Overexpression of Myc suppresses CCAAT transcription factor/nuclear factor 1-dependent promoters in vivo.

Overexpression of Myc in cells can suppress the transcription of specific genes. Because several of these genes have common transcriptional regulatory elements, we investigated the possibility that this effect of Myc is mediated through a specific transcription factor. In vitro DNA-binding assays detect only one form of CCAAT transcription factor/nuclear factor 1 (CTF/NF-1) in quiescent 3T3-L1 cells. By contrast, quiescent 3T3-L1 cells that stably overexpress either c-Myc or N-Myc contain at least three forms of CTF/NF-1. Biochemical characterization of the various CTF/NF-1 forms showed that they have the same native molecular weight but differ in charge density. The more negatively charged CTF/NF-1 forms present in Myc-overexpressing cells are converted into that found in normal cells by treatment with acid phosphatase, suggesting that they represent a more phosphorylated form of the CTF/NF-1 protein. The various CTF/NF-1 forms have a similar DNA-binding affinity. Transfection experiments demonstrated that transcription from CTF/NF-1-dependent promoters is specifically suppressed in cells that stably overexpress c-Myc. This effect requires CTF/NF-1 binding. CTF/NF-1-dependent promoter activity is also suppressed in 3T3-L1 cells during active growth (relative to the quiescent state). Interestingly, actively growing 3T3-L1 cells contain forms of CTF/NF-1 similar to those in quiescent cells that stably overexpress c-Myc. Thus, the CTF/NF-1 forms present in cells that express high amounts of c-Myc correlate with a lower transcription rate of CTF/NF-1-dependent promoters in vivo. Our results provide a basis for the suppression of specific gene transcription by c-Myc.