2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) modulates epidermal growth factor (EGF) binding to basal cells from a human keratinocyte cell line

TCDD induces dermal accumulation of keratinocyte-derived matrix metalloproteinase-10 in an organotypic model of human skin

The epidermis of skin is the first line of defense against the environment. A three dimensional model of human skin was used to investigate tissue-specific phenotypes induced by the environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Continuous treatment of organotypic cultures of human keratinocytes with TCDD resulted in intracellular spaces between keratinocytes of the basal and immediately suprabasal layers as well as thinning of the basement membrane, in addition to the previously reported hyperkeratinization. These tissue remodeling events were preceded temporally by changes in expression of the extracellular matrix degrading enzyme, matrix metalloproteinase-10 (MMP-10). In organotypic cultures MMP-10 mRNA and protein were highly induced following TCDD treatment. Q-PCR and immunoblot results from TCDD-treated monolayer cultures, as well as indirect immunofluorescence and immunoblot analysis of TCDD-treated organotypic cultures, showed that MMP-10 was specifically contributed by the epidermal keratinocytes but not the dermal fibroblasts. Keratinocyte-derived MMP-10 protein accumulated over time in the dermal compartment of organotypic cultures. TCDD-induced epidermal phenotypes in organotypic cultures were attenuated by the keratinocyte-specific expression of tissue inhibitor of metalloproteinase-1, a known inhibitor of MMP-10. These studies suggest that MMP-10 and possibly other MMP-10-activated MMPs are responsible for the phenotypes exhibited in the basement membrane, the basal keratinocyte layer, and the cornified layer of TCDD-treated organotypic cultures. Our studies reveal a novel mechanism by which the epithelial-stromal microenvironment is altered in a tissue-specific manner thereby inducing structural and functional pathology in the interfollicular epidermis of human skin.

Differential downregulation of e-cadherin and desmoglein by epidermal growth factor

Modulation of cell : cell junctions is a key event in cutaneous wound repair. In this study we report that activation of the epidermal growth factor (EGF) receptor disrupts cell : cell adhesion, but with different kinetics and fates for the desmosomal cadherin desmoglein and for E-cadherin. Downregulation of desmoglein preceded that of E-cadherin in vivo and in an EGF-stimulated in vitro wound reepithelialization model. Dual immunofluorescence staining revealed that neither E-cadherin nor desmoglein-2 internalized with the EGF receptor, or with one another. In response to EGF, desmoglein-2 entered a recycling compartment based on predominant colocalization with the recycling marker Rab11. In contrast, E-cadherin downregulation was accompanied by cleavage of the extracellular domain. A broad-spectrum matrix metalloproteinase inhibitor protected E-cadherin but not the desmosomal cadherin, desmoglein-2, from EGF-stimulated disruption. These findings demonstrate that although activation of the EGF receptor regulates adherens junction and desmosomal components, this stimulus downregulates associated cadherins through different mechanisms.

Sequential actions of myotubularin lipid phosphatases regulate endosomal PI(3)P and growth factor receptor trafficking

Two different human diseases, X-linked myotubular myopathy and Charcot-Marie-Tooth disease, result from mutant MTM1 or MTMR2 lipid phosphatases. Although events involved in endosomal PI(3)P and PI(3,5)P(2) synthesis are well established and pivotal in receptor signaling and degradation, enzymes involved in phosphoinositide degradation and their roles in trafficking are incompletely characterized. Here, we dissect the functions of the MTM1 and MTMR2 myotubularins and establish how they contribute to endosomal PI(3)P homeostasis. By mimicking loss of function in disease through siRNA-mediated depletion of the myotubularins, excess PI(3)P accumulates on early (MTM1) and late (MTMR2) endosomes. Surprisingly, the increased PI(3)P blocks the egress of epidermal growth factor receptors from early or late endosomes, suggesting that the accumulation of signaling receptors in distinct endosomes may contribute to the unique disease etiologies when MTM1 or MTMR2 are mutant. We further demonstrate that direct myotubularin binding to the type III PI 3-kinase complex hVps34/hVps15 leads to phosphatase inactivation. The lipid kinase-phosphatase interaction also precludes interaction of the PI 3-kinase with Rab GTPase activators. Thus, unique molecular complexes control kinase and phosphatase activation and locally regulate PI(3)P on discrete endosome populations, thereby providing a molecular rationale for related human myo- and neuropathies.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exposure of normal human dermal fibroblasts results in AhR-dependent and -independent changes in gene expression

