Multiple levels of control of the stage- and region-specific expression of rat intestinal lactase

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Molecular analysis and intestinal expression of SAR1 genes and proteins in Anderson's disease (Chylomicron retention disease)

BACKGROUND

Anderson's disease (AD) or chylomicron retention disease (CMRD) is a very rare hereditary lipid malabsorption syndrome. In order to discover novel mutations in the SAR1B gene and to evaluate the expression, as compared to healthy subjects, of the Sar1 gene and protein paralogues in the intestine, we investigated three previously undescribed individuals with the disease.

METHODS

The SAR1B, SAR1A and PCSK9 genes were sequenced. The expression of the SAR1B and SAR1A genes in intestinal biopsies of both normal individuals and patients was measured by RTqPCR. Immunohistochemistry using antibodies to recombinant Sar1 protein was used to evaluate the expression and localization of the Sar1 paralogues in the duodenal biopsies.

RESULTS

Two patients had a novel SAR1B mutation (p.Asp48ThrfsX17). The third patient, who had a previously described SAR1B mutation (p.Leu28ArgfsX7), also had a p.Leu21dup variant of the PCSK9 gene. The expression of the SAR1B gene in duodenal biopsies from an AD/CMRD patient was significantly decreased whereas the expression of the SAR1A gene was significantly increased, as compared to healthy individuals. The Sar1 proteins were present in decreased amounts in enterocytes in duodenal biopsies from the patients as compared to those from healthy subjects.

CONCLUSIONS

Although the proteins encoded by the SAR1A and SAR1B genes are 90% identical, the increased expression of the SAR1A gene in AD/CMRD does not appear to compensate for the lack of the SAR1B protein. The PCSK9 variant, although reported to be associated with low levels of cholesterol, does not appear to exert any additional effect in this patient. The results provide further insight into the tissue-specific nature of AD/CMRD.

Oral polyamine administration modifies the ontogeny of hexose transporter gene expression in the postnatal rat intestine

Gastrointestinal mucosal polyamines influence enterocyte proliferation and differentiation during small intestinal maturation in the rat. Studies in postnatal rats have shown that ornithine decarboxylase (ODC) protein and mRNA peak before the maximal expression of brush-border membrane (BBM) sucrase-isomaltase (SI) and the sugar transporters sodium-dependent glucose transporter 1 (SGLT1) and glucose transporter 2 (GLUT2). This study was undertaken to test the hypothesis that the oral administration of spermidine in postnatal rats upregulates the expression of ODC, thereby enhancing the expression of SI and SGLT1 in the brush-border membrane as well as basolateral membrane-facilitative GLUT2 and Na(+)-K(+)-ATPase. Northern and Western blot analyses were performed with antibodies and cDNA probes specific for SI, SGLT1, GLUT2, alpha(1)- and beta(1)-subunits of Na(+)-K(+)-ATPase, and ODC. Postnatal rats fed 6 mumol spermidine daily for 3 days from days 7 to 9 were killed either on postnatal day 10 (Sp10) or day 13 following a 3-day washout period (Sp13). Sp10 rats showed a precocious increase in the abundance of mRNAs for SI, SGLT1, and GLUT2 and Na(+)-K(+)-ATPase activity and alpha(1)- and beta(1)-isoform gene expression compared with controls. ODC activity and protein and mRNA abundance were also increased in Sp10 animals. The increased expression of these genes was not sustained in Sp13 rats, suggesting that these effects were transient. Thus, 3 days of oral polyamine administration induces the precocious maturation of glucose transporters in the postnatal rat small intestine, which may be mediated by alterations in ODC expression.

Decreased expression of Intestinal I- and L-FABP levels in rare human genetic lipid malabsorption syndromes

We investigated, for the first time, the expression of I- and L-FABP in two very rare hereditary lipid malabsorption syndromes as compared with normal subjects. Abetalipoproteinemia (ABL) and Anderson's disease (AD) are characterized by an inability to export alimentary lipids as chylomicrons that result in fat loading of enterocytes. Duodeno-jejunal biopsies were obtained from 14 fasted normal subjects, and from four patients with ABL and from six with AD. Intestinal FABP expression was investigated by immuno-histochemistry, western blot, ELISA and Northern blot analysis. In contrast to normal subjects, the cellular immunostaining for both FABPs was clearly decreased in patients, as the enterocytes became fat-laden. In patients with ABL, the intestinal contents of I- (60.7 +/- 13.38 ng/mg protein) and L-FABP (750.3 +/- 121.3 ng/mg protein) are significantly reduced (50 and 35%, P < 0.05, respectively) as compared to normal subjects (I-135.3 +/- 11.1 ng, L-1211 +/- 110 ng/mg protein). In AD, the patients also exhibited decreased expression (50%, P < 0.05; I-59 +/- 11.88 ng, L-618.2 +/- 104.6 ng/mg protein). Decreased FABP expression was not associated with decreased mRNA levels. The results suggest that enterocytes might regulate intracellular FABP content in response to intracellular fatty acids, which we speculate may act as lipid sensors to prevent their intracellular transport.

