Combinatorial selection of a single stranded DNA thioaptamer targeting TGF-beta1 protein.

A phosphorothioate single-stranded DNA aptamer (thioaptamer) targeting transforming growth factor-beta1 (TGF-beta1) was isolated by in-vitro combinatorial selection. The aptamer selection procedure was designed to modify the backbone of single-stranded DNA aptamers, where 5' of both A and C are phosphorothioates, since this provides enhanced nuclease resistance as well as higher affinity than that of a phosphate counterpart. The thioaptamer selected from a combinatorial library (5x10(14) sequences) binds to TGF-beta1 protein with an affinity of 90 nM. In this report, sequence, predicted secondary structure, and binding affinity of the selected thioaptamer (T18_1_3) are presented.

Magnesium ion is an effective inhibitor of the binding of Escherichia coli to heparin.

We investigated the effects of ions and temperature on the binding of E. coli to heparin using a chemiluminescence electrophoretic mobility shift assay. We found that magnesium ion is an effective inhibitor of the binding. The method can be readily applied to discover agents that can block the binding.

Combinatorial selection of a RNA thioaptamer that binds to Venezuelan equine encephalitis virus capsid protein.

A phosphorothioate RNA aptamer (thioaptamer) targeting the capsid protein of Venezuelan equine encephalitis virus (VEEV) was isolated by in vitro combinatorial selection. The selected thioaptamer had a strong binding affinity (approximately 7nM) and high specificity for the target protein. For the binding to the protein, the overall tertiary structure of the thioaptamer is required. We introduce two theoretical methods to examine the effect of phosphorothioate modification on the enhancement of binding affinity and one experimental method to examine the nature of the multiple bands of thioaptamer in a native gel.

Chemiluminescence-based electrophoretic mobility shift assay of RNA-protein interactions: application to binding of viral capsid proteins to RNA.

A chemiluminescence electrophoretic mobility shift assay was introduced for the study of RNA-protein interactions that include association of genomic RNA and viral capsid proteins. Binding of the capsid protein of Venezuelan equine encephalitis virus (VEEV) to several types of RNA was used as a model system to test the application of the method. The effects of RNA secondary structures and the significance of electrostatic interaction on binding were identified. This method may have wide application to the study of RNA-protein interactions.

Modulation of DNA-dependent protein kinase activity in chlorambucil-treated cells.

DNA-dependent protein kinase (DNA-PK) is activated in a two-step process whereby the Ku heterodimer first binds to the DNA double-strand breaks (dsbs) and then the DNA-PK catalytic subunit (cs) is recruited to form a repair complex. Oxidative stress is simultaneously generated along with DNA damage by ionizing radiation or chemotherapeutic agents whose impact on the DNA-PK activity has not previously been investigated. Here we show that the DNA damage-induced kinase activity of DNA-PK was modulated by oxidative stress, which was induced along with DNA dsbs in chlorambucil (Cbl)-exposed cells. Pretreatment with the antioxidants, 2(3)-t-butyl-4-hydroxyanisole or N-acetyl-l-cysteine enhanced the amount of DNA-PKcs phosphorylated at threonine 2609 (DNA-PK(pThr2609)) at the DNA dsbs and DNA-PK activity. Conversely, oxidative stress induced by l-buthionine (SR)-sulfoximine or glucose oxidase decreased the DNA-PK activity in Cbl-exposed cells. In addition, DNA-PK(pThr2609) was poorly detectable at the site of DNA dsbs, as shown by colocalization to DNA-end-binding pH2AX or p53BP1. There was no change in the protein levels of DNA-PKcs, Ku70, or Ku86. Data from these studies provide the first evidence that oxidative stress effects posttranslational modification and assembly of DNA-PK complex at DNA dsbs, and thereby repair of DNA dsbs.

Quantitative analysis of chemiluminescence signals using a cooled charge-coupled device camera.

Chemiluminescence has been considered an alternative to radioisotopic detection of materials in the life sciences. One application of this nonradioisotopic method is the electrophoretic mobility shift assay. The essential requirement for quantitative applications is that the chemiluminescence signal is linearly proportional to the concentration. However, the generation of chemiluminescence is a multi-step process, therefore linearity cannot be assumed. Therefore, it is important to verify linearity before applying the method as a quantitative tool. We used a commercial chemiluminescence generating system to evaluate the validity of quantitative measurements of biotin-labeled tRNA and single-stranded DNA. The results indicate that the relationship between the chemiluminescence signals and the quantity of biotin-labeled nucleic acids is hyperbolic rather than linear. However, it was found that with less than 50 fmol of biotin-labeled nucleic acid, which corresponds to 2.5 nM in 20 microl, linearity can be demonstrated within 5% error. Therefore, chemiluminescence-based quantitative measurements are a reliable method within these limitations.

