Transcription factor Sp3 antagonizes activation of the ornithine decarboxylase promoter by Sp1

In vitro and in vivo studies on exogenous polyamines and α-difluoromethylornithine to enhance bone formation and suppress osteoclast differentiation

Exogenous polyamines, including putrescine (PUT), spermidine (SPD), and spermine (SPM), and the irreversible inhibitor of the rate-limiting enzyme ornithine decarboxylase (ODC) of polyamine biosynthesis, α-difluoromethylornithine (DFMO), are implicated as stimulants for bone formation. We demonstrate in this study the osteogenic potential of exogenous polyamines and DFMO in human osteoblasts (hOBs), murine monocyte cell line RAW 264.7, and an ovariectomized rat model. The effect of polyamines and DFMO on hOBs and RAW 264.7 cells was studied by analyzing gene expression, alkaline phosphatase (ALP) activity, tartrate-resistant acid phosphatase (TRAP) activity, and matrix mineralization. Ovariectomized rats were treated with polyamines and DFMO and analyzed by micro computed tomography (micro CT). The mRNA level of the early onset genes of osteogenic differentiation, Runt-related transcription factor 2 (Runx2) and ALP, was significantly elevated in hOBs under osteogenic conditions, while both ALP activity and matrix mineralization were enhanced by exogenous polyamines and DFMO. Under osteoclastogenic conditions, the gene expression of both receptor activator of nuclear factor-κB (RANK) and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) was reduced, and TRAP activity was suppressed by exogenous polyamines and DFMO in RAW 264.7 cells. In an osteoporotic animal model of ovariectomized rats, SPM and DFMO were found to improve bone volume in rat femurs, while trabecular thickness was increased in all treatment groups. Results from this study provide in vitro and in vivo evidence indicating that polyamines and DFMO act as stimulants for bone formation, and their osteogenic effect may be associated with the suppression of osteoclastogenesis.

Magnesium Supplementation Attenuates Ultraviolet-B-Induced Damage Mediated through Elevation of Polyamine Production in Human HaCaT Keratinocytes

Magnesium ions (Mg²⁺) have favorable effects such as the improvement of barrier function and the reduction of inflammation reaction in inflammatory skin diseases. However, its mechanisms have not been fully understood. Microarray analysis has shown that the gene expressions of polyamine synthases are upregulated by MgCl₂ supplementation in human HaCaT keratinocytes. Here, we investigated the mechanism and function of polyamine production. The mRNA and protein levels of polyamine synthases were dose-dependently increased by MgCl₂ supplementation, which were inhibited by U0126, a MEK inhibitor; CHIR-99021, a glycogen synthase kinase-3 (GSK3) inhibitor; and Naphthol AS-E, a cyclic AMP-response-element-binding protein (CREB) inhibitor. Similarly, reporter activities of polyamine synthases were suppressed by these inhibitors, suggesting that MEK, GSK3, and CREB are involved in the transcriptional regulation of polyamine synthases. Cell viability was reduced by ultraviolet B (UVB) exposure, which was rescued by MgCl₂ supplementation. The UVB-induced elevation of reactive oxygen species was attenuated by MgCl₂ supplementation, which was inhibited by cysteamine, a polyamine synthase inhibitor. Our data indicate that the expression levels of polyamine synthases are upregulated by MgCl₂ supplementation mediated through the activation of the MEK/GSK3/CREB pathway. MgCl₂ supplementation may be useful in reducing the UVB-induced oxidative stress in the skin.

Functional activity of the porcine Gnrhr2 gene promoter in testis-derived cells is partially conferred by nuclear factor-κB, specificity protein 1 and 3 (SP1/3) and overlapping early growth response 1/SP1/3 binding sites

Unlike the classical gonadotropin-releasing hormone (GnRH1), the second mammalian isoform (GnRH2) is ubiquitously expressed, suggesting a divergent function. Indeed, we demonstrated that GnRH2 governs LH-independent testosterone secretion in porcine testes via interaction with its receptor (GnRHR2) on Leydig cells. Transient transfections with luciferase reporter vectors containing 3009bp of 5' flanking sequence for the porcine Gnrhr2 gene (-3009pGL3) revealed promoter activity in all 15 cell lines examined, including swine testis-derived (ST) cells. Therefore, ST cells were utilized to explore the molecular mechanisms underlying transcriptional regulation of the porcine Gnrhr2 gene in the testis. Reporter plasmids containing progressive 5' deletions of the Gnrhr2 promoter indicated that the -708/-490 region contained elements critical to promoter activity. Electrophoretic mobility shift assays (EMSAs) with radiolabeled oligonucleotides spanning the -708/-490bp region and ST nuclear extracts, identified specific binding complexes for the -513/-490, -591/-571 and -606/-581bp segments of promoter. Antibody addition to EMSAs indicated that the p65 and p52 subunits of nuclear factor-κB (NF-κB) comprised the specific complex bound to the oligonucleotide probe for the -513/-490bp promoter region, specificity protein (SP) 1 and 3 bound the -591/-571bp probe and early growth response 1 (EGR1), SP1 and SP3 bound the -606/-581 radiolabeled oligonucleotide. Transient transfections with vectors containing mutations of the NF-κB (-499/-493), SP1/3 (-582/-575) or overlapping EGR1/SP1/3 (-597/-587) binding sites reduced luciferase activity by 26%, 61% and 56%, respectively (P<0.05). Thus, NF-κB, SP1/3 and overlapping EGR1/SP1/3 binding sites are critical to expression of the porcine Gnrhr2 gene in ST cells.

A network of transcription factors operates during early tooth morphogenesis

Improving the knowledge of disease-causing genes is a unique challenge in human health. Although it is known that genes causing similar diseases tend to lie close to one another in a network of protein-protein or functional interactions, the identification of these protein-protein networks is difficult to unravel. Here, we show that Msx1, Snail, Lhx6, Lhx8, Sp3, and Lef1 interact in vitro and in vivo, revealing the existence of a novel context-specific protein network. These proteins are all expressed in the neural crest-derived dental mesenchyme and cause tooth agenesis disorder when mutated in mouse and/or human. We also identified an in vivo direct target for Msx1 function, the cyclin D-dependent kinase (CDK) inhibitor p19(ink4d), whose transcription is differentially modulated by the protein network. Considering the important role of p19(ink4d) as a cell cycle regulator, these results provide evidence for the first time of the unique plasticity of the Msx1-dependent network of proteins in conferring differential transcriptional output and in controlling the cell cycle through the regulation of a cyclin D-dependent kinase inhibitor. Collectively, these data reveal a novel protein network operating in the neural crest-derived dental mesenchyme that is relevant for many other areas of developmental and evolutionary biology.

Sp1 transcription factor and GATA1 cis-acting elements modulate testis-specific expression of mouse cyclin A1

Cyclin A1 is a male germ cell-specific cell cycle regulator that is essential for spermatogenesis. It is unique among the cyclins by virtue of its highly restricted expression in vivo, being present in pachytene and diplotene spermatocytes and not in earlier or later stages of spermatogenesis. To begin to understand the molecular mechanisms responsible for this narrow window of expression of the mouse cyclin A1 (Ccna1) gene, we carried out a detailed analysis of its promoter. We defined a 170-bp region within the promoter and showed that it is involved in repression of Ccna1 in cultured cells. Within this region we identified known cis-acting transcription factor binding sequences, including an Sp1-binding site and two GATA1-binding sites. Neither Sp1 nor GATA1 is expressed in pachytene spermatocytes and later stages of germ cell differentiation. Sp1 is readily detected at earlier stages of spermatogenesis. Site-directed mutagenesis demonstrated that neither factor alone was sufficient to significantly repress expression driven by the Ccna1 promoter, while concurrent binding of Sp1, and most likely GATA1 and possibly additional factors was inhibitory. Occupancy of Sp1 on the Ccna1 promoter and influence of GATA1-dependent cis-acting elements was confirmed by ChIP analysis in cell lines and most importantly, in spermatogonia. In contrast with many other testis-specific genes, the CpG island methylation status of the Ccna1 promoter was similar among various tissues examined, irrespective of whether Ccna1 was transcriptionally active, suggesting that this regulatory mechanism is not involved in the restricted expression of Ccna1.

