A bipartite suppressor: conjunction of two distinct factor-binding sites is essential for down-regulation in rat epoxide hydrolase gene expression

Identification and characterization of a negative regulatory element within the epidermal growth factor receptor gene first intron in hormone-dependent breast cancer cells

The epidermal growth factor receptor (EGFR) exhibits an inverse correlation with estrogen receptor (ER) expression in the majority of breast cancers, predicting a poor response to endocrine therapy and poor survival rate. Inappropriate overexpression of EGFR in breast cancer is associated with a more aggressive phenotype. Transcriptional regulation is the major regulatory mechanism controlling EGFR overexpression in breast cancer cells. We have identified a region within the first intron of the EGFR gene that mediates transcriptional repression of EGFR gene expression in ER +/low EGFR expressing but not in ER-/high EGFR expressing breast cancer cells. Utilizing transient transfections of homologous and heterologous promoter-reporter constructs, we localized optimal repressive activity to a 96 bp intron domain. The 96 bp fragment displayed differential DNA-protein complex formation with nuclear extracts from ER + vs. ER- breast cancer cells. Moreover, factors interacting with this intron negative regulatory element appear to be estrogen-regulated. Consequently, our results suggest that we have identified a potential mechanism by which maintenance of low levels of EGFR expression and subsequent EGFR upregulation may be attributed to the loss of transcriptional repression of EGFR gene expression in hormone-dependent breast cancer cells.

Two distinct factor-binding DNA elements in cardiac myosin light chain 2 gene are essential for repression of its expression in skeletal muscle. Isolation of a cDNA clone for repressor protein Nished

The expression of the cardiac myosin light chain 2 (MLC2) gene is repressed in skeletal muscle as a result of the negative regulation of its transcription. Two regulatory elements, the cardiac specific sequence (CSS) located upstream (-360 base pairs) and a downstream negative modulatory sequence (NMS), which function in concert with each other, are required for repression of the MLC2 promoter activity in skeletal muscle. Individually, CSS and NMS have no effect. Transient transfection analysis with recombinant plasmids indicated that CSS- and NMS-mediated repression of transcription is position- and orientation-dependent and is transferable to heterologous promoters. A minimal conserved motif, GAAG/CTTC, present in both CSS and NMS, is responsible for repression as the mutation in the core CTTC sequence alone was sufficient to abrogate its repressor activity. The DNA binding assay by gel mobility shift analysis revealed that one of the two complexes, CSSBP2, is significantly enriched in embryonic skeletal muscle relative to cardiac muscle. In extracts from adult skeletal muscle, where the cardiac MLC2 expression is suppressed, both complexes, CSSBP1 and CSSBP2, were present, whereas the cardiac muscle extracts contained CSSBP1 alone, suggesting that the protein(s) in the CSSBP2 complex accounts for the negative regulation of cardiac MLC2 in skeletal muscle. A partial cDNA clone (Nished) specific for the candidate repressor factor was isolated by expression screening of the skeletal muscle cDNA library by multimerized CSS-DNA as probe. The recombinant Nished protein binds to the CSS-DNA, but not to DeltaCSS-DNA where the core CTTC sequence was mutated. The amino acid sequence of Nished showed a significant structural similarity to the sequence of transcription factor "runt," a known repressor of gap and pair-rule gene expression in Drosophila.

Hepatic transcriptional up-regulator of the rat microsomal epoxide hydrolase gene

Rat microsomal epoxide hydrolase (mEH) is one of the detoxification enzymes and selectively expressed in liver. A 350-bp DNA fragment of the proximal promoter was found to contain information sufficient to express the mEH gene in hepatoma cells, however not in nonhepatoma cells. We identified two cis-acting elements, epoxide hydrolase proximal element 1 (EHP1) and 2 (EHP2), in this promoter region by using transient transfection assays. Each element is a new cell-type-specific transcriptional up-regulator. The cell-type-specific activity of EHP1 correlates to the limited cell distribution of its cognate transacting factor(s). In the case of EHP2, a similar or possibly the same cognate factor(s) binding to EHP2 was detected by DNase I footprinting and gel retardation assays in both hepatoma and nonhepatoma cells. However, EHP2 functions as an up-regulator only in hepatoma cells. Our finding adds repertoire to a battery of cis-regulatory elements that are required for liver-specific transcription.

Position and orientation-selective silencer in protein-coding sequences of the rat osteocalcin gene

Osteocalcin (OC) is a bone-specific protein which is expressed postproliferatively by osteoblasts during late stages of differentiation. We have found that a silencer element is present within the rat OC gene (between nt +39 and +104), overlapping the OC signal prepropeptide-coding sequence. The presence of this sequence in OC promoter-CAT reporter constructs suppresses promoter activity in transiently transfected proliferating osteoblasts, which do not express OC, by up to 50-fold. This is the first demonstration of contribution from protein-coding sequences to silencing of animal genes. The element appears to be bipartite; silencer activity requires both the protein-coding sequence +39 to +63 and the +93 to +104 exon 1/intron 1 border region. Both of these domains contain sequences highly similar to silencer motifs in several other genes, including chicken lysozyme as well as rat collagen type II, insulin, and growth hormone. OC silencer activity is fully retained when the element is placed outside the RNA-coding region, 3' but not 5' of the OC-CAT fusion gene. Repression activity is orientation independent in the native position but requires the native orientation when located in 3' extragenic positions. The silencer does not inhibit the activity of the heterologous SV40 early promoter. These results suggest interaction between the transcribed silencer and specific OC promoter element(s) residing farther upstream. The OC transcribed silencer may contribute to developmental control of OC expression.