Ca2+-dependent proteolysis of the Ah receptor

Recombinant expression of aryl hydrocarbon receptor for quantitative ligand-binding analysis

Recombinant expression of the aryl hydrocarbon receptor (AhR) yields small amounts of ligand-binding-competent AhR. Therefore, Spodoptera frugiperda (Sf9) cells and baculovirus have been evaluated for high-level and functional expression of AhR. Rat and human AhR were expressed as soluble protein in significant amounts. Expression of ligand-binding-competent AhR was sensitive to the protein concentration of Sf9 extract, and coexpression of the chaperone p23 failed to affect the yield of functional ligand-binding AhR. The expression system yielded high levels of functional protein, with the ligand-binding capacity (Bmax) typically 20-fold higher than that obtained with rat liver cytosol. Quantitative estimates of the ligand-binding affinity of human and rat AhR were obtained; the Kd for recombinant rat AhR was indistinguishable from that of native rat AhR, thereby validating the expression system as a faithful model for native AhR. The human AhR bound TCDD with significantly lower affinity than the rat AhR. These findings demonstrate high-level expression of ligand-binding-competent AhR, and sufficient AhR for quantitative analysis of ligand binding.

The aryl hydrocarbon receptor complex and the control of gene expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. The toxicity of the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin is almost exclusively mediated through this receptor. However, the key alterations in gene expression that mediate toxicity are poorly understood. It has been established through characterization of AhR-null mice that the AhR has a required physiological function, yet how endogenous mediators regulate this orphan receptor remains to be established. A picture as to how the AhR/ARNT heterodimer actually mediates gene transcription is starting to emerge. The AhR/ARNT complex can alter transcription both by binding to its cognate response element and through tethering to other transcription factors. In addition, many of the coregulatory proteins necessary for AhR-mediated transcription have been identified. Cross talk between the estrogen receptor and the AhR at the promoter of target genes appears to be an important mode of regulation. Inflammatory signaling pathways and the AhR also appear to be another important site of cross talk at the level of transcription. A major focus of this review is to highlight experimental efforts to characterize nonclassical mechanisms of AhR-mediated modulation of gene transcription.

Specific blockage of ligand-induced degradation of the Ah receptor by proteasome but not calpain inhibitors in cell culture lines from different species

To firmly establish the pathway involved in ligand-induced degradation of the AHR, cell lines derived from mouse rat or human tissues were exposed to inhibitors specific to the proteasome or calpain proteases and exposed to TCDD. The level of endogenous AHR and CYP1A1 protein was then evaluated by quantitative Western blotting. Treatment of cells with the calpain inhibitors: calpeptin, calpain inhibitor III, or PD150606 either individually or in combinations up to 75 microM did not reduce TCDD-induced degradation of the AHR, the induction of endogenous CYP1A1 or the nuclear accumulation of the AHR. The activity of the inhibitors was verified with an in vivo calpain assay. In contrast, exposure of cells to the specific proteasome inhibitors: epoxomicin (1-5 microM), proteasome inhibitor I (5-10 microM) or lactacystin (5-15 microM) completely inhibited TCDD-induced degradation of the AHR. Inhibition of AHR degradation with these compounds did not reduce the induction of endogenous CYP1A1. In addition, exposure of the Hepa-1 line to the various proteasome inhibitors caused an accumulation of the AHR in the nucleus in the absence of TCDD exposure. Finally, Western blot analysis of the DNA bound AHR showed that its molecular mass was unchanged in comparison to the unliganded (cytoplasmic) AHR. Thus, these studies conclusively implicate the proteasome and not calpain proteases in the ligand-induced degradation of the mouse, rat and human AHR and suggest that the pharmacological use of proteasome inhibitors may impact the time course and magnitude of gene regulatory events mediated through the AHR.