Functional implication of TRAIL -716 C/T promoter polymorphism on its in vitro and in vivo expression and the susceptibility to sporadic breast tumor

Recently, TRAIL function has been elucidated beyond its known classical role of mediating cellular homeostasis and immune surveillance against transformed cells. Here, we show how CC genotype of -716 TRAIL promoter SNP rendered risk for sporadic breast cancer as compared to the CT and TT genotypes (P (recessive model) = 0.018, OR = 1.4, 95% CI = 1.1-1.9; P (allele model) = 0.010, OR = 1.3, 95% CI = 1.1-1.7). The in silico prediction of the introduction of core Sp1/Sp3-binding motif suggested the functional significance of the SNP variation. This functional implication was validated by luciferase assay in HeLa (P = 0.026), MCF-7 (P = 0.022), HepG2 (P = 0.024), and HT1080 (P = 0.030) cells and also by real-time expression studies on tumor tissues (P = 0.01), revealing the transcriptionally repressed status of -716 T when compared to -716 C allele. The SNP-SNP interactions reflected an enhanced protective effect of CT and TT genotypes with the protective genetic backgrounds of TP53-BRCA2 (P = 0.002, OR = 0.2, 95% CI = 0.1-0.6), IFNG (P = 0.0000002, OR = 0.3, 95% CI = 0.2-0.4), and common variant Casp8 (P = 0.0003, OR = 0.5, 95% CI = 0.3-0.7). Interestingly, a comparison with clinical parameters showed overrepresented CT and TT genotypes in progressing (P = 0.041) and ER/PR negative tumors (P = 0.024/0.006). This was explained by increased apoptotic index, calculated as a ratio of selected pro-apoptotic and anti-apoptotic gene expression profiles, in CC genotyped tumors, favoring either intrinsic (P = 0.008,0.018) or extrinsic (P = 0.025,0.217) pathway depending upon the ER/PR status. Our study reveals for the first time that a promoter SNP of TRAIL functionally modulates the gene and consequently its role in breast cancer pathogenesis, cautioning to consider the -716 TRAIL SNP status in patients undergoing TRAIL therapy.

Involvement of FLIP in 2-methoxyestradiol-induced tumor regression in transgenic adenocarcinoma of mouse prostate model

PURPOSE

The purpose of this study is to investigate whether Fas-associated death domain interleukin-1 converting enzyme like inhibitory protein (FLIP) inhibition is a therapeutic target associated with 2-methoxyestradiol (2-ME2)-mediated tumor regression.

EXPERIMENTAL DESIGN

Expression and levels of FLIP were analyzed using (a) real-time PCR and immunoblot analysis in androgen-independent PC-3 cells treated with the newly formulated 2-ME2 and (b) immunohistochemistry in different Gleason pattern human prostate tumors. Transient transfections and chromatin immunoprecipitation (ChIP) assays were used to identify the transcription factors that regulate FLIP. Involvement of FLIP in 2-ME2-induced tumor regression was evaluated in transgenic adenocarcinoma mouse prostate (TRAMP) mice.

RESULTS

High Gleason pattern (5+5) human prostate tumors exhibit significant increase in FLIP compared with low Gleason pattern 3+3 (P=or<0.04). 2-ME2 reduced the levels and promoter activity of FLIP (P=0.001) in PC-3 cells. Transient expression assays show sequences between -503/+242 being sufficient for 2-ME2-induced inhibition of FLIP promoter activity. Cotransfection experiments show that overexpression of Sp1 activated, whereas Sp3 inhibited, Sp1 transactivation of FLIP promoter activity (P=0.0001). 2-ME2 treatment reduced binding of Sp1 to the FLIP promoter as evidenced by ChIP. Further, levels of FLIP associated with Fas or FADD decreased, whereas cleavage of caspase-8, levels of Bid, and apoptosis increased in response to 2-ME2 treatment in PC-3 cells. Administration of 2-ME2 regressed established prostate tumors in TRAMP mice that were associated with reduced expression of FLIP and Sp1.

CONCLUSION

Targeting Sp1-mediated FLIP signaling pathway may provide a novel approach for prostate cancer management.

Transcription of the LAT gene is regulated by multiple binding sites for Sp1 and Sp3

The LAT gene encodes an adaptor molecule that links receptor engagement to critical downstream signaling events. Previously, we identified the proximal promoter for the human LAT gene and found that it contains binding sites for members of the Ets and Runx transcription factor families. In the present study, we show that the promoter also contains 5 GC-rich elements that contribute to promoter activity and that are capable of binding the transcription factors Sp1 and Sp3. Overexpression of either Sp1 or full-length Sp3 was shown to augment LAT promoter activity, while siRNA-mediated knockdown of each transcription factor was demonstrated to have an inhibitory effect. We also discovered a cell-type specific DNase hypersensitive site that maps to the Sp1/Sp3 and adjacent Ets and Runx binding sites. Collectively, these results provide compelling data that implicates Sp1 and Sp3 in the transcriptional regulation of the human LAT gene.

Alleles of the bovine DGAT1 variable number of tandem repeat associated with a milk fat QTL at chromosome 14 can stimulate gene expression

A quantitative trait locus (QTL) affecting milk fat percentage has been mapped to the centromeric end of the bovine chromosome 14 (BTA14). This genomic area includes the DGAT1 gene, which encodes acyl-CoA:diacylglycerol acyltransferase 1, the key enzyme of triglyceride biosynthesis. Genetic and biochemical studies led to the identification of the nonconservative DGAT1-K232A polymorphism as a causal mutation for the QTL. In addition to this, another polymorphism in the 5'-regulatory region of this gene, the DGAT1 variable number of tandem repeat (VNTR), also showed a strong association with milk fat percentage. This promoter VNTR polymorphism affects the number of potential Sp1 binding sites and therefore might have an impact on DGAT1 expression and also milk fat content. Hence, the DGAT1 VNTR polymorphism might be another causal mutation for the BTA14 QTL. However, evidence for Sp1 binding to this polymorphic site and for the capability of DGAT1 VNTR alleles to stimulate gene expression was lacking. In the current work Sp1-VNTR interactions were analyzed by EMSA. In addition, effects of DGAT1 VNTR alleles on gene expression were measured with reporter gene analyses. Conclusions from the results are that 1) the DGAT1 VNTR sequence is indeed a target for Sp1 binding; 2) DGAT1 VNTR alleles can stimulate gene expression in vitro and probably in vivo as well; and 3) although the stimulating effects of the different DGAT1 VNTR alleles did not show significant differences in vitro, their effects on transcription might be different in the chromatin context existing in vivo.

Identification of a novel human zinc finger gene, ZNF438, with transcription inhibition activity

There were many different families of zinc finger proteins that contained multiple cysteine and/or histidine residues and used zinc to stabilize their folds. The classical C2H2 zinc finger proteins were the founding members of this superfamily and were among the most abundant proteins in eukaryotic genomes. C2H2 proteins typically contained several C2H2 fingers that made tandem contacts along the DNA. Here we reported a novel C2H2 type zinc finger gene, ZNF438, which encoded 828 amino acids that formed five zinc finger domains. Bioinformatics analysis revealed that the ZNF438 was mapped to human chromosome 10p11.2 and shared 62% identity with rat and mouse homologues. RT-PCR analysis indicated that it was ubiquitously expressed in 18 human adult tissues. With immunofluorescence assay, it was shown that the exogenous Flag-tagged ZNF438 was located in nucleus of COS-7 cells. To further explore the function of ZNF438, we examined the transcriptional activity of ZNF438 protein by transfecting recombinant pM-ZNF438 into mammalian cells. The subsequent analysis based on the duel luciferase assay system showed that ZNF438 was a transcriptional repressor.

Green tea catechin (-)-epicatechin gallate induces tumour suppressor protein ATF3 via EGR-1 activation

Epicatechin gallate (ECG) is the third major catechin component in green tea, but it shows strong biological activity in some aspects, including apoptosis, cell growth inhibition, and membrane transport system in various cells. We previously reported that ECG induces activating transcription factor 3 (ATF3), which is involved in pro-apoptosis in HCT-116 cells. In this report, we present a molecular mechanism by which ECG induces ATF3 expression at the transcriptional level. We found that Sp3 contributed to the basal expression of the ATF3 gene, whereas EGR-1 played an important role in ECG-induced ATF3 expression in HCT-116 cells, as assessed by EMSA and co-transfection experiments. These results suggested that EGR-1, a tumour suppressor protein, could substantiate ECG's role of ATF3 expression in human colorectal cancer cells. We also found that pro-oxidant activity of ECG contributed to ECG-induced ATF3 expression.

