1. Introduction
Essential trace elements are the indispensable nutrients for animals, and especially Cu, Fe, Zn, and Mn are required for the normal growth, development, and many physiological functions in animals [
1,
2,
3,
4]. Cu is a part of Cu-transporting P-type ATPase and Cu/Zn superoxide dismutase [
5]. Fe as the part of hemoglobin and myoglobin plays an important role in delivering the oxygen, and it also plays a vital role in the host immunity [
6]. Zn takes part in the growth, oxidation resistance and immunity [
7]. Mn as the part of phosphoenolpyruvate carboxykinase takes part in the gluconeogenesis, and it is related to the neuronal health [
8].
Pig (
Sus scrofa) is one of the most raised animals in the world. Piglets are weaned early to increase the reproductive performance of the sow and to reduce pathogen transmission [
9]. However, as the digestive system of piglets is immature, early weaning will lead to maldigestion [
10]. Meanwhile, because of rapid and dramatic change of the living environment and exposure to the bacteria [
11], early weaning piglets easily suffer from stress, which reduces the growth performance and feed intake [
12] and decreases the nutrient digestibility through digestive disorders [
13]. It leads to the resources waste and environment pollution and limits the sustainable development of animal husbandry. Thus, it is urgent to look for an effective additive to relieve early weaning stress and improve the absorption of nutrition. The absorption of nutrition is closely related to the intestinal health, however, early weaning stress increases the intestinal permeability of piglets which has a negative effect on the absorption of nutrients [
14]. Lipopolysaccharide (LPS) is the primary component of Gram-negative bacteria outer cell walls [
15] and it can induce severe bacterial diarrhea, apoptosis [
16], inflammatory responses [
17], intestinal barrier damage [
18], and then inhibits the growth performance and decreases the nutrients absorption of the animal [
19]. Due to its good repeatability, the LPS stress mode is widely used in research.
Many of growth factors exist in milk, such as insulin, nerve growth factor (NGF), and epidermal growth factor (EGF) which can improve the intestinal development of piglets and thus improve their growth performance [
20]. Early weaning prevents the supply of those growth factors from milk to piglets. Interestingly, EGF is one of the most abundant growth factors in milk [
21,
22], which indicates its important role for young mammals. EGF was first isolated by Dr. Cohen from the mouse (
Mus musculus) submaxillary gland in 1962 [
23]. It is a polypeptide comprising 53 amino acids [
24]. It is found in many body fluids such as the milk, blood, saliva, and intestinal fluid [
25], and it plays important roles in the regulation of cell growth, proliferation, apoptosis and tumorigenesis [
26,
27,
28]. Previous studies showed that EGF could improve the growth performance of broiler chicks (
Gallus gallus) [
27] and rats (
Rattus norvegicus) [
29]. Dietary EGF can augment the intestinal length and villus height by activating the phosphatidylinositol-3-kinases/protein-serine-threonine kinase (PI3K/AKT) and RAS/mitogen-activated protein kinase (RAS/MAPK) signaling pathways [
30,
31]. Meanwhile, EGF can also promote the proliferation of goblet cells [
10] and increase the activity of digestive enzymes in the intestine [
32]. However, the effect of EGF on growth performance and nutrients absorption in LPS challenged early-weaning pigs is unclear. Whether it can be added as an effective additive in the feed of early weaning piglets is still unknown. In this experiment, a model of LPS stress was established to examine how EGF attenuates the effect of LPS on the growth performance, nutrient digestibility, microelement absorption of early-weaned pigs, and study the underlying mechanism.
3. Results
3.1. Cu, Fe, Zn, and Mn Concentration in Diets
The concentration of essential microelements in diets 1 and 2 are shown in
Table 2. There were no significant differences of Cu, Fe, Zn, and Mn concentration between diet 1 and diet 2 (
p > 0.05).
3.2. Growth Performance
The effect of LPS and EGF levels on growth performance are shown in
Table 3. The LPS level affected ADG and FCR of piglets, and EGF level affected ADFI (
p < 0.05). The LPS and EGF levels displayed a significant interaction effect on ADFI (
p < 0.05). Piglets injected with H
LPS level significantly decreased the ADG and significantly increased the FCR compared with the piglets injected with Z
LPS level (
p < 0.05). The ADFI of piglets fed H
EGF level was 12% higher than the piglets fed Z
EGF level (
p < 0.05). The lowest ADFI was observed in the control group, which significantly differed from the other three groups (
p < 0.05)
3.3. Nutrient Apparent Digestibility
Except for the apparent digestibility of crude fat, no differences were observed in the nutrient apparent digestibility, and there were no interactions between LPS and EGF levels regarding the apparent digestibility (
p > 0.05,
Table 4). Compared with Z
LPS level, H
LPS level significantly decreased the apparent digestibility of crude fat (
p < 0.05). The lowest apparent digestibility of crude fat was observed in the LPS group, which was significantly differed from the other groups (
p < 0.05), and there was no significant difference between the EGF + LPS and control groups (
p > 0.05).
