Differentiation of 3T3T cells into adipocytes results in the progressive repression of growth factor responsiveness. This is associated with the transcriptional repression of the inducibility of c-jun and junB expression by serum. In contrast, differentiation of SV-40 large T antigen-transformed 3T3T cells (CSV3-1) does not repress growth factor responsiveness nor c-jun or junB inducibility even though CSV3-1 cells can differentiate into adipocytes. To better explain these observations, we have studied compositional changes in AP-1 DNA binding activity attributed to c-Jun, JunB, and JunD during the differentiation process in 3T3T and CSV3-1 cells. The results show that in nontransformed 3T3T cells, differentiation represses AP-1 DNA binding activity via a proportionate downregulation of c-Jun, JunB, and JunD. In contrast, in CSV3-1 cells, AP-1 DNA binding activity increases twofold during differentiation, which is accounted for by an increase in JunD with no change in c-Jun and JunB. If c-Jun and JunB serve as positive regulators and JunD serves as a negative regulator for cell proliferation as suggested by previous studies, the repression of JunD expression in differentiating CSV3-1 cells should be mitogenic because decreasing JunD/AP-1 DNA binding activity would allow c-Jun/AP-1 and JunB/AP-1 DNA binding activities to be dominant. The results confirm this prediction showing that antisense junD oligodeoxyribonucleotides are mitogenic for differentiating CSV3-1 cells whereas antisense c-jun and junB inhibit mitogenesis. These data support the conclusion that differentiation can regulate cellular proliferative potential by modulating the balance of positive and negative Jun/AP-1 DNA binding activities in distinct ways in nontransformed and transformed cells.

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