Myristylation-dependent transactivation by FBR v-fos is regulated by C/EBP

Viral oncogenes are generally believed to cause transformation through disregulated mimicry of their cellular homologues. However, here we show that FBR v-fos, unlike c-fos, transcriptionally activates unique genes in retrovirally induced chondro-osseous sarcomas. We show that FBR v-fos transactivates the collagen III and stromelysin promoters and that the unique transcriptional properties of transforming FBR depend upon its N-terminal myristylation and the differentiation state of the cell. Deletion or mutation of the myristylation site results in a loss of FBR v-fos transactivation in HeLa and undifferentiated 3T3-L1 preadipocyte cell lines. FBR v-fos transactivation of collagen III maps to a negative regulatory site which binds a key regulator of adipocyte differentiation, C/EBP alpha. Cotransfection of C/EBP alpha abolishes FBR v-fos transactivation of the alpha 1(III) collagen promoter. Furthermore, FBR v-fos cannot transactivate collagen III subsequent to adipocyte differentiation. We also demonstrate that collagen III transcription is reduced during adipocyte differentiation as the transcriptional activity of C/EBP alpha is concomitantly induced. Our results indicate that FBR v-fos transactivation depends upon its cotranslational myristylation and maps to a negative regulatory region which binds C/EBP alpha.