Comparisons of the effects of TCDD and hydrocortisone on growth factor expression provide insight into their interaction in the embryonic mouse palate

AhR promotes phosphorylation of ARNT isoform 1 in human T cell malignancies as a switch for optimal AhR activity

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a transcription factor present in immune cells as a long and short isoform, referred to as isoforms 1 and 3, respectively. However, investigation into potential ARNT isoform–specific immune functions is lacking despite the well-established heterodimerization requirement of ARNT with, and for the activity of, the aryl hydrocarbon receptor (AhR), a critical mediator of immune homeostasis. Here, using global and targeted transcriptomics analyses, we show that the relative ARNT isoform 1:3 ratio in human T cell lymphoma cells dictates the amplitude and direction of AhR target gene regulation. Specifically, shifting the ARNT isoform 1:3 ratio lower by suppressing isoform 1 enhances, or higher by suppressing isoform 3 abrogates, AhR responsiveness to ligand activation through preprograming a cellular genetic background that directs explicit gene expression patterns. Moreover, the fluctuations in gene expression patterns that accompany a decrease or increase in the ARNT isoform 1:3 ratio are associated with inflammation or immunosuppression, respectively. Molecular studies identified the unique casein kinase 2 (CK2) phosphorylation site within isoform 1 as an essential parameter to the mechanism of ARNT isoform–specific regulation of AhR signaling. Notably, CK2-mediated phosphorylation of ARNT isoform 1 is dependent on ligand-induced AhR nuclear translocation and is required for optimal AhR target gene regulation. These observations reveal ARNT as a central modulator of AhR activity predicated on the status of the ARNT isoform ratio and suggest that ARNT-based therapies are a viable option for tuning the immune system to target immune disorders.

Protein kinase C and chemical-induced abnormal palate development

The protein kinase C (PKC) family of proteins mediates the action of growth factors and other ligands by activating a network of transcription factors that bind to TRE sequences in the promoters of many genes that regulate cell proliferation, differentiation, extracellular matrix synthesis, apoptosis and others in a cell type-, isozymeand context-specific manner. The critical role of PKC in embryonic development is indicated by early death of embryos in which one or more of these isozymes are inactivated. Our studies together with others show that palatal PKC signalling is functional and may be essential for normal palate development. Although single gene knockouts have failed to exhibit the cleft palate (CP) phenotype, owing to compensation by other kinases, many chemicals including the mycotoxin, secalonic acid D, disrupt palatal PKC signalling leading to altered palatal mesenchymal gene expression. The potential relevance of such effects to chemical-induced CP is discussed.

My Winding Road: From Microbiology to Toxicology and Environmental Health

I would certainly never have predicted that I would become the director of the National Institute of Environmental Health Sciences (NIEHS) and the National Toxicology Program (NTP) when I was a Jewish girl growing up in Teaneck, New Jersey. My family stressed the importance of education. Yet for a girl there were many not-so-subtle suggestions that the appropriate careers were in teaching or nursing, and the most important thing was to be a wife and mother. Well, I can't disagree with the latter, although I would have to add grandmother to that list of achievements. My parents were both college graduates, but my mom only taught high school English for one year before leaving the field to start our family. My dad returned from World War II and joined his brother in accounting. After my first sister was born, my father joined my mother's family jewelry business and helped to open a second retail store. My mother helped my dad out during the busy times—Christmas and wedding season—but otherwise focused on our growing family of three girls and one boy. This became increasingly challenging when it became clear that my little brother was severely retarded and would require extra care.

Post-Vietnam military herbicide exposures in UC-123 Agent Orange spray aircraft

BACKGROUND

During the Vietnam War, approximately 20 million gallons of herbicides, including ~10.5 million gallons of dioxin-contaminated Agent Orange, were sprayed by about 34 UC-123 aircraft that were subsequently returned to the United States, without decontamination or testing, to three Air Force reserve units for transport operations (~1971-1982). In 1996, observed dioxin contamination led to withdrawal of these UC-123s from public auction and to their smelting in 2009. Current Air Force and Department of Veterans Affairs policies stipulate that "dried residues" of chemical herbicides and dioxin had not lead to meaningful exposures to flight crew and maintenance personnel, who are thus ineligible for Agent Orange-related benefits or medical examinations and treatment. Sparse monitoring data are available for analysis.

METHODS

Three complementary approaches for modeling potential exposures to dioxin in the post-Vietnam war aircraft were employed: (1) using 1994 and 2009 Air Force surface wipe data to model personnel exposures and to estimate dioxin body burden for dermal-oral exposure for dried residues using modified generic US Environmental Protection Agency intake algorithms; (2) comparing 1979 Air Force 2,4- dichlorophenoxyacetic acid and 2,4-5-trichlorophenoxyacetic acid air samples to saturated vapor pressure concentrations to estimate potential dioxin exposure through inhalation, ingestion and skin contact with contaminated air and dust; and (3) applying emission models for semivolatile organic compounds from contaminated surfaces to estimate airborne contamination.

