The contributions of William Money MRCS to smallpox vaccination and control - A little-known story

This history of vaccinology article outlines the work of William Money (1790-1843), who conducted a study related to smallpox disease, immunity, and vaccination. His hitherto unpublished study demonstrated that smallpox could be contracted more than once; notably, results from his studies showed that vaccination was not dangerous. He was also the author of a celebrated Vade Mecum in human anatomy. Here, we outline the work he conducted in England: from serving as the house surgeon at Northampton Infirmary to his post as a surgeon at the Royal Metropolitan Hospital in London.

Relationship between Changes in Thymic Emigrants and Cell-Associated HIV-1 Dna in HIV-1-Infected Children Initiating Antiretroviral Therapy

Objectives and methods

To investigate the relationship between cell-associated HIV-1 dynamics and recent thymic T-cell emigrants, HIV-1 DNA and T-cell receptor rearrangement excision circles (TREC, a marker of recent thymic emigrants) were measured in peripheral blood mononuclear cells in 181 samples from 33 HIV-1-infected children followed for 96 weeks after antiretroviral therapy (ART) initiation.

Results

At baseline, HIV-1 DNA was higher in children with higher TREC ( P=0.02) and was not related to age, CD4 or HIV-1 RNA in multivariate analyses ( P>0.3). Overall, TREC increased and HIV-1 DNA decreased significantly after ART initiation, with faster HIV-1 DNA declines in children with higher baseline TREC ( P=0.009). The greatest decreases in HIV-1 DNA occurred in children with the smallest increases in TREC levels during ART ( P=0.002). However, this inverse relationship between changes in HIV-1 DNA and TREC tended to vary according to the phase of HIV-1 RNA decline ( P=0.13); for the same increase in TREC, HIV-1 DNA decline was much smaller during persistent or transient viraemia compared with stable HIV-1 RNA suppression.

Conclusions

Overall, these findings indicate that TREC levels predict HIV-1 DNA response to ART and suggest that immune repopulation by thymic emigrants adversely affects HIV-1 DNA decline in the absence of persistent viral suppression, possibly by providing a cellular source for viral infection and replication.

An epizootic of highly virulent feline calicivirus disease in a hospital setting in New England

This article reports an outbreak of 24 cases of an unusually virulent feline calicivirus (FCV) infection in a small animal hospital. The circumstances and disease signs were very similar to those recently described in an outbreak of FCV hemorrhagic disease in Northern California (Vet. Microbiol. 73 (2000) 281). The virus entered the facility through shelter cats showing upper respiratory signs. Affected cats manifested high fever, anorexia, labored respirations, oral ulceration, facial and limb edema, icterus, and pancreatitis. The infection spread rapidly among the patients by contaminated animal caretakers and hospital equipment. One case of fomite transmission from an employee to a housecat was documented. Prior vaccination, even with multiple doses of FCV-F9-based live calicivirus vaccine, was not protective. Affected cats often required extensive supportive care for 7-10 days, and the overall mortality from death and euthanasia was 32%. The strain of FCV responsible for this outbreak was genetically and serologically distinct from the FCV strain responsible for a similar epizootic and the FCV-F9 strain contained in most vaccines. Outbreaks of this type are being reported with increasing frequency, and are often associated with the practice of treating sick shelter cats in private practices. Similar to the present epizootic, outbreaks of FCV hemorrhagic disease have been self-limiting, but require prompt application of strict quarantine, isolation, personnel sanitation, and disinfection procedures.

Clinical, microscopic, and molecular aspects of canine leproid granuloma in the United States

Leproid granulomas from seven dogs in the United States were evaluated. Gross characteristics included nodular and ulcerated dermal and subcutaneous lesions primarily on the caudal aspects of the pinnae and to a lesser extent on the muzzle, face, and forelimbs. In all except one dog, there was complete regression of the lesions within 6 months, either with no therapy or after surgical resection. Cytology or histopathology revealed pyogranulomatous inflammation with few to many acid-fast mycobacterial bacilli within macrophages. The organisms could not be cultivated in vitro. DNA sequencing of part of the 16S ribosomal RNA gene region revealed 99-100% homology among fragments from five of these dogs and fragments from dogs in the south Pacific. This syndrome occurs in dogs in North America and the prognosis is excellent, in contrast to the prognosis for rapid-growing or tuberculous mycobacteriosis.

Binding of aryl hydrocarbon receptor (AhR) to AhR-interacting protein. The role of hsp90

The aryl hydrocarbon receptor (AhR) has been shown to interact with an immunophilin-like molecule known as AhR-interacting protein (AIP) and to enhance AhR function. We show here that AIP associates with AhR homologues from mouse and fish, which can bind ligands such as dioxin, but nonligand binding homologues from Caenorhabditis elegans or Drosophila do not bind to AIP. However, a minimal ligand-binding domain of the AhR is incapable of binding AIP. The binding of AIP to AhR in reticulocyte lysate shows several of the characteristics of an hsp90-dependent process, including sensitivity to geldanamycin and temperature and a requirement for ATP or nonhydrolyzable analogues. Purified AIP binds to the C terminus of hsp90, and mutation of a conserved basic residue in the tetratricopeptide repeats of AIP (K266A, analogous to K97A in protein phosphatase 5) abolishes binding to hsp90. Mutation of K266A in AIP reduces binding to AhR by 75-80%; the geldanamycin sensitivity of this complex shows that AhR stabilizes the AIP-hsp90-AhR complex. The alpha-helical C terminus of AIP, which is outside the tetratricopeptide repeat domain, is absolutely required for binding to AhR as shown by deletions of the C-terminal 5 amino acids or alanine-scanning mutagenesis, but it is not required for binding of AIP to hsp90. The data support a model where 1) AIP binds to both hsp90 and AhR; 2) hsp90 is required for AhR-AIP binding; and 3) the binding of AhR to AIP stabilizes the AIP-hsp90-AhR complex.

An isolated epizootic of hemorrhagic-like fever in cats caused by a novel and highly virulent strain of feline calicivirus

An isolated epizootic of a highly fatal feline calicivirus (FCV) infection, manifested in its severest form by a systemic hemorrhagic-like fever, occurred over a 1-month period among six cats owned by two different employees and a client of a private veterinary practice. The infection may have started with an unowned shelter kitten that was hospitalized during this same period for a severe atypical upper respiratory infection. The causative agent was isolated from blood and nasal swabs from two cats; the electron microscopic appearance was typical for FCV and capsid gene sequencing showed it to be genetically similar to other less pathogenic field strains. An identical disease syndrome was recreated in laboratory cats through oral inoculation with tissue culture grown virus. During the course of transmission studies in experimental cats, the agent was inadvertently spread by caretakers to an adjoining room containing a group of four normal adult cats. One of the four older cats was found dead and a second was moribund within 48-72h in spite of symptomatic treatment; lesions in these animals were similar to those of the field cats but with the added feature of severe pancreatitis. The mortality in field cats, deliberately infected laboratory cats, and inadvertently infected laboratory cats ranged from 33-50%. This new isolate of calicivirus, named FCV-Ari, was neutralized at negligible to low titer by antiserum against the universal FCV-F9 vaccine strain. Cats orally immunized with FCV-F9, and then challenge-exposed shortly thereafter with FCV-Ari, developed a milder self-limiting form of disease, indicating partial protection. However, all of the field cats, including the three that died, had been previously immunized with parenteral FCV-F9 vaccine. FCV-Ari caused a disease that was reminiscent of Rabbit Hemorrhagic Disease, a highly fatal calicivirus infection of older rabbits.

