Dephosphorylation of Sp1 by protein phosphatase 1 is involved in the glucose-mediated activation of the acetyl-CoA carboxylase gene

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.

TRIB3 mediates glucose-induced insulin resistance via a mechanism that requires the hexosamine biosynthetic pathway

In the current study, we investigated the role of tribbles homolog 3 (TRIB3) in glucose-induced insulin resistance and whether the induction of TRIB3 by glucose is dependent on the nutrient-sensing hexosamine biosynthetic pathway (HBP) known to mediate glucose toxicity in diabetes. In diabetic rats, TRIB3 expression in skeletal muscle was increased after 10 days of hyperglycemia, and glycemia and muscle TRIB3 were both restored toward normal by insulin therapy. In L6 myocytes, the induction of TRIB3 by high glucose or glucosamine was reversible upon removal of these substrates. To assess the role of HBP in the induction of TRIB3, we demonstrated that the ability of high glucose to augment TRIB3 expression was prevented by azaserine, an inhibitor of glutamine: fructose-6-phosphate amidotransferase (GFAT), which is the rate-limiting enzyme in the HBP pathway. TRIB3 expression was also substantially stimulated by glucosamine, which bypasses GFAT, accompanied by a decrease in the insulin-stimulated glucose transport rate, and neither response was affected by azaserine. Further, knockdown of TRIB3 inhibited, and TRIB3 overexpression enhanced, the ability of both high glucose and glucosamine to induce insulin resistance. These data provide the mechanistic link between the HBP flux and insulin resistance and point to TRIB3 as a novel target for treatment of glucose-induced insulin resistance.

Transcription factors Sp1 and Hif2α mediate induction of the copper-transporting ATPase (Atp7a) gene in intestinal epithelial cells during hypoxia

Genes with G/C-rich promoters were up-regulated in the duodenal epithelium of iron-deficient rats including those encoding iron (e.g. Dmt1 and Dcytb) and copper (e.g. Atp7a and Mt1) metabolism-related proteins. It was shown previously that an intestinal copper transporter (Atp7a) was co-regulated with iron transport-related genes by a hypoxia-inducible transcription factor, Hif2α. In the current study, we sought to test the role of Sp1 in transcriptional regulation of Atp7a expression during iron deprivation/hypoxia. Initial studies in IEC-6 cells showed that mithramycin, an Sp1 inhibitor, reduced expression of Atp7a and iron transport-related genes (Dmt1, Dcytb, and Fpn1) and blocked their induction by CoCl2, a hypoxia mimetic. Consistent with this, overexpression of Sp1 increased endogenous Atp7a mRNA and protein expression and stimulated Atp7a, Dmt1, and Dcytb promoter activity. Site-directed mutagenesis and functional analysis of a basal Atp7a promoter construct revealed four functional Sp1 binding sites that were necessary for Hif2α-mediated induction of promoter activity. Furthermore, chromatin immunoprecipitation (ChIP) assays confirmed that Sp1 specifically interacts with the Atp7a promoter in IEC-6 cells and in rat duodenal enterocytes. This investigation has thus revealed a novel aspect of Atp7a gene regulation in which Sp1 may be necessary for the HIF-mediated induction of gene transcription during iron deficiency/hypoxia. Understanding regulation of Atp7a expression may help further clarify the physiological role of copper in the maintenance of iron homeostasis. Furthermore, this Sp1/Hif2α regulatory mechanism may have broader implications for understanding the genetic response of the intestinal epithelium to maintain whole-body iron homeostasis during states of deficiency.

Angiotensin II AT(2) receptor decreases AT(1) receptor expression and function via nitric oxide/cGMP/Sp1 in renal proximal tubule cells from Wistar-Kyoto rats

BACKGROUND

The renin-angiotensin (Ang) system controls blood pressure, in part, by regulating renal tubular sodium transport. In the kidney, activation of the angiotensin II type 1 (AT(1)) receptor increases renal sodium reabsorption, whereas the angiotensin II type 2 (AT(2)) receptor produces the opposite effect. We hypothesized that the AT(2) receptor regulates AT(1) receptor expression and function in the kidney.

METHODS AND RESULTS

In immortalized renal proximal tubule (RPT) cells from Wistar-Kyoto rats, CGP42112, an AT(2) receptor agonist, decreased AT(1) receptor mRNA and protein expression (P < 0.05), as assessed by reverse transcriptase-polymerase chain reaction and immunoblotting. The inhibitory effect of the AT(2) receptor on AT(1) receptor expression was blocked by the AT(2) receptor antagonist, PD123319 (10 (-6)mol/l), the nitric oxide synthase inhibitor N(w)-nitro-L-arginine methyl ester (10(-4) mol/l), or the nitric oxide-dependent soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo-[4,3-a] quinoxalin-1-one (10(-5) mol/l), indicating that both nitric oxide and cyclic guanosine monophosphate (cGMP) were involved in the signaling pathway. Furthermore, CGP42112 decreased Sp1 serine phosphorylation and reduced the binding of Sp1 to AT(1) receptor DNA. Stimulation with Ang II (10(-11) mol/l per 30 min) enhanced Na(+)-K(+)-ATPase activity in RPT cells, which was prevented by pretreatment with CGP42112 (10(-7) mol/l per 24 h) (P < 0.05). The above-mentioned results were confirmed in RPT cells from AT(2) receptor knockout mice; AT(1) receptor expression and Ang II-stimulated Na-K-ATPase activity were greater in these cells than in RPT cells from wild-type mice (P < 0.05). AT(1)/AT(2) receptors co-localized and co-immunoprecipitated in RPT cells; short-term CGP42112 (10 mol/l per 30 min) treatment increased AT(1)/AT(2) receptor co-immunoprecipitation (P < 0.05).

CONCLUSIONS

These results indicate that the renal AT(2) receptor, via nitric oxide/cGMP/Sp1 pathway, regulates AT(1 )receptor expression and function, which may be important in the regulation of sodium excretion and blood pressure.

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.