Lacking cytokine production in ES cells and ES-cell-derived vascular cells stimulated by TNF-alpha is rescued by HDAC inhibitor trichostatin A

Inflammation and TNF-alpha signaling play a central role in most of the pathological conditions where cell transplantation could be applied. As shown by initial experiments, embryonic stem (ES) cells and ES-cell derived vascular cells express very low levels of TNF-alpha receptor I (TNFRp55) and thus do not induce cytokine expression in response to TNF-alpha stimulation. Transient transfection analysis of wild-type or deletion variants of the TNFRp55 gene promoter showed a strong activity for a 250-bp fragment in the upstream region of the gene. This activity was abolished by mutations targeting the Sp1/Sp3 or AP1 binding sites. Moreover, treatment with trichostatin A (TSA) led to a pronounced increase in TNFRp55 mRNA and promoter activity. Overexpression of Sp1 or c-fos further enhanced the TSA-induced luciferase activity, and this response was attenuated by Sp3 or c-jun coexpression. Additional experiments revealed that TSA did not affect the Sp1/Sp3 ratio but caused transcriptional activation of the c-fos gene. Thus, we provide the first evidence that ES and ES-cell-derived vascular cells lack cytokine expression in response to TNF-alpha stimulation due to low levels of c-fos and transcriptional activation of Sp1 that can be regulated by inhibition of histone deacetylase activity.

The promoter of the rat 5alpha-reductase type 1 gene is bidirectional and Sp1-dependent

In many androgen target tissues, testosterone is reduced to the more potent androgen, dihydrotestosterone, by steroid 5alpha-reductase. Two isoforms of 5alpha-reductase, type 1 and type 2, have been cloned. They are differentially expressed and regulated. To determine the mechanisms of regulation of 5alpha-reductase type 1 expression, we have cloned its 5'upstream region and defined its promoter. The proximal 5'upstream region of 5alpha-reductase type 1 displays all the features of a CpG island and has numerous Sp1 binding sites. By transient transfection assays, we have identified a bidirectional promoter activity in this region; this activity was highest in the negative orientation, in the direction of the methyltransferase Nsun2 (predicted) gene. Promoter activity, in either orientation, was lost in Sp1 deficient cells but was rescued following co-transfection with a Sp1 expression vector. Thus, the 5'upstream region of rat 5alpha-reductase type 1 contains a bidirectional promoter with an activity that is Sp1-dependent.

Sp1/3 and NF-1 mediate basal transcription of the human P2X1 gene in megakaryoblastic MEG-01 cells

BACKGROUND

P2X1 receptors play an important role in platelet function as they can induce shape change, granule centralization and are also involved in thrombus formation. As platelets have no nuclei, the level of P2X1 expression depends on transcriptional regulation in megakaryocytes, the platelet precursor cell. Since nothing is known about the molecular mechanisms regulating megakaryocytic P2X1 expression, this study aimed to identify and functionally characterize the P2X1 core promoter utilized in the human megakaryoblastic cell line MEG-01.

RESULTS

In order to identify cis-acting elements involved in the transcriptional regulation of P2X1 expression, the ability of 4.7 kb P2X1 upstream sequence to drive luciferase reporter gene expression was tested. Low promoter activity was detected in proliferating MEG-01 cells. This activity increased 20-fold after phorbol-12-myristate-13-acetate (PMA) induced differentiation. A transcription start site was detected 365 bp upstream of the start codon by primer extension. Deletion analysis of reporter constructs indicated a core promoter located within the region -68 to +149 bp that contained two Sp1 sites (named Sp1a and Sp1b) and an NF-1 site. Individual mutations of Sp1b or NF-1 binding sites severely reduced promoter activity whereas triple mutation of Sp1a, Sp1b and NF-1 sites completely abolished promoter activity in both untreated and PMA treated cells. Sp1/3 and NF-1 proteins were shown to bind their respective sites by EMSA and interaction of Sp1/3, NF-1 and TFIIB with the endogenous P2X1 core promoter in MEG-01 cells was demonstrated by chromatin immunoprecipitation. Alignment of P2X1 genes from human, chimp, rat, mouse and dog revealed consensus Sp1a, Sp1b and NF-1 binding sites in equivalent positions thereby demonstrating evolutionary conservation of these functionally important sites.

CONCLUSION

This study has identified and characterized the P2X1 promoter utilized in MEG-01 cells and shown that binding of Sp1/3 and NF-1 to elements in the direct vicinity of the transcription start site is essential for basal transcription. Targeting the function of these transcription factors in megakaryocytes may therefore provide a basis for the future therapeutic manipulation of platelet P2X1 function.

Sodium butyrate-mediated Sp3 acetylation represses human insulin-like growth factor binding protein-3 expression in intestinal epithelial cells

OBJECTIVES

Butyrate concentrations in the gastrointestinal tract vary greatly with age. In intestinal epithelial cells, butyrate enhances gene transcription by increasing histone acetylation, rendering the nucleosome open to transcription factors. However, it inhibits human insulin-like growth factor binding protein (hIGFBP)-3 expression. We therefore hypothesized that butyrate also acts by regulating transcription factor acetylation.

METHODS

Gene regulation was examined in Caco-2 cells. RNA stability was measured after interruption of transcription. The activity of deletion mutations of the hIGFBP-3 promoter was examined in reporter assays. Transcription factor binding to promoter DNA was analyzed.

RESULTS

Butyrate did not increase the transcription of a repressor because it inhibited hIGFBP-3 mRNA in the absence of protein synthesis. Nor did butyrate decrease the stability of hIGFBP-3 mRNA. Analysis of the hIGFBP-3 promoter demonstrated a butyrate-response element that included the binding sites for p300 and Sp1/Sp3. Transfection of Caco-2 cells with E1A, an inhibitor of p300 acetyltransferase activity, reversed the butyrate-induced repression of hIGFBP-3. Because Sp3 represses the initiation of transcription, we studied whether butyrate induced Sp3 acetylation. Electrophoretic mobility shift assays of nuclei extracted from Caco-2 cells treated with 5 mmol/L butyrate demonstrated an extra, heavier band in addition to the Sp3-DNA binding in untreated cells. This corresponded to a protein, detected only in butyrate treated cells, that was identified both by an anti-Sp3 antibody and by an anti-acetyl lysine antibody.

CONCLUSIONS

This study demonstrates that butyrate increases the acetylation of a nonhistone protein, Sp3, catalyzed by p300 acetyltransferase activity.

Different roles of Sp family members in HIV-1 Tat-mediated manganese superoxide dismutase suppression in hepatocellular carcinoma cells

The expression of manganese superoxide dismutase (MnSOD) is regulated by agents associated with cancer development. It has been shown that infection with the human immunodeficiency virus type 1 (HIV-1) is associated with the development of liver cancer and that the transactivating transcriptional factor (Tat) of human HIV-1 reduces the expression of MnSOD in several cell types. However, the role of Tat in the expression of MnSOD in hepatocellular carcinoma is unknown. Furthermore, the precise mechanisms whereby Tat suppresses MnSOD expression in hepatocellular carcinoma cells remain unclear. In this report, we build on our original observations that Tat changes the distribution of Sp family members on the MnSOD promoter, which accounts for Tat-dependent changes in basal expression. In hepatic cells, Tat expression upregulates Sp1/Sp3, which play different roles in regulating MnSOD transcription. While overexpression of Sp1 stimulates, overexpression of Sp3 represses transcriptional activity. The transcription repression effect of Sp3 is not due to Sp3 competing for the binding site with Sp1 because only the full-length Sp3 but not the truncated Sp3 suppresses MnSOD promoter activity. These findings suggest a novel mechanism by which Tat modulates the repression of the MnSOD gene and establish a link between HIV infection and liver cancer.

