The homeobox gene Cdx1 belongs to the p53-p21(WAF)-Bcl-2 network in intestinal epithelial cells

Elucidation of the mechanism underlying CD44v6-induced transformation of IEC-6 normal intestinal epithelial cells

The transformation abilities of CD44s and CD44v6 in normal intestinal epithelial cells have not yet been reported. Herein, we established both CD44s and CD44v6 overexpressing stable clones from rat IEC-6 cells and demonstrated that the CD44v6 clones had higher saturation density and anchorage independence. Additionally, CD44v6 clones were more resistant to oxaliplatin and irinotecan which might be attributed to a significantly increased B-cell lymphoma 2 level and a reduced DNA damage response in these cells. Moreover, c-Met and vascular endothelial growth factor receptor 2 signalings were involved in modulating the saturation density in CD44v6 clones. Interestingly, higher activation of both AKT and extracellular-signal-regulated kinase (ERK) were detected in CD44v6 clones which might account in part for the cell density-independent nuclear localization of Yes-associated protein (YAP). To no surprise, increases of both saturation density and anchorage independence in CD44v6 clones were markedly diminished by PI3K, AKT, MEK, and ERK inhibitors as well as YAP knockdown. By contrast, overexpression of a constitutively active YAP robustly increased the aforementioned phenotypes in IEC-6 cells. Collectively, our results suggest that upregulation of CD44v6, but not CD44s, induces the transformation of normal intestinal epithelial cells possibly via activating the c-Met/AKT/YAP pathway which might also explain the important role of CD44v6 in the initiation of various carcinomas.

Homeodomain Protein Transforming Growth Factor Beta-Induced Factor 2 Like, X-Linked Function in Colon Adenocarcinoma Cells

Background:

TGIF2LX (transforming growth factor beta-induced factor 2 like, X-linked) is a homeodomain (HD) protein that has been implicated in the negative regulation of cell signaling pathways. The aim of this study was to investigate the possible functions of TGIF2LX in colon adenocarcinoma cells.

Methods:

The human SW48 cell line was transfected with cDNA for the wild-type TGIF2LX gene and gene/protein over-expression was confirmed by microscopic analysis, real time RT-PCR and Western blotting techniques. In vitro cell proliferation was evaluated by MTT and BrdU assays. After developing a colon tumor model in nude mice, immunohistochemical (IHC) staining of tumor tissue was carried out for Ki-67 (proliferation) and CD34 (angiogenesis) markers. To predict potential protein partners of TGIF2LX, in-silico analysis was also conducted.

Results:

Obtained results showed over-expression of TGIF2LX as a potential transcription factor could inhibit either proliferation or angiogenesis (P<0.05) in colon tumors. In-silico results predicted interaction of TGIF2LX with other proteins considered important for cellular development.

Conclusions:

Our findings provided evidence of molecular mechanisms by which TGIF2LX could act as a tumor suppressor in colon adenocarcinoma cells. Thus, this gene may potentially be a promising option for colon cancer gene-based therapeutic strategies.

Cell Fusion Connects Oncogenesis with Tumor Evolution

Cell fusion likely drives tumor evolution by undermining chromosomal and DNA stability and/or by generating phenotypic diversity; however, whether a cell fusion event can initiate malignancy and direct tumor evolution is unknown. We report that a fusion event involving normal, nontransformed, cytogenetically stable epithelial cells can initiate chromosomal instability, DNA damage, cell transformation, and malignancy. Clonal analysis of fused cells reveals that the karyotypic and phenotypic potential of tumors formed by cell fusion is established immediately or within a few cell divisions after the fusion event, without further ongoing genetic and phenotypic plasticity, and that subsequent evolution of such tumors reflects selection from the initial diverse population rather than ongoing plasticity of the progeny. Thus, one cell fusion event can both initiate malignancy and fuel evolution of the tumor that ensues.

Autophagy of cancer stem cells is involved with chemoresistance of colon cancer cells

BACKGROUND

Chemoresistance is a major cause of treatment failure in colon cancer, and cancer stem cells have been found to be involved in the chemoresistance of colon cancer. However, the mechanisms driving the chemoresistance of colon cancer stem cells have not been addressed.

METHODS

In this study, we investigated the cytotoxicity of paclitaxel in CD44(+)CD24(+) SW1222 colon cancer cells expressing Cdx1 (CD44(+)CD24(+)Cdx1(+) stem cells) and CD44(+)CD24(+) HCT116 colon cancer cells expressing wild-type p53 (CD44(+)CD24(+)p53wt stem cells).

