A limited upstream region of the human sucrase-isomaltase gene confers glucose-regulated expression on a heterologous gene

Fine-tuning and autoregulation of the intestinal determinant and tumor suppressor homeobox gene CDX2 by alternative splicing

On the basis of phylogenetic analyses, we uncovered a variant of the CDX2 homeobox gene, a major regulator of the development and homeostasis of the gut epithelium, also involved in cancer. This variant, miniCDX2, is generated by alternative splicing coupled to alternative translation initiation, and contains the DNA-binding homeodomain but is devoid of transactivation domain. It is predominantly expressed in crypt cells, whereas the CDX2 protein is present in crypt cells but also in differentiated villous cells. Functional studies revealed a dominant-negative effect exerted by miniCDX2 on the transcriptional activity of CDX2, and conversely similar effects regarding several transcription-independent functions of CDX2. In addition, a regulatory role played by the CDX2 and miniCDX2 homeoproteins on their pre-mRNA splicing is displayed, through interactions with splicing factors. Overexpression of miniCDX2 in the duodenal Brunner glands leads to the expansion of the territory of these glands and ultimately to brunneroma. As a whole, this study characterized a new and original variant of the CDX2 homeobox gene. The production of this variant represents not only a novel level of regulation of this gene, but also a novel way to fine-tune its biological activity through the versatile functions exerted by the truncated variant compared to the full-length homeoprotein. This study highlights the relevance of generating protein diversity through alternative splicing in the gut and its diseases.

Identification of an intestine-specific promoter and inducible expression of bacterial α-galactosidase in mammalian cells by a lac operon system

Background

α-galactosidase has been widely used in animal husbandry to reduce anti-nutritional factors (such as α-galactoside) in feed. Intestine-specific and substrate inducible expression of α-galactosidase would be highly beneficial for transgenic animal production.

Methods

To achieve the intestine-specific and substrate inducible expression of α-galactosidase, we first identified intestine-specific promoters by comparing the transcriptional activity and tissue specificity of four intestine-specific promoters from human intestinal fatty acid binding protein, rat intestinal fatty acid binding protein, human mucin-2 and human lysozyme. We made two chimeric constructs combining the promoter and enhancer of human mucin-2, rat intestinal trefoil factor and human sucrase-isomaltase. Then a modified lac operon system was constructed to investigate the induction of α-galactosidase expression and enzyme activity by isopropyl β-D-1-thiogalactopyranoside (IPTG) and an α-galactosidase substrate, α-lactose.

We declared that the research carried out on human (Zhai Yafeng) was in compliance with the Helsinki Declaration, and experimental research on animals also followed internationally recognized guidelines.

Results

The activity of the human mucin-2 promoter was about 2 to 3 times higher than that of other intestine-specific promoters. In the lac operon system, the repressor significantly decreased (P < 0.05) luciferase activity by approximately 6.5-fold and reduced the percentage of cells expressing green fluorescent protein (GFP) by approximately 2-fold. In addition, the expression level of α-galactosidase mRNA was decreased by 6-fold and α-galactosidase activity was reduced by 8-fold. In line with our expectations, IPTG and α-lactose supplementation reversed (P < 0.05) the inhibition and produced a 5-fold increase of luciferase activity, an 11-fold enhancement in the percentage of cells with GFP expression and an increase in α-galactosidase mRNA abundance (by about 5-fold) and α-galactosidase activity (by about 7-fold).

Conclusions

We have successfully constructed a high specificity inducible lac operon system in an intestine-derived cell line, which could be of great value for gene therapy applications and transgenic animal production.

Sugar sensing by enterocytes combines polarity, membrane bound detectors and sugar metabolism

Sugar consumption and subsequent sugar metabolism are known to regulate the expression of genes involved in intestinal sugar absorption and delivery. Here we investigate the hypothesis that sugar-sensing detectors in membranes facing the intestinal lumen or the bloodstream can also modulate intestinal sugar absorption. We used wild-type and GLUT2-null mice, to show that dietary sugars stimulate the expression of sucrase-isomaltase (SI) and L-pyruvate kinase (L-PK) by GLUT2-dependent mechanisms, whereas the expression of GLUT5 and SGLT1, did not rely on the presence of GLUT2. By providing sugar metabolites, sugar transporters, including GLUT2, fuelled a sensing pathway. In Caco2/TC7 enterocytes, we could disconnect the sensing triggered by detector from that produced by metabolism, and found that GLUT2 generated a metabolism-independent pathway to stimulate the expression of SI and L-PK. In cultured enterocytes, both apical and basolateral fructose could increase the expression of GLUT5, conversely, basolateral sugar administration could stimulate the expression of GLUT2. Finally, we located the sweet-taste receptors T1R3 and T1R2 in plasma membranes, and we measured their cognate G alpha Gustducin mRNA levels. Furthermore, we showed that a T1R3 inhibitor altered the fructose-induced expression of SGLT1, GLUT5, and L-PK. Intestinal gene expression is thus controlled by a combination of at least three sugar-signaling pathways triggered by sugar metabolites and membrane sugar receptors that, according to membrane location, determine sugar-sensing polarity. This provides a rationale for how intestine adapts sugar delivery to blood and dietary sugar provision.

