Regulation of lineage-specific transcription of the sucrase-isomaltase gene in transgenic mice and cell lines

Schlafen 12 mediates the effects of butyrate and repetitive mechanical deformation on intestinal epithelial differentiation in human Caco-2 intestinal epithelial cells

Intestinal epithelial differentiation may be stimulated by diverse pathways including luminal short-chain fatty acids and repetitive mechanical deformation engendered by villous motility and peristalsis. Schlafen 12 (SLFN12) is a cytosolic protein that stimulates sucrase-isomaltase (SI) expression. We hypothesized that two disparate differentiating stimuli, butyrate and repetitive deformation, would each stimulate SLFN12 expression in human Caco-2 intestinal epithelial cells and that increased SLFN12 expression would contribute to the differentiating activity of the human Caco-2 intestinal epithelial cells. We stimulated Caco-2 cells with 1-2 mM butyrate or repetitive mechanical deformation at 10 cycles/min at an average 10% strain, and measured SLFN12 and SI expression by qRT-PCR. Sodium butyrate enhanced SLFN12 expression at both 1 mM and 2 mM although SI expression was only significantly increased at 2 mM. Repetitive deformation induced by cyclic mechanical strain also significantly increased both SLFN12 and SI gene expression. Reducing SLFN12 by siRNA decreased basal, deformation-stimulated, and butyrate-stimulated SLFN12 levels, compared to control cells treated with non-targeting siRNA, although both deformation and butyrate were still able to stimulate SLFN12 expression in siRNA-treated cells compared to control cells treated with the same siRNA. This attenuation of the increase in SLFN12 expression in response to mechanical strain or butyrate was accompanied by parallel attenuation of SI expression. Butyrate stimulated SI-promoter activity, and reducing SLFN12 by siRNA attenuated butyrate-induced SI-promoter activity. These data suggest that SLFN12 mediates at least in part the stimulation by both butyrate and repetitive mechanical deformation of sucrase-isomaltase, a late stage differentiation marker in human intestinal epithelial cells.

The calcium-sensing receptor: A promising target for prevention of colorectal cancer

The inverse correlation between dietary calcium intake and the risk of colorectal cancer (CRC) is well known, but poorly understood. Expression of the calcium-sensing receptor (CaSR), a calcium-binding G protein-coupled receptor is downregulated in CRC leading us to hypothesize that the CaSR has tumor suppressive roles in the colon. The aim of this study was to understand whether restoration of CaSR expression could reduce the malignant phenotype in CRC. In human colorectal tumors, expression of the CaSR negatively correlated with proliferation markers whereas loss of CaSR correlated with poor tumor differentiation and reduced apoptotic potential. In vivo, dearth of CaSR significantly increased expression of proliferation markers and decreased levels of differentiation and apoptotic markers in the colons of CaSR/PTH double knock-out mice confirming the tumor suppressive functions of CaSR. In vitro CRC cells stably overexpressing wild-type CaSR showed significant reduction in proliferation, as well as increased differentiation and apoptotic potential. The positive allosteric modulator of CaSR, NPS R-568 further enhanced these effects, whereas treatment with the negative allosteric modulator, NPS 2143 inhibited these functions. Interestingly, the dominant-negative mutant (R185Q) was able to abrogate these effects. Our results demonstrate a critical tumor suppressive role of CaSR in the colon. Restoration of CaSR expression and function is linked to regulation of the balance between proliferation, differentiation, and apoptosis and provides a rationale for novel strategies in CRC therapy.

EIF3i promotes colon oncogenesis by regulating COX-2 protein synthesis and β-catenin activation

Translational control of gene expression has recently been recognized as an important mechanism controlling cell proliferation and oncogenesis, and it mainly occurs in the initiation step of protein synthesis that involves multiple eukaryotic initiation factors (eIFs). Many eIFs have been found to have aberrant expression in human tumors and the aberrant expression may contribute to oncogenesis. However, how these previously considered house-keeping proteins are potentially oncogenic remains elusive. In this study, we investigated the expression of eIF3i in human colon cancers, tested its contribution to colon oncogenesis and determined the mechanism of eIF3i action in colon oncogenesis. We found that eIF3i expression was upregulated in both human colon adenocarcinoma and adenoma polyps as well as in model inducible colon tumorigenic cell lines. Overexpression of ectopic eIF3i in intestinal epithelial cells causes oncogenesis by directly upregulating the synthesis of cyclooxygenase-2 (COX-2) protein and activates the β-catenin/T-cell factor 4 signaling pathway that mediates the oncogenic function of eIF3i. Together, we conclude that eIF3i is a proto-oncogene that drives colon oncogenesis by translationally upregulating COX-2 and activating the β-catenin signaling pathway. These findings imply that proto-oncogenic eIFs likely exert their tumorigenic function by regulating/altering the synthesis level of downstream tumor suppressor or oncogenes.

