The conserved role of Notch signaling in controlling intestinal cell fate specification and homeostasis has been extensively studied. Nevertheless, the precise identity of the cells in which Notch signaling is active and the role of different Notch receptor paralogues in the intestine remain ambiguous, due to the lack of reliable tools to investigate Notch expression and function in vivo. We generated a new series of transgenic mice that allowed us, by lineage analysis, to formally prove that Notch1 and Notch2 are specifically expressed in crypt stem cells. In addition, a novel Notch reporter mouse, Hes1-EmGFPSAT, demonstrated exclusive Notch activity in crypt stem cells and absorptive progenitors. This roster of knock-in and reporter mice represents a valuable resource to functionally explore the Notch pathway in vivo in virtually all tissues.

Aberrant activation of Notch receptors has been implicated in breast cancer; however, the mechanisms contributing to Notch-dependent transformation remain elusive because Notch displays dichotomous functional activities, promoting both proliferation and growth arrest. We investigated the cellular basis for the heterogeneous responses to Notch pathway activation in 3D cultures of MCF-10A mammary epithelial cells. Expression of a constitutively active Notch-1 intracellular domain (NICD) was found to induce two distinct types of 3D structures: large, hyperproliferative structures and small, growth-arrested structures with reduced cell-to-matrix adhesion. Interestingly, we found that these heterogeneous phenotypes reflect differences in Notch pathway activation levels; high Notch activity caused down-regulation of multiple matrix-adhesion genes and inhibition of proliferation, whereas low Notch activity maintained matrix adhesion and provoked a strong hyperproliferative response. Moreover, microarray analyses implicated NICD-induced p63 down-regulation in loss of matrix adhesion. In addition, a reverse-phase protein array-based analysis and subsequent loss-of-function studies identified STAT3 as a dominant downstream mediator of the NICD-induced outgrowth. These results indicate that the phenotypic responses to Notch are determined by the dose of pathway activation; and this dose affects the balance between growth-stimulative and growth-suppressive effects. This unique feature of Notch signaling provides insights into mechanisms that contribute to the dichotomous effects of Notch during development and tumorigenesis.

Germ cell tumors (GCTs) of the testis are the predominant cancer among young men. We analyzed gene expression profiles of 50 GCTs of various subtypes, and we compared them with 443 other common malignant tumors of epithelial, mesenchymal, and lymphoid origins. Significant differences in gene expression were found among major histological subtypes of GCTs, and between them and other malignancies. We identified 511 genes, belonging to several critical functional groups such as cell cycle progression, cell proliferation, and apoptosis, to be significantly differentially expressed in GCTs compared with other tumor types. Sixty-five genes were sufficient for the construction of a GCT class predictor of high predictive accuracy (100% training set, 96% test set), which might be useful in the diagnosis of tumors of unknown primary origin. Previously described diagnostic and prognostic markers were found to be expressed by the appropriate GCT subtype (AFP, POU5F1, POV1, CCND2, and KIT). Several additional differentially expressed genes were identified in teratomas (EGR1 and MMP7), yolk sac tumors (PTPN13 and FN1), and seminomas (NR6A1, DPPA4, and IRX1). Dynamic computation of interaction networks and mapping to existing pathways knowledge databases revealed a potential role of EGR1 in p21-induced cell cycle arrest and intrinsic chemotherapy resistance of mature teratomas. testicular cancer ͉ unknown primary tumors ͉ DNA microarrays ͉ molecular interaction networks H uman germ cell tumors (GCTs) are a diverse group of neoplasms that most commonly arise in the gonads, particularly in the testis. They account for up to 60% of all malignancies diagnosed in men between 20 and 40 years of age. Their incidence (6-11 per 100,000) has increased among Caucasians in recent decades, with an annual increase of 3-6% (1).The histopathological classification of GCTs has been controversial because of the different concepts of histogenesis of these neoplasms, as well as the pluripotent nature of transformed primordial germ cells. GCTs can mimic normal patterns of embryonic segregation and differentiation, giving rise to structures resembling embryonic (endoderm, mesoderm, ectoderm) and extra-embryonic (yolk sac, trophoblast) derivatives. On the basis of the presence of those elements, they are further divided into pure GCTs (seminoma, embryonal carcinoma, teratoma, choriocarcinoma, and yolk sac tumor) or mixed GCTs, if more than one element is present.Compared with most cancers of adults, GCTs are highly sensitive to chemotherapy (2). Even with metastases, 80% of GCT patients can be cured by cisplatin-based combination chemotherapy, followed by secondary resection of residual tumor lesions, which can contain necrotic cells, viable malignant cells, or mature teratoma. In contrast to the other histological subtypes, mature teratomas show a less aggressive clinical behavior, but are unresponsive to chemotherapy. The biological bases for the chemosensitivity of GCTs and the clinical behavior of mature teratomas are unclear (3).In this study, ...