Regulation of cyclin D2 and the cyclin D2 promoter by protein kinase A and CREB in lymphocytes

Decreased CREB phosphorylation impairs embryonic retinal neurogenesis in the Oa1-/- mouse model of Ocular albinism

Mutations in the human Ocular albinism type-1 gene OA1 are associated with abnormal retinal pigment epithelium (RPE) melanogenesis and poor binocular vision resulting from misrouting of ipsilateral retinal ganglion cell (iRGC) axons to the brain. We studied the latter using wild-type (WT) and Oa1-/- mouse eyes. At embryonic stages, the WT RPE-specific Oa1 protein signals through cAMP/Epac1-Erk2-CREB. Following CREB phosphorylation, a pCREB gradient extends from the RPE to the differentiating retinal amacrine and RGCs. In contrast to WT, the Oa1-/- RPE and ventral ciliary-margin-zone, a niche for iRGCs, express less pCREB while their retinas have a disrupted pCREB gradient, indicating Oa1's involvement in pCREB maintenance. Oa1-/- retinas also show hyperproliferation, enlarged nuclei, reduced differentiation, and fewer newborn amacrine and RGCs than WT retinas. Our results demonstrate that Oa1's absence leads to reduced binocular vision through a hyperproliferation-associated block in differentiation that impairs neurogenesis. This may affect iRGC axon's routing to the brain.

Thyroid hormone receptor alpha sumoylation modulates white adipose tissue stores

Thyroid hormone (TH) and thyroid hormone receptor (THR) regulate stem cell proliferation and differentiation during development, as well as during tissue renewal and repair in the adult. THR undergoes posttranslational modification by small ubiquitin-like modifier (SUMO). We generated the THRA (K283Q/K288R)^−/− mouse model for in vivo studies and used human primary preadipocytes expressing the THRA sumoylation mutant (K283R/K288R) and isolated preadipocytes from mutant mice for in vitro studies. THRA mutant mice had reduced white adipose stores and reduced adipocyte cell diameter on a chow diet, compared to wild-type, and these differences were further enhanced after a high fat diet. Reduced preadipocyte proliferation in mutant mice, compared to wt, was shown after in vivo labeling of preadipocytes with EdU and in preadipocytes isolated from mice fat stores and studied in vitro. Mice with the desumoylated THRA had disruptions in cell cycle G₁/S transition and this was associated with a reduction in the availability of cyclin D2 and cyclin-dependent kinase 2. The genes coding for cyclin D1, cyclin D2, cyclin-dependent kinase 2 and Culin3 are stimulated by cAMP Response Element Binding Protein (CREB) and contain CREB Response Elements (CREs) in their regulatory regions. We demonstrate, by Chromatin Immunoprecipitation (ChIP) assay, that in mice with the THRA K283Q/K288R mutant there was reduced CREB binding to the CRE. Mice with a THRA sumoylation mutant had reduced fat stores on chow and high fat diets and reduced adipocyte diameter.

Diosgenin exerts anti-tumor effects through inactivation of cAMP/PKA/CREB signaling pathway in colorectal cancer

Colorectal cancer (CRC) is the most fatal gastrointestinal tumor and it is urge to explore powerful drugs for the treatment. Diosgenin (DSG) as a new steroidal had been reported exerts anti-tumor activity in multiple cancers, including CRC. However, the potential mechanism of DSG suppresses CRC remains further to be revealed. Here, we reported that DSG inhibited proliferation of CRC cells in dose- and time-dependent manner, induced apoptosis by modulating p53 and Bcl-2 family proteins expression to mediate mitochondrial apoptosis pathway, suppressed migration and invasion by reducing MMP-9 (matrix metalloproteinase) and decreased aerobic glycolysis by mediating glucose transporter (GLUT) like GLUT3 and GLUT4, and pyruvate carboxylase PC downregulation. Intriguingly, mechanistic study suggests those phenotypes involved DSG inhibited cAMP/PKA/CREB pathway in CRC cells, and result to inhibit the phosphorylation of CREB to regulate the transcription of genes above-mentioned. Finally, nude mice xenograft tumor model further indicated that DSG could be a great agent to suppress the growth of CRC cells in vivo and have no obvious side effects. Taken together, we revealed a unique mechanism that DSG suppresses CRC cells through cAMP/PKA/CREB pathway and DSG is a promising candidate drug for CRC treatment.

The regulation of cyclin D promoters - review

Cyclins are key regulators of cell cycle progression and survival. Particularly cyclins D (cyclin D1, D2, and D3) act in response to the mitogenic stimulation and are pivotal mediators between proliferative pathways and the nuclear cell cycle machinery. Dysregulation of cyclins expression results in impaired development, abnormal cell growth or tumorigenesis. In this review we summarize current knowledge about regulatory role of the cyclin D promoters, transcriptional factors: regulators, co-activators and adaptor proteins necessary to their activation. We focused on the intracellular signaling pathways vital to cell growth, differentiation and apoptosis including transcription factor families: activator protein 1 (AP1), nuclear factor (NFκB), signal transducer and activator of transcription (STAT), cAMP response element-binding protein (CREB) and Sp/NF-Y, with a special insight into the tissue specific cyclin representation.

CREB acts as a common transcription factor for major epigenetic repressors; DNMT3B, EZH2, CUL4B and E2F6

CREB signaling is known for several decades, but how it regulates both positive and negative regulators of cell proliferation is not well understood. On the other hand functions of major epigenetic repressors such as DNMT3B, EZH2 and CUL4B for their repressive epigenetic modifications on chromatin have also been well studied. However, there is very limited information available on how these repressors are regulated at their transcriptional level. Here, using computational tools and molecular techniques including site directed mutagenesis, promoter reporter assay, chromatin immunoprecipitation (ChIP), we identified that CREB acts as a common transcription factor for DNMT3B, EZH2, CUL4B and E2F6. ChIP assay revealed that pCREB binds to promoters of these repressors at CREs and induce their transcription. As expected, the expression of these repressors and their associated repressive marks particularly H3K27me3 and H2AK119ub are increased and decreased upon CREB overexpression and knock-down conditions respectively in the cancer cells indicating that CREB regulates the functions of these repressors by activating their transcription. Since CREB and these epigenetic repressors are overexpressed in various cancer types, our findings showed the molecular relationship between them and indicate that CREB is an important therapeutic target for cancer therapy.

