CELL-TRANSFORMATION BY C-FOS REQUIRES AN EXTENDED PERIOD OF EXPRESSION AND IS INDEPENDENT OF THE CELL-CYCLE

The proto-oncogene transcription factors Fos and Jun form a heterodime ric complex that binds to DNA and regulates expression of specific tar get genes. Continuous expression of c-fos causes transformation of cul tured fibroblasts and induces osteogenic sarcoma in mice. To investiga te the molecular basis of fos-mediated oncogenesis, we developed a con ditional cell transformation system in which Fos expression was regula ted by isopropyl-beta-D-thiogalactopyranoside (IPTG). Synthesis or rep ression of Fos in L1-3c-fos cells occurred rapidly within 30 min, afte r the removal or addition of IPTG to the culture medium. However, ther e was a significant delay between the induction of Fos expression and the appearance of morphological transformation. No effect was observed after 12 h of Fos expression, partial transformation was detected aft er 24 h, and full transformation required approximately 3 days of cont inuous Fos expression. Similarly, the transformed cell morphology pers isted for at least 2 days after repression of Fos, and a normal phenot ype was observed only after 3 days. Fos-Jun complexes, capable of bind ing to AP-1 sequences, were present continuously during the delay in m orphological transformation. Furthermore, increased expression of seve ral candidate Fos target genes, including those encoding Fra-1, transi n (stromelysin), collagenase, and ornithine decarboxylase, was detecte d shortly after Fos induction. The induction of morphological transfor mation was not dependent on the cell cycle, as it occurred in both cyc ling and noncycling cells. Thus, the Fos-Jun complexes present before L1-3c-fos cells become fully transformed are transcriptionally active. These complexes disappeared, and the Fos target genes were repressed at least 2 days prior to reversion. Our results suggest that cell tran sformation by Fos requires increased expression of a target gene(s) wi th a long-lived product(s) that must reach a critical level.