Estrogen induction of the cyclin D1 promoter: involvement of a cAMP response-like element

Estrogens and progesterone promote persistent CCND1 gene activation during G1 by inducing transcriptional derepression via c-Jun/c-Fos/estrogen receptor (progesterone receptor) complex assembly to a distal regulatory element and recruitment of cyclin D1 to its own gene promoter

Transcriptional activation of the cyclin D1 gene (CCND1) plays a pivotal role in G(1)-phase progression, which is thereby controlled by multiple regulatory factors, including nuclear receptors (NRs). Appropriate CCND1 gene activity is essential for normal development and physiology of the mammary gland, where it is regulated by ovarian steroids through a mechanism(s) that is not fully elucidated. We report here that CCND1 promoter activation by estrogens in human breast cancer cells is mediated by recruitment of a c-Jun/c-Fos/estrogen receptor alpha complex to the tetradecanoyl phorbol acetate-responsive element of the gene, together with Oct-1 to a site immediately adjacent. This process coincides with the release from the same DNA region of a transcriptional repressor complex including Yin-Yang 1 (YY1) and histone deacetylase 1 and is sufficient to induce the assembly of the basal transcription machinery on the promoter and to lead to initial cyclin D1 accumulation in the cell. Later on in estrogen stimulation, the cyclin D1/Cdk4 holoenzyme associates with the CCND1 promoter, where E2F and pRb can also be found, contributing to the long-lasting gene enhancement required to drive G(1)-phase completion. Interestingly, progesterone triggers similar regulatory events through its own NRs, suggesting that the gene regulation cascade described here represents a crossroad for the transcriptional control of G(1)-phase progression by different classes of NRs.

Cell type-specific bidirectional regulation of the glucocorticoid-induced leucine zipper (GILZ) gene by estrogen

Estrogen has numerous beneficial physiological actions; however, by acting as a mitogen, it plays a significant role in the induction and maintenance of breast cancer. Although the positive effects of estrogen on gene expression are well described, negative gene regulation is not. Using microarray analysis, we identified 27 genes that were up-regulated and 20 that were down-regulated by estrogen in MCF-7 human breast cancer cells. One gene encoding GILZ (glucocorticoid-induced leucine zipper protein), a putative apoptosis-regulating transcription factor, is rapidly down-regulated by estrogen in these cells. Estrogen antagonists block the down-regulation. The region of the GILZ promoter between nucleotides -104 and -69 mediates both basal activity and estrogen-dependent down-regulation in MCF-7 cells. This region contains a functional Oct-1 binding site and a cyclic AMP response element binding protein (CREB) binding site. The same DNA region mediates up-regulation by estrogen in HeLa and HEK293 cells, indicating that cell-specific factors are involved in estrogen regulation of this gene. The estrogen receptor (ER) is present in GILZ promoter protein complexes, but it does not bind directly to the promoter itself, as the DNA-binding domain of the estrogen receptor is not required for down-regulation. Elimination of the CREB binding site blocks both basal activity and estrogen regulation. Our results suggest that ER action at the CRE may mediate estrogen-dependent, cell-specific regulation of this gene.

Ubiquitin and breast cancer

The regulation of protein stability by the ubiquitin-proteasome pathway is a critical issue central to the comprehension of the molecular basis of carcinogenesis. However, ubiquitin modification of target substrates signals many cellular processes other than proteolysis that are also important for the development of cancer. It is noteworthy that many proteins studied by clinical breast cancer researchers are involved in these ubiquitin pathways. This review summarizes recent works on such proteins including cyclins, CDK inhibitors, and the SCF in cell cycle control; the breast and ovarian cancer suppressor BRCA1-BARD1; ErbB2/HER2/Neu and its ubiquitin ligase c-Cbl or CHIP; and the estrogen receptor and its downstream target Efp. Understanding these pathways may provide some hints toward developing diagnostic tools and treatments for breast cancer patients.

STAT5 and Oct-1 form a stable complex that modulates cyclin D1 expression

Signal transducer and activator of transcription 5 (STAT5) is activated by numerous cytokines that control blood cell development. STAT5 was also shown to actively participate in leukemogenesis. Among the target genes involved in cell growth, STAT5 had been shown to activate cyclin D1 gene expression. We now show that thrombopoietin-dependent activation of the cyclin D1 promoter depends on the integrity of a new bipartite proximal element that specifically binds STAT5A and -B transcription factors. We demonstrate that the stable recruitment of STAT5 to this element in vitro requires the integrity of an adjacent octamer element that constitutively binds the ubiquitous POU homeodomain protein Oct-1. We observe that cytokine-activated STAT5 and Oct-1 form a unique complex with the cyclin D1 promoter sequence. We find that STAT5 interacts with Oct-1 in vivo, following activation by different cytokines in various cellular contexts. This interaction involves a small motif in the carboxy-terminal region of STAT5 which, remarkably, is similar to an Oct-1 POU-interacting motif present in two well-known partners of Oct-1, namely, OBF-1/Bob and SNAP190. Our data offer new insights into the transcriptional regulation of the key cell cycle regulator cyclin D1 and emphasize the active roles of both STAT5 and Oct-1 in this process.

Cell cycle genes in a mouse mammary hyperplasia model

Human mammary epithelial cells emerge spontaneously from senescence, exhibiting eroding telomeric sequences, and ultimately enter crisis to generate the type of chromosomal abnormalities seen in early stages of breast cancer. In a mouse mammary tumor model, the spontaneous escape of senescence can be observed as an increase in DNA synthesis that is reflected by alterations in the cell cycle profile and increases in the expression levels and activities of cell cycle molecular components. This review provides an overview of gene alterations in the cell cycle components in mouse mammary hyperplasia.

Cdc25B as a steroid receptor coactivator

The traditional role of the Cdc25 family of dual-specificity phosphatases is to activate cyclin-dependent kinases (CDKs) to enable progression through the cell cycle. This chapter reports that in addition to its cell cycle role, Cdc25B functions as a novel steroid receptor coactivator (SRC). When overexpressed in transgenic mammary glands, Cdc25B can up-regulate the expression of two estrogen receptor (ER)-target genes: cyclin D1 and Lactoferrin. In addition, when coexpressed with ER, Cdc25B can coactivate an ER-dependent reporter in the presence of estradiol. The coactivation of Cdc25B can be extended to the glucocorticoid receptor (GR), progesterone receptor (PR), and androgen receptor (AR). Because of the respective importance of ER and AR in breast and prostate cancer, this chapter focuses on the coactivation of both receptors by Cdc25B. We demonstrate that Cdc25B can interact directly with these nuclear receptors, recruit and enhance the activity of histone acetyltransferases (HATs), and potentiate cell-free transcription independent of its cell cycle regulatory function. Furthermore, because Cdc25B is up-regulated in highgrade and poorly differentiated prostate tumors, which are likely transiting from the hormone-dependent to hormone-independent state, we hypothesize that the coactivation of AR by Cdc25B may induce genes responsible for this progression. Taken together, it is highly conceivable that Cdc25B can promote neoplasia by its two disparate functions of (1) coactivation to induce higher levels of expression of steroid receptor target genes and (2) its role of activating CDKs to deregulate progression of the cell cycle, DNA replication, and mitosis.