Integrated expression profiling and genome-wide analysis of ChREBP targets reveals the dual role for ChREBP in glucose-regulated gene expression

The carbohydrate response element binding protein (ChREBP), a basic helix-loop-helix/leucine zipper transcription factor, plays a critical role in the control of lipogenesis in the liver. To identify the direct targets of ChREBP on a genome-wide scale and provide more insight into the mechanism by which ChREBP regulates glucose-responsive gene expression, we performed chromatin immunoprecipitation-sequencing and gene expression analysis. We identified 1153 ChREBP binding sites and 783 target genes using the chromatin from HepG2, a human hepatocellular carcinoma cell line. A motif search revealed a refined consensus sequence (CABGTG-nnCnG-nGnSTG) to better represent critical elements of a functional ChREBP binding sequence. Gene ontology analysis shows that ChREBP target genes are particularly associated with lipid, fatty acid and steroid metabolism. In addition, other functional gene clusters related to transport, development and cell motility are significantly enriched. Gene set enrichment analysis reveals that ChREBP target genes are highly correlated with genes regulated by high glucose, providing a functional relevance to the genome-wide binding study. Furthermore, we have demonstrated that ChREBP may function as a transcriptional repressor as well as an activator.

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.

Functional characterization of the proximal promoter of the murine pyruvate carboxylase gene in hepatocytes: role of multiple gc boxes

Pyruvate carboxylase (PC) catalyzes the first committed step in gluconeogenesis in the liver. The murine PC gene possesses two promoters, the proximal (P1) and the distal (P2) which mediate production of distinct tissue-specific mRNA isoforms. By comparing the luciferase activities of 5'-nested deletions of the P1-promoter in the AML12 mouse hepatocyte cell line, the critical cis-acting elements required for maintaining basal transcription were located within the 166 nucleotides proximal to the transcription start site. Three GC boxes were identified within this region and shown by gel shift and ChIP assays to bind Sp1/Sp3. Over-expression of Sp1/Sp3 in AML12 and NIH3T3 cells increased P1-promoter activity, with Sp1 being a stronger activator than Sp3. Mutation of any one of the three GC boxes dramatically reduced basal promoter activity by 60-80% suggesting that all three boxes are equally strong regulatory elements. In AML12 cells, over-expression of Sp1/Sp3 restored the transcriptional activity of GC1 and GC2 but not GC3 mutants to levels similar to that of the WT construct, suggesting that GC3 is particularly critical for Sp1/Sp3-mediated induction. In NIH3T3 cells, however, the three boxes were equally important, indicating that the GC boxes differentially contribute to transcriptional regulation of the P1-promoter in the two cell lines. Mutants harboring two disrupted GC boxes showed a further decrease in promoter activity similar to the triple GC box mutant. Neither Sp1 nor Sp3 was able to fully restore the promoter activities of these mutants to that the WT level.

Ischemia/reperfusion reduces transcription factor Sp1-mediated cystathionine beta-synthase expression in the kidney

Cystathionine beta-synthase (CBS) is a key enzyme that catalyzes the rate-limiting step for homocysteine (Hcy) metabolism via the trans-sulfuration pathway and is also responsible for the production of H(2)S through the desulfhydration reaction. Our recent studies demonstrate that renal ischemia/reperfusion decreased the CBS activity leading to Hcy accumulation and H(2)S reduction in the kidney, which in turn contributed to kidney injury. Both Hcy and H(2)S play important roles in physiological and pathological processes. In this study we investigated the molecular mechanism by which CBS activity was regulated in the kidney. The left kidney of Sprague-Dawley rat was subjected to 45 min of ischemia followed by 6 h of reperfusion. Ischemia/reperfusion caused a significant decrease in CBS mRNA and protein levels in the kidney. As a consequence, there was a marked reduction in the CBS enzyme activity. Transfection of kidney proximal tubular cells with transcription factor (Sp1) small interfering RNA caused a marked reduction in CBS mRNA, indicating a pivotal role for Sp1 in regulating CBS expression in kidney cells. Electrophoretic mobility shift assay and chromatin immunoprecipitation assay detected a lower Sp1 activity in kidneys subjected to ischemia/reperfusion as compared with that in a sham-operated group. ERK-mediated phosphorylation of Sp1 was responsible for a decreased transcriptional activity of Sp1 in the kidney upon ischemia/reperfusion. These results suggest that reduced kidney CBS gene expression during ischemia/reperfusion is mediated via a decrease in Sp1 transcriptional activity. Regulation of CBS-mediated Hcy and H(2)S homeostasis may offer a renal protective effect against ischemia/reperfusion injury.

Butyrate-induced phosphatase regulates VEGF and angiogenesis via Sp1

Sp1 is a ubiquitous transcription factor and master regulator of various eukaryotic gene expression. Better understanding of the role of increased Sp1 levels on angiogenic regulation and the regulatory regions of that transcription factor may act as a useful target in 'transcriptional therapy'. At the molecular level, butyrate inhibits Sp1-DNA binding activity by promoting Sp1 protein dephosphorylation in EAT cells. It also inhibits Sp1 binding activity and reduces expression of VEGF gene, thereby inhibiting angiogenesis. It was confirmed that butyrate induces expression of a tyrosine phosphatase by RT-PCR, cDNA sequence analysis, protein ESI-MS analysis and protein sequence homology comparison. Thus our result strongly suggests that inhibition of angiogenesis by butyrate involves Sp1 dephosphorylation and down-regulation of VEGF gene expression. Further, butyrate inhibits neoangiogenesis induced by tumor cells and VEGF in peritoneum of EAT bearing mice and rat cornea.

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.

Transcriptional regulation of phosphatidylcholine biosynthesis

Phosphatidylcholine biosynthesis in animal cells is primarily regulated by the rapid translocation of CTP:phosphocholine cytidylyltransferase alpha between a soluble form that is inactive and a membrane-associated form that is activated. Until less than 10 years ago there was no information on the transcriptional regulation of phosphatidylcholine biosynthesis. Research has identified the transcription factors Sp1, Rb, TEF4, Ets-1 and E2F as enhancing the expression of the cytidylyltransferase and Net as a factor that represses cytidylyltransferase expression. Key transcription factors involved in cholesterol or fatty acid metabolism (SREBPs, LXRs, PPARs) do not have a major role in transcriptional regulation of the cytidylyltransferase. Rather than being linked to cholesterol or energy metabolism, regulation of the cytidylyltransferase is linked to the cell cycle, cell growth and differentiation. Transcriptional regulation of phospholipid biosynthesis is more elegantly understood in yeast and involves responses to inositol, choline and zinc in the culture medium.

