Inhibition of p53-mediated transcriptional responses by mithramycin A

The Position of Indole Methylation Controls the Structure, DNA Binding, and Cellular Functions of Mithramycin SA-Trp Analogues

Mithramycin (MTM) is a polyketide anticancer natural product, which functions by noncovalent binding to DNA in the minor groove without intercalation, resulting in inhibiting transcription at G/C-rich promoters. MTM is a potent inhibitor of cancer cells, such as Ewing sarcoma, driven by abnormal fusions involving E26 transformation-specific (ETS) family transcription factors friend leukemia integration 1 (FLI1) and ETS-related gene (ERG). However, MTM is rather toxic and nonselective; therefore, safer, selective analogues of MTM are required for use in the clinic as anticancer drugs. Herein, by using a combination of X-ray crystallographic, biophysical, and cell and molecular biological techniques, the structural and functional consequences of 3-side chain methylation at positions 5, 6, and 7 of the indole ring of the potent analogue MTM SA-Trp are explored. The conformation of the analogues in complexes with DNA, their DNA binding function, cytotoxicity, selectivity, and potency as transcription antagonists depended on the position of the methylation. MTM SA-5-methyl-Trp emerged as the most selective analogue, presumably due to the right balance of the DNA binding and the solvent exposure of the 3-side chain. This study demonstrates that minor chemical changes can have strong effects in analogue development and paves the way to further development of next-generation MTM analogues.

Mithramycin selectively attenuates DNA-damage-induced neuronal cell death

DNA damage triggers cell death mechanisms contributing to neuronal loss and cognitive decline in neurological disorders, including traumatic brain injury (TBI), and as a side effect of chemotherapy. Mithramycin, which competitively targets chromatin-binding sites of specificity protein 1 (Sp1), was used to examine previously unexplored neuronal cell death regulatory mechanisms via rat primary neurons in vitro and after TBI in mice (males). In primary neurons exposed to DNA-damage-inducing chemotherapy drugs in vitro we showed that DNA breaks sequentially initiate DNA-damage responses, including phosphorylation of ATM, H2AX and tumor protein 53 (p53), transcriptional activation of pro-apoptotic BH3-only proteins, and mitochondrial outer membrane permeabilization (MOMP), activating caspase-dependent and caspase-independent intrinsic apoptosis. Mithramycin was highly neuroprotective in DNA-damage-dependent neuronal cell death, inhibiting chemotherapeutic-induced cell death cascades downstream of ATM and p53 phosphorylation/activation but upstream of p53-induced expression of pro-apoptotic molecules. Mithramycin reduced neuronal upregulation of BH3-only proteins and mitochondrial dysfunction, attenuated caspase-3/7 activation and caspase substrates' cleavage, and limited c-Jun activation. Chromatin immunoprecipitation indicated that mithramycin attenuates Sp1 binding to pro-apoptotic gene promoters without altering p53 binding suggesting it acts by removing cofactors required for p53 transactivation. In contrast, the DNA-damage-independent neuronal death models displayed caspase initiation in the absence of p53/BH3 activation and were not protected even when mithramycin reduced caspase activation. Interestingly, experimental TBI triggers a multiplicity of neuronal death mechanisms. Although markers of DNA-damage/p53-dependent intrinsic apoptosis are detected acutely in the injured cortex and are attenuated by mithramycin, these processes may play a reduced role in early neuronal death after TBI, as caspase-dependent mechanisms are repressed in mature neurons while other, mithramycin-resistant mechanisms are active. Our data suggest that Sp1 is required for p53-mediated transactivation of neuronal pro-apoptotic molecules and that mithramycin may attenuate neuronal cell death in conditions predominantly involving DNA-damage-induced p53-dependent intrinsic apoptosis.

Screening of Natural Products and Approved Oncology Drug Libraries for Activity against Clostridioides difficile

Clostridioides difficile is the most common cause of healthcare-associated diarrhea. Infection of the gastrointestinal tract with this Gram-positive, obligate anaerobe can lead to potentially life-threatening conditions in the antibiotic-treated populace. New therapeutics are urgently needed to treat this infection and prevent its recurrence. Here, we screened two libraries from the National Cancer Institute, namely, the natural product set III library (117 compounds) and the approved oncology drugs set V library (114 compounds), against C. difficile. In the two libraries screened, 17 compounds from the natural product set III library and 7 compounds from the approved oncology drugs set V library were found to exhibit anticlostridial activity. The most potent FDA-approved drugs (mitomycin C and mithramycin A) and a promising natural product (aureomycin) were further screened against 20 clinical isolates of C. difficile. The anticancer drugs, mitomycin C (MIC₅₀ = 0.25 μg/ml) and mithramycin A (MIC₅₀ = 0.015 μg/ml), and the naturally derived tetracycline derivative, aureomycin (MIC₅₀ = 0.06 μg/ml), exhibited potent activity against C. difficile strains. Mithramycin A and aureomycin were further found to inhibit toxin production by this pathogen. Given their efficacy, these compounds can provide a quick supplement to current treatment to address the unmet needs in treating C. difficile infection and preventing its recurrence.

DNA-BINDING and DNA-protecting activities of small natural organic molecules and food extracts

The review summarizes literature data on the DNA-binding, DNA-protecting and DNA-damaging activities of a range of natural human endogenous and exogenous compounds. Small natural organic molecules bind DNA in a site-specific mode, by arranging tight touch with the structure of the major and minor grooves, as well as individual bases in the local duplex DNA. Polyphenols are the best-studied exogenous compounds from this point of view. Many of them demonstrate hormetic effects, producing both beneficial and damaging effects. An attempt to establish the dependence of DNA damage or DNA protection on the concentration of the compound turned out to be successful for some polyphenols, daidzein, genistein and resveratrol, which were DNA protecting in low concentrations and DNA damaging in high concentrations. There was no evident dependence on concentration for quercetin and kaempferol. Probably, the DNA-protecting effect is associated with the affinity to DNA. Caffeine and theophylline are DNA binders; at the same time, they favor DNA repair. Although most alkaloids damage DNA, berberine can protect DNA against damage. Among the endogenous compounds, hormones belonging to the amine class, thyroid and steroid hormones appear to bind DNA and produce some DNA damage. Thus, natural compounds continue to reveal beneficial or adverse effects on genome integrity and provide a promising source of therapeutic activities.

Targeting Transcription Factors for Cancer Treatment

Transcription factors are involved in a large number of human diseases such as cancers for which they account for about 20% of all oncogenes identified so far. For long time, with the exception of ligand-inducible nuclear receptors, transcription factors were considered as “undruggable” targets. Advances knowledge of these transcription factors, in terms of structure, function (expression, degradation, interaction with co-factors and other proteins) and the dynamics of their mode of binding to DNA has changed this postulate and paved the way for new therapies targeted against transcription factors. Here, we discuss various ways to target transcription factors in cancer models: by modulating their expression or degradation, by blocking protein/protein interactions, by targeting the transcription factor itself to prevent its DNA binding either through a binding pocket or at the DNA-interacting site, some of these inhibitors being currently used or evaluated for cancer treatment. Such different targeting of transcription factors by small molecules is facilitated by modern chemistry developing a wide variety of original molecules designed to specifically abort transcription factor and by an increased knowledge of their pathological implication through the use of new technologies in order to make it possible to improve therapeutic control of transcription factor oncogenic functions.

An overview of recent advances in duplex DNA recognition by small molecules

As the carrier of genetic information, the DNA double helix interacts with many natural ligands during the cell cycle, and is amenable to such intervention in diseases such as cancer biogenesis. Proteins bind DNA in a site-specific manner, not only distinguishing between the geometry of the major and minor grooves, but also by making close contacts with individual bases within the local helix architecture. Over the last four decades, much research has been reported on the development of small non-natural ligands as therapeutics to either block, or in some cases, mimic a DNA–protein interaction of interest. This review presents the latest findings in the pursuit of novel synthetic DNA binders. This article provides recent coverage of major strategies (such as groove recognition, intercalation and cross-linking) adopted in the duplex DNA recognition by small molecules, with an emphasis on major works of the past few years.