Sumoylation of internally initiated Sp3 isoforms regulates transcriptional repression via a Trichostatin A-insensitive mechanism

Sp3 is a ubiquitously expressed member of the Sp family of transcription factors that encodes three proteins, Sp3, M1 and M2, with differing capacities to stimulate or repress transcription. As part of ongoing efforts to study the functions of Sp3 isoforms, we employed a yeast "two-hybrid" screen to identify Sp3-binding proteins. This screen resulted in the identification of Ubc9, a SUMO-1 conjugating enzyme, as an M2-binding protein, and consistent with these results sequence analyses identified consensus sumoylation motifs within several Sp family members. Western blots probed with anti-Sp3 detected a high molecular weight Sp3 isoform that is stabilized by a SUMO-1 hydrolase inhibitor, and this protein is also bound by anti-SUMO-1 antiserum. Transient transfection assays with epitope-tagged-SUMO-1 and GFP-SUMO-1 fusion proteins confirmed that Sp3, M1 and M2 proteins are sumoylated in vivo. Substitution of arginine for lysine at one putative site of sumoylation, lysine(551), blocked sumoylation of all Sp3 isoforms in vivo and led to a marginal increase in Sp3-mediated trans-activation in insect and mammalian cells. In contrast, introduction of this amino acid substitution within M1 converted it into a potent transcriptional trans-activator. We conclude that Sp3 isoforms are sumoylated in vivo and this post-translational modification plays an important role in the regulation of Sp3-mediated transcription.

Inhibition of glutaminase expression increases Sp1 phosphorylation and Sp1/Sp3 transcriptional activity in Ehrlich tumor cells

Tumor cells expressing antisense glutaminase RNA show a drastic inhibition of glutaminase activity and they acquire a more differentiated phenotype. We have studied the expression of Sp1 and Sp3 transcription factors in both Ehrlich tumor cells and their derivative 0.28AS-2 antisense glutaminase expressing cells. The expression of phosphorylated Sp1 in 0.28AS-2 cells was 3-fold the expression in EATC. Full length Sp3 was also incremented in 0.28AS-2 cells. Sp1 and Sp3 binding to a consensus Sp1 probe was higher in 0.28AS-2 nuclear extracts, as determined by supershift assays. Sp1-DNA binding was inhibited by phosphatase treatment, demonstrating that phosphorylation of Sp1 is critical for its DNA binding capacity. The Sp1 and Sp3 DNA binding found in 0.28AS-2 cells was also correlated with an increased Sp1 activity, as shown in transient transfections assays carried out with a luciferase reporter plasmid. Incubation of Ehrlich tumor cells with the differentiation agent PMA could not totally reproduce the Sp1/Sp3 changes observed in 0.28AS-2 cells. However, it was demonstrated that the intracellular concentration of glutamine, but not glutamate or aspartate, is increased in 0.28AS-2 cells. In conclusion, the antisense inhibition of glutaminase leads to an increased expression of phosphorylated Sp1 and that correlates with an increase in Sp1 activity.

Transcriptional regulation of the human UDP-galactose:ceramide galactosyltransferase (hCGT) gene expression: functional role of GC-box and CRE

UDP-galactose:ceramide galactosyltransferase (CGT, EC 2.4.1.45) is a key enzyme in the biosynthetic pathway of galactocerebroside (GalC), the most abundant glycolipid in myelin. Using a GalC expressing cell line, human oligodendroglioma (HOG), one which does not express GalC, human neuroblastoma (LAN-5), we previously demonstrated that the human CGT (hCGT) gene promoter functions in a cell-specific manner. Because the proximal (-292/-256) and distal (-747/-688) positive domains were shown to be critically involved in regulating the expression of several myelin-specific genes, we further investigated the functional roles of these two motifs in hCGT expression. Mutation analysis confirmed that a GC-box (-267/-259) and a CRE (-697/-690) were critical for hCGT expression. Electrophoretic mobility shift assay (EMSA) demonstrated that these motifs specifically bound to nuclear extracts from both cell lines. Using antibodies to Sp1, Sp3, pCREB-1, and ATF-1, these proteins were shown to be components of the EMSA complexes. However, the only difference between the HOG and LAN-5 cells was found in the EMSA profile of the CRE complexes. This difference may account for the differential transcription of the hCGT gene in the two cell types. Furthermore, the expression levels of ATF-1 detected were much higher in HOG cells than in LAN-5 cells. Thus, our data suggest that the GC-box and CRE function cooperatively, and that the CRE regulates the cell-specific expression of the hCGT gene.

Nkx2.1 transcription factor in lung cells and a transforming growth factor-beta1 heterozygous mouse model of lung carcinogenesis

The Nkx2.1 homeobox gene and transforming growth factor-beta1 (TGF-beta1) are essential for organogenesis and differentiation of the mouse lung. NKX2.1 is a marker of human lung carcinomas, but it is not known whether this gene participates in early tumorigenesis. Addition of TGF-beta1 to TGF-beta1-responsive nontumorigenic mouse lung cells cotransfected with a NKX2.1Luc luciferase reporter and either a Sp1 or Sp3 plasmid showed a significant increase or decrease, respectively, in NKX2.1Luc transcription. Cotransfection of Sp3 and dominant-negative TGF-beta type II receptor plasmids negated the effect of Sp1. Cotransfected Sp1 plasmid with either dominant-negative Smad2 or Smad3 or Smad4 plasmids significantly decreased NKX2.1Luc transcription. Electrophoretic mobility shift assays revealed binding of Sp1 and Smad4 to the NKX2.1 promoter. With a TGF-beta1 heterozygous mouse model, Nkx2.1 mRNA and protein in lungs of TGF-beta1 heterozygous mice were significantly lower compared to wildtype (WT) littermates. Competitive reverse transcription (RT)-polymerase chain reaction (PCR) and immunostaining showed that Nkx2.1 mRNA and protein decreased significantly in adenomas and adenocarcinomas compared to normal lung tissue. Our in vitro data showed that regulation of Nkx2.1 by TGF-beta1 occurs through TGF-beta type II receptor and Smad signaling, with Sp1 and Sp3 in lung cells. Our in vivo data showed reduced Nkx2.1 in lungs of TGF-beta1 heterozygous mice compared to WT mice, that is detectable in adenomas, and that is further reduced in carcinogenesis, and that correlates with reduction of Sp1, Sp3, and Smads in lung adenocarcinomas. Our findings suggest that reduced Nkx2.1 and TGF-beta1 signaling components may contribute to tumorigenesis in the lungs of TGF-beta1 heterozygous mice.

Estrogen-dependent regulation of ornithine decarboxylase in breast cancer cells through activation of nongenomic cAMP-dependent pathways

17beta-estradiol (E2) induces ornithine decarboxylase (ODC) activity in several E2-responsive tissues/cells, and this study investigated the mechanism of hormone-induced transactivation in MCF-7 human breast cancer cells. E2-induced reporter gene (luciferase) activity in MCF-7 cells transfected with a construct (pODC1) containing the -164 to +29 region of the human ODC gene promoter linked to bacterial luciferase. This promoter sequence contains GC-rich Sp1 binding sites, CAAT, LSF, cAMP response element (CRE), and TATA motifs. Deletion and mutational analysis of the ODC promoter showed that both CAAT and LSF sites were required for hormone-induced transactivation. Gel mobility shift and DNA footprinting assays indicated that NFYA and LSF bound the CAAT and LSF motifs, respectively, and GAL4-NFYA/GAL4-LSF chimeras were also activated by E2, 8-bromo-cAMP, and protein kinase A (PKA) expression plasmid. However, E2-induced transactivation of GAL4-NFYA and GAL4-LSF was blocked by the PKA inhibitor SQ22356 indicating that the mechanism of ODC induction by E2 involves upregulation of cAMP/PKA through nongenomic pathways of estrogen action.