The trouble with TEFs

Comments on Van den Berg, et al. Toxic equivalency factors (TEFs) for PCBs, PCDDs, PCDFs for humans and wildlife. Environ Health Perspect 106:775-792 (1998)

Outbreak of fatal salmonellosis in cats following use of a high-titer modified-live panleukopenia virus vaccine

A 14-week-old kitten from a private cattery was examined because of an acute onset of recumbency and epistaxis 10 days after receiving a high-titer modified-live virus vaccine containing panleukopenia virus, calicivirus, and herpesvirus components. The kitten died the following day, and intestinal crypt necrosis; hepatic, splenic, and lymph node inflammation and necrosis; and pneumonia were seen at necropsy. Salmonella typhimurium was isolated from mesenteric lymph nodes and the spleen. The breeder reported that 4 other kittens had died in the previous month, each within 1 to 2 weeks after being vaccinated with the same modified-live virus vaccine. Carcasses of 3 kittens were available for examination, and Salmonella sp was isolated from mesenteric lymph nodes of all 3. Villus crypt necrosis and secondary fibrosis were also found. Three of the remaining 12 kittens in the cattery were also found to be shedding Salmonella sp in their feces. Clinical and pathologic findings in these kittens were likely attributable to salmonellosis and panleukopenia, and suggest that mild immunosuppression induced by vaccination could have facilitated development of fatal salmonellosis in subclinical carrier kittens. However, we cannot prove that vaccination actually played any role.

Molecular, clinical, and pathologic comparison of two distinct strains of Haemobartonella felis in domestic cats

OBJECTIVE

To characterize 2 strains of Haemobartonella felis by use of molecular techniques.

ANIMALS

35 specific-pathogen-free cats, 6 months to 4 years old.

PROCEDURE

Intraperitoneal or IV inoculation with blood containing H felis small form (Hfsm, 18 cats) or H felis large form (Hflg, 11 cats); 6 cats were uninfected controls. Hfsm was evaluated for capability to cross-protect against the more virulent Hflg. Morphology of both strains was compared by light microscopy of Wright-Giemsa-stained blood smears, and the 16S rRNA genes were sequenced.

RESULTS

Infection with Hflg induced signs of depression, fever, and severe macrocytic normochromic anemia with nucleated erythrocytes. More than 95% of erythrocytes were parasitized. Inoculation with Hfsm and uninfected control blood induced mild or no clinical signs and no hematologic abnormalities. Anti-H felis IgG was first detected on postinoculation day (PID) 21, and increased to maximal titer of 400 by PID 28. Reactivated infection was observed in 8 of 29 cats (4 Hfsm and 4 Hflg), with 5% parasitized erythrocytes during the later attack. On PID 8, Hflg-inoculated cats had positive results of polymerase chain reaction analysis (PCR) that persisted until cats were treated with doxycycline or oxytetracycline; Hfsm-inoculated cats had positive PCR results that persisted for duration of observation (3 months).

CONCLUSIONS

Genetically and morphologically distinct strains of H felis infect cats in the field. The level of genetic difference suggested that these strains may be different species or genera.

CLINICAL RELEVANCE

PCR is a critical diagnostic aid to detect occult Haemobartonella spp infection, as well as response to treatment and clearance of the organism.

Diagnostic features of clinical neurologic feline infectious peritonitis

Feline infectious peritonitis (FIP) is a fatal Arthus-type immune response of cats to infection with FIP virus, a mutant of the ubiquitous feline enteric coronavirus (FECV). The disease may occur systemically or in any single organ system, and primary neurologic disease is a common subset of such manifestations. We examined 16 domestic cats with clinical neurologic FIP and 8 control cats with nonneurologic FIP, with the intention of identifying the ante- and postmortem diagnostic tests that most contribute to accurate diagnosis. Of the 16 cats with neurologic FIP, 15 were less than 2 years of age and all 16 originated from large multiple-cat households. The most useful antemortem indicators of disease were positive anti-coronavirus IgG titer in cerebrospinal fluid, high serum total protein concentration, and findings on magnetic resonance imaging suggesting periventricular contrast enhancement, ventricular dilatation, and hydrocephalus. Postmortem diagnosis was facilitated by FIP monoclonal antibody staining of affected tissue and coronavirus-specific polymerase chain reaction. Most cats with neurologic and ocular forms of FIP had patchy, focal lesions, suggesting that recently developed technologies described in this report may be useful for evaluation of cats with suspected FIP.

Ehrlichia spp. in cervids from California

Blood samples from six mule deer (Odocoileus hemionus hemionus), 15 black-tailed deer (O. hemionus columbianus), and 29 elk (Cervus elaphus nannodes) were assayed for human monocytic and human granulocytic ehrlichiosis (HGE) by polymerase chain reaction (PCR), DNA sequencing, and serology to determine whether or not cervids are involved in the maintenance of these potential human pathogens in California (USA). The deer were sampled in August to October 1992-95. The 29 tule elk from Point Reyes National Seashore were sampled in August 1997. All deer were seronegative for antibodies to HGE/Ehrlichia equi, while the E. equi seroprevalence among elk was 17%. The 16S rDNA PCR prevalence in deer was 38% (in mule deer and black-tailed deer) for Ehrlichia-like sp. of white-tailed deer, 5% (one black-tailed deer only) for E. equi, and 0% for E. chaffeensis. The PCR prevalence in elk was 0% for Ehrlichia-like sp. of white-tailed deer, 31% for E. equi, and 0% for E. chaffeensis. The E. equi from two positive elk samples was successfully propagated in HL-60 cell cultures. DNA sequencing confirmed that the Ehrlichia-like sp. sequences from deer in California were closely related to sequences reported from white-tailed deer from Oklahoma and Georgia. The E. equi strain from deer and elk resembled other E. equi strains from California. These results suggest that cervids may be important in the natural maintenance of E. equi in California.

Feline infectious peritonitis viruses arise by mutation from endemic feline enteric coronaviruses

Feline infectious peritonitis virus (FIPV) strains from six cats and three different geographic areas were compared genetically with feline enteric coronavirus (FECV) isolates obtained from cats inhabiting the same environments. Sequence comparisons were made from 1.2- to 8.9-kb segments on the 3' end of the genome. FECV/FIPV pairs from the same catteries or shelters were 97.3-99.5% related but were genetically distinct from FIPV and FECV strains obtained from cats living in geographically distinct environments. The high genetic similarity between FECVs and FIPVs from the same environment strongly suggested a common ancestry. Based on the presence of deletion mutations in the FIPVs and not in the FECVs, it was concluded that FIPVs evolved as mutants of FECVs. The mutations are deletions in the FIPVs and not insertions in the FECVs since similar sequences are present in other strains that have segregated earlier from a common ancestor. Therefore, the order of descent is form FECV to FIPV. Mutations unique to FIPVs were found in open reading frames (ORFs) 3c in 4 of 6 isolates and/or 7b in 3 of 6 isolates. When the study was extended to include 7 additional FIPV isolates, 11/13 of the FIPVs sequenced were found to have mutated 3c ORFs.

Patterns of feline coronavirus infection and fecal shedding from cats in multiple-cat environments

OBJECTIVE

To determine, by use of a reverse transcriptase-polymerase chain reaction (RT-PCR) test, patterns of fecal shedding of feline coronavirus among cats.

DESIGN

Prospective observational study.

ANIMALS

275 purebred cats from 6 private catteries and 40 specific-pathogen-free (SPF) laboratory-reared cats.

PROCEDURE

40 SPF cats were experimentally inoculated with crude fecal extract containing feline enteric coronavirus (FECV). Fecal and plasma samples were collected every 4 days and evaluated by use of RT-PCR and indirect immunofluorescence assays, respectively, to correlate RT-PCR results with fecal infectivity and to determine patterns of FECV shedding and anti-FECV IgG production in acutely infected cats. The 275 cats in private catteries were monitored for 1 year. Fecal and blood samples were collected every 1 to 3 months and assayed by use of RT-PCR and serologic tests to determine patterns of coronavirus shedding and cofactors for high frequency shedding.

