Cell cycle regulation of cyclin A gene expression by the cyclic AMP-responsive transcription factors CREB and CREM

Inhibition of DNA binding activity of cAMP response element-binding protein by 1,2-naphthoquinone through chemical modification of Cys-286

1,2-Naphthoquinone (1,2-NQ) is an atmospheric electrophile that reacts covalently with protein thiols. Our previous study revealed that exposure of bovine aortic endothelial cells to 1,2-NQ causes covalent modification of cAMP response element-binding protein (CREB), thereby inhibiting its DNA binding activity and substantial gene expression of B-cell lymphoma-2 (Bcl-2) that is regulated by this transcription factor. In this study, we identified the modification sites of CREB that are associated with the decreased transcriptional activity. Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF/MS) analysis indicated that three amino acids (Cys-286, Lys-290, and Lys-319) were irreversibly modified by 1,2-NQ. Mutational analysis revealed that electrophilic modification of Cys-286, but not the other two amino acids, at the DNA binding domain is essential for the reduced CREB activity. Substitution of Cys-286 with tryptophan (C286W), which mimics CREB modification by 1,2-NQ, supported this notion. These results suggest that the covalent interaction of CREB with 1,2-NQ through Cys-286 blocks the DNA binding activity of CREB, resulting in the repression of CREB-regulated genes.

PGE2 receptor EP2 mediates the antagonistic effect of COX-2 on TGF-beta signaling during mammary tumorigenesis

The molecular mechanisms that enable cyclooxygenase-2 (COX-2) and its mediator prostaglandin E2 (PGE2) to inhibit transforming growth factor-beta (TGF-beta) signaling during mammary tumorigenesis remain unknown. We show here that TGF-beta selectively stimulated the expression of the PGE2 receptor EP2, which increased normal and malignant mammary epithelial cell (MEC) invasion, anchorage-independent growth, and resistance to TGF-beta-induced cytostasis. Mechanistically, elevated EP2 expression in normal MECs inhibited the coupling of TGF-beta to Smad2/3 activation and plasminogen activator inhibitor-1 (PAI1) expression, while EP2 deficiency in these same MECs augmented Smad2/3 activation and PAI expression stimulated by TGF-beta. Along these lines, engineering malignant MECs to lack EP2 expression prevented their growth in soft agar, restored their cytostatic response to TGF-beta, decreased their invasiveness in response to TGF-beta, and potentiated their activation of Smad2/3 and expression of PAI stimulated by TGF-beta. More important, we show that COX-2 or EP2 deficiency both significantly decreased the growth, angiogenesis, and pulmonary metastasis of mammary tumors produced in mice. Collectively, this investigation establishes EP2 as a potent mediator of the anti-TGF-beta activities elicited by COX-2/PGE2 in normal and malignant MECs. Our findings also suggest that pharmacological targeting of EP2 receptors may provide new inroads to antagonize the oncogenic activities of TGF-beta during mammary tumorigenesis.-Tian, M., Schiemann, W. P. PGE2 receptor EP2 mediates the antagonistic effect of COX-2 on TGF-beta signaling during mammary tumorigenesis.

Evidence for phosphatase activity of p27SJ and its impact on the cell cycle

p27SJ, a novel protein isolated from St John's wort (Hypericum perforatum), belongs to an emerging family of DING proteins that are related to a prokaryotic phosphate-binding protein superfamily. Here we demonstrate that p27SJ exhibits phosphatase activity and that its expression in cells decreases the level of phosphorylated Erk1/2, a key protein of several signaling pathways. Treatment of p27SJ-expressing cells with phosphatase inhibitors including okadaic acid, maintained Erk1/2 in its phosphorylated form, suggesting that dephosphorylation of Erk1/2 is mediated by p27SJ. Further, expression of p27SJ affects Erk1/2 downstream regulatory targets such as STAT3 and CREB. Moreover, the level of expression of cyclin A that associates with active ERK1/2 and is regulated by CREB, was modestly reduced in p27SJ-expressing cells. Accordingly, results from in vitro kinase assays revealed a noticeable decrease in the activity of cyclin A in cells expressing p27SJ. Cell cycle analysis demonstrated dysregulation at S and G2/M phases in cells expressing p27SJ, supporting the notion that a decline in cyclin A activity by p27SJ has a biological impact on cell growth. These observations provide evidence that p27SJ alters the state of Erk1/2 phosphorylation, and impacts several biological events associated with cell growth and function.

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.

The role of protein synthesis in cell cycling and cancer

Cell cycling and protein synthesis are both key physiological tasks for cancer cells. Here we present a model for how the elongation phase of protein synthesis, governed by elongation factor 2 and elongation factor 2 kinase, both modulates and responds to cell cycling. Within this framework we also discuss survivin, a protein with both pro-mitotic and anti-apoptotic roles whose persistence in the cell is tied to protein synthesis due to its short half-life. Finally, we provide a brief overview of efforts of cancer researchers to target EF2 and EF2 kinase.

Inhibition of CREB function in mouse epidermis reduces papilloma formation

We used a double transgenic tetracycline system to conditionally express A-CREB, a dominant negative protein that prevents the DNA binding and function of cAMP-responsive element binding protein (CREB) family members, in mouse basal epidermis using the keratin 5 promoter. There was no phenotype in the adult. However, following a 7,12-dimethylbenz(a)anthracene (DMBA)/phorbol-12-myristate-13-acetate two-stage skin carcinogenesis experiment, A-CREB-expressing epidermis develop 5-fold fewer papillomas than wild-type controls. However, A-CREB expression one month after DMBA treatment does not prevent papilloma formation, suggesting that CREB functions at an early stage of papilloma formation. Oncogenic H-Ras genes with A-->T mutations in codon 61 were found in wild-type skin but not in A-CREB-expressing skin 2 days after DMBA treatment, suggesting that A-CREB either prevents DMBA mutagenesis or kills oncogenic H-Ras cells. In primary keratinocyte cultures, A-CREB expression induced apoptosis of v-Ras(Ha)-infected cells and suppressed the expression of cell cycle proteins cyclin B1 and cyclin D1. These results suggest that inhibiting CREB function is a valuable cancer prevention strategy.

Regulation of the spermatogonial stem cell niche

Spermatogonial stem cells (SSCs) reside within specialized microenvironments called 'niches', which are essential for their maintenance and self-renewal. In the mammalian testis, the main components of the niche include the Sertoli cell, the growth factors that this nursing cell produces, the basement membrane, and stimuli from the vascular network between the seminiferous tubules. This review focuses on signalling pathways maintaining SSCs self-renewal and differentiation and describes potential mechanisms of regulation of the spermatogonial stem cell niche.

