The retinoblastoma gene product regulates Sp1-mediated transcription

Crosstalk between TGF-beta and MAPK signaling during corneal wound healing

PURPOSE

The aim of this study was to elucidate the mechanisms governing epithelial cell migration and proliferation during wound healing.

METHODS

The authors used wound healing of mouse corneal epithelium to examine the role TGF-β signaling plays during the healing process. To achieve this goal, they used transgenic mice in which the TGF-β receptor type II (Tbr2) was conditionally ablated from the corneal epithelium. Epithelium debridement wounds were made, followed by the assessment of cell migration, proliferation, and immunostaining of various signaling pathway components.

RESULTS

The authors showed that in the absence of TGF-β signaling corneal epithelial wound healing is delayed by 48 hours; this corresponds to a delay in p38MAPK activation. Despite the delayed p38MAPK activation, ATF2, a substrate of p38MAPK, is still phosphorylated, leading to the suppression of cell proliferation at the leading edge of the wound. These data provide evidence that in the absence of TGF-β signaling, the suppression of cell proliferation during the early stages of wound healing is maintained through the JNK activation of ATF2. CONCLUSIONS; Together the data presented here demonstrate the importance of the TGF-β and MAPK signaling pathways in corneal epithelial wound healing.

Retinoblastoma protein modulates the inverse relationship between cellular proliferation and elastogenesis

The mechanism that leads to the inverse relationship between heightened cellular proliferation and the cessation of elastic fibers production, observed during formation of the arterial occlusions and dermal scars, is not fully understood. Because the retinoblastoma protein (Rb), responsible for cell cycle initiation, has also been implicated in insulin-like growth factor-I-mediated signaling stimulating elastin gene activation, we explored whether differential phosphorylation of Rb by various cyclin·cyclin-dependent kinase complexes would be responsible for promoting either elastogenic or pro-proliferative signals. We first tested cultures of dermal fibroblasts derived from Costello syndrome patients, in which heightened proliferation driven by mutated oncogenic H-Ras coincides with inhibition of elastogenesis. We found that Costello syndrome fibroblasts display elevated level of Rb phosphorylation on serine 780 (Ser(P)-780-Rb) and that pharmacological inhibition of Ras with radicicol, Mek/Erk with PD98059, or cyclin-dependent kinase 4 with PD0332991 not only leads to down-regulation of Ser(P)-780-Rb levels but also enhances Rb phosphorylation on threonine-821 (Thr(P)-821-Rb), which coincides with the recovery of elastin production. Then we demonstrated that treatment of normal skin fibroblasts with the pro-proliferative PDGF BB also up-regulates Ser(P)-780-Rb levels, but treatment with the pro-elastogenic insulin-like growth factor-I activates cyclinE-cdk2 complex to phosphorylate Rb on Thr-821. Importantly, we have established that elevation of Thr(P)-821-Rb promotes Rb binding to the Sp1 transcription factor and that successive binding of the Rb-Sp1 complex to the retinoblastoma control element within the elastin gene promoter stimulates tropoelastin transcription. In summary, we provide novel insight into the role of Rb in mediating the inverse relationship between elastogenesis and cellular proliferation.

Lipopolysaccharide-mediated IL-10 transcriptional regulation requires sequential induction of type I IFNs and IL-27 in macrophages

IL-10 is a potent anti-inflammatory molecule that regulates excessive production of inflammatory cytokines during an infection or tissue damage. Dysregulation of IL-10 is associated with a number of autoimmune diseases, and so, understanding the mechanisms by which IL-10 gene expression is regulated remains an important area of study. Macrophages represent a major source of IL-10, which is generated in response to TLR signaling as a feedback mechanism to curtail inflammatory response. In this study, we identify a signaling pathway in murine bone marrow-derived macrophages in which activation of TLR4 by LPS induces the expression of IL-10 through the sequential induction of type I IFNs followed by induction and signaling through IL-27. We demonstrate that IL-27 signaling is required for robust IL-10 induction by LPS and type I IFNs. IL-27 leads directly to transcription of IL-10 through the activation of two required transcription factors, STAT1 and STAT3, which are recruited to the IL-10 promoter. Finally, through systematic functional promoter-reporter analysis, we identify three cis elements within the proximal IL-10 promoter that play an important role in regulating transcription of IL-10 in response to IL-27.

Tandem E2F binding sites in the promoter of the p107 cell cycle regulator control p107 expression and its cellular functions

The retinoblastoma tumor suppressor (Rb) is a potent and ubiquitously expressed cell cycle regulator, but patients with a germline Rb mutation develop a very specific tumor spectrum. This surprising observation raises the possibility that mechanisms that compensate for loss of Rb function are present or activated in many cell types. In particular, p107, a protein related to Rb, has been shown to functionally overlap for loss of Rb in several cellular contexts. To investigate the mechanisms underlying this functional redundancy between Rb and p107 in vivo, we used gene targeting in embryonic stem cells to engineer point mutations in two consensus E2F binding sites in the endogenous p107 promoter. Analysis of normal and mutant cells by gene expression and chromatin immunoprecipitation assays showed that members of the Rb and E2F families directly bound these two sites. Furthermore, we found that these two E2F sites controlled both the repression of p107 in quiescent cells and also its activation in cycling cells, as well as in Rb mutant cells. Cell cycle assays further indicated that activation of p107 transcription during S phase through the two E2F binding sites was critical for controlled cell cycle progression, uncovering a specific role for p107 to slow proliferation in mammalian cells. Direct transcriptional repression of p107 by Rb and E2F family members provides a molecular mechanism for a critical negative feedback loop during cell cycle progression and tumorigenesis. These experiments also suggest novel therapeutic strategies to increase the p107 levels in tumor cells.

The retinoblastoma tumor suppressor Rb belongs to a family of cell cycle inhibitors along with the related proteins p107 and p130. Strong evidence indicates that the three family members have both specific and overlapping functions and expression patterns in mammalian cells, including in cancer cells. However, the molecular mechanisms underlying the functional differences and similarities among Rb, p107, and p130 are still poorly understood. One proposed mechanism of compensation is a negative feedback loop involving increased p107 transcription in Rb-deficient cells. To dissect the mechanisms controlling p107 expression in both wild-type and Rb-deficient cells, we have engineered inactivating point mutations into the E2F binding sites in the endogenous p107 promoter using gene targeting in mouse embryonic stem cells. Gene expression and DNA binding assays revealed that these two sites are essential for the control of p107 transcription in wild-type and Rb mutant cells, and cell cycle assays showed their importance for normal functions of p107. These experiments identify a key node in cell cycle regulatory networks.

Regulation of RB transcription in vivo by RB family members

In cancer cells, the retinoblastoma tumor suppressor RB is directly inactivated by mutation in the RB gene or functionally inhibited by abnormal activation of cyclin-dependent kinase activity. While variations in RB levels may also provide an important means of controlling RB function in both normal and cancer cells, little is known about the mechanisms regulating RB transcription. Here we show that members of the RB and E2F families bind directly to the RB promoter. To investigate how the RB/E2F pathway may regulate Rb transcription, we generated reporter mice carrying an eGFP transgene inserted into a bacterial artificial chromosome containing most of the Rb gene. Expression of eGFP largely parallels that of Rb in transgenic embryos and adult mice. Using these reporter mice and mutant alleles for Rb, p107, and p130, we found that RB family members modulate Rb transcription in specific cell populations in vivo and in culture. Interestingly, while Rb is a target of the RB/E2F pathway in mouse and human cells, Rb expression does not strictly correlate with the cell cycle status of these cells. These experiments identify novel regulatory feedback mechanisms within the RB pathway in mammalian cells.

MDM2 negatively regulates the human telomerase RNA gene promoter

Background

We have previously demonstrated that NF-Y and Sp1 interact with the human telomerase RNA (hTR) promoter and play a central role in its regulation. We have also shown that pRB activates the hTR promoter, but the mechanism of pRb directed activation is unknown. It has recently been reported that pRB induces Sp1 activity by relieving inhibition mediated by mdm2. The aim was to investigate possible roles for mdm2 in hTR promoter regulation.