Real-time PCR quantification of bovine lactase mRNA: localization in the gastrointestinal tract of milk-fed calves

Lactase is a disaccharidase that is present in the brush-border membrane of the small intestine, hydrolyzes lactose to glucose and galactose, and is therefore important in milk-fed animals. Assays based on quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) in the bovine species have not yet been described. Therefore, we have developed an RT-PCR assay for the quantification of lactase mRNA levels and have tested its suitability in the bovine gastrointestinal tract of seven 5-d-old milk-fed calves. Primers for RT-PCR amplification of bovine lactase mRNA were designed in the 100% identical regions of species (rats, rabbits, humans) from which lactase sequences were available. Lactase mRNA was expressed relative to mean levels of 4 housekeeping genes (glyceraldehyde-3-phosphate dehydrogenase, beta-actin, ubiquitin, and 18S). The presence of lactase mRNA along the entire gastrointestinal tract was evaluated in samples that consisted of whole gut walls (mucosa plus submucosa). Furthermore, mRNA levels of lactase were measured in fractionized layers of jejunal and ileal mucosa (mainly containing villus tips or crypts) and ileal lamina propria (mainly containing Peyer's patches). Agarose gel electrophoresis of the lactase PCR product revealed a single band that corresponded to the single-amplified product as predicted by the melting curve analysis of the PCR. The amplified partial-bovine lactase sequence showed 87% similarity with human and rabbit sequences and 82% similarity with the rat sequence. Lactase mRNA was present in whole walls (consisting of mucosa and submucosa) of the entire small intestine, but was absent in esophagus, rumen, fundus, pylorus, and colon. Furthermore, lactase mRNA was detected in fractionized villus and crypt layers of jejunum and ileum, but levels were higher in the jejunum in villus than in crypt fractions. No lactase mRNA was detectable in the lamina propria fraction of the ileum containing mainly Peyer's patches. In conclusion, the developed RT-PCR method allows study of lactase mRNA levels.

Adult-type hypolactasia and regulation of lactase expression

A common genetically determined polymorphism in the human population leads to two distinct phenotypes in adults, lactase persistence and adult-type hypolactasia (lactase non-persistence). All healthy newborn children express high levels of lactase and are able to digest large quantities of lactose, the main carbohydrate in milk. Individuals with adult-type hypolactasia lose their lactase expression before adulthood and consequently often become lactose intolerant with associated digestive problems (e.g. diarrhoea). In contrast, lactase persistent individuals have a lifelong lactase expression and are able to digest lactose as adults. Lactase persistence can be regarded as the mutant phenotype since other mammals down-regulate their lactase expression after weaning (the postweaning decline). This phenomenon does not occur in lactase persistent individuals. The regulation of lactase expression is mainly transcriptional and it is well established that adult-type hypolactasia is inherited in an autosomal recessive manner, whereas persistence is dominant. The recent findings of single nucleotide polymorphisms associated with lactase persistence have made it possible to study the potential mechanisms underlying adult-type hypolactasia. This work has led to the identification of gene-regulatory sequences located far from the lactase gene (LCT). The present review describes the recent advances in the understanding of the regulation of lactase expression and the possible mechanisms behind adult-type hypolactasia.

Abundance of mRNA encoding for components of the somatotropic axis and insulin receptor in different layers of the jejunum and ileum of neonatal calves1,2

Insulin-like growth factors-1 and -2, IGFBP-2 and -3, and receptors for IGF type-1 and type-2 (IGF-1R, IGF-2R), growth hormone (GHR), and insulin (InsR) in neonatal calves are variably expressed among gastrointestinal sites and thought to exert site-specific physiological functions. We studied by real-time reverse-transcription PCR, whether there are differences in the abundance of mRNA coding for IGF-I, IGF-2, IGFBP-2, IGFBP-3, IGF-1R, IGF-2R, GHR, and InsR in compartmentalized layers (fractions) of jejunum and ileum of 5-d-old calves fed colostrum. Samples of jejunum consisted primarily of villi and crypts; samples from ileum consisted mainly of villus tips, crypts, and lamina propria (LP; containing mainly Peyer's patches). After slaughter, segments of middle areas of jejunum and ileum were flushed with 154 mM NaCl. Pieces (5 mm x 5 mm) of jejunal (n = 9) and ileal walls (n = 5) were placed on glass slides and snap-frozen in liquid N before being cut horizontally into 10-mum-deep slices using a cryotome at -20 degrees C. Fifteen consecutive and morphologically similar slices were collected as fractions of villus, crypt, and LP layers, respectively. Fractions were characterized by use of 5'-bromo-2-deoxyuridine (BrdU) that labeled proliferating cells, and by expression of lactase mRNA. The BrdU-labeled cells were present in crypts and LP, but not in tips of villi. Lactase mRNA levels were greater in villus than crypt fractions, but lactase mRNA was absent in LP. In jejunum, mRNA levels, relative to levels of housekeeping genes (sum of levels of mRNA coding for ubiquitin, glyceraldehyde phosphate dehydrogenase, beta-actin, and ribosomal RNA), differed (P < 0.05) between fractions for InsR (crypts > villi), IGFBP-2 (crypts > villi), and IGFBP-3 (crypts > villi), and total RNA levels were greater (P < 0.05) in crypt than villus fractions. In ileum, mRNA levels, expressed relative to housekeeping genes, differed (P < 0.05) between fractions for IGF-I (LP > villi, crypts), IGF-2, and IGFBP-3 (villi > crypts, LP), GHR and InsR (crypts > LP), IGFBP-2 (crypts > villi, LP), and total RNA levels were greater (P < 0.05) in LP and crypt than in villus fractions. In conclusion, the tested fractionation technique is quite applicable for gene expression studies in the intestine of calves. Members of the somatotropic axis and of the insulin receptor are not equally expressed in different jejunal and ileal layers of neonatal calves.

