Neel H, Gondran P, Weil D, Dautry F
Laboratoire de Génétique Oncologique, CNRS URA 1967, Institut Gustave Roussy, Villejuif, France.
Curr Biol. 1995 Apr 1;5(4):413-22. doi: 10.1016/s0960-9822(95)00082-0.
Changes in gene expression in response to external signals provide a key mechanisms for the regulation of higher eukaryotic cell functions. The importance of transcriptional control in the response of cells to growth factors and cytokines has been extensively documented, but gene expression has also been shown to be controlled at other levels, such as the stability of mRNA in the cytoplasm, its localization and translation. By contrast to transcriptional control, little is known of the contribution of pre-mRNA nuclear processing to the regulation of gene expression, as most of our knowledge of pre-mRNA processing in vivo is indirect, being inferred from comparisons of transcription rates and levels of mRNA accumulation.
In this study, we have used as a model the well-characterized maturation pathway of transcripts of the cytokine, tumour necrosis factor beta (TNF beta). We have used the murine TNF beta gene as a reporter for pre-mRNA processing, using a co-transfection approach to investigate whether overproduction of proteins involved in signal transduction influences the processing of TNF beta transcripts. Although transfection of both activated ras and src genes led to an increase in RNA accumulation in the nuclear and cytoplasmic compartments, as expected from their transactivation of the TNF beta expression vector, only src induced a modification of RNA processing. Comparison of several modes of src activation indicated that two distinct effects of src on pre-mRNA processing can be coupled: one involves slowing down splicing and the other allows the export of partially spliced transcripts. These effects can be observed not only on the three introns of TNF beta but also on transcripts from a beta globin expression vector.
We have characterized how the processing of transcripts of TNF beta and beta globin is regulated by the signal transduction pathway that includes the Src protein, establishing that external signals have the capacity to regulate gene expression at a post-transcriptional level within the nucleus. Src seems to act on a general mechanism of splicing and/or mRNA transport, but its biologically relevant targets are likely to be restricted to genes for which either alternative processing pathways are in competition, or the kinetics of splicing is critical. This regulation could reflect a modulation by Src of the activity of components of the splicing and transport machineries, but could also involve RNA-binding proteins, which have been shown to interact with Src.
基因表达对外界信号的响应变化为高等真核细胞功能的调控提供了关键机制。转录控制在细胞对生长因子和细胞因子的响应中的重要性已得到广泛记载，但基因表达也已被证明在其他水平受到调控，如细胞质中mRNA的稳定性、其定位和翻译。与转录控制相比，人们对前体mRNA核加工对基因表达调控的贡献知之甚少，因为我们对体内前体mRNA加工的大部分了解都是间接的，是通过比较转录速率和mRNA积累水平推断出来的。
在本研究中，我们以细胞因子肿瘤坏死因子β（TNFβ）转录本的成熟途径为模型，该途径已得到充分表征。我们使用小鼠TNFβ基因作为前体mRNA加工的报告基因，采用共转染方法研究参与信号转导的蛋白质的过量表达是否会影响TNFβ转录本的加工。尽管激活的ras和src基因的转染都导致核和细胞质区室中RNA积累增加，正如它们对TNFβ表达载体的反式激活所预期的那样，但只有src诱导了RNA加工的改变。对几种src激活模式的比较表明，src对前体mRNA加工有两种不同的效应可以耦合：一种涉及减慢剪接，另一种允许部分剪接的转录本输出。这些效应不仅可以在TNFβ的三个内含子上观察到，也可以在β珠蛋白表达载体的转录本上观察到。
我们已经表征了TNFβ和β珠蛋白转录本的加工是如何由包括Src蛋白在内的信号转导途径调控的，确定了外部信号有能力在细胞核内的转录后水平调控基因表达。Src似乎作用于剪接和/或mRNA转运的一般机制，但其生物学相关靶点可能仅限于那些存在替代加工途径竞争或剪接动力学至关重要的基因。这种调控可能反映了Src对剪接和转运机制成分活性的调节，但也可能涉及已被证明与Src相互作用的RNA结合蛋白。
