Growth factors rapidly induce expression of the glucose transporter gene

The expression of the gene encoding the facilitated glucose transporter (GT) protein was studied in fibroblast cell lines. Addition of 15% calf serum to confluent BALB/c3T3, NIH3T3, or Rat-2 cells rapidly induced a 5-10-fold increase in GT mRNA, as determined by hybridization of size-fractionated total RNA to a rat brain GT cDNA. The rise in GT mRNA was maximal at 3-4 h after stimulation, and then returned to basal values by 16 h. The serum-stimulated increase in GT mRNA was not blocked by the protein synthesis inhibitors cycloheximide (10 micrograms/ml) or anisomycin (100 microM). In BALB/c3T3 cells, fibroblast growth factor (100 ng/ml), platelet-derived growth factor (5 units/ml), and epidermal growth factor (40 ng/ml) stimulated GT mRNA accumulation, although, when added individually, none of these growth factors increased DNA synthesis. The tumor promoter 12-O-tetradecanoyl phorbol-13-acetate (TPA), which activates the enzyme protein kinase C, also caused GT mRNA accumulation in BALB/c3T3 and NIH3T3 cells. Prolonged pretreatment of cells with TPA abolished the response to TPA but not fibroblast growth factor. The involvement of GT gene transcription was assessed by the nuclear run-on technique. Treatment of NIH3T3 cells with serum increased transcription at least 10-20-fold by 30 min and returned to near basal levels by 2 h. This rapid activation paralleled that of the c-fos gene, but preceded the increase in c-myc gene transcription. These data indicate the following: 1) serum growth factors increase glucose transporter mRNA levels by a process not requiring intermediary new protein synthesis and clearly dissociable from mitogenesis, 2) the changes in GT mRNA are preceded by a rapid and transient activation of GT gene transcription, and 3) there exist protein kinase C-dependent and independent pathways for regulation of GT gene expression.