An upstream polymorphism associated with lactase persistence has increased enhancer activity

BACKGROUND & AIMS

Intestinal lactase activity declines during childhood in some humans. This phenotypic polymorphism of lactase persistence or nonpersistence into adult life has been shown in a recent study to be 100% associated with a T/C nucleotide polymorphism at position -13910 and approximately 97% with an A/G nucleotide polymorphism at position -22018. The aim of this study was to investigate the role of these nucleotide polymorphisms for lactase-phlorizin hydrolase (LPH) gene expression.

METHODS

The -13910 and -22018 regions were cloned from lactase-persistent and -nonpersistent individuals, and the regions were analyzed for gene regulatory activity of a luciferase reporter gene by transfection experiments using the intestinal cell line Caco-2. Electrophoretic mobility shift assays (EMSAs) were used to investigate protein/DNA interactions with the -13910 sequence.

RESULTS

We show that the -13910 region contains a strong enhancer. The -13910 regions from both lactase persistent (-13910T variant) and lactase nonpersistent (-13910C variant) have enhancer activity. However, the -13910T variant enhances the LPH promoter approximately 4 times more than the -13910C variant when analyzed in differentiated Caco-2 cells. A nuclear factor from both an intestinal and a nonintestinal extract binds strongly to the -13910T variant whereas the binding to the -13910C variant is much weaker.

CONCLUSIONS

The discovery of a functional difference between the 2 alleles at position -13910 supports the notion that the molecular difference between lactase persistence and nonpersistence is caused by the mutation at position -13910.

Consequences of adrenalectomy on small intestine trophic parameters in aged and young rats: evidence of defective adaptation by aging and lack of corticoids

Previous study pointed to an important role of adrenals and glucocorticoids in the trophic status of the adult small intestine mucosa, with possible implications during stress events. Small intestine morphological and biochemical consequences of 10-day bilateral adrenalectomy and also sham-related laparotomy were determined in 23-month-old Sprague-Dawley rats. As described in young rats, adrenalectomy in old rats leads to partial atrophy and disorganization of the proximal small intestine epithelium, with an increase in the number of Paneth cells and reduced crypt cell proliferation. We also observed a decrease of goblet cell number and a reduction of all enzyme activities including disaccharidases, in contrast with the specific induced response shown in young rats. A number of marked biochemical effects have also been noted in aged rats subjected to solely laparotomy, suggesting age-related adaptation impairments. In conclusion, adrenalectomy modified the differentiation processes of the small intestinal mucosa in both young and aged rats, and some parameters underlined that the lack of corticoid-mediated adaptive process are exacerbated by cumulative surgical stress (event) and aging.

Chapter 16 Production and gene expression of brush border disaccharidases and peptidases during development in pigs and calves

This chapter reviews the expression of intestinal brush-border disaccharidases (maltase-glucoamylase, sucrase-isomaltase, lactase, and trehalase) and peptidases (aminopeptidases A and N and dipeptidyl peptidase IV) during development in growing animals. It describes the roles of intestinal enzymes, focussing on complementarity with salivary, gastric, and pancreatic digestive enzymes and their hydrolytic function in the process of absorption. Gene expression of the enzymes and nutritional regulation of their expression appear during postnatal development up to maturity. After translation of the specific mRNA, a single precursor of maltaseglucoamylase (pro-MG), rich in mannose, is produced in the rough endoplasmic reticulum (RER). In contrast to the relatively small number of carbohydrases, the number of peptidases found in enterocytes in the small intestine is large, because of the large number of different peptide bonds in oligopeptides produced by the action of pancreatic proteases. The digestive function (disaccharidase and peptidase activities) of the enterocytes and their microvilli begins when structural differentiation is complete, that is, during the period of migration over the cryptvillus junction. Modern techniques and investigations are expected to yield relevant data for elaborating feeding strategies that take into account the complex interactions between the diet, the microflora, the luminal milieu and the physiology of the small intestine, including the optimal functioning of the immunological and endocrine systems.

Loss of Hoxa5 gene function in mice perturbs intestinal maturation

The Hox gene family of transcription factors constitutes candidate regulators in the molecular cascade of events that governs establishment of normal terminal differentiation along the duodenum to colon axis. One member of this family, Hoxa5, displays a dynamic pattern of expression during gut development. Hoxa5 transcripts are present in midgut mesenchyme at the time of remodeling, supporting a role for this gene in digestive tract specification. To study the role of Hoxa5 in proper intestinal development and maturation, we examined whether Hoxa5 mutant mice exhibit any defect in this process. We report here that even though Hoxa5 is not required for midgut morphogenesis, its loss of function perturbs the acquisition of adult mode of digestion, which normally is temporally coordinated with the process of spontaneous weaning. Impaired maturation of the digestive tract might be related to altered specification of intestinal epithelial cells. Our findings provide evidence that Hoxa5 expression in the gut mesoderm is important for the region-specific differentiation of the adjacent endoderm.

