The retinoblastoma gene product RB stimulates Sp1-mediated transcription by liberating Sp1 from a negative regulator

Dual tandem promoter elements containing CCAC-like motifs from the tetrodotoxin-resistant voltage-sensitive Na+ channel (rSkM2) gene can independently drive muscle-specific transcription in L6 cells

cis-Elements in the -129/+124 promoter segment of the rat tetrodotoxin-resistant voltage-gated sodium channel (rSkM2) gene that are responsible for reporter gene expression in cultured muscle cells were identified by deletion and scanning mutations. Nested 5' deletion constructs, assayed in L6 myotubes and NIH3T3 cells, revealed that the minimum promoter allowing muscle-specific expression is contained within the -57 to +1 segment relative to the major transcription initiation site. In the context of the -129/+1 construct, however, scanning mutations in the -69/+1 segment failed to identify any critical promoter elements. In contrast, identical mutations in a minimal promoter (-57/+124) showed that all regions except -29/-20 are essential for expression, especially the -57/-40 segment, consistent with the 5' deletion analysis. Further experiments showed that the distal (-129/-58) and proximal promoter (-57/+1) elements can independently drive reporter expression in L6 myotubes, but not in NIH3T3 fibroblasts. This pair of elements is similar in sequence and contains Sp1 sites (CCGCCC), CCAC-like motifs, but no E-boxes or MEF-2 sites. The two segments form similarly migrating complexes with L6 myotube nuclear extracts in gel-shift assays. Critical elements within the distal promoter element were defined by 10 base pair scanning mutations in the -119 to -60 region in the context of the -129/+1 segment containing a mutated -59/-50 segment that inactivates the proximal promoter. Nucleotides in the -119/-90 region, especially -109/-100, were the most important regions for distal promoter function. We conclude that the -129/+1 segment contains two tandem promoter elements, each of which can independently drive muscle-specific transcription. Supershifts with antibodies to Sp1 and myocyte nuclear factor (MNF) implicate the involvement of Sp1, MNF, and other novel factors in the transcriptional regulation of rSkM2 gene expression.

Mathematical model of a telomerase transcriptional regulatory network developed by cell-based screening: analysis of inhibitor effects and telomerase expression mechanisms

Cancer cells depend on transcription of telomerase reverse transcriptase (TERT). Many transcription factors affect TERT, though regulation occurs in context of a broader network. Network effects on telomerase regulation have not been investigated, though deeper understanding of TERT transcription requires a systems view. However, control over individual interactions in complex networks is not easily achievable. Mathematical modelling provides an attractive approach for analysis of complex systems and some models may prove useful in systems pharmacology approaches to drug discovery. In this report, we used transfection screening to test interactions among 14 TERT regulatory transcription factors and their respective promoters in ovarian cancer cells. The results were used to generate a network model of TERT transcription and to implement a dynamic Boolean model whose steady states were analysed. Modelled effects of signal transduction inhibitors successfully predicted TERT repression by Src-family inhibitor SU6656 and lack of repression by ERK inhibitor FR180204, results confirmed by RT-QPCR analysis of endogenous TERT expression in treated cells. Modelled effects of GSK3 inhibitor 6-bromoindirubin-3′-oxime (BIO) predicted unstable TERT repression dependent on noise and expression of JUN, corresponding with observations from a previous study. MYC expression is critical in TERT activation in the model, consistent with its well known function in endogenous TERT regulation. Loss of MYC caused complete TERT suppression in our model, substantially rescued only by co-suppression of AR. Interestingly expression was easily rescued under modelled Ets-factor gain of function, as occurs in TERT promoter mutation. RNAi targeting AR, JUN, MXD1, SP3, or TP53, showed that AR suppression does rescue endogenous TERT expression following MYC knockdown in these cells and SP3 or TP53 siRNA also cause partial recovery. The model therefore successfully predicted several aspects of TERT regulation including previously unknown mechanisms. An extrapolation suggests that a dominant stimulatory system may programme TERT for transcriptional stability.

Tumour cells acquire the ability to divide and multiply indefinitely whereas normal cells can undergo only a limited number of divisions. The switch to immortalisation of the tumour cell is dependent on maintaining the integrity of telomere DNA which forms chromosome ends and is achieved through activation of the telomerase enzyme by turning on synthesis of the TERT gene, which is usually silenced in normal cells. Suppressing telomerase is toxic to cancer cells and it is widely believed that understanding TERT regulation could lead to potential cancer therapies. Previous studies have identified many of the factors which individually contribute to activate or repress TERT levels in cancer cells. However, transcription factors do not behave in isolation in cells, but rather as a complex co-operative network displaying inter-regulation. Therefore, full understanding of TERT regulation will require a broader view of the transcriptional network. In this paper we take a computational modelling approach to study TERT regulation at the network level. We tested interactions between 14 TERT-regulatory factors in an ovarian cancer cell line using a screening approach and developed a model to analyse which network interventions were able to silence TERT.

Immediate-early gene regulation by interplay between different post-translational modifications on human histone H3

In mammalian cells, induction of immediate-early (IE) gene transcription occurs concomitantly with histone H3 phosphorylation on Ser 10 and is catalyzed by mitogen-activated protein kinases (MAPKs). Histone H3 is an evolutionarily conserved protein located in the core of the nucleosome, along with histones H2A, H2B, and H4. The N-terminal tails of histones protrude outside the chromatin structure and are accessible to various enzymes for post-translational modifications (PTM). Phosphorylation, O-GlcNAc modification, and their interplay often induce functional changes, but it is very difficult to monitor dynamic structural and functional changes in vivo. To get started in this complex task, computer-assisted studies are useful to predict the range in which those dynamic structural and functional changes may occur. As an illustration, we propose blocking of phosphorylation by O-GlcNAc modification on Ser 10, which may result in gene silencing in the presence of methylated Lys 9. Thus, alternate phosphorylation and O-GlcNAc modification on Ser 10 in the histone H3 protein may provide an on/off switch to regulate expression of IE genes.

Elevated expression of vascular endothelial growth factor correlates with increased angiogenesis and decreased progression-free survival among patients with low-grade neuroendocrine tumors

BACKGROUND

Vascular endothelial growth factor (VEGF) is a critical proangiogenic factor in solid tumors. However, its expression and role in human neuroendocrine tumor development and progression remains unclear.

METHODS

Using immunohistochemistry, VEGF and Sp1 expression patterns were investigated in 50 cases of human gastrointestinal neuroendocrine tumor having various clinicopathologic characteristics.

RESULTS

It was found that strong VEGF expression was detected in tumor cells, whereas no or very weak VEGF expression was detected in stromal cells surrounding or within the tumors. The levels of VEGF expression directly correlated with the expression levels of Sp1 and microvessel density. Strong, weak, and negative VEGF expression was observed in 32%, 54%, and 14% of cases, respectively. Compared with the group with negative VEGF expression, VEGF (weak/strong) expression was associated with metastasis (14% versus 58%; P = .03). The median progression-free survival (PFS) durations of patients with strong and weak VEGF expression were 29 months and 81 months, respectively. With a median follow-up duration of 50 months, the median PFS duration for the group with negative VEGF expression has not been reached. Compared with the log-rank test, VEGF expression was associated with poor PFS (P = .02). Using in vitro and in vivo models, human carcinoid cell lines were treated with bevacizumab, a monoclonal antibody targeting VEGF. Bevacizumab did not inhibit the growth of carcinoid cells in vitro but significantly reduced tumor angiogenesis and impaired tumor growth in animals.

CONCLUSIONS

The data suggest that overexpression of VEGF promotes the growth of human neuroendocrine tumors in part through up-regulation of angiogenesis.