Involvement of Sp1 and Sp3 in differential regulation of human NHE3 promoter activity by sodium butyrate and IFN-gamma/TNF-alpha

Previously, we reported that IFN-gamma and TNF-alpha downregulate the expression of the human Na(+)/H(+) exchanger (NHE)3 gene by modulating Sp1/Sp3 transcription factors in C2BBe1 cells. It is reported that butyrate inhibits IFN-gamma and TNF-alpha signaling pathways. In this study, we have investigated the effect of sodium butyrate (NaB) and IFN-gamma/TNF-alpha on human NHE3 promoter activity. In transient transfection studies, NaB (5 mM) led to 10-fold stimulation of NHE3 promoter activity after incubation for 24 h. With 5'-deletion analysis, the NaB-responsive region was mapped to the NHE3 core promoter, bp -95 to + 5, which we had shown previously to confer responsiveness to IFN-gamma/TNF-alpha. The stimulatory effect of NaB on the NHE3 promoter was reduced by 60% in the presence of IFN-gamma/TNF-alpha. Mutually, the repressive effect of these cytokines was attenuated by NaB. Knockdown of Sp1 and Sp3 expression with small interfering RNA (siRNA) resulted in a significant resistance to NaB effects. NaB treatment showed no effect on Sp1 and Sp3 protein expression as assessed by Western blot analyses. Gel mobility shift assays with nuclear proteins from NaB-treated cells showed enhanced binding of Sp1 and Sp3 to the NHE3 promoter. The phosphatase inhibitor okadaic acid (200 nM) blocked the stimulatory effect of NaB on the NHE3 promoter. NaB effects on the NHE3 promoter were significantly attenuated by protein phosphatase (PP)1alpha- and PP2Aalpha-specific siRNA transfection. Our data suggest that the differential regulation of NHE3 gene expression by NaB and IFN-gamma/TNF-alpha is mediated through alternative pathways that converge on Sp1/Sp3.

Identification of a hydrogen peroxide-induced PP1-JNK1-Sp1 signaling pathway for gene regulation

Oxidative stress often results in changes in gene expression through the regulation of transcription factors. In this study, we examine how Sp1 phosphorylation is regulated by H2O2 in a human alveolar epithelial cell line (HAE). Treatment of HAE cells with H2O2 increases phosphorylation of Sp1 and activates JNK. To establish a relationship between JNK and Sp1, we show that JNK activator anisomycin increases Sp1 phosphorylation, and JNK inhibitors as well as dominant-negative JNK1 attenuate H2O2-induced Sp1 phosphorylation. Additionally, JNK1 directly phosphorylates Sp1 in vitro, reducing Sp1 binding to DNA. These results demonstrate the role of JNK in H2O2-induced Sp1 phosphorylation. Because H2O2 inhibits Ser/Thr protein phosphatase-1 (PP1), we examined the role of PP1 in the regulation of JNK. Similar to H2O2, inhibition of PP1 induces phosphorylation of Sp1 and activation of JNK in HAE cells. Inhibition of JNK activity using either inhibitors or dominant-negative mutant JNK1 suppresses PP1 inhibition-induced Sp1 phosphorylation. Furthermore, PP1 directly inactivates JNK1 in vitro. These data suggest that 1) H2O2 increases the phosphorylation level of Sp1, 2) Sp1 is a target of the JNK pathway, 3) PP1 regulates JNK activation, and 4) the “PP1-JNK” pathway plays a role in H2O2-induced Sp1 phosphorylation in lung epithelial cells.

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.

Tumor necrosis factor-alpha down-regulates human Cu/Zn superoxide dismutase 1 promoter via JNK/AP-1 signaling pathway

Overexpression of Cu/Zn superoxide dismutase 1 (SOD1) in monocytes blocks reactive oxygen species-induced inhibition of cell growth and apoptosis and renders cells resistant to the toxic effect of tumor necrosis factor (TNF)-alpha, suggesting that TNF-alpha represses the SOD1 gene in these cells. We herein show that TNF-alpha decreases SOD1 mRNA, protein, and promoter activity in U937 cells. Electrophoretic mobility-shift assays (EMSA) show that TNF-alpha decreased binding of three different complexes. Ectopic Sp1 overexpression markedly increased SOD1-basal promoter activity and partially antagonized the TNF-alpha inhibitory effect. In contrast, ectopic c-Jun overexpression mimics TNF-alpha inhibitory effects and antagonizes Sp1 stimulatory effects. In agreement with these findings, EMSA shows a TNF-alpha-induced increase in AP-1 and a decrease in Sp1 DNA binding. Disruption of the C/EBP site decreases, whereas mutation in the Sp1/Egr-1 site completely abolishes DNA-binding and promoter activity. A JNK inhibitor antagonized the negative effects of TNF-alpha on SOD1 promoter activity, suggesting that JNK signaling through c-Jun protein activation is critical for the TNF-alpha-dependent SOD1 repression. A greater understanding of the mechanisms of TNF-alpha-induced SOD1 repression could facilitate the design and development of novel therapeutic drugs for inflammatory conditions.

