The transforming growth factor βs (TGF-βs) are a group of multifunctional growth factors that inhibit cell cycle progression in many cell types. The TGF-β-induced cell cycle arrest has been partially attributed to the regulatory effects of TGF-β on both the levels and activities of the G1 cyclins and their cyclin-dependent kinase partners. The ability of TGF-β to inhibit the activity of these kinase complexes derives in part from its regulatory effects on the cyclin-dependent kinase inhibitors, p21/WAF1/Cip1, p27Kip1, and p15. Upon treatment of cells with TGF-β, these three inhibitors bind to and block the activities of specific cyclin-cyclin-dependent kinase complexes to cause cell cycle arrest. Little is known, however, on the mechanism through which TGF-β activates these cyclin-dependent kinase inhibitors. In the case of p21, TGF-β treatment leads to an increase in p21 mRNA. This increase in p21 mRNA is partly due to transcriptional activation of the p21 promoter by TGF-β. To further define the signaling pathways through which TGF-β induces p21, we have performed a detailed functional analysis on the p21 promoter. Through both deletion and mutation analysis of the p21 promoter, we have defined a 10-base pair sequence that is required for the activation of the p21 promoter by TGF-β. In addition, this sequence is sufficient to drive TGF-β-mediated transcription from a previously nonresponsive promoter. Preliminary gel shift assays demonstrate that this TGF-β responsive element binds specifically to several proteins in vitro. Two of these proteins are the transcription factors Sp-1 and Sp-3. These studies represent the initial steps toward defining the signaling pathways involved in TGF-β-mediated transcriptional activation of p21.

Functional Analysis of the Transforming Growth Factor βResponsive Elements in the WAF1/Cip1/p21 Promoter