Mithramycin A suppresses basal triple-negative breast cancer cell survival partially via down-regulating Krüppel-like factor 5 transcription by Sp1

As the most malignant breast cancer subtype, triple-negative breast cancer (TNBC) does not have effective targeted therapies clinically to date. As a selective Sp1 inhibitor, Mithramycin A (MIT) has been reported to have anti-tumor activities in multiple cancers. However, the efficacy and the mechanism of MIT in breast cancer, especially TNBC, have not been studied. In this study, we demonstrated that MIT suppressed breast cancer cell survival in a dosage-dependent manner. Interestingly, TNBC cells were more sensitive to MIT than non-TNBC cells. MIT inhibited TNBC cell proliferation and promoted apoptosis in vitro in time- and dosage-dependent manners. MIT suppressed TNBC cell survival, at least partially, by transcriptionally down-regulating KLF5, an oncogenic transcription factor specifically expressed in basal TNBC. Finally, MIT suppressed TNBC cell growth in a xenograft mouse model. Taken together, our findings suggested that MIT inhibits basal TNBC via the Sp1/KLF5 axis and that MIT may be used for TNBC treatment.

Tumor suppressor protein p53-mediated repression of human mitotic centromere-associated kinesin gene expression is exerted via down-regulation of Sp1 level

The repressive role of p53 on the human mitotic centromere-associated kinesin (MCAK) core promoter from ‒266 to +54, relative to the transcription start site, has been determined. The MCAK mRNA and protein levels were 2.1- and 3.0-fold higher, respectively, in HCT116 (p53^‒/‒) than in HCT116 (p53^+/+) cells. Enforced down-regulation of p53 levels either in HCT116 (p53^+/+) cells by p53 RNAi treatment or in MCF-7 cells using shRNA for p53 (shp53) resulted in a remarkable increase in the MCAK protein level. Site-directed mutagenesis and ChIP analyses showed that p53-mediated repression of the MCAK core promoter activity was not directly exerted by p53-binding to putative p53-response elements (p53-RE1 at −173/−166 and p53-RE2 at −245/−238), but indirectly by attenuating Sp1 binding to GC-motifs (GC1 at −93/−84 and GC2 at −119/−110). Treatment of HEK-293 cells bearing the MCAK core promoter-reporter (pGL2-320-Luc) with mithramycin A, which down-regulates Sp1 gene expression, reduced the promoter activity as well as endogenous MCAK levels. Exposure of HCT116 (p53^+/+) cells to nutlin-3a, a validated activator of p53, caused a simultaneous reduction in Sp1 and MCAK protein levels, but not in HCT116 (p53^−/−) cells. In contrast to wild-type (wt)-p53, tumor-derived p53 mutants (p53^V143A, p53^R248W, and p53^R273H) failed to repress the Sp1-dependent activation of the MCAK promoter and to down-regulate endogenous levels of Sp1 and MCAK proteins. Collectively, these findings demonstrate that p53 can repress MCAK promoter activity indirectly via down-regulation of Sp1 expression level, and suggest that MCAK elevation in human tumor cells might be due to p53 mutation.

Identification of miRNA-mRNA Modules in Colorectal Cancer Using Rough Hypercuboid Based Supervised Clustering

Differences in the expression profiles of miRNAs and mRNAs have been reported in colorectal cancer. Nevertheless, information on important miRNA-mRNA regulatory modules in colorectal cancer is still lacking. In this regard, this study presents an application of the RH-SAC algorithm on miRNA and mRNA expression data for identification of potential miRNA-mRNA modules. First, a set of miRNA rules was generated using the RH-SAC algorithm. The mRNA targets of the selected miRNAs were identified using the miRTarBase database. Next, the expression values of target mRNAs were used to generate mRNA rules using the RH-SAC. Then all miRNA-mRNA rules have been integrated for generating networks. The RH-SAC algorithm unlike other existing methods selects a group of co-expressed miRNAs and mRNAs that are also differentially expressed. In total 17 miRNAs and 141 mRNAs were selected. The enrichment analysis of selected mRNAs revealed that our method selected mRNAs that are significantly associated with colorectal cancer. We identified novel miRNA/mRNA interactions in colorectal cancer. Through experiment, we could confirm that one of our discovered miRNAs, hsa-miR-93-5p, was significantly up-regulated in 75.8% CRC in comparison to their corresponding non-tumor samples. It could have the potential to examine colorectal cancer subtype specific unique miRNA/mRNA interactions.

Dimerization and DNA recognition rules of mithramycin and its analogues

The antineoplastic and antibiotic natural product mithramycin (MTM) is used against cancer-related hypercalcemia and, experimentally, against Ewing sarcoma and lung cancers. MTM exerts its cytotoxic effect by binding DNA as a divalent metal ion (Me(2+))-coordinated dimer and disrupting the function of transcription factors. A precise molecular mechanism of action of MTM, needed to develop MTM analogues selective against desired transcription factors, is lacking. Although it is known that MTM binds G/C-rich DNA, the exact DNA recognition rules that would allow one to map MTM binding sites remain incompletely understood. Towards this goal, we quantitatively investigated dimerization of MTM and several of its analogues, MTM SDK (for Short side chain, DiKeto), MTM SA-Trp (for Short side chain and Acid), MTM SA-Ala, and a biosynthetic precursor premithramycin B (PreMTM B), and measured the binding affinities of these molecules to DNA oligomers of different sequences and structural forms at physiological salt concentrations. We show that MTM and its analogues form stable dimers even in the absence of DNA. All molecules, except for PreMTM B, can bind DNA with the following rank order of affinities (strong to weak): MTM=MTM SDK>MTM SA-Trp>MTM SA-Ala. An X(G/C)(G/C)X motif, where X is any base, is necessary and sufficient for MTM binding to DNA, without a strong dependence on DNA conformation. These recognition rules will aid in mapping MTM sites across different promoters towards development of MTM analogues as useful anticancer agents.

Sp1 transcription factor: A long-standing target in cancer chemotherapy

Sp1 (specificity protein 1) is a well-known member of a family of transcription factors that also includes Sp2, Sp3 and Sp4, which are implicated in an ample variety of essential biological processes and have been proven important in cell growth, differentiation, apoptosis and carcinogenesis. Sp1 activates the transcription of many cellular genes that contain putative CG-rich Sp-binding sites in their promoters. Sp1 and Sp3 proteins bind to similar, if not the same, DNA tracts and compete for binding, thus they can enhance or repress gene expression. Evidences exist that the Sp-family of proteins regulates the expression of genes that play pivotal roles in cell proliferation and metastasis of various tumors. In patients with a variety of cancers, high levels of Sp1 protein are considered a negative prognostic factor. A plethora of compounds can interfere with the trans-activating activities of Sp1 and other Sp proteins on gene expression. Several pathways are involved in the down-regulation of Sp proteins by compounds with different mechanisms of action, which include not only the direct interference with the binding of Sp proteins to their putative DNA binding sites, but also promoting the degradation of Sp protein factors. Down-regulation of Sp transcription factors and Sp1-regulated genes is drug-dependent and it is determined by the cell context. The acknowledgment that several of those compounds are safe enough might accelerate their introduction into clinical usage in patients with tumors that over-express Sp1.

Modulation of specificity protein 1 by mithramycin A as a novel therapeutic strategy for cervical cancer

Cervical cancer is the third most common cancer and the third leading cause of death among women. However, the standard treatment for cervical cancer includes cisplatin, which can cause side effects such as hematological damage or renal toxicity. New innovations in cervical cancer treatment focus on developing more effective and better-tolerated therapies such as Sp1-targeting drugs. Previous studies suggested that mithramycin A (Mith) inhibits the growth of various cancers by decreasing Sp1 protein. However, how Sp1 protein is decreased by Mith is not clear. Few studies have investigated the regulation of Sp1 protein by proteasome-dependent degradation as a possible control mechanism for the regulation of Sp1 in cancer cells. Here, we show that Mith decreased Sp1 protein by inducing proteasome-dependent degradation, thereby suppressing cervical cancer growth through a DR5/caspase-8/Bid signaling pathway. We found that prolonged Mith treatment was well tolerated after systemic administration to mice carrying cervical cancer cells. Reduction of body weight was minimal, indicating that Mith was a good therapeutic candidate for treatment of cancers in which Sp1 is involved in promoting and developing disease.

