Serum responsive gene expression mediated by Sp1

Role of Sp1 expression in gastric cancer: A meta-analysis and bioinformatics analysis

Sp1 (specificity protein 1) is an important transcription factor that regulates multiple cancer-related genes. A number of published studies have explored the relationship between Sp1 expression and prognosis in gastric cancer. Therefore, a deeper level of understanding is required into the molecular biological mechanism of gastric cancer. Finding new tumor biomarkers for the accurate prediction of occurrence, recurrence and metastasis of gastric cancer are of great significance. The present study uses a systematic meta-analysis and bioinformatics analysis to acquire evidence for a prognosis marker based on Sp1 expression in gastric cancer. A literature search was performed using PubMed and China National Knowledge Infrastructure on 8th June, 2018. A total of 13 studies were included in the meta-analysis. The meta-analysis showed that the expression of Sp1 was significantly higher in gastric cancer tissue, compared with that of normal mucosa [odds ratio (OR), −0.53; 95% CI, −0.62–0.44; P<0.0001] and dysplasia (OR, 0.24; 95% CI, 0.13–0.44; P<0.0001). A positive association was found Sp1 expression and depth of invasion (OR, 0.31; 95% CI, 0.11–0.86), lymph node metastasis (OR, 0.36; 95% CI, 0.22–0.59), TNM staging of gastric cancer (OR, 0.43; 95% CI, 0.24–0.79) and Lauren's classification (OR, 0.83; 95% CI, 0.51–1.36), but not with sex or tumor differentiation (OR, 1.34; 95% CI, 0.95–1.88). According to the Oncomine database, Sp1 mRNA expression is significantly higher in gastric cancer tissues compared with that in normal tissues (P<0.05), including that of intestinal, diffuse and mixed-type gastric carcinomas (P<0.05). Kaplan-Meier plots show that the expression of Sp1 mRNA is negatively associated with overall and progression-free survival rates of patients with gastric cancer, even when stratified according to expression level (P<0.05). The selected prediction parameter is overall survival or progressive-free survival rate. The expression level of Sp1 was divided into high expression group and low expression group according to the best cut off value provided on the Kaplan-Meier plotter. However, Sp1 protein expression is upregulated in gastric cancer tissues compared with normal tissues and is positively associated with depth of invasion and TNM stage of gastric cancer. The high protein expression of Sp1 might make it a good potential marker for the prognosis of patients with gastric cancer.

The Nutrient-Sensing Hexosamine Biosynthetic Pathway as the Hub of Cancer Metabolic Rewiring

Alterations in glucose and glutamine utilizing pathways and in fatty acid metabolism are currently considered the most significant and prevalent metabolic changes observed in almost all types of tumors. Glucose, glutamine and fatty acids are the substrates for the hexosamine biosynthetic pathway (HBP). This metabolic pathway generates the “sensing molecule” UDP-N-Acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc is the substrate for the enzymes involved in protein N- and O-glycosylation, two important post-translational modifications (PTMs) identified in several proteins localized in the extracellular space, on the cell membrane and in the cytoplasm, nucleus and mitochondria. Since protein glycosylation controls several key aspects of cell physiology, aberrant protein glycosylation has been associated with different human diseases, including cancer. Here we review recent evidence indicating the tight association between the HBP flux and cell metabolism, with particular emphasis on the post-transcriptional and transcriptional mechanisms regulated by the HBP that may cause the metabolic rewiring observed in cancer. We describe the implications of both protein O- and N-glycosylation in cancer cell metabolism and bioenergetics; focusing our attention on the effect of these PTMs on nutrient transport and on the transcriptional regulation and function of cancer-specific metabolic pathways.