RESULTS

SW1222 cells were more resistant to paclitaxel-induced cytotoxicity than HCT116 cells. Conversely, HCT-116 cells had higher matrigel colony formation ability than SW1222 cells. The isolated CD44(+)CD24(+)Cdx1(+) cells showed higher resistance to paclitaxel-induced cytotoxicity than CD44(+)CD24(+)p53wt cells. The resistance of CD44(+)CD24(+)Cdx1(+) cells to paclitaxel is associated with upregulation of Cdx1 and Bcl-2 expression, caspase-3 activity, and the ratio of LC3-II/LC3-I. The sensitivity of CD44(+)CD24(+)p53wt cells to paclitaxel is associated with the downregulation of Bcl-2 expression, upregulation of Bax levels, and upregulation of caspase-3 activity. Silencing of Cdx1 expression and treatment with lysosomal inhibitor bafilomycin A increased paclitaxel-induced cytotoxicity in CD44(+)CD24(+)Cdx1(+) cells. Conversely, overexpression of Cdx1 decreased cell death in CD44(+)CD24(+)p53wt cells. Intratumoral injection of Cdx1 siRNA significantly inhibited tumor growth in a xenograft tumor model inoculated with CD44(+)CD24(+)Cdx1(+) cancer cells.

CONCLUSION

Cdx1 exerts a protective role in colon cancer stem cells, which play a crucial role in chemoresistance to paclitaxel through activation of autophagy. Autophagy is activated though the Cdx1-Bcl-2-LC3 pathway. In contrast, p53 exerts a major role in apoptosis and inhibits autophagy in colon cancer stem cells.

A WNT/p21 circuit directed by the C-clamp, a sequence-specific DNA binding domain in TCFs

The lymphoid enhancer factor 1/T cell factor (LEF/TCF) family of transcription factors are downstream effectors of the WNT signaling pathway, which drives colon tumorigenesis. LEF/TCFs have a DNA sequence-specific high-mobility group (HMG) box that binds Wnt response elements (WREs). The "E tail" isoforms of TCFs are alternatively spliced to include a second DNA binding domain called the C-clamp. We show that induction of a dominant negative C-clamp version of TCF1 (dnTCF1E) induces p21 expression and a stall in the growth of DLD1 colon cancer cells. Induction of a C-clamp mutant did not efficiently induce p21, nor did it stall cell growth. Microarray analysis revealed that induction of p21 by wild-type dnTCF1E (dnTCF1E(WT)) correlated with a decrease in expression of multiple p21 suppressors that act at multiple levels from transcription (SP5, YAP1, and RUNX1), RNA stability (MSI2), and protein stability (CUL4A). We show that the C-clamp is a sequence-specific DNA binding domain that can make contacts with 5'-RCCG-3' elements upstream or downstream of WREs. The C-clamp-RCCG interaction was critical for TCF1E-mediated transcriptional control of p21-connected target gene promoters. Our results indicate that a rapid-response WNT/p21 circuit is driven by C-clamp target gene selection.

Methylation-dependent activation of CDX1 through NF-κB: a link from inflammation to intestinal metaplasia in the human stomach

The caudal homeobox factor 1 (CDX1) is an essential transcription factor for intestinal differentiation. Its aberrant expression in intestinal metaplasia of the upper gastrointestinal tract is a hallmark within the gastritis-metaplasia-carcinoma sequence. CDX1 expression is influenced by certain pathways, such as Wnt, Ras, or NF-κB signaling; however, these pathways alone cannot explain the transient expression of CDX1 in intestinal metaplasia or the molecular inactivation mechanism of its loss in cases of advanced gastric cancer. In this study, we investigated the epigenetic inactivation of CDX1 by promoter methylation, as well as the functional link of CDX1 promoter methylation to the inflammatory NF-κB signaling pathway. We identified methylation-dependent NF-κB binding to the CDX1 promoter and quantified it using competitive electrophoretic mobility shift assays and chromatin immunoprecipitation. A methylated CDX1 promoter was associated with closed chromatin structure, reduced NF-κB binding, and transcriptional silencing. Along the gastritis-metaplasia-carcinoma sequence, we observed a biphasic pattern of tumor necrosis factor-α (TNF-α) protein expression and an inverse biphasic pattern of CDX1 promoter methylation; both are highly consistent with CDX1 protein expression. The stages of hyper-, hypo-, and hyper-methylation patterns of the CDX1 promoter were inversely correlated with the NF-κB signaling activity along this sequence. In conclusion, these functionally interacting events drive CDX1 expression and contribute to intestinal metaplasia, epithelial dedifferentiation, and carcinogenesis in the human stomach.

