TGF‐β regulation of nuclear proto‐oncogenes and TGF‐β gene expression in normal human osteoblast‐like cells

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Abstract
Transforming growth factor‐β (TGF‐β) 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 TGF‐β on osteoblasts, the effects of TGF‐β 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‐β1 resulted in a rapid increase in c‐fos mRNA levels as early as 15 min post‐treatment. A maximum (10‐fold) increase was observed at 30 min after TGF‐β treatment followed by a decrease to control values. Similar responses were measured whether the cells were rapidly proliferating or quiescent. TGF‐β1 induced jun‐B mRNA levels more gradually with steady increase initially observed at 30 min and a maximum induction measured at 2 h post‐TGF‐β treatment. In contrast, TGF‐β 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‐β1 in the presence of actinomycin‐D abolished TGF‐β1 induction of c‐fos mRNA, suggesting that TGF‐β action is mediated via transcription. In the presence of cycloheximide, TGF‐β causes super‐induction of c‐fos mRNA at 30 min, indicating that the c‐fos expression by TGF‐β 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 17/2.8 cells was sufficient to be regulated by TGF‐β1. Interestingly, TGF‐β treatment also increased the mRNA levels of TGF‐β1 itself at 4 h post TGF‐β treatment, with a maximum increase observed at 14 h of treatment. TGF‐β1 treatment for 30 min were sufficient to cause a delayed increase in TGF‐β protein secretion within 24 h. These data support that TGF‐β 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.