Aflatoxin B1 (AFB1) is the most common carcinogen of aflatoxin, which contaminates many agricultural products in the daily diet of humans. More than 50% of patients with developing hepatocellular carcinoma (HCC) feature AFB1 exposure due to their shared consumption of contaminated food. One of the main mechanisms of AFB1-induced liver carcinogenesis is its biological activation and its interaction with DNA to produce AFB1-E-N7-dG adduct. This product may result in the formation of DNA damage and the mutations of tumor-associated genes such as TP53 and ras. In human, several pathways involving in AFB1 detoxification, including I- and II-type detoxification, DNA repair, have been reported. This study reviewed the detoxification mechanisms of AFB1 in human as well as AFB1 occurrence and toxification. Additionally, we also discussed prevention methods for AFB1 exposure.
- aflatoxin B1
Aflatoxin B1 (AFB1), an important mycotoxin, is first identified in animal feed in 1961 due to the death of 100,000 turkeys in the UK where these turkeys were feed using the peanut powder with the high concentration of AFB1. Until now, this mycotoxin has proved to come from the secondary metabolites of
2. AFB1 occurrence
2.1 Toxic fungi and their classifications
Toxic fungi often live in the human crops and produce mycotoxins such as aflatoxins. Toxic fungi in crops can be divided into two categories according to whether their mycotoxins are produced before or after crop harvest. The first category is termed as field fungi, which often invade crops and produce mycotoxins before harvest. The another, also called storage fungi, mostly occurs in the storage of crops after harvest. The sources of both types of toxigenic fungi are affected by environmental factors. Crops before harvest, fungi can invade crops to produce toxins by interacting with other organisms, such as insects. The harvested crops are regulated by factors such as nutrients, temperature and humidity in the air, and biological agents (insects, competitive interference). Furthermore, toxigenic fungi can be divided into four types according to their effects on crops:
The crop fungi inoculate the growing crop kernels in the field and proliferate in storage under suitable conditions. Among known crop fungi,
2.2 AFB1 occurrence
Several previous reviews have fully summarized the occurrence and biosynthesis of AFB1. Briefly, AFB1 are an important class of mycotoxins mainly produced by
3. AFB1 toxification and toxic mechanisms
3.1 The effects of AFB1 on the food chain
3.2 The toxic effects of AFB1 on human and animals
In 1963, Asao et al. completed the structural clarification of AFB1, a member of the aflatoxin family containing a fused difuranyl group . AFB1 is highly toxic to humans and several animals, and has three major characteristics: organophilic, genotoxic, and carcinogenic. Its pro-organism is mainly caused by damage to the liver, which can lead to hepatic hemorrhage and hepatocyte necrosis. The genotoxicity is mainly to induce the formation of AFB1-DNA adduct and the hot spot mutation of P53 gene. The carcinogenicity is mainly caused by hepatocellular carcinoma. The main toxicological effect of AFB1 is to induce DNA damage. AFB1 has been proven to be the main cause of liver cancer in patients with hepatitis B virus infection. It is a genotoxic liver cancer, which may cause cancer by inducing DNA adducts, leading to genetic changes in target cells, leading to DNA strand breaks and DNA base damage. And oxidative damage can eventually lead to cancer. AFB1 is mainly metabolized by the liver, and AFB1 taken from food is mainly metabolized by the cytochrome P450 enzyme to the final carcinogen AFB1-8-9-epoxide (AFBO). When AFBO reacts with DNA, it inhibits gene mutation in P53, a hotspot coding region of exon 249, by interacting with guanine bases, which may lead to HCC. AFB1 is metabolized by the P450 system into a number of hydroxylated products, including AFM1, AFQ1, AFP1, AFB2a [11, 20, 21, 22, 23, 24, 25]. After aflatoxin is ingested into the human body, it mainly manifests as an acute or chronic disease. Acute attacks usually involve high concentrations of aflatoxins. For example, 317 cases of acute liver failure occurred in Kenya in 2004. The main reason is the consumption of aflatoxin-contaminated corn, and the case of patients with AFB1 lysine in serum. The adduct concentration was the highest in history. Growth retardation, immunosuppression, and carcinogenicity are chronic effects, and the incidence of chronic attacks in developing countries is higher because of exposure to low levels of aflatoxin intake .