Mithramycin exerts an anti-myeloma effect and displays anti-angiogenic effects through up-regulation of anti-angiogenic factors

Mithramycin (MTM), a cytotoxic compound, is currently being investigated for its anti-angiogenic activity that seems to be mediated through an inhibition of the transcription factor SP1. In this study we evaluated its anti-myeloma effects in the syngenic 5TGM1 model in vitro as well as in vivo. In vitro, MTM inhibited DNA synthesis of 5TGM1 cells with an IC50 of 400 nM and induced an arrest in cell cycle progression at the G1/S transition point. Western-blot revealed an up-regulation of p53, p21 and p27 and an inhibition of c-Myc, while SP1 remained unaffected. In rat aortic ring assays, a strong anti-angiogenic effect was seen, which could be explained by a decrease of VEGF production and an up-regulation of anti-angiogenic proteins such as IP10 after MTM treatment. The administration of MTM to mice injected with 5TGM1 decreased 5TGM1 cell invasion into bone marrow and myeloma neovascularisation. These data suggest that MTM displays anti-myeloma and anti-angiogenic effects that are not mediated by an inhibition of SP1 but rather through c-Myc inhibition and p53 activation.

Estrogen receptor α (ESR1) over-expression mediated apoptosis in Hep3B cells by binding with SP1 proteins

Previous studies have reported that estrogen receptors (ERs) are expressed in normal human liver, chronic hepatitis, and benign hepatic tumor tissues. However, decreased expression of ERs can be observed in hepatocellular carcinoma (HCC) and the role of ERs in HCC is not fully understood. Thus, the present study aimed to investigate the molecular mechanism induced by the overexpression of ERα (ERα (ESR1)) in Hep3B cells. We first detected the induction of apoptosis in ER-negative Hep3B cells using DNA fragmentation assay and flow cytometry. We found that ERα and ERα plus 17β-estradiol treatment increased apoptosis in Hep3B cells. Additionally, western blotting showed increased expression of active caspase 3 and tumor necrosis factor α (TNFα (TNF)) in ERα-transfected cells. To further understand the importance of SP1-binding sites in the TNFα promoter, ERα-negative Hep3B cells were co-transfected with ERα and a wild-type TNFα plasmid or TNFα with deleted SP1 regions. Deletion of both distant and primal SP1 sites abolished the activity of ERα, and similar results were observed by blocking the expression of SP1 protein using mithramycin (MA). This result indicates that SP1 protein is essential for ERα-activated TNFα promoter activity. Co-immunoprecipitation assay further confirmed the binding interaction between ERα and SP1 in a ligand-dependent manner. In general, we demonstrate that the overexpression of ERα mediates apoptosis in ERα-negative Hep3B cells by the binding of ERα to SP1 protein. Additionally, this ERα-SP1 complex binds to the proximal and distal sites of the TNFα gene promoter and further induces the expression of active caspase 3 in a ligand-dependent manner.

Human rpL3 induces G₁/S arrest or apoptosis by modulating p21 (waf1/cip1) levels in a p53-independent manner

It is now largely accepted that ribosomal proteins may be implicated in a variety of biological functions besides that of components of the translation machinery. Many evidences show that a subset of ribosomal proteins are involved in the regulation of the cell cycle and apoptosis through modulation of p53 activity. In addition, p53-independent mechanisms of cell cycle arrest in response to alterations of ribosomal proteins availability have been described. Here, we identify human rpL3 as a new regulator of cell cycle and apoptosis through positive regulation of p21 expression in a p53-independent system. We demonstrate that the rpL3-mediated p21 upregulation requires the specific interaction between rpL3 and Sp1. Furthermore, in our experimental system, p21 overexpression leads to a dual outcome, activating the G₁/S arrest of the cell cycle or the apoptotic pathway through mitochondria, depending on its intracellular levels. It is noteworthy that depletion of p21 abrogates both effects. Taken together, our findings unravel a novel extraribosomal function of rpL3 and reinforce the proapoptotic role of p21 in addition to its widely reported ability as an inhibitor of cell proliferation.

A novel mithramycin analogue with high antitumor activity and less toxicity generated by combinatorial biosynthesis

Mithramycin is an antitumor compound produced by Streptomyces argillaceus that has been used for the treatment of several types of tumors and hypercalcaemia processes. However, its use in humans has been limited because of its side effects. Using combinatorial biosynthesis approaches, we have generated seven new mithramycin derivatives, which differ from the parental compound in the sugar profile or in both the sugar profile and the 3-side chain. From these studies three novel derivatives were identified, demycarosyl-3D-β-d-digitoxosylmithramycin SK, demycarosylmithramycin SDK, and demycarosyl-3D-β-d-digitoxosylmithramycin SDK, which show high antitumor activity. The first one, which combines two structural features previously found to improve pharmacological behavior, was generated following two different strategies, and it showed less toxicity than mithramycin. Preliminary in vivo evaluation of its antitumor activity through hollow fiber assays, and in subcutaneous colon and melanoma cancers xenografts models, suggests that demycarosyl-3D-β-d-digitoxosylmithramycin SK could be a promising antitumor agent worthy of further investigation.

Crosstalk of Sp1 and Stat3 signaling in pancreatic cancer pathogenesis

Pancreatic cancer progression is attributed to genetic and epigenetic alterations and a chaotic tumor microenvironment. Those diverse "upstream signal" factors appear to converge on specific sets of central nuclear regulators, namely, transcription factors. Specificity Protein 1 (Sp1) and signal transducer and activator of transcription 3 (Stat3) are central transcription factors that regulate a number of pathways important to tumorigenesis, including tumor cell-cycle progression, apoptosis, angiogenesis, metastasis, and evasion of the immune system. Recently, researchers demonstrated many types of crosstalk of Sp1 and Stat3 in tumor signal transduction and that these factors function cooperatively to activate targeted genes and promote tumorigenesis in pancreatic cancer. Therefore, targeting both Sp1 and Stat3 is a potential preventive and therapeutic strategy for pancreatic cancer.

Mechanisms of MEOX1 and MEOX2 regulation of the cyclin dependent kinase inhibitors p21 and p16 in vascular endothelial cells

Senescence, the state of permanent cell cycle arrest, has been associated with endothelial cell dysfunction and atherosclerosis. The cyclin dependent kinase inhibitors p21^CIP1/WAF1 and p16^INK4a govern the G₁/S cell cycle checkpoint and are essential for determining whether a cell enters into an arrested state. The homeodomain transcription factor MEOX2 is an important regulator of vascular cell proliferation and is a direct transcriptional activator of both p21^CIP1/WAF1 and p16^INK4a. MEOX1 and MEOX2 have been shown to be partially functionally redundant during development, suggesting that they regulate similar target genes in vivo. We compared the ability of MEOX1 and MEOX2 to activate p21^CIP1/WAF1 and p16^INK4a expression and induce endothelial cell cycle arrest. Our results demonstrate for the first time that MEOX1 regulates the MEOX2 target genes p21^CIP1/WAF1 and p16^INK4a. In addition, increased expression of either of the MEOX homeodomain transcription factors leads to cell cycle arrest and endothelial cell senescence. Furthermore, we show that the mechanism of transcriptional activation of these cyclin dependent kinase inhibitor genes by MEOX1 and MEOX2 is distinct. MEOX1 and MEOX2 activate p16^INK4a in a DNA binding dependent manner, whereas they induce p21^CIP1/WAF1 in a DNA binding independent manner.

