Changes in keratin expression during fetal and postnatal development of intestinal epithelial cells

Cytokeratin 18 is a specific marker of bovine intestinal M cell

Microfold (M) cells in the follicle-associated epithelium (FAE) of Peyer's patches have an important role in mucosal immune responses. A primary difficulty for investigations of bovine M cells is the lack of a specific molecular marker. To identify such a marker, we investigated the expression of several kinds of intermediate filament proteins using calf Peyer's patches. The expression patterns of cytokeratin (CK) 18 in jejunal and ileal FAE were very similar to the localization pattern of M cells recognized by scanning electron microscopy. Mirror sections revealed that jejunal CK18-positive cells had irregular and sparse microvilli, as well as pocket-like structures containing lymphocytes, typical morphological characteristic of M cells. However, CK18-negative cells had regular and dense microvilli on their surface, typical of the morphology of enterocytes. In contrast, CK20 immunoreactivity was detected in almost all villous epithelial cells and CK18-negative cells in the FAE. CK18-positive proliferating transit-amplifying cells in the crypt exchanged CK18 for CK20 above the mouth of the crypt and after moving to the villi; however, CK18-positive M cells in the crypt continued their expression of CK18 during movement to the FAE region. Terminal deoxynucleotidyl-transferase-mediated deoxyuridine-triphosphate-biotin nick-end labeling-positive apoptotic cells were specifically detected at the apical region of villi and FAE in the jejunum and ileum, and all were also stained for CK20. These data indicate that CK18 may be a molecular marker for bovine M cells in FAE and that M cells may transdifferentiate to CK20-positive enterocytes and die by apoptosis in the apex of the FAE.

K-ras mutation targeted to gastric tissue progenitor cells results in chronic inflammation, an altered microenvironment, and progression to intraepithelial neoplasia

Chronic infectious diseases, such as Helicobacter pylori infection, can promote cancer in a large part through induction of chronic inflammation. Oncogenic K-ras mutation in epithelial cells activates inflammatory pathways, which could compensate for a lack of infectious stimulus. Gastric histopathology and putative progenitor markers [doublecortin and calcium/calmodulin-dependent protein kinase-like 1 (Dcamkl1) and keratin 19 (K19)] in K19-K-ras-V12 (K19-kras) transgenic mice were assessed at 3, 6, 12, and 18 months of age, in comparison with Helicobacter felis-infected wild-type littermates. Inflammation was evaluated by reverse transcription-PCR of proinflammatory cytokines, and K19-kras mice were transplanted with green fluorescent protein (GFP)-labeled bone marrow. Both H. felis infection and K-ras mutation induced upregulation of proinflammatory cytokines, expansion of Dcamkl1(+) cells, and progression to oxyntic atrophy, metaplasia, hyperplasia, and high-grade dysplasia. K19-kras transgenic mice uniquely displayed mucous metaplasia as early as 3 months and progressed to high-grade dysplasia and invasive intramucosal carcinoma by 20 months. In bone marrow-transplanted K19-kras mice that progressed to dysplasia, a large proportion of stromal cells were GFP(+) and bone marrow-derived, but only rare GFP(+) epithelial cells were observed. GFP(+) bone marrow-derived cells included leukocytes and CD45(-) stromal cells that expressed vimentin or α smooth muscle actin and were often found surrounding clusters of Dcamkl1(+) cells at the base of gastric glands. In conclusion, the expression of mutant K-ras in K19(+) gastric epithelial cells can induce chronic inflammation and promote the development of dysplasia.

Identification of a bone marrow-derived mesenchymal progenitor cell subset that can contribute to the gastric epithelium