Targeting of Sp1 transcription factor: a novel therapeutic approach for keloids, an in vitro analysis

Keloid scars are fibroproliferative disorders characterized by the accumulation of extracellular matrix (ECM) components resulting in a fibrotic condition. Several ECM promoters are regulated by Sp1. Thus, our aim was to investigate the role of Sp1 in keloid pathogenesis and investigate the antiproliferative and antifibrotic effects of Wp631 and mitoxantrone, potent inhibitors of Sp1-activated transcription. An elevated level of Sp1 was observed in tissue extracts obtained from keloid tissue. Serum stimulation elevated Sp1 levels in keloid fibroblasts (KF). Under coculture conditions Sp1 seemed to be downregulated. Wp631 and mitoxanthrone in serum growth factors resulted in a reduced expression of ECM components in KF. Both Wp631 and mitoxanthrone were also able to inhibit the proliferation of normal and keloid keratinocytes and fibroblasts significantly. As Wp631 seems to be potent in downregulating the ECM components in KF and also inhibiting the proliferation of these cells it could be explored as a possible therapeutic agent in the treatment of keloids.

Transcriptional regulation of human papillomavirus type 18 P105 promoter by the co-activator CBP

Human papillomaviruses (HPVs) are the etiological agents of cervical cancer, with HPV-16 and 18 being the representative types of the higher risk group. The expression of the viral genes with transforming activity (E6 and E7) is controlled by the upstream regulatory region (URR), a segment of the viral genome that contains elements recognized by several transcription factors.

OBJECTIVE

We have analyzed the participation of the cellular co-activator CBP on the transcriptional regulation of the HPV-18 URR.

METHODS

We generated mutants and 5' end deletion constructs derived from the HPV-18 URR and evaluated their transcriptional activity performing transient co-transfection assays on C-33A cells with a plasmid that over-expresses the co-activator CBP. We also performed quantitative chromatin immunoprecipitation assays to analyze the participation of the co-activator CBP on the HPV-18 P105 promoter.

RESULTS

Our results demonstrate that in C-33A cells CBP acts as a strong activator of the HPV-18 P105 promoter by a mechanism that depends on the integrity of the SP1-binding site, directly correlating with the acetylation of the histone H3 that is involved in nucleosomal stability.

CONCLUSION

We propose a mechanism of regulation of the HPV-18 P105 promoter by the cellular co-activator CBP, recruited by the transcription factor SP1.

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.

Altered expression of transcription factor Sp1 critically impacts the angiogenic phenotype of human gastric cancer

Our recent study has shown that transcription factor Sp1 is an independent prognostic factor in gastric cancer. However, it is unclear how Sp1 impacts gastric cancer biology. Since Sp1 regulates multiple genes important to angiogenesis, we sought to evaluate the relationship between Sp1 expression and microvessel density (MVD) as well as their effects on cancer patient survival. The expression of Sp1 and status of MVD was determined using archival tissues of 86 cases of resected human gastric cancer. We found that MVD correlated highly with Sp1 expression (P < 0.001). Patients with strong Sp1 expression were 12 times more likely to have high MVD than were those with negative Sp1 expression. In univariate survival analyses, both elevated Sp1 expression (P = 0.007) and high MVD expression (P = 0.036) were associated with inferior survival. However, when Sp1 expression, MVD expression, disease stage, completeness of resection, Lauren's classification, and patient age were entered into a Cox proportional hazards model, only strong Sp1 expression (P = 0.047) and advanced stage (P < 0.01) were independently prognostic of poor survival. Furthermore, knocking down Sp1 expression significantly impaired the angiogenic potential of tumor cells in vitro and suppressed angiogenesis in vivo animal models. Therefore, we provided both clinical and experimental evidence to indicate that Sp1 might impact gastric cancer development and progression through regulating angiogenesis, a critical aspect of cancer biology.

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.

