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2
Departments of
*
Animal Science and
Medical Physiology, Texas A&M University, College Station, TX 77843 and
**
Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261
2To whom correspondence should be addressed.
 |
ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-(prop-1-ynyl)estra-4,9-dien-3-one
(RU486, an antagonist of glucocorticoid receptors), or from age-matched
suckling pigs. Activities and mRNA levels for type I and type II
arginases, argininosuccinate synthase (ASS) and argininosuccinate lyase
(ASL) were determined. Activities of arginase, ASL and ASS increased by
635, 56 and 106%, respectively, in weanling pigs, compared with
suckling pigs. RU486 treatment attenuated the increase in arginase
activity by 74% and completely prevented the ASL induction in weanling
pigs, but had no effect on ASS activity. Pig intestine expresses both
type I and type II arginases. On the basis of immunoblot analyses,
there was no significant difference in levels of intestinal type I
arginase among these three groups of pigs, indicating that changes in
arginase activity were due only to type II arginase. The mRNA levels
for type II arginase and ASL increased by 135 and 198%, respectively,
in weanling pigs compared with suckling pigs, and this induction was
completely prevented by RU486. In contrast, ASS mRNA levels did not
differ between suckling and weanling pigs. These results suggest that
intestinal type II arginase, ASS and ASL are regulated differentially
at transcriptional and post-translational levels and that
glucocorticoids play a major role in the induction of type II arginase
and ASL mRNAs in the small intestine of weanling pigs.
KEY WORDS: • arginase • intestine • weaning • pigs
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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).
Because intestinal amino acid catabolism plays an important role in
modulating dietary amino acid availability to extraintestinal tissues,
it has important implications for the efficiency of utilization of
dietary protein and amino acids (Burrin and Reeds 1997
,
Stoll et al. 1998
, Wu 1998a
). Thus, there
is growing interest in intestinal amino acid metabolism in animals and
humans. We recently demonstrated that activities of arginase,
argininosuccinate synthase
(ASS)3
and argininosuccinate lyase (ASL) were increased in enterocytes of
postweaning pigs (Flynn and Wu 1997b
). Arginase
hydrolyzes arginine to ornithine plus urea, whereas ASS and ASL convert
citrulline to arginine (Wu and Morris 1998
). Induction
of arginase resulted in enhanced arginine catabolism to ornithine,
which was subsequently utilized for proline biosynthesis by the small
intestine (Wu et al. 1996
). This new knowledge helps to
explain why proline is an essential amino acid for neonatal pigs
(Ball et al. 1986
) but not for postweaning pigs
(Chung and Baker 1993
). Intestinal expression of
arginase, together with other urea cycle enzymes, also makes it
possible for the small intestine to detoxify extracellular and
mitochondrially generated ammonia by forming urea (Wu 1995
). Despite recent advances in our understanding of
intestinal arginine metabolism, little is known about its regulation at
the molecular level (Wu 1998a
, and 1998b
).
The weanling pig is an excellent model for studying the role of
glucocorticoids in regulating intestinal arginine metabolism. Weaning
is associated with increased plasma concentrations of cortisol
(hydrocortisone), the major circulating glucocorticoid in pigs
(Worsae and Schmidt 1980
) and humans (Ganong 1991
). Because glucocorticoids are potent regulators of hepatic
urea cycle enzymes (Morris 1992
) and of arginine
metabolism in enterocytes (Flynn and Wu 1997b
), we
hypothesized that glucocorticoids mediate the increased expression of
intestinal arginine metabolic enzymes during weaning. This hypothesis
was tested in this study by using
17-ß-hydroxy-11ß-(4-dimethylaminophenyl)17-(prop-1-ynyl)estra-4,9-dien-3-one
(RU486, mifepristone), a potent antagonist of glucocorticoid receptors
(Baulieu 1989
). Because changes in activities of hepatic
urea cycle enzymes generally correlate with changes in their mRNA
levels (Morris 1992
), we also hypothesized that mRNA
levels of arginase, ASS and ASL are increased in the small intestine of
postweaning pigs.
