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Department of Nutrition, School of Food and Nutritional Sciences, The University of Shizuoka, Shizuoka 422-8526, Japan
2 To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • lactase-phlorizin hydrolase • sucrase-isomaltase • gene expression • villus • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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,
Quaroni and Isselbacher 1985
). In situ hybridization
analysis of rat jejunum revealed that SI mRNA levels were maximal in
lower and mid-villus cells with lower levels observed in the villus
tip (Traber 1990
). This is consistent with the report of
Riby and Kretchmer (1984)
who demonstrated that the rate
of synthesis of SI was maximal in the lower villus cells. LPH mRNA
levels were also detected in the villus cells of rat jejunum
(Duluc et al. 1993
). However, it is unclear whether
these two genes are concomitantly expressed in the absorptive cells at
a specific locus along the villus-crypt axis, while they
differentiate and/or mature. It was shown by a cryostat sectioning
technique that the distribution of lactase activity along the
villus-crypt columns of rat jejunum was localized more apically
than that of sucrase activity (Boyle et al. 1980
).
We previously showed that LPH mRNA levels (Goda et al. 1995
) as well as SI mRNA levels (Yasutake et al. 1995
) are elevated in rats fed a sucrose diet. These
diet-induced increases in LPH mRNA and SI mRNA levels occur within
12 h. In this study, we employed the cryostat slicing technique to
quantitatively determine the distribution of LPH mRNA and SI mRNA
levels along the villus-crypt axis of the jejunum of rats which
were force-fed either a low (5% as energy)-carbohydrate diet or
40% (as energy) sucrose diet.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Six-week-old male rats of Sprague-Dawley strain (Japan SLC,
Hamamatsu, Japan) had free access to a standard nonpurified diet (MF;
Oriental Yeast, Tokyo, Japan) and water until the experiment began. To
examine the basic localization of LPH mRNA and SI mRNA along the
villus-crypt columns, rats were killed at 1000 h. To
investigate the diet-induced changes in the gene expression of LPH
and SI, rats were fed a low-carbohydrate (5% energy as
cornstarch), high-fat (73% energy as corn oil) diet (Goda et al. 1995
) for 7 d, and they were subsequently
force-fed the low-carbohydrate, high-fat diet, or a diet
containing sucrose (40% energy as sucrose, 37% energy as corn oil,
22% energy as casein) for 3 h or 6 h via an
orogastric plastic tubing as described previously (Goda et al. 1985
, Kishi et al. 1999
). Force-feeding was
performed twice in the 6-h experiment at 1000 and 1300 h, and once
in the 3-h experiment at 1300 h. The rats were killed by
decapitation at 1600 h to prepare them uniformly for the
circadian rhythm of intestinal disaccharidases. Another experiment was
designed to investigate whether actinomycin D, an antibiotic which
inhibits transcription of genes, was able to abolish the
sucrose-induced increases in LPH and SI mRNA levels. The rats were
fed the low-carbohydrate diet for 7 d, and they were injected
intraperitoneally with either actinomycin D (50 µg/kg body weight) in
0.154 mol/L of NaCl or 0.154 mol/L of NaCl only (vehicle) at 30 min
prior to the force-feeding of the low-carbohydrate diet or the
sucrose diet. Force-feeding was repeated 3 h later, and all
animals were killed at 6 h after the initial force-feeding.
The experimental procedures used in the present study met the
guidelines of the animal usage committee of the University of Shizuoka.
Tissue Preparation
The entire small intestine was removed, and the duodenum
extending from the pylorus to the ligament of Treitz was discarded. The
jejunoileum was divided into three segments of equal length. The
proximal one-third (jejunum) of the jejunoileum was flushed with
diethylpyrocarbonate (DEPC)-treated ice-cold 0.154 mol/L of NaCl
solution. A 1-cm segment (100 mg) was excised from the middle region of
the jejunal segment, and total RNA was extracted from the jejunal
tissue. An adjacent segment (2 cm) was opened longitudinally and
flattened on a glass slide, serosal-side down, and then it was
frozen immediately in liquid nitrogen. The tissue was immediately
subjected to cryostat sectioning. A frozen tissue block of
approximately 7 x 7 mm2 was covered with a supporting
medium (OCT Compound, Miles Laboratories, Elkhart, IN) and then
transferred serosal-side down to a preflattened supporting surface
of 1 g/L of agar within a cryostat at -18°C. The tissue was then
sectioned transversely at 10-µm thickness through the submucosa into
the muscular layer as described previously (Goda et al. 1983
). At various depths in the villus-crypt unit, a
section was attached to a microscope slide for inspection of the
presence of villus and crypt architecture after staining with 3 g/L of
methylene blue.
