Identification of benzo[a]pyrene-inducible cis-acting elements within c-Ha-ras transcriptional regulatory sequences

Role of RAS signaling in ovarian cancer

The RAS family of proteins is among the most frequently mutated genes in human malignancies. In ovarian cancer (OC), the most lethal gynecological malignancy, RAS, especially KRAS mutational status at codons 12, 13, and 61, ranges from 6-65% spanning different histo-types. Normally RAS regulates several signaling pathways involved in a myriad of cellular signaling cascades mediating numerous cellular processes like cell proliferation, differentiation, invasion, and death. Aberrant activation of RAS leads to uncontrolled induction of several downstream signaling pathways such as RAF-1/MAPK (mitogen-activated protein kinase), PI3K phosphoinositide-3 kinase (PI3K)/AKT, RalGEFs, Rac/Rho, BRAF (v-Raf murine sarcoma viral oncogene homolog B), MEK1 (mitogen-activated protein kinase kinase 1), ERK (extracellular signal-regulated kinase), PKB (protein kinase B) and PKC (protein kinase C) involved in cell proliferation as well as maintenance pathways thereby driving tumorigenesis and cancer cell propagation. KRAS mutation is also known to be a biomarker for poor outcome and chemoresistance in OC. As a malignancy with several histotypes showing varying histopathological characteristics, we focus on reviewing recent literature showcasing the involvement of oncogenic RAS in mediating carcinogenesis and chemoresistance in OC and its subtypes.

Sulfur dioxide and benzo(a)pyrene modulates CYP1A and tumor-related gene expression in rat liver

Sulfur dioxide (SO(2)) and benzo(a)pyrene (B(a)P) are common industrial and environmental contaminants. However, few data are available on the effects of SO(2) on proto-oncogenes and tumor suppressor genes, as well as the interactions between SO(2) and other xenobiotics regulating proto-oncogenes or tumor suppressor genes expression. To investigate the interactions between SO(2) and B(a)P, male Wistar rats were exposed to intratracheally instilled with B(a)P or SO(2) inhalation alone or together. We detected mRNA expression of CYP1A1 and 1A2, 7-ethoxyresorufin O-deethylase (EROD), and methoxyresorufin O-demethylase (MROD) activities in livers. The mRNA and protein levels of several cancer-related genes were analyzed in livers by real-time RT-PCR and Western blot, respectively. The EROD/MROD activities and CYP1A1/2 expression were down-regulated by SO(2) but up-regulated by B(a)P alone. Exposure of SO(2) alone induced c-fos, c-jun, c-myc, H-ras, and p53 expression, and depressed p16 and Rb expression in livers. The effects of B(a)P on the above gene were similar to SO(2) except c-fos expression. Furthermore, SO(2) + B(a)P exposure increased the expression of c-fos, c-jun, c-myc, and p53, and decreased p16 and Rb expression in livers compared with exposed to SO(2) or B(a)P alone. However, no synergistic effects were observed on H-ras and CYP1A1/2 after SO(2) + B(a)P exposure. Our findings indicate that multiple cell cycle regulatory proteins play key roles in the toxicity of SO(2) and B(a)P in livers. It might involve the activation of c-fos, c-jun, c-myc, and p53. And p16-Rb pathway might also participate in the progress. Although the gene products we studied are classed as oncogenes and tumor suppressor genes, their functions actually relate to more general processes of control of cell proliferation, survival, and/or apoptosis.

Bioactivation of Polycyclic Aromatic Hydrocarbon Carcinogens within the vascular Wall: Implications for Human Atherogenesis

Atherogenesis is a complex pathogenetic process involving a variety of structural and functional deficits within the arterial wall that culminate in the formation of fibrous atherosclerotic plaques. Cigarette smoking is potentially the most remediable contributor to cardiovascular mortality and morbidity. Among the 4000 plus chemicals present in tobacco and tobacco smoke, polycyclic aromatic hydrocarbons (PAHs) have been firmly implicated in the etiology of atherosclerosis in experimental model systems. However, the molecular mechanisms responsible for PAH-induced vascular injury are not well understood. In this review, we have focused on the mechanisms of bioactivation of PAHs in the vas-culature, and the possible role(s) of cytochrome P4501A and 1B enzymes in the formation of PAH-DNA adducts within the vessel wall, a phenomenon that may contribute to the development of atherosclerotic plaques in humans.

The influence of diesel exhaust on polycyclic aromatic hydrocarbon-induced DNA damage, gene expression, and tumor initiation in Sencar mice in vivo