Dexras1 is a homeostatic regulator of exercise-dependent proliferation and cell survival in the hippocampal neurogenic niche

Adult hippocampal neurogenesis is highly responsive to exercise, which promotes the proliferation of neural progenitor cells and the integration of newborn granule neurons in the dentate gyrus. Here we show that genetic ablation of the small GTPase, Dexras1, suppresses exercise-induced proliferation of neural progenitors, alters survival of mitotic and post-mitotic cells in a stage-specific manner, and increases the number of mature newborn granule neurons. Dexras1 is required for exercise-triggered recruitment of quiescent neural progenitors into the cell cycle. Pharmacological inhibition of NMDA receptors enhances SGZ cell proliferation in wild-type but not dexras1-deficient mice, suggesting that NMDA receptor-mediated signaling is dependent on Dexras1. At the molecular level, the absence of Dexras1 abolishes exercise-dependent activation of ERK/MAPK and CREB, and inhibits the upregulation of NMDA receptor subunit NR2A, bdnf, trkB and vegf-a expression in the dentate gyrus. Our study reveals Dexras1 as an important stage-specific regulator of exercise-induced neurogenesis in the adult hippocampus by enhancing pro-mitogenic signaling to neural progenitor cells and modulating cell survival.

Identification of Upstream Kinases by Fluorescence Complementation Mass Spectrometry

Protein kinases and their substrates comprise extensive signaling networks that regulate many diverse cellular functions. However, methods and techniques to systematically identify kinases directly responsible for specific phosphorylation events have remained elusive. Here we describe a novel proteomic strategy termed fluorescence complementation mass spectrometry (FCMS) to identify kinase-substrate pairs in high throughput. The FCMS strategy employs a specific substrate and a kinase library, both of which are fused with fluorescence complemented protein fragments. Transient and weak kinase-substrate interactions in living cells are stabilized by the association of fluorescence protein fragments. These kinase-substrate pairs are then isolated with high specificity and are identified and quantified by LC-MS. FCMS was applied to the identification of both known and novel kinases of the transcription factor, cAMP response element-binding protein (CREB). Novel CREB kinases were validated by in vitro kinase assays, and the phosphorylation sites were unambiguously located. These results uncovered possible new roles for CREB in multiple important signaling pathways and demonstrated the great potential of this new proteomic strategy.

Stable knockdown of CREB, HIF-1 and HIF-2 by replication-competent retroviruses abrogates the responses to hypoxia in hepatocellular carcinoma

The fast proliferation of tumor cells develops faster than the vasculature, resulting, in most malignant tumors, in generation of hypoxic regions. Hypoxia renders solid tumors resistant to radiation and chemotherapeutics while providing opportunities for tumor-selective therapies targeting tumor hypoxia. Here we exploit two properties of tumors: propagation of tumor cells and ongoing generation of hypoxic regions to construct a system that preferentially leads to the death of tumor cells and thus hinders tumor growth. We constructed murine leukemia virus replication-competent (RCR) viruses that infect only propagating cells. These viruses express small hairpin RNAs (shRNAs) targeting cyclic AMP-response-element binding protein (CREB), hypoxia-inducible factors 1 (HIF)-1 or HIF-2 individually or all three together (X3). These viruses efficiently infected in vitro human hepatocellular carcinoma (HepG2 and FLC4) cells and established persistence of the virus and knocked down the expression of the regulators of the hypoxia-responding genes. Knockdown of either HIF-1 or CREB or both in hypoxia reduced the expression of hypoxia-response elements- and CRE-mediated gene expression, diminished cell proliferation and increased caspase-3 activity. We did not detect any significant effect of the efficiently knocked down HIF-2 on any of the functions tested in vitro. Moreover, severe combined immunodeficiency mice implanted subcutaneously with HepG2 stably infected with recombinant RCRs showed reduction of tumor growth and vascular endothelial growth factor expression, and no hypoxia-guided neovascularization. Combined treatment (RCRs+doxorubicin) improved efficacy in the context of in vitro hypoxia and in vivo (with either vACE-CREB or vACE-X3). This synergistic effect may lead to an improved efficacy and safety profile of the treatment that may result in fewer side effects.

The cAMP responsive element binding protein 1 transactivates epithelial membrane protein 2, a potential tumor suppressor in the urinary bladder urothelial carcinoma

In this study, we report that EMP2 plays a tumor suppressor role by inducing G₂/M cell cycle arrest, suppressing cell viability, proliferation, colony formation/anchorage-independent cell growth via regulation of G₂/M checkpoints in distinct urinary bladder urothelial carcinoma (UBUC)-derived cell lines. Genistein treatment or exogenous expression of the cAMP responsive element binding protein 1 (CREB1) gene in different UBUC-derived cell lines induced EMP2 transcription and subsequent translation. Mutagenesis on either or both cAMP-responsive element(s) dramatically decreased the EMP2 promoter activity with, without genistein treatment or exogenous CREB1 expression, respectively. Significantly correlation between the EMP2 immunointensity and primary tumor, nodal status, histological grade, vascular invasion and mitotic activity was identified. Multivariate analysis further demonstrated that low EMP2 immunoexpression is an independent prognostic factor for poor disease-specific survival. Genistein treatments, knockdown of EMP2 gene and double knockdown of CREB1 and EMP2 genes significantly inhibited tumor growth and notably downregulated CREB1 and EMP2 protein levels in the mice xenograft models. Therefore, genistein induced CREB1 transcription, translation and upregulated pCREB1(S133) protein level. Afterward, pCREB1(S133) transactivated the tumor suppressor gene, EMP2, in vitro and in vivo. Our study identified a novel transcriptional target, which plays a tumor suppressor role, of CREB1.