To determine the LPH and SI mRNA levels along the villus-crypt columns, 20 consecutive sections were combined and homogenized in 200 µL of 25 mmol/L of sodium citrate buffer (pH 7.0) containing 4 mol/L of guanidine thiocyanate, 17 mmol/L of sodium N-lauroylsarcosine, and 13 mmol/L of mercaptoethanol using a Polytron homogenizer at 15,000 rpm for 15 s. The homogenate was immediately subjected to RNA extraction.
To assay the activities of lactase and sucrase and the immunoreactive amounts of LPH and SI at different heights of the villus-crypt units, the tissue blocks were sectioned as described above, and 10 consecutive sections were combined and homogenized in 1 mL of 10 mmol/L of potassium phosphate buffer (pH 7.0). This homogenate preparation was used for assays of lactase and sucrase activities. An aliquot (400 µL) of the homogenate was treated with 10 g/L of Triton X-100 at 4°C for 90 min and centrifuged at 105,000x g at 4°C for 60 min. The Triton-solubilized supernatant was used for the assays of immunoreactive LPH and SI.
RNA extraction and RNA blot hybridization
Total RNA was extracted as described by Chomczynski and Sacchi (1987)
. For dot-blot analysis, aliquots (600 ng) of
total RNA were denatured with 2.2 mol/L of formaldehyde and spotted on
a nylon membrane (Hybond-N+; Amersham, Arlington Heights,
IL) using a dot-blot manifold (Immunodot; Atto, Tokyo, Japan). The
filters were treated with 0.05 mol/L of NaOH for 5 min, prehybridized
for 2 h in a solution containing 500 g/L of deionized formamide, 5
x sodium chloride/sodium phosphate buffer containing
ethylenediaminetetraacetic acid (SSPE) [1 x SSPE
= 0.18 mol/L of NaCl, 0.01 mol/L of sodium phosphate, 1 mmol/L of
EDTA (pH 7.7)], 5 x Denhardt's solution (1 g/L of Ficoll, 1 g/L
of bovine serum albumin, 1 g/L of polyvinylpyrrolidone), 5 g/L of SDS
at 42°C. The hybridization buffer consisted of the above buffer plus
20 mg/L of heat-denatured salmon sperm DNA and
32P-labeled probes. The cDNA probes for rat LPH and rat SI
were prepared and labeled with [
-32P] dCTP as
described previously (Goda et al. 1985
, Yasutake et al. 1995
). After hybridization (16 h at 42°C), the
membranes were washed twice with 2 x SSPE, 1 g/L of SDS at 60°C
for 15 min, once with 1 x SSPE, 1 g/L of SDS at 60°C for 30
min, and twice with 0.1 x SSPE, 1 g/L of SDS at room temperature
for 15 min. The washed membranes were exposed to an image plate (Fuji
Film, Tokyo, Japan) for 4 h at room temperature and analyzed with
an image analyzer (BAS 2000; Fuji Film). Control hybridizations were
carried out for 28S rRNA.
Enzyme and immunological assays
Lactase and sucrase activities were assayed according to the
method of Dahlqvist (1964)
. The lactase assay mixture
contained p-hydroxymercuribenzoate (Aldrich, Milwaukee, WI)
to inhibit any residual lysosomal acid ß-galactosidase activity
(Koldovsk
et al. 1969
). Protein was determined
according to the method of Lowry et al. (1951)
.
Immunoreactive LPH and SI were quantified by a sandwich-type
enzyme-linked immunosorbent assay using monoclonal antibodies
(YBB2-61/4/1 for LPH and BBC-35/11/2 for SI) as captive antibodies as
described previously (Goda et al. 1988
and
Goda et al. 1995
).
