SummaryTumor cells display increased metabolic autonomy in comparison to non-transformed cells, taking up nutrients and metabolizing them in pathways that support growth and proliferation. Classical work in tumor cell metabolism focused on bioenergetics, particularly enhanced glycolysis and suppressed oxidative phosphorylation (the 'Warburg effect'). But the biosynthetic activities required to create daughter cells are equally important for tumor growth, and recent studies are now bringing these pathways into focus. In this review, we discuss how tumor cells achieve high rates of nucleotide and fatty acid synthesis, how oncogenes and tumor suppressors influence these activities, and how glutamine metabolism enables macromolecular synthesis in proliferating cells.

Necrosis has been considered a passive form of cell death in which the cell dies as a result of a bioenergetic catastrophe imposed by external conditions. However, in response to alkylating DNA damage, cells undergo necrosis as a self-determined cell fate. This form of death does not require the central apoptotic mediators p53, Bax/Bak, or caspases and actively induces an inflammatory response. Necrosis in response to DNA damage requires activation of the DNA repair protein poly(ADP-ribose) polymerase (PARP), but PARP activation is not sufficient to determine cell fate. Cell death is determined by the effect of PARP-mediated ␤-nicotinamide adenine dinucleotide (NAD) consumption on cellular metabolism. Cells using aerobic glycolysis to support their bioenergetics undergo rapid ATP depletion and death in response to PARP activation. In contrast, cells catabolizing nonglucose substrates to maintain oxidative phosphorylation are resistant to ATP depletion and death in response to PARP activation. Because most cancer cells maintain their ATP production through aerobic glycolysis, these data may explain the molecular basis by which DNA-damaging agents can selectively induce tumor cell death independent of p53 or Bcl-2 family proteins.

Transactivating response region DNA binding protein (TDP-43) is the major protein component of ubiquitinated inclusions found in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) with ubiquitinated inclusions. Two ALS-causing mutants (TDP-43 Q331K and TDP-43 M337V ), but not wild-type human TDP-43, are shown here to provoke age-dependent, mutant-dependent, progressive motor axon degeneration and motor neuron death when expressed in mice at levels and in a cell typeselective pattern similar to endogenous TDP-43. Mutant TDP-43-dependent degeneration of lower motor neurons occurs without: (i) loss of TDP-43 from the corresponding nuclei, (ii) accumulation of TDP-43 aggregates, and (iii) accumulation of insoluble TDP-43. Computational analysis using splicing-sensitive microarrays demonstrates alterations of endogenous TDP-43-dependent alternative splicing events conferred by both human wild-type and mutant TDP-43 Q331K , but with high levels of mutant TDP-43 preferentially enhancing exon exclusion of some target pre-mRNAs affecting genes involved in neurological transmission and function. Comparison with splicing alterations following TDP-43 depletion demonstrates that TDP-43 Q331K enhances normal TDP-43 splicing function for some RNA targets but loss-of-function for others. Thus, adult-onset motor neuron disease does not require aggregation or loss of nuclear TDP-43, with ALS-linked mutants producing loss and gain of splicing function of selected RNA targets at an early disease stage.A myotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-U) are progressive, adult-onset neurodegenerative diseases with overlapping clinical and pathological features (1-3). ALS is characterized by the selective loss of upper and lower motor neurons, leading to progressive fatal paralysis and muscle atrophy. A large majority (∼90%) of ALS and FTLD-U cases are without a known genetic cause. Importantly, in these sporadic cases, the appearance of ubiquitinated inclusions within the affected neurons of the nervous system characterizes both ALS and FTLD-U patients, suggesting an overlapping mechanism underlying both diseases. Biochemical characterization of brains and spinal cords from ALS and FTLD-U patients identified transactivating response region (TAR) DNA binding protein (TDP-43) as the major protein component of these ubiquitinated inclusions (4, 5). The discovery of ALS-linked mutations in the glycine-rich C-terminal domain of TDP-43 (6-8) demonstrated a pathological role of TDP-43 in both diseases. The subsequent identification of mutations in a structurally and functionally related nucleic acid binding protein, FUS/ TLS (fused in sarcoma/translocated in liposarcoma) (9, 10), further implicated defects in RNA processing in ALS pathogenesis.TDP-43 is a multifunctional nucleic acid binding protein.Within the nervous system, TDP-43 binds to >6,000 pre-mRNAs and affects the levels of ∼600 mRNAs and the splicing patterns of another 950 (11). Structura...

Significance Direct conversion is a recently established method to generate neuronal progenitor cells (NPCs) from skin fibroblasts in a fast and efficient manner. In this study, we show that this method can be used to model neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Because the origin of ALS is mainly sporadic with unknown cause, methods to model the disease are urgently needed. The produced NPCs are differentiated into astrocytes, which are involved in motor neuron death in ALS. Strikingly, skin-derived astrocytes show similar toxicity toward motor neurons as astrocytes from autopsies of patients. This tool now allows studying ALS while the patient is still alive and can help in testing potential therapeutics for individual patients.

SUMMARY The activity and specificity of serine/threonine phosphatases is governed largely by their associated proteins. α4 is an evolutionarily conserved non-catalytic subunit for PP2A-like phosphatases. While α4 binds to only a minority of PP2A-related catalytic subunits, α4 deletion leads to progressive loss of all PP2A, PP4, and PP6 phosphatase complexes. In healthy cells, association with α4 renders catalytic (C) subunits enzymatically inactive while protecting them from proteasomal degradation until they are assembled into a functional phosphatase complex. During cellular stress, existing PP2A complexes can become unstable. Under such conditions, α4 sequesters released C subunits and is required for the adaptive increase in targeted PP2A activity that can dephosphorylate stress-induced phosphorylated substrates. Consistent with this, overexpression of α4 protects cells from a variety of stress stimuli, including DNA damage and nutrient limitation. These findings demonstrate that α4 plays a required role in regulating the assembly and maintenance of adaptive PP2A phosphatase complexes.

