A putative AP-2 binding site in the 5' flanking region of the mouse POMC gene

Pituitary tumor AP-2alpha recognizes a cryptic promoter in intron 4 of fibroblast growth factor receptor 4

Fibroblast growth factor receptors (FGFRs) have been implicated in a multitude of proliferative functions, and FGFR4 is expressed differentially in normal and neoplastic pituitary. Human pituitary tumors express a truncated FGFR4 isoform (ptd-FGFR4) for which transcription is initiated from a downstream alternative site. Analysis of FGFR4 intronic sequences predicted a possible promoter within intron 4 (In4) including a classic TATA box with a possible transcriptional start site in intron 5. We show here that the human In4 sequence can direct luciferase reporter activity in transfected pituitary GH4 cells. Four overlapping fragments (A1, A2, B1, and B2) of this intron were examined by electromobility shift assay using nuclear extracts from rat pituitary tumors. Of these, fragment B2 formed complexes with nuclear rat pituitary GH4 extracts that were competed specifically by wild type but not mutant oligonucleotides for the neural crest cell lineage-derived activating transcription factor AP-2. Conversely, an AP-2 consensus sequence probe was competed by the In4 B2 oligonucleotide but not by other fragments of the same intron. The In4 B2 complex was competed partially by NFkappaB, supershifted by an AP-2alpha-specific antibody, and co-migrated with the same probe incubated with recombinant AP-2alpha protein. We also examined the ability of primary human pituitary tumor extracts to interact with the In4 B2 fragment. Pituitary tumor-In4 B2 complexes were competed specifically by wild type AP-2 but not mutant AP-2 oligonucleotides. Western blotting revealed higher levels of AP-2alpha expression in primary human pituitary tumors than in nontumorous tissue. Mutagenesis of the putative AP-2 binding site in In4 B2 resulted in a marked loss of promoter activity in a luciferase assay. AP-2alpha transfection in the presence of the histone deacetylase inhibitor trichostatin-A resulted in enhanced expression of endogenous ptd-FGFR4. These data indicate that a cryptic promoter within intron 4 binds AP-2alpha. AP-2alpha and chromatin changes may contribute to the utilization of an alternative transcription start site leading to the genesis of the tumorigenic ptd-FGFR4 isoform.

Specific activation of the Na+/H+ exchanger gene during neuronal differentiation of embryonal carcinoma cells

We examined the regulation of the Na+/H+ exchanger gene during differentiation of the P19 mouse embryonal carcinoma cells. Treatment of P19 cells with retinoic acid induces the development of neurons, astroglia, and microglia cells. Upon retinoic acid-induced differentiation of P19 cells, there was an early and rapid 10-fold increase in NHE1 transcription. A proximal cis-acting AP-2 site of the NHE1 promoter was sufficient for stimulation of transcription of the gene by differentiation. Bandshift experiments demonstrated that in retinoic acid-treated cells there was an elevated level of AP-2 transcription factor binding to the AP-2 consensus site of the Na+/H+ exchanger gene. In the differentiation defective mutant RAC65, the effect of differentiation on Na+/H+ exchanger gene expression was reduced by 60%. Examination of Na+/H+ exchanger activity showed that retinoic acid-treated P19 cells recovered from an acid load at a rate approximately three times greater than untreated cells. The increases in gene expression and protein activity preceded major changes in cell morphology, suggesting that the initiation of differentiation is linked to NHE1 gene expression. Our findings show for the first time that the NHE1 gene is activated early in cell differentiation and that this activation may play an important role in the process of neuronal cell differentiation.

Genomic organization and promoter sequence of a gene encoding a rat liver-specific type-I transport protein

We report the isolation and characterization of a rat gene (designated TI-LTP) encoding a liver-specific cell-surface glycoprotein that belongs to the type-I transporter family. This gene, including 5' and 3' flanking domains, was cloned from a rat lambda DASH genomic library and its nucleotide sequence was determined. TI-LTP is a single-copy gene which spans over 6 kb, and contains nine introns ranging in size from 90 bp to over 1.8 kb. Two transcription start points were located using a nuclease S-1 protection assay. The TI-LTP promoter (pTI-LTP) lacks a canonical TATA-box element, but does contain five TAGA elements. Several putative transcription-factor-binding sites were identified in the pTI-LTP, including activator protein 2 (AP-2) sites and a peroxisome proliferator response element (PPRE).