Cdx4 is a Cdx2 target gene

The products of the Cdx genes, Cdx1, Cdx2 and Cdx4, play multiple roles in early vertebrate development, and have been proposed to serve to relay signaling information from Wnt, RA and FGF pathways to orchestrate events related to anterior-posterior vertebral patterning and axial elongation. In addition, Cdx1 and Cdx2 have been reported to both autoregulate and to be subject to cross regulation by other family members. We have now found that Cdx4 expression is significantly down regulated in Cdx2(-/-) mutants suggesting previously unrecognized cross-regulatory interactions. Moreover, we have previously shown that Cdx4 is a direct target of the canonical Wnt signaling pathway, and that Cdx1 physically interacts with LEF/TCF members in an autoregulatory loop. We therefore investigated the means by which Cdx2 impacted on Cdx4 expression and assessed potential interaction between Cdx2 and canonical Wnt signaling on the Cdx4 promoter. We found that the Cdx4 promoter was regulated by Cdx2 in transient transfection assays. Electrophoretic mobility shift assays showed that Cdx2 bound to predicted Cdx response elements in the Cdx4 promoter which, when mutated, significantly reduced activity. Consistent with these data, chromatin immunoprecipitation assays from embryos demonstrated occupancy of the Cdx4 promoter by Cdx2 in vivo. However, we failed to observe an interaction between Cdx2 and components of the canonical Wnt signaling pathway. These findings suggest that, while both canonical Wnt and Cdx2 can regulate the activity of the Cdx4 promoter, they appear to operate through distinct mechanisms.

Cdx2 regulates patterning of the intestinal epithelium

Cdx1, Cdx2 and Cdx4 encode homeodomain transcription factors that are involved in vertebral anterior-posterior (AP) patterning. Cdx1 and Cdx2 are also expressed in the intestinal epithelium during development, suggesting a role in this tissue. Intestinal defects have not been reported in Cdx1 null mutants, while Cdx2 null mutants die at embryonic day 3.5 (E3.5), thus precluding assessment of the null phenotype at later stages. To circumvent this latter shortcoming, we have used a conditional Cre-lox strategy to inactivate Cdx2 in the intestinal epithelium. Using this approach, we found that ablation of Cdx2 at E13.5 led to a transformation of the small intestine to a pyloric stomach-like identity, although the molecular nature of the underlying mesenchyme remained unchanged. Further analysis of Cdx1-Cdx2 double mutants suggests that Cdx1 does not play a critical role in the development of the small intestine, at least after E13.5.

The homeodomain transcription factor Cdx1 does not behave as an oncogene in normal mouse intestine

The Caudal-related homeobox genes Cdx1 and Cdx2 are intestine-specific transcription factors that regulate differentiation of intestinal cell types. Previously, we have shown Cdx1 to be antiproliferative and to promote cell differentiation. However, other studies have suggested that Cdx1 may be an oncogene. To test for oncogenic behavior, we used the murine villin promoter to ectopically express Cdx1 in the small intestinal villi and colonic surface epithelium. No changes in intestinal architecture, cell differentiation, or lineage selection were observed with expression of the transgene. Classic oncogenes enhance proliferation and induce tumors when ectopically expressed. However, the Cdx1 transgene neither altered intestinal proliferation nor induced spontaneous intestinal tumors. In a murine model for colitis-associated cancer, the Cdx1 transgene decreased, rather than increased, the number of adenomas that developed. In the polyps, the expression of the endogenous and the transgenic Cdx1 proteins was largely absent, whereas endogenous Villin expression was retained. This suggests that transgene silencing was specific and not due to a general Villin inactivation. In conclusion, neither the ectopic expression of Cdx1 was associated with changes in intestinal cell proliferation or differentiation nor was there increased intestinal cancer susceptibility. Our results therefore suggest that Cdx1 is not an oncogene in normal intestinal epithelium.