Evidence that Sp1 positively and Sp3 negatively regulate and androgen does not directly regulate functional tumor suppressor 15-lipoxygenase 2 (15-LOX2) gene expression in normal human prostate epithelial cells

In this project, we studied the gene regulation of 15-lipoxygenase 2 (15-LOX2), the most abundant arachidonate-metabolizing LOX in adult human prostate and a negative cell-cycle regulator in normal human prostate (NHP) epithelial cells. Through detailed in silico promoter examination and promoter deletion and activity analysis, we found that several Sp1 sites (i.e., three GC boxes and one CACCC box) in the proximal promoter region play a critical role in regulating 15-LOX2 expression in NHP cells. Several pieces of evidence further suggest that the Sp1 and Sp3 proteins play a physiologically important role in positively and negatively regulating the 15-LOX2 gene expression, respectively. First, mutations in the GC boxes affected the 15-LOX2 promoter activity. Second, both Sp1 and Sp3 proteins were detected in the protein complexes that bound the GC boxes revealed by electrophoretic mobility shift assay. Third, importantly, inhibition of Sp1 activity or overexpression of Sp3 both inhibited the endogenous 15-LOX2 mRNA expression. Since 15-LOX2 is normally expressed in the prostate luminal epithelial cells, we subsequently explored whether androgen/androgen receptor may directly regulate its gene expression. The results indicate that androgen does not directly regulate 15-LOX2 gene expression. Together, these observations provide insight on how 15-LOX2 gene expression may be regulated in NHP cells.

Ultraviolet radiation-induced DNA damage in promoter elements inhibits gene expression

Repair of DNA damage in gene promoters is slower than in actively transcribed genes. Persistent damage in gene promoters though transient can have significant biological effects on regulated gene expression. In this study we investigated the effect of ultraviolet radiation on gene promoter-associated functions when DNA damage is located within and outside transcription factor binding sites. Our results show that both cyclobutane pyrimidine dimers and (6-4) photoproducts inhibit DNA-protein interaction, in vitro transcript production and transactivation of reporter genes. The biological significance of transient DNA damage as a mechanism in carcinogenesis is discussed.

Tissue-specific activity of the proximal human calcitonin receptor promoter is mediated by Sp1 and an epigenetic phenomenon

To identify cis-acting sequences transcriptionally regulating the human calcitonin receptor (hCTR) gene, hCTR promoter/luciferase gene constructs were transiently or stably transfected into hCTR-positive and -negative cell lines. Luciferase assays demonstrated that the proximal hCTR promoter (hCTRP1) was transcriptionally active in all cell lines tested. High-level hCTRP1 activity depended on an 11 bp Sp1/Sp3 binding site. Electrophoretic mobility shift assay showed that this region bound the transcription factors Sp1 and Sp3. We further showed that hCTRP1 was strongly activated by the 11 bp Sp1/Sp3 binding site in hCTRP1/luciferase-, Sp1-transfected Drosophila S2 cells. Bisulphite-mediated sequencing of genomic DNA from hCTR-expressing and -non-expressing cell lines demonstrated that the endogenous hCTRP1 was hypomethylated in all cell lines tested. These results suggest that the hCTRP1 is activated by the tissue-ubiquitous transcription factor Sp1 and that an epigenetic process unrelated to CpG methylation represses its activity in hCTR-negative tissues.

Transcriptional regulation of mouse delta-opioid receptor gene by CpG methylation: involvement of Sp3 and a methyl-CpG-binding protein, MBD2, in transcriptional repression of mouse delta-opioid receptor gene in Neuro2A cells

Opioid receptors are expressed in a cell type-specific manner. Here we show that the mouse delta-opioid receptor (mDOR) gene is regulated by promoter region CpG methylation. The mDOR promoter containing a putative CpG island is highly methylated in Neuro2A cells, correlating with the repression of this gene in these cells. This is in contrast with the unmethylated state of the mDOR promoter in NS20Y cells, which express a high level of mDOR. Repression of mDOR transcription in Neuro2A cells could be partially relieved by chemically induced demethylation with 5-aza-2'-deoxycytidine. In addition, in vitro methylation of the luciferase reporter gene driven by the mDOR promoter resulted in an inhibition of transcription in NS20Y cells. Methyl-CpG-binding protein complex 1 (MeCP1) has been implicated in methylation-mediated transcriptional repression of several genes. Electrophoretic mobility shift assays showed that fully methylated, but not unmethylated, mDOR promoter fragment formed a MeCP1-like protein complex that contained methyl-CpG-binding domain protein 2 (MBD2) and Sp3. Furthermore, the expression level of Sp3 was decreased when Neuro2A cells were demethylated with 5-aza-2'-deoxycytidine, and increasing Sp3 levels in Schneider's Drosophila line 2 cells led to the repression of mDOR promoter activity when the promoter was methylated. These results demonstrate that Sp3 and MBD2 are involved in the transcriptional repression of mDOR in Neuro2A cells through binding to the methylated CpG sites in the promoter region and may play a role in the cell type-specific expression of mDOR.

Molecular characterization of the constitutive expression of the plasma platelet-activating factor acetylhydrolase gene in macrophages

Plasma platelet-activating factor acetylhydrolase (PAF-AH) is a phospholipase that inactivates platelet-activating factor (PAF) and PAF-like lipids to generate products with little or no biological activity. The levels of circulating PAF-AH correlate with several disease syndromes. We previously reported that mediators of inflammation regulate the expression of the human PAF-AH gene at the transcriptional level. In the present paper, we characterize the constitutive expression of plasma PAF-AH using the mouse gene as a model system, and we report comparative results obtained using human and mouse promoter constructs. We first cloned, sequenced and analysed the promoter region of the murine plasma PAF-AH (mPAF-AH) gene and found that this gene lacks a canonical TATA box. We demonstrated that the cis -elements required for basal transcription are localized within the -316 to -68 bp region. In vitro band-shift and supershift assays showed that Sp1 and Sp3 transcription factors from RAW264.7 and J774A.1 macrophage nuclear extracts bound strongly to a distal GC-rich site within -278/-243 [specificity protein (Sp-A)] and to a proximal TC-rich motif within -150/-114 (Sp-B). In addition, we observed weak binding to a GA-rich site within -110/-82 (Sp-C). The regions containing Sp-B and Sp-C are highly conserved between the human and mouse genes. Forced expression of Sp1 or Sp3 in Sp-lacking Drosophila SL2 cells induced markedly the activity of the exogenous mPAF-AH promoter in a dose-dependent manner, and this induction was dependent on the presence of intact Sp-A and Sp-B. Interestingly, we found that the Sp1- and Sp3-associated DNA-binding activities increased during the maturation of primary human monocytes into macrophages in cell culture. These results demonstrate that Sp1 and Sp3 are key factors that contribute to the basal, constitutive transcription of the plasma PAF-AH gene in macrophages.

Stability of the Sp3-DNA complex is promoter-specific: Sp3 efficiently competes with Sp1 for binding to promoters containing multiple Sp-sites

The transcription regulatory protein Sp3 shares more than 90% sequence homology with Sp1 in the DNA-binding domain and they bind to the same cognate DNA-element. However, the transcriptional activities of these two Sp-family factors are not equivalent. While Sp1 functions strictly as a transcriptional activator, Sp3 has been shown to be transcriptionally inactive for promoters containing multiple Sp-binding sites. In the present study, we show that the DNA-binding property of Sp3 is promoter dependent and is different from Sp1. The 116 kDa Sp3 polypeptide binds as a monomer to a single Sp-binding site but readily forms slower migrating complexes with adjacent Sp-binding sites. The slower migrating Sp3-DNA complexes are significantly more stable than monomeric Sp3-DNA complexes or multimeric Sp1-DNA complexes. As a consequence, Sp3 can efficiently compete with Sp1 for binding to regions containing multiple Sp sites. The transcription regulatory function of Sp3 is also significantly different from Sp1. Unlike Sp1, Sp3 does not synergistically activate transcription of promoters containing multiple Sp-binding sites. Therefore, although Sp3 is a transcription activator, Sp3 reduces Sp1-dependent transcription of promoters containing adjacent Sp-binding sites by competing with Sp1 for promoter occupancy and thereby blocking the synergistic transactivation function of Sp1. Taken together, this study provides a possible mechanism of the promoter-specific transcription repression function of Sp3.

