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A novel function of adenovirus E1A is required to overcome growth arrest by the CDK2 inhibitor p27(Kip1)
K Alevizopoulos1, B Catarin, J Vlach
1Swiss Institute for Experimental Cancer Research (ISREC), CH-1066 Epalinges, Switzerland.
Abstract:
We show here that the adenovirus E1A oncoprotein prevents growth arrest by the CDK2 inhibitor p27(Kip1) (p27) in rodent fibroblasts. However, E1A neither binds p27 nor prevents inhibition of CDK2 complexes in vivo. In contrast, the amount of free p27 available to inhibit cyclin E/CDK2 is increased in E1A-expressing cells, owing to reduced expression of cyclins D1 and D3. Moreover, E1A allows cell proliferation in the presence of supraphysiological p27 levels, while c-Myc, known to induce a cellular p27-inhibitory activity, is only effective against physiological p27 concentrations. E1A also bypasses G1 arrest by roscovitine, a chemical inhibitor of CDK2. Altogether, these findings imply that E1A can act downstream of p27 and CDK2. Retinoblastoma (pRb)-family proteins are known CDK substrates; as expected, association of E1A with these proteins (but not with p300/CBP) is required for E1A to prevent growth arrest by either p27 or the CDK4/6 inhibitor p16(INK4a). Bypassing CDK2 inhibition requires an additional function of E1A: the mutant E1A Delta26-35 does not overcome p27-induced arrest, while it binds pRb-family proteins, prevents p16-induced arrest, and alleviates pRb-mediated repression of E2F-1 transcriptional activity (although E1A Delta26-35 fails to restore expression of E2F-regulated genes in p27-arrested cells). We propose that besides the pRb family, E1A targets specific effector(s) of CDK2 in G1-S control.
Insights
Adenovirus E1A oncoprotein prevents cell growth arrest by p27(Kip1) (p27) and CDK2 inhibitors. E1A acts downstream of p27 and CDK2, interacting with pRb-family proteins to control cell cycle progression.
Area of Science:
- Molecular Biology
- Cell Biology
- Virology
Background:
- Cell cycle regulation is crucial for preventing uncontrolled cell proliferation.
- Cyclin-dependent kinases (CDKs) and their inhibitors, like p27(Kip1) and p16(INK4a), are key regulators of cell cycle checkpoints.
- Adenovirus E1A oncoprotein is known to interfere with host cell cycle control.
Purpose of the Study:
- To elucidate the mechanism by which adenovirus E1A oncoprotein overcomes cell cycle arrest induced by CDK inhibitors.
- To determine the specific targets and pathways affected by E1A in relation to p27(Kip1) and CDK2.
- To investigate the role of pRb-family proteins in E1A-mediated cell cycle bypass.
Main Methods:
- Fibroblast cell culture and manipulation of oncoprotein expression.
- Western blotting to assess protein levels and interactions.
- Inhibition assays using CDK inhibitors (p27, p16, roscovitine) and mutant E1A proteins.
- Analysis of cell proliferation and gene expression.
Main Results:
- E1A prevents growth arrest by p27(Kip1) without directly binding p27 or inhibiting CDK2.
- E1A increases free p27 levels by reducing cyclin D1/D3 expression, yet still promotes proliferation.
- E1A requires interaction with pRb-family proteins to block p27 and p16(INK4a) induced arrest, but an additional function is needed to bypass CDK2 inhibition.
Conclusions:
- Adenovirus E1A oncoprotein bypasses cell cycle arrest downstream of p27 and CDK2.
- E1A utilizes pRb-family proteins and additional effectors to override CDK2-mediated G1-S control.
- These findings reveal a complex mechanism of viral oncoprotein-induced cell cycle deregulation.