HNF-1α participates in glucose regulation of sucrase–isomaltase gene expression in epithelial intestinal cells

Sucrase-isomaltase (SI) gene expression is negatively regulated by glucose, but its molecular mechanism is not completely clear. The purpose of this study is to investigate whether HNF-1alpha and HNF-1beta contribute to glucose regulation of SI gene expression. To explore this question, we examined the association of gene expressions between SI and HNF-1alpha and HNF-1beta in Caco-2 cells cultured in medium containing 2.0 and 16.7 mM glucose. We found that gene expression of HNF-1alpha but not HNF-1beta exhibits a positive correlation with that of SI regulated by glucose. Moreover, to elucidate whether glucose regulation of SI gene expression is changed when HNF-1alpha and HNF-1beta are inhibited, we produced three stable cell lines, in which dominant-negative mutant HNF-1alphaT539fsdelC, mutant HNF-1betaR177X, and empty vector (as a control), respectively, were stably expressed. We found that the glucose regulation of SI gene expression was significantly attenuated in HNF-1alphaT539fsdelC cells, but it was well maintained in empty vector and HNF-1betaR177X cells. These results suggest that HNF-1alpha participates in glucose regulation of SI gene expression.

Phosphorylation of the homeotic tumor suppressor Cdx2 mediates its ubiquitin-dependent proteasome degradation

The Caudal-related homeodomain transcription factor Cdx2 plays a key role in intestinal cell fate determination. Reduction of Cdx2 expression is a feature of many human colon carcinomas and inactivation of one cdx2 allele facilitates the development of invasive adenocarcinoma in the murine colon. Here, we investigated the post-translational regulation of Cdx2. We showed that various forms of Cdx2 coexist in the intestine and colon cancer cell lines, some of them being phosphorylated forms. We found that cyclin-dependent kinase 2 phosphorylated Cdx2 in vitro and in vivo. Using site-specific mutagenesis, we identified serine 281 as a new key residue for Cdx2 phosphorylation. Intriguingly, serine 281 belongs to a conserved motif of four evenly spaced serines (the 4S motif) similar to the one controlling beta-catenin degradation by the proteasome pathway. A nonphosphorylated mutant Cdx2 lacking the 4S motif (4S>A) exhibited reduced polyubiquitination upon proteasome inhibition and increased stability compared to wild-type Cdx2. In addition, we found that this mutant was less efficient to suppress colony formation than wild-type Cdx2. Thus, our data highlight a novel post-translational mechanism controlling Cdx2 degradation via phosphorylation and polyubiquitination, which may be of importance for intestinal development and cancer.

Cyclic AMP stimulates fructose transport in neonatal rat small intestine

Intestinal fructose transporter (GLUT5) expression normally increases significantly after completion of weaning in neonatal rats. Increases in GLUT5 mRNA, protein, and activity can be induced in early weaning pups by precocious consumption of dietary fructose or by perfusion of the small intestine with fructose solutions. Little is known about the signal transduction pathway of the dietary fructose-mediated increase in GLUT5 expression during early intestinal development. Recent microarray results indicate that key gluconeogenic enzymes modulated by cAMP are markedly upregulated by fructose perfusion; hence, we tested the hypothesis that cAMP plays an important role in regulating intestinal fructose absorption by simultaneously perfusing adenylyl cyclase, phosphodiesterase, or protein kinase A (PKA) inhibitors along with fructose. Intestinal fructose uptake rates increased by 100% in rat pups perfused with 8-bromo-cAMP. Simultaneous fructose and dideoxyadenosine (DDA; inhibitor of adenylyl cyclase) perfusion completely inhibited increases in fructose uptake rate induced by perfusion with fructose alone. Fructose perfusion increased intestinal mucosal cAMP concentrations by 27%, but simultaneous perfusion of fructose and DDA inhibited the fructose-induced increase in cAMP. However, GLUT5 and sodium-glucose cotransporter (SGLT1) mRNA abundance and glucose transport rates were each not significantly affected by 8-bromo-cAMP and DDA. Moreover, simultaneous perfusion of the small intestine with fructose and PKA inhibitor or N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamid. 2HCl, both inhibitors of PKA, did not prevent the fructose-induced increases in GLUT5 mRNA abundance and fructose uptake rate. Cyclic AMP appears to modulate fructose transport without affecting GLUT5 mRNA abundance, and without involving PKA.

