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

KLF-1 orchestrates a xenobiotic detoxification program essential for longevity of mitochondrial mutants

Most manipulations that extend lifespan also increase resistance to various stress factors and environmental cues in a range of animals from yeast to mammals. However, the underlying molecular mechanisms regulating stress resistance during aging are still largely unknown. Here we identify Krüppel-like factor 1 (KLF-1) as a mediator of a cytoprotective response that dictates longevity induced by reduced mitochondrial function. A redox-regulated KLF-1 activation and transfer to the nucleus coincides with the peak of somatic mitochondrial biogenesis that occurs around a transition from larval stage L3 to D1. We further show that KLF-1 activates genes involved in the xenobiotic detoxification programme and identified cytochrome P450 oxidases, the KLF-1 main effectors, as longevity-assurance factors of mitochondrial mutants. Collectively, these findings underline the importance of the xenobiotic detoxification in the mitohormetic, longevity assurance pathway and identify KLF-1 as a central factor in orchestrating this response.

Pluripotency transcription factors in lung cancer-a review

Lung cancer is the leading cause of cancer-related mortality worldwide. Diagnosis of lung cancer in an early stage is still a challenge due to the asymptomatic course of early stages of the disease and the lack of a standard screening program for the population. Nowadays, learning about the mechanisms that lead to cancerogenesis in the lung is crucial for the development of new diagnostic and therapeutic strategies. Recently, many studies have proved that cancer stem cells (CSCs) are responsible for the initiation, progression, metastasis, recurrence, and even resistance of chemo- and radiotherapeutic treatment in patients with lung cancer. The expression of pluripotency transcription factors is responsible for stemness properties. In this review, we summarize the current knowledge on the role of CSCs and pluripotency transcription factors in lung carcinogenesis.

Regulation of EMT by KLF4 in gastrointestinal cancer

Gastrointestinal (GI) cancer is characterized by its aggressiveness, but the underlying mechanism is not fully understood. Studies reveal that epithelial to mesenchymal transition (EMT), which is regulated by a series of transcription factors and signaling pathways, is strongly associated with GI cancer cell proliferation, invasion and metastasis. Importantly, EMT is a product of crosstalk between signaling pathways. Krüppel-like factor 4 (KLF4), a zinc finger-type transcription factor, is decreased or lost in most GI cancers. By transcriptionally regulating its downstream target genes, KLF4 plays important roles of GI cancer tumorigenesis, proliferation and differentiation. In this review, we focus on the mechanism of KLF4 in GI cancer EMT, and demonstrate that through crosstalk with TGF-β, Notch, and Wnt signaling pathways, KLF4 negatively regulates EMT of GI cancers. Finally, we indicate the challenging new frontiers for KLF4 which contributes to better understanding of the mechanism of GI cancer aggressiveness.

Aryl hydrocarbon receptor ligands of widely different toxic equivalency factors induce similar histone marks in target gene chromatin

Posttranslational histone modifications are a critical regulatory mechanism of gene transcription. Previous studies from our laboratory have shown that contingent on binding to its cognate promoter motifs in the Cyp1a1 gene, activation of the aryl hydrocarbon receptor (AHR) by benzo[a]pyrene (BaP) treatment induces histone modifications in the Cyp1a1 promoter that are required for activation of gene transcription. Here, we have studied different AHR ligands, including polychlorinated biphenyls (PCBs) of different toxic equivalency factors (TEF), to determine whether changes in histone modifications are linked to different levels of Cyp1a1 expression or dependent on AHR-ligand affinity. We find that all ligands lead to the same pattern of histone modifications in a relationship that parallels the strength of their AHR-ligand affinity. Thus, whereas PCB126 (TEF 0.1), 3-methylcholanthrene, β-naphthoflavone, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) initiate a pattern of histone marks similar to those induced by BaP, PCB77 (TEF 0.0001) causes a lower level of change in the same marks and requires a longer activation time than PCB126, BaP, or TCDD. In contrast, the non-dioxin-like PCB153 recruits AHR to the Cyp1a1 enhancer causing a displacement of enhancer-associated histone H3 but does not cause the other observed histone mark changes nor does it induce transcription. These results indicate that AHR recruitment to the promoter is not sufficient to induce the histone modifications needed to activate gene expression and show that there is a good correlation between the regulatory chromatin changes associated with ligand-induced AHR target gene transcription and the resultant toxicity of the ligand.