Negative regulation of hormone signaling by RIP140

Receptor interacting protein (RIP) 140 is a negative transcriptional regulator of nuclear hormone receptors which is required for the maintenance of energy homeostasis and ovulation. Despite its recruitment by agonist-liganded receptors, this protein exhibits a strong repressive activity which was initially attributed to competition with coactivator binding on nuclear receptors. However, RIP140 also exerts active repression implicating the Carboxyl-terminal binding proteins (CtBPs) and histone deacetylases (HDACs). We recently demonstrated that the Carboxyl-terminal region of the molecule contains two additional silencing domains which require post-translational modifications to be fully active. In human breast cancer cells, RIP140 expression is up-regulated at the transcriptional level by various ligands of nuclear receptors. We have recently cloned the human RIP140 gene and defined the mechanism of its regulation by estrogens. In order to better characterize the role of RIP140 in hormone signaling, we have studied its interaction with the androgen receptor and demonstrated its ability to repress transcriptional regulation by androgens. RIP140 also inhibits transactivation by estrogen receptor-related receptors (ERRalpha, beta and gamma) on natural or artificial reporter genes containing different types of response elements. Surprisingly, RIP140 positively regulates ERR transactivation when the receptors are recruited to target promoters through interaction with the Sp1 transcription factor and this effect could involve titration of histone deacetylases. Altogether, these results underline that transcriptional regulation of hormone signaling by the cofactor RIP140 involves complex mechanisms relying on multiple domains and partners.

Genetic Variation and the Mitogen-Activated Protein Kinase (MAPK) Signaling Pathway

Non-synonymous single nucleotide polymorphisms (nsSNPs) are known to alter protein function, contributing to disease susceptibility. This report explores the nature of nsSNPs in the gene products of the highly conserved mitogen-activated protein kinase (MAPK) signaling pathways already implicated in cancer development. MAPK signaling pathways regulate cellular processes such as proliferation, differentiation, apoptosis, and survival mediated through interconnected signaling cascades. Using the dbSNP database, we have identified 25 nsSNPs in 17 out of 98 MAPK genes studied. Computational algorithms were used to predict whether the amino acid substitutions were evolutionarily tolerated, or affected putative functional units such as phosphorylation sites, protein motifs and domains. This study predicts that 36% of nsSNPs are likely to have functional consequences, based on evolutionary conservation analysis, and 36% based on phosphorylation prediction analysis. All such nsSNPs represent potentially functional and disease-causing/modifying alleles. More interestingly, the epistatic relationships discussed in this report represent potential synergistic/ antagonistic/additive effects of nsSNP combinations found within the same protein, or within members of the same protein complex and cascades. This strategy can effectively determine which nsSNPs potentially alter protein function, and can be utilized to study the genetic architecture and disease association of other biological protein complexes and networks.

Prostaglandin E2 enhances neurotrophin-4 production via EP3 receptor in human keratinocytes

Atopic dermatitis is characterized by increased skin innervation. The expression of neurotrophin-4 is enhanced in the epidermal keratinocytes of lesions with atopic dermatitis and may be related to hyperinnervation in these lesions. Prostaglandin E(2) (PGE(2)) levels are increased in lesions with atopic dermatitis; thus, PGE(2) may be involved in the development of this disease. We examined the in vitro effects of PGE(2) on neurotrophin-4 production in human keratinocytes. PGE(2) and EP1/EP3 agonist sulprostone increased neurotrophin-4 secretion and mRNA levels without altering its mRNA stability. Antisense Sp1 oligodeoxynucleotide and Sp1 inhibitor mithramycin A suppressed PGE(2) and sulprostone-induced neurotrophin-4 expression, indicating the requirement for Sp1 for expression. PGE(2) or sulprostone markedly enhanced the phosphorylation, DNA binding, and transcriptional activity of Sp1 and modestly increased Sp1 mRNA and protein levels. PGE(2) or sulprostone induced the membrane translocation of protein kinase Calpha and the phosphorylation of extracellular signal-regulated kinase (ERK). PGE(2)-induced increases in neurotrophin-4 expression, Sp1 transcriptional and DNA-binding activity, Sp1 mRNA and protein levels, and ERK phosphorylation were suppressed by antisense EP3 oligodeoxynucleotide, inhibitors of phosphatidylinositol-specific phospholipase C, conventional protein kinase C, and mitogen-activated protein kinase/ERK kinase 1 (MEK1). These results suggest that PGE(2) enhances neurotrophin-4 production by activating Sp1 via the EP3/phosphatidylinositol-specific phospholipase C/protein kinase Calpha/MEK1/ERK pathway. PGE(2) may promote innervation in skin lesions with atopic dermatitis via the induction of neurotrophin-4.

17beta-estradiol enhances heparin-binding epidermal growth factor-like growth factor production in human keratinocytes

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) enhances reepithelialization in wounds. Estrogen is known to promote cutaneous wound repair. We examined the in vitro effects of 17beta-estradiol (E2) on HB-EGF production by human keratinocytes. E2 or membrane-impermeable BSA-conjugated E2 (E2-BSA) increased HB-EGF secretion, mRNA level, and promoter activity in keratinocytes. E2 or E2-BSA enhanced in vitro wound closure in keratinocytes, and the closure was suppressed by anti-HB-EGF antibody. Activator protein-1 (AP-1) and specificity protein 1 (Sp1) sites on HB-EGF promoter were responsible for the E2- or E2-BSA-induced transactivation. Antisense oligonucleotides against c-Fos, c-Jun, and Sp1 blocked E2- or E2-BSA-induced HB-EGF transactivation. E2 or E2-BSA enhanced DNA binding and transcriptional activity of AP-1 and generated c-Fos/c-Jun heterodimers by inducing c-Fos expression. E2 or E2-BSA enhanced DNA binding and transcriptional activity of Sp1 in parallel with the enhancement of Sp1 phosphorylation. These effects of E2 or E2-BSA were not blocked by the nuclear estrogen receptor antagonist ICI-182,780 or anti-estrogen receptor-alpha or -beta antibodies but were blocked by inhibitors of G protein, phosphatidylinositol-specific PLC, PKC-alpha, and MEK1. These results suggest that E2 or E2-BSA may enhance HB-EGF production via activation of AP-1 and Sp1. These effects of E2 or E2-BSA may be dependent on membrane G protein-coupled receptors different from nuclear estrogen receptors and on the receptor-mediated activities of phosphatidylinositol-specific PLC, PKC-alpha, and MEK1. E2 may enhance wound reepithelialization by promoting HB-EGF production in keratinocytes.

Interplay between HIV-1 Vpr and Sp1 modulates p21(WAF1) gene expression in human astrocytes

The Vpr (viral protein R) of human immunodeficiency virus, type 1, which is expressed during the late stage of the viral infection, has received special attention because of its ability to control transcription of the human immunodeficiency virus, type 1, long terminal repeat and to influence cell cycle progression. Here we demonstrate that Vpr has the ability to regulate transcription of the cyclin-dependent kinase inhibitor, p21(WAF1) (p21), one of the key regulators of the cell cycle, in human astrocytic cells. The results from transcription assays demonstrated that Vpr augments promoter activity of p21 through the GC-rich region located between nucleotides -84 and -74 with respect to the +1 transcription start site. Activation of p21 by Vpr required cooperativity of Sp1, which binds to the DNA sequence spanning -84 to -74. Results from bandshift assay revealed an increased level of Sp1 DNA binding activity in the presence of Vpr. Furthermore, Vpr was able to associate with Sp1 via the zinc finger domain located in the C-terminal region of Sp1. Functional studies revealed that the cooperativity between Vpr and Sp1 requires the zinc finger domain at the C terminus and the glutamine-rich domain at the N terminus of Sp1. Expression of p53 further enhanced the level of Vpr-Sp1-mediated transcription activation of p21 through the sequence spanning -84 to -74 and increased the DNA binding activity of Sp1 in the presence of Vpr. Results from glutathione S-transferase pull-down assay showed the association of Vpr with p53 in extracts containing Sp1. Altogether, the outcome of our functional and binding studies suggested that the physical interaction of Vpr with Sp1 and p53 could modulate transcriptional activity of p21.

