FIG.7.
Expression of C/EBPβ or IGF-I promotes survival of J2-transformed C/EBPβ−/− bone marrow cells. (A) Bone marrow was collected from wt and C/EBPβ−/− mice, mock infected (−) or infected with J2 virus (+), cultured in M-CSF for 3 days, and assayed for colony formation in soft agar without M-CSF. Colonies were scored after 7 days. Each assay was performed in duplicate, and the data were averaged. (B) Using the protocol depicted, C/EBPβ−/− bone marrow cells were infected with either empty MSCV vector (Ctrl or C) or MSCV containing murine C/EBPβ, LIP (a truncated form of C/EBPβ lacking the transactivation domain), C/EBPβT217A (containing a disrupted Rsk phosphorylation site), C/EBPα, C/EBPδ, or rat IGF-I cDNA, followed by infection with the J2 virus (J2) or medium alone (Mock). Forty-eight hours after J2 infection, M-CSF was removed and 105 cells were plated in soft agar. After 7 days colonies (>50 cells) were counted. The data are means of two independent experiments, each performed in duplicate, with the indicated standard errors. Statistical analysis of the data in panels A and B was performed using the unpaired Student's t test (*, P < 0.05; **, P < 0.01). Lower panel: C/EBPβ−/− cells infected with the indicated C/EBP vectors and transformed with J2 were grown in liquid culture with M-CSF. After 10 days, nuclear extracts were prepared and analyzed by Western blotting to assess C/EBP protein expression. Proteins of interest are indicated with arrows; nonspecific, cross-reacting bands are indicated with asterisks. (C) Morphology of colonies from C/EBPβ−/− bone marrow infected with MSCV-IGF-I (IGF-I) or MSCV-C/EBPβ (C/EBPβ) and transformed with J2. Photographs were taken at 4× magnification and are representative of two to three experiments. IGF-I-expressing colonies were similar in size to those from C/EBPβ+/+ J2-infected cells (data not shown).