Methods

Chromatin immunoprecipitation was used to determine binding of mdm2 to the hTR promoter. Transfection and luciferase assays were used to investigate mdm2 repression of the promoter activity and interaction with known transcriptional modulators.

Results

Here we show using chromatin immunoprecipitation that mdm2 specifically binds the hTR promoter in vivo. Transient co-transfection experiments using an hTR promoter luciferase reporter construct show that hTR promoter activity is inhibited by over-expression of mdm2 in 5637 bladder carcinoma cells (p53 and pRB negative, low mdm2). Titration of mdm2 was able to antagonise activation of hTR promoter activity mediated by pRB or Sp1 over-expression, although in the presence of pRB, mdm2 could not repress promoter activity below basal levels. Using an Sp1 binding site mutation construct we showed that mdm2 repression did not absolutely require Sp1 binding sites in the hTR promoter, suggesting the possibility of pRB/Sp1 independent mechanisms of repression. Finally, we show that NF-Y mediated transactivation of the hTR promoter was also suppressed by mdm2 in a dose-dependent manner.

Conclusions

These studies suggest that mdm2 may inhibit the hTR promoter by multiple mechanisms. Mdm2 may directly repress activation by both pRB and Sp1, or activation by NF-Y. Furthermore, the ability of mdm2 to interact and interfere with components of the general transcription machinery might partly explain the general repressive effect seen here. Elucidation of new regulators affecting hTR basal promoter activity in cancer cells provides a basis for future studies aimed at improving our understanding of the differential hTR expression between normal and cancer cells.

Effects of altered expression and localization of cyclophilin A on differentiation of p19 embryonic carcinoma cells

1. The retinoblastoma susceptibility gene product, p105Rb (RB), is an important regulator in the control of cell proliferation, differentiation, and apoptosis. Several cellular factors that complex with RB and exert their cellular regulatory functions have been identified, such as the RB:cyclophilin A (CypA) complex. 2. CypA is a cytoplasmic immunophilin and known for its involvement in T-cell differentiation and proliferation. Although CypA has a pivotal role in the immune response, its function in other signaling pathways is largely unknown. 3. In this study, we used a model of neuronal differentiation to demonstrate that the nuclear translocation of CypA, the appearance of hypophosphorylated RB and the enhancement of RB: CypA complex formation correlates with retinoic acid induced neuronal differentiation. 4. Inhibition of CypA expression results in repression of both the hypophosphorylated RB and the neuron-specific differentiation marker, class III beta tubulin. 5. The evidence of enriched CypA and colocalization of RB with CypA in the nucleus of primary adult sensory neurons substantiated the important event of RB-mediated neuronal differentiation of p19 EC cells.

Galectin-3 enhances cyclin D(1) promoter activity through SP1 and a cAMP-responsive element in human breast epithelial cells

Galectin-3 is a multifunctional carbohydrate-binding protein found in the nucleus, cytoplasm and the extracellular milieu. Nuclear galectin-3 expression is associated with cell proliferation, and its role in pre-mRNA splicing has been suggested. In this report, we investigated the role of galectin-3 on cyclin D(1) gene expression, a critical inducer of the cell cycle and a potential oncogene in human cancer. We found that galectin-3 induces cyclin D(1) promoter activity in human breast epithelial cells independent of cell adhesion through multiple cis-elements, including the SP1 and CRE sites. We present evidence that galectin-3 induction of the cyclin D(1) promoter may result from enhancement/stabilization of nuclear protein-DNA complex formation at the CRE site of the cyclin D(1) promoter. We also show that galectin-3 co-operates with, but does not depend on, pRb for cyclin D(1) promoter activation. The present study reveals a growth promoting activity of galectin-3 through cyclin D(1) induction, and suggests a novel function of nuclear galectin-3 in the regulation of gene transcription.

The B-domain lysine patch of pRB is required for binding to large T antigen and release of E2F by phosphorylation

Cell cycle-dependent, site-specific phosphorylation of the retinoblastoma protein, pRB, is mediated by cyclin-dependent kinases (CDKs) and regulates the binding of pRB to many proteins. We previously showed that the interaction of pRB with E2F on DNA was regulated by the accumulation of phosphate groups on pRB. Here we show that positively charged lysine residues in the B domain of pRB are necessary for the release of pRB from E2F on DNA following phosphorylation by cyclin E-cdk2 kinase. These lysine residues are also important in the binding of the simian virus 40 large T antigen (TAg) to pRB, and mutation of these lysines to arginines alters the dependency of the pRB-TAg interaction on phosphorylation of pRB.

Sp1 as G1 cell cycle phase specific transcription factor in epithelial cells

Sp1 binding sites have been identified in enhancer/promoter regions of several growth and cell cycle regulated genes, and it has been shown that Sp1 is increasingly phosphorylated in G1 phase of the cell cycle. Interactions of Sp1 with proteins involved in control of cell cycle and tumor formation have been reported. Here we show that expression of Sp1 protein predominates in the G1 phase of the cell cycle in epithelial cells. This is achieved by proteasome-dependent degradation. Inhibition of endogeneous Sp1 activity by a dominant-negative Sp1 mutant was associated with a cell cycle arrest in G1 phase, a strongly reduced expression of cyclin D1, the EGF-receptor and increased levels of p27Kip1. We have thus identified Sp1 as an important regulator of the cell cycle in G1 phase.

pRB induces Sp1 activity by relieving inhibition mediated by MDM2

pRB activates transcription by a poorly understood mechanism that involves relieving negative regulation of the promoter specificity factor Sp1. We show here that MDM2 inhibits Sp1-mediated transcription, that MDM2 binds directly to Sp1 in vitro as well as in vivo, and that MDM2 inhibits the DNA-binding activity of Sp1. Forced expression of pRB relieves MDM2-mediated repression, and interaction of pRB with the MDM2-Sp1 complex releases Sp1 and restores DNA binding. These results suggest a model in which the opposing activities of MDM2 and pRB regulate Sp1 DNA-binding and transcriptional activity.

RB regulates transcription of the p21/WAF1/CIP1 gene

We have previously shown that RB plays an important role in the maintenance of the epithelial phenotype. p21 is also involved in several terminal differentiation systems including keratinocytes. We report here that p21 is an RB target gene in epithelial cells, but not in fibroblasts where RB is unable to transactivate p21 transcriptional expression. In epithelial cells, when RB family factors were inactivated by SV40 T antigen (LT), p21 expression was strongly repressed, whereas its expression was not affected when the cells were transformed by a mutated LT leaving RB active but inactivating p53. Moreover, retransformation by RB of LT transformed epithelial cells totally restored p21 expression. By cotransfection experiments and using deletions and point mutations of the p21 promoter, we show that the minimal region required for the RB-mediated transcriptional activation maps to a GC-rich region located between -83 and -74. This region is shown to interact specifically with the transcription factor Sp1 and Sp3. Thus for the first time, we show a positive transcriptional relationship between RB and p21 in epithelial cells. Since p21 keeps RB in a hypophosphorylated state important for its transcriptional activity during differentiation, our results imply an auto-loop of regulation between RB and p21 that may be essential for the maintenance of the differentiation state. We propose that this transcriptional relationship might be necessary of their roles in cell cycle arrest and in several differentiation pathways.