Recent studies with Helicobacter-infected mice have shown that bone marrow-derived cells can repopulate the gastric epithelium and progress to cancer. However, it has not been established which cellular subset can potentially contribute to the epithelium. The aim of this study was to investigate the ability of bone marrow-derived mesenchymal stem cells (MSCs) that express cytokeratin 19 (K19) to contribute to the gastric epithelium. MSCs cultures were established from whole bone marrow and expression of K19 was detected in a minority (1 of 13) of clones by real-time PCR and immunostaining. Transfection of a K19-green fluorescent protein (GFP) vector and isolation of GFP-expressing colonies generated high K19-expressing MSC clones (K19GFPMSC). Incubation of MSCs with gastric tissue extract markedly induced mRNA expression of gastric phenotypic markers and was observed to a greater extent in K19GFPMSCs compared with parental MSCs and mock transfectants. Both K19GFPMSCs and GFP-labeled control MSCs gave rise to gastric epithelial cells after injection into the murine stomach. In addition, after blastocyst injections, K19GFPMSCs gave rise to GFP-positive gastric epithelial cells in all 13 pups, whereas only 3 of 10 offspring showed GFP-positive gastric epithelial cells after injection of GFP-labeled control MSCs. Although K19 expression could not be detected in murine whole bone marrow, H. felis infection increased K19-expressing MSCs in the circulation. Taken together, our results show that bone marrow-derived MSCs can contribute to the gastric epithelium. The K19-positive MSC fraction that is induced by chronic H. felis infection appears to be the important subset in this process.

Colony-stimulating factor-1 promotes clonogenic growth of normal murine colonic crypt epithelial cells in vitro

The intestinal epithelium is a continuously renewing tissue. In the colon, stem cells are maintained at the base of highly organized crypts, where they undergo asymmetric division and give rise to daughter cells that proliferate and migrate up the crypt as they differentiate, then become senescent and are finally shed into the intestinal lumen. The growth factor requirements of fetal and prenatal colon cells for colony formation and that influence the establishment of cell lines from Immorto-mouse (Charles River, Wilmington, MA) transgenic embryos were explored. Single cell suspensions were isolated and cultured in a large range of growth factor combinations and conditions to determine their growth properties in soft agar. We report an important advance in the culture of mouse colonocytes by using macrophage colony-stimulating factor (CSF-1) and granulocyte-macrophage colony-stimulating factor (GM-CSF). A substantial proportion of colonies grown under low oxygen tension in the presence of CSF-1 and GM-CSF express intestinal epithelial A33 antigen, have the expected gene expression profile, including c-fms and transcription factor c-myb, and show an appropriate epithelial cell morphology and undetectable CD45. Confocal microscopy on isolated crypts displays basolateral expression of c-Fms and E-cadherin on most epithelial cells. Fetal colon cultures from the Immorto-mouse with CSF-1 produced rapid outgrowth and readily established cell lines, in contrast to cultures without CSF-1. These observations have implications for the understanding of colon epithelial development and recovery following cytotoxic damage as well as providing a basis for the observation that some colon (and other epithelial) tumor cells respond to CSF-1 and GM-CSF.

Expression of mucins and cytokeratins in primary carcinomas of the digestive system

To determine the most optimal treatment of cancer patients, it is fundamental to classify human carcinomas according to their primary anatomical site of origin. As for some patients, it is difficult to identify cancers occurring at obscure location and overlapping adjacent sites. The aim of this study is to partition the primary site of 486 patients in cancers of the digestive system by the expression pattern of the mucins and cytokeratins typifying each site. The expressions of MUC1, MUC2, MUC5AC, MUC6, CK7, CK8, CK13, CK14, CK18, CK19 and CK20 were evaluated immunohistochemically in 426 adenocarcinomas and 60 hepatocellular carcinomas using the tissue-array method. The finding of MUC series showed their characteristics in case of MUC2 in the appendix cancer and MUC1 and 5AC in pancreas cancer. As for CKs 7, 13, and 19, and 20 had a feature in cancers of common bile duct, liver, and appendix, respectively. We classified cancers in 11 sites by characteristic expression of antibodies. The sensitivity, specificity, positive predictive value, and diagnostic efficacy of significant antibodies were calculated with deducing the dichotomous tree made by SPSS 10.0. Six of 11 antibodies, CK 7, CK13, CK19, CK20, MUC1, and MUC5AC distinguished 6 groups from 11 sites. We also executed the clustering of cancers to investigate total relationship among cancers. They fell into three categories, which corresponded to embryologic origin. Unlike other sites, the small intestine and colorectum cancers expressed significantly different patterns to their sublocations. Mucins and CKs showed expression patterns to classify the primary sites of digestive cancers and may be helpful in predicting the primary sites of digestive cancers.

The keratin 19 promoter is potent for cell-specific targeting of genes in transgenic mice

BACKGROUND & AIMS

Keratins are intermediate filaments that are critical in cytoskeletal organization. Their roles in cellular processes are underscored by inherited human diseases in which germline mutations of keratins are found, as well as by transgenic and knockout mouse models that recapitulate those diseases. Keratin 19 (K19) has unique structural properties and developmental and spatial expression patterns. This suggests that K19 expression may correlate with important cell fate decisions in gastrointestinal tract epithelia.

