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Inactivation of p27Kip1 by the viral E1A oncoprotein in TGFbeta-treated cells
1Department of Molecular Biology, Cleveland Clinic Research Institute, Ohio 44195, USA.
Abstract:
The adenovirus oncoprotein E1A and the simian virus SV40 large T antigen can both reverse the strong growth-inhibitory effect of transforming growth factor(TGF)-beta on mink lung epithelial cells: exposure of TGF-beta causes these cells to arrest late in the G1 phase of the cell cycle (ref. 3). This arrest correlates with an increase in expression of the protein p15Ink4B (ref. 4), inactivation of the cyclin E/A-cdk2 complex by the inhibitory protein p27Kip1 (refs 5-7), and with the accumulation of unphosphorylated retinoblastoma protein. The rescue by E1A of cells from TGF-beta arrest is partly independent of its binding to retinoblastoma protein. Here we show that E1A directly affects the cyclin-dependent kinase inhibitor p27Kip1 in TGF-beta-treated cells by binding to it and blocking its inhibitory effect, thereby restoring the activity of the cyclin-cdk2 kinase complex. In this way, E1A can overcome the effect of TGF-beta and modulate the cell cycle. To our knowledge, E1A provides the first example of a viral oncoprotein that can disable a cellular protein whose function is to inhibit the activity of cyclin-dependent kinases.
Insights
Adenovirus E1A protein reverses TGF-beta
Area of Science:
- Cell Biology
- Virology
- Molecular Biology
Background:
- Transforming growth factor-beta (TGF-beta) inhibits mink lung epithelial cell growth by causing G1 phase cell cycle arrest.
- This arrest involves increased p15Ink4B expression, p27Kip1 inactivation of cyclin E/A-cdk2 complexes, and retinoblastoma protein accumulation.
- Adenovirus oncoprotein E1A and SV40 large T antigen can counteract TGF-beta's growth-inhibitory effects.
Purpose of the Study:
- To investigate the mechanism by which adenovirus E1A protein rescues cells from TGF-beta-induced cell cycle arrest.
- To determine if E1A directly interacts with cell cycle regulatory proteins involved in TGF-beta signaling.
Main Methods:
- Experiments were conducted on TGF-beta-treated mink lung epithelial cells.
- The study focused on the interaction between E1A and the cyclin-dependent kinase inhibitor p27Kip1.
- Cell cycle progression and kinase activity were monitored following E1A expression.
Main Results:
- Adenovirus E1A directly binds to the cyclin-dependent kinase inhibitor p27Kip1 in TGF-beta-treated cells.
- This binding blocks the inhibitory function of p27Kip1, restoring cyclin-cdk2 kinase complex activity.
- E1A's interaction with p27Kip1 is crucial for overcoming TGF-beta-mediated cell cycle arrest.
Conclusions:
- Adenovirus E1A modulates the cell cycle by directly disabling the cellular inhibitor p27Kip1.
- This represents the first identified viral oncoprotein that targets cyclin-dependent kinase inhibitors.
- E1A's mechanism provides insight into viral strategies for manipulating host cell cycle control.