RESULTS

Model (1): Body-burden estimates for dermal-oral exposure were 0.92 and 5.4pg/kg body-weight-day for flight crew and maintainers. The surface wipe concentrations were nearly two orders of magnitude greater than the US Army guidance level. Model (2): measured airborne concentrations were at least five times greater than saturated vapor pressure, yielding dioxin estimates that ranged from 13.2-27.0pg/m(3), thus supporting the likelihood of dioxin dust adsorption. Model (3): Theoretical models yielded consistent estimates to Model 2, 11-49pg/m(3), where the range reflects differences in experimental value of dioxin vapor pressure and surface area used. Model (3) results also support airborne contamination and dioxin dust adsorption.

CONCLUSIONS

Inhalation, ingestion and skin absorption in aircrew and maintainers were likely to have occurred during post-Vietnam use of the aircraft based on the use of three complementary models. Measured and modeled values for dioxin exceeded several available guidelines. Deposition-aerosolization-redeposition homeostasis of semivolatile organic compound contaminants, particularly dioxin, is likely to have continually existed within the aircraft. Current Air Force and Department of Veterans Affairs policies are not consistent with the available industrial hygiene measurements or with the widely accepted models for semivolatile organic compounds.

Perturbation of Fgf10 signal pathway in mouse embryonic palate by dexamethasone and vitamin B12 in vivo

BACKGROUND/PURPOSE

The Fgf10 signaling pathway plays an important role in early stages of mouse embryonic palatal development, which is associated with cell proliferation and differentiation. The objective of this study was to assess whether dexamethasone and vitamin B(12) affected the Fgf10 signal pathway of mouse embryonic palate.

MATERIALS AND METHODS

Immunohistochemical studies were performed for expression of Fgf10, Fgfr2b, and sonic hedgehog and for cell proliferation and apoptosis of mouse embryonic palate.

RESULTS

The expression of Fgf10, Fgfr2b, and sonic hedgehog was changed in mouse embryonic palate after dexamethasone and vitamin B(12) treatment, resulting in reduced and restored proliferation of mesenchymal cells.

CONCLUSIONS

Dexamethasone and vitamin B(12) affected the Fgf10 signaling pathway and cell proliferation of mouse embryonic palate. Cell apoptosis was not altered after dexamethasone and vitamin B(12) exposure.

Growth factors, cytokines and their receptors as downstream targets of arylhydrocarbon receptor (AhR) signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant, which causes a variety of severe health effects, e.g. immunosuppression, hepatotoxicity, and carcinogenesis. The main mediator of TCDD toxicity is the arylhydrocarbon receptor (AhR), which, upon activation, translocates into the nucleus and enforces gene expression. Since most of the pleiotropic effects caused by TCDD are associated with alterations in cell growth and differentiation, the analysis of the interference of the AhR with factors controlling these cellular functions seems to be a promising target regarding the prevention and treatment of chemical-provoked diseases. Cell growth and differentiation are regulated by numerous growth factors and cytokines. These multifunctional peptides promote or inhibit cell growth and regulate differentiation and other cellular processes, depending on cell-type and developmental stage. They are involved in the regulation of a broad range of physiological processes, including immune response, hematopoiesis, neurogenesis, and tissue remodeling. The complex network of growth factors and cytokines is accurately regulated and disturbances of this system are associated with adverse health effects. The molecular mechanisms by which the AhR interferes with this signaling network are multifaceted and the physiological consequences of this cross-talk are quite enigmatic. The investigation of this complex interaction is an exciting task, especially with respect to the recently described non-genomic and/or ligand-independent activities of AhR. Therefore, we summarize the current knowledge about the interaction of the AhR with three cytokine-/growth factor-related signal transducers -- the epidermal growth factor (EGF) family, tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta) -- with regard to pathophysiological findings.

Ah receptor: Dioxin-mediated toxic responses as hints to deregulated physiologic functions

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and member of the bHLH/PAS (basic Helix-Loop-Helix/Per-Arnt-Sim) family of chemosensors and developmental regulators. It represents a multifunctional molecular switch regulating endo- and xenobiotic metabolism as well as cell proliferation and differentiation. Physiologic functions of the AhR are beginning to be understood, including functions in vascular development, and in detoxification of endo- and xenobiotics. The AhR is also recognized as the culprit for most toxic responses observed after exposure to dioxins and related compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The non-metabolizable AhR agonist TCDD has to be distinguished from the myriad of metabolizable agonists present as dietary contaminants and plant constituents as well as endogenous toxins. The hypothesis is emerging that the diverse tissue-specific, TCDD-mediated toxicities are due to sustained and inappropriate AhR activation leading to deregulated physiologic functions. In support of this hypothesis recent observations in the context of some TCDD-mediated toxic responses are discussed, such as chloracne, cleft palate, thymus involution and in particular carcinogenesis. Major open questions are addressed, such as ligand-independent AhR activation by phosphorylation and the large differences in species-dependent susceptibility to toxic responses. Though important issues remain unresolved, the commentary is intended to stimulate efforts to understand dioxin-mediated toxic responses with emphasis on carcinogenesis in comparison with AhR-mediated physiologic functions.

