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The Ets dominant repressor En/Erm enhances intestinal epithelial tumorigenesis in ApcMin mice
© Jedlicka et al; licensee BioMed Central Ltd. 2009
Received: 23 December 2008
Accepted: 22 June 2009
Published: 22 June 2009
Ets transcription factors have been widely implicated in the control of tumorigenesis, with most studies suggesting tumor-promoting roles. However, few studies have examined Ets tumorigenesis-modifying functions in vivo using model genetic systems.
Using mice expressing a previously characterized Ets dominant repressor transgene in the intestinal epithelium (Villin-En/Erm), we examined the consequences of blocking endogenous Ets-mediated transcriptional activation on tumorigenesis in the ApcMin model of intestinal carcinoma.
En/Erm expression in the intestine, at levels not associated with overt crypt-villus dysmorphogenesis, results in a marked increase in tumor number in ApcMin animals. Moreover, when examined histologically, tumors from En/Erm-expressing animals show a trend toward greater stromal invasiveness. Detailed analysis of crypt-villus homeostasis in these En/Erm transgenic animals suggests increased epithelial turnover as one possible mechanism for the enhanced tumorigenesis.
Our findings provide in vivo evidence for a tumor-restricting function of endogenous Ets factors in the intestinal epithelium.
Members of the Ets transcription family, numbering up to 27 in humans, are widely expressed in developing and mature tissues, and regulate diverse cellular processes [1, 2]. Ets factors are also frequently misexpressed in the setting of neoplasia. Many Ets factors become overexpressed in tumors and appear to play tumor-promoting roles, while a limited number, notably the epithelial specific Ets, may perform tumor suppressor functions [1, 3–5]. However, to date, most information about Ets functions in tumorigenesis has come from cell culture and animal xenograft models. Indeed, very few studies have examined Ets functions in tumorigenesis in vivo using model genetic systems [6–8]. Ets factors are widely expressed in the intestine, and often misexpressed in carcinoma of the colon, but their tumor-modifying roles in intestinal epithelial neoplasia in vivo largely remain to be defined .
We have previously generated and characterized transgenic mice expressing an Ets dominant repressor (En/Erm) with broad Ets-blocking activity in the small intestinal epithelium . Nearly all members of the Ets transcription factor family are expressed in the mature mammalian intestine, but expression levels vary widely [9, 11]. Our previous study characterized the phenotypic consequences of transgene expression at immunohistochemically detectable levels, which resulted in marked disturbance of crypt-villus homeostasis . Additional transgenic lines, expressing En/Erm at levels detectable by RT-PCR but not immunohistochemistry ("low expressors"), did not manifest an overt dysmorphogenic phenotype under normal physiologic conditions, despite the fact that En/Erm is able to block Ets activity at substoichiometric levels in vitro . To determine whether this low-level En/Erm expression has phenotypic consequences under pathologic conditions, we tested its effect on intestinal epithelial tumorigenesis in the ApcMin mouse, a well-established model of multiple intestinal neoplasia [12–14].
We find that animals with low-level En/Erm expression develop more than twice as many tumors in the small intestine as non-transgenic ApcMin controls. Interestingly, while these animals do not manifest the overt crypt-villus dysmorphogenesis phenotype under conditions of homeostasis previously described for high-level En/Erm expressors , they do show a mild increase in epithelial transit. Thus, the increase in tumor number in the En/Erm animals may in part be due to increased crypt-villus epithelial turnover. Moreover, on histologic analysis, tumors from En/Erm-expressing animals show a trend toward greater stromal invasion. Our studies in a genetic tumor model thus uncover an unexpected role for epithelially expressed Ets factors in the restriction of tumorigenesis in the intestine.
