A Compendium of Mutational Signatures of Environmental Agents.

Whole-genome-sequencing (WGS) of human tumors has revealed distinct mutation patterns that hint at the causative origins of cancer. We examined mutational signatures in 324 WGS human-induced pluripotent stem cells exposed to 79 known or suspected environmental carcinogens. Forty-one yielded characteristic substitution mutational signatures. Some were similar to signatures found in human tumors. Additionally, six agents produced double-substitution signatures and eight produced indel signatures. Investigating mutation asymmetries across genome topography revealed fully functional mismatch and transcription-coupled repair pathways. DNA damage induced by environmental mutagens can be resolved by disparate repair and/or replicative pathways, resulting in an assortment of signature outcomes even for a single agent. This compendium of experimentally induced mutational signatures permits further exploration of roles of environmental agents in cancer etiology and underscores how human stem cell DNA is directly vulnerable to environmental agents. VIDEO ABSTRACT.

Tissue Organoid Cultures Metabolize Dietary Carcinogens Proficiently and Are Effective Models for DNA Adduct Formation.

Human tissue three-dimensional (3D) organoid cultures have the potential to reproduce in vitro the physiological properties and cellular architecture of the organs from which they are derived. The ability of organoid cultures derived from human stomach, liver, kidney, and colon to metabolically activate three dietary carcinogens, aflatoxin B1 (AFB1), aristolochic acid I (AAI), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), was investigated. In each case, the response of a target tissue (liver for AFB1; kidney for AAI; colon for PhIP) was compared with that of a nontarget tissue (gastric). After treatment cell viabilities were measured, DNA damage response (DDR) was determined by Western blotting for p-p53, p21, p-CHK2, and γ-H2AX, and DNA adduct formation was quantified by mass spectrometry. Induction of the key xenobiotic-metabolizing enzymes (XMEs) CYP1A1, CYP1A2, CYP3A4, and NQO1 was assessed by qRT-PCR. We found that organoids from different tissues can activate AAI, AFB1, and PhIP. In some cases, this metabolic potential varied between tissues and between different cultures of the same tissue. Similarly, variations in the levels of expression of XMEs were observed. At comparable levels of cytotoxicity, organoids derived from tissues that are considered targets for these carcinogens had higher levels of adduct formation than a nontarget tissue.

Metabolic Activation of Benzo[a]pyrene by Human Tissue Organoid Cultures.

Organoids are 3D cultures that to some extent reproduce the structure, composition and function of the mammalian tissues from which they derive, thereby creating in vitro systems with more in vivo-like characteristics than 2D monocultures. Here, the ability of human organoids derived from normal gastric, pancreas, liver, colon and kidney tissues to metabolise the environmental carcinogen benzo[a]pyrene (BaP) was investigated. While organoids from the different tissues showed varied cytotoxic responses to BaP, with gastric and colon organoids being the most susceptible, the xenobiotic-metabolising enzyme (XME) genes, CYP1A1 and NQO1, were highly upregulated in all organoid types, with kidney organoids having the highest levels. Furthermore, the presence of two key metabolites, BaP-t-7,8-dihydrodiol and BaP-tetrol-l-1, was detected in all organoid types, confirming their ability to metabolise BaP. BaP bioactivation was confirmed both by the activation of the DNA damage response pathway (induction of p-p53, pCHK2, p21 and γ-H2AX) and by DNA adduct formation. Overall, pancreatic and undifferentiated liver organoids formed the highest levels of DNA adducts. Colon organoids had the lowest responses in DNA adduct and metabolite formation, as well as XME expression. Additionally, high-throughput RT-qPCR explored differences in gene expression between organoid types after BaP treatment. The results demonstrate the potential usefulness of organoids for studying environmental carcinogenesis and genetic toxicology.

Mutagenicity of acrylamide and glycidamide in human TP53 knock-in (Hupki) mouse embryo fibroblasts.