Rescue of an in vitro palate nonfusion model using interposed embryonic mesenchyme

The authors previously established an in vitro palate nonfusion model on the basis of a spatial separation between prefusion embryonic day 13.5 mouse palates (term gestation, 19.5 days). They found that an interpalatal separation distance of 0.48 mm or greater would consistently result in nonfusion after 4 days in organ culture. In the present study, they interposed embryonic palatal mesenchymal tissue between embryonic day 13.5 mouse palatal shelves with interpalatal separation distances greater than 0.48 mm in an attempt to "rescue" this in vitro palate nonfusion phenotype. Because no medial epithelial bilayer (i.e., medial epithelial seam) could potentially form, palatal fusion in vitro was defined as intershelf mesenchymal continuity with resolution of the medial edge epithelia bilaterally. Forty-two (n = 42) palatal shelf pairs from embryonic day 13.5 CD-1 mouse embryos were isolated and placed on cell culture inserts at precisely graded distances (0, 0.67, and 0.95 mm). Positive controls consisted of shelves placed in contact (n = 6). Negative controls consisted of shelves placed at interpalatal separation distances of 0.67 mm (n = 6) and 0.95 mm (n = 7) with no interposed mesenchyme. Experimental groups consisted of embryonic day 13.5 palatal shelves separated by 0.67 mm (n = 11) and 0.95 mm (n = 12) with interposed lateral palatal mesenchyme isolated at the time of palatal shelf harvest. Specimens were cultured for 4 days (n = 19) or 10 days (n = 23), harvested, and evaluated histologically. All positive controls at 4 and 10 days in culture showed complete histologic palatal fusion. All negative controls at 4 days and 10 days in culture remained unfused. Five of six palatal shelves separated at 0.67 mm interpalatal separation distance with interposed mesenchyme were fused at 4 days, and all five were fused at 10 days. At an interpalatal separation distance of 0.95 mm with interposed mesenchyme (n = 12), no palates (zero of four) were fused at 4 days, but seven of eight were fused at 10 days. These data suggest that nonfused palatal shelves can be "rescued" with an interposed graft of endogenous embryonic mesenchyme to induce fusion in vitro.

An in vitro mouse model of cleft palate: defining a critical intershelf distance necessary for palatal clefting

It is unclear whether cleft palate formation is attributable to intrinsic biomolecular defects in the embryonic elevating palatal shelves or to an inability of the shelves to overcome a mechanical obstruction (such as the tongue in Pierre Robin sequence) to normal fusion. Regardless of the specific mechanism, presumably embryonic palatal shelves are ultimately unable to bridge a critical distance and remain unapproximated, resulting in a clefting defect at birth. We propose to use a palate organ culture system to determine the critical distance beyond which embryonic palatal shelves fail to fuse (i.e., the minimal critical intershelf distance). In doing so, we hope to establish an in vitro cleft palate model that could then be used to investigate the contributions of various signaling pathways to cleft formation and to study novel in utero treatment strategies. Palatal shelves from CD-1 mouse embryos were microdissected on day 13.5 of gestation (E13.5; term = 19.5 days), before fusion. Using a standardized microscope ocular grid, paired palatal shelves were placed on a filter insert at precisely graded distances ranging from 0 (in contact) to 1.9 mm (0, 0.095, 0.19, 0.26, 0.38, 0.48, 0.57, 0.76, 0.95, and 1.9 mm). A total of 68 paired palatal shelves were placed in serum-free organ culture for 96 hours (n = 68). Sample sizes of 10 were used for each intershelf distance up to and including 0.48 mm (n = 60). For intershelf distances of 0.57 mm and greater, two-paired palatal shelves were cultured (n = 8). All specimens were assessed grossly and histologically for palatal fusion. Palatal fusion occurred in our model only when intershelf distances were 0.38 mm or less. At 0.38 mm, eight of 10 palates appeared grossly adherent, whereas six of 10 demonstrated clear fusion histologically with resolution of the medial epithelial seam and continuity of the palatal mesenchyme. None of the 18 palates fused when placed at intershelf distances of 0.48 mm or greater. Using our selected intershelf distances as a guideline, we have established an approximate minimal critical intershelf distance (0.48 mm) at which we can reliably expect no palatal fusion. Culturing palatal shelves at intershelf distances of 0.48 mm or greater results in nonfusion or clefting in vitro. This model will allow us to study biomolecular characteristics of unfused or cleft palatal shelves in comparison with fused shelves. Furthermore, we plan to study the efficacy of grafting with exogenous embryonic mesenchyme or candidate factors to overcome clefting in vitro as a first step toward future in utero treatment strategies.