Runx2 regulates G protein-coupled signaling pathways to control growth of osteoblast progenitors

Runt-related transcription factor 2 (Runx2) controls lineage commitment, proliferation, and anabolic functions of osteoblasts as the subnuclear effector of multiple signaling axes (e.g. transforming growth factor-beta/BMP-SMAD, SRC/YES-YAP, and GROUCHO/TLE). Runx2 levels oscillate during the osteoblast cell cycle with maximal levels in G(1). Here we examined what functions and target genes of Runx2 control osteoblast growth. Forced expression of wild type Runx2 suppresses growth of Runx2(-/-) osteoprogenitors. Point mutants defective for binding to WW domain or SMAD proteins or the nuclear matrix retain this growth regulatory ability. Hence, key signaling pathways are dispensable for growth control by Runx2. However, mutants defective for DNA binding or C-terminal gene repression/activation functions do not block proliferation. Target gene analysis by Affymetrix expression profiling shows that the C terminus of Runx2 regulates genes involved in G protein-coupled receptor signaling (e.g. Rgs2, Rgs4, Rgs5, Rgs16, Gpr23, Gpr30, Gpr54, Gpr64, and Gna13). We further examined the function of two genes linked to cAMP signaling as follows: Gpr30 that is stimulated and Rgs2 that is down-regulated by Runx2. RNA interference of Gpr30 and forced expression of Rgs2 in each case inhibit osteoblast proliferation. Notwithstanding its growth-suppressive potential, our results surprisingly indicate that Runx2 may sensitize cAMP-related G protein-coupled receptor signaling by activating Gpr30 and repressing Rgs2 gene expression in osteoblasts to increase responsiveness to mitogenic signals.

Cyclic AMP response element-binding protein overexpression: a feature associated with negative prognosis in never smokers with non-small cell lung cancer

Lung cancer is the leading cause of cancer deaths worldwide. Recent advances in targeted therapies hold promise for the development of new treatments for certain subsets of cancer patients by targeting specific signaling molecule. Based on the identification of the transcription factor cyclic AMP response element-binding protein (CREB) as an important regulator of growth of several types of cancers and our recent findings of its importance in normal differentiation of bronchial epithelial cells, we hypothesized that CREB plays an important pathobiologic role in lung carcinogenesis. We conducted this initial study to determine whether the expression and activation status of CREB are altered in non-small cell lung cancer (NSCLC) and of any prognostic importance in NSCLC patients. We found that the expression levels of mRNA and protein of CREB and phosphorylated CREB (p-CREB) were significantly higher in most of the NSCLC cell lines and tumor specimens than in the normal human tracheobronchial epithelial cells and adjacent normal lung tissue, respectively. Analysis of CREB mRNA expression and the CREB gene copy number showed that CREB overexpression occurred mainly at the transcriptional level. Immunohistochemical analysis of tissue microarray slides containing sections of NSCLC specimens obtained from 310 patients showed that a decreased survival duration was significantly associated with overexpression of CREB or p-CREB in never smokers but not in current or former smokers with NSCLC. These are the first reported results illustrating the potential of CREB as a molecular target for the prevention and treatment of NSCLC, especially in never smokers.

Trichosanthin suppresses HeLa cell proliferation through inhibition of the PKC/MAPK signaling pathway

Trichosanthin (TCS) possesses a broad spectrum of biological and pharmacological activities, including anti-tumor activities. Our previous studies have shown that TCS inhibits HeLa cell proliferation by activating the apoptotic pathway. In particular, the transcriptional factor cAMP response element binding (CREB) protein plays a pivotal role in apoptotic HeLa cells. However, no information, to date, is available about the signaling pathways involved in the inhibition of cell proliferation induced by TCS. The present study showed that PKA and PKC activities were significantly inhibited by TCS treatment. However, specific inhibitor of PKA activity failed to affect the inhibition of HeLa cell proliferation induced by TCS, even in the presence of cAMP agonists. In contrast, PKC activator/inhibitor significantly attenuated/enhanced the inhibitory effect of TCS on cell proliferation. In particular, the reversed effect of cAMP agonist on cell proliferation was partly prevented by PKC, ERK1/2, and p38 MAPK blockade. Consistent with these results, the reversed effect of cAMP agonists on CREB phosphorylation was significantly decreased by inhibitors of these kinases, but not PKA inhibitor. Therefore, our results suggested that HeLa cell proliferation was inhibited by TCS via suppression of PKC/MAPK signaling pathway.

Multiple signaling pathways are responsible for prostaglandin E2-induced murine keratinocyte proliferation

Although prostaglandin E2 (PGE2) has been shown by pharmacologic and genetic studies to be important in skin cancer, the molecular mechanism(s) by which it contributes to tumor growth is not well understood. In this study, we investigated the mechanisms by which PGE2 stimulates murine keratinocyte proliferation using in vitro and in vivo models. In primary mouse keratinocyte cultures, PGE2 activated the epidermal growth factor receptor (EGFR) and its downstream signaling pathways as well as increased cyclic AMP (cAMP) production and activated the cAMP response element binding protein (CREB). EGFR activation was not significantly inhibited by pretreatment with a c-src inhibitor (PP2), nor by a protein kinase A inhibitor (H-89). However, PGE2-stimulated extracellularly regulated kinase 1/2 (ERK1/2) activation was completely blocked by EGFR, ERK1/2, and phosphatidylinositol 3-kinase (PI3K) pathway inhibitors. In addition, these inhibitors attenuated the PGE2-induced proliferation, nuclear factor-kappa B, activator protein-1 (AP-1), and CREB binding to the promoter regions of the cyclin D1 and vascular endothelial growth factor (VEGF) genes and expression of cyclin D1 and VEGF in primary mouse keratinocytes. Similarly, in vivo, we found that WT mice treated with PGE2 and untreated cyclooxygenase-2-overexpressing transgenic mice had higher levels of cell proliferation and expression of cyclin D1 and VEGF, as well as higher levels of activated EGFR, nuclear factor-kappa B, AP-1, and CREB, than vehicle-treated WT mice. Our findings provide evidence for a link between cyclooxygenase-2 overexpression and EGFR-, ERK-, PI3K-, cAMP-mediated cell proliferation, and the tumor-promoting activity of PGE2 in mouse skin.

Gdnf signaling pathways within the mammalian spermatogonial stem cell niche

Mammalian spermatogenesis is a complex process in which male germ-line stem cells develop to ultimately form spermatozoa. Spermatogonial stem cells, or SSCs, are found in the basal compartment of the seminiferous epithelium. They self-renew to maintain the pool of stem cells throughout life, or they differentiate to generate a large number of germ cells. A balance between SSC self-renewal and differentiation in the adult testis is therefore essential to maintain normal spermatogenesis and fertility. Maintenance and self-renewal are tightly regulated by extrinsic signals from the surrounding microenvironment, called the spermatogonial stem cell niche. By physically supporting the SSCs and providing them with growth factors, the Sertoli cell is the main component of the niche. In addition, adhesion molecules that connect the SSCs to the basement membrane and cellular components of the interstitium between the seminiferous tubules are important regulators of the niche function. This review mainly focuses on glial cell line-derived neurotrophic factor (Gdnf), which is produced by Sertoli cells to maintain SSCs self-renewal, and the downstream signaling pathways induced by this crucial growth factor. Interactions between Gdnf and other signaling pathways that maintain self-renewal, as well as the role of novel SSC- and Sertoli cell-specific transcription factors, are also discussed.