The induction of CYP1A1 by oltipraz is mediated through calcium-dependent-calpain

The induction of CYP1A1 expression by oltipraz, a synthetic chemo-preventive agent, which increases intracellular calcium concentration, has previously been shown to result from transcriptional activation of CYP1A1 gene mediated by the Ah receptor (AhR), although oltipraz does not bind the receptor. The present study investigated the possible mechanisms of oltipraz-induced activation of AhR and the subsequent induction of CYP1A1 transcription. Treatment of the human metastatic breast cancer cell line MT-2 with oltipraz results in a concentration-dependent increase in the activity of the calcium-dependent calpain, as measured towards the BOC-LM-CMAC fluorescent substrate. This increase in calpain activity was coupled with the AhR activation, as evidenced by its nuclear localization and increased transcription of CYP1A1 gene. Treatment of cells with calpain specific inhibitor MDL 28170 completely blocked the oltipraz-induced nuclear translocation of AhR and subsequent CYP1A1 expression. Furthermore, treatment with oltipraz resulted in the classical ligand-dependent down-regulation of AhR protein, in a concentration dependent manner. The presented data established for the first time a mechanism of activating AhR and its transcription of CYP1A1 by oltipraz through activation of calcium-dependent calpain.

Calpain Mediates the Dioxin-Induced Activation and Down-Regulation of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated basic-helix-loop-helix transcription factor that binds polyaromatic hydrocarbons (PAH), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and mediates their toxicity. Binding of PAH to AhR in the cytoplasm triggers a poorly defined transformation step of the receptor into a nuclear transcription factor. In this study, we show that the calcium-dependent cysteine protease calpain plays a major role in the ligand-induced transformation and signaling of AhR. Fluorescence imaging measurements showed that TCDD treatment elevates intracellular calcium, providing the trigger for calpain activation, as measured toward t-butoxycarbonyl-Leu-Met-chloromethylaminocoumarin, a calpain-specific substrate. Inhibition of calpain activity by the N-benzyloxycarbonyl-Val-Phe-aldehyde (MDL28170) blocked the TCDD-induced nuclear translocation of AhR in Hepa1c1c7 mouse hepatoma cell line. Treatment of the human metastatic breast carcinoma cell line MT-2 with MDL28170 and 3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid (PD 150606), two calpain-selective inhibitors, completely abolished the TCDD-induced transactivation of AhR as assessed by transcription of CYP1A1 gene. Previous studies have established that after TCDD-induced transactivation, the AhR undergoes a massive depletion; we show here that selective calpain inhibitors can block this step, which suggests that the ligand-induced down-regulation of the AhR is calpain-dependent. The data presented support a major role for calpain in the AhR transformation, transactivation, and subsequent down-regulation, and provide a possible explanation for many of the reported phenomena of ligand-independent activation of AhR.

2,3,7,8-tetrachlorodibenzo-p-dioxin-induced degradation of aryl hydrocarbon receptor (AhR) by the ubiquitin-proteasome pathway. Role of the transcription activaton and DNA binding of AhR

Activation of the aryl hydrocarbon receptor (AhR) by 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), a potent agonist of AhR, induces a marked reduction in steady state AhR. To analyze the mechanism of regulation of ligand-activated AhR, we examined the biochemical pathway and function of the down-regulation of the receptor by TCDD. Pulse-chase experiments reveal that TCDD shortens the half-life (t1/2) of AhR from 28 to 3 h in mouse hepatoma cells. Inhibitors of the 26 S proteasome, lactacystin and MG132, block the TCDD-induced turnover of AhR. The TCDD-induced degradation of AhR involves ubiquitination of the AhR protein, because (a) TCDD induces formation of high molecular weight, ubiquitinated AhR and (b) degradation of AhR is inhibited in ts20 cells, which bear a temperature-sensitive mutation in the ubiquitin-activating enzyme E1, at a nonpermissive temperature. Inhibition of proteasomal degradation of AhR increases the amount of the nuclear AhR.Arnt complex and "superinduces" the expression of endogenous CYP1A1 gene by TCDD, indicating that the proteasomal degradation of AhR serves as a mechanism for controlling the activity of the activated receptor. We also show that deletion of the transcription activation domain of AhR abolishes the degradation, whereas a mutation in the DNA-binding region of AhR or Arnt reduces the degradation; these data implicate the transcription activation domain and DNA binding in AhR degradation. Our findings provide new insights into the regulation of TCDD-activated AhR through ubiquitin-mediated protein degradation.

Expression of the aryl hydrocarbon receptor (AhR) and the aryl hydrocarbon receptor nuclear translocator (ARNT) in fetal, benign hyperplastic, and malignant prostate

BACKGROUND

Androgen-dependent tissue has been reported to be affected by chemical ligands of the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, which heterodimerizes with the aryl hydrocarbon receptor nuclear translocator protein (ARNT).

