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Mol. Cell. Biol., 06 1996, 2796-2801, Vol 16, No. 6
MF Roussel, RA Ashmun, CJ Sherr, RN Eisenman and DE Ayer
Mad1 is a basic helix-loop-helix-leucine zipper protein that is induced
upon differentiation of a number of distinct cell types. Mad1 dimerizes
with Max and recognizes the same DNA sequences as do Myc:Max dimers.
However, Mad1 and Myc appear to have opposing functions. Myc:Max
heterodimers activate transcription while Mad:Max heterodimers repress
transcription from the same promoter. In addition Mad1 has been shown to
block the oncogenic activity of Myc. Here we show that ectopic expression
of Mad1 inhibits the proliferative response of 3T3 cells to signaling
through the colony-stimulating factor-1 (CSF-1) receptor. The ability of
over-expressed Myc and cyclin D1 to complement the mutant CSF-1 receptor
Y809F (containing a Y-to-F mutation at position 809) is also inhibited by
Mad1. Cell cycle analysis of proliferating 3T3 cells transfected with Mad1
demonstrates a significant decrease in the fraction of cells in the S and
G2/M phases and a concomitant increase in the fraction of G1 phase cells,
indicating that Mad1 negatively influences cell cycle progression from the
G1 to the S phase. Mutations in Mad1 which inhibit its activity as a
transcription repressor also result in loss of Mad1 cell cycle inhibitory
activity. Thus, the ability of Mad1 to inhibit cell cycle progression is
tightly coupled to its function as a transcriptional repressor.
Copyright © 1996, American Society for Microbiology
Inhibition of cell proliferation by the Mad1 transcriptional repressor
Department of Tumor Cell Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA.
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