Molecular cloning and promoter analysis of downregulated in adenoma (DRA)

Downregulated in adenoma (DRA), also referred to as SLC26A3, is an intestinal anion transporter essential for intestinal chloride absorption. Mutations in DRA result in congenital chloride diarrhea. DRA expression has been shown to be induced by differentiation and to be modulated by cytokines. However, mechanisms of DRA gene transcription and its tissue-specific targeting have not yet been investigated. In this study, we cloned a 3,765-bp promoter fragment of human DRA gene and characterized its activity in human colonic LS174T and Caco-2 human colon cell lines. Primer extension identified a single transcriptional initiation site that was identical in both colon cancer cell lines and normal colon. Although hepatic nuclear factor HNF-4 is involved in the basal activity of DRA promoter, sodium butyrate induces its activity in LS174T cells via the binding of Yin Yang 1 (YY1) and GATA transcription factors to their respective cis-elements in promoter region. We also demonstrated a reduction in DRA promoter activity in Caco-2 cells by IFN-gamma, suggesting that regulation of DRA promoter by IFN-gamma may contribute to the pathophysiology of intestinal inflammation. Furthermore, we showed that the DRA promoter fragment is sufficient to drive human growth hormone transgene expression specifically in villus epithelial cells of the small intestine and in differentiated upper crypt and surface epithelial cells of the colon. Our studies provide evidence for the involvement of HNF-4, YY1, and GATA transcription factors in DRA expression in intestinal differentiated epithelial cells.

Gata4 and Hnf1alpha are partially required for the expression of specific intestinal genes during development

The terminal differentiation phases of intestinal development in mice occur during cytodifferentiation and the weaning transition. Lactase-phlorizin hydrolase (LPH), liver fatty acid binding protein (Fabp1), and sucrase-isomaltase (SI) are well-characterized markers of these transitions. With the use of gene inactivation models in mature mouse jejunum, we have previously shown that a member of the zinc finger transcription factor family (Gata4) and hepatocyte nuclear factor-1alpha (Hnf1alpha) are each indispensable for LPH and Fabp1 gene expression but are both dispensable for SI gene expression. In the present study, we used these models to test the hypothesis that Gata4 and Hnf1alpha regulate LPH, Fabp1, and SI gene expression during development, specifically focusing on cytodifferentiation and the weaning transition. Inactivation of Gata4 had no effect on LPH gene expression during either cytodifferentiation or suckling, whereas inactivation of Hnf1alpha resulted in a 50% reduction in LPH gene expression during these same time intervals. Inactivation of Gata4 or Hnf1alpha had a partial effect ( approximately 50% reduction) on Fabp1 gene expression during cytodifferentiation and suckling but no effect on SI gene expression at any time during development. Throughout the suckling period, we found a surprising and dramatic reduction in Gata4 and Hnf1alpha protein in the nuclei of absorptive enterocytes of the jejunum despite high levels of their mRNAs. Finally, we show that neither Gata4 nor Hnf1alpha mediates the glucocorticoid-induced precocious maturation of the intestine but rather are downstream targets of this process. Together, these data demonstrate that specific intestinal genes have differential requirements for Gata4 and Hnf1alpha that are dependent on the developmental time frame in which they are expressed.

Putative intestine-specific enhancers located in 5' sequence of the CDX1 gene regulate CDX1 expression in the intestine

CDX1 is a homeobox transcription factor that plays a critical role in intestinal epithelial cell growth and differentiation. CDX1 gene expression is tightly regulated in a temporal and cell-type specific manner. However, very little is known about the regulatory mechanisms that direct CDX1 gene expression in the intestine. To elucidate these mechanisms, we employed a series of transgenic mouse studies using the 5' flanking sequences of the human CDX1 gene. Transgenic mice containing nucleotides between -5667 and +68 relative to the transcription start site of the CDX1 gene demonstrated ectopic expression of the transgene in the brain and gastric smooth muscle. However, transgenic expression of the nucleotides -15601 to +68 of the CDX1 gene was restricted to the intestinal epithelium, which was identical to endogenous CDX1 gene expression. Taken together, the upstream sequences between -15601 and -5667 contain regulatory elements that direct transgene expression specifically to the intestinal epithelium. Furthermore, DNase I hypersensitivity assays revealed two active chromatin regions in the CDX1 gene (hypertensive sites 1 and 2) located at approximately -5.8 and -6.8 kb upstream of the CDX1 gene, respectively, which may function as potential intestine-specific enhancers.

