Mycotoxin is a naturally occurring substance produced by fungi. Consumption of low concentrations of mycotoxins in animals would result in severe hazardous symptoms [1]. The first credible evidence of the hazardous of mycotoxins effects dates back to the 11th century when ergot intoxication caused widespread human and animal poisoning and even mortality in Europe [2]. With detection and analysis technology advances, hundreds of mycotoxins have been discovered [3]. Five of these mycotoxins, aflatoxin B1 (AFB1), deoxynivalenol (DON), zearalenone (ZEA), fumonisin B1 (FB1), and ochratoxin A (OTA), have historically been major objects of mycotoxin study because of their high detection rates and significant toxicities in feed raw materials and foodstuffs [4]. Due to the extensive prevalence of fungi in the environment, grains have been contaminated with mycotoxins during the growth process, and almost all agricultural commodities are susceptible to fungi infection and the production of mycotoxins if improper storage [5]. Mycotoxins not only lower livestock productivity and result in significant economic losses, but also pose a threat to human health because of their accumulation along the food chain. Severe clinical symptoms occur during animal breeding with mycotoxin-contaminated feed, including diarrhea, liver and kidney damage, pulmonary edema, vomiting, bleeding, and tumors ^{[6][7][8][9][10][11]}[6,7,8,9,10,11]. Additionally, mycotoxins have a synergistic effect, typically including a combination of toxins, which results in human and animal poisoning upon consumption [12]. This complicates the toxicological mechanism of mycotoxins.

The gut serves as the first line of defense and protection against mycotoxins and the first location of mycotoxins’ absorption into the body ^[13][14][13,14]. The gut microbiota plays a critical role in forming the intestinal barrier and maintaining intestinal homeostasis [15]. With the introduction of new concepts such as the brain-gut axis, liver-gut axis, and kidney-gut axis ^{[16][17][18][19]}[16,17,18,19], as well as the widespread use and advancement of microbial sequencing technology, an increasing number of studies are focusing on the impact of mycotoxins in intestinal microbiota. Meanwhile, scientists are investigating the role of intestinal microbial changes in the process of mycotoxin poisoning and detoxification. The deleterious effects conferred by mycotoxins-induced microbial alterations would be brought to the forefront for investigating toxicological effects and response of the host.

2. Toxicity of Mycotoxins to Intestinal Epithelial Cells

As the “transit point” of animals ingesting mycotoxins, intestinal epithelial cells are the first barrier where mycotoxins contact the animal body [14]. Numerous animal studies have demonstrated that mycotoxins such as DON, ZEA, FB, OTA and AFB1 trigger direct epithelial cell damage (Table 1). The most noticeable symptom is that mycotoxin directly limits the growth and structural destruction of the small intestinal villi, the outer wall consisting of a single layer of epithelial cells ^{[20][21][22][23]}[20,21,22,23]. Recently, studies on the mechanism of directive injury of mycotoxins on the intestinal epithelium were well investigated. By studying changes in the physiological functions of intestinal cells exposed to mycotoxins, researchers initially explored the direct toxicological effects of mycotoxins in the absence of gut microbiota and have achieved considerable progress.

2.1. DON

The disclosure of the direct toxicological effects of DON on the intestinal epithelium has made great progress, and a systematic explanation mechanism has been constructed from multiple perspectives. It is assumed that the detrimental effects of DON on intestinal epithelial cells are mediated predominantly through the following three pathways: (Ι) DON activates DAO, up-regulates NF-κB signaling pathway, increases the levels of inflammatory cytokines in the intestine, and ultimately mediates intestinal epithelial apoptotic ^[24][25][26][24,25,26]. (II) DON decreases the expression of the trefoil factor family (TFFs) of peptides (a type of bioactive substance that could govern tissue regeneration, improve barrier function, and decrease proinflammatory expression) via triggering the MAPK signaling pathway, hence inhibiting intestinal epithelial cell self-repair. This mechanism has been validated in human intestinal cell line HT29-16E and swine intestinal explants ^[24][27][24,27]. (III) Recently, a new perspective on the enterotoxicity of DON has also been proposed. It was revealed that DON can also decrease the stability of intercellular compact proteins in the intestinal epithelium [28]. Along with accelerating tight junction protein degradation in the fusion medium, DON would activate the p38 (MAPK) signaling pathway, resulting in the swallowing and degradation of Occluding and ZO-1 in lysozyme, and eventually destroying the small intestine villus structure and increasing intestinal permeability. Collectively, DON can activate immune pathways and induce inflammatory responses in the intestinal epithelium. And it can activate lysosomes to engulf connexins between intestinal epithelial cells, resulting in structural collapse. In addition, DON also inhibits the self-repair process of the intestine, which eventually leads to the death and autophagy of intestinal epithelial cells.