IFN-gamma and TNF-alpha regulate human NHE3 gene expression by modulating the Sp family transcription factors in human intestinal epithelial cell line C2BBe1

Diarrhea associated with inflammatory bowel disease has been attributed to stimulated secretion of proinflammatory cytokines like IFN-gamma and TNF-alpha, which have been shown to downregulate the expression of the sodium-hydrogen exchanger-3 (NHE3) gene. In this study, we have investigated the mechanism of NHE3 gene regulation by IFN-gamma and TNF-alpha in C2BBe1 cells. In response to both IFN-gamma (30 ng/ml) and TNF-alpha (20 ng/ml), the construct containing the bp -95 to +5 region of the human NHE3 promoter, which harbors a number of cis-elements including four potential Sp1 binding sites, showed a maximum repression of 60%. Knockdown of Sp1 and Sp3 expression using small interfering RNA resulted in a significant inhibition of the NHE3 promoter activity and resistance to cytokines effects. These cytokines showed no effects on the expression of Sp1 and Sp3 mRNA and protein levels as assessed by RT-PCR and Western blot analyses, respectively. After treatment with cytokines, the binding of Sp1 and Sp3 proteins to NHE3 promoter decreased significantly, as seen by gel mobility shift assays and chromatin immunoprecipitation assays. The inhibitory effects of both cytokines on the NHE3 promoter were completely blocked by the broad-range kinase inhibitor staurosporine and the selective protein kinase A (PKA) inhibitor 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Rp-isomer. The binding affinity of Sp1 and Sp3 proteins for NHE3 Sp1 probe was significantly decreased after in vitro phosphorylation of nuclear proteins by the alpha-catalytic subunit of PKA. Our data indicate that IFN-gamma and TNF-alpha may repress the NHE3 promoter activity in C2BBe1 cells by PKA-mediated phosphorylation of Sp1 and Sp3 transcription factors.

Regulation of the ClC-2 lung epithelial chloride channel by glycosylation of SP1

Chloride channel-2 (ClC-2) is a pH- and voltage-activated chloride channel that is highly expressed in mammalian fetal airway epithelia during the period of maximal fluid secretion. A high level of luminal ClC-2 protein expression is maintained by the SP1 transcription factor until SP1 and ClC-2 decline rapidly at birth. Using fetal (preII-19) and adult (L2) rat lung Type 2 cell lines, we demonstrate that the active higher-molecular-weight 105-kD isoform of SP1 is phosphorylated and glycosylated. Exposure of either cell line to high-dose glutamine is sufficient to induce glycosylation of SP1 and to induce and maintain ClC-2. Exposure to tunicamycin to inhibit SP1 glycosylation reduces ClC-2 expression. We also demonstrate that in vivo ClC-2 expression is similarly regulated. SP1 from 6-wk-old murine lung (high ClC-2 expression) is hyperphosphorylated and hyperglycosylated compared with SP1 from 16-wk-old lung (low ClC-2 expression). Our results support the hypothesis that glycosylation of SP1 produces the 105-kD isoform of SP1 and is involved in regulating ClC-2 gene expression.

Transcriptional Repression of Telomerase RNA Gene Expression by c-Jun-NH2-Kinase and Sp1/Sp3

Telomerase is essential for immortalization of most human cancer cells. Expression of the core telomerase RNA (hTR) and reverse transcriptase (hTERT) subunits is mainly regulated by transcription. However, hTR transcriptional regulation remains poorly understood. We previously showed that the core hTR promoter is activated by Sp1 and is repressed by Sp3. Here, we show that the mitogen-activated protein kinase kinase kinase 1 (MEKK1)/c-Jun-NH(2)-kinase (JNK) pathway represses hTR expression by a mechanism that involves Sp1 and Sp3. Promoter activity was induced by the JNK inhibitor SP600125 and was repressed by activated MEKK1. Repression by MEKK1 was blocked by SP600125 or enhanced by coexpression of wild-type but not phosphoacceptor mutated JNK. SP600125 treatment also increased levels of endogenous hTR. Mutations in the hTR promoter Sp1/Sp3 binding sites attenuated SP600125-mediated promoter induction, whereas coexpression of MEKK1 with Sp3 enhanced hTR promoter repression. Chromatin immunoprecipitation showed that levels of immunoreactive Sp1 associated with the hTR promoter were low in comparison with Sp3 in control cells but increased after JNK inhibition with a reciprocal decrease in Sp3 levels. No corresponding changes in Sp1/Sp3 protein levels were detected. Thus, JNK represses hTR promoter activity and expression, apparently by enhancing repression through Sp3.

Upstream stimulatory factor 1 transactivates the human gene promoter of the cardiac isoform of acetyl-CoA carboxylase

E-box cis-elements act as binding sites for upstream stimulatory factors (USFs), putative glucose-responsive transcriptional modulators. Since four E-boxes were identified on the human ACCbeta promoter, we hypothesized that USF1 induces ACCbeta expression in a glucose-dependent manner. Here, murine cardiac ACCbeta expression was significantly increased in response to high carbohydrate re-feeding after fasting. However, transfection studies showed no difference in ACCbeta promoter activity in neonatal cardiomyocytes and CV-1 fibroblasts after low (5.5mM) and high (25 mM) glucose exposure. USF1 overexpression significantly increased ACCbeta promoter activity in both cell lines under low glucose conditions. With high glucose exposure, USF1 further induced ACCbeta promoter activity only in CV-1 fibroblasts. USF1-induced ACCbeta promoter responsiveness was markedly attenuated when co-transfecting cardiomyocytes with a -93/+65 or -38/+65 promoter deletion construct (lacking E-boxes 1-3). Thus, USF1 transactivates the human ACCbeta promoter in the heart, likely through an E-box cis-element located close to the transcription start site.

Glucose-induced lipogenesis in pancreatic beta-cells is dependent on SREBP-1

High concentrations of glucose induce de novo fatty acid synthesis in pancreatic beta-cells and chronic exposure of elevated glucose and fatty acids synergize to induce accumulation of triglycerides, a phenomenon termed glucolipotoxicity. Here we investigate the role of sterol-regulatory element binding proteins in glucose-induced lipogenesis in the pancreatic beta-cell line INS-1E. We show that glucose induces SREBP-1c expression and SREBP-1 activity independent of insulin secretion and signaling. Using adenoviral expression of SREBP-1c and a SREBP-mutant we show that lipogenic gene expression, de novo fatty acid synthesis and lipid accumulation are induced primarily through sterol-regulatory elements (SREs) and not E-Boxes. Adenoviral expression of a dominant negative SREBP compromises glucose induction of some lipogenic genes and significantly reduces glucose-induction of de novo fatty acid synthesis. Thus, we demonstrate for the first time that SREBP activity is necessary for full glucose induction of de novo fatty acid synthesis in pancreatic beta-cells.

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.

