Identification of repressor element 1 in cytochrome P450 genes and their negative regulation by RE1 silencing transcription factor/neuron-restrictive silencer factor

Internal genomic sequence of human CYP1A1 gene is involved in superinduction of dioxin-induced CYP1A1 transcription by cycloheximide

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 mRNA expression, and co-treatment with the protein synthesis inhibitor, cycloheximide (CHX) magnifies its expression severalfold further. Although this phenomenon has been reported as superinduction, its molecular mechanism is still obscure. In the present study, we analyzed the influence of the CYP1A1 internal genomic sequence on CHX-mediated superinduction. Partial sequences of the human CYP1A1 were inserted at the 5' end of the internal ribosomal entry site (IRES) connected to luciferase cDNA, and generated constructs were transiently transfected into CHO or HepG2 cells. Intron-1 deletion constructs showed higher inductivity than intron-1 intact constructs by TCDD. Quantitative RT-PCR analyses revealed that the superinduced levels by CHX of the intron-1 intact constructs were greater than those of the intron-1 deletion constructs. The present results indicate that internal genomic sequences of the human CYP1A1 gene, especially the internal sequence of intron-1, are involved in superinduction of the CYP1A1 gene by CHX.

A novel adenoviral vector which mediates hypoxia-inducible gene expression selectively in neurons

Selective gene expression in neurons is still a challenge. We have developed several expression vectors using a combination of neuron restrictive silencer elements (NRSEs), hypoxia responsive elements (HREs) and CMV minimal promoter (CMVmp). These elements were packaged into replication defective adenovirus to target gene expression selectively in neurons in a hypoxia-regulated manner. Neuronal selectivity and responsiveness to hypoxia of these novel constructs were determined empirically in both neural cell lines and primary cerebellar granule neurons (CGNs). The construct p5HRE-3NRSE exhibited not only the highest level of reporter gene expression in neuronal cells but also in an oxygen concentration-dependent manner when compared with all other constructs. As expected, this construct did not elicit reporter gene expression in non-neuronal cells including human HEK293A and HT29 cells, rat NRK cells, mouse 3T6 cells and 3T3 L1 cells. This construct was packaged into a replication defective adenoviral vector (Ad/5HRE-3NRSE) to determine neuron-selective and hypoxia-inducible gene expression in cultured mouse postmitotic primary CGNs and differentiated human NT2 neurons (NT2/Ns). Remarkably, in response to hypoxia, Ad/5HRE-3NRSE showed strong hypoxia-inducible gene expression selectively in neurons (12-fold induction in CGNs and 22-fold in NT2/Ns), but not in glial cells. Taken together, this vector with restricted gene expression to neurons under the regulation of hypoxia will be a useful tool for investigations of mechanisms of neuronal damage caused by ischemic insult.

Neuron-restrictive silencer factor (NRSF) functions as a repressor in neuronal cells to regulate the mu opioid receptor gene

The mu opioid receptor (MOR) is expressed in the central nervous system and specific cell lines with varying expression levels perhaps playing important roles. One of the neuronal-specific transcription regulators, neuron-restrictive silencer factor (NRSF), has been shown to repress the expression of neuron-specific genes in non-neuronal cells. However, we showed here that the neuron-restrictive silencer element (NRSE) of MOR functions as a critical regulator to repress the MOR gene expression in specific neuronal cells depending on NRSF expression level. Using co-transfection studies, we showed that the NRSE of the MOR promoter is functional in NRSF-positive cells (NS20Y and HeLa) but not in NRSF-negative cells (PC12). NRSF binds to the NRSE of the MOR gene in a sequence-specific manner confirmed by supershift and chromatin immunoprecipitation assays, respectively. The suppression of NRSF activity with either trichostatin A or a dominant-negative NRSF induced MOR promoter activity and transcription of the MOR gene. When the NRSF was disrupted in NS20Y and HeLa cells using small interfering RNA, the transcription of the endogenous target MOR gene increased significantly. This provides direct evidence the role of NRSF in the cells and also indicates that NRSF expression is regulated by post-translational modification in neuronal NMB cells. Our data suggested that NRSF can function as a repressor of MOR transcription in specific cells, via a mechanism dependent on the MOR NRSE.