Oncogenic cAMP responsive element binding protein 1 is overexpressed upon loss of tumor suppressive miR-10b-5p and miR-363-3p in renal cancer

Renal cell carcinoma (RCC) is the most common kidney cancer in adults and has a poor prognosis. cAMP responsive element binding protein 1 (CREB1) is a proto‑oncogenic transcription factor involved in malignancies of various organs. However, its functional role(s) have not yet been elucidated in RCC. We investigated the expression pattern, function and regulation of CREB1 in RCC. CREB1 was overexpressed in the RCC tissues and cell lines. Downregulation of CREB1 inhibited RCC tumorigenesis by affecting cell proliferation, migration and apoptosis. Multiple computational algorithms predicted that the 3'‑untranslated region (3'‑UTR) of human CREB1 mRNA is a target for miR‑10b‑5p and miR‑363‑3p. Luciferase reporter assay, qPCR and western blot analysis confirmed that miR‑10b‑5p and miR‑363‑3p bind directly to the 3'‑UTR of CREB1 mRNA and inhibit mRNA and protein expression of CREB1. qPCR data also revealed a significantly lower expression of miR‑10b‑5p and miR‑363‑3p in RCC tissues. Introduction of miR‑10b‑5p and miR‑363‑3p mimics led to suppressed expression of CREB1 and inhibited cell proliferation, migration and apoptosis reduction. Taken together, we propose that CREB1 is an oncogene in RCC and that upregulation of CREB1 by loss of tumor suppressive miR‑10b‑5p and miR‑363‑3p plays an important role in the tumorigenesis of RCC.

A new perspective on the role of the CREB family of transcription factors in memory consolidation via adult hippocampal neurogenesis

Adult neurogenesis is the process by which new neurons are generated in the brains of adults. Since its discovery 50 years ago, adult neurogenesis has been widely studied in the mammalian brain and has provided a new perspective on the pathophysiology of many psychiatric and neurodegenerative disorders, some of which affect memory. In this regard, adult hippocampal neurogenesis (AHN), which occurs in the subgranular zone (SGZ) of the dentate gyrus (DG), has been suggested to play a role in the formation and consolidation of new memories. This process involves many transcription factors, of which cyclic AMP (cAMP)-responsive element-binding protein (CREB) is a well-documented one. In the developing brain, CREB regulates crucial cell stages (e.g., proliferation, differentiation, and survival), and in the adult brain, it participates in neuronal plasticity, learning, and memory. In addition, new evidence supports the hypothesis that CREB may also participate in learning and memory through its involvement in AHN. This review examines the CREB family of transcription factors, including the different members and known signaling pathways. It highlights the role of CREB as a modulator of AHN, which could underlie its function in memory consolidation mechanisms.

Methamphetamine induces trace amine-associated receptor 1 (TAAR1) expression in human T lymphocytes: role in immunomodulation

The novel transmembrane G protein-coupled receptor, trace amine-associated receptor 1 (TAAR1), represents a potential, direct target for drugs of abuse and monoaminergic compounds, including amphetamines. For the first time, our studies have illustrated that there is an induction of TAAR1 mRNA expression in resting T lymphocytes in response to methamphetamine. Methamphetamine treatment for 6 h significantly increased TAAR1 mRNA expression (P < 0.001) and protein expression (P < 0.01) at 24 h. With the use of TAAR1 gene silencing, we demonstrate that methamphetamine-induced cAMP, a classic response to methamphetamine stimulation, is regulated via TAAR1. We also show by TAAR1 knockdown that the down-regulation of IL-2 in T cells by methamphetamine, which we reported earlier, is indeed regulated by TAAR1. Our results also show the presence of TAAR1 in human lymph nodes from HIV-1-infected patients, with or without a history of methamphetamine abuse. TAAR1 expression on lymphocytes was largely in the paracortical lymphoid area of the lymph nodes with enhanced expression in lymph nodes of HIV-1-infected methamphetamine abusers rather than infected-only subjects. In vitro analysis of HIV-1 infection of human PBMCs revealed increased TAAR1 expression in the presence of methamphetamine. In summary, the ability of methamphetamine to activate trace TAAR1 in vitro and to regulate important T cell functions, such as cAMP activation and IL-2 production; the expression of TAAR1 in T lymphocytes in peripheral lymphoid organs, such as lymph nodes; and our in vitro HIV-1 infection model in PBMCs suggests that TAAR1 may play an important role in methamphetamine -mediated immune-modulatory responses.

Quantitative nuclear proteomics reveals new phenotypes altered in lymphoblastoid cells

B-lymphocytes are essential for the production of antibodies to fight pathogens and are the cells of origin in 95% of human lymphomas. During their activation, and immortalisation by Epstein-Barr virus (EBV) which contributes to human cancers, B-lymphocytes undergo dramatic changes in cell size and protein content. This study was initiated to compare the proteome of two B-cell lines, from the same individual, that reflect different patterns of activation, one is EBV negative and the other is EBV positive. Using isobaric tags, LC-MALDI TOF-TOF and subcellular fractionation, we quantified 499 proteins from B-cells. From a detergent lysed protein extract, we identified 34 proteins that were differentially expressed in EBV-immortalised B-cells. By analysing a nuclear extract, we identified a further 29 differentially expressed proteins with only four proteins shared between the two extracts, illustrating the benefit of subcellular fractionation. This analysis has identified proteins involved in the cytoskeletal phenotype of activated B-cells and the increased antigen recognition in EBV-immortalised cells. Importantly, we have also identified new regulators of transcription and changes in ribonuclear proteins that may contribute to the increased cell size and immortalisation of lymphoblastoid cells.

