Differential activity of the RVF enhancer element in the decreased expression of the neu oncogene in NR‐6 cells versus parental Swiss Webster 3T3 cells

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Abstract
The rat neu oncogene encodes a growth factor receptor—like glycoprotein, termed p185, that shares structural similarity with epidermal growth factor receptor (EGFR), particularly in the tyrosine‐kinase domain. The Swiss Webster 3T3 murine embryo fibroblast (SW3T3) variant NR‐6, in contrast to the parental cell line, does not express EGFR mRNA. After transfection of an activated rat neu cosmid clone, we demonstrated in this report that whereas SW3T3 cells readily expressed the exogenous rat p 185 protein, NR‐6 cells did not express detectable levels of this protein product. By transfection of plasmids containing the chloramphenicol acetyltransferase (CAT) gene driven by the neu promoter and subsequent CAT assays, we also showed that neu gene promoter activity was significantly less in NR‐6 cells than in SW3T3 cells and that the neu promoter sequence responsible for this decreased transcription was a previously identified RVF enhancer element (Yan D‐H, Hung M‐C, Mol Cell Biol 11:1875–1882, 1991). That is to say, the RVF enhancer element of the neu promoter did not function as an enhancer in NR‐6 cells. To investigate the mechanism responsible for the inactivation of RVF in NR‐6 cells, we used southwestern blot analyses and demonstrated that the 60‐kDa RVF polypeptide was present in both NR‐6 and SW3T3 cell nuclear extracts. This result indicates that the DNA‐binding activity of RVF was similar in these two cell lines; therefore, loss of RVF enhancer activity in NR‐6 cells is probably due to inactivation of the trans‐activating function and not DNA binding activity of RVF.