The expression of retinoblastoma and Sp1 is increased by low concentrations of cyclin-dependent kinase inhibitors

We examined the effect of suboptimal concentrations of cyclin-dependent kinase inhibitors, which do not interfere with cell proliferation, on retinoblastoma expression in hamster (Chinese hamster ovary K1) and human (K562 and HeLa) cells. To achieve this, we used the chemical inhibitors roscovitine and olomoucine (which inhibit CDK2 preferentially), UCN-01 (which also inhibits CDK4/6) and p21 (as an intrinsic inhibitor). All chemical inhibitors and overexpression of p21 strongly induced retinoblastoma protein expression. UCN-01-mediated retinoblastoma expression was caused by an increase in both the levels of retinoblastoma mRNA and the stability of the protein. The expression of the transcription factor Sp1, a retinoblastoma-interacting protein, was also enhanced by all the cyclin-dependent kinase inhibitors tested. However, Sp1 expression was caused by an increase in the levels of Sp1 mRNA without modification in the stability of the protein. By using luciferase experiments, the transcriptional activation of both retinoblastoma and Sp1 promoters by UCN-01 was confirmed. Bisindolylmaleimide I, at concentrations causing a similar or higher inhibition of protein kinase C than UCN-01, provoked a lower activation of retinoblastoma and Sp1 expression. Finally, the effects of cyclin-dependent kinase inhibitors on dihydrofolate reductase gene expression were evaluated. Treatment with UCN-01 increased cellular dihydrofolate reductase mRNA levels, and dihydrofolate reductase enzymatic activity was enhanced by UCN-01, roscovitine, olomoucine and p21, in transient transfection experiments. These results support a mechanism for the self-regulation of retinoblastoma expression, and point to the need to establish the appropriate dose of cyclin-dependent kinase inhibitors as antiproliferative agents in anticancer treatments.

Differential patterns of human cytomegalovirus gene expression in various T-cell lines carrying human T-cell leukemia-lymphoma virus type I: role of Tax-activated cellular transcription factors

Replication of human cytomegalovirus (HCMV) was investigated in various T-cell lines expressing the tax gene product of human T-cell leukemia-lymphoma virus type I (HTLV-I). Differential patterns of HCMV replication were found in HTLV-I-carrying cell lines. HCMV gene expression was restricted to the immediate-early genes in MT-2 and MT-4 cells, whereas full replication cycle of the virus was observed in C8166-45 cells. Productive HCMV infection induced a cytopathic effect resulting in the lysis of infected cells. The results of electrophoretic mobility shift assay (EMSA) showed high levels of NF-kappaB-, CREB/ATF-1-, and SRF-specific DNA binding activity in all Tax-positive cell lines. In contrast, SP1 activity could be detected only in C8166-45 cells. Using an inducible system (Jurkat cell line JPX-9), a dramatic increase in NF-kappaB, CREB/ATF-1, SRF, and SP1 binding activity, as well as productive HCMV infection, were observed upon Tax expression. Overexpression of SP1 in MT-2 and MT-4 cells converted HCMV infection from an abortive to a productive one. These data suggest that the stimulatory effect of Tax protein on HCMV in T cells is accomplished through at least five host-related transcription factor pathways. The results of this study provide possible mechanisms whereby HCMV infections might imply suppression of adult T-cell leukemia.

Role for retinoblastoma protein family members in UV-enhanced expression from the human cytomegalovirus immediate early promoters

The expression from a reporter construct driven by a cytomegalovirus (CMV) immediate early (IE) promoter is strongly inducible by UV in human fibroblasts. This response is induced at lower UV fluences in transcription-coupled repair (TCR)-deficient fibroblasts compared with normal fibroblasts and is absent in their simian virus 40-transformed counterparts. In this study we demonstrate that expression of human papilloma virus (HPV) E7 (but not of HPV E6) can attenuate UV-induced expression from the human CMV-IE-driven reporter construct in human fibroblasts. Furthermore, UV-induced expression from the reporter construct appears impaired in murine fibroblasts harboring inactivating mutations in the retinoblastoma (Rb) gene family members p107 and pRb but not in fibroblasts harboring such mutations in the p53 gene. Taken together, these data suggest that one or more members of the pRb family (but not p53) play an essential role in mediating UV-induced expression from the CMV-IE promoter. In this study we report normal UV-upregulation of reporter expression in xeroderma pigmentosum (XP) group E fibroblasts, consistent with normal TCR. Because XP-E cells deficient in the p48 subunit of the damaged DNA-binding protein are impaired in E2F-1-activated transcription, these results also suggest that the (pRb-regulated) transcription factor E2F-1 does not play an essential role in UV-enhanced expression from the CMV-IE promoter.

NF-kappa B-dependent induction of cyclin D1 by retinoblastoma protein (pRB) family proteins and tumor-derived pRB mutants

The retinoblastoma protein (pRB) and its homologues, p107 and p130, prevent cell cycle progression from G(0)/G(1) to S phase by forming complexes with E2F transcription factors. Upon phosphorylation by G(1) cyclin-cyclin-dependent kinase (Cdk) complexes such as cyclin D1-Cdk4/6 and cyclin E-Cdk2, they lose the ability to bind E2F, and cells are thereby allowed to progress into S phase. Functional loss of one or more of the pRB family members, as a result of genetic mutation or deregulated phosphorylation, is considered to be an essential prerequisite for cellular transformation. In this study, we found that pRB family proteins have the ability to stimulate cyclin D1 transcription by activation of the NF-kappaB transcription factor. The cyclin D1-inducing activity of pRB is abolished by adenovirus E1A oncoprotein but not by the deletion of the A-box, the B-box, or the C-terminal region of the pocket, indicating that multiple pocket sequences are independently involved in cyclin D1 activation. Intriguingly, tumor-derived pRB pocket mutants retain the cyclin D1-inducing activity. Our results reveal a novel role of pRB family proteins as potential activators of NF-kappaB and inducers of G(1) cyclin. Certain pRB pocket mutants may give rise to a cellular situation in which deregulated E2F and cyclin D1 cooperatively promote abnormal cell proliferation.

Interaction of the retinoblastoma gene product, RB, with cyclophilin A negatively affects cyclosporin-inhibited NFAT signaling

The retinoblastoma susceptibility gene product, p105Rb (RB), is generally believed to be an important regulator in the control of cell growth, differentiation, and apoptosis. Several cellular factors that form complexes with RB and exert their cellular regulatory functions have been identified, such as the newly identified RB:cyclophilin A (CypA) complex. The physical interactions between RB and CypA were demonstrated by glutathione S-transferase affinity matrix binding assays and immunoprecipitation, followed by Western blot analyses. The N-terminal region of CypA mediated the interaction with RB, whereas the region upstream of the A-pocket of RB was required for binding to CypA. Ectopic expression of RB into Jurkat cells partially blocks the function of cyclosporin (CsA) to inhibit nuclear factor for activation of T cell (NFAT) activation by phorbol ester (PMA) plus ionomycin A (IA), suggesting that RB may prevent CsA inhibition of T lymphocyte activation. These results are further evidenced by the effect of RB on both calcineurin (CN) and NFAT binding activity in vitro, suggesting that the interaction of RB with CypA interferes with the CsA:CypA complex and blocks CsA-inhibited CN activity. These data reveal the functional link between RB and CypA and their involvement in T cell activation signaling.

Regulation of MSV and WDV virion-sense promoters by WDV nonstructural proteins: a role for their retinoblastoma protein-binding motifs

In this work we demonstrate that wheat dwarf virus (WDV) RepA can activate WDV and maize streak virus (MSV) virion (V)-sense expression in plant tissues. Rep alone does not have any effect on the silent WDV promoter and it represses the basal MSV promoter activity. MSV promoter activation by RepA depends on an intact RepA retinoblastoma protein (RB)-binding domain. Promoter repression by Rep also depends on this domain to some extent. Mutation of the RepA RB-binding domain has no effect on WDV promoter activation. The WDV promoter contains two sites that fit the consensus E2F-binding site. One, WDV1, binds human E2F-1 in one-hybrid assays in yeast. It also binds specifically to maize and wheat proteins in vitro and, when fused to a minimal 35S promoter, it confers responsiveness to RepA only when the RepA RB-binding domain and the WDV1 site are intact. In the whole WDV V-sense promoter context, mutations of this sequence have no effect, suggesting that additional sequences are important for RepA-mediated promoter activation.

