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Cell cycle arrest by FoxO transcription factors involves transcriptional repression of cyclin D, although the exact mechanism remains unclear. In this study, we used the BCR-ABL-expressing cell line BV173 as a model system to investigate the mechanisms whereby FoxO3a regulates cyclin D2 expression. Inhibition of BCR-ABL by STI571 results in down-regulation of cyclin D2 expression, activation of FoxO3a activity, and up-regulation of BCL6 expression. Using reporter gene assays, we demonstrate that STI571, FoxO3a, and BCL6 can repress cyclin D2 transcription through a STAT5/BCL6 site located within the cyclin D2 promoter. We propose that BCR-ABL inhibition leads to FoxO3a activation, which in turn induces the expression of BCL6, culminating in the repression of cyclin D2 transcription through this STAT5/BCL6 site. This process was verified by mobility shift and chromatin immunoprecipitation analyses. We find that conditional activation of FoxO3a leads to accumulation of BCL6 and down-regulation of cyclin D2 at protein and mRNA levels. Furthermore, silencing of FoxO3a and BCL6 in BCR-ABL-expressing cells abolishes STI571-mediated effects on cyclin D2. This report establishes the signaling events whereby BCR-ABL signals are relayed to cyclin D2 to mediate cell cycle progression and defines a potential mechanism by which FoxO proteins regulate cyclin D2 expression.In mammalian cells, the commitment to divide is made in the G 1 phase of the cell cycle in response to various stimuli, including growth factors. After passing the restriction point at mid-to late G 1 , cells become refractory to growth inhibition signals or do not require growth factors to progress into S phase (37). Progression of eukaryotic cells through the cell cycle is controlled by the two families of G 1 cyclins: (i) D-type cyclins (cyclins D1, D2, and D3) and cyclin E (cyclins E1 and E2) (29, 44) and (ii) the cyclin-dependent kinases (cdk's), their catalytic counterparts. The primary targets of the G 1 cyclin-cdk complexes are the retinoblastoma protein (pRb) family of pocket proteins, consisting of pRb, p107, and p130 (20,27,34). The phosphorylation state of pRb regulates the activity of the E2F family of transcription factors; in their hypophosphorylated forms, the pRb-related pocket proteins associate with members of the E2F family, negatively regulating transcription activity of E2F-regulated genes that are required for entry into the S phase of the cell cycle (15,35,40).In mammals, the phosphatidylinositol 3-kinase/protein kinase B (PI3-K/PKB) pathway is stimulated by a variety of growth factors and cytokines and by cell-matrix interactions, and it controls many biological functions, including cell proliferation, cell survival, and insulin responses (30). Importantly, constitutive activation of the PI3-K pathway facilitates tumor formation by two different mechanisms: it supports S-phase entry, and it confers resistance to apoptotic signals which normally restrict uncontrolled cell growth (49). Recently, it has been demonstrated that the member...

PolycombGroup (PcG) proteins are epigenetic silencers involved in maintaining cellular identity, and their deregulation can result in cancer [1]. Mice without the PcG gene Bmi1 are runted and suffer from progressive loss of hematopoietic and neural stem cells [2-4]. Here, we assess the effects of Bmi1 on stem cells and differentiation of an epithelial tissue in vivo. We chose the mammary gland because it allows limiting dilution transplantations [5, 6] and because Bmi1 is overexpressed in breast cancer [7, 8]. Our analyses show that Bmi1 is expressed in all cells of the mouse mammary gland and is especially high in luminal cells. Loss of Bmi1 results in a severe mammary-epithelium growth defect, which can be rescued by codeletion of the Ink4a/Arf locus or pregnancy. Even though mammary stem cells are present in the absence of Bmi1, their activity is reduced, and this is only partially due to Ink4a/Arf expression. Interestingly, loss of Bmi1 causes premature lobuloalveolar differentiation, whereas overexpression of Bmi1 inhibits lobuloalveolar differentiation induced by pregnancy hormones. Because Bmi1 affects not only mammary stem cells but also more committed cells, our data warrant a more detailed analysis of the different roles of Bmi1 in breast-cancer etiology.

Introduction Treatment of breast cancer is becoming more individualized with the recognition of tumor subgroups that respond differently to available therapies. Breast cancer 1 gene (BRCA1)-deficient tumors are usually of the basal subtype and associated with poor survival rates, highlighting the need for more effective therapy.