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) results in a variety of lesions in mammals including severe skin lesions. The majority of TCDD's biological effects are mediated through activation of the aryl hydrocarbon receptor (AhR). We have chosen to examine the effect of TCDD and the AhR pathway on dermal fibroblasts because this cell type plays an integral role in skin homeostasis through the production of cytokines and other factors that regulate epidermal proliferation and differentiation. Our data show that normal human dermal fibroblasts (NHDFs) are responsive to TCDD, as demonstrated by induction of cytochrome p450 1B1 (CYP1B1) expression. Further, our data demonstrate that TCDD treatment of NHDFs results in significant (75-90%) decrease in expression of Id-1 and Id-3, proteins that are involved in regulation of cell proliferation and differentiation. The Id (Inhibitor of DNA binding) proteins are transcriptional inhibitors that function by forming inactive heterodimers with other HLH proteins. TCDD-repression of Id-1 and -3 is independent of de novo protein synthesis; co-treatment with cycloheximide has no effect on TCDD inhibition of Id-1 and Id-3. Co-treatment with the AhR antagonist alpha-naphthoflavone also does not block inhibition of Id-1 and Id-3 by TCDD, suggesting that TCDD inhibition of Id-1 and Id-3 is, at least in part, mediated independently of the AhR pathway. Our data also show that TCDD inhibits expression of the cell cycle regulatory gene p16(ink4a), which is often linked to Id expression. TCDD-induced reduction of p16(ink4a) expression is also independent of protein synthesis and the AhR pathway.

Dioxin-induced chloracne--reconstructing the cellular and molecular mechanisms of a classic environmental disease

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is among the most toxic pollutants known to date that serves as a prototype for a group of halogenated hydrocarbon compounds characterized by extraordinary environmental persistence and unique ability to concentrate in animal and human tissues. TCDD can elicit a complex array of pleiotropic adverse effects in humans, although chloracne, a specific type of acne-like skin disease, is the only consistent manifestation of dioxin intoxication, thus representing a 'hallmark' of TCDD exposure. Chloracne is considered to be one of the most specific and sensitive biomarkers of TCDD intoxication that allows clinical and epidemiological evaluation of exposure level at threshold doses. The specific cellular and molecular mechanisms involved in pathogenesis of chloracne are still unknown. In this review, we summarize the available clinical data on chloracne and recent progress in understanding the role of the dioxin-dependent pathway in the control of gene transcription and discuss molecular and cellular events potentially involved in chloracne pathogenesis. We propose that the dioxin-induced activation of skin stem cells and a shift in differentiation commitment of their progeny may represent a major mechanism of chloracne development.

The aryl hydrocarbon receptor antagonist, 3'methoxy-4'nitroflavone, attenuates 2,3,7,8-tetrachlorodibenzo-p-dioxin-dependent regulation of growth factor signaling and apoptosis in the MCF-10A cell line

Previous studies have demonstrated that 2,3,7,8 tetracholorodibenzo-p-dioxin (TCDD) mimics epidermal growth factor receptor (EGFR) signaling in the MCF-10A human mammary epithelial cell line and protects cells from EGF withdrawal-induced apoptosis. These effects appear to be due to the ability of TCDD to increase the expression of transforming growth factor-alpha (TGFalpha), a known EGFR ligand. Because TCDD's effects occurred at concentrations as low as 1 nM, a role for the aryl hydrocarbon receptor (AhR) was hypothesized. In the present study, 3'methoxy-4'nitroflavone (MNF), a known AhR antagonist, was used to analyze AhR signaling in this cell line. MNF suppressed TCDD-dependent dioxin response element (DRE)-driven luciferase activity at concentrations as low as 10 nM. In addition, MNF attenuated TCDD's ability to inhibit apoptosis and to activate Akt and Erk1,2, two EGFR-dependent signaling molecules. Finally, the TCDD-dependent increase in TGFalpha mRNA was also suppressed by MNF. MNF's effects on TCDD action in the MCF-10A cell line occurred at concentrations ranging from 1 nM for Akt phosphorylation and TGFalpha expression to 100 nM for inhibition of apoptosis. Attenuation of TCDD-dependent luciferase activity occurred at concentrations as low as 10 nM, which suggests that TCDD inhibits apoptosis in human mammary epithelial cells by multiple mechanisms.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters the differentiation pattern of human keratinocytes in organotypic culture

