OVEREXPRESSION OF MYC SUPPRESSES CCAAT TRANSCRIPTION FACTOR NUCLEAR FACTOR-1-DEPENDENT PROMOTERS INVIVO

Overexpression of Myc in cells can suppress the transcription of speci fic 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 D NA-binding assays detect only one form of CCAAT transcription factor/n uclear factor 1 (CTF/NF-1) in quiescent 3T3-L1 cells. By contrast, qui escent 3T3-LI cells that stably overexpress either c-Myc or N-Myc cont ain at least three forms of CTF/NF-1. Biochemical characterization of the various CTF/NF-1 forms showed that they have the same native molec ular 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, suggest ing that they represent a more phosphorylated form of the CTF/NF-1 pro tein. 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 o verexpress c-Myc. This effect requires CTF/NF-1 binding. CTF/NF-1-depe ndent promoter activity is also suppressed in 3T3-L1 cells during acti ve growth (relative to the quiescent state). Interestingly, actively g rowing 3T3-L1 cells contain forms of CTF/NF-1 similar to those in quie scent cells that stably overexpress c-Myc. Thus, the CTF/NF-1 forms pr esent in cells that express high amounts of c-Myc correlate with a low er transcription rate of CTF/NF-1-dependent promoters in vivo. Our res ults provide a basis for the suppression of specific gene transcriptio n by c-Myc.