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Rapamycin and p53 act on different pathways to induce G1 arrest in mammalian cells
S M Metcalfe1, C E Canman, J Milner
1Department of Surgery, Addenbrooke's Hospital, University of Cambridge, UK.
Abstract:
Certain growth regulatory kinases contain a common domain related to the phospho-inositol 3 (PI-3) kinase catalytic site. These include the ATM gene product, DNA-PKcs, and the target of rapamycin (TOR in yeast; and FRAP in mammalian cells). Rapamycin inhibits growth factor signalling and induces G1 arrest in many cell types. Some growth regulatory PI-3 kinases appear functionally linked to p53 and we have explored potential links between cellular effects induced by rapamycin and p53. In p53 null cells rapamycin inhibited cell cycling but did not induce G1 arrest. In cells which showed selective G1 arrest in response to rapamycin, rapamycin had no effect on basal levels of p53 protein. Similarly p21(WAF1) protein was not induced by rapamycin. The kinetics of the cellular p53/p21(WAF1) response to ionising radiation was unaffected by rapamycin; and the ability of growth factor to protect against p53-mediated apoptosis in response to DNA damage was also unaffected by rapamycin. The ATM gene is mutated in the cancer susceptibility syndrome ataxia telangiectasia (AT) but such mutant cells showed a similar sensitivity to rapamycin compared to their normal counterparts. RKO cell lines of common genetic background, but with different levels of functional p53 protein, also responded similarly to rapamycin. Thus, although rapamycin and p53 are each able to induce G1 arrest, they appear to act through independent growth regulatory pathways.
Insights
Rapamycin, a drug targeting growth factor signaling, inhibits cell cycling but does not rely on the p53 protein pathway for its effects. This suggests independent mechanisms for rapamycin and p53 in regulating cell growth.
Area of Science:
- Cellular Biology
- Molecular Biology
- Oncology
Background:
- Growth regulatory kinases, including PI-3 kinase family members like ATM and TOR, are crucial in cell signaling.
- Rapamycin inhibits growth factor signaling and induces cell cycle arrest (G1 phase).
- A potential functional link between PI-3 kinases and the p53 tumor suppressor protein has been suggested.
Purpose of the Study:
- To investigate the relationship between the cellular effects of rapamycin and the p53 pathway.
- To determine if p53 is required for rapamycin-induced G1 arrest.
- To explore whether rapamycin influences p53 protein levels or activity.
Main Methods:
- Experiments were conducted using p53 null cells and cells with varying p53 functional levels.
- Rapamycin's effects on cell cycling, G1 arrest, p53 protein levels, and p21(WAF1) induction were assessed.
- The impact of rapamycin on cellular responses to ionizing radiation and growth factors was examined.
- Sensitivity to rapamycin in cells with mutated ATM (ataxia telangiectasia) and in RKO cell lines with different p53 levels was compared.
Main Results:
- Rapamycin inhibited cell cycling in p53 null cells but did not induce G1 arrest.
- Rapamycin did not alter basal p53 protein levels or induce p21(WAF1) in cells that underwent G1 arrest.
- Rapamycin did not affect the kinetics of the p53/p21(WAF1) response to ionizing radiation.
- Rapamycin did not alter the protective effect of growth factors against p53-mediated apoptosis.
- Cells with mutated ATM and RKO cells with varying p53 levels showed similar sensitivity to rapamycin.
Conclusions:
- Rapamycin and p53 appear to regulate cell growth through independent pathways.
- p53 is not required for rapamycin-induced G1 arrest.
- Rapamycin's effects on cell cycle progression are distinct from those mediated by the p53 pathway.