3.4. Concentration of Cu, Fe, Zn, Mn in the Gastrointestinal Chyme and Feces
The concentration of Cu, Fe, Zn, Mn in the gastrointestinal chyme and feces are shown in
Table 5. The present study revealed significantly interactions between the EGF and LPS levels regarding the Cu concentration in the stomach, jejunum and ileum chyme (
p < 0.05), and there was no interaction in the feces (
p > 0.05). Piglets injected with H
LPS level decreased the Cu concentration in the ileum chyme and increased the Cu concentration in the jejunum chyme compared the piglets injected with Z
LPS level (
p < 0.05). The Cu concentration in the jejunum, ileum chyme and feces of piglets fed H
EGF level were 18%, 55%, and 28% lower than those of piglets fed Z
EGF level, respectively (
p < 0.05). The LPS group had a significantly greater Cu concentration in the jejunum chyme compared with the other groups (
p < 0.05), while there was no significant difference between the EGF + LPS and control groups (
p > 0.05). The control and LPS groups had significantly greater Cu concentration in the ileum chyme compared with the EGF and EGF + LPS groups (
p < 0.05).
The LPS and EGF levels displayed a significant interaction effect on the Fe concentration in the feces (p < 0.05). Piglets injected with HLPS level decreased the Fe concentration in the ileum chyme and increased the Fe concentration in the jejunum chyme and feces compared with the piglets injected with ZLPS level (p < 0.05). The Fe concentration in the ileum chyme and feces of piglets fed HEGF level were 35%, and 31% lower than those of piglets fed ZEGF level, respectively (p < 0.05). In the feces, the LPS group had a significantly greater Fe concentration compared with the other groups (p < 0.05), and the EGF + LPS group had a significantly lower Fe concentration compared with the control group (p < 0.05).
The LPS and EGF levels displayed significant interactions on the Zn concentration in the stomach and ileum chyme, and feces (p < 0.05). The Zn concentration in the jejunum, ileum chyme and feces of piglets injected with HLPS level were 12%, 53%, and 19% higher than those of piglets fed ZLPS level, respectively (p < 0.05). The Zn concentration in the jejunum, ileum chyme and feces of piglets fed HEGF level were 12%, 68%, and 28% lower than those of piglets fed ZEGF level, respectively (p < 0.05). In the ileum chyme, a significantly greater Zn concentration was observed in the LPS group, which significantly differed from the other groups (p < 0.05), and the EGF + LPS group had a significantly lower Zn concentration compared with the control group (p < 0.05). In the feces, the LPS group had a significantly greater Zn concentration compared with the other groups (p < 0.05), while there was no significant difference between the EGF + LPS and control groups (p > 0.05).
Piglets injected with HLPS level significantly increased the Mn concentration in the stomach chyme and feces compared with the piglets injected with ZLPS level (p < 0.05). The Mn concentration in the jejunum, ileum chyme and feces of piglets fed HEGF level were 29%, 15%, and 27% lower than those of piglets fed ZEGF level, respectively (p < 0.05). In the stomach chyme, the LPS and EGF + LPS groups had significantly greater Mn concentration compared with the control and EGF groups (p < 0.05). In the feces, the LPS group had a significantly greater Mn concentration compared with the other groups (p < 0.05), and the EGF + LPS group had a significantly lower Mn concentration compared with the control group (p < 0.05).