Secalonic acid D alters the nature of and inhibits the binding of the transcription factors to the phorbol 12-O-tetradecanoate-13 acetate-response element in the developing murine secondary palate

Secalonic acid D (SAD), a mycotoxin produced by Penicillium oxalicum in corn, induces cleft palate (CP) in the offspring of exposed dams. Results of recent studies suggest that protein kinase C (PKC) inhibition by SAD may be relevant to its CP-induction. Downstream effects of PKC are determined by the nature of transcription factors (TF) that form the activator protein-1 (AP-1) and the binding of AP-1 (and other TF) to the phorbol 12-O-tetradecanoate-13 acetate-response element (TRE) to form AP-1-TRE complex, neither of which have been studied in the palate. The aims of the present study were to identify the components of the murine palatal AP-1-TRE complex during development and to uncover the effects of SAD on this complex. Western blots and gel mobility shift assays of control palatal nuclear extracts revealed that, although all relevant TF are present in the palate throughout development, only cyclic-AMP response element (CRE) binding protein (CREB) and CRE-modulator protein-1 (CREM-1) and activating transcription factor-1 bound to TRE on Gestation Day (GD) 12. The pattern shifted to c-Jun and c-Fos (known AP-1 components) on GD 13 and 14. In SAD-treated offspring, however, CREM-1 alone; c-Jun, c-Fos, and CREB; and c-Jun and c-Fos bound to TRE on GD 12, 13, and 14, respectively. Binding of TF to TRE was inhibited by SAD on both GD 12 and 13. These results suggest that a dynamic shift in the binding of TF to TRE from PKA- to PKC-responsive TF occurs during palate development and that teratogens such as SAD can alter both the nature and extent of TF binding to TRE.

Strain dependency of TGFbeta1 function during embryogenesis

There is incomplete penetrance to Tgfb1 knockout phenotypes. About 50% of Tgfb1 homozygous mutant (Tgfb1-/-) and 25% of Tgfb1 heterozygous (Tgfb1+/-) embryos die during embryogenesis. In a mixed NIH/Ola x C57BL/6J/Ola x 129 background partial embryonic lethality of the Tgfb1-/-embryos occurs due to defective yolk sac vasculopoiesis and/or hematopoiesis. We show here that on a predominantly CF-1 genetic background, lack of TGFbeta1 causes a pre-morula lethality in about 50% of the null embryos. This partial lethality is not reversed by transfer of Tgfb1-/- embryos to Tgfb1-/+ hosts. The extent of embryonic lethality in Tgfb1-/- embryos ranges in a background dependent manner from 20% to 100%. Based on these and other studies it is clear that TGFbeta1 acts at two distinct phases of embryogenesis: pre-implantation development and yolk sac vasculogenesis/hematopoiesis. The susceptibility for the pre-implantation lethality depends on a small number of genetic modifiers since a small number of backcrosses onto the high susceptibility strain C57BL/6 leads to complete penetrance of the lethality.

Cross-talk between signaling pathways in murine embryonic palate cells: effect of TGF beta and cAMP on EGF-induced DNA synthesis

Signaling pathways utilized by EGF, cAMP, and TGF beta have been demonstrated to play critical roles in normal palate development. Stimulation of these pathways has been shown in palate cells and numerous other systems to affect cell growth. Because proper regulation of cell growth is critical to palate development, we speculate that fine regulation of palatal cell growth may be accomplished through crosstalk between these signaling pathways. We therefore set out to determine the effects of cAMP and TGF beta on EGF-induced cell proliferation in murine embryonic palate cells. We found that both TGF beta and cAMP inhibited the proliferative response of cells to treatment with EGF, whereas H89, a serine/ threonine protein kinase inhibitor with selectivity towards cAMP-dependent protein kinase, increased the cells' proliferative response to EGF. Genestein, a selective inhibitor of tyrosine kinases, at high doses abrogated the cells' proliferative response to EGF, confirming that EGF's ability to induce cell proliferation is critically dependent upon tyrosine kinase activity. Lower doses of genestein, however, actually enhanced cellular response to EGF. The data suggest that both the TGF beta- and cAMP-mediated signaling pathways may be involved in modulation of the effects of EGF on palate cell growth in vivo.

Species-specific alteration of hepatic glucose 6-phosphate dehydrogenase activity with coplanar polychlorinated biphenyl: evidence for an Ah-receptor-linked mechanism

We examined the in vivo effect of a highly toxic coplanar polychlorinated biphenyl (PCB) on the hepatic activity of glucose 6-phosphate dehydrogenase (G6PDH) in aryl hydrocarbon (Ah)-responsive (C57/BL) and -less-responsive (DBA) strains of mice. The activity in the C57BL strain was moderately increased by 3,3',4,4',5-pentachlorobiphenyl (PCB 126) in a dose dependent manner. However, this was not observed in DBA mice although greater doses were injected. 2,2',5,5'-Tetrachlorobiphenyl (PCB 52) with a non-planar structure did not increase G6PDH activity. The increase in G6PDH activity with PCB 126 was also seen in rats, but not in guinea pigs. The activity in the latter species was decreased rather than increased. These results suggest that the induction of hepatic G6PDH by coplanar PCB is mediated by a mechanism involving the Ah receptor, and the response was highly species-specific.