A novel colonic repressor element regulates intestinal gene expression by interacting with Cux/CDP

Intestinal gene regulation involves mechanisms that direct temporal expression along the vertical and horizontal axes of the alimentary tract. Sucrase-isomaltase (SI), the product of an enterocyte-specific gene, exhibits a complex pattern of expression. Generation of transgenic mice with a mutated SI transgene showed involvement of an overlapping CDP (CCAAT displacement protein)-GATA element in colonic repression of SI throughout postnatal intestinal development. We define this element as CRESIP (colon-repressive element of the SI promoter). Cux/CDP interacts with SI and represses SI promoter activity in a CRESIP-dependent manner. Cux/CDP homozygous mutant mice displayed increased expression of SI mRNA during early postnatal development. Our results demonstrate that an intestinal gene can be repressed in the distal gut and identify Cux/CDP as a regulator of this repression during development.

Differential activation of intestinal gene promoters: functional interactions between GATA-5 and HNF-1 alpha

The effects of GATA-4, -5, and -6, hepatocyte nuclear factor-1 alpha (HNF-1 alpha) and -beta, and Cdx-2 on the rat and human lactase-phlorizin hydrolase (LPH) and human sucrase-isomaltase (SI) promoters were studied using transient cotransfection assays in Caco-2 cells. GATA factors and HNF-1 alpha were strong activators of the LPH promoters, whereas HNF-1 alpha and Cdx-2 were strong activators of the SI promoter, although GATA factors were also necessary for maximal activation of the SI gene. Cotransfection of GATA-5 and HNF-1 alpha together resulted in a higher activation of all three promoters than the sum of the activation by either factor alone, demonstrating functional cooperativity. In the human LPH promoter, an intact HNF-1 binding site was required for functional synergy. This study is the first to demonstrate 1) differential activation of the LPH and SI promoters by multiple transcription factors cotransfected singly and in combination and 2) that GATA and HNF-1 transcription factors cooperatively activate intestinal gene promoters. Synergistic activation is a mechanism by which higher levels of tissue-specific expression might be attained by overlapping expression of specific transcription factors.

The human Na(+)/H(+) exchanger NHE2 gene: genomic organization and promoter characterization

The Na(+)/H(+) exchanger (NHE) 2 belongs to a family of plasma membrane transporters involved in intracellular pH and cell volume regulation. We recently reported cloning of human NHE2 (hNHE2) from a colonic cDNA library. Northern blot analysis has identified NHE2 mRNA only in small intestine, prostate, kidney, colon, and skeletal muscle. In this study, we describe the structure and 5'-regulatory region of the hNHE2 gene. The hNHE2 gene spans >90 kb and is organized in 12 exons intervened by 11 introns. All introns contain the conserved GT and AG dinucleotides at the donor and acceptor sites, respectively. The hNHE2 gene was mapped to chromosome 2q11.2. Primer extension analysis revealed a single transcription initiation site in human colonic adenocarcinoma cell lines. Analysis of the DNA nucleotide sequences of a 1.4-kb fragment of the 5'-flanking region shows no canonical TATA or CAAT boxes. However, the promoter region contains several potential cis-regulatory elements such as Sp1, early growth response-1, activator protein-2, MyoD, p300, nuclear factor-kappaB, myeloid zinc finger protein-1, caudal-related homeobox (Cdx) gene A, and Cdx protein-2 binding sites. In transient transfection studies, a reporter construct containing the 1.4-kb promoter region exhibited low luciferase activity levels. However, after deletion upstream of -664, its activity increased approximately threefold. Thus our data suggest that an inhibitory element may exist in the NHE2 promoter 5'-upstream region.