RESULTS

Results of the RT-PCR test in SPF cats were directly correlated with fecal extract infectivity. Overall, 370 of 894 (41%) fecal samples collected from cattery and shelter cats contained infectious levels of coronavirus. Of 121 cats from which multiple samples were collected, 11 never shed virus and 35, 65, and 10, respectively, shed virus with low, moderate, and high frequency. High frequency shedding was associated with age and cattery of origin, but not with sex or concurrent disease. Stress associated with parturition and lactation did not induce shedding in queens. Kittens did not shed coronavirus before they were 10 weeks old, even when nursed by shedding mothers.

CLINICAL IMPLICATIONS

A large proportion of cats in multiple-cat environments shed coronavirus at any given time, but most undergo cycles of infection and shedding, recovery, and reinfection. Infection is acquired from chronically shedding cats and from infectious cats undergoing transient primary infection. Chronically shedding cats cannot be identified on the basis of antibody titer or signalment, but must be identified on the basis of the results of serial fecal RT-PCR tests.

Risk factors for feline infectious peritonitis among cats in multiple-cat environments with endemic feline enteric coronavirus

OBJECTIVE

To determine what risk factors, other than genetic predisposition, contribute to the incidence of feline infectious peritonitis (FIP) in private breeding catteries and animal shelters.

DESIGN

Cats from 7 catteries and a shelter were observed monthly for 1 year. At each visit, cats were examined, fecal samples were collected for determination of feline coronavirus shedding, and blood samples were collected for determination of coronavirus antibody titers. Diagnostic tests were performed on all cats that died of FIP.

ANIMALS

275 purebred or random-bred cats that were kept by private breeder-owners in homes.

RESULTS

24 cats died of FIP during the study. Development of FIP was not associated with cattery, mean cat number, mean age, sex, cattery median coronavirus antibody titer, husbandry and quarantine practices, caging and breeding practices, or prevalence of concurrent diseases. However, risk factors for FIP included individual cat age individual cat coronavirus titer, overall frequency of fecal coronarvirus shedding, and the proportion of cats in the cattery that were chronic coronavirus shedders. Deaths from FIP were more frequent in fall and winter, and on the basis of analysis of cattery records, the number of deaths varied yearly. Epidemics (> 10% mortality rate) were reported at least once in 5 years in 4 catteries.

CLINICAL IMPLICATIONS

Elimination of FIP from a cattery is only possible by total elimination of endemic feline enteric coronavirus (FECV) infection. The most important procedure to reduce FECV from catteries is elimination of chronic FECV shedders.

Analysis of the four alleles of the murine aryl hydrocarbon receptor

The cDNAs for the four murine aryl hydrocarbon (Ah) receptor alleles were cloned and sequenced, and the deduced amino acid sequences were compared. The Ahb-1 allele encodes a protein of 805 amino acids, the Ahd and Ahb-2 alleles encode proteins of 848 amino acids, and the Ahb-3 allele encodes a protein of 883 amino acids. The alleles differ by eight point mutations in the common open reading frame (the initial 805 amino acids) and by additional sequences at the carboxyl end. The amino halves of the proteins, containing a spliced leader sequence, a basic helix-loop-helix motif, and two 50-amino acid repeats (PAAS), have identical sequences except for a single amino acid change in the second PAAS box. The Ahd allele, which has a lower ligand binding affinity, differs from the Ahb-2 receptor by only two amino acids. Mutagenesis experiments with these cloned cDNAs, using in vitro transcription and translation and 2-[125I]iodo-7,8-dibromodibenzo-p-dioxin binding, indicate that the low ligand binding affinity of the Ahd allele is attributable to a valine at residue 375; changing this amino acid to an alanine, as in the Ahb-2 protein, enhances the affinity 4-fold. For in vitro translated Ahb-1 and Ahb-2 alleles the Kd values were approximately 6-10 pM and for Ahd the Kd value was approximately 37 pM. Using 5' truncation and mutations to produce 3' translation truncation sites, we mapped the ligand binding region for the Ahb-1 allele.

Photoaffinity labeling of the Ah receptor: phylogenetic survey of diverse vertebrate and invertebrate species

The mammalian aromatic hydrocarbon (Ah) receptor is a soluble protein involved in the regulation of gene expression by halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Little is known, however, about the presence and properties of this receptor in nonmammalian species. In these studies, we sought evidence for an Ah receptor in the liver or liver-equivalent of diverse species of invertebrate and vertebrate animals. Velocity sedimentation analysis of hepatic cytosol labeled with [3H]TCDD gave equivocal results with three species of marine fish. In subsequent studies, photoaffinity labeling with 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin was used to identify the Ah receptor. Specific labeling (labeling that could be displaced by an excess of unlabeled ligand) was observed in seven species of teleost and elasmobranch fish, including winter flounder (Pleuronectes americanus), killifish (Fundulus heteroclitus), scup (Stenotomus chrysops), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), and dogfish (Mustelus canis and Squalus acanthias). Specific labeling was also found in cytosolic fractions prepared from PLHC-1 fish hepatoma cells and livers of a turtle (Chrysemys picta) and a cetacean, the beluga whale Delphinapterus leucas. The fish Ah receptor was sensitive to conditions of tissue preparation; inclusion of proteinase inhibitors in the homogenization buffer stabilized the receptor in some species. There was heterogeneity in the apparent molecular mass of the largest specifically labeled band in each species; these ranged from 105 to 146 kDa, slightly larger on average than mammalian Ah receptors (95-130 kDa). In contrast to the results obtained with teleost and elasmobranch fish, no specifically labeled polypeptides were detectable in cytosol from two agnathan fish species (hagfish Myxine glutinosa and sea lamprey Petromyzon marinus), the tunicate Ciona intestinalis, or any of nine other invertebrate species representing eight classes in four phyla. Overall these results suggest that the Ah receptor evolved at least 450 million years ago, prior to the divergence of bony and cartilaginous fishes. Although the exact relationship between receptor presence and dioxin responsiveness in these species is uncertain, our data predict that the invertebrate species examined in this study, which appear to lack an Ah receptor protein like that seen in mammals and fish, may be less sensitive than vertebrates to the effects of environmental contaminants that act through this transcriptional regulator.

The aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator protein show distinct subcellular localizations in Hepa 1c1c7 cells by immunofluorescence microscopy

The aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (Arnt) protein were evaluated in the Hepa 1c1c7 (Hepa-1) cell line by indirect immunofluorescence microscopy and Western blot analysis. Wild-type (WT) Hepa-1 cells stained for AhR show intense cytoplasmic fluorescence with minimal nuclear reactivity. WT cells treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) show a time-dependent decrease in cytoplasmic AhR staining and a concomitant increase in nuclear fluorescence. WT cells stained for Arnt show nuclear fluorescence with minimal cytoplasmic reactivity, a pattern unchanged after TCDD treatment. Hepa-1 type II variants express normal levels of AhR but are defective in TCDD-mediated induction of cytochrome P4501A1. Type II variants stained for Arnt show reduced nuclear fluorescence, compared with WT cells, and express minimal levels of Arnt protein, as determined by Western blot analysis. Type II variants stained for the AhR show intense cytoplasmic fluorescence that becomes nuclear after TCDD treatment. Detailed evaluation by immunoelectron microscopy of the AhR and Arnt present in the nuclear compartment of WT cells shows that both proteins are uniformly distributed and do not appear to be associated with nuclear pores, membranes, or nucleoli. Western blot analysis of nuclei isolated from WT Hepa-1 cells fractionated with Nonidet P-40 shows that minimal levels of AhR or Arnt are retained in the nuclear fraction after TCDD treatment. Collectively, these results indicate that the unliganded AhR resides in the cytoplasm, Arnt is localized to the nucleus, and Hepa-1 cells defective in Arnt expression exhibit TCDD-mediated nuclear accumulation of the AhR.