Transcription profiling of estrogen target genes in young and old mouse uterus

The goal of this study was to identify age-related changes in the expression of estrogen target genes in mouse uterus. We developed a novel 'estrogen response element (ERE) Chip' microarray bearing 297 genes including both known estrogen target genes and genes identified by searching the mouse genome database to have EREs, AP-1 sites, and Sp1 sites, all targets of estrogen receptor (ER) regulation. 400-500 bp PCR products of these 297 genes were printed onto nylon membranes creating the 'ERE Chip' microarray. This microarray is unique because it is the first estrogen-responsive gene-specific microarray to identify changes in uterine gene expression in young versus old mice. Using this ERE microarray we identified 10 uterine genes whose expression was up-regulated in old mice, e.g. beta-actin, calcium binding protein 45a, Sp1, and COUP-TFII. In contrast, the expression of only 4 uterine genes, i.e., complement C3, lactoferrin, Muc-1, and 17-beta-hydroxysteroid dehydrogenase 8 (H2-Ke6) was down-regulated in old mice. These changes may reflect an increase in stromal and a decrease in glandular epithelial gene expression, and may be associated with age-related changes in these tissue compartments within the uterus, possibly leading to the decline in reproductive function in C57Bl/6 mice.

Beta-catenin/Tcf-1-mediated transactivation of cyclin D1 promoter is negatively regulated by thyroid hormone

Cyclin D1 is an oncogenic cyclin frequently over-expressed in cancer. To examine the effect of thyroid hormone (T3) and its receptor (TR) on the transcription of cyclin D1 gene, we co-transfected the chloramphenicol acetyl-transferase (CAT) reporter plasmid containing cyclin D1 promoter together with the expression plasmids for TRbeta1 and wild-type or mutant beta-catenin (SA) into 293T cells. In the presence of T3, beta-catenin-dependent transactivation of cyclin D1 promoter was suppressed by co-transfection of TRbeta1. The suppression by T3/TRbeta1 was in a dose-dependent manner. The CAT reporter gene in which Tcf/Lef-1 sites were fused to heterologous promoter was also suppressed by T3/TRbeta1. Furthermore, inhibition of endogenous wild-type beta-catenin by T3/TRbeta1 was observed in SW480 colon carcinoma cells with mutation of the adenomatous polyposis coli gene. These results indicate that the T3-bound TR inhibits the transcription of cyclin D1 through the Tcf/Lef-1 site, which is positively regulated by the Wnt-signaling pathway.

Biphasic estradiol-induced AKT phosphorylation is modulated by PTEN via MAP kinase in HepG2 cells

We reported previously in HepG2 cells that estradiol induces cell cycle progression throughout the G1-S transition by the parallel stimulation of both PKC-alpha and ERK signaling molecules. The analysis of the cyclin D1 gene expression showed that only the MAP kinase pathway was involved. Here, the presence of rapid/nongenomic, estradiol-regulated, PI3K/AKT signal transduction pathway, its modulation by the levels of the tumor suppressor PTEN, its cross-talk with the ERK pathway, and its involvement in DNA synthesis and cyclin D1 gene promoter activity have all been studied in HepG2 cells. 17beta-Estradiol induced the rapid and biphasic phosphorylation of AKT. These phosphorylations were independent of each other, being the first wave of activation independent of the estrogen receptor (ER), whereas the second was dependent on ER. Both activations were dependent on PI3K activity; furthermore, the ERK pathway modulated AKT phosphorylation by acting on the PTEN levels. The results showed that the PI3K pathway, as well as ER, were strongly involved in both G1-S progression and cyclin D1 promoter activity by acting on its proximal region (-254 base pairs). These data indicate that in HepG2 cells, different rapid/nongenomic estradiol-induced signal transduction pathways modulate the multiple steps of G1-S phase transition.

Transcription factor IID recruitment and Sp1 activation. Dual function of TAF1 in cyclin D1 transcription

Cyclin D1 is an oncogene that regulates progression through the G(1) phase of the cell cycle. A temperature-sensitive missense mutation in the transcription factor TAF1/TAF(II)250 induces the mutant ts13 cells to arrest in late G(1) by decreasing transcription of cell cycle regulators, including cyclin D1. Here we provide evidence that TAF1 serves two independent functions, one at the core promoter and one at the upstream activating Sp1 sites of the cyclin D1 gene. Using in vivo genomic footprinting, we have identified protein-DNA interactions within the cyclin D1 core promoter that are disrupted upon inactivation of TAF1 in ts13 cells. This 33-bp segment, which we termed the TAF1-dependent element 1 (TDE1), contains an initiation site that displays homology to the consensus motif and is sufficient to confer a requirement for TAF1 function. Electrophoretic mobility shift assays reveal that binding of ts13-TAF1-containing TFIID complexes to the cyclin D1 TDE1 occurs at 25 degrees C but not at 37 degrees C in vitro and involves the initiator element. Temperature-dependent DNA binding activity is also observed for TAF1-TAF2 heterodimers assembled with the ts13 mutant but not the wild-type TAF1 protein. These data suggest that a function of TAF is required for the interaction of TFIID with the cyclin D1 initiator. Our finding that recruitment of TFIID, by insertion of a TBP binding site upstream of the TDE1, restores basal but not activated transcription supports the model that TAF1 carries out two independent functions at the cyclin D1 promoter.

Differential regulation of gonadotropin-releasing hormone (GnRH)I and GnRHII messenger ribonucleic acid by gonadal steroids in human granulosa luteal cells

In humans, reproduction was generally believed to be controlled by only one form of GnRH (called mammalian GnRH or GnRHI). However, recently, a second form of GnRH, analogous to chicken GnRHII, was discovered in several tissues, including the human ovary. The regulation and function of GnRHI in the hypothalamus has been well studied. However, the function and regulation of GnRHI, and particularly GnRHII in the ovary, is less well understood. Because gonadal sex steroids are one of the main regulators of reproduction, we investigated, in the present study, the regulation of GnRHI and GnRHII mRNA expression by 17beta-estradiol (E2) and RU486 (a progesterone antagonist) in human granulosa luteal cells (hGLCs). The levels of the mRNA transcripts encoding the two GnRH forms were examined using semiquantitative RT-PCR followed by Southern blot analysis. With time in culture, GnRHI and GnRHII mRNA levels significantly increased, by 120% and 210%, at d 8 and d 1, respectively. The levels remained elevated until the termination of these experiments at d 10. A 24-h treatment of hGLCs with E2 (10(-9) to 10(-7) M) resulted in a dose-dependent decrease and increase in mRNA expression of GnRHI and GnRHII, respectively. E2 (10(-9) M) significantly decreased GnRHI mRNA levels (by 55%) and increased GnRHII mRNA levels (by 294%). Time-course studies demonstrated that E2 (10(-9) M) significantly decreased GnRHI mRNA levels in a time-dependent manner, with maximal inhibition of 77% at 48 h. In contrast, GnRHII mRNA levels significantly increased in a time-dependent fashion, reaching a maximum level of 280% at 24 h. Cotreatment of hGLCs with E2 and tamoxifen (an E2 antagonist) reversed the inhibitory and stimulatory effects of E2 on the mRNA expression of GnRHI and GnRHII, respectively. Time- and dose-dependent treatment with RU486 did not affect GnRHI mRNA levels in hGLCs. In contrast, RU486 treatment significantly increased GnRHII mRNA levels in hGLCs in a time- and dose-dependent fashion, with a maximum increase being observed at 24 h (with 10(-5)M RU486). In summary, the present study demonstrated that the expression of GnRHI and GnRHII at the transcriptional level is differently regulated by E2 and P4 in hGLCs.

