Forkhead transcription factors of the O class (FOXOs) are important targets of the phosphatidylinositol 3-kinase/Akt pathway, and are key regulators of the cell cycle, apoptosis and response to oxidative stress. FOXOs have been shown to have tumour suppressor function and are important for stem cell maintenance. We have performed a detailed analysis of the transcriptional programme induced in response to Forkhead-box protein O3a (FOXO3a) activation. We observed that FOXO3a activation results in the repression of a large number of nuclear-encoded genes with mitochondrial function. Repression of these genes was mediated by FOXO3a-dependent inhibition of c-Myc. FOXO3a activation also caused a reduction in mitochondrial DNA copy number, expression of mitochondrial proteins, respiratory complexes and mitochondrial respiratory activity. FOXO3a has been previously implicated in the detoxification of reactive oxygen species (ROS) through induction of manganese-containing superoxide dismutase (SOD2). We observed that reduction in ROS levels following FOXO3a activation was independent of SOD2, but required c-Myc inhibition. Hypoxia increases ROS production from the mitochondria, which is required for stabilisation of the hypoxia-inducible factor-1a (HIF-1a). FOXO3a activation blocked the hypoxia-dependent increase in ROS and prevented HIF-1a stabilisation. Our data suggest that FOXO factors regulate mitochondrial activity through inhibition of c-Myc function and alter the hypoxia response. 4 FOXO factors regulate the expression of genes involved in the inhibition of cell cycle progression and induction of apoptosis.5 FOXO also regulates detoxification of reactive oxygen species (ROS) through upregulation of mitochondrial superoxide dismutase (SOD2). 6Mitochondria are central hubs for cellular bioenergetics and are an important source of ROS within mammalian cells. ROS are produced by the respiratory complexes located in the inner mitochondrial membrane. Mitochondrial ROS can cause oxidative damage to the mitochondrial DNA (mtDNA), proteins and lipids, but are also involved in signalling from the mitochondria to the cytoplasm. The regulation of mitochondrial function is crucial for the survival of both normal and cancer cells. The mitochondrial genome encodes only 13 proteins. The majority of proteins required for the maintenance of the structure and function of mitochondria are nuclear encoded. The expression of these genes is regulated by a network of transcription factors, including the nuclear respiratory factors 1 and 2 (NRF1 and NRF2) and the oestrogen-related receptor-a (ERRa). These are bound to and activated by cofactors, peroxisome proliferator-activated receptor gamma co-activator-1a and 1b (PGC1a and PGC1b) and PGC-related 1 (PRC).8 In addition, the transcription factor c-Myc emerged as an important regulator of mitochondrial gene expression. 9,10

BackgroundRegulation of lipid metabolism via activation of sterol regulatory element binding proteins (SREBPs) has emerged as an important function of the Akt/mTORC1 signaling axis. Although the contribution of dysregulated Akt/mTORC1 signaling to cancer has been investigated extensively and altered lipid metabolism is observed in many tumors, the exact role of SREBPs in the control of biosynthetic processes required for Akt-dependent cell growth and their contribution to tumorigenesis remains unclear.ResultsWe first investigated the effects of loss of SREBP function in non-transformed cells. Combined ablation of SREBP1 and SREBP2 by siRNA-mediated gene silencing or chemical inhibition of SREBP activation induced endoplasmic reticulum (ER)-stress and engaged the unfolded protein response (UPR) pathway, specifically under lipoprotein-deplete conditions in human retinal pigment epithelial cells. Induction of ER-stress led to inhibition of protein synthesis through increased phosphorylation of eIF2α. This demonstrates for the first time the importance of SREBP in the coordination of lipid and protein biosynthesis, two processes that are essential for cell growth and proliferation. SREBP ablation caused major changes in lipid composition characterized by a loss of mono- and poly-unsaturated lipids and induced accumulation of reactive oxygen species (ROS) and apoptosis. Alterations in lipid composition and increased ROS levels, rather than overall changes to lipid synthesis rate, were required for ER-stress induction.Next, we analyzed the effect of SREBP ablation in a panel of cancer cell lines. Importantly, induction of apoptosis following SREBP depletion was restricted to lipoprotein-deplete conditions. U87 glioblastoma cells were highly susceptible to silencing of either SREBP isoform, and apoptosis induced by SREBP1 depletion in these cells was rescued by antioxidants or by restoring the levels of mono-unsaturated fatty acids. Moreover, silencing of SREBP1 induced ER-stress in U87 cells in lipoprotein-deplete conditions and prevented tumor growth in a xenograft model.ConclusionsTaken together, these results demonstrate that regulation of lipid composition by SREBP is essential to maintain the balance between protein and lipid biosynthesis downstream of Akt and to prevent resultant ER-stress and cell death. Regulation of lipid metabolism by the Akt/mTORC1 signaling axis is required for the growth and survival of cancer cells.