Statistical Analysis
All results were subjected to one way ANOVA. Differences in mean values at the same height of the villus-crypt columns among groups were tested using Tukey's multiple range test and were considered significant at P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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To quantitatively compare the expression of LPH and SI along the
villus-crypt axis of the jejunum, jejunal segments were
fractionated along the villus-crypt axis using a cryostat
sectioning technique, and the immunoreactive amounts of lactase and SI
as well as lactase and sucrase activities were determined in each
fraction at various heights of the villus-crypt columns. As shown
in Figure 1,
both lactase and sucrase activities were very low in crypts, and they
were gradually elevated from the base of the villi to the
mid-villus region. Lactase activity reached a maximal level at the
75% height of the villus-crypt columns, whereas maximal sucrase
activity was attained around the 55–65% heights of the
villus-crypt columns. Both sucrase and lactase activities decreased
toward the top of the villi. Thus, lactase activity was localized more
apically along the villus-crypt axis than sucrase activity (Fig. 1)
. The distribution of immunoreactive LPH protein along the
villus-crypt axis was almost identical to that of lactase activity;
the LPH protein was barely detected in crypts and gradually accumulated
from the base of the villi reaching a plateau at the 75% height of the
villus-crypt columns (Fig. 2
).The distribution of immunoreactive SI protein along the
villus-crypt axis was also similar to that of sucrase activity,
showing a maximal expression at the 55–65% heights of the
villus-crypt columns. Thus, not only the enzymatic activity but
also the immunoreactive amounts of LPH was localized more apically
along the villus-crypt axis than those of SI (Fig. 2)
.
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To examine whether the more apical localization of LPH protein
along the villus-crypt axis than SI protein was attributable to a
distinct villus-crypt distribution of LPH mRNA compared to that of
SI mRNA, we extracted total RNA from each fraction at various heights
of the villus-crypt columns and quantified the LPH mRNA and SI mRNA
by RNA-blot hybridization. The dot-blot analysis showed that
relatively large amounts of LPH mRNA accumulated in the mid- and
upper-villus regions, whereas SI mRNA was abundant in the
lower-villus (Fig. 3
A).To quantitatively compare the villus-crypt distributions of LPH
mRNA and SI mRNA, these mRNA levels were normalized for 28S rRNA. The
relative LPH mRNA abundance was gradually elevated from the base of the
villi and reached a maximal level only at the upper-villus (Fig. 3B)
. By contrast, SI mRNA level was abruptly elevated at the base of
the villi, reaching a maximal level at the lower-villus, and it
gradually declined toward the top of the villi (Fig. 3B)
.
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The rats force-fed the sucrose diet had a significantly greater LPH mRNA level in the jejunal segment than those fed the low-carbohydrate diet both at 3 h (87%, P < 0.01) and at 6 h (72%, P < 0.01) after the force-feeding (data not shown). The rats force-fed the sucrose diet also had more SI mRNA in the jejunal segment than those fed the low-carbohydrate diet both at 3 h (47%, P < 0.01) and at 6 h (55%, P < 0.01) after the force-feeding (data not shown).
To determine at which locus of the villus-crypt axis the LPH mRNA
and SI mRNA levels were elevated following the force-feeding of the
sucrose diet, RNA-blot hybridizations were performed using the
total RNA extracted from the homogenates of the cryostat sections
collected at various heights of the villus-crypt columns. The LPH
and SI mRNA levels at any height of the villus-crypt columns did
not differ between rats force-fed the low-carbohydrate diet
during the last 3 h and those force-fed during the last 6 h. Therefore, these two groups of rats force-fed the
low-carbohydrate diet were pooled and served as a control. In the
control, LPH mRNA level gradually increased from the lower- to
upper-villus regions, whereas SI mRNA level peaked at the
mid-villus, decreasing both in the lower- and the upper-villus
(Fig. 4
).Force-feeding the sucrose diet elevated the LPH mRNA level in
almost the entire villus except for the very top of the villus in rats
force-fed for 3 h (Fig. 4)
. The LPH mRNA level of rats
force-fed the sucrose diet during the last 6 h did not differ
from the level of rats force-fed the sucrose diet during the last
3 h in any region of the villus and were higher than in the
control throughout the villus except at the very top (Fig. 4)
.