Lactase-phlorizin hydrolase and sucrase-isomaltase genes are expressed differently along the villus-crypt axis of rat jejunum

Lactase-phlorizin hydrolase (LPH) and sucrase-isomaltase (SI) are two disaccharidases specifically expressed in small intestinal absorptive cells. We previously showed that the transcripts of both genes are elevated within 12 h of carbohydrate intake. To examine at which locus of villus-crypt axis this response to dietary carbohydrate occurs, 6-wk-old rats were fed a low-carbohydrate diet (5% energy) for 7 d, and then force-fed either the low-carbohydrate diet or a sucrose (40% energy) diet during the last 6 h. Cryostat sectioning of jejunal segments followed by RNA blot hybridizations of the transcripts revealed that, unlike SI mRNA which was expressed maximally in the lower villus, maximal LPH mRNA level was attained at the upper villus. The distribution of the respective immunoreactive protein and the enzymatic activity was shifted more toward the villus tips for LPH than for SI. Force-feeding the sucrose diet caused an abrupt increase in SI mRNA level in the lower villus within 3 h, while the rise in LPH mRNA level occurred in the mid- and upper-villus. The diet-induced increases in the LPH mRNA and SI mRNA levels were abolished along the entire villus by the administration of actinomycin D. These results suggest that LPH gene is maximally expressed in more apical villus cells than SI gene, and that dietary sucrose elicits enhancement of the gene expressions in the villus cells which are accumulating the respective transcripts.

Transcriptional regulation of pig lactase-phlorizin hydrolase: involvement of HNF-1 and FREACs

BACKGROUND & AIMS

One-kilobase sequence of the upstream fragment of the pig lactase-phlorizin hydrolase gene has been shown to control small intestinal-specific expression and postweaning decline of lactase-phlorizin hydrolase in transgenic mice. The aim of this study was to identify the regulatory DNA elements and transcription factors controlling lactase-phlorizin hydrolase expression.

METHODS

The activity of different lactase-phlorizin hydrolase promoter fragments was investigated by transfection experiments using Caco-2 cells. Electrophoretic mobility shift assays and supershift analyses were used to characterize the interaction between intestinal transcription factors and the identified regulatory elements.

RESULTS

Functional analysis revealed three previously undescribed regulatory regions in the lactase-phlorizin hydrolase promoter: a putative enhancer between -894 and -798 binding hepatocyte nuclear factor (HNF)-1 at position -894 to -880; a repressor-binding element between -278 to -264 to which an HNF-3-like factor is able to bind; and an element between -178 to -164 that binds an activating transcription factor.

CONCLUSIONS

Identification of three new regulatory regions and HNF-1 and HNF-3-like transcription factor as players in the regulation of lactase-phlorizin hydrolase gene transcription has an impact on the understanding of the molecular mechanisms behind age-dependent, tissue-specific, differentiation-dependent, and regional regulation of expression in the intestine.

Production of low-lactose milk by ectopic expression of intestinal lactase in the mouse mammary gland

We have investigated, in mice, an in vivo method for producing low-lactose milk, based on the creation of transgenic animals carrying a hybrid gene in which the intestinal lactase-phlorizin hydrolase cDNA was placed under the control of the mammary-specific alpha-lactalbumin promoter. Transgenic females expressed lactase protein and activity during lactation at the apical side of mammary alveolar cells. Active lactase was also secreted into milk, anchored in the outer membrane of fat globules. Lactase synthesis in the mammary gland caused a significant decrease in milk lactose (50-85%) without obvious changes in fat and protein concentrations. Sucklings nourished with low-lactose milk developed normally. Hence, these data validate the use of transgenic animals expressing lactase in the mammary gland to produce low-lactose milk in vivo, and they demonstrate that the secretion of an intestinal digestive enzyme into milk can selectively modify its composition.

Intestinal epithelial-mesenchymal cell interactions

Intestinal morphogenesis, as well as maintenance of the stem cell population and of the steady state between cell proliferation and differentiation, results from controlled cell interactions. There is growing evidence that the mesenchymal cells control epithelial cell behavior via their own expression and induction in the epithelial cells of key regulatory genes. This heterologous cross talk involves basement membrane molecules and paracrine factors. New in vitro/in vivo cellular models allowed us to analyze various mesenchymal cell phenotypes and to show that they exhibit different inductive properties on epithelial cells and that their proliferation and metabolic properties are differentially modulated by cytokines. Finally the epithelial-mesenchymal unit is controlled by hormonal and exogenous factors.

The murine Cdx1 gene product localises to the proliferative compartment in the developing and regenerating intestinal epithelium

The mouse Cdx1 gene encodes a homeobox-containing transcription factor and is one of the few homeobox genes known to be expressed in endodermally derived tissues of the intestine in fetal and adult mice. A detailed and systematic study of the expression of the Cdx1 protein was carried out during embryonic intestinal development, postnatal cytodifferentiation and in the regenerating (after radiation-induced damage) intestine of the mouse. Using antibodies directed against Cdx1, we show for the first time that the Cdx1 protein is localised in the proliferating immature epithelium during intestinal development. It becomes restricted to the proliferative crypt compartment during postnatal differentiation, as well as in the adult intestine. The mesenchymal layer was completely negative both during embryonic development and in the postnatal intestine. The expression of the protein was first clearly detected throughout the simple columnar epithelium at day 15 of development. This expression progressively became restricted to the regions of epithelial proliferation in the crypts of the adult mouse by day 40 of post-natal development. There were occasional cells that were Cdx1 positive in the villi. During regeneration of the epithelium after radiation-induced damage, Cdx1 expression diminished during the initial phase of cellular regression. The expression was then very strong in the regenerating epithelial foci, but not in the quiescent sterilised crypts between day 4 and 6. The normal pattern was restored between day 6 and 7. The Paneth cells were negative. The physical segregation of Cdx1 with the proliferative compartment and the hierarchy of cell renewal in the intestinal epithelium is an important example of how regulatory genes function in the maintenance and in the dysfunction of renewing tissues.

