C/EBPϵ represses c-Myc expression. (A) U937 cells stably transfected with either empty vector (pMT) or an inducible C/EBPϵ vector (pMTϵ) were treated with zinc for the indicated times. cDNA from these cells was used for real-time RT-PCR with c-Myc, C/EBPϵ, and 18S specific primers. The results are expressed in arbitrary units as a ratio of either c-Myc or C/EBPϵ transcripts/18S transcripts (each value represents the mean of 3 independent measurements of the sample). (B) Stably transfected U937 cells (pMT, empty vector, and pMTϵ, inducible-C/EBPϵ) were incubated either without (–) or with (+) zinc for 24 hours, followed by Western blot analysis with a c-Myc antibody. The blot was stripped and rehybridized sequentially with C/EBPϵ antibody and GAPDH antibodies. (C) Protein lysates were made from U937 cells stably transfected with empty vector (pMT) and U937 cells stably transfected with an inducible C/EBPϵ vector (pMTϵ) following 24 hours of exposure to zinc. Lysates were immunoprecipitated with an E2F1 antibody followed by Western analysis with a C/EBPϵ antibody. The blot was stripped and rehybridized with an E2F1 antibody.