A silencer inhibitor confers specific expression of intestinal trefoil factor in gobletlike cell lines

Intestinal trefoil factor (ITF) is selectively expressed in intestinal goblet cells. Previous studies identified cis-regulatory elements in the proximal promoter of ITF, but these were insufficient to recapitulate the exquisite tissue- and cell-specific expression of native ITF in vivo. Preliminary studies suggested that goblet cell-specific expression of murine ITF requires elements far upstream that include a silencer element that effectively prevents ITF expression in non-goblet cells. Transient transfection studies using native or mutant ITF 5'-flanking sequences identified a region that restores expression in goblet cells. This element, designated goblet cell silencer inhibitor (GCSI) element, enables human and murine goblet cell-like cell lines to override the silencing effect of more proximal elements. The GCSI has no intrinsic enhancer activity and regulates expression only when the silencer element is present. Ligation of GCSI and silencer elements to sucrase-isomaltase conferred goblet cell-specific expression. Goblet cells but not non-goblet cells possess a nuclear protein that binds to the GCSI regulatory element (GCSI binding protein; GCSI-BP). Both transient transfection and gel mobility shift assay studies localize the GCSI and GCSI-BP to -2216 to -2204. We conclude that goblet cell-specific transcription of ITF in vivo depends on a regulatory element designated GCSI.

Glucose and thyroid hormone co-regulate the expression of the intestinal fructose transporter GLUT5

Expression of the fructose transporter GLUT5 in Caco-2 cells is controlled by the carbohydrate content of the culture media [Mesonero, Matosin, Cambier, Rodriguez-Yoldi and Brot-Laroche (1995) Biochem. J. 312, 757-762] and by the metabolic status of the cells [Mahraoui, Takeda, Mesonero, Chantret, Dussaulx, Bell, and Brot-Laroche (1994) Biochem. J. 301, 169-175]. In this study we show that, in fully differentiated Caco-2/TC7 cells, thyroid hormone and glucose increase GLUT5 mRNA abundance in a dose-dependent manner. Using Caco-2/TC7 cells stably transformed with various fragments of the GLUT5 promoter inserted upstream of the luciferase reporter gene, we localized the sequences that confer 3,3',5-l-tri-iodothyronine (T3)- and/or glucose-sensitivity to the gene. Glucose responsiveness is conferred by the -272/+41 fragment of the promoter, but it is only with the -338/+41 region that transcription of the luciferase reporter gene is stimulated by T3. This 70 bp fragment from position -338 to -272 of the GLUT5 gene is able to confer T3/glucose-responsiveness to the heterologous thymidine kinase promoter. Electrophoretic-mobility-shift assays demonstrate that thyroid hormone receptors alpha and beta are expressed in Caco-2/TC7 cells. They further show that the -308/-290 region of the GLUT5 promoter binds thyroid hormone receptor/retinoid X receptor heterodimers, and that glucose and/or T3 exert a deleterious effect on the binding of the nuclear protein complex.

Glucose and thyroid hormone co-regulate the expression of the intestinal fructose transporter GLUT5

Expression of the fructose transporter GLUT5 in Caco-2 cells is controlled by the carbohydrate content of the culture media [Mesonero, Matosin, Cambier, Rodriguez-Yoldi and Brot-Laroche (1995) Biochem. J. 312, 757-762] and by the metabolic status of the cells [Mahraoui, Takeda, Mesonero, Chantret, Dussaulx, Bell, and Brot-Laroche (1994) Biochem. J. 301, 169-175]. In this study we show that, in fully differentiated Caco-2/TC7 cells, thyroid hormone and glucose increase GLUT5 mRNA abundance in a dose-dependent manner. Using Caco-2/TC7 cells stably transformed with various fragments of the GLUT5 promoter inserted upstream of the luciferase reporter gene, we localized the sequences that confer 3,3',5-l-tri-iodothyronine (T3)- and/or glucose-sensitivity to the gene. Glucose responsiveness is conferred by the -272/+41 fragment of the promoter, but it is only with the -338/+41 region that transcription of the luciferase reporter gene is stimulated by T3. This 70 bp fragment from position -338 to -272 of the GLUT5 gene is able to confer T3/glucose-responsiveness to the heterologous thymidine kinase promoter. Electrophoretic-mobility-shift assays demonstrate that thyroid hormone receptors alpha and beta are expressed in Caco-2/TC7 cells. They further show that the -308/-290 region of the GLUT5 promoter binds thyroid hormone receptor/retinoid X receptor heterodimers, and that glucose and/or T3 exert a deleterious effect on the binding of the nuclear protein complex.