On the holistic approach in cellular and cancer biology: nonlinearity, complexity, and quasi-determinism of the dynamic cellular network

A keystone of the molecular reductionist approach to cellular biology is a specific deductive strategy relating genotype to phenotype-two distinct categories. This relationship is based on the assumption that the intermediary cellular network of actively transcribed genes and their regulatory elements is deterministic (i.e., a link between expression of a gene and a phenotypic trait can always be identified, and evolution of the network in time is predetermined). However, experimental data suggest that the relationship between genotype and phenotype is nonbijective (i.e., a gene can contribute to the emergence of more than just one phenotypic trait or a phenotypic trait can be determined by expression of several genes). This implies nonlinearity (i.e., lack of the proportional relationship between input and the outcome), complexity (i.e. emergence of the hierarchical network of multiple cross-interacting elements that is sensitive to initial conditions, possesses multiple equilibria, organizes spontaneously into different morphological patterns, and is controlled in dispersed rather than centralized manner), and quasi-determinism (i.e., coexistence of deterministic and nondeterministic events) of the network. Nonlinearity within the space of the cellular molecular events underlies the existence of a fractal structure within a number of metabolic processes, and patterns of tissue growth, which is measured experimentally as a fractal dimension. Because of its complexity, the same phenotype can be associated with a number of alternative sequences of cellular events. Moreover, the primary cause initiating phenotypic evolution of cells such as malignant transformation can be favored probabilistically, but not identified unequivocally. Thermodynamic fluctuations of energy rather than gene mutations, the material traits of the fluctuations alter both the molecular and informational structure of the network. Then, the interplay between deterministic chaos, complexity, self-organization, and natural selection drives formation of malignant phenotype. This concept offers a novel perspective for investigation of tumorigenesis without invalidating current molecular findings. The essay integrates the ideas of the sciences of complexity in a biological context.

Asymmetrical localization of mRNAs in enterocytes of human jejunum

Intracellular localization of specific mRNAs is known to be a mechanism for targeting proteins to specific sites within the cell. Previous studies from this laboratory have demonstrated co-localization of mRNAs and proteins for a number of genes in absorptive enterocytes of fetal rat intestine. The present study was undertaken to examine in human enterocytes the intracellular localization patterns of mRNAs for the microvillous membrane proteins lactase-phlorizin hydrolase (LPH), sucrase-isomaltase (SI), and intestinal alkaline phosphatase (IAP), and the cytoskeletal protein beta-actin. In sections of human jejunum, mRNAs were localized by in situ hybridization using digoxigenin-labeled anti-sense RNA probes. Both LPH and SI mRNAs were localized to the apical region of villous enterocytes, whereas IAP and beta-actin mRNAs were detected both apically and basally relative to the nucleus. Therefore, in contrast to LPH, SI, and beta-actin mRNAs, which co-localize with their encoded proteins, that of IAP is present in the basal region of the cell where IAP protein has not directly been demonstrated to be present. Absorptive enterocytes from humans possess the mechanisms for intracellular mRNA localization, but not all mRNAs co-localize with their encoded proteins.

Expression of retinoid-responsive genes occurs in colorectal carcinoma-derived cells irrespective of the presence of resistance to all-trans retinoic acid

BACKGROUND AND OBJECTIVES

Retinoids are metabolized in human intestinal epithelial cells to all-trans retinoic acid; however, it is unknown whether these cells express retinoid receptors, and whether sensitivity or resistance to the hormone is associated with a particular pattern of expression of retinoid-responsive genes.

METHODS

Northern blot analysis and reverse transcriptase polymerase chain reaction (RT-PCR) were used to identify mRNAs for retinoid receptors. Both Relative RT-PCR and transfection of retinoid-inducible plasmid were applied to test functionality of the pathway in a model system for colorectal carcinoma progression (primary SW480, all-trans retinoic acid-sensitive cells vs. metastatic SW620, -insensitive cells).

RESULTS

Three colorectal carcinoma-derived cell lines were inhibited by the hormone. Retinoic acid receptor type alpha (hRAR alpha) and retinoid X receptor type alpha (hRXR alpha) mRNAs were detected in normal enterocytes, colonocytes, and in all colorectal carcinoma-derived cells studied. Primary carcinomas and metastatic lesions expressed high amounts of hRAR alpha receptor protein, showing no simple correlation between the amounts of mRNA and receptor protein. No pattern of expression of the retinoid-responsive genes was associated with sensitivity or resistance to the retinoid. Expression of the genes occurred irrespective of resistance to the hormone or inactivity of the pathway.

CONCLUSIONS

Colonocytes possess a molecular system for transduction of the retinoid signal. All-trans retinoic acid modifies gene expression and inhibits proliferation of these cells. Therefore, retinoids are likely to be effective in chemoprevention of colorectal carcinoma.

