Ah receptor: a novel ligand-activated transcription factor

Two years have passed since the Ah receptor (AhR) and the AhR nuclear translocator (Arnt) were revealed to constitute a novel ligand-activated transcription factor by molecular biological studies on cloned cDNAs. Many interesting properties of the AhR/Arnt system have begun to be elucidated. We present here a brief summary of the molecular biology of AhR and Arnt.

Disruption of aryl hydrocarbon receptor (AhR) induces regression of the seminal vesicle in aged male mice

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates diverse dioxin toxicities. Despite mediating the adverse effects, the AhR gene is conserved among animal species, suggesting important physiological functions for AhR. In fact, a recent study revealed that AhR has an intrinsic function in female reproduction, though its role in male reproduction is largely unknown. In this study, we show age-dependent regression of the seminal vesicles, probably together with the coagulating gland, in AhR(-/-) male mice. Knockout mice had abnormal vaginal plugs, low sperm counts in the epididymis, and low fertility. Moreover, serum testosterone concentrations and expression of steroidogenic 3betahydroxysteroiddehydrogenase (3betaHsd) and steroidogenic acute regulatory protein (StAR) in testicular Leydig cells were decreased in AhR(-/-) males. Taken together, our results suggest that impaired testosterone synthesis in aged mice induces regression of seminal vesicles and the coagulating glands. Such tissue disappearance likely resulted in abnormal vaginal plug formation, and eventually in low fertility. Together with previous findings demonstrating AhR function in female reproduction, AhR has essential functions in animal reproduction in both sexes.

Molecular mechanisms of AhR functions in the regulation of cytochrome P450 genes

AhR, a ligand-activated transcription factor, mediates xenobiotic signaling to enhance the expression of target genes, including drug-metabolizing cytochrome P450s. The recent development of several new techniques, including chromatin immunoprecipitation and RNA interference, has expanded and deepened our knowledge of AhR function in the xenobiotic signal transduction. In this review, we briefly summarize our current understanding of the activation and inactivation of AhR activities and discuss the future directions of AhR research.

Parathyroid hormone (PTH) down-regulates PTH/PTH-related protein receptor gene expression in UMR-106 osteoblast-like cells via a 3',5'-cyclic adenosine monophosphate-dependent, protein kinase A-independent pathway

Parathyroid hormone (PTH) regulates osteoblast function via a G protein-linked PTH/PTH-related protein (PTHrP) receptor. We have studied the mechanisms of PTH/PTHrP receptor gene repression by PTH in UMR-106 osteoblast-like cells. Inhibition of PTH/PTHrP receptor mRNA expression by rat (r) PTH(1-34) and Insulin-like growth factor-I (IGF-I) at 10(-7)M was significant at 1 h and 3 h, and maximal at 2 h and 6 h. A maximal decrease in receptor mRNA abundance by rPTH(1-34) and IGF-I was maintained for 24 h. Inhibition of receptor gene expression by rPTH(1-34) was mimicked in UMR-106 cells by the addition of forskolin (an adenylyl cyclase activator), or 8-(4-chlorophenylthio)-adenine 3',5'-cyclic monophosphate (8-pCPTcAMP; a cAMP analogue). Although H89, a selective protein kinase A (PKA) inhibitor, completely inhibited PKA activity stimulated by rPTH(1-34), forskolin or 8-pCPTcAMP, suppression of PTH/PTHrP receptor mRNA synthesis induced by these substances in UMR-106 cells was not affected by H89. In primary osteoblast cultures, rPTH(1-34) inhibited synthesis of PTH/PTHrP receptor mRNA irrespective of H89. The down-regulation effect of rPTH(1-34) was also unaltered by PD98059 (an extracellularly regulated kinase 1/2 mitogen-activated protein kinase pathway inhibitor). Pretreatment with cycloheximide, a protein synthesis inhibitor, did not alter the inhibition of PTH/PTHrP receptor mRNA expression by rPTH(1-34), indicating that receptor mRNA suppression does not require new protein synthesis. Transcriptional activation of PTH/PTHrP receptor gene promoter (U3P or U4P)-luciferase constructs was decreased by rPTH(1-34), forskolin and 8-pCPTcAMP irrespective of H89. Thus, PTH transcriptionally down-regulates PTH/PTHrP receptor gene expression in osteoblast-like cells via a cAMP-dependent, PKA-independent pathway.

Dioxin induces a novel nuclear factor, DIF-3, that is implicated in spermatogenesis

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin), a member of a class of environmental pollutants represented by polychlorinated dibenzo-p-dioxins and dibenzofurans, is one of the most toxic artificial compounds ever developed. In this study, we identified a novel TCDD target gene, DIF-3 (dioxin inducible factor-3), by cDNA representational difference analysis. DIF-3 protein is a nuclear factor and possesses a zinc-finger motif at its N-terminus. High DIF-3 mRNA expression in the testes was demonstrated by Northern blot analysis and abundant DIF-3 protein was detected during spermatogenesis. Thus, these results suggest that DIF-3 may be a target gene mediating the reproductive toxicity induced by TCDD.

Expression of human and mouse genes encoding polkappa: testis-specific developmental regulation and AhR-dependent inducible transcription

BACKGROUND

Human polkappa is a newly identified low-fidelity DNA polymerase. While the enzyme bypasses an abasic site and acetylaminofluorene-adduct in an error-prone manner, it bypasses benzo[a]pyrene-N2-dG lesions in a mostly error-free manner by incorporating predominantly dC opposite the bulky lesions. Benzo[a]pyrene (B[a]P) is activated through intracellular process mediated by the arylhydrocarbon receptor (AhR, also called the dioxin receptor), which is a ligand-activated transcription factor with high affinities for aromatic compounds such as B[a]P and dioxin.