Two HNF-1 binding sites govern the glucose repression of the human sucrase-isomaltase promoter

We have previously shown, using the Caco-2 clone PF11, that glucose represses transcription of the human sucrase-isomaltase (SI) gene and that the -370/+30 fragment of the SI gene conferred glucose-regulated expression on a heterologous gene. Different fragments beginning at the already characterized SI footprint (SIF) 1 (-53/-37), SIFR (-153/-129) or SIF3 (-176/-156) elements [Wu, Chen, Forslund and Traber (1994) J. Biol. Chem. 269, 17080-17085] were tested, in comparison with the -370/+30 fragment, for their capacity to inhibit reporter gene expression under high-glucose (25 mM) conditions. Unlike SIF1 and SIFR, the addition of the HNF (hepatocyte nuclear factor)-1-binding element SIF3 to the promoter fragment was required for repression under high-glucose conditions. This effect was enhanced when the SI promoter was extended to position -370, indicating that the -370/-176 region contains elements that may co-operate with SIF3 to increase the metabolic control of the SI promoter. We have characterized an additional HNF-1-binding site near to and upstream from SIF3; SIF4. By mutagenesis of the three HNF-1-binding elements we show that the two distal HNF-1-recognition sites are the most important for the glucose regulation of the SI gene. Moreover, this glucose regulation was abolished in PF11 cells overexpressing vHNF-1C (variant HNF, an isoform of the HNF-1 family). We thus propose that the differential binding of HNF-1-family proteins to their DNA targets on the SI promoter constitutes the molecular mechanism that controls the glucose regulation of the SI gene transcription.

Decreased expression of gamma-glutamyltranspeptidase in the intestinal cell line Caco-2 by inducers of cytochrome P450 1A1

Our purpose was to investigate whether inducers of cytochrome P450 1A1 (CYP1A1), which cause a decreased expression in Caco-2 cells, at both the mRNA and protein levels, of membrane proteins associated with the uptake and transport of hexoses, would also affect the expression of gamma-glutamyltranspeptidase (gammaGT) (EC 2.3.2.2). In Caco-2 clonal TC7 cells grown under standard conditions (25 mM glucose), exposure to beta-naphthoflavone (beta-NF), 2,3,7,8-tetrachlorodibenzo-p-dioxin, and 3-methylcholanthrene resulted in increased glucose consumption and decreased gammaGT activity in cells grown to confluence, i.e. when the differentiation is optimum. GammaGT activity was further analyzed during the time course of differentiation of TC7 cells treated or not with beta-naphthoflavone: while gammaGT activity in untreated cells showed a 10-fold increase from the exponential phase of growth until late postconfluence, gammaGT activity in beta-NF-treated cells, although increasing by 4-fold, remained at a much lower level (<25%). This decreased activity of gammaGT was associated with a decreased level of gammaGT mRNA. This inhibiting effect was not dependent on the CYP1A1 activity, as it also occurred in the presence of CYP1A1 inhibitors such as alpha-naphthoflavone, 8-methoxypsoralen or ellipticin. It was however dependent on glucose supply as it was not observed when the cells were cultured in low glucose (1 mM). These results raise the question of whether, in Caco-2 cells, CYP1A1 inducers or the signal transduction system which controls CYP1A1 are involved in the regulation of the expression of gammaGT through a mechanism involving glucose metabolism.

Primary culture and transfection of epithelial cells of human small intestine

BACKGROUND

So far, no techniques are available for primary culture and efficient transfection of human small-intestinal enterocytes, which would provide a valuable tool to investigate intestinal function.

METHODS

Human small-intestinal biopsy specimens were treated with collagenase and dispase. Resulting crypt units were cultured for several days. Using the intestinal epithelial cell lines Caco-2 and HT-29, we established optimal conditions for transfection of a control plasmid, which were then applied to primary cultured cells.

