2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces genetic changes in murine intestinal tumours and cells with ApcMin mutation

S-allylcysteine potently protects against PhIP-induced DNA damage via Nrf2/AhR signaling pathway modulation in normal human colonic mucosal epithelial cells

SCOPE

This study aimed to investigate whether S-allylcysteine (SAC) exerts chemoprophylactic effects on foodborne carcinogenicity caused by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in normal human colonic mucosal epithelial cells.

METHODS AND RESULTS

Cellular thermal shift assays showed that SAC had an affinity for the Keap1 protein. Moreover, SAC may also dampen the binding of Keap1 and NF-E2-related factor 2 (Nrf2) by inhibiting p-p38 and increasing the phosphorylation of ERK1/2 and AKT, thereby inducing Nrf2/HO-1 signaling and upregulating the ratio of GSH to GSH/GSSG, which inhibits PhIP-induced oxidative stress and DNA damage. In addition, SAC significantly downregulates the aryl hydrocarbon receptor signaling pathway, suggesting that SAC may potentially impede the metabolic transformation of carcinogens.

CONCLUSION

Collectively, these findings suggest that SAC protects against PhIP-induced reactive oxygen species production and DNA damage by modulating the Nrf2/AhR signaling pathway, which may have significant potential as a novel chemopreventive agent. This article is protected by copyright. All rights reserved.

Targeting Colon Luminal Lipid Peroxidation Limits Colon Carcinogenesis Associated with Red Meat Consumption

Red meat is probably carcinogenic to humans (WHO/IARC class 2A), in part through heme iron-induced lipoperoxidation. Here, we investigated whether red meat promotes carcinogenesis in rodents and modulates associated biomarkers in volunteers, speculating that an antioxidant marinade could suppress these effects via limitation of the heme induced lipid peroxidation. We gave marinated or non-marinated beef with various degrees of cooking to azoxymethane-initiated rats, Min mice, and human volunteers (crossover study). Mucin-depleted foci were scored in rats, adenoma in Min mice. Biomarkers of lipoperoxidation were measured in the feces and urine of rats, mice, and volunteers. The organoleptic properties of marinated meat were tested. Fresh beef increased colon carcinogenesis and lipoperoxidation in rats and mice and lipoperoxidation in humans. Without an adverse organoleptic effect on meat, marinade normalized peroxidation biomarkers in rat and mouse feces, reduced peroxidation in human feces and reduced the number of Mucin-depleted foci in rats and adenoma in female Min mice. This could lead to protective strategies to decrease the colorectal cancer burden associated with red meat consumption. Cancer Prev Res; 11(9); 569-80. ©2018 AACR.

Animal models of colorectal cancer

Colorectal cancer is a heterogeneous disease that afflicts a large number of people in the USA. The use of animal models has the potential to increase our understanding of carcinogenesis, tumor biology, and the impact of specific molecular events on colon biology. In addition, animal models with features of specific human colorectal cancers can be used to test strategies for cancer prevention and treatment. In this review, we provide an overview of the mechanisms driving human cancer, we discuss the approaches one can take to model colon cancer in animals, and we describe a number of specific animal models that have been developed for the study of colon cancer. We believe that there are many valuable animal models to study various aspects of human colorectal cancer. However, opportunities for improving upon these models exist.

Mouse models of colorectal cancer

Colorectal cancer is one of the most common malignancies in the world. Many mouse models have been developed to evaluate features of colorectal cancer in humans. These can be grouped into genetically-engineered, chemically-induced, and inoculated models. However, none recapitulates all of the characteristics of human colorectal cancer. It is critical to use a specific mouse model to address a particular research question. Here, we review commonly used mouse models for human colorectal cancer.

Environmental exposure of the mouse germ line: DNA adducts in spermatozoa and formation of de novo mutations during spermatogenesis

BACKGROUND

Spermatozoal DNA damage is associated with poor sperm quality, disturbed embryonic development and early embryonic loss, and some genetic diseases originate from paternal de novo mutations. We previously reported poor repair of bulky DNA-lesions in rodent testicular cells.

