of Dpep on glycolysis led us to explore combinations of Dpep with clinically approved drugs metformin and atovaquone that inhibit oxidative phosphorylation and that are in trials for cancer treatment. Dpep showed additive to synergistic activities in all lines tested. In summary, we find that Dpep induces TXNIP in a cell context-dependent manner that in turn suppresses glucose uptake and glycolysis and contributes to apoptotic death of a range of cancer cells.

Sh3rf2/POSHER protein promotes cell survival by ring-mediated proteasomal degradation of the c-Jun N-terminal kinase scaffold POSH (Plenty of SH3s) protein.

We report that Sh3rf2, a homologue of the pro-apoptotic scaffold POSH (Plenty of SH3s), acts as an anti-apoptotic regulator for the c-Jun N-terminal kinase (JNK) pathway. siRNA-mediated knockdown of Sh3rf2 promotes apoptosis of neuronal PC12 cells, cultured cortical neurons, and C6 glioma cells. This death appears to result from activation of JNK signaling. Loss of Sh3rf2 triggers activation of JNK and its target c-Jun. Also, apoptosis promoted by Sh3rf2 knockdown is inhibited by dominant-negative c-Jun as well as by a JNK inhibitor. Investigation of the mechanism by which Sh3rf2 regulates cell survival implicates POSH, a scaffold required for activation of pro-apoptotic JNK/c-Jun signaling. In cells lacking POSH, Sh3rf2 knockdown is unable to activate JNK. We further find that Sh3rf2 binds POSH to reduce its levels by a mechanism that requires the RING domains of both proteins and that appears to involve proteasomal POSH degradation. Conversely, knockdown of Sh3rf2 promotes the stabilization of POSH protein and activation of JNK signaling. Finally, we show that endogenous Sh3rf2 protein rapidly decreases following several different apoptotic stimuli and that knockdown of Sh3rf2 activates the pro-apoptotic JNK pathway in neuronal cells. These findings support a model in which Sh3rf2 promotes proteasomal degradation of pro-apoptotic POSH in healthy cells and in which apoptotic stimuli lead to rapid loss of Sh3rf2 expression, and consequently to stabilization of POSH and JNK activation and cell death. On the basis of these observations, we propose the alternative name POSHER (POSH-eliminating RING protein) for the Sh3rf2 protein.

Dpep Inhibits Cancer Cell Growth and Survival via Shared and Context-Dependent Transcriptome Perturbations.

Dpep is a cell-penetrating peptide targeting transcription factors ATF5, CEBPB, and CEBPD, and that selectively promotes the apoptotic death of multiple tumor cell types in vitro and in vivo. As such, it is a potential therapeutic. To better understand its mechanism of action, we used PLATE-seq to compare the transcriptomes of six cancer cell lines of diverse origins before and after Dpep exposure. This revealed a context-dependent pattern of regulated genes that was unique to each line, but that exhibited a number of elements that were shared with other lines. This included the upregulation of pro-apoptotic genes and tumor suppressors as well as the enrichment of genes associated with responses to hypoxia and interferons. Downregulated transcripts included oncogenes and dependency genes, as well as enriched genes associated with different phases of the cell cycle and with DNA repair. In each case, such changes have the potential to lie upstream of apoptotic cell death. We also detected the regulation of unique as well as shared sets of transcription factors in each line, suggesting that Dpep may initiate a cascade of transcriptional responses that culminate in cancer cell death. Such death thus appears to reflect context-dependent, yet shared, disruption of multiple cellular pathways as well as of individual survival-relevant genes.

Targeting Transcription Factors ATF5, CEBPB and CEBPD with Cell-Penetrating Peptides to Treat Brain and Other Cancers.

