c-Jun is a JNK-independent coactivator of the PU.1 transcription factor

The ETS domain transcription factor PU.1 is necessary for the development o f monocytes and regulates, in particular, the expression of the monocyte-sp ecific macrophage colony-stimulating factor (M-CSF) receptor, which is crit ical for monocytic cell survival, proliferation, and differentiation. The b ZIP transcription factor c-Jun, which is part of the AP-1 transcription fac tor complex, is also important for monocytic differentiation, but the monoc yte-specific M-CSF receptor promoter has no AP-1 consensus binding sites. W e asked the question of whether c-Jun could promote the induction of the M- CSF receptor by collaborating with PU.1. We demonstrate that c-Jun enhances the ability of PU.1 to transactivate the M-CSF receptor promoter as well a s a minimal thymidine kinase promoter containing only PU.1 DNA binding site s. c-Jun does not directly bind to the M-CSF receptor promoter but associat es via its basic domain with the ETS domain of PU.1, Consistent with our ob servation that AP-1 binding does not contribute to c-Jun coactivation is th e observation that the activation of PU.1 by c-Jun is blocked by overexpres sion of c-Fos. Phosphorylation of c-Jun by c-Jun NH2-terminal kinase on Ser -63 and -73 does not alter the ability of c-Jun to enhance PU.1 transactiva tion. Activated Ras enhances the transcriptional activity of PU.1 by up-reg ulating c-Jun expression without changing the phosphorylation pattern of PU .1. The activation of PU.1 by Ras is blocked by a mutant c-Jun protein lack ing the basic domain. The expression of this mutant form of c-Jun also comp letely blocks 12-O-tetradecanoylphorbol-13-acetate-induced M-CSF receptor p romoter activity during monocytic differentiation. We propose therefore tha t c-Jun acts as a c-Jun NH2-terminal kinase-independent coactivator of PU.1 , resulting in M-CSF receptor expression and development of the monocytic l ineage.