A mechanism for the inhibition of neural progenitor cell proliferation by cocaine

BACKGROUND

Prenatal exposure of the developing brain to cocaine causes morphological and behavioral abnormalities. Recent studies indicate that cocaine-induced proliferation inhibition and/or apoptosis in neural progenitor cells may play a pivotal role in causing these abnormalities. To understand the molecular mechanism through which cocaine inhibits cell proliferation in neural progenitors, we sought to identify the molecules that are responsible for mediating the effect of cocaine on cell cycle regulation.

METHODS AND FINDINGS

Microarray analysis followed by quantitative real-time reverse transcription PCR was used to screen cocaine-responsive and cell cycle-related genes in a neural progenitor cell line where cocaine exposure caused a robust anti-proliferative effect by interfering with the G1-to-S transition. Cyclin A2, among genes related to the G1-to-S cell cycle transition, was most strongly down-regulated by cocaine. Down-regulation of cyclin A was also found in cocaine-treated human primary neural and A2B5+ progenitor cells, as well as in rat fetal brains exposed to cocaine in utero. Reversing cyclin A down-regulation by gene transfer counteracted the proliferation inhibition caused by cocaine. Further, we found that cocaine-induced accumulation of reactive oxygen species, which involves N-oxidation of cocaine via cytochrome P450, promotes cyclin A down-regulation by causing an endoplasmic reticulum (ER) stress response, as indicated by increased phosphorylation of eIF2alpha and expression of ATF4. In the developing rat brain, the P450 inhibitor cimetidine counteracted cocaine-induced inhibition of neural progenitor cell proliferation as well as down-regulation of cyclin A.

CONCLUSIONS

Our results demonstrate that down-regulation of cyclin A underlies cocaine-induced proliferation inhibition in neural progenitors. The down-regulation of cyclin A is initiated by N-oxidative metabolism of cocaine and consequent ER stress. Inhibition of cocaine N-oxidative metabolism by P450 inhibitors may provide a preventive strategy for counteracting the adverse effects of cocaine on fetal brain development.

JunB breakdown in mid-/late G2 is required for down-regulation of cyclin A2 levels and proper mitosis

JunB, a member of the AP-1 family of dimeric transcription factors, is best known as a cell proliferation inhibitor, a senescence inducer, and a tumor suppressor, although it also has been attributed a cell division-promoting activity. Its effects on the cell cycle have been studied mostly in G1 and S phases, whereas its role in G2 and M phases still is elusive. Using cell synchronization experiments, we show that JunB levels, which are high in S phase, drop during mid- to late G2 phase due to accelerated phosphorylation-dependent degradation by the proteasome. The forced expression of an ectopic JunB protein in late G2 phase indicates that JunB decay is necessary for the subsequent reduction of cyclin A2 levels in prometaphase, the latter event being essential for proper mitosis. Consistently, abnormal JunB expression in late G2 phase entails a variety of mitotic defects. As these aberrations may cause genetic instability, our findings contrast with the acknowledged tumor suppressor activity of JunB and reveal a mechanism by which the deregulation of JunB might contribute to tumorigenesis.

Subversion of cell cycle regulatory pathways

Human cytomegalovirus (HCMV) has evolved numerous strategies to commandeer the host cell for producing viral progeny. The virus manipulates host cell cycle pathways from the early stages of infection to stimulate viral DNA replication at the expense of cellular DNA synthesis. At the same time, cell cycle checkpoints are by-passed, preventing apoptosis and allowing sufficient time for the assembly of infectious virus.

CREB is a critical regulator of normal hematopoiesis and leukemogenesis

The cAMP-responsive element binding protein (CREB) is a 43-kDa nuclear transcription factor that regulates cell growth, memory, and glucose homeostasis. We showed previously that CREB is amplified in myeloid leukemia blasts and expressed at higher levels in leukemia stem cells from patients with myeloid leukemia. CREB transgenic mice develop myeloproliferative disease after 1 year, but not leukemia, suggesting that CREB contributes to but is not sufficient for leukemogenesis. Here, we show that CREB is most highly expressed in lineage negative hematopoietic stem cells (HSCs). To understand the role of CREB in hematopoietic progenitors and leukemia cells, we examined the effects of RNA interference (RNAi) to knock down CREB expression in vitro and in vivo. Transduction of primary HSCs or myeloid leukemia cells with lentiviral CREB shRNAs resulted in decreased proliferation of stem cells, cell- cycle abnormalities, and inhibition of CREB transcription. Mice that received transplants of bone marrow transduced with CREB shRNA had decreased committed progenitors compared with control mice. Mice injected with Ba/F3 cells expressing either Bcr-Abl wild-type or T315I mutation with CREB shRNA had delayed leukemic infiltration by bioluminescence imaging and prolonged median survival. Our results suggest that CREB is critical for normal myelopoiesis and leukemia cell proliferation.

Gdnf upregulates c-Fos transcription via the Ras/Erk1/2 pathway to promote mouse spermatogonial stem cell proliferation

Glial cell line-derived neurotrophic factor (GDNF) plays a crucial role in regulating the proliferation of spermatogonial stem cells (SSC). The signaling pathways mediating the function of GDNF in SSC remain unclear. This study was designed to determine whether GDNF signals via the Ras/ERK1/2 pathway in the C18-4 cells, a mouse SSC line. The identity of this cell line was confirmed by the expression of various markers for germ cells, proliferating spermatogonia, and SSC, including GCNA1, Vasa, Dazl, PCNA, Oct-4, GFRalpha1, Ret, and Plzf. Western blot analysis revealed that GDNF activated Ret tyrosine phosphorylation. All 3 isoforms of Shc were phosphorylated upon GDNF stimulation, and GDNF induced the binding of the phosphorylated Ret to Shc and Grb2 as indicated by immunoprecipitation and Western blotting. The active Ras was induced by GDNF, which further activated ERK1/2 phosphorylation. GDNF stimulated the phosphorylation of CREB-1, ATF-1, and CREM-1, and c-fos transcription. Notably, the increase in ERK1/2 phosphorylation, c-fos transcription, bromodeoxyuridine incorporation, and metaphase counts induced by GDNF, was completely blocked by pretreatment with PD98059, a specific inhibitor for MEK1, the upstream regulator of ERK1/2. GDNF stimulation eventually upregulated cyclin A and CDK2 expression. Together, these data suggest that GDNF induces CREB/ATF-1 family member phosphorylation and c-fos transcription via the Ras/ERK1/2 pathway to promote the proliferation of SSC. Unveiling GDNF signaling cascades in SSC has important implications in providing attractive targets for male contraception as well as for the regulation of stem cell renewal vs. differentiation.

cAMP-responsive element-binding protein regulates vascular endothelial growth factor expression: implication in human prostate cancer bone metastasis

Aberrant expression of vascular endothelial growth factor (VEGF) is associated with human prostate cancer (PCa) metastasis and poor clinical outcome. We found that both phosphorylation of cyclic AMP-responsive element-binding protein (CREB) and VEGF levels were significantly elevated in patient bone metastatic PCa specimens. A PCa ARCaP progression model demonstrating epithelial-to-mesenchymal transition exhibited increased CREB phosphorylation and VEGF expression as ARCaP cells became progressively more mesenchymal and bone-metastatic. Activation of CREB induced, whereas inhibition of CREB blocked, VEGF expression in ARCaP cells. CREB may regulate VEGF transcription via a hypoxia-inducible factor-dependent mechanism in normoxic conditions. Activation of CREB signaling is involved in the coordinated regulation of VEGF and may pre-dispose to PCa bone metastasis.