HDAC1 bound to the Cyp1a1 promoter blocks histone acetylation associated with Ah receptor-mediated trans-activation

Metabolic bioactivation of polycyclic aromatic hydrocarbons, such as the environmental procarcinogen benzo[a]pyrene, is catalyzed by a cytochrome P450 monooxygenase encoded by the substrate-inducible Cyp1a1 gene. Cyp1a1 induction requires trans-activation by the heterodimeric transcriptional complex formed by the liganded Ah receptor (AHR) and its partner, ARNT. Previously, we showed that constitutively bound HDAC1 dissociates from Cyp1a1 promoter chromatin after ligand-mediated induction, concomitantly with the recruitment of AHR/ARNT complexes and p300. Here, we investigated the hypothesis that HDAC1 binding maintains the Cyp1a1 gene in a silenced state in uninduced cells. We find that Cyp1a1 induction by the AHR/ARNT is associated with modification of specific chromatin marks, including hyperacetylation of histone H3K14 and H4K16, trimethylation of histone H3K4, and phosphorylation of H3S10. HDAC1 and DNMT1 form complexes on the Cyp1a1 promoter of uninduced cells but HDAC1 inhibition alone is not sufficient to induce Cyp1a1 expression, although it allows for the hyperacetylation of H3K14 and H4K16 to levels similar to those found in B[a]P-induced cells. These results show that by blocking the modification of histone marks, HDAC1 plays a central role in Cyp1a1 expression and that its removal is a necessary but not sufficient condition for Cyp1a1 induction, underscoring the requirement for a concerted series of chromatin-remodeling events to complete the initial steps of gene trans-activation by the Ah receptor.

The family feud: turning off Sp1 by Sp1-like KLF proteins

Sp1 is one of the best characterized transcriptional activators. The biological importance of Sp1 is underscored by the fact that several hundreds of genes are thought to be regulated by this protein. However, during the last 5 years, a more extended family of Sp1-like transcription factors has been identified and characterized by the presence of a conserved DNA-binding domain comprising three Krüppel-like zinc fingers. Each distinct family member differs in its ability to regulate transcription, and, as a consequence, to influence cellular processes. Specific activation and repression domains located within the N-terminal regions of these proteins are responsible for these differences by facilitating interactions with various co-activators and co-repressors. The present review primarily focuses on discussing the structural, biochemical and biological functions of the repressor members of this family of transcription factors. The existence of these transcriptional repressors provides a tightly regulated mechanism for silencing a large number of genes that are already known to be activated by Sp1.

Functional analysis of basic transcription element (BTE)-binding protein (BTEB) 3 and BTEB4, a novel Sp1-like protein, reveals a subfamily of transcriptional repressors for the BTE site of the cytochrome P4501A1 gene promoter

The Sp1-like family of transcription factors is emerging as an integral part of the cellular machinery involved in the control of gene expression. Members of this family of proteins contain three highly homologous C-terminal zinc-finger motifs that bind GC-rich sequences found in the promoters of a diverse number of genes, such as the basic transcription element (BTE) in the promoter of the carcinogen-metabolizing cytochrome P4501A1 (CYP1A1) gene. In the present study, we report the molecular and functional characterization of BTE-binding protein (BTEB) 4, a novel ubiquitously expressed member of the Sp1-like proteins family. This protein represents a new homologue of BTEB1, originally described as a regulator of the BTE site in the CYP1A1 gene promoter. Similarly to the recently described BTEB3, we demonstrate that the N-terminal region of BTEB4 directly represses transcription and binds the co-repressor mSin3A. In addition, we show that the C-terminal zinc-finger domain of BTEB4 binds specifically the BTE site of the CYP1A1 promoter, similar to BTEB1 and BTEB3. Also, we show that both BTEB3 and BTEB4 repress the CYP1A1 gene promoter via the BTE site in HepG2 and BxPC3 cells. Thus the identification of this protein expands the repertoire of BTEB-like members of the Sp1-like protein family involved in transcriptional repression. Furthermore, our results demonstrate that the BTEB subfamily can repress the CYP1A1 gene promoter via the BTE site.