The carcinogenic effects of individual polycyclic aromatic hydrocarbons (PAH) are well established. However, their potency within an environmental complex mixture is uncertain. We evaluated the influence of diesel exhaust particulate matter on PAH-induced cytochrome P450 (CYP) activity, PAH-DNA adduct formation, expression of certain candidate genes and the frequency of tumor initiation in the two-stage Sencar mouse model. To this end, we monitored the effects of treatment of mice with diesel exhaust, benzo[a]pyrene (BP), dibenzo[a,l]pyrene (DBP), or a combination of diesel exhaust with either carcinogenic PAH. The applied diesel particulate matter (SRM(1975)) altered the tumor initiating potency of DBP: a statistically significant decrease in overall tumor and carcinoma burden was observed following 25 weeks of promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA), compared with DBP exposure alone. From those mice that were treated at the beginning of the observation period with 2 nmol DBP all survivors developed tumors (9 out of 9 animals, 100%). Among all tumors counted at the end, nine carcinomas were detected and an overall tumor incidence of 2.6 tumors per tumor-bearing animal (TBA) was determined. By contrast, co-treatment of DBP with 50mg SRM(1975) led to a tumor rate of only 66% (19 out of 29 animals), occurrence of only three carcinomas in 29 animals and an overall rate of 2.1 tumors per TBA (P=0.04). In contrast to the results with DBP, the tumor incidence induced by 200 nmol BP was found slightly increased when co-treatment with SRM(1975) occurred (71% vs. 85% after 25 weeks). Despite this difference in tumor incidence, the numbers of carcinomas and tumors per TBA did not differ statistically significant between both treatment groups possibly due to the small size of the BP treatment group. Since bioactivation of DBP, but not BP, predominantly depends on CYP1B1 enzyme activity, SRM(1975) affected PAH-induced carcinogenesis in an antagonistic manner when CYP1B1-mediated bioactivation was required. The explanation most likely lies in the much stronger inhibitory effects of certain PAHs present in diesel exhaust on CYP1B1 compared to CYP1A1. In the present study we also found molecular markers such as highly elevated AKR1C21 and TNFRSF21 gene expression levels in tumor tissue derived from animals co-treated with SRM(1975) plus DBP. Therefore we validate microarray data as a source to uncover transcriptional signatures that may provide insights into molecular pathways affected following exposure to environmental complex mixtures such as diesel exhaust particulates.

Activation profiles of HSPA5 during the glomerular mesangial cell stress response to chemical injury

Environmental injury has been associated with endoplasmic reticulum (ER) stress, a response characterized by activation of the unfolded protein response, proteasomal degradation of proteins, and induction of HSPA5, also known as GRP78 or BiP. Although HSPA5 has been implicated in the stress response to environmental injury in several cell types, its role in the glomerular ER stress response is unknown. In this study, we evaluated HSPA5 activation profiles in rat glomerular mesangial cells (rGMCs) challenged with heavy metals (HgCl2 or Pb2+ acetate) or polycyclic aromatic hydrocarbons (PAHs, ie, benzo(a)pyrene [BaP]). Challenge of rGMCs with 1 or 10 microM HgCl2 or Pb2+ acetate increased HSPA5 mRNA and protein levels. The induction response was sensitive to transcriptional and translational inhibition by actinomycin D (AD) and cyclohexamide, respectively. HSPA5 mRNA was induced by 3 microM BaP in an AD-sensitive manner, but this response was unaffected by the presence of heavy metals. A promoter construct containing sequences that mediate thapsigargin (TH) inducibility of the HSPA5 promoter was refractory to both heavy metals and BaP. The HSPA5 induction response in rGMCs is conserved because it was reproduced with fidelity in immunolocalization experiments of HSPA5 protein in M15 and HEK293 cells in embryonic lines of murine and human origin, respectively. Collectively, these findings identify HSPA5 in the stress response of rGMCs and implicate regulatory mechanisms that are distinct from those involved in TH inducibility.

Genetic and molecular mechanisms of chemical atherogenesis

Injury to the cellular components of the vascular wall and blood by endogenous and exogenous chemicals has been associated with atherosclerosis in humans and experimental systems. The genetic and molecular mechanisms responsible for initiation and promotion of atherosclerotic changes include modulation of extracellular matrix-integrin axis, genes involved in the regulation of growth and differentiation and possibly, genomic stability. This review summarizes seminal studies over the past 20 years that shed light on critical gene-gene and gene-environment interactions mediating the atherogenic response to chemical injury.

DNA sequence determinants of nuclear protein binding to the c-Ha-ras antioxidant/electrophile response element in vascular smooth muscle cells: identification of Nrf2 and heat shock protein 90 beta as heterocomplex components

The antioxidant/electrophile response element (ARE/EpRE) is a cis-acting element involved in redox regulation of c-Ha-ras gene. Protein binding to the ARE/EpRE may be credited to deoxyribonucleic acid sequence; therefore, studies were conducted to evaluate the influence of internal and flanking regions to the 10-bp human c-Ha-ras ARE/EpRE core (hHaras10) on nuclear protein binding in oxidant-treated vascular smooth muscle cells. A protein doublet bound to an extended oligonucleotide comprising the ARE/EpRE core in genomic context (hHaras27), whereas a single complex bound to hHarasl0. Protein binding involved specific interactions of 25- and 23-kDa proteins with hHarasl0, and binding of 80-, 65-, and 55-kDa proteins to hHaras27. Competition assays with hNQO1 and rGSTA2 confirmed the specificity of deoxyribonucleic acid-protein interactions and indicated preferred binding of p25 and p23 to the c-Ha-ras ARE/EpRE. "NNN" sequences within the core afforded unique protein-binding profiles to the c-Ha-ras ARE/EpRE. In addition, Nrf2 and heat shock protein 90beta (p80) were identified as components of the c-Ha-ras ARE/EpRE heterocomplex. We conclude that both internal bases and flanking sequences regulate nuclear protein recruitment and complex assembly on the c-Ha-ras ARE/EpRE.