Transcriptional regulation of the epiregulin gene in the rat ovary

Ovarian follicular development is initiated by FSH secreted from the pituitary gland. The FSH-induced follicular development involves granulosa cell proliferation and differentiation. We demonstrated that a growth factor of epidermal growth factor (EGF) family epiregulin was rapidly induced in the primary culture of rat ovarian granulosa cells by FSH within 1 h. Epiregulin gene expression was also observed in granulosa cells of antral ovarian follicles from pregnant mare's serum gonadotropin-primed rats in vivo. To analyze the regulation of gene expression of epiregulin, we isolated and characterized the rat epiregulin gene of 22.1 kb, including 3.8 kb of 5'-upstream region as well as all five exons and four introns. We determined the transcriptional start site of rat epiregulin gene by primer extension analysis and then characterized the upstream promoter region of the gene. By using a luciferase reporter system, deletion and mutation analyses of rat epiregulin gene promoter region revealed that 125 bp upstream of transcriptional start site was essential, and that two CT boxes and one GT box within this region were important for the gene expression. We also demonstrated by EMSAs that Sp1/Sp3 proteins were involved in the epiregulin gene expression via the upstream sequence. Involvement of Sp1/Sp3 was also demonstrated that transfection of Sp1 or Sp3 expression plasmids dramatically increased the epiregulin gene promoter activities about 90- or 7.9-fold, respectively, in Drosophila SL2 cells that lack endogenous Sp family proteins. Such an increase in the promoter activity was also observed in mammalian cells when NIH-3T3 cells were used. In conclusion, we demonstrated here for the first time that EGF-type growth factor epiregulin is rapidly and strongly induced in the ovarian granulosa cells by FSH stimulation, and that two CT boxes and one GT box present in the upstream region are essential for the promoter activity of rat epiregulin. We also demonstrated that Sp family members play crucial roles in the epiregulin promoter activity through the CT boxes. The restricted and hormonally regulated expression of epiregulin in the rat ovarian granulosa cells may correspond to the physiological relevance of this peptide growth factor to the FSH-induced ovarian follicular growth and maturation.

Regulation of the activity of Sp1-related transcription factors

The initiation of transcription is accomplished via interactions of many different proteins with common and gene-specific regulatory motifs. Clearly, sequence-specific transcription factors play a crucial role in the specificity of transcription initiation. A group of sequence-specific DNA-binding proteins, related to the transcription factor Sp1, has been implicated in the regulation of many different genes, since binding sites for these transcription factors (GC/GT boxes) are a recurrent motif in regulatory sequences such as promoters, enhancers and CpG islands of these genes. The simultaneous occurrence of several homologous GC/GT box-binding factors precludes a straightforward deduction of their role in transcriptional regulation. In this review, we focus on the connection between functional specificity and biochemical properties including glycosylation, phosphorylation and acetylation of Sp1-related factors.

Identification of two Sp1 phosphorylation sites for p42/p44 mitogen-activated protein kinases: their implication in vascular endothelial growth factor gene transcription

Sp1 regulates activation of many genes implicated in tumor growth and cell cycle progression. We have previously demonstrated its implication in the up-regulation of vascular endothelial growth factor (VEGF) gene transcription following growth factor stimulation of quiescent cells, a situation where p42/p44 mitogen-activate protein kinase (MAPK) activity is dramatically increased. Here we show that p42/p44 MAPK directly phosphorylates Sp1 on threonines 453 and 739 both in vitro and in vivo. Mutation of these sites to alanines decreases by half the MAPK-dependent transcriptional activity of Sp1, in the context of the VEGF promoter, in SL2 Drosophila cells devoid of the endogenous Sp1 protein. Moreover, inducible overexpression of the (T453A,T739A) Sp1 double mutant compromises MAPK-driven VEGF mRNA transcription in fibroblasts. These results highlight Sp1 as a key molecular link between elevated activation of the Ras >> p42/p44MAPK signaling pathway and increased VEGF expression, two major steps deregulated in tumor cells.

Ubiquitin (UbC) expression in muscle cells is increased by glucocorticoids through a mechanism involving Sp1 and MEK1

The muscle protein catabolism present in rats with insulin-dependent diabetes and other catabolic conditions is generally associated with increased glucocorticoid production and mRNAs encoding components of the ubiquitin-proteasome system. The mechanisms that increase ubiquitin (UbC) expression have not been identified. We studied the regulation of UbC expression in L6 muscle cells because dexamethasone stimulates the transcription of this gene and others encoding components of the ubiquitin-proteasome pathway. Results of in vivo genomic DNA footprinting experiments indicate that a protein(s) binds to Sp1 sites approximately 50 bp upstream from the UbC transcription start site; dexamethasone changes the methylation pattern at these sites. Sp1 binds to DNA probes corresponding to the rat or human UbC promoter, and treating cells with dexamethasone increases this binding. Deletion and mutation analyses of the rat and human UbC promoters are consistent with an important role of Sp1 in UbC induction by glucocorticoids. Dexamethasone-induced ubiquitin expression is blocked by mithramycin, an inhibitor of Sp1 binding. UO126, a pharmacologic inhibitor of MEK1, also blocks UbC transcriptional activation by dexamethasone; L6 cells transfected to express constitutively active MEK1 exhibit increased UbC promoter activity. Thus, glucocorticoids increase UbC expression in muscle cells by a novel transcriptional mechanism involving Sp1 and MEK1.

In vivo footprinting of the human 11beta-hydroxysteroid dehydrogenase type 2 promoter: evidence for cell-specific regulation by Sp1 and Sp3

11beta-Hydroxysteroid dehydrogenase type 2 is selectively expressed in aldosterone target tissues, where it confers aldosterone selectivity for the mineralocorticoid receptor by inactivating 11beta-hydroxyglucocorticoids with a high affinity for the mineralocorticoid receptor. The present investigation aimed to elucidate the mechanisms accounting for the rigorous control of the HSD11B2 gene in humans. Using dimethyl sulfate in vivo footprinting via ligation-mediated PCR, we identified potentially important regions for HSD11B2 regulation in human cell lines: two GC-rich regions in the first exon (I and II) and two upstream elements (III and IV). The footprints suggest a correlation between the extent of in vivo protein occupancy at three of these regions (I, II, and III) and the rate of HSD11B2 transcription in cells with high (SW620), intermediate (HCD, MCF-7, and HK-2), or low HSD11B2 mRNA levels (SUT). Moreover, gel shift assays with nuclear extracts from these cell lines revealed that decreased HSD11B2 expression is related to a decreased binding activity with oligonucleotides containing the putative regulatory elements. Antibody supershifts identified the majority of the components of the binding complexes as the transcription factors Sp1 and Sp3. Finally, transient transfections with various deletion mutant reporters define positive regulatory elements that might account for basal and selective expression of 11beta-hydroxysteroid dehydrogenase type 2.

Sp1 transcriptional activity is up-regulated by phosphatase 2A in dividing T lymphocytes

We have followed Sp1 expression in primary human T lymphocytes induced, via CD2 plus CD28 costimulation, to sustained proliferation and subsequent return to quiescence. Binding of Sp1 to wheat germ agglutinin lectin was not modified following activation, indicating that the overall glycosylation of the protein was unchanged. Sp1 underwent, instead, a major dephosphorylation that correlated with cyclin A expression and, thus, with cell cycle progression. A similar change was observed in T cells that re-entered cell cycle following secondary interleukin-2 stimulation, as well as in serum-induced proliferating NIH/3T3 fibroblasts. Phosphatase 2A (PP2A) appears involved because 1) treatment of dividing cells with okadaic acid or cantharidin inhibited Sp1 dephosphorylation and 2) PP2A dephosphorylated Sp1 in vitro and strongly interacted with Sp1 in vivo. Sp1 dephosphorylation is likely to increase its transcriptional activity because PP2A overexpression potentiated Sp1 site-driven chloramphenicol acetyltransferase expression in dividing Kit225 T cells and okadaic acid reversed this effect. This increase might be mediated by a stronger affinity of dephosphorylated Sp1 for DNA, as illustrated by the reduced DNA occupancy by hyperphosphorylated Sp factors from cantharidin- or nocodazole-treated cells. Finally, Sp1 dephosphorylation appears to occur throughout cell cycle except for mitosis, a likely common feature to all cycling cells.