Mesenchymal cell activity during 5-fluoro-2-deoxyuridine-induced cleft palate formation in the rat

Biosynthetic activity of the mesenchymal cells within the palatal shelves was determined during cleft palate formation induced by 5-fluoro-2-deoxyuridine (FUDR). The palatal shelves of 30 fetal rat heads with palatal clefts were surveyed at stages corresponding to normal palatogenesis, nucleolar organizer region (NOR) staining being employed to determine cell activity. Comparing cellular activity during normal and cleft palatogenesis, significantly lower counts were recorded for most stages of cleft formation. When anterior or posterior regions were compared, significant changes in NOR counts cell were found at a time corresponding to the pre-elevation stage of normal palatogenesis. At a time equivalent to normal fusion, the shelves in the anterior region showed signs of recovery, but posteriorly significantly lower activity occurred throughout all stages of palate dysmorphogenesis. The depressed level of cellular activity found after treatment with FUDR may be directly or indirectly related to the abolition of an intrinsic shelf-elevation force and, subsequently, cleft palate formation.

Developmental expression and CORT-regulation of TGF-beta and EGF receptor mRNA during mouse palatal morphogenesis: correlation between CORT-induced cleft palate and TGF-beta 2 mRNA expression

Glucocorticoids (CORT) have been shown to induce cleft palate in mice. Although the pathogenetic pathway of CORT-induced cleft palate has been investigated for several decades, the molecular details remain to be elucidated. Since growth factors have been shown to regulate palate morphogenesis, and the expression of several growth factors or their receptors, e.g. TGF-beta, EGF receptor (EGF-R), are known to be modulated by CORT, we postulate that CORT modulation of growth factor (or receptor) gene expression is a key mechanism involved in CORT-induced cleft palate. To test this hypothesis, we analyzed the steady-state levels (Northern and RNase protection) and developmental expression (in situ hybridization) of four CORT-responsive genes--TGF-Beta 1, TGF- beta 2, TGF-beta 3, and EGF receptor (EGF-R)--in developing mouse palates in the presence or absence of exogenous CORT. Pregnant B10.A dams were injected on day 12 of gestation with CORT or sham-injected and embryonic palates were collected at 1, 2, and 3 days postinjection (E13-E15). During mouse palate development, significant increases in TGF-beta 1 and TGF-beta 3 mRNA levels, as well as significant decrease in TGF-beta 2 mRNA levels, are detected; no significant difference in EGF-R transcript level is observed with progressive development. In CORT-exposed palates, we demonstrate no significant differences in the direction or magnitude of change with time in TGF-beta 1, TGF-beta 3, and EGF-R mRNA levels compared to controls. However, CORT delays by 1 day the down-regulation of palatal TGF-beta 2 transcript normally seen on day 14 of gestation. TGF-beta 2 is known to inhibit cell proliferation. The level of TGF-beta 2 mRNA, the only isoform primarily expressed in the palatal mesenchyme, significantly decreases with progressive palatal development; this down-regulation of TGF-beta 2 expression is associated with increased mesenchymal cell proliferation and palatal shelf growth. CORT, at a critical stage of palatogenesis, induces a delay in the normal down-regulation of TGF-beta 2 gene expression. Given that CORT is known to inhibit mesenchymal cell proliferation and palatal shelf growth, we conclude that the CORT-induced delay in the normal down-regulation of TGF-beta 2 gene expression is probably key event in the pathogenesis of CORT-induced cleft palate.

Effects of dexamethasone on the expression of transforming growth factor-beta in mouse embryonic palatal mesenchymal cells

The central role of TGF-beta in the development of the embryonic palate has been well characterized. TGF-beta inhibits mesenchymal cell proliferation, induces medial edge epithelial cell differentiation, and modulates the expression of extracellular matrix proteins as well as the proteases that act upon them. Mechanisms by which TGF-beta expression itself is regulated are less well understood. Glucocorticoids are recognized in several cellular systems as able to regulate the expression of TGF-beta. This study was therefore designed to examine whether glucocorticoids affect the expression of TGF-beta isoforms in embryonic palatal cells. Based on flow cytometric analysis and viability determination, confluent primary cultures of mouse embryonic palate mesenchymal (MEPM) cells exposed to up to 10(-6) M dexamethasone (dex) exhibited no signs of cytotoxicity after 24 hours of exposure. Northern blot analyses revealed that dexamethasone reduced steady-state mRNA levels of TGF-beta 3 in a dose-dependent manner as early as 4 hours after treatment but had little effect on TGF-beta 1 and TGF-beta 2 expression up to 24 hours of dex exposure. Dex also reduced the synthesis of both latent and mature forms of TGF-beta protein by approximately four-fold as determined by the mink lung epithelial cell growth inhibition bioassay. Assessment of the ratio of mature to latent protein found in conditioned medium of control compared to dex-treated cultures indicated that dexamethasone may reduce the activation of latent TGF-beta to mature biologically active TGF-beta. Dexamethasone inhibited the proliferation of MEPM cells despite the down-regulation of TGF-beta suggesting that dex-induced growth inhibition of MEPM cells is not mediated by TGF-beta. These data suggest that dex modulates TGF-beta signaling pathways directly by down-regulating TGF-beta expression and possibly indirectly by altering the availability of mature TGF-beta necessary to exert its biological effects in the developing palate.

