An Asp to Strike Out Cancer? Therapeutic Possibilities Arising from Aspartate\u27s Emerging Roles in Cell Proliferation and Survival.

A better understanding of the metabolic constraints of a tumor may lead to more effective anticancer treatments. Evidence has emerged in recent years shedding light on a crucial aspartate dependency of many tumor types. As a precursor for nucleotide synthesis, aspartate is indispensable for cell proliferation. Moreover, the malate-aspartate shuttle plays a key role in redox balance, and a deficit in aspartate can lead to oxidative stress. It is now recognized that aspartate biosynthesis is largely governed by mitochondrial metabolism, including respiration and glutaminolysis in cancer cells. Therefore, under conditions that suppress mitochondrial metabolism, including mutations, hypoxia, or chemical inhibitors, aspartate can become a limiting factor for tumor growth and cancer cell survival. Notably, aspartate availability has been associated with sensitivity or resistance to various therapeutics that are presently in the clinic or in clinical trials, arguing for a critical need for more effective aspartate-targeting approaches. In this review, we present current knowledge of the metabolic roles of aspartate in cancer cells and describe how cancer cells maintain aspartate levels under different metabolic states. We also highlight several promising aspartate level-modulating agents that are currently under investigation

Elevated CO2 suppresses specific Drosophila innate immune responses and resistance to bacterial infection

Elevated CO(2) levels (hypercapnia) frequently occur in patients with obstructive pulmonary diseases and are associated with increased mortality. However, the effects of hypercapnia on non-neuronal tissues and the mechanisms that mediate these effects are largely unknown. Here, we develop Drosophila as a genetically tractable model for defining non-neuronal CO(2) responses and response pathways. We show that hypercapnia significantly impairs embryonic morphogenesis, egg laying, and egg hatching even in mutants lacking the Gr63a neuronal CO(2) sensor. Consistent with previous reports that hypercapnic acidosis can suppress mammalian NF-kappaB-regulated innate immune genes, we find that in adult flies and the phagocytic immune-responsive S2* cell line, hypercapnia suppresses induction of specific antimicrobial peptides that are regulated by Relish, a conserved Rel/NF-kappaB family member. Correspondingly, modest hypercapnia (7-13%) increases mortality of flies inoculated with E. faecalis, A. tumefaciens, or S. aureus. During E. faecalis and A. tumefaciens infection, increased bacterial loads were observed, indicating that hypercapnia can decrease host resistance. Hypercapnic immune suppression is not mediated by acidosis, the olfactory CO(2) receptor Gr63a, or by nitric oxide signaling. Further, hypercapnia does not induce responses characteristic of hypoxia, oxidative stress, or heat shock. Finally, proteolysis of the Relish IkappaB-like domain is unaffected by hypercapnia, indicating that immunosuppression acts downstream of, or in parallel to, Relish proteolytic activation. Our results suggest that hypercapnic immune suppression is mediated by a conserved response pathway, and illustrate a mechanism by which hypercapnia could contribute to worse outcomes of patients with advanced lung disease, who frequently suffer from both hypercapnia and respiratory infections

Evolutionary Conserved Role of c-Jun-N-Terminal Kinase in CO₂-Induced Epithelial Dysfunction

<div><p>Elevated CO₂ levels (hypercapnia) occur in patients with respiratory diseases and impair alveolar epithelial integrity, in part, by inhibiting Na,K-ATPase function. Here, we examined the role of c-Jun N-terminal kinase (JNK) in CO₂ signaling in mammalian alveolar epithelial cells as well as in diptera, nematodes and rodent lungs. In alveolar epithelial cells, elevated CO₂ levels rapidly induced activation of JNK leading to downregulation of Na,K-ATPase and alveolar epithelial dysfunction. Hypercapnia-induced activation of JNK required AMP-activated protein kinase (AMPK) and protein kinase C-ζ leading to subsequent phosphorylation of JNK at Ser-129. Importantly, elevated CO₂ levels also caused a rapid and prominent activation of JNK in <em>Drosophila</em> S2 cells and in <em>C. elegans</em>. Paralleling the results with mammalian epithelial cells, RNAi against <em>Drosophila</em> JNK fully prevented CO₂-induced downregulation of Na,K-ATPase in <em>Drosophila</em> S2 cells. The importance and specificity of JNK CO₂ signaling was additionally demonstrated by the ability of mutations in the <em>C. elegans</em> JNK homologs, <em>jnk-1</em> and <em>kgb-2</em> to partially rescue the hypercapnia-induced fertility defects but not the pharyngeal pumping defects. Together, these data provide evidence that deleterious effects of hypercapnia are mediated by JNK which plays an evolutionary conserved, specific role in CO₂ signaling in mammals, diptera and nematodes.</p> </div