Association between expression of transcription factor Sp1 and increased vascular endothelial growth factor expression, advanced stage, and poor survival in patients with resected gastric cancer

The biological and clinical behaviors of cancer are affected by multiple molecular pathways that are under the control of transcription factors. Improved understanding of how transcription factors affect cancer biology may lead to improved ability to predict clinical outcome and discovery of novel therapeutic strategies. We evaluated the relationship between Sp1 and vascular endothelial growth factor (VEGF) expression, as well as their effect on survival in 86 cases of resected human gastric cancer. The degree of VEGF expression correlated highly with Sp1 expression (P < 0.01). Patients with high Sp1 expression were 98 times more likely to have high VEGF expression compared with those with negative Sp1 expression. Clinically, negative or weak Sp1 expression was associated with early stage (IA) in gastric cancer. Strong Sp1 expression was more frequently observed among patients with stage IB-IV disease (P = 0.035). Similarly, whereas strong Sp1 expression was uncommonly observed among patients with N0 or N1 disease (19 and 16%), N2/N3 gastric cancer was associated with strong Sp1 expression (48%; P = 0.034). Strong Sp1 expression was also associated with inferior survival. The median survival duration in patients who had a tumor with a negative, weak, and strong Sp1 expression was 44, 38, and 8 months (P = 0.0075), respectively, whereas patients with strong VEGF expression had a shorter survival duration; the difference was not statistically significant. When Sp1 and VEGF expression, stage, completeness of resection, histology, and patient age were entered in a Cox proportional hazards model, strong Sp1 expression (P = 0.021) and an advanced disease stage (P < 0.001) were independently prognostic of poor survival. Given the importance of Sp1 in the expression of VEGF, our data suggest that dysregulated Sp1 expression and activation play important roles in VEGF overexpression and, thus, gastric cancer development and progression.

The dominant role of Sp1 in regulating the cystathionine beta-synthase -1a and -1b promoters facilitates potential tissue-specific regulation by Kruppel-like factors

Cystathionine beta-synthase (CBS) catalyzes the condensation of serine with homocysteine to form cystathionine and occupies a crucial regulatory position between the methionine cycle and transsulfuration. The human cystathionine beta-synthase gene promoters -1a and -1b are expressed in a limited number of tissues and are coordinately regulated with proliferation through a redox-sensitive mechanism. Site-directed mutagenesis, DNase I footprinting and deletion analysis of 5276 bp of 5' proximal -1b flanking sequence revealed that this region does not confer tissue-specific expression and that 210 bp of proximal sequence is sufficient for maximal promoter activity. As little as 32 bp of the -1b proximal promoter region is capable of driving transcription in HepG2 cells, and this activity is entirely dependent upon the presence of a single overlapping Sp1/Egr1 binding site. Co-transfection studies in Drosophila SL2 cells indicated that both promoters are transactivated by Sp1 and Sp3 but only the -1b promoter is subject to a site-specific synergistic regulatory interaction between Sp1 and Sp3. Sp1-deficient fibroblasts expressing both Sp3 and NF-Y were negative for CBS activity. Transfection of these cells with a mammalian Sp1 expression construct induced high levels of CBS activity indicating that Sp1 has a critical and indispensable role in the regulation of cystathionine beta-synthase. Sp1 binding to both CBS promoters is sensitive to proliferation status and is negatively regulated by Kruppel-like factors in co-transfection experiments suggesting a possible mechanism for the tissue specific regulation of cystathionine beta-synthase.

Transcriptional control of the RECK metastasis/angiogenesis suppressor gene

The RECK gene is widely expressed in normal human tissues but is downregulated in tumor cell lines and oncogenically transformed fibroblasts. RECK encodes a membrane-anchored glycoprotein that suppresses tumor invasion and angiogenesis by regulating matrix-metalloproteinases (MMP-2, MMP-9 and MT1-MMP). Understanding of the transcriptional regulation of tumor/metastasis suppressor genes constitutes a potent approach to the molecular basis of malignant transformation. In order to uncover the mechanisms of control of RECK gene expression, the RECK promoter has been cloned and characterized. One of the elements responsible for the Ras-mediated downregulation of mouse RECK gene is the Sp1 site, to which Sp1 and Sp3 factors bind. Other regulatory events, such as DNA methylation of the RECK promoter and histone acetylation/deacetylation have been studied to understand the underlying mechanisms of RECK expression. Understanding of the mechanisms which control RECK gene transcription may lead to the development of new strategies for cancer prevention and treatment.

