The role of TGF-β signaling in tumorigenesis is paradoxical: it can be tumor suppressive or tumor promotional, depending on context. The metastatic regulator, Six1, was recently shown to mediate this switch, providing a novel means to explain this elusive “TGF-β paradox”. Herein, we identify a mechanism by which Six1 activates the tumor promotional arm of TGF-β signaling, via its ability to upregulate the miR-106b-25 microRNA cluster, and further identify a novel function for this cluster of microRNAs. While expression of the miR-106b-25 cluster is known to overcome TGF-β-mediated growth suppression via targeting p21 and BIM, we demonstrate for the first time that this same cluster can additionally target the inhibitory Smad7 protein, resulting in increased levels of the TGF-β type I receptor (TβRI) and downstream activation of TGF-β signaling. We further show that the miR-106b-25 cluster is sufficient to induce an epithelial to mesenchymal transition and a tumor initiating cell phenotype, and that it is required downstream of Six1 to induce these phenotypes. Finally, we demonstrate a significant correlation between miR-106b, Six1, and activated TGF-β signaling in human breast cancers, and further show that high levels of miR-106b and miR-93 in breast tumors significantly predicts shortened time to relapse. These findings expand the spectrum of oncogenic functions of miR-106b-25, and may provide a novel molecular explanation, through the Six1 regulated miR-106b-25 cluster, by which TGF-β signaling shifts from tumor suppressive to tumor promoting.

Six1 is a developmentally regulated homeoprotein with limited expression in most normal adult tissues and frequent misexpression in a variety of malignancies. Here we demonstrate, using a bitransgenic mouse model, that misexpression of human Six1 in adult mouse mammary gland epithelium induces tumors of multiple histological subtypes in a dose-dependent manner. The neoplastic lesions induced by Six1 had an in situ origin, showed diverse differentiation, and exhibited progression to aggressive malignant neoplasms, as is often observed in human carcinoma of the breast. Strikingly, the vast majority of Six1-induced tumors underwent an epithelial-mesenchymal transition (EMT) and expressed multiple targets of activated Wnt signaling, including cyclin D1. Interestingly, Six1 and cyclin D1 coexpression was found to frequently occur in human breast cancers and was strongly predictive of poor prognosis. We further show that Six1 promoted a stem/progenitor cell phenotype in the mouse mammary gland and in Six1-driven mammary tumors. Our data thus provide genetic evidence for a potent oncogenic role for Six1 in mammary epithelial neoplasia, including promotion of EMT and stem cell-like features.

The tumor suppressor p14 ARF gene is induced by ectopically expressed E2F, a positive regulator of the cell cycle. The gene is expressed at low levels in normally growing cells in contrast to high levels in varieties of tumors. How p14 ARF gene is regulated by E2F in normally growing cells and tumor cells remains obscure. Here we show that regulation of p14 ARF gene by E2F is distinct from that of classical E2F targets. It is directly mediated by E2F through a novel E2F-responsive element that varies from the typical E2F site. The element responds to E2F activity resulting from ectopic E2F1 expression, inactivation of pRb by adenovirus E1a or shRNA, but not to phosphorylation of pRb by serum stimulation or ectopic cyclin D1/cyclindependent kinase-4 expression in normal human fibroblasts. The element has activity in various tumor cells with defective pRb, but not in normally growing cells. These results indicate that the distinct regulation constitutes the basis of p14 ARF function as a tumor suppressor, discriminating abnormal growth signals caused by defects in pRb function from normal growth signals.

Initiation of DNA replication requires the function of MCM gene products, which participate in ensuring that DNA replication occurs only once in the cell cycle. Expression of all mammalian genes of the MCM family is induced by growth stimulation, unlike yeast, and the mRNA levels peak at G1/S boundary. In this study, we examined the transcriptional activities of isolated human MCM gene promoters. Human MCM5 and MCM6 promoters with mutation in the E2F sites failed in promoter regulation following serum stimulation and exogenous E2F expression. In addition, we identi®ed a novel E2F-like sequence in human MCM6 promoter which cooperates with the authentic E2F sites in E2F-dependent regulation. Forced expression of E2F1 could induce expression of all members of the endogenous MCM genes in rat embryonal ®broblast REF52 cells. Our results demonstrated that the growth-regulated expression of mammalian MCM5 and MCM6 genes, and presumably other MCM members, is primarily regulated by E2F through binding to multiple E2F sites in the promoters.

The transactivator protein Tax of human T-cell leukemia virus type I plays an important role in the development of adult T-cell leukemia probably through modulation of growth regulatory molecules including p16INK4a . The molecular mechanism of leukemogenesis induced by Tax has yet to be elucidated. We analyzed Tax