Dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced prostate carcinogenesis in CYP1A-humanized mice

Understanding the heterocyclic aromatic amines: An overview and recent findings

Heterocyclic aromatic amines (HAAs) constitute a group of highly toxic organic compounds strongly associated with the onset of various types of cancer. This paper aims to serve as a valuable resource for food scientists working towards a better understanding of these compounds including formation, minimizing strategies, analysis, and toxicity as well as addressing existing gaps in the literature. Despite extensive research conducted on these compounds since their discovery, several aspects remain inadequately understood, necessitating further investigation. These include their formation pathways, toxic mechanisms, effective mitigation strategies, and specific health effects on humans. Nonetheless, recent research has yielded promising results, contributing significantly to our understanding of HAAs by proposing new potential formation pathways and innovative strategies for their reduction.

Assessing cytochrome P450 function using genetically engineered mouse models

The ability to knock out and/or humanize different genes in experimental animals, globally or in cell- and tissue-specific patterns, has revolutionized scientific research in many areas. Genetically engineered mouse models, including knockout models, transgenic models, and humanized models, have played important roles in revealing the in vivo functions of various cytochrome P450 (CYP) enzymes. These functions are very diverse, ranging from the biotransformation of drugs and other xenobiotics, events that often dictate their pharmacokinetic or toxicokinetic properties and the associated therapeutic or adverse actions, to the metabolism of endogenous compounds, such as steroid hormones and other bioactive substances, that may determine susceptibility to many diseases, such as cancer and metabolic diseases. In this review, we provide a comprehensive list of Cyp-knockout, human CYP-transgenic, and CYP-humanized mouse models that target genes in the CYP1-4 gene families, and highlight their utility in assessing the in vivo metabolism, bioactivation, and toxicity of various xenobiotic compounds, including therapeutic agents and chemical carcinogens. We aim to showcase the advantages of utilizing these mouse models for in vivo drug metabolism and toxicology studies, and to encourage and facilitate greater utility of engineered mouse models to further improve our knowledge of the in vivo functions of various P450 enzymes, which is integral to our ability to develop safer and more effective therapeutics and to identify individuals predisposed to adverse drug reactions or environmental diseases.

The Cooked Meat Carcinogen 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine Hair Dosimeter, DNA Adductomics Discovery, and Associations with Prostate Cancer Pathology Biomarkers

Well-done cooked red meat consumption is linked to aggressive prostate cancer (PC) risk. Identifying mutation-inducing DNA adducts in the prostate genome can advance our understanding of chemicals in meat that may contribute to PC. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a heterocyclic aromatic amine (HAA) formed in cooked meat, is a potential human prostate carcinogen. PhIP was measured in the hair of PC patients undergoing prostatectomy, bladder cancer patients under treatment for cystoprostatectomy, and patients treated for benign prostatic hyperplasia (BPH). PhIP hair levels were above the quantification limit in 123 of 205 subjects. When dichotomizing prostate pathology biomarkers, the geometric mean PhIP hair levels were higher in patients with intermediate and elevated-risk prostate-specific antigen values than lower-risk values <4 ng/mL (p = 0.03). PhIP hair levels were also higher in patients with intermediate and high-risk Gleason scores ≥7 compared to lower-risk Gleason score 6 and BPH patients (p = 0.02). PC patients undergoing prostatectomy had higher PhIP hair levels than cystoprostatectomy or BPH patients (p = 0.02). PhIP-DNA adducts were detected in 9.4% of the patients assayed; however, DNA adducts of other carcinogenic HAAs, and benzo[a]pyrene formed in cooked meat, were not detected. Prostate specimens were also screened for 10 oxidative stress-associated lipid peroxidation (LPO) DNA adducts. Acrolein 1,N²-propano-2'-deoxyguanosine adducts were detected in 54.5% of the patients; other LPO adducts were infrequently detected. Acrolein adducts were not associated with prostate pathology biomarkers, although DNA adductomic profiles differed between PC patients with low and high-grade Gleason scores. Many DNA adducts are of unknown origin; however, dG adducts of formaldehyde and a series of purported 4-hydroxy-2-alkenals were detected at higher abundance in a subset of patients with elevated Gleason scores. The PhIP hair biomarker and DNA adductomics data support the paradigm of well-done cooked meat and oxidative stress in aggressive PC risk.

δ-Tocotrienol is the most potent vitamin E form in inhibiting prostate cancer cell growth and inhibits prostate carcinogenesis in Ptenp-/- mice

Vitamin E compounds, consisting of α, β, γ, and δ forms of tocopherols and tocotrienols, display different cancer preventive activities in experimental models. Tocotrienols may have higher potential for clinical use due to their lower effective doses in laboratory studies. However, most studies on tocotrienols have been carried out using cancer cell lines. Strong data from animal studies may encourage the use of tocotrienols for human cancer prevention research. To examine the cancer inhibitory activity of different vitamin E forms, we first investigated their inhibitory activities of different vitamin E forms in prostate cancer cell lines. We found that δ-tocotrienol (δT3) was the most effective form in inhibiting cell growth at equivalent doses. Because of this in vitro potency, δT3 was further studied using prostate specific Pten-/- (Ptenp-/-) mice. We found that 0.05% δT3 in diet reduced prostate adenocarcinoma multiplicity by 32.7%, featuring increased apoptosis and reduced cell proliferation. The inhibitory effect of 0.05% δT3 in diet was similar to that of 0.2% δ-tocopherol (δT) in diet reported previously. Our further study on the δT3-induced transcriptome changes indicated that δT3 inhibited genes in blood vessel development in the prostate of Ptenp-/- mice, which was confirmed by immunohistochemistry. Together, our results demonstrate that δT3 effectively inhibits the development of prostate adenocarcinoma in Ptenp-/- mice, which involves inhibition of proliferation and angiogenesis and promotion of apoptosis.