Functional diversity and interactions between the repeat domains of rat intestinal lactase

Lactase-phlorizin hydrolase (LPH), a major digestive enzyme in the small intestine of newborns, is synthesized as a high-molecular-mass precursor comprising four tandemly repeated domains. Proteolytic cleavage of the precursor liberates the pro segment (LPHalpha) corresponding to domains I and II and devoid of known enzymic function. The mature enzyme (LPHbeta) comprises domains III and IV and is anchored in the brush border membrane via a C-terminal hydrophobic segment. To analyse the roles of the different domains of LPHalpha and LPHbeta, and the interactions between them, we have engineered a series of modified derivatives of the rat LPH precursor. These were expressed in cultured cells under the control of a cytomegalovirus promoter. The results show that recombinant LPHbeta harbouring both domains III and IV produces lactase activity. Neither domain III nor IV is alone sufficient to generate active enzyme, although the corresponding proteins are transport-competent. Tandem duplication of domains III or IV did not restore lactase activity, demonstrating the separate roles of both domains within LPHbeta. Further, the development of lactase activity did not require LPHalpha; however, LPHalpha potentiated the production of active LPHbeta but the individual LPHalpha subdomains I and II were unable to do so. Lactase activity and targeting required the C-terminal transmembrane anchor of LPH; this requirement was terminal transmembrane anchor or LPH; this requirement was not satisfied by the signal/anchor region of another digestive enzyme: sucrase-isomaltase. On the basis of this study we suggest that multiple levels of intramolecular interactions occur within the LPH precursor to produce the mature enzyme, and that the repeat domains of the precursor have distinct and specific functions in protein processing, substrate recognition and catalysis. We propose a functional model of LPHbeta in which substrate is channelled from an entry point located within domain II to the active site located in domain IV.

Changing intestinal connective tissue interactions alters homeobox gene expression in epithelial cells

In segmented organs, homeobox genes are involved in axial patterning and cell identity. Much less is known about their role in non-segmented endoderm derivatives such as the digestive epithelium. Using a xenograft model of fetal intestinal anlagen implanted under the skin of nude mice, we have investigated whether the expression of five homeobox genes (HoxA-4, HoxA-9, HoxC-8, Cdx-1 and Cdx-2) is modified when intestinal epithelium undergoes normal development or displays heterodifferentiation in association with heterotopic mesenchyme. In homotypic associations of fetal endoderm and mesenchyme that recapitulate normal development, the overall pattern of homeobox gene expression was maintained: HoxA-9 and HoxC-8 were the highest in the colon and ileum, respectively, and HoxA-4 was expressed all along the intestine; Cdx-1 and Cdx-2 exhibited an increasing gradient of expression from small intestine to colon. Yet, grafting per se caused a faint upregulation of HoxA-9 and HoxC-8 in small intestinal regions in which these genes are not normally expressed, while the endoderm-mesenchyme dissociation-association step provoked a decay of Cdx-1 in the colon. In heterotopic associations of colonic endoderm with small intestinal mesenchyme, the colonic epithelium exhibited heterodifferentiation to a small intestinal-like phenotype. In this case, we observed a decay of HoxA-9 expression and an upregulation of HoxC-8. Additionally, heterodifferentiation of the colonic epithelium was accompanied by a downregulation of Cdx-1 and Cdx-2 to a level similar to that found in the normal small intestine. To demonstrate that mesenchyme-derived cells can influence Cdx-1 and Cdx-2 expression in the bowel epithelium, fetal jejunal endoderm was associated with intestinal fibroblastic cell lines that either support small intestinal-like or colonic-like morphogenesis. A lower expression of both homeobox genes was shown in grafts presenting the small intestinal phenotype than in those showing glandular colonic-like differentiation. Taken together, these results suggest that homeobox genes participate in the control of the positional information and/or cell differentiation in the intestinal epithelium. They also indicate that the level of Cdx-1 and Cdx-2 homeobox gene expression is influenced by epithelial-mesenchymal cell interactions in the intestinal mucosa.

Lactase phlorizin hydrolase synthesis is decreased in protein-malnourished pigs

We have examined the effect of protein malnutrition on brush border (BB) lactase phlorizin hydrolase (LPH) synthesis in young pigs. Two groups of four 3-wk-old pigs were fed diets containing either 19 g soy protein, 63 g carbohydrate and 5 g fat per 100 g diet (a protein-sufficient diet) or 3 g soy protein, 85 g carbohydrate and 5 g fat per 100 g diet (a protein-deficient diet). After 8 wk of consuming the diets, pigs were infused intravenously with 2H3-leucine for 8 h, then killed. The jejunum was collected for measurement of lactase activity, LPH mRNA abundance and the rate of LPH post-translational synthesis. Lactase activities did not differ between groups (mean 8.1 +/- 1.2 micromol x min(-1) x g mucosa(-1)). LPH mRNA abundance relative to elongation factor-1alpha mRNA (the constitutive/reference mRNA) was significantly (P < 0.05) higher in well-nourished pigs (0.36 +/- 0.03%) than in protein-malnourished pigs (0.21 +/- 0.02%). The rate constants of BB LPH post-translational synthesis were also significantly higher in the well-nourished (103 +/- 9% x d(-1)) than in the protein-malnourished pigs (66 +/- 8% x d(-1)). Further, the absolute synthesis rate of BB LPH, a measure of the amount of enzyme synthesized per gram of tissue, was significantly higher in well-nourished than in protein-malnourished pigs (in arbitrary units, 892 +/- 90 vs. 450 +/- 34, respectively). Thus, protein malnutrition affects both LPH mRNA abundance and post-translational processing in young pigs.