Genetic variation in ABCG1 and risk of myocardial infarction and ischemic heart disease

OBJECTIVE

ATP binding cassette transporter G1 (ABCG1) facilitates cholesterol efflux from macrophages to mature high-density lipoprotein particles. Whether genetic variation in ABCG1 affects risk of atherosclerosis in humans remains to be determined.

METHODS AND RESULTS

We resequenced the core promoter and coding regions of ABCG1 in 380 individuals from the general population. Next, we genotyped 10 237 individuals from the Copenhagen City Heart Study for the identified variants and determined the effect on lipid and lipoprotein levels and on risk of myocardial infarction (MI) and ischemic heart disease (IHD). g.-376C>T, g.-311T>A, and Ser630Leu predicted risk of MI in the Copenhagen City Heart Study, with hazard ratios of 2.2 (95% confidence interval: 1.2-4.3), 1.7 (1.0-2.9), and 7.5 (1.9-30), respectively. These results were confirmed for g.-376C>T in a case-control study comprising 4983 independently ascertained IHD cases and 7489 controls. Expression levels of ABCG1 mRNA were decreased by approximately 40% in g.-376C>T heterozygotes versus noncarriers (probability values: 0.005-0.009). Finally, in vitro specificity protein 1 (Sp1) bound specifically to a putative Sp1 binding site at position -382 to -373 in the ABCG1 promoter, and the presence of the -376 T allele reduced binding and transactivation of the promoter by Sp1.

CONCLUSIONS

This is the first report of a functional variant in ABCG1 that associates with increased risk of MI and IHD in the general population.

Opposite effects of HIF-1α and HIF-2α on the regulation of IL-8 expression in endothelial cells

Recently we have shown that hypoxia as well as overexpression of the stable form of hypoxia-inducible factor-1α (HIF-1α) diminished the expression of interleukin-8 (IL-8) by inhibition of the Nrf2 transcription factor in HMEC-1 cells. Because HIF isoforms may exert different effects, we aimed to examine the influence of HIF-2α on IL-8 expression in endothelial cells. In contrast to HIF-1α, overexpression of HIF-2α obtained by adenoviral transduction resulted in increased expression of IL-8 in an Nrf2-independent way. Importantly, HIF-2α augmented the activity of SP-1, a transcription factor involved in IL-8 regulation and known coactivator of c-Myc. Additionally, HIF-1 decreased, whereas HIF-2 increased, c-Myc expression, and silencing of Mxi-1, a c-Myc antagonist, restored IL-8 expression downregulated by HIF-1α or hypoxia. Accordingly, binding of c-Myc to the IL-8 promoter was abolished in hypoxia. Importantly, both severe (0.5% O(2)) and mild (5% O(2)) hypoxia diminished IL-8 expression despite the stabilization of both HIF-1 and HIF-2. This study reveals the opposite roles of HIF-1α and HIF-2α in the regulation of IL-8 expression in endothelial cells. However, despite stabilization of both isoforms in hypoxia the effect of HIF-1 is predominant, and downregulation of IL-8 expression in hypoxia is caused by attenuation of Nrf2 and c-Myc.

Combining betulinic acid and mithramycin a effectively suppresses pancreatic cancer by inhibiting proliferation, invasion, and angiogenesis

Both betulinic acid (BA) and mithramycin A (MIT) exhibit potent antitumor activity through distinct mechanisms of Sp1 inhibition. However, it is unknown whether a combination of these two compounds results in a synergistic inhibitory effect on pancreatic cancer growth and/or has a therapeutic advantage over gemcitabine. In xenograft mouse models of human pancreatic cancer, treatment with either BA or MIT alone showed dose-dependent antitumor activity but led to systemic side effects as measured by overall weight loss. Treatment with a nontoxic dose of either compound alone had only marginal antitumor effects. Importantly, combination treatment with nontoxic doses of BA and MIT produced synergistic antitumor activity, including inhibitory effects on cell proliferation, invasion, and angiogenesis. The treatment combination also produced less discernible side effects than therapeutic doses of gemcitabine. Moreover, combined treatment of BA and MIT resulted in drastic inhibition of Sp1 recruitment onto Sp1 and VEGF promoters, leading to transcriptional inhibition of both Sp1 and VEGF and downregulation of Sp1 and VEGF protein expression. Ectopic overexpression of Sp1 rendered tumor cells resistant to BA, MIT, and the combination of the two. Overall, our findings argue that Sp1 is an important target of BA and MIT and that their combination can produce an enhanced therapeutic response in human pancreatic cancer.

Identification of an inhibitor of the EWS-FLI1 oncogenic transcription factor by high-throughput screening

BACKGROUND

Chromosomal translocations generating oncogenic transcription factors are the hallmark of a variety of tumors, including many sarcomas. Ewing sarcoma family of tumors (ESFTs) are characterized by the t(11;22)(q24;q12) translocation that generates the Ewing sarcoma breakpoint region 1 and Friend leukemia virus integration 1 (EWS-FLI1) fusion transcription factor responsible for the highly malignant phenotype of this tumor. Although continued expression of EWS-FLI1 is believed to be critical for ESFT cell survival, a clinically effective small-molecule inhibitor remains elusive likely because EWS-FLI1 is a transcription factor and therefore widely felt to be "undruggable."

METHODS

We developed a high-throughput screen to evaluate more than 50 000 compounds for inhibition of EWS-FLI1 activity in TC32 ESFT cells. We used a TC32 cell-based luciferase reporter screen using the EWS-FLI1 downstream target NR0B1 promoter and a gene signature secondary screen to sort and prioritize the compounds. We characterized the lead compound, mithramycin, based on its ability to inhibit EWS-FLI1 activity in vitro using microarray expression profiling, quantitative reverse transcription-polymerase chain reaction, and immunoblot analysis, and in vivo using immunohistochemistry. We studied the impact of this inhibition on cell viability in vitro and on tumor growth in ESFT xenograft models in vivo (n = 15-20 mice per group). All statistical tests were two-sided.

RESULTS

Mithramycin inhibited expression of EWS-FLI1 downstream targets at the mRNA and protein levels and decreased the growth of ESFT cells at half maximal inhibitory concentrations between 10 (95% confidence interval [CI] = 8 to 13 nM) and 15 nM (95% CI = 13 to 19 nM). Mithramycin suppressed the growth of two different ESFT xenograft tumors and prolonged the survival of ESFT xenograft-bearing mice by causing a decrease in mean tumor volume. For example, in the TC32 xenograft model, on day 15 of treatment, the mean tumor volume for the mithramycin-treated mice was approximately 3% of the tumor volume observed in the control mice (mithramycin vs control: 69 vs 2388 mm(3), difference = 2319 mm(3), 95% CI = 1766 to 2872 mm(3), P < .001).

CONCLUSION

Mithramycin inhibits EWS-FLI1 activity and demonstrates ESFT antitumor activity both in vitro and in vivo.

Opposite regulation of the human apolipoprotein M gene by hepatocyte nuclear factor 1 and Jun transcription factors

HDL is a negative risk factor for atherosclerosis because of its multiple atheroprotective functions. Inflammation converts HDL particles from anti-atherogenic to pro-atherogenic, and this transformation is associated with changes in HDL structure and composition. Apolipoprotein M (apoM) has been recently shown to play a role in the maturation of HDL in plasma and to protect from atherosclerosis. ApoM gene is expressed primarily in the liver and kidney and is down-regulated by pro-inflammatory signals. We now show that the human apoM promoter harbors a dual specificity regulatory element in the proximal region that binds hepatocyte nuclear factor 1 (HNF-1) and members of the AP-1 family of pro-inflammatory transcription factors (c-Jun and JunB). Overexpression of c-Jun or JunB repressed both the basal and the HNF-1-mediated transactivation of the human apoM promoter. Treatment of HepG2 cells with potent inflammation-inducing phorbol esters or overexpression of PKCα was associated with a marked inhibition of apoM gene expression in a c-Jun/JunB-dependent manner. We provide evidence for a novel mechanism of inflammation-induced transcriptional repression that is based on the competition between HNF-1 and Jun proteins for binding to the same regulatory region. A similar mechanism accounts for the down-regulation of the liver-specific apolipoprotein A-II gene by Jun factors. Our studies provide novel insights on the mechanisms that control the expression of liver-specific apolipoprotein genes during inflammation and could affect the maturation and the functionality of HDL particles.