Can preclinical drug development help to predict adverse events in clinical trials?

The development of novel therapeutics is associated with high rates of attrition, with unexpected adverse events being a major cause of failure. Serious adverse events have led to organ failure, cancer development and deaths that were not expected outcomes in clinical trials. These life-threatening events were not identified during therapeutic development due to the lack of preclinical safety tests that faithfully represented human physiology. We highlight the successful application of several novel technologies, including high-throughput screening, organs-on-chips, microbiome-containing drug-testing platforms and humanised mouse models, for mechanistic studies and prediction of toxicity. We propose the incorporation of similar preclinical tests into future drug development to reduce the likelihood of hazardous therapeutics entering later-stage clinical trials.

Metabolism and biomarkers of heterocyclic aromatic amines in humans

Heterocyclic aromatic amines (HAAs) form during the high-temperature cooking of meats, poultry, and fish. Some HAAs also arise during the combustion of tobacco. HAAs are multisite carcinogens in rodents, inducing cancer of the liver, gastrointestinal tract, pancreas, mammary, and prostate glands. HAAs undergo metabolic activation by N-hydroxylation of the exocyclic amine groups to produce the proposed reactive intermediate, the heteroaryl nitrenium ion, which is the critical metabolite implicated in DNA damage and genotoxicity. Humans efficiently convert HAAs to these reactive intermediates, resulting in HAA protein and DNA adduct formation. Some epidemiologic studies have reported an association between frequent consumption of well-done cooked meats and elevated cancer risk of the colorectum, pancreas, and prostate. However, other studies have reported no associations between cooked meat and these cancer sites. A significant limitation in epidemiology studies assessing the role of HAAs and cooked meat in cancer risk is their reliance on food frequency questionnaires (FFQ) to gauge HAA exposure. FFQs are problematic because of limitations in self-reported dietary history accuracy, and estimating HAA intake formed in cooked meats at the parts-per-billion level is challenging. There is a critical need to establish long-lived biomarkers of HAAs for implementation in molecular epidemiology studies designed to assess the role of HAAs in health risk. This review article highlights the mechanisms of HAA formation, mutagenesis and carcinogenesis, the metabolism of several prominent HAAs, and the impact of critical xenobiotic-metabolizing enzymes on biological effects. The analytical approaches that have successfully biomonitored HAAs and their biomarkers for molecular epidemiology studies are presented.

The dietary carcinogen PhIP activates p53-dependent DNA damage response in the colon of CYP1A-humanized mice

Species differences in the metabolism of xenobiotics by cytochrome P450 are critical in evaluating the use of experimental animals in studying toxic compounds relevant to human diseases. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), which is produced by high-temperature cooking of fish and meat, is activated to become a carcinogen by cytochrome P4501A2 (CYP1A2) through N² -hydroxylation in humans, but is detoxified by Cyp1a2 through 4'-hydroxylation in mice. CYP1A-humanized (hCYP1A) mice, in which mouse Cyp1a is replaced with human CYP1A, show constitutive human xenobiotic metabolism by hCYP1A, thereby serving as a suitable model for studying PhIP. Previous studies have demonstrated that oral administration of PhIP induces colon tumors in hCYP1A mice; however, these studies used a super-high dose, raising concerns regarding the relevance of the mechanism to human cancer. Herein, we systematically investigated PhIP-induced colon carcinogenesis in hCYP1A mice treated with lower doses. We found that a dose 2000 times lower than that used previously, which is comparable to human daily intake levels, could induce colon tumors, albeit at a lower incidence rate. We further investigated the transcriptome changes in the colon of hCYP1A mice treated with PhIP and identified that PhIP treatment increased the expression of Bax, Btg2, Ccng1, Cdkn1a, and Trp53inp1 and decreased the expression of Igf1 and Ccnd1. Since these genes are key components of the p53-dependent DNA damage response, the altered expression patterns indicated PhIP-induced DNA damage in hCYP1A mice. Together, these results prove that hCYP1A mice are suitable for studying PhIP-induced carcinogenesis and show that PhIP is an important colorectal cancer carcinogen in human diet.

Shrimp as a substantial source of carcinogenic heterocyclic amines

In the present work, fifteen mutagenic/carcinogenic heterocyclicamines (HAs)were studied in cooked Caridean shrimp (pink) and Penaeid shrimp (tiger, white and brown). The cooking methods were used as stir-frying, broiling and steaming under controlled temperature and time, and HAs determination was performed by SPE/UPLC-ESI-MS/MS. HAs 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-Amino-1,6-dimethyl-imidazo[4,5-b]pyridine (DMIP), 1-Methyl-9H-pyrido[3,4-b]indole (Harman) and 9H-pyrido[3,4-b]indole (Norharman) were identified (0.05-22.48 ng/g) in all stir-fried and broiled shrimp, whereas 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) was only found (0.08-0.35 ng/g) in stir-fried shrimp. HAs 2-Amino-3,7,8-trimethyl-imidazo[4,5-f]quinoxaline (7,8-DiMeIQx) and, α-carbolines 2-Amino-9H-pyrido[2,3-b]indole (AαC) and 2-Amino-3-methyl-9H-pyrido[2,3-b]indole (MeAαC), γ-carbolines 3-Amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 3-Amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) and δ-carbolines 2-Amino-6-methyldipyrido[1,2-α:3́,2́-d]imidazole (Glu-P-1) and 2-Aminodipyrido[1,2-α:3́,2́-d]imidazole (Glu-P-2) were not detected or found below quantification limit. Brown shrimp (stir-fried) appeared to be more contaminated which constitutes total HAs (81.93 ng/g) followed by pink (70.41 ng/g), tiger (53.02 ng/g) and white (33.57 ng/g). Steaming method does not yield any HAs, and the cause might be elucidate that shrimp were not directly in contact with cooking pan or fire which affect the HAs formation. Food precursors (protein, moisture, fat, creatine and glucose) were also measured in raw and cooked shrimp to investigate the influence on HAs formation. Creatine (3.85 mg/g) and glucose (0.43 mg/g) were found at higher concentrations in brown shrimp, generates high amounts of HAs. Our findings have illustrated that the cooking method, shrimp types and precursors are the main contributors to the formation of HAs. The outcomes from this work could be applied to estimate the HAs human intake globally and add to steaming cooking method in such types of food products that diminish the risk of HAs exposure, and thus to get healthier food quality and security.

