Recently, we isolated a candidate tumor-suppressor gene, MYO18B, which was inactivated in approximately 50% of human lung cancers by deletion, mutation, and promoter methylation. However, more frequent reduction or loss of MYO18B expression and restoration of MYO18B expression by trichostatin A (TSA) treatment suggested the contribution of other mechanisms, especially histone deacetylation, for epigenetic inactivation of the MYO18B gene. In this study, we examined histone modification of the promoter region of the MYO18B gene in 8 human lung cancer cell lines by a chromatin immunoprecipitation assay. In 6 of 7 cell lines with reduced or silenced MYO18B expression, the levels of histones H3 and H4 acetylation surrounding the MYO18B promoter region were lower than those in a cell line with MYO18B expression. By treatment with TSA, the levels of histone H3 and H4 acetylation were increased in all 6 cell lines whose MYO18B expression was restored by TSA, whereas neither H3 nor H4 acetylation was increased in cells whose MYO18B expression was not restored by TSA. Significant correlations were observed between the levels of histone H3/H4 acetylation and MYO18B expression. These results suggest that acetylation of both histones H3 and H4 contributes to regulation of MYO18B expression in lung cancer cells and that histone deacetylation surrounding the promoter region plays an important role in MYO18B silencing and is involved in lung carcinogenesis.
The instability of the fushi tarazu (ftz) mRNA is essential for the proper development of the Drosophila embryo. Previously, we identified a 201-nucleotide instability element (FIE3) in the 3 untranslated region (UTR) of the ftz mRNA. Here we report on the identification of two additional elements in the protein-coding region of the message: the 63-nucleotide-long FIE5-1 and the 69-nucleotide-long FIE5-2. The function of both elements was position-dependent; the same elements destabilized RNAs when present within the coding region but did not when embedded in the 3 UTR of the hybrid mRNAs. We conclude that ftz mRNA has three redundant instability elements, two in the protein-coding region and one in the 3 UTR. Although each instability element is sufficient to destabilize a heterologous mRNA, the destabilizing activity of the two 5-elements depended on their position within the message.Drosophila embryonic development depends on the precise temporal and spatial expression of maternal and zygotic pattern-forming genes (1). Maternal pattern-forming genes are transcribed during oogenesis, and their mRNA abundance decreases rapidly in the early embryo. Moreover, many mRNAs encoded by zygotic pattern-forming genes undergo dramatic changes in abundance and spatial distribution during early embryogenesis. To achieve these rapid changes, especially for rapid down-regulation, transcriptional control alone is insufficient, and regulation at the level of mRNA stability is essential. For instance, the maternal bicoid mRNA is completely stable during the first 2 h of embryogenesis but is rapidly destabilized at cellularization of the blastoderm (2). As discussed in the following text, the zygotic fushi tarazu (ftz) mRNA is one of the most unstable eukaryotic mRNAs known. Given that most mRNAs in the Drosophila embryo are constitutively stable (30), the question arises how selected mRNAs in the same embryo cytoplasm are targeted for degradation. Recognition of the targeted RNAs by the RNA degrading machinery must involve cis-acting sequences. These sequences are the focus of the present study.ftz is a member of the pair-rule class of segmentation genes and one of the best characterized early zygotic genes. In early embryos ftz mRNA is detected only from about 1.5 to 4.5 h after fertilization. When first expressed, ftz mRNA is uniformly distributed through the embryo (4). As development progresses, its distribution first becomes restricted to a region comprising from 15 to 65% of egg length, then to four broad bands, and finally to seven narrow stripes that encircle the embryo (4 -6). The seven stripes are short-lived, and no ftz mRNA is detected by 5 h after fertilization. This rapid change of expression pattern and formation of stripes in a short time span can be attributed to the termination of transcription in interstripe regions coupled with rapid mRNA turnover. The need for rapid mRNA turnover is emphasized by the fact that the FTZ protein activates its own transcription in a positive feedback loop (7). Thus, it is impor...
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