3.5. Expression of Cu Transport-Relative Genes in the Mucosa of the Gastrointestinal Tissues
As shown in
Table 6, the expression levels of the Cu transport-related genes in the mucosa from the gastrointestinal tissues were affected by LPS and EGF levels (
p < 0.05). In the stomach, the LPS and EGF levels displayed significant interaction effects on the expression levels of Cox17, Atox1, ATP7A, and ATP7B (
p < 0.05). Piglets injected with H
LPS level significantly decreased the expression levels of Atox1 and ATP7B compared with the piglets injected with Z
LPS level (
p < 0.05), and piglets supplied with H
EGF level significantly increased the expression levels of Ctr1, Cox17, Atox1, ATP7A, and ATP7B compared with the piglets supplied with Z
EGF level (
p < 0.05). The LPS and EGF + LPS groups had a significantly lower expression level of Atox1 compared with the control and EGF groups (
p < 0.05), whereas there was no significant difference between the EGF + LPS and LPS groups (
p > 0.05). The LPS group had a significantly lowest expression level of ATP7A compared with the other groups (
p < 0.05), and EGF + LPS group had a significantly higher expression level compared with the LPS and control groups (
p < 0.05).
In the duodenum, there were significant interactions between LPS and EGF levels in the expression levels of Cox17, Atox1, and ATP7B (p < 0.05). Compared with the piglets injected with ZLPS level, piglets injected with HLPS level significantly decreased the expression levels of Cox17, ATP7A, ATP7B, and CCS (p < 0.05). Piglets supplied with HEGF level significantly increased the expression levels of Cox17, ATP7A, and ATP7B compared with the piglets supplied with ZEGF level (p < 0.05). The LPS group had the significantly lowest expression level of Cox17 compared with the other groups (p < 0.05), whereas there was no significant difference between the EGF + LPS and control groups (p > 0.05). The LPS group had a significantly lower expression level of ATP7A compared with the control and EGF groups (p < 0.05), while there was no significant difference between the EGF + LPS and control groups, or between the EGF + LPS and LPS groups (p > 0.05).
In the jejunum, the LPS and EGF levels displayed a significant interaction effect on the expression level of CCS (p < 0.05). The HLPS level significantly decreased the expression levels of Ctr1, Atox1, and CCS compared with the ZLPS level, and the HEGF level significantly increased the expression levels of Ctr1, Atox1, ATP7A, ATP7B, and CCS compared with the ZEGF level (p < 0.05). The LPS group had the significantly lowest expression levels of Ctrl and Atox1 compared with the other groups (p < 0.05), while there was no significant difference between the EGF + LPS and control groups (p > 0.05).
In the ileum, there was a significant interaction between LPS and EGF levels in the expression level of Atox1 (p < 0.05). Piglets injected with HLPS level significantly decreased the expression levels of Cox17, ATP7B, and CCS compared with the piglets injected with ZLPS level (p < 0.05), and piglets supplied with HEGF level significantly increased the expression level of Atox1 compared with the piglets supplied with ZEGF level (p < 0.05). The LPS and EGF + LPS groups had significantly lower expression levels of Cox17, ATP7B, and CCS compared with the control and EGF groups (p < 0.05), whereas there was no significant difference between the EGF + LPS and LPS groups (p > 0.05). The LPS group had the significantly lowest expression level of Atox1 compared with the other groups (p < 0.05), and the EGF + LPS group had a significantly greater expression level compared with the control group (p < 0.05).
3.6. Expression of Fe Transport-Relative Genes and DMT1 Gene in the Mucosa of the Gastrointestinal Tissues
The expression levels of Fe transport-related genes and DMT1 gene in the mucosa of the gastrointestinal tissues are shown in
Table 7. In the stomach, the LPS and EGF levels displayed a significant interaction effect on the expression level of CYTB (
p < 0.05). Piglets injected with H
LPS level significantly decreased the expression level of Tf compared with the piglets injected with Z
LPS level (
p < 0.05), and piglets supplied with H
EGF level significantly increased the expression levels of CYTB, Hp, Tf, and DMT1 compared with the piglets supplied with Z
EGF level (
p < 0.05).
In the duodenum, there were significant interactions between LPS and EGF levels in the expression levels of CYTB and DMT1 (p < 0.05). Compared with the piglets injected with ZLPS level, piglets injected with HLPS level significantly decreased the expression levels of Tf and DMT1 (p < 0.05). Supply with HEGF level significantly increased the expression levels of CYTB and DMT1 compared with the piglets supplied with ZEGF level (p < 0.05). The LPS and EGF + LPS groups had significantly lower expression levels of Tf and DMT1 compared with the control and EGF groups (p < 0.05), whereas there was no significant difference between the EGF + LPS and LPS groups (p > 0.05).
