Transforming Growth Factor ßlInhibits Mouse Keratinocytes Late in Independent of Effects on Gene Transcription1

Potential mechanisms by which transforming growth factor ßl i H.I /il i inhibits cell growth include suppression of transcription of genes required for proliferation and inactivation of enzymatic activities that regulate progression through G,. To clarify which events are important for I<,1 |! I signaling, we have defined more precisely the times in G, when the mouse keratinocyte cell line BALB/MK is sensitive to TGFßl-induced inhibition. TGFßl is capable of inhibiting BALB/MK cell growth when the inhibitory factor is added to cells at any point before the (.,-S transition. Synchronization at the G,-S boundary with mimosine, an inhibitor of initiation of origins of DNA replication, followed by release into S phase in the presence of TGFßl, indicated that cells lose sensitivity to TGFßl as they enter the replicative phase. It is interesting that TGFßl can inhibit BALB/MK cell growth late in G,, at a time when cell cycle progression is not blocked by an inhibitor of RNA polymerase H-mediated transcription, 5,6-dichlorobenzimidazole riboside (DRB). In addition, the ability of TGFßl to inhibit entry into S phase in late G, is unaffected by concurrent treatment with DRB. The data suggest that, at least when added to cultures that are in the latter few hours of G,, TGFßlcan inhibit cell cycle progression through mechanisms that do not involve stimulation or inhibition of gene expression at the transcriptional level.