3.2.1 Effect of aflatoxin on growth and development
An epidemiological survey was conducted in West Africa to measure exposure to aflatoxins in children between 9 months and 5 years of age, and their growth, development and height were examined against the reference population of World Health Organization (WHO) . Studies have shown a strong association between exposure to aflatoxin in children and dysplasia and underweight. In a field outbreak of aflatoxin, egg production fell by 5% . The study data showed that for every 1 mg/kg of aflatoxin AFB1 in the feed, the growth rate of pigs would be reduced by 16% and broilers by 5% .
In these animal studies, AFB1 has been shown to induce immunosuppression. For example, in studies of AFB1 exposed animals, it was found that the activity of B cells and T cells decreased, because T cells were more sensitive to AFB1 toxicity . Research data from GY et al. showed that chicken phagocytic cells were severely damaged during aflatoxosis, and the ability to remove foreign substances from the circulation decreased, which may reduce the ability to process antigenic components. Aflatoxell chickens are more susceptible to infection . In pigs, AFB1 exposure reduces lymphocyte response to mitogens, inhibits large phage migration and delayed skin allergic reactions . Although many data on AFB1 immune effects have been obtained from animal studies, there is little data on the effects of long-term consumption of food contaminated with AFB1 on the human immune system. The effect on the immune system by aflatoxins in the diet of Gambian children found a decrease in sIgA levels in saliva, probably due to the high level of exposure to aflatoxins in the diet . In a study of aflatoxin AFB1 exposure and cellular immune status in 64 Ghanaians, it was found that AFB1 exposure may result in a decrease in the major constituent cell T cells and B cells that cause lymphocyte subpopulations. High levels of AFB1 albumin adducts significantly reduced perforin- and granzyme a levels in CD8+ cytotoxic T cells compared to low levels of AFB1 albumin adduct. In participants with high levels of AFB1, changes in these immune parameters may result in impaired cellular immune function, thereby reducing host resistance to infection .
4. Detoxification of AFB1
Since the contamination of aflatoxins in food poses a risk to human health and leads to serious economic losses in crops, we have every reason to implement new methods to ensure the safety of food production. There are two main methods of implementation: (a) prevention of mold contamination and growth; (b) detoxification of contaminated products by opponents. Prevention of mycotoxin contamination can be achieved by storage before or after harvesting of the crop. However, the pollution of toxins is inevitable, and the detoxification pathway for contaminated food after harvest has been the subject of our in-depth research. Detoxification methods commonly used are physical methods and chemical methods. This article will focus on new research on detoxification of harvested contaminated crops.
4.1 Physical method
The most common way to remove AFB1 using physical methods is to heat and use gamma rays. Aflatoxins are highly thermostable. Studies have shown that AFB1 levels are significantly reduced by heating at 100 and 150°C for 90 minutes, respectively, at 41.9 and 81.2%. The AFB reduction rate of the soy milk after cooking was 97.9%, and the AFB1 reduction rate of the steamed soybeans after cooking was 33.6%. And studies have shown that high pressure cooking is better than ordinary cooking to remove AFB1. When the soybean is steamed or steamed in a pressure cooker, the reduction rate of the pressure cooker is about 10% higher than that of the steam. Using autoclave cooking in rice can reduce AFB1 levels by 72–83%. The high-pressure cooking method is low in cost and easy to handle, and one of the challenges it faces is how to ensure the integrity of the food after heating. To ensure the integrity of the food, the use of maximum temperatures is often limited [35, 36]. The gamma ray has a strong penetrating electromagnetic wave that can penetrate the material without leaving any residue, which is its advantage. There have been many reports of the increase, decrease, or even unaffected mycotoxin produced by fungi under different conditions. Studies have shown that the fungal structure on paper with a minimum radiation dose of 16 kGy has been altered to avoid fungal growth. Library and file management staff use gamma radiation protection technology to provide a powerful means for the preservation of ancient books, archives and other paper materials . A dose of gamma radiation exceeding 10 kGy can inhibit the germination of peanut seeds. Therefore, proper drying, packaging and environmental control measures with low relative humidity can reduce the growth of fungi and ensure safe, high quality peanuts . The DI Stefano study showed that a radiation dose of 0.5–15 KGy resulted in a decrease in aflatoxin levels in the feed, while a 15 kGy gamma ray did not completely destroy ochratoxin A and aflatoxin in the test feed, FAO/International The IAEA/WHO Expert Committee on Food Irradiation has concluded in its report that foods with an average radiation dose of 10 kGy will not cause toxicological hazards and that toxicologically tested foods do not require retreatment. It is necessary to irradiate the food with radiation before the mold produces toxins .
