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Discrete signaling pathways participate in RB-dependent responses to chemotherapeutic agents
Christopher N Mayhew1, Lisa M Perkin, Xiaoping Zhang
1Department of Cell Biology, Vontz Center for Molecular Studies, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0521, USA.
Abstract:
The retinoblastoma (RB) tumor suppressor has been proposed to function as a key mediator of cell cycle checkpoints induced by chemotherapeutic agents. However, these prior studies have relied on embryonic fibroblasts harboring chronic loss of RB, a condition under which compensation of RB functions is known to occur. Here we utilized primary adult fibroblasts derived from mice harboring loxP sites flanking exon 3 of the Rb gene to delineate the action of RB in the chemotherapeutic response. In this system we find that targeted disruption of Rb leads to little overt change in cell cycle distribution. However, these cells exhibited deregulation of RB/E2F target genes and became aneuploid following culture in the absence of RB. When challenged with both DNA damaging and antimetabolite chemotherapeutics, RB was required for primary adult cells to undergo DNA damage checkpoint responses and loss of RB resulted in enhanced aneuploidy following challenge. In contrast, following spontaneous immortalization and the loss of functional p53 signaling, the antimetabolite 5-fluorouracil (5-FU) failed to induce arrest despite the presence of RB. In these immortal cultures RB/E2F targets were deregulated in a complex, gene-specific manner and RB was required for the checkpoint response to camptothecin (CPT). Mechanistic analyses of the checkpoint responses in primary cells indicated that loss of RB leads to increased p53 signaling and decreased viability following both CPT and 5-FU treatment. However, the mechanism through which these agents act to facilitate cell cycle inhibition through RB were distinct. These studies underscore the critical role of RB in DNA-damage checkpoint signaling and demonstrate that RB mediates chemotherapeutic-induced cell cycle inhibition in adult fibroblasts by distinct mechanisms.
Insights
The retinoblastoma (RB) tumor suppressor is crucial for adult cells to respond to chemotherapy. Loss of RB impairs DNA damage checkpoints and increases aneuploidy, highlighting its role in cancer treatment response.
Area of Science:
- Oncology
- Cell Biology
- Molecular Biology
Background:
- The retinoblastoma (RB) protein is a key tumor suppressor involved in cell cycle regulation.
- Previous studies on RB's role in chemotherapy response used models with compensatory mechanisms.
- Understanding RB's function in primary adult cells is crucial for cancer therapy development.
Purpose of the Study:
- To investigate the specific role of RB in the response of primary adult fibroblasts to chemotherapeutic agents.
- To differentiate RB's function in normal adult cells versus immortalized cells with compromised p53 signaling.
- To elucidate the distinct mechanisms by which RB mediates cell cycle inhibition in response to different chemotherapeutics.
Main Methods:
- Utilized primary adult fibroblasts from mice with targeted deletion of the Rb gene (loxP sites flanking exon 3).
- Challenged cells with DNA damaging agents (camptothecin) and antimetabolites (5-fluorouracil).
- Analyzed cell cycle distribution, RB/E2F target gene expression, aneuploidy, p53 signaling, and cell viability.
Main Results:
- Targeted Rb disruption in primary adult fibroblasts caused RB/E2F target gene deregulation and aneuploidy, but not overt cell cycle arrest.
- RB was essential for DNA damage checkpoint responses and preventing aneuploidy in primary cells treated with chemotherapeutics.
- In immortalized cells lacking p53, 5-fluorouracil failed to induce arrest, while RB was still required for camptothecin-induced checkpoint response.
- Loss of RB increased p53 signaling and decreased viability in primary cells treated with camptothecin and 5-fluorouracil, via distinct mechanisms.
Conclusions:
- RB plays a critical role in DNA damage checkpoint signaling in primary adult fibroblasts.
- RB mediates chemotherapeutic-induced cell cycle inhibition through distinct mechanisms depending on the agent.
- These findings highlight RB's importance in the cellular response to genotoxic stress and chemotherapy.
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