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Postnatal adaptation of the gastrointestinal tract in neonatal
pigs: a possible role of milk-borne growth factors
a ,
*
a bR.J. Xu
, F. Wang , S.H. Zhang
a
Department of Zoology, The University of Hong Kong, Pokfulam Road, Hong Kong, China
b
Department of Human Physiology, Flinders University of South Australia, Bedford Park, SA, Australia
Abstract
During the postnatal period the gastrointestinal (GI) tract of the neonatal pig encounters numerous challenges and severe stress, particularly at the time of birth and weaning. In response to such challenges, the GI tract undergoes marked changes, including accelerated tissue growth and functional maturation during the immediate postnatal period, and intestinal villus atrophy and damage following weaning. The changes are apparently related to the onset of colostrum ingestion at the time of birth and the withdrawal of milk ingestion at the time of weaning. Porcine colostrum and milk contain not only highly digestible nutrients but also numerous bioactive compounds, including various types of growth factors, including epidermal growth factor, insulin-like growth factor-I (IGF-I), IGF-II, insulin and transforming growth factor-b. Experimental evidence summarized in the review indicates a regulatory role of these milk-borne growth factors in stimulating GI tissue growth and function maturation, and in enhancing repair of damaged GI mucosa in the suckling young. The findings imply that milk-borne growth factors may be used to enhance GI maturation in neonatal animals and be used to treat GI mucosal damages. Potential therapeutic applications of milk-borne growth factors in both agricultural animals and human infants warrant further investigation. 2000 Elsevier Science B.V. All rights reserved.
Keywords: Milk, Pig; Growth Factor; EGF; IGF; TGF-b
1. Introduction tract switches from handling a liquid nutritious diet,
milk, to solid food, often of plant origin. In response The gastrointestinal (GI) tract in neonatal animals to these challenges, the GI tract in healthy animals
encounters numerous challenges and severe physio- undergoes marked changes in structure and function
logical stresses during the postnatal period, particu- (Xu, 1996; Pluske et al., 1997). However, some
larly at the times of birth and weaning. At the time animals fail to adapt to these challenges, often
of birth, the GI tract is exposed, for the first time, to resulting in reduced growth, diarrhoea or death (Odle
nutrient dense colostrum as well as to various types et al., 1996).
of microorganisms. At the time of weaning, the GI The mechanisms involved in postnatal adaptation
of the GI tract are complex. Maternal milk apparent-ly plays an important role in facilitating postnatal
*Corresponding author. Tel.: 1852-2859-7014; fax: 1
852-adaptation of the GI tract in neonates. It has long
2559-9114.
E-mail address: [email protected] (R.J. Xu). been known that milk not only contains easily
Table 1
digestible nutrients but also bioactive compounds,
Changes in body weight (BW), stomach tissue weight (SW),
such as immunoglobulins and lysozymes, which
parietal cell volume density in the gastric glands (PCVD),
protect the GI tract from pathogenic microorganisms maximal acid output per unit gastric tissue mass (MAO), gastric (Xanthou, 1998). More recently, a number of growth acid secretory capacity (GASC), concentration of gastrin in antral
factors have been detected in maternal milk of tissue (TG), concentration of gastrin in plasma (PG), maximal proteolytic enzyme output per unit gastric tissue mass (MPO) and
various species (Xu, 1998). There is increasing
gastric proteolytic enzyme secretory capacity (GPSC) in neonatal
evidence showing that milk-borne growth factors can a
pigs during the immediate postnatal period
survive in the GI lumen of the suckling young, and
Birth Day 1 Day 3 Day 7
exogenous growth factors administered orally
stimu-late GI maturation in neonates (Odle et al., 1996; Xu, BW (kg) 1.34 1.44 1.65 2.10
SW (g) 5.7 7.2 8.7 11.1
1996). The biological significance of milk-borne
PCVD (%) 4.2 5.7 10.7 –
growth factors, however, remains to be fully
eluci-MAO (mM / g per h) 35 59 107 –
dated. Understanding the role of milk-borne growth GASC (mM / h) 198 422 932 –
factors in postnatal GI adaptation and their mecha- TG (pmol / g) 106 – – 459
nisms of action may help us to prevent and treat PG (fmol / ml) 44 – – 108
MPO (mg / g per h) 3.9 – – 29
many GI disorders in neonatal animals. This review
GPSC (mg / h) 22 – – 322
discusses the recent progress in the area of
milk-a
Data derived from Sangild et al. (1991), Xu and Cranwell
borne growth factors and regulation of postnatal
(1990, 1991) and Xu et al. (1992a).
adaptation of the GI tract in neonatal animals with a particular emphasis on neonatal piglets.
