Effects of Sodium Butyrate, D-Glucose, and Dibutyryl cAMP On Glut 4 Mediated Insulin-Stimulated Transport in Cultured Rat Skeletal Muscle Cells
Caruana, Crystal M.
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The effects of sodium butyrate, D-glucose, and dibutyryl cAMP on insulin-stimulated glucose transport in rat skeletal muscle cells (L-6 line) previously transfected with wild type GLUT 4 transporter protein were studied. 2-deoxy[3H]glucose (a glucose analog) uptake was measured in the presence of insulin, dibutryl cAMP, and D-glucose to investigate two basic problems. First, attempts were made to select a cell line transfected with wild type GLUT 4 (lRGT lines) that had insulin-stimulated 2-deoxy[3H]glucose transport that was at or near the level of the 243-6 line. 243-6 cells have a mutated form of the GLUT 4 protein and are overexpressors of GLUT 4 and 2-deoxy[3H]glucose insulin-stimulated transport. Incubation with sodium butyrate was used to enhance GLUT 4 mediated insulin-stimulated 2-deoxy[3H]glucose transport in IRGT lines that were chosen to be potential overexpressors of the protein. Increased transport was seen in some cell lines treated with sodium butyrate. GLUT 4 expression was measured by SD5-P AGE and immunoblotting using RBI, a GLUT 4 specific antibody, and I25I-labelled protein A. Increased membraneous expression of GLUT 4 in some cell lines was also observed. Enhanced 2-deoxy[3H]glucose transport was not accompanied by the expected increased membraneous expression of GLUT 4. It is proposed that sodium butyrate affects another mechanism of insulin stimulated glucose transport instead of GLUT 4. Second, the effects of glucose on insulin-stimulated transport were investigated. Dibutryl cAMP inhibited 2-deoxy[3H]glucose insulinstimulated transport. D-glucose reversed this inhibitory effect only in cells that contained GLUT 4. It was concluded that D-glucose interferes with normal mechanisms of dibutryl cAMP which have been shown previously to impede transporter activation through kinase activity. Glucose may reverse or prevent this activity thereby reducing cAMP's inhibitory effect.