Developing novel therapeutics often follows three steps: target identification, design of strategies to suppress target activity and drug development to implement the strategies. In this review, we recount the evidence identifying the basic leucine zipper transcription factors ATF5, CEBPB, and CEBPD as targets for brain and other malignancies. We describe strategies that exploit the structures of the three factors to create inhibitory dominant-negative (DN) mutant forms that selectively suppress growth and survival of cancer cells. We then discuss and compare four peptides (CP-DN-ATF5, Dpep, Bpep and ST101) in which DN sequences are joined with cell-penetrating domains to create drugs that pass through tissue barriers and into cells. The peptide drugs show both efficacy and safety in suppressing growth and in the survival of brain and other cancers in vivo, and ST101 is currently in clinical trials for solid tumors, including GBM. We further consider known mechanisms by which the peptides act and how these have been exploited in rationally designed combination therapies. We additionally discuss lacunae in our knowledge about the peptides that merit further research. Finally, we suggest both short- and long-term directions for creating new generations of drugs targeting ATF5, CEBPB, CEBPD, and other transcription factors for treating brain and other malignancies.

Activating Transcription Factor 5 Promotes Neuroblastoma Metastasis by Inducing Anoikis Resistance.

MYCN-amplified neuroblastoma often presents as a highly aggressive metastatic disease with a poor prognosis. Activating transcription factor 5 (ATF5) is implicated in neural cell differentiation and cancer cell survival. Here, we show that ATF5 is highly expressed in patients with stage 4 high-risk neuroblastoma, with increased expression correlating with a poorer prognosis. We demonstrated that ATF5 promotes the metastasis of neuroblastoma cell lines in vivo. Functionally, ATF5 depletion significantly reduced xenograft tumor growth and metastasis of neuroblastoma cells to the bone marrow and liver. Mechanistically, ATF5 endows tumor cells with resistance to anoikis, thereby increasing their survival in systemic circulation and facilitating metastasis. We identified the proapoptotic BCL-2 modifying factor (BMF) as a critical player in ATF5-regulated neuroblastoma anoikis. ATF5 suppresses BMF under suspension conditions at the transcriptional level, promoting anoikis resistance, whereas BMF knockdown significantly prevents ATF5 depletion-induced anoikis. Therapeutically, we showed that a cell-penetrating dominant-negative ATF5 peptide, CP-d/n-ATF5, inhibits neuroblastoma metastasis to the bone marrow and liver by inducing anoikis sensitivity in circulating tumor cells. Our study identified ATF5 as a metastasis promoter and CP-d/n-ATF5 as a potential antimetastatic therapeutic agent for neuroblastoma. SIGNIFICANCE: This study shows that resistance to anoikis in neuroblastoma is mediated by ATF5 and offers a rationale for targeting ATF5 to treat metastatic neuroblastoma.

Gata2 is required for migration and differentiation of retinorecipient neurons in the superior colliculus.

The superior colliculus (SC)/optic tectum of the dorsal mesencephalon plays a major role in responses to visual input, yet regulation of neuronal differentiation within this layered structure is only partially understood. Here, we show that the zinc finger transcription factor Gata2 is required for normal SC development. Starting at embryonic day 15 (E15) (corresponding to the times at which neurons of the outer and intermediate layers of the SC are generated), Gata2 is transiently expressed in the rat embryonic dorsal mesencephalon within a restricted region between proliferating cells of the ventricular zone and the deepest neuronal layers of the developing SC. The Gata2-positive cells are postmitotic and lack markers of differentiated neurons, but express markers for immature neuronal precursors including Ascl1 and Pax3/7. In utero electroporation with Gata2 small hairpin RNAs at E16 into cells along the dorsal mesencephalic ventricle interferes with their normal migration into the SC and maintains them in a state characterized by retention of Pax3 expression and the absence of mature neuronal markers. Collectively, these findings indicate that Gata2 plays a required role in the transition of postmitotic neuronal precursor cells of the retinorecipient layers of the SC into mature neurons and that loss of Gata2 arrests them at an intermediate stage of differentiation.

RTP801/REDD1 regulates the timing of cortical neurogenesis and neuron migration.

The generation, differentiation, and migration of newborn neurons are critical features of normal brain development that are subject to both extracellular and intracellular regulation. However, the means of such control are only partially understood. Here, we show that expression of RTP801/REDD1, an inhibitor of mTOR (mammalian target of rapamycin) activation, is regulated during neuronal differentiation and that RTP801 functions to influence the timing of both neurogenesis and neuron migration. RTP801 levels are high in embryonic cortical neuroprogenitors, diminished in newborn neurons, and low in mature neurons. Knockdown of RTP801 in vitro and in vivo accelerates cell cycle exit by neuroprogenitors and their differentiation into neurons. It also disrupts migration of rat newborn neurons to the cortical plate and results in the ectopic localization of mature neurons. On the other hand, RTP801 overexpression delays neuronal differentiation. These findings suggest that endogenous RTP801 plays an essential role in temporal control of cortical development and in cortical patterning.

