Glucose regulates the promoter activity of aldolase A and pyruvate kinase M2 via dephosphorylation of Sp1

Role of Pyruvate Kinase M2 (PKM2) in Cardiovascular Diseases

Cardiovascular diseases (CVDs) are the world's leading cause of death, accounting for 32% of all fatalities. Although therapeutic agents are available for CVDs, however, most of them have significant limitations such as the time-dependency effect, hypotension, and bradycardia. To overcome the limitations of current pharmacological therapies, new molecular targets and pathways need to be identified and investigated to provide better treatment options for CVDs. Recent evidence suggested the involvement of pyruvate kinase M2 (PKM2) and targeting PKM2 by its modulators (inhibitors and activators) has shown promising results in several CVDs. PKM2 regulates gene activation in the context of apoptosis, mitosis, hypoxia, inflammation, and metabolic reprogramming. PKM2 modulators might have a significant impact on the molecular pathways involved in CVD pathogenesis. Therefore, PKM2 modulators can be one of the therapeutic options for CVDs. This review provides an insight into PKM2 involvement in various CVDs along with their therapeutic potential.

SP and KLF Transcription Factors in Cancer Metabolism

Tumor development and progression depend on reprogramming of signaling pathways that regulate cell metabolism. Alterations to various metabolic pathways such as glycolysis, oxidative phosphorylation, lipid metabolism, and hexosamine biosynthesis pathway are crucial to sustain increased redox, bioenergetic, and biosynthesis demands of a tumor cell. Transcription factors (oncogenes and tumor suppressors) play crucial roles in modulating these alterations, and their functions are tethered to major metabolic pathways under homeostatic conditions and disease initiation and advancement. Specificity proteins (SPs) and Krüppel-like factors (KLFs) are closely related transcription factors characterized by three highly conserved zinc fingers domains that interact with DNA. Studies have demonstrated that SP and KLF transcription factors are expressed in various tissues and regulate diverse processes such as proliferation, differentiation, apoptosis, inflammation, and tumorigenesis. This review highlights the role of SP and KLF transcription factors in the metabolism of various cancers and their impact on tumorigenesis. A better understanding of the role and underlying mechanisms governing the metabolic changes during tumorigenesis could provide new therapeutic opportunities for cancer treatment.

Type-2 diabetes mellitus-associated cancer risk: In pursuit of understanding the possible link

BACKGROUND AND AIM

The insulin resistance-mediated abnormal gluconeogenesis when exceeds a given threshold culminates in type 2 diabetes mellitus (T2DM). This induces severe cellular oxidative stress that may eventually facilitate typical neoplastic transformations. This narrative review aims to portray some of the plausible key mechanistic links bridging T2DM and specific cancers.

METHODS

A thorough literature search was conducted in the PubMedCentral database to retrieve information from various reputed biomedical reports/articles published from the year 2000. The information regarding the key biochemical signaling pathways mediating the carcinogenic transformation, especially in T2DM patients, was extensively excavated to systematically compile and present a narrative review.

RESULTS

T2DM-associated insulin resistance is known to negatively influence certain crucial genetic and metabolic components (such as insulin/IGFs, PI-3K/Akt, AMPK, and AGEs/RAGE) that may eventually lead to neoplastic transformation. In particular, the risk of developing cancers like pancreatic, colorectal, breast, liver, endometrial, and bladder seems to be more significant in T2DM patients.

CONCLUSION

Despite the fact that several studies have suggested a possible correlation between T2DM and cancer mortality, a more detailed research at both pre-clinical and clinical levels is still required so as to fully understand the intricate relationship and make a precise conclusion.

Hyperglycemia Associated Metabolic and Molecular Alterations in Cancer Risk, Progression, Treatment, and Mortality

Cancer and diabetes are amongst the leading causes of deaths worldwide. There is an alarming rise in cancer incidences and mortality, with approximately 18.1 million new cases and 9.6 million deaths in 2018. A major contributory but neglected factor for risk of neoplastic transformation is hyperglycemia. Epidemiologically too, lifestyle patterns resulting in high blood glucose level, with or without the role of insulin, are more often correlated with cancer risk, progression, and mortality. The two conditions recurrently exist in comorbidity, and their interplay has rendered treatment regimens more challenging by restricting the choice of drugs, affecting surgical consequences, and having associated fatal complications. Limited comprehensive literature is available on their correlation, and a lack of clarity in understanding in such comorbid conditions contributes to higher mortality rates. Hence, a critical analysis of the elements responsible for enhanced mortality due to hyperglycemia-cancer concomitance is warranted. Given the lifestyle changes in the human population, increasing metabolic disorders, and glucose addiction of cancer cells, hyperglycemia related complications in cancer underline the necessity for further in-depth investigations. This review, therefore, attempts to shed light upon hyperglycemia associated factors in the risk, progression, mortality, and treatment of cancer to highlight important mechanisms and potential therapeutic targets.

HIF1 mediates a switch in pyruvate kinase isoforms after myocardial infarction

Alternative splicing of RNA is an underexplored area of transcriptional response. We expect that early changes in alternatively spliced genes may be important for responses to cardiac injury. Hypoxia inducible factor 1 (HIF1) is a key transcription factor that rapidly responds to loss of oxygen through alteration of metabolism and angiogenesis. The goal of this study was to investigate the transcriptional response after myocardial infarction (MI) and to identify novel, hypoxia-driven changes, including alternative splicing. After ligation of the left anterior descending artery in mice, we observed an abrupt loss of cardiac contractility and upregulation of hypoxic signaling. We then performed RNA sequencing on ischemic heart tissue 1 and 3 days after infarct to assess early transcriptional changes and identified 89 transcripts with altered splicing. Of particular interest was the switch in Pkm isoform expression (pyruvate kinase, muscle). The usually predominant Pkm1 isoform was less abundant in ischemic hearts, while Pkm2 and associated splicing factors (hnRNPA1, hnRNPA2B1, Ptbp1) rapidly increased. Despite increased Pkm2 expression, total pyruvate kinase activity remained reduced in ischemic myocardial tissue. We also demonstrated HIF1 binding to PKM by chromatin immunoprecipitation, indicating a direct role for HIF1 in mediating this isoform switch. Our study provides a new, detailed characterization of the early transcriptome after MI. From this analysis, we identified an HIF1-mediated alternative splicing event in the PKM gene. Pkm1 and Pkm2 play distinct roles in glycolytic metabolism and the upregulation of Pkm2 is likely to have important consequences for ATP synthesis in infarcted cardiac muscle.