METHODS

We used mouse K19 5' untranslated region and promoter sequences and fused it to the lacZ reporter gene in a transgene construct. Characterization was by beta-galactosidase expression and X-gal histochemistry in gastrointestinal epithelia. Because endogenous K19 protein is transcriptionally regulated by the Kruppel-like transcription factor 4 (KLF4), we determined the spatial expression patterns of KLF4 and K19 in relationship to the lacZ reporter gene product.

RESULTS

K19-lacZ transgenic mice were found to have reporter gene expression in an epithelial-specific pattern. Expression was restricted to ductal epithelial cells in the pancreas, surface colonocytes, small intestinal villi, and gastric isthmus cells. Transgene expression correlated with K19 and KLF4 protein expression in the pancreas and stomach and was overlapping in the small and large intestine.

CONCLUSIONS

The K19 promoter may be a useful tool to study epithelial cell biology and subsequent transdifferentiation programs, particularly the pancreas and stomach.

Characterization of colonic polyps by two-dimensional gel electrophoresis

To identify proteins that may specifically characterize colonic polyps we have investigated the abundance of numerous proteins in epithelial cells from 15 normal colon specimens and 13 colonic polyps, using two-dimensional gel analysis to detect possible differences in expression. Silver-stained digitized images of the gels were analyzed with Melanie II 2.1 software. We consistently detected more than 700 protein spots on each gel, and found that the intensity of 59 of them was significantly altered in polyp specimens (Wilcoxon test assuming P< or =0.05). Immunostaining, microsequencing and mass spectrometry (matrix-assisted laser desorption/ionization - time of flight; MALDI-TOF) techniques were used to identify these proteins and selected others that did not show differential regulation. The expression of numatrin (nucleophosphine/B23), hsp 70, and hsp60 was increased in polyps; levels of fatty acid binding protein (L-FABP), 14-3-3 sigma, citokeratin 20, cytochrome c oxidase polypeptide Va, Rho GDP-dissociation inhibitor (Rho GDI), and beta- and gamma-actins were decreased. Although the levels of expression of a given protein often varied among polyp specimens, it generally held true that the direction of variation (up or down) remained constant across the panel. We concluded that proteins showing constant differential regulation across all or most of the polyp specimens represent the most characteristic regulatory pathways in colon polyps, while more sporadic variations reflect characteristics of individual polyps.

The apical submembrane cytoskeleton participates in the organization of the apical pole in epithelial cells

In a previous publication (Rodriguez, M.L., M. Brignoni, and P.J.I. Salas. 1994. J. Cell Sci. 107: 3145-3151), we described the existence of a terminal web-like structure in nonbrush border cells, which comprises a specifically apical cytokeratin, presumably cytokeratin 19. In the present study we confirmed the apical distribution of cytokeratin 19 and expanded that observation to other epithelial cells in tissue culture and in vivo. In tissue culture, subconfluent cell stocks under continuous treatment with two different 21-mer phosphorothioate oligodeoxy nucleotides that targeted cytokeratin 19 mRNA enabled us to obtain confluent monolayers with a partial (40-70%) and transitory reduction in this protein. The expression of other cytoskeletal proteins was undisturbed. This downregulation of cytokeratin 19 resulted in (a) decrease in the number of microvilli; (b) disorganization of the apical (but not lateral or basal) filamentous actin and abnormal apical microtubules; and (c) depletion or redistribution of apical membrane proteins as determined by differential apical-basolateral biotinylation. In fact, a subset of detergent-insoluble proteins was not expressed on the cell surface in cells with lower levels of cytokeratin 19. Apical proteins purified in the detergent phase of Triton X-114 (typically integral membrane proteins) and those differentially extracted in Triton X-100 at 37 degrees C or in n-octyl-beta-D-glycoside at 4 degrees C (representative of GPI-anchored proteins), appeared partially redistributed to the basolateral domain. A transmembrane apical protein, sucrase isomaltase, was found mispolarized in a subpopulation of the cells treated with antisense oligonucleotides, while the basolateral polarity of Na+-K+ATPase was not affected. Both sucrase isomaltase and alkaline phosphatase (a GPI-anchored protein) appeared partially depolarized in A19 treated CACO-2 monolayers as determined by differential biotinylation, affinity purification, and immunoblot. These results suggest that an apical submembrane cytoskeleton of intermediate filaments is expressed in a number of epithelia, including those without a brush border, although it may not be universal. In addition, these data indicate that this structure is involved in the organization of the apical region of the cytoplasm and the apical membrane.