Human exposure to the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces a severe skin pathology known as chloracne. In these studies we employed a three-dimensional, organotypic model system to study the effects of TCDD on human skin. This model uses the spontaneously immortalized human keratinocyte cell line NIKS and recapitulates both the three-dimensional microenvironment and epithelial-mesenchymal interactions found in intact human skin. Treatment of the organotypic cultures with TCDD causes alterations in the pattern of keratinocyte terminal differentiation. Analysis at both the light and electron microscope levels reveals a fully differentiated cornified layer in TCDD-treated organotypic cultures at earlier time points than observed in vehicle (dimethyl sulfoxide)-treated controls. Furthermore, TCDD-treated organotypic cultures exhibit aberrant distribution of several differentiation-specific protein markers. Basal cells in TCDD- and DMSO-treated organotypic cultures show no differences in proliferation as measured by quantification of 5-bromo-2'-deoxyuridine (BrdU)-positive nuclei. No aberrant BrdU uptake was detected outside of the basal layer. Neither TUNEL labeling nor immunohistochemical staining with an antibody to active caspase-3 revealed increased apoptosis in TCDD-treated organotypic cultures relative to controls. These data clearly indicate that TCDD modulates homeostasis in a model of human stratifying epithelium independent of changes in proliferation and apoptosis, exclusively by impacting keratinocyte terminal differentiation. This TCDD-induced effect on differentiation-specific proteins results in profound changes in the tissue architecture.

Treatment of normal human keratinocytes with 2,3,7,8-tetrachlorodibenzo-p-dioxin causes a reduction in cell number, but no increase in apoptosis

We have examined the effect of TCDD on the growth of normal human keratinocytes. TCDD is a ubiquitous environmental toxicant that causes a severe dermatopathology in humans, which is known as chloracne. The cell biological basis of this pathology remains unknown. We conducted growth experiments in preconfluent normal human keratinocytes with both low (0.05 mM) and standard (0.66 mM) extracellular calcium concentrations in the media. TCDD treatment reduced the number of adherent keratinocytes relative to controls in media containing 0.05 or 0.66 mM calcium. Based on these observations, we speculated that the decrease in the cell number of TCDD-treated cultures might be the result of increased apoptosis. Analysis of nucleosomal fragmentation, nuclear morphology, and caspase-3 activity in keratinocytes reveals no increase in the characteristics of apoptosis in response to TCDD treatment. We therefore conclude that TCDD impacts on keratinocyte homeostasis independent of apoptosis.

Disruption of male reproductive tract development by administration of the xenoestrogen, nonylphenol, to male newborn rats

Nonylphenol (NP) treatment of neonatal male rat pups decreased the size of their testes, epididymis, seminal vesicle, and ventral prostate, and increased the frequency of cryptorchidism (60.7%, n = 56 vs 0% in vehicle-treated control, n = 58) when examined at 31 d of age. NP effects are dose-dependent. These effects were only seen when NP was given at > or =20.8 mg/kg daily for 15 d. There is a critical period of vulnerability to NP during male reproductive development in the neonatal stage. Changes were found when NPs were given to male pups before 13 d of age, but not when given at > or =13 d of age. NP acts on the male reproductive tissues through the estrogen receptor (ER), since concomitant treatment with ICI 182,780, a specific ER antagonist, blocked NP's effects on the testis and male accessory organs. NP-treated males in the neonatal period had greatly reduced their subsequent capacity to impregnate young fertile females. Our results suggest that exposure of neonatal male rats to NP is potentially deleterious to their reproductive development and affects their reproductive performance.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters retinoic acid receptor function in human keratinocytes

Actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on all-trans-retinoic acid (trans-RA) binding to retinoic acid receptors (RARs) in cultured human keratinocytes (SCC-12F) was investigated. TCDD and trans-RA elicited opposing actions on the production of biologically active TGF-beta. TCDD exposure caused concentration- and time-dependent decreases in trans-RA binding to SCC-12F RARs. The apparent half-maximal effective TCDD concentration = 1 nM. TCDD exerts its action via the aryl hydrocarbon receptor (AhR). TCDBF, a partial AhR agonist, reduced trans-RA binding, indicating AhR involvement (control = 0.33; TCDBF = 0.22; TCDD = 0.142 pmol trans-RA bound/mg nuclear protein). The dissociation constant (Kd) calculated from Eadie-Hofstee analysis of equilibrium binding for trans-RA was 0.13 nM in both TCDD-exposed and control cultures. Approximately half of the trans-RA binding sites were lost in TCDD-exposed cells (control = 0.195; TCDD = 0.108 pmol trans-RA bound/mg protein). The data suggest TCDD may exert its toxic action in human keratinocytes by directly modulating RAR action.