Ex vivo/in vitro protective effect of myricetin bulk and nano-forms on PhIP-induced DNA damage in lymphocytes from healthy individuals and pre-cancerous MGUS patients

2-Amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) is a central dietary mutagen, produced when proteinaceous food is heated at very high temperatures potentially causing DNA strand breaks. This study investigates the protective potential of a well-researched flavonoid, myricetin in its bulk and nano-forms against oxidative stress induced ex vivo/in vitro by PhIP in lymphocytes from pre-cancerous monoclonal gammopathy of undetermined significance (MGUS) patients and those from healthy individuals. The results from the Comet assay revealed that in the presence of myricetin bulk (10 µM) and myricetin nano (20 µM), the DNA damage caused by a high dose of PhIP (100 µM) was significantly (P < 0.001) reduced in both groups. However, nano has shown better protection in lymphocytes from pre-cancerous patients. Consistent results were obtained from the micronucleus assay where micronuclei frequency in binucleated cells significantly decreased upon supplementing PhIP with myricetin bulk (P < 0.01) and myricetin nano (P < 0.001), compared to the PhIP treatment alone. To briefly determine the cellular pathways involved in the protective role of myricetin against PhIP, we studied gene expression of P53 and ATR kinase (ATM- and Rad3-related), using the real-time PCR technique.

Metabolic Activation of the Cooked Meat Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine in Human Prostate

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), an heterocyclic aromatic amine (HAA) formed in cooked meat, is a rodent and possible human prostate carcinogen. Recently, we identified DNA adducts of PhIP in the genome of prostate cancer patients, but adducts of 2-amino-3, 8-dimethylmidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-9 H-pyrido[2,3-b]indole (AαC), other prominent HAAs formed in cooked meats, were not detected. We have investigated the bioactivation of HAAs by Phase I and II enzymes in the human prostate (LNCaP) cell line using cytotoxicity and DNA adducts as endpoints. PhIP, MeIQx, and 2-amino-3-methylimidazo[4,5-f]quinoline, another HAA found in cooked meats, were poorly bioactivated and not toxic. The synthetic genotoxic N-hydroxylated-HAAs were also assayed in LNCaP cells with Phase II enzyme inhibitors. Notably, 2-hydroxy-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), but not other HONH-HAAs, induced cytotoxicity. Moreover, PhIP-DNA adduct formation was 20-fold greater than adducts formed with other HONH-HAAs. Pretreatment of LNCaP cells with mefenamic acid, a specific inhibitor of sulfotransferase (SULT1A1), decreased PhIP-DNA adducts by 25%, whereas (Z)-5-(2'-hydroxybenzylidene)-2-thioxothiazolidin-4-one and pentachlorophenol, inhibitors of SULTs and N-acetyltransferases (NATs), decreased the PhIP-DNA adduct levels by 75%. NATs in cytosolic fractions of LNCaP cells and human prostate catalyzed DNA binding of HONH-PhIP by up to 100-fold greater levels than for SULT and kinase activities. Recombinant NAT2 is catalytically superior to recombinant NAT1 in the bioactivation of HONH-PhIP; however, the extremely low levels of NAT2 activity in prostate suggest that NAT1 may be the major isoform involved in PhIP-DNA damage. Thus, the high susceptibility of LNCaP cells recapitulates the DNA-damaging effect of HONH-PhIP in rodent and human prostate.

Dietary Carcinogens and DNA Adducts in Prostate Cancer

Prostate cancer (PC) is the most commonly diagnosed non-cutaneous cancer and the second leading cause of cancer-related to death in men. The major risk factors for PC are age, family history, and African American ethnicity. Epidemiological studies have reported large geographical variations in PC incidence and mortality, and thus lifestyle and dietary factors influence PC risk. High fat diet, dairy products, alcohol and red meats, are considered as risk factors for PC. This book chapter provides a comprehensive, literature-based review on dietary factors and their molecular mechanisms of prostate carcinogenesis. A large portion of our knowledge is based on epidemiological studies where dietary factors such as cancer promoting agents, including high-fat, dairy products, alcohol, and cancer-initiating genotoxicants formed in cooked meats have been evaluated for PC risk. However, the precise mechanisms in the etiology of PC development remain uncertain. Additional animal and human cell-based studies are required to further our understandings of risk factors involved in PC etiology. Specific biomarkers of chemical exposures and DNA damage in the prostate can provide evidence of cancer-causing agents in the prostate. Collectively, these studies can improve public health research, nutritional education and chemoprevention strategies.

P450-Humanized and Human Liver Chimeric Mouse Models for Studying Xenobiotic Metabolism and Toxicity

Preclinical evaluation of drug candidates in experimental animal models is an essential step in drug development. Humanized mouse models have emerged as a promising alternative to traditional animal models. The purpose of this mini-review is to provide a brief survey of currently available mouse models for studying human xenobiotic metabolism. Here, we describe both genetic humanization and human liver chimeric mouse models, focusing on the advantages and limitations while outlining their key features and applications. Although this field of biomedical science is relatively young, these humanized mouse models have the potential to transform preclinical drug testing and eventually lead to a more cost-effective and rapid development of new therapies.