Cooperation of E2F-p130 and Sp1-pRb complexes in repression of the Chinese hamster dhfr gene

In mammalian cells reiterated binding sites for Sp1 and two overlapping and inverted E2F sites at the transcription start site regulate the dhfr promoter during the cell growth cycle. Here we have examined the contributions of the dhfr Sp1 and E2F sites in the repression of dhfr gene expression. In serum-starved cells or during serum stimulation, the Chinese hamster dhfr gene was not derepressed by trichostatin A (TSA), an inhibitor of histone deacetylases (HDAC). Immunoprecipitation experiments showed that HDAC1 and hypophosphorylated retinoblastoma protein (pRb) are associated with Sp1 in serum-starved CHOC400 cells. In transfection experiments, reporter plasmids containing the reiterated dhfr Sp1 sites were stimulated 10-fold by TSA, while a promoter containing four dhfr E2F sites and a TATA box was responsive to E2F but was completely unaffected by TSA. HDAC1 did not coprecipitate with p130-E2F DNA binding complexes, the predominant E2F binding activity in cell extracts after serum starvation, suggesting that p130 imposes a TSA-insensitive state on the dhfr promoter. In support of this notion, recruitment of GAL4-p130 to a dihydrofolate reductase-GAL4 reporter rendered the promoter insensitive to TSA, while repression by GAL4-pRb was sensitive to TSA. Upon phosphorylation of pRb and p130 after serum stimulation, the Sp1-pRb and p130-E2F interactions were lost while the Sp1-HDAC1 interaction persisted into S phase. Together these studies suggest a dynamic model for the cooperation of pRb and p130 in repression of dhfr gene expression during withdrawal from the cell cycle. We propose that, during initial phases of cell cycle withdrawal, the binding of dephosphorylated pRb to Sp1-HDAC1 complexes and complexes of E2F-1 -to -3 with DP results in transient, HDAC-dependent suppression of dhfr transcription. Upon withdrawal of cells into G(0), recruitment of p130 to E2F-4-DP-1 complexes at the transcription start site results in a TSA-insensitive complex that cooperates with Sp1-HDAC-pRb complexes to stably repress dhfr promoter activity in quiescent cells.

Integration of the pRB and p53 cell cycle control pathways

Two fundamental molecular pathways, the pRB and p53 pathways, regulate cell growth and cell death. The importance of these pathways in cellular growth control is underscored by the observation that members of these pathways are found mutated in all human cancers. These two pathways have typically been studied and described independently. However, as we discuss here, recent data have revealed an intimate molecular and genetic interaction between the p53 and pRB pathways.

The H19 endodermal enhancer is required for Igf2 activation and tumor formation in experimental liver carcinogenesis

The expression of the linked but reciprocally imprinted Igf2 and H19 genes is activated in adult liver in the course of tumor development. By in situ hybridization analysis we have shown that both the Igf2 and H19 RNAs are expressed in the majority of the neoplastic nodules, and that hepatocellular carcinomas are developed in an experimental model of liver carcinogenesis. H19 is also highly activated in smaller and less distinct hyperplastic regions. The few neoplastic areas showing Igf2 but no H19 RNA display loss of the maternally inherited allele at the Igf2/H19 locus. These data are compatible with the existence of a common activation mechanism of these two genes during liver carcinogenesis and with a stronger H19 induction in the pre-neoplastic lesions. By using mice carrying a deletion of the H19 endodermal enhancer, we show that this regulatory element is necessary for the activation of the Igf2 and H19 genes upon induction of liver carcinogenesis. Furthermore, multiple sites of the H19 endodermal enhancer region become hypersensitive to DNase I when the carcinogenesis process is induced. Lastly, liver tumors developed in mice paternally inheriting the H19 enhancer deletion are found to have marked growth delays, increased frequency of apoptotic nuclei, and lack of Igf2 mRNA expression, thus indicating that this regulatory element plays a major role in the progression of liver carcinogenesis, since it is required for the activation of the anti-apoptotic Igf2 gene.

A novel Rb- and p300-binding protein inhibits transactivation by MyoD

The retinoblastoma protein (Rb) regulates both the cell cycle and tissue-specific transcription, by modulating the activity of factors that associate with its A-B and C pockets. In skeletal muscle, Rb has been reported to regulate irreversible cell cycle exit and muscle-specific transcription. To identify factors interacting with Rb in muscle cells, we utilized the yeast two-hybrid system, using the A-B and C pockets of Rb as bait. A novel protein we have designated E1A-like inhibitor of differentiation 1 (EID-1), was the predominant Rb-binding clone isolated. It is preferentially expressed in adult cardiac and skeletal muscle and encodes a 187-amino-acid protein, with a classic Rb-binding motif (LXCXE) in its C terminus. Overexpression of EID-1 in skeletal muscle inhibited tissue-specific transcription. Repression of skeletal muscle-restricted genes was mediated by a block to transactivation by MyoD independent of G(1) exit and, surprisingly, was potentiated by a mutation that prevents EID-1 binding to Rb. Inhibition of MyoD may be explained by EID-1's ability to bind and inhibit p300's histone acetylase activity, an essential MyoD coactivator. Thus, EID-1 binds both Rb and p300 and is a novel repressor of MyoD function.

Activation of the insulin-like growth factor II transcription by aflatoxin B1 induced p53 mutant 249 is caused by activation of transcription complexes; implications for a gain-of-function during the formation of hepatocellular carcinoma

Aflatoxin B1 (AFB1) induced mutation of the p53 gene at codon 249 (p53mt249) is critical during the formation of hepatocellular carcinoma (HCC) following hepatitis B virus (HBV) infection. p53mt249 markedly increases insulin-like growth factor II (IGF-II) transcription largely from promoter 4, accumulating the fetal form of IGF-II. Modulation of the transcription factor binding to IGF-II P4 by wild-type p53 and p53mt249 was identified. Wild-type p53 inhibited binding of transcription factors Sp1 and TBP on the P4 promoter, while p53mt249 enhanced the formation of transcriptional complexes through enhanced DNA-protein (Sp1 or TBP) and protein-protein (Sp1 and TBP) interactions. p53mt249 stimulates transcription factor Sp1 phosphorylation which might be a cause of increased transcription factor binding on the P4 promoter while wild-type p53 does not. Transfection of hepatocytes with p53mt249 impaired induction of apoptosis by the HBV-X protein and TNF-alpha. Therefore, the blocking of apoptosis through enhanced production of IGF-II should provide a favorable opportunity for the selection of transformed hepatocytes. These results explain the molecular basis for the genesis of HCC by p53mt249 which was found to be induced by a potent mutagen, AFB1.

Target gene specificity of E2F and pocket protein family members in living cells

E2F-mediated transcription is thought to involve binding of an E2F-pocket protein complex to promoters in the G(0) phase of the cell cycle and release of the pocket protein in late G(1), followed by release of E2F in S phase. We have tested this model by monitoring protein-DNA interactions in living cells using a formaldehyde cross-linking and immunoprecipitation assay. We find that E2F target genes are bound by distinct E2F-pocket protein complexes which change as cells progress through the cell cycle. We also find that certain E2F target gene promoters are bound by pocket proteins when such promoters are transcriptionally active. Our data indicate that the current model applies only to certain E2F target genes and suggest that Rb family members may regulate transcription in both G(0) and S phases. Finally, we find that a given promoter can be bound by one of several different E2F-pocket protein complexes at a given time in the cell cycle, suggesting that cell cycle-regulated transcription is a stochastic, not a predetermined, process.

Human papillomavirus type 16 E7 oncoprotein represses transcription of human fibronectin

The E7 oncoprotein encoded by human papillomavirus (HPV) type 16 repressed the transcription of fibronectin, a key component of the extracellular matrix. This repression, detected in several HPV-positive nontumorigenic and tumorigenic cell lines, was abolished when the Cys-X-X-Cys repeats in E7 were disrupted.

pRB is required for interferon-gamma-induction of the MHC class II abeta gene

pRB is required for IFN-gamma-induction of MHC class II in human tumor cell lines, providing a potential link between tumor suppressors and the immune system. However, other genes, such as cyclin D1, show pRB-dependency only in tumor cells, so by analogy, pRB may not be necessary for cII-regulation in normal cells. Here, we demonstrate that induction of the mouse MHC class II I-A heterodimer is normal in RB+/+ mouse embryonic fibroblasts (MEFs), but deficient in RB-/- MEFs. Inducibility is restored in RB-/- MEFs stably transfected with wild type RB cDNA or infected with an adenovirus expressing pRB. Thus, involvement of pRB in MHC class II expression is conserved in the mouse and is not an aberrant feature of tumorigenic, aneuploid, human tumor cells. Although cII genes are generally induced in a coordinate fashion, suggesting a common mechanism, we found that pRB was specifically required for induction of the Abeta, but not Aalpha or other MHC cII genes including Ebeta, Ii and H2-Malpha. Finally, IFN-gamma-induction of class II transactivator (CIITA), was pRB-independent, suggesting that pRB works downstream of this master-regulator of MHC class II expression.