Posttranslational Modifications of Pyruvate Kinase M2: Tweaks that Benefit Cancer

Cancer cells rewire metabolism to meet biosynthetic and energetic demands. The characteristic increase in glycolysis, i.e., Warburg effect, now considered as a hallmark, supports cancer in various ways. To attain such metabolic reshuffle, cancer cells preferentially re-express the M2 isoform of pyruvate kinase (PKM2, M2-PK) and alter its quaternary structure to generate less-active PKM2 dimers. The relatively inactive dimers cause the accumulation of glycolytic intermediates that are redirected into anabolic pathways. In addition, dimeric PKM2 also benefits cancer cells through various non-glycolytic moonlight functions, such as gene transcription, protein kinase activity, and redox balance. A large body of data have shown that several distinct posttranslation modifications (PTMs) regulate PKM2 in a way that benefits cancer growth, e.g., formation of PKM2 dimers. This review discusses the recent advancements in our understanding of various PTMs and the benefits they impart to the sustenance of cancer. Understanding the PTMs in PKM2 is crucial to assess their therapeutic potential and to design novel anticancer strategies.

miR-361-5p modulates metabolism and autophagy via the Sp1-mediated regulation of PKM2 in prostate cancer

Prostate cancer (PCa) is a leading cause of death among men. The dysregulation of metabolism and autophagy contributes to the progression of PCa. The transcription factor specificity protein 1 (Sp1) is implicated in the regulation of metabolism and autophagy. We confirmed that Sp1 is overexpressed in castration-resistant prostate cancer (CRPC) cells. However, the roles of Sp1 in PCa metabolism and autophagy remain unclear. Thus, in the present study, we retrieved the GSE35988 dataset from Gene Expression Omnibus (GEO) database to reinvestigate Sp1 expression and its role in PCa.We found that in PCa, Sp1 knockdown significantly inhibited cell growth, aerobic glycolysis, and hypoxia-induced autophagy, which were accompanied by an increased G1 cell cycle arrest. Pearson correlation indicated that pyruvate kinase isoenzyme type M2 (PKM2) is positively correlated with Sp1 expression. Western blot analysis demonstrated that Sp1 directly regulates PKM2; therefore, Sp1 modulates metabolism and autophagy in CRPC. Western blot analysis and luciferase reporter assay also indicated that the tumor suppressor miR-361-5p inversely regulates Sp1 by directly targeting the binding site in the 3'UTR of Sp1. miR-361-5p overexpression presented effects that are similar to Sp1 depletion in PCa. In summary, this study is the first to demonstrate that miR-361-5p suppresses the Sp1/PKM2 axis, consequently affecting the progression of PCa and the metabolism and autophagy of PCa cells. Therefore, targeting the miR-361-5p/Sp1/PKM2 pathway has considerable clinical significance in preventing the malignant progression of PCa.

Dysregulated glycolysis as an oncogenic event

Enhanced glycolysis in cancer, called the Warburg effect, is a well-known feature of cancer metabolism. Recent advances revealed that the Warburg effect is coupled to many other cancer properties, including adaptation to hypoxia and low nutrients, immortalisation, resistance to oxidative stress and apoptotic stimuli, and elevated biomass synthesis. These linkages are mediated by various oncogenic molecules and signals, such as c-Myc, p53, and the insulin/Ras pathway. Furthermore, several regulators of glycolysis have been recently identified as oncogene candidates, including the hypoxia-inducible factor pathway, sirtuins, adenosine monophosphate-activated kinase, glycolytic pyruvate kinase M2, phosphoglycerate mutase, and oncometabolites. The interplay between glycolysis and oncogenic events will be the focus of this review.

The multifaceted regulation and functions of PKM2 in tumor progression

Tumor cells undergo metabolic rewiring from oxidative phosphorylation towards aerobic glycolysis to maintain the increased anabolic requirements for cell proliferation. It is widely accepted that specific expression of the M2 type pyruvate kinase (PKM2) in tumor cells contributes to this aerobic glycolysis phenotype. To date, researchers have uncovered myriad forms of functional regulation for PKM2, which confers a growth advantage on the tumor cells to enable them to adapt to various microenvironmental signals. Here the richness of our understanding on the modulations and functions of PKM2 in tumor progression is reviewed, and some new insights into the paradoxical expression and functional differences of PKM2 in distinct cancer types are offered.

Multiple E-boxes in the distal promoter of the rat pyruvate carboxylase gene function as a glucose-responsive element

Pyruvate carboxylase (PC) is an anaplerotic enzyme that regulates glucose-induced insulin secretion in pancreatic islets. Dysregulation of its expression is associated with type 2 diabetes. Herein we describe the molecular mechanism underlying the glucose-mediated transcriptional regulation of the PC gene. Incubation of the rat insulin cell line INS-1 832/13 with glucose resulted in a 2-fold increase in PC mRNA expression. Transient transfections of the rat PC promoter-luciferase reporter construct in the above cell line combined with mutational analysis indicated that the rat PC gene promoter contains the glucose-responsive element (GRE), comprising three canonical E-boxes (E1, E3 and E4) and one E-box-like element (E2) clustering between nucleotides –546 and –399, upstream of the transcription start site. Mutation of any of these E-boxes resulted in a marked reduction of glucose-mediated transcriptional induction of the reporter gene. Electrophoretic mobility shift assays revealed that the upstream stimulatory factors 1 and 2 (USF1 and USF2) bind to E1, the Specificity Protein-1 (Sp1) binds to E2, USF2 and the carbohydrate responsive element binding protein (ChREBP) binds to E4, while unknown factors binds to E3. High glucose promotes the recruitment of Sp1 to E2 and, USF2 and ChREBP to E4. Silencing the expression of Sp1, USF2 and ChREBP by their respective siRNAs in INS-1 832/13 cells blunted glucose-induced expression of endogenous PC. We conclude that the glucose-mediated transcriptional activation of the rat PC gene is regulated by at least these three transcription factors.