δ-Tocopherol inhibits the development of prostate adenocarcinoma in prostate specific Pten-/- mice

The PTEN/PI3K/AKT axis plays a critical role in regulating cell growth, differentiation and survival. Activation of this signaling pathway is frequently found in human cancers. Our previous studies demonstrated that δ-tocopherol (δ-T) attenuates the activation of AKT by growth factor in prostate cancer cell lines, leading to inhibition of proliferation and induction of apoptosis. Herein, we investigated whether δ-T inhibits the development of prostate adenocarcinoma in prostate-specific Pten-/- (Ptenp-/-) mice in which the activation of AKT is the major driving force for tumorigenesis. By feeding Ptenp-/- mice with AIN93M or 0.2% δ-T supplemented diet starting at the age of 6 or 12 weeks, we found that δ-T treatment reduced prostate adenocarcinoma multiplicity at the age of 40 weeks by 53.3 and 42.7%, respectively. Immunohistochemical (IHC) analysis demonstrated that the phosphorylation of AKT (T308) was reduced in the prostate of the mice administered the δ-T diet. Consistently, proliferation was reduced and apoptosis was increased in prostate lesions of mice on the δ-T diet. Oxidative stress, as determined by IHC staining of 8-OH-dG, was not altered during prostate tumorigenesis, nor was it affected by administration of δ-T. In contrast, α-tocopherol (α-T) at 0.2% in the diet did not affect prostate adenocarcinoma multiplicity in the Ptenp-/- mice. This finding is consistent with data from our previous study that δ-T, but not α-T, inhibits the activation of AKT and the growth of prostate cancer cells. Together, these results demonstrate that δ-T inhibits the development of prostate adenocarcinoma in Ptenp-/- mice, mainly through inhibition of AKT activation.

Biomonitoring DNA Adducts of Cooked Meat Carcinogens in Human Prostate by Nano Liquid Chromatography-High Resolution Tandem Mass Spectrometry: Identification of 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine DNA Adduct

Epidemiologic studies have reported an association between frequent consumption of well-done cooked meats and prostate cancer risk. However, unambiguous physiochemical markers of DNA damage from carcinogens derived from cooked meats, such as DNA adducts, have not been identified in human samples to support this paradigm. We have developed a highly sensitive nano-LC-Orbitrap MS n method to measure DNA adducts of several carcinogens originating from well-done cooked meats, tobacco smoke, and environmental pollution, including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-9H-pyrido[2,3-b]indole (AαC), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), benzo[a]pyrene (B[a]P), and 4-aminobiphenyl (4-ABP). The limit of quantification (LOQ) of the major deoxyguanosine (dG) adducts of these carcinogens ranged between 1.3 and 2.2 adducts per 10 9 nucleotides per 2.5 μg of DNA assayed. The DNA adduct of PhIP, N-(deoxyguanosin-8-yl)-PhIP (dG-C8-PhIP) was identified in 11 out of 35 patients, at levels ranging from 2 to 120 adducts per 10 9 nucleotides. The dG-C8 adducts of AαC and MeIQx, and the B[a]P adduct, 10-(deoxyguanosin-N 2 -yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (dG-N 2 -B[a]PDE) were not detected in any specimen, whereas N-(deoxyguanosin-8-yl)-4-ABP (dG-C8-4-ABP) was identified in one subject (30 adducts per 10 9 nucleotides). PhIP-DNA adducts also were recovered quantitatively from formalin fixed paraffin embedded (FFPE) tissues, signifying FFPE tissues can serve as biospecimens for carcinogen DNA adduct biomarker research. Our biomarker data provide support to the epidemiological observations implicating PhIP, one of the most mass-abundant heterocyclic aromatic amines formed in well-done cooked meats, as a DNA-damaging agent that may contribute to the etiology of prostate cancer.

DNA damage response curtails detrimental replication stress and chromosomal instability induced by the dietary carcinogen PhIP

PhIP is an abundant heterocyclic aromatic amine (HCA) and important dietary carcinogen. Following metabolic activation, PhIP causes bulky DNA lesions at the C8-position of guanine. Although C8-PhIP-dG adducts are mutagenic, their interference with the DNA replication machinery and the elicited DNA damage response (DDR) have not yet been studied. Here, we analyzed PhIP-triggered replicative stress and elucidated the role of the apical DDR kinases ATR, ATM and DNA-PKcs in the cellular defense response. First, we demonstrate that PhIP induced C8-PhIP-dG adducts and DNA strand breaks. This stimulated ATR-CHK1 signaling, phosphorylation of histone 2AX and the formation of RPA foci. In proliferating cells, PhIP treatment increased the frequency of stalled replication forks and reduced fork speed. Inhibition of ATR in the presence of PhIP-induced DNA damage strongly promoted the formation of DNA double-strand breaks, activation of the ATM-CHK2 pathway and hyperphosphorylation of RPA. The abrogation of ATR signaling potentiated the cell death response and enhanced chromosomal aberrations after PhIP treatment, while ATM and DNA-PK inhibition had only marginal effects. These results strongly support the notion that ATR plays a key role in the defense against cancer formation induced by PhIP and related HCAs.

2-amino-1-methyl-6-phenylimidazo(4,5-b) pyridine (PhIP) induces gene expression changes in JAK/STAT and MAPK pathways related to inflammation, diabetes and cancer

Background

2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), a heterocyclic aromatic amine (HCA) formed in meat that is cooked at high temperatures and then ingested, can potentially be retained in human adipose tissues.