Increased transcription of ubiquitin-proteasome system components: molecular responses associated with muscle atrophy

Muscle atrophy is a common consequence of catabolic conditions like kidney failure, cancer, sepsis, and acute diabetes. Loss of muscle protein is due primarily to activation of the ubiquitin-proteasome proteolytic system. The proteolytic responses to catabolic signals include increased levels of mRNA that encode various components of the system. In the case of two genes, the proteasome C3 subunit and ubiquitin UbC, the higher levels of mRNA result from increased transcription but the mechanisms of transactivation differ between them. This review summaries the evidence that cachectic signals activate a program of selective transcriptional responses in muscle that frequently occurs coordinately with increased protein destruction.

Cystathionine beta-synthase is coordinately regulated with proliferation through a redox-sensitive mechanism in cultured human cells and Saccharomyces cerevisiae

Cystathionine beta-synthase (CBS) catalyzes the condensation of serine with homocysteine to form cystathionine and occupies a crucial regulatory position between the methionine cycle and the biosynthesis of cysteine by transsulfuration. Analysis of CBS activity under a variety of growth conditions indicated that CBS is coordinately regulated with proliferation in both yeast and human cells. In batch cultures of Saccharomyces cerevisiae, maximal CBS activities were observed in the exponential phase of cells grown on glucose, while growth-arrested cultures or those growing non-fermentatively on ethanol or glycerol had approximately 3-fold less activity. CBS activity assays and Western blotting indicated that growth-specific regulation of CBS is evolutionarily conserved in a range of human cell lines. CBS activity was found to be maximal during proliferation and was reduced two- to five-fold when cells became quiescent at confluence. In cultured HepG2 cells, the human CBS gene is induced by serum and basic fibroblast growth factor and is downregulated, but not abolished, by contact inhibition, serum-starvation, nutrient depletion, or the induction of differentiation. Consequently, for certain cell types, CBS may represent a novel marker of both differentiation and proliferation. The intracellular level of the CBS regulator compound, S-adenosylmethionine, was found to reflect the proliferation status of both yeast and human cells, and as such, constitutes an additional mechanism for proliferation-specific regulation of human CBS. Our data indicates that screening compounds for the ability to affect transsulfuration in cultured cell models must take proliferation status into account to avoid masking regulatory interactions that may be of significance in vivo.

Shear stress induces expression of vascular endothelial growth factor receptor Flk-1/KDR through the CT-rich Sp1 binding site

Fluid shear stress is 1 of the major factors that control gene expression in vascular endothelial cells. We investigated the role of shear stress in the regulation of the expression of fetal liver kinase-1/kinase domain region (Flk-1/KDR), a vascular endothelial growth factor receptor, by using human umbilical vein endothelial cells. Laminar shear stress (15 dyne/cm2) elevated Flk-1/KDR mRNA levels by approximately 3-fold for 8 hours, and the expression was upregulated within the range of 5 to 40 dyne/cm2. Deletion analysis of the 5'-flanking region of the Flk-1/KDR gene promoter by use of a luciferase reporter vector revealed that a shear stress-responsive element resided in the sequence between -94 and -31 bp, which contained putative nuclear factor-kappaB, activator protein-2, and GC-rich Sp1 and CT-rich Sp1 binding sites. Electrophoretic mobility shift assay demonstrated that nuclear extract was bound to the GC-rich Sp1 sites and the CT-rich Sp1 site with a similar pattern. However, shear stress enhanced the DNA-protein interactions only on the CT-rich Sp1 site but not on the GC-rich Sp1 sites. A 3-bp mutation in the CT-rich Sp1 site eliminated the response to shear stress in electrophoretic mobility shift assay and luciferase reporter assay. These results suggest that shear stress induces Flk-1/KDR expression through the CT-rich Sp1 binding site.