Physiological and genetic analyses of inbred mouse strains with a type I iodothyronine 5' deiodinase deficiency

Inbred mouse strains differ in their capacity to deiodinate iododioxin and iodothyronines, with strains segregating into high or low activity groups. Metabolism of iododioxin occurs via the type I iodothyronine 5'deiodinase (5'DI), one of two enzymes that metabolize thyroxine (T4) to 3,5,3'-triiodothyronine (T3). Recombinant inbred strains derived from crosses between high and low activity strains exhibit segregation characteristic of a single allele difference. Hepatic and renal 5'DI mRNA in a high (C57BL/6J) and low (C3H/HeJ) strain paralleled enzyme activity and concentration, in agreement with a recent report. 5'DI-deficient mice had twofold higher serum free T4 but normal free T3 and thyrotropin. Brown adipose tissue 5'DII was invariant between the two strains. Southern analyses using a 5'DI probe identified a restriction fragment length variant that segregated with 5'DI activity in 33 of 35 recombinant inbred strains derived from four different pairs of high and low activity parental strains. Recombination frequencies using previously mapped loci allowed assignment of the 5'DI gene to mouse chromosome 4 and identified its approximate chromosomal position. We propose the symbol Dio1 to denote the mouse 5'DI gene. Conserved linkage between this segment of mouse chromosome 4 and human HSA1p predicts this location for human Dio1.

Impairment of the selenoenzyme type I iodothyronine deiodinase in C3H/He mice

Type I iodothyronine deiodinase (ID-I) activity is impaired in C3H/He (C3H) mice compared with BALB/c and C57BL/6N (C57) mice. In this study we compared ID-I activity and protein labeling with N-bromoacetyl(-)[125I]T3 (BrAc[125I]T3) or 75Se in liver microsomes of C3H and C57 mice. Hepatic ID-I activity in C3H mice was highly variable with a median of only 18% of that in C57 mice. However, C3H mice had normal serum T4 and T3 levels, although serum reverse T3 was increased. The 28-kilodalton (kDa) ID-I protein was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of BrAc[125I]T3-labeled microsomes. Labeling of this protein was virtually undetectable in C3H samples with low enzyme activity. ID-I activity in liver microsomes was strongly decreased in Se-deficient mice, which was paralleled by a drastic decrease in BrAc[125I]T3-labeling of the 28-kDa band compared with control mice. Labeling of ID-I with 75Se was demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of liver microsomes of [75Se]selenite-injected mice. 75Se labeling of the 28-kDa band was markedly higher in Se-deficient than in control mice and was also markedly higher in C57 than in C3H mice. Finally, liver ID-I messenger RNA (mRNA) was measured on Northern blots using a rat ID-I complementary DNA probe. Messenger RNA levels correlated strongly with ID-I activity, showing a significant decrease in C3H mice. We conclude that in mice, like in rats and humans, ID-I is a selenoprotein. ID-I activity is impaired in C3H mice because of decreased transcription of the ID-I gene or reduced stability of the mRNA.

A brief review of the Ah locus

The Ah locus, first described as a functional polymorphism among inbred strains of mice, encodes the Ah receptor--a ligand dependent transcriptional activator. This paper reviews the work on the Ah receptor and its importance in the expression of cytochrome P-450IA1 and the pleiotropic effects of halogenated aromatic hydrocarbons.

Cloning of the Ah-receptor cDNA reveals a distinctive ligand-activated transcription factor

A cDNA encoding the murine Ah receptor (Ahb-1 allele for aromatic hydrocarbon responsiveness) has been isolated and characterized. Analysis of the deduced protein sequence revealed a region with similarity to the basic region/helix-loop-helix (BR/HLH) motif found in many transcription factors that undergo dimerization for function. In addition to the BR/HLH domain, the N-terminal domain of the Ah receptor has extensive sequence similarity to the human ARNT (aryl hydrocarbon receptor nuclear translocator) protein and two regulatory proteins of Drosophila, Sim and Per. Photoaffinity labeling and peptide mapping studies indicate that the Ah receptor binds agonist at a domain that lies within this conserved N-terminal domain. The Ah receptor appears to be a ligand-activated transcription factor with a helix-loop-helix motif similar to those found in a variety of DNA-binding proteins, including Myc and MyoD.

Cloning of the Ah-receptor cDNA reveals a distinctive ligand-activated transcription factor

A cDNA encoding the murine Ah receptor (Ahb-1 allele for aromatic hydrocarbon responsiveness) has been isolated and characterized. Analysis of the deduced protein sequence revealed a region with similarity to the basic region/helix-loop-helix (BR/HLH) motif found in many transcription factors that undergo dimerization for function. In addition to the BR/HLH domain, the N-terminal domain of the Ah receptor has extensive sequence similarity to the human ARNT (aryl hydrocarbon receptor nuclear translocator) protein and two regulatory proteins of Drosophila, Sim and Per. Photoaffinity labeling and peptide mapping studies indicate that the Ah receptor binds agonist at a domain that lies within this conserved N-terminal domain. The Ah receptor appears to be a ligand-activated transcription factor with a helix-loop-helix motif similar to those found in a variety of DNA-binding proteins, including Myc and MyoD.

Purification and N-terminal amino acid sequence of the Ah receptor from the C57BL/6J mouse

The Ah receptor is a presumed member of the superfamily of steroid/thyroid hormone receptors, a trace soluble protein present in a wide variety of vertebrate species that mediates the biological effects of halogenated aromatic hydrocarbons. In this paper, we report the purification to homogeneity of this protein (from the liver of C57BL/6J mice) and its N-terminal amino acid sequence. Selective covalent labeling of the Ah receptor in hepatic cytosol with the photoaffinity ligands 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin simplified identification and quantitation of the receptor and permitted purification under denaturing conditions. Photoaffinity-labeled hepatic cytosol was applied to a phosphocellulose column at 80 mM NaCl, and the fraction enriched with the Ah receptor eluted with 225 mM NaCl. The eluate was diluted to 150 mM NaCl and applied to a DEAE-cellulose column, and the enriched fraction eluted with 300 mM. These two ion exchange chromatography steps usually gave approximately 100-fold enrichment and 40-50% recovery of Ah receptor. The dilute protein in the eluate was precipitated with n-propanol/trichloroacetic acid and solubilized in formic acid. The sample was then subjected to three successive rounds of high performance liquid chromatography on C4 reverse phase columns. The final, shallow-gradient chromatography was able to resolve the unlabeled 95-kDa receptor protein from the later eluting 125I-photoaffinity-labeled protein. The pooled high performance liquid chromatography fractions subjected to electrophoresis on sodium dodecyl sulfate-polyacrylamide gels contained only the 95-kDa band upon staining with Coomassie blue R250 or silver. Using the above protocol, the Ah receptor was purified greater than 150,000-fold, to apparent homogeneity, with an overall yield of 3-5%. The N-terminal amino acid sequence of the purified peptide was determined to be ala/asp-ser-Arg-Lys-arg-Lys-Pro-Val-Gln-Lys-Thr-Val-Lys-Pro-Ile-Pro-Ala- Glu-Gly--Ile-Lys-ser-Asn-Pro-ser-Lys- (where the lowercase indicates a residue determined with less confidence).

Characterization of polyclonal antibodies to the Ah receptor prepared by immunization with a synthetic peptide hapten

A synthetic peptide based on the N-terminal amino acid sequence of the Ah receptor purified from C57BL/6J mice, linked to keyhole limpet hemocyanin, proved a remarkably good immunogen. All six rabbits that were immunized produced polyclonal antiserum that reacted with the synthetic peptide and the denatured and undenatured Ah receptor. Western blots were especially useful for antibody characterization; hepatic cytosol from C57BL/6J mice, in which the Ah receptor was photoaffinity labeled with 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin, was resolved by gel electrophoresis and electrotransferred to nitrocellulose. Co-incidence of the major band immunochemically stained with immunoaffinity-purified antibodies (with apparent Mr = 95,000) and the radiolabeled band on the autoradiograph indicated the specificity of the antibody. The estimated sensitivity of detection of the Ah receptor on a blot is 60 to 120 pg/200 micrograms of protein/gel lane. On Western blots, the antipeptide antibodies stained the photoaffinity-labeled Ah receptor from all four murine variants and all vertebrate forms examined (chicken, rodents, monkey, human), indicating conservation of these N-terminal epitopes. The immunoaffinity-purified antibodies also immunoprecipitated undenatured photoaffinity-labeled Ah receptor from diluted cytosol.

