O ‐GlcNAcylation of FoxO1 in pancreatic β cells promotes Akt inhibition through an IGFBP1‐mediated autocrine mechanism

Abstract

O-GlcNAcylation on serine/threonine is a post-translational modification that controls the activity of nucleocytoplasmic proteins according to glucose availability. We previously showed that O-GlcNAcylation of FoxO1 in liver cells increases its transcriptional activity. In the present study, we evaluated the potential involvement of FoxO1 O-GlcNAcylation in the context of pancreatic β-cell glucotoxicity. FoxO1 was O-GlcNAcylated in INS-1 832/13 β cells and isolated rat pancreatic islets. O-GlcNAcylation of FoxO1 resulted in a 2-fold increase in its transcriptional activity toward a FoxO1 reporter gene and a 3-fold increase in the expression of the insulin-like growth factor-binding protein 1 (Igfbp1) gene at the mRNA level, resulting in IGFBP1 protein oversecretion by the cells. Of note, increased IGFBP1 in the culture medium inhibited the activity of the insulin-like growth factor 1 receptor (IGF1R)/phosphatidyl inositol 3 kinase (PI3K)/Akt pathway. We reveal in this report a novel mechanism by which O-GlcNAcylation inhibits Akt activity through an autocrine mechanism. However, although inhibition of IGFBP1 expression using siRNA restored the PI3 kinase/Akt pathway, it did not rescue INS-1 832/13 cells from high-glucose- or O-glcNAcylation-induced cell death. In contrast, FoxO1 down-regulation by siRNA led to 30 to 60% protection of INS-1 832/13 cells from death mediated by glucotoxic conditions. Therefore, whereas FoxO1 O-GlcNAcylation inhibits Akt through an IGFBP1-mediated autocrine pathway, the deleterious effects of FoxO1 O-GlcNAcylation on cell survival appeared to be independent of this pathway.

Development of a Human Breast-Cancer Derived Cell Line Stably Expressing a Bioluminescence Resonance Energy Transfer (BRET)-Based Phosphatidyl Inositol-3 Phosphate (PIP3) Biosensor

“…We therefore hypothesized that IGF1 and/or IGF2 could be the active factors present in human serum. IGFBP1 (Insulin-like growth factor binding protein-1) is a secreted protein that binds to and inhibits the stimulatory activity of IGF1 and IGF2 on IGF-1R [26], [27]. To determine whether the PI3K stimulatory activity present in serum was mediated by IGFs, the serum was pre-incubated with recombinant IGFBP1 for 1 h prior to stimulation of MCF-7/B2 cells.…”

“…Therefore, our cell line should constitute an excellent tool to evaluate in clinical assays the PI3K stimulatory activity present in sera from large cohorts of patients treated with different insulin analogues or other medications. Finally, MCF-7/B2 cells could also be a useful tool to study PIP 3 -stimulatory activity present in conditioned media from normal or cancer cells, in order to evaluate whether proteins or small molecules secreted by these cells may affect the PI-3 kinase pathway through autocrine or paracrine mechanisms [27].…”

Urea impairs β cell glycolysis and insulin secretion in chronic kidney disease

“…nomena (27). In islets, although O-GlcNAcylation of specific β cell proteins such as PDX-1 and FOXO1 is essential for glucose regulation of gene expression (36) and insulin secretion (60), excessive protein O-GlcNAcylation reduces insulin secretion (28)(29)(30)61) and increases β cell apoptosis (62). Interestingly, protein O-GlcNAcylation in CKD mouse islets and in urea-treated islets was normalized by antioxidant treatment, implying that in the uremic context, ROS promote O-GlcNAcylation.…”

Pathogenesis of Chronic Hyperglycemia: From Reductive Stress to Oxidative Stress

“…Fructose 6-phosphate is the substrate of the enzyme glutamine-fructose 6-P amidotransferase (GFAT), which is the rate-limiting enzyme for this pathway. GFAT makes glucosamine 6-P from fructose 6-P and the former is further converted to UDP-N-acetylglucosamine, which is the substrate for specific O-GlcNAc transferase that catalyzes posttranslational modifications of proteins via O-GlcNAc on serine and threonine residues [133–135]. Increased glucose flux through this pathway has been shown to be involved in ROS generation and oxidative stress [136–138] and has been implicated in diabetic complications [139–142].…”