Differentiation status of cultured murine keratinocytes modulates induction of genes responsive to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Primary murine keratinocytes were cultured in a chemically defined, serum-free medium which facilitated manipulation of their differentiation status. Exposure of basal cell and differentiating cultures to >/= 0.1 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) preferentially elevated 7-ethoxyresorufin O-deethylase specific activities in differentiating cultures (28-fold versus 4-fold increases after 36 h of exposure). Semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analyses demonstrated the presence of constitutive mRNA transcripts corresponding to four known TCDD-inducible genes (e.g., Cyp1a1, Cyp1b1, Ahd4, and Nmo1) in both differentiating and proliferating cultures of murine keratinocytes. All four genes were induced in differentiating cultures following exposure to TCDD. No induction occurred in comparably treated basal cell cultures. Indirect immunofluorescence analyses demonstrated the presence of aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) proteins in both basal and differentiating keratinocytes. Both proteins appeared to be associated with the nucleus and their nuclear association was independent of prior exposure to TCDD. These studies suggest that AHR activation in murine skin is regulated as a function of the keratinocyte differentiation program.

2,3,7,8-Tetrachlorodibenzo-p-dioxin increases mRNA levels for interleukin-1beta, urokinase plasminogen activator, and tumor necrosis factor-alpha in human uterine endometrial adenocarcinoma RL95-2 cells

This study investigated the potential role of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in uterine growth utilizing a human endometrial adenocarcinoma cell line (RL95-2). Western immunoblot analysis showed a maximal induction of cytochrome P4501A1 (CYP1A1) at 1 nM TCDD, but no change in epidermal growth factor receptor (EGFR) protein level. Northern blot analysis showed that TCDD significantly increased the steady state mRNA level of CYP1A1 and CYP1B1 which was maximal at 1 nM. TCDD significantly increased mRNA levels for interleukin-1beta (IL-1beta) by 6h, and for urokinase plasminogen activator (uPA) and tumor necrosis factor-alpha (TNF-alpha) by 36h. Nuclear runoff analysis showed that transcription of CYP1A1 was significantly increased by TCDD with no effect on CYP1B1, uPA or IL-1beta. These results indicate that TCDD can differentially alter the expression of growth factor and cytokine gene products in uterine cells which may contribute to the promotion of uterine disease.

EGF receptor expression and growth of psoriatic and normal human keratinocytes are modulated by 1.25 (OH)2-vitamin D3 ex vivo

Calcitriol or 1.25 (OH)2-vitamin D3 is used in the treatment of psoriasis as an inhibitor of cell proliferation. We studied the action of calcitriol ex vivo on the growth of psoriatic and normal human keratinocytes, and on the expression of the EGF receptor. Third passaged normal and psoriatic keratinocytes were seeded (10(4)/cm2) in 24-well dishes in serum-free medium (MCDB supplemented with amino acids, with either 0.1 or 1.1 mM of calcium) and 10(-9) M calcitriol. When subconfluence was reached, cell counts and 125I-EGF binding studies were performed. Cell counts showed at least a 50% decrease in growth under all conditions studied (normal or psoriatic keratinocytes; 0.1 or 1.1 mM calcium) when calcitriol was added. 125I-EGF binding studies showed a decrease in total receptor numbers in the presence of calcitriol with acceleration of binding at low concentrations of 125I-EGF. Scatchard plot analysis showed only one type of high affinity receptor. Receptor sites were decreased (30% to 40% of controls) in the presence of calcitriol together with a decrease in the dissociation constant. In conclusion, at almost physiological concentrations ex vivo, calcitriol strongly decreased normal and psoriatic keratinocyte growth. This potent antiproliferative effect could in part be explained by the capacity of calcitriol to downregulate EGF receptor expression.

Enhanced modulation of keratinocyte motility by transforming growth factor-alpha (TGF-alpha) relative to epidermal growth factor (EGF)

Epidermal growth factor (EGF) and transforming growth factor (TGF)-alpha are high-affinity polypeptide ligands for the EGF receptor, which mediates their biologic activities. In this study, we directly compared the actions of both ligands in promoting keratinocyte motility. We found that normal and tumorigenic human keratinocytes responded to activation of the EGF receptor by either EGF or TGF-alpha; however, the two ligands did not elicit identical responses with regard to cell locomotion. TGF-alpha was more effective than EGF at promoting colony dispersion (cell scattering), in vitro wound closure, and single-cell migration as assessed by phagokinetic track analysis. In contrast, EGF and TGF-alpha evoked identical profiles for DNA synthesis with regard to concentration dependence and magnitude of response in normal keratinocytes and in a squamous cell carcinoma line. The overall pattern of tyrosine phosphorylation of intracellular substrates was similar when cells were stimulated with either growth factor; however, a limited number of differences in the kinetics or magnitude of protein phosphorylation were detected in subcellular fractions. These findings demonstrate that two growth factors implicated in promoting mitogenesis and locomotion may elicit divergent responses with regard to one biologic activity while retaining similar responses for other activities. This suggests that ligand-mediated mitogenic responses may not be tightly coupled to motogenic activity and further illustrates the multifunctional roles of polypeptide growth factors.