Resveratrol modulates gene expression associated with apoptosis, proliferation and cell cycle in cells with mutated human c-Ha-Ras, but does not alter c-Ha-Ras mRNA or protein expression

An accumulating body of evidence suggests that resveratrol can inhibit carcinogenesis through antiproliferative and apoptotic effects. One proposed mechanism for this is the modulation of genes, for example, Ras and p53, frequently associated with human cancer. To test the effect of resveratrol on gene expression, we used the WR-21 cell line because it contains a mutated human c-Ha-ras gene. Cells at > or =70% confluency were incubated with media alone or with increasing concentrations of trans-resveratrol (0.1-1000 microM) for 24 h. Resveratrol (30-100 microM) decreased cellular proliferation by 80% (bromodeoxyuridine incorporation) and increased apoptosis by 60% (TUNEL). Cells were then treated with media alone or with 50-microM resveratrol for 24 h. RNA was isolated for nylon-based macroarray analyses and protein for immunoblotting. Resveratrol increased (+) and decreased (-) gene expression associated with apoptosis (Birc5+, Cash+, Mcl-1+, Mdm2+, Rpa-like+), cellular proliferation (Ctsd+, Mdm2+, Egr1+, ODC+) and cell cycle (cyclin D+, cyclin g+, Gadd45a-, Mad2l-, Mdm2+). Resveratrol consistently increased by > or =6-fold Mdm2 expression and other downstream p53 effectors, but not p53 itself at 24 h. Subsequent cell cycle analysis indicated a significant accumulation of cells in G2/M, and a decrease in G1/G0 suggesting a G2/M blockade. Further RT-PCR and Western blot analyses indicated no differential changes in Ras mRNA expression or p21(ras) protein levels, respectively. These results suggest that resveratrol potently inhibits cellular proliferation, increases apoptosis, alters cell cycle dynamics and modulates associated gene expression. Furthermore, these effects appear mediated, in part, by p53 without direct modulation of mutant c-Ha-ras expression.

Altered gene expression patterns in MCF-7 cells induced by the urban dust particulate complex mixture standard reference material 1649a

Human exposures to polycyclic aromatic hydrocarbon (PAH) occur in complex mixtures. Here, gene expression patterns were investigated using standard reference material (SRM) 1649a (urban dust). MCF-7 cells were exposed to SRM 1649a alone or SRM 1649a with either benzo[a]pyrene (BP) or dibenzo[a,l]pyrene (DBP) for 24 hours. Global analyses of the gene expression data revealed alterations of 41 RNA transcripts with at least 2-fold change (signal log ratio </= -1 or >/= 1) in response to SRM 1649a exposure. Increase in expression of cytochrome P450 (CYP) genes was observed in response to BP exposure (CYP1A1 and CYP1B1; signal log ratio of 4.7 and 2.5, respectively). An additive induction of CYP1A1 and CYP1B1 was observed with cotreatment of SRM 1649a and BP. On the contrary, no change in gene expression of CYP1A1 and CYP1B1 was observed when the cells were exposed to DBP. Furthermore, to study the effect of complex PAH mixtures on the metabolic activation of carcinogenic PAH to DNA-binding derivatives and to relate this with gene expression studies, PAH-DNA adduct formation was determined. SRM 1649a decreased the total level of BP-DNA adducts in comparison with BP alone. No significant difference in adduct levels was observed in response to either DBP alone or in combination with SRM 1649a. These results provide a transcriptional signature for chemical carcinogen exposure; in addition, they suggest a major factor in carcinogenic activity of PAH within complex mixtures is their ability to promote or inhibit the activation of carcinogenic PAH by the induction of CYP enzymes.

Aryl hydrocarbon receptor, cell cycle regulation, toxicity, and tumorigenesis

Most effects of exposure to halogenated and polycyclic aromatic hydrocarbons are mediated by the aryl hydrocarbon receptor (AHR). It has long been recognized that the AHR is a ligand-activated transcription factor that plays a central role in the induction of drug-metabolizing enzymes and hence in xenobiotic detoxification. Of late, it has become evident that outside this well-characterized role, the AHR also functions as a modulator of cellular signaling pathways. In this Prospect, we discuss the involvement of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, immediate-early gene induction, and the functions of the RB protein. Ultimately, the toxicity of AHR xenobiotic ligands may be intrinsically connected with the perturbation of these pathways and depend on the many critical signaling pathways and effectors with which the AHR itself interacts.

A critical role of PI-3K/Akt/JNKs pathway in benzo[a]pyrene diol-epoxide (B[a]PDE)-induced AP-1 transactivation in mouse epidermal Cl41 cells

Mouse skin tumorigenicity studies indicate that benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) contributes to carcinogenesis as both a tumor initiator and promoter. However, the mechanisms that mediate B[a]PDE tumor promotion effects remain unclear. Our results demonstrated that in mouse epidermal Cl41 cells, B[a]PDE treatment resulted in marked activation of AP-1 and its upstream MAPKs, including ERKs, JNKs and p38K. B[a]PDE exposure also led to activation of phosphotidylinositol 3-kinase (PI-3K), Akt and p70 S6 kinase (p70S6k). B[a]PDE-induced AP-1 transactivation was inhibited by pretreatment of cells with PI-3K inhibitors, wortmannin or Ly294002. In contrast, inhibition of p70S6k with rapamycin did not show any inhibitory effects. An overexpression of dominant-negative mutant of PI-3K, Deltap85, impaired B[a]PDE-induced activation of PI-3K, Akt and AP-1 transactivation. Furthermore, an overexpression of dominant-negative Akt mutant, Akt-T308A/S473A, blocked B[a]PDE-induced activation of Akt, AP-1 and JNKs, while it did not affect the activation of p70S6k, ERKs and p38 kinase. These results demonstrated that B[a]PDE was able to induce AP-1 transactivation and this AP-1 induction was specific through PI-3K/Akt/JNKs-dependent and p70S6k-independent pathways. This study also indicated that Akt-T308A/S473A blocks B[a]PDE-induced AP-1 activation specific through impairing JNK pathway. These findings will help us to understand the signal transduction pathways involved in the carcinogenic effects of B[a]PDE.