Developmental expression of two members of a new class of transcription factors: II. Expression of aryl hydrocarbon receptor nuclear translocator in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix (bHLH) proteins involved in transcriptional regulation. The AhR is a ligand-activated partner of the ARNT protein. Both proteins are required to transcriptionally regulate gene expression. ARNT must be complexed to AhR to permit binding to the regulatory DNA sequence. The AhR-ligand complex is known to mediate a range of biological responses, such as developmental toxicity, induction of cleft palate, and hydronephrosis. AhR and ARNT are expressed in human embryonic palatal cells and AhR was recently shown to have a specific developmental pattern of expression in the mouse embryo. In the present study, expression of ARNT is characterized in C57Bl/6N mouse embryos from gestation day (GD) 10-16 using immunohistochemistry and in situ hybridization. Af affinity purified antibody against human ARNT (1.1 micrograms/ml) was detected with an avidin-biotin-peroxidase complex. ARNT mRNA was localized with a 35S-RNA probe from pBM5/NEO-M1-1. Specific spatial and temporal patterns of ARNT expression emerged and mRNA and protein expression correlated. The GD 10-11 embryos showed highest levels of ARNT in neuroepithelial cells of the neural tube, visceral arches, otic and optic placodes, and preganglionic complexes. The heart also had significant expression of ARNT with strong nuclear localization. After GD11, expression in heart and brain declined. In GD 12-13 embryos expression was highest in the liver where expression increased from GD 12-16. At GD 15-16 the highest levels of ARNT occurred in adrenal gland and liver, although ARNT was also detected in submandibular gland, ectoderm, tongue, bone, and muscle. In all of these tissues ARNT was cytoplasmic as well as nuclear, except in some of the cortical adrenal cells in which ARNT was strongly cytoplasmic with little or no nuclear localization. These specific patterns of ARNT expression, which differ in certain tissues from the expression of AhR, suggest that ARNT may have additional roles in normal embryonic development.

Developmental expression of two members of a new class of transcription factors: I. Expression of aryl hydrocarbon receptor in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with a basic region/helix-loop-helix (bHLH) motif. AhR has been sequenced and the functional domains defined and there is information on the formation of complexes with other peptides and interactions with DNA, although these areas continue to be investigated. AhR mediates many biological effects such as developmental toxicity, including induction of cleft palate and hydronephrosis. This regulatory protein is expressed in embryonic liver and has been immunohistochemically localized in cells of human and mouse secondary palate. The expression of AhR in embryonic tissues and its ability to disrupt development suggests a significant role for this protein in development. The present study examines the pattern of AhR expression in the C57BL/6N mouse embryo from gestation days (GD) 10-16, using in situ hybridization and immunohistochemical analysis. AhR mRNA was localized with 35S-RNA antisense riboprobe (cAh1 probe, 1.8 Kb amino terminal DNA). AhR protein was localized with purified monoclonal antibody (RPT-9) raised against the N-terminal peptide sequence. AhR mRNA and protein were expressed in GD 10-13 neuroepithelium, and as development progressed the levels in brain decreased. GD 10-12 embryos also showed AhR in branchial arches, heart, somites, and liver. AhR protein and mRNA in heart were highest at GD 10-11 and decreased with age. In liver, AhR mRNA and protein levels increased and nuclear localization became more pronounced with gestational age. In GD 14-16 embryos levels in liver and adrenal were highest, but AhR was present in ectoderm, bone, and muscle. AhR expression was specific for both cell type, organ/tissue, and developmental stage, suggesting that this novel ligand-activated transcriptional regulator may be important in normal embryonic development.

Elevated plasma transforming growth factor-beta 1 levels in breast cancer patients decrease after surgical removal of the tumor

OBJECTIVE

The authors determined whether untreated breast cancer patients have elevated plasma levels of transforming growth factor-beta 1 (TGF-beta 1).

SUMMARY BACKGROUND DATA

Increased plasma TGF-beta 1 levels recently were found after chemotherapy in patients with advanced breast cancer. However, it currently is unknown whether this elevation in plasma TGF-beta 1 is caused by chemotherapy-induced normal tissue damage or whether it results from the presence of the tumor.

METHODS

An enzyme-linked immunosorbent assay was used to measure plasma TGF-beta 1 levels in 26 newly diagnosed breast cancer patients before and after definitive surgery. Patients were grouped by postoperative tumor status: 1) negative lymph nodes (group 1); 2) positive lymph nodes (group 2); and 3) overt residual disease (group 3). The site of TGF-beta 1 production in the tumors was localized by immunohistochemistry and in situ hybridization.