Another objective of this study was to determine the identity of the
arginase expressed in pig small intestine. There are two distinct
isoforms of mammalian arginase, which are encoded by separate genes
(Jenkinson et al. 1996
). Type I arginase, a cytosolic
enzyme, is normally expressed almost exclusively in the liver as a
component of the urea cycle (Morris 1992
;
Takiguchi and Mori 1995
). In contrast, type II arginase,
a mitochondrial enzyme, is widely expressed at low levels in
extrahepatic tissues ( Gotoh et al. 1996
, Morris et al. 1997
, Vockley et al. 1996
). In the pig
small intestine, there exist two anionic forms of arginase
(M'Rabet-Touil et al. 1996
), and arginase activity is
present in both the cytosol and mitochondria (Davis and Wu 1998
). However, it has not been determined whether intestinal
arginase is comprised of only type I or type II arginase or both. We
have addressed this question by use of isoform-specific arginase cDNA
probes and antibody.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Chemicals.
HPLC-grade water and methanol were obtained from Fisher Scientific (Houston, TX). RU486 was provided by Research Biochemicals International as part of the Chemical Synthesis Program of the National Institute of Mental Health, Contract N01MH3 0003. Sesame oil, aprotinin, pepstatin A, chymostatin, phenylmethylsulfonyl fluoride, HEPES, Tris-HCl, EGTA, EDTA, Triton X-100, ATP, sucrose, dithiothreitol, L-amino acids, o-phthaldialdehyde and argininosuccinate were purchased from Sigma Chemicals (St. Louis, MO).
Animals.
Pigs were offspring of Yorkshire x Landrace sows and Duroc x Hampshire boars and housed in the Swine Center of Texas A&M
University. Piglets were freely nursed by their mothers. At 21 d
of age, 18 pigs were randomly assigned to one of three groups with six
pigs each. One group was allowed to continue to nurse freely (suckling
pigs), whereas the other two groups were weaned to a corn and soybean
meal–based diet that met NRC recommended requirements for nutrients
(Flynn and Wu 1997b
). Weanling pigs received
intramuscular injections of vehicle solvent (sesame oil) or RU486 (10
mg/kg body wt) 5 min before weaning and 24 and 72 h later. This
period of RU486 administration corresponded to the surge of cortisol in
weanling pigs (Borbolla 1994
). The dose of RU486 used
was based on previous in vivo studies with a number of species,
including rats, humans, guinea pigs (Baulieu 1989
) and
piglets (Flynn and Wu 1997a
). At 29 d of age, pigs
were anesthetized and killed by jugular puncture for the isolation of
jejunum, as previously described (Flynn and Wu 1997a
).
The jejunum was washed three times with saline to remove luminal
content and then stored at -80°C.
Determination of mRNA levels for arginase, ASS and ASL.
Total RNA was extracted from the jejunum (~500 mg) as described by
Chirgwin et al. (1979)
with modifications described by Puissant and Houdebine (1990)
. Total RNA was separated on a 1.1%
agarose-formaldehyde gel by electrophoresis, followed by transfer onto
a positively charged nylon membrane using transfer reagent from Biotecx
(Houston, TX). 32P-Labeled cDNA probes for rat
type I arginase (Kawamoto et al. 1987
), human type II
arginase (Morris et al. 1997
), rat ASS (Morris et al. 1989
) and rat ASL (Lambert et al. 1986
) were
generated using a Decaprime II kit from Ambion (Houston, TX). Mouse
ß-actin oligonucleotide probe was 5'-end labeled with
[
-32P]ATP using T4 polynucleotide kinase
(Oncogene Science, Uniondale, NY). Hybridization of RNA with denatured
probe was performed at 42°C for 16 h according to the
manufacturer's protocol (5 Prime
3 Prime, Boulder, CO).
Autoradiographs were subsequently obtained by exposure at -70°C for
8 h (ASS), 16 h (type I and type II arginases) and 60 h
(ASL). Autoradiograms were scanned using a Hewlett-Packard 4C Scanjet
(Palo Alto, CA) and analyzed by the BioImage Software IQ
system (Ann Harbor, MI). Band intensities were normalized on the basis
of the intensity of intestinal ß-actin mRNA levels. Relative
abundance of mRNA levels for enzymes was calculated by dividing
intensities of mRNA signals for enzymes by those for ß-actin
(arbitrary densitometry units). Values of enzyme mRNA/ß-actin
mRNA for suckling pigs were normalized to 1.00.
Determination of activities of arginase, ASS and ASL.
