Insulin suppresses transactivation by CAAT/Enhancer-binding proteins β (C/EBPβ). Signaling to p300/CREB-binding protein by protein kinase B disrupts interaction with the major activation domain of C/EBPβ
ABSTRACT CAAT/enhancer-binding proteins (C/EBPs) play an important role in the regulation of gene expression in insulin-responsive tissues. We have found that a complex containing C/EBPbeta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 (IGFBP-1) gene and that a C/EBP-binding site can mediate effects of insulin on promoter activity. Here, we examined mechanisms mediating this effect of insulin. The ability of insulin to suppress promoter activity via a C/EBP-binding site is blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor, but not by rapamycin, which blocks activation of p70(S6 kinase). Dominant negative phosphatidylinositol 3-kinase and protein kinase B (PKB) block the effect of insulin, while activated PKB suppresses promoter function via a C/EBP-binding site, mimicking the effect of insulin. Coexpression studies indicate that insulin and PKB suppress transactivation by C/EBPbeta, but not C/EBPalpha, and that N-terminal transactivation domains in C/EBPbeta are required. Studies with Gal4 fusion proteins reveal that insulin and PKB suppress transactivation by the major activation domain in C/EBPbeta (AD II), located between amino acids 31 and 83. Studies with E1A protein indicate that interaction with p300/CBP is required for transactivation by AD II and the effect of insulin and PKB. Based on a consensus sequence, we identified a PKB phosphorylation site (Ser(1834)) within the region of p300/CBP known to bind C/EBPbeta. Mammalian two-hybrid studies indicate that insulin and PKB disrupt interactions between this region of p300 and AD II and that Ser(1834) is critical for this effect. Signaling by PKB and phosphorylation of Ser(1834) may play an important role in modulating interactions between p300/CBP and transcription factors and mediate effects of insulin and related growth factors on gene expression.
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- "For instance, PKC-d-mediated p300 phosphorylation at serine 89 represses intrinsic acetyltransferase activity and transcriptional function (Yuan et al., 2002). In contrast, phosphorylation at serine 1835 by Akt/PKB enhances acetyltransferase activity (Huang and Chen, 2005), and also modulates p300 interaction with C/EBPb in response to insulin (Guo et al., 2001). Induction of phosphatidylinositol 3-kinase activity is associated with increased p300 stability and transcriptional activity (Chen et al., 2004). "
ABSTRACT: The transcriptional coactivator p300 is a ubiquitous nuclear phosphoprotein and transcriptional cofactor with intrinsic acetyltransferase activity. p300 controls the expression of numerous genes in cell-type and signal-specific manner, and plays a pivotal role in cellular proliferation, apoptosis, and embryogenesis. By catalyzing acetylation of histones and transcription factors, p300 plays a significant role in epigenetic regulation. Recent evidence suggests that abnormal p300 function is associated with deregulated target gene expression, and is implicated in inflammation, cancer, cardiac hypertrophy, and genetic disorders such as the Rubinstein-Taybi syndrome. The activity of p300 is regulated at multiple levels, including developmental stage-specific expression, post-translational modifications, subcellular localization, and cell-type and gene-specific interactions with transcription factors. Although p300 has been investigated extensively in epithelial and hematopoietic cells, its role in fibroblast biology and tissue repair has received little attention to date. Recent studies implicate p300 in the regulation of collagen synthesis by transforming growth factor-beta (TGF-beta). Both the acetyltransferase activity of p300 and its inducible interaction with Smad3 are essential for mediating TGF-beta-induced stimulation of collagen synthesis. As a signal integrator whose availability for intracellular interactions with transcription factors is strictly limiting, p300 mediates the antagonistic regulation of TGF-beta-induced collagen synthesis by IFN-gamma and TNF-alpha via intracellular competition for limiting amount of p300. Significantly, p300 is itself a direct transcriptional target of TGF-beta in normal fibroblasts, and its levels are significantly elevated in fibrotic lesions as well as in experimental models of fibrosis. The emerging appreciation of the importance of p300 in extracellular matrix (ECM) remodeling and fibrosis and novel insights concerning the regulation, mechanism of action, and significance of p300 in fibroblast biology are discussed in this minireview.Journal of Cellular Physiology 12/2007; 213(3):663-71. DOI:10.1002/jcp.21162 · 3.87 Impact Factor
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- "Glucocorticoid results in the binding of a complex of transcription factors, consisting of Foxo1, c/EBPb, and the coactivator CBP/p300, to this IRE sequence (Fig. 1B) (Ghosh et al. 2001; Guo et al. 1999, 2001; Nasrin et al. 2000). Insulin treatment results in the phosphorylation of Foxo proteins (Nasrin et al. 2000), their exclusion from the nucleus (Biggs et al. 1999; Brunet et al. 1999; Takaishi et al. 1999), and the phosphorylation of the coactivator CBP/p300 (Guo et al. 2001), leading to the dissociation of this transcription complex. Recently it has been shown that the effect of insulin on glucocorticoid-induced gene transcription can occur independent of this IRE (Gan et al. 2005). "
ABSTRACT: Insulin, after binding to its receptor, regulates many cellular processes and the expression of several genes. For a subset of genes, insulin exerts a negative effect on transcription; for others, the effect is positive. Insulin controls gene transcription by modifying the binding of transcription factors on insulin-response elements or by regulating their transcriptional activities. Different insulin-signaling cascades have been characterized as mediating the insulin effect on gene transcription. In this review, we analyze recent data on the molecular mechanisms, mostly in the liver, through which insulin exerts its effect. We first focus on the key transcription factors (viz. Foxo, sterol-response-element-binding protein family (SREBP), and Sp1) involved in the regulation of gene transcription by insulin. We then present current information on the way insulin downregulates and upregulates gene transcription, using as examples of downregulation phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes and of upregulation the fatty acid synthase and malic enzyme genes. The last part of the paper focuses on the signaling cascades activated by insulin in the liver, leading to the modulation of gene transcription.Canadian Journal of Physiology and Pharmacology 08/2006; 84(7):713-24. DOI:10.1139/y05-152 · 1.55 Impact Factor
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- "Another report indicates that serine 1834 in p300 is phosphorylated by protein kinase B/Akt, and that phosphorylation at this site disrupts the interaction with the transcription factor C/EBPß. At the present time, one cannot distinguish whether this is due to a loss of affinity for C/EBPß or due to an increased affinity for another cellular factor that competes with C/EBPß for binding to the same region of p300 encompassing serine 1834 (Guo et al., 2001). The p42/p44 MAPKs have been found to phosphorylate CBP in vitro and enhance the potency of its transactivation domains. "
ABSTRACT: p300 and CBP are highly homologous coactivators which promote gene transcription by bridging between DNA-binding transcription factors and the basal transcription machinery, by providing a scaffold for integrating transcription factors, and by modifying transcription factors and chromatin through acetylation. The p300/CBP cofactors are involved in a plethora of physiological processes, and their activity is essential for embryogenesis. Chromosomal translocations affecting the p300 and Cbp genes are the cause of hematological malignancies, and Cbp haploinsufficiency is a hallmark of the Rubinstein-Taybi syndrome. In addition, mutations in the Cbp or p300 gene, accompanied by loss of the other allele, have been found in various kinds of tumors. Furthermore, inhibition of CBP and p300 function in neurodegenerative diseases caused by polyglutamine expansion may be an underlying cause for cytotoxicity. Approaches to modulate p300/CBP function may be instrumental in the development of novel therapies directed against viral infections, cancer and neurodegenerative diseases.Histology and histopathology 05/2002; 17(2):657-68. · 2.24 Impact Factor