In vitro DNA adduct profiling to mechanistically link red meat consumption to colon cancer promotion

Riceberry Rice Bran Protein Hydrolyzed Fractions Induced Apoptosis, Senescence and G1/S Cell Cycle Arrest in Human Colon Cancer Cell Lines

Riceberry rice bran is the part of rice that has been scrubbed out during the coloring process. There are various health benefits to be gained from foods with a high protein content. This work aimed to study the effect of hydrolyzed riceberry rice bran extracts on colon cancer cell lines compared to normal cells. The MTT assay result showed that our extract has lower cytotoxicity effects on normal cells (PCS-201-010, IC50 = 6745 µg/mL) compared to colon cancer cell lines and has a greater effect on metastatic cancer cell lines (SW-620, IC50 = 5468 µg/mL) than non-metastatic cancer cell lines (HT-29, IC50 = 6054 µg/mL). The apoptotic inductive effects of HRBE on SW-620 were observed after 72 h at a maximum rate of 76% and at maximum concentration. According to the result of the cell senescence analysis after 24 h of HRBE treatment, the percentage of HT-29 (86%) expressing SA-β-gal was much higher than that of SW-620 (32%). Consequently, the decrease in the cell population in the S and M/G2 phases indicated cell cycle arrest in HT-29 cells after being treated with HRBE. Focusing on the peptide fraction size of HRBE, the largest (>50 kDa) fraction showed the highest anticancer activity compared to other fractions. In conclusion, the hydrolyzed riceberry rice bran extract induced the apoptosis process in the metastatic cancer cells and induced the senescence process in the non-metastatic cancer cells. This observed information will be useful and applicable for medical research and colon cancer treatment in the future.

A combination of direct reversion and nucleotide excision repair counters the mutagenic effects of DNA carboxymethylation

Distinct cellular DNA damage repair pathways maintain the structural integrity of DNA and protect it from the mutagenic effects of genotoxic exposures and processes. The occurrence of O6-carboxymethylguanine (O6-CMG) has been linked to meat consumption and hypothesized to contribute to the development of colorectal cancer. However, the cellular fate of O6-CMG is poorly characterized and there is contradictory data in the literature as to how repair pathways may protect cells from O6-CMG mutagenicity. To better address how cells detect and remove O6-CMG, we evaluated the role of two DNA repair pathways in counteracting the accumulation and toxic effects of O6-CMG. We found that cells deficient in either the direct repair protein O6-methylguanine-DNA methyltransferase (MGMT), or key components of the nucleotide excision repair (NER) pathway, accumulate higher levels O6-CMG DNA adducts than wild type cells. Furthermore, repair-deficient cells were more sensitive to carboxymethylating agents and displayed an increased mutation rate. These findings suggest that a combination of direct repair and NER circumvent the effects O6-CMG DNA damage.

Transcriptome Profiling and Metagenomic Analysis Help to Elucidate Interactions in an Inflammation-Associated Cancer Mouse Model

Simple Summary

Colitis-associated colorectal cancer is the third most significant condition that increases the overall risk of developing colorectal cancer. In this study, we examined normal colonic mucosa of tumor-bearing mice in the DSS/AOM mouse model by gene expression profiling and fecal samples by 16s rDNA amplicon sequencing. Gene set enrichment analysis revealed that genes associated with fatty acid metabolism, oxidative phosphorylation, and the PI3K-Akt-mTOR pathways were enriched colonic mucosa of DSS/AOM mice. Additionally, enrichment of the sphingolipid signal and lipoarabinomannan biosynthetic pathways were inferred from fecal microbial composition. Our findings provide insights into altered transcriptome and microbiome in a mouse model of colitis-induced carcinogenesis.

Abstract

Colitis is a risk factor for colorectal cancer (CRC) and can change the dynamics of gut microbiota, leading to dysbiosis and contributing to carcinogenesis. The functional interactions between colitis-associated CRC and microbiota remain unknown. In this study, colitis and CRC were induced in BALB/c mice by the administration of dextran sodium sulfate (DSS) and/or azoxymethane (AOM). Whole transcriptome profiling of normal colon was then performed, and gene set enrichment analysis (GSEA) revealed enriched fatty acid metabolism, oxidative phosphorylation, and PI3K-Akt-mTOR signaling in the tissues from DSS/AOM mice. Additionally, immunohistochemical staining showed increased expression levels of phosphorylated S6 ribosomal protein, a downstream target of the PI3K-Akt-mTOR pathway in the inflamed mucosa of DSS/AOM mice. Fecal microbes were characterized using 16S rDNA gene sequencing. Redundancy analysis demonstrated a significant dissimilarity between the DSS/AOM group and the others. Functional analysis inferred from microbial composition showed enrichments of the sphingolipid signal and lipoarabinomannan biosynthetic pathways. This study provides additional insights into alterations associated with DSS/AOM-induced colitis and associates PI3K-Akt-mTOR, sphingolipid-signaling and lipoarabinomannan biosynthetic pathways in mouse DSS/AOM-induced colitis.