METHODOLOGY/PRINCIPAL FINDINGS

We studied the fate of DNA lesions in the male germ line. B[a]PDE-N(2)-dG adducts were determined by liquid chromatography-tandem mass spectrometry, and de novo mutations were measured in the cII-transgene, in Big Blue mice exposed to benzo[a]pyrene (B[a]P; 3 x 50 mg/kg bw, i.p.). Spermatozoa were harvested at various time-points following exposure, to study the consequences of exposure during the different stages of spermatogenesis. B[a]PDE-N(2)-dG adducts induced by exposure of spermatocytes or later stages of spermatogenesis persisted at high levels in the resulting spermatozoa. Spermatozoa originating from exposed spermatogonia did not contain DNA adducts; however de novo mutations had been induced (p = 0.029), specifically GC-TA transversions, characteristic of B[a]P mutagenesis. Moreover, a specific spectrum of spontaneous mutations was consistently observed in spermatozoa.

CONCLUSIONS/SIGNIFICANCE

A temporal pattern of genotoxic consequences following exposure was identified, with an initial increase in DNA adduct levels in spermatozoa, believed to influence fertility, followed by induction of germ line de novo mutations with possible consequences for the offspring.

Escherichia coli induces DNA damage in vivo and triggers genomic instability in mammalian cells

Escherichia coli is a normal inhabitant of the human gut. However, E. coli strains of phylogenetic group B2 harbor a genomic island called "pks" that codes for the production of a polyketide-peptide genotoxin, Colibactin. Here we report that in vivo infection with E. coli harboring the pks island, but not with a pks isogenic mutant, induced the formation of phosphorylated H2AX foci in mouse enterocytes. We show that a single, short exposure of cultured mammalian epithelial cells to live pks(+) E. coli at low infectious doses induced a transient DNA damage response followed by cell division with signs of incomplete DNA repair, leading to anaphase bridges and chromosome aberrations. Micronuclei, aneuploidy, ring chromosomes, and anaphase bridges persisted in dividing cells up to 21 d after infection, indicating occurrence of breakage-fusion-bridge cycles and chromosomal instability. Exposed cells exhibited a significant increase in gene mutation frequency and anchorage-independent colony formation, demonstrating the infection mutagenic and transforming potential. Therefore, colon colonization with these E. coli strains harboring the pks island could contribute to the development of sporadic colorectal cancer.

Cyp1b1 exerts opposing effects on intestinal tumorigenesis via exogenous and endogenous substrates

Cytochrome P450 1B1 (Cyp1b1) metabolism contributes to physiologic functions during embryogenesis but also to carcinogenic activation of polycyclic aromatic hydrocarbons (PAH). We generated Cyp1b1-deficient mice carrying the Min allele of the adenomatous polyposis coli gene. These Cyp1b1-deficient Min mice developed twice as many tumors as Min controls, which, however, remained similar in size and histology. Tumors from older (130 days) Cyp1b1-deficient Min mice selectively exhibited focal areas of nuclear atypia associated with less organized epithelia. The metabolism of endogenous substrates by Cyp1b1, therefore, suppresses tumor initiation but also affects progression. Treatment of Min mice with 7,12-dimethylbenzanthracene (DMBA) doubled both tumor multiplicity and size within 20 days but not when mice lacked Cyp1b1. This was paralleled by an abnormal staining of crypts with beta-catenin, phospho-IkappaB kinase, and RelA, which may represent an early stage of tumorigenesis similar to aberrant crypt formation. Cyp1b1 deletion did not affect circulating DMBA and metabolites. Cyp1b1 expression was higher in the tumors compared with normal small intestines. Increased tumorigenesis may, therefore, arise from generation of DMBA metabolites by Cyp1b1 in the developing tumors. Benzo(a)pyrene (BP), which is similarly activated by Cyp1b1 in vitro, did not affect tumorigenesis in Min mice. By contrast, BP and DMBA each suppressed tumor multiplicity in the absence of Cyp1b1. Cyp1b1 metabolism of DMBA and endogenous oxygenation products may each affect a tumor-promoting nuclear factor-kappaB activation, whereas Ah receptor activation by PAH affects suppression. Tumorigenesis may, therefore, depend on activation of PAH by Cyp1b1 and on offsetting suppression by Cyp1b1 of endogenous tumor-enhancing substrates.

