REGULATION OF THE GLUT1 GLUCOSE-TRANSPORTER IN CULTURED MYOCYTES - TOTAL NUMBER AND SUBCELLULAR-DISTRIBUTION AS DETERMINED BY PHOTOAFFINITY-LABELING

We have used the impermeant photoaffinity label 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-[2-H-3]1,3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-[2-H-3]BMPA) to identify and quantify the glucose transporters on the surface of BC3H-1 cells, a continuously cultured skeletal-muscle cell line lacking the MyoD transcription factor required for cell fusion. ATB-[2-H-3]BMPA was used in combination with immunoprecipitation of the GLUT1 glucose transporter, the only isoform expressed in these cells. The total cellular GLUT1 content was also determined by photolabelling and immunoprecipitation after cell permeabilization with digitonin (0.025 %). In glucose-starved cells, 85 % of the glucose transporters were present at the cell surface in the basal state, with little change in response to insulin (200 nM), correlating with lack of additional 2-deoxyglucose uptake in response to insulin. Feeding the cells with glucose (25 mM) for 24 h resulted in an 80 % decrease in the total GLUT1 content relative to starved cells, of which only 25 % were present on the cell surface. This was associated with an 85 % decrease in 2-deoxyglucose uptake. In addition, acute stimulation of the fed cells with insulin or phorbol 12-myristate 13-acetate (PMA) led to an increase in GLUT1 at the cell surface, and, in correspondence, an increase in 2-deoxyglucose uptake by approx. 2- and 4-fold respectively. We conclude that exofacial photoaffinity labelling of glucose transporters with ATB-[2-H-3]BMPA in the presence and absence of digitonin, followed by specific immunoprecipitation, provides an accurate measure of total and cell-surface glucose transporters in differentiated BC3H-1 muscle cells. This technique demonstrates that glucose pre-feeding (1) decreases the total number of GLUT1 and (2) redistributes the majority of the remaining transporters to an intracellular site, where they can now be translocated to the cell surface in response to insulin and PMA.