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Updated: Jul 30, 2026
Two- and Three-Dimensional Live Cell Imaging of DNA Damage Response Proteins
Published on: September 28, 2012
p16INK4A participates in a G1 arrest checkpoint in response to DNA damage
G I Shapiro1, C D Edwards, M E Ewen
1Department of Adult Oncology, Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115, USA.
Abstract:
Members of the INK4 protein family specifically inhibit cyclin-dependent kinase 4 (cdk4) and cdk6-mediated phosphorylation of the retinoblastoma susceptibility gene product (Rb). p16INK4A, a prototypic INK4 protein, has been identified as a tumor suppressor in many human cancers. Inactivation of p16INK4A in tumors expressing wild-type Rb is thought to be required in order for many malignant cell types to enter S phase efficiently or to escape senescence. Here, we demonstrate another mechanism of tumor suppression by implicating p16INK4A in a G1 arrest checkpoint in response to DNA damage. Calu-1 non-small cell lung cancer cells, which retain Rb and lack p53, do not arrest in G1 following DNA damage. However, engineered expression of p16INK4A at levels compatible with cell proliferation restores a G1 arrest checkpoint in response to treatment with gamma-irradiation, topoisomerase I and II inhibitors, and cisplatin. A similar checkpoint can be demonstrated in p53-/- fibroblasts that express p16INK4A. DNA damage-induced G1 arrest, which requires the expression of pocket proteins such as Rb, can be abrogated by overexpression of cdk4, kinase-inactive cdk4 variants capable of sequestering p16INK4A, or a cdk4 variant incapable of binding p16INK4A. After exposure to DNA-damaging agents, there was no change either in overall levels of p16INK4A or in amounts of p16INK4A found in complex with cdks 4 and 6. Nonetheless, p16INK4A expression is required for the reduction in cdk4- and cdk6-mediated Rb kinase activity observed in response to DNA damage. During tumor progression, loss of p16INK4A expression may be necessary for cells with wild-type Rb to bypass this G1 arrest checkpoint and attain a fully transformed phenotype.
Insights
p16INK4A acts as a tumor suppressor by enforcing a G1 cell cycle arrest in response to DNA damage. Its restoration in cancer cells lacking p53 reactivates this critical DNA damage checkpoint, preventing uncontrolled proliferation.
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Oncology
Background:
- The INK4 protein family, including p16INK4A, inhibits cyclin-dependent kinases 4 and 6 (cdk4/6) to regulate cell cycle progression.
- p16INK4A is a known tumor suppressor, and its inactivation is crucial for malignant cell proliferation and senescence escape in many cancers.
- The role of p16INK4A in DNA damage response checkpoints, particularly in cells lacking p53, requires further elucidation.
Purpose of the Study:
- To investigate the role of p16INK4A in mediating G1 cell cycle arrest following DNA damage.
- To determine if p16INK4A can restore DNA damage-induced G1 arrest in cancer cells lacking p53.
- To elucidate the mechanism by which p16INK4A influences cdk4/6 activity and retinoblastoma protein (Rb) phosphorylation in response to DNA damage.
Main Methods:
- Utilized non-small cell lung cancer cells (Calu-1) lacking p53 but retaining Rb.
- Engineered p16INK4A expression in cancer cells and p53-/- fibroblasts.
- Treated cells with DNA-damaging agents including gamma-irradiation and various inhibitors.
- Assessed G1 arrest, cdk4/6-mediated Rb phosphorylation, and protein complex formation.
Main Results:
- Engineered p16INK4A expression restored G1 arrest in response to multiple DNA-damaging agents in Rb-proficient, p53-deficient cells.
- This p16INK4A-dependent G1 arrest was abrogated by overexpression of cdk4 or specific cdk4 variants.
- p16INK4A expression was essential for the observed reduction in cdk4/6-mediated Rb kinase activity after DNA damage, despite no change in p16INK4A or its complex levels with cdks.
Conclusions:
- p16INK4A functions as a critical mediator of the G1 arrest checkpoint in response to DNA damage, independent of p53.
- Loss of p16INK4A during tumor progression may allow cells with wild-type Rb to evade this checkpoint, contributing to transformation.
- Restoring p16INK4A function presents a potential therapeutic strategy for reactivating DNA damage response pathways in cancer.
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