Characterization and strain distribution pattern of the murine Ah receptor specified by the Ahd and Ahb-3 alleles

Two allelic forms of the Ah receptor have been previously identified by covalent labeling of the hepatic cytosol fractions of inbred strains of mice with the photoaffinity ligand 2-azido-3-[125I] iodo-7,8-dibromodibenzo-p-dioxin and resolution of the labeled protein by denaturing gel electrophoresis: 1) a Mr 95,000 protein encoded by the Ahb-1 allele carried by the C57 and C58 family of mice, and 2) a Mr 104,000 protein encoded by the Ahb-2 allele present in other common inbred strains that are responsive to aromatic hydrocarbons (e.g., C3H/He, BALB/cBy, and A). In this report, 125I-photoaffinity labeling is used to characterize two further murine variants and the strains that carry them: 1) the low affinity Ah receptor (Ahd allele) in strains that are nonresponsive to aromatic hydrocarbons and 2) a newly identified, high affinity variant (Ahb-3) found in several strains recently derived from feral mice. The low affinity Ah receptor has been recently characterized by reversible ligand binding by Okey et al. [Mol. Pharmacol. 35:823-830 (1989)], through the inclusion of sodium molybdate in the buffers during tissue preparation and ligand incubation to stabilize the receptor. Examination of the Ah receptor in hepatic cytosol from 18 strains of mice carrying the Ahd allele, by preparation in molybdate and photoaffinity labeling, revealed that all strains express a Mr 104,000 protein. Tissue preparation in 20 mM sodium molybdate and subsequent dilution of the molybdate to approximately 0.5 mM during ligand incubation was found to enhance photoaffinity labeling of the high and low affinity allelic forms of the Ah receptor. A new variant of the receptor (Ahb-3) expressing a Mr 105,000 protein was detected in Mus molossinus, hortulanus, pahari, spretus, and caroli but was absent from the strains of Mus musculus or domesticus that were examined. Allelic variants were also distinguishable by thermolability, i.e., the half-life of specific ligand binding capacity upon incubation at 35 degrees. For the Ahb-1 allele (Mr 95,000) the t 1/2 (thermostability) is 20-30 min, for the Ahb-2 allele (Mr 104,000) the t 1/2 is 3-6 min, and for the Ahb-3 allele (Mr 105,000) the thermolability is intermediate.

Pharmacokinetics of [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin in mice: analysis with a physiological modeling approach

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent inducer of hepatic microsomal enzymes. The influence of an inducing dose of TCDD on tissue distribution and other pharmacokinetic behavior of a TCDD analog in the mice was examined by employing a high specific activity radioligand. [125I]-2-iodo-3,7,8-trichlorodibenzo-p-dioxin (ITCDD). Female C57BL/6J mice were pretreated with 0.1 mumol/kg of TCDD or the vehicle only, followed by 0.1 nmol ITCDD/kg 3 days later. The control animals had the highest concentration of ITCDD-derived radioactivity in the fat, but the TCDD-pretreated animals had the highest concentration in their livers. Whole-body elimination of ITCDD approximated first-order behavior, and induction by pretreatment with the inducing dose of TCDD almost doubled the rate of excretion (control mice, t1/2 = 14.2 days; pretreated mice, t1/2 = 8.0 days). All disposition results in naive and pretreated mice were satisfactorily described by a consistent physiologically based pharmacokinetic model (Leung et al., 1988a) in which induction increased the amount of microsomal ITCDD-binding protein from 1.75 to 20 nmol/liver and increased the rate constant for metabolism of free ITCDD from 1 to 3/hr/kg liver. The binding affinity of the microsomal ITCDD-binding protein was the same (20 nM) in both induced and noninduced mice. Model simulations indicated a time delay in the elimination of nonparent ITCDD metabolites from the body and a more rapid absorption of the parent ligand in the pretreated mice. Consistent with previous physiological modeling with TCDD in different mouse strains, the primary factor influencing the liver/fat concentration ratio appears to be the affinity and capacity of the microsomal TCDD-binding proteins, which are altered by induction. These dose-dependent pharmacokinetic differences with ITCDD are important considerations for TCDD risk assessment in which data from high dose rodent experiments are extrapolated to predict behavior at much lower environmental concentrations in exposed humans.

Stimulation of in vivo hepatic uptake and in vitro hepatic binding of [125I]2-lodo-3,7,8-trichlorodibenzo-p-dioxin by the administration of agonist for the Ah receptor

[125I]2-lodo-3,7,8-trichlorodibenzo-p-dioxin ([125I]Cl3DpD), a radiolabeled, isosteric, analogue of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was synthesized and used to study in vivo tissue localization and in vivo tissue binding. Twenty-four hours after the administration of a tracer dose (1 X 10(-10) mol/kg) of [125I] Cl3DpD to C57BL/6J mice, the hepatic concentration of radioactivity was 1-2% of the administered dose, whereas in mice pretreated with TCDD (1 X 10(-7) mol/kg), the hepatic accumulation of radiolabel was 25-30% of that administered. Liver homogenate from TCDD-treated mice bound 4 to 10 times more [125I]Cl3DpD than homogenate from control mice. The enhancement of in vivo uptake and in vitro tissue binding of [125I]Cl3DpD by TCDD administration was confined to liver and was not observed in other tissues examined, kidney, lung, spleen, small intestines, and muscle. The administration of TCDD to C57BL/6J mice produces dose-related stimulation of in vivo hepatic uptake of [125I]Cl3DpD, binding of radioligand to liver homogenate, and hepatic aryl hydrocarbon hydroxylase activity, with the dose for half-maximal stimulation, ED50, varying from 1.5 to 4.0 x 10(-9) mol/kg. In congenic C57BL/6J (Ahd/Ahd) mice, which express the lower affinity Ah receptor, the ED50 values for all three responses were shifted to approximately 10-fold higher doses. 3,3',4,4',5,5'-Hexabromobiphenyl, a weak agonist for the Ah receptor produced a dose-related stimulation of these three responses in C57BL/6J mice (ED50 values of approximately 5 X 10(-7) mol/kg), but was without effect in C57BL/6J (Ahd/Ahd) mice. Stimulation of vivo hepatic uptake and in vitro liver homogenate binding of [125I]Cl3DpD was produced by administration of Ah agonists, such as 2,3,7,8-tetrachlorodibenzofuran and beta-naphthoflavone, but inactive congeners and other compounds that do not act via the Ah receptor, e.g. phenobarbital and pregnenolone-16 alpha-carbonitrile, did not evoke these effects. Thus, TCDD and other Ah agonists act through the Ah receptor to increase a liver binding species that increases the hepatic uptake of [125I]Cl3DpD in vivo and binding of this radioligand to liver homogenate in vitro.

[125I]2-iodo-3,7,8-trichlorodibenzo-p-dioxin-binding species in mouse liver induced by agonists for the Ah receptor: characterization and identification

The admininistration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to C57BL/6J mice produces a dose-related increase in the hepatic uptake of [125I]2-iodo-3,7,8-trichlorodibenzo-p-dioxin ([125I]Cl3DpD) in vivo and the binding of the radioligand to liver homogenate in vitro [Mol. Pharmacol. 36: 121-127 (1989)]. The TCDD-induced hepatic binding species was found to be predominantly in the microsomal fraction and was inactivated by heating at 60 degree, trypsin, and mercurials. The TCDD-induced binding species was found to have an apparent equilibrium dissociation constant, KD, ([125I]Cl3DpD) of 56 +/- 16 nM and a pool size, Bmax, of 22 +/- 5 nmol/g of liver. A number of halogenated dibenzo-p-dioxins, biphenyls, and polycyclic aromatic hydrocarbons compete with [125I]Cl3DpD binding to this species; all are aromatic and planar. The distinctive profile of this binding species, a protein of large pool size induced in the microsomal fraction of liver but not other tissues and induced by agonists for the Ah receptor, suggested that this moiety might be cytochrome P3-450. The coincidence of the major microsomal species covalently labeled with the photoaffinity ligand [125I]2-iodo-3-azido-7,8-dibromodibenzo-p-dioxin and that immunochemically stained with polyclonal antiserum that binds to cytochrome P3-450 confirms this hypothesis. This is a novel role for a cytochrome P-450 isozyme, as an induced sequestration site that enhances the hepatic localization of the agonist drug.

