Direct activation of lysosomal/endosomal enzyme-related genes by IRF-8. (A) Effect of cycloheximide (CHX) on IRF-8/ER–mediated induction of the indicated genes. IRF-8/ER–transduced Tot2 cells were treated with β-estradiol (Est) and/or CHX (10 μg/mL). CHX was added 10 minutes before addition of β-estradiol. Transcript levels were quantified in duplicate by real-time RT-PCR. Data were normalized by GAPDH levels and shown as values relative to those in untreated cells (mean ± standard deviation). (B) Expression of the indicated genes in IRF-8–transduced bone marrow lin^- cells. Irf-8^-/- bone marrow lin^- cells were cultured in the presence of SCF, IL-6, and IL-3 for 1 day and were transduced with MSCV–IRF-8 or control MSCV retrovirus on the following 2 days. Next day, cells were washed and reinoculated in the presence of SCF and M-CSF. Cells were harvested 2 and 7 days after addition of M-CSF. Transcript levels were determined as in panel A. (C) A model for transcriptional pathways activated by IRF-8. IRF-8 activates transcription regulators and macrophage proteases. IRF-8–induced transcription factors regulate their target genes, including those that regulate cell growth. The proteases are critical for the functionality of macrophages, and might also influence transcription factor activity.