Regulation of human apolipoprotein m gene expression by orphan and ligand-dependent nuclear receptors

Apolipoprotein M (apoM) plays an important role in the biogenesis and the metabolism of anti-atherogenic HDL particles in plasma and is expressed primarily in the liver and the kidney. We investigated the role of hormone nuclear receptors in apoM gene regulation in hepatic cells. Overexpression via adenovirus-mediated gene transfer and siRNA-mediated gene silencing established that hepatocyte nuclear factor 4 (HNF-4) is an important regulator of apoM gene transcription in hepatic cells. apoM promoter deletion analysis combined with DNA affinity precipitation and chromatin immunoprecipitation assays revealed that HNF-4 binds to a hormone-response element (HRE) in the proximal apoM promoter (nucleotides -33 to -21). Mutagenesis of this HRE decreased basal hepatic apoM promoter activity to 10% of control and abolished the HNF4-mediated transactivation of the apoM promoter. In addition to HNF-4, homodimers of retinoid X receptor and heterodimers of retinoid X receptor with receptors for retinoic acid, thyroid hormone, fibrates (peroxisome proliferator-activated receptor), and oxysterols (liver X receptor) were shown to bind with different affinities to the proximal HRE in vitro and in vivo. Ligands of these receptors strongly induced human apoM gene transcription and apoM promoter activity in HepG2 cells, whereas mutations in the proximal HRE abolished this induction. These findings provide novel insights into the role of apoM in the regulation of HDL by steroid hormones and into the development of novel HDL-based therapies for diseases such as diabetes, obesity, metabolic syndrome, and coronary artery disease that affect a large proportion of the population in Western countries.

From GC-rich DNA binding to the repression of survivin gene for quercetin nickel (II) complex: implications for cancer therapy

The DNA binding and cleavage properties of quercetin nickel (II) complex have been studied, but little attention has been devoted to the relationship between antitumor activity of this complex and DNA-binding properties. In the present study, we report that quercetin nickel (II) complex showed significant cytotoxicity against three tumor cell lines (HepG2, SMMC7721 and A549). Hoechst33258 and AO/EB staining showed HepG2 cells underwent the typical morphologic changes of apoptosis characterized by nuclear shrinkage, chromatin condensation, or fragmentation after exposure to quercetin nickel (II) complex. We also demonstrate that the levels of survivin and bcl-2 protein expression in HepG2 cells decreased concurrently, and the levels of p53 protein increased significantly after treatment with quercetin nickel (II) complex by immunocytochemistry analysis. The relative activity of caspase-3 and caspase-9 increased significantly after treatment with the complex. Furthermore, fluorescence measurements and molecular modeling were performed to learn that the complex could be preferentially bound to DNA in GC region. These results imply that quercetin nickel (II) complex may intercalate into the GC-rich core promoter region of survivin, down-regulating survivin gene expression and promoting tumor cells apoptosis. So our results suggest that antitumor activity of quercetin nickel (II) complex might be related to its intercalation into DNA and DNA-binding selectivity, and that the complex may be a promising agent for cancer therapy.

Therapeutic effects of the Sp1 inhibitor mithramycin A in glioblastoma

Mithramycin A (MitA) is a chemotherapeutic compound which has been used in the therapy of several types of cancer. For experimental cancer it has been shown that MitA mediates the expression of genes involved in tumor progression such as genes involved in immunosurveillance, cell motility or cell death. MitA works synergistically with Apo2L/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), and with antiangiogenic agents. We were therefore interested in analyzing whether MitA might be a suitable agent for glioma therapy. We demonstrate herein that the cell death sensitizing effects of MitA are cell line specific, independent of the endogenous status of the tumor suppressor p53 as well as of the endogenous expression of X-linked inhibitor of apoptosis (XIAP) or basal sensitivity towards death ligand-induced cell death. In glioma cells, MitA reduced the secretion and activity of the migration-involved matrix metalloproteinases (MMP), diminished vascular endothelial growth factor (VEGF), and increased recepteur d'origine nantais (RON) kinase messenger RNA (mRNA), paralleled by a significant reduction of glioma cell migration. In contrast to other cancer types, in glioma cells MitA did not alter the expression of the immunorelevant genes major histocompatibility complex I class related (MIC)-A, MIC-B or UL16 binding proteins (ULBP). We conclude that, whereas MitA-mediated reduction of XIAP expression and sensitization to Apo2L/TRAIL are cell line specific, its antimigratory effects are more general and might be the result of altered expression of MMP, VEGF, and/or RON kinase. Therefore, MitA might be a potential agent to reduce glioma cell migration.

Low-dose mithramycin exerts its anticancer effect via the p53 signaling pathway and synergizes with nutlin-3 in gynecologic cancers

MDM2 is a direct negative regulator of p53. The p53-independent mdm2-P1 and p53-dependent mdm2-P2 promoters have been recently shown to harbor Sp1 binding sites. Mithramycin, an inhibitor of Sp1 DNA binding, has been used clinically to treat hypercalcemia and some types of neoplastic disorders. In this study, we investigated the mechanisms behind the anticancer effect of mithramycin. In gynecologic cancer cells expressing wild-type p53, mithramycin stabilized p53 and increased the expression of the p53 downstream target genes PUMA and p21, arrested the cell cycle, and induced apoptosis. This activation of the p53 signaling pathway was a specific effect of MTH at concentrations <50 nm. Mithramycin temporally decreased transcription of both the mdm2-P1 and -P2 promoters. This was followed by a subsequent increase of mdm2-P2 promoter activity by activated p53. Up-regulated MDM2 was in its active form, and consequently attenuated p53 activity. Although mithramycin activated p53 and suppressed the growth of human gynecologic cancer cell xenografts in mice, this was accompanied with a secondary up-regulation of MDM2. Combined treatment with mithramycin and nutlin-3, a drug that inhibits MDM2-p53 interaction, overcame a secondary up-regulation of MDM2 and synergistically inhibited cancer cell growth by inducing apoptosis through activation of the p53 signaling pathway. These observations provide a better understanding of the mechanisms of mithramycin activity, and suggest a potential role for combining mithramycin and nutlin-3 as a chemotherapeutic treatment for gynecologic cancers.

Estrogen regulated expression of the p21 Waf1/Cip1 gene in estrogen receptor positive human breast cancer cells

The cyclin-dependent kinase inhibitor protein p21(Waf1/Cip1) is a potent tumor suppressor. Here, we demonstrate that estradiol regulates the p21(Waf1/Cip1) gene. Estradiol induces p21(Waf1/Cip1) mRNA expression within 30-60 min independent of new protein synthesis in the estrogen receptor alpha (ER alpha) positive human breast cancer cell line MCF-7. Similar to other estradiol responsive promoters, the p21(Waf1/Cip1) upstream promoter region has several estrogen response element (ERE) half-sites nestled in AP-1 binding sites, which are positioned upstream to Sp1 binding sites. Using the chromatin immunoprecipitation (ChIP) assay, we show that estradiol stimulation resulted in the recruitment of transcription factors ER alpha, Sp1, and Sp3 to the p21(Waf1/Cip1) upstream promoter element. The Sp1 inhibitor mithramycin A abrogated Sp1, and to a lesser extent Sp3 binding, and markedly reduced the estradiol stimulated p21(Waf1/Cip1) gene expression. However, ER alpha binding was not affected in the mithramycin A and estradiol treated cells. On closer examination of the half-site ERE/AP-1 sites upstream to the Sp1 sites in a separate ChIP experiment, we found a pronounced association of ER alpha upon estradiol treatment compared to almost negligible binding of Sp1 or Sp3. Together these studies provide evidence that ER alpha is recruited to the half-site ERE/AP-1 sites in the p21(Waf1/Cip1) upstream promoter element. Although Sp1/Sp3 is not involved in the recruitment of ER alpha to the promoter, Sp1 is necessary for estrogen-induced p21(Waf1/Cip1) promoter activity.