Characterization of a corticotropin releasing hormone responsive region in the murine proopiomelanocortin gene

The most potent, physiologic activator of proopiomelanocortin (POMC) gene transcription is corticotropin releasing hormone (CRH) and increased intracellular cAMP is critical for this effect. The 5'-flanking region of the murine POMC gene has several potential binding sites for regulatory proteins. To characterize the region between nucleotides -141 and -106, which includes a TRE-like site and an adjacent AP-2 consensus sequence, and to study its role in signal-transcription coupling, gel mobility shift assays and transient expression of CAT chimeras were performed. In transient transfections of AtT-20 cells with pCATp-141/-106, CRH treatment led to significant increases in CAT expression compared with CRH treatment of cells transfected with the enhancerless vector. However, no response to direct activation of cAMP dependent protein kinase or protein kinase C was detected. Despite the high homology of the sequence -137/-131 to the consensus AP-1 binding site (TRE), the nuclear factor(s) in AtT-20 cells binding to this region appears to be different than authentic AP-1 since neither a competitor oligonucleotide having the authentic TRE sequence nor antibodies against Jun or Fos affected the gel shift pattern of a probe having the -137/-131 sequence. We conclude that the -141 to -106 region of the murine POMC gene contains a functional CRH responsive element and that second messenger systems that transduce the CRH signal to this element do not exert their actions solely through activation of PKA or PKC.

Regulatory elements of the human proopiomelanocortin gene promoter

Proopiomelanocortin (POMC) is expressed predominantly in the corticotrophic cells of the pituitary. Regulatory sequences required for the expression of the human (h) POMC gene were investigated using transient expression of hPOMC-CAT fusion genes in pituitary and nonpituitary cells in combination with DNase I footprint and gel retardation assays. Gene transfer experiments revealed that the hPOMC promoter is more efficiently transcribed in AtT-20 pituitary cells than in HeLa cells. Using deletion analysis, negative regulatory elements between nucleotides -676 and -414 and positive regulatory elements between nucleotides -414 and -93 could be identified. When placed in front of the heterologous thymidine kinase (tk) promoter, nucleotides -414/-223 enhance transcription in AtT-20 cells and in primary cultures of human pituitary tumor cells, but not in various nonpituitary cell lines. In contrast, a -112/-93 element enhances transcription of the tk promoter in all cells tested. DNase I footprint analysis revealed five sites protected by nuclear extracts obtained from AtT-20 cells within nucleotides -414 and -83 of the hPOMC promoter region. In contrast, only one of these sites (between nucleotides -115 and -83) was protected by nuclear extracts from HeLa cells. Gel mobility-shift experiments revealed that an oligonucleotide comprising nucleotides -112/-93 binds a novel nuclear protein. This protein may contribute to the non-cell type-specific expression of the hPOMC gene outside the pituitary, whereas at least five transcription factors seem to be required for high basal transcription of the gene in corticotrophic cells.

Molecular cloning and primary structure of the chicken transforming growth factor-beta 2 gene

The chicken transforming growth factor-beta 2 (TGF-beta 2) gene and its flanking regions were cloned and characterized. The gene contains 7 exons and 6 introns spanning about 70 kb. Primer extension analysis identified one major and two minor starts of transcription. A comparison of the 5'-flanking regions for human and chicken TGF-beta 2 genes revealed limited sequence homology around the start of transcription, including conserved TATA-box, CRE, and AP-2 sequence motifs. A species comparison of the 5' untranslated region did not reveal any sequence homology beyond the coding region. In contrast, the 3' untranslated region was highly conserved, suggesting that this region may play an important role in the expression of the TGF-beta 2 gene.

Structural analysis of the entire proopiomelanocortin gene of Xenopus laevis

In the pars intermedia of the pituitary the prohormone proopiomelanocortin (POMC) is tissue-specifically processed to, among other peptides, alpha-melanotropin (alpha MSH). In the South African clawed toad Xenopus laevis this hormone mediates the process of background adaptation: release of alpha-MSH causes darkening of the animal, while inhibition of alpha-MSH release results in a pale toad. Elevated release of alpha-MSH coincides with a higher rate of POMC gene transcription. The present study aims to find possible transcriptional regulatory elements in the Xenopus POMC gene. For that purpose the complete nucleotide sequence of the POMC gene and its 5'- and 3'- flanking regions were determined and analyzed. The Xenopus POMC gene promoter contains several regions which may be regulatory DNA elements in view of their similarity with corresponding regions of mammalian POMC gene promoters. In the rat POMC gene promoter, many of these regions represent protein-binding sequences. Besides the promoter sequence and the protein-coding sequences, no other segments with significant identity between the Xenopus and human POMC genes were found. Intron A of the Xenopus POMC gene contains a simple sequence, (TATC)76, and a JH12 repetitive element, while the 3'-flanking region contains a repetitive-EcoRI-monomer-2 element. Comparison of the JH12 sequence of the POMC gene with JH12 sequences from other Xenopus genes revealed a 335-bp consensus sequence which is flanked by a 30-bp inverted repeat. This JH12 consensus sequence is significantly larger than the previously reported JH12 core region. Alignment of intron B of the Xenopus POMC gene with database sequences revealed a consensus sequence of a novel Xenopus repetitive element of 330 bp flanked by a nearly perfect inverted repeat, indicating that this element may be a transposon-like element.