RESULTS

We examined promoter structures of the human POLK and mouse Polk genes to study how their expressions are regulated. The mouse Polk gene is developmentally regulated in testis and utilizes two transcription start sites during spermatogenesis, while it utilizes only one site in tissues other than testis. Both of the mouse Polk and the human POLK genes have two AhR-binding sites in the promoter regions and the expression of the mouse Polk gene is indeed enhanced upon AhR-activation.

CONCLUSIONS

The AhR activation increases expression of the mouse Polk gene and probably the human POLK gene, the product of which bypasses benzo[a]pyrene-N2-dG lesions in a mostly accurate manner. Thus, polkappa seems to function to reduce mutagenesis at benzo[a]pyrene-adducts, although it may also have a role related to spermatogenesis.

Parathyroid hormone (PTH) suppresses rat PTH/PTH-related protein receptor gene promoter

Parathyroid hormone (PTH) regulates osteoblasts via a G protein-linked PTH/PTH-related protein (PTHrP) receptor. PTH effects on PTH/PTHrP receptor gene expression were studied in UMR 106 osteoblast-like cells. In heterogeneous nuclear RNA and Northern analysis, PTH suppressed PTH/PTHrP receptor transcription. We cloned the 7-kb promoter region of the rat PTH/PTHrP receptor gene and transiently transfected chimeric deletion constructs containing the 5'-flanking region and the luciferase gene into UMR 106 cells. In transfected cells the minimal region for basal promoter activity was between positions -128 and +103. The 5'-flanking region of exon U1 contained several putative-binding sites for Sp1 and the myc-associated zinc finger protein (MAZ). The minimal PTH-suppressive region (PTHSR) was between +1 and +25 in exon U1, but the 5'-flanking region or Sp1 and MAZ-binding sites also were required for PTH-mediated repression. By gel mobility shift assay PTH markedly decreased binding of PTHSR-protein complex in UMR 106 cells. The mutation experiments showed that the most critical sequence for the repression of PTH was 5'-GGGGGAGGGGAG-3' (+1 to +12) of PTHSR. This represents the first characterization of a PTH-suppressive region of the PTH/PTHrP receptor gene in rat.

Structure and expression of the Ah receptor repressor gene

The aryl hydrocarbon receptor (AhR) repressor (AhRR) gene has been isolated and characterized from a mouse genomic library. The gene is distributed as 11 exons in a total length of about 60 kilobase pairs. Fluorescence in situ hybridization analysis has shown that the AhRR gene is located at mouse chromosome 13C2, at rat chromosome 1p11.2, and at human chromosome 5p15.3. The AhRR gene has a TATA-less promoter and several transcription start sites. In addition, putative regulatory DNA sequences such as xenobiotic responsive element (XRE), GC box, and NF-kappaB-binding sites have been identified in the 5'-upstream region of the AhRR gene. Transient transfection analyses of HeLa cells with reporter genes that contain deletions and point mutations in the AhRR promoter revealed that all three XREs mediated the inducible expression of the AhRR gene by 3-methylcholanthrene treatment, and furthermore, GC box sequences were indispensable for a high level of inducible expression and for constitutive expression. Moreover, by using gel mobility shift assays we were able to show that the AhR/Arnt heterodimer binds to the XREs with very low affinity, which is due to three varied nucleotides outside the XRE core sequence. We have also shown that Sp1 and Sp3 can bind to the GC boxes. Finally, both transient transfection analysis and gel mobility shift assay revealed that the AhRR gene is up-regulated by a p65/p50 heterodimer that binds to the NF-kappaB site when the cells has been exposed to 12-O-tetradecanoylphorbol-13-acetate, and this inducible expression was further enhanced by cotreatment of 12-O-tetradecanoylphorbol-13-acetate and 3-methylcholanthrene.

Dioxin suppresses the checkpoint protein, MAD2, by an aryl hydrocarbon receptor-independent pathway

The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown recently to be carcinogenic, but little is currently known about the molecular mechanism of TCDD affecting cell proliferation and carcinogenesis. In this report, we demonstrate that TCDD suppresses the expression of the checkpoint protein, Mad2. Suppression of Mad2 was also observed in aryl hydrocarbon receptor-deficient mouse embryonic fibroblasts, suggesting that TCDD suppresses Mad2 by a novel TCDD receptor signaling mechanism. In addition, HeLa cells treated with TCDD failed to arrest in mitosis after nocodazole treatment. The Mad2 protein plays a significant role in accurate chromosome segregation in mitotic cells. Our data suggest that TCDD may increase chromosomal instability through the suppression of Mad2 expression.

Human arylhydrocarbon receptor repressor (AHRR) gene: genomic structure and analysis of polymorphism in endometriosis

The diversity of biological effects resulting from exposure to dioxin may reflect the ability of this environmental pollutant to alter gene expression by binding to the arylhydrocarbon receptor (AHR) gene and related genes. AHR function may be regulated by structural variations in AHR itself, in the AHR repressor (AHRR), in the AHR nuclear translocator (ARNT), or in AHR target molecules such as cytochrome P-4501A1 (CYP1A1) and glutathione S-transferase. Analysis of the genomic organization of AHRR revealed an open reading frame consisting of a 2094-bp mRNA encoded by ten exons. We found one novel polymorphism, a substitution of Ala by Pro at codon 185 (GCC to CCC), in exon 5 of the AHRR gene; among 108 healthy unrelated Japanese women, genotypes Ala/Ala, Ala/Pro, and Pro/Pro were represented, respectively, by 20 (18.5%), 49 (45.4%), and 39 (36.1%) individuals. We did not detect previously published polymorphisms of ARNT (D511N) or the CYP1A1 promoter (G-469A and C-459T) in our subjects, suggesting that these polymorphisms are rare in the Japanese population. No association was found between uterine endometriosis and any polymorphisms in the AHRR, AHR, ARNT, or CYP1A1 genes analyzed in the present study.

