Posttranslational regulation of Myc function in response to phorbol ester/interferon-gamma-induced differentiation of v-Myc-transformed U-937 monoblasts

The transcription factors of the Myc/Max/Mad network are important regulato rs of cell growth, differentiation, and apoptosis and are frequently involv ed in tumor development. Constitutive expression of v-Myc blocks phorbol es ter (TPA)-induced differentiation of human U-937 monoblasts. However, costi mulation with interferon-gamma (IFN-gamma) and TPA restores terminal differ entiation and G(1) cell-cycle arrest despite continuous expression of v-Myc . The mechanism by which TPA + IFN-gamma counteract v-Myc activity has not been unravelled. Our results show that TPA + IFN-gamma, treatment led to an inhibition of v-Myc- and c-Myc-dependent transcription, and a specific red uction of v-Myc:Max complexes and associated DNA-binding activity whereas t he steady state level of the v-Myc protein was only marginally affected. In contrast, TPA + IFN-gamma costimulation neither increased the expression o f Mad1 or other mad/mnt family genes nor altered heterodimerization or DNA- binding activity of Mad1, The reduced amount of v-Myc:Max heterodimers in r esponse to treatment was accompanied by partial dephosphorylation of v-Myc and c-Myc. Phosphatase treatment of Myc:Max complexes lead to their dissoci ation, thus mimicking the effect of TPA + IFN-gamma, In addition to modulat ion of the expression of Myc/Max/Mad network proteins, posttranslational ne gative regulation of Myc by external signals may, therefore, be an alternat ive biologically important level of control with potential therapeutic rele vance for hematopoietic and other tumors with deregulated Myc expression. ( C) 1999 by The American Society of Hematology.