4.2 Biological treatment
Studies using biotechnology to reduce AFB1 levels in contaminated foods fall into two main categories: one that uses plant extracts to degrade AFB1 and the other that inoculates bacterial strains in food substrates. In recent years, natural plant products have attracted much attention as synthetic antibacterial agents because of their biodegradability, biosafety, effectiveness, and regenerability. At the same time, they are conveniently used as an eco-friendly technology for detoxification. Mycotoxins. Many studies have shown that plant essential oils can inhibit the growth of microorganisms and reduce the production of toxins. Bluma et al. showed that the addition of essential oils in corn kernels has a significant effect on the growth rate, hysteresis and accumulation of AFB1 of aflatoxin molds. Depends on water activity, AFB1 concentration and incubation time . In addition to plant essential oils, water extracts of plants can also be used to dissolve AFB1. Another study by Vijayanandraj et al. also demonstrated the effect of different parameters on the detoxification of AFB1 aqueous extracts from different medicinal plants. They concluded that the leaf extract of Vasaka (
In another method, inoculation of the bacterial strain is to reduce AFB1 by physical binding or metabolism of the bacterial strain directly to AFB1. The biodegradation of aflatoxins has yielded some successful attempts, although most are carried out in sterile culture. The microbial degradation of aflatoxin is achieved by the activity of the enzyme, which is capable of decomposing the refractory polyheterocyclic molecules of aflatoxin. Brana et al. showed that
4.3 Chemical treatment
Mycotoxins can be removed or reduced chemically, and acids, bases, oxidizing agents, and reducing agents have been shown to destroy or extinguish mycotoxins. Acids are a natural part of foods that are added to the industry to add flavor to the food, and even some acids are used as preservatives or antioxidants. Organic acids in foods can degrade AFB1. AIKO et al. tested the degradation of AFB1 by various organic acids and considered that the effect of lactic acid was most effective in the organic acids tested. Since lactic acid is endogenous in the human body and is present in many foods, lactic acid is considered to be safe. Therefore, lactic acid can be recommended for food processing and as a preservative in fermented foods . Rushing and other studies have shown that under acidic conditions, organic acids and arginine can be mixed to treat contaminated foods, and AFB1 can be rapidly converted to AFB2a-Arg within 20 minutes, reducing toxicity [50, 51, 52, 53]. Aly et al. showed that HCL can effectively degrade AFB1 during acid hydrolysis . Alkaline cooking is also used in the commercial to remove AFB1 from corn. Amination under high temperature and pressure conditions can also reduce AFB1 in corn. There are also many studies on the degradation of AFB1 in foods using ozone. Ozone has been reported as an antibacterial agent because it has antibacterial effects against spores and bacteria of fungi, bacteria, viruses, protozoa and fungi, and has a wide range of antibacterial agents. Ozone inhibits or microbial growth by oxidizing cell membranes and cell wall complex processes . Diao et al. showed a significant decrease in AFB1 levels in peanut seeds at 13 and 21 mg/l ozone concentrations . Proctor et al. showed that the use of ozone oxidation can degrade AFB1 in peanuts, and at an increased temperature of 75°C, AFB1 degradation rate reached 77% in just 10 minutes .
