Bile Acids: Natural Defenders Against Aflatoxin B1

Aflatoxins (AFs) are highly toxic mycotoxins produced as secondary metabolites by Aspergillus flavus and Aspergillus parasiticus. AFS are widely distributed in various cereal crops and can cause contamination during crop maturation, harvest and storage. Feed crops including corn, cottonseed and peanuts are particularly vulnerable to aflatoxin contamination, especially under high-temperature and high-humidity conditions, which are conducive to the growth of Aspergillus flavus and the production of toxins. To date, more than 20 structurally distinct AFS have been identified, among which aflatoxin B1 (AFB1) possesses the strongest toxicity and represents a severe threat to both human and animal health. It has been classified as a Group 1 carcinogen by the World Health Organization. AFB1 intoxication can severely compromise growth performance and reproductive capacity in animals. The liver is the primary target organ of AFB1 in animals, and its marked hepatotropism results in prominent hepatic damage. In China, explicit regulatory limits for AFB1 are clearly specified in the national standard GB 13078-2017 Hygienical Standard for Feed.

Figure 1. Chemical structure of aflatoxin B1

The Lethal Impact of Aflatoxin B1 on the Liver

Due to its stable physicochemical properties, AFB1 is difficult to be removed from feed and mainly enters animals through contaminated feed. AFB1 is mainly absorbed in the intestinal tract, then enters the bloodstream and is transported to the liver. The liver is the most important organ for substance metabolism and detoxification in the body. When animals ingest feed containing AFB1, acute or chronic diseases will be induced, among which liver lesions are the most prominent.

In the liver, AFB1 undergoes a variety of metabolic reactions including reduction, oxidation, hydroxylation and demethylation. Part of it is converted into less toxic metabolites, which are excreted through milk, urine or bile; the other part is transformed into the toxic metabolite AFBO, which can react with RNA, DNA or liver proteins, causing DNA damage, protein inactivation or denaturation. It can also induce the production of reactive oxygen species (ROS) catalyzed by CYP450 enzymes, resulting in oxidative stress damage to the body and an increase in malondialdehyde (MDA) content in tissues.

The health impacts of AFB1 on animals are mainly reflected in liver injury and immune system suppression. Acute intoxication manifests as hepatotoxic symptoms such as listlessness, anorexia, jaundice and hemorrhage. Long-term exposure can lead to liver injury, impair the growth performance and reproductive capacity of animals, reduce body weight and production efficiency, and further cause severe economic losses.

Figure 2. Liver injury in meat ducks caused by aflatoxin B1

 

Application of Bile Acids

In addition to physical detoxification, chemical detoxification and biological detoxification, supplementation with exogenous substances in feed to implement exogenous regulation is an important approach to alleviate the toxic effects of AFB1 on the body. As a natural bioactive component, bile acids have been proven to exert a highly significant protective effect on the liver. Bile acids promote the biotransformation and excretion of AFB1 in the liver through multiple pathways, alleviating AFB1-induced liver injury. Experiments by Liang Chen have confirmed that dietary supplementation with bile acids can protect broilers from AFB1-induced hepatotoxicity, inflammation and oxidative stress, and significantly improve the metabolism of AFB1 and the excretion of AFB1/AFBO from the liver to the gallbladder and cecum in broilers.

1. Promotion of toxin metabolism

Bile acids upregulate the expression of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX) and glutathione S-transferase (GST). In liver detoxification-related reactions, GST can catalyze the conjugation of AFBO with GSH, converting it into non-toxic or low-toxic metabolites for excretion. The content of GSH in the body is a key indicator reflecting the redox status. AFB1 exposure reduces the content of GSH in the body, decreases the activities of antioxidant enzymes such as CAT, SOD and GPX, and increases the in vivo levels of peroxides such as MDA. By upregulating the expression of antioxidant enzymes including SOD, CAT, GPX and GST, bile acids assist the liver in converting AFB1 into non-toxic or low-toxic metabolites.

2. Promotion of toxin excretion

Bile acids promote the clearance of toxins through the hepatobiliary pathway. Free bile acids can promote bile secretion and enhance the flushing effect of bile on the bile ducts. Increased bile secretion can assist the liver in transporting detoxified toxins into the intestinal tract for excretion with feces, reducing the retention time of mycotoxins in the body and thereby alleviating toxin-induced damage to the body.

3. Alleviation of toxin-induced liver injury

Bile acids activate the FXR signaling pathway and exert anti-inflammatory, anti-apoptotic and anti-autophagic effects (Figure 3). In the liver, FXR is regarded as a multifunctional cytoprotective agent (Figure 4). FXR maintains the normal metabolism of bile acids, glucose and lipids in the liver; FXR promotes the regeneration and repair of the liver after injury; FXR protects hepatocytes from death and enhances cell viability; FXR counteracts hepatic inflammation by inhibiting NF-κB and STAT3-mediated signaling pathways, thereby preventing inflammatory damage; FXR can directly upregulate the expression of some tumor suppressor genes and inhibit the transcription of several oncogenes.

Bile acids alleviate oxidative stress. Bile acids reduce MDA levels by increasing the activities of CAT and SOD, thereby alleviating oxidative stress.

 

Figure 3 The FXR signaling pathway exerts anti-inflammatory, anti-apoptotic and anti-autophagic effects.

 

Figure 4 Hepatic protection mediated by FXR

Bile acids activate the TGR5 signaling pathway to exert anti-inflammatory effects, improve immunity and promote bile flow (Figure 5). A non-metabolic function of TGR5 is to protect the liver, reduce the exposure of the liver to inflammatory signals, and maintain normal biliary function.

TGR5 inhibits the activity of pro-inflammatory NF-κB and reduces the expression of the following cytokines: interferon-γ (IFN-γ), interleukin-13 (IL-13), IL-2, IL-6 and TNF-α, thereby achieving significant anti-inflammatory effects.

Activation of TGR5 promotes chloride ion secretion, which subsequently enters the bile duct in exchange for bicarbonate via anion exchanger 2 (AE2), maintaining bile flow and promoting the excretion of toxins.

 

Figure 5. Immune effects of TGR5 signaling pathway activated by bile acids