A competitive binding assay for 2,3,7,8-tetrachlorodibenzo-p-dioxin and related ligands of the Ah receptor

A sensitive competitive binding assay for the detection of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other ligands of the Ah receptor was developed using a stable preparation of the Ah receptor, the 40-55% ammonium sulfate fraction of liver cytosol from C57BL/6J mice, and the radioligand [125I]2-iodo-7,8-dibromodibenzo-p-dioxin (specific radioactivity, 2176 Ci/mmol, and binding affinity, KD = 6.5 pM). Conditions are described which maximize assay precision and sensitivity, while minimizing sample counting time, ensuring ligand solubility, and permitting attainment of binding equilibrium for competing ligands. Assay conditions were developed to allow calculation of the binding affinity for competing ligands and to ensure that an unknown competitor could be quantified in terms of "TCDD binding equivalents." Standard assay conditions consisted of incubation of 8 pM radioligand and 18-20 pM Ah receptor with 5-1000 pM TCDD, in a 1-ml volume, for 16 hr at 4 degrees. Statistical analysis of the standard curve of bound radioligand versus the log of the concentration of competing TCDD indicated the minimal detectable concentration of TCDD to be 10 pM (3.2 pg in a 1-ml assay alpha less than or equal to 0.01). The simplicity, sensitivity, and reproducibility of this competitive binding assay should prove useful as a screen to detect planar halogenated aromatic hydrocarbons and other ligands of the Ah receptor. The availability of this 125I-labeled dioxin congener also permitted the characterization of Ah receptor-ligand binding over a range of ligand and receptor concentrations not possible with currently available 3H-ligands.

Kinetic and equilibrium studies of Ah receptor-ligand binding: use of [125I]2-iodo-7,8-dibromodibenzo-p-dioxin

In this report, we have used the radioligand [125I]2-iodo-7,8-dibromo-dibenzo-p-dioxin to describe the kinetics of ligand binding to the Ah receptor prepared from C57BL/6J mouse liver. The higher specific activity of this radioligand (2176 Ci/mmol), compared with the usual tritiated ligand [1,6-3H]2,3,7,8-tetrachloro-dibenzo-p-dioxin (58 Ci/mmol) permitted the study of ligand-receptor interactions at much lower component concentrations. For this radioiodinated ligand, Scatchard analysis of saturation binding curves, determined at six different protein concentrations, indicated that the apparent equilibrium dissociation constant, KD, was directly related to the dilution of the receptor preparation; for example, at 1160 micrograms of protein/ml, KD = 1.6 x 10(-10) M; at 36 micrograms of protein/ml, KD = 1.2 x 10(-11) M. Extrapolation of this function to infinite receptor dilution yielded KD = 6 x 10(-12) M. The addition of 70 micrograms/ml of bovine serum albumin to a receptor preparation of 30 micrograms of protein/ml produced a 10-fold decrease in the slope of the Scatchard plot (i.e., 10-fold increase in the apparent KD). Conversely, enrichment of the receptor by high performance liquid chromatography led to an increased slope and thus decreased estimate of KD. The association rate constant (k1), calculated from the integrated second-order rate equation, was 2.8 x 10(10) M-1 hr-1 and, from the initial velocity equation, had a value of 5.25 x 10(10) M-1 hr-1. The dissociation rate constant was biphasic, consisting of a predominant fast component with a rate constant of 0.36 hr-1 (k-1) and a slower component with a rate constant of 4.2-9.4 x 10(-3) hr-1 (k-2). Higher protein concentrations produced a decrease in estimates of k1 but not k-1 or k-2. The KD determined from the ratio of the kinetic rate constant, k-1/k1 = 6.9 x 10(-12) M, is in excellent agreement with that derived from the results of equilibrium binding experiments extrapolated to infinite dilution, KD = 6 x 10(-12) M. The decrease in KD, observed in equilibrium binding studies upon dilution of the receptor preparation, is best explained by a more accurate classification of "free" radioligand at lower protein concentrations. Finally, ligand binding to the Ah receptor is best described by a two-step process, the formation of an initial complex, characterized by rapid ligand dissociation, which undergoes transformation to a second distinct complex displaying a much slower ligand dissociation rate.

Purification of the Ah receptor from C57BL/6J mouse liver

The photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, previously has been shown to selectively label two peptides in the cytosol fraction of C57BL/6J mouse liver: a 95-kDa peptide, the ligand binding moiety of the Ah receptor, and a 70-kDa proteolytic fragment formed from the larger peptide (Poland, A., Glover, E., Ebetino, F. H., and Kende, A.S. (1986) J. Biol. Chem. 261, 6352-6365). These two peptides were partially purified to an approximately 20,000-fold enrichment with a 15-20% yield by the following scheme: 1) photoaffinity labeling of the 35-55% ammonium sulfate fraction of liver cytosol; 2) chromatography on polyethyleneimine-Sepharose coupled at low charge density and heparin/Mn2+ precipitation of the dilute column eluate; 3) DEAE-Sepharose chromatography to remove heparin; 4) chromatography on heparin-Sepharose; 5) preparative sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis followed by electroelution of the protein and ion pair extraction to remove sodium dodecyl sulfate; and 6) high performance liquid chromatography on a reverse-phase C-4 column. Following initial chromatography on polyethyleneimine Sepharose, it was found that substantial subsequent purification could only be achieved under denaturing conditions.

Ca2+-dependent proteolysis of the Ah receptor

We previously reported (J. Biol. Chem. (1986) 261, 6352-6465) that the photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, upon incubation with the liver cytosol fraction from C57BL/6 mice, labeled in a 1:1 ratio two peptides that had apparent molecular masses of 95 and 70 kDa and similar proteolytic fragmentation patterns. In the cytosolic fraction of Hepa 1 cells, a cloned murine hepatoma cell line, the product of photoaffinity labeling is almost exclusively a 95-kDa peptide which is rapidly hydrolyzed by a Ca2+-dependent proteinase to a 70-kDa peptide as well as other fragments. Thus, the ligand binding unit of the Ah receptor in C57BL/6 mouse liver and Hepa 1 cell is a 95-kDa peptide, and the 70-kDa fragment is a proteolytic artifact. The Ca2+-dependent proteinase which hydrolyzes the 95-kDa peptide has the properties of calpain II: (i) an absolute requirement for Ca2+, with maximal activity at 0.5 to 1.0 mM Ca2+; (ii) a pH optimum of 7.5 to 8.0; (iii) inhibition by EDTA, iodoacetamide, leupeptin and L-trans-epoxysuccinylleucylamido(4-guanidino)butane, but not by soybean trypsin inhibitor, aprotinin, or phenylmethanesufonyl fluoride. Upon chromatographic separation of the liver cytosol of C57BL/6 mice on DEAE-Sephacel, Ca2+-dependent proteinase activity (using casein or the labeled 95-kDa peptide as substrates) elutes with 0.25 M NaCl, and a specific proteinase inhibitor elutes with 0.15 M NaCl. Ca2+-dependent proteinase activity that hydrolyzes the 95-kDa peptide is found in the liver cytosols of several mammalian species.