A CRE and the region occupied by a protein induced by growth factors contribute to up-regulation of cyclin D1 expression in hepatocytes

Induction of cyclin D1 expression is a critical feature of growth factor-induced cell proliferation in hepatocytes. To clarify the mechanisms regulating cyclin D1 gene expression, we isolated the rat cyclin D1 gene and analyzed the transcriptional regulatory elements in rat hepatoma cells and primary cultured rat hepatocytes. Two transcriptional regulatory regions were analyzed. One was mapped to a cAMP-responsive element (CRE) at position -41bp and was occupied by a CRE-binding protein (CREB), resulting in cyclin D1 expression. Another (CD1E0.7), located at -753bp, revealed high homology with binding sites for the Ets family, the hepatocyte nuclear factor-3beta, or the nuclear factor of activated T cells. However, CD1E0.7 did not interact with these nuclear factors and specific interaction with a protein extracted from growth factor-treated rat hepatocytes in primary cultures. These results indicate that CREB binds to the CRE and mediates activation of the cyclin D1 promoter, and suggest that CD1E0.7 may be possibly occupied by a protein induced by growth factors in hepatocytes.

Mechanism of action of a dominant negative c-jun mutant in inhibiting activator protein-1 activation

The dominant negative c-jun TAM-67 has been shown to inhibit tumor promotion induced by 12-O-tetradecanoylphorbol-13-acetate and okadaic acid (OA). To better understand this phenomenon, we investigated the mechanism of action of TAM-67 in response to OA. To identify the mechanism of action, we used a 6xHis-tagged TAM-67 as well as chimeric constructs of TAM-67 that either cannot bind DNA or cannot heterodimerize with wild-type transcription factors. The results of these studies indicated that TAM-67 acts by blocking or squelching. The results of elecrophoretic mobility-shift assays showed that TAM-67 must act by squelching in response to OA, as TAM-67 cannot be found in DNA-binding complexes. We then identified some of the proteins with which TAM-67 interacts. They include all members of the jun and fos families as well as the cAMP response element binding protein, activating transcription factor-1, activating transcription factor-2, and RelA (p65). Thus, we have shown that TAM-67 squelches the induction of activating transcription factor-1 transactivation in response to OA and that TAM-67 is capable of interacting with proteins that control transactivation by binding to the 12-O-tetradecanoylphorbol-13-acetate response element, cAMP response element and nuclear factor-kappaB sites.

Estrogen activates cyclin-dependent kinases 4 and 6 through induction of cyclin D in rat primary osteoblasts

Estrogen plays important roles in maintaining bone density and protecting against osteoporosis, but the underlying mechanisms of estrogen action via estrogen receptors (ERs) in bone remain to be clarified. In the present study, we isolated primary osteoblasts derived from transgenic rats harboring a dominant negative ER mutant, rat ERalpha (1-535) cDNA, and from their wild-type littermates. We observed that the rate of cell growth of osteoblasts from the transgenic rats was reduced compared to that of wild-type osteoblasts. Utilizing cDNA microarray analysis, we found that mRNA level of cyclin D2 was lower in the osteoblasts from the transgenic rats. D-type cyclins including cyclin D1, cyclin D2, and cyclin D3 are cell cycle regulators that promote progression through the early-to-mid G1 phase of the cell cycle. The protein levels of D-type cyclins including cyclin D2 and cyclin D3 but not cyclin D1 were elevated in wild-type osteoblasts with 17beta-estradiol treatment, resulting in the activation of cyclin-dependent kinases 4 and 6 (Cdk4/6) activities and the promotion of cell growth. Moreover, an anti-estrogen ICI 182,780 abolished the induction of the expression of D-type cyclins by 17beta-estradiol. Our findings indicate that estrogen and its receptors enhance Cdk4/6 activities through the induction of D-type cyclins, leading to the growth promotion of osteoblasts.

Cyclosporine inhibits growth through the activating transcription factor/cAMP-responsive element-binding protein binding site in the cyclin D1 promoter

The immunosuppressive agent cyclosporine affects proliferation depending on the cellular system used. In an attempt to study the inhibitory effect of cyclosporine on proliferation of pancreatic acinar cells, we used AR42J cells as a model system. Here we demonstrate that cyclosporine inhibits growth of these cells by inducing G(1) cell cycle arrest. This effect is mediated by the 5' regulatory region of the cyclin D1 gene and leads to a reduction of cyclin D1 mRNA expression and protein abundance. We show that in AR42J cells the proximal cyclin D1 promoter contains a cis-regulated element, which is important for the maintenance of basal transcriptional activity. This element overlaps the described cAMP-responsive element (CRE) and confers cyclosporine sensitivity to the cyclin D1 promoter. Furthermore, the DNA binding activity of the CRE-binding protein (CREB) decreases through cyclosporine treatment and this is mediated by cyclosporine-induced reduction of CREB steady-state levels. These results demonstrate that cyclosporine can inhibit proliferation of acinar cells by targeting the cyclin D1 promoter at the proximal CRE via a reduction of CREB protein abundance.

Determination of cyclin D1 and CD20 mRNA levels by real-time quantitative RT-PCR from archival tissue sections of mantle cell lymphoma and other non-Hodgkin's lymphomas

Cyclin D1 overexpression is a valuable marker for the diagnosis of mantle cell lymphoma (MCL). We used a real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) method to quantify levels of cyclin D1, CD20, and cyclophilin A mRNA in manually microdissected, paraffin-embedded tissue sections using an ABI 7700 qRT-PCR system. The study group included 21 cases of MCL and 37 cases of other types of B-cell non-Hodgkin's lymphoma. Cyclin D1 mRNA copy number was normalized to CD20 and cyclophilin A mRNA and evaluated statistically by analysis of variance. The relative cyclin D1 levels were similar whether normalized to CD20 or cyclophilin A, indicating that CD20 levels are stable and can be used as a B-cell-specific normalizer. Statistically significant differences were found in the median levels of cyclin D1 mRNA (expressed as % CD20 mRNA) among cases of MCL (87.6), small lymphocytic lymphoma (9.9), follicular lymphoma (2.4), diffuse large B-cell lymphoma (5.9), marginal zone B-cell lymphoma (39.8), and Burkitt lymphoma (7.1) (P < 0.05). We conclude that qRT-PCR can be used to quantify cyclin D1 mRNA levels in archival tissue sections. Normalization of cyclin D1 to a B-cell-specific marker more accurately reflects overexpression by MCL than other methods that normalize using constitutively expressed mRNA species.