Cumulative effect of phosphorylation of pRB on regulation of E2F activity

The product of the retinoblastoma susceptibility gene, pRB, is a nuclear phosphoprotein that controls cell growth by binding to and suppressing the activities of transcription factors such as the E2F family. Transactivation activity is inhibited when E2F is bound to hypophosphorylated pRB and released when pRB is phosphorylated by cyclin-dependent kinases (CDKs). To determine which of 16 potential CDK phosphorylation sites regulated the pRB-E2F interaction, mutant pRB proteins produced by site-directed mutagenesis were tested for the ability to suppress E2F-mediated transcription in a reporter chloramphenicol acetyltransferase assay. Surprisingly, no one CDK site regulated the interaction of pRB with E2F when E2F was bound to DNA. Instead, disruption of transcriptional repression resulted from accumulation of phosphate groups on the RB molecule.

The retinoblastoma susceptibility gene product/Sp1 signalling pathway is modulated by Ca2+/calmodulin kinases II and IV activity

To investigate the possible link between Ca2+ signalling and cell cycle control we analysed Ca2+/calmodulin kinases (CamK) interaction with the retinoblastoma susceptibility gene product/SP1 pathway. CamK II and IV activate c-fos transcription through a short promoter region (-99 to -53) containing the retinoblastoma control element (RCE) and a cAMP response element (CRE) related sequences. Deletion analysis revealed that the RCE is a major CamK responsive element and is sufficient to confer CamK and Ca2+ regulation to a minimal promoter. Direct interactions between SP1 and RCE were confirmed by gel shift experiments. Using transient transfection experiments, we show that CamK-dependent transcription is regulated by the retinoblastoma (Rb) susceptibility gene product and the p107 Rb related protein. However, the stimulatory effects of CamKs and Rb on c-fos are blocked by overexpression of both proteins. These effects appear to be directly mediated by SP1 as shown by the use of a Gal4/SP1 fusion proteins. In conclusion, CamK II and IV, two major Ca2+-dependent intracellular effectors, may represent a molecular link between this second messenger transduction pathway and effectors that control cell cycle progression through Rb/SP1 signalling pathway.

Neu differentiation factor stimulates phosphorylation and activation of the Sp1 transcription factor

Neu differentiation factors (NDFs), or neuregulins, are epidermal growth factor-like growth factors which bind to two tyrosine kinase receptors, ErbB-3 and ErbB-4. The transcription of several genes is regulated by neuregulins, including genes encoding specific subunits of the acetylcholine receptor at the neuromuscular junction. Here, we have examined the promoter of the acetylcholine receptor epsilon subunit and delineated a minimal CA-rich sequence which mediates transcriptional activation by NDF (NDF-response element [NRE]). Using gel mobility shift analysis with an NRE oligonucleotide, we detected two complexes that are induced by treatment with neuregulin and other growth factors and identified Sp1, a constitutively expressed zinc finger phosphoprotein, as a component of one of these complexes. Phosphatase treatment, two-dimensional gel electrophoresis, and an in-gel kinase assay indicated that Sp1 is phosphorylated by a 60-kDa kinase in response to NDF-induced signals. Moreover, Sp1 seems to act downstream of all members of the ErbB family and thus may funnel the signaling of the ErbB network into the nucleus.

Cyclin D1 associates with the TBP-associated factor TAF(II)250 to regulate Sp1-mediated transcription

We have previously shown that Sp1-mediated transcription is stimulated by Rb and repressed by cyclin D1. The stimulation of Sp1 transcriptional activity by Rb is conferred, in part, through a direct interaction with the TBP-associated factor TAF(II)250. Here we investigated the mechanism(s) through which cyclin D1 represses Sp1. We examined the ability of cyclin D1 to regulate transcription mediated by Gal4-Sp1 fusion proteins, which contain the Gal4 DNA-binding domain and Sp1 trans-activation domain(s). The domain of Sp1 sufficient to confer repression by cyclin D1 was mapped to a region important for interaction with TAF(II)110. We further demonstrate that TAF(II)250-cyclin D1 complexes can be immunoprecipitated from mammalian and baculovirus-infected insect cells and that recombinant GST-TAF(II)250 (amino acids 1-434) associates with cyclin D1 in vitro. Moreover, the overexpression of Rb or CDK4 reduced the level of TAF(II)250-cyclin D1 complex. The amino terminus of cyclin D1 (amino acids 1-100) was sufficient for association with TAF(II)250 and for repressing Sp1-mediated transcription. Taken together, the results suggest that cyclin D1 may regulate transcription by interacting directly or indirectly with TAF(II)250.

Rb inhibits the intrinsic kinase activity of TATA-binding protein-associated factor TAFII250

The retinoblastoma tumor suppressor protein, Rb, interacts directly with the largest TATA-binding protein-associated factor, TAFII250, through multiple regions in each protein. To define the potential role(s) of this interaction, we examined whether Rb could regulate the intrinsic, bipartite kinase activity of TAFII250. Here, we report that Rb is able to inhibit the kinase activity of immunopurified and gel-purified recombinant TAFII250. Rb inhibits the autophosphorylation of TAFII250 as well as its phosphorylation of the RAP74 subunit of TFIIF in a dose-responsive manner. Inhibition of TAFII250 kinase activity involves the Rb pocket (amino acids 379 to 928) but not its amino terminus. In addition, Rb appears to specifically inhibit the amino-terminal kinase domain of TAFII250 through a direct protein-protein interaction. We further demonstrate that two different tumor-derived Rb pocket mutants, C706F and Deltaex22, are functionally defective for kinase inhibition, even though they are able to bind the amino terminus of TAFII250. Our results suggest a novel mechanism of transcriptional regulation by Rb, involving direct interaction with TAFII250 and inhibition of its ability to phosphorylate itself, RAP74, and possibly other targets.

The Epstein-Barr virus immediate-early gene product, BRLF1, interacts with the retinoblastoma protein during the viral lytic cycle

Retinoblastoma protein (Rb) is a key regulator of cellular proliferation, controlling entry into G1/S in the cell cycle, largely through its action in binding the cellular transcription factor E2F, which activates genes important in DNA synthesis. Small DNA tumor viruses encode gene products which can functionally inactivate Rb, promoting cellular proliferation and viral DNA synthesis. In this study, the Epstein-Barr virus (EBV) immediate-early lytic gene product, BRLF1 (R), is shown to bind Rb in vivo, shortly after induction of the viral lytic cycle in EBV-infected Akata cells. Furthermore, the temporal kinetics of R-Rb interaction correlate with displacement of E2F1 from Rb. Mapping of the domains required for the interaction of R and Rb proteins reveals that R binds specifically to the N terminus of Rb, outside the Rb pocket, and that the first 200 amino acids of R are required for this interaction. The interaction of R and Rb may initiate cell cycle progression and facilitate viral DNA synthesis during lytic replication.