Soybean-derived gma-miR159a alleviates colon tumorigenesis by suppressing TCF7/MYC in mice

Previous reports have shown that plant-derived microRNAs (miRNAs) regulate mammalian gene expression through dietary intake. Our prior study found that gma-miR159a, which is abundant in soybean, significantly inhibited the proliferation of colon cancer cells. In the current study, dietary gma-miR159a was utilized to study its anti-colon cancer function in azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced colon cancer mice. Under processing conditions, gma-miR159a exhibited excellent stability in cooked soybean. In vitro, gma-miR159a suppressed the expression of the oncogene MYC downstream of the Wnt signaling pathway by targeting the TCF7 gene, significantly inhibiting the growth of colon cancer cells. The in vivo experiments showed that gma-miR159a and soybean RNA (total RNA extracted from soybean) significantly reduced tumor growth in AOM/DSS-induced colon cancer mice by gavage. This effect disappeared when anti-miR159a was present. In addition, gma-miR159a and soybean RNA significantly attenuated inflammation in colon cancer mice. These results showed that long-term dietary intake of soybean-derived gma-miR159a effectively prevented the occurrence of colon cancer and colitis, which provides novel evidence for the prevention function of soybean.

Direct Alkylation of Deoxyguanosine by Azaserine Leads to O6-Carboxymethyldeoxyguanosine

The O⁶-alkylguanosine adduct O⁶-carboxymethyldeoxyguanosine (O⁶-CMdG) has been detected at elevated levels in blood and tissue samples from colorectal cancer patients and from healthy volunteers after consuming red meat. The diazo compound l-azaserine leads to the formation of O⁶-CMdG as well as the corresponding methyl adduct O⁶-methyldeoxyguanosine (O⁶-MedG) in cells and is therefore in wide use as a chemical probe in cellular studies concerning DNA damage and mutation. However, there remain knowledge gaps concerning the chemical basis of DNA adduct formation by l-azaserine. To characterize O⁶-CMdG formation by l-azaserine, we carried out a combination of chemical and enzymatic stability and reactivity studies supported by liquid chromatography tandem mass spectrometry for the simultaneous quantification of O⁶-CMdG and O⁶-MedG. We found that l-azaserine is stable under physiological and alkaline conditions as well as in active biological matrices but undergoes acid-catalyzed hydrolysis. We show, for the first time, that l-azaserine reacts directly with guanosine (dG) and oligonucleotides to form an O⁶-serine-CMdG (O⁶-Ser-CMdG) adduct. Moreover, by characterizing the reaction of dG with l-azaserine, we demonstrate that O⁶-Ser-CMdG forms as an intermediate that spontaneously decomposes to form O⁶-CMdG. Finally, we quantified levels of O⁶-CMdG and O⁶-MedG in a human cell line exposed to l-azaserine and found maximal adduct levels after 48 h. The findings of this work elucidate the chemical basis of how l-azaserine reacts with deoxyguanosine and support its use as a chemical probe for N-nitroso compound exposure in carcinogenesis research, particularly concerning the identification of pathways and factors that promote adduct formation.

A Chemical Link between Meat Consumption and Colorectal Cancer Development?

O⁶-carboxymethylguanine (O⁶-CMG) is a mutagenic DNA adduct that forms at increased levels when people eat meat. It has been studied as a potential initiating event in colorectal carcinogenesis. It can arise from alkylation of guanine in DNA by electrophilic degradation products of N-nitroso compounds. There is significant data regarding biochemical and cellular process, including DNA repair and translesion DNA synthesis that control O⁶-CMG accumulation, persistence, and mutagenicity. Mutation spectra arising from the adduct closely resemble common mutations in colorectal cancer; however, gaps remain in understanding the biochemical processes that regulate how and where the damage persists in the genome. Addressing such questions relies on advances in chemistry such as synthesis approaches and bioanalytical methods. Results of research in this area help advance our understanding of the toxicological relevance of O⁶-CMG-modified DNA. Further attention should focus on understanding how a combination of genetic and environmental factors control its biological persistence and how this information can be used as a basis of biomoniotoring and prevention efforts to help mitigate colon cancer risk.