GC-rich elements flanking the transcription start site govern strong activation on the SNURF gene

To study the regulation of the murine small nuclear RING finger protein SNURF (RNF4) gene, approximately 0.7 kb of its TATA-less promoter was isolated. This fragment conferred strong activation in reporter gene assays, yielding > or = 30% of the activity of the SV40 virus promoter/enhancer construct. Interestingly, the short region from -38 to +36 flanking the transcription start site was sufficient for potent basal promoter activity in various mammalian cell lines. Mutation of the conserved GC box at +9 abolished nuclear protein binding to the proximal promoter and severely compromised promoter activity, suggesting that this element is critical for the assembly of the transcription apparatus to regulate SNURF gene expression. Furthermore, our results show that the Wilms' tumor 1 gene product is one of the potential activators of the SNURF gene.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Sp1 is the major fasl gene activator in abnormal CD4(-)CD8(-)B220(+) T cells of lpr and gld mice

The abnormal CD4(-)CD8(-)TCRalpha beta(+)B220(+) double-negative (DN) T cells that accumulate in lpr and gld mice are refractory to TCR cross-linking and IL-2 stimulation, yet they have an activated phenotype and express a high level of fasl mRNA. Specific binding sites for Sp1, NFAT, Egr, and NF-kappaB have been identified in the promoter region of the fasl gene. To determine the critical factor for fasl gene activation, fasl promoter reporter and mutant constructs were transiently transfected into the abnormal DN T cells. The data demonstrate that the Sp1 binding site is the major response element that regulates fasl promoter activity. Moreover, the abnormal DN T cells contain in their nuclei a high level of Sp1, a low level of NFAT and NF-kappaB, and a very low level of Egr. Ectopic expression of Egr-3 but not Sp1 protein in the abnormal DN T cells enhanced fasl promoter activity, suggesting that the Egr but not Sp1 was limiting for fasl gene activation. Comparison between the abnormal DN T cells and the Sertoli TM4 cells showed a strong correlation between Sp1 expression and fasl mRNA level and FasL function. Our study has identified Sp1 as the major transcription factor responsible for fasl gene activation in the abnormal DN T cells that are defective in signal transduction through TCR and IL-2R, thereby, implicating a novel regulatory pathway for fasl gene activation during the physiological development and elimination of the abnormal DN T cells.

Regulation of the DNA methylation machinery and its role in cellular transformation

DNA methylation, a covalent modification of the genome, is emerging as an important player in the regulation of gene expression. This review discusses the different components of the DNA methylation machinery responsible for replicating the DNA methylation pattern. Recent data have changed our basic understanding of the DNA methylation machinery. A number of DNA methyltransferases (DNMT) have been identified and a demethylase has recently been reported. Because the DNA methylation pattern is critical for gene expression programs, the cell possesses a number of mechanisms to coordinate DNA replication and methylation. DNMT1 levels are regulated with the cell cycle and are induced upon entry into the S phase of the cell cycle. DNMT1 also regulates expression of cell-cycle proteins by its other regulatory functions and not through its DNA methylation activity. Once the mechanisms that coordinate DNMT1 and the cell cycle are disrupted, DNMT1 exerts an oncogenic activity. Tumor suppressor genes are frequently methylated in cancer but the mechanisms responsible are unclear. Overexpression of DNMT1 is probably not responsible for the aberrant methylation of tumor suppressor genes. Unraveling how the different components of the DNA methylation machinery interact to replicate the DNA methylation pattern, and how they are disrupted in cancer, is critical for understanding the molecular mechanisms of 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.

Sp1 and krüppel-like factor family of transcription factors in cell growth regulation and cancer

The Sp/KLF family contains at least twenty identified members which include Sp1-4 and numerous krüppel-like factors. Members of the family bind with varying affinities to sequences designated as 'Sp1 sites' (e.g., GC-boxes, CACCC-boxes, and basic transcription elements). Family members have different transcriptional properties and can modulate each other's activity by a variety of mechanisms. Since cells can express multiple family members, Sp/KLF factors are likely to make up a transcriptional network through which gene expression can be fine-tuned. 'Sp1 site'-dependent transcription can be growth-regulated, and the activity, expression, and/or post-translational modification of multiple family members is altered with cell growth. Furthermore, Sp/KLF factors are involved in many growth-related signal transduction pathways and their overexpression can have positive or negative effects on proliferation. In addition to growth control, Sp/KLF factors have been implicated in apoptosis and angiogenesis; thus, the family is involved in several aspects of tumorigenesis. Consistent with a role in cancer, Sp/KLF factors interact with oncogenes and tumor suppressors, they can be oncogenic themselves, and altered expression of family members has been detected in tumors. Effects of changes in Sp/KLF factors are context-dependent and can appear contradictory. Since these factors act within a network, this diversity of effects may arise from differences in the expression profile of family members in various cells. Thus, it is likely that the properties of the overall network of Sp/KLF factors play a determining role in regulation of cell growth and tumor progression.

Transcriptional downregulation of ATM by EGF is defective in ataxia-telangiectasia cells expressing mutant protein

There is evidence that ATM plays a wider role in intracellular signalling in addition to DNA damage recognition and cell cycle control. In this report we show that activation of the EGF receptor is defective in ataxia-telangiectasia (A-T) cells and that sustained stimulation of cells with EGF downregulates ATM protein in control cells but not in A-T cells expressing mutant protein. Concomitant with the downregulation of ATM, DNA-binding activity of the transcription factor Sp1 decreased in controls after EGF treatment but increased from a lower basal level in A-T cells to that in untreated control cells. Mutation in two Sp1 consensus sequences in the ATM promoter reduced markedly the capacity of the promoter to support luciferase activity in a reporter assay. Overexpression of anti-sense ATM cDNA in control cells decreased the basal level of Sp1, which in turn was increased by subsequent treatment of cells with EGF, similar to that observed in A-T cells. On the other hand full-length ATM cDNA increased the basal level of Sp1 binding in A-T cells, and in response to EGF Sp1 binding decreased, confirming that this is an ATM-dependent process. Contrary to that observed in control cells there was no radiation-induced change in ATM protein in EGF-treated A-T cells and likewise no alteration in Sp1 binding activity. The results demonstrate that EGF-induced downregulation of ATM (mutant) protein in A-T cells is defective and this appears to be due to less efficient EGFR activation and abnormal Sp1 regulation.

Sodium butyrate induces transcription from the G alpha(i2) gene promoter through multiple Sp1 sites in the promoter and by activating the MEK-ERK signal transduction pathway

Sodium butyrate, an erythroid differentiation inducer and a histone deacetylase inhibitor, increases G alpha(i2) levels in differentiating K562 cells. Here we show that sodium butyrate induces G alpha(i2) gene transcription via sequences at -50/-36 and -92/-85 in the G alpha(i2) gene promoter. Both sequences contain core sequence motif for Sp1 binding; electrophoretic mobility shift as well as supershift assays confirmed binding to Sp1. Transcription from the G alpha(i2) gene promoter was also activated by two other histone deacetylase inhibitors, trichostatin A and Helminthsporium carbonium toxin (HC toxin), which also induce erythroblastic differentiation in K562 cells. However, hydroxyurea, a potent erythroid differentiation inducer in these cells, did not activate transcription from this gene promoter, indicating that promoter activation is inducer-specific. Mutations within the Sp1 sites at -50/-36 and -92/-85 in the G alpha(i2) gene promoter substantially decreased transcriptional activation by sodium butyrate, trichostatin A, or HC toxin. Transfection with constitutively activated ERKs indicated that this promoter can be activated through the MEK-ERK signal transduction pathway. Inhibition of the MEK-ERK pathway with U0126 or reduction in the expression of endogenous ERK with an antisense oligonucleotide to ERK significantly inhibited sodium butyrate- and HC toxin-induced transcription but had no effect on trichostatin A-induced transcription. Inhibition of the JNK and p38 MAPKs, using selective inhibitors, had no effect on sodium butyrate-induced transcription. In cells in which sodium butyrate induction of promoter activation had been inhibited by various concentrations of U0126, constitutively activated ERK2 reversed this inhibition. These results show that the MEK-ERK signal transduction pathway is important in butyrate signaling, which eventually converges in the cell nucleus.