Role of the aryl hydrocarbon receptor in cell cycle regulation

Traditionally, the aryl hydrocarbon receptor (AHR) is considered to be a ligand-activated receptor and transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was neatly established long before the first report of an AHR cDNA sequence was published. Only recently, other functions of this protein have begun to be recognized. This review addresses novel findings relating to AHR functions that have resulted from experimental approaches markedly outside traditional receptor analyses. Here we examine the aspects of AHR biology relevant to its role in cell cycle regulation, from the activation of mitogen-activated protein kinases to the cross-talk between AHR and the RAS pathway and the functional significance of the interaction between AHR and the retinoblastoma protein. We have attempted to provide the reader with a balanced interpretation of the evidence, highlighting areas of consensus as well as areas still being contested.

Identification of albumin precursor protein, Phi AP3, and alpha-smooth muscle actin as novel components of redox sensing machinery in vascular smooth muscle cells

Aerobic organisms are continually subjected to environmental stressors that compromise redox homeostasis and induce cellular injury. In vascular smooth muscle cells (vSMCs), the activation/repression of redox-regulated genes after environmental stress often involves protein binding to cis-acting antioxidant response elements (AREs). The present study was conducted to identify proteins that participate in redox-regulated protein binding to human c-Ha-ras and mouse glutathione S-transferase A1 AREs in vSMCs after oxidant injury. Challenge of vSMCs with 0.3 or 3 microM hydrogen peroxide, 3-methylcholanthrene, benzo[a]pyrene-7,8-diol, 3-hydroxy benzo[a]pyrene, and benzo[a]pyrene-3,6-quinone induced concentration-related increases in ARE protein binding. The profiles of ARE complex assembly were comparable, but exhibited chemical specificity. Pretreatment with 0.5 mM N-acetylcysteine inhibited activation of ARE protein binding in hydrogen peroxide-treated cells. Preparative electrophoretic mobility shift assays coupled to Western analysis identified NF-E2-related proteins 1 and 2 and JunD in complexes assembled on AREs. Polyethylenimine affinity and sequence-specific serial immobilized DNA affinity chromatography followed by N-terminal sequencing identified albumin precursor protein, phi AP3, and alpha-smooth muscle actin as members of the ARE signaling pathway. Sequence analysis of albumin protein revealed homology to the redox-regulated transcription factors Bach1 and 2, as well as cytoskeletal and molecular motor proteins. These results implicate albumin precursor protein, phi AP3, and alpha-smooth muscle actin as participants in redox sensing in vSMCs, and suggest that protein complex assembly involves interactions between leucine zipper and zinc finger transcription factors with cytoskeletal proteins.

Epigenetics of breast cancer: polycyclic aromatic hydrocarbons as risk factors

In the absence of a causal relationship between the incidence of sporadic breast cancer and occurrence of mutations in breast cancer susceptibility genes, efforts directed to investigating the contribution of environmental xenobiotics in the etiology of sporadic mammary neoplasia are warranted. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants, which have been shown to induce DNA damage and disrupt cell cycle progression. In this report we discuss published data pointing to PAHs as a risk factor in carcinogenesis, and present findings generated in our laboratory suggesting that the mammary tumorigenicity of PAHs may be attributable, at least in part, to disruption of BRCA-1 expression by reactive PAH-metabolites. We report that benzo[a]pyrene (B[a]P), selected as a prototype PAH, disrupts BRCA-1 transcription in estrogen receptor (ER)-positive but not ER-negative breast cancer cells. The reduced potential for BRCA-1 expression in B[a]P-treated cells coincides with disruption of cell cycle kinetics and accumulation of p53. These effects are counteracted by the AhR-antagonist alpha-naphthoflavone (ANF), and in breast cancer cells expressing mutant p53 or the E6 human papilloma virus protein. We suggest that exposure to PAHs may be a predisposing factor in the etiology of sporadic breast cancer by disrupting the expression of BRCA-1.

Induction of 78 kD glucose-regulated protein (GRP78) expression and redox-regulated transcription factor activity by lead and mercury in C6 rat glioma cells

Lead (Pb) and mercury (Hg) are widespread environmental contaminants that induce prominent neural toxicity. Although the brain is not the major Pb and Hg depot in the body, these metals preferentially accumulate in astroglia to exert toxic effects. In this study, we examined the effects of Pb acetate and HgCl(2) on the expression of GRP78, a molecular chaperone in the endoplasmic reticulum (ER) that may provide cytoprotection in response to cellular stresses in the C6 rat glioma cell line. We also evaluated the DNA binding activities of several redox-regulated transcription factors in metal-treated cells. Our results showed that mRNA levels of GRP78 were up-regulated by Pb and Hg at 0.1 and 1 micro M, but down-regulated at higher concentrations (10 micro M). GRP78 protein levels increased in a concentration- and time-dependent manner in Pb and/or Hg-treated cells. Pb increased protein binding to the GST- Upsilon a antioxidant/electrophile response element (ARE/EpRE) and to the NF- kappaB consensus binding sequence of the cytomegalovirus 2 (CMB2) promoter, but decreased protein binding to the Ha-ras ARE/EpRE or to the c-fos 12-O-tetradecanoyl-phorbol-13-acetate (TPA) response element (TRE). In contrast, Hg activated DNA binding by all redox-regulated transcription factors. These studies shed some light on the molecular mechanisms of Pb and Hg toxicity in C6 rat glioma cells and suggest that GRP78 and oxidative stress may participate in the neurotoxic response to these metals.