A case of localized primary sclerosing cholangitis mimicking intrahepatic bile duct cancer

A 75-year-old woman with chronic hepatitis was regularly followed-up in our hospital. A computed tomography (CT) scan revealed an obviously enlarged intrahepatic bile duct in the posterior branch of the left lateral segment. Percutaneous cholangiography revealed an enlarged posterior branch of the left lateral segment and a narrow stenotic region at the root of this branch. We diagnosed her as having intrahepatic bile duct cancer, and a left lateral segmentectomy of the liver was performed. However, microscopic examination of the resected specimens revealed peribiliary fibrosis in the stenotic bile duct and other areas of the intrahepatic bile duct with no malignant cells. Thus, the final diagnosis was made to be primary sclerosing cholangitis. We must consider primary sclerosing cholangitis in the differential diagnosis of localized stenosis of the intrahepatic bile duct.

Budd-Chiari syndrome and extrahepatic portal obstruction associated with congenital antithrombin III deficiency

We report a patient with Budd-Chiari syndrome (BCS) and extrahepatic portal obstruction (EHO) associated with congenital antithrombin (AT) III deficiency. A 35-year-old man was admitted to Nishi Kobe Medical Center for evaluation of abnormal intrahepatic veins. By various imaging modalities, BCS and EHO were diagnosed. Laboratory data revealed parallel decreases in activity and antigen concentration of AT III despite normal liver function. Taken together, the etiology of both BCS and EHO was considered to be thrombosis, associated with congenital AT III deficiency. Two years after beginning warfarin therapy, the patient has no symptoms and his liver function remains normal. Anticoagulant therapy is considered useful for preventing progression of the disease.

Vascularity of advanced gastric carcinoma: evaluation by using power Doppler imaging

BACKGROUND

We investigated the vascularity of advanced gastric adenocarcinomas by using percutaneous power Doppler imaging.

METHODS

Seventeen patients with gastric cancer and 10 without a gastric tumor, but with a slightly thick gastric wall in the B-mode ultrasound, were investigated with the use of power Doppler imaging. The color signals of the gastric lesion were graded as follows: 1, no color signals or the same as the surroundings; 2, color signals were slightly increasing; and 3, color signals were obviously increasing.

RESULTS

The color signals of three patients were graded 1, those of eight patients were graded 2 and those of six patients were graded 3 in the gastric cancer group. The color signals of all 10 patients without a gastric tumor were grade 1. This difference was statistically significant (P = 0.0002).

CONCLUSIONS

Power Doppler imaging showed vascularity of gastric cancer increasing in the majority of patients (14 of 17: 82%). Thus, power Doppler imaging might be a good screening examination method for gastric cancer.

Effect of angiotensin-converting enzyme inhibitor on cardiopulmonary baroreflex sensitivity in patients with acute myocardial infarction

We evaluated the effect of angiotensin-converting enzyme inhibition (quinapril) on cardiopulmonary baroreflex sensitivity in 30 patients with uncomplicated myocardial infarction (quinapril group, 15 patients; placebo group, 15 patients) at 5 and 10 days after the onset of myocardial infarction. This study indicates that quinapril improved cardiopulmonary baroreflex and thus reduced sympathetic outflow in patients with acute myocardial infarction.

Transactivation mechanisms of mouse clock transcription factors, mClock and mArnt3

BACKGROUND

The Arnt3 (also termed as BMAL1 or MOP3)/Clock heterodimer is a positive regulator of circadian rhythm and activates the transcription of target genes such as per1 and vasopressin.

RESULTS

We investigated the transcriptional mechanism of mArnt3/mClock heterodimer. While mClock did not possess any distinct activation domain, mArnt3 contained a transcriptional activation domain at the most C-terminal end, the activity of which was not expressed, even in the one hybrid system, until it was bound by mClock. It has been suggested that mClock plays a regulatory or structural role in exerting a transcription enhancing effect of the mArnt3/mClock heterodimer. Deletion proceeding from amino acids 559-492 of mClock markedly reduced the transactivation activity of mArnt3/mClock heterodimer, in consistence with the results of the Clock-delta 19 mutant. Yeast and mammalian two-hybrid systems revealed that CBP and p300 interacted with mArnt3 via the CREB binding domain. The In vivo interaction between mArnt3 and CBP was confirmed by the GST pull down assay.

CONCLUSION

Taken together, these results suggest that the mArnt3/mClock heterodimer exerted its transactivation activity via CBP or p300 interacting with mArnt3 in the heterodimer with mClock playing a structural or regulatory role in the transactivation process.

Aortic root dilatation among young competitive athletes: echocardiographic screening of 1929 athletes between 15 and 34 years of age

BACKGROUND

Aortic dilatation can be lethal for young competitive athletes. The prevalence among athletes is not known, however, and thus a reasonable approach to early recognition remains uncertain.

