Transforming growth factor$ (TGF-P) is present in high levels in bone and plays an important role in osteoblast growth and differentiation. In order to dissect the molecular mechanisms of action of TCF-P on osteoblasts, the effects of TGF-P on the steady state mRNA levels of c-fos, c-jun, and jun-B proto-oncogenes on normal human osteoblast-like cells (hOB) and a transformed human osteoblast cell line (MG-63) were measured. Treatment of hOBs with 2 ng/ml of TGF-P1 resulted in a rapid increase in c-fos mRNA levels as early as 15 rnin post-treatment. A maximum (10-fold) increase was observed at 30 min after TGF-P treatment followed by a decrease to control values. Similar responses were measured whether the cells were rapidly proliferating or quiescent. TGF-P1 induc~ed jun-B mRNA levels more gradually with steady increase initially observed at 30 min and a maximum induction measured at 2 h post-TGF-P treatment. In contrast, TGF-P treatment caused a time dependent decrease in the c-jun mRNA levels, an opposite pattern to that of jun-B mRNA. Treatment of hOBs with TGF-P, in the presence of actinomycin-D abolished TGF-PI induction of c-fos mRNA, suggesting that TGF-P action is mediated via transcription. In the presence of cycloheximide, JGF-P causes super-induction of c-fos mRNA at 30 min, indicating that the c-fos expression by TGF-P is independent of new protein synthesis. Further, transfection of 3 kb upstream region of jun-B promoter linked to a CAT reporter gene into ROS 1 7 / 2 3 cells was sufficient to be regulated by TGF-P1. Interestingly, TGF-P treatment also increased the mRNA levels of TGF-P1 itself at 4 h post TGF-P treatment, with a maximum increase observed at 14 h of treatment. TGF-P1 treatment for 30 min were sufficient to cause a delayed increase in TGF-P protein secretion within 24 h. These data support that TGF-P has major effects on hOB cell proto-oncogene expression and that the nuclear proto-oncogenes respond as rapid, early genes in a cascade model of hormone action.