Article

Interaction of FoxO1 and TSC2 induces insulin resistance through activation of the mammalian target of rapamycin/p70 S6K pathway

Department of Clinical Molecular Medicine, Division of Diabetes, Digestive and Kidney Disease, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan.
Journal of Biological Chemistry (Impact Factor: 4.6). 01/2007; 281(52):40242-51. DOI: 10.1074/jbc.M608116200
Source: PubMed

ABSTRACT Both TSC2 (tuberin) and forkhead transcription factor FoxO1 are phosphorylated and inhibited by Akt and play important roles in insulin signaling. However, little is known about the relationship between TSC2 and FoxO1. Here we identified TSC2 as a FoxO1-binding protein by using a yeast two-hybrid screening with a murine islet cDNA library. Among FoxOs, only FoxO1 can be associated with TSC2. The physical association between the C terminus of TSC2 (amino acids 1280-1499) and FoxO1 degrades the TSC1-TSC2 complex and inhibits GTPase-activating protein activity of TSC2 toward Rheb. Overexpression of wild type FoxO1 enhances p70 S6K phosphorylation, whereas overexpression of TSC2 can reverse these effects. Knockdown of endogenous FOXO1 in human vascular endothelial cells decreased phosphorylation of p70 S6K. Prolonged overexpression of wild type FoxO1 enhanced phosphorylation of serine 307 of IRS1 and decreased phosphorylation of Akt and FoxO1 itself even in the presence of serum. These data suggest a novel mechanism by which FoxO1 regulates the insulin signaling pathway through negative regulation of TSC2 function.

0 Bookmarks
 · 
163 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Exquisite regulation of insulin secretion by pancreatic β-cells is essential to maintain metabolic homeostasis. β-Cell mass must be accordingly adapted to metabolic needs and can be largely modified under different situations. The mammalian target of rapamycin (mTOR) complexes has been consistently identified as key modulators of β-cell mass. mTOR can be found into two different complexes, mTORC1 and mTORC2. Under systemic insulin resistance, mTORC1/mTORC2 signaling in β-cells is needed to increase β-cell mass and insulin secretion. However, type 2 diabetes arises when these compensatory mechanisms fail, being the role of mTOR complexes still obscure in β-cell failure. In this chapter, we introduce the protein composition and regulation of mTOR complexes and their role in pancreatic β-cells. Furthermore, we describe their main signaling effectors through the review of numerous animal models, which indicate the essential role of mTORC1/mTORC2 in pancreatic β-cell mass regulation.
    Vitamins & Hormones 01/2014; 95:425-69. DOI:10.1016/B978-0-12-800174-5.00017-X · 1.78 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Objective FoxO1 acts as a pivotal transcription factor in insulin signaling. However, in hyperglycemia induced cardiac complications, whether FoxO1 is involved remains unclear. The goal of this study was to delineate the potential role of FoxO1 under high-glucose condition. Materials/Methods We investigated insulin resistance and ROS generation in H9c2 cardiomyoblasts after high-glucose exposure. A series of autophagy biomarkers were measured and further confirmed by LC3 turnover assay. Using gene silencing and overexpression experiments we dissected the molecular mechanisms of FoxO1 regulated autophagy. We also tested the protective effect of (−)-epigallocatechin-3-gallate (EGCG, a green tea-derived polyphenol) in high-glucose treated H9c2 cardiomyoblasts. Results High-glucose elicited elevated reactive oxygen species (ROS), autophagy and FoxO1 abundance in cultured H9c2 cardiomyoblasts. Specifically, high-glucose significantly augmented the acetylated FoxO1 in cytosol. In line, compared with 3A-FoxO1 (majorly localized in nuclei with a strong transcriptional activity), over expression WT-FoxO1 led to more intense elevated autophagy with enhanced acetylation of FoxO1. In addition, FoxO1 RNAi brought down autophagy induced by high-glucose. Intriguingly, EGCG successfully reversed ROS, autophagy and acetylated FoxO1 in high-glucose treated H9c2 cells. Conclusion Our findings suggest that FoxO1, especially the acetylated form, regulates high-glucose induced autophagy in H9c2 cardiomyoblasts, which can be prevented by EGCG via a possible ROS-FoxO1 pathway.
    Metabolism 10/2014; 63(10). DOI:10.1016/j.metabol.2014.06.012 · 3.61 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: FoxO proteins are important regulators in cellular metabolism and are recognized to be nodes in multiple signaling pathways, most notably those involving PI3K/AKT and mTOR. FoxO proteins primarily function as transcription factors, but recent study suggests that cytosolic FoxO1 participates in the regulation of autophagy. In the current study, we find that cytosolic FoxO1 indeed stimulates cellular autophagy in multiple cancer cell lines, and that it regulates not only basal autophagy but also that induced by rapamycin and that in response to nutrient deprivation. These findings illustrate the importance of FoxO1 in cell metabolism regulation independent of its transcription factor function. In contrast to FoxO1, we find the closely related FoxO3a is a negative regulator of autophagy in multiple cancer cell lines, a previously unrecognized function for this protein, different from its function in benign fibroblast and muscle cells. The induction of autophagy by the knockdown of FoxO3a was found not to be mediated through the suppression of mTORC1 signaling; rather, the regulatory role of FoxO3a on autophagy was determined to be through its ability to transcriptionally suppress FoxO1. This complicated interplay of FoxO1 and FoxO3a suggests a complex checks- and balances-relationship between FoxO3a and FoxO1 in regulating autophagy and cell metabolism.
    PLoS ONE 12/2014; 9(12):e115087. DOI:10.1371/journal.pone.0115087 · 3.53 Impact Factor