Methods

To determine if PhIP is bioactive in the adipocyte, we exposed a human adipocyte cell line,HepG2 and Caco-2 cells to low dose PhIP. Uptake and retention of PhIP was determined and cytotoxicity was assessed by the TUNEL assay. Relative expression of PhIP-activating genes (CYP1A1, CYP1A2, SULT1A1 and UGT1A1) was determined by RT-PCR and global expression changes were also examined.

Results

The percent retention of 0.1 μCi [¹⁴C]-PhIP over a 24 h period was significantly higher in the adipocyte than the HepG2 (p = 0.0001) and Caco-2 (p = 0.0007) cell lines. Cytotoxicity rates were 14.4 and 2.6 % higher compared to controls in Caco-2 and HepG2 cells (p < 0.001 and 0.054, respectively); no significant differences were detected in adipocyte cells (p = 0.18). Caco-2 and HepG2 cells, respectively, had significantly higher basal expression of CYP1A1 (p = 0.001, p = 0.003), SULT1A1 (p = 0.04, p < 0.001) and UGT1A1 (p < 0.001, p = 0.01) compared to the adipocyte. Exposure to 5nM PhIP did not significantly induce expression of these genes in any of the cell lines. Global gene expression analysis of mature adipocytes exposed to 5nM PhIP for 72 h resulted in statistically significant changes in 8 genes (ANGPTL2, CD14, CIDEA, EGR1, FOS, IGFBP5, PALM and PSAT1). Gene-gene interaction and pathway analysis indicates that PhIP modulates genes controlled by the STAT3 transcriptional factor and initiates leptin signaling via the JAK/STAT and MAPK pathway cascades. Early growth response 1 (EGR1) and prostaglandin synthase 2 (COX-2) were down-regulated via c-Fos, while insulin binding protein 5 (IBP5) was up regulated. Expression of transcription factors (ANGPTL2, HP, LEP, SAA1, SAA2), genes related to inflammation (SAA1, LEP), diabetes (IGFBP5) and cancer risk (SAA2) were also elevated upon exposure to 5 nM PhIP..

Conclusions

PhIP mediates gene expression changes within the adipocyte, and the pathways most affected are related to cancer and other chronic diseases. Further studies are needed on the relationship between dietary carcinogens such as PhIP with cancer, obesity and diabetes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-016-0111-0) contains supplementary material, which is available to authorized users.

Environmentally induced epigenetic toxicity: potential public health concerns

Throughout our lives, epigenetic processes shape our development and enable us to adapt to a constantly changing environment. Identifying and understanding environmentally induced epigenetic change(s) that may lead to adverse outcomes is vital for protecting public health. This review, therefore, examines the present understanding of epigenetic mechanisms involved in the mammalian life cycle, evaluates the current evidence for environmentally induced epigenetic toxicity in human cohorts and rodent models and highlights the research considerations and implications of this emerging knowledge for public health and regulatory toxicology. Many hundreds of studies have investigated such toxicity, yet relatively few have demonstrated a mechanistic association among specific environmental exposures, epigenetic changes and adverse health outcomes in human epidemiological cohorts and/or rodent models. While this small body of evidence is largely composed of exploratory in vivo high-dose range studies, it does set a precedent for the existence of environmentally induced epigenetic toxicity. Consequently, there is worldwide recognition of this phenomenon, and discussion on how to both guide further scientific research towards a greater mechanistic understanding of environmentally induced epigenetic toxicity in humans, and translate relevant research outcomes into appropriate regulatory policies for effective public health protection.

Obesity promotes PhIP-induced small intestinal carcinogenesis in hCYP1A-db/db mice: involvement of mutations and DNA hypermethylation of Apc

Obesity is associated with an increased risk of cancer. To study the promotion of dietary carcinogen-induced gastrointestinal cancer by obesity, we employed 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) to induce intestinal tumorigenesis in CYP1A-humanized (hCYP1A) mice, in which mouse Cyp1a1/1a2 was replaced with human CYP1A1/1A2 Obesity was introduced in hCYP1A mice by breeding with Lepr(db/+) mice to establish the genetically induced obese hCYP1A-Lepr(db/db) mice or by feeding hCYP1A mice a high-fat diet. PhIP induced the formation of small intestinal tumors at the ages of weeks 28-40 in obese hCYP1A mice, but not in lean hCYP1A mice. No tumors were found in colon and other gastrointestinal organs in the lean or obese mice. Using immunohistochemistry (IHC), we found strong positive staining of NF-κB p65, pSTAT3 and COX2 as well as elevated levels of nuclear β-catenin (Ctnnb1) in small intestinal tumors, but not in normal tissues. By sequencing Apc and Ctnnb1 genes, we found that most PhIP-induced small intestinal tumors in obese mice carried only a single heterozygous mutation in Apc By bisulfite-sequencing of CpG islands of Apc, we found DNA hypermethylation in a CpG cluster located in its transcription initiation site, which most likely caused the inactivation of the wild-type Apc allele. Our findings demonstrate that PhIP-induced small intestinal carcinogenesis in hCYP1A-db/db mice is promoted by obesity and involves Apc mutation and inactivation by DNA hypermethylation. This experimental result is consistent with the association of obesity and the increased incidence of small intestinal cancer in humans in recent decades.