Combined treatment of pancreatic cancer with mithramycin A and tolfenamic acid promotes Sp1 degradation and synergistic antitumor activity

Mithramycin (MIT) and tolfenamic acid (TA) inhibit the activity of the transcription factor Sp1. In the present study, we investigated whether pancreatic cancer treatment with a combination of these compounds has a synergistic effect on Sp1 activity, tumor growth, and their underlying response mechanisms. Treatment of pancreatic tumor xenografts with MIT and TA produced dose-dependent antitumor activity, and significant antitumor activity of either compound alone was directly associated with systemic side effects. Combination treatment with nontoxic doses of both compounds produced synergistic antitumor activity, whereas treatment with a nontoxic dose of either compound alone lacked a discernible antitumor effect. Synergistic therapeutic effects correlated directly with synergistic antiproliferation and antiangiogenesis in vitro. Moreover, combination treatment resulted in Sp1 protein degradation, drastically downregulating expression of Sp1 and vascular endothelial growth factor. Our findings established that Sp1 is a critical target of TA and MIT in human pancreatic cancer therapy, rationalizing clinical studies to determine the effect of existing pancreatic cancer therapy regimens on Sp1 signaling in tumors and normal pancreatic tissue, and the ability of Sp1-targeting strategies to modify cancer responses.

Novel regulation of Smad3 oligomerization and DNA binding by its linker domain

Smad proteins are key effectors of the transforming growth factor beta (TGFbeta) signaling pathway in mammalian cells. Smads are composed of two highly structured and conserved domains called Mad homology 1 (MH1) and 2 (MH2), which are linked together by a nonconserved linker region. The recent identification of phosphorylation sites and binding sites for ubiquitin ligases in the linker regions of TGFbeta and bone morphogenetic protein (BMP) receptor-regulated Smads suggested that the linker may contribute to the regulation of Smad function by facilitating cross-talks with other signaling pathways. In the present study, we have generated and characterized novel Smad3 mutants bearing individual substitutions of conserved and nonconserved amino acid residues within a previously described transcriptionally active linker fragment. Our analysis showed that the conserved linker amino acids glutamine 222 and proline 229 play important roles in Smad functions such as homo- and hetero-oligomerization, nuclear accumulation in response to TGFbeta stimulation, and DNA binding. Furthermore, a Smad3 mutant bearing a substitution of the nonconserved amino acid asparagine 218 to alanine displayed enhanced transactivation potential relative to wild type Smad3. Finally, Smad3 P229A inhibited TGFbeta signaling when overexpressed in mammalian cells. In conclusion, our data are in line with previous studies supporting an important regulatory role of the linker region of Smads in their function as key transducers of TGFbeta signaling.

Regulation of human dUTPase gene expression and p53-mediated transcriptional repression in response to oxaliplatin-induced DNA damage

Deoxyuridine triphosphate nucleotidohydrolase (dUTPase) catalyzes the hydrolysis of dUTP to dUMP and PPi. Although dUTP is a normal intermediate in DNA synthesis, its accumulation and misincorporation into DNA is lethal. Importantly, uracil misincorporation is a mechanism of cytotoxicity induced by fluoropyrimidine chemotherapeutic agents including 5-fluorouracil (5-FU) and elevated expression of dUTPase is negatively correlated with clinical response to 5-FU-therapy. In this study we performed the first functional characterization of the dUTPase promoter and demonstrate a role for E2F-1 and Sp1 in driving dUTPase expression. We establish a direct role for both mutant and wild-type forms of p53 in modulating dUTPase promoter activity. Treatment of HCT116 p53(+/+) cells with the DNA-damaging agent oxaliplatin induced a p53-dependent transcriptional downregulation of dUTPase not observed in the isogenic null cell line. Oxaliplatin treatment induced enrichment of p53 at the dUTPase promoter with a concomitant reduction in Sp1. The suppression of dUTPase by oxaliplatin promoted increased levels of dUTP that was enhanced by subsequent addition of fluoropyrimidines. The novel observation that oxaliplatin downregulates dUTPase expression may provide a mechanistic basis contributing to the synergy observed between 5-FU and oxaliplatin in the clinic. Furthermore, these studies provide the first evidence of a direct transcriptional link between the essential enzyme dUTPase and the tumor suppressor p53.

Generation of new derivatives of the antitumor antibiotic mithramycin by altering the glycosylation pattern through combinatorial biosynthesis

Mithramycin is an antitumor drug produced by Streptomyces argillaceus. It consists of a tricyclic aglycone and five deoxyhexoses that form a disaccharide and a trisaccharide chain, which are important for target interaction and therefore for the antitumor activity. Using a combinatorial biosynthesis approach, we have generated nine mithramycin derivatives, seven of which are new compounds, with alterations in the glycosylation pattern. The wild-type S. argillaceus strain and the mutant S. argillaceus M7U1, which has altered D-oliose biosynthesis, were used as hosts to express various "sugar plasmids", each one directing the biosynthesis of a different deoxyhexose. The newly formed compounds were purified and characterized by MS and NMR. Compared to mithramycin, they contained different sugar substitutions in the second (D-olivose, D-mycarose, or D-boivinose instead of D-oliose) and third (D-digitoxose instead of D-mycarose) sugar units of the trisaccharide as well as in the first (D-amicetose instead of D-olivose) sugar unit of the disaccharide. All compounds showed antitumor activity against different tumor cell lines. Structure-activity relationships are discussed on the basis of the number and type of deoxyhexoses present in these mithramycin derivatives.

DAPK2 is a novel E2F1/KLF6 target gene involved in their proapoptotic function

Death-associated protein kinase 2 (DAPK2) belongs to a family of proapoptotic Ca(2+)/calmodulin-regulated serine/threonine kinases. We recently identified DAPK2 as an enhancing factor during granulocytic differentiation. To identify transcriptional DAPK2 regulators, we cloned 2.7 kb of the 5'-flanking region of the DAPK2 gene. We found that E2F1 and Krüppel-like factor 6 (KLF6) strongly activate the DAPK2 promoter. We mapped the E2F1 and KLF6 responsive elements to a GC-rich region 5' of exon 1 containing several binding sites for KLF6 and Sp1 but not for E2F. Moreover, we showed that transcriptional activation of DAPK2 by E2F1 and KLF6 is dependent on Sp1 using Sp1/KLF6-deficient insect cells, mithramycin A treatment to block Sp1-binding or Sp1 knockdown cells. Chromatin immunoprecipitation revealed recruitment of Sp1 and to lesser extent that of E2F1 and KLF6 to the DAPK2 promoter. Activation of E2F1 in osteosarcoma cells led to an increase of endogenous DAPK2 paralleled by cell death. Inhibition of DAPK2 expression resulted in significantly reduced cell death upon E2F1 activation. Similarly, KLF6 expression in H1299 cells increased DAPK2 levels accompanied by cell death that is markedly decreased upon DAPK2 knockdown. Moreover, E2F1 and KLF6 show cooperation in activating the DAPK2 promoter. In summary, our findings establish DAPK2 as a novel Sp1-dependent target gene for E2F1 and KLF6 in cell death response.