Colorectal Carcinogenesis: Connecting K-RAS-Induced Transformation and CREB Activity In Vitro and In Vivo

Oncogenic transformation is often associated with an increased expression of the cAMP response element binding (CREB) transcription factor controlling the expression of genes involved in cell proliferation, cell cycle, apoptosis, and tumor development, but a link between K-RAS(V12)-induced transformation and CREB has not yet been determined. Therefore, the constitutive and/or inhibitor-regulated mRNA and protein expression of CREB and signal transduction components and growth properties of parental fibroblasts, K-RAS(V12)-transformed counterparts, shCREB K-RAS(V12) transfectants and human colon carcinoma cells were determined. Increased CREB transcript and protein levels accompanied by an enhanced CREB activity was detected in K-RAS(V12)-transformed murine fibroblasts and K-RAS(V12)-mutated human tumor cells, which is dependent on the MAPK/MEK, PI3K, and/or PKC signal transduction. Immunohistochemical (IHC) staining of colorectal carcinoma lesions and murine tumors, with known KRAS gene mutation status, using antibodies specific for CREB and phospho-CREB, revealed a mechanistic link between CREB expression and K-RAS(V12)-mutated colorectal carcinoma lesions when compared with control tissues. Silencing of CREB by shRNA and/or treatment with a CREB inhibitor (KG-501) reverted the neoplastic phenotype of K-RAS(V12) transformants as demonstrated by a more fibroblast-like morphology, enhanced apoptosis sensitivity, increased doubling time, decreased migration, invasion and anchorage-independent growth, reduced tumorigenesis, and enhanced immunogenicity in vivo. The impaired shCREB-mediated invasion of K-RAS(V12) transformants was accompanied by a transcriptional downregulation of different matrix metalloproteinases (MMP) coupled with their reduced enzymatic activity.

IMPLICATIONS

CREB plays a key role in the K-RAS(V12)-mediated neoplastic phenotype and represents a suitable therapeutic target for murine and human K-RAS(V12)-induced tumors.

Cyproheptadine-induced myeloma cell apoptosis is associated with inhibition of the PI3K/AKT signaling

Recent studies revealed that the anti-allergic cyproheptadine displays anti-blood cancer activity. However, its mechanism is still elusive. In this study, cyproheptadine was found to decrease the expression of anti-apoptotic proteins, including Bcl-2, Mcl-1, and XIAP. More importantly, cyproheptadine-induced apoptosis was accompanied by suppressing AKT activation in myeloma cells. In the subsequent study, cyproheptadine was found to inhibit insulin-like growth factor 1-triggered AKT activation in a time- and concentration-dependent manner. Specifically, cyproheptadine blocked AKT translocation from nuclei for phosphorylation. This inhibition led to suppressed activation of p70S6K and 4EBP1, two key downstream signaling proteins in the PI3K/AKT pathway. However, cyproheptadine did not display inhibition on activation of IGF-1R or STAT3, possible upstream signals of AKT activation. These results further demonstrated that cyproheptadine suppresses the PI3K/AKT signaling pathway, which is probably critical for cyproheptadine-induced MM cell apoptosis.

Regulation of cyclin D2 expression and degradation by follicle-stimulating hormone during rat granulosa cell proliferation in vitro

Cyclin D2 (CCND2, encoded by Ccnd2) plays an important role in the induction of early-to-mid G1 phase transition and is required for granulosa cell proliferation during ovarian folliculogenesis. In the present study, we investigated the role of follicle-stimulating hormone (FSH) in the regulation of cyclin D2 expression and degradation during rat granulosa cell proliferation in vitro. FSH acutely increased granulosa cell Ccnd2 mRNA abundance and CCND2 protein content as well as proliferation. FSH-induced granulosa cell CCND2 protein content and proliferation were mimicked by forskolin and attenuated by inhibitors of protein kinase A (PKA; H89) and phosphatidylinositol 3-kinase (PI3K; LY294002) as well as PKA catalytic subunit (PRKACA) small interfering RNA (siRNA) and dominant-negative Akt (dn-Akt) but were not affected by mitogen-activated protein kinase kinase 1/2 (MEK1/2; U0126). Interestingly, FSH also enhanced CCND2 protein degradation in granulosa cells, a process involving a PKA-mediated ubiquitin-proteasome degradation pathway. Taken together, these results demonstrate that FSH acutely regulated CCND2 expression through both PKA and PI3K signaling pathways during granulosa cell proliferation and also accelerated its ubiquitination-proteasomal degradation, which may prevent overstimulation of granulosa cell proliferation and follicular growth.

A novel function of RNAs arising from the long terminal repeat of human endogenous retrovirus 9 in cell cycle arrest

The human genome contains approximately 50 copies of the replication-defective human endogenous retrovirus 9 (ERV-9) and thousands of copies of its solitary long term repeat (sLTR) element. While some sLTRs are located upstream of critical genes and have enhancer activity, other sLTRs are located within introns and may be transcribed as RNAs. We found that intronic RNAs arising from U3 sLTRs of ERV-9 were expressed as both sense (S) and antisense (AS) transcripts in all human cells tested but that expression levels differed in malignant versus nonmalignant cells. In nonmalignant cells, AS was expressed at higher levels than S and at higher levels than in malignant cells; in malignant cells, AS was expressed at amounts equivalent to those of S RNA. Critically, U3 AS RNA was found to physically bind to key transcription factors for cellular proliferation, including NF-Y, p53, and sp1, indicating that such RNA transcripts may function as decoy targets or traps for NF-Y and thus inhibit the growth of human cancer cells. Indeed, short U3 oligodeoxynucleotides (ODNs) based on these RNA sequences ably inhibited proliferation of cancer cell lines driven by cyclins B1/B2, the gene targets of NF-Y.

CREB and leukemogenesis

Acute myeloid leukemia (AML) is one of the most common leukemias with a 20% 5-year event-free survival in adults and 50% overall survival in children, despite aggressive chemotherapy treatment and bone marrow transplantation. The incidence and mortality rates for acute leukemia have only slightly decreased over the last 20 years, and therefore greater understanding of the molecular mechanisms associated with leukemic progression is needed. To this end, a number of transcription factors that appear to play a central role in leukemogenesis are being investigated; among them is the cAMP response element binding protein (CREB). CREB is a transcription factor that can regulate downstream targets involving in various cellular functions including cell proliferation, survival, and differentiation. In several studies, the majority of bone marrow samples from patients with acute lymphoid and myeloid leukemia demonstrate CREB overexpression. Moreover, CREB overexpression is associated with a poor outcome in AML patients. This review summarizes the role of CREB in leukemogenesis.