Negative regulatory role of Sp1 in metal responsive element-mediated transcriptional activation

Transcription of mammalian metallothionein (MT) genes is activated by heavy metals via multiple copies of a cis-acting DNA element, the metal-responsive element (MRE). Our previous studies have shown that certain MREs of the human MT-IIA gene (MREb, MREc, MREd, and MREf) are less active than the others (MREa, MREe, and MREg). Gel shift analysis of HeLa cell nuclear proteins revealed that whereas the active MREs strongly bind the transcription factor MTF-1 essential for metal regulation, the less active MREs bind another distinct protein, MREb-BF. This protein recognizes the GC-rich region of MREb rather than the MRE core required for MTF-1 binding. All the MREs recognized by MREb-BF contain the CGCCC and/or CACCC motif, suggesting that the MREb-BF.MRE complex contains Sp1 or related proteins. Supershift analysis using antibodies against Sp1 family proteins as well as gel shift analysis using the recombinant Sp1 demonstrated that Sp1 represents the majority of MREb-BF activity. An MREb mutant with reduced affinity to Sp1 mediated zinc-inducible transcription much more actively than the wild-type MREb. Furthermore, when placed in the native promoter, this mutant MREb raised the overall promoter activity. These results strongly suggest that Sp1 acts as a negative regulator of transcription mediated by specific MREs.

A novel regulatory element in the dnmt1 gene that responds to co-activation by Rb and c-Jun

Rb, c-Jun and dnmt1 play critical roles in the process of cellular differentiation. We demonstrate that a regulatory region of murine dnmt1 contains an element which is responsible for transactivation by Rb and c-Jun in P19 embryocarcinoma cells which is not observed in Y1 adrenocarcinoma cells. During differentiation of P19 cells, the induction of Rb and c-Jun coincides with an increase of dnmt1 mRNA. Using linker scanning mutagenesis we identify the element that is responsible for this activation to be a non-canonical AP-1 site. Our data is an example of how a proto-oncogene activates its downstream effectors by recruiting a tumor suppressor. This interaction of Rb and a proto-oncogene might play an important role in differentiation. The responsiveness of dnmt1 to this type of signal is consistent with an important role for regulated expression of dnmt1 during cellular differentiation.

Nuclear cathepsin B-like protease cleaves transcription factor YY1 in differentiated cells

Differentiation of pluripotent cells into differentiated cell types involves changes in many aspects of cellular biochemistry. Many of these changes result in alterations of gene expression, which may occur by changing the activity of transcription factors. The cell line NTERA-2 (NT2) can be differentiated into various cell types by incubation with retinoic acid. The differentiated cell type is also permissive for infection with the human herpesvirus cytomegalovirus (CMV). The transcription factor YY1 has been shown to regulate the immediate-early promoter of CMV in a differentiation specific manner by binding to one site at -958 to -950 and to at least two sites in the enhancer. It is demonstrated here that there is a second YY1 site in the modulator between -995 and -987. Levels of YY1 DNA binding activity and protein decrease in NT2 cells as they are differentiated with retinoic acid. This decrease in protein is due to the degradation of YY1 by a cathepsin B-like activity found in nuclear extracts. The cleavage products of YY1 include the intact C-terminal half of the protein, which contains the zinc fingers and the DNA binding activity. This suggests a mechanism that allows expression of the CMV immediate-early promoter in differentiated cells.

Mitogen-activated protein kinase pathway is involved in alpha6 integrin gene expression in androgen-independent prostate cancer cells: role of proximal Sp1 consensus sequence

Metastatic diseases of prostate cancer reveal high expression of alpha6 integrin and the activation of mitogen-activated protein kinases (MAP kinase). Therefore, the present study was conducted to examine whether MAP kinase pathway is involved in the alpha6 integrin gene expression in androgen-independent prostate cancer cell lines. alpha6 integrin mRNA expression, the alpha6 integrin promoter-induced luciferase activities and MAP kinase enzyme activities in androgen-independent LNCaP and PC-3 cell lines were higher than those in androgen-dependent LNCaP. Deletion and mutation analysis showed that Sp1 consensus sequence at -48 to -43 bp from the transcription start site was necessary for basal promoter activity. Binding of Sp1 to its consensus sequence in three cell lines was confirmed by electrophoretic mobility shift assays. Sp1 binding to its consensus sequence, as well as promoter activity and mRNA expression, were found to be inhibited by an inhibitor of MAP kinase kinase 1 and 2, U0126, in the androgen-independent cell lines. Our results indicate that the proximal Sp1 is necessary for basal promoter activity of the alpha6 integrin, suggesting that signal transduction from MAP kinases to activation of Sp1 might be involved in alpha6 integrin gene expression in androgen-independent prostate cancer cell lines.

The Sp1 transcription factor regulates cell type-specific transcription of MUC1

Because the MUC1 mucin is highly expressed in breast and other carcinomas, interest is focused on the MUC1 promoter, particularly in the context of the delivery of genes to carcinomas. Earlier in vitro studies showed that the region between -152 and -66 of the MUC1 promoter is required for transcriptional activity in MUC1-expressing cells. Experiments reported here showed that sequences -119/-62 within this region are able to modulate transcription of the heterologous constitutively active herpes simplex virus thymidine kinase promoter in a pattern consistent with MUC1 expression. Band-shift experiments showed that although several factors (including Sp1 and Sp3) bind to these sequences, the element important in directing this MUC1 pattern of expression was an Sp1 GC box at -97. The data also show that the positioning or phase of the GC box was crucial for directing expression. The importance of the Sp1 transcription factor was confirmed by demonstrating that overexpression of Sp1 in MUC1-nonexpressing cells increased, not only the expression of a reporter gene driven by the 1.4-kb MUC1 promoter, but also the expression of MUC1 from the endogenous gene. Together, these data define an important role for Sp1 in the cell type-specific transcription of MUC1.

Sp1-p53 heterocomplex mediates activation of HTLV-I long terminal repeat by 12-O-tetradecanoylphorbol-13-acetate that is antagonized by protein kinase C

We have previously demonstrated that 12-O-tetradecanoylphorbol-13-acetate (TPA) activates human T-cell leukemia virus type-I long terminal repeat (LTR) in Jurkat cells by a protein kinase C (PKC)-independent mechanism involving a posttranslational activation of Sp1 binding to an Sp1 site located within the Ets responsive region-1 (ERR-1). By employing the PKC inhibitor, bisindolylmaleimide I and cotransfecting the reporter LTR construct with a vector expressing PKC-alpha, we demonstrated, in the present study, that this effect of TPA was not only independent of, but actually antagonized by, PKC. Electrophoretic mobility shift assays together with antibody-mediated supershift and immuno-coprecipitation analyses, revealed that the posttranslational activation of Sp1 was exerted by inducing the formation of Sp1-p53 heterocomplex capable of binding to the Sp1 site in ERR-1. Furthermore, we demonstrated that Jurkat cells contain both wild-type (w.t.) and mutant forms of p53 and we detected both of them in this complex at variable combinations; some molecules of the complex contained either the w.t. or the mutant p53 separately, whereas others contained the two of them together. Finally, we showed that the Sp1-p53 complexes could bind also to an Sp1 site present in the promoter of another gene such as the cyclin-dependent kinase inhibitor p21(WAF-1), but not to consensus recognition sequences of the w.t. p53. Therefore, we speculate that there might be several other PKC-independent biological effects of TPA which result from interaction of such Sp1-p53 complexes with Sp1 recognition sites residing in the promoters of a wide variety of cellular and viral genes.

