CDK4 Gene Function
Cyclin Dependent Kinase 4
Overview
CDK4 partners with cyclin D to phosphorylate and inactivate the retinoblastoma protein (RB1), driving cells through the G1/S restriction point. CDK4/6 inhibitors (palbociclib) are approved for HR+ breast cancer.
Molecular Mechanism
Mechanism Summary
Mitogenic signals induce cyclin D1 accumulation, which binds CDK4 and enables T-loop autophosphorylation at Thr172. The active CDK4–cyclin D complex phosphorylates pRb at multiple residues, releasing E2F transcription factors to activate S-phase entry genes.
Step-by-Step Mechanism
The CDK4–cyclin D complex is assembled with p21/p27 as chaperone-like assembly factors. CAK (CDK-activating kinase, CDK7–cyclin H) phosphorylates CDK4 T-loop residue Thr172, completing kinase activation.
Active CDK4–cyclin D phosphorylates pRb at Ser780, partially disrupting pRb–HDAC interaction and allowing limited E2F activation. This initiates a mid-G1 transcriptional programme.
CDK2–cyclin E (activated by early E2F targets) hyperphosphorylates pRb at additional residues (Ser795, Ser807/Ser811), completing pRb inactivation and fully releasing E2F family members.
Freed E2F1/2/3 transcriptionally activate S-phase genes: cyclin E, PCNA, RRM1/2, thymidylate synthase, and DNA polymerase components — this is the irreversible commitment point to cell division.
p16INK4a (encoded by CDKN2A) competitively displaces cyclin D from CDK4/6, preventing T-loop phosphorylation and blocking pRb phosphorylation. Loss of p16 (common in many cancers) removes this brake.
Upstream Regulators
Obligate regulatory subunit; binds CDK4 to confer substrate specificity and partial activation
Competitive CDK4/6 inhibitor; disrupts cyclin D binding; frequently deleted in cancer
T-loop Thr172 phosphorylation required for full CDK4 kinase activity
Downstream Targets
Partial pRb inactivation; E2F de-repression initiation
Cytoplasmic sequestration blocks TGFβ growth-inhibitory signalling
CDK4 phosphorylates p21 Thr145, promoting p21 cytoplasmic relocalisation
Key Post-Translational Modifications
T-loop activation; required for substrate phosphorylation
Assembly factor at low concentrations; inhibitor at high concentrations
Disease Mechanism
CDK4 amplification on chromosome 12q13-15 co-occurs with MDM2 amplification in ~15% of well-differentiated and dedifferentiated liposarcomas, driving unrestrained pRb phosphorylation. CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) are approved for HR+/HER2− breast cancer, extending median progression-free survival by >12 months. RB1 loss confers intrinsic resistance by removing the CDK4/6 substrate.
Database References
Key Pathways
- ·Cell cycle G1/S transition
- ·RB tumor suppressor pathway
- ·Cyclin D signaling
Disease Associations
- ·Breast cancer
- ·Melanoma
- ·Glioblastoma
- ·Liposarcoma
Research Activity
CDK4 is an actively studied target: about 175+ clinical trials that mention it are currently recruiting on ClinicalTrials.gov. Trial activity reflects research interest, not proven benefit — designs, endpoints and populations vary widely.
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Functional Partners
Common Questions About CDK4
What does CDK4 do in the cell cycle?
CDK4 forms complexes with D-type cyclins during G1 phase and phosphorylates the retinoblastoma protein (pRb), releasing E2F transcription factors to activate S-phase entry genes. This CDK4/cyclin D phosphorylation step is the G1 restriction point — the irreversible commitment to cell division.
What drugs target CDK4?
CDK4/6 inhibitors occupy the kinase ATP-binding site, reduce pRb phosphorylation and can reimpose G1 arrest when the RB pathway remains functional. Regulatory indications and outcome estimates are regimen-, stage- and population-specific, so trial results should be read from the relevant label and primary publication rather than treated as one class-wide number.
What does CDK4 amplification mean in cancer?
CDK4 amplification on chromosome 12q13-15, often co-amplified with MDM2, leads to excessive pRb phosphorylation and unrestrained cell cycle entry. It is a key driver in well-differentiated and dedifferentiated liposarcoma, where CDK4 amplification is diagnostic and CDK4/6 inhibitors show activity.