Linking cyclins to transcriptional control

Cell cycle activation is coordinated by D-type cyclins which are rate limiting and essential for the progression through the G1 phase of the cell cycle. D-type cyclins bind to and activate the cyclin-dependent kinases Cdk4 and Cdk6, which in turn phosphorylate their downstream target, the retinoblastoma protein Rb. Upon Rb phosphorylation, the E2F transcription factors activate the expression of S-phase genes and thereby induce cell cycle progression. The raise of cyclin D levels in early G1 also serves to titrate Kip/Cip proteins away from cyclinE/Cdk2 complexes, further accelerating cell cycle progression. Therefore, cyclin D plays essential roles in the response to mitogens, transmitting their signal to the Rb/E2F pathway. Surprisingly, cyclin D1-deficient animals are viable and have developmental abnormalities limited to restricted tissues, such as retina, the nervous system and breast epithelium. This observation, combined with several other studies, have raised the possibility that cyclin D1 may have new activities that are unrelated to its function as a cdk regulatory subunit and as regulator of Rb. Effectively, cyclin D has been reported to have transcriptional functions since it interacts with several transcription factors to regulate their activity. Most often, this effect does not rely on the kinase function of Cdk4, indicating that this function is probably independent of cell cycle progression. Further extending its role in gene regulation, cyclin D interacts with histone acetylases such as P/CAF or NcoA/SRC1a but also with components of the transcriptional machinery such as TAF(II)250. Therefore, these studies suggest that the functions of cyclin D might need to be reevaluated. They have established a new cdk-independent role of cyclin D1 as a transcriptional regulator, indicating that cyclin D1 can act via two different mechanisms, as a cdk activator it regulates cell cycle progression and as a transcriptional regulator, it modulates the activity of transcription factors.

Distinct nongenomic signal transduction pathways controlled by 17beta-estradiol regulate DNA synthesis and cyclin D(1) gene transcription in HepG2 cells

Estrogens induce cell proliferation in target tissues by stimulating progression through the G1 phase of the cell cycle. Activation of cyclin D(1) gene expression is a critical feature of this hormonal action. The existence of rapid/nongenomic estradiol-regulated protein kinase C (PKC-alpha) and extracellular signal-regulated kinase (ERK) signal transduction pathways, their cross talk, and role played in DNA synthesis and cyclin D(1) gene transcription have been studied herein in human hepatoma HepG2 cells. 17Beta-estradiol was found to rapidly activate PKC-alpha translocation and ERK-2/mitogen-activated protein kinase phosphorylation in this cell line. These actions were independent of each other, preceding the increase of thymidine incorporation into DNA and cyclin D(1) expression, and did not involve DNA binding by estrogen receptor. The results obtained with specific inhibitors indicated that PKC-alpha pathway is necessary to mediate the estradiol-induced G1-S progression of HepG2 cells, but it does not exert any effect(s) on cyclin D(1) gene expression. On the contrary, ERK-2 cascade was strongly involved in both G1-S progression and cyclin D(1) gene transcription. Deletion of its activating protein-1 responsive element motif resulted in attenuation of cyclin D(1) promoter responsiveness to estrogen. These results indicate that estrogen-induced cyclin D(1) transcription can occur in HepG2 cells independently of the transcriptional activity of estrogen receptor, sustaining the pivotal role played by nongenomic pathways of estrogen action in hormone-induced proliferation.

Transcriptional activation by the oestrogen receptor alpha is modulated through inhibition of cyclin-dependent kinases

We have investigated the interaction between the expression of p21(WAF1/CIP1/SDI1), a stoichiometric inhibitor of Cdk, and the transcriptional activity of the oestrogen receptor alpha (ER(alpha). Transient transfection experiments demonstrated that the expression of p21(WAF1/CIP1/SDI1) amplified the transcriptional activation by ER(alpha). A dominant negative mutant of Cdk2 also enhanced the ER(alpha) transcriptional activity, indicating that the underlying mechanism relies on the inhibition of Cdk2 activity and cell cycle arrest. In agreement with this conclusion, experiments with p21(WAF1/CIP1/SDI1) mutants demonstrated that the domain involved in the binding of p21(WAF1/CIP1/SDI1) to Cdks was indispensable for the modulation of ER(alpha) activity. In addition, we show that expression of p21(WAF1/CIP1/SDI1) alleviates the block on CBP function mediated by Cdk2 and in turn stimulates transcriptional activation by ER(alpha) in a CBP-histone acetyltransferase (HAT)-dependent manner. These results suggest a novel mechanism by which p21(WAF1/CIP1/SDI1) functions as an enhancer of ER(alpha) activity through the modulation of CBP function.

Regulation of airway smooth muscle cyclin D1 transcription by protein kinase C-delta

The precise mechanism by which protein kinase C-delta (PKCdelta) inhibits cell cycle progression is not known. We investigated the regulation of cyclin D1 transcription by PKCdelta in primary bovine airway smooth muscle cells. Overexpression of the active catalytic subunit of PKCdelta attenuated platelet-derived growth factor (PDGF)-mediated transcription from the cyclin D1 promoter, whereas overexpression of a dominant-negative PKCdelta increased promoter activity. A PKCdelta-specific pseudosubstrate increased cyclin D1 protein abundance. To determine the transcriptional mechanism by which PKCdelta negatively regulates cyclin D1 expression, we transiently transfected cells with cDNAs encoding cyclin D1 promoter 5' deletions and site mutations in the context of a -66 promoter fragment. We found that the -57 to -52 CRE/ATF2 site functions as a basal level and PDGF enhancer, whereas the -39 to -30 nuclear factor-kappaB site functions as a basal level suppressor. Further, PDGF and PKCdelta responsiveness of the cyclin D1 promoter was maintained following 5' deletion to the Ets-containing -22 minimal promoter. Finally, using electrophoretic mobility gel shift and reporter assays, we determined that PKCdelta inhibits CRE/ATF2 binding and transactivation, activates nuclear factor-kappaB binding and transactivation, and attenuates Ets transactivation. These data suggest that PKCdelta attenuates cyclin D1 promoter activity via the regulation of three distinct cis-acting regulatory elements.

Opposing action of estrogen receptors alpha and beta on cyclin D1 gene expression

Induction of cyclin D1 gene transcription by estrogen receptor alpha (ERalpha) plays an important role in estrogen-mediated proliferation. There is no classical estrogen response element in the cyclin D1 promoter, and induction by ERalpha has been mapped to an alternative response element, a cyclic AMP-response element at -57, with possible participation of an activating protein-1 site at -954. The action of ERbeta at the cyclin D1 promoter is unknown, although evidence suggests that ERbeta may inhibit the proliferative action of ERalpha. We examined the response of cyclin D1 promoter constructs by luciferase assay and the response of the endogenous protein by Western blot in HeLa cells transiently expressing ERalpha, ERalphaK206A (a derivative that is superactive at alternative response elements), or ERbeta. In each case, ER activation at the cyclin D1 promoter is mediated by both the cyclic AMP-response element and the activating protein-1 site, which play partly redundant roles. The activation by ERbeta occurs only with antiestrogens. Estrogens, which activate cyclin D1 gene expression with ERalpha, inhibit expression with ERbeta. Strikingly, the presence of ERbeta completely inhibits cyclin D1 gene activation by estrogen and ERalpha or even by estrogen and the superactive ERalphaK206A. The observation of the opposing action and dominance of ERbeta over ERalpha in activation of cyclin D1 gene expression has implications for the postulated role of ERbeta as a modulator of the proliferative effects of estrogen.