1990). The percentage of mucosal volume occupied by parietal cells and the number of parietal cells per
2. Adaptive changes in the GI tract after birth unit volume of gastric mucosa increased significantly during the same period (Xu et al., 1992a). Gastric Before birth, the foetus lives in a sterile environ- secretory capacity for protease enzymes increased by
ment and obtains all the nutrients from the maternal 9-fold during the first 7 postnatal days (Sangild et
circulation via the placenta. Immediately after birth, al., 1991). It was also found that the concentration of
the GI tract of a newborn has to take over the gastrin in the gastric antral tissue increased by 3-fold
responsibility of absorbing nutrients, and at the same and the concentration of circulating gastrin increased
time the animal is exposed to various types of by about 1.5-fold in newborn pigs during the
post-micro-organisms. To adapt to the functional demands natal 7 days after birth (Xu and Cranwell, 1991).
and changes in the environment, the GI tract of a
Table 2
newborn undergoes dramatic tissue growth and
Changes in small intestinal tissue weight (IW), length (IL) and
functional maturation (see review by Xu, 1996). diameter (ID), crypt depth, villus height, intestinal total activities Changes of the GI tract during the immediate of lactase, sucrase, maltase and aminopeptidase, and intestinal total glucose absorption capacity in neonatal pigs during the
postnatal period in pigs have been examined in a
a
immediate postnatal period
number of recent studies (Xu et al., 1992a,b; Zhang
et al., 1997) and the results are summarised in Tables Birth Day 1 Day 3
1 and 2. It was observed that the stomach tissue IW (g) 35.7 63.4 61.5
weight in newborn piglets increased 26 and 54%, IL (cm) 343 426 443
ID (mm) 3.85 4.44 4.60
respectively, by the first and third day after birth in
Crypt depth (mm) 82 102 115
comparison with 7.5 and 23% increments in body
Villus height (mm) 883 1171 1077
weight (Xu et al., 1992a). Associated with the tissue
Lactase (mM / min) 257 800 –
weight gain was a dramatic functional maturation. Sucrase (mM / min) 100 314 –
The gastric acid secretory capacity per unit of gastric Maltase (mM / min) 103 223 –
Aminopeptidase (mM / min) 137 251 –
tissue mass increased 2-fold during the first 3 days
Glucose (mM / min) 174 158 –
after birth, and the total gastric acid secretory
a
The small intestine also undergoes a dramatic weaning (940–694 mm), and then continued to
tissue growth and functional change during the decline to approximately 50% of the pre-weaning
immediate postnatal period (Table 2). It was reported value 5 days after weaning. Villus height reduction is that the small intestine in piglets increased up to 70% suspected to be the result of an increased rate of cell
in total tissue weight, 115% in mucosal tissue loss, which subsequently leads to increased crypt cell
weight, 24% in length, 15% in diameter, 24% in production and increased crypt depth (Pluske et al.,
crypt depth and 33% in villus height during the first 1997). Associated with the reduction in villus height
postnatal day (Xu et al., 1992b). During the same and the increase in crypt depth, the morphology of
period, the intestinal brush border digestive enzyme the villi also change from long finger-like projections
activity increased 80–200%, although the specific before weaning to leaf- or tongue-like structures after
activity per unit mucosal tissue mass did not change weaning (Cera et al., 1988). The structural changes
significantly (Zhang et al., 1997). The intestinal in the small intestine vary along the intestinal tract
absorption capacity remained unchanged during the and are affected by the age of weaning. The changes
first day of life, although the specific absorption in the pig weaned at 14 days of age are more
capacity per unit mucosal tissue mass decreased conspicuous than those weaned at 21 days of age,
during this period (Zhang et al., 1997). The decline and the reduction of villus height is more prominent
of specific absorption capacity was unlikely due to at the proximal region while the increase in crypt
losses of brush membrane transporters, but rather to depth is more prominent at the distal region of the
a dilution effect resulting from transient retention of intestine (Pluske et al., 1997).
colostral protein in the epithelial cells (Xu et al., Concomitant with the structural changes, there are
1992b). marked alterations in intestinal functions following
A distinct feature of the newborn small intestine is weaning. Hampson and Kidder (1986) reported rapid
its ability to absorb macromolecules and, during the reductions in the specific activities of lactase and
first day of life, a large quantity of colostral protein, sucrase during the first 4–5 days after weaning.
