Flow cytometric analysis of glucose transport by rat brain cells

The fluorescent, non-metabolizable glucose analog 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-6-deoxyglucose (NBDG) was used to measure rates of hexose transport by dissociated brain cells from developing and adult rats. Flow cytometric analysis of glucose uptake and expression of glucose transporters was performed by mapping on size by granularity, which discriminated between neurons and astrocytes in a suspension of mixed brain cells. These mapped cell populations were identified by immunofluorescent staining with antisera to neuron-specific enolase (NSE) and glial fibrillary acidic protein (GFAP). Specific uptake of the analog by membrane glucose transporters was confirmed by its inhibition by D-glucose and by cytochalasin B. Both neurons and astrocytes expressed the GLUT1 and GLUT3 transporter isoforms. This was confirmed by the additive inhibition of NBDG uptake by antibodies to these transporter isoforms in both cell types. The advantages of flow cytometric analysis of glucose transport include continuous monitoring over extremely short periods of time, increased precision of cell-by-cell flow cytometric measurements versus average uptake rates obtained with radioisotopes, and simultaneous analysis of uptake by different cell populations. Moreover, both uptake rates and the abundance of specific transporters can be determined directly and rapidly on the same cell suspension.

Effects of ethanol on GLUT1 protein and gene expression in rat astrocytes

Effects of ethanol on glucose transporter gene expression were examined in cultured rat astrocytes. Exposure to 50 or 100 mM ethanol for 18 hours significantly inhibited hexose uptake and reduced the number of glucose transporters, as indicated by binding studies with cytochalasin B. Indirect immunofluorescence and immunoperoxidase staining showed marked reduction of the GLUT1 glucose transporter by exposure to 100 mM ethanol for 5 or 18 hours, but no obvious change in response to 50 mM ethanol. Western blot analysis showed GLUT1 protein levels to be decreased by 52 +/- 12% (p < 0.05) after exposure to 100 mM ethanol for 18 hours. In situ hybridization histochemistry indicated an increase in steady-state GLUT1 mRNA in astrocytes exposed to 50 or 100 mM ethanol for 5 or 18 hours. Quantitation of GLUT1 mRNA levels by northern blot analysis showed that GLUT1 mRNA levels were increased by 59 and 112% in cells treated for 5 h with 50 and 100 mM ethanol, respectively. A similar effect was observed after treatment for 18 hours, but ethanol did not alter actin gene expression. Experiments using actinomycin D to block RNA synthesis suggest that this increase in steady-state mRNA level results from increased message stability. These results suggest that ethanol acts on GLUT1 gene expression at the post-transcriptional level.

Ethanol-induced changes in insulin-like growth factors and IGF gene expression in the fetal brain

Brain growth retardation is a major feature of the fetal alcohol syndrome (FAS). Insulin-like growth factors (IGF-I and IGF-II) have been shown to exert significant metabolic and growth-promoting effects. Previously, we showed that circulating levels of IGF-I as well as hepatic gene expression of both IGFs were decreased in newborn offspring of rats fed ethanol during pregnancy. This study investigated the effects of maternal ethanol ingestion on fetal rat brain growth and on levels of IGF-I and IGF-II, as well as their mRNAs, in fetal brain. IGF-binding protein (IGFBP) levels also were determined. Rats were fed 5% w/v ethanol in a liquid diet during gestation (EF group). Weight-matched animals were pair-fed equicaloric control diet (PF group) or were fed ad libitum (AF group). The mean fetal brain weight of EF offspring was 13% and 16% lower (P < 0.01) than that of PF and AF offspring, respectively. Body weight of EF pups was decreased to a greater extent, resulting in higher brain to body weight ratios in EF pups than in either control group (P < 0.05). IGF-I levels in EF pups decreased by 33% and 41% compared with the corresponding PF and AF values (P < 0.01). IGF-I mRNA levels decreased by 27% and 40% compared with PF and AF values, respectively. A positive correlation was observed between brain IGF-I level and brain weight (r = 0.561, P < 0.01). IGF-II levels were not affected despite a 50% decrease in IGF-II expression. In PF animals, the fetal brain IGF-I and IGF-II mRNA levels were reduced by 28% and 21%, apparently in response to undernutrition. IGF-binding proteins levels were low in the EF group but not statistically significant compared with control values. The diminished fetal brain concentration of IGF-I and decreased gene expression of IGFs may play a role in brain growth retardation associated with FAS.

Ethanol and glucose-deprivation neurotoxicity in cortical cell cultures

The toxic effects of ethanol on rat cortical cell cultures were compared with neuronal damage induced by glucose deprivation. Exposure to decreased glucose concentrations produced dose-dependent neuronal injury, as indicated by the release of lactate dehydrogenase (LDH) into the culture medium. Complete glucose deprivation resulted in mean LDH release that was more than 60% greater than that from sister cultures incubated in the presence of 5.5 mmol/L glucose. Exposure to ethanol (25, 50, or 100 mmol/L) similarly resulted in dose-related LDH release. The degree of injury resulting from complete glucose deprivation or 100 mmol/L ethanol approximated that produced by exposure to 100 mmol/L glutamic acid. Ethanol did not significantly alter LDH release from cultures consisting of only astrocytes. Both effects were inhibited by the N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV). Glutamate levels were increased in the culture medium to 191% +/- 8% of the control value after glucose deprivation (P < .001) and to 186% +/- 16% after exposure to 100 mmol/L ethanol (P < .01). 3H-glutamate uptake by cultured astrocytes was reduced by glucose deprivation and by ethanol. This range of ethanol concentrations has previously been shown to inhibit hexose uptake by cultured astrocytes. The present results suggest that decreased glucose uptake by astrocytes in the presence of ethanol impairs their uptake of glutamate, which contributes to excitotoxic neuronal injury.