A review and meta-analysis of prospective studies of red and processed meat, meat cooking methods, heme iron, heterocyclic amines and prostate cancer

Prostate cancer remains a significant public health concern among men in the U.S. and worldwide. Epidemiologic studies have generally produced inconclusive results for dietary risk factors for prostate cancer, including consumption of red and processed meats. We aimed to update a previous meta-analysis of prospective cohorts of red and processed meats and prostate cancer with the inclusion of new and updated cohort studies, as well as evaluate meat cooking methods, heme iron, and heterocyclic amine (HCA) intake exposure data. A comprehensive literature search was performed and 26 publications from 19 different cohort studies were included. Random effects models were used to calculate summary relative risk estimates (SRREs) for high vs. low exposure categories. Additionally, meta-regression analyses and stratified intake analyses were conducted to evaluate dose-response relationships. The SRREs for total prostate cancer and total red meat consumption, fresh red meat consumption, and processed meat consumption were 1.02 (95% CI: 0.92-1.12), 1.06 (95% CI: 0.97-1.16), and 1.05 (95% CI: 1.01-1.10), respectively. Analyses were also conducted for the outcomes of non-advanced, advanced, and fatal prostate cancer when sufficient data were available, but these analyses did not produce significant results. No significant SRREs were observed for any of the meat cooking methods, HCA, or heme iron analyses. Dose-response analyses did not reveal significant patterns of associations between red or processed meat and prostate cancer. In conclusion, the results from our analyses do not support an association between red meat or processed consumption and prostate cancer, although we observed a weak positive summary estimate for processed meats.

Effect of Cytochrome P450 Reductase Deficiency on 2-Amino-9H-pyrido[2,3-b]indole Metabolism and DNA Adduct Formation in Liver and Extrahepatic Tissues of Mice

2-Amino-9H-pyrido[2,3-b]indole (AαC), a carcinogen formed during the combustion of tobacco and cooking of meat, undergoes cytochrome P450 (P450) metabolism to form the DNA adduct N-(deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole (dG-C8-AαC). We evaluated the roles of P450 expressed in the liver and intestine to bioactivate AαC by employing male B6 wild-type (WT) mice, liver-specific P450 reductase (Cpr)-null (LCN) mice, and intestinal epithelium-specific Cpr-null (IECN) mice. Pharmacokinetic parameters were determined for AαC, 2-amino-9H-pyrido[2,3-b]indol-3-yl sulfate (AαC-3-OSO3H), and N(2)-(β-1-glucosidurony1)-2-amino-9H-pyrido[2,3-b]indole (AαC-N(2)-Glu) with animals dosed by gavage with AαC (13.6 mg/kg). The uptake of AαC was rapid with no difference in the plasma half-lives (t1/2) of AαC, AαC-3-OSO3H, and AαC-N(2)-Glu among mouse models. The maximal plasma concentrations (Cmax) and the areas under concentration-time curve (AUC0-24h) of AαC and AαC-N(2)-Glu were 4-24-fold higher in LCN than in WT mice, but they were not different between WT and IECN mice. These findings are consistent with the ablation of hepatic P450 activity in LCN mice. However, the Cmax and AUC0-24h of AαC-3-OSO3H in plasma were not substantially different among the mouse models. Similar pharmacokinetic parameters were obtained with WT and LCN mice treated with a lower AαC dose (1.36 mg kg(-1)). dG-C8-AαC was detected at similar levels in the livers of all three mouse models at the high AαC dose; levels of dG-C8-AαC in colon, bladder, and lung were greater in LCN than in WT mice and were the same in colon of IECN and WT mice. At the low AαC dose, dG-C8-AαC occurred at ∼ 40% lower levels in liver of LCN mouse than in WT mouse liver, but adduct levels remained higher in extrahepatic tissues of LCN mice. Therefore, hepatic P450 plays an important role in detoxication of AαC, but other hepatic or extrahepatic enzymes contribute to the bioactivation of AαC. P450s expressed in the intestine do not appreciably contribute to bioactivation of AαC in mice.

Dietary tocopherols inhibit PhIP-induced prostate carcinogenesis in CYP1A-humanized mice

Tocopherols, the major forms of vitamin E, exist as alpha-tocopherol (α-T), β-T, γ-T and δ-T. The cancer preventive activity of vitamin E is suggested by epidemiological studies, but recent large-scale cancer prevention trials with high dose of α-T yielded disappointing results. Our hypothesis that other forms of tocopherols have higher cancer preventive activities than α-T was tested, herein, in a novel prostate carcinogenesis model induced by 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP), a dietary carcinogen, in the CYP1A-humanized (hCYP1A) mice. Treatment of hCYP1A mice with PhIP (200 mg/kg b.w., i.g.) induced high percentages of mouse prostatic intraepithelial neoplasia (mPIN), mainly in the dorsolateral glands. Supplementation with a γ-T-rich mixture of tocopherols (γ-TmT, 0.3% in diet) significantly inhibited the development of mPIN lesions and reduced PhIP-induced elevation of 8-oxo-deoxyguanosine, COX-2, nitrotyrosine, Ki-67 and p-AKT, and the loss of PTEN and Nrf2. Further studies with purified δ-T, γ-T or α-T (0.2% in diet) showed that δ-T was more effective than γ-T or α-T in preventing mPIN formations and p-AKT elevation. These results indicate that γ-TmT and δ-T could be effective preventive agents of prostate cancer.

Intake of Meat Mutagens and Risk of Prostate Cancer in a Cohort of U.S. Health Professionals

BACKGROUND

Evidence relating heterocyclic aromatic amines (HCA), associated with high-temperature cooking methods, to prostate cancer risk is inconsistent.

METHODS

In a large U.S. cohort study, intakes of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and a meat-derived mutagenicity (MDM) index were assessed using a cooking method questionnaire administered in 1996. Until 2010, 2,770 prostate cancer cases were observed among 26,030 participants.

