Abstract
On TGF-beta binding, the TGF-beta receptor directly phosphorylates and activates the transcription factors Smad2/3, leading to G(1) arrest. Here, we present evidence for a second, parallel, TGF-beta-dependent pathway for cell cycle arrest, achieved via inhibition of p70(s6k). TGF-beta induces association of its receptor with protein phosphatase-2A (PP2A)-Balpha. Concomitantly, three PP2A-subunits, Balpha, Abeta, and Calpha, associate with p70(s6k), leading to its dephosphorylation and inactivation. Although either pathway is sufficient to induce G(1) arrest, abrogation of both, the inhibition of p70(s6k), and transcription through Smad proteins is required for release of epithelial cells from TGF-beta-induced G(1) arrest. TGF-beta thereby modulates the translational and posttranscriptional control of cell cycle progression.