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BAD/BCL-[X(L)] heterodimerization leads to bypass of G0/G1 arrest
A Chattopadhyay1, C W Chiang, E Yang
1Department of Pediatrics, Vanderbilt-Ingram Cancer Center, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
Abstract:
The pro-apoptotic molecule BAD binds BCL-[X(L)] or BCL2 and inactivates their survival function. In addition to their anti-apoptotic function, BCL2 and BCL-[X(L)] also delay cell cycle entry from quiescence. We found that the BH3-only molecule BAD also exerted a cell cycle effect. BAD expression resulted in failure to cell cycle block in growth arrest conditions. In low serum and in confluence, fibroblasts constitutively or inducibly expressing BAD persisted in S phase, continued to incorporate BrdU, and exhibited sustained cyclin E/cdk2 activity. Mutation analysis indicated that the cell cycle effect of BAD was not dependent on its phosphorylation status or subcellular localization, but strictly co-segregated with BCL-[X(L)] binding. bclx(-/-) MEFs expressing BAD and bad(-/-) MEFs both arrested in G0/G1 in low serum similar to wild-type controls, suggesting that the ability to overcome the G0/G1 checkpoint resulted from the presence of BAD/BCL-x(L) heterodimers, rather than the absence of BCL-[X(L)] or BAD. These data provide evidence that in addition to regulating apoptosis, the BAD/BCL-[X(L)] heterodimer has a novel cell cycle function.
Insights
The pro-apoptotic molecule BAD, when bound to BCL-x(L), not only inhibits apoptosis but also prevents cells from entering the cell cycle from a quiescent state. This BAD/BCL-x(L) complex has a novel function in cell cycle regulation.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- The pro-apoptotic molecule BAD interacts with anti-apoptotic proteins BCL-x(L) and BCL2, inhibiting their survival functions.
- BCL2 and BCL-x(L) proteins are known to delay cell cycle entry from quiescence.
Purpose of the Study:
- To investigate the role of BAD in cell cycle regulation.
- To determine if BAD influences the cell cycle independently of its apoptotic function.
Main Methods:
- Fibroblast cell culture with constitutive or inducible BAD expression.
- Bromodeoxyuridine (BrdU) incorporation assays.
- Analysis of cyclin E/cdk2 activity.
- Studies using BCL-x(L) and BAD knockout mouse embryonic fibroblasts (MEFs).
Main Results:
- BAD expression led to a failure in cell cycle arrest under growth arrest conditions.
- Fibroblasts expressing BAD remained in S phase, incorporated BrdU, and showed sustained cyclin E/cdk2 activity in low serum and confluence.
- The cell cycle effects of BAD were dependent on its binding to BCL-x(L), not its phosphorylation or localization.
- BAD/BCL-x(L) heterodimers, not the absence of BAD or BCL-x(L), were responsible for overcoming the G0/G1 checkpoint.
Conclusions:
- The BAD/BCL-x(L) heterodimer possesses a novel function in cell cycle regulation, in addition to its role in apoptosis.
- BAD influences cell cycle progression by preventing arrest in growth conditions.
- These findings reveal a dual role for the BAD/BCL-x(L) complex in cellular fate determination.