METHODS AND RESULTS

Echocardiograms of 1929 normotensive athletes 15 to 34 years of age were analyzed. Five (0.26%) athletes had aortic dilatation; 4 of the 5 played basketball. This made the prevalence of aortic dilatation 0.96% (4 of 415) among basketball and volleyball players, who represented a population of especially tall athletes. Tallness aside, only 2 of the 5 athletes had features of Marfan syndrome. Among the athletes without aortic dilatation, the relation between body surface area and aortic root dimension was nonlinear and best described with a quadratic regression model. Athletes with aortic dilatation fell well outside the 95% confidence interval.

CONCLUSION

Because a higher incidence of aortic dilatation is to be anticipated among very tall athletes, inclusion of echocardiography in screening before participation in certain sports should be considered.

Benzo[a]pyrene carcinogenicity is lost in mice lacking the aryl hydrocarbon receptor

The contribution of the aryl hydrocarbon receptor (AhR) in induction of a battery of xenobiotic-metabolizing enzymes has been studied extensively. However, no direct proof has been obtained that it plays a role in modulating carcinogenesis. To address the question of whether AhR is required for tumor induction, we have investigated the response of AhR-deficient mice to benzo[a]pyrene (B[a]P), a widely distributed environmental carcinogen. B[a]P treatment induced expression of the cytochrome P450 gene Cyp1a1 in the skin and liver of AhR-positive mice bearing +/+ and +/- genotypes and did not induce expression of the cytochrome P450 gene Cyp1a1 in AhR-null mice in either skin or liver. In contrast, Cyp1a2 gene expression was positive in liver irrespective of the presence or absence of the AhR gene, or B[a]P treatment, although its inducibility was lost in the AhR(-/-) mouse. All AhR-positive male mice of both +/+ and +/- genotypes that received subcutaneous injection of B[a]P (2 mg) on the first and the eighth days had developed subcutaneous tumors at the site of injection at the end of the 18-week experiment. In contrast, no tumors were apparent in any of the AhR-deficient mice. Likewise, topical application of B[a]P (200 microg) at weekly intervals to the skin of female mice for 25 weeks produced skin tumors only in the AhR-positive mice. Thus the carcinogenic action of B[a]P may be determined primarily by AhR, a transcriptional regulator of the gene for CYP1A1. The results of the present study provide direct evidence that AhR is involved in carcinogenesis.

Endoscopic resection of small inflammatory fibroid polyp of the colon

Inflammatory fibroid polyp (IFP) is a solitary intestinal lesion of unknown etiology. Although IFP is benign, laparotomy for the resection of colonic IFP is performed in most cases because the polyp is usually large. We report a successful endoscopic resection of cecal IFP. It is considered that colonic IFP should be resected endoscopically if the polyp is small and is located submucosally.

Mild impairment of learning and memory in mice overexpressing the mSim2 gene located on chromosome 16: an animal model of Down's syndrome

Human Sim2 is a product of one of the genes located on human chromosome 21q22 and is a homolog of Drosophila single-minded ( sim ) which is a critical player in midline development of the central nervous system of the fly. Since Sim2 mRNA is expressed in facial, skull, palate and vertebra primordia in human and rodent embryos, features that are associated with phenotypes of Down's syndrome (DS), its trisomic state is suspected to contribute to the symptoms of DS. Here we describe that mSim2 mRNA is expressed in hippocampus and amygdala of adult mice, and that while mice overexpressing mSim2 under the control of the beta-actin promoter are viable and fertile and have superficially normal skeletal, brain and heart structures, they exhibit a moderate defect in context-dependent fear conditioning and a mild defect in the Morris water maze test. Taken together, our data show that overdosage of Sim2 may be important for the pathogenesis of Down's syndrome, especially mental retardation.

Portal venous volume flow: in vivo measurement by time-domain color-velocity imaging

The portal venous velocity and flow volume in 39 patients (16 with liver cirrhosis, 11 with chronic hepatitis, 12 without liver disease) were measured using both color velocity imaging quantification (CVI-Q) and conventional Doppler flowmetry. The average portal venous velocity and flow volume values obtained using the two methods were similar. The correlation coefficients for the paired measurements show positive correlations (velocity: 0.73, p < 0.0001; volume: 0.50, p = 0.001). However, the coefficients of variation between the two methods were not good (velocity: 14.9%, volume: 26.4%). In conventional Doppler flowmetry, the mean velocity to maximum velocity ratio (Vmean:Vmax) is assumed to be constant (Vmean:Vmax = 0.57 in this study). However, the Vmean:Vmax ratios calculated from the flow profile in CVI-Q were 0.67 +/- 0.13 in the patients with liver cirrhosis, 0.58 +/- 0.13 in the patients with chronic hepatitis, and 0.53 +/- 0.08 in the patients without liver disease. Therefore, a measurement method that takes the blood flow profile into account, such as CVI-Q, might be useful for the quantitative measurement of the portal venous velocity and volume.

Relationship between asthma and gastro-oesophageal reflux: significance of endoscopic grade of reflux oesophagitis in adult asthmatics

BACKGROUND

The association between asthma and gastro-oesophageal reflux disease (GERD) is well known. The aim of this study was to elucidate the causal relationship between reflux oesophagitis (RE) and asthma.

METHODS

Seventy-two adult asthmatics were examined regarding their GERD symptoms, and each underwent an endoscopic examination. According to the Los Angeles classification, we divided the patients into three groups: group 1 (n= 52), no mucosal break; group 2 (n= 15), RE corresponding to grades A or B; group 3 (n = 5), RE corresponding to grades C or D. The asthmatics in groups 2 and 3 received anti-reflux treatment for their GERD for 8 weeks. Their morning and evening peak expiratory flow rates (PEFR), daily variability of the PEFR and daily use of an inhalation bronchodilator were compared before and after this treatment.