Epidermal growth factor receptor dependence of radiation-induced transcription factor activation in human breast carcinoma cells

Ionizing radiation (1-5 Gy) activates the epidermal growth factor receptor (EGFR), a major effector of the p42/44 mitogen-activated protein kinase (MAPK) pathway. MAPK and its downstream effector, p90 ribosomal S6 kinase (p90RSK), phosphorylate transcription factors involved in cell proliferation. To establish the role of the EGFR/MAPK pathway in radiation-induced transcription factor activation, MDA-MB-231 human breast carcinoma cells were examined using specific inhibitors of signaling pathways. Gel-shift analysis revealed three different profile groups: 1) transcription factors that responded to both radiation (2 Gy) and epidermal growth factor (EGF) (CREB, Egr, Ets, and Stat3); 2) factors that responded to radiation, but not EGF (C/EBP and Stat1); and 3) those that did not respond significantly to either radiation or EGF (AP-1 and Myc). Within groups 1 and 2, a two- to fivefold maximum stimulation of binding activity was observed at 30-60 min after irradiation. Interestingly, only transcription factors that responded to EGF had radiation responses significantly inhibited by the EGFR tyrosine kinase inhibitor, AG1478; these responses were also abrogated by farnesyltransferase inhibitor (FTI) or PD98059, inhibitors of Ras and MEK1/2, respectively. Moreover, radiation-induced increases in CREB and p90RSK phosphorylation and activation of Stat3 and Egr-1 reporter constructs by radiation were all abolished by AG1478. These data demonstrate a distinct radiation response profile at the transcriptional level that is dependent on enhanced EGFR/Ras/MAPK signaling.

To ERR in the estrogen pathway

Estrogens control a variety of physiological and disease-linked processes, most notably reproduction, bone remodeling and breast cancer, and their effects are transduced through classic unclear receptors referred to as estrogen receptor-alpha (ER alpha) and ER beta. Recent results obtained using the estrogen-related receptors (ERR alpha, -beta and -gamma), a subfamily of orphan nuclear receptors closely related to the ERs, have shown that the ERRs share target genes, coregulatory proteins, ligands and sites of action with the ERs. In addition, the ERRs can actively influence the estrogenic response, suggesting that pharmacological modulation of ERR activity will be clinically useful to prevent and/or treat a variety of conditions related to women's health.

Molecular cloning and sequence analysis of the promoter region of mouse cyclin D1 gene: implication in phorbol ester-induced tumour promotion

Cyclin D1 is a cell cycle regulatory protein, which acts as a growth factor sensor to integrate extracellular signals with the cell cycle machinery, particularly during G1 phase of the cell cycle. Previous study using promotion-sensitive JB6 mouse epidermal cells, an in vitro model of the promotion stage of multistage carcinogenesis, showed that the expression of cyclin D1 is stimulated in the presence (but not in the absence) of 12-O-tetradecanoylphorbol-13-acetate (TPA) in these cells maintained under anchorage-independent culture conditions. In the present study, to explore the molecular basis of this observation, the promoter region of mouse cyclin D1 gene was cloned and sequenced (GenBank accession number AF212040). Dot matrix comparison of mouse, human and rat promoter sequences indicated that the mouse promoter is homologous to the human and more so to the rat promoters. The mouse promoter, like human and rat promoters, lacks canonical TATA-box or TATA-like sequence, but it has one or possibly two initiator (Inr) or Inr-like sequences. Energy dot plot analysis predicted that the mouse promoter consists of three domains: (1) the 3' domain contains NF-kappaB response element, cAMP-response element (CRE), Inr or Inr-like elements, Sp1 binding site and Oct 1 (2) the middle domain contains another Sp1 binding site, E-box and E2F binding site and (3) the 5' domain contains TPA-response element (TRE) and a tandem silencer element. The cyclin D1 promoter sequence of either promotion-sensitive or resistant JB6 mouse epidermal cells was, except for a few minor differences, essentially identical to the sequence determined for a mouse genomic clone. Since TPA is capable of stimulating the expression of cyclin D1 not only through TRE but also through CRE and NF-kappaB response element in the promoter, we tentatively propose a sequence of events that possibly leads to TPA-induced, anchorage-independent synthesis of cyclins D1 and A in the promotion-sensitive JB6 mouse epidermal cells.

Prognostic value of CCND1 gene status in sporadic breast tumours, as determined by real-time quantitative PCR assays

The CCND1 gene, a key cell-cycle regulator, is often altered in breast cancer, but the mechanisms underlying CCND1 dysregulation and the clinical significance of CCND1 status are unclear. We used real-time quantitative PCR and RT-PCR assays based on fluorescent TaqMan methodology to quantify CCND1 gene amplification and expression in a large series of breast tumours. CCND1 overexpression was observed in 44 (32.8%) of 134 breast tumour RNAs, ranging from 3.3 to 43.7 times the level in normal breast tissues, and correlated significantly with positive oestrogen receptor status (P=0.0003). CCND1 overexpression requires oestrogen receptor integrity and is exacerbated by amplification at 11q13 (the site of the CCND1 gene), owing to an additional gene dosage effect. Our results challenge CCND1 gene as the main 11q13 amplicon selector. The relapse-free survival time of patients with CCND1-amplified tumours was shorter than that of patients without CCND1 alterations, while that of patients with CCND1-unamplified-overexpressed tumours was longer (P=0.011). Only the good prognostic significance of CCND1-unamplified-overexpression status persisted in Cox multivariate regression analysis. This study confirms that CCND1 is an ER-responsive or ER-coactivator gene in breast cancer, and points to the CCND1 gene as a putative molecular marker predictive of hormone responsiveness in breast cancer. Moreover, CCND1 amplification status dichotomizes the CCND1-overexpressing tumors into two groups with opposite outcomes.

The transcription of the hGnRH-I and hGnRH-II genes in human neuronal cells is differentially regulated by estrogen

Gonadotropin releasing hormone-I (GnRH-I), a decapeptide serves as a key regulator of reproduction. Recently, several groups have identified in the mammalian brain a second form of GnRH, of unknown function, designated GnRH-II. The human neuronal medulloblastoma cells (TE-671) were recently demonstrated to express the two forms of GnRH (GnRH-I and GnRH-II). We used this cell line, as a model system, to investigate the regulation of human GnRH-I and GnRH-II genes by estrogen. Estrogen is one of the principal regulators of GnRH-I in hypothalamic neurons, acting as a classic homeostatic feedback molecule between the gonads and the brain. In this study, we investigated the regulation of the two GnRH forms by estrogen, in the human neuronal cell line TE-671. We demonstrate, for the first time, that the hGnRH-II and hGnRH-I genes are differentially regulated by estrogen. Using reverse transcriptase-polymerase chain reaction (RT-PCR) and Southern hybridization, we found that estrogen increases endogenous hGnRH-II mRNA levels and decreases endogenous hGnRH-I mRNA levels. Furthermore, we found these effects to be promoter-mediated. We cloned the hGnRH-I and hGnRH-II promoter constructs upstream to a luciferase reporter plasmid, and cotransfected these constructs with an estrogen receptor alpha into the TE-671 neuronal cells. Luciferase activity of GnRH promoter constructs treated with estrogen demonstrates that the differential regulation of the GnRH genes by estrogen is mediated at the transcription level.

