1: Toxicol Pathol. 2005;33(6):726-37. 

Oncogenic Signaling Pathways Activated in DMBA-Induced Mouse Mammary Tumors.

Currier N, Solomon SE, Demicco EG, Chang DL, Farago M, Ying H, Dominguez I,
Sonenshein GE, Cardiff RD, Xiao ZX, Sherr DH, Seldin DC.

University of California-Davis, Center for Comparative Medicine, Davis,
California, USA.

Only about 5% of human breast cancers can be attributed to inheritance of breast
cancer susceptibility genes, while the balance are considered to be sporadic in
origin. Breast cancer incidence varies with diet and other environmental
influences, including carcinogen exposure. However, the effects of environmental
carcinogens on cell growth control pathways are poorly understood. Here we have
examined oncogenic signaling pathways that are activated in mammary tumors in
mice treated with the prototypical polycyclic aromatic hydrocarbon (PAH)
7,12-dimethylbenz[a]anthracene (DMBA). In female FVB mice given 6 doses of 1 mg
of DMBA by weekly gavage beginning at 5 weeks of age, all of the mice developed
tumors by 34 weeks of age (median 20 weeks after beginning DMBA); 75% of the
mice had mammary tumors. DMBA-induced mammary tumors exhibited elevated
expression of the aryl hydrocarbon receptor (AhR), c-myc, cyclin D1, and
hyperphosphorylated retinoblastoma (Rb) protein. Because of this, the activation
of upstream regulatory pathways was assessed, and elements of the Wnt signaling
pathway, the NF-kappa B pathway, and the prolyl isomerase Pin-1 were found to be
frequently up-regulated in the tumors when compared to normal mammary gland
controls. These data suggest that environmental carcinogens can produce
long-lasting alterations in growth and anti-apoptotic pathways, leading to
mammary tumorigenesis.

PMID: 16263698 [PubMed - in process]


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2: Oncogene. 2005 Aug 1; [Epub ahead of print] 

The aryl hydrocarbon receptor constitutively represses c-myc transcription in
human mammary tumor cells.

Yang X, Liu D, Murray TJ, Mitchell GC, Hesterman EV, Karchner SI, Merson RR,
Hahn ME, Sherr DH.

1Department of Environmental Health, Boston University School of Public Health,
Boston, MA, USA.

The aryl hydrocarbon receptor (AhR) is an environmental carcinogen-activated
transcription factor associated with tumorigenesis. High levels of apparently
active AhR characterize a variety of tumors, even in the absence of
environmental ligands. Despite this association between transformation and AhR
upregulation, little is known of the transcriptional consequences of
constitutive AhR activation. Here, the effects of constitutively active and
environmental ligand-induced AhR on c-myc, an oncogene whose promoter contains
six AhR-binding sites (AhREs (aryl hydrocarbon response elements)), were
investigated. A reporter containing the human c-myc promoter, with its six AhREs
and two NF-kappaB-binding sites, was constructed. This vector, and variants with
deletions in the NF-kappaB and/or AhR-binding sites, was transfected into a
human breast cancer cell line, Hs578T, which expresses high levels of apparently
active, nuclear AhR. Results indicate that: (1) the AhR constitutively binds the
c-myc promoter; (2) there is a low but significant baseline level of c-myc
promoter activity, which is not regulated by NF-kappaB and is not affected by an
environmental AhR ligand; (3) deletion of any one of the AhREs has no effect on
constitutive reporter activity, while deletion of all six increases reporter
activity approximately fivefold; (4) a similar increase in reporter activity
occurs when constitutively active AhR is suppressed by transfection with an AhR
repressor plasmid (AhRR); (5) AhRR transfection significantly increases
background levels of endogenous c-myc mRNA and c-Myc protein. These results
suggest that the AhR influences the expression of c-Myc, a protein critical to
malignant transformation.Oncogene advance online publication, 1 August 2005;
doi:10.1038/sj.onc.1208938.

PMID: 16091746 [PubMed - as supplied by publisher]


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3: Oncogene. 2000 Nov 16;19(48):5498-506. 

The RelA NF-kappaB subunit and the aryl hydrocarbon receptor (AhR) cooperate to
transactivate the c-myc promoter in mammary cells.

Kim DW, Gazourian L, Quadri SA, Romieu-Mourez R, Sherr DH, Sonenshein GE.

Department of Biochemistry, Women's Health, Boston University School of
Medicine, Massachusetts 02118, USA.

