Mechanisms of adaptation of glucose transporters to changes in the oxidative chain of muscle and fat cells

Abstract

Cells in which glucose uptake is rate limiting respond to hypoxic insults with an increase in glucose transport activity. To understand the underlying cellular mechanisms involved in this adaptive response, the effects of an uncoupler of oxidative phosphorylation, 2,4-dinitrophenol (DNP), and of an inhibitor of the electron transport chain, rotenone, were compared with the effect of hypoxia in L6 muscle cells and 3T3-L1 adipocytes. All three conditions (DNP, rotenone, and 3% oxygen) elevated hexose uptake by approximately twofold in 4 h relative to control cells. All three insults decreased cellular ATP levels rapidly. A subsequent recovery was observed within 1-2 h in the presence of DNP or 3% oxygen, probably as a result of anaerobic production of ATP through increased glucose uptake and glycolysis. DNP and rotenone elevated the content of GLUT-1 protein in isolated plasma membranes and decreased it in intracellular light microsomes, suggestive of translocation of this transporter isoform. No change in GLUT-4 protein distribution was detected. In contrast, 3% oxygen caused a marked specific increase in GLUT-1 protein in both plasma membranes and microsomes. Consistently, cycloheximide had no effect on the hexose transport responses to DNP or rotenone, but prevented the response to hypoxia. However, GLUT-1 mRNA and the total cell content of GLUT-1 protein were elevated by all three treatments. It is proposed that within the time frame studied, reductions in the energy charge may activate the glucose transport system in L6 myotubes and 3T3-L1 adipocytes by GLUT-1 protein biosynthesis and translocation. When both responses exist, the biosynthetic pathway is dispensable, and posttranslational mechanisms, including transporter translocation suffice to sustain the adaptive elevation in glucose transport activity for several hours. s