The primary G1/S cell cycle checkpoint controls the commitment of eukaryotic cells to transition through the G1 phase to enter into the DNA synthesis S phase. Two cell cycle kinase complexes, CDK4/6-Cyclin D and CDK2-Cyclin E, work in concert to relieve inhibition of a dynamic transcription complex that contains the retinoblastoma protein (Rb) and E2F. In G1-phase uncommitted cells, hypo-phosphorylated Rb binds to the E2F-DP1 transcription factors forming an inhibitory complex with HDAC to repress key downstream transcription events. Commitment to enter S-phase occurs through sequential phosphorylation of Rb by Cyclin D-CDK4/6 and Cyclin E-CDK2 that dissociates the HDAC-repressor complex, permitting transcription of genes required for DNA replication. In the presence of growth factors, Akt can phosphorylate FoxO1/3, which inhibits their function by nuclear export, thereby allowing cell survival and proliferation. Importantly, a multitude of different stimuli exert checkpoint control, including TGF-β, DNA damage, replicative senescence, and growth factor withdrawal. These stimuli act though transcription factors to induce specific members of the INK4 or Kip/Cip families of cyclin dependent kinase inhibitors (CKIs). Notably, the oncogenic polycomb protein Bmi1 acts as a negative regulator of INK4A/B expression in stem cells and human cancer. In addition to regulating CKIs, TGF-β also inhibits cdc25A transcription, a phosphatase directly required for CDK activation. At a critical convergence point with the DNA- damage checkpoint, cdc25A is ubiquitinated and targeted for degradation via the SCF ubiquitin ligase complex downstream of the ATM/ATR/Chk-pathway. However, timely degradation of cdc25A in mitosis (M-phase) via the APC ubiquitin ligase complex allows progression through mitosis. Furthermore, growth factor withdrawal activates GSK-3β to phosphorylate Cyclin D, which leads to its rapid ubiquitination and proteasomal degradation. Collectively, ubiquitin/proteasome-dependent degradation and nuclear export are mechanisms commonly used to effectively reduce the concentration of cell cycle control proteins. Importantly, Cyclin D1/CKD4/6 complexes are explored as therapeutic targets for cancer treatment as researchers have found this checkpoint to be invariantly deregulated in human tumors.
We would like to thank Dr. Hans Widlund, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, for contributing to this diagram.
created November 2002
revised November 2012