Transactivation and growth suppression by the gut-enriched Krüppel-like factor (Krüppel-like factor 4) are dependent on acidic amino acid residues and protein-protein interaction

Gut-enriched Krüppel-like factor (GKLF or KLF4) is a pleiotropic (activating and repressive) transcription factor. This study characterizes the mechanisms of transactivation by GKLF. Using a GAL4 fusion assay, the activating domain of murine GKLF was localized to the 109 amino acid residues in the N-terminus. Site-directed mutagenesis showed that two adjacent clusters of acidic residues within this region are responsible for the activating effect. Transactivation by GKLF involves intermolecular interactions as demonstrated by the ability of wild-type, but not mutated, GKLF to compete with the N-terminal activation domain. In addition, wild-type adenovirus E1A, but not a mutated E1A that failed to bind p300/CBP, inhibited transactivation by the N-terminal 109 amino acids of GKLF, suggesting that p300/CBP are GKLF's interacting partners. A physical interaction between GKLF and CBP was demonstrated by glutathione- S -transferase pull-down and by in vivo co-immuno-precipitation experiments. We also showed that the two acidic amino acid clusters are essential for this interaction, since GKLF with mutations in these residues failed to co-immunoprecipitate with CBP. Importantly, the same mutations abrogated the ability of GKLF to suppress cell growth as determined by a colony suppression assay. These studies therefore provide plausible evidence for a structural and functional correlation between the transactivating and growth-suppressing effects of GKLF.

The biology of the mammalian Krüppel-like family of transcription factors

Recent advances in molecular cloning have led to the identification of a large number of mammalian zinc finger-containing transcription factors that exhibit homology to the Drosophila melanogaster protein, Krüppel. Although the amino acid sequences in the zinc finger domains of these Krüppel-like factors (KLFs) are closely related to one another, the regions outside the zinc fingers of the proteins are usually unique. KLFs display seemingly different and broad biological properties with each functioning as an activator of transcription, a repressor or both. This review article provides a current phylogenetic classification of the identified KLFs to date. More importantly, the currently known biological activities of the KLFs in regulating transcription, cell proliferation, differentiation and development are summarized and compared. Further characterization of this interesting protein family should provide additional insights into the their respective regulatory role in various important biological processes.

An autoregulatory loop controlling CYP1A1 gene expression: role of H(2)O(2) and NFI

Cytochrome P450 1A1 (CYP1A1), like many monooxygenases, can produce reactive oxygen species during its catalytic cycle. Apart from the well-characterized xenobiotic-elicited induction, the regulatory mechanisms involved in the control of the steady-state activity of CYP1A1 have not been elucidated. We show here that reactive oxygen species generated from the activity of CYP1A1 limit the levels of induced CYP1A1 mRNAs. The mechanism involves the repression of the CYP1A1 gene promoter activity in a negative-feedback autoregulatory loop. Indeed, increasing the CYP1A1 activity by transfecting CYP1A1 expression vectors into hepatoma cells elicited an oxidative stress and led to the repression of a reporter gene driven by the CYP1A1 gene promoter. This negative autoregulation is abolished by ellipticine (an inhibitor of CYP1A1) and by catalase (which catalyzes H(2)O(2) catabolism), thus implying that H(2)O(2) is an intermediate. Down-regulation is also abolished by the mutation of the proximal nuclear factor I (NFI) site in the promoter. The transactivating domain of NFI/CTF was found to act in synergy with the arylhydrocarbon receptor pathway during the induction of CYP1A1 by 2,3,7,8-tetrachloro-p-dibenzodioxin. Using an NFI/CTF-Gal4 fusion, we show that NFI/CTF transactivating function is decreased by a high activity of CYP1A1. This regulation is also abolished by catalase or ellipticine. Consistently, the transactivating function of NFI/CTF is repressed in cells treated with H(2)O(2), a novel finding indicating that the transactivating domain of a transcription factor can be targeted by oxidative stress. In conclusion, an autoregulatory loop leads to the fine tuning of the CYP1A1 gene expression through the down-regulation of NFI activity by CYP1A1-based H(2)O(2) production. This mechanism allows a limitation of the potentially toxic CYP1A1 activity within the cell.