In the jejunum, the HLPS level significantly decreased the expression levels of CYTB, Tf, and DMT1 compared with the ZLPS level, and the HEGF level significantly increased the expression levels of Hp and DMT1 compared with the ZEGF level (p < 0.05). The LPS and EGF + LPS groups had significantly lower expression level of CYTB compared with the control and EGF groups (p < 0.05), whereas there was no significant difference between the EGF + LPS and LPS groups (p > 0.05). The LPS group had a significantly lower expression level of DMT1 compared with the control and EGF groups (p < 0.05), while there was no significant difference between the EGF + LPS and control groups, or between the EGF + LPS and LPS groups (p > 0.05).
In the ileum, there were significant interactions between LPS and EGF levels in the expression levels of Hp and DMT1 (p < 0.05). Piglets injected with HLPS level significantly decreased the expression level of DMT1 compared with the piglets injected with ZLPS level (p < 0.05), and piglets supplied with HEGF level significantly increased the expression levels of CYTB and Hp compared with the piglets supplied with ZEGF level (p < 0.05). The LPS group had the significantly lowest expression level of Hp compared with the other groups (p < 0.05), and the EGF + LPS group had a significantly greater expression level compared with the control group (p < 0.05). The LPS group had a significantly lower expression level of DMT1 compared with the other groups (p < 0.05), but there was no significant difference between the EGF + LPS and control groups (p > 0.05).
3.7. Expression of Zn Transport-Relative Genes in the Mucosa of the Gastrointestinal Tissues
The expression levels of Zn transport-related genes in the mucosa from the gastrointestinal tissues are shown in
Table 8. The present study revealed significant interactions between EGF and LPS levels regarding the expression level of Zip4 in the stomach, and the expression levels of Zip7 in the stomach and ileum (
p < 0.05). Injected H
LPS level significantly decreased the expression levels of Zip4 and Zip7 in the stomach and ileum compared with the Z
LPS level (
p < 0.05) and supplied H
EGF level significantly increased the expression level of Zip4 in the stomach, jejunum and ileum compared with the Z
EGF level. The LPS group had a significantly lower expression level of Zip4 in the ileum compared with the other groups (
p < 0.05), whereas there was no significant difference between the EGF + LPS and control groups (
p > 0.05). The LPS and EGF + LPS groups had significantly lower expression levels of Zip7 in the stomach and ileum compared with the control group (
p < 0.05), while there was no significant difference between the LPS and EGF + LPS groups (
p > 0.05). For ZnT1, injected H
LPS level significantly increased the expression level of it in the stomach compared with the Z
LPS level (
p < 0.05), and supplied H
EGF level significantly increased the expression level of it in the stomach compared with Z
EGF level (
p < 0.05).
4. Discussion
Recently, the application of EGF has received increasing amounts of attention due to its positive impacts on animals [
26,
27,
28]. Previous studies had shown that LPS significantly decreased the ADG and ADFI of weaned piglets [
34], and EGF could increase body weight gain of broiler chickens and early-weaned mice [
27,
35], and increase gain/feed of early-weaned pigs [
36]. Our results indicated that injected H
LPS level significantly decreased the ADG and significantly increased the FCR, and dietary H
EGF level significantly increased the ADFI of early-weaned piglets, which were in agreement with the previous studies. Our results also indicated that injected H
LPS level significantly decreased the apparent digestibility of crude fat. The changes of growth performance induced by LPS was related to the changes of nutrients absorption. LPS leads to partial loss and sloughing of ileal villi and decreases the intestinal barrier function in mice [
37]. LPS also increases the intestinal epithelial cell permeability [
38]. LPS reduced the apparent digestibility maybe through reducing the intestinal health. Previous study had showed that dietary EGF had no significant influence on the apparent digestibility of crude protein, gross energy and P [
39]. Our results also indicated that dietary H
EGF level had no significant influence on the apparent digestibility, which was in agreement with the previous studies.
