The removal of intervening sequences from transcripts is catalyzed by the spliceosome, a multicomponent complex that assembles on the newly synthesized pre-mRNA. Pre-mRNA translation in the cytoplasm leads to the generation of aberrant proteins that are potentially harmful. Therefore, tight control to prevent undesired pre-mRNA export from the nucleus and its subsequent translation is an essential requirement for reliable gene expression. Here, we show that the natural product FR901464 (1) and its methylated derivative, spliceostatin A (2), inhibit in vitro splicing and promote pre-mRNA accumulation by binding to SF3b, a subcomplex of the U2 small nuclear ribonucleoprotein in the spliceosome. Importantly, treatment of cells with these compounds resulted in leakage of pre-mRNA to the cytoplasm, where it was translated. Knockdown of SF3b by small interfering RNA induced phenotypes similar to those seen with spliceostatin A treatment. Thus, the inhibition of pre-mRNA splicing during early steps involving SF3b allows unspliced mRNA leakage and translation.

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Six GEX1 compounds, GEX1A/herboxidiene and its related 5 novel compounds, were isolated from a culture broth of Streptomyces sp. GEX1 compounds induced both G1 and G2/M arrest in a human normal fibroblast cell line, WI-38. All six compounds up-regulated luciferase reporter gene expression directed by enhancer/promoter of various genes, such as cdc2, IL-2 and SV40 early genes. All GEX1 compounds showed cytotoxic activities in the same order of the up-regulating activities on gene expression, suggesting that these two activities are related. Despite the up-regulating activities on the reporter gene expression, GEX1A/herboxidiene did not enhance the expression of any endogenous genes involved in the cell cycle, proliferation and apoptosis. Although the unique effects of GEX1 compounds on cell cycle and the reporter gene expression were similar to those of trichostatin A (TSA), an inhibitor of histone deacetylase (HDAC), GEX1A/herboxidiene did not affect histone acetylation in cells. In addition, GEX1A/herboxidiene treatment gave rise to the shorter sized transcripts of the cdc25A and cdc2 genes as well as the normal sized ones. These results suggest that GEX1 compounds modulate gene expression by an unknown mechanism. New generation of anti-tumor agents exert inhibitory activities on cell proliferation by modulating the cell cycle and gene expression, e.g. histone deacetylase (HDAC) inhibitors1) and all-trans-retinoic acid2). Thus these pathways are attractive targets for anti-cancer drug discovery. We recently isolated six structurally related antitumor antibiotics, GEX1 compounds, from the culture broth of Streptomyces sp.3). A major compound GEX1A was identified as a herbicide, herboxidiene4), and GEX1 Q1-Q5 were novel compounds (Fig. 1). GEX1 compounds had cytotoxic activity, but the mechanism of action was unknown. In this paper, we report the effects of these compounds on the cell cycle and gene expression analyzed by flow cytometry, luciferase reporter assay and RT-PCR methods, and discuss the mode of action of GEX1 compounds. Materials and Methods Drugs GEX1 compounds were isolated from the culture broth of Streptomyces sp. GEX13). Trichostatin A was purchased from Wako Pure Chemical Industry, LTD. Test samples were dissolved in dimethyl sulfoxide (DMSO), diluted

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