RESULTS

Intake of PhIP from red meat was statistically significantly associated with total prostate cancer risk (top vs. bottom quintile HR, 1.18; 95% confidence intervals; CI, 1.03-1.35), but not other HCAs (MeIQx, 1.12; 0.98-1.27, PhIP from white meat, 1.08; 0.95-1.22, DiMeIQx, 1.09; 0.97-1.21) or MDM (1.13; 1.00-1.28). For high-grade (Gleason sum 7 with pattern 4+3 and Gleason sum 8-10, n = 483 cases) and advanced cancers (n = 281), we only observed positive associations for PhIP from red meat (top vs. bottom quintile: high grade: HR, 1.44; 95% CI, 1.04-1.98, Ptrend = 0.03; advanced: HR, 1.50; 95% CI, 0.99-2.26; Ptrend = 0.12), but associations for advanced cancers did not reach statistical significance. Observed associations remained similar after adjustment for total, unprocessed, or processed red meat intake.

CONCLUSION

Observed positive associations between PhIP intake from red meat and prostate cancer, particularly high-grade and possibly also advanced prostate cancer, need to be confirmed in other studies.

IMPACT

Results do not provide strong evidence that HCAs increase risk of prostate cancers.

Immune Infiltration and Prostate Cancer

It is becoming increasingly clear that inflammation influences prostate cancer (PCa) development and that immune cells are among the primary drivers of this effect. This information has launched numerous clinical trials testing immunotherapy drugs in PCa patients. The results of these studies are promising but have yet to generate a complete response. Importantly, the precise immune profile that determines clinical outcome remains unresolved. Individual immune cell types are divided into various functional subsets whose effects on tumor development may differ depending on their particular phenotype and functional status, which is often shaped by the tumor microenvironment. Thus, this review aims to examine the current knowledge regarding the role of inflammation and specific immune cell types in mediating PCa progression to assist in directing and optimizing immunotherapy targets, regimens, and responses and to uncover areas in which further research is needed. Finally, a summary of ongoing immunotherapy clinical trials in PCa is provided.

MicroRNA Regulating Glutathione S-Transferase P1 in Prostate Cancer

Glutathione S-transferase P1 (GSTP1), an enzyme involved in detoxification process, is frequently inactivated in prostate cancer due to epigenetic modifications. Through in silico analysis we identified a subset of miRNAs that are putative targets in regulating GSTP1. miRNAs are small endogenous non-coding RNA that are critical regulators of various physiologic and pathologic processes and their level of expression may play a precise role in early diagnosis and prognosis of cancer. These small molecules have been detected in a wide variety of human biological specimens including blood, serum, urine, ejaculate and tissues, which could be utilized as clinically useful biomarker in early detection and prognosis of prostate cancer. The chapter summarizes the current knowledge about miRNA involved in GSTP1 regulation in prostate cancer and their potential as useful biomarkers of disease for early detection and prognosis, along with challenges and limitations in this development.

Impact of prostate inflammation on lesion development in the POET3(+)Pten(+/-) mouse model of prostate carcinogenesis

Evidence linking prostatitis and prostate cancer development is contradictory. To study this link, the POET3 mouse, an inducible model of prostatitis, was crossed with a Pten-loss model of prostate cancer (Pten(+/-)) containing the ROSA26 luciferase allele to monitor prostate size. Prostatitis was induced, and prostate bioluminescence was tracked over 12 months, with lesion development, inflammation, and cytokine expression analyzed at 4, 8, and 12 months and compared with mice without induction of prostatitis. Acute prostatitis led to more proliferative epithelium and enhanced bioluminescence. However, 4 months after initiation of prostatitis, mice with induced inflammation had lower grade pre-neoplastic lesions. A trend existed toward greater development of carcinoma 12 months after induction of inflammation, including one of two mice with carcinoma developing perineural invasion. Two of 18 mice at the later time points developed lesions with similarities to proliferative inflammatory atrophy, including one mouse with associated carcinoma. Pten(+/-) mice developed spontaneous inflammation, and prostatitis was similar among groups of mice at 8 and 12 months. Analyzed as one cohort, lesion number and grade were positively correlated with prostatitis. Specifically, amounts of CD11b(+)Gr1(+) cells were correlated with lesion development. These results support the hypothesis that myeloid-based inflammation is associated with lesion development in the murine prostate, and previous bouts of CD8-driven prostatitis may promote invasion in the Pten(+/-) model of cancer.

Genetic analysis of colon tumors induced by a dietary carcinogen PhIP in CYP1A humanized mice: Identification of mutation of β-catenin/Ctnnb1 as the driver gene for the carcinogenesis

Replacing mouse Cyp1a with human CYP1A enables the humanized CYP1A mice to mimic human metabolism of the dietary carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), by N(2) -hydroxylation to a proximate carcinogen. Our previous study demonstrated that PhIP, combined with the dextrin sulfate sodium (DSS)-induced colitis, induces colon carcinogenesis in hCYP1A mice. Here, we employed whole exome sequencing and found multiple gene mutations in PhIP/DSS-induced colon tumors. Mutations in the exon 3 of Ctnnb1/β-catenin, however, were the predominant events. We further sequenced the key fragments of Apc, Ctnnb1, and Kras, because mutations of these genes in the humans are commonly found as the drivers of colorectal cancer. Mutations on either codon 32 or 34 in the exon 3 of Ctnnb1 were found in 39 out of 42 tumors, but no mutation was found in either Apc or Kras. The sequence context of codons 32 and 34 suggests that PhIP targets +3G in a TGGA motif of Ctnnb1. Since mutations that activate Wnt signal is a major driving force for human colorectal cancers, we conclude that the mutated β-catenin is the driver in PhIP/DSS-induced colon carcinogenesis. This result suggests that the colon tumors in hCYP1A mice mimic human colorectal carcinogenesis not only in the dietary etiology involving PhIP, but also in the aberrant activation of the Wnt signaling pathway as the driving force.

