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p16 INK4a can initiate an autonomous senescence program
1Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia 19104-6144, USA.
Abstract:
The tumor suppressor p16INK4a is a potent mediator of cell cycle arrest in transient expression studies, is induced in senescing cells, and can impose morphological features of senescence. Nonetheless, it is unclear whether p16INK4a can block cell proliferation irreversibly. We explored this issue using osteogenic sarcoma cell clones with inducible p16INK4a expression. Induction of p16INK4a for 1 day arrested most cells in G1 phase. If the induction was then interrupted, p16INK4a levels returned to baseline and robust growth resumed within 3-5 days. When p16INK4a was induced for 6 days DNA synthesis remained strongly inhibited and the cells acquired morphological features of senescence. Moreover, if p16INK4a induction was interrupted at this point and the cells were followed for 12 more days, most cells retained these morphologic features and either failed to divide or died. This occurred despite the prompt return of p16INK4a expression and retinoblastoma protein phosphorylation toward baseline levels. In fact, some senescing cells appeared to enter S phase. These results demonstrate that a sustained period of p16INK4a expression is sufficient in this setting to impose a durable block to cell proliferation and that this state becomes independent of p16INK4a expression, hypophosphorylation of pRB, or a strict G1 arrest.
Insights
A sustained p16INK4a expression can irreversibly block cell proliferation, inducing senescence. This cell cycle arrest becomes independent of p16INK4a levels and retinoblastoma protein phosphorylation.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- The tumor suppressor p16INK4a mediates cell cycle arrest and senescence.
- Its capacity for irreversible proliferation blockade remains unclear.
Purpose of the Study:
- To determine if sustained p16INK4a expression can irreversibly inhibit cell proliferation.
- To investigate the long-term effects of p16INK4a-induced senescence.
Main Methods:
- Osteogenic sarcoma cell clones with inducible p16INK4a expression were utilized.
- p16INK4a induction duration was varied (1 day vs. 6 days) followed by interruption.
- Cell cycle progression, morphology, and p16INK4a/pRB levels were monitored.
Main Results:
- Short p16INK4a induction (1 day) led to transient G1 arrest, with growth resuming upon interruption.
- Sustained induction (6 days) caused senescence and a durable proliferation block, even after p16INK4a withdrawal.
- Senescent cells maintained morphology and failed to divide or died, independent of p16INK4a levels or strict G1 arrest.
Conclusions:
- Sustained p16INK4a expression is sufficient to establish a durable, p16INK4a-independent block on cell proliferation.
- p16INK4a-induced senescence establishes a long-term state of growth arrest that persists beyond the initial trigger.