The gut-enriched Krüppel-like factor suppresses the activity of the CYP1A1 promoter in an Sp1-dependent fashion

The gut-enriched Krüppel-like factor (GKLF) is a newly identified zinc finger-containing transcription factor. Recent studies indicate that GKLF binds to a core DNA sequence of 5'-(G/A)(G/A)GG(C/T)G(C/T)-3', which is found in an endogenous cis element, the basic transcription element (BTE) of the cytochrome P-450IA1 (CYP1A1) promoter. The present study characterizes the ability of GKLF to regulate CYP1A1 expression. By electrophoretic mobility gel shift assay (EMSA) and methylation interference assay, GKLF was found to bind BTE in a manner similar to several other transcription factors known to interact with BTE including Sp1 and BTEB. Cotransfection studies in Chinese hamster ovary cells showed that GKLF inhibited the CYP1A1 promoter in a dose- and BTE-dependent manner. The same experiments also revealed that BTE was responsible for a significant portion of the CYP1A1 promoter activity. EMSA of nuclear extracts from Chinese hamster ovary cells showed that Sp1 and Sp3 were two major proteins that interacted with BTE. Additional cotransfection studies showed that GKLF inhibited Sp1-mediated activation of the CYP1A1 promoter. In contrast, GKLF enhanced Sp3-dependent suppression of the same promoter. Moreover, the ability of GKLF to inhibit Sp1-dependent transactivation was in part due to physical interaction of the two proteins. These findings indicate that GKLF is a negative regulator of the CYP1A1 promoter in a BTE-dependent fashion and that this inhibitory effect is in part mediated by physical interaction with Sp1.

Identification of the DNA sequence that interacts with the gut-enriched Krüppel-like factor

The gut-enriched Krüppel-like factor (GKLF) is a recently identified eukaryotic transcription factor that contains three C2H2zinc fingers. The amino acid sequence of the zinc finger portion of GKLF is closely related to several Krüppel proteins, including the lung Krüppel-like factor (LKLF), the erythroid Krüppel-like factor (EKLF) and the basic transcription element binding protein 2 (BTEB2). The DNA sequence to which GKLF binds has not been definitively established. In the present study we determined the DNA binding sequence of GKLF using highly purified recombinant GKLF in a target detection assay of an oligonucleotide library consisting of random sequences. Upon repeated rounds of selection and subsequent characterization of the selected sequences by base-specific mutagenesis a DNA with the sequence 5'-G/AG/AGGC/TGC/T-3' was found to contain the minimal essential binding site for GKLF. This sequence is present in the promoters of two previously characterized genes: the CACCC element of the beta-globin gene, which interacts with EKLF, and the basic transcription element (BTE) of the CYP1A1 gene, which interacts with Sp1 and several Sp1-like transcription factors. Moreover, the selected GKLF binding sequence was capable of mediating transactivation of a linked reporter gene by GKLF in co-transfection experiments. Our results establish GKLF as a sequence-specific transcription factor likely involved in regulation of expression of endogenous genes.