Characterization of protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in murine lymphocytes: indirect evidence for a role in the suppression of humoral immunity

Studies were undertaken to more thoroughly characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced stimulation of kinase activity in murine lymphocytes. In female B6C3F1 mice, TCDD-induced phosphorylation of 29, 45, 52 and 63 KDa proteins was selective for B cells, with little or no enhancement observed in T cells. When B cells were purified and separated by density on a percoll gradient, phosphorylation was only observed in the band composed of activated B cells, and was not enhanced in the band composed of resting B cells. TCDD-stimulated phosphorylation was associated with both the cytosol (45 and 52 KDa species) and membrane (52 KDa species) fractions. Purified B cells from both DBA/2 (Ahdd) and C57B16 (Ahbb) mice demonstrated equivalent enhancement of phosphorylation in response to TCDD. Administration of human gamma interferon (Hu-IFNg) at concentrations from 0.5 to 500 Units/ml produced a dose-related reversal of TCDD-induced suppression of in vitro antibody responses to both the polyclonal B cell activator, LPS, and the T-dependent antigen, sRBC in whole splenocytes isolated from female B6C3F1 mice. These concentrations of Hu-IFNg did not affect the magnitude of either response in the absence of TCDD, and did not reverse dexamethasone-induced suppression of either in vitro antibody response. TCDD-induced suppression of the T-dependent response was reversed only when Hu-IFNg was added to culture within the first 18 hours after treatment with TCDD and sRBC. These studies demonstrate that Hu-IFNg can reverse TCDD-induced in vitro Ab response suppression if it is administered during the period of susceptibility to TCDD. TCDD-induced phosphorylation in isolated B cells was also antagonized following co-incubation with Hu-IFNg. The profile of TCDD-induced increases in protein phosphorylation, including the selective effect on activated B cells, the general involvement of both cytosolic and membrane proteins, the lack of segregation with the Ah-dependent processes, and the ability of Hu-IFNg to reverse both the suppression of the Ab response and the increase in phosphorylation, supports the interpretation that such phosphorylation is involved in TCDD-induced suppression of the Ab response.

Induction of terminal differentiation in cultured human keratinocytes by polychlorinated aromatic hydrocarbons as measured by cell size analysis

Polychlorinated aromatic hydrocarbons modulate the proliferation and differentiation of human epidermal cells in vivo and in culture. One of the earliest events in the process of terminal differentiation is the increase in cell size. In this report the usefulness of morphometric cell size analysis as a quantifiable marker for chemical-induced differentiation was examined. Concentration-related increases in cell size distribution were induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and 2,3,4,7,8-pentachlorodibenzofuran in normal human keratinocytes and cells from an SV40-transformed keratinocyte cell line (SVK14) whereas the analog 1,2,3,4-tetrachlorodibenzo-p-dioxin did not affect the cell size distribution up to a concentration of 100 nM. The minimal effective concentrations of five 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins/dibenzofurans and a coplanar polychlorbiphenyl necessary to induce an increase in cell size distribution were determined in SVK14 cells. It was found that the potency of these compounds relative to that of 2,3,7,8-TCDD correlated well with the toxicity equivalency factors observed in other test systems. This indicates that the keratinocyte cell assay is a useful method for establishing the relative potency of various "dioxins" and their mixtures.

Targets for dioxin: genes for plasminogen activator inhibitor-2 and interleukin-1 beta

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD), a widespread environmental contaminant, may elicit its effects by altering gene expression in susceptible cells. Five TCDD-responsive complementary DNA clones were isolated from a human keratinocyte cell line. One of these clones encodes plasminogen activator inhibitor-2, a factor that influences growth and differentiation by regulating proteolysis of the extracellular matrix. Another encodes the cytokine interleukin-1 beta. Thus, TCDD alters the expression of growth regulatory genes and has effects similar to those of other tumor-promoting agents that affect both inflammation and differentiation.

