Actinomycin D and RNA transport

Toosendanin inhibits colorectal cancer cell growth through the Hedgehog pathway by targeting Shh

Colorectal cancer (CRC) is one of the most common gastrointestinal cancers worldwide. This complex and often fatal disease has a high mortality rate. The Hedgehog (Hh) signaling pathway is crucial in CRC. Many studies have indicated that Shh is overexpressed in cancer stem cells (CSCs), and shh overexpression is positively correlated with CRC tumorigenesis. New drugs that kill CRC cells through the Hh pathway are needed. Toosendanin (TSN), a natural triterpenoid saponin extracted from the bark or fruit of Melia toosendan Sieb. et Zucc, can inhibit various tumors. Here, we investigated the effects of TSN in CRC and explored the possible targets and mechanisms. Shh-Light Ⅱ cells were treated with TSN and tested by dual luciferase reporter assays to determine the relationship with the Hh pathway. Cell Counting Kit-8 (CCK-8) assays were used to test the inhibitory effects of TSN on CRC cells. The expression of Hh components after TSN treatment was detected using western blots and quantitative reverse transcription polymerase chain reaction. Cellular thermal shift assays confirmed the targets of TSN. The same effects of TSN on xenograft tumor growth were investigated in vivo. The average weight, volume of the finally resected tumor, and the expression of Shh in the TSN-treated groups were significantly lower than those of the control group. This result strongly suggested that TSN administration inhibited CRC growth in vivo. Our research preliminarily demonstrated that the target of TSN was Shh and that TSN inhibits CRC cell growth by inhibiting the Hh pathway, identifying a new anticancer molecular mechanism of TSN in CRC.

Evidence that all SC-35 domains contain mRNAs and that transcripts can be structurally constrained within these domains

A fundamental question of mRNA metabolism concerns the spatial organization of the steps involved in generating mature transcripts and their relationship to SC-35 domains, nuclear compartments enriched in mRNA metabolic factors and poly A+ RNA. Because poly A+ RNA in SC-35 domains remains after transcription inhibition, a prevailing view has been that most or all SC-35 domains do not contain protein-encoding mRNAs but stable RNAs with nuclear functions and thus that these compartments do not have direct roles in mRNA synthesis or transport. However, the transcription, splicing, and transport of transcripts from a specific gene have been shown to occur in association with two of these 15-30 nuclear compartments. Here we show that virtually all SC-35 domains can contain specific mRNAs and that these persist in SC-35 domains after treatment with three different transcription-inhibitory drugs. This suggests perturbation of an mRNA transport step that normally occurs in SC-35 domains and is post-transcriptional but still dependent on ongoing transcription. Finally, even after several hours of transcription arrest, these transcripts do not disperse from SC-35 domains, indicating that they are structurally constrained within them. Our findings importantly suggest a spatially direct role for all SC-35 domains in the coupled steps of mRNA metabolism and transport.

Effect of lipid composition on rat liver nuclear membrane fluidity

Nuclear membrane fluidity is measured in rat liver by use of the fluorescence anisotropy of two probes: diphenylhexatriene and its cationic derivative trimethylammonium-diphenylhexatriene. It has been shown that, in 2-month-old rat liver cells, the bilayer surface is less fluid than the hydrophobic core. The fluidity was higher in 6-day-old rat liver nuclei, in which both the amount of cholesterol and the cholesterol/phospholipid ratio decreased. The influence of the single phospholipids, and in particular of phosphatidylcholine, has been studied by increasing the phosphatidylcholine with a choline base exchange reaction in isolated nuclear membranes. After this reaction, the fluorescence anisotropy of the bilayer surface increased, whereas at the hydrophobic core it decreased. Analysis of fatty acid composition shows an increase of phosphatidylcholine unsaturated fatty acids. The results show that the fluidity of nuclear membranes changes in relation to the lipid content and to the fatty acid composition. The role of nuclear membrane fluidity in cell function is discussed.

Nucleocytoplasmic translocation of ribosomal RNA in the rabbit blastocyst: participation of sulfhydryl groups

The release of ribosomes from the nucleus in the rabbit blastocyst was investigated by pulse-labeling embryos to within 5 min of the earliest appearance of radiolabeled ribosomal RNA (rRNA) in the cytoplasmic fraction. The accumulation of radiolabeled 4.7 and 1.9 kilobase mature rRNA species in the cytoplasm was then followed during a 2 hour chase period, using polyacrylamide gel electrophoresis to identify the rRNAs. Colchicine, cytochalasin B, KCN, and EDTA were found to have no effect on the release of radiolabeled rRNA from the blastocyst nucleus during the 2 hour chase. Oligomycin, a known inhibitor of the nuclear envelope nucleoside triphosphatase, and the protein synthesis inhibitors puromycin and cycloheximide blocked rRNA release after a short delay. In contrast, actinomycin D and the sulfhydryl-reactive agents N-ethylmaleimide and diamide produced an abrupt and complete block to further rRNA release. The results indicate that ribosomes leave the nuclear compartment by an energy-dependent process. They further underscore the importance of reduced sulfhydryl groups in a rapidly growing blastocyst with a high level of oxidative metabolism.