METHODS

Fetal (n = 3), benign hyperplastic (BPH) (n = 10), and carcinomatous (CaP) (n = 19) prostate tissues were analyzed using immunohistochemistry. Western blot analysis was used to confirm the identity of the recognized proteins.

RESULTS

Immunoblotting of enriched prostatic epithelial cells (EC) and stromal cells revealed constitutive expression of bands at around 110 kDa and 90 kDa, using anti-AhR and anti-ARNT, respectively. Immunohistology of the fetal specimens revealed heterogeneous cytoplasmic and nuclear AhR expression of immature EC and mesenchymal cells. Constitutive expression of AhR (primarily cytoplasmic) and ARNT (nuclear and cytoplasmic) by the majority of adult basal and secretory EC, CaP, and smooth muscle cells was confirmed in situ. The most intense anti-AhR/-ARNT reactivity was found on smooth muscle cells, followed by EC and fibrocytes. Secretory BPH-EC revealed significantly decreased AhR expression when compared to normal tissue segments. By contrast, anti-AhR reactivity was frequently increased in the more dedifferentiated tumor areas.

CONCLUSIONS

These findings suggest that an undefined physiologic AhR ligand(s) as well as environmental factors may exert effects on EC and smooth muscle cells in the prostate through binding to these receptors.

Leukocyte activation induces aryl hydrocarbon receptor up-regulation, DNA binding, and increased Cyp1a1 expression in the absence of exogenous ligand

The aryl hydrocarbon receptor (AhR) functions as a transcription factor after ligand binding by halogenated aromatic hydrocarbons. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic halogenated aromatic hydrocarbon, is dependent on binding to the AhR to mediate a broad range of toxic effects. Immune suppression is one of the most sensitive sequela associated with TCDD exposure, yet, paradoxically, resting leukocytes express a relatively low amount of AhR. Here we report that activation of leukocytes produced a 6-fold increase in AhR steady state mRNA levels and a concordant increase in AhR protein expression. Furthermore, leukocyte activation induced AhR translocation, DNA binding to a dioxin response element, and CYP1A1 transcription in the absence of TCDD. Activated leukocytes exhibited an even greater enhancement of dioxin response element binding by the AhR in the presence of TCDD than in the absence of TCDD. These studies suggest that the mechanism responsible for the sensitivity of immunocompetent cells to TCDD may be directly associated with a marked increase in AhR expression, which accompanies leukocyte activation.

Identification of functional aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator in murine splenocytes

The objective of the present studies was to determine whether the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) protein are present and functional in B6C3F1 (C57BL/6 x C3H) mouse splenocytes. Northern analysis of poly(A) RNA isolated from splenocytes revealed transcripts of approximately 6.6 kb which hybridized to the AhR complementary DNA (cDNA) probe. Anti-AhR antibodies identified two major cytosolic forms of the AhR in splenocytes, approximately 95 and 104 kDa, corresponding to the codominately expressed Ahrb alleles in the B6C3F1 mice. Northern analysis utilizing an oligomer probe for ARNT identified three messenger RNA (mRNA) transcripts, approximately 5.6, 2.0, and 1.1 kb, in spleen which was consistent with the banding pattern observed in the B6C3F1 mouse liver. Western blotting confirmed the presence of the approximately 87 kDa ARNT protein in splenocytes. Protein quantitation by slot blot analysis demonstrated approximately 2.0-fold more AhR in liver than in splenocytes. Interestingly, ARNT was approximately 2.4-fold more abundant in splenocytes than in liver. Consistent with these results, comparison by quantitative reverse transcriptase-polymerase chain reaction analysis of AhR and ARNT transcripts in liver and splenocytes demonstrated approximately 2.3-fold more AhR transcripts in liver than in splenocytes and approximately 3.2-fold more ARNT transcripts in splenocytes than in liver. In addition, comparisons between AhR and ARNT transcripts isolated from the liver and splenocytes indicated a greater number of ARNT transcripts as compared with AhR in both preparations. TCDD treatment of splenocytes induced binding of the AhR nuclear complex to the dioxin-responsive enhancer (DRE) as detected by the electrophoretic mobility shift assay. These findings confirm that the AhR and ARNT are present in mouse splenocytes and are capable of binding to the DRE.

