Constitutive ectopic expression of truncated C/EBPβ renders ALCL cells resistent to RAD-mediated inhibition of proliferation. (A, left) Down-regulation of the mTOR target gene, ribosomal S6 protein, was demonstrated after treatment of cell lines with 0 nM to 10 nM RAD for 96 hours. (Right) Endogenous expression of C/EBPβ protein isoforms was determined in HL and ALCL cells, which were treated with 0 nM to 10 nM RAD for 96 hours. Fl indicates full-length isoform; Tr, truncated isoform. (B) Flag-tagged truncated C/EBPβ was ectopically expressed in Karpas 299 cells by retroviral transduction. Ectopic and endogenous expression of C/EBPβ protein isoforms was analyzed by immunoblotting with antibodies against C/EBPβ after treatment with 0 nM to 10 nM RAD for 96 hours. (C) Control and C/EBPβ-Tr-transfected Karpas 299 cells were incubated with 0 nM to 10 nM RAD for 48 hours. RAD-treated and untreated cells were counted and counts of untreated cells were set arbitrarily at 100%. Cell counts are given as the mean of 3 independent experiments. Error bars indicate SD.