Transforming growth factor-beta (TGF-beta) is a potent inhibitor of growth factor-stimulated hematopoiesis in normal and leukemic conditions. Using the factor-dependent myelogenous leukemia cell lines GF-D8 and Mo7, we show that TGF-beta interferes with stem cell factor (SCF)-induced proliferation by downmodulating c-jun gene expression. The ability of SCF to induce accumulation of c-jun transcripts was abolished when TGF-beta was present in culture. Transcriptional nuclear run-on assays indicated that TGF-beta relieved the capacity of SCF to enhance the transcriptional rate of the c-jun gene. Deletion analysis of the c-jun promoter furthermore showed that SCF was activating the c- jun promoter via the NF-jun transcription factor. Gel mobility shift assays showed that SCF increased the binding activity of NF-jun to its recognition site within 5 to 15 minutes. Binding activity peaked at 1 hour after exposure to SCF and declined to starting levels within 4 hours. The ability of SCF to enhance NF-jun binding activity was also dose-dependent in the range of 5 to 100 ng/mL. Exposure of GF-D8 and Mo7 cells to TGF-beta before the addition of SCF antagonized SCF- induced NF-jun binding. Moreover, whereas SCF was capable of functionally activating a heterologous promoter containing the NF-jun binding site, pretreatment of GF-D8 cells with TGF-beta abolished transcriptional activation of this heterologous promoter. These findings indicate that SCF-mediated activation of c-jun via NF-jun is crucial for the SCF-inducible proliferative response and is inhibited by TGF-beta. In additional experiments, the antisense technique was used. Treatment of GF-D8 and Mo7 cells with an antisense oligodeoxyribonucleotide directed against the translation initiation site of c-jun abolished the capacity of SCF to induce a proliferative response, whereas sense and nonsense oligomers had no effect. Taken together, our data indicate that the counteracting modulation of the binding activity of NF-jun by SCF and TGF-beta regulates the expression of the c-jun gene and thereby the proliferative state of the GF-D8 and Mo7 target.

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