Thiopental inhibits global protein synthesis by repression of eukaryotic elongation factor 2 and protects from hypoxic neuronal cell death.

Ischemic and traumatic brain injury is associated with increased risk for death and disability. The inhibition of penumbral tissue damage has been recognized as a target for therapeutic intervention, because cellular injury evolves progressively upon ATP-depletion and loss of ion homeostasis. In patients, thiopental is used to treat refractory intracranial hypertension by reducing intracranial pressure and cerebral metabolic demands; however, therapeutic benefits of thiopental-treatment are controversially discussed. In the present study we identified fundamental neuroprotective molecular mechanisms mediated by thiopental. Here we show that thiopental inhibits global protein synthesis, which preserves the intracellular energy metabolite content in oxygen-deprived human neuronal SK-N-SH cells or primary mouse cortical neurons and thus ameliorates hypoxic cell damage. Sensitivity to hypoxic damage was restored by pharmacologic repression of eukaryotic elongation factor 2 kinase. Translational inhibition was mediated by calcium influx, activation of the AMP-activated protein kinase, and inhibitory phosphorylation of eukaryotic elongation factor 2. Our results explain the reduction of cerebral metabolic demands during thiopental treatment. Cycloheximide also protected neurons from hypoxic cell death, indicating that translational inhibitors may generally reduce secondary brain injury. In conclusion our study demonstrates that therapeutic inhibition of global protein synthesis protects neurons from hypoxic damage by preserving energy balance in oxygen-deprived cells. Molecular evidence for thiopental-mediated neuroprotection favours a positive clinical evaluation of barbiturate treatment. The chemical structure of thiopental could represent a pharmacologically relevant scaffold for the development of new organ-protective compounds to ameliorate tissue damage when oxygen availability is limited

Inhibitors of protein synthesis reduce hypoxic neuronal damage.

<p>Cellular damage in human neuronal SK-N-SH cells was induced by oxygen deprivation (closed symbols) in an atmosphere containing 5% CO₂, 95% N₂ for 0–72 h and determined by an LDH release assay. Control cells were cultured in 5% CO₂, 21% O₂, and 74% N₂ (open symbols). In (A), cellular damage was measured in the presence (squares) or absence (rhombi) of fetal calf serum. In (B), 5 µg/ml cycloheximide (closed triangles) or 2 µg/ml actinomycin D (closed rhombi) were added to the cells in serum containing growth medium (squares). In (C), 0.1 mM thiopental (closed circles) or 0.5 mM thiopental (closed triangles) were added to the cells in serum containing growth medium (squares). Values represent the mean ± standard deviations of four separate experiments. Experimental groups were statistically analyzed by performing two-way ANOVA followed by the Bonferroni’s <i>post hoc</i> test. Statistically significant differences within groups shown for (A) are: serum treated oxygen deprived SK-N-SH cells versus serum treated normoxic control cells (***, p<0.001). Statistically significant differences within serum treated groups shown for (B) are: normoxic control cells versus oxygen deprived SK-N-SH cells (***, p<0.001); and oxygen deprived SK-N-SH cells versus oxygen-deprived cells treated with 2 µg/ml actinomycin D (^§§§, p<0.001) or versus oxygen-deprived cells treated with 5 µg/ml cycloheximide (^###, p<0.001). Statistically significant differences within serum treated groups shown for (C) are: normoxic control cells versus oxygen deprived SK-N-SH cells (***, p<0.001), versus oxygen-deprived cells treated with 0.1 mM thiopental (^{<>\raster(60%)="rg1"<>}, p<0.05; ^{<>\raster(60%)="rg1"<><>\raster(60%)="rg1"<><>\raster(60%)="rg1"<>}, p<0.001) or versus oxygen-deprived cells treated with 0.5 mM thiopental (^{<>\raster(60%)="rg2"<><>\raster(60%)="rg2"<><>\raster(60%)="rg2"<>}, p<0.001); and oxygen deprived SK-N-SH cells versus oxygen-deprived cells treated with 0.1 mM thiopental (^{<>\raster(60%)="rg3"<><>\raster(60%)="rg3"<><>\raster(60%)="rg3"<>}, p<0.001) or versus oxygen-deprived cells treated with 0.5 mM thiopental (^¥¥¥, p<0.001).</p

Thiopental induces eEF2 phosphorylation.

<p>SK-N-SH cells were treated with 10 µM –2 mM thiopental for 6 h (A) or with 0.5 mM thiopental for 10 min to 12 h (B) and analyzed by immunoblotting with an anti-human phospho-eEF2 threonine 56 antibody (upper blots) or an eEF2 antibody that detects endogenous levels of eEF2 independently of phosphorylation (lower blots). Data are representative of four independent experiments.</p

Thiopental increases intracellular calcium levels but not cAMP content.

