Epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) highlight crucial steps during embryogenesis and tumorigenesis. Induction of dramatic changes in gene expression and cell features is reflected by modulation of Cdh1 (E-cadherin) expression. We show that Cdh1 activity during MET is governed by two enhancers at +7.8 kb and at +11.5 kb within intron 2 that are activated by binding of Grhl3 and Hnf4α, respectively. Recruitment of Grhl3 and Hnf4α to the enhancers is crucial for activating Cdh1 and accomplishing MET in non-tumorigenic mouse mammary gland cells (NMuMG). Moreover, the two enhancers cooperate via Grhl3 and Hnf4α binding, induction of DNA-looping and clustering at the promoter to orchestrate E-cadherin re-expression. Our results provide novel insights into the cellular mechanisms whereby cells respond to MET signals and re-establish an epithelial phenotype by enhancer cooperativity. A general importance of our findings including MET-mediated colonization of metastasizing tumor cells is suggested.

Embryonic stem (ES) cell pluripotency and induced pluripotent stem (iPS) cell generation is dependent on a core transcriptional network and proper cell-cell adhesion mediated by E-cadherin (E-cad). Whereas E-cad is associated with pluripotency, N-cadherin (N-cad) expression is correlated with differentiation into mesodermal and neuroectodermal lineages. We investigated whether E-cad harbors unique molecular features in establishing or maintaining pluripotency. By using a gene replacement knock-in (ki) approach to express N-cadherin (N-cad) or E-cad/N-cad chimeric cadherins under the control of the E-cad locus, we show that all E-cad-depleted ki/ki ES cells are maintained in an undifferentiated state. Surprisingly, these cells retained key features of pluripotency, such as Nanog expression and full differentiation capacity in vitro and in vivo, whereas E-cad knockout (ko) ES cells irreversibly lost most of these features. Moreover, our results indicate that E-cad mediated adhesion is essential for iPS cell generation, since E-cad depleted fibroblasts were not reprogrammed. In contrast, N-cad efficiently supports somatic reprogramming similar to E-cad, and permits initiation of the crucial initial step of mesenchymal-epithelial transition. Thus, we show that cell adhesion and a robust pluripotent phenotype are ultimately connected. Since N-cad properly compensates for loss of E-cad, no specific 'cadherin code' is required.

The function of sodium iodide symporter (Na + /I À symporter, or NIS) in mammary epithelial cells is essential for the accumulation of I À in milk; the newborn's first source of I À for thyroid hormone synthesis. Furthermore, increased mammary gland NIS expression has previously been shown in human breast cancer. Several hormones and factors including all-trans-retinoic acid (tRA) regulate the expression of NIS. In this study, using breast cancer cell lines, we established that tRA-responsive NIS expression is confined to estrogen receptor-a (ERa) positive cells and we investigated the role of ERa in the regulation of NIS expression. We showed that the suppression of endogenous ERa by RNA interference downregulates NIS expression in ERa positive mammary cells. Besides, in an ERa negative cell line, reintroduction of ERa resulted in the expression of NIS in a ligand-independent manner. We also identified a novel estrogen-responsive element in the promoter region of NIS that specifically binds ERa and mediates ERa-dependent activation of transcription. Our results indicate that unliganded ERa (apo-ERa) contributes to the regulation of NIS gene expression. In mammary gland lactocytes, sodium/iodide (Na + /I À ) symport via NIS is required to secrete I À in mother's milk [1]. I À in milk is used by the newborn in thyroid hormone biosynthesis, and thus it plays an essential role in post-natal development of skeletal muscles, nervous system, and lungs [2]. In vivo experiments in mice have previously demonstrated that in normal physiology, NIS expression is strictly linked to mammary development in gestation, and to lactation [1]. Non-lactating mammary gland tissue in female mice does not express NIS unless animals receive subcutaneous oxytocin treatments for three consecutive days. On the other hand, a similar treatment in ovariectomized mice is not sufficient for NIS upregulation. In these surgically treated animals, administration of 17-b-estradiol (E2) together with oxytocin is essential for functional expression of NIS. The fact that E2 treatment was only essential in ovariectomized animals, whereas lactogenic hormones were sufficient for functional NIS expression in surgically untreated mice, suggested that ovary functions and endogenous estrogens are essential in upregulating NIS expression [1]. Unlike in non-lactating mammary gland tissue, in transgenic mice bearing experimental breast cancers triggered by Erb-B2/neu and ras oncogenes, functional expression of NIS significantly increases [1]. In the same study, human breast cancer specimens were also analyzed, and an increased NIS expression was detected in human invasive breast cancer and ductal carcinoma in situ, as compared to no expression of NIS in healthy breast samples obtained from reductive mammoplasty operations [1].Recent studies with an ERa+ mammary cell line model, MCF-7, have led to the identification of additional hormones or ligands that control transcriptional regulation of NIS. In this cell line, the symporter gene was shown to be indu...

The epithelial to mesenchymal transition (EMT) and the mesenchymal to epithelial transition (MET) are two critical biological processes that are involved in both physiological events such as embryogenesis and development and also pathological events such as tumorigenesis. They present with dramatic changes in cellular morphology and gene expression exhibiting acute changes in E-cadherin expression. Despite the comprehensive understanding of EMT, the regulation of MET is far from being understood. To find novel regulators of MET, we hypothesized that such factors would correlate with Cdh1 expression. Bioinformatics examination of several expression profiles suggested Elf3 as a strong candidate. Depletion of Elf3 at the onset of MET severely impaired the progression to the epithelial state. This MET defect was explained, in part, by the absence of E-cadherin at the plasma membrane. Moreover, during MET, ELF3 interacts with the Grhl3 promoter and activates its expression. Our findings present novel insights into the regulation of MET and reveal ELF3 as an indispensable guardian of the epithelial state. A better understanding of MET will, eventually, lead to better management of metastatic cancers.