RESULTS

The percentage of severe asthma and postprandial exacerbation of asthma in group 3 were significantly higher than those in the other two groups. In contrast, the number of eosinophiles and the serum level of immunoglobulin E in group 3 were significantly lower than those in the other two groups. After the antireflux treatment, significant improvements of both PEFR and daily use of the inhalation bronchodilator were observed only in group 3.

CONCLUSIONS

The endoscopic severity of RE is associated with the characteristics of adult asthmatics and the treatment of severe RE improved the asthmatics' condition.

Molecular mechanisms of transcription activation by HLF and HIF1alpha in response to hypoxia: their stabilization and redox signal-induced interaction with CBP/p300

Hypoxia-inducible factor 1 alpha (HIF1alpha) and its related factor, HLF, activate expression of a group of genes such as erythropoietin in response to low oxygen. Transfection analysis using fusion genes of GAL4DBD with various fragments of the two factors delineated two transcription activation domains which are inducible in response to hypoxia and are localized in the C-terminal half. Their sequences are conserved between HLF and HIF1alpha. One is designated NAD (N-terminal activation domain), while the other is CAD (C-terminal activation domain). Immunoblot analysis revealed that NADs, which were rarely detectable at normoxia, became stabilized and accumulated at hypoxia, whereas CADs were constitutively expressed. In the mammalian two-hybrid system, CAD and NAD baits enhanced the luciferase expression from a reporter gene by co-transfection with CREB-binding protein (CBP) prey, whereas CAD, but not NAD, enhanced beta-galactosidase expression in yeast by CBP co-expression, suggesting that NAD and CAD interact with CBP/p300 by a different mechanism. Co-transfection experiments revealed that expression of Ref-1 and thioredoxin further enhanced the luciferase activity expressed by CAD, but not by NAD. Amino acid replacement in the sequences of CADs revealed a specific cysteine to be essential for their hypoxia-inducible interaction with CBP. Nuclear translocation of thioredoxin from cytoplasm was observed upon reducing O2 concentrations.

Colonic ulceration caused by administration of loxoprofen sodium

A 54-year-old female with chronic headache was admitted to our hospital because of hematochezia. She had routinely taken loxoprofen sodium because of severe headache. Emergent colonoscopic examination revealed ulceration of the cecum. After administration of loxoprofen sodium was discontinued and administration of sulfasalazine was initiated, her intestinal bleeding subsided. Two months after discontinuation of loxoprofen sodium, the colonoscopic examination revealed scar formation at the site of cecal ulceration. In this case, it was conceivable that the administration of loxoprofen sodium might have induced colonic ulceration.

Identification of a novel mechanism of regulation of Ah (dioxin) receptor function

Ah receptor (AhR) is a ligand-activated transcription factor that mediates pleiotropic effects of environmental pollutants such as 2,3, 7,8-tetrachlorodibenzo-p-dioxin on host animals. In addition to induction of drug-metabolizing enzymes, the liganded AhR complex was found to activate gene expression of a factor designated AhR repressor (AhRR), which inhibits AhR function by competing with AhR for dimerizing with Arnt and binding to the XRE sequence. Thus, AhR and AhRR form a regulatory circuit in the xenobiotic signal transduction pathway and provide a novel mechanism of regulation of AhR function that may determine tissue-specific sensitivity to environmental pollutants.

Power Doppler imaging of hepatic tumours: differential diagnosis between hepatocellular carcinoma and metastatic adenocarcinoma

We evaluated the usefulness of power Doppler imaging in the differential diagnosis between hepatocellular carcinoma and metastatic adenocarcinoma. Forty-seven patients with hepatocellular carcinoma and 18 patients with metastatic adenocarcinoma were evaluated using power Doppler imaging. The colour signals of hepatic tumours were graded as follows: 1, colour signals only in the marginal area; 2, small dot or dotted line colour signals within the tumours; 3, continuous solid line colour signals within the tumours. The grade 3 colour signals were classified in the following three patterns; winding line pattern, stretched line pattern and mixed pattern. The colour signals of hepatocellular carcinoma were grade 1 in seven patients, grade 2 in 11 and grade 3 in 29. The colour signals of metastatic adenocarcinoma were grade 1 in three patients and grade 3 in 15. Of the 29 hepatocellular carcinoma patients with a grade 3 signal, 26 patients had winding line patterns and three had mixed patterns. Of the 15 metastatic adenocarcinoma patients with a grade 3 signal, 12 patients had stretched line patterns and three had mixed patterns. In conclusion, power Doppler imaging is useful in the differential diagnosis between hepatocellular carcinoma and metastatic adenocarcinoma to evaluate the colour signal pattern within the tumour.

Structure and expression of the mouse AhR nuclear translocator (mArnt) gene

Aryl hydrocarbon receptor (AhR) nuclear translocator (Arnt) gene has been isolated and characterized from a mouse genomic DNA library. The gene is about 60 kilobases long and split into 22 exons. An unusual exon/intron junctional sequence was found in the 11th intron of the gene that begins with GC at its 5'-end. The exon/intron arrangement of mArnt gene differs greatly from those of the other members of the same basic-helix-loop-helix/PAS family. The gene is TATA-less and has several transcription start sites. The promoter region of the mArnt gene is GC-rich and contains a number of putative regulatory DNA sequences such as two GC-boxes, a cAMP-responsive element, E-box, AP-1 site, and CAAT-box. Deletion experiments revealed that all these DNA elements made substantial contributions to a high level of expression of the gene, except for the cAMP-responsive element. Of all, two GC-boxes displayed the most dominant enhancing effects. It was demonstrated that there exist specific factors binding to these DNA elements in the nuclear extracts of HeLa cells. Among them, Sp1 and Sp3, and CAAT-box binding factor-A were identified to bind the GC-boxes and CAAT-box, respectively. Expression of MyoD in HeLa cells stimulated the Arnt promoter activity by binding to the E-box.