Insulin-like growth factors (IGF-I and IGF-II), IGF-binding proteins, and IGF gene expression in the offspring of ethanol-fed rats

Insulin-like growth factors I and II (IGF-I, IGF-II) and IGF-binding proteins (IGBPs) are important modulators of fetal growth. Fetal growth retardation is a major component of the fetal alcohol syndrome, which is associated with maternal alcoholism. This study examined the relationship of IGF-system components to growth retardation induced by ethanol in fetuses of rats fed equicaloric liquid diets (AF, ad libitum-fed controls; PF, pair-fed controls; EF, ethanol-fed) during gestation. The gene expression of IGF-I and IGF-II in fetal liver and the concentration of IGFs and IGFBPs in serum and liver were determined. The mean weight of EF fetuses was 13% and 16% less (p < 0.01) than that of PF and AF offspring, respectively. The serum concentration of IGF-I was decreased (p < 0.05) by 17% and 22% in EF as compared with PF and AF fetuses. Fetal body weight showed positive correlations with fetal serum IGF-I IGF-II (r = 0.566, p < 0.01, and r = 0.412, p < 0.05, respectively) Fetal liver weight correlated with fetal liver IGF-I and IGF-II, with r values of 0.514 (p < 0.01) and 0.493 (p < 0.01). Hepatic IGF-II mRNA abundance was decreased (p < 0.05) by 27% and 26% in EF as compared with PF and AF offspring. The level of fetal liver IGF-I mRNA expression was low but was also reduced comparably in EF pups. IGFBP content in EF fetal serum was increased (p < 0.05 vs AF), and correlated negatively with fetal body weight (r = -0.505, p < 0.01). The diminished IGF-I and IGF-II gene expression and the reduced tissue and circulating peptide levels, along with a converse change in serum IGFBP abundance, may have a role in the pathogenesis of fetal alcohol syndrome.

Differential removal of insulin-like growth factor binding proteins in rat serum by solvent extraction procedures

Solvent extraction of serum and other biological fluids at an acidic pH is a convenient method to remove the insulin-like growth factor binding proteins (IGFBPs); however, an incomplete removal of IGFBPs can occur and this can potentially interfere with the radioimmunoassay of insulin-like growth factors (IGFs). This study compared the removal of IGFBPs from normal adult rat serum and 5-day old neonatal rat serum by acid-gel filtration, and three solvent extraction methods, i.e., acid-ethanol (AE), acid-cryo-ethanol (ACE) and formic acid-acetone (FAA) treatments by western ligand blotting and slot-blotting analysis. In adult rat serum all three extraction methods removed nearly 75% of total IGFBPs present. For the neonatal serum, AE and FAA were very inefficient in eliminating the IGFBPs, while ACE was somewhat better, as it removed nearly 30% of IGFBPs. Ligand blots of extracted samples showed that IGFBPs of lower size range, 24 to 32 kDa (IGFBP-4, IGFBPs-1 and -2), were resistant to solvent extraction. Acid-gel filtration, in contrast, eliminated > 95% of IGF-binding components in both sera. Determination of IGF-I concentrations in samples after gel filtration and extraction methods revealed lower IGF-I values in neonatal serum in acid extracted samples. These data caution against using solvent extractions for IGFBP removal in fetal/neonatal serum.

Effects of ethanol on DNA, RNA, and protein synthesis in rat astrocyte cultures

Brain growth retardation is a major feature of the fetal alcohol syndrome (FAS). Insulin and insulin-like growth factors (IGF-I and IGF-II) exert significant growth-promoting effects on the central nervous system (CNS). The present study examined the effects of ethanol and its interactions with growth factors on the incorporation of labeled precursors into DNA, RNA, and protein in primary astrocyte cultures prepared from term fetal rats. Cultures were exposed to ethanol for 18hr in serum-free medium before measuring nucleoside or amino acid incorporation into acid-precipitable cell constituents. Under basal conditions, ethanol induced dose-dependent changes in the rates of incorporation of tritiated thymidine, uridine, and valine. The fraction of the total thymidine uptake that was incorporated into DNA was reduced in the presence of 100 and 200 mM ethanol. Effects on uridine and valine incorporation paralleled cell uptake. Insulin (10(-6) M) and IGF-I (10(-9) M) increased (p less than 0.01) incorporation of radiolabeled thymidine, uridine, and valine. Analysis of variance indicated highly significant interactions between ethanol and the effects of growth factors on incorporation of both nucleosides and valine. Interference with the action of neurotrophic factors may be a significant factor in fetal brain growth retardation associated with maternal ethanol ingestion.