Patrick Franken

β-catenin tyrosine 654 phosphorylation increases Wnt signalling and intestinal tumorigenesis

Objective Deregulation of the Wnt signalling pathway by mutations in the Apc or β-catenin genes underlies colorectal carcinogenesis. As a result, β-catenin stabilises, translocates to the nucleus, and activates gene transcription. Intestinal tumours show a heterogeneous pattern of nuclear β-catenin, with the highest levels observed at the invasion front. Activation of receptor tyrosine kinases in these tumour areas by growth factors expressed by surrounding stromal cells phosphorylate β-catenin at tyrosine residues, which is thought to increase β-catenin nuclear translocation and tumour invasiveness. This study investigates the relevance of β-catenin tyrosine phosphorylation for Wnt signalling and intestinal tumorigenesis in vivo. Design A conditional knock-in mouse model was generated into which the phospho-mimicking Y654E modification in the endogenous β-catenin gene was introduced. Results This study provided in vivo evidence that β-cateninE654 is characterised by reduced affinity for cadherins, increased signalling and strongly increased phosphorylation at serine 675 by protein kinase A (PKA). In addition, homozygosity for the β-cateninE654 targeted allele caused embryonic lethality, whereas heterozygosity predisposed to intestinal tumour development, and strongly enhanced Apc-driven intestinal tumour initiation associated with increased nuclear accumulation of βcatenin. Surprisingly, the expression of β-cateninE654 did not affect histological grade or induce tumour invasiveness. Conclusions A thus far unknown mechanism was uncovered in which Y654 phosphorylation of β-catenin facilitates additional phosphorylation at serine 675 by PKA. In addition, in contrast to the current belief that β-catenin Y654 phosphorylation increases tumour progression to a more invasive phenotype, these results show that it rather increases tumour initiation by enhancing Wnt signalling.

Paneth Cells in Intestinal Homeostasis and Tissue Injury

Adult stem cell niches are often co-inhabited by cycling and quiescent stem cells. In the intestine, lineage tracing has identified Lgr5 + cells as frequently cycling stem cells, whereas Bmi1 +, mTert +, Hopx + and Lrig1 + cells appear to be more quiescent. Here, we have applied a non-mutagenic and cell cycle independent approach to isolate and characterize small intestinal label-retaining cells (LRCs) persisting in the lower third of the crypt of Lieberkühn for up to 100 days. LRCs do not express markers of proliferation and of enterocyte, goblet or enteroendocrine differentiation, but are positive for Paneth cell markers. While during homeostasis, LR/Paneth cells appear to play a supportive role for Lgr5 + stem cells as previously shown, upon tissue injury they switch to a proliferating state and in the process activate Bmi1 expression while silencing Paneth-specific genes. Hence, they are likely to contribute to the regenerative process following tissue insults such as chronic inflammation.

Progesterone inhibition of Wnt/β-catenin signaling in normal endometrium and endometrial cancer

Purpose. Wnt signaling regulates the fine balance between stemness and differentiation. Here, the role of Wnt signaling to maintain the balance between estrogen-induced proliferation and progesterone-induced differentiation during the menstrual cycle, as well as during the induction of hyperplasia and carcinogenesis of the endometrium, was investigated. Experimental Design: Endometrial gene expression profiles from estradiol (E2) and E2 + medroxyprogesterone acetate–treated postmenopausal patients were combined with profiles obtained during the menstrual cycle (PubMed; GEO DataSets). Ishikawa cells were transfected with progesterone receptors and Wnt inhibitors dickkopf homologue 1 (DKK1) and forkhead box O1 (FOXO1), measuring Wnt activation. Expression of DKK1 and FOXO1 was inhibited by use of sequence-specific short hairpins. Furthermore, patient samples (hormone-treated endometria, hyperplasia, and endometrial cancer) were stained for Wnt activation using nuclear β-catenin and CD44. Results: In vivo, targets and components of the Wnt signaling pathway (among them DKK1 and FOXO1) are regulated by E2 and progesterone. In Wnt-activated Ishikawa cells, progesterone inhibits Wnt signaling by induction of DKK1 and FOXO1. Furthermore, using siRNA-mediated knockdown of both DKK1 and FOXO1, progesterone inhibition of Wnt signaling was partly circumvented. Subsequently, immunohistochemical analysis of the Wnt target gene CD44 showed that progesterone acted as an inhibitor of Wnt signaling in hyperplasia and in well-differentiated endometrial cancer. Conclusion: Progesterone induction of DKK1 and FOXO1 results in inhibition of Wnt signaling in the human endometrium. This Wnt inhibitory effect of progesterone is likely to play a rate-limiting role in the maintenance of endometrial homeostasis and, on its loss, in tumor onset and progression toward malignancy. (Clin Cancer Res 2009;15(18):5784–93)

