Monokine regulation of glucose transporter mRNA in L6 myotubes

Endotoxin-induced macrophage secretory proteins (monokines) have been shown to stimulate hexose uptake in L6 myotubes (1). In those studies a doubling of the Vmax for hexose uptake was observed which correlated with elevated numbers of glucose transporters (GT) in both plasma and microsomal membranes. To determine if these changes in transporter populations were due to increased GT mRNA, we performed Northern blot analysis using L6 cell RNA and a cDNA to the HepG2 glucose transporter. The L6 myotubes contained a single 2.8 kb species of GT mRNA that increased 2.5-fold after an 8h exposure to the monokine preparation. beta-Actin mRNA levels were unaltered by the treatment, indicating specificity of monokine action. Glucose transporter mRNA content appeared to reach a maximum 8 h after exposure to the monokine. Over the next 16 h the levels of this mRNA gradually decreased, approaching control levels. Data obtained from nuclear transcription run-on assays suggest that increased levels of CT mRNA are due to an increased rate of gene transcription. A second transporter, the insulin-sensitive glucose transporter, was also observed to be expressed in the L6 cells. Monokine treatment resulted in a 60% suppression of the mRNA coding for this protein.