Open AccessHypo-phosphorylation of the retinoblastoma protein (pRb) by cyclin D:Cdk4/6 complexes results in active pRb
Author(s) -
Sergei A. Ezhevsky,
Hikaru Nagahara,
Adita M. Vocero-Akbani,
David Gius,
Michael C. Wei,
Steven F. Dowdy
Publication year - 1997
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.94.20.10699
Subject(s) - retinoblastoma protein , cyclin dependent kinase 6 , e2f , phosphorylation , cyclin dependent kinase 2 , cancer research , cyclin dependent kinase , cyclin dependent kinase 4 , biology , kinase , cyclin d1 , microbiology and biotechnology , cyclin a , cell cycle , chemistry , protein kinase a , biochemistry , cell
In cycling cells, the retinoblastoma protein (pRb) is un- and/or hypo-phosphorylated in early G1 and becomes hyper-phosphorylated in late G1 . The role of hypo-phosphorylation and identity of the relevant kinase(s) remains unknown. We show here that hypo-phosphorylated pRb associates with E2Fin vivo and is therefore active. Increasing the intracellular concentration of the Cdk4/6 specific inhibitor p15INK4b by transforming growth factor β treatment of keratinocytes results in G1 arrest and loss of hypo-phosphorylated pRb with an increase in unphosphorylated pRb. Conversely, p15INK4b -independent transforming growth factor β-mediated G1 arrest of hepatocellular carcinoma cells results in loss of Cdk2 kinase activity with continued Cdk6 kinase activity and pRb remains only hypo-phosphorylated. Introduction of the Cdk4/6 inhibitor p16INK4a protein into cells by fusion to a protein transduction domain also prevents pRb hypo-phosphorylation with an increase in unphosphorylated pRb. We conclude that cyclin D:Cdk4/6 complexes hypo-phosphorylate pRb in early G1 allowing continued E2F binding.