Cyclin A is a prognostic indicator in early stage breast cancer with and without tamoxifen treatment

Overexpression of G1-S regulators cyclin D1 or cyclin A is frequently observed in breast cancer and is also to result in ligand-independent activation of oestrogen receptor in vitro. This might therefore, provide a mechanism for failure of tamoxifen treatment. We examined by immunohistochemical staining the effect of deregulation of these, and other cell cycle regulators on tamoxifen treatment in a group of 394 patients with early stage breast cancer. In univariate analysis, expression of cyclin A, Neu, Ki-67 index, and lack of OR expression were significantly associated with worse prognosis. When adjusted by the clinical model (for lymph node status, age, performance status, T-classification, grade, prior surgery, oestrogen receptor status and tamoxifen use), only overexpression of cyclin A and Neu were significantly associated with worse prognosis with hazard ratios of, respectively, 1.709 (P=0.0195) and 1.884 (P=0.0151). Overexpression of cyclin A was found in 86 out of the 201 OR-positive cases treated with tamoxifen, and was the only independent marker associated with worse prognosis (hazard ratio 2.024, P=0.0462). In conclusion, cyclin A is an independent predictor of recurrence of early stage breast cancer and is as such a marker for response in patients treated with tamoxifen.

Genomic structure and regulation of a novel human gene, Klp1

Klp1 (K562 cells-derived leucine zipper-like protein 1) is a transcription factor which binds to the coproporphyrinogen oxidase promoter regulatory element (GGACTACAG). In order to clarify the function of Klp1, we determined the complete human Klp1 genomic structure and regulatory element in the promoter region. The gene spans about 2.4 kb and has three exons. Its promoter region has multiple GC boxes, E2F binding site, one cAMP response element (CRE), and no TATA box with multiple transcription initiation sites, which is characteristic of housekeeping and growth regulating genes. Promoter analysis showed that the promoter was more active in K562 cells entered into the cell cycle by serum stimulation than quiescent cells. Further promoter analysis revealed that CRE at -42 is essential for full promoter activity, and c-Jun and activation transcription factor 1/cAMP response element binding protein 1 proteins bind to this element. These structural characteristics and the promoter function suggest that Klp1 may play a role in cell cycle regulation.

Nuclear receptors in cell life and death

The balance between cell proliferation and programmed cell death (apoptosis) determines body patterns during animal development and controls compartment sizes, tissue architecture and remodeling. The removal of primordial structures by apoptosis allows the organism to develop sex specifically and to adapt for novel functions at later stages; apoptosis also limits the size of evolving structures. It is a ubiquitous function that is essential for all cells. Although inappropriate regulation or execution of apoptosis leads to disease, such as cancer, there is now evidence for its great therapeutic potential. This would be particularly true if apoptosis could be targeted at defined cell compartments, rather than acting ubiquitously like chemotherapy. Here, we discuss the potential of nuclear receptor ligands, many of which act through their cognate receptors in defined body compartments as modulators of cell life and death, with special emphasis on the molecular pathways by which these receptors affect cell-cycle progression, survival and apoptosis.

Role of transcription factors in the pathogenesis of pituitary adenomas: a review

The diversity inherent in every organ has its roots in gene-expression variation and is revealed through distinctions in the molecular profile and hence the identity of individual cell type. Study into the molecular mechanisms of the development of individual cell type within the pituitary, which is under the control of transcription factors, has provided a basis for a deeper insight into the molecular mechanisms underlying the pathogenesis of a variety of hormone-producing pituitary tumors. Identification of some of these transcription factors in pituitary adenomas further supports their role in the pathogenesis of pituitary adenomas. Understanding the molecular mechanisms of regulation of proliferation of pituitary cell types by transcription factors offers a basis for hope that rational genetic or pharmacologic therapies for pituitary tumors can be designed in the future.

Cdc25B functions as a novel coactivator for the steroid receptors

We have previously demonstrated that overexpression of Cdc25B in transgenic mice resulted in mammary gland hyperplasia and increased steroid hormone responsiveness. To address how Cdc25B enhances the hormone responsiveness in mammary glands, we showed that Cdc25B stimulates steroid receptor-dependent transcription in transient transfection assays and in a cell-free assay with chromatin templates. Surprisingly, the effect of Cdc25B on steroid receptors is independent of its protein phosphatase activity in vitro. The direct interactions of Cdc25B with steroid receptors, on the other hand, were evidenced in in vivo and in vitro assays, suggesting the potential direct contribution of Cdc25B on the steroid receptor-mediated transcription. In addition, p300/CBP-associated factor and CREB binding protein were shown to interact and synergize with Cdc25B and further enhance its coactivation activity. Thus, we have uncovered a novel function of Cdc25B that serves as a steroid receptor coactivator in addition to its role as a regulator for cell cycle progression. This dual function might likely contribute to its oncogenic action in breast cancer.

Angiotensin II-induced transcriptional activation of the cyclin D1 gene is mediated by Egr-1 in CHO-AT(1A) cells

Cyclin D1 protein expression is regulated by mitogenic stimuli and is a critical component in the regulation of G(1) to S phase progression of the cell cycle. Angiotensin II (Ang II) binds to specific G protein-coupled receptors and is mitogenic in Chinese hamster ovary cells stably expressing the rat vascular Ang II type 1A receptor (CHO-AT(1A)). We recently reported that in these cells, Ang II induced cyclin D1 promoter activation and protein expression in a phosphatidylinositol 3-kinase (PI3K)-, SHP-2-, and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK)-dependent manner (Guillemot, L., Levy, A., Zhao, Z. J., Béréziat, G., and Rothhut, B. (2000) J. Biol. Chem. 275, 26349-26358). In this report, transfection studies using a series of deleted cyclin D1 promoters revealed that two regions between base pairs (bp) -136 and -96 and between bp -29 and +139 of the human cyclin D1 promoter contained regulatory elements required for Ang II-mediated induction. Mutational analysis in the -136 to -96 bp region provided evidence that a Sp1/early growth response protein (Egr) motif was responsible for cyclin D1 promoter activation by Ang II. Gel shift and supershift studies showed that Ang II-induced Egr-1 binding involved de novo protein synthesis and correlated well with Egr-1 promoter activation. Both U0126 (an inhibitor of the MAPK/ERK kinase MEK) and wortmannin (an inhibitor of PI3K) abrogated Egr-1 endogenous expression and Egr-1 promoter activity induced by Ang II. Moreover, using a co-transfection approach, we found that Ang II induction of Egr-1 promoter activity was blocked by dominant-negative p21(ras), Raf-1, and tyrosine phosphatase SHP-2 mutants. Identical effects were obtained when inhibitors and dominant negative mutants were tested on the -29 to +139 bp region of the cyclin D1 promoter. Taken together, these findings demonstrate that Ang II-induced cyclin D1 up-regulation is mediated by the activation and specific interaction of Egr-1 with the -136 to -96 bp region of the cyclin D1 promoter and by activation of the -29 to +139 bp region, both in a p21(ras)/Raf-1/MEK/ERK-dependent manner, and also involves PI3K and SHP-2.