Influenza virus: an NMR study of mechanisms involved in infection

High resolution 1H-NMR spectroscopy has been used to study the infection of chicken embryo fibroblasts by influenza virus. Marked changes in the NMR spectrum occur when infectious influenza virus is introduced into the fibroblasts and these changes appear to depend upon the presence of active neuraminidase (EC 3.2.1.18). A crude preparation of neuraminidase from Vibrio cholerae is able to effect similar changes. Only minor spectral changes are observed in the absence of culture medium or when the viral genome material is inactivated by beta-propiolactone. Similarly, little change is seen in the NMR spectrum when amantadine, which is thought to inhibit uncoating of the virus inside the cell, or actinomycin D, which inhibits cellular nucleic acid metabolism, are incubated with fibroblasts prior to the addition of virus. The results suggest that neuraminidase, in co-operation with a factor in the infectious process, initiates a cellular event which can be monitored by NMR. The nature of this cellular mechanism is unknown, but further studies are under way to determine its importance in viral infection.

Cordycepin and early effects of estradiol on the immature rat uterus

Estradiol induces the synthesis of a specific protein fraction (IP) in the uterus of the immature rat. The injection of cordycepin (3' deoxyadenosine), an inhibitor of poly A synthesis, inhibits the synthesis of IP. This fact suggests that one of the earliest effects of estrogen is the production of Hn-RNA poly-A relative to IP. Moreover, using electron microscopy, the stimulation by estradiol of the nucleolus of the immature rat uterine epithelium has been shown. Cordycepin does not affect this stimulation to any appreciable extent. Biochemical studies (incorporation of radioactive stracers into NRA, affinity chromatography on poly U-Sepharose) carried out in parallel with and under conditions comparable to those used in electron microscopy show that cordycepin does not greatly affect the increase in ribosomal RNA observed under the effect of estradiol. The blocking of IP by cordycepin and the lack of inhibition at the nucleolus level under the same conditions, show that the two early effects of the action of estrogen on the immature rat uterus are not directly correlated.

Quantitation of turnover and export to the cytoplasm of hnRNA transcribed in the Balbiani rings

Some quantitative parameters of the intranuclear metabolism and export to the cytoplasm of Balbiani ring 1 and 2 RNA molecules in salivary gland cells of Chironomus tentans have been determined. Growing RNA chains in the Balbiani rings attain uniform labeling with RNA precursors after 20 min of incorporation. The specific activity of 75S RNA released from the Balbiani rings into the nuclear sap increases rapidly and reaches a maximum level between 90 and 180 min of labeling. After 20 min, labeled 75S RNA enters the cytoplasm and accumulates at a linear rate. However, only a small proportion of the RNA produced at the Balbiani ring loci can subsequently be recovered in the nuclear sap (14-17%) or cytoplasm (4-7%) as 75S RNA; presumably the remainder is degraded entirely. Experiments using inhibitors of elongation (actinomycin D) or initiation (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) revealed that no significant quanity of the 75S RNA transcribed can be chased into the cytoplasm. Both the kinetics of entry of labeled 75S RNA into the cytoplasm-that is, a constant rate of increase after a brief lag-and chase data are incompatible with a precursor-product relationship between the great majority of nuclear 75S RNA and cytoplasmic 75S RNA with messenger characteristics. The results are discussed in relation to the possibility that a post-transcriptional control mechanism is operating in these cells.

Inhibition of Balbiani ring RNA synthesis at the initiation level

The nucleoside analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, a selective inhibitor of heterogeneous chromosomal RNA synthesis in salivary gland cells of the midge Chironomus tentans, blocks the initiation of transcription in Balbiani rings 1 and 2. The analogue seems to be without appreciable effect on the elongation of growing RNA chains and allows finished molecules to be detached from the chromosomal sites. The RNA chain growth rate, based on measurements of the time required for completion of synthesis of Balbiani ring 1 and 2 RNA molecules after addition of the inhibitor, was estimated to be around 25 nucleotides per second at 18 degrees.

The inhibition of nuclear RNA synthesis by the rifampicin derivative AF/013 in living cells

The rifampicin derivative, AF/013, completely inhibits synthesis of the nucleolar and chromosomal RNA in explanted salivary gland cells of Chironomus tentans. When the glands are preincubated in rifampicin AF/013 for a short period before the addition of the radioactive precursors, labelling of RNA is depressed in all size classes to the same extent. In contrast, if rifampicin is replaced by the nucleoside analogue, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, a tentative initiation inhibitor of heterogeneous nuclear RNA, the label is reduced preferentially in the lower molecular weight region of the heterogeneous nuclear RNA spectrum. In chase type experiments, when rifampicin AF/013 is added after an initial labelling period, the synthesis of heterogeneous nuclear RNA is suppressed equally in all size classes, a result analogous to that obtained with the elongation inhibitor, alpha-amanitin. 5,6-Dichloro-1-beta-D-ribofuranosylbenzimidazole, under similar chase conditions, preferentially inhibits the labelling of smaller heterogeneous nuclear RNA molecules, but later on abolishes labelling of molecules with higher S values, also. Rifampicin AF/013 prevents or affects seriously the normal processing of the prelabelled preribosomal RNA in the nucleolus. It further interferes with the export of nuclear RNA to the cytoplasm, and/or promotes a non-physiological breakdown of cytoplasmic RNA. The experimental data suggest that rifampicin AF/013 acts on RNA synthesis in living cells by interference with chain elongation.