FIG. 7.

Mnt knockdown triggers “Myc” transcriptional programs. (A) Immunoblot analyses of the indicated fibroblast strains infected with control (GFP) or MntRNAi-expressing retroviruses. The anti-Mnt and anti-ODC immunoblots demonstrate that the MntRNAi indeed ablates Mnt expression and up-regulates ODC. The absence of ODC and E2f1 signals in the c-myc^−/− cells appears to reflect the inability of these antibodies to recognize rat ODC and E2f1. In addition, Mnt loss was associated with a marked down-regulation in levels of p27^KipI and in MEFs with an up-regulation of E2f1. (B) Northern blot analysis demonstrates that Odc is up-regulated by MntRNAi in all three fibroblast strains. Evaluation of 18S rRNA levels served as a loading control. (C) The p19^Arf-p53 apoptotic pathway and the Bcl-X_L apoptotic pathway are triggered by Mnt loss. Knockdown of Mnt was associated with an increase in p53 levels in all fibroblasts and with the induction of Arf in MEFs. The loss of Mnt was also associated with the activation of caspase-9, since larger (inactive) proenzyme form of caspase-9 was lost in MntRNAi-expressing fibroblasts. Again, the absence of caspase-9 protein in the c-myc^−/− cells may reflect the inability of this antibody to recognize rat caspase-9. Bcl-X_L protein levels were dramatically reduced in all MntRNAi-expressing fibroblast strains.