Evolution of the coordinate regulation of glycolytic enzyme genes by hypoxia

Two billion years of aerobic evolution have resulted in mammalian cells and tissues that are extremely oxygen-dependent. Exposure to oxygen tensions outside the relatively narrow physiological range results in cellular stress and toxicity. Consequently, hypoxia features prominently in many human diseases, particularly those associated with blood and vascular disorders, including all forms of anemia and ischemia. Bioenergetic enzymes have evolved both acute and chronic oxygen sensing mechanisms to buffer changes of oxygen tension; at normal P(O) oxidative phosphorylation is the principal energy supply for eukaryotic cells, but when the P(O) falls below a critical mark metabolic switches turn off mitochondrial electron transport and activate anaerobic glycolysis. Without this switch cells would suffer an immediate energy deficit and death at low P(O). An intriguing feature of the switching is that the same conditions that regulate energy metabolism also regulate bioenergetic genes, so that enzyme activity and transcription are regulated simultaneously, albeit with different time courses and signaling pathways. In this review we explore the pathways mediating hypoxia-regulated glycolytic enzyme gene expression, focusing on their atavistic traits and evolution.

Molecular cloning and characterization of human acid sensing ion channel (ASIC)2 gene promoter

Acid sensing ion channel (ASIC)2 belongs to the amiloride-sensitive Na(+)-channel/ degenerin family. Our previous studies suggested that differential regulation of ASIC2 expression occurs between high-grade glial-derived tumor cells and normal astrocytes. To investigate the mechanisms involved in the regulation of ASIC2 gene expression, the human ASIC2 promoter region (-1551 to +117) was cloned and characterized. The ASIC2 promoter lacked a canonical TATA box, but contained one putative CCAAT box. Nucleotide sequencing of the promoter revealed the presence of a number of transcription factor-binding sites and a 404 bp CpG island upstream the transcription start site. Nested deletion mutants and transfection results showed that the construct between -133 and +117 base pairs conferred basal transcription specific activity. Mutation of Sp1 and CP2 sites in this region resulted in a 70 and 95% decrease in promoter activity, respectively. Gel shift assays demonstrated the existence of specific protein binding to the SP1 and CP2 elements. There was no mutation in the CpG island in six glioma cell lines, but methylation-specific PCR showed methylation in some of glioma cell lines and tumor tissues, and treatment with the methylation inhibitor 5-Aza-2'-deoxycytidine could partially restore ASIC2 expression in cell lines, suggesting that epigenetic mechanisms may contribute to dysregulated ASIC2 expression.

Gap junctional communication modulates gene transcription by altering the recruitment of Sp1 and Sp3 to connexin-response elements in osteoblast promoters

Loss-of-function mutations of gap junction proteins, connexins, represent a mechanism of disease in a variety of tissues. We have shown that recessive (gene deletion) or dominant (connexin45 overexpression) disruption of connexin43 function results in osteoblast dysfunction and abnormal expression of osteoblast genes, including down-regulation of osteocalcin transcription. To elucidate the molecular mechanisms of gap junction-sensitive transcriptional regulation, we systematically analyzed the rat osteocalcin promoter for sensitivity to gap junctional intercellular communication. We identified an Sp1/Sp3 containing complex that assembles on a minimal element in the -70 to -57 region of the osteocalcin promoter in a gap junction-dependent manner. This CT-rich connexin-response element is necessary and sufficient to confer gap junction sensitivity to the osteocalcin proximal promoter. Repression of osteocalcin transcription occurs as a result of displacement of the stimulatory Sp1 by the inhibitory Sp3 on the promoter when gap junctional communication is perturbed. Modulation of Sp1/Sp3 recruitment also occurs on the collagen Ialpha1 promoter and translates into gap junction-sensitive transcriptional control of collagen Ialpha1 gene expression. Thus, regulation of Sp1/Sp3 recruitment to the promoter may represent a potential general mechanism for transcriptional control of target genes by signals passing through gap junctions.

4-Hydroxy-3-methoxybenzoic acid methyl ester: a curcumin derivative targets Akt/NF kappa B cell survival signaling pathway: potential for prostate cancer management

Transcription factor NFkappaB and the serine/threonine kinase Akt play critical roles in mammalian cell survival signaling and have been shown to be activated in various malignancies including prostate cancer (PCA). We have developed an analogue of curcumin called 4-hydroxy-3-methoxybenzoic acid methyl ester (HMBME) that targets the Akt/NFkappaB signaling pathway. Here, we demonstrate the ability of this novel compound to inhibit the proliferation of human and mouse PCA cells. HMBME-induced apoptosis in these cells was tested by using multiple biochemical approaches, in addition to morphologic analysis. Overexpression of constitutively active Akt reversed the HMBME-induced growth inhibition and apoptosis, illustrating the direct role of Akt signaling in HMBME-mediated growth inhibition and apoptosis. Further, investigation of the molecular events associated with its action in LNCaP cells shows that: 1) HMBME reduces the level of activated form of Akt (phosphorylated Akt); and 2) inhibits the Akt kinase activity. Further, the transcriptional activity of NFkappaB, the DNA-binding activity of NFkappaB, and levels of p65 were all significantly reduced following treatment with HMBME. Overexpression of constitutively active Akt, but not the kinase dead mutant of Akt, activated the basal NFkappaB transcriptional activity. HMBME treatment had no influence on this constitutively active Akt-augmented NFkappaB transcriptional activity. These data indicate that HMBME-mediated inhibition of Akt kinase activity may have a potential in suppressing/decreasing the activity of major survival/antiapoptotic pathways. The potential use of HMBME as an agent that targets survival mechanisms in PCA cells is discussed.

Inhibitory effects of the ginsenoside Rg3 on phorbol ester-induced cyclooxygenase-2 expression, NF-kappaB activation and tumor promotion

Our previous studies demonstrated the anti-oxidant and anti-tumor promotional properties of the methanol extract of heat-processed Panax ginseng C.A. Meyer [Cancer Lett. 150 (2000) 41]. In the present work, we have evaluated anti-inflammatory as well as anti-tumor promoting effects of Rg(3), a major ginsenoside derived from heat-processed ginseng. Pretreatment of dorsal skins of female ICR mice with Rg(3) significantly inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ornithine decarboxylase activity and 7,12-dimethylbenz[a]anthracene-initiated papilloma formation. In another experiment, Rg(3) pretreatment abrogated the expression of cyclooxygenase-2 in TPA-stimulated mouse skin. Rg(3) also inhibited the TPA-induced activation of the eukaryotic transcription factor, NF-kappaB in both mouse skin and cultured human pro-myelocytic leukemia (HL-60) cells. Moreover, Rg(3) exerted potent inhibitory effects on the activation of another transcription factor, activator protein-1 (AP-1) that is responsible for c-jun and c-fos oncogenic transactivation. Based on these findings, it is likely that the anti-tumor promoting activity of Rg(3) is mediated possibly through down-regulation of NF-kappaB and AP-1 transcription factors.

Growth-inhibitory effects of the chemopreventive agent indole-3-carbinol are increased in combination with the polyamine putrescine in the SW480 colon tumour cell line

BACKGROUND

Many tumours undergo disregulation of polyamine homeostasis and upregulation of ornithine decarboxylase (ODC) activity, which can promote carcinogenesis. In animal models of colon carcinogenesis, inhibition of ODC activity by difluoromethylornithine (DFMO) has been shown to reduce the number and size of colon adenomas and carcinomas. Indole-3-carbinol (I3C) has shown promising chemopreventive activity against a range of human tumour cell types, but little is known about the effect of this agent on colon cell lines. Here, we investigated whether inhibition of ODC by I3C could contribute to a chemopreventive effect in colon cell lines.

METHODS

Cell cycle progression and induction of apoptosis were assessed by flow cytometry. Ornithine decarboxylase activity was determined by liberation of CO2 from 14C-labelled substrate, and polyamine levels were measured by HPLC.

RESULTS

I3C inhibited proliferation of the human colon tumour cell lines HT29 and SW480, and of the normal tissue-derived HCEC line, and at higher concentrations induced apoptosis in SW480 cells. The agent also caused a decrease in ODC activity in a dose-dependent manner. While administration of exogenous putrescine reversed the growth-inhibitory effect of DFMO, it did not reverse the growth-inhibition following an I3C treatment, and in the case of the SW480 cell line, the effect was actually enhanced. In this cell line, combination treatment caused a slight increase in the proportion of cells in the G2/M phase of the cell cycle, and increased the proportion of cells undergoing necrosis, but did not predispose cells to apoptosis. Indole-3-carbinol also caused an increase in intracellular spermine levels, which was not modulated by putrescine co-administration.