Differential regulation of intestinal alkaline phosphatase gene expression by Cdx1 and Cdx2

We have examined the role that the caudal-related homeobox transcription factors Cdx1 and Cdx2 play in activating the enterocyte differentiation marker gene intestinal alkaline phosphatase (IAP). Human colon cancer Caco-2 cells were transiently transfected with Cdx1 and/or Cdx2, and semiquantitative RT-PCR was used to study the effects on IAP mRNA expression. Transfections with a variety of IAP-luciferase reporter constructs were used to identify a Cdx response element located within the human IAP gene promoter. Protein-DNA interactions were examined by EMSA. Results showed that Cdx1 markedly induced IAP mRNA expression, whereas Cdx2 did not, and, in fact, inhibited the Cdx1 effects. Functional analysis revealed that Cdx1 transactivates (fourfold, P < 0.05) the IAP promoter through a novel Cdx response element (GTTTAGA) located between -2369 and -2375 upstream of the translational start site. EMSA showed that both Cdx1 and Cdx2 could bind to the cis element, but in cotransfection experiments, Cdx2 inhibited the Cdx1 effects by approximately 50%. Thus we have identified a previously unrecognized interaction between two important gut transcription factors, Cdx1 and Cdx2, in the context of IAP gene regulation. Cdx1 activates the IAP gene via a novel cis element, whereas Cdx2 inhibits the Cdx1 effects.

CDX1 expression is reduced in colorectal carcinoma and is associated with promoter hypermethylation

The CDX1 homeobox gene encodes a transcription factor specifically expressed in normal intestinal and colonic epithelia, and CDX1 gene expression is affected during colorectal tumour progression. In this study, real-time quantitative RT-PCR was used to investigate CDX1 expression in 26 colorectal carcinomas. Reduced expression of CDX1 was observed in 19 of 26 colon carcinomas compared to matched normal colonic mucosa: the decrease in CDX1 expression ranged between 0.10 and 0.79 (21-90% decrease; mean 64.75% +/-22; p = 0.001). Mutation and loss of heterozygosity (LOH) analyses were then used to determine if reduced CDX1 expression was due to genetic alteration. No CDX1 gene mutations, but two known polymorphisms in exon 1, were observed. LOH was observed in 33% of the tumours investigated but this was not related to CDX1 expression. Since aberrant promoter methylation is a well-known mechanism that participates in gene silencing, the methylation status of the CDX1 5' CpG island promoter was also investigated. PCR amplification of bisulphite-treated DNA followed by cloning was performed in 7 carcinomas that showed low expression of CDX1 and in 1 colonic carcinoma without reduced expression. Promoter hypermethylation occurred in carcinomas in which CDX1 reduced expression was present. These results suggest that CDX1 promoter hypermethylation is one of the molecular mechanisms that accounts for reduced CDX1 gene expression in colorectal carcinoma.

Cdx1 inhibits human colon cancer cell proliferation by reducing beta-catenin/T-cell factor transcriptional activity

The cessation of proliferation and the induction of differentiation are highly coordinated processes that occur continuously in the intestinal crypts. The homeodomain transcription factors Cdx1 and Cdx2 regulate intestine-specific gene expression and enterocyte differentiation. Their roles in regulating proliferation are recognized but remain poorly understood. Previously, we demonstrated that Cdx1 expression diminished the proliferation of human colon cancer cells in part by reducing cyclin D1 gene expression. In order to elucidate further the molecular mechanisms underlying this phenomenon, we first hypothesized that Cdx1 or Cdx2 expression reduces colon cancer cell proliferation by inhibiting beta-catenin/T-cell factor (TCF) transcriptional activity. We report that Cdx1 or Cdx2 expression does inhibit beta-catenin/TCF transcriptional activity in colon cancer cells. This inhibitory effect is dose-dependent and is observed in different colon cancer cell lines, and the degree of inhibition correlates with the ability of Cdx1 to reduce cell proliferation. Cdx1 expression does not alter beta-catenin protein levels or intracellular distribution nor does it induce an inhibitory TCF isoform. We also find that Cdx1 expression is lost in Min mouse polyps with increased nuclear localization of beta-catenin, suggesting that Cdx1 does not support beta-catenin-mediated transformation. Finally, we show that colon cancer cells effectively reduce Cdx2-mediated inhibition of Wnt/beta-catenin/TCF transcriptional activity when compared with other model systems. This suggests that colon cancer and possibly crypt epithelial cells can modulate the effects of Cdx2 on beta-catenin signaling and proliferation. We conclude that Cdx1 and Cdx2 inhibit colon cancer cell proliferation by blocking beta-catenin/TCF transcriptional activity.