CO₂-induced activation of JNK is dependent on Ser-129 phosphorylation downstream of PKC-ζ.

<p>(A) A549 cells transfected with a wild-type JNK (WT-JNK<b>₁</b>-HA) or with a mutant variant in which the Ser-129 residue was mutated to alanine (S129A-JNK<b>₁</b>-HA) were exposed to 40 (open bars) or 120 (closed bars) mmHg CO<b>₂</b> (pH<b>_e</b> 7.4) for 10 min. JNK was immunoprecipitated and incubated with c-Jun and p-c-Jun was measured by Western blot. n.s.: non-specific bands. (B) A549 cells transfected with WT-JNK<b>₁</b>-HA or S129A-JNK<b>₁</b>-HA were exposed to 40 (open bars) or 120 (closed bars) mmHg CO<b>₂</b> (pH<b>_e</b> 7.4) for 30 min. The amount of Na,K-ATPase protein at the plasma membrane was determined by biotinylation as described above. PM: plasma membrane, WCL: whole cell lysate. Values are expressed as mean ± SEM, n = 3. **, <i>p</i><0.01.</p

Activation of JNK by elevated CO₂ levels is required for endocytosis of Na,K-ATPase in alveolar epithelial cells.

<p>(A) ATII cells were exposed to 40, 60, 80 or 120 mmHg CO<b>₂</b> with a pH<b>_e</b> of 7.4 (open, light grey-, grey- and black-closed bars, respectively) for 1, 5 and 10 min and phosphorylation of JNK at Thr-183/Tyr-185 (p-JNK) and total JNK (JNK) were measured by Western blot. (B) ATII cells were infected with a null adenovirus (Ad-Null) or GFP-tagged Ad-DN JNK1 and were exposed to 40 (open bars) or 120 (closed bars) mmHg CO<b>₂</b> (pH<b>_e</b> 7.4) for 30 min. Na,K-ATPase at the plasma membrane was determined by biotin-streptavidin pull down and subsequent Western blot analysis. (C) ATII cells were exposed to 40 (open bars) or 120 (closed bars) mmHg CO₂ (pH_e 7.4) for 30 min in the presence or absence of SP600125 (5 µM, 30 min preincubation) and the amount of Na,K-ATPase protein at the plasma membrane was determined as in (B). Bars represent the mean ± SEM, n = 3. *, <i>p</i><0.05, **, <i>p</i><0.01. Representative Western blots of Na,K-ATPase α₁-subunit at the plasma membrane and total protein abundance are shown. PM: plasma membrane, WCL: whole cell lysate.</p

Activation of JNK by hypercapnia is required for inhibition of AFR in rat lungs.

<p>Isolated rat lungs were perfused for 1 h with 40 mmHg CO<b>₂</b> (pHe 7.4; open bars) or with 60 mmHg CO<b>₂</b> (pH<b>_e</b> 7.2; solid bars) in the presence or absence of SP600125 (5 µM, 30 min preincubation) and (A) AFR and (B) and passive fluxes of ²²Na⁺ (dark grey bars) and ³H-mannitol (light grey bars) were measured as described in the online supplementary material. Bars represent the mean ± SEM, n = 5, **, <i>p</i><0.01. AFR: alveolar fluid reabsorption.</p