CREB signalling in neural stem/progenitor cells: recent developments and the implications for brain tumour biology

This paper discusses the evidence for the role of CREB in neural stem/progenitor cell (NSPC) function and oncogenesis and how these functions may be important for the development and growth of brain tumours. The cyclic-AMP response element binding (CREB) protein has many roles in neurons, ranging from neuronal survival to higher order brain functions such as memory and drug addiction behaviours. Recent studies have revealed that CREB also has a role in NSPC survival, differentiation and proliferation. Recent work has shown that over-expression of CREB in transgenic animals can impart oncogenic properties on cells in various tissues and that aberrant CREB expression is associated with tumours in patients. It is the central position of CREB, downstream of key developmental and growth signalling pathways, which give CREB the ability to influence a spectrum of cell activities, such as cell survival, growth and differentiation in both normal and cancer cells.

Glucose regulates cyclin D2 expression in quiescent and replicating pancreatic β-cells through glycolysis and calcium channels

Understanding the molecular triggers of pancreatic β-cell proliferation may facilitate the development of regenerative therapies for diabetes. Genetic studies have demonstrated an important role for cyclin D2 in β-cell proliferation and mass homeostasis, but its specific function in β-cell division and mechanism of regulation remain unclear. Here, we report that cyclin D2 is present at high levels in the nucleus of quiescent β-cells in vivo. The major regulator of cyclin D2 expression is glucose, acting via glycolysis and calcium channels in the β-cell to control cyclin D2 mRNA levels. Furthermore, cyclin D2 mRNA is down-regulated during S-G(2)-M phases of each β-cell division, via a mechanism that is also affected by glucose metabolism. Thus, glucose metabolism maintains high levels of nuclear cyclin D2 in quiescent β-cells and modulates the down-regulation of cyclin D2 in replicating β-cells. These data challenge the standard model for regulation of cyclin D2 during the cell division cycle and suggest cyclin D2 as a molecular link between glucose levels and β-cell replication.

Enterovirus 71 modulates a COX-2/PGE2/cAMP-dependent viral replication in human neuroblastoma cells: role of the c-Src/EGFR/p42/p44 MAPK/CREB signaling pathway

Enterovirus 71 (EV71) has been shown to induce cyclooxygenase‐2 (COX‐2) expression in human neuroblastoma SK‐N‐SH cells through the action of MAPKs, NF‐κB, and AP‐1. On the other hand, the transcription factor CREB has also been implicated in the expression of COX‐2 in other cell lines. Here, we report that EV71‐induced COX‐2 expression and PGE₂ production were both inhibited by pretreatment with the PKA inhibitor H89 or by transfection with CREB siRNA. In addition, EV71‐induced COX‐2 expression and c‐Src/EGFR phosphorylation were both attenuated by transfection with c‐Src siRNA or pretreatment with the inhibitors of c‐Src (PP1) or EGF receptor (EGFR) (AG1478 and EGFR‐neutralizing antibody). We also observed that EV71‐induced p42/p44 MAPK phosphorylation was decreased following pretreatment with AG1478. Moreover, EV71‐induced COX‐2 expression was blocked by pretreatment with the p300 inhibitor GR343 or by transfection with p300 siRNA. Using immunoprecipitation and chromatin immunoprecipitation assays, we observed that EV71 stimulated the association of CREB and p300 with the COX‐2 promoter region. Notably, we also demonstrated that EV71‐induced COX‐2 expression and PGE₂ production promoted viral replication via cAMP signaling. In summary, this study demonstrates that EV71 activates the c‐Src/EGFR/p42/p44 MAPK pathway in human SK‐N‐SH cell, which leads to the activation of CREB/p300, and stimulates COX‐2 expression and PGE₂ release. J. Cell. Biochem. 112: 559–570, 2011. © 2010 Wiley‐Liss, Inc.

Transcriptional control of melanoma metastasis: the importance of the tumor microenvironment

The molecular changes associated with the transition of melanoma cells from radial growth phase (RGP) to vertical growth phase (VGP) and the metastatic phenotype are not very well defined. However, some of the genes involved in this process and their transcriptional regulation are beginning to be elucidated. For example, the switch from RGP to VGP and the metastatic phenotype is associated with loss of the AP-2α transcription factor. AP-2α regulates the expression of c-KIT, MMP-2, VEGF, and the adhesion molecule MCAM/MUC18. Recently, we reported that AP-2α also regulates two G-protein coupled receptors (GPCRs) PAR-1 and PAFR. In turn, the thrombin receptor, PAR-1, regulates the expression of the gap junction protein Connexin-43 and the tumor suppressor gene Maspin. Activation of PAR-1 also leads to overexpression and secretion of proangiogenic factors such as IL-8, uPA, VEGF, PDGF, as well certain integrins. PAR-1 also cooperates with PAFR to regulate the expression of the MCAM/MUC18 via phosphorylation of CREB. The ligands for these GPCRs, thrombin and PAF, are secreted by stromal cells, emphasizing the importance of the tumor microenvironment in melanoma metastasis. The metastatic phenotype of melanoma is also associated with overexpression and function of CREB/ATF-1. Loss of AP-2α and overexpression of CREB/ATF-1 results in the overexpression of MCAM/MUC18 which by itself contributes to melanoma metastasis by regulating the inhibitor of DNA binding-1 (Id-1). CREB/ATF-1 also regulates the angiogenic factor CYR-61. Our recent data indicate that CREB/ATF-1 regulates the expression of AP-2α, thus, supporting the notion that CREB is an important "master switch" in melanoma progression.