NF-kappaB/Rel transcription factors regulate many genes involved in control of
cellular proliferation, neoplastic transformation, and apoptosis, including the
c-myc oncogene. Recently, we have observed that levels of NF-kappaB and aryl
hydrocarbon receptor (AhR), which mediates malignant transformation by
environmental carcinogens, are highly elevated and appear constitutively active
in breast cancer cells. Rel factors have been found to functionally interact
with other transcription factors. Here we demonstrate a physical and functional
association between the RelA subunit of NF-kappaB and AhR resulting in the
activation of c-myc gene transcription in breast cancer cells. RelA and AhR
proteins were co-immunoprecipitated from cytoplasmic and nuclear extracts of
non-malignant MCF-10F breast epithelial and malignant Hs578T breast cancer
cells. In transient co-transfection, RelA and AhR gene products demonstrated
cooperation in transactivation of the c-myc promoter, which was dependent on the
NF-kappaB elements, and in induction of endogenous c-Myc protein levels. A novel
AhR/RelA-containing NF-kappaB element binding complex was identified by
electrophoretic mobility shift analysis of nuclear extracts from RelA and AhR
co-transfected Hs578T cells. Thus, the RelA and AhR proteins functionally
cooperate to bind to NF-kappaB elements and induce c-myc gene expression. These
findings suggest a novel signaling mechanism whereby the Ah receptor can
stimulate proliferation and tumorigenesis of mammary cells.

PMID: 11114727 [PubMed - indexed for MEDLINE]


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4: Biochem Biophys Res Commun. 1997 Dec 8;241(1):86-91. 

Transforming growth factor-beta1 coregulates mRNA expression of aryl hydrocarbon
receptor and cell-cycle-regulating genes in human cancer cell lines.

Dohr O, Abel J.

Department of Toxicology, Heinrich-Heine-University of Dusseldorf, Auf'm
Hennekamp 50, Dusseldorf, 40225, Germany.

Transforming growth factor (TGF)-beta1 down-regulates mRNA expression of the
aryl hydrocarbon receptor (AhR) and of AhR-inducible genes in A549 cells. Here,
we describe a dose-dependent inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin
(TCDD)-induced cytochrome P450 (CYP) 1A1, CYP1B1 and
NADPH-quinone-oxidoreductase (NMO-1) mRNA expression as well as TCDD-induced
7-ethoxyresorufin-O-deethylase (EROD) activity in MDA-MB 231 cells. The AhR mRNA
expression was not affected by TGF-beta1. TGF-beta-responsiveness was
investigated by examining the effect on the expression of responsive genes.
While TGF-beta1 up-regulates mRNA expression of TGF-beta1 and TIEG
(TGF-beta-inducible early gene) as well as luciferase activity of a responsive
reporter plasmid in both cell lines, a down-regulation of c-myc and cyclin A
mRNA expression was only found in A549 cells. Furthermore, TGF-beta1 inhibits
only cell proliferation of A549 but not of MDA-MB 231 cells. The results show a
coregulation of mRNA expression of AhR and cell-cycle regulating genes, and
further indicate that the AhR may be involved in regulation of cell
proliferation. Copyright 1997 Academic Press.

PMID: 9405238 [PubMed - indexed for MEDLINE]


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5: Biochem Biophys Res Commun. 1997 Feb 3;231(1):117-20. 

Benzo(a)pyrene, but not 2,3,7,8-tetrachlorodibenzo-p-dioxin, alters cell
proliferation and c-myc and growth factor expression in human placental
choriocarcinoma JEG-3 cells.

Zhang L, Shiverick KT.

Department of Pharmacology and Therapeutics, College of Medicine, University of
Florida, Gainesville 32610-0267, USA.

This study compared the effects of benzo(a)pyrene (BaP) and
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), two aryl hydrocarbon receptor (AhR)
agonists, on proliferation and gene expression in the human placental
choriocarcinoma JEG-3 cell line. BaP significantly inhibited [3H]thymidine
incorporation, whereas no effect of TCDD was observed over a 7 day period. TCDD
and BaP both showed induction of cytochrome P450 1A1 (CYP1A1), whereas only BaP
caused a significant loss of EGFRs. Exposure to 10 microM BaP significantly
increased the steady state mRNA level of transforming growth factor (TGF)-beta
1, while the c-myc mRNA levels were decreased by 61%. In contrast, TCDD showed
no changes in mRNA levels for TGF-beta 1 and c-myc. Thus, although both
compounds induce CYP1A1, only BaP inhibits cell proliferation which is
correlated with disruption of expression of significant regulators of
trophoblast growth.

PMID: 9070231 [PubMed - indexed for MEDLINE]


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