A targeted constitutive mutation in the APC tumor suppressor gene underlies mammary but not intestinal tumorigenesis.

Germline mutations in the adenomatous polyposis coli (APC) gene are responsible for familial adenomatous polyposis (FAP), an autosomal dominant hereditary predisposition to the development of multiple colorectal adenomas and of a broad spectrum of extra-intestinal tumors. Moreover, somatic APC mutations play a rate-limiting and initiating role in the majority of sporadic colorectal cancers. Notwithstanding its multifunctional nature, the main tumor suppressing activity of the APC gene resides in its ability to regulate Wnt/β-catenin signaling. Notably, genotype–phenotype correlations have been established at the APC gene between the length and stability of the truncated proteins encoded by different mutant alleles, the corresponding levels of Wnt/β-catenin signaling activity they encode for, and the incidence and distribution of intestinal and extra-intestinal tumors. Here, we report a novel mouse model, Apc1572T, obtained by targeting a truncated mutation at codon 1572 in the endogenous Apc gene. This hypomorphic mutant allele results in intermediate levels of Wnt/β-catenin signaling activation when compared with other Apc mutations associated with multifocal intestinal tumors. Notwithstanding the constitutive nature of the mutation, Apc +/1572T mice have no predisposition to intestinal cancer but develop multifocal mammary adenocarcinomas and subsequent pulmonary metastases in both genders. The histology of the Apc1572T primary mammary tumours is highly heterogeneous with luminal, myoepithelial, and squamous lineages and is reminiscent of metaplastic carcinoma of the breast in humans. The striking phenotype of Apc +/1572T mice suggests that specific dosages of Wnt/β-catenin signaling activity differentially affect tissue homeostasis and initiate tumorigenesis in an organ-specific fashion.

Cross-Species Comparison of Human and Mouse Intestinal Polyps Reveals Conserved Mechanisms in Adenomatous Polyposis Coli (APC)-Driven Tumorigenesis

Expression profiling is a well established tool for the genome-wide analysis of human cancers. However, the high sensitivity of this approach combined with the well known cellular and molecular heterogeneity of cancer often result in extremely complex expression signatures that are difficult to interpret functionally. The majority of sporadic colorectal cancers are triggered by mutations in the adenomatous polyposis coli (APC) tumor suppressor gene, leading to the constitutive activation of the Wnt/beta-catenin signaling pathway and formation of adenomas. Despite this common genetic basis, colorectal cancers are very heterogeneous in their degree of differentiation, growth rate, and malignancy potential. Here, we applied a cross-species comparison of expression profiles of intestinal polyps derived from hereditary colorectal cancer patients carrying APC germline mutations and from mice carrying a targeted inactivating mutation in the mouse homologue Apc. This comparative approach resulted in the establishment of a conserved signature of 166 genes that were differentially expressed between adenomas and normal intestinal mucosa in both species. Functional analyses of the conserved genes revealed a general increase in cell proliferation and the activation of the Wnt/beta-catenin signaling pathway. Moreover, the conserved signature was able to resolve expression profiles from hereditary polyposis patients carrying APC germline mutations from those with bi-allelic inactivation of the MYH gene, supporting the usefulness of such comparisons to discriminate among patients with distinct genetic defects.

