A specific cis-acting element regulates in vitro transcription of sex-dependent mouse steroid 16 alpha-hydroxylase (C-P450(16 alpha)) gene

Metabolism of human cytochrome P450 marker substrates in mouse: a strain and gender comparison

The aim was to characterize mouse gender and strain differences in the metabolism of commonly used human cytochrome (CYP) P450 probe substrates. Thirteen human CYP probe substrates (phenacetin, coumarin, 7-ethoxy-4-trifluoromethyl coumarin, amiodarone, paclitaxel, diclofenac, S-mephenytoin, bufuralol, dextromethorphan, chlorzoxazone, p-nitrophenol, testosterone and lauric acid) were used in activity measurements. The metabolism of the probe substrates was compared in liver microsomes from male and female NMRI, CBA, C57bl/6, 129/SvJ and CD1 strains. The expression of proteins identified on Western blots with commonly available antibodies selective for specific human and rat CYP enzymes were compared in the different mouse strains. Males had higher metabolism than corresponding females for phenacetin O-deethylation (human marker for CYP1A2 activity), and a high correlation was found between phenacetin activity and immunoreactivity in Western blots produced with rat CYP1A2 antibodies. Protein detected by antibodies cross-reacting with human CYP2B6 and rat CYP2B1/2 antibodies was female specific except for the 129/SvJ strain, where it was absent in both genders. Females generally had a higher metabolism of bufuralol 1'-hydroxylation and dextromethorphan O-demethylation (human markers for CYP2D activity). Bufuralol 1'-hydroxylation correlated with a female-dominant mouse CYP, which was detected with antibodies against rat CYP2D4. p-Nitrophenol 2-hydroxylation correlated better than chlorzoxazone 6-hydroxylation with the protein detected with antibodies against rat CYP2E1, indicating that p-nitrophenol is a more specific substrate for mouse CYP2E1.

Sexually dimorphic P450 gene expression in liver-specific hepatocyte nuclear factor 4alpha-deficient mice

Hepatocyte nuclear factor (HNF) 4alpha is a liver-enriched nuclear receptor that plays a critical role in regulating the expression of numerous hepatic genes, including members of the cytochrome P450 (CYP) superfamily, several of which are expressed in a sex-dependent manner. Presently, we use a liver-specific Hnf4alpha-deficient mouse model to investigate the role of HNF4alpha in regulating liver-enriched transcription factors and sexually dimorphic Cyps in liver in vivo. Real-time PCR analysis of RNA isolated from livers of wild-type and Hnf4alpha-deficient mice revealed the following: 1) HNF4alpha exerts both positive regulation (Hnfalpha, C/ebpalpha, and C/ebpbeta) and negative regulation (Hnf3alpha and the HNF4alpha coactivator Pgc-1alpha) on liver transcription factor expression; 2) a strong dependence on HNF4alpha characterizes several male-predominant Cyps (2d9 and 8b1), female-predominant Cyps (2b10, 2b13, 3a41, and 3a44) and Cyps, whose expression is sex independent (3a11, 3a25); 3) HNF4alpha confers a unique, positive regulation of two male-expressed genes (Cyp4a12 and GSTpi) and a negative regulation of several female-predominant genes (Cyp2a4, Cyp2b9, Hnf3beta, and Hnf6), both of which are manifest in male but not female mouse liver. These trends were confirmed at the protein level by Western blot analysis using antibodies raised to Cyp2a, Cyp2b, and Cyp3a family members. Thus, HNF4alpha is an essential player in the complex regulatory network of liver-enriched transcription factors and the sexually dimorphic mouse Cyp genes that they regulate. HNF4alpha is proposed to contribute to the sex specificity of liver gene expression by positively regulating a subset of male-specific Cyp genes while concomitantly inhibiting the expression of certain female-specific Cyps and liver transcription factors, by mechanisms that are operative in male, but not female, mouse liver.