Transcriptionally active heterodimer formation of an Arnt-like PAS protein, Arnt3, with HIF-1a, HLF, and clock

We isolated a cDNA clone encoding a polypeptide of 626 amino acids containing basic helix-loop-helix (bHLH) and PAS domains from a mouse cDNA library of P19 cells. This protein, termed Arnt3, showed the highest similarity to Arnt and Arnt2 in the bHLH and PAS regions. Arnt3 mRNA was expressed in brain, skeletal muscle, 13.5-day embryos, and P19 cells treated with retinoic acid. The partner PAS proteins of Arnt3 were searched for by the two-hybrid system in yeast, and HIF-1 alpha, HLF, and Clock among various bHLH/PAS proteins were found. Gel mobility shift analysis using nuclear extracts from 293T cells cotransfected with Arnt3 and HIF-1 alpha (or HLF) expression plasmids revealed that these complexes specifically bound the hypoxia-response element (HRE). Coexpression of Arnt3 and HIF-1 alpha (or HLF) in Arnt-deficient c4 cells enhanced transcription of a reporter gene driven by the HRE sequences. We also showed that Arnt3 contained an activation domain at the C-terminal region and a repression domain between the PAS-A and PAS-B regions.

cDNA cloning and predicted primary structure of scallop sarcoplasmic reticulum Ca(2+)-ATPase

Sarcoplasmic reticulum (SR) Ca(2+)-ATPase of the scallop cross-striated adductor muscle was purified with deoxycholate and digested with lysyl endopeptidase for sequencing of the digested fragments. Overlapping cDNA clones of the ATPase were isolated by screening the cDNA library with an RT-PCR product as a hybridization probe, which encodes the partial amino acid sequence of the ATPase. The predicted amino acid sequence of the ATPase contained all the partial sequences determined with the proteolytic fragments and consisted of the 993 residues with approximately 70% overall sequence similarity to those of the SR ATPases from rabbit fast-twitch and slow-twitch muscles. An outline of the structure of the scallop ATPase molecule is predicted to mainly consist of ten transmembrane and five 'stalk' domains with two large cytoplasmic regions as observed with the rabbit ATPase molecules. The sequence relationship between scallop and other sarco/endoplasmic reticulum-type Ca(2+)-ATPases is discussed.

Loss of teratogenic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice lacking the Ah (dioxin) receptor

BACKGROUND

The aryl hydrocarbon receptor (AhR or dioxin receptor) is a ligand-activated transcription factor that is considered to mediate pleiotropic biological responses such as teratogenesis, tumour promotion, epithelial hyperplasia and the induction of drug-metabolizing enzymes to environmental contaminants usually represented by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In contrast to the role of AhR in the regulatory mechanism of xenobiotic-metabolizing enzymes, there is no direct proof that the AhR is involved in the teratogenic effects of TCDD.

RESULTS

To gain insight into the physiological and teratogenic role of the AhR, we have used gene targeting in mice to disrupt the murine Ahr gene by homologous recombination. Ahr-null mice were viable and fertile and were apparently normal at birth, but displayed a slightly slower growth rate than wild-type mice for the first few weeks of life. When pregnant dams were administered with TCDD by gavage, at a dose of 40 microg/kg body weight at gestation day 12.5, none of the Ahr-null mutant foetuses were sensitive to the teratogenic effects of TCDD, although almost all wild-type foetuses suffered from cleft palate and hydronephrosis. In heterozygous Ahr+/- genotypes, nearly all foetuses suffered from hydronephrosis in response to TCDD treatment, while haplo-insufficiency was observed in the incidence of cleft palate.

CONCLUSION

These results clearly show that the AhR is involved in the malformation of the palate and kidney in mouse embryos caused by TCDD and suggests that the mechanism of its involvement differs between the cleft palate and hydronephrosis.

cDNA cloning and tissue-specific expression of a novel basic helix-loop-helix/PAS factor (Arnt2) with close sequence similarity to the aryl hydrocarbon receptor nuclear translocator (Arnt)

We isolated mouse cDNA clones (Arnt2) that are highly similar to but distinct from the aryl hydrocarbon receptor (AhR) nuclear translocator (Arnt). The composite cDNA covered a 2,443-bp sequence consisting of a putative 2,136-bp open reading frame encoding a polypeptide of 712 amino acids. The predicted Arnt2 polypeptide carries a characteristic basic helix-loop-helix (bHLH)/PAS motif in its N-terminal region with close similarity (81% identity) to that of mouse Arnt and has an overall sequence identity of 57% with Arnt. Biochemical properties and interaction of Arnt2 with other bHLH/PAS proteins were investigated by coimmunoprecipitation assays, gel mobility shift assays, and the yeast two-hybrid system. Arnt2 interacted with AhR and mouse Sim as efficiently as Arnt, and the Arnt2-AhR complex recognized and bound specifically the xenobiotic responsive element (XRE) sequence. Expression of Arnt2 successfully rescued XRE-driven reporter gene activity in the Arnt-defective c4 mutant of Hepa-1 cells. RNA blot analysis revealed that expression of Arnt2 mRNA was restricted to the brains and kidneys of adult mice, while Arnt mRNA was expressed ubiquitously. In addition, whole-mount in situ hybridization of 9.5-day mouse embryos showed that Arnt2 mRNA was expressed in the dorsal neural tube and branchial arch 1, while Arnt transcripts were detected broadly in various tissues of mesodermal and endodermal origins. These results suggest that Arnt2 may play different roles from Arnt both in adult mice and in developing embryos. Finally, sequence comparison of the currently known bHLH/PAS proteins indicates a division into two phylogenetic groups: the Arnt group, containing Arnt, Arnt2, and Per, and the AhR group, consisting of AhR, Sim, and Hif-1alpha.