Wnt Signaling Regulates the Lineage Differentiation Potential of Mouse Embryonic Stem Cells through Tcf3 Down-Regulation

Canonical Wnt signaling plays a rate-limiting role in regulating self-renewal and differentiation in mouse embryonic stem cells (ESCs). We have previously shown that mutation in the Apc (adenomatous polyposis coli) tumor suppressor gene constitutively activates Wnt signaling in ESCs and inhibits their capacity to differentiate towards ecto-, meso-, and endodermal lineages. However, the underlying molecular and cellular mechanisms through which Wnt regulates lineage differentiation in mouse ESCs remain to date largely unknown. To this aim, we have derived and studied the gene expression profiles of several Apc-mutant ESC lines encoding for different levels of Wnt signaling activation. We found that down-regulation of Tcf3, a member of the Tcf/Lef family and a key player in the control of self-renewal and pluripotency, represents a specific and primary response to Wnt activation in ESCs. Accordingly, rescuing Tcf3 expression partially restored the neural defects observed in Apc-mutant ESCs, suggesting that Tcf3 down-regulation is a necessary step towards Wnt-mediated suppression of neural differentiation. We found that Tcf3 down-regulation in the context of constitutively active Wnt signaling does not result from promoter DNA methylation but is likely to be caused by a plethora of mechanisms at both the RNA and protein level as shown by the observed decrease in activating histone marks (H3K4me3 and H3-acetylation) and the upregulation of miR-211, a novel Wnt-regulated microRNA that targets Tcf3 and attenuates early neural differentiation in mouse ESCs. Our data show for the first time that Wnt signaling down-regulates Tcf3 expression, possibly at both the transcriptional and post-transcriptional levels, and thus highlight a novel mechanism through which Wnt signaling inhibits neuro-ectodermal lineage differentiation in mouse embryonic stem cells.

Alterations in Wnt–β-catenin and Pten signalling play distinct roles in endometrial cancer initiation and progression

Endometrioid endometrial cancer arises through a gradual series of histological changes, each accompanied by specific alterations in gene expression and activity. Activation of the Wnt–β‐catenin pathway and loss of PTEN activity are frequently observed in endometrial cancers. However, the specific roles played by alterations in these pathways in the initiation and progression of endometrial cancer are currently unclear. Here, we investigated the effects of loss of Pten and Apc gene function in the mouse endometrium by employing tissue‐specific and inducible mutant alleles, followed by immunohistochemical (IHC) and loss of heterozygosity (LOH) analysis of their corresponding cancerous lesions. Loss of the Apc function in the endometrium leads to cytoplasmic and nuclear β‐catenin accumulation in association with uterine hyperplasia and squamous cell metaplasia, but without malignant transformation. Loss of Pten function also resulted in squamous metaplasia but, in contrast to loss of Apc function, it initiates endometrial cancer. On the other hand, loss of Apc function in the endometrium accelerates Pten‐driven endometrial tumourigenesis. Analysis of compound heterozygous mice confirmed that somatic loss of the wild‐type Pten allele represents the rate‐limiting initiation step in endometrial cancer. Simultaneous loss of Pten and Apc resulted in endometrial cancer characterized by earlier onset and a more aggressive malignant behaviour. These observations are indicative of the synergistic action between the Wnt–β‐catenin and Pten signalling pathways in endometrial cancer onset and progression. Copyright