Involvement of alpha-PAK-interacting exchange factor in the PAK1-c-Jun NH(2)-terminal kinase 1 activation and apoptosis induced by benzo[a]pyrene

Benzo[a]pyrene [B(a)P], a potent procarcinogen found in combustion products such as diesel exhaust and cigarette smoke, has been recently shown to activate the c-Jun NH(2)-terminal kinase 1 (JNK1) and induce caspase-3-mediated apoptosis in Hepa1c1c7 cells. However, the molecules of the signaling pathway that control the mitogen-activated protein kinase cascades induced by B(a)P and the interaction between those and apoptosis by B(a)P have not been well defined. We report here that B(a)P promoted Cdc42/Rac1, p21-activated kinase 1 (PAK1), and JNK1 activities in 293T and HeLa cells. Moreover, alpha-PAK-interacting exchange factor (alpha PIX) mRNA and its protein expression were upregulated by B(a)P. While overexpression of an active mutant of alpha PIX (DeltaCH) facilitated B(a)P-induced activation of Cdc42/Rac1, PAK1, and JNK1, overexpression of mutated alphaPIX (L383R, L384S), which lacks guanine nucleotide exchange factor activity, SH3 domain-deleted alphaPIX (Delta SH3), which lacks the ability to bind PAK, kinase-negative PAK1 (K299R), and kinase-negative SEK1 (K220A, K224L) inhibited B(a)P-triggered JNK1 activation. Interestingly, overexpression of alphaPIX (Delta CH) and a catalytically active mutant PAK1 (T423E) accelerated B(a)P-induced apoptosis in HeLa cells, whereas alphaPIX (Delta SH3), PAK1 (K299R), and SEK 1 (K220A, K224L) inhibited B(a)P-initiated apoptosis. Finally, a preferential caspase inhibitor, Z-Asp-CH2-DCB, strongly blocked the alphaPIX (Delta CH)-enhanced apoptosis in cells treated with B(a)P but did not block PAK1/JNK1 activation. Taken together, these results indicate that alphaPIX plays a crucial role in B(a)P-induced apoptosis through activation of the JNK1 pathway kinases.

Constitutive and inducible expression of Cyp1a1 and Cyp1b1 in vascular smooth muscle cells: role of the Ahr bHLH/PAS transcription factor

Ahr is a ligand-activated bHLH/PAS transcription factor involved in cytochrome P450 (CYP) gene regulation and murine susceptibility to atherogenic stimuli. The present studies were conducted to examine constitutive and inducible expression of Cyp1a1 and Cyp1b1 in vascular smooth muscle cells (VSMCs) from Ahr(+/+) and Ahr(-/-) mice. Cyp1a1 mRNA was not expressed constitutively in VSMCs irrespective of Ahr phenotype. Although Cyp1a1 was inducible in Ahr(+/+) by 3 micromol/L benzo(a)pyrene, a known hydrocarbon inducer, the protein was uninducible. In contrast, Cyp1b1 mRNA and protein were expressed under constitutive and inducible conditions irrespective of Ahr phenotype or growth status. CYP-encoded aryl hydrocarbon hydroxylase activity was higher in Ahr(-/-) VSMCs under constitutive conditions and induced by benzo(a)pyrene in Ahr(+/+) and Ahr(-/-) VSMCs. CYP expression was influenced by mitogenic status, because randomly cycling cells consistently exhibited higher levels than growth-arrested counterparts. Actinomycin D (2 microgram/mL) or cycloheximide (10 micromol/L) did not inhibit constitutive or hydrocarbon-inducible aryl hydrocarbon hydroxylase activity in VSMCs. These data indicate that in murine VSMCs, expression of Cyp1al and Cyp1b1 is differentially influenced by Ahr phenotype and mitogenic status, with patterns that may dictate inherent susceptibility to atherogenic stimuli.

Impact of cellular metabolism on the biological effects of benzo[a]pyrene and related hydrocarbons

Polycyclic aromatic hydrocarbons are ubiquitous contaminants in the environment. Benzo[a]pyrene (BaP), a prototypical member of this class of chemicals, has been extensively studied for its toxic effects in laboratory animals and human populations. BaP toxicity is often mediated by oxidative metabolism to reactive intermediates that interact with macromolecules leading to alterations in target cell structure and function. More recent evidence suggests that disruption of cellular signaling pathways involved in the regulation of growth and differentiation contribute significantly to the toxicity of BaP and its metabolites. This review summarizes recent advances in our understanding of biological mechanisms of BaP toxicity at the molecular level, and the role of metabolic intermediates in carcinogenesis, atherogenesis, and teratogenesis.

Tyrphostin [correction of Tryphostin] AG879, a tyrosine kinase inhibitor: prevention of transcriptional activation of the electrophile and the aromatic hydrocarbon response elements

To investigate a possible role of phosphorylation in the signal transduction pathways responsible for transcriptional regulation of drug-metabolizing enzymes, we tested seven specific tyrosine kinase inhibitors (tyrphostins) for their effects on NAD(P)H:quinone oxidoreductase-1 (NQO1) mRNA levels in mouse hepatoma Hepa-1c1c7 (Hepa-1) cells and chose to study AG879 further. The potent electrophile tert-butylhydroquinone (tBHQ) is known to activate NQO1 gene transcription via the electrophile response element (EPRE). Among the tyrphostins tested, tyrphostin AG879 was unique in preventing the accumulation of tBHQ-induced NQO1 mRNA; this effect was dependent on the AG879 dose and was also sensitive to the time when AG879 was added relative to the beginning of tBHQ treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is known to activate Cyp1a1 gene transcription by way of aromatic hydrocarbon response elements (AHREs). We found that AG879 also prevents, to a lesser extent, the AHRE-mediated induction of CYP1A1 and NQO1 mRNA by dioxin. Zinc or cadmium is known to activate metallothionein (Mt1) gene transcription via the metal response element (MRE). AG879 induced MT1 mRNA, and AG879 did not block zinc- or cadmium-induced MT1 mRNA, indicating that the effects of AG879 on NQO1 or CYP1A1 mRNA levels cannot be generalized to all transcripts. Using transient transfection of EPRE-, AHRE-, or MRE-driven luciferase reporter gene constructs in Hepa-1 cells, we showed that the inhibitory effects of AG879 occurred at the level of EPRE- and AHRE-mediated transcription, but that AG879 did not affect the MRE-driven transcriptional response. These data suggest that AG879 might inhibit an unknown tyrosine kinase(s) whose activity is essential for EPRE- and AHRE-mediated trans-activation of certain mammalian genes. These results also indicate that some sharing of common signal transduction pathways might exist in the regulation of genes involved in drug metabolism that also respond to oxidative stress.