Pyruvate kinase M2 and cancer: an updated assessment

Cancer cells are characterized by high glycolytic rates to support energy regeneration and anabolic metabolism, along with the expression of pyruvate kinase isoenzyme M2 (PKM2). The latter catalyzes the last step of glycolysis and reprograms the glycolytic flux to feed the special metabolic demands of proliferating cells. Besides, PKM2 has moonlight functions, such as gene transcription, favoring cancer. Accumulating evidence suggests a critical role played by the low-activity-dimeric PKM2 in tumor progression, supported by the identification of mutations which result in the down-regulation of its activity and tumorigenesis in a nude mouse model. This review discusses PKM2 regulation and the benefits it confers to cancer cells. Further, conflicting views on PKM2's role in cancer, its therapeutic relevance and future directions in the field are also discussed.

Regulation and function of pyruvate kinase M2 in cancer

Altered metabolism is fundamental to the growth and survival of cancer cells. Pyruvate kinase M2 (PKM2), a key enzyme in cancer metabolism, has been demonstrated to play a central role not only in metabolic reprogramming but also in direct regulation of gene expression and subsequent cell cycle progression. This review outlines the current understanding of PKM2 protein kinase activity and regulatory mechanisms underlying PKM2 expression, enzymatic activity, and nuclear localization, thus highlighting PKM2 as a potential therapeutic target.

Role of sp transcription factors in the regulation of cancer cell metabolism

Cancer cells exhibit altered metabolism characterized by the generation of adenosine triphosphate by glycolysis and generation of fatty acids by de novo synthesis. The majority of genes involved in these pathways have binding sites for specificity protein (Sp) transcription factors in their promoters. Studies showing that Sp transcription factors, particularly Sp1, are involved in the regulation in cancer cells of hexokinase, pyruvate kinase, lactate dehydrogenase, fatty acid synthase, and hypoxia-inducible factor-1α are reviewed. Glycolysis and lipogenesis in cancers are also known to be stimulated by the constitutive activation of the PI3K/Akt signaling pathway. Evidence is presented for the notion that Sp transcription factors may act in concert with Akt to regulate the abnormal metabolism of cancer cells.

Sp1 is a transcription repressor to stanniocalcin-1 expression in TSA-treated human colon cancer cells, HT29

Our previous study demonstrated that, stanniocalcin-1 (STC1) was a target of histone deacetylase (HDAC) inhibitors and was involved in trichostatin A (TSA) induced apoptosis in the human colon cancer cells, HT29. In this study, we reported that the transcriptional factor, specificity protein 1 (Sp1) in association with retinoblastoma (Rb) repressed STC1 gene transcription in TSA-treated HT29 cells. Our data demonstrated that, a co-treatment of the cells with TSA and Sp1 inhibitor, mithramycin A (MTM) led to a marked synergistic induction of STC1 transcript levels, STC1 promoter (1 kb)-driven luciferase activity and an increase of apoptotic cell population. The knockdown of Sp1 gene expression in TSA treated cells, revealed the repressor role of Sp1 in STC1 transcription. Using a protein phosphatase inhibitor okadaic acid (OKA), an increase of Sp1 hyperphosphorylation and so a reduction of its transcriptional activity, led to a significant induction of STC1 gene expression. Chromatin immunoprecipitation (ChIP) assay revealed that Sp1 binding on STC1 proximal promoter in TSA treated cells. The binding of Sp1 to STC1 promoter was abolished by the co-treatment of MTM or OKA in TSA-treated cells. Re-ChIP assay illustrated that Sp1-mediated inhibition of STC1 transcription was associated with the recruitment of another repressor molecule, Rb. Collectively our findings identify STC1 is a downstream target of Sp1.

Rat pancreatic level of cystathionine γ-lyase is regulated by glucose level via specificity protein 1 (SP1) phosphorylation

AIMS/HYPOTHESIS

Cystathionine γ-lyase (CSE) catalyses the endogenous production of hydrogen sulphide (H(2)S) in pancreatic beta cells, and H(2)S has been shown to inhibit insulin release from these cells. As altered pancreatic H(2)S production modulated by glucose has been previously shown, we hypothesised that the Cse gene could be regulated by glucose level in insulin-secreting cells.

METHODS

The effects of glucose on CSE protein level and mRNA level were analysed in INS-1E cells. Glucose effect on Cse promoter activity was tested by constructing a proximal Cse promoter vector including specificity protein 1 (Sp1) consensus sequence.

RESULTS

High glucose (20 mmol/l) inhibited H(2)S production in INS-1E cells and freshly isolated rat pancreatic islets. Cse mRNA expression, CSE activity and protein abundance were also profoundly reduced by high glucose. The involvement of SP1 in basal and high-glucose-regulated CSE production was demonstrated. Sp1-knockdown abolished a large portion of CSE production at basal glucose. Phosphorylation of SP1 stimulated by high glucose was inhibited by p38 mitogen-activated protein kinase (MAPK) inhibitors SB203580 and SB202190. After blocking p38 MAPK phosphorylation, the inhibitive effects of high glucose on CSE protein production and promoter activity in INS-1E cells were also virtually abolished.

CONCLUSIONS/INTERPRETATION

Glucose stimulates the phosphorylation of SP1 via p38 MAPK activation, which leads to decreased Cse promoter activity and subsequent downregulation of Cse gene expression. Inhibited H(2)S production through glucose-mediated CSE activity and production alterations may be involved in the fine control of glucose-induced insulin secretion.