Colon Carcinogenesis: The Interplay Between Diet and Gut Microbiota

Colorectal cancer (CRC) incidence increases yearly, and is three to four times higher in developed countries compared to developing countries. The well-known risk factors have been attributed to low physical activity, overweight, obesity, dietary consumption including excessive consumption of red processed meats, alcohol, and low dietary fiber content. There is growing evidence of the interplay between diet and gut microbiota in CRC carcinogenesis. Although there appears to be a direct causal role for gut microbes in the development of CRC in some animal models, the link between diet, gut microbes, and colonic carcinogenesis has been established largely as an association rather than as a cause-and-effect relationship. This is especially true for human studies. As essential dietary factors influence CRC risk, the role of proteins, carbohydrates, fat, and their end products are considered as part of the interplay between diet and gut microbiota. The underlying molecular mechanisms of colon carcinogenesis mediated by gut microbiota are also discussed. Human biological responses such as inflammation, oxidative stress, deoxyribonucleic acid (DNA) damage can all influence dysbiosis and consequently CRC carcinogenesis. Dysbiosis could add to CRC risk by shifting the effect of dietary components toward promoting a colonic neoplasm together with interacting with gut microbiota. It follows that dietary intervention and gut microbiota modulation may play a vital role in reducing CRC risk.

EP4 receptor as a novel promising therapeutic target in colon cancer

The most prevalent malignancy that can occur in the gastrointestinal tract is colon cancer. The current treatment options for colon cancer patients include chemotherapy, surgery, radiotherapy, immunotherapy, and targeted therapy. Although the chance of curing the disease in the early stages is high, there is no cure for almost all patients with advanced and metastatic disease. It has been found that over-activation of cyclooxygenase 2 (COX-2), followed by the production of prostaglandin E2 (PGE2) in patients with colon cancer are significantly increased. The tumorigenic function of COX-2 is mainly due to its role in the production of PGE2. PGE2, as a main generated prostanoid, has an essential role in growth and survival of colon cancer cell's. PGE2 exerts various effects in colon cancer cells including enhanced expansion, angiogenesis, survival, invasion, and migration. The signaling of PGE2 via the EP4 receptor has been shown to induce colon tumorigenesis. Moreover, the expression levels of the EP4 receptor significantly affect tumor growth and development. Overexpression of EP4 by various mechanisms increases survival and tumor vasculature in colon cancer cells. It seems that the pathway starting with COX2, continuing with PGE2, and ending with EP4 can promote the spread and growth of colon cancer. Therefore, targeting the COX-2/PGE2/EP4 axis can be considered as a worthy therapeutic approach to treat colon cancer. In this review, we have examined the role and different mechanisms that the EP4 receptor is involved in the development of colon cancer.

Sequence-Specific Quantitation of Mutagenic DNA Damage via Polymerase Amplification with an Artificial Nucleotide

DNA mutations can result from replication errors due to different forms of DNA damage, including low-abundance DNA adducts induced by reactions with electrophiles. The lack of strategies to measure DNA adducts within genomic loci, however, limits our understanding of chemical mutagenesis. The use of artificial nucleotides incorporated opposite DNA adducts by engineered DNA polymerases offers a potential basis for site-specific detection of DNA adducts, but the availability of effective artificial nucleotides that insert opposite DNA adducts is extremely limited, and furthermore, there has been no report of a quantitative strategy for determining how much DNA alkylation occurs in a sequence of interest. In this work, we synthesized an artificial nucleotide triphosphate that is selectively inserted opposite O⁶-carboxymethyl-guanine DNA by an engineered polymerase and is required for DNA synthesis past the adduct. We characterized the mechanism of this enzymatic process and demonstrated that the artificial nucleotide is a marker for the presence and location in the genome of O⁶-carboxymethyl-guanine. Finally, we established a mass spectrometric method for quantifying the incorporated artificial nucleotide and obtained a linear relationship with the amount of O⁶-carboxymethyl-guanine in the target sequence. In this work, we present a strategy to identify, locate, and quantify a mutagenic DNA adduct, advancing tools for linking DNA alkylation to mutagenesis and for detecting DNA adducts in genes as potential diagnostic biomarkers for cancer prevention.

Recent developments in DNA adduct analysis using liquid chromatography coupled with mass spectrometry

The formation of DNA adducts by genotoxic agents is an early event in cancer development, and it may lead to gene mutations, thereby initiating tumor development. The measurement of DNA adducts can provide critical information about the genotoxic potential of a chemical and its mechanism of carcinogenesis. In recent decades, liquid chromatography coupled with mass spectrometry has become the most important technique for analyzing DNA adducts. The improvements in resolution achievable with new chromatographic separation techniques coupled with the high specificity and sensitivity and wide dynamic range of new mass spectrometry systems have been used for both qualitative and quantitative analyses of DNA adducts. This review discusses the challenges in qualitative and quantitative analyses of DNA adducts by liquid chromatography coupled with mass spectrometry and highlights recent developments towards overcoming the limitations of liquid chromatography coupled with mass spectrometry methods. The key steps and new solutions, such as sample preparation, mass spectrometry fragmentation, and method validation, are summarized. In addition, the fundamental principles and latest advances in DNA adductomic approaches are reviewed.