Evidence for the mechanism of action of the 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated decrease of nuclear estrogen receptor levels in wild-type and mutant mouse Hepa 1c1c7 cells

Treatment of wild-type Hepa 1c1c7 cells with 1 nM [3H]-17 beta-estradiol resulted in the rapid accumulation of the nuclear estrogen receptor complex whose levels were maximized within 1 hr. Cotreatment of the cells with 10 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and [3H]-17 beta-estradiol did not affect the nuclear estrogen receptor levels 1 hr after addition of the radioligand; however, pretreatment of the cells for 1, 6, 24 or 42 hr with 10 nM TCDD prior to the addition of the radiolabeled hormone caused a greater than 50% decrease in nuclear estrogen receptor levels (determined by velocity sedimentation analysis) 1 hr after the addition of [3H]-17 beta-estradiol. In parallel experiments in which 10 nM TCDD was added 6 hr prior to the radiolabeled hormone, TCDD caused a 63 and 74% decrease in immunodetectable cytosolic and nuclear estrogen receptor protein levels, respectively, in the wild-type Hepa 1c1c7 cells. The nuclear estrogen receptor was also detected in two Hepa 1c1c7 mutant (class 1 and class 2) cell lines which have been characterized previously as TCDD non-responsive due to either decreased aryl hydrocarbon (Ah) receptor levels or a defect in the accumulation of transcriptionally active nuclear Ah receptor complexes, respectively. Treatment of these mutant cell lines with TCDD and [3H]-17 beta-estradiol (as described above) caused only a minimum (class 1) or non-detectable (class 2) decrease in nuclear estrogen receptor binding activity or immunodetectable protein levels. These results, coupled with the structure-dependent differences in the activities of TCDD (a strong Ah receptor agonist) and 2,8-dichlordibenzo-p-dioxin (a weak Ah receptor agonist) in this assay system, support a role for the Ah receptor in the TCDD-mediated decrease of the nuclear estrogen receptor in mouse Hepa 1c1c7 cells. In addition, actinomycin D and cycloheximide both inhibited the TCDD-mediated decrease of nuclear estrogen receptor levels in the Hepa 1c1c7 wild-type cells, and these results suggest that TCDD may induce specific gene products which are involved in this process.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on humoral immunity: I. Similarities to Staphylococcus aureus Cowan Strain I (SAC) in the in vitro T-dependent antibody response

We have determined that suppression of the in vitro T-dependent humoral immune response by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is dependent on the type and concentration of serum used in the culture media. Only five out of 23 commercial lots of serum (screened at both 5 and 10%) could support a suppression in the presence of 30 nM TCDD, with the remaining lots demonstrating an apparent 'protective-like' effect against the TCDD exposure. When log dose response curves were established with TCDD (0.3, 3.0, and 30 nM) in media containing each of the serum lots supporting a suppression (at both 5 and 10%), we determined that only three lots could support a full dose-responsive suppression. Subsequently, in a comparison study between the effects of TCDD and the polyclonal B-cell activator Staphylococcus aureus Cowan Strain I (SAC) on the in vitro T-dependent humoral immune response, we have found that SAC suppresses the antibody response to SRBC and demonstrates the same serum dependency for this effect as was previously noted for TCDD. Under serum-free culturing conditions, TCDD (30 nM) caused a 15-fold increase in the AFC response to SRBCs over controls, suggesting that direct addition of TCDD to whole splenocyte cultures in the absence of serum-derived growth factors results in an increase in B-cell activation. Likewise, under serum-free conditions, SAC dose-dependently increased the AFC response over media controls, and at doses which achieved the same degree of suppression of the humoral response aa TCDD. Taken together, these studies suggest that TCDD has actions that are similar to a T cell independent polyclonal B cell activator such as SAC, and selectively acts on the B cell to suppress the T-dependent humoral immune response by a mechanism which is unique to this series of compounds. This effect however, is only detectable under appropriate serum-supported (or serum-deficient) culture conditions as described.

Age-related DNA modifications (I-compounds): modulation by physiological and pathological processes

I-compounds are covalent DNA modifications that can be detected and measured by 32P-postlabeling assay because of their DNA-adduct like properties. They accumulate in an age-dependent, highly reproducible manner in tissue DNA of untreated animals in the absence of exogenous carcinogens and, therefore, appear to arise via the interaction of DNA with endogenous reactants formed in the course of normal metabolism. Chromatographically, they exhibit a wide range of polarities, indicative of structural diversity. In addition to age-dependent increases, I-compound profiles exhibit prominent species-, sex-, tissue- and diet-dependent qualitative and quantitative differences. Natural-ingredient (chow) diets produce qualitative differences as well as substantially higher I-compound levels in rat liver and kidney, when compared with purified diets. Modified purified diets containing high carbohydrate, protein, or fat concentrations further modulate I-compound profiles. During liver regeneration, I-compounds behave like DNA adducts rather than m5 C in that their levels are not quickly restored. Treatment of rats with the hepatocarcinogens 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), CCl4, and peroxisome proliferators as well as with a choline-devoid hepatocarcinogenic diet depressed the age-related increases of I-compound levels in liver, the target organ. Additional 32P-labeled derivatives were observed only with the peroxisome proliferators and presumably represent DNA adducts of exogenous origin. No I-compounds were detected in a series of Morris hepatomas with different degrees of differentiation. Thus, loss of I-compounds may be associated with altered gene expression/dedifferentiation. On the other hand, the age-dependent accumulation of I-compounds and their adduct-like character suggest potential relations to aging-associated dysdifferentiation and initiation of cancer. Structural complexity indicates different biological roles of I-compounds.