4.4 Sorbent additives
The above method of degrading AFB1 is to destroy or reduce the content of AFB1 in food, and the adsorbent is opposite thereto, which prevents AFB1 from entering the intestinal tract after ingestion by binding to AFB1, so as to prevent hepatotoxicity of AFB1. Novasil clay minerals and aflatoxins are highly affinitive and high-capacity combinations in the gastrointestinal tract. The study of NS has been shown to absorb AFB1 in vitro in both animal models and human studies, reducing the bioavailability of blood toxins, and its use in humans has not affected the utilization of vitamins and trace elements in the body, and has been determined through clinical trials. A safe dose of NS, a NS content of up to 2.0% (w/w) in the diet does not cause significant toxicity [58, 59, 60, 61, 62]. Xue and other studies have shown that the enteral nutrient NovaSil can effectively regulate the toxicity and carcinogenicity of co-exposure to AFB1 and fumonisin B1. When the concentration in the diet is as high as 0.5%, liver changes, liver glutathione S The number and size of -transferase (GST-P+) foci were significantly reduced . Another commonly used binder is chlorophyll. Studies in mammals and fish have shown that chlorophyll can inhibit the formation of carcinogens through the combination of AFB1, reduce the bioavailability of tissues, reduce DNA adduction, and reduce the incidence of tumors . Smimonich observed in the study that after adding chlorophyll to the contaminated AFB1 feed, the AFB1-DNA adduct was reduced by 42%, and the AFB1 albumin adduct was reduced by 65%. AFB n7 - guanine Urinary adducts are reduced by 90%. In the same study, it was also shown that chlorophyll reduced the volume of GSTP lesions in the liver by 74% and the mean number of abnormal crypt lesions in the colon by 63%. Studies have shown that chlorophyll can be used as an early biochemical and advanced pathophysiological marker for AFB1 carcinogenesis in the liver and colon .
4.5 DNA repair
In China, HCC is a common malignant tumor with a very poor prognosis, accounting for 55% of the world’s HCC cases and more than 340,000 cases per year. This area of tumor-prone is mainly concentrated in eastern and southeastern China. Clinical epidemiological studies have shown that exposure to AFB1 and/or chronic infection with HBV and HCV is a major risk factor for liver cancer. Studies on the toxicity of AFB1 indicate that AFB1 damage to DNA plays a central role in the carcinogenic process of HCC associated with this toxin [48, 66]. AFB1 is metabolized by the cytochrome P450 enzyme into a reactive AFB1-8,9-epoxide (AFB1-epoxide), which is covalently bound to DNA to induce DNA damage. AFB1-induced DNA damage includes AFB1-DNA adducts, oxidative DNA damage, and gene mutations. The AFB1-DNA adduct in AFB1-induced DNA damage is 9-hydroxy afb1 (AFB1-N7-Gua), which is the most common type. The formation of the AFB1-N7-Gua adduct is first performed by pre-covalent insertion of the complex electrophilic between the double-stranded DNA and the high-stranded DNA, and then on the imidazole moiety of the formed AFB1-N7-Gua adduct. The charge generates another desired DNA adduct, a ring-opened carboxamide pyridine AFB1 (AFB1-FAPy) adduct. These adducts are capable of forming subsequent anti-repair adducts, dislocations, or lead to error-prone DNA repair, resulting in single-strand breaks (SSBs), double-strand breaks (DSBs), and base pair substitutions. The mutations caused by AFB1 exposure, the current major research and experiments believe that the P53 gene is closely related, there is a common mutation hotspot at 249 of TP53 (AGG to AGT) codon [11, 23, 25, 48]. However, epidemiological evidence suggests that although many people are exposed to the same level of AFB1, only a small percentage of the exposed persons have toxicological effects of AFB1, such as genetic mutations and HCC. The Nucleic Acid Excision Repair Pathway (NER), which has been shown to repair aflatoxin-induced DNA adducts, is the major DNA repair pathway. The repair steps of NER are mainly divided into: damage perception, opening denatured bubbles, cutting damaged chains, transferring damaged oligonucleotides, filling gaps and ligation. There is increasing evidence that genetic polymorphisms in the NER gene are associated with DNA repair capacity and regulate the risk of cancer . In China, molecular epidemiological studies of afb1-related HCC have investigated the association of several genes associated with the NER pathway, such as xeroderma pigmentosum C (XPC) and xeroderma pigmentosum D (XPD). In the oxidative damage of DNA caused by AFB1 exposure, the formation of 8-oxodG is important because it is abundant, highly mutagenic and hepatocarcinogenesis occurs. 