Insulin Gene Transcription Is Mediated by Interactions between the p300 Coactivator and PDX-1, BETA2, and E47

Abstract

Pancreatic ␤-cell-type-specific expression of the insulin gene requires both ubiquitous and cell-enriched activators, which are organized within the enhancer region into a network of protein-protein and protein-DNA interactions to promote transcriptional synergy. Protein-protein-mediated communication between DNAbound activators and the RNA polymerase II transcriptional machinery is inhibited by the adenovirus E1A protein as a result of E1A's binding to the p300 coactivator. E1A disrupts signaling between the non-DNAbinding p300 protein and the basic helix-loop-helix DNA-binding factors of insulin's E-element activator (i.e., the islet-enriched BETA2 and generally distributed E47 proteins), as well as a distinct but unidentified enhancer factor. In the present report, we show that E1A binding to p300 prevents activation by insulin's ␤-cell-enriched PDX-1 activator. p300 interacts directly with the N-terminal region of the PDX-1 homeodomain protein, which contains conserved amino acid sequences essential for activation. The unique combination of PDX-1, BETA2, E47, and p300 was shown to promote synergistic activation from a transfected insulin enhancer-driven reporter construct in non-␤ cells, a process inhibited by E1A. In addition, E1A inhibited the level of PDX-1 and BETA2 complex formation in ␤ cells. These results indicate that E1A inhibits insulin gene transcription by preventing communication between the p300 coactivator and key DNA-bound activators, like PDX-1 and BETA2:E47.Pancreatic islet ␤-cell-specific expression of the insulin gene is due to a unique combination of factors that stimulate transcription through an enhancer located between nucleotides Ϫ340 and Ϫ90 relative to the transcription start site (2,5,55,62). These sequences can replicate the tissue-restricted and metabolically regulated expression pattern of the endogenous insulin gene when linked to a reporter construct in transgenic mice (3,6,20,66) and cell lines (14,25,32,56,57,59). C2 (Ϫ317 to Ϫ311 bp) (15,25,54), A3 (Ϫ201 to Ϫ196 bp) (15, 49, 50), C1 (Ϫ118 to Ϫ107 bp) (59, 73), and E1 (Ϫ100 to Ϫ91 bp) (9,25,71) are the cis-acting enhancer elements within the mammalian insulin gene that are essential for activation. (These insulin elements are labeled in accordance with the nomenclature proposed by German et al. [16].) Because each of these activator-binding motifs is found within the transcription unit of all characterized mammalian insulin genes (65), a common regulator strategy is likely to be involved in control.The PAX6 paired-homeodomain transcription factor activates insulin C2 element-stimulated transcription (64), and the PDX-1 homeodomain protein mediates A3 element-activated expression (42,47,49,50). PDX-1 is the name (i.e., pancreatic and duodenal homeobox-1) given by the International Committee on Standardized Genetic Nomenclature for Mice, but it has also been referred to as IPF-1 (42), IDX-1 (33), and STF-1 (28). The activator of E element-directed expression is a heterodimer of proteins in the basic helix-loop-helix (bHLH) f...

Chromatin-bound mitogen-activated protein kinases transmit dynamic signals in transcription complexes in β-cells

“…A second indirect effect may arise from synergistic interactions of transcription factors on the insulin gene promoter. Phosphorylation of one factor may promote the association of tightly bound partners that themselves may not be substrates (30).…”

A feat of metabolic proportions: Pdx1 orchestrates islet development and function in the maintenance of glucose homeostasis

“…These interacting factors include bHLH proteins that bind to Eboxes (BETA2/NeuroD1 and E2A proteins) [122,123,140,141], the basic leucine zipper factor MafA that binds to the C-box [73,142], the homeodomain proteins Pbx1 and MEIS2 [47,48,143,144], and high mobility group (HMG) proteins that bind to the DNA backbone [123]. The free energy gained from these interactions may increase the likelihood that Pdx1 targets genes with binding sites for these factors [123,144].…”

“…Apart from interaction with transcription factors, Pdx1 also appears to recruit transcriptional co-activators (proteins that bridge Pdx1 to the basal transcriptional machinery), including CBP/ p300 [68,113,141,145,146], Set7/9 [147], Bridge-1 [148], PCIF1 [149], and histone deacetylases (HDACs) [150]. Once recruited, cofactors are believed to serve as a physical or functional "bridge" that allows Pdx1 to communicate with components of the basal transcriptional machinery.…”

Notch1 confers thymocytes a resistance to GC-induced apoptosis through Deltex1 by blocking the recruitment of p300 to the SRG3 promoter

“…21 In addition, the interaction mechanism between E2A and p300/ CBP is reported in insulin gene transcription and in pre-B cells, respectively. 26,27 E2A/HEB binding to the E-box element on SRG3 promoter regulates the SRG3 expression and, thereby, controls the sensitivity to GC-induced apoptosis in thymocytes. 18 Therefore, it is possible that Notch-induced Deltex1 may inhibit the transcriptional activity of E2A by blocking the recruitment of a coactivator(s), and this will result in reduced expression of SRG3 and resistance to the GC-induced apoptosis in developing thymocytes.…”