Filtration assay for quantitation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) specific binding to whole cells in culture

A rapid and sensitive filtration assay for quantitating the specific binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to whole cells in culture is described. Cell monolayers are incubated with [3H]TCDD in the presence or absence of excess unlabeled ligand, detached from the culture dish with trypsin, filtered, and washed with cold (-78 degrees C) acetone to separate free and nonspecifically bound TCDD from specifically bound TCDD. TCDD receptor binding parameters were characterized in the murine hepatoma cell line Hepa1c1c7. The lower limit of detection of TCDD specific binding was in a sample equivalent to 10 micrograms of total cell protein. The equilibrium dissociation constant and stereospecificity for binding to the TCDD receptor were the same as those previously reported with other TCDD receptor assays on broken cell preparations. Analysis of binding in the murine hepatoma TCDD receptor variants TAO-c1BPrc1 and BPrc1 indicated that this assay will detect receptor number or affinity variants, but will not detect nuclear transfer deficient variants. The major advantage of the whole cell binding assay is that it provides the means to rapidly and reproducibly quantitate TCDD specific binding in small samples of whole cells in culture. In addition, this method eliminates loss or degradation of the receptor protein during the fractionation of cells required in previously reported methods. This method should prove useful in screening clonal cell populations for TCDD receptor number and affinity variants, and in screening for TCDD receptor binding activity in complementation studies of receptor deficient cells.

Effects of TCDD on embryonic ureteric epithelial EGF receptor expression and cell proliferation

The potent toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, producing hydronephrosis and cleft palate. Because of the long half-life of TCDD, the urinary tract is exposed throughout development after a single dose on gestation day (GD) 10 or earlier. TCDD-induced hydronephrosis is a consequence of occlusion of the ureter by epithelial cells. Since embryonic growth factors and the epidermal growth factor (EGF) receptor are probably involved in regulation of embryonic cell proliferation, this study examines the effects of TCDD on expression of EGF receptors and proliferation of ureteric epithelial cells in vivo and in culture. After exposure to TCDD by gavage (12, 24, or 30 micrograms/kg on GD 10; 6 or 24 micrograms/kg on GD 12) the mean cell depth of the ureteric and bladder epithelia was increased. EGF receptors were detected immunohistochemically in sectioned urinary tracts. The expression of receptors decreased with advancing development in control ureteric epithelia. However, after TCDD exposure the level of EGF receptors failed to decline. The incorporation of 3H-TdR was observed in sections by autoradiography, and after exposure to TCDD more epithelial cells showed incorporation than was apparent in controls. Transmission electron microscopy (TEM) of embryonic ureters from fetuses exposed to TCDD in vivo showed no cytotoxicity in basal cells and the cells remained undifferentiated, as in controls. Ureters taken from GD 12 embryos and cultured with 1 x 10(-10)M TCDD showed ureteric epithelial hyperplasia without cytotoxicity, but at 1 x 10(-8)M TCDD evidence of cytotoxicity was observed by TEM. The levels of TCDD found in fetuses after in vivo exposure (204-307 pg/fetus, with 1-2 pg in the urinary tract) compare well with the in vitro level (32 pg/ml), which was most effective in producing hyperplasia of the epithelial cells. The present study correlates a TCDD-induced increase in cell depth with altered regulation of EGF receptors and excessive proliferation, both in vivo and in cultured embryonic ureters.