The murine Ah locus: a new allele and mapping to chromosome 12

The Ah locus in mice, the presumed structural gene for the Ah receptor, is polymorphic in mice: some inbred strains carrying the Ahb allele express a high affinity receptor and are sensitive to receptor agonists, while other strains carrying the Ahd allele express a lower affinity receptor and are less sensitive to agonists. Using the photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, we have identified two allelic forms of the high affinity receptor (Ahb). In mouse strains of the C57 and C58 family and MA/MyJ, the photoaffinity labeled peptide in hepatic cytosol has an apparent molecular mass of 95 kDa (Ahb-1 allele), whereas in other responsive strains (e.g., C3H/HeJ, BALB/cByJ, A/J) the labeled peptide has an apparent mass of 104 kDa (Ahb-2 allele). In genetic crosses and backcrosses between C57BL/6J and C3H/HeJ mice, the expression of these peptides is consistent with codominant inheritance of two alleles at a single locus. From Scatchard plots of the specific binding of the reversible ligand, [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin, to hepatic cytosol, most strains of mice with the Ahb-1 allele were found to have an Ah receptor with a binding affinity, KD, of 0.4-0.7 nM, and a receptor concentration of 130-160 fmol/mg of protein, whereas most strains carrying the Ahb-2 allele have a slightly lower receptor affinity, KD = 0.8-1.3 nM, and a slightly lower receptor concentration, 80-110 fmol/mg of protein. From analysis of the strain distribution pattern of the Ahb-1 and Ahb-2 alleles in recombinant inbred strains, the Ah locus was linked to several unmapped loci (H-17, H-38, Ltw-2, Ly-18, D1Nyul, D1Nyu2), and this entire group mapped to chromosome 12.

Variation in the molecular mass of the Ah receptor among vertebrate species and strains of rats

The Ah receptor in eight vertebrate species was characterized by labeling the cytosolic fraction of tissue with the photoaffinity ligand, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, and analysis of the products by denaturing gel electrophoresis. The apparent molecular mass of the dominant labeled peptide showed appreciable species variation: mouse-95 kDa; chicken (embryo)-101 kDa; guinea pig-103 kDa; rabbit-104 kDa; rat-106 kDa; human-106 kDa; monkey-113 kDa, and hamster-124 kDa. Seven inbred strains of rats, had a Ah receptor ligand binding peptide of 106 kDa; however outbred Long-Evans rats were shown to be polymorphic expressing a 101 kDa and/or 106 kDa allelic forms. The notable frequency of structural variation in the Ah receptor is in contrast to the analogous highly conserved steroid hormone receptors.

Photoaffinity labelling of the Ah receptor

A series of halodibenzo-p-dioxins bearing the arylazide photolabile functional group were synthesized and tested as photoaffinity labels for the Ah receptor. 2-Azido-3-iodo-7,8-dibromodibenzo-p-dioxin (KD = 0.76 X 10(-9) M) was selected for radiosynthesis. Analysis of the 125I-photoaffinity-labelled proteins in mouse-liver cytosol by denaturing gel electrophoresis revealed two peptides which had apparent molecular masses of 95,000 and 70,000 daltons respectively, were labelled in an approximately 1:1 ratio and were selectively labelled at low concentrations of the photoaffinity ligand (0.05 KD = 0.04 X 10(-9) M). In addition, their labelling was inhibited by co-incubation with an excess of unlabelled ligand. On chromatographic separation under non-denaturing conditions, these two peptides co-migrated. These studies suggest that the Ah receptor in mouse liver cytosol is a heterodimer composed of two non-covalently bound peptides (95 K and 70 K) which each have a ligand binding site.

Photoaffinity labeling of the Ah receptor

A series of halodibenzo-p-dioxins with the photolabile aryl azide functional group were synthesized and screened as potential photoaffinity labels for the Ah receptor, and 2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin was selected for radiosynthesis with 125I (specific activity 2176 Ci/mmol, equilibrium dissociation constant, KD = 0.76 nM). Following incubation of this 125I-labeled photoaffinity ligand with the protamine sulfate-precipitated fraction of C57BL/6J mouse liver cytosol, and irradiation with long wavelength ultraviolet light, the radiolabeled macromolecules were precipitated with acetone and analyzed by denaturing gel electrophoresis and autoradiography. Among the labeled products, two peptides with apparent molecular masses of 95,000 and 70,000 daltons had the following properties: 1) they were selectively labeled at low ligand concentrations; 2) they were labeled in approximately a 1:1 ratio; 3) co-incubation with receptor agonists inhibited the photoaffinity labeling of both peptides to a similar extent, and structure activity relationship for inhibition of labeling by these agonists corresponded to that for their binding affinity to the Ah receptor; 4) upon nondenaturing chromatographic separation of photoaffinity labeled cytosol on high performance liquid chromatography size exclusion and anion exchange columns, the 95- and 70-kDa peptides coelute; 5) the migration of these peptides upon denaturing electrophoresis is the same in the presence or absence of a thiol reducing agent; and 6) proteolysis of the 95- and 70-kDa peptides produces a similar pattern of cleavage peptides. The simplest structure of the Ah receptor in mouse liver cytosol, appears to be a dimer composed of two noncovalently linked subunits of apparent molecular masses of 95 and 70 kDa, which have homologous structure and similar ligand binding sites, but other possibilities are discussed.

Structure-activity relationship of bispyridyloxybenzene for induction of mouse hepatic aminopyrine N-demethylase activity. Chemical, biological, and X-ray crystallographic studies

1,4-bis-[2-(3,5-Dichloropyridyloxy)]-benzene (TCPOBOP) was previously shown to be an extremely potent phenobarbital-like inducer of hepatic microsomal monooxygenase activity in the mouse. To examine the structure-activity relationship, 31 congeners of TCPOBOP were synthesized and tested for their potency to induce hepatic aminopyrine N-demethylase activity in B6D2F1/J mice. For biological activity, the minimum requirement is a) a central 1,4-dioxygenated benzene ring, b) lateral pyridine rings linked to the central ring by ether bonds, but with other lateral heteroaromatic rings, e.g., quinoline or pyrimidine, also active, c) 5,5'-substituents of Cl, Br, or NO2 on the pyridine rings. For a series of 5,5'-substituted and 3,3'-dichloro,5,5'-substituted bispyridyloxybenzenes, no correlation was observed for Hansch pi and sigma p values. To account for this lack of correlation and conformational variability produced by the two ether bonds, we performed x-ray structure determinations on three compounds: a) TCPOBOP, b) the 5,5'-dichloro analogue, and c) the biologically inactive, 3,3'-dichloro analogue. In the two biologically active congeners the positioning of the pyridine rings is anti to the plane of the central benzene ring, and the dihedral angle between the central ring and the pyridines is approximately 60 degrees. In the inactive analogue the pyridine rings are syn and the dihedral angle is 84 degrees. The x-ray crystallographic data are consistent with the ether oxygen having an sp2-bonding conjugating with the heterodipolar bond of the pyridine C(2)--N(1), which strongly restricts rotation about the ether bonds. The potency of TCPOBOP and other bispyridyloxybenzene analogues to induce a phenobarbital-like pleiotropic response and the sharply defined structure-activity relationship among these congeners support the hypothesis that they act by binding to a specific recognition site.

Histologic changes produced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in the skin of mice carrying mutations that affect the integument

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) produces epidermal hyperplasia and hyperkeratosis, squamous metaplasia of the sebaceous gland, and keratinized cyst formation in 8 strains of mice with the recessive mutation, hairless (hr/hr). The extent of these histologic changes is dependent on the genetic background. No cutaneous lesions are produced in haired (hr/+) mice. In examination of mice with 7 other mutations affecting the integument, TCDD produced similar histologic skin changes in cryptothrix, nude, plucked, and atrichosis; a marginal squamous metaplasia of sebaceous glands in Repeated epilation, and had no effect in fur deficient and Naked mutants. These genetically determined epidermal responses are discussed in light of the mechanism of action of TCDD.