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.

Activation of telomerase rna gene promoter activity by NF-Y, Sp1, and the retinoblastoma protein and repression by Sp3

Expression of the human telomerase RNA component gene, hTERC is essential for telomerase activity. The hTERC gene is expressed during embryogenesis and then downregulated during normal development, leaving most adult somatic cells devoid of hTERC expression. During oncogenesis, however, hTERC is re-expressed consequently contributing to the unrestricted proliferative capacity of many human cancers. Thus the identification of the molecular basis for the regulation of the telomerase RNA component gene in normal cells and its deregulation in cancer cells is of immediate interest. We have previously cloned the hTERC promoter and in this study have identified several transcription factors that modulate the expression of hTERC. We demonstrate that NF-Y binding to the CCAAT region of the hTERC promoter is essential for promoter activity. Sp1 and the retinoblastoma protein (pRb) are activators of the hTERC promoter and Sp3 is a potent repressor. These factors appear to act in a species-specific manner. Whereas Sp1 and Sp3 act on the human, bovine, and mouse TERC promoters, pRb activates only the human and bovine promoter, and NF-Y is only essential for the human TERC gene.

12-O-tetradecanoylphorbol-13-acetate (TPA)-induced c-Jun N-terminal kinase (JNK) phosphatase renders immortalized or transformed epithelial cells refractory to TPA-inducible JNK activity

c-Jun N-terminal kinase (JNK) regulates gene expression in response to various extracellular stimuli. JNK can be activated by the tumor promoting agent, 12-O-tetradecanoylphorbol-13-acetate (TPA) in normal human oral keratinocytes but not in human keratinocytes that have been immortalized (HOK-16B and HaCaT) or transformed (HOK-16B-Bap-T) nor in a cervical carcinoma cell line (HeLa). The refractory JNK activation response to TPA is not due a defect in the JNK pathway, because JNK can be activated by other stimuli, e.g. UV irradiation and an alkylating agent N-methyl-N'-nitrosoguanidine in these immortalized or transformed cells. More importantly, the refractory JNK and JNKK activation response to TPA can be restored by treatment of the cells with a combination of TPA and a protein-tyrosine phosphatase inhibitor, sodium orthovanadate. Furthermore, pretreatment of cells with TPA partially inhibited UV- or N-methyl-N'-nitrosoguanidine-induced JNK activity. These results suggest that a TPA-inducible, orthovanadate-sensitive protein-tyrosine phosphatase may specifically down-regulate JNK signaling pathway in these immortalized/transformed epithelial cells. In contrast, ERK and p38/Mpk2 are not regulated by this TPA-induced phosphatase. This putative protein-tyrosine phosphatase appears to be JNK pathway-specific.

Involvement of Sp1 in the transcriptional regulation of the rat insulin-like growth factor-1 gene

Most insulin-like growth factor-I (IGF-I) transcripts are initiated in exon 1, but mechanisms of regulation are not well understood. Since potential Sp1 sites are found in footprinted regions within approximately 360 bp upstream and downstream from the major initiation sites in exon 1, we explored the involvement of Sp1 and Sp3 in regulation of IGF-1 expression. Gel shift assays showed strong Sp1 binding to the downstream site, but binding to the upstream site was weak; Sp1 bound to a CCTGCCCA sequence in downstream footprint region V, and Sp3 binding was centered on the same sequence. IGF-I basal promoter constructs containing a mutation in the downstream Sp1 site exhibited a 32% decrease in expression in CHO cells and a 75% decrease in HepG2 cells, indicating the importance of Sp1 for expression in vivo. Sp1 and Sp3 expression vectors provided three- to five-fold stimulation of wild-type IGF-I constructs, but had little effect on a construct containing a mutation in the downstream Sp1 site, and Sp1 had comparable effects in Drosophila SL2 cells. IGF-I heterologous promoter constructs exhibited similar responses: in both SL2 cells and CHO cells, stimulation by Sp1 was enhanced with constructs containing downstream region V. Since Sp1 also stimulated expression of concatamers of putative cis-acting sites fused to the SV40 promoter enhancer in pGL3, the results in combination indicate that the presence of IGF-I region V is sufficient to permit stimulation by Sp1.

CONCLUSION

Sp1 and related factors may play an important role in the regulation of IGF-I gene transcription, through interactions with region V downstream from the major initiation sites in exon 1.

Collagen alpha1(I) gene contains an element responsive to tumor necrosis factor-alpha located in the 5' untranslated region of its first exon

The aims of the present study were to identify the cis-acting element through which tumor necrosis factor-alpha (TNFalpha) inhibits collagen alpha1(I) gene transcription and the trans-acting factors involved in this effect in cultured hepatic stellate cells. Deletion analysis of the collagen alpha1(I) promoter demonstrated that TNFalpha inhibited gene expression through an element located between -59 and + 116 bp relative to the transcription start site. DNase I protection assays revealed a footprint between +68 and +86 bp of the collagen first exon, the intensity of which decreased when the DNA probe was incubated with nuclear protein from TNFalpha-treated hepatic stellate cells. This footprint contained a G+C-rich box. Transfection experiments demonstrated that mutations in this G+C-rich element abrogated the inhibitory effect of TNFalpha on the collagen alpha1(I) promoter. Gel retardation experiments using a radiolabeled oligonucleotide containing sequences of this region confirmed that TNFalpha treatment decreased the formation of two complexes between nuclear proteins and DNA. These complexes were efficiently blocked with an oligonucleotide containing an Spl-binding site and were supershifted with specific Spl and Sp3 antibodies. These results suggest that TNFalpha inhibits collagen alpha1(I) gene expression by decreasing the binding of Spl to a G+C-rich box in the 5' untranslated region of its first exon.

The zinc finger repressor, ZBP-89, binds to the silencer element of the human vimentin gene and complexes with the transcriptional activator, Sp1

Vimentin is a component of the eukaryotic cytoskeleton belonging to the family of intermediate filament proteins. It exhibits a complex pattern of tissue- and development-specific expression. It is also a marker of the metastatic potential of many tumor cells. Previously, the human vimentin promoter was shown to contain several regions for the binding of positive and negative acting regulatory factors. Until now, the silencer element, which shuts down vimentin synthesis in selected tissues during development, was not precisely localized; nor was its binding protein known. In vivo DMS footprinting by ligation-mediated PCR delineated the position of guanine residues important to vimentin expression. Transient transfection assays in HeLa cells of various vimentin 5'-end promoter sequences and mutants thereof precisely defined two regulatory elements, a negative element and an adjoining positive acting element. Band shift assays, UV cross-linking, and Southwestern blot analysis confirm that the silencer element specifically binds a protein. Several lines of evidence show that ZBP-89, a zinc finger, Kruppel-like repressor protein is vimentin's silencer element binding factor. Co-immunoprecipitation and DNA affinity chromatography prove that Sp1 heterodimerizes with ZBP-89 when bound to the silencer element to yield a DNA-protein complex whose mobility is indistinguishable from that displayed by HeLa nuclear extract in band shift assays.