Villin-En/Erm transgenic animals were generated as previously described , and were maintained in an FVB/N background. ApcMin/+ animals were obtained from Jackson Laboratories and were maintained in a C57BL/6J genetic background. Experimental and control animals for the tumor study were both derived from a cross between Villin-En/Erm animals and ApcMin/+ animals. The studies were thus carried out in a hybrid (C57BL/6J × FVB/N) background, as done by others . In order to control for possible confounding effects of the modifier-of-Min locus (Mom1), the major modifier of tumor multiplicity in the ApcMin strain , the Mom1 genotype (resistant versus sensitive) was determined, as previously described [15, 17], and such analyses indicated that all animals in the study were heterozygous (Mom1S/R) for the Mom1 locus. The presence of the Villin-En/Erm transgene and ApcMin mutation were determined by PCR genotyping of tail-biopsy DNA, as previously described (; http://jaxmice.jax.org). Tumor number, size and histology in ApcMin and ApcMin;Villin-En/Erm animals were evaluated in H+E-stained sections of the complete length of the small intestine by a pathologist (PJ) blinded to the genotype. All animal work was carried out under protocols approved by the Institutional Animal Care and Use Committee.
Histology and immunohistochemistry
Animals were euthanized using CO2 followed by cervical dislocation. The small intestine was immediately harvested and cut into two to three segments of approximately equal length. Fecal contents were gently expelled, the lumen was injected with fixative (4% paraformaldehyde), and the intestine was rolled concentrically and placed in a histology cassette. Fixation was for 24 hours in 4% paraformaldehyde at 4°C, after which the tissues were placed in 70% ethanol, processed further on a standard histology processor and paraffin-embedded. Sections 4 um thick were stained with hematoxylin and eosin (H+E) or processed further for immunohistochemical staining. For immunohistochemical staining, sections were deparaffinized and rehydrated. Antigen retrieval was performed by incubating the slides in 10 mM sodium citrate buffer, pH 6.0, for 1 hour in a Biocare Medical Decloaker. Endogenous peroxidase activity was blocked by incubation in 3% H2O2 for 10 minutes. Immunohistochemical staining was performed using the M.O.M. (mouse on mouse) kit (Vector Laboratories), and developed using DAB (Dako or Sigma). Primary antibodies used were mouse anti-smooth muscle actin (Dako, 1:100) and mouse anti-E-cadherin (BD Biosciences, 1:100). Mcm6 and β-catenin immunohistochemical staining, and BrdU labeling and immunohistochemical staining were performed as described previously . All immunohistochemically stained slides were counterstained with hematoxylin, dehydrated, mounted and coverslipped.
Following euthanasia, the small intestine was removed, cut into multiple segments and fecal contents were gently expelled. The intestinal segments were opened lengthwise and stored in RNAlater (Ambion) at 4°C overnight. The tissue was removed from RNAlater and laid mucosal surface up onto Petri dish lids. The mucosa was gently scraped off with a razor blade and collected in a 1.5 ml tube. Trizol (1 ml; Invitrogen) was added, the tissue was homogenized with a disposable pestle (Fisher), and RNA was isolated per manufacturer protocol (Invitrogen) and stored at -80°C. Ten (10) ug of RNA were treated with DNase using a DNA-free kit (Ambion), and 1.6 ug of treated RNA were reverse transcribed using Superscript III reverse transcriptase (Invitrogen), per manufacturer protocol, using random primers in a 20 ul reaction. In parallel control reactions, reverse transcriptase was omitted. One ul of the reverse transcription reaction was then PCR-amplified using primers to Engrailed (for detection of the transgenic transcript) or Actin (control), as previously described , and the products were resolved on a 2% agarose gel stained with ethidium bromide.
Intestinal En/Erm expression increases tumor number in ApcMinmice
We thus examined the effect of low-level En/Erm expression on intestinal epithelial tumorigenesis. Our prior studies showed that the Villin-En/Erm transgene is expressed primarily in the small intestine . We therefore chose the ApcMin mouse as the tumor model, as these animals develop multiple epithelial tumors predominantly in the small intestine . The tumor studies were carried out in a hybrid (C57BL/6J × FVB/N) genetic background, since the Villin-En/Erm transgenic lines were generated in the FVB/N strain while the background of the ApcMin animals was C57BL/6J. As others have done in similar studies , we determined the genotype of the major modifier of tumor multiplicity in the ApcMin strain, Mom1, in order to control for possible genetic background differences between control (ApcMin) and experimental (ApcMin;Villin-En/Erm) groups. Such analyses indicated that all animals in both control and experimental groups were heterozygous for the Mom1 locus, and thus similarly susceptible to ApcMin-driven tumorigenesis.