Transcriptional regulation by HNF-4 of the steroid 15alpha-hydroxylase P450 (Cyp2a-4) gene in mouse liver

The mouse P450 gene Cyp2a-4 encodes the hepatic steroid 15apha-hydroxylase. We have defined in the 5'-flanking sequence of Cyp2a-4 gene, a composite regulatory element (-61AGACCAAAGTCCGGCCTTC-42) which contains a potential CpG methylation site at position -50. Gel-shift assays indicate that this element consists of overlapped binding sites for a hepatocyte-enriched transcription factor HNF-4 and a Sp1-like protein. Moreover, transcription of the Cyp2a-4 gene is activated by coexpression of HNF-4 in HepG2 cells. A mutation (C at -50 to A) abolishes the binding of HNF-4 to the element as well as the transcriptional activation by HNF-4. The methylated C at position -50, however, does not affect HNF-4 binding. Neither the mutation nor the methylation at position -50 affect the binding of Sp1-like protein to the element. It appears, therefore, that HNF-4 activates the hepatic transcription of Cyp2a-4 gene through its direct binding to the regulatory element regardless of the methylation at position -50.

Characterization of a phenobarbital-responsive enhancer module in mouse P450 Cyp2b10 gene

Induction of drug- and carcinogen-metabolizing cytochrome P450s by xenobiotic chemicals is a common cellular defense mechanism, usually leading to increased detoxification of xenobiotics but sometimes, paradoxically, to formation of more toxic and carcinogenic metabolites. Phenobarbital (PB) is an archetypal representative for chemicals including industrial solvents, pesticides, plant products, and clinically used drugs that induce several genes within CYP subfamilies 2B, 2A, 2C, and 3A in rodents and humans. Although the transcription of these CYP genes is activated by PB, the associated molecular mechanisms have not yet been elucidated. Here we have analyzed, in detail, enhancer activity of a far upstream region of mouse Cyp2b10 gene and report a 132-base pair PB-responsive enhancer module (PBREM) with a 33-base pair core element containing binding sites for nuclear factor I- and nuclear receptor-like factors. Mutations of these binding sites abolish the ability of PBREM to respond to inducers in mouse primary hepatocytes.

Interaction of a novel sex-dependent, growth hormone-regulated liver nuclear factor with CYP2C12 promoter

CYP2C12 is a steroid hydroxylase cytochrome P450 whose female-specific expression in adult rat liver is transcriptionally activated by the continuous plasma growth hormone (GH) profile characteristic of adult female rats. DNase I footprinting and gel mobility shift analysis of the 5'-flank of the CYP2C12 gene were carried out to identify cis-acting elements and trans-acting factors that may contribute to the GH-regulated, sex-dependent transcription of this P450 gene. DNase I footprinting analysis revealed sex- and GH-regulated DNase I hypersensitivity sites at the boundaries of several protein binding sites detected along a 1560-nucleotide upstream segment of CYP2C12. Five distinct sites bound a novel continuous GH-regulated nuclear factor, GHNF, which is enriched in adult female and continuous GH-treated male liver nuclear extracts compared to untreated male liver nuclear extracts. Two other footprinted sites correspond to binding sites for the liver transcription factors C/EBP and albumin D element-binding protein and a third to an HNF1 binding site. A specific binding site for GHNF was also found in the 5'-proximal promoter of CYP2C11, an adult male-specific liver P450 gene, suggesting that GHNF may contribute to the down-regulation of that gene by continuous GH. GHNF was distinguished from the nuclear factors that bind to a GH response element upstream of the rat Spi 2.1 gene and is also distinct from the GH-activatable latent cytoplasmic transcription factors STAT 1, STAT 3, and STAT 5. These findings support the hypothesis that continuous GH-activated transcription of CYP2C12 in adult female rat liver (a) involves the activation of a novel GH-regulated nuclear factor which binds to multiple sites along the 5'-flank of this cytochrome P450 gene, and (b) proceeds via a signaling pathway distinct from the GH pulse-activated STAT5 pathway proposed to induce CYP2C11 and other male-expressed liver genes.

Characterization of the human cytochrome P4502D6 promoter. A potential role for antagonistic interactions between members of the nuclear receptor family