Emerging Technologies in Mass Spectrometry-Based DNA Adductomics

The measurement of DNA adducts, the covalent modifications of DNA upon the exposure to the environmental and dietary genotoxicants and endogenously produced electrophiles, provides molecular evidence for DNA damage. With the recent improvements in the sensitivity and scanning speed of mass spectrometry (MS) instrumentation, particularly high-resolution MS, it is now feasible to screen for the totality of DNA damage in the human genome through DNA adductomics approaches. Several MS platforms have been used in DNA adductomic analysis, each of which has its strengths and limitations. The loss of 2′-deoxyribose from the modified nucleoside upon collision-induced dissociation is the main transition feature utilized in the screening of DNA adducts. Several advanced data-dependent and data-independent scanning techniques originated from proteomics and metabolomics have been tailored for DNA adductomics. The field of DNA adductomics is an emerging technology in human exposure assessment. As the analytical technology matures and bioinformatics tools become available for analysis of the MS data, DNA adductomics can advance our understanding about the role of chemical exposures in DNA damage and disease risk.

The Future of DNA Adductomic Analysis

Covalent modification of DNA, resulting in the formation of DNA adducts, plays a central role in chemical carcinogenesis. Investigating these modifications is of fundamental importance in assessing the mutagenicity potential of specific exposures and understanding their mechanisms of action. Methods for assessing the covalent modification of DNA, which is one of the initiating steps for mutagenesis, include immunohistochemistry, ³²P-postlabeling, and mass spectrometry-based techniques. However, a tool to comprehensively characterize the covalent modification of DNA, screening for all DNA adducts and gaining information on their chemical structures, was lacking until the recent development of "DNA adductomics". Advances in the field of mass spectrometry have allowed for the development of this methodology. In this perspective, we discuss the current state of the field, highlight the latest developments, and consider the path forward for DNA adductomics to become a standard method to investigate covalent modification of DNA. We specifically advocate for the need to take full advantage of this new era of mass spectrometry to acquire the highest quality and most reliable data possible, as we believe this is the only way for DNA adductomics to gain its place next to the other "-omics" methodologies as a powerful bioanalytical tool.

Carcinogenicity of consumption of red meat and processed meat: A review of scientific news since the IARC decision

In October 2015, the International Agency for Research on Cancer (IARC) issued a press release on the results of the evaluation of the carcinogenicity of red and processed meat. Based on the accumulated scientific literature, the consumption of red meat was classified as "probably carcinogenic to humans" and processed meat as "carcinogenic to humans". Given the importance of this topic, this review was aimed at revising the current state-of-the-art on the carcinogenicity of red and processed meat, some time after the IARC decision. Some new epidemiological studies and new reviews clearly supporting the IARC decision have been published during these months. However, a number of gaps still exist. It is basic to establish the mechanisms leading to the increased risk of colorectal cancer (CRC) and other cancers arising from red and processed meat consumption. Another important pending issue is to establish the role of known/suspected carcinogens contained in uncooked or unprocessed meats, as well as the influence of cooking. Finally, it would be highly recommended to conduct new epidemiological studies to elucidate whether the consumption of white meat, such as pork and/or poultry, are -positively or inversely-associated with an increased risk of CRC and other types of cancer.

DNA adductomics to study the genotoxic effects of red meat consumption with and without added animal fat in rats

Digestion of red and processed meat has been linked to the formation of genotoxic N-nitroso compounds (NOCs) and lipid peroxidation products (LPOs) in the gut. In this study, rats were fed a meat based diet to compare the possible genotoxic effects of red vs. white meat, and the interfering role of dietary fat. To this purpose, liver, duodenum and colon DNA adductomes were analyzed with UHPLC-HRMS. The results demonstrate that the consumed meat type alters the DNA adductome; the levels of 22 different DNA adduct types significantly increased upon the consumption of beef (compared to chicken) and/or lard supplemented beef or chicken. Furthermore, the chemical constitution of the retrieved DNA adducts hint at a direct link with an increase in NOCs and LPOs upon red (and processed) meat digestion, supporting the current hypotheses on the causal link between red and processed meat consumption and the development of colorectal cancer.