Glucose Deprivation Causes Posttranscriptional Enhancement of Brain Capillary Endothelial Glucose Transporter Gene Expression via GLUT1 mRNA Stabilization

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Abstract
The absence of neuroglucopenia symptoms in chronic hypoglycemia may be due to up-regulation of the blood-brain barrier glucose transporter type 1 (GLUT1). Therefore, we investigated the effect of glucose deprivation on the abundance of the GLUT1 transcript in bovine brain capillary endothelial cells in tissue culture (ECL). Northern blot analysis performed under high stringency conditions with 4–5 μg of ECL poly(A)+ mRNA showed that glucose deprivation (5 mg% glucose) caused a 2.4 ± 0.2-fold increase in the GLUT1/actin mRNA ratio versus control incubations (100 mg% glucose). This rise was dose and time dependent, and the maximum effect was observed 20–24 h after the hexose deprivation. Nuclear transcription run-on assay showed no changes in either the GLUT1 or actin gene transcription rate 24 h after glucose deprivation. To determine whether the increase in the abundance of the GLUT1 mRNA induced by glucose deprivation was due to increased stability of this transcript, the GLUT1 mRNA half-life was measured in ECL cells incubated with actinomycin D. The levels of the GLUT1 transcript continued to be augmented in glucose-deprived cells compared with controls 2 and 4 h after the transcription inhibitor was added to the media. Glucose deprivation induced a 78% increase in the t1/2 of the GLUT1 mRNA (from 3.6 to 6.4 h). Incubation of ECL cells with the protein synthesis inhibitor, cycloheximide, for 4 h partially reversed the effect of glucose deprivation on the abundance of the GLUT1 transcript. On the other hand, incubation with cycloheximide for 24 h completely blocked the effect of glucose deprivation on the GLUT1 transcript. Desensitization of cellular protein kinase C was performed by incubation of ECL cells with 1 μM phorbol ester for 24 h. This also blocked the effect of glucose deprivation on the abundance of the GLUT1 mRNA. Overall, the present study demonstrates that glucose deprivation increases the abundance of GLUT1 mRNA in brain capillary endothelial culture cells. This increase is due to enhanced stability of the GLUT1 mRNA without changes in gene transcriptional rate. This appears to be mediated by an ongoing protein synthesis and by a protein kinase C-dependent mechanism. The increase in the abundance of the GLUT1 gene transcript may represent the initial step in the up-regulation of this gene in hypoglycemia.

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