Localization of mRNAs for calpain and calpastatin in the adult rat brain by in situ hybridization histochemistry

The detailed localization of mRNAs for calpain II and calpastatin was examined in adult rat brain by in situ hybridization histochemistry. The expression patterns of the two mRNAs were similar to each other throughout the brain in terms of relative expression intensity, and almost all neurons expressed both mRNAs more or less. Among them, neurons in cranial nerve nuclei and some others in the brain stem expressed at relatively high levels, suggesting the high involvement of the non-lysosomal proteolytic system in the function of these neurons. On the other hand, the expression levels of the two mRNAs in non-neuronal cells including glia were basically low with the choroid plexuses expressing calpastatin mRNA relatively highly.

Early effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on rat thymocytes in vitro

TCDD is known to induce thymic atrophy in several mammalian species through activation of programmed cell death, or apoptosis. To investigate the time course of events which precede TCDD-induced thymic apoptosis in vitro, experiments were performed with thymocytes isolated from immature rats. Peak accumulation of both total and specifically bound [3H]TCDD was observed at 60 min post incubation. Incubation of cells with 10 nM TCDD resulted in significant increases in RNA polymerase activity and incorporation of [3H]uridine at 30 min, indicating increased RNA synthesis in response to TCDD. TCDD-induced stimulation of [3H]uridine incorporation was not significantly altered in the presence of cycloheximide, while this effect was abrogated in the presence of actinomycin D. Incubation of thymocytes with 10 nM TCDD also stimulated the activity of poly(A)polymerase, the enzyme catalyzing mRNA polyadenylation, at time points beyond 30 min. No significant increases in [35S] incorporation were observed in cells treated with 10 nM TCDD, although analysis of detergent and high salt extracted nuclear proteins by SDS-PAGE and coomassie blue staining revealed the increased abundance of at least two proteins with molecular masses of 52,000 and 42,000 Da, respectively. These studies reveal that thymocyte nuclei rapidly accumulated TCDD in vitro, leading to increased RNA synthesis, poly(A)polymerase activity and protein synthesis. These events correlate closely with the process of programmed cell death.

Quantitative measurement of calpain I and II mRNAs in differentiating rat muscle cells using a competitive polymerase chain reaction method

Levels of calpain I and calpain II mRNAs were analyzed at different stages of rat skeletal myoblast differentiation using a competitive polymerase chain reaction method. The results provide evidence that only calpain II mRNAs were present in significant quantities on the second day while calpain I mRNAs were identified on the fourth day of differentiation. If there is no compelling reason to believe that synthesis of calpains I and II is regulated at the level of mRNA, our results suggest that calpain II will be more particularly involved in Ca(2+)-mediated events accompanying myoblast fusion. On the other hand, calpain I, because of its later appearance may probably act on specific substrates such as myofibrillar proteins, associated myofibrillar proteins or the control of enzyme metabolism. Added factors such as insulin, which is known to induce enhancement of myoblast growth or myoblast fusion, had a significant effect on the amounts of calpain I and II mRNAs. In the presence of TGF-beta, a potent inhibitor of myoblast fusion, calpain I and II mRNAs were decreased. These results confirm first that a Ca(2+)-dependent proteolytic system is positively correlated with myoblast fusion (via calpain II) and second, that transcriptional regulation of calpains I and II may be negatively modulated during myoblast differentiation.

A comparison of the mouse versus human aryl hydrocarbon (Ah) receptor complex: effects of proteolysis