Acrylamide is a suspected human carcinogen formed during high-temperature cooking of starch-rich foods. It is metabolised by cytochrome P450 2E1 to its reactive metabolite glycidamide, which forms pre-mutagenic DNA adducts. Using the human TP53 knock-in (Hupki) mouse embryo fibroblasts (HUFs) immortalisation assay (HIMA), acrylamide- and glycidamide-induced mutagenesis was studied in the tumour suppressor gene TP53. Selected immortalised HUF clones were also subjected to next-generation sequencing to determine mutations across the whole genome. The TP53-mutant frequency after glycidamide exposure (1.1 mM for 24 h, n = 198) was 9% compared with 0% in cultures treated with acrylamide [1.5 (n = 24) or 3 mM (n = 6) for 48 h] and untreated vehicle (water) controls (n = 36). Most glycidamide-induced mutations occurred at adenines with A > T/T > A and A > G/T > C mutations being the most common types. Mutations induced by glycidamide occurred at specific TP53 codons that have also been found to be mutated in human tumours (i.e., breast, ovary, colorectal, and lung) previously associated with acrylamide exposure. The spectrum of TP53 mutations was further reflected by the mutations detected by whole-genome sequencing (WGS) and a distinct WGS mutational signature was found in HUF clones treated with glycidamide that was again characterised by A > G/T > C and A > T/T > A mutations. The WGS mutational signature showed similarities with COSMIC mutational signatures SBS3 and 25 previously found in human tumours (e.g., breast and ovary), while the adenine component was similar to COSMIC SBS4 found mostly in smokers' lung cancer. In contrast, in acrylamide-treated HUF clones, only culture-related background WGS mutational signatures were observed. In summary, the results of the present study suggest that glycidamide may be involved in the development of breast, ovarian, and lung cancer.

Nutlin-3a selects for cells harbouring TP53 mutations.

TP53 mutations occur in half of all human tumours. Mutagen-induced or spontaneous TP53 mutagenesis can be studied in vitro using the human TP53 knock-in (Hupki) mouse embryo fibroblast (HUF) immortalisation assay (HIMA). TP53 mutations arise in up to 30% of mutagen-treated, immortalised HUFs; however, mutants are not identified until TP53 sequence analysis following immortalisation (2-5 months) and much effort is expended maintaining TP53-WT cultures. In order to improve the selectivity of the HIMA for HUFs harbouring TP53 mutations, we explored the use of Nutlin-3a, an MDM2 inhibitor that leads to stabilisation and activation of wild-type (WT) p53. First, we treated previously established immortal HUF lines carrying WT or mutated TP53 with Nutlin-3a to examine the effect on cell growth and p53 activation. Nutlin-3a induced the p53 pathway in TP53-WT HUFs and inhibited cell growth, whereas most TP53-mutated HUFs were resistant to Nutlin-3a. We then assessed whether Nutlin-3a treatment could discriminate between TP53-WT and TP53-mutated cells during the HIMA (n = 72 cultures). As immortal clones emerged from senescent cultures, each was treated with 10 µM Nutlin-3a for 5 days and observed for sensitivity or resistance. TP53 was subsequently sequenced from all immortalised clones. We found that all Nutlin-3a-resistant clones harboured TP53 mutations, which were diverse in position and functional impact, while all but one of the Nutlin-3a-sensitive clones were TP53-WT. These data suggest that including a Nutlin-3a counter-screen significantly improves the specificity and efficiency of the HIMA, whereby TP53-mutated clones are selected prior to sequencing and TP53-WT clones can be discarded.

Carcinogenic polycyclic aromatic hydrocarbons induce CYP1A1 in human cells via a p53-dependent mechanism.

The tumour suppressor gene TP53 is mutated in more than 50 % of human tumours, making it one of the most important cancer genes. We have investigated the role of TP53 in cytochrome P450 (CYP)-mediated metabolic activation of three polycyclic aromatic hydrocarbons (PAHs) in a panel of isogenic colorectal HCT116 cells with differing TP53 status. Cells that were TP53(+/+), TP53(+/-), TP53(-/-), TP53(R248W/+) or TP53(R248W/-) were treated with benzo[a]pyrene (BaP), dibenz[a,h]anthracene and dibenzo[a,l]pyrene, and the formation of DNA adducts was measured by (32)P-postlabelling analysis. Each PAH formed significantly higher DNA adduct levels in TP53(+/+) cells than in the other cell lines. There were also significantly lower levels of PAH metabolites in the culture media of these other cell lines. Bypass of the need for metabolic activation by treating cells with the corresponding reactive PAH-diol-epoxide metabolites resulted in similar adduct levels in all cell lines, which confirms that the influence of p53 is on the metabolism of the parent PAHs. Western blotting showed that CYP1A1 protein expression was induced to much greater extent in TP53(+/+) cells than in the other cell lines. CYP1A1 is inducible via the aryl hydrocarbon receptor (AHR), but we did not find that expression of AHR was dependent on p53; rather, we found that BaP-induced CYP1A1 expression was regulated through p53 binding to a p53 response element in the CYP1A1 promoter region, thereby enhancing its transcription. This study demonstrates a new pathway for CYP1A1 induction by environmental PAHs and reveals an emerging role for p53 in xenobiotic metabolism.

