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Loss of Rb overrides the requirement for ERK activity for cell proliferation
Giovanna M D'Abaco1, Steven Hooper, Hugh Paterson
1Cancer Research UK Centre for Cell and Molecular Biology, Institute of Cancer Research, 237 Fulham Road, London SW3 6JB, UK.
Abstract:
The Ras GTPase is a critical transducer of mitogenic signals ultimately leading to inactivation of the retinoblastoma (Rb) protein, but the molecular basis underlying Ras-dependent control of cell cycle kinetics remains to a great extent unknown. In an effort to further elucidate the role of Ras activation in cell cycle control, we have studied the role of the downstream Mek-ERK pathway in facilitating exit from the quiescent G0 state and passage through the G1/S transition. We have adopted a genetic approach in combination with U0126, an inhibitor of Mek activation to study the role of Mek in cell cycle progression. Here we report that whereas wild-type (Wt) mouse embryo fibroblasts (MEFs) depend on ERK activation to enter the cell cycle, Rb-deficient (Rb(-/-)) MEFs have a reduced requirement for ERK signalling. Indeed in the presence of U0126 we found that Rb-null MEFs can exit G0, make the G1/S transition and proliferate. Analysis of Rb-deficient tumour cell lines also revealed a reduced requirement for ERK signalling in asynchronous growth. We discuss the molecular mechanism that may underlie this escape from MAP kinase signalling.
Insights
Ras GTPase signals control cell cycle entry via the Mek-ERK pathway. Retinoblastoma (Rb) protein deficiency reduces this dependency, allowing Rb-null cells to proliferate with Mek inhibition.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- Ras GTPase is a key signal transducer for mitogenic pathways.
- Ras signaling ultimately leads to the inactivation of the retinoblastoma (Rb) protein.
- The precise molecular mechanisms of Ras-dependent cell cycle control are not fully understood.
Purpose of the Study:
- To investigate the role of the Mek-ERK pathway in cell cycle progression from the quiescent G0 state.
- To elucidate the influence of retinoblastoma protein (Rb) status on Ras-Mek-ERK signaling and cell cycle kinetics.
Main Methods:
- Utilized a genetic approach using wild-type (Wt) and Rb-deficient (Rb(-/-)) mouse embryo fibroblasts (MEFs).
- Employed U0126, a specific inhibitor of Mek activation, to assess the requirement for ERK signaling.
- Analyzed cell cycle progression, including G0 exit and G1/S transition, and asynchronous growth in tumor cell lines.
Main Results:
- Wild-type MEFs require ERK activation for cell cycle entry.
- Rb-deficient MEFs exhibit a diminished requirement for ERK signaling to exit G0 and transition through G1/S.
- Rb-null MEFs can proliferate even when Mek activation is inhibited by U0126.
- Rb-deficient tumor cell lines show reduced dependence on ERK signaling for proliferation.
Conclusions:
- The retinoblastoma protein plays a significant role in mediating the cell cycle control exerted by the Ras-MAP kinase pathway.
- Loss of Rb function can lead to an escape from strict dependence on ERK signaling for cell cycle progression.
- Further investigation into the molecular mechanisms underlying this escape is warranted.