Indispensable microelements take part in the regulation of the body physiological functions, such as participating in the redox active [
1,
2,
3,
4], oxygen transport, DNA biosynthesis [
40], cellular signal recognition [
41,
42], and nutrients metabolism [
43,
44]. A higher concentration of microelements in the gastrointestinal chyme and feces means a lower absorption level. Cu as a cofactor plays an essential role in redox-active, pigmentation, oxidative phosphorylation and neuropeptide biogenesis [
45,
46]. Our results showed that injected H
LPS level increased the concentration of Cu in the jejunum chyme, and dietary H
EGF level decreased the concentration of Cu in the jejunum, ileum chyme, and feces. ATP7A, ATP7B, Cox17, Ctr1, Atox1, and CCS genes are Cu transport-related genes. Ctr1 is a major Cu extracellular uptake protein and involved Cu transport across membranes [
47]. Atox1, CCS, and Cox17 are metallochaperones: Cox17 transport Cu to the mitochondria, and CCS transport Cu to combine the SOD in the cytoplasm and mitochondria, and Cox17 transport Cu to combine the ATP7A and ATP7B [
48]. ATP7A and ATP7B are P-type Cu-ATPases which transport Cu to the ceruloplasmin and lysyl oxidase and take part in the Cu exportation from the cell [
49]. Our results showed that in the mucosa of gastrointestinal tissues, injected H
LPS level decreased the expression levels of the ATP7A, ATP7B, Cox17, Ctr1, Atox1, and CCS genes, while dietary H
EGF level increased the expression levels of these genes. These results explained how LPS and EGF regulated the absorption of Cu in the gastrointestinal tissues. However, injected H
LPS level decreased the concentration of Cu in the ileum, and the underlying reason needs further analysis.
Fe is the transporter of oxygen, and it takes part in the redox reaction, electron transport, cell growth, and energy production [
50,
51]. Our results showed that injected H
LPS level increased the concentration of Fe in the jejunum chyme and feces, while dietary H
EGF level decreased the concentration of Fe in the ileum chyme and feces. CYTB, Tf and HP are Fe transport-related genes. CYTB and HP are oxidoreductases: CYTB as ferric reductase changes ferric iron to ferrous iron [
52], while Hp is expressed in the enterocyte and oxidizes ferrous iron [
50,
53]. CYTB co-operate with DMT1 to transfer Fe from the duodenal lumen to the enterocyte, while Hp co-operate with ferroportin to transfer Fe from the basolateral membrane to the systemic circulation [
50]. Tf is synthesized almost in the liver [
54], and it can regulate iron homeostasis and erythropoiesis [
55]. As an important iron carrier in the blood [
53], Tf delivers iron to the tissues [
54]. Our results showed that in the mucosa of gastrointestinal tissues injected H
LPS level decreased the expression levels of the CYTB and Tf genes, and dietary H
EGF level increased the expression levels of the CYTB, Tf and HP genes. Consequently, LPS and EGF affected the absorption of Fe in the gastrointestinal tissues by regulating the expression levels of Fe transport-related genes.
Mn as enzymatic cofactors or structural centers takes part in a plethora of biological processes, such as glycosylation, signal transduction, phosphorylation, and hydrolysis [
56,
57]. It also takes part in the host immune system [
58]. Our results showed that injected H
LPS level increased the concentration of Mn in the stomach chyme and feces, while dietary H
EGF level decreased the concentration of Mn in the jejunum and ileum chyme and feces. DMT1 is a multiple divalent metals transport gene, and it can transfer the Cu, Mn, Zn [
59]. Our results showed that in the mucosa of gastrointestinal tissues injected with H
LPS level downregulated the expression level of the DMT1 gene, while dietary H
EGF level upregulated the expression level of the DMT1 gene. That was the one reason why the LPS and EGF changed the absorption of Cu, Mn, and Zn in the piglets.
Zn is the cofactor for many enzymes and takes part in many biological processes [
60], and it plays the important roles in protein synthesis, growth, and immunity [
61]. Our results showed that in the jejunum and ileum chyme, and feces, injected H
LPS level increased the concentration of Zn, while dietary H
EGF level decreased the concentration of Zn. In the body of vertebrates two kinds of zinc transporter family proteins exist, ZIP and ZnT family [
62]. Zip4, Zip7 and ZnT1 are the Zn transport-related genes. Zip4, Zip7 belong to the ZIP family and take part in the import of Zn to the cytoplasm. Zip4 is a primary importer for the absorption of Zn in the enterocyte, and it can transfer Zn from intestine lumen to the epithelial cells [
63]. Zip7 exists in the membrane of endoplasmic reticulum and Golgi apparatus, and transfers Zn to the cytosol [
64]. ZnT1 belongs to the ZnT family, which predominantly localizes in the basolateral membrane [
65]. In the intestinal epithelial cells, it takes part in the export of Zn from the cytoplasm to the portal vein [
65]. Our results showed that in the mucosa of gastrointestinal tissues, injected H
LPS level decreased the expression levels of the Zip4, Zip7 and ZnT1 genes, while dietary H
EGF level increased the expression levels of the Zip4 and ZnT1 genes. It is implied that the bioavailability of Zn was affected by LPS and EGF through regulating the expression of Zn transport-related genes.