RESULTS

Cells growing out of crypt units formed monolayer-like sheets and proliferated for several days. Most of the cells could be stained with antibodies against epithelial markers. Among seven different transfection reagents tested, Lipofectamine was the most potent, with transfection efficiencies up to 25% for primary enterocytes.

CONCLUSIONS

An easy technique was developed providing viable small-intestinal enterocytes that can be efficiently transfected.

Hypoxia and CYP1A1 induction-dependent regulation of proteins involved in glucose utilization in Caco-2 cells

Although induction of cytochrome P-450 1A1 (CYP1A1) in the Caco-2 clone TC7 alters glucose utilization and modifies the expression of sucrase-isomaltase (SI) and hexose transporters, nothing is known of the events that control these effects. In this study, we analyzed the effects of beta-naphthoflavone (beta-NF) and hypoxia on these parameters and expression of key enzymes of glucose metabolism. Both beta-NF and hypoxia induce similar changes: 1) induction of CYP1A1 mRNA; 2) increased glucose consumption and lactic acid production and lower glycogen content; 3) downregulation of SI and upregulation of GLUT1 mRNAs; 4) downregulation of fructose-1,6-bisphosphatase and pyruvate kinase mRNAs and upregulation of phosphoenolpyruvate carboxykinase, pyruvate dehydrogenase, lactate dehydrogenase, and phosphofructokinase mRNAs; and 5) upregulation of c-fos and c-jun mRNAs. Although addition of inhibitors of CYP1A1 catalytic activity to beta-NF-treated cells totally inhibits the enzyme activity, it does not modify CYP1A1 mRNA response and associated effects, thus excluding a direct role for the enzyme per se. These results point to a possible physiological implication of the signal-transduction pathway responsible for CYP1A1 induction.

Selecting agent hygromycin B alters expression of glucose-regulated genes in transfected Caco-2 cells

Incorporation into plasmids of genes conferring resistance to aminoglycoside antibiotics such as hygromycin B is currently utilized for selection in experiments involving gene transfer in eukaryotic cells. Using a subclone of Caco-2 cells stably transfected with an episomal plasmid containing the hygromycin resistance gene, we observed that transformed cells subcultured in the presence of hygromycin B exhibit, compared with the same cells subcultured in antibiotic-free medium, a sixfold increase in the rates of glucose consumption and lactic acid production and dramatic changes, at mRNA and protein level, of the expressions of sucrase-isomaltase and hexose transporter GLUT-2, which are downregulated, contrasting with an upregulation of hexose transporter GLUT-1. This occurs without significant modifications of the differentiation status of the cells, as demonstrated by the normal expression of villin, ZO-1, dipeptidyl peptidase IV, or Na(+)-K(+)-ATPase. The plasmid copy number is, however, the same, whether or not the cells are cultured in the presence of hygromycin B. These results draw attention to the need to consider antibiotic-dependent alterations of metabolism and gene expression in transfection experiments.

Dietary carbohydrates enhance lactase/phlorizin hydrolase gene expression at a transcription level in rat jejunum

We have previously shown that dietary sucrose stimulates the lactase/phlorizin hydrolase (LPH) mRNA accumulation along with a rise in lactase activity in rat jejunum [Goda, Yasutake, Suzuki, Takase and Koldovský (1995) Am. J. Physiol. 268, G1066-G1073]. To elucidate the mechanisms whereby dietary carbohydrates enhance the LPH mRNA expression, 7-week-old rats that had been fed a low-carbohydrate diet (5.5% of energy as starch) were given diets containing various monosaccharides or sucrose for 12h. Among carbohydrates examined, fructose, sucrose, galactose and glycerol elicited an increase in LPH mRNA accumulation along with a rise in lactase activity in the jejunum. By contrast, glucose and alpha-methylglucoside were unable to elicit a significant increase in LPH mRNA levels. To explore a transcriptional mechanism for the carbohydrate-induced increases in LPH mRNA levels, we employed two techniques currently available to estimate transcriptional rate, i.e. RNA protection assays of pre-mRNA using an intron probe, and nuclear run-on assays. Both assays revealed that fructose elicited an increase in transcription of the LPH gene, and that the transcription of LPH was influenced only slightly, if at all, by glucose intake. These results suggest that certain monosaccharides such as fructose or their metabolite(s) are capable of enhancing LPH mRNA levels in the small intestine, and that transcriptional control might play a major role in the carbohydrate-induced increase of LPH mRNA expression.