17beta-estradiol inhibits forskolin-induced vascular endothelial growth factor promoter in MCF-7 breast adenocarcinoma cells

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor whose expression is induced by the cAMP-dependent signalling pathway in several cell types, and by estrogens in some human breast cancer cells. Here, we investigated the cross-talk between estrogens and cAMP/PKA-dependent signalling pathway in human breast cancer MCF-7 cells. The results show that, in the absence of any CRE and ERE, forskolin induces whereas estrogens have no effect on VEGF promoter. Moreover, estrogens, through estrogen receptors, partly inhibit the forskolin-induced VEGF promoter in MCF-7 human breast cancer cells. Therefore, in breast cancers, estrogens could partly inhibit the effect of ligand-activated G protein-coupled receptors on VEGF expression.

GSK3 takes centre stage more than 20 years after its discovery

Identified originally as a regulator of glycogen metabolism, glycogen synthase kinase-3 (GSK3) is now a well-established component of the Wnt signalling pathway, which is essential for setting up the entire body pattern during embryonic development. It may also play important roles in protein synthesis, cell proliferation, cell differentiation, microtubule dynamics and cell motility by phosphorylating initiation factors, components of the cell-division cycle, transcription factors and proteins involved in microtubule function and cell adhesion. Generation of the mouse knockout of GSK3beta, as well as studies in neurons, also suggest an important role in apoptosis. The substrate specificity of GSK3 is unusual in that efficient phosphorylation of many of its substrates requires the presence of another phosphorylated residue optimally located four amino acids C-terminal to the site of GSK3 phosphorylation. Recent experiments, including the elucidation of its three-dimensional structure, have enhanced our understanding of the molecular basis for the unique substrate specificity of GSK3. Insulin and growth factors inhibit GSK3 by triggering its phosphorylation, turning the N-terminus into a pseudosubstrate inhibitor that competes for binding with the 'priming phosphate' of substrates. In contrast, Wnt proteins inhibit GSK3 in a completely different way, by disrupting a multiprotein complex comprising GSK3 and its substrates in the Wnt signalling pathway, which do not appear to require a 'priming phosphate'. These latest findings have generated an enormous amount of interest in the development of drugs that inhibit GSK3 and which may have therapeutic potential for the treatment of diabetes, stroke and Alzheimer's disease.

The renaissance of GSK3

Glycogen synthase kinase 3 (GSK3) was initially described as a key enzyme involved in glycogen metabolism, but is now known to regulate a diverse array of cell functions. The study of the substrate specificity and regulation of GSK3 activity has been important in the quest for therapeutic intervention.

Estrogens and cell-cycle regulation in breast cancer

Clinical and experimental data have established that the leading cause of sporadic female breast cancer is exposure to estrogens, predominantly 17beta-estradiol. Recent advances in the understanding of cell-cycle control mechanisms have been applied to outline the molecular mechanisms through which estrogens regulate the cell cycle in cultured breast cancer cells, in particular, in MCF-7 cells. Here, we discuss how estrogens exert control over several key G1 phase cell-cycle regulators, namely cyclin D1, Myc, Cdk2, Cdk4, Cdk inhibitors and Cdc25A. Although the molecular mechanisms underlying estrogenic regulation of G1 phase regulators are far from clear, current evidence indicates that estrogens might regulate several key molecules required for S phase entry, this regulation being independent of cell-cycle transit per se.

Estrogen regulation of cyclin D1 gene expression in ZR-75 breast cancer cells involves multiple enhancer elements

Cyclin D1 gene expression is induced by 17beta-estradiol (E2) in human breast cancer cells and is important for progression of cells through the G(1) phase of the cell cycle. The mechanism of activation of cyclin D1 is mitogen- and cell context-dependent, and this study describes the role of multiple promoter elements required for induction of cyclin D1 by E2 in estrogen receptor (ER)-positive ZR-75 breast cancer cells. Transcriptional activation of cyclin D1 by E2 was dependent, in part, on a proximal cAMP-response element at -66, and this was linked to induction of protein kinase A-dependent pathways. These results contrasted to a recent report showing that induction of cyclin D1 by E2 in ER-positive MCF-7 and HeLa cells was due to up-regulation of c-jun and subsequent interaction of c-Jun-ATF-2 with the CRE. Moreover, further examination of the proximal region of the cyclin D1 promoter showed that three GC-rich Sp1-binding sites at -143 to -110 were also E2-responsive, and interaction of ERalpha and Sp1 proteins at these sites was confirmed by electromobility shift and chromatin immunoprecipitation assays. Thus, induction of cyclin D1 by E2 in ZR-75 cells is regulated through nuclear ERalpha/Sp1 and epigenetic protein kinase A activation pathways, and our results suggest that this mechanism may be cell context-dependent even among ER-positive breast cancer cell lines.

Role of estrogen receptor beta in estrogen action

There was a time when the classification of sex hormones was simple. Androgens were male and estrogens female. What remains true today is that in young adults androgen levels are higher in males and estrogen levels higher in females. More recently we have learned that estrogens are necessary in males for regulation of male sexual behavior, maintenance of the skeleton and the cardiovascular system, and for normal function of the testis and prostate. The importance of androgen in females was never in doubt, it is after all the precursor of estrogen as the substrate for aromatase, the enzyme that produces estrogen. In addition, the tissue distribution of androgen receptors suggests that androgens themselves are important in the ovary, uterus, breast, and brain. New information promises to clarify some of the complex issues of the physiological roles of estrogen and the contribution of estrogen to the development of neoplastic diseases in humans. The discovery of the second estrogen receptor, the creation of mutant mice defective in both estrogen receptors and in the aromatase gene, the solution of the structures of the ligand-binding domains of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta), the finding of novel routes through which estrogen receptors can modulate transcription, and the identification of a man with a bi-allelic disruptive mutation of the ERalpha gene are but some of the milestones. This review focuses on the mechanistic aspects of signal transduction mediated by ERs and on the physiological consequences of deficiency of estrogen or estrogen receptor in the available mouse models.

Gender differences in autoimmunity: molecular basis for estrogen effects in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease that occurs primarily in women (9:1 compared to men). Estrogen is a female sex hormone that acts on target cells through specific receptor proteins and alters the rate of transcription of target genes. Experiments in our laboratory have shown that calcineurin steady-state mRNA levels and phosphatase activity increase when estrogen is cultured with SLE T cells. This estrogen-dependent increase is dose-dependent, hormone-specific and temporally regulated. Estrogen receptor antagonism by ICI 182,780 inhibits the increase in calcineurin mRNA and phosphatase activity, while cycloheximide has no effect suggesting that new protein synthesis is not required. Reverse transcription and polymerase chain amplification indicate that estrogen receptor-alpha and estrogen-beta are expressed in human T cells. However, calcineurin does not respond to estrogen stimulation in T cells from normal females, males and lupus males. Taken together, these results indicate a differential function of the estrogen receptor in women with lupus. A model is proposed that suggests estrogen, acting through the estrogen receptor, enhances T cell activation in women with lupus resulting in amplified T-B cells interactions, B cell activation and autoantibody production.