The genome as a record of environmental exposure.

Whole genome sequencing of human tumours has revealed distinct patterns of mutation that hint at the causative origins of cancer. Experimental investigations of the mutations and mutation spectra induced by environmental mutagens have traditionally focused on single genes. With the advent of faster cheaper sequencing platforms, it is now possible to assess mutation spectra in experimental models across the whole genome. As a proof of principle, we have examined the whole genome mutation profiles of mouse embryo fibroblasts immortalised following exposure to benzo[a]pyrene (BaP), ultraviolet light (UV) and aristolochic acid (AA). The results reveal that each mutagen induces a characteristic mutation signature: predominantly G→T mutations for BaP, C→T and CC→TT for UV and A→T for AA. The data are not only consistent with existing knowledge but also provide additional information at higher levels of genomic organisation. The approach holds promise for identifying agents responsible for mutations in human tumours and for shedding light on the aetiology of human cancer.

Cell cycle alterations induced by urban PM2.5 in bronchial epithelial cells: characterization of the process and possible mechanisms involved.

BACKGROUND: This study explores and characterizes cell cycle alterations induced by urban PM2.5 in the human epithelial cell line BEAS-2B, and elucidates possible mechanisms involved. METHODS: The cells were exposed to a low dose (7.5 µg/cm(2)) of Milan winter PM2.5 for different time points, and the cell cycle progression was analyzed by fluorescent microscopy and flow cytometry. Activation of proteins involved in cell cycle control was investigated by Western blotting and DNA damage by (32)P-postlabelling, immunostaining and comet assay. The formation of reactive oxygen species (ROS) was quantified by flow cytometry. The role of PM organic fraction versus washed PM on the cell cycle alterations was also examined. Finally, the molecular pathways activated were further examined using specific inhibitors. RESULTS: Winter PM2.5 induced marked cell cycle alteration already after 3 h of exposure, represented by an increased number of cells (transient arrest) in G2. This effect was associated with an increased phosphorylation of Chk2, while no changes in p53 phosphorylation were observed at this time point. The increase in G2 was followed by a transient arrest in the metaphase/anaphase transition point (10 h), which was associated with the presence of severe mitotic spindle aberrations. The metaphase/anaphase delay was apparently followed by mitotic slippage at 24 h, resulting in an increased number of tetraploid G1 cells and cells with micronuclei (MN), and by apoptosis at 40 h. Winter PM2.5 increased the level of ROS at 2 h and DNA damage (8-oxodG, single- and double stand breaks) was detected after 3 h of exposure. The PM organic fraction caused a similar G2/M arrest and augmented ROS formation, while washed PM had no such effects. DNA adducts were detected after 24 h. Both PM-induced DNA damage and G2 arrest were inhibited by the addition of antioxidants and α-naphthoflavone, suggesting the involvement of ROS and reactive electrophilic metabolites formed via a P450-dependent reaction. CONCLUSIONS: Milan winter PM2.5 rapidly induces severe cell cycle alterations, resulting in increased frequency of cells with double nuclei and MN. This effect is related to the metabolic activation of PM2.5 organic chemicals, which cause damages to DNA and spindle apparatus.

Mutagenicity of N-hydroxy-4-aminobiphenyl in human TP53 knock-in (Hupki) mouse embryo fibroblasts.