Rapamycin protects against neuron death in in vitro and in vivo models of Parkinson's disease.

We report that rapamycin, an allosteric inhibitor of certain but not all actions of the key cellular kinase mammalian target of rapamycin (mTOR), protects neurons from death in both cellular and animal toxin models of Parkinson's disease (PD). This protective action appears to be attributable to blocked translation of RTP801/REDD1/Ddit4, a protein that is induced in cell and animal models of PD and in affected neurons of PD patients and that causes neuron death by leading to dephosphorylation of the survival kinase Akt. In support of this mechanism, in PD models, rapamycin spares phosphorylation of Akt at a site critical for maintenance of its survival-promoting activity. The capacity of rapamycin to provide neuroprotection in PD models appears to arise from its selective suppression of some but not all actions of mTOR, as indicated by the contrasting finding that Torin1, a full catalytic mTOR inhibitor, is not protective and induces Akt dephosphorylation and neuron death.

Sertad1 plays an essential role in developmental and pathological neuron death.

Developmental and pathological death of neurons requires activation of a defined pathway of cell cycle proteins. However, it is unclear how this pathway is regulated and whether it is relevant in vivo. A screen for transcripts robustly induced in cultured neurons by DNA damage identified Sertad1, a Cdk4 (cyclin-dependent kinase 4) activator. Sertad1 is also induced in neurons by nerve growth factor (NGF) deprivation and Abeta (beta-amyloid). RNA interference-mediated downregulation of Sertad1 protects neurons in all three death models. Studies of NGF withdrawal indicate that Sertad1 is required to initiate the apoptotic cell cycle pathway since its knockdown blocks subsequent pathway events. Finally, we find that Sertad1 expression is required for developmental neuronal death in the cerebral cortex. Sertad1 thus appears to be essential for neuron death in trophic support deprivation in vitro and in vivo and in models of DNA damage and Alzheimer's disease. It may therefore be a suitable target for therapeutic intervention.

Akt as a victim, villain and potential hero in Parkinson's disease pathophysiology and treatment.

There are two major purposes of this essay. The first is to summarize existing evidence that irrespective of the initiating causes, neuron death and degeneration in Parkinson's disease (PD) are due to the common feature of failure of signaling by Akt, a kinase involved in neuron survival and maintenance of synaptic contacts. The second is to consider possible means by which such a failure of Akt signaling might be benignly prevented or reversed in neurons affected by PD, so as to treat PD symptoms, block disease progression, and potentially, promote recovery.

Cell-Penetrating CEBPB and CEBPD Leucine Zipper Decoys as Broadly Acting Anti-Cancer Agents.

Transcription factors are key players underlying cancer formation, growth, survival, metastasis and treatment resistance, yet few drugs exist to directly target them. Here, we characterized the in vitro and in vivo anti-cancer efficacy of novel synthetic cell-penetrating peptides (Bpep and Dpep) designed to interfere with the formation of active leucine-zipper-based dimers by CEBPB and CEBPD, transcription factors implicated in multiple malignancies. Both peptides similarly promoted apoptosis of multiple tumor lines of varying origins, without such effects on non-transformed cells. Combined with other treatments (radiation, Taxol, chloroquine, doxorubicin), the peptides acted additively to synergistically and were fully active on Taxol-resistant cells. The peptides suppressed expression of known direct CEBPB/CEBPD targets IL6, IL8 and asparagine synthetase (ASNS), supporting their inhibition of transcriptional activation. Mechanisms by which the peptides trigger apoptosis included depletion of pro-survival survivin and a required elevation of pro-apoptotic BMF. Bpep and Dpep significantly slowed tumor growth in mouse models without evident side effects. Dpep significantly prolonged survival in xenograft models. These findings indicate the efficacy and potential of Bpep and Dpep as novel agents to treat a variety of cancers as mono- or combination therapies.