The enhancement of PCR amplification of a random sequence DNA library by DMSO and betaine: application to in vitro combinatorial selection of aptamers.

A PCR method for uniform amplification of a random sequence DNA library is described. A combination of 1 M betaine and 5% DMSO improves the PCR amplification by increasing the ratio of full-length products to shortened products, which are a consequence of nonuniform amplification due to stable secondary structures in the templates. This method is expected to be beneficial for obtaining high-affinity aptamers with stable secondary structures.

TIS21/BTG2/PC3 is expressed through PKC-delta pathway and inhibits binding of cyclin B1-Cdc2 and its activity, independent of p53 expression.

Signal transduction pathway and a new function of TIS21/BTG2/PC3 were investigated in p53 null U937 cells; Expression of TIS21 by 12-O-tetradecanoyl phorbol-13-acetate (TPA) stimulation was mediated by PKC-delta activation, however, was strongly inhibited by cPKC isozymes. When U937 cells were treated with TPA+Go6976, but not TPA+Go6850, the level of TIS21 mRNA was maintained over that of TPA alone. When analyzed by FACS, TPA-induced G2/M arrest was significantly inhibited by Go6850, but not by Go6976, suggesting the involvement of TIS21 and nPKC isozymes. Indeed, PKC-delta was found to be a regulator of the G2/M arrest and TIS21 expression, confirmed by employing rottlerin and dnPKC-delta experiments. In vivo accumulation of TIS21 protein significantly induced cell death through caspase 3 activation, which was supported further by degradations of procaspase 3, full-length PKC-delta, pRB, and p21(WAF1) in TIS21DeltaC expresser. When the cells were synchronized by nocodazole, TIS21 overexpressers inhibited degradations of cyclin A and cyclin B1 in 3 h after release from the synchronization. Furthermore, TIS21 inhibited cyclin B1-Cdc2 binding and its kinase activity in vivo. In summary, TPA-induced TIS21 mRNA expression is mediated by PKC-delta, and TIS21 induces G2/M arrest and cell death by inhibiting cyclin B1-Cdc2 binding and the kinase activity through its binding to Cdc2.

Reduced DNA double strand breaks in chlorambucil resistant cells are related to high DNA-PKcs activity and low oxidative stress.

Modulation of DNA repair represents a strategy to overcome acquired drug resistance of cells to genotoxic chemotherapeutic agents, including nitrogen mustards (NM). These agents induce DNA inter-strand cross-links, which in turn produce double strand breaks (dsbs). These breaks are primarily repaired via the nonhomologous end-joining (NHEJ) pathway. A DNA-dependent protein kinase (DNA-PK) complex plays an important role in NHEJ, and its increased level/activity is associated with acquired drug resistance of human tumors. We show in this report that the DNA-PK complex has comparable levels and kinase activity of DNA-PK catalytic subunit (DNA-PKcs) in a nearly isogenic pair of drug-sensitive (A2780) and resistant (A2780/100) cells; however, treatment with chlorambucil (Cbl), a NM-type of drug, induced differential effects in these cells. The kinase activity of DNA-PKcs was increased up to 2h after Cbl treatment in both cell types; however, it subsequently decreased only in sensitive cells, which is consistent with increased levels of DNA dsbs. The decreased kinase activity of DNA-PKcs was not due to a change in its amount or the levels of Ku70 and Ku86, their subcellular distribution, cell cycle progression or caspase-mediated degradation of DNA-PK. In addition to DNA cross-links, Cbl treatment of cells causes a 2.2-fold increase in the level of reactive oxygen species (ROS) in both cell types. However, the ROS in A2780/100 cells were reduced to the basal level after 3-4h, while sensitive cells continued to produce ROS and undergo apoptosis. Pre-treatment of A2780 cells with the glutathione (GSH) precursor, N-acetyl-L-cysteine prevented Cbl-induced increase in ROS, augmented the kinase activity of DNA-PKcs, decreased the levels of DNA dsbs and increased cell survival. Depletion in GSH from A2780/100 cells by L-buthionine sulfoximine (BSO) resulted in sustained production of ROS, lowered DNA-PKcs kinase activity, enhanced levels of DNA dsbs, and increased cell killing by Cbl. We propose that oxidative stress decreases repair of DNA dsbs via lowering kinase activity of DNA-PKcs and that induction of ROS could be the basis for adjuvant therapies for sensitizing tumor cells to nitrogen mustards and other DNA cross-linking drugs.