Tissue-specific regulation of the insulin gene by a novel basic helix-loop-helix transcription factor.

Abstract

The insulin gene is one of the best paradigms of tissue-specific gene expression. It is developmentally regulated and is expressed exclusively in the pancreatic 13-cell. This restricted expression is directed by a tissue-specific enhancer, within the promoter, which contains an E-box sequence. The insulin E-box binds an islet-specific protein complex, termed 3al. E-boxes bind proteins belonging to the basic helix-loop-helix (bHLH) family of transcription factors. The bHLH proteins function as potent transcriptional activators of tissue-specific genes by forming heterodimers between ubiquitous and cell-restricted family members. In addition, the cell-restricted bHLH members play an important role in specifying cell fate. To isolate the tissue-specific bHLH factor controlling insulin gene expression and study its role in islet cell differentiation, a modified yeast two-hybrid system was utilized to clone a novel bHLH factor, BETA2 ([f-cell _E-box trans-activator 2_), from a hamster insulin tumor (HIT) cell cDNA library. Northern analysis demonstrates that high-level expression of the BETA2 gene is restricted to pancreatic o~-and [~-cell lines. As expected of tissue-specific bHLH members, BETA2 binds to the insulin E-box sequence with high affinity as a heterodimer with the ubiquitous bHLH factor E47. More importantly, antibody supershift experiments clearly show that BETA2 is a component of the native insulin E-box-binding complex. Transient transfection assays demonstrate that the BETA2/E47 heterodimer synergistically interacts with a neighboring 13-cell-specific complex to activate an insulin enhancer. In contrast, other bHLH factors such as MyoD and E47, which can bind to the insulin E-box with high affinity, fail to do so. Thus, a unique, cooperative interaction is the basis by which the insulin E-box enhancer discriminates between various bHLH factors to achieve tissue-specific activation of the insulin gene.

NeuroD Regulates Neuronal Migration

“…Similar to the results of previous reports in Bax single null mice (Knudson et al, 1995), ND +/-Bax +/-and ND +/-Bax -/-did not generate any ectopic phenotype except male infertility. Most of the ND -/-Bax +/+ , ND -/-Bax +/-and ND -/-Bax -/-mice died within 3 days postpartum, suggesting that the disruption of Bax did not rescue the pancreatic cell death and resulting severe diabetes caused by the mutation of NeuroD (Miyata et al, 1999;Naya et al, 1995).…”

Mouse MafA, homologue of zebrafish somite Maf 1, contributes to the specific transcriptional activity through the insulin promoter

“…The pancreatic islet restricted transcription factors BETA2/NeuroD and PDX1, which bind to the E1 and A3 elements, have been isolated. Mice deficient in these transcription factors revealed these factors to be indispensable for islet cell development and insulin gene expression [46][47][48][49]. In addition, mutations in the BETA2 and PDX1 genes were found in some patients with maturity-onset diabetes of the young (MODY) [50,51].…”

Transcriptional repression of the human p53 gene by hepatitis B viral X protein

“…The p53mCPE, p53mkB and p53mEbox plasmids carry the 85 bp promoter fragment from the p53 gene, with each mutation: the CPEp53 (767 -GCAGGTATTGATGCGCTTAGGGTTTT-736), NF-kB binding site (749 -ATCGTTTTAGG-739) and E-box element (734 -TCTGTG-729). The mutated sequences are underlined genes (Reisman and Rotter, 1993;Naya et al, 1995;Mutoh et al, 1997). The promoter of the human p53 tumor suppressor gene is also transactivated by the cMyc/Max heterodimer, which binds to the E-box element (Roy et al, 1994).…”