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Small contribution of G1 checkpoint control manipulation to modulation of p53-mediated apoptosis
1Johns Hopkins Oncology Center, Baltimore, Maryland 21205, USA.
Abstract:
Deregulation of the S-phase promoting E2F-1 transcription factor has been shown to cooperate with p53 to induce apoptosis. BaF3 cells undergo rapid, p53-dependent apoptosis when irradiated in the absence of IL-3. Rapid apoptosis induced by ionizing radiation (IR) coincides with attenuated p21(WAF1/Cip1) induction. Failure to adequately induce p21 could result in inappropriate release of E2F from Rb which may then cooperate with p53 to induce apoptosis in cells deprived of growth factor. We engineered BaF3 cells to express exogenous p21 and tested whether overexpressing p21 in cells irradiated in the absence of IL-3 protects from IR-induced apoptosis. Enforced p21 expression resulted in a consistent, but partial, protection of cells from undergoing IR-induced apoptosis. However, deregulating E2F activity through expression of HPV E7 failed to sensitize cells to IR-induced apoptosis in the presence of IL-3. Together, these data strongly suggest that the IL-3-responsive factors which modulate p53-mediated apoptosis in BaF3 cells are largely independent of G1 cell cycle checkpoint control mediated by p21.
Insights
Ionizing radiation (IR) induces apoptosis in BaF3 cells, partly mediated by p53 and E2F-1. Overexpressing p21 offered partial protection, suggesting IL-3-responsive apoptosis factors are independent of p21-mediated cell cycle control.
Area of Science:
- Cellular biology
- Molecular oncology
- Radiation biology
Background:
- Deregulation of E2F-1 transcription factor can cooperate with p53 to induce apoptosis.
- BaF3 cells exhibit rapid, p53-dependent apoptosis upon irradiation without IL-3, with attenuated p21(WAF1/Cip1) induction.
- Inadequate p21 induction may lead to E2F release, potentially cooperating with p53 in apoptosis.
Purpose of the Study:
- To investigate if overexpressing p21 protects BaF3 cells from IR-induced apoptosis in the absence of IL-3.
- To determine if deregulating E2F activity sensitizes cells to IR-induced apoptosis in the presence of IL-3.
Main Methods:
- Engineered BaF3 cells to express exogenous p21.
- Expressed HPV E7 to deregulate E2F activity.
- Irradiated cells in the presence and absence of IL-3.
- Assessed apoptosis rates and p21 induction.
Main Results:
- Enforced p21 expression provided partial protection against IR-induced apoptosis in IL-3-deprived cells.
- Deregulating E2F activity via HPV E7 did not sensitize cells to IR-induced apoptosis when IL-3 was present.
- IR-induced apoptosis in BaF3 cells showed attenuated p21(WAF1/Cip1) induction.
Conclusions:
- IL-3-responsive factors modulating p53-mediated apoptosis in BaF3 cells are largely independent of p21-mediated G1 cell cycle checkpoint control.
- p21 plays a role in mitigating IR-induced apoptosis, but other IL-3-dependent pathways are critical.