Regulation of mucin gene expression by CREB via a nonclassical retinoic acid signaling pathway

Vitamin A and its metabolite retinoic acid (RA) are essential elements for normal lung development and the differentiation of lung epithelial cells. We previously showed that RA rapidly activated cyclic AMP response element-binding protein (CREB) in a nonclassical manner in normal human tracheobronchial epithelial (NHTBE) cells. In the present study, we further demonstrated that this nonclassical signaling of RA on the activation of CREB plays a critical role in regulating the expression of airway epithelial cell differentiation markers, the MUC2, MUC5AC, and MUC5B genes. We found that RA rapidly activates the protein kinase Calpha isozyme and transmits the activation signal to CREB via the Raf/MEK/extracellular signal-regulated kinase/p90 ribosomal S6 kinase (RSK) pathway. Activated RSK translocated from the cytoplasm to the nucleus, where it phosphorylates CREB. Activated CREB then binds to a cis-acting replication element motif on the promoter (at nucleotides [nt] -878 to -871) of the MUC5AC gene. The depletion of CREB using small interfering RNA abolished not only the RA-induced MUC5AC but also RA-induced MUC2 and MUC5B. Taken together, our findings demonstrate that CREB activation via this nonclassical RA signaling pathway may play an important role in regulating the expression of mucin genes and mediating the early biological effects of RA during normal mucous differentiation in NHTBE cells.

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.

Inducible cAMP early repressor suppresses gastrin-mediated activation of cyclin D1 and c-fos gene expression

The gastric hormone gastrin and its precursors promote proliferation in several gastrointestinal cell types. Here we show that gastrin induces transcription of cell cycle gene cyclin D1 and protooncogene c-fos in the neuroendocrine pancreatic cell line AR42J and that this gastrin response is inhibited by endogenous inducible cAMP early repressor (ICER). The transcriptional repressor ICER is known to downregulate both its own expression and the expression of other genes containing cAMP-responsive elements (CREs). Using siRNA, we also show that CRE promoter elements are the targets of endogenous ICER in AR42J cells as well as in the neuroendocrine cell line RIN5F. Our results suggest that ICER plays an important role in molecular mechanisms governing gastrin-mediated growth by modulating gastrin's transcriptional activation of growth-related genes. Our finding that ICER modulates pituitary adenylate cyclase-activating polypeptide-activated gene expression also indicates a regulatory effect of ICER in the responses of neuroendocrine cells to peptides other than gastrin.

Phosphorylation of the cyclic AMP response element binding protein mediates transforming growth factor beta-induced downregulation of cyclin A in vascular smooth muscle cells

Transforming growth factor beta (TGFbeta), a multifunctional cytokine associated with vascular injury, is a potent inhibitor of cell proliferation. The current results demonstrate that the TGFbeta-induced growth arrest of vascular smooth muscle cells (VSMCs) is associated with cyclin A downregulation. TGFbeta represses the cyclin A gene through a cyclic AMP (cAMP) response element, which complexes with the cAMP response element binding protein (CREB). The CREB-cyclin A promoter interaction is hindered by TGFbeta, preceded by a TGFbeta receptor-dependent CREB phosphorylation. Induction of CREB phosphorylation with forskolin or 6bnz-cAMP mimics TGFbeta's inhibitory effect on cyclin A expression. Conversely, inhibition of CREB phosphorylation with a CREB mutant in which the phosphorylation site at serine 133 was changed to alanine (CREB-S133A) upregulated cyclin A gene expression. Furthermore, the CREB-S133A mutant abolished TGFbeta-induced CREB phosphorylation, cyclin A downregulation, and growth inhibition. Since we have previously shown that the novel PKC isoform protein kinase C delta (PKCdelta) is activated by TGFbeta in VSMCs, we tested the role of this kinase in CREB phosphorylation and cyclin A downregulation. Inhibition of PKCdelta by a dominant-negative mutant or by targeted gene deletion blocked TGFbeta-induced CREB phosphorylation and cyclin A downregulation. Taken together, our data indicate that phosphorylation of CREB stimulated by TGFbeta is a critical step leading to the inhibition of cyclin A expression and, thus, VSMC proliferation.

Transcriptional Regulation of Cyclin D2 by the PKA Pathway and Inducible cAMP Early Repressor in Granulosa Cells1

Cyclin D2 (Ccnd2) is an essential gene for folliculogenesis, as null mutation in mice impairs granulosa cell proliferation in response to FSH. Ccnd2 mRNA is induced during the estrus cycle by FSH and is rapidly inhibited by LH. Yet, the responsive elements and transcription factors accounting for the gene expression of cyclin D2 in the ovary have not been fully characterized. Using primary cultures of rat granulosa cells and immortalized mouse granulosa cells, we demonstrate a mechanism for the regulation of cyclin D2 at the level of transcription via a PKA-dependent signaling mechanism. The promoter activity of cyclin D2 was shown to be induced by FSH and the catalytic alpha subunit of PKA (PRKACA), and this activity was repressible by inducible cAMP early repressor (ICER), a cAMP response element (CRE) modulator isoform. In silico analysis of the mouse, rat, and human cyclin D2 promoters identified two CRE-binding protein sites, a conserved proximal element and a less conserved distal element relative to the translation start site. The mutation on the proximal element drastically decreases the effects of PRKACA and ICER on the promoter activity, whereas the mutation on the distal element did not contribute to the decrease in the promoter activity. Electrophoretic mobility shift assays and deoxyribonuclease footprint analysis confirmed ICER binding to the proximal element, and chromatin immunoprecipitation analysis demonstrated the occurrence of this binding in vivo. These results showed a CRE within the upstream region of Ccnd2 that is (at least partly) implicated in the stimulation and repression of cyclin D2 transcription. Finally, our data suggest that ICER involvement in the regulation of granulosa cell proliferation as overexpression of ICER results in the inhibition of PRKACA-induced DNA synthesis.