Down-regulation of ATM protein sensitizes human prostate cancer cells to radiation-induced apoptosis

Treatment with the protein kinase C activator 12-O-tetradecanoylphorbol 12-acetate (TPA) enables radiation-resistant LNCaP human prostate cancer cells to undergo radiation-induced apoptosis, mediated via activation of the enzyme ceramide synthase (CS) and de novo synthesis of the sphingolipid ceramide (Garzotto, M., Haimovitz-Friedman, A., Liao, W. C., White-Jones, M., Huryk, R., Heston, D. W. W., Cardon-Cardo, C., Kolesnick, R., and Fuks, Z. (1999) Cancer Res. 59, 5194-5201). Here, we show that TPA functions to decrease the cellular level of the ATM (ataxia telangiectasia mutated) protein, known to repress CS activation (Liao, W.-C., Haimovitz-Friedman, A., Persaud, R., McLoughlin, M., Ehleiter, D., Zhang, N., Gatei, M., Lavin, M., Kolesnick, R., and Fuks, Z. (1999) J. Biol. Chem. 274, 17908-17917). Gel shift analysis in LNCaP and CWR22-Rv1 cells demonstrated a significant reduction in DNA binding of the Sp1 transcription factor to the ATM promoter, and quantitative reverse transcription-PCR showed a 50% reduction of ATM mRNA between 8 and 16 h of TPA treatment, indicating that TPA inhibits ATM transcription. Furthermore, treatment of LNCaP, CWR22-Rv1, PC-3, and DU-145 human prostate cells with antisense-ATM oligonucleotides, which markedly reduced cellular ATM levels, significantly enhanced radiation-induced CS activation and apoptosis, leading to apoptosis at doses as a low as 1 gray. These data suggest that the CS pathway initiates a generic mode of radiation-induced apoptosis in human prostate cancer cells, regulated by a suppressive function of ATM, and that ATM might represent a potential target for pharmacologic inactivation with potential clinical applications in human prostate cancer.

Structure and regulation of acetyl-CoA carboxylase genes of metazoa

Acetyl-CoA carboxylase (ACC) plays a fundamental role in fatty acid metabolism. The reaction product, malonyl-CoA, is both an intermediate in the de novo synthesis of long-chain fatty acids and also a substrate for distinct fatty acyl-CoA elongation enzymes. In metazoans, which have evolved energy storage tissues to fuel locomotion and to survive periods of starvation, energy charge sensing at the level of the individual cell plays a role in fuel selection and metabolic orchestration between tissues. In mammals, and probably other metazoans, ACC forms a component of an energy sensor with malonyl-CoA, acting as a signal to reciprocally control the mitochondrial transport step of long-chain fatty acid oxidation through the inhibition of carnitine palmitoyltransferase I (CPT I). To reflect this pivotal role in cell function, ACC is subject to complex regulation. Higher metazoan evolution is associated with the duplication of an ancestral ACC gene, and with organismal complexity, there is an increasing diversity of transcripts from the ACC paraloges with the potential for the existence of several isozymes. This review focuses on the structure of ACC genes and the putative individual roles of their gene products in fatty acid metabolism, taking an evolutionary viewpoint provided by data in genome databases.

Protein phosphatase 1alpha is required for murine lung growth and morphogenesis

Protein phosphatase 1 (PP1) plays important roles in cell cycle control and apoptosis, two processes that impinge on morphogenesis and differentiation. Following the precedent set by other molecules regulating the cell cycle and apoptosis, we hypothesized that PP1 may have context-specific roles in development. Therefore, we have studied the spatial and temporal expression of PP1alpha during murine lung development and determined the consequences of loss of PP1alpha function on branching morphogenesis. By using an immunohistochemical approach, we show here that PP1alpha was expressed throughout the epithelium and mesenchyme upon the emergence of the lung primordium on embryonic day 10, with immunostaining exclusively extranuclear. During the late pseudoglandular stage, PP1alpha was predominantly expressed in the distal lung epithelium, whereas the mesenchyme contained very little or no PP1alpha protein. Peri- and postnatally, PP1alpha immunostaining was mostly nuclear in apparently differentiated cells, as judged by colocalization with well-known markers for lung differentiation. Exposure of fetal lung explants to antisense oligodeoxynucleotides against PP1alpha, resulted in decreased overall size of the cultured lung, a defect in forming new airways, lack of expression of surfactant protein C, and histologic signs of poor differentiation. These data suggest that PP1alpha is required for branching morphogenesis and differentiation.

Okadaic acid augments utrophin in myogenic cells

Duchenne muscular dystrophy is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A potential therapeutic approach is to increase utrophin levels in muscle. One way to achieve this aim is to increase the expression of the utrophin gene at a transcriptional level via promoter activation. In this study, we have shown that utrophin A mRNA levels can be induced by okadaic acid in murine myogenic C2C12 cells. We have found that a utrophin A promoter reporter can be induced by Sp1 in C2C12 myoblasts, but not in myotubes. This activation can be enhanced by okadaic acid treatment. Our data suggest that this induction is due to Sp1 phosphorylation during myogenesis and thus, utrophin expression in muscle could be regulated by treatment with phosphatase inhibitors. Control of utrophin promoter activation could then be used to increase the expression of utrophin, and thus ameliorate the symptoms of Duchenne muscular dystrophy.

Constitutive connective tissue growth factor expression in scleroderma fibroblasts is dependent on Sp1

Fibrotic diseases such as scleroderma (systemic sclerosis, SSc) are characterized by an excessive production of extracellular matrix and profibrotic proteins such as connective tissue growth factor (CTGF). In normal dermal fibroblasts, CTGF is not expressed unless induced by proteins such as tumor growth factor-beta (TGFbeta). Conversely, in fibroblasts cultured from fibrotic lesions CTGF mRNA and protein are constitutively expressed, even in the absence of exogenously added TGFbeta. Thus, studying the mechanism underlying CTGF overexpression in SSc fibroblasts is likely to yield valuable insights into the basis of the fibrotic phenotype of SSc and possibly other scarring disease. CTGF overexpression is mediated primarily by sequences in the CTGF promoter. In this report, we identify the minimal promoter element involved with the overexpression of CTGF in SSc fibroblasts. This element is distinct from the element necessary and sufficient for the induction of CTGF expression by TGFbeta in normal fibroblasts. Within this region is a functional Sp1 binding site. Blocking Sp1 activity reduces the elevated, constitutive levels of CTGF promoter activity and protein expression observed in SSc fibroblasts. Relative to those prepared from normal dermal fibroblasts, nuclear extracts prepared from SSc fibroblasts possess increased Sp1 binding activity. Removal of phosphate groups from nuclear extracts enhanced Sp1 binding activity, suggesting that phosphorylation of Sp1 normally reduces Sp1 binding to DNA. Thus, the constitutive overexpression of CTGF in SSc fibroblasts seems to be independent of TGFbeta signaling but dependent at least in part on Sp1.