A novel role of Sp1 and Sp3 in the interferon-gamma -mediated suppression of macrophage lipoprotein lipase gene transcription

The regulation of macrophage lipoprotein lipase by cytokines is of potentially crucial importance in the pathogenesis of atherosclerosis. We have shown previously that macrophage lipoprotein lipase expression is suppressed by interferon-gamma (IFN-gamma) at the transcriptional level. We investigated the regulatory sequence elements and the transcription factors that are involved in this response. We demonstrated that the -31/+187 sequence contains the minimal IFN-gamma-responsive elements. Electrophoretic mobility shift assays showed that the binding of proteins to two regions in the -31/+187 sequence was reduced dramatically when the cells were exposed to IFN-gamma. Both competition electrophoretic mobility shift assays and antibody supershift assays showed that the interacting proteins were composed of Sp1 and Sp3. Mutations of the Sp1/Sp3-binding sites in the minimal IFN-gamma-responsive elements abolished the IFN-gamma-mediated suppression of promoter activity, whereas multimers of the sequence were able to impart the response to a heterologous promoter. Western blot analysis showed that IFN-gamma reduced the steady state levels of Sp3 protein. In contrast, the cytokine decreased the DNA binding activity of Sp1 without affecting the protein levels. These studies therefore reveal a novel mechanism for IFN-gamma-mediated regulation of macrophage gene transcription.

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.

Floral transcription factor AGAMOUS interacts in vitro with a leucine-rich repeat and an acid phosphatase protein complex

We are interested in identifying potential protein interactors of MADS domain transcription factors during Arabidopsis thaliana flower development. We based our biochemical search on a conserved motif in the MADS domain that includes putative phosphatase and phosphorylation sites that may mediate protein interactions. An affinity column with this motif and a few surrounding hypervariable amino acids derived from the AGAMOUS sequence was prepared and used to isolate potential interactors from floral crude extracts. Only two proteins were specifically bound to the affinity column. The first corresponds to a carpel specific storage protein, VSP1, that presents acid phosphatase activity, and the second is a novel leucine-rich repeat protein that we have named FLOR1. Coimmunoprecipitation, two-hybrid yeast, and affinity column assays show that the FLOR1-VSP1 complex interacts with AGAMOUS and that this transcription factor directly interacts with FLOR1. This is the first assay to show an interaction between plant MADS domain factors and non-MADS proteins.

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.

A role for Sp1 in the transcriptional regulation of hepatic triacylglycerol hydrolase in the mouse

Microsomal triacylglycerol hydrolase (TGH) hydrolyzes stored triacylglycerol in cultured hepatoma cells (Lehner, R., and Vance, D. E. (1999) Biochem. J. 343, 1-10). We studied expression of TGH in murine liver and found both protein and mRNA increased dramatically at 27 days after birth. Nuclear run-on assays demonstrated that this was due to increased transcription. We cloned 542 base pairs upstream of the transcriptional start site of the murine TGH gene. Electrophoretic mobility shift assays demonstrated enhanced binding of hepatic nuclear proteins from 27-day-old mice to the murine TGH promoter, yielding three differentially migrating complexes. DNase I footprint analysis localized these complexes to two distinct regions: site A contains a putative Sp binding site, and site B contains a degenerate E box. We transfected primary murine hepatocytes with a series of 5'-deletion constructs upstream of the reporter luciferase cDNA. Positive control elements were identified in a segment containing site A. Competitive electrophoretic mobility shift assays and supershift assays demonstrated that site A binds Sp1 and Sp3. Transcriptional activation assays in Schneider SL-2 insect cells demonstrated that Sp1 is a potent activator of the TGH promoter. These experiments directly link increased TGH expression at the time of weaning to transcriptional regulation by Sp1.