Immunohistochemical evaluation of keratin 20 expression in intestinal metaplasia types I to III

AIMS

To investigate differences in expression of keratin 20, a cytoskeletal protein in gastrointestinal epithelial cells, in completely differentiated intestinal metaplasia (type I) and incomplete metaplasia (types II and III).

METHODS

Gastric biopsy specimens from 66 patients with intestinal metaplasia were analysed immunohistochemically. Expression of keratin 20 was quantified as the percentage of immunoreactive cells on the tips, the upper, and deep foveolae.

RESULTS

In all specimens keratin 20 was found on the tips and in the upper foveolae of intestinal metaplasia. Keratin 20 was not observed in the deep foveolae. No differences were seen between the antrum and the body. Expression patterns were comparable between types I and III. In type II, however, lower immunoreactivity was found. Both the differences between types I and II as well as between types II and III were significant (p < 0.05).

CONCLUSIONS

Keratin 20 is expressed in metaplastic mucosa as a result of intestinal differentiation. Positive staining found exclusively in juxtaluminal cells occurs only in mature cells containing keratin 20. Lowered immunoreactivity in type II compared with types I and III indicates the different nature of type II intestinal metaplasia. Further studies are needed to shed light on the basic fundamental mechanism responsible for this.

Switch in the expression of the K19/K18 keratin genes as a very early evidence of testicular differentiation in the rat

It has been shown previously that acidic K18 and K19 keratins display a differential immunohistochemical pattern of expression during sexual differentiation of the gonads in the rat (Fridmacher et al. (1992) Development 115, 503-517). The present results indicate that K18 and K19 gene expression is regulated at the transcriptional level. The analysis was performed by Northern Blot, reverse transcriptase polymerase chain reaction (RT-PCR) and in situ hybridization. PCR products were cloned, sequenced and used as species-specific K18 and K19 riboprobes for in situ hybridization. K19 mRNA but not K18 mRNA was detected in undifferentiated gonads and in somatic cells of ovarian cords throughout the fetal ovary development. K18 mRNA expression appeared in male gonads, at 13.5 days of gestation, at the onset of testicular differentiation, as the first Sertoli cells differentiated and aggregated to form seminiferous cords. As testicular differentiation progressed, K19 mRNA disappeared and, from 14.5 days of gestation on, fetal Sertoli cells expressed exclusively K18 mRNA. The changes in the transcriptional activity of K19 and K18 genes, observed in male gonads, occur characteristically at the very beginning of testicular differentiation. In the male pathway of sexual differentiation, the switch in K19/K18 gene expression is, in addition to the activation of the anti-Müllerian hormone gene, the most precocious regulative event occurring after the expression of the testis determining factor SRY.

Developmental, nutritional, and hormonal regulation of the pancreatitis-associated protein I and III gene expression in the rat small intestine

BACKGROUND

The pancreatitis-associated protein (PAP) I and III, but not PAP II, mRNAs are constitutively expressed in the small intestine of rats.

METHODS

We studied expression of both PAP I and PAP III mRNAs during development and on nutritional and hormonal manipulations.

RESULTS

Between day 20 of gestation and day 21 of age, PAP mRNAs could barely be detected. Their concentrations increased dramatically from day 21 to day 45 of age and remained constant thereafter. Rats adapted to a diet with low carbohydrate content showed a significant decrease in PAP mRNA concentrations. Finally, whereas thyroidectomy and ovariectomy induced a decrease in both mRNA concentrations, and adrenalectomy a limited decrease in PAP III mRNA only, diabetes and castration did not alter the expression of either gene.

CONCLUSION

Gene expression of PAP I and III mRNAs is regulated in a coordinate manner in the rat small intestine during development and on nutritional and hormonal manipulations.

