Hyperglycemia‐induced O‐GlcNAcylation of 4E‐BP1 enhances its expression through a reduced rate of degradation

Abstract

4E‐BP1 binds the mRNA cap‐binding protein eIF4E and thereby represses cap‐dependent mRNA translation. Phosphorylation of 4E‐BP1 releases it from eIF4E, allowing 4E‐BP1 to be poly‐ubiquitinated and subsequently degraded. Previous studies from our laboratory have established that 4E‐BP1 is also modified by addition of N‐acetylglucosamine to Ser and/or Thr residues (O‐GlcNAcylation) in the liver of diabetic mice concomitant with increased association with eIF4E. In the present study, we examined the hypothesis that increased O‐GlcNAcylation of 4E‐BP1 leads to an increase in its expression through a reduced rate of degradation. In support of this hypothesis, in both diabetic mice and in cells in culture exposed to 25 mM glucose, 4E‐BP1 protein expression was increased relative to controls. However, there was no significant difference in the rate of 4E‐BP1 synthesis between cells exposed to 25 mM compared to 5 mM glucose. In contrast, the rate of 4E‐BP1 degradation was significantly prolonged in cells maintained in 25 mM compared to 5 mM glucose. Together these findings suggest that the increased 4E‐BP1 expression observed in diabetic mice and in cells incubated in 25 mM glucose is the result of impaired degradation of the protein. These findings provide insight into the pathogenesis of metabolic abnormalities associated with diabetes. (Supported by NIH grant 13499 (LSJ) and an NIH Postdoctoral Fellowship (MDD))