Cell density regulated expression of transcription factor Sp1 in corneal stromal cultures

Sp1, a ubiquitously expressed transcription factor, has been implicated to have a role in cell differentiation and cell proliferation. In keratoconus, a corneal disease characterized by thinning and scarring of the central cornea, Sp1 is found up-regulated. In the present study, we examined the expression of Sp1 in stromal cells cultured from normal human and keratoconus-afflicted corneas and evaluated the influence of varying cell densities. Immunohistochemical staining, Western blotting and electrophoretic mobility shift assays indicated that in both normal human and keratoconus cultures, Sp1 protein levels and binding activities increased with the density of cells. The basal level of Sp1 in keratoconus cultures was higher than that in normals. These results demonstrate a marked density mediated up-regulation of Sp1 in corneal stromal cells, suggesting that the Sp1 expression may be regulated by differentiation states of the cells in the cornea. In addition, cells from keratoconus corneas in vitro appear to carry and retain the Sp1 abnormality as in vivo. The Sp1 defect may be an inborn error in keratoconus.

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.

Sp1 involvement in the 4beta-phorbol 12-myristate 13-acetate (TPA)-mediated increase in resistance to methotrexate in Chinese hamster ovary cells

4beta-Phorbol 12-myristate 13-acetate (TPA) increases the number of colonies resistant to methotrexate (MTX), mainly by amplification of the dihydrofolate reductase (dhfr) locus. We showed previously that inhibition of protein kinase C (PKC) prevents this resistance. Here, we studied the molecular changes involved in the development of TPA-mediated MTX resistance in Chinese hamster ovary (CHO) cells. TPA incubation increased the expression and activity of DHFR. Because Sp1 controls the dhfr promoter, we determined the effect of TPA on the expression of Sp1 and its binding to DNA. TPA incubation increased Sp1 binding and the levels of Sp1 protein. The latter effect was due to an increase in Sp1 mRNA. Dephosphorylation of nuclear extracts from control or TPA-treated cells reduced the binding of Sp1. Stable transfectants of PKCalpha showed increased Sp1 binding, and when treated with MTX, developed a greater number of resistant colonies than control cells. Seventy-five percent of the isolated colonies showed increased copy number for the dhfr gene. Transient expression of PKCalpha increased DHFR activity. Over-expression of Sp1 increased resistance to MTX, and inhibition of Sp1 binding by mithramycin decreased this resistance. We conclude that one mechanism by which TPA enhances MTX resistance, mainly by gene amplification, is through an increase in Sp1 expression which leads to DHFR activation.

Sp1 phosphorylation regulates inducible expression of platelet-derived growth factor B-chain gene via atypical protein kinase C-zeta

Platelet-derived growth factor (PDGF) is a broadly expressed mitogenic and chemotactic factor with diverse roles in a number of physiologic and pathologic settings. The zinc finger transcription factors Sp1, Sp3 and Egr-1 bind to overlapping elements in the proximal PDGF B-chain promoter and activate transcription of this gene. The anthracycline nogalamycin has previously been reported to inhibit the capacity of Egr-1 to bind DNA in vitro. Here we used electrophoretic mobility shift assays to show that nogalamycin added to cells in culture did not alter the interaction of Egr-1 with the PDGF-B promoter. Instead, it enhanced the capacity of Sp1 to bind DNA. Nogalamycin increased PDGF-B mRNA expression at the level of transcription, which was abrogated by mutation of the Sp1 binding site in the PDGF-B promoter or overexpression of mutant Sp1. Rather than increasing total levels of Sp1, nogalamycin altered the phosphorylation state of the transcription factor. Overexpression of dominant-negative PKC-zeta blocked nogalamycin-inducible Sp1 phosphorylation and PDGF-B promoter-dependent expression. Nogalamycin stimulated the phosphorylation of PKC-zeta (on residue Thr(410)). These findings demonstrate for the first time that PKC-zeta and Sp1 phosphorylation mediate the inducible expression of this growth factor.

Modulation of Sp1 activity by a cyclin A/CDK complex

Transcription factors of the Sp1 family are targets of several regulatory pathways and can induce or inhibit gene expression. Here we show that Sp1 is associated with a histone 1 kinase activity. This activity is growth regulated and correlates with the expression of cyclin A. Co-immunoprecipitation experiments demonstrate, that Sp1 interacts with cyclin A and can be phosphorylated by a cyclin A associated kinase. The interaction is direct and requires the zinc-finger region of Sp1 and the amino-terminal domain of cyclin A. Over-expression of cyclin A enhances the expression of a reporter gene controlled by an Sp1 responsive promoter. Addition of olomoucine, a specific inhibitor of CDK2 and CDC2 activity on the other hand reduces the expression of the reporter. Electrophoretic mobility shift assays suggest that this is due to a reduction of the DNA-binding ability of Sp1 family members. Our results indicate that phosphorylation of Sp1 and other members of the family by a cyclin A/CDK complex may play a role in the growth and cell cycle regulation of its transcriptional activity.