A small-molecule inhibitor of D-cyclin transactivation displays preclinical efficacy in myeloma and leukemia via phosphoinositide 3-kinase pathway

D-cyclins are universally dysregulated in multiple myeloma and frequently overexpressed in leukemia. To better understand the role and impact of dysregulated D-cyclins in hematologic malignancies, we conducted a high-throughput screen for inhibitors of cyclin D2 transactivation and identified 8-ethoxy-2-(4-fluorophenyl)-3-nitro-2H-chromene (S14161), which inhibited the expression of cyclins D1, D2, and D3 and arrested cells at the G(0)/G(1) phase. After D-cyclin suppression, S14161 induced apoptosis in myeloma and leukemia cell lines and primary patient samples preferentially over normal hematopoietic cells. In mouse models of leukemia, S14161 inhibited tumor growth without evidence of weight loss or gross organ toxicity. Mechanistically, S14161 inhibited the activity of phosphoinositide 3-kinase in intact cells and the activity of the phosphoinositide 3-kinases α, β, δ, and γ in a cell-free enzymatic assay. In contrast, it did not inhibit the enzymatic activities of other related kinases, including the mammalian target of rapamycin, the DNA-dependent protein kinase catalytic subunit, and phosphoinositide-dependent kinase-1. Thus, we identified a novel chemical compound that inhibits D-cyclin transactivation via the phosphoinositide 3-kinase/protein kinase B signaling pathway. Given its potent antileukemia and antimyeloma activity and minimal toxicity, S14161 could be developed as a novel agent for blood cancer therapy.

CREB inhibits AP-2alpha expression to regulate the malignant phenotype of melanoma

BACKGROUND

The loss of AP-2alpha and increased activity of cAMP-responsive element binding (CREB) protein are two hallmarks of malignant progression of cutaneous melanoma. However, the molecular mechanism responsible for the loss of AP-2alpha during melanoma progression remains unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Herein, we demonstrate that both inhibition of PKA-dependent CREB phosphorylation, as well as silencing of CREB expression by shRNA, restored AP-2alpha protein expression in two metastatic melanoma cell lines. Moreover, rescue of CREB expression in CREB-silenced cell lines downregulates expression of AP-2alpha. Loss of AP-2alpha expression in metastatic melanoma occurs via a dual mechanism involving binding of CREB to the AP-2alpha promoter and CREB-induced overexpression of another oncogenic transcription factor, E2F-1. Upregulation of AP-2alpha expression following CREB silencing increases endogenous p21(Waf1) and decreases MCAM/MUC18, both known to be downstream target genes of AP-2alpha involved in melanoma progression.

CONCLUSIONS/SIGNIFICANCE

Since AP-2alpha regulates several genes associated with the metastatic potential of melanoma including c-KIT, VEGF, PAR-1, MCAM/MUC18, and p21(Waf1), our data identified CREB as a major regulator of the malignant melanoma phenotype.

CCND2 polymorphisms associated with clearance of HBV infection

Cyclin D2s (CCND2s) are members of the D-type cyclin family. They interact and construct complexes with cyclin-dependent kinase (CDK)4 or 6. The cyclin D2/CDK4 or CDK6 complexes have key roles in controlling the progression of cell cycle from the Gap 1 (G1) phase to the synthesis (S) phase. Overexpression of cyclin D2 is associated with the development of tumors. In this study, we identified 16 sequence variants of CCND2 polymorphisms through direct DNA sequencing in 24 individuals, and 5 common variants were selected for genotyping in larger-scale subjects (n=1100). Genetic associations of those polymorphisms with hepatitis B virus (HBV) clearance and hepatocellular carcinoma (HCC) outcome among patients with HBV were analyzed. Although no significant association was observed between the polymorphisms and HCC outcome among HBV patients, one common polymorphism in the 5'-untranslated region (that is, rs1049606) and the most common haplotype (CCND-ht1 [T-C-T-A-T]), however, were significantly associated with HBV clearance (odds ratio=0.69, P=0.0002, Pcorr=0.001 and odds ratio=1.37, P=0.0009, Pcorr=0.004, respectively). The minor allele frequency of rs1049606 among the spontaneously recovered (SR) group was significantly higher than that of the chronic carrier (CC) group (frequency=0.403 vs 0.336, P=0.0002). In contrast, the frequency of CCND-ht1 was higher among the CC group than among the SR group (frequency=0.429 vs 0.374, P=0.0009). The information identified in this study might provide valuable insights into generating strategies for control of HBV.

Vav1 couples the T cell receptor to cAMP response element activation via a PKC-dependent pathway

The transcription factor cAMP-responsive element binding protein (CREB) is a regulator of the expression of several genes important for lymphocyte activation and proliferation. However, the proximal signaling events leading to activation of CREB in T cells upon antigen receptor stimulation remain unknown. Here we identify a role for Vav1 in the activation of the cAMP response element (CRE), the binding site for CREB. T cell receptor (TCR)/CD28 - induced costimulation of Jurkat T cells expressing Vav1 but not a GEF-deficient mutant showed increased CRE activation (7.2+/-2.4 fold over control), whereas Vav1 downregulation by siRNA reduced activation of CRE by 2.6+/-1.3 fold. Inhibition of PKC and MEK but not p38 could reduce Vav1-mediated CRE activation, suggesting that Vav1 transmits TCR and CD28 signals to activation of CRE via PKC and ERK signaling pathways. As a consequence, downregulation of Vav1 impaired the expression of several CRE-containing genes like cyclin D1, INFgamma and IL-2, whereas overexpression of Vav1 enhanced CRE-dependent gene expression. Furthermore, cAMP-induced CRE-dependent transcription and gene expression was also modulated by Vav1, but did not require activation of PKC and the GEF function of Vav1. Our data provide insights into the signal transduction events regulating CRE-mediated gene expression in T cells, which affects T cell development, proliferation and activation. We identify Vav1 as an essential component of TCR-induced CRE activation and gene expression, which underlines the central role for Vav1 as key player for TCR signal transduction and gene expression.

