A novel cis-acting DNA element required for a high level of inducible expression of the rat P-450c gene

Characterization of the rabbit CYP1A1 and CYP1A2 genes: developmental and dioxin-inducible expression of rabbit liver P4501A1 and P4501A2

In adult rabbits, the CYP1A1 and CYP1A2 genes are expressed constitutively. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) leads to elevations in both CYP1A1 and CYP1A2 gene products (S. T. Okino et al., 1985, Proc. Natl. Acad. Sci. USA 82, 5310-5314). In this report, we have characterized the rabbit CYP1A1 and CYP1A2 genes, and analyzed the pattern of expression of these genes in neonatal animals following exposure to TCDD. Genomic clones encoding the entire rabbit CYP1A1 and CYP1A2 genes were characterized. Restriction enzyme analysis and partial DNA sequence analysis identified the seven exons for the CYP1A1 and CYP1A2 genes. Primer extension analysis using mRNA from TCDD-treated neonatal rabbits helped confirm the start of transcription for the CYP1A genes. The length of the noncoding first exon of the CYP1A1 gene was 74 bases, compared to 90 and 88 bases for the human and rodent CYP1A1 genes. The length of the noncoding CYP1A2 gene first exon was 53 bases, similar to its counterpart in human and rodents. DNA sequence analysis of the 5' regulatory regions and comparison to the rodent and human CYP1 genes demonstrated that the rabbit CYP1A1 and CYP1A2 genes were most similar to their human orthologs. The 5' region of the CYP1A1 gene contained several consensus dioxin (Ah)-receptor responsive elements (XREs), while no functional XRE sequences were identified in the CYP1A2 gene. When expression of the two genes were monitored, a small amount of constitutive P4501A1 mRNA was detected in neonatal rabbits from the ages of 1 to 17 days, while P4501A2 mRNA levels could not be observed until 8-12 days postpartum. In response to TCDD treatment, P4501A1 mRNA levels were inducible at all neonatal time points, while P4501A2 mRNA levels could not be induced until the animals were 3-5 days postpartum. While the dioxin Ah-receptor most likely plays a major role in the induction of these genes by TCDD, early expression of the CYP1A1 and CYP1A2 genes is differentially regulated in a developmental fashion.

Characterization of the aryl hydrocarbon receptor in the human C-4II cervical squamous carcinoma cell line

Treatment of C-4II human cervical squamous carcinoma cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) gave a concentration-dependent increase in ethoxyresorufin O-deethylase (EROD) activity. The EC50 for this response was approximately 1 nM and the maximum induced activity was 27 pmol/min/mg protein. The molecular properties of the cytosolic and nuclear aryl hydrocarbon (Ah) receptor complex were determined by velocity sedimentation analysis, photoaffinity labeling, gel retardation using a consensus dioxin responsive element (DRE), and DNA-Sepharose, DRE-Sepharose and Sephacryl S-300 gel permeation column chromatography. The apparent molecular masses of the cytosolic and nuclear photoaffinity-labeled Ah receptor complexes were 110 kDa and differed from the corresponding values obtained for the Ah receptor from other animal species. In contrast, most of the other molecular properties of the Ah receptor were not significantly different from those previously reported for other species. The relative Ah-responsiveness of the C-4II cells was assessed by determining the ratio of the induced EROD activity/nuclear Ah receptor levels for a submaximal inducing dose of [3H]TCDD. The induced activity/binding ratio for the human C-4II cells was 0.77 and was at least one order of magnitude lower than the corresponding value for the Ah-responsive rat hepatoma H-4-II E cells.

Genomic organization of human fetal specific P-450IIIA7 (cytochrome P-450HFLa)-related gene(s) and interaction of transcriptional regulatory factor with its DNA element in the 5' flanking region

P-450IIIA7 is a form of cytochrome P-450 which was isolated from human fetal livers and termed P-450HFLa. This form has been clarified to be expressed during fetal life specifically (Komori, M., Nishio, K., Kitada, M., Shiramatsu, K., Muroya, K., Soma, M., Nagashima, K. and Kamataki, T. (1990) Biochemistry 29, 4430-4433). In the present study, we isolated five independent clones which probably corresponded to the human P-450IIIA7 gene. These clones were completely sequenced, all exons, exon-intron junctions and the 5' flanking region from the cap site to-869. Although the sequences in the coding region were completely identical to P-450IIIA7, it is possible that genomic fragments sequenced in this study encode portions of other P-450IIIA7-related genes since we could not obtain a complete overlapping set of genomic clones. Within its 5' flanking sequence, the putative binding sites of several transcriptional regulatory factors existed. Among them, it was shown that a basic transcription element binding factor (BTEB) actually interacted with the 5' flanking region of this gene.

P450 and human cancer

Most of the chemical carcinogens in our environment are activated mainly by a restricted number of cytochrome P450 species, P450 1A1, 1A2, 2E1, and 3A. This metabolic activation of procarcinogens is a crucial part of the initial host response to the environmental exposure, since most chemical carcinogens do not show any carcinogenicity by themselves. Inter-individual variability in the metabolic activity may thus be a key host factor to explain the differences in susceptibility to chemical carcinogenesis among individuals. Recent studies on P450s in cancer etiology have provided some valuable insights into this problem.

