Direct transcriptional regulation of MDM2 by Fli-1.

The Ets transcription factor, Fli-1, has been shown to play a pivotal role in the induction and progression of Friend Murine Leukemia Virus (F-MuLV)-induced erythroleukemia, with its overexpression leading to erythroblast survival, proliferation, and inhibition of terminal differentiation. P53 inactivation is an additional genetic alteration that occurs in late-stage leukemic progression associated with in vivo and in vitro immortalization. Since p53 protein expression levels are low, to undetectable, in primary erythroleukemic cells that express elevated levels of Fli-1, we investigated the potential regulation of p53 by Fli-1. We assessed whether the overexpression of Fli-1 could partially regulate p53 via modulation of its well-established regulator, MDM2. In this paper, we demonstrate that the promoter of MDM2 contains a consensus binding site for Fli-1 that is bound by this transcription factor in vitro and in vivo, resulting in MDM2 transcriptional regulation. We further substantiate these observations in vivo by demonstrating a positive correlation in the expression of Fli-1 and MDM2, and a negative correlation with p53 in leukemic tissues obtained from mice with Friend Disease. These observations depict a significant function of Fli-1 overexpression in the indirect control of p53, evidently capable of leading to an increasingly aggressive erythroleukemic clone in vivo.