Overexpression of the prostaglandin E2 receptor EP2 results in enhanced skin tumor development

We previously showed that the EP2 knockout mice were resistant to chemically induced skin carcinogenesis. The purpose of this study was to investigate the role of the overexpression of the EP2 receptor in mouse skin carcinogenesis. To determine the effect of overexpression of EP2, we used EP2 transgenic (TG) mice and wild-type (WT) mice in a DMBA (7,12-dimethylbenz[alpha]anthracene)/TPA (12-O-tetradecanoylphorbol-13-acetate) two-stage carcinogenesis protocol. EP2 TG mice developed significantly more tumors compared with WT mice. Overexpression of the EP2 receptor increased TPA-induced keratinocyte proliferation both in vivo and in vitro. In addition, the epidermis of EP2 TG mice 48 h after topical TPA treatment was significantly thicker compared to that of WT mice. EP2 TG mice showed significantly increased cyclic adenosine monophosphate levels in the epidermis after prostaglandin E2 (PGE2) treatment. The inflammatory response to TPA was increased in EP2 TG mice, as demonstrated by an increased number of macrophages in the dermis. Tumors and 7 x TPA-treated and DMBA-TPA-treated (6 weeks) skins from EP2 TG mice produced more blood vessels than those of WT mice as determined by CD-31 immunostaining. Vascular endothelial growth factor (VEGF) protein expression was significantly increased in squamous cell carcinoma (SCC) samples from EP2 TG mice compared that of WT mice. There was, however, no difference in the number of apoptotic cells in tumors from WT and EP2 TG mice. Together, our results suggest that the overexpression of the EP2 receptor plays a significant role in the protumorigenic action of PGE2 in mouse skin.

Mitosin/CENP-F in mitosis, transcriptional control, and differentiation

Mitosin/CENP-F is a large nuclear/kinetochore protein containing multiple leucine zipper motifs potentially for protein interactions. Its expression levels and subcellular localization patterns are regulated in a cell cycle-dependent manner. Recently, accumulating lines of evidence have suggested it a multifunctional protein involved in mitotic control, microtubule dynamics, transcriptional regulation, and muscle cell differentiation. Consistently, it is shown to interact directly with a variety of proteins including CENP-E, NudE/Nudel, ATF4, and Rb. Here we review the current progress and discuss possible mechanisms through which mitosin may function.

Cyclic AMP regulates the expression and nuclear translocation of RFC40 in MCF7 cells

We have previously shown that the regulatory subunit of PKA, RIalpha, functions as a nuclear transport protein for the second subunit of the replication factor C complex, RFC40, and that this transport appears to be crucial for cell cycle progression from G1 to S phase. In this study, we found that N(6)-monobutyryl cAMP significantly up-regulates the expression of RFC40 mRNA by 1.8-fold and its endogenous protein by 2.3-fold with a subsequent increase in the RIalpha-RFC40 complex formation by 3.2-fold. Additionally, the nuclear to cytoplasmic ratio of RFC40 increased by 26% followed by a parallel increase in the percentage of S phase cells by 33%. However, there was reduction in the percentage of G1 cells by 16% and G2/M cells by 43% with a concurrent accumulation of cells in S phase. Interestingly, the higher percentage of S phase cells did not correlate with a parallel increase in DNA replication. Moreover, although cAMP did not affect the expression of the other RFC subunits, there was a significant decrease in the RFC40-37 complex formation by 81.3%, substantiating the decrease in DNA replication rate. Taken together, these findings suggest that cAMP functions as an upstream modulator that regulates the expression and nuclear translocation of RFC40.

The proliferative status of thyrotropes is dependent on modulation of specific cell cycle regulators by thyroid hormone

In this report we have examined changes in cell growth parameters, cell cycle effectors, and signaling pathways that accompany thyrotrope growth arrest by thyroid hormone (TH) and growth resumption after its withdrawal. Flow cytometry and immunohistochemistry of proliferation markers demonstrated that TH treatment of thyrotrope tumors resulted in a reduction in the fraction of cells in S-phase that is restored upon TH withdrawal. This is accompanied by dephosphorylation and rephosphorylation of retinoblastoma (Rb) protein. The expression levels of cyclin-dependent kinase 2 and cyclin A, as well as cyclin-dependent kinase 1 and cyclin B, were decreased by TH, and after withdrawal not only did these regulators of Rb phosphorylation and mitosis increase in their expression but so too did the D1 and D3 cyclins. We also noted a rapid induction and subsequent disappearance of the type 5 receptor for the growth inhibitor somatostatin with TH treatment and withdrawal, respectively. Because somatostatin can arrest growth by activating MAPK pathways, we examined these pathways in TtT-97 tumors and found that the ERK pathway and several of its upstream and downstream effectors, including cAMP response element binding protein, were activated with TH treatment and deactivated after its withdrawal. This led to the hypothesis that TH, acting through increased type 5 somatostatin receptor, could activate the ERK pathway leading to cAMP response element binding protein-dependent decreased expression of critical cell cycle proteins, specifically cyclin A, resulting in hypophosphorylation of Rb and its subsequent arrest of S-phase progression. These processes are reversed when TH is withdrawn, resulting in an increase in the fraction of S-phase cells.

Lack of expression of the EP2 but not EP3 receptor for prostaglandin E2 results in suppression of skin tumor development

The EP2 receptor for prostaglandin E2 (PGE2) is a membrane receptor that mediates at least part of the action of PGE2. It has been shown that EP2 plays a critical role in tumorigenesis in mouse mammary gland and colon. However, the possibility that the EP2 receptor is involved in the development of skin tumors was unknown. The purpose of this study was to investigate the role of the EP2 receptor in mouse skin carcinogenesis. Unlike EP3 knockout mice, the EP2 knockout mice produced significantly fewer tumors and reduced tumor incidence compared with wild type (WT) mice in a 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) two-stage carcinogenesis protocol. EP2 knockout mice had significantly reduced cellular proliferation of mouse skin keratinocytes in vivo and in vitro compared with that in WT mice. In addition, the epidermis of EP2 knockout mice 48 hours after topical TPA treatment was significantly thinner compared with that of WT mice. The inflammatory response to TPA was reduced in EP2 knockout mice, based on a reduced number of macrophages in the dermis and a reduced level of interleukin-1alpha mRNA expression, compared with WT mice. EP2 knockout mice also had significantly reduced epidermal cyclic AMP levels after PGE2 treatment compared with WT mice. Tumors from WT mice produced more blood vessels and fewer apoptotic cells than those of EP2 knockout mice as determined by immunohistochemical staining. Our data suggest that the EP2 receptor plays a significant role in the protumorigenic action of PGE2 in skin tumor development.

Transactivation of the cyclin A promoter by bovine papillomavirus type 4 E5 protein

Bovine papillomavirus type 4 (BPV-4) E5 (formerly E8) is a 42-residue hydrophobic, membrane-localised protein that can transform NIH-3T3 cells by a poorly defined mechanism. In E5-expressing cells, the observed up-regulation of cyclin A is underpinned by transactivation of the cyclin A promoter. Here we show that E5 transactivates the minimal cell cycle-regulated cyclin A promoter in cells both stably and acutely expressing the viral protein. There are no detectable differences between control and E5 cells in protein complexes binding the E2F-like cell cycle-dependent element (CDE)/cell cycle-regulated element (CCRE) of the cyclin A promoter and E5 does not transactivate E2F reporter plasmids in an E2F-dependent manner in vivo. CCAAT box integrity and functional NF-Y complexes are required for E5-mediated transactivation and a Mr approximately 110 K CCAAT-box binding factor (p110 CBF) associates with NF-YA only in E5 cells. This suggests that E5 sets the extent of cyclin A promoter activation by a mechanism similar to other, structurally unrelated, DNA tumour virus oncoproteins but distinct from the action of serum factors and so is inconsistent with E5 acting through constitutive activation of tyrosine kinase growth factor receptors.