The jejunum (~500 mg) was homogenized in 4 mL of ice-cold medium
(0.33 mol/L sucrose, 5 mmol/L HEPES, 1 mmol/L EGTA, 1 mmol/L
dithiothreitol and 0.5% Triton X-100; pH 7.4) containing protease
inhibitors (5 mg/L aprotinin, 5 mg/L pepstatin A, 5 mg/L chymostatin
and 5 mg/L phenylmethylsulfonyl fluoride) (Wu 1997
,
Wu and Knabe 1995
). The homogenates were centrifuged at
3000 x g for 15 min, and the supernatant fluid was
used for enzyme assays. The activities of arginase, ASS and ASL were
determined at two levels of tissue protein concentration and three time
points to ensure that enzyme assays were linear with respect to protein
and time, as previously described (Wu 1995
). Briefly,
the arginase assay mixture (300 µL) consisted of 20 mmol/L arginine,
3.3 mmol/L MnCl2, 50 mmol/L Tris-HCl buffer (pH
7.5) and tissue extracts (0.5 and 1.0 mg protein), and was incubated at
37°C for 0, 10 or 20 min. The ASS assay mixture (200 µL) consisted
of 75 mmol/L potassium phosphate buffer (pH 7.5), 10 mmol/L citrulline,
5 mmol/L aspartate, 5 mmol/L ATP, 5 mmol/L MgSO4
and tissue extracts (0.5 and 1.0 mg protein), and was incubated at
37°C for 0, 10 and 20 min. The ASL assay mixture (200 µL) contained
129 mmol/L sodium phosphate buffer (pH 7.0), 10 mmol/L
argininosuccinate, 65 mmol/L EDTA and tissue extracts (0.5 and 1.0 mg
protein), and was incubated at 37°C for 0, 10 and 20 min. For all
enzyme assays, reactions were terminated by addition of 100 µL of 1.5
mol/L HClO4, and acidified medium was neutralized
with 50 µL of 2 mol/L
K2CO3. Neutralized extracts
were used for analysis of ornithine (for arginase), argininosuccinate
plus arginine (for ASS) and arginine (for ASL) by an HPLC method
involving precolumn derivatization with o-phthaldialdehyde
(Wu et al. 1996
).
Immunoblot analysis of type I arginase.
Preparation of tissue homogenates, immunoblot procedures and reagents
was exactly as described by Morris et al. (1998)
.
Statistical analysis.
Results were analyzed by one-way ANOVA, with the Student-Newman-Keuls
test for multiple comparison of means (Steel and Torrie
1980
). Probability < 0.05 was taken to indicate
statistical significance.
|
RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The activities of arginase, ASS and ASL were 635, 106 and 56% greater
(P < 0.05), respectively, in the jejunum of weanling
pigs compared with suckling pigs (Table 1). RU486 treatment attenuated (P < 0.05) the
increase in arginase activity by 74%, completely prevented
(P < 0.05) the induction of ASL, but had no effect
(P > 0.05) on ASS activity in weanling pigs.
|
and Table 2). The mRNA levels for type II arginase and ASL increased
(P < 0.05) by 135 and 198%, respectively, in
the jejunum of weanling pigs compared with age-matched suckling pigs.
RU486 not only prevented (P < 0.05) the induction of
type II arginase and ASL mRNAs in weanling pigs but actually decreased
(P < 0.05) ASL mRNAs to levels below those found in
suckling pigs, suggesting that glucocorticoids may be important for the
maintenance or induction of additional stimuli that induce ASL mRNAs in
the jejunum. No difference in intestinal ASS mRNA levels was detected
between weanling and suckling pigs (P > 0.05), and
RU486 treatment had no effect (P > 0.05) on the level
of this mRNA.
|
|
). These analyses clearly demonstrated the presence of type
I arginase in the jejunum of suckling and weanling pigs (Fig. 2
). Although there were minor sample-to-sample variations within
each group, there were no consistent differences in levels of type I
arginase among these three groups of pigs (Fig. 2)
.
|
|
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Weaning resulted in increases in both total
arginase activity and mRNA levels for type II arginase, whereas type I
arginase expression remained unchanged. However, total arginase
activity increased to a greater extent than did mRNA levels for type II
arginase (Tables 1 and
2)
. In contrast, intestinal ASL mRNA level
increased to a greater extent than ASL activity in postweaning pigs
(Tables 1 and
2)
. These results suggest that in weanling pigs, type II
arginase expression was regulated at both transcriptional and
translational or post-translational levels, whereas ASL expression was
regulated primarily at the transcriptional level. In contrast to
arginase and ASL, intestinal ASS activity increased, but its mRNA
levels did not change in postweaning pigs, suggesting that the
weaning-associated increase in intestinal ASS expression was regulated
primarily at the translational or post-translational level.
