R .J. Xu et al. Livestock Production Science 66 2000 95 –107
101
a receptor-mediated process. In our own laboratory, oral administration of iodine-labeled EGF to new-
we observed that following oral administration of born and 5-day-old pigs, over 95 of the recovered
iodinine-labeled IGF-I and fluorescent-labelled dex- radioactivity was found in the gastrointestinal tract,
tran with porcine colostrum as the delivery vehicle, of which 78–86 was found in the luminal contents
iodine-labelled IGF-I was detectable in the plasma of with the remaining found in the gastrointestinal wall
both newborn and 3-day-old piglets while fluorescen- Shen and Xu, 1998.
t-labelled dextran was detected in the plasma of only newborn pigs Xu and Wang, 1996. The finding
suggests that milk-borne IGF-I can be absorbed in
7. Effects of milk-borne growth factors on GI
neonatal pigs and the absorption is independent of function
gut closure. In newborn pigs, oral administration of a pharmacological dose of insulin led to a reduction of
Although direct evidence of milk-borne growth blood glucose level Shen and Xu, 2000b. In
factors affecting GI development and function in addition, Wester et al. 1998 reported that feeding
suckling piglets is lacking, numerous studies have colostrum increased circulating IGF-I concentration
shown that exogenous growth factors, particularly in newborn pigs and the increase was independent of
EGF, IGF-I and insulin, administered orally at a nutrient intake and endogenous growth hormone
physiological or pharmacological dose, affected GI secretion. The protein synthesis rate in liver, spleen
tissue growth and epithelial cell maturation. In the and skeletal muscle was greater in newborn piglets
following sections, we will discuss separately the fed colostrum than in those fed milk, milk formula or
effects of exogenous EGF, IGF-I, insulin and TGF-b water Burrin et al., 1992, and the increase was
on GI tissue growth and function maturation in partially due to nutrient-independent factors, most
neonatal pigs. likely colostrum-borne growth factors Burrin et al.,
1995. All the above findings indicate that milk- 7.1. Epidermal growth factors
borne IGF-I and insulin may be absorbed in newborn pigs.
Epidermal growth factor is a single-chain poly- In contrast to the above findings, Donovan et al.
peptide of 53 amino acids with a molecular weight of 1997 reported that iodine-labeled IGF-I adminis-
about 6 kDa. The peptide is highly homologous tered orally to newborn pigs using a milk replacer as
among species and elicits similar effects across the delivery vehicle was poorly absorbed. Oral
species Odle et al., 1996. It acts on target cells by administration of pharmacological doses 0.2–3.5
binding to a 170-kDa membrane-bound glycoprotein mg kg per day of IGF-I, as a supplement to milk
receptor that possesses intrinsic tyrosine kinase formula, did not increase circulating concentration of
activity Carpenter, 1984. IGF-I in newborn pigs Burrin et al., 1996; Houle et
Receptors for EGF have been identified on the al., 1997. The discrepancy among different reports
epithelial cells of the GI tract, from the oesophagus may be due to differences in the media used to
to the ileum, in 1- to 28-day-old pigs Jaeger and deliver the IGF-I colostrum versus milk formula. In
Lamar, 1992 and on the gastric parietal cells of a recent study with prenatal pigs, Sangild et al.
adult pigs Sjodin et al., 1992. Kelly et al. 1992 1999 reported that porcine colostrum enhanced
found that the abundance of EGF receptors on the both intestinal macromolecule absorption and onset
intestinal epithelial cells were greater in weaned pigs of gut closure. Although there is tangible evidence
than in newborn pigs, and the latter was much suggesting that colostrum-borne growth factors can
greater than in suckling pigs. The low abundance of be absorbed into blood circulation in suckling
EGF binding sites in suckling pigs was suspected to neonatal pigs, the current common view is that the
be the result of receptor down-regulation following absorption is limited and that milk-borne growth
exposure to milk-borne EGF. Using an autoradiog- factors are more likely to act locally in the GI tract
raphic technique, Kelly et al. 1992 found that EGF of the suckling young Burrin et al., 1997. Studies
receptors were located predominantly in the basal in our own laboratory also showed that 45 min after
regions of the villi and in the crypts, and that the
102 R
.J. Xu et al. Livestock Production Science 66 2000 95 –107
receptors were located at both the basolateral and newborn pigs for 8 days. Kingsnorth et al. 1990
apical brush-border membranes of the epithelial reported increased tensile strength of gastric wounds
cells. The latter finding suggests that both circulating in 20-kg pigs after 5 days of intraperitoneal infusion
and luminal EGF may influence intestinal epithelial of EGF at 0.5 mg kg per day. In 8-week-old rabbits,
cell development in neonatal pigs. Using an im- following a two-thirds resectioning of their proximal
munostaining technique, Playford et al. 1996 found small intestine, oral administration of EGF at 40
that EGF receptors were localised to the basolateral m
g kg per day for 5 days increased maltase specific surface but not to the apical brush-border membrane
activity and led to a 3- to 4-fold increase in glucose of the epithelial cells in the adult human GI tract. It
transportation capacity in the remaining small intes- is not clear whether these differences are the results
tine O’Loughlin et al., 1994. In adult rats with of variations in experimental techniques, species or
acute intestinal injury caused by methotrexate treat- age. In rats, EGF receptors were localised to the
ment, oral administration of EGF for 6 days in- basolateral membrane but not to the apical mem-
creased mucosal disaccharidase and leucine amino- brane of intestinal epithelial cells, irrespective of
peptidase activity Petschow et al., 1993. In 4-week- animal age, and orogastric administration of EGF led
old rabbits, orogastric EGF treatment daily, starting 3 to hepatic and intestinal EGF receptor phosphoryla-
days prior to infection with enteropathogenic E . coli,
tion in newborns but not in adults Thompson et al., prevented the occurrence of diarrhoea and reduction
1994. The authors suggested that EGF in the of body weight gain, inhibited E
. coli colonization in suckling rat intestine may bind to the basolateral
the small and large intestine, improved jejunal membrane receptors because of the mucosal per-
maltase and sucrase activities, and reduced microvil- meability of the immature gut to macromolecules.
