Feedback Regulation of O-GlcNAc Transferase through Translation Control to Maintain Intracellular O-GlcNAc Homeostasis

Abstract

Protein O-GlcNAcylation is a dynamic post-translational modification involving the attachment of N-acetylglucosamine (GlcNAc) to the hydroxyl groups of Ser/Thr residues on numerous nucleocytoplasmic proteins. Two enzymes are responsible for O-GlcNAc cycling on substrate proteins: O-GlcNAc transferase (OGT) catalyzes the addition while O-GlcNAcase (OGA) helps the removal of GlcNAc. O-GlcNAcylation modifies protein functions; therefore, dysregulation of O-GlcNAcylation affects cell physiology and contributes to pathogenesis. To maintain homeostasis of cellular O-GlcNAcylation, there exists feedback regulation of OGT and OGA expression responding to fluctuations of O-GlcNAc levels; yet, little is known about the molecular mechanisms involved. In this study, we investigated the O-GlcNAc-feedback regulation of OGT and OGA expression in lung cancer cells. Results suggest that, upon alterations in O-GlcNAcylation, the regulation of OGA expression occurs at the mRNA level and likely involves epigenetic mechanisms, while modulation of OGT expression is through translation control. Further analyses revealed that the eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) contributes to the downregulation of OGT induced by hyper-O-GlcNAcylation; the S5A/S6A O-GlcNAcylation-site mutant of 4E-BP1 cannot support this regulation, suggesting an important role of O-GlcNAcylation. The results provide additional insight into the molecular mechanisms through which cells may fine-tune intracellular O-GlcNAc levels to maintain homeostasis.

Disruption of O-GlcNAcylation Homeostasis Induced Ovarian Granulosa Cell Injury in Bovine

“…Enhancement of the O-GlcNAc level via the OGA inhibitor, PUGNAc, decreased the expression of the OGT mRNA and protein, but promoted OGA expression at the mRNA and protein level. Our findings are consistent with some previous reports which imply the feedback modulation between variations of O-GlcNAc levels and OGT or OGA expression [ 26 , 28 ]. Intriguingly, pharmacological inhibition of OGT did not lead to a compensatory induction of its own expression.…”

“…Increasing amounts of data from studies on animals and cell lines suggest that cells are sensitive to the perturbation of O-GlcNAc homeostasis due to altered nutritional availability and metabolic flux, which influence the cellular functions accordingly [ 26 , 27 , 28 ]. Blocked O-GlcNAc cycling by selective loss of OGA in hematopoietic stem cells impairs the stem cell self-renewal and repopulation capacity [ 29 ].…”

“…Human cell lines treated with an OGA inhibitor exhibit downregulated OGT protein expression and upregulated OGA protein expression, which indicates that O-GlcNAc homeostasis is correlated with the expression of OGT and OGA [ 26 ]. In cancer cells, OGA transcript levels displayed compensatory variations upon alterations in O-GlcNAc status, whereas augmenting overall cellular O-GlcNAc levels leads to decreased transcript and translational efficiency of OGT [ 28 , 32 ]. In the present study, repression of the O-GlcNAc level in GCs via the OGT inhibitor, BADGP, reduced the expressions of the OGT mRNA and protein, as well as the OGA protein.…”

“…Several studies have claimed that decreased O-GlcNAc levels are not necessarily accompanied by a particular increase in OGT abundance in all cell types [26,32]. As the mechanisms for maintenance of cellular O-GlcNAc homeostasis may vary in tumorous and non-tumorous cells [28], these seemingly discrepant findings observed in the current study further verify the notion that modulation of OGT expression for maintaining balanced O-GlcNAc levels for homeostasis depends on the cell type [32]. However, further studies are required to clarify the exact mechanism involved in this process.…”

A nexus of lipid and O-Glcnac metabolism in physiology and disease

“…Lipids appear to target OGT and OGA at multiple levels within this mechanistic diversity. It is important to acknowledge, however, that many counter-regulatory mechanisms exist to keep O -GlcNAcylation levels within a physiologically optimal range [see ( 75 , 76 )], such that effects on any particular element of the system may indirectly influence other elements.…”