N-MYC TRANSACTIVATES RCC1 GENE-EXPRESSION IN RAT FIBROBLAST CELLS TRANSFORMED BY N-MYC AND V-RAS

Oncogenic cooperation was found between the N-myc and v-ras oncogenes in a rat fibroblast cell line, 3Y1, To investigate the specific role o f N-myc in the transformation, we established transformed cell lines t hat expressed N-myc under a controllable promoter, Using these cells, we found that constitutive expression of N-myc is necessary to maintai n the transformation, and that the expression level of N-myc is closel y correlated with the transformation. Since another myc family gene, c -myc, directly activates expression of RCC1, which has important funct ions far eukaryotic cell proliferation, we focused on the relationship between N-myc and RCC1, Cells transformed by N-myc and v-ras expresse d several times more RCC1 mRNA than the parent 3Y1 cells, and the expr ession of RCC1 changed in a parallel with the expression of N-myc. Gel retardation analysis and experiments with reporter plasmids construct ed from a DNA fragment of the RCC1 gene indicated that the N-Myc prote in controls expression of RCC1 by binding directly to CACGTG elements in the RCC1 gene, These results suggest that N-myc can directly transa ctivate expression of RCC1, a c-myc target gene.