Dual role of MEK/ERK signaling in senescence and transformation of intestinal epithelial cells

The mitogen-activated protein kinase cascade operates downstream of Ras to convey cell-surface signals to the nucleus via nuclear translocation of ERK1 and ERK2. We and others have recently demonstrated that activation of ERK1/2 by growth factors is required for proliferation of intestinal epithelial crypt cells. However, it remained to be established whether ERK1/2 activation alone was sufficient to trigger intestinal epithelial cell (IEC) proliferation. To this aim, retrovirus encoding the hemagglutinin-tagged MAPK/ERK kinase (MEK)1 wild type (wtMEK), the upstream activator of ERK1/2, or a constitutively active mutant of MEK1 (MEK1-S218D/S222D; caMEK) were used to infect nonimmortalized human normal intestinal epithelial crypt cell cultures [human intestinal epithelial cells (HIEC)] and rodent immortalized intestinal crypt cells (IEC-6). Stable expression of caMEK but not wtMEK in HIEC led to the irreversible arrest of cellular proliferation (premature senescence). Concomitant with the onset of cell-cycle arrest was the induction of the cyclin-dependent kinase inhibitors p21(Cip), p53, and p16(INK4A). By contrast, overexpression of caMEK in IEC-6 cells induced growth factor relaxation for DNA synthesis, promoted morphological transformation and growth in soft agar, and did not affect expression of p21(Cip), p53, and p16(INK4A). We provided evidences that ERK1b, an alternatively spliced isoform of ERK1, is activated and may contribute to the deregulation of contact inhibition cell growth and transformation of these cells. Constitutive activation of MEK in IECs can produce either premature senescence or forced mitogenesis depending on the integrity of a senescence program controlled by the cell cycle inhibitors p53, p16(INK4A), and p21(CIP).

Cell and molecular biology of the small intestine: new insights into differentiation, growth and repair

PURPOSE OF REVIEW

This paper will discuss recent research that has provided new insights into the molecular and cell biology of the small intestine.

RECENT FINDINGS

Differentiation of the epithelial cell lineages, including the enterocytes, enteroendocrine, Goblet and Paneth cells, from the stem cells is better understood. Important interactions have been demonstrated between these cells, luminal bacteria, and underlying mesenchymal tissue. Intestine-specific gene expression is regulated by transcription factors that are becoming well characterized, including CDX1, CDX2 and HNF1. The actions of growth factors such as GLP-2 and EGF are now known to be complex, demonstrating multiple effects in this tissue at a number of levels.

SUMMARY

Progress in the cellular and molecular biology of the small intestine is producing many intriguing new findings.

Transcriptional control of the cell cycle in mammary gland development and tumorigenesis

Over the past several years it has become increasingly evident that normal development and cancer share many properties. Both processes involve alterations in cell proliferation and differentiation, cell death, neovascularization, and cell motility and invasion. Thus, genes involved in normal development are frequently utilized in neoplasia. During development, numerous transcriptional regulatory mechanisms are used to ensure tight control over cellular proliferation. In this review we focus on a number of transcription factor families (homeobox, STAT, and Ets), and on inhibitors of transcription factors (Id), which have been implicated in controlling the cell cycle not only in normal mammary gland development but also in breast tumorigenesis.

CDX2, a homeobox transcription factor, upregulates transcription of the p21/WAF1/CIP1 gene

The CDX2 homeobox transcription factor plays key roles in intestinal development and homeostasis. CDX2 is downregulated during colorectal carcinogenesis, whereas overexpression of CDX2 results in growth inhibition and differentiation of colon carcinoma and intestinal cells. However, the means by which CDX2 functions remain poorly understood. p21/WAF1/CIP1 is one of the cyclin-dependent kinase inhibitors. In addition to its role in cell cycle control, p21 plays critical roles in differentiation and tumor suppression. The overlapping in both the expression and function of CDX2 and p21 in the small intestine and colon strongly suggests a link between these two genes. By means of luciferase reporter and electrophoretic mobility shift assays, we show here that CDX2 transactivated and physically interacted with the promoter of p21 in a p53-independent manner. Moreover, overexpression of CDX2 increased the mRNA expression of p21 in HT-29 colon carcinoma cells, as demonstrated by reverse transcription-polymerase chain reaction. These data suggest that p21 is a transcriptional target of CDX2. Our results may thus provide a new mechanism underlying the functions of CDX2.