TP53 harbors somatic mutations in more than half of human tumors with some showing characteristic mutation spectra that have been linked to environmental exposures. In bladder cancer, a unique distribution of mutations amongst several codons of TP53 has been hypothesized to be caused by environmental carcinogens including 4-aminobiphenyl (4-ABP). 4-ABP undergoes metabolic activation to N-hydroxy-4-aminobiphenyl (N-OH-4-ABP) and forms pre-mutagenic adducts in DNA, of which N-(deoxyguanosin-8-yl)-4-ABP (dG-C8-4-ABP) is the major one. Human TP53 knock-in mouse embryo fibroblasts (HUFs) are a useful model to study the influence of environmental carcinogens on TP53-mutagenesis. By performing the HUF immortalization assay (HIMA) TP53-mutant HUFs are generated and mutations can be identified by sequencing. Here we studied the induction of mutations in human TP53 after treatment of primary HUFs with N-OH-4-ABP. In addition, mutagenicity in the bacterial lacZ reporter gene and the formation of dG-C8-4-ABP, measured by 32 P-postlabelling analysis, were determined in N-OH-4-ABP-treated primary HUFs. A total of 6% TP53-mutants were identified after treatment with 40 µM N-OH-4-ABP for 24 hr (n = 150) with G>C/C>G transversion being the main mutation type. The mutation spectrum found in the TP53 gene of immortalized N-OH-4-ABP-treated HUFs was unlike the one found in human bladder cancer. DNA adduct formation (~40 adducts/108 nucleotides) was detected after 24 hr treatment with 40 µM N-OH-4-ABP, but lacZ mutagenicity was not observed. Adduct levels decreased substantially (sixfold) after a 24 hr recovery period indicating that primary HUFs can efficiently repair the dG-C8-4-ABP adduct possibly before mutations are fixed. In conclusion, the observed difference in the N-OH-4-ABP-induced TP53 mutation spectrum to that observed in human bladder tumors do not support a role of 4-ABP in human bladder cancer development.

Mutagenicity of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP) in human TP53 knock-in (Hupki) mouse embryo fibroblasts.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a possible human carcinogen formed in cooked fish and meat. PhIP is bioactivated by cytochrome P450 enzymes to form 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP), a genotoxic metabolite that reacts with DNA leading to the mutation-prone DNA adduct N-(deoxyguanosin-8-yl)-PhIP (dG-C8-PhIP). Here, we studied N-OH-PhIP-induced whole genome mutagenesis in human TP53 knock-in (Hupki) mouse embryo fibroblasts (HUFs) immortalised and subjected to whole genome sequencing (WGS). In addition, mutagenicity of N-OH-PhIP in TP53 and the lacZ reporter gene were assessed. TP53 mutant frequency in HUF cultures treated with N-OH-PhIP (2.5 µM for 24 h, n = 90) was 10% while no TP53 mutations were found in untreated controls (DMSO for 24 h, n = 6). All N-OH-PhIP-induced TP53 mutations occurred at G:C base pairs with G > T/C > A transversions accounting for 58% of them. TP53 mutations characteristic of those induced by N-OH-PhIP have been found in human tumours including breast and colorectal, which are cancer types that have been associated with PhIP exposure. LacZ mutant frequency increased 25-fold at 5 µM N-OH-PHIP and up to ~350 dG-C8-PhIP adducts/108 nucleosides were detected by ultra-performance liquid chromatography-electrospray ionisation multistage scan mass spectrometry (UPLC-ESI-MS3) at this concentration. In addition, a WGS mutational signature defined by G > T/C > A transversions was present in N-OH-PhIP-treated immortalised clones, which showed similarity to COSMIC SBS4, 18 and 29 signatures found in human tumours.

Characterising Mutational Spectra of Carcinogens in the Tumour Suppressor Gene TP53 Using Human TP53 Knock-in (Hupki) Mouse Embryo Fibroblasts.

