Kip/Cip and Ink4 Cdk inhibitors cooperate to induce cell cycle arrest in response to TGF-β

Abstract

G1 progression in mammalian cells requites the activity of the cyclin D- dependent kinases Cdk4 and/or Cdk6 and the cyclin E-dependent kinase Cdk2. Proliferating Mv1Lu mink lung epithelial cells and human keratinocytes contain high levels of the universal Cdk inhibitor p27(Kip1) distributed in complexes with Cdk2, Cdk4, and Cdk6. Addition of the antimitogenic cytokine transforming growth factor-β (TGF-β) elevates expression of the Cdk4/6- specific inhibitor p15(Ink4B) and induces the release of p27 from Cdk4 and Cdk6. In Mv1Lu cells, this release of p27 coincides with increased binding of p27 to Cdk2. Recombinant p15 inhibits p27 binding to Cdk4 in vitro, and p15 overexpression induces the transfer of p27 from Cdk4 to Cdk2 in vivo, suggesting that the release of Cdk4-bound p27 in TGF-β-treated cells is caused by the surge in p15 levels. In keratinocytes, TGF-β increases not only p15 but also p21(Cip1), which binds to Cdk2. These events correlate with Cdk2 inhibition and cell cycle arrest and occur without a loss of G1 Cdk components. The results suggest that TGF-β induces G1 arrest in these two epithelial cell types by inhibiting various cyclin-Cdk kinases through the cooperative action of an Ink4 Cdk inhibitor and a Cip/Kip Cdk inhibitor. Subsequent to cell cycle arrest, Cdk2 and Cdk4 levels decline as part of a second set of events that may represent a program of cell adaptation to the quiescent state.