Signaling through CD31 protects endothelial cells from apoptosis

Endothelial damage has been implicated in the pathogenesis of chronic rejection. Conversely, expression of protective genes [including A20, A1, bcl-xl, and hemoxygenase-1 (HO-1)] in the endothelium has been associated with long-term graft survival. Overexpression of protective genes in cultured endothelial cells confers protection from apoptosis and prevents expression of inflammatory molecules through inactivation of NF-kappaB. CD31 (PECAM-1) expressed at endothelial cell junctions is ligated by leukocytes during transendothelial migration. Our laboratory has recently shown that cross-linking CD31 using a monoclonal antibody (LCI-4) triggers signaling events in endothelial cells. In this study, we demonstrate that treatment with LCI-4 protected serum-starved endothelial cells from apoptosis. CD31 cross-linking also led to elevation of A20 and A1 mRNA levels and activation of the transcription factor Sp-1. In summary, signaling through CD31 on endothelial cells leads to protection from apoptosis in association with up-regulation of two protective molecules, A20 and A1.

Distinct cellular factors regulate the c-myb promoter through its E2F element

Most E2F-driven promoters are transiently activated around the G(1)/S transition. Although the promoter for the c-myb proto-oncogene harbors an E2F element, it is induced early in G(1) following entry into the cell cycle. Furthermore, this promoter remains active throughout subsequent cell cycles. Since E2F sites function as repressor elements during G(1) (due to the association of pRb with E2F factors), we investigated whether the E2F element in the c-myb promoter is regulated differently than E2F elements in promoters that are repressed during G(1). By gel shift analysis, the E2F element from the c-myb promoter was found to form a unique complex, referred to as E2Fmyb-sp, which was not observed with E2F elements from several other promoters. Antibodies to DP-1, E2F1 to -5, p107, or pRb failed to either supershift or block E2Fmyb-sp complex formation. Methylation interference experiments indicate that the DNA contact residues for the E2Fmyb-sp complex are distinct from but overlapping with residues required for the binding of E2F proteins. In addition to the identification of E2Fmyb-sp, we have found that SP-1 binds to the c-myb E2F element. Functional studies revealed that E2Fmyb-sp and/or SP-1 are required to achieve full activation of the c-myb promoter in different cell types and to maintain elevated expression of the c-myb promoter during G(1) in NIH 3T3 cells. These studies demonstrate that E2F elements can be regulated differently through the binding of unique sets of proteins.

Enhanced Sp1 DNA-binding activity in murine keratinocyte cell lines and epidermal tumors

Altered regulation of ornithine decarboxylase (ODC) is frequently observed in epidermal tumors. We have shown that the transcription factor Sp1 is one of the regulators of ODC expression and that Sp3 antagonizes this Sp1-mediated activation of ODC expression. These results led us to examine the levels and binding activity of Sp1 and Sp3 in nuclear extracts prepared from cultured murine keratinocytes, transformed keratinocyte cell lines and epidermal tumors. Here we show that the Sp1 DNA-binding activity is higher in established keratinocyte cell line extracts than in primary keratinocyte extracts. Sp1 message levels and Sp1 DNA-binding activity was found to be low in 20-week papillomas and high in squamous cell carcinomas. These results suggest that increased levels of Sp1 and enhanced Sp1 DNA binding activity are correlated with epidermal tumor progression. Based on these results, we propose that increased Sp1 DNA binding may augment the proliferative capacity of tumor cells through overexpression of Sp1-responsive genes, possibly including ODC.