DNA in dividing cells is prone to mutagenesis, with mutations making key contributions to human disease including cancer. The tumour suppressor gene TP53 is the most frequently mutated gene in human tumours. Here, we present a robust protocol for studying TP53 mutagenesis utilising human TP53 knock-in (Hupki) mouse embryonic fibroblasts (HUFs). In the HUF immortalisation assay (HIMA), primary HUFs are treated with known or suspected carcinogens at 3% oxygen and then transferred to 20% atmospheric oxygen to induce senescence. Cells containing mutations (e.g., in TP53) that allow bypassing of senescence eventually emerge as immortalised clonal cell lines after 2-3 months of serial passaging. As not all immortalised HUF cells contain TP53 mutations, we developed a Nutlin-3a counter-screen to select for TP53-mutated clones prior to sequencing. TP53 mutation spectra generated can be compared with those of human tumours recorded in the International Agency for Research on Cancer TP53 mutation database. Environmental mutagens that have demonstrated and validated the utility of the HIMA include ultraviolet radiation, aristolochic acid, and benzo[a]pyrene. The TP53 mutation patterns induced by these mutagens in the HIMA corresponded to those found in human tumours from patients exposed to these mutagens. The approach presented helps to deepen our understanding of human cancer aetiology.

Preferential dependence of breast cancer cells versus normal cells on integrin-linked kinase for protein kinase B/Akt activation and cell survival.

The emerging paradigm of "oncogene addiction" has been called an Achilles' heel of cancer that can be exploited therapeutically. Here, we show that integrin-linked kinase (ILK), which is either activated or overexpressed in many types of cancers, is a critical regulator of breast cancer cell survival through the protein kinase B (PKB)/Akt pathway but is largely dispensable for the survival of normal breast epithelial cells and mesenchymal cells. We show that inhibition of ILK activity with a pharmacologic ILK inhibitor, QLT-0267, results in the inhibition of PKB/Akt Ser473 phosphorylation, stimulation of apoptosis, and a decrease in mammalian target of rapamycin (mTOR) expression in human breast cancer cells. In contrast, QLT-0267 treatment has no effect on PKB/Akt Ser473 phosphorylation or apoptosis in normal human breast epithelial, mouse fibroblast, or vascular smooth muscle cells. The inhibition of PKB/Akt Ser473 phosphorylation by QLT-0267 in breast cancer cells was rescued by a kinase-active ILK mutant but not by a kinase-dead ILK mutant. Furthermore, a dominant-negative ILK mutant increased apoptosis in the MDA-MB-231 breast cancer cell line but not in normal human breast epithelial cells. The inhibitor was active against ILK isolated from all cell types but did not have any effect on cell attachment and spreading. Our data point to an "ILK addiction" of breast cancer cells whereby they become dependent on ILK for cell survival through the mTOR-PKB/Akt signaling pathway and show that ILK is a promising target for the treatment of breast cancer.

Expression of the insulin-like growth factor I receptor and urokinase plasminogen activator in breast cancer is associated with poor survival: potential for intervention with 17-allylamino geldanamycin.

Urokinase plasminogen activator (uPA) expression in breast cancer is associated with relapse and a reduction in disease-specific survival. Thus, efforts are under way to identify uPA inhibitors. By screening a chemical library of >1000 compounds, 17-allyaminogeldanamycin (17AAG) was identified as a potent inhibitor of uPA by the National Cancer Institute and is now in Phase I clinical trials. At this time, it remains unclear how 17AAG blocks uPA; one possibility is through disruption of the insulin-like growth factor I receptor (IGF-IR) pathway. This would be consistent with studies from our laboratory showing that activation of IGF-IR results in the induction of uPA protein. In the study described herein, we observed that IGF-IR and uPA were highly expressed in 87 and 55% of breast cancer by screening tumor tissue microarrays representing 930 cases. A significant proportion (52.1% = 354 of 680 cases, P < 0.0001) of the patients had tumors expressing both proteins. uPA alone (P = 0.033) or in combination with IGF-IR (P = 0.0104) was indicative of decreased disease-specific survival. Next, we demonstrated that treating MDA-MB-231 cells with increasing concentrations of 17AAG resulted in IGF-IR degradation (IC(50) = 1.0 micro M) and blocked signal transduction through the Akt and mitogen-activated protein kinase pathways. Finally, we found that 17AAG had a robust inhibitory effect on the production of uPA mRNAand protein in the presence of IGF-I. Thus, our study raises the possibility that 17AAG could prove to be an effective therapeutic agent for a large number of breast cancer patients by inhibiting the IGF-IR and ultimately uPA.