Differences in deoxyribonuclease I hypersensitive sites in phenobarbital-inducible and constitutive rabbit P450IIC genes

DNase I hypersensitivity of nuclear chromatin near the rabbit cytochrome P450IIC genes was investigated by indirect end-labeling genomic Southern analysis. Major DNase I hypersensitive sites were observed in proximal (-200 base pairs) and distal (-2000 to -2200 base pairs) regions of the genes. The presence of the proximal site correlated well with the expression states of the individual genes. In contrast, the distal site was present in DNA from both liver and kidney nuclei and in untreated and phenobarbital-treated animals irrespective of the expression state of the genes. However, the distal site was present only in genes that respond to phenobarbital (cytochromes P450IIC1 and P450IIC2) and was not detected in the constitutive cytochrome P450IIC3 gene. The nucleotide sequences of 500 base pairs in the distal site regions of cytochromes P450IIC1 and P450IIC2 were 67% similar, and hepatocyte nuclear factor 1 like motifs were conserved in these two sequences. These results are consistent with the hypothesis that distal regions cooperate with the proximal promoter regions in the regulation of cytochrome P450IIC gene expression.

Structure and organization of the gene encoding rat S-adenosylmethionine decarboxylase

The gene for S-adenosylmethionine decarboxylase (AdoMetDC) was isolated from a rat genomic library using AdoMetDC cDNA as a probe. Nucleotide sequence analysis shows that the rat AdoMetDC gene consists of 8 exons which encode a protein identical to that inferred by a rat AdoMetDC cDNA sequence. The exons range in length from 43 to 1964 base pairs spanning 15672 bases of chromosomal DNA. All of the exon/intron junctions were found to conform to the consensus splice donor and acceptor sequences. Exon 8 corresponds to the 3' noncoding region of the 2 species of AdoMetDC mRNA which are formed by alternative utilization of 2 polyadenylation signals separated from each other by 1272 nucleotides. The transcription initiation site was located by S1 nuclease protection and by primer extension analysis, -325 nucleotides upstream of the translation initiation codon. The promoter region of the rat AdoMetDC gene contains a TATA box at -29 base pairs. No typical GC or CAAT boxes are located in the promoter, but six GC boxes and several putative binding sites for both tissue-specific and non-specific transcription factors are found in the proximal part of intron 1. Southern blot analyses reveal a complex hybridization pattern suggesting that there are multiple copies of the AdoMetDC gene in the rat genome.

Differences in 2,3,7,8-tetrachlorodibenzo-p-dioxin-inducible CYP1A1 expression in human breast carcinoma cell lines involve altered trans-acting factors

Differences in expression of the CYP1A1 gene have previously been observed in human breast carcinoma cell lines exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using an expression vector containing the functional 5'-regulatory region of human CYP1A1 (up to -1140) fused to the reporter gene CAT (for chloramphenicol acetyltransferase), the breast carcinoma cell lines, MCF-7, T47-D and ZR-75-1, classified as highly responsive to TCDD, were highly responsive to TCDD in the chloramphenicol acetyltransferase assay as well. Gel mobility shift assays have shown that these cell lines express a nuclear protein that binds the aryl hydrocarbon (Ah) receptor responsive element. The low or non-responsive cell lines, AL-1, BT-20 and CAMA-1, were low or non-responsive to TCDD in the chloramphenicol acetyltransferase assay, suggesting that the low-responsive phenotype is caused by altered trans-acting factors. However, the mechanism appears to differ among the cell lines. Whereas no induction was observed in AL-1, a fivefold induction in activity was observed in BT-20 and CAMA-1. The TCDD concentration giving half-maximum induction differed greatly between CAMA-1 and BT-20. The gel mobility shift assay showed the presence of a protein that bound specifically to the Ah responsive element in the non-responsive cell line AL-1, as well as the low-responsive cell lines, BT-20 and CAMA-1. The high basal activity but low induction observed in CAMA-1 may be due to an Ah receptor constitutively bound to the Ah responsive element.

The role of transcriptional regulation in steroid hormone biosynthesis

The regulated expression of the genes encoding the various steroidogenic enzymes is a crucial component in the control of steroid hormone biosynthesis. Tissue-specific transcription of each of the steroidogenic enzyme genes determines the array of enzymes present within a steroidogenic tissue, and therefore the types of steroid hormones the tissue produces. Transcriptional regulation also determines developmental changes in the steroid hormones synthesized by steroidogenic tissues and for the quantitative regulation of steroid hormones necessary for reproduction and for maintaining physiological homeostasis. The molecular mechanisms governing transcriptional regulation of steroidogenic enzyme genes is now being studied. The results so far indicate that, like most other genes, transcription of steroidogenic enzyme genes is regulated by cis-elements in the 5' flanking DNA of the genes that bind trans-acting proteins found in the nucleus. Several types of cis-elements have been identified: elements responsible for basal transcription, for induction by cAMP, and for both basal and cAMP induction. Some of the basal cis-elements identified may have a role in tissue-specific transcription of certain steroidogenic enzyme genes in steroidogenic tissues. We have also identified regions in both the human P450scc and human P450c17 promoters that repress transcription when activated by the Ca2+/protein kinase C intracellular second messenger system used by angiotensin II. This review summarizes our current understanding of transcriptional regulation of the steroidogenic enzyme genes.