Inhibition of cyclin D1 kinase activity is associated with E2F-mediated inhibition of cyclin D1 promoter activity through E2F and Sp1

Coordinated interactions between cyclin-dependent kinases (Cdks), their target "pocket proteins" (the retinoblastoma protein [pRB], p107, and p130), the pocket protein binding E2F-DP complexes, and the Cdk inhibitors regulate orderly cell cycle progression. The cyclin D1 gene encodes a regulatory subunit of the Cdk holoenzymes, which phosphorylate the tumor suppressor pRB, leading to the release of free E2F-1. Overexpression of E2F-1 can induce apoptosis and may either promote or inhibit cellular proliferation, depending upon the cell type. In these studies overexpression of E2F-1 inhibited cyclin D1-dependent kinase activity, cyclin D1 protein levels, and promoter activity. The DNA binding domain, the pRB pocket binding region, and the amino-terminal Sp1 binding domain of E2F-1 were required for full repression of cyclin D1. Overexpression of pRB activated the cyclin D1 promoter, and a dominant interfering pRB mutant was defective in cyclin D1 promoter activation. Two regions of the cyclin D1 promoter were required for full E2F-1-dependent repression. The region proximal to the transcription initiation site at -127 bound Sp1, Sp3, and Sp4, and the distal region at -143 bound E2F-4-DP-1-p107. In contrast with E2F-1, E2F-4 induced cyclin D1 promoter activity. Differential regulation of the cyclin D1 promoter by E2F-1 and E2F-4 suggests that E2Fs may serve distinguishable functions during cell cycle progression. Inhibition of cyclin D1 abundance by E2F-1 may contribute to an autoregulatory feedback loop to reduce pRB phosphorylation and E2F-1 levels in the cell.

Induction of Sp1 in differentiating human embryonal carcinoma cells triggers transcription of the fibronectin gene

Cells of the human embryonal carcinoma line NEC14 proliferate as densely packed clusters consisting of small, polygonal stem cells and do not express a detectable level of fibronectin (FN). Upon induction of differentiation by treatment with N,N'-hexamethylene bisacetamide (HMBA), the level of FN mRNA increased steeply within 24 h and FN began to be accumulated, along with the organization of actin filaments in the cells. The FN promoter elements required for the activation were analyzed in reference to a cluster of GC boxes by using the chloramphenicol acetyltransferase (CAT) gene fused to 5' sequential-deletion derivatives of the promoter and promoters carrying base substitutions in the GC boxes. Among four GC boxes, GC boxes 2 and 3 had the greatest effect on promoter activation, and base substitutions in these GC boxes resulted in 80% reduction in promoter activity. The pattern of DNA-protein complex formation with these GC boxes changed drastically after induction of differentiation. The extract prepared from undifferentiated NEC14 cells formed fast-migrating complexes (UnD complexes), while the extract prepared from NEC14 cells treated with HMBA for 24 h formed slow-migrating complexes containing Sp1. Both complexes were formed predominantly with GC box 2. Base substitutions within the GC boxes completely abolished the formation of both UnD and Sp1 complexes. Consistent with these changes, the Sp1 level increased steeply within 24 h. Induction of Sp1 expression in NEC14 cells effectively stimulated the promoter activity of the transfected FN promoter-CAT constructs. These results indicate that activation of the FN promoter in differentiating NEC14 cells occurs by the steep induction of Sp1, which prevents an undifferentiated cell factor from binding to the Sp1 sites.

The promyelocytic leukemia gene product (PML) forms stable complexes with the retinoblastoma protein

PML is a nuclear protein with growth-suppressive properties originally identified in the context of the PML-retinoic acid receptor alpha (RAR alpha) fusion protein of acute promyelocytic leukemia. PML localizes within distinct nuclear structures, called nuclear bodies, which are disrupted by the expression of PML-RAR alpha. We report that PML colocalizes with the nonphosphorylated fraction of the retinoblastoma protein (pRB) within nuclear bodies and that pRB is delocalized by PML-RAR alpha expression. Both PML and PML-RAR alpha form complexes with the nonphosphorylated form of pRB in vivo, and they interact with the pocket region of pRB. The regions of PML and PML-RAR alpha involved in pRB binding differ; in fact, the B boxes and the C-terminal region of PML, the latter of which is not present in PML-RAR alpha, are essential for the formation of stable complexes with pRB. Functionally, PML abolishes activation of glucocorticoid receptor-regulated transcription by pRB, whereas PML-RAR alpha further increases it. Our results suggest that PML may be part of transcription-regulatory complexes and that the oncogenic potential of the PML-RAR alpha protein may derive from the alteration of PML-regulated transcription.

HUB1, a novel Krüppel type zinc finger protein, represses the human T cell leukemia virus type I long terminal repeat-mediated expression

We have shown that human T-cell leukemia virus type I (HTLV-I) gene expression is negatively regulated by the U5 repressive element (U5RE) of its long terminal repeat (LTR). To isolate factors binding to U5RE, we screened a cDNA expression library by south-western blotting with a U5RE probe. Screening 2 x10(6) clones gave a positive clone with a 3.8 kb insert encoding a novel 671 residue polypeptide, named HTLV-I U5RE binding protein 1 (HUB1), with five zinc finger domains and a Krüppel-associated box like domain; HUB1 may be related to a repressor belonging to the Krüppel type zinc finger protein. A 4.0 kb mRNA for HUB1 is ubiquitously expressed among all human tissues tested. HUB1 recognizes the TCCACCCC sequence as a core motif and exerts a strong repressive effect on HTLV-I LTR-mediated expression. A new repressive domain, named HUB1 repressive (HUR) domain, was identified, rather than the Krüppel-associated box like domain. The N-terminal region upstream of HUR domain seemed to be also indispensable to the repression. Thus, we propose that HUB1 is a new type repressor and plays an important role in the HTLV-I U5-mediated repression.

Subunit composition determines E2F DNA-binding site specificity

The product of the retinoblastoma (Rb) susceptibility gene, Rb-1, regulates the activity of a wide variety of transcription factors, such as E2F, in a cell cycle-dependent fashion. E2F is a heterodimeric transcription factor composed of two subunits each encoded by one of two related gene families, denoted E2F and DP. Five E2F genes, E2F-1 through E2F-5, and two DP genes, DP-1 and DP-2, have been isolated from mammals, and heterodimeric complexes of these proteins are expressed in most, if not all, vertebrate cells. It is not yet clear whether E2F/DP complexes regulate overlapping and/or specific cellular genes. Moreover, little is known about whether Rb regulates all or a subset of E2F-dependent genes. Using recombinant E2F, DP, and Rb proteins prepared in baculovirus-infected cells and a repetitive immunoprecipitation-PCR procedure (CASTing), we have identified consensus DNA-binding sites for E2F-1/DP-1, E2F-1/DP-2, E2F-4/DP-1, and E2F-4/DP-2 complexes as well as an Rb/E2F-1/DP-1 trimeric complex. Our data indicate that (i) E2F, DP, and Rb proteins each influence the selection of E2F-binding sites; (ii) E2F sites differ with respect to their intrinsic DNA-bending properties; (iii) E2F/DP complexes induce distinct degrees of DNA bending; and (iv) complex-specific E2F sites selected in vitro function distinctly as regulators of cell cycle-dependent transcription in vivo. These data indicate that the specific sequence of an E2F site may determine its role in transcriptional regulation and suggest that Rb/E2F complexes may regulate subsets of E2F-dependent cellular genes.

Nerve growth factor induces transcription of the p21 WAF1/CIP1 and cyclin D1 genes in PC12 cells by activating the Sp1 transcription factor

The PC12 pheochromocytoma cell line responds to nerve growth factor (NGF) by gradually exiting from the cell cycle and differentiating to a sympathetic neuronal phenotype. We have shown previously () that NGF induces the expression of the p21 WAF1/CIP1/Sdi1 (p21) cyclin-dependent kinase (Cdk) inhibitor protein and the G1 phase cyclin, cyclin D1. In this report, we show that induction is at the level of transcription and that the DNA elements in both promoters that are required for NGF-specific induction are clusters of binding sites for the Sp1 transcription factor. NGF also induced a synthetic promoter with repeated Sp1 sites linked to a core promoter, and a plasmid regulated by a chimeric transactivator in which the Gal4 DNA binding domain is fused to the Sp1 transactivation domain, indicating that this transactivation domain is regulated by NGF. Epidermal growth factor, which is a weak mitogen for PC12, failed to induce any of these promoter constructs. We consider a model in which the PC12 cell cycle is arrested as p21 accumulates and attains inhibitory levels relative to Cdk/cyclin complexes. Sustained activation of p21 expression is proposed to be a distinguishing feature of the activity of NGF that contributes to PC12 growth arrest during differentiation