Silencing cAMP-response element-binding protein (CREB) identifies CYR61 as a tumor suppressor gene in melanoma

Metastatic progression of melanoma is associated with overexpression and activity of cAMP-response element-binding protein (CREB). However, the mechanism by which CREB contributes to tumor progression and metastasis remains unclear. Here, we demonstrate that stably silencing CREB expression in two human metastatic melanoma cell lines, A375SM and C8161-c9, suppresses tumor growth and experimental metastasis. Analysis of cDNA microarrays revealed that CREB silencing leads to increased expression of cysteine-rich protein 61 (CCN1/CYR61) known to mediate adhesion, chemostasis, survival, and angiogenesis. Promoter analysis and chromatin immunoprecipitation assays demonstrated that CREB acts as a negative regulator of CCN1/CYR61 transcription by directly binding to its promoter. Re-expression of CREB in CREB-silenced cells rescued the low CCN1/CYR61 expression phenotype. CCN1/CYR61 overexpression resulted in reduced tumor growth and metastasis and inhibited the activity of matrix metalloproteinase-2. Furthermore, its overexpression decreased melanoma cell motility and invasion through Matrigel, which was abrogated by silencing CCN1/CYR61 in low metastatic melanoma cells. Moreover, a significant decrease in angiogenesis as well as an increase in apoptosis was seen in tumors overexpressing CCN1/CYR61. Our results demonstrate that CREB promotes melanoma growth and metastasis by down-regulating CCN1/CYR61 expression, which acts as a suppressor of melanoma cell motility, invasion and angiogenesis.

Stem cell factor and interleukin-2/15 combine to enhance MAPK-mediated proliferation of human natural killer cells

Stem cell factor (SCF) promotes synergistic cellular proliferation in combination with several growth factors, and appears important for normal natural killer (NK)-cell development. CD34(+) hematopoietic precursor cells (HPCs) require interleukin-15 (IL-15) for differentiation into human NK cells, and this effect can be mimicked by IL-2. Culture of CD34(+) HPCs or some primary human NK cells in IL-2/15 and SCF results in enhanced growth compared with either cytokine alone. The molecular mechanisms responsible for this are unknown and were investigated in the present work. Activation of NK cells by IL-2/15 increases expression of c-kit whose kinase activity is required for synergy with IL-2/15 signaling. Mitogen-activated protein kinase (MAPK) signaling intermediaries that are activated both by SCF and IL-2/15 are enhanced in combination to facilitate earlier cell-cycle entry. The effect results at least in part via enhanced MAPK-mediated modulation of p27 and CDK4. Collectively the data reveal a novel mechanism by which SCF enhances cellular proliferation in combination with IL-2/15 in primary human NK cells.

Targeting transcription factors for therapeutic benefit

Transcription factors are a large class of biological molecules that are important for health and disease. Despite that there are challenges to targeting them therapeutically and most approaches alter their activity indirectly. Research at the chemical biology interface has led to the development of new ways of targeting transcription factors including blocking transcription factor dimerisation, targeting specific DNA sequences and DNA decoys. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of cancer.

Identification of kinetin riboside as a repressor of CCND1 and CCND2 with preclinical antimyeloma activity

Knockout and transgenic studies in mice demonstrate that normal somatic tissues redundantly express 3 cyclin D proteins, whereas tumor cells seem dependent on a single overexpressed cyclin D. Thus, selective suppression of the individual cyclin D deregulated in a tumor represents a biologically valid approach to targeted cancer therapy. In multiple myeloma, overexpression of 1 of the cyclin D proteins is a ubiquitous feature, unifying at least 7 different initiating genetic events. We demonstrate here that RNAi of genes encoding cyclin D1 and cyclin D2 (CCND1 and CCND2, respectively) inhibits proliferation and is progressively cytotoxic in human myeloma cells. By screening a chemical library using a cell-based assay for inhibition of CCND2 trans-activation, we identified the plant cytokinin kinetin riboside as an inhibitor of CCND2 trans-activation. Kinetin riboside induced marked suppression of CCND2 transcription and rapidly suppressed cyclin D1 and D2 protein expression in primary myeloma cells and tumor lines, causing cell-cycle arrest, tumor cell-selective apoptosis, and inhibition of myeloma growth in xenografted mice. Mechanistically, kinetin riboside upregulated expression of transcription repressor isoforms of cAMP-response element modulator (CREM) and blocked both trans-activation of CCND2 by various myeloma oncogenes and cis-activation of translocated CCND1, suggesting induction of an overriding repressor activity that blocks multiple oncogenic pathways targeting cyclin D genes. These data support targeted repression of cyclin D genes as a therapeutic strategy for human malignancies.

Characterization and function of CREB homologue from Crassostrea ariakensis stimulated by rickettsia-like organism

The cAMP response element-binding protein (CREB) is a transcription factor that plays important roles in cellular growth, proliferation and survival. Here, we report that a homologue of CREB transcription factor, Ca-CREB, was identified and functionally characterized in oyster, Crassostrea ariakensis. The full-length cDNA consists of 1397bp with an ORF encoding a 39.3kDa protein. Amino acid sequence analysis revealed that Ca-CREB shares conserved signature motifs with other CREB proteins. Ca-CREB was ubiquitously and constitutively expressed in oyster, and the expression level in hemocytes was higher than that in other tissues. The expression level of Ca-CREB was not modified after RLO stimulation, while tumor necrosis factor-alpha (TNF-alpha) expression was increased obviously, which was revealed by real-time reverse-transcriptase polymerase chain reaction (RT-PCR). Electrophoretic mobility shift assay (EMSA) and Western blotting showed that recombinant CREB proteins specifically bind the consensus CREB binding site, and DNA-binding activity and phosphorylation of Ca-CREB were induced by RLO. These results suggest that Ca-CREB is a CREB homologue and may be involved in immune responses against RLO.