|
. Effect of actinomycin D on the diet-induced increases in LPH mRNA and SI mRNA levels
To examine whether transcriptional control is involved in
the diet-induced increases in the accumulation of LPH and SI mRNA,
rats were injected intraperitoneally with actinomycin D before
force-feeding the low-carbohydrate and the sucrose diets. The
injection of neither vehicle nor actinomycin D affected the
distributions of LPH and SI mRNA along the villus-crypt axis in the
control. In the rats treated with 0.154 mol/L of NaCl (vehicle),
significantly greater LPH mRNA levels were detected at 35–85% heights
of the villus-crypt columns in rats force-fed the sucrose diet
than in the control, whereas significantly greater SI mRNA levels were
observed at 15–75% heights of the villus-crypt columns in those
force-fed the sucrose diet than in the control (data not shown). In
the groups of rats injected with actinomycin D, the LPH mRNA levels of
rats force-fed the sucrose diet did not differ from those in the
control at any height of the villus-crypt columns; the SI mRNA
levels did not differ between the groups at any height of the
villus-crypt columns, either (Fig. 5
).
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|
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Traber
1980
), may have a potential problem in admixture of the
cells of various heights of the villus as was suggested by
Hoffman and Chang (1993)
. This study extends the finding
of Boyle et al. (1980)
who demonstrated that lactase
activity was localized more apically along the villus-crypt axis
than sucrase activity in adult rat jejunum. We showed, using a
sensitive enzyme-linked immunosorbent assay, that the difference in
the distribution of enzyme activity was attributable to that in the
distribution of immunoreactive proteins, viz, maximal LPH-protein
content was seen in more apical region of the villus than that of SI
protein (Fig. 2)
. The RNA-blot hybridization of the RNA extracted
from the tissue slices which were obtained at various heights of
villus-crypt axis enabled us to directly compare the distribution
of LPH mRNA and SI mRNA. This analysis revealed a distinct mRNA
distribution along the villus-crypt axis between the two
enterocyte-specific genes.
The observation that both SI mRNA and LPH mRNA levels are minimal in
crypt regions, but they are abruptly elevated at the villus-crypt
junction is consistent with the previous in situ hybridization studies
(Duluc et al. 1993
, Markowitz et al. 1993
, Traber 1990
, Traber et al.
1992b
). Many other gene products that are characteristic of the
differentiated enterocyte are also known to be abruptly expressed at
the villus-crypt junction; these include the sodium-glucose
cotransporter 1 (Hwang et al. 1991
), GLUT5, the fructose
transporter (Rand et al. 1993
), aminopeptidase N
(Norén et al. 1989
), and liver fatty
acid-binding protein (Iseki et al. 1990
). Although
direct evidence for the involvement of transcriptional control at this
critical junction has not been obtained, the abrupt increase in mRNA
levels of a variety of enterocyte-specific genes suggest that
transcriptional induction of multiple genes should take place in this
region.
Transgenic studies showed that the enterocyte-specific genes
including intestinal fatty acid-binding protein (Cohn et al. 1992
), liver fatty acid-binding protein (Simon et al. 1993
), and SI (Traber and Silberg 1996
) have
short promoter regions immediately upstream of the start of
transcription capable of directing transcription of reporter genes
specifically to intestinal epithelial cells. Traber and colleagues
identified several transcriptionally functional elements in the short
promoter region of the SI gene (Traber et al. 1992a
,
Wu et al. 1992
, Wu et al. 1994
). Among
these elements, a 22-base region immediately upstream of the putative
TATA box, designated SIF1, was found to be a potent activator of SI
promoter, which binds to a caudal-related homeodomain-containing
protein, Cdx-2 (Suh et al. 1994
). The expression of
Cdx-2 was shown to be restricted to intestinal epithelium in adult mice
(James and Kazenwadel 1991
, Suh et al. 1994
). In this regard, it should be noted that the promoter of
LPH located immediately upstream of the start of transcription, which
carries the information required for both enterocyte-specific
expression and the post-weaning down-regulation (Troelsen et al. 1994a
), contains a cis-element capable of
binding not only a nuclear factor (Troelsen et al. 1992
), but Cdx-2 as well (Troelsen et al. 1994b
). Recently, we examined the distribution of Cdx-2 mRNA
along the villus-crypt axis of rat jejunum and found that Cdx-2
mRNA was expressed in both crypts and villus, with relatively even
distribution along the villus-crypt axis (Tanaka, Takase and Goda,
unpublished work). Thus, more studies are required to determine whether
Cdx-2 is associated with the abrupt induction of
enterocyte-specific genes at the villus-crypt junction.
