Augmented expression of normal c-myc is sufficient for cotransformation of rat embryo cells with a mutant ras gene

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.