2.2. ZEA

ZEA generates oxidative stress at the cellular level and increases lactate dehydrogenase activity, further impairing the organism’s scavenging of reactive oxygen species (ROS) and increasing the amount of oxidative stress in intestinal cells ^[29][30][29,30]. In addition, ZEA also mediates the activation of NLRP3 inflammasome in mouse intestine, which in turn promotes the level of Caspase-1, up-regulates inflammatory cytokines, leads to the expansion of inflammatory cells in intestinal epithelial cells, and produces apoptosis [31]. On the other hand, ZEA also leads to aberrant G2/M transition in IPEC-J2 cells by disrupting the cell cycle signaling system, consequently reducing cell proliferation and producing intestinal epithelial damage [32]. Intestinal epithelial cells have the shortest cell cycle and are the fastest-renewing somatic cells. Therefore, the perturbation of cell cycle signals by ZEA will greatly inhibit the development and self-repair process of the intestinal epithelium.

2.3. OTA

The mechanism of OTA cytotoxicity in the intestinal epithelium is mostly based on the production of ROS and the stimulation of apoptosis-regulating genes. Wang et al. [33] found that OTA may generate reactive oxygen species (ROS) in IPEC-J2 cells, which elevated the activity of the Ca²⁺ and MLCK Signaling pathways and ultimately resulted in barrier malfunction and destruction. Comparative transcriptomics demonstrated that OTA enhanced the expression of apoptosis-related genes such as casp3, cdc25B and egr1 in Caco-2 cells, elucidated the genome-wide biological reaction perspective of OTA controlling intestinal epithelial damage [34]. The researchers also explained the toxicological mechanism of OTA on intestinal epithelial cells from various aspects. It is worth noting that OTA appears to be a dose-dependent amplifier of apoptotic signaling to intestinal exposed-epithelial cells. OTA showed a perturbation of functional gene expression of human intestinal cells at a very low dose (0.0005 μg/mL).

2.4. FB1

FB1 regulates the secretion of mucin by stimulating the ERK phosphorylation pathway, lowering the expression level of intestinal tight junction protein, and inhibiting the viability of IPEC-J2 cells [35]. Mucin secreted by intestinal epithelial goblet cells is a glycoprotein composed of mucopolysaccharides that protects intestinal cells. The network of mucins makes it difficult for chemical irritants, digested foods, toxins, and bacteria to pass through, protects the intestinal epithelium from damage, and prevents pathogens from binding to the intestinal epithelium. The digestion of mucin by FB1 is likely to be an important inducement for it to enter the body and cause multi-organ toxicity. It is worth noting that FB1 could regulate the mouse intestinal aryl hydrocarbon receptor (AHR), the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), and downstream target genes (CYP450s) to disrupt nuclear xenobiotic receptor (NXR) homeostasis, and meanwhile induce intestinal villus and epithelial layer shedding, intestinal gland atrophy, and necrosis [36].

2.5. AFB1

There are few studies on the mechanism of AFB1 intestinal epithelial toxicity. By observing the ultrastructure of intestinal epithelial cells exposed to AFB1, the researchers found that AFB1 is significantly toxic to the organelles of intestinal epithelial cells. AFB1 causes mitochondrial vacuolization in small intestinal cells, leads to the disappearance of mitochondrial cristae, junctional complexes, and terminal reticulum, and then induces apoptosis [37]. Notably, AFB1 also reduced the ratio of goblet cells in epithelial cells in this researchtudy. This result indicates that FB1 and AFB1 also seem to have certain effects on the differentiation of intestinal epithelial cells, but whether the underlying mechanism is the toxic effect on goblet cells or the direct induction of the cell differentiation process remains to be further investigated. In addition, researchers have revealed that AFBI exposure increases p42/44 (MAPK) phosphorylation in Caco-2 cells, inhibiting tight junction protein synthesis between epithelial cells, increasing intestinal permeability, and weakening the intestinal barrier [38]. In conclusion, based on the results of direct mycotoxin exposure assays on intestinal epithelial cells, mycotoxins usually directly inhibit the normal division and proliferation of intestinal epithelial cells and cause apoptosis through modulation of cell signaling pathways.