The network organization and the phosphorylation of cytokeratins are concomitantly modified by forskolin in the enterocyte-like differentiated Caco-2 cell line

Confluent Caco-2 cells, originating from a human colon carcinoma, display morphological and functional characteristics of differentiated enterocytes such as the presence of a polarized monolayer covered by an apical brush border that express several hydrolases. The adaptation of these cells to grow in the continuous presence of forskolin, a drug known to stimulate adenylyl cyclase permanently, has been previously shown to result in a decreased apical expression of hydrolases and in morphological alterations including the disappearance of intercellular spaces and shortening of microvilli. In the present work we have analyzed the possibility that cytoskeletal proteins may be the target of forskolin in living Caco-2 cells. We show that forskolin initiates dramatic changes in the spatial organization of the cytokeratin network that correlate with an increased phosphorylation of cytokeratin molecules, whereas microtubules, microfilaments and vimentin remain mainly unaffected. Indirect immunofluorescence studies show that the cytokeratin network is redistributed from the cell periphery to the cytoplasm. Biochemical experiments indicate that forskolin doesn't interfere with the cytokeratin profile, since the three cytokeratins normally found in intestine (CK 8, CK 18, CK 19) are similarly expressed in both control and forskolin-Caco-2 cells. Analysis of 32P-labeled cytokeratin extracted from the two cell populations demonstrates that forskolin quantitatively increases the phosphorylation of type I cytokeratin (CK 18 and CK 19), whereas the phosphorylation of type II cytokeratin (CK 8) is altered both quantitatively and qualitatively with the emergence of a new phosphorylation site. These results provide a new cell system in which it is possible to control the subcellular distribution of cytokeratin by changing their phosphorylation status and therefore to study their potential cellular functions.

Identification of a 102 kDa protein (cytocentrin) immunologically related to keratin 19, which is a cytoplasmically derived component of the mitotic spindle pole

The mAb RK7, previously shown to recognize keratin 19, was also found to cross-react with a biologically unrelated 102 kDa protein, which becomes associated with the poles of the mitotic apparatus. This newly identified protein, called cytocentrin, is a stable cellular component, may be at least in part phosphorylated, and displays a cell cycle-dependent cellular localization. In interphase cells, it is diffusely distributed in the cytosol and shows no affinity for cytoplasmic microtubules. It becomes localized to the centrosome in early prophase, prior to nuclear envelope breakdown, separation of replicated centrosomes, and nucleation of mitotic apparatus microtubules. During metaphase, cytocentrin is located predominately at the mitotic poles, often appearing as an aggregate of small globular sub-components; it also associates with some polar microtubules. In late anaphase/early telophase cytocentrin dissociates entirely from the mitotic apparatus and becomes temporarily localized with microtubules in the midbody, from which it disappears by late telophase. In taxol-treated cells cytocentrin was associated with the center of the miniasters but also showed affinity for some cytoplasmic microtubules. Studies employing G2-synchronized cells and nocodazole demonstrated that cytocentrin can become associated with mitotic centrosomes independently of tubulin polymerization and that microtubules regrow from antigen-containing foci. We interpret these results to suggest that cytocentrin is a cytoplasmic protein that becomes specifically activated or modified at the onset of mitosis so that it can affiliate with the mitotic poles where it may provide a link between the pericentriolar material and other components of the mitotic apparatus.

Conditionally immortalized intestinal epithelial cells: novel approach for study of differentiated enterocytes

Clonal cell lines have been established from primary fetal rat intestinal epithelial cells by stable transfection with plasmids containing either the simian virus 40 (SV40) large T-antigen gene under the control of the heavy metal inducible metallothionein promoter (pMTWt) or the thermolabile SV40 T-antigen gene under the control of the SV40 early promoter (pZipSVtsa58). pMTWt-transfected cells produced sufficient T-antigen to allow them to proliferate both when the metallothionein promoter was induced and uninduced. No differences were observed in the pattern of intestinal epithelial markers expressed when the cells were cultured in the presence or absence of inducing agent (zinc). In contrast, fetal rat intestinal epithelial cells transfected with pZipSVtsa58 were immortalized conditionally; cells proliferated at 32 degrees C but ceased to proliferate between 48 and 72 h of culture at 39 degrees C. Four of these cell lines were characterized in detail; they showed microvilli and tight junctions as well as dome formation and expressed functional and biochemical markers of intestinal epithelial cells, including keratins 8, 19, and 21, aminopeptidase N, and dipeptidyl peptidase IV. One cell line, 2/4/A1, expressed in addition a low level of lactase and sucrase-isomaltase. The amount and/or activity of some of these markers changed during the switch from the proliferative to the nonproliferative state (switch from culture at 32 to 39 degrees C), resulting in a more differentiated phenotype and mimicking similar changes taking place during intestinal epithelial cell differentiation in vivo.