particularly immunoglobulins, are absorbed across Miller et al. (1986) reported that the specific
ac-the brush border membrane and a considerable tivities of sucrase, lactase and isomaltase fell by at
proportion of the protein is transiently retained in the least 50% during the first 5 days of after weaning in epithelial cells (Burrin et al., 1992; Xu et al., 1992b). pigs weaned at 28 or 42 days of age. On the other
The ability of macromolecule absorption diminishes hand, the activities of maltase and glucoamylase
within 2 days of postnatal life and the phenomena is increased in response to weaning (Kelly et al., 1991).
called gut closure (Westrom et al., 1984). Increases in these polysaccharidases are likely the
result of substrate induction. Several studies have shown that the decrease in villus height and the loss
3. Adaptive changes in the GI tract after of brush-border digestive enzyme activity after
weaning weaning coincided with a reduction in intestinal
absorption capacity for sugar and amino acids
(Ham-After weaning, the environment in the GI lumen pson and Smith, 1986; Miller et al., 1986; Nabuurs et
changes drastically with the replacement of highly al., 1994), although some other studies failed to
digestible milk by solid food, often of plant origin. detect a significant reduction in the ability of sugar
Associated with this, the GI tract in weaned pigs absorption (Kelly et al., 1991).
undergoes a marked change in structure and func-tion, particularly at the region of the small intestine
(Pluske et al., 1997). The most obvious changes in 4. Effects of colostrum and milk on GI function
the structure of the small intestine following weaning
are a reduction in villus height and an increase in The dramatic changes in the GI tract during the
crypt depth. Hampson (1986) reported that, follow- immediate postnatal period is apparently related to
ing weaning at 21 days after birth, villus height was the ingestion of colostrum while the changes during
to the withdrawal of milk. Laboratory studies have post-translational processing of intestinal lactase shown that tissue growth and functional changes in (Burrin et al., 1994). Kelly et al. (1993) reported that
the GI tract in newborn pigs would not occur if the colostrum feeding during the first 24 h of life in
animals were prevented from suckling (Widdowson newborn pigs enhanced intestinal epithelial cell
et al., 1976). Compared with piglets fed with water maturation, as indicated by an increased sucrase
or electrolyte solution, piglets fed with colostrum or activity and a decreased lactase activity at 1 week of
milk had a greater protein synthesis rate in the small age. A study in our own laboratory further showed
intestine and greater total mucosal brush border that colostrum feeding increased intestinal mucosal
lactase and maltase activities at 6–24 h after birth digestive enzyme activities and that pre-hydrolysis of
(Burrin et al., 1992, 1994; Zhang et al., 1998). colostrum with trypsin eliminated the stimulatory
Newborn piglets fed colostrum for 6 h showed an effect (Wang and Xu, 1996) (Fig. 1). The latter
increase in the relative abundance of pro-lactase finding suggests that colostrum contains
trypsin-protein and a decrease in the relative abundance of labile bioactive compounds which can modulate
the mature form of the enzyme in the intestinal intestinal function in neonates following oral
inges-mucosa, suggesting that feeding colostrum alters the tion. It has also been shown in both prenatal and
Fig. 1. Total lactase, maltase, alkaline phosphatase and aminopeptidase activities in the small intestinal mucosa of newborn unsuckled piglets (N) and piglets bottle fed for 3 days with either trypsinized porcine colostrum (TC) or intact porcine colostrum (C). Enzyme activity was expressed asmmoles of substrate hydrolyzed per minute. Significant differences between the corresponding mean values in bottle-fed groups and the newborn group are indicated by *P,0.05 or **P,0.01. Significant differences between the corresponding mean values in
[ [[
Table 3
postnatal pigs that feeding colostrum enhanced
in-Concentrations of EGF, IGF-I, IGF-II, insulin and TGF-b in
testinal macromolecule absorption and onset of gut
porcine colostrum and milk
closure (Westrom et al., 1985; Sangild et al., 1999).
