Cell Cycle Regulators in Cancer

Forms complexes with cyclin D during G1 to phosphorylate pRb, releasing E2F transcription factors for S-phase entry. Amplified in liposarcoma and glioblastoma; CDK4/6 inhibitors palbociclib, ribociclib, and abemaciclib are approved for HR+ breast cancer.

Encodes pRb, the G1/S restriction point enforcer that binds and represses E2F factors in its hypophosphorylated state. CDK4/6-cyclin D phosphorylation is the irreversible commitment step; virtually every cancer disrupts pRb either directly or through upstream kinase activation.

Encodes p16INK4a, which competitively inhibits CDK4/CDK6, and p14ARF, which stabilises p53. Deletion removes both the pRb and p53 checkpoint axes simultaneously; among the most commonly deleted loci (~50% of melanomas, ~30% of glioblastomas).

Transcriptionally activates p21 (CDKN1A) for CDK2 inhibition and G1 arrest, and GADD45 for G2/M arrest following DNA damage. p53-mediated cell cycle arrest buys time for DNA repair before S-phase replication amplifies mutations.

Cyclin D1, the regulatory subunit of CDK4/6; its expression is induced by mitogenic signals including RAS/MAPK and Wnt/β-catenin. Amplification in breast cancer, mantle cell lymphoma, and head and neck squamous cancers drives premature G1/S transition.

Transcriptionally drives cyclin D1 expression and suppresses CDK inhibitors p15, p21, and p27, directly promoting G1/S progression. MYC-driven cells require high CDK4/6 activity and are paradoxically sensitive to CDK inhibition in some models.

Activates the intra-S and G2/M checkpoints after DNA double-strand breaks through CHK2 and CDC25A degradation, preventing mitosis with unrepaired DNA. ATM loss disables these checkpoints, permitting replication of damaged templates.

Phosphorylates and inactivates GSK3β, preventing cyclin D1 degradation and promoting G1/S entry. AKT also phosphorylates p27Kip1 (a CDK inhibitor), targeting it for cytoplasmic sequestration and degradation.

PI3K activation by PIK3CA mutations drives cyclin D1 expression and CDK activity through AKT/mTOR and directly through RAC1-PAK signalling, providing a parallel route to G1/S bypass independent of direct CDK4 amplification.

mTORC1 promotes G1/S progression by driving cap-dependent translation of cyclin D1 and c-MYC. Rapamycin/everolimus inhibits mTORC1 and causes G1 arrest, the primary mechanism of mTOR inhibitor cytostasis in cancer.