The observation that SI mRNA levels decline at the upper half of the
villus-crypt columns indicates that the activation of the SI gene
transcription in the villus cells is temporal. This implicates the
presence of a second regulatory mechanism leading to a decrease in SI
gene expression which would be operative at upper regions of the
villus. It is likely that the decrease in SI mRNA level at the upper
villus is caused by cessation of transcription of SI gene. Along with
SI, several genes including liver fatty acid-binding protein
(Iseki et al. 1990
) and aminopeptidase N
(Norén et al. 1989
) were shown to exhibit
maximal accumulation of mRNAs at lower- to mid-villus regions,
followed by a marked decrease in levels at the top portions of the
villus. Thus it appears that the decrease in mRNA levels in the upper
villus cells is a common feature of most enterocyte-specific genes,
which may indicate the progress of cellular senescence, leading to
apoptosis or programmed cell death (Gavrieli et al. 1992
). However, we found in this study that, unlike SI mRNA,
LPH mRNA level did not reach a plateau until apical portions of the
villus. While this may well explain the more apical localization of LPH
protein along with lactase activity than SI protein and sucrase
activity, this observation suggests that there are the other types of
enterocyte-specific genes which are expressed persistently as the
enterocyte migrates up to the villus tip. Persistence of high levels of
mRNA in upper villus cells was reported for sodium-glucose
cotransporter-1 and the glucose transporter 2 (GLUT2) in rabbit jejunum
(Hwang et al. 1991
). Thus we speculate that there are at
least two groups of enterocyte-specific genes which exhibit
distinct mRNA distribution along the villus-crypt axis. Our data
also indicate that lactase activity and the amount of immunoreactive
LPH protein decline at the apical regions of the villus, despite
persistence of high levels of LPH mRNA. This may suggest that upper
villus cells are characteristic of reduced translation efficiency of
LPH transcript or elevated degradation of LPH protein or both.
The present study extends our previous studies which showed that
dietary sucrose elicited an enhancement of the levels of SI mRNA
(Yasutake et al. 1995
) as well as LPH mRNA (Goda et al. 1995
) in rat jejunum. The sucrose-induced increase
in these transcripts becomes prominent in as early as 3 h (Fig. 4)
, suggesting a rapid accumulation of these transcripts. The cryostat
sectioning technique enabled us to determine the precise locus of
villus cells where these rapid mRNA accumulations took place. This
study demonstrated that the initial rise in these mRNA levels was most
prominent in the cells which were accumulating these mRNAs, i.e., the
greatest extent of increase for SI mRNA level was observed at the lower
villus, whereas that for LPH mRNA level seen at the more apical and
broader locus of the villus. These results may suggest that dietary
sucrose enhances an efficiency of the transcription of both LPH and SI
genes. This hypothesis was supported by the experiments which showed
that the sucrose-induced increase in LPH and SI mRNA levels was
completely abolished by the pretreatment of the rats with actinomycin
D, only with a minor exception observed for the SI mRNA levels at the
upper villus, where the effect of actinomycin D was incomplete (Fig. 5)
. This may suggest that dietary sucrose not only stimulates
transcription of SI gene in the lower- and mid-villus cells, but
also stabilizes the SI mRNA in the upper villus cells. Recently we
gained more direct evidence by nuclear run-on assays that dietary
sucrose increased the rate of transcription of LPH gene (Tanaka et al. 1998
) and SI gene (Kishi et al. 1999
).
Thus, it seems most likely that, although a basic transcriptional
control which is operated in time- and space-specific manner along
the villus-crypt axis may be different between LPH and SI, the
sucrose-induced increases in LPH and SI gene expression might
involve common regulatory mechanisms presumably through the enhancement
of transcription rates of these genes.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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3 Abbreviations used: LPH, lactase-phlorizin
hydrolase; SI, sucrase-isomaltase; SSPE, sodium chloride/sodium
phosphate buffer containing ethylenediaminetetraacetic acid.
Manuscript received November 6, 1998. Initial review completed December 29, 1998. Revision accepted March 5, 1999.
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significantly different values from
rats force-fed the sucrose diet over the 3-h period (P
< 0.05).