Mj. Ravitz et al., DIFFERENTIAL REGULATION OF P27 AND CYCLIN D1 BY TGF-BETA AND EGF IN C3H 10T1 2 MOUSE FIBROBLASTS/, Journal of cellular physiology, 168(3), 1996, pp. 510-520
Previously, we found that stimulation of C3H 10T1/2 mouse fibroblasts
with TGF-beta leads to the striking and rapid down-regulation of p27(K
ip1) expression during G1 phase. Here, we demonstrate that TGF-beta tr
eatment of C3H 10T1/2 cells does not alter the steady-state level of K
ip1 message sufficiently to account for the observed down-regulation o
f p27. This demonstrates that TGF-beta-induced down regulation of p27(
Kip1) occurs at a post-transcriptional level, consistent with a degrad
ative mechanism of p27(Kip1) down-regulation. Epidermal growth factor
(EGF) does not lead to the rapid down-regulation of p27 observed follo
wing treatment of cells with TGF-beta. Also in contrast with TGF-beta,
EGF causes a strong upregulation of cyclin D1, while neither growth f
actor affects cdk4 protein levels. These results imply that in this ce
ll type TGF-beta overcomes an inhibitory threshold to cdk activation b
y cyclin-dependent kinase inhibitors primarily through down-regulation
of p27, while EGF overcomes this threshold predominantly through upre
gulation of cyclin D1 levels. This divergence in pathways may explain
why TGF-beta-induced cell cycle kinetics are slower than those of EGF
in these cells, and the ability of TGF-beta to delay EGF-induced cell
cycle kinetics to its own, slower kinetics. In support of this hypothe
sis, TGF-beta prevents EGF-induced upregulation of cyclin D1 levels, w
hile TGF-beta is still able to induce p27 down-regulation even in the
presence of EGF. In contrast to the case with p27 degredation, neither
TGF-beta nor EGF have an observable effect on the steady-state levels
of p21 in this cell type. (C) 1996 Wiley-Liss, Inc.