Benzo(a)pyrene activates L1Md retrotransposon and inhibits DNA repair in vascular smooth muscle cells

Benzo(a)pyrene (BaP) modulates vascular smooth muscle cells (vSMCs) from a quiescent to proliferative phenotype, a shift associated with activation of L1Md retrotransposon [K.P. Lu, K.S. Ramos, Biochem. Biophys. Res. Commun. 253 (1998) 828-833]. The present studies were conducted to evaluate L1Md activation profiles in murine vSMCs treated with BaP or its oxidative metabolites, and to screen for possible insertional mutations into p53 and retinoblastoma (RB) genes. We also sought to examine the profile of DNA damage and repair in BaP-treated vSMCs. Northern analysis revealed that BaP (0. 03-3microM), and its major reactive 7,8-diol metabolite (0. 03-3microM), activate L1Md gene in a concentration-dependent manner. Two other metabolites, 3-OH BaP and 3,6-BaP quinone (0.03-3microM), as well as hydrogen peroxide (25-75microM) also activated L1Md. No insertional mutations into either p53 or RB genes were observed in vSMCs treated with BaP in vitro, although a slight elevation of p53 mRNA was observed as early as 4h after chemical challenge. Treatment of vSMCs with 3 or 30microM BaP for 4h increased unscheduled DNA synthesis (UDS) 1.4- and 2.5-fold, respectively. Challenge with 0. 3microM BaP for 24h inhibited DNA repair capacity in vSMCs for up to 48h. These results demonstrate that BaP and its oxidative metabolites activate L1Md retrotransposon in vSMCs, which coupled to DNA damage and inhibition of DNA repair are part of the atherogenic response elicited by BaP and related hydrocarbons.

Profiles of antioxidant/electrophile response element (ARE/EpRE) nuclear protein binding and c-Ha-ras transactivation in vascular smooth muscle cells treated with oxidative metabolites of benzo[a]pyrene

Activation of nuclear protein binding to the antioxidant/electrophile response element (ARE/EpRE) by benzo[a]pyrene (BaP) in vascular smooth muscle cells (vSMCs) is associated with transcriptional deregulation of c-Ha-ras. This response may be mediated by oxidative intermediates of BaP generated during the course of cellular metabolism. To test this hypothesis, the profile of ARE/EpRE protein binding and transactivation elicited by BaP was compared with that of 3-hydroxy BaP (3-OH BaP) (0.03 to 3.0 microM), BaP 7,8-dihydrodiol (BaP 7,8-diol) (0.03 to 3.0 microM), BaP 3,6-quinone (BaP 3,6-Q) (0.0003 to 3.0 microM), and H(2)O(2) (25 to 100 microM). Specific protein binding to the consensus c-Ha-ras ARE/EpRE was observed in vSMCs treated with all BaP metabolites at concentrations considerably lower than those required for the parent compound. H(2)O(2), a by-product of BaP 3,6-Q redox cycling, also increased binding to the ARE/EpRE. Treatment of vSMCs with oxidative BaP metabolites or H(2)O(2) transactivated the c-Ha-ras promoter in all instances, but the response was consistently half of the maximal induction elicited by BaP. Similar proteins cross-linked specifically to the consensus c-Ha-ras ARE/EpRE sequence in cells treated with BaP or its oxidative intermediates. The protein binding profile in the c-Ha-ras promoter was similar to that in the NADPH:quinone reductase gene (NQO(1)) and the glutathione S-transferase Ya gene (GSTYa) promoters, but the relative abundance of individual complexes was promoter-specific. We conclude that oxidative intermediates of BaP mediate activation of nuclear protein binding to ARE/EpRE and contribute to transcriptional de-regulation of c-Ha-ras in vSMCs.

The transcriptional signature of dioxin in human hepatoma HepG2 cells

We have used a high density microarray hybridization approach to characterize the transcriptional response of human hepatoma HepG2 cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We find that exposure to 10 nM TCDD for 8 hr alters by at least a factor of 2.1 the expression of 310 known genes and of an equivalent number of expressed sequence tags. Treatment with TCDD in the presence of 20 microg/mL of cycloheximide blocked the effect on 202 of these genes, allowing us to distinguish between primary effects of TCDD exposure, which take place whether cycloheximide is present or not, and secondary effects, which are blocked by inhibition of protein synthesis. Of the 310 known genes affected by TCDD, 30 are up-regulated and 78 are down-regulated regardless of cycloheximide treatment, and 84 are up-regulated and 118 are down-regulated only when protein synthesis is not inhibited. Functional clustering of genes regulated by TCDD reveals many potential physiological interactions that might shed light on the multiple biological effects of this compound. Our results, however, suggest that arriving at a sound understanding of the molecular mechanisms governing the biological outcome of TCDD exposure promises to be orders of magnitude more complicated than might have been previously imagined.

