INHIBITION OF CELL-PROLIFERATION BY THE MAD1 TRANSCRIPTIONAL REPRESSOR

Mad1 is a basic helix-loop-helix-leucine zipper protein that is induce d upon differentiation of a number of distinct cell types. Mad1 dimeri zes with Max and recognizes the same DNA sequences as do Myc:Max dimer s. However, Mad1 and Myc appear to have opposing functions. Myc:Max he terodimers activate transcription while Mad:Max heterodimers repress t ranscription from the same promoter. In addition Mad1 has been shown t o block the oncogenic activity of Myc. Here we show that ectopic expre ssion of Mad1 inhibits the proliferative response of 3T3 cells to sign aling through the colony-stimulating factor-1 (CSF-1) receptor. The ab ility of overexpressed Myc and cyclin D1 to complement the mutant CSF- 1 receptor Y809F (containing a Y-to-F mutation at position 809) is als o inhibited by Mad1. Cell cycle analysis of proliferating 3T3 cells tr ansfected with Mad1 demonstrates a significant decrease in the fractio n of cells in the S and G(2)/M phases and a concomitant increase in th e fraction of G(1) phase cells, indicating that Mad1 negatively influe nces cell cycle progression from the G(1) to the S phase. Mutations in Mad1 which inhibit its activity as a transcription repressor also res ult in loss of Mad(1) cell cycle inhibitory activity. Thus, the abilit y of Mad(1) to inhibit cell cycle progression is tightly coupled to it s function as a transcriptional repressor.