Isolation of a promoter region in mouse cytochrome P450 3A (Cyp3A16) gene and its transcriptional control

An 11.5 kb fragment of the mouse Cyp3a16 gene containing the 5' flanking region was isolated from the lambda DASHII mouse genomic library. A part of the 5' flanking region and the first exon of Cyp3a16 gene were sequenced. S1 mapping analysis showed the presence of two transcriptional initiation sites. The first exon was completely identical to Cyp3a16 cDNA. The identity of 5' flanking sequences between Cyp3a16 and Cyp3a11 genes was about 69%. A typical TATA box and a basic transcription element (BTE) were found as seen with other CYP3A genes from various animal species Moreover, some putative transcriptional regulatory elements were also found in addition to the sequence motif seen for the formation of Z-type DNA. To examine the transcriptional activity of Cyp3a11 gene, DNA fragments in the 5'-flanking region of the gene were inserted front of the luciferase structural gene, and the constructs were transfected in primary hepatocytes. The analysis of the luciferase activity indicated that the region between -146 and -56 was necessary for the transcription of CYP3a16 gene.

Identification and characterization of a 44 kDa protein that binds specifically to the 3'-untranslated region of CYP2a5 mRNA: inducibility, subcellular distribution and possible role in mRNA stabilization

Stabilization of mRNA is important in the regulation of CYP2a5 expression but the factors involved in the process are not known [Aida and Negishi (1991) Biochemistry 30, 8041-8045]. In this paper, we describe, for the first time, a protein that binds specifically to the 3'-untranslated region of CYP2a5 mRNA and which is inducible by pyrazole, a compound known to increase the half-life of CYP2a5 mRNA. We also demonstrate that pyrazole treatment causes an elongation of the CYP2a5 mRNA poly(A) tail, and that phenobarbital, which is transcriptional activator of the CYP2a5 gene that does not affect the mRNA half-life, neither induces the RNA-binding protein nor affects the poly(A) tail size. SDS/PAGE of the UV-cross-linked RNA-protein complex demonstrated that the RNA-binding protein has an apparent molecular mass of 44 kDa. The protein-binding site was localized to a 70-nucleotide region between bases 1585 and 1655. Treatment of cytoplasmic extracts with an SH-oxidizing agent, diamide, an SH-blocking agent, N-ethylmaleimide or potato acid phosphatase abolished complex-formation, suggesting that the CYP2a5 mRNA-binding protein is subject to post-translational regulation. Subcellular fractionation showed that the 44 kDa protein is present in polyribosomes and nuclei, and that its apparent induction is much stronger in polyribosomes than in nuclear extracts. We propose that this 44 kDa RNA-binding protein is involved in the stabilization of CYP2a5 mRNA by controlling the length of the poly(A) tail.

cDNA cloning and transcriptional properties of a novel GC box-binding protein, BTEB2

We have cloned a cDNA for a novel GC box-binding protein designated BTEB2 from a human placenta cDNA library using rat BTEB cDNA (Imataka et al. (1992). EMBO J. 11,3663-3671. as a hybridization probe. BTEB2 consists of 219 amino acids and contains three contiguous zinc finger motifs at its C-terminus. The zinc finger domains showed 59% and 64% sequence similarity to those of Sp1 and BTEB, respectively. Adjacent to the N-terminal of the zinc finger motifs, a short sequence rich in basic amino acids is conserved between BTEB2 and Sp1. Furthermore, This basic sequence concurs with the N-terminal half of the consensus sequence for basic domains of the proteins containing both helix-loop-helix and leucine zipper motifs. The other region of BTEB2 is notably rich in proline, serine, threonine, and alanine residues. BTEB2 expressed in Escherichia coli showed DNA-binding activity whose specificity was closely similar to that of Sp1. Cotransfection experiments using Hepa-1 cells (a mouse hepatoma cell line) with a BTEB2 expression plasmid and GC box-containing reporter plasmids revealed that BTEB2 apparently activated the expression of the CAT activity. Moreover, when BTEB2 was fused to GAL4 DNA-binding domain, the chimeric protein could enhance the transcription through promoters containing GAL4-binding sites. Analysis of the BTEB2 mRNA by RNA blot analysis demonstrated that the mRNA was expressed specifically in testis and placenta with different sizes, 20S and 28S, respectively, among various organs examined.