CONCLUSION

While indole-3-carbinol decreased ornithine decarboxylase activity in the colon cell lines, it appears unlikely that this constitutes a major mechanism by which the agent exerts its antiproliferative effect, although accumulation of spermine may cause cytotoxicity and contribute to cell death. The precise mechanism by which putrescine enhances the growth inhibitory effect of the agent remains to be elucidated, but does result in cells undergoing necrosis, possibly following accumulation in the G2/M phase of the cell cycle.

The 5'-untranslated RNA of the human dhfr minor transcript alters transcription pre-initiation complex assembly at the major (core) promoter

The human dhfr minor transcript is distinguished from the predominant dhfr mRNA by an approximately 400 nucleotide extension of the 5'-untranslated region, which corresponds to the major (core) promoter DNA (its template). Based on its unusual sequence composition, we hypothesized that the minor transcript 5'-UTR might be capable of altering transcription pre-initiation complex assembly at the core promoter, through direct interactions of the RNA with specific regulatory polypeptides or the promoter DNA itself. We found that the minor transcript 5'-UTR selectively sequesters transcription factor Sp3, and to a lesser extent Sp1, preventing their binding to the dhfr core promoter. This allows a third putative transcriptional regulatory protein, which is relatively resistant to sequestration by the minor transcript RNA, the opportunity to bind the dhfr core promoter. The selective sequestration of Sp3 > Sp1 by the minor transcript 5'-UTR involves an altered conformation of the RNA, and a structural domain of the protein distinct from that required for binding to DNA. As a consequence, the minor transcript 5'-UTR inhibits transcription from the core promoter in vitro (in trans) in a concentration-dependent manner. These results suggest that the dhfr minor transcript may function in vivo (in cis) to regulate the transcriptional activity of the major (core) promoter.

SUMO-1 modification represses Sp3 transcriptional activation and modulates its subnuclear localization

The GC box binding transcription factor Sp3 both activates and represses transcription. We have found that Sp3 activity is regulated by SUMO-1 modification. Endogenous Sp3 is sumoylated and localized to the nuclear periphery and in nuclear dots. Removal of SUMO-1 from Sp3 by mutation of the SUMO acceptor lysines or expression of the SUMO-1 protease SuPr-1 converted Sp3 to a strong activator with a diffuse nuclear localization. Covalent attachment of SUMO-1 to Sp3 by gene fusion was sufficient to repress Sp3-dependent transcription and relocalize Sp3 to the nuclear periphery and nuclear dots. These studies reveal a direct effect of SUMO-1 modification on activity of a dual function transcription factor and provide a mechanism for functional specificity within the Sp transcription factor family.

Regulation of transcription of the Dnmt1 gene by Sp1 and Sp3 zinc finger proteins

The Sp family is a family of transcription factors that bind to cis-elements in the promoter regions of various genes. Regulation of transcription by Sp proteins is based on interactions between a GC-rich binding site (GGGCGG) in DNA and C-terminal zinc finger motifs in the proteins. In this study, we characterized the GC-rich promoter of the gene for the DNA methyltransferase (Dnmt1) that is responsible for methylation of cytosine residues in mammals and plays a role in gene silencing. We found that a cis-element (nucleotides -161 to -147) was essential for the expression of the mouse gene for Dnmt1. DNA-binding assays indicated that transcription factors Sp1 and Sp3 bound to the same cis-element in this region in a dose-dependent manner. In Drosophila SL2 cells, which lack the Sp family of transcription factors, forced expression of Sp1 or Sp3 enhanced transcription from the Dnmt1 promoter. Stimulation by Sp1 and Sp3 were independent phenomena. Furthermore, cotransfection reporter assays with a p300-expression plasmid revealed the activation of the promoter of the Dnmt1 gene in the presence of Sp3. The transcriptional coactivator p300 interacted with Sp3 in vivo and in vitro. Our results indicate that expression of the Dnmt1 gene is controled by Sp1 and Sp3 and that p300 is involved in the activation by Sp3.

Transcriptional regulation of human 3'-phosphoadenosine 5'-phosphosulphate synthase 2

Sulphonation is a fundamental process that is essential for normal growth and development as well as maintenance of the internal milieu. The universal sulphonate donor molecule essential for all sulphoconjugation reactions is adenosine 3'-phosphate 5'-phosphosulphate (PAPS), which is produced from ATP and inorganic sulphate by the action of bifunctional PAPS synthase. There are two isozymes encoded by genes located on chromosome 4 (PAPS synthase 1) and chromosome 10 (PAPS synthase 2). The promoter for PAPS synthase 2 contains neither a TATAAA nor a CCAAT box, although a consensus initiator motif is present. Three human cell lines were used to examine promoter activity after transfection with various lengths of the 5'-flanking region of the PAPS synthase 2 gene fused to a reporter gene. Proximal promoter activity was located between bp -84 and bp -124 upstream of the purported transcription start site. This region contains two GC/GT boxes that are essential for full promoter activity, as indicated by deletion analysis and supported further by mutagenesis. A nuclear extract of SW13 cells, which highly express PAPS synthase 2, contained proteins that bound to probes possessing promoter-specific GC/GT boxes. Furthermore, the presence of specificity protein (Sp) 1, Sp2 and Sp3 proteins in the nuclear extract was confirmed by supershift analysis. Co-transfection experiments using SL2 cells yielded additional support for the involvement of Sp1 in transcriptional regulation of the PAPS synthase 2 gene; the involvement of Sp2 and/or Sp3 remains to be clarified further.

Sp1- and Sp3-mediated transcriptional regulation of the fibroblast growth factor receptor 1 gene in chicken skeletal muscle cells

Expression of the fibroblast growth factor receptor 1 (FGFR1) gene in skeletal muscle is positively regulated in proliferating myoblasts and declines during differentiation. We have characterized the cis-regulatory elements in the proximal region of the FGFR1 promoter which render positive transcriptional activity. Multiple elements between -69 and -14 activate the FGFR1 promoter. Myoblast transfections revealed that potential Sp transcription factor binding sites are required for promoter activity. Electromobility shift assays indicated that myoblast nuclear proteins specifically bind to these cis-elements and that differentiated myotube nuclear extracts do not form these same complexes. In addition, Southwestern blot analysis detected binding of the most proximal Sp motif to a Sp1-like protein present in myoblast nuclear extracts but not in myotubes. In corroboration, Sp1 and Sp3 proteins were detected only in myoblasts and not in differentiated myotubes. Finally, transfection of Drosophila SL2 cells showed that Sp1 is a positive regulator of FGFR1 promoter activity and that Sp3 is a coactivator via the proximal Sp binding sites. These studies demonstrate that the FGFR1 promoter is activated by Sp transcription factors in proliferating myoblasts and demonstrate at least part of the mechanism by which FGFR1 gene expression is down-regulated in differentiated muscle fibers.

Characterization of regulatory elements on the promoter region of p16(INK4a) that contribute to overexpression of p16 in senescent fibroblasts

Cyclin-dependent kinase inhibitor p16(INK4a) is implicated in replicative senescence, cell immortalization, and tumor generation. However, the mechanism regulating its overexpression in senescent cells is unknown. We used the enhanced green fluorescent protein reporter system to scan regulatory elements in the upstream region of p16(INK4a). The results of 5'-deletion studies indicated that the transcription regulatory elements contributing to overexpression of p16(INK4a) in senescent cells were located in the region of the p16(INK4a) promoter from -622 to -280 bp. According to the results of in vitro DNase I footprinting, EMSA, and Southwestern blotting, we found a novel negative regulatory element, the INK4a transcription silence element (ITSE), at -491 to -485 bp of the p16(INK4a) promoter. A 24-kDa protein that was highly expressed in young cells may inhibit the expression of p16(INK4a) by interacting with the ITSE. The activity of the p16(INK4a) promoter increased significantly in young cells when the ITSE was deleted. The GC-rich region of the p16(INK4a) promoter from -466 to -451 was a positive transcription regulatory element. Deletion of this region showed 91.4% loss of p16(INK4a) promoter activity in senescent cells, and the promoter activity decreased by 41.2% in young cells comparably.