TP53 and lacZ mutagenesis induced by 3-nitrobenzanthrone in Xpa-deficient human TP53 knock-in mouse embryo fibroblasts.

3-Nitrobenzanthrone (3-NBA) is a highly mutagenic compound and possible human carcinogen found in diesel exhaust. 3-NBA forms bulky DNA adducts following metabolic activation and induces predominantly G:CT:A transversions in a variety of experimental systems. Here we investigated the influence of nucleotide excision repair (NER) on 3-NBA-induced mutagenesis of the human tumour suppressor gene TP53 and the reporter gene lacZ. To this end we utilised Xpa -knockout (Xpa-Null) human TP53 knock-in (Hupki) embryo fibroblasts (HUFs). As Xpa is essential for NER of bulky DNA adducts, we hypothesized that DNA adducts induced by 3-NBA would persist in the genomes of Xpa-Null cells and lead to an increased frequency of mutation. The HUF immortalisation assay was used to select for cells harbouring TP53 mutations following mutagen exposure. We found that Xpa-Null Hupki mice and HUFs were more sensitive to 3-NBA treatment than their wild-type (Xpa-WT) counterparts. However, following 3-NBA treatment and immortalisation, a similar frequency of TP53-mutant clones arose from Xpa-WT and Xpa-Null HUF cultures. In cells from both Xpa genotypes G:CT:A transversion was the predominant TP53 mutation type and mutations exhibited bias towards the non-transcribed strand. Thirty-two percent of 3-NBA-induced TP53 mutations occurred at CpG sites, all of which are hotspots for mutation in smokers' lung cancer (codons 157, 158, 175, 245, 248, 273, 282). We also examined 3-NBA-induced mutagenesis of an integrated lacZ reporter gene in HUFs, where we again observed a similar mutant frequency in Xpa-WT and Xpa-Null cells. Our findings suggest that 3-NBA-DNA adducts may evade removal by global genomic NER; the persistence of 3-NBA adducts in DNA may be an important factor in its mutagenicity.

Celecoxib analogues disrupt Akt signaling, which is commonly activated in primary breast tumours.

INTRODUCTION: Phosphorylated Akt (P-Akt) is an attractive molecular target because it contributes to the development of breast cancer and confers resistance to conventional therapies. Akt also serves as a signalling intermediate for receptors such as human epidermal growth factor receptor (HER)-2, which is overexpressed in 30% of breast cancers; therefore, inhibitors to this pathway are being sought. New celecoxib analogues reportedly inhibit P-Akt in prostate cancer cells. We therefore examined the potential of these compounds in the treatment of breast cancer. The analogues were characterized in MDA-MB-453 cells because they overexpress HER-2 and have very high levels of P-Akt. METHODS: To evaluate the effect of the celecoxib analogues, immunoblotting was used to identify changes in the phosphorylation of Akt and its downstream substrates glycogen synthase kinase (GSK) and 4E binding protein (4EBP-1). In vitro kinase assays were then used to assess the effect of the drugs on Akt activity. Cell death was evaluated by poly(ADP-ribose) polymerase cleavage, nucleosomal fragmentation and MTS assays. Finally, tumour tissue microarrays were screened for P-Akt and HER-2 expression. RESULTS: OSU-03012 and OSU-O3013 inhibited P-Akt and its downstream signalling through 4EBP-1 and GSK at concentrations well below that of celecoxib. Disruption of P-Akt was followed by induction of apoptosis and more than 90% cell death. We also noted that the cytotoxicity of the celecoxib analogues was not significantly affected by serum. In contrast, the presence of 5% serum protected cells from celecoxib induced death. Thus, the structural modification of the celecoxib analogues increased P-Akt inhibition and enhanced the bioavailability of the drugs in vitro. To assess how many patients may potentially benefit from such drugs we screened tumour tissue microarrays. P-Akt was highly activated in 58% (225/390) of cases, whereas it was only similarly expressed in 35% (9/26) of normal breast tissues. Furthermore, HER-2 positive tumours expressed high levels of P-Akt (P < 0.01), supporting in vitro signal transduction. CONCLUSION: We determined that Celecoxib analogues are potent inhibitors of P-Akt signalling and kill breast cancer cells that overexpress HER-2. We also defined an association between HER-2 and P-Akt in primary breast tissues, suggesting that these inhibitors may benefit patients in need of new treatment options.

