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Type beta transforming growth factor (TGF-beta) is a multifunctional regulator of cell growth and differentiation. In the BC3H1 muscle cell line, TGF-beta blocks the onset of differentiation when added to undifferentiated myoblasts and causes dedifferentiation when added to fully differentiated myocytes. The goal of the present study was to determine whether TGF-beta-dependent repression of muscle-specific genes was preceded by modulation in expression of members of the jun proto-oncogene family, which function as growth factor-inducible transcription factors. junB mRNA was expressed at a basal level in differentiated BC3H1 myocytes. Within 15 min following exposure of myocytes to TGF-beta, junB mRNA began to accumulate; a peak of expression 20-fold above basal levels was observed after 2 h with a gradual decline thereafter. Nuclear run-on transcription assays showed that induction of junB by TGF-beta occurred at the level of transcription through a mechanism independent of protein synthesis. junB was also induced by 20% fetal bovine serum, platelet-derived growth factor, and insulin, but the maximal level of expression in response to these growth factors was lower and less sustained than in the presence of TGF-beta. In contrast to the dramatic effects of TGF-beta on junB expression, c-jun showed only a 2.5-fold increase in expression in response to TGF-beta. In an effort to identify additional members of the jun family which might be regulated by TGF-beta, a cDNA library was prepared from the poly(A)+ mRNA of TGF-beta-stimulated BC3H1 myocytes and was screened under conditions of reduced stringency with a v-jun DNA probe. From this screen, a new jun-related gene product was identified which shared a high degree of homology with regions of c-jun and junB which have been implicated in transcriptional activation, dimerization, and DNA binding. The transcript for this jun-related gene was expressed constitutively in BC3H1 cells and was not regulated by TGF-beta. Three members of the jun family thus exhibit distinct responses to TGF-beta in BC3H1 cells. The rapid transcriptional induction of junB is among the earliest and most dramatic responses to TGF-beta yet described and suggests that junB may mediate certain of the diverse biological effects of this growth factor. |