Activation of nuclear protein binding to the antioxidant/electrophile response element in vascular smooth muscle cells by benzo(a)pyrene

This laboratory has previously shown that binding of nuclear proteins to the antioxidant/electrophile response element (ARE/EpRE) participates in deregulation of vascular gene expression by benzo(a)pyrene (BaP), a suspected atherogen. In the present study, oligonucleotides representing ARE/EpREs within the c-Ha-ras and glutathione-S-transferase (GST-Ya) promoters were employed to evaluate the role of flanking sequences in stabilizing protein:DNA interactions in BaP-treated vascular smooth muscle cells (vSMCs). We also wanted to define promoter-specific patterns of protein recognition to ARE/EpREs in this cell system. In electrophoretic mobility shift assays (EMSA), optimal protein binding to a human Ha-ras ARE/EpRE variant sequence fitted to match the extended mouse(m) GST-Ya ARE/EpRE core (5'-TMAnnRTGAYnnnGCR-3') was dependent on 5' nucleic acid sequence. Using immobilized DNA affinity chromatography (IDAC), we identified four nuclear proteins of M(r) 62, 60, 50, and 30 kDa that associated specifically with the mGSTYa ARE/EpRE. Photo crosslinking to a BrdU-substituted hHa-ras or mGST ARE/EpRE probe identified specific proteins of M(r) 80, 60, 55, 25, 23 kDa or 80, 60, 55, 27, 25, 23 kDa, respectively. Protein:DNA complexes detected using IDAC eluate overlapped with those observed in crude nuclear extracts. Chemical treatments known to modulate ARE/EpRE protein binding in vSMCs did not alter overall protein:DNA affinity and/or sequence recognition to either hHa-ras or mGST-Ya elements. We conclude that nucleotide sequences 5' to the core ARE/EpRE influence specific binding of nuclear proteins and that multiple proteins bind to ARE/EpREs in a promoter-specific manner in vSMCs.

Regulation of gamma-glutamylcysteine synthetase subunit gene expression by the transcription factor Nrf2

Exposure of HepG2 cells to beta-naphthoflavone (beta-NF) or pyrrolidine dithiocarbamate (PDTC) resulted in the up-regulation of the gamma-glutamylcysteine synthetase catalytic (GCS(h)) and regulatory (GCS(l)) subunit genes. Increased expression was associated with an increase in the binding of Nrf2 to electrophile response elements (EpRE) in the promoters of these genes. Nrf2 overexpression increased the activity of GCS(h) and GCS(l) promoter/reporter transgenes. Overexpression of an MafK dominant negative mutant decreased Nrf2 binding to GCS EpRE sequences, inhibited the inducible expression of GCS(h) and GCS(l) promoter/reporter transgenes, and reduced endogenous GCS gene induction. beta-NF and PDTC exposure also increased steady-state levels of MafG mRNA. In addition to Nrf2, small Maf and JunD proteins were detected in GCS(h)EpRE-protein complexes and, to a lesser extent, in GCS(l)EpRE-protein complexes. The Nrf2-associated expression of GCS promoter/reporter transgenes was inhibited by overexpression of MafG. Inhibition of protein synthesis by cycloheximide partially decreased inducibility by PDTC or beta-NF and resulted in significant increases in GCS mRNA at late time points, when GCS mRNA levels are normally declining. We hypothesize that, in response to beta-NF and PDTC, the GCS subunit genes are transcriptionally up-regulated by Nrf2-basic leucine zipper complexes, containing either JunD or small Maf protein, depending on the particular GCS EpRE target sequence and the inducer. Following maximal induction, down-regulation of the two genes is mediated via a protein synthesis-dependent mechanism.

Negative regulation of rat GST-Ya gene via Antioxidant/Electrophile response element is directed by a C/EBP-like site

The present studies were conducted to evaluate functional interactions between aryl hydrocarbon and antioxidant/electrophile response elements (AhRE and ARE/EpRE, respectively) in transcriptional regulation of the rat (r)GST-Ya gene. Transient transfection of an AhRECAT reporter construct into vascular smooth muscle cells (vSMCs) or HepG2 cells showed that benzo(a)pyrene (BaP) (0.3-30 microM) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (0. 1-10 nM), but not hydrogen peroxide (H(2)O(2)) (100-400 microM), increased gene transcription. ARE/EpRE did not mediate gene inducibility by any of the chemicals in vSMCs but increased transcription in HepG2 cells treated with BaP or H(2)O(2), but not TCDD. Gene inducibility in response to all chemicals was repressed in both cell types transfected with a 1.6CAT full-length promoter construct containing the AhRE and ARE/EpRE in genomic context. Site-directed mutagenesis of 1.6CAT showed that a CCAAT/enhancer-binding protein (C/EBP)-like site within the ARE/EpRE directed negative regulation of the rGST-Ya gene in vSMCs and HepG2 cells. These results show that ARE/EpRE in rGST-Ya does not function as a positive cis-acting regulatory element in all cell types, and that in the context of the full-length rGST-Ya promoter a C/EBP-like site directs negative regulation of the gene by BaP and related chemicals.

