Glucose Activates ChREBP by Increasing Its Rate of Nuclear Entry and Relieving Repression of Its Transcriptional Activity*S⃞

Carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that activates genes involved in de novo lipogenesis in mammals. The current model for glucose activation of ChREBP proposes that increased glucose metabolism triggers a cytoplasmic to nuclear translocation of ChREBP that is critical for activation. However, we find that ChREBP actively shuttles between the cytoplasm and nucleus in both low and high glucose in the glucose-sensitive β cell-derived line, 832/13. Glucose stimulates a 3-fold increase in the rate of ChREBP nuclear entry, but trapping ChREBP in the nucleus by mutagenesis or with a nuclear export inhibitor does not lead to constitutive activation. In fact, mutational studies targeting the nuclear export signal of ChREBP also identified a distinct function essential for glucose-dependent transcriptional activation. From this, we conclude that an additional event independent of nuclear translocation is required for activation. The N-terminal segment of ChREBP (amino acids 1-298) has previously been shown to repress activity under basal conditions. This segment has five highly conserved regions, Mondo conserved regions 1-5 (MCR1 to -5). Based on activating mutations in MCR2 and MCR5, we propose that these two regions act coordinately to repress ChREBP in low glucose. In addition, other mutations in MCR2 and mutations in MCR3 were found to prevent glucose activation. Hence, we conclude that both relief of repression and adoption of an activating form are required for ChREBP activation

Differential Regulation of STAT Family Members by Glycogen Synthase Kinase-3*

Excessive neuroinflammation contributes to many neurological disorders and is poorly controlled therapeutically. The signal transducer and activator of transcription (STAT) family of transcription factors has a central role in inflammatory reactions, being stimulated by multiple cytokines and interferons and regulating the expression of many proteins involved in inflammation. We found that STAT3 activation is highly dependent on glycogen synthase kinase-3 (GSK3). Inhibitors of GSK3 greatly reduced (>75%) the activating STAT3 tyrosine phosphorylation in mouse primary astrocytes, microglia, and macrophage-derived RAW264.7 cells induced by interferon-γ (IFNγ), IFNα, interleukin-6, or insulin. GSK3 inhibitors blocked STAT3 DNA binding activity and the expression of STAT3-induced GFAP and Bcl-3. GSK3 dependence was selective for activation of STAT3 and STAT5, whereas STAT1 and STAT6 activation were GSK3-independent. Knockdown of the two GSK3 isoforms showed STAT3 and STAT5 activation were dependent on GSK3β, but not GSK3α. The regulatory mechanism involved GSK3β binding STAT3 and promoting its association with the IFNγ receptor-associated intracellular signaling complex responsible for activating STAT3. Furthermore, GSK3β associated with the IFNγ receptor and was activated by stimulation with IFNγ. Thus, inhibitors of GSK3 reduce the activation of STAT3 and STAT5, providing a mechanism to differentially regulate STATs to modulate the inflammatory response