TP53 mutations induced by BPDE in Xpa-WT and Xpa-Null human TP53 knock-in (Hupki) mouse embryo fibroblasts.

Somatic mutations in the tumour suppressor gene TP53 occur in more than 50% of human tumours; in some instances exposure to environmental carcinogens can be linked to characteristic mutational signatures. The Hupki (human TP53 knock-in) mouse embryo fibroblast (HUF) immortalization assay (HIMA) is a useful model for studying the impact of environmental carcinogens on TP53 mutagenesis. In an effort to increase the frequency of TP53-mutated clones achievable in the HIMA, we generated nucleotide excision repair (NER)-deficient HUFs by crossing the Hupki mouse with an Xpa-knockout (Xpa-Null) mouse. We hypothesized that carcinogen-induced DNA adducts would persist in the TP53 sequence of Xpa-Null HUFs leading to an increased propensity for mismatched base pairing and mutation during replication of adducted DNA. We found that Xpa-Null Hupki mice, and HUFs derived from them, were more sensitive to the environmental carcinogen benzo[a]pyrene (BaP) than their wild-type (Xpa-WT) counterparts. Following treatment with the reactive metabolite of BaP, benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), Xpa-WT and Xpa-Null HUF cultures were subjected to the HIMA. A significant increase in TP53 mutations on the transcribed strand was detected in Xpa-Null HUFs compared to Xpa-WT HUFs, but the TP53-mutant frequency overall was not significantly different between the two genotypes. BPDE induced mutations primarily at G:C base pairs, with approximately half occurring at CpG sites, and the predominant mutation type was G:C>T:A in both Xpa-WT and Xpa-Null cells. Further, several of the TP53 mutation hotspots identified in smokers' lung cancer were mutated by BPDE in HUFs (codons 157, 158, 245, 248, 249, 273). Therefore, the pattern and spectrum of BPDE-induced TP53 mutations in the HIMA are consistent with TP53 mutations detected in lung tumours of smokers. While Xpa-Null HUFs exhibited increased sensitivity to BPDE-induced damage on the transcribed strand, NER-deficiency did not enhance TP53 mutagenesis resulting from damage on the non-transcribed strand in this model.

Comparison of the metabolic activation of environmental carcinogens in mouse embryonic stem cells and mouse embryonic fibroblasts.

We compared mouse embryonic stem (ES) cells and fibroblasts (MEFs) for their ability to metabolically activate the environmental carcinogens benzo[a]pyrene (BaP), 3-nitrobenzanthrone (3-NBA) and aristolochic acid I (AAI), measuring DNA adduct formation by (32)P-postlabelling and expression of xenobiotic-metabolism genes by quantitative real-time PCR. At 2 µM, BaP induced Cyp1a1 expression in MEFs to a much greater extent than in ES cells and formed 45 times more adducts. Nqo1 mRNA expression was increased by 3-NBA in both cell types but induction was higher in MEFs, as was adduct formation. For AAI, DNA binding was over 450 times higher in MEFs than in ES cells, although Nqo1 and Cyp1a1 transcriptional levels did not explain this difference. We found higher global methylation of DNA in ES cells than in MEFs, which suggests higher chromatin density and lower accessibility of the DNA to DNA damaging agents in ES cells. However, AAI treatment did not alter DNA methylation. Thus mouse ES cells and MEFs have the metabolic competence to activate a number of environmental carcinogens, but MEFs have lower global DNA methylation and higher metabolic capacity than mouse ES cells.

Role of IGF-1R in mediating breast cancer invasion and metastasis.