The functional mapping of the human cytochrome P4502D6 (CYP2D6) promoter in HepG2 cells revealed the presence of both positive and negative regulatory elements. One of these regulatory elements overlapped a sequence that is highly conserved in most members of the CYP2 family. This element, which consists of a degenerate AGGTCA direct repeat spaced by 1 base pair (DR1) and is known to be a target for members of the steroid receptor superfamily, was found to bind in vitro translated hepatocyte nuclear factor 4 (HNF4) in gel retardation analysis. Using HepG2 nuclear extracts, three protein-DNA complexes were formed on the DR1 element, one of which was confirmed to be dependent on the binding of HNF4. The other DR1 complexes were shown to be due to the interaction of the orphan receptor chicken ovalbumin upstream promoter transcription factor I (COUP-TFI). Experiments in COS-7 cells showed that HNF4 could activate the CYP2D6 promoter 30-fold. Surprisingly, mutation of the DR1 element produced a relatively minor 23% decrease in activity in HepG2 cells. Additionally, COUP-TFI was shown to inhibit HNF4 stimulation of the CYP2D6 promoter in COS-7 cells, suggesting that COUP-TFI could attenuate the effect of HNF4 in HepG2 cells. However, when HNF4 levels were increased in HepG2 cells by co-transfection, it resulted in the enhancement of CYP2D6 promoter activity, indicating that HNF4 could overcome the repressive effect of COUP-TFI. Therefore, the contribution of the DR1 element in controlling the transcription of the CYP2D6 gene depends on the balance between positively and negatively acting transcription factors.

Characterization of phenobarbital-inducible mouse Cyp2b10 gene transcription in primary hepatocytes

The mouse phenobarbital (PB)-inducible Cyp2b10 gene promoter has been isolated and sequenced, and control of its expression has been characterized. The 1405-base pair (bp) Cyp2bl0 promoter sequence is 83% identical to the corresponding region from the rat CYP2B2 gene. In addition to the lack of CA repeats, differences include insertion of 42 base pairs (-123/-82 bp) into the middle of a consensus sequence to the so-called "Barbie box." In this report, we have developed a primary mouse hepatocyte culture system in which endogenous 2B10 mRNA as well as Cyp2b10-driven CAT activity were induced by PB and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), but not by the 3-chloro derivative of TCPOBOP. Deletion analysis of the Cyp2b10 promoter identified a basal transcription element at -64/-34 bp and a negative element at -971/-775 bp. Sequences contained within the -1404/-971 bp region are responsible for the induced CAT activity. DNase I protection and gel shift assays detected five major protein binding sites within the -1404/-971 bp fragment, one of which shared high sequence identity with a portion of a regulatory element in CYP2B2 gene (Trottier, E., Belzil, A., Stoltz, C., and Anderson, A. (1995) Gene 158, 263-268). Our results indicate that sequences important for PB-induced transcription of Cyp2b10 gene are located in the distal promoter.

Differential protein binding and transcriptional activities of HNF-4 elements in three closely related CYP2C genes

A functional binding site for a liver-enriched transcription factor, hepatocyte nuclear factor-4 (HNF-4), has previously been identified around -100 in the CYP2C2 promoter and proposed to be a common regulatory motif for the hepatic expression of many CYP2 genes. The transcriptional activity of the proximal promoters of three closely related cytochrome P450 genes (CYP2C1, CYP2C2, and CYP2C3) have been compared in HepG2 cells and correlated with the relative binding affinities of the HNF-4 motifs in the genes for proteins in liver, kidney, and HepG2 extracts that react with antisera to HNF-4. Gel-shift assays suggested that these highly similar motifs bound HNF-4 with significantly different affinities. The relative binding affinities to the protein in liver extracts were estimated by competitive gel-shift binding, and the binding affinity of CYP2C2 was two-fold and eight-fold greater than that of CYP2C1 and CYP2C3, respectively. These affinity differences correlated well with the transcriptional activities of either the minimal hepatic promoters (117 to 135 bp of 5'-flanking region) of the genes in HepG2 cells or minimal CYP2C2 promoters containing the HNF-4 motif from each gene. Identification of the CYP2C3 site was somewhat indirect; the HNF-4 element in the CYP2C3 promoter could be converted to a motif with binding affinity and hepatic cell-specific transcriptional activity similar to that of CYP2C2 by a single nucleotide substitution. The activities of the minimal promoters did not correlate well with hepatic expression of each gene in vivo, as estimated previously by the concentration of mRNA in livers of untreated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver

The determination of sexually dimorphic hepatic steroid metabolism in rat liver has been shown to involve growth hormone. However, the mechanisms by which growth hormone controls the cytochrome P450 enzymes responsible for this dimorphic steroid metabolism is largely unknown. In this review we discuss different levels of growth hormone signal transduction, including receptor binding, signal transduction and activation of target genes by growth hormone.