Km. Ryan et Gd. Birnie, ANALYSIS OF E-BOX DNA-BINDING DURING MYELOID DIFFERENTIATION REVEALS COMPLEXES THAT CONTAIN MAD BUT NOT MAX, Biochemical journal, 325, 1997, pp. 79-85
It has been shown that during myeloid differentiation the levels of ma
d1 mRNA are induced before the loss of c-Myc protein. This suggests th
at inactivation of the differentiation-blocking activity of c-Myc migh
t not occur primarily through the loss of Myc protein, but through an
increase in the levels of its antagonist, Mad1. To investigate this qu
estion we have analysed the levels of mad1 mRNA during differentiation
of myeloid leukaemic HL60 cells. Although levels of mad1 mRNA were mo
derately increased after induction with phorbol ester, we also found t
hat differentiation could be achieved with other inducers without any
concomitant up-regulation of mad1 mRNA. In addition, analysis of E-box
DNA binding revealed that, although Myc-Max complexes were lost rapid
ly after differentiation induction, formation of Mad1-containing compl
exes only occurred during the later stages of the differentiation prog
ramme. Further analysis of these Mad-containing complexes revealed tha
t they were also unlikely to have the capacity to antagonize c-Myc fun
ction, as they did not contain Max. Therefore these data suggest that
an increase in the levels of mad1 mRNA or the formation of a Mad-Max c
omplex are unlikely to be essential or determining events for myeloid
differentiation. In addition, the discovery of DNA-binding complexes t
hat contain Mad1, but not Max, opens up this transcription factor netw
ork to include other Max-like proteins or proteins of an unrelated nat
ure.