Modulation of Sp1 and Sp3 in lung epithelial cells regulates ClC-2 chloride channel expression

ClC-2 is a pH- and voltage-activated chloride channel, which is highly expressed in fetal airways and downregulated at birth. The ClC-2 promoter contains consensus binding sites within the first 237 bp, which bind transcription factors Sp1 and Sp3(1). This study directly links Sp1 and Sp3 with ClC-2 protein expression by demonstrating: (i) induction of ClC-2 protein by transient overexpression of each transcription factor in adult rat Type II cells, which have low levels of ClC-2; and (ii) reduction of ClC-2 expression by incubation with a competitive inhibitor of Sp1 and Sp3 in fetal rat Type II cells, which have high levels of endogenous ClC-2. Endogenous fetal lung Sp1 is differentially expressed as two major species of 105 kD and 95 kD. Although low-level expression of Sp1 in adult cells is almost exclusively the 105-kD species, overexpression of Sp1 results in increased expression of the 95-kD band. These experiments suggest that the mechanism for postnatal reduction of ClC-2 expression in lung epithelia is based on decreased interaction of Sp1 and Sp3 with the ClC-2 promoter.

Proteasome Inhibition Induces Nuclear Translocation of the Dioxin Receptor Through an Sp1 and Protein Kinase C-Dependent Pathway

The dioxin receptor (AhR), in addition to its role in xenobiotic-induced carcinogenesis, appears to participate in cell proliferation, differentiation and organ homeostasis. Understanding potential mechanisms of activation of this receptor in the absence of exogenous ligands is therefore important to study its contribution to endogenous cellular functions. Using mouse embryo primary fibroblasts, we have previously shown that proteasome inhibition increased AhR transcriptional activity in the absence of xenobiotics. We suggested that proteasome inhibition-dependent AhR activation could involve an increase in the expression of the partner protein dioxin receptor nuclear translocator (ARNT). Since ARNT over-expression induced nuclear translocation of the AhR, and ARNT-deficient cells were unable to translocate this receptor to the nucleus upon proteasome inhibition, we have analyzed the effect of proteasome inhibition on the expression of regulatory proteins controlling ARNT levels. Treatment with the proteasome inhibitor MG132 increased endogenous Sp1 phosphorylation and its DNA-binding activity to the ARNT promoter. Sp1 phosphorylation and binding to the ARNT promoter, ARNT over-expression and AhR nuclear translocation were inhibited by GF109203X, a protein kinase C-specific inhibitor. In addition, MG132 stimulated protein kinase C activity in MEF cells with a pattern similar to that observed for ARNT expression. These data suggest that cellular control of protein kinase C activity, through Sp1 and ARNT, could regulate AhR transcriptional activity in the absence of xenobiotics.

Adjacent sequence controls the response polarity of nitric oxide-sensitive Sp factor binding sites

Nitric oxide (NO*) and cAMP-dependent protein kinase (PKA) inhibitors up-regulate tumor necrosis factor alpha (TNFalpha) by decreasing Sp1 binding to a proximal GC box element. Here, elements flanking GC boxes were tested for their role in determining whether Sp sites act as activators or repressors. Promoter studies in receptive human cell lines demonstrated that NO* down-regulated endothelial NO* synthase (eNOS) but up-regulated TNFalpha. Like TNFalpha, Sp1 binding to the eNOS promoter was decreased by NO* and a PKA inhibitor, H89, and increased by a PKA activator, dibutyryl cAMP (Bt2cAMP). For either promoter, mutation of Sp sites abolished NO* responses. In contrast, mutation of an upstream AP1 site in the TNFalpha promoter (not present in eNOS) maintained NO* responsiveness, but reversed the direction of NO* and cAMP effects. Using artificial constructs, NO* increased transcription when Sp and AP1 sites were both present (TNFalpha-like response), but decreased it when the adjacent AP1 site was disrupted (eNOS-like response). NO*, H89, and Bt2cAMP were found to produce reciprocal protein binding changes at contiguous AP1 and Sp sites (p < 0.0001 for an interaction). Chromatin immunoprecipitation assays demonstrated that Sp1 and to a lesser extent Sp3 bound to the GC box regions of eNOS and TNFalpha in intact cells. Thus, this NO*- and cAMP-responsive regulatory module has a Sp site sensor variably coupled to an adjacent element that determines response polarity. These results define a composite element that can utilize secondary inputs to convert off signals to on, thereby conferring complex functionalities to the same DNA binding motif.

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.

Sp family of transcription factors is involved in valproic acid-induced expression of Galphai2

Valproic acid-induced gene expression has been attributed to the DNA-binding activity of the transcription factor activator protein 1 (AP-1). Using K562 cells, we have studied valproic acid-induced transcription from the human Galpha(i2) gene promoter, which lacks AP-1-binding motifs. We find that valproic acid-induced expression of Galpha(i2) is inhibited by mithramycin A, a compound that interferes with Sp1 binding to GC boxes in DNA. Three Sp1-binding sequences, located at +68/+75, -50/-36, and -92/-85 in the promoter, accounted for about 60% of this transcriptional effect, as judged by transient transfection assays. Electrophoretic mobility shift assays indicated that these sites bind members of the Sp family of transcription factors. Binding to DNA was inhibited by mithramycin A and was greater in nuclear extracts from cells treated with valproic acid than in control cells. Okadaic acid, calyculin A, and fostriecin, which are potent inhibitors of protein phosphatase, suppressed the transcriptional response to valproic acid. This inhibitory effect was not observed when promoter constructs containing mutations in the referenced Sp1-binding sites were used for transfections. In nuclear extracts from cells cultured in the presence of these inhibitors, the binding of Sp1/Sp3 to DNA probes was much less than in control cells. Alkaline phosphatase treatment of nuclear extracts resulted in enhanced binding of Sp proteins to the DNA probes. These results are consistent with the idea that dephosphorylating conditions enhanced Sp binding to the DNA probes as well as Sp-mediated transcription induced by valproic acid. This study demonstrates that the gene expression-inducing effect of valproic acid occurs, in part, through the Sp family of transcription factors.