The differences in the molecular properties of the nuclear aryl hydrocarbon (Ah) receptor from human Hep G2 and mouse Hepa 1c1c7 cells were investigated by time-dependent partial proteolysis with chymotrypsin or trypsin followed by column chromatographic and velocity sedimentation analysis. The sedimentation coefficients, Stokes radii and apparent molecular weights of the untreated human and mouse Ah receptor complexes were similar. Treatment of the nuclear Ah receptor complexes from both cell lines with chymotrypsin for 10 or 60 min gave lower molecular weight proteolytic products which also exhibited comparable molecular properties and salt gradient elution profiles from Sepharose columns linked to DNA. Treatment of the human and mouse nuclear Ah receptor complexes with trypsin (5 micrograms/mg protein) for 10 or 60 min gave a minor low molecular weight (29.7- or 25.7-kDa) proteolysis product which was detected only with the mouse Hepa 1c1c7 Ah receptor complex. The time- and concentration-dependent proteolytic digest maps of the human and mouse Ah receptor were determined using receptor preparations which were photoaffinity labeled with [125I]7-iodo-2, 3-dibromodibenzo-p-dioxin. The human Ah receptor was significantly more resistant to proteolysis by trypsin or chymotrypsin than the mouse Ah receptor. At a low concentration of chymotrypsin (1 microgram/mg protein) the Hepa 1c1c7 receptor was degraded to two lower molecular weight fragments with apparent M(r) values at 71- and 48-kDa whereas the Hep G2 Ah receptor was relatively stable under these conditions. Although the human Ah receptor was more slowly hydrolyzed than the mouse receptor by trypsin, the major photolabeled breakdown products for the Ah receptor from both cell lines were observed at M(r) 48- and 45-kDa. The results of this study demonstrate that there were subtle but significant differences in the human and mouse Ah receptor complex; however, the proteolysis studies suggest that there are common structural features in their ligand binding sites.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Detection of the Ah receptor in rainbow trout: use of 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin in cell culture

The Ah receptor was detected in RTG-2 cells (rainbow trout embryonic gonad cells) following the addition of the photoaffinity ligand, [125I]2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, to cells in culture. Cytosolic and nuclear extracts were prepared and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and revealed one radiolabeled band. Very little non-specific binding was observed under the conditions employed when compared to photoaffinity labeling RTG-2 cytosolic extracts in vitro. The photoaffinity-labeled Ah receptor in RTG-2 cytosol was analyzed by sucrose density centrifugation. The cytosolic form was observed to sediment at approximately 9.8S and the high salt nuclear extract form at approximately 7.5S. The relative molecular weight of the Ah receptor was determined to be 145 kDa under denaturing conditions and is considerably larger than the Ah receptor from mammalian sources. Inhibition of photoaffinity ligand binding to the RTG-2 cytosolic Ah receptor by competing ligands revealed the same rank order of ligand affinity as that previously demonstrated for the mouse Ah receptor.

The Ah receptor and the mechanism of dioxin toxicity

Images

In situ and in vitro photoaffinity labeling of the nuclear aryl hydrocarbon receptor from transformed rodent and human cell lines

The photoaffinity labeling of the nuclear aryl hydrocarbon (Ah) receptor from mouse Hepa 1c1c7, rat hepatoma H-4-II E, and human liver Hep G2 cells was investigated using two high affinity ligands, namely 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) and 7-[125I]iodo-2,3,-dibenzo-p-dioxin ([125I]DBDD). Irradiation of nuclear [3H]TCDD-Ah receptor complexes from the three cell lines for 5 min gave 47, 38, and 62% yields of trichloroacetic acid-precipitable photoadducts from the Hepa 1c1c7, H-4-II E, and Hep G2 cell lines, respectively; denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation followed by autoradiography gave one major Ah receptor photoadduct for each cell line with apparent molecular masses at 97, 100, and 110 kDa, respectively. [125I]DBDD could also be used as a photoaffinity label for the nuclear Ah receptor from the three cell lines; although the maximum net yield of photoaffinity labeled nuclear Ah receptor from the rodent nuclear Ah receptor preparations was relatively low (0.5-2.5%), a greater than 15% yield of photoadduct was obtained from the human Hep G2 cells. Both [3H]TCDD and [125I]DBDD were utilized to photoaffinity label the nuclear Ah receptor in Hepa 1c1c7 cells in suspension and the net yield of photoadducts with these ligands was 94.6 and 3.0%, respectively. The cytosolic Ah receptor from the three cell lines was photolabeled with [125I]DBDD and the net yield of photoadducts varied from 3.3 to 14.7%. The functional activity of the photoaffinity-labeled nuclear TCDD-Ah receptor complexes from the cell lines was also determined by comparing relative binding affinities of the photolyzed and unphotolyzed complexes with a synthetic dioxin-responsive element (DRE) using a gel retardation assay. The photolyzed and unphotolyzed complexes from the three cell lines all bound with the DRE in the gel shift assay; however, the gel mobilities of the rodent and human nuclear receptor-DRE complexes were different. Quantitative analysis of the DRE binding showed that there were no significant differences between the photolyzed and unphotolyzed nuclear receptor complexes from the rodent cells, whereas there was a significant 27% decrease in the DRE binding of the photolyzed versus the unphotolyzed nuclear receptor complex from the human Hep G2 cells. These studies demonstrate the utility of [3H]TCDD and [125I]DBDD as photoaffinity labels for the Ah receptor and illustrate the structural and photochemical differences between the rodent and the human nuclear Ah receptor complexes.