Transcriptional and translational control over sodium-glucose-linked transporter (SGLT1) gene expression in adult rat small intestine

We have measured SGLT1 mRNA content and SGLT1-mediated glucose transport at different positions along the small intestine of control and streptozotocin diabetic rats and shown both parameters to be similar but higher in jejunal compared with ileal tissue. No such correlation was seen when comparing measurements of SGLT1 mRNA along jejunal villi with previous estimates of SGLT1 protein and SGLT1-mediated glucose transport [Debnam et al., Eur. J. Physiol. 430 (1995) 151-159]. This is the first time it has been possible to directly relate these three aspects of SGLT1 gene expression in a single species. Results are discussed in terms of a possible time rather than positional control over translation of SGLT1 mRNA.

Identification of homologues of the mammalian intestinal lactase gene in non-mammals (birds and molluscs)

Mammalian intestinal lactase hydrolyses a variety of beta-glycosides and is processed from a precursor comprising four tandem domains exhibiting sequence similarity, suggestive of multiple duplication events in the evolutionary past. The aim of the present study was to investigate whether genes homologous to the lactase gene exist in animals other than mammals. A reverse transcriptase-PCR strategy using a degenerate mixture of oligonucleotides was developed to search for the presence of transcripts similar in sequence to the mammalian lactase mRNA in the digestive tracts of a bird (the chicken) and an invertebrate (the mussel). Partial cDNAs corresponding to the 3' end of intestinal mRNAs were identified in both animals. In chicken, two cDNAs were isolated, corresponding to 6.5 kb transcripts that used two distinct polyadenylation sites. In mussels, three cDNAs were obtained and classified into two categories. One class of cDNA hybridized to a major mRNA of 3.5 kb and to minor species of 4.5 kb and 6 kb. The second class of cDNA hybridized to a 13 kb transcript, which was approximately twice as large as the mammalian lactase mRNA. Peptide sequences predicted from the chicken and mussel cDNAs confirmed that the proteins are related to mammalian lactase. They also suggested that the chicken protein and one mussel protein are integral molecules anchored in the cell membrane by a C-terminal transmembrane anchor, like lactase. These data provide evidence that proteins phylogenetically related to the mammalian-specific lactase are widespread in the animal kingdom, and that these proteins are expressed in the intestinal tract.

Heterogeneity of intestinal lactase activity in children: relationship to lactase-phlorizin hydrolase messenger RNA abundance

Despite extensive study in both humans and nonhuman mammals the mechanisms which regulate intestinal lactase activity, particularly during development, are incompletely understood. Our previous studies of human adults are consistent with an important role of lactase-phlorizin hydrolase (LPH) mRNA abundance in determining the lactase persistence/nonpersistence phenotypes. Our intent in the present study was to determine the role of LPH mRNA in the regulation of lactase in children. We therefore studied duodenal mucosal biopsies from 39 children undergoing diagnostic upper endoscopy in whom significant small intestinal and nutritional disease was excluded. We found no relationship between the level of LPH mRNA and lactase enzymatic activity. Our observations suggest the importance of posttranscriptional mechanisms in lactase regulation in human children.

Uncoordinated, transient mosaic patterns of intestinal hydrolase expression in differentiating human enterocytes

The heterogenous expression of brush border membrane hydrolases by the human enterocyte-like Caco-2 cell line during morphological and functional differentiation in vitro was investigated at the cellular level. Indirect immunofluorescence revealed that the heterogeneous ("mosaic") expression of sucrase-isomaltase, lactase, aminopeptidase N, and alkaline phosphatase was, in fact, transient in nature. The labeling indexes for each hydrolase gradually increased during culture at postconfluence in order to reach a maximum (> or = 90%) after 30 days, concomitant with an upregulation of their respective protein expression levels. In contrast, dipeptidylpeptidase IV labeling remained relatively constant. Backscattered electron imaging analysis in midstage (12 days postconfluence) monolayers demonstrated a lack of correlation between brush border membrane development and expression of each enzyme studied. Moreover, double immunostaining revealed that none of the other four hydrolases correlated directly with sucrase-isomaltase expression. Finally, immunodetection for the proliferation-associated antigen KI-67 revealed a transient mosaic pattern of proliferation which was inversely related to Caco-2 cell differentiation. These data indicate that enterocytic differentiation-related (as well as proliferation-related) gene expression in Caco-2 cells is regulated but uncoordinated at the cellular level, suggesting that an overall control mechanism is lacking.

