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dMax, a naturally occurring splice variant of the Myc binding protein Max, lacks the DNA binding basic region and helix 1 of the Helix – Loop – Helix domain; dMax interacts with c-Myc in vitro and in vivo, and inhibits E-box Myc site driven transcription in transient transfection assays. Here we have investigated the expression, function and interactions of dMax. RT/PCR analyses detected dmax mRNA in multiple tissues of the developing, newborn and adult mouse. Functionally, dMax reduced the ability of c-Myc to cooperate with the progression factor A-Myb to promote S phase entry of quiescent smooth muscle cells. In contrast, dMax failed to ablate inhibition of initiator element (Inr)-mediated transcription by c-Myc in Jurkat T cells. In in vitro protein : protein association assays, dMax interacted with c-Myc, N-Myc, L-Myc, Mad1, Mxi1, Mad3 and Mad4, but not with itself or wild-type Max. These interactions required an intact leucine zipper. Inhibition of E-box-mediated transactivation by induction of dMax overexpression resulted in apoptosis of WEHI 231 B cells. Thus, dMax is a widely expressed, naturally occurring protein, with the capacity to bind most members of the Myc/Max superfamily; dMax has little effect on Inr-mediated repression by c-Myc, but can significantly decrease E-box-mediated events promoting proliferation and cell survival. [ABSTRACT FROM AUTHOR]

Upon engagement of the B Cell Receptor (BCR) of WEHI 231 immature B cells, a drop in c-Myc expression is followed by activation of the cyclin-dependent kinase inhibitor (CKI) p27Kip1, which induces growth arrest and apoptosis. Here, we report inverse patterns of p27 and c-Myc protein expression follow BCR engagement. We present evidence demonstrating, for the first time, that the p27Kip1 gene is a target of transcriptional repression by c-Myc. Specifically, the changes in p27 protein levels correlated with changes in p27 mRNA levels, and gene transcription. Induction of p27 promoter activity followed BCR engagement of WEHI 231 cells, and this induction could be repressed upon co-transfection of a c-Myc expression vector. Inhibition of the TATA-less p27 promoter by c-Myc was also observed in Jurkat T cells, vascular smooth muscle, and Hs578T breast cancer cells, extending the observation beyond immune cells. Consistent with a putative Inr element CCAGACC (where +1 is underlined) at the start site of transcription in the p27 promoter, deletion of Myc homology box II reduced the extent of repression. Furthermore, enhanced repression was observed upon transfection of the c-Myc ‘super-repressor’, with mutation of Phe115 to Leu. The sequences mediating transcriptional activity and c-Myc repression were mapped to bp -20 to +20 of the p27 gene. Finally, binding of Max was shown to facilitate c-Myc binding and repression of p27 promoter activity. Overall, these studies identify the p27 CKI gene as a new target whereby c-Myc can control cell proliferation, survival and neoplastic transformation. Oncogene (2001) 20, 1688–1702. [ABSTRACT FROM AUTHOR]