PNUTS, a protein phosphatase 1 (PP1) nuclear targeting subunit. Characterization of its PP1- and RNA-binding domains and regulation by phosphorylation

PNUTS, Phosphatase 1 NUclear Targeting Subunit, is a recently described protein that targets protein phosphatase 1 (PP1) to the nucleus. In the present study, we characterized the biochemical properties of PNUTS. A variety of truncation and site-directed mutants of PNUTS was prepared and expressed either as glutathione S-transferase fusion proteins in Escherichia coli or as FLAG-tagged proteins in 293T cells. A 50-amino acid domain in the center of PNUTS mediated both high affinity PP1 binding and inhibition of PP1 activity. The PP1-binding domain is related to a motif found in several other PP1-binding proteins but is distinct in that Trp replaces Phe. Mutation of the Trp residue essentially abolished the ability of PNUTS to bind to and inhibit PP1. The central PP1-binding domain of PNUTS was an effective substrate for protein kinase A in vitro, and phosphorylation substantially reduced the ability of PNUTS to bind to PP1 in vitro and following stimulation of protein kinase A in intact cells. In vitro RNA binding experiments showed that a C-terminal region including several RGG motifs and a novel repeat domain rich in His and Gly interacted with mRNA and single-stranded DNA. PNUTS exhibited selective binding for poly(A) and poly(G) compared with poly(U) or poly(C) ribonucleotide homopolymers, with specificity being mediated by distinct regions within the domain rich in His and Gly and the domain containing the RGG motifs. Finally, a PNUTS-PP1 complex was isolated from mammalian cell lysates using RNA-conjugated beads. Together, these studies support a role for PNUTS in protein kinase A-regulated targeting of PP1 to specific RNA-associated complexes in the nucleus.

High glucose and insulin promote O-GlcNAc modification of proteins, including alpha-tubulin

Increased flux through the hexosamine biosynthesis pathway has been implicated in the development of glucose-induced insulin resistance and may promote the modification of certain proteins with O-linked N-acetylglucosamine (O-GlcNAc). L6 myotubes (a model of skeletal muscle) were incubated for 18 h in 5 or 25 mM glucose with or without 10 nM insulin. As assessed by immunoblotting with an O-GlcNAc-specific antibody, high glucose and/or insulin enhanced O-GlcNAcylation of numerous proteins, including the transcription factor Sp1, a known substrate for this modification. To identify novel proteins that may be O-GlcNAc modified in a glucose concentration/insulin-responsive manner, total cell membranes were separated by one- or two-dimensional gel electrophoresis. Selected O-GlcNAcylated proteins were identified by mass spectrometry (MS) analysis. MS sequencing of tryptic peptides identified member(s) of the heat shock protein 70 (HSP70) family and rat alpha-tubulin. Immunoprecipitation/immunoblot studies demonstrated several HSP70 isoforms and/or posttranslational modifications, some with selectively enhanced O-GlcNAcylation following exposure to high glucose plus insulin. In conclusion, in L6 myotubes, Sp1, membrane-associated HSP70, and alpha-tubulin are O-GlcNAcylated; the modification is markedly enhanced by sustained increased glucose flux.

Interaction of p38 and Sp1 in a mechanical force-induced, beta 1 integrin-mediated transcriptional circuit that regulates the actin-binding protein filamin-A

Connective tissue cells in mechanically active environments survive applied physical forces by modifying actin cytoskeletal structures that stabilize cell membranes. In fibroblasts, tensile forces induce the expression of filamin-A, a mechanoprotective actin-binding protein, but the mechanisms and protein interactions by which force activates filamin-A transcription are not defined. We found that in fibroblasts, application of tensile forces through collagen-coated magnetite beads to cell surface beta(1) integrins induced filamin-A expression. This induction required actin filaments and selective activation of the p38 mitogen-activated protein kinase. Force promoted the redistribution of p38 to the integrin/bead locus and the nucleus as well as enhanced binding of the transcription factor Sp1 to proximal, regulatory domains of the filamin-A promoter. Force application increased association of Sp1 with p38 and phosphorylation of Sp1. Transcriptional activation of filamin-A in force-treated fibroblasts was subsequently mediated by Sp1-binding sites on the filamin-A promoter. These results provide evidence for a mechanically coupled transcriptional circuit that originates at the magnetite bead/integrin locus, activates p38, tethers p38 to actin filaments, promotes binding of p38 to Sp1 in the nucleus, and induces filamin-A expression.

The trans-spliced variants of Sp1 mRNA in rat

trans-Splicing is the biological reaction that generates a mature mRNA from separate strands of pre-mRNAs. Previously, we reported that the trans-splicing between the two Sp1 pre-mRNA strands produced an mRNA with the exon 3-2-3 alignment in human HepG2 cells. Here we describe the rat counterpart as well as a newly identified variant with the exon 3-3 alignment in cultured rat cells. A qualitative evaluation of such alignments in poly(A)(+) RNA-rich preparation showed that both alignments arose from trans-splicing rather than circularization of a single strand. The identification of the trans-spliced products in both rat and human raises the possibility that trans-splicing on Sp1 pre-mRNA is rather common to mammals. It was observed that the level of the trans-spliced variants varies in different rat organs.