Retroviral gene transfer into the intestinal epithelium

The epithelial cells of the gastrointestinal tract may be attractive targets for somatic gene therapy. In these studies, we have used rats and mice to explore the feasibility of gene transfer into the small intestinal epithelium using retroviral vectors. The first series of experiments was conducted in mature Sprague-Dawley rats using an ecotropic retroviral vector that has bacterial beta-galactosidase (beta-Gal) as the reporter gene. The vector was introduced into the lumen of ligated segments of terminal ileum. After a 4-hr exposure period, the ligatures were removed. Sham-operated animals were subjected to the same ligation procedure but received only tissue culture medium in the ligated segment. All animals were sacrificed 6 days later, and tissue from both the experimental segment and an upstream control segment was assessed for cytoplasmic beta-Gal activity using X-Gal histochemistry. Expression of the reporter gene was observed in the crypt epithelium of tissue exposed to the vector. In the villus epithelium, high background staining precluded accurate assessment of reporter gene expression. To obviate the latter problem, we sought an alternative reporter gene for which there would be no background staining in control animals. We repeated the experiments with beta-glucuronidase as the reporter gene in MPS VII mutant mice, which are devoid of this enzyme. In these studies, ileal segments exposed to the vector demonstrated expression of the reporter gene in both the crypt and villus epithelium 4 days after exposure. These results indicate that genes can be transferred into the intestinal epithelium using retroviral vectors introduced luminally.(ABSTRACT TRUNCATED AT 250 WORDS)

Intestinal brush border glycohydrolases: structure, function, and development

The hydrolytic enzymes of the intestinal brush border membrane are essential for the degradation of nutrients to absorbable units. Particularly, the brush border glycohydrolases are responsible for the degradation of di- and oligosaccharides into monosaccharides, and are thus crucial for the energy-intake of humans and other mammals. This review will critically discuss all that is known in the literature about intestinal brush border glycohydrolases. First, we will assess the importance of these enzymes in degradation of dietary carbohydrates. Then, we will closely examine the relevant features of the intestinal epithelium which harbors these glycohydrolases. Each of the glycohydrolytic brush border enzymes will be reviewed with respect to structure, biosynthesis, substrate specificity, hydrolytic mechanism, gene regulation and developmental expression. Finally, intestinal disorders will be discussed that affect the expression of the brush border glycohydrolases. The clinical consequences of these enzyme deficiency disorders will be discussed. Concomitantly, these disorders may provide us with important details regarding the functions and gene expression of these enzymes under specific (pathogenic) circumstances.

The cis-element CE-LPH1 of the rat intestinal lactase gene promoter interacts in vitro with several nuclear factors present in endodermal tissues

We have shown by electrophoretic mobility shift assays that the nucleotide sequence CE-LPH1, centred at position -49 with respect to the transcription start site of the rat gene encoding intestinal lactase-phlorizin hydrolase, interacts in vitro with nuclear proteins present in the jejunum of suckling animals. Proteins binding to this element were also found in organs of endodermal origin that do not (or no longer) express lactase-phlorizin hydrolase, i.e. the colon, lung and the liver, but not in the brain. However, a DNA-protein interaction was hardly detected with nuclear extracts prepared from adult tissues, although typical factors binding to the Sp1 binding site were detected at the adult stage as in the sucklings. Southwestern blotting experiments conducted with nuclear extracts prepared from the tissues of suckling rats indicated that CE-LPH1 interacts with several factors in the jejunum, colon, lung and the liver. Some of these DNA-binding proteins are specifically expressed in the jejunum or in the liver, whereas others seem to be shared with the colon and the lung. Hence, the cis-element CE-LPH1 located in close vicinity to the pseudo-TATA-box of the intestinal lactase-phlorizin hydrolase gene promoter interacts in vitro with a family of nuclear proteins which may represent markers of the endodermal lineage predominantly expressed prior to weaning.

Fetal endoderm primarily holds the temporal and positional information required for mammalian intestinal development

In rodents, the intestinal tract progressively acquires a functional regionalization during postnatal development. Using lactase-phlorizin hydrolase as a marker, we have analyzed in a xenograft model the ontogenic potencies of fetal rat intestinal segments taken prior to endoderm cytodifferentiation. Segments from the presumptive proximal jejunum and distal ileum grafted in nude mice developed correct spatial and temporal patterns of lactase protein and mRNA expression, which reproduced the normal pre- and post-weaning conditions. Segments from the fetal colon showed a faint lactase immunostaining 8-10 d after transplantation in chick embryos but not in mice; it is consistent with the transient expression of this enzyme in the colon of rat neonates. Heterotopic cross-associations comprising endoderm and mesenchyme from the presumptive proximal jejunum and distal ileum developed as xenografts in nude mice, and they exhibited lactase mRNA and protein expression patterns that were typical of the origin of the endodermal moiety. Endoderm from the distal ileum also expressed a normal lactase pattern when it was associated to fetal skin fibroblasts, while the fibroblasts differentiated into muscle layers containing alpha-smooth-muscle actin. Noteworthy, associations comprising colon endoderm and small intestinal mesenchyme showed a typical small intestinal morphology and expressed the digestive enzyme sucrase-isomaltase normally absent in the colon. However, in heterologous associations comprising lung or stomach endoderm and small intestinal mesenchyme, the epithelial compartment expressed markers in accordance to their tissue of origin but neither intestinal lactase nor sucrase-isomaltase. A thick intestinal muscle coat in which cells expressed alpha-smooth-muscle actin surrounded the grafts. The results demonstrate that: (a) the temporal and positional information needed for intestinal ontogeny up to the post-weaning stage results from an intrinsic program that is fixed in mammalian fetuses prior to endoderm cytodifferentiation; (b) this temporal and positional information is primarily carried by the endodermal moiety which is also able to change the fate of heterologous mesodermal cells to form intestinal mesenchyme; and (c) the small intestinal mesenchyme in turn may deliver instructive information as shown in association with colonic endoderm; yet this effect is not obvious with nonintestinal endoderms.