RESULTS

Plasma TGF-beta 1 levels were elevated preoperatively in 81% of the patients; TGF-beta 2 and TGF-beta 3 were undetectable. The preoperative TGF-beta 1 levels in the three patient groups were similar; however, the postoperative plasma TGF-beta 1 levels differed by disease status. The mean plasma TGF-beta 1 level in group 1 (n = 12) normalized after surgery (19.3 +/- 3.2 vs. 5.5 +/- 1.0 ng/mL, p < 0.001). In contrast, the mean plasma TGF-beta 1 levels remained above normal in both group 2 (n = 9) and group 3 (n = 5) after surgery. Transforming growth factor-beta 1 expression was found to be preferentially increased in the tumor stroma.

CONCLUSIONS

Breast tumors result in increased plasma TGF-beta 1 levels in 81% of patients. After surgical removal of the primary tumor, the plasma TGF-beta 1 level normalizes in the majority of patients; persistently elevated levels correlate with the presence of lymph node metastases or overt residual tumor. These findings suggest that the usefulness of TGF-beta 1 as a potential plasma marker for breast tumors deserves further study.

Suppression of cytotoxic T lymphocyte activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin occurs in vivo, but not in vitro, and is independent of corticosterone elevation

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressive compound. In our laboratory, TCDD and structurally related polychlorinated biphenyls (PCBs) have been shown to suppress alloantigen-specific cytotoxic T lymphocyte (CTL) activity in C57B1/6 mice. PCB-induced CTL suppression occurs coincident with significant elevation of plasma glucocorticoid (GC) levels (> 500 ng/ml). Since GC elevation can cause immune suppression, this study was conducted to determine if TCDD-induced CTL suppression is correlated with elevation of plasma corticosterone (CS), the major GC in mice. Single oral doses of TCDD (2.5-40 micrograms/kg) induced a dose-dependent suppression of CTL activity with a calculated 50% immunosuppressive dose (ID50) occurring at 7.2 micrograms/kg. When total lytic units (LU)/spleen were calculated, the ID50 was 2.8 micrograms/kg. In contrast, plasma CS levels were not significantly altered at doses below 40 micrograms/kg. These data suggest that TCDD-induced CTL suppression is not dependent on CS elevation. The direct effect of TCDD on CTL generation was tested by adding TCDD at 10(-13)-10(-9) M to in vitro mixed lymphocyte-tumor cell (MLTC) cultures. No alteration of CTL activity was observed after 5 days of culture at any TCDD concentration. In contrast, CS alone significantly suppressed CTL activity in vitro. CS-induced CTL suppression in vitro was neither enhanced nor inhibited by the presence of TCDD. These results suggest that TCDD causes CTL suppression in vivo by a mechanism that does not involve CS.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

Immunohistochemical double-staining for Ah receptor and ARNT in human embryonic palatal shelves

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix-PAS (HLH) proteins involved in transcriptional regulation. Polycyclic aromatic halogenated chemicals, of which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent, bind to the AhR. In the cellular cytoplasm, the AhR exists as a complex with the heat shock protein HSP90 and other small peptides. This complex dissociates following ligand binding and then the ligand-bound AhR binds ARNT. The ligand-AhR-ARNT complex interacts with a specific, nuclear DNA sequence, the dioxin response element (DRE), altering transcription of a regulated gene. Studies in hepatoma cell lines indicate that both proteins are required for regulation of transcription. In this study, AhR and ARNT were localized immunohistochemically in human embryonic palatal cells and specific patterns of expression were seen for each protein. A double-staining protocol revealed that epithelial cells expressed both AhR and ARNT, but in mesenchyme and nasal spine cartilage individual cells were identified which expressed either AhR or ARNT. This heterogeneous pattern may be a means of suppressing transcriptional regulation and also suggests the existence of other, unidentified basic-helix-loop-helix partner(s). The heterogeneous expression pattern may also reflect a complex role for these HLH proteins as transcriptional regulators of embryonic development.

