Telomerase activity controls telomere length and plays a pivotal role in stem cells, aging, and cancer. Here, we report a molecular link between Wnt/β-catenin signaling and the expression of the telomerase subunit Tert. β-Catenin-deficient mouse embryonic stem (ES) cells have short telomeres; conversely, ES cell expressing an activated form of β-catenin (β-cat(ΔEx3/+)) have long telomeres. We show that β-catenin regulates Tert expression through the interaction with Klf4, a core component of the pluripotency transcriptional network. β-Catenin binds to the Tert promoter in a mouse intestinal tumor model and in human carcinoma cells. We uncover a previously unknown link between the stem cell and oncogenic potential whereby β-catenin regulates Tert expression, and thereby telomere length, which could be critical in human regenerative therapy and cancer.

The Ca(++)-dependent cell adhesion molecule E-cadherin is expressed throughout mouse development and in adult tissues. Classical gene targeting has demonstrated that E-cadherin-deficient embryos die at the blastocyst stage. To study the involvement of E-cadherin in organogenesis, a conditional gene inactivation scheme was undertaken using the bacteriophage P1 recombinase Cre/loxP system. Mice with homozygous loxP sites in both alleles of the E-cadherin (Cdh1) gene were generated and these mice were crossed with transgenic mice with the Cre recombinase under the control of the hormone-inducible MMTV promoter. This resulted in deletion of the E-cadherin gene in the differentiating alveolar epithelial cells of the mammary gland. The mutant mammary gland developed normally up to 16-18 days of pregnancy but exhibited a dramatic phenotype around parturition. The production of milk proteins was so drastically reduced that adult mutant mothers could not suckle their offspring. Thus, the lack of E-cadherin affected the terminal differentiation program of the lactating mammary gland. In concordance with this finding, the prolactin-dependent activation of the transcription factor Stat5a was initiated but not maintained in the mutant gland. Instead, without E-cadherin massive cell death was observed at parturition and the mutant mammary gland at this stage resembled that of the involuted gland normally seen after weaning. These results demonstrate an essential role for E-cadherin in the function of differentiated alveolar epithelial cells. No tumors were detected in mutant glands lacking E-cadherin.

Many components of the Wnt/β-catenin signaling pathway are expressed during mouse pre-implantation embryo development, suggesting that this pathway may control cell proliferation and differentiation at this time. We find no evidence for a functional activity of this pathway in cleavage-stage embryos using the Wnt-reporter line, BAT-gal. To further probe the activity of this pathway, we activated β-catenin signaling by mating a zona pellucida3-cre(Zp3-cre) transgenic mouse line with a mouse line containing an exon3-floxedβ-catenin allele. The result is expression of a stabilized form ofβ-catenin, resistant to degradation by the GSK3β-mediated proteasome pathway, expressed in the developing oocyte and in each cell of the resulting embryos. Nuclear localization and signaling function of β-catenin were not observed in cleavage-stage embryos derived from these oocytes. These results indicate that in pre-implantation embryos, molecular mechanisms independent of the GSK3β-mediated ubiquitination and proteasome degradation pathway inhibit the nuclear function of β-catenin. Although the mutant blastocysts initially developed normally, they then exhibited a specific phenotype in the embryonic ectoderm layer of early post-implantation embryos. We show a nuclear function of β-catenin in the mutant epiblast that leads to activation of Wnt/β-catenin target genes. As a consequence,cells of the embryonic ectoderm change their fate, resulting in a premature epithelial-mesenchymal transition.

IntroductionE-cadherin (E-cad; cadherin 1) and N-cadherin (N-cad; cadherin 2) are the most prominent members of the cadherin family of cell adhesion molecules. Although they share many structural and functional features, they are expressed in an almost mutually exclusive manner in vivo.MethodsTo explore functional differences between the two cadherins in vivo, we recently generated a knock-in line in which N-cad is expressed from the E-cad locus. In combination with a conditional gene inactivation approach, we expressed N-cad in the absence of E-cad (referred to as Ncadk.i.) in alveolar epithelial cells of the mammary gland starting in late pregnancy.ResultsWe found that the sole presence of N-cad induces constitutively active fibroblast growth factor (Fgf) signaling and a precocious involution resulting in massive apoptosis of alveolar cells. To block apoptosis, we conditionally deleted one allele of p53 in Ncadk.i. mice and observed a temporal rescue of alveolar morphology and function. However, an accumulation of fibrotic tissue and cysts with increasing age and lactation cycles was observed. This phenotype closely resembled fibrocystic mastopathy (FM), a common disorder in humans, which is thought to precede breast cancer. Concordantly, 55% of Ncadk.i. mice harboring a heterozygous p53 deletion developed malignant and invasive tumors.ConclusionsOur results demonstrate a possible role for N-cad in the formation of fibrosis and cysts in the mammary gland. Moreover, we show that these lesions precede the development of malignant tumors. Thus, we provide a new mouse model to investigate the molecular mechanisms of fibrocystic mastopathy and the transition from benign to malignant tumors.