2,3,7,8-Tetrachlorodibenzo-p-dioxin: examination of biochemical effects involved in the proliferation and differentiation of XB cells

XB, a cell line derived from a mouse teratoma, differentiates into stratified squamous epithelium when incubated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To examine the possible biochemical mediators of this response, we compared the effects produced by TCDD to those elicited by other compounds which stimulate epidermal proliferation and/or differentiation in mice. XB/3T3 cultures keratinize when incubated with cholera toxin, epidermal growth factor (EGF), or TCDD, but not 12-0-tetradecanoylphorbol-13-acetate (TPA). Incubation of XB cells with TCDD (10(-9)M) for 48 hours produces a 20% increase in thymidine incorporation, a response which is neither as large nor as rapid as that produced by cholera toxin, TPA, or EGF. Although both cholera toxin and TCDD stimulate differentiation and thymidine incorporation in XB/3T3 cultures, cholera toxin increases cAMP 30-fold in these cells, while TCDD does not affect cAMP accumulation at any of the times studies (15 min to 120 hours). Inhibitors of arachidonic acid metabolism, which block epidermal proliferative responses to TPA in vivo, do not prevent the differentiation of XB cells in response to TCDD. In XB/3T3 cultures, TPA stimulates arachidonic acid release at all times tested (1,6, and 24 hours) and increases the incorporation of 32Pi into total phospholipids and phosphatidylcholine after 3 hours. In contrast, TCDD affects neither arachidonic acid release nor the turnover of phosphatidylinositol or phosphatidylcholine at any of the times tested. Although we examined biochemical effects which have been suggested as part of the mechanism of TCDD and which are produced by other epidermal proliferative compounds in XB cells, no mediator of the TCDD-produced differentiation of XB/3T3 cultures was observed.

Effect of the brachymorphic trait in mice on xenobiotic sulfate ester formation

Mice carrying the recessive mutation brachymorphic have been shown previously to have a reduced capacity to synthesize 3'-phosphoadenosine-5'-phosphosulfate (PAPS), the required coenzyme in sulfation reactions [K. Sugahara and N. Schwartz, Proc. natn. Acad. Sci. U.S.A. 76, 6615 (1979)]. The capacity of the liver cytosol fractions from brachymorphic (bm/bm) mice or their phenotypically normal littermates (+/+ or +/bm) to catalyze the formation of sulfate esters of [3H]estrone and [14C]p-nitrophenol in vitro was determined. When PAPS was added to the reaction, the rates of sulfate ester formation catalyzed by the two cytosol fractions were similar. In contrast, when PAPS was generated in situ from ATP and SO(4)2-, the rates of sulfate ester formation catalyzed by the brachymorphic cytosol were only 4-22% of the rates catalyzed by the cytosol fraction from normal mice. The hepatic cytosol fraction from brachymorphic mice incorporated less 35SO(4)2- into PAPS than that catalyzed by cytosol of normal mice. [14C]p-Nitrophenol (1.5 mumoles/kg) was eliminated from brachymorphic and normal mice as urinary conjugates; in normal mice, 73% of the urinary radioactivity was p-nitrophenyl sulfate, while in the brachymorphic mice only 33% of the urinary excretion was the sulfate ester. Brachymorphic mice have a reduced capacity for synthesizing sulfate esters of xenobiotics in vitro and in vivo, which is attributable to their reduced synthesis of PAPS.

Strong evidence from studies with brachymorphic mice and pentachlorophenol that 1'-sulfoöxysafrole is the major ultimate electrophilic and carcinogenic metabolite of 1'-hydroxysafrole in mouse liver

The role of sulfation of 1'-hydroxysafrole in the formation of hepatic macromolecular adducts and in hepatic tumor formation in mice given 1'-hydroxysafrole was investigated by the use of: (a) mice treated with the specific sulfotransferase inhibitor pentachlorophenol; and (b) brachymorphic mice, which are characterized by a deficiency in the hepatic synthesis of 3'-phosphoadenosine 5'-phosphosulfate. Cytosolic sulfotransferase activity for 1'-hydroxysafrole in both mouse and rat liver was significantly inhibited by 10 microM pentachlorophenol, usually by greater than 90%. Prior administration of nontoxic amounts of pentachlorophenol, either in the diet of adult female CD-1 mice or by i.p. injection of 12-day-old male C57BL/6 X C3H F1 (hereafter called B6C3F1) mice, resulted in an 85% decrease in the level of adducts formed from 1'-hydroxysafrole in hepatic DNA and RNA as compared to those of non-pentachlorophenol-treated animals. Likewise, the chronic administration of a nontoxic level of pentachlorophenol in the diet of adult female CD-1 mice strongly inhibited hepatic tumor induction by long-term dietary administration of either safrole or 1'-hydroxysafrole. Initiation of hepatic tumors by a single i.p. injection of 1'-hydroxysafrole to 12-day-old male B6C3F1 mice was strongly inhibited by prior treatment with pentachlorophenol. Under these conditions, the hepatocarcinogenicity of diethylnitrosamine was not inhibited by pentachlorophenol. Supplementation with adenosine triphosphate and sulfate of hepatic cytosols from adult female or 12-day-old brachymorphic progeny of a B6C3 background outbred to B6C3F1 mice (B6C3F2), of either sex, resulted in 5- to 10-fold less binding of 1'-hydroxysafrole to added RNA than when cytosols from phenotypically normal B6C3F2 mice were used. On administration of [3H]-1'-hydroxysafrole to adult female or 12-day-old brachymorphic B6C3F2 mice of either sex, the levels of hepatic DNA and RNA adducts were 7- to 12-fold lower than those obtained in phenotypically normal B6C3F2 mice of the same age and sex. Brachymorphic mice were also much less responsive than their phenotypically normal littermates to the induction of liver tumors by 1'-hydroxysafrole; lower incidences were observed both when the carcinogen was fed chronically to adult females and when it was administered to males only prior to weaning. Thus, all of these data strongly support the conclusion that 1'-sulfoöxysafrole is the major ultimate electrophilic and tumor-initiating metabolite of 1'-hydroxysafrole.

Response of murine epidermis to 2,3,7,8-tetrachlorodibenzo-p-dioxin: interaction of the ah and hr loci

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons produce epidermal hyperplasia, hyperkeratosis and sebaceous gland metaplasia in the skin of mice bearing the recessive mutation (hr/hr) hairless. This response is mediated through the cytosol receptor protein: the structure-activity relationship for receptor binding corresponds to that for production of the skin lesion, and these histopathological changes segregate with the genetic polymorphism at the Ah locus, the locus determining the cytosol receptor. In HRS/J mice, an inbred strain segregating for the hr locus, both hairless (hr/hr) and haired (hr/+) mice possess the high-affinity cytosol receptor and respond to TCDD with the induction of epidermal aryl hydrocarbon hydroxylase activity, a receptor-mediated biochemical response; however, only hr/hr mice develop the proliferative/metaplastic skin response. We propose a genetic model for the interaction of the Ah and hr loci, to account for the differential response to TCDD observed in the skin of HRS/J hr/hr and hr/+ mice.

2,3,7,8-tetrachlorodibenzo-p-dioxin and related halogenated aromatic hydrocarbons: examination of the mechanism of toxicity

In this review, we have examined the biochemical and toxic responses produced by halogenated aromatic hydrocarbons and have tried to develop a model for their mechanism of action. These compounds bind to a cellular receptor and evoke a sustained pleiotropic response. In many tissues this response consists of the expression of a battery of enzymes which are, for the most part, involved in drug metabolism, but in other tissues, those which develop toxicity, an additional set of genes is expressed which effects cellular involution, division, and/or differentiation. The toxicity of these compounds appears to be due to the sustained expression of a normal cellular regulatory system, of which we were previously unaware. In future investigations it is hoped that we will learn the nature and physiologic role of this regulatory system. Only then can we hope to understand the mechanism of toxicity of these compounds.