Cell-cell contacts play a vital role in intracellular signaling, although the molecular mechanisms of these signaling pathways are not fully understood. E-cadherin, an important mediator of cell-cell adhesions, has been shown to be cleaved by ␥-secretase. This cleavage releases a fragment of E-cadherin, E-cadherin C-terminal fragment 2 (E-cad/CTF2), into the cytosol. Here, we study the fate and function of this fragment. First, we show that coexpression of the cadherin-binding protein, p120 catenin (p120), enhances the nuclear translocation of E-cad/CTF2. By knocking down p120 with short interfering RNA, we also demonstrate that p120 is necessary for the nuclear localization of E-cad/CTF2. Furthermore, p120 enhances and is required for the specific binding of E-cad/CTF2 to DNA. Finally, we show that E-cad/CTF2 can regulate the p120-Kaiso-mediated signaling pathway in the nucleus. These data indicate a novel role for cleaved E-cadherin in the nucleus.In multicellular organisms, individual cells are often connected with each other via cell-cell adhesions to form threedimensionally structured tissues or organs. Formation of tight and compact cell-cell adhesions suppresses free cell movement and provides cells with a positional cue for the establishment of cell polarity. In addition to the structural roles, it has been long known that cell-cell contacts play an important role in various signal transduction pathways (1, 2). In other words, through cell-cell contacts, cells can exchange a variety of information with their neighbors to behave properly in their community.Cadherins are a family of transmembrane cell-cell adhesion proteins that can be subdivided into several groups including classical cadherins and protocadherins (3). Classical cadherins, of which E-cadherin is the best characterized, play a crucial role in mediating cell-cell contacts at adherens junctions. The extracellular domains of classical cadherins form homophilic ligations. The cytoplasmic domain of cadherin includes two cadherin homology (CH) 2 domains: CH2 domain (located at the membrane proximal region) and CH3 domain (located at the distal region). These domains are conserved between classical cadherins. The CH2 and CH3 domains of cadherins interact with p120-catenin (p120) and ␤-catenin, respectively (4, 5). Furthermore, the CH2 domain of E-cadherin also interacts with Hakai (6, 7).Cadherins, especially classical cadherins, have been shown to be involved in several signaling pathways regulating cell proliferation, differentiation, and survival (8 -11). However, the molecular mechanisms whereby cadherins regulate varied cellular processes are not fully understood. The cytoplasmic domain of cadherins does not possess any intrinsic enzymatic activity that could directly mediate signaling pathways in the cytosol. However, cadherin-binding proteins, especially ␤-catenin and p120, have been shown to localize in the nucleus and play a key role in signal transduction. The crucial role of ␤-catenin in the canonical Wnt signaling pathway has been well...

Cadherins are the most crucial membrane proteins for the formation of tight and compact cell-cell contacts. Cadherin-based cell-cell adhesions are dynamically established and/or disrupted during various physiological and pathological processes. However, the molecular mechanisms that regulate cell-cell contacts are not fully understood. In this paper, we report a novel functional role of casein kinase 1 (CK1) in the regulation of cell-cell contacts. Firstly, we observed that IC261, a specific inhibitor of CK1, stabilizes cadherin-based cell-cell contacts, whereas the overexpression of CK1 disrupts them. CK1 colocalizes with E-cadherin and phosphorylates the cytoplasmic domain of E-cadherin in vitro and in a cell culture system. We show that the major CK1 phosphorylation site of E-cadherin is serine 846, a highly conserved residue between classical cadherins. Constitutively phosphorylated E-cadherin (S846D) is unable to localize at cell-cell contacts and has decreased adhesive activity. Furthermore, phosphorylated E-cadherin (S846D) has weaker interactions with ␤-catenin and is internalized more efficiently than wild-type E-cadherin. These data indicate that CK1 is a novel negative regulator of cadherin-based cell-cell contacts.