Sp3 encodes multiple proteins that differ in their capacity to stimulate or repress transcription

The product of the retinoblastoma (Rb) susceptibility gene ( RB-1 ) regulates expression of a variety of growth control genes via discrete promoter elements termed retinoblastoma control elements (RCEs). We have previously shown that RCEs are bound and regulated by a common set of ubiquitously expressed nuclear proteins of 115, 95 and 80 kDa, termed retinoblastoma control proteins (RCPs). We have also previously determined that Sp3 and Sp1, two members of the Sp family of transcription factors, encode the 115 and 95 kDa RCPs respectively and that Rb stimulates Sp1/Sp3-mediated transcription in vivo. In this report we have extended these results by determining that the 80 kDa RCP arises from Sp3 mRNA via translational initiation at two internal sites located within the Sp3 trans -activation domain. Internally initiated Sp3 proteins readily bind to Sp1 binding sites in vitro yet have little or no capacity to stimulate transcription of Sp-regulated genes in vivo. Instead, these Sp3-derived proteins function as potent inhibitors of Sp1/Sp3- mediated transcription. Since cell cycle- or signal- induced expression of a variety of genes, including p21 waf1/cip1, p15 INK4B, CYP11A, mdr1 and acetyl-CoA carboxylase, have been mapped to GC-rich promoter elements that bind Sp family members, we speculate that alterations of the protein and/or DNA binding activities of internally initiated Sp3 isoforms may account in part for the regulation of such differentially expressed genes.

The human cytomegalovirus UL55 (gB) and UL75 (gH) glycoprotein ligands initiate the rapid activation of Sp1 and NF-kappaB during infection

The cellular transcription factors Sp1 and NF-kappaB were upregulated shortly after the binding of purified live or UV-inactivated human cytomegalovirus (HCMV) to the cell surface. The rapid time frame of transcription factor induction is similar to that seen in other systems in which cellular factors are induced following receptor-ligand engagement. This similarity suggested that a cellular receptor-viral ligand interaction might be involved in Sp1 and NF-kappaB activation during the earliest stages of HCMV infection. To focus on the possible role viral ligands play in initiating cellular events following infection, we first used purified viral membrane extracts to demonstrate that constituents on the membrane are responsible for cellular activation. Additionally, these studies showed, through the use of neutralizing antibodies, that the viral membrane mediators of this activation are the major envelope glycoproteins gB (UL55) and gH (UL75). To confirm these results, neutralizing anti-gB and -gH antibodies were used to block the interactions of these glycoproteins on whole purified virus with their cell surface receptors. In so doing, we found that Sp1 and NF-kappaB induction was inhibited. Lastly, through the use of purified viral gB protein and an anti-idiotypic antibody that mimics the image of the viral gH protein, it was found that the engagement of individual viral ligands with their appropriate cell surface receptors was sufficient to activate cellular Sp1 and NF-kappaB. These results support our hypothesis that HCMV glycoproteins mediate an initial signal transduction pathway which leads to the upregulation of host cell transcription factors and suggests a model wherein the orderly sequence of virus-mediated changes in cellular activation initiates with viral binding via envelope glycoproteins to the cognate cellular receptor(s).

Expression of the murine RanBP1 and Htf9-c genes is regulated from a shared bidirectional promoter during cell cycle progression

The murine Htf9-a/RanBP1 and Htf9-c genes are divergently transcribed from a bidirectional promoter. The Htf9-a gene encodes the RanBP1 protein, a major partner of the Ran GTPase. The divergently transcribed Htf9-c gene encodes a protein sharing similarity with yeast and bacterial nucleic acid-modifying enzymes. We report here that both mRNA species produced by the Htf9-associated genes are regulated during the cell cycle progression, peak in S phase and decrease during mitosis. Transient expression experiments with reporter constructs showed that cell cycle expression is controlled at the transcriptional level, because the bidirectional Htf9 promoter is down-regulated in growth-arrested cells, is activated at the G1/S transition and reaches maximal activity in S phase, though with a different efficiency for each orientation. We have delimited specific promoter regions controlling S phase activity in one or both orientations: identified elements contain recognition sites for members belonging to both the E2F and Sp1 families of transcription factors. Together, the results suggest that the sharing of the regulatory region supports co-regulation of the Htf9-a/RanBP1 and Htf9-c genes in a common window of the cell cycle.

Induction of the transcription factor Sp1 during human cytomegalovirus infection mediates upregulation of the p65 and p105/p50 NF-kappaB promoters

During human cytomegalovirus (HCMV) infection, the promoters for the classical NF-kappaB subunits (p65 and p105/p50) are transactivated. Previously, we demonstrated that the viral immediate-early (IE) proteins (IE1-72, IE2-55, and IE2-86) were involved in this upregulation. These viral factors alone, however, could not account for the entirety of the increased levels of transcription. Because one of the hallmarks of HCMV infection is the induction of cellular transcription factors, we hypothesized that one or more of these induced factors was also critical to the regulation of NF-kappaB during infection. Sp1 was one such factor that might be involved because p65 promoter activity was upregulated by Sp1 and both of the NF-kappaB subunit promoters are GC rich and contain Sp1 binding sites. Therefore, to detail the role that Sp1 plays in the regulation of NF-kappaB during infection, we initially examined Sp1 levels for changes during infection. HCMV infection resulted in increased Sp1 mRNA expression, protein levels, and DNA binding activity. Because both promoters were transactivated by Sp1, we reasoned that the upregulation of Sp1 played a role in p65 and p105/p50 promoter activity during infection. To address the specific role of Sp1 in p65 and p105/p50 promoter transactivation by HCMV, we mutated both promoters. These results demonstrated that the Sp1-specific DNA binding sites were involved in the virus-mediated transactivation. Last, to further dissect the role of HCMV in the Sp1-mediated induction of NF-kappaB, we examined the role that the viral IE genes played in Sp1 regulation. The IE gene products (IE1-72, IE2-55, and IE2-86) cooperated with Sp1 to increase promoter transactivation and physically interacted with Sp1. In addition, the IE2-86 product increased Sp1 DNA binding by possibly freeing up inactive Sp1. These data supported our hypothesis that Sp1 was involved in the upregulation of NF-kappaB during HCMV infection through the Sp1 binding sites in the p65 and p105/p50 promoters and additionally demonstrated a potential viral mechanism that might be responsible for the upregulation of Sp1 activity.

Characterization of cell-specific modulatory element in the murine ornithine decarboxylase promoter

The promoter of the murine ornithine decarboxylase (ODC) gene contains, adjacent to the TATA box, a cAMP response element (CRE)-like motif that interacts with specific nuclear proteins. Here we examine the role of this CRE-like element (CREL) in ODC promoter activation in proliferating cells. Mutations that abolished binding of nuclear proteins to CREL influenced only marginally the cAMP induction of the reporter constructs driven by 1.6 kb of the ODC promoter. Instead, these mutations altered the basal promoter function in a cell-specific manner, in that they reduced the promoter activity in CV-1 cells, but increased it in NIH/3T3, CHO and HeLa cells. Thus, depending on the cell type, the CREL motif is able to confer either repression or activation on ODC gene transcription. In contrast with 1.6 kb promoter constructs, the same mutations in the context of a shorter sequence (proximal 133 nt) reduced the promoter strength in all cell types studied. The ability of the CREL element to attenuate transcription seems to be connected with the function of some upstream regulatory elements. Differences in nuclear proteins binding to CREL, as studied by electrophoretic mobility shift assays (EMSAs), did not explain the findings on cell-type specificity in transcriptional activation, as mutations in CREL abrogated formation of specific CREL-protein complexes in all cell lines examined. The protein complexes interacting with CREL were not recognized by antibodies specific for CRE-binding proteins CREB-1 and CREB-2, or activating transcription factors ATF-1, ATF-2 and ATF-3. EMSA experiments also demonstrated co-operative interactions between the CREL motif-binding proteins and other nuclear proteins, such as Sp1, interacting with CG-rich sequences of the promoter. In conclusion, the proximal ODC promoter contains a well-conserved regulatory element, which is clearly different from the CRE/ATF element. This motif acts in concert with other distal and proximal elements in a complex cell-specific manner.