Molecular cloning and characterization of human nonsteroidal anti-inflammatory drug-activated gene promoter. Basal transcription is mediated by Sp1 and Sp3

Nonsteroidal anti-inflammatory drug-activated gene (NAG-1) is known to be associated with anti-tumorigenic activity and belongs to the transforming growth factor-beta superfamily. In the present study, we cloned the promoter region (-3500 to +41) and investigated the transcriptional regulatory mechanisms of the basal expression of the human NAG-1 gene. Several potential transcription factor-binding sites in this region were identified. Based on the results from clones of nested deletions, the construct between -133 and +41 base pairs contains three Sp1-binding sites (Sp1-A, Sp1-B, and Sp1-C), which confer basal transcription specific activity of NAG-1 expression. When the Sp1-C site was mutated (GG to TT), a 60-80% decrease in promoter activity was observed in HCT-116 cells. Gel shift, co-transfection, and chromatin immunoprecipitation assays showed that the Sp transcription factors bind to the Sp1-binding sites and transactivate NAG-1 expression. In addition, chicken ovalbumin upstream promoter-transcription factor 1 can interact with the C-terminal region of Sp1 and Sp3 proteins and induce NAG-1 promoter activity through Sp1 and Sp3 transcription factors. These results identify the critical regulatory regions for the human NAG-1 basal promoter. Furthermore, the results suggest that the level of expression of the NAG-1 gene will depend on the availability of Sp proteins and on co-factors such as chicken ovalbumin upstream promoter-transcription factor 1.

5-azaC treatment enhances expression of transforming growth factor-beta receptors through down-regulation of Sp3

We have previously reported that Sp3 acts as a transcriptional repressor of transforming growth factor-beta receptors type I (RI) and type II (RII). We now present data suggesting that treatment of MCF-7L breast and GEO colon cancer cells with 5-aza cytidine (5-azaC) leads to down-regulation of Sp3 and the concomitant induction of RI and RII. Western blot and gel shift analyses on 5-azaC-treated MCF-7L and GEO nuclear extracts indicated reduced Sp3 protein levels and decreased binding of Sp3 protein to radiolabeled consensus Sp1 oligonucleotide. Southwestern analysis detected decreased binding of Sp3 to RI and RII promoters in 5-azaC-treated MCF-7L and GEO cells, suggesting a correlation between decreased Sp3 binding and enhanced RI and RII expression in these cells. Reverse transcription-polymerase chain reaction and nuclear run-on data from 5-azaC-treated MCF-7L and GEO cells indicated down-regulation of Sp3 mRNA as a result of decreased transcription of Sp3. We reported earlier that 5-azaC treatment induces RI and RII expression through increased Sp1 protein levels/activities in these cells. These studies demonstrate that the effect of 5-azaC involves a combination of effects on Sp1 and Sp3.

2-methoxyestradiol blocks cell-cycle progression at G(2)/M phase and inhibits growth of human prostate cancer cells

2-Methoxyestradiol (2-ME), an endogenous metabolite of 17beta-estradiol, is present in human blood and urine. Here we show for the first time that 2-ME significantly inhibited the growth of normal prostate epithelial cells and androgen-dependent LNCaP and androgen-independent DU145 prostate cancer cells. This growth inhibition was accompanied by a twofold increase in the G(2)/M population, with a concomitant decrease in the G(1) population, as shown by cell-cycle analysis. 2-ME treatment affected the cell-cycle progression of prostate cancer cells specifically by blocking cells in the G(2) phase. Immunoblot analysis of the key cell-cycle regulatory proteins in the G(2)/M phase showed a 14-fold increase in the expression of p21 and an eightfold increase in the expression of p34 cell division cycle 2 (cdc2). We also found an accumulation of phosphorylated cdc2 after 2-ME treatment. Furthermore, Wee 1 kinase was detectable after 2-ME treatment. 2-ME treatment also led to an increase in the activity of caspase-3, followed by apoptosis, as shown by terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate-biotin nick end-labeling and fluorescein isothiocyanate-poly(ADP-ribose) polymerase assay. Estrogen receptor levels did not change after treatment with 2-ME. Examination of the signaling pathways that mediate 2-ME-induced apoptosis showed reduction in the level of p53 expression and its DNA-binding activity. Given the fact that p53 mutations are common in patients with metastatic prostate cancer, our finding that 2-ME-mediated growth inhibition of human prostate cancer cells occurred in a p53-independent manner has considerable clinical significance. These findings, combined with the limited toxicity of 2-ME, may have significant implications for alternative treatment of advanced prostate cancer.

Transcriptional regulation of human 3'-phosphoadenosine 5'-phosphosulfate synthase 1

Sulfonation, which is essential for normal growth, development and maintenance of the internal milieu, requires the universal sulfonate donor molecule 3'-phosphoadenosine 5'-phosphosulfate (PAPS) produced from ATP and inorganic sulfate by two bifunctional PAPS synthase isozymes. The gene for PAPS synthase 1 containing neither a TATA nor a CCAAT box was found to be under the influence of the Sp1 family of transcription factors. Multiple GC/GT boxes are present in the proximal promoter region and deletion analysis implicated their involvement in transcription, a finding supported by mutational analysis of specific GC/GT boxes. Nuclear extract of SW13 cells, which highly express PAPS synthase 1, contains proteins that bind to probes possessing specific GC/GT boxes; furthermore, the presence of Sp1, Sp2, and Sp3 proteins in nuclear extracts was confirmed by supershift analysis. Cotransfection experiments using SL2 cells yielded additional support for the involvement of Sp1 in transcriptional regulation of the PAPS synthase 1 gene; the involvement of Sp2 and/or Sp3 is presently unclear.

Autostimulation of the Epstein-Barr virus BRLF1 promoter is mediated through consensus Sp1 and Sp3 binding sites

As an essential step in the lytic cascade, the Rta homologues of gammaherpesviruses all activate their own expression. Consistent with this biologic function, the Epstein-Barr virus (EBV) Rta protein powerfully stimulates the promoter of its own gene, Rp, in EBV-positive B cells in transient-transfection reporter-based assays. We analyzed the activity of RpCAT in response to Rta by deletional and site-directed mutagenesis. Two cognate Sp1 binding sites located at -279 and -45 relative to the transcriptional start site proved crucial for Rta-mediated activation. Previously described binding sites for the cellular transcription factor Zif268 and the viral transactivator ZEBRA were found to be dispensable for activation of RpCAT by Rta. Gel shift analysis, using extracts of B cells in latency or induced into the lytic cycle, identified Sp1 and Sp3 as the predominant cellular proteins bound to Rp near -45. During the lytic cycle, ZEBRA bound Rp near the Sp1/Sp3 site. The binding of Sp1 and Sp3 to Rp correlated with the reporter activities in the mutagenesis study, establishing a direct link between transcriptional activation of Rp by Rta and DNA binding by Sp1 and/or Sp3. The relative abundance or functional state of the cellular Sp1 and Sp3 transcription factors may be altered in response to stimuli that induce the BRLF1 promoter and thereby contribute to the activation of the viral lytic cycle.

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.

Sp3 is a transcriptional repressor of transforming growth factor-beta receptors

MCF-7E breast cancer cells express transforming growth factor-beta (TGF-beta) receptors RI and RII in comparison to MCF-7L cells. We present data showing that Sp3 acts as a transcriptional repressor of RI and RII in MCF-7L cells and GEO colon cancer cells. MCF-7L and GEO cells express high levels of Sp3 protein. Gel shift analysis indicated enhanced binding of Sp3 from MCF-7L cells to a consensus Sp1 oligonucleotide. Southwestern data indicated increased binding of Sp3 to RI and RII promoters in MCF-7L cells, suggesting a correlation between Sp3 binding and reduced expression of TGF-beta receptors in MCF-7L cells. Cotransfection of CMV-Sp3 cDNA with RI and RII promoter-luciferase reporter constructs decreased RI and RII promoter activities by 70% in MCF-7E and GEO cells. Southwestern analysis detected the binding of transiently expressed Sp3 to RI and RII promoters in MCF-7E cells. Significantly, ectopic Sp3 expression led to repression of RI and RII transcripts in MCF-7E cells. This report demonstrates that inappropriate overexpression of Sp3 is a mechanism that contributes to repression of TGF-beta receptors.