Identification and functional characterization of a cis-acting positive DNA element regulating CYP 2B1/B2 gene transcription in rat liver

A positive cis-acting DNA element in the near 5'-upstream region of the CYP2B1/B2 genes in rat liver was found to play an important role in the transcription of these genes. An oligonucleotide covering -69 to -98 nt mimicked the gel mobility shift pattern given by the fragment -179 to +29 nt, which was earlier found adequate to confer the regulatory features of this gene. Two major complexes were seen, of which the slower and faster moving complexes became intense under uninduced and Phenobarbitone-induced conditions respectively. Minigene cloned DNA plasmid covering -179 to +181 nt in pUC 19 and Bal 31 mutants derived from this parent were transcribed in whole nuclei and cell free transcription extracts and mutants containing only upto -75 nt of the upstream were poorly transcribed. Transcription extracts from phenobarbitone-injected rat liver nuclei were significantly more active than extracts from uninduced rats in transcribing the minigene constructs. Addition of the oligonucleotide (-69 to -98nt) specifically inhibited the transcription of the minigene construct (-179 to +181 nt) in the cell free transcription system. It is therefore, concluded that the region -69 to -98 nt acts as a positive cis-acting element in the transcription of the CYP2B1/B2 genes and in mediating the inductive effects of phenobarbitone.

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.

Liver cells contain constitutive DNase I-hypersensitive sites at the xenobiotic response elements 1 and 2 (XRE1 and -2) of the rat cytochrome P-450IA1 gene and a constitutive, nuclear XRE-binding factor that is distinct from the dioxin receptor

Dioxin stimulates transcription from the cytochrome P-450IA1 promoter by interaction with the intracellular dioxin receptor. Upon binding of ligand, the receptor is converted to a form which specifically interacts in vitro with two dioxin-responsive positive control elements located in close proximity to each other about 1 kb upstream of the rat cytochrome P-450IA1 gene transcription start point. In rat liver, the cytochrome P-450IA1 gene is marked at the chromatin level by two DNase I-hypersensitive sites that map to the location of the response elements and exist prior to induction of transcription by the dioxin receptor ligand beta-naphthoflavone. In addition, a DNase I-hypersensitive site is detected near the transcription initiation site and is altered in nuclease sensitivity by induction. The presence of the constitutive DNase I-hypersensitive sites at the dioxin response elements correlates with the presence of a constitutive, labile factor which specifically recognizes these elements in vitro. This factor appears to be distinct from the dioxin receptor, which is observed only in nuclear extract from treated cells. In conclusion, these data suggest that a certain protein-DNA architecture may be maintained at the response elements at different stages of gene expression.

Liver cells contain constitutive DNase I-hypersensitive sites at the xenobiotic response elements 1 and 2 (XRE1 and -2) of the rat cytochrome P-450IA1 gene and a constitutive, nuclear XRE-binding factor that is distinct from the dioxin receptor

Dioxin stimulates transcription from the cytochrome P-450IA1 promoter by interaction with the intracellular dioxin receptor. Upon binding of ligand, the receptor is converted to a form which specifically interacts in vitro with two dioxin-responsive positive control elements located in close proximity to each other about 1 kb upstream of the rat cytochrome P-450IA1 gene transcription start point. In rat liver, the cytochrome P-450IA1 gene is marked at the chromatin level by two DNase I-hypersensitive sites that map to the location of the response elements and exist prior to induction of transcription by the dioxin receptor ligand beta-naphthoflavone. In addition, a DNase I-hypersensitive site is detected near the transcription initiation site and is altered in nuclease sensitivity by induction. The presence of the constitutive DNase I-hypersensitive sites at the dioxin response elements correlates with the presence of a constitutive, labile factor which specifically recognizes these elements in vitro. This factor appears to be distinct from the dioxin receptor, which is observed only in nuclear extract from treated cells. In conclusion, these data suggest that a certain protein-DNA architecture may be maintained at the response elements at different stages of gene expression.

