Transforming growth factor β targeted inactivation of cyclin E:cyclin-dependent kinase 2 (Cdk2) complexes by inhibition of Cdk2 activating kinase activity

Research output: Contribution to journalArticle


Transforming growth factor β (TGF-β)-mediated G1 arrest previously has been shown to specifically target inactivation of cyclin D:cyclin-dependent kinase (Cdk) 4/6 complexes. We report here that TGF-β-treated human HepG2 hepatocellular carcinoma cells arrest in G1, but retain continued cyclin D:Cdk4/6 activity and active, hypophosphorylated retinoblastoma tumor suppressor protein. Consistent with this observation, TGF-β-treated cells failed to induce p15INK4b, down-regulate CDC25A, or increase levels of p21CIP1, p27KIP1, and p57KIP2. However, TGF-β treatment resulted in the specific inactivation of cyclin E:Cdk2 complexes caused by absence of the activating Thr160 phosphorylation on Cdk2. Whole-cell lysates from TGF-β-treated cells showed inhibition of Cdk2 Thr160 Cdk activating kinase (CAK) activity; however, cyclin H:Cdk7 activity, a previously assumed mammalian CAK, was not altered. Saccharomyces cerevisiae contains a genetically and biochemically proven CAK gene, CAK1, that encodes a monomeric 44-kDa Cak1p protein unrelated to Cdk7. Anti-Cak1p antibodies cross-reacted with a 45-kDa human protein with CAK activity that was specifically down-regulated in response to TGF-β treatment. Taken together, these observations demonstrate that TGF-β signaling mediates a G1 arrest in HepG2 cells by targeting Cdk2 CAK and suggests the presence of at least two mammalian CAKs: one specific for Cdk2 and one for Cdk4/6.


  • Hikaru Nagahara
  • Sergei A. Ezhevsky
  • Adamina M. Vocero-Akbani
  • Philipp Kaldis
  • Mark J. Solomon
  • Steven F. Dowdy
External organisations
  • Washington University in St. Louis
  • Yale University
Original languageEnglish
Pages (from-to)14961-14966
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number26
Publication statusPublished - 1999 Dec 21
Publication categoryResearch
Externally publishedYes