X radiation up-regulates the occurrence and the multiplicity of invasive carcinomas in the intestinal tract of Apc(min/+) mice

X rays are well known to cause genetic damage and to induce many types of carcinomas in humans. The Apc(min/+) mouse, an animal model for human familial adenomatous polyposis (FAP), contains a truncating mutation in the APC gene and spontaneously develops intestinal adenomas. To elucidate the role of X rays in the development of intestinal tumors, we examined the promotion of carcinogenesis in X-irradiated Apc(min/+) mice. Forty out of 77 (52%) X-irradiated Apc(min/+) mice developed adenocarcinomas that invaded the proprial muscle layer of the small intestine; 24 of 44 (55%) were in males, and 16 of 33 (49%) were in females. In contrast, invasive carcinomas were detected in the small intestines of only 13 of 64 (20%) nonirradiated Apc(min/+) mice; nine of 32 (28%) were in males and four of 32 (13%) were in females. These differences between X-irradiated and nonirradiated Apc(min/+) mice in the occurrence of invasive intestinal carcinomas were statistically significant (P < 0.05 for males, P < 0.005 for females). In wild-type mice, invasive carcinomas were not detected in either X-irradiated or nonirradiated mice. Apc(min/+) mice had many polyps in the large intestine with or without X irradiation; there was no difference in the number of polyps between the two groups. Also, invasive carcinomas were not detected in the large intestine with or without irradiation. The occurrence of mammary tumors, which was observed in Apc(min/+) mice, was found to be increased in irradiated Apc(min/+) mice (P < 0.01). Apc(min/+) mice had many polyps in the small and large intestines with or without X irradiation. X-irradiated Apc(min/+) mice had highly invasive carcinomas in the small intestine with multiplicities associated with invasiveness. Our results suggest that X radiation may promote the invasive activity of intestinal tumors in Apc(min/+) mice.

5-ASA affects cell cycle progression in colorectal cells by reversibly activating a replication checkpoint

BACKGROUND & AIMS

Individuals with inflammatory bowel disease are at risk of developing colorectal cancer (CRC). Epidemiologic, animal, and laboratory studies suggest that 5-amino-salicylic acid (5-ASA) protects from the development of CRC by altering cell cycle progression and by inducing apoptosis. Our previous results indicate that 5-ASA improves replication fidelity in colorectal cells, an effect that is active in reducing mutations. In this study, we hypothesized that 5-ASA restrains cell cycle progression by activating checkpoint pathways in colorectal cell lines, which would prevent tumor development and improve genomic stability.

METHODS

CRC cells with different genetic backgrounds such as HT29, HCT116, HCT116(p53-/-), HCT116+chr3, and LoVo were treated with 5-ASA for 2-96 hours. Cell cycle progression, phosphorylation, and DNA binding of cell cycle checkpoint proteins were analyzed.

RESULTS

We found that 5-ASA at concentrations between 10 and 40 mmol/L affects cell cycle progression by inducing cells to accumulate in the S phase. This effect was independent of the hMLH1, hMSH2, and p53 status because it was observed to a similar extent in all cell lines under investigation. Moreover, wash-out experiments demonstrated reversibility within 48 hours. Although p53 did not have a causative role, p53 Ser15 was strongly phosphorylated. Proteins involved in the ATM-and-Rad3-related kinase (ATR)-dependent S-phase checkpoint response (Chk1 and Rad17) were also phosphorylated but not ataxia telengectasia mutated kinase.

CONCLUSIONS

Our data demonstrate that 5-ASA causes cells to reversibly accumulate in S phase and activate an ATR-dependent checkpoint. The activation of replication checkpoint may slow down DNA replication and improve DNA replication fidelity, which increases the maintenance of genomic stability and counteracts carcinogenesis.