Post-translational control of sp-family transcription factors

Sp-family transcription factors are widely expressed in human tissues and involved in the regulation of many cellular processes and response to cellular microenvironment. These responses appear to be mediated by alterations in transcription factor affinity for DNA rather than altered protein level. How might such changes be effected? This review will identify the range of known post-translational modifications (PTMs) of Sp-factors and the sometimes conflicting literature about the roles of PTMs in regulating activity. We will speculate on the interaction between cell environment, chromatin microenvironment and the role of PTM in governing functionality of the proteins and the complexes to which they belong.

Specificity protein-1 as a critical regulator of human cystathionine gamma-lyase in smooth muscle cells

Cystathionine γ-lyase (CSE) is the major enzyme in vascular smooth muscle cells (SMCs) that catalyzes the endogenous production of H(2)S. Phenotypic switching of SMCs is affected by endogenous H(2)S level and alterations of this switching may result in vascular disorders. To date, the mechanisms underlying the alteration of CSE expression and H(2)S production in vascular proliferative diseases have been unclear. In the present study, we found that serum deprivation induced SMC differentiation marker gene expressions and increased CSE expression and H(2)S production in cultured human aorta SMCs (HASMCs). Carotid artery ligation in mice resulted in enhanced neointima formation and down-regulation of CSE expression, suggesting an important role of CSE in SMC differentiation. Transient transfection of HASMCs with human CSE (hCSE) promoter/luciferase reporter revealed that the region between -226 to +140 base pair contains the core promoter for the hCSE gene. Deletion and mutation analysis demonstrated that two specificity protein-1 (Sp1) consensus binding sites were present in the core promoter region of the hCSE gene. Incubation of HASMCs with Sp1 binding inhibitor mithramycin inhibited CSE mRNA expression in a dose-dependent manner. Overexpression of Sp1 alone was sufficient to increase the activity of the hCSE core promoter and CSE protein expression. Chromatin immunoprecipitation assay showed that the binding of Sp1 to the hCSE promoter was increased in differentiated HASMCs compared with that in proliferated HASMCs. Exogenously applied H(2)S at 100 μM stimulated SMC differentiation, which was reversed by p38 MAPK inhibitor SB203580. These results suggest that transcript factor Sp1 is a critical regulator of the hCSE expression during SMC differentiation, and CSE/H(2)S system is essential for maintenance of SMC phenotype.

A novel integrative network approach to understand the interplay between cardiovascular disease and other complex disorders

There is accumulating evidence that the proteins encoded by the genes associated with a common disorder interact with each other, participate in similar pathways and share GO terms. It has been anticipated that the functional modules in a disease related functional linkage network can be integrated with bibliomics to reveal association with other complex disorders. In this study, the cardiovascular disease functional linkage network (CFN) containing 1536 nodes and 3345 interactions was constructed using proteins encoded by 234 genes associated with the disease. Integration of CFN with bibliomics showed that 227 out of 566 functional modules are significantly associated with one or more diseases. Analysis of functional modules revealed the possible regulatory roles of SP1 and CXCL12 in the pathogenesis of cardiovascular disease (CVD) and modulation of their activities may be considered as potential therapeutic tools. The integration of CFN with bibliomics also indicated significant relations of CVD with other complex disorders. In a stratified map the members of 227 functional modules and 58 diseases in 15 disease classes were combined. In this map, leprosy, listeria monocytogenes, myasthenia, hemorrhagic diathesis and Protein S deficiency, which were not previously reported to be associated with CVD, showed significant associations. Several cancers arising from epithelial cells were also found to be linked to other diseases through hub proteins, VEGFA and PTGS2.

The complete control of glucose level utilizing the composition of ketogenic diet with the gluconeogenesis inhibitor, the anti-diabetic drug metformin, as a potential anti-cancer therapy

In the animal models of glucose dependent cancer growth, the growth is decreased 15-30% through the use of low-carbohydrate, calorically restricted and/or ketogenic diet. The remaining growth depends on glucose formed by the liver-kidney gluconeogenesis as is the case in the cancer cachexia. It is hypothesized that a new treatment for cancer diseases should be explored which includes the ketogenic diet combined with the inhibition of gluconeogenesis by the anti-diabetic drug metformin.

Role of obesity-associated dysfunctional adipose tissue in cancer: a molecular nutrition approach

Obesity is a complex disease caused by the interaction of a myriad of genetic, dietary, lifestyle and environmental factors, which favors a chronic positive energy balance, leading to increased body fat mass. There is emerging evidence of a strong association between obesity and an increased risk of cancer. However, the mechanisms linking both diseases are not fully understood. Here, we analyze the current knowledge about the potential contribution that expanding adipose tissue in obesity could make to the development of cancer via dysregulated secretion of pro-inflammatory cytokines, chemokines and adipokines such as TNF-α, IL-6, leptin, adiponectin, visfatin and PAI-1. Dietary factors play an important role in the risk of suffering obesity and cancer. The identification of bioactive dietary factors or substances that affect some of the components of energy balance to prevent/reduce weight gain as well as cancer is a promising avenue of research. This article reviews the beneficial effects of some bioactive food molecules (n-3 PUFA, CLA, resveratrol and lipoic acid) in energy metabolism and cancer, focusing on the molecular mechanisms involved, which may provide new therapeutic targets in obesity and cancer.

Sp1 is required for glucose-induced transcriptional regulation of mouse vesicular glutamate transporter 2 gene

BACKGROUND & AIMS

Vesicular glutamate transporter (VGLUT) has been reported to be involved in glucose-induced insulin secretion. It has been shown that glucose stimulates the expression of VGLUT isoform 2 (VGLUT2) in beta cells via transcriptional mechanism. In this study, we identified the mouse VGLUT2 (mVGLUT2) promoter and characterized the transcriptional mechanism of glucose-stimulated mVGLUT2 expression in beta-cells.

METHODS

A promoter region of mVGLUT2 was cloned by genomic polymerase chain reaction. The mechanism of Sp1 in glucose-induced transactivation of mVGLUT2 was investigated by luciferase assay, electrophoretic mobility shift assay, chromatin immunoprecipitation assay, and Western blot analysis.