The orphan nuclear estrogen receptor-related receptor alpha (ERRalpha) is expressed throughout osteoblast differentiation and regulates bone formation in vitro

The orphan nuclear estrogen receptor-related receptor alpha (ERRalpha), is expressed by many cell types, but is very highly expressed by osteoblastic cells in which it transactivates at least one osteoblast-associated gene, osteopontin. To study the putative involvement of ERRalpha in bone, we first assessed its expression in rat calvaria (RC) in vivo and in RC cells in vitro. ERRalpha mRNA and protein were expressed at all developmental stages from early osteoprogenitors to bone-forming osteoblasts, but protein was most abundant in mature cuboidal osteoblasts. To assess a functional role for ERRalpha in osteoblast differentiation and bone formation, we blocked its expression by antisense oligonucleotides in either proliferating or differentiating RC cell cultures and found inhibition of cell growth and a proliferation-independent inhibition of differentiation. On the other hand, ERRalpha overexpression in RC cells increased differentiation and maturation of progenitors to mature bone-forming cells. Our findings show that ERRalpha is highly expressed throughout the osteoblast developmental sequence and plays a physiological role in differentiation and bone formation at both proliferation and differentiation stages. In addition, we found that manipulation of receptor levels in the absence of known ligand is a fruitful approach for functional analysis of this orphan receptor and identification of potential target genes.

Estrogen receptor binding to DNA is not required for its activity through the nonclassical AP1 pathway

In the classical signaling pathway, the estrogen receptor (ER) binds directly to estrogen response elements (EREs) to regulate gene transcription. To test the hypothesis that the nonclassical pathway involves ER interactions with other proteins rather than direct binding to DNA, mutations were introduced into the DNA binding domain (DBD) of the mouse ERalpha. The effects of these DBD mutations were examined in DNA binding assays using reporter constructs containing either EREs (classical) or AP1 (nonclassical) response elements. Using the AP1 reporter, there was a reversal of ER action relative to that seen with the ERE reporter. Estradiol induced suppression, and the antiestrogen ICI 182,780 stimulated transcription of the AP1 reporter. DBD mutations in the proximal (P-box) of the first zinc finger of the ER (E207A/G208A and E207G/G208S) eliminated ERE binding. These mutants were inactive using the ERE reporter but retained partial or full activity with the AP1 reporter. The DBD mutant ERs interacted with Jun when tested in mammalian cell two-hybrid assays. Two mutations (K366D and I362R) in the ER ligand binding domain known to alter coactivator interactions impaired transcriptional responses using either the ERE or AP1 reporters. We concluded that ER action through the AP1 response element involves interactions with other promoter-bound proteins instead of, or in addition to, direct binding to DNA. Interactions with coactivators were required for both pathways. These data supported a model in which ER-mediated transcriptional activation or repression is dependent on the ligand and the nature of the response element in the target gene.

Reduction in cyclin D1/Cdk4/retinoblastoma protein signaling by CRE-decoy oligonucleotide

We have previously demonstrated that the activation of p53 signaling may contribute to tumor growth inhibition by the CRE-decoy oligonucleotide containing CRE sequence (5'-TGACGTCA-3') (Lee et al., Biochemistry 39, 4863-4868, 2000). However, growth inhibition by CRE-decoy treatment was also observed in tumor cells containing a mutant p53 (Park et al., J. Biol. Chem. 274, 1573-1580, 1999). To understand additional mechanisms of the decoy oligonucleotide, we investigated the effect on cyclin D1 expression and a cyclin D1/Cdk4/retinoblastoma protein (pRB) signaling pathway. Here we show that in MCF7 breast cancer cells the CRE-decoy competed with cyclin D1-CRE (5'-TAACGTCA-3') for binding transcription factors and reduced cyclin D1 gene expression (in reporter gene assay, Northern blotting and Western blotting) to modulate cyclin D1/Cdk4/pRB signaling and G1-S progression in a steady state and/or under estrogen stimulation. Decrease of cyclin D1 protein level by CRE-decoy treatment was also observed in p53-mutated cancer cells. Cyclin D1 expression was also diminished in MCF7 cells stably expressing dominant negative mutant CREB indicating that the nonspecific effect of oligonucleotide or its degradation products could be excluded. These data suggest that inhibition of cyclin D1 expression contributes to the growth inhibition induced by the decoy oligonucleotide in MCF7 cells through a cyclin D1/Cdk4/pRB signaling pathway. Downregulation of cyclin D1 expression also provides a mechanism of CRE-decoy-induced growth inhibition in tumor cells having p53 mutation.

Transcriptional activation of the cyclin D1 gene is mediated by multiple cis-elements, including SP1 sites and a cAMP-responsive element in vascular endothelial cells

In an attempt to examine the mechanisms by which transcriptional activity of the cyclin D1 promoter is regulated in vascular endothelial cells (EC), we examined the cis-elements in the human cyclin D1 promoter, which are required for transcriptional activation of the gene. The results of luciferase assays showed that transcriptional activity of the cyclin D1 promoter was largely mediated by SP1 sites and a cAMP-responsive element (CRE). DNA binding activity at the SP1 sites, which was analyzed by electrophoretic mobility shift assays, was significantly increased in the early to mid G(1) phase, whereas DNA binding activity at CRE did not change significantly. Furthermore, Induction of the cyclin D1 promoter activity in the early to mid G(1) phase depended largely on the promoter fragment containing the SP1 sites, whereas the proximal fragment containing CRE but not the SP1 sites was constitutively active. Finally, the increase in DNA binding and promoter activities via the SP1 sites was mediated by the Ras-dependent pathway. The results suggested that the activation of the cyclin D1 gene in vascular ECs was regulated by a dual system; one was inducible in the G(1) phase, and the other was constitutively active.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

A pure estrogen antagonist inhibits cyclin E-Cdk2 activity in MCF-7 breast cancer cells and induces accumulation of p130-E2F4 complexes characteristic of quiescence

Estrogen antagonists inhibit cell cycle progression in estrogen-responsive cells, but the molecular mechanisms are not fully defined. Antiestrogen-mediated G(0)/G(1) arrest is associated with decreased cyclin D1 gene expression, inactivation of cyclin D1-cyclin dependent kinase (Cdk) 4 complexes, and decreased phosphorylation of the retinoblastoma protein (pRb). We now show that treatment of MCF-7 breast cancer cells with the pure estrogen antagonist ICI 182780 results in inhibition of cyclin E-Cdk2 activity prior to a decrease in the G(1) to S phase transition. This decrease was dependent on p21(WAF1/Cip1) since treatment with antisense oligonucleotides to p21 attenuated the effect. Recruitment of p21 to cyclin E-Cdk2 complexes was in turn dependent on decreased cyclin D1 expression since it was apparent following treatment with antisense cyclin D1 oligonucleotides. To define where within the G(0) to S phase continuum antiestrogen-treated cells arrested, we assessed the relative abundance and phosphorylation state of pocket protein-E2F complexes. While both pRb and p107 levels were significantly decreased, p130 was increased 4-fold and was accompanied by the formation of p130.E2F4 complexes and the accumulation of hyperphophorylated E2F4, putative markers of cellular quiescence. Thus, ICI 182780 inhibits both cyclin D1-Cdk4 and cyclin E-Cdk2 activity, resulting in the arrest of MCF-7 cells in a state with characteristics of quiescence (G(0)), as opposed to G(1) arrest.