lus injury Buret et al., 1998. Okuyama et al. 1998 It has been shown in several studies that exogen-
found that newborn rabbit pups fed with milk ous EGF influences GI epithelial maturation and
formula had a high incidence of intestinal bacterial function. In 21-day-old newly weaned piglets, oral
translocation to mesenteric lymph nodes, liver and administration of EGF at 372 mg day increased
spleen compared with breast-fed newborn rabbits, jejunal lactase and sucrase specific activities by 77
and that subcutaneous administration of EGF at 1.5 and 97, respectively, after 3 days of treatment
m g g per day significantly reduced bacterial translo-
Jaeger et al., 1990. Subcutaneous administration of cation in artificially fed pups, which was associated
EGF at 60 mg kg per day to 3-day-old suckling with a significant increase in goblet cells in the small
piglets for 3 days increased the specific activities of intestinal mucosa.
sucrase and maltase in the middle and distal small intestine and reduced the specific activity of lactase
7.2. Insulin-like growth factors and insulin in the distal small intestine James et al., 1987.
Addition of EGF 0.5 mg l to the culture media Insulin-like growth factors are members of the
increased protein synthesis rate by 2-fold in jejunal insulin family, which consists of insulin, IGF-I, IGF-
explants from neonatal pigs Black and Ellinas, II and relaxin. Both IGF-I and IGF-II are single
1992. In addition, EGF inhibited porcine parietal chain polypeptides with 70 and 67 amino acid
cells in their uptake of aminopyrine, a process residues and molecular weights of about 7.6 and 7.5
involved in acid secretion Sjodin et al., 1992, and kDa, respectively. The primary structures of IGF-I
exogenous EGF inhibited gastric acid secretion in and IGF-II are highly conserved across species and
conscious dogs in vivo and acid secretion of isolated have identical sequences in pigs, humans and cattle.
rabbit gastric glands in vitro Konturek et al., 1984. The amino acid sequences of IGF-I and IGF-II share
In addition, exogenous EGF may aid in the about 70 structural homology and are about 40
recovery of traumatized gastric and intestinal tissues. homologous with insulin, the latter consisting of 51
Zijlstra et al. 1994 reported that supplementation of amino acids with a molecular weight of about 5.8
EGF at 0.5 or 1.0 mg l to milk replacer increased kDa.
villus height and lactase specific activity in a dose– Insulin-like growth factors exist in serum and milk
response fashion when fed to rotavirus-infected as high-molecular-weight complexes in association
R .J. Xu et al. Livestock Production Science 66 2000 95 –107
103
with one or more specific binding proteins called that study might have been due to the short treatment
IGF binding proteins, the latter acting as a reservoir period. In a subsequent report newborn pigs received
to protect IGFs from proteolysis, thereby prolonging formula containing 10–20 mg l of IGF-I 3.5 mg kg
their half-lives. Six types of IGF binding proteins per day for 4 days had an increased intestinal
have been identified in porcine milk Donovan et al., weight, and protein and DNA contents, and an
1994 and these binding proteins may modulate the increased villus height Burrin et al., 1996. New-
availability of the peptides to interact with their born pigs receiving a lower dose of IGF-I 0.5 mg l
target tissues. Recently, Elmlinger et al. 1999 in milk formula for 14 days had similar intestinal
found that human milk contains a protease which weight, length, and protein and DNA contents as
specifically cleaved the dominant type of IGF bind- those in control animals, but a significantly greater
ing protein in milk. The proteolysis occurred only in intestinal brush border enzyme activity and villus
the milk collected during the first 4 weeks of height Houle et al., 1997. Stimulatory effects of
lactation. It
was speculated
that the
specific oral IGF-I on GI tissue growth and epithelial matura-
proteolysis may increase biological availability of tion has also been observed in newborn rats Ma and
IGF in early milk and subsequently promote GI Xu, 1997; Staley et al., 1998 and calves Baum-
maturation in the suckling newborns. rucker et al., 1994.