Hepatic parenchymal replacement in mice by transplanted allogeneic hepatocytes is facilitated by bone marrow transplantation and mediated by CD4 cells

The lack of adequate donor organs is a major limitation to the successful widespread use of liver transplantation for numerous human hepatic diseases. A desirable alternative therapeutic option is hepatocyte transplantation (HT), but this approach is similarly restricted by a shortage of donor cells and by immunological barriers. Therefore, in vivo expansion of tolerized transplanted cells is emerging as a novel and clinically relevant potential alternative cellular therapy. Toward this aim, in the present study we established a new mouse model that combines HT with prior bone marrow transplantation (BMT). Donor hepatocytes were derived from human alpha(1)-antitrypsin (hAAT) transgenic mice of the FVB strain. Serial serum enzyme-linked immunosorbent assays for hAAT protein were used to monitor hepatocyte engraftment and expansion. In control recipient mice lacking BMT, we observed long-term yet modest hepatocyte engraftment. In contrast, animals undergoing additional syngeneic BMT prior to HT showed a 3- to 5-fold increase in serum hAAT levels after 24 weeks. Moreover, complete liver repopulation was observed in hepatocyte-transplanted Balb/C mice that had been transplanted with allogeneic FVB-derived bone marrow. These findings were validated by a comparison of hAAT levels between donor and recipient mice and by hAAT-specific immunostaining. Taken together, these findings suggest a synergistic effect of BMT on transplanted hepatocytes for expansion and tolerance induction. Livers of repopulated animals displayed substantial mononuclear infiltrates, consisting predominantly of CD4(+) cells. Blocking the latter prior to HT abrogated proliferation of transplanted hepatocytes, and this implied an essential role played by CD4(+) cells for in vivo hepatocyte selection following allogeneic BMT.

CONCLUSION

The present mouse model provides a versatile platform for investigation of the mechanisms governing HT with direct relevance to the development of clinical strategies for the treatment of human hepatic failure.

Therapeutic inhibition of Sp1 expression in growing tumors by mithramycin a correlates directly with potent antiangiogenic effects on human pancreatic cancer

BACKGROUND

Human pancreatic cancer over expresses the transcription factor Sp1. However, the role of Sp1 in pancreatic cancer angiogenesis and its use as target for antiangiogenic therapy remain unexplored.

METHODS

Archived human pancreatic cancer specimens were used to assess gene expression and microvessel density (MVD) status by immunohistochemistry: Small-interfering RNA (siRNA) was used to determine the impact of altered Sp1 expression on tumor growth and angiogenesis, and mithramycin A (MIT) was used to evaluate Sp1-targeted antiangiogenic treatment of human pancreatic cancer in animal models.

RESULTS

The expression level of Sp1 was correlated directly with the MVD status (P < .001) and the expression level of vascular endothelial growth factor (VEGF) (P < .05). Knockdown of Sp1 expression did not affect the growth of pancreatic cancer cells in vitro but inhibited their growth and metastasis in mouse models. This antitumor activity was consistent with the in vitro and in vivo antiangiogenic activity resulting from Sp1 knockdown. Subcutaneous and intraperitoneal injection of MIT significantly suppressed the growth of human pancreatic cancer in mouse models. This tumor suppression was correlated with the suppression of Sp1 expression in growing tumors but not in normal tissues. Moreover, treatment with MIT reduced tumor MVD, which was consistent with the down-regulation of VEGF, platelet-derived growth factor, and epidermal growth factor receptor.

CONCLUSIONS

Both clinical and experimental evidence indicated that Sp1 is a critical regulator of human pancreatic cancer angiogenesis and the antitumor activity of MIT is a result, at least in part, of the suppression of Sp1 expression and consequent down-regulation the downstream targets of Sp1 that are key to angiogenesis.

Mithramycin A inhibits DNA methyltransferase and metastasis potential of lung cancer cells

Abnormal CpG island hypermethylation of multiple tumor-suppressor genes (TSGs) can lead to the initiation and progression of human cancer. The cytosine of the CpG island on the promoter region is methylated by 5'-cytosine-methyltransferases (DNMTs). Pharmacologic inhibitors of CpG island methylation provide a rational approach to reactivate the TSGs in tumor cells and to restore the critical cellular pathways in cancer cells. Mithramycin A (MMA) is known to be a GC- and CG-rich DNA-binding agent. We sought to determine whether MMA could inhibit CpG island methylation and DNMT expression in lung cancer cells. We found that MMA reduced the CpG island methylation of antimetastasis TSGs, including SLIT2 and TIMP-3 genes, and was associated with the prevention of metastasis. When highly metastatic CL1-5 lung cancer cells were treated with low doses (10 nmol/l) of MMA for 14 days, they reexpressed mRNA levels for these genes. MMA also inhibited the invasion phenotypes of CL1-5 cells as indicated by its inhibition of cancer cell migration using wound-healing and transwell assays. Molecular docking of MMA onto the DNMT1 catalytic domain revealed that MMA might interact with the catalytic pocket of DNMT1. Western blots showed that DNMT1 protein levels were depleted after MMA. These data support the idea that MMA has demethylation and antimetastasis effects on lung cancer cells. This mechanism might be mediated by the interaction of MMA and DNMT1, leading to the depletion of the DNMT1 protein and the reversal of the metastasis phenotype in lung cancer cells.

Transcriptional control of human T-BET expression: the role of Sp1

Murine T-bet (T-box expressed in T cells) is a master regulator of IFN-gamma gene expression in NK and T cells. T-bet also plays a critical role in autoimmunity, asthma and other diseases. However, cis elements or trans factors responsible for regulating T-bet expression remain largely unknown. Here, we report on our discovery of six Sp1-binding sites within the proximal human T-BET promoter that are highly conserved among mammalian species. Electrophoretic mobility shift assays demonstrate a physical association between Sp1 and the proximal T-BET promoter with a direct dose response between Sp1 expression and T-BET promoter activity. Ectopic overexpression of Sp1 also enhanced T-BET expression and cytokine-induced IFN-gamma secretion in NK cells and T cells. Mithramycin A, which blocks the binding of Sp1 to the T-BET promoter, diminished both T-BET expression and IFN-gamma protein production in monokine-stimulated primary human NK cells. Collectively, our results suggest that Sp1 is a positive transcriptional regulator of T-BET. As T-BET and IFN-gamma are critically important in inflammation, infection, and cancer, targeting Sp1, possibly with mithramycin A, may be useful for preventing and/or treating diseases associated with aberrant T-BET or IFN-gamma expression.

Physical and functional interactions between liver X receptor/retinoid X receptor and Sp1 modulate the transcriptional induction of the human ATP binding cassette transporter A1 gene by oxysterols and retinoids

The lipid transporter ATP binding cassette transporter A1 (ABCA1) promotes the efflux of cellular phospholipids and cholesterol to lipid-free apolipoprotein A-I and thus initiates the biogenesis of high-density lipoprotein (HDL). The expression of the ABCA1 gene is controlled, coordinately with other genes of HDL metabolism, by liver X receptor/retinoid X receptor (LXR/RXR) heterodimers and their ligands oxysterols and retinoids. In the present study, we show that the oxysterol/retinoid-induced transcription of the ABCA1 gene is modulated by the ubiquitous transcription factor Sp1 that binds to the proximal ABCA1 promoter, adjacently to the LXR/RXR responsive element. The response of the ABCA1 gene to oxysterols/retinoids as well as the ligand-inducible recruitment of Sp1 and RXRalpha/LXRalpha heterodimers to the ABCA1 promoter was blocked by mithramycin A, a well-known Sp1 inhibitor. Using SL2 cells which lack endogenous Sp1, we showed that activation of the ABCA1 promoter by LXRalpha/RXRalpha heterodimers and their ligands requires Sp1. Functional interactions between these factors were demonstrated using the GAL4 transactivation system. Using both in vitro and in vivo assays, we show that physical interactions between Sp1 and LXRalpha require the N-terminal region of LXRalpha, which includes the AF1 and DNA binding domains and two different domains of Sp1: the transactivation domain B and the DNA binding domain. Overall, the present study revealed a novel mechanism of regulation of the human ABCA1 transporter which involves synergistic interactions between oxysterol/retinoid-inducible hormone nuclear receptors and the transcription factor Sp1.

