Exogenous expression of Myc inhibits ATRA-induced cell cycle arrest and the associated down-regulation of cyclin E and up-regulation of p27.

(A) The expression of c-Myc is down-regulated in response to ATRA treatment in U-937 cells. Cells were treated with ATRA for 0, 12, 24, 48, and 72 hours as indicated; whole cell lysates were prepared and subjected to immunoprecipitation using pan-Myc antibodies. The immunocomplexes were collected, separated by SDS-PAGE, and analyzed by Western blot using biotin-coupled pan-Myc antibodies and streptavidin-HRP. (B) U-937-myc-2 cells and the parental subline U-937-GTB were grown in the presence of ATRA, collected at the indicated times, and the distribution of cells in different cell cycle phases was determined by flow cytometry of PI-stained nuclei. Mean ± SD (n = 3). The asterisk indicates significant differences from the corresponding U-937-GTB controls. (C) The protein levels of cyclin E and p27 after 72 hours of ATRA treatment in U-937-GTB and U-937-myc-2 cells were analyzed by Western blot.