a a
Colostrum Milk Reference
Factors affecting GI structure and function after
weaning are complex and multiple, possibly includ- EGF (ng / ml) |1500 150–250 Jaeger et al., 1987
b
5 NR Vaughan et al., 1992
ing pathogenic bacterial interactions, psychological
IGF-I (ng / ml) 70–350 4–14 Simmen et al., 1988
stress, transient hypersensitivity to food components
72–136 10–27 Donovan et al., 1994
and withdrawal of maternal milk (Pluske et al., c
541 NR Xu et al., 1996
1997). At the time of weaning, maternal milk is IGF-II (ng / ml) 165–291 11–50 Donovan et al., 1994
b b
withdrawn abruptly and supplementation of sows’ Insulin (ng / ml) 12.4 1.6–3.3 Jaeger et al., 1987
b b
10.8–18.2 1.2–5.5 Westrom et al., 1987
milk or colostrum to piglets during the first week
b c
16.5 NR Wang and Xu, 1996
after weaning can prevent villus atrophy, diarrhoea c
TGFib(ng / ml) 9–12 ND Xu et al., 1999
and growth retardation (Mosenthin, 1998; Nabuurs,
a
Colostrum as the mammary secretion during the first 3 days
1998).
after parturition and milk as the mammary secretion thereafter.
b
Converted to ng / ml from the original published values using conversion factors of 7.175 pmol / mU and 5743 g / mol for insulin 5. Growth factors in colostrum and milk and 6040 g / mol for EGF.
c
NR, not reported; ND, not detected.
From the above discussion, it is apparent that
colostrum and milk have a profound effect on GI higher in colostrum than in milk, and often greater
structure and function in neonatal animals. Such than the concentration in the maternal blood
circula-effects may be partially attributable to various types tion (Donovan et al., 1994). of growth factors in the milk. The topic of
milk-borne growth factors has been discussed in a number
of recent reviews (Odle et al., 1996; Xu, 1996, 1998; 6. Stability and absorption of milk-borne
Zabielski et al., 1999). The knowledge in this area growth factors in the GI lumen
has increased rapidly in recent years and new growth
factors in maternal milk are reported each year. To Following oral ingestion, milk-borne growth
fac-date, more than 20 types of hormones, growth factors tors may act directly on the GI tract or act on
or regulatory peptides have been identified in milk of peripheral targets following absorption into the blood various species (Xu, 1998). In porcine colostrum and circulation. A prerequisite for either of these actions,
milk, epidermal growth factor (EGF), insulin-like however, is survival in the GI lumen. Experimental
growth factor-I (IGF-I), IGF-II, insulin and trans- studies showed that the degradation rate of EGF was
forming growth factor-b (TGF-b) have been de- about 5 and 15% after incubation for 20 min in the
tected and the concentrations are presented in Table gastric contents of suckling and weaned pigs, respec-3. From these data, it can be seen that there are large tively, while the degradation of IGF-I was negligible
discrepancies among different reports. The dis- (Fig. 2). The specific receptor binding ability of both
crepancies may be due to different assay methods EGF and IGF-I did not change significantly after
used by different researchers. Sample collection time incubation in gastric contents (Shen and Xu, 1996,
may also be an important factor affecting the con- 2000a). When incubated with intestinal fluids of
centrations of growth factors in milk. For example, suckling and weaned pigs, the degradation of both
IGF-I concentration in colostrum collected at the EGF and IGF-I was much greater in weaned than in
time of parturition was about twice that in samples suckling pigs, and in suckling pigs the degradation
collected 1 day after parturition and about five times was greater in the distal than in the proximal region
that in samples collected 2 days after parturition (Fig. 2). After oral administration of iodine-labeled
(Donovan et al., 1994). Despite the discrepancies EGF to newborn and 5-day-old suckling pigs, over
among different reports, it is generally agreed that 60% of the radioactivity recovered from GI contents
proteolytic enzymes, was low in suckling pigs (San-gild et al., 1991), and that the secretion capacity remained low by the age of weaning (Cranwell, 1995). In neonatal pigs, the major proteases pro-duced by the stomach are pepsin B and chymosin, both having a strong milk-clotting activity but little proteolytic activity (Cranwell, 1995). In an acidic gastric lumen with the presence of pepsin, EGF may undergo C-terminal truncation. Playford et al. (1995) reported that human EGF was truncated to EGF( 1 – 49 ) and EGF( 1 – 46 ) forms in human gastric juice with a pH of less than 4. Rao (1995) also reported that mouse EGF was truncated to EGF( 1 – 52 ), EGF( 1 – 48 ) and EGF( 1 – 47 ) forms in the GI lumen of mice. Both studies, however, found that all these truncated forms of EGF possessed biological activity as demonstrated by their receptor binding ability.