Molecular basis of the synergistic antiangiogenic activity of bevacizumab and mithramycin A

The impact of antiangiogenic therapy on the Sp1/vascular endothelial growth factor (VEGF) pathway and that of alteration of Sp1 signaling on the efficacy of antiangiogenic therapy is unclear, yet understanding their interactions has significant clinical implications. Treatment with bevacizumab, a neutralizing antibody against VEGF, suppressed human pancreatic cancer growth in nude mice. Gene expression analyses revealed that this treatment substantially up-regulated the expression of Sp1 and its downstream target genes, including VEGF and epidermal growth factor receptor, in tumor tissues, whereas it did not have this effect on pancreatic cancer cells in culture. Treatment with mithramycin A, an Sp1 inhibitor, suppressed the expression of Sp1 and its downstream target genes in both cell culture and tumors growing in nude mice. Combined treatment with bevacizumab and mithramycin A produced synergistic tumor suppression, which was consistent with suppression of the expression of Sp1 and its downstream target genes. Thus, treatment with bevacizumab may block VEGF function but activate the pathway of its expression via positive feedback. Given the fact that Sp1 is an important regulator of the expression of multiple angiogenic factors, bevacizumab-initiated up-regulation of Sp1 and subsequent overexpression of its downstream target genes may profoundly affect the potential angiogenic phenotype and effectiveness of antiangiogenic strategies for human pancreatic cancer. Therefore, this study is the first to show the significance and clinical implications of alteration of Sp1 signaling in antiangiogenic therapy for pancreatic cancer and other cancers.

Rad9 modulates the P21WAF1 pathway by direct association with p53

Background

Previous studies suggest that human RAD9 (hRad9), encoding a DNA damage checkpoint molecule, which is frequently amplified in epithelial tumor cells of breast, lung, head and neck cancer, participates in regulation of the tumor suppressor p53-dependent transactivation of pro-survival P21^WAF1. This study examined the exact mechanism of the hRad9 function, especially through the phosphorylation of the C-terminus, in the transcription regulation of P21^WAF1.

Results

The transfection of phosphorylation-defective hRAD9 mutants of C-terminus resulted in reduction of the p53-dependent P21^WAF1 transactivation; the knockdown of total hRad9 elicited an increased P21^WAF1 mRNA expression. Immunoprecipitation and a ChIP assay showed that hRad9 and p53 formed a complex and both were associated with two p53-consensus DNA-binding sequences in the 5' region of P21^WAF1 gene. The association was reduced in the experiment of phosphorylation-defective hRAD9 mutants.

Conclusion

The present study indicates the direct involvement of hRad9 in the p53-dependent P21^WAF1 transcriptional mechanism, presumably via the phosphorylation sites, and alterations of the hRad9 pathway might therefore contribute to the perturbation of checkpoint activation in cancer cells.

Targeted therapy for inherited GPI deficiency

Disrupted binding of the transcription factor Sp1 to the mutated promoter region of the mannosyl transferase-encoding gene PIGM causes inherited glycosylphosphatidylinositol (GPI) deficiency characterized by splanchnic vein thrombosis and epilepsy. We show that this results in histone hypoacetylation at the promoter of PIGM. The histone deacetylase inhibitor butyrate increases PIGM transcription and surface GPI expression in vitro as well as in vivo through enhanced histone acetylation in an Sp1-dependent manner. More important, the drug caused complete cessation of intractable seizures in a child with inherited GPI deficiency.

Entropically-driven binding of mithramycin in the minor groove of C/G-rich DNA sequences

The antitumour antibiotic mithramycin A (MTA) is a DNA minor-groove binding ligand. It binds to C/G-rich tracts as a dimer that forms in the presence of divalent cations such as Mg(2+). Differential scanning calorimetry, UV thermal denaturation, isothermal titration calorimetry and competition dialysis were used, together with computations of the hydrophobic free energy of binding, to determine the thermodynamic profile of MTA binding to DNA. The results were compared to those obtained in parallel using the structurally related mithramycin SK (MSK). The binding of MTA to salmon testes DNA determined by UV melting studies (K(obs) = 1.2 (+/-0.3) x 10(5) M(-1)) is tighter than that of MSK (2.9 (+/-1.0) x 10(4) M(-1)) at 25 degrees C. Competition dialysis studies showed a tighter MTA binding to both salmon testes DNA (42% C + G) and Micrococcus lysodeikticus DNA (72% C + G). The thermodynamic analysis of binding data at 25 degrees C shows that the binding of MTA and MSK to DNA is entropically driven, dominated by the hydrophobic transfer of the antibiotics from solution to the DNA-binding site. Direct molecular recognition between MTA or MSK and DNA through hydrogen bonding and van der Waals contacts may also contribute significantly to complex formation.

Regulation of expression of BIK proapoptotic protein in human breast cancer cells: p53-dependent induction of BIK mRNA by fulvestrant and proteasomal degradation of BIK protein

Induction of mRNA for BIK proapoptotic protein by doxorubicin or gamma-irradiation requires the DNA-binding transcription factor activity of p53. In MCF7 cells, pure antiestrogen fulvestrant also induces BIK mRNA and apoptosis. Here, we provide evidence that, in contrast to doxorubicin or gamma-irradiation, fulvestrant induction of BIK mRNA is not a direct effect of the transcriptional activity of p53, although p53 is necessary for this induction. It is known that p53 up-regulated modulator of apoptosis (PUMA) mRNA is induced directly by the transcriptional activity of p53. Whereas gamma-irradiation induced both BIK and PUMA mRNA, only BIK mRNA was induced by fulvestrant. Whereas both fulvestrant and doxorubicin induced BIK mRNA, only doxorubicin enhanced the DNA-binding activity of p53 and induced PUMA mRNA. Small interfering RNA (siRNA) suppression of p53 expression as well as overexpression of dominant-negative p53 effectively inhibited the fulvestrant induction of BIK mRNA, protein, and apoptosis. Transcriptional activity of a 2-kb BIK promoter, which contained an incomplete p53-binding sequence, was not affected by fulvestrant when tested by reporter assay. Fulvestrant neither affected the stability of the BIK mRNA transcripts. Interestingly, other human breast cancer cells, such as ZR75-1, constitutively expressed BIK mRNA even without fulvestrant. In these cells, however, BIK protein seemed to be rapidly degraded by proteasome, and siRNA suppression of BIK in ZR75-1 cells inhibited apoptosis induced by MG132 proteasome inhibitor. These results suggest that expression of BIK in human breast cancer cells is regulated at the mRNA level by a mechanism involving a nontranscriptional activity of p53 and by proteasomal degradation of BIK protein.

Transcriptional Regulation of ULBP1, a Human Ligand of the NKG2D Receptor

Tumor cells expressing ligands of the NKG2D receptor stimulate anti-tumor immunity mediated by natural killer and T cells. In humans, NKG2D ligands (NKG2DL) are encoded by MIC and ULBP proteins. NKG2DL exhibit highly restricted expression in healthy tissues but are widely expressed in tumors. However, regulation of each NKG2DL differs substantially in different cancer cells. In this study, we characterized the mechanisms that regulate the expression of ULBP1. We show that the transcription of ULBP1 strictly depends on the binding of Sp1 and Sp3 to a CRE(1) site located in the ULBP1 minimal promoter. The mutation or deletion of this Sp1/Sp3 binding site abolished the transcription of ULBP1. It also diminished the transactivation of ULBP1 promoter by Sp3 overexpression, but not by Sp1, indicating that Sp3 is the main transcription factor that regulates ULBP1 through the CRE(1) site. Experiments in SL2 cells showed that the ULBP1 promoter was inactive in the absence of the Sp proteins and indicate that Sp3 is the essential activator of ULBP1 transcription, because the overexpression of Sp3 up-regulated its promoter activity > 500-fold. Additionally, we demonstrated that AP-2alpha repressed the expression of ULBP1 in HeLa cells by interfering with the binding of Sp3 and Sp1 to the ULBP1 promoter. These data indicate that Sp1, Sp3, and AP-2alpha may play an important role in the immunosurveillance against cancer. Finally, the definition of ULBP1 regulation may have implications for development of new therapeutic strategies against cancer cells.