In this review we bring forward what is currently known about the role of type I insulin-like growth factor receptor (IGF-1R) in mediating breast cancer invasion and metastasis. We begin by addressing how activated IGF-1R could allow pre-cancerous cells to become invasive. To this effect, we discuss clinical reports suggesting that activation of IGF-1R could stimulate ductal carcinoma in situs to become invasive. In the same light, we review basic research from our laboratory showing that IGF-1R differentially regulates the expression of breast cancer progression genes when pre-malignant breast epithelial cells were stimulated with insulin-like growth factor-I (IGF-I) over time. The discussion then turns toward the ability of IGF-1R to stimulate invasion of breast cancer cells that have acquired a malignant phenotype. At this stage of breast cancer, it appears that IGF-I stimulates cells to invade in part by inducing urokinase plasminogen activator. Finally, we consider the potential role of IGF-1R in regulating breast cancer metastases by facilitating angiogenesis and lymphangiogenesis. In support of this idea, there is evidence for IGF-1R in both of these processes through the induction of vascular endothelial growth factors (VEGF(165) and VEGF(121)). Thus, IGF-1R affords breast cancer cells many opportunities to become invasive and eventually metastatic. We conclude that disrupting IGF-1R signaling has many important implications in the treatment and management of breast cancer.

Metabolic activation of diesel exhaust carcinogens in primary and immortalized human TP53 knock-in (Hupki) mouse embryo fibroblasts.

Approximately 50% of human tumors have a mutation in TP53. The pattern and spectra of TP53 mutations often differ between cancer types, perhaps due to different etiological factors. The Hupki (human TP53 knock-in) mouse embryo fibroblast (HUF) immortalization assay is useful for studying mutagenesis in the human TP53 gene by environmental carcinogens. Prior to initiating an immortalization assay, carcinogen treatment conditions must be optimized, which can require a large number of cells. As primary HUF cultures senesce within 2 weeks, restricting their use, we investigated whether immortalized HUFs retaining wild-type TP53 can be surrogates for primary HUFs in initial treatment optimization. DNA damage by eight compounds found in diesel exhaust, benzo[a]pyrene, 3-nitrobenzanthrone, 1-nitropyrene, 1,3-dinitropyrene, 1,6-dinitropyrene, 1,8-dinitropyrene, 6-nitrochrysene, and 3-nitrofluorene, was assessed by (32) P-postlabeling and the alkaline comet assay in primary HUFs and in an immortal HUF cell line J201. For most compounds, higher levels of DNA adducts accumulated in J201 cells than in primary HUFs. This difference was not reflected in the comet assay or by cell viability changes. Experiments in three additional immortal HUF cell lines (AAI49, U56, and E2-143) confirmed strong differences in DNA adduct levels compared with primary HUFs. However, these did not correlate with the protein expression of Nqo1 or Nat1/2, or with gene expression of Cyp1a1 or Cyp1b1. Our results show that using immortal HUFs as surrogates for primary HUFs in genotoxicity screening has limitations and that DNA adduct formation is the best measure of genotoxicity of the nitro-polycyclic aromatic hydrocarbons tested in HUFs.

Evidence of exposure to aristolochic acid in patients with urothelial cancer from a Balkan endemic nephropathy region of Romania.

Recently, chronic Aristolochia poisoning was found responsible for the aetiology of Balkan endemic nephropathy (BEN) in Croatia, Serbia, and Bosnia, and diet was the likely route of exposure to aristolochic acid (AA). BEN, often associated with an increased incidence of upper urinary tract carcinoma (UUC), also affects residents of certain rural villages in Romania. AA is a nephrotoxin and human carcinogen that forms DNA adducts after metabolic activation, which induce characteristic TP53 mutations in urothelial tumours. Here we present the first evidence linking AA exposure to UUC in residents of an endemic region in the Romanian Mehedinti County. DNA was extracted from kidney and tumour tissue of seven patients who underwent nephroureterectomy for UUC and resided in BEN villages (endemic group). Five patients with UUC from nonendemic villages served as controls. AA-DNA adducts (7-(deoxyadenosin-N(6) -yl)-aristolactam I), established biomarkers of AA exposure, were identified by (32)P-postlabelling in renal DNA of six patients from the endemic group and in one of the nonendemic group (adduct levels ranged from 0.3 to 6.5 adducts per 10(8) nucleotides). Additionally, an A to T transversion in TP53, a base substitution characteristic of AA mutagenic activity was found in urothelial tumour DNA of one patient from the endemic group. Our results provide a molecular link to the cause of urothelial tumours in BEN regions of Romania indicating that AA is the common aetiological agent for BEN across its numerous geographical foci.