Deciphering the role of GLUT4 N-glycosylation in adipocyte and muscle cell models.

GLUT4 (glucose transporter 4) is responsible for the insulin-induced uptake of glucose by muscle and fat cells. In non-stimulated (basal) cells, GLUT4 is retained intracellularly, whereas insulin stimulation leads to its translocation from storage compartments towards the cell surface. How GLUT4 is retained intracellularly is largely unknown. Previously, aberrant GLUT4 N-glycosylation has been linked to increased basal cell-surface levels, while N-glycosylation-deficient GLUT4 was found to be quickly degraded. As recycling and degradation of GLUT4 are positively correlated, we hypothesized that incorrect N-glycosylation of GLUT4 might reduce its intracellular retention, resulting in an increased cell-surface recycling, in increased basal cell-surface levels, and in enhanced GLUT4 degradation. In the present study, we have investigated N-glycosylation-deficient GLUT4 in detail in 3T3-L1 preadipocytes, 3T3-L1 adipocytes and L6 myoblasts. We have found no alterations in retention, insulin response, internalization or glucose transport activity. Degradation of the mutant molecule was increased, although once present at the cell surface, its degradation was identical with that of wild-type GLUT4. Our findings indicate that N-glycosylation is important for efficient trafficking of GLUT4 to its proper compartments, but once the transporter has arrived there, N-glycosylation plays no further major role in its intracellular trafficking, nor in its functional activity.