The human gene encoding cytokeratin 20 and its expression during fetal development and in gastrointestinal carcinomas

The differentiation of the predominant cell types of the mucosal epithelium of the mammalian gastrointestinal tract is characterized by increasing amounts of an intermediate-sized filament (IF) protein designated cytokeratin (CK) 20 which is a major cellular protein of mature enterocytes and goblet cells. Here we report the isolation of the human gene encoding CK 20, its complete nucleotide sequence and the amino acid sequence deduced therefrom that identifies this polypeptide (mol. wt. 48553) as a member of the type I-CK subfamily. Remarkable, however, is the comparably great sequence divergence of CK 20 from all other known type I-CKs, with only 58% identical amino acids in the conserved alpha-helical 'rod' domain of CK 20 and, e.g. CK 14. Using riboprobes corresponding to exon 6 of the gene in Northern blot and ribonuclease protection assays, we show that the approximately 1.75 kb mRNA encoding CK 20 is specifically produced in cells of the intestinal and gastric mucosa, including tumors and cell lines derived therefrom. The appearance of CK 20-positive cells in human embryonic and fetal development and in adult tissues has been studied using immunohistochemistry with CK 20-specific antibodies. CK 20 synthesis has first been recognized at embryonic week 8 in individual 'converted' simple epithelial cells of the developing intestinal mucosa. In later fetal stages, CK 20 synthesis extends over most goblet cells and a variable number of villus enterocytes. The distribution of CK 20-positive cells in the developing gastric and intestinal mucosa is similar to--but not identical with--the pattern in the adult intestine in which all enterocytes and goblet cells as well as certain 'low-differentiated' columnar cells contain CK 20, whereas the neuroendocrine ('enterochromaffin') and Paneth cells are negative. In gastrointestinal carcinomas similarly examined, CK 20 has been detected in almost all cases (50/52) of colorectal adenocarcinomas, including all grades of differentiation and malignancy and also metastatic tumors, whereas CK 20 immunostaining in gastric carcinomas has been found less consistent and more heterogeneous. The possible biological meaning of the specific expression of the CK 20 gene in certain cells of the gastrointestinal tract and carcinomas derived therefrom and the regulatory mechanisms involved in the integration of the protein in the IF cytoskeleton are discussed.

Differential localization by in situ hybridization of distinct keratin mRNA species during intestinal epithelial cell development and differentiation

The distribution of the major keratin mRNAs expressed during terminal differentiation and fetal development of the rat intestinal epithelium has been examined by in situ hybridization. We have obtained and characterized a partial cDNA clone encoding human keratin 20 whose sequence spans from the coil la region through the 3' poly(A) tail. Sequence data and immunoblot analysis demonstrated that keratin 20 is the human homologue of the rat keratin 21, suggesting the existence of a single type I keratin specifically expressed by differentiated intestinal epithelial cells. Four cRNA probes, specific for keratins 8, 18, 19, and 20 respectively, were prepared and found to specifically hybridize with their respective mRNA species from total intestinal RNA preparations. Analysis of frozen tissue sections by in situ hybridization revealed that, in the adult intestine, keratin 18 and 19 mRNAs are restricted to the region of the crypts, keratin 8 mRNA is found along the entire crypt-villus axis, and keratin 20 mRNA is expressed only by the differentiated villus cells. This pattern is established late during fetal rat intestinal development: in the undifferentiated stratified epithelium present at 16 days gestation (16dg) mRNAs coding for keratins 8, 18, and 19 are expressed by all epithelial cells and keratin 20 mRNA is absent. Upon completion of villus formation at 20dg (2 days before birth) keratin 18 and 19 mRNAs become strictly confined to cells at the base of the nascent villi and we observed the appearance of keratin 20 mRNA which, like keratin 8 mRNA, is expressed by the entire epithelium. These results strongly suggest that transcriptional regulation of keratin genes in the intestinal epithelium occurs at the level of both immature and terminally differentiated epithelial cells, and is tightly regulated during both fetal development and crypt-to-villus differentiation of the intestinal epithelium.