Nucleolar necklaces in chick embryo fibroblast cells. II. Microscope observations of the effect of adenosine analogues on nucleolar necklace formation.
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The round nucleoli of chick embryo fibroblast cells, when exposed to adenosine (2 mM)or to a number of adenosine analogues, lose material and unravel over a period of several hours to become beaded strands, 20 mu M in length, termed nucleolar necklaces (NN). Light microscope observations on this process are described. Biochemical experiments have revealed that most of these analogues interfere with both messenger RNA synthesis and ribosome synthesis, causing extensive degradation of the preribosome species containing 32S RNA although most of the preribosomes containing 18S RNA survive. We suggest that it is the depletion from the nucleolus of the adhesive 32S and 28S RNA preribosomes which allows the remaining nucleolar apparatus to spread apart into the NN configuration. Also required for the maintenance of the NN structure is the synthesis of some ribosomal RNA (rRNA) possibly present as rRNA "feathers" on the DNA. The addition of inhibitors of rRNA synthesis such as actinomycin D to the NN-containing cells causes loss of rRNA. Then a contraction and collapse of the NN structure into small dense spheres is observed. (+info)
Nucleolar necklaces in chick embryo fibroblast cells. I. Formation of necklaces by dichlororibobenzimidazole and other adenosine analogues that decrease RNA synthesis and degrade preribosomes.
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A number of chemicals, mostly adenosine analogues, cause the nucleolus of the chick embryo fibroblast to lose material and unravel over a period of several hours into beaded strands termed nucleolar necklaces (NN). The results of analyses of the fibroblasts, treated with the NN-forming chemical dichlororibobenzimidazole (DRB), suggests that the following biochemical alterations occur: DRB almost completely prevents the increase in both messenger RNA (mRNA) and heterogeneous nuclear RNA. It interferes with ribosome synthesis by decreasing the rate of 45S ribosomal RNA (rRNA) accumulation by 50%, slowing the rate of 18S rRNA appearance by 50%, and causing an extensive degradation (80%) of the 32S and 28S rRNA-containing preribisomes. Most of this preribosome degration probably occurs at or before the 32S rRNA preribosome stage. The degradation of these preribosomes appears to be due to the formation of defective 45S rRNA preribosomes rather than to a direct DRB interference with preribosome processing enzyme action. DRB inhibits total cellular RNA synthesis in less than 15 min, suggesting a direct interference with RNA synthesis. DRB also inhibits the uptake of nucleosides into the cell. DRB in the concentrations used does not appear to directly interfere with the translation of mRNA (i.e., protein synthesis). Other NN-forming adenoside analogues and high concentrations of adenosine (2 mM) cause biochemical alterations similar to those produced by DRB. To explain the preribosome degradation, we propose the hypothesis that DRB inhibits the synthesis of mRNA; as a consequence, some of the preribosomal proteins that normally coat the 32S rRNA portion of the 45S precursor RNA become limiting, and this defective portion is then subject to degradation by nucleases. (+info)
Induction of p16/INK4a gene expression and cellular senescence by toyocamycin.
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We constructed an assay system of a luciferase reporter with p16/lNK4a gene transcriptional regulatory domain to identify p16-inducing substances, and found toyocamycin to induce gene expression from the screening of culture fluids of Streptomyces. Toyocamycin is a nucleoside analog, and it increased the p16 mRNA level in human normal fibroblasts or synovial cells as assessed by Northern blot hybridization or real time RT-PCR. It also induced cellular senescence in normal human fibroblasts. The transcriptional regulatory regions of human p16 gene that were responsible for the induction were analyzed using deletion mutants of the transcriptional regulatory region of p16 linked to the luciferase gene. The DNA fragment -111 to +1 bp from the cap site was sufficient to drive toyocamycin-activated transcription of p16/luciferase reporter. Nucleotide sequences within this domain contained the Sp1- and Ets-binding sequences. Mutations were introduced into these sequences, and the Sp1 sequence was found to be critical for the induction, and this notion was confirmed from gel-mobility shift assay. (+info)
Identification of inhibitors of ribozyme self-cleavage in mammalian cells via high-throughput screening of chemical libraries.
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We have recently described an RNA-only gene regulation system for mammalian cells in which inhibition of self-cleavage of an mRNA carrying ribozyme sequences provides the basis for control of gene expression. An important proof of principle for that system was provided by demonstrating the ability of one specific small molecule inhibitor of RNA self-cleavage, toyocamycin, to control gene expression in vitro and vivo. Here, we describe the development of the high-throughput screening (HTS) assay that led to the identification of toyocamycin and other molecules capable of inhibiting RNA self-cleavage in mammalian cells. To identify small molecules that can serve as inhibitors of ribozyme self-cleavage, we established a cell-based assay in which expression of a luciferase (luc) reporter is controlled by ribozyme sequences, and screened 58,076 compounds for their ability to induce luciferase expression. Fifteen compounds able to inhibit ribozyme self-cleavage in cells were identified through this screen. The most potent of the inhibitors identified were toyocamycin and 5-fluorouridine (FUR), nucleoside analogs carrying modifications of the 7-position and 5-position of the purine or pyrimidine bases. Individually, these two compounds were able to induce gene expression of the ribozyme-controlled reporter approximately 365-fold and 110-fold, respectively. Studies of the mechanism of action of the ribozyme inhibitors indicate that the compounds must be incorporated into RNA in order to inhibit RNA self-cleavage. (+info)
Deciphering deazapurine biosynthesis: pathway for pyrrolopyrimidine nucleosides toyocamycin and sangivamycin.
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Herbicidal nucleosides from microbial sources.
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The structures of five naturally-occurring herbicidal nucleosides have been determined by spectral analysis. Three (5'-deoxyguanosine, coaristeromycin and 5'-deoxytoyocamycin) are novel natural products while the remaining two (coformycin and adenine 9-beta-D-arabinofuranoside) are known natural products which have not previously been reported to be herbicidal. (+info)
Light-activation of gene function in mammalian cells via ribozymes.
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Cell cycle arrest and cytochrome c-mediated apoptotic induction in human lung cancer A549 cells by MCS-C2, an analogue of sangivamycin.
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In the course of our screening for novel modulators on cell cycle progression and apoptosis as anticancer drug candidates, we generated an analogue of sangivamycin, MCS-C2, designated as 4-amino-6-bromo-7-cyclopentyl-7H-pyrrolo[2,3-d]pyrimidine-5-carboxamide. This study was aimed to evaluate the molecular mechanisms on cell cycle arrest and apoptotic induction of MCS-C2 in human lung cancer A549 cells. To investigate the effects of MCS-C2 on cell cycle progression in A549 cells, we measured DNA content of A549 cells treated with 5 microM of HY253 using flow cytometric analysis. The flow cytometric analysis revealed an appreciable G(2) phase arrest in A549 cells treated with 5 micronM of MCS-C2. This MCS-C2-induced G(2) phase arrest is associated with significant up-regulation of p53 and p21(Cip1) in A549 cells. Furthermore, TUNEL assay was used to examine apoptotic induction in A549 cells treated with 5 microM of MCS-C2 for 48 h. In addition, the effects of MCS-C2 on apoptosis-associated proteins in A549 cells were examined using Western blot analysis. The apoptotic induction in MCS-C2-treated A549 cells is associated with cytochrome c release from mitochondria which in turn resulted in the activation of caspase-9 and -3, and the cleavage of poly(ADP-ribose) polymerase (PARP). In conclusion, based on these results, we suggest that MCS-C2 may be a potent cancer chemotherapeutic candidate for use in treating human lung cancer cells via up-regulation and activation of p53. (+info)