Domains of type 1 protein phosphatase inhibitor-2 required for nuclear and cytoplasmic localization in response to cell-cell contact

Inhibitor-2 of type 1 protein phosphatase is a phosphoprotein conserved among all eukaryotes, and it appears in both the nucleus and cytoplasm of tissue culture cells. We discovered that endogenous inhibitor-2 is concentrated in the nucleus of cells cultured at low density, whereas cells growing at high density excluded inhibitor-2 from the nucleus. There was rapid redistribution of inhibitor-2 when cells were replated at low or high density. Localization of myc-tagged forms of inhibitor-2 showed that residues 119-197 were required for nuclear accumulation in low-density cells and residues 78-119 were required for cytoplasmic localization in high-density cells. Fusion of inhibitor-2 residues 78-119 to green fluorescent protein was sufficient to produce cytoplasmic retention. Inhibitor-2 fused to triple tandem green fluorescent protein (100 kDa) was imported into the nucleus of low-density cells but was not excluded from the nucleus when cells reached high density, implying that inhibitor-2 was actively imported into the nucleus but exited by passive diffusion instead of active export. We conclude that inhibitor-2 contains two separate domains that control its localization in the nucleus or cytoplasm. This change in inhibitor-2 localization may direct inhibitor-2 to different forms of protein phosphatase 1 or change the localization of protein phosphatase, as part of the cellular response to cell-cell contacts at high density.

Transcription factor Sp1 phosphorylation induced by shear stress inhibits membrane type 1-matrix metalloproteinase expression in endothelium

Membrane type 1-matrix metalloproteinase (MT1-MMP) plays a key role in endothelial cell migration, matrix remodeling, and angiogenesis. Previous studies demonstrated that a mechanical force, cyclic strain, increases MT1-MMP expression by displacing Sp1 with increased Egr-1 expression and binding to the promoter site. However, the effect of shear stress (SS) on MT1-MMP expression is poorly understood. Although Egr-1 mRNA transcription and protein was induced (7.6-fold) in response to SS (n = 5, 0-8 h, p < 0.05), SS decreased MT1-MMP mRNA transcription and protein levels in a time-dependent fashion (10, 50, and 90% reduction at 1, 4, and 8 h, respectively; n = 5, p < 0.05). Egr-1 protein was increased after SS and cyclic strain, but Sp1 was serine-phosphorylated only after SS. SS increased Sp1 DNA binding (3.8-, 5.8-, and 2.4-fold increase at 1, 4, and 8 h, respectively; n = 5, p < 0.05) that was inhibitable by calf intestinal phosphatase. Thus, SS inhibits MT1-MMP expression despite Egr-1 up-regulation by inducing the serine phosphorylation of Sp1, which in turn increases its binding affinity for its site on the MT1-MMP promoter, reducing the ability of Egr-1 to displace it. These data illustrate the complex control of microvascular endothelial cell MT1-MMP expression in response to distinct environmental stimuli (cyclic strain versus shear stress), consisting of both the modulation of specific transcription factor expression (Egr-1) as well as transcription factor post-translational modification (serine phosphorylation of Sp1).

The hexosamine pathway regulates the plasminogen activator inhibitor-1 gene promoter and Sp1 transcriptional activation through protein kinase C-beta I and -delta

Increased flux through the hexosamine biosynthesis pathway (HBP) has been shown to stimulate the expression of a number of genes. We previously demonstrated in glomerular mesangial and endothelial cells that both high glucose concentrations and glucosamine activated the plasminogen activator inhibitor-1 (PAI-1) gene promoter through the transcription factor, Sp1; and that the glutamine:fructose-6-phosphate amidotransferase inhibitor, 6-diazo-5-oxonorleucine, inhibited the effect of high glucose, but not that of glucosamine. Here, we examined the role of protein kinase C (PKC) isoforms in the regulation of the PAI-1 promoter and Sp1 transcriptional activity by the HBP. In transient transfections, exposure to 2 mm glucosamine or 20 mm glucose for 4 days increased the activities of a PAI-1 promoter-luciferase reporter gene as well as the Sp1 transcriptional activation domain fused to the GAL4 DNA-binding domain cotransfected with a GAL4 promoter-luciferase reporter. Cotransfected dominant negative PKC-betaI and -delta completely blocked the induction of PAI-1 promoter transcription by both sugars, whereas only dominant negative PKC-betaI interfered with Sp1-GAL4 activation. Both glucosamine and high glucose stimulated the in vitro kinase activity of immunoprecipitated PKC-betaI and -delta. Furthermore, 6-diazo-5-oxonorleucine suppressed high glucose-induced PKC kinase activity and Sp1-GAL4 transcriptional activation. These findings demonstrate a requirement for the PKC-betaI and -delta signal transduction pathways in HBP-induced transcription.

Promoter II of the bovine acetyl-coenzyme A carboxylase-alpha-encoding gene is widely expressed and strongly active in different cells

Three promoters express the bovine gene encoding the enzyme acetyl-CoA carboxylase-alpha (ACC alpha) known to be rate-limiting for fatty acid synthesis. Our sequence of promoter II shows that PII is evolutionary conserved, unlike PI or PIII. In vivo expression of PII reveals little tissue-specific restrictions. The proximal 133 bp of this promoter are sufficient for a strong basal expression in different cell lines.

Activation of dynamin I gene expression by Sp1 and Sp3 is required for neuronal differentiation of N1E-115 cells

Dynamin I is a key molecule required for the recycling of synaptic vesicles in neurons, and it has been known that dynamin I gene expression is induced during neuronal differentiation. Our previous studies established that neuronal restriction of dynamin I gene expression is controlled by Sp1 and nuclear factor-kappaB-like element-1. Here, using a series of deletion constructs and site-directed mutation, we found that transcription of dynamin I gene during neuronal differentiation of N1E-115 cells is controlled primarily by the Sp1 element located between -13 to -4 bp of the dynamin I promoter. Gel shift analysis demonstrated that in addition to Sp1, Sp3 could interact with this Sp1 element. The requirement for Sp family transcription factors in dynamin I gene expression was confirmed by using mithramycin, an inhibitor of Sp1/Sp3 binding. Mithramycin repressed dynamin I gene expression and resulted in blocking of neuronal differentiation of N1E-115 cells. The localization of the dynamin I protein was also restricted in the peripheral region of the nucleus by the mithramycin treatment. Thus, all of our results suggest that induction of dynamin I gene expression during N1E-115 cell differentiation is modulated by Sp1/Sp3 interactions with the dynamin I promoter, and its expression is important for neuronal differentiation of the N1E-115 cells.

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.