RESULTS

A promoter containing 2133 base pairs of upstream sequence of the 5'-flanking region of mVGLUT2 complementary DNA was cloned. Transient transfection of various 5'-end deletion constructs of the mVGLUT2 promoter/luciferase reporter indicated that the region between -96 to +68 base pair contains the basal promoter for mVGLUT2. Mutational analysis and electromobility shift assay showed an important role for the transcription factor Sp1 in both basal and glucose-induced mVGLUT2 transcription. The interaction between Sp1 and mVGLUT2 was confirmed by chromatin immunoprecipitation assays. Glucose stimulates the phosphorylation of Sp1 via mitogen-activated protein kinase P38 and P44/42. This leads to increased binding activity of Sp1 to the mVGLUT2 promoter and results in activation of the gene.

CONCLUSIONS

We cloned the mouse VGLUT2 promoter and showed a novel molecular mechanism of glucose-induced mVGLUT2 transcription.

Increased glucose availability activates chicken thymocyte metabolism and survival

Glucose metabolism in mammalian thymocytes is coupled to their development and selection in the thymus. In chickens, thymocytes develop in a low glucose concentration in ovo and a high glucose concentration posthatch. To determine the effect of glucose concentration on thymocyte glucose metabolism, embryonic thymic lobes were cultured in media containing varying glucose concentrations and thymocytes were isolated for analysis. Glucose transporter-1 (Glut-1) and Glut-3 mRNA abundance was at least 2-fold higher in thymocytes incubated with 10 and 20 mmol/L glucose than in those incubated with 0 mmol/L glucose (P < 0.05) and glucose uptake was greatest in thymocytes incubated with 20 mmol/L glucose (P < 0.05). Thymocytes incubated with 0 and 20 mmol/L glucose had 185% greater hexokinase activity than in those incubated with 10 mmol/L glucose (P < 0.05). Consequently, thymocyte glucose utilization was dependent upon glucose availability. Increased glucose utilization resulted in a higher mitochondrial membrane potential in thymocytes incubated with 15 mmol/L glucose than in those incubated with 5 mmol/L glucose (P < 0.05), indicating enhanced thymocyte energy status in response to high glucose concentrations. Additionally, thymocyte viability was lower in thymocytes incubated with 5 mmol/L glucose than in those incubated with 10 and 15 mmol/L glucose (P < 0.05) and rates of thymocyte apoptosis were higher in thymocytes incubated with 5 mmol/L glucose than in those incubated with 15 mmol/L glucose (P < 0.05). Glucose availability induced metabolic changes in thymocytes that altered their energy status and survival. Consequently, these data indicate that glucose availability may influence the development of naïve T cells in the chicken thymus.

Expression of the rat sterol regulatory element-binding protein-1c gene in response to insulin is mediated by increased transactivating capacity of specificity protein 1 (Sp1)

The induction of genes involved in lipid biosynthesis by insulin is mediated in part by the sterol regulatory element-binding protein-1c (SREBP-1c). SREBP-1c is directly regulated by insulin by transcriptional and post-transcriptional mechanisms. Previously, we have demonstrated that the insulin-responsive cis-acting unit of the rat SREBP-1c promoter is composed of several elements that include a sterol regulatory element, two liver X receptor elements, and a number of conserved GC boxes. Here we systematically dissected the role of these GC boxes and report that five bona fide Sp1-binding elements of the SREBP-1c promoter determine its basal and insulin-induced activation. Luciferase expression driven by the rat SREBP-1c promoter was accelerated by ectopic expression of Sp1, and insulin further enhanced the transactivation potential of Sp1. Introduction of a small interfering RNA against Sp1 reduced both basal and insulin-induced activation of the SREBP-1c promoter. We also found that Sp1 interacted with both SREBP-1c and LXRalpha proteins and that insulin promoted these interactions. Chromatin immunoprecipitation studies revealed that insulin facilitated the recruitment of the steroid receptor coactivator-1 to the SREBP-1c promoter. These studies identify a novel mechanism by which maximal activation of the rat SREBP-1c gene expression by insulin is mediated by Sp1 and its enhanced ability to interact with other transcriptional regulatory proteins.

Peptidoglycan enhances transcriptional expression of CCAAT/enhancer-binding protein delta gene in mouse macrophages

Peptidoglycan-activated gene expression is mediated through various transcription factors including CCAAT/enhancer-binding protein delta (C/EBPdelta). The purpose of the present study is to elucidate the mechanism of PGN-activated C/EBPdelta gene. PGN stimulated C/EBPdelta protein and mRNA expression in mouse macrophages RAW 264.7 cells. Analysis of C/EBPdelta promoter activity by luciferase reporter assay indicated that PGN-induced C/EBPdelta gene activation is partially mediated by the -345 to +24 bp of C/EBPdelta gene promoter. The in vitro protein-DNA binding assay showed that Sp1, c-Rel and c-Jun are the major protein binding to this PGN-response element of C/EBPdelta promoter, and the binding of c-Rel and c-Jun is increased after PGN treatment. All of these binding activities were abolished when Sp1-, NF-kappaB/APRE-, CRE-sites were mutated. Furthermore, analysis of this promoter region by site-directed mutants constructed in luciferase reporter vector indicated that two Sp1-sites, one NF-kappaB/APRE-site and one CRE-site are prominent for PGN-induced gene expression. In addition, when Sp1, c-Rel or c-Jun transcription factors were overexpressed in cells, all of them enhanced C/EBPdelta promoter activity. In summary, we suggest that Sp1, c-Rel and c-Jun transcription factors play important roles in activation of C/EBPdelta gene promoter under the stimulation of PGN. Given the importance of C/EBPdelta in inflammatory disease, these results reveal a clue as a potential therapeutic target for suppression of C/EBPdelta expression under PGN stimulation.