Transcriptional repression of p21((Waf1/Cip1/Sdi1)) gene by c-jun through Sp1 site

Previously, we found that c-jun represses the tumor suppressor p21((Waf1/Cip1/Sdi1)) (p21) gene expression. In this study, we further investigated the mechanism of the inhibitory effect of c-jun on p21. After analysis of a series of deletion and point mutants of p21 promoter, we found that Sp1-3 site (-77 and -83) relative to the transcription start site played an important role for c-jun-repressing-responsive element in the p21 promoter. Both Sp1 and Sp3 transcription factors were the key factors for this event. However, the data from electrophoretic mobility shift assay indicated that c-jun did not change the Sp1 DNA-binding affinity, suggesting that additional factors may be involved in the repression of p21 by c-jun. Furthermore, c-jun could inhibit butyrate-inducing p21 gene expression through Sp1, indicating at least one common pathway whereby p21 expression is affected by c-jun and butyrate in opposing actions. Moreover, the hyperphosphorylated retinoblastoma protein (Rb) increased in c-jun expressing cells, indicating that phosphorylated Rb may play a role in regulating Sp1 to repress p21 expression. This is the first demonstration of how housekeeping factors and oncogene product counteract the function of tumor suppressor genes to control cell cycle progression.

Structural and functional characterization of the leukocyte integrin gene CD11d. Essential role of Sp1 and Sp3

CD11d encodes the latest alpha-subunit of the leukocyte integrin family to be discovered, and it is expressed predominantly in myelomonocytic cells. We have isolated a genomic clone that contains CD11d and showed this gene to be 11,461 bp downstream and oriented in the same direction as the related CD11c gene. CD11d transcription begins 69-79 nucleotides upstream of the ATG codon. Transfection analysis of CD11d-luc reporter constructs revealed that the -173 to +74 region is sufficient to confer leukocyte-specific expression of luciferase in myelomonocytic cells (THP1 and HL60), B-cells (IM9), and T-cells (Jurkat). Transfection analysis showed that down-regulation of CD11d expression by phorbol ester was myelomonocyte-specific and is mediated by one or more cis-elements within the -173 to +74 region. In vitro DNase I footprint analysis and electrophoretic mobility shift analysis showed that Sp1 and Sp3 bind at -63 to -40. Deletion of the Sp-binding site significantly reduced CD11d promoter activity. Overexpression of either Sp1 or Sp3 in THP1 cells led to activation of the CD11d promoter even in the presence of phorbol ester, whereas down-regulation of either factor by antisense oligonucleotides decreased CD11d promoter activity. In contrast, overexpression of Sp3 in IM9 and Jurkat cells down-regulated CD11d promoter expression. In vivo genomic footprinting revealed that the -63 to -40 region is bound by a Sp protein in unstimulated HL60 cells but not in phorbol ester-stimulated HL60 cells. In contrast, this site is bound in both unstimulated and phorbol ester-stimulated IM9 and Jurkat cells. Together, these results show that myelomonocyte-specific phorbol ester down-regulation of CD11d is mediated through both Sp1 and Sp3.

Transcription of human cathepsin B is mediated by Sp1 and Ets family factors in glioma

Cathepsin B expression is increased at both the mRNA and protein levels in a wide variety of tumors. The mechanisms responsible for this regulation are not well elucidated. We have isolated a 2.2-kb cathepsin B genomic fragment that contains the 5'-flanking region of the cathepsin B gene. Using reporter gene analysis in human glioblastoma U87MG cells, we have mapped a 228-bp fragment (-172 to +56) having high promoter activity. This promoter region has a high G+C content; contains potential Spl, Ets, and USF binding motifs; and lacks canonical TATA and CAAT boxes immediately upstream of the major transcriptional initiation site. Cotransfection experiments demonstrated that Spl and Ets1 could trans-activate cathepsin B transcription, whereas Ets2 could not. Electrophoretic mobility shift assays and supershift assays revealed that three of the four putative Sp1 sites in this promoter region form a specific complex containing the Sp1 transcription factor. Mutating all four of the Spl binding sites individually markedly reduced the promoter activity of transfected reporter genes in U87 cells. Cotransfection of this cathepsin B promoter construct with Spl family expression vectors in Schneider's Drosophila line 2 (SL2) cells demonstrated that Spl and Sp3, but not Sp4, activated cathepsin B transcription. Taken together, these results suggest that Sp1, Sp3, and Ets1 are important factors in cathepsin B transcription. The regulation of cathepsin B transcription by Sp1- and Sp1-related factors is mediated through multiple GC boxes.

Coordinate modulation of Sp1, NF-kappa B, and p53 in confluent human malignant melanoma cells after ionizing radiation

Regulation of transcriptional responses in growth-arrested human cells under conditions that promote potentially lethal damage repair after ionizing radiation (IR) is poorly understood. Sp1/retinoblastoma control protein (RCP) DNA binding increased within 30 min and peaked at 2-4 h after IR (450-600 cGy) in confluent radioresistant human malignant melanoma (U1-Mel) cells. Increased phosphorylation of Sp1 directly corresponded to Sp1/RCP binding and immediate-early gene induction, whereas pRb remained hypophosphorylated. Transfection of U1-Mel cells with the human papillomavirus E7 gene abrogated Sp1/RCP induction and G(0)/G(1) cell cycle checkpoint arrest responses, increased apoptosis and radiosensitivity, and augmented genetic instability (i.e., increased polyploidy cells) after IR. Increased NF-kappaB DNA binding in U1-Mel cells after IR treatment lasted much longer (i.e., >20 h). U1-Mel cells overexpressing dominant-negative IkappaBalpha S32/36A mutant protein were significantly more resistant to IR exposure and retained both G(2)/M and G(0)/G(1) cell cycle checkpoint responses without significant genetic instability (i.e., polyploid cell populations were not observed). Nuclear p53 protein levels and DNA binding activity increased only after high doses of IR (>1200 cGy). Disruption of p53 responses in U1-Mel cells by E6 transfection also abrogated G(0)/G(1) cell cycle checkpoint arrest responses and increased polyploidy after IR, but did not alter radiosensitivity. These data suggest that abrogation of individual components of this coordinate IR-activated transcription factor response may lead to divergent alterations in cell cycle checkpoints, genomic instability, apoptosis, and survival. Such coordinate transcription factor activation in human cancer cells is reminiscent of prokaryotic SOS responses, and further elucidation of these events should shed light on the initial molecular events in the chromosome instability phenotype.-Yang, C.-R., Wilson-Van Patten, C., Planchon, S. M., Wuerzberger-Davis, S. M., Davis, T. W., Cuthill, C., Miyamoto, S., Boothman, D. A. Coordinate modulation of Sp1, NF-kappa B, and p53 in confluent human malignant melanoma cells after ionizing radiation.

Insulin-like growth factor (IGF)-I regulates IGF-binding protein-5 gene expression through the phosphatidylinositol 3-kinase, protein kinase B/Akt, and p70 S6 kinase signaling pathway

Expression of the insulin-like growth factor-binding protein 5 (IGFBP-5) gene in vascular smooth muscle cells is up-regulated by IGF-I through an IGF-I receptor-mediated mechanism. In this study, we studied the possible involvement of the mitogen-activated protein kinase (MAPK) and PI 3-kinase signaling pathways in mediating IGF-I-regulated IGFBP-5 gene expression. The addition of Des(1-3)IGF-I, an IGF analog with reduced affinity to IGFBPs, resulted in a transient activation of p44 and p42 MAPK. Inhibition of the MAPK activation by PD98059, however, did not affect IGF-I-stimulated IGFBP-5 expression. Des(1-3)IGF-I treatment also strongly activated PI 3-kinase. This activation was probably mediated through IRS-1, because IGF-I stimulation resulted in a significant increase in IRS-1- but not IRS-2-associated PI 3-kinase activity. This activation occurred within 5 min and was sustained at high levels for over 6 h. Likewise, Des(1-3)IGF-I caused a long lasting activation of PKB/Akt and p70(s6k). When LY294002 and wortmannin, two specific inhibitors of PI 3-kinase, were added with Des(1-3)IGF-I, the IGF-I-regulated IGFBP-5 expression was negated. The addition of rapamycin, which inhibits IGF-I-induced p70(s6k) activation, significantly inhibited IGF-I-regulated IGFBP-5 gene expression. These results suggest that the action of IGF-I on IGFBP-5 gene expression requires the activation of the PI 3-kinase-PKB/Akt-p70(s6k) pathway but not the MAPK pathway in vascular smooth muscle cells.