FoxO3a and BCR-ABL Regulatecyclin D2Transcription through a STAT5/BCL6-Dependent Mechanism

Abstract

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...

Many forks in the path: cycling with FoxO

“…Conversely, ectopic expression of cyclin D1 can partially overcome the FoxO-induced cell cycle arrest, implicating the downregulation of D-type cyclins as a mechanism of FoxO-induced cell cycle inhibition (Ramaswamy et al, 2002). At present, there is no evidence supporting FoxO proteins suppress D-type cyclin expression through direct binding to their gene promoters; however, FoxO proteins can indirectly downregulate D-type cyclins by upregulating the transcription repressor Bcl-6, a known transcriptional repressor of cyclin D2 gene (Fernandez de Mattos et al, 2004). It is also worth mentioning that the PI3K-PKB signalling pathway has been directly implicated in both cyclin D1 gene transcription and protein stabilization (Diehl et al, 1998;Muise-Helmericks et al, 1998;Gille and Downward, 1999).…”

Nodal enhances the activity of FoxO3a and its synergistic interaction with Smads to regulate cyclin G2 transcription in ovarian cancer cells

“…FoxO transcription factors regulate diverse cell functions (Accili and Arden, 2004) through regulation of genes involved in cell proliferation and apoptosis (Weidinger et al, 2008). Specifically, FoxO3a has been reported to activate genes that induce cell cycle arrest, such as p27 (Dijkers et al, 2000), p21 (Seoane et al, 2004), p15 and p19 (Katayama et al, 2008), and to suppress the expression of cyclin D2 (Fernandez de Mattos et al, 2004). FoxO3a is also involved in apoptosis by activating proapoptotic genes Bim (Sunters et al, 2003), Puma (You et al, 2006), TRAIL (Cornforth et al, 2008) and Fas ligand (Brunet et al, 1999).…”

FOXO3a Induces Differentiation of Bcr-Abl-transformed Cells through Transcriptional Down-regulation of Id1

“…Chromatin immunoprecipitation assays were performed as previously described by Fernandez de Mattos et al (33). Protein-DNA complexes were formaldehyde-cross-linked and immunoprecipitated with either the FOXO3a (Upstate, Lake Placid, NY) or an isotype control antibody (BabCO).…”