TGF-β induced G₁ cell cycle arrest requires the activity of the proteasome pathway

Transforming growth factor-β TGF-β induces a potent G₁/S-phase cell cycle arrest of epithelial cells by inhibiting the activities of cyclin D- and cyclin E-associated kinase complexes. Downregulation of the kinase activities is mediated by induction of cyclin dependent kinase (CDK) inhibitor p15^Ink4b which blocks CDK4 and CDK6 kinases and leads to binding of p27^Kip1 to CDK2-cyclin E complex. Levels of several of these factors are controlled by the ubiquitin-proteasome pathway. We demonstrate here that proteasomal inhibitors release the cells from TGF-β imposed G₁-phase arrest and instigate the entry of the cells into S-phase. Proteasomal inhibitors are shown to specifically increase the activity of the cyclin D-kinase complex by increasing the levels of p27^Kip1 and cyclin D and by maintaining CDK4/6 protein levels leading to phosphorylation of the retinoblastoma protein without increasing cyclin E-associated kinase activity. The results indicate caution in the potential therapeutic use of the proteasome inhibitors due to unscheduled initiation of DNA replication in the presence of a physiological growth inhibitor.