Selective inhibition of polymorphonuclear neutrophil activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Although the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), via its interaction with the Ah receptor, is an extremely potent carcinogen and immunosuppressive agent in experimental animals, its possible actions on polymorphonuclear (PMN) function have not been determined. In addition to their importance against infectious organisms, PMNs have been implicated in antitumor resistance. The present studies examined the effects of in vivo exposure to TCDD on PMN function in B6C3F1 (TCDD sensitive, presence of high affinity Ah receptor) and DBA/2N (TCDD resistant at low doses, defective Ah receptor) mice. Animals received a single oral exposure of 5 or 10 micrograms/kg of TCDD and PMNs were obtained 5 days later from the peritoneal cavity following elicitation with sodium caseinate. TCDD reduced the cytolytic and cytostatic activity of PMA-activated PMNs in B6C3F1, but not in DBA/2N mice, suggesting that this response segregates with the Ah locus. Furthermore, TCDD was found to bind specifically to PMNs from Ah-responsive mice. Neither the production of superoxide and hydrogen peroxide nor degranulation, the latter measured by beta-glucuronidase release, was impaired. Supernatants recovered from PMN cell cultures of TCDD-sensitive mice, but not from resistant DBA/2N mice, showed reduced killing capacity for actinomycin D-treated L929 tumor cells, while their ability to bind to tumor cells was not altered. These data suggest that TCDD interferes with PMN-mediated tumor cell killing by altering the production or secretion of a cytolytic factor. Examination of bone marrow stem cells revealed that granulocytic but not monocytic colonies were reduced after TCDD exposure in vivo and in vitro. Although mature PMNs had detectable levels of Ah receptor, exposure in vitro of these cells to TCDD had no effect on antitumor activity. Thus, it is possible that TCDD may affect PMNs at the level of hematopoiesis, via a direct interaction with granulocyte precursor cells, or modulate PMNs at different stages of maturation.

Evidence that 2,3,7,8-tetrachlorodibenzo-p-dioxin and thyroid hormones act through different mechanisms in human keratinocytes

It has been proposed [J. D. McKinney, J. Fawkes, S. Jordan, K. Chae, S. Oatley, R. E. Coleman, and W. Briner (1985). Environ. Health Perspect. 61, 41-53] that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces toxic responses through persistent occupancy of nuclear thyroxine (T4) receptors, and that maintenance of receptor occupancy by supraphysiologic concentrations of thyroid hormones mimics TCDD toxicity [L. H. Hong, J. D. McKinney, and M. I. Luster (1987). Biochem. Pharmacol., 36, 1361-1365]. TCDD induces hyperkeratinization in cultured normal human epidermal cells and the human keratinocyte line, SCC-12F. This response is associated with a decrease in high-affinity epidermal growth factor (EGF) receptors. These cell systems were used as models to compare the actions of TCDD with those of triiodothyronine (T3) and T4 on human target cells. Keratinocytes were treated simultaneously with T3 and T4 in a 4:1 molar ratio (T3/T4; Hong et al., 1987) and levels of EGF binding and 7-ethoxycoumarin O-deethylase activity (a marker for cytochrome P1-450 induction) were measured. T3/T4 (at concentrations up to 10 microM T3/2.5 microM T4) and T3 or T4 alone (0.1 to 10 microM) did not produce the hyperkeratinization, the decrease in EGF binding, or the increase in ECOD activity that are characteristic of TCDD exposure. Nonresponsiveness to T3/T4 was not due to metabolism of these hormones by the keratinocytes. T3 and T4 did not compete with [3H]TCDD for binding to cytosolic Ah receptor from C57BL6 mouse liver, SCC-12F, or normal human epidermal cells. TCDD and an active stereoisomer, 2,3,7,8-tetrachlorodibenzofuran, did not compete with [125I]T3 or [125I]T4 for binding to nuclear receptors from SCC-12F cells or C57BL6 mouse liver. Taken together, these data demonstrate that the actions of TCDD and thyroid hormones are mediated by distinct mechanisms in human keratinocytes.

Mechanisms of monooxygenase induction and inhibition

The mechanisms of monooxygenase induction and inhibition have been discussed from the standpoint of historic development, from the current concepts about the molecular mechanism of enzyme induction, and from the various possibilities by which inhibitors can interact with the complex cytochrome P-450 monooxygenase system. In detail the main features and phenomena of induction and of the induced new enzyme protein are briefly described, whereby general principles are emphasized. The current knowledge on the mechanism of induction is exemplified by a description of the inducing action of TCDD on mouse hepatoma cells. A special example of increase in the molecular activity towards 7-ethoxycoumarin-0-deethylation is given by the action of sulmazole on mouse liver cytochrome P-450. It possesses properties similar to that of cobaltous chloride in that it decreases the amount of cytochrome P-450 in the microsomal protein but at the same time increases the molecular activity to about a four-fold level. The mechanisms of inhibition of the microsomal monooxygenase are explained in general terms by outlining the various modes of inhibitory action that lead to a decrease in enzyme activity.