Characterization of multiple forms of the Ah receptor: recognition of a dioxin-responsive enhancer involves heteromer formation

We have employed a combination of gel retardation, protein-DNA cross-linking, and protein-protein cross-linking techniques to further examine the 2,3,7,8-tetrachlorodibenzo-p- dioxin-(TCDD-) dependent changes in the Ah receptor that result in a DNA-binding conformation. Gel retardation analysis of DNA-Sepharose chromatographic fractions of rat hepatic cytosol indicated that TCDD-dependent and sequence-specific DNA binding coeluted with a 200-kDa form of the Ah receptor (peak 2) previously characterized as being multimeric and having high affinity for calf thymus DNA. The TCDD-bound, 100-kDa form of the receptor (peak 1) bound weakly to the DNA recognition motif. These results indicated that the DNA-binding form of the Ah receptor is a multimer. SDS-polyacrylamide gel electrophoresis of peak 2 cross-linked to a bromodeoxyuridine-substituted DNA recognition motif indicated that this form of the receptor present in rat hepatic cytosol is composed of at least two DNA-binding proteins of approximately 100 and 110 kDa. Using the chemical cross-linking agent dimethyl pimelimidate, we further established that the 100-kDa form of the receptor (peak 1) associates with a different protein to generate the receptor form (peak 2) that binds to the dioxin-responsive enhancer. Photoaffinity-labeling studies indicated that only the 100-kDa protein (peak 1), and not the 110-kDa protein, binds ligand. Together, these observations imply that the DNA-binding form of the Ah receptor exists as a heteromer.

Equivalent molecular mass of cytosolic and nuclear forms of Ah receptor from Hepa-1 cells determined by photoaffinity labeling with 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin

The structure of the Ah receptor previously has been extensively characterized by reversible binding of the high affinity ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin. We report the use of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin as a photoaffinity ligand for Ah receptor from the mouse hepatoma cell line Hepa-1c1c9. Both cytosolic and nuclear forms of Ah receptor could be specifically photoaffinity-labeled, which allowed determination of molecular mass for the two forms under denaturing conditions. After analysis by fluorography of polyacrylamide gels run in the presence of sodium dodecyl sulfate, molecular mass for the cytosolic form of Ah receptor was estimated at 92,000 +/- 4,300 and that for the nuclear form was estimated at 93,500 +/- 3,400. Receptor in mixture of cytosol and nuclear extract (each labeled separately with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin) migrated as a single band. These results are consistent with the presence of a common ligand-binding subunit of identical molecular mass in both cytosolic and nuclear complexes.

Analysis of photoaffinity-labeled aryl hydrocarbon receptor heterogeneity by two-dimensional gel electrophoresis

The level of charge heterogeneity in the aryl hydrocarbon receptor (AhR) was examined by high-resolution denaturing two-dimensional (2D) gel electrophoresis. Hepa 1c1c7 cell cytosolic fraction was photoaffinity-labeled with 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin and applied to isoelectric focusing (IEF) tube gels. After optimization of focusing conditions a broad peak of radioactivity was detected in the apparent pI range of 5.2-5.7. IEF tube gels were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by visualization of the radiolabeled AhR by autoradiography; three distinct isoforms were detected. The same 2D electrophoretic isoform pattern was obtained when the AhR from Hepa 1c1c7 was photoaffinity-labeled in cell culture. BPrCl cells, a mutant line derived from Hepa 1c1c7 cells, contain an AhR that is unable to bind to DNA. Photoaffinity-labeled BPrCl cytosolic fractions were subjected to 2D gel electrophoretic analysis resulting in essentially the same molecular weight and isoform pattern as seen in Hepa 1c1c7 cytosol. This result would suggest that if a mutation is present in the BPrCl AhR it has not caused a significant change in its IEF pattern, although a small shift in the pI values was observed. Two-dimensional gel electrophoresis of photoaffinity-labeled cytosolic fractions from HeLa cells, the rat liver tumor cell line McA-RH7777, and buffalo rat thymus revealed three isoforms, essentially the same isoform pattern as in Hepa 1c1c7 cells. This would indicate that despite the considerable molecular weight polymorphism between species the level of charge heterogeneity is highly conserved.