8-oxodG lesions are mainly repaired by the BER pathway. The BER pathway promotes DNA repair through two common pathways: a. short patch BER pathway leading to a single nucleotide repair pathway; b. long patch BER pathway, resulting in at least two nucleotide repair pathways. Long et al. first reported DNA repair genes XRCC1, XRCC3, XRCC4, XRCC7, XPD, XPC (including rs25487, rs861539, rs7003908, rs28383151, rs3734091) by analyzing the AFB1-DNA adduct amount, TP53 gene mutation frequency and HCC risk. Genetic polymorphisms of (rs13181, rs2228001) and toxicological effects of AFB1 exposure. Studies have shown that the DNA repair gene XRCC1 gene mutation, XRCC3, XRCC4, XRCC7, XPC, and XPD may increase the AFB1-DNA adduct, the frequency of TP53M, and the risk of hepatocellular carcinoma, genetic mutations with lower DNA repair ability of these genes It should contribute to the toxicological effects of AFB1 and be of a preventive significance by identifying people with low DNA repair capacity [23, 67, 68].
5. AFB1-related legislation
In developing countries, AFB1 contamination of food is inevitable due to poor environmental and technical conditions, and it is not easy to be treated by high temperature, chemical, physical, etc. Humans can directly use contaminated foods (corn, peanuts, sorghum, rice, cashews, walnuts, pistachios, almonds) or animal products such as milk, eggs, etc. produced by using contaminated animals. The primary hazard of mycotoxin contamination in the food supply chain is human health, followed by animal health and productivity [69, 70]. Every country has strict controls on the mycotoxin contamination of food and feed to reduce human and animal exposure. Currently, Developed countries have access to federal regulatory bodies which set food safety standards and inspect domestic as well as imported/exported food products. Additionally, these countries have access to controlled storage conditions, which greatly reduces contamination post-harvest. These factors lead to lower overall contamination rates in developed countries. For example, the United States has reported acceptable AFB1 levels in corn (0–80 μg/kg during 1979–1983) and low daily intake of its citizens (0.34–197 ng/kg depending on the year and region of the country), which is much less than other undeveloped countries . The European Union (EU) has some of the world’s most stringent standards for mycotoxins in food and feed. Compared with the rest of the world, the European Union (EU) has the most extensive and detailed AFB1 presence in various foods and feeds provisions. It has been indicated that in many European countries the presence of AFM1 in milk and milk products was in lower range than the Asian and African countries [72, 73].
6. Summary and future direction
AFB1 is a kind of I-type chemical carcinogenic mycotoxin mainly produced by both
Conflicts of interest and source of funding
The authors declare no competing financial interests. This study was supported in part by the National Natural Science Foundation of China (Nos. 81,860,489, 81,760,502, 81,572,353, and 81,660,495), the Natural Science Foundation of Guangxi (Nos. 2018GXNSFAA281043, 2017GXNSFAA198002 and 2017GXNSFGA198002), Research Program of Guangxi “Zhouyue Scholar” (No. 2017-2038), Research Program of Guangxi Specially-invited Expert (No. 2017-6th), the ‘12th Five’ Planning Program of Guangxi Education Science (No. 2015C397), the Innovative Program of Guangxi Graduate Education (No. JGY2015139), Research Program of Guangxi Clinic Research Center of Hepatobiliary Diseases (No.AD17129025), and Open Research Program from Molecular Immunity Study Room Involving in Acute & Severe Diseases in Guangxi Colleges and Universities (Nos. kfkt20160062 and kfkt20160063).
|A. flavus||Aspergillus flavus|
|A. parasiticus||Aspergillus parasiticus|
|FAO||The Food and Agriculture Organization of the United Nations|
|WHO||World Health Organization|
|NER||Nucleic Acid Excision Repair Pathway|
|XPC||xeroderma pigmentosum C|
|XPD||xeroderma pigmentosum D|