Cdx1 homeobox gene during human colon cancer progression

The Cdx1 homeobox gene encodes an intestine-specific transcription factor with a pro-oncogenic function in vitro. Here we have analysed the pattern of Cdx1 in human colon cancer progression. Cdx1 expression remains at a high level in the majority of the polyps and it is even overexpressed in more than one-third of the specimens, consistent with the fact that the gene is an intestine-specific target of oncogenic pathways. However, Cdx1 decreases in one-fifth of the polyps, which is reminiscent of the loss of expression previously reported in the majority of carcinomas. Allelic imbalance analysis demonstrates that the Cdx1 locus located on chromosome 5q is a major site of genomic rearrangement in colorectal cancers, and that the frequency of the rearrangements increases during polyps to carcinoma progression. Allelic imbalance at the Cdx1 locus occurs in relation to, although not invariably in association with, the rearrangements at the APC locus on the same chromosomal arm. Xenografts of primary human colon carcinomas indicate that the level of Cdx1 mRNA correlates with the intensity of allelic imbalance. Together, these data show that Cdx1 exhibits a complex pattern during colorectal cancer progression. Given that Cdx1 has a pro-oncogenic function in vitro, the maintenance of a high level of expression in polyps, and even its overexpression in one-third of the specimens, suggest that this homeobox gene may be an important factor in the process toward malignant transformation during the first steps of tumorigenesis.

Effect of gastrin on differentiation of rat intestinal epithelial cells in vitro

AIM: To investigate the effect of gastrin on differentiation of IEC-6 cell line in vitro.

METHODS: IEC-6 cells were incubated with gastrin. On day 7 after treatment, cell morphology was examined by light microscope, and on day 20, the cellular ultrastructures were examined by electron microscope. After exposure to gastrin for 6 hours, villin mRNA was analyzed by reverse transcription-polymerase chain reaction, and on day 7, the expression of villin was examined by immunocytochemical analysis with laser confocal microscope.

RESULTS: After exposure to gastrin, IEC-6 cells showed differentiated phenotypes as villas enterocytes and contained an abundance of plasma, small nuclei with nucleoli, and were arranged regularly. There were numerous microvilli around edge of the cells, and several cells showed columnar structures. Villin mRNA expression in cytoplasm was increased in comparison with control.

CONCLUSION: Differentiated characteristics of villus enterocytes and phenotypic changes of rat intestinal epithelial cells (IEC-6) are induced by gastrin, and the effects of gastrin are correlated to increased villin expression.

Developmental regulation of apolipoprotein B mRNA editing is an autonomous function of small intestine involving homeobox gene Cdx1

Apolipoprotein B mRNA editing is developmentally regulated in the human and rodent small intestine, changing from <1% at day 14 to approximately 90% by day 20 in the rat fetus. This regulation is coincident with the developmental formation of the crypt-to-villus axis functional unit, a continuous and rapidly renewing system involving cell generation, migration, and differentiation. Utilizing small intestine isografts implanted into the subcutaneous tissue of adult recipients, apolipoprotein B mRNA editing was developmentally up-regulated, parallel to that seen with an intact control. In contrast, apoB mRNA expression remains nearly constant in the isograft, unlike the normal intact small intestine. Immunohistochemical analyses demonstrated that apoB-48 protein existed predominantly in well differentiated enterocytes along the villus surface whereas apoB-100 was in the lamina propria and crypts. ApoB mRNA editing levels were very low in the crypt-like rat intestinal cell line, IEC-6 ( approximately 0.3%), but very high in well differentiated enterocytes ( approximately 91.5%). The expression of homeobox gene Cdx1 increased 18-fold in small intestine in vivo during the same time course when apoB mRNA editing increased from approximately 2 to approximately 90%. The overexpression of Cdx1 in IEC-6 cells increased apoB mRNA editing over 10-fold compared with the vector control. This increase was associated with a significant increase of activating factor ACF, a component of the apoB mRNA editing complex. Taken together, these data suggest that the developmental regulation of apoB mRNA editing is an autonomous cytodifferentiation function of small intestine for which homeobox gene Cdx1 may play an important role.