Polymorphic forms of the Ah receptor and induction of the CYP1A1 gene

Induction of cytochrome P4501A1 (CYP1A1) by certain xenobiotics is mediated by the Ah receptor/Arnt complex. The present knowledge about the molecular process of induction is summarized with special attention to our recent work on characterization of the polymorphic forms of the Ah receptor.

A complete structure of the mouse Ah receptor gene

A complete sequence of the Ah receptor gene was cloned from a mouse genomic library by using the Ah receptor cDNA as a probe. The Ah receptor gene is 37.5 kb long and is split into 11 exons by 10 introns. Sequence analysis of the 5' flanking region of the Ah receptor gene reveals that there is neither a TATA box nor a CAAT box in the promoter region. Instead, this gene has a few GC boxes and other enhancer elements in the 5' upstream flanking region. Southern blot analysis indicated that the Ah receptor gene is a unique gene.

Dioxin binding activities of polymorphic forms of mouse and human arylhydrocarbon receptors

The genetic difference in the susceptibility of mice to environmental toxicities induced by dioxin and related chemicals is governed by polymorphism of the arylhydrocarbon receptor (AhR) (Poland, A., and Knutson, C. (1982) Annu. Rev. Pharmacol. Toxicol. 22, 517-554). cDNA cloning of AhR cDNA from responder (C57BL/6) and non-responder (DBA/2J) mice allowed us to analyze the structure and function of these AhRs. Both of the AhRs, which were expressed in COS-7 cells transfected with their expression plasmids, showed a clear 9 S complex with 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) in a linear glycerol gradient centrifugation, consistent with the result of the endogenously expressed AhR in Hepa-1 cells. This result provides the first direct evidence that the cDNA-encoded protein binds the ligand specifically. Scatchard plot analysis revealed that the dissociation constant (Kd) of C57BL AhR for TCDD is 0.27 nM, while that of DBA AhR is elevated up to six times that high. Chimeric plasmids between the two cDNAs and site-directed mutagenesis revealed two critical alterations responsible for the reduced ligand binding activity: an Ala375 to Val alteration and an elongated carboxyl-terminal sequence due to a T to C mutation at the first letter of the termination codon of C57BL AhR. Two variants with reduced and intermediate ligand binding activity were also found in human AhRs with amino acid alterations equivalent to those of the DBA AhR. Importance of the amino acid at position 381 of human AhR (equivalent to position 375 of mouse AhR) in the ligand binding was confirmed by the fact that the mutation of Val381 to Asp completely abolished the ligand binding activity of human AhR.

[Small nodular lesions of the pancreas: differential diagnosis with ultrasound angiography]

Ultrasound angiography (USAG), sonographic imaging of the blood flow in an organ or tissue obtained by carbon dioxide infusion into the supplying artery, was performed on 28 pancreatic nodular lesions less than 3 cm in diameter. The hemodynamics of tumors observed with USAG were divided into three groups: hypovascular, isovascular, and hypervascular, compared with the adjacent pancreatic tissue. Most of hypovascular nodules were duct cell carcinoma (sensitivity 94.1%, specificity 90.4%), while isovascular lesion was the characteristic of inflammatory masses (sensitivity 100%, specificity 95.8%). Hypervascular cases included all of the mucin producing tumors and islet cell tumors but only one case of duct cell carcinoma. So you can almost exclude duct cell carcinoma as an diagnosis in vascular rich tumors (negative predictive value 83.6%). These results were compared with those on conventional x-ray angiograms and incremental CT scans. Ultrasound angiography enabled us to detect more slight differences of tumor vascularity than the other modalities. Thus we conclude that USAG can be a useful diagnostic aid in small mass lesions of the pancreas.

[Clinical utility of receptor imaging in the assessment of liver function]

Technetium-99m diethylene triamine pentaacetic acid-galactosyl human serum albumin (TcGSA) is a newly developed receptor-binding radiopharmaceutical, specific for the asialoglycoprotein receptor, which resides exclusively on the plasma membrane of hepatocytes. Clinical utility of TcGSA was evaluated in 3 control subjects with normal livers and in 54 patients with various liver diseases. The parameter, Receptor Index, was derived from liver and heart time-activity data and is the ratio of radioactivity of the liver over the radioactivity of the liver plus heart at 15 min after the intravenous injection of 3 mg of TcGSA. Receptor concentration ([R]o) was obtained by kinetic analysis of liver and heart time-activity data using pharmacokinetic nonlinear modeling. Values for the Receptor Index and [R]o was statistically different in the control subjects and in patients with mild, moderate, and severe liver diseases. Good correlations were obtained between the Receptor Index, [R]o and conventional liver function tests, such as Child-Turcotte criteria score, prothrombin time, and indocyanine green test. Receptor Index and [R]o were properly estimated even in patients with obstructive jaundice or remarkable portocaval shunt. These data suggest that the receptor imaging as well as its parameters, Receptor Index and [R]o, is a potentially practical and reliable diagnostic method for estimating the functioning hepatocyte mass and for assessing liver function.