A novel BK virus-based episomal vector for expression of foreign genes in mammalian cells

A composite mammalian cell-E. coli shuttle vector was developed based on the human papova virus BK and pSV-neo. The vector contains a dioxin-responsive enhancer (DRE) controlling a mouse mammary tumor virus (MMTV) promoter for the inducible expression of inserted genes. In human cells the vector replicates episomally, presumably utilizing the BKV rather than the SV40 origin, and expresses the BK T/t antigens. A deletion in the late BK region precludes the expression of the core/capsid proteins VP1, VP2, and VP3, thereby preventing the infectious lytic cycle. HeLa cells which were transfected with this vector and selected for resistance to the antibiotic G418 maintained the construct primarily in episomal form during more than one year of continuous culture, with little or no integration into the host genome. Transformed cells cultured in higher concentrations of G418 contained higher copy numbers of the vector. This permits one to vary the dosage of an inserted gene easily and reversibly without the need of conventional amplification techniques and clonal analysis. Using a chloramphenicol acetyl transferase (CAT) reporter gene inserted downstream of the MMTV promoter, we found that CAT expression was greater in clones with higher vector copy number. CAT expression was inducible with 2,3,7,8-tetrachlorodibenzo-p-dioxin, but inducibility was found to be inversely proportional to the copy number. Transformation of bacteria with plasmid molecules retrieved from the mammalian host was efficient, making this vector well adapted for the screening of cDNA libraries for the ability to express a phenotype in mammalian cells. Moreover, DNA sequences were stable during long-term passage in mammalian cells; vector passaged continuously for more than one year retained fully functional bacterial genes for resistance to chloramphenicol and ampicillin.

Human P450scc gene transcription is induced by cyclic AMP and repressed by 12-O-tetradecanoylphorbol-13-acetate and A23187 through independent cis elements

Long-term regulation of mammalian steroid hormone synthesis occurs principally by transcriptional regulation of the gene for the rate-limiting cholesterol side-chain cleavage enzyme P450scc. Adrenal steroidogenesis is regulated primarily by two hormones: adrenocorticotropin, which works via cyclic AMP (cAMP) and protein kinase A, and angiotensin II, which works via Ca2+ and protein kinase C. Forskolin and 8-bromo-cAMP stimulated, while prolonged treatment with a phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA]) and a calcium ionophore (A23187) additively suppressed accumulation of endogenous P450scc mRNA in transformed murine adrenal Y1 cells. In Y1 cells transfected with 2,327 base pairs of the human P450scc promoter fused to the bacterial gene for chloramphenicol acetyltransferase (CAT), forskolin increased CAT activity 900% while combined TPA plus A23187 reduced CAT activity to 15% of the control level. Forskolin induced the P450scc promoter as rapidly as a promoter containing two cAMP-responsive elements fused to a simian virus 40 promoter, a system known to respond directly to cAMP. Basal expression was increased by sequences between -89 and -152 and was increased further by sequences between -605 and -2327. This upstream region also conferred inducibility by cAMP. TPA plus A23187 transiently increased CAT activity before repressing it, reflecting the complex actions of angiotensin II in vivo. Repression by prolonged treatment with TPA plus A23187 was mediated by multiple elements between -89 and -343. Induction of CAT activity by forskolin was not diminished by treatment with TPA plus A23187, nor were the regions of the promoter responsible for regulation by the two pathways coisolated. Thus, the human gene for P450scc is repressed by TPA plus A23187 by mechanisms and sequences independent of those that mediate induction by cAMP.

Characterization of a thyroid-specific enhancer located 5.5 kilobase pairs upstream of the human thyroid peroxidase gene