RU486, a glucocorticoid receptor antagonist, attenuated or completely
prevented the weaning-associated increases in the activities of
intestinal arginase and ASL, respectively (Table 1)
, as previously
shown for arginase activity in enterocytes (Flynn and Wu 1997b
). A novel finding of this study is that RU486
administration completely prevented increases in mRNA levels for
arginase and ASL in intestine of weanling pigs (Table 2)
. These results
suggest that glucocorticoids play an important role in regulating
intestinal expression of type II arginase and ASL in the pig small
intestine, as for other intestinal enzymes (Henning 1981
). Our finding that RU486 could not completely prevent the
weaning-associated increase in intestinal arginase activity suggests
that other factors may play a role in the induction of this enzyme
during weaning. We have previously suggested that thyroid hormone may
be one of these factors (Flynn and Wu 1997b
). In
contrast to their effects on arginase and ASL, glucocorticoids do not
appear to mediate increased ASS expression in the pig small intestine
during weaning, and the mechanism involved remains to be elucidated.
There are two differently charged species of arginase in the small
intestine of rats (Herzfeld and Raper 1976
) and pigs
(M'Rabet-Touil et al. 1996
). We recently reported that
arginase activities were present in both the cytosol and mitochondria
of pig enterocytes (Davis and Wu 1998
). Northern blot
analysis revealed the presence of mRNA for type II arginase in the pig
jejunum, but mRNA for type I arginase was not detectable, as reported
also for the rat small intestine (De Jonge et al. 1998
).
The failure to detect type I arginase mRNA in the pig jejunum, however,
was due to an inability of the rat type I arginase cDNA to hybridize
efficiently with the corresponding porcine mRNA because this cDNA probe
also was not efficient in detecting mRNA for type I arginase in total
RNA from pig liver (S. M. Morris, Jr., unpublished observations).
However, immunoblot analysis demonstrated the presence of type I
arginase protein in the jejunum of both suckling and weanling pigs
(Fig. 2)
. Unlike the effects on mRNA levels for type II arginase,
weaning had no effect on levels of type I arginase in the pig small
intestine (Fig. 2)
. Thus, the weaning-associated increase in jejunal
arginase activity was due to the induction of type II arginase. Our
recent findings that arginase activities in both cytosol and
mitochondria are markedly increased in the small intestine of weanling
pigs (Davis and Wu 1998
) suggest the presence of type II
arginase in both compartments. Both cytosolic and mitochondrial
arginases of pig enterocytes have similar kinetics when assayed at
37°C but differ in their sensitivity to heat inactivation
(Davis and Wu 1998
), suggesting a difference in
post-translational modifications. The induction of mitochondrial type
II arginase probably contributes to the ability to synthesize proline
from arginine in the small intestine of postweaning pigs (Wu et al. 1996
). We also have speculated that cytosolic arginase may
regulate the intestinal synthesis of polyamines and nitric oxide by
providing ornithine for ornithine decarboxylase and by limiting
availability of arginine for nitric oxide synthase (Davis and Wu 1998
).
In summary, our results indicated that porcine intestinal arginase consists of both type I and type II isoforms. Weaning enhanced expression of type II arginase but had no effect on type I arginase in the pig small intestine. The results of this study suggest that the expression of intestinal type II arginase, ASL and ASS is regulated differentially at transcriptional and translational levels during weaning. Glucocorticoids play a major role in the induction of type II arginase and ASL mRNAs in the small intestine of postweaning pigs.
|
ACKNOWLEDGMENTS |
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|
FOOTNOTES |
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2 Abbreviations used: ASL, argininosuccinate
lyase; ASS, argininosuccinate synthase; RU486 (mifepristone),
17-ß-hydroxy-11ß-(4-dimethylaminophenyl)17-(prop-1-ynyl)estra-4,9-dien-3-one.
Manuscript received July 30, 1998. Initial review completed September 10, 1998. Revision accepted December 15, 1998.
|
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