Dominant-Negative ATF5 Compromises Cancer Cell Survival by Targeting CEBPB and CEBPD.

The basic leucine zipper transcription factor ATF5 is overexpressed in many tumor types and interference with its expression or function inhibits cancer cell survival. As a potential therapeutic approach to exploit these findings, we created dominant-negative (DN) ATF5 forms lacking DNA-binding ability that retain the ATF5 leucine zipper, and thus associate with and sequester ATF5's requisite leucine zipper-binding partners. Preclinical studies with DN-ATF5, including a cell-penetrating form, show in vitro and in vivo efficacy in compromising cancer cell survival. However, DN-ATF5's targets, and particularly those required for tumor cell survival, have been unknown. We report that cells lacking ATF5 succumb to DN-ATF5, indicating that ATF5 itself is not DN-ATF5's obligate target. Unbiased pull-down assays coupled with mass spectrometry and immunoblotting revealed that DN-ATF5 associates in cells with the basic leucine zipper proteins CEBPB and CEBPD and coiled-coil protein CCDC6. Consistent with DN-ATF5 affecting tumor cell survival by suppressing CEBPB and CEBPD function, DN-ATF5 interferes with CEBPB and CEBPD transcriptional activity, while CEBPB or CEBPD knockdown promotes apoptotic death of multiple cancer cells lines, but not of normal astrocytes. We propose a two-pronged mechanism by which DN-ATF5 kills tumor cells. One is by inhibiting heterodimer formation between ATF5 and CEBPB and CDBPD, thus suppressing ATF5-dependent transcription. The other is by blocking the formation of transcriptionally active CEBPB and CEBPD homodimers as well as heterodimers with partners in addition to ATF5. IMPLICATIONS: This study indicates that the potential cancer therapeutic DN-ATF5 acts by associating with and blocking the transcriptional activities of CEBPB and CEBPD.

RTP801 is induced in Parkinson's disease and mediates neuron death by inhibiting Akt phosphorylation/activation.

Previously, we reported that RTP801, a stress regulated protein, is induced in multiple cellular models of Parkinson's disease (PD), in an animal model of PD and in dopaminergic neurons of PD patients. In cellular PD models, RTP801 is both sufficient and necessary for death. We further showed that RTP801 and PD mimetics such as 6-OHDA trigger neuron death by suppressing activation of the key kinase mammalian target of rapamycin (mTOR). Here, we report that as a consequence of mTOR signaling blockade, 6-OHDA suppresses the phosphorylation and activation of Akt, a major supporter of neuron survival. This effect is mediated by RTP801 and appears to underlie neuron death induced by 6-OHDA. Examination of postmortem dopaminergic neurons reveals a consistent depletion of phospho-Akt, but not of total Akt in PD patients. These observations support a sequential mechanism in which PD-associated stresses induce RTP801, suppress mTOR signaling, deplete phosphorylated/activated Akt and permit neuron degeneration and death.

The transcription factor ATF5: role in neurodevelopment and neural tumors.

We review recent findings regarding the properties of ATF5 and the major roles that this transcription factor plays in development of the nervous system and in survival of neural tumors. ATF5 is a widely expressed basic leucine zipper protein that has been subject to limited characterization. It is highly expressed in zones of neuroprogenitor cell proliferation. In vitro and in vivo studies indicate that it functions there to promote neuroprogenitor cell expansion and to suppress their differentiation into neurons or glia. ATF5 expression is down-regulated by trophic factors and this is required for their capacity to promote neuroprogenitor cell cycle exit and differentiation into either neurons, oligodendroglia or astrocytes. ATF5 is also highly expressed in a number of tumor types, including neural tumors such as neuroblastomas, medulloblastomas and glioblastomas. Examination of the role of ATF5 in glioblastoma cells indicates that interference with its expression or activity causes them to undergo apoptotic death. In contrast, normal astrocytes and neurons do not appear to require ATF5 for survival, indicating that it may be a selective target for treatment of glioblastomas and other neural neoplasias. Further studies are needed to identify the transcriptional targets of ATF5 and the mechanisms by which its expression is regulated in neuroprogenitors and tumors.