A 6.3-kbp segment of DNA, upstream of the human thyroid peroxidase gene, and various deletions thereof were linked to a promoterless bacterial chloramphenicol acetyltransferase reporter gene. These constructs were analyzed by transfection and expression in rat FRTL-5 thyroid cells and in human hepatoma HepG2 cells to localize sequences that are important for thyroid cell-specific expression of the thyroid peroxidase gene. A thyroid-specific enhancer element, capable of activating enhancerless simian virus 40 promoter expression in FRTL-5 cells, was localized to a 230-bp region approximately 5.5 kbp upstream of the human thyroid peroxidase gene transcription start site. DNase I footprinting, using nuclear extracts prepared from FRTL-5 cells, revealed three regions within the 230-bp fragment; none of these regions were protected by nuclear extracts from HepG2 cells. Gel mobility shift assays, using double-stranded oligonucleotides corresponding to the three protected regions, further confirmed the existence of factors in FRTL-5 cells, but not HepG2 cells, able to specifically bind to the enhancer sequences. These results suggest the presence of three cis-acting DNA elements in the human thyroid peroxidase gene enhancer that interact with thyroid-specific trans-acting factors.

Human P450scc gene transcription is induced by cyclic AMP and repressed by 12-O-tetradecanoylphorbol-13-acetate and A23187 through independent cis elements

Long-term regulation of mammalian steroid hormone synthesis occurs principally by transcriptional regulation of the gene for the rate-limiting cholesterol side-chain cleavage enzyme P450scc. Adrenal steroidogenesis is regulated primarily by two hormones: adrenocorticotropin, which works via cyclic AMP (cAMP) and protein kinase A, and angiotensin II, which works via Ca2+ and protein kinase C. Forskolin and 8-bromo-cAMP stimulated, while prolonged treatment with a phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA]) and a calcium ionophore (A23187) additively suppressed accumulation of endogenous P450scc mRNA in transformed murine adrenal Y1 cells. In Y1 cells transfected with 2,327 base pairs of the human P450scc promoter fused to the bacterial gene for chloramphenicol acetyltransferase (CAT), forskolin increased CAT activity 900% while combined TPA plus A23187 reduced CAT activity to 15% of the control level. Forskolin induced the P450scc promoter as rapidly as a promoter containing two cAMP-responsive elements fused to a simian virus 40 promoter, a system known to respond directly to cAMP. Basal expression was increased by sequences between -89 and -152 and was increased further by sequences between -605 and -2327. This upstream region also conferred inducibility by cAMP. TPA plus A23187 transiently increased CAT activity before repressing it, reflecting the complex actions of angiotensin II in vivo. Repression by prolonged treatment with TPA plus A23187 was mediated by multiple elements between -89 and -343. Induction of CAT activity by forskolin was not diminished by treatment with TPA plus A23187, nor were the regions of the promoter responsible for regulation by the two pathways coisolated. Thus, the human gene for P450scc is repressed by TPA plus A23187 by mechanisms and sequences independent of those that mediate induction by cAMP.

Functional analysis of the transcriptional promoter for the CYP1A1 gene

In mouse hepatoma cells, the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases the transcription rate of the CYP1A1 gene, which encodes a cytochrome P-450 enzyme. In this study, we analyzed the DNA region immediately upstream of the CYP1A1 gene. A domain that extends upstream to nucleotide--166 was found to function as a transcriptional promoter. The promoter was silent when uncoupled from the dioxin-responsive enhancer located farther upstream. DNase footprinting experiments indicated that nuclear proteins interact with distinct domains of the promoter in a TCDD-independent fashion. Mutational analyses indicated that the CYP1A1 promoter contains at least three functional domains, including a TATAAA sequence, a CCAAT box transcription factor/nuclear factor I-like recognition motif, and a guanine-rich G box.

Functional analysis of the transcriptional promoter for the CYP1A1 gene

In mouse hepatoma cells, the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases the transcription rate of the CYP1A1 gene, which encodes a cytochrome P-450 enzyme. In this study, we analyzed the DNA region immediately upstream of the CYP1A1 gene. A domain that extends upstream to nucleotide--166 was found to function as a transcriptional promoter. The promoter was silent when uncoupled from the dioxin-responsive enhancer located farther upstream. DNase footprinting experiments indicated that nuclear proteins interact with distinct domains of the promoter in a TCDD-independent fashion. Mutational analyses indicated that the CYP1A1 promoter contains at least three functional domains, including a TATAAA sequence, a CCAAT box transcription factor/nuclear factor I-like recognition motif, and a guanine-rich G box.