King L B, Lund F E, White D A, Sharma S, Corley R B
Department of Microbiology and Immunology, Duke University Medical Center, Durham, NC 27710.
J Immunol. 1990 Apr 15;144(8):3218-27.
We have previously shown that the steady state levels of transcripts encoded by an endogenous mouse mammary tumor virus (MMTV) increase during LPS-induced differentiation of both normal B lymphocytes and an inducible B cell lymphoma, CH12. A large body of evidence suggests that MMTV expression is primarily limited to mammary tissues and that expression in cell lines from nonmammary tissues is accompanied by viral amplification and alterations in the transcriptional control regions of the viral long terminal repeat. We have, therefore, carefully characterized MMTV expression in CH12 cells and in other cells of the B lineage in order to determine if the expression of MMTV transcripts in differentiating B cells results from the "abnormal" transcriptional regulation seen in other nonmammary tissue. In this manuscript, we present evidence that MMTV transcripts are expressed in a variety of cells of the B lineage and that the levels of constitutive expression vary among the different cells. On the other hand, T cell lymphomas lacking amplified MMTV do not contain proviral transcripts, suggesting that MMTV transcription may be preferentially expressed in B lymphocytes. We demonstrate that MMTV transcripts are up-regulated during cytokine-mediated as well as LPS-mediated differentiation, and that most, if not all, expression is due to the activity of a single proviral gene, Mtv-9, in CH12 cells. Furthermore, the expression of MMTV transcripts in CH12 cells neither requires nor is accompanied by amplification of the provirus. Sequence analysis demonstrates that the U3 region of the expressed Mtv-9 long terminal repeat contains neither deletions nor insertions, and the well-characterized enhancer and promoter sites in the glucocorticoid response element which are known to be involved in transcriptional regulation of MMTV in mammary tissues have not been disrupted. These data suggest that the Mtv-9 locus behaves as a normal somatic gene which is differentially regulated during B cell development and differentiation. Unlike the events which lead to MMTV expression in other nonmammary tissues, B cells may express transcription factor(s) which are capable of inducing expression of endogenous MMTV proviral genes during the natural course of differentiation. Analysis of the mechanisms which control the expression of this gene should be useful in characterizing the molecular events which govern B cell differentiation.
我们之前已经表明，内源性小鼠乳腺肿瘤病毒（MMTV）编码的转录本稳态水平在脂多糖（LPS）诱导正常B淋巴细胞和可诱导性B细胞淋巴瘤CH12分化的过程中会升高。大量证据表明，MMTV的表达主要局限于乳腺组织，且在来自非乳腺组织的细胞系中的表达伴随着病毒扩增以及病毒长末端重复序列转录控制区域的改变。因此，我们仔细地对CH12细胞以及B淋巴细胞系中的其他细胞的MMTV表达进行了表征，以确定在分化的B细胞中MMTV转录本的表达是否源于在其他非乳腺组织中所见的“异常”转录调控。在本论文中，我们提供证据表明MMTV转录本在多种B淋巴细胞系细胞中表达，且组成型表达水平在不同细胞之间存在差异。另一方面，缺乏扩增MMTV的T细胞淋巴瘤不包含前病毒转录本，这表明MMTV转录可能在B淋巴细胞中优先表达。我们证明MMTV转录本在细胞因子介导以及LPS介导的分化过程中上调，并且在CH12细胞中，大部分（如果不是全部）表达是由于单个前病毒基因Mtv-9的活性。此外，CH12细胞中MMTV转录本的表达既不需要前病毒的扩增，也不伴随着前病毒的扩增。序列分析表明，所表达的Mtv-9长末端重复序列的U3区域既没有缺失也没有插入，并且已知参与乳腺组织中MMTV转录调控的糖皮质激素反应元件中特征明确的增强子和启动子位点并未被破坏。这些数据表明，Mtv-9基因座表现为一个正常的体细胞基因，在B细胞发育和分化过程中受到差异调控。与导致MMTV在其他非乳腺组织中表达的事件不同，B细胞可能表达能够在分化的自然过程中诱导内源性MMTV前病毒基因表达的转录因子。对控制该基因表达机制的分析对于表征调控B细胞分化的分子事件应该是有用的。