Transcriptional regulation of cyclin A2 by RASSF1A through the enhanced binding of p120E4F to the cyclin A2 promoter

Recent advances in the study of RASSF1A, the candidate tumor suppressor gene, indicate a possible role of RASSF1A in cell cycle regulation; however, very little is known regarding molecular mechanisms underlying this control. Using small interfering RNA to knockdown endogenous RASSF1A in the breast tumor cell line HB2 and in the cervical cancer cell line HeLa, we identify that a key player in cell cycle progression, cyclin A2, is concomitantly increased at both protein and mRNA levels. In A549 clones stably expressing RASSF1A, cyclin A2 levels were diminished compared with vector control. A known transcriptional regulator of cyclin A2, p120(E4F) (a repressor of cyclin A2), has been shown previously by our group to interact with RASSF1A. We show that levels of p120(E4F) are not affected by RASSF1A small interfering RNA in HB2 and HeLa cells. However, electrophoretic mobility shift assays indicate that knockdown of endogenous RASSF1A in HB2 and HeLa cells leads to a reduction in the binding capacity of p120(E4F) to the cyclin A2 promoter, whereas in the A549 clone stably expressing RASSF1A the binding capacity is increased. These data are further corroborated in vitro by the luciferase assay and in vivo by chromatin immunoprecipitation experiments. Together, these data identify the cyclin A2 gene as a cellular target for RASSF1A through p120(E4F) and for the first time suggest a transcriptional mechanism for RASSF1A-dependent cell cycle regulation.

The role of CREB as a proto-oncogene in hematopoiesis and in acute myeloid leukemia

CREB is a transcription factor that functions in glucose homeostasis, growth factor-dependent cell survival, and memory. In this study, we describe a role of CREB in human cancer. CREB overexpression is associated with increased risk of relapse and decreased event-free survival. CREB levels are elevated in blast cells from patients with acute myeloid leukemia. To understand the role of CREB in leukemogenesis, we studied the biological consequences of CREB overexpression in primary human leukemia cells, leukemia cell lines, and transgenic mice. Our results demonstrate that CREB promotes abnormal proliferation and survival of myeloid cells in vitro and in vivo through upregulation of specific target genes. Thus, we report that CREB is implicated in myeloid cell transformation.

Induced ICER Iγ down-regulates cyclin A expression and cell proliferation in insulin-producing β cells

We have previously found that cyclin A expression is markedly reduced in pancreatic beta-cells by cell-specific overexpression of repressor inducible cyclic AMP early repressor (ICER Igamma) in transgenic mice. Here we further examined regulatory effects of ICER Igamma on cyclin A gene expression using Min6 cells, an insulin-producing cell line. The cyclin A promoter luciferase assay showed that ICER Igamma directly repressed cyclin A gene transcription. In addition, upon ICER Igamma overexpression, cyclin A mRNA levels markedly decreased, thereby confirming an inhibitory effect of ICER Igamma on cyclin A expression. Suppression of cyclin A results in inhibition of BrdU incorporation. Under normal culture conditions endogenous cyclin A is abundant in these cells, whereas ICER is hardly detectable. However, serum starvation of Min6 cells induces ICER Igamma expression with a concomitant very low expression level of cyclin A. Cyclin A protein is not expressed unless the cells are in active DNA replication. These results indicate a potentially important anti-proliferative effect of ICER Igamma in pancreatic beta cells. Since ICER Igamma is greatly increased in diabetes as well as in FFA- or high glucose-treated islets, this effect may in part exacerbate diabetes by limiting beta-cell proliferation.

Cyclin A is a c-Jun target gene and is necessary for c-Jun-induced anchorage-independent growth in RAT1a cells

Overexpression of c-Jun enables Rat1a cells to grow in an anchorage-independent manner. We used an inducible c-Jun system under the regulation of doxycycline in Rat1a cells to identify potential c-Jun target genes necessary for c-Jun-induced anchorage-independent growth. Induction of c-Jun results in sustained expression of cyclin A in the nonadherent state with only minimal expression in the absence of c-Jun. The promoter activity of cyclin A2 was 4-fold higher in Rat1a cells in which c-Jun expression was induced compared with the control cells. Chromatin immunoprecipitation demonstrated that c-Jun bound directly to the cyclin A2 promoter. Mutation analysis of the cyclin A2 promoter mapped the c-Jun regulatory site to an ATF site at position -80. c-Jun was able to bind to this site both in vitro and in vivo, and mutation of this site completely abolished promoter activity. Cyclin A1 was also elevated in c-Jun-overexpressing Rat1a cells; however, c-Jun did not regulate this gene directly, since it did not bind directly to the cyclin A1 promoter. Suppression of cyclin A expression via the introduction of a cyclin A antisense sequences significantly reduced the ability of c-Jun-overexpressing Rat1a cells to grow in an anchorage-independent fashion. Taken together, these results suggest that cyclin A is a target of c-Jun and is necessary but not sufficient for c-Jun-induced anchorage-independent growth. In addition, we demonstrated that the cytoplasmic oncogenes Ras and Src transcriptionally activated the cyclin A2 promoter via the ATF site at position -80. Using a dominant negative c-Jun mutant, TAM67, we showed that this transcriptional activation of cyclin A2 requires c-Jun. Thus, our results suggest that c-Jun is a mediator of the aberrant cyclin A2 expression associated with Ras/Src-induced transformation.

Mitosin/CENP-F as a negative regulator of activating transcription factor-4

Mitosin/CENP-F is a human nuclear matrix protein with multiple leucine zipper motifs. Its accumulation in S-G2 phases and transient kinetochore localization in mitosis suggest a multifunctional protein for cell proliferation. Moreover, its murine and avian orthologs are implicated in myocyte differentiation. Here we report its interaction with activating transcription factor-4 (ATF4), a ubiquitous basic leucine zipper transcription factor important for proliferation, differentiation, and stress response. The C-terminal portion of mitosin between residues 2488 and 3113 bound to ATF4 through two distinct domains, one of which was a leucine zipper motif. Mitosin mutants containing these domains were able to either supershift or disrupt the ATF4-DNA complex. On the other hand, ATF4, but not ATF1-3 or ATF6, interacted with mitosin through a region containing the basic leucine zipper motif. Moreover, overexpression of full-length mitosin repressed the transactivation activity of ATF4 in dual luciferase-based reporter assays, while knocking down mitosin expression manifested the opposite effects. These findings suggest mitosin to be a negative regulator of ATF4 in interphase through direct interaction.