Cancer-promoting and Inhibiting Effects of Dietary Compounds: Role of the Aryl Hydrocarbon Receptor (AhR)

Polyaromatic hydrocarbons, heterocyclic aromatic amines and dioxin-like compounds are environmental carcinogens shown to initiate cancer in a number of tissue types including prostate and breast. These environmental carcinogens elicit their effects through interacting with the aryl hydrocarbon receptor (AhR), a ligand activated transcription factor. Naturally occurring compounds found in fruits and vegetables shown to have anti-carcinogenic effects also interact with the AhR. This review explores dietary and environmental exposure to chemical carcinogens and beneficial natural compounds whose effects are elicited by the AhR.

Dietary chemoprevention of PhIP induced carcinogenesis in male Fischer 344 rats with tomato and broccoli

The heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-B]pyridine (PhIP), found in meats cooked at high temperatures, has been implicated in epidemiological and rodent studies for causing breast, prostate, and colorectal cancers. A previous animal study using a xenograft model has shown that whole tomato and broccoli, when eaten in combination, exhibit a marked effect on tumor reduction compared to when eaten alone. Our aim was to determine if PhIP-induced carcinogenesis can be prevented by dietary consumption of whole tomato + broccoli powders. Male Fischer 344 rats (n = 45) were randomized into the following treatment groups: control (AIN93G diet), PhIP (200 ppm in AIN93G diet for the first 20 weeks of the study), or tomato + broccoli + PhIP (mixed in AIN93G diet at 10% each and fed with PhIP for 20 weeks, and then without PhIP for 32 weeks). Study animals were monitored for 52 weeks and were euthanized as necessary based on a set of criteria for health status and tumor burden. Although there appeared to be some hepatic and intestinal toxicity due to the combination of PhIP and tomato + broccoli, these rodents had improved survival and reduced incidence and/or severity of PhIP-induced neoplastic lesions compared to the PhIP-alone treated group. Rats eating tomato + broccoli exhibited a marked decrease in the number and size of cribiform prostatic intraepitheilial neoplasia/carcinoma in situ (cribiform PIN/CIS) lesions and in the incidence of invasive intestinal adenocarcinomas and skin carcinomas. Although the apparent toxic effects of combined PhIP and tomato + broccoli need additional study, the results of this study support the hypothesis that a diet rich in tomato and broccoli can reduce or prevent dietary carcinogen-induced cancers.

DNA adduct formation of 2-amino-9H-pyrido[2,3-b]indole and 2-amino-3,4-dimethylimidazo[4,5-f]quinoline in mouse liver and extrahepatic tissues during a subchronic feeding study

Tobacco smoking is a risk factor for cancers of the liver and gastrointestinal (GI) tract, but the causal agents responsible for these cancers are uncertain. 2-Amino-9H-pyrido[2,3-b]indole (AαC) is an abundant heterocyclic aromatic amine present in tobacco smoke. AαC is a liver carcinogen and both a transgene mutagen and inducer of aberrant crypt foci in the colon of mice. We hypothesize that AαC may contribute to DNA damage and tumorigenesis in these organs of smokers. The potential of AαC to induce DNA adduct formation in liver, organs of the GI tract, lung, and urinary bladder, which are target organs of cancer in smokers, was examined using the C57BL/6 mouse as an animal model. AαC (400 or 800 ppm) and 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) (300 ppm), a liver and colon carcinogen in C57BL/6 mice, were given in the diet for up to 12 weeks. Liquid chromatography/mass spectrometry was employed to measure DNA adducts. The major DNA adducts of both carcinogens were identified as deoxyguanosine-C8 adducts. The levels of formation of AαC- and MeIQ-DNA adducts were similar in liver and extrahepatic tissues when adjusted for dose. The highest levels of adducts occurred in liver, followed by urinary bladder, and then in cecum and colon; lower DNA adduct levels were formed in the lung and pancreas following 12 weeks of feeding. The high levels of AαC adduct formed in liver, GI tract, and bladder of C57BL/6 mice reinforce the notion that AαC may contribute to DNA damage and cancer of these organs in smokers.

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-DNA adducts in benign prostate and subsequent risk for prostate cancer

Despite convincing evidence that 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)--a heterocyclic amine generated by cooking meats at high temperatures--is carcinogenic in animal models, it remains unclear whether PhIP exposure leads to increased cancer risk in humans. PhIP-DNA adduct levels were measured in specimens from 534 prostate cancer case-control pairs nested within a historical cohort of men with histopathologically benign prostate specimens. We estimated the overall and race-stratified risk of subsequent prostate cancer associated with higher adduct levels. PhIP-DNA adduct levels in benign prostate were significantly higher in Whites than African Americans (0.274 optical density units (OD) ±0.059 vs. 0.256 OD ±0.054; p<0.0001). Prostate cancer risk for men in the highest quartile of PhIP-DNA adduct levels was modestly increased [odds ratio (OR) = 1.25; 95% confidence interval (CI) = 0.76-2.07]. In subset analyses, the highest risk estimates were observed in White patients diagnosed more than 4 years after cohort entry (OR = 2.74; 95% CI = 1.01-7.42) or under age 65 (OR = 2.80; 95% CI = 0.87-8.97). In Whites, cancer risk associated with high-grade prostatic intraepithelial neoplasia combined with elevated PhIP-DNA adduct levels (OR = 3.89; 95% CI = 1.56-9.73) was greater than risk associated with either factor alone. Overall, elevated levels of PhIP-DNA adducts do not significantly increase prostate cancer risk. However, our data show that White men have higher PhIP-DNA adduct levels in benign prostate tissue than African American men, and suggest that in certain subgroups of White men high PhIP-DNA adduct levels may predispose to an increased risk for prostate cancer.