Approximately 50% of cancer patients receive radiation treatment, either alone or in combination with other therapies. Tumor hypoxia has long been associated with resistance to radiation therapy. Moreover, the expression of hypoxia inducible factors HIF1␣ and/or HIF2␣ correlates with poor prognosis in many tumors. Recent evidence indicates that HIF1␣ expression can enhance radiation-induced apoptosis in cancer cells. We demonstrate here that HIF2␣ inhibition promotes tumor cell death and, in contrast to HIF1␣, enhances the response to radiation treatment. Specifically, inhibiting HIF2␣ expression augments p53 activity, increases apoptosis, and reduces clonogenic survival of irradiated and non-irradiated cells. Moreover, HIF2␣ inhibition promotes p53-mediated responses by disrupting cellular redox homeostasis, thereby permitting reactive oxygen species (ROS) accumulation and DNA damage. These results correlate with altered p53 phosphorylation and target gene expression in untreated human tumor samples and show that HIF2␣ likely contributes to tumor cell survival including during radiation therapy.M any cellular responses to hypoxia are mediated by the hypoxia inducible factors (HIFs). These transcription factors promote the expression of over 200 genes (1) and are heterodimers consisting of either HIF1␣ or HIF2␣ bound to the HIF␤/ARNT subunit. While ARNT is constitutively expressed, both HIF␣ subunits are regulated by O 2 availability. Under normoxia, the von Hippel-Lindau (VHL) E3 ligase complex targets HIF␣ subunits for proteasomal degradation (2). When O 2 levels decline, the HIF␣ subunits are stabilized, bind ARNT, and activate the expression of target genes providing hypoxic adaptations.Solid tumors are characterized by oncogenic signaling and hypoxic microenvironments that promote HIF␣ accumulation (3). Moreover, HIF1␣ and/or HIF2␣ expression has been associated with increased tumor vascularization and poor prognosis of numerous cancers such as breast, ovarian, and non-small cell lung cancer (2, 4). Of note, in mouse xenograft models, HIF2␣ (and not HIF1␣) expression is crucial for growth of clear cell renal cell carcinoma (ccRCC) (5, 6) and neuroblastoma (7) tumors.TP53 is a tumor suppressor that is mutated or silenced in a majority of human cancers (8). It coordinates many cellular stress responses by regulating genes involved in DNA repair, cell cycle arrest, and apoptosis. Following stress stimuli, p53 is activated through a variety of post-translational modifications, including phosphorylation on serine 15 (9). For example, the ataxia telangiectasia mutated (ATM) and checkpoint kinase 2 (Chk2) kinases directly phosphorylate p53 in response to DNA damage, resulting in its activation (9). Although many tumors select for TP53 mutations, p53 pathway inhibition can also contribute to tumor progression (10).With the emergence of HIF inhibitors (11,12) and their potential use in cancer therapy, it is important to accurately predict the response of HIF␣-expressing tumor cells to treatment. HIF1␣ appears to enh...

Ubiquitous expression of amyotrophic lateral sclerosis (ALS)-causing mutations in superoxide dismutase 1 (SOD1) provokes noncell autonomous paralytic disease. By combining ribosome affinity purification and high-throughput sequencing, a cascade of mutant SOD1-dependent, cell type-specific changes are now identified. Initial mutant-dependent damage is restricted to motor neurons and includes synapse and metabolic abnormalities, endoplasmic reticulum (ER) stress, and selective activation of the PRKR-like ER kinase (PERK) arm of the unfolded protein response. PERK activation correlates with what we identify as a naturally low level of ER chaperones in motor neurons. Early changes in astrocytes occur in genes that are involved in inflammation and metabolism and are targets of the peroxisome proliferator-activated receptor and liver X receptor transcription factors. Dysregulation of myelination and lipid signaling pathways and activation of ETS transcription factors occur in oligodendrocytes only after disease initiation. Thus, pathogenesis involves a temporal cascade of cell type-selective damage initiating in motor neurons, with subsequent damage within glia driving disease propagation.

Mutations in superoxide dismutase 1 (SOD1) are linked to familial amyotrophic lateral sclerosis (ALS) resulting in progressive motor neuron death through one or more acquired toxicities. Involvement of wild-type SOD1 has been linked to sporadic ALS, as misfolded SOD1 has been reported in affected tissues of sporadic patients and toxicity of astrocytes derived from sporadic ALS patients to motor neurons has been reported to be reduced by lowering the synthesis of SOD1. We now report slowed disease onset and progression in two mouse models following therapeutic delivery using a single peripheral injection of an adeno-associated virus serotype 9 (AAV9) encoding an shRNA to reduce the synthesis of ALS-causing human SOD1 mutants. Delivery to young mice that develop aggressive, fatal paralysis extended survival by delaying both disease onset and slowing progression. In a later-onset model, AAV9 delivery after onset markedly slowed disease progression and significantly extended survival. Moreover, AAV9 delivered intrathecally to nonhuman primates is demonstrated to yield robust SOD1 suppression in motor neurons and glia throughout the spinal cord and therefore, setting the stage for AAV9-mediated therapy in human clinical trials.