The human vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor 1 (VPAC1) promoter: characterization and role in receptor expression during enterocytic differentiation of the colon cancer cell line Caco-2Cl.20

The basic organization of the human vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor (VPAC) 1 promoter was investigated after cloning the 5'-flanking region (1.4 kb) of the VPAC1 gene from a human genomic library. Subsequent functional analysis of various deletions of the 5'-flanking sequence, subcloned upstream of a luciferase reporter gene, was carried out in HT-29 cells. The minimal promoter region identified encompasses the -205/+76 sequence and contains a crucial CCAAT box (-182/-178) and a GC-rich sequence. Moreover a region (-1348/-933) containing a silencer element was identified. We previously showed that the expression of the VPAC1 receptor binding site is strictly dependent upon the enterocytic differentiation of human colon cancer Caco-2 cells [Laburthe, Rousset, Rouyer-Fessard, Couvineau, Chantret, Chevalier and Zweibaum (1987) J. Biol. Chem. 262, 10180-10184]. In the present study we show that VPAC1 mRNA increases dramatically when Caco-2Cl.20 cells differentiate, as measured by RNase protection assays and reverse transcriptase-PCR. A single transcript species of 3 kb is detected in differentiated cells by Northern-blot analysis. Accumulation of VPAC1 receptor mRNA is due to a 5-fold increase of transcription rate (run-on assay) without a change in mRNA half-life (9 h). Stable transfections of various constructs in Caco-2Cl.20 cells and subsequent analysis of reporter gene expression, during the enterocytic differentiation process over 25 days of culture, further indicated that the -254/+76 5'-flanking sequence is endowed with the regulatory element(s) necessary for transcriptional regulation of VPAC1 during differentiation. Altogether, these observations provide the first characterization of the basic organization of the human VPAC1 gene promoter and unravel the crucial role of a short promoter sequence in the strict transcriptional control of VPAC1 expression during differentiation of human colon cancer Caco-2 cells.

Glucocorticoids and dietary iron regulate postnatal intestinal heavy and light ferritin expression in rats

To cope with increasing dietary iron exposure, the intestinal epithelium of weaning rats must control intracellular labile iron pools. Intestinal expression of heavy (H) and light (L) ferritin subunits during early weaning and after cortisone administration and/or iron feeding was investigated. Changes in H and L ferritin gene expression were determined by nuclear runoff transcriptional assay, Northern blot analysis, and metabolic labeling of protein synthesis. H ferritin mRNA levels did not change between days 12 and 15, doubled on day 18, and tripled on day 24. L ferritin mRNA was reduced by 50% on days 18 and 24. The protein level of the H and L subunits paralleled the change in mRNAs. Cortisone treatment on day 12 induced a precocious increase of H and decrease of L mRNA expression on day 15. Nuclear runoff assays showed that cortisone did not change H and reduced L ferritin gene transcription. The increased level of H mRNA by cortisone was not translated, unless the rats were fed an iron-fortified diet, which reduced iron regulatory protein activity and stimulated a three- to sixfold increase of ferritin synthesis. Thus changes in intestinal H and L ferritin expression in weaning rats are modulated by glucocorticoids and iron; the former stabilizes H mRNA and suppresses L ferritin gene transcription, and the latter derepresses translation of ferritin mRNA.

Caco-2 cell differentiation is associated with a decrease in stat protein levels and binding

Novel proteins of the Stat (signal transducers and activators of transcription) family have been associated with proliferation and differentiation of certain cells; the role of these transcription factors in gut differentiation has not been examined. The purpose of this study was to determine whether the cellular levels and actual binding of the Stat proteins are altered with intestinal differentiation using the Caco-2 cell line that spontaneously differentiates to a small bowel phenotype after confluency. We found that both Stat3 and Stat5 protein levels were increased in preconfluent and confluent Caco-2 cells; levels then decreased with postconfluency. Mobility shift assays demonstrated maximal binding of Stat3 and Stat5 at confluency and, similar to protein levels, binding activity decreased with postconfluency. The intestinal differentiation marker gene sucrase-isomaltase was increased by postconfluent day 1 with maximal levels by day 6. The progressive decrease of Stat3 and Stat5 protein levels and binding activity, occurring at a time associated with increased Caco-2 cell differentiation, suggests that a decrease in the cellular levels of these proteins may potentially play a role in subsequent intestinal cell differentiation. Delineating the cellular mechanisms responsible for intestinal differentiation is crucial to a better understanding of both normal gut development and aberrant gut growth.

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.