The high stability of growth factors in the intesti-nal lumen of suckling pigs may reflect a limited luminal digestive capacity in neonatal animals (Cran-well, 1995). It has been shown that the activity of trypsin and chymotrypsin was low in the intestinal contents of suckling pigs when compared with that of weaned pigs (Shen and Xu, 1996). In addition, the high stability of EGF and IGF-I in the intestinal fluid
Fig. 2. Percentage degradation of EGF (upper panel) and IGF-I
of suckling pigs may also be partially attributable to
(lower panel) after incubation for 20 min in the contents of the
stomach (S), and the proximal (P), mid (M) and distal (D) the presence of milk in the fluid. It has been reported
sections of the intestine, of suckling and weaned pigs. Redrawn that porcine milk contains potent inhibitory activity from Shen and Xu (1996, 2000a).
against trypsin and chymotrypsin, and prevents EGF and IGF-I hydrolysis in pig intestinal fluids (Wes-precipitation characteristics to that of iodine-labeled trom et al., 1982; Shen and Xu, 1996, 2000a). These
EGF (Shen and Xu, 1998). The receptor binding findings suggest that colostrum, the natural carrier of
ability of the iodine-labeled compounds recovered milk-borne growth factors, may protect the peptides
from the gastric contents was almost 100% compar- from GI luminal digestion in the suckling young.
able with iodine-labeled EGF. The receptor binding Whether milk-borne growth factors can be
ab-ability remained high in the proximal small intestinal sorbed into the blood circulation in suckling pigs
lumen (40–58%) and it decreased to 15% in the following oral ingestion remains controversial. In
distal small intestinal lumen in newborn pigs (Shen newborn pigs, the gut epithelium is permeable to
and Xu, 1998). macromolecule transmission, thus allowing
absorp-The high survival rate of the growth factors in the tion of colostral immunoglobulins. The transmission
gastric lumen of suckling and weaned pigs may be is apparently non-selective as virtually all
macro-due to a limited secretion of gastric acid and protease molecules, such as bovine serum albumin and
dex-in those animals. It was reported that the pH value of tran, are absorbed when they come in contact with
gastric contents in both suckling and weaned pigs the intestinal mucosa (Westrom et al., 1984).
Cessa-was greater than 4 and no significant level of pepsin tion of such non-selective transmission (gut closure)
A activity was detected (Shen and Xu, 1996). It has occurs during the first 2 days after birth. However,
also been reported that the secretion capacity of transmission of certain macromolecules may
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
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 0.5mg / 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 mg / 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 mg / 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, 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
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-bis 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(b1,b2 andb3) have been
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 bsecreted 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).
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 asmmoles 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-bin neonatal mice strong- 8. Conclusion and implications
ly inhibited immunoresponse to an oral challenge
with sheep red blood cells (Ishizaka et al., 1994). In In this review we have described adaptive changes
nutrient-dependent factors stimulate protein synthesis in
colos-neonatal pigs during the postnatal period, particularly
trum-fed newborn pigs. Pediatr. Res. 37, 593–599.
at the times of birth and weaning. The changes are
Burrin, D.G., Wester, T.J., Davis, T.A., Amick, S., Heath, J., 1996.
apparently related to the onset of colostrum ingestion Orally administered IGF-I increases intestinal mucosal growth
at the time of birth and the withdrawal of milk in formula-fed neonatal pigs. Am. J. Physiol. 270, R1085–
R1091.
ingestion at the time of weaning. Porcine colostrum
Burrin, D.G., Davis, T.A., Fiorotto, M.L., Reeds, P.J., 1997. Role
and milk contain not only highly digestible nutrients
of milk-borne vs endogenous insulin-like growth factor I in
but also numerous bioactive compounds, including neonatal growth. J. Anim. Sci. 75, 2739–2743.
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IGF-II, insulin and TGF-b. Experimental evidence
Cera, K.R., Mahan, D.C., Cross, R.F., Reinhart, G.A., Whitmoyer,
presented in the review indicates a regulatory role for
R.E., 1988. Effect of age, weaning and postweaning diet on
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The findings imply that milk-borne growth factors
Karlsson, S., Wahl, S.M., 1994. Immune dysregulation in
TGF-may be used to enhance GI maturation in neonatal
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Potential therapeutic applications of milk-borne enzyme systems. In: Varley, M.A. (Ed.), The Neonatal Pig:
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