Genetic control of spermiogenesis: insights from the CREM gene and implications for human infertility

Male germ cell differentiation requires a highly cell-specific gene expression programme that is achieved by unique chromatin remodelling, transcriptional control, and the expression of testis-specific genes or isoforms. The regulatory processes governing gene expression in spermatogenesis have fundamentally unique requirements, including meiosis, ongoing cellular differentiation and a peculiar chromatin organization. The signalling cascades and the downstream effectors contributing to the programme of spermatogenesis are currently being unravelled, revealing the unique features of germ cell regulatory circuits. This paper reports on the unique role that CREM exerts as a master regulator. Targeted inactivation of the genes encoding CREM and ACT has been achieved. ACT selectively associates with KIF17b, a kinesin motor protein highly expressed in germ cells. It has been found that KIF17b directly determines the intracellular localization of ACT. Thus, the activity of a transcriptional co-activator is intimately coupled to the function of a kinesin via tight regulation of its intracellular localization. The conservation of these elements and of their regulatory functions in human spermatogenesis indicates that they are likely to provide important insights into understanding the molecular mechanisms of human infertility.

Distribution of transcription factor inducible cyclicAMP early repressor (ICER) in rodent brain and pituitary

In morphogenetic dynamics of neurons, and in adaptive physiology of brain function, transcription factors of the cyclicAMP signaling pathway, such as activator cyclicAMP responsive element binding protein (CREB) and inhibitor inducible cyclicAMP early repressor (ICER), play an important role. In particular, the presence of the transcription factor ICER in neurons or neuroendocrine cells suggests the need for the gating of an up-regulated gene expression. Little is known, however, about the natural distribution of the inhibitory transcription factor ICER. We, therefore, mapped the rodent brain and pituitary for an ICER immunoreaction and found a nuclear staining for this transcription factor. ICER-positive glial cells were found throughout the brain. ICER-positive neurons were found in sensory input centers, like the olfactory bulb, or sensory brain stem nuclei, and in hypothalamic nuclei involved in central neuroendocrine control. In addition, neuroendocrine/endocrine transducers, like the pituitary and the pineal gland showed a high basal presence of ICER. Our data show that a basic ICER level is required by many cell systems and can be seen as an anticipatory and/or a protective measure in systems with superior reactive dynamics.

HIP/PAP, a C-type lectin overexpressed in hepatocellular carcinoma, binds the RII alpha regulatory subunit of cAMP-dependent protein kinase and alters the cAMP-dependent protein kinase signalling

HIP/PAP is a C-type lectin overexpressed in hepatocellular carcinoma (HCC). Pleiotropic biological activities have been ascribed to this protein, but little is known about the function of HIP/PAP in the liver. In this study, therefore, we searched for proteins interacting with HIP/PAP by screening a HCC cDNA expression library. We have identified the RII alpha regulatory subunit of cAMP-dependent protein kinase (PKA) as a partner of HIP/PAP. HIP/PAP and RII alpha were coimmunoprecipitated in HIP/PAP expressing cells. The biological relevance of the interaction between these proteins was established by demonstrating, using fractionation methods, that they are located in a same subcellular compartment. Indeed, though HIP/PAP is a protein secreted via the Golgi apparatus we showed that a fraction of HIP/PAP escaped the secretory apparatus and was recovered in the cytosol. Basal PKA activity was increased in HIP/PAP expressing cells, suggesting that HIP/PAP may alter PKA signalling. Indeed, we showed, using a thymidine kinase-luciferase reporter plasmid in which a cAMP responsive element was inserted upstream of the thymidine kinase promoter, that luciferase activity was enhanced in HIP/PAP expressing cells. Thus our findings suggest a novel mechanism for the biological activity of the HIP/PAP lectin.

Disulfiram inhibits activating transcription factor/cyclic AMP-responsive element binding protein and human melanoma growth in a metal-dependent manner in vitro, in mice and in a patient with metastatic disease

The thiocarbamate alcoholism drug disulfiram blocks the P-glycoprotein extrusion pump, inhibits the transcription factor nuclear factor-κB, sensitizes tumors to chemotherapy, reduces angiogenesis, and inhibits tumor growth in mice. Thiocarbamates react with critical thiols and also complex metal ions. Using melanoma as the paradigm, we tested whether disulfiram might inhibit growth by forming mixed disulfides with critical thiols in a mechanism facilitated by metal ions. Disulfiram given to melanoma cells in combination with Cu2+ or Zn2+ decreased expression of cyclin A and reduced proliferation in vitro at lower concentrations than disulfiram alone. In electrophoretic mobility shift assays, disulfiram decreased transcription factor binding to the cyclic AMP-responsive element in a manner potentiated by Cu2+ ions and by the presence of glutathione, suggesting that thiocarbamates might disrupt transcription factor binding by inducing S-glutathionylation of the transcription factor DNA binding region. Disulfiram inhibited growth and angiogenesis in melanomas transplanted in severe combined immunodeficient mice, and these effects were potentiated by Zn2+ supplementation. The combination of oral zinc gluconate and disulfiram at currently approved doses for alcoholism also induced >50% reduction in hepatic metastases and produced clinical remission in a patient with stage IV metastatic ocular melanoma, who has continued on oral zinc gluconate and disulfiram therapy for 53 continuous months with negligible side effects. These findings present a novel strategy for treating metastatic melanoma by employing an old drug toward a new therapeutic use.

Neurotrophin-3 and a CREB-mediated signaling pathway regulate Bcl-2 expression in oligodendrocyte progenitor cells

Our previous results suggested that the transcription factor CREB mediates the actions of neuroligands and growth factor signals that coupled to different signaling pathways may play different roles along oligodendrocyte (OLG) development. We showed before that CREB phosphorylation in OLG progenitors is up-regulated by neurotrophin-3 (NT-3); and moreover CREB is required for NT-3 to stimulate the proliferation of these cells. We now show that treatment of OLG progenitors with NT-3 is also accompanied by an increase in the levels of the anti-apoptotic protein Bcl-2. Interestingly, the presence of a putative CREB binding site (CRE) in the Bcl-2 gene raised the possibility that CREB could also be involved in regulating Bcl-2 expression in the OLGs. Supporting this hypothesis, the NT-3 dependent increase in Bcl-2 levels is abolished by inhibition of CREB expression. In addition, transient transfection experiments using various regions of the Bcl-2 promoter and mutation of the CRE site indicate a direct role of CREB in regulating Bcl-2 gene activity in response to NT-3. Furthermore, protein-DNA binding assays show that the CREB protein from freshly isolated OLGs indeed binds to the Bcl-2 promoter CRE. Together with our previous results, these observations suggest that CREB may play an important role in linking proliferation and survival pathways in the OLG progenitors.