Control of gene expression in intestinal epithelial cells

Coordination of gene transcription is a critical regulatory step in orchestrating developmental, differentiation and adaptation processes in the mammalian intestinal epithelium. Insight into these mechanisms has been gained by the study of transcriptional regulation of the sucrase-isomaltase gene. An understanding of the regulatory network of nuclear proteins that direct transcriptional initiation of intestinal genes such as sucrase-isomaltase will provide insight into the mechanisms of normal development and differentiation as well as disease processes such as neoplasia.

T-cell subset-specific expression of the IL-4 gene is regulated by a silencer element and STAT6

During development of CD4+ T lymphocytes in the periphery, differential expression of cytokine genes, such as those of interleukin (IL)-2 and IL-4, occurs in distinct T-cell subsets. IL-4 is a cytokine produced by T-helper 2 (Th2) cells, and the IL-4 receptor (IL-4R)-mediated signaling pathway is thought to be required for commitment to the Th2 phenotype. However, the molecular basis for development of the Th subset-specific production of IL-4 remains unclear. We demonstrate here that the IL-4 promoter is functional in Th1 and B cells which do not normally form IL-4 transcripts as well as in IL-4-producing T cells. Based on studies of the effect of several different upstream and downstream regions of the IL-4 gene on IL-4 promoter activity, a Th1-specific IL-4 silencer element was identified in the 3'-untranslated region. The silencer region contained a consensus sequence for a transcriptional factor that is normally regulated by the IL-4 R signaling pathway, STAT6. Nuclear expression of STAT6 protein, which was shown to bind to the silencer region, was observed in Th2 cells but not in Th1 cells. Deletion of the STAT6-binding site from the silencer region and inhibition of STAT6 function resulted in the appearance of silencing function even in Th2 cells. These results provide evidence that the silencer element, and the binding of STAT6 to this element, play a permissive role in determining the commitment into Th2 phenotype.

Identification and characterization of the MUC2 (human intestinal mucin) gene 5'-flanking region: promoter activity in cultured cells

The initiation point for MUC2 gene transcription is located within a 7000-base GC-rich region of the mucin gene cluster found on chromosome 11p15.5. The promoter activity of the 5'-flanking region of the MUC2 gene was examined following its cloning into the luciferase-producing pGL2-Basic reporter vector. A short segment comprising bases -91 to -73 relative to the start of transcription was found to be important for basal promoter activity in all cell lines tested. Electrophoretic mobility shift assays demonstrated nuclear protein binding to this region, which contains the consensus CACCC motif (5'-GCCACACCC). This element has been shown to be functionally important in several promoters that are active in diverse cell types. Competition experiments using an Sp1 oligonucleotide and antibody supershift experiments indicated that both Sp1 and other Sp1 family members bind to this element. Inclusion of the region between bases -228 and -171 in pGL2-Basic constructs increased normalized luciferase reporter activity by almost 3-fold in C1a cells, which produce relatively high levels of MUC2 mRNA. Significantly lower levels of normalized luciferase activity resulted when the same construct was transfected into cultured cell lines that express low or undetectable levels of MUC2, suggesting a possible role for this region in conferring cell-type specificity of expression. We also demonstrate, using actinomycin D, that the MUC2 mRNA is long-lived, at least in cultured cells. Moreover, no evidence was found that the MUC2 mRNA turned over more rapidly in LS174T cells, which produce relatively low levels of MUC2 mRNA, as compared with C1a cells, which produce high levels of mRNA. Thus a long mRNA half-life appears to be an important mechanism involved in achieving elevated levels of MUC2 mRNA.

Regulation of lactase-phlorizin hydrolase gene expression by the caudal-related homoeodomain protein Cdx-2

Lactase-phlorizin hydrolase is exclusively expressed in the small intestine and is often used as a marker for the differentiation of enterocytes. The cis-element CE-LPH1 found in the lactase-phlorizin hydrolase promoter has previously been shown to bind an intestinal-specific nuclear factor. By electrophoretic mobility-shift assay it was shown that the factor Cdx-2 (a homoeodomain-protein related to caudal) binds to a TTTAC sequence in the CE-LPH1. Furthermore it was demonstrated that Cdx-2 is able to activate reporter gene transcription by binding to CE-LPH1. A mutation in CE-LPH1, which does not affect Cdx-2 binding, results in a higher transcriptional activity, indicating that the CE-LPH1 site contains other binding site(s) in addition to the Cdx-2-binding site.