<p>(A), SK-N-SH cells were left untreated or were incubated in the presence of 0.5 mM thiopental, 1 mM thiopental, or 40 µM thapsigargin in calcium containing (black bars) or calcium-free (white bars) RPMI medium for 1 h. For the last 30 min of the experiment cells were loaded with 2 µM fura-2 acetoxymethyl ester before intracellular calcium-complexes were excited at 340 nm and recorded at 510 nm. Values represent the means ± standard deviations. Experimental groups were statistically analyzed by performing one-way ANOVA followed by the Dunnett’s multiple comparisons test. ***, p<0.001 versus untreated control group, n = 3. In (B), SK-N-SH cells were left untreated or were incubated with 0.5 mM thiopental, 1 mM thiopental, or 100 µM forskulin for 15 min to 6 h before cell lysis. Cellular cAMP content was determined by a competitive cAMP immunoassay. Values represent the means ± standard deviations, n = 3. Experimental groups were statistically analyzed by performing two-way ANOVA followed by the Bonferroni’s <i>post hoc</i> test. Statistical differences versus untreated control cells are shown (***, p<0.001). In (C), immunoblots were performed using cells that were pretreated with 100 µM A484954, 10 µM camstatin, or 1 mM cAMPs-pR for 1 h before 0.5 mM thiopental were added for the last hour of the experiment. The immunoblots were analyzed with an anti-human phospho-eEF2 threonine 56 antibody or an anti-eEF2 antibody that detects endogenous levels of eEF2 independently of phosphorylation. Data are representative of three independent experiments.</p

Thiopental inhibits global protein synthesis.

<p>SK-N-SH cells were left untreated or were exposed to 0.5 mM thiopental, 1 mM thiopental, or 5 µg/ml cycloheximide for 6 h and then pulsed with [³⁵S]methionine for an additional 2 h. Cell lysates were separated by SDS-PAGE. The amounts of newly synthesized proteins were detected by autoradiography on dried electrophoresis gels (A) and quantified by densitometry (B). Values represent the means ± standard deviations. Statistical differences between experimental groups were determined by performing one-way ANOVA followed by the Dunnett’s multiple comparisons test. ***, p<0.001 versus untreated control group (first bar). The results shown are representative of three independent experiments.</p

Hypoxic neuronal damage is independent of caspase-3.

<p>SK-N-SH cells were cultured in the presence (grey bars) or absence (black bars) of 0.5 mM thiopental without oxygen for 0–72 h before caspase-3 like activity was assessed by a fluorogenic caspase-3 activity assay. Values are expressed as relative light units (RLU)/min and were normalized to protein content. As a control, apoptosis was induced by 40 µM thapsigargin in normoxic SK-N-SH cells for 30 min (white bar). Statistic evaluation of experimental groups was performed by one way ANOVA followed by the Dunnett’s multiple comparisons test. ***, p<0.001 versus untreated control cells (black bar, 0 h hypoxia). The results shown are representative of three independent experiments.</p

Inhibitors of protein synthesis preserve intracellular ATP-content during oxygen deprivation.

<p>Human neuronal SK-N-SH cells were cultured in an oxygen-free atmosphere for 12–72 h in the presence of 5 µg/ml cycloheximide (closed triangles), 0.1 mM thiopental (closed rhombi), 0.5 mM thiopental (asterisks), or left untreated (closed squares). Cells, cultured in a normoxic atmosphere (open squares) served as a control. ATP content of cells was measured in lysates by an ATP-driven luciferase assay. Determined relative light units (RLU) were normalized to protein content and represent the means ± standard deviations of three independent experiments. Experimental groups were statistically evaluated by performing two-way ANOVA followed by the Bonferroni’s <i>post hoc</i> test. Statistical differences of oxygen deprived SK-N-SH cells (closed squares) compared to oxygen-deprived cells treated with 5 µg/ml cycloheximide (^###, p<0.001), 0.5 mM thiopental (^{<>\raster(60%)="rg3"<>}, p<0.05; ^{<>\raster(60%)="rg3"<><>\raster(60%)="rg3"<>}, p<0.01), 0.1 mM thiopental (^¥¥, p<0.01), or untreated control cells (^§§, p<0.01; ^§§§, p<0.001) are shown.</p

Thiopental inhibits protein synthesis, ameliorates hypoxic damage, and maintains energy balance during oxygen deprivation in primary cortical neurons.

<p>In (A), cortical neurons were treated with 10 µM −2 mM thiopental for 30 min and analyzed for phosphorylation of eEF2 and AMPK by immunoblotting. In (B), cortical neurons were left untreated or were exposed to 0.1–2 mM thiopental for 6 h and then pulse labeled with 200 µCi of [³⁵S]methionine for an additional 2 h. Cellular lysates were separated by 10% SDS-PAGE and the amounts of newly synthesized proteins were detected by autoradiography on dried electrophoresis gels. In (C/D), cortical neurons were exposed to hypoxia for 48 h in the presence or absence of 0.5 mM thiopental. Cellular damage was determined by an LDH-release assay (C). The relative intracellular ATP-content was measured by an ATP-driven luciferase assay (D). Values represent the mean ± standard deviations of three independent experiments. Statistical evaluation of experimental groups was performed by one-way ANOVA followed by the Bonferroni’s <i>post hoc</i> test. The statistically significant difference of oxygen-deprived cortical neurons in the presence or absence of thiopental is shown (***, p<0.001).</p