Cancer stemness in Wnt-driven mammary tumorigenesis

Wnt signaling plays a central role in mammary stem cell (MaSC) homeostasis and in breast cancer. In particular, epigenetic alterations at different members of the Wnt pathway have been identified among triple-negative, basal-like breast cancers. Previously, we developed a mouse model for metaplastic breast adenocarcinoma, a subtype of triple-negative breast cancer, by targeting a hypomorphic mutations in the endogenous Apc gene (Apc (1572T/+)). Here, by employing the CD24 and CD29 cell surface antigens, we have identified a subpopulation of mammary cancer stem cells (MaCSCs) from Apc (1572T/+) capable of self-renewal and differentiation both in vivo and in vitro. Moreover, immunohistochemical analysis of micro- and macrolung metastases and preliminary intravenous transplantation assays suggest that the MaCSCs underlie metastasis at distant organ sites. Expression profiling of the normal and tumor cell subpopulations encompassing MaSCs and CSCs revealed that the normal stem cell compartment is more similar to tumor cells than to their own differentiated progenies. Accordingly, Wnt signaling appears to be active in both the normal and cancer stem cell compartments, although at different levels. By comparing normal with cancer mouse mammary compartments, we identified a MaCSC gene signature able to predict outcome in breast cancer in man. Overall, our data indicate that constitutive Wnt signaling activation affects self-renewal and differentiation of MaSCs leading to metaplasia and basal-like adenocarcinomas.

Barrett's oesophageal adenocarcinoma encompasses tumour-initiating cells that do not express common cancer stem cell markers

Accumulating evidence has suggested that tumours have a hierarchical organization in which only the cancer stem cells (CSCs) have tumour‐initiating properties. Several surface antigens have been employed to isolate CSCs from various malignancies, although not from oesophageal adenocarcinoma (EA). We tested whether Barretts oesophagus (BE) and EA might serve as a model for the CSC concept. In vivo assays were performed by transplantation of serially diluted bulk EA cells into NOD‐SCID mice to establish the presence and frequency of tumour‐initiating cells. These were found to be present as ca. 1 in 64 000 cells. The transplanted tumours fully recapitulated the primary lesions. Subsequently, a panel of previously established CSC markers was employed for immunohistochemistry. CD24, CD29 and CD44 showed heterogeneous staining in EA. Nuclear β‐catenin accumulation increased during progression from metaplasia to dysplasia and was often observed in the basal compartment with CD24 and CD29 staining. However, the overall staining patterns were not such to clearly point out specific candidate markers. Accordingly, all markers were employed to sort the corresponding subpopulations of cancer cells and transplant them at low multiplicities in NOD‐SCID mice. No increased tumour‐initiating capacity of sorted EA cells was observed upon transplantation. These results indicate that tumour‐initiating cells are present in EA, thus reflecting a hierarchical organization. However, antibodies directed against novel surface antigens are needed to detect subpopulations enriched for CSCs in EA by transplantation assays. Copyright

Generation of a tightly regulated doxycycline-inducible model for studying mouse intestinal biology.

To develop a sensitive and inducible system to study intestinal biology, we generated a transgenic mouse model expressing the reverse tetracycline transactivator rtTA2‐M2 under control of the 12.4 kb murine Villin promoter. The newly generated Villin‐rtTA2‐M2 mice were then bred with the previously developed tetO‐HIST1H2BJ/GFP model to assess inducibility and tissue‐specificity. Expression of the histone H2B‐GFP fusion protein was observed exclusively upon doxycycline induction and was uniformly distributed throughout the intestinal epithelium. The Villin‐rtTA2‐M2 was also found to drive transgene expression in the developing mouse intestine. Furthermore, we could detect transgene expression in the proximal tubules of the kidney and in a population of alleged gastric progenitor cells. By administering different concentrations of doxycycline, we show that the Villin‐rtTA2‐M2 system drives transgene expression in a dosage‐dependent fashion. Thus, we have generated a novel doxycycline‐inducible mouse model, providing a valuable tool to study the effect of different gene dosages on intestinal physiology and pathology. genesis 47:7–13, 2009.