SUPPRESSION OF MYC, BUT NOT E1A, TRANSFORMATION ACTIVITY BY MAX-ASSOCIATED PROTEINS, MAD AND MXI1

Mad and Mxi1, two members of the Myc-related basic-region helix-loop-h elix/leucine-zipper family of proteins, associate directly with Max to form sequence-specific DNA binding heterodimers that are transactivat ion-incompetent. Mad-Max complexes have been shown to exert a strong r epressive effect on Myc-induced transactivation, perhaps through the c ompetitive occupation of common promoter binding sites also recognized by active Myc-Max heterodimers. To place these recent biochemical obs ervations in a biological context, mad and mxi1 expression vectors wer e tested for their ability to influence Myc transformation activity in the rat embryo fibroblast cooperation assay. Addition of an equimolar amount of mad or mxi1 expression vector to mouse c-myc/ras cotransfec tions resulted in a dramatic reduction in both the number of foci gene rated and the severity of the malignant phenotype. Myc-specific suppre ssion by Mad and Mxi1 was demonstrated by their ability to affect c- a nd N-myc-, but not ela-, induced transformation. In contrast, mad and mxi1 expression constructs bearing deletions in the basic region exert ed only mild repressive effects on Myc transformation activity, sugges ting that occupation of common DNA binding sites by transactivation-in competent Mad-Max or Mxi1-Max complexes appears to play a more dominan t role in this suppression than titration of limited intracellular poo ls of Max away from active Myc-Max complexes. Thus, these biological d ata support a current model for regulation of Myc function in which re lative intracellular levels of Mad and Mxi1 in comparison to those of Myc may determine the degree of activation of Myc-responsive growth pa thways.