Effects of methanol on embryonic mouse palate in serum-free organ culture

Methanol has widespread applications in industry and manufacturing and is under consideration as an alternative automotive fuel. Human exposure to methanol would be expected to increase if applications expand in coming years. Methanol has been shown to be a reproductive and developmental toxicant in the rodent, producing cleft palate in the CD-1 mouse. Developmental toxicity has also been demonstrated in vitro for rat and mouse embryos in whole embryo culture. The present study examines the developmental toxicity of methanol in the palate using a serum-free organ culture model. Gestation day 12 CD-1 mouse embryos were dissected and mid-craniofacial tissues were cultured in BGJ medium at 37 degrees C for 4 days with medium changes at 24 hr intervals. Cultures were exposed to methanol from 0-20 mg/ml for 6 hr, 12 hr, 1 or 4 days. Some cultures were exposed to ethanol for 4 days at doses ranging from 0-15 mg/ml. All cultures were gassed with a 50% O2, 5% CO2, and 45% N2 upon addition of fresh medium and prior to the addition of alcohol. Following organ culture the craniofacial explants were examined for effects on morphology, fusion, proliferation, and growth. Incidence and completeness of palatal fusion decreased with increasing exposure. Depending on the concentration and duration of methanol exposure, the medial epithelium either degenerated completely or remained intact in unfused palates and either condition would interfere with fusion. Cellular proliferation appeared to be a specific and sensitive target for methanol as craniofacial tissues responded to methanol with reduction in total DNA content at an exposure that did not affect total protein. However, both DNA and protein decreased with increasing exposure to methanol. Incorporation of thymidine decreased significantly after 4 day exposure and autoradiography of 3H-thymidine (TdR) demonstrated exposure-dependent reduction in proliferation of palatal mesenchymal cells. Ethanol decreased fusion score, total protein, and DNA, but 3H-TdR/DNA was not significantly changed. In general the ethanol was more potent than methanol for inhibition of protein and DNA synthesis and palatal fusion. This study demonstrated that methanol can selectively affect specific sensitive cell populations and has effects on proliferation and cell fate.

Dioxin hepatic carcinogenesis: biologically motivated modeling and risk assessment

There are several key portions of the exposure-dose-response continuum with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) that have to be described quantitatively in developing a comprehensive mechanistically based dose-response model. These include: (i) the accumulation of TCDD in the target tissue, (ii) formation of a complex between dioxin and the Ah receptor, (iii) activation of transcription of growth regulatory genes by the TCDD-Ah receptor complex, (iv) cellular events on tumor initiation, promotion, and progression. Physiologically based pharmacokinetic (PBPK) models have been used as tools to integrate knowledge of the determinants of dioxin disposition, including specific binding to dioxin-inducible hepatic cytochromes, and to link TCDD tissue dosimetry with gene activation by pharmacodynamic (PD) models crafted to examine dioxin-regulated gene expression. Biological studies on growth factor regulation suggest hypotheses for the role of these gene products in transient cell proliferation, prolonged growth suppression, and hepatic tumor promotion. We have used these hypotheses as the basis for stochastic cell growth models of the promotional events with TCDD and to suggest experimental strategies for future research. The combination of PBPK, PBPD and stochastic cell growth models provides a seamless exposure-dose-response model for TCDD induction of liver tumors in rodents. This comprehensive exposure-dose-response model should prove useful for risk assessment, experimental design, and analysis of noncancer endpoints with this potent, ubiquitous environmental contaminant. This paper outlines progress in formulating and evaluating these models for TCDD.

Aryl hydrocarbon or dioxin receptor: biologic and toxic responses

1. The AhR represents a ligand-activated transcription factor. Receptor agonists include planar aromatic compounds, a variety of heterocyclic plant constituents, and PCDD/PCDF. The latter lead to persistent activation of the receptor due to their strong binding affinity and long biologic half-life of over 10 years in human blood and fat. Practically every person on earth is exposed to these compounds via the diet (> 90%) and by high concentrations in mother's milk. PCDD/PCDF produced toxic responses in exposed people (primarily chloracne and immunosuppression) in the past. However, the present PCDD/PCDF levels (basal levels) in the general population are below those warranting toxicologic concern. 2. The AhR has been characterized as a helix-loop-helix transcription factor related to the Drosophila developmental genes sim and per. The cytosolic form of the receptor is present as an inactive complex with two subunits of HSP90. After ligand binding HSP90 is released and the receptor enters the nucleus as a heterodimer together with a related protein ARNT. It binds with high affinity to certain enhancer elements in the upstream region of several genes such as cytochrome P4501A1 (CYP1A1). The AhR transcriptionally activates several drug-metabolizing enzymes and proteins involved in growth/differentiation, such as the plasminogen activator inhibitor PAI-2 and IL-1 beta. In addition, it modulates the action of a number of other nuclear transcription factors such as receptors of the steroid hormone receptor superfamily and of cell surface receptors such as EGF. With the exception of CYP1A1 induction, little is known about the mechanism of transcriptional activation of the AhR-controlled genes. Many AhR-modulated biologic responses (such as modulation of the estrogen and EGF receptor) appear to be indirect. 3. Persistent activation of the AhR is probably responsible for toxic responses in experimental animals and humans. They are markedly tissue and species specific. In rodents a wasting syndrome, immunosuppression, teratogenicity, chloracne, and carcinogenicity/tumor promotion have been well studied. There is good evidence for an involvement for the AhR in these responses. However, the chain of events from receptor activation to the diverse toxic endpoints is largely unknown. Alteration of growth and differentiation of epithelial tissues may underlie most of the toxic responses. A lot has already been achieved, mostly by characterizing the AhR and transcriptional activation of CYP1A1. Still more work lies ahead of us, for example, elucidation of the physiologic roles of the AhR and of the chains of events from receptor activation to the various biologic and toxic endpoints.