We report that exposure of mouse embryonic fibroblasts to transforming grow
th factor beta -1 (TGF beta -1) (5 ng/ml) results in a strong activation of
p8 mRNA expression that precedes the induction of cell growth. Involvement
of the p8 promoter in the regulation was demonstrated by using a p8-chlora
mphenicol acetyltransferase construct. We therefore speculated that p8 migh
t be a mediator of TGF beta -1 in these cells. The incorporation of [H-3]th
ymidine on treatment with TGF beta -1 was indeed significantly higher in p8
(+/+) fibroblasts than in p8(-/-) fibroblasts, Smad transcriptional activit
y was used as marker of the TGF beta -1 signalling pathway, to probe the lo
wer p8(-/-) response to TGF beta -1. Two Smad-binding elements (SBEs)-lucif
erase constructs were transfected into p8(-/-) and p8(+/+) embryonic fibrob
lasts before treatment with TGF beta -1. A lower level of Smad transactivat
ion was observed in p8(-/-) embryonic fibroblasts, under basal conditions a
nd after stimulation with TGF beta -1. To test whether Smad underexpression
in p8(-/-) cells was actually due to p8 depletion, p8(-/-) embryonic fibro
blasts were transfected with a human p8 expression plasmid together with an
SBE-luciferase construct. The expression of p8 restored Smad transactivati
on in unstimulated and TGF beta -1-treated cells to the level found in p8(-
/+) cells. We concluded that TGF beta -1 activates p8 expression. which in
turn enhances the Smad-transactivating function responsible for TGF beta -1
activity.