Adhesion-dependent cell cycle progression linked to the expression of cyclin D1, activation of cyclin E-cdk2, and phosphorylation of the retinoblastoma protein

Growth factors and cell anchorage jointly regulate transit through G1 in almost all cell types, but the cell cycle basis for this combined requirement remains largely uncharacterized. We show here that cell adhesion and growth factors jointly regulate the cyclin D1- and E-dependent kinases. Adhesion to substratum regulates both the induction and translation of cyclin D1 mRNA. Nonadherent cells fail to phosphorylate the retinoblastoma protein (Rb), and enforced expression of cyclin D1 rescues Rb phosphorylation and entry into S phase when G1 cells are cultured in the absence of substratum. Nonadherent cells also fail to activate the cyclin E-associated kinase, and this effect can be linked to an increased association of the cdk inhibitors, p21 and p27. These data describe a striking convergence in the cell cycle controls used by the two major signal transduction systems responsible for normal and abnormal cell growth. Taken together with our previous studies showing adhesion-dependent expression of cyclin A, they also establish the cell cycle basis for explaining the combined requirement for growth factors and the extracellular matrix in transit through the Rb checkpoint, entry into S phase, and anchorage-dependent growth.

Cell cycle-regulated association of E2F1 and Sp1 is related to their functional interaction

Because of the large number of growth-regulated genes containing binding sites for the transcription factors Sp1 and E2F and the reported ability of E2F to mediate cell cycle (growth) regulation, we studied interactions between E2F1 and Sp1. In transient transfection assays using Drosophila melanogaster SL2 cells, transfection with both Sp1 and E2F1 expression vectors resulted in greater than 85-fold activation of transcription from a hamster dihydrofolate reductase reporter construct, whereas cotransfection with either the Sp1 or E2F1 expression vector resulted in 30- or <2-fold activation, respectively. Therefore, these transcription factors act synergistically in activation of dihydrofolate reductase transcription. Transient transfection studies demonstrated that E2F1 could superactivate Sp1-dependent transcription in a promoter containing only Sp1 sites and that Sp1 could superactivate transcription of promoters through E2F sites, further demonstrating that these physically associated in Drosophila cells transfected with Sp1 and E2F1 expression vectors and in human cells, with maximal interaction detected in mid- to late G1. Additionally, E2F1 and Sp1 interact in vitro through specific domains of each protein, and the physical interaction and functional synergism appear to require the same regions. Taken together, these data demonstrate that E2F1 and Sp1 both functionally and physically interact; therefore this interaction, Sp1 and E2F1 may regulate transcription of genes containing binding sites for either or both factors.

Physical and functional sensitivity of zinc finger transcription factors to redox change

Redox regulation of DNA-binding proteins through the reversible oxidation of key cysteine sulfhydryl groups has been demonstrated to occur in vitro for a range of transcription factors. The direct redox regulation of DNA binding has not been described in vivo, possibly because most protein thiol groups are strongly buffered against oxidation by the highly reduced intracellular environment mediated by glutathione, thioredoxin, and associated pathways. For this reason, only accessible protein thiol groups with high thiol-disulfide oxidation potentials are likely to be responsive to intracellular redox changes. In this article, we demonstrate that zinc finger DNA-binding proteins, in particular members of the Sp-1 family, appear to contain such redox-sensitive -SH groups. These proteins displayed a higher sensitivity to redox regulation than other redox-responsive factors both in vitro and in vivo. This effect was reflected in the hyperoxidative repression of transcription from promoters with essential Sp-1 binding sites, including the simian virus 40 early region, glycolytic enzyme, and dihydrofolate reductase genes. Promoter analyses implicated the Sp-1 sites in this repression. Non-Sp-1-dependent redox-regulated genes including metallothionein and heme oxygenase were induced by the same hyperoxic stress. The studies demonstrate that cellular redox changes can directly regulate gene expression in vivo by determining the level of occupancy of strategically positioned GC-binding sites.

Nuclear localization of v-Abl leads to complex formation with cyclic AMP response element (CRE)-binding protein and transactivation through CRE motifs

Deregulated expression of v-abl and BCR/abl genes has been associated with myeloproliferative syndromes and myelodysplasia, both of which can progress to acute leukemia. These studies identify the localization of the oncogenic form of the abl gene product encoded by the Abelson murine leukemia virus in the nuclei of myeloid cells and the association of the v-Abl protein with the transcriptional regulator cyclic AMP response element-binding protein (CREB). We have mapped the specific domains within each of the proteins responsible for this interaction. We have shown that complex formation is a prerequisite for transcriptional potentiation of CREB. Transient overexpression of the homologous cellular protein c-Abl also results in the activation of promoters containing an intact CRE. These observations identify a novel function for v-Abl, that of a transcriptional activator that physically interacts with a transcription factor.

G10BP, an E1A-inducible negative regulator of Sp1, represses transcription of the rat fibronectin gene

Downregulation of the fibronectin (FN) gene in a rat 3Y1 derivative cell line, XhoC, transformed by the adenovirus E1A and E1B genes seems to be caused by the induction of a negative regulator, G10BP, which binds to three G-rich sequences in the promoter (T. Nakamura, T. Nakajima, S. Tsunoda, S. Nakada, K. Oda, H. Tsurui, and A. Wada, J. Virol. 66:6436-6450, 1992). These are the G10 stretch and two GC boxes consisting of the G10 stretch with one internal C residue insertion. The recognition sequences of G10BP and Sp1 (GGGCGG) overlap in these GC boxes. To analyze the mechanism of the downregulation, G10BP was purified by DNA affinity chromatography, and its molecular mass was estimated to be about 30 kDa. The promoter was modified by substituting the sequence GGGG with ATCC or CTTA in these G-rich sequences, leaving the Sp1 motif intact, and by replacing the Sp1 motif by the T stretch. Transcription of FN promoter-chloramphenicol acetyltransferase fusion genes carrying the base substitution in one or more of these G-rich sequences both in vivo and in vitro revealed that the base substitution in any G-rich sequence results in reduction of promoter activity, although the downstream GC box (GCd) plays a primary role. The addition of G10BP severely inhibited the activities of the FN promoters carrying the wild-type GCd in vitro, while the promoters carrying the mutant GCd were unaffected. The binding affinity of G10BP and Sp1 to each of the G-rich sequences, analyzed by gel shift assays, indicated that G10BP binds strongly to the GCd, moderately to the G10 stretch, and weakly to GCu, while Sp1 binds strongly to GCu, moderately to GCd, and weakly to the G10 stretch. Sp1 binding to GCd and the G10 stretch was inhibited by G10BP, while binding to GCu was unaffected. These results indicate that FN gene transcription is inhibited in XhoC cells primarily by exclusion of Sp1 binding to GCd by G10BP and that G10BP is a new class of Sp1 negative regulator.

Association of p107 with Sp1: genetically separable regions of p107 are involved in regulation of E2F- and Sp1-dependent transcription

The retinoblastoma-related protein p107 has been shown to be a regulator of the transcription factor E2F. p107 associates with E2F via its pocket region and represses E2F-dependent transcription. In this study, we provide evidence for a novel interaction between p107 and the transcription factor Sp1. We show that p107 can be found endogenously associated with Sp1 in the extracts of several different cell lines. Moreover, in transient transfection assays, expression of p107 represses Sp1-dependent transcription. This repression of Sp1-dependent transcription does not require the DNA-binding domain of Sp1. Transcription driven by a chimeric protein containing the Ga14 DNA-binding domain and the Sp1 activation domains is inhibited by p107. Interestingly, unlike the repression of E2F-dependent transcription, the repression of Sp1-dependent transcription does not depend on an intact pocket region. We show that distinct regions of p107 are involved in the control of Sp1 and E2F.