Inhibition of BRCA-1 expression by benzo[a]pyrene and its diol epoxide

The objective of this study was to investigate whether polycyclic aromatic hydrocarbons (PAHs) contribute to the etiology of sporadic breast cancer by altering the expression of BRCA-1. Acute exposure to the PAH benzo[a]pyrene (B[a]P) inhibited in a time- and dose-dependent fashion cell proliferation and levels of BRCA-1 mRNA and protein in estrogen receptor (ER)-positive breast MCF-7 and ovarian BG-1 cancer cells. Moreover, the acute exposure to B[a]P abrogated estrogen induction of BRCA-1 in MCF-7 cells. The loss of BRCA-1 expression was prevented by the aromatic hydrocarbon receptor (AhR) antagonist alpha-naphthoflavone, suggesting participation of the AhR pathway. BRCA-1 exon 1a transcripts were downregulated by B[a]P faster than exon 1b mRNA was. Long-term exposure to B[a]P (40 nM for 15 mo) lowered BRCA-1 mRNA levels in subclones of MCF-7 and BG-1 cells, whereas expression of BRCA-1 in these clones was reverted to normal levels by washing out of B[a]P. The mechanisms of BRCA-1 repression by B[a]P were further investigated by examining the effects of the halogenated aryl hydrocarbon 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and the B[a]P metabolite 7r, 8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE). While TCDD did not influence basal BRCA-1 mRNA and protein levels at any of the doses (from 10 nM to 1 microM) tested in this study, treatment with 50 nM BPDE drastically reduced BRCA-1 mRNA levels, indicating that metabolism of B[a]P to BPDE may contribute to downregulation of BRCA-1. Conversely, ER-negative breast MDA-MB-231 and HBL-100 cancer cells were refractory to treatment with B[a]P or TCDD and expressed constant levels of BRCA-1 mRNA and protein. We conclude that B[a]P may be a risk factor in the etiology of sporadic breast cancer.

Redox regulation of c-Ha-ras and osteopontin signaling in vascular smooth muscle cells: implications in chemical atherogenesis

Reduction/oxidation (redox) reactions play a central role in the regulation of vascular cell functions. Recent studies in this laboratory have identified c-Ha-ras and osteopontin genes as critical molecular targets during oxidant-induced atherogenesis. This review focuses on the deregulation of gene transcription by redox-activated trans-acting factors after benzo(a)pyrene challenge and the modulation of extracellular matrix signaling in vascular smooth muscle cells by allylamine-induced oxidative injury. The induction of atherogenic vascular smooth muscle cell phenotypes by chemical injury exhibits remarkable parallels with those seen in other forms of atherogenesis.

Identification of genes differentially expressed in vascular smooth muscle cells following benzo[a]pyrene challenge: implications for chemical atherogenesis

We have previously shown that serial passage of vascular smooth muscle cells (vSMCs) treated with a single low dose of benzo[a]pyrene (BaP) induces acquisition of highly proliferative (i.e. atherogenic) phenotypes. To define the molecular basis of this response, differential display polymerase chain reaction was used to identify early target genes in murine vSMCs challenged with 3 microM BaP for 8 hr. Of 170 differentially expressed cDNAs, 111 were re-amplified, and 64 examined for homology to known genes. Aac11 apoptosis inhibitor, aldose reductase, GalNAc transferase, TCP-1 chaperonin gene, and mouse mitochondrial gene, were downregulated in vSMCs treated with BaP. In contrast, enhanced expression of unique retrotransposon cDNAs were found in BaP-treated cells. This is the first report showing that BaP modulates the expression of these genes in mammalian cells. Of particular interest is the modulation of retrotransposon mRNAs which coupled to other genetic events, may play a significant role in the atherogenic response to this carcinogen.

Benzo(a)pyrene-induced alterations in growth-related gene expression and signaling in precision-cut adult rat liver and kidney slices

Benzo(a)pyrene (BaP) and related aromatic hydrocarbons are suspected carcinogens; however, the molecular basis underlying tumorigenesis remains unclear. To identify acute molecular targets of BaP within the liver and kidney, precision-cut slices harvested from naive, adult female Sprague-Dawley rats were challenged with BaP (0.3-30 microM) for 0.5 to 24 h. BaP did not elicit cytotoxicity, as assessed by intracellular K+ and ATP content and histological evaluation over the 24-h period. To determine if molecular signaling pathways were maintained in precision-cut slices, induction of the aryl hydrocarbon receptor (AhR) pathway was assessed following BaP challenge. Induction of cytochrome P450IA1 (P450IA1) mRNA and protein expression was observed in both liver and kidney slices. c-fos and c-Ha-ras gene expression was enhanced in liver, but not kidney, slices by BaP. c-jun mRNA levels were decreased in liver and kidney slices, although the effect was earlier (0.5 h) in liver slices compared to kidney slices. BaP increased the DNA binding of nuclear proteins to the AP-1 consensus recognition element in liver, but decreased DNA binding in kidney slices. In contrast, DNA binding of NF-kappa B was not affected by BaP in either liver or kidney slices. These results suggest that acute BaP challenge is associated with altered expression of several growth-related genes and AP-1 signaling and establish precision-cut slices as a useful in vitro system to investigate the molecular basis of BaP-induced tumorigenesis, including organ-specific differences.

Immunohistochemical detection of polycyclic aromatic hydrocarbon-DNA damage in human blood vessels of smokers and non-smokers

The presence of cigarette smoking-related polycyclic aromatic hydrocarbon (PAH)-DNA adducts was investigated in human arterial tissue using an immunoperoxidase staining method with a monoclonal antibody that recognizes benzo(a)pyrene and structurally related PAH diol epoxide-DNA adducts. This is the first time that immunohistochemical methods for detection of PAH-DNA damage have been applied to human endothelial and smooth muscle cells of blood vessels. Internal mammary artery specimens from a total of 37 smokers and non-smokers were tested. Positive nuclear staining was observed in both endothelial and smooth muscle cells, with higher staining in the endothelium. The correlation between smoking status, available for 33 subjects, and detectable PAH-DNA adducts in endothelium did not reach statistical significance (odds ratio = 3.38, 95% confidence interval is 0.47-27.60) in this small series. While no causal role can be inferred from our results, they support the theory that endothelial injury caused by cigarette smoking and other environmental exposures may be an early event in the process of atherosclerosis.