[Hepatic receptor imaging with Tc-99m GSA: estimates of liver function in acute liver disease]

Technetium-99m galactosyl human serum albumin (TcGSA) is a synthesized radiolabeled analog ligand to asialoglycoprotein receptor, which resides only at a mammalian hepatocyte. TcGSA studies were performed on 16 patients with various acute liver disease and 3 controls with normal livers. Dynamic data were obtained by a gamma camera during 35 minutes after an intravenous injection of 3 mg (185 MBq) of TcGSA. The parameters of TcGSA time activity curves were obtained by dividing radioactivity of the liver by that of the liver plus heart at 15 min (Receptor Index), and by dividing radioactivity of the liver at 15 min by that at 3 min post injection (Clearance Index). The two parameters correlated well with prothrombin time, clinically estimated staging, and severity of acute liver disease. We have concluded that liver function study by the newly developed receptor imaging with TcGSA can be a sensitive and promising tool in estimating the severity and prognosis of acute liver disease.

Sonography with intraarterial infusion of carbon dioxide microbubbles (sonographic angiography): value in differential diagnosis of hepatic tumors

Differential diagnosis of small liver tumors is important, but is not always possible, even with angiography. To solve this problem, we introduced sonographic angiography, which combines sonography and angiography. The vascular pattern of a variety of hepatic nodules was evaluated with sonographic angiography, and the results were compared with those of conventional angiography. Sonographic angiography (sonography performed during intraarterial infusion of carbon dioxide microbubbles) was performed in 184 patients with a total of 222 hepatic nodules: 139 hepatocellular carcinomas, nine adenomatous hyperplasias, seven regenerative nodules, 21 hemangiomas, 33 metastases, seven lymphomas, one granuloma, and five focal nodular hyperplasias. Sonographic angiography detected a hypervascular pattern with peripheral blood supply in cases of hepatocellular carcinoma (sensitivity, 90%; specificity, 89%). Typical vascular patterns of adenomatous hyperplasia, hemangioma, metastasis, and focal nodular hyperplasia on sonographic angiography were hypovascularity (sensitivity, 100%; specificity, 91%), spotty pooling (sensitivity, 100%; specificity, 100%), peripheral hypervascularity (sensitivity, 64%; specificity, 100%), respectively. The detectability of hypervascularity was greater with sonographic angiography than with conventional angiography in hepatocellular carcinoma, metastasis, and hemangioma. Our experience indicates that sonographic angiography depicts characteristic vascular features that reflect the vascular anatomy of specific types of hepatic tumors, and thus is useful in the differential diagnosis of these lesions.

Small hepatocellular carcinoma: diagnosis with US angiography with intraarterial CO2 microbubbles

Ultrasonographic (US) angiography enhanced with intraarterial CO2 microbubbles, a contrast material used in US imaging, was performed of 103 histologically proved hepatocellular carcinomas (HCCs) smaller than 3 cm in diameter in 95 patients. The detection rate for hypervascular HCC with US angiography was compared with the rate of detection with conventional angiography, digital subtraction angiography (DSA), and computed tomography (CT) after intraarterial injection of iodized oil. Sensitivity in detection of hypervascular HCCs with US angiography was 86% (89 of 103 HCCs), compared with 63% (44 of 70 HCCs) detected with conventional angiography, 70% (23 of 33 HCCs) with DSA, and 82% (75 of 91 HCCs) with CT with iodized oil. US angiography depicted small hypervascular HCCs, especially those less than 1 cm in diameter, and helped clarify vascularity as isovascular or hypovascular in angiographically undetectable HCCs. Findings at US angiography assisted the choice of a therapeutic strategy for treatment of HCC, such as transarterial therapy, percutaneous ethanol injection therapy, or resection.

[Tumor hemodynamics in hepatic nodules associated with liver cirrhosis: relationship between cancer progression and tumor hemodynamic change]

Tumor hemodynamics including arterial vascularity (AV) and portal perfusion (PP) were evaluated in histologically confirmed 55 hepatic nodules associated with cirrhosis using ultrasonographic (US) angiography during intraarterial carbon dioxide microbubbles injection and CT during arterial portography. Tumor hemodynamic patterns were classified into 6 types as follows: Type I (n = 10): PP (+), AV (hypo); Type I' (n = 2): PP (+), AV (iso); Type II (n = 5): PP (-), AV (hypo); Type III (n = 8): PP (-), AV (iso); Type IV (n = 25): PP (-), AV (hyper), Type V (n = 5): PP (partially +), AV (vascular spot in hypovascular). Eight nodules of Type I were diagnosed as benign nodules histologically including adenomatous hyperplasia (AH) (n = 6) and regenerative nodule (n = 2). Hundred percent (5/5) of Type II and 88% (7/8) of Type III nodules were well-differentiated HCC, in contrast to 8% (2/25) of Type IV nodules, typical HCCs. Fatty metamorphosis was observed in 75% (6/8) of Type III nodules, in contrast to 16% (4/25) of typical (classical) HCC nodules (Type IV). We concluded that at the malignant transformation from AH to HCC, reduction of portal blood flow in the nodule precedes the initiation of the increase of the arterial tumor vessel. Moreover, early stage HCC could exhibit hypovascular (Type I, II), isovascular (Type III), or vascular spot in hypovascular pattern (Type V) compared with a typical HCC (Type IV). It was also suggested that the more mature as a neoplasms the HCC becomes, the more the arterial tumor vessel in the nodule increases and fatty metamorphosis of well-differentiated HCC is highly related with tumor hemodynamic condition, i.e., hypoperfusion state from both arterial and portal vessel.