In L6 Skeletal Muscle Cells, Glucose Induces Cytosolic Translocation of Protein Kinase C-α and Trans-activates the Insulin Receptor Kinase

In L6 skeletal muscle cells expressing human insulin receptors (L6hIR), exposure to 25 mm glucose for 3 min induced a rapid 3-fold increase in GLUT1 and GLUT4 membrane translocation and glucose uptake. The high glucose concentration also activated the insulin receptor kinase toward the endogenous insulin receptor substrates (IRS)-1 and IRS-2. At variance, in L6 cells expressing kinase-deficient insulin receptors, the exposure to 25 mm glucose elicited no effect on glucose disposal. In the L6hIR cells, the acute effect of glucose on insulin receptor kinase was paralleled by a 2-fold decrease in both the membrane and the insulin receptor co-precipitated protein kinase C (PKC) activities and a 3-fold decrease in receptor Ser/Thr phosphorylation. Western blotting of the receptor precipitates with isoform-specific PKC antibodies revealed that the glucose-induced decrease in membrane- and receptor-associated PKC activities was accounted for by dissociation of PKCα but not of PKCβ or -δ. This decrease in PKCα was paralleled by a similarly sized increase in cytosolic PKCα. In intact L6hIR cells, inhibition of PKCα expression by using a specific antisense oligonucleotide caused a 3-fold increase in IRS phosphorylation by the insulin receptor. This effect was independent of insulin and accompanied by a 2.5-fold increase in glucose disposal by the cells. Thus, in the L6 skeletal muscle cells, glucose acutely regulates its own utilization through the insulin signaling system, independent of insulin. Glucose autoregulation appears to involve PKCα dissociation from the insulin receptor and its cytosolic translocation.