TGF‐β1 perturbation of the fibroblast cell cycle during exponential growth: switching between negative and positive regulation*

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Abstract
We have demonstrated previously that SV40 T antigen and serum regulate the length of G1 in exponentially growing NIH-3T3 cells in part by inhibiting density dependent negative cell cycle regulation. In these studies it was suggested that T antigen positively regulated G1 in a density independent manner as well. In this report we show that, 24 h after treatment, TGF-beta 1 perturbs the cell cycle of exponentially growing fibroblasts in a manner similar to T antigen. However, prior to 24 h, TGF-beta 1 produced a negative response, elongating the G1 phase of the cell cycle that was followed by a positive response, both of which were density independent. This biphasic response was measured between 0 and 12 h post-treatment and was relative to responses from serum. This switch from an early inhibitory effect to a late stimulatory effect was associated with changes in Rb phosphorylation, the timing and magnitude of which indicated that Rb may be directly regulating TG1 rather than reporting changes in the population. This is further substantiated by abrogation of the inhibitory effect by expression of wild-type SV40 T antigen and retention of the effect in cells that express an Rb-binding mutant of T antigen (K1). The biphasic regulatory effects of TGF-beta 1 were also displayed in WI-38 and IMR-90 human fibroblasts. This suggests that this biphasic effect is a property of fibroblasts.