Two types of IGF receptors type I and type II Milk-borne insulin may stimulate GI development
have been identified in the GI tract in pigs Schober in suckling animals via IGF type I receptors as well
et al., 1990; Morgan et al., 1996. The type I as directly through its own receptors. It has been
receptor, structurally homologous to the insulin reported that 2-day-old piglets bottle fed with milk-
receptor, binds IGF-I with greater affinity than IGF- based formula supplemented with 85 U l of porcine
II and binds insulin with the least affinity. The insulin for 6 days had a heavier small intestine,
receptor is a glycoprotein consisting of two extracel- greater intestinal mucosal weight, and protein and
lular a-subunits and two b-subunits. The type II RNA contents, and increased mucosal lactase and
receptor, a cation-independent mannose-6-phosphate maltase activities when compared with the control
receptor, consists of a single polypeptide chain animals Shulman, 1990. Studies in our own labora-
located almost entirely extracellularly; it binds IGF- tory also showed that newborn pigs bottle-fed with
II with greater affinity than IGF-I and does not bind milk formula containing 60 U l of insulin had
insulin. High levels of IGF-I binding were detected significantly higher levels of brush-border digestive
in the intestinal mucosa of newborn unsuckled enzyme activities than did the controls Fig. 3.
piglets and declined by about 40 at the third postnatal day due to uptake of IGF-I from ingested
7.3. Transforming growth factor-b milk Schober et al., 1990. Using an immuno-
histochemistry technique, Morgan et al. 1996 fur- Transforming growth factor-b is a dimeric protein
ther demonstrated that the type I receptors were with a molecular weight of 25 kDa. In mammals,
located on both apical and basolateral membranes of three isoforms of TGF-b b , b and b have been
1 2
3
the intestinal epithelium, suggesting that IGFs may identified; although highly homologous in structure
act on the intestinal mucosa either from the luminal and similar in biological activities in many bioassay
surface or via the blood circulation. systems, the isoforms are encoded by different genes
Several studies have shown that oral IGF-I stimu- and have different distributions and biological ac-
lates GI tissue growth and functional maturation in tions within the body Miyazono et al., 1993. TGF-
newborn animals. Newborn pigs bottle-fed for 24 h b
secreted from the producer cells is in a latent form with milk formula supplemented with 2 mg l of
of large molecular complexes, and thus requires IGF-I or IGF-II 0.44 mg kg had a mild increase in
activation for its biological actions. The latent TGF- pancreatic DNA content and cell proliferation in the
b complex can be activated in vitro by treatment
intestinal crypts compared with animals fed milk with extreme pH values, heating or urea Rogers et
formula only Xu et al., 1994. The lack of signifi- al., 1996.
cant effects of IGF treatment on GI tissue weight in Transforming growth factor-b has been detected in
104 R
.J. Xu et al. Livestock Production Science 66 2000 95 –107
Fig. 3. Activities of lactase, maltase, alkaline phosphatase and aminopeptidase in the small intestinal mucosa of newborn unsuckled piglets N and piglets bottle fed for 3 days with cow-milk formula M or cow-milk formula supplemented with insulin IM. Enzyme activity was
expressed as mmoles of substrate hydrolyzed per minute. Significant differences between the corresponding mean values in the newborn group and the bottle-fed groups are indicated by P , 0.05 or P , 0.01. Significant differences between the corresponding mean values in
the two bottle-fed groups are indicated by [P , 0.05 or [[P , 0.01 Wang and Xu, unpublished data.
bovine, porcine and human mammary secretions by the GI tract and can be recovered from the
Pakkanen and Aalto, 1997; Xu et al., 1999. Using a neonatal heart, lung and liver Letterio et al., 1994,
bioassay system, Xu et al. 1999 found that the suggesting possible actions at sites distant from the
concentration of TGF-b in porcine colostrum ranged GI tract. TGF-b-deficient mice remain physiological-
between 126 and 260 ng ml at the time of parturi- ly normal while on maternal milk but develop a
tion, and that the concentration declined to about 73 degenerative syndrome characterised by weight loss
ng ml 12 h after parturition. Most of the TGF-b and infiltration of inflammatory cells into the heart,
88 existed in the latent form which could be lungs and salivary glands soon after weaning Christ
activated by acid treatment pH 3 or lower. et al., 1994. It has also been proposed that TGF-b
The physiological significance of milk-borne TGF- may play a critical role in the repair of the mucosal
b is yet to be understood, but there is evidence
epithelium after injury Pakkanen and Aalto, 1997. suggesting its importance in neonatal development. It
has been shown that oral administration of human milk or recombinant TGF-b in neonatal mice strong-
8. Conclusion and implications