Downregulation of transcription factor, Sp1, during cellular senescence

We found that the protein level of Sp1 transcription factor decreases as normal human fibroblasts undergo replicative aging. Sp1 also undergoes a rapid decrease in the protein level and activity in MCF-7 cells that are induced to a state of cellular senescence. In the cells treated with other DNA damaging chemicals such as actinomycin D and H(2)O(2), the Sp1 level decreased progressively as well. Inhibition of ATM/ATR kinases prevented this downregulation, suggesting that DNA damage signaling is involved in the regulation of the Sp1. This decrease in Sp1 protein level is due to the accelerated proteasomal degradation since a proteasome inhibitor, ALLN, blocked this downregulation. Therefore, the global decrease in gene transcription frequently reported in aging cells and tissues could be attributed at least in part to the decrease in Sp1 level.

HGF protects rat mesangial cells from high-glucose-mediated oxidative stress

BACKGROUND

Oxidative stress has been considered to be a common pathogenetic factor of diabetic nephropathy. Recent observations suggested that hepatocyte growth factor (HGF) was an antioxidant growth factor; thus, its renoprotective effects in diabetic nephropathy might be related to antioxidant mechanism. The aim of the present study was to evaluate whether HGF could prevent rat mesangial cells (RMC) from high-glucose-mediated oxidative stress and explore its relevant mechanism.

METHODS

RMC were cultured in 5.6 mM (NG) or 30 mM (HG) glucose in the absence or presence of HGF (20 ng/ml) and c-met inhibitor SU11274 (5 microM) for 24 h.

RESULTS

c-met expression in HG was markedly increased. Enhanced oxidative stress was observed in HG as evidenced by elevated reactive oxygen species and malondialdehyde levels and decreased glutathione level, which was markedly attenuated by HGF. HGF also inhibited HG-induced p22(phox) and aldose reductase upregulation and prevented HG-reduced glutamate-cysteine ligase catalytic subunit (GCLC) expression through inhibiting USF binding to negative regulatory region of GCLC promoter. Reduced glucose-6-phosphate dehydrogenase activity and expression in RMC by HG was rescued by HGF.

CONCLUSION

HGF could function as an antioxidant factor and protect against HG-mediated oxidative stress by enhancing ROS scavenging and suppressing ROS production.

Increased chromatin association of Sp1 in interphase cells by PP2A-mediated dephosphorylations

Sp1 dephosphorylation by phosphatase 2A is related to sustained cellular proliferation and is illustrated by an enhanced electrophoretic migration shift. This event occurs concurrently with cell-cycle interphase and increases Sp1 transcriptional activity and in vitro affinity for DNA. We show here that dephosphorylated Sp1 is associated with chromatin more tightly than its phosphorylated counterparts from either resting or mitotic cells. Analysis of the expression of Sp1 point mutants and use of a phospho-specific antibody enabled identification of serine 59 as a major target of PP2A during cell-cycle interphase. Importantly, serine 59 dephosphorylation appeared to up-regulate Sp1 association with chromatin. Various studies suggested that this might occur through the control of the reciprocal O-phosphate/O-GlcNAc modification of other residues, some of which are likely to belong to the Sp1 C-terminal DNA-binding domain. In addition, we demonstrated by phosphopeptide mapping that threonine 681, which belongs to the latter region, is another target of PP2A, yet unrelated to serine 59. We propose that the coordinated dephosphorylation of several Sp1 residues, a general feature of dividing cells, is a required post-translational mechanism for Sp1-dependent transcription of genes related to cell division.

Glucose-6-phosphate dehydrogenase and NADPH-consuming enzymes in the rat olfactory bulb

The resistance to oxidative stress is a multifactorial reaction involving the clustering of transcriptionally regulated genes. Because glucose-6-phosphate dehydrogenase (G6PD), the principal enzyme responsible for reducing power, is highly expressed in the olfactory bulb (OB), it is of interest to verify whether other enzymes utilizing NADPH are also highly expressed. The level and localization of G6PD- and NADPH-consuming enzymes, such as NADPH-cytochrome P450 oxidoreductase (P450R), glutathione reductase (GR), and NADPH-diaphorase (NADPH-d), were analyzed in the rat olfactory bulb (OB) by quantitative histochemistry and immunohistochemistry. The highest concentration of G6PD, P450R, and GR was observed in the olfactory nerve layer (ONL), suggesting a correlation in the expression of these enzymes at the gene level. Correlation in staining intensity between G6PD and NADPH-d activities occurred only in part of the ONL, some glomeruli, and scattered periglomerular cells. This peculiar distribution of NADPH-d could reflect a spatial patterning of the nose to bulb projections. Taken together, these results indicate that G6PD expression in the ONL could be related to the importance of generating a substantial supply of NADPH to sustain the detoxifying systems represented by GR and P450R reactions and, only in discrete zones, by NADPH-d activity.

Sp1: regulation of gene expression by phosphorylation

As the prototype of a family of transcription factors, Sp1 has been extensively studied and widely reported for its role in gene regulation. The first evidence of Sp1 phosphorylation was reported more than a decade ago. Since then, an increasing number of Sp1 phosphorylation events have been characterized. Recent data demonstrate an important role for the phosphorylation state of Sp1 in the regulation of multiple genes. In this article, we review published literature in four specific areas relating to the phosphorylation of Sp1: (1) signal transduction pathways for Sp1 phosphorylation, (2) mechanisms of Sp1 dephosphorylation, (3) the functional implications of Sp1 phosphorylation, and (4) Sp1 phosphorylation in the lung.

Sp-family of transcription factors regulates human SHIP2 gene expression

We have characterized the regulation of human SH-2 containing inositol 5'-phosphatase 2 (SHIP2) gene expression. First, the transcription initiation sites and the sequence of the 5' upstream region of human SHIP2 gene were elucidated. Next, the minimal promoter of the human SHIP2 gene was identified by reporter gene assays in HL60 cells and differentiated human subcutaneous white adipocytes. An Sp1 element proximal to the transcription initiation site was indispensable for full promoter activity and bound specifically by Sp1 and Sp3 proteins. These findings suggest that human SHIP2 gene expression, like other housekeeping genes, is controlled by the Sp-family of transcription factors.