Overexpression of inducible cyclic AMP early repressor inhibits transactivation of genes and cell proliferation in pancreatic beta cells

Transcriptional control mediated by the cyclic AMP-responsive element (CRE) represents an important mechanism of gene regulation. To test our hypothesis that increased inducible cyclic AMP early repressor (ICER) Igamma inhibits function of CRE-binding proteins and thus disrupts CRE-mediated transcription in pancreatic beta cells, we generated transgenic mice with beta-cell-directed expression of ICER Igamma, a powerful repressor that is greatly increased in diabetes. Three transgenic lines clearly show that increased ICER Igamma expression in beta cells results in early severe diabetes. From birth islets were severely disorganized with a significantly increased proportion of alpha cells throughout the islet. Diabetes results from the combined effects of impaired insulin expression and a decreased number of beta cells. The decrease in beta cells appears to result from impaired proliferation rather than from increased apoptosis after birth. Cyclin A gene expression is impaired by the strong inhibition of ICER; the suppression of cyclin A results in a substantially decreased proliferation of beta cells in the postnatal period. These results suggest that CRE and CRE-binding factors have an important role in pancreatic beta-cell physiology not only directly by regulation of gene trans-activation but also indirectly by regulation of beta-cell mass.

Role of E2F and ERK1/2 in STI571-mediated smooth muscle cell growth arrest and cyclin A transcriptional repression

Platelet-derived growth factor (PDGF) ligand and receptors (PDGF-R) activate smooth muscle cell (SMC) proliferation, a key event during vascular obstructive disease. The PDGF-R tyrosine kinase inhibitor STI571 attenuates SMC proliferation and experimental neointimal thickening. Here, we investigated the molecular mechanisms underlying STI571-dependent SMC growth arrest. STI571 abrogates PDGF-BB-dependent cyclin D1 and cyclin A protein expression and inhibits transcriptional activation of reporter genes driven by the human cyclin A gene promoter. Repression of cyclin A promoter activity by STI571 requires a functional E2F-binding site, and forced expression of E2F overrides this inhibitory effect. Moreover, STI571 inhibits E2F DNA-binding activity in SMCs. We also found that STI571 abrogates PDGF-BB-dependent activation of extracellular-regulated kinase 1 and 2 (ERK1/2), and forced activation of these factors impaired STI571-dependent inhibition of both cyclin A promoter activity and SMC proliferation. Thus, E2F and ERK1/2 play an important role in STI571-mediated SMC growth arrest and cyclin A transcriptional repression. These findings may have importance in the development of novel therapeutic strategies for the treatment of neointimal hyperplasia.

A pivotal role of cyclic AMP-responsive element binding protein in tumor progression

Tumor microenvironment controls the selection of malignant cells capable of surviving in stressful and hypoxic conditions. The transcription factor, cyclic AMP-responsive element binding (CREB) protein, activated by multiple extracellular signals, modulates cellular response by regulating the expression of a multitude of genes. Previously, we have demonstrated that two cystein residues, at the DNA binding domain of CREB, mediate activation of CREB-dependent gene expression at normoxia and hypoxia. The construction of a dominant-positive CREB mutant, insensitive to hypoxia cue (substitution of two cystein residues at position 300 and 310 with serine in the DNA binding domain) and of a dominant negative CREB mutant (addition of a mutation in serine(133)), enabled a direct assessment, in vitro and in vivo, of the role of CREB in tumor progression. In this work, we demonstrate both in vitro and in vivo that CREB controls hepatocellular carcinoma growth, supports angiogenesis, and renders resistance to apoptosis. Along with the identification, by DNA microarray, of the CREB-regulated genes in normoxia and hypoxia, this work demonstrates for the first time that in parallel to other hypoxia responsive mechanisms, CREB plays an important role in hepatocellular carcinoma tumor progression.

Antimitogenic effects of HDL and APOE mediated by Cox-2-dependent IP activation

HDL and its associated apo, APOE, inhibit S-phase entry of murine aortic smooth muscle cells. We report here that the antimitogenic effect of APOE maps to the N-terminal receptor-binding domain, that APOE and its N-terminal domain inhibit activation of the cyclin A promoter, and that these effects involve both pocket protein-dependent and independent pathways. These antimitogenic effects closely resemble those seen in response to activation of the prostacyclin receptor IP. Indeed, we found that HDL and APOE suppress aortic smooth muscle cell cycle progression by stimulating Cox-2 expression, leading to prostacyclin synthesis and an IP-dependent inhibition of the cyclin A gene. Similar results were detected in human aortic smooth muscle cells and in vivo using mice overexpressing APOE. Our results identify the Cox-2 gene as a target of APOE signaling, link HDL and APOE to IP action, and describe a potential new basis for the cardioprotective effect of HDL and APOE.

E- and A-type cyclins as markers for cancer diagnosis and prognosis

Cyclin-dependent kinase (CDK)2 interacting cyclins perform essential functions for DNA replication and cellular proliferation. The human genome encodes two E-type cyclins (E and E2) and two A-type cyclins (A1 and A2). Dysregulation of the CDK2-bound cyclins plays an important role in the pathogenesis of cancer. Cyclin A2 is associated with cellular proliferation and can be used for molecular diagnostics as a proliferation marker. In addition, cyclin A2 expression is associated with a poor prognosis in several types of cancer. Cyclin A1 is a tissue-specific cyclin that is highly expressed in acute myeloid leukemia and in testicular cancer. High levels of cyclin E expression are found in many types of cancer. Overexpression of cyclin E at the mRNA level can be based on gene amplification and transcriptional mechanisms. In addition, proteolytically cleaved forms of cyclin E that show oncogenic functions have been described. Cyclin E plays a critical role for G1/S transition. Its overexpression is not only associated with proliferation but rather indicates a more malignant phenotype which is likely to be linked to the induction of chromosomal instability. These biological functions of cyclin E relate to a poor prognosis when high cyclin E levels are found. The link between cyclin E and poor prognosis is well established in breast and lung cancer but is likely to be observed in other cancers as well. The second E-type cyclin, cyclin E2, has been shown to be overexpressed in breast cancers although the potential role as a diagnostic or prognostic marker is unknown. This review provides an overview of the potential of cyclins E and A as markers for diagnosis and prognosis in human cancer.

Transcriptional activation of the cyclin A gene by the architectural transcription factor HMGA2

The HMGA2 protein belongs to the HMGA family of architectural transcription factors, which play an important role in chromatin organization. HMGA proteins are overexpressed in several experimental and human tumors and have been implicated in the process of neoplastic transformation. Hmga2 knockout results in the pygmy phenotype in mice and in a decreased growth rate of embryonic fibroblasts, thus indicating a role for HMGA2 in cell proliferation. Here we show that HMGA2 associates with the E1A-regulated transcriptional repressor p120(E4F), interfering with p120(E4F) binding to the cyclin A promoter. Ectopic expression of HMGA2 results in the activation of the cyclin A promoter and induction of the endogenous cyclin A gene. In addition, chromatin immunoprecipitation experiments show that HMGA2 associates with the cyclin A promoter only when the gene is transcriptionally activated. These data identify the cyclin A gene as a cellular target for HMGA2 and, for the first time, suggest a mechanism for HMGA2-dependent cell cycle regulation.