Dopamine D1 receptors of the calf parathyroid gland: identification of a ligand binding subunit with lower apparent molecular weight but similar primary structure to neuronal D1-receptors

The ligand binding subunit of the calf parathyroid D1 dopamine receptor was visualized by autoradiography following photoaffinity labeling with (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4'-azidophenyl)-2,3,4,5- tetrahydro-1H-benzazepine ([125I]IMAB) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein comprising the D1 binding subunit migrated with an apparent Mr approximately equal to 62,000. Photoincorporation of [125I]IMAB into the Mr approximately equal to 62,000 polypeptide required the presence of protease inhibitors and was stereoselectively antagonized by dopaminergic agonists and antagonists with an appropriate pharmacological specificity for D1 receptors. The electrophoretic mobility of the [125I]IMAB-labeled receptor was not altered by the absence or presence of urea or thiol-reducing/oxidizing reagents. The Mr approximately equal to 62,000 protein representing the ligand binding subunit of bovine parathyroid D1 receptors corresponds to one of three D1 receptor binding subunits (Mr = 74,000, 62,000, and 51,000) identified in bovine brain. Peptide map comparisons of radiolabeled D1 receptors from calf parathyroid and brain following limited proteolytic digestion with Staphylococcus aureus V8 and papain revealed marked structural similarities. These data suggest that, despite tissue-specific differences in overall molecular weight, both parathyroid and neuronal D1 dopamine binding subunits appear to be pharmacologically and structurally homologous and that the molecular mechanism(s) responsible for the apparent lack of a one to one correspondence in the subunit composition of the D1 receptor in these tissues probably reflect(s) tissue-specific posttranslational modifications.

Radioligand-dependent differences in the molecular properties of the mouse and rat hepatic aryl hydrocarbon receptor complexes

A comparison of the molecular properties of the male Long-Evans rat and male C57BL/6 mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor complex was determined using 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF) as radioligands. In low salt buffer, the sedimentation coefficients, Stokes radii, relative molecular masses, frictional ratios, axial ratios and gel permeation chromatographic properties of the rat receptor complexes were ligand independent. In contrast, there were several ligand-dependent differences in the mouse Ah receptor complexes formed after incubation in low salt buffer and these include: sucrose density gradient analysis of the 2,3,7,8-[3H]TCDF receptor complex gave a 9.5 S specifically bound peak and a 2.6 S nonspecifically bound peak whereas the corresponding 2,3,7,8-[3H]TCDD receptor complex gave a single 9.6 S specifically bound peak; sucrose density gradient analysis of the two major peaks eluted from a Sephacryl S-300 column chromatographic separation of the 2,3,7,8-[3H]TCDF receptor complex gave two specifically bound peaks at 9.2 and 5.1 S. The molecular properties of the rat hepatic cytosolic receptor complexes incubated in high salt (0.4 M KCl) buffer were ligand independent with one exception, namely the significant difference in the sedimentation coefficient of the specifically bound disaggregated 2,3,7,8-[3H]TCDD receptor complex (6.8 S) and the corresponding 2,3,7,8-[3H]TCDF receptor complex (5.0 S). The major ligand-dependent differences in the mouse receptor complexes incubated in high salt (0.4 M KCl) were associated with the sedimentation coefficients of the complexes derived after direct incubation and after gel permeation chromatography. For example, both ligands gave two specifically bound complexes after chromatography on Sephacryl S-300 column and centrifugation of these fractions gave both the approximately 9 and approximately 5 S peaks; this suggested that there was some equilibration between the aggregated and disaggregated receptor complexes. The behavior of the 2,3,7,8-[3H]TCDF mouse receptor complex was similar after incubation in low or high salt buffer except that sucrose density gradient analysis of the gel permeation chromatographic fractions gave an additional specifically bound peak which sedimented at 7.2 S. These studies demonstrate that the molecular properties of the Ah receptor were dependent on the source of the cytosolic receptor preparation, the ionic strength of the incubation media, and the structure of the radioligand.