NADPH-consuming enzymes correlate with glucose-6-phosphate dehydrogenase in Purkinje cells: an immunohistochemical and enzyme histochemical study of the rat cerebellar cortex

In cerebellum of the adult rat, glucose-6-phosphate dehydrogenase (G6PD) activity is particularly localized in Purkinje cells, showing lower activity in the molecular and granule cell layers. G6PD is the first and rate-limiting step of the hexose monophosphate shunt (HMS), which has the physiological role of providing NADPH for reductive biosynthesis and detoxifying reactions. In this study, we searched for a possible correlation between G6PD and other NADPH-consuming enzymes, such as NADPH-cytochrome P450 reductase (P450R), glutathione reductase (GR) and NADPH-diaphorase (NADPH-d). This study was performed by means of immunohistochemistry and enzyme histochemistry followed by quantitative densitometric and confocal laser scanning microscopic analyses. Our results demonstrated that G6PD, P450R and GR have a similar distribution pattern characterized by the highest concentration of these enzymes in the somata of Purkinje cells, and by lower concentrations in the molecular and the granule cell layers. Moreover, in Purkinje cells, G6PD colocalized with both P450R and GR. NADPH-d activity showed a different distribution pattern when compared to the other enzymes, revealing the highest activity in the molecular layer and the lowest in Purkinje cells. Our results suggest a coordinated regulative mechanism of G6PD, P450R and GR based on the request of NADPH or on specific transcription factors.

Down-regulation of Sp1 Activity through Modulation of O-Glycosylation by Treatment with a Low Glucose Mimetic, 2-Deoxyglucose

2-Deoxyglucose (2-DG), a nonmetabolizable glucose analogue, blocks glycolysis at the phosphohexose isomerase step and has been frequently used as a glucose starvation mimetic in studies of a wide variety of physiological dysfuctions. However, the effect of 2-DG on protein glycosylation and related signal pathways has not been investigated in depth. In HeLa, an HPV18-positive cervical carcinoma line, 2-DG treatment down-regulates human papillomavirus early gene transcription. This down-regulation was also achieved by low glucose supply or hypoxia, suggesting that this is a response commonly modulated by cellular glucose or energy level. We investigated how 2-DG and low glucose affect transcriptional activity. Human papillomavirus gene transcription was only marginally affected by the inhibition of ATP synthesis or the supplementation of pyruvate to 2-DG-treated cells, suggesting that poor ATP generation is involved only to a limited extent. 2-DG treatment also inhibited activation of p21 WAF1 promoter, which is controlled by p53 and/or Sp1. In a reporter assay using p21 WAF1 promoter constructs, 2-DG exerted a strong inhibitory effect on Sp1 activity. DNA binding activity of Sp1 in 2-DG-treated HeLa cells was intact, whereas it was severely impaired in cells incubated in a low glucose medium or in hypoxic condition. Unexpectedly, Sp1 was heavily modified with GlcNAc in 2-DG-treated cells, which is at least partially attributed to the inhibitory effect of 2-DG on N-acetyl-beta-D-glucosaminidase activity. Our results suggest that 2-DG, like low glucose or hypoxic condition, down-regulates Sp1 activity, but through hyper-GlcNAcylation instead of hypo-GlcNAcylation.

Glucose metabolism in cancer: importance of transcription factor-DNA interactions within a short segment of the proximal region og the type II hexokinase promoter

A common signature of many cancers is a high glucose catabolic rate frequently dependent on the overexpression of Type II hexokinase (HKII), a mitochondrial bound enzyme that also suppresses cell death. As the tumor HKII promoter plays a significant role in HKII overexpression, studies reported here were undertaken to identify both the major regions and transcription factors involved under tumor-like conditions. Reporter gene assays following transfection of hepatoma cells with decreasing segments of the HKII promoter traced its known strength to the proximal region (-281 to -35). Mutational analyses showed that in this short region GC boxes 1, 2, 5, and 6, a CCAAT box, an inverted CCAAT box, and CRE are involved in promoter activation. Other studies demonstrated binding of transcription factors Sp1, Sp2, and Sp3 to GC boxes 1 and 6, Sp1 and Sp2 to GC boxes 2 and 5, NF-Y to CCAAT boxes, and CREB, ATF1, and CREM to CRE. In addition, transfection studies involving Sp1, Sp2, Sp3, CREB, and NFY (dominant negative form) provided evidence that these proteins are promoter activators. Finally, alignment of available HK proximal promoters showed strong conservation only among HKII sequences. These findings implicate signaling pathways directed to a short segment of the proximal region of the HKII promoter as major contributors to HKII overexpression in many cancers.

Functional cooperation of simian virus 40 promoter factor 1 and CCAAT/enhancer-binding protein beta and delta in lipopolysaccharide-induced gene activation of IL-10 in mouse macrophages

Previous studies have revealed that LPS can activate transcription of the IL-10 gene promoter through an SV40 promoter factor 1 (Sp1) binding site in mouse macrophage RAW264.7. In this study, we determined that, in addition to Sp1, C/EBPbeta and delta were also involved in LPS-induced gene expression of IL-10. By transient transfection with 5'-deletion mutants of the IL-10 promoter, we found that there were two LPS-responsive elements in the promoter of the mouse IL-10 gene. Analysis of these two regions by gel shift assay suggested that Sp1 and C/EBPbeta and delta were bound to these two regions, respectively. By site-directed mutagenesis, we found that disruption at both the Sp1 and C/EBP binding sites almost completely blocked the LPS response. By gel shift assay and Western blotting, we found that the DNA binding complex and protein expression of C/EBPbeta and delta were increased by LPS treatment, but these results were not found for Sp1. Overexpression of C/EBPbeta or C/EBPdelta, respectively, activated the promoter of the IL-10 gene, and they were enhanced by LPS. Coimmunoprecipitation experiments in intact cells indicated that LPS stimulated interaction between Sp1 and C/EBPbeta and delta. These results suggested that the interaction between Sp1 and C/EBPbeta and delta induced by LPS cooperatively activated expression of the IL-10 gene. The increase of C/EBPbeta and delta proteins and the enhancement of transactivation activity of C/EBPbeta and delta by LPS treatment, at least in part, explain the activation of IL-10 gene expression.

