Recruitment of O-GlcNAc Transferase to Promoters by Corepressor mSin3A Coupling Protein O-GlcNAcylation to Transcriptional Repression

AbstractTranscription factors and RNA polymerase II can be modified by O-linked N-acetylglucosamine (O-GlcNAc) monosaccharides at serine or threonine residues, yet the precise functional roles of this modification are largely unknown. Here, we show that O-GlcNAc transferase (OGT), the enzyme that catalyzes this posttranslational modification, interacts with a histone deacetylase complex by binding to the corepressor mSin3A. Functionally, OGT and mSin3A cooperatively repress transcription in parallel with histone deacetylation. We propose that mSin3A targets OGT to promoters to inactivate transcription factors and RNA polymerase II by O-GlcNAc modification, which acts in concert with histone deacetylation to promote gene silencing in an efficient and specific manner

Proteolytic processing of TGFα redirects its mitogenic activity: the membrane-anchored form is autocrine, the secreted form is paracrine

AbstractWild-type transforming growth factor α (TGFα) expression in lactotrope cells in the pituitary gland led to lactotrope-specific pituitary hyperplasia and adenomata. To indicate whether the EGF receptor is involved in this TGFα-mediated phenotype, we bred TGFα mice with mice expressing the cytoplasmic truncated-EGF receptor (EGFR-tr), which is dominant-negative in other models. These bitransgenic mice developed pituitary pathology despite expression of the dominant-negative receptor. To further characterize this observation, we generated two lineages of transgenic mice that overexpress mutant forms of TGFα: a processed soluble form (s TGFα) and a cytoplasmic-deleted form (TGFαΔC). While sTGFα expression in lactotrope cells failed to induce autocrine lactotrope hyperplasia, the pituitary became very enlarged due to proliferation of neighboring interstitial cells. In contrast, the TGFαΔC mice did not develop a phenotype, although the mRNA and protein were present in the pituitary and this form of TGFα was confirmed to be biologically active and targeted properly to the plasma membrane of cultured CHO cells. The results suggest that the cytoplasmic domain of TGFα is required for autocrine parenchymal tumor formation in the pituitary gland. This signal cannot be inhibited by the EGFR-tr. Conversely, the released form of TGFα appears to have primarily paracrine activity

O-GlcNAc Modification Is an Endogenous Inhibitor of the Proteasome

AbstractThe ubiquitin proteasome system classically selects its substrates for degradation by tagging them with ubiquitin. Here, we describe another means of controlling proteasome function in a global manner. The 26S proteasome can be inhibited by modification with the enzyme, O-GlcNAc transferase (OGT). This reversible modification of the proteasome inhibits the proteolysis of the transcription factor Sp1 and a hydrophobic peptide through inhibition of the ATPase activity of 26S proteasomes. The Rpt2 ATPase in the mammalian proteasome 19S cap is modified by O-GlcNAc in vitro and in vivo and as its modification increases, proteasome function decreases. This mechanism may couple proteasomes to the general metabolic state of the cell. The O-GlcNAc modification of proteasomes may allow the organism to respond to its metabolic needs by controlling the availability of amino acids and regulatory proteins

Purification and characterization of TEF1, a transcription factor that controls the human transforming growth factor-α promoter

AbstractTransforming growth factor-α (TGF-α) is a member of the epidermal growth factor family. It activates signal transduction pathways leading to cell proliferation through the interaction with cell surface epidermal growth factor receptor. The overexpression of TGF-α has been found in many types of cancers and is thought to be involved in the genesis and maintenance of these tumors. Recent results also implicate this growth factor in the development of certain diabetic complications, such as atherosclerosis. The function of TGF-α can be tightly controlled at the level of transcription of its gene. We have previously characterized the proximal TGF-α promoter and identified two neighboring regulatory elements that appeared to cooperate with each other in the regulation of TGF-α transcription. The transcription factor that functions through the distal element was identified as AP-2, a protein that was found to be induced by the oncoprotein, Ras. However, what factor binds and controls the proximal regulatory element (PRE) is still unclear. Here, we report the purification and preliminary characterization of the PRE-binding transcription factor TEF1 by sequence-specific DNA-affinity chromatography from rat kidney nuclear extracts. The purified TEF1 migrates on the SDS–PAGE at a molecular mass of about 36 kDa. It specifically interacts with the PRE and was able to strongly activate transcription from the TGF-α promoter in HeLa cell nuclear extracts in an in vitro transcription assay. The UV cross-linking experiment confirmed that this 36 kDa protein is indeed the protein that specifically binds the PRE. We also show that the spacing between the AP-2 and the TEF1 sites in the TGF-α promoter has little effect on the transcription from the TGF-α promoter. The purification of TEF1 furthers our understanding of how TGF-α expression is regulated and may help us understand the upstream signaling events that lead to the elevated expression of this growth factor