Cis-acting sequences from the rat cytochrome P450 2B1 gene confer pulmonary and phenobarbital-inducible expression in transgenic mice

Constitutive expression and phenobarbital modulation of drug metabolizing enzymes and related nuclear receptors in cattle liver and extra-hepatic tissues

In humans and rodents, phenobarbital (PB) induces hepatic and extra-hepatic drug metabolizing enzymes (DMEs) through the activation of specific nuclear receptors (NRs). In contrast, few data about PB transcriptional effects in veterinary species are available. The constitutive expression and modulation of PB-responsive NR and DME genes, following an oral PB challenge, were investigated in cattle liver and extra-hepatic tissues (duodenum, kidney, lung, testis, adrenal and muscle). Likewise to humans and rodents, target genes were expressed to a lower extent compared to the liver with few exceptions. Phenobarbital significantly affected hepatic CYP2B22, 2C31, 2C87, 3A and UDP-glucuronosyltransferase 1A1-like, glutathione S-transferase A1-like and sulfotransferase 1A1-like (SULT1A1-like) mRNAs and apoprotein amounts; in extra-hepatic tissues, only duodenum showed a significant down-regulation of SULT1A1-like gene and apoprotein. Nuclear receptor mRNAs were never affected by PB. Presented data are the first evidence about the constitutive expression of foremost DME and NR genes in cattle extra-hepatic tissues, and the data obtained following a PB challenge are suggestive of species-differences in drug metabolism; altogether, these information are of value for the extrapolation of pharmacotoxicological data among species, the characterization of drug-drug interactions as well as the animal and consumer's risk caused by harmful residues formation.

Induction of cytochrome P4502B: Role of regulatory elements and nuclear receptors

Cytochrome P450 of the 2B subfamily is easily induced by many xenobiotics. In spite of intensive investigations, the molecular mechanisms of regulation of the CYP2B genes are not clear. The nuclear receptor CAR is shown to play a crucial role in the activation of CYP2B genes by xenobiotics, but many problems of CAR activation in different animal species and humans remain unsolved. This review focuses on signaling pathways involved in the control of CYP2B gene expression in mammals.

Generation of human hepatocytes by stem cell technology: definition of the hepatocyte

Since 1999, numerous articles have reported the generation of hepatocytes from different types of extrahepatic stem or precursor cells. This opens exciting new possibilities for pharmacology and toxicology, as well as for cell therapy. Hepatocyte marker expression, including albumin, cytokeratin 18, c-met, alpha-fetoprotein and cytochrome P450 3A4 and -2B6, has been observed after transplantation of different types of human stem cells into the liver of laboratory animals or in vitro after incubation with cytokines. These intriguing observations have prompted scientists to classify stem cell-derived cell populations as hepatocytes. However, this conclusion may be premature. It has been shown that factors of the liver microenvironment can induce expression of a limited number of hepatocyte marker genes in nonhepatic cell types. To conclude on the grounds of a limited number of markers that these cells are true hepatocytes is not indicated. In this case one should carefully evaluate crucial hepatocyte-defining enzymatic properties. The present article: i) reviews studies describing the fate of extrahepatic human stem and precursor cells in livers of laboratory animals, including the possibility of cell fusion; and ii) critically discusses the phenotype of stem cells after application of various differentiation protocols aimed at generating human hepatocytes. In addition, the necessary criteria needed for defining a true hepatocyte are suggested. Establishing the necessary properties for stem cell-derived hepatocytes is timely and reasonable, and thus avoids further misleading semantic confusion. Finally, it is essential to understand that the definition of a bona fide hepatocyte should not be limited to qualitative assays, such as reverse transcriptase polymerase chain reaction and immunohistochemistry, but has to include a quantitative analysis of enzymatic activities, which allows direct comparison with primary hepatocytes. Although the stem cell-derived-hepatocyte does not yet exist there is a good chance that this aim may be achieved in the future.

Regulation of CYP3A genes in the human respiratory tract

The CYP3A gene cluster consists of four members, CYP3A4, CYP3A5, CYP3A7 and CYP3A43. Especially the CYP3A4 and CYP3A5 enzymes play a significant role in the metabolism of numerous exogenous (drugs, pollutants, procarcinogens) and endogenous (steroids, bile acids) compounds. CYP3A5 protein is present in the liver and some extrahepatic tissues, such as the gut wall, kidney, adrenal gland, prostate and many cell types in the lung. In the lung, the highest amounts of CYP3A5 protein are present in bronchial and alveolar epithelial cells, bronchial glands and alveolar macrophages. The same cells types have little or no CYP3A4 expression. Cigarette smoking markedly represses CYP3A5 content in alveolar macrophages. CYP3A5 is upregulated by glucocorticoids via the glucocorticoid receptor (GR) in lung adenocarcinoma derived A549 cells. Tissue selective distribution of CYP3A4 is controlled by tissue enriched transcription factors, such as hepatic nuclear factor 4alpha (HNF4alpha), and ligand dependent nuclear receptors, most notably pregnane X receptor (PXR) and constitutive androstane receptor (CAR). The selective expression of CYP3A5 over CYP3A4 in specific lung cells is likely to be the sum of the effects of tissue-specific upregulating and downregulating transcription factors in these cells. Since the CYP3A4/5 enzymes mediate the metabolism of many exogenous and endogenous compounds with direct relevance to pulmonary physiology and pathology, the functions of these enzymes and factors controlling them should be elucidated in much more detail.

