Lee W M, Schwab M, Westaway D, Varmus H E
Mol Cell Biol. 1985 Dec;5(12):3345-56. doi: 10.1128/mcb.5.12.3345-3356.1985.
We studied the effect of altered c-myc structure and expression upon the ability of c-myc to promote the transformation of normal rat embryo cells when it was supplemented by EJras (the mutant c-H-ras1 gene from EJ/T24 bladder carcinoma cells). We tested several c-myc alleles cloned from normal and tumor tissues of chicken and human origin and found that only LL4myc (derived from a bursal lymphoma in which an avian leukosis virus long terminal repeat resides within the first c-myc intron in the same transcriptional orientation) had cotransforming activity. No activity was observed with normal chicken and human c-myc alleles, two other bursal lymphoma c-myc alleles (LL3myc and LL6myc), and two human c-myc genes (HSRmyc and DMmyc) from human neuroectodermal tumor cell line COLO320, in which c-myc is amplified. Some of these inactive alleles had the following alterations that are frequently found in tumor-derived c-myc: point mutations affecting the encoded protein (LL3myc); a truncated structure with loss of the first, noncoding exon (LL3myc and DMmyc); and proviral integration within or near the myc locus (LL3myc and LL6myc). The following two experimental approaches indicated that cotransforming activity was directly related to the transcriptional activity of the alleles in cultured rat cells: when cotransfected into Rat-2 cells, LL4myc was more highly expressed than the other (inactive) alleles; and augmented expression of HSRmyc, DMmyc, or normal human or normal chicken c-myc placed under the transcriptional control of retroviral long terminal repeats or increased expression of normal human c-myc under the influence of a retroviral enhancer element was accompanied by cotransformation activity. We concluded that augmented expression of even a normal c-myc gene is sufficient for cotransforming activity and that additional structural alterations frequently found in tumor-derived alleles are neither necessary nor sufficient for the gene to acquire rat embryo cell cotransforming properties.
我们研究了改变的c-myc结构和表达对其在由EJras（来自EJ/T24膀胱癌细胞的突变型c-H-ras1基因）补充时促进正常大鼠胚胎细胞转化能力的影响。我们测试了从鸡和人的正常及肿瘤组织中克隆的几个c-myc等位基因，发现只有LL4myc（源自一个法氏囊淋巴瘤，其中禽白血病病毒长末端重复序列以相同转录方向位于第一个c-myc内含子内）具有共转化活性。正常鸡和人的c-myc等位基因、另外两个法氏囊淋巴瘤c-myc等位基因（LL3myc和LL6myc）以及来自人神经外胚层肿瘤细胞系COLO320（其中c-myc被扩增）的两个人c-myc基因（HSRmyc和DMmyc）均未观察到活性。这些无活性等位基因中的一些具有以下在肿瘤衍生的c-myc中常见的改变：影响编码蛋白的点突变（LL3myc）；缺失第一个非编码外显子的截短结构（LL3myc和DMmyc）；以及在myc基因座内或附近的前病毒整合（LL3myc和LL6myc）。以下两种实验方法表明共转化活性与培养的大鼠细胞中等位基因的转录活性直接相关：当共转染到Rat-2细胞中时，LL4myc的表达比其他（无活性）等位基因更高；并且将HSRmyc、DMmyc或正常的人或鸡c-myc置于逆转录病毒长末端重复序列的转录控制下的增强表达，或在逆转录病毒增强子元件影响下正常的人c-myc的表达增加，均伴随着共转化活性。我们得出结论，即使是正常的c-myc基因的增强表达也足以产生共转化活性，并且在肿瘤衍生的等位基因中常见的其他结构改变对于该基因获得大鼠胚胎细胞共转化特性既不是必需的也不是充分的。