Beta cell-specific deficiency of the stimulatory G protein alpha-subunit Gsalpha leads to reduced beta cell mass and insulin-deficient diabetes

The G protein alpha-subunit G(s)alpha is required for hormone-stimulated cAMP generation. In pancreatic beta cells, G(s)alpha mediates the signaling of glucagon-like peptide 1 and other incretin hormones, which are implicated as important regulators of beta cell survival and insulin release. Studies have suggested that G(s)alpha/cAMP mediates these actions by stimulating insulin receptor substrate 2 (IRS2) expression. Mice with beta cell-specific G(s)alpha deficiency (betaGsKO) were generated by mating G(s)alpha-floxed mice to rat insulin II promoter-cre recombinase mice. betaGsKO mice had poor survival and postnatal growth with low serum insulin-like growth factor 1 levels. betaGsKO mice also developed severe hyperglycemia and glucose intolerance with severe hypoinsulinemia and reduced islet insulin content and glucose-stimulated insulin release. betaGsKO mice had markedly reduced average islet size and beta cell mass, which was partially explained by reduced beta cell size. In addition, betaGsKO mice had significantly reduced beta cell proliferation and increased beta cell apoptosis and markedly reduced expression of the cell cycle protein cyclin D2. The effects on beta cell mass and proliferation, but not apoptosis, were present from birth. Unexpectedly expression of Irs2 and the downstream gene Pdx1 were unaffected. These results show that G(s)alpha/cAMP pathways are critical regulators of beta cell function and proliferation that can work through IRS2-independent mechanisms.

Akt/cAMP-responsive element binding protein/cyclin D1 network: a novel target for prostate cancer inhibition in transgenic adenocarcinoma of mouse prostate model mediated by Nexrutine, a Phellodendron amurense bark extract

PURPOSE

Development of prostate cancer prevention strategies is an important priority to overcome high incidence, morbidity, and mortality. Recently, we showed that Nexrutine, an herbal extract, inhibits prostate cancer cell proliferation through modulation of Akt and cAMP-responsive element binding protein (CREB)-mediated signaling pathways. However, it is unknown if Nexrutine can be developed as a dietary supplement for the prevention of prostate cancer. In this study, we used the transgenic adenocarcinoma of mouse prostate (TRAMP) model to examine the ability of Nexrutine to protect TRAMP mice from developing prostate cancer.

EXPERIMENTAL DESIGN

Eight-week-old TRAMP mice were fed with pelleted diet containing 300 and 600 mg/kg Nexrutine for 20 weeks. Efficacy of Nexrutine was evaluated by magnetic resonance imaging at 18 and 28 weeks of progression and histologic analysis of prostate tumor or tissue at the termination of the experiment. Tumor tissue was analyzed for modulation of various signaling molecules.

RESULTS

We show that Nexrutine significantly suppressed palpable tumors and progression of cancer in the TRAMP model. Expression of total and phosphorylated Akt, CREB, and cyclin D1 was significantly reduced in prostate tissue from Nexrutine intervention group compared with tumors from control animals. Nexrutine also inhibited cyclin D1 transcriptional activity in androgen-independent PC-3 cells. Overexpression of kinase dead Akt mutant or phosphorylation-defective CREB inhibited cyclin D1 transcriptional activity.

CONCLUSIONS

The current study shows that Nexrutine-mediated targeting of Akt/CREB-induced activation of cyclin D1 prevents the progression of prostate cancer. Expression of CREB and phosphorylated CREB increased in human prostate tumors compared with normal tissue, suggesting their potential use as prognostic markers.

CREB--a real culprit in oncogenesis

The cAMP response element-binding protein (CREB) is a stimulus-induced transcription factor that responds rapidly to phosphorylation and/or coactivator activation. Regulated activation of CREB has a significant impact on cellular growth, proliferation and survival. To overturn the cellular control of these processes, tumor cells have developed various mechanisms to achieve constitutive activation of CREB, including gene amplification, chromosome translocation, interaction with viral oncoproteins, and inactivation of tumor suppressor genes. These mechanisms converge on the phosphorylation of CREB and/or the activation of transducer of regulated CREB activity (TORC) coactivators to effect uncontrolled proliferation of cells. This minireview summarizes the different lines of existing evidence that support a direct role of CREB in oncogenesis.

GSK3 beta mediates suppression of cyclin D2 expression by tumor suppressor PTEN

PTEN, encoding a lipid phosphatase, is a tumor suppressor gene and is mutated in various types of cancers. It is reported to regulate G1 to S phase transition of the cell cycle by influencing the expression, protein stability and subcellular location of cyclin D1. Here, we provide evidence that PTEN modulates the transcription and protein stability of cyclin D2. Targeted deletion of Pten in mouse embryonic fibroblasts (MEFs) endowed cells with greater potential to overcome G1 arrest than wild-type MEFs and led to the elevated expression of cyclin D2, which was suppressed by the introduction of PTEN. We further defined a pathway involving GSK3beta and beta-catenin/TCF in PTEN-mediated suppression of cyclin D2 transcription. LiCl, an inhibitor of GSK3beta, abolished inhibitory effect of PTEN on cyclin D2 expression, and TCF members could directly bind to the promoter of cyclin D2 and regulate its transcription in a CREB-dependent manner. Our results indicate that the downregulation of cyclin D2 expression by PTEN is mediated by the GSK3beta/beta-catenin/TCF pathway in cooperation with CREB, and suggest a convergence from the PI-3 kinase/PTEN pathway and the Wnt pathway in modulation of cyclin D2 expression.

Epstein-Barr virus represses the FoxO1 transcription factor through latent membrane protein 1 and latent membrane protein 2A

Epstein-Barr virus (EBV) infection is associated with the development of many B-cell lymphomas, including Burkitt's lymphoma, Hodgkin's lymphoma, and posttransplant lymphoproliferative disease. The virus alters a diverse range of cellular molecules, which leads to B-cell growth and immortalization. This study was initiated to investigate the interplay between EBV and a proapoptotic transcription factor target, FoxO1. In this report, we show that EBV infection of B cells leads to the downregulation of FoxO1 expression by phosphatidylinositol 3-kinase-mediated nuclear export, by inhibition of FoxO1 mRNA expression, and by alteration of posttranslational modifications. This repression directly correlates with the expression of the FoxO1 target gene Bcl-6 and inversely correlates with the FoxO1-regulated gene Cyclin D2. Expression of the EBV genes for latent membrane protein 1 and latent membrane protein 2A decreases FoxO1 expression. Thus, our data elucidate distinct mechanisms for the regulation of the proapoptotic transcription factor FoxO1 by EBV.