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.

Insulin stimulates expression of the pyruvate kinase M gene in 3T3-L1 adipocytes

M2-type pyruvate kinase (M2-PK) mRNA is produced from the PKM gene by an alternative RNA splicing in adipocytes. We found that insulin increased the level of M2-PK mRNA in 3T3-L1 adipocytes in both time- and dose-dependent manners. This induction did not require the presence of glucose or glucosamine in the medium. The insulin effect was blocked by pharmacological inhibitors of insulin signaling pathways such as wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), and PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) kinase. A stable reporter expression assay showed that the promoter activity of an about 2.2-kb 5'-flanking region of the rat PKM gene was stimulated by insulin, but the extents of these stimulations were lower than those of the mRNA stimulation. Thus, we suggest that insulin increases the level of M2-PK mRNA in adipocytes by acting at transcriptional and post-transcriptional levels through signaling pathways involving both PI3K and MAPK kinase.

Insulin induction of apolipoprotein AI, role of Sp1

Apolipoprotein AI (apo AI) is the major protein component of serum high-density lipoproteins. The abundance of apo AI correlates inversely with the risk of ischemic heart disease (IHD) and thus enhanced expression of the protein is expected to reduce the risk of IHD. Our previous studies show that insulin enhances apo AI promoter activity and this action requires the GC-rich insulin response core element (IRCE, -411 to -404). The motif binds to a ubiquitous transcription factor Sp1. We have extended studies that examine insulin induction of apo AI using a 41 bp (-425 to -385) fragment of apo AI DNA linked to the trout metallothionein TATA box and fused to luciferase (pIRCE-Luc). Luc activity in Hep G2 cells transfected with pIRCE-Luc was stimulated by insulin, an insulin mimetic bisperoxo (1,10-phenanthroline) oxovanadate (bpv) and the phorbol ester (PDBu). Our previous studies showed that insulin action on apo AI gene transcription flowed down two signaling pathways: Ras-raf and PI3K, leading to activation of the MAPK and PKC kinases, respectively. In contrast, PDBu activates only the PKC pathway. Although insulin and PDBu activation of apo AI were distinct, the cascades involved all appeared to target Sp1. Furthermore, exposure of transfected cells to okadaic acid or a phosphatase inhibitor also increased Luc activity and suggested a potential role for phosphorylation, likely involving Sp1. If true, then changes in the IRCE binding activity of Sp1 should be detected following exposure to MAPK, PKC, or the protein phosphatase I (PPI) alone and in various combinations followed by assaying the ability of Sp1 to bind the IRCE. Sp1 binding activity increased with either MAPK or PKC. Although exposure to PPI also affected IRCE binding activity of Sp1, whether it increased or decreased was dependent on the order of exposure to the protein. In summary, the IRCE alone can mediate the stimulatory effects of insulin, bpv, and PDBu, and Sp1 enhances these responses that may arise from phosphorylation of the protein.

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.

Metabolic cooperation between different oncogenes during cell transformation: interaction between activated ras and HPV-16 E7

The metabolism of tumor cells (tumor metabolome) is characterized by a high concentration of glycolytic enzymes including pyruvate kinase isoenzyme type M2 (M2-PK), a high glutaminolytic capacity, high fructose 1,6-bisphosphate (FBP) levels and a low (ATP+GTP):(CTP+UTP) ratio. The sequence of events required for the establishment of the tumor metabolome is presently unknown. In non-transformed rat kidney (NRK) cells we observed a high glutaminolytic flux rate and a low (ATP+GTP):(CTP+UTP) ratio, whereas FBP levels and M2-PK activity are still extremely low. After stable expression of oncogenic ras in NRK cells a strong upregulation of FBP levels and of M2-PK activity was observed. Elevated FBP levels induce a tetramerization of M2-PK and its migration into the glycolytic enzyme complex. AMP levels increase whereas UTP and CTP levels strongly decrease. Thus, ras expression completes the glycolytic part of tumor metabolism leading to the inhibition of nucleic acid synthesis and cell proliferation. The HPV-16 E7 oncoprotein, which cooperates with ras in cell transformation, directly binds to M2-PK, induces its dimerization and restores nucleic acid synthesis as well as cell proliferation. Apparently, the combination of the different metabolic effects of ras and E7 constructs the perfect tumor metabolome as generally found in tumor cells.

Spot 14 protein interacts and co-operates with chicken ovalbumin upstream promoter-transcription factor 1 in the transcription of the L-type pyruvate kinase gene through a specificity protein 1 (Sp1) binding site

In hepatocytes, the amount of the Spot 14 (S14) protein is closely related to the full expression of enzymes involved in the glycolytic and lipogenic pathways. In the present study we address the role played by this protein in the control of transcription of the L-type pyruvate kinase (L-PK) gene in primary hepatocytes. We show that human S14, which by itself does not bind to the L-PK promoter, physically interacts with the human chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1) and induces the switch of this factor from a repressor to an activator. However, the enhancing activity of S14 and COUP-TF1 depends on the presence of a proximal GC-rich box (the L0 element) that specifically binds nuclear proteins from the livers of rats fed a glucose-rich diet. Moreover, the L0 element, which strongly binds dephosphorylated specificity protein 1 (Sp1), loses all affinity when this factor is phosphorylated by cAMP-dependent protein kinase. Mutations that affect binding of Sp1 and nuclear proteins to the L0 box also decrease basal transcription and impair glucose responsiveness of the promoter. These results therefore shed light on the mechanism by which the S14 protein, whose concentration rapidly rises after glucose intake, contributes to the full activity of the L-PK promoter.

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.