Cervical cancer: is herpes simplex virus type II a cofactor?

In many ways, cervical cancer behaves as a sexually transmitted disease. The major risk factors are multiple sexual partners and early onset of sexual activity. Although high-risk types of human papillomaviruses (HPV) play an important role in the development of nearly all cases of cervical cancer, other sexually transmitted infectious agents may be cofactors. Herpes simplex virus type 2 (HSV-2) is transmitted primarily by sexual contact and therefore has been implicated as a risk factor. Several independent studies suggest that HSV-2 infections correlate with a higher than normal incidence of cervical cancer. In contrast, other epidemiological studies have concluded that infection with HSV-2 is not a major risk factor. Two separate transforming domains have been identified within the HSV-2 genome, but continued viral gene expression apparently is not necessary for neoplastic transformation. HSV infections lead to unscheduled cellular DNA synthesis, chromosomal amplifications, and mutations. These observations suggest that HSV-2 is not a typical DNA tumor virus. It is hypothesized that persistent or abortive infections induce permanent genetic alterations that interfere with differentiation of cervical epithelium and subsequently induce abnormal proliferation. Thus, HSV-2 may be a cofactor in some but not all cases of cervical cancer.

An alternatively spliced form of the transcription factor Sp1 containing only a single glutamine-rich transactivation domain

Protein-protein interactions involving specific transactivation domains play a central role in gene transcription and its regulation. The promoter-specific transcription factor Sp1 contains two glutamine-rich transcriptional activation domains (A and B) that mediate direct interactions with the transcription factor TFIID complex associated with RNA polymerase II and synergistic effects involving multiple Sp1 molecules. In the present study, we report the complementary DNA sequence for an alternatively spliced form of mouse Sp1 (mSp1-S) that lacks one of the two glutamine-rich activation regions present in the full-length protein. Corresponding transcripts were identified in mouse tissues and cell lines, and an Sp1-related protein identical in size to that predicted for mSp1-S was detected in mouse nuclear extracts. Cotransfection analysis revealed that mSp1-S lacks appreciable activity at promoters containing a single Sp1 response element but is active when multiple Sp1 sites are present, suggesting synergistic interactions between multiple mSp1-S molecules. The absence of a single glutamine-rich domain does not fully explain the properties of the smaller protein and indicates that additional structural features account for its unique transcriptional activity. The functional implications of this alternatively spliced form of Sp1 are discussed.

Direct transcriptional repression by pRB and its reversal by specific cyclins

It was recently shown that the E2F-pRB complex is a negative transcriptional regulator. However, it was not determined whether the whole complex or pRB alone is required for repression. Here we show that pRB and the related protein p107 are capable of direct transcriptional repression independent of E2F. When fused to the DNA binding domain of GAL4, pRB or p107 represses transcription of promoters with GAL4 binding sites. Thus, E2F acts as a tether for pRB or p107 but is not actively involved in repression of other enhancers. This function of pRB maps to the pocket and is abrogated by mutation of this domain. This result suggests an intriguing model in which the pocket has a dual function, first to bind E2F and second to repress transcription directly, possibly through interaction with other proteins. We also show that direct transcriptional repression by pRB is regulated by phosphorylation. Mutations which render pRB constitutively hypophosphorylated potentiate repression, while phosphorylation induced by cyclin A or E reduces repression ninefold.

Functional interactions between the retinoblastoma (Rb) protein and Sp-family members: superactivation by Rb requires amino acids necessary for growth suppression

The transient expression of the retinoblastoma protein (Rb) regulates the transcription of a variety of growth-control genes, including c-fos, c-myc, and the gene for transforming growth factor beta 1 via discrete promoter sequences termed retinoblastoma control elements (RCE). Previous analyses have shown that Sp1 is one of three RCE-binding proteins identified in nuclear extracts and that Rb functionally interacts with Sp1 in vivo, resulting in the "superactivation" of Sp1-mediated transcription. By immunochemical and biochemical criteria, we report that an Sp1-related transcription factor, Sp3, is a second RCE-binding protein. Furthermore, in transient cotransfection assays, we report that Rb "superactivates" Sp3-mediated RCE-dependent transcription in vivo and that levels of superactivation are dependent on the trans-activator (Sp1 or Sp3) studied. Using expression vectors carrying mutated Rb cDNAs, we have identified two portions of Rb required for superactivation: (i) a portion of the Rb "pocket" (amino acids 614-839) previously determined to be required for physical interactions between Rb and transcription factors such as E2F-1 and (ii) a novel amino-terminal region (amino acids 140-202). Since both of these regions of Rb are targets of mutation in human tumors, our data suggest that superactivation of Sp1/Sp3 may play a role in Rb-mediated growth suppression and/or the induction of differentiation.

The retinoblastoma-susceptibility gene product binds directly to the human TATA-binding protein-associated factor TAFII250

RB, the protein product of the retinoblastoma tumor-suppressor gene, regulates the activity of specific transcription factors. This regulation appears to be mediated either directly through interactions with specific transcription factors or through an alternative mechanism. Here we report that stimulation of Sp1-mediated transcription by RB is partially abrogated at the nonpermissive temperature in ts13 cells. These cells contain a temperature-sensitive mutation in the TATA-binding protein-associated factor TAFII250, first identified as the cell cycle regulatory protein CCG1. The stimulation of Sp1-mediated transcription by RB in ts13 cells at the nonpermissive temperature could be restored by the introduction of wild-type human TAFII250. Furthermore, we demonstrate that RB binds directly to hTAFII250 in vitro and in vivo. These results suggest that RB can confer transcriptional regulation and possibly cell cycle control and tumor suppression through an interaction with TFIID, in particular with TAFII250.

The transcription factor E2F-1 is a downstream target of RB action

Reintroduction of RB into SAOS2 (RB-/-) cells causes a G1 arrest and characteristic cellular swelling. Coexpression of the cellular transcription factor E2F-1 could overcome these effects. The ability of E2F-1 to bind to RB was neither necessary nor sufficient for this effect, and S-phase entry was not accompanied by RB hyperphosphorylation under these conditions. Furthermore, E2F-1 could overcome the actions of a nonphosphorylatable but otherwise intact RB mutant. These data, together with the fact that RB binds to E2F-1 in vivo, suggest that E2F-1 is a downstream target of RB action. Mutational analysis showed that the ability of E2F-1 to bind to DNA was necessary and sufficient to block the formation of large cells by RB, whereas the ability to induce S-phase entry required a functional transactivation domain as well. Thus, the induction of a G1 arrest and the formation of large cells by RB in these cells can be genetically dissociated. Furthermore, the ability of the E2F-1 DNA-binding domain alone to block one manifestation of RB action is consistent with the notion that RB-E2F complexes actively repress transcription upon binding to certain E2F-responsive promoters. In keeping with this view, we show here that coproduction of an E2F1 mutant capable of binding to DNA, yet unable to transactivate, is sufficient to block RB-mediated transcriptional repression.

Transcription of the E2F-1 gene is rendered cell cycle dependent by E2F DNA-binding sites within its promoter

The cell cycle-regulatory transcription factor E2F-1 is regulated by interactions with proteins such as the retinoblastoma gene product and by cell cycle-dependent alterations in E2F-1 mRNA abundance. To better understand this latter phenomenon, we have isolated the human E2F-1 promoter. The human E2F-1 promoter, fused to a luciferase cDNA, gave rise to cell cycle-dependent luciferase activity upon transfection into mammalian cells in a manner which paralleled previously reported changes in E2F-1 mRNA abundance. The E2F-1 promoter contains four potential E2F-binding sites organized as two imperfect palindromes. Gel shift and transactivation studies suggested that these sites can bind to E2F in vitro and in vivo. Mutation of the two E2F palindromes abolished the cell cycle dependence of the E2F-1 promoter. Thus, E2F-1 appears to be regulated at the level of transcription, and this regulation is due, at least in part, to binding of one or more E2F family members to the E2F-1 promoter.