C/EBP transcription factors in the lung epithelium

During recent years, the biological roles of CCAAT/enhancer binding proteins (C/EBPs) in the lung have started to be uncovered. C/EBPs form a family within the basic region-leucine zipper class of transcription factors. In the lung epithelium C/EBPalpha, -beta, and -delta are expressed. Lung-specific target genes for these transcription factors include the surfactant proteins A and D, the Clara cell secretory protein, and the P450 enzyme CYP2B1. As more information is gathered, a picture is emerging in which C/EBPalpha has a role in regulating proliferation as well as differentiation-dependent gene expression, whereas C/EBPbeta and -delta, in addition to a partly overlapping role in regulating expression of differentiation markers, also seem to be involved in responses to injury and hormones.

Human extrahepatic cytochromes P450: function in xenobiotic metabolism and tissue-selective chemical toxicity in the respiratory and gastrointestinal tracts

Cytochrome P450 (CYP) enzymes in extrahepatic tissues often play a dominant role in target tissue metabolic activation of xenobiotic compounds. They may also determine drug efficacy and influence the tissue burden of foreign chemicals or bioavailability of therapeutic agents. This review focuses on xenobiotic-metabolizing CYPs of the human respiratory and gastrointestinal tracts, including the lung, trachea, nasal respiratory and olfactory mucosa, esophagus, stomach, small intestine, and colon. Many CYPs are expressed in one or more of these organs, including CYP1A1, CYP1A2, CYP1B1, CYP2A6, CYP2A13, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP2F1, CYP2J2, CYP2S1, CYP3A4, CYP3A5, and CYP4B1. Of particular interest are the preferential expression of certain CYPs in the respiratory tract and the regional differences in CYP expression profile in different parts of the gastrointestinal tract. Current research activities on the characterization of CYP expression, function, and regulation in these tissues, as well as future research needs, are discussed.

Characterization of the human lung CYP2F1 gene and identification of a novel lung-specific binding motif

The CYP2F1 gene encodes a cytochrome P450 enzyme capable of bioactivating a number of pulmonary-selective toxicants. The expression of CYP2F1 is highly tissue-selective; the highest expression is observed in the lung with little or no hepatic expression. The objective of these studies was to elucidate the mechanisms that govern the unique tissue-specific regulation of CYP2F1. Cosmid and bacterial artificial chromosome clones were screened and sequenced to identify a gene that spanned 14 kbp containing 10 exons, including an untranslated exon 1. Primer extension analysis and 5'-rapid amplification of cDNA ends were used to identify the transcription start site. Several sequences homologous to known cis-elements were identified in the 5'-upstream region of the CYP2F1 promoter. Transient transfection studies with luciferase reporter constructs demonstrated a significant functional lung cell-specific CYP2F1 promoter region (from position -129 to +115). DNase footprinting analysis of 1.6 kbp of the upstream sequence with nuclear extracts from human lung tissues revealed one strong DNA-protein complex at -152 to -182. This nuclear protein (called lung-specific factor, LSF) was present only in lung but not liver or heart tissues. Competitive electrophoretic mobility shift assays characterized a DNA consensus site, within the LSF-binding domain, that was highly similar to two E box motifs, but no known "E box" trans-factors were identified. These studies identified a novel LSF and its consensus sequence that may control tissue-specific expression of CYP2F1.

Phenobarbital response elements of cytochrome P450 genes and nuclear receptors

Phenobarbital (PB) response elements are composed of various nuclear receptor (NR)-binding sites. A 51-bp distal element PB-responsive enhancer module (PBREM) conserved in the PB-inducible CYP2B genes contains two NR-binding direct repeat (DR)-4 motifs. Responding to PB exposure in liver, the NR constitutive active receptor (CAR) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates PBREM via binding to DR-4 motifs. For CYP3A genes, a common NR site [DR-3 or everted repeat (ER)-6] is present in proximal promoter regions. In addition, the distal element called the xenobiotic responsive module (XREM) is found in human CYP3A4 genes, which contain both DR-3 and ER-6 motifs. Pregnane X receptor (PXR) could bind to all of these sites and, upon PB induction, a PXR:RXR heterodimer could transactivate XREM. These response elements and NRs are functionally versatile, and capable of responding to distinct but overlapping groups of xenochemicals.

CYP2B1 is regulated by C/EBP alpha and C/EBP delta inlung epithelial cells

Pulmonary expression of several cytochrome P450 (CYP) monooxygenases is detected late in gestation. Little is known of the factors involved in this differentiation-dependent expression. C/EBP factors are known regulators of differentiation and differentiation-dependent gene expression in several tissues. In this study we demonstrate the importance of C/EBP alpha and C/EBP delta in pulmonary epithelial CYP2B1 gene expression. A 1.3 kb CYP2B1 promoter fragment which recently has been shown to confer lung tissue- and cell-specific expression of CYP2B1 in transgenic mice was used in transient transfection studies. Both C/EBP alpha and C/EBP delta transactivated the CYP2B1 promoter in the lung epithelial cell lines A549 and NCI-H441. C/EBP alpha in nuclear extracts from isolated rat primary bronchiolar Clara cells was capable of interacting with a C/EBP-binding site in the proximal CYP2B1 promoter. Site-directed mutagenesis studies showed that this proximal C/EBP-binding site is necessary for transactivation of the CYP2B1 gene by C/EBP alpha and C/EBP delta. Thi study shows that C/EBP factors have a role in pulmonary CYP2B1 expression and suggests that these transcription factors may be important for the differentiation-dependent expression of CYP2B1 in the lung.