Abstract 2340: Disentangling the Myc:Max from the Mad:Max network

Abstract The Myc family comprises three basic helix loop helix leucine zipper (bHLHLZ) transcription factors - C-myc, N-myc and L-myc - which regulate a plethora of intra and extracellular processes. Myc, when bound to Max, another bHLHLZ transcription factor, is able to bind specific DNA sequences, recruit coactivator complexes and promote transcriptional activation. Max can also promote transcriptional repression by dimerizing with distinct transcription factors such as Mad. Previous work has shown that Myc expression promotes proliferation whereas Mad expression leads to differentiation, implying that Myc and Mad have opposing roles in cells. Myc and Mad share the same heterodimeric partner, Max, suggesting that the cell proliferation/differentiation fate is determined by a Myc:Max:Mad network. To understand how this equilibrium is maintained, we are investigating the effects of separating the Myc:Max from the Mad:Max network. To separate these networks we have made use of specific Myc and Max mutant proteins in which reciprocal exchanges at the e and g positions of the leucine zipper have been introduced. Whereas the MycEG and MaxEG proteins dimerize with each other they are unable to associate with their wild type counterparts. Using these mutants we will determine whether cell fate is dictated by a balanced Myc:Max:Mad network or whether Max is simply a common binding partner for both Myc and Mad. In order to validate the EG mutants in vitro, we have used mouse embryonic fibroblasts derived from a c-Myc ER/ER mouse model generated in our laboratory, in which ER is fused to endogenous c-terminus, allowing Myc to be toggled on and off at will. These cells have to be cultured in the presence of the ER ligand, 4-hydroxytamoxifen (4OHT), so that C-myc is functional and the cells proliferate normally. Whereas MycEG or MaxEG alone are unable to support growth of these cells in the absence of 4OHT, the proliferative arrest phenotype observed without 4OHT is rescued by co-expression of MycEG and MaxEG. We are currently generating mouse models where C-mycEG and MaxEG will replace the endogenous proteins in order to separate the roles of Myc:Max and Mad:Max in development and adult tissues. Citation Format: Ana Paula Rebocho, Trevor Littlewood, Gerard I. Evan. Disentangling the Myc:Max from the Mad:Max network. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2340. doi:10.1158/1538-7445.AM2014-2340