Summary of animal tumor data
(n = 12)
(n = 11)
Animal age (months)
Tumor number (total)
Tumor size (total; cm)*
% tumors with stromal invasion
% histologically aggressive tumors
Effect of En/Erm expression on tumor invasiveness in ApcMinmice
Summary of genetic studies examining Ets factor functions in tumorigenesis
Effect on tumorigenesis
Subcompartment with effect
Inhibition (increased latency; decreased number and size)
Inhibition (increased latency; decreased number and size)
One extra gene copy
Inhibition (decreased number)
Most Ets factors function as transcriptional activators [1, 2]. Since En/Erm exerts its effect by blocking Ets-mediated transcriptional activation , our findings imply that the endogenous Ets factors blocked by En/Erm normally function to restrict tumorigenesis in the intestinal epithelium. Due to the high conservation of the Ets domain, the En/Erm protein is able to block transcriptional activation by multiple different Ets factors , making it difficult to determine which individual Ets is/are responsible for this tumorigenesis-modifying effect. We have previously shown that higher (immunohistochemically detectable) levels En/Erm expression result in small intestinal crypt-villus dysmorphogenesis, probably by interfering with the activity of relatively abundant Ets factors in the intestine, such as Elf3, Ehf and/or Ets2 . In contrast, possible candidate Ets factors responsible for the tumor phenotype include those with normally lower relative expression levels in the intestine, such as Pea3, Erm and/or Elf 1 . Alternatively, the tumor phenotype may be uncovering a differential requirement for a more highly expressed Ets. Interestingly, similar to our findings, Sussan et al recently observed an inverse relationship between Ets2 gene copy number and tumor number in the ApcMin model, suggesting that Ets2 normally functions to restrict intestinal tumor formation . Thus, while frequently overexpressed in colon cancer, at least some Ets factors, including Ets2 and those blocked by the En/Erm transgene in our studies, appear to normally restrict, rather than promote, epithelial neoplasia in the intestine. Indeed, it may be that the same Ets factors manifest different functions in neoplasia at different expression levels due to differential promoter binding and regulation.
Secondly, our studies suggest that the ApcMin mutation may provide a good model of human intestinal cancer in the C57BL/6J × FVB/N hybrid genetic background. In this background, the overall tumor burden is lower and the proportion of invasive lesions higher than in the pure C57BL/6J background, in which animals die relatively early from intestinal obstruction caused by numerous non-invasive adenomas. Thus, this mixed background model approximates the human disease, and could be useful for studying genetic parameters controlling carcinoma invasion and metastasis.
Expression of the Ets dominant repressor En/Erm in the small intestine, at levels that do not cause crypt-villus dysmorphogenesis, results in a marked increase in tumor number in the ApcMin model of intestinal carcinoma. Tumor size is relatively unaffected, indicating that this effect acts predominantly at the level of tumor initiation or/and early promotion. Histologic examination of the tumors suggests that En/Erm expression may also promote stromal invasion. Together, these findings from an animal genetic model provide in vivo evidence for an unexpected role for endogenous Ets factors in the restriction of epithelial tumorigenesis in the intestine. Moreover, our studies suggest that the ApcMin mutation may provide a good model for invasive human intestinal carcinoma in the C57BL/6J × FVB/N hybrid genetic background.
We wish to thank Jana Polzer, Janet Lieber, Michael Medina and Sara Garza-Williams for assistance with mouse histology, and Heide Ford for critical reading of the manuscript. This work was supported by the Cancer League of Colorado, The Children's Hospital/University of Colorado Denver Research Institute, NIH K08 DK74557 (to PJ), and NIH R01 DK37667 and DK46868 (to AGH).
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