Infection of mouse cell from a variety of lineages with retroviruses expressing high levels of avian v-myc was found to be invariably associated with a lack of c-myc expression. To distinguish between v-myc-induced shutdown versus a cell- programmed down regulation of c-myc expression, we have analyzed this phenomenon in three different cell lines in culture which express various levels of c-myc prior to infection. Extreme levels of v-myc expression (10- to 100-fold excess over c-myc) were achieved in a myeloid (FDC-P1) and a T-lymphoid (CTB-6) cell line. In both lines, c-myc expression was absent in the infected cells and, in the case of FDC-P1 cells, occurred at the level of transcription initiation and could not be induced by growth factor (IL-3) or inhibitors of protein synthesis (to remove a labile repressor). Moreover, DNase I hypersensitive sites typical for active c-myc alleles were absent in FDC-P1 v-myc-infected cells. c-myc expression was also suppressed in FDC-P1 cells infected with a c-myc retrovirus. In NIH 3T3 fibroblast cells, v- myc was expressed at levels 5 to 10 times higher than those of c- myc present in uninfected cells. Suppression of c-myc in these cells was not due to clonal variation nor to changes in c-myc gene structure, and occurred at the level of transcription initiation. The suppression of c-myc expression was mediated directly by v- myc, since cells infected with constructs containing frameshifts and deletions in v-myc had levels of c-myc mRNA and protein comparable to uninfected cells. Suppression of c-myc expression was not associated with any gross changes in chromatin structure and could be revised by treating infected cells with anisomycin or by stimulating growth factor-deprived cells with serum. Suppression of c-myc expression was also observed in fibroblasts transfected with an N-myc expression vector. These findings establish that myc proteins function in an auto-and cross- regulatory circuit which transcriptionally regulated myc family proto-oncogenes.
