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Updated: Aug 11, 2026
Analysis of Cell Cycle Position in Mammalian Cells
Published on: January 21, 2012
Retinoblastoma-protein-dependent cell-cycle inhibition by the tumour suppressor p16
Abstract:
D-type cyclins, in association with the cyclin-dependent kinases Cdk4 or Cdk6, promote progression through the G1 phase of the cell cycle by phosphorylating the retinoblastoma protein (RB). The activities of Cdk4 and Cdk6 are constrained by inhibitors such as p16, the product of the CDKN2 gene on human chromosome 9p21 (refs 12-14). The frequent deletion or mutation of CDKN2 in tumour cells suggests that p16 acts as a tumour suppressor. We show that wild-type p16 arrests normal diploid cells in late G1, whereas a tumour-associated mutant of p16 does not. Significantly, the ability of p16 to induce cell-cycle arrest is lost in cells lacking functional RB, including primary fibroblasts from Rb-/- mouse embryos. Thus, loss of p16, overexpression of D-cyclins and loss of RB have similar effects on G1 progression, and may represent a common pathway to tumorigenesis.
Insights
The CDKN2 gene product, p16, normally halts cell cycle progression in G1. Loss of p16 function, similar to D-cyclin overexpression or retinoblastoma protein (RB) loss, contributes to tumor development.
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Cancer Biology
Background:
- D-type cyclins and cyclin-dependent kinases Cdk4/Cdk6 drive G1 cell cycle progression by phosphorylating the retinoblastoma protein (RB).
- p16, encoded by the CDKN2 gene, inhibits Cdk4/Cdk6 activity and functions as a tumor suppressor.
- Frequent CDKN2 alterations in tumors suggest a critical role for p16 in preventing cancer.
Purpose of the Study:
- To investigate the role of wild-type and mutant p16 in regulating cell cycle progression.
- To determine the dependence of p16's cell-cycle arrest function on retinoblastoma protein (RB).
Main Methods:
- Treatment of normal diploid cells with wild-type and mutant p16.
- Analysis of cell-cycle arrest in cells with and without functional RB, including Rb-/- mouse embryonic fibroblasts.
Main Results:
- Wild-type p16 effectively arrested normal diploid cells in late G1 phase.
- A tumor-associated mutant p16 failed to induce cell-cycle arrest.
- The cell-cycle arrest function of p16 was abolished in RB-deficient cells.
Conclusions:
- p16 acts as a crucial regulator of the G1 cell cycle checkpoint.
- Loss of p16 function, alongside D-cyclin overexpression or RB loss, converges on G1 progression.
- These alterations represent a common mechanistic pathway contributing to tumorigenesis.
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